AHSS Ver3

Online at www.worldautosteel.
org
ADVANCED HIGH STRENGTH STEEL (AHSS)
APPLICATION GUIDELINES
Version 3
Prepared by
INTERNATIONAL IRON & STEEL INSTITUTE
Committee on Automotive Applications
September 2006
Online at www.worldautosteel.org

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Table of Contents
Preface............................................................................................................................ v
Section 1 General Description of AHSS .......................................................................1-1
1.A. Definitions .................................................................................................................... 1-1
1.B. Metallurgy of AHSS ...................................................................................................... 1-3
1.B.1. Dual Phase (DP) Steel ........................................................................................ 1-4
1.B.2. Transformation-Induced Plasticity (TRIP) Steel .................................................. 1-5
1.B.3. Complex Phase (CP) Steel ................................................................................. 1-6
1.B.4. Martensitic (MS) Steel ......................................................................................... 1-6
1.B.5. Ferritic-Bainitic (FB) Steel ................................................................................... 1-7
1.B.6. Twinning-Induced Plasticity (TWIP) Steel ........................................................... 1-7
1.B.7. Hot-Formed (HF) Steel........................................................................................ 1-8
1.B.8. Post-Forming Heat-Treatable (PFHT) Steel ........................................................ 1-8
1.B.9. Evolving AHSS Types...........................................................................................1-9
1.C. Conventional Low- & High-Strength Automotive Sheet Steels..................................... 1-9
1.C.1. Mild steels........................................................................................................... 1-9
1.C.2. Interstitial-free (IF) steels (Low strength and high strength) ............................... 1-9
1.C.3. Bake hardenable (BH) steels.............................................................................. 1-9
1.C.4. Isotropic (IS) steels ............................................................................................. 1-9
1.C.5. Carbon-manganese (CM) steels......................................................................... 1-9
1.C.6. High-strength low-alloy (HSLA) steels ................................................................ 1-9
Section 2 Forming ........................................................................................................2-1
2.A. General Comments...................................................................................................... 2-1
2.B. Computerized Forming-Process Development............................................................ 2-2
2.C. Sheet Forming............................................................................................................. 2-4
2.C.1. Mechanical Properties ........................................................................................ 2-4
2.C.1.a. Yield Strength - Total Elongation Relationships ......................................... 2-4
2.C.1.b. Tensile Strength - Total Elongation Relationships ...................................... 2-5
2.C.1.c. Work Hardening Exponent (n-value) .......................................................... 2-5
2.C.1.d. Stress-Strain Curves .................................................................................. 2-9
2.C.1.e. Normal Anisotropy Ratio ( or rm) ............................................................ 2-15
2.C.1.f. Strain Rate Effects .................................................................................... 2-15
2.C.1.g. Bake Hardening and Aging...................................................................... 2-17
2.C.1.h. Key Points................................................................................................ 2-18
2.C.2. Forming Limits .................................................................................................. 2-18
2.C.2.a. Forming Limit Curves (FLC)..................................................................... 2-18
2.C.2.b. Sheared Edge Stretching Limits............................................................... 2-20
2.C.2.c. Key Points ................................................................................................ 2-24
2.C.3. Forming Modes................................................................................................. 2-24
2.C.3.a. Stretching................................................................................................. 2-24
2.C.3.b. Deep Drawing (Cup Drawing) .................................................................. 2-28
2.C.3.c. Bending.................................................................................................... 2-30
2.C.3.d. Roll Forming ............................................................................................ 2-31
2.C.3.e. Key Points................................................................................................ 2-33
2.C.4. Tool Design....................................................................................................... 2-34
2.C.4.a. Tool Materials........................................................................................... 2-34

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2.C.4.b. Tool Design Issues................................................................................... 2-36
2.C.4.c. Prototype Tools ........................................................................................ 2-38
2.C.4.d. Key Points................................................................................................ 2-38
2.C.5. Springback........................................................................................................ 2-39
2.C.5.a. Origins of Springback............................................................................... 2-40
2.C.5.b. Types of Springback................................................................................. 2-41
2.C.5.c. Springback Correction.............................................................................. 2-46
2.C.6. Blanking, Shearing, and Trim Operations ......................................................... 2-55
2.C.6.a. General Comments.................................................................................. 2-55
2.C.6.b. Tool Wear, Clearances, and Burr Height.................................................. 2-55
2.C.6.c. Key Points ................................................................................................ 2-57
2.C.7. Press Requirements ......................................................................................... 2-57
2.C.7.a. Force versus Energy................................................................................ 2-57
2.C.7.b. Prediction of Press Forces Using Simulative Tests.................................. 2-60
2.C.7.c. Extrapolation From Existing Production Data .......................................... 2-60
2.C.7.d. Computerized Forming-Process Development........................................ 2-61
2.C.7.e. Case Study for Press Energy................................................................... 2-62
2.C.7.f. Setting Draw Beads .................................................................................. 2-64
2.C.7.g. Key Points................................................................................................ 2-64
2.C.8. Multiple Stage Forming..................................................................................... 2-64
2.C.8.a. General Recommendations: .................................................................... 2-64
2.C.8.b. Key Points................................................................................................ 2-65
2.C.9. In-service Requirements ................................................................................... 2-65
2.C.9.a. Crash Management ................................................................................. 2-66
2.C.9.b. Fatigue..................................................................................................... 2-67
2.C.9.c. Key Points ................................................................................................ 2-68
2.D. Tube Forming ............................................................................................................ 2-69
2.D.1. High Frequency Welded Tubes ........................................................................ 2-69
2.D.2. Laser Welded Tailored Tubes ........................................................................... 2-72
2.D.3. Key Points......................................................................................................... 2-74
Section 3 J oining..........................................................................................................3-1
3.A. General Comments...................................................................................................... 3-1
3.B. Welding Procedures..................................................................................................... 3-2
3.B.1. Resistance Welding ............................................................................................ 3-2
3.B.1.a. Weld Schedule........................................................................................... 3-2
3.B.1.b. Heat Balance - Material Balance - Thickness Balance .............................. 3-5
3.B.1.c. Welding Current Mode ............................................................................... 3-6
3.B.1.d. Electrode Geometry ................................................................................... 3-7
3.B.1.e. Part Fit-up .................................................................................................. 3-8
3.B.1.f. Factory Equipment Template ...................................................................... 3-8
3.B.1.g. Weld Evaluation by Carbon Equivalence................................................... 3-8
3.B.1.h. Zinc Penetration/Contamination................................................................. 3-8
3.B.1.i. Weld Integrity: Test Method and J oint Performance.................................... 3-9
3.B.2 High Frequency Induction Welding.................................................................... 3-14
3.B.3. Laser Welding - Fusion..................................................................................... 3-16
3.B.3.a. Butt Welds and Tailor Welded Products ................................................... 3-16
3.B.3.b. Assembly Laser Welding.......................................................................... 3-18
3.B.4. Arc Welding Uncoated Steels Fusion............................................................. 3-19
3.C. Brazing ...................................................................................................................... 3-22
Table of Contents

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3.D. Adhesive Bonding...................................................................................................... 3-23
3.E. Mechanical J oining .................................................................................................... 3-24
3.F. Hybrid J oining............................................................................................................. 3-26
3.G. Material Issues For Field Weld Repair and Replacement.......................................... 3-27
Section 4 Glossary .......................................................................................................4-1
Section 5 References ...................................................................................................5-1
Table of Contents
Preface

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Revised: 18 Jul 2006
Preface
Recent years have seen many new developments in steel technology and manufacturing processes
to build vehicles of reduced mass and increased safety with steel. The ULSAB (UltraLight Steel Auto Body)
and ULSAB-AVC (Advanced Vehicle Concepts) programmes, sponsored by the global steel industry, focused
attention on advances in lightweight design concepts and on more extensive use of Advanced High-Strength
Steel (AHSS) a key enabling factor in lightweight design. Along with the advantages of these newer steels
came the need to provide technical advice for forming and joining techniques.
AHSS Application Guidelines described in this document are the result of a cooperative effort by experts
from steel company members of the International Iron and Steel Institute (IISI). We gratefully acknowledge
the input of many people around the world in particular, those listed here who were the core working group
for creation of this latest version:
Dr Heiko Beenken ThyssenKrupp Steel
Mr Willie Bernert Dofasco
Mr Klaus Blmel ThyssenKrupp Steel
Dr Bjrn Carlsson SSAB Tunnplt
Dr J ayanth Chintamani Mittal Steel
Mr Bart DePompolo United States Steel Corporation
Mr Daniel Eriksson SSAB Tunnplt
Mr Peter Heidbchel ThyssenKrupp Steel
Mr Makoto Imanaka J FE Holdings
Dr Andre Krff Salzgitter Mannesmann Forschung
Dr Sree Harsha Lalam Mittal Steel
Mr Andy Lee Dofasco
Mr Stephen Lynes Dofasco
Mr Martin Munier Arcelor
Mr Tony Nilsson SSAB Tunnplt
Mr J uha Nuutinen Rautaruukki Oyj
Mr Chuck Potter American Iron and Steel Institute
Mr Pekka Ritakallio Rautaruukki Oyj
Dr Ming Shi United States Steel Corporation
Mr J ohn Szalla BlueScope Steel
Mr Andy Taylor Corus
Mr Muriali Tumuluru United States Steel Corporation
Mr Christian Walch voestalpine Stahl
Dr Kazumasa Yamazaki Nippon Steel Corporation
A special note of appreciation goes to Dr Stuart Keeler of Keeler Technologies LLC who is the Technical
Editor. Dr Keeler is a widely known expert, author and lecturer in the field of metal forming and application.
He provided valuable input and coordination of the complete AHSS Application Guidelines document.
These Guidelines and other IISI information can be found at www.worldautosteel.org. IISI steel companies
who sponsor this work are listed on the next page.
Edward G. Opbroek
Director, Automotive
International Iron and Steel Institute
Preface

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Company members of the International Iron and Steel Institute (IISI) who work through the commit-
tee on automotive applications (AutoCo) to sponsor this work are as follows:
Arcelor - Luxembourg
Baoshan Iron & Steel Co. Ltd. - China
BlueScope Steel - Australia
China Steel Corporation - China
Corus Group - The Netherlands/United Kingdom
Dofasco Inc. - Canada
Essar Steel Ltd. - India
JFE Holdings, Inc. - Japan
Kobe Steel, Ltd. - Japan
Mittal Steel - USA/South Africa
Nippon Steel Corporation - Japan
Nucor Corporation - USA
Pohang Iron and Steel Co., Ltd (POSCO) - Korea
Rautaruukki Oyj - Finland
Salzgitter AG - Germany
Severstal JSC - Russia
Severstal North American, Inc. - USA
SSAB Tunnplt AB - Sweden
Sumitomo Metal Industries, Ltd. - Japan
Tata Steel - India
ThyssenKrupp Stahl AG - Germany
Usinas Siderrgicas de Minas Gerais S.A. (USIMINAS) - Brazil
United States Steel Corporation - USA
voestalpine Stahl GmbH - Austria
Sec t i on 1 Sec t i on 1
Sec t i on 1 Sec t i on 1 Sec t i on 1
Gener Gener
Gener Gener Gener al al
al al al
Desc r i pt i on of Desc r i pt i on of
Desc r i pt i on of Desc r i pt i on of Desc r i pt i on of
AHSS AHSS
AHSS AHSS AHSS

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Revised: 18 Jun 2006
Section 1 - General Description of AHSS
The Advanced High-Strength Steel (AHSS) Application Guidelines focus on press-forming, fabri-
cation, and joining processes for automotive underbody, structural, and body panels designed for
higher strength steels. When selecting conventional high-strength steels to replace mild steel or
other traditional grades, reduced formability is often one of the unwelcome consequences. To
overcome this, and to further achieve lower mass automotive structures, recent steel develop-
ments have targeted improvements in formability. The AHSS family of multi-phase microstructures
typifies the steel industrys response to the demand for improved materials that utilize proven
production methods. These engineered materials address the automotive industrys need for steels
with both higher strength and enhanced formability.
1.A. Definitions
Automotive steels can be classified in several different ways. One is by a metallurgical designa-
tion. Common designations include low-strength steels (interstitial-free and mild steels); conven-
tional HSS (carbon-manganese, bake hardenable, high-strength interstitial-free, and high-strength,
low-alloy steels); and the newer types of AHSS (dual phase, transformation-induced plasticity,
complex phase, and martensitic steels). Additional higher strength steels for the automotive mar-
ket include ferritic-bainitic, twinning-induced plasticity, nano, hot-formed, and post-forming heat-
treated steels.
A second classification method important to part designers is strength of the steel. Therefore, this
document will use the general terms HSS and AHSS to designate all higher strength steels. In
contrast, much of the current literature uses narrowly defined ranges to categorize different steel
strength levels. One such system defines High-Strength Steels (HSS) as yield strengths from 210
to 550 MPa and tensile strengths from 270700 MPa, while Ultra-High-Strength Steels (UHSS)
steels have yield strengths greater than 550 MPa and tensile strengths greater than 700 MPa.
These arbitrary ranges suggest discontinuous changes when moving from one category to an-
other. However, data show property changes are a continuum across the entire span of steel
strengths. In addition, many steel types have a wide range of grades covering two or more strength
ranges.
A third classification method presents various mechanical properties or forming parameters of
different steels, such as total elongation, work hardening exponent n, or hole expansion ratio. As
an example, Figure 1-1 compares total elongations a steel property related to formability for the
different metallurgical types of steel. Figure 1-1A shows the lower strength steels in dark grey and
the traditional HSS steels in light grey. Some of the early AHSS steels are shown in colour. Figure
1-1B highlights some of the newer higher strength steels for the automotive market. Figures 1-1A
and 1-1B illustrate only the relative comparison of different steel grades not specific property
ranges of each type.

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Figure 1-1A - Schematic of AHSS steels (shown in colour) compared to low strength steels
(dark grey) and traditional HSS (light grey).
I-1
Figure 1-1B Schematic of newer higher strength steels utilizing unique chemistries,
processing, and microstructure to gain more specific properties and forming characteristics.
I-2
The principal difference between conventional HSS and AHSS is their microstructure. Conventional
HSS are single phase ferritic steels. AHSS are primarily multi-phase steels, which contain ferrite,
martensite, bainite, and/or retained austenite in quantities sufficient to produce unique mechanical
properties. Some types of AHSS have a higher strain hardening capacity resulting in a strength-
ductility balance superior to conventional steels. Other types have ultra-high yield and tensile
strengths and show a bake hardening behaviour.

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Since the terminology used to classify steel products varies considerably throughout the world,
this document uses a combination of methods to define the steels. Each steel grade is identified by
metallurgical type, yield strength (in MPa), and tensile strength (in MPa). As an example, DP 500/
800 means a dual phase steel type with 500 MPa minimum yield strength and 800 MPa minimum
ultimate tensile strength. This classification system was used in the ULSAB-AVC (UltraLight Steel
Auto Body Advanced Vehicle Concepts) Program.
I-1
Table 1-1 illustrates a range of AHSS grades used in the ULSAB-AVC body-structure concept
design.
Table 1-1 - Examples of Steel Grade Properties from ULSAB-AVC.
I-1
YS and UTS are minimum values
Tot. EL (Total Elongation) is a typical value for a broad range of thicknesses and gage lengths.
It is important to note that different specification criteria have been adopted by different automotive
companies throughout the world and that steel companies have different production capabilities
and commercial availability. Therefore, typical mechanical properties are shown above simply to
illustrate the broad range of AHSS grades that may be available. It is imperative to communicate
directly with individual steel companies to determine specific grade availability and the specific
associated parameters and properties, such as:
Mechanical properties and ranges.
Thickness and width capabilities.
Hot-rolled, cold-rolled, and coating availability.
Chemical composition specifications.
1.B. Metallurgy of AHSS
The fundamental metallurgy of conventional low- and high-strength steels is generally well
understood by manufacturers and users of steel products. Brief descriptions of common steel
types are given in Section 1.C. Since the metallurgy and processing of AHSS grades are somewhat
novel compared to conventional steels, they will be described here to provide a baseline
understanding of how their remarkable mechanical properties evolve from their unique processing
and structure. All AHSS are produced by controlling the cooling rate from the austenite or austenite

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plus ferrite phase, either on the runout table of the hot mill (for hot-rolled products) or in the cooling
section of the continuous annealing furnace (continuously annealed or hot-dip coated products).
1.B.1. Dual Phase (DP) Steel
DP steels consist of a ferritic matrix containing a
hard martensitic second phase in the form of
islands. Increasing the volume fraction of hard
second phases generally increases the strength.
DP (ferrite plus martensite) steels are produced
by controlled cooling from the austenite phase (in
hot-rolled products) or from the two-phase ferrite
plus austenite phase (for continuously annealed
cold-rolled and hot-dip coated products) to
transform some austenite to ferrite before a rapid
cooling transforms the remaining austenite to
martensite. Depending on the composition and
process route, hot-rolled steels requiring enhanced
capability to resist stretching on a blanked edge
(as typically measured by hole expansion capacity)
can have a microstructure containing significant
quantities of bainite.
Figure 1-2 shows a schematic microstructure of
DP steel,which contains ferrite plus islands of martensite. The soft ferrite phase is generally
continuous, giving these steels excellent ductility. When these steels deform, strain is concentrated
in the lower-strength ferrite phase surrounding the islands of martensite, creating the unique high
work-hardening rate exhibited by these steels.
Figure 1-2 - Schematic shows islands of
martensite in a matrix of ferrite.
The work hardening rate plus excellent elongation
give DP steels much higher ultimate tensile
strengths than conventional steels of similar yield
strength. Figure 1-3 compares the engineering
stress-strain curves for HSLA steel to a DP steel
of similar yield strength. The DP steel exhibits
higher initial work hardening rate, higher ultimate
tensile strength, and lower YS/TS ratio than the
similar yield strength HSLA.
Figure 1-3 - The DP 350/600 with higher TS than
the HSLA 350/450.
K-1
DP and other AHSS also have a bake hardening effect that is an important benefit compared to
conventional steels. The bake hardening effect is the increase in yield strength resulting from
elevated temperature aging (created by the curing temperature of paint bake ovens) after prestraining
(generated by the work hardening due to deformation during stamping or other manufacturing
process). The extent of the bake hardening effect in AHSS depends on the specific chemistry and
thermal histories of the steels. Additional bake hardening information is located in Section 2.C.1.g.
In DP steels, carbon enables the formation of martensite at practical cooling rates by increasing
the hardenability of the steel. Manganese, chromium, molybdenum, vanadium, and nickel, added

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individually or in combination, also help increase hardenability. Carbon also strengthens the
martensite as a ferrite solute strengthener, as do silicon and phosphorus. These additions are
carefully balanced, not only to produce unique mechanical properties, but also to maintain the
generally good resistance spot welding capability. However, when welding the highest strength
grade (DP 700/1000) to itself, the spot weldability may require adjustments to the welding practice.
1.B.2. Transformation-Induced Plasticity (TRIP) Steel
Figure 1-4 Bainite and retained austenite are
additional phases in TRIP steels.
The microstructure of TRIP steels is retained
austenite embedded in a primary matrix of ferrite.
In addition to a minimum of 5 volume percent of
retained austenite, hard phases such as
martensite and bainite are present in varying
amounts. TRIP steels typically require the use of
an isothermal hold at an intermediate temperature,
which produces some bainite. The higher silicon
and carbon content of TRIP steels also result in
significant volume fractions of retained austenite
in the final microstructure. A schematic of TRIP
steel microstructure is shown in Figure 1-4.
During deformation, the dispersion of hard second
phases in soft ferrite creates a high work hardening
rate, as observed in the DP steels. However, in
TRIP steels the retained austenite also
progressively transforms to martensite with
increasing strain, thereby increasing the work
hardening rate at higher strain levels. This is
illustrated in Figure 1-5, where the engineering
stress-strain behaviour of HSLA, DP and TRIP
steels of approximately similar yield strengths are
compared. The TRIP steel has a lower initial work
hardening rate than the DP steel, but the
hardening rate persists at higher strains where
work hardening of the DP begins to diminish.
The work hardening rates of TRIP steels are
substantially higher than for conventional HSS,
providing significant stretch forming. This is
particularly useful when designers take advantage
of the high work hardening rate (and increased
bake hardening effect) to design a part utilizing
the as-formed mechanical properties. The high
work hardening rate persists to higher strains in
TRIP steels, providing a slight advantage over DP
in the most severe stretch forming applications.
Figure 1-5 - TRIP 350/600 with a greater total
elongation than DP 350/600 and HSLA 350/450.
K-1

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TRIP steels use higher quantities of carbon than DP steels to obtain sufficient carbon content for
stabilizing the retained austenite phase to below ambient temperature. Higher contents of silicon
and/or aluminium are used to accelerate the ferrite/bainite formation. Thus these elements assist
in maintaining the necessary carbon content within the retained austenite. Suppressing the carbide
precipitation during bainitic transformation appears to be crucial for TRIP steels. Silicon and
aluminium are used to avoid carbide precipitation in the bainite region.
The strain level at which retained austenite begins to transform to martensite can be designed by
adjusting the carbon content. At lower carbon levels, the retained austenite begins to transform
almost immediately upon deformation, increasing the work hardening rate and formability during
the stamping process. At higher carbon contents, the retained austenite is more stable and begins
to transform only at strain levels beyond those produced during forming. At these carbon levels the
retained austenite persists into the final part. It transforms to martensite during subsequent
deformation, such as a crash event.
TRIP steels can therefore be engineered or tailored to provide excellent formability for manufacturing
complex AHSS parts or to exhibit high work hardening during crash deformation to provide excellent
crash energy absorption. The additional alloying requirements of TRIP steels degrade their resistance
spot-welding behaviour. This can be addressed somewhat by modification of the welding cycles
used (for example, pulsating welding or dilution welding).
1.B.3. Complex Phase (CP) Steel
CP steels typify the transition to steel with very
high ultimate tensile strengths. The microstructure
of CP steels contains small amounts of martensite,
retained austenite and pearlite within the ferrite/
bainite matrix. An extreme grain refinement is
created by retarded recrystallization or
precipitation of microalloying elements like Ti or
Cb. In comparison with DP- steels, CP steels
show significantly higher yield strengths at equal tensile strengths of 800 MPa and greater. CP-
steels are characterized by high energy absorption and high residual deformation capacity.
1.B.4. Martensitic (MS) Steel
To create MS steels, the austenite that exists during
hot-rolling or annealing is transformed almost
entirely to martensite during quenching on the run-
out table or in the cooling section of the continuous
annealing line. The MS steels are characterized
by a martensitic matrix containing small amounts
of ferrite and/or bainite. Within the group of
multiphase steels, MS steels show the highest
tensile strength level. This structure can also be developed with post-forming heat treatment. MS
steels provide the highest strengths, up to 1700 MPa ultimate tensile strength. MS steels are often
subjected to post-quench tempering to improve ductility, and can provide adequate formability
even at extremely high strengths.

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Carbon is added to MS steels to increase hardenability and for strengthening the martensite.
Manganese, silicon, chromium, molybdenum, boron, vanadium, and nickel are also used in various
combinations to increase hardenability. MS steels are produced from the austenite phase by rapid
quenching to transform most of the austenite to martensite. CP steels also follow a similar cooling
pattern, but here the chemistry is adjusted to produce less retained austenite and form fine
precipitates to strengthen the martensite and bainite phases.
1.B.5. Ferritic-Bainitic (FB) Steel
FB steels are sometimes called Stretch Flangeable
(SF) or High Hole Expansion (HHE) steels
because of their improved edge stretch capability.
FB steels have a microstructure of fine ferrite and
bainite. Strengthening is obtained by both grain
refinement and the second phase hardening with
bainite. FB steels are available as hot-rolled
products.
The primary advantage of FB steels over HSLA and DP steels is the improved stretchability of
sheared edges as measured by the hole expansion test. Compared to HSLA steels with the same
level of strength, FB steels also have a higher strain hardening exponent (n) and increased total
elongation.
Because of their good weldability, FB steels are considered for tailored blank applications. These
steels are characterized by both good crash performances and good fatigue properties.
1.B.6. Twinning-Induced Plasticity (TWIP) Steel
TWIP steels have high manganese content (17-
24%) that causes the steel to be fully austenitic at
room temperatures. This causes the principal
deformation mode to be twinning inside the grains.
The twinning causes a high value of the
instantaneous hardening rate (n value) as the
microstructure becomes finer and finer. The
resultant twin boundaries act like grain boundaries
and strengthen the steel. TWIP steels combine
extremely high strength with extremely high formability. The n value increases to a value of 0.4 at
an approximate engineering strain of 30% and then remains constant until a total elongation around
50%. The tensile strength is higher than 1000 MPa.

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1.B.8. Post-Forming Heat-Treatable (PFHT) Steel
Optimized part geometries with intricate shapes
and no springback issues are being accomplished
by hot-forming quench hardenable steels at
temperatures above the austenitic region (900-
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During processing, three states with different mechanical properties are important.
Ellipse 1: Tensile strengths up to 600 MPa at room temperature must be considered for
the design of blanking dies.
Ellipse 2: High elongations (more than 50%) and low strengths at deformation
temperatures allow forming of complex shapes. A special coating based on aluminium
and silicium is recommended to avoid surface oxidation of the product after forming.
Ellipse 3: Following forming, strengths above 1300 MPa are achieved after quenching
in the die. Special processes must be taken into account when finishing the product (no
additional forming, special cutting and trimming devices, etc.).
Typical cycle time is 20 to 30 seconds for each press cycle. However, several parts can be stamped
at the same time so that 2 or more parts can be obtained per cycle. Hot-forming boron steels are
most commonly used for safety and structural parts.
1.B.7. Hot-Formed (HF) Steel
One process is water quenching of inexpensive steels with chemistries that allow in-part strengths
between 900 and 1,400 MPa tensile strength. In addition, some zinc coatings can survive the heat
treating cycle because the time at temperature is very short. The wide assortment of chemistries to
meet specific part specifications requires extra special coordination with the steel supplier.
Another process is air-hardening of alloyed tempering steels that feature very good forming properties
in the soft-state (deep-drawing properties) and high strength after heat treatment (air-hardening). Apart
from direct application as sheet material, air-hardening steels are suitable for tube welding. These
Post-forming heat treatment is a general method
to develop an alternative higher strength steel. The
major issue holding back widespread
implementation of HSS typically has been
maintaining part geometry during and after the heat
treatment process. Fixturing the part and then
heating (furnace or induction) and immediate
quenching appear to be a solution with production
applications. In addition, the stamping is formed
at a lower strength (ellipse 1) and then raised to a
tubes are excellent for hydroforming applications. The components can be heat treated in the furnace
in a protective gas atmosphere (austenitized) and then hardened and tempered during natural cooling
in air or a protective gas. The very good hardenability and resistance to tempering is achieved by
adding, in addition to carbon and manganese, other alloying elements such as chrome, molybdenum,
vanadium, boron, and titanium. The steel is very easy to weld in both its soft and air-hardened states,
as well as in the combination of soft/air-hardened. This steel responds well to coating using standard
coating methods (conventional batch galvanizing and high-temperature batch galvanizing).
much higher strength by heat treatment (ellipse 2).

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1.C. Conventional Low- & High-Strength Automotive Sheet Steels
1.C.1. Mild steels
Mild steels have an essentially ferritic microstructure. Drawing Quality (DQ) and Aluminium Killed
(AKDQ) steels are examples and often serve as a reference base because of their widespread
application and production volume.
1.C.2. Interstitial-free (IF) steels (Low strength and high strength)
IF steels have ultra-low carbon levels designed for low yield strengths and high work hardening
exponents. These steels are designed to have more stretchability than Mild steels. Some grades
of IF steels are strengthened by a combination of elements for solid solution, precipitation of carbides
and/or nitrides, and grain refinement. Another common element added to increase strength is
phosphorous (a solid solution strengthener). The higher strength grades of IF steel type are widely
used for both structural and closure applications.
1.C.3. Bake hardenable (BH) steels
BH steels have a basic ferritic microstructure and are strengthened primarily by solid solution
strengthening. A unique feature of these steels is the chemistry and processing designed to keep
carbon in solution during steelmaking and then allowing this carbon to come out of solution during
paint baking. This increases the yield strength of the formed part.
1.C.4. Isotropic (IS) steels
IS steels have a basic ferritic type of microstructure. The key aspect of these steels is the delta r
value equal to zero, resulting in minimized earing tendencies.
1.C.5. Carbon-manganese (CM) steels
Higher strength CM steels are primarily strengthened by solid solution strengthening.
1.C.6. High-strength low-alloy (HSLA) steels
This group of steels are strengthened primarily by micro-alloying elements contributing to fine
carbide precipitation and grain-size refinement.
martensite, the ferrite matrix is strengthened with ultra-fine nano-sized (<10 Nm) particles. This is
accomplished in hot-rolled high strength steel with a tensile strength around 750 MPa. The resulting
steel has a high YS/TS ratio with an excellent balance of total elongation and local elongation (hole
expansion ratio). Other examples of these developing steels are ultrafine grain, low density, and
high Youngs modulus steels.
1.B.9. Evolving AHSS Types
In response to automotive demands for additional AHSS capabilities, steel industry research
continues to develop new types of steel. These steels are designed to reduce density, improve
strength, and/or increase elongation. For example, Nano steels are designed to avoid the low
values of edge stretch (local elongation) experienced by DP and TRIP steels. Instead of islands of
F F
F FFor or
or or or mi ng mi ng
mi ng mi ng mi ng

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Section 2 - Forming
2.A. General Comments
Forming of AHSS is not a radical change from forming conventional HSS. The major acquisition of
new knowledge and experience needed for forming higher strength steels in general has been
gained gradually over the years with ever-increasing strengths available in the HSLA grades. Now
new demands for improved crash performance, while reducing mass and cost, have spawned a
new group of steels that improve on the current conventional base of HSS.
The AHSS solve two distinct automotive needs by two different groups of steels. The first group as
a class has higher strength levels with improved formability and crash-energy absorption com-
pared to the current HSLA grades. This requirement is fulfilled by the DP and TRIP grades of steel,
which have increased values of the work hardening exponent. The second is to extend the avail-
ability of steel in strength ranges above the HSLA grades. This area is covered by the CP and MS
grades. Originally targeted only for chassis, suspension, and body-in-white components, AHSS
are now being applied to doors and other body panels. Additional steels highlighted previously in
Figure 1-1B are designed to meet specific process requirements. These include increased edge
stretch flangeability, strengthening after forming, or increased springback tolerances.
The improved capabilities the AHSS bring to the automotive industry do not bring new forming
problems but certainly accentuate problems already existing with the application of any higher
strength steel. These concerns include higher loads on presses and tools, greater energy require-
ments, and increased need for springback compensation and control. In addition, AHSS have
greater tendency to wrinkle due to lack of adequate hold-down and often a reduction in sheet
thickness.
The Applications Guidelines document utilizes a steel designation system to minimize regional
confusion about the mechanical properties when comparing AHSS to conventional high-strength
steels. The format is Steel Type YS/TS in MPa. Therefore, HSLA 350/450 would have minimum
yield strength of 350 MPa and minimum tensile strength of 450 MPa. The designation also high-
lights different yield strengths for steel grades with equal tensile strengths, thereby allowing some
assessment of the stress-strain curves and amount of work hardening.
Matching exact mechanical properties of the intended steel grade against the critical forming mode
in the stamping not only requires an added level of knowledge by steel suppliers and steel users,
but mandates an increased level of communication between them. A specific example is total
elongation versus local elongation. Total elongation has been the traditional measure of the steels
general stretchability over wide areas of the stamping required length of line deformation. Now,
local elongation over very small gauge lengths found in stretch flanging, hole expansion, and
blanked edge extension is as important as total elongation. The modification of microstructure to
create DP and TRIP steels for increased work hardening exponent, greater stretchability and crash
energy absorption, and higher total elongations reduces local elongation and edge stretchability
and vice versa.
New emphasis is being placed on determining specific needs of the stamping, highlighting critical
forming modes, and identifying essential mechanical properties. The interaction of all inputs to the
Section 2 - Forming

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forming system means the higher loads and energy needs of AHSS also place new requirements
on press capacity, tool construction/protection, lubricant capabilities, process design, and
maintenance.
To this end, the Forming Section of these Guidelines addresses the mechanical properties, forming
limits, and forming modes before covering the more traditional areas of tooling, springback, and
press loads. Most data and experience are available for DP steels that have been in production
and automotive use for some period of time. Less experience has been acquired with the TRIP
steels that are now transitioning from the research phase to production.
2.B. Computerized Forming-Process Development
Using software to evaluate sheet metal formability has been in industrial use (as opposed to university
and research environments) for more than a decade. The current sheet metal forming programs
are part of a major transition to virtual manufacturing that includes analysis of welding, casting
solidification, molding of sheet/fiber compounds, automation, and other manufacturing processes.
Computer simulation of sheet metal forming is more correctly identified as computerized forming-
process development or even computerized die tryout. The more highly developed software
programs closely duplicate the forming of sheet metal stampings as they would be done physically
in the press shop.
For conventional steels these programs have proven to be very accurate in blank movement,
strains, thinning, forming severity, wrinkles, and buckles. Prediction of springback generally provides
qualitatively helpful results. However, the magnitude of the springback probably will lack some
accuracy and will depend highly on the specific stamping, the input information, and user experience.
Traditionally the software uses the simple power law of work hardening that treats the n value as a
constant. For use with AHSS, the codes should treat the n value as a function of strain. Most
commercial software now have the ability to process the true stress true strain curve for the steel
being evaluated without the need for a constitutive equation. However, this capability is not present
in some proprietary industrial and university software and caution must be taken before using this
software to analyze stampings formed from AHSS.
Computerized forming-process development is ideally suited to the needs of current and potential
users of AHSS. A full range of analysis capabilities is available to evaluate AHSS as a new stamping
analysis or to compare AHSS stampings to conventional Mild steel stampings. These programs
allow rapid what-if scenarios to explore different grades of AHSS, alternative processing, or even
design optimization.
The potential involvement of software-based AHSS process development is shown in Figure 2-1.
At the beginning of the styling to production cycle, the key question is whether the stamping can
even be made. With only the CAD file of the final part and material properties, the One-Step or
Inverse codes can rapidly ascertain strain along section lines, thinning, forming severity, trim line-
to-blank, hot spots, blank contour, and other key information.
Section 2 - Forming

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Figure 2-1 - Schematic showing utilization of computerized forming-
process development to assist in forming stampings from AHSS.
During selection of process and die design parameters, the software will evaluate how each new
input not only affects the outputs listed in the previous paragraph, but also will show wrinkles and
generate a press-loading curve. The most useful output of the analysis is observing (like a video)
the blank being deformed into the final part through a transparent die. Each frame of the video is
equivalent to an incremental hit or breakdown stamping. Problem areas or defects in the final
increment of forming can be traced backwards through the forming stages to the initiation of the
problem. The most comprehensive software allows multi-stage forming, such as progressive dies,
transfer presses, or tandem presses. The effects of trimming and other offal removal on the
springback of the part are documented.
Since many applications of AHSS involve load bearing or crash analyses, computerized forming-
process development has special utilization in structural analysis. Previously the part and assembly
designs were analyzed for static and dynamic capabilities using CAD stampings with initial sheet
thickness and as-received yield strength. Often the tests results from real parts did not agree with
the early analyses because real parts were not analyzed. Now virtual parts are generated with
point to point sheet thickness and strength levels nearly identical to those that will be tested when
the physical tooling is constructed. Deficiencies of the virtual parts can be identified and corrected
by tool, process, or even part-design before tool construction has even begun.
Section 2 - Forming

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2.C. Sheet Forming
2.C.1. Mechanical Properties
By combining a number of different microstructures not traditionally found in conventional HSS, a
wide range of properties are possible with AHSS. This allows steel companies to tailor the processing
to meet the ever more focused application requirements demanded by the automotive industry.
Comparing these AHSS to their conventional HSS counterparts becomes much more difficult. The
same minimum tensile strength can be found with a variety of steel types having different yield
strengths. One example is TRIP 450/800, DP 500/800, and CP 700/800 steels with the same
minimum tensile strength with different yield strengths and typical total elongations in the range of
29%, 17%, and 13%, respectively. Some AHSS steels have their properties determined when the
steel is produced. However, the properties of TRIP change during deformation as the retained
austenite transforms to islands of martensite. The amount and rate of this transformation depends
on the type and amount of deformation, the strain rate, the temperature of the sheet metal, and
other conditions unique to the specific part, tool, and press. In contrast, a large range of HF and
PFHT steel types generate their final properties though some form of quench operation only after
forming has been completed.
AHSS property data contained in this section cover general trends and reasons why these trends
differ from conventional HSS. Specific data can only be obtained by selecting the exact type,
grade, and thickness of AHSS and then contacting the steel supplier for properties expected with
their processing of the order. Data for the newer TWIP, FB, and Nano steels are limited and therefore
can only be briefly noted.
2.C.1.a. Yield Strength - Total Elongation Relationships
A large range of yield strengths are available for the AHSS. Stretching is related to the total elongation
obtained in a standard tensile test. Figure 2-2 shows the general relationship between yield strength
and total elongation for AHSS compared to other high-strength steels.
Figure 2-2 - Relationship between yield strength and total elongation (50.8 mm
gauge length) for various types of steel.
I-1
Section 2 - Forming

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Note that the families of DP, CP, and TRIP steels generally have higher total elongations than
HSLA steels of equal yield strengths.
Most AHSS steels have no yield point elongation. Some samples of higher strength DP grades
and TRIP steels may show YPE but the value typically should be less than 1%. These values are
in contrast with various HSLA grades, which can have YPE values greater than 5%.
2.C.1.b. Tensile Strength - Total Elongation Relationships
The relationship between ultimate tensile strength and total elongation for the various types of
steels in Figure 2-3 parallels that observed in Figure 2-2.
Figure 2-3 - Relationship between ultimate tensile strength and total elongation
(50.8 mm gauge length) for various types of steel.
I-1
When ordering steel based on tensile strength, the DP, CP, and TRIP steels in general still have
higher total elongations than HSLA steels of equal tensile strengths. Total elongation information
for the newer TWIP, FB, HF, and PFHT steels are presented in Section 1, Figure 1-1B.
2.C.1.c. Work Hardening Exponent (n-value)
Sheet metal stretchability is strongly influenced by the work hardening exponent or n-value. The
capabilities of the n-value are schematically illustrated in Figure 2-4.
Section 2 - Forming

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Version
3 33 33
Page 2-6
Figure 2-4 - Schematic showing the safety margin between
allowable FLC strain for a higher n-value (solid line) and a
lower n-value (dashed line).
The n-value is the key parameter in determining the maximum allowable stretch as determined by
the Forming Limit Curve (FLC). The height of the FLC is directly proportional to the terminal n-
value as discussed later. The n-value also contributes to the ability of steel to distribute the strain
more uniformly in the presence of a stress gradient. The higher the n-value, the flatter the strain
gradient. A higher n-value (solid lines in Figure 2-4) compared to a lower n-value (dashed lines)
means a deeper part can be stretched for equal safety margins or a larger safety margin for equal
depth parts.
The decreasing n-value with increasing yield strength for conventional HSS (Figure 2-5) limits the
application of some HSS.
Figure 2-5 - Experimental relationship between n-value
(work hardening exponent measured from 10 to 20%
strain) and engineering yield stress for a wide range of
Mild steel and conventional HSS types and grades.
K-2
Section 2 - Forming

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Version
3 33 33
Page 2-7
Unfortunately, comparison of n-value for DP steel to HSLA steel requires more than comparing the
two single values of n for a given yield strength. The following tensile test data show why. In one
study the HSLA 350/450 has a 0.14 n-value and the DP 350/600 has an identical 0.14 n-value in a
standard test procedure measuring the n-value over a strain range of 5% to 15%.
I-1
No differences
are reported, which is contrary to increased stretchability gained when using DP steels. On the
other hand, a number of different DP steels showing a wide range of n-values were observed for a
given strength level.
Unlike the HSLA 350/450 steel that has an approximately constant n-value over most of its strain
range, the n-value for the DP 350/600 starts higher and then decreases with increasing strain as
the initial effect of the original martensite islands is diminished. To capture this behaviour, the
instantaneous n-value as a function of strain must be determined.
Figure 2-6 - Instantaneous n values versus strain for DP 350/600 and
HSLA 350/450 steels.
K-1
The instantaneous n-value curves for the HSLA 350/450 and DP 350/600 shown in Figure 2-6
clearly indicate this much higher n-value for DP steel for strain less than 7%. The higher initial n-
value tends to restrict the onset of strain localization and growth of sharp strain gradients.
Minimization of sharp gradients in the length of line also reduces the amount of localized sheet
metal thinning.
This reduction in thinning for a channel is presented in Figure 2-7. Substitution of DP 350/600 for
HSLA 350/450 reduced the maximum thinning from 25% to 20%. The instantaneous n-values for
these two steels are shown in Figure 2-6.
Section 2 - Forming

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Version
3 33 33
Page 2-8
Strain Measurement Location
Figure 2-7 - Thinning strain distribution for a channel produced
with DP and HSLA steels.
S-1
Unlike the DP steels where the increase in n-value is restricted to the low strain-values, the TRIP
steels constantly create new islands of martensite as the steel is deformed to higher strain-values.
These new martensite islands maintain the high value of n as shown in Figure 2-8.
Figure 2-8 - Instantaneous n values versus strain for TRIP, DP, and
HSLA steels.
K-1
Section 2 - Forming

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Version
3 33 33
Page 2-9
The continued high n-value of the TRIP steel relative to the HSLA steel contributes to the increase
in total elongation observed in Figures 2-2 and 2-3. The increased n-value at higher strain levels
further restricts strain localization and increases the height of the forming limit curve.
The n values for TWIP have been described
C-4
as increasing to 0.4 at an approximate strain of
30% due to the twinning mode of deformation and then remaining constant until a total elongation
of 52%. In contrast, the formability properties of the HF steels are only developed after the blanks
reach operating temperature.
2.C.1.d. Stress-Strain Curves
Stress-strain curves are extremely valuable for comparing different steel types and even different
grades within a single type of steel. Engineering stressengineering strain curves are developed
using initial gage length and initial cross-sectional area of the specimen. These curves highlight
yield point elongation, ultimate tensile strength, uniform elongation, total elongation, and other
strain events. In contrast, the true stresstrue strain curves are based on instantaneous gage
length and instantaneous cross-sectional area of the specimen. Therefore, the area under the
curve up to a specific strain is proportional to the energy required to create that level of strain or the
energy absorbed (crash management) when that level of strain is imparted to a part.
Figure 9 is a collection of typical stress-strain curves both engineering and true for different
grades of HSLA, DP, TRIP, CP, and MS steels. A typical stress-strain curve for Mild steel is included
in each graph for reference purposes. This will permit one to compare potential forming parameters,
press loads, press energy requirements, and other parameters when switching among different
steel types and grades.
Section 2 - Forming

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Version
3 33 33
Page 2-10
Figure 2-9A Engineering stress-strain (upper graphic) and true stress-strain
(lower graphic) curves for a series of cold-rolled HSLA steel grades.
S-5
Section 2 - Forming

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Version
3 33 33
Page 2-11
Figure 2-9B Engineering stress-strain (upper graphic) and true stress-strain
(lower graphic) curves for a series of DP steel grades.
S-5, V-1
Sheet thicknesses:
DP 250/450 and DP 500/800 =1.0mm. All other steels were 1.8-2.0mm.
Section 2 - Forming

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Version
3 33 33
Page 2-12
Figure 2-9C Engineering stress-strain (upper graphic) and true stress-strain
(lower graphic) curves for a series of TRIP steel grades.
V-1
Sheet thickness: TRIP
350/600 =1.2mm, TRIP 450/700 =1.5mm, TRIP 500/750 =2.0mm, and Mild
Steel =approx. 1.9mm.
Section 2 - Forming

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Version
3 33 33
Page 2-13
Figure 2-9D Engineering stress-strain (upper graphic) and true stress-strain
(lower graphic) curves for a series of CP steel grades.
V-1
Sheet thickness:
CP650/850 =1.5mm, CP 800/1000 =0.8mm, CP 1000/1200 =1.0mm, and Mild
Steel =approx. 1.9mm.
Section 2 - Forming

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Version
3 33 33
Page 2-14
Figure 2-9E Engineering stress-strain (upper graphic) and true stress-strain
(lower graphic) curves for a series of MS steel grades.
S-5
All Sheet thicknesses
were 1.8-2.0mm.
Section 2 - Forming

A
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A
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Page 2-15
2.C.1.e. Normal Anisotropy Ratio ( or r
m
)
The normal anisotropy ratio (r
m
) defines the ability of the metal to deform in the thickness direction
relative to deformation in the plane of the sheet. For r
m
values greater than 1, the sheet metal
resists thinning. Values greater than 1 improve cup drawing, hole expansion, and other forming
modes where metal thinning is detrimental.
High-strength steels with UTS greater than 450 MPa and hot-rolled steels have r
m
values
approximating 1. Therefore, HSS and AHSS at similar yield strengths perform equally in forming
modes influenced by the r
m
value.
2.C.1.f. Strain Rate Effects
To characterize the strain rate sensitivity, medium strain rate tests were conducted at strain rates
ranging from 10
-3
/sec (commonly found in tensile tests) to 10
3
/sec. For reference, 10
1
/sec
approximates the strain rate observed in a typical stamping. As expected, the results showed that
YS (Figure 2-10) and UTS (Figure 2-11) increase with increasing strain rate.
Figure 2-10 - Increase in yield stress as a function of strain rate.
Y-1
r
Section 2 - Forming

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Version
3 33 33
Page 2-16
Figure 2-11 - Increase in tensile stress as a function of strain rate.
Y-1
However, up to a strain rate of 10
1
/sec, both the YS and UTS only increased about 16-20 MPa per
order of magnitude increase in strain rate. These increases are less than those measured for low
strength steels. This means that the YS and UTS values active in the sheet metal are somewhat
greater than the reported quasi-static values traditionally reported. However, the change in YS and
UTS from small changes in press strokes per minute are very small and are less than the changes
experienced from one coil to another.
The change in n-value with increase in strain rate is shown in Figure 2-12. Steels with YS greater
than 300 MPa have an almost constant n-value over the full strain rate range, although some
variation from one strain rate magnitude to another is possible.
Figure 2-12 - Relationship between n-value and strain rate showing
relatively no overall increase.
Y-1
Section 2 - Forming

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Page 2-17
2.C.1.g. Bake Hardening and Aging
Strain aging was measured using a typical value for an automotive paint/bake cycle consisting of
2% uniaxial pre-strain followed by baking at 170
o
C for 30 minutes. Figure 2-13 defines the
measurement for work hardening (B minus A), unloading to C for baking, and reloading to yielding
at D for measurement of bake hardening (D minus B).
Figure 2-13 - Measurement of work hardening index and bake hardening index.
Figure 2-14 shows the work hardening and bake hardening increases for the prestrained and
baked tensile specimen. The HSLA shows little or no bake hardening, while AHSS such as DP and
TRIP steels show large positive bake hardening index. The DP steel also has significantly higher
work hardening than HSLA or TRIP steel because of higher strain hardening at low strains. No
aging behaviour of AHSS has been observed due to storage of as-received coils or blanks over a
significant length of time at normal room temperatures. Hence, significant mechanical property
changes of shipped AHSS products during normal storage conditions are unlikely.
Figure 2-14 - Comparison of work hardening (WH) and
bake hardening (BH) for TRIP, DP, and HSLA steels.
S-1, K-3
Section 2 - Forming

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Version
3 33 33
Page 2-18
2.C.1.h. Key Points
AHSS generally have greater total elongations compared to conventional HSS of equal
ultimate tensile strengths.
DP steels have increased n-values in the initial stages of deformation compared to HSS.
These higher n-values help distribute deformation more uniformly in the presence of a
stress gradient and thereby reduce local thinning.
TRIP steels have less initial increase in n-value than DP steels but sustain the increase
throughout the entire deformation process. These AHSS can have n-values comparable to
Mild steels.
Cold-rolled and coated AHSS and HSS steels with UTS greater than 450 MPa and all hot-
rolled steels have normal anisotropy values (r
m
) around a value of 1.
YS and UTS for AHSS increase only about 16-20 MPa per ten-fold increase in strain rate,
which is less than Mild steel increases. The n-value changes very little over a 10
5
increase
in strain rate.
As-received AHSS does not age-harden in storage.
DP and TRIP steels have substantial increase in YS due to a bake hardening effect, while
HSLA steels have almost none.
2.C.2. Forming Limits
Knowledge of forming limits is important throughout the entire product design to production cycle.
First is the computerized forming-process development (virtual die tryout), which requires forming
limits for the selected steel type and grade to assess the forming severity (hot spots) for each point
on the stamping. Next is the process and tool design stage where specific features of the tooling
are established and again computer-validated against forming limits for the specific steel.
Troubleshooting tools for die tryout on the press shop floor utilize forming limits to assess the final
severity of the part and to track process improvements. Finally, forming limits are used to track part
severity throughout the production life of the part as the tooling undergoes both intentional
(engineering) modifications and unintentional (wear) changes.
Two different types of forming limits are presented in this section. The first is the traditional forming
limit curves that apply to all modes of sheet metal forming. The second is sheared edge stretching
limits that apply strictly to the problem of stretching the cut edge of sheet metal.
2.C.2.a. Forming Limit Curves (FLC)
Forming limit curves (FLC) are used routinely in many areas around the world during the design,
tryout, and production stages of a stamping. An FLC is a map of strains that indicate the onset of
critical local necking for different strain paths, represented by major and minor strains. These
critical strains not only become the limit of useful deformation but are also the points below which
safety margins are calculated.
Experimental determination of FLCs involves forming sheet specimens of different widths to generate
different strain paths and measuring the different critical strains. Considerable prior work has been
done with respect to characterizing the minimum value of the FLC as a function on n-value and
thickness for different steel types and grades. One equation for FLC
0
is given in Figure 2-15.
Regional differences may be observed in the generation, shape, and application of forming limit
curves.
Section 2 - Forming

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Version
3 33 33
Page 2-19
Figure 2-15- Experimental FLCs for one sample each of Mild,
HSLA, and DP steels with thicknesses equal to 1.2 mm.
K-1
Determination of FLCs for TRIP and MS steels (Figure 2-16) present additional problems and
need further development. For example, the terminal n value of the TRIP steels depends strongly
on different chemistries and processing used by different steel producers. In addition, the terminal
n value is a function of the strain history of the stamping that determines the transformation of
retained austenite to martensite. Since different locations in a stamping follow different strain paths
(balanced biaxial, plane strain, uniaxial tension, compression, etc.) and varying amounts of
deformation, the terminal n for TRIP steel could vary not only from part design to part design but
also with location within the part. The MS steels have very little available deformation, which makes
generation of FLCs difficult.
Examples of experimental FLCs are shown in Figure 2-15 for Mild Steel 170/300, HSLA 350/450,
and DP 350/600 with sheet thicknesses equal to 1.2 mm. All three curves have approximately the
same shape and the minimum value of the major strain generally is predictable from the FLC
0
equation. Since the HSLA and DP steels have approximately the same terminal (high strain) n
value (Figure 2-6), the identical FLCs were expected. The Mild Steel has an elevated FLC because
its terminal n value is substantially higher than the HSLA and DP steels tested.
Section 2 - Forming

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Page 2-20
With only minor differences in sheet thickness, the height of the FLC
0
is primarily a function of the
terminal work hardening exponent (n). The measured properties of the steels are listed in Table 2-1.
Table 2-1 - Properties of steels in Figures 2-15 and 2-16.
K-1, C-1
Both HSLA 350/450 and DP 350/600 steels have terminal n-values (measured at high values of
strain) equal to 0.170. Therefore, the FLC
0
values are equal as shown in Figure 2-15. These two
steels have approximately the same YS and total elongations but the UTS values are very different.
More interesting are the Mild 170/300 and TRIP 400/600 steels. Both have terminal n values of
0.230. However, the FLC
0
equation shown in Figure 2-15 currently cannot be applied to TRIP
steels and must be further researched. The modified microstructures of the AHSS allow different
property relationships to tailor each steel type and grade to specific application needs. Even more
important is the requirement to obtain property data from the steel supplier for the types and
grades being considered for specific applications.
2.C.2.b. Sheared Edge Stretching Limits
Extensive work has been conducted in various parts of the world on the capability of AHSS to
withstand tensile stretching on sheared edges. This sheared edge can be created at many different
times during the transition from steel mill to final assembly. These include coil slitting, blanking
(straight and contour), offal trimming (external edges or internal cut-outs), hole punching, and
Figure 2-16 Preliminary experimental FLCs for a
t=1.2 mm TRIP steel and a t=1.5 mm MS steel.
K-1, C-1
Section 2 - Forming

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Version
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Page 2-21
other operations. Tensile stretching is most commonly created during hole expansion and stretch
flanging. In terms of deformation mode, the edge simulates a tensile test with similar width and
thickness reductions.
Sheared edge stretchability is generally evaluated by two different hole expansion test methods.
The first begins with clamping a flat blank containing a punched hole in the center. A flat bottom
punch with a diameter equal to the die opening is pushed into the blank. The circumference of the
hole expands as the metal slides across the bottom of the punch. The second test begins with the
clamping of the same flat blank with a punched hole in the center. In this test, however, a conical
punch is inserted into the hole. As the punch continues its travel, the circumference of the hole
expands as a flange of increasing height is generated. When a variety of steels was tested by both
methods, a correlation did exist between the two test methods. Either one could be used to compare
edge stretching of different metals. However, the hole expansion test utilizing a conical punch has
become the more common test because it is more simulative of a stretch flanging operation. The
increase in hole diameter (or circumference) is given the symbol lambda ( ). The key to consistent
data lies with the quality of the punched hole. Special efforts are needed to keep the tools sharp
and damage free. Hard, wear resistant tools, preferably coated PM grade, are highly recommended.
The same study also found that the hole expansion limits generated by a conical punch were
consistently higher than the hole expansion tests created by a flat-bottom punch.
A different study
C-1
evaluated the hole expansion ratio created by hole punching tools as they wore
in a production environment. The powder metallurgy (PM) tools had a 60 HRC. The tools were not
coated. Data were taken from newly ground punches and dies (shown as Sharp Tools in Figure 2-
17) and from worn tools shown in the same graphic. The radial clearance was 0.1 mm. Only rust
protection oil on the sheet was used during the punching. Aral Ropa oil was applied during the hole
expansion. The poor edge condition after punching was caused by tool wear and possible
microchipping. The clearance was hardly affected.
Figure 2-17 Production tooling used to evaluate hole expansion tests using
a conical punch with a 50 mm base diameter using sharp and worn tools.
C-1
Section 2 - Forming

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Page 2-22
The conclusions from this study were: 1) When exposing a DP steel to edge deformation, make
sure the best quality edge condition is utilized and the burr, if possible, should be facing inward,
and 2) Use hard wearing tooling , preferably coated PM grades, for punching.
Another test to assess sheared edge stretchability utilizes a half-tensile test specimen. One edge
of a tensile test blank is sheared. A standard reduced section is milled into the opposite edge. The
sample is pulled until the onset of edge checking, at which time the percent elongation is measured
between one of the sets of 2.5 mm gauge marks along the specimen.
A word of caution. The hole expansion test is highly dependent on the quality of the sheared hole.
Hilson
H-1
showed that a hole punched in 210 MPa (30 Ksi) YS AKDQ steel with the standard 8-
12% punch-to-die clearance allowed 80% hole expansion at failure. When a milled hole of the
same diameter was tested, the hole expansion at failure increased to 280% for 210 MPa Mild steel
and 180% for a 400 MPa HSS. The shearing can create a work hardened zone for a distance from
the sheared edge equal to one-half metal thickness. Therefore, careful reproducibility of the sheared
perimeter of the hole is required to run comparison tests on vastly different steels, such as AHSS
and HSS. The same severe work hardening generated during the edge shearing prevents the use
of traditional FLCs based on the as-received properties of the steel to determine allowable sheared
edge stretching.
Research
V-1
similar to Hilsons work but devoted to AHSS and other HSS is presented in Figure 2-
18. Note that the HE (%) can be significantly lower for punched holes compared to machined
holes. This probably is due to the reduced local elongation of these multiphase steels, which have
interfacial shearing between the ductile ferrite matrix and the hard martensitic islands.
Figure 2-18 Laboratory study shows the effect of damage (measured by a hole
expansion test) done to sheet metal stretchability when a hole is punched in
sheet metal compared to machining the hole.
V-1
To counteract this general trend, however, properties of the AHSS can be further tailored to increase
the sheared edge stretching limit. AHSS gain their well publicized improved total elongations from
microstructures with unique differences in morphology, hardness, and amounts of low temperature
transformation products (LTTPs). Unfortunately, these same microstructures reduce local
elongations or local ductility (measured by ) that affect hole expansion, stretch flanging, and
bending. This problem is shown in Figure 2-19.
Section 2 - Forming

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Page 2-23
Figure 2-19 - Schematic showing AHSS tailored to high total elongation
or high local elongation.
T-1
The key to improved sheared edge stretchability is homogeneous microstructure. Such metallurgical
trends include a single phase of bainite or multiple phases including bainite and removal of large
particles of martensite. This trend is shown in Figure 2-20.
Figure 2-20 - Improvements in hole expansion by modification of microstructure.
N-1
Section 2 - Forming

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Page 2-24
2.C.2.c. Key Points
Forming Limit Curves
Differences in determination and interpretation of FLCs exist in different regions of the
world. These Application Guidelines utilize one current system of commonly used FLCs
positioned by FLC
0
determined by terminal n and t.
This system of FLCs commonly used for low strength and conventional HSS is generally
applicable to experimental FLCs obtained for DP steels.
The left side of the FLC (negative minor strains) is in good agreement with experimental
data for DP and TRIP steels. The left side depicts a constant thinning strain as a forming
limit.
Data for 1.2 mm steels shows the FLCs for HSLA 350/450 and DP 350/600 overlap.
Determination of FLCs for TRIP, MS, TWIP, and other special steels present measurement
and interpretation problems and need further development.
Sheared Edge Stretching Limits
Sheared edge stretching limits (called local elongations) are important for hole expansion,
stretch flanging, and bending.
The microstructure of AHSS can be modified to enhance either total elongation for general
stretch forming or local elongation for sheared edge stretching limits. The same
microstructure generally does not provide high values for both total and local elongation
values. However, some increases in both can be created to provide a balance of total and
local elongation.
2.C.3. Forming Modes
Part designers are interested in the forming capabilities of the steels they specify. This is true of
HSS and even more so for AHSS. Unfortunately complex stampings are composed of several
different basic forming modes, which react to a different set of mechanical properties. Likewise,
formability of steel, and especially AHSS, cannot be characterized by a single number. Therefore,
formability comparisons of AHSS to conventional HSS must be done for each basic forming mode.
In this section, three general groups (stretching, cup drawing, bending and roll forming) are reviewed.
2.C.3.a. Stretching
As a general rule the depth of a part by stretching increases as the work hardening exponent (n)
increases. As discussed in 2.C.1., an increase in n value can increase:
1) The allowable stretch as determined by the forming limit curve (FLC).
2) The ability of steel to distribute the strain distribution more uniformly in the presence of a
stress gradient.
DP steels have an increased n value at low values of strain compared to HSS (Figure 2-6). Therefore,
DP steels have increased tendency to flatten strain gradients at their inception. Part designers can
benefit from AHSS for all stamping areas that are formed in pure stretch, such as embossments,
character lines, and other design features with localized strain gradients (Figure 2-21). Peak strain
Section 2 - Forming

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Version
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Page 2-25
reduction in these gradients also means less localized thinning for in-service requirements.
Figure 2-21 Stretch forming generated by a rounded or flat bottom punch.
By 2% strain the higher instantaneous n value of DP steels has been depleted (Figure 2-6) and the
n values are similar to conventional HSS. Therefore, traditional formulas used to set the height of
the FLC used for HSLA can be used for DP steels when compared at equal yield strengths (Figure
2-15). However, when comparisons are made between DP and HSLA steels with equal tensile
strengths, the DP steels do have higher FLCs. Caution must be taken when those stretch operations
(embossments and other design features) are performed on prior-deformed areas. Due to the
rapid work hardening rate for AHSS, the residual formability from the prior operation may be quite
different from that for conventional HSS.
TRIP steels have high n values compared to HSS throughout their entire strain range (Figure 2-8).
A continual high n means a steel is much more suitable to suppress localization of strain generated
by design features in the stamping. The higher terminal (high strain) n value also means a higher
FLC (Figure 2-16), where, for example, the FLC for the TRIP 350/600 steel approximates that of a
Mild steel.
In stretch forming, the TRIP steel has an additional advantage compared to DP and conventional
HSLA steels. As the strain begins to localize at the high stress locations in the stamping, the
deformation causes additional transformation from retained austenite to martensite. This further
strengthens the deformation zone and forces redistribution of deformation to areas of less strain.
Section 2 - Forming

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3 33 33
Page 2-26
The total effect of the higher n value and additional transformation to martensite is documented by
the Limiting Dome Height (LDH) test result shown in Figure 2-22.
N-1
The actual properties of the
two steels tested are:
TRIP 399/614, uniform elongation =26.3%, total elongation =35.3%
HSLA 413/564, uniform elongation =16.9%, total elongation =27.5%
Blanks were coated with conventional anti-rust oil and held with a circular lock bead of 165 mm
diameter. The minimum hemispherical dome height at failure is substantially higher for the TRIP
steel compared to the equivalent HSLA steel.
Figure 2-22 - Limiting Dome Height is greater for TRIP
than HSLA for the two steel grades tested.
T-2
The same tooling, steels, and lubricant from Figure 2-22 generated the thinning strains in Figure 2-
23. However, the 50 mm radius hemispherical punch stretched the dome height to only 25 mm for
both steels. The increased capability of the TRIP steel to minimize localized thinning is observed.
Section 2 - Forming

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Page 2-27
Figure 2-23 - The local thinning is smaller for
TRIP than HSLA at a constant dome height.
T-2
A series of hemispherical dome stretch forming tests showed the expected decrease in stretchability
as the yield and tensile strength increased (Figure 2-24).
Figure 2-24 - Dome stretch tests using a 100 mm
hemispherical punch and a clamped blank. Sheet thickness is
1.2 mm except for the MS thickness of 1.5 mm.
C-1
The maximum length of line that can be stretched depends on tool design, lubrication, and many
other inputs to the forming system. Computerized forming-process development is an important
procedure for assessing the benefits of AHSS over conventional HSS for specific stamping designs.
Section 2 - Forming

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Page 2-28
2.C.3.b. Deep Drawing (Cup Drawing)
Deep drawing is defined as radial drawing or cup drawing (Figure 2-25). The flange of a circular
blank is subjected to a radial tension and a circumferential compression as the flange moves in a
radial direction towards the circular die radius in response to a pull generated by a flat bottom
punch. In addition to forming cylindrical cups, segments of a deep drawn cup are found in corners
of box-shaped stampings and at the ends of closed channels.
Figure 2-25 - A circular blank is formed into a cylindrical cup by the
deep drawing, radial drawing, or cup drawing method of deformation.
The steel property that improves cup drawing or radial drawing is the normal anisotropy or r
m
value. Values greater than 1 allow an increase in the limiting draw ratio (LDR), which is the maximum
ratio of blank diameter to punch diameter allowed in the first draw. In contrast, the LDR is insensitive
to the strength of the steel and the n value.
High-strength steels with UTS greater than 450 MPa and hot-rolled steels have r
m
values
approximating 1 and the LDR averages around 2. Therefore, DP steels have an LDR similar to
HSS. However, the TRIP steels have a slightly improved LDR deep drawability.
T-2
Since the
martensite transformation is influenced by the deformation mode (Figure 2-26), the amount of
transformed martensite generated by shrink flanging in the flange area is less than the plane strain
deformation in the cup wall. This difference in transformation from retained austenite to martensite
makes the wall area stronger than the flange area, thereby increasing the LDR.
Section 2 - Forming

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Version
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Page 2-29
Figure 2-26 - The cup wall is strengthened more than the flange due to
increased amounts of transformed martensite in TRIP steels.
T-2
Excluding the special cup drawing features of the TRIP steels mentioned above, laboratory cup
drawing experiments show an approximate LDR of 2 for the DP steels tested (Figure 2-27).
Figure 2-27 - LDR tests for Mild, DP, and MS steels.
C-1
Section 2 - Forming

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The absolute value of the LDR, however, also depends on the lubrication, blank holder load, die
radius, and other system inputs.
2.C.3.c. Bending
The usual mode of bending is curvature around a straight line radius (Figure 2-28). Across the
radius is a gradient of strains from maximum outer fiber tension though a neutral axis to inner fiber
compression. No strain (plane strain) occurs along the bend axis.
Figure 2-28 - Typical three-point bend has outer
fiber tension and inner fiber compression with a
free bend neutral axis in the center.
Three-point Bending
A higher total elongation helps sustain a larger outer fiber stretch of the bend before surface fracture,
thereby permitting a smaller bend radius. Since total elongation decreases with increasing strength
for a given sheet thickness, the achievable minimum design bend radius must be increased (Figure
2-29).
Figure 2-29 Achievable minimum bend radius (r/t) in a three-point bend test
increases as the total elongation of the steel decreases.
S-5
Section 2 - Forming

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For equal strengths, most AHSS have greater total elongations than HSS (Figures 2-2 and 2-3).
The microstructure of DP and TRIP consists of a highly inhomogeneous combination of soft ferrite
matrix and hard martensite islands. This microstructure creates a larger total elongation due to the
increased work hardening. A smaller minimum design bend radius is expected. However, the
deformation can localize around the hard phases and create low local elongations or edge stretch
capability as measured by the hole expansion test (Figure 2-20). Several cases of early radii
cracking of production bending DP and TRIP steels have been attributed to this lower local
elongation.
2.C.3.d. Roll Forming
The roll forming process forms a flat metal strip by successive bending into the desired shape.
Each bending operation can be distributed along several sets of rolls to minimize strain localization
and compensate for springback. Therefore, roll forming is well suited for generating many complex
shapes from AHSS, especially those with low total elongations such as MS.
Roll forming can produce AHSS parts with:
Steels of all levels of mechanical properties and different microstructures.
Springback compensation without particularly complex tools.
Small radii depending on the thickness and mechanical properties of the steel.
Reduced number of forming stations compared with lower strength steel.
However, the forces on the rollers and frames are higher. A rule of thumb says that the force is
linear with the strength but square with the thickness. Therefore, structural strength ratings of the
roll forming equipment must be checked in order to avoid bending of the shafts.
Typical values of the minimum radius and springback can be determined for the different AHSS
with tests on simple U shapes performed with six stations (Figure 2-30)
Figure 2-30 - Comparison between the minimum radii made by roll forming
and bending a 2 mm MS 1050/1400.
S-5
Section 2 - Forming

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The value of minimum internal radius of a roll formed component depends primarily on the thickness
and the tensile strength of the steel (Figure 2-31). Roll forming allows smaller radii than a bending
process.
Figure 2-31 - Achievable minimum r/t values for bending and roll forming for
different strength and types of steel.
S-5
The main parameters having an influence on the springback are the radius of the component, the
thickness, and the yield strength of the steel. The effects of these parameters are shown in Figure
2-32. As expected, angular change increases for increased tensile strength and bend radius.
Figure 2-32 Angular change increases with increasing tensile strength and
bend radii.
A-4
Roll forming makes it possible to control the strains in the bend to minimise the springback (Figure
2-33).
Section 2 - Forming

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MS 1150/1400
MS 950/1200
DP 700/1000
Figure 2-33 - A profile made with the same tool setup for three steels having different strengths and the
same thickness. Even with the large difference in strength, the springback is almost the same.
S-5
2.C.3.e. Key Points
Stretching
DP steel has a higher initial n value than TRIP steel, which helps flatten emerging strain
gradients and localized thinning. Stretch form features such as embossments can be slightly
sharper or deeper. DP steel does not have a higher FLC compared to HSS with comparable
YS.
TRIP steels benefit from a higher n throughout the deformation process, which helps to
flatten emerging strain gradients and reduce localized thinning. In addition, the height of
the FLC is increased and higher values of strain are allowed before failure.
The limited stretchability of both HSS and AHSS (compared to Mild steels) increases the
importance of product design, change of forming mode, utilization of a preform stage,
lubricant selection, and other process design options.
Deep Drawing (Cup Drawing)
The LDR for both HSS and DP steels is approximately 2 because the r
m
values for most
HSS and AHSS are approximately 1.
The LDR for TRIP steel is slightly greater than 2 because transformation strengthening in
the cup wall is greater than equivalent strengthening in the deforming flange.
Bending
Since total elongation decreases with increasing strength for a given sheet thickness, the
minimum design bend radius must be increased.
For equal strengths, most AHSS have greater total elongations than HSS.
Roll forming can produce AHSS parts with steels of all levels of mechanical properties and
different microstructures.
Roll forming creates springback compensation primarily though overbending without
particularly complex tools.
Section 2 - Forming

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2.C.4. Tool Design
The primary concerns for tool design for forming AHSS are:
1) Increased forces required to form the sheet metal.
2) Need for additional tool features for increased springback compensation.
2.C.4.a. Tool Materials
In general, the existing tool and die shop procedures to select the appropriate die material can be
used to select dies made to stamp AHSS grades. However, the considerably higher strength level
of these grades exerts proportionally increased load on the die material. AHSS might reach hardness
values 4-5 times higher than Mild steel grades. This is partially due to the microstructure of the
sheet metal itself since some grades include martensitic phases for the required strength. For the
martensitic grades (MS), the basic structure is martensite with tensile strengths approaching 1700
MPa.
The higher forces required to form AHSS require increased attention to tool specifications. The
three primary areas are:
Stiffness and toughness of the tool substrate for failure protection.
Harder tool surface finishes for wear protection.
Surface roughness of the tool.
Lifetime and performance of a particular drawing die is primarily determined by the accepted amount
of wear/galling between maintenance periods. When selecting die material, some of the key elements
that affect the specification of the die material are:
Sheet metal: strength, thickness, surface coating.
Die: construction, machineability, radii sharpness, surface finish.
Lubrication.
Cost per part.
AHSS characteristics must be determined when designing tools. First is the initial, as-received
yield strength, which is the minimum yield strength throughout the entire sheet. Second is the
increase in strength level, which can be substantial for stampings that undergo high strain levels.
These two factors acting in tandem can greatly increase the local load. This local load increase
mostly accelerates the wear of draw radii with a less pronounced effect on other surfaces.
Counteracting this load increase can be a reduction in sheet thickness. Thickness reduction for
weight saving is one primary reason for applications of AHSS. Unfortunately, the combination of
reduced thickness and higher strength in the steel increases the tendency to wrinkle. Higher
blankholder loads are required to suppress these wrinkles. Any formation of wrinkles will increase
the local load and accelerate the wear effects.
Tool steel inserts for forming dies must be selected according to the work material and the severity
of the forming. Surface coatings are recommended for DP 350/600 and higher grades. When
coatings are used, it is important that the substrate has sufficient hardness/strength to avoid plastic
deformation of the tool surface - even locally. Therefore a separate surface hardening, such as
nitriding, can be used before the coating is applied. Before coating, it is important to use the tool as
a pre-production tool to allow the tool to set, and to provide time for tool to adjust. Surface roughness
Section 2 - Forming

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Page 2-35
must be as low as possible before coating. R
a

values below 0.2 mm are recommended. Steel
inserts of 1.2379 or 1.2382 with a TiC/TiN coating are recommended for local high-pressure die
areas wearing the zinc off galvanized blanks.
Tool steels for cutting, trimming, and punching tools must be selected in a similar way. A tool
hardness between 58 and 62 HRC is recommended. Coatings may be used to reduce tool wear,
but for the highest strength steels (above 1000 MPa tensile strength) use of coatings only generates
limited further improvements. At this level of steel strength, coating failures occur due to local
deformation of the die material substrate. Heat treated (hardened) cutter knives of 1.2379 or 1.2383
show minor wear of the cutter edge. The radial shear gap should be around 10% of the blank
thickness.
High performance tool steels, such as powder metallurgy (PM) grades, are almost always
economical, despite their higher price, because of their low wear rate. Figure 2-34 shows the
relative tool wear when punching Mild steel with conventional tool steel (A) and punching of DP
350/600 with an uncoated (B) and coated (C) PM tool steel.
A =Mild steel formed with 1.2363 tool steel dies (X100CrMoV5/1; US A2; J apan SKD 12)
B =DP 350/600 steel formed with 1.3344 tool steel dies (carbon 1.20%, vanadium 3%)
C =DP 350/600 steel formed with 1.3344 tool steel dies +hard surface CVD
Figure 2-34 - Tool wear results for different surface treatments using Mild steel (A) for a
reference of 1.
H-2
Research on different surface treatments for a hat-profile drawing with draw beads showed a
similar effect of coated surfaces on a cast iron die and a tool steel die (Figure 2-35).
Section 2 - Forming

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GGG70L =Spheroid graphite bearing cast iron, flame hardened
1.2379 =Tool steel (X155CrMo12/1; US D2; J apan SKD 11)
Figure 2-35 - Surface treatment effects on tool wear, DP steel EG, 1mm.
T-3
Ceramic tool inserts have extreme hardness for wear resistance, high heat resistance, and optimum
tribological behaviour, but have poor machineability and severe brittleness. High costs are offset
by reduced maintenance and increased productivity. While not commonly used, the ceramic tool
inserts offer a possible solution to high interface loading and wear.
Additional information on tool wear is contained in Section 2.C.6.B. Tool Wear, Clearances, and
Burr Height.
2.C.4.b. Tool Design Issues
Goals for springback compensation:
Design out springback in the first draw stage to eliminate additional costly corrective
operations.
Consider strain path and reduce the number of bend/unbend scenarios.
Adequate strain levels in the panel must be achieved to avoid greater springback and
sidewall curl.
Higher press forces are experienced on the structure of the tool.
Concerns for trim and pierce tool design:
Engineer trim tools to withstand higher loads since AHSS have higher tensile strength than
conventional high-strength steels.
Proper support for the trim stock during trim operation is very important to minimize edge
cracking.
Modify trim schedule to minimize elastic recovery.
Shedding of scrap can be a problem because springback of DP steel can cause scrap to
stick very firmly in the tool.
Section 2 - Forming

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Flange design:
Design more formable flanges to reduce need for extra re-strike operations.
Areas to be flanged should have a break-line or initial bend radius drawn in the first die to
reduce springback.
Adapt die radii for material strength and blank thickness.
Draw beads:
Draw beads can generate large amount work hardening and increased press loads.
Utilize draw beads to induce strain and therefore reduce elastic recovery.
Optimize the use of shape and size of blanks to reduce the reliance on draw beads, which
can excessively work harden the material before entering the die opening.
Guidelines to avoid edge cracking during stretch flanging:
Abrupt changes in flange length cause local stress raisers leading to edge cracks. Hence,
the transition of flange length should be gradual.
Use metal gainers in the draw die or in the die prior to stretch flange operation at a stretch
flange location to compensate for change in length of line that occurs to avoid edge cracking
of a stretch flange.
Avoid the use of sharp notch features in curved flanges.
Edge preparation (quality of cut) is a critical factor.
Correcting loose metal:
The higher strength of AHSS makes it more difficult to pull out loose metal or achieve a
minimum stretch in flat sections of stampings.
Increase the use of addendum, metal gainers (Figure 2-36), and other tool features to
balance lengths of line or to locally increase stretch.
Figure 2-36 - Insertion of metal gainers to avoid insufficient stretched areas
and eliminate buckles.
T-3
Section 2 - Forming

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Version
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Page 2-38
2.C.4.c. Prototype Tools
For prototyping tools, normally soft tool materials are used and tool surfaces are not protected by
wear resistant coatings during tool try out. When laser cut blanks of AHSS are used during try out,
the blank holder surface may be damaged due to the high hardness in the cut edges.
Measures to be taken:
Close control of laser cutting parameters in order to reduce burr and hardness.
Deburr the laser cut blanks.
Soft tools may be used for manufacturing prototype parts and the inserts may also be used to
eliminate local wrinkles or buckles. However, soft tools should not be used to assess
manufacturability and springback of AHSS parts.
2.C.4.d. Key Points
Areas of concern are the higher working loads that require better tool materials and coatings
for both failure protection and wear protection.
The higher initial yield strengths of AHSS, plus the increased work hardening of DP and
TRIP steels can increase the working loads of these steels by a factor 3 or 4 compared to
Mild steels.
AHSS hardness values might increase by a factor 4 or 5 over those of Mild steel.
Powder metallurgy (PM) tools may be recommended for some AHSS applications.
Parameters for normal tool design will have to be modified to incorporate more aggressive
springback compensation techniques.
Design process to minimize wrinkling. This leads to higher loads and more tool wear.
Section 2 - Forming

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Page 2-39
2.C.5. Springback
Decades ago the major concern in sheet metal forming was elimination of necks and tears. These
forming problems were a function of plastic strain and generally were addressed by maintaining
strain levels in the part below specific critical strains. These critical strains were dictated by various
forming limits, which included forming limit diagrams, sheared edge stretch tests, and in-service
structural requirements.
Today the primary emphasis has shifted to accuracy and consistency of product dimensions. These
dimensional problems are a function of the elastic stresses created during the forming of the part
and the relief of these stresses, or lack thereof, during the unloading of part after each forming
operation. These dimensional problems or springback are created in all parts. However, their
magnitude generally increases as the strength of the steel increases. Many companies have attacked
springback problems with proprietary in-house compensation procedures developed over years of
trial and error production of various parts. An example would be specific over-crowning of a hood
panel or over-bending a channel to allow the parts to springback to part print dimensions.
The introduction of AHSS creates additional challenges. First, many of the panels generate higher
flow stresses, which is the combination of yield strength and work hardening during deformation.
This creates higher elastic stresses in the part. Second, applying AHSS for weight reduction also
requires the application of thinner sheet metal that is less capable of maintaining part shape. Third,
very little or no prior experience has been generated in most companies relative to springback
compensation procedures for AHSS.
Many reports state that springback problems are much greater for AHSS than for traditional HSS
such as HSLA steels. However, a better description would be that the springback of AHSS is
different from springback of HSLA steels. Knowledge of different mechanical properties is required.
Certainly better communication between the steel supplier and the steel user is mandatory.
An example of this difference is shown in Figure 2-37. The two channels were made sequentially
in a draw die with a pad on the post. The draw die was developed to attain part print dimensions
with the HSLA 350/450 steel. The strain distributions between the two parts were very close with
almost identical lengths of line. However, the stress distributions were very different because of
the steel property differences between DP and HSLA steels (Figure 2-6).
Figure 2-37 - Two channels were made sequentially in the same die.
Section 2 - Forming

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Version
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Page 2-40
2.C.5.a. Origins of Springback
When sheet metal is plastically deformed into a part, the shape of the part always deviates somewhat
from the shape of punch and die after removal from the tooling. This dimensional deviation of the
part is known as springback. Springback is caused by elastic recovery of the part, which can be
illustrated simply on the stress-strain curve as shown in Figure 2-38.
Figure 2-38 - Schematic showing amount of springback is proportional to stress.
Unloading (by removing all external forces and moments) from the plastic deformation level A
would follow line AB to B, where OB is the permanent deformation (plastic) and BC is the recovered
deformation (elastic). Although this elastic recovered deformation at a given location is very small,
it can cause significant shape change due to its mechanical multiplying effect on other locations
when bending deformation and/or curved surfaces are involved.
The magnitude of springback is governed by the tooling and component geometry. When part
geometry prevents complete unloading (relaxing) of the elastic stresses, the elastic stresses
remaining in the part are called residual stresses. The part then will assume whatever shape it can
to minimize the total remaining residual stresses. If all elastic stresses cannot be relieved, then
creating a uniformly distributed residual stress pattern across the sheet and through the thickness
will help eliminate the source of mechanical multiplier effects and thus lead to reduced springback
problems.
In general, springback experienced in AHSS parts is greater than that experienced in mild or HSLA
steels. The expected springback is a function of the as-formed flow stress. Since AHSS have
higher as-formed flow stresses for equal part-forming strains, springback generally will be higher
for AHSS.
Section 2 - Forming

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Page 2-41
2.C.5.b. Types of Springback
Three modes of springback commonly found in channels and underbody components are angular
change, sidewall curl, and twist.
Angular Change
Angular change, sometimes called springback, is the angle created when the bending edge line
(the part) deviates from the line of the tool. The springback angle is measured off the punch radius
(Figure 2-39). If there is no sidewall curl, the angle is constant up the wall of the channel.
Figure 2-39 - Schematic showing difference between angular change and sidewall curl.
Angular/cross-section change is caused by stress difference in the sheet thickness direction when
a sheet metal bends and unbends over a die radius. This stress difference in the sheet thickness
direction creates a bending moment at the bending radius after dies are released, which results in
the angular change. The key to eliminating or minimizing the angular change is to eliminate or to
minimize this bending moment.
Sidewall Curl
Sidewall curl is the curvature created in the side wall of a channel (Figures 2-37 and 2-39). This
curvature occurs when a sheet of metal is drawn over a die/punch radius or through a draw bead.
The primary cause is uneven stress distribution or stress gradient through the thickness of the
sheet metal. This stress is generated during the bending and unbending process.
During the bending and unbending sequence, the deformation histories for both sides of the sheet
are unlikely to be identical. This usually manifests itself by flaring the flanges, which is an important
area for joining to other parts. The resulting sidewall curl can cause assembly difficulties for rail or
channel sections that require tight tolerance of mating faces during assembly. In the worst case, a
gap resulting from the sidewall curl can be so large that welding is not possible.
Section 2 - Forming

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Version
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Page 2-42
Figure 2-40 - Origin and mechanism of sidewall curl.
Figure 2-40 illustrates in detail what happens when sheet metal is drawn over the die radius (a
bending and unbending process). The deformation in side A changes from tension (A
1
) during
bending to compression (A
2
) during unbending. In contrast, the deformation in side B changes
from compression (B
1
) to tension (B
2
) during bending and unbending. As the sheet enters the
sidewall, side A is in compression and side B is in tension, although both sides may have similar
amounts of strain. Once the punch is removed from the die cavity (unloading), side A tends to
elongate and side B to contract due to the elastic recovery causing a curl in the sidewall.
This difference in elastic recovery in side A and side B is the main source of variation in sidewall
curl along the wall. The higher the strength of the deformed metal, the greater the magnitude and
difference in elastic recovery between sides A and B and the increase in sidewall curl. The strength
of the deformed metal depends not only on the as-received yield strength, but also on the work
hardening capacity. This is one of the key differences between conventional HSS and AHSS.
Clearly, the rule for minimizing the sidewall curl is to minimize the stress gradient through the sheet
thickness.
DIFFERENCE BETWEEN HSS AND AHSS - The difference in strain hardening between
conventional HSS and AHSS explains how the relationship between angular change and sidewall
curl can alter part behaviour. Figure 2-41 shows the crossover of the true stress true strain
curves when the two steels are specified by equal tensile strengths. The AHSS have lower yield
strengths than traditional HSS for equal tensile strengths. At the lower strain levels usually
encountered in angular change at the punch radius, AHSS have a lower level of stress and therefore
less springback.
Section 2 - Forming

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Version
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Page 2-43
Figure 2-41 - Schematic description of the effect of hardening
properties on springback.
K-4
This difference for steels of equal tensile strength (but different yield strengths) is shown in Figure
2-42. Of course, the predominant trend is increasing angular change for increasing steel strength.
Figure 2-42 - The AHSS have less angular change at the punch radius for
equal tensile strength steels.
K-4
Section 2 - Forming

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Page 2-44
Figure 2-43 - The AHSS have greater sidewall curl for equal tensile strength steels.
N-2
Sidewall curl is a higher strain event because of the bending and unbending of the steel going over
the die radius and any draw beads. For the two stressstrain curves shown in Figure 2-41, the
AHSS are at a higher stress level with increased elastic stresses. Therefore the sidewall curl is
greater for the AHSS (Figure 2-43).
Now assume that the comparison is made between a conventional HSS and an AHSS specified
with the same yield stress. Figure 2-41 would then show the stressstrain curve for the AHSS is
always greater (and sometimes substantially greater) than the curve for HSS. Now the AHSS
channel will have greater springback for both angular change and sidewall curl compared to the
HSS channel. This result would be similar to the channels shown in Figure 2-37.
These phenomena are dependant on many factors, such as part geometry, tooling design, process
parameter, and material properties, and in some cases they may not even appear. However, the
high work-hardening rate of the DP and TRIP steels causes higher increases in the strength of the
deformed steel for the same amount of strain. Therefore, any differences in tool build, die and
press deflection, location of pressure pins, and other inputs to the part can cause varying amounts
of springback - even for completely symmetrical parts.
Twist
Twist is defined as two cross-sections rotating differently along their axis. Twist is caused by torsion
moments in the cross-section of the part. The torsional displacement (twist) develops because of
unbalanced springback and residual stresses acting in the part to create a force couple, which
tends to rotate one end of the part relative to another. As shown in Figure 2-44 the torsional
moment can come from the in-plane residual stresses in the flange, the sidewall, or both.
Section 2 - Forming

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Page 2-45
Figure 2-44 - Torsion Moment created flange or sidewall residual stresses.
Y-2
The actual magnitude of twist in a part will be determined by the relationship between unbalanced
stresses on the part and the stiffness of the part in the direction of the twist. Low torsional stiffness
values in long, thin parts are the reason high aspect ratio parts have significantly higher tendencies
to twist. There is also a lever effect, whereby the same amount of twist will result in a larger
displacement in a long part than would be the case in a shorter part with a similar twist angle.
The tendency for parts to twist can be overcome by reducing the imbalance in the residual stresses
forming the force couple that creates the torsional movement. Unbalanced forces are more likely
in unsymmetrical parts, parts with wide flanges or high sidewalls, and in parts with sudden changes
in cross section. Parts with unequal flange lengths or non-symmetric cutouts will be susceptible to
twist due to unbalanced springback forces generated by these non-symmetrical features.
Even in geometrically symmetrical parts, unbalanced forces can be generated if the strain gradients
in the parts are non-symmetrical. Some common causes of non-symmetrical strains in symmetrical
parts are improper blank placement, uneven lubrication, uneven die polishing, uneven blankholder
pressure, misaligned presses, or broken/worn draw beads. These problems will result in uneven
material draw-in with higher strains and higher elastic recoveries on one side of the part compared
to the other, thereby generating a force couple and inducing twist.
Twist can also be controlled by maximizing the torsional stiffness of the part - by adding ribs or
other geometrical stiffeners or by redesigning or combining parts to avoid long, thin sections that
will have limited torsional stiffness.
Global Shape Change
Global shape changes, such as reduced curvature when unloading the panel in the die, are usually
corrected by springback compensation measures. The key problem is minimizing springback
variation during the run of the part and during die transition. One study showed that the greatest
global shape (dimensional) changes were generated during die transition.
A-1
Section 2 - Forming

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Version
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Page 2-46
Surface Disturbances
Surface disturbances develop from reaction to local residual stress patterns within the body of the
part. Common examples are high and low spots, oil canning, and other local deformations that
form to balance total residual stresses to their lowest value.
2.C.5.c. Springback Correction
Forming of a part creates elastic stresses unless the forming is performed at a higher temperature
range when stress relief is accomplished before the part leaves the die. An example of the latter
condition is HF steels. Therefore, some form of springback correction is required for bring the part
back to part print. This springback correction can take many forms.
The first approach is to apply an additional process that changes undesirable elastic stresses to
less damaging elastic stresses. One example is a post-stretch operation that reduces sidewall curl
by changing the tensile-to-compressive elastic stress gradient through the thickness of the sidewall
to a tensile elastic stress though the thickness. Another example is over-forming panels and channels
so that the release of elastic stresses brings the part dimensions back to part print instead of
becoming undersized.
A second approach is to modify the process and/or tooling to reduce the level of elastic stresses
actually imparted to the part during the forming operation. An example would be to reduce sidewall
curl by replacing sheet metal flowing through draw beads and over a die radius with a simple 90
degree bending operation.
A third approach for correcting springback problems is to modify product design to resist the release
of the elastic stresses. Mechanical stiffeners are added to the part design to lock in the elastic
stresses to maintain desired part shape.
All three approaches are discussed in detail in this unit. While most are applicable to all higher
strength steels, the very high flow stresses encountered with AHSS make springback correction
high on the priority list. In addition, most of the corrective actions presented here apply to angular
change and sidewall curl.
Change the Elastic Stresses
POSTSTRETCH: One of the leading techniques for significant reduction of both angular change
and sidewall curl is a Post-Stretch operation. An in-plane tension is applied after the bending
operations in draw beads and die radii to change tensile to compressive elastic stress gradients to
all tensile elastic stresses.
When the part is still in the die, the outer surface of the bend over the punch radius is in tension
(Point A in Figure 2-45), while the inner surface is in compression (B). Upon release from the
deforming force, the tensile elastic stresses (A) tend to shrink the outer layers and the compressive
elastic forces (B) tend to elongate the inner layers. These opposite forces form a mechanical
advantage to magnify the angular change. The differential stress can be considered the driver
for the dimensional change.
In the case of side wall curl this differential stress increases as the sheet metal is work hardened
going through draw beads and around the die radius into the wall of the part.
Section 2 - Forming

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Version
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Page 2-47
Figure 2-45 Sheet metal bent over a punch radius has elastic stresses
of the opposite sign creating a mechanical advantage to magnify angular
change. Similar effects create side wall curl for sheet metal pulled
through draw beads and over die radii.
To correct this angular change and side wall curl, a tensile stress is applied to the flange end of the
wall until an approximate tensile strain of 2% is generated within the sidewall of the stamping. The
sequence is shown in Figure 2-46. The initial elastic states are tensile (A1) and compressive (B1).
When approximately 2% tensile strain is added to A1, the strain point work hardens and moves up
slightly to A2. However, when 2% tensile strain is added to B1, the elastic stress state first decreases
to zero, then climbs to a positive level and work hardens slightly to point B2. The neutral axis is
moved out of the sheet metal. The differential stress now approaches zero. Instead of bending
or curving outward, the wall simply shortens by a small amount similar to releasing the load on a
tensile test sample. This shortening of the wall length can be easily corrected by an increased
punch stroke.
Figure 2-46 When subjected to a 2 percent tensile strain, the positive to
compressive stress differential shown in Figure 2-45 is now reduced to a
very small amount.
Section 2 - Forming

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Version
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Page 2-48
The common method used to create the 2% post-stretch is form the part with a pull-over plug. The
bottom blank holder contains movable beads and the upper blankholder contains the bead pockets.
An adjustable stop block is located directly under the movable beads. At the correct amount of
punch stroke, the movable beads hit the stop blocks and are forced into the sheet metal flange.
This creates a blank locking action while the punch continues to deform the part. In other applications,
the part is removed from the first die and inserted into a second die that locks the remaining flange.
The part is then further deformed by 2%.
These post-stretch forming operations normally require significantly higher forming forces to be
effective since the sidewalls have been strengthened by work hardening resulting from the forming
operation. This is especially true for AHSS. Therefore, the movable draw beads may have to be
replaced by movable lock beads. Even if the press is capable of generating the higher forces,
caution must be taken not to neck down and tear the sheet metal bent over the punch radius.
A restrike operation may be required after trimming to ensure dimensional precision. The restrike
die should sharpen the radius and provide sidewall stretch (post-stretch) of approximately 2%.
A case study on post-stretch was conducted on TRIP 450/800, DP 850/980, DP 450/750, DP 350/
600, CM 490/590, and HSLA 350/450 steels using two specially designed dies
L-1
. One die had
conventional metal flowing from the flange without a bead. The second die had a square lock bead
in the flange that created a post-stretch near the end of the stroke. As expected, the side wall curl
was very small with the post-stretch die. In addition, the material tensile strength did not have
much effect on the amount of springback in the post-stretch die. This translates into a more robust
process.
OVER-FORMING: Many angular change problems occur when the tooling is either constructed to
part print or has insufficient springback compensation. Over-forming or over-bending is required.
Rotary bending tooling should be used where possible instead of flange wipe dies. The
bending angle can be easily adjusted to correct for changes in springback due to variations
in steel properties, die set, lubrication, and other process parameters. In addition, the tensile
loading generated by the wiping shoe is absent.
Multiple stage forming processes may be desirable or even required depending on the part
shape. Utilize secondary operations to return a sprung shape back to part datum. Care
must be taken though to ensure that any subsequent operation does not exceed the work
hardening limit of the worked material. Use multi-stage computerized forming-process
development to confirm strain and work hardening levels. Try to fold the sheet metal over a
radius instead drawing or stretching.
Cross-section design for longitudinal rails, pillars, and cross members can permit greater
springback compensation. The rear longitudinal rail cross-section in sketch A of Figure 2-
47 does not allow over-bend for springback compensation in the forming die. In addition,
the forming will produce severe sidewall curl in AHSS channel-shaped cross sections.
These quality issues can be minimized by designing a cross section similar to sketch B that
allows for over-bend during forming. Sidewall curl is also diminished with the cross-sectional
design. Typical wall opening angles should be 3-degrees for Mild steel, 6 degrees for DP
350/600 and 10 degrees for DP 850/1000 or TRIP 450/800. In addition, the cross section in
sketch B will have the effect of reducing the impact shock load when the draw punch
contacts the AHSS sheet. The vertical draw walls shown in sketch A require higher binder
pressures and higher punch forces to maintain process control.
Section 2 - Forming

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Version
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Page 2-49
Figure 2-47 - Changing rail cross section from A to B allows easier
over-bending to reduce springback problems with AHSS.
N-3
If over-bend must be incorporated for some parts to minimize angular change, use tool/die
radii less than the part radius and use back relief for the die/punch (Figure 2-48).
Figure 2-48 Over-bending is assisted when back relief
is provided on the flange steel and lower die.
A-2
If necessary, add one or two extra forming steps. For example, use pre-crown in the bottom
of channel-type parts in the first step and flatten the crown in the second step to eliminate
the springback at sidewall (Figure 2-49).
Figure 2-49 - Schematic showing how bottom pre-crown can be flattened to
correct for angular springback.
A-3
Section 2 - Forming

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Page 2-50
Reduce or Minimize the Elastic Stresses
Many times the design of the process, and therefore the tooling design, can drastically affect the
level of the elastic stresses in the part.
FORMING THE CHANNEL WALL: Figure 2-50 shows four possible forming processes to create a
hat-profile channel with different blankholder actions.
Figure 2-50 Four processes for generating a channel for bumper reinforcement
create different levels of elastic stress and springback.
K-5
Descriptions of the four processes above:
Draw is the conventional forming type with continuous blankholder force and all blank
material undergoing maximum bending and unbending over the die radius. This forming
mode creates maximum sidewall curl.
Form-draw is a forming process in which the blank holder force is applied between the
middle and last stage of forming. It is most effective to reduce the sidewall curl because
bend-unbend deformation is minimized and during the last stage of forming a large tensile
stress (post-stretch) can be created.
Form process allows the flange to be formed in the last stage of forming and the material
undergoes only a slight amount of bend-unbend deformation.
Bend is a simple bending process to reduce the sidewall curl because the sidewall does
not undergo one or more sequences of bend and unbend. However, an angular change
must be expected.
GUIDELINES FOR DRAW AND STRETCH FORM DIES
Equalize depth of draw as much as possible.
Binder pressure must be increased for AHSS. For example, DP 350/600 requires a tonnage
factor 2.5 times greater than that required for AKDQ of comparable thickness. Higher binder
pressure will reduce panel springback.
Maintain a 1.1t maximum metal clearance in the draw dies.
Lubrication, upgraded die materials, and stamping process modification must be considered
when drawing AHSS.
Section 2 - Forming

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Version
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Page 2-51
Maintain die clearance as tight as allowed by formability and press capability to reduce
unwanted bending and unbending (Figure 2-51).
Figure 2-51 - Reducing die clearance restricts additional
bending and unbending as the sheet metal comes off the
die radius to minimize angular change.
Y-2
Stretch-forming produces a stiffer panel with less springback than drawing. Potential depth
of the panel is diminished for both processes as the strength of the material increases.
Deeper AHSS stampings will require the draw process.
An extensive American Iron and Steel Institute study
S-3
defined a number of tool parameters that
reduced angular change (Figure 2-52A) and side wall curl (Figure 2-52B).
Figure 2-52A The effect of tool parameters in angular change. The lower values are better.
S-3
Section 2 - Forming

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Page 2-52
Figure 2-52B - The effect of tool parameters in sidewall curl. Higher values of radius of
curl are better.
S-3
GUIDELINES FOR FORM DIE:
Set-up the die to allow for appropriate over-bend on sidewalls.
Equalize the depth of forming as much as possible.
Use a post-stretch for channel-shaped stampings. For less complex parts, one form die
should be sufficient. For more geometrically complex parts, the first die will form the part
with open sidewalls. The second die will finish the form in a restrike die with post-stretch of
the sidewalls. Part geometry will determine the required forming process.
Some complex parts will require a form die with upper and lower pressure pads. To avoid
upstroke deformation of the part, a delayed return pressure system must be provided for
the lower pad. When a forming die with upper pad is used, sidewall curl is more severe in
the vertical flange than in the angular flange.
Provide higher holding pressure. DP 350/600 requires a force double that needed for Mild
steel.
When using form dies, keep a die clearance at approximately 1.3t to minimize sidewall
curl. Die clearance at 1t is not desirable since the sidewall curl reaches the maximum at
this clearance.
Do not leave open spaces in the die flange steels at the corners of the flanges. Fit the
radius on both sides of metal at the flange break. Spank the flange radius at the bottom of
the press stroke.
Bottom the pad and all forming steels at the bottom of the press stroke.
Section 2 - Forming

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Page 2-53
PART DESIGN: Part design features should be considered as early as possible in the concept
stage to allow for proper process and tooling design decisions to be made.
Successful application of any material requires close coordination of part design and the
manufacturing process. Consult manufacturing process engineers when designing AHSS
parts to understand the limitations/advantages of the material and the proper forming process
to be employed.
Design structural frames (such as rails and crossbars) as open-end channels to permit
forming operations rather than draw die processes. AHSS stampings requiring draw
operations (closed ends) are limited to a reduced depth of draw. Half the draw depth permitted
for AKDQ is the rule of thumb for AHSS such as DP 350/600. Less complex, open-ended
stamped channels are less limited in depth.
Design AHSS channel shaped part depth as consistent as possible to avoid forming
distortions. All shape transitions should be gradual to avoid distortions, especially in areas
of metal compression. Minimize stretch/compression flanges whenever possible.
Design the punch radius as sharp as formability and product/style allow. Small bend radii
(<2t) will decrease the springback angle and variation (Figure 2-53). However, stretch
bending will be more difficult as yield strength increases. In addition, sharp radii contribute
to excessive thinning.
Figure 2-53- Angular changes are increased by YS and bend radius to
sheet thickness ratio.
S-2
Curved parts with unequal length sidewalls in the fore-aft direction will develop torsional
twist after forming. The shorter length wall can be under tension from residual forming
stresses. Torsional twist is more pronounced with the higher strength steels. Conventional
guidelines for normal steels can also be applied to AHSS to avoid asymmetry that
accentuates the possibility for part twist. However, the greater springback exhibited by
AHSS means extra caution should taken to ensure symmetry is maintained as much as
possible.
Section 2 - Forming

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Page 2-54
Inner and outer motor-compartment rails also require an optimized cross-section design
for AHSS applications. Sketch A in Figure 2-54 shows a typical rectangular box section
through the inner and outer rails. This design will cause many problems for production due
to sidewall curl and angular change. The hexagonal section in sketch B will reduce sidewall
curl and twist problems, while permitting over-bend for springback compensation in the
stamping dies.
Figure 2-54 - Changing rail cross section from A to
B reduces springback problems with AHSS.
N-3
Springback computer simulations should be used whenever possible to predict the trend of
springback and to test the effectiveness of solutions.
Design the part and tool in such a way that springback is desensitized to variations in
material, gauge, tools and forming processes (a robust system and process) and that the
effects of springback are minimized rather than attempt to compensate for it.
Lock In the Elastic Stresses
Where part design allows, mechanical stiffeners can be inserted to prevent the release of
the elastic stresses and reduce various forms of springback (Figure 2-55). However, all
elastic stresses not released remain in the part as residual or trapped stresses. Subsequent
forming, trimming, punching, heating, or other processes may unbalance the residual stress
and change the part shape. Twist also can be relieved by adding strategically placed beads,
darts, or other geometric stiffeners in the shorter length wall to equalize the length of line.
Figure 2-55 Mechanical stiffeners can be used to lock in the elastic stresses
and the part shape.
A-2
Section 2 - Forming

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Page 2-55
Key Points
Angular change and sidewall curl escalate with increasing as-formed yield strength and
decrease with increasing material thickness.
For equal yield strengths DP steels exhibit more angular change and sidewall curl than
conventional HSLA steels. The springback behaviours of TRIP steels are between DP and
HSLA steels.
The sidewall curl appears to be more sensitive to the material and set-up in a channel draw
test.
The angular change decreased with smaller tooling radii and tool gap, but sidewall curl
showed mixed results for smaller tooling radii and tool gap. Both angular change and sidewall
curl were reduced with a larger drawbead restraining force.
Numerous process modifications are available to remove (or at least minimize and stabilize)
the different modes of springback found in channels and similar configurations.
2.C.6. Blanking, Shearing, and Trim Operations
2.C.6.a. General Comments
AHSS exhibit high work hardening rates, resulting in improved forming capabilities compared to
conventional HSS. However, the same high work hardening creates higher strength and hardness
in sheared or punched edges. In addition, laser cutting samples will also lead to highly localized
strength and hardness increases in the cut edge. In general, AHSS can be more sensitive to edge
condition because of this higher strength. Therefore, it is important to obtain a good quality edge
during the cutting operation. With a good edge, both sheared and laser cut processes can be used
to provide adequate formability.
To avoid unexpected problems during a program launch, production intent tooling should be used
as early in the development as possible. For example, switching to a sheared edge from a laser-
cut edge may lead to problems if the lower ductility, usually associated with a sheared edge, is not
accounted for during development.
2.C.6.b. Tool Wear, Clearances, and Burr Height
Cutting and punching clearances should be increased with increasing sheet material strength. The
clearance range from about 6% of the sheet material thickness for Mild steel up to about 10 or 14%
for the highest grade with a tensile strength of about 1400 MPa.
Two hole punching studies.
C-2
were conducted with Mild steel and AHSS. The first measured tool
wear, while the second studied burr height formation. The studies showed that wear when punching
AHSS with surface treated high quality (PM) tool steels is comparable with punching Mild steel
with conventional tools. If burr height is the criterion, high quality tool steels may be used with
longer intervals between resharpening when punching AHSS, since the burr height does not increase
as quickly with tool wear as when punching Mild steel with conventional tool steels.
Tested were 1.0 mm sheet metals: mild 140/270, A80 =38%, DP 350/600, A80 =20%, DP 500/800,
A80=8%, and MS1150/1400, A80 =3%. Tool steels were W.Nr. 1.2363 / AISI A2 with a hardness of 61
HRC and a 6% clearance for Mild steel tests. PM tools with a hardness range of 60-62 HRC were used
Section 2 - Forming

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Page 2-56
for the AHSS tests.. For the DP 280/600, the punch was coated with CVD (TiC) and the clearance was
6%. Tool clearances were 10 % for the MS 1150/1400 and 14 % for DP 500/800.
Punching test results: The worn cross-section of the punch was measured after 200,000 punchings.
For comparison, relative tool wear with AHSS was compared to Mild steel with A2 tooling, which
was about 2000 m
2
for the 200,000 punchings. Test results are shown in Figure 2-56.
Figure 2-56 Punching up to DP 500/800 with surface treated high quality
tool steels can be comparable to Mild steel with conventional tools.
C-2
Burr height tests: The increasing burr height is often the reason for resharpening punching tools.
For Mild steels the burr height increases continuously with tool wear. This was found not to be the
case for the AHSS in Figure 2-57. Two AHSS tested were DP 500/800 and MS 1150/1400. The
burr heights were measured in four locations and averaged. The averages for the two AHSS were
so close that they are plotted as a single line.
Figure 2-57 - Burr height comparison for Mild steel and AHSS as a function of the
number of hits. Results for DP 500/800 and MS 1150/1400 are identical and
shown as the AHSS curve.
C-2
Section 2 - Forming

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Page 2-57
The plausible explanation for Figure 2-57 is that both materials initially have a burr height related
to the material strength and the sharpness of the tools. AHSS has a more brittle fracture and
therefore the burr height has a maximum possible height. This height is reached when the maximum
local elongation is obtained during the punching, after which the burr height does not increase.
The Mild steel, which is more formable, will continue to get higher burr height with increased tool
wear.
The burr height increased with tool wear and increasing die clearance when punching Mild steel.
AHSS may require a higher grade tool steel or surface treatment to avoid tool wear, but tool
regrinding because of burrs should be less of a problem.
2.C.6.c. Key Points
Clearances for blanking and shearing should increase as the strength of the material
increases.
Burr height increases with tool wear and increasing die clearances for shearing Mild steel,
but AHSS tends to maintain a constant burr height. This means extended intervals between
tool sharpening may be applicable to AHSS parts.
Laser cut blanks used during early tool tryout may not represent normal blanking, shearing,
and punching quality. Production intent tooling should be used as early as possible in the
development stage.
Additional information on tool wear is contained in Section 2.C.4.a. Tool Materials.
2.C.7. Press Requirements
2.C.7.a. Force versus Energy
Both mechanical and hydraulic presses require three different capacities or ratingsmaximum
force, energy, and power.
The most common press concern when forming higher strength steels is whether the press is
designed to withstand the maximum force required to form the stamping. Therefore press capacity
(for example, 1000 kN) is a suitable number for the mechanical characteristics of a stamping
press. Capacity, or tonnage rating, indicates the maximum force that the press can apply. However,
the amount of force available depends on whether the press is hydraulic or mechanically driven.
Hydraulic presses can exert maximum force during the entire stroke, whereas mechanical presses
exert their maximum force at a specific displacement just prior to bottom dead center. At increased
distances above bottom dead center, the press capacity is reduced.
Energy consumption inherent in sheet metal forming processes is related to the true stress-true
strain curve and it depends on the yield strength and the work hardening behaviour characterized
by the n-value. The energy required for plastically deforming a material (force times distance)
corresponds to the area under the true stress-true strain curve. Figure 2-58 shows the true stress-
strain curves for two materials with equal yield strength - HSLA 350/450 and DP 350/600. Many
other true stress-true strain curves can be found in Figure 2-9.
Section 2 - Forming

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Page 2-58
Figure 2-58 - True stress-strain curves for two materials with equal yield strength.
T-3
The higher work hardening of the DP grade requires higher press loads when compared to the
HSLA at the same sheet thickness. However, the use of AHSS is normally coupled with a reduced
thickness for the stamping and the required press load would be decreased or compensated. The
higher n values also tend to flatten strain gradients and further reduce the peak strains.
The required power is a function of applied forces, the displacement of the moving parts, and the
speed.
Predicting the press forces needed to initially form a part is known from a basic understanding of
sheet metal forming. Different methods can be chosen to calculate drawing force, ram force, slide
force, or blankholder force. The press load signature is an output from most computerized forming-
process development programs, as well as special press load monitors.
Example: Press Force Comparisons
The computerized forming process-development output (Figure 2-59) shows the press forces
involved for drawing and embossing Mild steel approximately 1.5 mm thick, conventional HSS,
and DP 350/600. It clearly shows that the forces required are dominated by the embossing phase
rather than by the drawing phase.
Section 2 - Forming

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Page 2-59
Figure 2-59 - Data demonstrates that embossing dominates
the required press force rather than the drawing force.
H-3
Sometimes the die closing force is an issue because of the variety of draw-bead geometries that
demand different closing conditions around the periphery of the stamping.
Example: Press Energy Comparisons
A similar analysis (Figure 2-60) shows the press energy required to draw and emboss the same
steels shown in Figure 2-59. The energy required is also dominated by the embossing phase
rather than by the drawing phase, although the punch travel for embossing is only a fraction of the
drawing depth.
Figure 2-60 - Data showing the energy required to emboss
a component is greater than for the drawing component.
H-3
Section 2 - Forming

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Page 2-60
2.C.7.b. Prediction of Press Forces Using Simulative Tests
Relative press forces from Marciniak stretching tests showed AHSS grades require higher punch
forces in stretch forming operations (Figure 2-61). However, applying the stretch forming mode for
CP grades is not common due to the lower stretchability of CP grades.
Figure 2-61 - Punch forces from Marciniak cup-stretch forming tests
for AHSS and conventional steel types.
H-3
2.C.7.c. Extrapolation From Existing Production Data
Relationships between thickness and UTS can be used as a quick extrapolation calculation of
press loads for simple geometries. Figure 2-62 shows the measured press loads for the production
of a cross member with a simple hat-profile made of HSLA 350/450 and DP 300/500 steels of the
same thickness.
Figure 2-62 - Measured press load for a Hat-Profile Cross Member.
T-3
Section 2 - Forming

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Page 2-61
Using the following equation, the press load F2 for DP 300/500 was estimated from the known
press load F1 from HSLA 350/450.
T-3
F2 is proportional to (F1) x (t
2
/t
1
) x (R
m2
/R
m1
)
Where: F1 Old Measured Drawing Force
F2 New Estimated Drawing Force
t
1
Old Material Thickness
t
2
New Material Thickness
R
m1
Old Tensile Strength
R
m2
New Tensile Strength
The data above compares the measured drawing force and the estimated drawing force for the DP
300/500 using the formula. A good correlation between measured and predicted drawing force
was obtained. While good force estimations are possible using this extrapolation technique, the
accuracy is rather limited and often overstates the load. Therefore, the calculation should be viewed
as an upper boundary.
2.C.7.d. Computerized Forming-Process Development
Rules of thumb are useful to estimate press loads. A better evaluation of press loads, such as draw
force, embossment force, and blank holding force, can be obtained from computerized tools. Many
of the programs enable the user to specify all of the system inputs. This is especially important
when forming AHSS because the high rate of work hardening has a major effect on the press
loads. In addition, instead of using a simple restraining force on blank movement, analyses of the
physical draw beads must be calculated.
Another important input to any calculation is the assumption that the tools are rigid during forming,
when in reality the tools deform elastically in operation. This discrepancy leads to a significant
increase in the determined press loads, especially when the punch is at home position. Hence, for
a given part, the draw depth used for the determination of the calculated press load is an important
parameter. For example, if the nominal draw depth is applied, press loads may be overestimated.
The deflection (sometimes called breathing) of the dies is accentuated by the higher work hardening
of the AHSS.
Similarly, the structure, platens, bolsters, and other components of the press are assumed to be
completely rigid. This is not true and causes variation in press loads, especially when physical
tooling is moved from one press to another.
Section 2 - Forming

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Page 2-62
If no proven procedure for computerized prediction is available, validation of the empirical calculations
is recommended. Practical pressing tests should be used to determine the optimum parameter
settings for the simulation. Under special situations, such as restrike operation, it is possible that
computerized analyses may not give a good estimation of the press loads. In these cases,
computerized tools can suggest forming trends for a given part and assist in developing a more
favourable forming-process design.
Most structural components include design features to improve local stiffness. Features requiring
embossing processes are mostly formed near the end of the ram cycle. Predicting forces needed
for such a process is usually based on press shop experiences applicable to conventional steel
grades. To generate comparable numbers for AHSS grades, computerized forming process-
development is recommended.
2.C.7.e. Case Study for Press Energy
The following study is a computerized analysis of the energy required to form a cross member with
a hat-profile and a bottom embossment at the end of the stroke (Figure 2-63).
Figure 2-63 - Cross-section of a component having a longitudinal embossment to
improve stiffness locally.
H-3
Increasing energy is needed to continue punch travel. The complete required energy curves are
shown in Figure 2-64 for mild, HSLA 250/350, and DP 350/600 steels. The three dots indicate the
start of the embossment formation at a punch movement of 85 mm.
Section 2 - Forming

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Page 2-63
Figure 2-64 - Computerized analysis showing the increase in energy needed to
form the component with different steel grades. Forming the embossment begins
at 85 mm of punch travel.
H-3
The last increment of punch travel to 98 mm requires significantly higher energy, as shown in
Figure 2-65. Throughout the punch travel however, the two higher strength steels appear to maintain
a constant proportional increase over the Mild steel.
Figure 2-65 - A further increase in energy is required to finish embossing.
H-3
Section 2 - Forming

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Page 2-64
2.C.7.f. Setting Draw Beads
A considerable force is required from a nitrogen-die cushion in a single-acting press to set draw
beads in AHSS before drawing begins. The nitrogen-die cushion may be inadequate for optimum
pressure and process control. In some cases, binder separation may occur because of insufficient
cushion tonnage, resulting in a loss of control for the stamping process.
The high impact load on the cushion may occur several inches up from the bottom of the press
stroke. Since the impact point in the stroke is both a higher velocity point and a derated press
tonnage, mechanical presses are very susceptible to damage due to these shock loads. Additional
flywheel energy is dissipated by the high shock loads well above bottom dead center of the stroke.
A double-action press will set the draw beads when the outer slide approaches bottom dead center
where the full tonnage rating is available and the slide velocity is substantially lower. This minimizes
any shock loads on die and press and resultant load spikes will be less likely to exceed the rated
press capacity.
2.C.7.g. Key Points
Press loads are increased for AHSS steels primarily because of their increased work
hardening.
More important than press force is the press energy required to continue production. The
required energy can be visualized as area under the true stresstrue strain curves.
High forming loads and energy requirements in a typical hat-profile cross member with a
strengthening bead in the channel base are due to the final embossing segment of the
punch stroke compared to the pure drawing segment.
DP 350/600 requires about twice the energy to form hat-profile cross member than the
same cross member formed from Mild steel.
While several punch force approximation techniques can be used for AHSS, the
recommended procedure is computerized forming-process development.
2.C.8. Multiple Stage Forming
2.C.8.a. General Recommendations:
1 If possible, form all mating areas in the first stage of a forming process and avoid reworking
the same area in the next stages.
2 Design stamping processes so the number of forming stages is minimized.
3 Address potential springback issues as early as possible in the product design stage (design
for springback):
Avoid right or acute angles.
Use larger open wall angles.
Avoid large transition radii between two walls.
Use open-end stamping (Figure 2-66) in preference to a close-end stamping.
Section 2 - Forming

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Page 2-65
Multiple stage forming is recommended for stamping rails or other parts with hat-like cross-section,
which consist of right angles. In this case, using a two-stage forming process gives much better
geometry control than a single stage process. An example of such a process is shown in the figure
below.
In the first operation (Figure 2-66) all 90-degree radii and mating surfaces are formed using gull-
wing processes with overbending to compensate for springback (note that a large radius is used
in the top of the hat area). In the second stage, the top of the rail is flattened. Certain cases may
require an overbending of the flat top section.
Part after first stage Cross section after first stage
Part after second stage Cross-section after second stage
Figure 2-66 - Two-stage forming to achieve a hat section with small radii.
R-1
Multiple forming is also recommended for parts that consist of small geometrical features of severe
geometry that can be formed only in the re-strike operation.
A part that has a variable cross section in combination with small geometrical features may need a
coining operation in the second or last stage of the forming process. This is the only way to control
the geometry.
2.C.8.b. Key Points
Minimize the number of multiple forming stages.
Address springback issues at the earliest possible stage.
Multiple stage forming can assist in producing a square channel cross section.
2.C.9. In-service Requirements
The microstructure of DP and TRIP steels increase the sheet metal forming capability, but also
improve energy absorption in both a crash environment and fatigue life.
Section 2 - Forming

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Page 2-66
2.C.9.a. Crash Management
DP and TRIP steels with ferrite as a major phase show higher energy absorbing property than
conventional high-strength steels, particularly after pre-deformation and paint baking treatments.
Two key features contribute to this high energy-absorbing property: high work hardening rate and
large bake hardening (BH) effect.
The relatively high work-hardening rate, exhibited by DP and TRIP steels, leads to a higher ultimate
tensile strength than that exhibited by conventional HSS of similar yield strength. This provides for
a larger area under the stress-strain curve, and results in greater energy absorption when deformed
in a crash event to the same degree as conventional steels. The high work hardening rate also
causes DP and TRIP steels to work harden during forming processes to higher in-panel strength
than similar YS HSS, further increasing the area under the stress-strain curve and crash energy
absorption. Finally, the high work-hardening rate better distributes strain during crash deformation,
providing for more stable, predictable axial crush that is crucial for maximizing energy absorption
during a front or rear crash event.
The relatively large BH effect also increases the energy absorption of DP and TRIP steels by
further increasing the area under the stress-strain curve. The BH effect adds to the work hardening
imparted by the forming operation. Conventional HSS do not exhibit a strong BH effect and therefore
do not benefit from this strengthening mechanism.
Figure 2-67 illustrates the difference in energy absorption between DP and TRIP steels as a function
of their static (traditional tensile test speed) yield strength.
Figure 2-67 - Absorbed energy for square tube as function of static yield strength.
T-2
Section 2 - Forming

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Figure 2-68 shows calculated absorbed energy plotted against total elongation for a square tube
component. The absorbed energy remains constant for the DP and TRIP steels but the increase in
total elongation allows for formation into complex shapes. For a given crash-critical component,
the higher elongations of DP and TRIP steels do not generally increase energy absorption compared
to conventional HSS if all materials under consideration have sufficient elongation to accommodate
the required crash deformation. In some applications the DP and TRIP grades could increase
energy absorption over that of a conventional HSS if the conventional steel does not have sufficient
ductility to accommodate the required crash deformation and splits rather than fully completing the
crush event. In the latter case, substituting DP or TRIP steel, with sufficient ductility to withstand
full crash deformation, will improve energy absorption by restoring stable crush and permitting
more material to absorb crash energy.
Figure 2-68 - Calculated absorbed energy for a square tube as a function of total
elongation.
T-2
2.C.9.b. Fatigue
The fatigue strength of DP steels is higher than that of precipitation-hardened steels or fully banitic
steels of similar yield strength for many metallurgical reasons. For example, the dispersed fine
martensite particles retard the propagation of fatigue cracks. For TRIP steels, the transformation of
retained austenite can relax the stress field and introduce a compressive stress that can also
improve fatigue strength. Figures 2-69 and 2-70 illustrate the improvements in fatigue capability.
Section 2 - Forming

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Page 2-68
Figure 2-69 - Fatigue characteristics of TRIP 450/780 steel compared to
conventional steels.
T-1
Figure 2-70 - Fatigue limit for AHSS compared to conventional steels.
T-2
2.C.9.c. Key Points
DP and TRIP steels have increased energy absorption in a crash event compared to
conventional HSS because of their high tensile strength, high work hardening rate, and
large BH effect.
The greater ductility of DP and TRIP steels permit use of higher strength, greater energy
absorbing capacity material in a complex geometry that could not be formed from
conventional HSS.
DP and TRIP steels have better fatigue capabilities compared to conventional HSS of
similar yield strength.
Section 2 - Forming

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Page 2-69
2.D. Tube Forming
2.D.1. High Frequency Welded Tubes
Welded tubes are commonly produced from flat sheet material by continuous roll forming and a
high frequency welding process. These types of tubes are widely used for automotive applications,
such as seat structures, cross members, side impact beams, bumpers, engine subframes, trailing
arms, and twist beams. Currently AHSS tubes up to grade DP 700/1000 are in commercial use in
automotive applications.
Tube manufacturing involves a sequence of processing steps (for example roll forming, welding,
calibration, shaping) that influence the mechanical properties of the tube. During the tube
manufacture process both the YS and the UTS are increased while the total elongation is decreased.
Subsequently, when manufacturing parts and components, the tubes are then formed by operations
such as flaring, flattening, expansion, reduction, die forming, bending and hydroforming. The actual
properties of the tube dictate the degree of success to which these techniques can be utilized.
Published data on technical characteristics of tubes made of AHSS is limited. For example, the
ULSAB-AVC programme deals only with those tubes and dimensions applied for the actual body
structure (Table 2.2).
Table 2-2 - Examples of properties for as-shipped straight tubes from ULSAB-AVC project.
I-1
The earlier ULSAC study resulted in design and manufacturing of demonstration hardware, which
included AHSS tubes made of DP 500/800 material. The ULSAC Engineering Report provides the
actual technical characteristics of those two tube dimensions used in the study: 55x30x1.5mm and
34x1.0mm (see http://www.worldautosteel.org/ulsac/ for more information).
The work hardening, which takes place during the tube manufacturing process, increases the YS
and makes the welded AHSS tubes appropriate as a structural material. Mechanical properties of
welded AHSS tubes (Figure 2-71) show welded AHSS tubes provide excellent engineering
properties.
In comparison with HSLA steel tubes, the AHSS tubes offer an improved combination of strength,
formability, and good weldability. AHSS tubes are suitable for structures and offer competitive
advantage through high energy absorption, high strength, low weight, and cost efficient
manufacturing.
Section 2 - Forming

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Figure 2-71 - Anticipated Total Elongation and Yield Strength of AHSS tubes.
R-1
The degree of work hardening, and consequently the formability of the tube, depends both on the
steel grade and the tube diameter/thickness ratio (D/T) as shown in Figure 2-71. Depending on the
degree of work hardening, the formability of tubular materials is reduced compared to the as-
produced sheet material.
Bending AHSS tubes follows the same laws that apply to ordinary steel tubes. One method to evaluate
the formability of a tube is the minimum bend radius, which utilizes the total elongation (A
5
) defined
with proportional test specimen by tensile test for the actual steel grade and tube diameter.
Section 2 - Forming

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Page 2-71
The minimum Centerline Radius (CLR) is defined as:
5
50
A
D
CLR =
Computerized forming-process development utilizes the actual true stress-true strain curve, which
is measured for the actual steel grade and tube diameter. Figure 2-72 contains examples of true
stress-true strain curves for AHSS tubes.
Figure 2-72 - Examples of true stress-true strain curves for AHSS tubes.
R-1
However, it is important to note that the bending behaviour of tube depends on both the tubular
material and the bending technique. The weld seam is also an area of non-uniformity in the tubular
cross section. Thus, the weld seam influences the forming behaviour of welded tubes. The first
recommended procedure is to locate the weld area in a neutral position during the bending operation.
The characteristics of the weld depend on the actual steel sheet material (that is chemistry,
microstructure, strength) and the set-up of the tube manufacturing process. The characteristics of
the high frequency welds in DP steel tubes are discussed in more detail in Section 3 - 3.B.2.
Figures 2-73 and 2-74 provide examples of the forming of AHSS tubes.
Section 2 - Forming

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Page 2-72
Figure 2-73 - Hydroformed Engine Cradle made from
welded DP 280/600 tube with YS 540 N/mm
2
;TS 710
N/mm
2
; Total Elong. 34%. Draw bending, Centreline
Bending Radius =1.6 x D, Bending Angle >90 Degrees.
R-1
Figure 2-74 - Bending test of welded DP 350/600 tube with
YS 610 N/mm
2
; TS 680 N/mm
2
; Total Elongation 27%.
Booster bending, Centerline Bending Radius =1.5 x D,
Bending Angle =45 Degrees.
R-1
2.D.2. Laser Welded Tailored Tubes
Tube products for chassis applications produced by conventional HF weld process (as previously
described) receive their properties during a traditional tube making processes (such as roll-forming,
and widely used HF-welding).
For body structures, thin-wall tube sections are recommended as a replacement for spot-welded
box-shape components. To meet further demands for even thinner gauges (with different metal
inner and outer surface coatings in all AHSS grades that are more sensitive to work hardening) an
alternative manufacturing process is required to maintain the sheet metal properties in the as-
rolled sheet conditions.
Laser welding, used extensively for tailored welded blanks, creates a very narrow weld seam.
Sheet metals with dissimilar thickness and/or strengths are successfully used to achieve required
weight savings by eliminating additional reinforcement parts. Further weld improvements have
been made during the steadily increasing series-production of laser welded blanks.
Section 2 - Forming

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Page 2-73
Part consolidation utilizing hydroforming is one strategy to simultaneously save both cost and
weight. With hydroforming technology, the next step in tubular components is to bring the sheet
metal into a shape closer to the design of the final component without losing tailored blank features
(Figure 2-75).
Figures 2-75 - Mechanical properties of tailored tubes are
close to the original metal properties in the sheet condition.
G-1
The tailored tube production process allows the designer to create complex variations in shape,
thickness, strength, and coating. (Figure 2-76). The shape complexity, however, is limited by the
steel grades and mechanical properties available.
Figure 2-76 - Laser welded, tailored tube examples and required pre-blank shapes.
F-1
A) 1-piece cylindrical tube
B) 2-piece tailored tube
C) Patchwork tube
D) 1-piece conical tube
Section 2 - Forming

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Conical tailored tubes, designed for front rail applications, with optimized lightweight and crash
management are one opportunity to cope with auto body-frame architecture issues. In frontal
crash and side impacts the load paths have a key importance on the body design as they have a
major bearing on the configuration of the structural members and joints. Figure 2-77 is an example
of a front-rail hydroformed prototype. The conical tailored tubes for this purpose take advantage of
the high work hardening potential of TRIP steel.
Figure 2-77- Front-rail prototype based on a conical tube
having 40 mm end to end difference in diameter.
F-2
2.D.3. Key Points
Due to the cold working generated during tube forming, the formability of the tube is reduced
compared to the as-received sheet.
The work hardening during tube forming increases the YS and TS, thereby allowing the
tube to be a structural member.
Laser welded tubes create a very narrow weld seam.
The weld seam should be located at the neutral axis of the tube, whenever possible during
the bending operation.
J J
J JJ oi ni ng oi ni ng
oi ni ng oi ni ng oi ni ng

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Section 3 - Joining
3.A. General Comments
The application of AHSS provides its largest benefit in the potential for safety and mass reduction.
With the application of thinner AHSS, material savings and/or improved crash energy manage-
ment can be achieved. AHSS are produced uncoated, EG, HDG, and GA. Unless differences are
highlighted, joining coated and uncoated AHSS is the same as conventional steels.
The Applications Guidelines document utilizes a steel designation system to minimize regional
confusion about the mechanical properties when comparing AHSS to conventional high strength
steels. The format is Steel Type YS/TS in MPa. Therefore, HSLA 350/450 would have minimum
yield strength of 350 MPa and minimum tensile strength of 450 MPa. The designation also high-
lights different yield strengths for steel grades with equal tensile strengths, thereby allowing some
assessment of the stress-strain curves and amount of work hardening.
AHSS can be satisfactorily welded for automotive applications. AHSS differ from Mild steels by
chemical composition and microstructure. In AHSS, higher strengths are achieved by modifying
the steel microstructure. The as-received microstructure will be changed while welding AHSS. The
higher the heat input, the greater the effect on the microstructure. At fast cooling, it is normal to see
martensite and/or bainite microstructures in the weld metal and in the HAZ.
When joining AHSS, production process control is more important for successful assembly. Manu-
facturers with highly developed joining control methodology will experience no major change in
their operations. Others may require additional checks and maintenance. In certain instances mi-
nor modifications to equipment or processing methodologies may be required for successful join-
ing of AHSS.
The coating methods for AHSS are similar to that for Mild steels. Welding of either AHSS or Mild
steels will generate fumes. The amount and nature of fumes will depend on the coating thickness,
coating composition, joining method, and fillers used to join these materials. The fumes may con-
tain some pollutants. The chemical composition of fumes and the relevant exhaust equipment
must meet appropriate regulatory standards. Thicker coatings and higher heat inputs cause more
fumes. Additional exhausters should be installed. While welding AHSS, with or without metallic or
organic coatings and oiled or not oiled, gases and weld fumes will arise that are similar to Mild
steels. The allowed fumes or gases have to comply with respective national rules and regulations.
The intent of these guidelines is to provide information regarding aspects of the joining processes
recognizing that more data is needed in some areas to be complete.
Section 3 - Joining

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Page 3-2
3.B. Welding Procedures
3.B.1. Resistance Welding
3.B.1.a. Weld Schedule
Application guidelines provided are of a general nature. Specific weld schedules and other detailed
information are not provided because there are many differences between each manufacturers
methods and equipment.
If any type of AHSS (DP, TRIP, CP, or MS) is used for the first time, the user should take the
welding schedules applied to Mild steel and then:
Increase the electrode force by 20% or more depending on yield strength.
Increase weld time when appropriate.
If these changes are insufficient, then try these additional changes:
Try a multi-pulse welding schedule (several pulses or post heating).
Larger tip diameter and/or change the type of electrode.
Increase minimum weld size.
When resistance welded, AHSS require less current than conventional Mild steel or HSLA as
AHSS has higher electrical resistivity. Therefore, current levels for AHSS are not increased and
may even be reduced depending on material chemical composition. However, AHSS may require
higher electrode forces for same thickness of Mild steels because electrode force depends on
material strength. If thick Mild steel or HSLA steel (of the same thickness) is replaced by an equivalent
thickness of AHSS, the same forces may be required during assembly welding.
AHSS often have tighter weld windows when compared with Mild steels, as shown in the Figure 3-1.
Figure 3-1 - Schematic weld lobes of AHSS, HSLA and Mild
steel with a shift to lower currents for increased strength grades.
Section 3 - Joining

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Version
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Page 3-3
Weld schedules (Figure 3-2) with pulsed current profiles for AHSS can have weld-current ranges
similar to Mild steel.
Even though weld expulsion is not usually increased with AHSS, weld expulsion should be avoided
with AHSS. Loss of nugget material can affect weld-nugget size.
Figure 3-2 Schematics of optimized weld schedules for AHSS.
B-1
Post annealing (tempering pulse weld schedule) of TRIP steel may alter weld fracture mode (Figure
3-3) and weld current range (Figure 3-4). However, since studies have shown that the occurrence
of partial or interfacial fractures does not necessarily indicate poor weld quality, the use of pulsed
current is not required to improve weld quality. Further, the effect of current pulsing on tensile and
fatigue properties, as well as the electrode tip life, is not known. Therefore, users should perform
their own evaluations regarding the suitability of such modified parameters.
Figure 3-3 - Effect of tempering pulse weld schedule on TRIP steels.
B-1
Section 3 - Joining

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Version
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Page 3-4
Figure 3-4 - Post-annealing may enlarge weld current range.
B-1
Section 3 - Joining

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Version
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Page 3-5
3.B.1.b. Heat Balance - Material Balance - Thickness Balance
Heat input is defined as:
Heat Input =I
2
Rt
Where: I = Welding current
R = Interfacial and bulk resistance between two sheets
t = Welding time
The heat input has to be changed depending on the gauge and grade of the steel. Compared to
low strength steel at a particular gauge, the AHSS at the same gauge will need less current.
Similarly, the thin gauge material needs less current than thick gauge. Controlling the heat input
according to the gauge and grade is called heat balance in resistance spot welding.
For constant thickness, Table 3-1 shows steel classification based on strength level. With increasing
group numbers, higher electrode force, longer weld time and lower current are required for
satisfactory resistance spot welding. Material combinations with one group difference can be welded
with little or no changes in weld parameters. Difference of two or three groups may require special
considerations in terms of electrode cap size, force or type of power source.
Table 3-1 - Steel classification for resistance spot weld purposes.
Section 3 - Joining

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Page 3-6
For a particular steel grade, changes in thickness may require adoption of special schedules to
control heat balance. When material type and gage are varied together, specific weld schedules
may need to be developed. Due to the higher resistivity of AHSS, the nugget growth is preferentially
in AHSS. Electrode life on the AHSS-side may be reduced due to higher temperature at this side.
In general, electrode life of welding AHSS may be similar to Mild steel because of lower operating
current requirement due to higher bulk resistivity in AHSS. This increase in electrode life may be
offset in production due to poor part fit up created by higher AHSS springback.
3.B.1.c. Welding Current Mode
AHSS can be welded with both AC and DC modes (Figure 3-5). Mid-frequency direct current
(MFDC) has an advantage over conventional alternate current (AC) due to both unidirectional and
continuous current. These characteristics assist in controlling and directing the heat generation at
the interface. Current mode has no significant difference in weld quality. Both AC and DC can
easily produce acceptable welds where thickness ratios are less than 2:1. Some advantage is
gained using DC where thickness ratios are over 2:1 but welding practices must be developed to
optimize the advantages. It also has been observed that nugget sizes are statistically larger when
using DC welding with the same secondary weld parameters. Some studies have shown that
welding with MFDC provides improvements in heat balance and weld process robustness when
there is a thickness differential in AHSS (as shown in Figure 3-6).
Consult safety requirements for your area when considering MFDC welding for manual weld gun
applications. The primary feed to the transformers contains frequencies and voltages higher than
for AC welding.
Figure 3-5 - Range for 1.4 mm DP 350/600 cold-rolled steel at
different current modes with single pulse.
L-2
Section 3 - Joining

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Version
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Page 3-7
(a) Insufficient fusion at the interface
with AC power source.
(b) Button size of 3.5 mm with DC power source
Figure 3-6 - Effect of current mode on dissimilar thickness stack-up.
L-2
3.B.1.d. Electrode Geometry
Although there are differences in weld process depending on weld tip material and shape (truncated
cone and dome shape), AHSS can be welded with all weld tip shapes and materials. Dome shaped
electrodes ensure buttons even at lower currents due to higher current densities at the centre of
the dome shape (Figure 3-7). The curve of dome-shaped electrodes will help to decrease the
effect of electrode misalignment. However, dome electrodes might have less electrode life on
coated steels without stepper/dressing. Due to round edges, the dome electrode will have fewer
tendencies to have surface cracks when compared with truncated electrode.
Figure 3-7 - The effect of electrode geometry on current range
using AC power mode and single pulse.
L-2
Section 3 - Joining

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Version
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Page 3-8
3.B.1.e. Part Fit-up
Resistance welding depends on the interfacial resistance between two sheets. Good and consistent
fit-up is important to all resistance welding. Fit-up is even more critical to the welding of AHSS due
to increased yield strength and greater springback. With inconsistent part fit-up, large truncated
cone electrodes are recommended for both AHSS and conventional steels. The larger cap size will
have large current range, which might compensate the poor part fit-up. Also progressive electrode
force and upslope can be used to solve poor part fit-up.
3.B.1.f. Factory Equipment Template
Equipment for welding AHSS requires higher electrode force than for welding Mild steel.
3.B.1.g. Weld Evaluation by Carbon Equivalence
Existing carbon equivalence formulas for resistance spot welds do not adequately predict weld
performance in AHSS. Weld quality depends on variables such as thickness, strength, loading
mode, and weld size. New formulae are proposed by various entities. Because there is no universally
accepted formula, using any CE equation is not recommended.
3.B.1.h. Zinc Penetration/Contamination
Surface quality of coated AHSS spot welds is similar to Mild steel spot welds. Surface cracking
propensity is less with rounded-edge dome electrodes than with sharp-edge truncated cone
electrodes. Fatigue performance is the same for spot welds with and without surface cracks
appearing within the weld indentation.
Section 3 - Joining

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Version
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Page 3-9
Figure 3- 8 - Load bearing capacity of various spot welds on cold-rolled steel.
L-2
Steel type, grade, and any coatings indicated on bars.
3.B.1.i. Weld Integrity: Test Method and Joint Performance
Acceptable Weld Integrity Criteria
Weld integrity criteria varies greatly among manufacturers and world regions. Each AHSS user
needs to establish their weld acceptance criteria and the characteristics of AHSS resistance spot
welds. AHSS spot weld strength is higher than that of the Mild steel for a given button size (Figure
3- 8). It is important to note that partial buttons (plugs) or interfacial fractures do not necessarily
characterize a failed spot weld in AHSS. Interfacial fracture or partial buttons can be eliminated by
using alternative welding procedures compared to those used for Mild steel. The weld fracture
mode will be specified by the user depending on the desired performance characteristics of the
final application. Interfacial fractures may be typical of smaller weld sizes in Mild steel or in all weld
sizes in AHSS. Reference is made to the new Specification ANS/ANSI D8.1 Weld Quality Acceptance
for additional details.
Section 3 - Joining

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Page 3-10
Destructive and In-Process Weld Testing
Peel and chisel testing of resistance spot welds in AHSS may produce fracture through the weld
during destructive or teardown testing. This type of fracture becomes more common with increasing
sheet thickness and base material strength. Weld metal fracture may accompany significant distortion
of the metal immediately adjacent to the weld during testing. Such distortion is shown in Figures 3-
9 and 3-10. Under these conditions weld metal fracture may not accurately predict serviceability of
the joint. Weld performance of AHSS depends on microstructure, loading mode, loading rate, and
degree of constraint.
Figure 3-9- Example of laboratory dynamic destructive
chisel testing of DP 300/500 EG 0.65 mm samples.
M-1
Figure 3-10 - Example of laboratory dynamic destructive chisel testing of
DP 350/600 GI 1.4 mm samples.
M-1
Section 3 - Joining

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Version
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Page 3-11
Additionally, because of inherent stiffness of AHSS sheets, non-destructive chisel testing (Figure
3-11) on AHSS spot welded panels will deform the panel permanently and may promote weld
metal fracture. Therefore, this type of in-process weld check method is not recommended for
AHSS with thicknesses greater than 1.0 mm.
Alternative test methods should be explored for use in field-testing of spot welds in AHSS.
Figure 3-11 - Semi-destructive chisel testing in DP300/500 EG 0.8 mm.
M-1
Ultrasonic non-destructive spot weld testing has gained acceptance with some manufacturers. It
still needs further development before it replaces destructive weld testing completely. Some on-
line real time systems to monitor the resistance welding are currently available and are being
accessed in experimental weld shops.
Mechanical Strength of Welds
The AHSS weld tensile strength is proportional to material tensile properties and is higher than
Mild steel spot weld strength (Figure 3-12).
Figure 3-12 - Tensile shear strength of single spot welds.
L-4
Section 3 - Joining

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Version
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Page 3-12
While testing thick AHSS spot welds (from small button size to expulsion button) the fracture mode
during tension shear testing will change from shear to button or plug (as shown in Figure 3-13).
This does not mean that the welds failed. The fracture is due to the thickness effect. Despite
interfacial fractures (Figure 3-13A) welds in AHSS may show high load bearing capacity. In thin
gauge steels the failure is often in a button or plug (Figure 3-14).
A B
Figure 3-13- Fracture modes in thick (1.87mm) DP 700/980 CR during tension shear
testing. (A) Shear fracture at interface (low currents). (B) Fracture in button or plug
(high currents).
L-2
Figure 3-14 - Fracture modes in thin (0.65 mm) DP 300/500 EG
during tension shear testing.
L-2
Section 3 - Joining

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Version
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Page 3-13
Similar to Mild steel, an increase in the number of welds in AHSS will increase the component
fatigue strength (Figure 3-15). Multiple welds on AHSS will increase the fatigue strength more than
Mild steel.
Figure 3-15 - Effect of increase in number of welds in Mild steel and DP
steel component.
S-4
Figure 3-16 is a best-fit curve through numerous data points obtained from Mild steel, DP steels
with tensile strengths ranging from 500 to 980 MPa, and a MS steel with a tensile strength of 1400
MPa. The curve indicates that the fatigue strength of single spot welds does not depend on the
base material strength.
Figure 3-16 - A best fit curve through many data points for
Mild steel, DP steels, and MS steel. Fatigue strength of
single spot welds does not depend on base metal strength.
L-2
Section 3 - Joining

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Page 3-14
3.B.2 High Frequency Induction Welding
High Frequency Induction Welding (HFIW) is the main welding technology for manufacturing cold-
formed welded steel tubes. Welded tubes are normally made from flat sheet material by continu-
ous roll forming and the HFIW process. The tubes are widely used for automotive applications,
including seat structures, cross members, side impact structures, bumpers, subframes, trailing
arms, and twist beams. A welded tube can be viewed as a sheet of steel having the shape of a
closed cross-section.
Two things distinguish the welded tube from the original sheet material:
1 The work hardening, which takes place during the tube forming process.
2 The properties and metallurgy of the weld seam differ from the base metal in the tubular cross
section.
Good weldability is one precondition for successful high frequency welding. Most DP steels are
applicable as feed material for manufacturing of AHSS tubes by continuous roll forming and the
HFIW process. The quality and the characteristics of the weld depend on the actual steel sheet
characteristics (such as chemistry, microstructure, and strength) and the set-up of the tube manu-
facturing process.
Table 3-2 provide some characteristics of the high frequency welds in tubes made of DP 280/600
steel.
For DP 280/600 the hardness of the weld area exceeds the hardness of the base material (Figure
3-17). There is a limited or no soft zone in the transition from HAZ to base material. The nonexist-
ent soft zone yields a high frequency weld that is stronger than the base material (Table 3-2). This
is an essential feature in forming applications where the tube walls and weld seam are subject to
transverse elongation, such as in radial expansion and in hydroforming.
Table 3-2 - Transverse tensile test data for HFIW DP 280/600 tube.
R-1
Section 3 - Joining

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Version
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Page 3-15
Figures 3-18 and 3-19 contain additional examples of the hardness distribution across high fre-
quency welds in different materials with comparison to Mild steel.
Figure 3-18 - Hardness variation across induction welds
for various types of steel.
M-1
Figure 3-19 - Hardness variation across induction welds
of DP 350/600 to Mild steel.
D-1
Figure 3-17 - Weld hardness of a high frequency weld in a DP 280/600 tube.
R-1
Section 3 - Joining

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Version
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Page 3-16
3.B.3. Laser Welding - Fusion
3.B.3.a. Butt Welds and Tailor Welded Products
AHSS can be laser butt-welded and is used in production of tailored products (that is, tailor welded
blanks and tubes). The requirements for edge preparation of AHSS are similar to Mild steels. In
both cases, a good quality edge and a good fit-up are needed to achieve good results after laser
welding. The blanking of AHSS needs higher shear loads than Mild steel sheets (see unit on
Blanking, shearing, and trim operations in Section 2.C.6.).
If a tailored product is intended for use in a forming operation, a general stretchability test such as
the Erichsen (Olsen) cup test can be used for assessment of the formability of the laser weld.
AHSS with tensile strengths up to 800 MPa show good Erichsen test values (Figure 3-20). The
percent stretchability in the Erichsen test =100 x the ratio of stretchability of weld to stretchability
of base metal.
Figure 3-20 - Hardness and stretchability of laser butt welds with two AHSS sheets of the same
thickness. Erichsen test values used for describing the stretchability.
B-1
Section 3 - Joining

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Version
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Page 3-17
The hardness of the laser welds for AHSS is higher than for Mild steels (Figure 3-20). However,
good stretchability ratios in the Erichsen test can be achieved when the difference in hardness
between weld metal and base metal is only slightly higher for AHSS compared to Mild steels. If the
hardness of the weld is too high, a post-annealing treatment (using HF-equipment or a second
laser scan) may be used to reduce the hardness and improve the stretchability of the weld (see
TRIP steel in Figures 3-20 and 3-21).
Figure 3-21 - Improved stretchability of AHSS laser welds with an induction heating
post heat treatment. Testing performed with Erichsen cup test.
T-3
Laser butt-welded AHSS of very high strength (for example, MS steels) have higher strength than
GMAW welded joints. The reason is that the high cooling rate in the laser welding process prompts
the formation of hard martensite and the lower heat input reduces the soft zone of the HAZ.
Laser butt-welding is also used for welding tubes (see unit on Tube Forming in Section 2.D.).
Section 3 - Joining

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Version
3 33 33
Page 3-18
3.B.3.b. Assembly Laser Welding
Laser welding is often used for AHSS overlap joints. This type of weld is either a conventional weld
with approximately 50% penetration in the bottom sheet or an edge weld. Welding is performed in
the same way as for Mild steels, but the clamping forces needed for a good joint fit-up are often
higher with AHSS than for Mild steels. To achieve good laser welded overlap joints for zinc-coated
AHSS, a small intermittent gap (0.1-0.2 mm) between the sheets is recommended, which is identical
to zinc coated Mild steels. In this way the zinc does not get trapped in the melt, avoiding pores and
other imperfections. An excessive gap can create an undesirable underfill on the topside of the
weld. Some solutions for lap joint laser welding zinc-coated material are shown in Figure 3-22.
Figure 3-22 - Laser welding of zinc coated steels to tubular hydroformed parts.
L-3
Recent studies have shown welding zinc-coated steels can be done without using a gap between
the overlapped sheets. This is accomplished through the use of dual laser beams.
L-5
While the first
beam is used to heat and evaporate the zinc coating, the second beam performs the welding. The
dual laser beam configuration combines two laser focussing heads through the use of custom-
designed fixtures.
Section 3 - Joining

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Version
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Page 3-19
3.B.4. Arc Welding Uncoated Steels Fusion
Conventional arc welding (for example GMAW, TIG, and plasma) can be used for AHSS in a similar
way to Mild steels. The same shielding gases can be used for both AHSS and Mild steels.
Despite the increased alloying content used for AHSS, there are no increased welding imperfec-
tions compared with Mild steel arc welds. The strength of the welds for AHSS increases with
increasing base metal strength and decreasing heat input. Depending on the chemical composi-
tion in AHSS of very high strength (for example MS and DP steels with high martensite content) the
strength of the weld joint may be reduced in comparison to the base metal strength due to small
soft zones in HAZ (Figure 3-23). For AHSS of the type CP and TRIP, no soft zones occur in HAZ
due to the higher alloying content for these steels in comparison to DP and MS steels.
Figure 3-23 Relationship between martensite content and reduction in true
ultimate tensile strength. Data obtained by Gleeble simulation of high heat input
GMAW HAZ.
D-1
An increased strength of the filler metal is recommended for the strength of the welds for AHSS
with very high strength levels (Figure 3-24 for single-sided welded lap joint and Figure 3-25 for butt
joints). It should be noted that higher strength filler is more expensive and less tolerant to the
presence of any weld imperfections. When welding AHSS to lower strength or Mild steel it is
recommended that filler wire with 70 Ksi (482 MPa) strength be used. Single-sided welded lap
joints are normally used in the automotive industry. Due to the unsymmetrical loading and the extra
bending moment associated with this type of joint, the strength of this lap joint is lower than the butt
joint.
Section 3 - Joining

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Version
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Page 3-20
Figure 3-24 - Influence of filler metal strength in arc welding of DP and MS. Tensile
strength is 560 MPa for low strength and 890 MPa for high strength fillers. Fracture
position in HAZ for all cases except DP 700/1000 and MS 1200/1400 combination
with low strength filler where fracture occurred in weld metal. Tensile strength equals
peak load divided by cross-sectional area of sample.
C-3
Figure 3-25 - Influence of filler metal strength in GMAW (butt) welding on
weld strength for MS steel. Filler metal tensile strength range is 510-950
MPa.
B-1
Arc welds are normally used in local areas of the vehicle where the loads are high. As required with
all GMAW of any grade of steel, care should be taken to control heat input and the resulting weld
metallurgy. The length of the welds is often quite short. The reduction in strength for some of the
AHSS welds, in comparison to base metal, can be compensated by increasing the length of the
weld.
By adjusting the number and length (that is the total joined area) of welds, the fatigue strength of
the joint can be varied. The fatigue strength of an arc welded joint can be better than a spot welded
joint (Figure 3-26).
Section 3 - Joining

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Version
3 33 33
Page 3-21
Figure 3-26 - Fatigue strength of GMAW welded DP 340/600 compared to spot welding.
L-2
Section 3 - Joining

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Version
3 33 33
Page 3-22
3.C. Brazing
Brazing can be used to join zinc-coated AHSS. Today there are many commercial grades of arc
braze materials that can be used for AHSS without any additional corrosion issues. The most
common braze material is SG-CuSi3 (Table 3-3) mainly due to the wide melting range, which
reduces the risk for imperfections during the brazing. To increase joint strength, braze materials
with a higher amount of alloying elements are available at higher costs.
Table 3-3 - Properties for the braze material SG-CuSi3 used in brazing.
T-3
Results from tensile-shear testing and peel testing of the braze material SG-CuSi3 (Figure 3-27)
show that the brazed joint strength for SG-CuSi3 is somewhat lower than the base metal, except
for DP 340/600 in tensile peel condition.
Figure 3-27 - Tensile shear (fillet weld on lap joint) and tensile peel tests (flange weld) for
the braze material SG-CuSi3 of DP 340/600 (1.0 mm), TRIP 400/700 (1.0 mm) and CP
680/800 (1.5 mm). Shielding gas: Argon.
T-3
Section 3 - Joining

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Version
3 33 33
Page 3-23
3.D. Adhesive Bonding
The bond strength of an adhesive is constant, and in design applications, is proportional to the
area covered by the adhesive. The adhesive joint strength will be unchanged and analysis of the
joint should be comprehensive. In general, the use of AHSS with high-strength structural adhesives
will result in higher bond strength than for Mild steel if the same sheet thickness is applied (Figure
3-28). Reduction of sheet thickness will decrease the strength because more peel load will occur.
The true mechanical load in the component part must be considered. If higher joint strengths are
needed, the overlapped area may be enlarged. Adhesives with even higher strength are under
development.
Figure 3-28 - The effect of material strength on bond strength. W is the integral of the
force/elongation curve.
B-2
J oining of AHSS with adhesive bonding is a good method to improve stiffness and fatigue strength
in comparison to other joining methods (spot welding, mechanical joining, arc welding, and laser
welding). Due to the larger bonding area with adhesive bonding, the local stresses can be reduced
and therefore the fatigue strength is increased. These improvements in stiffness and fatigue strength
are important factors to consider at the design stage, especially in those cases when AHSS is
used to decrease the weight of a component.
Section 3 - Joining

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Version
3 33 33
Page 3-24
3.E. Mechanical Joining
Examples of mechanical joining are clinching and rivets. A schematic drawing of a mechanical
joining system is shown in Figure 3-29. A simple round punch presses the materials to be joined
into the die cavity. As the force continues to increase, the punch side material is forced to spread
outwards within the die side material.
Figure 3-29 - Schematic drawing of a clinching system.
T-4
This creates an aesthetically round button, which joins cleanly without any burrs or sharp edges
that can corrode. Even with galvanized or aluminized sheet metals, the anti-corrosive properties
remain intact as the protective layer flows with the material. Table 3-4 shows characteristics of
different mechanical joining methods.
Table 3-4 - The characteristics of mechanical joining systems.
N-1
Section 3 - Joining

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Version
3 33 33
Page 3-25
Circular clinching without cutting and self-piercing riveting (existing half-hollow-rivets) are not
recommended for materials with less than 40% hole expansion ratio ( ) as shown in Figure 3-30.
Clinching with partial cutting may be applied instead.
Figure 3-30 - Balance between elongation and stretch flangeability of 980 MPa class AHSS
and surface appearance of mechanical joint at the back side.
N-1
Warm clinching and riveting are under investigation for material with less than 12 percent total
elongation. As with any steel, equipment size and clinch/pierce force are proportional to the material
strength and tool life is inversely proportional to material strength.
The strength of self-piercing riveted AHSS is higher than for Mild steels. Figure 3-31 shows an
example of a self piercing rivet joining two sheets of 1.5 mm thick DP 300/500. AHSS with tensile
strengths greater than 900 MPa cannot be self-piercing riveted by conventional methods today.
Figure 3-31 - Example of DP 300/500 with a self-piercing rivet.
G-2
Section 3 - Joining

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3.F. Hybrid Joining
As for Mild steels, AHSS-hybrid joints can be made by combining adhesive bonding with resistance
spot welding, clinching, or self-piercing riveting. These hybrid joints result in higher strength values
(static, fatigue, crash) than the spot joining techniques alone (Figure 3-32). If local deformation
and buckles can be avoided during in-service applications of weldbonding/adhesive hybrid joining,
the potential for component performance is enhanced.
Figure 3-32 - Comparison of bearing capacity for single and hybrid joints.
B-3
Section 3 - Joining

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3.G. Material Issues For Field Weld Repair and Replacement
The American Iron and Steel Institute (AISI), in cooperation with North American OEMs, has
undertaken studies to understand the influences of in field repair practices on some AHSS.
A-5
Studies have been completed for DP, MS, and TRIP steels. In particular, the effects of MIG (Metal
Inert Gas) welding and a practice called flame straightening were examined.
Test results indicate that GMAW welding is acceptable as a repair method for AHSS such as DP,
MS, and TRIP. Mechanical properties are within the expected range for each material in close
proximity to the repair weld and are therefore acceptable.
However, flame straightening consists of heating a portion of the body or frame structure that has
been deformed in a collision to 650 C (dull cherry red) for 90 seconds and then pulling the deformed
portion of the structure to its original position. This heating cycle then could be applied twice. Test
results indicate that flame straightening should NOT be used to repair AHSS such as DP, MS,
and TRIP. The heating cycle causes degradation to the mechanical properties of as-formed (work
hardened) body part.
Therefore, repair of AHSS parts using GMAW in the field may be acceptable. In any event, the
OEMs specific recommendations for the material and vehicle should be followed.
Gl ossar Gl ossar
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Section 4 - Glossary
Advanced High-Strength Steel (AHSS): A series of high-strength steel types with novel metal-
lurgy and processing compared to conventional high-strength steels. This results in different com-
binations of higher strength levels, improved formability, and/or increased crash energy absorp-
tion.
Aging: A change in material property or properties with time.
Angular change: Springback resulting from a change in radius at the punch with a resulting change
in flange position usually described as a springback angle.
Anisotropy: Variations in one or more physical or mechanical properties with direction in the
sheet metal. Related terms are normal anisotropy, planar anisotropy, and plastic strain ratio.
Austenite: Normally not found in steel at room temperature, austenite is a homogeneous phase
consisting of a solid solution of carbon in the gamma form of iron. It is formed when steel is heated
to temperature above the upper critical point. Rapid quenching of the austenite will produce mar-
tensite.
Bake Hardening steel (BH): Any high-strength steel that increases in strength as a result of a
combination of straining and aging at a temperature and time typical of the automotive paint cure
cycle.
Bake hardening: Generally means a change in mechanical properties created during a typical
automotive paint bake cycle.
Bend: A simple bending process to reduce the sidewall curl because the sidewall does not un-
dergo one or more sequences of bend and unbend.
Binder: Alternatively called a blank holder or holddown. The part of a forming die that holds the
blank by pressure against a mating surface of the die to control metal flow and prevent wrinkling.
Burr: The rough cut edge of metal.
Carbon equivalent: Various equations using percent concentrations of carbon, manganese, chro-
mium, molybdenum, and sometimes other elements to predict the weldability of a given steel.
Carbon Manganese steel (CM): High-strength steels primarily strengthened by solid solution
strengthening.
Clinching: Mechanical joining systems where the punch forces the two sheets of metal to spread
outward in the die and interlock.

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Complex Phase steel (CP): A steel with very fine microstructure of ferrite and higher volume
fractions of hard phases that are further strengthened by fine precipitates.
Computerized forming simulation: More accurately defined as computerized forming-process
development, where the forming of a stamping is accomplished in the computer without construction
of hard tooling. Used to determine if the initial product design can be formed, evaluate various
product and process design options, and obtain additional production requirements such as
maximum required press load.
Cup drawing: A press forming operation in which a cup shaped (often cylindrical) part is produced
from a sheet metal blank (often circular in shape).
Curl (sidewall): Springback resulting from metal moving over a radius. Curl is characterized by an
average radius of curvature.
Die clearance: The space, on each side, between the punch and die.
Draw bead: A ridge constructed around a portion of a die cavity to partially restrain metal flow. A
groove in the mating blankholder allows die closing. Sometimes called a die bead.
Draw: A conventional forming operation with continuous blankholder force.
Dual Phase steel (DP): A steel consisting of a ferrite matrix containing a hard second phase in the
form of islands.
Elastic deformation: Deformation that will return to its original shape and dimensions upon removal
of the load or stress.
Elastic limit: The maximum stress to which a material may be subjected and yet return to its
original shape and dimensions upon removal of the stress.
Elongation: The amount of permanent extension in a tensile test or any segment of a sheet metal
stamping.
Embossing: Displacing a section of metal a minor amount without noticeable reduction in sheet
metal thickness or metal flow from surrounding sheet metal.
Engineering strain: The unit elongation given by the change in length divided by the original
length. Sometimes called the nominal strain.
Engineering stress: The unit force obtained when the applied load is divided by the original
cross-sectional area. Sometime called the nominal stress.
Erichsen test: A test in which a piece of sheet metal, restrained except at the centre, is deformed
by a spherical punch until fracture occurs. The height of the cup at fracture is a measure of ductility.
Similar to the Olsen test.

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Ferritic-Bainitic (FB) steel: Steel with a microstructure containing ferrite and bainite. The bainite
provides strength and replaces the islands of martensite in DP and TRIP steel to provide improved
edge stretchability.
Filler metal: Available in the form of rods, spooled wire, or consumable inserts to improve the
quality of the welded part.
Form: A forming process that allows the flange to be created in the last stage of forming and the
sheet metal undergoes only a slight amount of bend-unbend deformation.
Form-draw: A forming process in which the blankholder force is applied from the middle to last
stage of forming.
Forming Limit Curve (FLC): An empirical curve showing the levels of different combinations of
biaxial strain beyond which failure (local necking) may occur in sheet metal forming. The strains
are given in terms of major and minor strains measured from deformed circles previously imprinted
as circles into the undeformed sheet metal.
Gas Metal Arc Welding (GMAW): An arc welding process that uses a continuously fed consumable
electrode and a shielding gas. Common GMAW processes are MIG (metal inert gas) welding and
MAG (metal active gas) welding.
Heat Affected Zone (HAZ): A zone paralleling the weld zone where a change in properties has
taken place as a result of the heat generated by the welding process.
Heat balance: The phenomenon in resistance spot welding of balancing the heat input during the
weld based on the gauge and grade of steel.
High Hole Expansion steel: See Ferritic-Bainitic steel (FB).
High-Strength, Low-Alloy steel (HSLA): Steels that generally contain microalloying elements
such as titanium, vanadium, or niobium, which increase strength by grain size control, precipitation
hardening, and solid solution hardening.
High-Strength steel (HSS): By International Iron and Steel Institute definition, any steel product
whose initial yield strength is specified between 210 and 550 MPa or whose tensile strength is
specified between 270 and 700 MPa.
Hole expansion: A formability test in which a tapered (usually conical) punch is forced through a
punch or drilled and reamed hole forcing the metal in the periphery of the hole to expand in a
stretching mode until fracture occurs.
Hot-Formed steel (HF): A quenchable steel that is heated to transform the microstructure to
austenite and then immediately hot-formed and in-die quenched. Final microstructure is martensite.
HF steel provides a combination of good formability, high tensile strength, and no springback
issues. Most common HF steels are boron based.
Hybrid joining: Combining adhesive bonding with resistance spot welding, clinching, or self-piercing
riveting to increase the strength value.

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Isotropic steel (IS): A ferritic type of microstructure modified so the delta r value is equal to zero to
minimize any earing tendencies.
Instantaneous n-value: For some AHSS the n-value changes with strain. For these steels, the n
value is plotted as a function of strain. The n value at any specific value of strain is called the
instantaneous n-value.
Interstitial-Free steel (IF): Steels with very low amounts of carbon and nitrogen to which are
added small amounts of elements such as titanium or niobium to combine with the remaining
interstitial elements such as carbon and nitrogen to remove their strengthening effects.
Local elongation: Elongation measured over a very short gage length is controlled by the
microstructure, particularly the frequency of interfaces between islands of hard martensite and the
soft matrix of ferrite. Local elongation is measured by a conical punch hole expansion test and
given the symbol .
Limiting Draw Ratio (LDR): An expression of drawability given by the highest drawing ratio (blank
diameter divided by punch diameter) attained in a series of tests such as the Swift Cupping Test.
MAG: Metal Active Gas - See Gas Metal Arc Welding (GMAW).
Martensitic steel (MS): During processing, the microstructure is transformed almost entirely to
hard martensite.
Major strain: Largest principal strain in the sheet surface. Often measured from the major axis of
the ellipse resulting from deformation of a circular grid. Usually called major stretch in the press
shop.
Metal gainer: A preformed area of the stamping that creates lengths of line used to feed metal into
an area that normally would be highly stretched and torn. Likewise, a post-formed area of the
stamping created in an area of the stamping that has excess metal and normally would generate
buckles.
Mid-Frequency Direct Current (MFDC): MFDC has the advantage of both unidirectional and
continuous current.
Microstructure: The different phases and structure of metals are shown when a flat ground surface,
highly polished, and etched (different enchants for different phases) is magnified and observed in
a microscope. A picture of the microstructure is called a photomicrograph.
Mild steel: Low strength steels with essentially a ferritic microstructure and some strengthening
techniques. Drawing Quality (DQ) and Aluminium-Killed Draw Quality (AKDQ) steels are examples
and often serve as a reference base because of their widespread application and production volume.
MIG: Metal Inert Gas - See Gas Metal Arc Welding (GMAW).

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Minor strain: The principal strain in the sheet surface in a direction perpendicular to the major
strain. Often measured from the minor axis of the ellipse resulting from deformation of a circular
grid. Usually called minor stretch in the press shop.
Multiple stage forming: Forming a stamping in more than one die or one operation. Secondary
forming stages can be redraw, ironing, restrike, flanging, trimming, hole expansion, and many
other operations.
n-value: A term commonly referred to as work hardening exponent derived from the relationship
between true stress and true strain. Except for AHSS, the n value usually is a constant for a given
steel.
Nano steel: Hot-rolled higher strength steel designed to avoid low values of blanked edge
stretchability by replacing islands of martensite with an array of ultra-fine nano-sized particles.
Overbend: Increasing the angle of bend to compensate for springback angular change. Upon
springback from overbend, the part will match part print.
Plastic deformation: Upon exceeding the elastic limit of the sheet metal, a permanent or plastic
increment of deformation is created.
Plastic strain ratio (r): A measure of anisotropy is defined by the ratio of the true width strain to the
true thickness strain in a tensile test.
Post-annealing: An annealing cycle given to a stamping or portion of the stamping to recrystallize
the microstructure and improve the properties for additional forming operations.
Post-Formed Heat-Treatable steel (PFHT): A broad category of steels having various chemistries
designed to be first formed and then heated and quenched off-line in fixtures to obtain higher
strengths.
Post-stretch: A stretch process added to near the end of the forming stroke to neutralize sidewall
curl and/or angular change resulting from the stamping process. Active lock beads, lock steps, or
other blank locking methods are used to prevent metal flow from the blank while generating a
minimum of 2% additional sidewall stretch at the end of the press stroke.
Quasi-static: Traditionally refers to the strain rate during a tensile test, which is very slow compared
to deformation rates during sheet metal forming or a crash event.
r value: The ratio of true width strain to true thickness strain. Often called the plastic strain ratio.
Residual stresses: Elastic stresses that remain in the stamping upon removal of the forming load.
Sometimes called trapped stresses because the final geometry of the stamping does not allow
complete release of all elastic stresses.
Restrike: A secondary forming operation designed to bring the stamping to part print by correcting
for springback or any other cause of dimensional variation.

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Retained austenite: With proper chemistry and heat treating, some austenite can be retained at
room temperature. With sufficient cold work, the retained austenite will transform into martensite.
Sheared edge stretchability: Reduced residual stretchability of an as-sheared edge due to the
high concentration of cold work and work hardening at the sheared interface.
Shrink flanging: A bending operation in which a narrow strip at the edge of a sheet is bent down
(or up) along a curved line that creates shrinking (compression) along the length of the flange.
Simulative formability tests: These tests provide very specific formability information that is
significantly dependent on deformation mode, tooling geometry, lubrication conditions, and material
behaviour. Examples include hemispherical dome tests, cup tests, flanging tests, and other focused
areas of formability.
Springback: The extent to which metal deviates from its designed or intended shape after
undergoing a forming operation. Also the angular amount a metal returns toward its former position
after being bent a specified amount.
Strain gradient: A change in strain along a line in a stamping. Some changes can be very severe
and highly localized and will have an accompanying increase in thickness strain.
Square lock bead: A square ridge constructed around a die cavity to completely restrict metal flow
into the die.
Strain rate: The amount of strain per unit of time. Used in this document to define deformation rate
in tensile tests, forming operations, and crash events.
Stretch flange: A bending operation in which a narrow strip at the edge of a sheet is bent down (or
up) along a curved line that creates stretching (tension) along the length of the flange.
Stretch Flangeable steel: See Ferritic-Bainitic steel (FB).
Tempering pulse: A post weld heat treatment or post annealing to improve the weld fracture mode
and the weld current range.
Tensile Strength (TS): Also called the ultimate tensile strength (UTS). In a tensile test, the strength
calculated by dividing the maximum load by the original cross-sectional area.
Terminal n-value: The n-value at high strain levels, which is a parameter influencing the height of
the forming limit curve. In the absence of an instantaneous n-value curve, the terminal n usually is
measured in a tensile test between 10% stretch and maximum load or ultimate tensile strength.
Total elongation: A parameter measured in a tensile test used as a measure of ductility. Defined
by the final gage length minus original gage length divided by the original gage length and times
100.
Transformation-Induced Plasticity steel (TRIP): A steel with a microstructure of retained austenite
embedded in a primary matrix of ferrite. In addition, hard phases of martensite and bainite are
present in varying amounts. The retained austenite progressively transforms to martensite with
increasing strain.

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True strain: The unit elongation given by the change in length divided by the instantaneous gage
length.
True stress: The unit force obtained when the applied load is divided by the instantaneous cross-
sectional area.
Twinning-Induced Plasticity steel (TWIP): A high manganese steel that is austenitic at all
temperatures. The twinning mode of deformation creates a very high n value, a tensile strength in
excess of 900 MPa, and a total elongation in excess of 50%.
Twist: Twist in a channel is two cross-sections rotating differently along their axis.
Ultimate Tensile Strength (UTS): See Tensile Strength.
Ultra-High-Strength steel (UHSS): By International Iron and Steel Institute definition, any steel
product whose initial yield strength is specified at 550 MPa or greater or whose tensile strength is
specified at 700 MPa or greater.
ULSAB-AVC: UltraLight Steel Auto Body Advanced Vehicle Concepts. Information is available at
www.worldautosteel.org.
ULSAC: UltraLight Steel Auto Closures. Information is available at www.worldautosteel.org.
Work hardening exponent: The exponent in the relationship where is the true
stress, K is a constant, and is the true strain.
Yield Strength (YS): The stress at which a steel exhibits a specified deviation (usually 0.2%
offset) from the proportionality of stress to strain and signals the onset of plastic deformation.
R R
R RRef ef
ef ef ef er er
er er er enc es enc es
enc es enc es enc es

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Section 5 - References
A-1. Body Systems Analysis Team, Automotive Sheet Steel Stamping Process Variation, Auto/Steel Partner-
ship (Summer 1999) www.a-sp.org.
A-2. High Strength Steel (HSS) Stamping Design Manual, Auto/Steel Partnership (2000).
A-3. High Strength Steel (HSS) Stamping Design Manual, Auto/Steel Partnership (1997).
A-4. Courtesy of M. Munier, Arcelor.
A-5. American Iron and Steel Institute, Advanced High-Strength Steel Repairability Studies: Phase I Final
Report and Phase II Final Report, www.autosteel.org.
B-1. H. Beenken, J oining of AHSS versus Mild Steel, Processing State-of-the-Art Multi-phase Steel; Euro-
pean Automotive Supplier Conference, Berlin (September 23, 2004).
B-2. H. Beenken et al, Verarbeitung Oberflchenveredelter Stahlfeinbleche mit Verschiedenen Fgetechniken,
Groe Schweitechnische Tagung 2000, Nrnberg, (September 27, 2000). DVS-Berichte Bd. 209, Schweien
und Schneiden (2000).
B-3. H. Beenken, Hochfeste Stahlwerkstoffe und ihre Weiterverarbeitung im Rohbau,
Fgetechnologien im Automobilleichtbau, AUTOMOBIL Produktion, Stuttgart, (March 20, 2002).
C-1. B. Carlsson, P. Bustard, D. Eriksson, Formability of High Strength Dual Phase Steels, Paper F2004F454,
SSAB Tunnplt AB, Borlnge, Sweden (2004).
C-2. B. Carlsson, Choice of Tool Materials for Punching and Forming of Extra- and Ultra High Strength Steel
Sheet, 3
rd
International Conference and Exhibition on Design and Production of Dies and Molds and 7
th
International Symposium on Advances in Abrasive Technology, Bursa, Turkey (J une 17-19, 2004).
C-3. V. Cuddy et al, Manufacturing Guidelines When Using Ultra High Strength Steels in Automotive Applica-
tions, EU Report (ECSC) R585 (J anuary 2004).
C-4, D. Corjette et al, Ultra High Strength FeMn TWIP Steels for Automotive Safety Parts, SAE Paper 2005-
01-1327 (2005).
D-1. Courtesy of A. Lee, Dofasco Inc.
F-1. T. Flehmig, K. Blmel, H. Stein, A New Method of Manufacturing Hollow Sections for Hydroformed Body
Components, International Body Engineering Conference, Detroit, USA (2000).
F-2. T. Flehmig, K. Blmel, M. Kibben, Thin Walled Steel Tube Pre-bending for Hydroformed Component
Bending Boundaries and Presentation of a New Mandrel Design, SAE Paper 2001-01-0642, Detroit, USA
(2001).
G-1. J . Gerlach, K. Blmel, U. Kneiphoff, Material Aspects of Tube-hydroforming, SAE Paper 1999-01-3204,
Detroit, USA (1999).

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G-2. S. Gkl, Innovative Fgetechnologien beim Einsatz Neuartiger Stahlwerkstoffe fr den
Schienenfahrzeugbau, Fgen und Konstruieren im Schienenfahrzeugbau, SLV Halle, (May 21, 1997).
G-3. S. Gkl et al, The Influence of Corrosion on the Fatigue Strength of J oined Components from Coated
Steel Plate, Materials and Corrosion 50, p.1 (1999).
H-1. R. Hilsen et al, Stamping Potential of Hot-Rolled, Columbium-Bearing High-Strength Steels, Proceedings
of Microalloying 75 (1977).
H-2. B. Hgman et al, Blanking in Docol Ultra High Strength Steels, Verschleischutztechnik, Schopfheim,
Germany (2004) and G. Hartmann Blanking and Shearing of AHS Steels Quality Aspects of Sheared Edges
and Prediction of Cutting Forces, ACI Conference; Processing State-of-the Art Multiphase Steels, Berlin,
Germany (2004).
H-3. G. Hartmann, Das Spektrum Moderner Stahlfeinbleche-Festigkeiten und Auswirkungen auf die Umformung
Verschleischutztechnik, Schopfheim, Germany (2004).
I-1. International Iron and Steel Institute, UltraLight Steel Auto Body - Advanced Vehicle Concepts (ULSAB
AVC) Overview Report (2002), www.worldautosteel.org.
I-2. AutoCo AHSS Working Group, International Iron and Steel Institute.
K-1. A. Konieczny, Advanced High Strength Steels Formability, 2003 Great Designs in Steel, American Iron
and Steel Institute (February 19, 2003), www.autosteel.org.
K-2. S. Keeler, Increased Use of Higher Strength Steels, PMA Metalforming magazine (J uly 2002).
K-3. A. Konieczny, On the Formability of Automotive TRIP Steels, SAE Technical Paper No. 2003-01-0521
(2003).
K-4. T. Katayama et al, Effects of Material Properties on Shape-Fixability and Shape Control Techniques in
Hat-shaped Forming, Proceedings of the 22
nd
IDDRG Congress, p.97 (2002).
K-5. Y. Kuriyama, The Latest Trends in Both Development of High Tensile Strength Steels and Press Forming
Technologies for Automotive Parts, NMS (Nishiyama Memorial Seminar), ISIJ , 175/176, p.1 (2001).
L-1. S-D. Liu, ASP HSS Load Beam Springback Measurement Data Analysis, Generalety Project Report
#001023 (May 27, 2004).
L-2. S. Lalam, B. Yan, Weldability of AHSS, Society of Automotive Engineers, International Congress,
Detroit (2004).
L-3. R. Laurenz, Bauteilangepasste Fgetechnologien, Fgetechnologien im Automobilbau, Ulm (February
11, 2004).
L-4. R. Laurenz, Spot Weldability of Advanced High Strength Steels (AHSS), Conference on Advanced
J oining, IUC, Olofstrm, (February 2,2004).

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L-5. F. Lu and M. Forrester, Proceedings of the 23
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International Congress on Applications of Lasers and
Electro Optics (2002).
M-1. Courtesy of S. Lalam, Mittal Steel.
N-1. Courtesy of K. Yamazaki, Nippon Steel Corporation.
N-2. M. F. Shi, Internal National Steel Corporation report.
N-3. J . Noel, HSS Stamping Task Force, Auto/Steel Partnership.
R-1. Courtesy of P. Ritakallio, Rautaruukki Oyj.
S-1. M. Shi, G. Thomas, X. Chen and J . Fekete, Formability Performance Comparison between Dual Phase
and HSLA Steels, Proceedings of 43
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Mechanical Working and Steel Processing, Iron & Steel Society, 39,
p.165 (2001).
S-2. M. Shi, Springback and Springback Variation Design Guidelines and Literature Review, National Steel
Corporation Internal Report (1994).
S-3. S. Sadagopan and D. Urban, Formability Characterization of a New Generation of High Strength Steels,
American Iron and Steel Institute (March 2003).
S-4. Singh et al, Selecting the Optimum J oining Technology, p.323 and Increasing the Relevance of Fatigue
Test Results, MP Materialprfung, 45, 7-8, p.330 (2003).
S-5. Courtesy of D. Eriksson, SSAB Tunnplt AB.
T-1. M. Takahashi et al, High Strength Hot-Rolled Steel Sheets for Automobiles, Nippon Steel Technical
Report No. 88 (J uly 2003).
T-2. M. Takahashi, Development of High Strength Steels for Automobiles, Nippon Steel Technical Report No.
88 (J uly 2003).
T-3. Courtesy of ThyssenKrupp Stahl.
T-4. Courtesy of TOX PRESSOTECHNIK GmbH & Co. KG, Weingarten.
U-1. M. Ueda and K. Ueno, A Study of Springback in the Stretch Bending of Channels, J ournal of Mechanical
Working Technology, 5, p.163 (1981).
V-1. Courtesy of C. Walch, voestalpine Stahl GmbH.
Y-1. B. Yan, High Strain Rate Behavior of Advanced High-Strength Steels for Automotive Applications, 2003
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Y-2. K. Yoshida, Handbook of Ease or Difficulty in Press Forming, Translated by J . Bukacek and edited by S-
D Liu (1987).

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