Metal Rolling process and Equipment

Chapter 13
 General purpose :

* Description of the flat-rolling process.

* Analyzing the force, torque, and power.

* Defects and their causes in rolled products.

 Introduction

Rolling: Is the process of reducing the thickness

or changing the cross section of a long
workpiece by compressive force applied
through a set of rolls .

(Figure 13.1)
 Flat-Rolling and
Shape-Rolling
Processes

Figure 13.1 Schematic

outline of various flat-
rolling and shape-rolling
processes. Source: After
the American Iron and
Steel Institute.
Typical products made by various rolling
process : Plates for ship ,bridges ,structures
,machines ,car bodies , aircraft fuselages
,appliances ,containers .

Alternative process: continuous casting, extrusion, drawing .

Rolling process represent about 90% of all
metal produced by metalworking processes .

Modern steelmaking practices and the

production of various ferrous and nonferrous
metals and alloys now involve combining
continuous casting with rolling processes.

This improves productivity and lower production

costs.
Nonmetallic materials also are rolled to reduce
their thickness and enhance their properties.

Typical applications are in the rolling of plastics,

powder metals, ceramic slurry, and hot glass.
•Rolling first is carried out at elevated
temperature (Hot rolling) ,in this phase the
coarse-grained ,brittle, and porous structure of
the ingot is broken down into a wrought
structure having a finer grain size and
enhanced properties, increased strength and
hardness.

-but when carried out at room temperature (cold

rolling) , the rolled product has higher strength
and hardness and a better surface finish.
 Plates and Sheets

Plates Sheets

thickness more than 6 mm Less than 6 mm

Application Ship hulls ,boilers Automobile, aircraft

,bridges ,machinery bodies, food and
,nuclear vessels. beverage containers,
kitchen and office
equipment.

Aircraft fuselages and trailer bodies usually are made of a minimum of

1-mm thick aluminum-alloy sheets .
 The flat-rolling process

A metal strip of thickness h0 enter the Roll gap

and is reduce to thickness hf by a pair of
rotating rolls .

(individually by electric motors)

 Flat-Rolling Process

whay Vf > Vunder roll

Figure 13.2 (a) Schematic illustration of the flat-rolling process. (b) Friction forces acting on
strip surfaces. (c) Roll force, F, and the torque, T, acting on the rolls. The width of the strip,
w, usually increases during rolling, as shown later in Fig. 13.5.
Vr : Surface speed of the rolls.
V0 : Velocity of the strip (entry value).
Vf : Velocity of the strip (exit from the roll gap).
L : Roll gap.

The surface speed of the rigid roll is constant.

Relative sliding occur between the roll and the strip
along the arc of contact in the roll gap (L).

Neutral point (no-slip point) : its one point along the

contact length.
In this point the velocity of the strip is the same as
that of the roll .
 To the left of Neutral point the roll moves faster
than the strip.
To the right of Neutral point the strip moves faster
than the roll.

The frictional force oppose motion between two sliding

bodies act on the strip.
The rolls pull the material into the roll gap through
a net frictional force on the material.
(the net frictional force must be to the right)

The frictional force to the left of the Neutral

point must be higher than the friction force to
the right.
•Increasing friction also increase rolling forces and
power requirements.

-High friction could damage the surface of the

rolled product.

-A compromise is made in practice :low and

controlled friction is induced in rolling through the
use of effective lubricants.
 Maximum possible draft : is the difference
between the initial and final strip thickness
( h0 – hf )

( h 0 – h f )= µ² R

R :roll radius
µ :coefficient of friction
higher friction and larger roll radius implies greater
maximum possible draft
hight losses of energy
hight corrogen
roll force, Torque, and power Requirements.

The rolls apply pressure on the flat strip in order to

reduce its thickness resulting in a roll force ,F,
(this force appears as perpendicular to the plane
of the strip rather than at an angle).

(The arc of contact is very small compared with the roll radius)
The roll force in flat rolling can be expressed as:

F = L w Yavg

L : roll-strip contact length. L=(R(t1-t2))^0.5

w : width of the strip.(initial width)
Yavg : average true stress of the strip in the roll gap.

(this equation is for frictionless situation)

 The torque on the roll is : torque = F*a
a=L/2
the power required per roll
(assume that force act in the middle of the arc of contact a=L/2)
the total power of two roll is :

Power (kW) = (2πFLN) / 60,000 (S.I. units)

F: in Newton's.
L: in meter.
N: revolutions per minute of the roll.
 the total power can be expressed as :

Power (hp) = (2πFLN) / 33,000 (English units)

F: in pound.
L: in feet.
N: revolutions per minute of the roll.
Reducing Roll Force.

Roll force can cause significant deflection and

flattening of the rolls.
The rolls have to be set closer than originally
calculated in order to compensate for this
deflection and to obtain the desired final thickness.
 Roll Arrangements

Figure 13.3 Schematic illustration of various roll arrangements: (a) four-high

rolling mill showing various features. The stiffness of the housing, the rolls, and the
roll bearings are all important in controlling and maintaining the thickness of the
rolled strip; (b) two-hill mill; (c) three-high mill; and (d) cluster (or Sendzimir) mill.
Roll force can be reduced by the following means:

* Reducing friction at the roll-work piece interface.

* Using smaller diameter rolls to reduce the contact area.
* Taking smaller reductions per pass to reduce the contact
area.
* Rolling at elevated temperatures to lower the strength of
the material.
* Applying front and/or back tension to the strip.
An effective method of reducing roll force is to
apply longitudinal tension to the strip during rolling
(as a result of which the compressive stress
required to plastically deform the material
become smaller) .

Because they require high roll force, tension are

important in rolling high-strength metals.
 Tension can be applied to the strip at:
entry zone (back tension)
exit zone (front tension)
“either or both”

back tension: applied to the sheet by applying a

braking action to the reel that supplies the sheet
into roll gap (pay-off reel) by some suitable means.

Front tension: applied by increasing the rotational

speed of the (tack-up reel).
Rolling can be carried out by front tension only,
with no power supplied to the rolls—a process
known as Steckel rolling.
Geometric considerations

because of the force acting on them, rolls undergo

changes in shape during rolling.
Just as a straight beam deflects under a
transverse load, roll forces tend to bend the rolls
elastically during rolling.
Figure 13.4 (a) Bending of
straight cylindrical rolls caused by
roll forces. (b) Bending of rolls
ground with camber, producing a
strip with uniform thickness
through the strip width.
Deflections have been
exaggerated for clarity.
As a result of roll bending , the rolled strip tends to be
thicker at its center than its edges (crown).

The usual method of avoiding this problem is to grind

the roll in such away that their diameter at the center is
slightly larger than at their edges (camber).

When the roll bends, the strip being rolled now has a
constant thickness along its width (figure 13.4.b)
For rolling sheet metals, the radius of the
maximum camber point is generally 0.25 mm
grater than that at the edges of the roll.

To reduce the effects of deflection, the rolls also

can be subjected to external bending by applying
moments at their bearings .
Rolls can become slightly barrel shaped
(thermal camber ); because of the heat generated by
plastic deformation during rolling.
(strips are thinner at the center than at the edges) .

The total camber can be controlled by:

adjusting the location and the flow rate of the coolant
along the length of the rolls during hot rolling.

Roll force also tend to flatten the rolls elastically

Spreading

In rolling plates and sheet with high width-to-thickness

ratios, the width of the strip remains effectively
constant during rolling.
With smaller ratios (such as a strip with a square cross
section) its width increase significantly as it passes
through the rolls, this increase in width is called
s preading .
(Figure 13.5)
 Spreading in Flat Rolling

Figure 13.5 Increase in strip width (spreading) in flat rolling. Note that
similar spreading can be observed when dough is rolled with a rolling pin.
Spreading increase with:

(a) decreasing width-to-thickness ratio of the entering

strip (because of reduction in the width constraint).

(b) increasing friction.

(c) decreasing ratio of the roll radius to the strip

thickness.
Vibration and Chatter

Vibration and Chatter can have significant effects on

product quality and the productivity of metalworking
operations.

Chatter, defined as self-excited vibration , can

occur in rolling as well as in extrusion, drawing,
machining, and grinding operations.
-Most two important parameters effect on chatter:

(a) rolling speed.

(b) lubrication Chatter can be reduced by:

-Chatter can be reduced by:

(a) increasing the distance between the stands of the rolling mill.
(b) increasing the strip width.

(c) decreasing the reduction per pass (draft).

(d) increasing the roll radius.

(e) increasing the strip-roll friction.

(f) incorporating dampers in the roll supports.

Flat-rolling Practice

The initial rolling steps (breaking down) of the

material typically is done by hot rolling.
A cast structure typically is dendritic, and it include
coarse and nonuniform grains; this structure is brittle
and may be porous.
Hot rolling converts the cast structure to a wrought
structure with finer grains and enhanced ductility.

(Figure 13.6)
 Effects of Hot Rolling

Figure 13.6 Changes in the grain structure of cast or of large-grain wrought

metals during hot rolling. Hot rolling is an effective way to reduce grain size
in metals for improved strength and ductility. Cast structures of ingots or
continuous castings are converted to a wrought structure by hot working.
Typical temperature range for hot rolling are about
450 °C for aluminum alloy, up to 1250 °C for alloy
steels, and up to 1650 °C for refractory alloys.

(table 14.3)
The product of the first hot-rolling operation is called
a Bloom, a slab, or a billet.

(see fig. 13.1)

 A bloom usually has a square cross section ,(at least 150 mm on
the side).
(blooms are processed further by shape-rolling into structural
shapes such as I-Beams and railroad rails )

A slab usually is rectangular in cross section ,(slabs are rolled

into plates and sheets)

A billet usually has a square cross section ,(with across-

sectional area smaller than blooms)
(billet rolled into various shapes such as round rods and bars)
In the hot rolling of Blooms, billets, and slabs, the
surface of the material usually is conditioned
(prepared for subsequent operation) prior to rolling
them.

Conditioning is often done by means of a torch

(scarfing) to remove heavy scale or by rough grinding
to smoothen surfaces.

Prior to cold rolling, the scale developed during hot

rolling maybe removed by pickling with acids (acid
etching), by such mechanical means as blasting with
water, or by grinding to remove other defects as well.
Cold rolling

Is carried out at room temperature and compared with

hot rolling, produces sheets and strips with a much
better surface finish (because of lack of scale), better
dimensional tolerances, and enhanced mechanical
properties (because of strain hardening)
Pack rolling

Is a flat-rolling operation in which two or more layers of

metal are rolled together, thus improving productivity.

For example Aluminum foil is pack rolled in two

layers, only the top and bottom outer layers have been
in contact with the rolls.

One side of aluminum foil is matte, while the other side

is shiny.
The foil-to-foil side has a matte and satiny finish.

The foil-to-roll side is shiny and bright.

(because it has been in contact under high contact
stresses with the polished rolls during rolling).
Rolled milled steel, when subsequently stretched
during sheet-forming operations, undergoes
yield-point elongation (section 16.3) a phenomenon
that causes surface irregularities called
stretcher strains or Lüder's bands.

To correct last situation, the sheet metal is subjected

to a final, light pass of 0.5 to 1.5% reduction known as
temper rolling or skin pass shortly before stretching.
A rolled sheet may not be sufficiently flat as it leaves
the roll gap, due to factors such as variations in the
incoming material or in the processing parameters
during rolling.

To improve flatness, the rolled strip typically goes

through a series of leveling rolls.

(Figure 13.7)
 Roller Leveling

Figure 13.7 (a) A method of roller leveling to flatten rolled sheets.

(b) Roller leveling to straighten drawn bars.
 Defects in Rolled Plates and Sheets.

Defects can be classified into two type:

(a) surface defect.

(b) internal structural defect.

Defects are undesirable not only * because they

compromise surface appearance but also * because
they may adversely affect strength, formability, and
other manufacturing characteristics.
Surface defects:

Have been identified in sheet metals such as

( scale, rust, scratches, gouges, pits, and cracks )

these defects maybe caused by inclusions and

impurities in the original cast material or by various
other conditions related to material preparation and to
the rolling operation.
•Wavy edges on sheets are the result of roll bending.
(Figure 13.8 a)

the strip is thinner along its edges than at its center,

the edges elongate more than the center .
(see figure 13.4 a)

the edges buckle because they are constrained by the

central region from expanding freely in the longitudinal
(rolling) direction.
Defects in Flat Rolling

Figure 13.8 Schematic

illustration of typical
defects in flat rolling: (a)
wavy edges; (b) zipper
cracks in the center of the
strip; (c) edge cracks; and
(d) alligatoring.
The cracks are the result of poor materials ductility at
the rolling temperature.
(figure 13.8 b, c)

Because the quality of the edges of the sheet may

affect sheet-metal-forming operations, edge defects in
rolled sheets often are removed by shearing and
slitting operations.
Alligatoring is a complex phenomenon and typically is
caused by nonuniform bulk deformation of the billet
during rolling or by the presence of defects in the
original cast material.

(figure 13.8 d)
Other characteristics of rolled metals.

Residual stresses.
Because of nonuniform deformation of the material in
the roll gap, residual stresses can developed in rolled
plates and sheets, especially during cold rolling.
 (a)
Small diameter rolls or small thickness reductions per
pass tend to plastically deform the metal more at its
surfaces than in the bulk.
(figure 13.9 a).

This situation results in compressive residual stresses

on the surfaces and tensile stress in the bulk.
 Residual Stresses Developed in Rolling

Figure 13.9 (a) Residual stresses developed in rolling with small-diameter rolls or at small
reductions in thickness per pass. (b) Residual stresses developed in rolling with large-
diameter rolls or at high reductions per pass. Note the reversal of the residual stress patterns.
 (b)
Large diameter or high reductions per pass tend to
deform the bulk more than the surfaces.
(figure 13.9 b).

This situation due to the higher frictional constraint at

the surfaces along the arc contact.
A situations that produces residual stress distributions
that are the opposite of those with small-diameter rolls.
 Dimensional Tolerances.

Thickness Flatness
tolerances tolerances

Cold-rolling ± 0.1 to 0.35 ± 15 mm/m.

mm

Hot-rolling ± 55 mm/m.

Thickness tolerances depending on the thickness.

Tolerance are much grater for hot-rolled plates.
 Surface Roughness.

Cold rolling produce very fine surface finish

(products not require additional finishing operations)

Hot rolling and sand casting produce the same range

of surface roughness.
 Various Rolling processes

Shape-Rolling Operations

• Various shapes can be produced by shape rolling

– Bars
– Channels
– I-beams
– Railroad rails

• Roll-pass design requires considerable experience

in order to avoid external and internal defects
 Stages in Shape Rolling of an H-section part. Various other
structural sections such as channels and I-beams, are rolled by this
kind of process.
RING ROLLING

(a) Schematic illustration of

Ring-rolling operation.
Thickness reduction
results in an increase in
the part diameter.
(b) Examples of cross-
sections that can be
formed by ring-rolling
 Thread Rolling

• Cold-forming process
• Straight or tapered threads are formed on round rods by passing
the pipe though dies
• Typical products include:
– Screws
– Bolts
 Thread Rolling Con’t
• Threads may then be heat treated, and subjected to final machining or
grinding
 Production of Seamless Pipe & Tubing
• Rotary tube piercing (Mannesmann process)
– Hot-working process
– Produces long thick-walled seamless pipe
– Carried out by using an arrangement of rotating rolls
• Tensile stresses develop at the center of the bar when it is subjected to compressive
forces