EP2569112A1

EP2569112A1 - Method for producing a structural part from an iron-manganese-steel sheet

Info

Publication number: EP2569112A1
Application number: EP11720423A
Authority: EP
Inventors: Ludovic Samek; Martin Peruzzi; Enno Arenholz
Original assignee: Voestalpine Stahl GmbH
Current assignee: Voestalpine Stahl GmbH
Priority date: 2010-05-12
Filing date: 2011-05-06
Publication date: 2013-03-20
Anticipated expiration: 2031-05-06
Also published as: CN103003002B; WO2011141367A1; KR101567132B1; CN103003002A; DE102010020373A1; ES2764790T3; US9138797B2; EP2569112B1; KR20130036250A; US20130125607A1

Abstract

In a method for producing a structural part from an iron-manganese-steel sheet (1), a sheet-metal workpiece (2) is cold-formed in a forming die (3). The formed sheet-metal workpiece is heated to a temperature between 500°C and 700°C (4) and calibrated in a calibrating die (5).

Description

description

Process for producing a component from an iron-manganese steel sheet

The invention relates to a method for producing a component from an iron-manganese steel sheet.

Iron-manganese steels are lightweight steels that can have high strength and high ductility at the same time. This makes iron-manganese steels a material with great potential in vehicle construction. A high material strength allows a reduction of the body weight, where ^¬ by the fuel consumption can be reduced. High extensibility and stability of the steels is important both for the production of the body parts by deep-drawing processes and for their crash behavior. For example, structure and / or safety parts such as

Door impact beams, A- and B-pillars, bumpers or longitudinal and transverse beams can realize complex component geometries and at the same time achieve the weight targets and safety requirements.

It is already known to produce body parts made of iron-manganese steel sheet by cold forming. However, the cold forming through-hardening in the reshaped areas in a decrease in ductility and thus to a reduction of the energy absorption potential in Bela ^¬ stungsfall (crash). Such inhomogeneous mechanical component properties caused by work hardening can lead to the component not meeting the safety requirements. Further disadvantages of the cold forming technique are that it increases the risk of delayed cracking due to hydrogen embrittlement, that the formed part has a significant springback behavior (so-called "spring back"). Effect) shows and cold formed components have insufficient numerical simulability of the component behavior in Bela ^¬ stungsfall. Hot forming offers a well-known alternative to

Cold forming. Conventional hot forming processes are carried out at high temperatures of about 900 ° C or above. Hot working reduces both springback of the formed component and strain hardening in formed areas. Thus, with the hot forming technique complex deep drawn parts can be produced without appreciable spring-back in one go. However, a disadvantage of hot forming is the high process temperatures and the material-dependent reduction in the strength of the component after the cooling process, caused by the hot forming.

To avoid the reduction in strength, hot forming is often combined with hardening technology. This is based on the known possibility of strengthening of steel materials by martensite formation. Upon curing an austenitic structure is generated by a heating of the component on the so-called hardness ^¬ temperature above Ac3, which is subsequently converted by rapid cooling TENSIT completely in Mar-. Condition for the full Mar- TENSIT conversion is that a critical Abkühlge ^¬ speed is exceeded. For this purpose, it requires cooled pressing tools that allow a sufficiently rapid cooling of the workpiece by contact of the hot workpiece surface with the cold tool surface.

An object of the invention is based can be seen to provide a method with which the production of formed parts made of iron-manganese steel sheet with good mechanical properties is made possible cost. In particular, the method should be the Manufacture of formed sheet metal workpieces with complex component geometry and favorable material properties even in formed component areas. The problem underlying the invention is solved by the features of claim 1. Advantageous ^embodiments and refinements are specified in the dependent claims. There is provided a method of manufacturing a ferro-manganese steel sheet member in which a sheet metal workpiece is cold-formed in a molding die, the formed sheet metal workpiece is heated to a temperature between 500 ° C and 700 ° C, and heated

Sheet metal workpiece is calibrated in a calibration tool. By calibrating the formed sheet metal workpiece at the indicated, elevated temperatures can be achieved that a cold work hardening occurred in the cold forming is degraded again in the formed areas. In particular, homogenization of the mechanical properties over the entire component can thereby be achieved. Further advantages of the inventive method is to se ^¬ hen that embrittlement than the calibration of the heated component both the risk of delayed cracking by hydrogen-also the resilience of the component after removal from the calibration are substantially reduced.

It should be noted that in the above-mentioned tempera ^¬ ren the austenitizing temperature Ac3 is not exceeded, that is to say that structure occurs in a fully austenitic structure when heated no conversion of the workpiece.

The degree of degradation of strain hardening in the reformed part regions can be controlled by the choice of temperature. At high temperatures, the strength of the molded areas are even lowered below the strength in not or we ^¬ niger heavily deformed areas. In order to avoid excessive degradation of work hardening, a temperature between 600 ° C and 680 ° C may be advantageous. For ER of the formed sheet metal work piece to the required warming at Kali ^¬ brieren elevated temperature may be heated, the formed sheet metal work piece in an oven and be inserted after the heating in the calibration. It is also conceivable that the heating of the sheet metal workpiece takes place directly in the calibration tool. In both cases, the initial temperature may be Tempe ^¬ during calibration also in the specified range between 500 ° C and 700 ° C. During calibration, then a cooling of the formed sheet metal workpiece takes place in a held or fixed state.

The residence time of the sheet metal workpiece in the furnace can be so ge be ^¬ selected so that a homogeneous heating of the sheet metal workpiece is ensured, taking into account that with increasing thickness of the sheet metal workpiece typischerwei- se an extension of the time duration for the warming-up operation to be estimated is.

In the calibration tool is a rapid cooling of the

Blechwerkstückes made in the held state. Since no such required the so-called press hardening structural transformation from austenite structure must be effected in the Marten- sit-structure during cooling does not have to be adhered to the critical from the press ^¬ cure known minimum cooling rate, ie the rate of cooling in the Kalibrierwerk- convincing can according to other aspects (for example, cycle times, operating costs, tool costs, etc.).

For the reduction of strain hardening in formed sections of the sheet metal workpiece, the heating temperature of the converted shaped sheet metal workpiece of importance. In an execution ^¬ for example it can be adjusted so that the Kaltver ^¬ fortification in the reshaped portions of the (transformed)

Blechwerkstückes by the calibration by at least 70%, in particular at least 80%, is degraded.

According to a further embodiment, the Erwär ^¬ mung temperature of the sheet metal workpiece can be adjusted so that the calibrated sheet workpiece over its entire geometry of a maximum fluctuation range of the tensile strength of

20%, in particular 10%. In other words, a substantial homogenization of the mechanical component properties in terms of tensile strength is achievable. The invention will be explained in more detail below with reference to the description with reference to the drawings in an exemplary manner. In the drawings show:

Fig. 1 is a schematic representation of a sequence of

Method steps according to an embodiment of the invention; and

Fig. 2 is a graph in which the hardness of a formed component is plotted against a distance from the Umformort.

In the following embodiments are described for a herstel ^¬ averaging method of a component of iron-manganese steel sheet. The component may be, for example, a body component for vehicle construction. The chassis ^¬ riebauteil may have a complex part geometry. It may be a structural and / or safety part, which may be required to meet special security requirements at Be ^¬ lastungsfall (crash). For example, the component may be an A or B pillar, a side pillar Impact protection zträger in doors, a sill, a frame part, a bumper, a cross member for floor and roof or to act on a front or rear side member.

The component consists of an iron-manganese (FeMn) steel. FeMn components are known in the vehicle and may have a manganese ^¬ content of about 12 to 35 wt%. For example, TWIP, TRIP / TWIP and TRIPLEX steels as well as mixed forms of these steels can be used.

TWining steels (TWining Induced Plasticity) are austenitic steels. They are characterized by a high manganese content (for example over 25%) and relatively high alloying additions of aluminum and silicon. In cold plastic deformation, intensive twin formation takes place, which solidifies the steel. TWIP steels have a high elongation at break. They are therefore particularly suitable for the production of structural or safety components in accident-relevant areas of the body.

TRIP / TWIP steels are combinations of TWIP and TRIP steels (TRANSformation Induced Plasticity). TRIP steels are ^¬ be essentially of a plurality of phases of iron-carbon alloys, namely ferrite, bainite and carbon-rich residual austenite. The TRIP effect is based on the deformation-induced transformation of residual austenite into the high-strength martensitic phase (CC martensite). In TRIP / TWIP steels a double TRIP effect occurs because the austenitic structure is first converted into the hexagonal and then into the ku ^¬ bisch centered martensite. Due to the two martensitic transformations TRIP / TWIP steels have a double expansion reserve. TRIPLEX steels consist of a multiphase microstructure of 0C ferrite and γ-austenite mixed crystals with a martensitic s-phase and / or K-phase. They have good formability.

Furthermore, combinations of said steels may be used in embodiments of the invention. The list of examples of the above steel is not from ^¬ closing, other FeMn steels can likewise for the invention if used.

Fig. 1 shows schematically an embodiment of a method according to the invention, wherein also optional Ver ^¬ drive steps are shown. The starting point of procedural rensablaufs a coil of steel strip 1 as example ^¬ example is made in a steel mill and to a customer (eg vehicle manufacturer or supplier) is shipped. The FeMn strip steel may be, for example, a cold-rolled and annealed steel. However, it is also possible to use a hot-rolled steel. The manufacturing process of FeMn strip steel in the steel plant should be designed from ^¬ that a good cold formability of the steel is guaranteed. The steel strip is then cut, for example, in the vehicle manufacturer or supplier in FeMn boards 2. The cutting takes place in a cutting station.

One or more boards 2 are then placed in a cold forming tool 3 and cold formed. The temperatures in the cold forming tool can be in the usual range, for example at about 70 ° C to 80 ° C. Furnaces are not used to realize these temperatures. The residence time of the workpiece in the cold forming tool 3 is typically without any significant influence on the workpiece properties. During cold forming, locally different strengths are achieved depending on the component geometry. The greater the local degree of deformation, the higher the corresponding strength value. This effect is also called cold work hardening. It can strong Kaltverfesti ^¬ conditions occur up to about 1800 MPa. The tensile strength of the

Starting material (board 2), for example, at R _m ~ 1100

MPa are, the yield strength, for example, R _p0 .2 ~ 600 MPa and the elongation at break A of the starting material may be, for example, 40% or more (A> 40%). During cold forming, springback can be accommodated and the workpiece can be reshaped beyond its final geometric dimension. However, this is not absolutely necessary due to the subsequent process steps. The Kaltumformwerkzeug 3 can be implemented in the form ei ^¬ ner deep drawing press.

Furthermore, it is possible for a trimming of the workpiece to be carried out simultaneously in the cold forming tool 3. This crop may be the final crop of the

Act component. Furthermore, any necessary punching or the creation of a hole pattern in the cold forming tool 3 can be made. Ie, after the cold forming a component with fully ready meals imputed component shape in terms of material removing Prozes ^¬ se may already exist.

It is also possible that material removing processes

(Trimming, hole imaging, etc.) in a cutting line (not shown) are made that (which is located in the so-called ^¬ press line is located) is arranged outside and behind the Kaltumformwerkzeug. 3 Even in the case ^¬ sem already the Endbau- part may be after trimming or hole imaging in terms of material removing processes. The cold formed and optionally trimmed workpiece is then fed to a furnace 4 and heated there to a temperature between 500 ° C and 700 ° C. The heating should be carried out so long that the component is brought homogeneously to a uniform temperature (T = 500 ° C - 700 ° C). Upon reaching the uniform temperature, it can be kept at this temperature for a certain time. For example, the residence time may be minutes in the oven 10, where 5 min used for the achievement of homogeneous Temperaturvertei ^¬ lung and another 5 min for holding the component at this homogeneous temperature. However, since with the temperature increase, no crucial for the component properties microstructure transformation is connected, the heating step should be feasible without holding time. It is possible that the oven temperature is significantly higher than the desired target temperature T = 500 ° C - 700 ° C of the workpiece and the workpiece temperature over the residence time in the oven 4 is controlled.

As a furnace 4, a radiation furnace can be used or it can be provided furnaces that supply the workpiece in another way energy. For example, a convective heating, an inductive heating or an infrared heating, as well as combinations of said mechanisms may be used.

The heated to the target temperature between 500 ° C and 700 ° C, formed workpiece is then removed from the oven 4, placed in a calibration tool 5 and fixed there in the desired shape and cooled. The temperature of the workpiece at the beginning of the calibration process may also be lower than the temperature of the workpiece during removal from the furnace, it may in particular be between 400 ° C and 700 ° C. The calibration tool 5 may be, for example act a sizing press. Calibration ensures the dimensional accuracy of the workpiece. The surface geometry of the pressing surfaces of the tool corresponds to the final shape of the workpiece or is very close to the final shape, since the calibration in the calibration tool significantly reduces springback. By holding the workpiece in the calibration tool in the desired shape thus the workpiece is given the final shape. The cooling of the workpiece takes place in the calibration tool 5 when the workpiece is fixed, ie when the workpiece surfaces abut the tool surfaces. The heat dissipation takes place via the tool. The cooling rate may be, for example unge ^¬ ferry 30 ° C / s, however, is likely to be critical since Toggle DERS than the press hardening no critical Abkühlgeschwindig ^¬ ness must be exceeded. For example, the Ab ^¬ cooling rate may be less than 50 ° C / s, which is achievable without size ^¬ ren tool technical effort and in many cases allows sufficient short cycle times. Higher cooling rates, for example in the range from 50 ° C / s to 150 ° C / s, are also possible. The calibration tool 5 may have a cooling device (eg water cooling). By heating and subsequent "held" cooling of the work ^¬ tee in fixed workpiece geometry, the surfaces in the areas of strong expansion achieved hardening is reduced, thus decreasing, leveled or possibly even überkompen ^¬ Siert, as later in connection with FIG. 2. The temperature of the heated workpiece at the beginning of the calibration may also be within the stated range of T = 500 ° C to 700 ° C, or only slightly less, This may be ensured by the transport path between the oven 4 and the calibration tool 5 is short and / or that the heated workpiece on the transport path between the oven 4 and the calibration tool 5 is heated or kept warm, for example, by thermal radiation. Another possibility is to realize the furnace 4 and the calibration tool 5 in one and the same ^¬ press station, ie to provide a calibration tool 5, which is coupled to a furnace.

Described with reference to FIG. 1, embodiments of the invention ^¬ He can be modified in many ways and trained. For example, coated FeMn steels can be used for the process. The sheet metal workpiece can be coated with an organic and / or inorganic or metallic coating, in particular an alloy based on zinc or aluminum. The coating can be done before cold forming or at another time, eg after calibration.

A cathodic corrosion protection is effected for example by a galvanizing. The coating can be carried out electrolytically or by a hot-dip method before the cold-forming step 3 (for example, already at the steel manufacturer on the coil 1) or after the cold-forming step 3 and before heating in the furnace 4. By the heat treatment before or during the calibration, a mixed crystal layer forms between the FeMn steel and the Zn coating in the case of a Zn coating, which ensures good adhesion of the Zn layer on the component. It is also possible to ^{carry out the} coating (eg galvanizing) only on the finished component, ie after the calibration in the calibration tool 5. Fig. 2 relates to further embodiments of the hand at ^¬ Fig. 1 method exemplified and illu strated ^¬ the reduction of work-hardening, depending on the reached upon heating the workpiece temperature. The Vickers hardness Hv is shown as a function of the distance from the place of transformation. Was used a circuit board 2, which was cut from a cold-rolled, annealed FeMn steel strip. The board 2 had a tensile strength R _m ~ 1100 MPa, which corresponded to the tensile strength of the strip steel. The elongation at break was A = 60%. From a plurality of blanks 2 by means of a cold forming 3 more identical

Cups deep-drawn, whose diameter was D = 50 mm. The wells were then heated in an oven 4 to the different temperatures T = 500 ° C, 600 ° C, 650 ° C and 700 ° C. The residence time in the oven 4 was 10 minutes, so that a complete and homogeneous heating of the wells was guaranteed. Immediately thereafter and at substantially the same temperature T, the hot wells were fixed in a calibration tool 5 in the final shape and cooled down there ^¬ . The cooling rate in this example was about 30 ° C / s.

The Vickers hardness Hv can be used as a measure of the tensile strength R _m , wherein the conversion factor is about 3.1, ie a Vickers hardness Hv = 350 corresponds approximately to a tensile strength R _m ~ 1100 MPa of the starting material, see Be ^¬ zugszeichen 6. Fig. 2 shows for the cold-drawn, not him ^¬ warmed cups work hardening in the range of

R _m = 1600 MPa (corresponds to Hv = 520), see reference numeral 7, which leads to strongly inhomogeneous mechanical properties in the component. Furthermore, the risk of delayed Rissbil ^¬ dung is increased by hydrogen embrittlement, as this particular occurs where a high Kaltverfe ^¬ stigungsgradient is observed during cold forming. The hot calibration according to the invention leads to the reduction of strain hardening in the wells. At a temperature T = 500 ° C the tensile strength is in the vicinity of the Umformor ^¬ tes nor R _m = 1490 MPa (Hv = 480), at T = 600 ° C is the ma ^¬ ximum hardening (already on R _m = 1330 MPa Hv = 430) decreased, T = 650 ° C leads almost to a leveling of the mechanical properties (R _m = 1120 MPa, equivalent to Hv = 360) in formed and non-formed sections of the component, and at T = 700 ° C results in overcompensation, ie the workpiece strength in the section close to the transformation is R _m ~ 870 MPa (Hv = 280) and thus lies significantly below the tensile strength in sections of the workpiece

(Wells) that were not or only slightly reshaped. From FIG. 2 it can be seen that, by selecting a suitable temperature T for the hot calibration, the work hardening in the area of a component near the forming area can be selectively influenced and, if desired, reduced to a specific value. For example, homogeneous mechanical properties in terms of tensile strength can be achieved with a variation of less than 20%, or even 10%, in terms of reshaped and unformed portions of the component. It is also possible to reduce the strain hardening, for example by 70% or 80%. FIG. 2 shows that only affects caused by hardening increased strength values by the heat treatment and the hot calibration and down ^¬ be built, while the mechanical properties in the remaining portions of the workpiece to be subjected to forming, hardly change. In other words, it can be achieved that a component having complex component ^¬ geometry over its entire extent has homogeneous mechanical properties or that it acquires specifically increased or decreased strength compared to non-deformed portions at locations forming.

Claims

claims

1. A method for producing a component from an iron-manganese steel sheet, comprising the steps of:

Cold forming a sheet metal workpiece (2) in a mold (3);

Heating the shaped sheet-metal workpiece to a temperature Tem ^¬ between 500 ° C and 700 ° C; and

Calibrating the heated sheet metal workpiece in a calibration tool (5).

2. The method of claim 1, wherein the temperature is between 600 ° C and 680 ° C.

3. The method of claim 1 or 2, comprising the step:

Heating the formed sheet metal workpiece in an oven (4); and

Inserting the heated sheet metal workpiece in the Kali ^¬ brierwerkzeug (5).

4. The method of claim 3, wherein the residence time of the sheet metal workpiece in the oven (4) is chosen so that a substantially homogeneous heating of the sheet metal workpiece is guaranteed.

5. The method according to any one of the preceding claims, wherein the iron-manganese steel sheet is a TWIP steel, TRIP / TWIP steel or TRIPLEX steel.

6. The method according to any one of the preceding claims, wherein the manganese content of the iron-manganese steel sheet is between 12 and 35% by weight.

7. The method according to any one of the preceding claims, wherein the temperature is adjusted so that a Kaltverfesti- The calibration shall be reduced by at least 70%, in particular 80%, in formed sections of the formed sheet metal work piece by the calibration.

8. The method according to any one of the preceding claims, wherein the temperature is adjusted so that the calibrated sheet metal workpiece over its entire geometry has a maximum variation in tensile strength of 20%, in particular 10.

9. The method according to any one of the preceding claims, comprising the step:

Coating of the sheet metal workpiece with an organic and / or inorganic or metallic coating, in particular a special alloy based on zinc or aluminum, before cold forming.

10. The method according to any one of the preceding claims, comprising the step:

Coating of the sheet metal workpiece with an organic and / or inorganic or metallic coating, in particular a special alloy based on zinc or aluminum, after calibration.