EP2569112B1 - Method for producing a structural part from an iron-manganese-steel sheet - Google Patents

Description

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 in body weight, which can reduce fuel consumption. 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 security parts such as e.g. 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. Cold working, however, leads to a reduction in ductility due to strain hardening in formed areas and thus to a reduction in the energy absorption potential in the event of a load (crash). Such inhomogeneous mechanical component properties caused by the work hardening can lead to the component not meeting the safety requirements. Other disadvantages of the cold forming technique are that it increases the risk of delayed cracking by hydrogen embrittlement, the formed part a significant springback (so-called "spring back" effect) shows and cold-formed components have an insufficient numerical simulability of the component behavior in the load case.

Hot forming offers a well-known alternative to the cold forming process. 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. A disadvantage of hot forming, however, are the high process temperatures and the material-dependent reduction in the strength of the component caused by the hot working after the cooling process.

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. During hardening, an austenitic structure is produced by heating the component to the so-called hardening temperature above Ac3, which is then completely converted into martensite by rapid cooling. Condition for the complete martensite transformation is that a critical cooling rate 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.

From the DE 10 2008 050 315 A1 A method of manufacturing a ferro-manganese steel sheet member is known, comprising the steps of cold working a sheet metal workpiece in a mold and heating the formed sheet metal workpiece to a temperature between 500 ° C and 700 ° C.

One object of the invention is to 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 further developments 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 the heated sheet metal workpiece 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 method according to the invention can be seen in the fact that by calibrating the heated component, both the risk of delayed cracking due to hydrogen embrittlement and the springback of the component after removal from the calibration tool are substantially reduced.

It should be noted that at the stated temperatures the austenitizing temperature Ac3 is not exceeded, i. that during heating no transformation of the workpiece structure into a fully austenitic structure occurs.

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 formed Even under the strength in areas not or less heavily deformed areas can be lowered ranges. In order to avoid excessive degradation of work hardening, a temperature between 600 ° C and 680 ° C may be advantageous. For heating the deformed sheet metal workpiece to the required during calibration elevated temperature, the formed sheet metal workpiece can be heated in an oven and inserted after 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 during calibration can also be 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 oven can be chosen so that a homogeneous heating of the sheet metal workpiece is ensured, it should be noted that with increasing thickness of the sheet metal workpiece is typically to be estimated an extension of the time for the warm-up.

In the calibration tool a rapid cooling of the sheet metal workpiece is carried out in the held state. Since no cooling of the austenite microstructure into the martensite microstructure, as required in the case of so-called press-hardening, has to be effected during cooling, the critical minimum cooling rate known from press-hardening does not have to be adhered to, ie. the cooling rate in the calibration tool may be determined according to other considerations (for example, cycle times, operating costs, tooling costs, etc.).

For the reduction of strain hardening in formed sections of the sheet metal workpiece, the heating temperature of the formed Blechwerkstückes of importance. In one embodiment, this can be adjusted so that the work hardening in formed portions of the (formed) sheet metal workpiece by the calibration by at least 70%, in particular at least 80%, degraded.

According to a further embodiment, the heating temperature of the sheet metal workpiece can be adjusted so that the calibrated sheet metal workpiece over its entire geometry has a maximum variation in 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.

Fig. 1

eine schematische Darstellung einer Abfolge von Verfahrensschritten nach einem Ausführungsbeispiel der Erfindung; und

Fig. 2

ein Schaubild, in welchem die Härte eines umgeformten Bauteils gegenüber einer Distanz vom Umformort aufgetragen ist.

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

a schematic representation of a sequence of method steps according to an embodiment of the invention; and

Fig. 2

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

Embodiments of a manufacturing method of an iron-manganese steel sheet member will be described below. The component may be, for example, a body component for vehicle construction. The body component may have a complex component geometry. It may be a structural and / or safety part, which may have to meet special safety requirements in the event of a load (crash). For example, the component may be an A or B pillar, a side impact protector in doors, a sill, a frame part, a bumper, a cross member for floor and roof or act on a front or rear side member.

The component consists of an iron-manganese (FeMn) steel. FeMn components are known in the automotive industry 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 (TWining Induced Plasticity) steels 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 parts in accident-relevant areas of the body.

TRIP / TWIP steels are combinations of TWIP and TRIP steels (TRANSformation Induced Plasticity). TRIP steels essentially consist of several 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 (a-martensite). With TRIP / TWIP steels, a double TRIP effect occurs because the austenitic structure is first converted into hexagonal and then cubic body-centered martensite. Due to the two martensitic transformations TRIP / TWIP steels have a double expansion reserve.

TRIPLEX steels consist of a multi-phase structure of α-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 exemplary list of the above-mentioned steels is not exhaustive, other FeMn steels can also be used for the invention.

Fig. 1 shows a schematic embodiment of an embodiment of a method according to the invention, wherein also optional method steps are shown. Starting point of the procedure is a coil 1 made of strip steel, as it is produced for example in a steel mill and delivered to a customer (eg vehicle manufacturers or suppliers). 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 the FeMn steel strip in the steel mill should be designed to ensure good cold workability of the steel.

The steel strip will then be e.g. cut at 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 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. Strong work hardening up to about 1800 MPa can occur. The tensile strength of the starting material (board 2) may be, for example, at R _m ≈ 1100 MPa, the yield strength eg R _p0.2 ≈ 600 MPa and the elongation at break A of the starting material may be 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 cold forming tool 3 can be realized in the form of a 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 bleed can be the final trim of the part. Furthermore, any necessary punching or the creation of a hole pattern in the cold forming tool 3 can be made. That is, after the cold forming step, a component having a completely finished component shape may already exist with respect to material removing processes.

It is also possible that material removing processes (trimming, hole patterning etc.) are performed in a cutting line (not shown) located outside and behind the cold forming tool 3 (which is in the so-called press line). In this case too, the final component may already be present after trimming or hole pattern generation with respect to 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 in the oven may be 10 minutes, with 5 minutes being used for achieving the homogeneous temperature distribution and the remaining 5 minutes 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, convective heating, inductive heating or 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 close to the final shape, since the calibration in the calibration tool significantly reduces the 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, about 30 ° C / s, but should not be critical because unlike the press hardening no critical cooling rate must be exceeded. For example, the cooling rate may be less than 50 ° C / s, which can be achieved without major tooling effort and in many cases sufficiently short cycle times possible. Higher cooling rates, for example in the range of 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 the subsequent "held" cooling of the workpiece in a fixed workpiece geometry, the work hardening achieved in the areas of high elongation is reduced, ie reduced, equalized or possibly even overcompensated, as later in connection with Fig. 2 is explained.

The temperature of the heated workpiece at the beginning of calibration may also be within the stated range of T = 500 ° C to 700 ° C, or only slightly less. This can be ensured that 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 furnace 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.

The basis Fig. 1 described embodiments of the invention can be modified and developed in many ways. 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 may be carried out electrolytically or by a hot dip process prior to the cold forming step 3 (e.g., already at the steelmaker on coil 1) or even after the cold forming step 3 and before heating in the furnace 4. The heat treatment before or during calibration forms a mixed crystal layer between the FeMn steel and the Zn coating in the case of a Zn coating, which ensures good adhesion of the Zn layer to the component. It is also possible to coat the coating (e.g., galvanizing) only on the finished component, i. after calibration in the calibration tool 5 make.

Fig. 2 refers to further embodiments of the basis Fig. 1 exemplified method and illustrates the reduction of work hardening as a function of the workpiece temperature achieved during heating. 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 several blanks 2, several identical wells were deep drawn by means of a cold forming tool 3, 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 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 reference numeral 6. Fig. 2 shows for the cold-drawn, unheated cup a strain hardening in the range of R _m = 1600 MPa (corresponds to Hv = 520), see reference numeral 7, which leads to strong inhomogeneous mechanical properties in the component. In addition, the risk of delayed cracking due to hydrogen embrittlement is increased, as this occurs especially where a high work hardening gradient is observed during cold working.

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 near the forming site is still R _m ≈ 1490 MPa (Hv = 480), at T = 600 ° C the maximum strain hardening is already at R _m ≈ 1330 MPa (Hv = 430). decreased, T = 650 ° C leads almost to a leveling of the mechanical properties (R _m ≈ 1120 MPa, corresponds to Hv = 360) in formed and non-formed sections of the component, and at T = 700 ° C results in overcompensation, ie Workpiece strength in the section close to the forming section is R _m ≈ 870 MPa (Hv = 280) and is thus significantly below the tensile strength in sections of the workpiece (wells) which were not or only slightly reshaped.

Out Fig. 2 It can be seen that by the choice of a suitable temperature T for the hot calibration, the strain hardening in the region of a component close to the forming area can be selectively influenced and, if desired, reduced to a certain 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 illustrates that the heat treatment and the hot calibration only affect and degrade the increased strength values caused by work hardening, while the mechanical properties hardly change in the remaining sections of the work piece which are not subjected to forming. In other words, it can be achieved that a component with complex component geometry has homogeneous mechanical properties over its entire extension or that it attains specifically increased or reduced strength at transformation sites in comparison to non-deformed sections.

Claims (10)

1. Method for producing a structural part from an iron-manganese steel sheet, comprising the steps:

   cold-forming a sheet metal workpiece (2) in a forming die (3) ;

   heating the formed sheet metal workpiece to a temperature of between 500°C and 700°C; and

   calibrating the heated formed sheet metal workpiece by fixing the formed sheet metal workpiece in a calibrating die (5) in the final shape and held cooling of the heated formed workpiece with fixed workpiece geometry at an initial temperature for the calibration in the range of between 500°C and 700°C.
2. Method according to claim 1, wherein the temperature is between 600°C and 680°C.
3. Method according to claim 1 or 2, comprising the step:

   heating the formed sheet metal workpiece in a furnace (4); and

   inserting the heated sheet metal workpiece into the calibrating die (5).
4. Method according to claim 3, wherein the residence time of the sheet metal workpiece in the furnace (4) is chosen so as to ensure substantially homogeneous through-heating of the sheet metal workpiece.
5. Method according to one of the preceding claims, wherein the iron-manganese steel sheet is a TWIP steel, TRIP/TWIP steel or TRIPLEX steel.
6. Method according to one of the preceding claims, wherein the manganese content of the iron-manganese steel sheet is between 12 and 35% by weight.
7. Method according to one of the preceding claims, wherein the temperature is set such that cold work-hardening in formed portions of the formed sheet metal workpiece is reduced by at least 70%, in particular 80%, by the calibration.
8. Method according to one of the preceding claims, wherein the temperature is set such that the calibrated sheet metal workpiece has a maximum tensile strength fluctuation range of 20%, in particular 10%, over its entire geometry.
9. Method according to one of the preceding claims, further comprising:
   coating the sheet metal workpiece with an organic and/or inorganic or metallic coating, in particular an alloy based on zinc or aluminum, before the cold forming.
10. Method according to one of the preceding claims, further comprising:
    coating the sheet metal workpiece with an organic and/or inorganic or metallic coating, in particular an alloy based on zinc or aluminum, after the calibration.

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