METHOD FOR PRODUCING A FORMED COMPONENT FROM A STEEL BLANK, USE OF SUCH A COMPONENT, AND CORRESPONDING BLANK AND COMPONENT

Abstract

A method for producing a component from a blank made of a medium manganese steel having 4 to 12 wt. % Mn and a TRIP effect at room temperature, in which method the blank is mechanically cut to make a prepared blank having the desired dimensions, cut edges are produced on the prepared blank by means of mechanical cutting, and the prepared blank with the cut edges is cold-formed to obtain the component at room temperature or at a temperature above room temperature but below 60° C. The method is distinguished by cost-effective production, improved formability with reduced cracking at the formed cut edges, while simultaneously reducing the forming forces. The mechanical cutting is performed at a pre-heating temperature in the range of 60° C. to less than 250° C.

Description

CROSS REFERENCE TO RELATED APPLICATION

The present application claims the priority benefits of International Patent Application No. PCT/EP2022/051161, filed Jan. 20, 2022, and claims benefit of German patent application no. 10 2021 101 267.8, filed on Jan. 21, 2021.

BACKGROUND AND FIELD OF THE INVENTION

The invention relates to a method for producing a component from a steel plate with a TRIP effect at room temperature. The invention also relates to the use of such a component. The steel is a medium manganese-containing steel having 4 to 12 wt. % Mn, preferably having more than 5 to less than 10 wt. % Mn.

The invention also relates to the use of a component produced in this manner and to a corresponding plate and a corresponding component.

A component is understood hereinafter to mean a component produced from a steel plate by forming by means of a forming tool at room temperature. The steel plates can be provided uncoated or with a metallic and/or organic corrosion-protection coat.

Such components are used mainly in vehicle body manufacture but can also be used in the household appliance industry, in mechanical engineering or construction.

The intensely competitive automobile market constantly forces producers to find solutions to reducing their fleet consumption whilst maintaining the highest possible level of comfort and passenger protection. On the one hand, the weight saving across all of the vehicle components plays a decisive role as does, on the other hand, the most favourable possible behaviour of the individual components in the event of high static and dynamic loading during operation and even in the event of a crash.

The suppliers of precursor materials attempt to take the necessary material requirements into consideration in that by providing high-strength and ultra-high-strength steels the wall thicknesses can be reduced whilst at the same time achieving improved component behaviour during manufacture and operation.

Therefore, these steels must satisfy comparatively stringent requirements in terms of strength, extensibility, toughness, energy consumption and corrosion-resistance and their processability e.g. during cold-forming and welding.

Typically, in order to produce a component a sheet metal plate is initially cut to size from a hot or cold strip at room temperature. The cutting methods used are mostly mechanical separation methods, such as e.g. shearing or punching, shear-cutting or trimming, and less frequently also thermal separation methods, such as laser cutting, are used.

Thermal separation methods are significantly more cost-intensive compared to mechanical separation methods, and so they are used only in exceptional cases. Reference is made hereinafter only to mechanical separation methods.

After cutting, the cut plate is placed in a forming tool and the finished component, such as e.g. a chassis beam, is produced in single-stage or multi-stage forming steps.

Before the steel plate is formed, various further manufacturing steps are carried out on a case by case basis, such as e.g. punching and cutting operations on the plate and combined flanging operations on perforated portions during the forming procedure. The above operations include, in particular, an operation within the plate blank.

The edges or separation surfaces produced by means of mechanical separation methods are summarised hereinafter as separation edges.

During forming, the separation edges, especially when they are placed upright or raised, are subjected to particular loading, e.g. in collar operations in perforated plates.

Various types of preliminary damage can be present at the separation edges. On the one hand, this is due to cold hardening of the material caused by mechanical separation which represents total formation up to material separation. On the other hand, a notch effect can occur which is caused by the topography of the separation surface.

Medium manganese-containing steels have a multi-phase microstructure with residual austenite proportions. Further phase constituents can be ferrite, bainite and martensite as well as tempered martensite. The residual austenite content in the steel is adjusted by heat treatment in the two-phase region by means of intercritical annealing. At room temperature, deformation-induced twinning (TWIP effect) or deformation-induced martensite formation (TRIP effect) can occur in these steels. The TRIP effect which occurs predominantly during forming at RT causes the conversion of austenite into martensite, whereby the material hardens, the forming forces are correspondingly high. In addition to the hardening as a result of the increase in dislocation density, the hardening occurs by reason of the microstructure conversion of austenite into martensite. The martensite proportion simultaneously reduces the remaining residual deformation capability and the resistance to delayed cracking by hydrogen by reason of the low hydrogen solubility in martensite compared to austenite.

The cutting and punching of medium manganese-containing steels at room temperature leads to mechanical stress on the separation edge, which can initiate local stress-induced and/or deformation-induced martensite formation and can cause small incipient cracks in the strip edge. The TRIP martensite newly formed by the mechanical stress on the separating edge reduces the deformation capability of the edges, whereby, during subsequent forming procedures, a poorer hole expansion capability and lower edge formability are achieved in general. Furthermore, in the presence of hydrogen, increased hydrogen-induced delayed cracking or hydrogen embrittlement can occur.

Therefore, especially in the case of materials with a TRIP effect not only but also at room temperature, there is an increased probability of cracking in the edge regions of these separation edges during subsequent forming at corresponding temperatures.

The aforementioned preliminary damage at the separation edges can lead to premature failure during subsequent forming operations or during operation of the component. The testing of the forming behaviour of cut metal sheet edges with regard to their edge crack sensitivity is performed with a hole expansion test according to ISO 16630.

In the hole expansion test, a circular hole is introduced into the metal sheet by means of shear cutting, the hole is then expanded by means of a conical punch. The measurement variable is the change in hole diameter, related to the initial diameter, at which the first crack through the metal sheet occurs at the edge of the hole.

Basically, in order to improve the formability of medium manganese-containing steels, it is known e.g. from laid-open document DE 10 2016 117 494 A1 to perform the forming step at a temperature of the flat steel product of 60° C. to below Ac3, preferably of 60° C. to 450° C. By way of the formation with preheating of the flat steel product prior to the first forming step, conversion of metastable austenite into martensite (TRIP effect) is to be completely or partially suppressed during the forming procedure, wherein deformation twins (TWIP effect) can form in the austenite. This is intended to prevent hardening and achieve a reduction in the forming forces, and thus increase the overall forming capability. The flat steel product has substantially the following chemical composition in wt. %): C: 0,0005 to 0.9, Mn: 4 to 12; with the remainder being iron including unavoidable steel-associated elements, However, this approach is relatively cost-intensive and logistically complex in Improving the formability and avoiding the problem of cracks at the separation edges of the steel plate, since the forming is linked directly to the heating of the steel plate for the forming process.

SUMMARY OF THE INVENTION

The present invention provides a cost-effective method for producing a component from a steel plate with a TRIP effect at room temperature, a use therefor as well as a corresponding plate and a corresponding component, which are characterised by cost-effective production, improved formability with reduced cracking of the formed separation edges with a simultaneous reduction in the forming forces.

In accordance with an embodiment of the invention, a method for producing a component from a plate consisting of a medium manganese-containing steel with 4 to 12 wt. % Mn and with a TRIP effect at room temperature, wherein (i) the plate is mechanically separated to form a prepared plate with desired dimensions, (ii) separation edges are produced on the prepared plate by means of the mechanical separation, and (iii) the prepared plate with the separation edges is cold-formed to form the component at room temperature T_R or at a temperature above room temperature T_R and below 60° C. (T_R≤T<60° C.), the formability of the separation or cut edges is significantly increased during cold forming at room temperature between 5° C. and 30° C. and the formation of cracks at the separating edges is significantly reduced by effecting mechanical separation at a preheating temperature T_V in the range of 60° C.≤T_V<250° C.

The medium manganese-containing steel in question is a steel which, in addition to the TRIP effect, also has a temperature-dependent TWIP effect (in short: TRIP/TWIP steel). In the case of this medium manganese-containing steel, the preheating temperature T_V is then limited to a temperature range, namely 60° C.≤T_V<250° C., at which a TWIP effect caused by the mechanical separation occurs at the separation edge.

Mechanical separation according to the method in accordance with the invention at a preheating temperature T_V in the extended range of 60 to 400° C. already has the great advantage that deformation-induced martensite formation (TRIP effect) at the separation edges is avoided during the subsequent forming. This also achieves a reduction in the forming forces during the cold forming and thereby increases the overall forming capability of the plate. By decoupling the preheating step during mechanical separation from the forming of the plate into a component, more economical production of the component can also be achieved, as the plate can now be formed at room temperature, i.e. without prior heating of the entire plate.

The forming capability of the separation edges of the steel plate can also be significantly improved by adjusting the process temperature. It is thus essential that the mechanical separation procedure, such as cutting, is effected with a separation or cutting region heated to preheating temperature T_V in order to avoid martensite formation as a result of a TRIP effect during cutting.

In accordance with the invention, provision is made that the preheating temperature is less than 250° C., since up to this temperature a TWIP effect is achieved at the separation edges during mechanical separation. Above this temperature, the TWIP effect no longer occurs, but martensite formation (TRIP effect), which impairs the formability of the separation edges, is still avoided. From 400° C. upwards, the material becomes brittle due to blue brittleness and an optionally present zinc layer liquefies. Both are associated with a significant impairment in the properties of the material.

In conjunction with the present invention, room temperature T_R is defined as being in the range between 5 to 30° C. The cold forming of the prepared plate into the component is effected in particular at room temperature. The cold forming at room temperature T_R into the component can advantageously be effected in one or more steps.

In an advantageous manner, provision is made that the steel is a medium manganese-containing steel, with more than 5 to less than 10 wt. % Mn. Such a medium manganese-containing steel is particularly suitable for the method for producing a component.

According to an embodiment, the plate is heated locally to the preheating temperature only in regions of the separation edges (said separation or cut regions) to be produced by the mechanical separation. This heating is therefore not heating over a large area but is instead targeted local heating, and is effected in particular inductively, i.e. this is inductive heating. From a cost-benefit perspective, it is preferred if the preheating temperature T_V is 100 to 200° C.

In one advantageous further development, provision is made that the separation edges are heated to preheating temperature in a heating device arranged in the cutting or punching tool. Alternatively, provision can also be made that the separation edges are heated to preheating temperature in a separate heating device. The heating of the separation edges to preheating temperature can advantageously be effected inductively, conductively or via radiant heat.

The component produced from such a steel plate by cold forming and consisting of TRIP (TRansformation Induced Plasticity) and/or TWIP (TWinning Induced Plasticity) steel has excellent cold-formability and warm-formability, increased resistance to hydrogen-induced delayed crack formation (delayed fracture), to hydrogen embrittlement after formation and to liquid metal embrittlement (LME) during welding.

The invention also relates to a prepared plate for producing a component by cold forming the prepared plate at room temperature, having at least one separation edge of a mechanical separation from an original plate consisting of a medium manganese-containing steel with 4 to 12 wt. % Mn and with a TRIP effect at room temperature, wherein the at least one separation edge determines or at least co-determines the dimensions of the prepared plate. Provision is made that TWIP effect-induced deformation twins are present in the microstructure at the separation edge, which improve the forming capability of the edge for forming at room temperature.

The presence of TWIP effect-induced deformation twins can be detected by means of microscopy (e.g. by means of light microscopy and/or scanning electron microscopy) at the separation edge. These TWIP effect induced deformation twins are also a sure indication that the formability of the separation edge is significantly increased during subsequent cold forming at room temperature T_R between 5° C. to 30° C. and that crack formation at the separation edge is significantly reduced.

The embodiments of the invention and the advantages thereof as stated in connection with the production method in accordance with the invention are also provided accordingly for the prepared plate in accordance with the invention for producing a component and the component in accordance with the invention stated hereinafter.

The invention also relates to a component consisting of a plate of a steel with a TRIP effect at room temperature. Provision is made that the plate is an aforementioned prepared plate. The component is then a component which is produced from this plate by forming at room temperature T_R or at a temperature above room temperature T_R and below 60° C. (T_R≤T<60° C.). In particular, the component is a component produced by means of the aforementioned method. Preferably, the component is a component for at least one of the applications listed hereinafter: automobile construction, rail vehicle construction, shipbuilding, plant construction, infrastructure construction, mining, aerospace industry and household appliance technology.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of hole expansion tests;

FIG. 2a is a scanning microscopy image of the formed sample of A1 shown in the graph of FIG. 1; and

FIG. 2b is a scanning microscopy image of the formed sample of A3 shown in the graph of FIG. 1.

BRIEF DESCRIPTION OF THE DRAWINGS

The good results for the formability of separation edges produced in accordance with the invention become apparent from FIG. 1 for results of hole expansion tests according to ISO 16630, as illustrated in the appendix.

A TRIP/TWIP steel in wt. % with 0.14 C, 6 Mn, 0.15 Si and 1.2 Al, which has a TRIP effect at room temperature between 5 to 30° C., was selected for the tests. Holes were produced in a steel metal sheet by punching at different preheating temperatures and these were expanded at different temperatures in the course of the hole expansion test according to ISO 16630.

Sample A1 (light grey) was punched at room temperature (25° C.) and the expansion by means of the hole expansion test was also effected at room temperature (25° C.).

Sample A2 (dark grey) was punched at room temperature (25° C.) and expanded at 150° C.

Sample A3 (black) was punched at 150° C. and expanded at room temperature (25° C.).

Sample A4 (spotted) was punched at 150° C. and expanded at 150° C.

It is clearly apparent that the expansion value A (lambda) increases at a preheating temperature of the punching edge of 150° C. compared to punching at room temperature of 25° C. The value for sample A3 of λ=31.8% is significantly higher than the value of sample A1 of λ=14.18.

However, forming by hole expansion at an elevated temperature of 150° C. compared to forming at room temperature does not provide any significant improvement to the hole expansion ratio. The value for sample A2 with λ=16.92% is only slightly above the value of A1. The value for sample A4 with λ=35.40% is only slightly above the value of A3.

In addition, the micrographs obtained by scanning electron microscopy in FIG. 2a (sample A1, without preheating) and FIG. 2b (sample A3, with preheating) prove that significantly fewer and smaller cracks are produced on the formed cut edges when the punching procedure was effected on preheated samples.

In FIG. 2a which shows a scanning microscopy image of the formed sample A1 without preheated separation edges, cracks are very clearly visible as dark lines at the concavely curved separation edges. However, in FIG. 2b which shows a scanning microscopy image of the formed sample A3 with preheated separation edges, no such clear cracks are visible at the concavely curved separation edges.

In accordance with the invention, a use of a component produced according to the previously described method is advantageously provided in the automobile construction, rail vehicle construction, shipbuilding, plant construction, infrastructure construction, the aerospace industry, household appliance technology.

Provision is preferably made that the component is produced from a medium manganese-containing steel with the following chemical composition (in wt. %) in order to achieve in particular the described advantages:

• • C: 0.0005 to 0.9, preferably 0.05 to 0.35;
  • Mn: 4 to 12, preferably greater than 5 to less than 10 and with the remainder being iron including unavoidable steel-associated elements, with the optional addition by alloying of the following elements in wt. %:
  • Al: 0 to 10, preferably 0.05 to 5, particularly preferably greater than 0.5 to 3;
  • Si: 0 to 6, preferably 0.05 to 3, in a particularly preferred manner 0.1 to 1.5;
  • Cr: 0 to 6, preferably 0.1 to 4, particularly preferably greater than 0.5 to 2.5;
  • Nb: 0 to 1, preferably 0.005 to 0.4, in a particularly preferred manner 0.01 to 0.1;
  • V: 0 to 1.5, preferably 0.005 to 0.6, in a particularly preferred manner 0.01 to 0.3;
  • Ti: 0 to 1.5, preferably 0.005 to 0.6, in a particularly preferred manner 0.01 to 0.3;
  • Mo: 0 to 3, preferably 0.005 to 1.5, in a particularly preferred manner 0.01 to 0.6;
  • Sn: 0 to 0.5, preferably less than 0.2, in a particularly preferred manner less than 0.05;
  • Cu: 0 to 3, preferably less than 0.5, in a particularly preferred manner less than 0.1;
  • W: 0 to 5, preferably 0.01 to 3, in a particularly preferred manner 0.2 to 1.5;
  • Co: 0 to 8, preferably 0.01 to 5, in a particularly preferred manner 0.3 to 2;
  • Zr: 0 to 0.5, preferably 0.005 to 0.3, in a particularly preferred manner 0.01 to 0.2;
  • Ta: 0 to 0.5, preferably 0.005 to 0.3, in a particularly preferred manner 0.01 to 0.1;
  • Te: 0 to 0.5, preferably 0.005 to 0.3, in a particularly preferred manner 0.01 to 0.1;
  • B: 0 to 0.15, preferably 0.001 to 0.08, in a particularly preferred manner 0.002 to 0.01;
  • P: less than 0.1, preferably less than 0.04;
  • S: less than 0.1, preferably less than 0.02; and
  • N: less than 0.1, preferably less than 0.05.

This composition is provided both for the plate and for the component produced therefrom. The plate preferably has a microstructure with the following proportions: 10 to 80 vol. % austenite, 20 to 90 vol. % martensite, ferrite and bainite, wherein at least vol. % of the martensite is present as tempered martensite. In a particularly preferred manner, the microstructure has 40 to 80 vol. % austenite, less than 20 vol. % ferrite/bainite, with the rest being martensite.

In the case of the microstructure of the resulting component, the corresponding proportions are preferably present approximately in the same limits as in the case of the plate.

The information regarding composition and microstructure corresponds to that from the document DE 10 2016 117 494 A1 mentioned in the introduction. Effects of the alloy elements used can be found in this document.

Claims

1. A method for producing a component from a plate comprising a medium manganese-containing steel with 4 to 12 wt. % Mn and with a TRIP effect at room temperature, wherein the method comprises:

mechanically separating the plate to form a prepared plate with desired dimensions, and wherein separation edges are produced on the prepared plate by said mechanically separating the plate; and

cold forming the prepared plate with the separation edges to form the component at room temperature or at a temperature above room temperature and below 60° C.;

wherein said mechanically separating the plate is effected at a preheating temperature in the range of 60° C. to less than 250° C.

2. The method as claimed in claim 1, wherein the steel is a medium manganese-containing steel, with more than 5 to less than 10 wt. % Mn.

3. The method as claimed in claim 1, wherein the plate is heated locally to the preheating temperature only in regions of the separation edges to be produced by said mechanically separating.

4. The method as claimed in claim 1, wherein the preheating temperature is 100 to 200° C.

5. The method as claimed in claim 1, wherein the separation edges are heated to the preheating temperature in a heating device arranged in a cutting or punching tool.

6. The method as claimed in claim 1, wherein the separation edges are heated to the preheating temperature in a separate heating device.

7. The method as claimed in claim 5, wherein the separation edges are heated inductively, conductively or via radiant heat.

8. Use of a component produced according to claim 1 in at least one of automobile construction, rail vehicle construction, shipbuilding, plant construction, infrastructure construction, mining, the aerospace industry, and household appliance technology.

9. A prepared plate for producing a component by cold forming the prepared plate at room temperature, said prepared plate comprising:

at least one separation edge of a mechanical separation from an original plate comprising a medium manganese-containing steel with 4 to 12 wt. % Mn and with a TRIP effect at room temperature;

wherein the at least one separation edge determines or at least co-determines the dimensions of the prepared plate, and wherein TWIP effect-induced deformation twins are present in the microstructure at the separation edge.

10. A component comprising a plate of a steel with a TRIP effect at room temperature, wherein the plate is a prepared plate as claimed in claim 9.

11. The component as claimed in claim 10, produced by:

mechanically separating the plate to form a prepared plate with desired dimensions, and wherein separation edges are produced on the prepared plate by said mechanically separating the plate; and

cold forming the prepared plate with the separation edges to form the component at room temperature or at a temperature above room temperature and below 60° C.;

wherein said mechanically separating the plate is effected at a preheating temperature in the range of 60° C. to less than 250° C.

12. The component as claimed in claim 10, wherein the component is a component for at least one of automobile construction, rail vehicle construction, shipbuilding, plant construction, infrastructure construction, mining, the aerospace industry, and household appliance technology.

13. The method as claimed in claim 6, wherein the separation edges are heated inductively, conductively or via radiant heat.

14. The method as claimed in claim 3, wherein the separation edges are heated to the preheating temperature.

15. The method as claimed in claim 14, wherein the separation edges are heated inductively, conductively or via radiant heat.

16. The method as claimed in 14, wherein the separation edges are heated to the preheating temperature in a heating device arranged in a cutting or punching tool, or the separation edges are heated to the preheating temperature in a separate heating device.