Method for producing components from lightweight steel

Abstract

A method is disclosed for producing components from an austenitic lightweight steel which is metastable in its initial state, by forming of a sheet, a circuit board or a pipe in one or more steps, exhibiting a temperature-dependent TRIP and/or TWIP effect during forming. To obtain a component with, in particular, high toughness, the forming is carried out at a temperature above room temperature, at 40 to 160 C., which avoids the TRIP/TWIP effect, and to achieve in particular high component strength, the forming is carried out at a temperature below room temperature, at 65 to 0 C., which enhances the TRIP/TWIP effect.

Claims

1. A method for producing a steel component from an austenitic lightweight steel which is metastable in its initial state and exhibits a temperature-dependent TRIP and/or TWIP effect during forming, comprising: providing a sheet, plate or tube made of the metastable austenitic light lightweight steel; forming the sheet, plate or tube in one or more steps in at least one of two ways, a first way in which the sheet, plate or tube is formed at a forming temperature above room temperature at 40 to 160 C. so as to avoid the TRIP-/TWIP effect, resulting in a high tenacity of the steel component, and a second way in which the forming is performed at a forming temperature below room temperature at 65-0 C., so as to enhance the TRIP-/TWIP effect resulting in a high strength of the steel component.

2. The method of claim 1, wherein the forming is a rolling.

3. The method of claim 1, wherein the forming is a deep drawing.

4. The method of claim 1, wherein the forming is an internal high pressure forming (IHU).

5. The method of claim 1, wherein the forming is performed in multiple steps, the method further comprising varying the forming temperature and/or a degree of forming and/or a forming speed between individual ones of the multiple steps.

6. The method of claim 5, wherein in a first one of the multiple steps or in a further one of the multiple steps subsequent to the first step, the forming is performed above room temperature, and wherein in a final one of the multiple steps the forming is performed at a temperature below room temperature.

7. Components made from an austenitic, metastable lightweight steel which is metastable in its initial state and exhibits a temperature-dependent TRIP and/or TWIP effect during forming, said components being produced by forming a sheet, plate or tube made of the metastable austenitic lightweight steel in one or more steps in at least one of two ways, a first way in which the sheet, plate or tube is formed at a forming temperature above room temperature at 40 to 160 C. so as to avoid the TRIP-/TWIP effect, resulting in a high tenacity of the steel component, and a second way in which the forming is performed at a forming temperature below room temperature at 65-0 C., so as to enhance the TRIP-/TWIP effect resulting in a high strength of the steel component, wherein the components have a metallic coating.

8. The components of claim 7, wherein the sheet plate or tube used in the forming has a metallic coating.

Description

SUMMARY OF THE INVENTION

(1) It is an object of the invention to provide a method for producing components from metastable austenitic lightweight steel with TRIP and TWIP properties with which it is possible to produce components in a simple and cost-effective manner by using a single material with which different demands during operation can be met.

(2) This object is solved with the preamble and the characterizing features of the independent method claim.

(3) According to the teaching of the invention, for achieving an in particular high tenacity of the component the forming is performed at a temperature above room temperature, at 40 to 160 C. which avoids the TRIP-/TWIP effect, and for achieving in particular a high component strength the forming is performed at a temperature below room temperature at 65 to 0 C. that enhances the TRIP-/TVVIP effect.

(4) In the following, the term room temperature means a temperature range from 19 C to 27 C.

(5) The basic idea of the invention is that he required forming temperatures are set in a targeted manner in correspondence to the demands placed on the component. Hereby the temperature dependence of the hardening mechanism in metastable austenitic lightweight steels, which have a TRIP-/TVVIP effect, is utilized. Consequently it is now possible to use a single material for producing components with different material properties which, corresponding to the demands, are produced with different forming temperatures.

(6) According to the invention the sheets, plates or tubes used for the components can be metallically blank or provided with a metallic coating.

(7) From the state of the art it is known that the TRIP effect is based on the difference between the energies of the individual phases. When the forming temperature exceeds the difference of the energies, the microstructure correspondingly transforms. In the case of a metastable austenite the phase at room temperature is the stable phase, however, it has a very low energy difference with regard to the or phase (Figure).

(8) By using different temperatures during forming the TRIP effect can thus be enhanced at low temperatures, because the energy that has to be overcome is low. When the forming is performed at temperatures above room temperature, the austenite is stabilized because the energy that has to be overcome strongly increases.

(9) For example, the temperature increase occurring in the component during the forming can be used in a targeted manner. Hereby, starting from room temperature, the temperature of the component increases to about 40 to 160 C. While the tools usually have to be cooled during manufacturing in order to prevent an influencing the material properties of the component, in the instant case according to the invention cooling is not performed or the tools are set to a temperature of 40 to 16 C. in a targeted manner. In this way components are produced which have a stable austenitic microstructure with high ductility.

(10) This process can be used for example for producing crash relevant components such as airbag mounts which, due to the increased tenacity, can absorb a much higher amount of energy in the event of an abrupt stress than components that were produced at room temperature.

(11) On the other hand when the material is for example formed between 65 to 0 C., an increased TRIP effect occurs. In particular it was found that a significantly higher yield strength of the component is achieved than when forming with higher temperatures.

BRIEF DESCRIPTION OF THE DRAWING

(12) Correspondingly, this process is relevant for components that (also locally) undergo a small degree of forming and with this solid state hardening, and at the same time require a high strength in the regions that are formed to a small degree, such as cross members or longitudinal members.

(13) For achieving a high tenacity of the component during operation, the forming into a component should therefore occur at temperatures of about 40-160 C., and for achieving a high strength of the component between about 65 and 0 C.

(14) With this innovative manufacturing method the cost-disadvantages of the state of the art can be overcome in a simple manner. In particular for example no expensive highly alloyed austenitic CrNi materials are needed when components with extremely high tenacity are required. On the other hand, this manufacturing method also enables producing components, which have a very high strength and high tenacity during operation, which is not possible with the known material concepts.

(15) On one hand, the high forming capability of austenitic materials without additionally adding alloy elements can be optimized by suppressing the TRIP- or TWIP effect in the first forming steps, and thus retaining the forming capability of the basic material prior to the last forming step. On the other hand, the TRIP- or TWIP effect can be enhanced by forming at low temperature. Thus the strength of the component can be increased even without addition of further alloy elements.

(16) For example in the first step or in a further step the forming can occur at a temperature above room temperature, which avoids the deformation-induced TRIP-/TWIP effect, in order to retain the ductility of the starting material, and in the subsequent step the forming can occur at a temperature below room temperature which enhances the TRIP-/TWIP effect, in order to produce a component with high strength.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(17) Possible forming methods for producing the components are for example different rolling methods, deep drawing or also the forming by means of internal high pressure.

(18) In addition, the method according to the invention enables producing components, which have to be subjected to extreme forming degrees. This is achieved by suppressing the TRIP-/TWIP effect at elevated forming temperatures.

(19) According to an advantageous refinement of the invention, the forming is performed in multiple stages, wherein in the individual stages the forming temperature and/or the degree of forming and/or the forming speed can be varied. This enables providing the component with very different material characteristics in the different forming stages, which offers many possibilities to meet many different demands placed on the component.

(20) Hereby it is not only possible to impinge the entire component with the corresponding forming temperature but also to form the component with partially different temperatures, so that even different material properties within one component can be realized.