Method for manufacturing a complex-formed component

Abstract

The present invention relates to a method for manufacturing a complex-formed component by using austenitic steels in a multi-stage process where cold forming and heating are alternated for at least two multi-stage process steps. The material during every process step and a component produced has an austenitic microstructure with non-magnetic reversible properties.

Claims

1. A method for manufacturing a complex-formed component, comprising: subjecting austenitic steel to a multi-stage process where cold forming steps and heating steps are alternated for at least two multi-stage process steps, wherein the cold forming steps of the multi-stage process are carried out by deep-drawing, plunging, bulging, bending, spinning, stretch forming, or a hydro-mechanical deep-drawing process, the austenitic steel maintains an austenitic microstructure with non-magnetic reversible properties during every process step and the component produced has an austenitic microstructure with non-magnetic reversible properties, the austenitic steel is a stable full-austenitic steel exhibiting a twinning induced plasticity (TWIP) hardening mechanism with a defined stacking fault energy of 20-30 mJ/m.sup.2, the austenitic steel has an initial elongation of A.sub.80 that is greater than or equal to 30%, and the heating temperature of the heating steps is 750-1150° C.

2. The method according to claim 1, wherein during heating, twins in the microstructure of the austenitic steel are dissolved, and during forming, the twins in the microstructure of the austenitic steel are rebuilt.

3. The method according to claim 1, wherein the austenitic steel is a sheet having an initial thickness of less than 3.0 mm.

4. The method according to claim 1, wherein a sum of the carbon and nitrogen in the austenitic steel is 0.4-1.2 weight %.

5. The method according to claim 1, wherein the component is in the form of a sheet, a tube, a profile, a wire or a joining rivet.

6. The method according to claim 1, wherein the austenitic steel has a manganese content of 10-26 weight %.

7. The method according to claim 1, wherein the austenitic steel is a stainless steel with more than 10.5 weight % chromium.

8. The method according to claim 1, wherein the heating steps of the multi-staged process are carried out by induction heating, conduction heating or infrared heating.

9. The method according to claim 1, wherein a forming process is integrated into the multi-staged process as a non-final step before a subsequent heating step.

10. The method according to claim 1, wherein an upset forming treatment on the surface is integrated into the multi-staged process to create a scratch-resistant and compressive-loaded surface of the component which is also non-magnetic.

11. The method according to claim 1, wherein a nitriding or carburizing surface heat treatment with a heating temperature between 500 and 650° C. is integrated into the multi-staged process to create a scratch-resistance and non-magnetic surface of the component.

12. The method according to claim 1, wherein the component is a white good appliance, a domestic appliance, an automotive component, a mounting part for a transportation system, a part of a fuel injection system, or a battery case.

13. A method for manufacturing a complex-formed component, comprising: subjecting austenitic steel to a multi-stage process where cold forming steps and heating steps are alternated for at least two multi-stage process steps, wherein the austenitic steel maintains an austenitic microstructure with non-magnetic reversible properties during every process step and the component produced has an austenitic microstructure with non-magnetic reversible properties, the austenitic steel is a stable full-austenitic steel exhibiting a twinning induced plasticity (TWIP) hardening mechanism with a defined stacking fault energy of 20-30 mJ/m.sup.2, the austenitic steel is a stainless steel with more than 10.5 weight % chromium, the austenitic steel has an initial elongation of A.sub.80 that is greater than or equal to 30%, and the heating temperature of the heating steps is 750-1150° C.

14. The method according to claim 13, wherein during heating, twins in the microstructure of the austenitic steel are dissolved, and during forming, the twins in the microstructure of the austenitic steel are rebuilt.

15. The method according to claim 13, wherein the austenitic steel is a sheet having an initial thickness of less than 3.0 mm.

16. The method according to claim 13, wherein a sum of the carbon and nitrogen in the austenitic steel is 0.4-1.2 weight %.

17. The method according to claim 13, wherein the component is in the form of a sheet, a tube, a profile, a wire, or a joining rivet.

18. The method according to claim 13, wherein the austenitic steel has a manganese content of 10-26 weight %.

19. The method according to claim 13, wherein the forming steps of the multi-staged process are carried out by deep-drawing, pressing, plunging, bulging, bending, spinning, stretch forming, or a hydro-mechanical deep-drawing process.

20. The method according to claim 13, wherein the heating steps of the multi-staged process are carried out by induction heating, conduction heating, or infrared heating.

21. The method according to claim 13, wherein a forming process is integrated into the multi-staged process as a non-final step before a subsequent heating step.

22. The method according to claim 13, wherein an upset forming treatment on the surface is integrated into the multi-staged process to create a compressive-loaded surface of the component which is also non-magnetic.

23. The method according to claim 13, wherein a nitriding or carburizing surface heat treatment with a heating temperature between 500 and 650° C. is integrated into the multi-staged process to create a scratch-resistance and non-magnetic surface of the component.

24. The method according to claim 13, wherein the component is a white good appliance, a domestic appliance, an automotive component, a mounting part for a transportation system, a part of a fuel injection system, or a battery case.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention is illustrated in more details referring to the attached drawings where

(2) FIG. 1 shows hardness-comparison of different process,

(3) FIG. 2 shows the formation of twins as a metallographic inspection,

(4) FIG. 3 shows forming degree diagram of a an austenitic TWIP steel,

(5) FIG. 4 shows effect of hardening from a stamped edge,

(6) FIG. 5 shows effect of surface hardening by shot peening,

(7) FIG. 6 shows effect of surface nitriding heat treatment on the mechanical properties of an austenitic TWIP steel, and

(8) FIG. 7 shows a multi-stage metal-forming process.

DESCRIPTION OF THE INVENTION

(9) FIG. 1 shows the result of a hardness measured component after such a forming and heating operation. Hardness-comparison of different process steps of the multi-staged forming operation: Initial, base material (left), after first forming step with a forming degree of 20% (middle) and after heating process (right); for every state 10 hardness point per measured.

(10) In FIG. 2 the formation of twins is shown as a metallographic inspection in FIG. 2, related to the hardness measurement in FIG. 1.

(11) FIG. 3 shows the forming degree diagram of austenitic TWIP steel with 12-17% of chromium and manganese.

(12) In FIG. 4 is shown the effect of hardening from a stamped edge for a 12-17% chromium and manganese alloyed TWIP steel.

(13) FIG. 5 shows the effect of surface hardening by shot peening on full-austenitic TWIP steel.

(14) In FIG. 6 is shown the effect of surface nitriding heat treatment on the mechanical properties of an austenitic TWIP steel in annealed condition R.sub.p0,2=yield strength, A.sub.80=elongation after fracture, A.sub.g=uniform elongation, sample definition: A=sampled in initial annealed condition, N=sample after nitriding treatment.

(15) In FIG. 7 a multi-stage metal-forming process consists of different heating and forming steps with utilization of the TWIP hardening effect.

(16) The material used in the method will be hardened during the forming operation because of the TWIP effect, but the material will maintain the austenitic microstructure. For an austenitic TWIP material the forming degree shall be less than or equal to 60%, preferably less than or equal to 40%. If the forming potential, defined by the forming degree of the material is at the end of the method or if high tooling forces for forming are required, the second step, a heating step can be started. During the following heating step, the twins are dissolved and the material will be softened again. Because of the before defined material characteristics, the method is a reversible process. The heating process can be integrated into one forming tool with induction or conduction. The heating temperature must be between 750 and 1150° C., preferably between 900 and 1050° C. The process can be repeated as many times as required to establish the desired complex geometry.

(17) The initial thickness of the sheet used for the multi-staged process shall be less than 3.0 mm, preferably between 0.25 and 1.5 mm. It is also possible to use flexible rolled sheets with the present invention, too.

(18) The component is in the form of a sheet, a tube, a profile, a wire or a joining rivet.

(19) The formations of twins are shown as a metallographic inspection in FIG. 2, related to the hardness measurement in FIG. 1. The formation of twins by forming and dissolving by heating can be pointed out very well. With a further forming step after heating, the formation of twins is restarted again and the component will be hardened again. This process can be used alternated and repeated as many times as required to reach the geometry as well as target mechanical values for strength and elongation. Therefore the last step of the multi-staged forming operation can be a forming step with a defined forming degree as well as a locally heating step. For the use of a TWIP-steel which is alloyed with 12-17% of chromium as well as manganese, the forming diagram is used to adjust the sufficient values of the finished component, FIG. 3. As seen in FIG. 3, the invention is especially suitable for high or ultra-high strength steels having a minimum yield strength level more or equal than 500 MPa. The heating steps can be designed with induction, conduction or also infrared technology. Heating-up rates of 20K/s are possible and do not influence the behavior of the twins.

(20) Additionally forming operations can be integrated to the forming tool. As a result the hardening effect for state of the art operations can be reached over 160% of the base material. This drawback of edge hardening can be solved also by a following heating step. As a result the edge crack sensitive can be reduced significantly.

(21) A further positive aspect of the invention is the possibility to create a compressive stress value on the surface by an upset forming operation such as shot peening, grit blasting or high frequency pounding to reduce edge crack or surface crack sensitivity as well as a better fatigue behavior when the multi-stage formed component is under fatigue stressed conditions e.g. automotive component. Such surface treatment is in general well-known but the combination with the pointed out material characteristic shows new properties because the microstructure and therefore the material properties (e.g. non-magnetic) will be constant. The combination of process and material results in the values are shown in table 1, where the effect of surface hardening (shot peening) and subsequent heat treatment are on the residual stress level of full-austenitic TWIP steels.

(22) TABLE-US-00001 TABLE 1 Residual stresses on the surface [MPa] Yield strength Initial After shot After an subsequent material [MPa] state peening heat treament TWIP steel 515 28 −811 −560 annealed condition TWIP steel 811 102 −889 −589 strain hardened

(23) In table 1, a plus sign means tensile stresses on the surface; a minus sign means a compressive stress level.

(24) The general deviation of the measuring method can be +/−30 MPa. It can be shown with table 1. that the material stresses in initial state, especially for the strain hardened cold-rolled variants, can be transferred by an upset forming operation into uncritical compressive values. Such an operation can be also integrated into the multi-stage forming process because a high compressive load level can be also maintained after a subsequent heat treatment.

(25) A multi-staged complex-formed component can be used as an automotive component, like a wheel-house, bumper system, channel or as a chassis component e.g. suspension arm. Furthermore a multi-staged complex-formed component as a mounting part can be used in transportation systems like a door, a flap, a flender beam or a load-bearing flank, a interior part of a transport system like a seat structure component e.g. seat backrest.

(26) There are also possibilities to create a multi-staged complex-formed component as a part of a fuel injection system like a filler neck or as a tank or storage for cars, trucks, transport systems, railway, agricultural vehicles as well as for automotive industry, and further in building and a pressure vessel or boiler or to be used of a multi-staged complex-formed component as battery electric vehicles or hybrid cars like a battery case.

(27) An additional surface effect like an upset forming operation can be reached with a nitriding or carburizing heat treatment. Both elements, nitrogen and carbon, operate as austenite formers and therefore this elements stabilize the local stacking fault energy and the resulting hardening effect, TWIP mechanism. The effect of nitriding or carburizing is in a hardening of the near surface structure of the component as shown in FIG. 5. Furthermore, the near surface structure influence for the mechanical values of the TWIP steel, represent as shown the mechanical values in FIG. 6.

(28) A nitriding or carburizing surface treatment with a heating temperature between 500 and 650° C., preferably between 525 and 575° C., is integrated into the multi-staged process to create a scratch-resistance and at the same time non-magnetic surface of the component.

(29) A multi-stage metal-forming process can be seen in FIG. 7, which includes a sheet, plate, tube 1 at least two different (or independent from each other) steps where at least one step is a forming step 2. The next step 3 is heat treatment. The number of multi-stage process 4 steps depends on the forming complexity 5. As a final result of the method is a complex-formed component 6.