Method for producing workpieces from lightweight steel having material properties that are adjustable across the wall thickness

Abstract

A method for producing a workpiece having properties which are adjustable across a wall thickness or strip thickness of the workpiece, includes the steps of subjecting the workpiece to a decarburizing annealing treatment under an oxidizing atmosphere and to an accelerated cooling and/or a cold forming for generating a property gradient of the workpiece, wherein the workpiece is made of an austenitic lightweight steel which has an alloy composition which includes by weight percent 0.2% to 1% carbon, 0.05% to<15% aluminum, 0.05% to 6.0% silicon, 9% to<30% manganese, and at least one element selected from the group consisting of chromium, copper, boron, titanium, zirconium, vanadium and niobium, wherein chromium=4.0%; titanium+zirconium=0.7%; niobium+vanadium=0.5%, boron=1%, the remainder iron including common steel companion elements.

Claims

1. A Method for producing a workpiece having properties which are adjustable across a wall thickness or strip thickness of the workpiece, said workpiece being made of an austenitic lightweight steel which has an alloy composition comprising by weight percent 0.2% to 1% carbon, 0.05% to <15% aluminum, 0.05% to 6.0% silicon, 9% to <30% manganese, and at least one element selected from the group consisting of chromium, copper, boron, titanium, zirconium, vanadium and niobium, chromium6.5%; titanium+zirconium0.7%; niobium+vanadium0.5%, boron1%, the remainder iron including common steel companion elements, said method comprising the steps of: subjecting the workpiece to a decarburizing annealing treatment under an oxidizing atmosphere thereby causing a ferritic or meta-stable austenitic structure to form in a surface proximate region of the workpiece; and subjecting the workpiece to an accelerated cooling and/or a cold forming for generating an increased hardness in the surface proximate region and a property gradient of the workpiece, wherein a layer thickness and properties of the austenitic structure are adjustable via variation of at least one annealing parameter selected from the group consisting of temperature and holding time and via variation of at least one of a gas composition and a partial pressure of the atmosphere.

2. The method of claim 1, further comprising forming the workpiece before, during or after the annealing treatment.

3. The method of claim 2, wherein the forming is a hot or cold forming.

4. The method of claim 3, wherein the forming is a hot or cold rolling.

5. The method of claim 4, further comprising after the rolling process, subjecting the workpiece to an annealing process under a reducing or inert atmosphere to adjust a depth and a degree of a decarburization generated in the workpiece by the annealing treatment before and/or after individual rolling passes of the hot rolling.

6. The method of claim 4, further comprising adjusting a depth and a degree of a decarburization caused by the decarburizing annealing treatment in a targeted manner by reheating the workpiece between individual rolling passes of the hot rolling.

7. The method of claim 3, wherein the forming is a hydroforming.

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

9. The method of claim 3, wherein the forming is a pressing.

10. The method of claim 3, wherein the forming is a press hardening.

11. The method of claim 2, wherein the forming is performed after the annealing treatment, and wherein the accelerated cooling is performed during the forming.

12. The method of claim 1, wherein the accelerated cooling is a quenching.

13. The method of claim 1, wherein the oxidizing annealing atmosphere is ambient air.

14. The method of claim 3, wherein oxygen or oxygen containing gases are added to the ambient air.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which:

(2) FIG. 1a shows a photograph of the structure of a workpiece according to the invention;

(3) FIG. 1b shows another a photograph of the structure of a workpiece according to the invention;

(4) FIG. 1c shows another a photograph of the structure of a workpiece according to the invention;

(5) FIG. 1d shows another a photograph of the structure of a workpiece according to the invention;

(6) The following alloy compositions were used in the operating tests In weight %:

(7) TABLE-US-00002 FIG. 1a FIG. 1b FIG. 1c FIG. 1d C 0.7 0.7 0.7 0.7 Al 2.5 2.5 2.5 2.5 Si 2.5 0.2 0.3 0.3 Mn 15 15 15 15
remainder iron including usual steel tramp elements

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(8) Photographs of structures of workpieces which were treated according to the invention for martensite formation and corresponding measurements of hardness are shown in two pictures of structures (FIG. 1a, 1b). The materials differ here with regard to their Si-content. The pictures of the structure show a layer of martensite of different thickness in the surface proximate regions and the significant increase in hardness associated therewith compared to the austenite structure in the matrix. Here, the steel according to FIG. 1a shows a significantly greater increase in hardness than the steel according to FIG. 1b.

(9) The oxidizing annealing treatment of the samples of FIGS. 1a and 1b which is required for decarburization was carried out under ambient pressure (air) at an annealing temperature of 1150 C. and an annealing time of 1 h. In the present case, the samples were not quenched after the annealing treatment but only subjected to a cold forming for verifying the TRIP-effect (formation of forming-induced martensite).

(10) FIGS. 1c and 1d show that, depending on the degree of the decarburization, border regions with local twin formation can also be adjusted. A variation of the carbide formation across the plate thickness can also be adjusted in dependence on the degree of the decarburization.

(11) The annealing treatment which is required for the decarburization of the samples in FIGS. 1c and 1d occurred during the hot rolling. After the subsequent cold rolling a reducing annealing retreatment with different temperatures (FIG. 1c: 750 C.border layer 30 m with twins, FIG. 1d: 700 C.border layer 60 m with twins).

(12) In addition, work pieces made of lightweight steel have to satisfy relatively high demands with regard to workability for example by cold forming, welding and/or corrosion protection (for example zinc containing coatings).

(13) When welding zinc-plated austenitic lightweight steels, the so called liquid metal embrittlement can cause problems. Here, the heating up of the basic material during welding leads to an infiltration of the grain boundaries by liquefied zinc material of the coating. This causes the basic material in the vicinity of the welding zone to lose strength and ductility so that the welding connection or the basic material which borders the welding connection no longer satisfies the demands on the mechanical properties which increases the risk of premature failure of the welding connection.

(14) Tests have shown that when welding steels with high manganese content the grain border attack by the molten zinc material can be effectively avoided by formation of a martensitic or martensitic-austenitic mixed structure in the de-carbonized surface proximate regions. The surface proximate de-carbonized border layer is very well suited as intermediate layer to avoid the liquid metal embrittlement in zinc-plated lightweight steels.

(15) The inventive idea is not only applicable for flat products such as hot and cold strip but also for profiled sections and pipes and components produced therefrom. All known methods of the cold, hot and warm forming can be used for the forming such as bending, deep-drawing, compressing, widening and so on. But also the known hydroforming or press form hardening. Thus, the production of gradient materials according to the invention can be achieved using the following process routes: Cold or hot forming of a workpiece such as for example a cut sheets to a component with subsequent oxidizing annealing of the component and subsequent targeted cooling for hardening the surface by transformation of he de-carbonized regions to martensite. Hydroforming of a pipe at elevated temperature, which allows a decarburization of the surface, with a final fast cooling (hardening). Hydroforming of a pipe at room temperature with final oxidizing annealing of the already formed component and subsequent fast cooling (hardening). Press form hardening of a work piece with an oxidizing annealing before the forming; forming at elevated temperature in the austenitic structure state and subsequent fast cooling for martensitic transformation of the surface proximate, de-carbonized regions. Oxidizing annealing for establishing a de-carbonized layer, for example of a steel plate with subsequent targeted cooling (without hardening) with subsequent cold forming. Oxidizing annealing for establishing a de-carbonized layer, for example of a steel plate with subsequent targeted cooling (without hardening) with subsequent cold rolling for targeted establishment of the hardening layer thickness via the formation of forming martensite. Oxidizing annealing of for example a steel plate with subsequent targeted cooling (hardening) and direct application without further forming technical stress. Oxidizing annealing in the course of the hot rolling process for establishing a de-carbonized layer with subsequent cold rolling. Oxidizing annealing in the course of the hot rolling process for establishing of a de-carbonized layer with subsequent cold rolling and annealing under oxidizing atmosphere for further decarburization. Oxidizing annealing in the course of the hot rolling process for establishing a de-carbonized layer with subsequent cold rolling and annealing under reducing or inert atmosphere fro decreasing or establishment of the decarburization by compensation processes.

(16) The method according to the invention can generally be used for all alloys which are austenitic at room temperature, in particular however of high alloyed lightweight steels.

(17) Advantageously, the method according to the invention for the first time offers the possibility to accommodate the specific demands on the material properties of the finished component by adjusting these properties across the strip thickness.

(18) In summary, the following advantages result from the invention: Establishment of required material properties via the wall thickness by simple decarburizing annealing with subsequent hardening or mechanical forming It can be influenced in a targeted manner: Wear/abrasion/tribology Scaling resistance Corrosion protection Coating properties Bonding properties Electrical properties Weldability (for example resistance spot weldability) Thermal properties (bimetal) Optical properties (appearance) Damping Realization of combinations of different surfaces and material properties