HOT-FORMING COMPOSITE MATERIAL, PRODUCTION THEROF, COMPONENT, AND USE THEREOF

Abstract

The invention relates to a hot-forming composite material (1) composed of an at least three-layer material composite comprising a core layer (1.1) of a hardenable steel and two outer layers (1.2), cohesively bonded to the core layer (1.1), of a ferritic, transformation-free FeAlCr steel.

Claims

1. A hot-forming composite material comprising: an at least three-layer material composite comprising a core layer of a hardenable steel and two outer layers, cohesively bonded to the core layer, of a ferritic, transformation-free FeAlCr steel.

2. The hot-forming composite material as claimed in claim 1, wherein the ferritic, transformation-free FeAlCr steel of the outer layers, aside from Fe and unavoidable impurities from the production, consists of, in % by weight, C: up to 0.15%, Al: 2% to 9%, Cr: 0.1% to 12%, Si: up to 2%, Mn: up to 1%, Mo: up to 2%, Co: up to 2% P: up to 0.1%, S: up to 0.03%, Ti: up to 1%, Nb: up to 1%, Zr: up to 1%, V: up to 1%, W: up to 1%.

3. The hot-forming composite material as claimed in claim 2 wherein the hardenable steel of the core layer, aside from Fe and unavoidable impurities from the production, consists of, in % by weight, C: 0.06-0.8%, Si: up to 0.5%, Mn: 0.4-3%, P: up to 0.06%, S: up to 0.03%, Al: up to 0.2%, Cr+Mo: up to 1%, Cu: up to 0.2%, N: up to 0.01%, Nb+Ti: up to 0.2%, Ni: up to 0.4%, V: up to 0.2%, B: up to 0.01%, As: up to 0.02%, Ca: up to 0.01%, Co: up to 0.02%, Sn: up to 0.05%.

4. The hot-forming composite material as claimed in claim 3 wherein the steel of the core layer has a carbon content between 0.28-0.75% by weight.

5. The hot-forming composite material as claimed in claim 3 wherein the outer layers each have a material thickness between 1% and 22%, based on the total material thickness of the hot-forming composite material.

6. The hot-forming composite material as claimed in claim 5 wherein the material composite has been produced by means of one of cladding and casting.

7. The hot-forming composite material as claimed in claim 5 wherein the hot-forming composite material is part of one of a tailored product, a tailored welded blank and a tailored rolled blank.

8. A method for producing a hot-rolling-clad hot-forming composite material comprising at least three-layer material composite comprising a core layer of a hardenable steel and two outer layers, cohesively bonded to the core layer, of a ferritic, transformation-free FeAlCr steel, the method comprising the following steps: providing a layer of a hardenable steel and at least two layers of a ferritic, transformation-free FeAlCr steel, stacking the layers provided in such a way that the layer of the hardenable steel forms a core layer and the two layers of the ferritic, transformation-free steel as outer layers receive the core layer between them, cohesively bonding the edges at least in some regions between the individual layers to produce a preliminary composite, especially by means of welding, heating the preliminary composite in a furnace to at least 1200 C., and; hot rolling the heated preliminary composite in one or more steps to give a coilable hot strip.

9. A component produced from a hot-forming composite material by means of one of a press hardening and a multistage hot-forming process, the component comprising: a core layer of a hardenable steel and two outer layers cohesively bonded to the core layer of a ferritic, transformation-free FeAlCr steel wherein the ferritic transformation-free FeAlCr steel of the outer layers, aside from Fe and unavoidable impurities from the production, consists of, in % weight, C: 0.06-0.8%, Si: up to 0.5%, Mn: 0.4-3%, P: up to 0.06%, S: up to 0.03%, Al: up to 0.2%, Cr+Mo: up to 1%, Cu: up to 0.2%, N: up to 0.01%, Nb+Ti: up to 0.2%, Ni: up to 0.4%, V: up to 0.2%, B: up to 0.01%, As: up to 0.02%, Ca: up to 0.01%, Co: up to 0.02%, Sn: up to 0.05%.

10. The component as claimed in claim 9, wherein the component after the press hardening or multistage hot-forming process has an aluminum oxide layer.

11. The component as claimed in claim 10, wherein the component is one of a bodywork and chassis of a land vehicle.

12. The hot-forming composite material as claimed in claim 4 wherein the steel of the core layer has a carbon content between 0.33-0.68% by weight.

13. The hot-forming composite material as claimed in claim 5 wherein the outer layers each have a material thickness between 2% and 17% based on the total material thickness of the hot-forming composite material.

14. The hot-forming composite material as claimed in claim 13 wherein the outer layers each have a material thickness between 4% and 12% based on the total material thickness of the hot-forming composite material.

15. The method of claim 8, further comprising: cold rolling the hot strip in at least one step to give a cold strip.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0080] The invention is elucidated in detail hereinafter with reference to a drawing. The drawing shows:

[0081] FIG. 1) a schematic section through a hot-forming composite material of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0082] The sole FIGURE shows a schematic section diagram through a hot-forming composite material (1) of the invention. The hot-forming composite material (1) of the invention comprises a core layer (1.1) of a hardenable steel having a carbon content C of at least 0.06% by weight, which, in the press-hardened state, has a tensile strength >500 MPa and/or a hardness >170 HV10, especially a tensile strength >1300 MPa and/or a hardness >450 HV10, preferably a tensile strength >1700 MPa and/or a hardness >520 HV10, further preferably a tensile strength >1900 MPa and/or a hardness >575 HV10, two outer layers (1.2), cohesively bonded to the core layer (1.1), of a ferritic, transformation-free FeAlCr steel having an aluminum content Al between 3% and 7% by weight and a chromium content Cr between 0.1% and 12% by weight. The material thickness of the outer layers (1.3) is at least 1% and at most 22%, preferably at least 4% and at most 12%, per side, based on the total material thickness of the hot-forming composite material (1), where the hot-forming composite material (1) may have, for example, a total material thickness between 0.5 and 8 mm.

[0083] Commercial flat steel products were used to produce, by means of hot roll cladding, a hot-forming composite material that had a three-layer material composite. The outer layers used were a steel designated Fe-5.4Al-6Cr-0.04Ti, and the core layer used was a hardenable steel designated 37MnB5.

[0084] In each case, sheet blanks (slabs) were stacked one on top of another to form a core layer with two outer layers, which were cohesively bonded to one another, preferably by means of welding, along their edges at least in some regions to form a preliminary composite. By virtue of the lower Cr content compared to chemically stable steels (chromium steels), it was possible to produce the pack construction in a less complex manner. The preliminary composite was brought to a temperature of >1200 C. in a furnace and hot-rolled in multiple steps to give a material composite having a total material thickness of 3 mm and then processed further to give a 1.5 mm cold strip.

[0085] Blanks were divided off from the hot-formed composite material. The blanks were heated or through-heated by means of induction to austenitization temperature, especially above A.sub.c3 (based on the core layer), and then, in a cooled mold, hot-formed to give components and cooled. The cooling rates were >30 K/s.

[0086] By means of EDX analysis by scanning electron microscope, the components produced were examined in detail, and essentially no increase in hardening, i.e. no increase in the concentration of carbon in the outer layers, was detected. Over the cross section of the core layer, a carbon profile had formed with an essentially higher concentration of carbon in the edge region (close to the interface) than in the middle of the core layer. At the transition between the two layers, there was enrichment of a C-rich phase. By virtue of the outer layers consisting of a ferritic, transformation-free lattice structure with corresponding carbon solubility, it was possible to freeze diffusion of the carbon out of the core layer by means of the free chromium in the outer layers, essentially close to the interface in the form of chromium carbides. In the region further from the interface in the direction of the center or middle of the core layer, there was essentially no change in the chemical alloy elements by comparison with the original state or state as supplied.

[0087] The core layer was composed essentially entirely of martensite over the thickness and, at the transition to the outer layer, the microstructure additionally contained proportions of bainite and/or ferrite. The outer layer essentially retained its original microstructure that it had at the time of provision prior to the manufacture of the material composite and the further processing to form a component, and so there was no transformation. The outer layers of FeAlCr steel used have a positive influence on the bending properties of the material composite or hot-forming composite material since, in addition to intrinsic low strength and hence high ductility, they offer the option of influencing ongoing diffusion processes such that regions having lower strength are formed in the core layer of the material composite that previously had high strength throughout. The material thickness of the outer layers was 6% per side, based on the total material thickness of the hot-forming composite material, such that the core layer had a material thickness of 88% based on the total material thickness. The thickness of the aluminum oxide layer formed on the surface of the in the course of press hardening was less than 150 nm.

[0088] The invention is not limited to the working example shown in the drawing. Instead, the hot-forming composite material of the invention may also be part of a tailored product, for example part of a tailored welded blank and/or tailored rolled blank, and may also have more than three layers. In addition, a component may also be produced by means of a multistage hot-forming process.