METHOD FOR PRODUCING STEEL COMPOSITE MATERIALS

Abstract

The invention relates to a method for producing a steel composite in which at least two steel sheets that consist of different steel grades are placed against each other, hot rolled together, and then possibly cold rolled and in which after the rolling, the composite material, which is thus produced from at least two layers with different steel compositions, is diffusion annealed, wherein the annealing temperature is set so as to select the chemical potential of the steel materials to correspond to the following equation:

.sub.C, material 1>.sub.C, material 2,

where material 1 has a lower carbon content than material 2 so that an uphill diffusion of carbon takes place between material 1 and material 2.

Claims

1. A method for producing a steel composite in which at least two steel sheets that consist of different steel grades are placed against each other, hot rolled together, and then possibly cold rolled and in which after the rolling, the composite material, which is thus produced from at least two layers with different steel compositions, is diffusion annealed, characterized in that the annealing temperature is set so as to select the chemical potential of the steel materials to correspond to the following equation:
.sub.C, material 1>.sub.C, material 2, where material 1 has a lower carbon content than material 2 so that an uphill diffusion of carbon takes place between material 1 and material 2.

2. The method according to claim 1, characterized in that the diffusion annealing takes place below the lowest Ac1 temperature of the steel materials used, at temperatures of 650 to 720 C., preferably 670 to 700 C.

3. The method according to claim 1, characterized in that the chemical potential difference of the two materials at the annealing temperature is greater than 500 J/mol, preferably 1000 J/mol, more preferably greater than 2000 J/mol.

4. The method according to claim 1, characterized in that three steel sheets are rolled together, the central steel material having a different carbon content and a different chemical potential than the outer steel materials.

5. The method according to claim 1, characterized in that the outer regions make up a total thickness of 25%, preferably less than 10%, more preferably 5% of the total thickness of the steel sheet.

6. The method according to claim 1, characterized in that at least one side of the composite is provided with a metallic coating with aluminum or an alloy containing essentially aluminum or an alloy composed of aluminum and zinc and/or another zinc alloy containing essentially zinc and/or zinc and/or other coating metals.

7. The method according to claim 1, characterized in that the steel composite composed of two, three, four, or more different steel materials is rolled, the steel composite being composed of three, four, five, or more layers.

8. A steel sheet composite, particularly produced using a method according to claim 1, characterized in that the composite is press hardened or form hardened; after the press hardening or form hardening of the composite, the softer sheet layers develop a tensile strength Rm of 0.3 to 0.7 GPa and the harder sheet layers develop a tensile strength Rm of between 1 GPa and 2.5 GPa.

9. The steel sheet composite according to claim 8, characterized in that the steel sheet composite has softer steel types on the outside and a harder steel type between them or vice versa.

10. A use of a steel sheet composite according to claim 8 for producing components that are partially press hardened or form hardened in terms of the thickness, in which the component is cold-formed, austenitized, and then quench-hardened or is austenitized, formed, and quench-hardened.

11. A hardened component made of a steel sheet composite according to claim 8, produced with a method according to claim 1.

Description

[0059] The invention will be explained by way of example based on the drawings. In the drawings:

[0060] FIG. 1 shows the chemical potentials for different materials;

[0061] FIGS. 2a-2p show the carbon distribution in composite materials according to the invention in the initial state and after the annealing treatment according to the invention;

[0062] FIGS. 3a-3c show a three-layer sheet structure according to the prior art with the corresponding carbon distribution.

[0063] FIGS. 3a to 3c show that in the prior art, a material was produced having thin outer regions that consist of a high carbon steel material while the inner region consists of a so-called low carbon steel.

[0064] In this case, the edge regions each make up about 10% of the thickness while the central region makes up about 80% of the thickness.

[0065] FIG. 3a shows the expected carbon diffusion between the high carbon steel in the edge region and the low carbon steel in the inner region; over a width of 60 micrometers, a carbon drop and thus also a hardness drop can be observed.

[0066] In embodiments according to the invention (FIGS. 2a-2p), it is clear that for example with a material pairing of 340 LA on the outside and 29MnB6 on the inside, the carbon content in the initial state (dotted lines) shows a sharply defined difference, with the carbon content in the core region being approximately three times as high as in the edge region.

[0067] After this material is rolled and has been annealed in the coil at 680 C. for 10 hours, this yields the values shown with the solid lines. It is clear that the edge regions are almost completely decarburized while the carbon content in the edge region of the central material has nearly doubled and then falls toward the inside. The carbon has thus diffused uphill from the carbon-poorer material into the carbon-richer material. The diffusion behaves the same, but in a somewhat weaker way with different material combinations (FIGS. 2c and 2b), in which the same annealing was performed, but the inner material was changed to 34MnB5 (FIGS. 2b) and 22 MnB5 (FIG. 2c). The effects are visible in the same way, even though the absolute carbon contents may vary somewhat.

[0068] FIG. 2d shows an extreme case in which the chemical potential between the edge material 500 LA and the core material 22MnB5 is not sufficient to achieve the effect according to the invention at the predetermined annealing temperature. This is even more extreme with the material pairing in FIG. 2E in which 340 LA was used in the edge region and 29MnB6 was used in the core region and after the annealing, an equalization of the carbon content took place, i.e. the opposite of what is desired.

[0069] The effect according to the invention is also not achieved with the material pairing in FIG. 2f.

[0070] This is also true for the material pairing in FIGS. 2g and 2h, in which IF steels and boron-manganese steels have been combined and the desired effects do not occur.

[0071] With somewhat higher annealing temperatures and the material combination 500 LA and 22MnB5, a carbon depletion of the carbon-richer material and an edge carburization even occur.

[0072] But if the chemical potential is widened again even further, for example with the use of 500 LA in the edge region and 39MnCrB6-2, the effect according to the invention is once again achieved.

[0073] This is also true for the combination of 340 LA with 22MnB5 and of 340 LA with 22MnB5 at an annealing temperature of 800 and 680 C., respectively.

[0074] FIG. 2o shows a material combination of 22MnB5 in the edge region and 340 LA in the core region, i.e. a combination of hardenable steel on the outside and softer steel on the inside. Here, there is a clear carbon difference between the outer material and the inner material in the initial state (dotted line) and after 20 hours of annealing at 680 C. In this case, the central material is almost completely decarburized in the outer regions, while the carbon-richer material in the edge regions has been carburized to almost triple the value.

[0075] FIG. 1 shows the different chemical potentials for different materials and at different temperatures; in this case, it is clear that up to the corresponding annealing temperatures, a potential difference should be present and in particular, materials [sic] in which the potential decreases too sharply with increasing temperatures or crosses other lines is not suitable because it changes its potential properties so to speak during the annealing.

[0076] According to the invention, it is thus possible through a suitable materials selection and suitable annealing temperatures to further intensify the differences in carbon content between two adjoining steel materials so that by means of an uphill diffusion, it is possible to selectively influence the materials and as a result, a steel composite can be produced, which has very different properties.

[0077] The composite produced according to the invention can be press hardened or form hardened; the component produced in this way is in particular a body component for motor vehicles and in particular a structural component such as an A, B, or C pillar, a longitudinal member, a cross-member, or the like.