FLAT STEEL PRODUCT WITH AN AL-COATING, METHOD FOR PRODUCING THE SAME, STEEL COMPONENT AND METHOD FOR PRODUCING THE SAME

Abstract

A flat steel product for hot forming may be produced from a steel substrate that includes a steel comprising 0.1-3% by weight Mn and up to 0.01% by weight B, along with a protective coating that is applied to the steel substrate. The protective coating may be based on Al and may contain up to 20% by weight of other alloy elements. Also disclosed are methods for producing such flat steel products, steel components, and methods for producing steel components. Absorption of hydrogen is minimized during heating necessary for hot forming. This is achieved at least in part through an alloy constituent of 0.1-0.5% by weight of at least one alkaline earth or transition metal in the protective coating, wherein an oxide of the alkaline earth or transition metal is formed on an outer surface of the protective coating during hot forming of the flat steel product.

Claims

1.-14. (canceled)

15. A flat steel product for hot forming that comprises: a steel substrate from a steel that includes 0.1-3% by weight Mn and up to 0.01% by weight B; and a protective coating applied to the steel substrate, wherein the protective coating is Al-based, wherein the protective coating comprises an alloy constituent of 0.1-0.5% by weight of at least one alkaline earth or transition metal, wherein an oxide of the at least one alkaline earth or transition metal is formed on an outer surface of the protective coating, wherein the oxide is formed during hot forming of the flat steel product.

16. The flat steel product of claim 15 wherein the protective coating comprises an alloy constituent of 0.1-0.5% by weight Mg, wherein an Mg oxide is formed on the outer surface of the protective coating, the Mg oxide being formed after the hot forming of the flat steel product at a heating temperature of at least 700° C.

17. The flat steel product of claim 16 wherein an Mg content of the protective coating is up to 0.4% by weight.

18. The flat steel product of claim 16 wherein the protective coating includes up to 20% by weight of other alloy elements.

19. The flat steel product of claim 15 wherein the protective coating is an AlSi coating having an Si content of 3-15% by weight.

20. The flat steel product of claim 15 wherein the protective coating includes up to 5% by weight Fe.

21. The flat steel product of claim 15 wherein the protective coating is a hot dip coating.

22. A steel component produced by hot press forming the flat steel product of claim 15.

23. The steel component of claim 22 wherein the alloy constituent is present in a layer that forms a finish of the protective coating relative to surroundings and that has a thickness of up to 200 nm.

24. A method for producing a flat steel product, the method comprising: providing a steel substrate in a form of a flat steel product produced from a steel that comprises 0.1-3% by weight Mn and up to 0.01% by weight B; and coating the steel substrate with an Al-based protective coating that comprises 0.1-0.5% by weight of at least one alloy constituent taken from a group of alkaline earth or transition metals.

25. The method of claim 24 further comprising forming an oxide of the at least one alloy constituent on an outer surface of the Al-based protective coating during hot forming of the flat steel product.

26. The method of claim 24 wherein the at least one alloy constituent comprises Mg.

27. The method of claim 26 wherein the Al-based protective coating is applied to the steel substrate by hot dip coating.

28. The method of claim 24 wherein the Al-based protective coating further comprises up to 20% by weight of other alloy elements.

29. A method for producing a steel component produced by hot press forming a flat steel product that comprises a steel substrate comprising steel that includes 0.1-3% by weight Mn and up to 0.01% by weight B, and a protective coating applied to the steel substrate, wherein the protective coating is Al-based, wherein the protective coating comprises an alloy constituent of 0.1-0.5% by weight of Mg, wherein an oxide of the Mg is formed on an outer surface of the protective coating, wherein the alloy constituent is present in a layer that forms a finish of the protective coating relative to surroundings and that has a thickness of up to 200 nm, the method comprising: heating the flat steel product to a heating temperature for hot forming, wherein the heating occurs under an ambient atmosphere or under an H.sub.2O-reduced atmosphere; and hot forming the flat steel product to create the steel component.

30. The method of claim 29 wherein the oxide is formed during the hot forming of the flat steel product.

31. The method of claim 29 wherein a level of the heating temperature is such that at a start of forming the flat steel product has a hot forming temperature at which a microstructure of the steel substrate is austenitic, wherein the flat steel product is quenched after or during the hot forming so that a hardened microstructure forms in the microstructure of the steel substrate of the flat steel product.

32. The method of claim 31 wherein the heating temperature is at least 700° C.

33. The method of claim 29 wherein the protective coating includes up to 20% by weight of other alloy elements.

Description

[0047] Below, the invention is elucidated in more detail, using working examples. In the drawings:

[0048] FIG. 1 shows a layer construction of a protective coating present on an inventive steel component, in a schematic representation;

[0049] FIG. 2 shows a diagram showing the outcome of GDOES measurement of the near-surface layers of an inventive flat steel product after hot dip coating but before hot press hardening;

[0050] FIG. 3 shows a diagram showing the outcome of GDOES measurement of the near-surface layers of a flat steel product of the invention after hot press hardening;

[0051] FIG. 4 shows a diagram contrasting the hydrogen uptake over various annealing times of flat steel products provided with a conventional protective AlSi coating and with inventive protective coatings.

[0052] The experiments carried out to verify the effect of the invention were conducted using four samples, E1, E2, E3, and V, of a steel strip consisting of an MnB steel, with a composition as reported in table 1.

TABLE-US-00001 TABLE 1 C Si Mn P S Al Ti Cr + Mo B 0.22 0.25 1.16 0.014 0.002 0.038 0.023 0.21 0.0026 balance iron and unavoidable impurities, figures in wt %

[0053] Samples E1, E2, E3, and V, which at this point in time are hydrogen-free, are each provided by hot dip coating with a protective AlSi coating having a composition as reported in table 2.

TABLE-US-00002 TABLE 2 Sample Si Mg Fe E1 10 0.4 up to 3 E2 10 0.3 up to 3 E3 10 0.05 up to 3 V 10 — up to 3 balance aluminum and unavoidable impurities, figures in wt %

[0054] The minimum add-on weight of the protective coating was 120 g/m.sup.2 in each case.

[0055] The flat steel product samples E1, E2, E3, and V thus provided with a protective coating were each heated on travel through an oven, for an annealing time GD of 360 s, 600 s or 800 s (see FIG. 4), under a standard atmosphere with a dew point of +14.0° C., to 900° C. Subsequently they were transported within a transfer time of 4-6 seconds into a hot press mold, where they were hot-formed to give a component. In the course of the hot forming, the respective sample was cooled at a rate such that hardened microstructure was formed in the microstructure of its steel substrate.

[0056] As shown schematically in FIG. 1, in the case of inventive samples E1, E2, and E3, whose protective coating “C”, present on the steel substrate “S” and comprising FeAl(Si) compounds contains Mg as an additional alloy constituent in each case, came over the course of the annealing treatment to have a thin MgO layer “M” adjoining the surface O of the protective coating S and containing Al.sub.2O.sub.3 as well as MgO. The layer M shields the underlying AlSi layer “A”, comprising the FeAl(Si) compounds, from the moist oven atmosphere “P” and so prevents excessive oxidation of the aluminum in the layer A.

[0057] For the sample E1, FIG. 2 shows the outcome of GDOES analysis of the protective coating after hot dip coating and before hot press hardening, and FIG. 3 shows the outcome of GDOES analysis of the protective coating after hot press hardening. It is evident that both before and after the hot press hardening, a thin Mg layer is present on the AlSi coating.

[0058] A comparison of FIGS. 2 and 3 shows that the magnesium fraction prior to press hardening is distributed essentially uniformly within the layer (FIG. 2), whereas after press hardening it has accumulated in a near-surface layer (FIG. 3).

[0059] For the steel components generated in this way from the samples E1, E2, E3, and V, the hydrogen content Hdiff was determined, being the amount of hydrogen diffused into the steel substrate in the course of the heat treatment.

[0060] FIG. 4 compares the results of these analyses for the samples E1, E2, E3, and V, calcined for different periods of time. It is found that the protective coatings of the invention produce a decisive reduction in hydrogen uptake even when Mg is present only in very small amounts in the protective coating.