Improving the cold-forming suitability of aluminium-based coatings by adding alkaline earth metals

Abstract

Steel flat product for cold forming consisting of a steel substrate coated with a protective coating, wherein the protective coating consists of an inner Al-Si-containing and an outer alkaline earth oxide and / or transition metal oxide-containing layer, and wherein the steel substrate has a composition containing (in wt.%): - up to 0.09 carbon - up to 0.3 silicon - up to 0.9 manganese - up to 0.025 phosphorus - up to 0.02 sulfur - up to 0.1 aluminum - up to 0.14% titanium and / or niobium - up to 0.001 boron, and possibly - further alloying elements selected from the group up to 0.01 N, up to 0.15 Cu, up to 0.15 Cr, up to 0.15 Ni, up to 0.025 Mo, and / or up to 0.05 Sn, and - Residual iron and unavoidable impurities.

Description

Technical field

[0001] The present invention relates to a steel flat product for cold forming and to a method for producing the steel flat product according to the invention. According to a further aspect, the present invention relates to a method for producing a steel component and to the steel component obtained according to the method of the invention. Furthermore, the present invention relates to the use of at least one alkaline earth or transition metal to improve the tribological properties and to reduce the coefficient of friction of a cold-formable steel flat product.

[0002] For the purposes of the present invention, the term "flat steel product" refers to all rolled products whose length is significantly greater than their thickness. This includes steel strips and sheets, as well as blanks and sheets derived therefrom.

[0003] In the context of the present invention, the term "inner Al-Si-containing layer and outer alkaline earth oxide and / or transition metal oxide-containing layer" means that the layer structure does not have a sharply defined layer structure, but rather an alloy in which - as a result of the oxygen affinity of the respective components - these are present in increased concentrations in the inner or outer layer. Technical background

[0004] Aluminum-silicon protective coatings are known from the state of the art and have been used for years in various applications. For example, in the automotive industry as a temperature- and corrosion-resistant coating, for components such as exhaust systems or heat shields.

[0005] The prior art includes various coating and forming technologies, primarily concerned with corrosion protection, hot forming behavior, or the creation of oxide functional layers, as illustrated by the following documents: WO 2009 / 047183 A1 describes a two-layer AlSi / Zn coating system for use in hot press hardening. A similar two-layer AlSi / Zn coating system is also described in DE 10 2009 007 909 A1. WO 2015 / 036151 A1 discloses Al-Zn-Mg-based coatings for hot-formable steels, where Mg is used in significant quantities to adjust corrosion behavior. Thematically related is WO 2016 / 034476 A1, which describes Al or AlSi coatings with 0.1–0.5 wt.% alkaline earth or transition metal content. WO 2015 / 117754 A1 concerns plasma electrolytic oxidation for the production of hard ceramic, micrometer-thick oxide layers from Al, Mg, Ti or Zr alloys.DE 10 2004 059 566 B3 deals with a process for hot-dip coating high-strength steels, in which an iron oxide layer is selectively created and subsequently reduced to avoid adhesion problems in the hot-dip bath. DE 10 2009 007 909 A1 discloses coating variants and process parameters for stabilizing hot-formed, coated components.

[0006] One advantage of aluminum-silicon-containing protective coatings is that, due to the passive aluminum oxide layer that forms, they exhibit better corrosion properties in many atmospheres than zinc-based protective coatings and are also significantly more resistant to temperatures of up to 800 °C.

[0007] The application range of currently existing aluminium-silicon-containing protective systems is severely limited due to their relatively high brittleness, which, in addition to adhesion problems, leads in particular to higher coefficients of friction. Summary of the invention

[0008] The invention is therefore based on the objective of providing a steel flat product, in particular for cold forming, which has improved tribological properties compared to cold-formable steel flat products known from the prior art.

[0009] This problem is solved by a flat steel product with the features of claim 1.

[0010] Advantageous embodiments and variants of the invention will result from the dependent claims and the following description.

[0011] The steel flat product according to the invention consists of a steel substrate coated with a protective coating, wherein the protective coating consists of an inner Al-Si-containing and an outer alkaline earth oxide and / or transition metal oxide-containing layer, and wherein the steel substrate has a composition containing (in wt.%): - up to 0.09 carbon - up to 0.3 silicon - up to 0.9 manganese - up to 0.025 phosphorus - up to 0.02 sulfur - up to 0.1 aluminum - up to 0.14% titanium and / or niobium - up to 0.001 boron, and possibly - further alloying elements selected from the group up to 0.01 N, up to 0.15 Cu, up to 0.15 Cr, up to 0.15 Ni, up to 0.025 Mo, and / or up to 0.05 Sn, and - Residual iron and unavoidable impurities.

[0012] Preferably, the steel flat product has a composition containing (in wt.%): - up to 0.06 carbon, - up to 0.12 silicon, - up to 0.3 manganese, preferably up to 0.2 manganese, - up to 0.017 phosphorus, - up to 0.018 sulfur, - up to 0.07 aluminium, - up to 0.12% titanium and / or niobium, - up to 0.0008 boron, and possibly - further alloying elements selected from the group up to 0.007 N, up to 0.11 Cu, up to 0.1 Cr, up to 0.1 Ni, up to 0.02 Mo, and / or up to 0.03 Sn, and - Residual iron and unavoidable impurities.

[0013] The inner Al-Si-containing layer of the protective coating according to the invention preferably contains 3–15 wt.% silicon in addition to aluminum as the main component. Particularly preferred Si contents of Al-Si protective coatings of this type are in the range of 7–12 wt.%, with a standard Al-Si-containing protective coating known from practice most preferably containing 9–10 wt.% Si.

[0014] According to the invention, the protective coating further comprises an outer layer containing alkaline earth oxides and / or transition metal oxides. Magnesium and calcium are particularly preferred alkaline earth metals, although lithium, strontium, sodium, or barium may also be used as alternatives or supplements.

[0015] Preferably, the outer alkaline earth oxide and / or transition metal oxide-containing layer has a layer thickness of up to 200 nm, more preferably up to 100 nm and most preferably up to 50 nm.

[0016] Preferably, the protective coating has a total thickness in the range of 5 to 60 µm, more preferably in the range of 9 to 40 µm.

[0017] Transition metals such as zirconium and titanium can be used.

[0018] Surprisingly, it has now been shown that the steel flat product according to the invention, intended for cold forming, has improved tribological properties compared to an aluminium-silicon coated steel flat product as a result of the outer layer containing alkaline earth oxide and / or transition metal oxide.

[0019] Furthermore, it was surprisingly found that the steel flat product according to the invention also exhibits improved corrosion behavior.

[0020] According to a preferred embodiment, the steel substrate has a yield strength in the range of 100 to 400 MPa and a tensile strength in the range of 250 to 600 MPa. A steel substrate with such mechanical properties has proven to be particularly suitable for cold forming in practice.

[0021] According to a preferred embodiment, the protective coating has an alkaline earth and / or transition metal content of 0.05 to 2 wt.%, more preferably 0.1 to 0.5 wt.% and most preferably 0.15 to 0.4 wt.%.

[0022] Experiments have shown that even small amounts of an alkaline earth and / or transition metal within the protective coating can be sufficient for the purposes according to the invention.

[0023] Therefore, the concentrations are particularly preferably limited to less than 0.5 wt.%, in particular less than 0.45 wt.% or at most 0.4 wt.%. This is especially true given that tests have shown that excellent effects, sufficient for the purposes of the invention, are achieved even at concentrations of up to 0.3 wt.%.

[0024] To achieve the purposes of the invention, magnesium and calcium have proven particularly suitable for forming the outer layer containing alkaline earth oxides and / or transition metal oxides, as they can be readily incorporated into aluminum protective coatings of the type discussed here. The added amount of Mg and / or Ca is optimally adjusted so that the protective coating contains at least 0.05 wt.% and at most 2 wt.% Mg and / or Ca, whereby, for the reasons already explained above, Mg and / or Ca contents of less than 0.5 wt.%, in particular less than 0.45 wt.% or up to 0.4 wt.% or up to 0.3 wt.%, have proven particularly advantageous in practice.

[0025] For the same reasons, strontium can also be present as an additional alloying element in a protective coating according to the invention in amounts of 0.005 to 0.25 wt.%. Here, too, it can be advantageous to limit the Sr content to up to 0.15 wt.%, in particular up to 0.10 wt.%.

[0026] Barium can be added to the protective coating according to the invention as an additional alloying component for the purposes of the invention in amounts of 0.005 to 0.25 wt.%, whereby, for the reasons explained, Ba contents of 0.005 to 0.05 wt.% prove to be particularly advantageous here.

[0027] If zirconium and titanium are to be added to the protective coating as additional alloying elements for the purposes of the invention, this can be done in amounts of 0.15 to 0.7 wt.% each, whereby, for the reasons already explained, contents of at most 0.5 wt.%, in particular at most 0.4 wt.% or at most 0.3 wt.%, are particularly advantageous.

[0028] According to a further preferred embodiment, the protective coating according to the invention contains up to 5 wt.% Fe, which is present as a further alloying component in the Al-Si-containing protective coating in addition to or as an alternative to the Si content.

[0029] According to another aspect, the present invention relates to a method for producing the steel flat product according to the invention, comprising the steps of: - Providing a steel substrate having a composition containing a maximum (in wt.%) of: - up to 0.09 carbon - up to 0.3 silicon - up to 0.9 manganese - up to 0.025 phosphorus - up to 0.02 sulfur - up to 0.1 aluminum - up to 0.14% titanium and / or niobium - up to 0.001 boron, and possibly - further alloying elements selected from the group up to 0.01 N, up to 0.15 Cu, up to 0.15 Cr, up to 0.15 Ni, up to 0.025 Mo, and / or up to 0.05 Sn, and - Residual iron and unavoidable impurities, - Annealing of the steel substrate at a temperature in the range of 650 - 950 °C, - Cooling the steel substrate to a temperature in the range of 460-650 °C, and - Hot-dip coating of the steel substrate with a melt containing aluminium, silicon and alkaline earth metals and / or transition metals.

[0030] To adjust the thickness of the protective coating, which preferably has a total thickness in the range of 5 to 60 µm, more preferably in the range of 9 to 40 µm, the excess melt applied to the flat steel product is blown off, for example by means of a nozzle bar.

[0031] Preferably, the steel flat product has a composition containing (in wt.%): - up to 0.06 carbon, - up to 0.12 silicon, - up to 0.3 manganese, preferably up to 0.2 manganese, - up to 0.017 phosphorus, - up to 0.018 sulfur, - up to 0.07 aluminium, - up to 0.12% titanium and / or niobium, - up to 0.0008 boron, and possibly - further alloying elements selected from the group up to 0.007 N, up to 0.11 Cu, up to 0.1 Cr, up to 0.1 Ni, up to 0.02 Mo, and / or up to 0.03 Sn, and - Residual iron and unavoidable impurities.

[0032] According to a preferred embodiment, the melt has an alkaline earth and / or transition metal content in the range of 0.05 to 2 wt.%, more preferably 0.1 to 0.5 wt.% and most preferably 0.15 to 0.4 wt.%.

[0033] The content of alkaline earth and / or transition metals in the melt should not exceed 2 wt%, as this can lead to undesirable oxidation reactions in the melt bath. The resulting slag particles or oxide films can negatively affect wetting and appearance, thus increasing surface defects.

[0034] Furthermore, it has been shown that a content of alkaline earth and / or transition metals in the melt of < 0.1 wt.% has proven too low to have a sufficient effect. The resulting flat steel products therefore do not exhibit a completely covering oxide layer.

[0035] Alternatively, the steel flat product according to the invention can also be produced by a process in which the inner Al-Si-containing and / or the outer alkaline earth oxide and / or transition metal oxide-containing layer are successively applied to the steel substrate using suitable methods. In principle, any known method is suitable that allows the deposition of sufficiently thin layers on the bare steel substrate or on a steel substrate coated with an Al-Si-containing protective layer. Examples include the known PVD (Physical Vapor Deposition) or CVD (Chemical Vapor Deposition) processes.

[0036] Another aspect of the present invention relates to a method for manufacturing a steel component, wherein a flat steel product according to the invention is first provided and then subjected to a cold pressing process to obtain a desired steel component.

[0037] Furthermore, the present invention relates in another aspect to a steel component which is manufactured according to the inventive method. Preferably, such a component manufactured in this way surprisingly exhibits a coefficient of friction in the range of 0.05 to 0.2, more preferably 0.1 to 0.15.

[0038] Further aspects of the present invention relate to the use of at least one alkaline earth or transition metal in an amount of 0.05 to 2 wt.% as an additional alloying element in an aluminium-silicon-containing protective coating to reduce the tribological properties of a cold-formable steel flat product, as well as the use of at least one alkaline earth or transition metal in an amount of 0.05 to 2 wt.% as an additional alloying element in an aluminium-silicon-containing protective coating to reduce the corrosion behavior of a cold-formable steel flat product.

[0039] The present invention is explained in more detail below with reference to figures and examples: Fig. Figure 1 shows a schematic representation of a steel flat product according to the invention, Fig. Figure 2 shows metallographic images of a polished section of a conventional cold-formable steel flat product (a) and of the steel flat product according to the invention (b), Fig. Figure 3 shows images of the surfaces after corrosion testing of a conventional cold-formable steel flat product (a) and of the steel flat product according to the invention (b).

[0040] Fig.Figure 1 shows a schematic representation of the steel flat product according to the invention, which consists of a steel substrate “S” and a protective coating “C” arranged thereon. The protective coating “C” in turn consists of an inner Al-Si-containing layer “A” and an outer alkaline earth oxide and / or transition metal oxide-containing layer “M”. During the annealing process, some of the iron diffuses from the steel substrate “S” into the overlying Al-Si-containing layer “A”, and some of the Al₂O₃ diffuses into the alkaline earth oxide and / or transition metal oxide-containing layer “M”.

[0041] Fig. Figure 2 shows metallographic images of a cross-section of a conventional cold-formable steel flat product (a) and of the steel flat product according to the invention (b). It can be seen that the surface of the reference sample develops grooves. In contrast, the steel flat product according to the invention has a uniform layer thickness across its cross-section.

[0042] Fig. Figure 3 shows images of the surfaces after corrosion testing of a conventional cold-formable steel flat product (a) and of the steel flat product according to the invention (b). It is evident that the surface of the reference sample has a significantly higher surface area covered with red rust than the steel flat product according to the invention. Example

[0043] Several steel substrates with the composition shown in Table 1 were hot-dip coated with a melt containing aluminum, silicon, and magnesium. For this process, the steel substrate was subjected to a hot-dip coating process in which it was first recrystallized at a temperature of 780–850 °C, then cooled to a strip immersion temperature of 600–640 °C, and subsequently passed through the molten metal bath. The molten metal bath had a composition of (by weight) 10% Si, 3% Fe, 0.4% Mg, and the remainder aluminum. The resulting coating had a density of 30 to 120 g / m². 2 per side. The concentration of the respective components in the melt was adjusted so that the resulting protective coating had a total thickness in the range of 9 to 40 µm.

[0044] The samples were subsequently characterized. For this purpose, the samples were exposed to a corrosive sulfur dioxide atmosphere and then metallographically examined to determine the corrosion depth. The forming properties were determined both using a strip drawing machine and by forming a cup. Both tests were carried out with a low oil coating of 0.5 g / m². 2 Instead, a standard surface oil was used. Table 1: C Si Mn P S Al Ti B N Percentage [wt.%) 0,004 0,05 0,15 0,01 0,011 0,04 0,06 0,0005 0,002 Residual iron and unavoidable impurities. Table 2: sample Corrosion depth in µm after SO2 test Friction force in kN reference 125 2 1 55 1,5 2 56 1,5 3 54 1,5 4 55 1,5

Claims

[1] Steel flat product for cold forming consisting of a steel substrate coated with a protective coating, wherein the protective coating consists of an inner Al-Si-containing and an outer alkaline earth oxide and / or transition metal oxide-containing layer and wherein the steel substrate has a composition containing (in wt.%): - up to 0.09 carbon - up to 0.3 silicon - up to 0.9 manganese - up to 0.025 phosphorus - up to 0.02 sulfur - up to 0.1 aluminum - up to 0.14% titanium and / or niobium - up to 0.001 boron, and possibly - further alloying elements selected from the group up to 0.01 N, up to 0.15 Cu, up to 0.15 Cr, up to 0.15 Ni, up to 0.025 Mo, and / or up to 0.05 Sn, and - Residual iron and unavoidable impurities. [2] Steel flat product according to claim 1, wherein the steel substrate has a yield strength in the range of 100 to 400 MPa and a tensile strength in the range of 250 to 600 MPa. [3] Steel flat product according to claim 1 or 2, wherein the protective coating has an alkaline earth and / or transition metal content of 0.05 to 2 wt.%, preferably 0.1 to 0.5 wt.%, more preferably 0.15 to 0.4 wt.%. [4] Steel flat product according to any one of the preceding claims 1 to 3, wherein the protective coating contains up to 5 wt.% Fe. [5] Method for producing a steel flat product according to any one of claims 1 to 4 comprising the steps: - Providing a steel substrate having a composition containing a maximum (in wt.%) of: - up to 0.09 carbon - up to 0.3 silicon - up to 0.9 manganese - up to 0.025 phosphorus - up to 0.02 sulfur - up to 0.1 aluminum - up to 0.14% titanium and / or niobium - up to 0.001 boron, and possibly - further alloying elements selected from the group up to 0.01 N, up to 0.15 Cu, up to 0.15 Cr, up to 0.15 Ni, up to 0.025 Mo, and / or up to 0.05 Sn, and balance iron as well as unavoidable impurities, - Annealing of the steel substrate at a temperature in the range of 650 - 950 °C, - Cooling the steel substrate to a temperature in the range of 550 - 640 °C, and - Hot-dip coating of the steel substrate with a melt containing aluminium, silicon and alkaline earth metals and / or transition metals. [6] Method according to claim 5, wherein the alkaline earth and / or transition metal content in the melt is 0.05 to 2 wt.%, preferably 0.1 to 0.5 wt.%, more preferably 0.15 to 0.4 wt.%. [7] Method for manufacturing a steel component, comprising the steps: - Providing a steel flat product according to any one of claims 1 to 4, and - Cold pressing of the steel flat product into the steel component. [8] Steel component manufactured according to claim 7, having a coefficient of friction in the range of 0.05 to 0.2, more preferably 0.1 to 0.

15. [9] Use of at least one alkaline earth or transition metal in an amount of 0.05 to 2 wt.% as an additional alloying element in an aluminium-silicon protective coating to reduce the tribological properties of a cold-formable steel flat product. [10] Use of at least one alkaline earth or transition metal in an amount of 0.05 to 2 wt.% as an additional alloying element in an aluminium-silicon protective coating to reduce the corrosion behavior of a cold-formable steel flat product.

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