Flat steel product with colour change

Abstract

The present invention relates to a steel flat product for the production of a sheet metal part by hot forming, comprising a. a steel substrate consisting of a steel having 0.1-3 wt.% Mn and optionally up to 0.01 wt.% B, and b. an aluminum-based coating arranged on at least one side of the steel substrate, wherein the coating has an Al base layer consisting of 1.0-15 wt.% Si, optionally 2-4 wt.% Fe, 0.1-5.0 wt.% alkali or alkaline earth metals, and optional further components, the sum of which amounts to a maximum of 2.0 wt.%.-% are limited, and the remainder consists of aluminium, wherein the alkali or alkaline earth metals in the Al base layer have such a distribution that the steel flat product, when cooled, undergoes a colour change after annealing in a furnace with a furnace temperature of 900°C for an annealing period of 9 minutes compared to the cooled state after a reference annealing period of 4 minutes, wherein the colour change has a colour dynamic of greater than 100.

Description

[0001] The invention relates to a flat steel product for producing a sheet metal part by hot forming, a method for producing such a flat steel product and a method for producing a sheet metal part.

[0002] When we refer to a "flat steel product" or a "sheet metal product" below, we mean rolled products, such as steel strips or sheets, from which "sheet metal blanks" (also called blanks) are cut for the production of, for example, car body components. "Formed sheet metal parts" or "sheet metal components" are made from such sheet metal blanks, whereby the terms "formed sheet metal part" and "sheet metal component" are used synonymously here.

[0003] All information regarding the contents of the steel compositions specified in this application is based on weight, unless expressly stated otherwise. All unspecified "%" figures relating to a steel alloy are therefore to be understood as "wt%." Information given in this text regarding the contents of the components of an atmosphere refers to the volume (indicated in "vol%").

[0004] Flat steel products with similar aluminum-based coatings and processes for their production are known from WO 2022 / 048990 A1, EP 2 993 248 B1, US 2021 / 402740 A1 and WO 2019 / 016041 A1.

[0005] Such flat steel products have an aluminum-based coating and are further processed into sheet metal parts by hot forming. Blanks from the flat steel products are heated to a hot forming temperature (e.g., 900°C) for a specific annealing time (e.g., 4 minutes). During this annealing time, iron diffuses from the steel substrate into the aluminum-based coating. This creates a coating that provides highly effective protection against corrosion. The hot blank is then formed into a sheet metal part in a forming tool and rapidly cooled, forming a hardened structure (e.g., martensite) in the steel substrate. The result is a sheet metal part with high strength and a coating that provides very effective protection against corrosion.

[0006] The problem, however, is that the desired corrosion protection can only be achieved if the processing conditions, especially the annealing time, are adhered to very precisely. An annealing time that is too long leads to increased diffusion of iron into the coating, which severely disrupts the layer structure. In industrial production, however, deviations can occasionally occur due to disruptions in the operational process. Therefore, it is necessary to identify and sort out sheet metal parts that were inadvertently manufactured under incorrect conditions. In principle, this is possible, for example, by taking a metallographic section or by chemically analyzing the surface. However, such examinations are very complex and not non-destructive.

[0007] Therefore, the object of the present invention is to further develop a flat steel product in such a way that it can be determined without great effort on the formed sheet metal part whether the desired manufacturing conditions have been met.

[0008] This object is achieved by a flat steel product for producing a sheet metal part by hot forming, according to claim 1.

[0009] Typically, coatings of this type are produced by hot-dip coating. Surprisingly, it has been shown that the addition of 0.1–1.0 wt.% alkaline earth metals to the melt and specific cooling conditions, which will be explained in detail later, results in a coating with a specific distribution of the alkaline earth metals in the aluminum base layer. This distribution leads to a color change in the flat steel product after annealing times of more than 4 minutes. Further diffusion processes change the near-surface concentration of alkaline earth metals, which in turn causes the color change. Comparative tests clearly show that this color change is significantly more pronounced than with coatings without the addition of alkaline earth metals.Therefore, by measuring color values on the formed sheet metal part and comparing them with reference color values of a cooled reference sheet metal part, it is possible to determine whether the sheet metal part has undergone the desired annealing process. The reference sheet metal part was annealed for 4 minutes in a furnace at a temperature of 900°C. A cooled sheet metal part (or reference sheet metal part) is understood to mean that the sheet metal part (or reference sheet metal part) has reached room temperature.

[0010] The color dynamics of a color change are determined as follows: Using a spectrophotometer and a light source with CIE standard illuminant A (can be realized with sufficient accuracy by a gas-filled tungsten filament lamp with a distribution temperature of TV≈2856 K) and a 10° field of view according to the CIE 1964 convention, the color of the sheet metal part is determined in the CIE Lab color space. This is standardized according to EN ISO 11664-4 "Colorimetry - Part 4: CIE 1976 L*a*b* Color space." The values from the CIE Lab color space are converted to the sRGB 8 color space. The sRGB 8 color space used is the 8-bit variant, in which the color values assume integers from 0-255. The conversion is familiar to the expert and can be used, for example, on https: / / convertingcolors.com / cielab-color-1.00 1.00 1.00.html can be carried out online.

[0011] The result is unique RGB values for the sheet metal part. The reference color values (R ref G ref B ref ) of a correctly processed sheet metal part (reference sheet metal part) are determined in a similar way.

[0012] A color dynamic D of a color change from the reference sheet metal part to the sheet metal part is now defined as the sum of the amounts of the differences of the RGB values, ie: D = R − R ref + G − G ref + B − B ref

[0013] It has been shown that the color dynamics defined in this way are particularly well suited to quantify the color change.

[0014] Such a flat steel product according to the invention is produced by means of the method according to the invention described below according to claim 2.

[0015] In step a), a semi-finished product composed of the alloy specified for the steel flat product according to the invention is provided. This can be a slab produced by conventional continuous slab casting or by thin slab casting.

[0016] In step b), the semi-finished product is thoroughly heated to a temperature (T1) of 1000–1400°C. If the semi-finished product has cooled after casting, it is first reheated to 1000–1400°C for thorough heating. The thorough heating temperature should be at least 1000°C to ensure good formability for the subsequent rolling process. The thorough heating temperature should not exceed 1400°C to avoid the presence of molten phases in the semi-finished product.

[0017] In the optional step c), the semi-finished product is pre-rolled into an intermediate product. Thin slabs are not usually subjected to pre-rolling. Thick slabs to be rolled into hot strip can be pre-rolled if required. In this case, the temperature of the intermediate product (T2) at the end of pre-rolling should be at least 1000°C to ensure that the intermediate product retains sufficient heat for the subsequent finish-rolling step. However, high rolling temperatures can also promote grain growth during the rolling process, which has a detrimental effect on the mechanical properties of the flat steel product. To keep grain growth low during the rolling process, the temperature of the intermediate product at the end of pre-rolling should not exceed 1200°C.

[0018] In step d), the slab or thin slab, or, if step c) has been performed, the intermediate product, is rolled into a hot-rolled flat steel product. If step c) has been performed, the intermediate product is typically finish-rolled immediately after rough rolling. Typically, finish rolling begins no later than 90 seconds after the end of rough rolling. The slab, the thin slab, or, if step c) has been performed, the intermediate product, are rolled to a final rolling temperature (T3). The final rolling temperature, i.e., the temperature of the finished hot-rolled flat steel product at the end of the hot rolling process, is 750-1000°C. At finish rolling temperatures below 750°C, the amount of free vanadium decreases because larger amounts of vanadium carbides are precipitated. The vanadium carbides precipitated during finish rolling are very large.They typically have an average grain size of 30 nm or more and are no longer dissolved in subsequent annealing processes, such as those performed before hot-dip coating. The final rolling temperature is limited to a maximum of 1000°C to prevent coarsening of the austenite grains. Furthermore, final rolling temperatures of a maximum of 1000°C are relevant for the process to achieve coiling temperatures (T4) below 700°C.

[0019] Hot rolling of the steel flat product can be carried out as continuous hot strip rolling or reversing rolling. In the case of continuous hot strip rolling, step e) provides for optional coiling of the hot-rolled steel flat product. For this purpose, the hot strip is cooled to a coiling temperature (T4) within less than 50 seconds after hot rolling. The cooling medium used for this purpose can be water, air, or a combination of both. The coiling temperature (T4) should not exceed 700°C to avoid the formation of large vanadium carbides. In principle, there is no lower limit on the coiling temperature. However, coiling temperatures of at least 500°C have proven favorable for cold rolling. The coiled hot strip is then cooled to room temperature in air using the conventional method.

[0020] In step f), the hot-rolled flat steel product is optionally descaled in a conventional manner by pickling or by another suitable treatment.

[0021] The scale-cleaned hot-rolled flat steel product can optionally be subjected to cold rolling before annealing in step g), for example, to meet more stringent thickness tolerance requirements. The cold rolling degree (CTC) should be at least 30% to inject sufficient deformation energy into the flat steel product for rapid recrystallization. The CTC is defined as the quotient of the thickness reduction during cold rolling ΔdCTC divided by the hot strip thickness d: KWG = Δd KW / d

[0022] Where Δd KW = thickness reduction during cold rolling in mm and d = hot strip thickness in mm, the thickness reduction Δd KW results from the difference between the thickness of the flat steel product before cold rolling and the thickness of the flat steel product after cold rolling. The flat steel product before cold rolling is typically a hot strip with a hot strip thickness of d. The flat steel product after cold rolling is also commonly referred to as cold strip. The cold rolling degree can, in principle, be very high, exceeding 90%. However, cold rolling degrees of no more than 80% have proven to be beneficial for preventing strip breakage.

[0023] In step h), the flat steel product undergoes an annealing treatment at annealing temperatures (T5) of 650–900°C. For this purpose, the flat steel product is first heated to the annealing temperature within 10–120 seconds and then held at that temperature for 30–600 seconds. The annealing temperature is at least 650°C, preferably at least 720°C. Annealing temperatures above 900°C are undesirable for economic reasons.

[0024] In step i), the flat steel product is cooled to an immersion temperature (T6) after annealing to prepare it for subsequent coating treatment. The immersion temperature is lower than the annealing temperature and is adjusted to the temperature of the molten bath. The immersion temperature is 600-800°C, preferably at least 650°C, more preferably at least 670°C, and most preferably at most 700°C. For a particularly homogeneous boundary layer formation, it is important that sufficient thermal energy is present in the boundary layer between the steel substrate and the aluminum melt. This is not the case at temperatures lower than 600°C, so that undesirable compounds can form, the subsequent reconversion of which can lead to pores.Above the preferred immersion temperatures, the diffusion rate of iron into aluminum increases significantly again, allowing increased iron diffusion into the still-liquid boundary layer right at the beginning of the coating process. The cooling time of the annealed flat steel product from the annealing temperature T5 to the immersion temperature T6 is preferably 10-180 seconds. In particular, the immersion temperature T6 should not deviate from the temperature of the molten bath T7 by more than 30°C, especially not more than 20°C, and preferably not more than 10°C.

[0025] The flat steel product is subjected to a coating treatment in step j). The coating treatment is preferably carried out by continuous hot-dip coating. The coating can be applied to just one side, both sides, or all sides of the flat steel product. The coating treatment is preferably carried out as a hot-dip coating process, in particular as a continuous process. The flat steel product usually comes into contact with the molten bath on all sides, so that it is coated on all sides. The molten bath, which contains the alloy to be applied to the flat steel product in liquid form, typically has a temperature (T7) of 660-800°C, preferably 670-740°C, particularly preferably 670-710°C. Aluminum-based alloys have proven particularly suitable for coating ageing-resistant flat steel products with a corrosion-protective coating.In such a case, the molten bath contains 1.0-15 wt.% Si, optionally 2-4 wt.% Fe, 0.1-1.0 wt.% alkaline earth metals, and the remainder aluminum. In a preferred variant, the Si content of the melt is 1.0-3.5 wt.% or 7-12 wt.%, in particular 8-10 wt.%. The alkaline earth metal content consists of 0.1-1.0 wt.% Mg, or the alkaline earth metal content consists of 0.1-0.7 wt.% Mg and at least 0.0015 wt.% Ca.

[0026] After leaving the molten bath, the flat steel product is blown off using a gas stream to adjust the thickness of the coating.

[0027] After coating treatment, the coated flat steel product is cooled to room temperature in step k). The average cooling rate between 660°C and 570°C is at least 15 K / s, preferably at least 20 K / s. This corresponds to the range between the start and end of solidification of the coating. Upon cooling to 660°C, solidification of the coating begins, and upon further cooling to 570°C, the coating is fully solidified. The average cooling rate is preferably a maximum of 50 K / s. It has been shown that this rapid cooling creates a specific distribution of the alkaline earth metals in the Al base layer of the coating, which leads to the advantageous color change described during the subsequent forming process. In general, the alkaline earth metals tend to concentrate close to the surface.While the alkaline earth metals are still evenly distributed in the melt, they diffuse toward the surface in the coating. This is possible as long as sufficient thermal energy is available for diffusion processes. If the cooling rate between the beginning and end of solidification is too slow, the majority of the alkaline earth metals will have already diffused to the surface. Therefore, during subsequent annealing for the forming process, no significant diffusion of the alkaline earth metals to the surface occurs. Therefore, in such a case, no significant color change of the surface occurs. Therefore, diffusion must be stopped quickly enough after the coating treatment to allow sufficient diffusion processes to occur during the subsequent annealing for the forming process, which cause the color change.This is achieved by an average cooling rate of at least 15 K / s between 660°C and 570°C, i.e. between the start of hardening and the end of hardening. Diffusion must not end too quickly, however. A certain proportion of the alkaline earth metals must already be concentrated on the surface. The purpose of adding the alkaline earth metals is to ensure that oxides of the alkaline earth metals form on the surface of the coating during the subsequent annealing process before forming, instead of aluminum oxides. This has the advantage that less free hydrogen is produced than during the formation of Al 2 O 3 . (The oxygen bound during oxidation usually comes from water molecules in the atmosphere, so that the remaining hydrogen is inevitably released during oxidation.) The free hydrogen diffuses into the steel substrate and leads to undesirable hydrogen embrittlement.The addition of alkaline earth metals leads to a reduction in free hydrogen and thus to a reduction in hydrogen embrittlement. However, the alkaline earth metals can only achieve this effect if they are close to the surface of the substrate during the annealing process before forming. Therefore, it is advantageous if a certain proportion of the alkaline earth metals has already diffused to the surface during solidification. This ensures that a sufficient proportion of alkaline earth metals is available for the described oxidation process immediately from the start of the annealing process before forming. Therefore, it is advantageous if the cooling rate is a maximum of 50 K / s. This ensures that a sufficient proportion of the alkaline earth metals has diffused to the surface of the coating.Through this targeted choice of the cooling process, a distribution of the alkaline earth metals in the coating (especially in the Al base layer of the coating) is achieved which, on the one hand, has sufficient alkaline earth metals close to the surface to reduce the free hydrogen immediately from the beginning of the subsequent annealing and, on the other hand, still allows sufficient diffusion which leads to the color change according to the invention.

[0028] The coated flat steel product can optionally be subjected to skin passing with a skin passing degree of up to 2% to improve the surface roughness of the flat steel product.

[0029] The steel used in the flat steel product and in the process for producing a flat steel product is a steel containing 0.1-3 wt.% Mn and optionally up to 0.01 wt.% B. The same naturally applies to the steel of the hot-formed sheet metal part. In particular, the steel structure can be converted into a martensitic or partially martensitic structure through hot forming. The structure of the steel substrate of the sheet metal part is therefore preferably martensitic or at least partially martensitic, as this exhibits particularly high hardness.

[0030] In addition to iron and unavoidable impurities (in wt%), the steel consists of: C: 0.04 - 0.45 wt.% Si: 0.02 - 1.2 wt.% Mn: 0.5 - 2.6 wt.% Al: 0.02 - 1.0 wt.% P: ≤ 0.05 wt% S: ≤ 0.02 wt% N: ≤ 0.02 wt% Sn: ≤ 0.03 wt% Ace: ≤ 0.01 wt% Approx: ≤ 0.005 wt% and optionally one or more of the elements "Cr, B, Mo, Ni, Cu, Nb, Ti, V" in the following contents: Cr: 0.08-1.0 wt% B: 0.001-0.005 wt% Mon: ≤0.5 wt% No: ≤0.5 wt% Cu: ≤0.2 wt% Note: 0.02-0.08 wt.%, Ti: 0.01-0.08 wt% V: ≤0.2 wt%

[0031] The elements P, S, N, Sn, As, and Ca are impurities that cannot be completely avoided during steel production. In addition to these elements, other elements may also be present as impurities in the steel. These additional elements are summarized under "unavoidable impurities." The total content of unavoidable impurities is preferably a maximum of 0.2 wt.%, preferably a maximum of 0.1 wt.%. The optional alloying elements Cr, B, Nb, and Ti, for which a lower limit is specified, may also be present as unavoidable impurities in the steel substrate in amounts below the respective lower limit. In this case, they are also counted as unavoidable impurities, with their total content limited to a maximum of 0.2 wt.%, preferably a maximum of 0.1 wt.%. The individual upper limits for the respective impurities of these elements are preferably as follows: Cr: Cr: ≤ 0.050 wt%, B: ≤ 0.0005 wt% Note: ≤ 0.005 wt%, Ti: ≤ 0.005 wt%

[0032] These preferred upper limits should be considered alternatively or jointly. Preferred steel variants therefore meet one or more of these four conditions.

[0033] In a preferred embodiment, the C content of the steel is a maximum of 0.37 wt.% and / or at least 0.06 wt.%. In particularly preferred embodiments, the C content is in the range of 0.06-0.09 wt.%, or in the range of 0.12-0.25 wt.%, or in the range of 0.33-0.37 wt.%.

[0034] In a preferred embodiment, the Si content of the steel is a maximum of 1.00 wt% and / or at least 0.06 wt%.

[0035] In a preferred variant, the Mn content of the steel is a maximum of 2.4 wt.% and / or at least 0.75 wt.%. In particularly preferred embodiments, the Mn content is in the range of 0.75-0.85 wt.% or in the range of 1.0-1.6 wt.%.

[0036] In a preferred variant, the Al content of the steel is a maximum of 0.75 wt.%, in particular a maximum of 0.5 wt.%, preferably a maximum of 0.25 wt.%. Alternatively or additionally, the Al content is preferably at least 0.02%.

[0037] It has also been shown that it can be helpful if the sum of the silicon and aluminum contents is limited. In a preferred variant, the sum of the Si and Al contents (commonly referred to as Si+Al) is therefore a maximum of 1.5 wt.%, preferably a maximum of 1.2 wt.%. Additionally or alternatively, the sum of the Si and Al contents is at least 0.06 wt.%, preferably at least 0.08 wt.%.

[0038] The elements P, S, and N are typical impurities that cannot be completely avoided during steel production. In preferred variants, the P content is a maximum of 0.03 wt.%. Irrespective of this, the S content is preferably a maximum of 0.012%. Additionally or supplementarily, the N content is preferably a maximum of 0.009 wt.%.

[0039] Optionally, the steel also contains chromium at a content of 0.08–1.0 wt.%. The Cr content is preferably a maximum of 0.75 wt.%, in particular a maximum of 0.5 wt.%.

[0040] In the case of an optional alloying of chromium, the sum of the chromium and manganese contents is preferably limited. The sum is a maximum of 3.3 wt.%, in particular a maximum of 3.15 wt.%. Furthermore, the sum is at least 0.5 wt.%, preferably at least 0.75 wt.%.

[0041] Preferably, the steel optionally also contains boron at a content of 0.001-0.005 wt.%. In particular, the boron content is a maximum of 0.004 wt.%.

[0042] Optionally, the steel may contain molybdenum in a maximum content of 0.5 wt%, in particular a maximum of 0.1 wt%.

[0043] Furthermore, the steel may optionally contain nickel with a content of maximum 0.5 wt.%, preferably maximum 0.15 wt.%.

[0044] Optionally, the steel may also contain copper with a content of maximum 0.2 wt.%, preferably maximum 0.15 wt.%.

[0045] In addition, the steel can optionally contain one or more of the microalloying elements Nb, Ti, and V. The optional Nb content is at least 0.02 wt.% and a maximum of 0.08 wt.%, preferably a maximum of 0.04 wt.%. The optional Ti content is at least 0.01 wt.% and a maximum of 0.08 wt.%, preferably a maximum of 0.04 wt.%. The optional V content is a maximum of 0.2 wt.%, in particular a maximum of 0.1 wt.%, preferably a maximum of 0.05 wt.%.

[0046] In the case of an optional alloying of several of the elements Nb, Ti, and V, the sum of the contents of Nb, Ti, and V is preferably limited. The sum is a maximum of 0.1 wt.%, in particular a maximum of 0.068 wt.%. Furthermore, the sum is preferably at least 0.015 wt.%.

[0047] The above explanations regarding preferred steel substrates naturally also apply to the steel substrate of the flat steel product as well as to the steel substrates in the described manufacturing processes and also to the process for inspecting a sheet metal part described below.

[0048] The flat steel product according to the invention comprises an aluminum-based coating arranged on at least one side of the steel substrate, wherein the coating has an Al base layer consisting of 1.0-15 wt% Si, optionally 2-4 wt% Fe, 0.1-1.0 wt% alkaline earth metals and the remainder aluminum, wherein the content of alkaline earth metals consists of 0.1-1.0 wt% Mg or the content of alkaline earth metals consists of 0.1-0.7 wt% Mg and at least 0.0015 wt% Ca.

[0049] Such a coating serves to protect the steel substrate from oxidation and corrosion during hot forming and during use of the resulting steel component. Therefore, the coating is also synonymously referred to as a corrosion protection coating.

[0050] The corrosion protection coating can be applied to one or both sides of the flat steel product. The two large, opposing surfaces of the flat steel product are referred to as the two sides. The narrow surfaces are referred to as the edges.

[0051] Such a corrosion protection coating is preferably created by hot-dip coating the flat steel product. The flat steel product is passed through a liquid melt consisting of 1.0-15 wt.% Si, optionally 2-4 wt.% Fe, 0.1 to 1.0 wt.% alkaline earth metals, and the remainder aluminum. The alkaline earth metal content consists of 0.1-1.0 wt.% Mg, or the alkaline earth metal content consists of 0.1-0.7 wt.% Mg and at least 0.0015 wt.% Ca.

[0052] In a preferred variant, the Si content of the melt is 1.0-3.5 wt.% or 7-12 wt.%, in particular 8-10 wt.%.

[0053] In a first variant, the alkaline earth metal content consists of 0.1-1.0 wt% Mg, in particular 0.1-0.7 wt% Mg, preferably 0.1-0.5 wt% Mg.

[0054] In a second variant, the alkaline earth metal content consists of 0.1-0.7 wt% Mg, preferably 0.1-0.5 wt% Mg and at least 0.0015 wt% Ca, in particular at least 0.01 wt% Ca.

[0055] During hot-dip coating, iron diffuses from the steel substrate into the liquid coating, so that the anti-corrosive coating of the flat steel product, upon solidification, comprises, in particular, an alloy layer and an Al base layer. The alloy layer lies on top of the steel substrate and directly borders it. The alloy layer is essentially formed from aluminum and iron. The remaining elements from the steel substrate or the melt composition do not accumulate significantly in the alloy layer. The alloy layer preferably consists of 35-60 wt.% Fe, preferably α-iron, optional further constituents whose total contents are limited to a maximum of 5.0 wt.%, preferably 2.0%, and the remainder aluminum, with the Al content preferably increasing towards the surface.The optional additional components include in particular the remaining components of the melt (i.e. silicon and optionally alkaline earth metals, in particular Mg or Ca) and the remaining components of the steel substrate in addition to iron.

[0056] The Al base layer lies on top of the alloy layer and directly adjoins it. The composition of the Al base layer preferably corresponds to the composition of the melt in the molten bath. Preferred compositions of the Al base layer correspond to the preferred melt compositions.

[0057] In a further preferred variant of the corrosion protection coating, the Si content in the alloy layer is lower than the Si content in the Al base layer.

[0058] The corrosion protection coating preferably has a thickness of 5-60 µm, in particular 10-40 µm. The coating weight of the corrosion protection coating is in particular 30-360 g / m² for double-sided corrosion protection coatings or 15-180 g / m² for the single-sided variant. The coating weight of the corrosion protection coating is preferably 100-200 g / m² for double-sided coatings or 50-100 g / m² for single-sided coatings. The coating weight of the corrosion protection coating is particularly preferably 120-180 g / m² for double-sided coatings or 60-90 g / m² for single-sided coatings.

[0059] The thickness of the alloy layer is preferably less than 20 µm, more preferably less than 16 µm, particularly preferably less than 12 µm, and especially less than 10 µm. The thickness of the Al base layer results from the difference between the thicknesses of the anti-corrosive coating and the alloy layer. The thickness of the Al base layer is preferably at least 1 µm, even with thin anti-corrosive coatings.

[0060] In a preferred variant, the flat steel product comprises an oxide layer arranged on the corrosion protection coating. The oxide layer is located in particular on the aluminum base layer and preferably forms the outer edge of the corrosion protection coating.

[0061] The oxide layer consists, in particular, of more than 80 wt.% oxides, with the majority of the oxides (i.e., more than 50 wt.% of the oxides) being aluminum oxide. In particular, in addition to aluminum oxide, hydroxides and / or magnesium oxide are present in the oxide layer, either alone or as a mixture. Preferably, the remainder of the oxide layer not occupied by the oxides and optionally present hydroxides consists of silicon, aluminum, iron, and / or magnesium in metallic form.

[0062] Preferably, the oxide layer of the flat steel product has a thickness greater than 50 nm. In particular, the maximum thickness of the oxide layer is 500 nm.

[0063] The invention further relates to a method for producing a sheet metal part, comprising the following steps: a. Providing a sheet metal blank from a previously described flat steel product; b. Heating the sheet metal blank in such a way that the AC3 temperature of the blank is at least partially exceeded and the temperature T EinIg of the blank when inserted into a forming tool intended for hot press forming (work step c)) at least partially has a temperature above Ms+100°C, where Ms denotes the martensite start temperature; c. Inserting the heated sheet metal blank into a forming tool, wherein the transfer time t Trans required for removing the blank from the heating device and inserting it is at most 20s, preferably at most 15s; d.Hot-pressing the sheet metal blank into the formed sheet metal part, wherein the blank is cooled to the target temperature T target during the hot-pressing over a period t WZ of more than 1 s at a cooling rate r WZ of at least partially more than 30 K / s and is optionally held there; e. removing the sheet metal part cooled to the target temperature T target from the tool; f. cooling the sheet metal part to room temperature.

[0064] In the method according to the invention, a blank is prepared which consists of a steel suitably composed in accordance with the above explanations (working step a)), which is then heated in a manner known per se such that the AC3 temperature of the blank is at least partially exceeded and the temperature T EinIg of the blank when placed in a forming tool intended for hot press forming (working step c)) is at least partially above Ms+100°C. Partially exceeding a temperature (here AC3 or Ms+100°C) is understood in the context of this application to mean that at least 30%, in particular at least 60%, of the volume of the blank exceeds a corresponding temperature. When placed in the forming tool, therefore, at least 30% of the blank has an austenitic structure, i.e.The transformation from a ferritic to an austenitic structure does not necessarily have to be complete when the blank is placed in the forming tool. Rather, up to 70% of the volume of the blank when placed in the forming tool can consist of other microstructural components, such as tempered bainite, tempered martensite and / or non- or partially recrystallized ferrite. For this purpose, certain areas of the blank can be kept at a lower temperature than others during heating. To do this, the heat can be specifically directed only at certain sections of the blank, or the parts that are to be heated less can be shielded from the heat. In the part of the blank material whose temperature remains lower, no or significantly less martensite is formed during forming in the tool, so that the structure there is significantly softer than in the other parts that have a martensitic structure.In this way, a softer area can be specifically set in the respective formed sheet metal part, for example by providing optimal toughness for the respective intended use, while the other areas of the sheet metal part have maximized strength.

[0065] Maximum strength properties of the obtained sheet metal part can be achieved by ensuring that the temperature reached at least partially in the sheet metal blank is between Ac3 and 1000°C, preferably between 850°C and 950°C.

[0066] The minimum temperature Ac3 to be exceeded is determined according to the formula given by HOUGARDY, HP in "Werkstoffkunde Stahl Volume 1: Grundlagen", Verlag Stahleisen GmbH, Düsseldorf, 1984, p. 229. Ac 3 = 902 − 225 * % C + 19 * % Si − 11 * % Mn − 5 * % Cr + 13 * % Mo − 20 * % Ni + 55 * % V ° C with %C = respective C content, %Si = respective Si content, %Mn = respective Mn content, %Cr = respective Cr content, %Mo = respective Mo content, %Ni = respective Ni content and %V = respective V content of the steel from which the blank is made.

[0067] An optimally uniform distribution of properties can be achieved by heating the blank completely in step b).

[0068] In a preferred embodiment, the average heating rate r furnace of the sheet metal blank during heating in step b) is at least 3 K / s, preferably at least 5 K / s, in particular at least 10 K / s, preferably at least 15 K / s. The average heating rate r furnace is understood to be the average heating rate from 30°C to 700°C.

[0069] In a preferred embodiment, the heating takes place in a furnace with a furnace temperature T furnace of at least 850°C, preferably at least 880°C, particularly preferably at least 900°C, in particular at least 920°C, and at most 1000°C, preferably at most 950°C, particularly preferably at most 930°C.

[0070] The dew point in the oven is preferably at least -20°C, preferably at least -15°C, in particular at least -5°C, particularly preferably at least 0°C, in particular at least 5°C and at most +25°C, preferably at most +20°C, in particular at most +15°C.

[0071] In a specific embodiment, the heating in step b) takes place stepwise in areas with different temperatures. In particular, the heating takes place in a roller hearth furnace with different heating zones. Heating takes place in a first heating zone at a temperature (so-called furnace inlet temperature) of at least 650°C, preferably at least 680°C, in particular at least 720°C. The maximum temperature in the first heating zone is preferably 900°C, in particular a maximum of 850°C. Furthermore, the maximum temperature of all heating zones in the furnace is preferably a maximum of 1200°C, in particular a maximum of 1000°C, preferably a maximum of 950°C, and particularly preferably a maximum of 930°C.

[0072] The total time in the furnace t furnace , which consists of a heating time and a holding time, is preferably at least 2 minutes, in particular at least 3 minutes, preferably at least 4 minutes for both variants (constant furnace temperature, step-by-step heating). Furthermore, the total time in the furnace for both variants is preferably a maximum of 20 minutes, in particular a maximum of 15 minutes, preferably a maximum of 12 minutes, in particular a maximum of 8 minutes. Longer total times in the furnace have the advantage of ensuring uniform austenitization of the sheet metal blank. On the other hand, holding for too long above Ac3 leads to grain coarsening, which has a negative effect on the mechanical properties.

[0073] The blank heated in this way is removed from the respective heating device, which can be, for example, a conventional heating furnace, an equally known induction heating device or a conventional device for keeping steel components hot, and transported into the forming tool so quickly that its temperature upon arrival in the tool is at least partially above Ms+100°C, preferably above 600°C, in particular above 650°C, particularly preferably above 700°C. Here, Ms denotes the martensite start temperature. In a particularly preferred variant, the temperature is at least partially above the AC1 temperature. In all of these variants, the temperature is in particular a maximum of 900°C. These temperature ranges ensure good formability of the material overall.

[0074] In step c), the transfer of the austenitized blank from the heating device used to the forming tool is completed within preferably a maximum of 20 seconds, in particular within a maximum of 15 seconds. Such rapid transport is necessary to avoid excessive cooling prior to forming.

[0075] When the blank is inserted, the tool typically has a temperature between room temperature (RT) and 200°C, preferably between 20°C and 180°C, in particular between 50°C and 150°C. Optionally, in a special embodiment, the tool can be tempered at least partially to a temperature T WZ of at least 200°C, in particular at least 300°C, in order to only partially harden the component. Furthermore, the tool temperature T WZ is preferably a maximum of 600°C, in particular a maximum of 550°C. It only needs to be ensured that the tool temperature T WZ is below the desired target temperature T Ziel. The residence time in the tool t WZ is preferably at least 2s, in particular at least 3s, particularly preferably at least 5s. The maximum residence time in the tool is preferably 25s, in particular a maximum of 20s.

[0076] The target temperature T target of the sheet metal part is at least partially below 400°C, preferably below 300°C, in particular below 250°C, preferably below 200°C, particularly preferably below 180°C, in particular below 150°C. Alternatively, the target temperature T target of the sheet metal part is particularly preferably below Ms-50°C, where Ms denotes the martensite start temperature. Furthermore, the target temperature of the sheet metal part is preferably at least 20°C, particularly preferably at least 50°C.

[0077] The martensite start temperature of a steel within the scope of the invention is according to the formula Ms ° C = 490 , 85 − 302 , 6 % C − 30 , 6 % Mn − 16 , 6 % Ni − 8 , 9 % Cr + 2 , 4 % Mo − − 11 , 3 % Cu + 8 , 58 % Co + 7 , 4 % W − 14 , 5 % Si ° C / Gew . − % to be calculated, where C% is the C content, %Mn is the Mn content, %Mo is the Mo content, %Cr is the Cr content, %Ni is the Ni content, %Cu is the Cu content, %Co is the Co content, %W is the W content and %Si is the Si content of the respective steel in wt.%.

[0078] The AC1 temperature and the AC3 temperature of a steel within the scope of the specifications according to the invention are according to the formulas AC 1 ° C = 739 − 22 * % C − 7 * % Mn + 2 * % Si + 14 * % Cr + 13 * % Mo − 13 * % Ni + 20 * % V ° C / Gew . − % and AC 3 ° C = 902 − 225 * % C + 19 * % Si − 11 * % Mn − 5 * % Cr + 13 * % Mo − 20 * % Ni + 55 * % V ° C / Gew . − % to be calculated, where %C denotes the C content, %Si denotes the Si content, %Mn denotes the Mn content, %Cr denotes the Cr content, %Mo denotes the Mo content, %Ni denotes the Ni content and +%V denotes the vanadium content of the respective steel (Brandis H 1975 TEW-Techn. Ber. 1 8-10)

[0079] In the tool, the blank is not only formed into the sheet metal part, but is also simultaneously quenched to the target temperature. The cooling rate in the tool to the target temperature is in particular at least 20 K / s, preferably at least 30 K / s, in particular at least 50 K / s, and in special designs at least 100 K / s.

[0080] In step f), after the sheet metal part has been removed in step e), the sheet metal part is cooled to room temperature within a cooling time t AB of 0.5-600 s. This is usually done by air cooling.

[0081] During the heating of the flat steel product prior to hot forming, further iron diffusion into the corrosion protection coating occurs. This results in a corrosion protection coating alloyed with iron and having an Fe content of at least 35 wt.% within a short heating period. Furthermore, a special structure of the corrosion protection coating of the formed sheet metal part is preferably obtained, which is described below: In a specific embodiment, the formed sheet metal part preferably comprises an aluminum-based corrosion protection coating. The corrosion protection coating of the formed sheet metal part preferably comprises an alloy layer and an Al base layer.

[0082] Preferably, the corrosion protection coating of the steel component comprises an alloy layer and an Al base layer.

[0083] The alloy layer lies directly on the steel substrate. The alloy layer of the sheet metal part preferably consists of 35-90 wt.% Fe, 0.1-12 wt.% Si, optionally up to 0.5 wt.% Mg, and optional additional components, the total contents of which are limited to a maximum of 2.0 wt.%, with the remainder being aluminum. Due to the further diffusion of iron into the alloy layer, the Si and Mg contents are correspondingly lower than their respective contents in the melt of the molten bath.

[0084] The alloy layer preferably has a ferritic structure.

[0085] The Al base layer of the sheet metal part lies on top of the alloy layer of the steel component and directly borders it. The Al base layer of the steel component preferably consists of 35-55 wt.% Fe, 0.4-10 wt.% Si, 0.1-5.0 wt.% alkaline earth metals, and optional additional components, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder aluminum, with the alkaline earth metal content consisting of 0.1-1.0 wt.% Mg, or the alkaline earth metal content consisting of 0.1-0.7 wt.% Mg and at least 0.0015 wt.% Ca.

[0086] In a preferred embodiment, the alkaline earth metal content consists of 0.1-0.7 wt% Mg, preferably 0.1-0.5 wt% Mg.

[0087] In a preferred embodiment, the alkaline earth metal content consists of 0.1-0.5 wt% Mg and at least 0.0015 wt% Ca, in particular at least 0.01 wt% Ca.

[0088] The Al base layer can have a homogeneous silicon distribution, with local silicon contents varying by no more than 10%. Preferred variants of the Al base layer, however, have silicon-poor phases and silicon-rich phases. Silicon-poor phases are regions whose average Si content is at least 20% less than the average Si content of the Al base layer. Silicon-rich phases are regions whose average Si content is at least 20% higher than the average Si content of the Al base layer.

[0089] In a preferred variant, the silicon-rich phases are arranged within the silicon-poor phase. In particular, the silicon-rich phases form at least a 40% continuous layer bordered by silicon-poor regions. In an alternative embodiment, the silicon-rich phases are arranged in islands within the silicon-poor phase.

[0090] For the purposes of this application, "island-shaped" means an arrangement in which discrete, unconnected areas are enclosed by another material - i.e., "islands" of a particular material are located within another material.

[0091] In a preferred variant, the steel component comprises an oxide layer arranged on the corrosion protection coating. The oxide layer is located, in particular, on the Al base layer and preferably forms the outer edge of the corrosion protection coating.

[0092] The oxide layer of the steel component consists, in particular, of more than 80 wt.% oxides, with the majority of the oxides (i.e., more than 50 wt.% of the oxides) being aluminum oxide. Optionally, in addition to aluminum oxide, hydroxides and / or magnesium oxide are present in the oxide layer, either alone or as a mixture. Preferably, the remainder of the oxide layer not occupied by the oxides and optionally present hydroxides consists of silicon, aluminum, iron, and / or magnesium in metallic form.

[0093] The oxide layer preferably has a thickness of at least 50 nm, in particular of at least 100 nm. Furthermore, the thickness is preferably a maximum of 4 µm, in particular a maximum of 2 µm.

[0094] Advantages arise when inspecting a hot-formed sheet metal part with an aluminum-based coating, particularly a sheet metal part developed as described above and / or produced according to the method described above. The inspection method comprises at least the following steps: 1. Determining one or more color values of the coating 2. Comparing one or more color values with one or more reference values 3. Sorting out the sheet metal part based on the result of the comparison

[0095] Using a simple process—determining one or more color values and comparing them with corresponding reference values—it is possible to automatically determine whether the component has undergone the desired production process. This allows each component to be easily inspected during series production. This increases the certainty that no defective components are reused. Previous testing methods required chemical analysis and / or the creation of a metallographic section. Such methods are therefore not non-destructive. Consequently, only sample samples of the components could be tested. This method therefore enables the testing of the entire component production.

[0096] In a preferred development of the method, the one or more reference values are one or more color values of a cooled reference sheet metal part that has undergone annealing in a furnace with a furnace temperature of 900°C for a reference annealing time of 4 minutes. Tests have shown that annealing at 900°C for 4 minutes often produces a component with the desired properties. Therefore, it is advisable to use this as a reference process and to use the color values obtained from such a reference component as reference values.

[0097] In a special embodiment of the method, the determination of at least one color value or multiple color values is carried out using a photometer, in particular a spectrophotometer. The use of photometers has proven particularly useful because they operate very reliably and can be easily integrated into an industrial series process. For example, it is easily possible to measure many components consecutively with a photometer at short intervals.

[0098] Particularly preferably, determining the one color value or the multiple color values of the coating comprises determining at least three color values in a color space. In particular, determining the one color value or the multiple color values of the coating consists of determining at least three color values in a color space. The three color values in a color space can, for example, be the values L*a*b in the standardized CIE Lab color space. Alternatively, a selected RGB color space can also be used. In this case, the three color values are the corresponding RGB values. The three color values in a color space are, of course, three independent color values, so that by specifying the three color values, the color can be clearly identified in this color space.By recording at least three color values in a color space, the color change can be better characterized because more information is available and the color can be clearly identified in the color space.

[0099] In a particularly preferred variant, comparing the one or more color values with one or more reference values comprises calculating a color dynamic range. In particular, the color values and the reference values are RGB values, and the color dynamic range D is calculated as the sum of the absolute values of the differences between the RGB values and the RGB reference values: D = R − R ref + G − G ref + B − B ref

[0100] Tests have shown that such a color dynamic D is particularly sensitive to the color changes occurring here. Therefore, such a color dynamic is well suited to sorting out the sheet metal part based on the color dynamic calculated during the comparison, for example, if the color dynamic is greater than 270.

[0101] In a preferred embodiment of the method for producing a sheet metal part described above, the described method steps a) - f) are followed, in particular directly, by step g): g. Checking the cooled sheet metal part using the method for checking a sheet metal part described above

[0102] The manufactured sheet metal parts are therefore checked as part of a quality control process to ensure that they have undergone the desired manufacturing process.

[0103] To demonstrate the effectiveness of the invention, several tests were conducted. Slabs with the compositions specified in Table 1, a thickness of 240 mm and a width of 1200 mm, were produced and heated in a pusher-type furnace to a temperature T1 of 1200°C. The slabs were then held at T1 for between 30 and 450 minutes until the temperature T1 in the core of the slabs was reached and the slabs were thus thoroughly heated. The slabs were discharged from the pusher-type furnace at their respective through-heating temperature T1 and subjected to hot rolling. The tests were carried out as continuous hot strip rolling. For this purpose, the slabs were first pre-rolled to an intermediate product with a thickness of 40 mm. At the end of the pre-rolling phase, the intermediate products, which in hot strip rolling can also be referred to as pre-strips, each had an intermediate product temperature T2 of 1100°C.The pre-rolled strips were fed to the finish rolling immediately after rough rolling, so that the intermediate product temperature T2 corresponds to the initial rolling temperature for the finish rolling phase. The pre-rolled strips were rolled into hot strips with a final thickness of 4 mm and a final rolling temperature T3 of 890°C, cooled to the respective coiling temperature, and wound into coils at a coiling temperature T4 of 580°C and then cooled in still air. The hot strips were descaled in the conventional manner by pickling before being subjected to cold rolling until the thickness specified in Table 3 was achieved. The cold-rolled flat steel products were heated in a continuous annealing furnace to an annealing temperature T5 of 870°C and held at annealing temperature for 100s each before being cooled at a cooling rate of 1 K / s to the immersion temperature T6 of 690°C.The cold-rolled strips were passed through a molten coating bath at temperature T7 of 676°C at their respective immersion temperature T6. The strip speed was 76 m / min in all cases. The composition of the coating bath is given in Table 2. After coating, the coated strips were blown off to adjust the coating weights. An air stream was used for this purpose. The temperature of the air stream was 70°C in all cases. The thickness of the coating is given in Table 3. The strips were first cooled to 660°C at an average cooling rate of 10-15 K / s. Between 660°C and 570°C, i.e. between the start of solidification and the end of solidification of the coating, the cooling rate was 21 K / s. During the further cooling process between 570°C and room temperature, the strips were cooled at a cooling rate of 5-12 K / s in each case.

[0104] Blanks were cut from the steel strips produced in this way and used for further tests. In these tests, sheet metal part samples in the form of 200 x 300 mm 2< plates were hot-pressed from the respective blanks. For this purpose, the blanks were heated in a heating device, for example a conventional heating furnace, from room temperature with an average heating rate r furnace (between 30°C and 700°C) into a furnace with a furnace temperature T furnace of 900°C. The annealing time in the furnace, which includes heating and holding, is designated t furnace. The dew point of the furnace atmosphere was -5°C in all cases. The blanks were then removed from the heating device and placed in a forming tool which has the temperature T wz. At the time of removal from the furnace, the blanks had reached the furnace temperature.The transfer time t Trans , comprising the time required for removal from the heating device, transport to the tool, and insertion into the tool, was 8 s. The temperature T EinIg of the blanks upon insertion into the forming tool was above the respective martensite start temperature of +100°C in all cases. The blanks were formed into the respective sheet metal parts in the forming tool, with the sheet metal parts being cooled in the tool at a cooling rate r WZ to a target temperature T Target . The residence time in the tool is designated t WZ . Finally, the samples were cooled in air to room temperature. Table 4 summarizes these parameters again, where "RT" stands for room temperature.

[0105] In the described process, the coating was varied. Sheet metal types 1 and 2 were used, differing only in the coating type α or β (see Table 3). Second, the annealing time in the furnace was varied between 4 minutes and 11 minutes. For the sheet metal parts produced in this way, the color in the CIE Lab color space was determined using a spectrophotometer and a light source with CIE standard illuminant A and a 10° field of view. From this data, the color values in the RGB space were calculated. From this data, the color dynamics D were determined. The results are shown in Table 5. Coating variant α from Table 2 and its subsequent use represent reference examples. Table 1 (steel grades) Steel C Si Mn Al Cr Nb Ti B P S N Sn Ace Cu Mon Ca Ni CEV A 0,235 0,3 1,3 0,05 0,28 0,003 0,040 0,0035 0,02 0,003 0,007 0,03 0,01 0,03 0,03 0,005 0,025 0,52 The remainder is iron and unavoidable impurities. All values are in wt.%. Table 2 (coating variants) Coating variant Melt analysis Si Fe Mg Other Al α* 9,5 3 <0,01 <1% rest β 10 3 0,3 <1% rest * non-inventive reference examples Table 3 (structure) Construction Stahl's places Coating Sheet thickness [mm] Coating thickness [µm] 1* A α 1,5 27 2 A β 1,5 27 * non-inventive reference examples Table 4 (hot forming parameters) Average heating rate r furnace [30 - 700 °C] [K / s] T oven [°C] Transfer time [s] Dew point oven [°C] T EinIg [°C] T WZ [°C] t WZ [s] Cooling rate r WZ [K / s] T Target [°C] 8 900 8 -5 800 RT 15 50 50 Table 5 (Results) Construction Annealing time T furnace [minutes] L a b R G B D 1* 4 39,94 1,35 3,09 99 93 89 0 (reference) 1* 5 37,22 1,58 0,39 91 87 87 16 1* 6 34,02 0,33 -5,74 76 80 89 40 1* 7 34,46 -1,16 -7,99 72 82 94 51 1* 8 34,14 -2,93 -10,77 65 82 97 61 1* 9 35,70 -5,00 -12,61 61 87 104 77 1* 10 37,23 -6,33 -13,00 61 92 108 86 1* 11 37,27 -4,63 -14,67 63 91 111 92 2 4 38,41 -0,78 -6,38 84 91 101 0 (reference) 2 5 36,70 -3,78 -11,66 68 89 105 22 2 6 38,65 -1,61 -4,82 84 92 99 47 2 7 42,00 -5,76 -10,23 78 103 116 81 2 8 53,68 -5,76 -9,13 108 132 144 168 2 9 45,29 -4,77 -5,68 93 110 116 233 2 10 45,31 -0,46 6,10 111 107 97 273 2 11 50,79 -2,87 6,99 121 122 109 310 * non-inventive reference examples

Claims

1. A flat steel product for producing a sheet metal part by hot forming, comprising a. a steel substrate and b. an aluminum-based coating arranged on at least one side of the steel substrate, wherein the steel substrate consists of a steel which, in addition to iron and unavoidable impurities in % by weight, consists of C:0.04-0.45% by weight,Si:0.02-1.2% by weight,Mn:0.5-2.6 wt.%,Al:0.02-1.0 wt.%,P:≤ 0.05 wt.%,S:≤ 0.02 wt.%,N:≤ 0.02 wt.%,Sn:≤ 0.03 wt.%,As:≤ 0.01 wt.%,Ca:≤ 0.005 wt.%, and optionally one or more of the elements "Cr, B, Mo, Ni, Cu, Nb, Ti, V" in the following contents Cr:0.08-1.0% by weight,B:0.001-0.005% by weight,Mo:≤0.5 wt.%,Ni:≤0.5 wt.%,Cu:≤0.2 wt.%,Nb:0.02-0.08 wt.%,Ti:0.01-0.08 wt.%,V:≤0.2 wt.%, and wherein the coating has an Al base layer which consists of 1.0-15 wt.% Si, optionally 2-4 wt.% Fe, 0.1-1.0 wt.% alkaline earth metals and the remainder aluminium, characterized in that the content of alkaline earth metals consists of 0.1-1.0% by weight of Mg or the content of alkaline earth metals consists of 0.1-0.7% by weight of Mg and at least 0.0015% by weight of Ca, wherein the alkaline earth metals in the Al base layer have such a distribution that the steel flat product in the cooled state after annealing in a furnace with a furnace temperature of 900°C for an annealing time of 9 minutes undergoes a color change compared to the cooled state after a reference annealing time of 4 minutes, wherein the color change has a color dynamic of greater than 100, wherein the color dynamic is determined according to the description.

2. Method for producing a steel flat product according to claim 1 for hot forming with a coating, comprising the following steps: a) providing a slab or a thin slab consisting of a steel which, in addition to iron and unavoidable impurities in % by weight, consists of C:0.04-0.45% by weight,Si:0.02-1.2% by weight,Mn:0.5-2.6 wt.%,Al:0.02-1.0 wt.%,P:≤ 0.05 wt.%,S:≤ 0.02 wt.%,N:≤ 0.02 wt.%,Sn:≤ 0.03 wt.%,As:≤ 0.01 wt.%,Ca:≤ 0.005 wt.%, and optionally one or more of the elements "Cr, B, Mo, Ni, Cu, Nb, Ti, V" in the following contents Cr:0.08-1.0% by weight,B:0.001-0.005% by weight,Mo:≤0.5 wt.%,Ni:≤0.5 wt.%,Cu:≤0.2 wt.%,Nb:0.02-0.08 wt.%,Ti:0.01-0.08 wt.%,V:≤0.2 wt.%; b) through-heating of the slab or thin slab at a temperature T1 of 1000-1400°C; c) optional pre-rolling of the through-heated slab or thin slab to an intermediate product with an intermediate product temperature T2 of 1000-1200°C; d) hot rolling to a hot-rolled flat steel product, whereby the final rolling temperature T3 is 750-1000°C; e) optional coiling of the hot-rolled flat steel product, the coiling temperature T4 being at most 700°C; f) Optional descaling of the hot-rolled steel flat product; g) optional cold rolling of the steel flat product, whereby the degree of cold rolling is at least 30%; h) annealing of the flat steel product at an annealing temperature (T5) of 650-900°C; i) cooling the flat steel product to an immersion temperature (T6), which is 600-800°C, preferably 680-720°C; j) coating the steel flat product cooled to the immersion temperature with a coating by i. Immersion in a molten bath with a melting temperature (T7) of 660-800°C, preferably 670-710°C, the molten bath containing 1.0-15 wt.% Si, optionally 2-4 wt.% Fe, 0.1-1.0 wt.% alkaline earth metals and as well as 1.0-1.0 wt.% alkaline earth metals.% alkaline earth metals and the remainder aluminum, wherein the content of alkaline earth metals consists of 0.1-1.0 % by weight Mg or the content of alkaline earth metals consists of 0.1-0.7 % by weight Mg and at least 0.0015 % by weight Ca ; ii. Blowing off the steel flat product after leaving the molten bath by means of a gas stream; k) cooling the coated steel flat product to room temperature; l) optionally skin-passing the coated steel flat product wherein in step k) an average cooling rate between 660°C and 570°C is at least 15 K / s and at most 50 K / s.

3. A method of manufacturing a sheet metal blank, comprising the following steps: a. Providing a sheet metal blank from a flat steel product according to claim 1; b. heating the sheet metal blank in such a way that the AC3 temperature of the blank is at least partially exceeded and the temperature TEinIg of the blank on insertion into a forming tool provided for hot press forming (step c)) has at least partially a temperature above Ms+100°C, where Ms is the martensite start temperature; c. inserting the heated sheet blank into a forming tool, the transfer time tTransrequired for removal from the heating device and insertion of the blank being at most 20 s, preferably at most 15 s; d. hot-press forming of the sheet metal blank to form the sheet metal part, the blank being cooled to the target temperature Ttargetduring the hot-press forming over a period tWZof more than 1s at a cooling rate rWZwhich is at least partially greater than 30 K / s and optionally being held there; e. Removing the sheet metal part cooled to the target temperature Ttargetfrom the mold; f. Cooling the sheet metal part to room temperature.

Images