Steel flat product with color change

Abstract

A method for inspecting a hot-formed sheet metal part with an aluminum-based coating, comprising at least the following steps: a. Determining one or more color values ​​of the coating; b. Comparing the one or more color values ​​with one or more reference values; c. Sorting out the sheet metal part based on the result of the comparison.

Description

The invention relates to a steel flat product for manufacturing a sheet metal part by hot forming, a method for manufacturing such a steel flat product, a method for checking a sheet metal part and a method for manufacturing a sheet metal part. When the terms "flat steel product" or "sheet metal product" are used below, these refer to rolled products such as steel strips or sheets, from which "sheet blanks" (also called blanks) are cut for the production of, for example, body parts. "Sheet metal forming parts" or "sheet metal components" of the type according to the invention are manufactured from such sheet metal blanks, whereby the terms "sheet metal forming part" and "sheet metal component" are used synonymously. All information regarding the concentrations of the steel compositions specified in this application is based on weight, unless expressly stated otherwise. All unspecified percentages relating to a steel alloy are therefore to be understood as values ​​in wt.%. Information in this text regarding the concentrations of the constituents of an atmosphere refers to volume (expressed in vol.%). Steel flat products with similar aluminium-based coatings and methods for their production are known from WO 2022 / 048990 A1 and EP 2 993 248 B1. WO 2019 / 016041 A1 discloses a coated steel flat product suitable for press hardening, which has particularly good aging resistance, and a method for its production. The steel of the flat steel product consists, in addition to iron and unavoidable impurities (in wt.%), of 0.10–0.4% C, 0.05–0.5% Si, 0.5–3.0% Mn, 0.01–0.2% Al, 0.005–1.0% Cr, 0.001–0.2% V, ≤ 0.1% P, ≤ 0.05% S, ≤ 0.02% N, and optionally one or more of the elements B, Ti, Nb, Ni, Cu, Mo, W in the following amounts: B: 0.0005–0.01%, Ti: 0.001–0.1%, Nb: 0.001–0.1%, Ni: 0.01–0.4%, Cu: 0.01–0.8%, Mo: 0.002– 1.0%, W: 0.001 - 1.0%. The steel flat product has a yield strength with a continuous profile or a yield strength with a difference between the upper and lower yield strength values ​​of no more than 45 MPa and a uniform elongation Ag of at least 11.5%. These flat steel products have an aluminum-based coating and are further processed into sheet metal parts using hot forming. In this process, blanks cut 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 results in a coating that provides very effective protection against corrosion. The hot blank is then formed into a sheet metal part in a forming tool and rapidly cooled, which causes a hardened microstructure (e.g., martensite) to form in the steel substrate. The result is a sheet metal part with high strength and a coating that offers excellent corrosion protection. The problem, however, is that the desired corrosion protection is only achieved if the processing conditions, especially the annealing time, are adhered to very precisely. An excessively long annealing time leads to increased iron diffusion into the coating, which significantly disrupts the layer structure. In industrial production, however, deviations can always occur due to disruptions in the production process. Therefore, it is necessary to identify and reject sheet metal parts that were inadvertently manufactured under incorrect conditions. In principle, this is possible, for example, by creating a metallographic section or by chemical analysis of the surface. However, such investigations are very complex and also not non-destructive. Therefore, the object of the invention is to provide a method by which it can be determined, without great effort, whether the desired manufacturing conditions have been met on the formed sheet metal part. In such a process, a steel flat product is used for the production of a sheet metal part by hot forming, comprising: a) a steel substrate consisting of a steel containing 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 other components, the total content of which is limited to a maximum of 2.0 wt.%, with aluminum as the remainder. The alkali or alkaline earth metals in the Al base layer exhibit such a distribution that the steel flat product, when cooled after annealing in a furnace at a temperature of 900°C for a duration of 9 minutes, undergoes a color change compared to the cooled state after a reference annealing duration of 4 minutes, with the color change having a color dynamic of greater than 100. Typically, coatings of this type are produced by hot-dip coating. Surprisingly, it has been found that by adding 0.1–5.0 wt% alkali or alkaline earth metals to the melt and applying specific cooling conditions, which will be explained in detail later, a coating is produced with a specific distribution of the alkali or alkaline earth metals in the aluminum base layer. This distribution leads to a color change in the steel flat product when heated for more than 4 minutes. Further diffusion processes alter the near-surface concentration of alkali and 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 alkali or alkaline earth metals.Therefore, by measuring the color values ​​of 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. The cooled sheet metal part (or reference sheet metal part) is defined as one that has reached room temperature. The color dynamics of a color change are determined as follows: Using a spectrophotometer and a light source with CIE standard illuminant type A (which can be achieved with sufficient accuracy by a gas-filled tungsten incandescent lamp with a distribution temperature 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* Colour space". The values ​​are then converted from the CIE Lab color space to the sRGB8 color space. The sRGB8 color space used is the 8-bit variant, in which the color values ​​are integers from 0 to 255. This conversion is familiar to those skilled in the art and can be performed online, for example, at https: / / convertingcolors.com / cielab-color-1.00_1.00_1.00.html. The result is unique RGB values ​​for the sheet metal part. Similarly, the reference color values ​​(RrefGrefBref) of a correctly processed sheet metal part (reference sheet metal part) are determined. A color dynamic D of a color change from reference sheet metal part to sheet metal part is now defined as the sum of the magnitudes of the differences of the RGB values, i.e.: It has been shown that the color dynamics defined in this way are particularly well suited to quantifying color change. A steel flat product for such a process according to the invention is produced using the process described below. The process for producing a steel flat product for hot forming with a coating comprises the following steps: a) Providing a slab or a thin slab consisting of a steel containing 0.1-3 wt.% Mn and optionally up to 0.01 wt.% Mn.-% B; 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 steel flat product, wherein the final rolling temperature (T3) is 750-1000°C; e) Optional coiling of the hot-rolled steel flat product, wherein the coiling temperature (T4) is at most 700°C; f) Optional descaling of the hot-rolled steel flat product; g) Optional cold rolling of the steel flat product, wherein the degree of cold rolling is at least 30%; h) Annealing the steel flat product at an annealing temperature (T5) of 650-900°C; i) Cooling the steel flat product to an immersion temperature (T6) of 650-800°C, preferably 670-720°C; j) Coating the steel flat product cooled to the immersion temperature with a coating by i.Immersion in a melt bath with a melt temperature (T7) of 660-800°C, preferably 670-710°C; ii. Blowing off the steel flat product after exiting the melt bath by means of a gas stream; k) Cooling the coated steel flat product to room temperature, wherein a cooling rate between 660°C and 570°C is at least 15 K / s; l) Optionally, dressing the coated steel flat product. In step a), a semi-finished product composed according to 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 continuous slab casting. In step b), the semi-finished product is heated through at 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 heating temperature should be at least 1000°C to ensure good formability for the subsequent rolling process. The heating temperature should not exceed 1400°C to avoid the presence of molten phases in the semi-finished product. In the optional step c), the semi-finished product is pre-rolled to an intermediate product. Thin slabs are not usually pre-rolled. Thick slabs intended for hot-rolled strip can be pre-rolled if necessary. In this case, the temperature of the intermediate product (T2) at the end of pre-rolling should be at least 1000°C to ensure it contains sufficient heat for the subsequent finish rolling step. However, high rolling temperatures can also promote grain growth during the rolling process, which negatively impacts the mechanical properties of the finished steel product. To minimize grain growth during rolling, the temperature of the intermediate product at the end of pre-rolling should not exceed 1200°C. In step d), the slab or thin slab, or, if step c) was performed, the intermediate product, is rolled into a hot-rolled steel flat product. If step c) was performed, the intermediate product is typically finish-rolled immediately after rough rolling. Finish rolling typically begins no later than 90 seconds after the end of rough rolling. The slab, thin slab, or, if step c) was performed, the intermediate product, is rolled to a final rolling temperature (T3). The final rolling temperature, that is, the temperature of the finished hot-rolled steel flat product at the end of the hot rolling process, is 750–1000°C. At final rolling temperatures below 750°C, the amount of free vanadium decreases because larger quantities of vanadium carbides are precipitated. The vanadium carbides precipitated during finish rolling are very large.They typically have a mean grain size of 30 nm or more and are not 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 no more than 1000°C are relevant for process engineering purposes in maintaining coil temperatures (T4) below 700°C. Hot rolling of the steel flat product can be carried out as continuous hot strip rolling or as reversing rolling. Step e) provides for optional coiling of the hot-rolled steel flat product in the case of continuous hot strip rolling. For this purpose, the hot strip is cooled to a coiling temperature (T4) within less than 50 seconds after hot rolling. Water, air, or a combination of both can be used as the cooling medium. The coiling temperature (T4) should not exceed 700°C to prevent the formation of large vanadium carbides. In principle, there is no lower limit to the coiling temperature. However, coiling temperatures of at least 500°C have proven advantageous for cold rolling. Subsequently, the coiled hot strip is cooled to room temperature in the conventional manner using air. In step f), the hot-rolled steel flat product is optionally descaled in a conventional manner by pickling or by another suitable treatment. The hot-rolled steel flat product, cleaned of scale, can optionally undergo cold rolling before the annealing treatment in step g) to meet, for example, higher requirements for the thickness tolerances of the steel flat product. The cold rolling degree (CW) should be at least 30% to introduce sufficient deformation energy into the steel flat product for rapid recrystallization. The cold rolling degree (CW) is defined as the quotient of the thickness reduction during cold rolling ΔdCW divided by the hot strip thickness d: With ΔdKW = thickness reduction during cold rolling in mm and d = hot strip thickness in mm, the thickness reduction ΔdKW results from the difference between the thickness of the steel flat product before cold rolling and the thickness of the steel flat product after cold rolling. The steel flat product before cold rolling is usually hot strip with a hot strip thickness d. The steel flat product after cold rolling is also commonly referred to as cold strip. The degree of cold rolling can, in principle, reach very high values ​​of over 90%. However, degrees of cold rolling of no more than 80% have proven advantageous for preventing strip cracking. In step h), the steel flat product undergoes an annealing treatment at annealing temperatures (T5) of 650–900°C. For this purpose, the steel flat product is first heated to the annealing temperature within 10–120 s and then held at the annealing temperature for 30–600 s. The annealing temperature is at least 650°C, preferably at least 720°C. Annealing temperatures above 900°C are not desirable for economic reasons. In step i), the steel flat 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 melt pool. The immersion temperature is 600–800°C, preferably at least 650°C, particularly 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 in aluminum increases significantly again, so that even at the beginning of the coating process, more iron can diffuse into the still-liquid boundary layer. The cooling time of the annealed steel flat product from the annealing temperature T5 to the immersion temperature T6 is preferably 10–180 s. In particular, the immersion temperature T6 deviates from the temperature of the melt bath T7 by no more than 30 K, more specifically no more than 20 K, and preferably no more than 10 K. In step j), the steel flat product undergoes a coating treatment. This coating treatment is preferably carried out by continuous hot-dip coating. The coating can be applied to one side, both sides, or all sides of the steel flat product. The coating treatment is preferably carried out as a hot-dip coating process, particularly as a continuous process. In this process, the steel flat product typically comes into contact with the molten metal bath on all sides, so that it is coated on all sides. The molten metal bath, which contains the alloy to be applied to the steel flat product in liquid form, typically has a temperature (T7) of 660–800°C, preferably 670–740°C, and particularly preferably 670–710°C. Aluminum-based alloys have proven to be particularly suitable for coating age-resistant steel flat products with a corrosion-resistant coating.In such a case, the melt contains 1.0–15 wt.% Si, optionally 2–4 wt.% Fe, optionally 0.1–5.0 wt.% alkali or alkaline earth metals, preferably up to 1.0 wt.% alkali or alkaline earth metals, and optional further components, the total content of which is limited to a maximum of 2.0 wt.%, with aluminum as the remainder. In a preferred embodiment, the Si content of the melt is 1.0–3.5 wt.% or 7–12 wt.%, particularly 8–10 wt.%. In a preferred embodiment, the optional content of alkali or alkaline earth metals in the melt comprises 0.1–1.0 wt.% Mg, particularly 0.1–0.7 wt.% Mg, preferably 0.1–0.5 wt.% Mg. Furthermore, the optional content of alkali or alkaline earth metals in the melt can particularly include at least 0.0015 wt.% Ca, particularly at least 0.01 wt.% Ca. After exiting the molten bath, the flat steel product is blown off using a gas stream to adjust the thickness of the coating. After coating, the coated steel flat 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 beginning and end of the coating's hardening. The coating begins to harden upon cooling to 660°C and is fully hardened upon further cooling to 570°C. Preferably, the average cooling rate is a maximum of 100 K / s, and particularly preferably a maximum of 50 K / s. It has been shown that this rapid cooling results in a specific distribution of the alkali or alkaline earth metals in the aluminum base layer of the coating, leading to the described advantageous color change during the subsequent forming process. Generally, alkali or alkaline earth metals tend to concentrate near the surface.While the alkali or alkaline earth metals are still evenly distributed in the melt, they diffuse towards the surface in the coating. This is possible as long as sufficient thermal energy is available for diffusion. If the cooling rate between the beginning and end of the solidification process is too slow, the majority of the alkali or alkaline earth metals will have already diffused to the surface. Therefore, during the subsequent annealing for the forming process, no significant diffusion of the alkali or alkaline earth metals to the surface occurs. Consequently, no significant color change of the surface takes place in such a case. Diffusion must therefore be stopped quickly enough after the coating treatment so that sufficient diffusion processes can still occur during the subsequent annealing for the forming process to cause the color change.This is achieved by a mean cooling rate between 660°C and 570°C, that is, between the beginning and end of the hardening process, which is at least 15 K / s. However, the diffusion must not be stopped too quickly. A certain proportion of the alkali or alkaline earth metals must already be concentrated at the surface. The purpose of adding the alkali or alkaline earth metals is that, during the subsequent annealing process before forming, oxides of the alkali or alkaline earth metals form on the surface of the coating instead of aluminum oxides. This has the advantage that less free hydrogen is produced than during the formation of Al₂O₃. (The oxygen bound during oxidation usually originates 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 there.The addition of alkali or alkaline earth metals leads to a reduction in free hydrogen and thus to a reduction in hydrogen embrittlement. However, these alkali or alkaline earth metals can only have this effect if they are located near the surface of the substrate during the annealing process prior to forming. Therefore, it is advantageous if a certain proportion of the alkali or alkaline earth metals has already diffused to the surface during solidification. This ensures that a sufficient proportion of alkali or alkaline earth metals is available for the described oxidation process immediately from the start of the annealing process prior to forming. Therefore, it is advantageous if the cooling rate is a maximum of 100 K / s, preferably a maximum of 50 K / s. This ensures that a sufficient proportion of the alkali or alkaline earth metals has diffused to the surface of the coating.This targeted choice of cooling process achieves a distribution of the alkali or alkaline earth metals in the coating (especially in the Al base layer of the coating) that, on the one hand, has sufficient near-surface alkali or alkaline earth metals to immediately reduce the free hydrogen from the beginning of the subsequent annealing, and on the other hand, still allows sufficient diffusion to lead to the color change according to the invention. The coated steel flat product can optionally be subjected to a dressing process with a dressing grade of up to 2% to improve the surface roughness of the steel flat product. The steel used in the flat steel product and in the process for manufacturing a flat steel product is a steel containing 0.1–3 wt.% manganese and optionally up to 0.01 wt.% brine. The same applies, of course, to the steel of the hot-formed sheet metal part. In particular, the microstructure of the steel can be transformed into a martensitic or partially martensitic microstructure by hot forming. The microstructure of the steel substrate of the sheet metal part is therefore preferably a martensitic or at least partially martensitic microstructure, as this exhibits particularly high hardness. The steel preferably consists, in addition to iron and unavoidable impurities (in wt.%), 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.% 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 amounts: Cr: 0.08 - 1.0 wt.% B: 0.001 - 0.005 wt.% 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.% 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 other elements are collectively referred to as "unavoidable impurities." Preferably, the total content of unavoidable impurities is a maximum of 0.2 wt.%, more 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 occur as unavoidable impurities in the steel substrate at levels below the respective lower limit. In this case, they are also counted among the unavoidable impurities, the total content of which is limited to a maximum of 0.2 wt.%, more preferably a maximum of 0.1 wt.%. The preferred individual upper limits for the respective impurities of these elements are as follows: Cr: ≤ 0.050 wt.%, B: ≤ 0.0005 wt.%, Nb: ≤ 0.005 wt.%-%,Ti: ≤ 0.005 wt.%. These preferred upper limits should be considered as alternatives or in combination. Preferred steel variants therefore fulfill one or more of these four conditions. In a preferred embodiment, the carbon content of the steel is a maximum of 0.37 wt.% and / or a minimum of 0.06 wt.%. In particularly preferred embodiments, the carbon 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.%. In a preferred embodiment, the Si content of the steel is a maximum of 1.00 wt.% and / or a minimum of 0.06 wt.%. In a preferred embodiment, the manganese content of the steel is a maximum of 2.4 wt.% and / or a minimum of 0.75 wt.%. In particularly preferred embodiments, the manganese content is in the range of 0.75–0.85 wt.% or in the range of 1.0–1.6 wt.%. In a preferred variant, the aluminum content of the steel is a maximum of 0.75 wt.%, in particular a maximum of 0.5 wt.%, and preferably a maximum of 0.25 wt.%. Alternatively or additionally, the aluminum content is preferably at least 0.02%. Furthermore, it has been shown that limiting the sum of silicon and aluminum contents can be beneficial. Therefore, in a preferred embodiment, the sum of Si and Al contents (usually referred to as Si+Al) is a maximum of 1.5 wt.%, preferably a maximum of 1.2 wt.%. Additionally or alternatively, the sum of Si and Al contents is at least 0.06 wt.%, preferably at least 0.08 wt.%. The elements P, S, and N are typical impurities that cannot be completely avoided in steel production. In preferred variants, the P content is a maximum of 0.03 wt.%. Independently of this, the S content is preferably a maximum of 0.012 wt.%. Additionally or supplementarily, the N content is preferably a maximum of 0.009 wt.%. Optionally, the steel also contains chromium in a content of 0.08–1.0 wt.%. Preferably, the chromium content is a maximum of 0.75 wt.%, and particularly a maximum of 0.5 wt.%. In the case of optional chromium alloying, 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.%. Preferably, the steel also optionally contains boron in a content of 0.001–0.005 wt.%. In particular, the boron content is a maximum of 0.004 wt.%. Optionally, the steel may contain molybdenum with a maximum content of 0.5 wt.%, in particular a maximum of 0.1 wt.%. Furthermore, the steel may optionally contain nickel with a content of a maximum of 0.5 wt.%, preferably a maximum of 0.15 wt.%. Optionally, the steel may also contain copper with a content of a maximum of 0.2 wt.%, preferably a maximum of 0.15 wt.%. Furthermore, the steel may optionally contain one or more of the microalloying elements Nb, Ti, and V. The optional Nb content is at least 0.02 wt.% and at most 0.08 wt.%, preferably at most 0.04 wt.%. The optional Ti content is at least 0.01 wt.% and at most 0.08 wt.%, preferably at most 0.04 wt.%. The optional V content is at most 0.2 wt.%, in particular at most 0.1 wt.%, preferably at most 0.05 wt.%. In the case of optional alloying with several of the elements Nb, Ti, and V, the sum of the Nb, Ti, and V contents is preferably limited. The sum is a maximum of 0.1 wt.%, particularly a maximum of 0.068 wt.%. Furthermore, the sum is preferably at least 0.015 wt.%. 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 method described below for checking a sheet metal part. The steel flat 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, 0.1-5.0 wt.% alkali or alkaline earth metals, and optional further components, the total content of which is limited to a maximum of 2.0 wt.%, and aluminum as the remainder. Such a coating serves to protect the steel substrate from oxidation and corrosion during hot forming and during the use of the produced steel component. Therefore, the coating is also referred to synonymously as a corrosion protection coating. The corrosion protection coating can be applied to one or both sides of the steel flat product. The two sides of the steel flat product are defined as the two opposing large surfaces. The narrow surfaces are referred to as edges. Such a corrosion protection coating is preferably produced by hot-dip coating the steel flat product. The steel flat product is passed through a liquid melt consisting of 0.1–15 wt.% Si, preferably more than 1.0 wt.% Si, optionally 2–4 wt.% Fe, 0.1 to 5.0 wt.% alkali or alkaline earth metals, preferably up to 1.0 wt.% alkali or alkaline earth metals, and optional further components, the total content of which is limited to a maximum of 2.0 wt.%, with aluminum as the remainder. 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.%. In a preferred embodiment, the alkali or alkaline earth metal content in the melt comprises 0.1–1.0 wt.% Mg, in particular 0.1–0.7 wt.% Mg, preferably 0.1–0.5 wt.% Mg. Furthermore, the alkali or alkaline earth metal content in the melt can, in particular, comprise at least 0.0015 wt.% Ca, in particular at least 0.01 wt.% Ca. In a preferred embodiment, the content of alkali or alkaline earth metals consists of 0.1-1.0 wt.% Mg, in particular 0.1-0.7 wt.% Mg, preferably 0.1-0.5 wt.% Mg. In a preferred embodiment, the alkali or 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 and at least 0.0015 wt.% Ca, in particular at least 0.01 wt.% Ca. During hot-dip coating, iron diffuses from the steel substrate into the liquid coating, so that upon solidification, the corrosion protection coating of the flat steel product consists primarily of an alloy layer and an aluminum base layer. The alloy layer lies on top of the steel substrate and is directly adjacent to it. The alloy layer is essentially composed of aluminum and iron. The remaining elements from the steel substrate or the melt composition do not accumulate significantly in the alloy layer. Preferably, the alloy layer consists of 35–60 wt.% iron, preferably α-iron, optional additional components whose total content is limited to a maximum of 5.0 wt.%, preferably 2.0%, and aluminum as the remainder, with the aluminum content preferably increasing towards the surface.The optional additional components include in particular the remaining components of the melt (i.e. silicon and, if applicable, alkali or alkaline earth metals, especially Mg or Ca) and the remaining components of the steel substrate in addition to iron. The aluminum base layer lies on top of the alloy layer and is directly adjacent to it. Preferably, the composition of the aluminum base layer corresponds to the composition of the melt of the molten pool. That is, it consists of 1.0–15 wt.% Si, optionally 2–4 wt.% Fe, 0.1–5.0 wt.% alkali or alkaline earth metals, preferably up to 1.0 wt.% alkali or alkaline earth metals, and optionally other components, the total content of which is limited to a maximum of 2.0 wt.%, with aluminum as the remainder. Preferred compositions of the aluminum base layer correspond to the preferred melt compositions. In a preferred embodiment of the Al base layer, the optional content of alkali or alkaline earth metals comprises 0.1–1.0 wt.% Mg, in particular 0.1–0.7 wt.% Mg, preferably 0.1–0.5 wt.% Mg. Furthermore, the optional content of alkali or alkaline earth metals in the Al base layer can in particular comprise at least 0.0015 wt.% Ca, in particular at least 0.1 wt.% Ca. 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. The corrosion protection coating preferably has a thickness of 5-60 µm, particularly 10-40 µm. The coating weight is preferably 30-360 g / m² for double-sided coatings and 15-180 g / m² for single-sided coatings. Preferably, the coating weight is 100-200 g / m² for double-sided coatings and 50-100 g / m² for single-sided coatings. Particularly preferably, the coating weight is 120-180 g / m² for double-sided coatings and 60-90 g / m² for single-sided coatings. The thickness of the alloy layer is preferably less than 20 µm, particularly preferably less than 16 µm, particularly preferably less than 12 µm, and especially less than 10 µm. The thickness of the aluminum base layer is the difference between the thicknesses of the corrosion protection coating and the alloy layer. Preferably, the thickness of the aluminum base layer is at least 1 µm, even with thin corrosion protection coatings. In a preferred embodiment, the steel flat 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 layer of the corrosion protection coating. The oxide layer consists in particular of more than 80 wt.% oxides, wherein the main proportion of the oxides (i.e., more than 50 wt.% of the oxides) is 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. Preferably, the oxide layer of the steel flat product has a thickness greater than 50 nm. In particular, the thickness of the oxide layer is a maximum of 500 nm. Such a steel flat product is used in a process for manufacturing a sheet metal part, comprising the following steps: a. Providing a sheet metal blank from a previously described steel flat product; b. Heating the sheet metal blank such that at least partially the AC3 temperature of the blank is exceeded and the temperature TInput of the blank when inserted into a forming tool intended for hot pressing (step c)) is at least partially 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 tTrans required for removing the blank from the heating device and inserting it is at most 20 s, preferably at most 15 s; d.Hot pressing of the sheet metal blank into the sheet metal part, wherein the blank is cooled to the target temperature Ttarget during the hot pressing process over a duration tWZ of more than 1 s at a cooling rate rWZ of at least partially more than 30 K / s and optionally held there; e. Removal of the sheet metal part cooled to the target temperature Ttarget from the tool; f. Cooling of the sheet metal part to room temperature. In this process, a blank consisting of steel appropriately composed according to the preceding explanations is provided (step a)), which is then heated in a manner known per se such that at least partially the AC3 temperature of the blank is exceeded and the temperature TInput of the blank when inserted into a forming tool intended for hot pressing (step c)) is at least partially above Ms+100°C. For the purposes of this application, "partially exceeding a temperature" (here AC3 or Ms+100°C) means that at least 30%, and in particular at least 60%, of the volume of the blank exceeds the corresponding temperature. Therefore, when inserted into the forming tool, at least 30% of the blank has an austenitic microstructure, i.e.,The transformation from a ferritic to an austenitic microstructure need not be complete when the blank is placed in the forming tool. Up to 70% of the blank's volume may consist of other microstructural constituents, such as tempered bainite, tempered martensite, and / or non- or partially recrystallized ferrite. To achieve this, certain areas of the blank can be deliberately kept at a lower temperature than others during heating. This can be accomplished by selectively directing the heat input to specific sections of the blank or by shielding the parts that are to be heated less. In the portion of the blank that remains at a lower temperature, little or no martensite forms during the forming process, resulting in a significantly softer microstructure compared to the other parts, which exhibit a martensitic microstructure.In this way, a softer area can be selectively set in the respective sheet metal part, for example by providing an optimal toughness for the respective application, while the other areas of the sheet metal part have maximized strength. Maximum strength properties of the resulting 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. The minimum temperature Ac3 to be exceeded is determined according to the formula given by HOUGARDY, HP. in “Materials Science Steel Volume 1: Fundamentals”, Verlag Stahleisen GmbH, Düsseldorf, 1984, p. 229, where %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. An optimally uniform distribution of properties can be achieved by thoroughly heating the cut piece in step b). In a preferred embodiment, the average heating rate of the ovened sheet metal blank during heating in step b) is at least 3 K / s, preferably at least 5 K / s, particularly at least 10 K / s, and preferably at least 15 K / s. The average heating rate of the oven is to be understood as the average heating rate from 30°C to 700°C. In a preferred embodiment, the heating takes place in an oven with an oven temperature TOfen 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. Preferably the dew point in the oven is at least -20°C, preferably at least -15°C, in particular at least -5°C, especially 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. In a specific embodiment, the heating in step b) takes place in stages in areas with different temperatures. Specifically, the heating is carried out in a roller hearth furnace with different heating zones. Here, the heating in a first heating zone occurs at a temperature (so-called furnace inlet temperature) of at least 650°C, preferably at least 680°C, and particularly at least 720°C. The maximum temperature in the first heating zone is preferably 900°C, and particularly preferably 850°C. Furthermore, the maximum temperature of all heating zones in the furnace is preferably at most 1200°C, particularly preferably at most 1000°C, more preferably at most 950°C, and most preferably at most 930°C. The total oven time, consisting of a heating time and a holding time, is preferably at least 2 minutes, particularly at least 3 minutes, and preferably at least 4 minutes for both variants (constant oven temperature, stepwise heating). Furthermore, the total oven time for both variants is preferably a maximum of 20 minutes, particularly a maximum of 15 minutes, preferably a maximum of 12 minutes, and particularly a maximum of 8 minutes. Longer total oven times have the advantage of ensuring uniform austenitization of the sheet metal blank. On the other hand, holding the blank above Ac3 for too long leads to grain coarsening, which negatively affects the mechanical properties. The pre-heated blank is removed from the respective heating device, which may be, for example, a conventional heating furnace, a known induction heating device, or a conventional device for keeping steel components warm, and transported so quickly into the forming tool that its temperature upon arrival in the tool is at least partially above Ms+100°C, preferably above 600°C, particularly above 650°C, and most 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 these variants, the temperature is, in particular, a maximum of 900°C. These temperature ranges ensure good formability of the material. In step c), the transfer of the austenitized blank from the heating device used to the forming tool is completed within preferably no more than 20 s, and in particular within a maximum of 15 s. Such rapid transport is necessary to prevent excessive cooling before forming. 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, and particularly between 50°C and 150°C. Optionally, in a particular embodiment, the tool can be pre-heated, at least in certain areas, to a temperature TWZ of at least 200°C, and particularly at least 300°C, in order to harden the component only partially. Furthermore, the tool temperature TWZ is preferably a maximum of 600°C, and particularly a maximum of 550°C. It is only necessary to ensure that the tool temperature TWZ is below the desired target temperature TZiel. The dwell time in the tool tWZ is preferably at least 2 s, particularly at least 3 s, and most preferably at least 5 s. The maximum dwell time in the tool is preferably 25 s, and particularly a maximum of 20 s. The target temperature Ttarget of the sheet metal part is at least partially below 400°C, preferably below 300°C, particularly below 250°C, preferably below 200°C, and especially preferably below 180°C, particularly below 150°C. Alternatively, the target temperature Ttarget 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, and particularly preferably at least 50°C. The martensite start temperature of a steel within the parameters of the invention is to be calculated according to the formula, where C% denotes the C content, %Mn the Mn content, %Mo the Mo content, %Cr the Cr content, %Ni the Ni content, %Cu the Cu content, %Co the Co content, %W the W content and %Si the Si content of the respective steel in wt.%. The AC1 temperature and the AC3 temperature of a steel within the parameters of the invention are to be calculated according to the formulas and where %C denotes the C content, %Si the Si content, %Mn the Mn content, %Cr the Cr content, %Mo the Mo content, %Ni the Ni content and %V the vanadium content of the respective steel (Brandis H 1975 TEW-Techn. Ber. 1 8-10 ) In the tool, the blank is not only formed into the sheet metal part, but also simultaneously quenched to the target temperature. The cooling rate in the tool rWZ to the target temperature is, in particular, at least 20 K / s, preferably at least 30 K / s, especially at least 50 K / s, and in a special embodiment at least 100 K / s. In step f), after the removal of the sheet metal part in step e), the sheet metal part is cooled to room temperature within a cooling period tAB of 0.5-600 s. This is usually done by air cooling. During the heating of the steel flat product prior to hot forming, further iron diffusion occurs into the corrosion protection coating. Thus, within a short heating period, an iron-alloyed corrosion protection coating forms, exhibiting an iron content of at least 35 wt.%. Furthermore, a specific structure of the corrosion protection coating of the sheet metal part is preferably achieved, which is described below: In a particular embodiment, the sheet metal part preferably comprises an aluminum-based corrosion protection coating. Preferably, the corrosion protection coating of the sheet metal part comprises an alloy layer and an aluminum base layer. Preferably, the corrosion protection coating of the steel component comprises an alloy layer and an Al base layer. The alloy layer lies directly on the steel substrate and borders directly upon it. Preferably, the alloy layer of the sheet metal part consists of 35-90 wt.% Fe, 0.1-12 wt.% Si, optionally up to 0.5 wt.% Mg, and optional other components, the total content of which is limited to a maximum of 2.0 wt.%, with aluminum as the remainder. Due to the further diffusion of iron into the alloy layer, the proportions of Si and Mg are correspondingly lower than their respective proportions in the melt of the molten pool. The alloy layer preferably has a ferritic structure. The aluminum base layer of the sheet metal part lies on top of the alloy layer of the steel component and is directly adjacent to it. Preferably, the aluminum base layer of the steel component consists of 35-55 wt.% Fe, 0.4-10 wt.% Si, 0.1-5.0 wt.% alkali or alkaline earth metals, and optionally other components, the total content of which is limited to a maximum of 2.0 wt.%, with aluminum as the remainder. In a preferred embodiment, the alkali or alkaline earth metal content comprises 0.1–1.0 wt.% Mg, in particular 0.1–0.7 wt.% Mg, preferably 0.1–0.5 wt.% Mg. Furthermore, the alkali or alkaline earth metal content in the melt can comprise in particular at least 0.0015 wt.% Ca, in particular at least 0.01 wt.% Ca. In a preferred embodiment, the content of alkali or alkaline earth metals consists of 0.1-1.0 wt.% Mg, in particular 0.1-0.7 wt.% Mg, preferably 0.1-0.5 wt.% Mg. In a preferred embodiment, the alkali or 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 and at least 0.0015 wt.% Ca, in particular at least 0.01 wt.% Ca. The aluminum base layer can exhibit a homogeneous silicon distribution, with local silicon contents varying by no more than 10%. Preferred variants of the aluminum base layer, however, feature silicon-poor and silicon-rich phases. Silicon-poor phases are defined as regions whose average silicon content is at least 20% lower than the average silicon content of the aluminum base layer. Silicon-rich phases are defined as regions whose average silicon content is at least 20% higher than the average silicon content of the aluminum base layer. In a preferred embodiment, the silicon-rich phases are arranged within the silicon-poor phase. In particular, the silicon-rich phases form a layer that is at least 40% continuous and is bounded by silicon-poor regions. In an alternative embodiment, the silicon-rich phases are arranged in island-like formations within the silicon-poor phase. For the purposes of this application, "island-shaped" means an arrangement in which discrete, unconnected areas are enclosed by another material – i.e., there are "islands" of a certain material within another material. In a preferred embodiment, the steel component 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 layer of the corrosion protection coating. The oxide layer of the steel component consists in particular of more than 80 wt.% oxides, wherein the main proportion of the oxides (i.e., more than 50 wt.% of the oxides) is 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. The oxide layer preferably has a thickness of at least 50 nm, particularly at least 100 nm. Furthermore, the thickness is preferably a maximum of 4 µm, particularly a maximum of 2 µm. The central aspect of the invention is the inspection of a hot-formed sheet metal part with an aluminum-based coating, in particular a sheet metal part that is further 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 the 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. A simple process, namely determining one or more color values ​​and comparing them with corresponding reference values, allows for the automated determination of whether the component has undergone the desired manufacturing process. In this way, every component can be easily inspected during series production. This increases the assurance that no defective components are reused. Previous testing methods required chemical analysis and / or the preparation of a metallographic section. Such methods are therefore not non-destructive. Consequently, only samples of the components could be tested. With the method according to the invention, it is therefore possible to inspect the entire component production run. In a preferred further development of the method, 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 at a temperature of 900°C for a reference annealing time of 4 minutes. Tests have shown that annealing at 900°C for 4 minutes often yields a component with the desired properties. Therefore, it is advantageous to use this as the reference process and to use the color values ​​found on such a reference component as reference values. In a specific embodiment of the process, 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 advantageous because they are highly reliable and can be easily integrated into industrial serial production processes. For example, it is readily possible to measure many components in quick succession using a single photometer. Preferably, determining the single or multiple color values ​​of the coating involves determining at least three color values ​​in a color space. Specifically, determining the single or multiple color values ​​of the coating consists of determining at least three color values ​​in a color space. These three color values ​​in a color space could, for example, be the values ​​L*a*b in the standardized CIE Lab color space. Alternatively, a selected RGB color space can be used. In this case, the three color values ​​are the corresponding RGB values. Naturally, these three color values ​​are independent, so that by specifying the three color values, the color in that color space can be uniquely identified.By including at least three color values ​​in a color space, the color change can be better characterized, as more information is available and the color can be clearly identified in the color space. In a particularly preferred variant, comparing one or more color values ​​with one or more reference values ​​involves calculating a color dynamic range. Specifically, the color values ​​and 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. 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 within the comparison, for example, if the color dynamic is greater than 270. In a preferred embodiment of the previously described method for manufacturing a sheet metal part, step g) follows the described process steps a) - f), in particular directly: g. Checking the cooled sheet metal part using the described method for checking a sheet metal part The manufactured sheet metal parts are therefore checked as part of a quality control process to ensure they have undergone the desired manufacturing procedure. To demonstrate the effectiveness of the invention, several tests were conducted. For this purpose, slabs with the compositions specified in Table 1, measuring 240 mm thick and 1200 mm wide, were produced and heated to a temperature T1 of 1200°C in a pusher furnace. The slabs were then held at T1 for 30 to 450 minutes until the core temperature T1 was reached and the slabs were thoroughly heated. The slabs were then removed from the pusher furnace at their respective core 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. These intermediate products, which can also be referred to as rough strips in hot strip rolling, each had an intermediate product temperature T2 of 1100°C at the end of the pre-rolling phase.The roughing strips were fed to the finish rolling line immediately after pre-rolling, so that the intermediate product temperature T2 corresponded to the starting rolling temperature for the finish rolling phase. The roughing strips were hot-rolled to 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, then cooled in still air. The hot-rolled strips were descaled conventionally by pickling before being cold-rolled to the thickness specified in Table 3. The cold-rolled steel flat products were heated in a continuous annealing furnace to an annealing temperature T5 of 870°C and held at this temperature for 100 seconds at a time before being cooled at a rate of 1 K / s to the immersion temperature T6 of 690°C.The cold-rolled strips, at their respective immersion temperatures T6, were passed through a molten coating bath at temperature T7 of 676°C. 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 coating thickness is given in Table 3. The strips were initially cooled to 660°C at an average cooling rate of 10–15 K / s. Between 660°C and 570°C, i.e., between the beginning and end of the coating hardening, the cooling rate was 21 K / s. During the subsequent cooling phase between 570°C and room temperature, the strips were cooled at a rate of 5–12 K / s. From the steel strips produced in this way, blanks were cut off and used for further tests. In these tests, sheet metal samples in the form of 200 × 300 mm² 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 at a mean heating rate (between 30°C and 700°C) to a furnace temperature of 900°C. The heating time in the furnace, which includes heating and holding, is denoted by tfurnace. The dew point of the furnace atmosphere was -5°C in all cases. Subsequently, the blanks were removed from the heating device and placed in a forming tool at temperature Twz. At the time of removal from the furnace, the blanks had reached the furnace temperature.The transfer time tTrans, comprising the time required for removal from the heating unit, transport to the tool, and insertion into the tool, was 8 s. The temperature TInling of the blanks upon insertion into the forming tool was above the respective martensite start temperature of +100°C in all cases. In the forming tool, the blanks were formed into the respective sheet metal parts, which were cooled to a target temperature TTinc within the tool at a cooling rate rWZ. The residence time in the tool is denoted by tWZ. Finally, the samples were cooled to room temperature in air. Table 4 summarizes the aforementioned parameters, where "RT" abbreviates room temperature. 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). The annealing time in the oven was also varied between 4 and 11 minutes. The color of the resulting sheet metal parts was determined in the CIE Lab color space using a spectrophotometer and a light source with CIE standard illuminant A and a 10° field of view. The color values ​​in the RGB space were calculated from these data. The color dynamics D were then determined from these calculations. The results are shown in Table 5. Residual iron and unavoidable impurities. All values ​​in wt.%; Table 1 (Steel grades) Table 2 (Coating options) Table 2 (Coating options) α9.53<0.01<1%Rest β1030.3<1%Rest Table 3 (Structure) Table 3 (Structure) 1*AA1,527 2Aβ1,527 * Non-inventive reference examples Table 4 (hot forming parameters) Table 4 (Hot forming parameters) 89008-5800RT155050 Table 5 (Results) Table 5 (Results) 1*439,941,353,099993890 (reference) 1*537,221,580,3991878716 1*634,020,33-5,7476808940 1*734.46-1.16-7.9972829451 1*834,14-2,93-10,7765829761 1*935,70-5,00-12,61618710477 1*1037,23-6,33-13,00619210886 1*1137,27-4,63-14,67639111192 2438.41-0.78-6.3884911010 (reference) 2536.70-3.78-11.66688910522 2638.65-1.61-4.8284929947 2742.00-5.76-10.237810311681 2853.68-5.76-9.13108132144168 2945,29-4,77-5,6893110116233 21045,31-0,466,1011110797273 21150.79 - 2876.99121122109310

Claims

A method for inspecting a hot-formed sheet metal part with an aluminum-based coating, comprising at least the following steps: a. Determining one or more color values ​​of the coating; b. Comparing the one or more color values ​​with one or more reference values; c. Sorting out the sheet metal part based on the result of the comparison. Method according to claim 1, characterized in that the one or more reference values ​​are one or more color values ​​of a cooled reference sheet metal part that has undergone annealing in an oven with an oven temperature of 900°C for a reference annealing time of 4 minutes. Method according to one of claims 1 to 2, characterized in that the determination of the at least one color value or the several color values ​​is carried out by means of a photometer, in particular by means of a spectrophotometer. Method according to one of claims 1 to 3, characterized in that determining the one color value or the several color values ​​of the coating comprises determining at least three color values ​​in a color space. Method according to one of claims 1 to 4, characterized in that comparing the one color value or the several color values ​​with one or more reference values ​​includes the calculation of a color dynamic. A method for producing a sheet metal part comprising the following steps: a. Providing a sheet blank from a steel flat product for the production of a sheet metal part by hot forming, comprising: di. a steel substrate consisting of a steel having 0.1-3 wt.% Mn and optionally up to 0.01 wt.% B, and ii. 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 does not exceed 2.0 wt.%.-% are limited, and the remainder consists of aluminum, and wherein the alkali or 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 at a furnace temperature of 900°C for a duration of 9 minutes, undergoes a color change compared to the cooled state after a reference annealing duration of 4 minutes, wherein the color change has a color dynamic of greater than 100; b. Heating the sheet blank such that at least partially the AC3 temperature of the blank is exceeded and the temperature TInput of the blank when inserted into a forming tool intended for hot pressing (step c)) has at least partially a temperature above Ms+100°C, where Ms denotes the martensite start temperature; c.d. Inserting the heated sheet metal blank into a forming tool, wherein the transfer time tTrans required for removing the blank from the heating device and inserting it is at most 20 s, preferably at most 15 s; d. Hot forming the sheet metal blank into the sheet metal part, wherein, during hot forming, the blank is cooled to the target temperature TZiel over a period tWZ of more than 1 s at a cooling rate rWZ of at least partially more than 30 K / s and optionally held there; e. Removing the sheet metal part cooled to the target temperature TZiel from the tool; f. Cooling the sheet metal part to room temperature; g. Checking the cooled sheet metal part using the method according to any one of claims 1 to 5. Method according to claim 6, wherein the temperature reached at least partially in the sheet metal blank in step b) is between Ac3 and 1000°C, preferably between 850°C and 950°C. Method according to one of claims 6 to 7, wherein the target temperature Ttarget of the sheet metal forming part is at least partially below 400°C, preferably below 300°C.