Efficient forming process

Abstract

The invention relates to a method for producing a sheet metal part, wherein a strip-shaped flat steel product is provided. After uncoiling and straightening, a sheet metal blank is cut from this and provided with an absorption layer. The sheet metal blank is then subjected to hot forming. The invention further relates to a sheet metal blank with an absorption layer.

Description

[0001] The invention relates to a method for producing a sheet metal part from a flat steel product by means of hot forming and a correspondingly prepared sheet metal blank.

[0002] When reference is made below to a "flat steel product" or a "sheet metal product," this refers to rolled products, such as steel strips or sheets, whose length is much greater than their thickness, from which "sheet metal blanks" (also called blanks) are cut for the production of, for example, body components. "Sheet metal formed parts" of the type according to the invention are produced from such sheet metal blanks by forming.

[0003] All information regarding the contents of the steel compositions specified in this application is based on weight, unless expressly stated otherwise. All unspecified "%" information in connection with a steel alloy should therefore be understood as "wt%."

[0004] During hot forming, also known as hot forming, press hardening or hot press hardening, flat steel products, such as steel blanks, cut from cold- or hot-rolled steel strip are heated to a deformation temperature that is usually above the austenitizing temperature (AC3) of the respective steel and then placed in the tool of a forming press in the heated state. During the subsequent forming process, the sheet metal blank or the component formed from it undergoes rapid cooling due to contact with the cool tool. The cooling rates are adjusted to create a hardened microstructure in the steel substrate. This transforms the microstructure into an at least partially martensitic microstructure. The result of hot forming is a hardened sheet metal part.

[0005] The heating of the flat steel product typically takes place in a preheated roller hearth furnace through which the steel flat product passes. In practice, this results in the problem that the radiant heat is largely reflected by the smooth and reflective surfaces of the metallic corrosion protection coating applied to the flat steel product. This leads to a significant delay in the heating process, resulting in increased time and energy expenditure for the heating process.

[0006] From the perspective of optimal energy utilization, it would therefore be desirable to achieve the most effective possible transfer of the thermal energy, usually introduced as thermal radiation, into the flat steel product during heating. A shortened heating time of the flat steel product to the processing temperature would mean that the corresponding roller hearth furnaces could be designed smaller, which would have a positive impact on both the required space and the cost of purchasing them. Furthermore, a shortened heating time could shorten the process duration. The CO2 emissions generated during the process could also be reduced. Overall, this would result in an optimized process control option.

[0007] Flat steel products with various corrosion protection coatings are known from WO 2012 / 120081 A2. To improve heating behavior, various coatings containing a metallic compound from the group of oxide, nitride, sulfide, sulfate, carbide, carbonate, fluoride, hydrate, hydroxide, or phosphate compounds are proposed. The disadvantage of these coatings is that they negatively impact product properties, such as scaling protection, corrosion susceptibility, weldability, and / or paintability.

[0008] When selecting a suitable material for a top layer to improve heating behavior, it is important to ensure that the top layer ideally burns without residue during heating. This is necessary to avoid negatively affecting further processing properties (e.g., paintability, weldability, adhesive adhesion). Furthermore, combustion should not produce any harmful substances to avoid environmental pollution and to avoid the need for extensive filtration.

[0009] An example of an absorption coating that meets these criteria is known from WO 2024 / 084018 A1. This application discloses an aluminum-based corrosion protection coating applied to the steel substrate, with an absorption layer comprising carbon particles arranged on the corrosion protection coating.

[0010] By omitting additional additives such as adhesion promoters, it is possible to ensure that the top layer burns without leaving any residue and without producing any pollutants, but this can lead to difficulties in processing in the further processing plant.

[0011] Typically, steel flat products for hot forming are delivered by the steel supplier to the processing plant ready for further processing in the form of a coiled steel strip. At the processing plant, these coils are uncoiled and straightened to ensure the necessary flatness. Sheet metal blanks are cut from the straightened steel flat products, dimensioned according to the desired sheet metal parts. These are then heated and formed in a conventional manner.

[0012] When processing flat steel products with a corrosion protection coating and the absorption layer described above, the lack of additional additives results in a low abrasion resistance of the absorption layer. This leads to various problems. It has been shown that significant abrasion of the absorption layer occurs during uncoiling and straightening in the processing plant. This causes severe contamination in the straightening mill of the processing plant. The same problem arises during handling and intermediate storage of the cut sheet blanks. These must be transported and stored with great care to prevent abrasion of the layer.In addition to the unwanted contamination caused by abrasion (during straightening, cutting, and / or storing the sheet metal blanks), abrasion also leads to the absorption layer thickness no longer being consistent across all parts of the sheet metal blank. This causes inhomogeneous heating behavior during the hot forming process, which is also disadvantageous.

[0013] The object of the present invention is to enable the use of an absorption layer which burns residue-free during the hot forming process and produces small amounts of pollutants.

[0014] This object is achieved by a method for producing a sheet metal part comprising the following work steps: a) providing a steel coil from a strip-shaped flat steel product consisting of a steel substrate consisting of a steel having 0.1 - 3 wt% Mn and optionally up to 0.01 wt% B; b) uncoiling and straightening the strip-shaped flat steel product; c) performing one of steps c1 or c2, wherein c1) separating at least one sheet blank from the strip-shaped flat steel product and applying an absorption layer to at least one side of the sheet blank; c2) applying an absorption layer to at least one side of the strip-shaped flat steel product and separating at least one sheet blank from the strip-shaped flat steel product; d) 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 Einlg the blank, when placed in a forming tool intended for hot press forming (working step e)), at least partially has a temperature above Ms + 100 °C, where Ms denotes the martensite start temperature; e) Inserting the heated sheet metal blank into a forming tool, whereby the transfer time required for removing the blank from the heating device and inserting it is t Trans at most 20 s, preferably at most 15 s; f) hot-press forming the sheet metal blank to form the sheet metal part, wherein the blank is subjected to a hot-press forming process for a period of time t wz of more than 1 s to a target temperature T Ziel cooled and optionally kept there, whereby the cooling depends on the temperature T Einlg at least up to the martensite start temperature with a cooling rate r at least partially exceeding 25 K / s wz occurs; g) Remove the sample to the target temperature T Ziel cooled sheet metal part from the tool.

[0015] Contrary to the usual procedure of modifying the properties of the surface layer through additives, the inventors recognized that it is more expedient to change the sequence of processing steps. For example, in step a), the steel coil is first provided to a processing plant. In this process step, the flat steel product does not yet have any absorption layer. In step a), the flat steel product therefore only has a corrosion protection coating on at least one side, with the corrosion protection coating representing the coating closest to the surface of the flat steel product. Optionally, the flat steel product has an oil film on the corrosion protection coating to protect it from atmospheric influences during transport.

[0016] The strip-shaped flat steel product is then uncoiled and straightened in the further processing plant. This prepares the strip-shaped flat steel product for the subsequent processing steps by achieving the required flatness. Because the strip-shaped flat steel product does not yet have an absorption layer at this stage, it can be transported and stored in the usual way. No special care is required to prevent abrasion. The corrosion coating itself, as the layer closest to the surface, is highly resistant to abrasion. Therefore, excessive abrasion of an absorption layer does not occur in the straightening mill either, as it has not yet been applied.

[0017] Only in the subsequent step c) immediately before further processing is an absorption layer applied. Next, sheet metal blanks are cut from the strip-shaped flat steel product, which are dimensioned according to the desired sheet metal parts. Step c) can be carried out in two alternatives. According to alternative c1), at least one sheet metal blank is first cut from the strip-shaped flat steel product. An absorption layer is then applied to at least one side of this sheet metal blank. One of the advantages of this is that the material of the absorption layer is used very economically. The offcuts that inevitably arise when cutting the sheet metal blanks are not coated. This means that less material is used. In addition, the offcuts can easily be remelted later.Furthermore, the absorption layer cannot be abrasioned when the sheet metal blank is separated, for example by punching, because the layer has not yet been applied at this point. In alternative c2), however, an absorption layer is first applied to at least one side of the strip-shaped flat steel product. Subsequently, at least one sheet metal blank is separated from the thus coated strip-shaped flat steel product. This alternative has the advantage that the application of the absorption layer can be carried out very efficiently. For example, the entire strip-shaped flat steel product can be coated in a continuous process, e.g. by dipping. This can usually be better integrated into a manufacturing process than individually coating each separated sheet metal blank.

[0018] In both variants, the absorption layer is preferably applied over the entire surface, i.e. not only selectively but seamlessly over the entire surface of a larger area (e.g. at least 1 cm 2 ).

[0019] In both variants of step c), the result is a sheet blank from the strip-shaped flat steel product with an absorption layer on at least one side.

[0020] This sheet metal blank is heated in step d) in a manner known per se so that the AC3 temperature of the blank is at least partially exceeded and the temperature T Einlg of the blank when placed in a forming tool intended for hot pressing (work step e)) is at least partially at a temperature above Ms + 100 °C.

[0021] Preferably, the average heating rate in step d) is greater than 15 Kmm / s. Such high heating rates can be achieved by the absorption layer by reducing reflection in the infrared range.

[0022] For the purposes of this application, partially exceeding a temperature (here AC3 or Ms + 100 °C) means 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, at least 30% of the blank therefore has an austenitic structure, i.e. the transformation from a ferritic to an austenitic structure does not have to be complete when 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 structural components, such as tempered bainite, tempered martensite and / or non- or partially recrystallized ferrite. For this purpose, certain areas of the blank can be deliberately kept at a lower temperature level than others during heating.To achieve this, the heat supply can be specifically directed to specific sections of the blank, or the parts that require less heating can be shielded from the heat supply. In the part of the blank material whose temperature remains lower, no or only significantly less martensite is formed during forming in the tool, so that the microstructure there is significantly softer than in the other parts, which have a martensitic microstructure. In this way, a softer area can be specifically adjusted within the formed sheet metal part, for example by achieving optimal toughness for the respective intended use, while the other areas of the sheet metal part have maximized strength.

[0023] 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.

[0024] 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. Ac3=(902−225*%C+19*%Si−11*%Mn−5*%Cr+13*%Mo−20*%Ni+55*%V)°C 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.

[0025] An optimally uniform distribution of properties can be achieved by heating the blank completely in step d) and preferably 100% of the volume is above the stated temperatures (AC3 or Ms + 100 °C) or in the stated temperature intervals.

[0026] The average heating rate is the product of the average heating rate from 30°C to 700°C and the sheet thickness. The average heating rate is more than 15 Kmm / s, in particular more than 20 Kmm / s, preferably more than 25 Kmm / s, in particular more than 30 Kmm / s. The heating in step d) preferably takes place in a furnace, in particular a roller hearth furnace. Therefore, thermal radiation dominates over thermal conduction when heating the sheet metal blanks. The absorption layer according to the invention increases the proportion of absorbed thermal radiation, resulting in the advantageous high average heating rates.

[0027] In a preferred embodiment, heating takes place in an oven with an oven temperature T Ofen 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.

[0028] 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.

[0029] In a special embodiment, the heating in step d) takes place stepwise in regions with different temperatures. In particular, the heating takes place in a roller hearth furnace with different heating zones. Here, the heating takes place in a first heating zone with 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, particularly preferably a maximum of 930 °C.

[0030] The total time in the oven t Ofen, which consists of a heating time and a holding time, is preferably at least 1 minute, in particular at least 2 minutes, preferably at least 3 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.

[0031] The blank heated in this way is removed from the respective heating device, which can be a conventional heating furnace, for example, 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.

[0032] In step e), 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.

[0033] When inserting the blank, 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 heated 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 preferably a maximum of 600 °C, especially a maximum of 550 °C. It is only necessary to ensure that the tool temperature T WZbelow the desired target temperature T Ziel The residence time in the tool t WZ is preferably at least 2 s, in particular at least 3 s, particularly preferably at least 5 s. The maximum residence time in the tool is preferably 25 s, in particular a maximum of 20 s.

[0034] The target temperature T Ziel 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 Ziel 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.

[0035] 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 / wt.−%] 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.%.

[0036] The AC1 temperature and the AC3 temperature of a steel within the scope of the invention specifications are according to the formulas: AC1[°C]=(739−22*%C−7*%Mn+2*%Si+14*%Cr+13*%Mo−13*%Ni+20*%V)[°C / wt.−%] and AC3[°C]=(902−225*%C+19*%Si−11*%Mn−5*%Cr+13*%Mo−20*%Ni+55*%V)[°C / wt.− %] to be calculated, 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).

[0037] In the tool, the blank is not only formed into the sheet metal part, but simultaneously quenched to the target temperature. The cooling rate in the tool r wz is in particular at least 25 K / s, preferably at least 30 K / s, in particular at least 50 K / s, in a special embodiment at least 100 K / s. The cooling rate r WZ is defined as the average cooling rate between the temperature T Einlg when inserted into the forming tool and the martensite start temperature.

[0038] In a preferred embodiment, the cooling rate r is also wz from the insertion temperature T Einlgto the martensite finish temperature of at least 25 K / s, preferably at least 30 K / s, in particular at least 50 K / s, and in special designs at least 100 K / s. The martensite finish temperature is determined using dilatometer measurements.

[0039] The further cooling from the martensite start temperature or the martensite finish temperature to the target temperature T Ziel can also be carried out with lower cooling rates, since this cooling no longer has a significant effect on the microstructure formation.

[0040] After removing the sheet metal part in step g), the sheet metal part is cooled to a cooling temperature T AB of less than 100 °C within a cooling time t AB from 0.5 to 600 s. This is usually done by air cooling.

[0041] The sheet metal part according to the invention is preferably a component for a land vehicle, marine vehicle, or aircraft. It is particularly preferably an automotive part, in particular a body part. The component is preferably a B-pillar, side member, A-pillar, sill, or cross member.

[0042] In a preferred embodiment of the method, the flat steel product (already in step a)) has a corrosion protection coating on at least one side, wherein the corrosion protection coating is in particular an aluminum-based corrosion protection coating and preferably comprises an alloy layer and an Al base layer. Since the flat steel product preferably has such a corrosion protection coating, this also applies to the sheet blank cut from the flat steel product.

[0043] In step c), in particular, the absorption layer is applied to the corrosion protection coating that the flat steel product or sheet blank comprises.

[0044] The corrosion protection coating can be applied to one or both sides of the flat steel product or sheet metal blank. The two large, opposing surfaces of the flat steel product or sheet metal blank are referred to as the two sides. The narrow surfaces are referred to as the edges.

[0045] Such a corrosion protection coating is preferably produced by hot-dip coating the flat steel product. The flat steel 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, optionally up to 5 wt.% alkali or alkaline earth metals, preferably up to 1.0 wt.% alkali or alkaline earth metals, and optionally up to 15 wt.% Zn, preferably up to 10 wt.% Zn, and optionally further constituents, the total contents of which are limited to a maximum of 2.0 wt.%, with aluminum as the remainder. The optional content of alkali or alkaline earth metals is preferably at least 0.1 wt.%.

[0046] In a preferred variant, the Si content of the melt is 0.5 - 3.5 wt.% or 7 - 12 wt.%, in particular 8 - 10 wt.%.

[0047] In a preferred variant, the optional content of alkali or alkaline earth metals 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 optional content of alkali or alkaline earth metals in the melt can comprise in particular at least 0.0015 wt.% Ca, in particular at least 0.01 wt.% Ca.

[0048] Furthermore, the optional content of alkali or alkaline earth metals in the melt preferably consists of 0.1 - 1.0 wt.% Mg, in particular 0.1 - 0.7 wt.% Mg, preferably 0.1 - 0.5 wt.% Mg and optionally at least 0.0015 wt.% Ca, in particular at least 0.01 wt.% Ca.

[0049] Furthermore, the content of optional further constituents is preferably limited to 1.5 wt. %, in particular to 1.0 wt. %, preferably to 0.5 wt. %, in particular to 0.3 wt. %, preferably to 0.10 wt. %, in particular to 0.05 wt. %. In particular, the optional further constituents correspond to unavoidable impurities and are present only in technically unavoidable amounts. This applies to all melt compositions explained above.

[0050] During hot-dip coating, iron diffuses from the steel substrate into the liquid coating, so that the corrosion protection coating of the flat steel product has, in particular, an alloy layer and an Al base layer upon solidification.

[0051] The alloy layer lies on the steel substrate and is directly adjacent to it. The alloy layer is essentially formed from aluminum and iron. The alloy layer preferably consists of 25-50 wt.% Fe, 5-20 wt.% Si, optional further components whose total content is limited to a maximum of 5.0 wt.%, preferably 2.0 wt.%, and the remainder being aluminum. The optional further components include, in particular, the remaining components of the melt (i.e., optionally alkali or alkaline earth metals, in particular Mg or Ca) and the remaining components of the steel substrate in addition to iron. In a further variant (variant with an Si content in the melt of 0.5-3.5 wt.%), the alloy layer consists of 25-50 wt.% Fe, 0.5-5.0 wt.% Si, optional further components whose total content is limited to a maximum of 5.0 wt.%, preferably 2.0 wt.%, and the remainder being aluminum.The optional additional components also include in particular the remaining components of the melt (i.e. alkali or alkaline earth metals, in particular Mg or Ca) and the remaining components of the steel substrate in addition to iron.

[0052] The Al base layer lies on the alloy layer and is directly adjacent to it. The composition of the Al base layer preferably corresponds to the composition of the melt of the molten bath. This means that it consists of 1.0-15 wt.% Si, optionally 2-4 wt.% Fe, optionally up to 5 wt.% alkali or alkaline earth metals, preferably up to 1.0 wt.% alkali or alkaline earth metals, optionally up to 15 wt.% Zn and optional further components, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder being aluminum. Preferred compositions of the Al base layer correspond to the preferred melt compositions. Accordingly, the content of optional further components in the Al base layer is preferably limited to 1.5 wt.%, in particular to 1.0 wt.%, preferably to 0.5 wt.%, in particular to 0.3 wt.%, preferably to 0.10 wt.%, in particular to 0.05 wt.%.In particular, the optional additional components correspond to unavoidable impurities and are only present in technically unavoidable amounts.

[0053] In a preferred variant 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. Further preferably, the optional 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 and optionally at least 0.0015 wt.% Ca, in particular at least 0.1 wt.% Ca.

[0054] 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.

[0055] The corrosion protection coating preferably has a thickness of 5 - 60 µm, in particular 10 to 40 µm. The application weight of the corrosion protection coating is in particular 30−360gm2 with corrosion protection coatings on both sides or 15−180gm2 in the one-sided variant. The preferred application weight of the corrosion protection coating is 100−200gm2 for double-sided coatings or 50−100gm2 for one-sided coatings. The application weight of the corrosion protection coating is particularly preferred 120−360gm2 for double-sided coatings or 60−90gm2 for one-sided covers.

[0056] 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 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.

[0057] 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.

[0058] 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. Optionally, in addition to aluminum oxide, hydroxides and / or magnesium oxide are present in the oxide layer, 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. For the optional embodiment with zinc as a component of the Al base layer, zinc oxide components are also present in the oxide layer.

[0059] Preferably, the oxide layer of the flat steel product has a thickness greater than 50 nm. In particular, the thickness of the oxide layer is a maximum of 500 nm.

[0060] In an alternative design, the flat steel product or sheet metal blank includes a zinc-based corrosion protection coating. 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.

[0061] Such a zinc-based corrosion protection coating preferably comprises 0.20-6.0 wt.% Al, 0.1-10.0 wt.% Mg, optionally 0.1-40 wt.% manganese or copper, optionally 0.1-10.0 wt.% Cr, optionally at most 0.2 wt.% other elements, unavoidable impurities, and the remainder zinc. In particular, the Al content is a maximum of 2.0 wt.%, preferably a maximum of 1.5 wt.%. The Mg content is in particular a maximum of 3.0 wt.%, preferably a maximum of 1.0 wt.%. The corrosion protection coating can be applied by hot-dip coating, by physical vapor deposition, or by electrolytic processes.

[0062] In a preferred embodiment, the absorption layer comprises carbon particles, in particular selected from the group consisting of graphite, fullerenes, graphene, carbon nanotubes and mixtures thereof.

[0063] For the purposes of the invention, carbon particles are particles of elemental carbon. Particles within the meaning of the invention consist of a solid and have a hydrodynamic diameter of 1 to 500 nm. Several of these primary particles can be combined to form aggregates with a hydrodynamic diameter of 50 to 1000 nm. Typically, these aggregates are combined to form further agglomerates with diameters of 1 to 100 µm. The hydrodynamic diameter of the particles is determined using dynamic light scattering (DLS).

[0064] The carbon particles are preferably those selected from the group consisting of graphite, fullerenes, graphene, carbon nanotubes and mixtures thereof.

[0065] Carbon particles have the advantage that even small amounts in the absorption layer result in significant absorption in the infrared range, thus reducing reflectivity in the infrared range. The infrared range is precisely the region of the radiation spectrum in which the furnace interior emits radiation, which essentially serves to heat the sheet metal blanks to the forming temperature. The reduction in the reflectivity of the flat steel product or the increase in absorption in the infrared range by the absorption layer comprising carbon particles thus leads to faster heating of the flat steel product to the forming temperature.In addition, the absorption layer comprising carbon particles is almost completely burned during hot forming at 920 °C, so that the absorption layer does not adversely affect the properties, such as weldability, corrosion susceptibility, scaling protection and paintability, of the sheet metal part obtained from the flat steel product after hot forming.

[0066] For the purposes of this application, the infrared range is defined as the wavenumber range from 667 - 10000 cm -1 This corresponds to the wavelength range of 1 - 15 µm.

[0067] In addition, an absorption layer with carbon particles can be easily created by immersing the flat steel product in an aqueous dispersion containing carbon particles, spraying it with such a dispersion, applying it using a (coil) coating process, chemical or physical vapor deposition (CVD or PVD), or screen printing. During subsequent drying of the flat steel product, an absorption layer with carbon particles forms on the aluminum-containing surface of the corrosion protection coating, concealing the highly reflective aluminum-based corrosion protection coating.

[0068] The absorption layer lies on top of the corrosion protection coating and is directly adjacent to it. In particular, the absorption layer is a cover layer that completes the layer structure formed on the flat steel product according to the invention on each of its outer sides.

[0069] It has proven particularly practical if the absorption layer has a coating weight (dry) of 0.09 to 10 g / m 2 , especially 0.5 to 5 g / m 2 , per side of the flat steel product. Coating weights of less than 0.09 g / m 2 do not show sufficient reduction of the reflectance, while at coating weights of more than 10 g / m 2 , especially more than 5 g / m 2 , the effect reaches saturation. Applying a higher coating weight is therefore possible, but uneconomical.

[0070] In a preferred variant of the flat steel product according to the invention, the thickness of the absorption layer is 0.05 to 5 µm. The thickness refers to the thickness per side of the flat steel product. The two sides of the flat steel product are referred to as the two large, opposing surfaces of the flat steel product. The narrow surfaces are referred to as edges. For flat steel products coated on both sides with an absorption layer on both sides, the thickness on each of the two sides is therefore 0.05 to 5 µm. It has been shown that even such small absorption layer thicknesses lead to a significant reduction in the degree of reflection.

[0071] For the purposes of this application, the reflectance R̅ in the infrared range is determined using a blackbody radiator as a reference. The blackbody has a temperature of T = 920 °C, which corresponds to an average furnace temperature. Thus, the spectral radiant power i λ (T) of the blackbody radiator at temperature T with the measured spectral reflectivity ρ λ multiplied and integrated over the wavelength range. This integral is standardized to the spectral radiant power integrated over the same wavelength range. Therefore, R¯(T)=∫λ1λ2ρλ⋅iλ(T)dλ∫λ1λ2iλ(T)dλ

[0072] This results in i λ (T) from Planck's radiation law iλ(T)=2πhc2λ51hceλkBT−1 with the speed of light c, Planck's constant h and Boltzmann's constant k BThe integration is carried out over the wavelength range corresponding to the wavenumbers 667 - 10000 cm -1 corresponds to, i.e. from λ 1 = 1 µm to λ 2 = 15 µm. The reflectance R̅ used below is defined as R̅ (920 °C).

[0073] In the preferred embodiment of the method, the absorption layer comprises carbon particles. The proportion of carbon particles in the dry absorption layer can be 10-99 wt. %, preferably 30-99 wt. %, particularly preferably 50-99 wt. %. The effect according to the invention occurs even with very small amounts of carbon particles in the absorption layer and increases with increasing amounts of carbon particles. Particularly high absorption in the infrared range, and thus a particularly advantageous reduction in reflectivity in the infrared range, is achieved when the proportion of carbon particles in the dry absorption layer is 60-99 wt. %, in particular 80-99 wt. %, preferably 90-99 wt. % and particularly preferably 95-99 wt. %.

[0074] The further design of the carbon particles in the absorption layer has already been described above.

[0075] According to a preferred embodiment of the method according to the invention, the absorption layer comprises at least one surfactant. The presence of the at least one surfactant leads to improved wettability of the absorption layer on the corrosion protection coating.

[0076] Suitable surfactants include anionic, cationic, zwitterionic and non-ionic surfactants and mixtures thereof.

[0077] According to a preferred embodiment of the flat steel product according to the invention, the at least one surfactant is selected from the group consisting of alkyl sulfates, alkyl sulfonates, alkyl phosphonates, alkoxylated fatty alcohols, alkoxylated fatty acids, alkoxylated fatty acid amines, alkoxylated alkylphenols, or alkyl polyglycosides.

[0078] It has proven particularly practical if the “alkyl” of the aforementioned alkyl sulfates, alkyl sulfonates, alkyl phosphonates, alkylphenols and / or alkyl polyglycosides has a chain length of 8 to 22 carbon atoms.

[0079] Preferably, alkoxylated fatty alcohols, fatty acids and / or fatty acid amines are used which have an alkyl chain length of 6 to 22 carbon atoms.

[0080] The alkoxylated fatty alcohols, fatty acids, fatty acid amines, and / or alkylphenols can be ethoxylated, propoxylated, or butoxylated fatty alcohols, fatty acids, fatty acid amines, and / or alkylphenols. The degree of ethoxylation, propoxylation, or butoxylation can be from 1 to 18, preferably from 3 to 10.

[0081] The proportion of at least one surfactant in the dry absorption layer is 0.01 to 5 wt.%, in particular 0.5 to 2 wt.%. A minimum content of 0.01 wt.% has proven necessary to ensure the wettability of the aluminum-based anti-corrosive coating. A proportion of more than 5 wt.% surfactant in the absorption layer does not lead to any further improvement in wettability and is therefore not economically viable.

[0082] In order to improve the wettability of the sheet surface and the adhesion of the absorption layer to the aluminum-based anti-corrosive coating, the absorption layer may contain at least one polymer in addition to the surfactant. An embodiment of the flat steel product according to the invention with an absorption layer comprising at least one surfactant and at least one polymer is therefore particularly preferred.

[0083] Suitable polymers include polyalkylene glycols and their mixtures.

[0084] According to a preferred embodiment of the flat steel product according to the invention, the at least one polymer is selected from polyethylene glycols or polypropylene glycols. Good results have been achieved particularly with polyethylene glycols or polypropylene glycols whose molecular weight is in the range of 400 to 5000 g / mol.

[0085] The proportion of the at least one polymer in the dry absorption layer can range from 1 to 90 wt.%. A minimum content of 1 wt.% is required to achieve the aforementioned advantageous adhesion improvement. Adding more than 90 wt.% has a detrimental effect on the reflectance and drying of the coating.

[0086] In a preferred variant of the method, the absorption layer has an average reflectance R̅ in the infrared range that is less than 0.55, in particular less than 0.50, preferably less than 0.45, in particular less than 0.40, preferably less than 0.35, particularly preferably less than 0.30, in particular less than 0.25, preferably less than 0.20, in particular less than 0.15. The lower the reflectance in the infrared range, the higher the heating rate during the subsequent production of a shaped sheet metal part. The reflectance R̅ of the absorption layer is understood to mean the absorbance of the flat steel product or sheet metal blank on the side provided with the absorption layer.

[0087] In a specific embodiment of the method, the application of the absorption layer in step c) comprises at least one of the following steps: i. Immersion in a coating solution or ii. Spraying with a coating solution or iii. Coating with a coating solution using the (coil) coating process or iv. Depositing the absorption coating by chemical or physical vapor deposition or v. Spraying an adhesive powder.

[0088] The coating solution in variants i, ii, and iii is preferably an aqueous dispersion comprising carbon particles. This treatment distributes the aqueous dispersion containing carbon particles evenly over the entire surface, forming a homogeneous, surface-covering absorption layer comprising carbon particles. Alternatively, the homogeneous, surface-covering absorption layer is formed by chemical or physical vapor deposition. In another variant, the absorption layer is formed by spraying on adhesive powder.

[0089] In a preferred embodiment of the process according to the invention, the pH of the aqueous dispersion is at most 14, in particular at most 13, preferably at most 12, particularly preferably at most 10. This ensures good wettability of the aluminum-based corrosion protection coating and thus a particularly uniform distribution of the aqueous dispersion and the carbon particles contained therein. It has proven particularly practical if the pH of the aqueous dispersion is at least 8 and at most 12, in particular at least 8 and at most 10. This ensures good wettability of the aluminum-based corrosion protection coating and thus a particularly uniform distribution of the aqueous dispersion and the carbon particles contained therein.

[0090] In particular, the aqueous dispersion contains 1 - 70 wt.%, in particular 2 - 50 wt.% carbon particles based on the total weight of the aqueous dispersion.

[0091] In a preferred embodiment of the process according to the invention, the aqueous dispersion additionally contains at least one surfactant. This can improve the stability of the dispersion. Furthermore, the presence of the at least one surfactant in the aqueous dispersion also has a beneficial effect on the wettability of the aluminum-based anti-corrosion coating. By additionally adding at least one polymer to the aqueous dispersion, improved adhesion of the absorption layer to the anti-corrosion coating can be achieved.

[0092] The above statements in connection with the flat steel product according to the invention apply accordingly to the respective configuration of the surfactant and the polymer.

[0093] The proportion of at least one surfactant in the aqueous dispersion is 0 to 5 wt.% based on the total weight of the aqueous dispersion. A minimum content of 0.1 wt.% is required to achieve the aforementioned beneficial effects. Adding more than 5 wt.% is not economically viable, as an increase in the beneficial effects can no longer be observed.

[0094] The proportion of the at least one polymer in the aqueous dispersion is 0 to 50 wt.% based on the total weight of the aqueous dispersion. A minimum content of 1 wt.% is required to achieve the aforementioned beneficial effects. The addition of more than 50 wt.% has a detrimental effect on the reflectance and drying of the absorption layer obtained from the aqueous dispersion.

[0095] It goes without saying that the proportion of carbon particles as well as the proportion of the optionally present at least one surfactant and the optionally additionally present at least one polymer in the aqueous dispersion can be varied depending on the type of application in order to adjust the desired proportion of carbon particles as well as the optionally present at least one surfactant and the optionally additionally present at least one polymer in the dry absorption layer. For example, it may be practical to use an aqueous dispersion with a significantly lower proportion of carbon particles for immersion, whereas a higher proportion of carbon particles in the aqueous dispersion is required for coating in order to achieve the same desired proportion of carbon particles in the dry absorption layer.

[0096] In the case of immersion according to (i) of step c) of the process according to the invention, the immersion is preferably carried out for a time of 0.5 to 30 s, preferably 1 to 5 s. A longer immersion time has the advantage of ensuring wetting of the flat steel product. However, for industrial production, a shorter immersion time is advantageous in order to make the manufacturing process efficient. The times mentioned have proven to be a good compromise in this regard.

[0097] As previously explained, there is a preferred embodiment in which the flat steel product has an oil film on the anti-corrosive coating. Such an oil film is particularly advantageous in combination with the "spraying an adhering powder" method. Powder particles, which are sprayed on, for example, using compressed air as a carrier, penetrate at least partially into the oil film and thereby adhere to the anti-corrosive coating. This is particularly the case with carbon particles. Carbon particles sprayed on using compressed air as a carrier adhere to the oil film, thus forming an absorption layer.

[0098] However, if an aqueous dispersion containing carbon particles is used, an oil film is rather detrimental. Therefore, the application weight of the oil film in such a case is preferably less than 3 g / m 2 .

[0099] In a preferred development of the method, the flat steel product has a temperature of 40°C to 100°C, preferably 50°C to 80°C, during application of the aqueous dispersion, in particular during immersion, spraying, or coating in the (coil) coating process or during coating by chemical vapor deposition. A higher temperature accelerates the drying of the absorption layer and thus the layer formation when applying the aqueous dispersion. However, at too high a temperature, the aqueous dispersion evaporates too quickly, so that the layer formation is not reliably completed. For chemical vapor deposition, the aforementioned temperature ranges are advantageous in order to accelerate the reactions on the surface.

[0100] In the case of coating according to (iv) of step c) of the method according to the invention by means of chemical vapor deposition (CVD) or physical vapor deposition (PVD), the coating can be carried out in such a way that, for example, the combustion of gas, for example methane, propane, butane or acetylene, takes place on a hot flat steel product surface formed by the aluminum-based corrosion protection coating, or the coating is carried out via liquid feed flame spray pyrolysis, in which a carbon-containing precursor, for example methane, propane, butane or acetylene, is incompletely burned and the carbon particles produced thereby adhere to the surface of the flat steel product, or the coating is carried out from a carbon target, for example graphite or amorphous carbon, by a sputtering PVD process in a vacuum.

[0101] In an alternative preferred variant of the process according to the invention, the flat steel product is subjected to an activation treatment prior to the application of the absorption layer in step c), wherein an adhesion promoter is applied to the aluminum-based anti-corrosive coating. The same polymers that have already been described in detail above with regard to the flat steel product according to the invention can serve as adhesion promoters. The configurations of these polymers described therein are analogously applicable to the process according to the invention. By applying an adhesion-promoting layer to the anti-corrosive coating, particularly good adhesion of the carbon particles contained in the absorption layer can be ensured, even if these are applied via CVD or PVD.

[0102] In a preferred embodiment, a drying step is performed after applying the absorption layer in step c) and before heating the sheet metal blank in step d). This drying step serves to evaporate any solvent residues of the coating solution—i.e., water in the case of an aqueous dispersion. Alternatively, drying takes place immediately during heating in step d).

[0103] In a preferred embodiment of the method, a time period t1 from the completion of step c) to the beginning of step d) is a maximum of 60 s, preferably a maximum of 30 s.

[0104] After applying the absorption layer in step c) and before heating the sheet metal blank in step d), as few additional work steps as possible are performed. In particular, the sheet metal blanks are not stored between steps c) and d). In particular, after step c), the sheet metal blank is moved without interruption by means of transport rollers to a heating unit, which is used to carry out step d). The entire process is therefore preferably carried out "in-line," in which the sheet metal blanks produced in step c) are continuously processed.

[0105] In a preferred embodiment, the absorption layer is applied only to the side facing away from the transport rollers. This further reduces the abrasion of the absorption layer, as the number of mechanical components (e.g., rollers) that come into contact with the absorption layer prior to heating in step d) is reduced.

[0106] In a special preferred embodiment, the application weight of the absorption layer after step c) varies across the sheet metal blank. This makes it possible to deliberately vary the energy input through absorption of infrared radiation across the sheet metal blank. As explained above, it can be advantageous if certain areas of the sheet metal blank are not or at least not fully austenitized when placed in the forming tool. Such areas of the sheet metal blank consist of other microstructure components when placed in the forming tool, such as tempered bainite, tempered martensite and / or non- or partially recrystallized ferrite. During cooling in the forming tool, no or at least less martensite forms in these areas. These areas of the sheet metal part are therefore softer.In this way, a softer area can be specifically set in the respective formed 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.

[0107] In a further preferred embodiment, the sheet metal blank has a first region with a first thickness d 1 and a second region having a second thickness d 2 wherein the second thickness is greater than the first thickness and wherein the application weight of the absorption layer is greater in the second region than in the first region, so that the average reflectance R̅ in the infrared range is smaller in the second region than in the first region.

[0108] This also includes the case where the application weight of the absorption layer in the first region is zero, i.e., the absorption layer is applied in the second region but not in the first region. In such a preferred embodiment, the application weight of the absorption layer in the first region is therefore zero.

[0109] Because the reflectance R̅ is lower in the second region than in the first, more energy is absorbed in the second region than in the first. At the same time, the second region is thicker than the first, so more energy is required to increase the temperature in the second region. The different coating weight or the different reflectance R̅ thus lead to a uniform temperature increase for sheet metal blanks with varying thickness.

[0110] Areas of different thickness of the sheet metal blank (so-called “tailored blanks”) can be created in various ways: - One or more special cold rolling passes, in which individual areas are rolled more intensively or more frequently, result in a lower material thickness and thus a lower thickness in these areas (so-called “tailor rolled blanks”); - Sheet metal blanks of different thicknesses are joined together by welding (typically by laser welding) to create a continuous sheet metal blank with areas of different thicknesses (so-called “tailor welded blanks”); - Using resistance spot welding or laser welding, patches are applied to an existing sheet metal blank to thicken it in certain areas. Alternatively, the patches can extend beyond the existing sheet metal blank or overlap only a fraction of the sheet metal blank and be joined using resistance spot welding or laser welding, resulting in a partial or complete variation of tailor-welded blanks using resistance spot welding or laser welding. Alternatively, the patches can also be applied using structural adhesives.

[0111] In the latter two cases, sheet metal blanks made of different materials can also be used and joined together.

[0112] Areas of varying thickness have the advantage that individual regions of the final sheet metal part can be specifically reinforced or given greater ductility. This makes it possible to design those sections subject to particular stresses (for example, during a crash) with greater stability, while making other sections thinner to reduce the weight of the component. The result is a weight-optimized component with targeted reinforcements in areas subject to high stress. At the same time, more ductile areas of the component absorb energy over a greater distance in a crash, reducing the impact on passengers.

[0113] Areas with different application weights of the absorption layer can be created, for example, by performing several of the steps i. - v. in succession, with certain areas being covered with masks during individual steps. As a result, less absorption layer material is applied to the areas covered in the meantime, resulting in a lower application weight in these areas. Preferably, no absorption layer is applied to the areas covered in the meantime.

[0114] Areas with different application weights of the absorption layer can also be created when spraying with a coating solution or when spraying an adhesive powder by spraying the corresponding areas for a longer time or by increasing the density of the spray points per area.

[0115] The steel substrate consists of a steel containing 0.1–3 wt.% Mn and optionally up to 0.01 wt.% B. 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 a martensitic or at least partially martensitic structure, as this exhibits particularly high hardness.

[0116] Particularly preferably, the steel substrate consists of a steel which, in addition to iron and unavoidable impurities (in wt.%), consists of C: 0.04 - 0.45 wt.%, Si: 0.02 - 1.2 wt.%, Mn: 0.5 - 2.6 wt.%, AI: 0.02 - 1.0 wt.%, P: ≤ 0.05 wt.%, S: ≤ 0.02 wt.%, N: ≤ 0.02 wt.%, Sn: ≤ 0.03 wt.%, Ace: ≤ 0.010 wt.%, Approx: ≤ 0.005 wt.%, and optionally one or more of the elements “Cr, B, Mo, Ni, Cu, Nb, Ti, V, W” in the following contents Cr: 0.08 - 1.0 wt.%, B: 0.001 - 0.010 wt.%, Mon: ≤ 0.5 wt%, No: ≤ 0.5 wt%, Cu: ≤ 0.2 wt.%, Note: 0.01 - 0.2 wt.%, T: 0.008 - 0.10 wt.%, V: ≤ 0.3 wt%, W: 0.001 - 1.00 wt%. consists.

[0117] The elements P, S, N, Sn, As, and Ca are impurities that cannot be completely avoided during steel production. Occasionally, Ca is deliberately added to alloy steel to bind sulfur. In such cases, the Ca content is at least 0.001 wt.%. The maximum Ca content in this case is also 0.005 wt.%.

[0118] In addition to these elements, other elements may also be present as impurities in the steel. These additional elements are summarized under the term "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: ≤ 0.050 wt.%, B: ≤ 0.0005 wt%, Note: ≤ 0.005 wt.%, T: ≤ 0.005 wt%.

[0119] These preferred upper limits should be considered alternatively or jointly. Preferred steel variants therefore meet one or more of these four conditions.

[0120] 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 0.11-0.25 wt.%, or in the range of 0.32-0.37 wt.%.

[0121] Carbon in flat steel products according to the invention delays the formation of ferrite and bainite. At the same time, residual austenite is stabilized and the Ac3 temperature is reduced. A carbon content of at least 0.06 wt.% is advantageous to ensure the hardenability of the flat steel product and the tensile strength of the press-hardened product of at least 1000 MPa. If a higher strength level is desired, C contents of > 0.12 wt.% are preferred. If the C content is further increased to values ​​of at least 0.19 wt.%, hardenability can also be improved, so that the flat steel product exhibits a very good combination of hardenability and strength. However, carbon contents greater than 0.45 wt.% have a detrimental effect on the mechanical properties of the flat steel product, since C contents greater than 0.45 wt.% promote the formation of brittle martensite during press hardening.High carbon contents can also negatively impact weldability. To improve weldability, the carbon content can preferably be adjusted to values ​​below 0.40 wt.%, especially 0.3 wt.%. Especially with carbon contents < 0.25 wt.%, weldability can be significantly improved, and a good ratio of force absorption to maximum bending angle can be achieved in the bending test according to VDA 238-100 in the press-hardened state.

[0122] In a preferred embodiment, the Si content of the steel is a maximum of 1.00 wt% and / or at least 0.06 wt%.

[0123] Silicon is used to further increase the hardenability of the flat steel product and the strength of the press-hardened product through solid solution strengthening. Silicon also enables the use of ferro-silicon-manganese as an alloying agent, which has a beneficial effect on production costs. A hardening effect is already evident at a Si content of 0.06 wt.%. A significant increase in strength occurs at a Si content of at least 0.15 wt.%. Si contents above 0.5 wt.% have a detrimental effect on coating behavior, particularly with Al-based coatings. Si contents of a maximum of 0.4 wt.% are preferred to improve the surface quality of the coated flat steel product.

[0124] 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.%.

[0125] Manganese acts as a hardening element by significantly delaying the formation of ferrite and bainite. At manganese contents below 0.5 wt.%, ferrite and bainite are formed during press hardening, even at very rapid cooling rates, which should be avoided. Mn contents greater than 0.75 wt.%, in particular 0.9 wt.%, are preferred if a martensitic microstructure is to be ensured, particularly in areas of greater deformation. Manganese contents greater than 2.6 wt.% have a detrimental effect on processing properties. In particular, weldability is severely limited, which is why the Mn content of flat steel products according to the invention is limited to a maximum of 2.4 wt.%, in particular to a maximum of 1.6 wt.%. Manganese contents of less than 1.6 wt.% are also preferred for economic reasons.

[0126] 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%.

[0127] Aluminum is used as a deoxidizer to capture oxygen. Aluminum also inhibits cementite formation. To reliably capture oxygen, at least 0.02 wt.% Al is required in the steel. However, since the Ac3 temperature also increases significantly with increasing Al alloy content, the Al content is preferably limited to 0.25 wt.%. Above a content of 0.25 wt.%, Al severely impedes the transformation to austenite prior to press hardening, meaning that austenitization can no longer be carried out in a time- and energy-efficient manner. For typical furnace temperatures between 850 and 950 °C in hot forming, an Al content of no more than 0.1 wt.% is preferred to ensure complete austenitization of the steel.

[0128] 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 (usually 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.%.

[0129] 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.%. Independently 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.%.

[0130] Phosphorus (P) and sulfur (S) are elements that are introduced into steel as impurities through iron ore and cannot be completely eliminated in the large-scale steelmaking process. The P and S contents should be kept as low as possible, since mechanical properties such as impact energy deteriorate with increasing P and S contents. Furthermore, at P contents of 0.1 wt.% and above, increasing embrittlement of the martensite occurs, which is why the P content of a flat steel product according to the invention is limited to a maximum of 0.05 wt.%, preferably a maximum of 0.03 wt.%. The S content of a flat steel product according to the invention is limited to a maximum of 0.02 wt.%, preferably a maximum of 0.012 wt.%.

[0131] Nitrogen (N) is present in small amounts in steel due to the steelmaking process. The N content should be kept as low as possible and should be less than 0.02 wt.%. Nitrogen is particularly harmful to alloys containing boron, as it inhibits the transformation-retarding effect of boron by forming boron nitrides. Therefore, the nitrogen content in this case should be a maximum of 0.010 wt.%, preferably a maximum of 0.009 wt.%.

[0132] The Sn content is a maximum of 0.03 wt.%, preferably a maximum of 0.02 wt.%. The As content is a maximum of 0.010 wt.%, in particular a maximum of 0.005 wt.%.

[0133] Optionally, the steel also contains chromium in a concentration 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.%.

[0134] Chromium is added to the steel of a flat steel product according to the invention in amounts of 0.08 - 1.0 wt.%. Chromium influences the hardenability of the flat steel product by slowing the diffusive transformation during press hardening. In flat steel products according to the invention, chromium has a beneficial effect on hardenability at a content of 0.08 wt.% and above, with a Cr content of > 0.1 wt.% being preferred for reliable process control, especially to prevent bainite formation. If the steel contains more than 1.0 wt.% chromium, the coating behavior deteriorates. To maintain good surface quality, the Cr content can preferably be limited to a maximum of 0.75 wt.%, in particular a maximum of 0.5 wt.%.

[0135] 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.%.

[0136] Preferably, the steel optionally also contains boron in a content of 0.001-0.010 wt.%. In particular, the boron content is a maximum of 0.005 wt.%, preferably a maximum of 0.004 wt.%.

[0137] Boron can optionally be added to the alloy to improve the hardenability of the steel flat product. Boron atoms or boron precipitates deposited on the austenite grain boundaries reduce the grain boundary energy, thereby suppressing ferrite nucleation during press hardening. A significant effect on hardenability occurs at B contents of at least 0.001 wt.%. At B contents above 0.010 wt.%, however, increased formation of boron carbides, boron nitrides, or boron nitrocarbides occurs, which in turn represent preferred nucleation sites for ferrite nucleation and reduce the hardening effect. For this reason, the B boron content is limited to a maximum of 0.010 wt.%.

[0138] When adding boron, titanium is preferably also added to bind nitrogen. In this case, the Ti content should preferably be at least 3.42 times the nitrogen content in wt.%.

[0139] Optionally, the steel may contain molybdenum with a maximum content of 0.5 wt%, in particular a maximum of 0.1 wt%.

[0140] Molybdenum (Mo) can optionally be added to improve process stability, as it significantly slows ferrite formation. Starting at concentrations of 0.002 wt.%, dynamic molybdenum-carbon clusters, including ultrafine molybdenum carbides, form at the grain boundaries. These clusters significantly slow grain boundary mobility and thus diffusive phase transformations. Furthermore, molybdenum reduces grain boundary energy, which slows the nucleation rate of ferrite. Due to the high costs associated with a molybdenum alloy, the Mo content should be no more than 0.5 wt.%, preferably no more than 0.3 wt.%.

[0141] Optionally, the steel may also contain copper with a maximum content of 0.2 wt.%, preferably a maximum of 0.15 wt.%.

[0142] Copper (Cu) can be optionally added to the alloy to increase hardenability at a minimum addition of 0.01 wt.%. Furthermore, copper improves the resistance to atmospheric corrosion of uncoated sheets or cut edges.

[0143] Furthermore, the steel may optionally contain nickel with a content of maximum 0.5 wt.%, preferably maximum 0.15 wt.%.

[0144] Nickel (Ni) stabilizes the austenitic phase and can optionally be added to the alloy to reduce the Ac3 temperature and suppress the formation of ferrite and bainite. Nickel also has a positive influence on hot rollability, especially when the steel contains copper. Copper impairs hot rollability. To counteract the negative influence of copper on hot rollability, at least 0.01 wt.% nickel can be added to the steel. For economic reasons, the nickel content should be limited to a maximum of 0.5 wt.%, preferably a maximum of 0.4 wt.%.

[0145] 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 0.01 wt.%, in particular at least 0.02 wt.% and at most 0.2 wt.%, preferably at most 0.08 wt.%, preferably at most 0.04 wt.%. The optional Ti content is at least 0.01 wt.% and at most 0.10 wt.%, in particular at most 0.08 wt.%, preferably at most 0.04 wt.%. The optional V content is at most 0.3 wt.%, preferably at most 0.2 wt.%, in particular at most 0.1 wt.%, preferably at most 0.05 wt.%.

[0146] Niobium (Nb) can optionally be added to the alloy to contribute to grain refinement at a content of 0.01 wt.% and above, particularly at 0.010 wt.%. However, niobium impairs the recrystallizability of the steel. At an Nb content above 0.1 wt.%, the steel can no longer be recrystallized in conventional continuous furnaces prior to hot-dip coating.

[0147] Titanium (Ti) is a microalloying element that can optionally be added to contribute to grain refinement. Furthermore, titanium forms coarse titanium nitrides with nitrogen, which is why the Ti content should be kept comparatively low. Titanium binds nitrogen, allowing boron to exert its strong ferrite-inhibiting effect. Adequate nitrogen binding requires at least 3.42 times the nitrogen content, with at least 0.008 wt.% Ti being added to ensure sufficient availability. The titanium content is preferably at least 0.010 wt.%, more preferably at least 0.015 wt. Above 0.1 wt.% Ti, cold rollability and recrystallizability deteriorate significantly, which is why higher Ti contents should be avoided.

[0148] Vanadium (V) has a high affinity for carbon. When vanadium is free, i.e., in an unbound or dissolved state, it can bind supersaturated dissolved carbon in the form of carbides or clusters, or at least reduce its diffusion rate. The decisive factor here is that V is present in a dissolved state. Surprisingly, very low V contents have proven particularly favorable for aging resistance. At higher V contents, larger vanadium carbides can precipitate even at higher temperatures, which then no longer dissolve at temperatures of 800–900 °C, which are typical for continuous annealing in hot-dip coating systems. Even the smallest amounts of vanadium, as small as 0.001 wt.%, can inhibit the attachment of free carbon to dislocations. Above a V content of 0.2 wt.%, vanadium no longer improves aging resistance.The anti-aging effect of vanadium is particularly pronounced at V contents of up to 0.009 wt.%, with a maximum effect occurring at a preferred V content of 0.002 wt.%. At V contents greater than 0.009 wt.%, vanadium carbides form more frequently. Vanadium carbides cannot be dissolved at a vanadium content in steel of 0.009 wt.% or more at temperatures of 860°C, which are typical annealing temperatures in a hot-dip coating plant, for example. The vanadium content of the steel of a flat steel product according to the invention is limited to a maximum of 0.1 wt.% for cost reasons, on the one hand. On the other hand, higher V contents do not result in a significant improvement in the mechanical properties.

[0149] 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.%.

[0150] Tungsten (W) can optionally be added to the alloy in concentrations of 0.001–1.0 wt.% to slow ferrite formation. A positive effect on hardenability is already achieved at W contents of at least 0.001 wt.%. For cost reasons, a maximum of 1.0 wt.% tungsten is added.

[0151] Implementation of the above-described method according to the invention results in a sheet metal blank for producing a shaped sheet metal part by hot forming, comprising a steel substrate consisting of a steel containing 0.1-3 wt.% Mn and optionally up to 0.01 wt.% B, and an aluminum-based anti-corrosive coating applied to the steel substrate, wherein an absorption layer is arranged on the anti-corrosive coating, characterized in that the absorption layer comprises carbon particles. Such a sheet metal blank is produced according to the work steps a) - c) described above. Specifically, such a sheet metal blank is produced in particular according to one of the preferred methods of work steps a) - c), such as, for example, by means of one of the described aqueous solutions and / or one of the described application methods.

[0152] The preferred embodiments previously explained in connection with the manufacturing process apply accordingly to the sheet metal blank. This includes, in particular, the preferred configurations of the corrosion protection coating and the absorption layer, such as the proportion of carbon particles in the dry absorption layer.

[0153] In a special design of the sheet metal blank, the absorption layer is arranged on one side of the blank. This has the advantage that the blank can be moved using transport rollers without the risk of damaging the absorption layer.

[0154] In a further preferred embodiment, the sheet metal blank has a first region with a first thickness d 1 and a second region having a second thickness d 2wherein the second thickness is greater than the first thickness and wherein the coating weight of the absorption layer is greater in the second region than in the first region, so that the average reflectance R̅ in the infrared range is smaller in the second region than in the first region. The advantages of this embodiment and its preferred variants have already been explained with regard to the method.

[0155] In a further preferred embodiment of the sheet metal blank, the sheet metal blank is straightened. The sheet metal blank is thus obtained, for example, by uncoiling and straightening a strip-shaped flat steel product in a further processing plant.

[0156] In the following, the invention is explained in more detail using exemplary embodiments.

[0157] The figures show: Fig. 1 Reflectance as a function of wavelength for Comparative Example V and for Inventive Example 7; Fig.2 Heating diagram of Comparative Example V and Inventive Example 7; Fig. 3 Reflectance as a function of the coating weight of the absorption layer; Fig. 4 Heating rate as a function of the application weight of the absorption layer.

[0158] To demonstrate the effect of the invention, Examples 1 to 7 according to the invention and a Comparative Example V were carried out. For this purpose, a strip-shaped flat steel product with a thickness of 1.5 mm and a steel composition according to Table 1 was coated with an aluminum-based anti-corrosive coating by hot-dip coating. After coating, the strip was wound into a coil.

[0159] After a certain period of storage, the steel coil was uncoiled and straightened. From the thus straightened strip-shaped flat steel product, sheet blanks measuring 100 x 200 mm were cut.

[0160] The melt analysis of the corrosion protection coating is shown in Table 2. The resulting corrosion protection coating had an Al base layer whose composition corresponded to the melt analysis. The one-sided thickness of the corrosion protection coating was 25 µm.

[0161] The sheet metal blanks thus prepared were treated with an aqueous dispersion comprising carbon particles to create an absorption layer on the corrosion protection coating (inventive examples 1 to 7). The respective composition of the aqueous dispersion is shown in Table 3.

[0162] Table 3 further provides details of the treatment method. These include the application method, the pH value of the aqueous dispersion, the application time, and the temperature of the steel blanks during treatment. The resulting properties, such as the application weight of the absorption layer and the layer thickness of the absorption layer after drying, the average reflectance in the infrared range, and the heating rate, are also listed in Table 3. In tests 1-7, the application time corresponds to an immersion time. In tests 8 and 9, the aqueous dispersion is applied with a coating roller, and in tests 10 and 11, the aqueous dispersion is sprayed on. In these cases, specifying the application time is therefore meaningless. In test 10, the aqueous dispersion was only sprayed on one side, so the absorption layer was only formed on one side.

[0163] It can be seen from Table 3 that the inventive examples 1 to 11 have significantly lower mean reflectances in the infrared range compared to Comparative Example V (see also Fig. 3). Furthermore, the heating rate for Examples 1 to 11 according to the invention is significantly higher than for Comparative Example V (see also Fig. 4). The average reflectance decreases with increasing application weight or increasing layer thickness of the dry absorption layer, considered for the use of an aqueous dispersion with the same proportion of carbon particles, while the heating rate increases accordingly (see comparison of inventive examples 3 to 7). A comparison of inventive examples 6 and 7 shows that, above a certain application weight or a certain layer thickness of the absorption layer, saturation of the inventive effect occurs (see also Fig. 3 and Fig. 4).

[0164] The resulting sheet metal blanks were then processed into a sheet metal part by hot forming. For this purpose, the blanks were heated in a roller hearth furnace at room temperature with an average heating rate r Ofen (between 30 °C and 700 °C) with a furnace temperature of 920 °C. The average heating rate is given in Table 3.

[0165] The blanks were then further processed conventionally. For this purpose, the blanks were removed from the roller hearth furnace and placed in a forming tool. Upon removal from the furnace, the blanks had reached the furnace temperature. The transfer time, consisting of removal from the heating device, transport to the tool, and insertion into the tool, was approximately 10 seconds. The temperature of the blanks upon insertion into the forming tool was above the respective AC1 temperature in all cases and thus also above Ms + 100 °C.

[0166] The blanks were formed into the respective sheet metal parts in the forming tool, which was tempered to room temperature. The sheet metal parts were cooled in the tool at a cooling rate of approximately 50 K / s for approximately 15 s. Finally, the samples were removed from the tool and cooled to room temperature. Cooling took place in still air at a cooling rate of 7 K / s. Table 2: Melt analysis Si Fe Mg Other A l 9,5 3 0,3 < 1% rest

[0167] Melt analysis data in wt.%. Table 3: Attempt Layer thickness (µm) Coating weight [g / m 2 ] Proportion of carbon particles in dispersion [wt.%] Proportion of carbon particles in dry absorption layer [wt.%] Proportion of surfactant in dispersion [wt.%] Proportion of polymer in dispersion [wt.%] Application method Application time [s] Temperature of the flat steel product [°C] PH value Heating rate between 30 and 700°C [Kmm / s] Average reflectance R̅ V - - - - - - - - - - 12,3 0,68 1 0,33 0,6 4 99,3 0,03 - Immerse yourself 2 25 8 27,6 0,2 2 0,43 0,78 4 99,3 0,03 - Immerse yourself 5 25 8 28,65 0,14 3 0,067 0,12 20 92,4 0,15 1,5 Coil coating < 2 30 9 15,9 0,43 4 0,17 0,3 20 92,4 0,15 1,5 Coil coating < 2 30 9 22,14 0,3 5 0,39 0,7 20 92,4 0,15 1,5 Coil coating < 2 30 9 30,15 0,17 6 0,94 1,7 20 92,4 0,15 1,5 Coil coating < 2 30 9 31,65 0,12 7 2,06 3,7 20 92,4 0,15 1,5 Coil coating < 2 30 9 31,95 0,11 8 0,5 0,9 10 95 0,05 - coating roll - Ambient temperature 8 30,5 0,14 9 0,40 0,7 10 95 0,05 - coating roll - Ambient temperature 8 28,9 0,17 10 0,38 0,35 10 95 0,05 - Spraying (one-sided) - Ambient temperature 8 24,6 0,18 11 0,4 0,81 10 95 0,05 - Spray on - Ambient temperature 8 29,5 0,15

Claims

[1] Method for producing a sheet metal part comprising the following steps: a) providing a steel coil from a strip-shaped flat steel product consisting of a steel substrate consisting of a steel having 0.1 - 3 wt% Mn and optionally up to 0.01 wt% B; b) uncoiling and straightening of the strip-shaped flat steel product; c) performing one of steps c1 or c2, wherein c1) separating at least one sheet blank from the strip-shaped flat steel product and applying an absorption layer to at least one side of the sheet blank; c2) applying an absorption layer to at least one side of the strip-shaped flat steel product and separating at least one sheet blank from the strip-shaped flat steel product; d) 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 Einlgthe blank, when placed in a forming tool intended for hot press forming (working step e)), at least partially has a temperature above Ms + 100 °C, where Ms denotes the martensite start temperature; e) Inserting the heated sheet metal blank into a forming tool, whereby the transfer time t required for removing the blank from the heating device and inserting it Trans at most 20 s, preferably at most 15 s; f) hot-press forming the sheet metal blank to form the sheet metal part, wherein the blank is subjected to a hot-press forming process for a period of time t WZ of more than 1 s to a target temperature T Ziel cooled and optionally kept there, whereby the cooling depends on the temperature T Einlg at least up to the martensite start temperature with a cooling rate r of at least partially more than 25 K / s wz occurs; g) Remove the sample to the target temperature TZiel cooled sheet metal part from the tool. [2] Method according to claim 1, characterized by that the flat steel product in step a) has a corrosion protection coating on at least one side, wherein the corrosion protection coating represents the coating of the flat steel product closest to the surface. [3] Method according to one of claims 1 to 2, characterized by that the flat steel product has a corrosion protection coating on at least one side, wherein the corrosion protection coating is in particular an aluminum-based corrosion protection coating and preferably has an alloy layer and an Al base layer. [4] Method according to one of claims 1 to 3, characterized by that the absorption layer comprises carbon particles, in particular selected from the group consisting of graphite, fullerenes, graphene, carbon nanotubes and mixtures thereof. [5] Method according to one of claims 1 to 4, characterized bythat the absorption layer has an average reflectance R in the infrared range which is less than 0.55, preferably less than 0.50, in particular less than 0.40, preferably less than 0.

35. [6] Method according to one of claims 1 to 5, characterized by that the application of the absorption layer in step c) comprises at least one of the following steps: i. Immersion in a coating solution or ii. Spraying with a coating solution or iii. Coating with a coating solution using the coil coating process or iv. Depositing the absorption coating by chemical or physical vapor deposition or v. Spraying an adhesive powder. [7] Method according to one of claims 1 to 6, characterized bythat a drying step is carried out after the application of the absorption layer in step c) and before the heating of the sheet metal blank in step d). [8] Method according to one of claims 1 to 7, characterized by that a time period t1 from the completion of step c) to the beginning of step d) is a maximum of 60 s, preferably a maximum of 30 s. [9] Method according to one of claims 1 to 8, characterized by that the sheet metal blank is moved after step c) by means of transport rollers without interruption to a heating unit with which step d) is carried out. [10] Method according to claim 9, characterized by that the absorption layer is only applied on the side facing away from the transport rollers. [11] Method according to one of claims 1 to 10, characterized by that an application weight of the absorption layer after step c) varies over the sheet metal blank. [12] Method according to claim 11, characterized bythat the sheet metal blank has a first area with a first thickness d 1 and a second region having a second thickness d 2 wherein the second thickness is greater than the first thickness and wherein the application weight of the absorption layer is greater in the second region than in the first region, so that the average reflectance R̅ in the infrared range is smaller in the second region than in the first region. [13] Method according to one of claims 1 to 12, characterized by that the steel, in addition to iron and unavoidable impurities (in wt%), consists of C: 0.04 - 0.45 wt.% Si: 0.02 - 1.2 wt.%, Mn: 0.5 - 2.6 wt.%, AI: 0.02 - 1.0 wt.%, P: ≤ 0.05 wt.%, S: ≤ 0.02 wt.%, N: ≤ 0.02 wt.%, Sn: ≤ 0.03 wt.%, Ace: ≤ 0.010 wt.%, Approx: ≤ 0.005 wt.%, and optionally one or more of the elements “Cr, B, Mo, Ni, Cu, Nb, Ti, V, W” in the following contents Cr: 0.08 - 1.0 wt.%, B: 0.001 - 0.010 wt.%, Mon: ≤ 0.5 wt.%, No: ≤ 0.5 wt.%, Cu: ≤ 0.2 wt.%, Note: 0.01 - 0.2 wt.%, T: 0.008 - 0.10 wt.%, V: ≤ 0.3 wt%, W: 0.001 - 1.00 wt.%, consists.

Images