Flat steel product for producing a steel component by hot forming, method for the production thereof and method for producing the steel component

Abstract

The invention relates to a flat steel product for producing a steel component by hot forming, comprising a steel substrate, which consists of a steel having 0.1-3 wt.% Mn and optionally up to 0.01 wt.% B, and an aluminium-based anticorrosion coating deposited on the steel substrate, wherein an absorption layer comprising carbon particles is arranged on the anticorrosion coating. The invention further relates to a method for producing a flat steel product according to the invention, and to the use of carbon particles in an absorption layer on a flat steel product coated with an aluminium-based anticorrosion coating, in order to reduce the reflectivity in the infrared region. The invention also relates to a method for producing a sheet metal part from a flat steel product according to the invention.

Description

[0001] Flat steel product for the production of a steel component by hot forming, process for its production and process for the production of the steel component

[0002] The invention relates to a flat steel product for producing a steel component by hot forming, a method for producing such a flat steel product, a method for producing a sheet metal part from such a flat steel product and the use of carbon particles in an absorption layer on a flat steel product coated with an aluminum-based corrosion protection coating for reducing reflectivity in the infrared range.

[0003] The term "flat steel product" refers to all rolled products whose length is many times greater than their thickness. This includes steel strips and sheets, as well as blanks and plates obtained from them.

[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 generally 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 steel component.

[0005] The heating of the steel flat 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 reflected by the smooth and reflective surfaces of the metallic corrosion protection coating applied to the steel flat product. This leads to a significant delay in the heating process, resulting in increased time and energy consumption.

[0006] From the perspective of optimal energy utilization, it would therefore be desirable to achieve the most effective 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 forming temperature would result in the corresponding roller hearth furnaces being able to be shortened, 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 are proposed that contain a metallic compound from the group of oxide, nitride, sulfide, sulfate, carbide, carbonate, fluoride, hydrate, hydroxide, or phosphate compounds. The disadvantage of these coatings is that they negatively impact product properties, such as scaling protection, corrosion susceptibility, weldability, and / or paintability.

[0008] Against this background, the object underlying the invention was to provide a flat steel product that can be heated to the initial temperature required for hot forming within shorter heating times. Furthermore, a method allowing the production of such a flat steel product and a method for producing a sheet metal part made from such a flat steel product were to be specified.

[0009] This object is achieved by a steel flat product for producing a steel component by hot forming, comprising a steel substrate consisting of a steel having 0.1-3 wt.% Mn and optionally up to 0.01 wt.% B, and an aluminum-based corrosion protection coating applied to the steel substrate, wherein an absorption layer comprising carbon particles is arranged on the corrosion protection coating.

[0010] 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 bonded 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 pm. The hydrodynamic diameter of the particles is determined using dynamic light scattering (DLS).

[0011] The carbon particles are preferably those selected from the group consisting of graphite, fullerenes, graphene, carbon nanotubes and mixtures thereof.

[0012] 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 interior of the furnace 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.

[0013] 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 pm.

[0014] In addition, a well-adhering 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, or coating it using a coil coating process or chemical or physical vapor deposition (CVD or PVD). 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.

[0015] 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.

[0016] 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 in absorption, while at application 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.

[0017] 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.

[0018] For the purposes of this application, the reflectance R 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 z (T) of the blackbody radiator at temperature T is multiplied by the measured spectral reflectivity p . and integrated over the wavelength range. This integral is standardized to the spectral radiant power integrated over the same wavelength range. Therefore,

[0019] This results in i z (T)from Planck's radiation law with the speed of light c, Planck's constant h and Boltzmann's constant kB. The integration is carried out over the wavelength range corresponding to the wavenumbers 667 - 10000 cm -1, ie from Xi= 1 pm to Ä2= 15 pm. The reflectance R used below is defined as R (920°C).

[0020] According to the invention, 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.%.

[0021] The further design of the carbon particles in the absorption layer has already been described above.

[0022] According to a preferred embodiment of the flat steel product 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.

[0023] Suitable surfactants include, for example, anionic, cationic, zwitterionic, and nonionic surfactants, as well as mixtures thereof. 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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 can 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.

[0029] Suitable polymers include polyalkylene glycols and their mixtures.

[0030] 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 weights are in the range of 400 to 5000 g / mol.

[0031] 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.

[0032] The aluminum-based anti-corrosive coating can be applied to one or both sides of the flat steel product. "Aluminum-based anti-corrosive coating," as used here, means that the anti-corrosive coating consists of more than 50% aluminum by weight.

[0033] 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 up to 15 wt.% Si, preferably more than 1 wt.%, in particular more than 1.0 wt.% Si, optionally 2 to 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.%, and the remainder being aluminum. In a preferred variant of the flat steel product according to the invention, the Si content of the melt is 1-3.5 wt.% or 7-12 wt.%, in particular 8-10 wt.%.

[0034] In a further preferred variant of the flat steel product according to the invention, 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.%. Furthermore, the optional content of alkali or alkaline earth metals in the melt can comprise in particular at least 0.0015 wt.% Ca, preferably at least 0.01 wt.% Ca.

[0035] 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 aluminum base layer (Al base layer) upon solidification.

[0036] 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 remaining elements from the steel substrate or the composition of the melt do not accumulate significantly in the alloy layer. The alloy layer preferably consists of 35-60 wt.% Fe, preferably α-iron, optional further constituents whose total contents are limited to a maximum of 5.0 wt.%, preferably 2.0 wt.%, and the remainder aluminum, with the Al content preferably increasing towards the surface. The optional further constituents include in particular the remaining constituents of the melt (i.e., silicon and optionally alkali or alkaline earth metals, in particular Mg or Ca) and the remaining portions of the steel substrate in addition to iron.

[0037] The Al base layer lies on top of the alloy layer and directly adjoins it. The composition of the Al base layer preferably corresponds to the composition of the melt of the molten bath. This means that it consists of 1-15 wt.%, in particular 1.0-15 wt.%, Si, optionally 2-4 wt.% Fe, optionally

[0038] 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.

[0039] 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 comprise in particular at least 0.0015 wt.% Ca, in particular at least 0.1 wt.% Ca.

[0040] 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.

[0041] The corrosion protection coating preferably has a thickness of 5 - 60 pm, in particular 10 - 40 pm. The coating weight of the corrosion protection coating is in particular 30 - 360 g / m 2 with corrosion protection coatings on both sides or 15 - 180 g / m 2 in the one-sided variant. The preferred coating weight of the corrosion protection coating is 100 - 200 g / m 2for double-sided coatings or 50 - 100 g / m 2 for one-sided coatings. The coating weight of the corrosion protection coating is particularly preferably 120 - 180 g / m 2 for double-sided coatings or 60 - 90 g / m 2 for one-sided covers.

[0042] The thickness of the alloy layer is preferably less than 20 pm, particularly preferably less than 16 pm, particularly preferably less than 12 pm, in particular less than 10 pm. The thickness of the Al base layer results from the difference between the thicknesses of the corrosion protection coating and the alloy layer. The thickness of the Al base layer is preferably at least 1 pm, even with thin corrosion protection coatings. In a preferred variant of the flat steel product, the average reflectance R in the infrared range 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 steel component.

[0043] The invention also relates to the use of carbon particles in an absorption layer on a flat steel product coated with an aluminum-based anti-corrosive coating to reduce reflectivity in the infrared range. This use offers the same advantages as explained above with respect to the flat steel product. Furthermore, the invention also relates to the use of the aforementioned specially developed absorption layers and, in particular, to the use of a mixture of carbon particles with at least one surfactant and / or polymer in an absorption layer on an anti-corrosive coating containing 0.1-1.0 wt.% Mg in the Al base layer.

[0044] The steel substrate is made 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 by hot forming. The structure of the steel substrate of the steel component is therefore preferably a martensitic or at least partially martensitic structure, as this exhibits particularly high hardness.

[0045] Particularly preferably, the steel substrate is a steel which, in addition to iron and unavoidable impurities (in wt%), consists of

[0046] C: 0.04 - 0.45 wt%,

[0047] Si: 0.02 - 1.2 wt.%,

[0048] Mn: 0.5 - 2.6% by weight, Al: 0.02 - 1.0% by weight,

[0049] P: < 0.05 wt%,

[0050] S: < 0.02 wt%,

[0051] N: < 0.02 wt%,

[0052] Sn: < 0.03 wt%

[0053] As: < 0.01 wt%

[0054] Ca: < 0.005 wt.% and optionally one or more of the elements "Cr, B, Mo, Ni, Cu, Nb, Ti, V" in the following contents

[0055] Cr: 0.08 - 1.0 wt%,

[0056] B: 0.001 - 0.005 wt.%

[0057] Mo: <0.5 wt%

[0058] Ni: <0.5 wt%

[0059] Cu: <0.2 wt%

[0060] Nb: 0.02 - 0.08 wt.%,

[0061] Ti: 0.01 - 0.08 wt%

[0062] V: <0.1 wt%.

[0063] 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 elements, which may be present as impurities in the steel in addition to the elements P, S, N, Sn, As, and Ca, 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 in the steel substrate as unavoidable impurities in amounts below the respective lower limit. In this case, they are also counted as unavoidable impurities, whose total content is limited to a maximum of 0.2 wt.%, preferably a maximum of 0.1 wt.%.The preferred individual upper limits for the respective contamination of these elements are as follows:.

[0064] Cr: < 0.050 wt%,

[0065] B: < 0.0005 wt%

[0066] Nb: < 0.005 wt%,

[0067] Ti: < 0.005 wt%

[0068] These preferred upper limits should be considered alternatively or jointly. Preferred steel variants therefore meet one or more of these four conditions.

[0069] In a preferred embodiment, the C content of the steel is a maximum of 0.37 wt.% and / or at least 0.06 wt.%. In particularly preferred embodiments, the C content is in the range of 0.06-0.09 wt.%, or in the range of 0.12-0.25 wt.%, or in the range of 0.33-0.37 wt.%.

[0070] In a preferred embodiment, the Si content of the steel is a maximum of 1.00 wt% and / or at least 0.06 wt%.

[0071] 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.%. 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%.

[0072] It has also been shown that it can be helpful to limit the sum of the silicon and aluminum contents. In a preferred variant, the sum of the Si and Al contents (commonly referred to as Si+Al) is therefore a maximum of 1.5 wt.%, preferably a maximum of 1.2 wt.%. Additionally or alternatively, the sum of the Si and Al contents is at least 0.06 wt.%, preferably at least 0.08 wt.%.

[0073] The elements P, S, and N are typical impurities that cannot be completely avoided during steel production. In preferred variants, the P content is a maximum of 0.03 wt.%. Irrespective of this, the S content is preferably a maximum of 0.012%. Additionally or supplementarily, the N content is preferably a maximum of 0.009 wt.%.

[0074] Optionally, the steel also contains chromium at a content of 0.08–1.0 wt.%. The Cr content is preferably a maximum of 0.75 wt.%, in particular a maximum of 0.5 wt.%.

[0075] 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.%.

[0076] Preferably, the steel optionally also contains boron in a content of 0.001–0.005 wt.%. In particular, the boron content is a maximum of 0.004 wt.%.

[0077] Optionally, the steel may contain molybdenum at a maximum content of 0.5 wt.%, in particular at a maximum content of 0.1 wt.%. Furthermore, the steel may optionally contain nickel at a maximum content of 0.5 wt.%, preferably at a maximum content of 0.15 wt.%.

[0078] Optionally, the steel may also contain copper with a content of maximum 0.2 wt.%, preferably maximum 0.15 wt.%.

[0079] In addition, the steel can optionally contain one or more of the microalloying elements Nb, Ti, and V. The optional Nb content is at least 0.02 wt.% and a maximum of 0.08 wt.%, preferably a maximum of 0.04 wt.%. The optional Ti content is at least 0.01 wt.% and a maximum of 0.08 wt.%, preferably a maximum of 0.04 wt.%. The optional V content is a maximum of 0.1 wt.%, preferably a maximum of 0.05 wt.%.

[0080] 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.%.

[0081] The above explanations regarding preferred steel substrates naturally also apply to the steel substrates in the manufacturing processes according to the invention described below.

[0082] The invention further relates to a method for producing a flat steel product according to the invention, comprising at least the following work steps: a) providing a flat steel product 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, b) applying an absorption layer comprising carbon particles to the flat steel product, in particular by

[0083] (i) immersing the flat steel product in an aqueous dispersion comprising carbon particles or

[0084] (ii) spraying the flat steel product with an aqueous dispersion comprising carbon particles or

[0085] (iii) coating the flat steel product with an aqueous dispersion comprising carbon particles by coil coating or by chemical or physical vapor deposition.

[0086] Through this treatment, the aqueous dispersion with carbon particles is evenly distributed over the entire surface, so that a homogeneous, area-covering absorption layer comprising carbon particles is formed.

[0087] Alternatively, the homogeneous, area-covering absorption layer is formed by chemical or physical vapor deposition.

[0088] 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, and particularly preferably at most 10. This ensures good wettability of the aluminum-based anti-corrosive 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.

[0089] 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.

[0090] 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.

[0091] 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.

[0092] 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 feasible, as an increase in the beneficial effects can no longer be observed.

[0093] 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 advantageous 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.

[0094] It goes without saying that the proportion of carbon particles as well as the proportion of any at least one surfactant present and any additionally present at least one polymer in the aqueous dispersion can be varied depending on the type of application in order to achieve a desired proportion of carbon particles as well as any at least one surfactant present and any 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, while 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.

[0095] In the case of immersion according to (i) of step b) 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.

[0096] In a preferred development of the process, 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 accelerating the reactions on the surface.

[0097] In the case of coating according to (iii) of step b) 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 combusted 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.

[0098] In an alternative preferred variant of the process according to the invention, the flat steel product is subjected to an activation treatment prior to step b), 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.

[0099] The invention further relates to a method for producing a shaped sheet metal part, comprising the following work steps: a) providing a sheet metal blank from a flat steel product according to the invention; b) heating the sheet metal blank such that the AC3 temperature of the blank is at least partially exceeded and the temperature TEinig of the blank when placed in a forming tool provided for hot press forming (work step c)) at least partially has a temperature above Ms+100°C, where Ms denotes the martensite start temperature, wherein the average heating rate is greater than 15 Kmm / s; c) placing the heated sheet metal blank in a forming tool, wherein the transfer time trtrans required for removing the blank from the heating device and placing it in is at most 20 s, preferably at most 15 s;d) hot-press forming the sheet metal blank into the formed sheet metal part, wherein the blank is cooled during the hot-press forming process over a period twz of more than 1 s at a cooling rate rwz of at least partially more than 30 K / s to the target temperature Tziei and optionally held there; e) removing the sheet metal part cooled to the target temperature Tziei from the tool.

[0100] In the method according to the invention, a blank consisting of a previously explained flat steel product according to the invention is thus provided (step a)). This blank is heated at an average heating rate greater than 15 mm / s such that the AC3 temperature of the blank is at least partially exceeded and the temperature TEinig of the blank when inserted into a forming tool intended for hot press forming (step c)) is at least partially above Ms+100°C.

[0101] 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 a) 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.

[0102] 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 created in 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.

[0103] 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.

[0104] The minimum temperature AC 3 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.

[0105] AC3 = (902 - 225*%C + 19*%Si - 11*%Mn - 5*%Cr + 13*%Mo - 20*%Ni + 55*%V) °C with %C = respective C content, %Si = respective Si content, %Mn = respective Mn content, %Cr = respective Cr content, %Mo = respective Mo content, %Ni ^respective Ni content and %V = respective V content of the steel from which the blank is made.

[0106] An optimally uniform distribution of properties can be achieved by heating the blank completely in step b).

[0107] In a preferred embodiment, the heating takes place in a furnace with a furnace 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.

[0108] The dew point in the oven is preferably at least -20°C, preferably at least -15°C, in particular at least -5°C, preferably at least 0°C, particularly preferably at least +5°C and at most +25°C, preferably at most +20°C, in particular at most +15°C.

[0109] In a specific embodiment of the process according to the invention, the heating in step b) 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 at a temperature (so-called furnace inlet temperature) of at least 650°C, preferably at least 680°C, in particular at least 720°C. The maximum temperature in the first heating zone is preferably 900°C, in particular a maximum of 850°C. Furthermore, the maximum temperature of all heating zones in the furnace is preferably a maximum of 1200°C, in particular a maximum of 1000°C, preferably a maximum of 950°C, particularly preferably a maximum of 930°C.

[0110] The total furnace time, which consists of a heating time and a holding time, is preferably at least 1 minute, in particular at least 2 minutes, and preferably at least 3 minutes for both variants (constant furnace temperature, stepwise heating). Furthermore, the total furnace time for both variants is preferably a maximum of 12 minutes, in particular a maximum of 10 minutes, preferably a maximum of 8 minutes, and in particular a maximum of 6 minutes. Longer total furnace times 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 impact on the mechanical properties.

[0111] The heated blank is removed from the respective heating device 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, and particularly preferably above 700°C. Ms denotes the martensite start temperature. In a particularly preferred variant, the temperature is at least partially above the ACl temperature. In all of these variants, the maximum temperature is, in particular, 900°C. These temperature ranges ensure good formability of the material overall.

[0112] In step c), the transfer of the austenitized blank from the heating device used to the forming tool is completed within preferably a maximum of 20 seconds, in particular within a maximum of 15 seconds. Such rapid transport is necessary to avoid excessive cooling prior to forming.

[0113] When the blank is inserted, the tool typically has a temperature between room temperature (RT) and 200°C, preferably between 20°C and 180°C, in particular between 50°C and 150°C. Optionally, in a particular embodiment, the tool can be tempered at least partially to a temperature Twz of at least 200°C, in particular at least 300°C, in order to only partially harden the component. Furthermore, the tool temperature Twz is preferably a maximum of 600°C, in particular a maximum of 550°C. It only needs to be ensured that the tool temperature Twz is below the desired target temperature Tziei. The residence time in the tool twz is preferably at least 2s, in particular at least 3s, particularly preferably at least 5s. The maximum residence time in the tool is preferably 25s, in particular a maximum of 20s.

[0114] The target temperature Tziei 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 Tziei 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.

[0115] The martensite start temperature of a steel within the scope of the invention is according to the formula:

[0116] 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.%], 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.%.

[0117] The ACl temperature and the AC3 temperature of a steel within the scope of the invention specifications are according to the formulas:

[0118] AC1[°C] = (739 — 22*%C - 7*%Mn + 2*%Si + 14*%Cr + 13*%Mo - 13*%Ni +20*%V )[°C / wt.%] AC3[°C] = (902 - 225*%C + 19*%Si - 11*%Mn - 5*%Cr + 13*%Mo - 20*%Ni +55*%V)[°C / wt.%], 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).

[0119] In the tool, the blank is not only formed into the sheet metal part, but is also simultaneously quenched to the target temperature. The cooling rate in the tool (rwz) to the target temperature is in particular at least 20 K / s, preferably at least 30 K / s, in particular at least 50 K / s, and particularly preferably at least 100 K / s.

[0120] After removal of the sheet metal part in step e), the sheet metal part is cooled to a cooling temperature TAB of less than 50°C within a cooling time tAB of 0.5 to 600 s. This is usually done by air cooling.

[0121] In the following, the invention is explained in more detail using exemplary embodiments.

[0122] The figures show:

[0123] Figure 1 Reflectance as a function of wavelength for Comparative Example V and for Example 7 according to the invention;

[0124] Figure 2 Heating diagram of Comparative Example V and Example 7 according to the invention;

[0125] Figure 3 Reflectance as a function of the applied weight of the absorption layer; Figure 4 Heating rate as a function of the applied weight of the absorption layer.

[0126] 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, steel blanks measuring 100x200 mm and having a thickness of 1.5 mm with a steel composition according to Table 1 were coated with an aluminum-based anti-corrosive coating by hot-dip coating.

[0127] 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 pm.

[0128] The steel blanks thus prepared were treated with an aqueous dispersion containing carbon particles to create an absorption layer on the corrosion protection coating (inventive examples 1 to 7). The respective composition of the aqueous dispersion can be found in Table 3.

[0129] Table 3 further provides details of the treatment method. These include the application method, the pH value of the aqueous dispersion, the immersion time, and the temperature of the steel blanks during treatment. The resulting properties, such as the coating 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.

[0130] It can be seen from Table 3 that inventive examples 1 to 7 have significantly lower average reflectances in the infrared range compared to comparative example V (see also Figure 3). Furthermore, the heating rate for inventive examples 1 to 7 is significantly higher than for comparative example V (see - ZI - also Figure 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 above a certain layer thickness of the absorption layer, saturation of the inventive effect occurs (see also Figures 3 and 4).

[0131] The resulting steel blanks were then processed into a sheet metal part by hot forming. For this purpose, the blanks were heated in a roller hearth furnace from room temperature with an average heating rate (between 30°C and 700°C) to a furnace temperature of 920°C. The average heating rate is given in Table 3.

[0132] 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 ACl temperature in all cases and thus also above Ms+100°C.

[0133] 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 1:

[0134] The remainder is iron and unavoidable impurities. All values ​​are in wt.%.

[0135] Table 2:

[0136] Melt analysis data in wt.%.

[0137] Table 3:

Claims

Patent claims Flat steel product for producing a steel component by hot forming, comprising a steel substrate consisting of a steel which has 0.1 - 3 wt. % Mn and optionally up to 0.01 wt. % B, and an aluminum-based corrosion protection coating lying on the steel substrate, wherein an absorption layer is arranged on the corrosion protection coating, characterized in that the absorption layer comprises carbon particles. Flat steel product according to claim 1, characterized in that the carbon particles are selected from the group consisting of graphite, fullerenes, graphene, carbon nanotubes and mixtures thereof. Flat steel product according to one of claims 1 to 2, characterized in that the thickness of the absorption layer is 0.05 to 5 pm. Flat steel product according to one of claims 1 to 3, characterized in that the aluminum-based corrosion protection coating consists of 1 - 15 wt. % Si, optionally 2-4 wt.-% Fe, optionally up to 5 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. Flat steel product according to claim 4, characterized in that the aluminum-based anti-corrosive coating contains up to 1.0 wt.% alkali or alkaline earth metals and preferably comprises 0.1-1.0 wt.% Mg. Flat steel product according to one of claims 1 to 5, characterized in that the mean reflectance in the infrared range is less than. is less than 0.45, in particular less than 0.35, preferably less than 0.

25. Use of carbon particles in an absorption layer on a flat steel product coated with an aluminum-based anti-corrosive coating to reduce reflectivity in the infrared range. Process for producing a flat steel product according to one of the claims 1 to 6 comprising the following work steps: a) Providing a flat steel product comprising a steel substrate consisting of a steel having 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, b) Applying an absorption layer comprising carbon particles to the flat steel product, in particular by (i) immersing the flat steel product in an aqueous dispersion comprising carbon particles or (ii) spraying the flat steel product with an aqueous dispersion comprising carbon particles or (iii) coating the flat steel product with an aqueous dispersion comprising carbon particles using a coil-coating process or by chemical or physical vapor deposition. The process according to claim 8, characterized in that the pH of the aqueous dispersion is at least 8 and at most 12, in particular at most 10.

10. The method according to claim 8 or 9, characterized in that the immersion in an aqueous dispersion comprising carbon particles is carried out for an immersion time of 0.5 to 30 s, preferably 1 to 5 s.

11. Method according to one of claims 8 to 10, characterized in that the flat steel product has a temperature of 40°C to 100°C, preferably 50°C to 80°C, when immersed in, sprayed or coated with the aqueous dispersion comprising carbon particles or when coated by chemical vapor deposition.

12. The method according to any one of claims 8 to 11, characterized in that the aqueous dispersion further contains at least one surfactant, and preferably additionally contains at least one polymer.

13. The method according to claim 12, characterized in that 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, and the preferably additionally contained at least one polymer is selected from polyethylene glycols or polypropylene glycols.

14. Process according to one of claims 8 to 13, characterized in that the aqueous dispersion has a solids content of 1 to 70 wt.%, in particular up to 50 wt.%.

15. A method for producing a sheet metal part comprising the following steps: a) providing a sheet metal blank from a flat steel product according to one of claims 1 to 6; b) heating the sheet metal blank in such a way that the AC3 temperature of the blank is at least partially exceeded and the temperature T of the blank when inserted into a forming tool intended for hot press forming (work step c) at least partially has a temperature above Ms+100°C, where Ms denotes the martensite start temperature, the average heating rate being greater than 15 Kmm / s; c) inserting the heated sheet metal blank into a forming tool, the transfer time trtrans required for removing the blank from the heating device and inserting it being at most 20 s, preferably at most 15 s; d) hot-press forming the sheet metal blank to form the sheet metal part, wherein the blank is cooled during the hot-press forming process over a period twz of more than 1 s at a cooling rate rwz which is at least partially more than 30 K / s to the target temperature Tziei and is optionally held there;e) Removing the sheet metal part cooled to the target temperature Tziei from the tool.;

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