Steel flat product with fast-heating coating

Abstract

Steel flat product for the manufacture of a steel component by hot forming, comprising a steel substrate consisting 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.%, Al: 0.02 - 1.0 wt.%, P: ≤ 0.05 wt.%, S: ≤ 0.02 wt.%, N: ≤ 0.02 wt.%, Sn: ≤ 0.03 wt.%, As: ≤ 0.01 wt.%, Ca: ≤ 0.005 wt.%, and optionally one or more of the elements "Cr, B, Mo, Ni, Cu, Nb, Ti, V" in the following amounts: Cr: 0.08 - 1.0 wt.%, B: 0.001 - 0.005 wt.% Mo:≤0.5 wt.% Ni:≤0.5 wt.% Cu:≤0.2 wt.% Nb:0.02 - 0.08 wt.% Ti:0.01 - 0.08 wt.% V:≤0.1 wt.%, and an aluminum-based corrosion protection coating resting on the steel substrate, wherein an absorption layer is arranged on the corrosion protection coating, the aluminum-based corrosion protection coating having an Al base layer consisting of 1.0 - 15 wt.% Si, optionally 2-4 wt.% Fe, 0.1 to 5 wt.%-% alkali or alkaline earth metals, optionally up to 15 wt% Zn and optional other components, the total content of which is limited to a maximum of 2.0 wt%, and the remainder being aluminium, wherein the content of alkali or alkaline earth metals comprises 0.1-1.0 wt% Mg and wherein the absorption layer comprises a mixture of hydrotalcite and lithium hydrotalcite.

Description

[0001] The invention relates to a flat steel product for the production of 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 lying on the steel substrate.

[0002] The term "flat steel product" here refers to all rolled products whose length is much greater than their thickness. This includes steel strips and sheets, as well as blanks and sheets derived from them.

[0003] In 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 forming temperature generally above the austenitizing temperature (AC3) of the respective steel and placed in the die of a forming press while heated. During the subsequent forming process, the sheet blank or the component formed from it experiences rapid cooling through contact with the cool die. The cooling rates are set so that a hardened microstructure develops in the steel substrate. This microstructure is transformed into at least a partially martensitic structure. The result is a hardened steel component.

[0004] The heating of the flat steel product typically takes place in a preheated roller hearth furnace through which the product passes. From the perspective of optimal energy utilization, including reduced CO2 emissions, minimized process duration, and optimized process control, the requirement here is for the most efficient possible transfer of the heat energy, usually introduced as thermal radiation, into the flat steel product. A shorter heating time allows for smaller roller hearth furnaces, which has a positive impact on costs and space requirements.

[0005] Steel flat products with various corrosion protection coatings are known from WO 2012 / 120081 A2. Various coatings are proposed to improve heating behavior.

[0006] The object of the present invention is to provide a particularly suitable composition of the absorption layer for flat steel products with an aluminum-based corrosion protection coating.

[0007] This problem is solved by a flat steel product for the manufacture of a steel component 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 corrosion protection coating applied to the steel substrate. An absorption layer comprising lithium aluminum hydroxide is arranged on the corrosion protection coating.

[0008] According to the invention, lithium aluminum hydroxides are mixed hydroxides or double hydroxides in which, within the crystal structure, metal cations share hydroxide ions with their respective neighbors. The positive excess charge is compensated by anions.

[0009] These anions are preferably carbonate anions. The latter are referred to as hydroxycarbonates or hydroxide carbonates. The absorption layer therefore preferably comprises carbonates of lithium aluminum hydroxides, in particular lithium hydrotalcite.

[0010] Lithium hydrotalkite is the simplified name for lithium aluminum hydroxide carbonate with the chemical formula [LiAl2(OH)6]2(CO3) · 4H2O.

[0011] In particular, lithium hydrotalcite exists in crystalline form.

[0012] In a specific embodiment of the invention, the lithium aluminum hydroxides form layers in the crystal structure with interlayers of anions that may contain water of crystallization. The H₂O molecules in the interlayer stabilize the linkage of the layers via hydrogen bonds.

[0013] In particular, the absorption layer consists of 80 wt.% to 100 wt.% lithium hydrotalcite or of 80 wt.% to 100 wt.% a mixture of lithium hydrotalcite and hydrotalcite and optional components, the total content of which does not exceed 20 wt.%.

[0014] Lithium aluminum hydroxides, especially their carbonates, preferably lithium hydrotalcite, have the advantage that even small quantities result in significant absorption in the infrared range and thus a reduction in reflectivity in the infrared range, i.e., the part of the radiation spectrum in which the furnace interior emits radiation that primarily heats the sheet metal blanks. Furthermore, lithium-based salt layers passivate the surface and thus improve corrosion protection.

[0015] For the purposes of this application, the infrared range is defined as the range of wavenumbers from 1000 to 10000 cm⁻¹. -1Understood. This corresponds to the wavelength range of 1 - 10 µm.

[0016] Furthermore, a well-adhering absorption layer with lithium aluminum hydroxides can be easily created by immersing the steel flat product in an aqueous solution containing lithium carbonate (Li₂CO₃), spraying it with such a solution, or coating it using a coil-coating process. During the subsequent drying of the steel flat product, lithium hydrotalcite crystals form on the aluminum-containing surface of the corrosion protection coating. Thus, an absorption layer with lithium hydrotalcite crystals is formed.

[0017] The absorption layer lies on top of the corrosion protection coating and is directly adjacent to it. In particular, the absorption layer is a top layer that completes the layer structure formed on the steel flat product on its outer side.

[0018] In a preferred variant, the surface weight of the absorption layer is 20 - 750 mg / m². 2 The contact weight refers to the contact weight per side. For double-sided coated steel flat products with an absorption layer on both sides, the contact weight on each side is therefore 20–750 mg / m². 2 It has been shown that even such low tracking force weights lead to a significant reduction in the reflectance.

[0019] The reflectance R in the infrared range is determined, as described in this application, by using a blackbody radiator as a reference. The blackbody has a temperature of T = 920°C, which corresponds to an average oven temperature. Thus, the spectral radiant power i is determined. λ (T) of the blackbody radiator at temperature T with the measured spectral reflectivity ρ λThe values ​​are multiplied and integrated over the wavelength range. This integral is then normalized to the spectral radiant power integrated over the same wavelength range. Therefore, the following applies: R¯(T)=∫λ1λ2ρλ⋅iλ(T)dλ∫λ1λ2iλ(T)dλ

[0020] This results in i λ (T) from Planck's radiation law iλ(T)=2πhc2λ51ehcλkBT−1 with the speed of light c, Planck's constant h and the Boltzmann constant k B The integration is performed over the wavelength range corresponding to wavenumbers from 1000 to 10000 cm⁻¹. -1 This corresponds to λ1 = 1 µm to λ2 = 10 µm. The subsequently used reflectance R is defined as R(920°C).

[0021] The aluminum-based corrosion protection coating can be applied to one or both sides of the steel flat product. The two sides of the steel flat product are defined as the two opposing large surfaces. The narrow surfaces are referred to as edges.

[0022] Such a corrosion protection coating is preferably produced by hot-dip coating the steel flat product. The steel flat product is passed through a liquid melt consisting of up to 15 wt.% Si, preferably more than 1.0%, 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 other components, the total content of which is limited to a maximum of 2.0 wt.%, with aluminum as the remainder. In a preferred embodiment, the Si content of the melt is 1.0-3.5 wt.% or 7-12 wt.%, in particular 8-10 wt.%.

[0023] In a preferred embodiment, 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 in particular comprise at least 0.0015 wt.% Ca, in particular at least 0.01 wt.% Ca.

[0024] In 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 when it solidifies.

[0025] The alloy layer lies on top of the steel substrate and is directly adjacent to it. The alloy layer is essentially composed of aluminum and iron. The remaining elements from the steel substrate or the melt composition do not accumulate significantly in the alloy layer. Preferably, the alloy layer consists of 35–60 wt.% Fe, preferably α-iron, optional additional components whose total content is limited to a maximum of 5.0 wt.%, preferably 2.0%, and aluminum as the remainder, with the Al content preferably increasing towards the surface. The optional additional components include, in particular, the remaining components of the melt (i.e., silicon and optionally alkali or alkaline earth metals, especially Mg or Ca) and the remaining portions of the steel substrate in addition to iron.

[0026] The aluminum base layer lies on top of the alloy layer and is directly adjacent to it. Preferably, the composition of the aluminum base layer corresponds to the composition of the melt in the molten pool. That is, 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% Zn, and optionally other components, the total content of which is limited to a maximum of 2.0 wt.%, with aluminum as the remainder.

[0027] In a preferred embodiment of the Al base layer, the optional content of alkali or alkaline earth metals comprises 0.1–1.0 wt.% Mg, in particular 0.1–0.7 wt.% Mg, preferably 0.1–0.5 wt.% Mg. Furthermore, the optional content of alkali or alkaline earth metals in the Al base layer can comprise, in particular, at least 0.0015 wt.% Ca, in particular at least 0.1 wt.% Ca.

[0028] 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 AI ​​base layer.

[0029] The corrosion protection coating preferably has a thickness of 5–60 µm, particularly 10–40 µm. The coating weight of the corrosion protection coating is particularly 30-360gm2 in the case of corrosion protection coatings on both sides or 15-180gm2 in the single-sided version. Preferably, the bearing weight of the corrosion protection coating is 100-200gm2 in the case of double-sided coatings or 50-100gm2 For single-sided coatings. The coating weight of the corrosion protection layer is particularly preferred. 120-180gm2 in the case of double-sided coatings or 60-90gm2 for one-sided coatings.

[0030] The thickness of the alloy layer is preferably less than 20 µm, particularly preferably less than 16 µm, particularly preferably less than 12 µm, and especially less than 10 µm. The thickness of the aluminum base layer is the difference between the thicknesses of the corrosion protection coating and the alloy layer. Preferably, the thickness of the aluminum base layer is at least 1 µm, even with thin corrosion protection coatings.

[0031] In the preferred variant with 0.1 - 1.0 wt% Mg in the Al base layer, the absorption layer preferably comprises not only lithium hydrotalcite, but also hydrotalcite Mg6Al2[(OH) 16Hydrotalcite forms when the steel flat product is immersed in an aqueous solution containing lithium carbonate (Li₂CO₃), sprayed with such a solution, or coated using a coil-coating process, and the described Mg content is also present in the AI ​​base layer. In such a case, the absorption layer comprises a mixture of hydrotalcite and lithium hydrotalcite. In particular, the absorption layer consists of 80 wt.% to 100 wt.% of a mixture of lithium hydrotalcite and hydrotalcite, along with optional components, the total content of which does not exceed 20 wt.%. Preferably, the proportion of hydrotalcite in the mixture of hydrotalcite and lithium hydrotalcite is at least 5 wt.%, more preferably at least 10 wt.%. In particular, the proportion of hydrotalcite in the mixture of hydrotalcite and lithium hydrotalcite is a maximum of 90 wt.%, more preferably a maximum of 75 wt.%.

[0032] In a preferred embodiment of the steel flat product, the infrared reflectance R is less than 0.55, in particular less than 0.50, more preferably less than 0.45, more preferably less than 0.40, more preferably less than 0.35, more preferably less than 0.30, more preferably less than 0.25, more preferably less than 0.20, more preferably less than 0.15. The lower the infrared reflectance, the greater the heating during the subsequent production of a steel component.

[0033] The invention further relates to the use of lithium aluminum hydroxide, in particular its carbonates, preferably lithium hydrotalcite, in an absorption layer on an aluminum-based corrosion protection coating for reducing infrared reflectivity. This use offers the same advantages as described above with regard to the steel flat product. The invention also relates to the use of the aforementioned specially developed absorption layers, and in particular the use of a mixture of lithium hydrotalcite and hydrotalcite in an absorption layer on a corrosion protection coating containing 0.1–1.0 wt% Mg in the aluminum base layer.

[0034] The steel substrate is made of a steel containing 0.1–3 wt.% manganese and optionally up to 0.01 wt.% brine. In particular, the microstructure of the steel can be transformed into a martensitic or partially martensitic microstructure by hot forming. The microstructure of the steel substrate of the steel component is therefore preferably a martensitic or at least partially martensitic microstructure, as this exhibits particularly high hardness.

[0035] The steel substrate is particularly preferred if it consists of a steel which, in addition to iron and unavoidable impurities (in wt.%), of C: 0.04 - 0.45 wt.%, Si: 0.02 - 1.2 wt.%, Mn: 0.5 - 2.6 wt.% Al: 0.02 - 1.0 wt.%, P: ≤ 0.05 wt.%, S: ≤ 0.02 wt.%, N: ≤ 0.02 wt.%, Sn: ≤ 0.03 wt.% As: ≤ 0.01 wt.% Ca: ≤ 0.005 wt.% and optionally one or more of the elements “Cr, B, Mo, Ni, Cu, Nb, Ti, V” in the following amounts Cr: 0.08 - 1.0 wt.%, B: 0.001 - 0.005 wt.% Mo: ≤ 0.5 wt.% Ni: ≤ 0.5 wt.% Cu: ≤ 0.2 wt.% Note: 0.02 - 0.08 wt.% Ti: 0.01 - 0.08 wt.% V: ≤ 0.1 wt.% consists.

[0036] The elements P, S, N, Sn, As, and Ca are impurities that cannot be completely avoided during steel production. In addition to these elements, other elements may also be present as impurities in the steel. These other elements are collectively referred to as "unavoidable impurities." Preferably, the total content of unavoidable impurities is a maximum of 0.2 wt.%, more preferably a maximum of 0.1 wt.%. The optional alloying elements Cr, B, Nb, and Ti, for which a lower limit is specified, may also occur as unavoidable impurities in the steel substrate at levels below the respective lower limit. In this case, they are also counted among the unavoidable impurities, the total content of which is limited to a maximum of 0.2 wt.%, more preferably a maximum of 0.1 wt.%. The 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.% Ti: ≤ 0.005 wt.%

[0037] These preferred upper limits should be considered as alternatives or in combination. Preferred steel variants therefore fulfill one or more of these four conditions.

[0038] In a preferred embodiment, the carbon content of the steel is a maximum of 0.37 wt.% and / or a minimum of 0.06 wt.%. In particularly preferred embodiments, the carbon content is in the range of 0.06–0.09 wt.%, or in the range of 0.12–0.25 wt.%, or in the range of 0.33–0.37 wt.%.

[0039] In a preferred embodiment, the Si content of the steel is a maximum of 1.00 wt.% and / or a minimum of 0.06 wt.%.

[0040] In a preferred embodiment, the manganese content of the steel is a maximum of 2.4 wt.% and / or a minimum of 0.75 wt.%. In particularly preferred embodiments, the manganese content is in the range of 0.75–0.85 wt.% or in the range of 1.0–1.6 wt.%.

[0041] In a preferred variant, the aluminum content of the steel is a maximum of 0.75 wt.%, in particular a maximum of 0.5 wt.%, and preferably a maximum of 0.25 wt.%. Alternatively or additionally, the aluminum content is preferably at least 0.02%.

[0042] Furthermore, it has been shown that limiting the sum of silicon and aluminum contents can be beneficial. Therefore, in a preferred embodiment, the sum of Si and Al contents (usually referred to as Si+Al) is a maximum of 1.5 wt.%, preferably a maximum of 1.2 wt.%. Additionally or alternatively, the sum of Si and Al contents is at least 0.06 wt.%, preferably at least 0.08 wt.%.

[0043] The elements P, S, and N are typical impurities that cannot be completely avoided in steel production. In preferred variants, the P content is a maximum of 0.03 wt.%. Independently of this, the S content is preferably a maximum of 0.012 wt.%. Additionally or supplementarily, the N content is preferably a maximum of 0.009 wt.%.

[0044] Optionally, the steel also contains chromium in a content of 0.08–1.0 wt.%. Preferably, the chromium content is a maximum of 0.75 wt.%, and particularly a maximum of 0.5 wt.%.

[0045] In the case of optional chromium alloying, the sum of the chromium and manganese contents is preferably limited. The sum is a maximum of 3.3 wt.%, in particular a maximum of 3.15 wt.%. Furthermore, the sum is at least 0.5 wt.%, preferably at least 0.75 wt.%.

[0046] Preferably, the steel also optionally contains boron in a content of 0.001–0.005 wt.%. In particular, the boron content is a maximum of 0.004 wt.%.

[0047] Optionally, the steel may contain molybdenum with a maximum content of 0.5 wt.%, in particular a maximum of 0.1 wt.%.

[0048] Furthermore, the steel may optionally contain nickel with a content of a maximum of 0.5 wt.%, preferably a maximum of 0.15 wt.%.

[0049] Optionally, the steel may also contain copper with a content of a maximum of 0.2 wt.%, preferably a maximum of 0.15 wt.%.

[0050] Furthermore, the steel may optionally contain one or more of the microalloying elements Nb, Ti, and V. The optional Nb content is at least 0.02 wt.% and at most 0.08 wt.%, preferably at most 0.04 wt.%. The optional Ti content is at least 0.01 wt.% and at most 0.08 wt.%, preferably at most 0.04 wt.%. The optional V content is at most 0.1 wt.%, preferably at most 0.05 wt.%.

[0051] In the case of optional alloying with several of the elements Nb, Ti, and V, the sum of the Nb, Ti, and V contents is preferably limited. The sum is a maximum of 0.1 wt.%, particularly a maximum of 0.068 wt.%. Furthermore, the sum is preferably at least 0.015 wt.%.

[0052] The above explanations regarding preferred steel substrates naturally also apply to the steel substrates in the manufacturing processes described below.

[0053] The invention further relates to a method for producing a previously described flat steel product comprising at least the following steps: a) Providing a steel flat 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 aluminium-based corrosion protection coating applied to the steel substrate, b) Applying an aqueous solution containing lithium carbonate (Li2CO3) to the steel flat product, in particular immersing the steel flat product in an aqueous solution containing lithium carbonate (Li2CO3) or spraying the steel flat product with an aqueous solution containing lithium carbonate (Li2CO3) or coating the steel flat product with an aqueous solution containing lithium carbonate (Li2CO3) in the coil-coating process.

[0054] This treatment ensures that the aqueous solution containing lithium carbonate is evenly distributed over the entire surface, forming a homogeneous, surface-covering absorption layer comprising lithium aluminum hydroxides, in particular their carbonates, preferably lithium hydrotalkite.

[0055] In a preferred further development, the pH value of the aqueous solution is at least 11 and at most 15, particularly at most 13, preferably at most 12. This ensures that sufficient Li₂CO₃ is dissolved so that enough lithium aluminum hydroxides can form. Furthermore, the pH value in this range leads to good wettability and thus to a particularly uniform distribution.

[0056] In particular, the aqueous solution contains 0 to 20 g / L Li₂CO₃. Preferably, the Li₂CO₃ content is at least 5 g / L. More preferably, the Li₂CO₃ content is a maximum of 15 g / L. Optionally, the solution may contain LiOH. The LiOH content is particularly 0 to 10 g / L. Preferably, the LiOH content is at least 1 g / L. More preferably, the LiOH content is a maximum of 5 g / L.

[0057] In a preferred embodiment, the aqueous solution additionally contains an oxidizing agent to improve homogeneity and accelerate the reaction. The oxidizing agent is preferably nitrate at a concentration of 0.1–15 g / L, more preferably 2–10 g / L, or chlorate at a concentration of 0.05–2 g / L, more preferably 0.2–1.5 g / L. Suitable nitrates and chlorates are the salts of alkali metals, preferably LiNO3 and KClO3.

[0058] In the case of immersion, the immersion time is preferably 60 to 300 s, preferably 120 to 240 s. A longer immersion time has the advantage that the reaction to form lithium hydrotalcite can occur reliably. However, for industrial production, a shorter immersion time is advantageous to make the manufacturing process more efficient. The times mentioned have proven to be a good compromise in this respect.

[0059] In a preferred embodiment of the process, the flat steel product has a temperature of 40°C to 100°C, preferably 50°C to 80°C, when the aqueous solution is applied, particularly during immersion, spraying, or coating in the coil-coating process. A higher temperature accelerates the chemical reaction during coating formation; however, at excessively high temperatures, the aqueous solution evaporates too quickly, so that coating formation is not reliably completed.

[0060] In a preferred variant, the steel flat product undergoes an activation treatment analogous to a phosphating process before step b). Furthermore, the surface can be activated before treatment, also analogous to a phosphating process. For this purpose, an activation solution based on phosphates, in particular sodium titanyl phosphate, or on copper, is applied to the steel flat product. Suitable activators are, for example, under the trade names SurTec® 145, SurTec® 610 V, SurTec® 615 V, SurTec® 616 V, Fixodine®X, Fixodine®50, Fixodine®50CF (now Bonderite® M-AC 50CF), Fixodine®950 (now Bonderite® M-AC 950), Fixodine®G 3039, Fixodine®C 5020 A, Fixodine®G 5020 B, Fixodine®C 9114, Fixodine®9112, Gardolene® Z26, Gardolene®V 6599, Gardolene® V 6560 A, Gardolene® V 6559, Gardolene® V 6526, Gardolene® V 6522, Gardolene® V 6520, Gardolene® V 6518, Gardolene® V 6513, Prepalene® X ® 163 available.

[0061] The invention further relates to a method for manufacturing a sheet metal forming part comprising the following steps: a) Providing a sheet metal blank from a previously explained steel flat product; b) Heating the sheet metal blank such that at least partially the AC3 temperature of the blank is exceeded and the temperature of the blank when placed in a forming tool intended for hot pressing (step c)) is at least partially above Ms+100°C, where Ms denotes the martensite start temperature, wherein the average heating rate is greater than 15 Kmm / s; c) Inserting the heated sheet metal blank into a forming tool, wherein the transfer time t required for removing the blank from the heating device and inserting it Trans at most 20s, preferably at most 15s; d) Hot pressing of the sheet metal blank to form the sheet metal part, wherein the blank is cooled to the target temperature T during hot pressing for a duration twz of more than 1 s at a cooling rate rwz of at least partially more than 30 K / s Ziel cooled down and optionally kept there; e) Removing the sheet metal part, cooled to the target temperature, from the tool.

[0062] In the inventive method, a blank consisting of a steel composed in a suitable manner according to the preceding explanations with an absorption layer as described above is provided (step a)), which is then heated with a mean heating rate greater than 15 Kmm / s such that at least partially the AC3 temperature of the blank is exceeded and the temperature T Einigthe blank, when placed in a forming tool intended for hot pressing (step c)), is at least partially at a temperature above Ms+100°C.

[0063] The mean heating rate is understood to be the product of the mean heating rate from 30°C to 700°C and the sheet thickness. The mean heating rate is more than 15 mm / s, particularly more than 20 mm / s, preferably more than 25 mm / s, particularly more than 30 mm / s, and most preferably more than 35 mm / s. The heating in step a) preferably takes place in an oven, in particular a roller hearth oven. Therefore, thermal radiation dominates over thermal conduction during the heating of the sheet metal blanks. The absorption layer according to the invention increases the proportion of absorbed thermal radiation, resulting in such high mean heating rates.

[0064] For the purposes of this application, "partially exceeding a temperature" (here AC3 or Ms+100°C) means that at least 30%, and in particular at least 60%, of the blank's volume exceeds the corresponding temperature. Therefore, when inserted into the forming tool, at least 30% of the blank exhibits an austenitic microstructure; that is, the transformation from a ferritic to an austenitic microstructure need not yet be complete when inserted into the forming tool. Rather, up to 70% of the blank's volume may consist of other microstructural constituents, such as tempered bainite, tempered martensite, and / or non- or partially recrystallized ferrite. To this end, certain areas of the blank can be deliberately kept at a lower temperature level than others during heating.For this purpose, the heat input can be selectively directed only to specific sections of the blank, or the parts that are to be heated less can be shielded from the heat input. In the part of the blank whose temperature remains lower, no or significantly less martensite forms during the forming process in the die, so that the microstructure there is considerably softer than in the other parts where a martensitic microstructure is present. In this way, a softer area can be selectively created in the formed sheet metal part, for example, by providing optimal toughness for the respective application, while the other areas of the sheet metal part have maximized strength.

[0065] 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.

[0066] The minimum temperature to be exceeded, AC3, is determined according to the formula given by HOUGARDY, HP. in Werkstoffkunde Stahl Band 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 with %C = respective C content, %Si = respective Si content, %Mn = respective Mn content, %Cr = respective Cr content, %Mo = respective Mo content, %Ni = respective Ni content and %V = respective V content of the steel from which the blank is made.

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

[0068] In a preferred embodiment, the 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.

[0069] Preferably the dew point in the oven is 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.

[0070] In a specific embodiment, the heating in step b) takes place in stages in areas with different temperatures. Specifically, the heating is carried out in a roller hearth furnace with different heating zones. Here, the heating in a first heating zone occurs at a temperature (so-called furnace inlet temperature) of at least 650°C, preferably at least 680°C, and particularly at least 720°C. The maximum temperature in the first heating zone is preferably 900°C, and particularly preferably 850°C. Furthermore, the maximum temperature of all heating zones in the furnace is preferably at most 1200°C, particularly preferably at most 1000°C, more preferably at most 950°C, and most preferably at most 930°C.

[0071] The total time in the oven t OfenThe heating time, which consists of a heating time and a holding time, is preferably at least 1 minute, particularly at least 2 minutes, and preferably at least 3 minutes for both variants (constant oven temperature, stepwise heating). Furthermore, the total oven time for both variants is preferably a maximum of 12 minutes, particularly a maximum of 10 minutes, preferably a maximum of 8 minutes, and particularly a maximum of 6 minutes. Longer total oven times have the advantage of ensuring uniform austenitization of the sheet metal blank. On the other hand, holding the blank above AC3 for too long leads to grain coarsening, which negatively affects the mechanical properties.

[0072] The pre-heated blank is removed from the heating unit and transported so quickly into the forming tool that its temperature upon arrival in the tool is at least partially above Ms + 100°C, preferably above 600°C, particularly above 650°C, and most preferably above 700°C. Here, Ms denotes the martensite start temperature. In a particularly preferred variant, the temperature is at least partially above the AC1 temperature. In all these variants, the temperature is, in particular, a maximum of 900°C. These temperature ranges ensure good formability of the material.

[0073] In step c), the transfer of the austenitized blank from the heating device used to the forming tool is completed within preferably no more than 20 s, and in particular within a maximum of 15 s. Such rapid transport is necessary to prevent excessive cooling before forming.

[0074] When the blank is inserted, the tool typically has a temperature between room temperature (RT) and 200°C, preferably between 20°C and 180°C, and particularly between 50°C and 150°C. Optionally, in a particular embodiment, the tool can be heated at least in certain areas to a temperature Twz of at least 200°C, and particularly at least 300°C, in order to harden the component only partially. Furthermore, the tool temperature Twz is preferably a maximum of 600°C, and particularly a maximum of 550°C. It is only necessary to ensure that the tool temperature Twz remains below the desired target temperature T. ZielThe residence time in the tool twz is preferably at least 2s, particularly at least 3s, and most preferably at least 5s. The maximum residence time in the tool is preferably 25s, and more particularly at most 20s.

[0075] The target temperature T Ziel The temperature of the sheet metal part is at least partially below 400°C, preferably below 300°C, particularly below 250°C, preferably below 200°C, most preferably below 180°C, and particularly below 150°C. Alternatively, the target temperature T is Ziel The sheet metal part is preferably heated to a temperature of 50°C (Ms), where Ms denotes the martensite start temperature. Furthermore, the target temperature of the sheet metal part is preferably at least 20°C, and particularly preferably at least 50°C.

[0076] The martensite start temperature of a steel that meets the requirements of the invention is given by 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 calculate, where here too, C% denotes the C content, %Mn the Mn content, %Mo the Mo content, %Cr the Cr content, %Ni the Ni content, %Cu the Cu content, %Co the Co content, %W the W content and %Si the Si content of the respective steel in wt.%.

[0077] The AC1 temperature and the AC3 temperature of a steel within the parameters of the invention are determined according to the following formulas: 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.−%] to calculate, 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)

[0078] In the tool, the blank is not only formed into the sheet metal part, but also simultaneously quenched to the target temperature. The cooling rate in the tool rwz to the target temperature is in particular at least 20 K / s, preferably at least 30 K / s, more preferably at least 50 K / s, and most preferably at least 100 K / s.

[0079] After removing the sheet metal part in step e), the sheet metal part is cooled to a cooling temperature T. AB of less than 50°C within a cooling period t AB from 0.5 to 600s. This usually happens through air cooling.

[0080] The invention will be explained in more detail below using exemplary embodiments.

[0081] The figures show: Fig. 1a a scanning electron microscope image of a top view of an absorption layer at a first resolution; Fig. 1b a scanning electron microscope image of a top view of an absorption layer at a second resolution; Fig. 2. Determine the reflectance R(k) as a function of the wavenumber k for experiments 1-6.

[0082] To demonstrate the effectiveness of the invention, several tests were conducted. Steel blanks with a steel composition according to Table 1 were hot-dip coated with an aluminum-based corrosion protection coating. The melt analysis is given in Table 2. The resulting corrosion protection coating each had an aluminum base layer whose composition corresponds to the melt analysis. The thickness of the corrosion protection coating on one side is also listed in Table 2. The steel blanks thus prepared were treated with an aqueous solution containing lithium carbonate to create an absorption layer on the corrosion protection coating. The solution contained 13 g / L Li₂CO₃ (saturated solution) and 2 g / L LiOH. The pH of the solution was 12. The steel blanks were immersed in the aqueous solution for a specified immersion time. The details of the treatment method are given in Table 3.These are the application method, the pH value of the aqueous solution, the immersion time, and the temperature of the steel blanks during treatment. The resulting properties are listed in Table 2. These include the coating weight of the absorption layer and the average reflectance in the infrared range.

[0083] Fig. 1a and Fig. Figure 1b shows an example of a top view of the absorption layer from experiment 4. The crystalline form of the lithium hydrotalcite, which contributes to the increased absorption, is clearly visible.

[0084] Fig. Figure 2 shows a measurement of the spectral reflectivity ρ k as a function of the wavenumber k for experiments 1–6 according to Tables 2 and 3. The experiment number is given as a reference symbol in each case. It is clearly visible that the spectral reflectivity ρ k In the infrared range, the absorption layer significantly reduces the effect compared to reference example 1.

[0085] The resulting steel blanks were then hot-formed into sheet metal parts. For this process, the blanks were heated from room temperature in a roller hearth furnace at a medium heating rate r. Ofen (between 30°C and 700°C) were heated to an oven temperature of 920°C. The average heating rate and average heating rate are given in Table 2. It is clearly evident that the average heating rate was more than 15 Kmm / s in all cases according to the invention.

[0086] The blanks were then processed using conventional methods. For this purpose, the blanks were removed from the roller hearth furnace and placed in a forming die. Upon removal from the furnace, the blanks had already reached the furnace temperature. The transfer time, comprising the time for removal from the heating unit, transport to the die, and placement in the die, was approximately 10 seconds. The temperature of the blanks upon placement in the forming die was, in all cases, above the respective AC1 temperature and thus also above Ms+100°C. In the forming die, which was at room temperature, the blanks were formed into the respective sheet metal parts. The sheet metal parts were then cooled in the die at a rate of approximately 50 K / s for about 15 seconds. Finally, the samples were removed from the die and cooled to room temperature.The cooling process took place in still air at a cooling rate of 7 K / s. Table 1 (Steel grades) Steel C Si Mn Al Cr Note Ti B P s N Sn As Cu Mon Approx Ni A 0,22 0,145 1,1 0,18 0,2 0,032 0,017 0,0024 0,004 0,0007 0,0034 0,03 B 0,35 0,16 1,1 0,21 0,118 0,026 0,0096 0,0025 0,005 <0,0005 0,0035 0,005 0,003 0,019 0,005 0,001 0,032 C 0,235 0,3 1,3 0,05 0,28 0,003 0,040 0,0035 0,02 0,003 0,007 0,03 0,01 0,03 0,03 0,005 0,025 Residual iron and unavoidable impurities. All values ​​in wt.%; Table 2 (Properties of steel flat products) Attempt Steel grade Melt analysis Layer thickness [µm] Surface weight [mg / m²] 2 ] Reflectance Heating rate [K / s] Thickness of cut pieces [mm] Heating [Kmm / s] Si Fe Mg Other AI 1* A 8,1 3,9 <0,01 <1% rest 27 - 0.58 7.1 1.8 12.8 2 A 8,1 3,9 <0,01 <1% rest 27 83 0.43 10.2 1.8 18.4 3 A 8,1 3,9 <0,01 <1% rest 27 106 0.35 14.0 1.8 25.2 4 A 8,1 3,9 <0,01 <1% rest 27 148 0.20 16.1 1.8 28.9 5 A 8,2 3,8 0,25 <1% rest 28 295 0.11 20.6 1.8 37.0 6 A 8,1 3,9 <0,01 <1% rest 27 534 0.09 22.8 1.8 41.0 7 B 8,1 3,9 <0,01 <1% rest 29 158 0.18 16.6 1.8 29.9 8 C 8,1 3,9 <0,01 <1% rest 26 145 0.21 15.9 1.8 28.6 * Non-inventive reference examples Table 3 (Manufacturing conditions for steel flat products) Attempt Application method PH value Dive time [s] Temperature [°C] 1* - - - - 2 Immersion 12 60 60 3 Immersion 12 120 60 4 Immersion 12 180 60 5 Immersion 12 300 60 6 Immersion 12 600 60 7 Immersion 12 180 60 8 Immersion 12 180 60 * Non-inventive reference examples QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] WO 2012 / 120081 A2

[0005]

Claims

[1] Steel flat product for the manufacture of 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 aluminium-based corrosion protection coating resting on the steel substrate, wherein an absorption layer is arranged on the corrosion protection coating. characterized by , that the absorption layer comprises lithium aluminum hydroxide. [2] Steel flat product according to claim 1, characterized by , that the absorption layer comprises carbonates of lithium aluminium hydroxides, in particular lithium hydrotalkite ([LiAl2(OH)6]2(CO3) · 4H2O). [3] Steel flat product according to one of claims 1 to 2, characterized by that the surface weight of the absorption layer is 20 - 750 mg / m² 2 amounts. [4] Steel flat product according to any one of claims 1 to 3, characterized by, that the aluminum-based corrosion protection coating has an Al base layer consisting of 1.0 - 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 other components, the total content of which is limited to a maximum of 2.0 wt.%, and aluminum as the remainder. [5] Steel flat product according to claim 4, characterized by , that the Al base layer contains up to 1.0 wt.% alkali or alkaline earth metals, comprising 0.1 - 1.0 wt.% Mg, and wherein the absorption layer comprises a mixture of hydrotalcite and lithium hydrotalcite. [6] Steel flat product according to claim 5 characterized by , that the proportion of hydrotalcite in the mixture of hydrotalcite and lithium hydrotalcite is at least 5 wt.% and at most 90 wt.%. [7] Steel flat product according to any one of claims 1 to 6, characterized bythat the reflectance R in the infrared range is less than 0.55, preferably less than 0.50, in particular less than 0.40, preferably less than 0.

30. [8] Use of lithium aluminium hydroxides in an absorption layer on an aluminium-based corrosion protection coating to reduce infrared reflectivity. [9] A method for producing a steel flat product according to claim 1 comprising the following steps: a) Providing a steel flat 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 aluminium-based corrosion protection coating applied to the steel substrate, b) Applying an aqueous solution containing lithium carbonate (Li2CO3) to the steel flat product, in particular immersing the steel flat product in an aqueous solution containing lithium carbonate (Li2CO3) or spraying the steel flat product with an aqueous solution containing lithium carbonate (Li2CO3) or coating the steel flat product with an aqueous solution containing lithium carbonate (Li2CO3) in the coil-coating process [10] Method according to claim 9, characterized by that the pH value of the aqueous solution is at least 11 and at most 15, in particular at most 13. [11] Method according to any one of claims 9 to 10, characterized by , that the application of an aqueous solution comprising lithium carbonate (Li2CO3) to the steel flat product is carried out by immersing the steel flat product in the aqueous solution for an immersion time of 60s to 300s, preferably 120s to 240s. [12] Method according to any one of claims 9 to 11, characterized bythat the steel flat product has a temperature of 40°C to 100°C, preferably 50°C to 80°C, when the aqueous solution is applied. [13] Method for producing a sheet metal forming part comprising the following steps: a) Providing a sheet metal blank made from a flat steel product according to any one of claims 1 to 7; b) Heating the sheet metal blank in such a way that at least partially the AC3 temperature of the blank is exceeded and the temperature T Einlg the blank, when placed in a forming tool intended for hot pressing (work step c)), has at least a partial temperature above Ms+100°C, where Ms denotes the martensite start temperature, where the mean heating rate is greater than 15 Kmm / s; c) Inserting the heated sheet metal blank into a forming tool, wherein the transfer time t required for removing the blank from the heating device and inserting itTrans at most 20 s, preferably at most 15 s; d) Hot pressing of the sheet metal blank to form the sheet metal part, wherein the blank is cooled to the target temperature T during hot pressing for a duration twz of more than 1 s at a cooling rate rwz of at least partially more than 30 K / s Ziel cooled down and optionally kept there; e) Extraction of the product at the target temperature T Ziel cooled sheet metal part from the tool.

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