Flat steel product with a protective layer against scale

Abstract

The invention relates to a steel flat product for manufacturing a sheet metal part by hot forming. The steel flat product comprises a steel substrate and a protective coating arranged on at least one side of the steel substrate. The protective coating includes a protective layer comprising Mg with a thickness of 20–80 nm or a protective layer comprising Mn with a thickness of 350–800 nm. The invention further relates to a manufacturing method for such a steel flat product and a forming method for manufacturing a sheet metal part from such a steel flat product.

Description

[0001] The invention relates to a flat steel product with a protective layer against scale, a method for producing such a flat steel product, a sheet metal part produced from such a flat steel product and a method for producing such a sheet metal part from a flat steel product.

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

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

[0004] Flat steel products for hot forming into sheet metal parts are known from the prior art and feature an aluminum-based or zinc-based coating. For example, WO 2022 / 048990 A1 and EP 2 993 248 B1 show such flat steel products. Such coatings are typically thicker than 20 µm and offer good corrosion protection.

[0005] Likewise, flat steel products for hot forming without coating are known from the prior art. Such flat steel products are known, for example, from EP 3 924 528 A1. Sheet metal parts formed from such flat steel products are typically used in applications that are less susceptible to corrosion.

[0006] Uncoated flat steel products for hot forming have the disadvantage that scaling of the surface can occur during heating for hot forming. EP 3 924 528 A1 discloses how to suppress such scaling by using a protective gas atmosphere (typically nitrogen) during the furnace process. However, the use of a nitrogen-based protective gas leads to effusion of carbon during austenitization on the surface of a flat steel product. This phenomenon is known as edge decarburization and leads to a decrease in material hardness at the edge. Depending on the application, this can be undesirable. Furthermore, it is difficult to completely prevent scaling using a protective gas atmosphere, since the heated sheet metal blank must be transported to a forming tool. At this point at the latest, the heated surface comes into contact with ambient air, resulting in scale formation.Therefore, the surface of the sheet metal part is usually descaled by blasting. All granular blasting media are suitable, such as steel balls, steel scrap, blast furnace slag, sand, corundum, glass, etc.

[0007] The object of the present invention is to provide a cost-effective flat steel product that can be further processed into a sheet metal part in a conventional manner without the formation of scale, thus avoiding additional blasting. Furthermore, the flat steel product should be less complex to manufacture than the known flat steel products with a thick aluminum-based or zinc-based coating.

[0008] This object is achieved by a flat steel product for producing a sheet metal part by hot forming, comprising a) a steel substrate consisting of a steel containing 0.1 - 3 wt% Mn and optionally up to 0.01 wt% B, and b) a protective coating arranged on at least one side of the steel substrate, characterized in that the protective coating contains a protective layer comprising Mg with a thickness of 20 - 80 nm or a protective layer comprising Mn with a thickness of 350 - 800 nm.

[0009] The protective layer preferably consists of a maximum of 10 wt.% of other alloying elements, in particular a maximum of 5 wt.% of other alloying elements, preferably a maximum of 2 wt.% of other alloying elements and the remainder of magnesium or manganese.

[0010] In particular, the protective coating consists of the protective layer described above. The protective layer contacts the substrate on one side and forms the outer boundary of the flat steel product with its other surface.

[0011] Tests have shown that even a thin layer of magnesium or manganese prevents scale formation. It has been found that there are strict limits to the thickness of this protective layer. If the layer is too thin, the scale protection is not sufficient and scale will still form. If the layer is too thick, however, delamination occurs during the subsequent forming process. The protective layer or the oxides of the protective layer begin to peel off. This, of course, must also be avoided. In the case of magnesium, the thickness is at least 20 nm, in particular at least 30 nm, preferably at least 40 nm. Likewise, the thickness is a maximum of 80 nm, preferably a maximum of 70 nm, in particular a maximum of 60 nm. In the case of manganese, the thickness is at least 350 nm, in particular at least 400 nm, preferably at least 450 nm. Likewise, the thickness is a maximum of 800 nm, preferably a maximum of 700 nm, in particular a maximum of 600 nm.

[0012] In addition, it has been shown that weldability is given for resistance spot welding of the produced sheet metal part at the above-mentioned thicknesses without the need for a subsequent blasting process.

[0013] Weldability is determined according to SEP 1220 Part 2 (2001).

[0014] 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 sheet metal part described below is therefore preferably a martensitic or at least partially martensitic structure, as this exhibits particularly high hardness.

[0015] Particularly preferably, the steel substrate is 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% Ace: ≤ 0.01 wt% Approx: ≤ 0.005 wt% and optionally one or more of the elements "Cr, B, Mo, Ni, Cu, Nb, Ti, V, W" in the following contents Cr: 0.08 - 1.0 wt.%, B: 0.001 - 0.010 wt.% Mon: ≤ 0.5 wt% No: ≤ 0.5 wt% Cu: ≤ 0.2 wt% Note: 0.01 - 0.2 wt.%, Ti: 0.01 - 0.10 wt.% V: ≤ 0.3 wt% W: 0.001 - 1.00 wt.% consists.

[0016] The elements P, S, N, Sn, As, and Ca are impurities that cannot be completely avoided during steel production. Occasionally, Ca is also deliberately added to the alloy to bind sulfur. In such a case, the Ca content is at least 0.001 wt.%. The maximum Ca content in this case is also 0.005 wt.%.

[0017] In addition to these elements, other elements may also be present as impurities in the steel. These additional elements are summarized under "unavoidable impurities." The total content of unavoidable impurities is preferably a maximum of 0.2 wt.%, preferably a maximum of 0.1 wt.%. The optional alloying elements Cr, B, Nb, and Ti, for which a lower limit is specified, may also be present as unavoidable impurities in the steel substrate in amounts below the respective lower limit. In this case, they are also counted as unavoidable impurities, with their total content limited to a maximum of 0.2 wt.%, preferably a maximum of 0.1 wt.%. The individual upper limits for the respective impurities of these elements are preferably as follows: Cr: ≤ 0.050 wt.%, B: ≤ 0.0005 wt% Note: ≤ 0.005 wt%, Ti: ≤ 0.005 wt%

[0018] These preferred upper limits should be considered alternatively or jointly. Preferred steel variants therefore meet one or more of these four conditions.

[0019] In a preferred embodiment, the C content of the steel is a maximum of 0.37 wt.% and / or at least 0.06 wt.%. In particularly preferred embodiments, the C content is in the range of 0.06-0.09 wt.%, or in the range of 0.11-0.25 wt.%, or in the range of 0.32-0.37 wt.%.

[0020] In a preferred embodiment, the Si content of the steel is a maximum of 1.00 wt% and / or at least 0.06 wt%.

[0021] 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.%.

[0022] 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%.

[0023] It has also been shown that it can be helpful if the sum of the silicon and aluminum contents is limited. In a preferred variant, the sum of the Si and Al contents (usually referred to as Si+Al) is therefore a maximum of 1.5 wt.%, preferably a maximum of 1.2 wt.%. Additionally or alternatively, the sum of the Si and Al contents is at least 0.06 wt.%, preferably at least 0.08 wt.%.

[0024] 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.%.

[0025] 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.%.

[0026] 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.%.

[0027] Preferably, the steel optionally also contains boron in a content of 0.001 - 0.010 wt.%. In particular, the B content is a maximum of 0.005 wt.%, preferably a maximum of 0.004 wt.%.

[0028] Optionally, the steel may contain molybdenum in a maximum content of 0.5 wt%, in particular a maximum of 0.1 wt%.

[0029] Furthermore, the steel may optionally contain nickel with a content of maximum 0.5 wt.%, preferably maximum 0.15 wt.%.

[0030] Optionally, the steel may also contain copper with a content of maximum 0.2 wt.%, preferably maximum 0.15 wt.%.

[0031] In addition, the steel can optionally contain one or more of the microalloying elements Nb, Ti and V. The optional Nb content is at least 0.01 wt.%, in particular at least 0.02 wt.% and at most 0.2 wt.%, preferably at most 0.08 wt.%, preferably at most 0.04 wt.%. The optional Ti content is at least 0.01 wt.% and at most 0.10 wt.%, in particular at most 0.08 wt.%, preferably at most 0.04 wt.%. The optional V content is at most 0.3 wt.%, preferably at most 0.2 wt.%, in particular at most 0.1 wt.%, preferably at most 0.05 wt.%.

[0032] 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.%.

[0033] Tungsten (W) can optionally be added to the alloy in amounts of 0.001–1.0 wt.% to slow ferrite formation. A positive effect on hardenability is already achieved at W contents of at least 0.001 wt.%. For cost reasons, a maximum of 1.0 wt.% tungsten is added.

[0034] The above explanations regarding preferred steel substrates naturally also apply to the steel substrate of the sheet metal part described below, as well as the steel substrates in the manufacturing processes described.

[0035] Such a flat steel product is produced using one of the manufacturing processes described below, which includes the following steps: a) providing a slab or a thin slab consisting of steel containing 0.1 - 3 wt.% Mn and optionally up to 0.01 wt.% B; b) through-heating the slab or thin slab at a temperature (T1) of 1000 - 1400 °C; c) optionally pre-rolling the through-heated slab or thin slab to an intermediate product with an intermediate product temperature (T2) of 1000 - 1200 °C; d) hot-rolling to a hot-rolled flat steel product, wherein the final rolling temperature (T3) is 750 - 1000 °C; e) optionally coiling the hot-rolled flat steel product, wherein the coiling temperature (T4) is not more than 700 °C; f) optionally descaling the hot-rolled flat steel product; g) optionally cold rolling the steel flat product, wherein the degree of cold rolling is at least 30%; h) coating the steel flat product with a protective layer comprising Mg with a thickness of 20-80 nm or a protective layer comprising Mn with a thickness of 350-800 nm.

[0036] In step a), a semi-finished product composed according to the alloy specified for the flat steel product according to the invention is provided. This can be a slab produced by conventional continuous slab casting or by thin slab casting. The alloy preferably has the composition explained above with regard to the substrate of the flat steel product.

[0037] In step b), the semi-finished product is thoroughly heated to a temperature (T1) of 1000–1400 °C. If the semi-finished product has cooled after casting, it is first reheated to 1000–1400 °C for thorough heating. The thorough heating temperature should be at least 1000 °C, preferably at least 1100 °C, to ensure good formability for the subsequent rolling process. The thorough heating temperature should not exceed 1400 °C to avoid the presence of molten phases in the semi-finished product.

[0038] In the optional step c), the semi-finished product is pre-rolled into an intermediate product. Thin slabs are not usually subjected to pre-rolling. Thick slabs to be rolled into hot strip can be pre-rolled if required. In this case, the temperature of the intermediate product (T2) at the end of pre-rolling should be at least 1000 °C to ensure that the intermediate product retains sufficient heat for the subsequent finish rolling step. However, high rolling temperatures can also promote grain growth during the rolling process, which has a detrimental effect on the mechanical properties of the flat steel product. To keep grain growth low during the rolling process, the temperature of the intermediate product at the end of pre-rolling should not exceed 1200 °C.

[0039] In step d), the slab or thin slab, or, if step c) has been performed, the intermediate product, is rolled into a hot-rolled flat steel product. If step c) has been performed, the intermediate product is typically finish-rolled immediately after rough rolling. Typically, finish rolling begins no later than 90 seconds after the end of rough rolling. The slab, the thin slab, or, if step c) has been performed, the intermediate product, are rolled to a final rolling temperature (T3). The final rolling temperature, i.e., the temperature of the finished hot-rolled flat steel product at the end of the hot rolling process, is 750–1000 °C. At finish rolling temperatures below 750 °C, the amount of free vanadium decreases because larger amounts of vanadium carbides are precipitated. The vanadium carbides precipitated during finish rolling are very large.They typically have an average grain size of 30 nm or more and are no longer dissolved in subsequent annealing processes, such as those performed before hot-dip coating. The final rolling temperature is limited to a maximum of 1000 °C to prevent coarsening of the austenite grains. Furthermore, final rolling temperatures of a maximum of 1000 °C are relevant for the process to achieve coiling temperatures (T4) below 700 °C.

[0040] Hot rolling of the steel flat product can be carried out as continuous hot strip rolling or reversing rolling. In the case of continuous hot strip rolling, step e) provides for the optional coiling of the hot-rolled steel flat product. For this purpose, the hot strip is cooled to a coiling temperature (T4) within less than 50 seconds after hot rolling. The cooling medium used for this purpose can be water, air, or a combination of both. The coiling temperature (T4) should not exceed 700 °C to avoid the formation of large vanadium carbides. In principle, there is no lower limit on the coiling temperature. However, coiling temperatures of at least 500 °C have proven favorable for cold rolling. The coiled hot strip is then cooled to room temperature in air using the conventional method.

[0041] In step f), the hot-rolled flat steel product is optionally descaled conventionally by pickling or another suitable treatment. Alternatively or additionally, further cleaning steps such as annealing or plasma cleaning are also possible.

[0042] The scale-cleaned hot-rolled flat steel product can optionally be subjected to cold rolling before annealing in step g), for example, to meet more stringent thickness tolerance requirements. The cold rolling degree (CTC) should be at least 30% to inject sufficient deformation energy into the flat steel product for rapid recrystallization. The CTC is defined as the quotient of the thickness reduction during cold rolling ΔdCTC divided by the hot strip thickness d: KWG = Δ dKW / d where ΔdKW = thickness reduction during cold rolling in mm and d = hot strip thickness in mm, where the thickness reduction ΔdKW results from the difference between the thickness of the flat steel product before cold rolling and the thickness of the flat steel product after cold rolling. The flat steel product before cold rolling is usually a hot strip with a hot strip thickness of d. The flat steel product after cold rolling is also commonly referred to as cold strip. The cold rolling degree can, in principle, assume very high values ​​of over 90%. However, cold rolling degrees of up to 80% have proven to be beneficial for preventing strip breakage.

[0043] In step h), the steel flat product is coated with a protective layer comprising Mg with a thickness of 20 - 80 nm or a protective layer comprising Mn with a thickness of 350 - 800 nm.

[0044] In a preferred embodiment, the coating is carried out by physical vapor deposition, in particular by a method as described in WO 2016 / 042079 A1. In an alternative embodiment, the coating is carried out by electrolytic deposition.

[0045] The invention further relates to a sheet metal part, in particular formed from a previously explained flat steel product, comprising a) a steel substrate consisting of a steel containing 0.1-3 wt% Mn and optionally up to 0.01 wt% B, b) and a protective coating arranged on at least one side of the steel substrate, characterized in that the protective coating contains a protective layer comprising Mg with a thickness of 20 - 80nm or a protective layer comprising Mn with a thickness of 350 - 800nm.

[0046] The protective layer preferably consists of a maximum of 10 wt.% of other alloying elements, in particular a maximum of 5 wt.% of other alloying elements, preferably a maximum of 2 wt.% of other alloying elements and the remainder of magnesium or manganese.

[0047] In particular, the protective coating consists of the protective layer described above. The protective layer contacts the substrate on one side and forms the outer boundary of the flat steel product with its other surface, or borders the optional oxide layer described below.

[0048] In addition to the advantages explained above, that scale formation can be avoided during production, such sheet metal parts have excellent weldability.

[0049] In a preferred variant, the sheet metal part comprises an oxide layer arranged on the anti-corrosive coating. The oxide layer is located, in particular, on the protective layer and preferably forms the outer edge of the anti-corrosive coating.

[0050] The oxide layer of the sheet metal part consists in particular of more than 80 wt.% oxides, the main proportion of the oxides (i.e. more than 50 wt.% of the oxides) being magnesium oxide in the first variant or manganese oxide in the second variant.

[0051] The oxide layer preferably has a thickness of at least 50 nm, in particular of at least 100 nm. Furthermore, the thickness is a maximum of 4 µm, in particular a maximum of 2 µm.

[0052] In a specific refinement, the steel substrate of the sheet metal part has a structure with at least partially more than 90% martensite, in particular at least partially more than 95%, particularly preferably at least partially more than 98%. In this context, "partially having" means that there are regions of the sheet metal part that have the mentioned structure. In addition, there may also be regions of the sheet metal part that have a different structure. Thus, the sheet metal part has the mentioned structure in sections or regions.

[0053] The steel of the steel substrate of the sheet metal part is preferably designed as explained above with respect to the steel substrate of the flat steel product.

[0054] The invention further relates to a method for producing a sheet metal part, in particular a sheet metal part as described above. The method comprises the following steps: a. Providing a sheet metal blank from a previously explained flat steel product; b. Heating the sheet metal blank in such a way that the AC3 temperature of the blank is at least partially exceeded and the temperature T Einlg of the blank when inserted into a forming tool intended for hot press forming (work step c)) at least partially has a temperature above Ms+100 °C, where Ms denotes the martensite start temperature; c. Inserting the heated sheet metal blank into a forming tool, wherein the transfer time t Trans required for removing the blank from the heating device and inserting it is at most 20 s, preferably at most 15 s; d.Hot-pressing the sheet metal blank into the formed sheet metal part, wherein the blank is cooled to the target temperature T target during the hot-pressing over a period t WZ of more than 1 s at a cooling rate r WZ that is at least partially more than 30 K / s and is optionally held there; e. removing the sheet metal part cooled to the target temperature T target from the tool; f. cooling the sheet metal part to room temperature.

[0055] In the method according to the invention, a blank is thus provided which consists of a flat steel product suitably assembled in accordance with the above explanations (working step a)), which is then heated in a manner known per se such that the AC3 temperature of the blank is at least partially exceeded and the temperature T Einlg of the blank when placed in a forming tool intended for hot press forming (working step c)) is at least partially above Ms+100 °C. Partially exceeding a temperature (here AC3 or Ms+100 °C) is understood in the context of this application to mean that at least 30%, in particular at least 60%, of the volume of the blank exceeds a corresponding temperature. When placed in the forming tool, therefore, at least 30% of the blank has an austenitic structure, i.e.The transformation from a ferritic to an austenitic structure does not necessarily have to be complete when the blank is placed in the forming tool. Rather, up to 70% of the volume of the blank when placed in the forming tool can consist of other 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 kept at a lower temperature than others during heating. To do this, the heat can be specifically directed only at certain sections of the blank, or the parts that are to be heated less can be shielded from the heat. In the part of the blank material whose temperature remains lower, no or significantly less martensite is formed during forming in the tool, so that the structure there is significantly softer than in the other parts that have a martensitic structure.In this way, a softer area can be specifically set in the respective formed sheet metal part, for example by providing optimal toughness for the respective intended use, while the other areas of the sheet metal part have maximized strength.

[0056] Maximum strength properties of the obtained sheet metal part can be achieved by ensuring that the temperature reached at least partially in the sheet metal blank is between Ac3 and 1000 °C, preferably between 850 °C and 950 °C.

[0057] The minimum temperature Ac3 to be exceeded is determined according to the formula given by HOUGARDY, HP. in Werkstoffkunde Stahl Volume 1: Grundlagen, Verlag Stahleisen GmbH, Düsseldorf, 1984, p. 229. Ac3 = 902 − 225 * % C + 19 *%Si − 11 *%Mn − 5 *%Cr + 13 *%Mo − 20 *%Ni + 55 *%V ° C 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.

[0058] An optimally uniform distribution of properties can be achieved by heating the blank completely in step b).

[0059] In a preferred embodiment, the average heating rate r furnace of the sheet metal blank during heating in step b) is at least 3 K / s, preferably at least 5 K / s, in particular at least 10 K / s, preferably at least 15 K / s. The average heating rate r furnace is understood to be the average heating rate from 30 °C to 700 °C.

[0060] In a preferred embodiment, the heating takes place in a furnace with a furnace temperature T furnace of at least 850 °C, preferably at least 880 °C, particularly preferably at least 900 °C, in particular at least 920 °C, and at most 1000 °C, preferably at most 950 °C, particularly preferably at most 930 °C.

[0061] The dew point in the oven is preferably at least -20 °C, preferably at least -15 °C, in particular at least -5 °C, particularly preferably at least 0 °C, in particular at least 5 °C and at most +25 °C, preferably at most +20 °C, in particular at most +15 °C.

[0062] In a specific embodiment, the heating in step b) takes place stepwise in areas with different temperatures. In particular, the heating takes place in a roller hearth furnace with different heating zones. 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.

[0063] The total time in the furnace t furnace , which consists of a heating time and a holding time, is preferably at least 2 minutes, in particular at least 3 minutes, preferably at least 4 minutes for both variants (constant furnace temperature, step-by-step heating). Furthermore, the total time in the furnace for both variants is preferably a maximum of 20 minutes, in particular a maximum of 15 minutes, preferably a maximum of 12 minutes, in particular a maximum of 8 minutes. Longer total times in the furnace have the advantage of ensuring uniform austenitization of the sheet metal blank. On the other hand, holding for too long above Ac3 leads to grain coarsening, which has a negative effect on the mechanical properties.

[0064] The blank heated in this way is removed from the respective heating device, which can be, for example, a conventional heating furnace, an equally known induction heating device or a conventional device for keeping steel components hot, and transported into the forming tool so quickly that its temperature upon arrival in the tool is at least partially above Ms+100 °C, preferably above 600 °C, in particular above 650 °C, particularly preferably above 700 °C. Here, Ms denotes the martensite start temperature. In a particularly preferred variant, the temperature is at least partially above the AC1 temperature. In all of these variants, the temperature is in particular a maximum of 900 °C. These temperature ranges ensure good formability of the material overall.

[0065] 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.

[0066] When the blank is inserted, the tool typically has a temperature between room temperature (RT) and 200 °C, preferably between 20 °C and 180 °C, in particular between 50 °C and 150 °C. Optionally, in a special embodiment, the tool can be tempered at least partially to a temperature T WZ of at least 200 °C, in particular at least 300 °C, in order to only partially harden the component. Furthermore, the tool temperature T WZ is preferably a maximum of 600 °C, in particular a maximum of 550 °C. It only needs to be ensured that the tool temperature T WZ is below the desired target temperature T Ziel. The residence time in the tool t WZ is preferably at least 2 s, in particular at least 3 s, particularly preferably at least 5 s. The maximum residence time in the tool is preferably 25 s, in particular a maximum of 20 s.

[0067] The target temperature T target of the sheet metal part is at least partially below 400°C, preferably below 300°C, in particular below 250°C, preferably below 200°C, particularly preferably below 180°C, in particular below 150°C. Alternatively, the target temperature T target of the sheet metal part is particularly preferably below Ms-50°C, where Ms denotes the martensite start temperature. Furthermore, the target temperature of the sheet metal part is preferably at least 20°C, particularly preferably at least 50°C.

[0068] The martensite start temperature of a steel which is within the scope of the specifications according to the invention is to be calculated according to the formula: where C% denotes the C content, %Mn the Mn content, %Mo the Mo content, %Cr the Cr content, %Ni the Ni content, %Cu the Cu content, %Co the Co content, %W the W content and %Si the Si content of the respective steel in wt.%.

[0069] The AC1 temperature and the AC3 temperature of a steel within the scope of the specifications according to the invention are to be calculated according to the formulas:, where %C denotes the C content, %Si denotes the Si content, %Mn denotes the Mn content, %Cr denotes the Cr content, %Mo denotes the Mo content, %Ni denotes the Ni content and +%V denotes the vanadium content of the respective steel (Brandis H 1975 TEW-Techn. Ber. 1 8-10).

[0070] In the tool, the blank is not only formed into the sheet metal part, but is also simultaneously quenched to the target temperature. The cooling rate in the tool to the target temperature is in particular at least 20 K / s, preferably at least 30 K / s, in particular at least 50 K / s, and in special designs at least 100 K / s.

[0071] After removal of the sheet metal part in step e), the sheet metal part is cooled to room temperature within a cooling time t AB of 0.5 to 600 s. This is usually done by air cooling.

[0072] The sheet metal part according to the invention is preferably a component for a land vehicle, marine vehicle, or aircraft. It is particularly preferably an automotive part, in particular a body part. The component is preferably a B-pillar, side member, A-pillar, sill, or cross member.

[0073] The invention is explained in more detail in connection with the following tables.

[0074] To demonstrate the effectiveness of the invention, several tests were conducted. Slabs with the compositions listed in Table 1, thicknesses of 200–280 mm and widths of 1000–1200 mm, were produced, heated in a pusher-type furnace to a respective temperature T1, and held at T1 for between 30 and 450 minutes until the temperature T1 in the core of the slabs was reached and the slabs were thus thoroughly heated. The production parameters are listed in Table 2. The slabs were discharged from the pusher-type furnace at their respective thorough heating temperatures T1 and subjected to hot rolling. The tests were carried out as continuous hot strip rolling. For this purpose, the slabs were first pre-rolled to an intermediate product with a thickness of 40 mm. The intermediate products, which in hot strip rolling can also be referred to as pre-strips, each had an intermediate product temperature T2 at the end of the pre-rolling phase.The pre-rolled strips were fed to the finish rolling immediately after rough rolling, so that the intermediate product temperature T2 corresponds to the initial rolling temperature for the finish rolling phase. The pre-rolled strips were rolled into hot strips with a final thickness of 3–7 mm and the respective final rolling temperatures T3 specified in Table 2, cooled to the respective coiling temperature, and wound into coils at the respective coiling temperatures T4 and then cooled in still air. The hot strips were descaled conventionally by pickling before being subjected to cold rolling with the cold rolling degrees specified in Table 2. The strips were then coiled into coils using a tensile force specified in Table 2. The thickness of the steel strips produced was 1.6 mm in all tests.

[0075] Blanks were cut from the steel strips produced in this way and used for further tests. The blanks were then coated with different protective layers, although one sample was left uncoated for reference. This was done in all cases by physical vapor deposition. For further tests, sheet metal specimens in the form of 200 x 300 mm 2< plates were hot-pressed from the respective blanks. For this purpose, the blanks were heated in a heating device, for example a conventional heating furnace, from room temperature at an average heating rate r furnace (between 30 °C and 700 °C) in a furnace with a furnace temperature T furnace. The total time in the furnace, which includes heating and holding, is designated t furnace. The dew point of the furnace atmosphere was -5 °C in all cases.The blanks were then removed from the heating device and placed in a forming tool at a temperature of T WZ . At the time of removal from the furnace, the blanks had reached the furnace temperature. The transfer time t Trans , comprising the time required for removal from the heating device, transport to the tool, and insertion into the tool, was between 5 and 14 s. The temperature T Einlg of the blanks upon insertion into the forming tool was above the respective martensite start temperature of +100 °C in all cases. The blanks were formed into the respective sheet metal parts in the forming tool, with the sheet metal parts being cooled in the tool at a cooling rate r WZ . The residence time in the tool is denoted by t WZ . Finally, the samples were cooled in air to room temperature. These parameters are summarized again in Table 3, where "RT" stands for room temperature.

[0076] After the forming process, the mass of the respective samples was determined and compared with the mass before heating and forming.

[0077] Furthermore, after forming, the weldability was determined using the above-mentioned method.

[0078] The overall results are summarized in Table 4. Column 4 indicates the coating material. The first three samples were coated with magnesium, resulting in a protective layer consisting of magnesium and unavoidable impurities (<1 wt%). Samples 4-6 were coated with manganese, resulting in a protective layer consisting of manganese and unavoidable impurities (<1 wt%). Column 6 shows the relative mass change during the forming process. Ideally, heat treatment only results in a slight increase in mass due to oxidation of the protective layer. With mass increases of 0.5% or more, the oxidation is too severe and effective scale protection is not provided. In the case of the Mg 100nm and Mn 1000nm variants, the mass decreases despite oxidation. In the samples, delamination of oxide coating components occurred during the forming process. These layers are therefore also unsuitable as scale protection.However, between the two extremes, there is a window of application for both coatings, where only a moderate and desired mass increase occurs. Furthermore, it was found that the weldability (WPS) of the samples with a small mass increase improves (indicated in Table 4 with "iO" = "good" as opposed to "niO" = "not good"). Table 1 (steel grades) Steel C Si Mn Al Cr Nb Ti B P S N Sn Ace Cu Mon Ca Other A 0,235 0,3 1,3 0,05 0,28 0,003 0,04 0,0035 0,02 0,003 0,007 0,03 0,01 0,03 0,03 0,005 Table 2 (Manufacturing conditions for flat steel products) Process variant T1 [°C] T2 [°C] T3 [°C] T4 [°C] KWG [%] Tensile force of reels [daN] a 1205 1060 820 550 55 1000 Figures partially rounded Table 3 (Hot forming parameters) Hot forming variant Average heating rate r furnace [30 - 700 °C] [K / s] T oven [°C] t oven [min.] Transfer time [s] Dew point oven [°C] T Einlg [°C] T WZ [°C] t WZ [s] Cooling rate r WZ [K / s] T Target [°C] I 8 920 4 8 5 800 RT 15 50 50 Figures partially rounded Table 4 (Results) Experiment No. Steel Thickness of the steel strip [mm] Coating material Thickness of the protective layer [nm] Relative mass change Δ m m [%] Welding suitability 1* A 1,6 Mg 10 0,5 not OK 2 A 1,6 Mg 50 0,2 OK 3* A 1,6 Mg 100 -0,4 not OK 4* A 1,6 Mn 250 0,5 not OK 5 A 1,6 Mn 500 0,3 OK 6* A 1,6 Mn 1000 -0,3 OK 7* A 1,6 - -0,03 not OK * non-inventive reference examples

Claims

1. A flat steel product for producing a sheet metal part by hot forming, comprising a) a steel substrate consisting of a steel containing 0.1 - 3 wt.% Mn and optionally up to 0.01 wt.% B, and b) a protective coating arranged on at least one side of the steel substrate, characterized in that the protective coating contains a protective layer comprising Mg with a thickness of 20 - 80 nm or a protective layer comprising Mn with a thickness of 350 - 800 nm.

2. Flat steel product according to claim 1, characterized in that the steel, in addition to iron and unavoidable impurities (in wt%), C: 0.04 - 0.45 wt.%, Si: 0.02 - 1.2 wt.%, Mn: 0.5 - 2.6 wt.%, Al: 0.02 - 1.0 wt.%, P: ≤ 0.05 wt%, S: ≤ 0.02 wt.%, N: ≤ 0.02 wt.%, Sn: ≤ 0.03 wt%, Ace: ≤ 0.01 wt.%, Approx: ≤ 0.005 wt%, and optionally one or more of the elements "Cr, B, Mo, Ni, Cu, Nb, Ti, V, W" in the following contents Cr: 0.08 - 1.0 wt.%, B: 0.001 - 0.010 wt.%, Mon: ≤ 0.5 wt%, No: ≤ 0.5 wt%, Cu: ≤ 0.2 wt%, Note: 0.01 - 0.2 wt.%, Ti: 0.01 - 0.10 wt.%, V: ≤ 0.3 wt%, W: 0.001 - 1.00 wt.%, consists.

3. A process for producing a steel flat product for hot forming with a coating, comprising the following steps: a) providing a slab or a thin slab consisting of a steel containing 0.1 - 3 wt.% Mn and optionally up to 0.01 wt.-% B; b) through-heating the slab or thin slab at a temperature (T1) of 1000 - 1400 °C; c) optionally pre-rolling the through-heated slab or thin slab to an intermediate product with an intermediate product temperature (T2) of 1000 - 1200 °C; d) hot rolling to a hot-rolled flat steel product, wherein the final rolling temperature (T3) is 750 - 1000 °C; e) optionally coiling the hot-rolled flat steel product, wherein the coiling temperature (T4) is at most 700 °C; f) optionally descaling the hot-rolled flat steel product; g) optionally cold rolling the flat steel product, wherein the cold rolling degree is at least 30%; h) Coating the flat steel product with a protective layer comprising Mg with a thickness of 20 - 80 nm or a protective layer comprising Mn with a thickness of 350 - 800 nm.

4. Method according to claim 3 characterized in that the coating is carried out by means of physical vapor deposition or electrolytic deposition.

5. Sheet metal part, in particular formed from a flat steel product according to one of claims 1 to 2, comprising a) a steel substrate consisting of a steel containing 0.1 - 3 wt.% Mn and optionally up to 0.01 wt.% B, b) and a protective coating arranged on at least one side of the steel substrate, characterized in that the protective coating contains a protective layer comprising Mg with a thickness of 20 - 80nm or a protective layer comprising Mn with a thickness of 350 - 800nm.

6. Sheet metal part according to claim 5, characterized in that the sheet metal part comprises an oxide layer arranged on the corrosion protection coating.

7. Sheet metal part according to one of claims 5 to 6 characterized in that the steel substrate of the sheet metal part has a structure with at least partially more than 80% martensite, preferably at least partially more than 90% martensite.

8. Sheet metal part according to one of claims 5 to 7, characterized in thatthe steel, in addition to iron and unavoidable impurities (in wt%), C: 0.04 - 0.45 wt.%, Si: 0.02 - 1.2 wt.%, Mn: 0.5 - 2.6 wt.%, Al: 0.02 - 1.0 wt.%, P: ≤ 0.05 wt%, S: ≤ 0.02 wt.%, N: ≤ 0.02 wt.%, Sn: ≤ 0.03 wt%, Ace: ≤ 0.01 wt.%, Approx: ≤ 0.005 wt%, and optionally one or more of the elements "Cr, B, Mo, Ni, Cu, Nb, Ti, V, W" in the following contents Cr: 0.08 - 1.0 wt.%, B: 0.001 - 0.010 wt.%, Mon: ≤ 0.5 wt%, No: ≤ 0.5 wt%, Cu: ≤ 0.2 wt%, Note: 0.01 - 0.2 wt.%, Ti: 0.01 - 0.10 wt.%, V: ≤ 0.3 wt%, W: 0.001 - 1.00 wt.%, consists.

9. 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 2; b. Heating the sheet metal blank such that the AC3 temperature of the blank is at least partially exceeded and the temperature T Einlg the blank, when placed in a forming tool intended for hot press forming (working step c)), at least partially has a temperature above Ms+100 °C, where Ms denotes the martensite start temperature; c. Placing the heated sheet metal blank in a forming tool, wherein the transfer time required for removing the blank from the heating device and placing it in the forming tool is t Transat 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 hot-press formed over a period t WZ of more than 1 s with a cooling rate r of at least partially more than 30 K / s WZ to the target temperature T Ziel cooled and optionally kept there; e. Removing the sample cooled to the target temperature T Ziel cooled sheet metal part from the tool; f. cooling the sheet metal part to room temperature.

10. The method according to claim 9, wherein the temperature at least partially reached in the sheet metal blank in step b) is between Ac3 and 1000 °C, preferably between 850 °C and 950 °C.

11. Method according to one of claims 9 to 10, wherein the target temperature T Ziel of the sheet metal part is at least partially below 400 °C, preferably below 300 °C.

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