Molded sheet-metal part with improved hardness curve

Abstract

The invention relates to a molded sheet-metal part which is formed from a steel sheet-metal blank, comprising a steel substrate (1) which consists of steel that has 0.1-3 wt.% of Mn and optionally up to 0.01 wt.% of B. The molded sheet-metal part has an aluminum-based corrosion protection coating on at least one face and is characterized in that the value of the hardness gradient of the corrosion protection coating and the steel substrate perpendicularly to the surface of the steel substrate is less than 1.7 GPa / µm. The invention additionally relates to a method for producing such a molded sheet-metal part.

Description

[0001] Sheet metal part with improved hardness profile

[0002] The invention relates to a method for producing a sheet metal part by hot forming a steel sheet blank.

[0003] "Steel sheet blanks" or "sheet metal blanks" are understood here to mean blanks from flat steel products, such as blanks. When a "steel flat product" or "sheet metal product" is mentioned, 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" are made from such sheet metal blanks, whereby the terms "formed sheet metal part" and "sheet metal component" are used synonymously here.

[0004] All information regarding the contents of the steel compositions specified in this application is based on weight, unless expressly stated otherwise. All unspecified "%" data relating to a steel alloy are therefore to be understood as data in "wt. %." With the exception of the data relating to the residual austenite content of the microstructure of a sheet metal part according to the invention, which is based on volume (specified in "vol. %"), data on the contents of the various microstructure components (e.g., martensite) refer to the area of ​​a microsection of a sample of the respective product (specified in area percent, "area %"), unless expressly stated otherwise. Information provided in this text regarding the contents of the components of an atmosphere refers to the volume (specified in "vol. %").

[0005] Where formulas or conditions are mentioned in this text in which values ​​are calculated or formed on the basis of contents of certain alloying elements, the respective contents of alloying elements are entered in these formulas or conditions in wt.%, unless otherwise stated.

[0006] From WO 2022 / 048990 A1, flat steel products and formed sheet metal parts and methods for their production are known. The flat steel products and formed sheet metal parts have an aluminum-based corrosion protection coating produced by hot-dip coating. The melt used has a Si content of 0.05-3 wt.%. Such flat steel products have an aluminum-based coating and are further processed into formed sheet metal parts by hot forming. In this process, blanks from the flat steel products are heated to a hot forming temperature (e.g., 900°C) for a certain annealing time (e.g., 4 minutes). During this annealing time, iron diffuses from the steel substrate into the aluminum-based coating. This results in a coating that provides very effective protection against corrosion. The hot blank is then formed into a formed sheet metal part in a forming tool and quickly cooled, whereby a hardened microstructure (e.g.,Martensite) forms in the steel substrate. The result is a sheet metal part with high strength and a coating that provides excellent protection against corrosion.

[0007] The object of the present invention is to further develop such sheet metal parts and their manufacturing processes to achieve improved mechanical properties. In particular, crack formation during subsequent cold forming (e.g., in the event of a crash) is to be reduced.

[0008] This object is achieved by a method for producing a steel flat product for hot forming with a coating comprising the following work steps: a) Providing a slab or a thin slab consisting of a steel which contains 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) annealing the flat steel product at an annealing temperature (T5) of 650-900°C; i) cooling the flat steel product to an immersion temperature (T6) which is 600-800°C, preferably 680-720°C; j) coating the flat steel product cooled to the immersion temperature with a coating by i.Immersion in a molten bath with a melting temperature (T7) of 660-800°C, preferably 670-710°C, wherein the molten bath consists of 0.5-4 wt.% Si, optionally 2-4 wt.% Fe, optionally up to 5.0 wt.% alkali or alkaline earth metals, optionally up to 10% Zn and optionally further components, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder being aluminum; ii. Blowing off the flat steel product after it has left the molten bath by means of a gas stream; k) Cooling the coated flat steel product to room temperature, wherein an average cooling rate between 660°C and 570°C is at least 15 K / s; l) Optionally skin passing the coated flat steel product.

[0009] Surprisingly, it has been shown that the Si content of the melt in the range of 0.5–4 wt.% Si, especially in the range of 0.5–1.5 wt.%, in combination with the specific cooling rate in step k), results in a flat steel product that can be formed into a sheet metal part with improved properties. The mechanism involved is explained in detail below.

[0010] In step a), a semi-finished product composed of the alloy specified for the steel flat product according to the invention is provided. This can be a slab produced by conventional continuous slab casting or by thin slab casting.

[0011] In step b), the semi-finished product is subjected to a temperature CT 1) of 1000–1400°C. If the semi-finished product has cooled after casting, it is first reheated to 1000–1400°C for through-heating. The through-heating temperature should be at least 1000°C to ensure good formability for the subsequent rolling process. The through-heating temperature should not exceed 1400°C to avoid the presence of molten phases in the semi-finished product. In the optional step c), the semi-finished product is pre-rolled into an intermediate product. Thin slabs are not usually subjected to pre-rolling. Thick slabs to be rolled into hot strip can be subjected to pre-rolling if required. In this case, the temperature of the intermediate product (T2) at the end of rough rolling should be at least 1000°C so that the intermediate product contains sufficient heat for the subsequent finish rolling step.However, high rolling temperatures can also promote grain growth during the rolling process, which adversely affects the mechanical properties of the flat steel product. To minimize grain growth during the rolling process, the temperature of the intermediate product at the end of pre-rolling should not exceed 1200°C.

[0012] 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.

[0013] 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 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.

[0014] In step f), the hot-rolled flat steel product is optionally descaled in a conventional manner by pickling or by another suitable treatment.

[0015] The scale-cleaned hot-rolled flat steel product can optionally be subjected to cold rolling prior to annealing in step g), for example, to meet more stringent thickness tolerance requirements. The cold rolling degree (KWG) should be at least 30% to inject sufficient deformation energy into the flat steel product for rapid recrystallization. The cold rolling degree KWG is defined as the quotient of the thickness reduction during cold rolling (AdKW) divided by the hot strip thickness d:

[0016] KWG = AdKW / d, where AdKW = thickness reduction during cold rolling in mm and d = hot strip thickness in mm, where the thickness reduction AdKW 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 typically 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, be very high, exceeding 90%. However, cold rolling degrees of no more than 80% have proven to be beneficial for preventing strip breakage.

[0017] In step h), the flat steel product is subjected to an annealing treatment at annealing temperatures (T5) of 650-900°C. For this purpose, the flat steel product is first heated to the annealing temperature within 10 to 120 seconds and then held at that temperature for 30 to 600 seconds. The annealing temperature is at least 650°C, preferably at least 720°C. Annealing temperatures above 900°C are undesirable for economic reasons.

[0018] In step i), the flat steel product is cooled to an immersion temperature (T6) after annealing to prepare it for subsequent coating treatment. The immersion temperature is lower than the annealing temperature and is adjusted to the temperature of the molten bath. The immersion temperature is 600-800°C, preferably at least 650°C, more preferably at least 670°C, and most preferably at most 700°C. For a particularly homogeneous boundary layer formation, it is important that sufficient thermal energy is present in the boundary layer between the steel substrate and the aluminum melt. This is not the case at temperatures lower than 600°C, so that undesirable compounds can form, the subsequent reconversion of which can lead to pores.Above the preferred immersion temperatures, the diffusion rate of iron into aluminum increases significantly again, allowing increased iron diffusion into the still-liquid boundary layer right at the beginning of the coating process. The cooling time of the annealed flat steel product from the annealing temperature T5 to the immersion temperature T6 is preferably 10-180 seconds. In particular, the immersion temperature T6 should not deviate from the temperature of the molten bath T7 by more than 30°C, especially not more than 20°C, and preferably not more than 1°C.

[0019] The flat steel product is subjected to a coating treatment in step j). The coating treatment is preferably carried out by continuous hot-dip coating. The coating can be applied to just one side, both sides, or all sides of the flat steel product. The coating treatment is preferably carried out as a hot-dip coating process, in particular as a continuous process. The flat steel product usually comes into contact with the molten bath on all sides, so that it is coated on all sides. The molten bath, which contains the alloy to be applied to the flat steel product in liquid form, typically has a temperature (T7) of 660-800°C, preferably 670-740°C, particularly preferably 670-710°C. Aluminum-based alloys have proven particularly suitable for coating ageing-resistant flat steel products with a corrosion-protective coating.In such a case, the molten bath contains 0.5 to 4 wt.% Si, in particular 0.5-1.5 wt.% Si, optionally 2-4 wt.% Fe, optionally 0.1-5.0 wt.% alkali or alkaline earth metals, preferably up to 1.0 wt.% alkali or alkaline earth metals, and optional further constituents whose total contents are limited to a maximum of 2.0 wt.%, and the remainder aluminum. In a preferred variant, the optional content of alkali or alkaline earth metals in the melt comprises 0.1-1.0 wt.% Mg, in particular 0.1-0.7 wt.% Mg, preferably 0.1-0.5 wt.% Mg. Furthermore, the optional content of alkali or alkaline earth metals in the melt can comprise in particular at least 0.0015 wt.% Ca, in particular at least 0.01 wt.% Ca. In particular, the optional content of alkali or alkaline earth metals in the melt consists of 0.1-1.0 wt% Mg, in particular 0.1-0.7 wt% Mg, preferably 0.1-0.5 wt% Mg and optionally at least 0.0015 wt% Ca, preferably at least 0.01 wt% Ca.

[0020] After leaving the molten bath, the flat steel product is blown off using a gas stream to adjust the thickness of the coating.

[0021] After the coating treatment, the coated flat steel product is cooled to room temperature in step k). The average cooling rate between 660°C and 570°C is at least 15 K / s, preferably at least 20 K / s. This corresponds to the range between the beginning and end of solidification of the coating. Upon cooling to 660°C, solidification of the coating begins, and upon further cooling to 570°C, the coating is fully solidified. The average cooling rate is preferably a maximum of 100 K / s, particularly preferably a maximum of 50 K / s.

[0022] Even during hot-dip coating and cooling, a certain amount of iron diffusion from the steel substrate into the coating occurs. This diffusion is restarted when the sheet blanks of the resulting flat steel product are reheated to temperatures above AC3 prior to forming. While diffusion during hot-dip coating and cooling occurs at temperatures ranging from approximately 750°C down to 570°C (at 570°C, diffusion essentially stops due to the solidification of the coating), diffusion occurs during reheating prior to forming at temperatures around 900°C. Surprisingly, it has been shown that the interface between the steel substrate and the coating develops differently depending on the temperature at which the diffusion occurs.In the lower temperature range of 750°C to 570°C, diffusion is comparatively slow, resulting in a relatively sharp transition between the steel substrate and the coating. In contrast, at temperatures of 900°C, diffusion processes occur significantly faster. In addition, molten phases form in the coating, so that in addition to the uniform diffusion, a certain amount of mixing occurs due to convection. Such convection zones are naturally randomly distributed. As a result, these two effects lead to a less sharp transition between the coating and the steel substrate.

[0023] The inventors have recognized that it is advantageous to stop the diffusion processes after hot-dip coating as quickly as possible by setting an average cooling rate between 660°C and 570°C of at least 15 K / s, preferably at least 20 K / s. This shifts the diffusion to the later process step during reheating, where it occurs at higher temperatures. The result is a more uneven transition between the steel substrate and the coating in the resulting sheet metal part. This more uneven transition manifests itself as a flatter average hardness gradient in the transition between the steel substrate and the coating. The advantages of this hardness gradient are explained below with reference to the sheet metal part.

[0024] The coated flat steel product can optionally be subjected to skin passing with a skin passing degree of up to 2% to improve the surface roughness of the flat steel product.

[0025] The steel used in the process for manufacturing a flat steel product, in the process for manufacturing a sheet metal part, and in the sheet metal part itself is a steel containing 0.1-3 wt.% Mn and optionally up to 0.01 wt.% B. The same applies, of course, to the steel of the hot-formed sheet metal part.

[0026] In particular, the steel structure can be converted into a martensitic or partially martensitic structure through hot forming. The structure of the steel substrate of the sheet metal part is therefore preferably a martensitic or at least partially martensitic structure, as this exhibits particularly high hardness.

[0027] Particularly preferably, the steel substrate is a steel which, in addition to iron and unavoidable impurities (in wt%), consists of

[0028] C: 0.04-0.45 wt%,

[0029] Si: 0.02-1.2 wt%,

[0030] Mn: 0.5-2.6 wt%,

[0031] AI: 0.02-1.0 wt%,

[0032] P: < 0.05 wt%,

[0033] S: < 0.02 wt%,

[0034] N: < 0.02 wt%,

[0035] Sn: < 0.03 wt%,

[0036] As: < 0.01 wt%,

[0037] Ca: < 0.005 wt.%, and optionally one or more of the elements “Cr, B, Mo, Ni, Cu, Nb, Ti, V” in the following contents

[0038] Cr: 0.08-1.0 wt.%,

[0039] B: 0.001-0.005 wt%,

[0040] Mo: <0.5 wt%,

[0041] Ni: <0.5 wt%,

[0042] Cu: <0.2 wt%,

[0043] Nb: 0.01-0.08 wt%,

[0044] Ti: 0.01-0.08 wt%,

[0045] V: <0.3 wt%.

[0046] 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.%.

[0047] In addition to these elements, other elements may also be present as impurities in the steel. These additional elements are summarized under the term "unavoidable impurities." The total content of unavoidable impurities is preferably a maximum of 0.2 wt.%, preferably a maximum of 0.1 wt.%. The optional alloying elements Cr, B, Nb, and Ti, for which a lower limit is specified, may also be present as unavoidable impurities in the steel substrate in amounts below the respective lower limit. In this case, they are also counted as unavoidable impurities, with their total content limited to a maximum of 0.2 wt.%, preferably a maximum of 0.1 wt.%. The individual upper limits for the respective impurities of these elements are preferably as follows:

[0048] Cr: < 0.050 wt%,

[0049] B: < 0.0005 wt%

[0050] Nb: < 0.005 wt%,

[0051] Ti: < 0.005 wt%

[0052] These preferred upper limits should be considered alternatively or jointly. Preferred variants of the steel therefore meet one or more of these four conditions. 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.%.

[0053] In a preferred embodiment, the Si content of the steel is a maximum of 1.00 wt.% and / or at least 0.06 wt.%.

[0054] 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.%.

[0055] 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%.

[0056] 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 (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.%.

[0057] 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.%.

[0058] 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.%.

[0059] 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.%.

[0060] Preferably, the steel optionally also contains boron at a content of 0.001-0.005 wt.%. In particular, the boron content is a maximum of 0.004 wt.%.

[0061] Optionally, the steel may contain molybdenum in a maximum content of 0.5 wt%, in particular a maximum of 0.1 wt%.

[0062] Furthermore, the steel may optionally contain nickel with a content of maximum 0.5 wt.%, preferably maximum 0.15 wt.%.

[0063] Optionally, the steel may also contain copper with a content of maximum 0.2 wt.%, preferably maximum 0.15 wt.%.

[0064] 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 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 0.3 wt.%, preferably a maximum of 0.2 wt.%, in particular a maximum of 0.1 wt.%, preferably a maximum of 0.05 wt.%.

[0065] 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.%.

[0066] The above explanations regarding preferred steel substrates naturally also apply to the steel substrate of the flat steel product described below, as well as the steel substrates in the manufacturing processes described.

[0067] The resulting flat steel product includes an aluminum-based corrosion protection coating. The corrosion protection coating can be applied to one or both sides of the flat steel product. The two large, opposing surfaces of the flat steel product are referred to as the two sides. The narrow surfaces are referred to as the edges.

[0068] As already explained, during hot-dip coating, iron diffuses from the steel substrate into the liquid coating, so that the corrosion protection coating of the flat steel product has, in particular, an alloy layer and an Al base layer upon solidification.

[0069] The alloy layer lies on the steel substrate and directly borders it. The alloy layer is essentially formed from aluminum and iron. The remaining elements from the steel substrate or the melt composition 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%, 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.

[0070] 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 0.5-4 wt.% Si, optionally 2-4 wt.% Fe, optionally up to 5.0 wt.% alkali or alkaline earth metals, optionally up to 10% Zn, and optionally other components whose total contents are limited to a maximum of 2.0 wt.%, with the remainder being aluminum. Preferred compositions of the Al base layer correspond to the preferred melt compositions.

[0071] 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 can in particular comprise at least 0.0015 wt.% Ca, in particular at least 0.01 wt.% Ca. In particular, the optional content of alkali or alkaline earth metals in the melt consists of 0.1-1.0 wt.% Mg, in particular 0.1-0.7 wt.% Mg, preferably 0.1-0.5 wt.% Mg and optionally at least 0.0015 wt.% Ca, preferably at least 0.01 wt.% Ca. In a further preferred variant of the corrosion protection coating, the Si content in the alloy layer is lower than the Si content in the Al base layer.

[0072] The corrosion protection coating preferably has a thickness of 5-60 μm, in particular 10 to 40 μm. The coating weight of the corrosion protection coating is in particular 30 - 360 μm for double-sided corrosion protection coatings or 15 - 180 μm for the single-sided variant. The coating weight of the corrosion protection coating is preferably 100-200^ for double-sided coatings or 50-100^ for single-sided coatings. The coating weight of the corrosion protection coating is particularly preferably 120-180^ for double-sided coatings or 60-90^ for single-sided coatings.

[0073] The thickness of the alloy layer is preferably less than 20 μm, particularly preferably less than 16 μm, particularly preferably less than 12 μm, and especially less than 10 μm. The thickness of the Al base layer results from the difference between the thicknesses of the anti-corrosive coating and the alloy layer. The thickness of the Al base layer is preferably at least 1 μm, even with thin anti-corrosive coatings.

[0074] In a preferred variant, the flat steel product comprises an oxide layer arranged on the corrosion protection coating. The oxide layer is located in particular on the aluminum base layer and preferably forms the outer edge of the corrosion protection coating.

[0075] The oxide layer consists in particular of more than 80 wt.% oxides, with the majority of the oxides (i.e., more than 50 wt.% of the oxides) being aluminum oxide. Optionally, in addition to aluminum oxide, hydroxides and / or magnesium oxide are present in the oxide layer, alone or as a mixture. Preferably, the remainder of the oxide layer not occupied by the oxides and optionally present hydroxides consists of silicon, aluminum, iron, and / or magnesium in metallic form. For the optional embodiment with zinc as a constituent of the aluminum base layer, zinc oxide components are also present in the oxide layer.

[0076] The oxide layer of the flat steel product preferably has a thickness greater than 50 nm. In particular, the maximum thickness of the oxide layer is 500 nm. The invention also relates to a method for producing a shaped sheet metal part. The shaped sheet metal part is particularly designed as described in detail below. The method comprises the following steps: a. Producing a flat steel product according to the method described above; b. Separating a sheet metal blank from the flat steel product; c. Heating the sheet metal blank in a furnace with a furnace temperature T O f en during a glow time t G such that the AC3 temperature of the blank is at least partially exceeded and the temperature T Einigthe 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; d. 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 Tr ans is at most 20 s, preferably at most 15 s; e. hot-press forming the sheet metal blank to form the sheet metal part, wherein the blank is subjected to a heat treatment over a period t wz of more than ls with a cooling rate r of at least partially more than 30 K / s wz to the target temperature T Ziei cooled and optionally kept there; f. Removing the cooled sample to the target temperature T Ziei cooled sheet metal part from the tool.

[0077] In the method according to the invention, a flat steel product is first produced as described above. A sheet metal blank is cut from this flat steel product. This blank, which consists of a steel suitably composed according to the above explanations, is then heated in a conventional manner such that the AC3 temperature of the blank is at least partially exceeded and the temperature T Einigof the blank when placed in a forming tool intended for hot press forming (working step d)) is at least partially at a temperature above Ms+100°C. For the purposes of this application, partially exceeding a temperature (here AC3 or Ms+100°C) is understood 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, at least 30% of the blank therefore has an austenitic structure, i.e. the transformation from the ferritic to the 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 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 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 that have a martensitic microstructure. In this way, a softer area can be specifically set in the formed sheet metal part, for example by ensuring optimal toughness for the respective intended use, while the other areas of the sheet metal part have maximized strength.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] An optimally uniform distribution of properties can be achieved by heating the blank completely in step b).

[0082] In a preferred embodiment, the average heating rate r O f en of the sheet metal blank during heating in step b) 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 O f en is to be understood as the average heating rate from 30°C to 700°C.

[0083] In a preferred embodiment, heating takes place in an oven with an oven temperature Tof en 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.

[0084] 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.

[0085] 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. 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, and particularly preferably a maximum of 930°C.

[0086] The total furnace time, which consists of a heating time and a holding time, is preferably at least 2 minutes, in particular at least 3 minutes, and preferably at least 4 minutes for both variants (constant furnace temperature, stepwise heating). Furthermore, the total furnace time for both variants is preferably a maximum of 20 minutes, in particular a maximum of 15 minutes, preferably a maximum of 12 minutes, and in particular a maximum of 8 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.

[0087] 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 maximum temperature is in particular 900°C. These temperature ranges ensure good formability of the material overall.

[0088] In step d), the transfer of the austenitized blank from the heating device used to the forming tool is completed within preferably no more than 20 seconds, especially no more than 15 seconds. Such rapid transport is necessary to avoid excessive cooling prior to forming.

[0089] When inserting the blank, the tool typically has a temperature between room temperature (RT) and 200°C, preferably between 20°C and 180°C, in particular between 50°C and 150°C. Optionally, in a special embodiment, the tool can be heated at least partially to a temperature T wz of at least 200°C, especially at least 300°C, in order to only partially harden the component. Furthermore, the tool temperature T W z preferably a maximum of 600°C, especially a maximum of 550°C. It is only necessary to ensure that the tool temperature T wzbelow the desired target temperature T Zjei The residence time in the tool t wz is preferably at least 2 seconds, in particular at least 3 seconds, particularly preferably at least 5 seconds. The maximum residence time in the tool is preferably 25 seconds, in particular a maximum of 20 seconds.

[0090] The target temperature T Ziei 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 Ziei 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. The martensite start temperature of a steel within the scope of the inventive specifications is according to the formula

[0091] 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.%.

[0092] The ACl temperature and the AC3 temperature of a steel within the scope of the specifications according to the invention are according to the formulas

[0093] AC1[°C] = (739 — 22*%C - 7*%Mn + 2*%Si + 14*%Cr + 13*%Mo - 13*%Ni +20*%V )[°C / wt.-%] and

[0094] 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 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)

[0095] In the tool, the blank is not only formed into the sheet metal part, but simultaneously quenched to the target temperature. The cooling rate in the tool r wz 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, in a special embodiment at least 100 K / s.

[0096] After removing the sheet metal part in step e), the sheet metal part is cooled to a cooling temperature T A B of less than 100°C within a cooling time t AB from 0.5 to 600 s. This is usually done by air cooling. The invention further relates to a sheet metal part formed from a steel sheet blank comprising a steel substrate made of a steel containing 0.1-3 wt.% Mn and optionally up to 0.01 wt.% B. The sheet metal part has an aluminum-based anti-corrosion coating on at least one side, wherein the hardness gradient between the anti-corrosion coating and the steel substrate perpendicular to the surface of the steel substrate is less than 1.7 GPa / pm.

[0097] Such a sheet metal part can be manufactured using the method described above, among others.

[0098] The hardness gradient is determined according to the following procedure:

[0099] - embedding at least part of the sheet metal part in an embedding compound and producing a cross-section of the sheet metal part;

[0100] - Polishing of the cross-section and light etching with 3% Nital (alcoholic nitric acid);

[0101] - defining a measuring field on the cross-section of at least 50 pm width parallel to the surface and a Cartesian measuring grid on the measuring field, with the Cartesian x-axis running perpendicular to the surface of the steel substrate and the Cartesian y-axis running parallel to the surface of the steel substrate and with the grid spacing being 1.5 pm in both directions;

[0102] - Measurement of nanohardness at the grid points of the measuring grid using a nanoindenter with a calibrated Berkovich pyramid as test syringe and with a load function with a maximum load of 2000 pN;

[0103] - Calculating a hardness curve as a function of the Cartesian x-coordinate by assigning to each x-coordinate the mean value of the nanohardness over all grid points with this x-coordinate (i.e. the hardness values ​​of all points with the same distance to the surface of the steel substrate are averaged);

[0104] - Calculate the hardness gradient as a difference quotient of the hardness curve.

[0105] The "Hysitron TI Premier" device from Bruker, for example, is used as a nanoindenter. Details about the device can be obtained from Bruker or accessed, for example, at the following link: https: / / www.bruker.com / en / products-and-solutions / test-and-measurement / nanomechanical-test-systems / hysitron-ti-premier-nanoindenter.html. With the nanoindenter, a specific measuring tip, such as a Berkovich tip (made of diamond), is pressed into a sample to be tested. Based on the measured force-indentation curve, a hardness can be determined, preferably using the Oliver & Pharr evaluation method (the method can be accessed at the following link: https: / / www.sciencedirect.com / topics / engineering / oliver-pharr-method).

[0106] A hardness gradient smaller than 1.7 GPa / pm means that the hardness averaged parallel to the surface of the steel substrate changes more slowly.

[0107] A narrower hardness gradient has the advantage of greater resistance to cracking. A sharp hardness gradient is always an indicator of a "predetermined breaking point." However, if the hardness transition occurs more slowly, cracking only occurs at higher stress intensities during deformation (e.g., in a crash). The material can thus withstand higher external forces before failing.

[0108] Preferably, the corrosion protection coating of the sheet metal part comprises an alloy layer and an Al base layer.

[0109] The alloy layer lies on the steel substrate and borders directly on it. The alloy layer of the sheet metal part preferably consists of 35-95 wt.% Fe, preferably 60-95 wt.% Fe, 0.1-4 wt.% Si and optional further components, the total contents of which are limited to a maximum of 3.5 wt.%, preferably 2.0 wt.%, with aluminum as the remainder. The optional further components are preferably the elements present in the steel of the steel substrate alongside iron and the other elements from the melt, such as Zn and alkali or alkaline earth metals. These elements from the melt only accumulate in the alloy layer to a very small extent. The alloy layer preferably has a ferritic structure.

[0110] The Al base layer of the sheet metal part lies on the alloy layer of the steel component and is directly adjacent to it. The Al base layer of the steel component preferably consists of 35-55 wt.% Fe, preferably 40-50 wt.% Fe, 0.4-4 wt.% Si, in particular 0.4-1.5 wt.% Si, optionally 3 wt.% alkali or alkaline earth metals, preferably up to 1.0 wt.% alkali or alkaline earth metals, optionally up to 10% Zn and optional further constituents, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder aluminum. The optional content of alkali or alkaline earth metals is preferably at least 0.1 wt.%.

[0111] In a preferred variant of the Al base layer, the optional content of alkali or alkaline earth metals comprises 0.1-1.0 wt.% Mg, in particular 0.1-0.7 wt.% Mg, preferably 0.1-0.5 wt.% Mg. Furthermore, the optional content of alkali or alkaline earth metals in the Al base layer can in particular comprise at least 0.0015 wt.% Ca, in particular at least 0.1 wt.% Ca. Further preferably, the optional content of alkali or alkaline earth metals consists of 0.1-1.0 wt.% Mg, in particular 0.1-0.7 wt.% Mg, preferably 0.1-0.5 wt.% Mg and optionally at least 0.0015 wt.% Ca, in particular at least 0.1 wt.% Ca.

[0112] The Al base layer can have a homogeneous element distribution, with local element contents varying by no more than 10%. Preferred variants of the Al base layer, however, have silicon-poor phases and silicon-rich phases. Silicon-poor phases are regions whose average Si content is at least 20% lower than the average Si content of the Al base layer. Silicon-rich phases are regions whose average Si content is at least 20% higher than the average Si content of the Al base layer.

[0113] In a preferred variant, the silicon-rich phases are arranged within the silicon-poor phase. In particular, the silicon-rich phases form a layer that is at least 40% continuous and bordered by silicon-poor regions. A continuous layer of silicon-rich phases is understood to mean that, in the vertical micrograph, a line can be drawn parallel to the surface of the steel substrate such that it runs completely through the silicon-rich phases. In contrast, a layer that is at least X% continuous is understood to mean that, in the vertical micrograph, a line can be drawn parallel to the surface of the steel substrate such that it runs at least X% within the silicon-rich phases.In this case, the silicon-rich phases are arranged in such a coherent manner that, in the vertical micrograph, a line can be drawn parallel to the surface of the steel substrate, such that at least 40% of it runs within the silicon-rich phases. In an alternative design variant, the silicon-rich phases are arranged in islands within the silicon-poor phase.

[0114] For the purposes of this application, “island-shaped” is understood to mean an arrangement in which discrete, unconnected regions are enclosed by another material - i.e. “islands” of a particular material are located within another material. In a preferred variant, the steel component comprises an oxide layer arranged on the corrosion protection coating. The oxide layer is located in particular on the Al base layer and preferably forms the outermost layer of the corrosion protection coating. The oxide layer of the steel component consists in particular of more than 80 wt.% oxides, with the main proportion of the oxides (i.e. more than 50 wt.% of the oxides) being aluminum oxide. Optionally, hydroxides and / or magnesium oxide are present in addition to aluminum oxide, alone or as a mixture.Preferably, the remainder of the oxide layer not occupied by the oxides and optionally present hydroxides consists of silicon, aluminum, iron and / or magnesium in metallic form.

[0115] The oxide layer preferably has a thickness of at least 50 nm, in particular of at least 100 nm. Furthermore, the thickness is preferably a maximum of 4 pm, in particular a maximum of 2 pm.

[0116] 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.

[0117] The invention is explained in more detail with reference to the figures. They show:

[0118] Fig. 1 a scanning electron micrograph of the area examined in Experiment 1 with the indenter tip,

[0119] Fig. 2 shows the hardness curve of the sample according to the invention,

[0120] Fig. 3 shows the hardness gradient of the sample according to the invention,

[0121] Fig. 4 the hardness curve of the comparison sample and

[0122] Fig. 5 shows the hardness gradient of the comparison sample.

[0123] To demonstrate the effectiveness of the invention, several tests were conducted. Slabs with the compositions specified in Table 1, a thickness of 240 mm and a width of 1200 mm, were produced and heated in a pusher-type furnace to a temperature TI of 1200°C. The slabs were then held at TI for between 30 and 450 minutes until the temperature TI in the core of the slabs was reached and the slabs were thus thoroughly heated. The slabs were discharged from the pusher-type furnace at their respective through-heating temperatures TI 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. At the end of the pre-rolling phase, the intermediate products, which in hot strip rolling can also be referred to as pre-strips, each had an intermediate product temperature T2 of 1100°C.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 4 mm and a final rolling temperature T3 of 890°C, cooled to the respective coiling temperature, and wound into coils at a coiling temperature T4 of 580°C and then cooled in still air. The hot strips were descaled in a conventional manner by pickling before being subjected to cold rolling until the thickness specified in Table 3 was achieved. The cold-rolled flat steel products were heated in a continuous annealing furnace to an annealing temperature T5 of 870°C and held at annealing temperature for 100s each before being cooled at a cooling rate of 1 K / s to the immersion temperature T6 of 690°C.The cold-rolled strips were passed through a molten coating bath at temperature T7 of 676°C at their respective immersion temperature T6. The strip speed was 76 m / min in all cases. The composition of the coating bath is given in Table 2. After coating, the coated strips were blown off to adjust the coating weights. An air stream was used for this purpose. The temperature of the air stream was 70°C in all cases. The thickness of the coating is given in Table 3. The strips were first cooled to 660°C at an average cooling rate of 10-15 K / s. Between 660°C and 570°C, i.e. between the start of solidification and the end of solidification of the coating, the cooling rate in test 1 according to the invention was 21 K / s. In contrast, the cooling rate in reference test 2 between 660°C and 570°C was only 13 K / s.During the further cooling process between 570°C and room temperature, the strips were cooled at a cooling rate of 5 - 12 K / s each.

[0124] From the steel strips produced in this way, blanks were cut and used for further tests. In these tests, sheet metal specimens measuring 200 x 300 mm were prepared from the blanks. 2 large sheets were hot-pressed. For this purpose, the blanks were heated in a heating device, for example in a conventional heating furnace, from room temperature with an average heating rate r O f en (between 30°C and 700°C) in an oven with an oven temperature T O f en of 900°C. The annealing time in the furnace, which includes heating and holding, is t O f enThe 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, which maintains the temperature T W z. At the time of removal from the furnace, the blanks had reached the furnace temperature. The transfer time t, which consists of the removal from the heating device, transport to the tool, and insertion into the tool, Trans was 8s. The temperature T Einig The temperature of the blanks when placed in the forming tool was in all cases above the respective martensite start temperature +100°C. In the forming tool, the blanks were formed into the respective sheet metal parts, with the sheet metal parts in the tool cooling at a rate of r wz to a target temperature T Ziei The residence time in the tool is given by t Wz. Finally, the samples were cooled in air to room temperature. Table 4 summarizes the parameters mentioned, where "RT" stands for room temperature.

[0125] Cross-sections were made of each of the sheet metal parts produced in this way. For this purpose, a section of the sheet metal part was embedded in an embedding compound. A cross-section of the sheet metal part was then made. This cross-section was polished and lightly etched with 3% nital. On this cross-section, a measuring field of at least 50 pm wide was defined parallel to the surface and a Cartesian measuring grid was defined on the measuring field, with the Cartesian x-axis running perpendicular to the surface of the steel substrate and the Cartesian y-axis running parallel to the surface of the steel substrate and the grid spacing being 1.5 pm in both directions. As an example, Figure 1 shows a scanning electron microscope image of the area examined with the indenter tip in test 1. The section shown corresponds to 70 pm x 70 pm. The steel substrate 1 is clearly visible. The alloy layer 3 is arranged on the steel substrate.The Al base layer 5 is arranged on the alloy layer 3. Above the alloy layer is the embedding compound 7, which is required for producing the cross-section. The cross-section was also lightly etched with 3% Nital. On the cross-section, a measuring field of at least 50 pm wide was defined parallel to the surface and a Cartesian measuring grid was defined on the measuring field, with the Cartesian x-axis running perpendicular to the surface of the steel substrate and the Cartesian y-axis running parallel to the surface of the steel substrate, and the grid spacing being 1.5 pm in both directions. The nanohardness was then determined at the grid points of the measuring grid using a nanoindenter with a Berkovich pyramid as the test probe and a load function with a maximum load of 2000 pN. The indentations left by the test probe at the grid points are clearly visible in Figure 1.

[0126] From the measured nanohardness values ​​at the grid points, a hardness curve was then determined as a function of the Cartesian x-coordinate. For this purpose, the mean nanohardness value across all grid points with this x-coordinate was assigned to each x-coordinate. Thus, the average was calculated across all points at the same distance from the surface of the steel substrate. In Figure 1, these are all grid points that lie vertically above one another.

[0127] Figure 2 shows the resulting hardness curve of Test 1. The coating hardness of approximately 12-13 GPa is clearly visible, decreasing to 5-6 GPa towards the substrate. From this hardness curve, the hardness gradient was then determined by calculating the difference quotient of two adjacent x-values ​​and plotting it in the middle of the interval. The result is shown in Figure 3.

[0128] Figures 4 and 5 show identical values ​​for comparison sample 2. Comparing Figures 3 and 5, it is clearly evident that the hardness gradient is significantly greater in the comparison sample. The magnitude of the hardness gradient assumes values ​​of more than 2 GPa / pm. In contrast, the maximum magnitude of the hardness gradient for sample 1 according to the invention is less than 1.5 GPa / pm.

[0129] hyssenKrupp Steel Europe AG 227047P00DE

[0130] December 12, 2022

[0131] 26 / 27 table 1 (steel grades) Iron and unavoidable impurities. All values ​​are given in wt.%; Table 2 (coating variants)

[0132] hyssenKrupp Steel Europe AG 227047P00DE

[0133] December 12, 2022

[0134] 27 / 27 table 3 (structure) Non-inventive reference examples Table 4 (hot forming parameters)

Claims

Patent claims 1. 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) annealing the flat steel product at an annealing temperature (T5) of 650-900°C; i) cooling the flat steel product to an immersion temperature (T6) which is 600-800°C, preferably 680-720°C; j) coating the flat steel product cooled to the immersion temperature with a coating by i.Immersion in a molten bath with a melting temperature (T7) of 660-800°C, preferably 670-710°C, wherein the molten bath consists of 0.5-4 wt.% Si, optionally 2-4 wt.% Fe, optionally up to 5.0 wt.% alkali or alkaline earth metals, optionally 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; ii. Blowing off the flat steel product after exiting the molten bath using a gas stream; k) cooling the coated flat steel product to room temperature, with an average cooling rate between 660°C and 570°C being at least 15 K / s; l) optionally tempering the coated flat steel product. Process according to claim 1, characterized in that the molten bath consists of 0.5-1.5 wt.% Si, optionally 2-4 wt.% Fe, optionally up to 5.0 wt.% alkali or alkaline earth metals, optionally up to 10% Zn and optional further components, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder being aluminum. Process according to one of claims 1 to 2, characterized in that the steel, in addition to iron and unavoidable impurities (in wt.%), consists of C: 0.04-0.45 wt%, Si: 0.02-1.2 wt%, Mn: 0.5-2.6 wt%, AI: 0.02-1.0 wt%, P: < 0.05 wt%, S: < 0.02 wt%, N: < 0.02 wt%, Sn: < 0.03 wt%, 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 contents 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: 0.01-0.08 wt%, Ti: 0.01-0.08 wt%, V: <0.3 wt%. A method for producing a sheet metal part, in particular according to one of claims 7 to 12, comprising the following steps: a. Producing a flat steel product according to the method according to one of claims 1 to 2; b. Separating a sheet metal blank from the flat steel product; c. Heating the sheet metal blank in a furnace with a furnace temperature T O f en during a glow time t G such that the AC3 temperature of the blank is at least partially exceeded and the temperature T Einigthe blank, when placed in a forming tool intended for hot press forming (working step d)), at least partially has a temperature above Ms+100°C, where Ms denotes the martensite start temperature; d. Placing the heated sheet metal blank in a forming tool, wherein the transfer time t required for removing the blank from the heating device and placing it in the forming tool Tr ans is at most 20 s, preferably at most 15 s; e. hot-press forming the sheet metal blank to form the sheet metal part, wherein the blank is subjected to a heat treatment during the hot-press forming process for a duration t wz of more than ls with a cooling rate r of at least partially more than 30 K / s wz to the target temperature T Ziei cooled and optionally kept there; f. removing the product heated to the target temperature T Zieicooled sheet metal part from the tool. Method according to claim 4, 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. Method according to one of claims 4 to 5, wherein the target temperature T Ziei of the sheet metal part is at least partially below 400°C, preferably below 300°C. Sheet metal part formed from a steel sheet blank comprising a steel substrate (1) which consists of a steel which has 0.1-3 wt.% Mn and optionally up to 0.01 wt.% B, wherein the sheet metal part has an aluminum-based corrosion protection coating on at least one side, characterized in that the amount of the hardness gradient of the corrosion protection coating and the steel substrate perpendicular to the surface of the steel substrate is less than 1.7 GPa / pm. Sheet metal part according to claim 7, the steel in addition to iron and unavoidable impurities (in wt.%) from C: 0.04-0.45 wt%, Si: 0.02-1.2 wt%, Mn: 0.5-2.6 wt%, AI: 0.02-1.0 wt%, P: < 0.05 wt%, S: < 0.02 wt%, N: < 0.02 wt%, Sn: < 0.03 wt%, 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 contents 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: 0.01-0.08 wt%, Ti: 0.01-0.08 wt%, V: <0.3 wt.%. A sheet metal part according to one of claims 7 to 8, characterized in that the anti-corrosive coating comprises an alloy layer (3) and an Al base layer (5). A sheet metal part according to claim 9, characterized in that the alloy layer (3) consists of 35-95 wt.% Fe, 0.1-4 wt.% Si, and optional further components, the total contents of which are limited to a maximum of 3.5 wt.%, with the remainder being aluminum. A sheet metal part according to one of claims 9 to 10, characterized in that the Al base layer (5) consists of 35-55 wt.% Fe, 0.4-4 wt.% Si, optionally 3 wt.% alkali or alkaline earth metals, optionally up to 10% Zn and optionally further components, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder being aluminum. A sheet metal part according to one of claims 9 to 11, the Al base layer (5) consists of 35-55 wt.% Fe, 0.4-1.5 wt.% Si, optionally 3 wt.% alkali or alkaline earth metals, optionally up to 10% Zn and optionally further components, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder being aluminum.

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