Flat steel product with coating for producing a sheet metal component by means of hot forming

Abstract

The invention relates to a flat steel product for producing a sheet metal component by means of hot forming. The flat steel product comprises a steel substrate made of a steel containing 0.1-3 wt. % Mn and optionally up to 0.01 wt. % B. Furthermore, the flat steel product comprises an aluminium-based coating arranged on at least one side of the steel substrate. The coating has an Al base layer consisting of 1.0-15 wt. % Si, optionally 2-4 wt. % Fe, 0.030-0.060 wt. % Ca, optionally 0.1-5.0 wt.% further alkali or alkaline earth metals, optionally up to 15 wt. % Zn, preferably up to 10 wt. % Zn and optional further components, the contents of which in total are limited to at most 2.0 wt. %, and the remainder being aluminum. The invention further relates to a method for producing a flat steel product of this type, to a shaped sheet metal part formed from a flat steel product of this type, and to a corresponding forming method.

Description

[0001] ThyssenKrupp Steel Europe AG 237118P10WO

[0002] August 27, 2025 1 / 39

[0003] Steel flat product with improved processing properties

[0004] The invention relates to a flat steel product for hot forming and a method for producing such a flat steel product. Furthermore, the invention relates to a sheet metal part with improved processing properties and a method for producing such a sheet metal part from a flat steel product.

[0005] When the terms "flat steel product" or "sheet metal product" are used below, these refer to rolled products such as steel strips or sheets, from which "sheet blanks" (also called blanks) are cut for the production of, for example, body parts. "Sheet metal forming parts" or "sheet metal components" of the type according to the invention are manufactured from such sheet metal blanks, whereby the terms "sheet metal forming part" and "sheet metal component" are used synonymously.

[0006] All information regarding the content of the steel compositions specified in this application is based on weight, unless expressly stated otherwise. All unspecified percentages relating to a steel alloy are therefore to be understood as wt.%. With the exception of the information relating to the retained austenite content of the microstructure of a sheet metal part according to the invention, which is based on volume (expressed in vol.%), information regarding the content of the various microstructural constituents refers to the area of ​​a section of a sample of the respective product (expressed in area percent, "area %"), unless expressly stated otherwise. Information in this text regarding the content of the constituents of an atmosphere refers to volume (expressed in vol.%).

[0007] The microstructure was determined from longitudinal sections that had been etched with 3% Nital (alcoholic nitric acid). The proportion of retained austenite was determined by X-ray diffraction.

[0008] The automotive industry uses various types of press-hardened components. In addition to uncoated components, components with an aluminum-silicon coating are also frequently used.

[0009] For example, EP 2 086 755 Bl describes a steel flat product for hot forming with an aluminum-silicon coating of 20–33 pm thickness. Such a product is registered with ThyssenKrupp Steel Europe AG (237118P10WO).

[0010] August 27, 2025

[0011] 2 / 39

[0012] Thick coatings, however, have several disadvantages. Firstly, a relatively long curing time in the furnace is required before hot forming. This is because sufficient iron must diffuse from the steel substrate into the coating. The thicker the coating, the longer this process takes. EP 2 086 755 Bl describes furnace curing times of at least 3 minutes. Secondly, problems arise when rapidly heating such coatings using induction before hot forming. With such rapid heating, the coating liquefies very quickly before sufficient iron can diffuse into it, which lowers the melting point again. The liquid aluminum-silicon melt then agglomerates in certain areas under the influence of the electromagnetic fields. The result is a significant variation in the coating thickness of the resulting sheet metal part.

[0013] Alternatively, EP 3 611 288 Al describes a steel flat product for hot forming with an aluminum-silicon coating of 3–19 pm thickness. This reduced thickness reduces the furnace dwell time but also results in less corrosion protection.

[0014] EP 4 174 207 Al also describes a steel flat product for hot forming with a thin aluminium-silicon coating.

[0015] The object of the present invention is to provide a coating which has improved processing properties during hot forming.

[0016] This problem is solved by a steel flat product for the production of a sheet metal component 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) an aluminum-based coating arranged on at least one side of the steel substrate, wherein the coating has an Al base layer consisting of 1.0–15 wt.% Si, optionally 2–4 wt.% Fe, 0.030–0.060 wt.% Ca, optionally 0.1–5.0 wt.% other alkali or alkaline earth metals, optionally up to 15 wt.% Zn, preferably up to 10 wt.% Zn, and the remainder being aluminum and unavoidable impurities. ThyssenKrupp Steel Europe AG 237118P10WO

[0017] August 27, 2025

[0018] 3 / 39

[0019] Such a coating is typically produced by hot-dip coating, in which the flat steel product is passed through a molten metal bath whose composition corresponds to that of the subsequent aluminum base layer. Diffusion processes occur at the interface between the steel substrate and the molten metal bath until both the steel substrate and the melt adhering to the steel substrate have completely cooled. As a result, an alloy layer forms at the interface, consisting of 25–60 wt% iron, optional additional components (the total content of which is limited to a maximum of 5.0 wt%), and the remainder being aluminum and unavoidable impurities. Surprisingly, it has been shown that adding at least 0.030 wt% calcium to the melt produces a thicker alloy layer. Apparently, the proportion of at least 0.030 wt% calcium...-% calcium leads to the formation of diffusion channels, which accelerate diffusion processes at the interface. As a result, the thickness of the alloy layer increases.

[0020] A thicker alloy layer offers several advantages. For example, with the same overall coating thickness, the holding time in the furnace during subsequent hot forming is reduced. Since the coating consists of the alloy layer and the aluminum base layer, a thicker alloy layer results in a correspondingly thinner aluminum base layer, which then alloys more quickly. A sheet metal part with a coating of a predefined thickness can therefore be produced faster by adding at least 0.030 wt% calcium to the coating than a comparable sheet metal part with the same coating thickness. This is because some of the necessary diffusion processes occur during hot-dip coating, allowing the holding time in the furnace before hot forming to be reduced accordingly. This diffusion behavior can also be further enhanced by increasing the melt temperature.If induction heating is used for pre-heating before hot forming, either instead of or in addition to a furnace, a thick alloy layer reduces the problem of layer thickness variation. The alloy layer does not liquefy during rapid induction heating and therefore does not contribute to layer thickness variation due to agglomeration. This effect only occurs for the aluminum base layer. Thus, the smaller the proportion of the aluminum base layer in the coating, the lower the layer thickness variation.

[0021] In a preferred embodiment of the steel flat product, the coating has a thickness of 9–15 µm, preferably 9–12 µm. ThyssenKrupp Steel Europe AG 237118P10WO

[0022] August 27, 2025 4 / 39

[0023] The invention is particularly relevant for coatings of medium thickness. To minimize the holding time in the furnace before hot forming, and to largely eliminate the problem of agglomeration during rapid induction heating, coatings with a very thin aluminum base layer have been used to date. This is achieved, for example, in EP 4 174 207 Al by blow-off after hot-dip coating at a flow pressure of 300 mbar. The thickness of the aluminum base layer is then approximately 3 pm. The total coating thickness of such a non-inventive coating is then approximately 9 pm, since an alloy layer thickness of approximately 5–6 pm is achieved. By adding at least 0.030 wt% calcium to the melt according to the invention, a thicker alloy layer is obtained, and thus, with an identical thickness of the aluminum base layer, a thicker coating is achieved.Such a flat steel product is very easy to process further. This is because the aluminum base layer is very thin, allowing for a very short holding time in the furnace before hot forming, or enabling the flat steel product to be heated by induction without risk of layer thickness variations. At the same time, this results in a greater coating thickness compared to conventional products with a thin aluminum base layer. This provides improved corrosion protection and better weldability of the resulting sheet metal part. The improved weldability arises because, in contrast to the very thin overall thickness, a favorable layer structure develops after hot forming, featuring a partially interrupted, near-surface ternary phase. This ternary phase locally penetrates the oxide layer, creating channels where the current flow can initiate during welding.However, with very thin overall thicknesses, too much Si quickly accumulates in the near-surface area during annealing, which then makes the proportion of the poorly conducting phase near the surface too large.

[0024] The steel flat product according to the invention thus combines the good hot forming properties of the low layer thicknesses described in EP 3 611 288 Al and EP 4 174 207 Al with the good processing properties of the sheet metal part produced at high layer thicknesses described in EP 2 086 755 Bl.

[0025] In a preferred variant, the Si content of the Al base layer is 0.5 - 3.5 wt.% or 7 - 12 wt.%, in particular 8 - 10 wt.%.

[0026] In a preferred variant, the calcium (Ca) content is at least 0.035 wt.%, in particular at least 0.040 wt.%. Furthermore, preferably, the maximum content is ThyssenKrupp Steel Europe AG 237118P10WO

[0027] August 27, 2025 5 / 39

[0028] Calcium at most 0.055 wt.%, in particular at most 0.050 wt.%, preferably at most 0.045 wt.%. Calcium is particularly affine for oxygen. Therefore, an excessively high calcium content leads to an unsuitable oxide layer thickness. Furthermore, variations in the local calcium content lead to variations in the oxide layer thickness and thus to a patchy appearance. This can be avoided by limiting the calcium content as explained above.

[0029] The optional content of 0.1–5.0 wt.% of other alkali or alkaline earth metals is to be understood as the sum of the contents of all alkali or alkaline earth metals except calcium. Preferably, one or more of the elements Be, Mg, Sr, Li, Na, K, Rb are present, wherein the total content of all these elements is 0.1–5.0 wt.%.

[0030] In a preferred variant, the optional content of further 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.

[0031] Furthermore, the optional content of other alkali or alkaline earth metals in the Al base layer preferably consists of 0.1 - 1.0 wt.% Mg, in particular 0.1 - 0.7 wt.% Mg, preferably 0.1 - 0.5 wt.% Mg.

[0032] In principle, all alkali or alkaline earth metals, especially the elements Be, Mg, Sr, Li, Na, K, Rb, and Ca, have the property of having a higher affinity for oxygen than aluminum. Even in the presence of small amounts of one of these elements, a thin oxide layer forms on the surface of the coating, covering the aluminum between it and the steel substrate. This thin layer prevents the aluminum from reacting with the moisture present in the atmosphere of the furnace used for heating the steel flat product during the heating process required for hot forming. Oxidation of the aluminum in the coating and the associated release of hydrogen, which could diffuse into the coating and the steel substrate of the flat product, are thus effectively prevented.Surprisingly, this is especially true when the Al-based coating becomes locally molten as a result of heating and its surface cracks, allowing molten coating material to come into contact with the furnace atmosphere.

[0033] The combination of the calcium content according to the invention of 0.030 - 0.060 wt.% Ca with 0.1 - 1.0 wt.% Mg, in particular 0.1 - 0.7 wt.% Mg, has proven to be particularly advantageous. ThyssenKrupp Steel Europe AG 237118P10WO

[0034] August 27, 2025

[0035] 6 / 39 preferably 0.1 - 0.5 wt% Mg proved effective. Both elements have the property of oxidizing faster than aluminum, with calcium oxidizing even faster than magnesium. The aforementioned combination of calcium and magnesium offers several advantages. The calcium content accelerates diffusion processes at the interface with the steel substrate, thereby increasing the thickness of the alloy layer. Simultaneously, magnesium forms a thin, covering oxide layer that appears optically homogeneous and allows for further processing.

[0036] The term "unavoidable impurities" refers to technically unavoidable levels of foreign elements introduced during hot-dip coating. This applies in all cases where the term "residual aluminum and unavoidable impurities" is used below. The total content of unavoidable impurities is preferably limited to 1.0 wt.%, in particular 0.5 wt.%, in particular 0.3 wt.%, preferably 0.10 wt.%, and in particular 0.05 wt.%.

[0037] In a preferred embodiment of the flat steel product, the coating has an alloy layer that rests on the steel substrate and on which the aluminum base layer is arranged. The alloy layer lies on the steel substrate and is directly adjacent to it. The alloy layer is essentially composed of aluminum and iron. The remaining elements from the steel substrate or the melt composition do not accumulate significantly in the alloy layer. Preferably, the alloy layer consists of 25–60 wt.% Fe, preferably iron, optional additional components whose total content is limited to a maximum of 5.0 wt.%, preferably 2.0 wt.%, and the remainder being aluminum and unavoidable impurities, with the aluminum content preferably increasing towards the surface. The alloy layer is preferably ferritic.

[0038] The optional components of the alloy layer, the total content of which is limited to a maximum of 5.0 wt.%, preferably 2.0 wt.%, include, on the one hand, the other elements that diffuse from the steel substrate into the alloy layer in addition to iron. These are, in particular, manganese and chromium. On the other hand, this category also includes the remaining elements of the melt, such as zinc and alkali or alkaline earth metals (especially calcium and magnesium). These elements do not accumulate significantly in the alloy layer but can be detected even in small amounts. In particular, the total content of the optional components is limited to a maximum of 1.0 wt.%, preferably a maximum of 0.5 wt.%. ThyssenKrupp Steel Europe AG 237118P10WO

[0039] August 27, 2025

[0040] 7 / 39

[0041] In a preferred embodiment, the alloy layer consists of 25 - 60 wt.% Fe, preferably iron, 1.0 - 3.0 wt.% Si, optional further components whose total content is limited to a maximum of 3.0 wt.%, preferably 2.0 wt.%, and as a remainder of aluminum and unavoidable impurities, wherein the Al content preferably increases towards the surface.

[0042] In a further development, the alloy layer consists of 25 - 60 wt.% Fe, preferably iron, 1.0 - 3.0 wt.% Si, 0.5 - 3.0 wt.% Zn, optional further components whose total content is limited to a maximum of 2.0 wt.%, and as a remainder of aluminum and unavoidable impurities, wherein the Al content preferably increases towards the surface.

[0043] Due to the increased diffusion caused by calcium, the iron content of the alloy layer is preferably at least 35 wt.%, and particularly at least 40 wt.%. This applies to all previously described embodiments of the alloy layer.

[0044] In a preferred embodiment, the alloy layer has a thickness of at least 7 pm, preferably at least 8 pm, and particularly at least 9 pm. As already explained, a thicker alloy layer results in good processing properties of the subsequent sheet metal part, even with a thinner Al base layer, due to the greater overall coating thickness.

[0045] In a preferred variant, the steel substrate contains a proportion of diffusible hydrogen H dThe diffusible hydrogen content of the steel substrate, as defined in this application, must be determined within 48 hours of coating application. The diffusible hydrogen content is determined according to ISO 16575-1:2020.

[0046] The steel substrate is made of a steel containing 0.1–3 wt.% manganese and optionally up to 0.01 wt.% brine. In particular, the microstructure of the steel can be transformed into a martensitic or partially martensitic microstructure by hot forming. The microstructure of the steel substrate of the steel component is therefore preferably a martensitic or at least partially martensitic microstructure, as this exhibits particularly high hardness.

[0047] The steel substrate is particularly preferred if it consists of a steel which, in addition to iron and unavoidable impurities (in wt.%), is made from ThyssenKrupp Steel Europe AG 237118P10WO

[0048] August 27, 2025

[0049] 8 / 39

[0050] C: 0.04 - 0.45 wt.%,

[0051] Si: 0.02 - 1.2 wt.%,

[0052] Mn: 0.5 - 2.6 wt.%

[0053] AI: 0.02 - 1.0 wt.%,

[0054] P: < 0.05 wt.%,

[0055] S: < 0.02 wt.%,

[0056] N: < 0.02 wt.%,

[0057] Sn: < 0.03 wt.%,

[0058] As: < 0.010 wt.%,

[0059] Ca: < 0.005 wt.%, and optionally one or more of the elements “Cr, B, Mo, Ni, Cu, Nb, Ti, V, W” in the following amounts

[0060] Cr: 0.08 - 1.0 wt. -%,

[0061] B: 0.001 - 0.010 wt.%,

[0062] Mo: < 0.5 wt.%,

[0063] Ni: < 0.5 wt.%,

[0064] Cu: < 0.2 wt.%,

[0065] Note: 0.01 - 0.2 wt.%

[0066] Ti: 0.008 - 0.10 wt.%

[0067] V: < 0.3 wt.%,

[0068] W: 0.001 - 1.00 wt.%. consists of.

[0069] The elements P, S, N, Sn, As, and Ca are impurities that cannot be completely avoided in steel production. Occasionally, Ca is also deliberately added to bind sulfur. In such cases, the Ca content is at least 0.001 wt.%. The maximum Ca content, even in this case, is 0.005 wt.%. ThyssenKrupp Steel Europe AG 237118P10WO

[0070] August 27, 2025

[0071] 9 / 39

[0072] In addition to these elements, other elements may also be present as impurities in the steel. These other elements are collectively referred to as "unavoidable impurities." Preferably, the total content of unavoidable impurities is a maximum of 0.2 wt.%, more preferably a maximum of 0.1 wt.%. The optional alloying elements Cr, B, Nb, and Ti, for which a lower limit is specified, may also occur as unavoidable impurities in the steel substrate at levels below the respective lower limit. In this case, they are also counted among the unavoidable impurities, the total content of which is limited to a maximum of 0.2 wt.%, more preferably a maximum of 0.1 wt.%. Preferably, the individual upper limits for the respective impurities of these elements are as follows:

[0073] Cr: < 0.050 wt.%,

[0074] B: < 0.0005 wt.%,

[0075] Nb: < 0.005 wt.%,

[0076] Ti: < 0.005 wt.%.

[0077] These preferred upper limits should be considered as alternatives or in combination. Preferred steel variants therefore fulfill one or more of these four conditions.

[0078] In a preferred embodiment, the carbon content of the steel is a maximum of 0.37 wt.% and / or a minimum of 0.06 wt.%. In particularly preferred embodiments, the carbon content is in the range of 0.06–0.09 wt.%, or in the range of 0.11–0.25 wt.%, or in the range of 0.32–0.37 wt.%.

[0079] In the steel flat products according to the invention, carbon has a retarding effect on the formation of ferrite and bainite. Simultaneously, retained austenite is stabilized and the Ac3 temperature is reduced. A carbon content of at least 0.06 wt.% is advantageous to ensure the hardenability of the steel flat product and the tensile strength of the press-hardened product of at least 1000 MPa. If a higher strength level is desired, carbon contents > 0.12 wt.% are preferred. If the carbon content is further increased to values ​​of at least 0.19 wt.%, the hardenability can also be improved, so that the steel flat product exhibits a very good combination of hardenability and strength. However, carbon contents greater than 0.45 wt.% have a detrimental effect on the mechanical properties of the steel flat product, since carbon contents greater than 0.45 wt.% promote the formation of brittle martensite during press hardening.Furthermore, high carbon content can negatively affect weldability. (See ThyssenKrupp Steel Europe AG 237118P10WO.)

[0080] August 27, 2025

[0081] To improve weldability, the carbon content can preferably be adjusted to values ​​below 0.40 wt.%, particularly 0.3 wt.%. Especially with carbon contents below 0.25 wt.%, weldability can be significantly improved again, and a good ratio of force absorption to maximum bending angle can also be achieved in the bending test according to VDA 238-100 in the press-hardened condition.

[0082] In a preferred embodiment, the Si content of the steel is a maximum of 1.00 wt.% and / or a minimum of 0.06 wt.%.

[0083] Silicon is used to further increase the hardenability of the steel flat product and the strength of the press-hardened product via solid solution strengthening. Silicon also enables the use of ferro-silicon manganese as an alloying agent, which has a positive effect on production costs. A hardening effect is already noticeable at a silicon content of 0.06 wt.%. A significant increase in strength occurs at a silicon content of > 0.15 wt.%. Silicon contents above 0.5 wt.% have a detrimental effect on coating behavior, especially with aluminum-based coatings. Silicon contents below 0.4 wt.% are preferred to improve the surface quality of the coated steel flat product.

[0084] In a preferred embodiment, the manganese content of the steel is a maximum of 2.4 wt.% and / or a minimum of 0.75 wt.%. In particularly preferred embodiments, the manganese content is in the range of 0.75–0.85 wt.% or in the range of 1.0–1.6 wt.%.

[0085] Manganese acts as a hardening element by significantly delaying ferrite and bainite formation. At manganese contents below 0.5 wt.%, ferrite and bainite form during press hardening, even at very rapid cooling rates, which should be avoided. Mn contents above 0.75 wt.%, particularly 0.9 wt.%, are preferred when a martensitic microstructure is desired, especially in areas of significant deformation. Manganese contents above 2.6 wt.% adversely affect processing properties. In particular, weldability is severely restricted, which is why the Mn content of the steel flat products according to the invention is limited to a maximum of 2.4 wt.%, particularly 1.6 wt.%. Manganese contents below 1.6 wt.% are also preferred for economic reasons. ThyssenKrupp Steel Europe AG 237118P10WO

[0086] August 27, 2025

[0087] 11 / 39

[0088] In a preferred variant, the aluminum content of the steel is a maximum of 0.75 wt.%, in particular a maximum of 0.5 wt.%, and preferably a maximum of 0.25 wt.%. Alternatively or additionally, the aluminum content is preferably at least 0.02%.

[0089] Aluminum is used as a deoxidizing agent to bind oxygen. Furthermore, aluminum inhibits cementite formation. At least 0.02 wt% aluminum is required in the steel to reliably bind oxygen. However, since the Ac3 temperature also increases significantly with rising aluminum alloy content, the aluminum content is preferably limited to 0.25 wt%. Above a content of 0.25 wt%, aluminum hinders the conversion to austenite before press hardening too much, making austenitization inefficient in terms of time and energy. For typical furnace temperatures between 850 and 950 °C in hot forming, an aluminum content of no more than 0.1 wt% is preferably maintained to ensure complete austenitization of the steel.

[0090] Furthermore, it has been shown that limiting the sum of the silicon and aluminum contents can be beneficial. In a preferred embodiment, 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.%.

[0091] The elements P, S, and N are typical impurities that cannot be completely avoided in steel production. In preferred variants, the P content is a maximum of 0.03 wt.%. Independently of this, the S content is preferably a maximum of 0.012 wt.%. Additionally or supplementarily, the N content is preferably a maximum of 0.009 wt.%.

[0092] Phosphorus (P) and sulfur (S) are elements that are introduced into steel as impurities from iron ore and cannot be completely removed in large-scale steelmaking processes. The P and S content should be kept as low as possible, since mechanical properties such as impact strength deteriorate with increasing P and S content, respectively. Furthermore, martensite becomes increasingly brittle at P contents above 0.1 wt.%, which is why the P content of a flat steel product according to the invention is limited to a maximum of 0.05 wt.%, preferably a maximum of 0.03 wt.%. The S content of a flat steel product according to the invention is limited to a maximum of 0.02 wt.%, preferably a maximum of 0.012 wt.%. ThyssenKrupp Steel Europe AG 237118P10WO

[0093] August 27, 2025

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[0095] Nitrogen (N) is present in small quantities in steel due to the steelmaking process. The N content should be kept as low as possible and should be less than 0.02 wt.%. Nitrogen is particularly detrimental in alloys containing boron, as it inhibits the conversion-retarding effect of boron by forming boron nitrides. Therefore, in this case, the nitrogen content should preferably be a maximum of 0.010 wt.%, and more preferably a maximum of 0.009 wt.%.

[0096] The tin content is a maximum of 0.03 wt.%, preferably a maximum of 0.02 wt.%. The arsenic content is a maximum of 0.010 wt.%, particularly a maximum of 0.005 wt.%.

[0097] Optionally, the steel also contains chromium in a content of 0.08–1.0 wt.%. Preferably, the chromium content is a maximum of 0.75 wt.%, and particularly a maximum of 0.5 wt.%.

[0098] Chromium is added to the steel of a flat steel product according to the invention in amounts of 0.08–1.0 wt.%. Chromium influences the hardenability of the flat steel product by slowing down the diffusive transformation during press hardening. In flat steel products according to the invention, chromium has a beneficial effect on hardenability from a content of 0.08 wt.%, whereby a Cr content > 0.1 wt.% is preferred for reliable process control, especially to prevent bainite formation. If the steel contains more than 1.0 wt.% chromium, the coating properties deteriorate. To obtain good surface quality, the Cr content can preferably be limited to a maximum of 0.75 wt.%, and in particular to a maximum of 0.5 wt.%.

[0099] In the case of optional chromium alloying, the sum of the chromium and manganese contents is preferably limited. The sum is a maximum of 3.3 wt.%, in particular a maximum of 3.15 wt.%. Furthermore, the sum is at least 0.5 wt.%, preferably at least 0.75 wt.%.

[0100] Preferably, the steel also optionally contains boron in a content of 0.001–0.010 wt.%. In particular, the boron content is a maximum of 0.005 wt.%, preferably a maximum of 0.004 wt.%.

[0101] Boron can be optionally added to improve the hardenability of the flat steel product by depositing boron atoms or boron precipitates on the austenite grain boundaries. ThyssenKrupp Steel Europe AG 237118P10WO

[0102] August 27, 2025

[0103] 13 / 39 reduces the limiting energy, thereby suppressing ferrite nucleation during press hardening. A significant effect on hardenability occurs at boron contents of at least 0.001 wt.%. However, at boron contents above 0.010 wt.%, boron carbides, boron nitrides, or boron nitrocarbides are formed in greater quantities, which in turn represent preferred nucleation sites for ferrite and reduce the hardening effect again. For this reason, the boron content is limited to a maximum of 0.010 wt.%.

[0104] When boron is added, titanium is preferably also added to bind nitrogen. In this case, the titanium content should preferably be at least 3.42 times the nitrogen content in wt.%.

[0105] Optionally, the steel may contain molybdenum with a maximum content of 0.5 wt.%, in particular a maximum of 0.1 wt.%.

[0106] Molybdenum (Mo) can be added optionally to improve process stability, as it significantly slows down ferrite formation. From contents of 0.002 wt.%, molybdenum-carbon clusters, up to and including ultrafine molybdenum carbides, form dynamically at the grain boundaries, significantly slowing down grain boundary mobility and thus diffusive phase transformations. Furthermore, molybdenum reduces the grain boundary energy, which decreases the nucleation rate of ferrite. Due to the high costs associated with a molybdenum alloy, the Mo content should be no more than 0.5 wt.%, preferably no more than 0.3 wt.%.

[0107] Optionally, the steel may also contain copper with a content of a maximum of 0.2 wt.%, preferably a maximum of 0.15 wt.%.

[0108] Copper (Cu) can be added as an optional alloy to increase hardenability at additions of at least 0.01 wt%. Furthermore, copper improves the resistance to atmospheric corrosion of uncoated sheets or cut edges.

[0109] The steel may optionally contain nickel with a maximum content of 0.5 wt.%, preferably a maximum of 0.15 wt.%. ThyssenKrupp Steel Europe AG 237118P10WO

[0110] August 27, 2025

[0111] 14 / 39

[0112] Nickel (Ni) stabilizes the austenitic phase and can be optionally added to lower the Ac3 temperature and suppress the formation of ferrite and bainite. Nickel also has a positive effect on hot rollability, especially when the steel contains copper. Copper impairs hot rollability. To counteract the negative effect of copper on hot rollability, at least 0.01 wt% nickel can be added to the steel. For economic reasons, the nickel content should be limited to a maximum of 0.5 wt%, preferably a maximum of 0.4 wt%.

[0113] Furthermore, the steel may optionally contain one or more of the microalloying elements Nb, Ti, and V. The optional Nb content is at least 0.01 wt.%, in particular at least 0.02 wt.% and at most 0.2 wt.%, preferably at most 0.08 wt.%, and more 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.%, and more 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.%, and more preferably at most 0.05 wt.%.

[0114] Niobium (Nb) can be added as an optional alloying element to contribute to grain refinement from a content of 0.01 wt.%, and particularly from 0.010 wt.%. However, niobium impairs the recrystallizability of the steel. At an Nb content above 0.1 wt.%, the steel can no longer be recrystallized in conventional continuous furnaces before hot-dip coating.

[0115] Titanium (Ti) is a microalloying element that can be optionally added to contribute to grain refinement. Titanium also forms coarse titanium nitrides with nitrogen, which is why the Ti content should be kept relatively low. Titanium binds nitrogen, thus enabling boron to exert its strong ferrilytic effect. Sufficient nitrogen binding requires at least 3.42 times the nitrogen content, with at least 0.008 wt% Ti being added to ensure adequate availability. Preferably, the titanium content is at least 0.010 wt%, more preferably at least 0.015 wt%. From 0.1 wt% Ti, cold rollability and recrystallizability deteriorate significantly, which is why higher Ti contents should be avoided. ThyssenKrupp Steel Europe AG 237118P10WO

[0116] August 27, 2025

[0117] 15 / 39

[0118] In the case of optional alloying with several of the elements Nb, Ti, and V, the sum of the Nb, Ti, and V contents is preferably limited. The sum is a maximum of 0.1 wt.%, particularly a maximum of 0.068 wt.%. Furthermore, the sum is preferably at least 0.015 wt.%.

[0119] Vanadium (V) is a highly carbon-affine element. When vanadium is free, that is, in an unbound or dissolved state, it can bind supersaturated dissolved carbon in the form of carbides or clusters, or at least reduce its diffusion rate. Crucially, the presence of V in a dissolved state is essential. Surprisingly, very low V contents have proven particularly beneficial for aging resistance. At higher V contents, larger vanadium carbides can precipitate even at elevated temperatures, and these carbides then do not dissolve at temperatures of 800–900 °C, which are typical for continuous annealing in hot-dip coating systems. Even minute amounts of vanadium, as low as 0.001 wt.%, can hinder the attachment of free carbon to dislocations. Above a V content of 0.2 wt.%, vanadium no longer improves aging resistance.The anti-aging effect of vanadium is particularly pronounced at vanadium contents up to 0.009 wt.%, with a maximum effect occurring at a preferred vanadium content of 0.002 wt.%. At vanadium contents greater than 0.009 wt.%, vanadium carbides are increasingly formed. Vanadium carbides cannot be dissolved at temperatures of 860 °C, which are typical, for example, for annealing temperatures in a hot-dip coating plant, when the vanadium content in the steel exceeds 0.009 wt.%. The vanadium content of the steel in a flat steel product according to the invention is limited to a maximum of 0.1 wt.% for cost reasons. Furthermore, higher vanadium contents do not result in a significant improvement in the mechanical properties.

[0120] Tungsten (W) can optionally be added in amounts of 0.001–1.0 wt.% to slow down ferrite formation. A positive effect on hardenability is already observed with W contents of at least 0.001 wt.%. For cost reasons, a maximum of 1.0 wt.% tungsten is added.

[0121] The foregoing explanations regarding preferred steel substrates naturally also apply to the steel substrate or steel of the sheet metal part described below, as well as to the steel substrates or steels used in the described manufacturing processes. ThyssenKrupp Steel Europe AG 237118P10WO

[0122] August 27, 2025

[0123] 16 / 39

[0124] The inventive method for producing a steel flat product for hot forming with a coating comprises 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 of the slab or thin slab at a temperature (TI) of 1000 - 1400 °C; c) Optional pre-rolling of 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 steel flat product, wherein the final rolling temperature (T3) is 750 - 1000 °C; e) Optional coiling of the hot-rolled steel flat product, wherein the coiling temperature (T4) is not more than 700 °C; f) Descaling of the hot-rolled steel flat product; g) Optional cold rolling of the steel flat product, wherein the degree of cold rolling is at least 30%; h) Annealing the steel flat product at an annealing temperature (T5) of 650–900 °C; i) Cooling the steel flat product to an immersion temperature (T6) of 650–800 °C, preferably 680–720 °C; j) Coating the steel flat product cooled to the immersion temperature with a coating 9–15 pm thick by i.Immersion in a melt bath with a melt temperature (T7) 660 - 800 °C, preferably 670 - 710 °C, wherein the melt bath contains the coating to be applied to the steel flat product in liquid form, which ThyssenKrupp Steel Europe AG 237118P10WO.

[0125] August 27, 2025

[0126] 17 / 39 consisting of 1.0 - 15 wt.% Si, optionally 2 - 4 wt.% Fe, 0.030 - 0.060 wt.% Ca, optionally 0.1 - 5.0 wt.% other alkali or alkaline earth metals, optionally up to 15 wt.% Zn, preferably up to 10 wt.% Zn, and the remainder consisting of aluminum and unavoidable impurities; ii. Blowing off the steel flat product after exiting the molten bath by means of a gas stream, in particular with a flow pressure of 100 - 1000 mbar, preferably 200 - 750 mbar; k) Cooling the coated steel flat product to room temperature, wherein the first cooling time t mT in the temperature range between 600 °C and 450 °C is more than 10 s, in particular more than 14 s, and the second cooling time in the temperature range between 400 °C and 300 °C is more than 8 s, in particular more than 12 s; l) optional dressing of the coated steel flat product.

[0127] In step a), a semi-finished product composed according to 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 continuous slab casting.

[0128] In step b), the semi-finished product is heated through at a temperature (TI) 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 heating temperature should be at least 1000 °C to ensure good formability for the subsequent rolling process. The heating temperature should not exceed 1400 °C to avoid the presence of molten phases in the semi-finished product.

[0129] In the optional step c), the semi-finished product is pre-rolled to an intermediate product. Thin slabs are not usually pre-rolled. Thick slabs intended for hot-rolled strip can be pre-rolled if necessary. In this case, the temperature of the intermediate product (T2) at the end of pre-rolling should be at least 1000 °C to ensure sufficient heat for the subsequent step. ThyssenKrupp Steel Europe AG 237118P10WO

[0130] August 27, 2025

[0131] 18 / 39 of the finish rolling process is included. However, high rolling temperatures can also promote grain growth during the rolling process, which adversely affects the mechanical properties of the steel flat product. To keep grain growth during the rolling process low, the temperature of the intermediate product at the end of pre-rolling should not exceed 1200 °C.

[0132] In step d), the slab or thin slab, or, if step c) was performed, the intermediate product, is rolled into a hot-rolled steel flat product. If step c) was performed, the intermediate product is typically finish-rolled immediately after pre-rolling. Finish rolling typically begins no later than 90 seconds after the end of pre-rolling. The slab, thin slab, or, if step c) was performed, the intermediate product, is rolled to a final rolling temperature (T3). The final rolling temperature, that is, the temperature of the finished hot-rolled steel flat product at the end of the hot rolling process, is 750–1000 °C. At final rolling temperatures below 750 °C, the amount of free vanadium decreases because larger quantities of vanadium carbides are precipitated. The vanadium carbides precipitated during finish rolling are very large.They typically have a mean grain size of 30 nm or more and are not 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 no more than 1000 °C are process-related for setting coil temperatures (T4) below 700 °C.

[0133] Hot rolling of the steel flat product can be carried out as continuous hot strip rolling or as reversing rolling. Step e) provides for optional coiling of the hot-rolled steel flat product in the case of continuous hot strip rolling. For this purpose, the hot strip is cooled to a coiling temperature (T4) within less than 50 seconds after hot rolling. Water, air, or a combination of both can be used as the cooling medium. The coiling temperature (T4) should not exceed 700 °C to prevent the formation of large vanadium carbides. In principle, there is no lower limit to the coiling temperature. However, coiling temperatures of at least 500 °C have proven advantageous for cold rolling. Subsequently, the coiled hot strip is cooled to room temperature in the conventional manner using air. ThyssenKrupp Steel Europe AG 237118P10WO

[0134] August 27, 2025 19 / 39

[0135] In step f), the hot-rolled flat steel product is descaled in a conventional manner by pickling or by another suitable treatment.

[0136] The hot-rolled steel flat product, cleaned of scale, can optionally undergo cold rolling before the annealing treatment in step g) to meet, for example, higher requirements for the thickness tolerances of the steel flat product. The cold rolling degree (CW) should be at least 30% to introduce sufficient deformation energy into the steel flat product for rapid recrystallization. The cold rolling degree (CW) is defined as the quotient of the thickness reduction during cold rolling (AdW) divided by the hot strip thickness (d).

[0137] KWG = AdKW / d, where AdKW = thickness reduction during cold rolling in mm and d = hot-rolled strip thickness in mm. The thickness reduction AdKW is the difference between the thickness of the steel flat product before cold rolling and the thickness of the steel flat product after cold rolling. The steel flat product before cold rolling is typically hot-rolled strip with a thickness d. The steel flat product after cold rolling is also commonly referred to as cold-rolled strip. In principle, the degree of cold rolling can reach very high values ​​of over 90%. However, degrees of cold rolling of no more than 80% have proven advantageous for preventing strip cracking.

[0138] In step h), the flat steel product undergoes 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 s and then held at that temperature for 30 to 600 s. The annealing temperature is at least 650 °C, preferably at least 720 °C. Annealing temperatures above 900 °C are not desirable for economic reasons.

[0139] In step i), the steel flat product is cooled to an immersion temperature (T6) after annealing to prepare it for subsequent coating treatment. The pre-cooling temperature is lower than the annealing temperature and is adjusted to the temperature of the melt pool. The immersion temperature is 600–800 °C, preferably at least 650 °C, particularly preferably at least 680 °C, and particularly preferably at most 700 °C. Preferably, the immersion temperature T6 is a maximum of 750 °C, and particularly preferably a maximum of 720 °C. For a particularly homogeneous boundary layer formation, it is important that sufficient thermal energy is available at the boundary. ThyssenKrupp Steel Europe AG 237118P10WO

[0140] August 27, 2025 20 / 39 A layer exists between the steel substrate and the aluminum melt. This is not the case at temperatures below 600 °C, so undesirable compounds can form, the subsequent reconversion of which can lead to pores. Above the preferred immersion temperatures, the diffusion rate of iron in aluminum increases significantly again, so that more iron can diffuse into the still-liquid boundary layer right from the start of the coating process. The cooling time of the annealed steel flat product from the annealing temperature T5 to the immersion temperature T6 is preferably 10–180 s. In particular, the immersion temperature T6 deviates from the temperature of the melt pool T7 by no more than 30 K, especially no more than 20 K, and preferably no more than 1000 K.

[0141] In step j), the steel flat product undergoes a coating treatment. This coating treatment is preferably carried out by continuous hot-dip coating. The coating can be applied to one side, both sides, or all sides of the steel flat product. The coating treatment is preferably carried out as a hot-dip coating process, particularly as a continuous process. In this process, the steel flat product typically comes into contact with the molten metal bath on all sides, so that it is coated on all sides. The molten metal bath, which contains the alloy to be applied to the steel flat product in liquid form, typically has a temperature (T7) of 630–800°C, particularly 660–800°C, and preferably 670–710°C. Preferably, the melt temperature is at least 20 K, and particularly preferably at least 30 K, above the melting point of the alloy in the melt.In particular, the melting temperature T7 is preferably at least 670 °C, more specifically at least 680 °C. Furthermore, the melting temperature is preferably at most 750 °C, more specifically at most 730 °C, more preferably at most 710 °C. The melt consists of 1.0–15 wt.% Si, optionally 2–4 wt.% Fe, 0.030–0.060 wt.% Ca, optionally 0.1–5.0 wt.% other alkali or alkaline earth metals, optionally up to 15 wt.% Zn, more preferably up to 10 wt.% Zn, and the remainder being aluminum and unavoidable impurities. The preferred compositions of the melt correspond to the previously described preferred compositions of the Al base layer of the steel flat product. In particular, the melt consists of 8 - 10 wt.% Si, optionally 2-4 wt.% Fe, 0.030 - 0.060 wt.% Ca, 0.1 - 1.0 wt.% Mg, in particular 0.1 - 0.7 wt.% Mg, preferably 0.1 - 0.5 wt.% Mg, and as a remainder of aluminium and unavoidable impurities.

[0142] Due to the calcium content of at least 0.030 wt.%, increased diffusion processes occur at the interface between the steel substrate and the liquid melt. A thicker layer forms. ThyssenKrupp Steel Europe AG 237118P10WO

[0143] August 27, 2025

[0144] 21 / 39

[0145] Alloy layer at the interface compared to the identical process without calcium content.

[0146] After exiting the molten pool, the steel flat product is blown off using a gas stream. A gas stream with a flow pressure of 100–1000 mbar, preferably 200–750 mbar, has proven particularly advantageous. Since the viscosity of the melt, and thus its adhesion to the steel flat product, changes depending on the melt temperature T7, it is necessary to adjust the flow pressure of the gas stream appropriately so that a coating with a thin aluminum base layer remains on the steel flat product. It has been shown that the combination of a melt temperature T7 of 670–710 °C with a flow pressure of 200–750 mbar is particularly well suited for this purpose. This results in a total coating thickness of 9–15 µm. This thickness is greater than in comparative tests without calcium, because calcium accelerates the diffusion processes, so that a thicker alloy layer with a significant iron content forms even at this stage.Such an alloy layer has a higher melting point, so it solidifies and is therefore not removed during blow-off. The blow-off only affects the remaining molten portion of the coating.

[0147] After the coating treatment, the coated steel flat product is cooled to room temperature in step k). The initial cooling time is t. m T in the temperature range between 600 °C and 450 °C (mean temperature range mT) is more than 10 s, in particular more than 14 s, and a second cooling period t n T in the temperature range between 400 °C and 300 °C (low temperature range nT) more than 8s, especially more than 12s.

[0148] The first cooling period t can be mTemperature T in the temperature range between 600 °C and 450 °C (mean temperature range mT) can be achieved through slow, continuous cooling or by holding the product at a temperature within this range for a certain period of time. Intermediate heating is also possible. The only important factor is that the flat steel product remains cooled for at least a certain period of time t. m The temperature range T remains between 600 °C and 450 °C. Within this temperature range, there is a significant diffusion rate of iron into aluminum, while the diffusion of aluminum into steel is inhibited because the temperature is below half the melting point of steel. This allows iron to diffuse into the coating without significant diffusion of aluminum into the steel substrate.

[0149] The diffusion of iron into the coating has several advantages: ThyssenKrupp Steel Europe AG 237118P10WO

[0150] August 27, 2025 22 / 39

[0151] Firstly, the melting of the coating during austenitizing prior to press hardening is delayed. Secondly, the coefficients of thermal expansion of the coating and substrate become more homogeneous. This means that the transition zone between the thermal expansion coefficient of the substrate and the surface becomes wider, which reduces thermal stresses during reheating.

[0152] At the same time, the diffusion of aluminum into the steel substrate would have significant disadvantages: Due to aluminum's very high affinity for nitrogen, a high aluminum content can lead to nitrogen dissolving from fine precipitates, such as niobium carbonitrides or titanium carbonitrides, and instead, coarse precipitates, such as aluminum nitrides, preferentially forming at the grain boundaries. These would impair crash performance and reduce the bending angle. Furthermore, this destabilizes the fine precipitates (e.g., the niobium-containing precipitates) in the uppermost substrate layer, which are important for many desirable properties. Additionally, the inhomogeneous diffusion rate of aluminum in the steel substrate, in ferrite compared to pearlite / bainite / martensite, would lead to an uneven distribution of aluminum in the surface layer of the steel substrate. This should also be prevented to improve crash and bending performance.These disadvantages of aluminum diffusing into the steel substrate are therefore reduced or avoided by inhibition.

[0153] Due to the preferred first cooling duration t m At T (14s) the iron concentration in the transition boundary layer increases to such an extent that the activity of aluminum in the coating directly at the substrate boundary is further reduced. This then leads to an even further reduced aluminum uptake into the substrate during austenitization prior to press hardening, with the associated advantages described above.

[0154] The second cooling period in the temperature range between 400 °C and 300 °C (low temperature range nT) can also be achieved through slow, continuous cooling or by holding the product at a specific temperature within this range for a certain period. Intermediate heating is also possible. The only important factor is that the flat steel product remains in the temperature range between 400 °C and 300 °C for at least a cooling period tnT.

[0155] In this temperature range, carbon still diffuses somewhat into the steel substrate, while its thermodynamic solubility is very low. Thus, ThyssenKrupp Steel Europe AG 237118P10WO

[0156] August 27, 2025

[0157] 23 / 39

[0158] Carbon forms lattice defects and accumulates there, for example as dissolved Nb atoms. Due to their significantly larger atomic volume, these expand the atomic lattice, thus enlarging the tetrahedral and octahedral voids, thereby increasing the local solubility of carbon. This results in clusters of carbon and Nb, which then transform into very fine precipitates during the austenitization step of hot forming, leading to a refined austenite microstructure and thus also a hardened microstructure, as well as a reduction in the free hydrogen content.

[0159] With a preferred holding time of more than 12s, very fine iron carbides (so-called transition carbides) are also formed, which dissolve very quickly during austenitizing and lead to additional austenite nuclei and thus an even finer austenite structure and therefore also a hardened structure.

[0160] The coated steel flat product can optionally be subjected to a dressing process with a dressing degree of up to 2% to improve the surface roughness of the steel flat product.

[0161] In a preferred embodiment of the process, the gas stream is an air stream, preferably with a temperature between room temperature and 130 °C, and more preferably between 50 and 90 °C. These temperature ranges have proven particularly advantageous for appropriately influencing the surface temperature of the coating. On the one hand, this prevents the surface from solidifying too quickly, allowing sufficient diffusion to occur. On the other hand, it slightly cools the surface of the coating to prevent adhesion to subsequent rollers by low-viscosity phases of the coating.

[0162] In a preferred further development of the process, the dew point temperature TP during the through-heating carried out in step b) is 30 - 80 °C, due to the flames of the burners used for through-heating.

[0163] The invention further relates to a sheet metal forming part, in particular formed from a previously described steel flat product, comprising a) a steel substrate consisting of a steel having 0.1 - 3 wt.% Mn and optionally up to 0.01 wt.% B, and ThyssenKrupp Steel Europe AG 237118P10WO

[0164] August 27, 2025

[0165] 24 / 39 b) an aluminium-based coating arranged on at least one side of the steel substrate, characterized in that the coating consists of 1.0 - 15 wt.% Si, 35 - 90 wt.% Fe, 0.010 - 0.050 wt.% Ca, optionally 0.1 - 5.0 wt.% other alkali or alkaline earth metals, optionally up to 15 wt.% Zn, preferably up to 10 wt.% Zn and optional further components, the total content of which is limited to a maximum of 2.0 wt.%, and the remainder being aluminium and unavoidable impurities.

[0166] The optional components of the coating, whose total content is at most 2.0 wt.%, include the other elements that diffuse from the steel substrate into the coating in addition to iron. These are, in particular, manganese and chromium. The total content of the optional components is preferably at most 1.5 wt.%, more preferably at most 1.0 wt.%, and most preferably at most 0.5 wt.%.

[0167] Preferably, the coating has a thickness of 9 - 25 pm.

[0168] Preferably, the silicon content is a maximum of 10 wt.%, in particular a maximum of 8 wt.%, and preferably a maximum of 7 wt.%. Regardless of the above, the silicon content is preferably at least 3 wt.%, in particular at least 4 wt.%, and preferably at least 5 wt.%.

[0169] The optional content of 0.1–5.0 wt.% of other alkali or alkaline earth metals is to be understood as the sum of the contents of all alkali or alkaline earth metals except calcium. Preferably, one or more of the elements Be, Mg, Sr, Li, Na, K, Rb are present, wherein the total content of all these elements is 0.1–5.0 wt.% and corresponds to the content of other alkali or alkaline earth metals.

[0170] In a preferred variant, the optional content of further alkali or alkaline earth metals comprises 0.1 - 1.0 wt.% Mg, in particular 0.1 - 0.4 wt.% Mg, preferably 0.1 - 0.3 wt.% Mg.

[0171] Furthermore, the optional content of other alkali or alkaline earth metals in the Al base layer preferably consists of 0.1–1.0 wt.% Mg, in particular 0.1–0.4 wt.% Mg, preferably 0.1–0.3 wt.% Mg. ThyssenKrupp Steel Europe AG 237118P10WO

[0172] August 27, 2025

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[0174] In a special embodiment, the coating of the sheet metal part has an Al base layer and an alloy layer, wherein the alloy layer rests on the steel substrate and the Al base layer rests on the alloy layer.

[0175] The thickness of the coating increases during the heat treatment described below prior to hot pressing, as additional iron diffuses into the coating. For this reason, a flat steel product with a coating thickness of 15 µm can be further processed into a component with a coating thickness of up to 25 µm.

[0176] Preferably, the alloy layer of the sheet metal part consists of 35–90 wt.% Fe, 0.1–12 wt.% Si, and optional additional components, the total content of which is limited to a maximum of 5.0 wt.%, in particular a maximum of 3.5 wt.%, preferably 2.0 wt.%, with the remainder being aluminum and unavoidable impurities. In particular, the alloy layer of the sheet metal part consists of 55–90 wt.% Fe, 0.1–12 wt.% Si, and optional additional components, the total content of which is limited to a maximum of 5.0 wt.%, in particular a maximum of 3.5 wt.%, preferably 2.0 wt.%, with the remainder being aluminum and unavoidable impurities. Due to the further diffusion of iron into the alloy layer, the proportions of Si and Mg are correspondingly lower than their respective proportions in the melt of the molten pool.

[0177] The optional components of the alloy layer, the total content of which is limited to a maximum of 5.0 wt.%, 3.5 wt.%, and preferably 2.0 wt.%, include, on the one hand, the other elements that diffuse from the steel substrate into the alloy layer in addition to iron. These are, in particular, manganese and chromium. On the other hand, this category also includes the remaining elements of the melt, such as zinc and alkali or alkaline earth metals (especially calcium and magnesium). These elements do not accumulate significantly in the alloy layer but can be detected even in small amounts. In particular, the total content of the optional components is limited to a maximum of 1.0 wt.%, and preferably a maximum of 0.5 wt.%.

[0178] The alloy layer preferably has a ferritic structure.

[0179] Preferably, the alloy layer of the sheet metal part has a thickness corresponding to 60–95% of the coating thickness, particularly 70–90% of the coating thickness. ThyssenKrupp Steel Europe AG 237118P10WO

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[0182] The alloy layer can be determined, for example, on longitudinal sections that have been etched with 3% Nital (alcoholic nitric acid). The thickness of the alloy layer is also determined on such longitudinal sections. For this purpose, a light microscopic image is taken with a width of at least 50 pm and a resolution of at least 500 pixels per 50 pm. Using computer-aided image processing, the thickness of the alloy layer is determined as the average thickness across the 50 pm wide image field.

[0183] The aluminum base layer of the sheet metal part lies on top of the alloy layer and is directly adjacent to it. Preferably, the aluminum base layer of the sheet metal part consists of 35–55 wt.% Fe, 0.4–10 wt.% Si, up to 0.040 wt.% Ca, optionally up to 3 wt.% other alkali or alkaline earth metals, preferably up to 1.0 wt.% other alkali or alkaline earth metals, optionally up to 10 wt.% Zn, preferably up to 7 wt.% Zn, and the remainder being aluminum and unavoidable impurities. Preferably, the optional content of other alkali or alkaline earth metals is at least 0.1 wt.%. Preferably, the Ca content is at least 0.010 wt.%, in particular at least 0.015 wt.%, and more preferably at least 0.020 wt.%.

[0184] The aluminum base layer can exhibit a homogeneous elemental distribution, with local element concentrations varying by no more than 10%. Preferred variants of the aluminum base layer, however, feature silicon-poor and silicon-rich phases. Silicon-poor phases are regions whose average silicon content is at least 20% lower than the average silicon content of the aluminum base layer. Silicon-rich phases are regions whose average silicon content is at least 20% higher than the average silicon content of the aluminum base layer.

[0185] In a preferred embodiment, 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 is bordered by silicon-poor regions. In an alternative embodiment, the silicon-rich phases are arranged in island-like formations within the silicon-poor phase.

[0186] For the purposes of this application, "island-like" means an arrangement in which discrete, non-contiguous areas are enclosed by another material – i.e., there are "islands" of a particular material within another material. ThyssenKrupp Steel Europe AG 237118P10WO

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[0189] In a preferred embodiment, the steel component comprises an oxide layer arranged on the coating. The oxide layer is located, in particular, on the aluminum base layer and preferably forms the outer layer of the coating.

[0190] The oxide layer of the steel component consists, in particular, of more than 80 wt.% oxides, wherein the main proportion of the oxides (i.e., more than 50 wt.% of the oxides, in particular more than 70 wt.%, preferably more than 80 wt.%) are alkali and alkaline earth metal oxides (preferably magnesium oxides). Optionally, in addition to the oxides of the alkali and alkaline earth metals, hydroxides and / or aluminum oxide, alone or as a mixture, are present in the oxide layer. Preferably, the remainder of the oxide layer not occupied by the oxides and optionally present hydroxides consists of silicon, aluminum, iron, and / or the alkali and alkaline earth metals (preferably magnesium) in metallic form.

[0191] The oxide layer preferably has a thickness of at least 50 nm, particularly at least 100 nm. Furthermore, the thickness is at most 4 pm, particularly at most 2 pm.

[0192] In a specific further development, the steel substrate of the sheet metal part has a microstructure with at least partially more than 80% martensite, preferably at least partially more than 90% martensite, particularly preferably at least partially more than 95%, and most preferably at least partially more than 99%. "Partially" in this context means that there are areas of the sheet metal part that exhibit the aforementioned microstructure. Additionally, there may also be areas of the sheet metal part that exhibit a different microstructure. Thus, the sheet metal part exhibits the aforementioned microstructure in sections or areas.

[0193] The high martensite content allows for very high tensile strengths and yield strengths.

[0194] The sheet metal component according to the invention is preferably a component for a land vehicle, sea vehicle, or aircraft. It is particularly preferably an automotive component, especially a body panel; preferably, the component is a B-pillar, longitudinal member, A-pillar, sill, or cross member. ThyssenKrupp Steel Europe AG 237118P10WO

[0195] August 27, 2025

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[0197] In the inventive method for producing a sheet metal part of the invention as described above, at least the following steps are carried out: a) providing a sheet metal blank from a previously described flat steel product; b) heating the sheet metal blank such that at least partially the AC3 temperature of the blank is exceeded and the temperature T E ini gof the blank when placed in a forming tool intended for hot pressing (step c)) has at least a partial temperature above Ms+100 °C, where Ms denotes the martensite start temperature; c) Place the heated sheet blank into a forming tool, wherein the transfer time t required for removing the blank from the heating device and placing it Tr a maximum of 20 s, preferably a maximum of 15 s; d) Hot pressing of the sheet metal blank to form the sheet metal part, wherein the blank is subjected to a duration t during hot pressing. wz is cooled to a target temperature Tziei for more than 1 s and optionally held there, whereby the cooling from temperature T E ini g at least up to the martensite start temperature with a cooling rate r that is at least partially more than 25 K / s Wz; e) Removal of the sheet metal part cooled to target temperature from the tool.

[0198] In the inventive method, a blank consisting of a steel flat product suitably assembled according to the preceding explanations is provided (step a)), which is then heated in a manner known per se such that the AC3 temperature of the steel is at least partially exceeded and the temperature of the blank when placed in a forming tool intended for hot pressing (step c)) is at least partially above Ms+100 °C, preferably AC1. For the purposes of this application, "partially exceeding a temperature (here AC3 or Ms+100 °C)" means that at least 30%, in particular at least 60%, of the volume is at least partially above the temperature of the steel. ThyssenKrupp Steel Europe AG 237118P10WO

[0199] August 27, 2025 29 / 39 of the blank must exceed a corresponding temperature. Therefore, when inserted into the forming tool, at least 30% of the blank must have an austenitic microstructure; that is, the transformation from a ferritic to an austenitic microstructure need not yet be complete when inserted into the forming tool. Rather, up to 70% of the blank's volume may consist of other microstructural constituents, such as tempered bainite, tempered martensite, and / or non- or partially recrystallized ferrite. For this purpose, certain areas of the blank can be deliberately kept at a lower temperature level than others during heating. This can be achieved by selectively directing the heat input only to specific sections of the blank or by shielding the parts that are to be heated less from the heat input.In the portion of the blank material where the temperature remains lower, little or no martensite forms during the forming process in the die. As a result, the microstructure there is significantly softer than in the other parts, which exhibit a martensitic structure. This allows for the targeted creation of a softer zone within the formed sheet metal part, for example, by providing optimal toughness for a specific application, while the other areas of the part possess maximized strength.

[0200] Maximum strength properties of the resulting sheet metal part can be achieved by ensuring that the temperature reached at least partially in the sheet metal blank is between Ac3 and 1000 °C, preferably between 850 °C and 950 °C.

[0201] The minimum temperature Ac3 to be exceeded is determined according to the formula given by HOUGARDY, HP. in Werkstoffkunde Stahl Band 1 : Grundlagen, Verlag Stahleisen GmbH, Düsseldorf, 1984, p. 229.

[0202] Ac3 = (902 - 225*%C + 19*%Si - 11*%Mn - 5*%Cr + 13*%Mo - 20*%Ni + 55*%V) °C where %C = respective C content, %Si = respective Si content, %Mn = respective Mn content, %Cr = respective Cr content, %Mo = respective Mo content, %Ni = respective Ni content and %V = respective V content of the steel from which the blank is made.

[0203] Optimally uniform property distribution can be achieved by thoroughly heating the blank in step b). ThyssenKrupp Steel Europe AG 237118P10WO

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[0206] In a preferred embodiment, the average heating rate of the sheet metal blank in the oven during heating in step b) is at least 3 K / s, preferably at least 5 K / s, particularly at least 10 K / s, and preferably at least 15 K / s. The average heating rate roven is understood to be the average heating rate from 30 °C to 700 °C.

[0207] In a preferred embodiment, the heating takes place in an oven with an oven temperature 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.

[0208] Preferably the dew point in the oven is at least -20 °C, preferably at least -15 °C, in particular at least -5 °C, preferably at least 0 °C, particularly preferably at least 5 °C and at most +25 °C, preferably at most +20 °C, in particular at most +15 °C.

[0209] In a specific embodiment, the heating in step b) takes place in stages in areas with different temperatures. Specifically, the heating is carried out in a roller hearth furnace with different heating zones. Here, the heating in a first heating zone occurs at a temperature (so-called furnace inlet temperature) of at least 650 °C, preferably at least 680 °C, and particularly at least 720 °C. The maximum temperature in the first heating zone is preferably 900 °C, and particularly preferably 850 °C. Furthermore, the maximum temperature of all heating zones in the furnace is preferably at most 1200 °C, particularly at most 1000 °C, preferably at most 950 °C, and most preferably at most 930 °C.

[0210] The total time in the oven t O f enThe heating time, which consists of a heating time and a holding time, is preferably at least 50 s, preferably at least 100 s, for sheet thicknesses of 1.5 mm or less in both variants (constant oven temperature, stepwise heating). Furthermore, the total time in the oven for such sheets is preferably a maximum of 600 s, particularly a maximum of 480 s, preferably a maximum of 360 s, and most preferably a maximum of 240 s in both variants.

[0211] For sheet thicknesses greater than 1.5 mm (especially up to 5 mm sheet thickness), the total time in the oven is t O f en In particular, at least 75 s, preferably at least 100 s, preferably at least 150 s. Furthermore, the total time in the furnace for such sheets is preferably a maximum of 720 s for both variants, in particular a maximum of 600 s, preferably a maximum of 480 s, and most preferably a maximum of 360 s. ThyssenKrupp Steel Europe AG 237118P10WO

[0212] August 27, 2025

[0213] 31 / 39

[0214] Longer overall oven times have the advantage of ensuring uniform austenitization of the sheet metal blank. On the other hand, prolonged exposure above Ac3 leads to grain coarsening, which negatively affects the mechanical properties.

[0215] The pre-heated blank is removed from the respective heating device, which may be, for example, a conventional heating furnace, a known induction heating device, or a conventional device for keeping steel components warm, and transported so quickly into the forming tool that its temperature upon arrival in the tool is at least partially above Ms + 100 °C, preferably above 600 °C, particularly above 650 °C, and most preferably above 700 °C. Here, Ms denotes the martensite start temperature. In a particularly preferred variant, the temperature is at least partially above the ACl temperature. In all these variants, the temperature is, in particular, a maximum of 900 °C. These temperature ranges ensure good formability of the material.

[0216] In step c), the transfer of the austenitized blank from the heating device used to the forming tool is completed within preferably no more than 20 s, and in particular within a maximum of 15 s. Such rapid transport is necessary to prevent excessive cooling before forming.

[0217] When the blank is inserted, the tool typically has a temperature between room temperature (RT) and 200 °C, preferably between 20 °C and 180 °C, and particularly between 50 °C and 150 °C. Optionally, in a particular embodiment, the tool can be heated at least in certain areas to a temperature Twz of at least 200 °C, and particularly at least 300 °C, in order to harden the component only partially. Furthermore, the tool temperature Twz is preferably a maximum of 600 °C, and particularly a maximum of 550 °C. It is only necessary to ensure that the tool temperature Twz is below the desired target temperature Tziei. The dwell time in the tool twz is preferably at least 2 s, and particularly preferably at least 3 s, and particularly preferably at least 5 s. The maximum dwell time in the tool is preferably 25 s, and particularly a maximum of 20 s.

[0218] The target temperature Tziei of the sheet metal part is at least partially below 400 °C, preferably below 300 °C, particularly below 250 °C, preferably below 200 °C, especially ThyssenKrupp Steel Europe AG 237118P10WO

[0219] August 27, 2025

[0220] 32 / 39 preferably below 180 °C, particularly below 150 °C. Alternatively, the target temperature Tzid 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.

[0221] The martensite start temperature of a steel that meets the specifications of the invention is determined according to the formula

[0222] Ms [°C] = (490.85 - 302.6 %C - 30.6 %Mn - 16.6 %Ni - 8.9 %Cr + 2.4 %Mo - 11.3 %Cu + 8.58 %Co + 7.4 %W - 14.5 %Si) [°C / wt.%] to be calculated, where here too, C% denotes the C content, %Mn the Mn content, %Mo the Mo content, %Cr the Cr content, %Ni the Ni content, %Cu the Cu content, %Co the Co content, %W the W content and %Si the Si content of the respective steel in wt.%.

[0223] The AC1 temperature and the AC3 temperature of a steel within the parameters of the invention are determined according to the formulas

[0224] AC1[°C] = (739 - 22*%C - 7*%Mn + 2*%Si + 14*%Cr + 13*%Mo - 13*%Ni +20*%V )[°C / wt%],

[0225] AC3[°C] = (902 - 225*%C + 19*%Si - 11*%Mn - 5*%Cr + 13*%Mo - 20*%Ni +55*%V)[°C / wt%] to be calculated, where %C denotes the C content, %Si the Si content, %Mn the Mn content, %Cr the Cr content, %Mo the Mo content, %Ni the Ni content and +%V the vanadium content of the respective steel (Brandis H 1975 TEW-Techn. Ber. 1 8 - 10).

[0226] In the tool, the blank is not only formed into the sheet metal part, but the target temperature is also simultaneously cooled. The cooling rate in the tool r W z is in particular at least 25 K / s, preferably at least 30 K / s, in particular at least 50 K / s, and in a special embodiment at least 100 K / s. The cooling rate r W z is defined as the average cooling rate ThyssenKrupp Steel Europe AG 237118P10WO

[0227] August 27, 2025 33 / 39 between the temperature T E ini gduring insertion into the forming tool and the martensite start temperature.

[0228] In a preferred embodiment, the cooling rate r is also W z' from the insertion temperature T E ini g The martensite finish temperature must be at least 25 K / s, preferably at least 30 K / s, in particular at least 50 K / s, and in a special embodiment at least 100 K / s. The martensite finish temperature is determined by means of dilatometer measurements.

[0229] Further cooling from the martensite start temperature or the martensite finish temperature to the target temperature Tziei can also be carried out with lower cooling rates, since this cooling no longer has a significant effect on the microstructure formation.

[0230] After the removal of the sheet metal part in step e), the sheet metal part is cooled to a cooling temperature TAB of less than 100 °C within a cooling period t. Aß from 0.5 to 600 s. This usually occurs through air cooling.

[0231] The invention will be explained in more detail below using exemplary embodiments.

[0232] To demonstrate the effectiveness of the invention, several tests were conducted. For this purpose, a slab with the compositions specified in Table 1, measuring 240 mm thick and 1200 mm wide, was produced and heated to a temperature TI of 1200 °C in a pusher furnace. The slab was then held at TI for 30 to 450 minutes until the core temperature T1 was reached and the slab was thoroughly heated. The slabs were then removed from the pusher furnace at their through-heating temperature TI and subjected to hot rolling. The test was carried out as continuous hot strip rolling. For this, the slab was first pre-rolled to an intermediate product with a thickness of 40 mm. This intermediate product, which can also be referred to as roughing strip in hot strip rolling, had an intermediate product temperature T2 of 1100 °C at the end of the pre-rolling phase.The roughing strip was fed to finish rolling immediately after pre-rolling, so that the intermediate product temperature T2 corresponds to the starting rolling temperature for the finish rolling phase. The roughing strip was hot-rolled to a final thickness of 4 mm and a final rolling temperature T3 of 890 °C, cooled to the coiling temperature, and wound into coils at a coiling temperature T4 of 580 °C. It was then cooled in still air. The hot-rolled strip was descaled conventionally by pickling before being cold-rolled. ThyssenKrupp Steel Europe AG 237118P10WO.

[0233] August 27, 2025 34 / 39 The thickness shown in Table 4 was achieved. The cold-rolled steel flat products were heated in a continuous annealing furnace to an annealing temperature T5 of 870 °C and held at this temperature for 100 s at a time before being cooled at a rate of 1 K / s to the immersion temperature T6 of 690 °C (Examples B1-B7) or 670 °C (Examples B8 and B9). The cold-rolled strips were then passed through a molten coating bath at temperature T7 at their respective immersion temperatures T6. (The respective bath temperature T7 is given in Table 2.) 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 airflow with a flow pressure given in Table 2 was used for this purpose. The temperature of the airflow was 70 °C in all cases.In this process, all strips were coated on both sides, with the coating thickness specified in Table 3. The strips were initially cooled to 600 °C at an average cooling rate of 10–15 K / s. During the subsequent cooling phases between 600 °C and 450 °C and between 400 °C and 300 °C, the strips were cooled for the cooling durations T (600 °C–450 °C) and T (400 °C–300 °C), respectively, as specified in Table 2. Between 450 °C and 400 °C and below 220 °C, the strips were cooled at a cooling rate of 5–15 K / s.

[0234] Table 3 summarizes the properties of the steel flat product. The layer thicknesses were determined from metallographic cross-sections. The composition of the alloy layer was determined using EDX. It is clearly evident that in tests B4–B9, the thickness of the alloy layer is significantly increased compared to the reference tests B1–B3. Likewise, the iron content of the alloy layer is increased in the samples according to the invention. The composition of the aluminum base layer in tests B1–B9 corresponds to the composition of the melt listed in Table 2.

[0235] From the steel strips produced in this way, blanks were cut off and used for further tests. In these tests, sheet metal samples 1-9, measuring 200 x 300 mm², were hot-pressed from the respective blanks. For this purpose, the blanks were heated in a heating device, for example in a conventional heating oven, from room temperature at a medium heating rate r. O f en The sample was heated at a rate of 6 K / s (in the temperature range between 30 °C and 700 °C) in an oven with an oven temperature of (see Table 4). The total time in the oven, including heating and holding, is denoted by t. O The blanks are designated and listed in Table 4. The following are the blanks from ThyssenKrupp Steel Europe AG 237118P10WO.

[0236] August 27, 2025

[0237] 35 / 39 were removed from the heating unit and placed in a forming tool. Upon removal from the oven, the blanks had reached oven temperature. The transfer time, comprising the time for removal from the heating unit, transport to the tool, and placement in the tool, was approximately 10 seconds. The temperature of the blanks upon placement in the forming tool was above the respective AC1 temperature in all cases. The forming tool had a temperature Twz of 60 °C. In the forming tool, the blanks were formed into the respective sheet metal parts, with the sheet metal parts cooling in the tool at a rate r. W The samples were cooled at a rate of 50 K / s. Finally, the samples were cooled to room temperature. Cooling was carried out in still air at a rate of 7 K / s.

[0238] Table 5 also lists the properties of the sheet metal parts obtained in this way. The coating thickness ranged from 9 to 14 µm.

[0239] A wet chemical analysis of the entire coating yielded the composition shown in Table 5 for the experiments according to the invention. It is clearly evident that in examples 4-9 according to the invention, the iron content in the coating is comparable to, and in some cases even higher than, the iron content of comparative examples 1-3. However, comparative examples 1-3 were processed in the furnace for a significantly longer time (see t O f en (in Table 4). Despite a shorter furnace time t O f en The examples according to the invention thus achieve comparable or higher iron contents. This results from the diffusion behavior enhanced by calcium.

[0240] In addition to chemical analysis, the coating was also analyzed in cross-section. All tests showed a structure with an aluminum base layer and an alloy layer, with the alloy layer resting on the steel substrate and the aluminum base layer resting on the alloy layer.

[0241] The microstructure of the steel substrate was also determined based on the cross-sections. In all cases, a martensite content of more than 95% of the surface area was observed. hyssenKrupp Steel Europe AG 237118P10WO

[0242] August 27, 2025

[0243] 36 / 39

[0244] Residual iron and unavoidable impurities. All values ​​in wt.%;

[0245] Table 1 (Steel grades) Non-inventional reference examples table 2 (coating variant)

[0246] hyssenKrupp Steel Europe AG 237118P10WO

[0247] August 27, 2025

[0248] 37 / 39 Non-inventive reference examples table 3 (overview test results)

[0249] hyssenKrupp Steel Europe AG 237118P10WO

[0250] August 27, 2025

[0251] 38 / 39 Table 4 (Overview of trial results)

[0252] hyssenKrupp Steel Europe AG 237118P10WO

[0253] August 27, 2025

[0254] 39 / 39 Non-inventive reference examples e table 5 (overview test results after hot forming)

Claims

ThyssenKrupp Steel Europe AG 237118P10WO August 27, 2025 1 / 5 Patent claims 1. Steel flat product for the production of a sheet metal component 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) an aluminum-based coating arranged on at least one side of the steel substrate, characterized in that the coating has an Al base layer consisting of 1.0 - 15 wt.% Si, optionally 2-4 wt.% Fe, 0.030 - 0.060 wt.% Ca, optionally 0.1 - 5.0 wt.% other alkali or alkaline earth metals, optionally up to 15 wt.% Zn, preferably up to 10 wt.% Zn, and the remainder being aluminum and unavoidable impurities.

2. Steel flat product according to claim 1, characterized in that the coating has a thickness of 9 - 15 pm.

3. Steel flat product according to one of claims 1 to 2, characterized in that the coating has an alloy layer which rests on the steel substrate and on which the Al base layer is arranged, wherein the alloy layer consists of 25 - 60 wt.% Fe, optional further components whose content in total is limited to a maximum of 5.0 wt.%, preferably 2.0 wt.%, and consists as a remainder of aluminium and unavoidable impurities.

4. Steel flat product according to one of claims 1 to 3, characterized in that the coating has an alloy layer which rests on the steel substrate and on which the Al base layer is arranged, wherein the alloy layer has a thickness of at least 7pm.

5. Steel flat product according to one of the preceding claims, characterized in that the content of further alkali or alkaline earth metals in the Al base layer comprises 0.1 - 1.0 wt.% Mg, in particular 0.1 - 0.7 wt.% Mg, preferably 0.1 - 0.5 wt.% Mg. ThyssenKrupp Steel Europe AG 237118P10WO August 27, 2025 2 / 5 6. Method for producing a steel flat product, in particular according to one of claims 1-5, 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.% Mn.-% B; b) Through-heating of the slab or thin slab at a temperature (TI) of 1000 - 1400 °C; c) Optional pre-rolling of 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 steel flat product, wherein the final rolling temperature (T3) is 750 - 1000 °C; e) Optional coiling of the hot-rolled steel flat product, wherein the coiling temperature (T4) is not more than 700 °C; f) Descaling of the hot-rolled steel flat product; g) Optional cold rolling of the steel flat product, wherein the degree of cold rolling is at least 30%; h) Annealing the steel flat product at an annealing temperature (T5) of 650 - 900 °C; i) Cooling the steel flat product to an immersion temperature (T6) of 650 - 800 °C, preferably 680 - 720 °C; j) Coating the steel flat product cooled to the immersion temperature with a coating 9 - 15 pm thick. ThyssenKrupp Steel Europe AG 237118P10WO August 27, 2025 3 / 5 i. Immersion in a melt bath with a melt temperature (T7) of 630–800 °C, preferably 670–710 °C, wherein the melt bath contains the coating to be applied to the steel flat product in liquid form, which consists of 1.0–15 wt.% Si, optionally 2–4 wt.% Fe, 0.030–0.060 wt.% Ca, optionally 0.1–5.0 wt.% other alkali or alkaline earth metals, optionally up to 15 wt.% Zn, preferably up to 10 wt.% Zn, and the remainder being aluminum and unavoidable impurities; ii. Blowing off the steel flat product after exiting the melt bath by means of a gas stream, in particular with a flow pressure of 100–1000 mbar, preferably 200–750 mbar; k) Cooling the coated steel flat product to room temperature, wherein the first cooling time t mT in the temperature range between 600 °C and 450 °C is more than 10 s, in particular more than 14 s, and the second cooling time in the temperature range between 400 °C and 300 °C is more than 8 s, in particular more than 12 s; l) optional dressing of the coated steel flat product.

7. Method according to claim 6, characterized in that the gas stream is an air stream which preferably has a temperature of room temperature to 130 °C, preferably of 50 - 90 °C.

8. Sheet metal forming part comprising a) a steel substrate consisting of a steel having 0.1 - 3 wt.% Mn and optionally up to 0.01 wt.% B, and b) an aluminium-based coating arranged on at least one side of the steel substrate, ThyssenKrupp Steel Europe AG 237118P10WO August 27, 2025 4 / 5 characterized in that the coating consists of 1.0 - 15 wt.% Si, 35 - 90 wt.% Fe, 0.010 - 0.050 wt.% Ca, optionally 0.1 - 5.0 wt.% other alkali or alkaline earth metals, optionally up to 15 wt.% Zn, preferably up to 10 wt.% Zn and optional further components, the total content of which is limited to a maximum of 2.0 wt.%, and as a remainder aluminium and unavoidable impurities.

9. Sheet metal forming part according to claim 8, characterized in that the coating has a thickness of 9 - 25pm.

10. Sheet metal forming part according to one of claims 8 to 9, characterized in that the coating has an Al base layer and an alloy layer, wherein the alloy layer rests on the steel substrate and the Al base layer rests on the alloy layer.

11. Sheet metal forming part according to one of claims 8 to 10, characterized in that the steel substrate of the sheet metal forming part has a structure with at least partially more than 80% martensite, preferably more than at least partially more than 90% martensite.

12. A method for manufacturing a sheet metal part according to any one of claims 8 to 11, comprising the following steps: a) providing a sheet metal blank from a flat steel product according to any one of claims 1 to 5; b) heating the sheet metal blank such that at least partially the AC3 temperature of the blank is exceeded and the temperature T E ini gof the blank when placed in a forming tool intended for hot pressing (step c)) has at least a partial temperature above Ms+100 °C, where Ms denotes the martensite start temperature; c) placing the heated sheet blank into a forming tool, wherein the transfer time t required for removing the blank from the heating device and placing it Tr at most 20s, preferably at most 15s; ThyssenKrupp Steel Europe AG 237118P10WO August 27, 2025 5 / 5 d) Hot pressing of the sheet metal blank to form the sheet metal part, wherein the blank is cooled to a target temperature Tzid during hot pressing for a duration twz of more than ls and optionally held there, wherein the cooling from the temperature T E ini g at least up to the martensite start temperature with a cooling rate r that is at least partially more than 25 K / s Wz; e) Removal of the sheet metal part cooled to the target temperature Tziei from the tool.

13. Method according to claim 12, wherein the temperature reached at least partially in the sheet metal blank is between Ac3 and 1000 °C, preferably between 850 °C and 950 °C.

14. Method according to one of claims 12 to 13, wherein the target temperature Tziei of the sheet metal part is at least partially below 400 °C, preferably below 300 °C.

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