Steel having excellent local and overall ductility and simultaneously low LME susceptibility, and method for manufacturing same

Abstract

The present invention relates to a high-strength coated flat steel product which exhibits good LME properties. The flat steel product has a microstructure which consists of at least 75% tempered martensite and / or lower bainite, at least 5% residual austenite, at most 10% ferrite, and at most 10% untempered martensite, and has a coating with an aluminium concentration from 0.20% to 0.50%. The invention also relates to the manufacture of such a flat steel product and to a component made from such a flat steel product.

Description

[0001] ThyssenKrupp Steel Europe AG 237003P10WO

[0002] 1 / 28 September 1, 2025

[0003] Steel with excellent local and global elongation and simultaneously low LME sensitivity and manufacturing process

[0004] The invention relates to a high-strength, coated steel flat product with excellent local and global elongation and at the same time good LME (Liquid-Metal-Embrittlement) properties.

[0005] The flat steel products described in the invention are typically rolled products, such as steel strips or sheets, as well as blanks and sheets produced therefrom.

[0006] Mechanical properties, such as tensile strength R m , yield strength R p0 ,2, Elongation at break A 80The values ​​reported here were determined in tensile tests according to DIN EN ISO 6892-1:2017, unless explicitly stated otherwise. The hole expansion HER is determined according to ISO 16630.

[0007] In this application, all information regarding steel composition is based on weight, unless expressly stated otherwise. Therefore, all unspecified "%" values ​​relating to a steel alloy should be understood as "wt%".

[0008] With the exception of the information relating to volume (specified in "Vol.-%") regarding the retained austenite content of the microstructure of a sheet metal part according to the invention, information on the contents of the various microstructural constituents refers to the area of ​​a polished section of a sample of the respective product (specified in area percent "area %"), unless expressly stated otherwise.

[0009] The microstructure is determined on longitudinal sections etched with 3% Nital (alcoholic nitric acid). Microstructure analysis is performed using a scanning electron microscope at 5000x magnification to determine the proportion of plate-like and other non-plate-like bainite, and at 20,000x to 50,000x magnification to determine plate length, width, and spacing. The proportion of retained austenite is determined by X-ray diffraction (XRD) according to ASTM E975.

[0010] The Al content in the coating was determined using wet chemical methods in accordance with DIN EN ISO 10111:2019-04 and DIN EN ISO 11885:2009-09.

[0011] The spatially resolved concentrations of Si, Cr and Mn in a near-surface layer were determined using glow discharge optical emission spectroscopy (GD- ThyssenKrupp Steel Europe AG 237003P10WC).

[0012] 2 / 28 1 September 2025

[0013] The oxygen concentration (OES) was determined. For this purpose, a GD-OES measuring device from Leco, for example, can be used. GD-OES allows for the quantitative determination of elements in layered structures along the layer thickness. The GD-OES measurements were performed according to the standards ISO 16962 (2017-02); ISO 14707 (2021-03); and ISO 11505 (2012-12). Based on the obtained concentration profiles, the layer thickness d of the near-surface layer was determined. For this purpose, starting from the surface and extending into the base material, the point at which the oxygen concentration first falls below 1% was determined. The distance between the surface and the point at which the oxygen concentration first falls below 1% is the layer thickness d. In a particular embodiment, the layer thickness can be a maximum of 8 pm, preferably 7 pm, and most preferably 6 pm. In another special embodiment, the layer thickness can be a minimum of 1 pm, preferably 2 pm.In each layer, the average concentration profile of Si, Cr, and Mn was determined and compared to the average concentration in the core. The core of the steel flat product begins at least 50 pm, preferably 100 pm, from both surfaces of the steel flat product.

[0014] All strip temperatures in the process can be determined, for example, with a commercially available pyrometer.

[0015] In the present application, "same atmosphere" (i.e., A=Aj) means that the dew points are identical within the limits of measurement accuracy and preferably that the proportions of hydrogen, oxygen and nitrogen are identical within the limits of measurement uncertainty.

[0016] In the present invention, the Karagoulis test was used to determine the LME (Liquid Metal Embrittlement) properties. For this purpose, the test procedure "Rapid LME Test Procedure for Coated Sheet Steels," Revision 2.0 dated March 10, 2020, published by General Motors through the Auto / Steel Partnership of the American Iron and Steel Institute, was used. Good LME properties mean that no cracks greater than 30%, preferably greater than 20%, form in the Karagoulis Type D test.

[0017] High-strength steels with good forming properties are known from the state of the art for applications such as in automotive engineering. High-strength steels are characterized by a high proportion of alloying elements, which contribute to increased strength. At the same time, the steel must exhibit good ductility. (ThyssenKrupp Steel Europe AG 237003P10WC)

[0018] 3 / 28 1 September 2025

[0019] The high proportion of alloying elements, especially silicon, has the disadvantage that a sensitivity to LME (Liquid-Metal-Embrittlement) arises during thermal joining processes in subsequent processing steps.

[0020] Liquid metal embrittlement (LME) is a phenomenon in which metals become brittle through contact with liquid metals or alloys. This effect occurs during the thermal joining of a coated steel sheet. In conventional joining methods, the components of the surface coating, such as zinc, are melted. The molten zinc from the surface coating can attack the grain boundaries of the steel sheet at areas of particularly high stress, penetrating between them and leading to separation and thus to cracking.

[0021] EP 3 647 452 Bl discloses a steel sheet with good LME properties containing 0.04% to 0.35% C, 0.99% or less Al+Si, 3.5% to 10% Mn, 0.05% or less P (excluding 0%), 0.02% or less S (excluding 0%), and 0.02% or less N (excluding 0%), with a residue of Fe and other unavoidable impurities, and exhibiting a manganese-depleted surface layer. The good LME properties are achieved by limiting the sum of aluminum and silicon. Sufficient mechanical properties can only be achieved with a high manganese content. However, manganese has the disadvantage that a high Mn content severely restricts weldability and reduces corrosion resistance.

[0022] WO 2019 / 097440 Al discloses a coated steel sheet containing 0.10% to 0.40% C, 1.5% to 3.0% Mn, 0.7% to 3.0% Si, 0.05% to 1.0% Al, 0.75% to 3.0% Si+Al, the optional elements Nb < 0.5%, B < 1.0%, Mo < 0.50%, Ni < 0.5%, Ti < 0.5%, and the remainder iron. Low metal oxide (LME) resistance is achieved through a double coating. The first coating, applied at wavelengths from 600 nm to 1400 nm, is nickel-based, and the second coating is zinc-based and contains no nickel. However, this has the disadvantage of requiring two coating steps.

[0023] The object of the present invention is to provide a coated steel sheet and a method for its production which overcomes the disadvantages of the prior art, i.e., has excellent LME resistance but also good mechanical properties.

[0024] The problem is solved using a high-strength, coated flat steel product with a tensile strength of R. m = 900 MPa-1500 MPa, a yield strength R p02 > 700 MPa and an elongation A 80 = 10%-24%, which includes a steel with ThyssenKrupp Steel Europe AG 237003P10WC)

[0025] 4 / 28 1 September 2025

[0026] 0.10%–0.5% C

[0027] 1.0%–3.0% Mn

[0028] 0.7%–2.5% Si

[0029] 0.05% - 1.0% Cr

[0030] < 0.020 % P,

[0031] < 0.005 % S,

[0032] < 0.02% N, and optionally one or more of the following elements:

[0033] 0.01-1.5% AI,

[0034] 0.05-0.5% Mo,

[0035] 0.0004-0.002% B

[0036] 0.005% < Ti+Nb+V < 0.2%, and as a remainder of iron and unavoidable elements, wherein the steel flat product has a microstructure consisting of at least 75% tempered martensite and / or lower bainite, at least 5% retained austenite, a maximum of 10% ferrite and a maximum of 10% untempered martensite, and wherein the coated steel flat product has at least one surface with a coating wherein the Al concentration in the coating is 0.20%-0.50%, preferably 0.25%-0.45%.

[0037] The composition and microstructure according to the invention, together with the aluminum content in the coating according to the invention, leads to a high-strength, coated steel flat product with excellent LME properties. The inventors have surprisingly discovered that a steel flat product with a microstructure consisting of at least 75% tempered martensite and / or lower bainite, at least 5% retained austenite, a maximum of 10% ferrite, and a maximum of 10% untempered martensite, and with an aluminum concentration in the coating of 0.20%–0.50%, preferably 0.25%–0.45%, and particularly preferably 0.25%–0.40%, exhibits excellent LME properties and excellent local and global elongation. If the aluminium content in the coating is greater than 0.50%, preferably greater than 0.45%, and especially preferably greater than 0.40%, the LME properties deteriorate, i.e., the risk of LME cracking increases.The exact mechanism of LME is not yet fully understood, but the inventors assume that it is due to the increased melting temperature caused by aluminum or the reduced solubility of the molten coating in the substrate. (ThyssenKrupp Steel Europe AG 237003P10WC).

[0038] 5 / 28 1 September 2025

[0039] If the aluminum content is less than 0.20%, preferably less than 0.25%, sufficient adhesion of the coating cannot be achieved. In a particular embodiment, the coating comprises an Fe₂Al₅ interface layer and a Zn layer. In this case, with an aluminum content of less than 0.20%, preferably less than 0.25%, a thick Fe₂Al₅ interface layer does not form, and therefore sufficient adhesion of the coating is not achieved.

[0040] Particularly preferably, the coating consists of an Fe₂Al₅ interface layer and a Zn layer. For the purposes of this application, Zn layer means a zinc-based layer, i.e., it comprises more than 50% zinc. In a particular embodiment, the coating is applied directly to the substrate. This eliminates the need for a further process step to apply another layer and is more cost-effective than applying an additional layer.

[0041] In a particular embodiment, the coating contains no nickel, i.e., the nickel content is less than 2.0%, preferably less than 1.0%, particularly preferably less than 0.50%, and particularly preferably less than 0.10%.

[0042] In a particular embodiment, the steel flat product has a near-surface layer extending into the steel flat product with a layer thickness d, wherein the concentration of Si is at most 80% compared to the core of the steel flat product, preferably at most 70%, particularly preferably at most 65%, preferably at least 50%, particularly preferably at least 55%, and the concentration of Mn is at most 70% compared to the core of the steel flat product, preferably at most 65%, particularly preferably at most 60%, preferably at least 50%, particularly preferably at least 55%, and the concentration of Cr is at most 75% compared to the core of the steel flat product, preferably at most 70%, particularly preferably at most 65%, preferably at least 50%, particularly preferably at least 55%.

[0043] By selectively adjusting the Si, Mn, and Cr contents in the near-surface layer, the LME properties are further improved. Si, Mn, and Cr have a negative effect on the LME properties. However, they are necessary in the steel flat product according to the invention to achieve the desired microstructure and the desired mechanical properties. In a preferred embodiment, this negative effect can be reduced by layer d, in which the elements are present in a lower concentration under the coating. Surprisingly, it has been shown that only a reduction of all three elements in the preferred areas of the steel flat product according to the invention leads to a further advantageous effect on the LME properties without impairing the mechanical properties of the steel flat product. ThyssenKrupp Steel Europe AG 237003P10WO

[0044] 6 / 28 1 September 2025

[0045] The steel flat product according to the invention is described in more detail below.

[0046] Carbon “C” is present in the steel according to the invention in contents of 0.10% to 0.5%. In the steel according to the invention, carbon supports the formation and stabilization of austenite. In particular, stabilization occurs during quenching and the subsequent annealing treatment. Furthermore, the addition of C gives the steel high strength, as the strength of the martensite, which forms during the process, is increased. Therefore, the C content should be at least 0.10%, preferably 0.12%, particularly preferably 0.13%, and most preferably 0.15%. On the other hand, the martensite start temperature is shifted to increasingly lower temperatures with increasing C content, so that it may be that no or only an insufficient proportion of low-temperature phases can be formed. For this reason, the C content in the steel according to the invention should be a maximum of 0.5%, preferably 0.45%, and most preferably 0.35%.

[0047] Silicon (Si) is required to achieve the specific microstructure in this invention because it delays cementite formation. An excessively high cementite content would bind the carbon in carbides, making it unavailable for stabilizing the retained austenite during the process and resulting in reduced elongation. Therefore, the steel according to the invention must contain at least 0.7%, preferably at least 0.9%, particularly preferably at least 1.05%, and most preferably at least 1.10% silicon. Conversely, an excessively high silicon content leads to poor surface quality, so the steel according to the invention contains a maximum of 2.5%, preferably 1.7%, and most preferably at most 1.5% silicon.

[0048] The steel according to the invention contains manganese (Mn). With a content of 1.0% or higher, Mn enables martensite formation by suppressing pearlite formation. A content of at least 1.2% has proven advantageous, and a content of at least 1.5% is particularly advantageous. However, an excessively high Mn content can lead to strong segregation, which is why the Mn content is limited to 3.0%. Furthermore, a high Mn content significantly restricts weldability and reduces corrosion resistance. Therefore, a Mn content of preferably a maximum of 2.7%, particularly preferably 2.5%, and especially preferably 2.3% has proven to be particularly advantageous.

[0049] Chromium (Cr) is an effective inhibitor of pearlite and contributes to its strength. A chromium content of at least 0.05% has proven particularly advantageous. However, chromium can lead to grain boundary oxidation through the formation of Cr oxides. Therefore, the chromium content is limited to 1.0%, preferably 0.9%, and most preferably 0.6%. (ThyssenKrupp Steel Europe AG 237003P10WC)

[0050] 7 / 28 September 1, 2025

[0051] The addition of phosphorus “P” severely restricts weldability and should therefore be limited to 0.020%, with contents of 0.018%, and in particular 0.015%, being especially advantageous. In the steel according to the invention, it has been found that a P content of at least 0.002%, and in particular 0.006%, can be advantageous, as this strengthens the solid solution hardening.

[0052] Sulfur (S) can lead to the formation of manganese sulfides, which significantly impair the formability properties. Therefore, in the steel according to the invention, the sulfur content is limited to 0.005%, with a limitation to 0.004% and, in particular, to 0.003% being advantageous. Sulfur contamination cannot be completely avoided during steel production.

[0053] Nitrogen (N) concentrations above 0.02% can lead to the formation of coarse nitrides, resulting in impaired formability. A maximum concentration of 0.008% has proven particularly advantageous in preventing these nitrides. Nitrogen contamination cannot be completely avoided during steel production.

[0054] In addition to the previously explained impurities P, S, and N, 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 these "unavoidable impurities" is a maximum of 0.2%, more preferably a maximum of 0.1%. The optional alloying elements "Al, Cr, Mo, B, Ti, Nb, V" described below, 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%, more preferably a maximum of 0.1%.

[0055] Aluminum “Al” can be added to the steel according to the invention for deoxidation and to bind any nitrogen that may be present. Aluminum can also be used to increase the retained austenite content. A higher retained austenite content results from the addition of aluminum by delaying the formation of cementite precipitates. For this purpose, an aluminum content of at least 0.01%, preferably 0.03%, has proven advantageous in the flat steel product according to the invention. On the other hand, an excessively high aluminum content can lead to the formation of coarse Al nitrides, which have an embrittlement effect and thus to poorer formability. Furthermore, higher Al contents can lead to poorer casting properties, as aluminum compounds can cause clogging. (ThyssenKrupp Steel Europe AG 237003P10WC)

[0056] The invention described in section 8 / 28 dated September 1, 2025, therefore provides for a limitation of the aluminium content to 1.5%, preferably 0.8%, and particularly preferably 0.4%.

[0057] Molybdenum (“Mo”) also forms fine, strength-enhancing carbon nitrides even in small quantities. Therefore, an addition of at least 0.05% has proven advantageous. However, the strength-enhancing effect of the carbon nitrides diminishes as soon as the molybdenum content becomes too high. Furthermore, high molybdenum contents can impair cold formability and weldability. Here, a content of at most 0.5%, preferably 0.2%, particularly preferably 0.10%, and especially preferably 0.07% has proven advantageous.

[0058] The addition of boron (B) leads to a fine-grained microstructure because boron segregates at the phase boundaries and blocks their movement. At least 0.0004%, and particularly preferably at least 0.0005%, can be added to the steel according to the invention for this purpose. The effect of boron is saturated at a maximum content of 0.002%.

[0059] In a particular embodiment, microalloying elements (MLE) (preferably Ti and / or Nb and / or V) can be added to the steel according to the invention. For the purposes of this invention, boron is not considered a microalloying element. These elements contribute to increased strength through the formation of very finely dispersed carbides. A minimum total MLE content of 0.005% leads to the freezing of grain and phase boundaries during annealing. Conversely, an excessively high concentration of MLE, which strongly promotes carbide formation and phase boundary immobility, is detrimental to the stabilization of the retained austenite. Therefore, the total MLE concentration should be limited to a maximum of 0.2%.

[0060] The microstructure of the steel flat product according to the invention consists of at least 75% tempered martensite and / or lower bainite, at least 5% retained austenite, a maximum of 10% ferrite and a maximum of 10% untempered martensite.

[0061] In a particular embodiment, the steel flat product has a microstructure with a maximum of 85% tempered martensite and / or lower bainite, preferably bainitic ferrite.

[0062] The microstructure of a flat steel product according to the invention contains at least 5% retained austenite, preferably 10%. Retained austenite has a beneficial effect on the formability and elongation of martensitic steels. The austenite, stabilized down to room temperature, can be elongated more than other microstructural constituents by utilizing the TRIP effect, while simultaneously exhibiting higher work hardening. (ThyssenKrupp Steel Europe AG 237003P10WC)

[0063] 9 / 28 1 September 2025 For weldability reasons, a retained austenite content greater than 20% is not possible with the described manufacturing process.

[0064] In the microstructure according to the invention, ferrite is present at a maximum of 10%, preferably at a maximum of 5%, and particularly preferably at a maximum of 3%, in order to ensure the required high strengths. In a preferred embodiment, the ferrite present is polygonal ferrite.

[0065] In the microstructure according to the invention, a maximum of 10%, particularly preferably 8%, particularly preferably 5% of the martensite is present untempered.

[0066] The steel flat product according to the invention exhibits excellent mechanical properties with a tensile strength R m of at least 900 MPa, preferably 1000 MPa and at most 1500 MPa, preferably 1350 MPa. Furthermore, the steel flat product according to the invention has a yield strength of R p02 of at least 700 MPa, preferably 820 MPa and an elongation of A 80 = 10%-24%, preferably 12%-18%.

[0067] In a preferred embodiment, the hole enlargement HER is at least 20%. In another preferred embodiment, the product of hole enlargement and strength is at least 3500% MPa.

[0068] Furthermore, the problem is solved by a component for structural lightweight construction in automotive engineering, formed from the steel flat product according to the invention.

[0069] In a preferred embodiment, the component is manufactured using a thermal joining process. Thermal joining processes describe all joining methods in which the flat steel product undergoes thermal heating during the joining process. Examples of thermal joining processes include welding and / or brazing according to DIN 8593.

[0070] Furthermore, the object of the present invention is achieved by a method for producing a high-strength, coated steel flat product, comprising at least the following steps: a) Providing a cold-rolled steel flat product comprising a steel consisting of the following elements:

[0071] 0.10%–0.5% C

[0072] 1.0%-3.0% Mn, ThyssenKrupp Steel Europe AG 237003P10WC)

[0073] 10 / 28 September 1, 2025

[0074] 0.7%–2.5% Si

[0075] 0.05% - 1.0% Cr

[0076] < 0.020 % P,

[0077] < 0.005 % S,

[0078] < 0.02% N, and optionally one or more of the following elements:

[0079] 0.01-1.5% AI,

[0080] 0.05-0.5% Mo,

[0081] 0.0004-0.002% B

[0082] 0.005% < Ti+Nb+V < 0.2%, and as a remainder of iron and unavoidable elements; b) Heating and pre-oxidizing the cold-rolled steel flat product under the following conditions: i. Heating the cold-rolled steel flat product in a reducing atmosphere Ai from room temperature to a temperature Ti, where L = 650°C–750°C; ii. Heating the cold-rolled steel flat product to a temperature T2 and then pre-oxidizing and heating the cold-rolled steel flat product from T2 to a temperature T3 in an oxidizing atmosphere A2 for t2 = 1 s–30 s, where Ti < T2 < T3 with T3 = 750°C–850°C; c) Heating the cold-rolled steel flat product from a temperature T3 to a temperature T4 and heating through at a temperature T4 for t4 = 5 s - 300 s, where T3 < T4 < 950 °C and T4 > A c3 -30 °C, heating and warming through in a reducing atmosphere A4 takes place; d) Cooling of the cold-rolled steel flat product in a time t Lk to a temperature TLK, where t Lk = 30 s - 120 s and T L K>T4-175°C; ThyssenKrupp Steel Europe AG 237003P10WC)

[0083] 11 / 28 1 September 2025 e) Cooling the cold-rolled steel flat product with a cooling rate 5 > 30 K / s from a temperature TLK to a temperature T5, where T5 = (TMS + 40 °C) - (TMS - 175 °C); f) Setting and holding the cold-rolled steel flat product at a temperature T6 for a holding time t6 = 1 s - 60 s, where T6 = T M s -( T MS - 175°C); g) Reheating the cold-rolled steel flat product at a heating rate of 4°C / s to 1000°C / s to a temperature T B , where TMS <T B <510 °C; h) Heating the strip surface to a temperature T9 by means of a purge gas with a temperature TSP = 500 °C-650 °C, where T B = T 9 + A T9 and A T9 > 10 °C; i) Coating and cooling of the cold-rolled steel flat product under the following conditions: i. Coating of the steel flat product at a melt bath temperature T ZN with a coating bath consisting of:

[0084] 0.10%–0.20% AI

[0085] Fe-saturated,

[0086] Residual Zn and unavoidable impurities, whereby

[0087] TZN = 450°C-520°C; ii. Cooling the coated steel flat product to a temperature Tw, where Tw < 60°C.

[0088] In a preferred embodiment, the method for producing a high-strength, uncoated steel flat product includes no further work steps and consists of steps a) to i).

[0089] In a particular embodiment, no further temperature changes occur between two successive steps; this is especially preferred for all pairs of successive steps. This means, for example, that after step e) (cooling to T5), T6 is directly set and maintained. ThyssenKrupp Steel Europe AG 237003P10WO

[0090] 12 / 28 September 1, 2025

[0091] The inventive method makes it possible to produce a steel flat product according to the invention which has good LME properties but still has high strength.

[0092] The following section describes each step in detail:

[0093] Step a)

[0094] The provided cold-rolled steel flat product is manufactured using conventional methods. These conventional methods include casting the steel into a slab, reheating the slabs, hot rolling, coiling the hot-rolled strip, pickling the hot-rolled strip, and cold rolling the hot-rolled strip. The same guidelines apply to the slab composition according to the invention and the optional variations as those already given in connection with the composition of the steel flat product according to the invention.

[0095] Step b)

[0096] The steel flat product according to the invention is preferably heated from room temperature to a temperature Ti in an atmosphere Ai. The temperature Ti is at least 650 °C, preferably 670 °C. The temperature Ti is at most 750 °C, preferably 730 °C, since above this temperature recrystallization processes begin and the process conditions must be adjusted according to step c).

[0097] The atmosphere Ai according to the invention is reducing, thereby preventing uncontrolled pre-oxidation. The atmosphere Ai preferably comprises at least 2% hydrogen “H2”, more preferably 3% H2, and particularly preferably 5% H2. The hydrogen content ensures that the atmosphere is reducing, especially with respect to iron. This prevents uncontrolled oxidation, allowing a thin oxide layer, particularly preferably thinner than 300 nm, to be established during the subsequent pre-oxidation. For economic reasons, the H2 content should be limited to a maximum of 20%, preferably 10%. Furthermore, up to 0.5% oxygen “O2”, particularly trace amounts of O2, and up to 0.5% water “H2O” can be added to the atmosphere. Both proportions must be limited to minimize the selective oxidation of base alloying elements.The remainder of the preferred atmosphere is composed of nitrogen “N2”, preferably 80%, particularly preferably 85%, and particularly preferably 90%. In a particular embodiment, the atmosphere consists of the described proportions of H2, O2, H2O, and N2. The dew point of the T. Pi The temperature is preferably at least -60 °C, preferably -55 °C, and particularly preferably -45 °C. Furthermore, the ThyssenKrupp Steel Europe AG 237003P10WO

[0098] 13 / 28 1 September 2025

[0099] The dew point should preferably not exceed -5 °C, particularly preferably -10 °C, and especially preferably -15 °C, as otherwise selective oxidation of base alloying elements may occur. This specific dew point control prevents coating and adhesion problems.

[0100] The steel flat product is heated to a temperature T2. It is then heated from temperature T2 to temperature T3 and pre-oxidized in an oxidizing atmosphere A2 for t2 = 1 s–30 s, where T3 < T2 < T3 = 750 °C–850 °C. T3 is at least 750 °C, preferably 800 °C, and at most 850 °C, preferably 840 °C. This pre-oxidation results in a covering FeO layer of defined thickness, preferably 50 nm–300 nm. In subsequent process steps, this covering layer prevents or at least significantly inhibits the selective oxidation of the oxygen-affine alloying elements on the external steel surface. Furthermore, in a particular embodiment, the oxygen-affine elements Mn and Si can be oxidized below the surface in this process step, which, together with the other process steps according to the invention, can lead to a depletion of the elements below the surface in the final product.To some extent, Mn and Si can also undergo internal oxidation. However, due to the strongly oxidizing atmosphere, the layer in which internal oxides occur is preferably less than 2 pm, particularly preferably less than 1 pm, and especially preferably less than 100 nm thick, since strong oxide formation is counteracted by a depletion zone d below the surface. This depletion can lead to an advantageous concentration distribution of Mn, Si, and Cr in layer d, which can further improve the LME properties.

[0101] The minimum value of T2 results from the fact that pre-oxidation occurs at T2<TI nicht ausreichend sicher eine im Wesentlichen deckendende FeO-Schicht, dicker > 50nm, generated. The maximum value of T2<T3ergibt sich daraus, dass die Voroxidation bei T2> T3 may tend to produce an FeO layer >300 nm thick, which can only be insufficiently reduced back to metallic Fe during the subsequent reduction according to step c). The exposure time during pre-oxidation should be at least 1 s, preferably 5 s, so that the pre-oxidation conditions according to the invention are sufficiently reliable to form a substantially opaque FeO layer thicker than >50 nm. The maximum value of t2 results from the fact that with an exposure time >30 s compared to the pre-oxidation conditions according to the invention, an FeO layer >300 nm thick is produced, which can only be insufficiently reduced back to metallic Fe during the subsequent reduction according to step c).This pre-oxidation takes place in an atmosphere A2, which is adjusted so that the conditions in this furnace zone always have an oxidizing effect on iron in order to achieve a targeted pre-oxidation of the steel surface to a FeO layer of at least 50 nm, preferably 60 nm and a maximum of 300 nm, preferably 200 nm (ThyssenKrupp Steel Europe AG 237003P10WC).

[0102] 14 / 28 1 September 2025

[0103] To adjust the oxide layer thickness. Various methods are known for setting oxidizing atmospheres. For example, atmosphere A2 can comprise at least 0.5% oxygen "O2", preferably 0.6% O2, particularly preferably 0.8% O2, and a maximum of 5% O2, preferably 2% O2, the remainder being N2 with traces of H2O, and possibly technically unavoidable residues of H2, CO2, and CO. Alternatively, moist N2 can be blown into this furnace zone as the oxidizing medium, in which case A2 reaches a dew point T. P2exhibits a temperature range of >0°C to <+60°C with an H2O / H2 ratio of >0.957 to ensure sufficient Fe oxidation. Preferably, in both variants mentioned, the dew point T P2 The dew point range should be at least +5 °C, particularly preferably +10 °C, and preferably a maximum of +30 °C, particularly preferably +25 °C. The inventors have found that the preferred dew point ranges are particularly advantageous for achieving a complete FeO layer between 70 nm and 180 nm. This thickness, in particular, avoids selective oxidation such as strong oxidation of Mn.

[0104] In a particular embodiment, the heating rate Ji in step b) between 500 °C and Ti is at least 2 °C / s, preferably 4 °C / s and a maximum of 50 °C / s, preferably 10 °C / s. The heating rate of at least 2 °C / s delays and further minimizes the selective oxidation of the base alloying elements until Ti is reached.

[0105] In a particular embodiment, the heating of Ti to T4 takes place with a mean heating rate2 = 0.5 °C / s - 10 °C / s.

[0106] Step c)

[0107] Subsequently, in step c), the steel flat product is heated from a temperature Ts to a temperature T4 and heated through at temperature T4 in a reducing atmosphere A4. The steel flat product is heated and heated through for t4 > 5 s, preferably b > 10 s, and particularly preferably b > 15 s.

[0108] The temperature T4 must not fall below the maximum pre-oxidation temperature T3. The minimum value of b results from the fact that the pre-oxidized steel surface is not sufficiently reduced back to metallic Fe under the reduction conditions according to the invention during an exposure time of < 5 s.

[0109] Setting the T4 temperature and allowing sufficient time (b) results in adequate austenitization. However, the time should be limited and should not exceed t4 by more than 300 s, preferably b by more than 180 s, as otherwise the austenite grains will become coarser, negatively affecting the mechanical properties. ThyssenKrupp Steel Europe AG 237003P10WO

[0110] 15 / 28 1 September 2025

[0111] For the temperature T4, in this step T3 < T4 < 950 °C and T4 > A c3 -30 °C, especially T4>

[0112] The minimum temperature to be exceeded is A. c3 according to the formula given by HOUGARDY, HP. in Werkstoffkunde Stahl, Band 1: Grundlagen, Verlag Stahleisen GmbH, Düsseldorf, 1984, p. 229:

[0113] A C3= (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.

[0114] The atmosphere A4 according to the invention is adjusted to ensure the targeted reduction of the previously formed FeO layer to metallic Fe. This allows the pre-oxidized steel surface to be sufficiently reduced. In a particular embodiment, the oxide layer formed in step c), in particular the FeO layer, is completely reduced to metallic iron.

[0115] In a preferred embodiment, the dew point is controlled within a narrow range, as this allows for the adjustment of Mn, Si, and Cr in the near-surface layer d, thereby achieving advantages in the LME properties. In a particular embodiment, the dew point T should be P4 Therefore, it should be set to at least -50 °C, preferably -45 °C, and particularly preferably -40 °C. Furthermore, the dew point should preferably not be greater than 0 °C, particularly preferably -3 °C, and especially preferably -11 °C, as otherwise unwanted oxide formation may occur.

[0116] The preferably set atmosphere A4 comprises at least 2% hydrogen “H2”, preferably 3% H2, and particularly preferably 5% H2. In a particular embodiment, AI can be equal to A4. The set hydrogen content ensures that the atmosphere is reducing, particularly with respect to iron. For economic reasons, the H2 content should preferably be limited to a maximum of 20%, preferably 10%. Furthermore, up to 0.5% oxygen “O2”, particularly traces of O2, and up to 0.5% water “H2O” can be added to the atmosphere. Both proportions must be limited to minimize the selective oxidation of base alloying elements. The remainder of the preferred atmosphere is composed of nitrogen “N2”, preferably 80%, particularly preferably 85%, and particularly preferably 90%. In a particular embodiment, the atmosphere consists of the described proportions of H2, O2, H2O, and N2. (ThyssenKrupp Steel Europe AG 237003P10WC)

[0117] 16 / 28 1 September 2025

[0118] In a particular embodiment, the heating of the steel flat product in step c) can be achieved by maintaining it at a constant temperature T4. A constant temperature is understood to mean a fluctuation of a maximum of ±5 °C, as a more precise setting is not technically feasible. This particular embodiment is especially recommended when a relatively low T4 temperature has been set for analytical reasons.

[0119] Step d)

[0120] The steel flat product is produced in a time t Lk cooled to a temperature TLK, where t Lk = 30 s-120 s and T LK > T4-175 °C. Slow cooling to no more than 175 °C below T4 should be carried out to avoid ferrite formation. The cooling time t Lkmust be at least 30 s, preferably 35 s and a maximum of 120 s, preferably 100 s, to avoid unwanted ferrite formation.

[0121] In a particular embodiment, cooling takes place in a reducing atmosphere. In a particular embodiment, the dew point T should be PL K should therefore be set to at least -50 °C, preferably at least -45 °C, and particularly preferably at least -4 °C. Furthermore, the dew point should preferably not be greater than 0 °C, particularly preferably -3 °C, and especially preferably -11 °C, as otherwise unwanted oxide formation may occur.

[0122] Step e)

[0123] Cooling the cold-rolled steel flat product to a temperature T5 at a cooling rate of 5 > 30 K / s, preferably 5 > 35 K / s, particularly preferably 5 > 40 K / s, especially preferably 5 > 45 K / s. The temperature T5 is at most (T M s + 40 °C), preferably (T Ms + 20 °C), particularly preferably TMS, to ensure a sufficient martensite content or sufficient nucleation of bainite in the final microstructure. The temperature T5 should be at least (TMS-175 °C), preferably (TMS-120 °C). In this step, the so-called primary martensite is formed. TMS can be determined using the following equation:

[0124] TMS (°C) = 539 °C + (- 423 [%C] - 30.4 [%Mn] - 7.5 [%Si] + 30 [%AI]) °C / wt%

[0125] Furthermore, the temperature T5 must be < 550 °C to avoid selective re-oxidation on the steel surface.

[0126] In a particular embodiment, the atmosphere A5 is reducing in step e). In a particular embodiment, A5 = A L K and / or A5, comprising at least 2% hydrogen “H2”, preferably 3% H2, particularly preferably 5% H2. Due to the adjusted hydrogen content, ThyssenKrupp Steel Europe AG 237003P10WQ

[0127] 17 / 28 1 September 2025, it can be ensured that the atmosphere is reducing, particularly with respect to iron. This prevents uncontrolled oxidation and reoxidation on the surface. The H₂ content should preferably be limited to a maximum of 80%, preferably 50%, for economic reasons. Furthermore, up to 0.5% oxygen "O₂", particularly trace amounts of O₂, and up to 0.5% water "H₂O" can be added to the atmosphere. Both proportions must be limited to minimize the selective oxidation of base alloying elements. The remainder of the preferred atmosphere is composed of nitrogen "N₂", preferably 80%, particularly preferably 85%, and particularly preferably 90%. In a particular embodiment, the atmosphere consists of the described proportions of H₂, O₂, H₂O, and N₂. The dew point T P5The ambient temperature is preferably at least -60 °C, particularly preferably -40 °C. Furthermore, the dew point should preferably not be greater than 0 °C, particularly preferably -10 °C, and especially preferably -15 °C, as otherwise selective oxidation of base alloying elements may occur.

[0128] In a particular embodiment, A4 is not equal to A5, as this prevents the uncontrolled transfer of hydrogen from atmosphere A4 into atmosphere A5. This allows the hydrogen content in atmosphere A5 to be precisely adjusted to the required amount to prevent selective oxidation, and there is no excess hydrogen that could unintentionally diffuse into the steel and lead to hydrogen embrittlement. This can be achieved through a structural separation, in particular an airlock system.

[0129] work step

[0130] In step f), the steel flat product is set to a temperature T6 in one atmosphere A6 and held for a holding time t6 = l s - 60 s. The minimum value for T6 is (T M s-175°C), preferably (TMS-150°C). The maximum value is TMS, preferably (TMS-75°C). T M S denotes the martensite start temperature, which can be estimated using the following equation:

[0131] TMS (°C) = 539 °C + (- 423 [%C] - 30.4 [%Mn] - 7.5 [%Si] + 30 [%AI]) °C / wt%, where the element concentrations are to be used in weight percent.

[0132] In a particular embodiment, T6 = T5. In a particular embodiment, T P6 = T P5The steel flat product should be held at T6 for at least 1 s, preferably at least 4 s, and particularly preferably at least 9 s, as this ensures a homogeneous temperature distribution in the material according to the invention, which guarantees the formation of a particularly fine and uniform microstructure of primary martensite and retained austenite across the cross-section of the steel flat product. For economic reasons, the holding time t6 is limited to 60 s. (ThyssenKrupp Steel Europe AG 237003P10WO)

[0133] 18 / 28 1 September 2025 special embodiment for thicknesses of the steel flat product > 1.0 mm the holding time is 10 s-60 s.

[0134] The preferably configured atmosphere A6 in step e) comprises at least 2% hydrogen “H2”, preferably 3% H2, and particularly preferably 5% H2. The hydrogen content ensures that the atmosphere is reducing, especially with respect to iron. This prevents uncontrolled oxidation and reoxidation on the surface. For economic reasons, the H2 content should preferably be limited to a maximum of 20%, preferably 10%. Furthermore, up to 0.5% oxygen “O2”, particularly trace amounts of O2, and up to 0.5% water “H2O” may be added to the atmosphere. Both of these proportions must be limited to minimize the selective oxidation of base alloying elements. The remainder of the preferred atmosphere is composed of nitrogen “N2”, preferably 80%, particularly preferably 85%, and particularly preferably 90%.In a particular embodiment, the atmosphere consists of the described proportions of H2, O2, H2O and N2. The dew point of the T. P6 The ambient temperature is preferably at least -60 °C, particularly preferably -40 °C. Furthermore, the dew point should preferably not be greater than -5 °C, particularly preferably -10 °C, and especially preferably -15 °C, as otherwise selective oxidation of base alloying elements may occur.

[0135] In a particular embodiment, A6 is not equal to A5, as this prevents uncontrolled transfer of hydrogen from atmosphere A4 into atmosphere A5. This allows the hydrogen content in atmosphere A5 to be precisely adjusted to the required amount to prevent selective oxidation, and there is no excess hydrogen that could unintentionally diffuse into the steel and lead to hydrogen embrittlement. This can be achieved through structural separation, in particular an airlock system.

[0136] Step g)

[0137] In step g), the steel flat product is heated to a temperature T at a rate J7 = 4 °C / s - 1000 °C / s in an atmosphere A7. B heated, whereby TMS <T B <510 °C. The temperature T B The temperature is at most 510 °C, particularly preferably 500 °C, and especially preferably 490 °C, as otherwise an unwanted decrease in the strength of the steel flat product will occur.

[0138] The temperature T B should be larger than TMS, preferably larger than T M The temperature must be s+50 °C to enrich the retained austenite in the base material microstructure with carbon from the supersaturated primary martensite or bainite. ThyssenKrupp Steel Europe AG 237003P10WO

[0139] 19 / 28 1 September 2025

[0140] The heating rate J7 should be at least 4 °C / s, preferably 6 °C / s, as otherwise unwanted carbides may form, which bind the carbon and are not available for enrichment in the retained austenite. The cooling rate J7 should be limited to a maximum of 1000 °C / s, preferably 50 °C / s.

[0141] The preferably configured atmosphere A7 in step g) comprises at least 2% hydrogen “H2”, preferably 3% H2, and particularly preferably 5% H2. The hydrogen content ensures that the atmosphere is reducing, especially with respect to iron. This prevents uncontrolled oxidation and reoxidation on the surface. For economic reasons, the H2 content should preferably be limited to a maximum of 20%, preferably 10%. Furthermore, up to 0.5% oxygen “O2”, particularly trace amounts of O2, and up to 0.5% water “H2O” may be added to the atmosphere. Both of these proportions must be limited to minimize the selective oxidation of base alloying elements. The remainder of the preferred atmosphere is composed of nitrogen “N2”, preferably 80%, particularly preferably 85%, and particularly preferably 90%.In a particular embodiment, the atmosphere consists of the described proportions of H2, O2, H2O and N2. The dew point of the T. PB The ambient temperature is preferably at least -60 °C, particularly preferably -40 °C. Furthermore, the dew point should preferably not be greater than 0 °C, particularly preferably -10 °C, and especially preferably -15 °C, as otherwise selective oxidation of base alloying elements may occur.

[0142] In a particular embodiment, A7 is equal to A6.

[0143] Step h)

[0144] The surface of the strip of steel flat product is then cooled to a temperature T9 by means of a purge gas with a temperature T S p = 500 °C-650 °C heated, where T B= T9+A T9 applies. This heats the strip surface to a temperature almost equal to the bath temperature of the coating bath. Preferably, the flat steel product is coated with a zinc-based coating. Advantageously, this process prevents the strip core from heating up, which could otherwise negatively affect the mechanical properties of the final product. Furthermore, this process step and the subsequent process steps according to the invention allow for optimal adjustment of the aluminum concentration in the coating. (ThyssenKrupp Steel Europe AG 237003P10WC)

[0145] 20 / 28 1 September 2025

[0146] Step i)

[0147] In step i), the steel flat product is coated in a coating bath at a melt bath temperature TZN. The coating bath consists of 0.15%–2.0% aluminum, is saturated with iron and the remainder with zinc and unavoidable impurities. The coating bath contains at least 0.15%, preferably 0.17%, aluminum "Al", as otherwise the formation of brittle Fe-Zn phases at the steel / coating interface cannot be sufficiently prevented, or only an insufficiently formed Fe₂Al₅ interface layer will form. The aluminum content should be a maximum of 2.0%, preferably 1.8%, and particularly preferably 0.94%, especially preferably 0.24%, as otherwise the weldability of the resulting coating will be negatively affected. The melt bath is saturated with iron "Fe". Optionally, the coating bath may contain magnesium “Mg” at a minimum of 0.25%, preferably 0.28% and a maximum of 8.0%, preferably 2.0%.In a preferred embodiment, the aluminum content is less than the magnesium content. Coating is carried out at a melt bath temperature (TZN) of at least 450 °C, preferably 460 °C, and at most 520 °C, preferably 500 °C, and particularly preferably 480 °C. The minimum value of TZN results from insufficient Fe₂Al₅ boundary layer formation at coating bath temperatures below 450 °C. The maximum value of TZN is limited by the increased iron dissolution into the coating bath, accompanied by increased slag formation at coating bath temperatures above 520 °C. Furthermore, the associated heating of the steel strip to a temperature above T₈ can be avoided as much as possible to prevent unwanted carbide formation in the base material.

[0148] The coating process is preferably carried out in a reducing atmosphere A9 to prevent re-oxidation. Therefore, the coating is preferably performed in a closed nozzle design to further prevent contact with the ambient air. To prevent coating defects caused by slag formation on the coating bath surface or by the precipitation of coating bath vapors, the nozzle is flooded with an additional atmosphere A9. In a particular embodiment, the atmosphere A9 can contain N2 and optionally an H2 content of 5%–10%, as well as unavoidable impurities, in particular H2O and O2. The addition of H2, if any, depends on the technically unavoidable residual O2 content in A9, which should always be <10 ppm. In a particular embodiment, the dew point of atmosphere A9 is at least -60 °C, preferably -40 °C, and at most +30 °C, preferably 0 °C. The minimum value of T P9This results from the fact that at low dew points, the evaporation of coating bath components is no longer sufficiently prevented, and setting such low dew points requires a disproportionately large technical effort. The maximum value of T P9 This results from the fact that higher dew points lead to both strong slag formation and ThyssenKrupp Steel Europe AG 237003P10WO

[0149] 21 / 28 1 September 2025

[0150] This can promote oxide layer formation on the coating bath surface and further lead to unwanted (selective) oxidation of the steel surface. The dew point can be controlled by adding moistened N2, or alternatively, moistened N2-H2.

[0151] After exiting the coating bath, excess molten zinc is removed from the steel strip by spraying it with air, nitrogen, or a mixture of air and nitrogen. A higher nitrogen content compared to air can be advantageous for preventing coating defects. The inventive design of the annealing and coating steps results—despite the high alloying content of base elements in the steel composition—in a zinc-phase-based coating as a well-adhering layer, which is bonded to the predominantly reduced steel surface via a predominantly opaque Fe₂Al₅ interface layer. Furthermore, the inventive process establishes an inventive aluminum concentration in the coating with an inventive microstructure, leading to a flat steel product with good mechanical properties and good low-molecular-weight (LME) properties.

[0152] In step i) ii, the flat steel product is cooled to a temperature Tw, where Tw < 60 °C. In a particular embodiment, the cooling rate Jw > 5 °C / s. The minimum value of Jw results from technical and economic considerations to avoid making the necessary cooling section unnecessarily long. If Tw > 60 °C, preferably Tw > 40 °C, this can lead to surface defects during the optional subsequent tempering process.

[0153] In a particular embodiment, the atmosphere A4 and the dew point T can be P4 In step d), the H2O / H2 ratio is adjusted so that it is <0.957, preferably <0.90, and particularly preferably <0.80. This will further reduce the thin oxide layer that forms in the atmosphere A4.

[0154] The following describes the laboratory testing of the inventive method. First, various cold-rolled steel flat products were provided, manufactured from different steels according to Table 1. These different steel flat products were then tested using the methods according to Tables 2 and 3. The resulting properties of the steel flat products are shown in Tables 4 and 5.

[0155] Steel alloys B and DF exhibit the steel composition according to the invention. Steel alloy B was tested under different manufacturing parameters (B2-B4).

[0156] In example B2, a reducing atmosphere A2 was set, and a temperature TLK was tested with a large margin relative to temperature T4. This leads to a low ThyssenKrupp Steel Europe AG 237003P10WC)

[0157] 22 / 28 1 September 2025

[0158] The proportion of tempered martensite and / or lower bainite, and an excessively high near-surface silicon concentration compared to the core, result in poor liquid electrochemical (LM) properties.

[0159] In example B4, annealing was performed at a low T4 temperature, which was below A. c3 -30 °C. This leads to a low proportion of tempered martensite and / or lower bainite and a high ferrite content not in accordance with the invention.

[0160] The inventive example B3 exhibits the inventive microstructure and an Al concentration in the coating of 0.38%. The coating consists of an Fe₂Al₅ interface layer and a Zn layer. Furthermore, a defined, reduced concentration of Mn, Si, and Cr is present in the near-surface layer compared to the core. The example thus exhibits good LME properties (see Table 5). Example B3 was heated from 720 °C to 840 °C in an oxidizing atmosphere and cooled in a controlled manner using TLK.

[0161] Example D7 also exhibits very good LME properties with a low maximum crack length. The Al content in the coating is 0.28%. The coating consists of an Fe₂Al₅ interface layer and a Zn layer. Example D7 was produced according to the inventive process and shows good mechanical properties and the inventive microstructure.

[0162] Example D8, however, has too high a ferrite content, which results in a low R p0 A value of 0.2 results. This is due to a TLK temperature that is too low. Furthermore, the T9 temperature is set too low, which negatively affects the LME characteristics of the example.

[0163] In example D9, a reducing atmosphere A2 was set. As a result, D9 exhibits the microstructure according to the invention; however, the Al content in the coating is too high, and the steel sheet exhibits poor LME properties.

[0164] Examples F12 and F13 exhibit very good mechanical properties combined with good LME properties and were produced using the method according to the invention.

[0165] In the examples B3, D7, F12 and F13 according to the invention, the hole enlargement HER was over 20%.

[0166] Steel alloys A and E have a silicon content that is not in accordance with the invention. Due to the low silicon content, a high proportion of carbides forms in the microstructure. This results in a low retained austenite content and / or a high ferrite content. Therefore, examples Al, E10, and Eil are not in accordance with the invention. While these examples exhibit good LME properties, they do not display the required mechanical properties such as Rc. p0 ,2, A 80 , Rm value. ThyssenKrupp Steel Europe AG 237003P10WO

[0167] 23 / 28 1 September 2025

[0168] The steel alloy C has a carbon and silicon content that does not conform to the invention, because too much fresh martensite, i.e., untempered martensite, is formed. Both examples C5 and C6 therefore exhibit a proportion of tempered martensite and lower bainite that does not conform to the invention and an excessively high ferrite content.

[0169] hyssenKrupp Steel Europe AG 237003P10WC)

[0170] 24 / 28 1. September 2025 abelle 1

[0171] hyssenKrupp Steel Europe AG 237003P10WC)

[0172] 25 / 28 1. September 2025 abelle 2

[0173] hyssenKrupp Steel Europe AG 237003P10WC)

[0174] 26 / 28 1. September 2025 abelle 3

[0175] hyssenKrupp Steel Europe AG 237003P10WC)

[0176] 27 / 28 1. September 2025

[0177] hyssenKrupp Steel Europe AG 237003P10WC)

[0178] 28 / 28 1. September 2025

Claims

ThyssenKrupp Steel Europe AG 237003P10WO 1 / 5 September 1, 2025 Patent claims 1. High-strength, coated steel flat product with a tensile strength R m = 900 MPa-1500 MPa, a yield strength R p02 > 700 MPa and an elongation A 80 = 10%-24%, which includes a steel with: 0.10%-0.5% C, 1.0%-3.0% Mn, 0.7%–2.5% Si 0.05% - 1.0% Cr < 0.020 % P, < 0.005 % S, < 0.02% N, and optionally one or more of the following elements: 0.01-1.5% AI, 0.05-0.5% Mo, 0.0004-0.002% B 0.005% < Ti+Nb+V < 0.2%, and as a remainder of iron and unavoidable elements, wherein the steel flat product has a microstructure consisting of at least 75% tempered martensite and / or lower bainite, at least 5% retained austenite, a maximum of 10% ferrite and a maximum of 10% untempered martensite, and wherein the coated steel flat product has at least one surface with a coating, characterized in that the Al concentration in the coating is 0.20%-0.50%.

2. High-strength coated steel flat product according to claim 1, characterized in that the coating consists of an Fe2Al5 interface layer and a Zn layer.

3. High-strength coated steel flat product according to one of the preceding claims, characterized in that in a near-surface layer extending from the interface between the coating and the steel flat product into the steel flat product with a layer thickness d, wherein the layer thickness d is determined according to the description, ThyssenKrupp Steel Europe AG 237003P10WO 2 / 5 1 September 2025 the concentration of Si compared to the core of the steel flat product is a maximum of 80%, and the concentration of Mn compared to the core of the steel flat product is a maximum of 70%, and the concentration of Cr compared to the core of the steel flat product is a maximum of 75%, the concentrations being determined according to the description.

4. High-strength coated steel flat product according to one of the preceding claims, characterized in that the steel flat product has a microstructure with at least 10% retained austenite.

5. High-strength, coated steel flat product according to one of the preceding claims, characterized in that the tensile strength R m = 1000 MPa-1500 MPa, the yield strength R p o2> 820 MPa and the strain A 80 = 12%-18%.

6. High-strength, coated steel flat product according to one of the preceding claims, characterized in that the hole enlargement HER is at least 20%.

7. High-strength coated steel flat product according to one of the preceding claims, characterized in that the steel flat product has a microstructure with a maximum of 85% tempered martensite and / or lower bainite.

8. Method for producing a high-strength coated steel flat product, comprising at least the following steps: a) Providing a cold-rolled steel flat product comprising a steel consisting of the following elements: 0.10%–0.5% C 1.0%–3.0% Mn 0.7%–2.5% Si ThyssenKrupp Steel Europe AG 237003P10WO 3 / 5 1 September 2025 0.05% - 1, 0% Cr, < 0.020 % P, < 0.005 % S, < 0.02% N, and optionally one or more of the following elements: 0.01-1.5% AI, 0.05-0.5% Mo, 0.0004-0.002% B 0.005% < Ti+Nb+V < 0.2%, and as a remainder of iron and unavoidable elements; b) Heating and pre-oxidizing the cold-rolled steel flat product under the following conditions: i. Heating the cold-rolled steel flat product in a reducing atmosphere Ai, preferably from room temperature to a temperature Ti, where Ti = 650°C–750°C; ii. Heating the cold-rolled steel flat product to a temperature T2 and then pre-oxidizing and heating the cold-rolled steel flat product from T2 to a temperature T3 in an oxidizing atmosphere A2 for t2 = 1 s–30 s, where Ti < T2 < T3 with T3 = 750°C–850°C; c) Heating the cold-rolled steel flat product from a temperature T3 to a temperature T4 and heating through at a temperature T4 for b = 5 s - 300 s, where T3 < T4 < 950 °C and T4 > A c3 -30 °C, heating and warming through in a reducing atmosphere A4 takes place; d) Cooling of the cold-rolled steel flat product in a time t Lk to a temperature TLK, where tL k = 30 s - 120 s and T L K> T4-175 °C; e) Cooling of the cold-rolled steel flat product with a cooling rate &5>30 K / s from a temperature TLK to a temperature T5, where T5= (TMS + 40 °C) - (TMS-175 °C); ThyssenKrupp Steel Europe AG 237003P10WO 4 / 5 1 September 2025 f) Setting and holding the cold-rolled steel flat product at a temperature T6 for a holding time t6 = 1 s - 60 s, where T6 = T M s -( T MS - 175°C); g) Reheating the cold-rolled steel flat product at a heating rate of $7 = 4 °C / s - 1000 °C / s to a temperature T B , where TMS <T B <510 °C; h) Heating the strip surface to a temperature T9 by means of a purge gas with a temperature TSP = 500 °C-650 °C, where T B = T 9 + A T9 and A T9 > 10 °C; i) Coating and cooling of the cold-rolled steel flat product under the following conditions: i. Coating of the steel flat product at a melt bath temperature T ZN with a coating bath consisting of 0.10%–2.0% AI Fe-saturated, Residual Zn and unavoidable impurities, wherein TZN = 450°C - 520°C; ii. Cooling the coated steel flat product to a temperature Tw, wherein Tw < 60°C.

9. Method according to claim 8, characterized in that for the atmosphere A2 in step b) ii. H2O / H2>0.957 applies and the dew point T P2 in the range (0 °C) to (+60 °C).

10. Method according to one of claims 8 to 9, characterized in that in step b) the heating rate is between 500 °C and TI 2 °C / s - 50 °C / s.

11. Method according to one of claims 8 to 10, characterized in that in step c) the heating is carried out by holding at a constant temperature T4.

12. Method according to one of claims 8 to 11, characterized in that in step c) the heating of Ti to T4 is carried out with a mean heating rate $2= 0.5 °C / s - 10 °C / s. ThyssenKrupp Steel Europe AG 237003P10WO 5 / 5 1 September 2025 13. Method according to one of claims 8 to 12, characterized in that the reducing atmosphere A4 has a dew point T P4 in the range of (-40 °C) to (-11 °C).

14. Component for structural lightweight construction in automotive engineering formed from a flat steel product according to claim 1.

15. Component according to claim 14, characterized in that the component is manufactured using a thermal joining process.

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