Steel having excellent local and global elongation and simultaneously low lme sensitivity, and method for producing same

Abstract

The present invention relates to a high-strength, coated steel flat product exhibiting good LME properties. 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, with an aluminum concentration of 0.20% to 0.50% in the coating. The invention further relates to the production of such a steel flat product and a component made from such a steel flat product.

Description

[0001] The invention relates to a high-strength, coated steel flat product with excellent local and global elongation and simultaneously good LME (Liquid-Metal-Embrittlement) properties.

[0002] 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.

[0003] Mechanical properties such as tensile strength Rm, yield strength Rp0.2, and elongation at break A80, reported here, were determined in tensile tests according to DIN EN ISO 6892-1: 2017, unless explicitly stated otherwise. Hole expansion HER is determined according to ISO 16630.

[0004] 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 are to be understood as values ​​in "wt.%".

[0005] 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.

[0006] The microstructure is determined on longitudinal sections etched with 5% 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.

[0007] 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.

[0008] The spatially resolved concentrations of Si, Cr, and Mn in a near-surface layer were determined using glow discharge optical emission spectroscopy (GD-OES). A GD-OES instrument from Leco, for example, can be used for this purpose. 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). The layer thickness d of the near-surface layer was determined based on the obtained concentration profiles. For this purpose, the point at which the oxygen concentration first falls below 1% was determined, starting from the surface and extending into the base material. The distance between the surface and the point where 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 µm, preferably 7 µm, and particularly preferably 6 µm. In a further particular embodiment, the layer thickness can be a minimum of 1 µm, preferably 2 µm. The average concentration profile of Si, Cr, and Mn in the layer was determined and compared to the average concentration in the core. The core of the steel flat product begins at least 50 µm, preferably 100 µm, away from the two surfaces of the steel flat product.

[0009] All strip temperatures in the process can be determined, for example, with a commercially available pyrometer.

[0010] In the present application, "same atmosphere" (i.e., A i =A j) means that the dew points are within the limits of measurement accuracy and preferably the proportions of hydrogen, oxygen and nitrogen are the same within the limits of measurement uncertainty.

[0011] 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 50%, preferably greater than 20%, form in the Karagoulis Type D test.

[0012] 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.

[0013] 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.

[0014] 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. With conventional joining methods, the components of the surface coating, e.g., 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.

[0015] EP 3 647 452 B1 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.

[0016] WO 2019 / 097440 A1 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 balance 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.

[0017] 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.

[0018] The problem is solved using a high-strength, coated steel flat product with a tensile strength of Rm = 900 MPa-1500 MPa, a yield strength Rp02 ≥ 700 MPa and an elongation A80 = 10%-24%, which comprises 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% Al, 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 wherein the Al concentration in the coating is 0.20%-0.50%, preferably 0.25%-0.45%.

[0019] 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 aluminum content in the coating is greater than 0.50%, preferably greater than 0.45%, and particularly 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 this is due to the increase in the melting temperature by aluminium or to the reduced solubility of the molten coating in the substrate.

[0020] 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.

[0021] 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.

[0022] 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%.

[0023] 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%.

[0024] 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 simply reducing 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.

[0025] The steel flat product according to the invention is described in more detail below.

[0026] 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%, and particularly 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 particularly preferably 0.35%.

[0027] Silicon ("Si") is required to achieve the specific microstructure in this invention because it delays cementite formation. An excessively high cementite content would cause the carbon to be bound in carbides and thus unavailable for stabilizing the retained austenite during the process, 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 silicon content of 2.5%, preferably 1.7%, and most preferably at most 1.5%.

[0028] 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.

[0029] 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 here to 1.0%, preferably 0.9%, and most preferably 0.6%.

[0030] 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.

[0031] 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 0.005% being advantageous. Sulfur contamination cannot be completely avoided during steel production.

[0032] 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.

[0033] 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%, preferably a maximum of 0.1%.

[0034] 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 0.05%, 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 cause embrittlement and thus poorer formability. Furthermore, higher Al contents can lead to poorer casting properties, as aluminum compounds can cause clogging. Therefore, in the present invention, the aluminum content is limited to 1.5%, preferably 0.8%, and particularly preferably 0.4%.

[0035] 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.

[0036] The addition of boron "B" leads to a fine-grained microstructure, as boron segregates at the phase boundaries and blocks their movement. For this purpose, at least 0.0004%, and particularly preferably at least 0.0005%, can be added to the steel according to the invention. The effect of boron is saturated at a maximum content of 0.002%.

[0037] 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%.

[0038] 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.

[0039] 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.

[0040] 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. Due to the limitation of austenite-stabilizing alloying elements such as carbon and manganese for weldability reasons, a retained austenite content greater than 20% is not possible with the described manufacturing process.

[0041] 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.

[0042] In the microstructure according to the invention, a maximum of 10%, particularly preferably 8%, particularly preferably 5% of the martensite is present untempered.

[0043] The steel flat product according to the invention exhibits excellent mechanical properties with a tensile strength Rm of at least 900 MPa, preferably 1000 MPa and a maximum of 1500 MPa, preferably 1350 MPa. Furthermore, the steel flat product according to the invention has a yield strength Rp02 of at least 700 MPa, preferably 820 MPa, and an elongation of A80 = 10%–24%, preferably 12%–18%.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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: 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% Al, 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. 1. Heating the cold-rolled steel flat product in a reducing atmosphere A1 from room temperature to a temperature T1, where T1 = 650–750 °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 T1 ≤ T2 ≤ T3 with T3 = 750–850 °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, the heating and heating through taking place in a reducing atmosphere A4; d) Cooling the cold-rolled steel flat product in a time t Lk to a temperature T LK , where t Lk = 30 s - 120 s and T LK ≥ T4 - 175 °C; e) Cooling the cold-rolled steel flat product with a cooling rate ϑ 5 ≥ 30 K / s from a temperature T LK to a temperature T 5, where T 5 = (T MS + 40 °C) - (T MS - 175 °C); f) Setting and holding the cold-rolled steel flat product at a temperature T 6 for a holding time t 6 = 1 s - 60 s, where T 6 = T MS - (T MS - 175 °C); g) Reheating the cold-rolled steel flat product with a heating rate ϑ 7 = 4 °C / s - 1000 °C / s to a temperature TB, where T MS <T B ≤510 °C;h) Heating the strip surface to a temperature T9 using a purge gas with a temperature TSP = 500–650 °C, where TB = T9 + ΔT9 and ΔT9 ≥ 10 °C; i) Coating and cooling the cold-rolled steel flat product under the following conditions: i. Coating the steel flat product at a melt bath temperature TZN with a coating bath consisting of: 0.10–0.20% Al, Fe-saturated, balance Zn and unavoidable impurities, where TZN = 450–520 °C; ii. Cooling the coated steel flat product to a temperature T10, where T10 ≤ 60 °C.

[0048] In a preferred embodiment, the method for producing a high-strength, uncoated steel flat product does not include any further work steps and the method consists of steps a) to i).

[0049] 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 T 5 ), T 6 is directly set and maintained.

[0050] 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.

[0051] The following section describes each step in detail: Step a)

[0052] 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. Step b)

[0053] The steel flat product according to the invention is preferably heated from room temperature to a temperature T1 in an atmosphere A1. The temperature T1 is at least 650 °C, preferably 670 °C. The temperature T1 is at most 750 °C, preferably 750 °C, since above this temperature recrystallization processes begin and the process conditions must be adjusted according to step c).

[0054] The atmosphere A1 set according to the invention is reducing, thereby preventing uncontrolled pre-oxidation. Atmosphere A1 preferably comprises at least 2% hydrogen "H2", more preferably 3% H2, and particularly preferably 5% H2. The set hydrogen content ensures that the atmosphere is reducing, particularly with respect to iron. This prevents uncontrolled oxidation, allowing a thin oxide layer, particularly preferably thinner than 300 nm, to be set 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 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. The dew point of T P1 is preferably at least -60 °C, preferably -55 °C, and particularly preferably -45 °C. Furthermore, the dew point should preferably not exceed -5 °C, particularly preferably -10 °C, and particularly preferably -15 °C, as otherwise selective oxidation of base alloying elements can occur. This specific dew point control ensures that coating and adhesion problems of the coating can be avoided.

[0055] 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 T₁ ≤ T₂ ≤ T₃, with 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 µm thick, particularly preferably less than 1 µm thick, 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.

[0056] The minimum value of T2 results from the fact that pre-oxidation occurs at T2 <T 1 nicht ausreichend sicher eine im Wesentlichen deckendende FeO-Schicht, dicker ≥50nm, erzeugt. Der Maximalwert von T 2 <T 3 ergibt sich daraus, dass die Voroxidation bei T 2 >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 ≥50 nm thick. 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 such that the conditions in this furnace zone are always oxidizing to iron, in order to achieve targeted pre-oxidation of the steel surface to a FeO layer as opaque as possible, with an oxide layer thickness of at least 50 nm, preferably 60 nm and a maximum of 300 nm, preferably 200 nm. Various methods for creating oxidizing atmospheres are known. 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 optionally, technically unavoidable traces of H₂, CO₂, and CO. As an alternative to O 2, moist N 2 can also be blown into this furnace zone as an oxidative medium, whereby in this case A 2 has a dew point T P2 of ≥0°C - ≤+60°C at an H 2 O / H 2 ratio of ≥0.957 to ensure sufficient Fe oxidation.Preferably, in both of the aforementioned variants, the dew point TP2 can be at least +5 °C, more preferably +10 °C, and more preferably a maximum of +30 °C, more 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.

[0057] In a particular embodiment, the heating rate ϑ 1 in step b) between 500 °C and T 1 is at least 2 °C / s, preferably 4 °C / s and at most 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 T 1 is reached.

[0058] In a particular embodiment, the heating from T 1 to T 4 takes place with a mean heating rate ϑ 2 = 0.5 °C / s - 10 °C / s. Step c)

[0059] Subsequently, in step c), the steel flat product is heated from a temperature T3 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 t4 ≥ 10 s, and particularly preferably t4 ≥ 15 s.

[0060] The temperature T4 must not fall below the maximum pre-oxidation temperature T3. The minimum value of T4 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 if the exposure time is < 5 s.

[0061] Setting the temperature T4 and ensuring a sufficient time t4 results in adequate austenitization. However, the time should be limited and ideally be no more than t4 ≤ 300 s, preferably t4 ≤ 180 s, as otherwise the austenite grains will become coarsened, negatively affecting the mechanical properties.

[0062] For the temperature T 4 in this step, T 3 ≤ T 4 ≤ 950 °C and T 4 ≥ Ac3 - 30 °C, in particular T 4 ≥ Ac3.

[0063] The minimum temperature A c3 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: A c 3 = 902 − 225 * % C + 19 * % Si − 11 * % Mn − 5 * % Cr + 13 * % Mo − 20 * % Ni + 55 * % V ° C %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.

[0064] The atmosphere A4 set 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.

[0065] In a preferred embodiment, the dew point is controlled within a narrow range, as this allows for precise adjustment of Mn, Si, and Cr concentrations in the near-surface layer d, thereby improving the LME properties. In a particular embodiment, the dew point TP4 should therefore be set to at least -50 °C, preferably -45 °C, and most preferably -40 °C. Furthermore, the dew point should preferably not exceed 0 °C, more preferably -5 °C, and most preferably -11 °C, as otherwise undesired oxide formation may occur.

[0066] The preferably set atmosphere A4 comprises at least 2% hydrogen "H2", preferably 5% H2, and particularly preferably 5% H2. In a particular embodiment, A1 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.

[0067] 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. Step d)

[0068] The steel flat product is cooled to a temperature TLK over a time tLk, where tLk = 30 s - 120 s and TLK ≥ T4 - 175 °C. Slow cooling to no more than 175 °C below T4 should be carried out to prevent ferrite formation. The cooling time tLk must be at least 30 s, preferably 35 s, and at most 120 s, preferably 100 s, to avoid undesirable ferrite formation.

[0069] In a particular embodiment, cooling takes place in a reducing atmosphere. In this particular embodiment, the dew point TPLK 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 -5 °C, and most preferably -11 °C, as otherwise unwanted oxide formation may occur. Step e)

[0070] Cooling the cold-rolled steel flat product to a temperature T5 at a cooling rate ϑ5 ≥ 30 K / s, preferably ϑ5 ≥ 35 K / s, particularly preferably ϑ5 ≥ 40 K / s, and especially preferably ϑ5 ≥ 45 K / s. The temperature T5 is at most (TMS + 40 °C), preferably (TMS + 20 °C), and 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: T MS ° C = 539 ° C + − 423 % C − 30,4 % Mn − 7,5 % Si + 30 % Al ° C / wt % Furthermore, the temperature T 5 ≤ 550 °C must be to avoid selective re-oxidation on the steel surface.

[0071] In a particular embodiment, the atmosphere A5 in step e) is reducing. In this particular embodiment, A5 = ALK and / or A5 comprises at least 2% hydrogen "H2", preferably 3% H2, and particularly preferably 5% H2. The adjusted 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 80%, more preferably 50%. 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. The dew point TP5 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 particularly preferably -15 °C, since otherwise selective oxidation of base alloying elements may occur.

[0072] 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. Step f)

[0073] In step f), the steel flat product is heated to a temperature T6 in an atmosphere A6 and held for a holding time t6 = 1 s - 60 s. The minimum value for T6 is (TMS - 175°C), preferably (TMS - 150°C). The maximum value is TMS, preferably (TMS - 75°C). TMS denotes the martensite start temperature, which can be estimated using the following equation: T MS ° C = 539 ° C + − 423 % C − 30,4 % Mn − 7,5 % Si + 30 % Al ° C / wt % , where the element concentrations are to be given in weight percent.

[0074] In a particular embodiment, T6 = T5. In a particular embodiment, TP6 = TP5. The 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, since 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. In a particular embodiment, for steel flat product thicknesses ≥ 1.0 mm, the holding time is 10 s to 60 s.

[0075] The preferably configured atmosphere A 6 in step e) comprises at least 2% hydrogen "H 2", preferably 3% H 2, and particularly preferably 5% H 2. The hydrogen content ensures that the atmosphere is reducing, particularly with respect to iron. This prevents uncontrolled oxidation and reoxidation on the surface. For economic reasons, the H 2 content should preferably be limited to a maximum of 20%, preferably 10%. Furthermore, up to 0.5% oxygen "O 2", particularly trace amounts of O 2, and up to 0.5% water "H 2 O" 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 "N 2", 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 T P6 is preferably at least -60 °C, particularly preferably -40 °C. Furthermore, the dew point should preferably not exceed -5 °C, particularly preferably -10 °C, and most preferably -15 °C, since otherwise selective oxidation of base alloying elements may occur.

[0076] In a particular embodiment, A6 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 by a structural separation, in particular an airlock system. Step g)

[0077] In step g), the steel flat product is heated to a temperature TB in an atmosphere A 7 with a heating rate ϑ 7 = 4 °C / s-1000 °C / s, where T MS <T B ≤510 °C. Die Temperatur T B beträgt höchstens 510 °C besonders bevorzugt 500 °C, insbesondere bevorzugt 490 °C, da ansonsten ein ungewollte Festigkeitsabnahme des Stahlflachprodukts erfolgt.

[0078] The temperature TB should be greater than T MS, preferably greater than T MS +50 °C, in order to enrich the retained austenite in the base material structure with C from the supersaturated primary martensite or bainite.

[0079] The heating rate ϑ 7 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 ϑ 7 should be limited to a maximum of 1000 °C / s, preferably 50 °C / s.

[0080] The preferably configured atmosphere A 7 in step g) comprises at least 2% hydrogen "H 2", preferably 3% H 2, and particularly preferably 5% H 2. The hydrogen content ensures that the atmosphere is reducing, particularly with respect to iron. This prevents uncontrolled oxidation and reoxidation on the surface. For economic reasons, the H 2 content should preferably be limited to a maximum of 20%, preferably 10%. Furthermore, up to 0.5% oxygen "O 2", particularly trace amounts of O 2, and up to 0.5% water "H 2 O" 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 "N 2", 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 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, since otherwise selective oxidation of base alloying elements may occur.

[0081] In a particular embodiment, A 7 is equal to A 6 . Step h)

[0082] The surface of the steel strip is then heated to a temperature T9 using a purge gas with a temperature TSP = 500 °C–650 °C, where TB = T9 + ΔT9. This heats the strip surface to a temperature almost equal to the bath temperature of the coating bath. Preferably, the steel strip is coated with a zinc-based coating. An advantage of this process is that the strip core does not heat 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. Step i)

[0083] 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% Al, is saturated with Fe and the remainder with Zn 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 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 Al content is ≤ the Mg 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 <450 °C. The maximum value of TZN is limited by the increased Fe dissolution into the coating bath, accompanied by increased slag formation at coating bath temperatures >520 °C. Furthermore, the associated heating of the steel strip to a temperature >T8 can be avoided as much as possible to prevent unwanted carbide formation in the base material.

[0084] The coating process is preferably carried out in a reducing atmosphere A 9 to prevent re-oxidation. Therefore, the coating is preferably performed in a closed nozzle assembly 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 A 9. In a particular embodiment, the atmosphere A 9 can contain N 2 and optionally an H 2 content of 5%–10%, as well as unavoidable impurities, in particular H 2 O and O 2. The addition of H 2, if any, depends on the technically unavoidable residual O 2 content in A 9, which should always be ≤10 ppm. In a particular embodiment, the dew point of the atmosphere A 9 is at least -60 °C, preferably -40 °C, and at most +30 °C, preferably 0 °C.The minimum value of TP9 results from the fact that at low dew points, the evaporation of coating bath components is no longer sufficiently prevented, and maintaining such low dew points requires a disproportionately high level of technical effort. The maximum value of TP9 results from the fact that higher dew points promote both strong slag and oxide layer formation on the coating bath surface and can also lead to unwanted (selective) oxidation of the steel surface. The dew point can be controlled by adding moistened N2 or, alternatively, moistened N2-H2.

[0085] After exiting the coating bath, excess molten zinc is removed from the steel strip by spraying it with air, nitrogen (N₂), or a mixture of air and nitrogen. A higher proportion of nitrogen 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.

[0086] In step i) ii, the flat steel product is cooled to a temperature T10, where T10 ≤ 60 °C. In a particular embodiment, the cooling rate ϑ10 > 5 °C / s. The minimum value of ϑ10 results from technical and economic considerations to avoid making the necessary cooling section unnecessarily long. If T10 > 60 °C, preferably T10 > 40 °C, this can lead to surface defects during the optional subsequent tempering process.

[0087] In a particular embodiment, the atmosphere A4 and the dew point TP4 can be adjusted in step d) such that H2O / H2 <0.957, preferably H2O / H2 <0.90, and particularly preferably H2O / H2 <0.80. This will further reduce the thin oxide layer that forms in the atmosphere A4.

[0088] 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 described in Tables 2 and 5. The resulting properties of the steel flat products are shown in Tables 4 and 5.

[0089] Steel alloys B and DF exhibit the steel composition according to the invention. Steel alloy B was tested under different manufacturing parameters (B2-B4).

[0090] In example B2, a reducing atmosphere A2 was set, and a temperature TLK was tested at a large margin relative to temperature T4. This leads to a low proportion of tempered martensite and / or lower bainite and to an excessively high near-surface silicon concentration compared to the core. This results in poor LME properties.

[0091] In example B4, annealing was performed at a low T4 temperature, which is below A c3 -30 °C. This results in a low proportion of tempered martensite and / or lower bainite and a high ferrite content not in accordance with the invention.

[0092] 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 via TLK.

[0093] 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.

[0094] Example D8, however, has an excessively high ferrite content, resulting in a low Rp0.2 value. This is due to an excessively low TLK temperature. Furthermore, the T9 temperature is set too low, negatively impacting the LME properties of the example.

[0095] In example D9, a reducing atmosphere A 2 was established. 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.

[0096] Examples F12 and F13 exhibit very good mechanical properties combined with good LME properties and were produced using the method according to the invention.

[0097] In the examples B3, D7, F12 and F13 according to the invention, the hole enlargement HER was over 20%.

[0098] 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 A1, E10, and E11 are not in accordance with the invention. While these examples exhibit good LME properties, they do not show the required mechanical properties such as Rp0.2, A80, and Rm values.

[0099] 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. Table 1 C [wt%] Si [wt%] Mn [wt%] P [wt%] S [wt%] Al [wt%] Cr [wt%] B [wt%] Mo [wt%] N [wt%] Ti+Nb+V [wt%] T MS [°C] A C3 [°C] A 0,156 0,24 1,63 0,012 0,0027 0,031 0,780 0,0011 0,003 0,0027 0,041 423 850 B 0,209 1,47 2,17 0,016 0,0023 0,024 0,174 0,0013 0,010 0,0046 0,011 374 858 C 0,079 0,26 2,59 0,013 0,0021 0,029 0,690 0,0013 0,110 0,0025 0,085 426 859 D 0,158 0,98 1,99 0,014 0,0020 0,280 0,135 0,0021 0,005 0,0016 0,017 413 863 E 0,153 0,42 2,35 0,013 0,0025 0,710 0,720 0,0014 0,010 0,0042 0,053 421 846 F 0,274 1,43 2,37 0,005 0,0021 0,022 0,161 0,0017 0,099 0,0013 0,091 341 842 Table 2 Nr. ϑ 1 [K / s] T 1 [ °C] T P1 [ °C] t 2 [s] A 2 T P2 [ °C] T 3 [ °C] t 4 [s] T 4 [ °C] T P4 [ °C] T LK [ °C] t LK [S] T 5 [°C] ϑ 5 [K / S] T 6 [ °C] t 6 [S] A1 5 675 -20 12 Ox. 10 815 130 890 -25 720 45 370 37 365 9 B2 4 700 -20 16 Editor's note. 5 825 160 895 -30 700 50 340 40 325 12 B3 4 720 -20 17 Ox. 20 840 95 905 -40 735 42 330 42 310 12 B4 6 670 -20 15 Ox. 15 800 85 820 -40 693 35 345 36 330 11 C5 7 650 -20 16 Ox. 15 810 130 855 -35 670 37 325 34 310 18 C6 5 650 -20 45 Ox. 15 810 90 850 -35 650 38 380 37 370 13 D7 4 680 -20 16 Ox. 20 830 110 905 -45 740 31 310 42 305 22 D8 6 680 -20 17 Ox. 20 830 170 895 -45 680 56 340 38 325 10 D9 6 695 -20 18 Editor's note. -5 830 65 895 -45 725 52 290 43 285 12 E10 8 650 -20 12 Ox. 10 825 65 850 -30 690 48 390 39 370 13 E11 4 660 -20 16 Ox. 10 840 180 860 -30 630 59 365 45 345 8 F12 5 705 -20 20 Ox. 20 850 65 905 -40 735 50 295 49 275 17 F13 4 705 -20 14 Ox. 20 850 80 895 -40 725 43 315 42 310 13 Table 3 Nr. ϑ 7 [K / s] TB [ °C] T PB [ °C] T Zn [ °C] T Sp [ °C] T 9 [ °C] T P9 [ °C] T 10 [ °C] A1 8 455 -20 460 505 462 -40 50 B2 12 450 -25 462 525 465 -40 45 B3 15 450 -15 461 520 467 -40 50 B4 10 457 -20 465 515 465 -45 45 C5 16 460 -30 461 500 462 -45 50 C6 12 455 -30 463 500 461 -35 45 D7 9 448 -20 461 520 468 -40 55 D8 13 470 -30 460 590 470 -35 50 D9 16 451 -40 465 550 469 -30 45 E10 14 455 -35 463 550 467 -35 40 E11 13 450 -30 462 520 465 -35 45 F12 12 445 -20 466 540 463 -40 45 F13 19 450 -25 460 540 464 -35 45 Table 4 Nr. Lower bainite + tempered martensite retained austenite ferrite Untreated martensite Rp0.2 Rm A80 A1 50 2 40 8 643 997 10 B2 65 15 10 10 742 1205 16 B3 84 12 0 4 941 1198 15 B4 60 12 25 3 741 1203 10 C5 40 7 35 18 712 1081 12 C6 27 8 43 22 677 1124 15 D7 85 13 0 2 820 1065 16 D8 69 11 12 8 697 1015 18 D9 90 8 0 2 890 1051 13 E10 65 9 20 6 714 1238 9 E11 80 4 10 6 869 1213 6 F12 80 14 0 6 1295 1497 13 F13 75 16 0 9 1137 1471 15 Table 5 Nr. Concentration in the surface layer compared to the core coating Sheet thickness Max. crack length [µm] Crack length [%] Mn[%] Si[%] Cr[%] Al [%] A1 60 75 60 0,29 1,5 75 5% B2 70 85 75 0,42 1,5 650 43% B3 55 65 70 0,38 1,5 150 10% B4 55 70 65 0,36 1,5 90 6% C5 65 75 70 0,47 1,8 130 7% C6 65 75 70 0,47 1,8 150 8% D7 60 65 70 0,28 1,6 95 6% D8 65 65 70 0,39 1,6 180 11% D9 75 90 70 0,51 1,6 510 32% E10 65 70 60 0,41 1,4 75 5% E11 65 70 60 0,36 1,4 60 4% F12 60 65 65 0,3 1,5 210 14% F13 60 65 65 0,32 1,5 180 12%

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 comprises a steel with: 0.10%–0.5% C, 1.0%–3.0% Mn, 0.05%–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% Al, 0.05–0.5% Mo, 0.0004–0.002% B, 0,005 % ≤ Ti + Nb + V ≤ 0,2 % , and as a residue 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 by the fact that The Al concentration in the coating is 0.20%–0.50%.

2. High-strength, coated steel flat product according to claim 1, characterized by the fact thatthe coating consists of an Fe2Al5 boundary layer and a Zn layer.

3. High-strength, coated steel flat product according to any of the preceding claims, characterized by the fact 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, the concentration of Si is a maximum of 80% compared to the core of the steel flat product, and the concentration of Mn is a maximum of 70% compared to the core of the steel flat product, and the concentration of Cr is a maximum of 75% compared to the core of the steel flat product, wherein the concentrations are determined according to the description.

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

5. High-strength, coated steel flat product according to any of the preceding claims, characterized by the fact that the tensile strength R m = 1000 MPa-1500 MPa, the yield strength R p02 ≥ 820 MPa and the strain A 80 = 12%-18%.

6. High-strength, coated steel flat product according to any of the preceding claims, characterized by the fact that The hole enlargement HER is at least 20% 7. High-strength coated steel flat product according to one of the preceding claims characterized by the fact that The steel flat product has a microstructure with a maximum of 85% tempered martensite and / or lower bainite.

8. 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 steel consisting of the following elements: 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% Al, 0.05–0.5% Mo, 0.0004–0.002% B, 0,005 % ≤ Ti + Nb + V ≤ 0,2 % , and as a residue 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 A1, preferably from room temperature to a temperature T1, where T1 = 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 T1 ≤ 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 T LK , where tLk = 30 s-120 s and T LK ≥ T4-175 °C; e) Cooling of the cold-rolled steel flat product with a cooling rate ϑ5≥30 K / s from a temperature T LK to a temperature T5, where T5 = (T MS +40 °C) - (T MS -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 MS -( T MS - 175°C); g) Reheating the cold-rolled steel flat product with a heating rate ϑ7 = 4 °C / s - 1000 °C / s to a temperature T B , where T MS <T B ≤510 °C; h) Heating the strip surface to a temperature T9 using a purge gas with a temperature T SP = 500 °C-650 °C, where T B = T 9 + ΔT9 and ΔT9 ≥ 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 ZNwith a coating bath consisting of 0.10%-2.0% Al, Fe-saturated, balance Zn and unavoidable impurities, wherein T ZN =450°C -520°C; ii. Cooling the coated steel flat product to a temperature T 10 , where T 10 ≤ 60 °C.

9. Method according to claim 8, characterized by the fact that for 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 by the fact that In step b) between 500 °C and T1 the heating rate ϑ1= 2 °C / s - 50 °C / s is.

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

12. Method according to any one of claims 8 to 11, characterized by the fact that In step c) the heating from T1 to T4 takes place with an average heating rate ϑ2= 0.5 °C / s - 10 °C / s.

13. Method according to any one of claims 8 to 12, characterized by the fact that the reducing atmosphere A4 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 by the fact that the component is manufactured using a thermal joining process.

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