Highly resistant steel with good shaping and surface properties

Abstract

The present invention relates to an uncoated, high-strength steel with good forming and surface properties, a manufacturing process for the steel, and a component made from the steel. The steel has a good surface with a roughness Ra = 0.5 µm - 1.8 µm and a peak count RPC ≥ 40 cm-1.

Description

[0001] The invention relates to a high-strength steel with good forming and surface properties and its manufacturing process, as well as a component made of the steel.

[0002] The flat steel products described in the invention are typically rolled products, such as steel strips or sheets, as well as blanks and plates made therefrom.

[0003] Mechanical properties, such as tensile strength R m , yield strength R p0.2 , elongation at break A 80 , reported here, were determined in tensile tests according to DIN-EN ISO 6982-1:2017, unless explicitly stated otherwise.

[0004] In this application, all information regarding steel composition is based on weight unless expressly stated otherwise. All unspecified "%" information relating to a steel alloy is therefore to be understood as "wt%."

[0005] In the following, the term uncoated steel flat product is to be understood as meaning that there is no corrosion coating, for example through hot-dip coating or electrolytically produced coatings, on the surface of the steel flat product.

[0006] With the exception of the data relating to the volume (specified in "vol.%) on the residual austenite content of the microstructure of a sheet metal part according to the invention, data on the contents of the various microstructure components refer in each case to the area of a microsection of a sample of the respective product (specified in area percentage "area %"), unless expressly stated otherwise.

[0007] The microstructure is determined on cross-sections subjected to etching with 3% Nital (alcoholic nitric acid). The microstructure is determined 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 the plate length, width, and plate spacing. The proportion of retained austenite is determined by X-ray diffraction (XRD) according to ASTM E975.

[0008] All strip temperatures in the process can be determined, for example, using a commercially available pyrometer.

[0009] In the present application, the same atmosphere (ie: A i = A j ) means that the dew points and preferably the proportions of hydrogen, oxygen and nitrogen are the same within the measurement uncertainty.

[0010] High-strength steels with good forming properties are known from the state of the art for applications in, for example, 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 elongation properties.

[0011] From EP 2 710 158 A1 an ultra-high-strength, cold-rolled steel sheet is known which contains in mass% 0.10 - 0.50% C, 0.01 - 2.5% Si, 1.0 - 3.5% Mn, not more than 2.5% Al, not more than 0.020% P, not more than 0.005% S, and not more than 0.02% N, optionally further one or more elements from 0.01 - 0.50% Cr, 0.01 - 0.5% Mo, 0.01 - 0.1% V, 0.001 - 0.15% Ti, 0.02 - 0.05% Nb, where for the sum of V, Ti, Nb ≤ 0.2% applies, 0.0005 - 0.005% B, not more than 0.01% Ca and a microstructure of less than 5% ferrite, less than 5% bainite, 5-70% untempered martensite, 5-30% retained austenite, and 25-80% tempered martensite, with at least 99% of the iron carbides contained in the tempered martensite having a size of less than 500 nm. These properties result in good yield strength and strength.

[0012] High-strength steels are often electrolytically coated or hot-dip coated, e.g., as in EP 2 258 886 B1. However, this demonstrates the need to provide an ultra-high-strength, cold-rolled steel sheet that can be used uncoated. Due to the high alloy content, particularly the oxygen-affine elements Si and Al, many surface defects are apparent.

[0013] Therefore, the object of the present invention is to provide a high-strength flat steel product with good elongation properties and good surface properties, as well as a manufacturing process for it. Furthermore, it may be necessary for the flat steel product with the good surface properties to also have optimized forming properties.

[0014] The task is solved by a process for the production of a high-strength flat steel product in which the dew points and thus the various oxide layers are specifically controlled in the individual production steps. This results in a flat steel product with particularly good surface properties and a roughness R a = 0.5 µm - 1.8 µm and a peak number R PC ≥ 40 cm -1<.

[0015] The manufacturing process comprises at least the following steps: a. Providing a cold-rolled flat steel product comprising a steel consisting of the following elements: C: 0,10 - 0,5 %; Mn: 1,0 - 3,0 %; Si: 0,9 -1,7 %; P: ≤ 0,020 %; S: ≤ 0,005 %; N: ≤ 0,008 % and optionally one or more of the following elements AI: 0,01 - 1,5 %; Cr: 0,05 - 1 %; Mon: 0,05 - 0,2 %; B: 0,0004 - 0,002 %; Cu: 0,05 - 0,2 %; 0.005% ≤ Ti+Nb+V ≤ 0.2%; and the remainder consists of iron and unavoidable elements. b. Heating the cold-rolled flat steel product to a furnace inlet temperature. c. Heating the cold-rolled flat steel product from T 0 in an atmosphere A 1 to a temperature T 1 , where T 1 = 650 °C - 750 °C and the dew point T P1 the atmosphere A 1 in the range (-60 °C) to (-5 °C). d. Heating and soaking the cold-rolled flat steel product at a temperature T 4 for t 4 = 5 s - 300 s, where T 1 ≤ T 4 ≤ 950 °C and T 4 ≥ A c3 -30 °C, the Warming up in an atmosphere A 4 occurs and the dew point T P4 the atmosphere A 4 in the range (-60 °C) to (-0 °C). e. Cooling the cold-rolled flat steel product at a cooling rate ϑ 5 = 10 °C / s - 100 °C / s in one atmosphere A 5 to a temperature T 5 , where T 5 = (T Ms + 40 °C) - (T MS - 175 °C) and T 5 ≤ 550 °C and the dew point T P5 the atmosphere A 5 in the range (-60 °C) to (-0 °C). f. Setting and maintaining the cold-rolled flat steel product at a temperature T 6 in an atmosphere A 6 for a holding time t 6 = 1 s - 60 s, where T 6 = T MS -(T MS - 175 °C), and the dew point T P6 the atmosphere A 6 in the range (-60 °C) to (-5 °C). g. Reheating the cold-rolled flat steel product at a heating rate ϑ 7 = 2 °C / s - 100 °C / s in one atmosphere A 7 to a temperature T 7 , where T MS < T 7 ≤ 510 °C and the dew point T P7 the atmosphere A 7 in the range (-60 °C) to (-5 °C). h. Setting and maintaining the cold-rolled flat steel product at a temperature T 8 , where T MS < T 8 ≤ 510 °C, in an atmosphere A 8 for a total time for reheating, setting and holding t 8 = 10 s - 600 s, where the dew point T P8 the atmosphere A 8 in the range (-60 °C) to (-5 °C). i. Cooling the cold-rolled flat steel product at a cooling rate ϑ 10 > 5 °C / s to a temperature T 10 , where T 10 ≤ 60 °C. j. Skin-passing of the flat steel product with at least one forming pass and a total skin-pass degree D = 0.1% - 0.8%.

[0016] In a preferred embodiment, the method for producing a high-strength, uncoated flat steel product comprises no further working steps and the method consists of steps a to j.

[0017] In a particular embodiment, no further temperature changes occur between two consecutive steps; this particularly preferably applies to all pairs of consecutive steps. This means, for example, that after step e) (cooling to T 5 ), T 6 is set and maintained directly.

[0018] The individual work steps are described in detail below: Work step a)

[0019] The provided cold-rolled flat steel product is manufactured in a conventional manner. This conventional method includes casting the steel into a slab, reheating the slabs, hot rolling, coiling the hot strip, pickling the hot strip, and cold rolling the hot strip. The following explanations regarding the composition apply to the inventive composition of the slab and the inventive flat steel product and their optional variations.

[0020] Carbon "C" is present in the steel according to the invention in amounts of 0.10% to 0.5%. Carbon supports the formation and stabilization of austenite in the steel according to the invention. Stabilization occurs particularly during quenching and the subsequent annealing treatment. Furthermore, the addition of C imparts high strength to the steel, as the strength of the martensite formed during the process is increased. Therefore, the C content should be at least 0.10%, preferably 0.12%, particularly preferably 0.15%. On the other hand, the martensite initiation temperature shifts to increasingly lower temperatures with increasing C content, so that possibly 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 0.4%, particularly preferably 0.5%.

[0021] Silicon ("Si") is required to achieve the special microstructure in this invention because it delays cementite formation. An excessively high cementite content would result in the carbon being bound in carbides, making it unavailable to stabilize the residual austenite during the process, and elongation would deteriorate. Therefore, silicon must be present in the steel according to the invention at a level of at least 0.9%, preferably at least 1.05%, particularly preferably at least 1.10%. On the other hand, an excessively high silicon content leads to poor surface quality, so the steel according to the invention should contain a maximum of 1.7%, particularly preferably a maximum of 1.5%.

[0022] In a particular embodiment, the steel according to the invention comprises C ≤ 0.16% and Si ≤ 1.2%, preferably C ≤ 0.15% and Si ≤ 1.2%, particularly preferably C ≤ 0.15% and Si ≤ 1.1%. In this particular embodiment, it can particularly preferably have C ≥ 0.12%, particularly preferably C ≥ 0.15%, and preferably Si ≥ 0.9%, particularly preferably Si ≥ 1.05%.

[0023] In an alternative embodiment, the steel according to the invention has C > 0.16% and Si > 1.2%, preferably C > 0.16% and Si ≥ 1.25%, particularly preferably C ≥ 0.18% and Si ≥ 1.3%, especially preferably C ≥ 0.20% and Si ≥ 1.4%. In this particular embodiment, it can particularly preferably have C ≤ 0.5%, particularly preferably C ≤ 0.5% and preferably Si ≤ 1.7%, particularly preferably Si ≤ 1.5%.

[0024] The steel according to the invention contains manganese (Mn). At a content of 1.0% or more, 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 severe segregation, which is why the Mn content is limited to 3.0%. Furthermore, a high Mn content severely limits weldability and reduces corrosion resistance. Therefore, a maximum Mn content of 2.5% and, in particular, 2.3% has proven particularly advantageous.

[0025] The addition of phosphorus ("P") severely limits weldability and should therefore be limited to 0.020%, with contents of 0.018%, especially 0.015%, being particularly advantageous. In the steel according to the invention, it has been found that it can be advantageous to include P in amounts of at least 0.002%, especially 0.006%, as this strengthens solid solution hardening.

[0026] Sulfur "S" can lead to the formation of Mn sulfides, which severely impair formability properties. Therefore, in the steel according to the invention, the content is limited to 0.005%, although a restriction to 0.004% and especially to 0.005% may be advantageous. Sulfur contamination cannot be completely avoided during steelmaking.

[0027] Nitrogen "N" can lead to the formation of coarse nitrides at levels above 0.010%, resulting in impaired formability. To avoid these nitrides, a maximum content of 0.008% has proven particularly advantageous. Nitrogen contamination cannot be completely avoided during steelmaking.

[0028] In addition to the previously discussed impurities P, S, and N, other elements may also be present as impurities in the steel. These additional elements are summarized under the term "unavoidable impurities." The total content of these "unavoidable impurities" is preferably a maximum of 0.2%, preferably a maximum of 0.1%. The optional alloying elements "Al, Cr, Mo, B, Cu, Ti, Nb" described below, for which a lower limit is specified, may also be present as unavoidable impurities in the steel substrate in amounts below the respective lower limit. In this case, they are also counted as "unavoidable impurities," whose total content is limited to a maximum of 0.2%, preferably a maximum of 0.1%.

[0029] 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 residual austenite content. A higher residual 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 aluminum nitrides, which have an embrittling effect and thus to poorer formability. Furthermore, higher aluminum contents can lead to poorer casting behavior, as aluminum compounds can lead to clogging. Therefore, the present invention provides for a limitation of the aluminum content to 1.5%, preferably 0.8%, particularly preferably 0.4%.

[0030] Chromium (Cr) is an effective pearlite inhibitor and contributes to strength. A chromium content of at least 0.10% 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%.

[0031] Molybdenum (Mo) also forms fine, strength-enhancing carbon nitrides in small amounts. Therefore, an addition of at least 0.05% has proven beneficial. However, the strength-enhancing effect of carbon nitrides is exhausted as soon as the molybdenum content becomes too high. Furthermore, high molybdenum contents can impair cold formability and weldability. A maximum content of 0.2%, preferably 0.10%, and particularly preferably 0.07%, has proven advantageous in this case.

[0032] 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%, particularly preferably at least 0.0005%, can be added to the steel according to the invention. The effect of B is saturated at a maximum content of 0.002%.

[0033] The addition of copper ("Cu") to the flat steel product according to the invention can form very fine strength-enhancing Cu precipitates. Therefore, an addition of at least 0.05%, preferably 0.10%, can be advantageous in the present invention. However, the copper content should be limited to 0.2%, as otherwise, so-called red brittleness, i.e., cracks in the slab, can occur during the hot rolling process.

[0034] In a particular embodiment, microalloying elements (=MLEs) (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 distributed carbides. A minimum MLE content of 0.005% in total leads to the freezing of grain and phase boundaries during annealing. However, an excessively high MLE concentration, which strongly promotes carbide formation and phase boundary immobility, is detrimental to the stabilization of the residual austenite. Therefore, the total MLE concentration should be limited to a maximum of 0.2%. Work step b)

[0035] The cold-rolled flat steel product is heated to a furnace inlet temperature. The furnace inlet temperature is the temperature at the center of the sheet when entering the furnace. The furnace inlet temperature is preferably at least 10 °C, more preferably 15 °C. The furnace inlet temperature should preferably not exceed 100 °C, more preferably 50 °C, and especially preferably 35 °C. Work step c)

[0036] The flat steel product according to the invention is heated from T 0 to a temperature T 1 in an atmosphere A 1. The temperature T 1 is at least 650 °C, preferably 670 °C. The temperature T 1 is a maximum of 750 °C, preferably 750 °C, since recrystallization processes begin above this temperature and the process conditions must be adjusted according to step d).

[0037] The atmosphere A1 set according to the invention is reducing. It preferably comprises at least 2% hydrogen "H2", preferably 3% H2, particularly preferably 5% H2. The set hydrogen content ensures that the atmosphere is reducing, particularly with respect to iron. This prevents uncontrolled oxidation and allows a thin oxide layer, particularly preferably thinner than 300 nm, to be set. For economic reasons, the H2 content should be limited to a maximum of 20%, preferably 10%. In addition, up to 0.5% oxygen "O2", in particular 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 alloy elements. The remainder of the preferred atmosphere is nitrogen "N2", preferably 80%, particularly preferably 85%, especially preferably 90%.In a particular embodiment, the atmosphere consists of the described proportions of H 2 , O 2 , H 2 O, and N 2 . The dew point of the T P1 is at least -60 °C, preferably -55 °C, particularly preferably -45 °C. Furthermore, the dew point should not exceed -5 °C, preferably -10 °C, particularly preferably -15 °C, since otherwise selective oxidation of base alloy elements may occur. This would lead to undesirable surface defects on the subsequent steel surface.

[0038] In a particular embodiment, the heating rate ϑ 1 in step c) between 500 °C and T 1 in step c) 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 can delay the selective oxidation of the base alloying elements until T 1 is reached and further minimize it. Work step d)

[0039] Subsequently, in step d), the flat steel product is heated from a temperature T 1 to a temperature T 4 and soaked at a temperature T 4 in a reducing atmosphere A 4 . The flat steel product is heated and soaked for at least t 4 ≥ 5 s, preferably 10 s, particularly preferably 15 s. The minimum value of t 4 results from the fact that residual oxides on the starting material surface are not sufficiently reduced back to metallic Fe at an exposure time of < 5 s compared to the reduction conditions according to the invention.

[0040] Setting the T4 temperature and a sufficient time t4 results in sufficient austenitization. However, the time should be limited to t4 ≤ 300 s, preferably 180 s, as otherwise coarsening of the austenite grain will occur, which will negatively affect the mechanical properties.

[0041] For the temperature T 4 in this step, T 1 ≤ T 4 ≤ 950 °C and T 4 ≥ A c3 -30 °C, in particular T 4 ≥ A c3 .

[0042] The minimum temperature to be exceeded, A c3, is determined according to the formula given by HOUGARDY, HP in Werkstoffkunde Stahl, Volume 1: Grundlagen, Verlag Stahleisen GmbH, Düsseldorf, 1984, p. 229. A c3 = 902 − 225 * % C + 19 * % Si − 11 * % Mn − 5*%Cr + 13 * % Mo − 20 * % Ni + 55 * % V ° C where %C = respective C content, %Si = respective Si content, %Mn = respective Mn content, %Cr = respective Cr content, %Mo = respective Mo content, %Ni = respective Ni content and %V = respective V content of the steel from which the blank is made.

[0043] This prevents uncontrolled oxidation and allows the formation of a thin oxide layer, preferably with FeO as the main component. In a particular embodiment, the oxide layer has a thickness of less than 100 nm; particularly preferably, the oxide layer formed in step c), in particular the FeO layer, is completely reduced to metallic iron.

[0044] The preferably set atmosphere A 4 comprises at least 2% hydrogen "H 2 ", preferably 3% H 2 , particularly preferably 5% H 2 . In a particular embodiment, A 1 = A 4 . The set hydrogen content can ensure that the atmosphere is reducing, particularly with regard to iron. The H 2 content should preferably be limited to a maximum of 20%, preferably 10%, for economic reasons. In addition, up to 0.5% oxygen "O 2 ", in particular traces of O 2 , and up to 0.5% water "H 2 O" can be added to the atmosphere. Both proportions must be limited in order to minimize the selective oxidation of base alloy elements. The remainder of the preferred atmosphere is added to nitrogen "N 2 ", preferably 80%, particularly preferably 85%, especially preferably 90%. In a particular embodiment, the atmosphere consists of the described proportions of H 2 , O 2 , H 2 O and N 2 .The dew point of the T P2 is at least -60 °C, preferably -40 °C, particularly preferably -35 °C. Furthermore, the dew point should not be greater than 0 °C, preferably -10 °C, particularly preferably -16 °C, since otherwise unwanted oxide formation may occur.

[0045] In a special embodiment, the heating of the flat steel product in step d) can be achieved by maintaining it at a constant temperature T 4 . A constant temperature is defined as a maximum fluctuation of ±5 °C, since a more precise setting is not possible due to process technology. This special embodiment is particularly suitable if a relatively low T 4 temperature has been set for analytical reasons.

[0046] In a particular embodiment, heating from T 1 to T 4 can be carried out at a heating rate ϑ 2 = 0.5 °C / s - 10 °C / s. The heating rate should not exceed 10 °C / s, preferably 8 °C / s, particularly preferably 4 °C / s. In this alternative embodiment, the heating rate should be at least 0.5 °C / s, preferably 1.0 °C / s, particularly preferably 2.5 °C / s.

[0047] In a special embodiment, heating can take place in atmosphere A 2 , in particular A 2 = A 1 . The preferably set atmosphere A 2 comprises at least 2% hydrogen "H 2 ", preferably 3% H 2 , particularly preferably 5% H 2 . The set hydrogen content ensures that the atmosphere is reducing, especially with regard to iron. This prevents uncontrolled oxidation and allows a thin oxide layer, in particular less than 100 nm, to be achieved. The H 2 content should preferably be limited to a maximum of 20%, preferably 10%, for economic reasons. In addition, up to 0.5% oxygen "O 2 ", in particular traces of O 2 , and up to 0.5% water "H 2 O" can be added to the atmosphere. Both proportions must be limited in order to minimize the selective oxidation of base alloy elements.The remainder of the preferred atmosphere is added nitrogen "N 2 ", preferably 80%, particularly preferably 85%, especially preferably 90%. In a particular embodiment, the atmosphere consists of the described proportions of H 2 , O 2 , H 2 O and N 2 . The dew point of the T P2 of the atmosphere A 2 is at least -60 °C, preferably -30 °C. Furthermore, the dew point should not be greater than -5 °C, preferably -10 °C, particularly preferably -15 °C, since otherwise selective oxidation of base alloy elements can occur. Work step e)

[0048] The flat steel product is cooled to a temperature T 5 at a cooling rate of ϑ 5 = 10 °C / s - 100 °C / s in a reducing atmosphere A 5 . The temperature T 5 is at most (T MS + 40 °C), preferably (T MS + 20 °C), particularly preferably T MS , to ensure a sufficient martensite content or sufficient nucleation for bainite in the final structure. The temperature T 5 should be at least (T MS - 175 °C). In this step, the so-called primary martensite is formed. T MS can be determined using the following equation: T MS ° C = 539 ° C + − 423 % C − 30,4 % Mn − 7,5 % Si + 30 % AI ° C / wt%

[0049] In addition, the temperature T 5 must be ≤ 550 °C to avoid selective re-oxidation on the steel surface. The cooling rate ϑ 5 should be at least 10 °C / s, particularly preferably 20 °C / s. The cooling rate ϑ 5 should be limited to a maximum of 100 °C / s, preferably 50 °C / s, particularly preferably 30 °C / s. The minimum value of u 5 results from the fact that if the cooling rate is too low, an unwanted ferritic and / or bainitic transformation cannot be ruled out. The maximum value of u 5 is limited by the fact that there is an excessively high risk of unwanted (selective) re-oxidation of the steel surface.

[0050] The preferably set atmosphere A5 in step e) comprises at least 2% hydrogen "H2", preferably 3% H2, particularly preferably 5% H2. The set hydrogen content ensures that the atmosphere is reducing, particularly with regard to iron. This avoids uncontrolled oxidation and re-oxidation on the surface. The H2 content should preferably be limited to a maximum of 80%, preferably 50%, for economic reasons. In addition, up to 0.5% oxygen "O2", in particular traces of O2, and up to 0.5% water "H2O" can be added to the atmosphere. Both proportions must be limited in order to minimize the selective oxidation of base alloy elements. The remainder of the preferred atmosphere is nitrogen "N2", preferably 80%, particularly preferably 85%, especially preferably 90%.In a particular embodiment, the atmosphere consists of the described proportions of H 2 , O 2 , H 2 O, and N 2 . The dew point of the T P5 is at least -60 °C, preferably -40 °C. Furthermore, the dew point should not be greater than 0 °C, preferably -10 °C, particularly preferably -15 °C, since otherwise selective oxidation of base alloy elements may occur.

[0051] In a special embodiment, A 4 is not equal to A 5 , as this prevents uncontrolled entrainment of hydrogen from atmosphere A 4 into atmosphere A 5 . This allows the hydrogen content in atmosphere A 5 to be precisely adjusted to the required amount to prevent selective oxidation, and there is no excess hydrogen present that could undesirably diffuse into the steel and lead to hydrogen embrittlement. This can be achieved by structural separation, particularly a lock system. Work step f)

[0052] In step f), the steel flat product is heated to a temperature T 6 in an atmosphere A 6 and held for a holding time t 6 = 1 s - 60 s. The minimum value for T 6 is (T MS - 175 °C), preferably (T MS - 150 °C). The maximum value is T MS , preferably (T MS - 75 °C). T MS 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 % AI ° C / wt% , where the element concentrations are to be used in weight percent.

[0053] In a particular embodiment, T 6 = T 5 . In a particular embodiment, T P6 = T P5 . The flat steel product should be held at T 6 for at least 1 s, preferably at least 4 s, particularly preferably at least 9 s, since this achieves a homogeneous temperature distribution in the material according to the invention, which ensures the formation of a particularly fine and uniform microstructure of primary martensite and residual austenite across the cross-section of the flat steel product. The holding time t 6 is limited to 60 s for economic reasons. In a particular embodiment, for flat steel product thicknesses ≥ 1.0 mm, the holding time is 10 s - 60 s.

[0054] The preferably adjusted atmosphere A6 in step f) comprises at least 2% hydrogen "H2", preferably 3% H2, particularly preferably 5% H2. The adjusted hydrogen content ensures that the atmosphere is reducing, particularly with regard to iron. This avoids uncontrolled oxidation and re-oxidation on the surface. The H2 content should preferably be limited to a maximum of 20%, preferably 10%, for economic reasons. In addition, up to 0.5% oxygen "O2", in particular traces of O2, and up to 0.5% water "H2O" can be added to the atmosphere. Both proportions must be limited in order to minimize the selective oxidation of base alloy elements. The remainder of the preferred atmosphere is nitrogen "N2", preferably 80%, particularly preferably 85%, especially preferably 90%.In a particular embodiment, the atmosphere consists of the described proportions of H 2 , O 2 , H 2 O, and N 2 . The dew point of the T P6 is at least -60 °C, preferably -40 °C. Furthermore, the dew point should not be greater than (-5) °C, preferably -10 °C, particularly preferably -15 °C, since otherwise selective oxidation of base alloy elements may occur.

[0055] In a special embodiment, A 6 is not equal to A 5 , as this prevents uncontrolled entrainment of hydrogen from atmosphere A 4 into atmosphere A 5 . This allows the hydrogen content in atmosphere A 5 to be precisely adjusted to the required amount to prevent selective oxidation, and there is no excess hydrogen present that could undesirably diffuse into the steel and lead to hydrogen embrittlement. This can be achieved by structural separation, particularly a lock system. Work step g)

[0056] In step g), the flat steel product is heated to a temperature T 7 at a heating rate of ϑ 7 = 2 °C / s - 100 °C / s in an atmosphere A 7 . The temperature T 7 is at most 510 °C, particularly preferably 500 °C, since otherwise an undesirable decrease in the strength of the flat steel product occurs.

[0057] In a particular embodiment, T 7 can be ≤ 400 °C, preferably 350 °C, since this allows the desired strength and elongation to be better achieved.

[0058] The temperature T 7 should be greater than T MS , preferably greater than T MS +50 °C, in order to enrich the residual austenite in the base material structure with C from the supersaturated primary martensite or bainite.

[0059] The heating rate ϑ 7 should be at least 2 °C / s, particularly preferably 4 °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 100 °C / s, preferably 50 °C / s.

[0060] The preferably adjusted atmosphere A7 in step g) comprises at least 2% hydrogen "H2", preferably 3% H2, particularly preferably 5% H2. The adjusted hydrogen content ensures that the atmosphere is reducing, particularly with regard to iron. This prevents uncontrolled oxidation and re-oxidation on the surface. The H2 content should preferably be limited to a maximum of 20%, preferably 10%, for economic reasons. In addition, up to 0.5% oxygen "O2", in particular traces of O2, and up to 0.5% water "H2O" can be added to the atmosphere. Both proportions must be limited in order to minimize the selective oxidation of base alloy elements.

[0061] The remainder of the preferred atmosphere is nitrogen "N 2 ", preferably 80%, particularly preferably 85%, especially preferably 90%. In a particular embodiment, the atmosphere consists of the described proportions of H 2 , O 2 , H 2 O, and N 2 . The dew point of the T P7 is at least -60 °C, preferably -40 °C. Furthermore, the dew point should not be greater than 0 °C, preferably -10 °C, particularly preferably -15 °C, since otherwise selective oxidation of base alloy elements can occur.

[0062] In a particular embodiment, A 7 is equal to A 6 . Work step h)

[0063] The flat steel product is then held at a temperature T 8 in a reducing atmosphere A 8 for a total time of reheating (step g), setting the temperature to T 8 and holding it at t 8 = 10 s - 600 s. The temperature T 8 is a maximum of 510 °C, particularly preferably 500 °C, since otherwise an undesirable decrease in the strength of the flat steel product would occur.

[0064] In a particular embodiment, T 8 can be ≤ 400 °C, preferably 350 °C, since this allows the desired strength and elongation to be better achieved. In this embodiment, t 8 can particularly preferably be set to > 400 s to provide sufficient energy to enrich the retained austenite with C from the supersaturated primary martensite and bainite.

[0065] The temperature T 8 should be greater than T MS , preferably greater than T MS +50 °C, in order to enrich the residual austenite in the base material structure with C from the supersaturated primary martensite and bainite. In a special embodiment, T 7 = T 8 .

[0066] The flat steel product should be held at T 8 for at least 10 s, preferably at least 15 s, and particularly preferably at least 20 s, since otherwise there is insufficient diffusion time for C to accumulate in the residual austenite. The holding time t 8 is limited to 120 s, preferably 100 s, since otherwise an undesirably high carbide content would form in the basic structure.

[0067] The preferably adjusted atmosphere A8 in step h) comprises at least 2% hydrogen "H2", preferably 3% H2, particularly preferably 5% H2. The adjusted hydrogen content ensures that the atmosphere is reducing, particularly with regard to iron. This avoids uncontrolled oxidation and re-oxidation on the surface. The H2 content should preferably be limited to a maximum of 20%, preferably 10%, for economic reasons. In addition, up to 0.5% oxygen "O2", in particular traces of O2, and up to 0.5% water "H2O" can be added to the atmosphere. Both proportions must be limited in order to minimize the selective oxidation of base alloy elements. The remainder of the preferred atmosphere is nitrogen "N2", preferably 80%, particularly preferably 85%, especially preferably 90%.In a particular embodiment, the atmosphere consists of the described proportions of H 2 , O 2 , H 2 O, and N 2 . The dew point of the T P8 is at least -60 °C, preferably -40 °C. Furthermore, the dew point should not be greater than (-5) °C, preferably -10 °C, particularly preferably -15 °C, since otherwise selective oxidation of base alloy elements may occur.

[0068] In a particular embodiment, A 8 =A 7 , in particular A 8 = A 7 = A 6 . Work step i)

[0069] In step i), the flat steel product is cooled at a cooling rate of ϑ 10 > 5 °C / s to a temperature T 10 , where T 10 ≤ 60 °C. The minimum value of u 10 is determined for technical and economic reasons to avoid making the required cooling section unnecessarily long. If T 10 > 60 °C, preferably T 10 > 40 °C, this can lead to surface defects during the subsequent skin-passing process. Work step i)

[0070] Skin-passing the flat steel product with at least one forming pass and a total skin-pass degree of D = 0.1% - 0.8%, preferably D = 0.1% - 0.5%, particularly preferably D = 0.2% - 0.5%. In a particular embodiment, the skin-passing can be carried out with at least two forming passes. Skin-passing serves to improve flatness, fine-tune the mechanical properties by ultimately increasing strength, and to imprint a defined fine surface structure into the surface via the skin-pass roll structure. In addition to the other process steps described, the skin-passing according to the invention achieves the desired roughness and peak count of the surface.

[0071] The minimum value of D is determined by the fact that with a skin pass degree of < 0.1%, preferably 0.2%, insufficient rolling force is applied to optimize the flatness and to meet the inventive minimum requirements for R a and R pc. The maximum value of D is limited by the fact that the product properties cannot be further improved by a D degree > 0.8%, preferably 0.5%, but the technical effort increases disproportionately due to the necessary rolling force or multi-pass skin pass. For economic and logistical reasons, skin pass is preferably carried out in-line, i.e. in a continuous process in the same plant together with the upstream annealing treatment.Alternatively, skin-passing can also be performed in a subsequent process on a stand-alone skin-pass mill or in a combination of in-line and offline skin-passing, particularly if more than one forming pass is necessary to achieve the inventive limits of D, R a , and R pc . The applied surface texturing can be based on a deterministic or stochastic fine structure. A preferred embodiment is to apply stochastic surface texturing during skin-passing in order to optimize the friction behavior between the steel surface and the tool during forming into the component in the oiled or greased state.Under the high compressive loads resulting from the high forming forces required for high-strength steels, a stochastic surface structure offers the advantage that, under high compressive loads, the lubricant can flow out of the stress zone via microchannels that open up between the peaks and valleys of the surface texture. This allows for a more even distribution of the lubricant over the entire surface where contact occurs between the tool and the flat steel product during the forming process. Furthermore, a stochastic basic structure ensures flow and adhesion properties for organic or metallic coatings, which can be additionally applied to the flat steel product according to the invention if necessary.

[0072] Skin-passing can be performed either wet (under water or oil) or dry (without liquid media). Dry skin-passing offers the advantage that no liquid residues can be carried over, which later lead to surface defects in the form of corrosion. Nevertheless, if necessary, dry skin-passing can be supported by applying a small amount of a rapidly evaporating forming aid, especially if a high degree of skin-passing is to be achieved.

[0073] In a particular embodiment, the steel flat product in the process according to the invention is moved through a furnace in steps b) - h) by means of furnace rollers, wherein at least one of the furnace rollers used, preferably all of the furnace rollers, has a coating with a microhardness ≥ 750 HV0.3, preferably ≥ 900 HV0.3, and a roughness R a = 3.0 µm - 8 µmThe microhardness is determined according to DIN EN ISO 6507. In the annealing process according to the invention, interactions between the various metallic and oxide components of the steel surface and the furnace rollers can occur. This can lead to growths on the furnace rollers, which in turn can cause surface defects in the steel strip. It has been shown that in the particular embodiment, a furnace roller, preferably all furnace rollers with a coating with a microhardness ≥ 750 HV0.3, preferably ≥ 900 HV0.3, and a roughness t R a = 3.0 µm - 8 µm should be provided so that these surface defects can be avoided.

[0074] In a particular embodiment, the uncoated flat steel product can be provided with an additional coating or conversion layer, in a particular embodiment chromating or phosphating.

[0075] In another embodiment, the flat steel product can be coated with a metallic zinc-based corrosion protection layer. The coating can be applied using a PVD process or by electrolytic deposition. This coating is preferably applied by electrolytic deposition with a zinc layer thickness d of ≥ 2 - ≤ 10 µm. The minimum value of d is based on the fact that at a layer thickness of < 2 µm, the desired cathodic corrosion protection cannot be adequately ensured. At d > 10 µm, however, the forming and welding properties can be negatively affected.

[0076] Carrying out the process according to the invention leads to the steel flat product according to the invention: A high-strength steel flat product, with a tensile strength Rm = 900 MPa - 1500 MPa, a yield strength Rp02 ≥ 680 MPa and an elongation A80 = 7% - 25%, which comprises a steel consisting of the following elements: C: 0,10 - 0,5 %; Mn: 1,0 - 3,0 %; Si: 0,9 - 1,7 %; P: ≤ 0,020 %; S: ≤ 0,005 %; N: ≤ 0,010 %; and optionally one or more of the following elements AI: 0,01 - 1,5 %; Cr: 0,05 - 1 %; Mon: 0,05 - 0,2 %; B: 0,0004 - 0,002 %; Cu: 0,05 - 0,2 %; 0.005%≤TI+NB+V≤0.2% and the remainder being iron and unavoidable elements, wherein the steel flat product has a structure consisting of ≥ 80 % bainite and / or martensite, of which at least 75 % of the martensite is tempered, ≥ 5 % retained austenite and ≤ 10 % ferrite with a surface of the steel flat product which has a roughness R a = 0.5 µm - 1.8 µm and a peak number R PC ≥ 40 cm -1 has.

[0077] The surface according to the invention is characterized by a roughness R a = 0.5 µm - 1.8 µm, in particular R a = 0.7 µm - 1.8 µm and a peak number R PC≥ 40 cm -1< , especially R PC ≥ 50 cm -1< . A roughness Ra of both < 0.5 µm and > 1.8 µm should be avoided, since such values can lead to adverse friction behavior during subsequent forming into the component. R PC should not be < 40cm -1< to ensure sufficiently good optical properties even after painting.

[0078] The flat steel product according to the invention has a tensile strength R m = 900 MPa -1500 MPa, a yield strength R p02 ≥ 680 MPA and an elongation A 80 = 7% - 25%.

[0079] In a particular embodiment with C ≤ 0.16% and Si ≤ 1.2%, preferably C ≤ 0.15% and Si ≤ 1.2%, particularly preferably C ≤ 0.15% and Si ≤ 1.1%, the tensile strength R m is < 1000 MPa, preferably R m ≤ 980 MPa. In this particular embodiment, the steel according to the invention can particularly preferably have C ≥ 0.12%, particularly preferably C ≥ 0.15% and preferably Si ≥ 0.9%, particularly preferably Si ≥ 1.05%.

[0080] In an alternative embodiment with C > 0.16 and Si > 1.2, preferably C > 0.16% and Si ≥ 1.25%, particularly preferably C ≥ 0.18% and Si ≥ 1.3%, particularly preferably C ≥ 0.20% and Si ≥ 1.4%, the tensile strength R m ≥ 1000MPa, preferably R m ≥ 1080MPa. In this particular embodiment, the steel according to the invention can particularly preferably have C ≤ 0.5%, particularly preferably C ≤ 0.5% and preferably Si ≤ 1.7%, particularly preferably Si ≤ 1.5%.

[0081] In a special embodiment, the flat steel product has a bending angle > 80% and a hole expansion > 25%.

[0082] The flat steel product according to the invention has a structure consisting of ≥ 80% bainite and / or martensite, of which at least 75% of the martensite is tempered, ≥ 5% residual austenite and ≤ 10% ferrite.

[0083] The present microstructure consists of 80% bainite and / or martensite. The bainite is preferably bainitic ferrite. 75%, preferably 80%, particularly preferably 90% of the martensite is tempered during the process according to the invention. Preferably, a maximum of 25%, particularly preferably 20%, particularly preferably 10% of the martensite in the microstructure according to the invention is untempered.

[0084] The microstructure of a flat steel product according to the invention contains at least 5% residual austenite. Residual austenite has a positive effect on the formability and elongation of martensite-containing steels. Austenite stabilized down to room temperature can be elongated to a greater extent than other microstructure components by utilizing the TRIP effect, while simultaneously achieving higher work hardening. Due to the limitation of austenite-stabilizing alloying elements such as C and Mn for weldability reasons, a residual austenite content greater than 20% is not possible with the described manufacturing process.

[0085] The flat steel product according to the invention has a microstructure containing a maximum of 10% ferrite, preferably 5%, particularly preferably 3%, to ensure the required high strength. In a preferred embodiment, the ferrite present is polygonal ferrite.

[0086] In a particular embodiment, the atmosphere A 4 and the dew point T P4 in step d) can be adjusted so that H 2 O / H 2 < 0.957, preferably H 2 O / H 2 < 0.90, particularly preferably H 2 O / H 2 < 0.80. This further reduces the thin oxide layer that forms in the atmosphere A 4. Furthermore, the surface layer, i.e. the portion that is at a maximum distance of 10 µm from the surface, is chemically changed. In particular, carbon diffuses out of the material and the surface layer becomes depleted of carbon. As a result, no carbon is present in step h) to stabilize the residual austenite. The chemical change in the surface region can be supported by the targeted inflation of NH 3 , which can have a positive effect on the mechanical properties close to the surface and additionally inhibits the external selective oxidation of the base alloy elements.Therefore, in this preferred embodiment, the ratio of the residual austenite content in the RA Bulk material compared to the residual austenite content in the surface layer RA Surface is RA Surface / RA Bulk ≥ 80%. The surface layer is defined as the area of the steel that has a maximum distance of 10 µm from the surface. The surface area is therefore softer than the material, resulting in good forming properties. In this particular embodiment, a flat steel product with the inventive good surface and good formability can be achieved.

[0087] In a particular embodiment, a component for structural lightweight construction in automotive engineering can be formed from a flat steel product according to the invention.

[0088] The following describes the laboratory testing of the process according to the invention. First, various cold-rolled flat steel products made from different steels according to Table 1 were provided. The various flat steel products were then tested using the processes shown in Tables 2 and 3. The achieved flat steel product properties are shown in Table 4.

[0089] Additionally, samples F112 and B102 underwent stochastic surface texturing during the skin-passing step. Sample A1 was subsequently coated with a conversion layer.

[0090] For samples D107 and F113, a furnace roller was coated with a coating with a microhardness of 8001 HV0.3 and a roughness of 4 µm.

[0091] Steel alloys B and DF have the steel composition according to the invention. Steel alloy B was tested under different manufacturing parameters (B102 - B106). Tests B102, BIOS and B106 were carried out under process conditions according to the invention and show good surface conditions with good mechanical properties. Test B104 did show good surface properties, but due to a T4 temperature < Ac3 -30 °C the carbon cannot be distributed homogeneously in the austenite structure, which results in an excessively low proportion of tempered martensite. In example B105, on the other hand, the structure according to the invention is achieved, but the example has poor surface properties, i.e. no roughness according to the invention. These poor surface properties are due to the dew points Tp6 and Tp7 not according to the invention.

[0092] Examples D107, D108, F112, and F113 were produced according to the inventive process and exhibited good surface properties, i.e., roughness. Furthermore, the residual austenite in the surface layer had a RA_Surface / RA_Bulk ratio of ≥ 80%. In contrast, in Example D9, a good surface could not be achieved due to a non-inventive skin-passing degree. In Example D110, however, the non-inventive dew points T p6 and T p7 lead to the selective oxidation of base alloying elements and poor surface properties.

[0093] Steel alloys A and E have a silicon content that is not in accordance with the invention; all further process steps are within the inventive range. Due to the low silicon content, a high proportion of bainite and carbides forms in the microstructure. This results in a low residual austenite content and a high proportion of tempered martensite. Therefore, examples A101, E110, and E111 are not in accordance with the invention.

[0094] Steel alloy C has a carbon and silicon content that is not in accordance with the invention because too much fresh martensite is formed. Both examples C105 and C106 therefore have a non-inventive proportion of tempered martensite. C106 also has a non-inventive proportion of ferrite and retained austenite. Table 1 Nr. C Si Mn P S Al Cr Cu Nb Mon N TI V B Ti+Nb+V Ac3 MS A 0,142 0,21 1,63 0,012 0,0027 0,031 0,780 0,051 0,002 0,003 0,0027 0,037 0,002 0,0011 0,041 808 429 B 0,218 1,478 2,21 0,016 0,0023 0,024 0,173 0,047 0,001 0,01 0,0046 0,007 0,003 0,0004 0,011 834 369 C 0,072 0,26 2,59 0,013 0,0021 0,029 0,690 0,090 0,001 0,110 0,0025 0,079 0,005 0,0013 0,085 816 429 D 0,158 1,18 1,99 0,014 0,0020 0,017 0,022 0,008 0,001 0,005 0,0016 0,015 0,001 0,0015 0,017 840 403 E 0,153 0,42 2,35 0,013 0,0025 0,710 0,720 0,061 0,027 0,010 0,0042 0,023 0,003 0,0014 0,053 799 421 F 0,274 1,47 2,31 0,005 0,0021 0,022 0,132 0,036 0,001 0,099 0,0013 0,086 0,004 0,0003 0,091 823 343 Table 2 Nr. ϑ 1 T1 T P1 t 4 T 4 T P4 T P5 T 5 ϑ 5 T P6 T 6 t 6 A101 5 670 -20 130 893 -25 -35 375 31 -20 375 8 B102 4 695 -20 160 892 -30 -15 331 34 -25 331 12 B103 4 725 -20 95 902 -40 -15 326 36 -25 326 10 B104 6 665 -20 85 793 -35 -20 341 31 -20 341 11 C105 7 645 -20 110 857 -35 -20 329 30 -5 329 20 C106 5 645 -20 170 851 -35 -30 389 31 -10 389 9 D107 4 675 -20 65 904 -45 -10 311 35 -35 311 10 D108 6 675 -20 65 892 -45 -20 333 32 -60 333 13 D109 6 690 -20 180 893 -45 -15 291 37 -60 291 8 E110 8 645 -20 80 849 -30 -35 397 32 -55 397 12 E111 4 655 -20 80 858 -30 -30 374 39 -25 374 8 F112 5 700 -20 45 901 -40 -15 290 44 -50 290 7 F113 4 700 -20 15 896 -40 -15 321 37 -45 321 13 B105 5 670 -20 130 897 -25 -20 336 31 5 336 18 B106 5 670 -20 90 888 -25 -35 319 31 -20 319 17 D110 6 675 -20 65 893 -45 -20 337 32 5 337 15 Table 3 Nr. ϑ7 T 7 T P7 D A101 6 450 -20 0,1 B102 9 443 -25 0,3 B103 7 446 -25 0,2 B104 13 451 -20 0,5 C105 20 453 -5 0,2 C106 17 450 -10 0,3 D107 16 441 -35 0,5 D108 18 453 -60 0,6 D109 80 447 -60 0,95 E110 19 450 -55 0,1 E111 21 451 -25 0,4 F112 97 443 -50 0,3 F113 14 447 -45 0,25 B105 15 451 5 0,2 B106 18 454 -20 0,15 D110 14 459 5 0,45 Table 4 Nr. Bainite + Martensite Proportion of tempered martensite retained austenite ferrite Rp0.2 Rm A80 Roughness Ra Peak number Rpc A101 77 40% 3 20 585 883 17 0,65 55 B102 84 88% 12 4 880 1199 16 0,8 52 B103 88 91% 11 1 940 1181 14 0,7 54 B104 84 50% 8 8 719 1245 10 1,3 58 C105 94 56% 6 0 759 1091 10 0,8 50 C106 82 31% 3 15 673 1101 13 0,9 51 D107 88 80% 10 2 900 1055 16 1,2 57 D108 90 76% 9 1 853 1021 13 1,4 60 D109 81 81% 14 5 843 1098 17 2,1 71 E110 82 47% 3 15 679 1209 9 0,7 55 E111 93 54% 3 4 873 1219 6 1,1 56 F112 84 89% 16 0 1239 1482 17 1 54 F113 78 93% 17 5 1158 1489 22 1 53 B105 80 88% 15 5 891 1197 16 1,9 61 B106 88 97% 12 0 922 1184 15 0,65 55 D110 86 78% 2 12 849 1001 8 2,1 59

Claims

1. A method for producing a high-strength flat steel product comprising at least the following steps: a. Providing a cold-rolled flat steel product comprising a steel consisting of the following elements: C: 0,10 - 0,5 %; Mn: 1,0 - 3,0 %; Si: 0,9 - 1,7 %; P: ≤ 0,020 %; S: ≤ 0,005 %; N: ≤ 0,010 %; and optionally one or more of the following elements: Al: 0,01 - 1,5 %; Cr: 0,05 - 1 %; Mon: 0,05 - 0,2 %; B: 0,0004 - 0,002 %; Cu: 0,05 - 0,2 %; 0,005 % ≤ Ti + Nb + V ≤ 0 ,2 % and the remainder being iron and unavoidable elements. b. Heating the cold-rolled flat steel product to a furnace inlet temperature. c. Heating the cold-rolled flat steel product from T0 in an atmosphere A1 to a temperature T1, where T1 = 650 °C - 750 °C and the dew point T P1 the atmosphere A1 is in the range (-60 °C) to (-5 °C). d. Heating and soaking the cold-rolled flat steel product at a temperature T4 for t4 = 5 s - 300 s, where T1 ≤ T4 ≤ 950 °C and T4 ≥ A c3 - 30 °C, the heating takes place in an atmosphere A4 and the dew point T P4the atmosphere A4 is in the range (-60 °C) to (-0 °C). e. Cooling the cold-rolled flat steel product at a cooling rate υ5 = 10 °C / s - 100 °C / s in an atmosphere A5 to a temperature T5, where T5 = (T MS +40 °C) - (T MS -175 °C) and T5 ≤ 550 °C and the dew point T P5 the atmosphere A5 is in the range (-60 °C) to (-0 °C). f. Setting and holding the cold-rolled flat steel product at a temperature T6 in an atmosphere A6 for a holding time t6 = 1 s - 60 s, where T6 = T MS - (T MS - 175 °C), and the dew point T P6 the atmosphere A6 is in the range (-60 °C) to (-5 °C). g. Reheating the cold-rolled flat steel product at a heating rate υ7 = 2 °C / s - 100 °C / s in an atmosphere A7 to a temperature T7, where T MS < T7 ≤ 510 °C and the dew point T P7the atmosphere A7 is in the range (-60 °C) to (-5 °C). h. Setting and maintaining the cold-rolled flat steel product at a temperature T8, where T MS < T8 ≤ 510 °C, in an atmosphere A8 for a total time for reheating, setting and holding of t8 = 10 s - 600 s, where the dew point T P8 the atmosphere A8 is in the range (-60 °C) to (-5 °C). i. Cooling the cold-rolled flat steel product at a cooling rate υ 10 > 5 °C / s to a temperature T 10 , where T 10 ≤ 60 °C. j. Skin-passing of the flat steel product with at least one forming pass and a total skin-pass degree D = 0.1% - 0.8%.

2. Method according to claim 1, characterized in that in step c) between 500 °C and T1 the heating rate υ1 = 2 °C / s - 50 °C / s.

3. Process according to claims 1 and 2, characterized in that in step d) the heating is carried out by holding at a constant temperature T4.

4. Process according to claims 1 to 3, characterized in that in step d) the heating from T1 to T4 takes place at a heating rate υ2 = 0.5 °C / s - 10 °C / s.

5. Process according to claims 1 to 4, characterized in that for the atmosphere A4 of H2O / H2 < 0.957 applies.

6. Process according to claims 1 to 5, characterized in that the flat steel product is moved through a furnace in steps b) to h) by means of furnace rollers, wherein at least one of the furnace rollers used has a coating with a microhardness ≥ 750 HV0.3 and a roughness R a = 3.0 µm - 8 µm.

7. Process according to claims 1 to 6, characterized in that the flat steel product is provided with an additional coating or conversion layer.

8. Process according to claims 1 to 7, characterized in that the flat steel product is coated with a metallic Zn-based corrosion protection layer.

9. Process according to claims 1 to 8, characterized in thatthe reheating temperature T7 ≤ 400 °C.

10. Process according to claims 1 to 9, characterized in that during the skin-pass step j a stochastic surface texturing is applied.

11. High-strength steel flat product, with a tensile strength Rm = 900 MPa - 1500 MPa, a yield strength Rp02≥ 680 MPa and an elongation A80=7% - 25%, which comprises a steel consisting of the following elements: C: 0,10 - 0,5 %; Mn: 1,0 - 3,0 %; Si: 0,9 - 1,7 %; P: ≤ 0,020 %; S: ≤ 0,005 %; N: ≤ 0,010 %; and optionally one or more of the following elements Al: 0,01 - 1,5 %; Cr: 0,05 - 1 %; Mon: 0,05 - 0,2 %; B: 0,0004 - 0,002 %; Cu: 0,05 - 0,2 %; 0,005 % ≤ Ti + Nb + V ≤ 0 ,2 %; and the remainder being iron and unavoidable elements, the steel flat product having a microstructure consisting of ≥ 80 % bainite and / or martensite, of which at least 75 % of the martensite is tempered, ≥ 5 % retained austenite and ≤ 10 % ferrite characterized in that the surface of the flat steel product has a roughness R a = 0.5 µm -1.8 µm and a peak number R PC ≥ 40 cm -1 has.

12. High-strength flat steel product according to claim 11 characterized in that the ratio of retained austenite in the material RA Bulk compared to the residual austenite content in the surface layer RA Surface , which starts 10 µm from the surface, RA _Surface / Attorney Bulk ≥ 80%.

13. High-strength flat steel product according to claims 11 to 12, characterized in that it has a bending angle > 80% and a hole expansion > 25%.

14. Component for structural lightweight construction in automotive engineering, formed from a flat steel product according to claims 11 to 13.

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