Fire-coated high strength steel with good surface and deformation properties with zinc-iron based coating

Abstract

The present invention relates to a hot-dip coated, high-strength steel with good forming and surface properties, its manufacturing process, and a component made from the steel. The coated steel exhibits good results in the powdering test.

Description

[0001] The invention relates to a hot-dip galvanized, 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 Rm, yield strength Rp0.2, elongation at break A80, 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] 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.

[0006] 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.

[0007] All strip temperatures in the process can be determined, for example, using a commercially available pyrometer.

[0008] In the present application, the same atmosphere (ie: Ai=Aj) means that the dew points are the same within the measurement accuracy and preferably the proportions of hydrogen, oxygen and nitrogen are the same within the measurement uncertainty.

[0009] 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.

[0010] For many applications, high-strength steels are treated with corrosion protection. For this purpose, the steels can be hot-dip coated with a zinc-iron-based coating. The challenge with steels containing high amounts of alloying elements is applying an adhesive coating to the surface. The alloying elements silicon, manganese, and chromium, in particular, lead to adhesion problems.

[0011] In EP 3 856 936A1, improved adhesion parameters for zinc-based coatings on high-strength steels are achieved by controlling the distribution of silicon, manganese, and chromium in the boundary layer. This distribution function is achieved by the atmosphere at the highest temperature in the annealing treatment of the cold-rolled flat steel product, the austenitization temperature.

[0012] When coating with zinc-iron-based coatings, this control is not sufficient; further adjustment of the process is required to achieve good adhesion of a zinc-iron-based coating.

[0013] Therefore, it is an object of the present invention to provide a method which enables good adhesion of a zinc-iron-based coating to a high-strength steel and does not adversely affect the strength properties.

[0014] This task is solved by a process in which the flat steel product is deliberately pre-oxidized during heating. This results in a predominantly external Fe oxide layer. This oxide layer is then selectively removed in a later step by re-oxidation. However, as long as the surface is protected by the generated Fe oxide layer, selective oxidation of the elements silicon, manganese, and chromium cannot occur. Therefore, no oxides form on the surface, which could lead to surface defects and poor adhesion in individual areas. In addition, special atmospheric and temperature conditions are established shortly before and during the coating process to further minimize defects.

[0015] The manufacturing process according to the invention comprises at least the following steps: a. Providing a cold-rolled flat steel product comprising a steel consisting of the following elements: 0.10-0.5% C, 1.0-3.0% Mn, 0.9-1.7% Si, <0.020% P, <0.005% S, <0.010% N, and optionally one or more of the following elements: 0.01-1.5% Al, 0.05-1% Cr, 0.05-0.2% Mo, 0.0004-0.002% B, 0.05-0.2% Cu, 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 and pre-oxidation of the cold-rolled flat steel product under the following conditions: 1. Heating the cold-rolled flat steel product from T 0 in a reducing atmosphere A 1 to a temperature T 1 , where T1 = 650 °C-750 °C and the dew point T P1 of the atmosphere A 1 is in the range (-60 °C) to (-5 °C). 2.Heating the cold-rolled flat steel product to a temperature T 2 and then pre-oxidising and heating the cold-rolled flat steel product from T 2 to a temperature T 3 in an oxidising atmosphere A 2 for t 2 = 1 s -30 s, where T 1 ≤ T 2 ≤ T 3 with T 3 = 750° C-850° C. d. Heating the cold-rolled flat steel product from a temperature T 3 to a temperature T 4 and soaking at a temperature T 4 for t 4 = 5 s - 300 s, where T 3 ≤ T 4 ≤ 950 °C and T 4 ≥ A c3 -30 °C, the heating and soaking takes place in an atmosphere A 4 and the dew point T P4 of the atmosphere A 4 is in the range (-60 °C) to (-0 °C); e. Cooling the cold-rolled flat steel product at a cooling rate υ 5 = 10 °C / s-100 °C / s in a reducing 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 of the atmosphere A 5 is in the range (-60 °C) to (-0 °C); f.Setting and holding the cold-rolled flat steel product at a temperature T 6 in a reducing 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 of the atmosphere A 6 is in the range (-60°C) to (-5°C); g. Reheating the cold-rolled flat steel product at a heating rate υ 7 = 4°C / s-1000°C / s in a reducing atmosphere A 7 to a temperature T 7 , where T MS <T 7 ≤510 °C und der Taupunkt T P7 der Atmosphäre A 7 im Bereich (-60 °C) bis (-5 °C) liegt; h. Einstellen und Halten des kaltgewalzten Stahlflachprodukts auf einer Temperatur T 8 in einer reduzierenden Atmosphäre A 8 für eine Gesamtzeit für das Wiederaufheizen, Einstellen und Halten von t 8 = 10 s-120 s, und der Taupunkt T P8 der Atmosphäre A 8 im Bereich (-60 °C) bis (-5 °C) liegt; i. Beschichten, Glühen und Abkühlen des kaltgewalzten Stahlflachprodukts unter den Bedingungen: 1.Coating the steel flat product at a molten bath temperature T 9 in a reducing atmosphere A 9 with a coating bath consisting of: 0.10%-0.20% Al, saturated Fe, balance Zn and unavoidable impurities where T 9 = 450 °C-520 °C and the dew point T P8 of the atmosphere A 8 is in the range (-60 °C) to (+30 °C). 2. Annealing the coated steel flat product at T 9 * for a time t 9 * = 2 s-15 s, where T 9 *≥ T 9 and T 9 *≤ 580 °C. 3. Cooling the coated steel flat product to a temperature T 10 , where T 10 ≤ 60 °C. j. Skin-passing of the coated flat steel product with at least one forming pass and a total skin-pass ratio 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 T5), T6 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 conventionally. 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 same information already provided in connection with the composition of the flat steel product according to the invention applies to the composition of the slab according to the invention and the 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 because it increases the strength of the martensite that forms during the process. Therefore, the C content should be at least 0.10%, preferably 0.12%, particularly preferably 0.13%. On the other hand, with increasing C content, the martensite initiation temperature shifts to increasingly lower temperatures, so that potentially 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 concentration 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 must contain at least 1.7%, particularly preferably at least 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 ≥ 0.13%, 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 ≤ 0.3% 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 manganese sulfides, which severely impair formability. Therefore, in the steel according to the invention, the content is limited to 0.005%, although a restriction to 0.004% and especially to 0.003% may be advantageous. Sulfur contamination cannot be completely avoided during steel production.

[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 0.03%, has proven advantageous in the flat steel product according to the invention. On the other hand, an excessively high aluminum content can lead to the formation of coarse Al nitrides, which have an embrittling effect and thus to poorer formability. Furthermore, higher Al 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 0.1%, 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 (= 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 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 730 °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 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 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 can occur. This special dew point control prevents coating and adhesion problems of the coating.

[0038] The steel flat product is heated to a temperature T 2 . The steel flat product is then heated from a temperature T 2 to a temperature T 3 and pre-oxidized in an oxidizing atmosphere A 2 for t 2 = 1 s-30 s, where T 1 ≤ T 2 ≤ T 3 with T 3 = 750 °C-850 °C. This pre-oxidation results in a covering FeO layer of a defined thickness, preferably 50 nm-300 nm. In the subsequent process steps, this covering layer prevents or at least strongly inhibits the selective oxidation of the oxygen-affine alloying elements on the external steel surface. The minimum value of T 2 results from the fact that the pre-oxidation at T 2 <T 1 nicht ausreichend sicher eine im Wesentlichen deckendende FeO-Schicht, dicker ≥ 50 nm, erzeugt. Der Maximalwert von T 2 <T 3 ergibt sich daraus, dass die Voroxidation bei T 2 >T 3 may tend to produce a FeO layer > 300 nm, which may be deposited during the subsequent reduction according to step d.can only be insufficiently reduced back to metallic Fe. The exposure time during the 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 covering FeO layer thicker ≥ 50 nm. The maximum value of t 2 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 is produced, which can only be insufficiently reduced back to metallic Fe during the subsequent reduction according to step d. This pre-oxidation takes place in an atmosphere A 2 , which is set such that the conditions in this furnace zone always have an oxidizing effect on iron in order to achieve targeted pre-oxidation of the steel surface to form the most covering FeO layer possible of at least 50 nm, preferably 60 nm and a maximum of 300 nm, preferably 200 nm oxide layer thickness.Various methods are known for setting oxidizing atmospheres. For example, atmosphere A2 can comprise at least 0.5% oxygen "O2", preferably 0.6% O2, particularly preferably 0.8% O2 and a maximum of 5% O2, preferably 2% O2, the remainder being N2 with traces of H2O, and possibly technically unavoidable residues of H2, CO2, and CO. As an alternative to O2, moist N2 can also be blown into this furnace zone as an oxidizing medium. In this case, A2 has a dew point Tp2 of ≥ 0 °C - ≤ + 60 °C at an H2O / H2 ratio of ≥ 0.957 to ensure sufficient Fe oxidation.

[0039] In a particular embodiment, the heating rate υ 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.

[0040] Subsequently, in step d., the steel flat product is heated from a temperature T 3 to a temperature T 4 and soaked at a temperature T 4 in a reducing atmosphere A 4. The steel flat product is heated and soaked for at least t 4 ≥ 5 s, preferably 10 s, particularly preferably 15 s.

[0041] The temperature T 4 must not be below the maximum pre-oxidation temperature T 3 . The minimum value of t 4 results from the fact that the pre-oxidized steel surface is not sufficiently reduced back to metallic Fe at an exposure time of < 5 s compared to the reduction conditions according to the invention.

[0042] 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.

[0043] For the temperature T 4 in this step, T 3 ≤ T 4 ≤ 950 °C and T 4 ≥ A c3 -30 °C, in particular T 4 ≥ A c3 .

[0044] 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 c 3 = 902 − 225 * % C + 19 * % Si − 11 * % Mn − 5 * % Cr + 13 * % Mo − 20 * % Ni + 55 * % V ° C with %C = respective C content, %Si = respective Si content, %Mn = respective Mn content, %Cr = respective Cr content, %Mo = respective Mo content, %Ni = respective Ni content and %V = respective V content of the steel from which the blank is made.

[0045] This allows the pre-oxidized steel surface to be sufficiently reduced. In a special embodiment, the oxide layer formed in step c, in particular the FeO layer, is completely reduced to metallic iron.

[0046] 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.

[0047] In a special embodiment, the steel flat product can be thoroughly heated in step d. 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.

[0048] In a particular embodiment, heating from T 1 to T 4 can be carried out at a heating rate υ 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. Work step e.

[0049] The steel flat product is cooled to a temperature T 5 at a cooling rate of υ 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: TMS ° C = 539 ° C + − 423 % C − 30,4 % Mn − 7,5 % Si + 30 % Al ° C / wt %

[0050] In addition, the temperature T 5 must be ≤ 550 °C to avoid selective re-oxidation on the steel surface. The cooling rate υ 5 should be at least 10 °C / s, particularly preferably 20 °C / s. The cooling rate υ 5 should be limited to a maximum of 100 °C / s, preferably 50 °C / s, particularly preferably 30 °C / s. The minimum value of υ 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 υ 5 is limited by the fact that there is an excessively high risk of unwanted (selective) re-oxidation of the steel surface.

[0051] The preferably set atmosphere A 5 in work step e. 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, particularly with regard to iron. This avoids uncontrolled oxidation and re-oxidation on the surface. The H 2 content should preferably be limited to a maximum of 80%, preferably 50%, 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 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 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.

[0052] 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.

[0053] 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: TMS ° C = 539 ° C + − 423 % C − 30,4 % Mn − 7,5 % Si + 30 % Al ° C / wt % , where the element concentrations are to be used in weight percent.

[0054] 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.

[0055] The preferably set atmosphere A 6 in work step f. 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, particularly with regard to iron. This avoids uncontrolled oxidation and re-oxidation on the surface. 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 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 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.

[0056] 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.

[0057] In step g., the flat steel product is heated to a temperature T 7 at a heating rate of υ 7 = 4 °C / s-1000 °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.

[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 υ 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 υ 7 should be limited to a maximum of 100 °C / s, preferably 50 °C / s.

[0060] The preferably set atmosphere A7 in step g. 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 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. Nitrogen "N2", preferably 80%, particularly preferably 85%, especially preferably 90%, is added to the remainder of the preferred atmosphere.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 may occur.

[0061] In a particular embodiment, A 7 is equal to A 6 . Work step h.

[0062] 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.), adjusting the temperature to T 8, and holding it for t 8 = 10 s-600 s. The maximum temperature T 8 is 510 °C, particularly preferably 500 °C, since otherwise an undesirable decrease in the strength of the flat steel product would occur.

[0063] 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 .

[0064] 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.

[0065] The preferably set atmosphere A8 in work step h. 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 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.

[0066] In a particular embodiment, A 8 = A 7 , in particular A 8 = A 7 = A 6 .

[0067] In a particular embodiment, after t 8 has elapsed, the flat steel product is briefly brought to a temperature T 8 * in the atmosphere A 8 of T 8 , in particular for a period of less than 10 s. In this case, 460 °C < T 8 * ≤ 510 °C, preferably 460 °C < T 8 * ≤ 500 °C, applies. This can be advantageous for avoiding wetting or adhesion problems during the subsequent coating and for stabilizing the strip immersion temperature. Work step i.

[0068] In step i1, the flat steel product is coated in a coating bath at a melt bath temperature T9 in an atmosphere A9. The coating bath, consisting of 0.10%–0.20% Al, is saturated with Fe and the remainder Zn and unavoidable impurities. The coating bath contains at least 0.10%, preferably 0.12%, aluminum ("Al"), as this content allows a sufficient Al2O3 skin to form on the coating bath to inhibit Zn slag formation. The aluminum content should be a maximum of 0.20%, preferably 0.17%, as otherwise the desired Fe-Zn phase formation is inhibited during the subsequent heat treatment. The melt bath is saturated with iron ("Fe"). Coating takes place at a melt bath temperature T9 of at least 450°C, preferably 460°C, and a maximum of 520°C, preferably 500°C.The minimum value of T 9 results from the insufficient alloy formation between the steel surface and the coating bath at coating bath temperatures < 450 °C, preferably < 460 °C. The maximum value of T 9 is limited by the increased Fe dissolution into the coating bath, accompanied by increased slag formation at coating bath temperatures > 520 °C, preferably > 500 °C.

[0069] Coating takes place in a reducing atmosphere A 9 to avoid re-oxidation. Therefore, coating is preferably carried out in a closed nozzle construction to further prevent contact with the ambient air. To avoid coating defects due to slag formation on the coating bath level or due to precipitation of coating bath vapors, the nozzle is flooded with a purge gas atmosphere A 9. The purge gas atmosphere preferably comprises 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 possible addition of H 2 depends on the technically unavoidable proportion of residual O 2 in A 9, which should always be ≤10 ppm. In a special embodiment, the dew point of the atmosphere A 9 is at least -60 °C, preferably -40 °C and a maximum of +30 °C, preferably 0 °C.The minimum value of T P9 results from the fact that at low dew points, the evaporation of coating bath components is no longer sufficiently prevented, and the adjustment of such low dew points requires disproportionate technical effort. The maximum value of T P9 results from the fact that higher dew points promote both the formation of heavy slag and oxide layers 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 humidified N 2 or, alternatively, humidified N 2 -H 2 .

[0070] In the special embodiment in which the strip is raised to T 8 * in step h, the purge gas atmosphere is preferably preheated to a temperature of 500 °C–650 °C to support stabilization of the strip immersion temperature. The minimum value is justified by the fact that a purge gas temperature of < 500 °C only inadequately prevents the deposition of evaporated coating bath components. The maximum value is limited by the fact that a purge gas temperature of > 650 °C leads to disproportionately high technical expenditure. This process is particularly advantageous for demanding annealing and coating treatments for the steel alloy according to the invention.

[0071] After leaving the coating bath, excess zinc melt is wiped off the steel strip by spraying with air, N2, or a mixture of air and N2. A higher N2 content than that of air can be advantageous for avoiding defects in the coating. Subsequently, in step i2, the coated steel strip is held at a temperature T9*, where T9* ≥ T9 and T9* ≤ 580°C, for a duration of t9* of at least 2 s, preferably 3 s, and a maximum of 15 s, preferably 10 s. Inductive heat input is typically used. A temperature > 580°C should be avoided if possible to prevent unwanted carbide formation in the base material. During annealing of the coated flat steel product, the coating is alloyed to form Fe-Zn phases (FeZn 10 or δ phase, FeZn 13 or ζ phase, Fe 5 Zn 21 or Fe 3 Zn 10 or Γ / Γ1 phase).In a typical coating formation, the ζ phase can completely disappear in favor of the δ phase, resulting in a steel / coating interface consisting of the Γ / Γ1 phase. Due to the similarity in chemical and physical properties between Fe and Mn, Mn atoms from the steel alloy can substitute for Fe atoms in this phase formation. This alloy formation can also cause any residual oxides or spinels of the base alloying elements to detach from the steel surface and subsequently become present in the solidified coating. This deliberate alloying improves, among other things, the stone chip resistance and spot weldability of the coating, although this may result in an acceptable reduction in formability.

[0072] In step i3, the flat steel product is cooled to a temperature T 10 , where T 10 ≤ 60 °C. In a special embodiment, the cooling rate is υ 10 > 5 °C / s. The minimum value of υ 10 results from technical and economic considerations to avoid making the necessary 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 j.

[0073] 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 special embodiment, skin-passing can be performed with at least two forming passes. Skin-passing serves to improve flatness, fine-tune the mechanical properties by finally increasing the strength, and imprint a defined fine surface structure into the coating via the skin-pass roll structure.

[0074] 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 re-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.

[0075] 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, one furnace roller, preferably all furnace rollers, should be provided with a coating with a microhardness ≥ 750 HV0.3, preferably ≥ 900 HV0.3, and a roughness R a = 3.0 µm-8 µm so that these surface defects can be avoided.

[0076] In a particular embodiment, the atmosphere A4 and the dew point Tp4 in step d. can be adjusted such that H2O / H2 is <0.957, preferably H2O / H2 <0.90, particularly preferably H2O / H2 <0.80. This further reduces the thin oxide layer that forms in the atmosphere A4.

[0077] Carrying out the process according to the invention leads to the steel flat product according to the invention: High-strength, coated steel flat product, with a tensile strength R m = 900 MPa-1500 MPa, a yield strength R p02 ≥ 680 MPa and an elongation A 80 = 7%-25%, which comprises a steel consisting of the following elements: 0.10-0.5% C, 1.0-3.0% Mn, 0.9-1.7% Si, ≤ 0.020% P, ≤ 0.005% S, ≤ 0.010% N, and optionally one or more of the following elements: 0.01-1.5% Al, 0.05-1% Cr, 0.05-0.2% Mo, 0.0004-0.002% B, 0.05-0.2% Cu, 0,005 % ≤ Ti + Nb + V ≤ 0,2 % and the remainder being iron and unavoidable elements, wherein the steel flat product has 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 and it has a class 1 or 2 powdering test according to SEP1933.

[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 ≥ 0.13% 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] The ZF coating has a thickness of at least 5 µm and a maximum of 10 µm.

[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. The furnace inlet temperature was room temperature in all examples.

[0089] Furthermore, stochastic surface texturing was applied to samples F212 and B202 during the tempering step.

[0090] For samples D207 and F213, 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 B202-B206. In case B202, the atmosphere A2 was reducing. A coating with good powdering values could not be achieved. Surprisingly, a controlled setting of an oxidizing atmosphere A2 in example B203 led to significantly improved coating properties. This is reflected in a powdering value of class 1. Test B204 did show good surface properties, but due to a T4 temperature < Ac3-30 °C and T4 < T3, the carbon cannot be distributed homogeneously in the austenite structure, resulting in an excessively low proportion of tempered martensite. In example B205, on the other hand, the inventive structure is achieved, but the example has poor surface properties, such as the powdering value.These poor surface properties are due to the dew point T p8, which is not in accordance with the invention. Poor coating properties were also observed in the process according to Example B206. This is due to an excessively long time t 2 , which leads to an increased oxidation layer that cannot be sufficiently reduced in the subsequent steps.

[0092] Examples D207, D208, F212, and F213 are prepared according to the inventive process and exhibit good surface properties in the coating, i.e., the inventive powdering value. In contrast, in Example D209, the inventive powdering value could not be achieved because the atmosphere A2 was reducing. In Example D210, however, the non-inventive dew point T P8 and the elevated T 8 temperature lead to the selective oxidation of base alloy elements, resulting in poor surface properties and a high ferrite content in the microstructure.

[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 proportion of residual austenite and a high proportion of tempered martensite. Therefore, examples A201, E210, and E211 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 C205 and C206 therefore have a non-inventive proportion of tempered martensite. C206 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 Tl V B Tl+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. J1 T1 T P1 T 2 t 2 A 2 T 3 t 4 T 4 T P4 T P5 T 5 J 5 T P6 T 6 t 6 A201 5 675 -20 710 12 Ox. 815 130 890 -25 -35 380 31 -20 380 8 B202 4 700 -20 730 16 Ed. 825 160 895 -30 -15 335 34 -25 335 12 B203 4 730 -20 770 17 Ox. 840 95 905 -40 -15 325 36 -25 325 10 B204 6 670 -20 675 15 Ox. 800 85 795 -35 -20 340 31 -20 340 11 C205 7 650 -20 700 16 Ox. 810 110 855 -35 -20 330 30 -5 330 20 C206 5 650 -20 700 17 Ox. 810 170 850 -35 -30 390 31 -10 390 9 D207 4 680 -20 760 18 Ox. 830 65 905 -45 -10 315 35 -35 315 10 D208 6 680 -20 760 12 Ox. 830 65 895 -45 -20 335 32 -60 335 13 D209 6 695 -20 760 16 Ed. 830 180 895 -45 -15 295 37 -60 295 8 E210 8 650 -20 735 14 Ox. 825 80 850 -30 -35 395 32 -55 395 12 E211 4 660 -20 775 20 Ox. 840 80 860 -30 -30 375 39 -25 375 8 F212 5 705 -20 815 12 Ox. 850 45 905 -40 -15 290 44 -50 290 7 F213 4 705 -20 810 12 Ox. 850 15 895 -40 -15 320 37 -45 320 13 B205 5 675 -20 710 16 Ox. 815 130 895 -25 -20 335 31 5 335 18 B206 5 675 -20 715 45 Ox. 815 90 890 -25 -35 320 31 -20 320 17 D210 6 680 -20 760 20 Ox. 830 65 895 -45 -20 335 32 5 335 15 Table 3 Nr. J 7 T 7 T P7 t 8 T 8 T P8 T 9 T 9 * T 10 D A201 6 455 -20 110 465 -20 460 480 55 0,1 B202 9 450 -25 60 467 -25 462 480 58 0,3 B203 7 450 -25 100 468 -25 461 480 52 0,2 B204 13 457 -20 80 472 -20 465 480 40 0,5 C205 20 460 5 120 479 -5 461 480 40 0,2 C206 17 455 -20 110 465 -10 463 485 58 0,15 D207 16 448 -5 80 469 -35 461 480 54 0,2 D208 18 460 -10 10 471 -60 460 490 50 0,3 D209 80 451 -35 50 483 -60 465 480 45 0,5 E210 19 455 -60 50 465 -55 463 490 54 0,6 E211 21 450 -60 80 454 -25 462 490 53 0,95 F212 97 445 5 30 472 -50 466 490 56 0,45 F213 14 450 -55 30 467 -45 460 490 53 0,1 B205 15 455 -25 110 505 5 460 485 51 0,4 B206 18 455 -50 110 505 -20 460 485 49 0,3 D210 14 460 -45 5 520 5 460 485 50 0,25 Table 4 Nr. Bainite + Martensite Proportion of tempered martensite retained austenite ferrite Rp0.2 Rm A80 Powdering A201 77 40% 3 20 590 886 17 4 B202 84 88% 12 4 883 1205 16 4 B203 88 91% 11 1 942 1185 14 1 B204 84 50% 8 8 715 1243 10 5 C205 94 56% 6 0 767 1089 10 4 C206 82 31% 3 15 679 1108 13 4 D207 88 80% 10 2 897 1060 16 1 D208 90 76% 9 1 854 1027 13 2 D209 81 81% 14 5 848 1091 17 4 E210 82 47% 3 15 675 1231 9 4 E211 93 54% 3 4 869 1213 6 4 F212 84 89% 16 0 1264 1493 17 1 F213 78 93% 17 5 1149 1469 22 1 B205 80 88% 15 5 894 1198 16 4 B206 88 97% 12 0 937 1190 15 5 D210 86 78% 2 12 856 997 8 4

Claims

1. A method for producing a high-strength, coated flat steel product, comprising at least the following steps: a. Providing a cold-rolled flat steel product which comprises a steel consisting of the following elements: 0.10-0.5% C, 1.0-3.0% Mn, 0.9-1.7% Si, ≤ 0.020% P, ≤ 0.005% S, ≤ 0.010% N, and optionally one or more of the following elements: 0.01-1.5% Al, 0.05-1% Cr, 0.05-0.2% Mo, 0.0004-0.002% B, 0.05-0.2% Cu, 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 and pre-oxidizing the cold-rolled flat steel product under the following conditions:

1. heating the cold-rolled flat steel product from T0 in a reducing atmosphere A1 to a temperature T1, where T1 = 650 °C-750 °C and the dew point T P1the atmosphere A1 is in the range (-60 °C) to (-5 °C); 2. Heating the cold-rolled flat steel product to a temperature T2 and then pre-oxidizing and heating the cold-rolled flat steel 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; d. Heating the cold-rolled flat steel product from a temperature T3 to a temperature T4 and soaking at a temperature T4 for t4 = 5 s-300 s, where T3 ≤ T4 ≤ 950 °C and T4 ≥ A c3 -30 °C, the heating and soaking takes place in an atmosphere A4 and the dew point T P4 the 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 a reducing atmosphere A5 to a temperature T 5, where T5 = (T MS + 40 °C)-(T MS -175 °C) and T5 ≤ 550 °C and the dew point T P5the 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 a reducing 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 = 4 °C / s-1000 °C / s in a reducing atmosphere A7 to a temperature T7, where T MS < T7 ≤ 510 °C and the dew point T P7 the 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 in a reducing atmosphere A8 for a total time for reheating, setting and holding of t8 = 10 s-600 s, and the dew point T P8the atmosphere A8 is in the range (-60 °C) to (-5 °C); i. Coating, annealing and cooling of the cold-rolled flat steel product under the following conditions:

1. Coating of the flat steel product at a molten bath temperature T9 in a reducing atmosphere A9 with a coating bath consisting of 0.10%-0.20% Al, Fe-saturated, balance Zn and unavoidable impurities where T9 = 450 °C-520 °C and the dew point T P8 the atmosphere A8 is in the range (-60 °C) to (+30 °C); 2. Annealing the coated flat steel product at T9* for a time t9* = 2 s-15 s, where T9* ≥ T9 and T9* ≤ 580 °C; 3. Cooling the coated flat steel product to a temperature T 10 , where T 10 ≤ 60 °C; j. Skin-passing of the coated flat steel product with at least one forming pass and a total skin-pass ratio D = 0.1%-0.8%; 2. Method according to claim 1, characterized in that for the atmosphere A2 in step c2. H2O / H2 ≥ 0.957 applies and the dew point TP2 in the range (0 °C) to (+60 °C).

3. Method according to one of the preceding claims, characterized in that In step c. between 500 °C and T1 the heating rate υ1 = 2 °C / s-50 °C / s.

4. Method according to one of the preceding claims, characterized in that In step d. the heating is carried out by holding at a constant temperature T4.

5. Method according to one of the preceding claims, 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.

6. Method according to one of the preceding claims, characterized in that for the atmosphere A4 of H2O / H2 < 0.957 applies.

7. Method according to one of the preceding claims, characterized in that the flat steel product is moved through a furnace in steps b.-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.

8. Method according to one of the preceding claims, characterized in that during the tempering step j. a stochastic surface texturing is applied.

9. High-strength coated flat steel product, with a tensile strength R m = 900 MPa-1500 MPa, a yield strength R p02 ≥ 680 MPA and an elongation A 80 = 7%-25%, which comprises a steel consisting of the following elements: 0.10-0.5% C, 1.0-3.0% Mn, 0.9-1.7% Si, ≤ 0.020% P, ≤ 0.005% S, ≤ 0.010% N, and optionally one or more of the following elements: 0.01-1.5% Al, 0.05-1% Cr, 0.05-0.2% Mo, 0.0004-0.002% B, 0.05-0.2% Cu, 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 thatit has a class 1 or 2 rating in the powdering test according to SEP1933.

10. Component for structural lightweight construction in automotive engineering formed from a flat steel product according to claim 9.

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