EP4592406A1 - Fire-coated high strength steel with good surface and deformation properties with zinc-based coating - Google Patents

Abstract

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

Description

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.

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.

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.

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%."

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.

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.

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

In the present application, the same atmosphere (ie: A _i = A _j ) 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.

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.

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

In the EP 3 856 936 A1 Better 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. Nevertheless, oxide formation occurs in the remaining process steps, leading to surface defects and a subsequent incomplete coating, as well as poor adhesion in individual areas.

Therefore, it is an object of the present invention to provide a method which leads to a well-adhering, zinc-based hot-dip coating on a high-strength flat steel product and preferably has few uncoated areas.

The 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 selectively removed in a later step by re-oxidation. However, as long as the surface is protected by the generated Fe oxide layer, no selective oxidation of the elements silicon, manganese, and chromium can take place. Therefore, among other things, no oxides form on the surface, which could lead to surface defects and a later incomplete coating and poor adhesion in individual areas. In addition, special Atmospheric and temperature conditions are adjusted shortly before and during the coating process to further minimize errors.

1. a. Bereitstellen eines kaltgewalzten Stahlflachprodukt, welcher einen Stahl umfasst, der aus den folgenden Elemente besteht:
   • C: 0,10 - 0,5 %;
   • Mn: 1,0 - 3,0 %;
   • Si: 0,9-1,7%;
   • P: ≤ 0,020 %;
   • S: ≤ 0,005 %;
   • N: ≤ 0,010 %;
   • sowie optional eines oder mehrere der folgenden Elemente
   • AI: 0,01 - 1,5 %;
   • Cr: 0,05 - 1 %;
   • Mo: 0,05 - 0,2 %;
   • B: 0,0004 - 0,002 %;
   • Cu: 0,05 - 0,2 %; $$\mathrm{0,005}\%\le \mathrm{Ti}+\mathrm{Nb}+\mathrm{V}\le \mathrm{0,2}\%;$$ und als Rest aus Eisen und unvermeidbaren Elementen.
2. b. Aufheizen des kaltgewalzten Stahlflachprodukts auf eine Ofeneintrittstemperatur
3. c. Aufheizen und Voroxidieren des kaltgewalzten Stahlflachprodukts unter den Bedingungen:
   1. 1. Aufheizen des kaltgewalzten Stahlflachprodukts von T₀ in einer reduzierenden Atmosphäre A₁ auf eine Temperatur T₁, wobei T₁= 650 °C - 750 °C und der Taupunkt T_P1 der Atmosphäre A₁ im Bereich (-60 °C) bis (-5 °C) liegt.
   2. 2. Aufheizen des kaltgewalzten Stahlflachprodukts auf eine Temperatur T₂ und dann Voroxidieren und Aufheizen des kaltgewalzten Stahlflachprodukts von T₂ auf eine Temperatur T₃ in einer oxidierenden Atmosphäre A₂ für t₂= 1 s - 30 s, wobei T₁ ≤ T₂ ≤ T₃ mit T₃ = 750 °C - 850 °C.
4. d. Aufheizen des kaltgewalzten Stahlflachprodukts von einer Temperatur T₃ auf eine Temperatur T₄ und Durchwärmen bei einer Temperatur T₄ für t₄ = 5 s - 300 s, wobei T₃ ≤ T₄ ≤ 950 °C und T₄ ≥ A_c3 - 30 °C, das Aufheizen und Durchwärmen in einer Atmosphäre A₄ erfolgt und der Taupunkt T_P4 der Atmosphäre A₄ im Bereich (-60 °C) bis (-0 °C) liegt.
5. e. Abkühlen des kaltgewalzten Stahlflachprodukts mit einer Abkühlrate ϑ₅ = 10 °C/s - 100 °C/s in einer reduzierenden Atmosphäre A₅ auf eine Temperatur T₅, wobei T₅ = (T_MS + 40 °C) - (T_MS - 175 °C) und T₅ ≤ 550 °C und der Taupunkt T_P5 der Atmosphäre A₅ im Bereich (-60 °C) bis (-0 °C) liegt.
6. f. Einstellen und Halten des kaltgewalzten Stahlflachprodukts auf einer Temperatur T₆ in einer reduzierenden Atmosphäre A₆ für eine Haltezeit t₆ = 1 s - 60 s, wobei T₆ = T_MS - (T_MS - 175 °C), und der Taupunkt T_P6 der Atmosphäre A₆ im Bereich (-60 °C) bis (-5 °C) liegt.
7. g. Wiederaufheizen des kaltgewalzten Stahlflachprodukts mit einer Aufheizrate ϑ₇ = 4 °C/s - 1000 °C/s in einer reduzierenden Atmosphäre A₇ auf eine Temperatur T₇, wobei T_MS < T₇ ≤ 510 °C und der Taupunkt T_P7 der Atmosphäre A₇ im Bereich (-60 °C) bis (-5 °C) liegt.
8. h. Einstellen und Halten des kaltgewalzten Stahlflachprodukts auf einer Temperatur T₈ in einer reduzierenden Atmosphäre A₈ für eine Gesamtzeit für das Wiederaufheizen (Schritt g), Einstellen und Halten von t₈ = 10 s - 600 s, und der Taupunkt T_P8 der Atmosphäre A₈ im Bereich (-60 °C) bis (-5 °C) liegt.
9. i. Beschichten und Abkühlen des kaltgewalzten Stahlflachprodukts unter den Bedingungen:
   1. 1. Beschichten des kaltgewalzten Stahlfachprodukts bei einer Schmelzbadtemperatur T₉ in einer reduzierenden Atmosphäre A₉ mit einem Beschichtungsbad bestehend aus
      • AI: 0,15 % -2,0 %;
      • Fe-gesättigt
      • Rest Zn und unvermeidbare Verunreinigungen
      • optional 0,25 % - 8,0 % Mg
      • wobei T₉ = 450 °C - 520 °C
   2. 2. Abkühlen des beschichteten Stahlflachprodukts auf eine Temperatur T₁₀, wobei T₁₀ ≤ 60 °C.
10. j. Dressieren des beschichteten Stahlflachprodukts mit mindestens einem Umformstich und einem Gesamtdressiergrad D = 0,1 % - 0,8 %.

The manufacturing process according to the invention comprises at least the following steps:

1. 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
   • AI: 0.01 - 1.5%;
   • Cr: 0.05 - 1%;
   • Mo: 0.05 - 0.2%;
   • B: 0.0004 - 0.002%;
   • Cu: 0.05 - 0.2%; $$\mathrm{0,005}\%\le \mathrm{Ti}+\mathrm{Nb}+\mathrm{V}\le \mathrm{0,2}\%;$$ and the remainder consists of iron and unavoidable elements.
2. b. Heating the cold-rolled flat steel product to a furnace inlet temperature
3. c. Heating and pre-oxidation of the cold-rolled flat steel product under the following conditions:
   1. 1. Heating the cold-rolled flat steel product from T ₀ in a reducing atmosphere A ₁ to a temperature T ₁ , where T ₁ = 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. 2. Heating the cold-rolled flat steel product to a temperature T ₂ and then pre-oxidizing and heating the cold-rolled flat steel product from T ₂ to a temperature T ₃ 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.
4. 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).
5. e. Cooling the cold-rolled flat steel product at a cooling rate ϑ 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).
6. 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).
7. g. Reheating the cold-rolled flat steel product at a heating rate ϑ 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 and the dew point T _P7 of the atmosphere A ₇ is in the range (-60 °C) to (-5 °C).
8. h. Setting and holding the cold-rolled flat steel product at a temperature T ₈ in a reducing atmosphere A ₈ for a total reheating time (step g), setting and holding t ₈ = 10 s - 600 s, and the dew point T _P8 of the atmosphere A ₈ is in the range (-60 °C) to (-5 °C).
9. i. Coating and cooling the cold-rolled flat steel product under the following conditions:
   1. 1. Coating the cold-rolled steel product at a melt bath temperature T ₉ in a reducing atmosphere A ₉ with a coating bath consisting of
      • AI: 0.15% -2.0%;
      • Fe-saturated
      • Rest Zn and unavoidable impurities
      • optionally 0.25% - 8.0% Mg
      • where T ₉ = 450 °C - 520 °C
   2. 2. Cooling the coated flat steel product to a temperature T ₁₀ , where T ₁₀ ≤ 60 °C.
10. j. Skin-passing of the coated flat steel product with at least one forming pass and a total skin-pass degree D = 0.1% - 0.8%.

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

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.

The individual work steps are described in detail below:

Work step a)

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.

Carbon "C" is present in the steel according to the invention in amounts ranging from 0.10% to 0.5%. Carbon supports the formation and stabilization of austenite in the steel according to the invention. In particular, stabilization occurs during quenching and the subsequent annealing treatment. Furthermore, the addition of C imparts high strength to the steel by increasing 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%.

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%.

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%.

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%.

The steel according to the invention contains manganese "Mn." With a content of 1.0% or more, Mn enables martensite formation because pearlite formation is suppressed. A content of at least 1.2% has proven advantageous, and a content of at least 1.5% has proven particularly advantageous. However, too high a 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 especially 2.3%, has proven particularly advantageous.

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.

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.

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.

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%.

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. It has 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 aluminum nitrides, which have an embrittling effect and thus to poorer formability. Furthermore, higher aluminum contents can lead to poorer casting behavior, since 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%.

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%.

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.

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%.

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.

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 The formation of very finely dispersed carbides contributes to increased strength. A minimum total MLE content of 0.005% 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 retained austenite. Therefore, the total MLE concentration should be limited to a maximum of 0.2%.

Work step b)

The cold-rolled flat steel product is heated to a furnace inlet temperature T ₀ . The furnace inlet temperature is the temperature at the center of the steel product upon 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)

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).

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, as otherwise selective oxidation of base alloy elements can occur. This special dew point control prevents coating and adhesion problems.

The flat steel product is heated to a temperature T ₂ . The flat steel 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 creates a covering FeO layer of a defined thickness, preferably 50 nm - 300 nm. In later 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 pre-oxidation at T ₂ < T ₁ does not sufficiently reliably produce a substantially covering FeO layer ≥ 50 nm thick. The maximum value of T ₂ < T ₃ results from the fact that the pre-oxidation at T ₂ > T ₃ can tend to produce an FeO layer > 300 nm, which can only be inadequately reduced back to metallic Fe during the subsequent reduction according to step d). 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 ≥ 50 nm thick. 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 inadequately 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 adjusted so 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, with an oxide layer thickness of at least 50 nm, preferably 60 nm and a maximum of 300 nm, preferably 200 nm. Various ways of adjusting oxidizing atmospheres are known. For example, the atmosphere A ₂ can comprise at least 0.5% oxygen "O ₂ ", preferably 0.6% O ₂ , particularly preferably 0.8% O ₂ and a maximum of 5% O ₂ , preferably 2% O ₂ , the remainder N ₂ with traces of H ₂ O, and possibly technically unavoidable residues of H ₂ and CO ₂ and CO. As an alternative to O ₂ , moist N ₂ can also be blown into this furnace zone as an oxidative medium, whereby in this case A ₂ has a dew point T _P2 of ≥ 0 °C - ≤ +60 °C at an H ₂ O/H ₂ ratio of ≥ 0.957 to ensure sufficient Fe oxidation.

In a particular embodiment, the heating rate ϑ ₁ in step c) between 500 °C and T ₁ 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)

Subsequently, in step d), the flat steel 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 flat steel product is heated and soaked for at least t ₄ ≥ 5 s, preferably 10 s, particularly preferably 15 s.

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.

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.

For the temperature T ₄ in this step $${\mathrm{T}}_3\le {\mathrm{T}}_4\le 950°\mathrm{C}\mathrm{und}{\mathrm{T}}_4\ge {\mathrm{A}}_{\mathrm{c3}}-30°\mathrm{C},\mathrm{insbesondere}{\mathrm{T}}_4\ge {\mathrm{A}}_{\mathrm{c3}}.$$

The minimum temperature A _c3 to be exceeded is determined according to the HOUGARDY, HP. in Materials Science of Steel, Volume 1: Fundamentals, Verlag Stahleisen GmbH, Düsseldorf, 1984, p. 229 .,given formula $${\mathrm{A}}_{\mathrm{c3}}=\left(902-225*\%\mathrm{C}+19*\%\mathrm{Si}-11*\%\mathrm{Mn}-5*\%\mathrm{Cr}+13*\%\mathrm{Mo}-20*\%\mathrm{Ni}+55*\%\mathrm{V}\right)°\mathrm{C}$$ 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.

The atmosphere _A4 set according to the invention is adjusted to ensure the targeted reduction of the previously formed FeO layer back to metallic iron. This allows the pre-oxidized steel surface to be sufficiently reduced. In a particular embodiment, the oxide layer formed in step c), in particular the FeO layer, is completely reduced to metallic iron.

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.

In a special embodiment, the steel flat product can be thoroughly heated in step d) by maintaining it at a constant temperature _T4 . 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 _T4 temperature was set for analytical reasons.

Work step e)

The steel flat 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: $${\mathrm{T}}_{\mathrm{MS}}\left(°\mathrm{C}\right)=539°\mathrm{C}+\left(-423\left[\%\mathrm{C}\right]-\mathrm{30,4}\left[\%\mathrm{Mn}\right]-\mathrm{7,5}\left[\%\mathrm{Si}\right]+30\left[\%\mathrm{Al}\right]\right)°\mathrm{C}/\mathrm{wt}\%$$

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 ϑ 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 ϑ ₅ is limited by the fact that there is an excessively high risk of unwanted (selective) re-oxidation of the steel surface.

The preferably set atmosphere A ₅ in 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 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 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.

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)

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 initiation temperature, which can be estimated using the following equation: $${\mathrm{T}}_{\mathrm{MS}}\left(°\mathrm{C}\right)=539°\mathrm{C}+(-423\left[\%\mathrm{C}\right]-\mathrm{30,4}\left[\%\mathrm{Mn}\right]-\mathrm{7,5}\left[\%\mathrm{Si}\right]+30\left[\%\mathrm{Al}\right]°\mathrm{C}/\mathrm{wt}\%,$$ where the element concentrations are to be used in weight percent.

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.

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, especially 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 to prevent the selective oxidation of base alloy elements. to minimize. Nitrogen "N ₂ ", 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 _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 can occur.

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)

In step g), the flat steel product is heated to a temperature T ₇ at a heating rate ϑ 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.

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.

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.

The preferably adjusted atmosphere A ₇ in step g) comprises at least 2% hydrogen "H ₂ ", preferably 3% H ₂ , particularly preferably 5% H ₂ . The adjusted hydrogen content ensures that the atmosphere is reducing, especially with regard to iron. This prevents uncontrolled oxidation and reoxidation to the surface can be avoided. 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 _H2 , _O2 , _H2O and _N2 . 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. In a particular embodiment, A ₇ is equal to A ₆ .

Work step h)

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 for t ₈ = 10 s - 600 s. The maximum temperature T ₈ is 510 °C, otherwise an undesirable decrease in the strength of the flat steel product would occur. In a special embodiment, T ₈ is a maximum of 500 °C to avoid wetting or adhesion problems during the subsequent coating process.

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 .

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 600 s, preferably 120 s, and particularly preferably 100 s, since otherwise an undesirably high carbide content would form in the basic structure.

The preferably adjusted atmosphere A ₈ in step h) comprises at least 2% hydrogen "H ₂ ", preferably 3% H ₂ , particularly preferably 5% H ₂ . The adjusted hydrogen content can ensure that the atmosphere is reducing, in particular with regard to Iron. This prevents uncontrolled oxidation and re-oxidation on the surface can be avoided. 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 alloying elements. Nitrogen "N ₂ ", 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 T _P8 is at least -60 °C, preferably -40 °C, since otherwise zinc dust will deposit on the surface. 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. In a particular embodiment, A ₈ = A ₇ , in particular A ₈ = A ₇ = A ₆ .

Work step i)

In step i)1, the flat steel product is coated in a coating bath at a molten bath temperature T ₉ in a reducing atmosphere A _9. The coating bath consists of 0.15% - 2.0% Al, is saturated with Fe, and the remainder is Zn, along with unavoidable impurities. The coating bath contains at least 0.15%, preferably 0.17%, aluminum ("Al"), because otherwise the formation of brittle Fe-Zn phases at the steel/coating interface cannot be sufficiently prevented, or only an insufficiently developed Fe2Al5 interface is formed. The aluminum content should be a maximum of 2.0%, preferably 1.8%, particularly preferably 0.94%, particularly preferably 0.24%, because otherwise the weldability of the resulting coating would be negatively affected. The molten bath is saturated with iron ("Fe"). Optionally, the coating bath can contain magnesium "Mg" with at least 0.25%, preferably 0.28% and a maximum of 8.0%, preferably 2.0%. In a preferred embodiment, Al content ≤ Mg content. Coating is carried out at a melt bath temperature T ₉ 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 Fe2Al5 boundary layer formation at coating bath temperatures < 450 °C. The maximum value of T ₉ is limited by the increased Fe dissolution in the coating bath accompanied by increased slag formation at coating bath temperatures > 520 °C. Furthermore, the associated Heating of the steel strip to a temperature > T ₈ should be avoided as far as possible in order to avoid unwanted carbide formation in the base material.

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. In a special embodiment, the atmosphere A ₉ can comprise 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 +50 °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 .

After leaving the coating bath, excess zinc melt is stripped from the steel strip by spraying with air, _N2 , or a mixture of air and _N2 . A higher _N2 content than air can be advantageous for preventing defects in the coating. The inventive execution of the annealing and coating steps results in a Zn-phase-based coating as a well-adhering layer, despite the high alloying proportions of base elements in the steel composition. This coating is bonded to the predominantly reduced steel surface via a predominantly covering Fe2Al5 boundary layer. Furthermore, the inventive process prevents coating defects and adhesion problems. Likewise, networks of internal oxides of the base alloy elements are avoided, so that even the grain layers close to the surface possess sufficient cohesion with the base material, which also prevents adhesion problems. In a special embodiment, the annealing treatment can be adjusted in relation to the multi-phase base material, in particular by changing reductive / oxidative / reductive furnace zone atmospheres, that at least one first grain layer on the steel surface is at least partially ferritized, i.e., > 50% ferrite is present. This has also proven to be an advantage, since this near-surface edge layer is capable of absorbing crack growth during or after forming into the component more ductilely than the base material structure.

In step i)2, the flat steel product is cooled to a temperature T ₁₀ , where T ₁₀ ≤ 60 °C. In a special embodiment, the cooling rate is ϑ ₁₀ > 5 °C/s. The minimum value of u 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)

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 finally increasing the strength, and to imprint a defined fine surface structure into the coating 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.

The minimum value of D is determined by the fact that at 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-passing. For economic and logistical reasons, skin-passing 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 carried out in a subsequent process on a stand-alone skin-pass mill or in a combination of in-line and offline skin-passing, especially 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 a stochastic surface texturing during skin passing in order to optimize the friction behavior between the steel surface and the tool in the oiled or greased state during forming into the component. Under the high compressive load due to the high forming forces necessary for high-strength steels, a stochastic surface structure has the advantage that, at 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 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 leveling and adhesion properties for organic or metallic coatings, which can be additionally applied to the flat steel product according to the invention if necessary.

In a particular embodiment, in the process according to the invention, the flat steel product is moved through a furnace by means of furnace rollers in steps b) - h), 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 µm . The microhardness is determined according to DIN EN ISO 6507. In the annealing process according to the invention, interactions between the various metallic and oxidic 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, should be provided with a coating with a microhardness ≥ 750 HV0.3, preferably ≥ 900HV0.3, and a roughness R _a = 3.0 µm - 8 µm , so that these surface defects can be avoided.

In a particular embodiment, the atmosphere A ₄ and the dew point T _P4 in step d) can be adjusted such 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 will further reduce the thin oxide layer that forms in the atmosphere A ₄ .

• C: 0,10 - 0,5 %;
• Mn: 1,0 - 3,0 %;
• Si: 0,9 -1,7 %;
• P: ≤ 0,020 %;
• S: ≤ 0,005 %;
• N: ≤ 0,010 %;
• sowie optional eines oder mehrere der folgenden Elemente
• AI: 0,01-1,5 %;
• Cr: 0,05 - 1 %;
• Mo: 0,05 - 0,2 %;
• B: 0,0004 - 0,002 %;
• Cu: 0,05 - 0,2 %; $$\mathrm{0,005}\%\le \mathrm{Ti}+\mathrm{Nb}+\mathrm{V}\le \mathrm{0,2}\%;$$ und als Rest aus Eisen und unvermeidbaren Elementen, wobei das Stahlflachprodukt ein Gefüge aufweist, das aus ≥ 80 % Bainit und/oder Martensit, wovon mindestens 75 % des Martensits angelassen ist, ≥ 5 % Restaustenit und ≤ 10 % Ferrit besteht und das Haftungsergebnis im Kugelschlagtest nach SEP 1931 der Klasse 1 oder 2, vorzugsweise Klasse 1, vorliegt.

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 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%;
• Mo: 0.05 - 0.2%;
• B: 0.0004 - 0.002%;
• Cu: 0.05 - 0.2%; $$\mathrm{0,005}\%\le \mathrm{Ti}+\mathrm{Nb}+\mathrm{V}\le \mathrm{0,2}\%;$$ 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% residual austenite and ≤ 10% ferrite and the adhesion result in the ball impact test according to SEP 1931 is Class 1 or 2, preferably Class 1.

In a particular embodiment, the coating layer is a continuous layer. A coating layer is considered continuous if ≥ 95%, preferably ≥ 98%, particularly preferably ≥ 99%, of the flat steel product surface is covered with the coating. This can be verified using scanning electron microscopy.

The surface according to the invention preferably has a roughness of R _a = 0.5 µm - 1.8 µm , in particular R _a = 0.7 µm - 1.8 µm and a peak count R _PC ≥ 40 cm ^-1 , in particular R _PC ≥ 50 cm ^-1 . A roughness R _a of both < 0.5 µm and > 1.8 µm should be avoided, since such These values can lead to adverse friction behavior during subsequent forming into the component. R _Pc should preferably not be < 40 cm ^-1 to ensure sufficiently good optical properties even after painting.

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%.

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%.

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%.

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

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.

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.

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.

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.

In a particular embodiment, the coating is zinc-based, in particular it has a layer thickness of at least 5 µm and a maximum of 25 µm.

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.

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.

To determine whether a layer was closed, scanning electron microscopy (SEM) was performed at a voltage of 25 keV on an image section measuring 300 µm × 300 µm. A 1 µm × 1 µm grid was applied to the surface. All grid fields where coverage was not complete were counted, and a percentage of how closed the layer was was calculated ([total number of grid fields - number of grid fields not covered] / total number of grid fields).

Furthermore, stochastic surface texturing was applied to samples F12 and B2 during the tempering step.

For samples D7 and F13, a furnace roller was coated with a coating with a microhardness of 8001 HV0.3 and a roughness of 4 µm.

Steel alloys B and DF have the steel composition according to the invention. Steel alloy B was tested under different manufacturing parameters B2 - B6. In case B2, atmosphere _A2 was reducing. Although a continuous coating layer was achieved, poor adhesion results were observed with a Class 3 value. The top grain layer of the flat steel product was not ferritized. Surprisingly, a controlled setting of an oxidizing atmosphere _A2 in example B3 led to significantly improved coating properties. These show only a Class 2 value for a continuous coating layer and adhesion in the ball impact test. Furthermore, the surface displays low roughness (see _Ra and _RPC values). Test B4 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 B5, however, the inventive structure is achieved, but the example exhibits poor surface properties, such as coating quality, adhesion, and roughness. These poor surface properties are due to the non-inventive dew point T _p8 . Poor coating and adhesion properties were also observed in the process according to Example B6. 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.

Examples D7, D8, F12, and F13 were prepared according to the process according to the invention and exhibited good surface properties in terms of coating, adhesion, and roughness. Furthermore, the top grain layer of the flat steel product was ferritized. In contrast, in Example D9, good adhesion could not be achieved because the atmosphere _A2 was reducing. Furthermore, D9 exhibits a roughness that does not conform to the invention. In Example D10, however, the dew point T _P8 and the elevated T ₈ temperature, which are not conform to the invention, lead to the selective oxidation of base alloying elements, resulting in poor surface properties and a high ferrite content in the microstructure. The top grain layer of the flat steel product was not ferritized.

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 A1, E10, and E11 are not in accordance with the invention.

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 C5 and C6 therefore have a non-inventive proportion of tempered martensite. C6 also has a non-inventive proportion of ferrite and retained austenite. Table 1 No. C Si Mn P S Al Cr Cu Nb Mon N Tl 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 No. ϑ ₁ T1 T _{P1 T2 t2 A2 T3 t4 T4} T _P4 T _{P5 T5} ϑ ₅ T _{P6 T6 t6} A1 5 675 -20 710 12 Ox. 815 130 890 -25 -35 380 31 -20 380 8 B2 4 700 -20 730 16 Ed. 825 160 895 -30 -15 335 34 -25 335 12 B3 4 730 -20 770 17 Ox. 840 95 905 -40 -15 325 36 -25 325 10 B4 6 670 -20 675 15 Ox. 800 85 795 -35 -20 340 31 -20 340 11 B5 5 675 -20 710 16 Ox. 815 130 895 -25 -20 335 31 5 335 18 B6 5 675 -20 715 45 Ox. 815 90 890 -25 -35 320 31 -20 320 17 C5 7 650 -20 700 16 Ox. 810 110 855 -35 -20 330 30 -5 330 20 C6 5 650 -20 700 17 Ox. 810 170 850 -35 -30 390 31 -10 390 9 D7 4 680 -20 760 18 Ox. 830 65 905 -45 -10 315 35 -35 315 10 D8 6 680 -20 760 12 Ox. 830 65 895 -45 -20 335 32 -60 335 13 D9 6 695 -20 760 16 Ed. 830 180 895 -45 -15 295 37 -60 295 8 D10 6 680 -20 760 20 Ox. 830 65 895 -45 -20 335 32 5 335 15 E10 8 650 -20 735 14 Ox. 825 80 850 -30 -35 395 32 -55 395 12 E11 4 660 -20 775 20 Ox. 840 80 860 -30 -30 375 39 -25 375 8 F12 5 705 -20 815 12 Ox. 850 45 905 -40 -15 290 44 -50 290 7 F13 4 705 -20 810 12 Ox. 850 15 895 -40 -15 320 37 -45 320 13 No. _{ϑ7 T7} T _{P7 t8 T8} T _{P8 T9 T10} D A1 6 455 -20 110 465 -20 460 55 0.1 B2 9 450 -25 60 467 -25 462 58 0.3 B3 7 450 -25 100 468 -25 461 52 0.2 B4 13 457 -20 80 472 -20 465 40 0.5 B5 15 455 5 110 505 5 460 51 0.2 B6 18 455 -20 110 505 -20 460 49 0.15 C5 20 460 -5 120 479 -5 461 40 0.2 C6 17 455 -10 110 465 -10 463 58 0.3 D7 16 448 -35 80 469 -35 461 54 0.5 D8 18 460 -60 10 471 -60 460 50 0.6 D9 80 451 -60 50 483 -60 465 45 0.95 D10 14 460 5 5 520 5 460 50 0.45 E10 19 455 -55 50 465 -55 463 54 0.1 E11 21 450 -25 80 454 -25 462 53 0.4 F12 97 445 -50 30 472 -50 466 56 0.3 F13 14 450 -45 30 467 -45 460 53 0.25 No. Bainite + Martensite Proportion of tempered martensite retained austenite ferrite Rp0.2 Rm A80 Coating value Roughness Ra Peak number Rpc Ball impact test class A1 77 40% 3 20 590 886 17 good, no uncoated areas 0.65 55 1 B2 84 88% 12 4 883 1205 16 good, no uncoated areas 0.8 52 3 B3 88 91% 11 1 942 1185 14 good, no uncoated areas 0.7 54 2 B4 84 50% 8 8 715 1243 10 good, no uncoated areas 1.3 58 2 B5 80 88% 15 5 894 1198 16 bad, many uncoated areas 1.9 61 3 B6 88 97% 12 0 937 1190 15 bad, many uncoated areas 0.65 55 4 C5 94 56% 6 0 767 1089 10 good, no uncoated areas 0.8 50 1 C6 82 31% 3 15 679 1108 13 good, no uncoated areas 0.9 51 1 D7 88 80% 10 2 897 1060 16 good, no uncoated areas 1.2 57 1 D8 90 76% 9 1 854 1027 13 good, no uncoated areas 1.4 60 1 D9 81 81% 14 5 848 1091 17 good, no uncoated areas 2.1 71 3 D10 86 78% 2 12 856 997 8 bad, many uncoated areas 2.1 59 4 E10 82 47% 3 15 675 1231 9 good, no uncoated areas 0.7 55 1 E11 93 54% 3 4 869 1213 6 good, no uncoated areas 1.1 56 2 F12 84 89% 16 0 1264 1493 17 good, no uncoated areas 1 54 1 F13 78 93% 17 5 1149 1469 22 good, no uncoated areas 1 53 1

Claims (11)

Process for producing a high-strength coated 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%; $$\mathrm{0,005}\%\le \mathrm{Ti}+\mathrm{Nb}+\mathrm{V}\le \mathrm{0,2}\%;$$ and the remainder consists of iron and unavoidable elements. b. Heating the cold-rolled flat steel product to a furnace inlet temperature T ₀ . 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-oxidizing and heating the cold-rolled flat steel product from T ₂ to a temperature T ₃ 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. 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 ϑ 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 ϑ 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 and the dew point T _P7 of the atmosphere A ₇ is in the range (-60 °C) to (-5 °C). h. Setting and maintaining the cold-rolled flat steel product at a temperature T ₈ in a reducing atmosphere A ₈ for a total reheating time (step g), setting and maintaining t ₈ = 10 s - 600 s, wherein the dew point T _P8 of the atmosphere A ₈ is in the range (-60 °C) to (-5 °C). i. Coating and cooling the cold-rolled flat steel product under the following conditions: 1. Coating the cold-rolled steel product at a melt bath temperature T ₉ with a coating bath consisting of: Al: 0.15% - 2.0%; Fe-saturated; Rest Zn and unavoidable impurities; optional Mg: 0.25% - 8.0%; where T ₉ = 450 °C - 520 °C. 2. 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 degree D = 0.1% - 0.8%. Method according to one of the preceding claims, characterized in that in step c) between 500 °C and T1 the heating rate ϑ ₁ = 2 °C/s - 50 °C/s. Method according to one of the preceding claims, characterized in that in step d) the heating takes place as holding at a constant temperature T ₄ . Method according to one of the preceding claims, 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. Method according to one of the preceding claims, characterized in that a stochastic surface texturing is applied during the tempering step j). 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 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%; $$\mathrm{0,005}\%\le \mathrm{Ti}+\mathrm{Nb}+\mathrm{V}\le \mathrm{0,2}\%;$$ and the remainder being iron and unavoidable elements, wherein the flat steel product 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 , characterized in that the adhesion result in the ball impact test according to SEP 1931 is class 1 or 2. High-strength, coated flat steel product according to claim 6 , 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 ^-11 . High-strength, coated flat steel product according to claims 6 and 7 , characterized in that the coating layer is a closed layer. High-strength, coated flat steel product according to claims 6 to 8 , characterized in that the uppermost grain layer of the flat steel product is partially ferritized. High-strength, coated flat steel product according to claims 6 to 9 , characterized in that the coating is zinc-based and has a layer thickness of at least 5 µm and a maximum of 25 µm. Component for structural lightweight construction in automotive engineering formed from a flat steel product according to claims 6 to 10.