EP4043603A1 - Flat steel product and method for its production - Google Patents

Abstract

The invention relates to a high-strength flat steel product suitable for bake-hardening treatment and a method for producing such a flat steel product. The steel flat product consists of a steel consisting of (in % by weight) 0.1 - 0.5% C, 1.0 - 3.0% Mn, 0.5 - 2.0% Si, 0.01 - 1.5% Al, 0.001 - 0.008% N, up to 0.02% P, up to 0.005% S and optionally one or more of the following elements 0.01 - 1.0% Cr, 0.01 - 0, 2% Mo, 0.001 - 0.01% B and optionally a total of 0.005 - 0.2% V, Ti and Nb, with the Ti content not exceeding 0.10%, and the remainder being iron and unavoidable impurities , and wherein the flat steel product has a microstructure consisting of no more than 15% by area ferrite, no more than 5% by area bainite, at least 5% by volume retained austenite and at least 80% by area martensite, of which at least 75 areas -% is tempered martensite.

Description

The invention relates to a high-strength flat steel product suitable for bake-hardening treatment and a method for producing such a flat steel product.

Steel flats suitable for bake hardening (BH) treatment are also referred to as bake hardening (BH) steel flats and are commonly used in automotive applications such as body panels.

When flat steel products are discussed here, they are understood to be steel strips, steel sheets or blanks produced from them, such as blanks.

BH steel flat products have a lower strength level before BH treatment than after BH treatment. This circumstance is used to carry out the forming of flat steel products to be formed before the BH treatment and thus with lower yield points and better forming capacity. The strength level is increased by the BH treatment, in which the material is subjected to heat treatment. The BH treatment is typically performed for 3 to 40 minutes within a temperature range of 120 to 250°C. The BH treatment stimulates atoms of interstitially dissolved elements to diffuse, whereby they can attach to dislocations. This hinders the dislocations in their movement, which leads to an increase in the yield point. This effect of the increase in yield point is also referred to as the bake hardening effect (BH effect) and the difference in the yield points before and after the BH treatment is also referred to as the bake hardening value (BH value). The higher the BH value, the greater the increase in yield point due to the BH treatment.

If a flat steel product has a pronounced yield point, the term yield point is the characteristic value referred to as the upper yield point ReH meant. BH steel flat products often do not have a pronounced yield point before the BH treatment, but only a yield point. If the increase in yield point or the BH value is discussed here, this means the difference between the yield point Rp0.2 before the BH treatment and the yield point for steel flat products that do not have a pronounced yield point before the BH treatment, but rather a yield point ReH understood after BH treatment.

A high BH value has a positive effect on the buckling resistance of components made from BH flat steel products. As a result, it is possible to reduce the thickness of the component by using flat steel products, which have a high BH value, while maintaining the rigidity of the components.

The BH effect has so far been used for soft steels, which often have a predominantly ferritic matrix, only low martensite content and tensile strengths below 700 MPa.

Out of US 2013/0240094 A1 cold-rolled bake-hardening sheets for automotive applications are known. The sheets should consist of a steel which, in addition to iron and unavoidable impurities, contains 0.0010 - 0.0040% by mass C, 0.005 - 0.05% by mass Si, 0.1 - 0.8% by mass Mn, 0. 01 - 0.07% by mass P, 0.001 - 0.01% by mass S, 0.01 - 0.08% by mass Al, 0.0010 - 0.0050% by mass N, 0.002 - 0.02% by mass -% Nb and 0.005 - 0.050% by mass Mo, the value of the quotient of the proportions of Mn and P [Mn%]/[P%] should be between 1.6 and 45, and the amount present in solid solution Carbon, which is obtained from [C%]-(12/95)×[Nb%], should be between 0.0005 and 0.0025% by mass. The cold-rolled sheets suitable for bake-hardening should satisfy the equation X(222)/{X(119)+X(200)} > 3.0. Here, X(222), X(110) and X(200) are the integrated X-ray diffraction intensity of the {222} plane, the {110} plane and the {200} plane which are parallel to a plane which starting from the sheet surface is ¼ of the sheet thickness. The sheets should have good deep-drawing properties and tensile strengths of 300 to 450 MPa.

High-strength flat steel products are usually used for body parts in order to be able to realize small component thicknesses with good dent resistance. High-strength steels are characterized by a high proportion of martensite in the structure. Martensite is a carbon-rich structural component from which carbon can diffuse into other structural components when thermally activated. The higher the proportion of martensite in the structure, the more pronounced the BH effect is typically. However, high proportions of martensite go hand in hand with poor deformability.

Out of U.S. 2017/009315 A1 high-strength, hot-dip galvanized steel sheets are known. The sheets should contain 0.05 - 0.30% by mass C, 0.5 - 3.0% by mass Si, 0.2 - 3.0% by mass Mn, up to 0.10% by mass P, up to 0.010% by mass S, up to 0.010% by mass N and 0.001 - 0.10% by mass Al, the rest iron and unavoidable impurities. The structure should contain 50 - 85% by area martensite, less than 5% by area ferrite and the rest bainite and have a dislocation density of at least 5.0 x 10 ¹⁵ m ^-2 and at least 0.08% by mass of dissolved carbon. The sheets should be suitable for bake-hardening and have good bending properties and tensile strengths of 1180 MPa or more. The sheets are produced using conventional continuous casting, hot rolling and cold rolling. The cold-rolled sheets should be heated to annealing temperatures of Ac3+50°C up to 930°C, held at this annealing temperature for 30 to 1200 s, then at an average speed of 15°C/s or more to a cooling stop temperature between 450° C to 550 °C, then immersed in a molten bath for 10 to 60 s at 480 to 525 °C within 30 s or less from reaching the cooling stop temperature and then to 200 ° at an average cooling rate of 15 °C/s or more C to be cooled.

Against this background, the object of the invention was to specify a high-strength flat steel product with optimized properties, in particular very good bake-hardening properties and very good forming properties both before and after a BH treatment.

In addition, a method for producing such a flat steel product should be specified.

With regard to the flat steel product, the object was achieved by a product which has at least the features specified in claim 1.

With regard to the method, the object was achieved in that at least the method steps specified in claim 10 are completed in the production of a flat steel product according to the invention.

• 0,1 - 0,5 % C,
• 1,0 - 3,0% Mn,
• 0,5 - 2,0% Si,
• 0,01 - 1,5 % Al,
• 0,001 - 0,008 % N,
• bis zu 0,02 % P,
• bis zu 0,005 % S

• sowie optional aus einem oder mehreren der folgenden Elemente
  • 0,01 - 1,0 % Cr,
  • 0,01 - 0,2 % Mo,
  • 0,001 - 0,01 % B
• sowie optional aus in Summe 0,005 - 0,2 % V, Ti und Nb, wobei der Ti-Anteil nicht mehr als 0,10 % beträgt, und als Rest Eisen und unvermeidbaren Verunreinigungen besteht, und wobei das Stahlflachprodukt ein Gefüge aufweist, das aus
  • nicht mehr als als 15 Flächen-% Ferrit,
    • nicht mehr als als 5 Flächen-% Bainit,
    • mindestens 5 Volumen-% Restaustenit und
    • mindestens 80 Flächen-% Martensit, von welchem
  • mindestens 75 Flächen-% angelassener Martensit ist,
    besteht.

A steel flat product according to the invention consists of a steel which consists of (in % by weight)

• 0.1 - 0.5% C,
• 1.0 - 3.0% Mn,
• 0.5 - 2.0% Si,
• 0.01 - 1.5% Al,
• 0.001 - 0.008% N,
• up to 0.02% P,
• up to 0.005% S

• and optionally one or more of the following
  • 0.01 - 1.0% Cr,
  • 0.01 - 0.2% Mo,
  • 0.001 - 0.01% B
• and optionally from a total of 0.005 - 0.2% V, Ti and Nb, the Ti content being no more than 0.10%, and the remainder being iron and unavoidable impurities, and the flat steel product having a structure that consists of
  • not more than 15% by area ferrite,
    • not more than 5% by area bainite,
    • at least 5% by volume retained austenite and
    • at least 80% by area martensite, of which
  • at least 75% by area is tempered martensite,
    consists.

If ferrite is spoken of here, then polygonal ferrite is spoken of in each case. Based on the total proportion of martensite in the microstructure, at least 90% of the martensite has a martensite lancet length of no more than 7.5 μm and a martensite lancet width of no more than 1000 nm.

A flat steel product according to the invention is characterized in that, before a BH treatment, it has a yield point Rp0.2 of over 700 MPa or a yield point ReH of over 700 MPa, a tensile strength Rm of 950-1500 MPa and an elongation A80 of 7-25% and has a high bake hardening (BH) potential. The BH potential is expressed in the flat steel product having an increase in yield point of at least 80 MPa after a BH treatment and an elongation A80_BH which is at least half as high as the elongation A80 before the BH treatment.

If information on alloy contents and compositions is given here, this refers to the weight or the mass, unless otherwise expressly stated.

The carbon content of the steel of a flat steel product according to the invention is 0.1-0.5% by weight. On the one hand, carbon contributes to the formation and stabilization of austenite. C contents of at least 0.1 wt. %, preferably at least 0.12 wt is, on the invention Flat steel product to ensure a residual austenite content of at least 5% by volume. The residual austenite can be stabilized particularly reliably if the C content is at least 0.14% by weight. On the other hand, the C content has a strong influence on the strength of the martensite. This applies to both the strength of the martensite formed during the first quench and the strength of the martensite formed during the second quench that begins after the partitioning anneal. In order to utilize the influence of the carbon on the strength of the martensite, the C content should be at least 0.1% by weight. In addition, a minimum content of 0.1% by weight is required in order to provide sufficient carbon atoms for diffusion to the dislocations present in the material during a later BH treatment and thus to ensure a pronounced BH effect. Particularly high BH values are obtained when the C content is at least 0.14% by weight. However, with increasing C content, the martensite start temperature Ms is also shifted to lower temperatures. A C content above 0.5% by weight could therefore lead to insufficient martensite formation during quenching. The workability, in particular the weldability, is also impaired at higher C contents, which is why the C content should be at most 0.5% by weight, preferably at most 0.4% by weight.

As an alloying element, manganese is important for the hardenability of the steel and for avoiding the formation of the microstructure component pearlite during the first quenching. The Mn content of the steel of a flat steel product according to the invention is therefore at least 1.0% by weight, preferably at least 1.9% by weight, in order to provide a pearlite-free structure for the further process steps after the first quenching. However, as the Mn content increases, the weldability deteriorates and the risk of severe segregation occurring increases. Segregations are chemical inhomogeneities in the composition formed during the solidification process in the form of macroscopic or microscopic demixing. In order to reduce segregation and ensure good weldability, the Mn content of the steel of a flat steel product according to the invention is limited to a maximum of 3.0% by weight, preferably a maximum of 2.7% by weight.

The Si content of the steel of a flat steel product according to the invention is limited to 0.5-2.0% by weight. Si as an alloying element helps to suppress cementite formation. Cementite is an iron carbide. The formation of cementite binds carbon in the form of iron carbide and is no longer available in atomic form for solution in the iron lattice. However, atomic carbon, which is interstitially dissolved in the iron lattice, contributes significantly to the stabilization of retained austenite on the one hand and to the improvement of the BH effect on the other. In turn, retained austenite helps improve formability, especially elongation, both before and after BH treatment. A similar effect in terms of stabilizing the retained austenite can also be achieved by alloying aluminum. If the steel contains at least 0.2% by weight Al, the minimum Si content required to obtain a flat steel product according to the invention can be reduced to 0.5% by weight. If the Al content is less than 0.2% by weight, the Si content should preferably be at least 0.9% by weight. However, since a high Si content can have a negative effect on the surface quality of the steel flat product, the steel should contain no more than 2.0% by weight, preferably no more than 1.6% by weight.

Aluminum is present in the steel of a flat steel product according to the invention in contents of 0.01-1.5% by weight. Al is added for deoxidation and grain refinement. Grain refinement occurs through the formation of AlN clusters and AlN precipitations, each of which inhibits grain growth during austenitizing annealing, which is also referred to as austenitizing for short. AlN clusters are generally understood to be accumulations of aluminum and nitrogen atoms which, in contrast to AlN precipitations, do not have a sharp phase boundary to the matrix. In order to effectively inhibit austenite grain growth, the Al content should be at least 0.01% by weight. Particularly fine austenite grains can thus be set by the addition of Al and N to lattice defects and their subsequent cluster formation or precipitation. The finer austenite grain size results in fine martensite with a small lancet length being formed during the first quench. In cases where the duration of austenitizing is to be shortened, increased Al contents are required of at least 0.02% by weight is particularly advantageous. A further advantage for the formation of AlN clusters and AlN precipitations is a high number of lattice defects that are available during heating to the austenitizing temperature (THZ). These lattice defects can be introduced into the material before austenitizing, for example in the form of dislocations. For flat steel products according to the invention, it has proven advantageous to introduce lattice defects by cold rolling with a degree of cold rolling of at least 37%. Aluminum, like silicon, contributes to the suppression of cementite formation. However, Al is not as effective as Si in suppressing cementite formation. However, since Si has a negative effect on scaling and coatability and thus on the surface quality of the steel flat products, Al can be used as a substitute for Si when selecting the alloy composition. Al contents of at least 0.1% by weight have proven to be particularly effective for the steel composition according to the invention. At lower Al contents, the influence of Al on cementite suppression is not significant. In addition, aluminum contributes to increasing the carbon activity in martensite. This applies both to the martensite formed after the first quench, which takes place after austenitizing, and to the martensite formed after the second quench, which takes place after partitioning annealing. In the martensite formed after the first quench, aluminum contributes to accelerating the partitioning of the carbon from martensite to austenite during the partitioning anneal. This can shorten the duration of the partitioning anneal. But the aging resistance of the end product is also improved, since increased carbon activity means that individual carbon atoms can already diffuse at dislocations during partitioning annealing and are then no longer available for aging at room temperature.

The increase in carbon activity also shows a positive effect on the BH effect. In a BH treatment, high carbon activity also increases the driving force for attachment of carbon atoms to dislocations, resulting in the increase in BH value. Al contents of at least 0.02% by weight have proven to be particularly advantageous for increasing the carbon activity in the martensite. Since aluminum has the required for complete austenitizing If the annealing temperature is increased and complete austenitization is only possible with difficulty at Al contents above 1.5% by weight, the Al content of the steel of the flat steel product according to the invention is limited to a maximum of 1.5% by weight. If a low austenitizing temperature is to be set in order to improve energy efficiency, Al contents of at most 0.2% by weight have proven to be expedient.

In a preferred embodiment, to improve the BH value, the sum of the Si content and half of the Al content is at least 0.9% by weight.

mit

• %Si: jeweiliger Si-Gehalt des Stahls in Gew.-%
• %Al : jeweiliger Al-Gehalt des Stahls in Gew.-%.

The following relationship then applies: $$\%\mathrm{si}+0.5\%\mathrm{Al}\ge \mathrm{0}\mathrm{.9wt}\%$$

With

• %Si: respective Si content of the steel in % by weight
• %Al : Al content of the steel in % by weight.

Values of less than 0.9% by weight increase the risk of cementite forming, through which carbon is bound and is no longer available for diffusion into the retained austenite during partitioning annealing and is therefore no longer available for stabilization of the retained austenite.

The N content in the steel of a flat steel product according to the invention is limited to 0.001-0.008% by weight. In the steel of a flat steel product according to the invention, nitrogen forms nitrides, for example with aluminum or titanium. For effective grain refinement by means of AlN clusters or AlN precipitations, the steel should contain at least 0.001% by weight of N. In order to increase the thermodynamic driving force of precipitation formation and thus to stabilize the process, a preferred N content of at least 0.002% by weight can be set. Increasing N contents tend to lead to the formation of larger precipitates. In order to avoid coarse precipitates, which can have an adverse effect on formability, the N content is limited to a maximum of 0.008% by weight.

Phosphorus has a negative effect on the weldability in flat steel products according to the invention. For this reason, the P content should be as low as possible and in particular should not exceed 0.02% by weight.

At sufficiently high levels, sulfur leads to the formation of sulfides such as MnS or (Mn,Fe)S. These sulfide precipitates impair the elongation of a steel flat product according to the invention, which is why the S content is limited to at most 0.005% by weight.

Optionally, chromium can be present in the steel in contents of up to 1.0% by weight. Chromium is an effective inhibitor of pearlite and contributes to strength. This applies in particular to Cr contents of at least 0.01% by weight. With Cr contents of more than 1.0% by weight, however, the risk of pronounced grain boundary oxidation, which leads to a deterioration in the surface quality, is increased.

Molybdenum can likewise optionally be contained in the steel of a steel flat product according to the invention in amounts of at least 0.01% by weight in order to prevent the formation of pearlite. For reasons of cost, the Mo content is limited to levels of up to 0.2% by weight.

Boron can be contained as an optional alloying element in amounts of 0.001 to 0.01% by weight in the steel of a flat steel product according to the invention. Boron segregates onto the phase boundaries and thus blocks their movement. This supports the formation of a fine-grain structure, which improves the mechanical properties of the steel flat product. When alloying boron, sufficient aluminum should be available so that AlN forms preferentially. In a preferred embodiment, an Al/B ratio of at least 10 is therefore set. However, no further improvement can be achieved by adding boron in excess of 0.01% by weight.

Optionally, steels of flat steel products according to the invention can also contain one or more micro-alloying elements from the group Ti, Nb and V. Micro-alloying elements can be combined with carbon or nitrogen carbides, form nitrides or carbonitrides. In the form of very finely distributed precipitations, these contribute to greater strength. The sum of the micro-alloying elements should be at least 0.005% by weight, so that the precipitation of carbides, nitrides or carbonitrides can lead to the freezing of grain and phase boundaries during austenitizing and thus counteract grain coarsening. At the same time, however, carbon, which in atomic form is favorable for stabilizing the retained austenite, is bound as carbide or carbonitride. In order to ensure adequate stabilization of the retained austenite, the total concentration of the micro-alloying elements should not exceed 0.2% by weight. To avoid coarse titanium nitride precipitation, the titanium concentration should not be more than 0.10%.

A flat steel product according to the invention preferably has a yield strength Rp02 of more than 700 MPa or a yield strength ReH of more than 700 MPa, a tensile strength Rm of 950-1500 MPa and an elongation A80 of 7-25%, with the yield strength Rp02 or the yield strength ReH the tensile strength Rm and the elongation A80 are determined according to DIN EN ISO 6892:2009. At the same time, a flat steel product according to the invention preferably has a high bake-hardening potential (BH potential). A measure of the BH potential is the BH2 value, which is determined after pre-deformation of 2% and tempering for 20 minutes at 170° C. in accordance with DIN EN 10325:2006 and is at least 80 MPa for flat steel products according to the invention. The elongation A80_BH present after a BH treatment for 20 minutes at 170° C. on flat steel products according to the invention preformed by 2% is at least half as high as the elongation A80 before the BH treatment. The elongation values A80 and A80_BH are determined according to DIN EN ISO 6892:2009.

The flat steel product according to the invention has a structure that contains no more than 15% by area of ferrite in order to ensure the required high strength.

In addition, due to the way the process is carried out, the microstructure does not have more than 5% by area of bainite.

The microstructure of a flat steel product according to the invention contains at least 5% by volume of retained austenite. Residual austenite has a beneficial effect on the formability and elongation of martensite steels. The austenite, which has been stabilized down to room temperature, can be stretched more than other structural components using the TRIP effect with simultaneous higher hardening. With the limitation of the austenite-stabilizing alloying elements such as C and Mn for reasons of weldability, a residual austenite content greater than 20% by volume is not possible with the manufacturing process described.

In addition, the flat steel product according to the invention contains at least 80% by area of martensite, of which at least 75% by area is tempered martensite.

The martensite formed in the course of the method according to the invention after the partitioning by the second quenching in step j) is also referred to as untempered martensite. The martensite resulting from the first quench after austenitizing, which undergoes partitioning, is also known as tempered martensite. All of the martensite present in the structure is composed of tempered and untempered martensite, with the possibility that there is no untempered martensite.

The total proportion of martensite, ie the sum of tempered and non-tempered martensite, should be at least 80% by area, preferably at least 90% by area. This high proportion of martensite contributes to the high strength of the flat steel product. In addition, martensite is a carbon-rich structural component. As such, the martensite serves as a source for the diffusion of carbon both during the partitioning anneal and during the BH treatment. The residual austenite present is stabilized by the carbon diffusion from the martensite into the austenite during the partitioning annealing, which makes it possible to set a residual austenite proportion of at least 5% by volume. The carbon diffusion during the BH treatment increases the BH effect, resulting in an increase in the BH value.

At least 75% of the martensite present in the flat steel product is tempered martensite, because only then is there enough martensite available for sufficient residual austenite stabilization during partitioning annealing. At least 90% of the martensite lancets have a martensite lancet width of at most 1000 nm. The narrow lancet width of at most 1000 nm leads to short diffusion paths during partitioning annealing, which enables targeted local stabilization of the retained austenite.

The martensite lancet length is limited to a maximum of 7.5 µm to ensure good formability. Since the lancets grow with a defined ratio of length to width, the width is limited, which has an advantageous effect on the diffusion of the carbon.

Unless otherwise stated, the information on the microstructural proportions for the microstructural components martensite, ferrite and bainite are based on area % and for retained austenite on vol. %. Due to the fineness of the microstructure, it is advisable to carry out the microstructure investigations, including the determination of the martensite lancet length and width, using a scanning electron microscope (SEM) at 5000x magnification. An examination using X-ray diffraction (XRD) according to ASTM E975 is recommended as a suitable method for the quantitative determination of retained austenite.

1. a) Bereitstellen eines warmgewalzten Stahlflachprodukts, welches aus einem Stahl besteht, der neben Eisen und unvermeidbaren Verunreinigungen aus (in Gew.-%) 0,1 - 0,5 % C, 1,0 - 3,0 % Mn, 0,5 - 2,0 % Si, 0,01 - 1,5 % Al, 0,001 - 0,008 % N, bis zu 0,02 % P, bis zu 0,005 % S sowie optional aus einem oder mehreren der folgenden Elemente: 0,01 - 1,0 % Cr, 0,01 - 0,2 % Mo, 0,001 - 0,01 % B sowie optional aus in Summe 0,005 - 0,2 % V, Ti und Nb besteht, wobei der Ti-Anteil nicht mehr als 0,10% beträgt;
2. b) Beizen des warmgewalzten Stahlflachprodukts;
3. c) Kaltwalzen des Stahlflachprodukts mit einem Kaltwalzgrad von mindestens 37 %;
4. d) Erwärmen des kaltgewalzten Stahlflachprodukts auf eine Haltezonentemperatur THZ, welche oberhalb der A3-Temperatur des Stahls liegt und höchstens 950 °C beträgt, wobei das Aufheizen bis zu einer 200 - 400 °C betragenden Wendetemperatur TW mit einer Aufheizgeschwindigkeit ThetaH1 von 5 - 50 K/s und oberhalb der Wendetemperatur TW mit einer Aufheizgeschwindigkeit ThetaH2 von 2 - 10 K/s erfolgt;
5. e) Halten des Stahlflachprodukts für 5 - 15 s auf der Haltezonentemperatur THZ;
6. f) optionales Abkühlen des Stahlflachprodukts innerhalb von 30 - 300 Sekunden von der Haltezonentemperatur THZ auf eine mindestens 620 und höchstens 720°C betragende Zwischentemperatur TLK;
7. g) Abkühlen des Stahlflachprodukts mit einer Abkühlrate ThetaQ von mehr als durchschnittlich 5 K/s auf eine Kühlstopptemperatur TAB, welche zwischen der Martensitstarttemperatur TMS und einer Temperatur, die bis zu 175 °C kleiner als TMS ist, liegt;
8. h) Halten des Stahlflachprodukts auf der Kühlstopptemperatur TAB für 10 - 60 Sekunden;
9. i) Erwärmen des Stahlflachprodukts mit einer Aufheizrate ThetaB1, welche 1 - 80 K/s beträgt, auf eine 350 - 500 °C betragende Behandlungstemperatur TB und optionales isothermes Halten des Stahlflachprodukts auf der Behandlungstemperatur TB, wobei die Zeit für das Erwärmen und das optionale isotherme Halten insgesamt 10 - 1000 Sekunden beträgt;
10. j) Abkühlen des Stahlflachprodukts mit einer Abkühlrate ThetaB2 von mehr als 5 K/s und weniger als 500 K/s auf Raumtemperatur;
11. k) optionales Beschichten des Stahlflachprodukts in einem Schmelzbad entweder
    • k1) mittels Schmelztauchbeschichten vor dem Abkühlen in Arbeitsschritt j)
      oder
    • k2) mittels elektrolytischem Beschichten nach dem Abkühlen in Arbeitsschritt j).

The method according to the invention for producing a high-strength flat steel product suitable for bake-hardening treatment comprises at least the following work steps:

1. a) Providing a hot-rolled flat steel product, which consists of a steel which, in addition to iron and unavoidable impurities, consists of (in % by weight) 0.1-0.5% C, 1.0-3.0% Mn, 0.5 - 2.0% Si, 0.01 - 1.5% Al, 0.001 - 0.008% N, up to 0.02% P, up to 0.005% S and optionally one or more of the following elements: 0.01 - 1.0% Cr, 0.01 - 0.2% Mo, 0.001 - 0.01% B and optionally a total of 0.005 - 0.2% V, Ti and Nb, with the Ti content not exceeding 0 is .10%;
2. b) pickling the hot-rolled flat steel product;
3. c) cold rolling of the steel flat product with a degree of cold rolling of at least 37%;
4. d) Heating of the cold-rolled flat steel product to a holding zone temperature THZ, which is above the A3 temperature of the steel and is at most 950 °C, with the heating up to a turning temperature TW of 200 - 400 °C at a heating rate ThetaH1 of 5 - 50 K /s and above the turning temperature TW with a heating rate ThetaH2 of 2 - 10 K/s;
5. e) holding the steel flat product for 5-15 s at the holding zone temperature THZ;
6. f) optional cooling of the steel flat product within 30-300 seconds from the holding zone temperature THZ to an intermediate temperature TLK of at least 620 and at most 720°C;
7. g) cooling the flat steel product at a cooling rate ThetaQ of more than an average of 5 K/s to a cooling stop temperature TAB, which is between the martensite start temperature TMS and a temperature that is up to 175 °C lower than TMS;
8. h) holding the steel flat product at the cooling stop temperature TAB for 10 - 60 seconds;
9. i) heating the steel flat product at a heating rate ThetaB1, which is 1 - 80 K/s, to a treatment temperature TB of 350 - 500 °C and optionally isothermally holding the steel flat product at the treatment temperature TB, the time for the heating and the optional isotherm total hold is 10 - 1000 seconds;
10. j) cooling the flat steel product to room temperature at a cooling rate ThetaB2 of more than 5 K/s and less than 500 K/s;
11. k) optional coating of the flat steel product in a molten bath either
    • k1) by means of hot-dip coating before cooling in step j)
      or
    • k2) by means of electrolytic coating after cooling in step j).

In step a), a hot-rolled flat steel product is provided, which consists of a steel of the composition mentioned in step a).

The hot-rolled flat steel product is pickled before cold rolling. The pickling in step b) is carried out in a conventional manner.

wobei h0 die Dicke des Stahlflachprodukts vor dem Kaltwalzen in mm ist und h1 die Dicke des Stahlflachprodukts nach dem Kaltwalzen in mm ist. Sollte das Stahlflachprodukt nach dem Beizen und vor dem Aufheizen in Arbeitsschritt d) mehrere Kaltwalzvorgänge oder Kaltwalzstiche erfahren, so ist KWG auf den Gesamtkaltwalzgrad bezogen, d.h. h0 ist die Dicke des Stahlflachprodukts vor dem ersten Kaltwalzvorgang oder Kaltwalzstich in mm und h1 ist die Dicke des Stahlflachprodukts nach dem letzten Kaltwalzvorgang oder Kaltwalzstich in mm. Durch ein Kaltwalzen mit einem KWG von mindestens 37 % wird eine mechanische Homogenisierung sowie eine Reduzierung der Korngröße und damit ein feinkörniges Gefüge eingestellt. Aufgrund des hohen Kaltwalzgrades sowie der Ausscheidungsvorgänge und des daraus resultierenden feinen Ausgangsgefüges vor dem Glühen liegt bereits vor der Abkühlung ein sehr feinkörniges Austenitgefüge vor. Hierbei fungieren die Korngrenzen als Hindernis für das Wachstum der Martensitlanzetten und der kurze Abstand zwischen den Korngrenzen in einem feinen Gefüge führt zu kürzeren und schmaleren Lanzetten. Beim Abschrecken entsteht so ein Gefüge aus feinsten Martensitlanzetten mit dazwischen eingelagertem Restaustenit. Dies führt im anschließenden Behandlungsschritt zu kurzen Diffusionswegen, wodurch eine gezielte lokale Stabilisierung des Restaustenits möglich ist.According to the invention, the cold rolling in step c) should be carried out with a degree of cold rolling of at least 37%. In the present case, the degree of cold rolling KWG is understood to mean the reduction in thickness that occurs as a result of the cold rolling of the flat steel product. The KWG can be described with the following relationship: $$\mathrm{KWG}=\left(\mathrm{H}0-\mathrm{h1}\right)/\mathrm{h0}$$

where h0 is the thickness of the steel flat product before cold rolling in mm and h1 is the thickness of the steel flat product after cold rolling in mm. If the flat steel product undergoes several cold rolling processes or cold rolling passes after pickling and before heating in step d), KWG is related to the total degree of cold rolling, i.e. h0 is the thickness of the flat steel product before the first cold rolling process or cold rolling pass in mm and h1 is the thickness of the flat steel product after the last cold rolling process or cold rolling pass in mm. Cold rolling with a KWG of at least 37% results in mechanical homogenization and a reduction in grain size, resulting in a fine-grained structure. Due to the high degree of cold rolling and the precipitation processes and the resulting fine initial structure before annealing, a very fine-grained austenite structure is already present before cooling. Here, the grain boundaries act as an obstacle to the growth of martensite lancets, and the short distance between grain boundaries in a fine structure results in shorter and narrower lancets. During quenching, a structure of the finest martensite lancets with residual austenite embedded in between is created. This leads to the following Treatment step to short diffusion paths, whereby a targeted local stabilization of the retained austenite is possible.

It has been recognized that cold rolling grades of 37% or more provide many nucleation sites for austenite formation during austenitizing annealing, resulting in a fine-grained austenitic structure during austenitizing. The grain size of the austenitic structure can be further reduced if the degree of cold rolling is at least 42%. For technical reasons, the degree of cold rolling is typically limited to 85%.

The heating of the cold-rolled flat steel product in step d) to a holding zone temperature THZ takes place initially until a turning temperature TW is reached, which is 200-400° C., at a heating rate ThetaH1 of 5-50 K/s. Above the turning temperature TW, heating takes place at a heating rate ThetaH2 of 2 - 10 K/s until the holding zone temperature THZ is reached. The heating can also take place in one step, i.e. the heating speeds ThetaH1 and ThetaH2 are set to the same value.

mit %C=C-Gehalt des Stahls in Gew.-%, %Ni=Ni-Gehalt des Stahls in Gew.-%, %Si=Si-Gehalt des Stahls in Gew.-%, %Mo=Mo-Gehalt des Stahls in Gew.-%, %Mn=Mn-Gehalt des Stahls in Gew.-%.The steel flat product is heated to a holding zone temperature THZ, which is above the A3 temperature of the steel, in order to enable a complete transformation into austenite. The A3 temperature is analysis dependent and can be estimated using the following empirical equation: $$\mathrm{A3}\left[°\mathrm{C}\right]=910-203*\surd \mathrm{\%C}-15.2\%\mathrm{no}+44.7\%\mathrm{si}+31.5\%\mathrm{Mon}-\mathrm{21:1}\%\mathrm{Mn}$$

with %C=C content of the steel in % by weight, %Ni=Ni content of the steel in % by weight, %Si=Si content of the steel in % by weight, %Mo=Mo content of the steel in % by weight, %Mn=Mn content of the steel in % by weight.

The holding zone temperature THZ can also be referred to as the austenitizing temperature and annealing at THZ can also be referred to as austenitizing. For reasons of cost, the holding zone temperature is limited to a maximum of 950 °C.

In step e), the flat steel product is held at the holding zone temperature THZ for a holding time tHZ of at least 5 seconds in order to ensure complete austenitization. The holding time tHZ should not exceed 15 seconds in order to avoid the formation of a coarse austenite grain and irregular austenite grain growth. The aim of austenitizing is to set a fine and regular austenite grain, since such a structure has a favorable effect on the BH value.

From the holding zone temperature THZ, the flat steel product can optionally first be slowly cooled in step f) to an intermediate temperature TLK, which is 620° C. or more. TLK is not lower than 620 °C to avoid phase transformation into ferrite. For the same reason, the duration tLK of the cooling from THZ to TLK is limited to 30 - 300 seconds.

After the optional slow cooling of the flat steel product in step f) or after holding the flat steel product at the holding zone temperature THZ in step e), the flat steel product is cooled in step g) at a higher cooling rate ThetaQ than the cooling rate in step f) of more than 5 K/s cooled to a cooling stop temperature TAB. Because of the high cooling rate, such cooling is also referred to as quenching or, to distinguish between quenching after partitioning annealing, the quenching in work step g) is also referred to as first quenching. The cooling rate from the intermediate temperature TLK to the cooling stop temperature TAB is more than 5 K/s in order to avoid both the transformation of the austenite into ferrite and into bainite for the steel compositions according to the invention. This is even more reliable with higher cooling rates, which is why the ThetaQ cooling rate is preferably set to more than 20 K/s. The cooling rate ThetaQ is technically limited to values of at most 500 K/s, preferably at most 100 K/s.

mit %C=C-Gehalt des Stahls in Gew.-%, %Mn=Mn-Gehalt des Stahls in Gew.-%, %Si=Si-Gehalt des Stahls in Gew.-%, %Al=Al-Gehalt des Stahls in Gew.-%.The cooling stop temperature TAB is between the martensite start temperature TMS and a temperature that is up to 175°C lower than TMS ((TMS-175°C)<TAB<TMS). The martensite start temperature TMS is the temperature at which the transformation from austenite to martensite begins. The martensite start temperature can be estimated using the following equation: $$\mathrm{TMS}\left[°\mathrm{C}\right]=539°\mathrm{C}+\left(-423\%\mathrm{C}-30.4\%\mathrm{Mn}-7.5\%\mathrm{si}+30\%\mathrm{Al}\right)°\mathrm{c/wt}\%$$

with %C=C content of the steel in % by weight, %Mn=Mn content of the steel in % by weight, %Si=Si content of the steel in % by weight, %Al=Al content of the steel in % by weight.

Since the transformation of austenite into martensite does not take place suddenly but as a function of time, the extent of the transformation, i.e. the proportion of martensite, can be controlled via the holding time tQ, with which the steel flat product is held at the cooling stop temperature TAB. The holding time tQ in step h) is at least 10 seconds to ensure sufficient conversion of the austenite into martensite. In relation to the entire microstructure, the proportion of martensite produced by the first quenching after austenitizing should be at least 60% by area. The holding time tQ should not be more than 60 seconds in order to avoid complete transformation into martensite and to ensure a residual austenite content of at least 5% by volume in the structure of the steel flat product at room temperature.

In step i), the flat steel product is heated to a treatment temperature TB at a heating rate ThetaB1 and optionally maintained at TB in order to enrich the residual austenite present after step h) with carbon from the supersaturated martensite, which was formed by the first quenching. The redistribution of the carbon, which can also be referred to as partitioning, takes place during the heating phase on TB. If the flat steel product is then also held isothermally on TB, partitioning also takes place during the optional isothermic holding. The heating to the treatment temperature TB and the subsequent optional holding at the treatment temperature TB are also referred to as partitioning annealing or partitioning. In order to enable sufficient redistribution of the carbon, heating is carried out at a heating rate of at least 1 K/s and at most 80 K/s. The treatment temperature TB is 350 - 500 °C to avoid the formation of carbides and the decomposition of retained austenite. In addition, the total treatment time tBT is at least 10 and at most 1000 seconds, also to ensure sufficient redistribution of the carbon. The total treatment time tBT is made up of the time required for heating and, if applicable, the time used for the optional isothermal hold.

The flat steel product is then cooled to room temperature in step j) at a cooling rate ThetaB2. The cooling rate ThetaB2 is more than 5 K/s, preferably more than 20 K/s to allow the formation of martensite. This cooling step can also be referred to as quenching due to the high cooling rate. To distinguish it from the first quenching carried out in step g), the quenching in step j) is also referred to as second quenching. The cooling rate ThetaB2 is technically limited to values of at most 500 K/s, preferably at most 100 K/s.

The flat steel product can additionally optionally be subjected to a coating treatment (step k)). The coating treatment can be carried out either as a hot-dip coating (step k1)) or as an electrolytic coating (step k2)). If hot-dip coating takes place (work step k1)), the flat steel product, after partitioning in work step i) and before cooling in work step j), runs through a coating bath with a zinc-based molten bath composition. The temperature of the molten bath is preferably 450-500.degree.

As an alternative to applying a coating by means of hot-dip coating, the flat steel product can be subjected to electrolytic coating (work step k2)). The electrolytic coating takes place here in contrast to Hot-dip coating not before, but only after the cooling of the flat steel product in step j).

The coating treatment of steps k1) or k2) preferably takes place in a continuous process. A possible molten bath composition can consist of up to 1% by weight Al, the remainder zinc and unavoidable impurities. Another possible molten bath composition can consist of 1-2% by weight Al, 1-2% by weight Mg, the remainder zinc and unavoidable impurities. By the coating treatment, an anti-corrosion coating is applied to the flat steel product on at least one side of the flat steel product. The coated flat steel product can also optionally be subjected to a galvannealing treatment.

The process according to the invention can be carried out continuously in annealing plants or coil coating plants which are usually provided for this purpose.

When proceeding according to the invention, in particular when maintaining the degree of cold rolling KWG, the cooling rate ThetaQ of the rapid cooling after austenitizing and the holding time tQ, a microstructure results which has a very fine martensite structure. This martensite structure is characterized by a particularly fine-grained structure with a narrow lancet width. The high degree of cold rolling and the carbide and nitride precipitations lead to a fine-grained starting structure for austenitizing annealing. With the procedure according to the invention, coarsening of the grains during the austenitizing is avoided, so that a very fine-grained structure is already present before the cooling that follows the austenitizing. The numerous grain boundaries of the fine structure hinder the growth of the martensite lancets. The short distances between the grain boundaries of the fine-grain structure lead to short and narrow martensite lancets. The rapid cooling with cooling rates ThetaQ of more than 5 K/s results in a structure of very fine martensite lancets with residual austenite embedded in between. Such a structure provides short for the annealing process of the subsequent step i). Diffusion paths for the carbon available and thus enables a targeted local stabilization of the retained austenite.

At the same time, however, there is still sufficient carbon from the untempered martensite and from the deformation-induced martensite formed during forming for the subsequent BH treatment to attach to the dislocations. Due to the fine structure present after the partitioning annealing, the diffusion paths into the tempered martensite are short enough during the later BH treatment to be able to achieve a high BH effect even at low BH temperatures and short BH treatment times.

The flat steel products made available by the present invention are particularly suitable for further processing that includes a cold forming process and subsequent heat treatment at temperatures below 300.degree. The production of components for automotive applications is mentioned here as an example. Flat steel products are formed into components, for example painted using cathodic dip painting (KTL) and then subjected to heat treatment in a further process step, for example during paint baking. The heat treatment usually takes the form of heating within a temperature range of typically 120 to 250°C for a period of typically 3 to 40 minutes. The flat steel products according to the present invention are particularly suitable for such applications. However, the advantageous properties of the flat steel products according to the invention can also be used for products that have not been subjected to any pre-deformation.

The invention is explained in more detail below using exemplary embodiments:
For testing, five melts AE of the compositions given in Table 1 were produced, from which 10 hot strips with a thickness of 1.8-2.5 mm were produced in a conventional manner. The melts correspond to C and E meet the specifications for the steel composition according to the invention, whereas the melts A, B and D have too low Si contents.

The hot strips were pickled in a conventional manner and processed into cold strips with the cold rolling degrees "KWG" given in Table 2a. The further production of the cold strips took place in accordance with the information given in Table 2a and Table 2b. The cold strips were each heated to a turning temperature "TW" at a first, faster heating rate "ThetaH1" and then brought to the holding zone temperature "THZ" at a second, slower heating rate "ThetaH2", at which they were held for the duration "tHZ". . The cold strips from tests 1-9 were first slowly cooled to an intermediate temperature "TLK" within a period of time "tLK", then quickly quenched from the intermediate temperature "TLK" at a cooling rate "ThetaQ" to a cooling stop temperature "TAB". on which they were held for a duration "tQ". The cold strip of test 10 was quickly quenched directly to the cool stop temperature "TAB" at a cooling rate "ThetaQ" without slow cooling and held at this temperature for the duration "tQ". The flat steel products were then subjected to partitioning for a time "tBT", with them being heated to the partitioning temperature "TB" at a heating rate "ThetaB1". Finally, the steel flat products were quenched to room temperature with a cooling rate "ThetaB2". 10 tests were carried out, of which tests 4, 8 and 10 meet the requirements of the invention.

Samples were taken from tests 1-10, on which the structure was examined and the mechanical properties tested. The results of the structural tests are given in Table 3 and the results of the mechanical property tests are given in Table 4. "MA" denotes the proportion of tempered martensite in the entire structure, "M" the proportion of untempered martensite in the entire structure, "F" the proportion of ferrite, "B" the proportion of bainite, "RA" the proportion of residual austenite. The terms lancet length and lancet width refer to the structures of the martensite.

The structural investigations were carried out on cross sections at 1/3t layer, i.e. on sections which were taken at a third of the sheet thickness. The sections were prepared for scanning electron microscopy (SEM) examination and treated with a 3% Nital etch. Due to the fineness of the microstructure, the microstructure was characterized by means of REM observation at a magnification of 5000x. The quantitative determination of the retained austenite was carried out by means of X-ray diffraction (XRD) according to ASTM E975.

The mechanical properties yield strength "Rp02", tensile strength "Rm" and elongation "A80" were tested according to DIN EN ISO 6892:2009 on samples that were not subjected to any BH treatment. To test the BH properties, samples were taken from the same steel flat products and subjected to a pre-deformation of 2% and tempered at 170 °C for 20 minutes. The bake hardening value "BH2" was tested in accordance with DIN EN 10325:2006. The test of the elongation "A80_BH" present after the BH treatment, which is also referred to as the residual elongation, was carried out according to DIN EN ISO 6892:2009.

The tests show that the difference between the yield point Rp02 before the BH treatment and the yield point after the BH treatment tends to increase with increasing strength of the steel flat product. This can be attributed to the higher proportion of martensite in the higher-strength samples. With comparable strength and elongation A80, samples 4 and 8 according to the invention have a higher BH value "BH2" and a significantly better residual elongation "A80_BH" than their comparison samples 5 and 9, which were not produced according to the invention from the same melt. <b>Table 1</b> melt C si Mn P S Al Cr Nb Mon N Ti B A 0.142 0.210 1.63 0.012 0.0027 0.031 0.780 - - 0.0027 0.037 0.0011 B 0.072 0.260 2.59 0.013 0.0021 0.029 0.690 - 0.11 0.0025 0.079 0.0013 C 0.158 1,180 1.99 0.014 0.0020 0.017 - - - 0.0016 0.015 0.0015 D 0.153 0.420 2.35 0.013 0.0025 0.710 0.720 0.027 - 0.0042 0.023 0.0014 E 0.218 1,478 2:21 0.016 0.0023 0.024 0.173 - - 0.0046 - - Data in % by weight, remainder iron and unavoidable impurities.
Underlined values are outside the specifications according to the invention. sample melt A3 [°C] TMS [°C] KWG [%] Theta H1 [K/s] TW [°C] ThetaH2 [K/s] THZ [°C] tHZ [s] 1 A 808 429 42 12 350 3.5 780 14 2 B 816 429 39 10 375 4.0 820 7 3 B 816 429 25 10 375 4.0 820 18 4 C 840 403 45 12 350 5.0 850 8th 5 C 840 403 29 12 350 5.0 850 6 6 D 799 421 48 15 360 3.5 875 5 7 D 799 421 33 15 360 3.5 875 12 8th E 834 369 42 12 325 3.3 895 9 9 E 834 369 25 12 325 3.3 895 13 10 E 834 369 44 12 325 3.3 895 9 Underlined values are outside the specifications according to the invention sample melt DC [°C] tLK [s] ThetaQ [K/s] TAB [°C] tQ [s] TB [°C] ThetaB1 [K/s] tBT [s] ThetaB2 [K/s] 1 A 685 415 33 410 25 425 65 15 20 2 B 690 320 34 390 15 440 35 30 15 3 B 690 340 34 390 55 440 50 25 33 4 C 710 129 38 355 30 455 25 150 25 5 C 710 75 38 355 65 455 15 80 29 6 D 715 185 31 340 20 437 70 120 15 7 D 715 290 31 340 45 437 45 70 34 8th E 685 45 33 330 55 445 30 200 35 9 E 685 25 33 330 10 445 5 15 27 10 E * * 39 330 50 450 30 180 33 Underlined values are outside the specifications according to the invention
* No slow cooling performed sample MA M f B RA Lancet length [µm] Lancet width [nm] 1 25 30 40 Sp. 2 3.5 400 2 45 25 27 0 3 7.5 600 3 35 32 26 0 7 8.5 850 4 70 15 7 Sp. 7 4.5 150 5 60 28 5 Sp. 6 9.0 700 6 70 20 8th Sp. 2 8.0 450 7 55 35 9 0 1 11.5 650 8th 80 5 Sp. Sp. 12 6.0 250 9 65 25 Sp. Sp. 8th 9.5 750 10 85 0 Sp. Sp. 13 6.5 275 Underlined values are outside the specifications according to the invention. "Sp." stands for traces, ie microstructure fractions of at most 2% by area. Information on MA, M, F, B in area %. Information on RA in % by volume. sample Rp02 [MPa] Rm [MPa] A80 [%] BH2 [MPa] A80_BH [%] 1 475 810 21 28.3 19.3 2 750 1050 12 83.78 5.1 3 720 1065 13 65 5 , 1 4 823 1060 18.1 85 15.3 5 791 1073 18.6 80 7.5 6 912 1217 7.8 123 1.8 7 940 1235 8.2 137.2 2.4 8th 907 1204 13.3 158 12 9 825 1229 12.7 148 5.1 10 935 1225 13.1 151 10.4 Underlined values are outside the specifications according to the invention.

Claims (15)

High-strength flat steel product suitable for bake-hardening treatment, which consists of a steel which, in addition to iron and unavoidable impurities (in % by weight), consists of C: 0.1 - 0.5%, Si: 0.5 - 2.0%, Mn: 1.0 - 3.0%, Al: 0.01 - 1.5%, N: 0.001 - 0.008%, P: ≤ 0.02%, S: ≤ 0.005%, and optionally one or more of the following elements Cr: 0.01 - 1.0%, Mo: 0.01 - 0.2%, B: 0.001 - 0.01%, and optionally from a total of 0.005 - 0.2% V, Ti and Nb, with the Ti content not exceeding 0.10%, consists, wherein the steel flat product has a structure that consists of - not more than 15% by area ferrite, - not more than 5% by area bainite, - at least 5% by volume retained austenite and - at least 80% by area of martensite, of which at least 75% by area is tempered martensite, consists, based on the total proportion of martensite in the microstructure, this has a martensite lancet length of at most 7.5 μm and a martensite lancet width of at most 1000 nm for at least 90% of the martensite. Flat steel product according to Claim 1 , characterized in that the carbon content is at least 0.14% by weight. Flat steel product according to Claim 1 or 2 , characterized in that the sum of the Si content and half of the Al content is at least 0.9% by weight. Flat steel product according to one of the preceding claims , characterized in that the Si content is at least 0.5% by weight. Flat steel product according to one of the preceding claims , characterized in that the Si content is at least 0.9% by weight. Flat steel product according to one of the preceding claims , characterized in that the Al content of the steel of the flat steel product is at least 0.02% by weight. Flat steel product according to one of the preceding claims , characterized in that the Al content of the steel of the flat steel product is at most 0.2% by weight. Flat steel product according to one of the preceding claims , characterized in that it has a yield point Rp02 of over 700 MPa or a yield point ReH of over 700 MPa, a tensile strength Rm of 950 - 1500 MPa, an elongation A80 of 7 - 25% and a BH2 value which is determined after a pre-deformation of 2% and tempering for 20 minutes at 170°C according to DIN EN 10325:2006, is at least 80 MPa. Flat steel product according to one of the preceding claims , characterized in that it is provided with a zinc-based anti-corrosion coating on at least one side. Process for producing a high-strength steel flat product suitable for bake-hardening treatment, comprising the following work steps: a) Provision of a hot-rolled flat steel product, which consists of a steel which, in addition to iron and unavoidable impurities, consists of (in % by weight) 0.1-0.5% C, 1.0-3.0% Mn, 0.5 - 2.0% Si, 0.01 - 1.5% Al, 0.001 - 0.008% N, up to 0.02% P, up to 0.005% S and optionally one or more of the following elements: 0.01 - 1.0% Cr, 0.01 - 0.2% Mo, 0.001 - 0.01% B and optionally a total of 0.005-0.2% V, Ti and Nb, with the Ti content not exceeding 0.10%; b) pickling the hot-rolled flat steel product; c) cold rolling of the steel flat product with a degree of cold rolling of at least 37%; d) Heating of the cold-rolled flat steel product to a holding zone temperature THZ, which is above the A3 temperature of the steel and is at most 950 °C, with the heating up to a turning temperature TW of 200 - 400 °C at a heating rate ThetaH1 of 5 - 50 K /s and above the turning temperature TW with a heating rate ThetaH2 of 2 - 10 K/s; e) holding the steel flat product for 5-15 s at the holding zone temperature THZ; f) optional cooling of the steel flat product within 30-300 seconds from the holding zone temperature THZ to an intermediate temperature TLK of at least 620 and at most 720°C; g) cooling the flat steel product at a cooling rate ThetaQ of more than an average of 5 K/s to a cooling stop temperature TAB, which is between the martensite start temperature TMS and a temperature that is up to 175 °C lower than TMS; h) holding the steel flat product at the cooling stop temperature TAB for 10 - 60 seconds; i) heating the steel flat product at a heating rate ThetaB1, which is 1 - 80 K/s, to a treatment temperature TB of 350 - 500 °C and optionally isothermally holding the steel flat product at the treatment temperature TB, the time for the heating and the optional isotherm total hold is 10 - 1000 seconds; j) cooling the flat steel product to room temperature at a cooling rate ThetaB2 of more than 5 K/s and less than 500 K/s k) optional coating of the flat steel product in a molten bath either k1) by means of hot-dip coating before cooling in step j) or k2) by means of electrolytic coating after cooling in step j). Method according to Claim 10 , characterized in that the degree of cold rolling in step c) is at least 42%. Method according to Claim 10 or 11 , characterized in that the flat steel product runs through a zinc-based molten bath in work step k). Process according to Claim 12 , characterized in that the molten bath composition in step k) consists of 1-2% by weight Al, 1-2% by weight Mg, the remainder zinc and unavoidable impurities. Method according to one of Claims 10 to 13 , characterized in that the optional coating treatment in step k) takes place in a continuous process. Method according to one of Claims 10 to 14 , characterized in that the cooling rate ThetaB2 in step j) is more than 20 K/s.