Flat steel product and method for the production thereof

Abstract

The present invention relates to a flat steel product which has good deep-drawing ability, low edge-crack sensitivity and good bending behaviour. To this end, the flat steel product contains a steel which consists of (in wt%) 0.1-0.5% C, 1.0-3.0% Mn, 0.9-1.5% Si, up to 1.5% AI, up to 0.008% N, up to 0.020% P, up to 0.005% S, 0.01-1% Cr and optionally one or more of the following elements: up to 0.2% Mo, up to 0.01% B, up to 0.5% Cu, up to 0.5% Ni and optionally a total of 0.005-0.2% microalloying elements, the remainder being iron and unavoidable impurities, wherein 75 < (Mn2 + 55*Cr) / Cr < 3000 where Mn is the Mn content of the steel in wt% and Cr is the Cr content of the steel in wt%. The steel has a structure which consists of at least 80 area % martensite, of which at least 75 area % is tempered martensite and at most 25 area % is non-tempered martensite, at least 5 volume % residual austenite, 0.5 to 10 area % ferrite and at most 5 area % bainite, wherein in the region of the phase boundary between tempered martensite and residual austenite there is a low-Mn ferrite seam which has a width of at least 4 nm and at most 12 nm and the Mn content of which is at most 50% of the average Mn content of the flat steel product. The flat steel product contains carbides with a length of less than or equal to 250 nm.The invention also relates to a method for producing a flat steel product according to the invention, in which method the structural characteristics of the flat steel product according to the invention are set by suitable heat treatment.

Description

[0001] The present application relates to a cold-rolled flat steel product, in particular a cold-rolled flat steel product for automotive construction, which has good deep-drawing properties, low edge crack sensitivity and good bending behavior, as well as to a method for producing such a flat steel product.

[0002] In automotive engineering, high- and ultra-high-strength steels are preferred to reduce vehicle weight. These steels must combine high strength with good formability. Sheet metal subjected to shear cutting processes has a significantly reduced formability in the edge area, increasing the risk of edge cracking during further processing. One method for characterizing edge crack susceptibility is the hole expansion test according to ISO 16630. In bending testing, however, the bending strength and maximum deflection up to the first crack initiation are determined. The angle obtained after the bent specimen springs back is called the bend angle and is a measure of the formability of the tested material. Particularly for complex component geometries, high demands are placed on the deep-drawability of the steels.One method for assessing deep-drawability is the cup-drawing test according to DIN 8584-3, which provides information about the material's deep-drawability by determining the maximum deep-drawing ratio (limiting draw ratio β max ). Typically, both the elongation at break and the maximum deep-drawing ratio decrease with increasing strength.

[0003] When we refer to flat steel products in this case, we mean steel strips, steel sheets or cut pieces made from them, such as blanks.

[0004] WO 2012 / 156428 A1 discloses a method for producing flat steel products. The flat steel products are subjected to a heat treatment in which, after austenitizing, they are cooled to a cooling stop temperature, held at that temperature, and then reheated in a single step at a heating rate Theta_P1 to a temperature TP. The flat steel products have a yield strength of 600 to 1400 MPa, a tensile strength of at least 1200 MPa, an elongation A50 of 10 to 30%, a hole expansion of 50 to 120%, and a bending angle of 100 to 180°. The flat steel products consist of 0.10 - 0.50 wt.% C, 0.1 - 2.5 wt.% Si, 1.0 - 3.5 wt.% Mn, up to 2.5 wt.% Al, up to 0.020 wt.% P, up to 0.003 wt.% S, up to 0.02 wt.% N, and optionally 0.1 - 0.5 wt.% Cr, 0.1 - 0.3 wt.% Mo, 0.0005 - 0.005 wt.% B, up to 0.01 wt.% Ca, 0.01 - 0.1 wt.% V, 0.001 - 0.15 wt.% Ti, 0.02 - 0.05 wt.-% Nb, with the sum of the contents of V, Ti, and Nb being less than or equal to 0.2 wt.%. The microstructure of the flat steel products has less than 5% ferrite, less than 10% bainite, 5-70% untempered martensite, 5-30% retained austenite, and 25-80% tempered martensite. However, WO 2012 / 156428 A1 does not disclose how high strength and good deep-drawability can be achieved simultaneously.

[0005] EP 3 128 023 describes a flat steel product containing Ti carbides and a bainite content of preferably 15% or more and 50% or less and a tempered martensite content of between 30% and more or 70% and less, which is produced by a process having different heating rates, cooling rates and holding times than the process according to the invention.

[0006] WO 2018 / 055695 A1 describes a flat steel product with a microstructure containing more than 70% tempered martensite and bainite. Specific amounts for the individual contents of martensite and bainite in the microstructure are not disclosed. The manufacturing process according to WO 2018 / 055695 A1 provides, among other things, a significantly longer holding time at a holding zone temperature above the Ac3 temperature compared to the process according to the invention.

[0007] WO 2017 / 179372 A1 describes a flat steel product with a microstructure comprising a total of 85% tempered martensite and bainite. The bainite content should preferably be in the range of 20 to 50%, and the martensite content should preferably be in the range of 35 to 85%. In contrast to the process according to the invention, a significantly longer holding time at a holding zone temperature above the Ac3 temperature is used in the production of the flat steel product, and the cooling rate is significantly lower than in the process according to the invention.

[0008] JP H06 93340 A describes a high-strength flat steel product with good formability and a process for its production.

[0009] WO 2019 / 063081 A1 is a subsequently published document and describes a flat steel product produced by a process which differs from the process according to the invention at least in the absence of two-stage heating to treatment temperature.

[0010] Any information provided herein regarding alloy contents and compositions refers to weight or mass, unless expressly stated otherwise. Unless otherwise stated, the information regarding the microstructure components martensite, ferrite, and bainite is based on area percent and the information regarding retained austenite is based on volume percent.

[0011] Against the background of the prior art, the object of the invention was to provide a high-strength flat steel product with optimized mechanical properties, in particular with very good forming properties, in particular good deep-drawing ability and at the same time high strength.

[0012] A further object of the invention was to provide a method for producing such a flat steel product. This method should be particularly suitable for integration into a hot-dip coating process.

[0013] With regard to the flat steel product, the object was achieved by a product having at least the features specified in claim 1. With regard to the method, the object was achieved by completing at least the method steps specified in claim 8 during the production of a flat steel product according to the invention.

[0014] A flat steel product according to the invention contains a steel consisting of (in wt%) 0,1 - 0,5 % C, 1,0 - 3,0 % Mn, 0,9 - 1,5 % Si, up to 1.5% Al, up to 0.008% N, up to 0.020% P, up to 0.005% S 0,01 - 1 % Cr, and optionally one or more of the following elements up to 0.2% Mon, up to 0.01% B, up to 0.5% Cu, up to 0.5% Ni and optionally consisting of a total of 0.005 - 0.2% microalloying elements selected from Ti, Nb and V, and the remainder of iron and unavoidable impurities, where: 75≤Mn2+55*Cr / Cr≤3000 with Mn: Mn content of the steel in wt.%, Cr: Cr content of the steel in wt.%.

[0015] A flat steel product according to the invention has a structure consisting of at least 80 area% martensite, of which at least 75 area% is tempered martensite and at most 25 area% is untempered martensite, at least 5 volume% retained austenite, 0.5 to 10 area% ferrite and at most 5 area% bainite consists.

[0016] Essential for the good mechanical properties is the presence of a low-Mn ferrite seam in the area of ​​the phase boundaries between tempered martensite and retained austenite. The Mn content in this ferrite seam is at most 50% of the average total Mn content of the flat steel product. The width of the low-Mn ferrite seam is at least 4 nm, preferably more than 8 nm, and at most 12 nm, preferably less than 10 nm. Furthermore, a flat steel product according to the invention contains carbides whose length is equal to or less than 250 nm, preferably less than 175 nm.

[0017] A flat steel product according to the invention is characterized by a tensile strength Rm of 900 to 1500 MPa, a yield strength Rp02 which is equal to or more than 700 MPa and less than the tensile strength of the flat steel product, an elongation A80 of 7 to 25%, a bending angle which is greater than 80°, a hole expansion which is greater than 25% and a maximum deep drawing ratio β max for which applies: β max ≥ -1.9 · 10 -6< × ( R m ) 2< + 3.5 · 10 -3< × R m + 0.5 with Rm: tensile strength of the flat steel product in MPa, where the tensile strength, the yield strength and the elongation in the tensile test are determined according to DIN EN ISO 6892-1 (sample shape 2) from 02 / 2017, the bending angle according to VDA238-100 from 12 / 1010, the hole expansion according to ISO 16630 from 10 / 2017 and the maximum deep drawing ratio, ß max according to DIN 8584-3 from 09 / 2003.

[0018] The carbon content of the steel of a flat steel product according to the invention is 0.1–0.5 wt.%. In the steel of a flat steel product according to the invention, carbon contributes to the formation and stabilization of austenite. Especially during the first cooling after austenitizing and during the subsequent partitioning annealing, C contents of at least 0.1 wt.%, preferably at least 0.12 wt.%, contribute to the stabilization of the austenitic phase, making it possible to ensure a residual austenite content of at least 5 vol.% in the flat steel product according to the invention. Furthermore, the C content has a strong influence on the strength of the martensite. This applies both to the strength of the martensite formed during the first quenching and to the strength of the martensite formed during the second quenching following partitioning annealing.To utilize the influence of carbon on martensite strength, the C content is at least 0.1 wt.%. As the C content increases, the martensite initiation temperature Ms shifts to lower temperatures. A C content above 0.5 wt.% could therefore result in insufficient martensite formation during quenching. Furthermore, a high C content can lead to the formation of large, brittle carbides. Processability, especially weldability, is also impaired at higher C contents, which is why the C content should be no more than 0.5 wt.%, preferably no more than 0.4 wt.%.

[0019] Manganese (Mn) is an important alloying element for the hardenability of the steel and for preventing the formation of the microstructural component pearlite during cooling. The Mn content of the steel of a flat steel product according to the invention is at least 1.0 wt.%, preferably at least 1.9 wt.%, in order to provide a pearlite-free microstructure consisting of martensite and retained austenite for the subsequent process steps after the first quenching. An excessively low Mn content would also prevent the formation of a low-Mn ferrite seam. The positive effects of Mn can be utilized particularly reliably at contents of preferably at least 1.9 wt.%. However, with increasing Mn content, the weldability of a flat steel product according to the invention deteriorates and the risk of severe segregation increases.Segregations are chemical inhomogeneities in the composition formed during the solidification process in the form of macroscopic or microscopic demixing. To reduce segregations 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 wt.%, preferably to a maximum of 2.7 wt.%.

[0020] Silicon (Si) as an alloying element helps 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 as interstitially dissolved carbon to stabilize the residual austenite. This impairs the elongation of the flat steel product, since residual austenite contributes to improving elongation. A similar effect with regard to stabilizing the residual austenite can also be achieved by alloying with aluminum. To utilize the positive effect of Si, at least 0.9 wt.% Si should be present in the steel of the flat steel product according to the invention. However, since a high Si content can have a negative effect on the surface quality of the flat steel product, the steel should not contain more than 1.5 wt.%, preferably less than 1.5 wt.% Si.

[0021] Aluminum (Al) can be added to the steel of a flat steel product according to the invention in amounts of up to 1.5 wt.% for deoxidation and to bind nitrogen, if nitrogen is present in the steel. Aluminum can also be added to suppress cementite formation. However, Al increases the austenitizing temperature of the steel. If higher annealing temperatures are to be set for austenitizing, Al can be added in alloys up to 1.5 wt.%. Since aluminum increases the annealing temperature required for complete austenitizing and complete austenitizing is difficult at Al contents above 1.5 wt.%, the Al content of the steel of a flat steel product according to the invention is limited to a maximum of 1.5 wt.%, preferably a maximum of 1.0 wt.%. If a low austenitizing temperature is to be set, Al contents of at least 0.01 wt.%, in particular from 0.01 to 0.1 wt.%, have proven advantageous.

[0022] Phosphorus (P), sulfur (S), and nitrogen (N) have a negative effect on the mechanical and technological properties of flat steel products according to the invention. P has an adverse effect on weldability, which is why the P content should be a maximum of 0.02 wt.%, preferably less than 0.02 wt.%. At higher concentrations, S leads to the formation of MnS or the formation of (Mn, Fe)S, which have a negative effect on elongation. Therefore, the S content is limited to values ​​of a maximum of 0.005 wt.%, preferably less than 0.005 wt.%. Nitrogen bound to form nitrides can have a negative effect on formability, which is why the N content should be limited to a maximum of 0.008 wt.%, preferably less than 0.008 wt.%.

[0023] Chromium (Cr) is present in steel in amounts ranging from 0.01 to 1.0 wt.%. Chromium is an effective pearlite inhibitor and contributes to strength. Therefore, the steel according to the invention should contain at least 0.01 wt.% Cr, preferably at least 0.1 wt.% Cr. At Cr contents of more than 1.0 wt.%, the weldability of a flat steel product according to the invention deteriorates and the risk of severe grain boundary oxidation, which leads to a deterioration in surface quality, is increased. Therefore, the Cr content is limited to a maximum of 1.0 wt.%, preferably a maximum of 0.50 wt.%, and particularly preferably to less than 0.2 wt.%.

[0024] Furthermore, the invention is based on the finding that maintaining a specific ratio of Mn and Cr has a favorable effect on the formation of a Mn-poor ferrite seam along the phase boundary between retained austenite and tempered martensite. Thus, a Mn-poor ferrite seam is established along the phase boundary between retained austenite and tempered martensite if the following condition is met: 75 ≤ Mn 2 + 55 * Cr / Cr ≤ 3000 with Mn: Mn content of the steel in wt.% and Cr: Cr content of the steel in wt.%. If the chromium content is too high compared to the Mn content, the grain boundaries may become coated with chromium carbides. This is undesirable because the formation of the Mn-poor ferrite seam would be prevented by reduced mobility of the phase boundary. However, if the Mn content is too high compared to the chromium content, premature saturation of the austenite with Mn occurs and the diffusion of manganese comes to a halt. No Mn-poor ferrite seam can form due to the still high local Mn concentration. The absence of the ferrite seam would impair the forming properties and in particular the maximum deep drawing ratio β max.

[0025] Optionally, one or more elements from the group of molybdenum (Mo), boron (B) and copper (Cu) may be present in the steel of a flat steel product according to the invention to improve the mechanical-technological properties.

[0026] Molybdenum (Mo) may also optionally be contained in the steel of a flat steel product according to the invention in amounts of up to 0.2 wt.%, preferably less than 0.2 wt.%, in order to prevent the formation of pearlite.

[0027] Boron (B) can be included as an optional alloying element in amounts of up to 0.01 wt.% in the steel of a flat steel product according to the invention. Boron segregates at the phase boundaries, thus blocking their movement. This promotes the formation of a fine-grained microstructure, which improves the mechanical properties of the flat steel product. When adding boron, sufficient Ti should be available to bind N, which prevents the formation of harmful boron nitrides, namely Ti>3.42*N. From a technical perspective, the lower limit for boron is 0.0003%.

[0028] Copper (Cu) can be included as an optional alloying element in the flat steel product according to the invention in amounts of up to 0.5 wt.%. The yield strength and strength can be increased by Cu. To effectively utilize the strength-enhancing effect of Cu, Cu can preferably be added in amounts of at least 0.03 wt.%. Furthermore, at these levels, resistance to atmospheric corrosion is increased. At the same time, however, increasing Cu contents lead to a significant decrease in elongation at break. Furthermore, weldability is significantly reduced at Cu contents greater than 0.5 wt.%, and the tendency toward red brittleness increases, which is why the Cu content is up to 0.5 wt.%, preferably 0.2 wt.%.

[0029] Nickel (Ni) can be included as an optional alloying element in the steel of a flat steel product according to the invention in amounts of up to 0.5 wt.%. Like chromium, it is a pearlite inhibitor and effective even in small amounts. This supporting effect can be achieved with optional alloying with nickel, preferably at least 0.02 wt.%, in particular at least 0.05 wt.%. At the same time, with regard to the desired adjustment of the mechanical properties, it is expedient to limit the Ni content to 0.5 wt.%, whereby Ni contents of a maximum of 0.2 wt.%, in particular 0.1 wt.%, have proven particularly practical.

[0030] Optionally, steels of flat steel products according to the invention can contain one or more microalloying elements. In this context, microalloying elements are understood to be the elements titanium (Ti), niobium (Nb), and vanadium (V). Titanium and / or niobium are preferably used. The microalloying elements can form carbides with carbon, which contribute to increased strength in the form of very finely distributed precipitates. With a total microalloying element content of at least 0.005 wt.%, precipitates can form which lead to the freezing of grain and phase boundaries during austenitization. At the same time, however, carbon, which in atomic form is beneficial for the stabilization of the residual austenite, is bound as carbide. To ensure sufficient stabilization of the residual austenite, the total concentration of the microalloying elements should not exceed 0.2 wt.%.In a preferred embodiment, the sum of Ti and / or Nb is 0.005 - 0.2 wt.%.

[0031] In a preferred embodiment, the flat steel product according to the invention is a cold-rolled flat steel product.

[0032] In a further preferred embodiment, the flat steel products can optionally be provided with a metallic coating for corrosion protection. Zn-based coatings are particularly suitable for this purpose. The coating can be applied, in particular, by hot-dip coating.

[0033] The method according to the invention for producing a high-strength flat steel product comprises at least the following steps: a) Providing a slab which consists of a steel which, in addition to iron and unavoidable impurities, comprises (in wt. %) 0.1 - 0.5% C, preferably 0.12 - 0.4% C, 1.0 - 3.0% Mn, preferably 1.9 - 2.7% Mn, 0.9 - 1.5% Si, up to 1.5% Al, up to 0.008% N, up to 0.020% P, up to 0.005% S, 0.01 to 1% Cr and optionally one or more of the following elements: up to 0.2% Mo, up to 0.01% B, up to 0.5% Cu, up to 0.5% Ni and optionally a total of 0.005 - 0.2% microalloying elements selected from Ti, Nb and V, preferably a total of 0.005 - 0.2% Ti and / or Nb, where: 75 ≤ (Mn 2< + 55* Cr) / Cr ≤ 3000, with Mn: Mn content of the steel in wt.%, Cr: Cr content of the steel in wt.%; b) heating the slab to temperatures of 1000-1300°C and hot rolling the slab to a hot strip, wherein the final rolling temperature T_ET is greater than 850°C;c) cooling the hot-rolled strip within a maximum of 25 seconds to a coiling temperature T_HT of 400 to 620 °C and winding the hot-rolled strip into a coil; d) pickling the hot-rolled flat steel product; e) cold-rolling the hot-rolled flat steel product; f) heating the cold-rolled flat steel product to a holding zone temperature T_HZ which is at least 15 °C above the A3 temperature of the steel and not more than 950 °C, wherein the heating is carried out either f1) in one stage with an average heating rate of 2 - 10 K / s or f2) in two stages with a first heating rate Theta_H1 of 5 - 50 K / s up to a turning temperature T_W of 200 - 400 °C and above the turning temperature T_W with a second heating rate Theta_H2 of 2 - 10 K / s; g) holding the flat steel product at the holding zone temperature T_HZ for a period t_HZ of 5 - 15 s;h) cooling the steel flat product from the holding zone temperature T_HZ to a cooling stop temperature T_Q which lies between the martensite start temperature T_MS and a temperature which is 175 °C lower than T_MS, with a first cooling rate Theta_LK of less than 30 K / s for a first cooling to an intermediate temperature T_LK which is not lower than 650 °C, and a second cooling rate Theta_Q2 for a second cooling from T_LK to T_Q, wherein Theta_Q2 is at least 30 K / s; i) holding the steel flat product at the cooling stop temperature T_Q for 1 - 60 seconds;j) heating the steel flat product at a first heating rate Theta_B1 of between 5 and 100 K / s to a first treatment temperature T_B1 of at least T_Q + 10 °C and at most 450 °C, holding the steel flat product at the first treatment temperature T_B1 for a duration t_B1 of between 8.5 s and 245 s, heating the steel flat product at a second heating rate Theta_B2 of between 2 and 50 K / s to a second treatment temperature T_B2 of at least T_B1 + 10 °C and at most 500 °C, optionally holding the steel flat product at the treatment temperature T_B2 for a duration t_B2 of up to 34 s, the total treatment time t_BT for heating and isothermal holding being between 10 and 250 s; k) optionally coating the flat steel product in a Zn-based coating bath; l) cooling the flat steel product to room temperature with a cooling rate Theta_B3 of at least 5 K / s.

[0034] In step a), a slab produced in a conventional manner is provided, which consists of a steel of the composition specified in step a).

[0035] In step b), the slab is heated to temperatures of 1000-1300°C and rolled into a hot strip. Hot rolling is carried out with a final rolling temperature T_ET greater than 850°C in the otherwise conventional manner. The final rolling temperature T_ET should be higher than 850°C to prevent the formation of coarse, polygonal ferrite grains during the rolling process.

[0036] In step c), the hot strip is cooled after hot rolling and before coiling and then wound into a coil at the coiling temperature T_HT. In order to reduce the formation of polygonal ferrite or, preferably, to completely suppress it, cooling takes place within a time period t_RG of less than or equal to 25 s, i.e., within a maximum of 25 s. t_RG is the time period that begins after the completion of the rolling process, i.e., after the last rolling pass, and ends after the completion of the cooling process, i.e., when the coiling temperature T_HT is reached. The formation of polygonal ferrite can be minimized particularly effectively if t_RG is at most 18 s, preferably at most 15 s. For process-related reasons, t_RG is typically at least 2 s, usually at least 5 s.

[0037] To prevent the formation of the undesirable microstructural component pearlite, coiling takes place at a coiling temperature T_HT of 620 °C or less. In a preferred embodiment, the coiling temperature T_HT is set to 600 °C or less, which also has a positive effect on avoiding polygonal ferrite. Coiling temperatures of 580 °C or less are particularly preferred in order to increase the proportion of bainite in the microstructure of the hot strip. If the coiling temperature is selected to be between 620 °C and 580 °C, the proportion of bainite and bainitic ferrite increases with decreasing coiling temperature. This allows a uniform microstructure without large hardness differences to be achieved, which enables compliance with tight thickness and width tolerances in the subsequent cold rolling step. A further positive effect of the low coiling temperatures is the reduced sensitivity to grain boundary oxidation.In general, the higher the coiling temperature, the more likely oxygen-affine elements, such as Si, Cr, or Mn, will diffuse to the grain boundary and form stable oxides there, reducing surface quality and complicating optional subsequent coating. However, the coiling temperature T_HT should not be set lower than 400 °C, as lower coiling temperatures impair cold rollability due to extensive martensite formation. Martensite is a particularly hard and brittle phase that negatively impacts cold rollability. Furthermore, at lower coiling temperatures, there is no longer sufficient thermal energy available to redistribute the Mn.

[0038] If the cooling time t_RG and coiling temperature T_HT according to the invention are observed, a largely bainitic microstructure is produced in the first few minutes of coiling. This consists primarily of very finely distributed bainitic ferrite and very finely distributed austenite, with the grain sizes of the ferrite and austenite each being in the nanometer range. The shortest distance between two phases is typically less than or equal to 20 µm. Mn is a strong austenite former, which is why there is a driving force for the redistribution of Mn atoms from the ferritic microstructure components into the austenite grains. During cooling in the coil, which is very slow, Mn diffuses from the ferrite into the austenite. As a result, the ferritic microstructure components become depleted of Mn in a region located directly behind the ferrite-austenite phase interface. This Mn-depleted region is a few nanometers wide.At the same time, Mn accumulates in the austenite grains directly behind the phase boundary. The diffusion process is locally limited to a region a few nanometers wide around the phase boundary between austenite and ferrite, as volume diffusion of Mn occurs very slowly in a temperature range between 620 °C and 400 °C. As cooling progresses to temperatures below 400 °C, the austenite partially decomposes into iron carbides. However, this has no influence on the redistribution of Mn, as the diffusion rate of Mn is too low below 400 °C, and no thermodynamic driving force is available for homogenization.

[0039] The Mn diffusion process is supported by very low cooling rates and correspondingly long holding times. In a preferred embodiment, low cooling rates can be achieved by cooling the hot strip in the coil in air, especially in stagnant air.

[0040] In another preferred embodiment, the coil weight can be used to influence the cooling process in the coil. The heavier a coil, the slower the cooling process, because the ratio of coil mass to coil surface area increases. Thus, slow cooling and thus a redistribution of Mn in the hot strip can be promoted if the coil mass m_CG is at least 10 t, particularly preferably at least 15 t, and most preferably at least 20 t.

[0041] After cooling in the coil, the hot-rolled flat steel product is pickled in a conventional manner (step d)) and then subjected to cold rolling in a conventional manner (step e)).

[0042] The cold-rolled flat steel product is heated in step f) to an annealing temperature T_HZ, which can also be referred to as the holding zone temperature. Heating takes place either in a single stage with an average heating rate of 2 - 10 K / s, preferably 5 - 10 K / s. Alternatively, heating can also take place in two stages. The flat steel product is first heated at a heating rate Theta_H1 of 5 - 50 K / s until a turning temperature T_W, which is 200 - 400 °C, is reached. Above the turning temperature T_W, heating takes place at a heating rate Theta_H2 of 2 - 10 K / s until the holding zone temperature T_HZ is reached. In two-stage heating, the first heating rate Theta_H1 is not the same as the second heating rate Theta_H2. Theta_H2 is preferably smaller than Theta_H1.

[0043] In a preferred embodiment, the steel flat product is heated in a continuous furnace. In a particularly preferred embodiment, the steel flat product is heated in a furnace equipped with ceramic radiant tubes, which is particularly advantageous for achieving strip temperatures above 900 °C.

[0044] The holding zone temperature T_HZ is at least 15 °C, preferably more than 15 °C, above the steel's A3 temperature to allow complete microstructural transformation to austenite. The A3 temperature depends on the analysis and can be estimated using the following empirical equation: A 3 ° C = 910 − 15,2 % Ni + 44,7 % Si + 31,5 % Mo − 21,1 % Mn − 203 * √ % C with %C=C content of the steel in wt.%, %Ni=Ni content of the steel in wt.%, %Si=Si content of the steel in wt.%, %Mo=Mo content of the steel in wt.%, %Mn=Mn content of the steel in wt.%.

[0045] The holding zone temperature T_HZ is limited to a maximum of 950 °C, since at higher temperatures and longer holding times, the Mn enrichment in the austenite and the Mn depletion in the ferrite already generated in the hot strip could be rehomogenized. Furthermore, operating costs can be reduced by limiting the annealing temperature to 950 °C.

[0046] In step g), the steel flat product is held at the holding zone temperature T_HZ for a holding time t_HZ of 5 - 15 s. The holding time t_HZ should not exceed 15 seconds to avoid the formation of coarse austenite grains and irregular austenite grain growth, which would negatively impact the formability of the steel flat product. The holding time should last at least 5 s to achieve complete transformation to austenite and a homogeneous C distribution in the austenite. The formation of the Mn-poor zone is also negatively influenced by a long t_HZ and the associated Mn homogenization. A holding time t_HZ that is too long leads to a uniform distribution of the manganese and thus not to the formation of the Mn-poor ferrite seam.

[0047] In step h), the flat steel product is cooled from the holding zone temperature T_HZ to a cooling stop temperature T_Q. The cooling in step h) produces martensite, also known as primary martensite. Cooling occurs in two stages. Rapid cooling occurs over at least part of the temperature range between T_HZ and T_Q, with a cooling rate Theta_Q of at least 30 K / s. In the case of two-stage cooling, the rapid cooling rate Theta_Q is referred to as Theta_Q2.

[0048] In two-stage cooling, the flat steel product is first cooled to an intermediate temperature T_LK at a first cooling rate Theta_LK of less than 30 K / s. In a preferred embodiment, Theta_LK is greater than 0.1 K / s to avoid the formation of ferrite contents exceeding 10% as far as possible. T_LK is less than T_HZ and not lower than 650 °C to avoid the formation of ferrite contents exceeding 10%. After reaching the intermediate temperature T_LK, further cooling to the cooling stop temperature T_Q takes place without interruption at a second cooling rate Theta_Q2 of at least 30 K / s. The maximum value for Theta_Q2 is 1000 K / s, preferably a maximum of 500 K / s, particularly preferably a maximum of 200 K / s, to ensure uniform temperature distribution.The two-stage cooling is carried out at temperatures below 650 °C at a minimum of 30 K / s to prevent both the formation of ferrite fractions exceeding 10% and a bainitic transformation. Ferritic and bainitic transformations can be particularly reliably limited if, in addition, the cooling time t_LK from T_HZ to T_LK does not exceed 30 seconds.

[0049] To control martensite formation, the cooling stop temperature T_Q is selected so that T_Q lies between the martensite start temperature, T_MS, and a temperature that is up to 175 °C lower than T_MS. The following applies: T_MS − 175 ° C < T_Q < T_MS .

[0050] In a preferred embodiment, T_Q can be chosen such that T_Q lies between a temperature which is 75 °C lower than T_MS and a temperature which is 150 °C lower than T_MS: (T_MS-150°C) < T_Q < (T_MS-75°C).

[0051] The martensite start temperature (T_MS) is the temperature at which the transformation from austenite to martensite begins. The martensite start temperature can be estimated using the following equation: T_MS ° C = 539 ° C + − 423 % C − 30,4 % Mn − 7,5 % Si + 30 % Al ° C / Gew . − % with %C=C content of the steel in wt.%, %Mn=Mn content of the steel in wt.%, %Si=Si content of the steel in wt.%, %Al=Al content of the steel in wt.%.

[0052] Manganese reduces the martensite start temperature because Mn, as an austenite former, inhibits the thermodynamic driving force for martensite formation. Thus, martensite formation is promoted by reduced Mn contents. For this reason, the first martensite lancets form preferentially in areas depleted in Mn, whereas areas with increased Mn content remain predominantly austenitic. Thus, the phase boundaries between austenite and martensite are preferentially located at locations of local Mn enrichment and local Mn depletion. These locations of local Mn enrichment and local Mn depletion were already created during the hot strip production process and are finely distributed throughout the material. Typically, the locations of local Mn enrichment and local Mn depletion are distributed within the material at intervals of less than 5 µm, preferably less than 1 µm, from each other.

[0053] In step i), the flat steel product cooled to T_Q is held at the cooling stop temperature T_Q for a duration t_Q, which is 1 - 60 seconds, in order to achieve homogenization of the temperature distribution in the flat steel product across both the thickness and the width. A homogeneous temperature distribution across the thickness and width of the flat steel product promotes the formation of a particularly fine microstructure. Typically, the average grain size is less than 20 µm. In some cases, microstructures with average grain sizes of less than 15 µm or even less than 10 µm can occur. Typically, a uniform microstructure of primary martensite and retained austenite is present across the thickness and width of the flat steel product, which has a positive effect on the formability of the cold-rolled and annealed end product, in this case the coil or the cut sheets.The temperature distribution can be achieved particularly reliably if the flat steel product is kept at T_Q for at least 5 s, particularly preferably at least 10 s.

[0054] After holding at T_Q, the flat steel product is reheated in step j). During heating, the flat steel product is first heated at a first heating rate Theta_B1, which is between 5 and 100 K / s, to a first treatment temperature T_B1, which is at least 10 °C above the cooling stop temperature T_Q. The treatment temperature T_B1 is at least T_Q +10 °C, preferably T_Q + 15 °C, particularly preferably T_Q + 20 °C, and at most 450 °C. The flat steel product is then heated at a second heating rate Theta_B2, which is between 2 and 50 K / s, to a second treatment temperature T_B2, which is at least 10 °C above the first treatment temperature T_B1. The second treatment temperature T_B2 is at least T_B1 + 10 °C, preferably at least T_B1 + 15 °C, particularly preferably at least T_B1 + 20 °C. The second treatment temperature T_B2 is at most 500 °C.In a subsequent optional treatment step, the flat steel product can be held isothermally at the second treatment temperature T_B2 for a duration t_B2 of up to 34 s. The total treatment duration t_BT, which includes heating to T_B1, isothermal holding at T_B1, heating to T_B2, and optional holding at T_B2, is between 10 and 250 seconds.

[0055] During heating to the first treatment temperature T_B1, the retained austenite is enriched with carbon from the supersaturated primary martensite. In a preferred embodiment, the ratio of primary martensite to retained austenite is greater than 2:1, as this ratio has proven particularly favorable for achieving good forming behavior. With a ratio of primary martensite to retained austenite greater than 2:1, the effect of an increased thermodynamic driving force can be used to support the displacement of carbon into the retained austenite. Due to the comparatively low atomic mass and the high diffusibility of carbon, particularly in the body-centered cubic lattice of martensite, the diffusion process begins as early as the cooling stop temperature T_Q and thus with the onset of the martensitic transformation.Since the diffusibility of carbon in the face-centered cubic lattice of austenite is significantly lower than in martensite, C atoms accumulate at the phase boundary between the primary martensite and the austenite. This accumulation leads to a local increase in the C concentration at this point, which can amount to several weight percent. To ensure sufficient enrichment of C atoms at the phase boundary between the primary martensite and the austenite, the first treatment temperature T_B1 should be at least 10 °C, preferably at least 15 °C, and particularly preferably at least 20 °C above the cooling stop temperature T_Q. In order to prevent an excessive local increase in the C concentration at this point, T_B1 should not exceed 450 °C, preferably not exceed 430 °C, and the duration of the isothermal holding at T_B1 should not exceed 245 s, preferably at most 200 s, particularly preferably at most 150 s.

[0056] Heating to the second treatment temperature T_B2 increases the thermodynamic stability of the retained austenite to such an extent that the austenite phase expands locally. The accumulated C atoms are initially absorbed by the retained austenite. As the temperature continues to rise, so does the diffusion of carbon in the retained austenite. This reduces the concentration gradient of the C content at the phase boundary between primary martensite and austenite, so that the carbon in the retained austenite is distributed almost evenly and homogeneously. To ensure adequate homogenization, the second treatment temperature T_B2 is at least 10°C, preferably at least 15°C, and particularly preferably at least 20°C higher than the first treatment temperature T_B1, and does not exceed 500°C.As the carbon becomes more homogeneous, the grain boundaries of the retained austenite retreat, reducing the proportion of retained austenite formed during isothermal holding at treatment temperature T_B1. The moving phase boundary transports the carbon into the retreated retained austenite formed during heating to the second treatment temperature T_B2. At the same time, the heating increases the diffusibility of manganese in the region of the phase boundary, leading to an enrichment of manganese in the retreated retained austenite. An optional holding at treatment temperature T_B2 for a duration of up to 34 s has also proven beneficial for carbon and manganese diffusion. A seam of low-manganese ferrite forms along the retreating austenite phase boundary, with a width of a few nanometers, in particular equal to or less than 12 nm.The low-Mn ferrite seam forms primarily in the low-Mn areas already formed during the production of the hot strip in steps b) and c), as ferrite formation is particularly favored in these areas. The low-Mn ferrite seam is significantly more ductile than the remaining structural components. In the final product, this ductile ferrite serves as a compensation zone between structural components with different degrees of plasticization, such as tempered and untempered martensite. The low-Mn ferrite seam, together with the retained austenite, counteracts the propagation of microcracks, thereby improving hole expansion in particular.

[0057] The heating time to T_B1 is referred to here as t_BR1. t_BR1 can be determined from the quotient of the difference between the treatment temperature T_B1 and the cooling stop temperature T_Q divided by the heating rate Theta_B1: t_BR1 = T_B1 − T_Q / Theta_B 1 with t_BR1 = heating time in s; T_B1 = treatment temperature in °C; T_Q = cooling stop temperature in °C; Theta_B1 = heating rate in K / s.

[0058] With faster heating, with heating rates Theta_B1 greater than 100 K / s, it is difficult to achieve a uniform treatment temperature T_B1 across the entire range due to process and control technology. With very slow heating, with heating rates Theta_B1 less than 5 K / s, the process is very slow, and increased carbides form. However, the carbides bind carbon, which is then no longer available to stabilize the residual austenite. Furthermore, these carbides are brittle, which prevents flow in the material, which in turn leads to a deterioration of subsequent macroscopic properties, such as the deep-drawing ratio, elongation at fracture, and hole expansion.

[0059] Completely preventing carbide formation is generally not possible due to process technology. However, the length of the carbides, which influences the mechanical and technological properties of the flat steel product, can be controlled by the heating rate. The heating rate Theta_B1 is between 5 and 100 K / s to adjust the carbide length to a maximum of 250 nm, preferably a maximum of 175 nm. In this context, the length of the carbides is defined as the longest axis of the carbides.

[0060] The average heating rate Theta_B2, with which the steel flat product is heated from the first treatment temperature T_B1 to the second treatment temperature T_B2 during the two-stage heating process, is 2 to 50 K / s. The time in which the steel flat product is heated from T_B1 to T_B2 is referred to here as t_BR2. t_BR2 is 0 to 35 s. The average heat treatment rate Theta_B2 can be determined as Theta_B2 = T_B2 − T_B1 / t_BR2 where Theta_B2 = heat treatment rate in K / s; t_BR2 = time in which the steel flat product is brought from T_B1 to T_B2, in s; T_B1 or T_B2 = treatment temperature in °C.

[0061] In principle, heating can be carried out using conventional heating devices. However, the use of jet pipes or a booster has proven particularly effective.

[0062] In step j), the steel flat product is held isothermally at the treatment temperature T_B1 and optionally at the treatment temperature T_B2. Isothermal holding at T_B1 and optionally at T_B2 can be used to support the redistribution of the carbon. The steel flat product is held at the treatment temperature T_B1 for a duration t_B1 of between 8.5 and 245 s and optionally at the treatment temperature T_B2 for a duration t_B2 of up to 34 s. In a preferred embodiment, the duration of heating to T_B2 and the holding time at temperature T_B2 total a maximum of 35 s, i.e. (t_B2+t_BR2) ≤ 35 s, preferably less than 25 s, and particularly preferably less than 20 s.

[0063] The total treatment time t_BT, during which the steel flat product is heated to T_B1, held at T_B1, heated to T_B2, and optionally held at T_B2, should be between 10 and 250 s. Treatment times shorter than 10 s have a detrimental effect on carbon redistribution. Treatment times longer than 250 s promote undesirable carbide formation.

[0064] During the holding time or directly during heating in step j), the steel flat product can be subjected to a hot-dip coating in a Zn-based coating bath in an optional step k). The duration for which the steel flat product is passed through the coating bath is included in the holding time t_B2 or the heating time t_BR2.

[0065] To avoid strength losses, it has proven advantageous to keep the duration t_BR2 for heating to the second treatment temperature T_B2 and the holding time t_B2 short. In particular, it has proven advantageous if the holding time t_B2 is zero seconds, so that the flat steel product passes directly from the second heating phase t_BR2 into the coating bath. Thus, high strength values ​​can be achieved particularly reliably if the duration t_BR2 for heating to T_B2 and the optional holding time t_B2 together are a maximum of 35 s, preferably less than 25 s, and particularly preferably less than 20 s.

[0066] Coating baths suitable for hot-dip coating have the following composition: ≥ 96 Gew . − % Zn , 0,5 − 2 Gew . − % Al , 0 − 2 Gew . − % Mg .

[0067] The coating baths typically have temperatures of 450 - 500 °C.

[0068] After the optional coating in step k) or - if step k) is omitted - after heating and optionally holding at treatment temperature T_B2 in step j), the flat steel product is cooled in a further step l) at a cooling rate Theta_B3 of more than 5 K / s. The cooling rate should be more than 5 K / s to enable the formation of secondary martensite. Secondary martensite is understood to be the martensite formed during cooling in step l). Since the secondary martensite does not undergo any heating treatment, it is also referred to herein as untempered martensite.

[0069] The flat steel product produced according to the invention has a particularly fine-grained structure with an average grain size of less than 20 µm, which contains a total martensite content of at least 80 area%, of which at least 75 area% is tempered martensite and at most 25 area% is untempered martensite, at least 5 vol.% residual austenite, 0.5 to 10 area% ferrite and at most 5 area% bainite.

[0070] The microstructure contains carbides with a length of less than or equal to 250 nm, in particular less than 250 nm, and preferably less than 175 nm. The residual austenite is surrounded by a low-Mn ferrite rim. This rim forms a low-Mn zone in a region of the phase boundary between tempered martensite and residual austenite, the Mn content of which is at most 50%, in particular less than 50% of the average total Mn content of the flat steel product, preferably at most 30%, in particular less than 30% of the average total Mn content of the flat steel product. The width of the low-Mn ferrite rim is at least 4 nm, in particular more than 4 nm, and preferably at least 8 nm, in particular more than 8 nm. The width of the low-Mn ferrite rim is at most 12 nm, in particular less than 12 nm, and preferably at most 10 nm, in particular less than 10 nm.

[0071] In this case, the average total Mn content of the flat steel product is equated with the average Mn content of the steel melt from which the flat steel product was produced.

[0072] Martensite: The total martensite content in the microstructure of a flat steel product according to the invention is at least 80 area%. The martensite present in the microstructure of a flat steel product according to the invention is formed firstly during the first cooling in step h) and secondly during the second cooling in step l). The martensite formed during the first cooling is also referred to as primary martensite, and the martensite formed during the second cooling is also referred to as secondary martensite. The primary martensite is heated in step j). The heated primary martensite is also referred to as tempered martensite or primary tempered martensite. The sum of the martensite contents of the tempered and secondary martensite is also referred to as the total martensite content. As a hard structural component, martensite contributes significantly to the strength of the flat steel product.The total martensite content is at least 80 area% to obtain a flat steel product with a tensile strength Rm of at least 900 MPa.

[0073] Tempered martensite: The primary martensite formed prior to the heating process performed in step j) is the source of the carbon that diffuses into the retained austenite during the heating treatment and stabilizes it. After the heating treatment, this martensite is referred to as tempered martensite. Its proportion should be at least 75% of the total martensite by area to ensure a bending angle greater than 80° and a hole expansion greater than 25%.

[0074] Secondary martensite: Secondary martensite is formed from the residual austenite that was insufficiently stabilized in treatment step j) and contributes to strength. In amounts greater than 25% of the total martensite content, secondary martensite leads to premature crack formation during forming and should therefore be kept below 25%.

[0075] Retained austenite: The microstructure of a flat steel product according to the invention contains retained austenite at room temperature. Retained austenite contributes to improving elongation properties. To ensure sufficient elongation, the proportion of retained austenite should be at least 5 vol.%.

[0076] Ferrite: Ferrite is less strong than martensite, but can support formability in small amounts. Therefore, the ferrite content in the microstructure of a flat steel product according to the invention is limited to 0.5 to 10 area%. Due to the low-Mn ferrite seam formed during reheating (step j), a minimum ferrite content of 0.5 area% is present in the microstructure.

[0077] Bainite: In principle, bainite can also form during the phase transformation of austenite. During the transformation of austenite to bainite, a portion of the dissolved carbon is incorporated into the bainite and is thus no longer available for carbon enrichment in the austenite. To ensure as much carbon as possible for austenite enrichment, the bainite content should be limited to a maximum of 5 area%. The lower the bainite content, the more reliably the mechanical properties of the flat steel product can be achieved. The mechanical properties can be achieved with particular reliability if the formation of bainite can be completely suppressed and the bainite content is reduced to as little as 0 area%.

[0078] Low-Mn ferrite rim: In the flat steel product according to the invention, the residual austenite grains are surrounded by a narrow, low-Mn ferrite rim. During heating to treatment temperatures T_B1 or T_B2 and during holding at T_B1 or T_B2, a low-Mn zone consisting of a low-Mn ferrite rim forms around the residual austenite grains. The low-Mn ferrite rim is significantly more ductile than the surrounding structural components. It represents a compensation zone between structural components with different degrees of plasticization and thus counteracts the propagation of microcracks. This leads to an improvement in the forming behavior of the final product, particularly hole expansion and the maximum deep-drawing ratio. The Mn content in the Mn-poor zone is at most 50%, in particular less than 50% of the average total Mn content of the flat steel product in order to achieve a hole expansion of more than 25% and a bending angle of more than 80°.This effect can be achieved particularly reliably if the Mn content in the Mn-poor zone is at most 30%, in particular less than 30% of the average Mn content of the flat steel product. The width of the Mn-poor ferrite rim is at least 4 nm, in particular more than 4 nm, since ductile compensation can only occur from a width of 4 nm. If the Mn-poor ferrite rim were narrower, the zone would no longer contribute effectively to ductility compensation, but the forming would already be influenced by grain boundary effects. Ductility compensation can be achieved particularly reliably if the Mn-poor ferrite rim is preferably at least 8 nm, in particular more than 8 nm wide. The width of the Mn-poor ferrite rim increases with increasing treatment time during treatment step j).Since the positive contribution of the seam is saturated at 12 nm and the risk of carbide formation increases with increasing treatment time during step j), the width of the seam should be no more than 12 nm, especially less than 12 nm. This effect can be achieved particularly reliably if the low-Mn ferrite seam is preferably no more than 10 nm wide, especially less than 10 nm.

[0079] Carbides: Carbides bind carbon. The carbon bound in carbide form is not available for redistribution into the austenite. Furthermore, carbides exhibit brittle fracture behavior. The brittle behavior of carbides prevents plastic flow in the material, which leads to a deterioration of macroscopic properties, such as the maximum deep drawing ratio and / or hole expansion. The maximum length of the carbides should be equal to or less than 250 nm to avoid a deterioration in fracture elongation and / or hole expansion. The mechanical and technological properties can be achieved with particular reliability if the length of the carbides is preferably less than 175 nm. The length of a carbide is understood to be its longest axis. In this context, the term "carbides" generally refers to carbon precipitates.These are precipitates in which carbon, together with elements present in the flat steel product, forms compounds such as iron carbides, chromium carbides, titanium carbides, niobium carbides or vanadium carbides.

[0080] The method according to the invention enables the production of a steel flat product with a tensile strength Rm of 900 to 1500 MPa, a yield strength Rp02 equal to or greater than 700 MPa and less than the tensile strength of the steel flat product, an elongation A80 of 7 to 25%, a bending angle greater than 80°, a hole expansion greater than 25%, and a maximum deep drawing ratio β max , for which the following relationship applies: β max ≥ − 1,9 ⋅ 10 − 6 × R m 2 + 3,5 ⋅ 10 − 3 × R m + 0,5 with Rm: tensile strength of the flat steel product in MPa.

[0081] In a preferred embodiment, the flat steel product exhibits a balanced ratio of high tensile strength and good deep-drawing properties. In this case, the maximum deep-drawing ratio β max is at least 1.475. A flat steel product according to the invention thus exhibits both good strength and forming properties.

[0082] The method according to the invention enables in particular the production of a flat steel product according to one of claims 1 to 7.

[0083] The invention is explained in more detail below using exemplary embodiments and figures.

[0084] Figure 1schematically shows a method not according to the invention. The cold-rolled and uncoated flat steel product is heated to a holding temperature T_HZ and held there before being cooled in a single stage at a cooling rate Theta_Q1 to a cooling stop temperature T_Q. After an isothermal hold at T_Q, the flat steel product is heated in a first heating step to the treatment temperature T_B1, where it is held isothermally. It is then heated to a second treatment temperature T_B2, where it is held again before being cooled to room temperature.

[0085] Figure 2shows a schematic representation of a method according to the invention. The cold-rolled and uncoated flat steel product is also heated to a holding temperature T_HZ and held there, before being cooled at a first, slower cooling rate Theta_LK to an intermediate temperature T_LK and then at a second, faster cooling rate Theta_Q2 to the cooling stop temperature T_Q. The flat steel product is then Figure 1 As already explained, it is heated in two stages and then cooled to room temperature.

[0086] The described inventive method can also be combined with a hot-dip coating treatment. The hot-dip coating is included in the isothermal holding at treatment temperature T_B2 or in the time period t_BR2 during heating to treatment temperature T_B2, before the flat steel product is cooled to room temperature.

[0087] The invention was tested using several exemplary embodiments. 14 tests were conducted. Samples of 14 cold-rolled and coated steel strips produced from the steels A - G listed in Table 1 were examined. For this purpose, slabs were first produced in a conventional manner from melts with the compositions listed in Table 1. Before hot rolling, the slabs were heated to a temperature of 1000-1300°C and, under the conditions listed in Table 2, rolled into hot strips in an otherwise conventional manner and wound into hot strip coils. The hot strips were pickled in a conventional manner and then cold-rolled in a conventional manner.

[0088] Table 3 shows the conditions under which the samples were heat treated. The cold-rolled flat steel products were each heated in a single stage at the heating rate Theta_H1 specified in Table 3 to the holding zone temperature T_HZ and held at temperature T_HZ for 5 to 15 s. The flat steel products were then cooled in two stages, first at a first cooling rate Theta_LK, which was more than 0.1 K / s and equal to or less than 30 K / s, to the intermediate temperature T_LK and then at a second cooling rate Theta_Q2 to the cooling stop temperature T_Q. The flat steel products were held at T_Q for between ≥1 second and ≤60 seconds and then heated to a first treatment temperature T_B1 at a first heating rate Theta_B1 for a duration t_BR1.After heating, the steel flat products were held at T_B1 for a duration t_B1 and then heated at a second heating rate Theta_B2 for a duration t_BR2 to the second treatment temperature T_B2, at which they were directly introduced into a Zn-based coating bath. The steel flat products were continuously passed through a coating bath with a composition of ≥ 96% Zn, 0.5-2% Al, 0-2% Mg. The time t_B2, which also includes the passage of the steel flat products through the coating bath, as well as the total treatment time, are also given in Table 3. After coating, the steel flat products were cooled at a cooling rate Theta_B3 of more than 5 K / s.

[0089] After cooling, samples were taken for microstructure analysis and to determine the mechanical properties. The microstructure was examined on three cross-sections, each taken equidistantly across the width of the flat steel product. The microstructure was examined at at least three equidistant locations across the thickness of the flat steel product. Due to the very fine-grained microstructure, microstructural assessment using conventional light-optical examination methods was not possible. Therefore, the proportions of primary, tempered martensite (M(PRI) M_1), secondary martensite (M(SEK) M_2), ferrite (F), and bainite (B) were examined using a scanning electron microscope (SEM) at a magnification of at least 5000x. The quantitative determination of the retained austenite content was carried out using X-ray diffraction (XRD) according to ASTM E975.The Mn-poor ferrite rim and the Mn content of the Mn-poor ferrite rim were described using atom probe tomography (APT). The width of the Mn-poor ferrite rim, referred to as the Mn rim in Table 4, was also determined in this way. To determine the Mn content of the Mn-poor ferrite, the number of atoms in a defined volume element, e.g., a cylinder or a cuboid, was determined. To determine the width of the Mn-poor ferrite rim, the width of the rim was measured at at least three different locations on a sample. The individual values ​​were arithmetically averaged and represent the value referred to as the width of the Mn-poor ferrite rim. The Mn content of the Mn-poor ferrite is referred to as the Mn content rim in Table 4. The length of the carbides was determined using TEM. The results of the microstructure investigations are shown in Table 4.

[0090] The results of the mechanical property tests are shown in Table 5. The mechanical properties were tested on samples taken from three locations equidistantly distributed along the length of the flat steel product, each at the center of the width of the flat steel product. The yield strength Rp02, the tensile strength Rm, and the elongation A80 were determined in the tensile test according to DIN EN ISO 6892-1 (sample shape 2) from February 2017. The bending angle was determined according to VDA238-100 from December 2010, the hole expansion (HER) was determined according to ISO 16630 from October 2017, and the maximum deep-drawing ratio β max was determined according to DIN 8584-3 from September 2003.

[0091] The results demonstrate that tests conducted using the process according to the invention lead to high strength combined with good forming behavior. Specimens B2, B3, D7, D9, F12, F13, and G14 exhibit bending angles greater than 80° and hole expansion values ​​greater than 25%. Test A1 demonstrates that the microstructure according to the invention could not be achieved with a silicon content not according to the invention. The high proportion of secondary martensite and the high proportion of ferrite led to a comparatively low yield strength and tensile strength. Furthermore, only a very narrow, low-Mn ferrite seam was present, so that only a small bending angle and low hole expansion were achieved.

[0092] Test B4 shows that, despite the steel composition according to the invention, formability deteriorates if the final rolling temperature T_ET and the cooling stop temperature T_Q are not in accordance with the invention and the Mn-poor ferrite seam is too narrow. Although the yield strength and tensile strength are sufficiently high, the bending angle and hole expansion are too low due to the insufficient Mn depletion in the Mn-poor ferrite seam and the insufficient Mn enrichment in the zone adjacent to the Mn-poor ferrite seam, respectively.

[0093] Tests C5 and C6 show that when the carbon and silicon content is too low, the proportion of bainite (Test C5) or secondary martensite and ferrite (Test C6) is too high, and the width of the Mn-poor ferrite rim is too small to achieve sufficiently high hole expansion (Test C5) or sufficient yield strength, bending angle, and hole expansion (Test C6). Test D8 shows that, despite the steel composition according to the invention, formability is impaired by excessively long carbides if the coiling temperature T_HT is too high, the heating rate Theta_B1 is too low, and the overall heat treatment time t_BT is too long. A t_BT that is too long leads to exceeding the maximum carbide length, which negatively affects hole expansion.

[0094] Test E10 shows that if the silicon content is too low and the cooling time after hot rolling to coiling temperature, t_RG, is too long, the proportion of secondary martensite and the proportion of ferrite increases, which leads to an inhomogeneous microstructure and thus to an insufficient bending angle and insufficient hole expansion.

[0095] Test E11 shows that with too low a silicon content and a coiling temperature not in accordance with the invention, the proportion of secondary martensite increases and the carbides become too long, which impairs the A80 elongation and hole expansion. Test E11 also shows that both an excessively low coiling temperature and an exceeding the treatment time to T_B2, i.e., t_BR2+t_B2 > 35 seconds, have a negative impact on the properties of the flat steel product. If carbide formation cannot be sufficiently suppressed, excessively long carbides form, leading to premature crack formation and correspondingly poor hole expansion values. Table 1 melt C Si Mn P S Al Cr Cu Nb Mon N Ti V Ni B 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 0,034 0,0011 B 0,218 1,48 2,21 0,016 0,0023 0,024 0,173 0,047 0,001 0,010 0,0046 - - 0,036 0,0004 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 0,030 0,0013 D 0,158 1,18 1,99 0,014 0,0020 0,017 0,022 - - - 0,0016 0,015 0,001 - 0,0015 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 0,041 0,0014 F 0,246 1,47 2,26 0,011 0,0022 0,023 0,153 - - 0,054 0,0030 - - - - G 0,202 1,40 2,80 0,011 0,0022 0,023 0,030 0,039 - - 0,0037 0,021 - 0,030 0,0007 Values ​​in wt.%, the remainder being iron and unavoidable impurities. Underlined values ​​are outside the specifications of the invention. Table 2 sample melt T_ET [°C] t_RG [s] T_HT [°C] m_CG [1000kg] A1 A 910 19 610 24 B2 B 900 21 570 22 B3 B 900 22 560 23 B4 B 830 17 560 20 C5 C 920 20 570 12 C6 C 930 29 520 17 D7 D 920 18 540 27 D8 D 930 19 650 28 D9 D 900 14 580 25 E10 E 910 27 510 28 E11 E 870 18 380 11 F12 F 905 20 550 17 F13 F 895 17 575 18 G14 G 920 22 515 15 Underlined values ​​are outside the specifications of the invention. Table 3 sample Theta_H [K / s] T_HZ [°C] T_LK [°C] T_Q [°C] Theta_Q2 [K / s] T_B1 [°C] Theta_B1 [Class] t_BR1 [s] t_B1 [s] T_B2 [°C] Theta_B2 [K / s] t_BR2 [s] t_B2 [s] t_BT [s] A1 5 890 689 403 31 420 3 5,7 70 460 50 0,8 15 91,5 B2 4 895 651 335 34 395 5,5 10,9 45 455 42 1,4 22 79,3 B3 4 905 657 325 36 405 20,5 3,9 65 455 45 1,1 25 95,0 B4 6 890 693 421 31 438 12 1,4 23 463 35 0,7 28 53,1 C5 7 855 670 390 30 413 3 7,7 75 459 10 4,6 26 113,3 C6 5 790 650 390 26 441 29 1,8 15 462 25 0,8 17 34,6 D7 4 882 690 286 35 389 34 3,0 80 454 29 2,2 21 106,3 D8 6 890 700 320 32 402 0,5 164 156 467 21 3,1 35 358,1 D9 6 880 705 295 37 393 43 2,3 110 451 32 1,8 30 144,1 E10 8 810 650 405 32 435 6 5,0 90 462 38 0,7 31 126,7 E11 4 895 630 375 39 405 4,5 6,7 85 449 30 1,5 50 143,1 F12 4 905 755 327 38 410 11 7,5 57 462 12 4,3 0 68,8 F13 8 891 683 295 43 349 5 10,8 54 447 15 6,5 25 96,3 G14 6 905 679 267 47 395 18 7,1 59 456 24 2,5 12 80,6 Underlined values ​​are outside the specifications of the invention. Table 4 sample M(PRI) M_1 [area-%] M(SEK) M_2 [area-%] F [area%] B [area%] RA [vol.%] Mn edge [nm] Mn content edge [%] Carbide length [nm] A1 35 45 15 2 3 1 0,31 50 B2 70 10 8 0 12 7 0,58 120 B3 80 8 1 0 11 9 0,64 150 B4 50 40 0 2 8 2 0,41 90 C5 45 20 0 29 6 2 0,62 130 C6 25 55 15 2 3 1 0,61 150 D7 70 17 2 1 10 10 0,57 140 D8 80 11 0 0 9 12 0,79 310 D9 65 15 5 1 14 9 0,86 130 E10 40 42 15 0 3 2 1,02 195 E11 45 47 5 0 3 4 1,13 265 F12 70 15 3 0 12 11 0,51 140 F13 85 5 2 0 8 8 0,65 125 G14 90 0 4 0 6 8 0,63 90 Underlined values ​​are outside the specifications of the invention. Table 5 sample Rp02 [MPa] Rm [MPa] A80 [%] Bending [°] HER [%] ß max [-] A1 580 897 15 67 12 2,0 B2 870 1199 16 95 37 2,1 B3 935 1185 14 116 42 2,0 B4 728 1254 11 58 7 1,9 C5 715 1103 11 81 24 2,1 C6 685 1124 14 72 19 2,1 D7 902 1075 15 139 49 2,2 D8 867 1027 12 63 4 1,7 D9 848 1091 18 128 37 2,1 E10 714 1238 9 76 12 1,8 E11 869 1213 6 92 18 1,6 F12 1005 1379 18 97 32 1,8 F13 1283 1358 16 119 35 1,9 G14 1098 1189 17 112 31 2,1 Underlined values ​​are outside the specifications of the invention.

Claims

1. Flat steel product, characterised in that it contains a steel, which consists of (in wt%) 0.1 - 0.5%C,1.0 - 3.0%Mn,0.9 - 1.5%Si,up to 1.5%Al,up to 0.008%,N,up to 0.020%,P,up to 0.005%,S,0.01- 1%Cr, as well as optionally consisting of one or more of the following elements up to 0.2%Mo,up to 0.01%B,up to 0.5%Cu,up to 0.5%Ni as well as optionally consisting of in total 0.005 - 0.2% microalloying elements, selected from Ti, Nb and V, and iron as a remainder and unavoidable impurities, wherein the following applies: 75 ≤ Mn 2 + 55 * Cr / Cr ≤ 3000 where Mn is the Mn content of the steel in wt%, Cr is the Cr content of the steel in wt% and has a structure, which consists of - at least 80 area% martensite, of which at least 75 area% is tempered martensite and at most 25 area% is non-tempered martensite, - at least 5% by volume of residual austenite, - 0.5 to 10 area% ferrite and - at most 5 area% bainite, wherein in the region of the phase boundary between tempered martensite and residual martensite there is a low-Mn ferrite seam which has a width of at least 4 nm and at most 12 nm and the Mn content of which is at most 50% of the average total Mn content of the flat steel product and the flat steel product has carbides, the length of which is equal to or less than 250 nm and the flat steel product has a tensile strength of 900 to 1500 MPa, a yield strength Rp02 of more than 700 MPa, an elongation A80 of 7 to 25%, a bending angle of greater than 80°, a hole expansion of greater than 25% and a maximum deep-drawing ratio ßmax, determined as disclosed in the description, for which the following applies: β max ≥ − 1.9 ⋅ 10 − 6 × R m 2 + 3.5 ⋅ 10 − 3 × R m + 0.5 where Rm is the tensile strength of the flat steel product in MPa.

2. Flat steel product according to claim 1, characterised in that the width of the low-Mn ferrite seam is at least 8 nm.

3. Flat steel product according to any one of the preceding claims, characterised in that the width of the low-Mn ferrite seam is at most 10 nm.

4. Flat steel product according to any one of the preceding claims, characterised in that the Mn-content of the low-Mn ferrite seam is at most 30% of the average total Mn content in the flat steel product.

5. Flat steel product according to any one of the preceding claims, characterised in that the length of the carbides is shorter than 175 nm.

6. Flat steel product according to any one of the preceding claims, characterised in that the flat steel product has a maximum deep-drawing ratio βmax of at least 1.475.

7. Flat steel product according to any one of the preceding claims, characterised in that the flat steel product is provided with a metallic coating.

8. Method for producing a high-strength flat steel product, comprising at least the following work steps: a) Providing a slab, which consists of a steel, which, in addition to iron and unavoidable impurities, consists of (in wt%) 0.1 - 0.5% C, 1.0 - 3.0% Mn, 0.9 - 1.5% Si, up to 1.5% Al, up to 0.008% N, up to 0.020% P, up to 0.005% S, 0.01 to 1% Cr, as well as optionally consisting of one or more of the following elements: up to 0.2% Mo, up to 0.01% B, up to 0.5% Cu, up to 0.5% Ni as well as optionally consisting of in total 0.005 - 0.2% of microalloying elements, selected from Ti, Nb and V, wherein the following applies: 75 ≤ (Mn2 + 55* Cr) / Cr ≤ 3000 where Mn is the Mn content of the steel in wt%, Cr is the Cr content of the steel in wt%; b) Heating the slab to temperatures of 1000 - 1300°Cand hot rolling the slab into a hot strip, wherein the end rolling temperature T_ET is greater than 850°C; c) Cooling the hot strip within at most 25 seconds to a coiling temperature T_HT of 400 to 620°C, and winding the hot strip into a coil; d) Pickling the hot-rolled flat steel product; e) Cold-rolling the hot-rolled flat steel product; f) Heating the cold-rolled flat steel product to a holding zone temperature T_HZ of at least 15°C above the A3 temperature of the steel and which is at most 950°C, wherein the heating takes place either f1) in one phase at an average heating rate of 2 - 10 K / s or f2) in two phases at a first heating speed Theta_H1 of 5 - 50 K / s up to a conversion temperature T_W of 200 - 400°C and above the conversion temperature T_W at a second heating speed Theta_H2 of 2 - 10 K / s; g) Holding the flat steel product for a duration t_HZ of 5 - 15 seconds at the holding zone temperature T_HZ; h) Cooling the flat steel product from the holding zone temperature T_HZ to a cooling stop temperature T_Q that is between the martensite start temperature T_MS and a temperature that is 175°C lower than T_MS, at a first cooling rate Theta_LK of less than 30 K / s for a first cooling to an intermediate temperature T_LK of not lower than 650°C, and a second cooling rate Theta_Q2 for a second cooling from T_LK to T_Q, wherein Theta_Q2 is at least 30 K / s; i) Holding the flat steel product at the cooling stop temperature T_Q for 1 - 60 seconds; j) Heating the flat steel product at a first heating rate Theta_B1 of between 5 and 100 K / s, to a first treatment temperature T_B1 of at least T_Q+ 10°C and at most 450°C, holding the flat steel product at the first treatment temperature T_B1 for a duration t_B1 of 8.5 seconds to 245 seconds, heating the flat steel product at a second heating rate Theta_B2 of between 2 and 50 K / s, to a second treatment temperature T_B2 of at least T_B1 + 10°C and at most 500°C, optionally holding the flat steel product at the treatment temperature T_B2 for a duration t_B2 of up to 34 seconds, wherein the entire treatment time t_B2 for the heating and the isothermic holding is in total between 10 and 250 seconds; k) Optionally coating the flat steel product in a Zn-based coating bath; l) Cooling the flat steel product to room temperature at a cooling rate Theta_B3 of at least 5 K / s;9. Method according to claim 8, characterised in that the weight of the hot strip wound into a coil is at least 10 t.

10. Method according to claim 8 or 9, characterised in that the hot strip is cooled in work step c) within at most 18 seconds.

11. Method according to any one of claims 8 to 10, characterised in that the coiling temperature T_HT is at most 600°C.

12. Method according to any one of claims 8 to 11, characterised in that the hold time t_Q is at least 5 seconds.

13. Method according to any one of claims 8 to 12, characterised in that the cooling stop temperature T_Q is between a temperature that is lower than the martensite start temperature T_MS by 75°C, and a temperature that is lower than T_MS by 150°C.

14. Method according to any one of claims 8 to 13, characterised in that the duration t_BR2 for the heating to T_B2 and the duration for the optional holding at T_B2 is together at most 35 seconds.

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