METHOD FOR MANUFACTURING A STEEL FLAT PRODUCT, STEEL FLAT PRODUCT AND USE OF SUCH A STEEL FLAT PRODUCT

Description

[0001] The invention relates to a coated steel flat product suitable for press hardening, which has particularly good aging resistance, and to a method for its production.

[0002] Here, "flat steel products" refers to rolled products whose length and width are significantly greater than their thickness. This includes, in particular, steel strips, steel sheets, and blanks or plates derived from them. Blanks and plates are defined as sheet metal panels separated from the steel strips or sheets, which have more complex contours than the steel strips or sheets and possess a shape suitable for forming into a component.

[0003] Unless explicitly stated otherwise, all information regarding the content of alloying elements in this text is given in wt.%.

[0004] The aging of steel is caused by free carbon in the ferrite. At temperatures above 300 °C, the solubility of carbon in ferrite is significantly higher than at room temperature, resulting in a certain free carbon content. Temperatures above 300 °C are typically reached during coating processes such as hot-dip coating. With the temperature and time profiles typical of coating processes, carbon can diffuse into the steel. The proportion of free carbon at room temperature is then significantly higher than the equilibrium content, because approaching thermodynamic equilibrium takes longer than the cooling time available after coating to room temperature. At room temperature, the ferrite is then highly supersaturated with carbon.As an interstitial alloying element, carbon can diffuse very slowly even at room temperature and accumulates at defects, including dislocations. This phenomenon is known as aging, and the interstitially dissolved atoms deposited at these defects are called Cottrell clouds. The carbon blocks the dislocations, resulting in a pronounced yield strength, which is highly undesirable for cold forming. Among other things, the discontinuous deformation behavior makes straightening the steel product more difficult. The increased resistance to deformation leads to increased tool wear during blank trimming, and any subsequent deep-drawing cold forming results in an uneven, irregular surface. Therefore, aging of the steel by free carbon should be prevented or at least mitigated whenever possible.

[0005] From EP 2 848 709 A1, a steel flat product is known which is formed from a steel containing 0.2 - 0.5 wt.% C, 0.5 - 3.0 wt.% Mn, 0.002 - 0.004 wt.% B, and optionally one or more elements of the group "Si, Cr, Al, Ti" in the following amounts: 0.1 - 0.3 wt.% Si, 0.1 - 0.5 wt.% Cr, 0.02 - 0.05 wt.% Al, 0.025 - 0.04 wt.% Ti. The steel flat product is coated with a corrosion protection coating formed from an aluminum-zinc alloy. The coated steel flat product is intended for the manufacture of a component by press hardening. Steel flat products of this composition have only limited resistance to aging and exhibit a pronounced yield strength after coating and aging.

[0006] Furthermore, EP 2 631 307A1 discloses a steel sheet consisting of, in wt.%, 0.18–0.35% C, 1.0–3.0% Mn, 0.01–1.0% Si, 0.001–0.02% P, 0.0005–0.01% S, 0.001–0.01% N, 0.01–1.0% Al, 0.005–0.2% Ti, 0.0002–0.005% B, and 0.002–2.0% Cr, with the remainder being Fe and unavoidable impurities. The microstructure of the steel sheet consists, in vol.%, of 50% ferrite, with 30% being non-recrystallized ferrite.At the same time, the ratio Crθ / CrM formed from the parameters Crθ and CrM satisfies the condition Crθ / CrM ≤ 2, where Crθ is the respective content of Cr present in solid solution in iron carbide, and CrM is the content of Cr present in solid solution in the base material of the steel flat product, and where the ratio Mnθ / MnM formed from the parameters Mnθ and MnM satisfies the condition Mnθ / MnM ≤ 10, where Mnθ is the content of Mn present in a solid solution in iron carbide, and MnM is the content of Mn in solid solution in the base material of the steel flat product.

[0007] Furthermore, EP 2 703 511 A1 discloses a steel sheet for hot pressing, consisting of, in wt.%, 0.10–0.35% C, 0.01–1.0% Si, 0.3–2.3% Mn, 0.01–0.5% Al, ≤ 0.03% P, ≤ 0.02% S, ≤ 0.1% N, and the remainder being iron and unavoidable impurities, wherein the standard deviation of the diameter of the iron carbides present in a thickness range extending from the surface of the sheet to one-quarter of the thickness of the steel sheet is less than or equal to 0.8 µm. EP 2 562 286 A1 discloses a galvanized steel flat product for automotive manufacturing.

[0008] In the production of automotive components, steels are typically cold-formed (e.g., during cutting, straightening, and forming). For this process, good dimensional accuracy, high-quality cut edges, and a smoother surface finish on the cold-formed parts are desirable. To achieve the desired permanent plastic deformation ("forming"), the upper yield strength (ReH) and the lower yield strength (ReL) must be exceeded. It is advantageous for the forming process if ReH and ReL have the same value or differ only very slightly, as this allows for a uniform and defined deformation, which has a positive effect on machinability.For flat steel products that do not have a pronounced yield strength, where the range of elastic deformation transitions continuously into the range of plastic deformation, the so-called Rp0.2 proof stress is used as a substitute characteristic value, i.e., the stress at which deformation causes a permanent elongation of exactly 0.2%.

[0009] In addition to optimized forming behavior, flat steel products intended for the production of automotive body parts should also exhibit good crash performance. To meet this requirement, high ductility combined with high strength is desirable. In a crash, both lateral and axial loads cause bending stress in the sheet metal, ultimately leading to wrinkling. The more pronounced a fold can be (given the same material strength) without the material failing, the better the energy absorption of the corresponding component.

[0010] For further processing in car body construction, good suitability for spot welding is also desirable. This means that a good strength-to-ductility ratio should be maintained in the weld area, ideally at the level of the unwelded microstructure or the microstructure located in the heat-affected zone. Small grain sizes in the microstructure contribute to the strength and ductility of flat steel products of the type discussed here.

[0011] Against this background, the task arose to create a process that enables the production of flat steel products particularly suitable for forming into sheet metal components, exhibiting good weldability and, after forming into a body panel, demonstrating excellent crash performance. Likewise, a flat steel product ideally suited for forming into a sheet metal component was to be specified.

[0012] With regard to the method, the invention proposes to solve this problem by carrying out at least the process steps specified in claim 8 during the production of flat steel products. It is understood that, when carrying out the method according to the invention, the person skilled in the art not only carries out the process steps mentioned in the claims and explained here, but also performs all other steps and activities that are regularly carried out in the practical implementation of such methods in the prior art, if the need arises.

[0013] A steel flat product solving the aforementioned problem has, according to the invention, at least the features specified in claim 1.

[0014] A steel flat product of this type has a range of properties that make it particularly suitable for forming into a sheet metal component.

[0015] Advantageous embodiments of the invention are specified in the dependent claims and, like the general concept of the invention, are explained in detail below.

[0016] In step a) of the production of a flat steel product according to the invention, a steel melt is thus produced, consisting of, in wt.%, C: 0.10 - 0.4%, Si: 0.05 - 0.5%, Mn: 0.5 - 3.0%, Al: 0.01 - 0.2%, Cr: 0.005 - 1.0%, V: 0.001 - 0.2%, and optionally one or more elements from the group "B, Ti, Nb, Ni, Cu, Mo, W" in the following amounts: B: 0.0005 - 0.01%, Ti: 0.001 - 0.1%, Nb: 0.001 - 0.1%, Ni: 0.01 - 0.4%, Cu: 0.01 - 0.8%, Mo: 0.002 - 1.0%, W: 0.001 - 1.0%, and as a remainder The steel consists of iron and unavoidable impurities, including up to 0.1% phosphorus (P), up to 0.05% sulfur (S), and up to 0.02% nitrogen (N). In the steel flat products according to the invention, carbon has a retarding effect on the formation of ferrite and bainite. Simultaneously, austenite is stabilized and the Ac3 temperature is reduced. The carbon content of the steel in a steel flat product according to the invention is limited to between 0.10% and 0.4% by weight.A carbon content of at least 0.10 wt.% is required to ensure the hardenability of the steel flat product and a tensile strength of at least 1000 MPa for the press-hardened product. Adjusting the carbon content to a value of at least 0.19 wt.%, and in particular at least 0.205 wt.%, further improves hardenability, resulting in a steel flat product with a very good combination of hardenability and strength. However, carbon contents greater than 0.4 wt.% have a detrimental effect on the mechanical properties of the steel flat product, as these promote the formation of brittle martensite during press hardening. High carbon contents can also negatively affect weldability. To improve weldability, the carbon content can preferably be adjusted to a maximum of 0.3 wt.%. At carbon contents of at most 0.25 wt.%, and in particular at most 0.235 wt.%, the weldability can be significantly reduced.-% silicon content can significantly improve weldability and achieve a good ratio of force absorption to maximum bending angle in the bending test according to VDA 238-100 in the press-hardened condition. Silicon is used to further increase the hardenability of the flat steel product and the strength of the press-hardened product via solid solution strengthening. Silicon also enables the use of ferro-silicon manganese as an alloying agent, which has a positive effect on production costs. A hardening effect occurs even with a silicon content of 0.05 wt.%. A significant increase in strength occurs with a silicon content of at least 0.15 wt.%, and especially at least 0.20 wt.%. Silicon contents above 0.5 wt.% have a detrimental effect on coating behavior, especially with aluminum-based coatings. Silicon contents of at most 0.4 wt.%, and especially at most 0.30 wt.%, are not recommended.-% are preferably set to improve the surface quality of the coated steel flat product.

[0017] Manganese acts as a hardening element by significantly delaying ferrite and bainite formation. At manganese contents below 0.5 wt.%, ferrite and bainite form during press hardening, even at very rapid cooling rates, which should be avoided. Mn contents of at least 0.9 wt.%, and in particular at least 1.10 wt.%, are preferred when a martensitic microstructure is to be ensured, especially in areas of significant deformation. Manganese contents above 3.0 wt.% adversely affect processing properties, which is why the Mn content of the steel flat products according to the invention is limited to a maximum of 3.0 wt.%. Weldability, in particular, is severely restricted, which is why the Mn content is preferably limited to a maximum of 1.6 wt.%, and in particular to 1.30 wt.%. Manganese contents of 1.6 wt.% or less are also preferred for economic reasons.

[0018] Aluminum is used as a deoxidizing agent to bind oxygen. It also inhibits cementite formation. For reliable oxygen binding, at least 0.01 wt%, and preferably at least 0.02 wt%, of aluminum is required in the steel. However, since the Ac3 temperature also increases significantly with rising aluminum alloy content, the aluminum content is limited to 0.2 wt%. Above 0.2 wt%, aluminum significantly hinders the conversion to austenite before press hardening, making austenitization inefficient in terms of time and energy. For typical furnace temperatures between 850 and 950 °C, which are used for austenitizing before press hardening, an aluminum content of at most 0.1 wt%, and preferably at most 0.05 wt%, is preferred to ensure complete austenitization of the steel.

[0019] Chromium is added to the steel of a flat steel product according to the invention in amounts of 0.005 to 1.0 wt.%. Chromium influences the hardenability of the flat steel product by slowing down the diffusive transformation during press hardening. In flat steel products according to the invention, chromium has a beneficial effect on hardenability from a content of 0.005 wt.%, whereby a Cr content of at least 0.1 wt.%, in particular at least 0.18 wt.%, is preferred for reliable process control, especially to prevent bainite formation. If the steel contains more than 1.0 wt.% chromium, the coating properties deteriorate. To obtain good surface quality, the Cr content can preferably be limited to a maximum of 0.4 wt.%, in particular to a maximum of 0.28 wt.%.

[0020] Vanadium (V) plays a particularly important role in the steel of a flat steel product according to the invention. Vanadium is a highly carbon-affine element. When vanadium is free, i.e., in an unbound or dissolved state, it can bind supersaturated dissolved carbon in the form of carbides or clusters, or at least reduce its diffusion rate. Crucially, V must be present in a dissolved state. Surprisingly, very low V contents have proven particularly advantageous for aging resistance. At higher V contents, larger vanadium carbides can form even at higher temperatures, which then no longer dissolve at temperatures of 650–900 °C, typical for continuous annealing in hot-dip coating systems. Even minute amounts of vanadium, as low as 0.001 wt.%, can hinder the attachment of free carbon to dislocations. From a V content of 0.2 wt.%...Below a vanadium content of 0.009 wt.%, no further improvement in aging resistance occurs due to vanadium. The anti-aging effect of vanadium is particularly pronounced at contents up to 0.009 wt.%, with a maximum effect occurring at the required content of 0.002 wt.%. At contents above 0.009 wt.%, vanadium carbides increasingly form. Vanadium carbides cannot be dissolved at temperatures of 700 to 900 °C, which are typical for annealing temperatures in a hot-dip coating system, for example, when the vanadium content in the steel exceeds 0.009 wt.%. Increasing the vanadium content does not necessarily mean that more free vanadium is available, as the precipitation kinetics of vanadium carbides are continuously accelerated. Thus, while the vanadium carbides become larger and more stable, the proportion of dissolved vanadium does not increase further. This effect occurs particularly at concentrations above 0.030 wt.%, which is why the concentration is preferably set to values ​​of no more than 0.030 wt.%.The vanadium content is set to -%. Since vanadium contributes to increased strength through precipitation hardening in addition to reducing aging effects, higher contents of up to 0.2 wt.% can preferably be used to increase strength. The vanadium content of the steel in a flat steel product according to the invention is limited to a maximum of 0.2 wt.% for cost reasons. Furthermore, higher contents do not result in a significant improvement in the mechanical properties.

[0021] Phosphorus (P) and sulfur (S) are elements that are introduced into steel as impurities from iron ore and cannot be completely removed in large-scale steelmaking processes. Phosphorus is a highly segregating element. However, a melt completely free of phosphorus and sulfur is technically unrealistic, so a certain P and S content greater than zero (P content > 0 wt.%) must always be assumed. Phosphorus present in the melt forms segregations that adversely affect the mechanical properties of the steel. Higher S contents also lead to a deterioration of the mechanical properties due to embrittlement. The P and S content of a steel processed according to the invention should therefore be kept as low as possible to achieve optimal toughness, which is reflected in good impact strength values. From P contents of 0.1 wt.Furthermore, an increasing embrittlement of the martensite occurs at -%, which is why the P content of a steel flat product according to the invention is limited to a maximum of 0.1 wt.%, in particular a maximum of 0.02 wt.%. The S content of a steel flat product according to the invention is simultaneously limited to a maximum of 0.05 wt.%, in particular a maximum of 0.003 wt.%, in order to reliably avoid negative effects from the technically unavoidable presence of S.

[0022] Nitrogen (N) is present in small quantities in steel due to the steelmaking process. The N content should be kept as low as possible and should not exceed 0.02 wt.%. Nitrogen is particularly detrimental in alloys containing boron, as it inhibits the conversion-retarding effect of boron by forming boron nitrides. Therefore, in this case, the nitrogen content should preferably be no more than 0.01 wt.%, and more specifically, no more than 0.007 wt.%.

[0023] Boron, titanium, niobium, nickel, copper, molybdenum and tungsten can be optionally alloyed individually or in combination with each other to the steel of a flat steel product according to the invention.

[0024] Boron can be optionally added to improve the hardenability of the steel flat product. Boron atoms or boron precipitates deposited at the austenite grain boundaries reduce the grain boundary energy, thereby suppressing ferrite nucleation during press hardening. A significant effect on hardenability occurs at contents of at least 0.0005 wt.%, and particularly at least 0.0020 wt.%. However, at contents above 0.01 wt.%, boron carbides, boron nitrides, or boron nitrocarbides are formed more frequently. These, in turn, represent preferred nucleation sites for ferrite and reduce the hardening effect. For this reason, the boron content is limited to a maximum of 0.01 wt.%, and particularly at most 0.0035 wt.%. When boron is added, titanium is preferably also added to bind nitrogen. In this case, the titanium content should preferably be at least 3.42 times the nitrogen content.

[0025] Titanium (Ti) is a microalloying element that can be optionally added to contribute to grain refinement. Titanium also forms coarse titanium nitrides with nitrogen, which is why the Ti content should be kept relatively low. Titanium binds nitrogen, thus enabling boron to exert its strong ferrilytic effect. Sufficient nitrogen binding requires at least 3.42 times the nitrogen content, with at least 0.001 wt% Ti, preferably at least 0.023 wt% Ti, being added to ensure adequate availability. From 0.1 wt% Ti onwards, cold rollability and recrystallizability deteriorate significantly, which is why higher Ti contents should be avoided. To improve cold rollability, the Ti content can preferably be limited to 0.038 wt%.

[0026] Niobium (Nb) can be optionally added to the alloy to contribute to grain refinement from a content of 0.001 wt.%. However, niobium impairs the recrystallizability of the steel. At an Nb content above 0.1 wt.%, the steel can no longer be recrystallized in conventional continuous furnaces before hot-dip coating. To reduce the risk of impaired recrystallizability, the Nb content can preferably be limited to 0.003 wt.%.

[0027] Copper (Cu) can be optionally added to increase hardenability at additions of at least 0.01 wt.%. Furthermore, copper improves the resistance to atmospheric corrosion of uncoated sheets or cut edges. From a content of 0.8 wt.%, hot rollability deteriorates significantly due to low-melting-point copper phases on the surface; therefore, the copper content is limited to a maximum of 0.8 wt.%, preferably a maximum of 0.10 wt.%.

[0028] Nickel (Ni) stabilizes the austenitic phase and can be added optionally to lower the Ac3 temperature and suppress the formation of ferrite and bainite. Nickel also has a positive effect on hot rollability, especially when the steel contains copper. Copper impairs hot rollability. To counteract the negative effect of copper on hot rollability, 0.01 wt% nickel can be added to the steel. For economic reasons, the nickel content should be limited to a maximum of 0.4 wt%, and in particular, a maximum of 0.10 wt%.

[0029] Molybdenum (Mo) can be added optionally to improve process stability, as it significantly slows down ferrite formation. From contents of 0.002 wt.%, molybdenum-carbon clusters, up to and including ultrafine molybdenum carbides, form dynamically at the grain boundaries, significantly slowing down grain boundary mobility and thus diffusive phase transformations. Furthermore, molybdenum reduces the grain boundary energy, which decreases the nucleation rate of ferrite. Due to the high costs associated with a molybdenum alloy, the content should be no more than 1.0 wt.%, preferably no more than 0.1 wt.%.

[0030] Tungsten (W) can optionally be added in amounts of 0.001–1.0 wt.% to slow down ferrite formation. A positive effect on hardenability is already observed at W contents of at least 0.001 wt.%. For cost reasons, a maximum of 1.0 wt.% tungsten is added.

[0031] The composition and the work steps and process parameters carried out in the production of a steel flat product according to the invention are selected such that optimal bending properties are achieved on the top and bottom surfaces of the sheet. The invention is based on the knowledge that formability as well as crash behavior are determined by the ductility of a steel flat product according to the invention. When forming at tight radii or in a crash, the ability to withstand high bending loads is desirable. Considering the cross-section reveals a stress profile across the sheet thickness. A neutral axis, which experiences no significant stresses, lies in the center of the sheet. Compressive stresses predominate on the inside of the bend, while tensile stresses are induced at the outer fiber – the greater the distance from the neutral axis, the greater the tensile stresses.Since steels are generally more sensitive to failure under tensile stresses than under compressive stresses, a crack initially forms in the outer fiber region of the sheet. Therefore, an increase in ductility in the outer fiber has a particularly positive effect on the permissible bending load. According to the invention, the casting parameters are adjusted during the casting process such that only minimal segregation of Si, Mn, and P occurs in the outer edge regions of a steel flat product according to the invention. These elements serve only as examples to demonstrate that segregation of other elements that could impair formability is also minimized in the edge regions of a steel flat product produced according to the invention, which are crucial for formability.Examples include sulfur, nitrogen, molybdenum, niobium, titanium, nickel, magnesium, lead, antimony, bismuth, cerium, tellurium, aluminum, arsenic, tin, boron, copper, zinc, copper and tungsten.

[0032] In contrast, higher concentrations of elements present in segregations are permitted in the core area of ​​a steel flat product according to the invention, since this area plays only a subordinate role with regard to the bending properties of a steel flat product according to the invention.

[0033] In step b) of the process according to the invention, the molten steel is cast into a strand, wherein the molten steel is first poured into a tundish, from which the melt flows into a continuous casting mold to form the strand, wherein the molten steel has a superheat temperature TUE when poured into the tundish, which is 5–60 °C above the liquidus temperature of the molten steel, and wherein the following applies to the product a formed from a thickness DS of the strand formed in the continuous casting mold and a casting velocity VS at which the melt flows into the continuous casting mold: a min < a < a max with a min = 0.05 m² / min, a max = 0.7 m² / min and DS: 20 - 500 mm

[0034] The larger the solidification bath in the mold, the more pronounced the resulting segregations. A large value of 'a' leads to a large solidification bath. Therefore, the solidification bath should be as small as possible; this determines the upper limit 'amax. However, the smaller the value of 'a', the lower the productivity. According to the invention, the limits 'amin and 'amax are adjusted to achieve optimal operating conditions from a technological and economic perspective.

[0035] Particularly good results can be achieved if, during casting of the melt, the casting speed is determined as a function of the specified thickness of the strand to be cast (or vice versa) such that the product α of casting speed VS and strand thickness DS is selected such that the product α still fulfills condition 1 even when a min = 0.1 m² / min and a max = 0.42 m² / min. The effects targeted by the invention are achieved particularly reliably when condition 1 is set to a min = 0.18 m² / min and a max = 0.33 m² / min. In practice, casting speeds VS that meet the requirements of the invention can be 0.3–2.0 m / min, and in particular 0.5–1.5 m / min, with casting speeds of 0.8–1.3 m / min proving to be particularly advantageous.

[0036] The strand thickness DS is typically in the range of 20 - 500 mm, with particularly practical strand thicknesses DS being 50 - 300 mm, especially 180 - 300 mm.

[0037] The superheating temperature (Tue), the temperature at which molten steel is poured from the ladle into the tundish, is at least 5 °C and at most 60 °C above the liquidus temperature of the molten steel being cast. Superheating temperatures (Tue) that are 10–50 °C, and especially at least 18 °C or at least 20 °C above the liquidus temperature of the steel being cast, are particularly practical, with superheating temperatures (Tue) of no more than 30 °C proving to be especially suitable.

[0038] The casting conditions specified according to the invention for step b) take into account that manganese sulfides bind to MnS in larger local proportions and, as such, indirectly serve as an indicator of the presence of sulfur. Due to their low strength and the disruption of fiber orientation they cause in the component, manganese sulfide compounds must be avoided as far as possible. Under bending stress, MnS occurring in the edge regions of the material is particularly detrimental, while it is less harmful in the core. The casting method according to the invention makes it possible to distribute the Mn sulfides over the cross-section of the flat steel product produced according to the invention in such a way that they do not impair the flexibility of the flat steel product. This has the particular advantage that the melt cast according to the invention does not need to be completely desulfurized.In combination with a slow casting speed, this leads to an accumulation of MnS in the core area, while the levels in the outer area of ​​the steel flat product are minimized.

[0039] During welding, the MnS and TiN precipitates prove to be very stable. They are not dissolved, or only minimally dissolved, even as a result of the heat input during welding. If they do dissolve, they precipitate so quickly after resolidification that they prevent austenite grain growth in the weld pool. This results in finer austenite grains in the heat-affected zone, which in turn leads to finer martensite (smaller former austenite grain size). This increases the fracture toughness of the weld. For this effect, MnS is only required in the core of the base material. During welding, melt convection occurs, causing the MnS precipitates to spread over a wider area. At the same time, particularly in the core, they contribute to grain refinement, which improves the mechanical properties of the flat steel product.With the inventive method for casting (step b)), it is possible to control the segregation of Mn and S in such a way that MnS precipitates are predominantly present in the core area of ​​a steel flat product produced according to the invention, while they occur only in low concentration in the outer area.

[0040] Phosphorus also proves detrimental to the deformability of a steel flat product of the type under discussion, as it can form a fine network at the grain boundaries. Accordingly, high local phosphorus contents indicate the presence of such a network. However, an increased phosphorus content in the central region, combined with a low phosphorus content in the outer regions of a steel flat product according to the invention, offers particular advantages, especially in conjunction with the vanadium contents provided for in the invention. Phosphorus significantly increases the activity of carbon, resulting in increased efficiency of the vanadium alloying. Due to the increased carbon activity, carbon is more readily bound in carbides within the temperature range of 550–300 °C, which enhances the positive effect of vanadium on the yield strength. In the outer region, however, phosphorus is particularly detrimental, as it deposits on the austenite and ferrite grain boundaries and weakens them.Without the measures according to the invention to prevent a corresponding accumulation in the edge region, an increased sensitivity to intergranular cracks would occur both in the pre-product and in the press-hardened component.

[0041] Due to its affinity for oxygen, silicon forms silicon oxides on its surface, which are very difficult to dissolve. These non-metallic inclusions degrade the surface quality and can cause cracks. The casting process according to the invention also prevents this by ensuring that silicon is present only to a reduced extent in the outer surfaces of a steel flat product according to the invention.

[0042] To determine the segregation coefficients SSi,OS, SMn,OS, SPOS, SSi,US, SMn,US, SPUS, SSi,MS, SMn,MS, SPMS according to the invention, an ESMA scan is performed of three thickness ranges OS, US, MS of the steel substrate and over the entire thickness GS of the steel substrate, of which the thickness range OS is assigned to the top, the second thickness range US to the bottom and the third thickness range MS to the middle of the steel substrate.

[0043] The resolution of the ESMA scan is 2 x 2 µm, i.e. 2 µm in the longitudinal direction and 2 µm in the thickness direction of the examined area section.

[0044] The area section examined in each case is a section of a polished section of the steel substrate oriented lengthwise to its rolling direction. The length LP of each area section under consideration is 300 µm.

[0045] For example, if the thickness DP of the steel substrate is 2 mm, and consequently the thickness DFos and the thickness DF US are each 300 µm (= 15% DP), and the thickness DF MS is 600 µm (= 30% DP), then in step i) two matrices are obtained with Si, Mn, and P content measurements determined at 150 x 150 measuring points M L_n ,DF OS_m , M L_n ,DF US_m (n = 1 ... 300 µm / 2 µm = 1 ... 150; m = 1 ... 300 µm / 2 µm = 1 ... 150) for the upper thickness range OS and the lower thickness range US, as well as a matrix with the Si, Mn, and P content measurements determined at 150 x 300 measuring points M L_n ,DF MS_m (n = 1 ... 600 µm / 2 µm = 1 ... 300; m = 1 ... 300 µm / 2 µm = 1 ... 150) for the medium thickness range (MS). In step ii), the arithmetic mean is calculated for each series n of the Si, Mn, P measurement matrices thus determined for the thickness ranges OS, US, MS.

[0046] Following step ii), there are therefore three lists for each thickness range OS, US with 150 mean values ​​of the Si, Mn and P content measurements each, and for the medium thickness range MS three lists with 300 mean values ​​of the Si, Mn and P measurements each, which now only represent the contents in the thickness direction of the sheet substrate.

[0047] In step iii), the largest mean values ​​of the Si, Mn and P contents are determined from these values ​​for the three thickness ranges OS, US, MS.

[0048] These maximum values ​​form the numerator in the ratio calculated in step v) for the respective segregation coefficients SSi,OS, SMn,OS, SPOS, SSi,US, SMn,US, SPUS, SSi,MS, SMn,MS, SPM. The denominator of the relevant ratio is determined in step iv) as the arithmetic mean of all content measurements determined in step i) for the Si, Mn, and P contents across the entire thickness GS of the steel substrate.

[0049] In step c) of the process according to the invention, semi-finished products in the form of slabs or thin slabs are separated from the cast strand according to the invention in a conventional manner and fed to further processing.

[0050] In step d), the respective slab or thin slab is heated thoroughly at a temperature (T1) of 1100–1400 °C. If the slab or thin slab has cooled down too much after casting, it is first reheated to 1100–1400 °C and then held at temperature T1 until a homogeneous temperature distribution is achieved. The heating temperature should be at least 1100 °C to ensure good formability for the subsequent rolling process. The heating temperature should not exceed 1400 °C to prevent the formation of molten phases.

[0051] In the optional step e), the thoroughly heated slab or thin slab is pre-rolled to an intermediate product, if necessary. Thin slabs, due to their already relatively small thickness, are usually not pre-rolled. However, this may be necessary for conventional slabs because of their greater thickness. In this case, the temperature of the intermediate product (T2) at the end of pre-rolling should be at least 1000 °C so that the intermediate product contains sufficient heat for the subsequent finish rolling step. However, high rolling temperatures can also promote grain growth during the rolling process, which has a detrimental effect on the mechanical properties of the flat steel product. To keep grain growth low during the rolling process, the temperature of the intermediate product at the end of pre-rolling should not exceed 1250 °C, and in particular, not exceed 1200 °C.

[0052] In step f), the slab or thin slab, or the intermediate product obtained in the optional step e), is rolled into a hot-rolled steel flat product. If the optional step e) was performed, the intermediate product is finish-rolled after pre-rolling. Finish rolling typically begins no later than 90 seconds after the end of pre-rolling. The final rolling temperature of hot rolling, that is, the temperature of the finished hot-rolled steel flat product at the end of the hot rolling process, is 750–1000 °C. At final rolling temperatures below 750 °C, the amount of free vanadium would decrease because larger quantities of vanadium carbides would precipitate. The vanadium carbides precipitated during finish rolling are very large. They typically have an average grain size of 30 nm or more and are not dissolved in subsequent annealing processes, such as those carried out before hot-dip coating.The final rolling temperature is limited to a maximum of 1000 °C to prevent coarsening of the austenite grains.

[0053] The hot rolling of the steel flat product can be carried out as continuous hot strip rolling or as reversing rolling. Step g) provides for the optional coiling of the hot-rolled steel flat product into a coil in the case of continuous hot strip rolling. For this purpose, the hot strip is cooled to a coiling temperature (T4) after hot rolling, preferably within less than 50 s. Water, air, or a combination of both can be used as the cooling medium. The coiling temperature (T4) should not exceed 700 °C to prevent the formation of large vanadium carbides. In principle, there is no lower limit to the coiling temperature. However, coiling temperatures of at least 500 °C have proven advantageous for cold rolling. Subsequently, the hot strip is cooled to room temperature in air using conventional methods.

[0054] If necessary, the hot-rolled steel flat product is descaled in step h) in a conventional manner by pickling or by another suitable treatment.

[0055] If thin steel flat products are required, the hot-rolled steel flat product, optionally cleaned of scale, can now be subjected to cold rolling in step i). Such cold rolling can also be carried out, for example, to meet higher requirements for the thickness tolerances of the steel flat product. The cold rolling ratio (CRR) should be at least 25%, and preferably at least 30%, to introduce sufficient deformation energy into the steel flat product for rapid recrystallization. The cold rolling ratio (CRR) is defined as the quotient of the thickness reduction during cold rolling (ΔdCRR) divided by the hot strip thickness (d) (CRR = ΔdCRR / d, where ΔdCRR = thickness reduction during cold rolling in mm and d = hot strip thickness in mm, and the thickness reduction ΔdCRR is the difference between the thickness of the steel flat product before cold rolling and the thickness of the steel flat product after cold rolling).The steel flat product before cold rolling is typically a hot-rolled strip with a thickness of d. The steel flat product after cold rolling is also commonly referred to as cold-rolled strip. The degree of cold rolling can, in principle, reach very high values ​​of over 90%. However, degrees of cold rolling of no more than 80% have proven advantageous in preventing strip cracking.

[0056] In step j), the steel flat product undergoes an annealing treatment at annealing temperatures (T5) of 650–900 °C. For this purpose, the steel flat product can first be heated to the annealing temperature within 10–120 s and then held at the annealing temperature for 30–600 s. The annealing temperature is at least 650 °C, preferably at least 720 °C, to keep the vanadium in solution. Thermodynamically, vanadium carbide precipitates at vanadium contents of 0.002 wt.% and temperatures above 650 °C, or vanadium carbides already formed do not dissolve. However, very fine vanadium carbides are thermodynamically unstable due to their high surface energy. This effect is used in the present invention to dissolve vanadium at temperatures of 650 - 900 °C or to keep already dissolved vanadium in solution, which has a positive effect on the aging resistance of the steel flat product.Annealing temperatures above 900 °C do not improve aging resistance, which is why the annealing temperature is limited to 900 °C for economic reasons as well.

[0057] After annealing (step j)), the steel flat product is cooled to room temperature in step k). The cooling rates are adjusted such that the largest possible proportion of supersaturated dissolved carbon can be bound by vanadium. For this purpose, the average cooling rate (CR1) in a first critical temperature range, which is optimal for the precipitation kinetics of vanadium and which is 600 °C to 450 °C for steel flat products with the composition according to the invention, should be at most 25 K / s, and in particular at most 18 K / s, with cooling rates of 12 K / s proving to be particularly practical.

[0058] The extent to which free carbon is bound by vanadium increases when cooling in a second critical temperature range between 400 °C and 220 °C occurs at a lower cooling rate (CR2) than in the temperature range between 600 °C and 450 °C. The average cooling rate (CR2) should therefore be no more than 20 K / s, preferably 14 K / s, and particularly no more than 9.5 K / s, between 400 °C and 220 °C. In the temperature range from 400 °C to 220 °C, the free carbon in the flat steel product still possesses a diffusion rate sufficient for recombination with vanadium, which promotes the binding of free carbon. Furthermore, the driving force for the growth of vanadium carbides is particularly high in this temperature range, which also binds free carbon. This applies especially to vanadium contents of 0.002–0.009 wt.%.Furthermore, the driving force for the formation of iron carbides is particularly high in the temperature range between 400 °C and 220 °C. These carbides preferentially germinate on existing carbides of microalloying elements such as vanadium, niobium, or titanium. The formation of iron carbides also binds free carbon, which has a beneficial effect on aging behavior.

[0059] In the temperature range between the annealing temperature and 600 °C, between 450 °C and 400 °C, and between 220 °C and room temperature, the cooling rate has no significant influence on the aging resistance. For process-related reasons, a mean cooling rate of at most 25 K / s is preferably set between the annealing temperature and 600 °C and between 450 °C and 400 °C, and a mean cooling rate of at most 20 K / s is preferably set between 220 °C and room temperature. At the same time, for economic reasons, the mean cooling rate is preferably at least 0.1 K / s in each of the various temperature ranges considered here.

[0060] The mean cooling rate CRn, n = 1, 2, ..., is understood here to be the average cooling rate, which is the quotient of the difference [TAnfang - TEnd] between the initial temperature TAnfang and the final temperature TEnd as the dividend and the time Δt required for cooling over this temperature difference [TAnfang - TEnd] as the divisor (CRn = [TAnfang - TEnd] / Δt).

[0061] In principle, the cooling process can be carried out arbitrarily slowly, as the proportion of free carbon decreases continuously, which improves the aging tendency. However, due to technical limitations and for economic reasons, the cooling rate of the entire cooling process—that is, the cooling of the coated steel flat product after exiting the coating bath until it reaches room temperature—can be limited to values ​​of typically at least 0.1 K / s.

[0062] Provided that the cooling requirements according to step k) are met, a hot-dip coating of the steel flat product with a corrosion protection coating can optionally be integrated into the cooling process (optional step I)). In this variant, the steel flat product is first cooled from the respective annealing temperature to a bath inlet temperature (T6) of 440–800 °C, which is equal to the temperature at which the steel flat product is introduced into the respective melt bath for hot-dip coating. If, for example, in a first variant the respective bath inlet temperature T6 is above 600 °C, the cooling to the respective bath inlet temperature T6 in the first cooling stage can be carried out at any desired speed, in accordance with the requirements of step k).If the steel flat product leaves the molten bath at a temperature above 600 °C, the second cooling stage must ensure that the cooling rate in the first critical temperature range of 600–450 °C does not exceed 25 K / s, and in the second critical temperature range of 400–220 °C does not exceed 20 K / s. Similarly, in the first cooling stage, the first critical temperature range may only be traversed at a maximum rate of 25 K / s if, according to a second variant, the bath inlet temperature T6 is below 450 °C. However, if, according to a third variant, the bath inlet temperature T6 is within the first critical temperature range, any cooling rate can be selected in the first cooling stage until the steel flat product has cooled to 600 °C. Once this limit temperature is reached, the cooling to the respective bath inlet temperature T6 must again only occur at a maximum rate of 25 K / s.If the temperature of the steel flat product is in the first critical temperature range of 600 - 450 °C when leaving the melt bath, the cooling of the steel flat product down to 450 °C in the subsequent second cooling stage may only continue to take place with a maximum cooling rate CR1 of 25 K / s, whereas in the second cooling stage, the cooling rate CR2 in the second critical temperature range of 400 - 220 °C must never exceed 20 K / s.

[0063] It follows from the foregoing that the selected bath inlet temperature T6 is lower than the annealing temperature and is matched to the temperature of the coating bath. The bath inlet temperature is 440–800 °C, particularly at least 470 °C, preferably at least 600 °C or at least 640 °C, and most preferably at most 700 °C. The cooling time of the annealed steel flat product from the annealing temperature T5 to the bath inlet temperature T6 is preferably 10–180 s. The steel flat product, cooled in this way, can be directly transferred to a hot-dip coating process, in which the steel flat product passes through a suitably composed melt bath. The melt bath, which contains the alloy to be applied to the steel flat product in liquid form, typically has a temperature (T7) of 640–720 °C. This applies particularly if an aluminum-based alloy is used for the corrosion protection coating.Aluminum-based protective coatings have proven particularly suitable for coating age-resistant flat steel products. The molten bath containing the corrosion protection coating to be applied to the flat steel product in liquid form contains, for example, 3–15 wt.% silicon, in particular 9–12 wt.% silicon, up to 5 wt.% iron, up to 30 wt.% zinc, up to 5 wt.% magnesium, up to 0.5 wt.% unavoidable impurities, and aluminum as the remainder, with the sum of each component totaling 100 wt.%. Unavoidable impurities can include, for example, unavoidable amounts of chromium, manganese, calcium, or tin. The coating composition of the finished flat steel product can be determined, for example, using glow discharge electron spectroscopy (GDOES).

[0064] The resulting flat steel product can optionally be subjected to a tempering process with a tempering degree of up to 2% to improve the surface roughness of the flat steel product.

[0065] A flat steel product produced according to the invention is suitable for press hardening and optionally features a corrosion protection coating, a high uniform elongation Ag of at least 11.5%, and a continuous yield strength Re or a pronounced yield strength where the difference between the upper ReH and the lower yield strength ReL is at most 45 MPa. The press hardening can be carried out in a single-stage process, in which a sheet blank is heated to the respective forming temperature and then formed into the respective component in a single operation in the press die while simultaneously being quenched, or in a two-stage process, in which a component is first cold-formed from a blank, then heated to hardening temperature and quenched. In this variant, too, the component can be placed in a die adapted to the shape of the component for quenching.

[0066] Typical thicknesses of steel flat products produced according to the invention range from 0.5 mm to 10 mm, preferably from 0.6 mm to 6 mm, and particularly preferably from 0.8 mm to 3.5 mm.

[0067] The invention is explained below using an exemplary embodiment.

[0068] Melts S1 - S5 were melted in a conventional manner, the

[0069] The complete analyses are given in Table 1. The concentrations of the elements P, S, N, Sn, and As are to be attributed to impurities. The same applies to the Mo concentrations given in examples S1 and S2, and to the Nb concentration given in example S1.

[0070] Melts S1–S5 were cast into strands with a thickness DS and a width Bs in a conventional continuous casting machine comprising a tundish and a continuous casting mold. For this purpose, melts S1–S5 were poured from a ladle at a superheat temperature that was above the liquidus temperature of the steel by a difference Tue (superheat temperature = liquidus temperature + Tue), and then flowed from the tundish into the continuous casting mold at a pouring rate VS. Table 2 lists the thickness Ds, width BS, pouring rate VS, the product a = DS x VS, and the difference Tue achieved during the casting of melts S1–S5.

[0071] Slabs were cut from the strands produced from the melts S1 - S5 and then heated to a temperature T1 in a conventional pusher furnace.

[0072] The slabs heated in this way were pre-rolled in a conventional manner to form a strip, which at the end of the pre-rolling had an intermediate product temperature T2.

[0073] The resulting strips were then hot-rolled into strip using conventional methods. The hot rolling process was terminated at a final rolling temperature of T3.

[0074] The resulting hot strips were then wound into coils in the conventional manner at a coiling temperature of T4.

[0075] After descaling, which was also carried out conventionally to remove scale present on the hot strips, the hot strips were cold-rolled in an equally conventional manner to a total cold rolling degree KGW achieved via cold rolling, each resulting in a cold strip.

[0076] The cold-rolled strips obtained were then heated through in a continuous process at an annealing temperature T5.

[0077] After annealing, the cold-rolled strips were cooled to room temperature in two stages, with a conventionally formulated aluminum-based coating being hot-dip coated between each cooling stage. Accordingly, in the first cooling stage, the annealed cold-rolled strips were cooled at an average cooling rate CR' of 2.5 K / s to 50 K / s to a bath inlet temperature T6, at which point they entered a molten bath at 675 °C. The molten bath was conventionally alloyed with (in wt%) 8–12% Si, 1–4% Fe, and 0–0.5% Mg, the balance being aluminum and unavoidable impurities, such that the conventionally formulated aluminum-based coating formed on the respective cold-rolled strip. Upon exiting the molten bath, the temperature of the cold-rolled strips was approximately the same as the bath temperature, but in any case above 600 °C.

[0078] The cold-rolled strips emerging from the molten bath, coated with the aluminum-based protective layer, were initially cooled to 600 °C in the second cooling stage at a cooling rate of CR' and then cooled at a medium cooling rate of CR1 in the first critical temperature range of 600–450 °C. After reaching the lower limit of this first critical temperature range, the cold-rolled strips were cooled at a medium cooling rate of CR2 in the temperature range of 400–250 °C.

[0079] The temperatures T1 - T6 set during the processing of the slabs produced from melts S1 - S5, the degree of cold rolling achieved in each case, and the cooling rates CR1, CR2 maintained in each case are given in Table 3.

[0080] On two samples each of the steel flat products, which were obtained as described above in the form of cold-rolled strips produced from melts S1-S5 and coated with an aluminum-based finish, a 300 µm long section of a longitudinal section of the cold-rolled strips was scanned at measuring points M L_n ,DF OS_m , M L_n ,DF US_m , M L_n ,DF MS_m , M L_n ,DF OS_m , M L_n ,DF US_m , M L_n ,DF MS_m using an ESMA scan for a thickness range OS, extending from the top of the steel substrate of the cold-rolled strip over a thickness of 225 µm (corresponding to 15% of the sheet thickness of 1.5 mm), and for a thickness range US, extending from the bottom of the steel substrate of the cold-rolled strip also over a thickness of 225 µm. extended, and for a third thickness range MS,which was symmetrically aligned to the thickness center of the steel substrate of the cold-rolled strip and extended from the thickness center over 225 µm in the direction of the top and bottom of the steel substrate of the cold-rolled strip (total thickness of the middle thickness range MS = 450 µm) the contents of Si, Mn and P,

[0081] The resolution of the ESMA scan is 2 x 2 µm, i.e., 2 µm in the thickness direction and 2 µm in the longitudinal direction of the examined area section. Each examined area section is a segment of a polished section of the steel substrate oriented longitudinally to its rolling direction. The length of each examined area section is 300 µm.

[0082] For example, if the thickness DP of the steel substrate is 2 mm, and consequently the thicknesses DF OS and DF US are each 300 µm, and the thickness DF MS is 600 µm, then in step i) a matrix with 150 x 150 values ​​for the Si, Mn, and P contents is generated for the thickness ranges OS,US, and a matrix with 150 x 300 values ​​for the Si, Mn, and P contents is generated for the thickness range MS at the respective measurement points. From this, the segregation coefficients SSi,OS, SSi,MS, SSi,US, SMn,OS, SMn,MS, SMn,US, SPOS, SPMS, SPUS ...

[0083] Table 4a shows, as examples, the largest values ​​xMax,Si,OS, xMax,Mn,OS, and xMa for the cold-rolled strip produced from melt 1. x,P,OS ; x Max,Si,US , x Max,Mn,US , x Max,P,US ; x Max,Si,MS , x Max,Mn,MS , x Max,P,MS of the arithmetic means x Mean,Si,OS_n , x Mean,Mn,OS_n , x Mean,P,OS_n ; x Mean,Si,US_n , x Mean,Mn,US_n , x Mean,P,US_n ; x Mean,Si,MS_n , x Mean,Mn,MS_n , x Mean,P,MS_n are given, which were calculated for the series n from the contents of Si, Mn, P of the respective series n measured at the measuring points M L_n ,DF OS_m , M L_n ,DF US_m , M L_n ,DF MS_m (n = 1, 2, 3, ..., 150 for the thickness ranges OS,US; n = 1, 2, 3, ..., 300 for the thickness range MS; m = 1, 2, 3, ..., 150).

[0084] Furthermore, Table 4a shows the arithmetic mean Xmean,Si, calculated from all Si contents determined at measuring points ML_n, DFGS_m, the arithmetic mean Xmean,Mn, calculated from all Mn contents determined at measuring points ML_n, DFGS_m, and the arithmetic mean Xmean,P, calculated from all P contents determined at measuring points ML_n, DFGS_m (n = 1, 2, 3, ..., 1000 for the total sheet thickness GS; m = 1, 2, 3, ..., 150).

[0085] Finally, Table 4a also shows the arithmetic means of the values ​​listed in Table 4a for the cold strip produced from melt 1: x Mean,Si,OS_n , x Mean,Mn,OS_n , x Mean,P,OS_n ; x Mean,Si,US_n , x Mean,Mn,US_n , x Mean,P,US_n ; The calculated segregation coefficient SSi,OS, SSi,MS_n, SMn,MS_n, XMean,P,MS_n and XMean,Si, XMean,Mn, XMean,P is given for the thickness ranges SSi,OS, SSi,MS, SSi,US, SMn,OS, SMn,MS, SMn,US, SPOS, SPMS, SPMS, SPUS (n = 1, 2, 3, ..., 150 for the thickness ranges OS,US; n = 1, 2, 3, ..., 300 for the thickness range MS; m = 1, 2, 3, ..., 150).

[0086] Table 4b contains the segregation coefficients SSi,OS, SSi,MS, SSi,US, SMn,OS, SMn,MS, SMn,US, SP,OS, SP,MS, SP,US, SP,US, determined accordingly for the melts S2 - S5.

[0087] In addition, the yield strength R, the type of yield strength (REL = pronounced yield strength, RP02 = continuous), in the case of the pronounced yield strength REL the respective upper yield strength ReH and the difference ΔRe between the upper and lower yield strength, the tensile strength Rm, the uniform elongation Ag and the elongation at break A80 were determined on the cold-rolled strips obtained from melts S1 - S5 in the manner described above, in accordance with DIN EN ISO 6892-1:2017-02. The relevant mechanical properties are given in Table 5.

[0088] Furthermore, the bending angle of transverse specimens was determined according to VDA 238-100 after a standard press hardening of the cold strips produced from melt S1 at maximum force. It was 50.1 ± 1.3° in three measurements. Table 1 melt C Si Mn P S Al Cr Cu Note S1 0,219 0,265 1,14 0,013 0,0023 0,032 0,183 0,016 0,001 S2 0,139 0,148 2,156 0,016 0,0013 0,177 0,288 0,446 - S3 0,324 0,197 0,868 0,015 0,0027 0,021 0,952 - 0,069 S4 0,210 0,095 1,284 0,017 0,0014 0,075 0,025 0,062 - S5 0,247 0,408 1,521 0,017 0,0029 0,002 0,263 0,238 0,039 Mon N Ti V Ni B Sn As S1 0,004 0,004 0,025 0,002 0,019 0,0026 0,004 0,002 S2 0,009 0,005 0,047 0,128 0,258 0,0041 - - S3 0,186 0,001 - 0,060 0,024 - - - S4 0,537 0,003 0,048 0,194 - 0,0023 0,028 - S5 - 0,005 0,072 0,036 - 0,0022 - - *) Values ​​in wt.%, balance iron and unavoidable impurities Table 2 Size Unit melt S1 S2 S3 S4 S5 Strand width BS m 1,63 1,78 1,13 1,92 1,29 Strand thickness DS mm 257 411 284 207 38 Casting speed VS m / min 0,63 1,28 1,37 0,39 4,24 Product a = DS x VS m² / min 0,16 0,53 0,39 0,08 0,16 Superheating temperature above liquid temperature °C 19 7 14 38 52 Table 3 melt T1 T2 T3 T4 KWG T5 T6 CR1 CR2 [°C] [%] [°C] [K / s] S1 1240 1091 805 647 60 785 696 11,82 8,05 S2 1239 1096 848 598 60 791 694 15,11 10,29 S3 1261 1111 798 647 40 789 706 7,28 4,96 S4 1291 1093 802 599 60 786 704 17,23 11,74 S5 1248 1105 797 652 50 788 697 7,43 5,06 Table 4a X Medium,Si 0,273 X Max, Si, OS 0,292 S Si,OS = X Max,Si,OS / X Medium,Si 1,073 X Max, Si, MS 0,379 S Si,MS = X Max,Si,MS / X Medium,Si 1,391 X Max, Si, US 0,288 S Si,US = X Max,Si,US / X Medium,Si 1,055 X Medium,Mn 1,145 X Max, Mn, OS 1,225 S Mn,OS = X Max,Mn,OS / X Medium,Mn 1,070 X Max, Mn, MS 1,570 S Mn,MS = X Max,Mn,MS / X Medium,Mn 1,363 X Max, Mn, US 1,238 S Mn,US = X Max,Mn,US / X Mean,Mn 1,081 X Medium,P 0,0124 X Max, P, OS 0,0177 SP,OS = X Max,P,OS / X Medium,P 1,424 X Max, P, MS 0,0608 SP,MS = X Max,P,MS / X Medium,P 4,909 X Max, P, US 0,0179 SP,US = X Max,P,US / X Medium,P 1,447 Table 4b melt S2 S3 S4 S5 S Si,OS 1,069 1,1012 1,0515 1,2490 S Si,MS 1,289 1,2945 1,1287 1,8281 S Si,US 1,074 1,1048 1,0916 1,2513 S Mn,OS 1,473 1,1405 1,0351 1,1003 S Mn,MS 2,109 1,6056 1,1922 1,2930 S Mn,US 1,343 1,2071 1,0588 1,0863 SP,OS 1,312 1,4038 1,1856 1,8087 SP,MS 3,780 2,2054 1,9562 6,0950 SP,US 1,371 1,3120 1,3260 1,7032 Table 5 melt R Type of yield strength Deer ΔRe Rm AG A80 [MPa] [MPa] [MPa] [%] [%] S1 439 REL 445 6 548 13,7 23,8 S2 375 REL 386 11 511 15,1 26,3 S3 503 RP02 656 11,0 19,0 S4 405 REL 425 20 530 14,2 24,8 S5 419 RP02 601 13,0 19,6

Claims

1. Flat steel product comprising a steel substrate consisting of, in wt.%, C: 0.19 - 0.4%, Si: 0.05 - 0.5%, Mn: 0.5 - 3.0%, Al: 0.01 - 0.2%, Cr: 0.005 - 1.0%, V: 0.002 - 0.2%, and optionally, in each case, one or more elements from the group "B, Ti, Nb, Ni, Cu, Mo, W" in the following contents: B: 0.0005 - 0.01%, Ti: 0.001 - 0.1%, Nb: 0.001 - 0.1%, Ni: 0.01 - 0.4%, Cu: 0.01 - 0.8%, Mo: 0.002 - 1.0%, W: 0.001 - 1.0%, and as the remainder iron and unavoidable impurities, wherein the impurities include up to 0.1% P, up to 0.05% S and up to 0.02% N, and the segregation coefficients SSi,OS, SSi,MS, SSi,US determined for Si, the segregation coefficients SMn,OS, SMn,MS, SMn,US determined for Mn and the segregation coefficients SP,OS, SP,MS, SP,US determined for P meet the following conditions: S Si , OS < 1.30 , S Mn , OS < 1.5 , S P , OS < 2.2 , S Si , US < 1.30 , S Mn , US < 1.5 , S P , US < 2.2 , 1.05 < S Si , MS < 2.5 , 1.10 < S Mn , MS < 3 , 1.5 < S P , MS < 10 , wherein - the segregation coefficients SSi,OS, SMn,OS, SP,OS are assigned to an upper thickness region OS of the steel substrate of the flat steel product, which region starts from the top of the steel substrate and of which the thickness DFOS is 15% of the thickness DP of the steel substrate, - the segregation coefficients SSi,US, SMn,US, SP,US are assigned to a lower thickness region US of the steel substrate of the flat steel product, which region starts from the bottom of the steel substrate and of which the thickness DFUS is 15% of the thickness DP of the steel substrate, and - the segregation coefficients SSi,MS, SMn,MS, SP,MS are assigned to a medium thickness region MS of the steel substrate of the flat steel product, which region is symmetrically aligned to the thickness center and of which the thickness DFMS is 30% of the thickness DP of the steel substrate, and wherein the segregation coefficients SSi,OS, SMn,OS, SP,OS, SSi,US, SMn,US, SP,US, SSi,MS, SMn,MS, SP,MS are determined by the fact that i) at a section, extending over a length LP of 300 µm measured parallel to the top of the steel substrate, of a ground part of the steel substrate of the steel product that is aligned longitudinally with the rolling direction of the steel substrate and is taken from a portion of the steel substrate which is arranged centrally in relation to the longitudinal axis of the belt and which extends over 70% of the width of the steel substrate, over each thickness region OS, MS, US and the entire thicknesses GS of the steel substrate, at each measurement point ML_n,DFOS_m, ML_n,DFUS_m, ML_n,DFMS_m, ML_n,DFGS_m, the concentrations of the elements Si, Mn and P present at the relevant measurement point ML_n,DFOS_m, ML_n,DFUS_m, ML_n,DFMS_m, ML_n,DFGS_m are determined by means of electron probe microanalysis with a resolution A x A of 2 x 2 µm, wherein n designates the rows of measurement values extending in parallel with the top of the steel substrate (n = 1, 2, 3, ... integer[Dx / A], where Dx = thickness DFOS, DFUS, DFMS, DFGS of each thickness region OS, MS, US and of the entire thickness GS of the steel substrate) and m designates the columns of measurement values extending in the thickness direction (m = 1, 2, 3, ..., integer[LP / A]), ii) for each row n of the upper, lower and central thickness region OS, US, MS and the total thickness GS, each of the arithmetical averages xMittel,Si,OS_n, xMittel,Mn,OS_n, xMittel,P,OS_n; XMittel,Si,US_n, XMittel,Mn,US_n, xMittel,P,US_n; XMittel,Si,MS_n, xMittel,Mn,MS_n, xMittel,P,MS_n; xMittel,Si,GS_n, xMittel,Mn,GS_n, xMittel,P,GS_n of the concentrations of Si, Mn and P ascertained at the measurement points ML_n,DFOS_m, ML_n,DFUS_m, ML-n,DFMS_m, ML_n,DFGS_m assigned to the relevant row n are formed, iii) for each thickness region OS, US, MS, from the arithmetical averages xMittel,Si,OS_n, xMittel,Mn,OS_n, xMittel,P,OS_n; xMittel,Si,US_n, xMittel,Mn,US_n, xMittel,P,US_n; xMittel,Si,MS_n, xMittel,Mn,MS_n, xMittel,P,MS_n ascertained in step ii), the greatest value xMax,Si,OS, xMax,Mn,OS, xMax,P,OS; xMax,Si,US, xMax,Mn,US, xMax,P,US; xMax,Si,MS, xMax,Mn,MS, xMax,P,MS is determined, iv) the arithmetical average XMittel,Si is formed from the totality of all concentrations of Si ascertained at the measurement points ML_n,DFGS_m, the arithmetical average XMittel,Mn is formed from the totality of all concentrations of Mn ascertained at the measurements points ML_n,DFGS_m, and the arithmetical average XMittel,P is formed from the totality of all concentrations of P ascertained at the measurement points ML_n,DFGS_m, and v) with the maximum values xMax,Si,OS, xMax,Mn,OS, xMax,P,OS; xMax,Si,US, xMax,Mn,US, xMax,P,US; xMax,Si,MS, xMax,Mn,MS, xMax,P,MS obtained in step iii) and the averages XMittel,Si, XMittel,Mn, XMittel,P obtained in step iv), the segregation coefficients SSi,OS, SMn,OS, SP,OS, SSi,US, SMn,US, SP,US, SSi,MS, SMn,MS, SP,MS are calculated as follows: S Si , OS = X Max , Si , OS / X Mittel , Si , S Si , MS = X Max , Si , MS / X Mittel , Si , S Si , US = X Max , Si , US / X Mittel , Si , S Mn , OS = X Max , Mn , OS / X Mittel , Mn , S Mn , MS = X Max , Mn , MS / X Mittel , Mn , S Mn , US = X Max , Mn , US / X Mittel , Mn , S P , OS = X Max , P , OS / X Mittel , P , S P , MS = X Max , P , MS / X Mittel , P , S P , US = X Max , P , US / X Mittel , P 2. Flat steel product according to claim 1, characterized in that it has a yield strength with a difference (ΔRe) between the upper yield strength value (ReH) and the lower yield strength value (ReL) of at most 45 MPa.

3. Flat steel product according to claim 1 or 2, characterized in that it has a uniform elongation Ag of at least 11.5%.

4. Flat steel product according to any of claims 1 - 3, characterized in that its thickness is 0.5 - 10 mm.

5. Flat steel product according to any of claims 1 - 4, characterized in that it is coated with a corrosion protection coating.

6. Use of a flat steel product according to any of claims 1 - 5 to produce a component by press-forming.

7. Use according to claim 6, characterized in that the press-forming is carried out as press mold hardening.

8. Method for producing a flat steel product according to any of claims 1 to 5, comprising the following work steps: a) melting a steel melt which consists of, in wt.%, C:0.19 - 0.4%,Si:0.05 - 0.5%,Mn:0.5 - 3.0%,Al:0.01 - 0.2%,Cr:0.005 - 1.0%,V:0.002 - 0.2%, as well as optionally, in each case, one or more elements from the group "B, Ti, Nb, Ni, Cu, Mo, W" in the following contents B:0.0005 - 0.01%,Ti:0.001 - 0.1%,Nb:0.001 - 0.1%,Ni:0.01 - 0.4%,Cu:0.01 - 0.8%,Mo:0.002 - 1.0%,W:0.001 - 1.0% and as the remainder iron and unavoidable impurities, wherein the impurities include up to 0.1% P, up to 0.05% S and up to 0.02% N, b) casting the steel melt to form a strand, - wherein the steel melt is first poured into a tundish, from which the melt flows into a continuous casting mold to form the strand, - wherein upon being poured into the tundish, the steel melt has an overheating temperature TUE that is 5 - 60°C above the liquidus temperature of the steel melt, and - wherein for the product a formed from a thickness DS of the strand formed in the continuous casting mold and from a casting speed VS at which the melt flows into the continuous casting mold, the following applies: a min < a < a max where a min = 0.05 m 2 / min a max = 0.7 m 2 / min DS: 20 - 500 mm c) separating a slab or thin slab from the strand; d) heating the slab or thin slab thoroughly at a temperature (T1) of 1100 - 1400°C; e) optionally pre-rolling the thoroughly heated slab or thin slab to form an intermediate product having an intermediate product temperature (T2) of 1000 - 1250°C; f) hot rolling the slab or thin slab or intermediate product to form a hot-rolled flat steel product, wherein the final rolling temperature (T3) is 750 - 1000°C; g) optionally coiling the hot-rolled flat steel product, wherein the coiling temperature (T4) is at most 700°C; h) optionally descaling the hot-rolled flat steel product; i) optionally cold rolling the flat steel product, wherein the degree of cold rolling achieved by the cold rolling is at least 25%; j) annealing the flat steel product at an annealing temperature (T5) of 650 - 900°C; k) cooling the flat steel product to room temperature, wherein the cooling takes place in the temperature range of 600°C to 450°C with a mean cooling rate (CR1) of 0.1 to 25 K / s and in the temperature range of 400°C to 220°C with a mean cooling rate (CR2) of 0.1 to 20 K / s; l) wherein optionally the cooling carried out according to work step k) is completed in two stages and a hot-dip coating of the flat steel product is carried out between the two stages of the cooling as follows: - In the first step of the cooling, the flat steel product is cooled to a bath inlet temperature (T6) of 440 - 800°C. - The flat steel product, cooled to the bath inlet temperature (T6), is passed through a molten bath in order to coat it with the metallic protection coating. - In the second step of the cooling, the flat steel product is cooled to room temperature, starting from the temperature at which it leaves the molten bath. m) optionally dressing the flat steel product.

9. Method according to claim 8, characterized in that in the condition 1 (work step b)) amin = 0.1 m2 / min and amax = 0.42 m2 / min.

10. Method according to claim 9, characterized in that in the condition 1 (work step b)) amin = 0.18 m2 / min and amax = 0.33 m2 / min.

11. Method according to any of claims 8 - 10, characterized in that the thickness DS of the strand formed in the continuous casting mold is 50 - 300 mm.

12. Method according to claim 10 or 11, characterized in that the thickness DS of the strand formed in the continuous casting mold is at least 180 mm.

13. Method according to any of claims 8 - 12, characterized in that the overheating temperature TUE is 10 - 50°C above the liquidus temperature of the steel melt.

14. Method according to claim 13, characterized in that the overheating temperature TUE is 18 - 30°C above the liquidus temperature of the steel melt.