Hot-rolled flat steel product and method for producing a hot-rolled flat steel product

Abstract

The invention relates to a high-strength hot-rolled flat steel product, which has a KAM value of less than 2.0° at a step width of 250 nm. The invention further relates to a method for producing a hot-rolled flat steel product.

Description

[0001] Hot-rolled flat steel product and method for producing a hot-rolled flat steel product

[0002] The invention relates to a high-strength flat steel product having a KAM value at a step size of 250 nm of less than 2.0° in order to obtain improved cutting quality and to reduce the probability of crack formation during cutting and punching operations, as well as to a use of such a high-strength flat steel product.

[0003] Flat steel products of the type in question here are typically rolled products such as steel strips or sheets, as well as blanks and plates made from them.

[0004] All information regarding the contents of the steel compositions specified in this application is based on weight, unless expressly stated otherwise. All unspecified "%" figures relating to a steel alloy are therefore to be understood as "wt%."

[0005] High-strength flat steel products are becoming increasingly important, especially in commercial vehicle construction, as they enable a reduction in the vehicle's tare weight and an increase in payload. Low weight not only contributes to the optimal utilization of the respective drive unit's technical performance, but also supports resource efficiency, cost optimization, and climate protection.

[0006] A significant reduction in the dead weight of sheet steel structures can be achieved by increasing the mechanical properties, especially the strength, of the processed flat steel product. In addition to high strength, modern flat steel products intended for commercial vehicle construction are also expected to offer good cutting properties and crack-free cut edges.

[0007] Micro-alloyed high-strength flat steel products are suitable for cold forming and are characterized by a combination of high strength and formability with relatively low alloy contents. They achieve their high strength through the addition of micro-alloying elements such as titanium, niobium, or vanadium, combined with controlled rolling and cooling processes in the hot rolling mill. The high strength of flat steel products enables the construction of components that can withstand high mechanical loads while maintaining a low component weight. These steels are used for various component geometries, primarily in automotive and commercial vehicle construction, electromobility, and lightweight automotive construction.

[0008] As component geometries become increasingly complex, especially for chassis components and car seat parts, and material strengths increase, coupled with increasingly stringent demands on component dimensional accuracy, the automotive industry's requirements for flat steel products have also become significantly more stringent. Customers are increasingly demanding flat steel products with a consistent property profile tailored to their intended use, coupled with increased forming and processing potential.

[0009] Increasing efficiency is a constant goal in the further processing of flat steel products. Reducing material waste through clean cut edges during blank cutting plays a key role in this process. The tendency or sensitivity to cracking of flat steel products typically increases with higher microalloy contents and rising strengths, which is why preventing cracks during processing is becoming increasingly important and is associated with the development of ever higher strengths. Crack-free cut edges can be achieved through correct tool settings or by using cutting-optimized materials. Since a component can be manufactured from various flat steel products with different microstructures, different cutting parameters are usually required. However, regularly changing the cutting parameters is unpopular with customers due to cost and time constraints.The customer's expectations therefore lie with the supplied material, which should avoid recurring changes in cutting parameters. Good cuttability of the flat steel product is expected regardless of the cutting parameters.

[0010] As strength increases, cutting quality decreases and the likelihood of cracks forming during cutting and punching operations increases. Due to the deterioration of forming and cutting properties of high-strength steels, the number of microscopic and macroscopic cracks after cutting and punching operations increases. This reduces the effectiveness of further processing of flat steel products. EP 2 407 572 B1 describes a process for producing a high-strength, hot-dip galvanized flat steel product characterized by good corrosion resistance. For this purpose, the dew point of the atmosphere in the tempering furnace is regulated to -40°C or lower at tempering temperatures above 600°C. There is no evidence whatsoever of good cuttability of the flat steel product.

[0011] EP 3 015 562 B1 discloses a high-strength flat steel product featuring a ferritic microstructure with carbide precipitation. During the process, after reaching the final rolling temperature, the temperature difference between the center of the sheet and the edge of the sheet must not exceed 50°C. This ensures a uniform strength distribution across the sheet plane. There is no indication whatsoever of good cuttability of the flat steel product.

[0012] EP 3 544 808 B1 discloses a high-strength flat steel product that exhibits improved coating adhesion of aluminum and aluminum alloys after hot forming. To achieve this, the intergranular oxidation in a ferritic-pearlitic flat steel product is specifically adjusted. For this purpose, the maximum coiling temperature is regulated. There is no indication whatsoever of good cutting properties of the flat steel product.

[0013] WO 2018 / 096387 A1 discloses a hot-rolled and coated flat steel product, with a surface coating having a thickness between 10 and 30 μm and improved adhesion. To this end, the depth of intergranular oxidation is limited to 4 μm below the surface of the flat steel product, and the coiling temperature is specifically adjusted. There is no indication whatsoever of good cuttability of the flat steel product.

[0014] EP 2 412 842 B1 discloses a high-strength hot-dip galvanized flat steel product that offers a good surface coating under severe stress conditions. To this end, oxides are introduced into direct contact with the zinc plating layer, and the oxygen partial pressure in the annealing furnace is regulated between 600°C and 900°C. There is no indication whatsoever of good cuttability of the flat steel product. The object of the present invention is to provide a high-strength flat steel product that has improved forming and cutting properties. A further object is to provide an efficient process for producing this flat steel product.

[0015] This task is solved by a hot-rolled flat steel product consisting of a steel with the following composition (in wt.%):

[0016] C: 0.02 - 0.15%

[0017] Mn: 0.2 - 2.5%

[0018] AI: 0.01 - 0.5%

[0019] Nb: 0.002 - 0.15%

[0020] Optionally one or more of the following elements in the weight percentage specified below:

[0021] Ti: 0.0015 - 0.24%

[0022] Si: 0.01 - 0.6%

[0023] Cr: < 1.5%

[0024] Mo: < 1.0%

[0025] V: < 0.5%

[0026] Cu: < 0.5%

[0027] Ni: < 0.5%

[0028] B: < 0.0025%

[0029] Ca: < 0.01%

[0030] The remainder is iron and unavoidable impurities, and has a KAM value at a step size of 250 nm of less than or equal to 2.0°. Preferably, the KAM value at a step size of 250 nm is less than or equal to 1.5°, and particularly preferably less than or equal to 1.0°.

[0031] The KAM value stands for kernel average misorientation and is also called local misorientation. The KAM value is a measure of local strain in crystalline materials and can be determined using the electron backscatter diffraction (EBSD) method. The KAM value quantifies the average misorientation around a measurement point with respect to neighboring measurement points.

[0032] During the measurement, the local orientation of each point within a given measurement field is determined. In the subsequent evaluation, the misorientation relative to its surroundings is calculated for each measurement point based on the orientation information. The surroundings are determined by specifying the neighbor; preferably, the first neighbor is measured. When measuring the second or third neighbor, the local misorientations are determined over larger distances in the structure, resulting in a larger KAM value.

[0033] The KAM value is a value that correlates with plastic deformation. The KAM value represents the local deformation and dislocation density at the microscopic level; therefore, by determining the KAM value, local plastic deformation can be measured at the microscopic level. Generally, the KAM value is high in deformed grains due to the higher dislocation density. Since KAM analysis describes local lattice distortions or local deformations, it can provide information about the energy stored in the deformed structure.

[0034] Adjusting the KAM value means that structural stresses in the material are limited. These structural stresses can be limited by various mechanisms in the structure. On a two-dimensional level, the limitation of structural stress can be achieved by homogenizing the grain structure and / or by reducing unwanted second phases and / or mixed structures. On a one-dimensional level, the limitation of structural stress can be achieved by arranging atoms and / or precipitates in energetically favorable positions and reducing the general dislocation density in the material. This leads to a reduction in crack formation during cutting and punching. Adjusting the KAM value has the advantage of limiting grain stretching and thus establishing a more homogeneous structure across the strip thickness. Undesirable second phases and / or mixed structures are avoided.This allows the overall structural stress in the material to be reduced. This leads to a reduction in crack formation during cutting and punching.

[0035] In particular, the hot-rolled flat steel product has a maximum crack sensitivity of 20%. Crack sensitivity is measured by quantifying macroscopic cracks at cut edges. It is therefore used to assess the robustness of the material. In order to compare the cut surface formation, standardized cutting tests are carried out and evaluated. Any gap with a minimum length of 1 mm and a length-to-width ratio of greater than or equal to 5 is considered a macroscopic crack. A macroscopic crack (3) is shown as an example in Fig. 2. Cut edges are typically created during a mechanical cutting process with an open cutting line, preferably with a cutting gap of 8%. The sample position is preferably transverse to the rolling direction. A created cut edge is at least 200 mm and a maximum of 400 mm long.The cut edge is defined as the entire surface, consisting of the smooth cut portion (1) and the broken portion (2), as shown in Fig. 1. The cut edges produced are then evaluated by macroscopic examination. If at least one macroscopic crack is observed on a cut edge, the cut edge is considered to be crack-sensitive. According to the invention, the crack sensitivity of cut edges due to macroscopic cracks is a maximum of 20%, preferably a maximum of 10%, particularly preferably a maximum of 5%. As a comparable benchmark, 100 cut edges are examined. According to the invention, macroscopic cracks occur at fewer than 20 cut edges. The examination of crack sensitivity can be carried out on uncoated and coated hot-rolled strips.

[0036] The various components of the steel are explained in detail below, with preferred minimum and maximum contents for their alloying elements also being given.

[0037] Carbon (C) is primarily present in the steel substrate to increase tensile strength and yield strength. The carbon contained in the steel alloy according to the invention is primarily bound in the precipitates. The concentration of C dissolved in the solid solution is thereby minimized. A C content of more than 0.02 wt.% is required to achieve a high precipitate density and thus ensure the required tensile strength of at least 550 MPa.

[0038] To enable the utilization of the positive effect of carbon on strength properties while simultaneously ensuring particularly good weldability, an upper limit of 0.15 wt.% is recommended. Since ductility and toughness are also influenced by the C content, a further upper limit is advisable. To limit the reduction in ductility of the flat steel product according to the invention, the C content should preferably be limited to a maximum of 0.12 wt.%. To further prevent a reduction in toughness, the C content should particularly preferably be set to a maximum of 0.08 wt.%.

[0039] Manganese (Mn) occupies regular lattice sites in the steel substrate as an alloying element. Due to their atomic radii, which differ from those of the iron atoms, the substitution atoms distort the cubic lattice and thus increase strength. To enable this solid solution strengthening effect, Mn should be present in the flat steel product according to the invention in amounts of at least 0.2 wt.%. Mn also exhibits a high affinity for sulfur (S), which, due to manufacturing processes, is usually present in the flat steel product according to the invention in the form of unavoidable impurities. By preferentially adding Mn contents of at least 0.5 wt.%, this affinity can bind the sulfur to MnS, thus preventing the formation of embrittling phases (e.g., FeS).Furthermore, at higher contents, Mn suppresses the formation of pearlite and cementite and thus promotes the formation of Cr-containing carbonitride precipitates based on the contents of Nb and optionally Ti provided according to the invention. For this reason, a Mn content of at least 1.4 wt.% is particularly preferred in the steel according to the invention.

[0040] Mn tends to form segregations throughout the material thickness, which impair the mechanical and technological properties of the flat steel product according to the invention. Such segregations can be contained by limiting the Mn content to a maximum of 2.5 wt.%. Furthermore, higher Mn contents can negatively impact the weldability and forming behavior of the flat steel product according to the invention. The negative effects on joinability can be largely suppressed by limiting the Mn content to a maximum of 2.2 wt.%.

[0041] Like silicon (Si), aluminum (Al) can also be used as an alloying element. In steelmaking, Al is typically used to calm the melt. By binding the oxygen to Al2O3, the rising of oxygen bubbles is prevented. To utilize this effect, an Al content of at least 0.01 wt.% is required in the flat steel product according to the invention.

[0042] In addition to its function as a deoxidizer in the steel melt, which is due to aluminum's high oxygen affinity, Al is also used for grain refinement. Al binds the optional alloying element nitrogen (N), forming aluminum nitrides. These improve nucleation and, due to the resulting high nucleation density, inhibit grain growth, thereby producing more small grains and increasing the toughness of the flat steel product according to the invention. For grain refinement, an Al content of at least 0.02 wt.% is preferably required.

[0043] Due to their high affinity for oxygen, the resulting Al2O3 particles become coarser at high Al contents. To prevent the precipitation of coarse particles, which have a negative impact on the mechanical properties and the degree of purity, an Al content of a maximum of 0.5 wt.% should not be exceeded. During the production of a flat steel product according to the invention, the Al content affects castability. To ensure good castability, an Al content of a maximum of 0.2 wt.% should preferably be set. A flat steel product according to the invention with an Al content of preferably a maximum of 0.06 wt.% leads to optimal utilization of the alloyed aluminum if there are no requirements for density reduction.

[0044] The microalloying element niobium (Nb) is present in the steel flat product according to the invention to achieve an increase in strength and toughness through grain refinement and age hardening. Titanium (Ti) can also be added to increase strength and toughness through grain refinement and age hardening.

[0045] The underlying mechanisms include the obstruction of austenite grain growth, the delay of recrystallization during hot rolling, and the formation of precipitates in the form of titanium and / or niobium nitrides, titanium and / or niobium carbides, and titanium and / or niobium carbonitrides. Austenite growth is already hindered during the heating of the slab at the beginning of hot rolling by the stable niobium nitrides and optionally by titanium nitrides. As hot rolling progresses, the formation of new temperature-dependent and / or deformation-induced precipitates leads to an additional growth blockage. Recrystallization is delayed, on the one hand, by the slowing of the movement of dislocations, grain boundaries, and subgrain boundaries due to the entrainment of dissolved niobium atoms and optionally dissolved titanium atoms.On the other hand, the deformation-induced precipitation of very fine niobium particles and optionally very fine titanium particles leads to a delay in recrystallization, since grain boundaries and dislocations are fixed and thus the nucleation of recrystallization is hindered.

[0046] Hardening occurs on the coiler through further precipitation. Depending on the coiler temperature, varying numbers and sizes of incoherent, partially coherent, or coherent niobium particles and, optionally, titanium particles form, which contribute to a varying degree of strength enhancement.

[0047] To ultimately achieve a grain refinement and hardening effect via the above-mentioned mechanisms, at least 0.002 wt.% Nb, preferably 0.005 wt.% Nb, particularly preferably 0.01 wt.% Nb must be added. A titanium content of at least 0.0015 wt.%, preferably 0.002 wt.%, particularly preferably 0.001 wt.% can also promote the grain refinement and hardening effect via the above-mentioned mechanisms.

[0048] When the Nb content exceeds 0.15 wt%, preferably 0.12 wt%, particularly preferably 0.08 wt%, the above effect is saturated and the economic efficiency is reduced due to the increasing cost.

[0049] The Ti content is limited to 0.24 wt%, preferably to 0.22 wt%, particularly preferably to 0.2 wt% due to the saturated effect and economic efficiency.

[0050] The total content of Ti and Nb is desirably in the range of 0.05 wt.% to 0.22 wt.%, preferably in the range of 0.06 wt.% to 0.20 wt.%, particularly preferably in the range of 0.09 wt.% to 0.15 wt.%. Silicon (Si) is optionally used as a deoxidizer in the production of the flat steel product according to the invention and contributes to improving the strength properties of the flat steel product according to the invention. For this purpose, at least 0.01 wt.% Si is required. Si contents of more than 0.6 wt.% would impair the surface quality and the toughness properties of the material according to the invention, in particular the toughness in the heat-affected zone of the weld seam produced in the steel flat product according to the invention, so that contents above 0.6 wt.% are undesirable. Furthermore, excessively high Si contents could impair weldability.To reliably avoid these negative influences and, in particular, to ensure optimized surface quality, the Si content should preferably be limited to 0.3 wt.%. To completely avoid the formation of red scale, which significantly reduces the efficiency of the cooling water due to its insulating effect, the Si content should preferably be limited to 0.06 wt.%.

[0051] Chromium (Cr), as an optional element, can contribute to increased strength in certain concentrations. The mechanisms of precipitation and solid solution strengthening are primarily effective. At a concentration of at least 0.05 wt.%, these mechanisms become apparent, and a measurable increase in strength is achieved.

[0052] In addition, chromium has the property of preventing the coarsening of other precipitates, thus increasing the yield strength and simultaneously improving toughness. To effectively utilize this additional mechanism, a Cr content of at least 0.08 wt.% is required.

[0053] From a joining technology perspective, an excessively high total Cr content is detrimental, as weldability decreases noticeably with increasing content. To ensure the joinability of the flat steel product according to the invention, the Cr content must be limited to a maximum of 1.5 wt.%. Since coarse carbides negatively influence the mechanical and technological properties of the flat steel product according to the invention, the Cr content should preferably be limited to a maximum of 1.2 wt.%. In order to reduce adhesion problems in galvanized steels and limit the number of ungalvanized areas, pronounced grain boundary oxidation on the coiler after hot rolling and pronounced selective oxidation, which can occur during heat treatment, must be reduced. A reduction to a non-critical level is possible if the Cr content is particularly preferably limited to a maximum of 0.7 wt.%.

[0054] The optional alloying element molybdenum (Mo) exhibits similar properties to chromium. To achieve the strength level of a flat steel product according to the invention, a Mo content of at least 0.01 wt.% must be set. The mechanisms of precipitation and solid solution strengthening are primarily effective here. Molybdenum (Mo), like chromium, has the property of preventing the coarsening of other precipitates, thus increasing the yield strength and simultaneously improving toughness. To utilize this mechanism effectively, a Mo content of at least 0.05 wt.% is preferably required.

[0055] Increasing the Mo content above 1.0 wt.% is discouraged for economic reasons, as this provides no mechanical-technological benefit and unnecessarily increases costs. The increased strength through the addition of Mo correlates with a decrease in ductility, which significantly influences forging and other forming processes when machining a flat steel product according to the invention. The Mo content should preferably be limited to a maximum of 0.7 wt.% in order not to jeopardize the forgeability of the flat steel product. An increased Mo content also has an increasing effect on the tensile strength Ai, preferably A ri which, with preferred Mo contents of a maximum of 0.30 wt.%, does not lead to any problems in the production of the flat steel product according to the invention.

[0056] Vanadium (V) can optionally be used to increase the yield strength and strength levels through the formation of carbonitrides. The precipitation of vanadium carbonitrides leads to grain refinement and hardening. To achieve an initial visible effect from vanadium, contents of > 0.008% are required. From an economic perspective, V contents exceeding 0.5 wt.% are not recommended, as the slight improvement in properties does not justify the significant cost increase. To prevent coarsening of the precipitates, V contents above 0.2 wt.% are preferably avoided. For optimal utilization of the active mechanisms, V contents of a maximum of 0.15 wt.% should preferably be added. Copper (Cu) is a trace element and can optionally be limited to a maximum of 0.5%. At excessively high contents, it impairs weldability and, due to its strong tendency to segregate in the steel, can lead to a defective surface.In addition, Cu has a negative impact on castability. To avoid any negative influence of Cu, the permissible upper limit of the Cu content in the flat steel product according to the invention is preferably 0.2 wt.%, particularly preferably less than 0.1 wt.%. However, at lower contents, copper can contribute to increased strength in the form of very fine precipitates. To achieve this positive effect, contents of > 0.02% are preferably used.

[0057] The optional alloying element nickel (Ni) increases the adhesion between the scale layer and the steel substrate at the surface and is therefore particularly undesirable in pickling processes. Furthermore, it increases material costs and is therefore not added, or only in very small quantities, unless necessary to achieve the mechanical and technological properties, where it is primarily used to improve toughness. To avoid unnecessarily increasing material costs, the nickel content is limited to a maximum of 0.5%. To facilitate the pickling process and limit adhesion between the scale layer and the steel substrate, the nickel content is preferably kept below 0.25%, particularly preferably below 0.15%. To utilize the toughness-enhancing effect of nickel, at least 0.04% Ni is preferably added.

[0058] Boron (B) is optionally present in the inventive flat steel product in amounts of up to 0.0025 wt.%. B has a beneficial effect on the strength properties and hardenability of the steel from which the inventive flat steel product is made. This beneficial effect of B can be utilized by using B contents of preferably at least 0.0005 and particularly preferably at least 0.0015 for the inventive flat steel product, while simultaneously limiting B contents to preferably a maximum of 0.0020 wt.%. B contents of more than 0.0025 wt.% would impair the toughness properties.

[0059] Calcium (Ca) can optionally be present in the steel of a flat steel product according to the invention in order to form non-metallic inclusions in the microstructure of the flat steel product in order to improve toughness. This effect becomes visible from a Ca content of 0.0005%. If the Ca content is above 0.01 wt.%, however, this can have a negative effect on the purity of the melt and lead to defects in the shell of the cast intermediate product during casting of the steel. In order to reliably avoid a negative effect on the mechanics and rollability, Ca contents of less than or equal to 0.005 wt.%, particularly preferably less than or equal to 0.002 wt.%, should preferably be set.

[0060] Phosphorus (P) and sulfur (S) are undesirable impurities in the flat steel product according to the invention because they impair its mechanical properties, particularly the notched bar impact energy and formability. To avoid any influence of these accompanying elements, which are unavoidable due to production reasons, an upper limit of 0.03 wt.%, preferably 0.02 wt.%, particularly preferably 0.015 wt.%, is set for the P content, and an upper limit of 0.008 wt.%, preferably 0.005 wt.%, particularly preferably 0.003 wt.% is set for the S content.

[0061] Nitrogen (N) is also an unavoidable impurity due to manufacturing processes and should be limited to a maximum of 0.02 wt.%, since excessive nitrogen levels impair the toughness of a flat steel product according to the invention. If titanium is alloyed, high levels also lead to the formation of numerous coarse, degenerate titanium nitrides, which, due to their size and shape, act as sharp-edged fracture points and can thus negatively impact formability. Limiting the N content to 0.012 wt.%, particularly preferably to 0.008 wt.%, is sensible from this perspective.

[0062] Since nitrogen can form very fine aluminum nitrides in the presence of aluminum, which improve nucleation and hinder grain growth, it is particularly preferable to set a minimum content of 0.002 wt.%.

[0063] In addition to the mandatory and optional alloying elements of the flat steel product according to the invention, the remaining portion consists of iron and elements whose presence is unavoidable due to the manufacturing process. The contents of such impurities are to be kept as low as possible within an economically feasible and reasonable technical framework. In particular, the hot-rolled flat steel product has a thickness of d w from 1.0 mm to 6 mm, especially up to 4 mm. Due to the intended application profile, the thickness is preferably at least 1.5 mm to enable sufficiently rigid constructions.

[0064] The tensile strength R mThe tensile strength of the flat steel product according to the invention, which is determined according to DIN-EN ISO 6892, is, in a preferred embodiment, at least 550 MPa to ensure sufficient strength for structural applications. In particular, the tensile strength is at least 650 MPa to enable efficient designs. The tensile strength is preferably determined on longitudinal specimens. For final thicknesses d w less than 3 mm is measured according to DIN-EN ISO 6892-1, specimen shape 2 (Annex B Tab. Bl) (as of 2020-06), for final thicknesses d w of greater than or equal to 3 mm, the proportional sample size is calculated according to D.2.3.1 according to DIN-EN ISO 6892-1 (as of 2020-06).

[0065] In a special development, the flat steel product according to the invention has a breaking elongation A for thicknesses dw of less than 3.0 mm 80of at least 14% to ensure sufficient forming. The elongation at break A80 is measured according to DIN-EN ISO 6892-1 and reflects the deformability of the flat steel product.

[0066] In particular, the flat steel product according to the invention for thicknesses dw greater than or equal to 3.0 mm has an elongation at break A pr op of at least 17%. The elongation at break A pr op is measured according to DIN-EN ISO 6892-1 and represents the deformability of the flat steel product. The elongation at break A pr op is preferably determined on longitudinal samples.

[0067] In a preferred development, the flat steel product according to the invention has a hole expansion of at least 40%. The hole expansion test is a method for determining the forming properties of sheet metal edges and serves to determine edge crack susceptibility. The hole expansion can be measured according to ISO 16630. The hole expansion is preferably at least 45%, particularly preferably at least 50%. The time between punching the hole and forming using a conical punch must be a maximum of one hour. In a specific development, the flat steel product according to the invention has a surface coating. The surface coating serves as corrosion protection and can change the visual appearance of the surface of the hot-rolled flat steel product. A zinc-based coating is particularly preferably applied as the surface coating.A zinc-based coating contains a zinc content of greater than or equal to 90 wt.%, in particular greater than or equal to 92 wt.%. Magnesium and / or aluminum are preferably included as additional alloying elements in the coating, each in amounts up to a maximum of 5 wt.%, with the remainder being zinc and unavoidable impurities. The zinc-based coating may contain unavoidable impurities. The levels of such impurities should be kept as low as possible within an economically feasible and technically feasible framework.

[0068] The object of the invention is also achieved by a method for producing a previously described flat steel product. The method comprises the following steps:

[0069] - Producing a steel melt with the following composition (in wt.%):

[0070] C: 0.02 - 0.15%

[0071] Mn: 0.2-2.5%

[0072] AI: 0.01 - 0.5%

[0073] Nb: 0.002 - 0.15%

[0074] Optionally one or more of the following elements in the weight percentage specified below:

[0075] Ti: 0.0015 - 0.24%

[0076] Si: 0.01 - 0.6%

[0077] Cr: < 1.5%

[0078] Mo: < 1.0%

[0079] V: < 0.5%

[0080] Cu: < 0.5%

[0081] Ni: < 0.5%

[0082] B: < 0.0025%

[0083] Ca: < 0.01% remainder iron and unavoidable impurities;

[0084] - Pouring the molten steel into a precursor product in the form of an ingot, a slab or a thin slab;

[0085] - Optional heating of the pre-product to an austenitizing temperature TWE between 1100°C and 1350°C;

[0086] - Hot rolling of the preliminary product to the hot-rolled flat steel product with successive individual pass reductions, whereby the individual pass reductions £< RL T below the recrystallization temperature RLT; and the individual samplings E> RL T above the recrystallization temperature RLT; and for the ratio of the sums of E> RL T TO E< RL T applies:

[0087] - Setting a hot rolling end temperature TEW of at least 840 °C and a maximum of 960 °C;

[0088] - Coiling the hot-rolled flat steel product cooled to the cooling stop temperature into a coil at a coiling temperature of at least 500°C and a maximum of 650°C;

[0089] Tempering of the flat steel product at an annealing temperature T G of at least 630°C and a holding time t Gof at least 20 seconds. Said steel melt may preferably also contain one or more optional elements or have preferred element contents, which have been explained in detail with reference to the flat steel product.

[0090] The hot rolling of the hot-rolled flat steel product according to the invention can be carried out in a conventional hot strip mill, with slabs being rolled to the required thickness in a multi-stage reversing roughing process and a subsequent multi-stand finish rolling process.

[0091] The hot rolling of the hot-rolled flat steel product according to the invention can also be carried out in a Compact Strip Production (CSP) plant directly from continuously cast starting material, which is fed immediately after solidification via an equalizing furnace into a multi-stand hot rolling mill and there rolled to the required finished thickness in several rolling passes.

[0092] During finish rolling, a finished strip with the desired strip thickness dw is rolled from the preliminary strip or thin slab in a multi-stand rolling process. A "multi-stand" rolling process refers to at least three consecutive pass reductions. The number of pass reductions is selected depending on the finished strip thickness.

[0093] In order to produce the flat steel product according to the invention, it is important to control the processes in the finishing train during hot rolling.

[0094] The hot rolling of the preliminary product to the hot-rolled flat steel product is carried out with successive individual pass reductions, whereby the individual pass reductions £< RL T below the recrystallization temperature RLT and the individual samplings E> RL T above the recrystallization temperature RLT are summed and for the ratio of the sums of £>RLT TO E <RLT gilt:

[0095] For the ratio of the sums of £> RL T TO £< RL T is preferably at least 1.0 and particularly preferably at least 1.5. If rolling is carried out entirely above RLT, the value in equation (1) is divided by one.

[0096] £ is the dimensional change per pass in the thickness direction in [%]. This is calculated as follows:

[0097] Here, hi represents the thickness after the stitch and h0 represents the thickness before the stitch and since the result of the respective thickness reduction is negative, the respective amount of the thickness reduction s is used for equation (1).

[0098] The recrystallization temperature (RLT) represents the temperature below which complete recrystallization is no longer possible between two samplings. The RLT is calculated using the following formula:

[0099] The equation for RLT was based on the equation by Bai 2011, which can be found in GORNI, Antonio Augusto: Steel Forming and Heat Treating Handbook, 2019, pp. 14 to 15. If the ratio of the individual pass reductions £< RL T below the recrystallization temperature RLT and the individual samplings E> RL If the T above the recrystallization temperature RLT according to equation (1) is not met, a microstructure that is too inhomogeneous across the strip thickness with an excessively high proportion of dislocation-rich and elongated grains can develop after hot rolling, resulting in excessive structural stress. From such an initial microstructure, regardless of compliance with the conditions during the tempering process, the material properties according to the invention cannot be ensured. By adhering to equation (1), the KAM value according to the invention is less than 2.0° for a step size of 250 nm.

[0100] The final hot rolling temperature TE w should be at least 840°C and at most 960°C, preferably at least 850°C and particularly preferably at least 860°C. If the final hot rolling temperature TEW is undershot, excessive forming below RLT may occur, whereby compliance with equation (1) can no longer be guaranteed. The reason for this is an excessive reduction in rolling speed. The total temperature loss during forming between the individual roll stands is too high. This higher temperature loss leads to too many rolling passes taking place below RLT, so that equation (1) can no longer be reliably maintained.

[0101] Another reason for limiting the hot rolling end temperature (TEW) is flatness. If the final stand is formed at temperatures that are too low, there is an increased susceptibility to center and / or edge waves, which negatively impact the quality of the strip. By limiting the hot rolling end temperature (TEW) upwards, the rolling speed is limited, thus ensuring compliance with the break times (X).

[0102] Through the targeted control of the individual stitch reductions E< RL T below and / or the individual stitch reductions E> RLT above the recrystallization temperature RLT and the selection of the hot rolling end temperature TEW during the hot rolling process are intended to limit grain stretching, which in turn leads to better microstructure homogeneity across the strip thickness and to the KAM value according to the invention. In a special embodiment of the process, a cumulative pause time X is maintained between all pass reductions performed above RLT, for which the following applies:

[0103] This represents a single pause time between two stitch reductions, 3 i and n represent the number of sampling points above RLT and n > 2.

[0104] By adhering to the accumulated pause time according to equation (5), a relatively homogeneous microstructure is created throughout the strip thickness during hot rolling, with a low proportion of dislocation-rich and elongated grains. This results in reduced microstructural stress and contributes to the inventive KAM value of less than 2.0° at a step size of 250 nm. Adhering to the pause times X is important to give the mechanism of grain regeneration and growth the necessary time to develop the KAM value of less than 2.0° at a step size of 250 nm.

[0105] The hot-rolled flat steel product, cooled to the cooling stop temperature, is coiled into a coil at a coiling temperature of at least 500°C and at most 650°C, preferably at least 520°C and at most 630°C, particularly preferably at least 540°C. The coiling temperature influences transformation hardening and precipitation hardening and thus significantly affects the mechanical and technological properties of the flat steel product. Cooling from the final rolling temperature (TEW) to the coiling temperature (HT) is preferably carried out by water cooling.

[0106] A coiling temperature that is too low (less than 500°C) results in the precipitation potential freezing, meaning the strength and elongation at break according to the invention can no longer be reliably achieved. Furthermore, a coiling temperature that is too low carries the risk of undesirably high bainite contents forming, resulting in a mixed microstructure and a failure of the high-strength material properties. A coiling temperature that is too high (greater than 650°C) leads to undesirable grain growth, which, while positive for the elongation at break, results in an excessive loss of strength. Furthermore, hard, coarse pearlite islands can form, which exhibit a different level of local misorientation than the main phase and thus jeopardize a KAM value of less than 2.0° at a step size of 250 nm. Furthermore, the hard, coarse pearlite islands can also negatively impact cutting quality by forming notches.

[0107] The specified coiling temperature is intended to prevent or limit the formation of an undesirable second phase or mixed structure. The coiling temperature increases precipitation hardening, which also serves to establish the required tensile strength Rm.

[0108] The tempering of the flat steel product takes place at an annealing temperature T G of at least 630°C, preferably at an annealing temperature T G of maximum 750°C, particularly preferably at an annealing temperature T G of at least 650°C and a maximum of 750°, and a holding time t G of at least 20 seconds, preferably at least 30 seconds, particularly preferably at least 40 seconds, and a maximum of 200 seconds. The annealing process is necessary to achieve the KAM value according to the invention.

[0109] If the tempering temperature T G and / or holding time t GIt cannot be guaranteed that the KAM value will be set at a step size of 250 nm to less than 2.0°. This is because a too low annealing temperature T G or too short holding time t G does not cause sufficient dislocation reduction within the not yet fully recrystallized grains. The existing structural stress would be too high without adherence to the tempering parameters, making it impossible to reliably achieve the KAM value. In the tempering step, structural recovery is achieved through controlled heat input.

[0110] If the tempering temperature T G or if the holding time t is exceeded GThis can lead to a very strong structural change, which can lead to the formation of undesirable microstructures such as pearlite and / or cementite, and can cause a loss of strength. Hard and coarse pearlite islands should be reduced or avoided because they exhibit a different level of local misorientation or KAM value and thus jeopardize the achievement of the material properties according to the invention. Furthermore, the hard, coarse pearlite and / or cementite islands can also negatively impact cutting quality by forming notches.

[0111] In a special design, the tempering annealing can be carried out separately in a continuous annealing plant, such as a continuous annealing line.

[0112] In a preferred variant, the hot-rolled flat steel product is tempered and coated during the coating process; in particular, a zinc-based coating is applied, whereby the zinc-based coating can be applied by hot-dip galvanizing. The application of a zinc-based coating is called galvanizing. The zinc-based coating has a zinc content of greater than or equal to 90 wt.%, in particular greater than or equal to 92 wt.%. Preferably, magnesium and / or aluminum are included as additional alloying elements, each in contents of up to a maximum of 5 wt.%, with the remainder being zinc and unavoidable impurities. The zinc-based coating may contain unavoidable impurities. The contents of such impurities must be kept as low as possible within an economically viable framework and with reasonable technical effort.

[0113] In an alternative embodiment, the tempering step can be carried out before the coating process, in particular a zinc-based coating is applied after the tempering step, wherein the zinc-based coating can be applied in particular by electrolytic galvanizing.

[0114] Galvanizing involves applying a metallic coating to the flat steel product to provide effective corrosion protection. A distinction is made between hot-dip galvanizing and electrolytic galvanizing. The zinc-based coating preferably has a thickness between 2 and 20 μm.

[0115] During hot-dip galvanizing, the flat steel product is first heated to the required coating temperature in an annealing furnace under protective gas before being coated with a metallic coating in a molten metal bath. The exact coating thickness is adjusted by various configurations of the wiping nozzles. In electrolytic galvanizing, the metallic coating is applied to the flat steel product by electroplating zinc. The thickness of the zinc layer is controlled by the current flow.

[0116] To improve the surface appearance and precisely adjust the mechanical properties, a special version optionally includes a skin-passing process after the tempering and / or coating step. Skin-passing can be performed inline in a continuous annealing line or in a galvanizing line, but is also possible after tempering and / or coating in a separate skin-passing stand.

[0117] The setting of the individual pass reductions above and below the RLT, the setting of the hot rolling end temperature TEW as well as the coiling and tempering temperatures have a decisive influence on limiting grain elongation and achieving the most homogeneous microstructure possible across the strip thickness, on avoiding or reducing undesirable second phases or mixed structures, and on microstructural recovery through reduced structural stress in the material. A targeted hot rolling process can reduce the formation of grain elongations of varying degrees with varying degrees of intra-grain dislocation densities. A subsequent tempering process leads to the removal of further dislocations and the relocation of atoms to energetically more favorable positions. This limits the KAM value to less than 2.0° at a step size of 250 nm, thereby reducing crack formation and crack susceptibility during cutting and punching.The result is good cutting edges.

[0118] The hot-rolled flat steel product is used, for example, in chassis parts, structural components or seats for the automotive industry, particularly in the field of electromobility.

[0119] The invention is described below with reference to figures and exemplary embodiments:

[0120] Fig. 1 shows an intact cut edge consisting of a smooth cut portion (1) and a broken portion (2). This is a microscopic image with 25x magnification. The image shows an uncoated flat steel product. Fig. 2 shows a cut edge with a macroscopic tear (3). This is a macroscopic image of an uncoated flat steel product.

[0121] Fig. 3 shows a schematic representation to describe the KAM value determination.

[0122] In systematic laboratory and operational tests, a total of 10 steel melts were produced, the chemical compositions of which are listed in Table 1. All data are in wt. %. All steel melts were cast either as slabs in a conventional continuous caster or as thin slabs in a casting-rolling mill and then hot-rolled. After hot rolling, the resulting flat steel products were tempered in a continuous annealing line and, in some cases, subsequently electrolytically galvanized, or, in some cases, tempered and galvanized after hot rolling in a hot-dip galvanizing line. The production parameters are listed in Table 2.

[0123] Production route A was carried out via the hot strip mill. This includes the steps of heating the cooled slab, rough rolling and descaling, finish rolling in several stands taking into account the inventive parameters, cooling with water, and winding on the coiler into a coil at the inventive coiling temperature (HT). Tests on the casting-rolling mill are designated as production route B and were carried out via the production of thin slabs with direct reheating after complete solidification, whereby the reheating is generally lower than in a conventional hot strip mill. After descaling, the slabs are rolled in several passes and then cooled with water and coiled at a coiling temperature (HT).

[0124] After hot rolling, a tempering step follows. Tempering involves reheating in a continuous annealing furnace to the inventive annealing furnace temperature T Gand holding this according to the inventive starting time t G followed by cooling.

[0125] Optionally, the steel flat product is galvanized after tempering. If the steel flat product is to be hot-dip galvanized, the tempering process described above takes place in a hot-dip galvanizing plant. After tempering, the steel flat product is first cooled to the galvanizing temperature, where it is immersed in a zinc-based melt to provide the steel flat product with a metallic surface coating and create effective corrosion protection. This is followed by a second cooling to room temperature.

[0126] If electrolytic galvanizing is to be carried out, after tempering in a continuous annealing line, the coating with a metallic surface coating is carried out by the galvanic deposition of a zinc-based coating in a separate electrolytic galvanizing plant.

[0127] The abbreviation Z stands for a zinc-based coating with more than 98 wt% zinc, the abbreviation ZM stands for a zinc-based coating with more than 90 wt% zinc and a maximum of 5 wt% magnesium and / or a maximum of 5 wt% aluminum and the remainder zinc and unavoidable impurities and the abbreviation EG stands for electrolytically galvanized with a zinc-based coating with more than 90 wt% zinc and a maximum of 5 wt% magnesium and / or a maximum of 5 wt% aluminum and the remainder zinc and unavoidable impurities.

[0128] In Table 3, in addition to the values ​​of the Kernel Average Misorientation (KAM), the tensile strength (R m ), elongation at break (A 80and A pr op), crack sensitivity, and hole expansion (LA).

[0129] Inventive experiments 1 and 2 demonstrate that by adhering to the inventive hot rolling and tempering parameters, it is possible to keep the crack sensitivity below 20% for composition A. In comparison, non-inventive experiment 3 with composition A did not meet the required ratio of the total deformation degree above RLT to below RLT. This resulted in an excessively high KAM value. This high KAM value, which was not in accordance with the invention, ultimately led to significantly more cut edges with macroscopic cracks. Furthermore, only very low, non-inventive hole expansion values ​​were determined.

[0130] In tests 4 to 7 according to the invention with chemical compositions B and C, a low KAM value was again achieved by adhering to the hot rolling and annealing parameters according to the invention, and the proportion of cut edges with macroscopic cracks, as well as the hole expansion, were maintained in accordance with the invention. In test 9, which is not according to the invention and has chemical composition D, the pause time "X" was not met. The result was that the KAM value was exceeded due to an excessively inhomogeneous microstructure and thus excessive crack sensitivity and low hole expansion. In tests 8 and 10, also with composition D, the pause time "X" was maintained, and thus ultimately the product properties according to the invention were also maintained.

[0131] In the inventive experiments 11 to 14 it becomes clear that the inventive material properties are also achieved with the chemical compositions E and F if the required ratio of the total degree of deformation above RLT to below RLT and the pause time "X" during hot rolling, as well as the tempering conditions in the continuous annealing line or in the hot-dip galvanizing plant are observed.

[0132] In Experiments 15 to 17 according to the invention with chemical composition G, it becomes clear again that adherence to one of the inventive manufacturing parameters is important in this analysis as well to obtain the material properties according to the invention. In Experiment 16, the tempering temperature of 630°C was not reached, resulting in poorer KAM values ​​and crack susceptibility.

[0133] Inventive tests 18 to 22, it becomes clear that the importance of chemical analysis is not as high as that of the manufacturing parameters. The tests with chemical compositions H to J show that, despite different alloying elements and alloy contents, the inventive properties are achieved in the flat steel products when the hot rolling and tempering conditions according to the invention are observed.

[0134] Examples not according to the invention are marked with a “*”, whereby values ​​of the respective embodiments which lie outside the specifications according to the invention are underlined.

[0135] To determine the mechanical properties such as tensile strength (R m ) and elongation at break (A 80 and Aprop), tensile tests according to DIN-EN ISO 6892-1 were carried out on longitudinal specimens of the hot-rolled steel TI flat products. The elongation at break for thicknesses less than 3.0 mm is defined as A 80-Elongation at break measured (gauge length Lo = 80 mm). The elongation at break for thicknesses greater than or equal to 3.0 mm is defined as A pr op-break strain is measured, where the measuring length is given by the following equation and So represents the initial cross-section:

[0136] The hole expansion is tested according to ISO 16630.

[0137] The KAM value (Kernel Average Misorientation) can be determined using the Electron Backscatter Diffraction (EBSD) method, using a scanning electron microscope with an electron backscatter diffraction imaging system. To determine a KAM value, the local misorientations or local orientation differences compared to all neighboring measuring points are calculated for each measuring point. For the EBSD measurement, a piece of the center of the sheet of the flat steel product is prepared as a longitudinal section. The center of the sheet represents the entire length across the strip, excluding the first 30 m at the beginning and the last 30 m at the end of the strip, and the entire width across the strip, excluding the first 30 mm from the outer strip edge on both sides.In order to remove the deformation stresses generated by the preparation, the samples are preferably polished after a final polishing with 1 pm diamond suspension and particularly preferably additionally polished with a colloidal silica suspension, in particular the OPS polish from Struers is used.

[0138] Fig. 3 shows a schematic representation for describing a method for determining the KAM value. In order to obtain the KAM value, a plurality of polygonal, in particular hexagonal, measuring point areas P are preferably defined in a measuring area of ​​the flat steel product. The measuring field size of a measuring area is 230 pm x 185 pm, preferably 90 pm x 90 pm. The measuring point areas P are measuring points for the KAM value. The measuring area is scanned by an electron beam from measuring point to measuring point with a step size of 250 nm, preferably with a step size of 100 nm. The KAM value is calculated for each measuring point area P contained in the measuring area. All orientation differences are included as absolute values.Accordingly, the measuring point areas P form a key figure for indicating the distribution of local misorientations, and an arithmetic mean is calculated across all measuring point areas P in a measuring area to calculate the KAM value. A cut-off value of 5° is defined, which means that local misorientations of a measuring point P greater than 5° are not taken into account when calculating the KAM value in order to reduce the disruptive influence of grain boundaries and small-angle grain boundaries. The measuring areas are selected at least 30 mm away from the strip edges of the flat steel product, preferably in the 1 / 4 layer of the flat steel product relative to the strip width. The measuring areas are preferably positioned in the 1 / 3 layer area across the thickness.

[0139] In the EBSD analysis, point-by-point diffraction patterns from the microstructure are evaluated to determine the crystallographic orientation at that point in the microstructure. For the KAM analysis, the misorientations relative to its neighbors are determined for each measurement point, and the magnitudes are averaged. This average is assigned to the measurement point under investigation as the KAM value. The individual KAM values ​​of all measurement points in a measurement range result in the average KAM value. Since the measurement points in the EBSD measurement are arranged such that consecutive rows of measurement points are offset from each other by half a step size, hexagonal regions result for each measurement point.

[0140] As shown in Fig. 3, to obtain the KAM value of the hatched measurement point area PI, the local misorientations between the measurement point area PI and the measurement point areas P2, P3, P4, P5, P6, and P7 surrounding the measurement point area PI are measured. That is, the local misorientation between the measurement point area PI and the measurement point area P2, the local misorientation between the measurement point area PI and the measurement point area P3, the local misorientation between the measurement point area PI and the measurement point area P4, the local misorientation between the measurement point area PI and the measurement point area P5, the local misorientation between the measurement point area PI and the measurement point area P6, and the local misorientation between the measurement point area PI and the measurement point area P7 are measured. Then, the magnitudes of the six local misorientations are averaged. The resulting value is the KAM value of the measurement point area PI.From the individual KAM values ​​of each measurement point area, an average KAM value is determined across the entire measurement field and the entire measurement range. The EBSD camera "Digiview" from the manufacturer "TSL" is the preferred camera. The KAM values ​​are preferably determined using the measurement software "OIM Data Collection V5.2," and the data analysis is preferably performed using the software "OIM Analysis V8.0."

[0141] The flat steel product according to the invention was characterized with regard to its cutting behavior, both in uncoated and coated versions. Cracks perpendicular to the sheet thickness direction, also frequently referred to as delamination, are fundamentally critical for its performance. To compare the robustness of different steel compositions against this unfavorable cutting surface formation, standardized cutting tests are conducted and evaluated.

[0142] Before the actual cutting test, sample strips with a width of approximately 19.5 mm and a length ranging from 200 to 400 mm are cut from the flat steel product to be tested using guillotine shears. Cutting is preferably carried out on a mechanical Schuler high-speed press type PD80-280. The cut is a cut-off process with an open cutting line. The waste is not supported by a counterholder. The sample position is transverse to the rolling direction from the center of the sheet, with the center of the sheet representing the entire length of the strip, excluding the first 30 m at the beginning of the strip and the last 30 m at the end of the strip, and the entire width across the strip width, excluding the first 30 mm from the outer strip edge on both sides. A cutting gap of 8% must be set based on the sheet thickness. The cut surfaces produced are then evaluated using macroscopic observation.The cut area is defined as the entire area consisting of the smooth cut portion and the fracture portion, as shown in Fig. 1. If a macroscopic crack is detected, it can be measured, for example, using a ruler. A macroscopic crack is defined as a crack with a minimum length of 1 mm and a length-to-width ratio of greater than 5.

[0143] The flat steel product according to the invention does not exhibit macroscopic cracks as defined above in a maximum of 20%, preferably a maximum of 10%, particularly preferably a maximum of 5% of the cut sheets out of a total of 100 sheet cuts carried out in order to meet the requirements of crack sensitivity.

[0144]

[0145] Data in wt.%, remainder iron and unavoidable impurities, with unstated element contents below the detection limit

[0146]

[0147] Examples not according to the invention are marked with a “*”, whereby values ​​of the relevant exemplary embodiments which lie outside the specifications according to the invention are underlined.

[0148]

[0149] Examples not according to the invention are marked with a “*”, whereby values ​​of the respective embodiments which lie outside the specifications according to the invention are underlined.

Claims

Patent claims 1. Hot-rolled flat steel product consisting of a steel with the composition specified below (in % by weight): C: 0.02 - 0.15%, Mn: 1.00 - 2.00%, AI: 0.01 - 0.5%, Nb: 0.002 - 0.15% optionally one or more of the following elements with the weight part specified below: Ti: 0.0015 - 0.24% Si: 0.01 - 0.6% Cr: < 1.5% Mo: < 1.0% V: < 0.5% Cu: < 0.5% Ni: < 0.5% B: < 0.0025% Ca: < 0.01% - Rest iron and unavoidable impurities, and has a KAM value at a step size of 250 nm of less than 2.0°.

2. Hot-rolled flat steel product according to claim 1, characterized in that it has a crack sensitivity of maximum 20%.

3. Hot-rolled flat steel product according to one of claims 1 to 2, characterized in that it has a tensile strength R m of at least 550 MPa.

4. Hot-rolled flat steel product according to one of claims 1 to 3, characterized in that for thicknesses of less than 3.0 mm it has an elongation at break A 80 of at least 14%.

5. Hot-rolled flat steel product according to one of claims 1 to 3, characterized in that for thicknesses greater than or equal to 3.0 mm it has an elongation at break A prop of at least 17%.

6. Hot-rolled flat steel product according to one of claims 1 to 5, characterized in that it has a hole expansion of at least 40%.

7. Hot-rolled flat steel product according to one of claims 1 to 5, characterized in that it has a surface coating, wherein the surface coating is preferably a zinc-based coating.

8. A method for producing a hot-rolled flat steel product according to one of the preceding claims, comprising the following steps: - Producing a steel melt with the following composition (in wt.%): C: 0.02 - 0.15% Mn: 0.2 - 2.5% AI: 0.01 - 0.5% Nb: 0.002 - 0.15% - optionally one or more of the following elements in the proportions by weight specified below: Ti: 0.0015 - 0.24% Si: 0.01 - 0.6% Cr: < 1.5% Mo: < 1.0% V: < 0.5% Cu: < 0.5% B: < 0.0025% Ca: < 0.01% - Residual iron and unavoidable impurities; - casting of the molten steel into a precursor product in the form of an ingot, a slab, a thin slab or a cast strip; - Optional heating of the pre-product to an austenitizing temperature T WEbetween 1100 °C and 1350 °C; - Hot rolling of the preliminary product to the hot-rolled flat steel product with successive individual pass reductions, whereby the individual pass reductions £< rlt below the recrystallization temperature RLT are summed and the individual samplings £> RL T above the recrystallization temperature RLT are summed and for the ratio of the sums of £> RLT to E< RL T applies: 0.75, - Setting a hot rolling end temperature T E w, which is at least 840°C and at most 960°C; - coiling the hot-rolled flat steel product cooled to the cooling stop temperature into a coil at a coiling temperature of at least 500 °C and a maximum of 650 °C; - Tempering the flat steel product at an annealing temperature T G of at least 630 °C and a holding time t G of at least 20 seconds.

9. Method according to claim 8, characterized in that between all samplings carried out above RLT, a cumulative pause time X is maintained, for which the following applies: and where t 1 F ( r i— 1) _ 1 F t represents the individual pause time between two samplings, i and n represent the number of samplings above RLT and n > 2.

10. A method according to any one of claims 8 to 9, characterized in that the tempering step takes place in a continuous annealing plant.

11. Method according to one of claims 8 to 10, characterized in that the hot-rolled flat steel product is coated after the tempering step, in particular a zinc-based coating is applied, wherein the zinc-based coating is preferably carried out by electrolytic galvanizing.

12. The method according to any one of claims 8 to 10, characterized in that the tempering step is carried out as part of a coating process, wherein the coating is preferably a zinc-based coating and the zinc-based coating is preferably carried out by hot-dip galvanizing.

13. Method according to one of claims 8 to 12, characterized in that after the tempering step and / or coating step, a tempering step is carried out.

14. Use of the hot-rolled flat steel product according to one of claims 1 to 7 in chassis parts, structural components or seats for the automotive industry, in particular in the field of electromobility.

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