Hot-rolled flat steel product and method for its production

Description

[0001] The invention relates to a hot-rolled flat steel product with a particularly high uniformity of the mechanical-technological properties over its length and width, excellent forming and toughness properties and low springback.

[0002] Furthermore, the invention relates to a method for producing such a hot-rolled flat steel product.

[0003] "Flat steel products" are defined here as rolled products whose length and width are each significantly greater than their thickness. These include, in particular, steel strips, steel sheets, and cut pieces made from them, such as blanks and the like.

[0004] In this text, unless explicitly stated otherwise, information on alloy components is always given in wt.%.

[0005] The proportions of certain components in the structure of a flat steel product are, however, given in area % unless otherwise stated.

[0006] Flat steel products made from micro-alloyed steels have been manufactured for over 40 years. They are used for applications that require a combination of high strength and good formability. Due to their low alloying element content, such steels also have excellent weldability and can be produced relatively inexpensively. The production of flat steel products from micro-alloyed steels is usually based on a melt analysis that contains at least one of the micro-alloying elements niobium, titanium or vanadium. By processing the precursor cast from an alloyed steel melt into a hot-rolled strip in a controlled thermo-mechanical rolling and cooling process, the hot-rolled flat steel product obtained in this way has a very fine-grained microstructure. This typically consists predominantly of ferrite and / or bainite and small amounts of pearlite or ferrite.Cementite (FeS) and fine and very fine precipitates ensure the fine-grained structure of the microstructure. The fine grain structure and the high density of fine and very fine precipitates, together with the solid solution strengthening caused by the presence of manganese and silicon, lead to the high strength of such flat steel products.

[0007] From a material engineering perspective, the fineness of the microstructure significantly impedes the migration of dislocations during deformation, as substituted foreign atoms (e.g., manganese) must be circumvented and numerous grain boundaries and large amounts of fine and very fine precipitates must be overcome. Although elongation is known to decrease with increasing strength, it nevertheless remains at a high level in flat steel products made of microalloyed steels.

[0008] EP 2 924 140 A1 discloses a process for producing a flat steel product with a yield strength of at least 700 MPa and with a bainitic structure of at least 70 vol.%, in which a steel melt is melted in the first step, which consists of (in wt.%) C: 0.05 - 0.08%, Si: 0.015 - 0.500%, Mn: 1.60 - 2.00%, P: up to 0.025%, S: up to 0.010%, Al: 0.020 - 0.050%, N: up to 0.006%, Cr: up to 0.40%, Nb: 0.060 - 0.070%, B: 0.0005 - 0.0025%, Ti: 0.090 - 0.130%, and the remainder of iron and technically unavoidable impurities, including up to 0.12% Cu, up to 0.100% Ni, up to 0.010% V, up to 0.004% Mo, and up to 0.004% Sb. The resulting melt is cast into a slab, which is then heated to a reheating temperature of 1200-1300 °C.The slab is then rough-rolled at a rough-rolling temperature of 950–1250 °C with a total pass reduction of at least 50% achieved during rough-rolling, and then finish-rolled hot, with hot rolling ending at a final hot-rolling temperature of 800–880 °C. Within a maximum of 10 seconds after finish-rolling, the resulting hot strip is then intensively cooled at a cooling rate of at least 40 K / s to a coiling temperature of 550–620 °C, where it is finally wound into a coil. The alloying elements of the steel are closely matched to one another in a process that can be carried out reliably. US 2019 / 085421 A1 discloses a hot-rolled flat steel product with good forming properties.

[0009] It is known that niobium, titanium, and vanadium contribute to strength increases to varying degrees. Niobium has the strongest strength-enhancing effect, followed by titanium and vanadium. Alloy concepts based on titanium without niobium and vanadium are generally favorable because titanium is inexpensive. However, pure titanium microalloyed concepts only achieve high strengths with very high titanium contents. Due to its high affinity for nitrogen, titanium forms titanium nitrides, which remain stable even at temperatures above 1200 °C. Due to their usually sharp-edged structure, these can have a detrimental effect on the forming of such alloyed flat steel products into components.

[0010] Vanadium-based alloy concepts usually fail to achieve the desired strengths, even with high vanadium contents, because precipitation hardening with vanadium is significantly weaker than with niobium and titanium. Furthermore, vanadium is very expensive, which is why only low contents are usually used.

[0011] A combination of micro-alloying elements (e.g., titanium and vanadium) to achieve high strength is possible and common practice. However, these concepts often tend to result in fluctuations in the mechanical properties across the length and width of the flat steel product produced from the correspondingly alloyed steel. The reason for this is that, for technical reasons, during cooling in the coil wound from the hot-rolled steel strip obtained after hot rolling, the temperature is not constant across the entire strip length and width. As a result, the formation of micro-alloying element precipitates, which is dependent on the respective temperature, varies to a greater or lesser extent across the strip length and width, with the result that the mechanical properties of the steel strip, which are directly influenced by these precipitates, exhibit correspondingly strong fluctuations.

[0012] Against this background, the task was to develop a hot-rolled flat steel product that is characterized by a particularly uniform distribution of its mechanical properties across its length and width and has good formability and a low tendency to spring back, which make it suitable for the production of complex-shaped components with a particularly wide range of requirements.

[0013] A flat steel product which achieves this object according to the invention has at least the features specified in claim 1.

[0014] In addition, a process should be specified that enables the targeted production of such a hot-rolled flat steel product.

[0015] A method for solving this problem comprises at least the method steps specified in claim 9.

[0016] Advantageous embodiments of the invention are specified in the dependent claims and, like the general inventive concept, are explained in detail below.

[0017] A flat steel product according to the invention therefore consists of (in wt. %) 0.02 - 0.1% C, 0.1 - 2.5% Mn, 0.02 - 0.1% Al, 0.04 - 0.12% Nb and optionally one or more elements from the group "Si, Ti, V, Cr, B, Ca, Mo" with the proviso that the Si content is at most 0.25%, the Ti content at most 0.12%, the V content at most 0.2%, the Cr content at most 0.2%, the B content at most 0.0025%, the Ca content at most 0.01% and the Mo content at most 0.3%, and the remainder of iron and unavoidable impurities, wherein the impurities include up to 0.05% P, up to 0.03% S, up to 0.01 % N, up to 0.2% Ni and up to 0.15% Cu.

[0018] The microstructure of a flat steel product according to the invention consists of at least 60 area-% ferrite and / or bainite, the remainder being pearlite, carbide or carbonitride precipitates, and a maximum of 2 area-% other microstructure components. At the same time, the microstructure has a grain aspect ratio of 0.2 - 0.7, with the α-fiber portions present in the ferrite <110> parallel to the rolling direction in a proportion of not more than 30% and the γ-fibre components present in the ferrite <111> are formed parallel to the sheet metal normal in a proportion of no more than 20%.

[0019] It has proven particularly advantageous if the microstructure of a flat steel product according to the invention also has an average ferrite grain size of 15 µm or less. The grain stretch ratios that can be reliably achieved in the microstructure, particularly through the inventive manufacturing method, contribute significantly to the good toughness properties of a flat steel product according to the invention. Furthermore, the fine microstructure characterized by small ferrite grain sizes contributes to a further improvement in the toughness properties.

[0020] Due to its special composition and microstructure, which is a result of its manufacturing process, a flat steel product according to the invention possesses excellent forming properties coupled with optimized mechanical properties. Thus, the yield strength Re of flat steel products according to the invention is Re > RE_BER, where: Re _ BER = 400 + 2243 * % Nb / d 0 , 15 with %Nb: respective Nb content of the flat steel product in wt.% and d: respective thickness of the flat steel product in mm Typically, the yield strengths Re of flat steel products according to the invention are more than 350 MPa, in particular more than 400 MPa, 450 MPa, or more than 500 MPa. Furthermore, the yield strengths Re are typically below 800 MPa, in particular below 750 MPa or below 700 MPa.

[0021] At the same time, notched-bar impact bending tests conducted in accordance with DIN EN ISO 148-1 in the "longitudinal" test direction on flat steel products according to the invention at -20°C and -30°C each resulted in an impact energy of > 100 J, with impact energy values of more than 125 J, and in particular more than 150 J, being regularly achieved. Using the same test setup, but at test temperatures of -60°C, the determined impact energy in the "longitudinal" test direction at -60°C was more than 27 J, with impact energy values of more than 80 J being regularly achieved. Even at a test temperature of -80 °C, flat steel products according to the invention still achieved an impact energy of more than 27 J in the "longitudinal" test direction. Their accordingly optimized impact toughness and their high brittle fracture resistance make flat steel products according to the invention particularly suitable for applications where lower temperatures, ie in particular temperatures below -40 °C, prevail.

[0022] The springback determined according to DIN EN 10149-2 with a bending mandrel diameter of 8 mm is less than 20% for hot-rolled flat steel products according to the invention, although in practice springback values of less than 17%, in particular less than 15%, are regularly achieved.

[0023] Hot-rolled flat steel products according to the invention develop their properties through the presence of niobium as the sole mandatory microalloying element, without the need to add additional microalloying elements, namely titanium and vanadium. Although these elements can optionally be present in a flat steel product according to the invention in effective amounts to support the effect of the niobium content that is always present according to the invention, as explained below, the alloying concept of a flat steel product according to the invention is based on the presence of niobium, the contents of which are adjusted according to the invention such that, in principle, no additional microelements are required to meet all requirements placed on the formability, strength, toughness, notch impact behavior, and springback behavior of flat steel products according to the invention.During forming, it is shown that the risk of crack formation in the steel flat products according to the invention is delayed to such an extent due to their special microstructure (i.e. their special grain stretching and fiber orientation) that even complex components such as spring strut mounts for vehicle chassis can be manufactured without problems.

[0024] The method according to the invention for producing a flat steel product according to the invention comprises the following steps: a) Producing a steel melt consisting of (in wt. %) 0.02 - 0.1% C, 0.1 - 2.5% Mn, 0.02 - 0.1% Al, 0.04 - 0.12% Nb and optionally one or more elements from the group "Si, Ti, V, Cr, B, Ca, Mo" with the proviso that the Si content is not more than 0.25%, the Ti content is not more than 0.12%, the V content is not more than 0.2%, the Cr content is not more than 0.2%, the B content is not more than 0.0025%, the Ca content is not more than 0.01% and the Mo content is not more than 0.3%, and the remainder is iron and unavoidable impurities, the impurities including up to 0.05% P, up to 0.03% S, up to 0.01% N, up to 0.2% Ni and up to 0.15% Cu; b) casting the melt into a precursor product which is a slab with a thickness of 70 mm - 350 mm or a thin slab with a thickness of 30 - 70 mm; c) austenitizing the precursor product in such a way that the precursor product is heated to an austenitizing temperature of 1150 - 1320 °C;(d) where the precursor is a slab: rough-rolling of the austenitized precursor in two or more rolling passes to a thickness of at least 30 mm and at most 70 mm at a rough-rolling temperature which is at most equal to the austenitizing temperature but at least 30 °C above the recrystallization stop temperature T NR , calculated as follows: ; T NR ° C = 913 ° C + 910 ° C / Gew . − % * % Nb with %Nb = respective Nb content of the steel flat product; e) finish hot rolling of the precursor product to a hot-rolled steel flat product in several rolling passes, whereby a number nw of rolling passes is greater than or equal to a value nw' rounded to an integer, which is calculated according to the formula n W ′ ≥ 7 * Wurzel d EW / 6 * Z + 2 where d EW : final rolling thickness of the flat steel product Z: thickness of the preliminary product is calculated, at a temperature below the recrystallization stop temperature T NR calculated according to formula (1), whereby the degree of deformation Δh ges achieved overall over the final hot rolling applies Δh ges = d Einlauf − d Auslauf / d Einlauf > 65 % with d inlet: thickness of the steel flat product at the entry into the finish hot rolling, C outlet: thickness of the steel flat product at the end of the finish hot rolling, where the degree of deformation Δh LG achieved in the last rolling pass of the finish hot rolling is: Δ h LG ist > Δ h LG min with Δ h LG ist = d Einlauf LG − d Auslauf LG d Einlauf LG d Inlet LG :Thickness of the flat steel product at the inlet into the last rolling stand, d Outlet LG :Thickness of the flat steel product at the outlet from the last rolling stand Δ h LG min = 1 , 8 ∗ 10 − 5 ∗ e 0 , 0135 ∗ TEW + 11 + 3 Δh LG min : minimum degree of deformation in the last pass of finish hot rolling TEW: final hot rolling temperature in °C, and where the final hot rolling temperature is 760 - 940 °C; f) cooling the hot-rolled flat steel product to a coiling temperature of 520 - 650 °C; g) coiling the hot-rolled flat steel product into a coil and cooling the hot-rolled flat steel product in the coil to room temperature.

[0025] The alloying constituents of a flat steel product according to the invention and the contents of these constituents are selected according to the invention as follows: In addition to iron, C, Mn, Al, and Nb are mandatory elements of the alloy of a flat steel product according to the invention. All other elements explained below are optionally present in the flat steel product according to the invention to develop certain properties or to be assigned to the unavoidable impurities whose presence is undesirable but unavoidable for manufacturing reasons. The contents of these unavoidable accompanying elements are limited in a flat steel product according to the invention so that they have no negative influence on the properties of the flat steel product.

[0026] Carbon "C" is present in a flat steel product according to the invention in amounts of 0.02 - 0.1 wt.%. At least 0.02 wt.% is required for a flat steel product according to the invention to achieve the required strength properties. This effect can be achieved particularly reliably with C contents of at least 0.4 wt.%. At the same time, the C content is limited to a maximum of 0.1 wt.% to avoid a negative impact on weldability and formability. Negative effects of the presence of C, such as a reduction in toughness, can be particularly reliably avoided by setting a maximum C content of 0.08 wt.%.

[0027] Silicon ("Si") is optionally used as a deoxidizer in the production of the steel of a flat steel product according to the invention and contributes to improving the strength properties of the flat steel product according to the invention. To reliably utilize this effect of Si, the Si content can be at least 0.01 wt.%. Si contents of more than 0.6 wt.% would impair the surface quality and toughness properties of the material according to the invention, in particular the toughness in the heat-affected zone of a weld seam produced on a flat steel product according to the invention. Furthermore, excessively high Si contents could impair the weldability of flat steel products according to the invention. To reliably avoid these negative influences and, in particular, to ensure optimized surface quality, the Si content is limited to 0.25 wt.%.

[0028] Manganese ("Mn") is present in a flat steel product according to the invention in amounts of 0.1–2.5 wt.% to ensure good mechanical properties, particularly high toughness, and S-bonding. At amounts of less than 0.1 wt.%, sufficient strength would not be achieved. However, Mn contents of more than 2.5 wt.% would negatively impact weldability and formability. To reliably utilize the strength-enhancing effect of Mn in the flat steel product according to the invention, the Mn content can be increased to at least 0.5 wt.%. To avoid negatively affecting segregation behavior and toughness, the Mn content can be limited to a maximum of 2.0 wt.%.

[0029] Aluminum ("Al") is present in the inventive flat steel product in amounts of 0.02–0.1 wt.%. It is used as a deoxidizer in the production of the steel for a flat steel product according to the invention. It inhibits the coarsening of the austenite grain by forming AlN precipitates during heating ("austenitizing") during the processing of the steel into the inventive flat steel product. If the aluminum content is below 0.02 wt.%, the deoxidation processes do not complete during steel production. However, if the Al content exceeds the upper limit of 0.1%, undesirable Al 2 O 3 inclusions may form. These would negatively impact the purity and toughness properties of a flat steel product according to the invention.If restrictions on the castability of the steel melt are to be avoided particularly reliably during the production of flat steel products according to the invention, the Al content of a flat steel product according to the invention can be limited to a maximum of 0.05 wt.%.

[0030] Niobium ("Nb") is present in the inventive flat steel product in amounts of 0.04–0.12 wt.% to achieve optimized strength properties through precipitation hardening during the coiling process carried out according to the invention. If the Nb content is less than 0.02 wt.%, the required strength properties would not be achieved. To ensure grain refinement of the austenite structure during the temperature-controlled rolling process according to the invention, Nb is added in amounts of at least 0.04 wt.%. If Nb contents of more than 0.12 wt.% were provided, this would impair the weldability and toughness in the heat-affected zone of a welded joint produced on a flat steel product according to the invention. The effects achieved by the inventively provided Nb contents can be achieved particularly economically with Nb contents of a maximum of 0.08 wt.%.

[0031] Even though the alloying concept of the invention aims to use only Nb as a microalloying element, titanium ("Ti") can be present in the inventive flat steel product in amounts of up to 0.12 wt.% to support the strength properties by preventing grain growth during austenitization and by precipitation hardening during coiling. These beneficial effects of the presence of Ti can be utilized particularly reliably by setting the Ti content of a flat steel product according to the invention to at least 0.005 wt.%. At Ti contents above 0.12 wt.%, there is a risk that the formability, weldability, and toughness of the flat steel product will deteriorate due to the formation of coarse Ti precipitates. This risk can be minimized by limiting the Ti content to a maximum of 0.10 wt.%.In order to ensure the most uniform strength properties possible across length and width, a Ti alloy is omitted entirely or the content is preferably limited to a maximum of 0.010 wt.%, particularly preferably to a maximum of 0.006 wt.%.

[0032] Vanadium "V" can be added to a flat steel product according to the invention in amounts of up to 0.2 wt.% to enhance strength through the formation of carbonitrides. If this effect is to be specifically utilized in a flat steel product according to the invention, a content of at least 0.005 wt.% V can be provided for this purpose. At contents of more than 0.2 wt.%, no further increase in the positive effects of the optional presence of V in a flat steel product according to the invention occurs. The strength-enhancing effect of V can be optimally utilized if up to 0.15 wt.% V is present in a flat steel product according to the invention. In principle, however, the addition of V is to be considered purely optional; thus, the addition of V can be completely omitted because the alloy concept according to the invention is primarily based on the presence of the microelement Nb.To ensure the most uniform strength properties possible across length and width, according to a particularly practical variant of the invention, the V content in a flat steel product according to the invention is reduced to technically ineffective levels. For this purpose, the V content can be limited to a maximum of 0.010 wt.%, in particular a maximum of 0.006 wt.%.

[0033] Chromium ("Cr") is optionally present in the inventive flat steel product in amounts of up to 0.2 wt.%. The strength properties of a flat steel product according to the invention can also be improved by adding Cr. However, if the chromium content is too high, the weldability and toughness in the heat-affected zone of a weld made on a flat steel product according to the invention are negatively affected. The positive effects of the optional presence of Cr in a flat steel product according to the invention can be reliably utilized by ensuring that its Cr content is at least 0.02 wt.%.

[0034] 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 a flat steel product according to the invention is made. This beneficial effect of B can be utilized by providing B contents of at least 0.0005 wt.% B, in particular at least 0.0015 wt.% B, for a flat steel product according to the invention. However, B contents of more than 0.0025 wt.% would impair the toughness properties.

[0035] Calcium ("Ca") can optionally be present in the steel of a flat steel product according to the invention to form non-metallic inclusions in the microstructure of the flat steel product, thereby improving toughness. However, if the Ca content exceeds 0.01 wt.%, this can have a negative impact on the purity of the melt and lead to defects in the shell of the cast intermediate product during casting of the steel from which a flat steel product according to the invention is produced. The positive effects of the optional presence of Ca in a flat steel product according to the invention can be reliably utilized by ensuring that its Ca content is at least 0.0005 wt.%.

[0036] Molybdenum ("Mo") can optionally be present in the steel of a flat steel product according to the invention to achieve higher strength properties. The positive effects of the optional presence of Mo can be achieved from a Mo content of at least 0.02%. If the Mo content is too high, the elongation at break and the formability of the material are negatively affected. For this purpose, the Mo content is limited to a maximum of 0.3 wt.%, in particular to a maximum of 0.25 wt.%, or, particularly advantageously, to a maximum of 0.1 wt.%.

[0037] A flat steel product according to the invention exhibits an optimized combination of strength and toughness without the need for expensive alloying elements such as nickel (Ni) and copper (Cu). However, these elements can also enter the steel as unavoidable impurities due to manufacturing processes, for example, through the use of scrap in steel production. In any case, the Ni and Cu contents in a flat steel product according to the invention are kept so low that they have no influence on the properties of a flat steel product according to the invention. For the same reasons, the Ni content of a flat steel product according to the invention is a maximum of 0.2 wt.%, and its Cu content a maximum of 0.15 wt.%.

[0038] Phosphorus ("P" and sulfur "S" are also undesirable impurities in the flat steel product according to the invention because they impair its mechanical properties, particularly the notch impact energy and formability. To avoid any influence of these accompanying elements, which are unavoidable due to production reasons, the invention sets an upper limit for the P content of 0.05 wt.%, in particular of at most 0.025 wt.% or, particularly preferably, of at most 0.015 wt.%, and an upper limit for the S content of at most 0.03 wt.%, in particular of at most 0.01 wt.% or, particularly preferably, of at most 0.003 wt.%.

[0039] Nitrogen ("N") is also an unavoidable impurity due to manufacturing processes, which, if present at excessive levels, impairs the toughness properties of a flat steel product according to the invention. Therefore, the N content of a flat steel product according to the invention is limited to a maximum of 0.01 wt.%, in particular 0.008 wt.%, or, particularly preferably, a maximum of 0.006 wt.%. Typical N contents of a flat steel product according to the invention are at least 0.004 wt.%. These N contents, which are typically present at least in the flat steel product according to the invention, lead to the formation of AlN and, if Ti is present, TiN precipitates, which, as already explained above, have a positive effect on the fineness of the microstructure of a flat steel product according to the invention.

[0040] The microstructure of a flat steel product according to the invention consists predominantly of ferrite and / or bainite. Thus, the ferrite and / or bainite content of the microstructure of a flat steel product according to the invention is typically at least 60 area% ferrite and / or bainite, whereby the ferrite can be present as polygonal ferrite and / or quasi-polygonal ferrite and / or strongly dislocation-strengthened ferrite. To ensure the best possible formability, a proportion of at least 80 area% is preferably set; to increase toughness, more preferably at least 90 area%, and particularly preferably at least 95 area% ferrite and / or bainite. The remainder of the microstructure of a flat steel product according to the invention is made up of pearlite and the aforementioned precipitates in the form of carbides or carbonitrides, as well as a maximum of 2 area% of other microstructural components, including martensite or retained austenite.

[0041] The microstructure of a flat steel product according to the invention is further characterized by a grain aspect ratio between 0.2 and 0.7. Setting a grain aspect ratio within this range ensures good toughness properties. If the grain aspect ratio is above 0.7, elongation is negatively affected. If the ratio is below 0.2, the effect on strength is too small.

[0042] In addition, the structure of the flat steel product according to the invention is characterized by the fact that the α-fiber portions present in the ferrite <110> parallel to the rolling direction in a proportion of not more than 30% and the proportions of the γ-fibre present in the ferrite <111> parallel to the sheet standard in a proportion of no more than 20%. This characteristic ensures good flow behavior during forming as well as good elongation at break properties, thereby achieving overall excellent forming behavior of the flat steel products according to the invention.

[0043] The working steps of the process according to the invention and the process parameters set according to the invention are specially adapted to the alloying concept according to the invention in such a way that the special combination of properties of flat steel products according to the invention is achieved in an operationally reliable manner.

[0044] The alloying concept based on the use of niobium as the preferred sole microalloying element allows high strengths to be achieved even with comparatively low Nb contents. Due to the fact that niobium significantly retards recrystallization, the hot rolling process during the production of steel flat products according to the invention results in significant austenite grain elongation, resulting in a very fine-grained, dislocation-rich ferrite or bainite structure after coiling. Compared to titanium and vanadium, niobium significantly retards recrystallization at the same element contents and also contributes particularly effectively to grain refinement of the microstructure after coiling.

[0045] This results in the formation of fine Nb precipitates in the coil into which the flat steel product is wound after hot rolling in step g). These precipitates form in a stable manner and are insensitive to non-uniform temperature distribution across the entire length and width of the flat steel product. Consequently, a flat steel product according to the invention is characterized by minimal fluctuations in mechanical properties across its length and width.

[0046] It goes without saying that when carrying out the method according to the invention, the person skilled in the art not only completes the method steps mentioned in the claims and explained here, but also carries out all other steps and activities which are regularly carried out in the practical implementation of such methods in the prior art, if the need arises.

[0047] For the production of flat steel products according to the invention, a steel melt composed according to the invention is produced in step a) in a manner known per se with regard to the procedure to be used, in accordance with the above explanations.

[0048] This molten steel is then cast under similarly known conditions to form a precursor product, which can be a slab or a thin slab. The pouring temperature of the molten steel during continuous casting should exceed 1500 °C to ensure that the steel does not solidify in the transport ladle. This is especially true if a thin slab is produced as the precursor product.

[0049] The casting of the melt into a slab can be carried out in any manner known in the art for this purpose.

[0050] For the production of thin slabs, the "CSP process" (CSP = Compact Strip Production, see https: / / www.sms-group.com / de / anlagen / alle-anlagen / csp-technologie / ) is available in practice. In this process, the melt is cast in a continuous process into a strand, from which the thin slabs are subsequently separated. The liquid steel is fed via a tundish into a mold, from which two strands emerge parallel to each other, each of which begins solidification by forming a so-called outer "strand shell." After exiting the mold, solidification progresses from the strand shell in the center of the strand (core) until the core area is also solidified. The fully solidified strand has a thickness of at least 30 mm, typically 35-70 mm, and a temperature typically exceeding 600 °C.After solidification, the thin slabs are separated from the strand.

[0051] The resulting precursors (slab or thin slab) are heated in a preheating or soaking furnace to an austenitizing temperature above 1150 °C for the so-called "austenitizing" process (step c)), at which they acquire a fully austenitic microstructure. The high austenitizing temperature is important to ensure that the coarse precipitates formed during solidification of the respective precursor are dissolved during the austenitizing process. According to the invention, the upper limit of the austenitizing temperature range is 1320 °C to prevent coarsening of the austenite grain and increased scale formation.

[0052] If the precursor product is a thin slab, it can be fed directly to the final hot rolling (step e). This, like the austenitization in the soaking furnace and the preceding production of the thin slab, can be carried out in a conventional manner in a continuous work sequence.

[0053] If the precursor product is a slab, however, its thickness must be reduced before finish hot rolling. For this purpose, the slab in question is pre-rolled after austenitizing, for example in one or more reversing rolling stands available for this purpose in the state of the art, in more than one pass to produce a pre-strip with a thickness of at least 30 mm, but no more than 70 mm. The temperature of the slab at the start of pre-rolling is at most equal to the austenitizing temperature and at least 1100 °C. The temperature of the pre-strip pre-rolled from the slab after completion of pre-rolling is referred to as the pre-rolling temperature TVW. The pre-rolling temperature is at most equal to the austenitizing temperature. The pre-rolling temperature is preferably below 1150 °C, in particular below 1120 °C.At a pre-rolling temperature above this limit, on the one hand, a coarser austenite grain would form as a result of grain growth after recrystallization. On the other hand, an increased number of discontinuities, i.e. defects, would appear on the surface of the finished hot-rolled product. However, the pre-rolling temperature must not be lower than a temperature TVW min that is at least 30 °C above the recrystallization stop temperature T NR calculated in the manner specified above as a function of the Nb content %Nb. This ensures that the recrystallization processes run to completion, thus maintaining a fine austenite grain and limiting subsequent grain growth. This has a positive effect on the toughness properties and the elongation at break of flat steel products according to the invention. The elongation at break is typically at least 14%.If the pre-rolling temperature were to fall below the TVW min, this would lead to an undesirable mixed structure as a result of the incomplete recrystallization, which would impair the toughness and elongation at break properties of the flat steel product and could also result in a greater scatter of the mechanical properties.

[0054] In the finish hot rolling process according to the invention (work step e)), the pre-rolled strip produced from the pre-rolled strip or the thin slab produced as a pre-rolled product is hot-rolled into a finished hot-rolled flat steel product, which is typically a hot-rolled strip, in a multi-stand hot rolling mill, typically comprising five, six, or seven rolling stands. The hot-rolling parameters are adjusted so that the resulting flat steel product has the required thickness and, upon completion of the process according to the invention, exhibits the structural features already explained. According to the invention, a "multi-stand" rolling process means at least three consecutive pass reductions.

[0055] The number of passes actually required in each case is selected depending on the thickness required for the finished hot-rolled flat steel product. A certain number nw of rolling passes must be carried out in the finishing hot rolling mill at a temperature below the recrystallization stop temperature T NR (equation (1)) calculated as described above. This number nw is determined as the result nw' of the above equation (2), rounded to the nearest whole number.

[0056] According to the invention, the final hot rolling is carried out in a so-called "thermomechanical rolling" manner. The degrees of deformation, also known as "pass reductions," are specifically adjusted according to the invention and maintained throughout the entire final hot rolling process and the individual hot rolling passes, as well as precise temperature control, resulting in the fiber orientation of the microstructure and its grain stretching. This positively influences not only the strength properties, but also the toughness, elongation at break, and the resulting forming properties, especially springback, compared to typical microalloying concepts based on two microalloying elements.

[0057] The total degree of deformation Δh ges to be achieved according to the invention over the entire finish hot rolling process, as well as the degree of deformation Δh LG ist to be achieved over the last rolling pass, are adjusted so that a highly elongated austenite grain is established in the microstructure of a steel flat product according to the invention. The finish rolling is ended below a final rolling temperature of 940 °C, in particular below 920 °C or, particularly preferably, below 890 °C. The low final hot rolling temperature according to the invention enhances the effect of thermo-mechanical rolling, so that dislocation-rich austenite is present in the microstructure of the steel flat product according to the invention at the end of hot rolling. By keeping the final hot rolling temperature according to the invention at a maximum of 940 °C, it is ensured that no further recrystallization processes occur, even locally, after the end of hot rolling.In this way it is ensured that the grain stretching required according to the invention is achieved and that the α-fiber portions present in the ferrite are aligned in a way that is favorable for the forming process. <110> parallel to the rolling direction and γ-fiber <111> parallel to the sheet normal. At the same time, however, the final hot rolling is terminated at a temperature above 760 °C to ensure that phase transformation is avoided during final hot rolling, particularly in the surface area.

[0058] According to the invention, the total degree of deformation Δh ges achieved in the finishing hot rolling mill should be at least 65%. In the last stand, however, the degree of deformation Δh LG ist should at least meet the condition according to equation (4). In this case, the degree of deformation Δh LG ist should preferably be at least 4%. If the requirements established by the invention with regard to the degrees of deformation Δh ges and Δh LG ist are not met, the grain stretching according to the invention and the orientation according to the invention of the portions of the α-fiber present in the ferrite <110> parallel to the rolling direction and γ-fiber <111> parallel to the sheet normal is not achieved.

[0059] After hot rolling, the resulting hot-rolled flat steel product is cooled in step f) to a coiling temperature of 520–650 °C. Cooling can be carried out in a conventional manner. Typical cooling rates suitable for the purposes of the invention are in the range of 10–300 K / s. Cooling should begin within a maximum of 20 s after the end of hot rolling.

[0060] By adhering to the coiling temperature range specified in the invention, optimal transformation and precipitation hardening is achieved. The coiling temperature range is selected so that the number of carbonitride precipitations is maximized. A coiling temperature that is too low would result in the precipitation potential freezing, and thus the minimum yield strength required of a flat steel product according to the invention would not be achieved. A coiling temperature that is too high would lead to undesirable precipitation and / or grain growth, which in turn could result in a loss of toughness and yield strength.

[0061] Due to their combination of properties, hot-rolled flat steel products according to the invention are suitable for forming into complex components of all kinds, particularly for components used in passenger or truck construction, which typically include chassis or running gear parts such as strut mounts, axle supports, cross members, or longitudinal members, or automotive seat parts such as seat rails. They are also suitable for painted components, which are manufactured, for example, using laser cutting.

[0062] The components of the structure mentioned in this text can be determined using a light microscope, a scanning electron microscope and electron back scattered diffraction (EBSD).

[0063] To determine the microstructure components, samples were taken from a quarter of the width of the flat steel product at a third of the sheet thickness, prepared as longitudinal sections, and etched with alcoholic nitric acid containing 3 vol.% nitric acid (also known as "Nital" in technical terms) or sodium disulfite. The respective proportions of the microstructure components were then determined using light or scanning electron microscopy in a conventional manner using surface analysis.

[0064] The grain aspect ratio and the proportion of α-fiber present in the ferrite <110> parallel to the rolling direction and γ-fiber <111> parallel to the sheet standard can be determined using EBSD on a longitudinal section. For this purpose, a measuring field of 800 x 800 µm is positioned in 1 / 3 position across the sheet thickness and scanned with a step size of 0.9 µm. Twin grain boundaries are not considered as grain boundaries. Within the grains, a misorientation of up to 5° between adjacent measuring points is permitted. A minimum of ten connected measuring points has been selected as the minimum grain size. To determine the grain aspect ratio, the grains are approximated as ellipses so that the grain aspect ratio can then be specified as the length ratio of the semi-axes of the ellipse. A value of 1 therefore corresponds to a circular grain, and the smaller the value, the more elongated the grain is.To quantify the proportions of texture components from the α-fiber and the γ-fiber, an orientation tolerance of 10° is applied – i.e., orientations are counted as belonging to the corresponding fiber if they deviate by up to 10° from the ideal fiber orientation. The measurements can be performed, for example, on a LEO 1530 field emission scanning electron microscope from Carl Zeiss Microscopy GmbH with an EBSD system from EDAX Inc. with the Digiview camera. The data analysis and creation of the grain size distributions, as well as the quantification of the proportions of the α-fiber <110> parallel to the rolling direction and γ-fiber <111> parallel to the sheet metal standard can be carried out, for example, with the software OIM Analysis V 8 from EDAX Inc.

[0065] The tensile tests to determine the yield strength (Re) were carried out according to DIN EN ISO 6892-1 on longitudinal specimens of the hot-rolled strip.

[0066] The notched bar impact tests to determine the impact energy Av at -20 °C, -30 °C, -60 °C, and -80 °C were carried out on longitudinal specimens according to DIN EN ISO 148-1. The results described here always refer to full specimens.

[0067] The springback was determined in the 180° bending test according to DIN EN 10149-2, with a bending mandrel diameter of 8 mm.

[0068] The invention is explained in more detail below using exemplary embodiments: For the experiments 1 - 32 explained below, alloyed steel melts A - M were produced according to the invention, the compositions of which are given in Table 1. Table 1 also shows the recrystallization stop temperature T NR of the respective steel, determined according to the formula T NR [°C] = 913 °C + 910 °C / wt.% * %Nb as a function of its Nb content %Nb.

[0069] Part of these heats A - M was cast into slabs (variant "A"). The other part of the heats A - M was processed in a continuous process in a CSP plant, first into thin slabs and then directly into a hot-rolled flat steel product in the form of hot strip (variant "B").

[0070] The cast slabs were austenitized at an austenitizing temperature TA. In variant A (= processing of slabs as a preliminary product), the slabs were then rough-rolled in several passes. At the start of the rough-rolling process, the slabs each had a temperature approximately 30 °C below the austenitizing temperature TA. After rough-rolling was completed at a rough-rolling temperature TVW specified in the table, 30 - 70 mm thick pre-strips were then finish-rolled. In variant B (= processing of thin slabs as a preliminary product), however, the 30 - 70 mm thick thin slabs were fed directly into the hot rolling mill, i.e. without intermediate rough-rolling.

[0071] In the hot rolling mill, the respective pre-strip or slab was rolled into a hot-rolled flat steel product with a thickness d in 5 to 7 hot rolling passes α. A total deformation degree Δh total was achieved across all hot rolling passes, and a deformation degree Δh LG actual was achieved across the final hot rolling pass.

[0072] The hot rolling of the flat steel products was completed with a final hot rolling temperature TEW.

[0073] The flat steel products leaving the hot rolling mill, at the final hot rolling temperature (TEW), were cooled by water cooling at cooling rates of 10 to 300 K / s to a coiling temperature (HT), at which they were coiled into a coil. Finally, they were cooled to room temperature in the coil.

[0074] For tests 1 - 32, Table 2 shows the steel used in each case, the variant run through in each case, the thickness "d" of the finished hot-rolled steel flat product, the austenitizing temperature "TA", the respective pre-rolling temperature "TVW", the respective final hot-rolling temperature "TEW", the total degree of deformation "Δh ges " and the degree of deformation achieved over the last pass of hot rolling "Δh LG ist " as well as the degree of deformation calculated according to equation (4) "Δh LG min ".

[0075] The non-inventive tests 3 (degree of deformation "Δh LG is too low"), 7 (pre-rolling temperature and final hot rolling temperature too high), 18 (austenitizing temperature too high) and 28 (coiling temperature too low) are highlighted by a suffix "*".

[0076] Table 3 shows the yield strength "Re", impact energy "Av" at -20 °C, -30 °C, -60 °C and -80 °C, the grain stretch ratio, the position of the α-fiber portions present in the ferrite and the <110> parallel to the rolling direction and the γ-fiber <111> parallel to the sheet standard, the elongation at break A and the springback are specified, with the specimen position in which the relevant characteristic values were determined also being noted. For the information on elongation at break A, the following applies: for specimens made from sheets with a thickness of 3 mm or greater, the elongation at break A5 was determined in accordance with DIN EN ISO 6892-1 (December 2009), and for specimens made from sheets with a thickness of less than 3 mm, the elongation at break A80 was determined in accordance with DIN EN ISO 6892-1 (December 2009).

[0077] It is shown that the steel flat products obtained in the tests, which are designed and produced according to the invention, have a combination of high yield strength, high impact energy, i.e. high toughness, pronounced grain elongation, good alignment of the α-fiber components present in the ferrite <110> parallel to the rolling direction and γ-fiber <111> parallel to the sheet metal normal, as well as good springback behavior, which ensure optimal forming behavior with simultaneously optimized usage properties. Table 1 Steel C Si Mn Al Nb Ti V Cr No Cu Mon B Ca P S N TNR [°C] A 0,063 0,017 1,46 0,029 0,061 - - - - - - - - 0,014 0,002 0,0059 968,51 B 0,061 0,018 1,45 0,031 0,063 0,008 - - - - - - - 0,011 0,002 0,0040 970,33 C 0,051 0,18 1,41 0,035 0,058 - 0,05 - - - - - - 0,010 0,001 0,0099 965,78 D 0,054 0,21 1,42 0,031 0,055 - - - - - - - - 0,010 0,001 0,0036 963,05 E 0,028 0,031 1,54 0,031 0,091 - - - - - - - - 0,014 0,001 0,0056 995,81 F 0,026 0,032 1,59 0,034 0,093 - - - - - 0,2 - - 0,015 0,002 0,0050 997,63 G 0,049 0,025 1,32 0,043 0,069 - 0,009 - - - - - - 0,014 0,002 0,0078 975,79 H 0,048 0,025 1,33 0,047 0,072 - - - - - - - - 0,010 0,003 0,0053 978,52 I 0,086 0,019 1,42 0,034 0,046 - - - - - - - - 0,014 0,002 0,0056 954,86 J 0,082 0,018 1,47 0,030 0,047 - - - - - - - - 0,019 0,003 0,0050 955,77 K 0,061 0,24 0,55 0,029 0,054 0,054 - - - - - - - 0,014 0,002 0,0051 962,14 L 0,059 0,23 0,58 0,028 0,051 - - - - - 0,08 0,002 - 0,018 0,001 0,0065 959,41 M 0,060 0,016 1,35 0,036 0,059 - - - - - - - - 0,025 0,004 0,0040 966,69 all contents in wt.%, remainder iron and unavoidable impurities Table 2 Attempt Steel variant d TA TVW TEW HT Δh total Δh LG is Δh LG min [mm] [°C] [%] [%] [%] 1 A B 2 1280 - 880 580 95 6,76 6,01 2 A A 12 1240 1040 840 590 76 8,92 4,76 3* A* A 20 1260 1060 800 580 67 3,41 4,02 4 B A 4 1220 1040 930 590 90 15,45 8,92 5 B A 8 1250 1050 910 580 80 14,89 7,52 6 C A 2 1290 1120 900 610 95 9,52 6,95 7* C A 4 1300 1160 950 610 90 11,33 10,76 8 C A 10 1250 1060 850 600 80 15,89 5,01 9 D A 5 1180 1040 880 580 88 7,85 6,01 10 D A 8 1160 1030 860 580 80 10,02 5,30 11 E A 2 1280 1090 880 590 95 7,69 6,01 12 E A 12 1220 1040 840 580 76 12,65 4,76 13 E A 20 1240 1030 810 570 67 15,70 4,17 14 F A 8 1270 1080 840 530 80 12,45 4,76 15 F B 12 1260 - 840 550 76 15,78 4,76 16 F A 15 1250 1070 820 540 70 14,01 4,34 17 G A 5 1260 1080 880 600 88 9,92 6,01 18* G A 20 1330 1100 880 590 67 16,35 6,01 19 H A 7 1250 1070 860 580 83 10,76 5,30 20 H A 3 1270 1090 890 580 93 7,85 6,45 21 I A 6 1240 1050 850 580 85 11,49 5,01 22 I A 9 1240 1060 830 570 82 6,32 4,53 23 I A 18 1210 1040 800 580 70 19,52 4,02 24 J A 2 1300 1110 870 620 95 7,41 5,63 25 J B 8 1290 - 870 600 80 10,71 5,63 26 K A 5 1260 1090 880 570 88 9,82 6,01 27 K A 9 1250 1080 860 550 82 12,32 5,30 28* K A 16 1230 1080 830 510 68 17,35 4,53 29 L A 4 1280 1090 880 580 90 9,28 6,01 30 L A 8 1270 1060 870 580 80 11,50 5,63 31 L A 12 1250 1070 850 570 76 13,89 5,01 32* L A 18 1250 1040 820 570 60 15,23 4,34 *) - not according to the invention Table 3 Attempt Steel Sample position re Av**-20°C Av**-30°C Av**-60°C Av**-80°C A Grain aspect ratio α-fiber <110> γ-fiber <111> springback [MPa] [J] (full sample) [%] [-] [%] [%] [°] 1 A BA 561 - - - - 23 0,45 12 7 [°] 13 1a A BM 557 - - - - 24 0,47 11 7 13 2 A BA 435 220 205 167 148 37 0,43 19 10 4 3* A BA 420 96 72 25 3 41 0,18 35 9 7 4 B BA 548 - - - - 28 0,62 7 8 12 5 B BA 481 253 238 200 177 32 0,37 10 12 5 6 C BA 539 - - - - 23 0,41 14 8 12 7* C BA 529 - - - - 12 0,73 5 3 26 8 C BA 483 208 192 169 141 33 0,36 15 10 4 9 D BA 557 - - - - 27 0,39 12 7 10 10 D BA 530 278 261 235 219 28 0,38 19 9 5 11 E BA 576 - - - - 21 0,48 13 10 14 12 E BA 464 196 183 144 125 35 0,40 22 6 4 13 E BA 436 182 178 136 122 39 0,42 19 12 2 14 F BA 460 221 216 171 139 33 0,37 18 4 8 15 F BA 421 187 171 138 111 38 0,51 14 6 5 16 F BA 413 261 235 199 171 40 0,48 16 8 4 17 G BA 541 - - - - 29 0,52 10 11 9 18* G BA 438 90 80 20 6 42 0,18 33 12 7 19 H BA 552 198 175 121 100 28 0,49 14 7 7 20 H BA 568 - - - - 24 0,51 17 10 11 21 I BA 523 176 162 108 85 27 0,44 15 9 11 22 I BA 505 189 175 126 97 30 0,52 19 8 9 23 I BA 472 213 198 178 161 38 0,36 21 9 3 23a I BM 465 209 192 169 149 38 0,37 20 9 3 24 J BA 567 - - - - 22 0,41 18 12 13 25 J BA 548 164 148 102 78 29 0,51 17 10 8 26 K BA 562 - - - - 26 0,49 12 5 11 27 K BA 554 187 172 128 96 29 0,45 17 5 7 28* K BA 361 220 208 130 117 13 0,29 33 5 23 29 L BA 540 - - - - 26 0,39 16 10 12 30 L BA 549 267 258 210 186 32 0,47 14 7 5 30a L BM 554 260 251 204 174 31 0,49 12 7 5 31 L BA 445 273 264 206 174 34 0,41 16 7 5 32* L BA 401 120 105 40 16 40 0,18 35 8 10 *)- not according to the invention **) - converted to full sample BA - beginning of strip BM - middle of strip

Claims

1. Flat steel product consisting of, in wt. %, C:0.02 - 0.1%,Mn:0.1 - 2.5%,Al:0.02 - 0.1 %,Nb:0.04 - 0.12%, and optionally consisting of one element or a plurality of elements from the group "Si, Ti, V, Cr, B, Ca, Mo", with the proviso that the Si content is at most 0.25%, the Ti content is at most 0.12%, the V content is at most 0.2%, the Cr content is at most 0.2%, the B content is at most 0.0025%, the Ca content is at most 0.01% and the Mo content is at most 0.3%, and the remainder consists of iron and unavoidable impurities, wherein the impurities include up to 0.05% P, up to 0.03% S, up to 0.01% N, up to 0.2% Ni, and up to 0.15% Cu, wherein the structure of the flat steel product - consists of at least 60% by area of ferrite and / or bainite and the remainder consists of pearlite and carbide precipitates or carbonitride precipitates and at most up to 2% by area of other structural constituents, and - has a grain aspect ratio of 0.2 - 0.7, wherein the proportions of the α-fiber <110> present in the ferrite are formed in parallel with the rolling direction in a proportion of at most 30%, and the proportions of the γ-fiber <111> present in the ferrite are formed in parallel with the sheet normal in a proportion of at most 20%, the grain aspect ratio, the α-fiber and the γ-fiber, measured as specified in the description, and wherein the flat steel product - has a yield strength Re for which Re > RE_BER applies, where Re_BER = 400 + 2243 * % Nb / d 0.15 where %Nb:relevant Nb content of the flat steel product in wt.%, and d: relevant thickness of the flat steel product in mm.

2. Flat steel product according to claim 1, characterized in that the structure has an average ferrite grain size of at most 15 µm, measured as specified in the description.

3. Flat steel product according to any of the preceding claims, characterized in that it has a notched bar impact work Av in the "longitudinal" test direction of more than 27 J at -60°C.

4. Flat steel product according to any of the preceding claims, characterized in that it has a springback of less than 20%, determined in a 180° folding test according to DIN EN 10149-2 with a bending mandrel diameter of 8 mm.

5. Flat steel product according to any of the preceding claims, characterized in that it contains at least 0.005 wt.% Ti.

6. Flat steel product according to any of the preceding claims, characterized in that it contains at least 0.005 wt.% V.

7. Flat steel product according to any of the preceding claims, characterized in that it contains at most 2.0 wt.% Mn.

8. Flat steel product according to any of the preceding claims, characterized in that it contains at least 0.5 wt.% Mn.

9. Method for producing a flat steel product formed according to any of the preceding claims, comprising the following work steps: a) producing a steel melt consisting of, in wt.%, 0.02 - 0.1% C, 0.1 - 2.5% Mn, 0.02 - 0.1% Al, 0.04 - 0.12% Nb, and optionally consisting of one element or a plurality of elements from the group "Si, Ti, V, Cr, B, Ca, Mo", with the proviso that the Si content is at most 0.25%, the Ti content is at most 0.12%, the V content is at most 0.2%, the Cr content is at most 0.2%, the B content is at most 0.0025%, the Ca content is at most 0.01% and the Mo content is at most 0.3%, and the remainder consists of iron and unavoidable impurities, wherein the impurities include up to 0.05% P, up to 0.03% S, up to 0.01% N, up to 0.2% Ni, and up to 0.15% Cu; b) casting the melt to form a precursor, which is a slab which has a thickness of 70 - 350 mm or a thin slab which has a thickness of 30 - 70 mm; c) austenitizing the precursor such that the precursor is heated through to an austenitizing temperature of 1150 - 1320°C; d) if the precursor is a slab: pre-rolling the austenitized precursor in two or more rolling passes to a thickness of at least 30 mm and at most 70 mm at a pre-rolling temperature which is at most equal to the austenitizing temperature but at least 30°C above the recrystallization stop temperature TNR, which is calculated as follows: T NR ° C = 913 ° C + 910 ° C / wt . % * % Nb where %Nb = relevant Nb content of the flat steel product; e) finish hot rolling the precursor to form a hot-rolled flat steel product in a plurality of rolling passes, - wherein a number nwof rolling passes equal to a value nW', rounded to a whole number, which is calculated according to the formula n W ′ = 7 * root d EW / 6 * Z + 2 where dEW final rolled thickness of the flat steel product Z: thickness of the precursor is carried out at a temperature below the recrystallization stop temperature TNR calculated according to formula (1), - wherein for a total degree of deformation Δhgesachieved by the finish hot rolling, the following applies Δh ges = d entry − d exit / d entry > 65 % where dentry: thickness of the flat steel product when entering the finish hot rolling, dexit: thickness of the flat steel product at the end of the finish hot rolling, - wherein for the degree of deformation ΔhLG actual achieved in the last rolling pass of the finish hot rolling, the following applies: Δh LG actual > Δh LG min where Δh LG actual = d entry LG − d exit LG / d entry LG dentry LG: thickness of the flat steel product when entering the last rolling stand, dexit LG: thickness of the flat steel product when exiting the last rolling stand Δh LG min = 1.8 * 10 − 5 * e 0.0135 * TEW + 11 + 3 ΔhLG min: minimum degree of deformation in the last rolling pass of the finish hot rolling TEW: hot rolling end temperature in °C, and - wherein the hot rolling end temperature is 760 - 940°C; f) cooling the hot-rolled flat steel product to a coiling temperature of 520 - 650°C; g) coiling the hot-rolled flat steel product into a coil and cooling the hot-rolled flat steel product in the coil to room temperature.

10. Method according to claim 9, characterized in that the pre-rolling temperature in work step d) is at most 1150°C.

11. Method according to any of claims 9-10, characterized in that the hot-rolling end temperature in work step e) is at most 920°C.

12. Method according to claim 11, characterized in that the hot rolling end temperature in work step e) is at most 890°C.

13. Use of a flat steel product according to claim 1 for producing a component.