HOT-ROLLED STEEL FLAT PRODUCT AND METHOD FOR ITS MANUFACTURE

Description

[0001] The invention relates to a hot-rolled flat steel product that possesses optimally coordinated mechanical properties, such as high tensile strengths Rm, high yield strengths Rp, and high elongation at break A, combined with good formability, which is characterized by a high hole expansion value, for which the abbreviation "λ" ("lambda") has been introduced. Furthermore, hot-rolled flat steel products according to the invention are characterized by good fatigue strength and wear resistance.

[0002] The invention also relates to a method for producing such a flat steel product.

[0003] When we talk about flat steel products here, we mean products produced by rolling technology, such as strips, sheets or cut blanks and cut pieces, which each have a width and length significantly greater than their thickness.

[0004] Information on alloy contents provided here refers to weight or mass, unless explicitly stated otherwise. Information on the contents of structural components, with the exception of the information on retained austenite contents, which is given in vol. %, generally refers to the area examined in the cross-section, unless otherwise stated. Information on the composition of an atmosphere, however, refers to the respective volume examined, unless explicitly stated otherwise.

[0005] So-called "quench & partitioning" flat steel products are characterized by high strength combined with high elongation and optimized formability. In practice, such flat steel products have so far been used as cold-rolled products with low sheet thicknesses.

[0006] However, WO 2013 / 004910 A1 (EP 2 726 637) discloses a process for producing high-strength structural steels and products made from them. In this process, slabs made of a suitably selected steel alloy are first heated to 950–1300°C and held there until a uniform temperature distribution is achieved throughout the slabs. Typically, the steel from which the slabs are produced should consist of (in wt. %) 0.17–0.23% C, 1.4–2.0% Si, or a total of 1.2–2.0% Al and Si, if Al is present, 1.4–2.3% Mn, and 0.4–2.0% Cr, optionally up to 0.7% Mo, the remainder being iron and unavoidable impurities. After annealing, the slabs undergo hot rolling, where they are rolled in a temperature range below the recrystallization temperature but above the A3 temperature.After hot rolling, the resulting hot strip is quenched at a quenching rate of at least 20 °C / s to a quench stop temperature that lies in the temperature range between the temperature Ms, at which martensite formation begins, and the temperature Mf, at which martensite formation is complete. Typically, the quench stop temperature is in the range of more than 200 °C and less than 400 °C. The quenched hot strip is subjected to a so-called "partitioning treatment" to transfer carbon from the martensitic to the austenitic microstructure components. Finally, the treated hot strip is cooled to room temperature. Key parameters of the quenching and partitioning treatment remain open. JP 2016 194158 A discloses a high-strength steel sheet with hole expansion properties.

[0007] Against the background of the prior art explained above, the object of the invention was to provide a flat steel product with a greater sheet thickness and an optimized combination of properties.

[0008] A process for the cost-effective, reliable manufacture of such a product should also be specified.

[0009] With regard to the product, the invention has solved this problem by the hot-rolled flat steel product specified in claim 1.

[0010] With regard to the method, the solution according to the invention to the above-mentioned object consists in that the work steps specified in claim 7 are carried out during the production of a flat steel product according to the invention.

[0011] Advantageous embodiments of the invention are specified in the dependent claims and are explained in detail below, as is the general inventive concept.

[0012] The invention provides a hot-rolled flat steel product and a process suitable for its production.

[0013] Carbon "C" is present in the steel melt processed according to the invention in concentrations of 0.1–0.3 wt.%. C primarily plays a major role in austenite formation. A sufficient C concentration enables full austenitization at temperatures of up to 930 °C, which are below the final rolling temperatures typically used for hot rolling steels of the type in question. During quenching, a portion of the residual austenite is already stabilized by the carbon provided according to the invention. Furthermore, further stabilization occurs during the subsequent partitioning step. The strength of the martensite formed during the first cooling step (θQ) or during the final cooling step (θP2) also depends strongly on the C content of the steel composition processed according to the invention. At the same time, however, the martensite initiation temperature shifts to increasingly lower temperatures with increasing C content.An excessively high C content would therefore lead to difficulties in production, as the quench temperature to be achieved would be shifted to very low temperatures. Furthermore, the C content of a steel processed according to the invention contributes most significantly to a higher CE value compared to other alloying elements, which negatively impacts weldability. The CE value indicates which alloying elements negatively influence the weldability of the steel. The CE value can be calculated as follows: . CE = % C + % Si + % Mn / 6 + % Cr + % Mo + % V / 5 + % Cu + % Ni / 15 with (each in wt.%) %C = C content of the steel, %Si = Si content of the steel, %Mn = Mn content of the steel, %Cr = Cr content of the steel, %Mo = Mo content of the steel, %V = V content of the steel, %Cu = Cu content of the steel, %Ni = Ni content of the steel.

[0014] The strength level of the final product can be specifically influenced by the C content specified in the invention.

[0015] Manganese (Mn) is an important element for the hardenability of steel. At the same time, manganese reduces the tendency toward undesirable pearlite formation during cooling. These properties enable the creation of a suitable initial microstructure of martensite and retained austenite after the first quenching with cooling rates < 100 K / s according to the process of the invention. An excessively high Mn concentration negatively affects elongation and the CE value, i.e., weldability. Therefore, the Mn content is limited to 1.5–3.0 wt.%. An optimized coordination of the strength properties can be achieved by setting the Mn content to 1.9–2.7 wt.%.

[0016] Silicon "Si" plays an important role in suppressing pearlite formation and controlling carbide formation. The formation of cementite would bind carbon, making it unavailable for further stabilization of the residual austenite. On the other hand, an excessively high Si content impairs elongation at break and surface quality through accelerated red scale formation. A similar effect can be triggered by alloying with Al. A minimum of 0.7 wt.% Si is required to achieve the product properties intended according to the invention. The desired microstructure can be achieved particularly reliably if Si contents of at least 1.0 wt.% are present in the flat steel product according to the invention. Due to the desired elongation at break, the upper limit of the Si content is specified at 1.8 wt.% Si. Limiting this to a maximum of 1.6 wt.% Si results in flat steel products with optimized surface quality.Depending on the respective Al content of the flat steel product according to the invention, the Si content can also be set to 0.5 - 1.1 wt.%, in particular 0.7 - 1.0 wt.%, as explained in the following paragraph.

[0017] Aluminum ("Al") is used for deoxidation and to bind any nitrogen that may be present. Furthermore, as already mentioned, Al can also be used to suppress cementite, but it is not as effective as Si. However, an increased Al addition significantly increases the austenitizing temperature, which is why cementite suppression is preferably achieved solely with Si. In this case, an Al content of 0–0.03 wt.%, which is favorable with regard to the austenitizing temperature, is provided if Si is simultaneously present in amounts of at least 1.0 wt.%. However, if the Si content is limited, for example, to achieve optimized surface quality, i.e., adjusted to values ​​between 0.5–1.1 wt.%, preferably 0.7–1.0 wt.%, then Al must be added with a minimum content of 0.5 wt.% to suppress cementite.In a preferred embodiment, the Al content can be adjusted to values ​​of at least 0.01 wt.% for particularly reliable production of deoxidized melts. The Al content is limited to a maximum of 1.5 wt.%, preferably a maximum of 1.3 wt.%, to avoid problems during casting of the steel.

[0018] Phosphorus "P" has a negative impact on weldability. Its content in the hot-rolled strip according to the invention or in the melt processed according to the invention is therefore limited to a maximum of 0.1 wt.%, although P contents of up to 0.02 wt.%, especially less than 0.02 wt.%, can be advantageous.

[0019] At higher concentrations, sulfur (S) leads to the formation of MnS or (Mn, Fe)S, which negatively affects elongation. To avoid this effect, the S content is limited to a maximum of 0.03 wt.%, although limiting the S content to a maximum of 0.003 wt.%, especially less than 0.003 wt.%, can be advantageous.

[0020] Nitrogen ("N") leads to the formation of nitrides, which negatively impact formability. The N content should therefore be less than 0.008 wt.%. With considerable technical effort, very low N contents of, for example, less than 0.0010 wt.% can be achieved. To reduce the technical effort, the N content can preferably be set to at least 0.0010 wt.%, and particularly preferably to at least 0.0015 wt.%.

[0021] The alloying elements summarized in the group "Cr,Mo,Ni,Nb,Ti,V,B" can optionally be added individually, together or in various combinations in accordance with the instructions explained below in order to adjust certain properties of the flat steel product according to the invention.

[0022] Chromium ("Cr") is an effective pearlite inhibitor and can thus reduce the required minimum cooling rate. To achieve this, Cr is added to the steel processed according to the invention or to the steel of the hot-rolled flat steel product according to the invention. To effectively achieve this effect, a minimum content of 0.10 wt.% Cr, preferably 0.15 wt.% Cr, is required. At the same time, the addition of Cr greatly increases strength and also poses the risk of severe grain boundary oxidation. Furthermore, the formation of chromium oxides near the surface of the steel impedes potential coatability and can lead to undesirable surface defects. These surface defects can lead to reduced fatigue strength and thus premature failure of the material under cyclic loading.Furthermore, an excessively high Cr content impairs the formability of the steel; in particular, a good hole expansion λ of greater than 20% cannot be guaranteed. Therefore, the Cr content is limited to a maximum of 0.30 wt.%, preferably a maximum of 0.25 wt.%.

[0023] Molybdenum (Mo) is also a very effective element for suppressing pearlite formation. To achieve this effect, at least 0.05 wt.%, especially at least 0.1 wt.%, can be optionally added to the steel. Additions of more than 0.25 wt.% are not advisable from an effectiveness perspective.

[0024] Nickel ("Ni"), like Cr, is a pearlite inhibitor and effective even in small amounts. This supporting effect can be achieved with optional alloying with Ni of at least 0.05 wt.%, in particular at least 0.1 wt.%, at least 0.2 wt.%, or at least 0.3 wt.%. At the same time, with regard to the desired mechanical properties, it is advisable to limit the Ni content to a maximum of 2.0 wt.%, although Ni contents of no more than 1.0 wt.%, in particular 0.5 wt.%, have proven particularly practical.

[0025] The steel of a flat steel product according to the invention can optionally also contain microalloying elements such as vanadium (V), titanium (Ti), or niobium (Nb), which contribute to increased strength through the formation of very finely distributed carbides (or carbonitrides in the presence of nitrogen (N). Furthermore, the presence of Ti, V, or Nb leads to the freezing of grain and phase boundaries after the hot rolling process during the partitioning step, which promotes the desired combination of strength and formability through grain refinement. The minimum content at which a significant effect is noticeable is 0.02 wt.% for Ti, 0.01 wt.% for Nb, and 0.1 wt.% for V. However, an excessively high concentration of the microalloying elements leads to the formation of too many and coarse carbides and thus to the binding of carbon, which is then no longer available for the inventive stabilization of the retained austenite.In addition, the formation of excessively coarse carbides has a negative impact on the desired high fatigue strength. Therefore, depending on the effect of the individual elements, the upper limit is set at 0.07 wt.% for Ti, 0.06 wt.% for Nb, and 0.3 wt.% for V.

[0026] Likewise, optionally added amounts of boron ("B") segregate at the phase boundaries and impede their movement. This leads to a fine-grained microstructure, which can have a beneficial effect on the mechanical properties. Therefore, when using this alloying element, a minimum B content of 0.0008 wt.% must be maintained. However, when adding B, sufficient Ti must be present to bind the N. The effect of B is saturated at a content of approximately 0.0020 wt.%, which is also considered the upper limit.

[0027] A hot-rolled steel flat product according to the invention has a tensile strength Rm of 800 - 1500 MPa, a yield strength Rp of more than 700 MPa and an elongation at break A of 7 - 25%, wherein the tensile strength Rm, the yield strength Rp and the elongation at break A are determined according to DIN EN ISO 6892-1-2009-12.

[0028] At the same time, the hot strip according to the invention is characterized by very good formability, which is reflected in a hole expansion λ of more than 20%, determined according to DIN ISO 16630.

[0029] Hot-rolled strip manufactured according to the invention and, in particular, produced by the process according to the invention has a microstructure of tempered and untempered martensite with portions of retained austenite, whereby bainite, polygonal ferrite, non-polygonal ferrite, and cementite may also be present in small portions in the microstructure. The martensite portion of the microstructure is at least 85 area%, preferably at least 90 area%, of which at least half is tempered martensite. The proportion of retained austenite in a hot-rolled flat steel product according to the invention is accordingly at most 15 vol. Likewise, up to 15 area% of bainite, up to 15 area% of polygonal ferrite, up to 5 area% of cementite, and / or up to 5 area% of non-polygonal ferrite may be present in the microstructure, each at the expense of the retained austenite.In a preferred embodiment, the proportion of polygonal ferrite and the proportion of non-polygonal ferrite is 0 area%, since in this case the values ​​for hole expansion are particularly high due to the delayed crack formation in a predominantly martensitic structure with uniform hardness.

[0030] The microstructure of the hot-rolled strip according to the invention is very fine, making its assessment using conventional optical microscopy almost impossible. Therefore, assessment using scanning electron microscopy (SEM) and a magnification of at least 5000x is recommended. However, the maximum permissible retained austenite content is difficult to determine even at high magnification. Therefore, quantitative determination of retained austenite using X-ray diffraction (XRD) according to ASTM E975 is recommended.

[0031] The microstructure of the hot-rolled flat steel product according to the invention is characterized by a defined, local misorientation in the crystal lattice. This applies in particular to the desired proportion of primary martensite, i.e., the martensite fraction formed during the initial cooling. This local misorientation is quantified by the so-called "Kernel Average Misorientation," or "KAM" for short, which is greater than or equal to 1.50°, preferably greater than 1.55°. The KAM value should be at least 1.50°, because then a homogeneous deformation resistance is present due to uniform lattice distortion in the grain. This prevents locally limited pre-damage to the multiphase microstructure at the beginning of deformation. If the KAM value is below 1.50°, the microstructure is excessively tempered, resulting in strength properties outside the desired range of the invention.

[0032] In addition to the pure phase components, the distortion of the crystal lattice is crucial for the mechanical properties of a steel product designed and manufactured according to the invention. This lattice distortion represents a measure of the initial resistance to plastic deformation, which determines the properties due to the desired strength ranges. A suitable method for measuring and thus quantifying lattice distortion is electron backscatter diffraction (EBSD). EBSD generates and combines a large number of local diffraction measurements to detect small differences and patterns, as well as local misorientations in the microstructure. A common EBSD evaluation method in practice is the aforementioned kernel average misorientation (KAM), in which the orientation of a measurement point is compared with that of neighboring points.Below a threshold, which is typically 5°, neighboring points are assigned to the same (distorted) grain. Above this threshold, the neighboring points are assigned to different (sub-)grains. Due to the very fine microstructure, a maximum step size of 100 nm is recommended for the EBSD evaluation method. To assess the steels presented in this invention report, the KAM is evaluated in relation to the current measuring point and its third closest neighboring point. A product according to the invention must then have a KAM mean value from a measuring range of at least 75 µm x 75 µm of ≥ 1.50°, preferably > 1.55°. A more detailed description of how the KAM value is determined can be found in Wright, SI, Nowell, MM, Fielda, DA, Review of Strain Analysis Using Electron Backscatter Diffraction, Microsc. Microanal. 17, 2011: 316-329.

[0033] A method according to the invention for producing a hot-rolled flat steel product according to the invention comprises at least the following steps: a) Melting a steel alloy, the composition and variants of which have already been explained above in connection with the hot-rolled flat steel product according to the invention and which accordingly has the following composition (in wt. %): 0.1 - 0.3% C, 1.5 - 3.0% Mn, 0.5 - 1.8% Si, up to 1.5% Al, up to 0.1% P, up to 0.03% S, up to 0.008% N, optionally one or more elements from the group "Cr, Mo, Ni, Nb, Ti, V, B" with the following contents: 0.1 - 0.3% Cr, 0.05 - 0.25% Mo, 0.05 - 2.0% Ni, 0.01 - 0.06% Nb, 0.02 - 0.07% Ti, 0.1 - 0.3% V, 0.0008 - 0.0020% B, the remainder being iron and unavoidable impurities resulting from the manufacturing process; b) pouring the melt into a precursor product, such as a slab or thin slab; c) heating the precursor product to a heating temperature TWE of 1000 - 1300 °C;d) hot rolling of the soaked precursor product into a hot strip with a thickness of 1.5 - 10 mm, wherein the hot rolling is terminated at a hot rolling end temperature TET for which TET ≥ (A3 - 100°C), where "A3" denotes the respective A3 temperature of the steel; e) first quenching of the hot strip starting from the hot rolling end temperature TET at a cooling rate θQ of more than 30 K / s to a quench temperature TQ for which RT ≤ TQ ≤ (TMS + 100°C), where "RT" denotes the room temperature and "TMS" the martensite start temperature of the steel, and where the martensite start temperature TMS is determined as follows: ; TMS ° C = 462 − 273 % C − 26 % Mn − 13 % Cr − 16 % Ni − 30 % Mo where (each in wt.%) %C = C content of the steel, %Mn = Mn content of the steel, %Cr = Cr content of the steel, %Ni = Ni content of the steel, %Mo = Mo content of the steel; f) optionally winding the flat steel product quenched to the quench temperature TQ into a coil; g) holding the flat steel product cooled to the quench temperature TQ within a temperature range of TQ - 80°C to TQ + 80°C for a period of 0.1 - 48 hours; h) heating the steel flat product to a partitioning temperature TP or maintaining the steel flat product at a partitioning temperature TP which is at least equal to the temperature TQ+ / -80°C of the steel flat product after step g) and not exceeding 500°C, for a partitioning time tPT of 0.5 - 30 hours, whereby in the event of heating, the heating rate θP1 is not more than 1 K / s; i) cooling the steel flat product to room temperature;j) optional descaling of the flat steel product k) optional coating of the flat steel product. ;

[0034] The process-technical production of hot strip according to the invention is shown schematically in Fig. 1 and is explained in detail below. Work step a):

[0035] The same information as has already been given above in connection with the composition of the product according to the invention naturally applies to the alloying of the steel melt melted according to the invention and its possible variations. Step b):

[0036] A pre-product is cast from the melt alloyed according to the invention, which will typically be a slab or thin slab. Step c):

[0037] The precursor product is heated to a heating temperature TWE that lies within the temperature range in which austenite forms in the steel according to the invention. The heating temperature TWE of the steels according to the invention should therefore be at least 1000 °C in the process according to the invention, since excessive hardening occurs at lower heating temperatures during the subsequent hot rolling process. At the same time, the heating temperature should not exceed 1300 °C to avoid partial melting of the slab surfaces.

[0038] The heating temperature TWE is preferably at least 1150 °C, because in this way structural inhomogeneities that could arise, for example, from manganese segregation can be reliably avoided.

[0039] By limiting the heating temperature TWE to a maximum of 1250 °C, the heating itself and further process steps starting from this temperature range can be operated economically.

[0040] In addition, by setting the heating temperature TWE to 1150 - 1250 °C, a defined microstructure is established and a targeted dissolution of precipitates is achieved.

[0041] Heating to the temperature TWE can be carried out in a conventional pusher or walking beam furnace. When applying the inventive method in a conventional thin-slab caster, in which the steel composition according to the invention is cast into thin slabs with a thickness of typically 40-120 mm (see DE 4104001 A1), heating can also take place in the furnace directly connected to the caster after casting. Step d):

[0042] After heating, the precursor product is hot-rolled into hot strip with final thicknesses between 1.5 and 10 mm. Depending on the available plant technology, hot rolling may involve roughing, possibly in reverse, in a roughing stand, followed by finish rolling in a so-called finishing stage, which consists of several, typically five or seven, rolling stands passed through in a continuous sequence. The final rolling temperature TET of the hot rolling process must be set according to the requirement TET ≥ (A3 - 100°C). In practice, it has proven advantageous to set the final rolling temperature TET at least equal to the A3 temperature of the respective steel composition being processed or above the A3 temperature. For example, it may be advantageous to set the final rolling temperature TET in the range of 850 - 950°C.However, if the process according to the invention is to be carried out in such a way that certain amounts of polygonal ferrite are reliably formed in the microstructure, this can be achieved by selecting final rolling temperatures TET that are up to 100 °C below the respective A3 temperature of the steel. The A3 temperature of the respective processed steel composition can be estimated according to equation (1) published by Andrews, J. in Iron and Steel Institute (203), pp. 721-727, 1965: . A 3 ° C = 910 − 203 %C − 15 , 2 % Ni + 44 , 7 % Si + 31 , 5 % Mo − 30 % Mn + 11 % Cr with (each in wt.%) %C = C content of the steel, %Ni = Ni content of the steel, %Si = Si content of the steel, %Mo = Mo content of the steel, %Mn = Mn content of the steel, %Cr = Cr content of the steel. Step e):

[0043] After hot rolling, the steel is quenched in a first quenching step starting from the hot rolling end temperature TET at a high cooling rate to a quench temperature TQ.

[0044] The cooling rate θQ is more than 30 K / s.

[0045] The quench temperature TQ targeted during cooling is not lower than room temperature. On the other hand, it is at most 100 °C higher than the martensite start temperature TMS, at which the martensitic transformation begins.

[0046] The martensite start temperature TMS can be estimated using the following equation (2) developed by van Bohemen: TMS ° C = 462 − 273 % C − 26 % Mn − 13 % Cr − 16 % Ni − 30 % Mo with %C = C content of the steel, %Mn = Mn content of the steel, %Cr = Cr content of the steel, %Ni = Ni content of the steel, %Mo = Mo content of the steel, each in wt.%;

[0047] At a quench temperature TQ above the martensite start temperature TMS, the desired proportion of primary martensite would not form. Instead, excessive proportions of ferrite, pearlite, or bainite would form, each of which exceeds the proportions specified in the invention for the flat steel product according to the invention. If the proportions of these microstructural components are too high, the stabilization of the residual austenite during the partitioning treatment following cooling is prevented. In addition, the primary martensite formed would relax so much during further cooling through self-tempering that the KAM values ​​​​desired according to the invention would not be achieved. In addition, at quench temperatures TQ above the limit of TMS + 100 °C specified in the invention, inhomogeneities and thus segregation of individual elements can occur more frequently, which in turn could lead to the formation of an undesirable linear microstructure.

[0048] An optimal microstructure with regard to the desired formability of the final product can thus be achieved, particularly with regard to the primary martensite formed during quenching, by ensuring that the quench temperature TQ is at most 100 °C higher than the martensite start temperature TMS and at least equal to the martensite start temperature TMS - 250 °C, so that: TMS − 250 ° C ≤ TQ ≤ TMS + 100 ° C .

[0049] A quench temperature TQ between the martensite start temperature TMS and the martensite start temperature TMS -150 °C has proven particularly favorable. TMS − 150 ° C ≤ TQ ≤ TMS proven.

[0050] However, if a maximum martensite content is to be achieved in the microstructure of the flat steel product according to the invention, it may also be expedient to select low temperatures, such as a temperature in the range of room temperature, as the quench temperature TQ. Work step f):

[0051] Optionally, the flat steel product quenched to the quench temperature TQ can be wound into a coil after step e) in order to ensure temperature constancy and homogeneity throughout the material.

[0052] However, it should be noted that the temperature of the flat steel product may not fall below the quench temperature TQ by more than 80 °C. Step g):

[0053] After cooling, the hot-rolled flat steel product, cooled to the quench temperature TQ, is kept in a temperature range of TQ - 80 °C to TQ + 80 °C for a period of 0.1 - 48 hours in order to ensure the targeted transformations and, when using micro-alloying elements, the formation of finely distributed carbides.

[0054] The aim of this work step is to form a martensitic structure which can contain up to 15 vol.% residual austenite. Practical tests have shown that with hot-rolled strip made from the steel specified according to the invention, this result is generally already achieved with holding times of up to 2.5 hours. Therefore, with regard to energy utilization, it may be expedient to limit the holding time to a maximum of 2.5 hours. Longer holding times are harmless and are therefore selected if this makes sense taking into account the available plant technology or its utilization. Holding times of at least 1 hour have also proven expedient in order to achieve complete temperature homogeneity of the material and the associated formation of up to 15 vol.% residual austenite in the martensitic structure.

[0055] Maintaining within the temperature range from TQ -80 °C to TQ +80 °C can be done isothermally, i.e. at a constant temperature, or non-isothermally, ie at a decreasing or increasing or oscillating temperature.

[0056] If system-related cooling occurs during holding, the maximum permissible cooling rate is 0.05 K / s.

[0057] However, the redistribution and transformation processes occurring during the holding period can also be exothermic, releasing heat of transformation that leads to an increase in the temperature of the flat steel product. This heat of transformation then counteracts any possible cooling. The self-heating rates for this non-isothermal microstructure development are a maximum of 0.01 K / s.

[0058] The rate at which temperature changes occur during the hold is typically in the range of -0.05 K / s to +0.01 K / s, based on the respective quench temperature TQ.

[0059] The holding conditions must be selected so that the specified temperature window of TQ + / -80 °C is not exceeded despite the temperature changes that occur. Work step h):

[0060] The aim of this process step, also known as "partitioning", is to create a microstructure with martensite, tempered martensite and, if necessary, retained austenite.

[0061] In step h), the steel flat product is brought to a partitioning temperature TP from its temperature set in step g) or, if the partitioning temperature TP lies within a range fluctuating by + / - 80°C around the quench temperature TQ, is held there to enrich the residual austenite with carbon from the supersaturated martensite. The partitioning temperature TP should advantageously be at least as high as the quench temperature TQ, but preferably at least 50°C higher, in particular at least 100°C higher.

[0062] If the partitioning temperature TP is lower than the temperature after step g) (quench temperature TQ + / -80 °C), the carbon mobility is too low to stabilize the retained austenite. Furthermore, the tempering effect of the primary martensite does not occur to the desired extent.

[0063] The partitioning temperature TP for the steels according to the invention is a maximum of 500 °C, in particular a maximum of 470 °C, in order to achieve the optimal tempering state.

[0064] The partitioning time tPT is between 30 minutes and 30 hours to allow sufficient redistribution of carbon without causing the decomposition of the residual austenite present in the structure.

[0065] The partitioning time tPT is composed of the time tPR (heating ramp) required for the heating process and the time tPl provided for the isothermal holding, where tPl can also be zero.

[0066] The proportions of the times tPR and tPl in the partitioning time tPT are variable as long as the total partitioning time tPT specified according to the invention is maintained.

[0067] If heating in step h) occurs with a coiled steel flat product, the hot strip is ideally heated at a heating rate θP1 of up to 0.075 K / s. Heating rates θP1 below 0.005 K / s do not appear practical. At heating rates θP1 > 1 K / s, unacceptable temperature differences may occur between the outer, middle, and inner turns of the coiled hot strip. These differences should not exceed 85 °C to ensure homogeneous material properties across the entire length of the hot-rolled flat steel product produced according to the invention.

[0068] The formation of pearlite and the decomposition of retained austenite are specifically suppressed by an adjusted holding time at a defined temperature.

[0069] It has been found to be advantageous from a process engineering perspective if the time tPl is zero. In this case, the desired microstructure is achieved solely during the heating process, i.e., during the time tPR.

[0070] As already mentioned, the partitioning temperature can also be equal to the temperature of the flat steel product after step g) (quenching temperature TQ+ / -80°C), so that no time tPR is required for heating the flat steel product.

[0071] Partitioning (step h) is carried out batchwise in a bell annealing furnace, which allows for slow heating of the hot strip, which in this case is necessarily wound into a coil.

[0072] Annealing in a bell-type annealing furnace offers the following advantages: Smaller temperature gradients occur during heating, resulting in more homogeneous heating of the material. The maximum heating rate depends on the target temperature and the respective charging weight in the bell-type furnace. If heating is too rapid, the strip is not heated completely and evenly. This leads to an uneven microstructure, in particular to different martensite morphologies, which influence the further partitioning behavior and thus the final microstructure. This is particularly the case with heating units that are directly integrated into the hot strip mill (continuous annealing or induction inline annealing as in US 2014 / 0299237). An uneven microstructure leads to poor formability, in particular to poorer hole expansion.

[0073] Slow heating, on the other hand, leads to a uniform carbon redistribution from the martensite into the austenite, which on the one hand prevents the undesirable formation of coarse carbides and on the other hand allows the proportion of carbon-enriched austenite in the final microstructure to be adjusted. Heating too quickly causes a build-up of carbon at crystallographic defects, such as phase boundaries and dislocations, and thus promotes the precipitation of transition carbides and / or cementite. This leads to a reduction in the proportion of carbon available for stabilizing the austenite during the partitioning step and thus to a non-uniform microstructure. Adjusting the heating conditions during the partitioning step to the kinetics of carbon redistribution thus enables the creation of a uniform microstructure with improved forming properties, in particular with improved hole expansion.

[0074] The maximum heating rate θP1 during the partitioning step is 0.075 K / s to achieve uniform properties across both the length and width of the flat steel product. Otherwise, local irregularities associated with reduced forming properties, particularly impaired hole expansion, occur. It is particularly advantageous to heat at a maximum heating rate θP1 of 0.03 K / s to ensure optimal homogeneity of the final microstructure and thus the best hole expansion and fatigue strength properties.

[0075] For reasons of economy, the minimum heating rate θP1 is 0.005 K / s, preferably 0.01 K / s.

[0076] A further advantage of using a bell annealing furnace is that the target annealing temperatures can be set more precisely than in continuous annealing furnaces. Annealing also takes place in a protective gas mixture, which prevents harmful effects on the hot strip surface, such as oxidation. Hydrogen, nitrogen, and mixtures of hydrogen and nitrogen are used as protective gases. Furthermore, partitioning the process in a separate bell annealing furnace allows for cycle time decoupling from the hot rolling mill. This enables better utilization of hot rolling capacities.

[0077] If a bell annealing furnace is used in step h), the flat steel product should be transported to the bell annealing furnace during step g) taking into account the requirements explained above with regard to maintaining the temperature TQ.

[0078] After step h), the hot-rolled flat steel product is cooled to room temperature. Cooling in step i) should be carried out at a cooling rate θP2 of no more than 1 K / s to control the stress in the flat steel product. For economic reasons, a minimum cooling rate of 0.01 K / s may be used.

[0079] It goes without saying that the flat steel product, if it is in strip form and has been wound into a coil in the optional step f), can now be unwound and divided into so-called strip sheets for logistical reasons.

[0080] Depending on the intended use, it may be appropriate to subject the flat steel product obtained or obtained according to the invention to a surface treatment such as descaling, pickling or the like.

[0081] It may also be appropriate to apply a metallic coating to the flat steel product in a conventional manner to protect against corrosion. This can be achieved, for example, by electrolytic galvanizing.

[0082] A flat steel product according to the invention or produced according to the invention is processed in the hot-rolled state. This allows for flat steel product thicknesses of 1 mm and more, with typical thicknesses in the range of 1.5 - 10 mm.

[0083] The hot-rolled flat steel product according to the invention is particularly suitable for structural lightweight construction, as the higher strength allows for a reduction in material thickness. Conventional higher- and ultra-high-strength grades are unsuitable for more heavily formed parts because they lack the necessary formability.

[0084] In addition, the flat steel product according to the invention enables component integration, since the good formability despite high strength makes it possible to replace several components of an assembly with a component made of the hot-rolled flat steel product according to the invention.

[0085] The increased hole expansion is particularly advantageous for automotive chassis components, significantly facilitating the formation of through-holes. Insufficient hole expansion was previously considered a disqualification criterion for use in chassis components for grades with strengths above 800 MPa. The cyclic loading typical of chassis components requires that the material also ideally exhibit good fatigue strength.

[0086] Furthermore, the improved formability enables new component geometries while reducing material thickness for lightweight construction reasons.

[0087] The advantages of flat steel products according to the invention can also be used in motor vehicles in the areas of drive trains as well as for interior and transmission parts.

[0088] In the metalworking industry, the mechanical properties of the inventive flat steel products can be used for the lightweight construction of stamped parts. Component integration also offers the possibility of eliminating joining operations, thus simultaneously increasing production reliability and generating cost advantages.

[0089] The use of the flat steel products according to the invention in the construction industry is also advantageous, as they offer improved formability while maintaining high strength. Furthermore, they have a higher yield strength ratio compared to other flat steel products of comparable strength levels. These properties ensure improved structural stability under unforeseen load conditions such as earthquakes, impact loads, or exceeding the maximum load specified in the design.

[0090] The invention is explained in more detail below using exemplary embodiments.

[0091] In the tables explained below, the examples not according to the invention are marked with an "*", whereby values ​​of the respective examples which lie outside the specifications according to the invention are underlined.

[0092] To test the invention, test melts A - O with the compositions given in Table 1 were melted.

[0093] Table 2 shows the A3-determined temperatures according to equation (1) and the martensite start temperatures TMS determined according to equation (2) for steels A - O.

[0094] For 47 tests, melts A - O were cast into slabs, each of which was subsequently heated to a reheating temperature (TWE). The heated slabs were then conventionally rolled into hot strip with a thickness of 2 - 3 mm. The hot rolling, in a conventional manner, comprised a roughing and a finish rolling step, each of which was terminated at a final hot rolling temperature (TET).

[0095] Within a maximum of 5 seconds after the end of hot rolling, i.e., in the technical sense, immediately after hot rolling, the resulting hot-rolled steel strips were quenched at a cooling rate θQ to a quench temperature TQ, at which they were subsequently held for a period tQ. Those hot strips that were subsequently subjected to batch annealing were wound into a coil between quenching and holding.

[0096] After holding, the hot strips were heated at a heating rate θP1 over a duration tPR to a partitioning temperature TP and held there for a duration tPl.

[0097] Finally, the hot strips obtained in tests 1 - 47 were cooled to room temperature.

[0098] The parameters reheating temperature "TWE", hot rolling end temperature "TET", cooling rate "θQ", quench temperature "TQ", holding time "tQ", heating rate "θP1", holding time "tPl", partitioning temperature "TP" and heating time "tPR" are given for each of the tests 1 - 47 in Table 3.

[0099] Additionally, Table 3 lists the aggregate used for the partitioning treatment (step h)) and the respective difference between the quenching temperature TQ and the partitioning temperature TP for each of the experiments. When a bell-type annealer was used, it is also indicated whether it was used to increase ("heat") the temperature or to maintain ("maintain") the temperature.

[0100] The mechanical-technological properties "yield strength RP0.2", "tensile strength Rm", "ratio RP0.2 / Rm", "elongation A" and "hole expansion value λ" of the hot-rolled steel strips obtained in tests 1 - 47 after production are given in Table 4.

[0101] Table 5 shows the proportions of polygonal ferrite "pF", non-polygonal ferrite "npF", tempered martensite "AM", cementite "Z", retained austenite "RA", untempered martensite "M" and bainite "B" in the microstructure as well as the KAM value of the hot strips obtained in tests 1 - 47.

[0102] In test 7, which is not inventive, the value required for hole expansion according to the invention was not achieved because the quenching was stopped at temperatures that were too high.

[0103] In contrast, tests 3-6 resulted in an increase in hole expansion of 7% to 38% compared to the non-inventive comparative test 7, while simultaneously avoiding an excessive bainite content. Thus, in tests 3-5, only traces of bainite were present, and in test 6, 10 area% bainite was present, whereas in test 7, 20 area% bainite was present in the microstructure.

[0104] Tests 11 - 13 show the necessity of rolling above the A3 temperature and of maintaining a sufficiently long holding time t Q .

[0105] With melts D and E, it was possible to produce a material with a strength of 1028 - 1500 MPa and a hole expansion of 22 - 87%.

[0106] However, the manufacturing parameters in test 24, which is not according to the invention, lead to the formation of too high a proportion of bainite.

[0107] With melt F not according to the invention, the formation of cementite could not be prevented despite a sufficiently long holding time (see test 29).

[0108] Melt M, as an example of a variant with optimized surface quality, exhibits a reduced Si content and a simultaneously increased Al content. With a simultaneously low TET (see test 45), a 5 area % polygonal ferrite content forms in the microstructure, allowing for low yield strengths combined with good hole expansion.

[0109] While melts AM and O were produced under conventional operating conditions, melt N was produced as a laboratory melt in a vacuum furnace. With the high-purity melt N, it was possible to produce a material with very good hole expansion (see test 46).

[0110] Test 47 with melt analysis O shows that if all production parameters are observed, a material with just sufficient values ​​for elongation and hole expansion can be produced. Table 1 melt C Si Mn Al P S N Cr V Mon Ti Nb B N A* 0,145 0,24 2,15 0,660 0,011 0,0017 0,0033 0,71 - - 0,028 0,027 - - B 0,186 1,52 2,54 0,025 0,009 0,0021 0,0021 0,25 - - 0,041 - 0,0019 - C 0,249 1,71 1,89 0,019 0,011 0,0015 0,0025 0,17 - 0,102 0,027 - - - D 0,201 1,46 1,98 0,028 0,013 0,0013 0,0032 - - 0,100 0,017 - - - E 0,179 1,51 2,05 0,021 0,007 0,0025 0,0029 0,14 - - - - - 0,13 F* 0,150 0,29 1,82 0,027 0,015 0,0027 0,0041 0,37 - 0,101 0,047 - 0,0010 - G 0,174 1,10 1,62 0,017 0,006 0,0019 0,0052 - - - - - - - H 0,242 0,75 1,74 0,920 0,005 0,0014 0,0018 - 0,150 - - - - - I* 0,152 0,74 1,27 0,017 0,007 0,0014 0,0045 0,32 - - - - - - J 0,204 1,23 2,49 0,012 0,010 0,0008 0,0022 0,14 - - - - - 0,321 K 0,123 1,37 2,62 0,023 0,008 0,0012 0,0019 - - 0,224 - 0,035 - 0,820 L 0,166 1,49 2,01 0,024 0,011 0,0015 0,0025 0,105 - - 0,028 - 0,0011 - M 0,177 0,90 2,02 1,47 0,008 0,0012 0,0016 0,12 - - - - - 0,52 N 0,166 1,55 2,01 - - - - - - - - - - - O 0,183 1,47 2,51 0,026 0,092 0,026 0,0076 0,18 - - - - 0,0008 - Data in wt.%, remainder iron and unavoidable impurities * = not according to the invention Table 2 melt A3 [°C] TMS [°C] A* 787 357 B 817 342 C 834 340 D 828 352 E 832 357 F* 797 366 G 826 372 H 792 351 I* 829 383 J 795 335 K 816 341 L 835 363 M 798 351 N 836 364 O 816 344 * = not according to the invention Table 3 Attempt melt TWE [°C] TET [°C] ΘQ [K / s] TQ [°C] tQ [s] ΘP1 [K / s] tPI [s] TP [°C] tPR [s] TP-TQ [°C] Annealing unit According to the invention? 1 A 1230 910 45 345 3000 0,075 10800 410 867 65 annealing furnace (heating) NO 2 A 1230 920 50 295 950 0,03 10200 425 4333 130 annealing furnace (heating) NO 3 B 1250 900 50 195 4500 0,05 18600 300 2100 105 annealing furnace (heating) YES 4 B 1240 890 50 205 7200 0,08 14200 450 3063 245 annealing furnace (heating) YES 5 B 1250 905 45 255 5400 0,04 16000 400 3625 145 annealing furnace (heating) YES 6 B 1270 900 40 345 6300 0,02 18400 350 250 5 Hood annealing (hold) NO 7 B 1250 905 38 475 12600 - =tQ 395 - -80 Hood annealing (hold) NO 8 B 1160 845 52 165 2400 0,02 85200 280 5750 115 annealing furnace (heating) YES 9 B 1230 910 62 325 4100 2,5 2200 385 24 60 Continuous annealing NO 10 C 1240 850 37 350 9000 0,02 14500 425 3750 75 annealing furnace (heating) NO 11 C 1230 890 43 245 3500 0,03 21300 400 5167 155 annealing furnace (heating) YES 12 C 1240 895 51 195 8500 0,04 21600 410 5375 215 annealing furnace (heating) YES 13 C 1210 915 58 265 0 5 14800 400 27 135 Continuous annealing NO 14 D 1250 920 25 350 12100 - =tQ 350 - 0 Hood annealing (hold) NO 15 D 1250 920 41 320 5500 0,025 21900 405 3400 85 annealing furnace (heating) YES 16 D 1250 920 48 290 3100 0,045 12300 450 3556 160 annealing furnace (heating) YES 17 D 1180 880 58 28 19900 0,01 12700 255 22700 227 annealing furnace (heating) NO 18 D 1230 905 42 25 3000 0,01 12800 445 42000 420 annealing furnace (heating) NO 19 D 1200 910 41 290 160000 0,06 12500 395 1750 105 annealing furnace (heating) YES 20 D 1250 890 48 380 8700 - 12900 400 - 20 Hood annealing (hold) NO 21 E 1200 910 35 335 7900 0,03 21500 390 1833 55 annealing furnace (heating) YES 22 E 1190 895 42 295 4050 0,06 14400 420 2083 125 annealing furnace (heating) YES 23 E 1220 890 50 240 6020 0,04 14900 405 4125 165 annealing furnace (heating) YES 24 E 1210 895 42 365 10500 0,03 8900 525 5333 160 annealing furnace (heating) NO 25 E 1250 855 35 26 7200 0,03 21500 455 14300 429 annealing furnace (heating) NO 26 E 1260 895 42 170 4200 0,06 14400 245 1250 75 annealing furnace (heating) YES 27 E 1210 915 50 230 6700 0,04 14900 450 5500 220 annealing furnace (heating) YES 28 E 1270 920 42 375 2200 0,075 19400 390 200 15 Hood annealing (hold) NO 29 F 1250 925 35 350 19900 - =tQ 370 - 20 Hood annealing (hold) NO 30 F 1250 925 46 275 3000 0,03 16400 400 4167 125 annealing furnace (heating) NO 31 G 1240 920 39 305 160000 0,07 12300 395 1286 90 annealing furnace (heating) YES 32 G 1220 900 37 315 8700 0,035 12600 380 1857 65 annealing furnace (heating) YES 33 G 1250 915 31 525 7200 -0,02 13100 405 6000 -120 without NO 34 H 1240 900 36 325 4200 0,04 12300 415 2250 90 annealing furnace (heating) YES 35 H 1210 895 24 365 6700 0,02 12100 395 1500 30 annealing furnace (heating) NO 36 H 1220 890 35 335 170000 0,01 12700 380 4500 45 annealing furnace (heating) YES 37 I 1240 905 41 315 8400 0,01 12800 375 6000 60 annealing furnace (heating) NO 38 J 1230 910 45 260 7100 0,06 12500 400 2333 140 annealing furnace (heating) YES 39 K 1240 905 37 315 9300 0,035 12600 450 3857 135 annealing furnace (heating) YES 40 K 1250 915 42 345 2450 0,02 0 405 3000 60 annealing furnace (heating) NO 41 L 1260 850 35 290 123000 - =tQ 260 - -30 Hood annealing (hold) YES 42 L 1160 920 46 340 2350 0,03 10200 405 2167 65 annealing furnace (heating) YES 43 L 1240 910 39 390 4500 - 17100 390 - 0 Hood annealing (hold) NO 44 L 1230 915 37 45 7200 0,03 21500 245 6667 200 annealing furnace (heating) NO 45 M 1200 795 39 331 8000 0,02 22000 395 3200 64 annealing furnace (heating) YES 46 N 1150 950 45 345 2300 0,03 11000 410 2167 65 annealing furnace (heating) YES 47 O 1220 910 52 310 7500 0,07 15000 440 1857 130 annealing furnace (heating) YES Table 4 Attempt melt R P02 [MPa] R m [MPa] R P02 / R m A [%] λ [%] According to the invention? 1 A 601 1128 0,53 14,5 16 NO 2 A 759 1134 0,67 12,8 17 NO 3 B 1281 1482 0,85 7,9 34 YES 4 B 1125 1214 0,93 10,2 43 YES 5 B 1177 1317 0,89 8,8 55 YES 6 B 1027 1325 0,78 9 23 YES 7 B 807 1270 0,64 12,7 17 NO 8 B 1210 1446 0,84 9,2 27 YES 9 B 1170 1345 0,87 6,1 35 NO 10 C 865 1220 0,71 16,2 32 YES 11 C 1090 1380 0,79 13,1 27 YES 12 C 1209 1412 0,86 10,9 23 YES 13 C 1232 1441 0,85 5,9 31 NO 14 D 690 1253 0,55 13,2 13 NO 15 D 974 1124 0,87 12 54 YES 16 D 876 1056 0,83 15,6 47 YES 17 D 1299 1500 0,87 9,1 22 YES 18 D 1052 1102 0,95 12,7 36 YES 19 D 1178 1241 0,95 11 55 YES 20 D 1054 1149 0,92 13,3 49 YES 21 E 836 1187 0,7 16,8 34 YES 22 E 851 1072 0,79 14 56 YES 23 E 913 1059 0,86 12,3 67 YES 24 E 680 1015 0,67 17,1 16 NO 25 E 975 1028 0,95 12,3 41 YES 26 E 1189 1431 0,83 9 66 YES 27 E 1028 1064 0,97 12,4 51 YES 28 E 999 1059 0,94 11,9 87 YES 29 F 945 1104 0,86 5,8 18 NO 30 F 1067 1189 0,90 4,9 43 NO 31 G 857 1017 0,84 12,7 49 YES 32 G 821 1043 0,79 13,5 38 YES 33 G 457 984 0,46 11,3 5 NO 34 H 868 1109 0,78 14 63 YES 35 H 523 1061 0,49 15,9 7 NO 36 H 824 1197 0,69 13,6 29 YES 37 I 670 965 0,69 10,8 17 NO 38 J 1043 1267 0,82 9,5 47 YES 39 K 804 1029 0,78 14,1 25 YES 40 K 871 1040 0,84 11,2 22 YES 41 L 1209 1420 0,85 8,1 24 YES 42 L 1043 1107 0,94 11,4 48 YES 43 L 935 1071 0,87 8,6 42 YES 44 L 1211 1396 0,87 7,1 21 YES 45 M 822 1176 0,7 17,2 29 YES 46 N 1055 1121 0,94 9,8 51 YES 47 O 1194 1221 0,98 7,2 27 YES Table 5 Attempt melt pF [area%] npF [area%] AM [area%] Z [area%] RA [vol.%] M [area%] B [area%] CAME [°] According to the invention? 1 A 0 20 65 - 8,5 5 Sp. 1,19 NO 2 A 0 25 70 - 4,5 0 Sp. 1,14 NO 3 B 0 0 80 - 1 16 Sp. 1,51 YES 4 B 0 0 80 - 0 19 Sp. 1,53 YES 5 B 0 0 75 - 2 21 Sp. 1,54 YES 6 B 0 0 65 - 0 24 10 1,5 YES 7 B 0 0 60 - 10,5 9,5 20 1,48 NO 8 B 0 0 85 - 2 13 Sp. 1,62 YES 9 B 0 0 30 - 2,5 65 Sp. 1,57 NO 10 C 5 0 65 - 5 20 5 1,5 YES 11 C 0 0 80 - 8 10 Sp. 1,53 YES 12 C 0 0 85 - 4,5 10 Sp. 1,56 YES 13 C 0 0 35 - 0 65 Sp. 1,49 NO 14 D 20 0 35 - 8,5 20,5 15 1,42 NO 15 D 0 0 70 - 3 25 Sp. 1,55 YES 16 D 0 0 75 - 0 25 0 1,51 YES 17 D 0 0 75 5,00 3,5 15 0 1,5 YES 18 D 0 0 85 - 1,5 13 Sp. 1,56 YES 19 D 0 0 75 - 5,5 15 2 1,6 YES 20 D 0 0 60 Sp. 1,5 25 12 1,58 YES 21 E 0 0 60 - 7,5 30 Sp. 1,51 YES 22 E 0 0 75 - 2 20 Sp. 1,54 YES 23 E 0 0 85 - 0 15 0 1,57 YES 24 E 0 Sp. 35 Sp. 1,5 38 25 1,37 NO 25 E 0 3 65 - 1,5 30 0 1,53 YES 26 E 0 0 80 - 2 15 Sp. 1,61 YES 27 E 0 0 70 Sp. 10,5 15 2 1,52 YES 28 E 0 0 70 - 2 15 13 1,53 YES 29 F 0 5 35 15 4,5 20 20 1,45 NO 30 F 0 Sp. 60 5 6 20 7 1,47 NO 31 G 0 0 75 Sp. 8,5 15 Sp. 1,53 YES 32 G 0 0 70 - 5,5 23 Sp. 1,51 YES 33 G 20 0 0 Sp. 6 30 42 1,38 NO 34 H 0 0 50 - 8 38 4 1,53 YES 35 H 25 0 Sp. 5 Sp. 6,5 60 Sp. 1,32 NO 36 H 0 0 50 Sp. 11,5 37 Sp. 1,51 YES 37 I 18 0 55 - 1,5 20 5 1,41 NO 38 J 3 0 70 Sp. 3 20 2 1,62 YES 39 K 0 60 - 4,5 35 0 1,51 YES 40 K 0 2 50 - 1,5 46 Sp. 1,55 YES 41 L 10 Sp. 75 2,50 1 10 Sp. 1,5 YES 42 L 0 0 60 Sp. 8,5 30 0 1,55 YES 43 L 0 0 70 - 2,5 25 Sp. 1,52 YES 44 L 0 0 85 Sp 0,5 12 Sp. 1,57 YES 45 M 0 5 60 - 6 27 Sp. 1,52 YES 46 N 0 0 75 - 5 20 0 1,63 YES 47 O 0 0 82 - 1 13 Sp. 1,5 YES

Claims

1. A hot-rolled flat steel product consisting of a steel having the following composition (in wt%): C:0.1 - 0.3%Mn:1.5 - 3.0%Si:0.5 - 1.8%Al:up to 1.5%P:up to 0.1%S:up to 0.03%N:up to 0.008%, optionally one or more elements of the "Cr, Mo, Ni, Nb, Ti, V, B" group having levels as follows: Cr:0.1 - 0.3%Mo:0.05 - 0.25%Ni:0.05 - 2.0%Nb:0.01 - 0.06%Ti:0.02 - 0.07%V:0.1 - 0.3%B:0.0008 - 0.0020%, the balance being iron and production-relatedly unavoidable impurities, - wherein the flat steel product has a tensile strength Rm of 800 - 1500 MPa, a yield strength Rp of more than 700 MPa, an elongation at break A of 7 - 25%, and a hole expansion λ of more than 20%, - wherein the structure of the flat steel product consists to an extent of at least 85 area% of martensite, of which at least half is tempered martensite, with the respective remainder of the structure consisting of up to 15 vol% residual austenite, of up to 15 area% bainite, of up to 15 area% polygonal ferrite, of up to 5 area% cementite and / or of up to 5 area% non polygonal ferrite, - wherein the structure of the flat steel product has a kernel average misorientation KAM of at least 1.50° measured as described in the description, and - wherein the flat steel product has a thickness in the range of 1.5 - 10 mm.

2. The hot-rolled flat steel product as claimed in claim 1, characterized in that its Al content is at most 0.03 wt%.

3. The hot-rolled flat steel product as claimed in either of the preceding claims, characterized in that its Si content is at least 1.0 wt%.

4. The hot-rolled flat steel product as claimed in claim 1, characterized in that its Al content is at least 0.5 wt%.

5. The hot-rolled flat steel product as claimed in any of the preceding claims, characterized in that its Si content is at most 1.1 wt%.

6. A process for producing a flat steel product constituted as claimed in any of the preceding claims, comprising the following operations: a) melting of a steel alloy having the following composition (in wt%): C:0.1 - 0.3%Mn:1.5 - 3.0%Si:0.5 - 1.8%Al:up to 1.5%P:up to 0.1%S:up to 0.03%N:up to 0.008%, optionally one or more elements of the "Cr, Mo, Ni, Nb, Ti, V, B" group having levels as follows: Cr:0.1 - 0.3%Mo:0.05 - 0.25%Ni:0.05 - 2.0%Nb:0.01 - 0.06%Ti:0.02 - 0.07%V:0.1 - 0.3%B:0.0008 - 0.0020%, the balance being iron and production-relatedly unavoidable impurities; b) casting of the melt to give a semi-finished product, such as a slab or thin slab; c) heating-through of the semi-finished product to a heating temperature TWE of 1000 - 1300°C; d) hot-rolling of the heated-through semi-finished product to give a hot strip having a thickness of 1.5 - 10 mm, the hot-rolling being ended at a hot-rolling end temperature TET for which TET ≥ (A3 - 100°C), where "A3" designates the respective A3 temperature of the steel; e) first quenching of the hot strip, starting from the hot-rolling end temperature TET, at a cooling rate θQ of more than 30 K / s, to a quench temperature TQ, for which (TMS - 250°C) ≤ TQ ≤ TMS, where "RT" designates the room temperature and "TMS" the martensite start temperature of the steel, and where the martensite start temperature TMS is determined as follows: TMS ° C = 462 − 273 % C − 26 % Mn − 13 % Cr − 16 % Ni − 30 % Mo where %C = C content of the steel, %Mn = Mn content of the steel, %Cr = Cr content of the steel, %Ni = Ni content of the steel, %Mo = Mo content of the steel, in each case in wt%; f) optional coiling of the flat steel product, quenched to the quench temperature TQ, to give a coil; g) holding of the flat steel product, cooled to the quench temperature TQ, within a temperature range from TQ -80°C to TQ +80°C over a time of 0.1 - 48 hours; h) heating of the flat steel product to a partitioning temperature TP or holding of the flat steel product at a partitioning temperature TP which is at least equal to the temperature TQ+ / -80°C of the flat steel product as present after the operation g), and is at most 500°C, over a partitioning time tPT of 0.5 - 30 hours; in the event that heating takes place, the heating rate θP1 is at most 0.075 K / s, and wherein operation h) is carried out in a batch annealing furnace; i) cooling of the flat steel product to room temperature; j) optional descaling of the flat steel product; k) optional coating of the flat steel product.

7. The process as claimed in claim 6, characterized in that the heating rate θP1 is not more than 0.03 K / s.

8. The process as claimed in any of claims 6 or 7, characterized in that in the operation c) the heating temperature TWE is 1150 - 1250°C.

9. The process as claimed in any of claims 6 to 8, characterized in that the quench temperature TQ lies between the martensite start temperature TMS, and a temperature that is lower by at most 150 °C than the martensite start temperature TMS.

10. The process as claimed in any of claims 6 to 9, characterized in that the holding time in the operation g) is not more than 2.5 hours.

11. The process as claimed in any of claims 6 to 10, characterized in that the partitioning temperature TP in the operation h) is at least 50 °C higher than the quench temperature TQ.