Component produced by forming a steel sheet board and method for producing same

Abstract

The invention provides a weight-reduced component which, in the tempered and / or hot-formed state, exhibits an optimal combination of strength and toughness and meets the highest requirements for mechanical properties or resistance to abrasive wear. The component is manufactured by forming a sheet of steel and consists of a steel with the following composition (in mass %): C: 0.1–0.6%, Mn: 0.1–2%, Al: 0.05–0.2%, Nb: 0.01–0.06%, B: 0.0005–0.005%, Cr: 0.05–0.8%, Si: up to 0.8%, Mo: up to 1.5%, Cu: up to 0.5%, Ni: up to 1.5%, V: up to 0.2%, SEM: up to 0.05%, Ti: up to 0.02%, Ca: up to 0.005%, the remainder being iron and unavoidable impurities, including impurities of up to 0.03% P, up to 0.03% S, up to 0.01% N, and less than 0.05% Sn, less than 0.05% As and less than 0.05% Co count,wherein the ratio %Al / %N*14 / 27 ≥ 8, formed from the respective Al content %Al and the respective N content %N, and wherein the component has a microstructure consisting of at least 95 areal % of martensite and the remainder of other microstructural constituents, and in which a maximum of 150 areal ppm of particles are present in a homogeneous distribution over the strip thickness, the mean circular equivalent particle size of which is 0.2–10 µm and which consist of oxide-based Al compounds, Al₂A₂, TiN, or conglomerates formed from these particles. The invention also relates to a method for manufacturing such a component. The abstract is to be published without a figure.

Description

[0001] The invention relates to a component which is produced by forming a sheet steel blank, and to a method for producing such a component.

[0002] In this text, unless explicitly stated otherwise, information on alloy components is always given in mass%.

[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 blanks obtained from them, such as blanks and the like. Flat steel products of the type in question are used for cold forming with subsequent quenching and tempering to adjust the mechanical properties of the resulting component, or for hot forming to create a component in order to adjust the mechanical properties of the resulting component.

[0004] "Hot forming" is also referred to as "die hardening" or "press hardening." Strictly speaking, press hardening refers to the hardening of a workpiece or component in a cooled tool, while hot forming also includes the preliminary shaping in a heated state. However, these three terms are often used synonymously.

[0005] "Quenching and tempering" refers to a treatment known per se from the state of the art, which initially involves heating to a temperature at which the steel of the processed flat steel product (component) has a completely austenitic structure. This heating serves to bring the respective component to the appropriate temperature. This heating is carried out as a separate work step on the component previously cold-formed from the flat steel product. After heating, the component is cooled at an accelerated rate so that the steel of the flat steel product from which the component is formed forms a hardened structure, resulting in the component achieving significantly increased strength. After quenching, the component can be tempered to reduce the internal stresses that may arise in the component's structure as a result of the quenching process.In general, the aim is to achieve the highest possible temperatures in order to shorten the cycle time and thus the costs.

[0006] The Ac3 temperatures given in this text, i.e. the temperature above which the transformation of the steel structure into the austenitic state is complete upon heating, were estimated using the following formula: where in this formula, %C denotes the respective carbon content, %Si the respective silicon content, %Mn the respective manganese content, %P the respective phosphorus content, %Al the respective aluminum content, %Cr the respective chromium content, %Cu the respective copper content, %Mo the respective molybdenum content, %Ni the respective nickel content and %V the respective vanadium content of the steel composition whose Ac3 temperature is to be determined, and where the contents of the relevant elements, where present, are inserted into the formula in mass%.

[0007] The Ar3 temperatures given in this text, i.e. the temperature at which the transformation of the previously austenitic structure of the steel begins after cooling, were estimated using the following formula: where in this formula too, %C denotes the respective carbon content, %Si the respective silicon content, %Mn the respective manganese content, %Cr the respective chromium content, %Mo the respective molybdenum content and %Ni the respective nickel content of the steel composition whose Ar3 temperature is to be determined, and where the contents of the relevant elements, where present, are inserted into the formula in mass%.

[0008] Flat steel products of the type in question here are required in particular for the manufacture of components for passenger or commercial vehicles, the mechanical properties of which are subject to the highest demands, and of components which are exposed to high abrasive loads in practical use, such as components for machines and vehicles used in agriculture, road construction, mining or the like.

[0009] Since the early 1980s, there has been a continuously increasing demand for weight reduction, particularly in automobile bodies, for environmental reasons. Reducing vehicle weight is intended to reduce moving masses, so that less fuel is required to power the vehicle and, consequently, fewer climate-damaging gases are emitted.

[0010] In the steel processing industry, the trend has become established to achieve sheet thickness reduction and thus the desired weight reduction by using steel grades with ever-increasing strengths, without compromising the performance of the corresponding flat steel products. On the one hand, this is possible through the use of higher-strength steels that can be formed even in the cold state. On the other hand, steel concepts have become established that can be hardened through heat treatment, in which they undergo austenitization followed by controlled cooling. The strength of steels processed in this way can be further increased through martensite transformation. Optionally, a tempering treatment following hardening can reduce internal stresses.

[0011] The invention is based on the object of creating a weight-reduced component which, in the tempered and / or hot-formed state, has an optimal combination of strength and toughness and is, as such, suitable for applications which place the highest demands on mechanical properties or resistance to abrasive wear.

[0012] In addition, the invention should also mention a method for producing such a component.

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

[0014] A method according to the invention that achieves the above-mentioned object comprises at least the work steps specified in claim 9. It goes without saying that a person skilled in the art, when carrying out the method according to the invention, will supplement the work steps not explicitly mentioned here, which he knows, based on his practical experience, are regularly used in carrying out such methods.

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

[0016] A component according to the invention is accordingly formed from a flat steel product which consists of a steel which, in mass%, consists of C: 0.1 - 0.6%, in particular 0.10 - 0.60% C, Mn: 0.1 - 2.0%, in particular 0.1 - 2.0% Mn, Al: 0.05 - 0.20%, in particular 0.050 - 0.20% Al, Nb: 0.01 - 0.06%, in particular 0.010 - 0.060% Nb, B: 0.0005 - 0.005%, Cr: 0.05 - 0.8%, Si: up to 0.8%, Mo: up to 1.5%, Cu: up to 0.5%, Ni: up to 1.5%, V up to 0.2%, REM up to 0.05%, Ti: up to 0.02%, in particular 0.020% Ti, Ca: up to 0.005%, the remainder being iron and unavoidable impurities, the impurities including contents of up to 0.03% P, up to 0.03% S, up to 0.01% N, less than 0.05% Sn, less than 0.05% As and less than 0.05% Co, and the ratio %Al / %N*14 / 27 formed from the respective Al content %Al and the respective N content %N is ≥ 8.

[0017] A flat steel product according to the invention has a structure in which, in a homogeneous distribution across the strip thickness, at most 150 area ppm of particles of high hardness are present, whose mean circle-equivalent particle size is 0.2 - 10 µm and which consist of Al compounds on an oxide basis, of AIN, TiN or of conglomerates formed on the basis of these particles.

[0018] Carbon "C" is present as a mandatory element in the steel of the flat steel product from which a component according to the invention is formed, in amounts of 0.1–0.6 mass%, in particular 0.10–0.60 mass%. The presence of C controls the level of hardening potential. With increasing C content, both the martensite content and the hardness of the martensite obtained in the microstructure of a component according to the invention increase after austenitization and accelerated cooling, with a single-phase martensite structure representing the target microstructure of the finished processed component. The increase in hardness is synonymous with an increase in strength in the tensile test. This enables a reduction in sheet thickness and thus a reduction in weight in force-transmitting component cross-sections, as is the goal in modern automotive structural engineering with regard to resource-efficient lightweight body construction.To efficiently increase component hardness and strength, a C content of at least 0.1 mass%, in particular at least 0.10 mass%, is required. The beneficial effects of the presence of C can be achieved particularly reliably in the flat steel product according to the invention with a C content of at least 0.12 mass%, in particular at least 0.15 mass%. However, with C contents of more than 0.60 mass%, the hardness or strength after accelerated quenching would be so high that the toughness of the flat steel product from which a component according to the invention is formed, as reflected in the elongation at break or area at break, would be significantly reduced. At the same time, the tendency to crack formation would increase and weldability would deteriorate. Negative effects of the presence of C can be particularly reliably prevented by limiting the C content to a maximum of 0.55 mass%, in particular a maximum of 0.50 mass%.Therefore, the optimal C content is 0.12–0.55 mass%, especially 0.15–0.50 mass%. However, for certain applications, especially those involving high abrasive loads and where the potentially negative effects of higher C contents play only a minor role, it may also be advisable to provide C contents of at least 0.5 mass% due to the associated high hardness.

[0019] Silicon ("Si") can optionally be present in the steel of the flat steel product from which a component according to the invention is formed, in amounts of up to 0.8 mass%. Si inhibits cementite and pearlite transformation and thereby increases the martensite hardenability of the flat steel product. Si reduces the cooling rate critical for the desired martensite formation and thus increases the hardening of a flat steel product produced according to the invention. Si also exhibits opposite segregation behavior than Mn and thus overall improves the segregation resistance of the steel from which the flat steel product from which a component according to the invention is formed. Minimizing segregation across the cross-section is particularly important when a component according to the invention is a pipe or similar material.Reduced segregation sensitivity can prevent cracks in longitudinally welded pipes, particularly at higher C contents. To utilize the positive effects of the presence of Si, Si contents of at least 0.1 mass%, in particular at least 0.15 mass%, can be provided. However, excessively high Si contents could impair the wetting behavior of the flat steel product from which a component according to the invention is formed, particularly if alloyed flat steel products are to be hot-dip coated. Si tends to form external oxides during the annealing of the flat steel product carried out in this case. To prevent this, the Si content of a flat steel product used for a component according to the invention is a maximum of 0.8 mass%. Negative effects of the presence of Si can be particularly reliably avoided if the Si content is limited to a maximum of 0.5 mass%.Optimally, the Si content is therefore 0.1 - 0.8 mass%, especially 0.15 - 0.5 mass%.

[0020] Manganese "Mn" is present in the steel of the flat steel product from which a component according to the invention is formed, in amounts of 0.1–2 mass%, particularly 0.10–2.0 mass%. Mn increases the hardenability of the steel by lowering the A3 transformation temperature (i.e., the Ac3 and / or Ar3 temperature) from ferrite to austenite. This allows the furnace temperature for complete transformation to austenite during heat treatment of the flat steel product from which a component according to the invention is formed to be reduced during heating. In particular, the formation of the diffusion-controlled transformation phases ferrite, pearlite, and bainite is shifted to longer times. Therefore, in this respect, manganese is a similarly effective alloying element to carbon. Compared to carbon, manganese has the advantage of achieving greater deformability in the hardened state, which is expressed, for example, in higher notched impact toughness.Reducing the critical cooling rate with increasing manganese content also increases the hardening capacity. Fluctuations in the cooling conditions or different contact conditions during the cooling of components made from steel alloyed according to the invention can be better compensated for, and property variation is limited. However, excessively high Mn contents increase C segregation behavior and can lead to inhomogeneous hardening behavior across the cross-section of the respective product and the formation of hardening cracks. Furthermore, increasing Mn contents increase the risk of external Mn oxides or Mn-based mixed oxides forming on the surface of the product made from steel alloyed according to the invention.As in the case of excessive Si contents, this would trigger the risk of a deterioration in the wetting behavior of a flat steel product made from a steel material alloyed according to the invention during hot-dip coating. In the case of batch annealing processes, excessive Mn contents on the hot or cold strip surface would also lead to undesirable discoloration or so-called "manganese haze" due to the formation of manganese oxides. To avoid these negative effects, the Mn content of a flat steel product intended for forming a component according to the invention is limited to a maximum of 2 mass%, in particular a maximum of 2.0 mass%. Adverse effects of the presence of Mn can be particularly reliably avoided by limiting the Mn content to a maximum of 1.5 mass%, in particular 1.50 mass%.In contrast, the positive influences of Mn on the properties of a flat steel product intended for forming a component according to the invention can be utilized particularly reliably if the Mn content is at least 0.4 mass%, in particular at least 0.40 mass%. Therefore, the Mn content is optimally 0.4-1.5 mass%, preferably 0.40-1.50 mass%, in particular 0.6-1.3 mass% or 0.6-1.2 mass%, preferably 0.60-1.30 mass% or 0.60-1.20 mass%.

[0021] Phosphorus ("P") is one of the unavoidable steel components present during production. P segregates particularly at the grain boundaries and reduces grain boundary strength. Higher P contents would therefore contribute to a weakening of the microstructure, which in turn would lead to a deterioration in the toughness of the material. The P content of a flat steel product intended for forming a component according to the invention is therefore limited to a maximum of 0.03 mass%, with the P content being kept as low as possible. Therefore, the P content of the flat steel product is preferably a maximum of 0.025 mass%, in particular a maximum of 0.02 mass%.

[0022] Sulfur "S" is also an accompanying element, the presence of which is fundamentally undesirable in the steel flat product intended for forming a component according to the invention. Due to the Mn contents provided for in the invention, non-metallic MnS precipitates would form at higher S contents. Due to their low hardness, these precipitates would be present in an elongated form after rolling the steel flat product and would negatively influence the fracture behavior. During deformation, initial microscopic material separations could form through crack initiation and crack propagation on elongated MnS, expand, and coalesce until they deteriorate the material behavior macroscopically in the form of reduced notched impact toughness and increasing material anisotropy.In order to exclude the negative effects of the presence of S in the steel alloyed according to the invention, the S content is limited to a maximum of 0.03 mass%, with low S contents of less than 0.006 mass%, in particular less than 0.003 mass%, being particularly favorable.

[0023] Aluminum ("Al") is present in the steel of the flat steel product from which a component according to the invention is formed, in amounts of 0.05–0.2 mass%, in particular 0.050–0.20 mass%. Al traditionally serves as a deoxidizer, for which purpose it is typically alloyed in amounts of 0.02–0.05 mass%. According to the invention, in contrast, increased Al contents of 0.050–0.20 mass%, in particular 0.050–0.20 mass%, in combination with optionally low Ti contents of up to 0.02 mass%, in particular up to 0.020 mass%, are provided in the steel alloyed according to the invention. In this way, the formation of AlN or NbN is promoted in competition with the nitrogen binding by TiN which is classically known in heat-treatable steels and, insofar as Ti is present in the steel of a flat steel product intended for forming a component according to the invention, the formation of comparatively coarse TiN is avoided.The aim is to prevent the formation of boron nitrides so that B, as explained below, can exert its beneficial influence on delaying the transformation in dissolved form in the crystal lattice. Furthermore, the presence of Al within the content limits specified by the invention achieves grain refinement. At Al contents of less than 0.05 mass%, in particular less than 0.050 mass%, the precipitation pressure for the formation of AlN would be too low. In order to reliably utilize the positive effects of Al in the flat steel product according to the invention, the Al content can be set to at least 0.06 mass%, in particular at least 0.060 mass%, or at least 0.07 mass%, in particular at least 0.070 mass%.However, at Al contents of more than 0.2 mass%, in particular more than 0.20 mass%, there is a risk of external Al oxides forming on the surface of a product made from a steel material alloyed according to the invention, which would impair the wetting behavior during hot-dip coating. Furthermore, higher Al contents would promote the formation of non-metallic Al-based inclusions, which essentially consist of alumina (Al 2 O 3 ) and aluminum nitride (AlN), which also have a high hardness (Mohs hardness 9) and are therefore undesirable with regard to avoiding the risk of crack initiation and propagation during plastic deformation and cyclic loading. Another disadvantage is that the oxidic Al precipitates can form conglomerates with other precipitate types such as sulfides and silicates, thus forming larger precipitates that may have a higher crack initiation and failure potential.This can prove particularly risky for flat steel products produced from steel alloyed according to the invention, which can achieve strengths of up to 2500 MPa after quenching and tempering or hot forming. In addition, high Al contents cause longitudinal cracks to develop in the slabs cast from the steel alloyed according to the invention during processing. Furthermore, Al causes a drastic increase in the Ac3 transformation temperature, so that at higher Al contents, the temperature that must be exceeded for complete austenitization would be unnecessarily increased.According to the invention, the Al content of a flat steel product according to the invention is therefore limited to a maximum of 0.2 mass%, in particular a maximum of 0.20 mass%, whereby negative effects of the presence of Al in the flat steel product intended for forming a component according to the invention can be particularly reliably avoided by limiting the Al content to a maximum of 0.15 mass%, in particular a maximum of 0.150 mass%, or a maximum of 0.13 mass%, in particular a maximum of 0.130 mass%. Therefore, the Al content of a flat steel product intended for forming a component according to the invention is optimally 0.06-0.15 mass%, in particular 0.07-0.13 mass%, whereby Al contents of 0.060-0.150 mass%, in particular 0.070-0.130 mass%, have proven particularly effective.

[0024] Chromium ("Cr") is present as a mandatory element in the steel of the flat steel product from which a component according to the invention is formed, in amounts of 0.05–0.8 mass% to increase hardenability through its transformation-retarding effect. Chromium effectively suppresses the formation of ferrite and pearlite during accelerated cooling of the flat steel product and enables complete martensite formation even at lower cooling rates, thereby increasing hardenability. The presence of Cr in the amounts provided for in the invention thus contributes to the through-hardenability of the steel used to form a flat steel product according to the invention by means of suitable cooling and reduces the scattering of local product properties. Cr also increases the tensile strength without significantly impairing elongation. This is also explained by the formation of chromium carbides, which have a strength-enhancing effect and can increase tempering resistance.To achieve the desired effect on transformation, a minimum Cr content of 0.05 mass% is required, with this effect being particularly reliable at Cr contents of at least 0.15 mass%. At contents of more than 0.8 mass% Cr, no further increase in the positive influence of Cr can be observed due to the overall alloy layer selected according to the invention. The transformation-retarding effect of Cr can be utilized particularly effectively at contents of up to 0.55 mass%. Therefore, the Cr contents of a flat steel product intended for forming a component according to the invention are optimally 0.15 - 0.55 mass%.

[0025] Nitrogen ("N") can be present in the alloy concept according to the invention as a fundamentally undesirable accompanying element in amounts of up to 0.01 mass%. At higher N contents, increased contents of nitride formers such as Ti, Nb, and Al are necessary to bind N as nitride. At the same time, the risk of forming coarser, toughness-impairing TiN precipitates increases if Ti is optionally added. In particular, there would then be a risk that B would no longer be available in dissolved form. BN formation must be avoided, as otherwise the desired transformation-retarding effect of free boron could not be utilized.In order to limit the alloying measures required for this purpose, the N content is limited to a maximum of 0.01% by mass, whereby the negative influence of N on the properties of a flat steel product intended for forming a component according to the invention can be particularly reliably avoided by limiting the N content to a maximum of 0.007% by mass, in particular to a maximum of 0.005% by mass.

[0026] Niobium (Nb) is present in contents of 0.01–0.06 mass%, particularly 0.010–0.060 mass%, as a mandatory element in the steel of the flat steel product from which a component according to the invention is formed. Nb has a strong grain-refining effect because, even as a dissolved alloying element in austenite, it can inhibit grain growth. Furthermore, Nb forms fine carbide or nitride precipitates, which, in the case of nitrides, are significantly finer than, for example, TiN. Grain refinement and precipitate formation contribute to increasing the strength of the final product manufactured from the steel alloyed according to the invention and also improve toughness. Furthermore, fine precipitates contribute to the prevention of cracks. Furthermore, fine precipitates are more favorable than coarse precipitates in terms of preventing crack initiation and crack propagation.Furthermore, a finer austenite grain size reduces the martensite package size, resulting in a more homogeneous hardness and strength distribution in the product produced from a steel material alloyed according to the invention. Furthermore, the presence of Nb positively influences the segregation behavior, since the finer grain structure already in the austenite state promotes a refinement of the segregation structure. To achieve these positive effects, the minimum Nb content of a flat steel product intended for forming a component according to the invention is 0.010 mass%, with Nb contents of at least 0.015 mass% or at least 0.020 mass% having proven particularly favorable.The upper limit of the Nb content for the flat steel product used according to the invention is 0.060 mass%, since with increasing Nb content, a clogging effect can occur during the casting of the steels alloyed according to the invention that were melted to produce the flat steel product. Furthermore, particularly with simultaneously higher C contents, there is a risk that increased Nb contents may no longer be completely dissolved when heating slabs cast from steel material alloyed according to the invention at minimum furnace temperatures of 1100 °C. However, the complete dissolution of Nb-based precipitates during slab preheating is advantageous in order to optimally utilize grain refinement and to form finely distributed, strength-relevant Nb precipitates during hot rolling or in later process phases (recrystallization annealing, hot forming furnace) of the processing of steel material alloyed according to the invention.Excessively high Nb contents can also negatively impact the coating behavior during the hot-dip process. The beneficial effects of the presence of Nb in the flat steel product intended for forming a component according to the invention can be utilized particularly reliably at contents of up to 0.05 mass% Nb, in particular up to 0.050 mass% Nb, or up to 0.04 mass% Nb, in particular up to 0.040 mass% Nb. The optimal Nb content of the flat steel product is therefore 0.015 - 0.05 mass%, in particular 0.015 - 0.050 mass%, with contents of 0.020 - 0.04 mass%, in particular 0.020 - 0.040 mass%, having proven particularly effective.

[0027] Titanium "Ti" can optionally be added to the steel of the flat steel product from which a component according to the invention is formed, in amounts of up to 0.02 mass%, in particular up to 0.020 mass%, in order to bind the nitrogen inevitably present in the steel and to ensure that B is retained in an unbound, interstitially dissolved form. At the same time, the Ti content must be limited so as to avoid the formation of coarse TiN precipitates in order to minimize the risk of crack initiation and propagation, particularly under cyclic and dynamic loading, in high-strength products produced from steel alloyed according to the invention. The beneficial effects of Ti on the properties of a flat steel product intended for forming a component according to the invention are reliably achieved if the Ti content is at least 0.001 mass%, in particular at least 0.004 mass% or at least 0.010 mass%.For the purposes of the invention, a Ti concentration of 0.004 mass% and above, in particular of at least 0.005 mass%, is to be classified as a deliberately added element. Ti contents below the minimum limit of 0.004 mass%, in particular 0.005 mass%, specified in the invention for the Ti content are considered to be unavoidable contamination introduced by the raw materials used in the production of the steel. At the same time, negative effects of Ti can be particularly reliably avoided by limiting the Ti content to a maximum of 0.020 mass%. Therefore, the Ti content of the steel from which a component according to the invention is made is optimally between 0.004 and 0.016 mass%.

[0028] Optionally, the respective Ti content %Ti can be adjusted to the respective N content %N of the flat steel product from which a component according to the invention is formed, so that the ratio % Ti / %N applies: % Ti / % N < 4 .

[0029] According to a particularly advantageous embodiment of the invention in this context with regard to the desired setting of N, the Ti content %Ti, the N content %N as well as the Al content %AI and the remaining N content %Nrest, which is still present after setting of N by Ti to TiN (%Nrest = %N -% Ti*14 / 48), are coordinated in such a way that not only the condition %Ti / %N < 4 is met, but that the difference between the respective Al content %Al and the respective stoichiometrically determined value 27 / 14*%Nrest applies: UG Al_Nrest ≤ % Al − 27 / 14 * % Nrest ≤ OG Al_Nrest

[0030] Here, UG Al_Nrest is equal to 0.070 mass%, in particular equal to 0.075 mass%, preferably equal to 0.080 mass%, in particular equal to 0.081 mass%, and OG Al_Nrest is equal to 0.150 mass%, in particular equal to 0.135 mass%, preferably equal to 0.125 mass%, particularly preferably equal to 0.121 mass%. Accordingly, according to a particularly advantageous embodiment of the invention: 0 , 081 Masse − % ≤ % Al − 27 / 14 * % Nrest ≤ 0,121 Masse − %

[0031] Boron "B" is present as a mandatory component in the steel of the flat steel product from which a component according to the invention is formed, in amounts of 0.0005 - 0.005 mass%. B is an effective hardening element that, even at very low concentrations, can significantly delay transformation and thus significantly increase hardenability. Furthermore, B improves grain boundary strength by primarily accumulating at grain boundaries and thus displacing harmful elements, such as P, from there. In this way, toughness and fracture reduction are improved. Below 0.0005 mass% B, however, the transformation delay is too small. The beneficial effect of B in the flat steel product from which a component according to the invention is formed can therefore be utilized particularly reliably at B contents of at least 0.001 mass%. However, at contents of more than 0.005 mass% boron, a saturation effect occurs.The beneficial effects of the presence of B can therefore be utilized particularly effectively at B contents of no more than 0.0035 mass%, in particular no more than 0.0030 mass%. The optimal B content of a flat steel product intended for forming a component according to the invention is thus 0.001 - 0.0035 mass%, in particular 0.001 - 0.003 mass%.

[0032] Molybdenum ("Mo") can optionally be present in the steel of the steel product according to the invention in amounts of up to 1.5 mass%. Like chromium, Mo suppresses the formation of ferrite and pearlite during cooling and enables increased martensite or bainite formation even at lower cooling rates, thereby increasing hardenability. The hardenability-enhancing effect of Mo is significantly greater than that of Cr. Therefore, Mo can effectively increase strength in large thicknesses and cross-sections where, due to dimensional or structural reasons, only relatively low cooling rates are possible. Mo also reduces temper embrittlement of quenched and tempered steels. Mo is also a strong carbide former and can thus also contribute to increasing strength through precipitation formation.These beneficial effects of Mo occur with optional Mo contents of at least 0.03 mass%, whereby the hardness-increasing contribution of Mo can be particularly reliably utilized at Mo contents of at least 0.1 mass%. At too high Mo contents, however, the hot formability of the steel would be severely restricted. In addition, at too high contents, Mo could form low-melting sulfide compounds with S, which locally increase the risk of cracking during hot forming and could thus, for example, promote defects near the surface. Therefore, the Mo content is limited according to the invention to a maximum of 1.5 mass%. Negative effects of the presence of Mo can be particularly reliably avoided by limiting the Mo content to a maximum of 0.5 mass%.

[0033] One or more elements from the group "Cu", "Ni", "V", and "REM" can also optionally be present in the steel of the flat steel product from which a component according to the invention is formed, in accordance with the requirements explained below: Copper "Cu" and nickel "Ni" can optionally be provided in the steel of the flat steel product from which a component according to the invention is formed, to increase hardenability. Suitable Cu contents for this purpose are up to 0.5 mass%, with the effect of Cu occurring from an optional content of at least 0.1 mass%. Ni can be provided in contents of up to 1.5 mass% if not only the hardenability but also the toughness of the component manufactured from a steel product alloyed according to the invention is to be improved. For this purpose, optional Ni contents of at least 0.15 mass% are required.

[0034] Vanadium "V" can also optionally be present in the steel of the flat steel product from which a component according to the invention is formed to effect precipitation strengthening. Suitable V contents for this purpose are up to 0.2 mass%, whereby the effect of V can be utilized by optional contents of at least 0.03 mass%.

[0035] Rare earth elements (REMs), such as cerium and lanthanum, can cause grain refinement in the steel used in the steel flat product from which a component according to the invention is formed, thus increasing toughness and strength. To utilize this effect, REM contents of at least 0.02 mass% can optionally be present. This effect can be utilized particularly effectively at REM contents of up to 0.05 mass%.

[0036] Calcium ("Ca") is optionally present in the steel of the flat steel product from which a component according to the invention is formed in amounts of up to 0.005 mass%. Ca can be added to the steel to influence the sulfide shape. For example, it also forms sulfides in competition with manganese. Due to the higher hardness of CaS, a round precipitate shape is maintained during the rolling process, resulting in a smaller interface with the substrate. This prevents the development of a preferred direction during crack initiation and propagation. In conjunction with a reduction in the sulfur content, this improves material toughness and isotropy. To ensure safe use, the Ca content can be adjusted to at least 0.001 mass%.However, excessive Ca contents would increase the likelihood of other non-metallic inclusion types forming with Ca, which would impair the steel's purity and toughness. For this reason, an upper limit of 0.005 mass%, preferably 0.003 mass%, should be maintained.

[0037] For example, tin (Sn), arsenic (As), and cobalt (Co), as well as all other potential alloying elements not mentioned here, are present in the steel of the flat steel product from which a component according to the invention is formed, at most as unavoidable impurities. Their contents must be kept as low as possible, but in any case minimized so that they have no influence on the properties of the flat steel product and the products manufactured from it. To this end, the following upper limits for the contents of Sn, As, and Co must be observed: Sn: < 0.05 mass%, As: < 0.05 mass%, Co: < 0.05 mass%.

[0038] As already explained, B contributes significantly to the hardenability of the steel in a flat steel product intended for forming a component according to the invention by delaying the microstructural transformation during cooling. At the same time, B improves the toughness and fracture reduction of the flat steel product. The Al and Nb contents provided according to the invention in the steel of the flat steel product of a component according to the invention ensure that the nitrogen, which is undesirable but inevitably present in the steel in certain quantities due to production, is bound before boron nitrides can form. To this end, the invention provides that in each case, sufficient Al is present in the steel of the flat steel product that the ratio formed from the respective Al content %Al and the respective N content %N satisfies the following condition: % Al / % N * 14 / 27 ≥ UG % Al / % N with UG %Al%N = 8, especially 8.0. If the %Al / %N*14 / 27 ratio is below this limit, there is not enough Al available to sufficiently promote N binding via the desired formation of AIN compared to BN due to the higher Al supply. This risk can be mitigated by raising the lower limit UG %Al%N for the %Al / %N*14 / 27 ratio to UG %Al%N = 8.3, especially UG %Al%N = 8.6, or, particularly preferably, to UG %Al%N = 9.0.

[0039] By simultaneously applying the condition % Al / % N * 14 / 27 ≤ OG % Al / % N If the OG %Al / %N = 15, especially OG %Al / %N = 15.0, is maintained, excessive Al availability in the steel can be prevented. This would otherwise result in the risk of coarser, oxidic Al precipitates of the type Al 2 O 3 and their conglomerates with silicates and sulfides growing. If OG %Al / %N = 13.5, especially OG %Al / %N = 12.0, this risk can be minimized particularly reliably. It is therefore particularly advantageous if the following applies to the %Al / %N ratio: 9 , 0 ≤ % Al / % N * 14 / 27 ≤ 12 , 0

[0040] It goes without saying that all optional elements can be present individually or in combination as impurities in the steel of the flat steel product intended for forming a component according to the invention. In this case, the contents of the elements in question are so low that they are below the minimum limits above which, according to the above explanations, the effect of the respective element can be utilized. At contents of the optionally present alloying elements below these minimum limits, these elements have no effect on the properties of the flat steel product and can therefore be tolerated as an impurity.

[0041] Endogenous or exogenous inclusions (particles, precipitates) that arise during steel production generally lead to a reduction in the degree of purity, which can lead to premature failure of components. This can be an increasing problem, especially for high-strength components. This is especially true when such components are exposed to cyclic or dynamic loads. Of interest here are the endogenous inclusions that arise during the steel production process due to the thermodynamic conditions arising from the chemical composition and process control. Exogenous inclusions are generally isolated cases and originate, for example, from ladle slag or refractory material, but play no role here and are therefore not considered here.

[0042] Based on these findings, one of the objectives of the inventive adjustment of the steel alloy of a component according to the invention was to reduce the proportion of coarse and hard TiN-, AlN-, and oxidic Al-based particles, as well as conglomerates of these compounds, for toughness reasons, while still reliably binding the nitrogen present in each case in order to achieve complete transformation to martensite, even with relatively low cooling rates of at least 30 °C / s to a maximum of 120 °C / s, even with larger strip thicknesses and component cross-sections, via the strong transformation-retarding effect of interstitially dissolved B. As already explained above, Al and the optionally present Ti form hard precipitates, which, in components formed from flat steel products alloyed according to the invention, could be the source of cracks and their propagation due to the notch effect and the stress fields surrounding the particles.The angular and cube-shaped TiN particles in particular prove to be harmful due to their shape and size.

[0043] The invention has matched the contents of the alloying elements and the conditions during the production of flat steel products intended for forming components according to the invention in such a way that in the microstructure of a flat steel product according to the invention and of a component produced therefrom, a maximum of up to 150 area ppm of hard TiN particles and Al-based oxide particles as well as AIN with an average, circle-equivalent particle size of 0.2 - 10 µm are present, homogeneously distributed over the strip thickness.

[0044] The definition of "hard particles" here essentially includes particles of AlN, Al2O3, and Al2O3-based spinels, as well as TiN particles and conglomerates formed on the basis of these particles. Such particles each exhibit a high Mohs hardness of approximately 9. Due to their high hardness, they are hardly deformable during rolling or forming processes and lead to local stress fields in their surroundings, which can contribute to premature material failure. Conglomerates (mixed forms) are particularly referred to as particle composites in which additional particles form through heterogeneous nucleation on existing particles, e.g., Al2O3 with MnS, where the basis represents one of the previously mentioned hard particle types.

[0045] Furthermore, the alloying concept according to the invention has achieved that the total number of hard TiN-based precipitates and their mixed forms falling within this particle size range in a component formed from a flat steel product alloyed according to the invention is reduced to less than 30% of the particles in the 0.2 - 10 µm size class present in the structure of a component. At the same time, the absolute number of precipitates falling within the relevant particle size range is reduced compared to conventional flat steel products, for example, those made from a steel with higher Ti contents, whereby the average spacing of the 0.2 - 10 µm precipitates in the component formed from a flat steel product alloyed according to the invention is significantly increased.It was found that in the comparison concept, the proportion of hard TiN particles and their mixed forms accounts for more than 45 - over 80% of the volume fraction of the particles present in the 0.2 - 10 µm size class. Due to this high proportion, a reduction in the Ti mass fraction makes sense, which correspondingly leads to a reduction in the proportion of hard TiN particles in the inventive concept. Due, among other things, to the optional, but in any case limited, addition of titanium alloying, coarse particles such as TiN occur significantly less frequently in a flat steel product intended for forming a component according to the invention than is the case with conventional concepts that provide for higher Ti contents. By reducing the proportion of coarse precipitates, an improvement in toughness is achieved, which prevents the formation and propagation of cracks.Surprisingly, the moderate increase in the Al mass content does not, in turn, lead to a significant increase in the proportion of similarly hard, oxide-based Al-based precipitates, as well as AlN and their conglomerates. As a result, the risk of premature material failure is reduced in the flat steel products intended for forming a component according to the invention. The optimization of toughness achieved by the invention is noticeable in an improvement in the reduction of area at fracture of the component according to the invention in the hot-formed, tempered state, which is of particular interest to the component manufacturer.

[0046] During the production of a component according to the invention, a microstructure consisting entirely of martensite in the technical sense is created through full austenitization, followed by quenching and optional tempering. This naturally includes, according to expert understanding, the possibility that up to 5% of other components may be present in the microstructure of a component according to the invention, which, however, are ineffective with regard to the properties of a component according to the invention determined by the martensite content.

[0047] As already mentioned, the Al and Nb contents provided in the steel according to the invention result in additional microstructure refinement. Thus, during the production and heat treatment of the flat steel product made from the alloyed steel according to the invention and the component manufactured therefrom, Nb and Al in dissolved and precipitated form reduce austenite grain growth and, after transformation, reduce the martensite package size. This results in further relevant precipitates in the flat steel product, such as NbN, NbC, and AlN, which, as monolithic particles without nucleation on previously formed precipitates, generally only reach a maximum size of approximately 100 nm. In this way, more homogeneous precipitate fractions with narrower particle size ranges are achieved. These prove particularly effective with regard to controlling austenite grain size.Thus, the steel used to produce the flat steel product from which a component according to the invention is formed has an austenite grain size during austenitization that is up to half an ASTM grain size finer than that of conventional steel concepts belonging to the type of steel according to the invention. In addition, the grain sizes of a flat steel product alloyed and processed according to the invention are within a narrower range, i.e. with a reduced standard deviation. At the same time, there is reduced variation in the original austenite grain size across the strip thickness. This leads to finer martensite packages and a high homogeneity of the martensite microstructure, which is advantageous for the toughness of components manufactured from such a flat steel product according to the invention in the quenched and tempered or press-hardened state. This also results in better dimensional stability of the component, since strength fluctuations across the strip thickness can be reduced.

[0048] An important material parameter for determining the final properties is the former austenite grain size. This is the grain size of the austenite that has changed after the completion of the austenitization process in the furnace as a result of

[0049] Recrystallization and grain growth occur, i.e., they predominate in the microstructure shortly before the start of quenching. The finer this austenite grain size on average, the finer the resulting martensite package size and the more beneficial it is for the toughness of the martensite and thus of the material or component.

[0050] Likewise, it is beneficial for the homogeneity of the local strength properties if the grain size of the austenite fluctuates only slightly, thus resulting in only minimal local fluctuations in the martensite hardness after transformation. This also helps prevent springback effects on a press-hardened or tempered component caused by locally inhomogeneous microstructures.

[0051] Components produced in accordance with the invention are therefore characterized, after hot forming or quenching during tempering, by the fact that the product KG = KA x Ks, referred to as "grain size quality KG", of the former austenite grain size KA, used in µm, and the simple standard deviation Ks of the former austenite grain size, also used in µm and averaged at three points over half the strip thickness, applies to the structure of the resulting component: KG < 30 μ m 2

[0052] With such a qualified grain size quality KG, it is possible to achieve, compared to the state of the art, not only a small grain size which is advantageous for toughness, but also a high homogeneity of the microstructure across the component cross-section due to a small scatter of the grain size.

[0053] After a suitable heat treatment as explained below, components according to the invention achieve a tensile strength of at least 1000 MPa at C contents of 0.1 mass%, in particular 0.10 mass%, or

[0054] Tensile strengths up to 2500 MPa at C contents of 0.6%, especially 0.60 mass%.

[0055] The area of ​​reduction in area at fracture ε(epsilon)3 was investigated here as a measure of toughness, since the investigation of the notched bar impact toughness determined according to DIN EN ISO 148-1 according to Charpy is limited to thicknesses of 10 mm or so-called undersize specimens (thicknesses of 2.5, 5 and 7.5 mm) and is therefore only suitable for the investigation of correspondingly thick specimens, which were not available here. The toughness properties were therefore not determined here according to DIN EN ISO 148-1 according to Charpy. The area of ​​reduction in area at fracture was evaluated as a measure of toughness or local elongation, since contrary to the requirements of the notched bar impact test according to DIN EN ISO 148-1, different thicknesses were available, in particular thicknesses < 2.5 mm, meaning that no uniform, so-called undersize specimens could be used and therefore the application of this standard was not suitable.Although the local strain from the fracture necking correlates with the hole expansion, it represents an extended description of the local deformation behavior.

[0056] Accordingly, components according to the invention are simultaneously characterized by excellent toughness for this strength class, which, depending on the tensile strength, also results in a percentage improvement in reduction of area at fracture (ΔBE) of at least 5 to 45% compared to a Ti / B-based tempering concept with increasing tensile strength from 1000 to 2500 MPa, respectively. The absolute reduction of area at fracture in the thickness direction ε(epsilon)3 for components made from steel concepts according to the invention is 10 to 65%.

[0057] The components formed from flat steel products obtained according to the invention in the manner explained above, using inventive processing, are particularly suitable for weight-reduced component applications in the automotive and truck sectors, including longitudinally welded tubes for stabilizers, rotor shafts, camshafts, or tubular components used in the steering and chassis sectors. In particular, a flat steel product according to the invention can be cold-formed to form a seam-welded steel tube suitable for use, for example, as a stabilizer for vehicle suspension, a steering shaft, or a drive shaft in motor vehicles. A subsequent tempering treatment can achieve a significant increase in the strength of the formed tube.

[0058] The thickness of flat steel products intended for forming components according to the invention is typically 1-16 mm, although sheets with a thickness of 2-9 mm, in particular 4-7 mm, can be used for automotive applications, although thicknesses of up to 5 mm can be of particular importance in practice. If special requirements are placed on the resistance to abrasive wear of such flat steel products, it has proven advantageous, due to the associated high hardness, if the C content of the flat steel products intended for forming a component according to the invention is at least 0.5% by mass, in particular 0.50% by mass.

[0059] Furthermore, flat steel products manufactured according to the invention can also be used as hot-rolled or cold-rolled strip for forming components according to the invention. For example, structural components for automobile bodies can be hot-formed from such hot-rolled or cold-rolled strip, and their high strength can be retained by subsequent targeted cooling from the forming heat. Flat steel products intended for forming components according to the invention, which are provided for this processing route, typically have a thickness of 0.5–3.5 mm, in particular 0.5–3 mm, 1–3 mm, or 1.2–2.5 mm. Examples of components according to the invention that can be formed from flat steel products manufactured according to the invention are beams of automotive structures subject to bending stress, such as the B-pillars or seat cross members of passenger vehicles, as well as longitudinal and cross members of passenger or commercial vehicle chassis, and generally body structural components.The flat steel product according to the invention is also particularly suitable for processing into components that move during use, such as parts of shock absorbers, camshafts or their parts, piston rods, or shafts, especially electric motor shafts. Thus, it is possible to obtain a high-strength component for an automobile body through hot forming or press hardening.

[0060] To protect against corrosion, flat steel products used according to the invention and components produced from them can be coated with a metallic protective layer. Zinc- or aluminum-based metallic protective layers, such as AlSi coatings, are particularly suitable for this purpose. These can be applied conventionally by hot-dip coating. Electrolytic coatings are also conceivable.

[0061] By specifying the chemical composition of the steel of a component according to the invention and by carefully controlling the steelmaking process, the endogenous inclusions can be influenced in terms of type, size, and distribution. In addition to solidification, this influence extends particularly to the hot rolling production stage, as explained below.

[0062] The method according to the invention for producing a component with a structure consisting of at least 95% by area of ​​martensite and the remainder of other structural components, therefore comprises the following work steps: A) A hot-rolled strip is produced by a) melting steel which consists of (in mass%) C: 0,1 - 0,6 %, Mn: 0,1 - 2 %, Al: 0,05 - 0,2 %, Note: 0,01 - 0,06 %, B: 0,0005 - 0,005 %, Cr: 0,05 - 0,8 %, Si: up to 0.8%, Mon: up to 1.5%, Cu: up to 0.5%, No: up to 1.5%, V: up to 0.2%, REM: up to 0.05% Ti: up to 0.02%, Approx: up to 0.005%, Rest iron and unavoidable impurities, where the impurities include contents of up to 0.03% P, up to 0.03% S, up to 0.01% N, less than 0.05% Sn, less than 0.05% As and less than 0.05% Co, and where the ratio %Al / %N*14 / 27 formed from the respective Al content %Al and the respective N content %N is ≥ 8, b) the molten steel is cast into a precursor product, namely a slab, a thin slab or a cast strip, c) the preliminary product is, if necessary, heated to a preheating temperature of 1100 - 1350 °C, (d) the precursor product is hot-rolled into a hot-rolled strip with a thickness of 1-16 mm, with hot rolling being terminated at a final hot-rolling temperature which is at least 50 °C and at most 150 °C higher than the Ar3 temperature of the steel, e) the hot strip obtained is cooled to a coiling temperature of 450 - 700 °C, with the cooling taking place in the temperature range of 800 - 650 °C at a cooling rate of 20 - 200 °C / s, f) the hot strip cooled to the coiling temperature is coiled into a coil and the hot strip is cooled to room temperature in the coiled state, and g) optionally: the hot-rolled strip cooled in the coiled state is pickled and h) also optional: batch annealing is carried out at a core temperature of the hot strip of 500 - 720 °C for a period of 5 - 50 hours. B) From the hot strip obtained, a cold strip is optionally produced by i) the hot-rolled strip is cold-rolled into a cold-rolled strip with a thickness of 0.5–3.5 mm in one or more cold-rolling steps. j) Optionally, the cold-rolled strip can be annealed in a batch annealing line or a continuous annealing line. C) A component is formed from the hot strip or the optionally produced cold strip by k) a blank is cut from the hot or cold strip and according to alternative 1: I.1) the blank is heated to an austenitizing temperature which is at most 100 °C lower than the Ac3 temperature of the steel from which the hot or cold strip is produced and at most 950 °C, I.2) within 1 - 20 s after the end of the heating to the austenitizing temperature, the blank is placed in a cooled hot forming tool in which the blank is hot formed into the component, and I.3) the component is press hardened by accelerated cooling at a cooling rate of 30 - 120 °C / s until the martensite start temperature of the steel from which the respective hot or cold strip is made is reached, so that the component acquires a fully martensitic structure, or according to alternative 2: m.1) the blank is cold formed into the component, m.2) the cold-formed component is heated to an austenitizing temperature which is no more than 100 °C lower than the Ac3 temperature of the steel from which the hot-rolled or cold-rolled strip is produced, and no higher than 950 °C, and m.3) the component heated to the austenitizing temperature is cooled at an accelerated rate of 30 - 120 °C / s until the martensite start temperature of the steel from which the respective hot-rolled or cold-rolled strip is made is reached, so that the component acquires a fully martensitic structure. n) Optionally, the component obtained after steps I.1 - I.3 or m.1 - m.3 can be tempered at temperatures of 150 - 700 °C for an annealing time of 5 - 60 minutes.

[0063] In the process according to the invention, a melt is produced in step a) whose composition corresponds to the above explanations regarding the alloying of the steel of a flat steel product intended for forming a component according to the invention. Naturally, the above information regarding advantageous configurations of the steel of a flat steel product intended for forming a component according to the invention applies equally to the melt produced and processed in the course of the process according to the invention.

[0064] The melt produced in step a) is cast in the conventional manner into slabs, thin slabs, or strip (step b)). Slabs typically have thicknesses of 180 mm to 260 mm. Thin slabs are typically available in thicknesses of 40 to 60 mm, and cast strip in thicknesses of 2 to 5 mm.

[0065] In step c), the precursors are thoroughly heated for subsequent hot rolling (step d). This thorough heating typically takes place in pusher or walking beam furnaces available for this purpose in the state of the art. The conditions established during the preheating of the slabs, thin slabs, or strips cast from a steel melt alloyed according to the invention are of particular importance for the development of the properties of a flat steel product composed and produced according to the invention. It has been shown that, as a result of the inventively only minimal addition of titanium to the steel processed according to the invention, even relatively low furnace temperatures are process-safe in order to produce little or no nucleation effects on coarse particles such as TiN, since these particles occur much less frequently in the steel concepts according to the invention."Nucleation" refers to the formation of precipitates on previously formed precipitates based on heterogeneous nucleation, as opposed to homogeneous precipitate formation without extraneous nucleation sites. The alloy of the steel used in the invention, from which the flat steel products processed according to the invention and the components formed therefrom are made, statistically reduces nucleation effects on previously formed precipitates. The nucleation of TiC, NbN, NbC, and AlN on TiN would reduce the probability of these precipitates forming at lower formation temperatures, thus impairing their effectiveness with regard to the microstructure refinement sought by the invention. Surprisingly, it has been found that, by maintaining a suitable preheating temperature, austenite grain size refinement can be achieved even at the intended, comparatively low Nb contents.

[0066] The preheating temperatures used according to the invention are 1100–1350 °C, and preferably 1150–1280 °C. Below 1100 °C, coarsening and nucleation effects of the particles during preheating must be expected. Temperatures above 1350 °C should be avoided to limit coarsening of the austenite grain, reduce material loss due to scaling, and, from an economic perspective, reduce energy costs.

[0067] Of equal importance are the holding times for preheating the precursors. These are required for the complete dissolution of the precipitates present in the precursors to be preheated after casting. The total holding time for slabs specified in the invention is 150–400 minutes, including the time required for heating to the respective target preheating temperature and for thorough heating of the precursors. With total holding times of less than 150 minutes, there is a risk that the relevant microalloy precipitates will not dissolve completely. However, holding times of more than 400 minutes should also be avoided to limit austenite grain coarsening. Thin slabs are preheated in an equalizing furnace for significantly shorter times of 10–90 minutes.

[0068] Strips produced by strip casting are generally not preheated, but are hot-rolled directly in one or more hot-roll stands to final hot-rolled thicknesses of 1 - 4 mm.

[0069] The slabs or thin slabs thoroughly heated in accordance with the invention, taking into account the above-explained requirements, can be hot-rolled in a conventional manner in an equally conventional hot-rolling or casting-rolling mill into a so-called "hot strip." Hot rolling may include roughing, in which the slabs are typically rolled in a reversing manner in a so-called "roughing stand" to an intermediate thickness of approximately 35 to 60 mm. The roughed slab then enters a multi-stand finishing hot rolling mill, where it is continuously hot-rolled step by step into a hot strip.

[0070] With a thin slab, pre-rolling is unnecessary. It can be fed directly into the finishing hot rolling mill after preheating (if necessary).

[0071] According to the invention, hot rolling in step d) is terminated at hot rolling end temperatures that are at least 50 °C higher than the Ar3 temperature of the steel, but no more than 150 °C above this temperature. Hot rolling is thus terminated at a hot rolling end temperature at which the resulting hot strip still possesses a fully austenitic microstructure. Such a rolling strategy is referred to as "normalizing rolling." According to the invention, the hot rolling end temperature is selected such that the tendency of Nb and Al to form deformation-induced precipitates is reduced and a larger proportion of precipitation potential is available for inhibiting grain growth during austenitization in the subsequent quenching and tempering process or during hot forming. Suitable hot rolling end temperatures for the alloying concept according to the invention are typically above 830 °C.The final rolling temperature is preferably at least 60 °C and at most 130 °C higher than the Ar3 temperature, with hot rolling final temperatures no more than 110 °C above the Ar3 temperature having proven particularly effective in limiting austenite grain growth. Normalizing rolling is preferred here because the hot rolling forces are comparatively low and the precipitation of deformation-induced, relatively coarse precipitates is avoided. Thus, the precipitation potential for reducing the original austenite grain size can be maximized in subsequent austenitizing stages of quenching and tempering and hot forming. This has a positive effect on toughness.

[0072] In order to preserve the precipitation potential of Al and Nb in the hot-rolled strip obtained after hot rolling for subsequent process steps, it is necessary, in step e), to cool the hot-rolled strip after hot rolling in the temperature range of 800 °C to 650 °C at a cooling rate of more than 20 °C / s to the coiling temperature. The actual coiling temperature achieved is determined by the cooling in the cooling section. According to the invention, it is significantly below the A1 temperature of the steel from which the flat steel product according to the invention is produced in order to avoid relatively coarse pearlite precipitation in the hot-rolled strip. In the iron-carbon diagram, the temperature "A1" is the temperature at which austenite decomposes into pearlite from high temperatures. In the pure iron-carbon binary system, A1 is 723 °C, whereby this transformation occurs at carbon contents > 0.02 mass%, which is the case with the steel concepts according to the invention.According to empirical formulas that reflect the influence of alloying elements on A1 (see, for example, Hougardy, HP "Werkstoffkunde Stahl Volume 1: Grundlagen", Verlag Stahleisen GmbH, Düsseldorf, 1984, p. 229), the A1 temperature is 722–727 °C, and thus within a narrow range. In the case of the invention, coiling temperatures of ≤ 720 °C are used in particular. At such low coiling temperatures, the solution state and precipitation form of the carbon are influenced to achieve a finely distributed C precipitation for subsequent quenching and tempering or hot forming treatments, in order to accelerate C dissolution for the hardening process. This makes flat steel products produced or obtained according to the invention particularly suitable for quenching and tempering and heating treatments in which rapid heating by means of induction heating or other rapid heating processes is to be used.

[0073] The rapid cooling of the resulting hot strip according to the invention in the temperature range of 800 - 650 °C thus suppresses the precipitation formation of Nb and Al. This can be ensured in particular by a cooling rate of at least 20 °C / s. It should be noted that during cooling after hot rolling, reheating of up to 30 °C can occur due to the phase transformation. In practice, the strip can be sprayed with water for the inventively controlled cooling downstream of the hot rolling mill in which the hot rolling takes place. Cooling lines known from the prior art, which combine laminar and spray cooling devices, are particularly suitable for this purpose. These should be capable of achieving cooling rates of preferably more than 20 °C / s, in particular at least 50 °C / s, and a maximum of 200 °C / s, especially in the temperature range of 800 - 650 °C.

[0074] The coiling temperature to which the hot strip is cooled after hot rolling and at which the hot strip is coiled into a coil in step f) is 450 - 720 °C. The upper limit of 720 °C is advantageous in order to be able to set a sufficiently low tensile strength for subsequent cold forming at C contents ≥ 0.4 %. The coiling temperature is particularly preferably lower than 650 °C in order to further suppress the precipitation formation of Nb and Al and to achieve the most finely distributed C dissolution state possible. An upper coiling temperature of 650 °C proves to be particularly advantageous because it largely prevents coarse-structured pearlite formation. At coiling temperatures of less than 450 °C, significant strength development would occur in the hot strip, for which subsequent cold forming or cold rolling would represent a significant increase in the deformation forces and is therefore avoided.The hot strip is then cooled to room temperature in the coil in the conventional manner.

[0075] The hot-rolled steel flat product obtained according to the invention after coiling typically has a tensile strength of less than 700 MPa. Only through the subsequent quenching and tempering treatment or through the processing completed during hot forming are the largely fully martensitic microstructure according to the invention and, consequently, the optimized mechanical properties of a component according to the invention achieved.

[0076] After coiling, in step g), which is only performed optionally if necessary, the hot-rolled strip can be subjected to pickling for further processing to remove any adhering scale. This processing step is advantageous when the hot-rolled strip is formed in a cold-forming tool and contamination or damage to the tool can be avoided by abrasion of the scale. There are no special requirements for pickling. It can be carried out using any method known for this purpose.

[0077] The resulting hot-rolled strip has a microstructure consisting of pearlite with a small amount of ferrite (< 5%). The ferrite can be linear or network-like.

[0078] Likewise optionally, the hot strip can be subjected to a batch annealing process in step h) in order to reduce the strength of the steel for subsequent cold forming. The core temperatures of the coiled steel flat product set during batch annealing are 500 - 720 °C. A core temperature of at least 500 °C is required to ensure sufficient strength reduction. However, annealing temperatures of more than 720 °C would result in the formation of new pearlite due to the A1 temperature being exceeded in all areas of the coil being reliably avoided during batch annealing. A batch annealing time at core temperature level of at least 5 h is required to also significantly reduce the strength level, i.e., to < 700 MPa tensile strength. However, batch annealing should not last longer than 50 h, as the deformation and coagulation of the pearlite due to the ongoing diffusion processes would then lead to coarse pearlite particles.Optimally, the annealing conditions for batch annealing are selected so that only partial deformation of the cementite occurs, with a degree of deformation ≤ 85%. In practice, the optional batch annealing according to the invention can be carried out at core temperatures of max. 720°C under a protective gas atmosphere. The protective gas atmosphere can consist of a pure hydrogen atmosphere (H2) or a mixture of N2 and up to 12 vol.% H2 ("HNX"). Typical mixtures are 95% N2 and 5% H2. HNX annealing results in longer total annealing times of up to 50 hours, as heat transfer is slower than with a pure H2 atmosphere. The core temperature of the batch annealing should be below 720°C, in particular around 680°C, but in any case below the A1 temperature of the steel from which the flat steel product is made. This restriction prevents new pearlite from forming during the annealing process.Instead, cementite particles (carbide particles) are partially incorporated from the hot strip structure present at the beginning of annealing, particularly due to carbon diffusion and carbon redistribution. At the same time, the structure can become coarser as a result of coagulation. During the bell annealing process, which is optionally carried out according to the invention, cementite forms in a partially incorporated, globulitic form, which is largely homogeneous and randomly distributed in a ferritic matrix, with the degree of inclusion being ≤ 85% according to the invention. The restriction of the bell annealing temperature and holding time serves to limit the degree of inclusion. A limited degree of inclusion reduces the time required for complete C dissolution during austenitization.The microstructure in the hot-rolled condition thus consists predominantly of partially formed cementite, pearlite in a proportion of up to 90% and a proportion of non-polygonal ferrite of up to 10%.

[0079] If a cold-rolled flat steel product is to be produced from the hot-rolled strip produced in the manner described above, the following further steps can be carried out following the work steps a) - h), of which the work steps g) and h) are only carried out as required, i.e. optionally: i) Cold rolling of the hot strip into a cold strip with a thickness of 0.5 - 3 mm in one or more cold rolling steps; j) Recrystallization annealing of the cold strip, which can take place in a batch annealing furnace or a continuous annealing furnace. If annealing takes place in a batch annealing furnace, it can be carried out under the conditions already specified for step h) above. If annealing is to be carried out in a continuous annealing facility, no special requirements are required for the annealing parameters. Accordingly, heating can take place at rates of up to 30 °C / s until the annealing temperature is reached, which can be in the range Ac1 to Ac3 + 30 °C. The cooling rate to room temperature can be achieved via gas jet or roller cooling and can be up to 20 °C / s. Hot-dip coating can be integrated into the continuous annealing process after the actual annealing. Additionally, a coating can be applied in an electrolytic coating system;

[0080] The production of the cold strip can be completed in the usual way by a skin pass with usual degrees of deformation of 0.5 - 1.5%, although here too no special requirements are imposed.

[0081] Two alternative approaches are available to form components according to the invention from hot-rolled or cold-rolled strip produced according to the invention, which possess an optimized combination of high strength and toughness. According to the first alternative, a blank cut from the respective hot-rolled or cold-rolled strip is heated and press-hardened according to steps I.1 - I.3 of the inventive method. According to the second alternative, the blank is first cold-formed and then tempered according to steps m.1 - m.3 of the inventive method.

[0082] For the quenching and tempering / hot forming of components formed from flat steel products according to the invention by cold forming, or for hot forming into components, the respective component (quenching and tempering) or flat steel product (hot forming or press hardening) is first heated to a suitably high austenitizing temperature ("austenitizing"). In practice, this can be done, for example, in a conventional manner, first in a furnace in which the respective flat steel product (work step I.1) of the first alternative) or the component (work step m.2) of the second alternative) is heated to the respective austenitizing temperature for a sufficient total time, in particular (including the heating time).

[0083] In the first alternative, the blank is heated in step I.1, and in the second alternative, the component is heated in step m.2 to an austenitizing temperature that is no more than 100°C below the Ac3 temperature of the steel from which the hot-rolled or cold-rolled strip, or from which the blank or component is made (austenitizing temperature ≥ (Ac3 - 100°C)). Austenitizing temperatures that are no more than 75°C lower than the Ac3 temperature (austenitizing temperature ≥ (Ac3 - 75°C)), in particular no more than 50°C lower than the Ac3 temperature of the steel of the hot-rolled or cold-rolled sheet from which the blank or component is made (austenitizing temperature ≥ (Ac3 - 50°C)), lead to the desired result in a particularly reliable manner in practice.Austenitizing temperatures that are at least equal to the Ac3 temperature of the steel from which the respective blank or component is made are particularly suitable. The upper austenitizing temperature is limited to a maximum of 950 °C. The austenitizing temperature maintained in steps I.1 and m.2 is accordingly within a range from (Ac3 - 100 °C) to 950 °C, in particular (Ac3 - 75 °C) to 950 °C, or, particularly advantageously, from (Ac3 - 100 °C) to 950 °C, with austenitizing temperatures of Ac3 - 950 °C being particularly practical.

[0084] A total heating time of typically 1 second to 20 minutes is required for the through-heating of the blank or component, although in practice, total times of at least 10 seconds, especially at least 1 minute, are suitable to reliably achieve through-heating. The total heating time includes the time required to heat up to the austenitizing temperature.

[0085] For heating circuit boards in step I.1, total times (including heating time) of 1 - 20 minutes are particularly suitable.

[0086] In the case of quenching and tempering of a component (process steps m.2 and m.3 of the second alternative), total times of 1 - 20 min or 2 - 10 min, in particular 5 - 10 min, are typically provided for piecewise heating of the component to the austenitizing temperature.

[0087] For faster heating of the component, commercially available inductive flow heating devices can be used. The component to be heated passes through these devices in a continuous flow, so that within a short time, the section of the component located within the effective range of an electromagnetic field induced by the heating device is thoroughly heated. In this way, the component is gradually heated to the austenitizing temperature along its entire length. Such flow heating devices are particularly suitable for the continuous heating of components such as pipes or profiles that require high dimensional accuracy.

[0088] In the case of hot forming according to the first alternative, the respective flat steel product is placed after austenitizing within a transfer time of 1 - 20 seconds into a hot forming device known for this purpose from the state of the art, in which it is then press-hardened into a component in an equally known manner, wherein the average cooling rate to room temperature is 30 - 120 °C / s.

[0089] In the case of tempering according to the second alternative, the component heated to the austenitizing temperature is quenched to room temperature after austenitizing, also at an average cooling rate of 30–120 °C / s. For this purpose, the component can be immersed in a suitable quenching medium in a conventional manner or exposed to the quenching medium using equally conventional devices, such as nozzles or jet devices. If a continuous heating device of the type described above, particularly an inductive one, is used to heat the component, the section of the blank heated to the austenitizing temperature can also be cooled in a continuous flow using a suitable quenching device upon exiting the respective heating device.

[0090] Quenching occurs within 1-20 seconds after removal from the equipment used to heat the component to the austenitizing temperature (quenching and tempering) or through contact with the tool at the end of the press hardening process (hot forming). In practice, an oil bath can be used for quenching during quenching and tempering, in which the component is quenched to room temperature within 1-30 seconds while moving. Typical transfer times between the furnace, where heating to the austenitizing temperature takes place, and the oil bath are 1-20 seconds.

[0091] Due to their special property profile, flat steel products processed according to the invention are particularly suitable for the production of highly stressed components for vehicle bodies, in particular for supports, structural parts, frames, bumpers, battery boxes, and the like. In particular, the components according to the invention are tubular components, in the production of which blanks of hot-rolled or cold-rolled strip produced according to the invention are formed into a tubular body and then longitudinally welded.

[0092] The microstructure of the flat steel products in question here and the components according to the invention manufactured from them was examined as follows: The proportions of hard oxide and nitride particles in the microstructure of a flat steel product are given in area ppm, unless otherwise stated. The exact procedure for determining them is described below. According to ASTM E2142 from 2008, the area fraction of inclusions can be equated with the volume fraction. Likewise, the phase fractions of the microstructure given in this text refer to the evaluated ground surface and are therefore given in area %.

[0093] The inclusions were examined on longitudinal sections across the strip thickness using a Zeiss scanning electron microscope (SEM) (model GeminiSEM 500) equipped with the Oxford Xmax EDX system from Oxford Instr. for energy-dispersive elemental analysis. Data were evaluated using the Aztec 3.3 SPI, Feature Analysis software from Oxford Instr. Inclusions with a size of approximately 0.2 µm or larger were recorded. The element contents of the precipitates were determined using calibration samples. The inclusions were classified based on the stoichiometry of the known precipitates, with a classification into oxides, sulfides, and TiN. Quantification and standardization of the measured elements, excluding Fe, C, and Ag, were performed. The recorded elements were converted into oxides (excluding S, P, Cl, F) and normalized to 100%.In addition, a calculation of the subsystem<Al 2 O 3 -SiO 2 -CaO> and normalization to 100%. Subsequently, classification tables of the analyzed inclusions were created using computer-assisted analysis from the raw data obtained in this way. Inclusions that could not be clearly classified were listed separately. These inclusions were examined individually. The particle size was idealized as a circle-equivalent diameter, regardless of particle shape.

[0094] The homogeneity of the microstructure of the former austenite and the distribution of its constituents was determined using electron backscatter diffraction (EBSD) in the fully martensitic state after tempering or press hardening on longitudinal sections across the strip thickness. The samples were polished with the polishing compound "OP-S Suspension" from the manufacturer "Struers." For this purpose, a measuring field measuring 140 µm x 140 µm was positioned at different layers across the strip thickness and scanned with a step size of 0.15 µm. Several layers across the strip thickness were also examined (1 / 6, 1 / 3, 1 / 2) to obtain information about the homogeneity of the microstructure. The data obtained from the EBSD investigations on the martensitic microstructure were subsequently used to reconstruct the austenitic parent microstructure using the software "ARPGE 2.0, Reconstruction of Parent Grains from EBSD Data" (described in C.Cayron, Acta Cryst. A62 (2006) 21-40; C. Cayron, J. Appl. Cryst. 40 (2007), pp. 1183-1188). The Nishiyama-Wassermann orientation relationship was used for the transformation of body-centered cubic crystals into face-centered cubic crystals (Z. Nishiyama, Sci. Rep. Res. Inst. Tohuku Univ. Vol. 23 (1934-1935), p. 647).

[0095] The quantitative estimation of the microstructure components ferrite, pearlite, cementite, bainite and martensite, which goes beyond the inclusion structure, was carried out using light microscopy on the basis of longitudinal sections in the 1 / 3 zone in the strip thickness direction at 500 - 1000 times magnification.

[0096] The mechanical parameters of flat steel products or components made from them mentioned in this text are the tensile test values ​​(tensile strength, yield strength, modulus of elasticity, uniform elongation and elongation at break) determined according to DIN EN ISO 6892-1.

[0097] The reduction in area at fracture, also referred to as "absolute strain in the thickness direction" ε(epsilon)3 = (t0 - tf) / t0, expressed in % was used as a measure of toughness (Larour, P., Freudenthaler, J., Weissböck, T.: Reduction of cross section area at fracture in tensile test: measurement and applications for flat sheet steels, IDDRG 2017, 36th International Deep Drawing Research Group Conference, Materials Modelling and Testing for Sheet Metal Forming, Munich, DE, July 2-6, 2017, Volume 896 (2017), p. 012073 / 1-8). Where t0 is the initial thickness of the specimen, tf is the thickness of the thinnest points in the reduction in area of ​​the fracture cross-section determined from four measurements across the specimen width. The "absolute elongation in the thickness direction" or "necking at fracture" was measured on tensile specimens after quenching and tempering using an optical system (microscope).For this purpose, the thickness tf was determined at four locations across the fracture cross-section (1 mm to the right of the left edge, center, minimum, 1 mm to the left of the right edge). Three parallel tensile specimens were tested in each case to obtain a representative result for the respective condition under investigation. A total of six fracture cross-sections were measured. The average value for one specimen was calculated from the six measured values. The tensile specimens were oriented longitudinally to the rolling direction.

[0098] The invention is explained in more detail below using exemplary embodiments: To demonstrate the effectiveness of the invention, three steels 1 - 3a according to the invention were melted, the compositions of which are given in Table 1. For comparison, three further steels 4 - 6 were melted which were not alloyed according to the invention and whose compositions are also given in Table 1.

[0099] The production of flat steel products from steel grades 1-6 was carried out conventionally in an integrated steelworks, in which the process chain "pig iron and crude steel production," "steel production," and the various stages of semi-finished product production, such as "preheating" and "hot rolling," as well as optional "pickling" and "batch annealing" for the hot strip stage, as well as "pickling," "cold rolling," "continuous annealing," and optional "AISi coating" and "skin-passing" for the cold strip stage according to the invention, are represented. The inventive specifications and measures apply to the production of uncoated hot or cold strip for quenching and tempering or hot forming processes, as well as to the production of an AlSi-coated, alloyed and cold-rolled sheet for hot forming.

[0100] Steel grades 1-6 were each melted and cast into slabs. The slabs were then heated to a preheating temperature and then hot-rolled into a hot strip. The hot strip obtained during hot rolling was cooled to a coiling temperature at which it was coiled into a coil. The coil was then cooled to room temperature.

[0101] The flat steel product produced in this way as unpickled hot-rolled strip from steel 2 was made available for further processing without any further treatment.

[0102] After cooling in the coil, the hot strips produced from steels 1 and 3 - 6 were subjected to a pickling treatment in order to remove any scale adhering to them.

[0103] The hot-rolled strips produced from steels 1 and 4 were then cold-rolled into cold-rolled strips without intermediate annealing. The resulting cold-rolled strips each underwent continuous annealing, were coated with an AlSi coating by hot-dip coating, and finally skin-pass rolled. The flat steel products, available as cold-rolled strips with an AlSi coating, were prepared for further processing into components.

[0104] The hot strips produced from steels 3, 3a and 6 were subjected to batch annealing and were made available in this condition as flat steel products for further processing.

[0105] The flat steel product produced from steel 5 as hot-rolled strip was prepared for further processing after pickling.

[0106] Table 1 lists the chemical compositions of steels 1-6. The contents of the elements P, S, and N, which are present during production but are attributable to impurities, are given here because they are of particular importance for the quality of the steels produced according to the invention, and especially for steels 1-3a according to the invention, it must be ensured that the contents of these elements comply with the requirements of the invention.

[0107] The final thickness D of the flat steel products produced from steels 1 - 6 is given in Table 2. This means that for the steel strips produced from steels 1 and 4, the thickness D is given in the finished cold-rolled and AlSi-coated state, and for the hot-rolled steel strips produced from steels 2, 3, 3a, 5 and 6, the thickness is given after coiling (hot strip produced from steel 2) or after descaling (hot strip produced from steels 3, 3a, 5 and 6).

[0108] Likewise, Table 2 shows for steels 1 - 6 the result of the equation %Ti-48 / 14*%N, the ratio %Ti / %N, the content %Nrest of the nitrogen not bound by Ti, the result of the equation %Al-27 / 14*%Nrest, the ratio %Al / %N and the result of the equation %Al / / %N*14 / 27, where %Ti is the Ti content, %N is the N content and %Al is the Al content of the respective steel.

[0109] Each of the steels was alloyed with B, with the B contents being at least 0.001 mass%.

[0110] The inventive steels 1 - 3a each have Ti contents that are insufficient, or at best only just sufficient, to bind the N content present in the respective steel. The stoichiometric ratio %Ti / %N to be maintained for theoretically complete binding of the nitrogen present by Ti is 48 / 14 = 3.43. In the inventive steel 1, the %Ti / %N ratio is significantly below this value. Likewise, in the inventive steels 2, 3, and 3a, the %Ti / %N ratio is still below the stoichiometric ratio of 3.43. In any case, the %Ti / %N ratio for the inventive steels was less than 4. In contrast, all comparative steels 4 - 6 had a %Ti / %N ratio > 5.

[0111] To compensate for the low Ti contents, the Al content of the inventive steels 1-3a was increased to achieve AlN precipitation through the higher Al contents, i.e., through a higher precipitation pressure, and to avoid BN formation. The %Al / %N ratios of the inventive steels 1-3 are significantly higher than those of the comparative steels 4-6, each exceeding 15. They are thus also significantly above the stoichiometric %Al / %N ratio, which is 27 / 14 = 1.93. In the comparative variants 4-6, the %Al / %N ratio reaches a maximum of 12.3.

[0112] The melts composed according to the invention were cast into slabs in a conventional continuous caster. After being thoroughly heated to a preheating temperature "VWT" over a holding time "LIZ," these slabs were first pre-rolled in a conventional hot strip mill to form a pre-strip with a thickness "VBD." The pre-strips leaving the roughing mill at a temperature "VBT" were then finished hot-rolled in a continuous, conventional hot-rolling process to form hot strip with a hot strip thickness "WBD." The finished hot-rolled hot strips leaving the hot rolling mill were cooled to a coiler temperature (HT) of less than 650 °C, with a cooling rate (ABK) of at least 50 °C / s being set in the temperature range of 800 - 650 °C.

[0113] The exemplary process parameters "preheating temperature VWT", "holding time LIZ", "pre-strip temperature VBT", "pre-strip thickness VBD", "hot rolling end temperature WET", "cooling rate ABK", "coiler temperature HT" and "hot strip thickness WBD" set for the production and further processing of slabs made of steels 1 - 6 are given in Table 3, in addition to the temperature "Ar3" calculated according to the formula given above.

[0114] The hot-rolled strips produced from the inventive steel 1 and the comparative steel 4 were rolled to their final thickness "D" in cold rolling mills. The degree of cold rolling achieved through cold rolling is not a decisive factor. It is determined solely by the given hot-rolled strip thickness and the required cold-rolled strip thickness, so that cold rolling can be carried out according to the usual procedure in the state of the art. Cold rolling causes the strip to undergo plastic deformation, which results in significant material hardening and a reduction in further deformability. Therefore, after cold rolling, a recrystallization annealing is also carried out in a conventional manner. This softens the strip and makes it suitable for forming into a component.In the case of hot-dip coating, as in the example of the cold-rolled strip produced from steel 1, the annealing can be integrated into the conventional hot-dip coating process, which is usually carried out in a continuous process. Alternatively, a batch annealing can also be used. Likewise, an electrolytic coating can be performed instead of the hot-dip coating.

[0115] To determine the behavior of the flat steel products manufactured from the inventive steels 1-3a and from the comparative steels 4-6 during quenching and tempering or during a hot forming process as described above, samples of the flat steel products in question were subjected to a simulation of a conventional quenching and tempering or hot forming process. The samples were each heated to an austenitizing temperature "T_aust" that was approximately 60 °C higher than the Ac3 temperature of the respective steels 1-6. The austenitizing time required for heating and soaking at the austenitizing temperature T_aust was 7-10 minutes, including the heating time in a salt bath furnace. Following austenitizing, the samples were quenched in oil at an average cooling rate of 70-120 °C / s to room temperature.These process parameters correspond to the usual conditions encountered in practice when quenching and tempering components cold-formed from flat steel products made from steel grades 1-6, or when press-hardening such flat steel products into components. The austenitizing parameters are listed in Table 4.

[0116] After quenching, the samples were tempered at 170–200 °C for 20 minutes. This tempering corresponds both to a heat treatment typically performed at the end of quenching and tempering, and to the conditions prevailing during cathodic dip coating in a typical automotive painting process. Temperatures of 150–700 °C combined with holding times of 5–60 minutes are also common in industrial practice for tempering a component.

[0117] From the individual values ​​of the reduction in area at fracture ("absolute elongation in the thickness direction") ε(epsilon)3 determined as explained above, a linear regression calculation for ε(epsilon)3 as a function of the tensile strength was created for the inventive concept and the comparison concept. With a coefficient of determination of > 90%, statistically significant influences were obtained. The toughness improvement was defined as ΔBE = (ε(epsilon)3_Req. - ε(epsilon)3 Comp.) / ε(epsilon)3_Req. as a function of the tensile strength of both regression calculations. Req. means according to the invention, while com. means according to comparison. This value, like the respective determined value ε(epsilon)3 of the reduction in area at fracture, is listed as the value ΔBE in Table 4.

[0118] After these final heat treatment processes, the mechanical tensile test parameters "Young's modulus", "yield strength Rp0.2", "tensile strength Rm", "uniform elongation Ag", and "elongation at break A" were determined on the specimens produced from steels 1 - 6 in the manner described above, in accordance with DIN EN ISO 6892-1. For cold-rolled strip thicknesses ≤ 3 mm, the elongation at break A refers to specimen shape 2 with cross-sections 20 mm wide and an initial gauge length of 80 mm. For thicknesses > 3.0 mm, an initial gauge length of 50 - 65 mm (proportional tensile specimens) was used. The determination was carried out at three locations on each of the tested specimens in the longitudinal direction at room temperature. The results of these tests, averaged from the three measurements, are also summarized in Table 4.

[0119] Likewise, the inclusion state in the microstructure of the samples produced from steels 1 - 6 in the manner explained above was examined, firstly in the as-delivered condition, i.e. before quenching and tempering, and secondly after quenching and tempering. No significant differences in the non-metallic precipitates were found, so an average value was calculated from both measurements. The examination methods explained above were used for this purpose. These investigations confirmed that the samples made from the steels 1 - 3a alloyed according to the invention had a significantly reduced proportion of less than 150 area ppm of hard TiN, AIN and Al-based, oxide precipitates in their microstructures, the average particle size of which was 0.2 - 10 µm. Furthermore, the precipitates were homogeneously distributed across the strip thickness.The measuring area covered by the respective investigation, the corresponding microstructural parameters "number of TiN per cm 2"< , average diameter of the TiN precipitates "TiN-Ø", % share of TiN precipitates (including TiN particles as a conglomerate with softer particles) in the total sum of hard particles "TiN share+conglomerates" as well as the total sum of hard particles of the TiN, AIN and Al 2 O 3 precipitates and their conglomerates with softer particles "TiN, Al 2 O 3 , AIN+conglomerates" are listed in Table 5.

[0120] Furthermore, it was shown that the samples produced and processed according to the invention from the inventive steels 1 - 3a exhibit a reduced former austenite grain size compared to the non-inventive variants produced from steels 4 - 6, combined with a likewise reduced austenite grain size dispersion across the strip thickness, also averaged at three locations of 1 / 6, 1 / 3, and 1 / 2 across the strip thickness. The parameters determined in these investigations: mean diameter of the former austenite grains "KA=Ø former A-KG," standard deviation (relative to the sample size) of the diameter of the former austenite grains "Ks=σ former A-KG," and the derived grain size grade "KG grade" are summarized in Table 6. The KG grade is calculated by multiplying the mean former austenite grain size by the standard deviation of the diameter of the former austenite grain size.The smaller the grain size, the more favorable the effects on toughness and local elongation are. Toughness is known to improve with decreasing grain size. In addition, a smaller grain size variation ensures increased homogeneity of deformation behavior and thus a delayed onset of instability due to fracture reduction, as there are fewer local differences. Table 1 Steel C Si Mn Al Cr Nb Ti B Ca P*) S*) N*) V Cu Mon Ni According to the invention? 1 0,13 0,20 1,17 0,130 0,21 0,021 0,004 0,0021 0,0012 0,011 0,005 0,0059 0,001 0,04 0,026 0,09 YES 2 0,15 0,21 1,06 0,092 0,21 0,024 0,015 0,0023 0,0008 0,014 0,002 0,0046 0,006 0,04 0,009 0,05 YES 3 0,45 0,19 0,67 0,090 0,30 0,026 0,016 0,0027 0,0005 0,010 0,001 0,0048 0,002 0,01 0,005 0,02 YES 3a 0,47 0,27 1,27 0,081 0,32 0,025 0,013 0,0023 0,0005 0,008 0,001 0,0038 0,001 0,02 0,005 0,03 YES 4 0,21 0,27 1,17 0,048 0,21 0,001 0,034 0,0025 0,0004 0,015 0,001 0,0039 0,003 0,08 0,027 0,05 NO 5 0,41 0,22 1,16 0,036 0,41 0,001 0,029 0,0023 0,0005 0,017 0,003 0,0051 0,003 0,01 0,002 0,02 NO 6 0,38 0,19 1,25 0,036 0,31 0,001 0,029 0,0012 0,0011 0,020 0,002 0,0057 0,002 0,01 0,003 0,02 NO *) Impurities in mass%, remainder iron and other unavoidable impurities Table 2 Steel D %Ti-48 / 14*%N %Ti / %N %Nrest %Al-27 / 14*%Nrest %Al / %N %Al / %N*14 / 27 [mm] [%] [%] [%] 1 1,45 -0,0162 0,678 0,0047 0,1209 22,034 11,425 2 3,03 -0,0008 3,261 0,0002 0,0916 20,000 10,370 3 4,04 -0,0005 3,333 0,0001 0,0897 18,750 9,722 3a 4,02 -0,0004 3,333 0,0001 0,0808 21,316 11,053 4 2,20 0,0206 8,718 0,0000 0,0480 12,308 6,382 5 4,05 0,0115 5,686 0,0000 0,0360 7,059 3,660 6 3,10 0,0095 5,088 0,0000 0,0360 6,316 3,275 %Ti = Ti content, %N = N content, %Nrest = N content not bound by Ti, %Al = Al content of the respective steel Table 3 Steel VWT LIZ VBT VBD WET Ar3 ABK* HT WBD [°C] [min.] [°C] [mm] [°C] [°C] [°C / s] [°C] [mm] 1 1287 341,6 1084 37,4 869 808 61 579 3,30 2 1265 222,2 1093 39,6 892 807 76 592 3,03 3 1205 216,3 1072 40,3 861 758 53 612 4,04 3a 1261 224,0 1036 38,1 859 741 35 612 4,02 4 1273 236,9 1090 40,5 840 791 41 586 3,11 5 1260 247,6 1080 40,2 849 751 35 634 4,05 6 1259 359,4 1088 39,7 910 752 47 717 3,10 *: average cooling rate in the temperature range between 800 °C and 650 °C Table 4 Steel Ac3 T_aust Young's modulus Rp0.2 Rm Ag A ε(Epsilon)3 ΔBE [°C] [°C] [MPa] [MPa] [MPa] [%] [%] [%] [%] 1 888 940 193721 930 1183 3,6 5,3 59,9 7,5 2 881 940 195068 937 1213 3,5 6,1 58,5 7,8 3 815 875 173333 1515 2243 4,9 9,1 16,7 53,0 3a 803 870 192402 1555 2159 4,5 8,1 13,5 42,2 4 857 915 196419 1105 1455 3,8 6,8 43,5 -11,5 5 812 875 203333 1462 2188 3,9 5,0 8,0 -45,5 6 818 880 192333 1312 1862 4,0 8,7 27,5 -22,6 Table 5 Steel measuring area Number of TiN per cm 2< TiN-Ø TiN content + conglomerates TiN, Al 2 O 3 , AIN + conglomerates - [mm 2 ] [N / cm 2 ] [µm] [% share of area ppm] [Area ppm] 1 22,8 62 0,4 0 115 2 39,0 5221 1,0 24 121 3 39,1 1408 1,1 11 116 3a 39,2 3235 1,1 16 135 4 35,2 13450 1,2 79 326 5 35,9 13298 1,0 48 357 6 32,7 12502 1,2 63 373 Table 6 Steel Position KA= Ø former A-KG Ks= σ former A-KG KG quality [µm] [µm] [µm 2< ] 1 1 / 2 5,6 2,4 13,2 1 1 / 6 6,8 3,3 22,4 1 1 / 3 7,2 3,6 25,7 2 1 / 2 8,3 3,4 28,6 3 1 / 6 6,4 3,1 19,6 3 1 / 3 6,7 4,0 26,7 3 1 / 2 7,5 3,8 28,4 3a 1 / 6 7,1 3,3 23,4 3a 1 / 3 7,3 3,2 23,4 3a 1 / 2 6,9 3,5 24,2 4 1 / 2 9,1 6,0 54,5 5 1 / 6 7,7 4,3 32,7 5 1 / 3 7,5 5,7 42,5 5 1 / 2 8,5 4,5 38,1 6 1 / 2 8,2 4,7 38,6

[0121] The invention further relates to the embodiments of the invention described in the following sentences: 1. Component which is produced by forming from a sheet steel blank, - which consists of a steel which (in mass %) consists of C: 0,1 - 0,6 %, Mn: 0,1 - 2 %, Al: 0,05 - 0,2 %, Note: 0,01 - 0,06 %, B: 0,0005 - 0,005 %, Cr: 0,05 - 0,8 %, Si: up to 0.8%, Mon: up to 1.5%, Cu: up to 0.5%, Ni: up to 1.5%, V: up to 0.2%, REM: up to 0.05% Ti: up to 0.02%, Approx: up to 0.005%, Rest iron and unavoidable impurities, where the impurities include contents of up to 0.03% P, up to 0.03% S, up to 0.01% N, less than 0.05% Sn, less than 0.05% As and less than 0.05% Co, where the ratio %Al / %N*14 / 27 formed from the respective Al content %Al and the respective N content %N is ≥ 8, and - the component has a structure which consists of at least 95% by area of ​​martensite and the remainder of other structural components and in which, in a homogeneous distribution across the strip thickness, a maximum of 150 ppm by area of ​​particles is present, the mean circle-equivalent particle size of which is 0.2 - 10 µm and which consist of Al compounds on an oxide basis, of AIN, TiN or of conglomerates formed on the basis of these particles. 2. Component according to sentence 1, characterized by , that its Al content is 0.07 - 0.13 mass%. 3. Component according to one of the preceding sentences, characterized by , that its B content is 0.001 - 0.0035 mass%. 4. Component according to one of the preceding sentences, characterized by , that in its microstructure for the product KA x Ks of former austenite grain size KA, used in µm, and simple standard deviation Ks of the former austenite grain size, also used in µm and averaged over the one-sixth, one For the austenite grain sizes determined from the thickness of the respective wall section of the component, the following applies: KA×Ks<30μm2. 5. Component according to one of the preceding sentences, characterized by , that in the quenched and tempered or press-hardened condition it has a tensile strength Rm of 1000 - 2500 MPa. 6. Component according to one of the preceding sentences, characterized by , that in the tempered or press-hardened condition it has a reduction in area at fracture in the thickness direction ε3 of 10 - 65%. 7. Component according to one of the preceding sentences, characterized by , that it is a hollow tube formed from a longitudinally welded blank of a component according to one of claims 0 - 3. 8. Component according to one of the preceding sentences, characterized by , that it is a body structural part, a stabilizer for a vehicle suspension, a steering shaft or a drive shaft of motor vehicles. 9. A process for producing a component with a structure consisting of at least 95% martensite and the remainder of other structural components, wherein A) a hot strip is produced by a) steel is melted which consists of (in mass%) C: 0,1 - 0,6 %, Mn: 0,1 - 2%, Al: 0,05 - 0,2 %, Note: 0,01 - 0,06 %, B: 0,0005 - 0,005 %, Cr: 0,05 - 0,8 %, Si: up to 0.8%, Mon: up to 1.5%, Cu: up to 0.5%, Ni: up to 1.5%, V: up to 0.2%, REM: up to 0.05% Ti: up to 0.02%, Approx: up to 0.005%, Rest iron and unavoidable impurities, where the impurities include contents of up to 0.03% P, up to 0.03% S, up to 0.01% N, less than 0.05% Sn, less than 0.05% As and less than 0.05% Co, and where the ratio %Al / %N*14 / 27 formed from the respective Al content %Al and the respective N content %N is ≥ 8, b) the molten steel is cast into a precursor product, namely a slab, a thin slab or a cast strip, c) the preliminary product is, if necessary, heated to a preheating temperature of 1100 - 1350 °C, (d) the precursor product is hot-rolled into a hot-rolled strip with a thickness of 1-16 mm, with hot rolling being terminated at a final hot-rolling temperature which is at least 50 °C and at most 150 °C higher than the Ar3 temperature of the steel, e) the hot strip obtained is cooled to a coiling temperature of 450 - 700 °C, with the cooling taking place in the temperature range of 800 - 650 °C at a cooling rate of 20 - 200 °C / s, f) the hot strip cooled to the coiling temperature is coiled into a coil and cooled to room temperature in the coiled state, and g) optionally: the hot-rolled strip cooled in the coiled state is pickled and h) optionally: batch annealing is carried out at a core temperature of the hot strip of 500 - 720 °C for a period of 5 - 50 hours; B) optionally producing a cold strip from the hot strip obtained by i) the hot-rolled strip is cold-rolled to a cold-rolled strip with a thickness of 0.5 to 3.5 mm in one or more cold-rolling steps; j) optionally: annealing of the cold-rolled strip in a batch annealing line or in a continuous annealing line; C) a component is formed from the hot strip or the cold strip optionally produced from it by k) a blank is cut from the hot-rolled or cold-rolled strip and, according to alternative 1: I.1) the blank is heated to an austenitizing temperature that is no more than 100 °C lower than the Ac3 temperature of the steel from which the hot-rolled or cold-rolled strip is produced and no more than 950 °C, I.2) within 1 - 20 s after the end of the heating to the austenitizing temperature, the blank is placed in a cooled hot-forming tool in which the blank is hot-formed into the component, and I.3) the component is press-hardened by accelerated cooling at a cooling rate of 30 - 120 °C / s until the martensite start temperature of the steel from which the respective hot-rolled or cold-rolled strip is reached, so that the component acquires a fully martensitic structure, or according to alternative 2: m.1) the blank is cold-formed into the component will, m.2) the cold-formed component is heated to an austenitizing temperature which is no more than 100 °C lower than the Ac3 temperature of the steel from which the hot-rolled or cold-rolled strip is produced and no more than 950 °C, and m.3) the component heated to the austenitizing temperature is cooled at an accelerated rate of 30 - 120 °C / s until the martensite start temperature of the steel from which the respective hot-rolled or cold-rolled strip is made is reached, so that the component acquires a fully martensitic structure; and n) optionally: the component obtained after steps I.1 - I.3 or m.1 - m.3 is tempered at temperatures of 150 - 700 °C for an annealing time of 5 - 60 min. 10. Procedure according to sentence 9, characterized by , that the coiling temperature is 550 - 630 °C. 11. Procedure under sentence 9 or 10, characterized by , that the total time provided for heating the blank in step 1.1 or for heating the component in step m.2 is 1 second to 20 minutes, whereby the total time includes the heating time required for heating to the respective austenitizing temperature.

Claims

1. Steel flat product consisting of a steel which (in mass %) consists of C: 0,1 - 0,6 %, Mn: 0,1 - 2%, Al: 0,05 - 0,2 %, Note: 0,01 - 0,06 %, B: 0,0005 - 0,005 %, Cr: 0,05 - 0,8 %, Si: up to 0.8%, Mon: up to 1.5%, Cu: up to 0.5%, No: up to 1.5%, V: up to 0.2%, REM: up to 0.05% Ti: up to 0.02%, Approx: up to 0.005%, the remainder being iron and unavoidable impurities, - the impurities including contents of up to 0.03% P, up to 0.03% S, up to 0.01% N, less than 0.05% Sn, less than 0.05% As and less than 0.05% Co, - the ratio %Al / %N*14 / 27 formed from the respective Al content %Al and the respective N content %N being ≥ 8, and - the flat steel product having a structure in which, in a homogeneous distribution across the strip thickness, a maximum of 150 area ppm of particles are present, the mean circle-equivalent particle size of which is 0.2 - 10 µm and which consist of oxide-based Al compounds, of AIN, TiN or of conglomerates formed on the basis of these particles.

2. Flat steel product according to claim 1, characterized in that its Al content is 0.07 - 0.13 mass%.

3. Flat steel product according to one of the preceding claims, characterized in that its B content is 0.001 - 0.0035 mass%.

4. Flat steel product according to one of the preceding claims, characterized in that the total number of particles falling within the particle size range of 0.2 - 10 µm consists of TiN-based precipitates and their mixed forms and is reduced to less than 30% of the particles in the size class 0.2-10 µm present in the structure of a component formed from the flat steel product.

5. Flat steel product according to one of the preceding claims, characterized in that which has a metallic protective layer based on zinc or aluminum.

6. Flat steel product according to one of the preceding claims, characterized in that the particles are selected from AIN, Al2O3 and Al2O3-based spinels as well as TiN particles and conglomerates formed on the basis of the said particles.

7. Flat steel product according to claim 6, characterized in that The particles have a Mohs hardness of about 9.

8. A process for producing a steel flat product, wherein the steel flat product is a hot-rolled strip or optionally a cold-rolled strip, wherein A) a hot-rolled strip is produced by a) melting steel which consists of (in mass%) C: 0,1 - 0,6 %, Mn: 0,1 - 2%, Al: 0,05 - 0,2 %, Note: 0,01 - 0,06 %, B: 0,0005 - 0,005 %, Cr: 0,05 - 0,8 %, Si: up to 0.8%, Mon: up to 1.5%, Cu: up to 0.5%, No: up to 1.5%, V: up to 0.2%, REM: up to 0.05% Ti: up to 0.02%, Approx: up to 0.005%, the remainder being iron and unavoidable impurities, - the impurities including contents of up to 0.03% P, up to 0.03% S, up to 0.01% N, less than 0.05% Sn, less than 0.05% As and less than 0.05% Co, and - the ratio %Al / %N*14 / 27 formed from the respective Al content %Al and the respective N content %N is ≥ 8, b) the steel melt is cast to form a precursor, namely a slab, a thin slab or a cast strip, c) the precursor, if necessary, is heated to a preheating temperature of 1100 - 1350 °C, d) the precursor is hot-rolled to form a hot-rolled strip with a thickness of 1 - 16 mm, the hot rolling being terminated at a final hot-rolling temperature which is at least 50 °C and at most 150 °C higher than the Ar3 temperature of the steel, e) the hot strip obtained is cooled to a coiling temperature of 450 - 700 °C,wherein the cooling takes place in the temperature range of 800 - 650 °C at a cooling rate of 20 - 200 °C / s, f) the hot strip cooled to the coiling temperature is coiled into a coil and cooled to room temperature in the coiled state, and g) optionally: the hot strip cooled in the coiled state is pickled and h) optionally: batch annealed at a core temperature of the hot strip of 500 - 720 °C for a period of 5 - 50 hours; B) wherein a cold strip is optionally produced from the hot strip obtained by i) cold rolling the hot strip into a cold strip with a thickness of 0.5 - 3.5 mm in one or more cold rolling steps; j) optionally: annealing the cold strip in a batch annealing line or in a continuous annealing line.

9. Method according to claim 8, characterized in that the coiling temperature is 550 - 630 °C.

10. Method according to claim 8 or 9, characterized in that the final rolling temperature is at least 60°C and at most 130°C higher than the Ar3 temperature.

11. Method according to one of the preceding claims, characterized in that a cold strip is produced from the hot strip and the cold strip is annealed in a bell annealer at 500-720°C.

12. Method according to one of the preceding claims, characterized in that a cold strip is produced from the hot strip and the annealing of the cold strip takes place in a continuous annealing line with integrated hot-dip coating.

Images