Hot-rolled flat steel product and method for producing a flat steel product of this type

Abstract

The invention relates to a hot-rolled flat steel product having a tensile strength Rm of > 950 MPa, the flat steel product being made of a steel having the following composition in % by weight: C: 0.08 to 0.20, preferably 0.08 to 0.13; Si: 0.2 to 0.6, preferably 0.2 to 0.5; Mn: 1.6 to 2.5, preferably 1.9 to 2.2; N: up to 0.03, preferably up to 0.01; P: up to 0.10, preferably up to 0.02; S: up to 0.010, preferably up to 0.002; Al: up to 0.10, preferably up to 0.05; Cu: up to 0.50, preferably up to 0.05; Cr: up to 1.0, preferably 0.3 to 0.8; Mo: up to 0.50, preferably 0.05 to 0.35, particularly preferably 0.20 to 0.30; Ni: up to 0.50, preferably up to 0.05; Ti: up to 0.20, preferably 0.01 to 0.06, particularly preferably 0.02 to 0.03; V: up to 0.30, preferably up to 0.07; Nb: up to 0.08, preferably up to 0.04; remainder iron, including usual steel-accompanying smelting-related impurities, wherein the steel has a microstructure composed of the following proportions: more than 60% by volume but less than 90% by volume of a matrix consisting of bainite and ferrite, and 10 to 40% by volume of a second phase consisting of tempered martensite, on the one hand, and of MA constituent, on the other hand, wherein the proportion by volume of the tempered martensite makes up at least half of the proportion by volume of the second phase, i.e. is ≥ 50% by volume of the second phase, and the austenite in the MA constituent makes up a proportion of ≤ 10% by volume based on the total microstructure made of the matrix and the second phase. The invention also relates to a corresponding method for producing a hot-rolled flat steel product.

Description

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

[0002] The invention is based on a hot-rolled flat steel product with a tensile strength Rm > 950 MPa and a method for producing such a hot-rolled flat steel product.

[0003] Demand for high-strength steel sheets with a tensile strength of at least 950 MPa and improved fatigue strength and formability has increased in recent years. High-strength steel sheets have been used, for example, to manufacture chassis parts, bumper components, suspension parts, and impact beams for vehicles. They reduce vehicle body weight and thus fuel consumption, and suppress deformation of passenger compartments during collisions, thus improving safety. The high strength of the steel sheets, combined with their improved fatigue and formability, makes them particularly suitable for fatigue-stressed components, where the high strength of the steel allows for the use of thinner thicknesses.

[0004] The document EP 3492 611 B1 describes a hot-rolled steel flat product with a tensile strength Rm > 950 MPa, which consists of a steel with the following composition in weight %: C: 0.09, Si: 0.18, Mn: 1.8, Al: 0.035, Cu: 0.15, Cr: 0.75, Mo: 0.10, Ni: 0.15, Ti: 0.045, V: 0 and Nb: 0.030, the remainder being iron, including usual steel-accompanying smelting-related impurities, wherein the steel has a microstructure comprising bainite in a bainite matrix with an area ratio of 70% or more, the difference being as follows: martensite with an area ratio of 30% or less and optionally ferrite with an area ratio of 20% or less, wherein a major part of the bainite is upper bainite and islands of martensite are embedded in the bainite matrix.

[0005] For further processing of the flat steel product, for example into the aforementioned vehicle components, it is also important that the flat steel product has good characteristics in terms of elongation at break and hole expansion.

[0006] The object of the invention is to provide a hot-rolled flat steel product with a tensile strength Rm > 950 MPa and a method for producing such a hot-rolled flat steel product, in which the flat steel product can be easily further processed into steel products.

[0007] According to the invention, the object is achieved by the features of the independent claims. Preferred embodiments of the invention are specified in the subclaims, each of which may represent an aspect of the invention individually or in combination.

[0008] The hot-rolled flat steel product according to the invention with a tensile strength Rm > 950 MPa is intended to consist of a steel with the following composition in weight %:

[0009] C: 0.08 to 0.20, preferably 0.08 to 0.13,

[0010] Si: 0.2 to 0.6, preferably 0.2 to 0.5,

[0011] Mn: 1.6 to 2.5, preferably 1.9 to 2.2,

[0012] N: up to 0.03, preferably up to 0.01, P: up to 0.10, preferably up to 0.02, S: up to 0.010, preferably up to 0.002, Al: up to 0.10, preferably up to 0.05, Cu: up to 0.50, preferably up to 0.05, Cr: up to 1.0, preferably 0.3 to 0.8, Mo: up to 0.50, preferably 0.05 to 0.35, particularly preferably 0.20 to 0.30 Ni: up to 0.50, preferably up to 0.05,

[0013] Ti: up to 0.20, preferably 0.01 to 0.06, particularly preferably 0.02 to 0.03 V: up to 0.30, preferably up to 0.07, Nb: up to 0.08, preferably up to 0.04,

[0014] The remainder is iron, including impurities normally associated with the smelting process, the steel having a microstructure composed of the following proportions: more than 60 volume% but less than 90 volume% of a matrix of bainite and ferrite, 10 - 40 volume% of a second phase of tempered martensite on the one hand, and of MA constituent on the other, where the volume fraction of tempered martensite makes up at least half the volume fraction of the second phase, i.e. > 50 volume% of the second phase, and the austenite in the MA constituent makes up < 10 volume% based on the total microstructure consisting of matrix and second phase. The MA constituent is a microstructure component consisting of martensite and austenite MA (martensite-austenite constituent). The martensite of the MA constituent is a different martensite than the tempered martensite of the second phase and can therefore also be called untempered martensite.

[0015] The hot-rolled flat steel product according to the invention differs primarily from the hot-rolled flat steel product described in the aforementioned document EP 3492 611 B1 in that the second phase contains a proportion of tempered martensite (aM), which has a positive effect on the further processability of the flat steel product. The absolute proportion of tempered martensite (aM) is 5% to 40% by volume.

[0016] According to a preferred embodiment of the invention, both the tempered martensite (aM) and the martensite and austenite (MA) of the second phase are each embedded in the matrix of bainite and ferrite in the form of islands. It is preferably provided that the embedded islands of martensite and austenite (MA) have an area-weighted average diameter D that is < 1.2 pm, preferably < 1.0 pm, i.e. D < 1.2 pm, preferably D < 1.0 pm. The second phase has greater strength and lower formability than the matrix. As a result, with high local forming, stress concentrations occur in the interfaces between the second phase and matrix, which lead to crack initiation and premature failure. If the islands are small in diameter, failure will only occur later in the forming process and the local formability of the flat steel product is consequently increased.High local formability is necessary for a high hole expansion ratio.

[0017] It is advantageous to include a detectable proportion of austenite in the MA constituent. Thus, the austenite content in the MA constituent is > 0 volume% relative to the overall microstructure.

[0018] According to a further preferred embodiment of the invention, the proportion of austenite relative to the overall structure, which consists of matrix and second phase, is less than 5 volume% (<5 volume%). The austenite is transformed into martensite during forming. This transformation can contribute positively to uniform elongation due to the simultaneous increase in volume. However, the transformed martensite exhibits particularly high strength and thus significantly reduces local formability. Comparatively low proportions of austenite are therefore advantageous for achieving a balanced ratio of elongation at fracture and hole expansion.

[0019] According to yet another preferred embodiment of the invention, the grain boundary misorientation index (KMI) in the matrix of bainite and ferrite is at least 0.5. In other words, for the matrix of bainite and ferrite, the KMI is > 0.5. The basic strength and formability can be positively influenced by the microstructure of the matrix by adjusting a sufficiently high proportion of fine bainite. The KMI represents a useful indicator for the bainite ratio in the matrix. Details can be found in the following scientific article: Skemer, P., Katayama, I., Jiang, Z., & Karato, SI: 'The misorientation index: Development of a new method for calculating the strength of lattice-preferred orientation.' Tectonophysics, 411 (1-4), 157-167 (2005).

[0020] In particular, the hot-rolled flat steel product has an elongation at break A5 > 11% or a hole expansion ratio LA > 18%.

[0021] Characteristic values ​​such as elongation at break and hole expansion ratio (LA) are established parameters for describing cold formability. To describe cold formability, it is necessary to consider both global and local formability. For the purposes of the invention, the elongation at break parameter is used as a representative of predominantly global formability, and the hole expansion ratio (LA) is used as a representative of predominantly local formability. The technical (percentage) values ​​are specified.

[0022] It is preferably provided that the hot-rolled flat steel product has an elongation at break A5 > 11% and a hole expansion ratio LA > 18%.

[0023] The hot-rolled flat steel product has in particular a yield strength Rpo,2 s

[0024] 720 MPa.

[0025] Furthermore, it is preferably provided that when shearing with a cutting gap of 8% - 12%, a cut edge is produced that is free of fracture cracks and / or that when tearing a tensile specimen, a fracture edge is created that is free of fracture cracks. Accordingly, the flat steel product is designed such that when shearing with a cutting gap of 8% - 12%, a cut edge can be produced that is free of fracture cracks and / or that when tearing a tensile specimen made from the flat steel product, a fracture edge is created that is free of fracture cracks. In the case of fracture cracks, cracks run parallel to the surface of the flat product and have a length that (i) is typically in the range of % to % based on the thickness of the flat product and / or (ii) is typically between 1 mm and several centimeters long. If no such crack is visible on the edge, the edge is free of fracture cracks.

[0026] The method according to the invention for producing the hot-rolled flat steel product with a tensile strength Rm > 950 MPa comprises the following steps:

[0027] Providing a steel slab with the following composition in % by weight: C: 0.08 to 0.20, preferably 0.08 to 0.13, Si: 0.2 to 0.6, preferably 0.2 to 0.5, Mn: 1.6 to 2.5, preferably 1.9 to 2.2, N: up to 0.03, preferably up to 0.01, P: up to 0.10, preferably up to 0.02, S: up to 0.010, preferably up to 0.002, Al: up to 0.10, preferably up to 0.05, Cu: up to 0.50, preferably up to 0.05, Cr: up to 1.0, preferably 0.3 to 0.8, Mo: up to 0.50, preferably 0.05 to 0.35, particularly preferably 0.20 to 0.30 Ni: up to 0.50, preferably up to 0.05, Ti: up to 0.20, preferably 0.01 to 0.06, particularly preferably 0.02 to 0.03, V: up to 0.30, preferably up to 0.07 and Nb: up to 0.08, preferably up to 0.04, balance iron, including usual steel-accompanying impurities caused by melting,

[0028] Heating the steel slab to an austenitizing temperature in the range of 1100 °C to 1400 °C;

[0029] Hot rolling to produce a hot-rolled steel strip product of a desired thickness at a temperature in the range from the final rolling temperature EWT to 1300 °C, the final rolling temperature being in the range from 800 °C to 960 °C, preferably from 850 °C to 930 °C; subsequent rapid cooling of the hot-rolled steel strip product to a coiling temperature of 410 °C to a maximum of 650 °C, preferably from 450 °C to a maximum of 550 °C, in a first cooling phase with a cooling rate of on average > 50 K / s in at least one time period of at least 2 s of the first cooling phase; subsequent slow cooling of the product in a second cooling phase during and / or after coiling with an average of < 50 K / h in at least one time period of at least 4 h of the second cooling phase; optionally stretching or stretch bending or skin passing; and optional heat treatment in the form of tempering at a temperature T in the range of 150 °C to 700 °C.

[0030] With the above-mentioned production parameters, self-tempering during cooling of the hot-rolled steel strip product, optionally supplemented by subsequent tempering, produces a significant proportion of tempered martensite in the microstructure of the resulting flat steel product, ranging from 5% to 40% by volume. At the same time, embedded islands of MA constituent with a particularly small diameter are formed. Both of these factors have a positive effect on the further processability of this flat steel product.

[0031] It is preferably provided that the heat treatment is carried out as continuous annealing or alternatively as batch annealing.

[0032] According to a preferred embodiment of the method according to the invention, the duration t and the temperature T of the heat treatment in the form of tempering are as follows:

[0033] 7.5 < HJP < 17.0, with HJP = T(20+Iog t) / 1,000, where time t is given in h and temperature T in K and log is the natural logarithm.

[0034] Furthermore, the invention relates to the use of an above-mentioned hot-rolled flat steel product and / or a hot-rolled flat steel product produced according to the above-mentioned method in the automotive industry or vehicle construction industry.

[0035] In the following, the significance of the individual chemical elements in the composition of the steel will be discussed, whereby the use of the term "up to" in the definition of the content ranges, such as 0.01 to 1 wt%, means that the key values ​​- in the example 0.01 and 1 - are included, i.e. 0.01 < X < 1.

[0036] Carbon C: Required for the formation of carbides, especially in conjunction with the so-called microalloying elements Nb, V, and Ti. It promotes the formation of martensite and bainite, stabilizes austenite, and generally increases strength. Higher C contents impair welding properties and lead to a reduction in formability, which is why a maximum content of 0.20 wt.% is specified. To achieve sufficient material strength, a minimum addition of 0.08 wt.% is required.

[0037] Silicon (Si): Is one of the elements that enables the cost-effective increase in steel strength through solid solution strengthening. However, Si reduces the surface quality of the hot strip by promoting firmly adhering scale on the reheated slabs, which can only be removed with great effort or insufficiently at high Si contents. Therefore, the Si content is limited to a maximum of 0.6 wt.%. A lower limit of 0.2 wt.% is considered reasonable for the effectiveness of Si.

[0038] Manganese Mn: Stabilizes austenite, increases strength and toughness, and extends the temperature window for hot rolling below the recrystallization stop temperature. Higher contents of > 2.5 wt% Mn increase the risk of center segregation, which significantly reduces ductility and thus product quality. Lower contents of < 1.6 wt% do not allow the required strength and toughness to be achieved while maintaining the desired moderate analysis costs. Therefore, the Mn content is set at 1.6 to 2.5 wt%.

[0039] Nitrogen N: Is a byproduct of steel production. Even at high temperatures, nitrogen forms nitrides with aluminum, niobium, and titanium. The nitrides formed at high temperatures are coarse and therefore reduce the ductility and toughness of the flat steel product. Furthermore, these alloying elements are no longer available for the subsequent formation of small, highly effective precipitates in terms of strength. For the reasons stated above, the nitrogen content is limited to a maximum of 0.03% by weight.

[0040] Phosphorus P: Is a trace element from iron ore and is dissolved in the iron lattice as a substitution atom. Phosphorus increases hardness through solid solution strengthening and improves hardenability. However, attempts are generally made to reduce the phosphorus content as much as possible because, among other things, it is highly susceptible to segregation and significantly reduces toughness. The accumulation of phosphorus at grain boundaries can cause cracks along the grain boundaries during hot rolling or forming of the flat steel product. For the reasons stated above, the phosphorus content is limited to a maximum of 0.10% by weight.

[0041] Sulfur S: Like phosphorus, it is bound as a trace element in iron ore. It is generally undesirable in steel because it leads to unwanted inclusions of MnS, which impair elongation and toughness properties. Therefore, efforts are made to minimize the amount of sulfur in the melt and, if necessary, to convert the elongated inclusions into a more favorable geometric shape through a so-called Ca treatment. For the reasons stated above, the sulfur content is limited to a maximum of 0.010% by weight.

[0042] Aluminum (AI): Used for deoxidation in the steelmaking process. The amount of AI used depends on the process. Therefore, no minimum Al content is specified. An Al content greater than 0.10 wt.% significantly impairs casting performance in continuous casting. This results in increased casting effort. Therefore, the AI ​​content is set at a maximum of 0.10 wt.%.

[0043] Copper (Cu): Can optionally be added to the steel flat product according to the invention to increase hardenability. Cu can, in particular, cause solid solution strengthening and contribute to precipitation hardening. If added in excessive amounts, Cu impairs weldability and the toughness of the heat-affected zone (HAZ). Therefore, the upper limit for Cu is set at 0.50 wt%.

[0044] Chromium Cr: As an optional alloying element, Cr improves strength, reduces the corrosion rate, and delays ferrite and pearlite formation. The maximum content is set at 1.0 wt.%, as higher contents result in a deterioration in ductility. For sufficient effectiveness, a content of more than 0.3 wt.% is recommended, resulting in a preferred content of 0.3 to 0.8 wt.%.

[0045] Molybdenum Mo: As an optional alloying element, Mo increases hardenability and reduces the critical cooling rate, thus promoting the formation of fine, bainitic structures. Furthermore, even the use of small amounts of Mo delays the coarsening of fine precipitates, which should be as fine as possible to increase the strength of micro-alloyed structures. A minimum content of 0.05 wt.% is recommended for sufficient effectiveness. The use of this element is limited to a maximum of 0.50 wt.% for cost reasons.

[0046] Nickel Ni: The optional use of even small amounts of Ni promotes ductility while maintaining strength. Due to its comparatively high cost, the Ni content is limited to a maximum of 0.5 wt%.

[0047] Titanium Ti: Acts as a carbide former, resulting in grain refinement, thereby simultaneously improving strength, toughness, and elongation properties. Ti contents above 0.20 wt.% impair ductility and hole expansion due to the formation of very coarse, primary TiN precipitates, which is why a maximum content of 0.20 wt.% is specified.

[0048] Vanadium V: Contributes to strength and high formability through precipitation strengthening and the promotion of fine bainite. In this alloy concept, the addition of vanadium is not mandatory. The vanadium content is limited to a maximum of 0.30 wt.% for cost reasons.

[0049] Niobium Nb: The addition of niobium to the alloy has a grain-refining effect, particularly through the formation of carbides during the rolling process, which simultaneously improves strength, toughness, and elongation properties. In addition, after the phase transformation in the range of approximately 550 °C - 650 °C, very fine precipitates can form, which contribute significantly to the strength of the product. At contents above 0.08 wt.%, saturation occurs, which is why a maximum content of less than or equal to 0.08 wt.% is recommended.

[0050] The invention is explained below by way of example with reference to the attached tables and drawings, wherein the features presented below can represent an aspect of the invention both individually and in combination. They show:

[0051] Table 1 shows the respective chemical composition of the alloys A to E investigated.

[0052] Table 2 shows for the individual examples # 1 to 11 (a) the process parameters during production, (b) the microstructure properties, (c) the resulting product characteristics and (d) a classification into the categories example (inv.) and counterexample (comp.) with regard to the invention.

[0053] In the process parameters, the final rolling temperature is designated as EWT, the coiling temperature as HT, and the HJT value for describing the tempering annealing as HJT. If no HJT value is specified (notation: thus, no heat treatment in the form of tempering annealing was carried out. In the microstructure description, [F+B] indicates the structural proportion of a matrix of bainite and ferrite in volume %, [aM+MA] the structural proportion of a second phase of tempered martensite aM on the one hand, and the MA constituent consisting of martensite and austenite MA on the other hand in volume %, KMI the grain boundary misorientation index (as known from the aforementioned article by Skemmer et al.) in the matrix of bainite and ferrite, the ratio aM / [aM+MA] the relative structural proportion of the tempered martensite aM in the second phase [aM+MA], the proportion of austenite based on the total structure consisting of matrix and second phase is AA and the mean diameter of the MA islands is 0MA. The characteristic values ​​are given as Rm, the tensile strength Rpo,2 as Rp0,2, the elongation at break A5 as A5, the hole expansion ratio as LA and the fracture cracks as BA.

[0054] To determine the microstructural proportions of the second-phase components, i.e., tempered martensite aM on the one hand and martensite and austenite MA on the other, metallographic longitudinal sections are etched with 3% nitrate acid and examined in a scanning electron microscope (SEM). At least three secondary electron (SE) images are taken in a range between % and % sheet thickness at a magnification of 4,000:1. In the SE images, MA appears as a raised microstructure component with a uniform surface texture.

[0055] In contrast, aM appears as a raised microstructure component with a distinct substructure of lath structures and carbides with typical diameters of 50 - 200 nm - see Figure 1.

[0056] The area fractions and mean grain size of MA and aM are determined using a suitable image analysis algorithm (e.g., with a trainable classifier such as WEKA segmentation in ImageJ). The mean area fractions of MA and aM are determined from the results of at least three SE images. The mean Feret diameter of the grains is calculated for the mean grain size of MA and aM. For both the area fractions and the grain size, only grains with a minimum Feret diameter of 100 nm are considered.

[0057] While Examples #5 and 7 to 11 serve as counterexamples, Examples #1 to 4 and 6 represent individual embodiments of the invention.

[0058] Examples 1 and 2 are based on alloy A, with the production of the samples differing essentially in that in example 2, heat treatment in the form of tempering was carried out, whereas in example 1, this was not the case. The same applies to examples 3 and 4: These are based on alloy B, with the production of the samples differing essentially in that in example 4, heat treatment in the form of tempering was carried out, whereas in example 3 this was not the case. The samples of examples 1 to 4 all exhibit the "correct" microstructure properties and excellent product characteristics.

[0059] Examples 5 and 6 are based on alloy C, whereby the production of the samples differs essentially in that in example 6 a heat treatment in the form of tempering was carried out and in example 5 not. While the sample of example 6 shows the "right" microstructure properties and excellent product characteristics, in the non-tempered sample of example 5 the volume fraction of the tempered martensite (with 0.3 times the volume fraction of the total second phase) is too low, so that as a result the elongation at break A5 is quite low at 9.6%, which can be seen as an indicator of a poorer

[0060] Cold formability is to be considered.

[0061] Examples 7 and 8 are based on alloy D, with the production of the samples differing essentially in that Example 8 underwent heat treatment in the form of tempering annealing, whereas Example 7 did not. In contrast to all other examples, the production process for both examples prior to heat treatment consisted of austenitizing and quenching to room temperature. While the untempered sample of Example 7 has a completely martensitic microstructure, the tempered sample of Example 8 has a microstructure consisting entirely of tempered martensite. Examples 7 and 8 are therefore counterexamples in which - due to the different microstructure - the elongation at break A5 is still quite low at 7.4% (Example 7) and 6.7% (Example 8), which can be seen as an indicator of poorer cold formability.

[0062] Examples 9 and 10 are based on alloy E, with the production of the samples differing essentially in that in Example 10, heat treatment in the form of tempering was carried out, whereas in Example 9, this was not the case. In both examples, the coiling temperature HT of 408 °C was selected, below a threshold value of 410 °C. As a result, the samples from both Examples 9 and 10 exhibit a completely bainitic-ferritic microstructure. Examples 9 and 10 are therefore counterexamples in which - due to the different microstructure - the elongation at break A5 is also quite low at 8.8% (Example 9) and 10.5% (Example 10), which can be seen as an indicator of poorer cold formability.

[0063] The last example, Example 11, is again based on Alloy A, whereby the sample underwent a heat treatment in the form of tempering during production - similar to the sample in Example 2. In contrast to Example 2, Example 11 was coiled at a very low coiling temperature HT of 73 °C, and the HJP value of 17.2 was selected during tempering. As a result, the sample in Example 11 exhibits a microstructure of tempered martensite aM with coarse carbides. The product parameters of tensile strength Rm, yield strength Rpo,2, and elongation at fracture A5 are consistently good. However, fracture cracking occurs, so Example 11 should be considered a counterexample.

[0064] Finally, it should be noted that the listed examples 1 to 4 and 6 have an elongation at break of A5 > 13% and a hole expansion ratio LA > 20%.

[0065] The following outlines advantageous procedures for producing samples or flat steel products and their effects on the samples / flat steel products (Feature -> Effect). Regarding hot rolling, the following applies:

[0066] Heat a steel slab with a thickness of 250 mm to 1230 °C — >

[0067] Austenitizing and dissolving precipitations of microalloying elements (Ti, Nb),

[0068] Pre-rolling in 5 passes to 45 mm,

[0069] Finish rolling in 7 passes with starting temperature 1000 - 1130 °C, final rolling temperature EWT 850 - 930 °C — > high dislocation density for fine structure,

[0070] Final thickness 3.5 mm,

[0071] Cooling with water in 5 s to 530 - 590 °C with > 50 K / s and in 12 s to 480 - 530 °C — > the proportion and properties of the microstructure matrix as well as the carbon content of the second phase are adjusted, and

[0072] Coiling at 480 - 520 °C, cooling to room temperature RT in the steel coil with a cooling rate < 50 K / h — > the composition of the second phase is set, which depends on the carbon content and the cooling rate.

[0073] Regarding the optional heat treatment:

[0074] Implementation as continuous annealing or batch annealing — > Increasing the hole expansion ratio LA by increasing the proportion of tempered martensite aM — > Increasing the yield strength by carbon diffusion into the dislocations,

[0075] Temperature range: 100 °C - 700 °C — > below this no effect, above this there is disintegration of the structure, and as a rule: the duration decreases with increasing temperature; the following applies: 7.5 < HJP < 17.0, where HJP = T(20+logt) / 1,000, where t is in h and T is in K and log is the natural logarithm (in German also In) — > below this no effect, above this there is disintegration of the structure and / or coarsening of the carbides, which is detrimental with regard to fracture cracking BA.

[0076] In the following, the production of a cut edge on the flat steel product / sample by shear cutting with a specified cutting gap and its evaluation with regard to fracture cracks will be discussed in brief with reference to Figures 2a and 2b:

[0077] Fracture cracks BA occur during shear cutting at typical cutting gaps of 8 - 15%, the corresponding cracks run parallel to the surface of the flat product and have a length that (i) is typically in the range of % to % in relation to the sheet thickness and / or (ii) is typically in the range of 1 mm to several centimeters,

[0078] The tendency to fracture cracking BA can be demonstrated by tensile testing, as shown in Figures 2a and 2b. The corresponding cracks run parallel to the surface; at least one crack, usually several, is present, and the length of the cracks is 0.25 (denoted by B) to 1 (denoted by A) times the specimen width (tensile test according to ISO 6892-1).

[0079]

[0080] Table 1 : chemical composition of the alloys A to E investigated

[0081] Table 2: Process parameters during production, microstructure properties and resulting product characteristics for the individual examples

Claims

Patent claims 1. Hot-rolled flat steel product with a tensile strength Rm > 950 MPa, which consists of a steel with the following composition in weight %: C: 0.08 to 0.20, preferably 0.08 to 0.13, Si: 0.2 to 0.6, preferably 0.2 to 0.5, Mn: 1.6 to 2.5, preferably 1.9 to 2.2, N: up to 0.03, preferably up to 0.01, P: up to 0.10, preferably up to 0.02, S: up to 0.010, preferably up to 0.002, Al: up to 0.10, preferably up to 0.05, Cu: up to 0.50, preferably up to 0.05, Cr: up to 1.0, preferably 0.3 to 0.8, Mo: up to 0.50, preferably 0.05 to 0.35, particularly preferably 0.20 to 0.30 Ni: up to 0.50, preferably up to 0.05, Ti: up to 0.20, preferably 0.01 to 0.06, particularly preferably 0.02 to 0.03 V: up to 0.30, preferably up to 0.07, Nb: up to 0.08, preferably up to 0.04, The remainder is iron, including impurities normally associated with steel melting, the steel having a structure composed of the following proportions: more than 60 volume% but less than 90 volume% of a matrix of bainite and ferrite, 10 to 40 volume% of a second phase of tempered martensite on the one hand, and of MA constituent on the other hand, the volume proportion of the tempered martensite being at least half the volume proportion of the second phase, i.e. > 50 volume% of the second phase, and the austenite in the MA constituent making up a proportion of < 10 volume% based on the total structure consisting of matrix and second phase.

2. Flat steel product according to claim 1, characterized in that both the tempered martensite and the MA constituent of the second phase are embedded in the matrix of bainite and ferrite in the form of islands.

3. Flat steel product according to claim 2, characterized in that the embedded islands of MA constituent have an area-weighted average diameter D which is < 1.2 pm, i.e. D < 1.2 pm.

4. Steel flat product according to claim 2 or 3, characterized in that the proportion of austenite is < 5 volume % based on the total structure consisting of matrix and second phase.

5. Steel flat product according to one of claims 1 to 4, characterized in that the grain boundary misorientation index, KMI for short, in the matrix of bainite and ferrite is at least 0.5, i.e. for the matrix of bainite and ferrite the KMI is > 0.

5.

6. Steel flat product according to one of claims 1 to 5, characterized by an elongation at break A5 > 11% or a hole expansion ratio LA > 18%.

7. Flat steel product according to claim 6, characterized by an elongation at break A5 > 11% and a hole expansion ratio LA > 18%.

8. Flat steel product according to one of claims 1 to 7, characterized by a yield strength Rpo,2 s 720 MPa.

9. Steel flat product according to one of claims 1 to 8, characterized in that it is designed in such a way that when shearing with a cutting gap of 8% -12% a cut edge can be produced which is free from fracture cracks and / or that when tearing as a tensile test a fracture edge is produced which is free from fracture cracks.

10. A method for producing the hot-rolled flat steel product with a tensile strength Rm > 950 MPa, in particular a hot-rolled flat steel product according to one of claims 1 to 9, comprising the following steps Providing a steel slab with the following composition in weight %: C: 0.08 to 0.20, preferably 0.08 to 0.13, Si: 0.2 to 0.6, preferably 0.2 to 0.5, Mn: 1.6 to 2.5, preferably 1.9 to 2.2, N: up to 0.03, preferably up to 0.01, P: up to 0.10, preferably up to 0.02, S: up to 0.010, preferably up to 0.002, Al: up to 0.10, preferably up to 0.05, Cu: up to 0.50, preferably up to 0.05, Cr: up to 1.0, preferably 0.3 to 0.8, Mo: up to 0.50, preferably 0.05 to 0.35, particularly preferably 0.20 to 0.30 Ni: up to 0.50, preferably up to 0.05, Ti: up to 0.20, preferably 0.01 to 0.06, particularly preferably 0.02 to 0.03, V: up to 0.30, preferably up to 0.07 and Nb: up to 0.08, preferably up to 0.04, balance iron, including usual steel-accompanying impurities caused by melting, Heating the steel slab to an austenitizing temperature in the range of 1100 °C to 1400 °C; Hot rolling to produce a hot-rolled steel strip product of a desired thickness at a temperature in the range from the final rolling temperature to 1300°C, the final rolling temperature being in the range from 800°C to 960°C, preferably from 850°C to 930°C; subsequent rapid cooling of the hot-rolled steel strip product to a coiling temperature of 410°C to a maximum of 650°C, preferably from 450°C to a maximum of 550°C, in a first cooling phase with a cooling rate of on average > 50 K / s in at least a time period of at least 2 s of the first cooling phase; subsequent slow cooling of the product in a second cooling phase during and / or after coiling with an average of < 50 K / h in a time period of at least 4 h of the second cooling phase; optionally stretching or stretch bending or skin passing; and optional heat treatment in the form of tempering at a temperature T in the range of 150 °C to 700 °C.

11. The method according to claim 10, characterized in that the optional heat treatment is carried out as continuous annealing.

12. The method according to claim 10, characterized in that the optional heat treatment is carried out as a bell annealing.

13. Method according to one of claims 10 to 12, characterized in that the duration t and the temperature T of the heat treatment in the form of tempering are as follows: 7.5 < HJP < 17.0, with HJP = T(20+Iog t) / 1,000, where time t is given in h and temperature T in K and log is the natural logarithm.

14. Use of a hot-rolled flat steel product according to one of claims 1 to 9 and / or produced by a process according to any one of claims 10 to 13 in the automotive or vehicle construction industry.

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