Hot-rolled flat steel product and process for production thereof

Abstract

The invention relates to a hot-rolled flat steel product having a Brinell hardness in the range from 330 HBW to 485 HBW, preferably in the range from 390 HBW to 485 HBW. This flat steel product consists of a steel having the following composition in % by weight: C: 0.05 to 0.35, preferably 0.09 to 0.31, more preferably 0.12 to 0.28, Si: 0.20 to 0.60, preferably 0.20 to 0.55, more preferably 0.25 to 0.55, Mn: 1.65 to 2.8, preferably 1.80 to 2.6, more preferably 2.05 to 2.4, Al: 0.01 to 0.10, and optionally one or more elements from: Mo: up to 0.315, preferably 0.018 to 0.295, more preferably 0.040 to 0.265, N: up to 0.018, P: up to 0.020, S: up to 0.010, Cr: up to 1.0, preferably 0.10 to 0.85, more preferably 0.15 to 0.75, Cu: up to 1.0, preferably 0.05 to 1.0, Ni: up to 0.30, preferably 0.02 to 0.25, Ti: up to 0.033, preferably 0.01 to 0.031, Nb: up to 0.06, preferably 0.005 to 0.06, V: up to 0.10, preferably 0.005 to 0.10, B: up to 0.0004, preferably up to 0.00035, Ca: up to 0.005, preferably 0.0005 to 0.005, W: up to 0.2, Co: up to 0.2, Sn: up to 0.05, Sb: up to 0.05 and Zr: up to 0.010, the balance being iron, including customary steel-accompanying melting-related impurities, wherein the steel has a microstructure composed of the following proportions: at least 70% by volume of martensite, up to 30% by volume of bainite, wherein a remaining balance accounts for a proportion of not more than 1.5% by volume and is formed by one or more of the following phases: ferrite, pearlite and residual austenite. The invention further relates to use of such a flat steel product and to a process for producing a corresponding flat steel product.

Description

[0001] Hot-rolled flat steel product and process for its production

[0002] The invention relates to a hot-rolled flat steel product with a Brinell hardness in the range of 330 HBW to 485 HBW.

[0003] The invention further relates to a corresponding method for producing such a hot-rolled flat steel product and a use of such a hot-rolled flat steel product for producing components for construction and work machines and / or construction and work equipment.

[0004] For components for machinery or equipment in the mining, construction, agriculture, and forestry sectors, it is important that the steel used has high wear resistance. The high hardness of a flat steel product directly impacts its wear resistance; the higher the hardness, the better the wear resistance. In the context of this invention, high hardness means that the Brinell hardness is 330 HBW to 390 HBW for simpler applications and in the range of 390 HBW to 485 HBW, i.e., approximately 450 HBW, for more demanding applications.

[0005] Wear-resistant steels are also known as abrasion-resistant steels. They are used in applications requiring high resistance to abrasive and impact wear. Such applications include mining, earthmoving equipment and machinery, and waste and debris transportation. Wear-resistant steels are used, for example, in excavator buckets and dump truck bodies. Their high hardness results in a longer service life for vehicle components. The advantages of wear-resistant steels are even more crucial when a paint coating applied to the exterior surface is frequently exposed to mechanical stresses such as impacts, which cause long-term damage to the paint coating, such as scratches.

[0006] Another important property for these applications is the toughness and, in particular, the impact strength of the steel, i.e., its resistance to fracture or crack propagation, especially under impact loads. Since the steel used in the flat steel product usually requires significant deformation for these applications, the formability of the steel, such as its flexibility, is another important property. Due to the application areas, very good weather resistance of the flat steel product is also important in many cases, both in the blasted state, i.e., after removal of the scale, and in the unblasted state, i.e., with scale.

[0007] Document EP 3 719 148 B1 describes a hot-rolled steel strip product with a Brinell hardness in the range of 420 HBW to 580 HBW, which consists of a steel with the following composition in weight %:

[0008] C: 0.17 to 0.38; Si: 0.01 to 0.5; Mn: 0.1 to 0.4; Al: 0.015 to 0.15; Cu: 0.1 to 0.6; Ni: 0.2 to 0.8; Cr: 0.1 to 1; Mo: 0.01 to 0.3; Nb: 0 to 0.005; Ti: 0 to 0.05; V: 0 to 0.06; B: 0.0005 to 0.005; P: 0 to 0.025; S: 0 to 0.008; N: 0 to 0.01; Ca: 0.0008 to 0.003; The remainder is Fe and unavoidable impurities, whereby the steel is a martensitic steel whose microstructure is composed of the following proportions: martensite: at least 90% by volume, retained austenite: 0 to 1% by volume, the remainder being bainite, ferrite and / or pearlite. Since the impact toughness of a steel with a Mn content of 1.0% by weight and more is assumed to be comparatively low, the Mn content in the steel is limited to 0.1 to 0.4% by weight. Document EP 3 719 149 B1 describes a very similar hot-rolled steel strip product with a Brinell hardness in the range of 420 HBW to 580 HBW. Here, the Mn content in the steel is limited to 0.05 to 0.4% by weight.

[0009] The object of the invention is therefore to provide a hot-rolled flat steel product with a sufficiently high Brinell hardness, preferably in the range of approximately 450 HBW, and a method for its production, in which the hot-rolled flat steel product has a sufficiently high resistance to abrasive and impact wear and the production is possible using conventional hot-rolling plants.

[0010] 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.

[0011] The hot-rolled flat steel product according to the invention with a Brinell hardness in the range of 330 HBW to 485 HBW, preferably in the range of 390 to 485 HBW, is intended to consist of a steel with the following composition in weight %:

[0012] C: 0.05 to 0.35, preferably 0.09 to 0.31, particularly preferably 0.12 to 0.28, Si: 0.20 to 0.60, preferably 0.20 to 0.55, particularly preferably 0.25 to 0.55, Mn: 1.65 to 2.8, preferably 1.80 to 2.6, particularly preferably 2.05 to 2.4, Al: 0.01 to 0.10, and optionally one or more elements from:

[0013] Mo: up to 0.315, preferably 0.018 to 0.295, particularly preferably 0.040 to 0.265,

[0014] N: up to 0.018, P: up to 0.020, S: up to 0.010,

[0015] Cr: to 1.0, preferably 0.10 to 0.85, particularly preferably 0.15 to 0.75,

[0016] Cu: up to 1 ,0, preferably 0.05 to 1 ,0,

[0017] Ni: up to 0.30, preferably 0.02 to 0.25,

[0018] Ti: up to 0.033, preferably 0.01 to 0.031,

[0019] Nb: up to 0.06, preferably 0.005 to 0.06,

[0020] V: up to 0.10, preferably 0.005 to 0.10,

[0021] B: up to 0.0004, preferably up to 0.00035,

[0022] Ca: up to 0.005, preferably 0.0005 to 0.005,

[0023] W: up to 0.2, Co: up to 0.2, Sn: up to 0.05, Sb: up to 0.05 and Zr: up to 0.010,

[0024] The remainder is iron, including usual impurities associated with steel melting, whereby the steel has a structure consisting of the following

[0025] proportions: at least 70 volume% martensite, up to 30 volume% bainite, with any remaining residue amounting to a maximum of 1.5 volume% and being formed by one or more of the following phases: ferrite, pearlite and retained austenite.

[0026] The hot-rolled flat steel product according to the invention exhibits not only a comparatively high (Brinell) hardness but also sufficiently high resistance to abrasive and impact wear. The hot-rolled flat steel product according to the invention can be readily manufactured on conventional hot-rolling mills, since the required manufacturing parameters, in particular a cooling rate after completion of hot rolling, can generally be easily achieved using conventional hot-rolling mills. The good resistance to abrasive and impact wear is due, among other things, to the fact that (i) one phase (here, martensite) clearly dominates the microstructure with > 70% by volume, (ii) a Mn content of at least 1.65% by weight is present, and (iii) the Ti content is limited to a maximum of 0.033% by weight. The use of the relatively cost-intensive alloying element Ni, on the other hand, is only optional and can be kept very low.The steel for such a hot-rolled flat steel product can be produced either via the blast furnace route or as electrical steel with a significant scrap content.

[0027] Although the hot-rolled flat steel product according to the invention has a significantly higher Mn content than the hot-rolled steel strip products described in the documents EP 3 719 148 B1 and EP 3 719 149 B1 mentioned at the outset, this does not result in a significant cost difference, since inexpensive ferromanganese can be used for alloying.

[0028] In particular, the steel has a microstructure composed of the following components: at least 90% martensite by volume, up to 10% bainite by volume, with a remaining portion of no more than 1.5% by volume consisting of one or more of the following phases: ferrite, pearlite, and retained austenite. In this case, one can already speak of a single-phase steel, namely martensitic steel. Such a single-phase steel tends to exhibit higher toughness than a multi-phase steel.

[0029] According to a preferred embodiment of the invention, the flat steel product has a notched impact strength at -20 °C of at least 65 J / cm 2Such a high notched impact strength indicates high resistance to shock wear, which is particularly advantageous for the applications mentioned above. A flat steel product with a tensile strength difference of no more than 15% across the entire strip length, preferably 12%, and particularly preferably 10%.

[0030] According to a further preferred embodiment of the invention, the flat steel product has an abrasion resistance which is characterized by at least one of the following statements about an abrasion value in the form of a loss of mass or volume:

[0031] (i) Volume loss as abrasion value, determined by abrasion wheel test according to ASTM G65-16: removal < 280 mm 3 , preferably < 260 mm 3 , particularly preferably < 240 mm 3 ,

[0032] (ii) Mass loss as abrasion value, determined by abrasion wheel test according to ASTM G65-16: removal < 2,200 mg, preferably <2,050 mg, particularly preferably <1,900 mg,

[0033] (iii) Volume loss as abrasion value, determined by Miller test according to ASTM G75-15: removal < 220 mm 3 , preferably < 200 mm 3 , particularly preferably <185 mm 3 and

[0034] (iv) Mass loss as abrasion value, determined by the Miller test according to ASTM G75-15: material removed < 1,700 mg, preferably < 1,620 mg, particularly preferably < 1,500 mg. Such abrasion values ​​indicate high resistance to abrasive wear, which is particularly advantageous for the applications mentioned above.

[0035] The abrasion values ​​were determined according to ASTM G65-16 and ASTM G75-15. The following test parameters were used for ASTM G65-16 (Method A):

[0036] Friction wheel: chlorobutyl rubber,

[0037] Abrasive material: Quartz sand GL23 (average grain size d50 of 335 pm), mass flow: 305 g / min, friction distance method A: 4309 m, test force: 130 N

[0038] Speed: 200 min' 1 .

[0039] Furthermore, the following test parameters were used in ASTM G75-15:

[0040] Test force 22.24 N,

[0041] Abrasive material Corundum F220, used as a solid-liquid mixture: 150 g Corundum

[0042] F220 and 150 g distilled water, test duration 6 h,

[0043] Friction travel per stroke 200 mm,

[0044] Friction speed 20 m / min, test temperature RT

[0045] Overlap material neoprene.

[0046] Preferably, the steel of the flat steel product has a microstructural dislocation density in the range 2 * 10 16 m / m 3 up to 5 * 10 16 m / m 3 Such a microstructural dislocation density indicates high strength combined with high hardness.

[0047] The density of dislocation lines in the material is determined using X-ray diffraction (XRD). For this purpose, the sample is mechanically ground and polished in a plane parallel to the rolling and transverse direction of the sheet. Subsequently, the top 10–20 pm of the sample surface are removed by electrolytic polishing to remove deformation residues from the mechanical sample preparation. A diffractogram is measured from the sample, and the integral width of the diffraction reflections from at least three different lattice planes is determined. The measured integral widths l int are corrected by the previously determined instrumental width hnstr, and from this the dislocation density is calculated according to the procedure described in the article "Kapoor, K.; Lahiri, D.; Rao, SVR; Sanyal, T.; Kashyap, BP (2004): X-ray diffraction line profile analysis for defect study in Zr-2.5% Nb material. In: Bulletin of Materials Science 27 (1), pp. 59-67." [A]. In addition, contrast factors are used for this calculation, which can be found in the article "Borbely, Andräs, et al. "Computer program ANIZC for the calculation of diffraction contrast factors of dislocations in elastically anisotropic cubic, hexagonal and trigonal crystals." Journal of applied crystallography 36.1 (2003): 160-162."

[0048] In order to ensure good and even hardness, the following must be given in particular: 0.5 > V / A < 3, where A is the numerical value of the former austenite grain size as ECD (Equivalent Circle Diameter) in pm and V is the numerical value of the dislocation density in 10 16 m / m 3 is.

[0049] The flat steel product preferably has a tensile strength of > 980 MPa, preferably > 1,150 MPa, and particularly preferably e 1,250 MPa. Flat steel products with such strength values ​​are well suited for the applications mentioned above.

[0050] According to a further preferred embodiment of the invention, the properties of the steel of the flat steel product - even after forming into an application-specific component - change upon subsequent heating to up to 250 °C with regard to tensile strength, Brinell hardness and / or abrasion values ​​as follows:

[0051] Tensile strength: reduced by a maximum of 15%, Brinell hardness: reduced by a maximum of 15% and one or more of the abrasion values ​​mentioned: increased by a maximum of 15%.

[0052] Furthermore, it is preferably provided that the flat steel product has a hardness difference between minimum Vickers hardness and maximum Vickers hardness, based on the average Vickers hardness, of < 24%, preferably < 15%, particularly preferably < 12%, over its entire thickness.

[0053] The HV 0.5 hardness measurement is carried out according to DIN EN ISO 6507-1. The sample is mechanically ground and polished in a plane parallel to the rolling and transverse direction of the sheet and then etched with 3% nitrate acid. To evaluate the homogeneity of the sample's hardness, it is determined at at least five, preferably 20-30, positions evenly distributed across the sheet thickness. The positions are spaced at least 200 μm apart. The relative hardness difference is calculated by (H m ax-H min) / H m ean.

[0054] The flat steel product, in particular, has a previous austenite grain size with an average equivalent grain diameter of < 5.0 pm, preferably < 3.8 pm, particularly preferably < 3.4 pm. Electron backscatter diffraction (EBSD) is used to determine the grain size. For this purpose, the sample is mechanically ground and polished in a plane parallel to the rolling and transverse direction of the sheet. The sample is then polished with a polishing agent containing colloidal silica (OP-S). For the EBSD measurement, an area of ​​at least 100 pm x 100 pm is examined with a step size of 100-200 nm at 1 / 3 sheet thickness. The former austenite grain structure can be reconstructed from the EBSD data using the orientation relationship of the martensitic transformation (e.g., Kurdjumov-Sachs). Corresponding reconstruction algorithms are offered by many EBSD manufacturers or can be carried out, for example, with the Matlab toolbox MTEX, as shown here.From the reconstructed EBSD data, the equivalent circular diameter (ECD) of the former austenite grains is determined with a segmentation angle of 10°.

[0055] Furthermore, the flat steel product preferably has no edge decarburization on its surface or edge decarburization with a limited decarburization depth, where the decarburization extends only < 300 pm, preferably only < 250 pm, particularly preferably only < 200 pm and most preferably < 0.1 mm into the steel, so that good properties such as hardness and abrasion resistance are also present close to the surface. The decarburization depth is determined in accordance with DIN EN ISO 3887. The sample is mechanically ground and polished in a plane parallel to the rolling and transverse direction of the sheet and then etched with 3% nitrate. The total decarburization depth is determined as the deepest uniform decarburization depth on the top and bottom sides of the sheet. An average total decarburization depth of the sample is calculated from the two values ​​determined in this way.

[0056] According to yet another preferred embodiment of the invention, the flat steel product has, with respect to its roughness / roughness

[0057] (i) an Ra value of < 8 pm, preferably < 6 pm, particularly preferably < 5 pm and / or

[0058] (ii) an Rz value of at most 45 pm, preferably < 35 pm, particularly preferably < 32 pm.

[0059] The mean roughness value Ra is the calculated mean of all deviations of the roughness profile from the mean line along the reference section. The average roughness depth Rz is the mean of individual roughness depths from five consecutive individual measurement sections in the roughness profile.

[0060] It is preferably provided that the product, after removal of scale, has an Ra value of at most 8 pm, preferably < 6 pm, particularly preferably < 5 pm.

[0061] The values ​​Ra and RPc are determined according to DIN EN 10049:2006 using the following parameters:

[0062] Type of probe system: FRT Mahr, cutoff wavelength Ac: 2.5 mm, measuring length: 17.5 mm, measuring direction: longitudinal and transverse to the rolling direction, number of measurements: 3 and removal of scale: HCl:H2O 1:1+hexamethylenetetramine at 80°C for up to a maximum of 2 minutes until scale is visibly removed.

[0063] Furthermore, it is preferably provided that the flat steel product is readily weldable and, when welded with appropriate weld metal, has a sufficient hardness, in particular > 330 HBW. Finally, it is preferably provided that the flat steel product has such resistance to cyclic corrosion conditions that, when tested according to ISO 11997-3 (VDA 233-102, DIN 55635), a mass removal averaged over 3 cycles of < 1,100 g / m 2 , preferably < 1,080 g / m 2 and particularly preferably < 1,060 g / m 2 results.

[0064] The invention further relates to the use of an aforementioned flat steel product for the production of components for construction and work machines and / or construction and work equipment in thicknesses of 2-12 mm, particularly preferably also in thicknesses of 2-11 mm, and very particularly preferably in thicknesses of 2-10 mm, for example for components in mobile construction and work machines. A particular embodiment here is the thickness of 2-8 mm for receptacles for construction and work materials. In this case, the flat steel product has an elongation at break of at most 14%, preferably 12%, and very particularly preferably 11%.

[0065] Another intended use is as security steel, i.e., as a material for the ballistic protection of living beings, vehicles (both civilian and military), devices, or structures. For this purpose, it is important to possess very good resistance to ballistic impacts and to thrown and possibly fast-moving objects.

[0066] The method according to the invention for producing a hot-rolled flat steel product, in particular a flat steel product mentioned above, comprises the following steps:

[0067] Providing a steel slab having a chemical composition as mentioned above in connection with the flat steel product according to the invention;

[0068] (a) heating said steel slab to an austenitizing temperature in the range of 1100 °C to 1400 °C;

[0069] (b) optional pre-rolling of the soaked steel slab to an intermediate product with an intermediate product temperature (T2) of 1000 - 1200 °C with a pre-strip thickness of 30-60 mm;

[0070] (c) hot rolling to the desired thickness of 2 to 12 mm at a temperature in the range of Ar3 to 1300 °C, wherein the final rolling temperature is in the range of 885 °C to 990 °C, preferably 895 °C to 960 °C, more preferably 905 °C to 930 °C; (d) subsequently cooling the hot-rolled steel strip product to a coiling temperature of at most 450 °C, preferably at most 250 °C, more preferably at most 150 °C, and even more preferably at most 100 °C;

[0071] (e) optional heat treatment in the form of tempering at a temperature in the range of 150 °C to 250 °C;

[0072] (f) optional heat treatment in the form of a tempering treatment in the form of heating to the austenite region, holding at temperature to equalise the temperature in the steel strip product (workpiece), and subsequent very rapid cooling (quenching) taking into account the critical cooling rate, followed by optional tempering in a second process step at a temperature in the range of 150 °C to 250 °C in order to achieve the desired properties of hardness, tensile strength and impact strength (Note: This form of heat treatment, in particular the water quenching process, requires additional expenditure in terms of transport, time and energy for heating and cooling);

[0073] (g) optional straightening and cutting into a sheet product and

[0074] (h) optional pickling and

[0075] (i) Coating the surface(s), in particular hot-dip coating the surface^), with a zinc-based coating which, in addition to Zn, optionally contains one or more of the elements Al, Mg, Si in the form of up to 4.0% by weight Al; up to 5.0% by weight Mg and up to 8.0% by weight Si.

[0076] In one embodiment, step (d) is immediately followed by cooling the hot-rolled steel strip product, in particular wound into a coil, to room temperature in air. In a variant of this embodiment, straightening and slab forming into a sheet metal product can then take place according to step (g).

[0077] The resulting flat steel product is thus preferably either a hot-rolled steel strip product - preferably wound into a coil - or a cut-to-size steel strip product, in particular a sheet product.

[0078] The steel slab is prepared primarily by producing an iron-containing melt using a conventional production route via a blast furnace and a blast furnace, or alternatively, via an electric melter. This second alternative involves the following process steps:

[0079] (i) Feeding the melter with solids such as scrap or ferrous material, e.g. DRI material, pellets, iron ore, and slag formers,

[0080] (ii) melting the solids by means of an electric melter to produce a molten iron and a liquid slag arranged on the molten iron,

[0081] (iii) tapping of the liquid slag and the molten iron, the feeding of the solids being carried out in such a way that the process gas extracted from the electric melter is used to heat the fed solids,

[0082] (iv) Subsequently, optional optimization of the alloy composition via secondary metallurgy and pouring of the melt into the steel slab or other starting material.

[0083] (v) Optionally, the starting material, such as the slab, may be kept at a temperature of at least 300 °C after casting until it is used in the slab heating unit for rolling.

[0084] The advantages and embodiments of the invention mentioned in connection with the hot-rolled flat steel product according to the invention also apply completely analogously to the method according to the invention for producing a hot-rolled flat steel product.

[0085] In particular, it is intended that the cooling / quenching of the hot-rolled steel strip product from a final rolling temperature to the coiling temperature takes place at an average cooling rate of at least 45 °C / s.

[0086] The following discusses the importance of the individual chemical elements in the composition of steel.

[0087] Carbon (C) is an element necessary to increase the hardness of martensite and ensure excellent abrasion resistance. Furthermore, C increases the strength of the material through solid solution strengthening. To achieve these effects, its content must be 0.05 wt% or more. C contents exceeding 0.35 wt% excessively increase hardness and therefore have adverse effects on weldability, impact toughness, formability or bendability, and resistance to stress corrosion cracking. Therefore, depending on the desired hardness, C is used in the range of 0.05 wt% to 0.35 wt%, preferably 0.09 wt% to 0.31 wt%, and more preferably 0.12 wt% to 0.28 wt%.

[0088] Silicon (Si) acts as a deoxidizer and is therefore necessary for steelmaking. Si also promotes the hardening of steel sheets through solid solution strengthening and improving austenite hardenability. Furthermore, Si delays the formation of coarse carbide, thus effectively improving the formability and impact resistance of a steel sheet. To achieve this effect, its content must be 0.2 wt% or more. However, a silicon content of more than 0.6 wt% can unnecessarily increase the carbon equivalent (CE), thereby impairing weldability. If the Si content is too high, red scale forms on the surface of a steel sheet during hot rolling, which significantly deteriorates the surface quality of the steel sheet and leads to poor weldability.Therefore, in the present invention, the Si content is set in the range of 0.20 wt% to 0.60 wt%, preferably 0.20 wt% to 0.55 wt%, particularly preferably 0.25 wt% to 0.55 wt%.

[0089] Manganese (Mn): Similar to Si, alloying with Mn increases the strength of steel sheet through solid solution strengthening. Furthermore, Mn increases the hardenability of steel, so that a martensite or bainite phase easily forms during cooling after heat treatment. At excessively high Mn contents, a significant segregation part develops in the mid-thickness section of a slab during slab casting in a continuous casting process, or an uneven structure develops across the sheet thickness during cooling after heat treatment. This can contribute to a deterioration of impact strength in a low-temperature range. Furthermore, excessively high Mn contents impair the toughness, ductility, and weldability of the base material, promote the intergranular segregation of P, and the occurrence of stress corrosion cracking.Therefore, Mn is added in an amount of at least 1.65 wt% to ensure high hardenability, but not more than 2.8 wt% to avoid the adverse effects described above and to ensure excellent mechanical properties such as impact resistance and flexibility. Manganese is therefore used in the range of 1.65 wt% to 2.8 wt%, preferably 1.80 wt% to 2.6 wt%, and more preferably 2.05 wt% to 2.4 wt%.

[0090] Aluminum (Al) acts as a deoxidizer and is most commonly used in deoxidation processes for molten steel for steel sheets and strips. Al has the effect of fixing dissolved N in the steel to form AlN, thus suppressing grain coarsening and toughness deterioration. When the Al content is less than 0.01 wt%, the deoxidation effect may not be sufficiently achieved. However, when the content is more than 0.10 wt%, it will contaminate the weld metal during welding and impair the toughness of the weld metal. An excess of Al can also lead to more non-metallic inclusions, thereby deteriorating cleanliness and leading to related defects. Excessive Al precipitates also promote the formation of edge cracks during continuous casting. Therefore, the content is limited to 0.01 wt% to 0.10 wt% or less.The content is preferably 0.08% by weight or less.

[0091] Chromium (Cr) may optionally be present in the flat steel product according to the invention. Cr contributes to the solid solution strengthening of steel and serves to delay ferrite formation during cooling, thus promoting the formation of a martensite or bainite phase. A Cr alloy also provides better resistance to pitting corrosion, thereby preventing stress corrosion cracking at an early stage. Cr also promotes the formation of a protective oxide layer under corrosive climatic conditions, which offers good resistance to atmospheric corrosion. To achieve the above-mentioned effects, it is necessary to add Cr in a content of 0.10 wt% or more. However, if the Cr content is greater than 1.0 wt%, similar to Mn, significant segregation occurs in the mid-thickness region of the slab, forming an uneven structure in the thickness direction, which impairs impact toughness in a low-temperature range.Weldability and HAZ toughness can also be adversely affected by excessive Cr additions. Therefore, Cr is used in the range of up to 1.0 wt%, preferably 0.10 wt% to 0.85 wt%, particularly preferably 0.15 wt% to 0.75 wt%.

[0092] Molybdenum (Mo) is an element that significantly increases the hardenability of steel and facilitates the formation of the martensite or bainite phases. It also improves impact toughness, cold toughness, and tempering resistance. To achieve these effects, the content is preferably 0.018 wt% or more. However, if the content exceeds 0.315 wt%, the toughness of the base material, ductility, and weld crack resistance are negatively affected. Furthermore, if the Mo content is too high, the precipitates formed during coiling immediately after hot rolling become coarse during heat treatment, which impairs impact toughness in the low-temperature range. At the same time, Mo is a cost-intensive alloying element, so it should be used cost-optimally. Therefore, Mo is used in the range up to 0.315 wt%, preferably 0.018 wt% to 0.295 wt%, particularly preferably 0.040 wt% to 0.265 wt%.

[0093] In order to achieve a sufficient strength-enhancing effect of the elements Cr, Mn and Mo and at the same time avoid possible harmful effects of excessive contents, the combination of the contents of these three elements should preferably correspond to an interval according to the formula

[0094] 2.9 < (3*Cr+2*Mn+5*Mo)*0.85 < 9.65.

[0095] In the present invention, in case of increasing strength requirements, one or more of Cu, Ni, Ti, Nb, V, and / or B may be additionally included. These elements contribute to increasing the strength of steel and can be included accordingly depending on the desired strength.

[0096] Copper (Cu) can optionally be present in the flat steel product according to the invention to increase hardenability. In particular, Cu can contribute to solid solution strengthening and precipitation hardening. Cu can also have a beneficial effect in inhibiting stress corrosion cracking. In addition, Cu can be added to facilitate the formation of a protective oxide layer under corrosive climatic conditions, which offers good resistance to climatic corrosion and increases the durability of a paint layer that can be easily damaged or removed from machine surfaces due to wear. A suitable Cu content for this purpose is 0.05 wt% to 1.0 wt%. The effect of Cu in the flat steel product according to the invention can be used particularly effectively if Cu is optionally present in contents of up to 0.5 wt%.If Cu is added in excessive amounts, it deteriorates weldability and the toughness of the heat-affected zone (HAZ). Therefore, the upper limit for Cu is set at 1.0 wt%, preferably 0.05 to 0.5 wt%.

[0097] Similar to copper, nickel (Ni) can also be optionally added to the flat steel product according to the invention to increase hardenability. Ni is an alloying element that improves the hardenability of austenite and thereby increases strength without or only slightly losing impact toughness and / or HAZ toughness. Ni is also used to prevent quenching-induced cracks and to improve toughness at low temperatures. Ni also improves surface quality and thus prevents pitting corrosion, the starting point for stress corrosion cracking. Suitable Ni contents for this purpose are 0.02 wt.% to 0.30 wt.%. The effect of Ni in the flat steel product can be particularly effectively utilized if Ni is optionally present in a proportion of up to 0.25 wt.%. An excess of Ni can produce highly viscous iron oxide deposits that impair the surface quality of the steel product.Higher Ni contents also negatively impact weldability due to the increased carbon equivalent and crack sensitivity coefficient. Higher nickel contents without significant technical improvement would also increase alloying costs excessively. Therefore, the upper limit for Ni is set at 0.30 wt%, preferably 0.02 to 0.25 wt%.

[0098] In the flat steel product according to the invention, titanium (Ti) is generally not a deliberately added alloying element. It is accepted as a component of scrap additions or other additives during the manufacturing process within the limits specified here. However, Ti can also be optionally added to the steel of a flat steel product according to the invention. The titanium nitrides (TiN), which form at high temperatures or directly from the melt, inhibit grain growth during reheating of the slab prior to hot rolling and thus promote a finer-grained microstructure and thus higher toughness values. TiN precipitates can also prevent grain coarsening in the HAZ during welding and thus further improve toughness. Ti can also contribute to grain refinement during the rolling process and to precipitation strengthening through the precipitation of titanium carbonitrides.These precipitates are capable of deeply trapping a significant amount of hydrogen H, which reduces the H diffusivity in the material and removes some of the harmful H from the microstructure to prevent stress corrosion cracking. In the case of optional addition of boron (B), Ti suppresses the precipitation of BN through the formation of TiN, leaving B free to make its contribution to hardenability. To exploit these effects, at least 0.005 wt.% can be added to the flat steel product according to the invention. However, excessively high titanium contents would lead to the formation of coarse carbides, nitrides, and / or carbonitrides, resulting in a reduction in toughness and fatigue strength. Furthermore, excessively fine grained austenite microstructures can accelerate ferrite nucleation and thus reduce austenite hardenability.Therefore, it is necessary to limit the titanium content to a maximum of 0.033% by weight, preferably from 0.01 to 0.031 and particularly preferably from 0.01 to 0.020.

[0099] Niobium (Nb): In flat steel products, Nb is generally not a deliberately added alloying element. It is accepted as a component of scrap additions or other additives during the manufacturing process within the limits specified here. However, Nb can also be optionally added to the steel of a flat steel product according to the invention. At relatively high temperatures, Nb forms niobium carbides and / or niobium carbonitrides, which inhibit grain growth before, after, and during the hot rolling process, thus causing grain refinement and thus increasing notch impact toughness. Furthermore, these precipitates can increase strength through precipitation hardening, which is utilized according to the invention to prevent excessive softening of the heat-affected zone in the area of ​​a weld made on a flat steel product according to the invention.To utilize the effects of niobium explained here, optional minimum Nb contents of 0.005 wt.% are provided in the steel flat product according to the invention. However, an excess of Nb can impair the bendability of the material, particularly when direct quenching is used and / or when Mo is present in the composition. Furthermore, Nb can be detrimental to HAZ toughness in the case of parallel Ti addition, as Nb can promote the formation of a coarse upper bainite structure through the formation of relatively unstable TiNbN or TiNb(C,N) precipitates. Excessive fine-grained austenite microstructures can also accelerate ferrite nucleation and thus reduce austenite hardenability. At the same time, Nb is a cost-intensive alloying element and should therefore be used cost-optimally.Therefore, it is necessary to limit the niobium content to a maximum of 0.06% by weight, preferably 0.005 to 0.06% by weight and particularly preferably 0.005 to 0.03% by weight.

[0100] Vanadium (V): In flat steel products, V is generally not a deliberately added alloying element. It is accepted as a component of scrap additions or other additives during the manufacturing process within the limits specified here. However, V can optionally be added to the steel of a flat steel product according to the invention. V has essentially the same, but less pronounced, effects as Nb. V4C3 precipitates are capable of deeply capturing a significant amount of hydrogen H, which reduces the H diffusivity in the material and removes some of the harmful H from the microstructure to prevent hydrogen-induced cracking. V is a strong carbide and nitride former, but mixed precipitates of the V(C,N) type can also form. V solubility in austenite is higher than that of Nb or Ti.Thus, the V alloy has the potential for precipitation hardening in subsequent tempering processes, as large amounts of V are dissolved and available for precipitation in the microstructure. However, an addition of more than 0.10 wt.% V has negative effects on weldability and hardenability. At the same time, V is a cost-intensive alloying element, which should therefore be used cost-optimally. Therefore, it is necessary to limit the vanadium content to less than 0.10 wt.%, preferably to 0.005 to 0.10, and particularly preferably 0.005 to 0.05.

[0101] Boron (B): In the flat steel product according to the invention, B is generally not a deliberately added alloying element. It is accepted as a component of scrap additions or other additives during the manufacturing process within the limits specified here. However, B can also be optionally added to the steel of a flat steel product according to the invention. B is a proven microalloying element for increasing hardenability. Coming from the rolling heat, B segregates to the austenite grain boundaries and suppresses the nucleation of ferrite there. In this way, the ferritic-pearlitic transformation is shifted to longer cooling times, and a martensitic transformation can be achieved at lower cooling rates. For these effects of boron to occur in the flat steel product according to the invention, it must be ensured that boron is dissolved in the microstructure of the flat steel product and is not bound by the formation of boron nitride.This can be achieved by the parallel addition of Ti in an amount of at least 3.42*N. If the content exceeds 0.005 wt.%, the toughness, ductility, and weld crack resistance of the base material are negatively affected. Furthermore, the presence of TiN precipitates in the microstructure can impair the toughness of the material. Therefore, TiN is only an optionally added alloying element in the flat steel product according to the invention and is limited to less than 0.0004 wt.%, preferably 0.00035.

[0102] In the present invention, the higher the contents of C, Mn, Cr, Mo, and optionally Ni, the higher the hardenability of the steel microstructure, and thus, even at a lower cooling rate, a martensite phase is easily formed, which is advantageous in ensuring strength. However, C, Mn, Cr, Mo, and optionally Ni are locally enriched in the middle thickness portion of the steel sheet, resulting in the microstructure being uneven throughout the thickness of the flat steel product of the present invention. Since a microstructure and a material of a surface layer portion vary, bending formability and impact strength deteriorate accordingly in a low-temperature region. Therefore, center segregation must be reduced, and the accumulated content of the aforementioned alloying elements must be limited.Furthermore, as described above, excessively high contents of C, Mn, Cr, Mo, and Al impair the toughness, ductility, and weldability of the base material. Studies have shown that the positive influences of the alloying elements C, Mn, Cr, Mo, Al, and optionally Ni in the flat steel product according to the invention can be utilized particularly effectively and economically, while avoiding the aforementioned negative effects, if the ratio for the carbon equivalent used is

[0103] CEA = 3*C+Mn / 4.8+(Cr+Mo) / 2.3+(Ni+Al) / 17) <1.9, in particular CEA <1.7, preferably CEA <1.5 applies.

[0104] As described above, in the case of optional addition of Ti and / or B, increased contents of Al, Ti, and B can contribute to a deterioration in the toughness, ductility, and weld cracking resistance of the material. Therefore, the total content of these three elements is preferably adjusted according to the relationship 0.1 < 3*Al + 10*B + 2*Ti < 0.5, taking into account the amount of Al required for deoxidation of the molten steel.

[0105] Calcium (Ca) is used in steels for deoxidation, desulfurization, and to control the shape, size, and distribution of oxide and sulfide inclusions. The rounded shape significantly reduces the negative effect of the inclusions on hot formability, fatigue strength, and toughness. To utilize this effect in a flat steel product according to the invention, a flat steel product according to the invention can optionally contain 0.0005–0.005 wt.% Ca. To ensure a high degree of steel purity, an excessive amount of Ca should be avoided. In particular, the formation of calcium sulfide (CaS) or calcium oxide (CaO), or a mixture thereof (CaOS), should be prevented, as these can impair mechanical properties such as flexibility and resistance to stress corrosion cracking.Ca is therefore optionally used in an amount of up to 0.005 wt%, preferably 0.0005 to 0.005, to ensure excellent mechanical properties such as impact resistance and flexibility.

[0106] Unavoidable impurities may include, in particular, nitrogen (N), phosphorus (P), sulfur (S), tin (Sn), antimony (Sb), zirconium (Zr), tungsten (W), cobalt (Co), zinc (Zn), oxygen (O), and hydrogen (H). Their content is preferably defined as follows: N: up to 0.018 wt%, P: up to 0.020 wt%, S: up to 0.010 wt%, Sn: up to 0.05 wt%, Sb: up to 0.05 wt%, Zr: up to 0.010 wt%, W: up to 0.20 wt%, Co: up to 0.20 wt%, O: up to 0.005 wt%, and H: up to 0.005 wt%.

[0107] Nitrogen (N) as an accompanying element has roughly the same effect on the hardening of steel as the alloying element carbon. In its unbound form (interstitially dissolved), N can accumulate at the grain boundaries and negatively influence toughness over the course of the aging process. This phenomenon is associated with an increasing susceptibility to intergranular stress corrosion cracking. The nitrogen is usually bound with the help of aluminum; with the optional addition of other nitride-forming elements such as Ti, Nb, V or B, the nitrogen can be bound by these elements. Taking into account the practical conditions prevailing in economical steel production and the alloying measures taken according to the invention to bind the nitrogen present in the flat steel product, N contents of up to 0.018 wt.% can be accepted as an unavoidable contamination.

[0108] Phosphorus (P) has a strong effect on solid solution strengthening, but can lead to brittleness and thus poor fracture toughness due to grain boundary segregation. Therefore, its content in the flat steel product according to the invention is limited to <0.020, preferably <0.018, wt.%.

[0109] At contents above 0.01 wt.%, sulfur (S) forms sulfides with iron or manganese (FeS or MnS, respectively). These have a negative impact on formability and toughness. Therefore, the sulfur content in the flat steel product according to the invention is limited to a maximum of 0.010 wt.%.

[0110] Tin (Sn) can accumulate at grain boundaries at elevated temperatures and thereby cause embrittlement, for example, of the weld metal. Furthermore, it can increase the embrittling effect of copper described above. To prevent these negative effects, the Sn content in the flat steel product according to the invention is limited to a maximum of 0.05 wt.%. Antimony (Sb), similar to tin, can accumulate at grain boundaries at elevated temperatures and thereby cause embrittlement, for example, of the weld metal. To prevent these negative effects, the Sb content in the flat steel product according to the invention is limited to a maximum of 0.05 wt.%. Zirconium (Zr) can cause grain refinement in the flat steel product according to the invention comparable to the addition of Ti, Nb, or V. Excessive fineness of the austenite microstructure can accelerate ferrite nucleation and thus reduce the hardenability of the austenite.In the flat steel product according to the invention, a Zr content of at most 0.01 wt.% is tolerable, since the negative effects of zirconium generally only occur above 0.01 wt.%.

[0111] Tungsten (W) significantly increases the hardenability of the material, but forms a Laves phase with molybdenum at certain concentrations. This can have a negative impact on the notched impact toughness and weld crack resistance of the material. However, for technical reasons, traces of tungsten usually remain in steels. In the flat steel product according to the invention, a W content of no more than 0.2 wt.% is tolerable, since the negative effects of tungsten generally only occur above 0.2 wt.%.

[0112] Cobalt (Co) has a negative impact on the hardenability and toughness of the material. However, for technical reasons, traces of cobalt usually remain in steels. In the flat steel product according to the invention, a Co content of no more than 0.2 wt.% is tolerable, since the negative effects of cobalt generally only occur above 0.2 wt.%.

[0113] Oxygen (O) combines, particularly with aluminum, to form oxide inclusions of the type Al2O3. These reduce both toughness and fatigue strength. Therefore, the oxygen content is limited to a maximum of 0.005% by weight.

[0114] Excessive levels of hydrogen (H) can lead to the formation of cracks in the material. To prevent this, its content in the flat steel product according to the invention is limited to a maximum of 0.005% by weight.

[0115] The harmful effects of various trace and accompanying elements, especially Zr, Co, Sn, W, and Sb, can accumulate and potentially reinforce each other. Therefore, the total content of these elements in the flat steel product according to the invention must be limited to a maximum of 0.50% by weight.

[0116] In order to achieve very good weather resistance or outdoor weather resistance, for example

[0117] - different humidity levels in the air

[0118] - Rain,

[0119] - Snow,

[0120] - changing conditions, for example in terms of temperature, wetness and humidity, etc.

[0121] Under these conditions, it was determined that the interaction of the following alloying elements is very important. The following relationships have proven advantageous for the numerical values ​​of the proportions of the corresponding alloying elements in weight percent:

[0122] 2.9 < (3 * Cr+2 * Mn+5 * Mo) * 0.85 < 9.64 and at the same time

[0123] 0.3 < (9.75 * Cu + 3.2 * Ni + 1.56 * Mo + 2.52 * Cr - 61.3 * B) / 1.72 The invention will be explained below using selected examples.

[0124] Table 1 shows the respective chemical composition (alloy) of samples 1 to 14 (#1 to #14) and of sample 15, which serves only as a comparison sample for determining resistance under cyclic corrosion conditions (Table 14). Samples #1 (steel A), #2 (steel B), #5 (steel D), #6 (steel E), #7 (steel F) and #14 (steel I) fall outside the range of the composition according to the invention (-). Samples #1, 5 and 14 fall outside the range of the composition according to the invention because of too high a titanium content, sample #2 because of too low a manganese content and sample #6 because of significantly too high a chromium content and too high a nickel content. Samples #1, 6 and 7 also fall outside the range of the composition according to the invention because of too low a manganese content. All other samples (samples #3, 4 and 8 to 13) have a respective composition that lies within the limits of the composition according to the invention (+).

[0125] Table 2 shows the production routes for samples #1 to 14: With the exception of sample #7, which was produced as electric steel using an electric melter, all other samples (#1 to 6 and 8 to 14) were produced via the blast furnace route. After hot rolling and coiling, samples #1, 2, and 7 underwent a subsequent heat treatment in the form of quenching and tempering. This quenching and tempering treatment comprised heating to an austenitizing temperature, holding at that temperature, and subsequent very rapid cooling (quenching) while observing the critical cooling rate, optionally followed by tempering in a further process step.

[0126] Table 3 shows the corresponding manufacturing parameters for samples 1 to 14, particularly during hot rolling and subsequent coiling. Samples 1, 2, 5, 6, and 14 were hot rolled at a final rolling temperature (FRT) of less than 885°C, which is not within the inventive range. Samples 3, 4, 7, and 13, however, were hot rolled at a final rolling temperature of at least 885°C, which is within the inventive range. Samples 5, 6, and 14 were coiled at a coiling temperature (CT) well above 150°C (535°C to 630°C). Samples 1 to 4 and 7 to 13 were coiled at coiling temperatures CT below 120°C.The cooling / quenching of the hot-rolled steel strip product from the final rolling temperature to the coiling temperature CT takes place at an average cooling rate of at least 45 °C / s or 45 K / s, preferably at an average cooling rate of at least 55 °C / s or 55 K / s, particularly preferably at least 60 °C / s or 60 K / s.

[0127] The resulting microstructures of samples 1 to 14 are shown in Table 4. Only samples 5 and 14 have a microstructure composition that does not match the specifications for the inventive microstructure. For these two samples, neither the chemical composition nor the coiling temperature used matched the specifications for the inventive process / steel product. For sample 6, despite a coiling temperature significantly higher than the specifications for the inventive process, a martensitic steel is produced. This is due to the different chemical composition, more specifically, the very high Cr content.

[0128] Table 5 shows the resulting hardness values ​​(Brinell hardness HBW), notched impact toughnesses (KBZ), and dislocation densities of specimens 1 to 8 and 10 to 13. All of the specimens listed exhibit a Brinell hardness of 370 HBW and above. Only specimen #6 exhibits a Brinell hardness below 390 HBW. Samples #1 and 6, which already serve as counterexamples due to their chemical composition (sample #6 also due to the coiling temperature), also exhibit very weak notched impact toughness values. All other specimens tested exhibit acceptable notched impact toughness values.

[0129] Table 6 shows the resulting yield strength (Rp02), tensile strength (Rm), and elongation at break (A5) for specimens #2 to 8, 11, and 12. Specimen #5, which already serves as a counterexample due to its chemical composition, shows significantly weak values. For the applications discussed above, a tensile strength of > 980 MPa is generally desired, but specimen #5 (at 805 MPa) does not achieve this. Sample #6, which also serves as a counterexample due to its chemical composition, does achieve a tensile strength of > 980 MPa, but at 1159 MPa, this value is only just within the preferred range of > 1150 MPa tensile strength. All other specimens tested show excellent values ​​for yield strength (Rp02), tensile strength (Rm), and elongation at break (A5). Table 7 lists the resulting abrasion values ​​for specimens 2, 3, 5 to 7, and 12.Compared to the examples #2, 3, 7, and 12, the counterexamples #6 and especially #5 show a relatively high mass and / or volume loss (mass and / or volume removal) as abrasion values ​​compared to the other samples (#2, 3, 7, and 12). The mass loss and volume loss were determined according to both ASTM G65-16 and ASTM G75-15. The associated abrasion resistance is therefore lower for samples #5 and 6 than for the other samples. However, the values ​​for example sample #3 and counterexample sample #6 are quite close to each other.

[0130] Table 8 shows the respective hardness, tensile strength and yield strength as a function of heat treatment for samples # 2, 3, 8 and 12. In addition to a first alternative where there is no heat treatment / no tempering, there is a second alternative with a heat treatment in which at least 200 °C was reached over 30 minutes and then cooled in air to room temperature RT, a third alternative with a heat treatment in which at least 220 °C was reached over 30 minutes and then cooled in air to room temperature RT and finally a fourth alternative with a heat treatment in which at least 250 °C was reached over 30 minutes and then cooled in air to room temperature RT.While hardness and tensile strength remain largely constant in the specimens tested regardless of heat treatment, a slight tendency toward an increase in yield strength is evident with heat treatment at increasingly higher temperatures. Overall, however, the parameters are quite constant.

[0131] Table 9 shows the characteristics of former austenite grains for samples 3, 8 and 12, namely the mean size of the former austenite grains as ECD, a yield ratio of the former austenite grains and a weighted yield ratio of the former austenite grains.

[0132] Table 10 shows the maximum differences in Vickers hardness across sheet thicknesses for specimens 1 to 3, 5 to 7, and 12. Samples 5 and 6, which serve as counterexamples, have the lowest hardness values ​​of well below 400 HV 0.5. All other specimens have hardness values ​​between 415 and 520 HV 0.5. The relative hardness difference between maximum Vickers hardness H max and minimum Vickers hardness HV m in, which is measurable along the sheet thickness of a sample, is related to the average hardness HV me between 5.2% (sample # 2) and 22.6% (sample # 7).

[0133] Table 11 shows the decarburization depths of samples 2, 3, 7 and 11 to 13.

[0134] Table 12 shows roughness values ​​Ra, Rz, RPc for samples 2 to 4 and 11 to 13 with and without scale.

[0135] Finally, Table 13 summarizes the most important parameters for samples 1 to 8 and 10 to 14.

[0136] Furthermore, welding tests were conducted in which samples were butt-welded using MAG (metal active gas) welding. More specifically, a butt joint was welded with and without a gap (up to 3 mm) and without preheating at room temperature. The weld seam is either single-layer or double-layer. The tests showed that the flat steel product is easy to weld and, when combined with appropriate weld metal, exhibits a sufficiently high hardness, particularly > 330 HBW, as well as being highly bendable.

[0137] Table 14 shows the test results for determining "resistance under cyclic corrosion conditions" for samples #3, 8, 10, 15, and a control sample, tested according to ISO 11997-3 (VDA 233-102, DIN 55635). For samples #3, 8, and 10 according to the invention, as well as the control sample, tested according to ISO 11997-3, an average mass loss after 3 cycles of < 1,060 g / m² was observed. 2 Only the control sample (sample # 15 with a Mn content of only 1.02 wt%) has an average mass removal after 3 cycles of > 1,100 g / m 2 , which indicates significantly lower resistance to cyclic corrosion conditions. The results of the control sample confirm that the test was conducted correctly.

[0138] Table 1: Chemical composition of the examined samples # 1 to 14 and 15 (essential components)

[0139]

[0140] Table 2: Manufacturing route of samples 1 to 14

[0141]

[0142] Table 3: Manufacturing parameters of samples 1 to 14

[0143]

[0144] Table 4: Resulting microstructure of samples 1 to 14; data in volume %

[0145]

[0146]

[0147]

[0148] Table 7: Resulting mass and volume losses of samples 2, 3, 5 to 7 and 12 as characteristic values ​​for abrasion (abrasion values)

[0149]

[0150] Table 8: Hardness, tensile strength and yield strength as a function of heat treatment for samples 2, 3, 8 and 12.

[0151]

[0152] Table 10: Hardness across the sheet thickness of samples 1 to 3, 5 to 7 and 12.

[0153]

[0154]

[0155] Table 13: Summary of the most important parameters for samples 1 to 8 and 10 to 15

[0156]

Claims

Patent claims 1. Hot-rolled flat steel product with a Brinell hardness in the range of 330 HBW to 485 HBW, preferably in the range of 390 HBW to 485 HBW, which consists of a steel with the following composition in % by weight: C: 0.05 to 0.35, preferably 0.09 to 0.31, particularly preferably 0.12 to 0.28, Si: 0.20 to 0.60, preferably 0.20 to 0.55, particularly preferably 0.25 to 0.55, Mn: 1.65 to 2.8, preferably 1.80 to 2.6, particularly preferably 2.05 to 2.4, Al: 0.01 to 0.10, and optionally one or more elements from: Mo: up to 0.315, preferably 0.018 to 0.295, particularly preferably 0.040 to 0.265, N: up to 0.018, P: up to 0.020, S: up to 0.010, Cr: up to 1.0, preferably 0.10 to 0.85, particularly preferably 0.15 to 0.75, Cu: up to 1 ,0, preferably 0.05 to 1 ,0, Ni: up to 0.30, preferably 0.02 to 0.25, Ti: up to 0.033, preferably 0.01 to 0.031, Nb: up to 0.06, preferably 0.005 to 0.06, V: up to 0.10, preferably 0.005 to 0.10, B: up to 0.0004, preferably up to 0.00035, Ca: up to 0.005, preferably 0.0005 to 0.005, W: up to 0.2, Co: up to 0.2, Sn: up to 0.05, Sb: up to 0.05 and Zr: up to 0.010, The remainder is iron, including impurities normally present in steel during the melting process, the steel having a structure composed of the following proportions: at least 70% by volume martensite, up to 30% by volume bainite, with a remainder amounting to a maximum of 1.5% by volume and being formed by one or more of the following phases: ferrite, pearlite and retained austenite.

2. Steel flat product according to claim 1, having a notched impact strength at - 20 °C of at least 65 J / cm 2 .

3. Steel flat product according to claim 1 or 2, having an abrasion resistance characterized by at least one of the following statements about an abrasion value: Volume loss as abrasion value, determined by friction wheel test according to ASTM G65-16: Removal in mm 3 < 280, preferably < 260, particularly preferably < 240, Mass loss as abrasion value, determined by friction wheel test according to ASTM G65-16: Removal in mg < 2,200, preferably < 2,050, particularly preferably < 1,900, Volume loss as abrasion value, determined by Miller test according to ASTM G75-15: Removal in mm 3 < 220, preferably < 200, particularly preferably < 185 and Mass loss as abrasion value, determined by Miller test according to ASTM G75-15: Removal in mg < 1,700, preferably < 1,620, particularly preferably < 1,500.

4. Flat steel product according to one of claims 1 to 3, with a tensile strength > 980 MPa, preferably > 1,150 MPa, particularly preferably e 1,250 MPa.

5. Steel flat product according to one of claims 1 to 4, wherein the steel has a structural dislocation density in the range 2 * 10 16 m / m 3 up to 5 * 10 16 m / m 3 has.

6. Steel flat product according to one of claims 1 to 5, wherein the properties of the steel of the steel flat product - even after forming into an application-specific component - change upon subsequent heating to up to 250 °C with respect to tensile strength, Brinell hardness and / or abrasion values ​​as follows: Tensile strength: reduced by a maximum of 15% based on the flat steel product, Brinell hardness: reduced by a maximum of 15% based on the flat steel product and one or more of the abrasion values ​​mentioned: increased by a maximum of 15% based on the flat steel product.

7. Steel flat product according to one of claims 1 to 6, wherein the steel flat product has over its entire thickness a relative hardness difference between maximum Vickers hardness HV ma x and minimum Vickers hardness HV m in, related to the average hardness HV me of < 24%, preferably < 15%, particularly preferably < 12%.

8. Flat steel product according to one of claims 1 to 7, wherein the flat steel product has, with regard to its roughness, an Ra value in pm of < 8, preferably < 6, particularly preferably < 5 and / or an Rz value in pm of at most 45, preferably < 35, particularly preferably < 32.

9. Flat steel product according to one of claims 1 to 8, characterized by a thickness of 2 mm to 12 mm, particularly preferably a thickness of 2 mm to 11 mm and most preferably a thickness of 2 mm to 10 mm.

10. Flat steel product according to one of claims 1 to 9, with an elongation at break of at most 14%, preferably 12% and particularly preferably 11%.

11. Steel flat product according to one of claims 1 to 10, which has no edge decarburization on its surface or a limited edge decarburization with respect to its decarburization depth determined according to DIN EN ISO 3887, in which the decarburization extends only < 300 pm, preferably only < 250 pm and particularly preferably only < 200 pm into the steel.

12. Steel flat product according to one of claims 1 to 11, characterized by such resistance to cyclic corrosion conditions that, when tested according to ISO 11997-3, a mass removal averaged over 3 cycles is < 1,100 g / m 2 , preferably < 1,080 g / m 2 and particularly preferably < 1,060 g / m 2 results.

13. Use of a flat steel product according to one of claims 1 to 12 for the production of components for construction and work machines and / or construction and work equipment.

14. A method for producing a hot-rolled flat steel product, in particular according to one of claims 1 to 12, comprising the following steps: Providing a steel slab with a chemical composition according to claim 1; Heating the steel slab to an austenitizing temperature in the range of 1100 °C to 1400 °C; Hot rolling to the desired thickness at a temperature in the range of Ar3 to 1300 °C, wherein the final rolling temperature is in the range of 885 °C to 990 °C, preferably 895 °C to 960 °C, more preferably 905 °C to 930 °C; subsequent cooling of the hot-rolled steel strip product to a coiling temperature of at most 450 °C, preferably at most 250 °C, more preferably at most 150 °C, and even more preferably at most 100 °C; optional heat treatment in the form of tempering at a temperature in the range of 150 °C to 250 °C;optional heat treatment in the form of a tempering treatment in the form of heating to an austenitizing temperature in the range of 1100 °C to 1400 °C, holding at temperature to equalize the temperature in the steel strip product, and subsequent very rapid cooling, i.e. quenching, taking into account the critical cooling rate, and subsequently optional tempering at a temperature in the range of 150 °C to 250 °C, followed by optional straightening and cutting of the flat steel product into a sheet product; 15. The method according to claim 14, with the following additional steps: pickling and coating the surface(s) of the steel strip product or the steel flat product, in particular hot-dip coating said surface(s), with a zinc-based coating which, in addition to Zn, optionally contains one or more of the elements Al, Mg, Si in the form of up to 4.0% by weight Al; up to 5.0% by weight Mg and up to 8.0% by weight Si.

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