Hot rolled flat steel product and method for its production

Abstract

The invention relates to a hot-rolled steel flat product, the microstructure of which may contain a maximum of 9% retained austenite and a maximum of 11% martensite, the remainder being bainitic ferrite with precipitation in the form of carbides, nitrides and / or carbonitrides, and the steel flat product having a tensile strength Rm of at least 950 MPa, a yield strength Re of at least 670 MPa and a bake-hardening effect BH of at least 40 MPa, and a method for its production.

Description

[0001] The invention relates to a hot-rolled steel flat product whose microstructure essentially comprises bainitic ferrite with precipitates, wherein retained austenite and martensite may be present, and the steel flat product has a tensile strength Rm of at least 950 MPa, a yield strength Re of at least 670 MPa, and a bake-hardening effect BH of at least 40 MPa. The invention also relates to a method for producing a hot-rolled steel flat product.

[0002] Electromobility is one of the drivers of lightweight automotive construction, particularly in the chassis. The vehicle weight increases due to the battery, and the chassis must therefore withstand correspondingly higher loads. At the same time, the chassis plays a more significant role in crash situations, meaning that the energy generated in a crash must be absorbed more effectively by the chassis in the future. These changing requirements place greater emphasis on the formability of high-strength complex-phase steels, enabling lightweight construction not only in terms of materials but also in terms of design.

[0003] Especially in the area of ​​chassis components, there is a demand for flat steel products that possess both high strength and good formability at a low weight. Furthermore, OEMs require excellent cutting properties combined with high strength for further processing. Specifically, OEMs demand an additional increase in yield strength after moderate heating during the e-coating process.

[0004] Complex phase steels for cold forming are characterized by a combination of high strength and good local formability. They achieve their high strength primarily through the adjustment of phase proportions in the microstructure via a controlled rolling and cooling process in a hot rolling mill. These steels are used for component geometries mainly in the automotive and commercial vehicle industries.

[0005] As component shapes, especially for chassis parts but also for automotive seats, become increasingly complex, and material strengths rise in parallel with ever-increasing demands on dimensional accuracy, the automotive industry's requirements for steels have also become significantly more demanding. OEMs are increasingly requiring flat steel products with a uniform property profile tailored to the intended use, combined with enhanced forming and processing potential.

[0006] Increasing efficiency is a continuous goal in the further processing of flat steel products. Reducing material waste through clean cut edges during blank cutting plays a crucial role in this. The tendency to crack typically increases with higher microalloy content and increasing strength, which is why preventing cracks during processing is of growing importance and goes hand in hand with the development of ever higher strengths.

[0007] Additionally, it is advantageous for automotive manufacturers to use a flat steel product that is easily formable while maintaining high strength. Therefore, it is particularly beneficial to increase this strength only after successful forming, for example, by leveraging the ability of a flat steel product to achieve increased strength through heating, a process known as bake-hardening, as seen, for instance, in cathodic dip painting (e-coating).

[0008] Crack-free cut edges can be achieved through correct tool settings or by using material optimized for cutting. Since a component can be manufactured from various steels with different microstructures, different cutting parameters are usually required. In this case, recurring changes to the cutting parameters would be necessary, but this is unpopular with OEMs due to cost and time considerations. Therefore, the focus is on the supplied material. Good cutting performance is expected regardless of the cutting parameters.

[0009] High-strength hot-rolled steel flat products and methods for their production are disclosed by way of example in documents EP 3 353 555 B1, EP 3 516 085 B1 and EP 1 607 489 B1.

[0010] The object of this invention is therefore to provide a hot-rolled steel flat product with very good or high formability, very good or high edge crack resistance, and outstanding cutting ability. Furthermore, the steel flat product should have a surface finish that allows, using a simple manufacturing process, the produced steel flat product, in its uncoated state or with a corrosion-protective coating, to be cold-formed into a complexly shaped component, such as a chassis part.

[0011] This problem is solved by a steel flat product having the features of claim 1 and by a method having the features of claim 10. Further embodiments are described in the dependent claims.

[0012] The first teaching concerns a hot-rolled flat steel product consisting of a steel with the following composition (in wt.%): C: 0,05-0,12 %, Themselves: 0,1-0,6 %, Mn: 2,1-3,5 %, Cr: 0,1-1,2 %, You: 0,01-0,15 %,

[0013] Optionally, one or more of the following elements with the weight percentage specified below: Nb: < 0,05 %, V: < 0,5 %, Al: < 0,2 %, In: < 0,5 %, B: < 0,005 %, With: < 0,5 %, Neither: < 0,5 %, The: < 0,005 %, P: < 0,02 %, S: < 0,01 %, N: < 0,012 %,

[0014] The remaining iron and unavoidable impurities, and whose microstructure may contain a maximum of 9% retained austenite and a maximum of 11% martensite, the remainder being bainitic ferrite with precipitates, and has a tensile strength Rm according to DIN-EN ISO 6892-1 of at least 950 MPa, a yield strength Re according to DIN-EN ISO 6892-1 of at least 670 MPa and a bake-hardening effect BH according to DIN EN 10325 of at least 40 MPa.

[0015] A hot-rolled steel flat product according to the invention is characterized by high tensile strengths Rm of at least 950 MPa, in particular 970 MPa, whereby regular tensile strengths of 1000 MPa and more can be achieved to ensure sufficient strength for structural applications. At the same time, the hot-rolled steel flat product according to the invention has a yield strength Re of at least 670 MPa, in particular at least 700 MPa, and exhibits an elongation at break A80 or Aprop of at least 9%, in particular at least 10%.

[0016] Furthermore, the hot-rolled steel flat product exhibits a bake-hardening effect of at least 40 MPa, in particular at least 45 MPa, with regular strength increases of 55 MPa or more achievable, as determined according to DIN EN 10325 (2006-10). In addition, the hot-rolled steel flat product has an excellent cut edge.

[0017] The tensile strength Rm can be limited, for example, to a maximum of 1400 MPa, in particular to a maximum of 1500 MPa. The yield strength Re can be limited, for example, to a maximum of 1100 MPa, in particular to a maximum of 1050 MPa. The bake-hardening effect BH can be limited, for example, to a maximum of 150 MPa, in particular to a maximum of 120 MPa.

[0018] A hot-rolled steel flat product according to the invention can have a thickness dw of 1.0 mm to 6.0 mm, in particular at least 1.2 mm, preferably at least 1.5 mm, in order to enable a sufficiently stiff construction due to the desired application profile. The thickness can in particular be a maximum of 5.0 mm, preferably a maximum of 4.0 mm.

[0019] The tensile strength Rm is preferably determined on longitudinal specimens. For thicknesses / final thicknesses dw less than 3.0 mm, measurements are taken according to DIN EN ISO 6892-1, specimen form 2 (Annex B, Table B1) (as of 2020-06). For thicknesses / final thicknesses dw greater than or equal to 3.0 mm, the proportional specimen size is calculated according to DIN EN ISO 6892-1, section D.2.3.1 (as of 2020-06). The yield strength Re is measured analogously.

[0020] For thicknesses / final thicknesses dw less than 3.0 mm, an elongation at break A80 of at least 9%, and preferably at least 10%, is desirable to ensure sufficient forming capacity. The elongation at break A80 is measured according to DIN EN ISO 6892-1 and represents the deformability of the flat steel product.

[0021] If the hot-rolled steel flat product according to the invention has thicknesses / final thicknesses dw of greater than or equal to 3.0 mm, then it has an elongation at break Aprop of at least 8%, in particular at least 9%, preferably at least 10%. The elongation at break Aprop is measured according to DIN EN ISO 6892-1 and represents the deformability of the steel flat product.

[0022] The bending angle is determined according to VDA standard 238-100 (version 2020-07) for the maximum force. For the purposes of this application, the bending angle is understood to be the bending angle along the rolling direction. The flat steel product according to the invention has a bending angle of at least 60°, in particular at least 70°, preferably at least 80°.

[0023] The steel flat product according to the invention exhibits a hole expansion of at least 20%. The hole expansion test is a method for determining the forming properties of sheet metal edges and serves to determine edge crack susceptibility. The hole expansion can be measured according to ISO 16650 (September 2017 edition). The hole expansion is particularly at least 25%, preferably at least 30%. A maximum of one hour must elapse between punching the hole and forming it using a conical die.

[0024] Due to the combination of high strength and good formability, hot-rolled flat steel products according to the invention are particularly suitable for the production of complexly shaped components that are subject to high loads in practical use, such as those required, for example, in the field of chassis construction for automobiles.

[0025] The general term "steel flat product" typically includes steel strips and sheets of the type according to the invention.

[0026] The advantageous combination of properties of a steel flat product according to the invention is achieved, among other things, by the fact that, despite its high strength, it possesses a complex phase microstructure. The alloy of a steel flat product according to the invention is composed such that it has a bainitic ferrite content of at least 80% to a maximum of 98%, with retained austenite content of up to 9%, and in particular up to 7%, being present. Precipitations are contained within the bainitic ferrite matrix. Martensite content of up to 11%, and in particular up to 9%, and preferably up to 7%, may also be present. For example, the martensite content may be at least 1%. For example, the retained austenite content may be at least 1%, and in particular at least 2%.

[0027] Precipitations are understood to be carbides, nitrides, and carbonitrides, which can occur together or individually. Therefore, precipitates are present in the form of carbides, nitrides, and / or carbonitrides, particularly based on Ti, Cr, and optionally V, Mo, and Nb.

[0028] The main components of the microstructure can be identified using light microscopy (LOM) at 200x to 2000x magnification. The microstructure of the bainitic ferrite contains fine carbide-nitride-carbonitride precipitates based on Ti, Cr, and optionally V, Mo, and Nb. These precipitates can have a mean diameter of at most 200 nm, particularly at most 100 nm, preferably at most 60 nm, and most preferably at most 50 nm. Due to their fineness, these precipitates are not detectable using LOM but can only be identified using transmission electron microscopy (TEM) at 50,000x to 500,000x magnification.

[0029] The high strength, good elongation properties, high bake-hardening effect, and excellent cutting properties are achieved through the inventive adjustment of the complex phase structure in conjunction with the process control. This is made possible by a precise selection of the individual contents of the alloying elements, in addition to iron and unavoidable impurities, that are present in a flat steel product according to the invention.

[0030] All information regarding the content of the steel compositions specified in this application is based on weight, unless expressly stated otherwise. All unspecified "%" values ​​relating to a steel alloy are therefore to be understood as values ​​in "wt.%".

[0031] Unless expressly stated otherwise, the information regarding the concentrations of the various microstructural constituents refers to the area of ​​a polished section of a sample of the respective product. The exception is the information regarding the retained austenite content of the microstructure of a flat steel product according to the invention, since the microstructure refers to a volume and can be determined by X-ray diffraction.

[0032] Carbon (C) is primarily present in the steel flat product according to the invention to increase tensile strength and yield strength. The carbon provided in the steel alloy is partially bound, mainly in the precipitates. The concentration of C dissolved in the solid solution is thereby reduced and minimized. A C content of more than 0.05 wt.% is required to provide a sufficient supply of carbon and thus ensure the required tensile strength of at least 950 MPa. To enable the utilization of the positive effect of carbon on the strength properties while simultaneously ensuring particularly good weldability, an upper limit of 0.12 wt.% is recommended. Since ductility and toughness are also influenced by the C content, a further upper limit is advisable.To limit the reduction in ductility of the steel flat product according to the invention, the carbon content should in particular be limited to a maximum of 0.11 wt.%. Furthermore, to also preclude a reduction in toughness, the carbon content should preferably be set to a maximum of 0.10 wt.%.

[0033] Silicon (Si) is optionally used as a deoxidizing agent in the production of the steel flat product according to the invention and contributes to improving the strength properties of the steel flat product. At least 0.01 wt.% Si is required for this purpose. Si contents exceeding 0.6 wt.% would impair the surface finish and toughness properties of the steel flat product according to the invention, particularly the toughness in the heat-affected zone of the weld seam produced, so contents above 0.6 wt.% are undesirable. Furthermore, excessively high Si contents could impair weldability. To reliably avoid these negative effects and, in particular, to ensure optimized surface quality, the Si content can be limited to 0.5 wt.%.

[0034] Manganese (Mn) occupies regular lattice sites as an alloying element in the steel flat product according to the invention. The substitution atoms, with their atomic radius differing from that of the iron atoms, distort the cubic lattice and thus increase the strength. To enable this solid solution strengthening effect, Mn should be present in the steel flat product in contents of at least 2.1 wt.%, and in particular at least 2.2 wt.%. Mn tends to form segregations across the thickness, which impair the mechanical and technological properties of the steel flat product according to the invention. Such segregations can be limited by a maximum Mn content of 3.5 wt.%. Furthermore, higher Mn contents can negatively affect the weldability and formability of the steel flat product according to the invention. In particular, limiting the Mn content to a maximum of 5.5 wt.% can prevent this.-% the negative effects on the suitability are largely suppressed.

[0035] Chromium (Cr) also contributes to increased strength. The primary mechanisms at play here are precipitation and solid solution strengthening. These mechanisms become clearly evident and a measurable increase in strength occurs at a content of at least 0.1 wt.%. Furthermore, chromium has the property of preventing the coarsening of other precipitates, thereby increasing the yield strength and simultaneously improving toughness. To effectively utilize this additional mechanism, a Cr content of at least 0.2 wt.% is particularly necessary. From a joining technology perspective, an excessively high total Cr content is detrimental, as weldability decreases noticeably with increasing content. To ensure the joinability of the steel flat product according to the invention, the Cr content must be limited to a maximum of 1.2 wt.%.Since coarse carbides negatively affect the mechanical and technological properties of the steel flat product according to the invention, the Cr content must be limited, in particular, to a maximum of 1.1 wt.%. To reduce adhesion problems and limit the number of uncoated areas in steel flat products, for example, those provided with a zinc-based corrosion protection coating, pronounced grain boundary oxidation at the coil after hot rolling and pronounced selective oxidation that can occur during heat treatment must be reduced. A reduction to a non-critical level is possible if the Cr content is preferably limited to a maximum of 1.0 wt.%.

[0036] The microalloying element titanium (Ti) is present in the flat steel product according to the invention to form the desired carbides and / or nitrides and / or carbonitrides. Therefore, a titanium content of at least 0.01 wt.%, in particular at least 0.015 wt.%, preferably at least 0.02 wt.%, must be maintained. Due to the saturation effect and for economic reasons, the Ti content is limited to a maximum of 0.15 wt.%, in particular to a maximum of 0.15 wt.%, preferably to a maximum of 0.11 wt.%.

[0037] Niobium (Nb) can optionally be added to increase strength and toughness through grain refinement and hardening, as well as to form carbides and / or nitrides and / or carbonitrides.

[0038] The underlying mechanisms include the inhibition of austenite grain growth, the delay of recrystallization during hot rolling, and the formation of precipitates in the form of titanium, chromium, and optionally vanadium, molybdenum, niobium nitrides and / or titanium, chromium, and optionally vanadium, molybdenum, niobium carbides and / or titanium, chromium, and optionally vanadium, molybdenum, niobium carbonitrides.

[0039] Austenite growth is inhibited as early as the initial heating of the slab during hot rolling by the stable titanium and chromium nitrides, and optionally by vanadium, molybdenum, and niobium nitrides. During the further course of hot rolling, the formation of new temperature-dependent and / or deformation-induced precipitates leads to an additional growth inhibition. Recrystallization is delayed, firstly, by the slowing of dislocation, grain boundary, and subgrain boundary movement due to the entrainment of dissolved titanium and chromium atoms, and optionally dissolved vanadium, molybdenum, and niobium atoms. Secondly, the deformation-induced precipitation of very fine titanium and chromium particles, and optionally very fine vanadium, molybdenum, and niobium particles, delays recrystallization because grain boundaries and dislocations are fixed, thus hindering recrystallization nucleation.

[0040] At the coil, further hardening occurs through precipitation. Depending on the coil temperature, varying numbers and sizes of incoherent, partially coherent, or coherent titanium and chromium particles, and optionally vanadium, molybdenum, and niobium particles, are formed, contributing to varying degrees to the increase in strength.

[0041] Niobium as an optional alloying element should be limited to a maximum content of 0.05 wt.%, particularly 0.05 wt.%. If the Nb content exceeds 0.05 wt.%, this leads to a significant increase in rolling forces, an incompletely recrystallized microstructure, and thus inhomogeneous properties. The Nb content should therefore preferably be limited to a maximum of 0.02 wt.%. Preferably, the flat steel product according to the invention can also be niobium-free.

[0042] Vanadium (V) can optionally be used to increase yield strength and strength levels through the formation of carbonitrides. Grain refinement and hardening can occur, particularly through the precipitation of vanadium carbonitrides. To achieve an initial visible effect from vanadium, contents ≥ 0.008 wt.% would be required. From an economic perspective, vanadium contents exceeding 0.5 wt.% are not recommended, as the minimal improvement in properties does not justify the significant increase in cost. To prevent coarsening of the precipitates, vanadium contents above 0.2 wt.% are preferably avoided. For optimal utilization of the mechanisms of action, vanadium contents of up to 0.15 wt.% are preferably added.

[0043] Like silicon (Si), aluminum (Al) can also be used as an optional alloying element for deoxidation and / or grain refinement. In steel production, Al is typically used to stabilize the melt. By binding oxygen to form Al₂O₃, the formation of oxygen bubbles is prevented. To exploit this effect, the steel flat product according to the invention can contain an Al content of at least 0.005 wt.%. Al also binds the optional alloying element nitrogen (N), forming aluminum nitrides. These can prevent nucleation and, due to the resulting high nucleation density, can inhibit grain growth, leading to the formation of more small grains and increasing the toughness of the steel flat product according to the invention. Due to their high affinity for oxygen, the resulting Al₂O₃ particles become coarser at high Al contents.To prevent the precipitation of coarse particles, which negatively affect the mechanical and technological properties and the degree of purity, the aluminum content should not exceed 0.2 wt.%. In the production of a steel flat product according to the invention, the aluminum content affects its castability. To ensure good castability, an aluminum content of no more than 0.15 wt.% is particularly recommended. A steel flat product with an aluminum content of preferably no more than 0.1 wt.% results in optimal utilization of the optionally added aluminum if there are no requirements for density reduction.

[0044] The optional alloying element molybdenum (Mo) exhibits similar properties to chromium. To achieve the strength level of a steel flat product according to the invention, a Mo content of at least 0.02 wt.% can be adjusted. The mechanisms of precipitation and solid solution strengthening are primarily responsible for this. Similar to chromium, molybdenum (Mo) has the property of preventing the coarsening of other precipitates, thereby increasing the yield strength and simultaneously improving toughness. To effectively utilize this mechanism, a content of at least 0.05 wt.% can be particularly advantageous. Increasing the Mo content above 0.5 wt.% is not recommended for economic reasons, as this offers no mechanical or technological benefit and unnecessarily increases costs.The increased strength resulting from the addition of molybdenum correlates with a decrease in ductility, which significantly affects forging and other forming manufacturing processes when machining a flat steel product according to the invention. Therefore, the molybdenum content should be limited to a maximum of 0.45 wt.% to avoid impairing the forgeability of the flat steel product, preferably to a maximum of 0.4 wt.%.

[0045] Boron (B) can be present in the steel flat product according to the invention as an optional alloying element in contents up to 0.003 wt.%. B can have a beneficial effect on the strength properties and the hardenability of the steel. This beneficial effect of B can be utilized by using B contents, in particular at least 0.0005 wt.%, preferably at least 0.0009 wt.%, in the steel flat product according to the invention, while at the same time limiting the B contents, in particular, to a maximum of 0.0025 wt.%. B contents of more than 0.005 wt.% would impair the toughness properties.

[0046] Copper (Cu) is an optional alloying element and should be limited to a maximum of 0.5 wt.%. In excessively high concentrations, it impairs weldability and, due to its strong tendency to segregate within the steel, can lead to surface defects. Furthermore, Cu negatively affects castability. To avoid any negative influence of Cu, the permissible upper limit of the Cu content in the steel flat product according to the invention is, in particular, a maximum of 0.45 wt.%, preferably a maximum of 0.4 wt.%. However, in lower concentrations, copper can contribute to increased strength in the form of very fine precipitates. To achieve this positive effect, concentrations of at least 0.005 wt.% can be used.

[0047] The optional alloying element nickel (Ni) increases the adhesion between the scale layer and the flat steel product at the surface and is therefore undesirable, especially in pickling processes. Furthermore, it increases material costs and is therefore not added, or only in very small quantities, unless necessary to achieve the desired mechanical properties, where it is primarily used to improve toughness. To avoid unnecessarily increasing material costs, the nickel content is limited to a maximum of 0.5 wt.%. To facilitate the pickling process and limit the adhesion between the scale layer and the flat steel product, the nickel content is preferably kept below 0.4 wt.%, and ideally below 0.5 wt.%. Preferably, the flat steel product according to the invention can be nickel-free.

[0048] Calcium (Ca) can optionally be present in the flat steel product according to the invention to form non-metallic inclusions in the microstructure of the flat steel product, thereby improving its toughness. This effect can become visible from a Ca content of 0.0005 wt.%. However, if the Ca content is above 0.005 wt.%, this can have a negative effect on the purity of the melt and lead to defects in the shell of the cast intermediate product during casting. In order to reliably avoid a negative effect on the mechanical properties and rollability, Ca contents of less than or equal to, in particular, 0.004 wt.%, preferably less than or equal to, 0.0025 wt.%, should be used.

[0049] Phosphorus (P) and sulfur (S) are undesirable impurities in the steel flat product according to the invention because they impair its mechanical properties, in particular its impact strength and formability. To avoid any influence of these accompanying elements, which are unavoidable due to the manufacturing process, an upper limit of a maximum of 0.02 wt.%, in particular a maximum of 0.018 wt.%, preferably a maximum of 0.015 wt.%, is set for the P content, and an upper limit of a maximum of 0.01 wt.%, in particular a maximum of 0.009 wt.%, preferably a maximum of 0.008 wt.%, is set for the S content.

[0050] Nitrogen (N) is also an unavoidable impurity due to the manufacturing process and should be limited to a maximum of 0.012 wt.%, as nitrogen impairs the toughness of a flat steel product according to the invention at excessively high concentrations. If N is present, Ti, Cr, and optionally V, Mo, and Nb, in the simultaneous presence of C, preferably form nitrides or carbonitrides with N. Therefore, in practice, under technically and economically feasible conditions, the absorption of N in the precipitates is unavoidable. However, the lowest possible concentrations should generally be aimed for, as N-dominated carbonitrides are often very coarse and angular, which is why they do not contribute to work hardening but act as crack initiators. To avoid the formation of N-dominated carbonitrides, the concentration should be limited, in particular, to a maximum of 0.01%, and more specifically, a maximum of 0.008%.Since nitrogen can form very fine aluminum nitrides in the presence of aluminum, which improve nucleation and hinder grain growth, it is preferable to set a minimum content of 0.002 wt.%.

[0051] In addition to the mandatory and optional alloying elements of the steel flat product according to the invention, the remaining content consists of iron and elements whose presence is unavoidable due to the manufacturing process. The levels of such impurities should be kept as low as possible within economically feasible limits and with reasonable technical effort.

[0052] The alloy concept preferably fulfills the following formula: C 12 + N 14 0,2 ∗ Cr 52 + Nb 93 + Ti 48 + 0,3 ∗ Mo 96 > 1,0 with C, N, Cr, Nb, Ti, and Mo in wt.%. If the aforementioned ratio is met, and in particular if it is greater than 1.20, preferably greater than 1.50, this can promote the achievement of a sufficiently high bake-hardening effect, especially if sufficient free carbon is available. The free carbon, in conjunction with the high coil temperature TH, further promotes the conversion of retained austenite to low-carbon martensite / bainitic ferrite and leads to an excellent bake-hardening effect and particularly good cutting properties.

[0053] In a specific embodiment, the hot-rolled steel flat product according to the invention is provided with a metallic corrosion protection coating. Besides providing good corrosion protection, the metallic corrosion protection coating can also alter the visual appearance of the surface of the hot-rolled steel flat product. A zinc-based corrosion protection coating is particularly preferred. A zinc-based coating contains a zinc content of greater than or equal to 90 wt.%, and preferably greater than or equal to 92 wt.%. Magnesium and / or aluminum are preferably included as further alloying elements in the coating, each in contents of up to a maximum of 5 wt.%, with the remainder being zinc and unavoidable impurities. The zinc-based coating may contain unavoidable impurities. The levels of such impurities should be kept as low as possible within economically feasible limits and with reasonable technical effort.The zinc-based corrosion protection coating can have a thickness between 2 and 30 µm.

[0054] The second teaching concerns processes for the production of a hot-rolled steel flat product, comprising the following steps: Melting of a steel containing in wt.%: C: 0.05-0.12%, Si: 0.1-0.6%, Mn: 2.1-3.5%, Cr: 0.1-1.2%, Ti: 0.01-0.15%

[0055] Optionally, one or more of the following elements with the weight percentage specified below: Nb: < 0.05%, V: < 0.5%, Al: < 0.2%, Mo: < 0.5%, B: < 0.005%, Cu: < 0.5%, Ni: < 0.5%, Ca: < 0.005%, P: < 0.02%, S: < 0.01%, N: < 0.012%,

[0056] Residual iron and unavoidable impurities, Pouring the melt into a semi-finished product in the form of a block, slab, or thin slab; heating the semi-finished product to a temperature and / or holding the semi-finished product at a temperature between 1100 °C and 1500 °C; hot rolling the semi-finished product into a hot-rolled flat steel product at a final hot rolling temperature TE of more than 850 °C and less than 960 °C, with a degree of deformation ε1 of less than 15% in the last hot rolling pass; cooling the resulting hot-rolled flat steel product to a coiling temperature TH below 650 °C; skin-treating the hot-rolled flat steel product.

[0057] The aforementioned steel melt may preferably also contain one or more optional elements or have preferred element contents, which have been explained in detail with reference to the steel flat product.

[0058] The hot rolling of the hot-rolled steel flat product according to the invention can be carried out in a conventional hot strip mill, wherein slabs are rolled to the required thickness in a multi-stage, for example reversing, pre-rolling process and a subsequent multi-stand finishing rolling process.

[0059] The hot rolling of the hot-rolled flat steel product according to the invention can also be carried out in a Compact Strip Production plant (CSP plant) directly from continuously cast pre-material, which is fed immediately after solidification via a compensating furnace into a multi-stand hot rolling mill and rolled there to the required finished thickness in several rolling passes.

[0060] In finish rolling, a finished strip with the desired strip thickness dw is rolled from the roughing strip or thin slab in a multi-stand rolling process. A "multi-stand" rolling process means at least five consecutive passes. The number of passes is selected depending on the desired finished strip thickness.

[0061] The hot rolling temperature TE should be at least 850 °C and at most 960 °C, in particular at least 860 °C and preferably at least 880 °C. One reason for limiting the hot rolling temperature TE downwards is to ensure flatness. If forming occurs at excessively low temperatures in the final stand, there is an increased susceptibility to central and / or edge waviness, which negatively affects the quality of the flat steel product. Limiting the hot rolling temperature TE upwards also limits the rolling speed.

[0062] The hot rolling temperature TE is measured at the exit of the last rolling stand using means known to those skilled in the art, in particular in an area with a distance of up to 15 m behind the roll gap of the last rolling stand, for example with pyrometers.

[0063] The coiling of the cooled, hot-rolled steel flat product into a coil takes place at a coiling temperature TH of at least 500 °C and at most 650 °C, in particular at least 520 °C and at most 640 °C, preferably at least 550 °C and at most 630 °C. The coiling temperature influences the transformation hardening, as well as the precipitation and precipitation hardening, and thus substantially affects the mechanical and technological properties of the steel flat product. Cooling from the rolling end temperature TE to the coiling temperature TH is preferably carried out by means of water cooling.

[0064] The coiling temperature TH in the inlet of the reel is measured using means known to those skilled in the art, in particular in an area with a distance of up to 10 m in front of the reel, for example with pyrometers.

[0065] A coil temperature TH that is too low (less than 500 °C) causes the precipitation potential to freeze, and thus the strength and elongation at break according to the invention can no longer be reliably set.

[0066] Excessively high coil temperatures (TH greater than 650 °C) lead to undesirable grain growth, which, while positively impacting elongation at break, results in a significant loss of strength. Furthermore, it can lead to the formation of hard, coarse, undesirable pearlite islands, which, acting as notches, further impair cutting quality.

[0067] The cooling rate rK for cooling the steel flat product to coil temperature TH is at least 10 K / s, in particular at least 15 K / s, preferably at least 20 K / s and can be limited to a maximum of 60 K / s, in particular a maximum of 50 K / s, preferably a maximum of 40 K / s.

[0068] The specified coiling temperature TH is intended to prevent or limit the formation of an undesirable second phase or a mixed structure. The coiling temperature TH increases precipitation hardening, which also serves to achieve the required tensile strength Rm and yield strength Re.

[0069] Preferably, a degree of deformation ε1 of less than 15%, particularly less than 13%, and preferably less than 11%, is achieved in the last rolling stand, i.e., in the last hot rolling pass, as this leads to sufficient homogenization in the microstructure. ε1 is at least 1%, particularly at least 3%, and preferably at least 5%.

[0070] The steel flat product can be pickled to remove oxides and other interfering components from its surface. The steel flat product can be subjected to known pickling baths before undergoing further processing steps.

[0071] The steel flat product can be tempered, for example, at an annealing temperature TG of at least 630 °C and at most 750 °C, with a holding time tG of at least 20 seconds, in particular at least 30 seconds, preferably at least 40 seconds and at most 200 seconds. The annealing can be carried out in a continuous annealing furnace, such as a continuous annealing furnace. In a preferred embodiment, the hot-rolled steel flat product is annealed as part of the coating process and coated with a metallic corrosion protection coating, in particular a zinc-based corrosion protection coating, which can be applied by hot-dip galvanizing. Alternatively, electrolytic deposition on zinc or a zinc-based alloy would also be conceivable.

[0072] The tempering process takes place with a degree of deformation ε2 between 0.1% and 1.8%, in particular between 0.1% and 1.5%, preferably between 0.2% and 1.1%, in order to achieve the desired mechanical-tribological properties in the steel flat product according to the invention.

[0073] ε is the dimensional change per stitch in the thickness direction in [%]. This is calculated as follows: ε = h 1 − h 0 / h 0 ∗ 100 %

[0074] Here, h1 represents the thickness after the stitch and h0 the thickness before the stitch, and since the result of the respective thickness reduction is negative, the respective amount of the thickness reduction ε is used.

[0075] Preferably, the ratio ϕ = ε1 / ε2 is used, where 3 < ϕ < 150, in particular 4 < ϕ < 120, preferably 5 < ϕ < 100, preferably 6 < ϕ < 80, and most preferably 8 < ϕ < 45. If ϕ ≤ 3, few nucleation sites for precipitates are formed during hot rolling, or the material undergoes excessive work hardening during temper hardening. While this results in a high bake-hardening effect due to the readily available carbon, the elongation at break decreases and the desired yield strength is not achieved. If ϕ ≥ 150, the degree of cold forming from temper hardening is too low, and the desired bake-hardening effect is not achieved. Furthermore, poor hot strip flatness is to be expected.

[0076] The invention is explained in more detail using the following exemplary embodiments.

[0077] In systematic laboratory and operational trials, a total of 14 steel melts were produced, the chemical compositions of which are given in Table 1. All values ​​are in wt.%. Furthermore, the aforementioned formula of the alloy concept (FL) was also calculated. Table 1 Schmelze C Themselves Mn Cr You Nb Al In V B Cu Neither Ca P S N FL 1 0,087 0,15 2,21 0,61 0,048 0,013 0,007 0,11 0,08 0,0011 0 0 0 0,013 0,004 0,005 1,99 2 0,071 0,29 2,53 0,66 0,071 0 0,034 0,09 0 0,0013 0,09 0 0,001 0,012 0,001 0,004 1,44 3 0,09 0,23 1,68 0,53 0,078 0 0,035 0,09 0,09 0,0013 0 0 0 0,002 0,002 0,005 1,99 4 0,088 0,22 2,32 0,64 0,082 0 0,041 0,35 0 0 0,34 0 0 0,002 0,002 0,004 1,45 5 0,09 0,31 2,56 0,92 0,051 0 0,038 0,15 0 0,001 0 0,27 0,0011 0,009 0,003 0,004 1,25 6 0,06 0,25 2,62 0,24 0,06 0 0,036 0,14 0 0,0011 0 0,19 0 0,002 0,002 0,005 2,05 7 0,092 0,35 3,2 0,96 0,09 0 0,032 0 0 0,0017 0 0 0,0009 0,003 0,002 0,004 1,43 8 0,065 0,5 2,85 0,42 0,025 0 0,034 0,09 0 0 0 0 0 0,002 0,003 0,003 2,33 9 0,04 0,71 2,44 0,66 0,072 0,021 0,03 0,08 0 0 0 0 0 0,002 0,003 0,003 0,79 10 0,131 0,54 1,94 0,89 0,063 0 0,044 0,22 0 0 0 0 0 0,004 0,002 0,005 2,08 11 0,072 0,37 3,25 0,22 0,031 0,012 0,02 0 0 0,0019 0 0,11 0 0,009 0,002 0,005 3,92 12 0,126 0,66 2,23 0,71 0,101 0 0,041 0 0 0 0 0 0 0,007 0,003 0,004 2,23 13 0,083 0,42 2,67 0,12 0,073 0 0,037 0,37 0 0,0021 0 0 0 0,012 0,003 0,005 2,32 14 0,102 0,63 3,89 1,13 0,063 0 0,032 0,21 0 0 0,15 0 0 0,008 0,002 0,003 1,38

[0078] All molten steel was cast into slabs in a conventional continuous casting plant. The cooled slabs were heated in a furnace to a temperature of 1220 °C and then hot-rolled. This process includes descaling and rough rolling, finish rolling in several stands taking into account the parameters according to the invention, at a hot-rolling end temperature (TE), cooling with water at a cooling rate (rK), and coiling at a coiling temperature (HT). Subsequently, all strips were coated with a metallic corrosion protection layer and skin-treated. The parameters for producing the hot-rolled strips are given in Table 2. Table 2 StahI TEA TH rK ε1 ε2 Φ 1 901 622 22 11 0,7 15,7 2 921 601 29 9 0,3 30 3 889 541 28 13 0,3 43,3 4 955 586 31 7 0,8 8,8 5 895 620 18 16 0,2 80 6 893 564 33 14 1,5 9,3 7 940 651 25 17 0,1 170 8 881 591 19 11 0,1 110 9 911 580 25 15 0,3 50 10 926 556 40 13 0,5 26 11 956 582 29 8 2,1 3,8 12 906 522 27 10 1,1 9,1 13 885 599 37 4 0,9 4,4 14 916 641 21 8 0,2 40

[0079] Mechanical properties such as tensile strength, yield strength, and elongation reported here were determined in tensile tests according to DIN EN ISO 6982-1, specimen type 2 (Annex B, Table B1) (dated June 2020), unless explicitly stated otherwise. The bending angle is determined according to VDA standard 238-100 for the maximum force. The results are listed in Table 3. Table 3 Stahl dw Rm King A80 / Aprop BH Widow Win Verhaltnis Matrix RA Martensite Ausscheidung (Dutch.) 1 3 1001 826 12 50 78 32 1,89 bait. Ferrite 2 1 < 40 2 2 992 834 10 35 84 35 1,44 bait. Ferrite 4 1 < 40 3 3 815 690 14 51 101 41 1,99 Bathe 6 2 < 40 4 2,5 961 736 11 43 98 29 1,45 bait. Ferrite 5 2 < 40 5 5,5 1040 760 12 33 69 30 1,25 bait. Ferrite 5 1 < 40 6 4 1056 801 12 55 63 26 2,05 bait. Ferrite 4 2 < 40 7 2 933 668 13 19 112 31 1,43 Bathe 8 3 < 100 8 2 1098 820 10 66 71 23 2,33 bait. Ferrite 3 1 < 40 9 4 794 638 17 16 131 68 0,79 Ferrite 1 1 < 200 10 4 944 694 13 67 114 41 2,08 Bathe 6 2 < 100 11 5,5 1056 803 9 74 64 21 3,92 bait. Ferrite 4 3 < 40 12 3 1126 901 8 57 56 18 2,23 Bathe 6 7 < 100 13 2 964 748 10 52 88 41 2,32 bait. Ferrite 2 1 < 40 14 2,5 998 801 12 44 53 16 1,38 Bathe 9 1 < 100

[0080] The microstructure was determined from longitudinal sections etched with 3% Nital (alcoholic nitric acid). The proportion of retained austenite was determined by X-ray diffraction, with other microstructural components identified using LOM and precipitates determined using TEM.

[0081] It is clearly evident that steels 3, 7, 9, 10, 12, and 14 are not embodiments of the invention, as not all requirements are met. Steel 3 has a low Mn content and does not meet the required tensile strength. Steel 7 was subjected to an excessively high coiling temperature, and neither the required tensile strength nor yield strength was achieved. Furthermore, the ratio ϕ was also too high, preventing the desired bracing effect from being achieved. Steel 9 contained too little C and too much Si, and the required tensile strength and yield strength were not reached. Steel 10, on the other hand, contained too much C and too little Mn, and the required tensile strength could not be achieved. Steel 12 contained too much C and too much Si, and the required elongation, hole expansion, and bending angle could not be achieved. Steel 14 contained too much Si and too much Mn, and the required hole expansion and bending angle could not be achieved.

[0082] The steel flat product was further characterized with regard to its cutting behavior. Cracks perpendicular to the sheet thickness direction, often referred to as delamination, are fundamentally critical for its serviceability. To compare the robustness of different steel compositions against this unfavorable cut surface formation, standardized cutting tests were conducted and evaluated.

[0083] Before the actual cutting test, one sample strip, 20 mm wide and between 200 and 400 mm long, was cut from the steel flat product under investigation using a guillotine shear. The cutting was performed on a Schuler PD80-280 high-speed mechanical press. The cut was made as a shear with an open cut line. The waste material was not supported by a counter-holder. The sample was positioned perpendicular to the rolling direction, starting from the center of the sheet. The center of the sheet represented the entire length of the strip, excluding the first 30 m at the beginning and the last 30 m at the end, and the entire width, excluding the first 30 mm from the outer edge on both sides. A cutting gap of 8% relative to the sheet thickness was set. The resulting cut surfaces were then evaluated by macroscopic inspection.The cut surface was defined as the entire area consisting of the smooth cut portion (2) and the fracture portion (1), as in . Figure 1 As shown, if a macroscopic crack was detected, it could be measured, for example, with a ruler. A macroscopic crack was defined as a crack with a minimum length of 1 mm and a length-to-width ratio greater than 5.

[0084] The hot-rolled steel flat product shows no macroscopic cracks as defined above on a maximum of 80%, in particular a maximum of 90%, preferably a maximum of 95% of the cut sheets out of a total of 20 sheet cuts, in order to meet in particular the requirements of crack sensitivity.

Claims

1. Hot-rolled flat steel product consisting of a steel with the following composition (in wt.%): C: 0,05-0,12 %, To: 0,1-0,6 %, Mn: 2,1-3,5 %, Cr: 0,1-1,2 %, Of: 0,01-0,15 %, Optionally, one or more of the following elements with the weight percentage specified below: Note: < 0,05 %, V: < 0,5 %, Al: 0,005-0,2 %, Know: < 0,5 %, B: < 0,005 %, Cu: < 0,5 %, In: < 0,5 %, Ca: < 0,005 %, P: < 0,02 %, S: < 0,01 %, N: < 0,012 %, Residual iron and unavoidable impurities, and whose microstructure may contain a maximum of 9% retained austenite and a maximum of 11% martensite, the remainder being bainitic ferrite with precipitation in the form of carbides, nitrides and / or carbonitrides, and a tensile strength R m according to DIN-EN ISO 6892-1 of at least 950 MPa, a yield strength R e according to DIN-EN ISO 6892-1 of at least 670 MPa and a bake-hardening effect BH according to DIN EN 10325 of at least 40 MPa.

2. Steel flat product according to claim 1, wherein it has an elongation at break A80 or Aprop according to DIN-EN ISO 6892-1 of at least 8%.

3. Steel flat product according to one of the preceding claims, wherein it contains Ti in 0.02 to 0.011 wt.%.

4. Steel flat product according to one of the preceding claims, wherein it contains Cr in 0.2 to 1.0 wt.%.

5. Steel flat product according to one of the preceding claims, wherein it comprises Al in 0.005 to 0.2 wt.%.

6. Steel flat product according to one of the preceding claims, wherein the alloy concept satisfies the following formula: C 12 + N 14 0,2 ∗ Cr 52 + Nb 93 + Ti 48 + 0,3 ∗ Mo 96 > 1,0 with C, N, Cr, Nb, Ti and Mo in wt.%.

7. Steel flat product according to one of the preceding claims, wherein it is provided with a metallic corrosion protection coating.

8. Steel flat product according to one of the preceding claims, wherein it has a bending angle according to VDA 238-100 of at least 60°.

9. Steel flat product according to one of the preceding claims, wherein it has a hole expansion according to ISO 16650 of at least 20%.

10. Method for producing a hot-rolled steel flat product, comprising the steps of: melting a steel containing in wt.%: C: 0,05- 0,12 %, To: 0,1-0,6 %, Mn: 2,1-3,5 %, Cr: 0,1-1,5 %, Of: 0,01-0,12 %, Optionally, one or more of the following elements with the weight percentage specified below: Note: < 0,05 %, V: < 0,5 %, Al: < 0,2 %, Know: < 0,5 %, B: < 0,005 %, Cu: < 0,5 %, In: < 0,5 %, Ca: < 0,005 %, P: < 0,02 %, S: < 0,01 %, N: < 0,012 %, Residual iron and unavoidable impurities; - Casting the melt into a semi-finished product in the form of a block, slab, or thin slab; - Heating the semi-finished product to a temperature and / or holding the semi-finished product at a temperature between 1100 °C and 1500 °C; - Hot rolling the semi-finished product into the hot-rolled flat steel product at a final hot rolling temperature TE of more than 850 °C and less than 960 °C, with a degree of deformation ε1 of less than 15% in the last hot rolling pass; - Cooling the resulting hot-rolled flat steel product to a coiling temperature TH below 650 °C; - Skin-treating the hot-rolled flat steel product.

11. Method according to claim 10, wherein the dressing takes place with a degree of deformation ε2 between 0.1% and 1.8%.

12. Method according to claim 10 or 11, wherein the degree of deformation ε1 in the last hot rolling pass and the degree of deformation ε2 during the tempering process has a ratio ϕ = ε1 / ε2, wherein for ϕ: 5 < ϕ < 150.

13. Method according to one of claims 10 to 12, wherein cooling to the coil temperature TH between 500 °C and 650 °C takes place with a cooling rate rK of at least 10 K / s.

14. Method according to any one of claims 10 to 13, wherein the steel flat product is pickled.

15. Method according to any one of claims 10 to 14, wherein the steel flat product is coated with a metallic corrosion protection coating.

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