METHOD FOR PRODUCING A HOT-ROLLED FLAT STEEL PRODUCT AND FLAT STEEL PRODUCT

Description

[0001] The invention relates to a method for producing a hot-rolled flat steel product with a microstructure whose main components are tempered or freshly formed martensite and ferrite, the remainder of the microstructure being filled with retained austenite, bainite and / or cementite.

[0002] The invention also relates to a flat steel product with a corresponding structure, wherein the flat steel product can be produced in particular by the method according to the invention.

[0003] In this text, unless explicitly stated otherwise, information on the contents of alloying components is always given in mass %. The microstructure components of a flat steel product are given here in vol % unless otherwise stated.

[0004] Image analysis for quantitative microstructure determination is performed using optical light microscopy ("LOM") with 1000x resolution and a field emission scanning electron microscope ("FE-SEM") with 20,000x resolution. The imaging and measurement of the retained austenite rim present on the martensite islands of the microstructure in flat steel products according to the invention, as explained below, was also performed using the FE-SEM at 20,000x magnification.

[0005] The strength and elongation properties mentioned here, such as tensile strength Rm, uniform elongation Ag, and elongation at break A50 of flat steel products, were determined in tensile tests according to DIN EN 6892-1, specimen form 1, transverse to the rolling direction (WR), unless otherwise noted. The elongation at break A80 was calculated according to DIN EN 2566-1 (Sept. 1999, Chapter 9.3).

[0006] The hole expansion behavior or the achievable hole expansion HER of the flat steel products were determined on 100*100 mm 2< samples according to ISO 16630.

[0007] The qualitative C distribution in the microstructure of the flat steel products was determined using an FE microprobe, as described by H. Farivar et al. in the article "Experimental quantification of carbon gradients in martensite and its multiscale effects in a DP steel", MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING 718 (2018) 250-259. An example of the evaluation used here can be found in Fig. 2 .

[0008] EP 2 690 183 A1 discloses a hot-rolled flat steel product which, in mass%, is composed of C: 0.10 - 0.60%, Si: 0.4 - 2.0%, Al: up to 2.0%, Mn: 0.4 - 2.5%, Ni: up to 1%, Cu: up to 2.0%, Mo: up to 0.4%, Cr: up to 2%, Ti: up to 0.2%, Nb: up to 0.2%, V: up to 0.5% and the remainder of iron and unavoidable impurities. In addition to optionally present proportions of up to 5 vol.% ferrite and up to 10 vol.% martensite, the microstructure of the flat steel product consists of at least 60 vol.% bainite and the remainder of retained austenite, with at least part of the retained austenite in blocky form and the blocks of the retained austenite in blocky form having an average diameter of less than 5 µm to at least 98%.Such a steel flat product can be manufactured by producing a precursor in the form of a slab, thin slab or cast strip from a melt composed in the specified manner, which is then hot-rolled into a hot strip in one or more rolling passes, whereby the hot strip obtained has a final hot rolling temperature of at least 880 °C upon leaving the last rolling pass. The hot-rolled steel flat product thus obtained is cooled at an accelerated rate of at least 5 °C / s to a coiling temperature which lies between the martensite start temperature MS and 600 °C, and wound into a coil at this temperature.The flat steel product is then cooled in the coil, with the temperature of the coil being maintained during cooling to form bainite within a temperature range whose upper limit is equal to the bainite start temperature BS, above which bainite forms in the microstructure of the hot strip, and whose lower limit is equal to the martensite start temperature MS, above which martensite forms in the microstructure of the hot strip, until at least 60 vol.% of the microstructure of the hot strip consists of bainite. A hot-rolled flat steel product produced in this way regularly has tensile strengths Rm of more than 1000 MPa, in particular at least 1200 MPa, with elongations A80 that are also regularly above 17%, in particular above 19%. Accordingly, the quality Rm*A80 of known flat steel products is regularly in the range of 18,000 - 30,000 MPa*%.

[0009] Furthermore, US 2014 / 0261914 A1 discloses a process for producing hot-rolled high-strength dual-phase steels consisting, in weight percent, of 0.06–0.09% C, 0.8–2.0% Mn, min. 0.4% Cr, max. 0.08% Si, max. 0.05% P, 0.005–0.010% N, Al in a content for which (Al / N) ≤ 10.0, a total content of Nb+Ti+Mo of a maximum of 0.015%, and the remainder consisting of Fe and unavoidable impurities. The contents of Nb, Ti, and Mo are minimized to within the impurity range. According to a first variant, appropriately alloyed steel strip is continuously cooled to less than 100°C after hot rolling at a cooling rate of 40-70°C / s and then coiled. In another variant, the hot-rolled strip is cooled to less than 100°C by a stepwise cooling process before coiling.In a third variant, the hot-rolled strip is continuously cooled to less than 100°C before coiling, using a cooling rate of between 70 and 100°C / second. The hot-rolled strip subjected to a cooling rate of between 40 and 70°C / second or to stepwise cooling should have a tensile strength of more than 590 MPa. The hot-rolled strip subjected to a cooling rate of 70 and 100°C / s should achieve a tensile strength of more than 690 MPa. The hot-rolled steel sheet exhibits a ferrite and martensite structure, free of pearlite and bainite.

[0010] Finally, JP 2006-316301 A discloses a process which is also intended to produce high-strength hot-rolled steel sheets with a tensile strength of ≥ 590 MPa and excellent ductility, bending processability, fatigue properties and surface properties. For this purpose, a steel composition is provided which, in addition to iron and unavoidable impurities, can contain, in mass%, > 0.01 - 0.25% C, > 0.2 - < 1.0% Si, 0.5 - 2.5% Mn, 0.003 - 0.03% P, ≤ 0.02% S, 0.005 - 1.0% Al, ≤ 0.01% N, ≤ 0.2% Ti, ≤ 0.1% Nb, ≤ 0.5% V, ≤ 0.5% W, as well as optionally further contents of Cr, Cu, Mo, Ni and B. The microstructure of the steel sheet consists of 50 to 95 area% ferrite, with the remainder of the microstructure being filled with martensite and less than 5% other phases.

[0011] Based on the state of the art explained above, the object was to provide a method for producing a flat steel product which is further improved with regard to its mechanical properties and is characterized in particular by favorable hole expansion behavior.

[0012] Likewise, a hot-rolled flat steel product should be specified with a property spectrum that offers an optimized combination of high strength and good formability, in particular good hole expansion behavior.

[0013] With regard to the method, the invention has achieved this object by completing at least the work steps specified in claim 1 during the production of a hot-rolled flat steel product. It goes without saying that when carrying out the method according to the invention, the person skilled in the art not only completes the process steps mentioned in the claims and explained in detail here, but also carries out all other steps and activities that are regularly carried out in the practical implementation of such methods in the prior art, if the need arises.

[0014] A hot-rolled flat steel product which achieves the above-mentioned object has at least the features specified in claim 7.

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

[0016] In the case of the inventive production of a hot-rolled flat steel product with a structure consisting of 5 - 40 vol.% of tempered or freshly formed martensite and 50 - 95 vol.% of ferrite, and in which the portion not occupied by martensite and ferrite is filled with up to 45 vol.% of residual austenite, bainite and / or cementite, at least the following work steps are carried out: a) Casting of a steel melt to form a precursor in the form of a slab, thin slab or a cast strip, wherein the melt consists of, in mass%, C: 0,05 - 0,15 %, Si: < 0,5 %, Mn: 0,7 - 2,1 %, Al: < 0,1 %, Cr: 0,2 - 1 %, at least one element from the group "Nb, Ti, V" with the proviso that the sum of the contents of Nb, Ti and / or V is 0.01 - 0.1%, B: < 0,0015 %, Mon: < 0,2 %, Cu: < 0,2 %, No: < 0,2 % P: < 0,05 % N: < 0,01 % and the remainder consists of Fe and a total of not more than 0.1% by mass of unavoidable impurities; b) heating the precursor product at a temperature of 1150 - 1380 °C for a period of 60 min to 960 min; c) optional: descaling of the precursor product; d) in the case of the pre-product being a slab: pre-rolling of the pre-product at temperatures of 1020 - 1150 °C to a thickness of 30 - 50 mm; e) Hot rolling of the optionally pre-rolled intermediate product in one or more rolling passes to a hot-rolled strip with a thickness of 1.4 - 6.4 mm, whereby the final hot rolling temperature ET, at which the hot rolling is terminated, is at least equal to the Ar 3 -temperature of the steel and at most 200 °C higher than the Ar 3 -temperature of the steel; f) Cooling the hot strip obtained to a coiling temperature HT which lies in a range below the martensite start temperature T MS of the steel and ends at room temperature, wherein the cooling rate dT1 is 20 K / s to 90 K / s in a temperature range extending from the hot rolling end temperature ET to an intermediate temperature Tz which is 50 °C below the Ar 1 temperature of the steel, wherein after reaching the intermediate temperature Tz, according to a first variant, the hot strip is cooled at a cooling rate dT2' of 5 - 100 K / s to the coiling temperature HT lying in the range between the martensite start temperature T MS of the steel and room temperature, or according to a second variant, the hot strip is cooled at a cooling rate dT2" of 10 - 130 K / s, first to a cooling stop temperature ZT1 lying in the range 550 - 770 °C and at which the hot strip is optionally held for a holding time tH of up to 5 s, and then to the coiling temperature HT lying in the range between the martensite start temperature T MS of the steel and room temperature becomes; g) coiling the hot strip cooled to the coiling temperature HT into a coil; h) optionally cooling the hot-rolled flat steel product in the coil to room temperature or keeping the hot-rolled flat steel product at the coiling temperature HT.

[0017] A hot-rolled flat steel product according to the invention accordingly comprises a steel substrate, which consists of, in mass%, C: 0.05 - 0.15%, Si: < 0.5%, Mn: 0.7 - 2.1%, Al: < 0.1%, Cr: 0.2 - 1%, at least one element from the group "Nb, Ti, V" with the proviso that the sum of the contents of Nb, Ti and / or V is 0.01 - 0.1%, B: < 0.0015%, Mo: < 0.2%, Cu: < 0.2%, Ni: < 0.2%, P: < 0.05%, N: < 0.01% and the remainder of Fe and a total of not more than 0.1 mass% of unavoidable impurities and whose structure consists of 5 - 40 vol.% of tempered or freshly formed martensite and 50 - 95 vol.% of ferrite and in which the portion not occupied by martensite and ferrite from up to 45 vol.% retained austenite, bainite and / or cementite, wherein the martensite portions of the structure are island-shaped and at least 70% of all martensite islands have a central region covering the center of the respective martensite island, which is bordered by an edge region of the respective martensite island bordering the edge of the respective martensite island, wherein the C content of the edge region is higher at least in one section than the C content of the central region and wherein the martensite islands are bordered by a rim consisting of retained austenite at least over part of their circumference.

[0018] Typically, the section or sections of the edge region of the martensite islands in which a higher C content is present than in the central region of the respective martensite island, in total, takes up at least 30 - 70 % of the circumference of the respective martensite island, as can be seen from FIG 2 visible.

[0019] The inventive manufacturing method makes it possible to produce a hot-rolled flat steel product according to the invention in which most martensite islands with an average diameter of more than 3 µm have a higher C content in at least one section of their edge regions than in the center region of the respective martensite island. Such a flat steel product is characterized by particularly good hole expansion properties.

[0020] The so-called "martensite islands" in this text are also referred to as "martensite grains" in technical terms.

[0021] With regard to the mechanical properties of a flat steel product according to the invention, it proves particularly advantageous if the martensite islands of the microstructure of a flat steel product according to the invention are bordered by a rim consisting of retained austenite over at least part of their circumference. The width of this rim is typically 10 nm to 1 µm, but can also be up to one-third of the diameter of the respective martensite island.

[0022] The improved tensile strength and elongation in a flat steel product according to the invention are achieved by the presence of several phases and the associated high hardening, and the good hole expansion by the reduction of shear stresses compared to pure dual-phase structures.

[0023] Thus, a flat steel product according to the invention achieves tensile strengths Rm, hole expansions HER and uniform elongations Ag, the product of which Rm x HER x Ag is regularly at least 200,000 MPa% 2< , in particular at least 300,000 MPa% 2< .

[0024] The tensile strength Rm of a hot-rolled flat steel product according to the invention regularly reaches values ​​of at least 530 MPa, the hole expansion HER regularly reaches values ​​of at least 20% and the uniform elongation Ag regularly reaches values ​​of at least 5%, in particular at least 8%.

[0025] The alloy of the melt produced to manufacture a flat steel product according to the invention, and thus of the steel substrate of a flat steel product according to the invention, was selected as follows: Carbon ("C") is present in the flat steel product according to the invention in amounts of 0.05 - 0.15 mass% to achieve the required strength level. At least 0.05 mass% C is required for this. The effects of the presence of C utilized according to the invention are achieved particularly reliably when the C content is at least 0.065 mass%. Limiting the C content to a maximum of 0.15 mass%, in particular less than 0.15 mass%, ensures that a sufficient amount of ferrite forms in the microstructure of a flat steel product according to the invention and that the martensite formed can actually deform in some areas, thus reducing shear stresses.This effect can be achieved in particular if the C content is limited to a maximum of 0.14 mass%, in particular a maximum of 0.12 mass%.

[0026] Silicon ("Si") can be present in the steel of a flat steel product according to the invention to strengthen the steel. This effect can be reliably achieved with Si contents of at least 0.01 mass%, in particular 0.04 mass%. However, excessively high Si contents would increase the Ar3 temperature. This would complicate the hot rolling process desired by the invention in a temperature range in which the flat steel product has a completely austenitic microstructure. The invention avoids this by limiting the Si content to less than 0.5 mass%, in particular less than 0.4 mass%.

[0027] Manganese ("Mn") is present in the steel of a flat steel product according to the invention in contents of 0.7 - 2.1 mass% to minimize the concentration of C in the microstructure and the associated formation of undesirable hard martensite. This effect is particularly reliably achieved with Mn contents of at least 0.7 mass%. At contents of more than 2.1 mass%, there is a risk of Mn segregations forming in the microstructure of the flat steel product according to the invention, which would impair the mechanical properties. This negative influence of the presence of Mn can be particularly reliably excluded by limiting the Mn content to a maximum of 2.0 mass%.

[0028] The aluminum ("Al") content in a steel flat product according to the invention is limited to less than 0.1 mass% to avoid the effects of this alloying element on the Ar3 temperature and to ensure optimized castability of the molten steel. However, Al can be used for deoxidation during steel production. This typically requires Al contents of at least 0.02 mass%. Negative effects caused by the presence of Al can be avoided, in particular, by limiting the Al content to less than 0.05 mass%.

[0029] Chromium ("Cr") is present in the steel of a flat steel product according to the invention in amounts of 0.2 - 1 mass% to increase hardenability and suppress pearlite formation. For this purpose, at least 0.2 mass% Cr is required, whereby the beneficial effects of the presence of Cr can be particularly reliably utilized at Cr contents of at least 0.25 mass%. At the same time, the Cr content is a maximum of 1 mass% to enable the formation of ferrite in the microstructure of the flat steel product according to the invention, as desired by the invention. This can be particularly reliably ensured by limiting the Cr content to a maximum of 0.9 mass%.

[0030] The steel of a flat steel product according to the invention contains at least one of the microalloying elements niobium ("Nb"), vanadium ("V"), and titanium ("Ti") to increase fine grain size and strength. The sum of the contents of these elements is 0.01–0.1 mass% according to the invention. The intended content of the microalloying elements can be provided by one of the microelements alone, or two or three of the aforementioned microalloying elements can be present in combination. The positive influences of the microalloying elements on the mechanical properties of a flat steel product according to the invention can be utilized particularly reliably if their sum total is at least 0.01 mass%. At the same time, the contents of the microalloying elements are limited to a maximum of 0.1 mass%, in particular a maximum of 0.05 mass%, in order to avoid precipitation and enable accelerated recrystallization.

[0031] Boron ("B") can optionally be present in the steel of a flat steel product according to the invention in amounts of up to 0.0015 mass%. It significantly increases hardenability. However, this must not be too high to allow the formation of sufficient amounts of ferrite in the microstructure of a flat steel product according to the invention. Negative effects of the presence of B can be particularly reliably avoided by limiting the B content to a maximum of 0.0008 mass%.

[0032] Molybdenum ("Mo") can optionally be added to the steel of a flat steel product according to the invention in amounts of less than 0.2 mass%, in particular less than 0.20 mass%, to increase hardenability. In practice, at least 0.01 mass% Mo can be provided for this purpose. An alloy of a steel according to the invention that is particularly balanced from a cost / benefit perspective contains up to 0.18 mass% Mo or up to 0.1 mass% Mo, in particular up to 0.05 mass% Mo or up to 0.018 mass% Mo.

[0033] Copper ("Cu") can also optionally be added to the steel of a flat steel product according to the invention in amounts of less than 0.2 mass% to further increase strength (precipitation and solid crystal strengthening). The positive effect of the presence of Cu can be reliably utilized at Cu contents of at least 0.1 mass%.

[0034] Nickel ("Ni") can also optionally be added to the steel of a flat steel product according to the invention in amounts of less than 0.2 mass% to further increase strength through precipitation and solid crystal strengthening. The positive effect of the presence of Ni can be reliably utilized at contents of at least 0.1 mass% Ni.

[0035] Phosphorus ("P") can also optionally be present in the steel of the invention at levels of less than 0.05 mass% to further increase strength and control transformation behavior. The positive effect of the presence of P can be reliably utilized at levels of at least 0.002 mass% P.

[0036] Nitrogen ("N") is one of the unavoidable impurities present in steel due to the manufacturing process. In the steel of the invention, concentrations of less than 0.01 mass% are permitted as harmless to the properties. Higher concentrations would lead to coarse precipitates, which could negatively impact the forming behavior.

[0037] In step b), the precursor product, cast in an otherwise conventional manner from a melt composed according to the above remarks, is thoroughly heated at a temperature of 1150–1380°C for a period of 60–960 minutes. The maximum temperature and duration of the thorough heating must be such that all carbides contained in the precursor product are dissolved. For this purpose, the heating temperature is preferably below 1380°C. If a conventional slab is processed as the precursor product, a thorough heating time of at least 60 minutes has proven particularly effective; however, in practice, a maximum heating time of 8 hours is sufficient for thorough heating with conventional slab dimensions.The lower limit of the temperature range of the soaking process specified according to the invention is at least 1150 °C, preferably more than 1200 °C, in order to prevent the formation of precipitates and other undesirable phases in the structure of the precursor product.

[0038] In order to produce a flat steel product with an optimal surface finish in the subsequent hot rolling process, the preliminary product can optionally be descaled if necessary before it is fed into the hot rolling process.

[0039] In any case, if the precursor is a slab, it is pre-rolled at temperatures of 1020–1150 °C to a thickness of 30–50 mm. Pre-rolling compacts the cast structure of the slab, creating the best conditions for the subsequent final hot rolling. If the precursor is a thin slab or a cast strip, pre-rolling is not necessary.

[0040] Hot rolling of the optionally pre-rolled precursor to a thickness of 1.5–6.4 mm can be carried out conventionally in one or more steps. The only critical requirement is that the final hot rolling temperature (ET), at which hot rolling is terminated, is at least equal to the Ar 3 temperature of the steel and no more than 200 °C higher than the Ar 3 temperature of the steel. Final hot rolling temperatures of 820–900 °C are particularly practical.

[0041] The Ar 3 temperature of steels of the type processed according to the invention can be determined experimentally in a conventional manner or according to the formula (1) given in CHOQUET, P. et al.: Mathematical Model for Predictions of Austenite and Ferrite Microstructures in Hot Rolling Processes. IRSID Report, St. Germain-en-Laye, 1985, p. 7. Ar 3 = Ar 3 = 902 − 527 * C − 62 * Mn + 60 * Si with %C = C content, %Mn = Mn content, %Si = Si content, of the steel

[0042] be estimated in a manner sufficient for the invention. The hot rolling end temperature is selected according to the invention such that hot rolling takes place as exclusively as possible in a temperature range in which an austenitic microstructure is present in the hot-rolled flat steel product. For this purpose, the hot rolling end temperature can be set to at least 820 °C. At the same time, the hot rolling end temperature is at most 200 °C, in particular less than 200 °C, above the Ar 3 temperature in order to support the development of a fine-grained austenite microstructure in which as many nucleation sites as possible are present for the subsequent ferrite formation. Particularly suitable hot rolling end temperatures are therefore in the range of 820 - 900 °C.

[0043] Crucial to the success of the invention is the strategy for cooling the hot-rolled strip to the respective coiling temperature. Above all, the cooling rate dT1 between the final hot-rolling temperature and the intermediate temperature of Ar1 at -50 °C must be at least 20 K / s to ensure that a concentration profile of C in the austenite is created during ferrite formation, which is later converted to martensite. Cooling rates dT1 of at least 30 K / s are particularly suitable for this purpose. In practice, the cooling rate dT1 is limited to 90 K / s for efficiency reasons.Through the cooling down to the intermediate temperature Tz controlled according to the invention, the cooling rate is controlled in such a way that, on the one hand, sufficient ferrite is formed and, on the other hand, a sufficiently high diffusion of carbon from the ferrite into the adjacent austenite is enabled, through which the retained austenite, which later forms the rim region of the martensite islands, is enriched with carbon. In this temperature range, C diffusion from the forming ferrite into the adjacent retained austenite can occur and diffuse therein.

[0044] The Ar 1 temperature can be determined experimentally in a conventional manner or according to the formula (2) Ar 1 = 741 , 7 − 7 , 13 × % C − 14 , 09 × % Mn + 16 , 26 × % Si + 11 , 54 × % Cr − 49 , 69 × % Ni where %C = C content, %Mn = Mn content, %Si = Si content, %Cr = Cr content and %Ni = Ni content of the steel, which was given by LUTSENKO, A. et al. in the article "The Definition and Use of Technological Reserves - An Effective Way to Improve the Production Technology of Rolled Metal", published in 9th International Rolling Conference, Associazione Italiana di Metallurgia, Venice, June 2013, 8 p.

[0045] In a flat steel product according to the invention, the section or sections of the edge region of the martensite islands in which a higher C content is present than in the middle region of the respective martensite island and this C concentration at the edge is so high that the retained austenite can be seen at 20,000x magnification (see FIG. 1 ), in total at least 30% of the circumference of the martensite island in question.

[0046] The "higher C content" is defined as at least 0.05 wt.%, although higher C contents of at least 0.1 wt.%, especially at least 0.15 wt.%, have proven particularly advantageous in practice. The C concentration is determined using an FE microprobe within a range of 300 x 300 nm 2< .

[0047] Through the cooling controlled according to the invention, island-like martensite is obtained in the microstructure of a flat steel product according to the invention, which usually has an inhomogeneous distribution of the carbon content throughout its volume. In some cases, retained austenite remains at the martensite edges, forming a rim surrounding the respective martensite island. This rim typically has a width of 10 nm - 1 µm, although its width can also be up to 1 / 3 of the island diameter. The carbon concentration, which increases towards the edge region, extends in a flat steel product according to the invention by at least 30% of the circumference of the martensite islands (see, for example, the martensite islands shown in Fig. 1shown) and is present in at least 70% of all martensite islands. The C concentration profile produced according to the invention can be observed on all martensite islands that have a diameter ØMI of more than 3 µm, determined according to the formula ØMI = half the shortest length of the martensite island + half the longest length of the martensite island (see Fig. 1 , Martensite island M).

[0048] The C gradient created in the martensite islands of the microstructure according to the invention increases the hole expansion HER by preventing the formation of large martensite islands with homogeneous carbon distribution, which would increase the shear stress in a ferritic matrix and thus minimize hole expansion. Furthermore, the residual austenite present between the ferrite matrix and the respective martensite island, according to the invention, achieves smoother transitions between the soft ferrite matrix and the hard martensite islands and facilitates deformation in partial areas of the martensite island. The C distribution allows areas in the martensite to deform earlier under external loading, thereby reducing steep hardness jumps that are detrimental to the hole expansion HER. Nevertheless, a high degree of hardening of the microstructure remains due to the hardness differences.This results in good elongation combined with good hole expansion values ​​HER at high strength values.

[0049] The further cooling strategy subordinately promotes the advantageous product properties: After the intermediate temperature Tz is reached, in a second cooling phase the cooling rate in the temperature range up to the martensite start temperature T MS is controlled in such a way that the diffusion length of C in austenite remains as limited as possible.

[0050] The martensite start temperature T MS can be determined experimentally in a conventional manner or according to the formula (3) published by SMC Van Bohemen in the article "Bainite and martensite start temperature calculated with exponential carbon dependence", Mater. Sci. Technol. 28 (2012) 487-495 T MS = 565 − 600 × 1 − EXP − 0 , 96 × % C − 31 × % Mn − 13 × % Si − 10 × % Cr − 12 × % Mo with %C = C content, %Mn = Mn content, %Si = Si content, %Cr = Cr content and %Mo = Mo content of the steel.

[0051] According to a first variant of the second cooling stage, cooling is carried out starting from the intermediate temperature Tz at a cooling rate dT2' of at least 5 K / s, in particular more than 5 K / s or at least 20 K / s, until the range between the martensite start temperature TMS and room temperature is reached, so that carbon diffusion does not homogenize all of the adjacent residual austenite with carbon. In this variant, the cooling rate is limited to a maximum of 100 K / s to ensure that carbon can diffuse from the previously formed ferrite into the adjacent austenite. This can be particularly reliably ensured by limiting the cooling rate dT2' to a maximum of 70 K / s. It is therefore particularly practical if the cooling rate dT2' is 20 - 70 K / s.

[0052] According to the second variant of the second cooling stage, cooling to the martensite start temperature TMS is completed at a cooling rate dT2" of 10 - 130 K / s. A cooling rate dT2" of at least 10 K / s, in particular at least 30 K / s, also limits carbon diffusion from the ferrite into the austenite. The diffusion of a sufficient amount of carbon can be supported by interrupting cooling at a cooling stop temperature of 550 - 700 °C for up to 5 s.

[0053] A pause of at least 1 s is particularly practical here. At the same time, the cooling rate dT2" should be no more than 130 K / s, and especially less than 100 K / s, to allow sufficient carbon diffusion length into the austenite. This can be particularly reliably ensured by limiting the cooling rate dT2" to a maximum of 80 K / s. It is therefore particularly practical if the cooling rate dT2" is 30 - 80 K / s.

[0054] The third cooling stage, in which the hot-rolled flat steel product reaches the coiling temperature (HT), is non-critical and can be carried out at a cooling rate in still air. The coiling temperature (HT) is lower than the martensite initiation temperature and can reach room temperature. In practice, the coiling temperature (HT) is typically between 20 and 80 °C.

[0055] The cooled hot-rolled flat steel product is coiled into a coil. If the coiling temperature HT is above room temperature, the flat steel product is then cooled to room temperature in the coil.

[0056] The invention is explained below using exemplary embodiments. Fig. 1 shows a section of a structure of a flat steel product according to the invention in 20,000x magnification; Fig. 2 shows an enlarged section of Fig. 1; Fig. 3 a graphic representation of the section according to Fig. 2 .

[0057] To test the invention, four melts E1 - E4 composed according to the provisions of the invention and a comparison melt V1 not composed according to the invention were melted, the compositions of which are given in Table 1.

[0058] In addition, Table 1 lists the martensite start temperatures Tmst, Ar 3 temperatures and Ar 1 temperatures estimated according to the formulas (1) - (3) explained above for the melts E1 - E4 and V1.

[0059] The melts E1 - E4 and V1 were cast in a conventional manner into slabs, which were each heated at 1150 - 1380 ° for a period of 60 - 240 min.

[0060] The slabs thus thoroughly heated were subjected to a pre-rolling process in which they were hot-rolled in the temperature range of 1020 - 1150 °C to form a pre-strip with a thickness of 30 - 50 mm.

[0061] The pre-rolled slabs were then hot-rolled in seven passes in a conventional manner to produce hot-rolled strips ("hot strip") W1 - W11 and WV with a thickness Dw. Upon leaving the last pass of hot rolling, the hot strips W1 - W11 and WV had a final hot-rolling temperature Twe that was above the Ar 3 temperature of the steels E1 - E4 and V1 from which the hot strips W1 - W11 and WV were each made.

[0062] After hot rolling, the hot strips W1 - W11, WV obtained were cooled from their respective final hot rolling temperature Twe at a cooling rate dT1 to an intermediate temperature Tz which was 50 °C below the Ar 1 temperature of the steel E1 - E4 and V1 from which the hot strips W1 - W11, WV were each made.

[0063] After reaching the intermediate temperature Tz, the hot strips W1 - W11, WV were cooled at a cooling rate dT2' to the martensite start temperature Tmst of the steel.

[0064] Starting from the martensite starting temperature T MS , the hot-rolled strips W1 - W11, WV were cooled to the respective coiling temperature HT at a cooling rate dT3, at which they were coiled into a coil. Finally, they were cooled to room temperature in the coil.

[0065] For the hot strips W1 - W11, WV thus obtained, the thickness Dw, as well as the hot rolling end temperatures ET, cooling rates dT1, cooling rates dT2', dT2" and coiling temperatures HT set during their production are listed in Table 2.

[0066] For hot-rolled strips W1 - W11, WV, the tensile strength Rm, the yield strength Re, the uniform elongation Ag, the elongation A50, the elongation A80, and the hole expansion HER were determined. The corresponding properties, as well as the product Rm x HER x Ag and the ratio Re / Rm, are listed in Table 3 for hot-rolled strips W1 - W11, WV.

[0067] For hot-rolled strips W1 - W11, WV, the martensite, ferrite, bainite, pearlite, and retained austenite fractions of the microstructure were determined. In addition, for some of the hot-rolled strips W1 - W11, the martensite and retained austenite grain sizes as well as the width Bras of the retained austenite rim surrounding the martensite islands (=martensite grains) in the microstructure of hot-rolled strips W1 - W11, WV were determined. The relevant values ​​are listed in Table 4.

[0068] It can be seen that the hot-rolled strips W1 - W3 and W6 - W11 produced according to the invention and alloyed according to the invention have reliably high mechanical properties Rm, Re, Ag, A50, A80 and HER, which lead to high values ​​for the product Rm x HER x Ag. In contrast, the hot-rolled strip WV produced from the comparative steel V1, which is not alloyed according to the invention, in a non-inventive manner does not reach the minimum limit specified in the invention for the product Rm x HER x Ag. The same applies to the hot-rolled strips W4 (cooling rate dT1 too high) and W5 (cooling rate dT1 too low), which are alloyed according to the invention but not produced according to the invention.

[0069] In Fig. 1is a picture of a martensite island M present in the microstructure of the hot strip W1, which is embedded in the ferritic microstructure F. Clearly visible is the rim RAS surrounding the martensite island M, consisting of retained austenite, which separates the martensite island M from the surrounding ferrite F.

[0070] As shown by the Fig. 2 reproduced, in Fig. 3 schematically abstracted enlargement of section A of Fig. 1 As can be easily understood, the central region MMB of the martensite island M is bordered by a peripheral region MRB, around which the retained austenite seam RAS runs. Table 1 Steel Data in mass%, balance iron and unavoidable impurities °C C Si Mn P S Al Cr Mon N Ti+Nb+V B T MS A r3 A r1 V1 0,08 0,6 1,7 0,009 0,0014 0,04 0,3 0,018 0,0048 0,124 0,0002 456 788 730 E1 0,08 0,11 1,0 0,009 0,0008 0,027 0,5 0,021 0,005 0,044 0,0002 485 807 735 E2 0,07 0,06 1,0 0,003 0,0007 0,035 0,4 0,012 0,0087 0,038 0,0001 489 806 733 E3 0,12 0,30 1,7 0,003 0,0007 0,036 0,8 0,014 0,0051 0,044 0,0002 435 751 731 E4 0,14 0,10 1,0 0,003 0,0006 0,033 0,4 0,01 0,0052 0,038 0,0001 455 774 733 Table 2 Hot-rolled strip Steel thickness Oven temperature Laytime Pre-tape stitch Pre-strip thickness ET HT dT1 dT2' dT2" ZT1 thH mm [min] [°C] [mm] [°C] [C / s] [C / s] [°C] [s] WV V1 3,2 1240 130 1030 41 820 520 30 2 W1 E1 4 1260 140 1080 40 860 60 40 50 690 2 W2 E1 2 1280 240 1140 41 850 50 35 30 W3 E1 1,5 1260 130 1130 42 850 30 30 40 W4 E1 2 1230 150 1050 46 910 50 135 50 W5 E2 1,6 1180 160 1040 47 850 30 2 30 W6 E2 1,6 1290 120 1140 44 860 50 51 60 W7 E2 2 1260 145 1130 43 855 90 62 40 W8 E2 3 1230 165 1110 42 900 30 30 40 W9 E3 1,5 1260 125 1130 41 860 40 80 70 675 3 W10 E3 2 1250 135 1125 42 850 80 31 30 W11 E4 3,4 1270 140 1120 38 860 40 42 30 Table 3 Hot-rolled strip Steel thickness re Rm Re / Rm Ag A050 A80 HER Rm*HER*Aq According to the invention? mm MPa % MPa*%*% WV V1 3,2 762 856 0,89 7,9 15,5 14,1 26 175822 NO W1 E1 4 501 702 0,71 12,6 20,7 18,8 34 300737 YES W2 E1 2 474 711 0,67 12,4 19,4 17,6 38 335023 YES W3 E1 1,5 562 752 0,75 11,8 18,5 16,8 31 275082 YES W4 E1 2 750 934 0,80 12,4 15,2 13,8 15 173724 NO W5 E2 1,6 390 590 0,66 12 20,5 18,6 41 290280 NO W6 E2 1,6 438 652 0,67 11,8 20,0 18,2 48 369293 YES W7 E2 2 401 612 0,66 12,6 20,6 18,7 56 431827 YES W8 E2 3 480 687 0,70 11,5 18,4 16,7 45 355523 YES W9 E3 1,5 750 1014 0,74 8,5 13,4 12,2 25 215475 YES W10 E3 2 689 975 0,71 9,1 13,9 12,6 27 239558 YES W11 E4 3,4 508 804 0,63 10,3 20,8 20,8 30 248436 YES Table 4 Hot-rolled strip Steel Martensite ferrite Bainite Perlite RA KG Bras [Area-%] [µm] WV V1 5 10 75 5 2 - - W1 E1 15 80 5 - 2 2 0,3 W2 E1 20 75 5 - 2 - - W3 E1 25 75 0 - 2 2 0,3 W4 E1 35 62 5 - 3 2 nb W5 E2 5 80 - 15 0 - - W6 E2 15 80 5-10 <1 1,5 4 0,4 . W7 E2 10 85 5 - 1,5 - - W8 E2 15 85 - - 1,5 - - W9 E3 35 60 5 - 1 3 0,9 W10 E3 35 65 - - <1 - - W11 E3 30 70 5 - 1,0 - "-": Not determined

Claims

1. Method for producing a hot-rolled flat steel product having a microstructure consisting of 5 - 40 vol.% tempered or freshly formed martensite and 50 - 95 vol.% ferrite, and in which the portion not occupied by martensite and ferrite is filled with up to 45 vol.% residual austenite, bainite and / or cementite, comprising the following steps: a) casting a steel melt to form a precursor in the form of a slab, a thin slab or a cast strip, wherein the melt consists of, in mass%, C:0.05 - 0.15%,Si:< 0.5%,Mn:0.7 - 2.1%,Al:< 0.1%,Cr:0.2 - 1%, at least one element from the group "Nb, Ti, V" with the proviso that the total of the contents of Nb, Ti and / or V is 0.01 - 0.1%, B:< 0.0015%,Mo:< 0.2%,Cu:< 0.2%,Ni:< 0.2%P:< 0.05%N:< 0.01% and, as the remainder, Fe and a total of not more than 0.1 mass% of unavoidable impurities; b) heating the precursor at a temperature of 1150 - 1380°C for a period of 60 - 960 min; c) optionally: descaling of the precursor; d) if the precursor is a slab: pre-rolling the precursor at temperatures of 1020 - 1150°C to a thickness of 30 - 50 mm; e) hot-rolling the optionally pre-rolled precursor in one or more rolling passes to form a hot-rolled strip with a thickness of 1.4 - 6.4 mm, wherein the final hot-rolling temperature ET, at which the hot rolling is terminated, is at least equal to the Ar3 temperature of the steel and at most 200°C higher than the Ar3 temperature of the steel; f) cooling the obtained hot strip to a coiling temperature HT which lies in a range which starts below the martensite start temperature TMS of the steel and ends at room temperature, - wherein the cooling rate dT1 is 20 K / s to 90 K / s in a temperature range which extends from the final hot-rolling temperature ET to an intermediate temperature Tz, which is 50°C below the Ar1 temperature of the steel, - wherein after reaching the intermediate temperature Tz - according to a first variant, the hot strip is cooled at a cooling rate dT2' of 5 - 100 K / s to the coiling temperature HT in the range between the martensite start temperature TMS of the steel and room temperature, or - according to a second variant, the hot strip is cooled at a cooling rate dT2" of 10 - 130 K / s first to a cooling stop temperature ZT1, which is in the range 550 - 770°C and at which the hot strip is optionally held for a holding time tH of up to 5 s, and then to the coiling temperature HT in the range between the martensite start temperature TMS of the steel and room temperature; g) coiling the hot strip, which is cooled to the coiling temperature HT, to form a coil; h) optionally cooling the hot-rolled flat steel product in the coil to room temperature or keeping the hot-rolled flat steel product at the coiling temperature HT.

2. Method according to claim 1, characterized in that the final hot-rolling temperature ET is 820 - 900°C and the cooling rate dT1 is not more than 70 K / s.

3. Method according to claim 1 or claim 2, characterized in that the cooling rate dT1 is at least 30 K / s.

4. Method according to any one of the preceding claims, characterized in that during the cooling according to the first variant (step f), the cooling rate is 20 - 70 K / s.

5. Method according to any one of claims 1 - 3, characterized in that during cooling according to the second variant (step f), the cooling rate is 30 - 80 K / s and the holding time is 1 - 3 s.

6. Method according to any of the preceding claims, characterized in that the coiling temperature HT is in a temperature range from room temperature to 100°C.

7. Hot-rolled flat steel product with a steel substrate, - which consists of, in mass%, C: 0.05 - 0.15%, Si: < 0.5%, Mn: 0.7 - 2.1 %, Al: < 0.1 %, Cr: 0.2 - 1%, at least one element from the group "Nb, Ti, V" with the proviso that the total of the contents of Nb, Ti and / or V is 0.01 - 0.1%, B: < 0.0015%, Mo: < 0.2%, Cu: < 0.2%, Ni: < 0.2%, P: < 0.05%, N: < 0.01% and, as the remainder, Fe and a total of not more than 0.1 mass% of unavoidable impurities and - of which the microstructure consists of 5 - 40 vol.% tempered or freshly formed martensite and 50 - 95 vol.% ferrite, and in which the portion not occupied by martensite and ferrite consists of up to 45 vol.% residual austenite, bainite and / or cementite, wherein the martensite portions of the microstructure are island-shaped and at least 70% of all martensite islands have a central region covering the center of the particular martensite island, which is bordered by an edge region of the martensite island in question bordering the edge of the particular martensite island, - wherein the C content of the edge region is, at least in one portion, higher than the C content of the central region and - wherein the martensite islands are bordered by a rim consisting of residual austenite over at least part of their periphery.

8. Flat steel product according to claim 7, characterized in that the portion or portions of the edge region of the martensite islands in which there is a higher C content than in the central region of the particular martensite island, and this C concentration at the edge is so high that the residual austenite can be seen at 20,000x magnification, in total occupies or occupy at least 30% of the periphery of the martensite island in question.

9. Flat steel product according to one of claims 7 or 8, characterized in that in all martensite islands with a mean diameter of more than 3 µm, a higher C content is present in at least one portion of their edge regions than in the central region of the particular martensite island.

10. Flat steel product according to claim 9, characterized in that the width of the residual austenite rim is 10 nm to 1 µm.

11. Flat steel product according to one of claims 7 - 10, characterized in that the product Rm x HER x Ag formed from its tensile strength Rm, its hole expansion HER and its uniform elongation Ag is at least 200,000 MPa%2.

12. Flat steel product according to claims 7 - 11, characterized in that its tensile strength Rm is at least 530 MPa.

13. Flat steel product according to one of claims 7 - 12, characterized in that its hole expansion HER is at least 20%.

14. Flat steel product according to one of claims 7 - 13, characterized in that its uniform elongation Ag is at least 5%.