Method for producing a cold-rolled flat steel product

Abstract

The present invention relates to a method for producing a cold-rolled flat steel product having a ferritic primary structure and carbide precipitates based on Ti, Nb and / or V embedded in the ferritic primary structure.

Description

[0001] Process for producing a cold-rolled flat steel product

[0002] The invention relates to a method for producing a cold-rolled flat steel product with a ferritic basic structure and carbide precipitates based on Ti, Nb and / or V embedded in the ferritic basic structure.

[0003] Energy consumption represents by far the largest contributor to the economic and environmental costs of steel production. In large-scale steel production, energy is predominantly provided in the form of fossil fuels (e.g., coal and natural gas). Therefore, steel production is associated with very high CO2 emissions. The steadily rising costs of CO2 certificates (so-called "carbon offset credits"), particularly within the EU, mean that the steel industry's dependence on fossil fuels will no longer be sustainable in the future. For this reason, alternatives to the use of fossil fuels in steel production are being urgently sought.

[0004] In sheet metal production, fossil fuels are used in hot-dip coating plants and continuous annealing plants to heat cold-rolled flat steel products to very high temperatures. Such plants consist of several furnaces heated by natural gas-fired burners, through which the flat products continuously pass. Annealing the cold-rolled flat steel product is necessary to transform the microstructure into desired phases, eliminate the negative effects of cold rolling, and, if necessary, prepare the flat steel product for hot-dip coating.

[0005] Cold rolling is typically performed to achieve both a lower thickness and high surface quality and dimensional tolerance on the flat product. However, cold rolling leads to both severe work hardening and the formation of strong texture (anisotropy) in terms of grain orientation and mechanical properties. These effects generally have a negative impact on the mechanical and technological properties of the final product and are eliminated by annealing at high temperatures (typically in the range 650–950 °C). At these temperatures, recovery and recrystallization, or transformation, of the microstructure occur. Recovery is the annihilation of crystallographic defects (e.g., dislocations) in the microstructure that otherwise cause work hardening. This typically occurs at lower temperatures compared to recrystallization and transformation.During recrystallization, new ferrite grains form in place of old, work-hardened and anisotropic ferrite grains. This can begin even at temperatures below Acl. During the transformation, the ferrite transforms into austenite; depending on the temperature (between Acl and Ac3) and the chemical analysis of the flat steel product, the microstructure can be partially or completely transformed. In multiphase steels, the newly formed austenite transforms into hard, carbon-rich phases such as martensite and / or bainite upon subsequent cooling to low temperatures. The mechanical properties of multiphase steels can be adjusted by combining hard phases, together with ferrite and retained austenite.

[0006] Cold-rolled multiphase steels, especially those with a zinc-based anti-corrosive coating, are used in the construction of automotive components. Multiphase steels offer a number of advantages for such applications. Due to their low alloying element content, multiphase steels are characterized by lower costs, good reproducibility, good recyclability, and good suitability for processing (e.g., welding) compared to other materials. Furthermore, by adjusting the chemical analysis and process conditions, multiphase steels can be used to achieve a wide range of microstructure components and, consequently, mechanical properties. This flexibility enables the use of multiphase steels for components with different requirements regarding mechanical properties.

[0007] For many multi-phase steels, the diffusion of alloying elements, especially carbon (C), during the annealing cycle is essential for the formation of microstructure components and thus for the adjustment of mechanical properties. C has different solubilities in different iron allotropes. Accordingly, C atoms must redistribute between the newly formed phases during microstructural transformation. The redistribution of C has a decisive influence on the formation of the new phases during cooling and consequently on the mechanical properties of the final product. In contrast, the diffusion of alloying elements (including C) is a relatively slow process. For this reason, the cold-rolled flat steel product must be aged for an extended period (typically several minutes) at a specified annealing temperature.

[0008] DE 10 2021 105 357 A1 describes a method for producing a generic cold-rolled flat steel product. The object of the invention is to provide a generic method that reduces or substantially eliminates the use of fossil fuels.

[0009] The teaching of the invention relates to a method for producing a cold-rolled flat steel product with a ferritic basic structure and carbide precipitates based on Ti, Nb and / or V embedded in the ferritic basic structure, comprising the steps: a) melting a steel consisting, in addition to Fe and unavoidable impurities in wt.% of

[0010] C: 0.020 to 0.20%,

[0011] Mn: 0.10 to 4.00%,

[0012] P: up to 0.020%,

[0013] S: up to 0.010%,

[0014] N: up to 0.010%, with at least one or more microalloying elements from the group (Ti, Nb, V):

[0015] Ti: at least 0.040%,

[0016] Nb: at least 0.040%,

[0017] V: at least 0.040%, subject to the following conditions:

[0018] I) 0.04% <= X <= 0.3% with X = Ti + V / 1.06 + Nb / 1.94

[0019] II) 0.3 <= Y <= 1.0 with Y = 0.25 * X / (C + 0.86 * N) optionally one or more alloying elements from the group (Si, AI, Cr, Mo, W, Cu) with

[0020] Si: up to 1.50%,

[0021] AI: up to 1.50%,

[0022] Cr: up to 1.50%,

[0023] Mon: up to 0.50%,

[0024] W: up to 0.50%,

[0025] Cu: up to 0.10%; b) casting the melt to form a precursor product; c) preheating the precursor product to a temperature and / or maintaining the precursor product at a temperature between 1150 and 1350°C; d) hot rolling the precursor product to form a hot-rolled flat steel product with a final hot-rolling temperature between 850 and 980°C; e) cooling the resulting hot-rolled flat steel product at a cooling rate of between 20 and 400°C / s to a coiling temperature of between 400 and 700°C; f) coiling the hot-rolled flat steel product cooled to the coiling temperature to form a coil; g) uncoiling the coil and cold rolling with a cold rolling degree of between 5 and 80% to form a cold-rolled flat steel product; h) coiling the cold-rolled flat steel product into a coil; i) uncoiling the coil and annealing the cold-rolled flat steel product in a continuous process, comprising the steps of:

[0026] 11) Heating at an average heating rate between > 100 and 1000 °C / s to a temperature between 800 and 900 °C and holding at 800 to 900 °C for a duration between 0.1 and 18 s;

[0027] (12) cooling at an average cooling rate of between 100 and 1000 °C / s to a temperature not exceeding 550 °C and, optionally, holding at that temperature for a maximum of 100 s; (j) coiling the cold-rolled flat steel product into a coil.

[0028] The molten steel with an alloy composition within the ranges specified above is cast to form a precursor, which in the classic production process can be a slab of usual dimensions. However, the steel can also be produced from a continuous cast product into a thin slab by direct hot rolling in a casting and rolling mill, or into a cast strip precursor in a strip casting mill. For example, in a casting and rolling mill or strip casting mill, the precursor can be further processed directly, i.e. coming directly from the casting heat, so that the precursor is kept at a temperature or, if necessary, preheated to a temperature, for example in an equalizing or preheating furnace, which ensures the most complete homogenization possible and in which any precipitates formed during casting are (re)dissolved as completely as possible.If the melt is cast into a precursor product in a continuous casting plant, for example, the cast and fully solidified strand is separated into several slabs of finite dimensions. The slabs are then allowed to cool to ambient temperature, particularly through natural cooling. The precursor product or slab is reheated to a temperature for further processing, for example, in a walking beam furnace or by other suitable means. Otherwise, the slab is placed under a hood after casting, at a lower temperature, or directly into a walking beam furnace.

[0029] The temperature for preheating and / or holding the precursor product is at least 1150 °C, in particular at least 1200 °C, to ensure the most complete dissolution possible of any unwanted precipitates in the form of carbides / carbonitrides and / or nitrides in the precursor product. The temperature for preheating and / or holding should not exceed 1350 °C to avoid partial melting and / or excessive scaling of the precursor product. For ecological and economic reasons, the temperature for preheating and / or holding is limited to a maximum of 1275 °C.

[0030] The precursor product is hot-rolled into a hot-rolled flat steel product in one or more rolling stands (hot rolling mill) with a hot-rolling end temperature between 850 and 980 °C. A hot-rolling end temperature of at least 850 °C, in particular at least 870 °C, is chosen to produce the hot-rolled flat steel product in order to prevent excessively high forming resistance. To avoid undesirable coarse grain formation, the hot-rolled flat steel product's end temperature is limited to a maximum of 980 °C.

[0031] The resulting hot-rolled flat steel product is cooled at a cooling rate between 20 and 400 °C / s to a coiling temperature between 400 and 700 °C. A cooling rate of at least 20 °C / s is required to largely prevent the formation of pearlite and cementite and the formation of coarse precipitates that cannot be dissolved in subsequent process steps. A cooling rate exceeding 400 °C / s offers no further advantages. The coiling temperature is at least 400 °C, in particular at least 410 °C, to prevent martensite formation and promote the formation of a microstructure of bainite, bainitic ferrite, and / or ferrite in the hot-rolled flat steel product. Martensite in the microstructure of the hot-rolled flat steel product would be transferred to the microstructure of the cold-rolled and annealed flat steel product and would be an undesirable phase in the microstructure of the cold-rolled flat steel product.In addition, the martensite in the microstructure of the hot-rolled flat steel product negatively affects both the cold-rollability of the hot-rolled flat steel product and the isotropy of the microstructure of the cold-rolled and annealed flat steel product. To limit the diffusion of oxygen-affine alloying elements to the surface during the coiling process, the coiling temperature is limited to a maximum of 700 °C, in particular a maximum of 660 °C. At a coiling temperature above 700 °C, the Ti-, Nb-, and / or V-containing precipitates would coarsen, preventing the desired precipitate size in the cold-rolled flat steel product and, consequently, high tensile strength and a high hole expansion ratio from being achieved. For hot-rolled flat steel products consisting of ferrite and / or bainitic ferrite reinforced with Ti-, Nb- and / or V-containing precipitates, see among others:WO 2020 / 048599 Al and EP 1 338 665 Al, the coiling temperature is usually set to at least 550 to 600 °C so that Ti-, Nb-, and / or V-containing carbides can precipitate during the cooling of the coiled hot strip product. At a lower coiling temperature, the carbides do not precipitate, so that the desired mechanical properties cannot be achieved. In the present case, it is irrelevant whether the precipitates already form in the hot-rolled flat steel product. If no precipitates form in the hot-rolled flat steel product during the cooling of the hot strip product, they form during the subsequent annealing of the cold-rolled flat steel product. Thus, the desired mechanical-technological properties can also be achieved by coiling at a coiling temperature of at least 400 °C, in particular at least 410 °C.

[0032] The hot-rolled flat steel product, cooled to the coiling temperature, is coiled into a coil.

[0033] The hot-rolled flat product is uncoiled from the coil and cold-rolled to a cold-rolled steel flat product with a cold-rolling degree of between 5 and 80%. Cold rolling is required to achieve high surface quality and dimensional tolerances, which are essential for the intended use of the cold-rolled flat steel product in thin-walled components (e.g., body-in-white components). However, cold rolling leads to work hardening, which negatively affects the ductility and hole expansion ratio of the steel. Furthermore, cold rolling results in a dominant rolling texture, which leads to a pronounced anisotropy of the mechanical properties and, consequently, to a reduction in the hole expansion ratio. The influence of work hardening and rolling texture on the mechanical and technological properties cannot be fully recovered by subsequent annealing.The cold rolling degree KWG is calculated using the formula: KWG = 100 * (LWB - LKB) / LWB, where LWB is the thickness of the hot-rolled flat steel product (hot strip) and LKB is the thickness of the cold-rolled flat steel product (cold strip). When cold rolling with a cold rolling degree that is too low, the surface quality and dimensional tolerance required for the target application cannot be achieved. For this reason, the cold rolling degree is at least 5%, in particular at least 10%. When cold rolling with a cold rolling degree that is too high, the influences of work hardening and rolling texture are so great that the required mechanical and technological properties cannot be achieved. For this reason, the cold rolling degree is limited to a maximum of 80%, in particular a maximum of 70%, preferably a maximum of 50%.

[0034] The cold-rolled flat steel product is coiled into a coil.

[0035] The cold-rolled flat steel product (cold strip) can have a thickness between 0.5 and 4 mm.

[0036] The cold-rolled flat steel product is uncoiled from the coil and annealed continuously. Annealing can be performed, for example, in a multi-stage continuous annealing line, which includes the following steps:

[0037] Heating at an average heating rate between > 100 and 1000 °C / s to a temperature between 750 and 900 °C and holding at 750 to 900 °C for a duration between 0.1 and 18 s;

[0038] Cooling at an average cooling rate between 100 and 1000 °C / s to a maximum temperature of 550 °C.

[0039] Annealing cold-rolled flat steel products has a significant influence on the mechanical and technological properties of the final product. In general, the mechanical and technological properties of the final product are influenced by two changes in the microstructure during annealing: the recovery or recrystallization of ferrite and / or bainitic ferrite grains, and the coarsening of Ti, Nb, and / or V-containing precipitates. Recovery or recrystallization and precipitation coarsening have opposing effects on the hole expansion ratio. Recovery or recrystallization of the microstructure is necessary to at least partially correct the work hardening caused by cold rolling, which would otherwise impair the hole expansion ratio and increase the tensile strength.Recrystallization only occurs at such high temperatures that the precipitates can also rapidly coarsen. This has a negative impact on both the hole expansion ratio and the tensile strength.

[0040] A key feature of the invention is that the annealing of the cold-rolled flat steel product is faster and takes place in a significantly shorter period of time compared to the annealing of conventional multi-phase steel. The short annealing time takes advantage of the different kinetics of recovery or recrystallization and precipitation coarsening. Depending on the temperature, recovery or recrystallization occurs in a few seconds, whereas precipitation coarsening would require a comparatively longer time. In the production of the cold-rolled flat steel product, the annealing time is sufficiently long to largely recover or recrystallize the microstructure, but short enough to prevent excessive coarsening of the precipitates.

[0041] A short annealing time requires both a fast average heating and cooling rate and a short holding time. In the first stage of the annealing process, the cold-rolled flat steel product is heated to a temperature between 800 and 900 °C at an average heating rate in the range of > 100 to 1000 °C / s. Such a high heating rate can be achieved, for example, through induction heating. Induction heating is powered by electricity, which can preferably be generated from renewable energies (wind, water, solar), thus having a beneficial effect on the CO2 footprint. Induction heating enables much higher heating rates to be achieved compared to those achievable with a gas-fired burner.The heating rate is at least > 100 °C / s, 120 °C / s, 150 °C / s, in particular at least 200 °C / s, 220 °C / s, 250 °C / s, preferably at least 300 °C / s, 320 °C / s, 350 °C / s, to avoid excessive coarsening of the Ti-, Nb-, and / or V-containing precipitates. Heating rates exceeding 1000 °C / s offer no advantage. A restriction to, in particular, a maximum of 900 °C / s, 800 °C / s, preferably a maximum of 700 °C / s, 600 °C / s, or 500 °C / s would be possible.

[0042] The "average" heating or cooling rate is to be understood as the difference between an initial temperature (actual temperature) and a target temperature (setpoint temperature) in relation to the time required between the initial temperature and reaching the target temperature. As a rule, the heating and cooling rates are not constant. As already noted, existing annealing processes in steel production are very energy-intensive and associated with high CO2 emissions. Therefore, alternatives that do not consume fossil fuels are being pursued with great intensity. One possible alternative for annealing thin sheet is induction heating. This process is very fast: the strip is heated at rates that are significantly higher than those encountered in conventional plants. In contrast to conventional annealing processes, the strip is held at the annealing temperature for a few seconds instead of minutes.Such a short annealing time is sufficient for recovery or recrystallization of the microstructure, but leaves insufficient time for the diffusion of alloying elements. Therefore, induction heating is unsuitable for annealing many multiphase steels consisting of Fe- and C-based phases such as martensite, bainite, and austenite.

[0043] The use of inductive heating in steel production requires steel concepts that can achieve mechanical properties similar to those of modern multi-phase steels through very short annealing times. One possibility is ferritic steels reinforced by very fine precipitates such as carbides or carbonitrides or intermetallic particles. The carbides or carbonitrides form either during the cooling of the coiled hot-rolled strip product or in the initial annealing stage of the cold-rolled flat steel product. The formation of the final microstructure and, consequently, the adjustment of the mechanical and technological properties is therefore not dependent on slower diffusion processes. Due to the very short annealing time, excessive coarsening of the precipitates can be avoided. During annealing in conventional annealing or hot-dip coating plants, however, the annealing time is so long that the precipitates can coarsen.The coarsening of the precipitates negatively impacts the mechanical properties of the final product. Due to their highly isotropic or homogeneous microstructure, such concepts exhibit both high tensile strength and high ductility, as well as very high resistance to edge cracking. Edge cracking can occur during the forming of stamped sheet metal and is therefore a critical factor in automotive engineering. In the laboratory, the edge cracking sensitivity of a steel is assessed using the so-called "hole expansion test," in which a hole punched into a sheet metal sample is expanded with a mandrel until the first crack appears (see ISO 16630:2017).

[0044] After heating, the cold-rolled flat steel product is held in the second stage of the annealing process at a temperature in the range of 800 to 900 °C for a period of 0.1 to 18 s. The adjustment of the annealing temperature and time in the second step is crucial both for sufficient recovery or recrystallization of the microstructure and for minimizing precipitation coarsening. At an annealing temperature of less than 800 °C or a duration of less than 0.1 s, the microstructure would not recover or recrystallize sufficiently. Adequate recovery or recrystallization is necessary to eliminate the work hardening and anisotropy resulting from cold rolling. For this reason, the annealing temperature is at least 800 °C, in particular at least 810 °C, 820 °C, and preferably at least 830 °C, 840 °C, or 850 °C.A temperature exceeding 900 °C and a duration exceeding 18 s would, in turn, lead to excessive coarsening of the Ti-, Nb-, and / or V-containing precipitates and consequently to a deterioration of the mechanical and technological properties. Therefore, the duration of annealing after heating can be limited to a maximum of 16 s, 15 s, preferably a maximum of 13 s, 11 s, and preferably a maximum of 10 s, 8 s.

[0045] In the third stage of the annealing process, the cold-rolled flat steel product is cooled to a maximum temperature of 550 °C at an average cooling rate of 100 to 1000 °C / s. The cooling rate can be limited, in particular, to 900 °C / s, 800 °C / s, and preferably to 700 °C / s, or 600 °C / s. Such a high cooling rate can be achieved, for example, by water quenching. Conventionally, air blowers are used for cooling, which are less efficient than water quenching. Similar to the average heating rate in the first stage, an excessively slow average cooling rate would lead to excessive coarsening of the Ti-, Nb-, and / or V-containing precipitates, making it impossible to achieve the desired precipitate size and, consequently, the required mechanical and technological properties.For this reason, the average cooling rate is at least 100 °C / s, 120 °C / s, 150 °C / s, in particular at least 200 °C / s, 220 °C / s, 250 °C / s, preferably at least 300 °C / s, 320 °C / s, 350 °C / s.

[0046] At temperatures below 550 °C, neither recovery or recrystallization of the microstructure nor coarsening of the Ti, Nb, and / or V-containing precipitates occurs. For this reason, no special requirements are imposed for further cooling after the end of the third stage. If necessary, the cold-rolled flat steel product can be cooled directly after the third stage at an average cooling rate of 100-1000 °C / s to a temperature of no more than 100 °C. Alternatively, the cold-rolled flat steel product can be held at an average cooling rate of 100-1000 °C / s to a temperature of less than 550 °C and higher than 100 °C for a maximum of 1000 s and then cooled further at an average cooling rate of 100-1000 °C / s to a temperature of no more than 100 °C.

[0047] According to one embodiment, the hot-rolled flat steel product can be pickled directly before cold rolling. Alternatively, the hot-rolled flat steel product can be coiled into a coil after pickling and unwound before cold rolling.

[0048] According to one embodiment, the cold-rolled flat steel product can be coated with a Zn-based anti-corrosive coating after step i2). The Zn-based anti-corrosive coating can be applied to the cold-rolled flat steel product either during cooling or holding, particularly during or after the third stage of the annealing process in the temperature range 100-550 °C, or subsequently. The Zn-based anti-corrosive coating can be applied either by immersion in a molten bath (hot-dip coating) or following annealing in an electrogalvanizing plant.

[0049] Optionally, the hot-dip galvanizing process can be followed by a further heat treatment ("galvannealing"), in which the hot-dip galvanized steel flat product is heated to up to 550 °C to bake in the zinc layer. Either immediately after leaving the molten bath or after the further heat treatment, the resulting cold-rolled steel flat product can be cooled to a temperature of less than 100 °C at a cooling rate between 0.5 and 1000 °C / s.

[0050] There are no special requirements regarding the composition of the corrosion protection coating and the molten bath through which the cold-rolled flat steel product passes during its hot-dip coating process. The corrosion protection coating, in particular, consists primarily of zinc (Zn) and can otherwise be composed conventionally. Accordingly, in addition to Zn and unavoidable impurities (in wt. %), the corrosion protection coating can contain up to 20% Fe, up to 5% Mg, and up to 10% Al. Typically, at least 5% Fe, at least 1% Mg, and / or at least 1% Al, where present, are required to achieve optimal performance properties of the corrosion protection. The coated or uncoated cold-rolled flat steel product thus obtained can optionally be subjected to conventional skin passing to optimize its dimensional stability and surface finish.The skin-pass ratio used is typically at least 0.1% and at most 2.0%, with a skin-pass ratio of at least 0.3% and at most 1.0% being particularly preferred. A skin-pass ratio of less than 0.1% would have no significant impact on dimensional accuracy and surface quality and would result in an excessively low surface roughness in a cold-rolled flat steel product optionally coated with a metal coating, which would have a negative impact on the formability of the flat steel product. A skin-pass ratio of more than 2.0% would negatively affect both the mechanical properties (yield strength and elongation at break) and the hole expansion ratio.

[0051] The alloying elements of the melt or the steel (flat steel product) are given as follows:

[0052] Carbon (C) is mainly bound in the precipitates. The concentration of C dissolved in the solid solution is minimized to avoid the formation of undesirable iron-based phases. A content of at least 0.020%, in particular at least 0.030%, preferably at least 0.040% is required to achieve a high precipitate density and thus the required tensile strength. An excessively high content, in turn, would lead to the formation of undesirable phases such as martensite, bainite, austenite, retained austenite, pearlite and / or cementite in the microstructure, which would reduce ductility and increase edge crack susceptibility. Therefore, the content is limited to a maximum of 0.20%, in particular a maximum of 0.150%, preferably a maximum of 0.120%. Negative effects of the presence of C can be particularly reliably avoided if the content is preferably a maximum of 0.110%.

[0053] Manganese (Mn) contributes to the increase in strength through solid solution strengthening of the ferrite. Furthermore, Mn suppresses the formation of pearlite and thus promotes the formation of Ti, Nb, and / or V-containing precipitates. To achieve these effects of Mn, a Mn content of at least 0.10%, in particular at least 0.20%, preferably at least 0.40%, is required. However, an excessively high content negatively affects weldability and increases the risk of dominant segregations (chemical inhomogeneities in the microstructure that develop during solidification). Therefore, the upper content limit is set at a maximum of 4.0%, although lower contents, in particular at a maximum of 3.0%, preferably at a maximum of 2.50%, can prevent potential negative effects of the presence of Mn.

[0054] Phosphorus (P) is broadly considered an impurity that is introduced into steel by iron ore and cannot be completely eliminated in the large-scale steelmaking process. Its content should be kept as low as possible, with a maximum of 0.020%, especially 0.010%, for reliable weldability. The lower limit can be as low as 0.0002%.

[0055] Sulfur (S) is also considered an impurity in the broadest sense and must therefore be controlled to a maximum content of 0.010% to avoid a strong tendency toward segregation and negative effects on formability or elongation due to excessive formation of sulfides (FeS; MnS; (Mn, Fe)S), particularly 0.0050%. Calcium can generally be added to the alloy for desulfurization and to adjust the S content depending on the Ca content. The lower limit can be 0.0002%, in particular.

[0056] Nitrogen (N) is also an unavoidable impurity due to manufacturing processes. If N is present, Ti, Nb, and / or V, in the presence of C, preferentially form nitrides or carbonitrides with N. Therefore, in practice, under technically and economically feasible conditions, the inclusion of N in the precipitates is unavoidable. In principle, however, the lowest possible contents should be aimed for, since N-dominated carbonitrides are often very coarse and angular, which is why they do not contribute to strengthening but act as crack initiators. To avoid the formation of N-dominated carbonitrides, the content must be limited to a maximum of 0.010%, in particular a maximum of 0.0050%. The lower limit can be, in particular, 0.0002%.

[0057] Titanium (Ti), niobium (Nb) and vanadium (V) are considered microalloying elements and are added either individually or in combination (Ti and Nb; or Ti and V; or Nb and V; or Ti, Nb and V). The microalloying elements are essential for the formation of the "carbide" precipitates. The required density of the precipitates can be achieved if at least one of the microalloying elements from the group (Ti, Nb, V) is added in a content of at least 0.040% each. In addition, condition I) with 0.04% <= X <= 0.3% must be met, where X can be calculated using the formula X = Ti + V / 1.06 + Nb / 1.94. If the contents of Ti, Nb and / or V are too low, which would result in an X value of less than 0.04%, the required density of the precipitates and consequently the required tensile strength would not be achieved. X is therefore at least 0.04% and may in particular be at least 0.05%, preferably at least 0.07%.To avoid the negative effects of excessively high contents of microalloying elements, X is limited to a maximum of 0.3%. This prevents, for example, increased Nb contents in the presence of Nb, from leading to cracking during (continuous) casting or slab cooling and / or preheating. At the same time, only a certain content of microalloying elements is required to achieve the desired strength. If this content is exceeded, only a slight further increase in strength occurs. In addition, the mean diffusion distances decrease, increasing the risk of undesirably large precipitates forming. For these reasons, contents of Ti, Nb and / or V are added such that X is no more than 0.3%, in particular no more than 0.25%, preferably no more than 0.2%. X in % means wt.%.

[0058] When adjusting or adding the micro-alloying elements, it must be ensured that condition II) is still met with 0.3 <= Y <= 1.0, where Y can be calculated using the formula Y = 0.25 xx / (C + 0.86 x N). A Y < 0.3 would result from an excessively high C or N content. An excess of C would lead to the excessive formation of undesirable phases such as martensite, bainite, austenite, retained austenite, pearlite and / or cementite, which would have a negative impact on the hole expansion ratio. An excessively high N content would lead to the formation of coarse N-dominated carbonitrides, which would also have a negative impact on the hole expansion ratio. For these reasons, the contents of the alloying elements determining condition II) are adjusted so that Y is at least 0.3, in particular at least 0.35, preferably at least 0.4. A Y > 1.0 results from an excessively high total content of Ti, Nb and V.An excess of Ti, Nb, and V in total does not contribute to a further increase in strength, but rather increases the risk of the formation of undesirable precipitates, which negatively impact the mechanical and technological properties. For this reason, the contents of the alloying elements determining condition b) are adjusted so that Y is no more than 1.0, in particular no more than 0.8, and preferably no more than 0.7. Y has no unit of measurement.

[0059] Furthermore, one or more alloying elements from the group (Si, Al, Cr, Mo, W, Ca, B, Cu, Ni, Sn, As, Co, Zr, La, Ce, Nd, Pr, O, H) can optionally be included: Silicon (Si) can be added as an optional alloying element to suppress the formation of pearlite in the microstructure. In addition to Mn, Si contributes to the increase in strength through solid solution strengthening of the ferrite. To achieve these effects of Si, a content of at least 0.050% is required. If the Si content is too high, the rollability of the steel would be adversely affected and growths could occur on the rolls in the cold rolling mill or cold rolling stand during rolling. Furthermore, if the Si content is too high, the surface quality of the flat steel product can be impaired during optional hot-dip galvanizing.To avoid these negative effects of Si, the Si content is limited to a maximum of 1.50%, with contents of no more than 1.00%, preferably no more than 0.70%, proving particularly advantageous in terms of avoiding the negative effects of the presence of Si. If special requirements exist regarding batch galvanization capability, Si alloying can be omitted and a maximum content of 0.03% is permitted.

[0060] Aluminum (Al) can be added as an optional alloying element to suppress pearlite. Because Al is typically used to deoxidize the melt, a content of at least 0.010% is unavoidable in conventional steel production. However, excessively high levels can negatively impact castability. Therefore, the upper limit of the Al content is limited to a maximum of 1.50%, in particular a maximum of 1.00%, and preferably a maximum of 0.70%.

[0061] Chromium (Cr), molybdenum (Mo), and / or tungsten (W) can be added as optional alloying elements, either individually or in combination (Cr and Mo; or Cr and W; or Mo and W; or Cr, Mo, and W). When Cr, Mo, or W are present, the precipitates containing Ti, Nb, and / or V are partially or completely bound and slow down or prevent their coarsening. The fineness of the precipitates is essential for achieving the desired mechanical and technological properties of the cold-rolled flat steel product, in particular the tensile strength and the hole expansion ratio. With very short annealing times, coarsening of the precipitates can be avoided or reduced. In such cases, alloying with Cr, Mo, and / or W is not absolutely necessary to slow down the coarsening of the precipitates.With somewhat longer annealing times, alloying with Cr, Mo, and / or W can prevent excessive coarsening of the precipitates and consequently a deterioration in tensile strength and hole expansion ratio. To achieve these effects of Cr, Mo, and / or W, a content of at least 0.050% each, in particular at least 0.08 wt.%, preferably at least 0.10 wt.%, is required. If the content of Cr, Mo, and / or W is too high, the risk of undesirable phases forming, which could impair the mechanical and technological properties, increases. In addition, an excess of Cr would cause undesirable, pronounced grain boundary oxidation. For these reasons, the Cr content is limited to a maximum of 1.5 wt.%, in particular a maximum of 1.2 wt.%, preferably a maximum of 1.0 wt.%. Mo and W are among the most expensive alloying elements; therefore, high contents of Mo and W should be avoided for economic reasons.For these reasons, the contents of Mo and W are each limited to 0.5 wt%, in particular not more than 0.3 wt%, preferably not more than 0.2 wt%.

[0062] Copper (Cu) can precipitate in the form of coarse particles, which negatively impact mechanical properties. Furthermore, Cu negatively influences castability. To avoid any influence of Cu, the content is limited to a maximum of 0.10%, preferably a maximum of 0.05%. The lower limit can be as low as 0.0002%.

[0063] Calcium (Ca), boron (B), nickel (Ni), tin (Sn), arsenic (As), cobalt (Co), zirconium (Zr), lanthanum (La), cerium (Ce), neodymium (Nd), praseodymium (Pr), oxygen (O), and hydrogen (H), as well as all other conceivable alloying elements not explicitly listed here, are classified as unavoidable impurities during production. These impurities are components of the raw material from which the steel is produced, or can enter the steel during steel melting and processing. The contents of these elements must be kept so low that they have no technical effect on the properties of the underlying steel.

[0064] Ca is typically added to the melt during steelmaking for deoxidation and desulfurization, as well as to improve castability. Excessive levels can lead to the formation of undesirable inclusions, which negatively impact mechanical properties and rollability. Therefore, the upper limit is limited to a maximum of 0.0050%, and in particular, a maximum of 0.0020%.

[0065] Carbides form at moving phase boundaries in cold-rolled flat steel products. The movement of phase boundaries can be slowed by B segregated at them. This can prevent the formation of carbides. To avoid this effect, the B content is limited to a maximum of 0.0010%, in particular a maximum of 0.0006%, preferably a maximum of 0.0004%.

[0066] Ni, Sn, As, Co, Zr, and rare earths such as La, Ce, Nd, and Pr are also optional alloying elements and are not required. Even if they are detectable, they are considered unavoidable impurities. Accordingly, the Ni content is limited to a maximum of 0.10%; the Sn content to a maximum of 0.050%; the As content to a maximum of 0.020%; the Co content to a maximum of 0.020%; the Zr content to a maximum of 0.0002%; the La content to a maximum of 0.0002%; the Ce content to a maximum of 0.0002%; the Nd content to a maximum of 0.0002%; and the Pr content to a maximum of 0.0002%.

[0067] 0 is also undesirable in the melt or in the steel, as oxide deposition would negatively affect both the mechanical properties and the castability and rollability. The maximum permissible content is therefore set at 0.0050%, specifically 0.0020%.

[0068] As the smallest atom, H is highly mobile at interstitial sites in steel and can lead to cracking in the core, particularly in high-strength steels, during cooling from hot rolling. Its content should therefore be as low as possible, in any case no more than 0.0010%, in particular no more than 0.0006%, preferably no more than 0.0004%, with a preferred content of no more than 0.0002%.

[0069] The information in % in connection with the above-mentioned alloying elements refers to weight%.

[0070] The cold-rolled flat steel product comprises a ferritic basic structure. A ferritic basic structure is therefore understood to be a structure containing at least 90% ferrite. The proportion of the ferritic structure can in particular be at least 92%, preferably at least 94%, more preferably at least 96%, more preferably at least 98%. The main components of the structure can be determined using light-optical microscopy (LOM) at a magnification of 200 to 2000 times. Fine "carbide" precipitates based on Ti, Nb and / or V are embedded in the ferritic structure. The precipitates have an average precipitation diameter of at most 10 nm, in particular of at most 7 nm, preferably of at most 5 nm. If precipitates are present, the average precipitation diameter is > 0 nm.Due to their fineness, these are not detectable by LOM but can only be determined by transmission electron microscopy (TEM) at a magnification of 50,000 to 500,000 times. Hard iron-based phases such as martensite, bainite, austenite, retained austenite, pearlite, and / or cementite are detrimental to the desired mechanical and technological properties, particularly for the hole expansion ratio, and are therefore undesirable phases. However, depending on the alloying elements, they can be present in the above-mentioned ranges in total amounts of less than 10%, in particular less than 7%, preferably less than 5%, and more preferably less than 3%. Furthermore, the microstructure may contain other unavoidable microstructural components due to manufacturing, up to a maximum of 1%, in particular up to a maximum of 0.5%.

[0071] Precipitations are defined as “carbides” with a NaCl (Bl) crystal structure, which consist predominantly of C and at least one of the alloying elements from the group Ti, Nb and V. If one or more of the alloying elements from the group Mo, W or Cr are present, they may also be present in the precipitates. In addition, the precipitates may contain a small amount of N. Precipitations with a significant proportion of N are often referred to as “carbonitrides”, but have the same crystal structure and effect on the mechanical and technological properties of the steel as carbides. The sum of the contents of Ti, Nb and V (each in atom%) based on the chemical analysis of the precipitates is therefore at least 20 atom%. To convert wt% to atom%, the usual formula KAT = 100 * (KG / mK) / sum of (iG / mi) is used, where KAT and KG are the concentrations of element K in atom% and wt.-%; mK is the atomic mass of the element K; and iG is the content (in wt%) and mi is the atomic mass of component i in the mixture of components. If the contents of Ti, Nb and / or V in the precipitates are too low, these are not "carbides" but other precipitates that are significantly coarser than the Ti-, Nb and / or V-based precipitates and which do not meet the requirements regarding the size of the precipitates. These requirements are essential for achieving the desired mechanical properties; if the precipitates are too coarse, the requirements regarding tensile strength R. m and hole expansion ratio X cannot be achieved.

[0072] The cold-rolled flat steel product has a tensile strength R mof at least 550 MPa, in particular at least 580 MPa, preferably at least 610 MPa, preferably at least 650 MPa, whereby tensile strengths of at least 780 MPa or even 960 MPa can also be achieved. The maximum tensile strength can be, for example, 1300 MPa. The cold-rolled flat steel product has a low yield strength ratio. The yield strength ratio is determined by the ratio of the yield strength R p0 ,2ZU the tensile strength R m The yield strength ratio is at least 0.6, in particular at least 0.7, and at most 0.9, in particular at most 0.85.

[0073] The elongation at break A 50 in the cold-rolled flat steel product is at least 9%, in particular at least 11%, preferably at least 12%.

[0074] The tensile strength R m , the yield strength R p0 ,2and the elongation at break A 50 can be determined in tensile tests according to DIN EN ISO 6892-1:2017.

[0075] The hole expansion ratio X in the cold-rolled flat steel product is at least 40%, in particular at least 45%, preferably at least 50%, preferably at least 55%, more preferably at least 60%, wherein the hole expansion ratio X can be determined according to DIN EN ISO 16630:2017. It has been found that the cold-rolled flat steel product has a particularly favorable ratio of hole expansion ratio X to tensile strength R m Thus, in a cold-rolled flat steel product according to the invention, high hole expansion ratios are achieved even at high tensile strengths, which are reflected in high values ​​for the product of tensile strength R m and hole expansion ratio X. Therefore, values ​​of at least 30,000 MPa*%, in particular at least 40,000 MPa*%, preferably at least 45,000 MPa*%, preferably at least 50,000 MPa*% are achieved.

[0076] The invention is explained in more detail below using exemplary embodiments. These are summarized in Tables 1-4. Table 1 shows the chemical compositions as well as the sum X and the ratio Y of the exemplary embodiments. The production specifications with regard to hot and cold rolling, as well as annealing and galvanizing, are given in Tables 2 and 3, respectively. Table 4 shows both the mechanical and technological properties and the microstructure characteristics of the exemplary embodiments.

[0077] To test the invention, melts A - AE alloyed according to the compositions specified in Table 1 were produced and cast into slabs. Melts not according to the invention and their contents of certain alloying elements that deviate from the specifications of the invention are underlined in Table 1. Alloying element contents that are so low that they are "0" in the technical sense, i.e., so low that they have no influence on the properties of the steel, are designated by the entry "-" in Table 1.

[0078] The slabs produced from steel grades A - AE were thoroughly heated in a preheating furnace at a preheating temperature ("VWO"). The preheated slabs were then hot-rolled in a conventional manner into a hot-rolled flat steel product (hot strip). The resulting hot-rolled steel strip left the hot-rolling mill at a hot-rolling final temperature ("WET") and was subsequently cooled at a cooling rate ("KR1") to a coiling temperature ("HT"), at which it was coiled into a coil. After the coil had cooled to room temperature, the hot-rolled flat steel product was pickled in a conventional manner and then cold-rolled at a cold-rolling grade ("KWG") into a cold-rolled flat steel product.

[0079] To demonstrate the effect of the invention, one of the combinations I - XII of VWO, WET, KR1, HT, and KWG listed in Table 2 was selected for the production of the cold-rolled flat steel products. The non-inventive combinations of I - XII and the specifications that did not meet the requirements of the invention are highlighted in Table 2 by underlining.

[0080] After cold rolling, the cold-rolled flat steel product was annealed in a continuous process in an induction annealing plant. During annealing, the cold-rolled steel strip was heated at a medium heating rate ("HR") to a (medium) annealing temperature ("GT"), at which it was held for a holding time ("HZ"). The cold-rolled steel strip was then cooled at a medium cooling rate ("KR2") to a maximum temperature of 550 °C. The resulting flat steel product was optionally skin-passed ("DG") and / or coated with a Zn-based corrosion protection coating.

[0081] In the exemplary embodiments, annealing and, if applicable, galvanizing are carried out according to a combination a - i of HR, GT, HZ, KR2, and DG specified in Table 3. For combinations a - i, it is also indicated whether the cold-rolled flat steel product has been coated with a Zn coating. The non-inventive combinations of a - i and the specifications that did not comply with the requirements of the invention are highlighted in Table 3 by underlining.

[0082] The resulting flat steel products were tested for mechanical and technological properties as well as structural characteristics. The results of these tests, such as the yield strength R p0 ,2, the tensile strength R m , the yield strength ratio R p o,2 / R m , the elongation at break A 50, the hole expansion ratio X, the product Rm * X, the mean precipitation diameter DM and the ferrite content F of the microstructure are summarized in Table 4. For steel flat products produced on the basis of the hot-rolled steel flat products Al - AE1, it is also stated which of the steels A -AE the steel substrate of the respective steel flat product consisted of (column "Analysis"), which of the combinations I - XII of hot strip production (column "Rolling") and which of the variants a - i of annealing treatment and melt deposition the respective steel substrate has undergone (column "Annealing").

[0083] Example Al is an example according to the invention, consisting of a steel substrate with chemical composition A, produced according to rolling specification I and annealing specification a. This resulted in an optimal combination of mechanical-technological properties and microstructure characteristics.

[0084] Examples A2 - A4 are comparable to Example A1, except that the hot rolling conditions deviate from the specifications required according to the invention. In this sense, they serve as counterexamples. In Example A2, the slab was heated at a preheating temperature VWO that was too low, so that the slab was not fully annealed. As a result, the alloying elements and the manufacturing processes had no effect on the mechanical properties. In Example A3, the final rolling temperature WET was set too low, so that the desired isotropy of the material was lost due to the effects of thermomechanical rolling. In Example A4, the hot-rolled steel strip was cooled at a cooling rate KR1 that was too low, so that coarse precipitates were formed before the coiler. As a result, the required precipitate size and consequently the required mechanical-technological properties could not be achieved.

[0085] Chemical analyses B - G are variations of analysis A, where Ti was replaced by various combinations of Ti, Nb or V. Chemical analyses H - J are also variations of analysis A, additionally containing Cr, Mo or Mo. For all analyses B - J, other alloy components as well as the sum X and the quantitative ratio Y were kept the same as in analysis A. The resulting examples B1 - J1 were processed under the same conditions as example Al. This resulted in mechanical and technological properties at the same level as example Al.

[0086] Examples Kl - Ml are counterexamples, also based on example Al. In example Kl, the sum X and consequently the quantity ratio Y are too low. Example LI has too low a C content and consequently a too high quantity ratio Y. In contrast, example Ml has a very high C content and consequently a too low quantity ratio Y. Examples Kl - Ml were produced using the same process conditions as example Al, but due to the different chemical analyses, their mechanical and technological properties are outside the target range.

[0087] Steels N and O are low-alloy concepts that differ only in their C content and consequently in the quantity ratios Y. Using steel N, the influence of the cold rolling degree KWG was investigated in examples NI - N4. This showed that with decreasing KWG, the tensile strength Rm decreases and the hole expansion ratio X increases. Steel O has a slightly higher C content and consequently a lower Y compared to steel N. This resulted in an increased R m and a reduced X compared to, for example, N2, which was processed under the same conditions.

[0088] Steels P and Q are very low-alloy concepts in which high ratios Y were achieved by adjusting the C and Nb contents. The resulting examples PI and Ql were processed under the same conditions as examples N2 and 01. Compared to examples N2 and 01, examples PI and Ql exhibited lower but still acceptable tensile strengths R. m on.

[0089] The influence of Mn on the mechanical properties was investigated using steels R - U. Steels R - U have lower Mn contents than steel A. Furthermore, unlike steel A, steels R - U are alloyed with Cr and Mo. The resulting examples RI - U1 were processed under the same conditions as examples N2 and 01 - Q1. This showed that the tensile strength increases with increasing Mn content. Steel U was used in examples U1 - U5 to investigate the influence of the heating rate HR, the holding time HZ, and the cooling rate KR2. This shows that with decreasing HR and KR2 and increasing HZ, the precipitate size increases and consequently the R m and remove the X. In example U2, HR and KR2 were set too low and the HZ was set too long. This led to an excretion size that was too high and consequently, to an R m and an X outside the required ranges. Example U2 therefore serves as a counterexample.

[0090] Steels V and W are medium-strength variants with higher contents of Al and Si, respectively. These were used in Examples VI - W3 to investigate the influence of the coiling temperature HT. This shows that with increasing HT, the precipitate size increases and, consequently, the R m and decrease the X. In the counterexamples V3 and W3, the HT was set too high. This led to an excessively high precipitate size and consequently to a too low product R m x X.

[0091] Steels X and Y are higher-strength variants that have even higher contents of Al and Si, respectively. These were hot-rolled in Examples XI - Y4 at a low rolling end temperature WET, cooling rate KR1, and coiling temperature HT. In Examples XI - Y4, the influence of the annealing temperature GT was investigated. This shows that with increasing GT, the R mdecreases and X increases. In counterexamples XI and Y1, the GT was set too low. This led to insufficient recovery or recrystallization of the microstructure and consequently to an X that was too low.

[0092] Steels Z and AA are ultra-high-strength variants with very high Si and Al contents, respectively. The resulting examples, ZI and AA1, exhibited a high X and a very high R. m Steel AB is similar to steel Z, except for its excessive C content. The resulting steel, ABI, was processed under the same conditions as Example ZI. Due to the excess C, Example ABI contained too little ferrite. This led to a significant deterioration in its mechanical and technological properties. Therefore, Example ABI serves as a counterexample.

[0093] Steels AC - AE are comparable to reference example A, but have different concentrations of impurities P, S, N and Cu. These were processed in examples AC1 - AE1 under the same conditions as example Al. Steel AC has a very low concentration of impurities. Example AC1 produced from it has mechanical and technological properties comparable to those of example Al. Steel AD has a higher but still acceptable concentration of impurities compared to steel A. Example ADI produced from it has poorer mechanical and technological properties than example Al, but they were still within the required ranges. Steel AE has too high a concentration of impurities. As a result, example AE1 produced from it has too low a ferrite content and too large a precipitate diameter and consequently too low an X.

[0094]

[0095] Table 1

[0096] Table 2

[0097] Table 3

[0098] Table 4

Claims

Claims Process for producing a cold-rolled flat steel product with a tensile strength R m of at least 550 MPa, an elongation at break A 50 of at least 9%, R m and A 50 determined according to DIN EN ISO 6892-1:2017, with a hole expansion ratio X of at least 40% determined according to DIN EN ISO16630:2017, with a ferritic basic structure with at least 90% ferrite and carbide precipitates based on Ti, Nb and / or V embedded in the ferritic basic structure with an average precipitate diameter of at most 10 nm comprising the steps: a) melting a steel consisting of Fe and unavoidable impurities in wt.% of C: 0.020 to 0.20%, Mn: 0.10 to 4.00%, P: up to 0.020%, S: up to 0.010%, N: up to 0.010%, with at least one or more microalloying elements from the group (Ti, Nb, V): Ti: at least 0.040%, Nb: at least 0.040%, V: at least 0.040%, subject to the following conditions: I) 0.04% <= X <= 0.3% with X = Ti + V / 1.06 + Nb / 1.94 II) 0.3 <= Y <= 1.0 with Y = 0.25 * X / (C + 0.86 * N) optionally one or more alloying elements from the group (Si, AI, Cr, Mo, W, Cu) with Si: up to 1.50%, AI: up to 1.50%, Cr: up to 1.50%, Mon: up to 0.50%, W: up to 0.50%, Cu: up to 0.10%, b) casting the melt to form a precursor product; c) preheating the precursor product to a temperature and / or maintaining the precursor product at a temperature between 1150 and 1350 °C; d) hot rolling the precursor product to form a hot-rolled flat steel product with a final hot-rolling temperature of between 850 and 980 °C; e) cooling the resulting hot-rolled flat steel product at a cooling rate of between 20 and 400 °C / s to a coiling temperature of between 400 and 700 °C; f) coiling the hot-rolled flat steel product cooled to the coiling temperature to form a coil; g) uncoiling the coil and cold rolling it with a cold rolling degree of between 5 and 80% to form a cold-rolled flat steel product; h) coiling the cold-rolled flat steel product to form a coil; i) Uncoiling the coil and annealing the cold-rolled flat steel product in a continuous process comprising the steps: 11) Heating at an average heating rate between > 100 and 1000°C / s to a temperature between 800 and 900°C and holding at 800 to 900°C for a duration between 0.1 and 18 s; 12) cooling at an average cooling rate between 100 and 1000 °C / s to a temperature of at most 550 °C and optionally holding at this temperature for a maximum of 100 s; j) coiling the cold-rolled flat steel product into a coil. The method according to claim 1, wherein the hot-rolled flat steel product is pickled between steps f) and g). The method according to claim 1 or 2, wherein after step i2), the cold-rolled flat steel product is hot-dip coated with a Zn-based anti-corrosive coating. The method according to claim 3, wherein the hot-dip coated flat steel product is heated to a temperature of up to 550 °C. The method according to claim 1 or 2, wherein, after step i2), the cold-rolled flat steel product is electrolytically coated with a Zn-based corrosion protection coating. The method according to any one of the preceding claims, wherein the cold-rolled flat steel product is skin-passed with a skin-pass ratio of between 0.1 and 2.0%. A cold-rolled flat steel product produced according to any one of the preceding claims with a yield strength ratio R p0 ,2 / R m of at least 0.6 and at most 0.

9. Steel flat product according to claim 7 with a yield strength ratio R p0 ,2 / R m of at least 0.7 and at most 0.

85. Steel flat product according to claim 7 or 8, wherein the product of tensile strength R mand the hole expansion ratio X reaches at least 30,000 MPa*%. A flat steel product according to any one of claims 7 to 9, wherein the ferrite in the microstructure is at least 94%. A flat steel product according to any one of claims 7 to 10, wherein the average precipitation diameter is at most 7 nm.

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