Cold-rolled flat steel product and method for producing same

Abstract

The invention relates to a cold-rolled flat steel product having a tensile strength Rm of at least 590 MPa and a maximum of 900 MPa, a yield limit Rp0.2 of at least 330 MPa and a maximum of 600 MPa, and an elongation at break A80 of at least 14%, and to a method for the production thereof.

Description

Technical Field

[0001] The present invention relates to a cold-rolled flat steel product having a high tensile strength R m , a low yield limit R p0.2 and a high elongation at break A 80 , and a method for manufacturing the same. Background Art

[0002] The growing demand for fuel-efficient vehicles has driven the need to reduce weight through lightweight design. An economically effective weight reduction measure is to use high-strength steel. This makes it possible to produce thin-walled components that can withstand high mechanical loads despite their low weight. In order for the steel to be formed into components with higher geometric complexity, they must have high ductility at the same time. Generally, the strength and ductility of steel are inversely related, that is, the higher the strength, the lower the ductility. Therefore, the application of high-strength steel in such components is limited.

[0003] The goal of a new generation of high-strength steels, so-called "Advanced High Strength Steels", is to achieve extremely high formability while maintaining high strength. This includes so-called dual-phase (DP) steels, which usually mainly consist of ferrite and martensite, but may also contain other phases such as bainite and / or retained austenite. During the plastic deformation of DP steel, mainly the softer ferrite deforms, while the harder martensite increases the strength. During plastic forming, martensite can be additionally generated from the possible retained austenite. This also contributes to the increase in strength without affecting the initial stage of plastic deformation. Therefore, in laboratory tests, DP steels are characterized by a relatively low yield limit ratio compared to complex-phase (CP) steels, that is, the ratio of the yield limit R p0.2 to the tensile strength R m , and a relatively high elongation at break A80. The latter mainly consists of medium-hard phases such as bainite and / or tempered martensite.

[0004] Compared with other high-strength steels, one disadvantage of DP steel is its high sensitivity to edge cracking during the stamping sheet forming process. In the laboratory, the edge crack sensitivity of steel is evaluated by the so-called "hole expansion test", in which a punch is used to expand the hole punched in the sheet sample until the first crack appears, see ISO 16630:2017 "Metallic materials - Sheet and strip - Hole expansion test". The hole expansion rate λ is calculated from the diameter D0 of the hole before the test and the diameter D R at the time of the first crack formation, and the formula is as follows:

[0005] λ = 100×(D R - D0) / D0.

[0006] The relatively high edge crack sensitivity of DP steels mainly stems from the inhomogeneity of their microstructure. In DP steels, martensite exists in the form of coarse bundles, which are usually arranged linearly. Under the action of an external mechanical load, due to the large hardness difference between the ferrite phase and the martensite phase, high internal stresses will be generated at the interface between the two phases. This promotes crack formation in the flanging test because extremely high degrees of deformation occur at the punching edge. In contrast, CP steels have a relatively isotropic and uniform microstructure, as well as relatively fine precipitates. Therefore, under the action of an external mechanical load, the local stresses in CP steels are more uniform and lower than those generated in DP steels. Therefore, compared with DP steels, CP steels are characterized by higher edge crack resistance.

[0007] There is now an optimization need to produce cold-rolled DP steels with lower sensitivity to edge cracks. One possible way to achieve this goal is to reduce the internal hardness gradients that can lead to crack formation. For example, this can be achieved by partially or completely replacing hard martensite with softer bainite in the microstructure, or by reducing the hardness of the martensite. Another method to reduce the edge crack sensitivity of DP steels is to suppress the cold-rolling texture, which otherwise leads to anisotropy of the microstructure. Such a method and the corresponding flat steel substrates produced are described, for example, in DE 10 2021 121 997 A1. Summary of the Invention

[0008] Therefore, the object of the present invention is to provide a cold-rolled flat steel product with reduced edge crack sensitivity and a corresponding manufacturing method therefor.

[0009] In a first aspect of the present invention, this object is achieved by a cold-rolled flat steel product having the features of claim 1.

[0010] According to the present invention, a cold-rolled flat steel product is provided, which, apart from Fe and inevitable impurities due to the production process, has the following composition by weight %:

[0011] C: 0.050 to 0.170%,

[0012] Si: 0.080 to 0.350%,

[0013] Mn: 1.50 to 2.40%,

[0014] Cr: 0.20 to 0.950%,

[0015] Al: 0.010 to 0.50%,

[0016] And optionally at least one alloying element from the following group (Ti, Mo, B, Mo, Cu, Ni):

[0017] Ti: at most 0.060%,

[0018] B: up to 0.0015%,

[0019] Mo: up to 0.20%,

[0020] Cu: up to 0.20%,

[0021] Ni: up to 0.20%,

[0022] wherein the impurities include alloying elements (P, S, N, Nb, V, W, Sn, As, Co, Ca, O, H) of the following groups, and their contents may be:

[0023] P: up to 0.020%,

[0024] S: up to 0.010%,

[0025] N: up to 0.010%,

[0026] Nb: up to 0.030%,

[0027] V: up to 0.020%,

[0028] W: up to 0.10%,

[0029] Sn: up to 0.050%,

[0030] Sb: up to 0.0004%,

[0031] As: up to 0.020%,

[0032] Co: up to 0.020%,

[0033] Ca: up to 0.0050%,

[0034] O: up to 0.0050%,

[0035] H: up to 0.0010%.

[0036] In addition, the flat steel product according to the present invention has such a microstructure except for the inevitable microstructure components limited by production, which includes the following phases: ferrite and bainite together account for 75% to 96%, and martensite and retained austenite together account for 4% to 25%.

[0037] Except that the data of the content of retained austenite in the microstructure of the flat steel product according to the present invention are based on volume (the data are expressed in "volume %"), the data of the contents of other microstructure components are respectively based on the area of the ground section of the corresponding product sample (the data are expressed in "area %"), unless otherwise specified.

[0038] The flat steel product according to the invention herein has a tensile strength R of 590 to 900 MPa determined according to DIN EN ISO 6892-1:2017 (specimen form 2, longitudinal specimen). m , a yield limit R of 330 to 600 MPa p0.2 , and an elongation at break A of at least 14%. 80 . The yield limit R p0.2 can be specifically limited to a maximum of 580 MPa, preferably a maximum of 570 MPa. The tensile strength R m can be specifically limited to a maximum of 880 MPa, preferably a maximum of 860 MPa. The minimum allowable hole expansion rate λ (%) determined according to DIN EN ISO 16630:2017 can be approximately calculated by the following formula:

[0039] λ = (-0.08 × R m ) × % / MPa + X,

[0040] where the constant X is at least 85%, especially at least 87%, preferably at least 88%. For some high-strength steel grades, especially multiphase steels, it has been found that the hole expansion rate λ depends on the time delay between punching and conducting the test. ISO 16630:2017 does not specify the time range of the test. The values given here are based on tests conducted in accordance with ISO 16630:2017 within 5 hours after punching.

[0041] The high hole expansion rate λ of the flat steel product according to the invention, together with the high tensile strength R m , low yield strength R p0.2 , and high elongation at break A 80 , is especially attributed to the composition of its microstructure.

[0042] Except for the retained austenite content, the main components of the microstructure can be determined by scanning electron microscopy (REM) at a magnification of 3000 to 7500 times after etching with a suitable etchant (such as nitric acid alcohol solution). For this purpose, 3 to 5 scanning electron microscopy images need to be acquired, and the visible structures are assigned to each phase and then quantified pixel by pixel. Image acquisition is preferably carried out at a low acceleration voltage and a small working distance, for example, using an Inlens SE detector, so as to clearly distinguish each metallographic phase by its etched structure. The specific implementation methods are well-known to those skilled in the art. The proportion of retained austenite is determined by X-ray diffraction (XRD) according to ASTM E975 standard.

[0043] For a low yield limit, a relatively high proportion of ferrite in the microstructure is necessary. In the initial stage of plastic deformation, ferrite deforms preferentially, while the harder carbon-rich phases, such as bainite, retained austenite and martensite, play a strengthening role. Therefore, the yield limit is mainly affected by the ferrite component. It has been found that in order to achieve the desired range of the yield limit, the ferrite proportion should be at least 25%, especially at least 28%, preferably 30%. Since the hardness of ferrite is relatively low, a certain proportion of bainite, retained austenite and martensite is required to achieve a tensile strength R of at least 590 MPa. m , so the proportion of ferrite in the microstructure is correspondingly limited to a maximum of 85%, especially a maximum of 80%, preferably a maximum of 75%.

[0044] Bainite also has a positive effect on the tensile strength. However, different from martensite, bainite also has a positive effect on the hole expansion rate. A higher hole expansion rate is the result of a higher bainite proportion and a lower total proportion of martensite and austenite compared with other DP steels with the same tensile strength. Compared with martensite, bainite has a lower hardness, so the hardness difference between ferrite and bainite is smaller than the hardness difference between ferrite and martensite. Therefore, under the action of an external mechanical load, the ferrite-bainite boundary is less sensitive to cracks. Therefore, a high bainite proportion and a corresponding low martensite proportion are beneficial to the cutting edge sensitivity. However, the hardness of bainite is lower than that of martensite, which means that in order to achieve the same tensile strength, a higher proportion of bainite is needed to replace a certain proportion of martensite. In the flat steel product according to the present invention, it has been found that the required mechanical properties can be achieved by using a bainite proportion of 5% to 60%. If the bainite proportion is too low, the minimum requirements for tensile strength and hole expansion rate cannot be met. Therefore, a bainite content of at least 5%, especially at least 8%, preferably at least 10% is required. An excessive bainite proportion will lead to too high a yield limit. Therefore, the bainite proportion is limited to a maximum of 60%, especially a maximum of 58%, preferably a maximum of 56%.

[0045] The fractions of martensite and retained austenite in the structure have a particularly strong influence on the mechanical-technological properties of the flat steel products according to the invention. The martensite fraction has a positive influence on the tensile strength. A high martensite fraction is in turn associated with a low hole expansion rate, since under the action of an applied mechanical load, the boundaries between adjacent ferrite grains and martensite bundles become crack initiation points. The high density of potential crack initiation points has an adverse effect on the cutting edge sensitivity. During plastic deformation, the retained austenite in the structure partially or completely transforms into martensite (the so-called TRIP effect). Therefore, the retained austenite fraction also has a positive influence on the tensile strength. In the hole expansion rate test according to ISO 16630:2017, the plastic deformation caused by the stamping process causes most of the retained austenite next to the stamping edge to transform into martensite, thus having an adverse effect on the hole expansion rate. Therefore, the total fraction of martensite and retained austenite is crucial for the hole expansion rate. The total martensite fraction is limited to a maximum of 25%, in particular a maximum of 23%, preferably a maximum of 22%. To achieve the effect, the sum of the fractions of martensite and retained austenite is at least 4%, in particular at least 5%, preferably at least 6%, more preferably at least 7%, particularly preferably at least 8%, further preferably at least 9%. For example, martensite and retained austenite are present at >0% respectively. Preferably, the martensite content in the structure is at least 1% and the retained austenite content is at least 1%.

[0046] As previously mentioned, the influence of martensite on the hole expansion rate is mainly due to the high hardness difference between martensite and adjacent ferrite. Therefore, the hole expansion rate depends on both the fraction of martensite in the structure and the hardness of martensite. The martensite hardness can be measured by nanoindentation. For example, for this purpose, a calibrated cube corner indenter is used to perform indentation tests on a finely polished and etched surface with a maximum load of 1000 μN. It has been found that a martensite nano-hardness measured by this method exceeding 10 GPa is particularly adverse to the hole expansion rate. Therefore, the nano-hardness of martensite in the flat steel products according to the invention does not exceed 10 GPa, in particular does not exceed 9 GPa, preferably does not exceed 8 GPa. The values given here are the corresponding averages of at least 25 indentations in martensite.

[0047] The average nanohardness and the area fraction are determined using a nanoindenter, such as the "HysitronTI Premier" device from Bruker. Details of the device can be obtained from Bruker or, for example, can be accessed via the link: https: / / www.bruker.com / en / products--and--solutions / test-and-measurement / nanomechanical-test-systems / hysitron-ti-premier-nanoindenter.html. In a nanoindenter, a specific measuring indenter, such as a cubic pyramid indenter (made of diamond), is pressed into the sample to be tested, and the hardness is determined by the measured force-indentation curve, preferably using the evaluation method according to Oliver & Pharr (the method can be accessed at the following link: https: / / www.sciencedirect.com / topics / engineering / oliver-pharr-method).

[0048] Furthermore, the flat steel product according to the invention can have a relatively low rolling texture. In the cold rolling of ferrite-containing steels, a texture with significant α fibers is usually formed. The α fibers are <110> The orientation composition is oriented parallel to the rolling direction. The orientation tolerance shown here is 10°. The significant α fibers are therefore associated with an anisotropic microstructure and therefore with anisotropic material properties. In the hole expansion test according to ISO 16630:2017, the punch is deformed upward by the punch. This leads to high tensile stresses tangential to the punch edge. Therefore, the test results are affected by the mechanical material properties of all orientations in the plane of the sheet. In anisotropic microstructures, weak points are formed on the cut edges, where cracks preferentially occur. Therefore, anisotropic microstructures, especially significant α fibers, have an adverse effect on hole expansion. It has been found that the hole expansion rate is particularly unfavorable when the proportion of α fibers in the microstructure exceeds 30%. Therefore, the proportion of microstructures with α fibers in the flat steel product according to the invention is a maximum of 30%, particularly a maximum of 28%, preferably a maximum of 26%. It is inevitable that the proportion of microstructures with α fibers is at least 5%, particularly at least 10%.

[0049] Another indicator of the texture strength is the orientation density distribution function f(g). It corresponds to the area proportion of the microstructure with orientation g. The maximum f(g) therefore corresponds to the area proportion of the most frequent orientation in the microstructure. It has been found that a maximum f(g) of more than 10% is particularly disadvantageous for the hole expansion rate. Therefore, the maximum f(g) of the flat steel product according to the invention is a maximum of 10%, in particular a maximum of 9%, preferably a maximum of 8%. A maximum f(g) of at least 2%, in particular at least 3%, is unavoidable.

[0050] The proportion of the tissue structure with α or γ fibers and the orientation density distribution function f(g) can both be measured by electron backscatter diffraction (EBSD). The values given here were measured in the area at the 1 / 3 position relative to the strip thickness on samples that had been treated with diamond grinding paste and subsequent OPS polishing. A measurement field of 90×90 μm was positioned on each sample and scanned in steps of 0.1 m.

[0051] Other phases may be present in isolation or in combination in the form of pearlite, cementite that is not part of the bainite component, non-metallic inclusions such as MnS or AlO, and coarse carbides and nitrides such as NbCN or TiCN with a precipitation diameter exceeding 50 nm. These are harmful to the mechanical properties and the hole expansion rate and are therefore undesirable. These other phases are thus regarded as inevitable tissue structure components due to production limitations. Their total should not exceed 4%, especially 3%, preferably 2%, and more preferably 1% at most.

[0052] This object is achieved according to the second aspect of the invention by a method having the features of claim 9.

[0053] The method according to the invention for producing cold-rolled flat steel products comprises the following steps:

[0054] a) Melting a steel which, in addition to Fe and inevitable impurities, has the following composition in % by weight:

[0055] C: 0.050 to 0.170%,

[0056] Si: 0.080 to 0.350%,

[0057] Mn: 1.50 to 2.40%,

[0058] Cr: 0.20 to 0.950%,

[0059] Al: 0.010 to 0.50%,

[0060] and optionally at least one alloying element from the following group (Ti, Mo, B, Mo, Cu, Ni):

[0061] Ti: at most 0.060%,

[0062] B: at most 0.0015%,

[0063] Mo: at most 0.20%,

[0064] Cu: at most 0.20%,

[0065] Ni: at most 0.20%,

[0066] Among them, the impurities include alloying elements (P, S, N, Nb, V, W, Sn, As, Co, Ca, O, H) in the following groups, and their contents can be as follows:

[0067] P: at most 0.020%,

[0068] S: at most 0.010%,

[0069] N: at most 0.010%,

[0070] Nb: at most 0.030%,

[0071] V: at most 0.020%,

[0072] W: at most 0.10%,

[0073] Sn: at most 0.050%,

[0074] Sb: at most 0.0004%,

[0075] As: at most 0.020%,

[0076] Co: at most 0.020%,

[0077] Ca: at most 0.0050%,

[0078] 0: at most 0.0050%,

[0079] H: at most 0.0010%,

[0080] b) Cast the melt into a pre-product;

[0081] c) Preheat the pre-product to and / or maintain the pre-product at a temperature between 1150 and 1350 °C;

[0082] d) Hot-roll the pre-product into a hot-rolled flat steel product, especially with a thickness between 1.8 and 5.0 mm, where the hot-rolling end temperature is between 850 and 980 °C;

[0083] e) Cool the obtained hot-rolled flat steel product to a coiling temperature of 450 to 600 °C at an average cooling rate of 20 to 400 °C / s;

[0084] f) Coil the hot-rolled flat steel product cooled to the coiling temperature into a coil;

[0085] g) Unroll the coil of the hot-rolled flat steel product and cold-roll it into a cold-rolled flat steel product, especially with a thickness between 0.6 and 2.4 mm, where the cold-rolling rate is between 30% and 80%;

[0086] h) Coil the cold-rolled flat steel product into a coil;

[0087] i) Unroll a coil of cold-rolled flat steel product and anneal the cold-rolled flat steel product in a continuous process, including the following steps:

[0088] i1) Heat to a holding temperature at an average heating rate between 0.5 and 20 °C / s, wherein the average heating rate between 700 °C and the holding temperature is between 1 and 10 °C / s;

[0089] i2) Hold for a holding time between 30 and 300 s, wherein the temperature T1 at the end of the holding time is between 840 and 900 °C, and the holding temperature is at least T1 - 30 °C;

[0090] i3) Cool to a temperature T2 at an average cooling rate between 0.5 and 100 °C / s, which is equal to or greater than the martensite start temperature Ms and less than 490 °C, wherein the average cooling rate between 700 and 600 °C is between 10 and 50 °C / s, and the temperature difference DT between T1 and T2 is greater than 385 °C;

[0091] i4) Hold for a holding time between 1 and 1000 s at a temperature T3 that is equal to or greater than the martensite start temperature Ms and less than 455 °C;

[0092] To obtain a flat steel product having such a microstructure, in addition to inevitable microstructural components, the microstructure contains the following phases: ferrite and bainite together account for 75% to 96%, and martensite and retained austenite together account for 4% to 25%.

[0093] The molten steel having an alloy composition within the ranges given above is cast into a pre-product. After melting, the steel is cast into a pre-product, which in a conventional production route can be a slab of standard dimensions. However, it is also possible to produce a thin slab as a pre-product from the steel in a continuous casting and rolling plant by direct hot rolling of the continuous casting billet, or to produce a cast strip as a pre-product in a strip casting plant. For example, in a continuous casting and rolling plant or a strip casting plant, the pre-product can be directly further processed, i.e., directly processed from the casting hot state, such that the pre-product is maintained at a certain temperature or preheated to a certain temperature if necessary, for example, in a soaking furnace or a preheating furnace, at which temperature as complete a homogenization as possible is ensured, and the precipitates formed during the casting process are dissolved as completely as possible (again). For example, if the melt is cast into a pre-product in a continuous casting plant, the fully solidified continuous casting billet is divided into a plurality of slabs of limited size, and then the slabs are allowed to cool to ambient temperature, especially by natural cooling. The pre-product or slab is reheated to a certain temperature, for example, in a pusher furnace (Hubbalkenofen) or by other suitable means, for further processing.

[0094] The temperature of the pre-product during preheating and / or holding is at least 1150 °C, in particular at least 1200 °C, to ensure that any unwanted precipitates that may be present in the pre-product in the form of carbides / carbonitrides and / or nitrides are dissolved as completely as possible. The temperature of preheating and / or holding should not exceed 1350 °C to avoid partial melting of the pre-product and / or excessive scale formation. For environmental and economic reasons, the temperature of preheating and / or holding can be particularly limited to a maximum of 1300 °C.

[0095] The pre-product is hot-rolled into a hot-rolled flat steel product in one or more rolling mill stands (hot rolling plant) at a hot rolling finishing temperature between 850 and 980 °C. A hot rolling finishing temperature of at least 850 °C, in particular at least 880 °C, is selected for the production of the hot-rolled flat steel product so that the deformation resistance does not increase significantly. Too low a hot rolling finishing temperature leads to a disproportionately high rolling force and, due to the thermo-mechanical rolling effect, the desired material isotropy is lost. To avoid the formation of unwanted coarse grains, the rolling finishing temperature for the production of the hot-rolled flat steel product is limited to a maximum of 980 °C.

[0096] The thickness of the hot-rolled flat steel product (hot-rolled strip) is preferably between 2.0 and 4.5 mm.

[0097] The resulting hot-rolled flat steel product is cooled to a coiling temperature between 450 and 600 °C at an average cooling rate between 20 and 400 °C / s. An average cooling rate of at least 20 °C / s is necessary to substantially avoid the formation of pearlite and cementite, as well as the formation of coarse precipitates that cannot be dissolved in subsequent process steps. An average cooling rate exceeding 400 °C / s does not bring additional benefits. The coiling temperature is at least 450 °C, in particular at least 480 °C, to prevent the formation of martensite and to promote the formation of a microstructure consisting 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 transfers to the microstructure of the cold-rolled and annealed flat steel product and becomes an unwanted phase in the microstructure of the cold-rolled flat steel product. In addition, martensite in the microstructure of the hot-rolled flat steel product has an adverse effect on 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. Too high a coiling temperature increases the risk of pearlite formation and subsequent manganese segregation during the cooling of the coil. In addition, too high a coiling temperature poses a significant risk of grain boundary oxidation. To prevent this, the coiling temperature is limited to a maximum of 600 °C, in particular a maximum of 580 °C.

[0098] The hot-rolled flat steel product cooled to the coiling temperature is coiled into a coil.

[0099] Optionally, the hot-rolled flat steel product can be uncoiled from the coil and conveyed to a conventional pickling operation, either through a reel-to-reel process, i.e., uncoiling - pickling - recoiling, or preferably directly before cold rolling, i.e., uncoiling - pickling - cold rolling. Pickling can remove the scale present on the hot-rolled flat product and / or prepare or activate the surface of the hot-rolled flat product for subsequent steps.

[0100] The hot-rolled flat product is uncoiled from the coil and cold-rolled into a cold-rolled flat steel product at a cold rolling rate of 30% to 80%. The cold rolling rate KWG is calculated by the following formula:

[0101] KGW = 100×(L WB - L KB ) / L WB ,

[0102] where L WB is the thickness of the hot-rolled flat steel product (hot-rolled strip), and L KB is the thickness of the cold-rolled flat steel product (cold-rolled strip).

[0103] Cold rolling is necessary to obtain high surface quality and dimensional tolerances, which are required for the intended application of the cold-rolled flat steel product in thin-walled components (such as body components). However, cold rolling causes work hardening, which has an adverse effect on the ductility and hole expansion rate of the steel. In addition, cold rolling forms a dominant rolling texture, which leads to significant anisotropy in mechanical properties, resulting in a reduced hole expansion rate. The effects of work hardening and rolling texture on mechanical-process properties cannot be completely eliminated by subsequent annealing.

[0104] When cold rolling is carried out at too low a cold rolling rate, the surface quality and dimensional tolerances required for the target application cannot be achieved. Therefore, the cold rolling rate is at least 30%, especially at least 40%, preferably at least 50%, and more preferably at least 60%. When cold rolling is carried out at too high a cold rolling rate, the effects of work hardening and rolling texture are too great to achieve the required mechanical-process properties. Therefore, the cold rolling rate is limited to a maximum of 80%, especially a maximum of 77%.

[0105] The thickness of the cold-rolled flat steel product (cold-rolled strip) is preferably between 0.7 and 2.1 mm.

[0106] The cold-rolled flat steel product is wound into a coil.

[0107] The cold-rolled flat steel product is uncoiled from a coil and annealed in a continuous process. The annealing of the cold-rolled flat steel product has a decisive influence on the formation of the structure components and thus on the establishment of the mechanical and technological properties of the final product. For the production of the flat steel product according to the invention, the annealing conditions need to be set such that the proportion of bainite in the structure is increased and the proportion of martensite in the structure is reduced. In this way, high tensile strength and high hole expansion rate can be achieved. Bainite is formed by the decomposition of austenite during the cooling process. However, this only occurs within a narrow temperature range. Ferrite or pearlite is usually formed at temperatures above this range, while martensite is formed at temperatures below this range (i.e., below the martensite start temperature Ms, which is usually about 400 °C). In addition, the decomposition of austenite into bainite within this temperature range is promoted by the instability of austenite. The stability of austenite mainly depends on its carbon content. Therefore, the formation of bainite can be achieved by adjusting the carbon concentration or distribution in austenite.

[0108] Annealing can be carried out in a conventional manner in a multi-stage continuous annealing plant or in a hot-dip galvanizing plant in a multi-stage annealing plant in a conventional manner.

[0109] The cold-rolled flat steel product is heated at an average heating rate of 0.5 to 20 °C / s in the first stage of annealing. The heating of the cold-rolled flat steel product in the first stage can be carried out individually or in combination by induction heating or by heating in a furnace or furnace section heated by a gas burner. The final temperature of the first stage depends on the holding temperature of the next stage, and the holding temperature in turn depends on the temperature T1. An average heating rate of at least 0.5 °C / s is necessary to avoid excessive coarsening of the structure, which has an adverse effect on the tensile strength of the material. When the average heating rate exceeds 20 °C / s, the structure may not be fully recrystallized. Sufficient recrystallization is to minimize or eliminate the work hardening and anisotropy caused by cold rolling, which have an adverse effect on the elongation at break and the hole expansion rate. Therefore, the average heating rate is limited to a maximum of 20 °C / s.

[0110] In addition, the average heating rate in the range from 700 °C to the holding temperature, which is dominant in the first stage, needs to be more precisely controlled. It has been found that the average heating rate in this temperature range has a particularly strong influence on the coarsening or recrystallization of the structure in the second stage. Therefore, an average heating rate between 1 and 10 °C / s, particularly between 1.5 and 8 °C / s, and preferably between 2 and 6 °C / s, is required in the range from 700 °C to the holding temperature to achieve the desired mechanical and technological properties.

[0111] In the second stage, the cold-rolled flat steel product is held for a holding time of 30 to 300 s at a specific holding temperature, for example, in a furnace or furnace section heated by a gas burner. The holding time also affects the uniformity of the carbon distribution and thus the mechanical-technological properties of the final product. In the redistribution of carbon that takes place during the holding time, diffusion processes that depend on temperature and time are involved. If the holding time is too short, there is insufficient time for the redistribution of carbon, and thus a uniform carbon distribution cannot be achieved. A holding time of at least 30 s is necessary for a sufficiently uniform carbon distribution. In contrast, too long a holding time leads to excessive coarsening of the microstructure. Therefore, the holding time is limited to a maximum of 300 s.

[0112] The holding temperature is determined by the temperature T1, which is measured, for example, at the transition between the heated furnace section or furnace and the subsequent cooling section. More precisely, it corresponds to the temperature at the transition between the second and third stages. The conditions of the second stage affect both the composition and properties of the microstructure components of the final product and the completeness of recrystallization, and thus the strength of the rolling texture generated by cold rolling. These are crucial for achieving the desired mechanical-technological properties.

[0113] The holding temperature in the second stage corresponds to the critical range (between the Ac1 and Ac3 temperatures) or higher. At this holding temperature, the microstructure of the cold-rolled flat steel product partially or completely transforms into austenite. The completeness of austenitization depends mainly on the holding temperature but also partially on time. In this temperature range, the diffusion rate of carbon atoms is fast enough to enable the redistribution of carbon between ferrite and austenite during the holding time. Since the solubility of carbon in austenite is much higher than in ferrite, carbon accumulates in austenite. Therefore, the higher the proportion of austenite, the more uniform the carbon distribution. The uniformity of the carbon distribution has a decisive influence on the formation and properties of other microstructure components, such as new ferrite, martensite, and / or bainite, during the subsequent cooling process, and thus on the mechanical-technological properties of the final product. Depending on the cooling conditions, microstructural regions with a high carbon content tend to transform into martensite, while regions with a lower carbon content tend to transform into bainite. New ferrite forms only in a very low carbon content temperature range. Therefore, a more uniform carbon distribution promotes the formation of bainite rather than martensite, which has a positive effect on the hole expansion rate, provided that the cooling conditions support this.

[0114] The formation of bainite in place of martensite benefits from a more uniform carbon distribution, which in turn requires a higher holding temperature. In common practice, DP steels are annealed at temperatures below 840 °C, at which temperature the microstructure still contains a large amount of untransformed constituents, such as old ferrite, martensite and / or bainite. As a result, the carbon distribution remains relatively non-uniform. Therefore, annealing at a temperature of at least 840 °C and up to 900 °C is pursued. At a temperature of at least 840 °C and below the Ac3 temperature of approximately 860 °C, the microstructure mainly consists of austenite, and thus the carbon distribution is relatively uniform. Therefore, the annealing temperature must be at least 840 °C. Annealing above the Ac3 temperature results in a fully austenitic microstructure, and thus the carbon distribution is even more uniform. At 900 °C annealing, the carbon distribution is fully homogenized. Therefore, annealing at higher temperatures does not have a further effect on the mechanical-process characteristics of the final product and may instead lead to an undesired coarsening of the microstructure. Therefore, the annealing temperature is limited to a maximum of 900 °C.

[0115] A uniform carbon distribution also requires that the holding temperature be kept constant during the holding time. Therefore, the holding temperature of the cold-rolled flat steel product during the holding time must not be lower than a temperature 30 °C lower than T1. In addition, the setting of the holding temperature needs to meet the requirements for temperatures T1 and especially T2. The temperature T1 at the end of the holding time is between 840 and 900 °C.

[0116] The transfer of the rolling texture generated during cold rolling to the final product is also affected by the temperature T1. A high temperature T1 ensures that the microstructure is fully recovered or recrystallized, thus substantially suppressing the cold-rolled microstructure. However, the recovery or recrystallization of the structure occurs in a time interval much shorter than that of the redistribution of carbon. Therefore, the holding time has no significant effect on the rolling texture.

[0117] The carbon distribution at the end of the holding time also affects the characteristics of the microstructure constituents formed during the subsequent cooling process. In particular, the hardness of martensite depends on its carbon content. Therefore, a uniform carbon distribution promotes the formation of relatively soft martensite, which has a positive effect on the hole expansion rate of the final product.

[0118] In the third stage, the cold-rolled flat steel product is transferred to the cooling section and cooled to a temperature T2 at an average cooling rate of 0.5 to 100 °C / s, which is measured at the outlet of the cooling section. Both the temperature T1 and T2 affect the composition of the microstructure constituents. In particular, the difference between T1 and T2, called DT, is crucial for the adjustment of the microstructure constituents, especially the formation of bainite. Bainite is formed by the decomposition of austenite in a narrow temperature range, usually equal to or higher than the martensite start temperature Ms and lower than 490 °C. The martensite start temperature Ms (°C) is calculated based on the contents of C, Mn and Cr in weight %, respectively, for example by the following formula:

[0119] Ms = 539 - 423×C - 30.4×Mn - 12.1×Cr。

[0120] If the temperature T2 is lower than Ms, a part of the remaining austenite will transform into martensite. This is disadvantageous for forming a microstructure with a high bainite ratio and thus for the combination of high tensile strength and high hole expansion ratio. Therefore, the lower limit of the temperature T2 is limited to Ms. When the temperature T2 is too high, too high a proportion of austenite will be retained instead of forming bainite, and most of this austenite will transform into martensite when the cold-rolled flat steel product is subsequently cooled to a temperature of up to 100 °C, for example, room temperature. In addition, when the temperature T2 is too high, a part of the existing austenite may transform into pearlite, which has an adverse effect on the hole expansion ratio.

[0121] However, the prerequisite for forming a high proportion of bainite is the uniform distribution of carbon, which weakens the stability of austenite. Therefore, the formation of bainite replacing martensite requires both a high temperature T1 and a low temperature T2, provided that all other requirements for T1 and T2 are met. Therefore, the difference DT between T1 and T2 has a particularly strong influence on the mechanical and technological properties of the final product. A high difference DT ensures the uniform distribution of carbon and high recrystallinity, and promotes the formation of bainite, in which the remaining martensite is softer. Therefore, DT must be greater than 385 °C, especially greater than 400 °C, preferably greater than 410 °C, and more preferably greater than 415 °C.

[0122] The average cooling rate dominated in the third stage is 0.5 to 100 °C / s. The average cooling rate of at least 0.5 °C / s is to avoid the formation of unwanted phases due to austenite decomposition, such as pearlite and excessive coarsening of the microstructure during cooling. An excessively high average cooling rate will prevent the formation of new ferrite during cooling. A high ferrite ratio is necessary to achieve the required mechanical and technological properties. Therefore, the upper limit of the average cooling rate is set to a maximum of 100 °C / s.

[0123] In addition, the average cooling rate in the temperature range of 700 to 600 °C dominated in the third stage needs to be more precisely controlled. It has been found that the average cooling rate in this temperature range has a particularly strong influence on the formation of the microstructure components. Therefore, the average cooling rate in the temperature range of 700 to 600 °C needs to be between 10 and 50 °C / s to achieve the required mechanical and technological properties.

[0124] In the fourth stage, the cold-rolled flat steel product is held at a temperature T3 which is greater than or equal to the martensite start temperature Ms and lower than 455 °C for a holding time of 1 to 1000 s. In particular, T3 = T2 ± 20 °C may hold. When the holding time is less than 1 s, it is not sufficient to form a significant proportion of bainite. When the holding time is too long, an excessive proportion of pearlite may be formed, which has an adverse effect on the tensile strength and the hole expansion rate. Therefore, the holding time is limited to a maximum of 1000 s, in particular a maximum of 800 s, preferably a maximum of 500 s, more preferably a maximum of 300 s.

[0125] After the fourth stage of annealing, the cold-rolled flat steel product can be cooled to a temperature of at most 100 °C at an average cooling rate of 0.5 to 20 °C / s in the fifth stage.

[0126] Alternatively and preferably, after the fourth stage, the cold-rolled flat steel product can be provided with a zinc-based anti-corrosion coating by immersion in a molten bath. The bath entry temperature is at least 10 °C lower and at most 20 °C higher than the molten bath temperature to prevent a significant change in the molten bath temperature due to the introduction of the cold-rolled flat steel product. The required bath entry temperature can be achieved by controlling the corresponding temperature of the cold-rolled flat steel product in the fifth stage.

[0127] There are no special requirements for the anti-corrosion coating and thus for the composition of the molten bath through which the cold-rolled flat steel product passes during its hot-dip coating. For example, the anti-corrosion coating or the molten bath consists mainly of zinc (Zn) and essentially has a conventional composition. Thus, the anti-corrosion coating or the molten bath may contain at most 20 wt% Fe, at most 5 wt% Mg and at most 10 wt% Al in addition to Zn and unavoidable impurities. If present, typically at least 1 wt% Mg and / or at least 1 wt% Al are set here to achieve the best service properties of the anti-corrosion protection.

[0128] After the hot-dip coating operation, a further heat treatment ("galvanizing annealing") can be carried out optionally, in which the hot-dip coated flat steel product is heated to at most 550 °C to bake the zinc-based coating.

[0129] Immediately after leaving the hot-dip coating operation, or after an additional optional heat treatment, the obtained cold-rolled flat steel product can be cooled to a temperature below 100 °C at an average cooling rate of 0.5 to 20 °C / s.

[0130] The "average" heating or cooling rate should be understood as the difference between the starting temperature (actual temperature) and the target temperature (desired temperature) divided by the time required to reach the target temperature from the starting temperature. Usually, the heating and cooling rates are not constant parameters.

[0131] The flat steel products obtained thereby can optionally be further subjected to a conventional skin pass rolling operation to optimize their dimensional accuracy and surface characteristics. The skin pass rolling rate set herein is at least 0.1% and at most 1.0%, and it is particularly preferably set at least 0.2%. A skin pass rolling rate lower than 0.1% results in too low surface roughness of the cold-rolled flat steel products selectively coated with a metal coating, which has an adverse effect on the forming properties of the flat steel products. A skin pass rolling rate exceeding 1.0% has an adverse effect on both mechanical properties (yield limit and elongation at break) and the hole expansion rate.

[0132] The flat steel products obtained by cold rolling, selective coating, selective heat treatment and selective skin pass rolling are preferably wound into coils or cut into blanks.

[0133] In the flat steel products according to the present invention, the required combination of microstructure components is achieved by adjusting the chemical analysis composition of the steel and its manufacture using the above production method. The alloying elements of the melt or steel (flat steel product) are specified as follows:

[0134] Carbon (C) is crucial for the formation of harder microstructure components such as martensite, retained austenite and bainite. The proportion of these microstructure components has a great influence on the mechanical properties of the steel. The combination of high tensile strength and high hole expansion rate given according to the present invention requires a specific proportion of these microstructure components. Too low a proportion of martensite, retained austenite or bainite results in insufficient tensile strength, while too high a proportion of martensite or retained austenite results in insufficient hole expansion rate. To achieve the desired microstructure and thus the desired combination of tensile strength and hole expansion rate, the C content must be between 0.050% and 0.170% by weight, particularly between 0.055% and 0.160% by weight. When the C content is lower than 0.050% by weight, insufficient martensite, retained austenite and bainite are formed, and thus the required minimum tensile strength of 590 MPa cannot be achieved. When the C content exceeds 0.170% by weight, too much martensite or retained austenite is formed, and thus the minimum requirement for the hole expansion rate cannot be met.

[0135] Silicon (Si) prevents the formation of pearlite, which is harmful to the ductility and formability of the material. Otherwise, the carbon combined in the form of pearlite promotes the formation of carbon-rich phases such as martensite, bainite and retained austenite. Therefore, the alloying addition of Si indirectly affects the mechanical properties. In particular, the alloying addition of Si promotes the formation of retained austenite, which is for the elongation at break A 80There is a positive impact. To achieve this effect, a Si content of at least 0.080% by weight, especially at least 0.10% by weight, preferably at least 0.120% by weight is necessary. An excessive Si content will lead to the formation of too much retained austenite, which has an adverse effect on the hole expansion rate. In addition, an excessive Si content is harmful to the surface quality of hot-dip coated steel. Therefore, the upper limit of the Si content is restricted to a maximum of 0.350% by weight, especially a maximum of 0.310% by weight, preferably a maximum of 0.270% by weight.

[0136] Manganese (Mn) is a key alloying element, and its distribution is uneven in the strip thickness direction. Mn has a strong influence on the solubility of C in Fe, and thus has a strong influence on the local forming characteristics of the microstructure. Therefore, the Mn content has a decisive influence on the formation of carbon-rich microstructure components during annealing and thus on the mechanical properties and hole expansion rate of the annealed material. It has been found that a Mn content between 1.50% and 2.40% by weight, especially between 1.60% and 2.350% by weight, preferably between 1.70% and 2.30% by weight can achieve the best combination of mechanical properties and hole expansion rate. When the Mn content is less than 1.50% by weight, the minimum tensile strength requirement of 590 MPa cannot be achieved. When the Mn content exceeds 2.40% by weight, it will lead to the formation of too much martensite and retained austenite, and thus the minimum requirements for hole expansion rate and elongation at break cannot be met.

[0137] Chromium (Cr) affects the formation of microstructure components during annealing in a similar way to Mn, thus affecting the mechanical properties and hole expansion rate of the final material. In addition, Cr can slow down the coarsening of Nb-based carbides, etc. In this way, Cr can also contribute to the increase in strength through precipitation hardening. To achieve these effects, a Cr content of at least 0.20% by weight, especially at least 0.250% by weight is necessary. However, an excessive amount of Cr will increase the risk of obvious grain boundary oxidation, which will reduce the surface quality. Therefore, the upper limit of the Cr content is restricted to a maximum of 0.950% by weight, especially a maximum of 0.90% by weight.

[0138] In addition to Si, aluminum (Al) can be added to inhibit the formation of pearlite and thus inhibit the formation of retained austenite. Since Al is usually used for deoxidation of the melt, a content of at least 0.010% by weight is usually inevitable in the production of conventional steel. In contrast to Si, Al has a smaller adverse effect on the surface quality of hot-dip coated flat steel products. However, an excessive Al content will lead to the excessive formation of retained austenite, which has an adverse effect on the hole expansion rate. Therefore, the upper limit of the Al content is restricted to a maximum of 0.50% by weight, especially not exceeding 0.30% by weight, preferably not exceeding 0.10% by weight, more preferably not exceeding 0.080% by weight.

[0139] In addition, at least one of the following alloying elements from the group (Ti, Mo, B, Mo, Cu, Ni) can be selectively added:

[0140] Titanium (Ti), as a selective alloying element that precipitates in the form of carbides, can refine the microstructure, especially at a minimum content of 0.0010 wt%. A finer microstructure can have a positive impact on the tensile strength without significantly impairing ductility. It has been found that a finer microstructure has a positive effect on the hole expansion rate of the material. Ti-based carbides can also directly affect the tensile strength of the material through the precipitation hardening effect. This effect is particularly significant when the carbides do not coarsen excessively during annealing in a continuous annealing facility. However, too high a titanium content can lead to cracks during continuous casting or slab cooling / reheating. To avoid the negative effects of Ti, the upper limit of the Ti content is restricted to a maximum of 0.060 wt%, especially a maximum of 0.055 wt%.

[0141] Boron (B), as a selective alloying element, can enrich at grain boundaries during the austenitization of the microstructure and prevent the formation of ferrite there during cooling, thus selective alloying addition can be used to increase strength, especially at a minimum content of 0.0002 wt%. However, too high a B content can lead to excessive formation of martensite, which is not conducive to the desired microstructure composition. Therefore, the B content should not exceed 0.0015 wt%, especially not exceed 0.0014 wt%. When a particularly high ferrite ratio is required and strength improvement by adding B is not needed, the B content should not exceed 0.0005 wt% or not added at all.

[0142] Molybdenum (Mo), as a selective alloying element, can retard the diffusion of C, thereby hindering the homogenization of C distribution during annealing. Therefore, the Mo content needs to be restricted to a maximum of 0.20 wt%, especially a maximum of 0.10 wt%. A minimum content of 0.0010 wt% can be added or present if necessary.

[0143] Copper (Cu), as a selective alloying element, can precipitate in the form of coarse grains, thus having an adverse effect on mechanical properties. In addition, Cu also has a negative impact on casting properties. To avoid any effects caused by Cu, the Cu content should be restricted to a maximum of 0.20 wt%, especially a maximum of 0.10 wt%. A minimum content of 0.0010 wt% can be added or present if necessary.

[0144] Nickel (Ni), as a selective alloying element, can be alloyed and added, for example, at a minimum content of 0.0010 wt% to improve ductility. However, at the same time, the presence of nickel may also lead to an undesired decrease in the strength of the material. Therefore, to prevent the adverse effect of Ni on strength, the Ni content is restricted to a maximum of 0.20 wt%, especially a maximum of 0.10 wt%.

[0145] All other alloying elements not specifically listed here are considered unavoidable impurities limited by production. These impurities may be part of the steelmaking raw materials or enter the steel due to process limitations during steel melting and processing. Impurity elements include alloying elements in the groups of (P, S, N, Nb, V, W, Sn, Sb, As, Co, Ca, O, H), and their contents must be controlled at a sufficiently low level to ensure that they do not have any technical impact on the properties of the steel.

[0146] Phosphorus (P) is generally considered an impurity. It mainly enters the steel through iron ore and cannot be completely removed during industrial-scale steelmaking. Its content should be reduced as much as possible. To ensure reliable welding performance of the process, its content should be at most 0.020%, especially at most 0.0170%.

[0147] Sulfur (S) is also generally considered an impurity. Therefore, its content must be adjusted to at most 0.010 wt%, especially at most 0.0050%, preferably at most 0.0030 wt%, to avoid obvious segregation tendency caused by excessive formation of sulfides (FeS, MnS, (Mn, Fe)S) and adverse effects on formability or ductility. Usually, desulfurization treatment can be carried out by alloying addition of calcium, and the S content can be adjusted accordingly according to the Ca content.

[0148] Nitrogen (N) is also an unavoidable impurity limited by production. As long as N exists, Ti, Nb and / or V will preferentially form nitrides or carbonitrides with N in the presence of C. Therefore, during the actual production process, it is inevitable for N to be absorbed by precipitates under technically and economically acceptable conditions. But in principle, as low a content as possible should be pursued, because carbonitrides mainly composed of N are usually very coarse and angular, so they not only cannot play a strengthening role, but will instead become crack initiation sources. To avoid the formation of carbonitrides mainly composed of N, its content should be limited to at most 0.010%, especially at most 0.0080%.

[0149] Niobium (Nb), vanadium (V), tungsten (W), tin (Sn), antimony (Sb), arsenic (As), cobalt (Co), zirconium (Zr) and rare earth elements such as lanthanum (La), cerium (Ce), neodymium (Nd) and praseodymium (Pr) are not alloying elements required for the steel according to the invention and, if detected, are all considered inevitable impurities. Thus: the Nb content is limited to a maximum of 0.030 wt%, in particular a maximum of 0.0280 wt%; the V content is limited to a maximum of 0.020 wt%, in particular a maximum of 0.010 wt%; the W content is limited to a maximum of 0.10 wt%, in particular a maximum of 0.050 wt%; the Sn content is limited to a maximum of 0.050 wt%; the Sb content is limited to a maximum of 0.0004 wt%; the As content is limited to a maximum of 0.020 wt%; the Co content is limited to a maximum of 0.020 wt%.

[0150] Calcium (Ca) is usually added to the melt during steel production, both for deoxidation and desulphurization and for improving castability. Excessive amounts lead to the formation of undesired inclusions which have an adverse effect on the mechanical properties and rollability. Its upper limit is therefore restricted to a maximum of 0.0050%, in particular a maximum of 0.0030%.

[0151] Oxygen (O) is likewise undesirable in the melt or in the steel, since oxide coatings have an adverse effect on the mechanical properties, castability and rollability. Its maximum allowable content is therefore set at a maximum of 0.0050%, in particular a maximum of 0.0020%.

[0152] Hydrogen (H), being the smallest atom, has a high mobility in the interstitial spaces of the steel lattice and can cause core cracks during hot rolling cooling, especially in high-strength steels. Its content should therefore be as low as possible, in any case a maximum of 0.0010%, in particular a maximum of 0.0006%, preferably a maximum of 0.0004%, with a content of preferably a maximum of 0.0002% being pursued. Detailed Description of the Invention

[0153] The present invention will be described in detail below by way of examples. These examples are summarized in Tables 1 to 4. Table 1 lists the chemical composition of the examples. The specifications for hot rolling, cold rolling, annealing treatment and selective hot dip coating are given in Tables 2 and 3. Table 4 shows both the mechanical-process characteristics and the microstructural characteristics of the examples.

[0154] To verify the present invention, alloy melts A - F were prepared according to the compositions listed in Table 1 and cast into slabs. The melts not in accordance with the present invention and the contents of specific alloying elements deviating from the specifications of the present invention are underlined in Table 1. For certain alloying elements with extremely low contents which can be regarded as "0" at the technical level, i.e., low enough not to affect the properties of the steel, they are marked with a "-" symbol in Table 1.

[0155] The slabs prepared from melts A - F are fully heated in a preheating furnace at a temperature of (“VWO”). Subsequently, these preheated slabs are hot - rolled in a conventional manner to produce hot - rolled flat steel products (hot - rolled strips). The resulting hot - rolled strips leave the hot - rolling mill at the hot - rolling end temperature (“WET”) and are then cooled to the coiling temperature (“HT”) at an average cooling rate (“AKR”), at which temperature they are wound into coils respectively. After the coils are cooled to room temperature, the hot - rolled strips are pickled in a conventional manner and then cold - rolled in a conventional manner at a cold - rolling degree (“KWG”) to produce cold - rolled flat steel products (cold - rolled strips).

[0156] To verify the effects of the present invention, one of the combinations I - VII of VWO, WET, AKR, HT, and KWG listed in Table 2 was selected respectively during the production of cold - rolled strips. The combinations in I - VII that do not conform to the present invention and the items that do not match the data of the present invention respectively have been marked with underlines in Table 2.

[0157] After cold - rolling, the cold - rolled strips are annealed in a multi - stage continuous annealing device in a conventional manner, or in a hot - dip coating device in a multi - stage continuous annealing device in a conventional manner. The annealing process is carried out in multiple stages. In the first stage, the cold - rolled strips are heated to a holding temperature at an average heating rate (“HR”), which is set with reference to the reference temperature (“T1”) and corresponds to a temperature between T1 - 30°C and T1. The cold - rolled strips are held at this holding temperature for a certain holding time (“HZ1”). Subsequently, the cold - rolled strips are cooled to a certain holding temperature at an average cooling rate (“KR1”), where the difference DT between T1 and T2 is given in Table 3. The cold - rolled strips are held at the holding temperature set with reference to the reference temperature (“T2”) for a certain holding time (“HZ2”). After the holding time (“HZ2”), the cold - rolled strips are selectively coated with a zinc - based anti - corrosion coating by immersion in a molten bath. The cold - rolled strips are cooled to room temperature at an average cooling rate (“KR2”) immediately after the end of the holding time (“HZ2”), or after leaving the molten bath.

[0158] The cold - rolled strips (cold - rolled flat steel products) thus produced can be selectively leveled at a flatness (“DG”).

[0159] In the examples, the annealing and the selective hot - dip coating treatment are carried out according to one of the combinations a - i of HR, T1, HZ1, KR1, T2, HZ2, KR2, and DG listed in Table 3. For the combinations a - i, it is also noted whether the cold - rolled strips are coated with a zinc - based anti - corrosion coating. The combinations in a - i that do not conform to the present invention and the corresponding items that do not match the data of the present invention have been marked with underlines in Table 3.

[0160] The mechanical and technological properties as well as the microstructural characteristics were determined on the cold-rolled strip produced thereby. The results of these studies are summarized in Table 4. These results include the yield limit R P0.2 , the tensile strength R m , the elongation at break A 80 and the hole expansion ratio λ, as well as the constant X in the formula λ = -0.08×Rm + X. Table 4 also gives the proportions of ferrite F, bainite B and their sum F + B, as well as the total proportion M + A of martensite M and retained austenite RA and the other microstructural constituent "others". In addition, the microhardness HM of martensite, the area proportion "α" of α-fibers and the maximum orientation density distribution function "f(g)max" are given. In addition, for the cold-rolled flat steel product Al-F3, it is also additionally given which steel / melt A-F the steel matrix of each corresponding flat steel product consists of ("analysis composition" column), which of the combinations I-VII of hot-rolled strip used in the production of each corresponding flat steel product ("rolling" column), and which of the variants a-i of annealing treatment and optional coating treatment ("annealing" column).

[0161] Example A1 consists of a steel matrix with chemical composition A and is prepared according to rolling specification I and annealing specification a. This achieves the best combination of mechanical and technological properties and microstructural characteristics.

[0162] Example A2 was prepared under conditions of too low VWO, but the remaining conditions are the same as those of Example A1. The deviation of VWO results in the absence of the bainite fraction and too high a proportion of unwanted microstructural constituents. Therefore, the elongation at break A 80 and the value of the constant X of Example A2 are too low. Thus, Example A2 serves as a counterexample here.

[0163] Example A3 was prepared with lower but acceptable VWO and WET values than Example A1. In addition, compared with Example A1, Example A3 also has a low AKR, a high HT and a low KWG. This keeps its mechanical and technological properties and microstructural characteristics within the required range. While Example A4 was prepared under conditions of too low WET, and the remaining conditions are the same as those of Example A3. Too low WET results in high α and f(g)max values, which in turn cause R p0.2 to be too high, A 80 to be too low and the constant X to be too low. Therefore, Example A4 serves as a counterexample.

[0164] Steel B is similar to steel A, but has too high an Mn content. In Example B1, steel B was produced under the same conditions as Example A1. The deviation of the chemical analysis composition results in the absence of the bainite fraction and too high a total proportion of martensite and retained austenite. Therefore, the mechanical and technological properties of this example exceed the target range. Thus, Example B1 serves as a counterexample.

[0165] Steel C was used in Examples C1 - C3 to study the effect of the temperature difference T1 - T2. Example C1 was prepared under the same conditions as Example A1, and its mechanical - technological characteristics and microstructural features were all within the required ranges. Example C2 used a significantly lower but still acceptable value of T1 - T2. This led to a lower proportion of bainite and a higher total proportion of martensite and austenite, and thus a higher R m and a lower constant X compared to Example C1. In Example C3, an overly low value of T1 - T2 was set. This led to the absence of a bainite fraction, which in turn led to an overly low constant X. Therefore, Example C3 serves as a counterexample.

[0166] Steel D, as a low - strength variant, was used in Examples D1 to D3 to study the effects of AKR and HT. In addition, Examples D1 to D3 were all prepared with extremely high values of WET and KWG. In Example D1, low AKR and high HT values were set. In Example D2, high AKR and lower HT values were set. However, the mechanical - technological characteristics and microstructural features of both Example D1 and D2 were within the required ranges. In Example D3, an overly low HT value was set. Therefore, the microstructural features and mechanical - technological characteristics were outside the intended range. Thus, Example D3 is a counterexample.

[0167] In Examples D4 - D6, the T1 and T2 values were varied while keeping T1 - T2 constant. Example D4 was prepared with an overly low T1. This led to the absence of a bainite fraction, and H M and α and f(g)max were too high, and thus the constant X was too low. Therefore, Example D4 serves as a counterexample.

[0168] Example D5 was prepared with a lower but still acceptable T1. Therefore, the proportion of bainite was lower, and H M and α and f(g)max were higher but still within the required ranges. This led to a lower but acceptable constant X. In Example D6, an extremely high T1 was set. This led to a high proportion of bainite and low H M and α and f(g)max, which had a positive effect on the constant X.

[0169] Steel E is similar to Steel D, but has too low C and Mn contents. In Example E1, Steel E was produced under the same conditions as Example D1. Due to the extremely low alloy content, the ferrite content exceeded the required level, while R p0.2 and R m were below the required levels. Therefore, Example E1 serves as a counterexample.

[0170] Compared with Steels A, C, and D, Steel F has a lower C content and a higher Mn content. Steel F was used in Examples F1 - F3 to study the effect of HZ1. In addition, Examples F1 - F3 were all prepared with extremely high T1 - T2, extremely long HZ2, and no zinc coating. In Example F1, an overly short HZ1 value was used. This led to the absence of a bainite fraction, and H M, α and f(g)max are too high. Therefore, the mechanical-process characteristics exceed the required range. Thus, Example F1 serves as a counterexample.

[0171] Although HZ1 of Examples F2 and F3 is shorter or longer than that of other examples, it is still within the acceptable range. This results in mechanical-process characteristics and microstructural features within the desired range.

[0172]

[0173] Table 1: Chemical analysis composition of the examples (all data are in wt%)

[0174]

[0175]

[0176] Table 2: Process conditions of the examples involving hot rolling and cold rolling

[0177]

[0178] Table 3: Process conditions of the examples involving annealing and selective hot dip coating

[0179]

Claims

1. A cold-rolled flat steel product having a tensile strength R of 590 MPa to 900 MPa m , a yield limit R of 330 MPa to 600 MPa p0.2 , and an elongation at break A of at least 14%, determined in accordance with DIN EN ISO 6892-1:2017, wherein the cold-rolled flat steel product, apart from Fe and unavoidable impurities due to production, consists by weight % of: 80 ​ C: 0.050 to 0.170%, Si: 0.080 to 0.350%, Mn: 1.50 to 2.40%, Cr: 0.20 to 0.950%, Al: 0.010 to 0.50%, and optionally at least one alloying element selected from the group (Ti, Mo, B, Mo, Cu, Ni): Ti: at most 0.060%, B: at most 0.0015%, Mo: at most 0.20%, Cu: at most 0.20%, Ni: at most 0.20%, wherein the impurities include the following alloying elements selected from the group (P, S, N, Nb, V, W, Sn, As, Co, Ca, O, H), and their contents are: P: at most 0.020%, S: at most 0.010%, N: at most 0.010%, Nb: at most 0.030%, V: at most 0.020%, W: at most 0.10%, Sn: at most 0.050%, Sb: at most 0.0004%, As: at most 0.020%, Co: at most 0.020%, Ca: at most 0.0050%, O: at most 0.0050%, H: at most 0.0010%, wherein the cold-rolled flat steel product has a microstructure comprising the following phases, excluding the inevitable microstructural components due to production: the sum of ferrite and bainite is 75% to 96%, and the sum of martensite and retained austenite is 4% to 25%.

2. The flat steel product according to claim 1, wherein the minimum allowable hole expansion ratio λ in % measured according to DIN EN ISO 16630:2017 is calculated by the following approximate formula: λ = (-0.08 × Rm)*% / MPa + X, where the constant X is at least 85%.

3. The flat steel product according to any one of the preceding claims, wherein the ferrite content in the microstructure is 25% to 85%.

4. The flat steel product according to any one of the preceding claims, wherein the bainite content in the microstructure is 5% to 60%.

5. The flat steel product according to any one of the preceding claims, wherein the martensite content in the microstructure is at least 1% and the retained austenite content is at least 1%.

6. The flat steel product according to any one of the preceding claims, wherein the proportion of the microstructure having α fibers measured by electron backscatter diffraction is at least 5% and at most 30%.

7. The flat steel product according to any one of the preceding claims, wherein the orientation density distribution function f(g) is measured by electron backscatter diffraction, and the maximum f(g) corresponding to the area proportion of the most frequent orientation in the microstructure is determined such that the maximum f(g) is at least 2% and at most 10%.

8. The flat steel product according to any one of the preceding claims, wherein the cold-rolled flat steel product has a zinc-based anti-corrosion coating.

9. A method for producing a cold-rolled flat steel product, comprising the following steps: a) melting a steel, the composition of which, except for Fe and inevitable impurities, in wt% is: C: 0.050 to 0.170%, Si: 0.080 to 0.350%, Mn: 1.50 to 2.40%, Cr: 0.20 to 0.950%, Al: 0.010 to 0.50%, and optionally at least one alloying element from the group (Ti, Mo, B, Mo, Cu, Ni): Ti: at most 0.060%, B: at most 0.0015%, Mo: at most 0.20%, Cu: at most 0.20%, Ni: at most 0.20%, wherein the impurities include alloying elements from the group (P, S, N, Nb, V, W, Sn, As, Co, Ca, O, H), and their contents may be: P: at most 0.020%, S: at most 0.010%, N: at most 0.010%, Nb: at most 0.030%, V: at most 0.020%, W: at most 0.10%, Sn: at most 0.050%, Sb: at most 0.0004%, As: at most 0.020%, Co: at most 0.020%, Ca: at most 0.0050%, O: at most 0.0050%, H: at most 0.0010%; b) casting the melt into a pre-product; c) preheating the pre-product to and / or maintaining the pre-product at a temperature between 1150 and 1350 °C; d) hot rolling the pre-product into a hot-rolled flat steel product, wherein the hot rolling finishing temperature is between 850 and 980 °C; e) cooling the obtained hot-rolled flat steel product to a coiling temperature between 450 and 600 °C at an average cooling rate between 20 and 400 °C / s; f) coiling the hot-rolled flat steel product cooled to the coiling temperature into a coil; g) uncoiling the hot-rolled flat steel product and cold rolling it into a cold-rolled flat steel product, wherein the cold rolling rate is between 30% and 80%; h) coiling the cold-rolled flat steel product into a coil; i) uncoiling the cold-rolled flat steel product and annealing the cold-rolled flat steel product in a continuous process, including the following steps: i1) heating at an average heating rate between 0.5 and 20 °C / s to a holding temperature between 840 and 900 °C, wherein the average heating rate between 700 °C and the holding temperature is 1 to 10 °C / s; i2) holding for a holding time between 30 and 300 s, wherein the temperature T1 at the end of the holding time is between 840 and 900 °C, and the holding temperature is at least T1 - 30 °C; i3) cooling at an average cooling rate between 0.5 and 100 °C / s to a temperature T2, which is equal to or greater than the martensite start temperature Ms and less than 490 °C, wherein the average cooling rate between 700 and 600 °C is 10 to 50 °C / s, and the temperature difference DT between T1 and T2 is greater than 385 °C; i4) holding for a holding time between 1 and 1000 s at a temperature T3, which is equal to or greater than the martensite start temperature Ms and less than 455 °C; to obtain a flat steel product having such a microstructure except for inevitable microstructural components, the microstructure comprising the following phases: the sum of ferrite and bainite accounts for 75% to 96%, and the sum of martensite and retained austenite accounts for 4% to 25%.

10. The method according to claim 9, wherein after step i4), the cold-rolled flat steel product is hot-dip coated with a zinc-based anti-corrosion coating.

11. The method according to claim 10, wherein the hot-dip galvanized flat steel product is heated to a temperature of up to 550 °C.

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