DE102021121997A1 - Cold-rolled flat steel product and method for its manufacture - Google Patents

Abstract

The invention relates to a correspondingly cold-rolled flat steel product with a tensile strength Rm of at least 780 MPa, a yield point Rp0.2 of at least 440 MPa and a maximum of 600 MPa and an elongation at break A80 of at least 18%, and a method for its production.

Description

Technical Field

The invention relates to a cold-rolled flat steel product with a high tensile strength R _m , a low yield point R _p0.2 and a high elongation at break A ₈₀ , and a method for its manufacture.

Technical background

The ever-increasing demand for fuel-efficient cars is driving an increasing need for weight reduction through lightweight construction. A cost effective weight reduction measure is the use of high strength steels. This enables the production of thin-walled components which, despite their low weight, can withstand high mechanical loads. So that the steels can also be formed into components with greater geometric complexity, these must also have greater ductility. Typically, the strength and ductility of steels are anti-correlated, i.e. the higher the strength, the lower the ductility. For this reason, the use of high-strength steels for such components is only possible to a limited extent.

The latest generation of high-strength steels, so-called "Advanced High Strength Steels", aim for very high formability with higher strength at the same time. These include so-called dual-phase (DP) steels, which typically consist largely of ferrite and martensite, but may also contain other phases such as bainite and/or retained austenite. During the plastic deformation of DP steels, the soft ferrite is usually deformed, while the hard martensite increases strength. During the plastic deformation, martensite can also be formed from optionally present retained austenite. This also contributes to the increase in strength without affecting the first stages of plastic deformation. When tested in the laboratory, DP steels are characterized by a relatively lower yield point ratio, ie the ratio between yield point R _p0.2 and tensile strength R _m , and a relatively high elongation at break A ₈₀ compared to complex phase (CP) steels. The latter consist mainly of phases with a medium hardness such as bainite and/or tempered martensite.

DP steels with a relatively high content of retained austenite, typically more than 5% based on the measuring surface, are characterized by a relatively high elongation at break compared to conventional DP steels. The higher elongation at break results from the elongation-induced transformation of the retained austenite, ie the so-called "transformation induced plasticity" or TRIP effect. Such steels are therefore often referred to as "TRIP-aided" DP steels. An example of a cold-rolled flat steel product is given in the patent application WO 2013/182621 A1 disclosed, in which a residual austenite content of up to 15% by volume is described by the alloying of Al. In one production variant, the cold-rolled flat steel product is produced in a conventional manner by casting to form a preliminary product, by hot and cold rolling and annealing in a hot-dip coating plant. Annealing takes place in several steps, whereby the cold-rolled flat steel product is heated to a maximum annealing temperature in the range of 750 - 870 °C and then cooled to a temperature in the range of 455 - 550 °C.

A disadvantage of DP steels relative to other high-strength steels is their high susceptibility to edge cracking, which can occur during the forming of stamped sheet metal. In the laboratory, the edge crack sensitivity of a steel is evaluated with the so-called "hole expansion test", in which a hole punched in a sheet metal sample is widened with a mandrel until the first crack appears, see ISO 16630:2017, "Metallic materials --Sheet and strip -- Get expanding test". From the diameter of the hole before testing, Do, and at first crack initiation, D _R , the hole expansion ratio, λ, is calculated using the following formula: $$\mathrm{\lambda }=100*\left({\text{D}}_{\text{R}}-{\text{D}}_0\right)/{\text{D}}_0.$$

The relatively high sensitivity to edge cracks in DP steels essentially depends on the inhomogeneity of the microstructure. In DP steels, martensite forms in the form of coarse clusters, which are often arranged in rows. Due to the high difference in hardness between the ferrite and martensite phases, external mechanical loads lead to high internal stresses at the interfaces between the two phases. This promotes crack formation during the hole expansion test, since very high degrees of deformation occur there at the edge of the punched hole. In contrast, CP steels are characterized by a relatively isotropic and homogeneous structure with relatively fine precipitations. For this reason, the local stresses in CP steels are homo under external mechanical loads higher and lower than those generated in DP steels. Therefore, CP steels are characterized by higher resistance to edge cracks compared to DP steels.

Generic methods and correspondingly produced flat steel substrates are, for example, in the published specifications WO 2013/037485 A1 , WO 2014/159625 A1 described.

Summary of the Invention

The object of the invention is therefore to provide a cold-rolled flat steel product with an optimized combination of high tensile strength R _m , a low yield point R _p0.2 and a high elongation at break A ₈₀ , and to specify a corresponding method for its production.

According to a first aspect of the invention, this object is achieved by a cold-rolled flat steel product having the features of patent claim 1.

According to the invention, a cold-rolled flat steel product is provided which, in addition to Fe and unavoidable impurities in wt C: 0.12 to 0.18%, Si: 0.05 to 0.4%, Mn: 1.9 to 2.5%, Al: 0.2 to 0.5%, Cr: 0.05 to 0.2%, Nb: 0.01 to 0.06% consists of the following alloying elements from the group (P, S, N, Ti, V, Mo, B, Cu, W, Ni, Sn, As, Co, Zr, La, Ce, Nd, Pr, Ca, O, H) and can be present with the following contents: P: up to 0.02%, S: up to 0.01%, N: up to 0.01%, Ti: up to 0.01%, V: up to 0.02%, Mon: up to 0.1%, B: up to 0.0007%, Cu: up to 0.1%, W: up to 0.1%, Ni: up to 0.1%, Sn: up to 0.05%, Sb: up to 0.0004% As: up to 0.02%, Co: up to 0.02%, Zr: up to 0.0002%, La: up to 0.0002%, Ce: up to 0.0002%, Nd: up to 0.0002%, Pr: up to 0.0002%, Approx: up to 0.005%, O: up to 0.005%, H: up to 0.001%.

Furthermore, the flat steel product according to the invention has a structure together with unavoidable structural components, which comprises the following phases (in %): Ferrite: 20 to 85%, bainite: 10 to 70%, retained austenite: 5 to 15%, martensite: up to 20%, (including 0).

All information on the content of the alloying elements specified in the present description is based on the weight, unless expressly stated otherwise. All contents are therefore to be understood as percentages by weight. The specified structural components are determined by evaluation of light or electron microscopic investigations and are therefore to be understood as percentages of area, unless expressly stated otherwise. An exception to this is the structural component austenite or residual austenite, which is given as a volume percentage in % by volume, unless expressly stated otherwise.

wobei die Konstante A mindestens 75 %, insbesondere mindestens 78 %, vorzugsweise mindestens 80 % beträgt. Bei einigen hochfesten Stählen, insbesondere Mehrphasenstählen hat es sich herausgestellt, dass das Lochaufweitungsverhältnis λ von der Zeitverzögerung zwischen den Stanzen des Lochs und die Durchführung der Prüfung abhängig ist. Ein Zeitrahmen für die Prüfung ist in ISO 16630:2017 nicht vorgegeben. Die hier angegebenen Werte beziehen sich auf Prüfungen, die innerhalb von 5 Stunden nach dem Stanzen des Lochs gemäß ISO 16630:2017 durchgeführt wurden.The flat steel product according to the invention has a tensile strength R _m determined according to DIN EN ISO 6892-1:2017 (specimen shape 2, longitudinal samples) of at least 780 MPa, yield point R _p0.2 of a maximum of 600 MPa and elongation at break A ₈₀ of at least 18%. The yield point R _p0.2 can in particular be limited to a maximum of 580 MPa, preferably to a maximum of 550 MPa. The minimum yield point is at least 440 MPa. The lowest permissible hole expansion ratio λ (in %) determined according to DIN EN ISO 16630:2017 can be calculated using the following formula: $$\mathrm{\lambda }=-0,067\times \text{rm}+\text{A}$$

where the constant A is at least 75%, in particular at least 78%, preferably at least 80%. For some high-strength steels, particularly multiphase steels, it has been found that the hole expansion ratio λ depends on the time delay between punching the hole and performing the test. A time frame for the test is not specified in ISO 16630:2017. The values given here refer to tests carried out within 5 hours after punching the hole according to ISO 16630:2017.

The tensile strength R _m is in particular a maximum of 920 MPa, preferably a maximum of 900 MPa.

The combination of a high hole expansion ratio λ in relation to the tensile strength R _m , a low yield point R _p0.2 and a high elongation at break A ₈₀ of the flat steel product according to the invention is due to the composition of the microstructure. The main components of the structure can be determined by means of light-optical microscopy (LOM) at a magnification of 200 to 2,000 times after etching with a suitable etchant (eg Nital or sodium metabisulfite).

A relatively high proportion of ferrite is required for a low yield point R _p0.2 . In the first stage of plastic deformation, the ferrite is preferentially deformed, while the harder, carbon-rich phases such as bainite, retained austenite and martensite serve as reinforcement. As a result, the yield point is mainly influenced by the ferrite content. It has been found that for a yield point in the desired range there should be at least 20% by area, in particular at least 30% by area, preferably 40% by area. Due to the relatively low hardness of the ferrite, a certain proportion of bainite, retained austenite and optionally martensite is required in order to be able to achieve a tensile strength R _m of at least 780 MPa, so that the proportion of ferrite in the structure is limited to a maximum of 85% by area, in particular is limited to a maximum of 80% by area, preferably to a maximum of 75% by area.

Bainite and optionally martensite are essential for increasing strength. For this reason, a certain proportion of bainite and optionally martensite must be present in order to achieve the minimum requirements in terms of tensile strength _Rm . However, the proportions of bainite and optionally martensite have a decisive influence on the hole expansion ratio λ. The relatively high hole expansion ratio λ of the steel flat product according to the invention is a consequence of the relatively high proportion of bainite and relatively low proportions of optional martensite compared to other DP steels, cf. WO 2013/182621 A1 , the same train fes activity. A high proportion of martensite is generally associated with a low hole expansion ratio λ, since the boundaries between adjacent ferrite grains and martensite packets serve as crack initiation points under external mechanical loads. A high density of potential crack initiation points has a negative effect on the sensitivity of the cut edges. For this reason, the proportion of martensite in the structure should be minimized as much as possible. Accordingly, the proportion of martensite is limited to a maximum of 20% by area, in particular to a maximum of 17% by area, preferably to a maximum of 15% by area, preferably to a maximum of 10% by area. The martensite content can be 0. In order to be able to achieve the minimum requirement in relation to the tensile strength _Rm , it may be desirable to provide a martensite content of at least 2% by area. If a higher tensile strength R _m is desired, a martensite content of in particular at least 5% by area can be provided.

The high crack sensitivity of ferrite-martensite boundaries is due to the relatively high hardness of martensite compared to ferrite and consequently the high hardness difference between the phases. Compared to martensite, bainite is less hard. Therefore, the hardness difference between ferrite and bainite is lower than that between ferrite and martensite. Consequently, ferrite-bainite boundaries are less prone to cracking under external mechanical loads. Accordingly, a high proportion of bainite and a correspondingly low proportion of martensite is advantageous for the sensitivity of the cut edges. However, the lower hardness of bainite compared to martensite means that some proportion of martensite must be replaced with a higher proportion of bainite to achieve the same tensile strength. In the case of the flat steel product according to the invention, it has been found that the required mechanical properties can be achieved with a bainite content of between 10 and 70% by area. If the proportion of bainite is too low, the minimum requirements in terms of tensile strength R _m and hole expansion ratio λ are not met. For this reason, a bainite content of at least 10% by area, in particular at least 15% by area, preferably at least 20% by area, is required. On the other hand, if the proportion of bainite is too high, the yield point R _p0.2 is too high. For this reason, the bainite content is limited to a maximum of 70% by area, in particular to a maximum of 60% by area, preferably to a maximum of 50% by area.

The relatively high elongation at break A ₈₀ in relation to the tensile strength R _m of the flat steel product according to the invention is mainly a consequence of the relatively high proportion of retained austenite in the structure. Due to the strain-induced transformation of the retained austenite (ie the so-called TRIP effect), this has a positive effect on the elongation at break A ₈₀ without influencing the first stages of plastic deformation (characterized by the yield point R _p0.2 ). In order to achieve the minimum requirement in terms of elongation at break A ₈₀ , a residual austenite content of at least 5%, in particular at least 6%, preferably at least 7%, is required. When punching the hole in a hole expansion specimen, the retained austenite next to the punching edge is transformed into martensite due to the very high plastic deformation. In this way, retained austenite, like martensite, has a negative effect on the hole expansion ratio λ. For this reason, the flat steel product according to the invention may contain a maximum of 15%, in particular a maximum of 15%, preferably a maximum of 12%.

Since the residual austenite is measured by volume diffractometry, for example by means of XRD, it can also be a component of bainite and/or martensite, so that the addition of the structural components can sometimes result in more than 100%. However, depending on how coarse the retained austenite is, it can also be evaluated as a separate structural component.

Fine precipitations (carbides or carbonitrides) based on Nb can also be embedded in the microstructure. Due to their fineness, these cannot be detected using LOM, but can only be detected using transmission electron microscopy (TEM) at a magnification of 50,000 to 500,000 times. Precipitations mean carbides or carbonitrides with a NaCl (B1) crystal structure, which mainly consist of Nb and C. Depending on the concentration of the impurities that are unavoidable during production, the precipitates can also contain a small concentration of Ti, V, Mo, Cr, W or N. Fine Nb-based precipitates promote a fine grain structure and can therefore have a positive effect on the mechanical properties and the hole expansion ratio. In order to be able to achieve this effect, the Nb-based precipitates should not exceed a certain size. For this reason, the mean precipitation diameter is at most 20 nm, in particular at most 15 nm, preferably at most 10 nm. The mean precipitation diameter can also be determined using TEM.

Other phases can be present individually or together in the form of pearlite, cementite -which is not part of bainite-, non-metallic inclusions such as MnS or AlO, and coarse carbonitrides such as NbCN or TiCN with a precipitation diameter of more than 50 nm. which are detrimental to the mechanical properties and the hole expansion ratio λ and are therefore undesirable. The other phases are therefore among the (manufacturing-related) unavoidable structural components. These should total a maximum of 5% by area, in particular a maximum of 3% by area, preferably a maximum of 2% by area.

According to a second aspect of the invention, this object is achieved by a method having the features of patent claim 7.

1. a) Erschmelzen eines Stahls bestehend neben Fe und unvermeidbaren Verunreinigungen (in Gew.-%) aus

C: 0,12 bis 0,18 %, Si: 0,05 bis 0,4 %, Mn: 1,9 bis 2,5 %, AI: 0,2 bis 0,5 %, Cr: 0,05 bis 0,2 %, Nb: 0,01 bis 0,06 % besteht, wobei zu den Verunreinigungen folgende Legierungselemente aus der Gruppe (P, S, N, Ti, V, Mo, B, Cu, W, Ni, Sn, As, Co, Zr, La, Ce, Nd, Pr, Ca, O, H) zählen und mit folgenden Gehalten vorhanden sein können: P: bis 0,02 %, S: bis 0,01 %, N: bis 0,01 %, Ti: bis 0,01 %, V: bis 0,02 %, Mo: bis 0,1 %, B: bis 0,0007 %, Cu: bis 0,1 %, W: bis 0,1 %, Ni: bis 0,1 %, Sn: bis 0,05 %, Sb: bis 0,0004 % As: bis 0,02 %, Co: bis 0,02 %, Zr: bis 0,0002 %, La: bis 0,0002 %, Ce: bis 0,0002 %, Nd: bis 0,0002 %, Pr: bis 0,0002 %, Ca: bis 0,005 %, O: bis 0,005 %, H: bis 0,001 %,

• b) Vergießen der Schmelze zu einem Vorprodukt;
• c) Vorwärmen des Vorprodukts auf eine Temperatur und/oder Halten des Vorprodukts bei einer Temperatur zwischen 1150 und 1350 °C;
• d) Warmwalzen des Vorprodukts zu einem warmgewalzten Stahlflachprodukt insbesondere mit einer Dicke zwischen 1,8 und 5 mm, wobei die Warmwalzendtemperatur zwischen 850 und 980 °C beträgt;
• e) Abkühlen des erhaltenen warmgewalzten Stahlflachprodukts mit einer zwischen 20 und 400 °C/s betragenden Abkühlgeschwindigkeit auf eine zwischen 450 und 600 °C betragende Haspeltemperatur;
• f) Haspeln des auf die Haspeltemperatur abgekühlten warmgewalzten Stahlflachprodukts zu einem Coil;
• g) Abhaspeln des Coils und Kaltwalzen zu einem kaltgewalzten Stahlflachprodukt insbesondere mit einer Dicke zwischen 0,6 und 2,4 mm, wobei der Kaltwalzgrad zwischen 30 und 80 % beträgt;
• h) Haspeln des kaltgewalzten Stahlflachprodukts zu einem Coil;
• i) Abhaspeln des Coils und Glühen des kaltgewalzten Stahlflachprodukts im kontinuierlichen Durchlauf umfassend die Schritte:
  • i1) Aufheizen mit einer mittleren Aufheizgeschwindigkeit zwischen 0,5 und 20 °C/s auf eine Glühtemperatur zwischen 840 und 900 °C und Halten bei 840 bis 900 °C für eine Dauer zwischen 30 und 500 s;
  • i2) Abkühlen mit einer mittleren Abkühlgeschwindigkeit zwischen 0,5 und 20 °C/s auf eine Temperatur gleich oder höher Ms und weniger als 455 °C und Halten bei dieser Temperatur für eine Dauer zwischen 1 und 1000 s, um ein Stahlflachprodukt mit einem Gefüge nebst unvermeidbaren Gefügebestandteilen zu erhalten, welches folgende Phasen (in %) umfasst:

Ferrit: 20 bis 85 %, Bainit: 10 bis 70 %, Restaustenit: 5 bis 15 %, Martensit: bis 20 %, (einschließlich 0),

• j) Haspeln des kaltgewalzten Stahlflachprodukts zu einem Coil.

The method according to the invention for producing a cold-rolled flat steel product with a ferritic matrix comprises the steps:

1. a) Melting of a steel consisting of, in addition to Fe and unavoidable impurities (in % by weight).

C: 0.12 to 0.18%, Si: 0.05 to 0.4%, Mn: 1.9 to 2.5%, AI: 0.2 to 0.5%, CR: 0.05 to 0.2%, Nb: 0.01 to 0.06% consists of the following alloying elements from the group (P, S, N, Ti, V, Mo, B, Cu, W, Ni, Sn, As, Co, Zr, La, Ce, Nd, Pr, Ca, O, H) and can be present with the following contents: P: up to 0.02%, S: up to 0.01%, N: up to 0.01%, Ti: up to 0.01%, V: up to 0.02%, Mon: up to 0.1%, B: up to 0.0007%, Cu: up to 0.1%, W: up to 0.1%, Ni: up to 0.1%, Sn: up to 0.05%, Sb: up to 0.0004% As: up to 0.02%, Co: up to 0.02%, Zr: up to 0.0002%, La: up to 0.0002%, Ce: up to 0.0002%, Nd: up to 0.0002%, Pr: up to 0.0002%, Approx: up to 0.005%, O: up to 0.005%, H: up to 0.001%,

• b) pouring the melt into a preliminary product;
• c) preheating the precursor to a temperature and/or maintaining the precursor at a temperature between 1150 and 1350°C;
• d) hot-rolling of the preliminary product to form a hot-rolled flat steel product, in particular with a thickness of between 1.8 and 5 mm, the final hot-rolling temperature being between 850 and 980° C.;
• e) cooling the hot-rolled steel flat product obtained at a cooling rate of between 20 and 400°C/s to a coiling temperature of between 450 and 600°C;
• f) coiling the hot-rolled flat steel product, which has been cooled to the coiling temperature, into a coil;
• g) Uncoiling the coil and cold rolling to form a cold-rolled flat steel product, in particular with a thickness between 0.6 and 2.4 mm, the degree of cold-rolling being between 30 and 80%;
• h) coiling the cold-rolled flat steel product into a coil;
• i) Uncoiling the coil and annealing the cold-rolled steel flat product in a continuous flow, comprising the steps:
  • i1) heating with an average heating rate between 0.5 and 20 °C/s to an annealing temperature between 840 and 900 °C and holding at 840 to 900 °C for a duration between 30 and 500 s;
  • i2) cooling at an average cooling rate of between 0.5 and 20 °C/s to a temperature equal to or higher than Ms and less than 455 °C and holding at this temperature for a period of between 1 and 1000 s to obtain a flat steel product with a microstructure along with unavoidable structural components, which includes the following phases (in %):

Ferrite: 20 to 85%, bainite: 10 to 70%, residual austenite: 5 to 15%, martensite: up to 20%, (including 0),

• j) Coiling the cold-rolled flat steel product into a coil.

The molten steel with an alloy composition within the ranges given above is cast into an intermediate product. After it has been melted, the steel is cast into a preliminary product which, in the classic production method, can be a slab of the usual dimensions. However, the steel can also be produced from the steel by direct hot rolling of a continuous cast in a casting-rolling plant as a preliminary product of a thin slab or in a strip casting plant as a preliminary product of a cast strip. For example, in a casting-rolling plant or strip casting plant, the primary product can be further processed directly, i. H. coming directly from the casting heat, so that the preliminary product is kept at a temperature or, if necessary, preheated to a temperature, for example in an equalizing or preheating furnace, in which the most complete possible homogenization is ensured and in which any precipitates that may have formed during the casting process are eliminated as much as possible completely (re)dissolve. If, for example, the melt is cast in a continuous casting plant to form a preliminary product, the cast and completely solidified strand is separated into several slabs of finite size and the slabs are then allowed to cool down to ambient temperature, in particular by natural cooling. The preliminary product or the slab is reheated to a temperature for further processing, for example in a walking beam furnace or by other suitable means.

The temperature during preheating and/or when holding the preliminary product is at least 1150° C., in particular at least 1200° C., in order to ensure the most complete possible dissolution of any unwanted precipitations present in the form of carbides/carbonitrides and/or nitrides in the preliminary product. The temperature for preheating and/or holding should not exceed 1350 °C in order to avoid partial melting and/or excessive scaling of the pre-product. For ecological and economic reasons, the temperature for preheating and/or holding can be limited to a maximum of 1300° C. in particular.

The pre-product is hot-rolled in one or more roll stands (hot rolling mill) with a final hot-rolling temperature of between 850 and 980 °C to form a hot-rolled flat steel product. A final hot-rolling temperature for producing the hot-rolled flat steel product of at least 850° C., in particular at least 880° C., is chosen so that the deformation resistance does not increase too much. If the final hot rolling temperatures are too low, the rolling forces would increase disproportionately and the The desired isotropy of the material was lost due to the effects of thermomechanical rolling. In order to avoid undesirable coarse grain formation, the final rolling temperature for producing the hot-rolled flat steel product is limited to a maximum of 980 °C.

The hot-rolled flat steel product (hot strip) has a thickness of between 1.8 and 5 mm.

The hot-rolled flat steel product obtained is cooled at a cooling rate of between 20 and 400°C/s to a coiling temperature of between 450 and 600°C. The cooling rate of at least 20 °C/s is required in order to avoid as far as possible the formation of perlite and cementite and the formation of coarse precipitates that cannot be dissolved in the later process steps. A cooling rate of more than 400 °C/s is technically not feasible. The coiling temperature is at least 450° C., in particular at least 480° C., in order to prevent martensite formation and to promote the formation of a microstructure of bainite, bainitic ferrite and/or ferrite in the hot-rolled flat steel product. Martensite in the structure of the hot-rolled flat steel product would be transferred to the structure of the cold-rolled and annealed flat steel product and would be an undesirable phase in the structure of the cold-rolled flat steel product. In addition, the martensite in the microstructure of the hot-rolled flat steel product has a negative effect both on the cold-rollability of the hot-rolled flat steel product and on the isotropy of the microstructure of the cold-rolled and annealed flat steel product. If the coiling temperature is too high, the risk of pearlite formation and consequently manganese segregation during coil cooling increases. Furthermore, too high a coiling temperature would raise the risk of pronounced grain boundary oxidation. In order to prevent this, the coiling temperature is limited to a maximum of 600°C, in particular a maximum of 580°C.

The hot-rolled flat steel product, which has been cooled to the coiling temperature, is coiled into a coil.

Optionally, the hot-rolled flat steel product can be uncoiled from the coil and sent to conventional pickling, either in the coil-to-coil process, i.e. uncoiling-pickling-coiling, or preferably directly before cold rolling, i.e. uncoiling-pickling-cold-rolling. By pickling, scale present on the hot-rolled flat product can be removed and/or the surface of the hot-rolled flat product can be prepared or activated for the next steps.

wobei L_WB die Dicke des warmgewalzten Stahlflachprodukts (Warmband) und L_KB die Dicke des kaltgewalzten Stahlflachprodukts (Kaltband) ist.The hot-rolled flat product is unwound from the coil and cold-rolled with a degree of cold rolling of between 30 and 80% to form a cold-rolled flat steel product. The degree of cold rolling KWG is calculated using the formula: $$\text{KWG}=100*\left({\text{L}}_{\text{WB}}-{\text{L}}_{\text{KB}}\right)/{\text{L}}_{\text{WB}}$$

where L _WB is the thickness of the hot-rolled flat steel product (hot strip) and L _KB is the thickness of the cold-rolled flat steel product (cold strip).

Cold rolling is necessary for a high surface quality and dimensional tolerance, which is necessary for the intended use of the cold-rolled steel flat product for thin-walled components (e.g. body shell components). However, cold rolling leads to strain hardening, which adversely affects the ductility and hole expansion ratio of the steel. In addition, cold rolling results in a dominant rolling texture, which leads to a marked anisotropy of the mechanical properties and consequently a reduction in the hole expansion ratio. The influence of strain hardening and rolling texture on the mechanical-technological properties cannot be fully recovered by subsequent annealing.

Cold rolling at too low a cold rolling degree will not achieve the surface quality and dimensional tolerance required for the target application. For this reason, the degree of cold rolling is at least 30%. When cold rolling with a degree of cold rolling that is too high, the influences of work hardening and the rolling texture are so great that the required mechanical and technological properties cannot be achieved. For this reason, the degree of cold rolling is limited to a maximum of 80%, in particular a maximum of 70%.

The cold-rolled flat steel product (cold strip) has a thickness of between 0.6 and 2.4 mm.

The cold-rolled steel flat product is coiled. Alternatively, cutting to length to form circuit boards would also be conceivable.

The cold-rolled flat steel product is unwound from the coil and annealed in a continuous process. The annealing of the cold-rolled flat steel product has a significant influence on the formation of the structural components and consequently on the setting of the mechanical-technological properties of the end product. To produce the flat steel products according to the invention, the annealing conditions are set in such a way that the proportion of bainite in the microstructure is increased and the proportion of martensite in the microstructure is reduced and possibly even avoided. As a result, both high tensile strength and a high hole expansion ratio can be achieved. Bainite is formed by the decomposition of austenite during cooling. However, this only takes place in a narrow temperature range. At temperatures above this range, ferrite or pearlite usually form, whereas at temperatures below this range; ie below the martensite start temperature Ms, which is usually around 400 °C; martensite forms. Furthermore, the decomposition of austenite into bainite in this temperature range is favored by the destabilization of the austenite. The stability of austenite depends primarily on its carbon content. Accordingly, the formation of bainite could be achieved by adjusting the concentration or the distribution of carbon in the austenite.

The annealing can be carried out in a conventional manner in a multi-stage continuous annealing line or in a conventional manner in a multi-stage continuous annealing line in a hot-dip coating line.

In a first annealing stage, the cold-rolled flat steel product is heated to an annealing temperature of between 840 and 900 °C at an average heating rate of between 0.5 and 20 °C/s. A mean heating rate of at least 0.5 °C/s is required to avoid excessive coarsening of the structure, which would otherwise adversely affect tensile strength. A mean heating rate of over 20 °C/s can prevent complete recrystallization of the structure. Sufficient recrystallization is required to minimize work hardening and anisotropies resulting from cold rolling, which would adversely affect elongation at break and hole expansion ratio. For this reason, the average heating rate is limited to a maximum of 20 °C/s.

In a second annealing stage, the cold-rolled flat steel product is held at an annealing temperature of between 840 and 900 °C for a period of between 30 and 500 s. The prior art annealing of DP steels involves heating to an annealing temperature in the intercritical range, ie between the Ac1 and Ac3 temperatures, at which the structure partially transforms to austenite, and holding at this annealing temperature for a certain length of time. During this holding time, the carbon in the structure is partitioned or redistributed between the ferrite and austenite. Due to the higher solubility of carbon in austenite, carbon is enriched in austenite. For this reason, the homogeneity of the distribution of carbon in the microstructure depends on the annealing temperature. The homogeneity of the carbon distribution has a significant influence on the formation and properties of other structural components, such as new ferrite, martensite or bainite, during the subsequent cooling and consequently on the mechanical-technological properties of the end product. At temperatures above, but close to, the Ac1 temperature, typically around 710 °C, the proportion of austenite is low and consequently the carbon distribution is very inhomogeneous. In common practice, DP steels are annealed at annealing temperatures below 840°C, at which the microstructure contains a significant proportion of untransformed microstructural components such as old ferrite, martensite or bainite. Accordingly, the carbon distribution is still relatively inhomogeneous. In contrast, according to the invention, annealing takes place at annealing temperatures between at least 840° C. and at most 900° C. At an annealing temperature of at least 840 °C and, for example, less than the Ac5 temperature, usually around 860 °C, the microstructure consists predominantly of austenite and consequently the carbon distribution is comparatively homogeneous. For this reason, the annealing temperature must be at least 840 °C. Annealing at annealing temperatures higher than the Ac3 temperature leads to a fully austenitized structure and consequently an improved homogeneous carbon distribution. In conjunction with adjusting the temperature in the third stage, a more homogeneous carbon distribution promotes the formation of bainite in the microstructure, which is essential for the required combination of mechanical properties and hole expansion ratio λ. When annealed at 900 °C, the carbon distribution is completely homogenized. This leads to an excessive formation of bainite and consequently to _a too high yield point R _p0.2 . The homogeneity of the carbon distribution and consequently the mechanical-technological properties of the end product is also influenced by the holding time. The redistribution of carbon during the holding time involves diffusion processes that are dependent on both the annealing temperature and the holding time. If the hold time is too low, there would be insufficient time to redistribute the carbon and consequently achieve a homogeneous carbon distribution. There is a holding time for a sufficiently homogeneous carbon distribution of at least 30 s required. On the other hand, too long a holding time leads to an excessive coarsening of the structure. For this reason, the holding time is limited to a maximum of 300 s.

In a third stage of annealing, the cold-rolled steel flat product is cooled to a temperature equal to or higher than Ms and less than 455 °C at an average cooling rate of between 0.5 and 20 °C/s. A mean cooling rate of at least 0.5 °C/s is required to avoid the formation of undesired phases such as pearlite due to the decomposition of austenite and excessive coarsening of the microstructure during cooling. On the other hand, too high an average cooling rate prevents the formation of new ferrite during cooling. A high proportion of ferrite is required to achieve the desired mechanical and technological properties. For this reason, the upper limit of the mean cooling rate is set to a maximum of 20 °C/s.

The cold-rolled steel flat product is held in a fourth stage of annealing at the final temperature coming out of the third stage equal to or higher than Ms and less than 455 °C for a period of 1 to 1000 s. The setting of the temperature and time in the fourth stage is of crucial importance for the formation of a high proportion of bainite, which is necessary for achieving the required mechanical and technological properties of the end product. Temperatures equal to or higher than Ms and less than 455 °C correspond to the range in which bainite is formed by the decomposition of austenite, provided that the austenite is obtained by adjusting the annealing temperature in the range between 840 and 900 °C and the duration between 30 and has been destabilized for 300 s. The martensite start temperature Ms (in °C) is calculated from the C, Mn and Cr contents (each in % by weight) using the formula: $$\text{Ms}=539-423\times \text{C}-30,4\times \text{Mn}-12,1\times \text{Cr}$$

If the temperature falls below Ms in the fourth stage, some of the austenite still present would convert into martensite. This would not be conducive to the formation of a structure with a high bainite content and therefore a combination of high tensile strength and high hole expansion ratio. For this reason, the lower limit of the temperatures in the fourth stage is limited to Ms. At a temperature equal to or higher than 455 °C, bainite is no longer formed, but an excessive proportion of austenite would remain, which would largely convert into martensite when the cold-rolled steel flat product later cooled to room temperature, for example. Furthermore, a temperature of 455°C and higher would transform some of the austenite present into pearlite, which would affect the hole expansion ratio. For these reasons, the fourth stage temperature must be less than 455 °C. The formation of bainite also requires that the cold-rolled steel flat product is held at a temperature between Ms and less than 455 °C for a period of at least 1 s. Times of less than 1 s are not sufficient for the formation of a significant proportion of bainite. If the time is too long, an excessive amount of pearlite may be formed, which adversely affects the tensile strength and the hole expansion ratio. For this reason, the duration is limited to a maximum of 1000 s, in particular a maximum of 800 s, preferably a maximum of 500 s, preferably a maximum of 300 s.

After the fourth annealing stage, the cold-rolled flat steel product can be tempered in a fifth stage such that the cold-rolled flat steel product is cooled to a maximum temperature of 100 °C at an average cooling rate of between 0.5 and 20 °C/s.

Alternatively and preferably, the cold-rolled flat steel product is coated with a Zn-based anti-corrosion coating after the fourth stage by immersion in a molten bath. The bath inlet temperature is at least 10 °C lower and at most 20 °C higher than the melt bath temperature in order to prevent the melt bath temperature from changing significantly as a result of the entry of the hot-rolled steel strip. The required bath inlet temperature can be set in the fifth stage by appropriately tempering the cold-rolled flat steel product.

No special requirements are placed on the composition of the anti-corrosion coating and the associated molten bath through which the cold-rolled flat steel product passes during its hot-dip coating. The anti-corrosion coating or the molten bath consists mainly of zinc (Zn) and can otherwise be composed in a conventional manner. Accordingly, in addition to Zn and unavoidable impurities, the anti-corrosion coating or the molten bath can contain up to 20% by weight Fe, up to 5% by weight Mg and up to 10% by weight Al. Typically, if present, at least 2% by weight Fe, at least 1% by weight Mg and / or at least 1 wt .-% Al provided in order to achieve optimal performance properties of the corrosion protection.

Optionally, the hot-dip coating can be followed by a further heat treatment ("galvannealing"), in which the hot-dip coated steel flat product is heated to up to 550 °C in order to burn in the zinc-based coating.

Either immediately after exiting the hot-dip coating or following the additional optional heat treatment, the resulting cold-rolled steel flat product can be cooled to a temperature of less than 100 °C at an average cooling rate of 0.5 to 20 °C/sec.

The "average" heating or cooling rate is to be understood as meaning that it is related to the difference between an initial temperature (actual temperature) and a target temperature (setpoint temperature) for the required duration between the initial temperature and reaching the target temperature. As a rule, the heating and cooling rates are not constant.

The steel flat product obtained in this way can optionally be subjected to conventional skin-passing in order to optimize its dimensional accuracy and surface quality. The skin-pass degree set is at least 0.1% and at most 1.0%, with a skin-pass degree of at least 0.5% being particularly preferred. A degree of tempering of less than 0.1% would lead to a lower surface roughness in a cold-rolled steel flat product optionally coated with a metal coating, which would have a negative impact on the formability of the steel flat product. With a degree of skin pass of more than 1.0%, both the mechanical properties (yield point and elongation at break) and the hole expansion ratio would be adversely affected.

• Kohlenstoff (C) ist wesentlich für die Bildung von härteren Gefügebestandteilen wie Martensit, Restaustenit und Bainit. Die Anteile an diesen Gefügebestandteilen haben einen sehr großen Einfluss auf die mechanischen Eigenschaften des Stahls. Die erfindungsgemäß vorgegebene Kombination aus hoher Zugfestigkeit R_m und hohem Lochaufweitungsverhältnis λ erfordert ein gewisses Verhältnis an solchen Gefügebestandteilen. Ein zu niedriger Anteil an Martensit, Restaustenit bzw. Bainit führt zu einer unzureichenden Zugfestigkeit R_m, wohingegen ein zu hoher Anteil an Martensit bzw. Restaustenit zu einem unzureichenden Lochaufweitungsverhältnis λ führt. Zur Erreichung des erforderlichen Gefüges und daher der gewünschten Kombination aus Zugfestigkeit R_m und Lochaufweitungsverhältnis λ muss der Gehalt an C im Bereich zwischen 0,12 und 0,18 Gew.-%, insbesondere zwischen 0,15 und 0,17 Gew.-%, liegen. Bei einem C-Gehalt von weniger als 0,12 Gew.-% wird nicht genügend Martensit, Restaustenit und Bainit gebildet, so dass die erforderliche Mindestfestigkeit von 780 MPa nicht erreicht werden kann. Bei einen C-Gehalt von mehr als 0,18 Gew.-% wird zu viel Martensit bzw. Restaustenit gebildet, so dass die Mindestanforderung hinsichtlich des Lochaufweitungsverhältnisses λ nicht erreicht werden können.

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

• Carbon (C) is essential for the formation of harder structural components such as martensite, retained austenite and bainite. The proportions of these structural components have a very large influence on the mechanical properties of the steel. The combination of high tensile strength R _m and high hole expansion ratio λ specified according to the invention requires a certain ratio of such structural components. Too low a proportion of martensite, retained austenite or bainite leads to insufficient tensile strength R _m , whereas too high a proportion of martensite or retained austenite leads to an insufficient hole expansion ratio λ. In order to achieve the required microstructure and therefore the desired combination of tensile strength R _m and hole expansion ratio λ, the C content must be in the range between 0.12 and 0.18% by weight, in particular between 0.15 and 0.17% by weight. , lay. With a C content of less than 0.12% by weight, not enough martensite, retained austenite and bainite are formed so that the required minimum strength of 780 MPa cannot be achieved. With a C content of more than 0.18% by weight, too much martensite or residual austenite is formed, so that the minimum requirement with regard to the hole expansion ratio λ cannot be met.

Silicon (Si) prevents the formation of pearlite, which is detrimental to the ductility and forming behavior of the material. The carbon C, which is otherwise bound in the form of pearlite, instead leads to the formation of C-rich phases such as martensite, bainite and retained austenite. As a result, alloying with Si has an indirect effect on the mechanical properties. In particular, alloying with Si promotes the formation of residual austenite, which has a positive effect on the elongation at break A ₈₀ . In order to achieve this effect, a content of at least 0.05% by weight, in particular at least 0.10% by weight, preferably at least 0.15% by weight, is required. Too high a Si content leads to excessive formation of retained austenite, which has a negative effect on the hole expansion ratio λ. Furthermore, too high a Si content is detrimental to the surface quality of hot-dip coated steels. For these reasons, the upper limit of the Si content is restricted to a maximum of 0.4% by weight, in particular a maximum of 0.5% by weight, preferably a maximum of 0.25% by weight.

Manganese (Mn) is an essential alloying element, which is inhomogeneously distributed in rows across the strip thickness. Mn has a very strong effect on the solubility of C in Fe and therefore on the local transformation behavior of the microstructure. As a result, a Mn content has a dominant influence on the formation of the C-rich microstructure components during annealing and consequently on the mechanical properties and the hole expansion ratio λ of the annealed material. It has been found that a Mn content in the range between 1.9 and 2.5% by weight, in particular between 1.95 and 2.35% by weight, preferably between 2.0 and 2.3% by weight, leads to an optimal combination of mechanical properties and hole expansion ratio λ. With a Mn content of less than 1.9% by weight, the required lower limit of the tensile strength of 780 MPa is not reached. A Mn content of more than 2.5% by weight leads to excessive formation of martensite and retained austenite, so that the minimum requirement with regard to the hole expansion ratio λ is not met.

Aluminum (Al) is added along with Si for suppression of pearlite and consequent formation of retained austenite. In this way, the Al content also contributes to the ductility of the steel. In order to achieve the minimum requirements with regard to the proportion of retained austenite and the elongation at break A ₈₀ , an Al content of at least 0.2% by weight, in particular at least 0.25% by weight, preferably at least 0.27% by weight necessary. In contrast to Si, Al has fewer negative effects on the surface quality of hot-dip coated steel flat products. Nevertheless, too high an Al content leads to excessive formation of retained austenite, which negatively affects the hole expansion ratio λ. For this reason, the upper limit of the Al content is restricted to a maximum of 0.5% by weight, in particular a maximum of 0.45% by weight, preferably a maximum of 0.4% by weight.

Similar to Mn, chromium (Cr) influences the formation of the microstructure components during annealing and consequently the mechanical properties and the hole expansion ratio λ of the final material. In addition, Cr slows down the coarsening of Nb-based carbides. In this way, Cr can also contribute to strengthening through precipitation hardening. In order to achieve these effects, a Cr content of at least 0.05% by weight, in particular at least 0.07% by weight, is required. However, an excess of Cr increases the risk of pronounced grain boundary oxidation, which degrades the surface quality. For these reasons, the upper limit of the Cr content is limited to at most 0.2% by weight, particularly at most 0.15% by weight.

Niobium (Nb) separates out in the form of carbides, which lead to a refinement of the structure. A finer microstructure has a positive effect on tensile strength without greatly affecting ductility. It has been found that a finer structure has a positive effect on the hole expansion ratio λ of the material. Nb-based carbides also directly affect the tensile strength of the material through the precipitation hardening effect. This effect is particularly pronounced when the carbides do not coarsen excessively during annealing in a continuous annealing line. In order to achieve the positive effects of Nb, an Nb content of at least 0.01% by weight, in particular at least 0.015% by weight, is required. Too high a Nb content, in turn, can lead to cracking during continuous casting or during slab cooling or reheating. For this reason, the upper limit of the Nb content is limited to at most 0.06% by weight, particularly at most 0.05% by weight, preferably at most 0.04% by weight.

All other conceivable alloying elements not explicitly listed here are to be attributed to the production-related unavoidable impurities that can get into the steel as components of the starting material from which the steel is produced or as a result of the process during steel melting and processing. The impurities include the following alloying elements from the group (P, S, N, Ti, V, Mo, B, Cu, W, Ni, Sn, Sb, As, Co, Zr, La, Ce, Nd, Pr, Ca, O, H) and can be present in levels that are to be kept so low that they have no technical effect on the properties of the steel.

In the broadest sense, phosphorus (P) is an impurity that is carried into the steel by iron ore and cannot be completely eliminated in the large-scale steelworks process. The content should be set as low as possible, whereby the content should be a maximum of 0.02%, in particular a maximum of 0.017%, for process-reliable weldability.

Sulfur (S) is also an impurity in the broadest sense and must therefore be adjusted to a maximum content of 0.01% in order to avoid a strong tendency to segregation and a negative influence on formability or ductility as a result of excessive formation of sulfides (FeS; MnS; (Mn,Fe)S) to be avoided, in particular at most 0.005%, preferably at most 0.003% by weight. As a rule, calcium can be alloyed for desulfurization and adjustment of the S content depending on the Ca content.

Nitrogen (N) is also an impurity that is unavoidable during production. If N is present, Ti, Nb and/or V preferably form nitrides or carbonitrides with N if C is present at the same time. Therefore, in practice, under the conditions that can be technically and economically represented there genes, the uptake of N in the excretions 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 hardening but act as crack initiators. In order to avoid the formation of N-dominated carbonitrides, the content should be limited to a maximum of 0.01%, in particular a maximum of 0.006%.

Titanium (Ti) and vanadium (V) are considered micro-alloying elements and occur in the form of carbides. It has been found that, in contrast to Nb, these have no significant effect on the properties of the steel according to the invention. For this reason, they are among the undesirable contaminants. Therefore, the maximum allowable content of Ti is set to at most 0.01 wt%, specifically, at most 0.007 wt%, and the maximum allowable content of V is set to at most 0.02 wt%, specifically, at most 0.01 wt% .

Molybdenum (Mo) slows down the diffusion of C and therefore prevents the homogenization of the C distribution during annealing. Therefore, the Mo content must be limited to at most 0.1% by weight, particularly at most 0.05% by weight.

Boron (B) accumulates at grain boundaries during austenitization of the structure and prevents the formation of ferrite there during cooling. This is not expedient for the formation of a structure with a high proportion of ferrite. For this reason, the B content must not exceed 0.0007% by weight, in particular 0.0005% by weight, preferably 0.0004% by weight.

Copper (Cu) can separate out in the form of coarse particles, which have a negative effect on the mechanical properties. In addition, Cu has a negative impact on castability. In order to avoid any influence of Cu, the Cu content is limited to at most 0.1% by weight, particularly at most 0.05% by weight.

Tungsten (W), Nickel (Ni), Tin (Sn), Antimony (Sb), Arsenic (As), Cobalt (Co), Zircon (Zr), and rare earths such as Lanthanum (La), Cerium (Ce) , neodymium (Nd) and praseodymium (Pr) are not required as alloying elements in the steel according to the invention and, if they are nevertheless detectable, are among the unavoidable impurities. Accordingly: the W content is limited to at most 0.1% by weight, in particular at most 0.05% by weight; the Ni content to at most 0.1% by weight, in particular at most 0.05% by weight; the Sn content to at most 0.05 wt%; the Sb content to at most 0.0004% by weight; the As content to a maximum of 0.02% by weight; the Co content to a maximum of 0.02% by weight; the Zr content to a maximum of 0.0002% by weight; the La content to a maximum of 0.0002% by weight; the Ce content is limited to not more than 0.0002 wt%, the Nd content to not more than 0.0002 wt%, and the Pr content to not more than 0.0002 wt%.

Calcium (Ca) is commonly added to the melt in steelmaking for deoxidation and desulfurization as well as to improve castability. Too high a content can lead to the formation of undesirable inclusions, which have a negative effect on mechanical properties and rollability. Therefore, the upper limit is limited to a maximum of 0.005%, in particular a maximum of 0.002%.

Oxygen (O) is also undesirable in the melt or in the steel, since an oxide coating would have a negative effect on both the mechanical properties and the castability and rollability. The maximum permissible content is therefore set at a maximum of 0.005%, in particular a maximum of 0.002%.

As the smallest atom, hydrogen (H) is very mobile in the interstitial spaces in the steel and can lead to cracking in the core, particularly in high-strength steels when it cools down after hot rolling. The content should therefore be as low as possible, in any case not more than 0.001%, in particular not more than 0.0006%, preferably not more than 0.0004%, with contents of preferably not more than 0.0002% being aimed for.

Description of the Preferred Embodiments

The invention is explained in more detail below using exemplary embodiments. These are summarized in Tables 1 to 4. Table 1 shows the chemical compositions of the working examples. The specifications related to hot and cold rolling as well as annealing and the optional hot dip coating are given in Tables 2 and 3. Table 4 shows both the mechanical-technological properties and the structural characteristics of the exemplary embodiments.

To test the invention, the melts A - Y alloyed according to the compositions given in Table 1 were produced and cast to form slabs. The melts not according to the invention and their contents of certain alloying elements, which deviate from the requirements of the invention, are underlined in Table 1. Contents of an alloying element that were so low that they were "0" in the technical sense, i.e. so low that they had no influence on the properties of the steel, are indicated in Table 1 by the entry "-".

The slabs produced from steels A - Y were thoroughly heated in a preheating furnace in which a temperature ("VWO") prevailed. The preheated slabs were then hot-rolled in a conventional manner to form a hot-rolled flat steel product (hot strip). The hot strip obtained in each case left the hot rolling mill at a hot rolling end temperature ("WET") and was then cooled at a medium cooling rate ("AKR") to a coiling temperature ("HT"), at which they were each coiled. After cooling in the coil to room temperature, the hot strip was pickled in a conventional manner and then cold-rolled in a conventional manner with a cold-rolling grade (“KWG”) to form a cold-rolled flat steel product (cold strip).

In order to demonstrate the effect of the invention, one of the combinations I - XIII of VWO, WET, AKR, HT and KWG given in Table 2 was selected for the production of the cold strip. The combinations of I-XIII that are not according to the invention and the specifications, which in each case did not correspond to the requirements of the invention, are highlighted in Table 2 by underlining.

After cold rolling, the cold strip has been annealed in a conventional manner in a multi-stage continuous annealing plant or in a conventional manner in a multi-stage continuous annealing plant in a hot-dip coating plant. The annealing took place in several stages. In the first stage, the cold strip was heated at an average heating rate ("HR") to an annealing temperature ("HT1"), at which it was held for a duration or holding time ("HZ1"). The cold strip is then an average cooling rate (“KR1”) to a temperature (“HT2”) at which it was held for a duration or holding time (“HZ2”), followed by the duration or holding time (“HZ2”) the cold strip has optionally been coated with a Zn-based anti-corrosion coating by immersion in a melt bath. Either immediately after the end of the duration or holding time ("HZ2") or after exiting the melt bath, the cold strip is cooled at a medium rate ("KR2 ’) cooled to room temperature.

The cold strip produced in this way (cold-rolled flat steel product) has optionally been skin-passed with a skin-pass grade (“DG”).

In the exemplary embodiments, the annealing and optional hot-dip coating took place according to one of the combinations a-I of HR, HT1, HZ1, KR1, HT2, HZ2, KR2 and DG given in Table 3. For the combinations a - I it is also specified whether the cold strip has been coated with a Zn-based corrosion coating. The combinations of a-I not according to the invention and the specifications, which in each case did not correspond to the requirements of the invention, are underlined in Table 3.

Both the mechanical-technological properties and the structural characteristics of the cold strips produced in this way were determined. The results of these investigations are summarized in Table 4. These include yield strength R _P0.2 , tensile strength R _m ., elongation at break A ₈₀ and hole expansion ratio λ as well as the constant A from the formula λ = - 0.067 × Rm + A. Table ₄ also shows the proportions of ferrite F, bainite B, and martensite M, retained austenite RA and other structural components otherwise indicated. For steels A1 - Y1, it is also specified which of the steels A - Y the steel substrate of the respective cold strip consisted of (column "Analysis"), which of the combinations I - XIII of hot strip production (column "Rolling") and which of the variants a - I of the annealing and the application of a Zn-based coating has passed through the respective steel substrate (column "Annealing").

Example A1 is a reference example consisting of a steel with chemical composition A, and was produced according to rolling specification I and annealing specification a. This resulted in an optimal combination of mechanical-technological properties and structural characteristics.

Example A2 was produced with too low a VWO, but is otherwise the same as example A1. The deviating VWO resulted in too low a proportion of bainite, too low a proportion of retained austenite and too high a proportion of undesirable microstructure components. As a result, example A2 has a to low tensile strength R _m , an elongation at break A ₈₀ that is too low and a constant A that is too low. Example A2 is therefore used here as a counter-example. Steel A was also used in examples A3 - A6 to study the influence of the annealing temperature HT1. With the exception of the HT1 (and consequently the mean heating rate HR and the mean cooling rate KR1), examples A3 to A6 are otherwise the same as example A1. Too low HT1, as in example A3, resulted in too low a bainite content and too high a martensite content, consequently in a very low λ and too low a constant A. In contrast, too high HT1, as in example Example A6, too high a proportion of bainite and _too low a proportion of ferrite, consequently too low R _m and too high R _p0.2 .

Examples B1 - E1 were produced with a low but allowable VWO and WET compared to those of example A1. In addition, the examples B1-E1 differed in their C content. This showed that the R _m increases and the A ₈₀ decreases with increasing C content. With examples C1 and D1, it was possible to achieve mechanical-mechanical-technological properties as well as structural characteristics in the desired range. Steel B has too low a C content. As a result, Example B1 has an insufficient R _m . Steel E, on the other hand, has too high a C content. This led to the formation of too high a proportion of undesirable structural components, and consequently to an A ₈₀ that was too low and a constant A that was too low in example E1. Examples B1 and E1 therefore serve as counterexamples.

Examples C2 and C3 are comparable to Example C1 but were made with even lower WET. For Example C2, the WET was below the acceptable range. This led to a low A ₈₀ and a constant A that was too low. C2 therefore serves as a counterexample. For Example C3, the WET was very low but within acceptable limits. As a result, the mechanical-technological properties of the example were within the desired range.

Steels F - J differ primarily in their Mn content, but are otherwise comparable to steel A. Examples F1 - J1 showed that the R _m increases with increasing Mn content. Examples F1-H1 were also produced with a relatively low HT1. Examples I1 - J1 were again created with a relatively high HT1. In example F1, the Mn content was below the required minimum content. This resulted in too low an _Rm and too low a constant A. In contrast, sample J1 has too high a Mn content. This led to an excessive formation of martensite and retained austenite, which had a negative effect on the λ and the constant A. Examples I1 and J1 therefore serve as counterexamples.

Examples F1 through J1 were also produced with a relatively low AKR. The influence of the AKR can be seen by comparing Examples I1, I2 and I5. Example I2 was produced with an ASR that was too low, but is otherwise the same as example 11. The ASR that was too low led to the formation of undesirable structural components and consequently to mechanical-technological properties outside the target range. Example I2 therefore serves as a counter-example. Example I5 was generated with a very high AKR. However, this had not affected the mechanical-technological properties in any way.

Steels K to O have different Al and Si contents, but are otherwise comparable to steel A. These were processed with a relatively short HZ1 and a relatively high DG. Example K1 has a very low Si content and a very high Al content. These resulted in structural characteristics and mechanical-technological properties similar to example A1. Example L1 has a low Si content and too high an Al content. These resulted in an excessively high proportion of retained austenite, which had a negative effect on the λ. In contrast, example M1 has a high Si content and too low an Al content. These resulted in too low a proportion of retained austenite and consequently an insufficient A ₈₀ . Examples L1 and M1 therefore serve as counterexamples. Example N1 has a very high Si content and a low Al content. These resulted in a relatively low λ, but it was within the target range. Example O1 is also one of the counterexamples with too high a Si content. In a similar way to example L1, this resulted in too high a proportion of retained austenite, which had a negative impact on the λ.

Examples N2-N4 are variations of example N1, with the duration or holding time HZ1 being varied. Too short HZ1 in counterexample N2 resulted in too high a proportion of martensite and too low a proportion of bainite, which resulted in a low λ and consequently a constant A that was too low. Example N3 had a longer HZ1 compared to example N1, but this had no significant effect on the structural characteristics and the mechanical-technological properties. In the counter-example N4, an excessively high HZ1 was set, which led to an excessively high proportion _of bainite and an excessively low proportion of ferrite, and consequently to an R _p0.2 that was too high.

Steels P to S have different Nb contents, but are otherwise comparable to steel A. In examples P1-S1, these were processed with a low HT2, a low DG and without a Zn-based anti-corrosion coating. The comparison between Examples P1 to S1 shows that the R _m increases and the A ₈₀ decreases with increasing Nb content. Example P1 has too low Nb content and consequently insufficient R _m , whereas example S1 has too high Nb content and consequently too low A ₈₀ . Examples P1 and S1 therefore serve as counterexamples.

Examples R2 and R3 are similar to example R1 but have been annealed with different HT2. Example R2 has a higher HT2 compared to example R1. This led to a higher proportion of martensite and a lower proportion of bainite, which had a negative effect on the λ and the constant A. However, the resulting mechanical and technological properties were within the target range. In example R3, an even higher HT2 was set. This has an even stronger effect on the martensite and bainite contents, so that the λ and the constant A were outside the target range. For this reason example R3 serves as a counterexample.

Steels T - V have different Cr contents, but are otherwise comparable to steel A. Examples T1 - V1 have been processed with a relatively lower HT. The comparison between examples T1-C1 shows that the proportion of martensite increases and the proportion of bainite decreases with increasing Cr content. This had a negative effect on the λ and the constant A. Example V1 contains too high a Cr content, so that the proportions of martensite and bainite and consequently the constant A are outside the target range. Therefore, example V1 serves as a counter-example.

Examples T2 to T4 are similar to example T1, but were processed with different HT. In example T2, the HT was set too low. This resulted in too high a martensite content and too low a bainite content and consequently too low A ₈₀ and constant A. Example T3 had a higher HT compared to example T1; however, this had no significant influence on either the structural characteristics or the mechanical-technological properties. In example T4, the HT was set too high, which resulted in excessive formation of undesirable structural components and consequently a deterioration in the mechanical-technological properties. Therefore, examples T2 and T4 serve as counter-examples.

Example T5 is also similar to Example T1, but was created with a KWG that was too high. This resulted in an A ₈₀ that was too low and a constant A that was too low. Therefore, example T5 serves as a counterexample.

Steels W through Y differ in their levels of impurity elements V, Ti, Mo, Cu, Ni, P, S, N and B, but are otherwise comparable to Steel A. Steel W contains very low levels of impurity elements and was tested in Example W1 processed analogously to example A1. The structural characteristics and the mechanical-technological properties of example W1 are comparable to those of example A1. Compared to steel A, steel X has a higher but still acceptable concentration of impurity elements. The resulting mechanical-technological properties, which were produced under the same process conditions, were worse than example A1, but still within the target range. Steel Y has too high a concentration of impurity elements. As a result, example Y1, which was also produced using the same process conditions as example A1, has too high a proportion of undesirable structural components and consequently too low an A₈₀ and a constant A that is too low. For this reason, example Y1 serves as a counterexample. Table 1: Chemical analysis of the exemplary embodiments (all data in % by weight) C si Al Mn Cr Nb V Ti Mon Cu no P S N B A 0.151 0.199 0.351 2:13 0.117 0.021 0.0095 0.0066 0.055 0.020 0.054 0.0075 0.0016 0.0028 0.00035 B 0.118 0.208 0.354 2:19 0.120 0.023 0.0095 0.0028 0.053 0.037 0.030 0.0110 0.0015 0.0030 0.00040 C 0.121 0.198 0.354 2:21 0.120 0.023 0.0052 0.0040 0.055 0.023 0.021 0.0185 0.0022 0.0024 0.00025 D 0.175 0.208 0.336 2:19 0.121 0.021 0.0104 0.0074 0.023 0.053 0.054 0.0069 0.0021 0.0043 0.00042 E 0.181 0.202 0.350 2.06 0.121 0.023 0.0104 0.0038 0.019 0.021 0.020 0.0135 0.0033 0.0059 0.00027 f 0.155 0.210 0.368 1.88 0.118 0.022 0.0074 0.0038 0.033 0.020 0.033 0.0068 0.0030 0.0026 0.00047 G 0.155 0.210 0.354 1.93 0.117 0.023 0.0109 0.0055 0.043 0.043 0.025 0.0091 0.0015 0.0055 0.00042 H 0.155 0.200 0.561 1.99 0.109 0.023 0.0068 0.0072 0.033 0.055 0.051 0.0130 0.0011 0.0064 0.00045 I 0.150 0.202 0.361 2.42 0.120 0.022 0.0103 0.0030 0.045 0.044 0.024 0.0167 0.0029 0.0056 0.00045 J 0.155 0.204 0.336 2.55 0.118 0.023 0.0054 0.0047 0.025 0.045 0.020 0.0184 0.0029 0.0025 0.00032 K 0.155 0.060 0.470 2:11 0.109 0.022 0.0098 0.0040 0.027 0.049 0.039 0.0068 0.0028 0.0055 0.00041 L 0.150 0.120 0.520 2.26 0.116 0.021 0.0087 0.0073 0.024 0.039 0.041 0.0152 0.0051 0.0064 0.00032 M 0.144 0.270 0.180 2:19 0.109 0.023 0.0040 0.0038 0.033 0.027 0.035 0.0152 0.0021 0.0055 0.00044 N 0.158 0.390 0.220 2:11 0.120 0.022 0.0077 0.0073 0.030 0.040 0.055 0.0173 0.0015 0.0042 0.00042 O 0.143 0.410 0.350 2:19 0.113 0.022 0.0100 0.0044 0.033 0.023 0.045 0.0158 0.0026 0.0046 0.00036 P 0.146 0.196 0.361 2.06 0.118 0.008 0.0067 0.0028 0.033 0.048 0.042 0.0191 0.0028 0.0036 0.00033 Q 0.146 0.210 0.357 2.04 0.110 0.012 0.0077 0.0064 0.019 0.054 0.041 0.0173 0.0014 0.0032 0.00024 R 0.156 0.206 0.350 2:21 0.115 0.056 0.0076 0.0032 0.038 0.026 0.050 0.0122 0.0029 0.0063 0.00021 S 0.144 0.200 0.333 2:11 0.120 0.070 0.0044 0.0048 0.033 0.053 0.036 0.0191 0.0023 0.0040 0.00044 T 0.147 0.204 0.340 2:11 0.060 0.022 0.0113 0.0038 0.023 0.023 0.032 0.0082 0.0016 0.0038 0.00051 u 0.147 0.192 0.336 2:11 0.170 0.023 0.0070 0.0070 0.037 0.054 0.037 0.0135 0.0034 0.0040 0.00042 V 0.150 0.190 0.368 2.24 0.210 0.022 0.0050 0.0041 0.055 0.024 0.053 0.0119 0.0018 0.0036 0.00032 W 0.146 0.204 0.350 2:15 0.120 0.023 - - - - - - - - - X 0.149 0.196 0.343 2:21 0.114 0.023 0.0189 0.0098 0.097 0.095 0.099 0.0180 0.0095 0.0089 0.00060 Y 0.153 0.202 0.350 2.04 0.120 0.023 0.0210 0.0115 0.102 0.109 0.132 0.0350 0.0156 0.0192 0.00110 Table 2: Process conditions of the exemplary embodiments with regard to hot and cold rolling VWO WET AKR HT KWG [°C] [°C] [°C/s] [°C] % I 1250 930 40 530 50 II 1140 930 40 530 50 III 1170 845 40 530 33 IV 1170 860 40 530 33 V 1170 890 40 530 33 VI 1280 950 15 530 66 vii 1280 950 20 530 66 viii 1280 950 200 530 66 IX 1320 970 100 445 75 X 1320 970 100 460 75 XI 1320 970 100 590 75 XII 1320 970 100 610 75 XIII 1320 970 100 460 85 Table 3: Process conditions of the exemplary embodiments with regard to the annealing and the optional hot-dip coating MR HT1 HZ1 KR1 HT2 HZ2 KR2 DG Zn [°C/sec] [°C] [n] [°C/sec] [°C] [n] [°C/sec] [%] a 4 865 100 4 435 15 4 0.5 X b 3 835 100 3 435 15 3 0.5 X c 3 840 100 3 435 15 3 0.5 X i.e 5 895 100 5 435 15 5 0.5 X e 6 910 100 6 435 15 6 0.5 X f 12 865 20 12 435 15 12 0.9 X G 10 865 50 10 455 15 10 0.9 X H 0.5 865 500 0.5 435 15 0.5 0.9 X i 0.5 865 310 0.5 435 15 0.5 0.9 X j 4 865 100 4 415 15 4 0.2 k 4 865 100 4 454 15 4 0.2 l 4 865 100 4 460 15 4 0.2 Table 4: Mechanical-technological properties and structural characteristics of the exemplary embodiments Rp0.2 rm A80 λ A f B M RA Otherwise analysis rolls Glow [MPa] [MPa] [%] [%] [%] [%] [%] [%] [%] [%] Al A I a 511 826 21 28 83.3 54 29 7 10 < 1 A2 A II a 487 712 12 13 607 77 0 12 3 8th A3 A I b 526 874 19 15 73.6 68 5 22 5 < 1 A4 A I c 513 854 20 19 76.2 63 12 17 8th < 1 A5 A I i.e 584 795 22 46 99.3 24 65 2 9 <1 A6 A I e 609 772 23 57 108.7 19 74 0 7 < 1 B1 B V a 454 768 19 29 80.5 76 13 7 1 < 1 C1 C V a 489 784 22 30 82.5 66 19 9 6 < 1 D1 D V a 543 869 20 23 81.2 47 31 11 11 < 1 E1 E V a 592 907 17 13 73.8 38 28 14 13 7 C2 C III a 497 801 17 17 70.7 67 16 11 6 < 1 C3 C IV a 475 795 18 22 75.3 65 19 10 6 < 1 F1 f vii c 462 775 21 22 73.9 73 10 12 5 < 1 G1 G vii c 475 786 20 23 75.7 66 12 16 6 < 1 H1 H vii c 493 812 20 22 76.4 59 14 19 8th <1 I1 I vii i.e 593 835 22 41 96.9 23 57 9 11 <1 J1 J vii i.e 561 912 19 12 73.1 18 44 22 16 < 1 I2 I VI i.e 612 806 14 17 71 32 33 18 8th 9 I3 I viii i.e 586 831 21 39 94.7 22 55 11 12 <1 K1 K I G 510 831 20 26 81.7 53 28 8th 11 <1 L1 L I G 541 849 25 17 73.9 45 27 12 16 < 1 M1 M I G 515 825 17 28 83.3 54 31 11 4 < 1 N1 N I G 521 829 24 21 76.5 50 29 7 14 < 1 O1 O I G 538 856 26 16 73.4 39 30 13 18 < 1 N2 N I f 502 823 21 17 72.1 62 6 21 11 < 1 N3 N I H 513 825 23 24 79.3 50 29 8th 13 < 1 N4 N I j 610 791 22 38 91 15 72 5 8th < 1 P1 P I j 481 775 23 32 83.9 42 43 6 9 < 1 Q1 Q I j 492 799 22 34 87.5 40 45 5 10 < 1 R1 R I j 512 836 19 32 88 43 41 7 9 < 1 S1 S I j 518 862 17 23 80.8 44 37 9 9 1 R2 R I k 507 846 19 21 77.7 59 13 18 10 < 1 R3 R I l 513 859 18 16 73.6 61 8th 23 8th < 1 T1 T X a 508 805 22 26 79.9 55 30 6 9 < 1 U1 u X a 540 843 20 22 78.5 57 16 17 10 < 1 V1 V X a 554 863 18 16 73.8 61 9 22 8th < 1 T2 T IX a 520 822 17 18 73.1 67 6 21 6 < 1 T3 T XI a 512 803 20 32 85.8 54 31 5 10 < 1 T4 T XII a 505 791 18 20 73 55 13 18 7 7 T5 T XIII a 562 792 17 19 72.1 52 29 11 8th < 1 w1 W I a 509 832 23 29 84.7 55 30 4 11 < 1 X1 X I a 492 807 19 23 77.1 56 26 7 7 4 Y1 Y I a 472 798 17 17 70.5 58 17 12 5 8th

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• WO 2013182621 A1 [0004, 0017]
• WO 2013037485 A1 [0007]
• WO 2014159625 A1 [0007]

Claims (13)

Cold-rolled flat steel product with a tensile strength R _m of at least 780 MPa, a yield point R _p0.2 of at least 440 MPa and a maximum of 600 MPa and an elongation at break A ₈₀ of at least 18%, determined according to DIN EN ISO 6892-1:2017, with the cold-rolled Flat steel product in addition to Fe and unavoidable production-related impurities in % by weight from: C: 0.12 to 0.18%, Si: 0.05 to 0.4%, Mn: 1.9 to 2.5%, AI: 0.2 to 0.5%, Cr: 0.05 to 0.2%, Nb: 0.01 to 0.06% consists of the following alloying elements from the group (P, S, N, Ti, V, Mo, B, Cu, W, Ni, Sn, As, Co, Zr, La, Ce, Nd, Pr, Ca, O, H) and can be present with the following contents: P: up to 0.02%, S: up to 0.01%, N: up to 0.01%, Ti: up to 0.01%, V: up to 0.02%, Mon: up to 0.1%, B: up to 0.0007%, Cu: up to 0.1%, W: up to 0.1%, Ni: up to 0.1%, Sn: up to 0.05%, Sb: up to 0.0004% As: up to 0.02%, Co: up to 0.02%, Zr: up to 0.0002%, La: up to 0.0002%, Ce: up to 0.0002%, Nd: up to 0.0002%, Pr: up to 0.0002%, Approx: up to 0.005%, O: up to 0.005%, H: up to 0.001% consists, whereby the cold-rolled flat steel product has a microstructure together with unavoidable microstructural components, which comprises the following phases (in %): Ferrite: 20 to 85%, bainite: 10 to 70%, residual austenite: 5 to 15%, martensite: up to 20%, (including 0). flat steel product claim 1 , whereby the lowest permissible hole expansion ratio λ (in %) determined according to DIN EN ISO 16630:2017 can be calculated using the following formula: λ = -0.067 × Rm + A, with the constant A being at least 75%. Flat steel product according to one of the preceding claims, wherein the Si content is at least 0.15% by weight and at most 0.25% by weight Flat steel product according to one of the preceding claims, wherein the Al content is at least 0.27% by weight and at most 0.4% by weight. Flat steel product according to one of the preceding claims, wherein the proportion of bainite in the microstructure is at least 20% by area and at most 50% by area. Flat steel product according to one of the preceding claims, wherein the proportion of residual austenite in the structure is at least 7% and at most 15%. Flat steel product according to one of the preceding claims, in which the proportion of martensite in the microstructure is at most 10% by area. Steel flat product according to one _of the preceding claims, wherein the cold-rolled steel flat product has a yield point R _p0.2 of at most 550 MPa. Flat steel product according to one of the preceding claims, wherein the cold-rolled flat steel product has a Zn-based anti-corrosion coating. Process for the production of a cold-rolled flat steel product, comprising the steps: a) Melting of a steel consisting of Fe and unavoidable impurities (in % by weight) from C: 0.12 to 0.18%, Si: 0.05 to 0.4%, Mn: 1.9 to 2.5%, AI: 0.2 to 0.5%, Cr: 0.05 to 0.2%, Nb: 0.01 to 0.06% consists of the following alloying elements from the group (P, S, N, Ti, V, Mo, B, Cu, W, Ni, Sn, As, Co, Zr, La, Ce, Nd, Pr, Ca, O, H) and can be present with the following contents: P: up to 0.02%, S: up to 0.01%, N: up to 0.01%, Ti: up to 0.01%, V: up to 0.02%, Mon: up to 0.1%, B: up to 0.0007%, Cu: up to 0.1%, W: up to 0.1%, Ni: up to 0.1%, Sn: up to 0.05%, Sb: up to 0.0004% As: up to 0.02%, Co: up to 0.02%, Zr: up to 0.0002%, La: up to 0.0002%, Ce: up to 0.0002%, Nd: up to 0.0002%, Pr: up to 0.0002%, Approx: up to 0.005%, O: up to 0.005%, H: up to 0.001%, b) pouring the melt into a preliminary product; c) preheating the precursor to a temperature and/or maintaining the precursor at a temperature between 1150 and 1350°C; d) hot rolling of the preliminary product to form a hot-rolled flat steel product, the final hot-rolling temperature being between 850 and 980° C.; e) cooling the hot-rolled steel flat product obtained at a cooling rate of between 20 and 400°C/s to a coiling temperature of between 450 and 600°C; f) coiling the hot-rolled flat steel product, which has been cooled to the coiling temperature, into a coil; g) uncoiling the coil and cold rolling to form a cold-rolled steel flat product, the degree of cold-rolling being between 30 and 80%; h) coiling the cold-rolled flat steel product into a coil; i) Uncoiling the coil and annealing the cold-rolled steel flat product in a continuous flow, comprising the steps: i1) Heating at an average heating rate between 0.5 and 20 °C/s to an annealing temperature between 840 and 900 °C and holding at 840 to 900 °C C for a duration between 30 and 500 s; i2) cooling at an average cooling rate of between 0.5 and 20 °C/s to a temperature equal to or higher than Ms and less than 455 °C and holding at this temperature for a period of between 1 and 1000 s to obtain a flat steel product with a microstructure along with unavoidable structural components, which includes the following phases (in %): Ferrite: 20 to 85%, bainite: 10 to 70%, residual austenite: 5 to 15%, martensite: up to 20%, (including 0), j) Coiling the cold-rolled flat steel product into a coil. procedure after claim 10 , wherein the hot-rolled flat steel product is pickled between steps f) and g). procedure after claim 10 or 11 , wherein after step i2) the cold-rolled flat steel product is hot-dip coated with a Zn-based anti-corrosion coating. procedure after claim 12 , whereby the hot-dip coated flat steel product is heated to a temperature of up to 550 °C.