EP3781717B1 - Cold-rolled flat steelproduct and use, and method for producing such a flat steel product - Google Patents

Description

The invention relates to a cold-rolled flat steel product which has an optimal combination of its mechanical properties for forming into components, in particular vehicle components and the like.

The invention further relates to a method for producing such a cold-rolled flat steel product.

The invention also names particularly suitable uses for such flat steel products.

If contents of a steel or a flat steel product are mentioned in this text, these contents always refer to the weight (in% by weight), unless expressly stated otherwise. If information on the composition of atmospheres or gas mixtures is given in this text, the content information given for the individual components always refers to the volume (in% by volume), unless expressly stated otherwise. If information is given in this text about the content of individual components in the structure of a flat steel product, these always refer to the proportion in area % determined on the microstructure in accordance with DIN EN 13925 (2003.07) (specification in area %), unless expressly stated otherwise is.

In this text, the term “flat steel product” refers to steel strips or steel sheets produced in a rolling process and those obtained from them Blanks, boards and comparable products are understood, the thickness of which is significantly smaller than their width and length.

When we talk about the “n-value” here, we mean the n-value determined as “n _10-20 ” in tensile tests according to DIN 10275. The tensile test itself is carried out in accordance with DIN EN ISO 6892-1 (sample form 1).

When we talk about the “r value” here, we mean the r value determined as “r _10-20 ” in tensile tests according to DIN 10113. The tensile test itself is also carried out here according to DIN EN ISO 6892-1 (sample form 1).

Higher-strength, cold-formable steels and flat steel products made from them are required in particular for the production of body components for motor vehicles. It is precisely there that there is a requirement that the sheets from which the components are made not only be easily deformable with an optimally low weight, but also have sufficient strength in order to be able to make an effective contribution to the stability of the body with small sheet thicknesses.

From the US 2017/0114433 A9 a cold-rolled steel sheet is known which, after annealing, should have TRIP properties and a tensile strength of at least 1000 MPa, in particular at least 1180 MPa, with a total elongation of at least 15%, in particular at least 20%. The steel sheet consists of (in wt.%) C: 0.1 - 0.3%; Mn: 4 - 10%, Al: 0.05 - 5%, Si: 0.05 - 5%; and Nb: 0.008 - 0.1, the balance iron and unavoidable impurities, with Mn contents of 4 - 7%, in particular 6 - 7%, Si contents of 0.1 - 1.1 and Al contents of 0.5 - 1.6% are particularly preferred. Likewise, an Nb content of 0.02% and C content of 0.15 - 0.17% are considered particularly favorable. The microstructure of the cold-rolled sheet consists of at least 20% retained austenite and more than 50% lath-type ferrite. At the same time, the retained austenite and ferrite have an ultra-fine grain size of less than 5 µm. To achieve the desired structure and In order to achieve strength properties, an appropriately composed melt melted by vacuum induction melting was cast into slabs on a laboratory scale, which were then pre-rolled into 20 mm thick plates. The plates were held at a temperature of 1230 ° C for 3 hours to effect homogenization and were then hot rolled in three steps into plates with a thickness of 4 mm. The hot-rolled plates leaving the finishing hot rolling mill at a final temperature of 900 °C were then accelerated to cool down to a coiling temperature of 750 °C, at which they were then held for one hour. The plates were then kept at room temperature in the oven. In this way, the processes that occur when a hot strip produced in a corresponding manner is wound into a reel and cools to room temperature in the reel should be simulated. After annealing, the hot-rolled plates were cold-rolled with a thickness reduction of 40 - 50%. The cold-rolled plates finally passed through a continuous annealing device in which an annealing cycle was simulated in which the plates were heated at a heating rate of 10 ° C / s to 650 - 750 ° C, in particular 680 - 740 ° C, and for 180 seconds Temperature have been maintained in order to then be cooled to room temperature at a cooling rate of 10 ° C / s.

In addition to the prior art explained above, from the KR 101 481 069 B1 a steel sheet with high specific strength and excellent ductility is known, which contains 0.15 to 0.5% by weight of carbon (C), 6.0 to 8.0% by weight of manganese (Mn), 5.0 to 6, 0% by weight aluminum (Al), 0.05 to 0.5% by weight silicon (Si), less than 0.02% by weight (except 0) sulfur (S), the balance being iron (Fe) and unavoidable impurities. Optionally, the steel can contain 0.005 - 1.0% by weight of Ti, V, Nb or Zr. The yield strength of the steel is greater or equal to 550 MPa. The tensile strength multiplied by the elongation is greater than or equal to 28,000 MPa%.

Furthermore, from the CN 104 694 816 A a process for producing manganese steel with a high aluminum content is known. For this purpose, a flat steel product is made from a steel with (in mass%) 0.10 to 0.35% C, 5.0 to 9.0% Mn, 4.0 to 7.5% Al, less than 0.003% P, less than 0.002% S, the rest Fe and unavoidable impurities, cast into a starting material, which is then heated to 1200 ° C over 1.5 hours and then in several stages starting from an initial temperature of 1050 - 1100 ° C and with a Final temperature of 850 - 900 °C is hot rolled. The degree of deformation achieved during hot rolling is 80 to 85%. The hot strip obtained in this way is wound into a coil at a coiling temperature of 600 - 700 °C and then cold rolled with a cold rolling ratio of 65 to 70%. Finally, a final annealing is carried out at 860 - 930 °C for a period of 0.5 hours, which is followed by quenching with water. The cold-rolled steel sheet produced in this way should have high strength and ductility with a high aluminum content.

The advantage of flat steel products of the type explained above is their combination of low weight, high strength and good elongation properties due to their comparatively low density. However, the production of such flat steel products on a large industrial scale proves to be problematic. For example, there is a high sensitivity to the formation of edge cracks during hot rolling. In practice, it also often proves to be difficult to form complex shaped components, such as those required in particular in the field of vehicle construction, by deep drawing or comparable forming operations in which high degrees of deformation can be achieved.

Against the background of the state of the art, the task has arisen of creating a cold-rolled flat steel product that meets the practical requirements for its usability with a high degree of safety and that can also be produced reliably on a large industrial scale.

In addition, a practical method for producing and at least one advantageous use of such a flat steel product should be specified.

With regard to the flat steel product, the invention has solved this problem in that such a flat steel product is designed according to claim 1.

A method that solves the above-mentioned problem according to the invention is specified in claim 12.

Flat steel products according to the invention are particularly suitable for the production of vehicle components, in particular for chassis or body parts of motor vehicles, such as cars or trucks. These parts include, for example, steering components, wheel rims, vehicle chassis components and the like. However, flat steel products according to the invention can also be used particularly well for the production of devices that are intended to offer ballistic protection. Due to their good formability, flat steel products according to the invention are also particularly suitable for internal or external high-pressure forming processes. For example, flat steel products according to the invention can also be used to produce components for internal combustion engines, such as camshafts, piston rods and the like. Furthermore, flat steel products according to the invention can be used successfully in the field of container construction or other applications in which large-volume bodies with complex shapes have to be produced.

Advantageous embodiments of the invention are specified in the dependent claims and are explained in detail below, as is the general idea of the invention.

• wobei für das Verhältnis %Mn/%AI gilt %Mn/%AI > 1,2
• und für $${\mathrm{Al}}_{\mathrm{eq}}=\%\mathrm{Al}+\mathrm{0,4}\times {\left(\%\mathrm{Si}\right)}^3-3\times {\left(\%\mathrm{Si}\right)}^2+\mathrm{8,3}\times \%\mathrm{Si}\mathrm{gilt}3\le {\mathrm{Al}}_{\mathrm{eq}}\le 8$$
  
• mit %Mn: jeweiliger Mn-Gehalt des Stahls
  • %AI: jeweiliger Al-Gehalt des Stahls
  • %Si: jeweiliger Si-Gehalt des Stahls
• wobei das Gefüge des Stahlflachprodukts zu 10 - 60 Flächen-% aus Austenit und zu 40 - 90 Flächen-% aus Ferrit bei einer mittleren Korngröße des Austenits von 0,85 - 3 µm besteht.

A flat steel product according to the invention is therefore characterized by the fact that in the cold-rolled state it has an n value of at least 0.21, the flat steel product being made from a steel which consists of (in wt.%) C: 0.08 - 0.25%, Al: 3 - 5.4%, Mn: 9 - 14%, B: 0 - 0.1%, CR: 0 - 2%, Si: 0 - 0.4%, P: 0 - 0.1%, S: 0 - 0.3%, Ta: 0 - 0.5%, W: 0 - 0.5%, Ni: 0 - 2%, Cu: 0 - 2%, Approx.: 0 - 0.15%, N: 0 - 0.02% Co: 0 - 2% and one or more elements from the group "Ti, Nb, V, Mo" with the proviso that the sum of the contents of these elements is at least 0.05% and at most 1%, and / or one or more elements the group "Zr, La, Ce, Y" with the proviso that the sum of the contents of these elements is at least 0.05% and at most 0.3%, as well as the remainder iron and unavoidable impurities
consists,

• where the ratio %Mn/%AI is %Mn/%AI > 1.2
• and for $${\mathrm{Al}}_{\mathrm{eq}}=\%\mathrm{Al}+0.4\times {\left(\%\mathrm{Si}\right)}^3-3\times {\left(\%\mathrm{Si}\right)}^2+8.3\times \%\mathrm{Si}\mathrm{applies}3\le {\mathrm{Al}}_{\mathrm{eq}}\le \mathrm{8th}$$
  
• with %Mn: respective Mn content of the steel
  • %AI: respective Al content of the steel
  • %Si: respective Si content of the steel
• whereby the structure of the flat steel product consists of 10 - 60 area % austenite and 40 - 90 area % ferrite with an average austenite grain size of 0.85 - 3 µm.

The flat steel product according to the invention has a composition and a structural state that give it properties optimized for the respective intended use.

The invention provides a carbon ("C") content of 0.08 - 0.25% by weight in order to ensure a sufficient amount of austenite in the structure of the flat steel product. The C content is limited to 0.25% by weight in order to avoid the tendency to form brittle kappa carbides at the grain boundaries. Such carbides would impair the hot and cold formability of the flat steel products according to the invention. In the content range specified according to the invention, the carbon has a strengthening effect through the formation of mixed crystals and increases the stacking fault energy, which in turn contributes to increasing the strength. In addition to the negative effects on formability, higher C contents would impair weldability. Therefore, according to the invention, the upper limit of the C content is limited to 0.25% by weight, with C contents of at most 0.22% by weight, in particular at most 0.19% by weight or very particularly preferably at most 0.17% by weight have proven to be particularly favorable in terms of avoiding negative effects of the C content. On the other hand, the positive effects of the presence of C in the flat steel product according to the invention can be exploited by ensuring that the C content is at least 0.08% by weight, whereby: Contained at least 0.11% by weight of C, in particular at least 0.13% by weight of C, the advantageous effects are particularly reliably adjusted.

Aluminum ("Al") is present in the flat steel product according to the invention in amounts of 3 - 5.4% by weight in order, on the one hand, to reduce the density and thus the weight of the flat steel product and, on the other hand, to increase the tensile strength through solid solution hardening. Al also contributes to the formation of the ferrite content in the structure of the flat steel product according to the invention. These favorable effects occur with Al contents of at least 3% by weight, with levels of at least 4% by weight being particularly favorable in this regard. However, in order to avoid negative influences, the Al content of a flat steel product according to the invention is limited to a maximum of 5.4% by weight. Al contents above this limit would lead to a deterioration in cold formability due to the formation of intermetallic phases. Al contents of at most 5.1% by weight have proven to be particularly suitable for achieving good cold formability. In addition, Al, when present in higher levels, forms embrittling particles with nitrogen and carbon, reduces thermal conductivity and lowers the Young's modulus.

Manganese ("Mn") is present in the flat steel product according to the invention in levels of 9 - 14% by weight. Mn contributes to the formation of a sufficient amount of austenite in the structure of the flat steel product and its stability. At the same time, the presence of Mn in the amounts prescribed according to the invention improves the hot formability, weldability and strength of the flat steel product. Mn has a deoxidizing effect during the production of the steel from which the flat steel product according to the invention is made. Excessively high Mn contents are avoided in order not to impair the corrosion resistance of the flat steel product. In addition, Mn contents that are too high lead to too high a stability of the austenite, so that the TRIP effect is prevented or limited, which has a negative effect on the formability of the flat steel product. The positive influences of Mn can be used particularly safely in a flat steel product according to the invention at levels of at least 9% by weight. Negatives Influences of the presence of Mn while at the same time optimizing the cost-benefit ratio can be avoided by limiting the Mn content to a maximum of 12% by weight, in particular a maximum of 11.0% by weight. Mn contents of at most 10.7% by weight have proven to be particularly favorable for achieving good formability.

Boron ("B") can optionally be present in the flat steel product according to the invention in order to support the development of a fine structure. For this purpose, up to 0.1% by weight of B can be provided. This results in a positive contribution of B to the properties of a flat steel product according to the invention even at levels of 0.005% by weight. The positive influence of B can be used particularly effectively with B contents of less than 0.02% by weight.

Chromium ("Cr") can optionally be added to the flat steel product according to the invention in amounts of up to 2% by weight in order to improve the corrosion and oxidation resistance. At levels above 2% by weight, Cr carbides can form, which can impair the deformability of the flat steel product. The positive influence of Cr can be used particularly effectively with Cr contents of less than 0.8% by weight. The positive effects of Cr can be used particularly safely in the flat steel product according to the invention if the Cr content is at least 0.05% by weight, in particular at least 0.10% by weight.

Optional contents of up to 0.4% by weight of silicon ("Si") can be added to the flat steel product according to the invention in order to increase the strength and corrosion resistance. At the same time, Si, like Al, reduces the density of the material and contributes to deoxidation. However, too high Si contents can worsen the ductility and reduce the weldability of the flat steel product according to the invention. These negative influences can be reliably avoided by limiting the Si content to a maximum of 0.3% by weight in a flat steel product according to the invention. At the same time, the effects of Si in the flat steel product according to the invention already occur when the Si content is at least 0.05% by weight, in particular at least 0.1% by weight.

Phosphorus ("P") is generally undesirable in the flat steel product of the invention because it causes segregation and impairs cold workability, weldability and oxidation resistance. Therefore, the P content is limited to at most 0.1% by weight, in particular at most 0.05% by weight.

Sulfur ("S") is also an undesirable accompanying element in the flat steel product according to the invention, since if the content is too high, it reduces the hot formability during hot rolling and worsens the corrosion resistance.

Tantalum ("Ta") and tungsten ("W") can be added to the flat steel product according to the invention to increase strength by forming carbide in amounts of up to 0.5% by weight. An optimal cost-benefit ratio results from contents of up to 0.1% by weight.

Nickel ("Ni") optionally present in amounts of up to 2% by weight increases the amount and stability of the austenite present in the structure of a flat steel product according to the invention. At the same time, Ni contributes to the strength and toughness of the flat steel product. Ni also increases corrosion resistance. These advantageous effects can be used in particular when the Ni content of a flat steel product according to the invention is at least 0.05% by weight. The positive influence of Ni can be used particularly effectively with Ni contents of less than 1.0% by weight.

The addition of copper ("Cu") in amounts of up to 2% by weight improves the corrosion resistance of the flat steel product. However, higher Cu contents worsen hot formability and weldability. The positive influence of Cu can be used particularly effectively with Cu contents of less than 0.1% by weight.

Calcium ("Ca") can optionally be added to the flat steel product in amounts of up to 0.15% by weight to bind harmful S contents. Ca proves to be particularly effective in concentrations of up to 0.05% by weight, which are therefore preferred if Ca is present at all. Harmful effects of Ca can be reliably avoided by limiting the Ca content to less than 0.01% by weight.

Nitrogen ("N") is present in the flat steel product according to the invention in the lowest possible levels in order to avoid the formation of undesirable Al nitrides, which would particularly impair the mechanical properties and deformability of the flat steel product. Therefore, according to the invention, the N content of the flat steel product is limited to a maximum of 0.02% by weight, in particular 0.01% by weight, with practical N contents in the range of 0.001 - 0.008% by weight.

Cobalt ("Co") can optionally be added to the flat steel product according to the invention in amounts of up to 2% by weight in order to increase the amount and stability of the austenite present in the structure. At the same time, Co increases the recrystallization temperature. Optimal effects of the presence of Co occur when the Co content is limited to a maximum of 1% by weight, with Co contents of less than 0.01% by weight having proven to be sufficient in many cases.

Antimony ("Sb") negatively influences the properties of the flat steel product according to the invention and is therefore one of the undesirable but unavoidable impurities for production reasons. Its content should be as low as possible, but in any case, like that of all the other elements not listed here and therefore considered unavoidable impurities, should be in the technically ineffective range. 0.002% by weight has proven to be a practical upper limit below which the Sb content should be.

The elements assigned to the groups “Ti, Nb, V, Mo” and “Zr, La, Ce, Y” are of particular importance. According to the invention, at least one element from at least one of these groups is present in the flat steel product according to the invention in order to support edge stability, i.e. the avoidance of the formation of cracks, during hot rolling through the formation of precipitates. As mentioned, the elements from the group “Ti, Nb, V, Mo” can be present alone, i.e. without the presence of an element from the group “Zr, La, Ce, Y”, or in combination with at least one of the elements Group "Zr, La, Ce, Y". Accordingly, the elements of the group "Zr, La, Ce, Y" can be present alone, i.e. without the presence of an element of the group "Ti, Nb, V, Mo", or in combination with at least one of the elements of the group "Ti, Nb, V, Mo".

In the event that only one element from the group "Ti, Nb, V, Mo" is present in the flat steel product according to the invention, the content of this one element is 0.05 - 1% by weight. If, on the other hand, two or more elements from the group “Ti, Nb, V, Mo” are present, the sum of the contents is also 0.05 - 1% by weight. Particularly favorable influences of the elements of the group "Ti, Nb, V, Mo" arise when their content is individual in the case that only one element of this group is present or in the case that two or more elements of this group are present , in total is at least 0.08% by weight. An optimal cost-benefit ratio can be obtained if, in the case of only one element of this group being present, individually or in the case of two or more elements of this group being present, in the sum of the contents of the elements of the group present in each case at most 0.5% by weight, in particular at most 0.35% by weight.

Likewise, if only one element from the group “Zr, La, Ce, Y” is present in the flat steel product according to the invention, its content is 0.05 - 0.3% by weight. If, on the other hand, two or more elements from the group “Zr, La, Ce, Y” are present, the sum of the contents of these elements is also 0.05 - 0.3% by weight. Particularly favorable influences of the elements of the group "Zr, La, Ce, Y" arise if their content is at least 0.08% by weight if only one element of this group is present, individually or if two or more elements of this group are present, in total. An optimal cost-benefit ratio can be obtained if, in the case where only one element of this group is present, individually or in the case where two or more elements of this group are present, in total, the content of the elements of the group present in each case at most 0.2% by weight, in particular at most 0.15% by weight.

In addition to improving edge stability, titanium ("Ti") increases strength through the formation of Ti carbides and improves the r-value. This has a positive effect on the edge stability and strength of Ti, especially if Nb is also present at the same time. The simultaneous presence of Ti and Nb is therefore preferred. The addition of Ti also improves the low-temperature toughness and high-temperature strength of the material. However, excessive Ti contents should be avoided in order to avoid negative influences on deformability and welding properties. The advantageous effect of Ti can be used in particular when the Ti content is at least 0.06% by weight.

Like Ti, niobium ("Nb"), in addition to improving edge stability, which results in particular when Ti and Nb are present at the same time, causes an increase in strength through the formation of Nb carbides and improves the r-value, which also has this effect occurs particularly safely when Ti is present at the same time. The addition of Nb also improves the low-temperature toughness and high-temperature strength of the material. However, excessive Nb contents should be avoided in order to avoid negative influences on deformability and welding properties. The advantageous effect of Nb can be used in particular when the Nb content is at least 0.03% by weight.

Vanadium ("V") forms carbides and, in addition to its positive influence on edge stability, contributes to the strength of the flat steel product according to the invention. At the same time, the addition of V improves the r-value of the flat steel product. The positive influences of V can be used particularly safely at levels of at least 0.03% by weight.

Molybdenum ("Mo") not only improves edge stability, but also contributes to tensile strength and forms fine carbides with C, which support the development of a fine structure. However, if the content is too high, Mo impairs the hot and cold formability of the flat steel product according to the invention. The positive effects of Mo can be used particularly when the Mo content is at least 0.05% by weight.

On the one hand, cerium ("Ce"), lanthanum ("La"), zirconium ("Zr") and yttrium ("Y") improve edge stability. On the other hand, they also have a desulfurizing and deoxidizing effect. They also increase the r-value and improve the deep-drawing ability. In this way, Ce, La, Zr and Y compensate for the negative influences that high Al contents can have on these properties of the flat steel product. The effects of Ce, La, Zr and Y are the same, so that these elements can be exchanged for each other.

The additional requirement according to which the aluminum equivalent Al _eq formed from the respective Al and Si content must be in the range of 3 - 8% by weight ensures that the contents of the similarly acting alloy elements Al and Si are limited to such an extent that the formation of extremely brittle intermetallic precipitates is prevented. In order to counteract this danger particularly reliably, the range specified for Al _eq can be limited according to the invention to a maximum of 7.6% by weight, in particular a maximum of 7% by weight or a maximum of 6.5% by weight. At the same time, the minimum value for Al _eq can be set to 4.8% by weight be raised in order to safely utilize the positive effects of the simultaneous presence of AI and Si.

According to the invention, the ratio %Mn/%Al of the contents of Mn ("%Mn") and Al ("%Al") should be more than 1.2% by weight, so that there is a sufficient amount of austenite despite the high Al content achieved in the structure of the flat steel product.

The alloy engineering measures explained above and the procedural measures explained below ensure that the austenite content in the structure of the flat steel product according to the invention is 10 - 60% by area. With such austenite contents, the steel of a flat steel product according to the invention has TRIP properties and the flat steel product has a high n value of at least 0.21.

In addition to the technically unavoidable other structural components, which, however, are present in such small quantities that they have no influence on the properties, the rest of the structure of the flat steel product according to the invention that is not occupied by austenite consists of ferrite as a result of the high Al content.

Also as a result of the alloying measures and the method of production according to the invention, a flat steel product according to the invention has an n-value of at least 0.21, with flat steel products according to the invention regularly achieving n-values of at least 0.25, in particular at least 0.26. High n-values represent a high formability associated with a high elongation at break A50 and therefore allow the formation of complex components. However, too high n-values should be avoided as this would result in high deformation forces required. As a result of their composition and production, according to the invention Flat steel products have n values that are regularly not higher than 0.5, in particular not higher than 0.4.

The austenite grain size in the structure of a flat steel product according to the invention is on average 0.85 - 3 µm. Grains that are too small would inhibit the TRIP effect. The desired n value would not be achieved. However, grains that are too large would lead to a large decrease in yield strength and tensile strength. Optimally, the average grain size of the austenite is at least 0.9 µm. It has also proven to be advantageous if the maximum size of the austenite grains in the structure of a flat steel product according to the invention is limited to 1.5 μm.

According to a first embodiment of the invention, which contributes to further improved deformability, in a flat steel product according to the invention, in addition to a high austenite grain size, a high degree of recrystallization in the ferrite grains is sought so that they can support the deformation of the austenite grains. This degree of recrystallization can be quantified using the so-called “ Kernal Average Misorientation ” (“KAM”). The KAM is a measure of the dislocation density. A low KAM in the ferrite ultimately means a high deformability of the ferrite. Since the structure of a flat steel product according to the invention consists of austenite and ferrite ("duplex structure"), a low KAM of the ferrite can compensate for a low austenite grain size. For the quotient KG/(KAM 1 5°) formed from the respective mean grain size KG of the austenite and the "KAM 1 5° value" determined on the ferrite, KG/(KAM 1 5°) > 2.7 µm/ °. The KAM of the ferrite is measured in the longitudinal section as “KAM 1 5°”. The sample was scanned using EBSD with a resolution of 100 nm and then evaluated with a step size of 1 (only 1st neighbor) and a cutoff value of 5°, as in Nafisi S., Szpunar J., Vali H., Ghomashchi R., 2009, Grain misorientation in thixo-billets prepared by melt stirring. Mater Char 60:938-945 ).

1. a) Erschmelzen einer Stahlschmelze, die aus (in Gew.-%) C: 0,08 - 0,25 %, AI: 3 - 5,4 %, Mn: 9 - 14 %, B: 0 - 0,1 %, Cr: 0 - 2 %, Si: 0 - 0,4 %, P: 0 - 0,1 %, S: 0 - 0,3 %, Ta: 0 - 0,5 %, W: 0 - 0,5 %, Ni: 0 - 2 %, Cu: 0 - 2 %, Ca: 0 - 0,15 %, N: 0 - 0,02 %, Co: 0 - 2 %, sowie einem Element oder mehreren Elementen aus der Gruppe "Ti, Nb, V, Mo" mit der Maßgabe, dass die Summe der Gehalte an diesen Elementen mindestens 0,05 % und höchstens 1 % beträgt, undloder einem Element oder mehreren Elementen aus der Gruppe "Zr, La, Ce, Y" mit der Maßgabe, dass die Summe der Gehalte an diesen Elementen mindestens 0,05 % und höchstens 0,3 % beträgt, sowie Rest Eisen und unvermeidbaren Verunreinigungen besteht, wobei für das Verhältnis %Mn/%AI gilt %Mn/%AI > 1,2 und für Al_eq = %AI + 0,4 × (%Si)³ - 3 × (%Si)² + 8,3 × %Si gilt 3 ≤ Al_eq ≤ 8 mit %Mn: jeweiliger Mn-Gehalt des Stahls, %Al: jeweiliger Al-Gehalt des Stahls, %Si: jeweiliger Si-Gehalt des Stahls;
2. b) Vergießen der Stahlschmelze zu einem Vorprodukt, wie einem Gussblock, einer Bramme, einer Dünnbramme oder einem gegossenen Band;
3. c) Halten oder Erwärmen des Vorprodukts bei einer Vorwärmtemperatur von 1100 - 1350 °C;
4. d) Warmwalzen des Vorprodukts zu einem Warmband mit einer Warmwalzendtemperatur, die 850 - 1050 °C beträgt;
5. e) Abkühlen des Warmbands auf eine Haspeltemperatur, die 400 - 900 °C beträgt;
6. f) Haspeln des auf die Haspeltemperatur abgekühlten Warmbands zu einem Coil;
7. g) optional: Glühen des Warmbands bei einer Warmbandglühtemperatur von 700 - 1000 °C;
8. h) optional: Beizen des Warmbands;
9. i) Kaltwalzen des Warmbands zu einem kaltgewalzten Stahlband mit einem Gesamtumformgrad von 25 - 90 %;
10. j) Schlussglühen des kaltgewalzten Stahlbands, wobei das Stahlband auf eine Schlussglühtemperatur aufgeheizt wird und das Schlussglühen
    • entweder als Durchlaufglühung im kontinuierlichen Durchlauf durch einen Durchlaufglühofen absolviert wird, bei dem das kaltgewalzte Stahlband über eine Dauer von mindestens 20 sec und weniger als 10 min auf eine Schlussglühtemperatur, die mindestens 950 °C und höchstens 1070 °C beträgt, gehalten wird, wobei die Aufheizrate, mit der das kaltgewalzte Stahlflachprodukt auf die Schlussglühtemperatur erwärmt wird, von 1 - 100 Kls beträgt,
    • oder als Haubenglühung über eine Dauer von 0,5 - 60 h bei einer Schlussglühtemperatur durchgeführt wird, die mehr als 800 °C und bis zu 950 °C beträgt, wobei die Aufheizrate, mit der das kaltgewalzte Stahlflachprodukt auf die Schlussglühtemperatur erwärmt wird, von 0,001 - 0,5 Kls beträgt.

The method for producing a cold-rolled flat steel product according to the invention is designed in such a way that the desired property profile of the flat steel product is achieved based on the alloying specification according to the invention. For this purpose, the method according to the invention includes the following work steps:

1. a) Melting a steel melt consisting of (in wt.%) C: 0.08 - 0.25%, Al: 3 - 5.4%, Mn: 9 - 14%, B: 0 - 0.1% , Cr: 0 - 2%, Si: 0 - 0.4%, P: 0 - 0.1%, S: 0 - 0.3%, Ta: 0 - 0.5%, W: 0 - 0, 5%, Ni: 0 - 2%, Cu: 0 - 2%, Ca: 0 - 0.15%, N: 0 - 0.02%, Co: 0 - 2%, as well as one or more elements from the Group "Ti, Nb, V, Mo" with the proviso that the sum of the contents of these elements is at least 0.05% and at most 1%, and/or one or more elements from the group "Zr, La, Ce, Y " with the proviso that the sum of the contents of these elements is at least 0.05% and at most 0.3%, and the balance consists of iron and unavoidable impurities, whereby the ratio %Mn/%AI applies to %Mn/%AI > 1.2 and for Al _eq = %AI + 0.4 × (%Si) ³ - 3 × (%Si) ² + 8.3 × %Si 3 ≤ Al _eq ≤ 8 with %Mn: respective Mn content of the steel, %Al: respective Al content of the steel, %Si: respective Si content of the steel;
2. b) casting the molten steel into a preliminary product, such as a cast block, a slab, a thin slab or a cast strip;
3. c) holding or heating the preliminary product at a preheating temperature of 1100 - 1350 °C;
4. d) hot rolling the preliminary product into a hot strip with a final hot rolling temperature of 850 - 1050 ° C;
5. e) cooling the hot strip to a coiling temperature of 400 - 900 °C;
6. f) coiling the hot strip cooled to the coiling temperature into a coil;
7. g) optional: annealing the hot strip at a hot strip annealing temperature of 700 - 1000 °C;
8. h) optional: pickling of the hot strip;
9. i) cold rolling the hot strip into a cold-rolled steel strip with a total degree of deformation of 25 - 90%;
10. j) final annealing of the cold-rolled steel strip, whereby the steel strip is heated to a final annealing temperature and the final annealing
    • either as continuous annealing in a continuous run through a continuous annealing furnace, in which the cold-rolled steel strip is held at a final annealing temperature of at least 950 ° C and at most 1070 ° C for a period of at least 20 seconds and less than 10 minutes, whereby the Heating rate at which the cold-rolled flat steel product is heated to the final annealing temperature is from 1 - 100 Kls,
    • or as a hood annealing is carried out over a period of 0.5 - 60 h at a final annealing temperature that is more than 800 ° C and up to 950 ° C, the heating rate at which the cold-rolled flat steel product is heated to the final annealing temperature being 0.001 - 0.5 Kls.

The work steps listed here and explained below are those that have an influence on the properties of the flat steel product produced according to the invention. It goes without saying that further work steps are used when carrying out the method according to the invention. However, these can be carried out by a person skilled in the art in the usual way in the production of flat steel products, so that no further explanation is required.

Before hot rolling, the preliminary product, which is typically a slab, a thin slab or It is a cast strip and needs to be heated through. Incomplete, inhomogeneous heating would result in the risk of cracks forming in the subsequent hot rolling process. The preheating temperatures suitable for thorough heating are 1100 - 1300 °C, although in practice preheating temperatures of at least 1150 °C have proven to be particularly reliable. By limiting the preheating temperature to a maximum of 1250 °C, negative effects of preheating, such as an excessively doughy consistency of the preliminary product and an associated tendency to stick during hot rolling, can be counteracted.

The hot rolling of the preliminary product into a hot strip can be carried out in a conventional manner on a hot rolling train that is available for this purpose in practice. The only important thing is that the hot strip still has a final hot rolling temperature of 850 - 1050 °C at the end of hot rolling. Hot rolling end temperatures below 850 °C would require such high hot rolling forces that the desired degrees of forming cannot be achieved on conventional hot rolling stands. Optimal hot rolling end temperatures are therefore in the range of 900 - 1050 °C.

Starting from the final hot rolling temperature, the hot strips obtained are, if necessary, cooled in a conventional manner to the coiling temperature of 400 - 900 ° C specified according to the invention, with which they are then coiled into a coil. The minimum coiling temperature of 400 ° C is due to the poor thermal conductivity of the steel of a flat steel product according to the invention necessary because otherwise high temperature gradients in the flat steel product would lead to tensions and the resulting poor flatness of the hot strip. Cooling too quickly in the cooling section could interrupt the recrystallization processes that begin at the end of hot rolling. Optionally, the hot strip wound into a coil can be cooled using a coil shower in order to shorten the cooling time and have a positive effect on scale formation.

In order to complete the recrystallization and to produce an optimal condition for the subsequent cold rolling, the hot strip can optionally undergo an annealing treatment after cooling in the coil, in which it is at a temperature of 700 - 1000 ° C, in particular, for a period of time sufficient for complete recrystallization 700 - 900 °C. Optionally, the hot strip annealing is preferably carried out as a hood annealing, in which case the annealing time typically lasts 0.5 - 60 hours.

To clean its surface, the hot strip can be subjected to a pickling treatment before cold rolling. The scale formation on the hot strip surface caused by oxygen-affinous elements during the manufacturing process can be reduced by extending the residence time of the hot strip in the pickling medium. Hot strip pickling can be carried out using different pickling media such as hydrochloric acid.

The cold rolling of the hot-rolled strip into a cold-rolled strip takes place with a total degree of deformation of 25 - 90%, in particular at least 40% or at least 50%. The minimum degree of deformation provided according to the invention is necessary in order to initiate the recrystallization, in particular of the ferrite contained in the structure of the flat steel product, during the final annealing. However, too high a degree of deformation should be avoided, as this would result in too large a degree of deformation Work hardening and the associated risk of belt breaks would result.

The mechanical properties of the flat steel product according to the invention are decisively influenced by the final annealing. The final annealing can be carried out in a continuous process through a conventional continuous furnace or as a hood annealing in batch operation.

In the event that the final annealing is completed in one run, the cold-rolled flat steel product is heated to a final annealing temperature that is at least 950 ° C and at a rate that is typically 1 - 100 K/s, in particular 10 - 80 K/s or 20 - 50 Kls is heated to a maximum of 1070 °C. Heating up too quickly can have an unfavorable effect on the homogeneity of the heating and thus on the homogeneous distribution of properties of the flat steel product.

The final annealing temperature is selected so that the necessary austenite grain size is achieved, which is a prerequisite for the TRIP properties and the n value of the flat steel product according to the invention. The necessary austenite grain size can be set particularly reliably if the final annealing temperature is at least 970 °C. Optimally practical final annealing temperatures are in the range of 1000 - 1070 °C. By setting such high final annealing temperatures, flat steel products according to the invention can be produced with austenite grain sizes of 0.9 - 1.5 μm and an n value that is regularly above 0.25, for example above 0.26.

The holding times at the final annealing temperature in the continuous annealing furnace are, depending on the level of the final annealing temperature, not more than 10 minutes, in particular not more than 5 minutes or 3 minutes, with higher continuous final annealing temperatures enabling shorter holding times and minimum holding times of at least 20 seconds each are practical.

In the event that the final annealing is carried out as a hood annealing, the cold-rolled strip placed in the coil under the hood is heated to the final annealing temperature at a heating rate of 0.001 - 0.5 Kls, in particular 0.002 - 0.5 Kls or 0.008 - 0.5 Kls heated, which is in each case more than 800 ° C, in particular more than 850 ° C. A final annealing temperature of 950 °C should not be exceeded in order to avoid excessive grain growth. The annealing times after reaching the final annealing temperature are in the range of 0.5 - 60 hours, in particular 1 - 30 hours. In this way, flat steel products according to the invention with an n value of at least 0.21 and austenite grain sizes of 0.85 - 3 µm can be produced particularly reliably.

To protect against corrosion and external influences, the flat steel product according to the invention can be provided with a protective coating. This can, for example, be applied in a conventional manner using coating systems available in practice as a zinc or aluminum-based coating.

The invention is explained in more detail below using exemplary embodiments.

To test the effects of the invention, eight melts A - H were produced, the wet chemical analyzes of which are shown in Table 1. The analysis data for the individual elements refer to the determined contents in% by weight. If "-" is entered in the respective table field, this means that the content of the respective element was below the respective detection limit and the element in question was therefore ineffective with regard to the properties. (Note: The detection limits for P and Ti are 0.005 each % by weight, for Cr, Co, S and Ca each at 0.001% by weight, for V, Mo and Ni each at 0.01% by weight, for Cu and Nb each at 0.02% by weight , for B at 0.0004% by weight and for Ce, La, Y, Zr, As and Sn each at 0.002% by weight. Elements that were present in such low levels are to be classified as unavoidable impurities, just like the elements not listed. These include, for example, contents of As, Sn, Mg and H. The sum of the microalloying elements Ti, Nb, V, Mo, the sum of the rare earth metals Ce, La, Y, Zr, the ratio of Mn to Al and the Al Equivalent of the individual melts are given in Table 2.

Of the melts A - H, melts A and B are not according to the invention since they meet the requirements of the invention for the presence of at least one element from the groups "Ti, Nb, V, Mo" and "Zr, La, Ce, Y" cannot be fulfilled.

For 27 experiments 1 - 27, melts A - H were cast in a conventional manner into blocks, which were then heated through at a preheating temperature VWT for a period of VWD.

After heating, the blocks were rolled into slabs in an equally conventional manner in a conventional hot rolling stand. The slabs were rolled in a hot rolling mill into hot strip with a thickness of 2.5 - 3.0 mm. The hot rolling was each ended with a hot rolling final temperature WET.

After hot rolling, the resulting hot strips were cooled in air to a coiling temperature HT, at which they were coiled into a coil.

After cooling in the coil, an assessment of the strip edges for the formation of cracks would be carried out on each of the hot strips obtained in tests 1 - 27. These are considered “strong” by Edge cracks affect those hot strips in which regular indentations measuring more than 15 mm were found. Hot strips classified as “slightly” affected by edge cracks are those in which regular notches measuring 2 - 15 mm in size were found. The rating "none" was given to the hot strips in which the maximum number of indentations present was less than 2 mm.

Some of the hot strips were then subjected to hot strip annealing, which was carried out as a hood annealing with an annealing temperature of 850 ° C and an annealing time of 6 hours.

The annealed hot strips, like the non-annealed hot strips, were cold-rolled into cold-rolled steel strip in a conventional cold rolling facility with a total cold forming degree of KGW.

Finally, the cold-rolled steel strips obtained were final annealed in a continuous annealing furnace (“Conti”) or in batches in a hood annealing furnace (“hood”). They were each heated to the respective final annealing temperature Tsg at an average heating rate HR, at which they were each maintained for a period of time tsg.

The melt composition "steel" of the steel strips processed in experiments 1 - 27, the parameters used in experiments 1 - 27: preheating temperature VWT, preheating time VWD, hot strip thickness WB, hot rolling end temperature WET, coiling temperature HT, the result of the assessment of the edge cracks on the respective hot strip, The information as to whether hot strip annealing was carried out in the manner explained above, the total cold forming degree KGW, the type of final annealing, the heating rate HR of the final annealing, the final annealing temperature Tsg and the final annealing duration tsg are given in Table 3a and Table 3b.

In Table 4a and Table 4b, as far as recorded, the upper and lower yield strengths ReH and ReL, the yield strength Rp0.2, the tensile strength Rm, the elongation at break A50, the n-value n _10-20 are determined on the cold-rolled and finally annealed steel strips obtained , the r value r _10-20 , the austenite grain size KG, the KAM value determined with a step size of 100 nm and a deposition value of 5 °, and the quotient KG/KAM. The yield strengths ReH and ReL, the yield strength Rp0.2, the tensile strength Rm and the elongation at break A50 were determined in accordance with DIN EN ISO 6892-1:2009 on the cold-rolled strip in the transverse direction. To determine the austenite grain size KG, the mean circle-equivalent grain diameter was determined using EBSD (Electron Back Scattering Diffraction). To calculate the grains in the EBSD representation, a tolerance angle of 5° was chosen with a minimum grain size of five contiguous neighboring measuring points with a step size of 0.15 µm. The n _10-20 and the r _10-20 values as well as the KAM value were determined in the manner already explained above. Values that were not recorded are marked with “-” in Table 3.

Examples 10, 11, 15, 16, 20, 23 were final annealed in a continuous furnace with a final annealing temperature of 830 ° C, which was outside the temperature range of 950 - 1070 ° C specified according to the invention for a final annealing that takes place in a continuous process. The flat steel products produced in Examples 10, 11, 15, 16, 20, 23 accordingly had very low n values, which are below the minimum value of 0.21 provided for in the invention.

The experiments prove that with the alloy according to the invention and the method of production according to the invention, cold-rolled flat steel products can be produced reliably, which show at most slight edge cracking and have a combination of mechanical properties that make them optimally suitable for uses in which components are formed from the flat steel products at high degrees of deformation become. <b>Table 1</b> steel C Si Mn P S AI N Cu Cr v Mo Ni Ti Nb b Ce La Co Approx A*) 0.148 0.18 10.0 0.009 - 5.0 0.0032 0.05 0.7 - 0.04 0.17 - - 0.001 - - 0.007 0.002 B*) 0.152 0.06 10.2 - 0.004 5.0 0.0019 0.06 0.44 - - 0.06 - - - - - 0.003 - C 0.151 0.08 10.1 - 0.004 5.1 0.0017 0.05 0.44 - - 0.06 0.07 0.03 - - - 0.002 - D 0.155 0.08 10.1 - 0.004 5.1 0.0016 0.06 0.44 - - 0.07 0.21 0.10 - - - - - E 0.159 0.07 10.7 - - 4.7 0.0015 0.05 0.21 0.18 0.12 0.06 - - - - - 0.002 - F 0.147 0.09 9.6 - 0.005 4.5 0.0019 - - - - 0.06 - - - 0.08 0.04 0.003 - G 0.223 0.04 10.1 0.005 0.004 5.0 0.0012 - 0.02 - - 0.02 0.20 0.10 - - - - - H 0.145 0.06 11.1 - 0.004 5.1 0.0012 0.05 0.41 - - - 0.19 0.09 - - - 0.002 - Residual iron and unavoidable impurities
Information on alloy contents in wt.%
*) not according to the invention steel Ti+Nb+V+Mo Ce+La+Y+Zr %Mn/%Al Al _eq A*) 0.04 - 2 6.4 B*) - - 2.04 5.49 C 0.1 - 1.98 5.75 D 0.31 - 1.98 5.75 E 0.3 - 2.28 5.27 F - 0.12 2.13 5.22 G 0.30 - 2.02 5.33 H 0.28 - 2.18 5.59 Al _eq = %Al + 0.4 × (%Si) ³ - 3 × (%Si) ² + 8.3 × %Si
%Mn = respective Mn content,
%AI = respective Al content,
%Si = respective Si content
*) not according to the invention Attempt steel VWT [°C] VWD [min] WET [°C] HT [°C] Edge cracks? Hot strip annealing KWG Final annealing HR [K/s] Tsg [°C] tsg 1 A 1240 90 1020 550 strong without 64% Accounts Not recorded 1000 3min 2 without 64% 6min 3 without 82% 3min 4 b 1200 950 800 without 66% 1min 5 without 80% 830 3min with 6 7 without Hood 0.04 850 24 hours 8th with 9 C light without 66% Accounts 10 1000 1min 10 without 80% 830 3min 11 with 12 without Hood 0.04 850 24 hours 13 with 14 D no without 66% Accounts 10 1000 1min 15 without 80% 830 3min 16 with 17 without Hood 0.04 850 24 hours 18 with Attempt steel VWT [°C] VWD [min] WET [°C] HT [°C] Edge cracks? Hot strip annealing KWG Final annealing HR [Kls] Tsg [°C] tsg 19 E 1200 90 950 800 no without 66% Accounts 10 1000 1min 20 80% 830 3min 21 Hood 0.04 850 24 hours 22 F without 66% Accounts 10 1000 1min 23 80% 830 3min 24 Hood 0.04 850 24 hours 25 G without 66% Accounts 10 1000 1min 26 Hood 0.04 850 24 hours 27 H without 66% Accounts 10 970 1min Attempt steel Edge cracks? Deer ReL 1 R _p0.2 Rm _A50 n _10-20 r _10-20 KG [µm] KAM [Ferrite/°] KG/KAM [µm/°] [MPa] [%] 1 A strong - - 484 658 34.3 0.214 0.20 - - - 2 - - 501 662 32.0 0.203 0.18 - - - 3 - - 498 707 40.0 0.230 0.36 - - - 4 b - - 430 743 38.2 0.326 0.66 1.14 0.37 3.08 5 624 611 - 733 30.2 0.199 0.61 0.92 0.35 2.63 6 - - 543 692 29.8 0.206 0.62 - - - 7 - - 395 649 36.9 0.235 0.73 2.78 0.39 7.13 8th - - 393 631 32.9 0.235 0.75 2.94 0.39 7.54 9 C light - - 431 738 37.5 0.327 0.82 0.90 0.32 2.81 10 623 616 - 738 29.9 0.190 0.59 0.85 0.36 2.36 11 - - 539 695 30.9 0.202 0.59 - - - 12 - - 407 653 36.0 0.230 0.73 2.58 0.35 7.37 13 - - 396 638 35.2 0.230 0.76 2.71 0.35 7.74 14 D no - - 434 705 34.5 0.295 0.95 0.87 0.30 2.90 15 - - 580 714 29.8 0.181 0.68 0.83 0.35 2.37 16 - - 521 678 27.7 0.191 0.70 - - - 17 - - 412 638 35.8 0.213 0.79 2.33 0.36 6.47 18 - - 405 632 31.8 0.217 0.84 2.55 0.35 7.29 Attempt steel Edge cracks? _Deer R _{eL Rp0.2} Rm _A50 n 10-20 r _10-20 KG [µm] KAM [Ferrite/°] KG/KAM [µm/°] [MPa] [%] 19 E no - - 475 755 33.7 0.279 0.83 1.10 0.31 3.55 20 - - 601 739 28.5 0.180 0.51 0.90 0.37 2.43 21 - - 433 688 34.5 0.212 0.81 2.62 0.36 7.28 22 F - - 413 701 39.1 0.305 0.89 0.98 0.31 3.16 23 - - 551 699 32.8 0.195 0.52 0.92 0.37 2.49 24 - - 392 628 36.8 0.233 0.75 2.76 0.36 7.67 25 G - - 451 803 34.1 0.400 0.70 0.89 0.32 2.78 26 - - 398 652 31.3 0.239 0.73 2.48 0.36 6.89 27 H - - 466 612 20.8 0.214 0.69 0.88 0.32 2.75

Claims (15)

1. Cold-rolled flat steel product which has an n-value, which is at least 0.21, measured as described in the description, and which is made of a steel which consists of (in wt.%) C: 0.08 - 0.25%, Al: 3 - 5.4%, Mn: 9 - 14%, B: 0 - 0.1%, Cr: 0 - 2%, Si: 0 - 0.4%, P: 0 - 0.1%, S: 0 - 0.3%, Ta: 0 - 0.5%, W: 0 - 0.5%, Ni: 0 - 2%, Cu: 0 - 2%, Ca: 0 - 0.15%, N: 0 - 0.02% Co: 0 - 2%,

   as well as of an element or a plurality of elements from the group "Ti, Nb, V, Mo" with the proviso that the sum of the contents of these elements is at least 0.05% and at most 1%, and/or an element or a plurality of elements from the group "Zr, La, Ce, Y" with the proviso that the sum of the contents of these elements is at least 0.05% and at most 0.3%, the remainder being iron and unavoidable impurities,

   wherein for the %Mn/%AI ratio, the following applies $$\%\mathrm{Mn}/\%\mathrm{Al}>1.2$$

   

   and for $${\mathrm{Al}}_{\mathrm{eq}}=\%\mathrm{Al}+0.4\times {\left(\%\mathrm{Si}\right)}^3-3\times {\left(\%\mathrm{Si}\right)}^2+8.3\times \%\mathrm{Si},$$

   

   the following applies $$3\le {\mathrm{Al}}_{\mathrm{eq}}\le 8$$

   

   with %Mn being the corresponding Mn content of the steel

   %AI being the corresponding Al content of the steel

   %Si being the corresponding Si content of the steel

   wherein the structure of the flat steel product consists of 10 - 60 area % austenite and 40 - 90 area % ferrite with an average particle size of the austenite of 0.85 - 3 µm.
2. Flat steel product according to claim 1, characterized in that for the quotient KG/(KAM 1 5°) of the corresponding average particle size KG of the austenite and the KAM 1 5°-value determined on the ferrite, KG/(KAM 1 5°) > 2.7 µm/° applies.
3. Flat steel product according to any of the preceding claims,
   characterized in that its n-value is at least 0.25.
4. Flat steel product according to any of the preceding claims,
   characterized in that the average particle size of the austenite is 0.9 - 1.5 µm.
5. Flat steel product according to any of the preceding claims,
   characterized in that its Al content is 4 - 5.4 wt.%.
6. Flat steel product according to any of the preceding claims,
   characterized in that its C content is 0.11 - 0.19 wt.%.
7. Flat steel product according to any of the preceding claims,
   characterized in that its Mn content is 9 - 12 wt.%.
8. Flat steel product according to any of the preceding claims,
   characterized in that its Si content is at least 0.01 wt.%.
9. Flat steel product according to any of the preceding claims,
   characterized in that for Al_eq, Al_eq ≤ 7 applies.
10. Flat steel product according to any of the preceding claims,
    characterized in that its tensile strength Rm is at most 1050 MPa, in particular at most 950 MPa.
11. Use of a cold-rolled flat steel product specified according to any of the preceding claims for the production of vehicle parts.
12. Method for producing a cold-rolled flat steel product specified according to any of the claims 1 - 10, comprising the following work steps:

    a) melting a steel melt which consists of (in wt. %) C: 0.08 - 0.25%, Al: 3 - 5.4%, Mn: 9 - 14%, B: 0 - 0.1 %, Cr: 0 - 2%, Si: 0 - 0.4%, P: 0 - 0.1%, S: 0 - 0.3%, Ta: 0 - 0.5%, W: 0 - 0.5%, Ni: 0 - 2%, Cu: 0 - 2%, Ca: 0 - 0.15%, N: 0 - 0.02%, Co: 0 - 2%, as well as an element or a plurality of elements from the group "Ti, Nb, V, Mo" with the proviso that the sum of the contents of these elements is at least 0.05% and at most 1%, or an element or a plurality of elements from the group "Zr, La, Ce, Y" with the proviso that the sum of the contents of these elements is at least 0.05% and at most 0.3%, the remainder being iron and unavoidable impurities, wherein for the %Mn/%AI ratio, the following applies: %Mn/%AI > 1.2, and for Al_eq = %AI + 0.4 × (%Si)³ - 3 × (%Si)² + 8.3 × %Si, the following applies 3 ≤ Al_eq ≤ 8, with %Mn being the corresponding Mn content of the steel, %AI being the corresponding Al content of the steel, %Si being the corresponding Si content of the steel;

    b) casting the steel melt into a preliminary product, such as a cast ingot, a slab, a thin slab or a cast strip;

    c) holding or heating the preliminary product at a preheating temperature of 1100 - 1350°C;

    d) hot rolling the preliminary product into a hot strip having a hot rolling end temperature which is 850 - 1050°C;

    e) cooling the hot strip to a coiling temperature which is 400 - 900°C;

    f) coiling the hot strip, which is cooled to the coiling temperature, to form a coil;

    g) optionally annealing the hot strip at a hot strip annealing temperature of 700 - 1000°C;

    h) optionally pickling the hot strip;

    i) cold rolling the hot strip to a cold-rolled steel strip having a total forming degree of 25 - 90%;

    j) final annealing of the cold-rolled steel strip, wherein the final annealing

    - is either completed as continuous annealing in a continuous pass through a continuous annealing furnace, in which the cold-rolled steel strip is kept at a final annealing temperature which is at least 950°C and at most 1070°C for a duration of at least 20 seconds and less than 10 minutes, wherein the heating rate at which the cold-rolled flat steel product is heated to the final annealing temperature is 1 - 100 K/s,

    - or is carried out in a bell furnace at a final annealing temperature which is more than 800°C and up to 950°C for a duration of 0.5 - 60 hours, wherein the heating rate at which the cold-rolled flat steel product is heated to the final annealing temperature is 0.001 - 0.5 K/s.
13. Method according to claim 12, characterized in that the hot rolling end temperature is at least 900°C.
14. Method according to claim 12 or claim 13, characterized in that if final annealing is carried out as continuous annealing, the final annealing temperature is at least 1000°C, or if final annealing is carried out in a bell furnace, the final annealing temperature is more than 850°C.
15. Method according to any of claims 12 - 14, characterized in that the coiling temperature is 600 - 850°C.