High tensile steel containing Mn, steel surface product made from such steel and method for producing same

Abstract

Steel with an elongation at break A80 of minimum 4% and a tensile strength of 900-1500 MPa, comprises iron and unavoidable impurities comprising carbon (up to 0.5%), manganese (4-12%), silicon (up to 1%), aluminum (up to 3%), chromium (0.1-4%), copper (up to 2%), nickel (up to 2%), nitrogen (up to 0.05%), phosphorus (up to 0.05%), and sulfur (up to 0.01%), and optionally at most 0.5% of one or more elements comprising vanadium, niobium or titanium. Independent claims are included for: (1) a flat rolled steel product made of the steel, comprising 30-100% of martensite, tempered martensite or bainite and residual quantity of austenite; and (2) making the flat rolled steel products, comprising melting composite molten steel, producing an starting product for subsequent hot rolling, in which the molten steel is poured into a strand of which at least a slab or a thin slab partitioned as a starting material for hot rolling, or a cast strip provided as a starting material for hot rolling, heat treating the starting product at a hot rolling start temperature of 1000-1150[deg] C, hot rolling the starting product to a hot strip with a thickness of at most 2.5 mm, where the hot rolling is performed at 800-1050[deg] C, coiling the hot strip into a coil at a coiling temperature of = 700[deg] C, and optionally annealing the hot strip at 250-950[deg] C, cold rolling the annealed hot strips in one or more steps to a cold-rolled strip with a thickness of at most 60% of the thickness of the hot strips, annealing the cold-rolled strip at 450-950[deg] C, coating the surface of the hot strip or cold-rolled strip with a metallic corrosion protective coating, and coating the surface of the hot strip or the cold strip with an organic coating.

Description

[0001] For modern vehicle construction, higher-strength steels such as dual-phase (DP) steels, complex-phase (CP) steels, TRIP steels or martensitic (MS) steels are increasingly being used.

[0002] The high strength of these steels increases driving safety. At the same time, increasingly lighter car bodies can be designed, which, due to their reduced weight and the associated savings in required drive energy, are particularly environmentally friendly.

[0003] One problem in the development of high-strength steels is that their forming properties (elongation at break) typically deteriorate with increasing strength. An example of this effect is a high-strength dual-phase steel that, at a strength of 1000 MPa, can only be expected to have an A80 elongation at break of approximately 12%. This comparatively low elongation at break can lead to material failure during component forming.

[0004] The development of high-manganese steels, i.e., steels with Mn contents exceeding 15 wt.%, therefore aimed to combine high strength with excellent formability. At a strength of 1000 MPa, this material concept offers an elongation at break (A80) of 50%. However, these material concepts are very expensive due to the high manganese content and the comparatively complex production processes.

[0005] Examples of high-strength austenitic-martensitic lightweight steels alloyed with chromium, silicon, nickel, manganese, and aluminum are known from WO 2007 / 000156 A1. These steels exhibit a tensile strength of > 800–1200 MPa and an elongation at break of > 25%. For Mn contents (in wt%) > 2.5% and < 30%, Cr contents > 0.5% and < 18%, Si contents > 1% and < 4%, and Al contents > 0.05% and < 4%, a chromium and a nickel equivalent are to be adjusted, depending on the respective contents of Cr, Mo, Si, W, Mn, N, Co, Cu, and Al, such that the specified limit pairs for both equivalents are met. Specifically, the examples that meet these requirements each exhibit high Si contents in combination with high Ni contents and varied Cr contents.

[0006] A method for producing hot-rolled strips from a formable, in particular cold-deep-drawable, lightweight structural steel, which is intended to possess high tensile strength and TRIP and / or TWIP properties, is known from WO 2005 / 061152 A1. According to this method, molten steel is cast in a horizontal strip casting plant to near-net-shape, with controlled flow and no bending, to form a pre-strip in the range of 6 to 15 mm and is then subjected to further processing.

[0007] Specifically, a horizontal strip casting process is used. The steel used contains, in addition to iron and unavoidable impurities (in wt.%), C: 0.04–1.0%, Al: 0.05–<4.0%, Si: 0.05–6.0%, Mn: 9.0–30.0%, and optionally Cr: up to 6.5%, with Cr contents of 0.2–0.3% being preferred. Nb and V may be present in total contents of up to 0.06%, and Ti and Zr in total contents of up to 0.7%. The effect of chromium is seen in its stabilization of the ε-martensite and its improvement of corrosion resistance. For this purpose, higher Cr contents are recommended at Mn contents of 9–18%, while lower Cr contents are considered sufficient at Mn contents above 18%. However, nowhere in WO 2005 / 061152 A1 is it specified how this ratio is to be specifically adjusted.

[0008] Another way to produce high-strength components is through the hot press hardening of conventional hot-forming steels. After press hardening – following prior full austenitization – these steels exhibit a martensitic microstructure, which, however, has a relatively low residual ductility.

[0009] In addition to the prior art described above, EP 0 425 058 A1 discloses the use of a killed-cast steel containing 0.15–0.25% C, 3.40–6.10% Mn, 0–1.0% Ni, 0–1.0% Cr, 0–1.0% Mo, 0–0.15% V, max. 0.03% P, max. 0.03% S, max. 0.6% Si, max. 0.05% Al, balance iron and usual impurities, as a material for manufacturing tubes for reinforcing motor vehicle doors, provided that the following relationship for the sum of the alloying elements (in wt.%) is satisfied: Mn + Ni + Cr + Mo + 10 × V ≧ 4 , 5 Gewichts − %

[0010] Against the background of the prior art explained above, the object of the invention was to create a flat steel product with good strength and good formability from a steel that can be produced more cost-effectively than the known high-manganese steels and at the same time has high elongation at break values ​​and thus significantly improved formability.

[0011] With regard to the steel, this problem has been solved according to the invention by the steel specified in claim 1.

[0012] Advantageous embodiments of the invention are specified in the dependent claims and are explained in detail below, as is the general concept of the invention.

[0013] The invention proposes a material concept according to which a steel, in addition to iron and unavoidable impurities, consists of (in wt.%) C: 0,02 - 0,5 %, Mn: 7 - 12,0 %, Yes: 0,05 - 1,0 %, Al: up to 3.0% Cr: 1 - 4,0 %, Cu: up to 2.0% Ni: up to 2.0% N: up to 0.05% P: up to 0.05% S: up to 0.01% consists of and optionally contains one or more elements from the group "V, Nb, Ti", where the sum of the contents of these elements is at most 0.5%.

[0014] The microstructure of a flat steel product produced from such a steel according to the invention typically consists of 30 - 100% hardened microstructure (martensite, tempered martensite or bainite), while the remainder of the microstructure is austenitic.

[0015] Compared to known high-manganese steels, a steel according to the invention, due to its medium-range manganese content, can be produced at significantly reduced alloying and manufacturing costs, both by continuous casting and by strip casting. In a steel according to the invention, carbon determines, on the one hand, the strength of martensite and, on the other hand, the quantity and stability of the retained austenite. Excessively high carbon contents negatively affect the weldability and toughness of the steel, for example, through the formation of chromium carbides. Ideally, the carbon content of manganese steels of the type according to the invention is therefore below 0.5 wt.%, with optimal properties being achieved when the carbon content is limited to less than 0.2 wt.%, and in particular less than 0.1 wt.%. However, excessively low carbon contents impair the quantity and stability of the remaining retained austenite.Therefore, the carbon content of a steel according to the invention is at least 0.02 wt.%, in particular at least 0.03 wt.%, for example at least 0.05 wt.%.

[0016] Manganese is an austenite former. It delays the transformation of ferrite, pearlite, and bainite, thus stabilizing austenite up to the martensite start temperature. Manganese promotes the formation of cubically or hexagonally distorted martensite (α- or ε-martensite). These manganese martensites are characterized by high strength and significantly higher toughness compared to carbon-induced, cubically distorted α-martensite. If the manganese content is too low, bainite forms upon cooling, resulting in lower strength and elongation at break. Conversely, if the manganese content is too high, there is a risk that the entire austenite will remain stable until room temperature. The manganese content of 7–12% specified according to the invention, however, enables the creation of a martensite matrix with a retained austenite component in the microstructure. This effect is particularly reliable when the Mn content is at least 7 wt.-%, whereby an optimization of the positive effects of manganese in a steel according to the invention can be achieved by limiting the upper limit of the Mn content to 10 wt.%, in particular to less than 9 wt.%, for example to up to 8.5 wt.%.

[0017] Aluminum and silicon are strong ferrite formers. Both elements counteract the influence of the austenite formers carbon and manganese. The essential function of the elements silicon (Si) and aluminum (Al) in a steel according to the invention is to suppress carbide precipitation in the martensite matrix and thus promote the stability of the retained austenite. At the same time, Si and Al lead to solid solution hardening and reduce the specific gravity of the steel. However, if the Si and Al content is too low, carbide precipitation may not be effectively suppressed. Conversely, if the Si and Al content is too high, processing becomes more difficult in both continuous casting and strip casting processes.

[0018] Therefore, the invention provides for limiting the Si content to a maximum of 1 wt.%, whereby the positive effects of the presence of Si can be effectively utilized if the Si content of the steel according to the invention is at least 0.05 wt.%, in particular 0.1 wt.%. The negative effects of Si can be particularly reliably excluded by limiting the Si content to 0.7 wt.%, in particular 0.5 wt.%.

[0019] In order to be able to safely utilize the advantageous effect of Al, the Al content can be set at a minimum of 0.01 wt. %, in particular 0.02 wt. %, while negative influences of Al can be particularly reliably excluded if the Al content of a steel according to the invention is limited to 2 wt. %, in particular 1 wt. %,.

[0020] The presence of copper, chromium, and nickel generally improves the resistance of a steel according to the invention to various corrosion mechanisms. The positive effect of Cu and Ni can be utilized particularly reliably by adding these elements to the steel according to the invention in amounts totaling at least > 0 wt.%, and in particular 0.1 wt.%. Conversely, negative effects of the presence of Cu and / or Ni in steels according to the invention are avoided by limiting the Cu and Ni content to a maximum of 1 wt.% each, or by limiting the total Cu and Ni content to a maximum of 2 wt.%, and in particular 1 wt.%.

[0021] The presence of chromium (Cr) in a steel according to the invention specifically reduces the risk of stress corrosion cracking. Cr also contributes to increased strength. These positive effects can be observed from a Cr content of 0.1 wt.%, with the positive effect of Cr being particularly reliable when the Cr content, as in the steel according to the invention, is at least 1 wt.%. The Cr content of a steel according to the invention is limited to a maximum of 4 wt.% because higher contents can lead to the formation of Cr carbides, which can negatively affect the ductility of the steel. Such negative effects can be particularly reliably prevented by limiting the Cr content to a maximum of 2 wt.%. The presence of Cr in a steel according to the invention has an optimal effect when the Cr content is 1–2 wt.%.

[0022] Ti, Nb, and V, which can be present in a total of up to 0.5 wt.% in a steel according to the invention, contribute to grain refinement and increased strength. Total contents of Ti, Nb, and V exceeding 0.5 wt.% do not increase this effect. The strength-enhancing effect of Ti, Nb, and V can be utilized particularly effectively and efficiently when the total content of these microalloying elements in a steel according to the invention is limited to 0.3 wt.%, particularly 0.2 wt.%. The positive effect of the microalloying elements mentioned here is already achieved when the total content is at least 0.025 wt.%. In the case of Ti, its content is advantageously limited to a maximum of 0.15 wt.% to prevent coarse Ti precipitates. The addition of nitrogen in contents of up to 0.05 wt.%, particularly 0.03 wt.%, further enhances this effect.-%, the austenitic microstructure can be further stabilized. This effect occurs even when the nitrogen content of a steel according to the invention is at least 0.002 wt.%, in particular at least 0.0025 wt.%, with an optimal effect being achieved when the nitrogen content is limited to a maximum of 0.025 wt.%.

[0023] The phosphorus content of a steel according to the invention is limited to a maximum of 0.05 wt.%, preferably 0.03 wt.%, in order to reliably exclude negative influences of this element.

[0024] For the same reason, the sulfur content of a steel according to the invention is limited to a maximum of 0.01 wt.%, in particular 0.005 wt.%.

[0025] In principle, the alloy concept according to the invention is designed to enable the formation of hardened microstructures with or without retained austenite in hot-rolled strip. This means that the martensite start temperature MS of a steel alloyed according to the invention is above room temperature, and the martensite finish temperature MF of a steel composed according to the invention is below room temperature.

[0026] The alloy concept according to the invention enables the creation of a hardened microstructure with up to 70% austenite. Depending on the alloy layer, the following phases can occur: Stable austenite, metastable austenite with the ability to form stress-induced martensite (TRIP effect), C- and / or Mn- distorted cubic α-martensite, hexagonal distorted ε-martensite, bainite.

[0027] A process for manufacturing a flat steel product comprises the following steps: Melting a steel melt composed according to the invention, producing a starting product for subsequent hot rolling by casting the steel melt into a strand from which at least one slab or thin slab is cut off as the starting product for hot rolling, or by casting a strip via two-roll strip casting, which is fed to the hot rolling process as the starting product, heat treating the starting product to bring it to a hot rolling start temperature of 1150–1000 °C, hot rolling the starting product into a hot strip with a thickness of at most 2.5 mm, wherein the hot rolling is terminated at a hot rolling end temperature of 1050–800 °C, coiling the hot strip into a coil at a coiling temperature ≤ 700 °C, wherein the following steps may optionally follow the coiling: annealing the hot strip at a temperature of 250–950 °C Hot strip annealing temperatureCold rolling of the annealed hot strip in one or more steps to a cold strip with a thickness of no more than 60% of the thickness of the hot strip, annealing of the cold strip at a cold strip annealing temperature of 450–950 °C, coating of the surface of the hot strip or the cold strip with a metallic corrosion protection coating, coating of the surface of the hot strip or the cold strip with an organic coating.

[0028] The steel flat product according to the invention is an uncoated hot-rolled strip. The possibilities for producing hot-rolled or cold-rolled strips made of manganese steel are summarized in the accompanying diagram. Specifically, they comprise the following processing steps: Hot strip production

[0029] Compared to high-Mn steels, the castability of Mn steels according to the invention is improved as a result of the reduction in Mn content.

[0030] One method for producing hot-rolled strip is conventional continuous casting. In this process, a steel according to the invention proves particularly advantageous because it allows for a reduced hot-rolled strip thickness of less than 2.5 mm. This is due to the fact that its forming resistance is significantly reduced compared to conventional high-manganese steels as a result of the lower manganese content.

[0031] It is also possible to produce manganese steels by strip casting. Hot-cast strip thicknesses of less than 2.0 mm are achievable. Hot strip annealing

[0032] The higher austenite content is achieved by annealing the hot-rolled strip. This reduces the strength and significantly increases the elongation at break. After hot-rolled strip annealing, up to 70% austenite content is achieved, depending on the analytical approach; this is primarily responsible for the improved elongation at break. Since a martensite matrix is ​​present in unannealed hot-rolled strip, it is difficult to process it directly into cold-rolled strip. Therefore, hot-rolled strip annealing can also serve the purpose of softening the hot-rolled strip for cold rolling. Both hood annealing and continuous annealing are suitable methods for hot-rolled strip. Cold rolling and annealing

[0033] Cold rolling of the annealed or unannealed hot-rolled strip (then with an optimized coiling temperature) further reduces the strip thickness and improves strip flatness. Subsequent annealing eliminates work hardening for component manufacturing and results in an optimal microstructure with an increased austenite content. Surface finishing

[0034] Both hot-annealed and cold-annealed strip can be finished either electrolytically, by hot-dip galvanizing (following cold-annealing), or by other strip coatings. It is also possible to apply an organic coating to the resulting steel strip. Hot forming

[0035] The desired microstructure of a steel according to the invention, typically comprising 30-100% hardened microstructure (martensite, tempered martensite or bainite) and the remainder being austenite, can be achieved by hot forming and quenching the steel.

[0036] Based on the steels according to the invention, it is therefore possible to produce extremely strong components by hot forming followed by hardening, whose residual deformation capacity is significantly improved compared to conventional high-strength steels due to the formation of hard, but comparatively tough phases. Examples of implementation Example 1

[0037] A steel melt containing, in addition to iron and unavoidable impurities (in wt.%), 0.1% C, 10% Mn, 0.4% Si, 0.008% N, 1.6% Al, and 2% Cr, was continuously cast and hot-rolled at a hot rolling temperature (ET) of 900°C to produce a hot strip, which was then coiled at a coiling temperature (HT) of 650°C. The resulting hot strip exhibited a tensile strength (Rm) of 1400 MPa and an elongation at break (A80) of 7%. The retained austenite content of its microstructure was 14%. Example 2

[0038] A steel melt containing, in addition to iron and unavoidable impurities (in wt.%), 0.1% C, 10% Mn, 0.4% Si, 0.008% N, 1.6% Al, and 1.6% Cr, was cast into a strip in a strip casting machine and hot-rolled at a hot rolling temperature (ET) of 900 °C. This strip was then coiled at a coiling temperature (HT) of 650 °C. A hood anneal was subsequently performed. The resulting strip exhibited a tensile strength (Rm) of 990 MPa and an elongation at break (A50) of 27.5%. The retained austenite of the hot-rolled strip after annealing was 60%. Example 3

[0039] A hot-rolled strip, consisting of iron and unavoidable impurities, and containing (in wt%) 0.1% C, 7% Mn, 0.13% Si, 0.02% Al, 1.5% Cr, 0.18% Ni, 0.13% Cu, 0.02% N, and 0.079% V, was subjected to hood annealing at a temperature of 650°C for 40 hours. The annealed hot-rolled strip exhibited a tensile strength Rm of 1030 MPa and an elongation at break A50 of 23%. The austenite content of its microstructure was 30%. Example 4 (not according to the invention)

[0040] A hot-rolled strip containing, in addition to iron and unavoidable impurities (in wt.%), 0.1% C, 7% Mn, 0.13% Si, 0.02% Al, 0.6% Cr, 0.18% Ni, 0.13% Cu, 0.02% N, and 0.079% V, was cold-rolled with a total deformation of 50% and subsequently annealed continuously at a temperature of 680 °C. The tensile strength Rm of the resulting cold-rolled strip was 1120 MPa with an elongation at break A50 of 21%. The austenite content of the microstructure was 30%. Example 5 (not according to the invention)

[0041] A steel melt containing, in addition to iron and unavoidable impurities (in wt.%), 0.11% C, 5% Mn, 0.39% Si, 0.008% N, 1.5% Al, and 0.6% Cr, was continuously cast and hot-rolled at a hot rolling temperature (ET) of 900 °C to produce a hot strip, which was then coiled at a coiling temperature (HT) of 650 °C. The resulting hot strip exhibited a tensile strength (Rm) of 1345 MPa and an elongation at break (A80) of 5%. The retained austenite content of its microstructure was 5.5%.

[0042] The hot-rolled strip obtained according to Example 5 was subjected to hot-roll annealing at 300 °C for a period of 10 minutes. The annealed hot-rolled strip exhibited a tensile strength Rm of 1100 MPa and an elongation at break A80 of 8%. Example 7

[0043] A hot-rolled strip, as described in Example 2, was subjected to hot-roll annealing at 300 °C for 10 minutes. The annealed hot-rolled strip exhibited a tensile strength Rm of 1300 MPa and an elongation at break A80 of 8%. Example 8

[0044] A steel melt containing, in addition to iron and unavoidable impurities (in wt.%), 0.12% C, 7% Mn, 0.11% Si, 1.6% Al, 0.3% Ni, 0.1% Cu, 0.007% N, 0.01% V, and 0.5% Cr, was cast into a strip. The cast strip exhibited a tensile strength Rm of 1380 MPa and an elongation at break A50 of 6%. The proportion of retained austenite in the microstructure of the resulting cast strip was 2%. After hood annealing, its tensile strength Rm was 1050 MPa and its elongation at break A50 was 22%. The proportion of retained austenite in the microstructure of the strip after annealing was 35%. Example 9 Example 6 (not according to the invention)

[0045] A hot-rolled strip, consisting of iron and unavoidable impurities, with (in wt%) 0.1% C, 7% Mn, 0.20% Si, 0.01% N, and 2.6% Cr, was annealed at 920 °C for three minutes, then transferred to a quenching tank within 7 seconds and quenched in water. Alternatively, quenching in oil would have yielded the same result. After quenching, its tensile strength Rm was 1450 MPa with an elongation at break A80 of 11%. The product RmxA80 was therefore approximately 16,000 MPa x%. The microstructure of the hot-rolled strip obtained in this way consisted of cubically distorted α-martensite and small volume fractions of approximately 5% each of austenite and hexagonally distorted ε-martensitanium. Example 10 (not according to the invention)

[0046] Hot-rolled strip containing, in addition to iron and unavoidable impurities (in wt.%), 0.1% C, 7% Mn, 0.13% Si, 0.02% Al, 1.5% Cr, 0.18% Ni, 0.13% Cu, 0.002% N, and 0.08% V, was cold-rolled and subsequently hot-dip galvanized. The galvanized cold-rolled strip exhibited a tensile strength Rm of 1300 MPa and an elongation at break A50 of 15%. The retained austenite content of the resulting cast strip was 20%. Example 11 (not according to the invention)

[0047] A hot-rolled strip containing, in addition to iron and unavoidable impurities (in wt.%), 0.08% C, 8% Mn, 0.15% Si, 0.02% Al, 1% Cr, 0.2% Ni, 0.15% Cu, 0.015% N, and 0.06% V, was cold-rolled and subsequently subjected to a hood annealing process at an annealing temperature of 550 °C. After hood annealing, its tensile strength Rm was 1080 MPa and its elongation at break A50 was 25%. The proportion of retained austenite in the microstructure of the cast strip after annealing was 30%. Example 12 (not according to the invention)

[0048] A steel sheet containing, in addition to iron and unavoidable impurities (in wt%), 0.05% C, 0.06% Si, 1.1% Cr, 0.01% N, and 10% Mn, was heated to 920 °C within three minutes. The sheet was then transferred within 7 seconds to quenching tanks, where it was quenched in either oil or water. The oil-quenched steel exhibited a tensile strength Rm of 1390 MPa at an elongation at break A80 of 12%. The product Rm*A was therefore 16680 MPa%. The water-quenched steel exhibited a tensile strength Rm of 1350 MPa at an elongation at break A80 of 12%. The product Rm*A for the water-quenched steel was therefore 16200 MPa%. After oil or water quenching, the microstructure of the steel consisted of cubically distorted α-martensite and small volume contents of tough austenite (approx. 4%) and hexagonally distorted ε-martensite (approx. 6%). Example 13 (not according to the invention)

[0049] A steel sheet containing, in addition to iron and unavoidable impurities (in wt%), 0.05% C, 10% Mn, 0.06% Si, 0.009% N, 1.1% Cr, and 1% Ni, was heated to 920 °C within three minutes. The sheet was then transferred within 7 seconds to quenching tanks, where it was quenched in either oil or water. The steel quenched in oil exhibited a tensile strength Rm of 1315 MPa and an elongation at break A80 of 12.1%. The product Rm*A was therefore 15910 MPa%. The steel quenched in water exhibited a tensile strength Rm of 1285 MPa and an elongation at break A80 of 12.3%. The product Rm*A for the water-quenched steel was therefore 15810 MPa%. After oil or water quenching, the microstructure of the steel consisted of cubically distorted α-martensite and small volume contents of tough austenite (approx. 7%) and hexagonally distorted ε-martensite (approx. 5%). Example 14 (not according to the invention)

[0050] A steel sheet containing, in addition to iron and unavoidable impurities (in wt%), 0.1% C, 10% Mn, 0.06% Si, 0.009% N, 1.1% Cr, and 1.5% Al, was heated to 920 °C within three minutes. The sheet was then transferred within 7 seconds to quenching tanks, where it was quenched in either oil or water. The steel quenched in oil exhibited a tensile strength Rm of 1350 MPa and an elongation at break A80 of 10.8%. The product Rm*A was therefore 14580 MPa%. The steel quenched in water exhibited a tensile strength Rm of 1350 MPa and an elongation at break A80 of 10.6%. For the water-quenched steel, the product Rm*A was therefore 14310 MPa%. After oil or water quenching, the microstructure of the steel consisted of cubically distorted α-martensite and small volume contents of tough austenite (approx. 12%).

[0051] Overall, the inventive method achieves an improved combination of component strength and residual deformation capacity compared to the prior art for hot-formed high-strength materials, which is characterized by high values ​​of the product of tensile strength and respective elongation at break.

Claims

1. Flat steel product having a thickness of a maximum of 2.5 mm and an elongation at break A80 which is at least 4% and a tensile strength which is from Rm of 900 to 1500 MPa and which in addition to iron and inevitable impurities consists of (in % bei weight) C: 0.02 - 0.5%, Mn: 7 - 12.0%, Si: 0.05 - 1.0%, Al: up to 3.0%, Cr: 1 - 4.0%, Cu: up to 2.0%, Ni: up to 2.0%, N: up to 0.05%, P: up to 0.05%, S: up to 0.01 %, and optionally one or more elements from the group "V, Nb, Ti", wherein the sum of the contents of these elements is at a maximum equal to 0.5%, wherein the structure of the steel consists of 30 - 100% of hardening structure (martensite, tempered martensite or bainite), whilst the remainder of the structure is austenitic, wherein the flat steel product is an uncoated hot strip.

2. Flat steel product according to claim 1, characterized in that its C content is at least 0.03 % by weight.

3. Flat steel product according to any one of the preceding claims, characterized in that the Mn content thereof is a maximum of 10 % by weight.

4. Flat steel product according to any one of the preceding claims, characterized in that the Mn content thereof is less than 9.5 % by weight.

5. Flat steel product according to any one of the preceding claims, characterized in that the Si content thereof is a maximum of 0.5 % by weight.

6. Flat steel product according to any one of the preceding claims, characterized in that the Al content thereof is a maximum of 2 % by weight.

7. Flat steel product according to any one of the preceding claims, characterized in that the Cr content thereof is at least 0.5 % by weight.

8. Flat steel product according to any one of the preceding claims, characterized in that the Cr content thereof is a maximum of 3 % by weight.

9. Flat steel product according to any one of the preceding claims, characterized in that the Cr content thereof is a maximum of 2 % by weight.

10. Flat steel product according to any one of the preceding claims, characterized in that the Cu content thereof is a maximum of 1 % by weight.

11. Flat steel product according to any one of the preceding claims, characterized in that the Ni content thereof is a maximum of 1 % by weight.

12. Flat steel product according to any one of the preceding claims, characterized in that the N content thereof is at least 0.0025 % by weight.

13. Flat steel product according to any one of the preceding claims, characterized in that the N content thereof is a maximum of 0.03 % by weight.

14. Flat steel product according to any one of the preceding claims, characterized in that the sum of the contents of the optionally present elements from the group "V, Nb, Ti" is at a maximum equal to 0.3 % by weight.

15. Flat steel product according to any one of the preceding claims, characterized in that the optionally present content of Ti is at a maximum equal to 0.15 % by weight.

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