EP3512968B1 - Method for producing a flat steel product made of a manganese-containing steel, and such a flat steel product - Google Patents

Description

The invention relates to a method for producing a flat steel product from a medium-manganese steel with a TRIP / TWIP effect, and a use for a flat steel product produced by this method.

From the European patent application EP 2 383 353 A2 a flat steel product made from a manganese-containing steel is known which has a tensile strength of 900 to 1500 MPa and consists of the following elements (contents in percent by weight and based on the steel melt): C: up to 0.5; Mn: 4 to 12.0; Si: up to 1.0; AI: up to 3.0; Cr: 0.1 to 4.0; Cu: up to 4.0; Ni: up to 2.0; N: up to 0.05; P: up to 0.05; S: up to 0.01 plus the remainder iron and unavoidable impurities. Optionally, one or more elements from the group “V, Nb, Ti” are provided, the sum of the contents of these elements being at most equal to 0.5. This steel is said to be characterized by the fact that it is more cost-effective to manufacture than steels with a high manganese content and at the same time has high elongation at break and, as a result, significantly improved formability.

Also are in the German patent application DE 10 2012 013 113 A1 So-called TRIP steels have already been described, which have a predominantly ferritic basic structure with embedded retained austenite, which can convert to martensite during forming (TRIP effect). Because of its strong work hardening, TRIP steel achieves high values of uniform elongation and tensile strength. TRIP steels are suitable for use in structural, chassis and crash-relevant components of vehicles, as sheet metal blanks and as welded blanks.

the German patent application DE 10 2015 111 866 A1 discloses a formable lightweight steel with a manganese content of 3 to 30% by weight and TRIP / TWIP properties, which is obtained by adding up to 0.8% by weight of antimony (Sb) and targeted heat treatment at 480 to 770 ° C exhibits improved material properties for 1 minute to 48 hours. In particular, this steel has, in addition to improved tensile strength and elongation at break, increased resistance to hydrogen-induced cracking and hydrogen embrittlement.

From the German patent application DE 10 2005 052 774 A1 a method for producing hot strips with TRIP and / or TWIP properties and high tensile strengths is known. The lightweight steel, consisting of the main elements Fe, Mn, Si and Al, is cast under protective gas to form a pre-strip close to its final dimensions, which then passes through a homogenization zone. This is followed by hot rolling until the specified overall degree of deformation of greater than 70% is reached. The hot strip is then annealed in a recrystallizing manner before cold forming. The finished hot strip is then cooled and cold-rolled several times, with intermediate anneals being carried out between the individual cold-rolling processes if necessary.

Furthermore, from the German patent DE 10 2004 054 444 B3 a method for producing metal components or semi-finished products with high strength and plasticity by cold forming of steels is known. Their cold forming should lead to solidification through TWIP (Twinning Induced Plasticity) or SIP (Shearband Induced Plasticity) effects. Here, the degrees of deformation are in the range of 10 to 70% for a total elongation. After a final or crystallization annealing, the deformation takes place until an increase in strength of at least 30% of the initial value is achieved and the remaining tensile elongation of the metal does not drop below 20%. This forming process with high elongation should have the advantage that, despite the high strength values, a plasticity reserve is retained, which enables subsequent final shaping into a finished component using conventional forming technology. The steels selected for this are characterized by an Mn content in% by weight of 10 to 30. Such high-manganese alloyed steels are more costly than medium-manganese steels due to their high alloying element content.

It is also from the international published application WO 2016/067626 A1 a method for producing a high-strength flat steel product from a steel with a manganese content between 4.20 and 6% by weight and with a TRIP effect is already known. In the course of the process, a hot strip with a degree of rolling of more than 30% is cold-rolled into a cold strip and then annealed for 900 to 21,600 s at a temperature between the Ac1 transformation temperature and the Ac1 transformation temperature + 100 ° C. The annealed cold strip can with a The degree of rolling between 0.1 and 2.0% can be re-rolled.

Furthermore, in the Japanese patent application JP 2015 175051 A Another method for producing a high-strength flat steel sheet from a steel with a manganese content between 1.5 and 4.0% by weight has already been described. Here, a produced hot strip with a degree of rolling of more than 30% is cold-rolled into a cold strip and then annealed for 3 to 200 s at a temperature between the Ac3 temperature -60 ° C and 900 ° C. The annealed cold strip is hot rolled with a degree of rolling between 0.1 and 0.8%.

Proceeding from this, the present invention is based on the object of creating a method for producing a flat steel product from a steel with medium manganese content, and a use for a flat steel product produced by this method, which is achieved by improving the yield point while maintaining a sufficient residual deformability of the flat steel product produced distinguish.

This object is achieved by a method for producing a flat steel product from a medium-manganese steel with TRIP / TWIP effect having the features of claim 1 and a use for a flat steel product produced by this method according to claim 11. Advantageous refinements of the invention are specified in the subclaims.

According to the invention, a method for producing a flat steel product from a medium-manganese steel with a manganese content of 4 to 12, preferably greater than 5 to less than 10, wt .-% and with TRIP / TWIP effect, comprising the steps: Cold strip, - Annealing the cold-rolled hot or cold strip at 500 to 840 ° C for 1 min. To 24 h, - Rerolling or skin-passing the annealed hot or cold strip to a flat steel product with a degree of deformation between 0.3% and 60% and with a yield point that is at least 50 MPa higher than that achieved before re-rolling or skin-passing, so that the re-rolling or skin-passing of the flat steel product increases its yield point. The degree of deformation is usually related to the thickness direction of the flat steel product. By increasing the yield point, this flat steel product can be converted into optimized components with lower Sheet thickness can be produced. The rerolling or skin passage causes a partial conversion of the metastable austenite of the annealed hot or cold strip into deformation twins (TWIP effect) and martensite (TRIP effect), whereby at least 3% of the austenite has to be converted into martensite and at least a proportion of 10% of the austenite is retained as a face-centered cubic phase.

With regard to the re-rolling, it is preferably provided that the annealed hot or cold strip is re-rolled with a degree of deformation between 10 to 40%.

With regard to skin passaging, it is preferably provided that the annealed hot or cold strip is passaged with a degree of deformation between 0.6 and 2.2%.

It is preferably provided that the annealed hot or cold strip is re-rolled or skin-pass at a temperature of 0 to 400 ° C. As a result, deformation twins are formed (TWIP effect) which, analogous to the dislocation density of other types of steel, increase the yield strength and / or yield strength.
It is particularly preferably provided that the flat steel product has a tensile strength of greater than 1300 MPa and an elongation at break A80 of greater than 3%.

In an advantageous embodiment of the method, the hot or cold strip is cold-rolled with a first rolling pass at a temperature of the hot or cold strip from 60 ° C to below Ac3, preferably from 60 ° C to 450 ° C. Optionally, the hot or cold strip is then intermediately heated or intermediately cooled between the further rolling passes following the first rolling pass to temperatures from 60 ° C to below Ac3, preferably from 60 ° C to 450 ° C. With the increase in temperature before the first rolling pass, there is also a reduction in the required forming forces. An increase in the residual deformability of the cold-rolled hot or cold strip with tensile strengths of greater than 800 MPa to 2000 MPa with elongations at break of greater than 3% is brought about in the most heavily formed areas. The hot or cold strip can be preheated for a coil or unwound strip or sheet material. Cold rolling with preheating of the hot or cold strip prior to the first forming step converts metastable austenite into martensite (TRIP effect) during the rolling process. completely or partially suppressed, whereby deformation twins (TWIP effect) can form in the austenite. This results in an advantageous reduction in the rolling forces and increases the overall formability. Through the further rolling passes at elevated temperatures, deformation twins are specifically introduced, which further convert to martensite at room temperature and thus increase the energy absorption capacity and allow a higher degree of deformation.

The flat steel product mentioned is to be understood as meaning cold re-rolled heavy plate, hot and / or cold strip.

• C: 0,0005 bis 0,9, vorzugsweise 0,05 bis 0,35
• Mn: 4 bis 12, vorzugsweise größer 5 bis kleiner 10
• Rest Eisen einschließlich unvermeidbarer stahlbegleitender Elemente, mit optionaler Zulegierung von:
  • AI: 0 bis 10, bevorzugt 0,05 bis 5, insbesondere bevorzugt größer 0,5 bis 3
  • Si: 0 bis 6, bevorzugt 0,05 bis 3, insbesondere bevorzugt 0,1 bis 1,5
  • Cr: 0 bis 6, bevorzugt 0,1 bis 4, insbesondere bevorzugt größer 0,5 bis 2,5
  • Nb: 0 bis 1, bevorzugt 0,005 bis 0,4, insbesondere bevorzugt 0,01 bis 0,1
  • V: 0 bis 1,5, bevorzugt 0,005 bis 0,6, insbesondere bevorzugt 0,01 bis 0,3
  • Ti: 0 bis 1,5, bevorzugt 0,005 bis 0,6, insbesondere bevorzugt 0,01 bis 0,3
  • Mo: 0 bis 3, bevorzugt 0,005 bis 1,5, insbesondere bevorzugt 0,01 bis 0,6
  • Sn: 0 bis 0,5, bevorzugt kleiner 0,2, insbesondere bevorzugt kleiner 0,05
  • Cu: 0 bis 3, bevorzugt kleiner 0,5, insbesondere bevorzugt kleiner 0,1
  • W: 0 bis 5, bevorzugt 0,01 bis 3, insbesondere bevorzugt 0,2 bis 1,5
  • Co: 0 bis 8, bevorzugt 0,01 bis 5, insbesondere bevorzugt 0,3 bis 2
  • Zr: 0 bis 0,5, bevorzugt 0,005 bis 0,3, insbesondere bevorzugt 0,01 bis 0,2
  • Ta: 0 bis 0,5, bevorzugt 0,005 bis 0,3, insbesondere bevorzugt 0,01 bis 0,1
  • Te: 0 bis 0,5, bevorzugt 0,005 bis 0,3, insbesondere bevorzugt 0,01 bis 0,1
  • B: 0 bis 0,15, bevorzugt 0,001 bis 0,08, insbesondere bevorzugt 0,002 bis 0,01
  • P: kleiner 0,1, bevorzugt kleiner 0,04
  • S: kleiner 0,1, bevorzugt kleiner 0,02
  • N: kleiner 0,1, bevorzugt kleiner 0,05.

It is particularly preferred that the flat steel product is produced with the following chemical composition (in% by weight) in order to achieve the advantages described in particular:

• C: 0.0005 to 0.9, preferably 0.05 to 0.35
• Mn: 4 to 12, preferably greater than 5 to less than 10
• Remaining iron including unavoidable steel-accompanying elements, with optional addition of:
  • AI: 0 to 10, preferably 0.05 to 5, particularly preferably greater than 0.5 to 3
  • Si: 0 to 6, preferably 0.05 to 3, particularly preferably 0.1 to 1.5
  • Cr: 0 to 6, preferably 0.1 to 4, particularly preferably greater than 0.5 to 2.5
  • Nb: 0 to 1, preferably 0.005 to 0.4, particularly preferably 0.01 to 0.1
  • V: 0 to 1.5, preferably 0.005 to 0.6, particularly preferably 0.01 to 0.3
  • Ti: 0 to 1.5, preferably 0.005 to 0.6, particularly preferably 0.01 to 0.3
  • Mo: 0 to 3, preferably 0.005 to 1.5, particularly preferably 0.01 to 0.6
  • Sn: 0 to 0.5, preferably less than 0.2, particularly preferably less than 0.05
  • Cu: 0 to 3, preferably less than 0.5, particularly preferably less than 0.1
  • W: 0 to 5, preferably 0.01 to 3, particularly preferably 0.2 to 1.5
  • Co: 0 to 8, preferably 0.01 to 5, particularly preferably 0.3 to 2
  • Zr: 0 to 0.5, preferably 0.005 to 0.3, particularly preferably 0.01 to 0.2
  • Ta: 0 to 0.5, preferably 0.005 to 0.3, particularly preferably 0.01 to 0.1
  • Te: 0 to 0.5, preferably 0.005 to 0.3, particularly preferably 0.01 to 0.1
  • B: 0 to 0.15, preferably 0.001 to 0.08, particularly preferably 0.002 to 0.01
  • P: less than 0.1, preferably less than 0.04
  • S: less than 0.1, preferably less than 0.02
  • N: less than 0.1, preferably less than 0.05.

This flat steel product made from the medium-manganese-containing TRIP (TRansformation Induced Plasticity) and / or TWIP (TWinning Induced Plasticity) steel has excellent cold and warm formability, increased resistance to hydrogen-induced delayed fracture, and hydrogen embrittlement. as well as against liquid metal embrittlement when welding in the galvanized state.

• Erschmelzen einer Stahlschmelze mit der vorstehend beschriebenen chemischen Zusammensetzung in einem über die Prozessroute Hochofen-Stahlwerk oder Elektrolichtbogenofen-Stahlwerk mit optionaler Vakuumbehandlung der Schmelze;
• Vergießen der Stahlschmelze zu einem Vorband mittels eines endabmessungsnahen horizontalen oder vertikalen Bandgießverfahrens oder Vergießen der Stahlschmelze zu einer Bramme oder Dünnbramme mittels eines horizontalen oder vertikalen Brammen- oder Dünnbrammengießverfahrens,
• Erwärmen des Vorbandes auf eine Walztemperatur von 1050 bis 1250°C oder Inlinewalzen aus der Gießhitze (erste Hitze) heraus,
• Warmwalzen des Vorbandes oder der Bramme oder der Dünnbramme zu einem Warmband mit einer Dicke von 20 bis 0,8 mm mit einer Walzendtemperatur von 1050 bis 800°C,
• Aufhaspeln des Warmbandes bei einer Temperatur von mehr als 100 bis 800°C,
• Beizen des Warmbandes,
• Glühen des Warmbandes in einer Durchlauf- oder Haubenglühanlage beziehungsweise in einer kontinuierlichen oder diskontinuierlichen Glühanlage bei einer Glühzeit von 1 min. bis 24 h und Temperaturen von 500 °C bis 840°C,
• Kaltwalzen des Warmbandes bei Raumtemperatur, bevorzugt mit einem Vorwärmen auf 60 °C bis unterhalb Ac3-Temperatur, bevorzugt 60 °C bis 450 °C vor dem ersten Walzstich zur Verringerung der Walzkräfte und Bildung von Verformungszwillingen im Austenit und bedarfsweisem Kühlen oder Erwärmen zwischen den Walzstichen auf 60 °C bis unterhalb der Ac3-Temperatur, bevorzugt 60 °C bis 450 °C,
• Glühen des kaltgewalzten Warm- oder Kaltbands bei 500 bis 840 °C für 1 min bis 24 h über Durchlauf- oder Haubenglühung,
• Nachwalzen oder Dressieren des geglühten Warm- oder Kaltbands zur Erhöhung der Streckgrenze mit glatten oder texturierten Walzen (beispielsweise mit PretexTexturieru ng),
• optionales elektrolytisches Verzinken oder Feuerverzinken des Stahlbandes oder Aufbringen einer anderweitigen organischen oder anorganischen Beschichtung,
• optionales Glühen bei 500 bis 840 °C für 1 min bis 24 h in einer Durchlaufglühanlage, Haubenglühanlage oder sonstigen kontinuierlichen oder diskontinuierlichen Glühanlagen.

The flat steel product described above is produced in the usual way with a production route listed below:

• Melting a steel melt with the chemical composition described above in a blast furnace steelworks or electric arc furnace steelworks via the process route with optional vacuum treatment of the melt;
• Pouring the steel melt into a pre-strip by means of a near-net-shape horizontal or vertical strip casting process or casting the steel melt into a slab or thin slab by means of a horizontal or vertical slab or thin slab casting process,
• Heating of the pre-strip to a rolling temperature of 1050 to 1250 ° C or inline rolling from the casting heat (first heat),
• Hot rolling of the pre-strip or the slab or the thin slab to form a hot strip with a thickness of 20 to 0.8 mm with a final rolling temperature of 1050 to 800 ° C,
• Coiling of the hot strip at a temperature of more than 100 to 800 ° C,
• Pickling of the hot strip,
• Annealing the hot strip in a continuous or hood annealing plant or in a continuous or discontinuous annealing plant with an annealing time of 1 min. To 24 hours and temperatures of 500 ° C to 840 ° C,
• Cold rolling of the hot strip at room temperature, preferably with preheating to 60 ° C to below Ac3 temperature, preferably 60 ° C to 450 ° C before the first rolling pass to reduce the rolling forces and formation of deformation twins in the austenite and, if necessary, cooling or heating between the rolling passes to 60 ° C to below the Ac3 temperature, preferably 60 ° C to 450 ° C,
• Annealing the cold-rolled hot or cold strip at 500 to 840 ° C for 1 min to 24 hours using continuous or hood annealing,
• Rerolling or skin-passing of the annealed hot or cold strip to increase it the yield point with smooth or textured rollers (e.g. with pretex texturing),
• optional electrolytic galvanizing or hot-dip galvanizing of the steel strip or application of another organic or inorganic coating,
• optional annealing at 500 to 840 ° C for 1 min to 24 h in a continuous annealing system, hood annealing system or other continuous or discontinuous annealing systems.

The usual thickness ranges for pre-strip are 1 mm to 35 mm and for slabs and thin slabs 35 mm to 450 mm. It is preferably provided that the slab or thin slab is hot rolled into a hot strip with a thickness of 20 mm to 0.8 mm or the pre-strip cast close to its final dimensions is hot rolled into a hot strip with a thickness of 8 mm to 0.8 mm. The cold strip has a thickness of usually less than 3 mm, preferably 0.1 to 1.4 mm.

In connection with the above method according to the invention, a pre-strip produced near net dimensions using the two-roll casting method with a thickness of less than or equal to 3 mm, preferably 1 mm to 3 mm, is already understood as hot strip. The pre-strip produced in this way as hot strip does not have a cast structure due to the reshaping of the two counter-rotating rolls. Hot rolling thus already takes place inline during the two-roll casting process, so that separate heating and hot rolling can optionally be omitted.

The cold rolling of the hot strip can take place at room temperature or advantageously at an elevated temperature with heating before the first rolling pass and / or heating in a further or between several rolling passes. Cold rolling at elevated temperatures is advantageous in order to reduce the rolling forces and to promote the formation of deformation twins (TWIP effect). Advantageous temperatures of the rolling stock before the first rolling pass are 60 ° C to below the Ac3 temperature, preferably 60 to 450 ° C.

If the cold rolling takes place in several rolling passes, it is advantageous to temporarily heat or cool the steel strip between the rolling passes to a temperature of 60 ° C to below Ac3 temperature, preferably 60 ° C to 450 ° C, since the TWIP effect in this area particularly advantageous for Carry comes. Depending on the rolling speed and degree of deformation, both intermediate heating, e.g. at very low degrees of deformation and rolling speeds, as well as additional cooling, due to the heating of the material during fast rolling and high degrees of deformation, can be carried out.

After cold rolling the hot strip at room temperature, the steel strip should be annealed in a continuous annealing plant, hood annealing plant or other continuous or discontinuous annealing plant with an annealing time of 1 min. To 24 h and temperatures of 500 to 840 ° C to restore sufficient forming properties. If necessary to achieve certain material properties, this annealing process can also be carried out with the steel strip rolled at an elevated temperature.

After the annealing treatment, the steel strip is advantageously cooled to a temperature of 250 ° C to room temperature and then, if necessary, to set the required mechanical properties, in the course of an aging treatment, heated again to a temperature of 300 to 450 ° C at this temperature held for up to 5 minutes and then cooled to room temperature. The aging treatment can advantageously be carried out in a continuous annealing plant.

The flat steel product produced in this way can optionally be electrolytically galvanized or hot-dip galvanized. In an advantageous development, the steel strip produced in this way receives a coating on an organic or inorganic basis instead of or after electrolytic galvanizing or hot-dip galvanizing. These can be, for example, organic coatings, plastic coatings or lacquers or other inorganic coatings such as iron oxide layers.

According to the invention, a component manufactured according to the above-described method can advantageously be used in motor vehicle construction, rail vehicle construction, shipbuilding, plant construction, infrastructure construction, in aerospace, domestic appliance technology and in welded blanks (tailored welded blanks).

A flat steel product produced by the process according to the invention advantageously has a yield strength Rp0.2 of 300 to 1350 MPa, a tensile strength Rm of 1100 to 2200 MPa and an elongation at break A80 of more than 4 to 41%, with high strengths tending to be associated with lower elongations at break and vice versa: - Rm from over 1100 to 1200 MPa: Rm x A80 ≥ 25,000 up to 45,000 - Rm from over 1200 to 1400 MPa: Rm x A80 ≥ 20,000 up to 42,000 - Rm from over 1400 to 1800 MPa: Rm x A80 ≥ 10,000 up to 40,000 - Rm of over 1800 MPa: Rm x A80 ≥ 7200 up to 20000

For the elongation at break investigations, specimen form 2 with an initial gauge length of A80 was used in accordance with DIN 50 125.
The use of the term “to” in the definition of the content ranges, such as 0.01 to 1% by weight, means that the benchmarks - in the example 0.01 and 1 - are included.

Alloy elements are usually added to steel in order to specifically influence certain properties. An alloy element can influence different properties in different steels. The effect and interaction generally depends heavily on the amount, the presence of other alloying elements and the state of solution in the material. The relationships are varied and complex. In the following, the effect of the alloying elements in the alloy according to the invention will be discussed in more detail. The positive effects of the alloying elements used according to the invention are described below.

Carbon C: Is required for the formation of carbides, stabilizes the austenite and increases the strength. Higher contents of C worsen the welding properties and lead to a deterioration in the elongation and toughness properties, which is why a maximum content of 0.9% by weight, preferably 0.35% by weight, is specified. In order to achieve the desired combination of strength and elongation properties of the material, a minimum addition of 0.0005% by weight, preferably 0.05% by weight, is required.

Manganese Mn: Stabilizes austenite, increases strength and toughness and enables deformation-induced martensite and / or twin formation in the alloy according to the invention. Contents of less than 4% by weight are not sufficient to stabilize the austenite and thus worsen the elongation properties, while with contents of 12% by weight and more the austenite is too strongly stabilized and thus the strength properties, in particular the 0.2% yield strength, be reduced. For the manganese steel according to the invention with medium manganese contents, a range from greater than 5 to less than 10% by weight is preferred.

Aluminum Al: Al improves the strength and elongation properties, lowers the specific density and influences the transformation behavior of the alloy according to the invention. Too high a content of Al worsens the elongation properties. Higher Al contents also significantly worsen the casting behavior in continuous casting. This results in a higher effort when potting. High Al contents delay the precipitation of carbides in the alloy according to the invention. An Al content of 0 to 10% by weight, preferably 0.05 to 5% by weight, particularly preferably greater than 0.5 to 3% by weight, is therefore specified.

Silicon Si: The optional addition of Si in higher contents hinders the carbon diffusion, reduces the specific density and increases the strength and the elongation and toughness properties. Furthermore, an improvement in cold rollability could be observed through the addition of Si. Higher Si contents lead to embrittlement of the material and have a negative effect on hot and cold rollability and coatability, for example through galvanizing. An Si content of 0 to 6% by weight, preferably 0.05 to 3% by weight, particularly preferably 0.1 to 1.5% by weight, is therefore specified.

Chromium Cr: The optional addition of Cr improves the strength and reduces the corrosion rate, delays the formation of ferrite and pearlite and forms carbides. Higher contents lead to a deterioration in the elongation properties. A Cr content of 0 to 6% by weight, preferably 0.1 to 4% by weight, particularly preferably greater than 0.5 to 2.5% by weight, is therefore specified.

Micro-alloy elements are usually only used in very small quantities admitted. In contrast to the alloying elements, they work mainly through the formation of precipitates, but can also influence the properties in a dissolved state. Even small amounts of the micro-alloying elements have a considerable influence on the processing and final properties. In hot forming in particular, micro-alloy elements have an advantageous effect on the recrystallization behavior and cause grain refinement.

Typical micro-alloy elements are vanadium, niobium and titanium. These elements can be dissolved in the iron lattice and form carbides, nitrides and carbonitrides with carbon and nitrogen.

Vanadium V and niobium Nb: These have a grain-refining effect due to the formation of carbides, which at the same time improves strength, toughness and elongation properties. Contents of more than 1.5% by weight or 1% by weight have no further advantages. For vanadium and niobium, a minimum content of 0.005% by weight and a maximum content of 0.6% by weight or 0.4% by weight, particularly preferably a minimum content of 0.01% by weight and a maximum content, are optionally preferred of 0.3% by weight and 0.1% by weight, respectively, are provided.

Titanium Ti: Has a grain-refining effect as a carbide former, which at the same time improves strength, toughness and elongation properties and reduces intergranular corrosion. Contents of Ti of more than 1.5% by weight deteriorate the elongation properties, which is why a maximum content of 1.5% by weight, preferably 0.6% by weight, particularly preferably 0.3% by weight, is optionally specified . Minimum contents of 0.005% by weight, preferably 0.01% by weight, can be provided in order to bind nitrogen and advantageously precipitate Ti.

Molybdenum Mo: Acts as a carbide former, increases the strength and increases the resistance against delayed crack formation and hydrogen embrittlement. High contents of Mo impair the elongation properties. An Mo content of 0 to 3% by weight, preferably 0.005 to 1.5% by weight, particularly preferably greater than 0.01 to 0.6% by weight, is therefore optionally specified.

Tin Sn: Tin increases the strength, but, like copper, accumulates under the scale and at the grain boundaries at higher temperatures. Penetrating into the grain boundaries leads to the formation of low-melting phases and the associated cracks in the structure and solder brittleness, which is why an optional maximum content of 0.5% by weight, preferably less than 0.2% by weight, particularly preferably less 0.05 wt%, is provided.

Copper Cu: Reduces the rate of corrosion and increases strength. Contents above 3 wt .-% worsen the manufacturability by the formation of low-melting phases during casting and hot rolling, which is why optionally a maximum content of 3 wt .-%, preferably less than 0.5 wt .-%, particularly preferably less than 0.1 wt. -%, is determined.

Tungsten W: Acts as a carbide former and increases strength and heat resistance. W contents of more than 5% by weight impair the elongation properties, which is why a maximum content of 5% by weight is optionally specified. A content of 0.01% by weight to 3% by weight is preferably provided, and particularly preferably 0.2 to 1.5% by weight.

Cobalt Co: Increases the strength of the steel, stabilizes the austenite and improves the high temperature strength. Contents of more than 8% by weight impair the elongation properties. The Co content is therefore set at a maximum of 8% by weight, preferably from 0.01 to 5% by weight, particularly preferably from 0.3 to 2% by weight.

Zirconium Zr: Acts as a carbide former and improves strength. Zr contents of more than 0.5% by weight deteriorate the elongation properties. A Zr content of 0 to 0.5% by weight, preferably 0.005 to 0.3% by weight, particularly preferably 0.01 to 0.2% by weight, is therefore specified.

Tantalum Ta: Like niobium, tantalum has a grain-refining effect as a carbide former and thereby improves strength, toughness and elongation properties at the same time. Contents of more than 0.5% by weight do not bring about any further improvement in the properties. A maximum content of 0.5% by weight is therefore optionally specified. A minimum content of 0.005 and a maximum content of 0.3% by weight are preferably specified, in which the grain refinement can advantageously be effected. To improve the A content of 0.01% by weight to 0.1% by weight is particularly preferred for economy and optimization of the grain refinement.

Tellurium Te: Tellurium improves the corrosion resistance and the mechanical properties as well as the machinability. Furthermore, Te increases the strength of manganese sulfides (MnS), which is less elongated in the rolling direction during hot and cold rolling. Contents above 0.5% by weight impair the elongation and toughness properties, which is why a maximum content of 0.5% by weight is specified. Optionally, a minimum content of 0.005% by weight and a maximum content of 0.3% by weight are specified, which advantageously improve the mechanical properties and increase the strength of the MnS present. Furthermore, a minimum content of 0.01% by weight and a maximum content of 0.1% by weight are preferred, which enable the mechanical properties to be optimized with a simultaneous reduction in alloy costs.

Boron B: Boron retards the austenite transformation, improves the hot forming properties of steels and increases the strength at room temperature. It develops its effect even with very low alloy contents. Contents above 0.15% by weight greatly impair the elongation and toughness properties, which is why the maximum content is set at 0.15% by weight. Optionally, a minimum content of 0.001% by weight and a maximum content of 0.08, preferably a minimum content of 0.002% by weight and a maximum content of 0.01, are specified in order to advantageously use the strength-increasing effect of boron.

Phosphorus P: is a trace element, comes mainly from iron ore and is dissolved in the iron lattice as a substitution atom. Phosphorus increases hardness through solid solution strengthening and improves hardenability. As a rule, however, the aim is to lower the phosphorus content as much as possible, since, among other things, due to its low diffusion rate, it is very susceptible to segregation and to a great extent reduces the toughness. The accumulation of phosphorus at the grain boundaries can cause cracks to appear along the grain boundaries during hot rolling. In addition, phosphorus increases the transition temperature from tough to brittle behavior by up to 300 ° C. For the aforementioned reasons, the phosphorus content is less than 0.1% by weight, preferably less than 0.04% by weight, limited.

Sulfur S: Like phosphorus, it is bound in coke as a trace element in iron ore but especially during the production route via the blast furnace process. It is generally undesirable in steel because it tends to segregate strongly and has a strong embrittling effect, as a result of which the elongation and toughness properties are impaired. Attempts are therefore made to achieve the lowest possible amounts of sulfur in the melt (for example by means of deep desulphurisation). For the reasons mentioned above, the sulfur content is limited to values less than 0.1% by weight, preferably less than 0.02% by weight.

Nitrogen N: N is also an accompanying element in steel production. In the dissolved state, it improves the strength and toughness properties of steels with a higher manganese content with greater than or equal to 4% by weight Mn. Lower Mn-alloyed steels with less than 4% by weight tend to have a strong aging effect in the presence of free nitrogen. The nitrogen diffuses at dislocations even at low temperatures and blocks them. It thus causes an increase in strength combined with a rapid loss of toughness. The nitrogen can be set in the form of nitrides, for example, by adding titanium or aluminum to the alloy, with aluminum nitrides in particular having a negative effect on the forming properties of the alloy according to the invention. For the reasons mentioned above, the nitrogen content is limited to less than 0.1% by weight, preferably less than 0.05% by weight.

Claims (11)

1. Method for producing a flat steel product from a medium manganese steel having a manganese content of 4 to 12, preferably more than 5 to less than 10 wt.% and with a TRIP/TWIP effect, comprising the steps of:

   - cold-rolling a hot or cold strip,

   - annealing the cold-rolled hot or cold strip at 500 to 840°C for 1 min. to 24 h,

   - rerolling or skin-pass rolling the annealed hot or cold strip to form a flat steel product with a degree of deformation between 0.3% and 60% and with a yield strength that is at least 50 MPa higher than before the rerolling or skin-passing.
2. Method according to claim 1, characterised in that the annealed hot or cold strip is rerolled with a degree of deformation between 10 and 40%.
3. Method according to either claim 1 or claim 2, characterised in that the annealed hot or cold strip is skin-pass rolled with a degree of deformation between 0.6 and 2.2%.
4. Method according to at least one of claims 1 to 3, characterised in that the hot or cold strip is cold-rolled with a first rolling pass at a temperature of the hot or cold strip of from 60°C to below Ac3, preferably from 60°C to 450°C.
5. Method according to claim 4, characterised in that the hot or cold strip is intermediately heated or intermediately cooled to temperatures of from 60°C to below Ac3, preferably from 60°C to 450°C, between the further rolling passes following the first rolling pass.
6. Method according to at least one of claims 1 to 5, characterised in that the annealed hot or cold strip is rerolled or skin-pass rolled at a temperature of from 0°C to 400°C.
7. Method according to at least one of claims 1 to 6, characterised in that the flat steel product has a tensile strength of more than 1300 MPa and an elongation at break A80 of greater than 3%.
8. Method according to at least one of claims 1 to 7, characterised in that the metastable austenite is partially converted into deformation twins (TWIP effect) and martensite (TRIP effect) by rerolling or skin-pass rolled to form a flat steel product, at least a proportion of 3% of the metastable austenite being converted into martensite and at least a proportion of 10% of the metastable austenite being retained as a face-centred cubic phase.
9. Method according to at least one of claims 1 to 8, characterised in that the flat steel product is produced with the following chemical composition (in % by weight):

   C: 0.0005 to 0.9, preferably 0.05 to 0.35

   Mn: 4 to 12, preferably more than 5 to less than 10

   Remainder iron including unavoidable steel-accompanying elements, with optional addition of:

   Al: 0 to 10, preferably 0.05 to 5, particularly preferably more than 0.5 to 3

   Si: 0 to 6, preferably 0.05 to 3, particularly preferably 0.1 to 1.5

   Cr: 0 to 6, preferably 0.1 to 4, particularly preferably greater than 0.5 to 2.5

   Nb: 0 to 1, preferably 0.005 to 0.4, particularly preferably 0.01 to 0.1

   V: 0 to 1.5, preferably 0.005 to 0.6, particularly preferably 0.01 to 0.3

   Ti: 0 to 1.5, preferably 0.005 to 0.6, particularly preferably 0.01 to 0.3

   Mo: 0 to 3, preferably 0.005 to 1.5, particularly preferably 0.01 to 0.6

   Sn: 0 to 0.5, preferably less than 0.2, particularly preferably less than 0.05

   Cu: 0 to 3, preferably less than 0.5, particularly preferably less than 0.1

   W: 0 to 5, preferably 0.01 to 3, particularly preferably 0.2 to 1.5

   Co: 0 to 8, preferably 0.01 to 5, particularly preferably 0.3 to 2

   Zr: 0 to 0.5, preferably 0.005 to 0.3, particularly preferably 0.01 to 0.2

   Ta: 0 to 0.5, preferably 0.005 to 0.3, particularly preferably 0.01 to 0.1

   Te: 0 to 0.5, preferably 0.005 to 0.3, particularly preferably 0.01 to 0.1

   B: 0 to 0.15, preferably 0.001 to 0.08, particularly preferably 0.002 to 0.01

   P: less than 0.1, preferably less than 0.04

   S: less than 0.1, preferably less than 0.02

   N: less than 0.1, preferably less than 0.05.
10. Method according to any of claims 1 to 9, characterised in that the flat steel product is coated with metal, in an inorganic manner, or in an organic manner.
11. Use of a flat steel product produced according to a method according to at least one of the preceding claims 1 to 10 in motor vehicle construction, rail vehicle construction, shipbuilding, plant construction, infrastructure construction, mining, in aerospace, household appliance technology and in welded blanks.