EP3469108B1 - Method for producing a cold-rolled steel strip having trip-characteristics made of a high-strength mangan-containing steel - Google Patents

Description

The invention relates to a method for producing a cold-rolled steel strip from a high-strength, manganese-containing steel. Steel strip is understood below to mean steel strips in particular, but also steel sheets. Typical tensile strengths Rm for these steels are around 800 MPa to 2000 MPa. The elongations at break A80 have values of around 3% to 40%.

From the European patent application EP 2 383 353 A2 A high-strength manganese-containing steel, a steel strip made of this steel and a process for producing this steel strip are known. The steel consists of the elements (contents in weight % and based on the steel melt): C: up to 0.5; Mn: 4 to 12.0; Si: up to 1.0; Al: 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 and the remainder iron and unavoidable impurities. One or more elements from the group "V, Nb, Ti" are optionally provided, with the sum of the contents of these elements being at most 0.5. The steel is said to be characterized by the fact that it is cheaper to produce than steels with a high manganese content and at the same time has high elongation at break values and thus significantly improved formability.

• Erschmelzen der vorbeschriebenen Stahlschmelze,
• Erzeugen eines Ausgangsprodukts für ein anschließendes Warmwalzen, indem die Stahlschmelze zu einem Strang, von dem mindestens eine Bramme oder Dünnbramme als Ausgangsprodukt für das Warmwalzen abgeteilt wird oder zu einem gegossenen Band vergossen wird, das als Ausgangsprodukt dem Warmwalzen zugeführt wird,
• Wärmebehandeln des Ausgangsprodukts, um das Ausgangsprodukt auf eine Warmwalzstarttemperatur von 1150 bis 1000°C zu bringen,
• Warmwalzen des Ausgangsprodukts zu einem Warmband mit einer Dicke von höchstens 2,5 mm, wobei das Warmwalzen bei einer 1050 bis 800°C betragenden Warmwalzendtemperatur beendet wird,
• Haspeln des Warmbandes zu einem Coil bei einer Haspeltemperatur von ≤ 700°C, optionales Glühen des Warmbandes und anschließendes Kaltwalzen auf eine Dicke von höchstens 60 % der Dicke des Warmbandes.

A process for producing a steel strip from the above-described high-strength manganese-containing steel comprises the following steps:

• Melting the previously described steel melt,
• 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 separated as a starting product for hot rolling or into a cast strip which is fed to hot rolling as a starting product,
• Heat treating the starting product to bring the starting product to a hot rolling starting temperature of 1150 to 1000°C,
• Hot rolling of the starting product into a hot strip with a thickness of not more than 2.5 mm, whereby the hot rolling is terminated at a final hot rolling temperature of 1050 to 800°C,
• Coiling the hot strip into a coil at a coiling temperature of ≤ 700°C, optional annealing of the hot strip and subsequent cold rolling to a thickness of not more than 60 % of the thickness of the hot strip.

Depending on the alloy layer, this steel can exhibit a metastable austenite with the ability to form stress-induced martensite (TRIP effect).

The international patent application also WO 2005/061152 A1 a process for producing hot strip from a formable, easily cold-drawn lightweight steel with a Mn content of 9 to 30 wt.% is described. The hot strip has TRIP properties in addition to high tensile strength. From the German publication DE 197 27 759 A1 is a well-drawable, ultra-high-strength austenitic lightweight steel with a tensile strength of up to 1100 MPa, which also has TRIP and TWIP properties. The German publication EN 10 2012 111 959 A1 describes a high manganese steel material with TRIP and TWIP properties, which experiences an increase in hardness and formability through cold forming below room temperature, preferably in the range from +25°C to -200°C. In the German laid-open specification EN 10 2009 030 324 A1 A high-manganese steel with a low tendency to hydrogen embrittlement and high tensile strengths combined with high values of elongation at break is described. The patent application US 2012/0059196 A1 discloses a method for producing hot strip using a horizontal strip casting plant. The hot strip consists of the main components Fe, Mn, Si and Al, has TRIP and/or TWIP properties and is suitable for deep drawing. The patent US 6 358 338 B1 relates to a process for producing a steel strip made of high-manganese steel. To increase the tensile strength and ductility, the steel strip is subjected to recrystallization annealing after cold rolling. In the patent application US 2009/0074605 A1 A high-manganese steel strip with excellent crash behavior and high tensile strength and elongation values is produced by cold rolling the steel strip after hot rolling and then annealing it at 600°C.

Furthermore, the German disclosure document EN 10 2012 013 113 A1 TRIP steels are described, which have a predominantly ferritic basic structure with embedded residual austenite. Due to its strong work hardening, TRIP steel achieves high values of uniform elongation and tensile strength.

The disadvantage of these manganese-containing steels with TRIP effect is that the achievable degree of deformation is limited when producing a cold-rolled steel strip due to the strong work hardening of the material during cold rolling and the associated high load on the rolling stands. In order to achieve high degrees of cold forming, several cold rolling steps with correspondingly low degrees of deformation are therefore often required, whereby a recrystallization annealing must be carried out before a further cold rolling step in order to soften the material again and thus make it suitable for cold rolling. This procedure with several cold rolling steps with intermediate recrystallization annealing is very time-consuming and costly and is associated with additional CO2 emissions.

The WO 2014/180456 A1 concerns sheets or tubes made of a manganese-containing lightweight steel with a metastable austenitic structure in the initial state with temperature-dependent TRIP/TWIP properties. Accordingly, forming the corresponding sheets or tubes in a temperature range of 40 to 160 °C leads to an avoidance of the TRIP/TWIP effects and in a temperature range of -65 to 0 °C to an intensification of the TRIP/TWIP effects.

From the CN 102 912 219 A is a high-strength manganese steel with TRIP properties and a structure with a martensite content of 30 to 90% and an austenite content of 5 to 30%. Further processing takes place by hot rolling at temperatures of at least 850 to 1100 °C.

The EN 10 2004 061284 A1 shows hot-rolled strips made of a formable, particularly cold-deep-drawable, lightweight steel consisting of the main elements Si, Al and Mn, which has a high tensile strength and TRIP and/or TWIP properties.

In the WO 2013/124283 A1 High-strength cast parts are described which are made from austenitic cast steel with the following contents in mass%: C: 0.4 to 1.2; Mn: 12 to 25; P: 0.01 to 1.5; Si: <= 3; Al: <= 3 and the remainder Fe as well as accompanying steel elements caused by melting. The cast parts are finished by cold forming at more than 10% in the temperature range below 200 °C.

Furthermore, the WO 2015/195062 A1 a process is known for producing a cold strip from an ultra-high-strength steel with the following chemical composition in mass%: C: 0.01 to 0.8; Mn: 10 to 22; B: 0.001 to 0.02; Wo: 0.001 to 0.4; Co: 0.001 to 0.4; Ta: 0.001 to 0.3 and the remainder Fe. A hot strip produced from this is cold rolled at room temperature and then annealed.

The republished EN 10 2015 112 886 A1 relates to a high-strength aluminum-containing manganese steel with the following chemical composition (in % by weight): C: 0.01 to <0.3; Mn: 4 to <10; AI: > 1 to 4; Si: 0.01 to 1; Cr: 0.1 to 4; Mo; 0.02 to 1; P: <0.1; S: <0.1; N: <0.3; remainder iron including unavoidable steel-accompanying elements, with optional alloying of one or more of the following elements (in % by weight): V: 0.01 to 1; Nb: 0.01 to 1; Ti: 0.01 to 1; Sn: 0 to 0.5; Cu: 0.005 to 3; W: 0.03 to 3; Co: 0.05 to 3; Zr: 0.03 to 0.5 and Ca: 0.0005 to 0.1, which has a good combination of strength, elongation and forming properties. In the course of manufacturing a flat steel product from this steel, hot rolling at temperatures in the range of 800 to 1200 °C and cold rolling are carried out.

Based on this, the present invention is based on the object of specifying a method for producing a cold-rolled steel strip from a high-strength manganese-containing steel with TRIP properties, with which cold rolling to a required final thickness can be made more economical and ecological. In addition, a production route from the melting of the steel to the steel strip cold-rolled to the required final thickness should be specified.

This object is achieved by a method for producing a steel strip with the features of claim 1. Advantageous embodiments of the invention are specified in the respective subclaims.

• C: 0,05 bis 0,42
• Mn: mehr als 5,0 bis <10
• sowie eines oder mehrere der folgenden Elemente (in Gewichts-%)
• Al: 0,1 bis 5, insbesondere > 0,5 bis 3
• Si: 0,05 bis 3, insbesondere > 0,1 bis 1,5
• Cr: 0,1 bis 4, insbesondere > 0,5 bis 2,5

The process according to the invention for producing a cold-rolled steel strip from a high-strength manganese-containing steel with TRIP properties containing (in % by weight):

• C: 0.05 to 0.42
• Mn: more than 5.0 to <10
• and one or more of the following elements (in weight %)
• Al: 0.1 to 5, especially > 0.5 to 3
• Si: 0.05 to 3, especially > 0.1 to 1.5
• Cr: 0.1 to 4, especially > 0.5 to 2.5

• Nb: bis 1,5
• V: bis 1,5
• Ti: bis 1,5
• Mo: bis 3
• Cu: bis 3
• Sn: bis 0,5
• W: bis 5
• Co: bis 8
• Zr: bis 0,5
• Ta: bis 0,5
• Te: bis 0,5
• B: bis 0,15
• P: max. 0,1, insbesondere < 0,04
• S: max. 0,1, insbesondere < 0,02
• N: max. 0,1, insbesondere < 0,05
• Ca: bis 0,1
• ist dadurch gekennzeichnet, dass unter Vermeidung des Kaltwalzens bei Raumtemperatur das Walzen auf eine geforderte Enddicke bei einer Temperatur von 70°C bis 250°C erfolgt.

Remainder iron including unavoidable steel-accompanying elements, with optional alloying of one or more of the following elements (contents in weight % and based on the steel melt):

• Note: up to 1.5
• V: up to 1.5
• Ti: up to 1.5
• Mon: until 3
• Cu: up to 3
• Sn: up to 0.5
• W: up to 5
• Co: up to 8
• Zr: up to 0.5
• Ta: up to 0.5
• Te: up to 0.5
• B: up to 0.15
• P: max. 0.1, especially < 0.04
• S: max. 0.1, especially < 0.02
• N: max. 0.1, especially < 0.05
• Approx: up to 0.1
• is characterized in that, avoiding cold rolling at room temperature, rolling to a required final thickness is carried out at a temperature of 70°C to 250°C.

In the context of the present invention, high-strength steels are understood to be steels with a tensile strength of 800 MPa to 2000 MPa.

The reason for the strong strain hardening of these high-strength manganese-containing steels with a TRIP effect is the proportion of retained austenite contained in the structure alongside martensite and/or ferrite and/or bainite and/or pearlite. This retained austenite can transform into martensite at appropriate ambient temperatures (TRIP effect, both ε and α'-martensite), whereby at room temperature up to about 50°C a significant proportion of martensite formation always takes place due to the TRIP effect. This leads to a hardening of the material and thus to a strong increase in the rolling forces during cold rolling already during the first pass and is accompanied by a reduction in the maximum degree of deformation. The cold-rolled strip then has a high strength and a low residual formability. In addition, the effect of mechanical stresses can cause deformation twins (TWIP effect).

According to the invention, by raising the forming temperature before the first pass to 70°C to 250°C, the TRIP transformation mechanism from austenite to martensite is completely or partially suppressed, so that significantly higher degrees of deformation are possible during rolling in only one rolling pass.

The term "cold rolling" is usually often used to refer to cold rolling at room temperature. In connection with the present invention, the term "cold rolling" is also used for cold rolling at elevated temperature. In contrast to hot rolling, this elevated temperature in cold rolling according to the invention is significantly below the AC1 transformation temperature associated with a structural transformation. The cold rolling according to the invention also preferably takes place below a homologous temperature at which no creep processes occur in the steel sheet.

In the single report presented in the appendix Figure 1 , the influence of the forming temperature during rolling on the hardening behavior of the material is shown using the parameters of tensile tests. Compared to forming at room temperature of 20°C, forming temperatures of 100°C or 200°C achieve significantly higher elongation values with a significantly lower increase in tensile strength.

Preferably, it is provided that a hot strip or a pre-strip is heated to a temperature of 70°C to 250°C or that a hot strip or a pre-strip already has a temperature of 70°C to 250°C and is then cold rolled to the required final thickness at a temperature before the first pass of 70°C to 250°C. By having a temperature is meant that the Temperature already comes from a previous process step or is maintained at this temperature. The previous process step can mean reheating, continuous or discontinuous processing using the heat present in the hot strip or preliminary strip, in particular a hot rolling process, or maintaining the temperature in a furnace.

By heating the hot strip to a temperature of 70°C to 250°C before cold rolling, the transformation of austenite into martensite is significantly reduced or avoided by increasing the stacking fault energy in the first rolling pass, so that the strip hardens less during the cold rolling process and more deformation twins (TWIP effect) are formed in the austenite. This results in both lower rolling forces and a significantly improved formability of the strip during the rolling process. In order to compensate for the additional heating of the strip due to the forming work during cold forming and to keep the strip temperature in the optimal range for the TWIP effect, the strip can optionally be cooled between the individual rolling passes, for example using compressed air or other liquid or gaseous media.

Furthermore, the steel strip has a considerable residual formability after rolling, since the deformation twins formed in the austenite and any residual austenite that may be present can be completely or partially converted into martensite at room temperature due to the TRIP effect, which is associated with an increase in the maximum elongation and thus an improvement in the formability for component production from the flat product even without additional annealing following the cold rolling process.

In addition, the formation of deformation twins results in improved behavior during subsequent forming processes against hydrogen-induced delayed cracking and hydrogen embrittlement compared to cold rolling without prior heating with an optional annealing process.

The steel used for the process according to the invention has a multiphase structure consisting of ferrite and/or martensite and/or bainite and/or pearlite as well as residual austenite/austenite. The proportion of residual austenite/austenite can be 5% to 80 %. When mechanical stresses are applied, the residual austenite/austenite can be partially or completely converted into martensite due to the TRIP effect.

The alloy on which the invention is based exhibits a TRIP and/or TWIP effect when subjected to appropriate mechanical stress. Due to the strong hardening (analogous to work hardening) at room temperature induced by the TRIP and/or TWIP effect and by the increase in the dislocation density, the steel achieves very high values of elongation at break, in particular uniform elongation, and tensile strength. This property is advantageously only achieved at manganese contents of over 3% by weight due to the residual austenite present.

The use of the term "up to" in the definitions of the content ranges, such as 0.01 wt% to 1 wt%, means that the key values, in the example 0.01 and 1, are included.

The steel according to the invention is particularly suitable for producing high-strength steel strip, which can be provided with a metallic or non-metallic coating, for example based on zinc. Applications in vehicle construction, shipbuilding, plant construction, infrastructure construction, aerospace and household appliance technology are conceivable, among others. Due to a high austenite content, the steel produced according to the invention is suitable for low-temperature stresses.

Advantageously, the steel has a tensile strength Rm of > 800 to 2000 MPa and an elongation at break A80 of 3 to 40%, preferably > 8 to 40%.

• C: 0,05 bis 0,42
• Mn: > 5 bis < 10
• sowie eines oder mehrere der folgenden Elemente (in Gewichts-%)
• Al: 0,1 bis 5, insbesondere > 0,5 bis 3
• Si: 0,05 bis 3, insbesondere > 0,1 bis 1,5
• Cr: 0,1 bis 4, insbesondere > 0,5 bis 2,5

Particularly uniform and homogeneous material properties can be achieved according to the invention if the steel has the following
Alloy composition in weight %:

• C: 0.05 to 0.42
• Mn: > 5 to < 10
• and one or more of the following elements (in weight %)
• Al: 0.1 to 5, especially > 0.5 to 3
• Si: 0.05 to 3, especially > 0.1 to 1.5
• Cr: 0.1 to 4, especially > 0.5 to 2.5

• Nb: 0,005 bis 0,4, insbesondere 0,01 bis 0,1
• B: 0,001 bis 0,08, insbesondere 0,002 bis 0,01
• Ti: 0,005 bis 0,6, insbesondere 0,01 bis 0,3
• Mo: 0,005 bis 1,5, insbesondere 0,01 bis 0,6
• Sn: < 0,2, insbesondere < 0,05
• Cu: < 0,5, insbesondere < 0,1
• W: 0,01 bis 3, insbesondere 0,2 bis 1,5
• Co: 0,01 bis 5, insbesondere 0,3 bis 2
• Zr: 0,005 bis 0,3, insbesondere 0,01 bis 0,2
• Ta: 0,005 bis 0,3, insbesondere 0,01 bis 0,1
• Te: 0,005 bis 0,3, insbesondere 0,01 bis 0,1
• V: 0,005 bis 0,6, insbesondere 0,01 bis 0,3
• Ca: 0,005 bis 0,1

Remainder iron including unavoidable steel-accompanying elements, with optional alloying of one or more of the following elements (in weight %):

• Nb: 0.005 to 0.4, especially 0.01 to 0.1
• B: 0.001 to 0.08, in particular 0.002 to 0.01
• Ti: 0.005 to 0.6, especially 0.01 to 0.3
• Mo: 0.005 to 1.5, especially 0.01 to 0.6
• Sn: < 0.2, especially < 0.05
• Cu: < 0.5, especially < 0.1
• W: 0.01 to 3, especially 0.2 to 1.5
• Co: 0.01 to 5, especially 0.3 to 2
• Zr: 0.005 to 0.3, especially 0.01 to 0.2
• Ta: 0.005 to 0.3, especially 0.01 to 0.1
• Te: 0.005 to 0.3, especially 0.01 to 0.1
• V: 0.005 to 0.6, especially 0.01 to 0.3
• Approx: 0.005 to 0.1

Alloying elements are usually added to steel to specifically influence certain properties. An alloying 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. The effect of the alloying elements in the alloy according to the invention will be discussed in more detail below. 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 strength. Higher C contents impair 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 is specified. The minimum content is set at 0.0005% by weight. A content of 0.05 to 0.42% by weight is preferred, since the ratio of residual austenite to other phase components can be adjusted particularly advantageously in this range.

Manganese Mn: Stabilizes the austenite, increases the strength and toughness and enables deformation-induced martensite and/or twinning in the alloy according to the invention. Contents ≤ 3% by weight are not sufficient to stabilize the austenite and thus impair the elongation properties, while contents of more than 12% by weight stabilize the austenite too much and thus reduce the strength properties, in particular the yield strength. For the manganese steel according to the invention with medium manganese contents, a range of more than 5 to < 10% by weight is preferred, since in this range the ratio of the phase proportions to one another and the transformation mechanisms during rolling to final thickness can be advantageously influenced.

Aluminium Al: Improves the strength and elongation properties, lowers the specific density and influences the transformation behavior of the alloy according to the invention. Contents of more than 10% by weight Al impair the elongation properties and cause predominantly brittle fracture behavior. For the manganese steel according to the invention with medium manganese contents, an Al content of 0.1 to 5% by weight is preferred in order to increase the strength while at the same time maintaining good elongation. In particular, contents of > 0.5 to 3% by weight enable particularly high strength and elongation at break.

Silicon Si: Prevents carbon diffusion, reduces the specific density and increases the strength and the elongation and toughness properties. Contents of more than 6% by weight prevent further processing by cold rolling due to embrittlement of the material. Therefore, a maximum content of 6% by weight is specified. Optionally, a content of 0.05 to 3% by weight is specified, as contents in this range have a positive effect on the forming properties. Si contents of > 0.1 to 1.5% by weight have proven to be particularly advantageous for the forming and transformation properties.

Chromium Cr: Improves strength and reduces the corrosion rate, delays the formation of ferrite and pearlite and forms carbides. The maximum content is set at 6% by weight, as higher contents result in a deterioration in the elongation properties and significantly higher costs. For the manganese steel according to the invention with medium manganese contents, a Cr content of 0.1 to 4 % by weight is preferred to reduce the precipitation of coarse Cr carbides. In particular, contents of > 0.5 to 2.5 wt. % have proven to be beneficial for stabilizing the austenite and the precipitation of fine Cr carbides. To achieve the beneficial properties of adding Al and Si in addition to Cr, the total content of Al + Si + Cr should be more than 1.2 wt. %.

Molybdenum Mo: Acts as a carbide former, increases strength and increases resistance to delayed cracking and hydrogen embrittlement. Mo contents of more than 3% by weight impair the elongation properties, which is why a maximum content of 3% by weight is specified. For the manganese steel according to the invention with medium manganese contents, a Mo content of 0.005 to 1.5% by weight is preferred in order to avoid the precipitation of excessively large Mo carbides. In particular, contents of 0.01 to 0.6% by weight cause the precipitation of desired Mo carbides while at the same time reducing alloy costs.

Phosphorus P: Is a trace element from iron ore and is dissolved in the iron lattice as a substitution atom. Phosphorus increases hardness through solid solution strengthening and improves hardenability. However, attempts are generally made to reduce the phosphorus content as much as possible because, among other things, its low diffusion rate makes it highly susceptible to segregation and greatly reduces 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 reasons stated above, the phosphorus content is limited to a maximum of 0.1% by weight, with contents of < 0.04% by weight being advantageous for the reasons stated above.

Sulfur S: Like phosphorus, it is bound as a trace element in iron ore. It is generally undesirable in steel because it tends to segregate strongly and has a strong embrittling effect, which impairs the elongation and toughness properties. Attempts are therefore made to achieve the lowest possible amounts of sulfur in the melt (e.g. by means of deep vacuum treatment). For the reasons mentioned above, the sulfur content is limited to a maximum of 0.1% by weight. The limitation to < 0.2% by weight is particularly advantageous in order to To reduce the excretion of MnS.

Nitrogen N: Is also an accompanying element from steel production. In its dissolved state, it improves the strength and toughness properties of steels with a high manganese content and greater than or equal to 4% by weight Mn. Low-Mn alloyed steels with < 4% by weight Mn, which contain free nitrogen, tend to have a strong aging effect. The nitrogen diffuses to 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 bound in the form of nitrides, for example, by alloying with aluminum, vanadium, niobium or titanium. For the reasons mentioned above, the nitrogen content is limited to a maximum of 0.1% by weight, with contents of < 0.05% by weight being preferred to largely avoid the formation of AIN.

Microalloying elements are usually only added in very small quantities (< 0.1% by weight per element). In contrast to alloying elements, they mainly act by forming precipitations, but can also influence properties in a dissolved state. Despite the small quantities added, microalloying elements have a strong influence on the manufacturing conditions as well as the processing and final properties.

Typical microalloying 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, particularly through the formation of carbides, which simultaneously improves strength, toughness and elongation properties. Contents of more than 1.5% by weight do not bring any further advantages. For vanadium and niobium, a minimum content of greater than or equal to 0.005% by weight and a maximum content of 0.6 (V) or 0.4 (Nb)% by weight is optionally preferred, in which the alloying elements advantageously bring about grain refinement. To improve economic efficiency while at the same time achieving optimal grain refinement, the V content can be further limited to 0.01% by weight to 0.3% by weight and the Nb content to 0.01 to 0.1% by weight.

Tantalum Ta: Tantalum acts as a carbide former in a similar way to niobium and refines grain, thereby simultaneously improving strength, toughness and elongation properties. Contents of more than 0.5% by weight do not result in any further improvement in properties. Therefore, a maximum content of 0.5% by weight is optionally specified. A minimum content of 0.005% by weight and a maximum content of 0.3% by weight are preferred, in which grain refinement can be advantageously achieved. To improve cost-effectiveness and optimize grain refinement, a content of 0.01% by weight to 0.1% by weight is particularly preferred.

Titanium Ti: acts as a carbide former to refine the grain, thereby simultaneously improving strength, toughness and elongation properties and reducing intergranular corrosion. Ti contents of more than 1.5% by weight impair the elongation properties, which is why a maximum Ti content of 1.5% by weight is specified. Optionally, a minimum content of 0.005 and a maximum content of 0.6% by weight is specified, in which Ti is advantageously precipitated. A minimum content of 0.01% by weight and a maximum content of 0.3% by weight is preferred, which ensures optimal precipitation behavior at low alloy costs.

Tin Sn: Tin increases strength, but like copper, it accumulates under the scale layer and at the grain boundaries at higher temperatures. By penetrating the grain boundaries, it leads to the formation of low-melting phases and, as a result, to cracks in the structure and brittle solder, which is why a maximum content of ≤ 0.5% by weight is optionally provided. For the reasons stated above, contents of < 0.2% by weight are preferred. Contents of < 0.05% by weight are particularly advantageous for avoiding low-melting phases and cracks in the structure.

Copper Cu: Reduces the corrosion rate and increases strength. Contents of more than 3% by weight impair manufacturability by forming low-melting phases during casting and hot rolling, which is why a maximum content of 3% by weight is specified. Optionally, a maximum content of < 0.5% by weight is provided, at which the occurrence of cracks during casting and hot rolling can be advantageously prevented. Cu contents of < 0.1% by weight have proven to be particularly advantageous for avoiding low-melting phases and for preventing cracks.

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 specified. Optionally, a maximum content of 3% by weight and a minimum content of 0.01% by weight are specified, in which the precipitation of carbides advantageously takes place. In particular, a minimum content of 0.2% by weight and a maximum content of 1.5% by weight are preferred, which enables optimal precipitation behavior at low alloy costs.

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, which is why a maximum content of 8% by weight is specified. Optionally, a maximum content of ≤ 5% by weight and a minimum content of 0.01% by weight are specified, which advantageously improve the strength and high-temperature strength. A minimum content of 0.3% by weight and a maximum content of 2% by weight are preferred, which, in addition to the strength properties, has a beneficial effect on the austenite stability.

Zirconium Zr: Acts as a carbide former and improves strength. Zr contents of more than 0.5% by weight impair the elongation properties, which is why a maximum content of 0.5% by weight is specified. Optionally, a maximum content of 0.3% by weight and a minimum content of 0.005% by weight are specified, since carbides are advantageously precipitated in this range. A minimum content of 0.01% by weight and a maximum content of 0.2% by weight are preferred, which advantageously enable optimal carbide precipitation at low alloy costs.

Boron B: Delays the austenite transformation, improves the hot forming properties of steels and increases the strength at room temperature. It is effective even at very low alloy contents. Contents above 0.15% by weight significantly 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% by weight are set, in which the strength-increasing effect of boron is used to advantage. A minimum content of 0.002% by weight and a maximum content of 0.01% by weight are preferred, which enable optimal use to increase strength while simultaneously improving the transformation behavior.

Tellurium Te: Improves corrosion resistance and mechanical properties as well as machinability. Furthermore, Te increases the strength of MnS, which is therefore 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 existing MnS. Furthermore, a minimum content of 0.01% by weight and a maximum content of 0.1% by weight are preferred, which enable optimization of the mechanical properties while reducing alloy costs.

Calcium Ca: Is used to modify non-metallic oxidic inclusions, which could otherwise lead to undesirable failure of the alloy due to inclusions in the structure, which act as stress concentration points and weaken the metal bond. Furthermore, Ca improves the homogeneity of the alloy according to the invention. In order to develop a corresponding effect, a minimum content of 0.0005% by weight is optionally necessary. Contents of more than 0.1% by weight do not provide any further advantage in the inclusion modification, impair manufacturability and should be avoided due to the high vapor pressure of Ca in steel melts. A maximum content of 0.1% by weight is therefore provided.

• Erschmelzen einer Stahlschmelze enthaltend (in Gewichts-%):
  • C: 0,05 bis 0,42
  • Mn: mehr als 5,0 bis <10
  • sowie eines oder mehrere der folgenden Elemente (in Gewichts-%)
  • Al: 0,1 bis 5, insbesondere > 0,5 bis 3
  • Si: 0,05 bis 3, insbesondere > 0,1 bis 1,5
  • Cr: 0,1 bis 4, insbesondere > 0,5 bis 2,5
  • Rest Eisen einschließlich unvermeidbarer stahlbegleitender Elemente, mit optionaler Zulegierung von einem oder mehreren der folgenden Elemente (in Gewichts-%):
    • Nb: bis 1,5
    • V: bis 1,5
    • Ti: bis 1,5
    • Mo: bis 3
    • Cu: bis 3
    • Sn: bis 0,5
    • W: bis 5
    • Co: bis 8
    • Zr: bis 0,5
    • Ta: bis 0,5
    • Te: bis 0,5
    • B: bis 0,15
    • P: max. 0,1, insbesondere < 0,04
    • S: max. 0,1, insbesondere < 0,02
    • N: max. 0,1, insbesondere < 0,05
    • Ca: bis 0,1
• 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,
• Wiedererwärmen der Bramme oder Dünnbramme auf 1050°C bis 1250°C und anschließendes Warmwalzen der Bramme oder Dünnbramme zu einem Warmband oder Wiedererwärmen des endabmessungsnah erzeugten Vorbandes auf 1000°C bis 1200°C und anschließendes Warmwalzen des Vorbandes zu einem Warmband oder Warmwalzen des Vorbandes ohne Wiedererwärmen aus der Gießhitze zu einem Warmband mit optionalem Zwischenerwärmen zwischen einzelnen Walzstichen des Warmwalzens,
• Aufhaspeln des Warmbandes bei einer Haspeltemperatur zwischen 820°C und Raumtemperatur,
• Optionales Glühen des Warmbandes mit folgenden Parametern:
  Glühtemperatur: 580°C bis 820°C, Glühdauer: 1 Minute bis 48 Stunden,
• Unter Vermeidung des Kaltwalzens bei Raumtemperatur, Walzen des Warmbandes mit einer geforderten Enddicke von weniger als 10 mm zu einem gewalzten Stahlband bei einer Temperatur vor dem ersten Stich von 70°C bis 250°C.
• Optionales Glühen des Stahlbandes mit folgenden Parametern:
  Glühtemperatur: 580°C bis 820°C, Glühdauer: 1 Minute bis 48 Stunden.
• Optionales Beizen und/oder Dressieren des Stahlbandes
• Optionales Beschichten des Stahlbandes mit einer Korrosionsschutzbeschichtung

A production route according to the invention from the melting of the steel to the finished steel strip with a required final thickness of less than 10 mm, preferably less than 4 mm, made of a high-strength manganese-containing steel comprises the steps:

• Melting of a steel melt containing (in % by weight):
  • C: 0.05 to 0.42
  • Mn: more than 5.0 to <10
  • and one or more of the following elements (in weight %)
  • Al: 0.1 to 5, especially > 0.5 to 3
  • Si: 0.05 to 3, especially > 0.1 to 1.5
  • Cr: 0.1 to 4, especially > 0.5 to 2.5
  • Remainder iron including unavoidable steel-accompanying elements, with optional alloying of one or more of the following elements (in weight %):
    • Note: up to 1.5
    • V: up to 1.5
    • Ti: up to 1.5
    • Mon: until 3
    • Cu: up to 3
    • Sn: up to 0.5
    • W: up to 5
    • Co: up to 8
    • Zr: up to 0.5
    • Ta: up to 0.5
    • Te: up to 0.5
    • B: up to 0.15
    • P: max. 0.1, especially < 0.04
    • S: max. 0.1, especially < 0.02
    • N: max. 0.1, especially < 0.05
    • Approx: up to 0.1
• Casting the molten steel into a pre-strip by means of a near-net-shape horizontal or vertical strip casting process or casting the molten steel into a slab or thin slab by means of a horizontal or vertical slab or thin slab casting process,
• Reheating the slab or thin slab to 1050°C to 1250°C and then hot rolling the slab or thin slab to a hot strip or reheating the near-net-shape pre-strip to 1000°C to 1200°C and then hot rolling the pre-strip to a hot strip or hot rolling the pre-strip without reheating from the casting heat to a hot strip with optional intermediate heating between individual rolling passes of the hot rolling,
• Coiling the hot strip at a coiling temperature between 820°C and room temperature,
• Optional annealing of the hot strip with the following parameters:
  Annealing temperature: 580°C to 820°C, annealing time: 1 minute to 48 hours,
• Avoiding cold rolling at room temperature, rolling the hot strip with a required final thickness of less than 10 mm into a rolled steel strip at a temperature before the first pass of 70°C to 250°C.
• Optional annealing of the steel strip with the following parameters:
  Annealing temperature: 580°C to 820°C, annealing time: 1 minute to 48 hours.
• Optional pickling and/or skin-passing of the steel strip
• Optional coating of the steel strip with an anti-corrosion coating

For further advantages, please refer to the above statements.

Typical thickness ranges for preliminary strip are 1 mm to 35 mm and for slabs and thin slabs 35 mm to 450 mm. Preferably, the slab or thin slab is hot rolled to a hot strip with a thickness of 20 mm to 1.5 mm or the preliminary strip cast close to the final dimension is hot rolled to a hot strip with a thickness of 8 mm to 1 mm. The cold-rolled steel strip produced according to the invention has a thickness of, for example, > 0.15 mm to 10 mm.

For hot rolling the preliminary strip from the casting heat to a hot strip with optional intermediate heating between the individual rolling passes of the hot rolling, reheating temperatures in the range of 720°C to 1200°C are provided. If only a few rolling passes are required, the reheating temperature can be selected at the lower end of the range.

The hot strip can optionally be subjected to heat treatment in the temperature range between 580°C and 820°C for 1 minute to 48 hours, with higher temperatures being assigned to shorter treatment times and vice versa. Annealing can be carried out both in a bell annealing furnace (longer annealing times) and, for example, in a continuous annealing furnace (shorter annealing times). The optional annealing serves to reduce the strength and/or increase the residual austenite content of the hot strip before the cold rolling process, which Forming properties can be advantageously improved for the subsequent process.

Following the hot rolling process, cold rolling of the hot strip takes place at an increased temperature according to the invention with the aim of achieving the thickness of the steel strip required for the end application of ≥ 0.15 mm to 10 mm. This can optionally be followed by a further annealing process, possibly coupled with a coating process and finally a skin-pass process, with which the surface structure required for the end application is achieved.

Preferably, the steel strip is hot-dip or electrolytically galvanized or coated with a metallic, inorganic or organic coating.

A steel strip produced by the process according to the invention has a tensile strength Rm > 800 to 2000 MPa and an elongation at break A80 of 3 to 40%, preferably > 8 to 40%. High strengths tend to be associated with lower elongations at break and vice versa.

The cold-rolled steel strip produced according to the invention can then be processed into a component, for example as a sheet section, coil or panel, by cold forming at room temperature or by warm forming at temperatures of 60°C to below AC3, preferably < 450°C, whereby the considerable residual formability means that intermediate annealing can be dispensed with depending on the application.

In further processing stages, the cold-rolled steel strip produced according to the invention can be processed into longitudinally or spirally welded pipes, whereby here too, due to the considerable residual formability of the steel strip, intermediate annealing can be dispensed with depending on the application. The pipe can have an external and/or internal metallic, organic or inorganic coating.

The tube produced in this way can then be further deformed, for example drawn or expanded or formed using internal high pressure and further processed into a component.

The main areas of application are the automotive and commercial vehicle industries, mechanical engineering, white goods, construction, and applications at temperatures below 0°C, as well as safety steel. Safety steels are used to protect vehicles and buildings against bullets and explosions, and are extremely hard and tough.

Tests were carried out to investigate the mechanical properties of the steel strips produced according to the invention using exemplary alloys 1 to 4. Alloys 1 to 4 contain the following elements in the listed contents in wt.%: alloy C Mn Al Si Cr Mon 1 0.2 7.0 2.0 0.5 1.0 - 2 0.2 7.0 0.9 0.5 - - 3 0.27 7.4 2.2 0.5 1.2 - 4 0.21 7.2 2.5 0.5 1.2 0.16

For comparison, the steel strips made from the above-mentioned alloys 1 to 4 were cold-rolled, ie at room temperature and thus below 50°C, and also rolled according to the invention at 250°C. The measured rolling forces are given below: alloy Rolling force [kN] cumulative - cold rolling Rolling force [kN] cumulative - at 250 °C Degree of deformation (e=Δd/d0) [%] Rolling force reduction [%] 1 103000 59000 44 approx. 43 2 144000 55000 44 approx. 62 3 161000 63000 44 approx. 60 4 107000 56000 44 approx. 43

Cumulative rolling force is the addition of the rolling forces of the individual passes in order to obtain a comparable measure of the force required. The rolling force was standardized to a strip width of 1000 mm. The degree of deformation e is defined as the quotient of the change in thickness Δd of the steel strip under investigation by the Initial thickness d0 of the steel strip under investigation. The rolling force reduction is the calculated reduction in the rolling force at 250 °C compared with the rolling force during cold rolling.

The elongation at break A50 was also evaluated: alloy Elongation at break A50 [%] cold rolled Elongation at break A50 [%] rolled at 250 °C 1 2.0 15.5 2 2.5 20.5 3 3.5 19.0 4 3.0 18.5

The elongation values represent the elongation in the rolling direction.

Claims (7)

1. Process for producing a cold-rolled steel strip from a high-strength manganesecontaining steel having TRIP properties containing (in wt.%):

   C: 0.05 to 0.42

   Mn: more than 5.0 to < 10

   and one or more of the following elements (in wt.%)

   Al: 0.1 to 5, in particular > 0.5 to 3

   Si: 0.05 to 3, in particular > 0.1 to 1.5

   Cr: 0.1 to 4, in particular > 0.5 to 2.5

   the remainder being iron, including unavoidable steel-accompanying elements, with the optional alloying addition of one or more of the following elements (in wt.%):

   Nb: up to 1.5

   V: up to 1.5

   Ti: up to 1.5

   Mo: up to 3

   Cu: up to 3

   Sn: up to 0.5

   W: up to 5

   Co: up to 8

   Zr: up to 0.5

   Ta: up to 0.5

   Te: up to 0.5

   B: up to 0.15

   P: max. 0.1, in particular < 0.04

   S: max. 0.1, in particular < 0.02

   N: max. 0.1, in particular < 0.05

   Ca: up to 0.1,

   characterised in that the cold rolling to a required final thickness takes place at a temperature, prior to the first pass of the cold rolling, of 70°C to 250°C.
2. Process according to claim 1, characterised in that a hot strip or a near-net strip is heated to a temperature of above 50°C to 400°, preferably from 70°C to 250°C, or a hot strip or a near-net-strip already has a temperature of above 50°C to 400°, preferably from 70°C to 250°C, and is then cold-rolled to the required final thickness at a temperature, prior to the first pass, of above 50°C to 400°, preferably from 70°C to 250°C.
3. Process according to either claim 1 or 2, characterised in that the strip is cooled to a temperature of 50°C to 400°C, in particular to a temperature of 70°C to 250°C, between the rolling passes of the cold rolling.
4. Process according to claim 1, characterised in that the steel contains (in wt.%): $$\mathrm{Al}+\mathrm{Si}+\mathrm{Cr}>1.2.$$

   
5. Process according to any of claims 1 to 4, characterised in that the steel contains one or more of the following elements (in wt.%):

   Nb: 0.005 to 0.4, in particular 0.01 to 0.1

   V: 0.005 to 0.6, in particular 0.01 to 0.3

   Ti: 0.005 to 0.6, in particular 0.01 to 0.3

   Mo: 0.005 to 1.5, in particular 0.01 to 0.6

   Sn: < 0.2, in particular < 0.05

   Cu: < 0.5, in particular < 0.1

   W: 0.01 to 3, in particular 0.2 to 1.5

   Co: 0.01 to 5, in particular 0.3 to 2

   Zr: 0.005 to 0.3, in particular 0.01 to 0.2

   Ta: 0.005 to 0.3, in particular 0.01 to 0.1

   Te: 0.005 to 0.3, in particular 0.01 to 0.1

   B: 0.001 to 0.08, in particular 0.002 to 0.01

   Ca: 0.005 to 0.1.
6. Process according to any of claims 1 to 5, characterised in that the cold-rolled steel strip is produced having a required final thickness of less than 10 mm, preferably less than 4 mm, comprising the steps of:

   - smelting a steel melt according to any of claims 1 and 5 to 9,

   - casting the melt to form a near-net strip by means of a near-net-shape horizontal or vertical strip-casting process or casting the steel melt to form a slab or thin slab by means of a horizontal or vertical slab-casting or thin slab-casting process,

   - reheating the slab or thin slab to 1050°C to 1250°C and subsequently hot rolling the slab or thin slab to form a hot strip or reheating the near-net strip produced in a near-net-shape process to 1000°C to 1200°C and subsequently hot rolling the near-net strip to form a hot strip or hot rolling the near-net strip without reheating from the casting heat to form a hot strip having optional intermediate heating between individual rolling passes of the hot rolling,

   - coiling the hot strip at a coiling temperature between 820°C and room temperature,

   - optionally annealing the hot strip with the following parameters:
   annealing temperature: 580 to 820°C, annealing time: 1 minute to 48 hours,

   - cold rolling the hot strip or the near-net strip in accordance with the steps according to claims 1 to 3,

   - optionally annealing the steel strip with the following parameters:
   annealing temperature: 580°C to 820°C, annealing time: 1 minute to 48 hours,

   - optional pickling and/or skin-passing of the steel strip,

   - optional coating of the steel strip with a metallic, organic or inorganic anti-corrosion coating.
7. Use of a steel strip produced in accordance with the process according to at least one of claims 1 to 6 for producing components by hot working, cold working or warm working, or for producing longitudinally or spiral-welded pipes, or for producing components for the automotive and commercial vehicle industry and for mechanical engineering, white goods and the construction industry, or for use in the low temperature range below 0°C to -273°C, or as safety steel.

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