EP3469108A1

EP3469108A1 - Method for producing a cold-rolled steel strip having trip-characteristics made of a high-strength mangan-containing steel

Info

Publication number: EP3469108A1
Application number: EP17730110.8A
Authority: EP
Inventors: Peter PALZER; Thomas Dr. Evertz; Martin Schubert
Original assignee: Salzgitter Flachstahl GmbH
Current assignee: Salzgitter Flachstahl GmbH
Priority date: 2016-06-09
Filing date: 2017-06-08
Publication date: 2019-04-17
Also published as: KR20190020694A; RU2711696C1; WO2017211952A1; US20190256943A1; DE102016110661A1

Abstract

The invention relates to a method for producing a cold-rolled steel strip made of a high-strength mangan-containing steel with TRIP-characteristics, containing (in wt.%) C: 0.0005 to 0.9, Mn: more than 3.0 to 12, with the remaining portion being iron including unavoidable steel-associated elements, with the optional addition of one or more of the following elements (in wt.%): AI: up to 10; Si: up to 6; Cr: up to 6; 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. According to the invention, in order to improve a corresponding method, the cold-rolling to a required end thickness occurs at a temperature of over 50°C to 400°C before the first impact.

Description

METHOD FOR PRODUCING A COLD-ROLLED STEEL STRIP WITH TRIP PROPERTIES OF A HIGH-TERM, MANG-SAVED STEEL

The invention relates to a method for producing a cold-rolled steel strip from a high-strength, manganese-containing steel. Under steel strip are understood in particular steel bands but also steel sheets below. typical

Tensile strengths Rm are about 800 MPa to 2000 MPa for these steels. The elongations at break A80 have values of about 3% to 40%. European Patent Application EP 2 383 353 A2 discloses a high-strength manganese-containing steel, a steel strip made from this steel and a method for producing this steel strip. The steel consists of the elements (contents in% by weight and based on the molten steel): 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 and balance iron and unavoidable

Impurities. Optionally, one or several elements from the group "V, Nb, Ti" are provided, the sum of the contents of these elements being at most equal to 0.5 The steel should be characterized in that it is cheaper to produce than high manganese steels and at the same time high Elongation at break and associated therewith has a significantly improved formability.

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

Melting the above-described molten steel,

- Producing a starting product for subsequent hot rolling, by the molten steel into a strand of which at least one slab or

Thin slab is separated as a starting material for the hot rolling or is cast into a cast strip, which is fed as a starting product to the hot rolling,

Heat treating the starting product to bring the starting product to a hot rolling start temperature of from 1 150 to 1000 ° C,

Hot rolling the starting product into a hot strip having a thickness of at most 2.5 mm, the hot rolling being terminated at a hot rolling end temperature of 1050 to 800 ° C,

- winding the hot strip into a coil at a reel temperature of <700 ° C, optional annealing of the hot strip followed by cold rolling to a thickness of at most 60% of the thickness of the hot strip.

Depending on the alloy layer, this steel may have a metastable austenite with the capability of stress-induced martensite formation (TRIP effect).

Also, International Patent Application WO 2005/061152 A1 discloses a method for producing hot strips of a formable, readily cold-deep-drawable lightweight structural steel having an Mn content of 9 to 30% by weight. described. The

Hot-rolled strip has TRIP properties in addition to a high tensile strength. German Offenlegungsschrift DE 197 27 759 A1 discloses a deep-drawable, ultra-high-strength austenitic lightweight structural steel with a tensile strength of up to 1100 MPa, which likewise has TRIP and TWIP properties. The German patent application DE 10 2012 1 1 1 959 A1 describes a high-manganese steel material with TRIP and TWIP properties, which by cold forming below room temperature, preferably in the range of + 25 ° C to -200 ° C, an increase in hardness and formability experiences. In the German

Offenlegungsschrift DE 10 2009 030 324 A1 describes a high manganese steel with a low tendency to hydrogen embrittlement and with high tensile strengths with simultaneously high values of the elongation at break. The patent application US

2012/0059196 A1 discloses a method for producing hot strip with 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 also relates to a method for

Production of a steel strip made of high manganese steel. To increase the tensile strength and stretchability, 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 with high tensile strength and elongation values is produced by cold rolling the steel strip after hot rolling and then annealing at 600 ° C.

Furthermore, German published patent application DE 10 2012 013 1 13 A1 describes TRIP steels which have a predominantly ferritic basic structure with retained austenite intercalated. Because of its high work hardening, TRIP steel achieves high levels of uniform elongation and tensile strength. A disadvantage of these manganese-containing steels with TRIP effect is that in the production of a cold-rolled steel strip, the achievable degree of deformation is limited because of the high 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 working, therefore, often several cold rolling steps with correspondingly low degrees of deformation are required, in each case a recrystallization annealing must be performed before a new cold rolling step to re-solidify the material and thus make cold-rollable. This procedure with several cold rolling steps with interposed recrystallizing annealing is very time-consuming and costly and associated with additional CO 2 emissions.

On this basis, the present invention based on the object to provide a method for producing a cold rolled steel strip of a high-strength manganese steel with TRIP properties, with which the cold rolling can be designed to a required final thickness more economical and ecological. In addition, a production route from the melting of the steel to the cold rolled steel strip to the required final thickness shall be indicated. This object is achieved by a method for producing a steel strip having the features of claim 1. Advantageous embodiments of the invention are specified in the respective subclaims.

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

C: 0.0005 to 0.9

Mn: more than 3.0 to 12

Remainder of iron, including unavoidable steel-supporting elements, with optional addition of one or more of the following elements (contents by weight):

% and based on the molten steel):

AI: until 10

Si: to 6

Cr: to 6

Nb: to 1, 5 V: to 1, 5

Ti: to 1, 5

Mo: to 3

Cu: to 3

Sn: up to 0.5

W: to 5

Co: to 8

Zr: to 0.5

Ta: to 0.5

Te: up to 0.5

B: 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: to 0.1

is characterized in that while avoiding cold rolling at

Room temperature, the rolling to a required final thickness at a temperature of above 50 ° C to 400 ° C. In the context of the present invention, high-strength steels are understood as steels having 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 present in the microstructure in addition to martensite and / or ferrite and / or bainite and / or perlite. This retained austenite can convert into martensite at appropriate ambient temperatures (TRIP effect, both ε and α ^' -Martensit), wherein at room temperature to about 50 ° C always takes a significant amount of martensite formation by the TRIP effect. This leads to a solidification of the material and, consequently, 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 Resumability.

In addition, by the action of mechanical stresses

Deformation twins (TWIP effect) arise. According to the TRIP conversion mechanism of austenite in martensite is now completely or partially suppressed by raising the forming temperature before the first pass to above 50 ° C to 400 ° C, so that much higher degrees of deformation during rolling in only one roll pass are possible.

The term "cold rolling" is commonly associated with cold rolling

Room temperature related. In the context of the present invention, the term "cold rolling" is also used for elevated temperature cold rolling, which is significantly below the AC1 transformation temperature associated with microstructure transformation, unlike hot rolling in cold rolling according to the invention preferably below a homologous temperature, at which just no creeping processes occur in the steel sheet In the single figure 1 shown in the appendix, the influence of the

Forming temperature during rolling on the solidification behavior of the material based on the characteristics of tensile tests. Compared to forming at room temperature of 20 ° C, significantly higher elongation values are achieved at forming temperatures of 100 ° C or 200 ° C with a significantly lower increase in tensile strength.

Preferably, it is provided that a hot strip or a pre-strip to a temperature of above 50 ° C to 400 ° C, preferably from 70 ° C to 250 ° C, heated or a hot strip or a pre-strip a temperature of above 50 ° C to 400 ° , preferably from 70 ° C to 250 ° C, already having and then cold rolled to the required final thickness at a temperature before the first pass of above 50 ° C to 400 °, preferably from 70 ° C to 250 ° C. By having a temperature, it is understood that the temperature already comes from or is maintained at a previous process step. The previous process step may mean reheating, continuous or discontinuous processing utilizing the existing heat in the hot strip or pre-strip, in particular a hot rolling process, or maintaining the temperature in an oven. By heating the hot strip before cold rolling to the temperature of above 50 ° C to 400 ° C, preferably 70 ° C to 250 ° C, the conversion of austenite to martensite by increasing the stacking fault energy is already substantially reduced or avoided in the first pass, so that the band less strongly solidified during the cold rolling process and more Deformation twins (TWIP effect) are formed in austenite. This results in both lower rolling forces and a significantly improved forming capacity of the strip during the rolling process. To the additional warming of the band due to the

To compensate forming work during cold forming and to keep the strip temperature in the optimum range for the TWIP effect, can be optionally between the rolling passes, a cooling of the strip, for example by compressed air or other liquid or gaseous media, take place.

The steel strip also has a considerable amount after rolling

Restumformvermögen on, since the deformation twins formed in austenite and any residual austenite can be converted by the TRIP effect at room temperature completely or partially in martensite, resulting in an increase in the maximum elongation and thus an improvement in the formability for the component production from the flat product without a additional, the

Cold rolling process connected annealing is connected.

In addition, the formation of strain twins results in improved behavior in subsequent reformations against hydrogen induced delayed cracking and hydrogen embrittlement as compared to cold rolling without prior heating with optional annealing process.

The steel used for the process according to the invention has a multiphase microstructure consisting of ferrite and / or martensite and / or bainite and / or perlite and retained austenite / austenite. The content of retained austenite / austenite may be 5% to 80%. The retained austenite austenite can partially or completely convert to martensite when mechanical stress is applied due to the TRIP effect.

The invention of the underlying alloy has corresponding

mechanical stress on a TRIP and / or TWIP effect. Because of the TRIP and / or TWIP effect and by increasing the

Dislocation density induced strong solidification (analogous to strain hardening) at room temperature, the steel achieves very high values of elongation at break, in particular of uniform elongation, and tensile strength. Advantageously, this property is achieved by the existing retained austenite only at manganese contents of about 3% by weight.

The use of the term "bis" in the definitions of the content ranges, such as 0.01% by weight to 1% by weight, means that the basic values, in the example 0.01 and 1, are included in particular for the production of

high-strength steel strip, which can be provided with a metallic or non-metallic coating, for example based on zinc. An application, inter alia, in vehicle construction, shipbuilding, plant construction, infrastructure construction, aerospace and home appliance technology is conceivable. 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 of> 8 to 40%.

Particularly uniform and homogeneous material properties can be achieved if the steel has the following alloy composition in% by weight:

C: 0.05 to 0.42

Mn:> 5 to <10

Remaining iron including unavoidable steel-accompanying elements, with optional

Admission of one or more of the following elements (in% by weight):

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, especially> 0.5 to 2.5

Nb: 0.005 to 0.4, especially 0.01 to 0.1

B: 0.001 to 0.08, especially 0.002 to 0.01

Ti: 0.005 to 0.6, especially 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, 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

Ca: 0.005 to 0.1 alloying elements are usually added to the steel in order to specifically influence certain properties. An alloying element in different steels can influence different properties. The effect and interaction generally depends strongly on the amount, the presence of other alloying elements and the dissolution state in the material. The

Connections are versatile 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 deteriorate the welding properties and lead to the deterioration of the elongation and toughness properties, therefore, a maximum content of 0.9% by weight is set. The minimum salary is set at 0.0005% by weight. A content of 0.05 to 0.42% by weight is preferred, since in this range the ratio of retained austenite to other phase fractions can be set particularly advantageously.

Manganese Mn: Stabilizes austenite, increases strength and toughness, and allows for strain-induced martensite and / or twin formation in the alloy of the present invention. Contents <3% by weight are insufficient to stabilize the austenite and thus worsen the elongation properties, while at levels above 12% by weight the austenite is excessively stabilized and thereby the strength properties, in particular the yield strength, are reduced. For the manganese steel of the invention with average manganese contents, a range of more than 5 to <10% by weight is preferred, since in this range the Ratio of the phase components to each other and the conversion mechanisms during rolling to final thickness can be favorably influenced.

Aluminum AI: Improves the strength and elongation properties, lowers the specific gravity and influences the conversion behavior of the

alloy according to the invention. Contents of more than 10% by weight of Al

worsen the elongation properties and cause a predominantly brittle fracture behavior. For manganese manganese according to the invention with medium

If manganese is used, an Al content of 0.1 to 5% by weight is preferred in order to increase the strength while maintaining good elongation. In particular, contents of> 0.5 to 3% by weight allow a particularly high strength and elongation at break.

Silicon Si: hinders carbon diffusion, reduces specific gravity and increases strength and 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 set. Optionally, a content of 0.05 to 3% by weight is determined, since contents in this range have the forming properties positive

influence. As particularly advantageous for the forming and

Conversion properties have proved Si contents of> 0.1 to 1, 5% by weight.

Chromium Cr: Improves strength and reduces corrosion rate, retards ferrite and pearlite formation and forms carbides. The maximum content is set at 6% by weight because higher contents result in deterioration in elongation properties and significantly higher costs. For the invention

Manganese steel with average manganese content has a Cr content of 0.1 to 4

% By weight preferred to reduce the precipitation of coarse Cr carbides. In particular, contents of> 0.5 to 2.5% by weight have proven to be advantageous for the stabilization of austenite and the precipitation of fine Cr carbides. In order to achieve the advantageous properties of addition of Al and Si in addition to Cr, the total content of Al + Si + Cr should be more than 1.2% by weight. Molybdenum Mo: acts as a carbide former, increases strength and increases Resistance to delayed cracking and hydrogen embrittlement. Contents of Mo exceeding 3% by weight deteriorate the elongation properties, therefore, a maximum content of 3% by weight is set. For the manganese steel of the present invention having average manganese contents, a Mo content of 0.005 to 1.5% by weight is preferable in order to avoid the precipitation of large Mo carbides. In particular, contents of 0.01 to 0.6% by weight cause the excretion of desired Mo carbides with simultaneously reduced alloying costs.

Phosphorus P: is a trace element from iron ore and is dissolved in the iron lattice as a substitution atom. Phosphorus increases hardness by solid solution strengthening and improves hardenability. However, it is usually tried to

Reduce phosphorus content as much as possible, since it is highly susceptible to segregation, inter alia due to its low diffusion rate and greatly reduces the toughness. The addition of phosphorus to the grain boundaries can cause cracks 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 limited to a maximum of 0.1% by weight, with contents of <0.04% by weight being advantageously sought 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 as it tends to segregate and has a strong embrittlement, resulting in elongation and toughness properties

be worsened. It is therefore trying to minimize quantities

To reach sulfur in the melt (eg by a deep vacuum treatment). For the aforementioned reasons, the sulfur content is limited to a maximum of 0.1% by weight. Particularly advantageous is the limitation to <0.2% by weight to the

To reduce excretion of MnS. Nitrogen N: Is also an accompanying element of steelmaking. In the dissolved state, it improves the strength and toughness properties of steels containing more than 4% by weight of manganese-containing Mn. Low Mn-alloyed steels with <4% by weight Mn containing free nitrogen tend to be strong

Aging effect. The nitrogen diffuses at low temperatures at dislocations and blocks them. He thus causes a strength increase associated with a rapid loss of toughness. Curing of the nitrogen in the form of nitrides is possible, for example, by alloying aluminum, vanadium, niobium or titanium. For the aforementioned reasons, the nitrogen content is limited to a maximum of 0.1% by weight, with contents of <0.05% by weight being preferred in order largely to avoid the formation of AIN.

Microalloying elements are usually added only in very small amounts (<0.1% by weight per element). They work in contrast to the

Alloy elements mainly by excretion formation can also affect the properties in a dissolved state. Despite the low

Additions of quantities strongly influence micro-alloying elements on the production conditions as well as on the processing and final properties.

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

Vanadium V and niobium Nb: These have a grain-refining effect, in particular due to the formation of carbides, which at the same time strength, toughness and

Elongation properties are improved. Levels of over 1, 5% by weight bring no further benefits. 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 provided, in which the alloying elements advantageously bring about grain refinement. In order to improve the economic efficiency and at the same time optimum grain refining, the contents of V can continue to be 0.01

Weight% to 0.3% by weight and the contents of Nb be limited to 0.01 to 0.1% by weight.

Tantalum Ta: Like niobium, tantalum acts as a carbide-forming agent, enhancing its strength, toughness and elongation properties. Contents of more than 0.5% by weight cause no further improvement in the properties. Therefore, a maximum content of 0.5% by weight is optionally set. Preference is given to a minimum content of 0.005% by weight and a maximum content of 0.3% by weight, in which the grain refining can be advantageously effected. To improve the economy and optimize the grain refining is In particular, a content of from 0.01% by weight to 0.1% by weight is desired.

Titanium Ti: As a carbide former, it refines grain, improving its strength, toughness, and elongation properties while reducing intergranular corrosion. Contents of Ti more than 1.5% by weight deteriorate the elongation properties, therefore, a maximum content of Ti of 1.5% by weight is determined. Optionally, a minimum content of 0.005 and a maximum content of 0.6% by weight is determined, in which Ti is advantageously eliminated. A minimum content of 0.01% by weight and a maximum content of 0.3 is preferred

Weight% provided, which ensures optimum precipitation behavior at low alloying costs.

Tin Sn: Tin increases the strength but, like copper, accumulates at higher temperatures below the scale and grain boundaries. It leads by penetration into the grain boundaries to the formation of low-melting phases and associated with cracks in the structure and solder brittleness, which is why optionally a maximum content of <0.5% by weight is provided. For reasons mentioned above, contents of <0.2% by weight are preferably set. In particular, to avoid low-melting phases and cracks in the structure, contents of <0.05% by weight are preferred.

Copper Cu: Reduces the corrosion rate and increases strength. Contents of more than 3% by weight deteriorate the manufacturability by forming low-melting phases during casting and hot rolling, therefore a maximum content of 3

Weight% is set. Optionally, a maximum content of <0.5% by weight is provided, in which the occurrence of cracks during casting and hot rolling can be advantageously prevented. Cu contents of <0.1% by weight have proven particularly advantageous for avoiding low-melting phases and for preventing cracks.

Tungsten W: acts as a carbide former and increases strength and heat resistance. Contents of W of more than 5% by weight deteriorate the elongation properties, and therefore a maximum content of 5% by weight is set. Optionally, a maximum content of 3% by weight and a minimum content of 0.01% by weight determined in which advantageously takes place the precipitation of carbides.

In particular, a minimum content of 0.2% by weight and a maximum content of 1.5% by weight are preferred, which enables optimum precipitation behavior at low alloying costs.

Cobalt Co: Increases the strength of the steel, stabilizes the austenite and improves the heat resistance. Contents of over 8% by weight worsen the

Elongation properties, which is why a maximum content of 8% by weight is set. Optionally, a maximum content of <5% by weight and a minimum content of 0.01% by weight are determined, which advantageously improve the strength and heat resistance. Preference is given to a minimum content of 0.3% by weight and a

Maximum content of 2% by weight provided, in addition to the

Strength properties favorably influences austenite stability. Zirconium Zr: acts as a carbide former and improves strength. Contents of Zr exceeding 0.5% by weight deteriorate the elongation properties, and therefore a maximum content of 0.5% by weight is set. Optionally, a

Maximum content of 0.3% by weight and a minimum content of 0.005% by weight, since carbides are advantageously eliminated in this area.

Preferably, a minimum content of 0.01% by weight and a maximum content of 0.2% by weight are provided, which advantageously allow optimal carbide precipitation at low alloying costs.

Boron B: Delays the austenite transformation, improves the hot working properties of steels and increases the strength at room temperature. It unfolds its effect even at very low alloy contents. Contents above 0.15% by weight greatly deteriorate the elongation and toughness properties, therefore, the maximum content is set to 0.15% by weight. Optionally, a minimum content of 0.001% by weight and a maximum content of 0.08% by weight is determined, in which the strength-increasing effect of boron is advantageously used. Preferably, a minimum content of 0.002% by weight and a maximum content of 0.01% by weight, which is an optimal use for increasing the strength at

allow simultaneous improvement of the conversion behavior. Tellurium Te: Improves corrosion resistance and mechanical Properties as well as the machinability. Furthermore, Te increases the strength of MnS, which is less elongated in the rolling direction during hot and cold rolling. Contents above 0.5% by weight deteriorate the elongation and toughness properties, and therefore a maximum content of 0.5% by weight is determined. Optionally, a minimum content of 0.005% by weight and a

Maximum content of 0.3% by weight determined by the mechanical

Advantageously improve 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 allow an optimization of the mechanical properties while reducing the alloying costs.

Calcium Ca: Used to modify non-metallic oxide inclusions, which could otherwise lead to unwanted alloy failure due to inclusions in the microstructure, which act as stress concentration sites and weaken the metal composite. 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 above 0.1% by weight bring no further advantage in the inclusion modification, deteriorate the manufacturability and should be avoided due to the high vapor pressure of Ca in molten steel. Therefore, a maximum content of 0.1% by weight is provided.

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, of a high-strength manganese-containing steel comprises the steps:

Melting of a steel melt containing (in% by weight):

C: 0.0005 to 0.9

Mn: more than 3.0 to 12

Remaining iron including unavoidable steel-accompanying elements, with optional

Admission of one or more of the following elements (in% by weight):

AI: until 10

Si: to 6

Cr: to 6

Nb: to 1, 5 V: to 1, 5

Ti: to 1, 5

Mo: to 3

Cu: to 3

Sn: up to 0.5

W: to 5

Co: to 8

Zr: to 0.5

Ta: to 0.5

Te: up to 0.5

B: 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: to 0.1

- Pouring the molten steel to a Vorband by means of a

horizontal or vertical strip casting close to the final dimensions or pouring 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 final near net produced Vorbandes to 1000 ° C to 1200 ° C and then hot rolling of the pre-strip to a hot strip or hot rolling of 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 of the hot strip at a reel 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 to a rolled steel strip at a temperature before the first pass of above 50 ° C to 400 ° 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 dressing of the steel strip

- Optional coating of the steel strip with a corrosion protection coating With regard to further advantages, reference is made to the above statements.

Typical thickness ranges for pre-strip are 1 mm to 35 mm and for slabs and thin slabs 35 mm to 450 mm. Preferably, it is provided that the slab or thin slab is hot rolled to a hot strip having a thickness of 20 mm to 1, 5 mm or hot rolled near the endabmessungs close cast stock 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 of the pre - strip from the casting heat to a hot strip with optional intermediate heating between the individual rolling passes of the

Hot rolling is intended for reheating temperatures in the range of 720 ° C to 1200 ° C. If only a few rolling passes are required, the reheat temperature can be selected at the lower end of the range. The hot strip may optionally be subjected to a heat treatment in the temperature range between 580 ° C and 820 ° C for 1 minute to 48 hours, with higher temperatures being associated with shorter treatment times and vice versa. The annealing can be carried out both in a bell annealer (longer annealing times), as well as, for example, in a continuous annealing (shorter annealing times). The optional annealing serves to reduce the strength and / or the increase of the

Restaustenitanteils the hot strip before the cold rolling process, whereby the forming properties for the subsequent process are advantageously improved.

Following the hot rolling process, the cold rolling takes place according to the invention increased temperature of the hot strip with the aim to adjust the required thicknesses for the end application of> 0.15 mm to 10 mm of the steel strip. Following this, optionally, a further annealing process can be carried out optionally coupled with a coating process and finally a skin-pass process with which the surface structure required for the end application is adjusted. Preferably, the steel strip is hot-dip or electrolytically galvanized or metallic, inorganic or organic coated. 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, for example, as a sheet metal section, coil or sheet by cold forming at room temperature or by warm forging at temperatures of 60 ° C to below the AC3, preferably <450 ° C to form a component, which due to the considerable Resumability on a 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 helically welded tubes, whereby here too, the considerable residual deformability of the steel strip can be dispensed with an intermediate annealing, depending on the application. The tube may have an outer and / or inner metallic, organic or inorganic coating.

The tube produced in this way can then be further deformed, for example drawn or expanded, or shaped by means of hydroforming and further processed into a component.

Areas of application are primarily the automotive and commercial vehicle industry as well as mechanical engineering, white goods, construction, and applications

Temperatures below 0 ° C and as security steel. Safety steels are used to protect vehicles and buildings against fire and impact, and have high hardness and toughness.

Attempts were made to investigate the mechanical properties of the steel strips produced according to the invention with exemplary alloys 1 to 4 carried out. The alloys 1 to 4 contain the following elements

listed contents in wt .-%:

The steel strips produced from the abovementioned alloys 1 to 4 were cold-rolled for comparison, 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:

Cumulative rolling force is understood to mean adding up 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 bandwidth of 1000 mm. The degree of deformation e is defined as the quotient of the change in thickness Ad of the steel strip examined by the initial thickness dO of the steel strip examined. The rolling force reduction is the calculated reduction in rolling force at 250 ° C as compared with the cold rolling force.

Also, the elongation at break A50 was evaluated:

Alloy Elongation at break A50 Elongation at break A50

[%] cold rolled [%] 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 characteristics stand for the elongation in the rolling direction.

Claims

Patent claims

1 . Process for producing a cold-rolled steel strip from a high-strength manganese-containing steel with TRIP properties containing (in% by weight):

C: 0.0005 to 0.9

Mn: more than 3.0 to 12

Rest of iron including unavoidable steel-accompanying elements, with optional

Addition of one or more of the following elements (in percent by weight):

AI: until 10

Si: until 6

Cr: up to 6

Nb: 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: until 5

Co: until 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, in particular <0.05

Approximately: up to 0.1

characterized in that cold rolling to a required final thickness takes place at a temperature before the first pass of above 50 ° C to 400 ° C.

2. The method according to claim 1, characterized in that the cold rolling to the required final thickness takes place at a temperature before the first pass of cold rolling of 70 ° C to 250 ° C.

3. The method according to claim 1 or 2, characterized in that a hot strip or a pre-strip is heated to a temperature of above 50°C to 400°C, preferably from 70°C to 250°C, or a hot strip or a pre-strip is heated to a temperature of above 50°C to 400°C, preferably of 70°C up to 250 ° C, and is then cold rolled to the required final thickness at a temperature before the first pass of above 50 ° C to 400 ° C, preferably from 70 ° C to 250 ° C.

4. The method according to any one of claims 1 to 3, characterized 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 cold rolling .

5. Method according to one of claims 1 to 4, characterized in that the steel contains (in percent by weight):

C: 0.05 to 0.42

6. Method according to one of claims 1 to 5, characterized in that the steel contains (in percent by weight):

Mn: > 5 to < 10

7. Method according to one of claims 1 to 6, characterized in that the steel contains (in percent by weight):

AI: 0.1 to 5, especially > 0.5 to 3

8. Method according to one of claims 1 to 7, characterized in that the steel contains (in percent by weight):

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

9. Method according to one of claims 1 to 8, characterized in that the steel contains (in percent by weight):

Cr: 0.1 to 4, especially >0.5 to 2.5

10. Method according to claims 7 to 9, characterized in that the steel contains (in percent by weight):

Al + Si + Cr > 1,2

1 1 . Method according to one of claims 1 to 10, characterized in that the steel contains (in% by weight):

Nb: 0.005 to 0.4, especially 0.01 to 0.1

12. The method according to any one of claims 1 to 1 1, characterized in that the steel contains (in% by weight):

V: 0.005 to 0.6, especially 0.01 to 0.3

13. The method according to any one of claims 1 to 12, characterized in that the steel contains (in% by weight):

Ti: 0.005 to 0.6, especially 0.01 to 0.3

14. The method according to any one of claims 1 to 13, characterized in that the steel contains (in percent by weight):

Mon: 0.005 to 1.5, especially 0.01 to 0.6

15. The method according to any one of claims 1 to 14, characterized in that the steel contains (in percent by weight):

Sn: <0.2, especially <0.05

16. The method according to any one of claims 1 to 15, characterized in that the steel contains (in% by weight):

Cu: <0.5, especially <0.1

17. The method according to any one of claims 1 to 16, characterized in that the steel contains (in% by weight):

W: 0.01 to 3, in particular 0.2 to 1.5

18. The method according to any one of claims 1 to 17, characterized in that the steel contains (in percent by weight):

Co: 0.01 to 5, especially 0.3 to 2

19. The method according to any one of claims 1 to 18, characterized in that the steel contains (in percent by weight): Zr: 0.005 to 0.3, especially 0.01 to 0.2

20. The method according to any one of claims 1 to 19, characterized in that the steel contains (in percent by weight):

Ta: 0.005 to 0.3, especially 0.01 to 0.1

21. Method according to one of claims 1 to 20, characterized in that the steel contains (in% by weight):

Te: 0.005 to 0.3, especially 0.01 to 0.1

22. The method according to any one of claims 1 to 21, characterized in that the steel contains (in% by weight):

B: 0.001 to 0.08, especially 0.002 to 0.01

23. Method according to one of claims 1 to 22, characterized in that the steel contains (in percent by weight):

Approx: 0.005 to 0.1

24. The method according to any one of claims 1 to 23, characterized in that the cold-rolled steel strip is produced with a required final thickness of less than 10 mm, preferably less than 4 mm, comprising the steps:

- Melting a melt of steel according to one of claims 1, 5 to 23,

- casting the melt into a pre-strip using a horizontal or vertical strip casting process close to the final dimension or casting the molten steel into a slab or thin slab using 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 into a hot strip or reheating the pre-strip produced close to the final dimension to 1000°C to 1200°C and then hot rolling the pre-strip into 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 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,

- Cold rolling of the hot strip or the pre-strip according to the steps according to claims 1 to 3,

- 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 tempering of the steel strip,

- Optional coating of the steel strip with a metallic, organic or inorganic corrosion protection coating.

25. Use of a steel strip produced by the method according to at least one of claims 1 to 24 for the production of components by hot, cold or warm forming or for the production of longitudinally or spirally welded pipes or for the production of components for the automobile and

Commercial vehicle industry as well as mechanical engineering, white goods and construction or for use in the low temperature range below 0°C to -273°C or as safety steel.