METHOD FOR PRODUCING A COLD-ROLLED STEEL STRIP HAVING TRIP-CHARACTERISTICS MADE OF A HIGH-STRENGTH MANGAN-CONTAINING STEEL

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. %): Al: 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.

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Description

The invention relates to a method for producing a cold-rolled steel strip made of a high-strength, manganese-containing steel. Steel strip is understood to mean hereinunder in particular steel strips but also steel sheets. Typical tensile strengths Rm in these steels are about 800 MPa to 2000 MPa. Elongations at fracture 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 are in % by weight and relate to the steel melt): C: 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, with the remainder being iron and unavoidable impurities. Optionally, one or more elements from the group “V, Nb, Ti” are provided, wherein the sum of the contents of these elements is at most equal to 0.5. The steel is said to be characterised in that it can be produced in a more cost-effective manner than steels with a high content of manganese and at the same time has high elongation at fracture values and, associated therewith, a considerably improved deformability.

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

    • melting the above-described steel melt,
    • producing a starting product for subsequent hot-rolling, in that the steel melt is cast into a string, from which at least one slab or thin slab is separated off as a starting product for the hot-rolling, or into a cast strip which is supplied to the hot-rolling process as a starting product,
    • heat-treating the starting product in order to bring the starting product to a hot-rolling starting temperature of 1150 to 1000° C.,
    • hot-rolling the starting product to form a hot strip having a thickness of at most 2.5 mm, wherein the hot-rolling is terminated at a hot-rolling end temperature of 1050 to 800° C.,
    • reeling the hot strip to form a coil at a reeling temperature of ≤700° C., optionally annealing the hot strip and then cold-rolling it to a thickness of at most 60% of the thickness of the hot strip.

Depending on the alloy point, this steel can have a metastable austenite with the capacity for stress-induced martensite formation (TRIP effect).

The international patent application WO 2005/061152 A1 also describes a method for producing hot strips from a deformable lightweight steel, which can be successfully cold-deep-drawn, with a Mn content of 9 to 30 wt. %. In addition to a high level of tensile strength, the hot strip has TRIP properties. The German laid-open document DE 197 27 759 A1 discloses an ultra-high-strength austenitic lightweight steel which can be successfully deep-drawn with a tensile strength of up to 1100 MPa, which likewise has TRIP and TWIP properties. The German laid-open document DE 10 2012 111 959 A1 describes a high manganese-content steel material with TRIP and TWIP properties, which undergoes an increase in harness and deformability by virtue of cold-forming below ambient temperature preferably in the range of +25° C. to −200° C. The German laid-open document DE 10 2009 030 324 A1 describes a high manganese-content steel with a low tendency towards hydrogen embrittlement and with high tensile strengths while at the same time having high elongation at fracture values. The patent application US 2012/0059196 A1 discloses a method for producing a hot strip with a horizontal strip casting installation. 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 U.S. Pat. No. 6,358,338 B1 also relates to a method for producing a steel strip from a high manganese-content steel. In order to increase the tensile strength and extensibility, the steel strip is subjected to recrystallisation annealing after cold-rolling. In the patent application US 2009/0074605 A1 a high manganese-content steel strip with excellent crash behaviour and with high tensile strength and elongation values is produced in that the steel strip is cold-rolled after hot-rolling and then annealed at 600° C.

Furthermore, German laid-open document DE 10 2012 013 113 A1 describes TRIP steels which have a predominantly ferritic basic microstructure having incorporated residual austenite. Owing to its intense cold-hardening, the TRIP steel achieves high values for uniform elongation and tensile strength.

A disadvantage with these manganese-containing steels with a TRIP effect is that, during production of a cold-rolled steel strip, the achievable degree of deformation is limited owing to the intense cold-hardening of the material during cold-rolling and the high loading on the roll stand associated therewith. In order to achieve high degrees of cold-forming, a plurality of cold-rolling steps with correspondingly low degrees of deformation are often required, wherein prior to a renewed cold-rolling step, in each case recrystallisation annealing must be carried out in order to loosen the material and therefore render it capable of being cold-rolled. This procedure with a plurality of cold-rolling steps with intermediate recrystallising annealing is very time-consuming and expensive and associated with additional CO2 emissions.

On the basis of the above, the object of the present invention is to provide a method for producing a cold-rolled steel strip made of a high-strength, manganese-containing steel with TRIP properties, with which the cold-rolling to the required end thickness can be effected in a more economical and ecologically-friendly manner. In addition, a production route from the melting of the steel to the steel strip cold-rolled to the required end thickness is to be provided.

This object is achieved by a method for producing a steel strip having the features of claim 1. Advantageous embodiments of the invention are described in the respective dependent claims.

The method in accordance with the invention for producing a cold-rolled steel strip from high-strength, manganese-containing steel with TRIP properties containing (in wt. %):

C: 0.0005 to 0.9

Mn: more than 3.0 to 12
with the remainder being iron including unavoidable steel-associated elements, with optional addition by alloying of one or more of the following elements (contents in wt. % and in relation to the steel melt):

Al: to 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: to 0.5 W: to 5 Co: to 8 Zr: to 0.5 Ta: to 0.5 Te: 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 characterised in that in avoiding cold-rolling at ambient temperature, the rolling to a required end thickness takes place at a temperature above 50° C. to 400° C.

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

The cause of the intense cold-hardening of these high-strength, manganese-containing steels with a TRIP effect is the proportion of residual austenite contained in the microstructure in addition to martensite and/or ferrite and/or bainite and/or perlite. This residual austenite can be converted at appropriate ambient temperatures into martensite (TRIP effect both ε as well as α′ martensite), wherein at ambient temperature up to about 50° C. a substantial proportion of martensite formation always takes place owing to the TRIP effect. This leads to a hardening of the material and, associated therewith, to an intense increase in the rolling forces during cold-rolling, even during the first pass and is associated with a reduction in the maximum degree of deformation. The cold-rolled strip then has a high level of strength and low residual deformation capability. In addition, the influence of mechanical stresses can cause deformation twins (TWIP effect).

In accordance with the invention, by means of the raising of the deformation temperature prior to the first pass to above 50° C. to 400° C., the TRIP conversion mechanism from austenite to martensite is now wholly or partly suppressed and so substantially higher degrees of deformation are possible during rolling in only one rolling pass.

The term “cold-rolling” is conventionally frequently related to cold-rolling at ambient temperature. In conjunction with the present invention, the term “cold-rolling” is also used for cold-rolling at raised temperature. In contrast to hot-rolling, this raised temperature in the case of cold-rolling in accordance with the invention is clearly below the AC1 conversion temperature associated with a microstructure conversion. The cold-rolling in accordance with the invention also preferably takes place below a homologous temperature, at precisely which creeping processes still do not occur in the steel sheet.

In the single FIGURE enclosed herewith, FIG. 1, the influence of the deformation temperature during rolling on the hardening behaviour of the material is illustrated with the aid of the characteristic values of tensile tests. In comparison to the deformation at ambient temperature of 20° C., at deformation temperatures of 100° C. or 200° C., clearly greater elongation values are achieved while having a clearly lower increase in tensile strength.

Provision is preferably made that a hot strip or a pre-strip is heated to a temperature above 50° C. to 400° C., preferably from 70° C. to 250° C., or a hot strip or a pre-strip is already at a temperature above 50° C. to 400° C., preferably from 70° C. to 250° C., and is then cold-rolled to the required end thickness at a temperature, prior to the first pass, above 50° C. to 400° C., preferably from 70° C. to 250° C. “Being at a temperature” is understood to mean that the temperature is the result of a preceding process step or this temperature has been maintained. The preceding process step can mean a reheating step, a continuous or discontinuous processing step using the available heat in the hot strip or pre-strip, in particular a hot-rolling process, or maintenance of the temperature in a furnace.

By heating the hot strip, prior to cold-rolling, to the temperature above 50° C. to 400° C., preferably 70° C. to 250° C., the conversion of austenite into martensite by increasing the stacking fault energy in the first rolling pass is substantially reduced or avoided and so the strip hardens less intensely during the cold-rolling process and more deformation twins (TWIP effect) are formed in the austenite. This results in both lower rolling forces and also a substantially improved deformation capability for the strip during the roll process. In order to compensate for the additional heating of the strip by reason of the deformability during cold-forming and to keep the strip temperature in the range which is optimal for the TWIP effect, cooling of the strip, e.g. by compressed air or other liquid or gaseous media, can optionally take place between the individual rolling passes.

Furthermore, after rolling, the steel strip comprises a considerably residual deformation capability since the deformation twins formed in the austenite and residual austenite which may be present can wholly or partially convert into martensite at ambient temperature owing to the TRIP effect, this is associated with an increase in the maximum elongation and therefore an improvement in the deformation capability for the production of components from the flat product even without additional annealing associated with the cold-rolling process.

In addition, the formation of deformation twins brings about improved behaviour during subsequent deformations with respect to hydrogen-induced delayed crack formation and hydrogen embrittlement compared with cold-rolling without prior heating with an optionally associated annealing process.

The steel used for the method in accordance with the invention has a multi-phase microstructure, including ferrite and/or martensite and/or bainite and/or perlite and residual austenite/austenite. The proportion of residual austenite/austenite can be 5% to 80%. The residual austenite/austenite can partially or completely convert into martensite owing to the TRIP effect when mechanical stresses are present.

The alloy forming the basis of the invention has a TRIP and/or TWIP effect when subjected to the relevant mechanical stress. Owing to the intense hardening (similar to cold-hardening) at ambient temperature, induced by the TRIP and/or TWIP effect and by the increase in the dislocation density, the steel achieves very high values in terms of elongation at fracture, in particular uniform elongation, and tensile strength. In an advantageous manner, this property is achieved owing to the residual austenite present, only in the case of manganese contents of over 3 wt. %.

The use of the word “to” in the definitions of the content ranges, such as e.g. 0.01 wt. % to 1 wt. %, means that the limit values, 0.01 and 1 in the example, are also included.

The steel in accordance with the invention is suitable in particular for producing a high-strength steel strip which can be provided with a metallic or non-metallic coating, e.g. a zinc-based coating. It may feasibly be used inter alia in the automotive industry, shipbuilding, plant design, infrastructure, the aerospace industry and in household appliances. The high proportion of austenite means that the steel produced in accordance with 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 fracture A80 of 3 to 40%, preferably >8 to 40%.

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

C: 0.05 to 0.42 Mn: >5 to <10

with the remainder being iron including unavoidable steel-associated elements, with optional addition by alloying of 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
Nb: 0.005 to 0.4, in particular 0.01 to 0.1
B: 0.001 to 0.08, in particular 0.002 to 0.01
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
V: 0.005 to 0.6, in particular 0.01 to 0.3

Ca: 0.005 to 0.1

Alloy elements are generally added to the steel in order to influence specific properties in a targeted manner. An alloy element can thereby influence different properties in different steels. The effect and interaction generally depend greatly upon the quantity, presence of further alloy elements and the solution state in the material. The correlations are varied and complex. The effect of the alloy elements in the alloy in accordance with the invention will be discussed in greater detail hereinafter. The positive effects of the alloy elements used in accordance with the invention will be described hereinafter:

Carbon C: is required to form carbides, stabilises the austenite and increases the strength. Higher contents of C impair the welding properties and result in the impairment of the elongation and toughness properties, for which reason a maximum content of 0.9 wt. % is set. The minimum content is set at 0.0005 wt. %. A content of 0.05 to 0.42 wt. % is preferred because in this range the ratio of residual austenite to other phase proportions can be set in a particularly advantageous manner.

Manganese Mn: stabilises the austenite, increases the strength and the toughness and renders possible a deformation-induced martensite formation and/or twinning in the alloy in accordance with the invention. Contents ≤3 wt. % are not sufficient to stabilise the austenite and therefore impair the elongation properties whereas with contents of over 12 wt. % the austenite is stabilised too much and as a result the strength properties, in particular the yield strength, are reduced. For the manganese steel in accordance with the invention having average manganese contents, a range of over 5 to <10 wt. % is preferred because in this range the ratio of the phase proportions to each other and the conversion mechanisms can be advantageously influenced during rolling to the end thickness.

Aluminium Al: improves the strength and elongation properties, decreases the relative density and influences the conversion behaviour of the alloy in accordance with the invention. Al contents of more than 10 wt. % impair the elongation properties and cause predominantly brittle fracture behaviour. For the manganese steel in accordance with the invention with average manganese contents, an Al content of 0.1 to 5 wt. % is preferred in order to increase the strength while having a good degree of elongation. In particular, contents of >0.5 to 3 wt. % render possible a particularly high level of strength and elongation at fracture.

Silicon Si: impedes the diffusion of carbon, reduces the relative density and increases the strength and elongation properties and toughness properties. Contents of more than 6 wt. % prevent further processing by cold-rolling by reason of embrittlement of the material. Thus, a maximum content of 6 wt. % is set. Optionally, a content of 0.05 to 3 wt. % is set because contents in this range positively influence the deformation properties. Si contents of >0.1 to 1.5 wt. % have proved to be particularly advantageous for the deformation and conversion properties.

Chromium Cr: improves the strength and reduces the rate of corrosion, delays the formation of ferrite and perlite and forms carbides. The maximum content is set to 6 wt. % since higher contents result in an impairment of the elongation properties and substantially higher costs. For the manganese steel in accordance with the invention having average manganese contents, a Cr content of 0.1 to 4 wt. % is preferred in order to reduce the precipitation of coarse Cr carbides. In particular, contents of >0.5 to 2.5 wt. % have proved to be advantageous for stabilising the austenite and precipitating fine Cr carbides. In order to achieve the advantageous properties of an addition of 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-forming agent, increases the strength and increases the resistance to delayed crack formation and hydrogen embrittlement. Mo contents of more than 3 wt. % impair the elongation properties, for which reason a maximum content of 3 wt. % is set. For the manganese steel in accordance with the invention having average manganese contents, a Mo content of 0.005 to 1.5 wt. % is preferred in order to avoid the precipitation of excessively large Mo carbides. In particular, contents of 0.01 wt. % to 0.6 wt. % bring about the precipitation of desired Mo carbides while incurring reduced alloy costs.

Phosphorus P: is a trace element from iron ore and is dissolved in the iron lattice as a substitution atom. Phosphorous increases the hardness by means of solid solution hardening and improves the hardenability. However, attempts are generally made to lower the phosphorous content as much as possible because inter alia it exhibits a strong tendency towards segregation owing to its low diffusion rate and greatly reduces the level of toughness. The attachment of phosphorous to the grain boundaries can cause cracks along the grain boundaries during hot-rolling. Moreover, phosphorous increases the transition temperature from tough to brittle behaviour by up to 300° C. For the aforementioned reasons, the phosphorous content is limited to a maximum of 0.1 wt. %, wherein contents <0.04 wt. % are advantageously sought for the aforementioned reasons.

Sulphur S: like phosphorous, is bound as a trace element in the iron ore. It is generally not desirable in steel because it exhibits a strong tendency towards segregation and has a greatly embrittling effect, whereby the elongation and toughness properties are impaired. An attempt is therefore made to achieve amounts of sulphur in the melt which are as low as possible (e.g. by deep vacuum treatment). For the aforementioned reasons, the sulphur content is limited to a maximum of 0.1 wt. %. In a particularly advantageous manner the limit is <0.2 wt. % in order to reduce the precipitation of MnS.

Nitrogen N: is likewise an associated element from steel production. In the dissolved state, it improves the strength and toughness properties in steels with a high manganese content of greater than or equal to 4 wt. % Mn. Lower Mn-alloyed steels with <4 wt. % Mn, which contain free nitrogen, tend to have a strong aging effect. The nitrogen diffuses even at low temperatures to dislocations and blocks the same. It thus produces an increase in strength associated with a rapid loss of toughness. Binding of the nitrogen in the form of nitrides is possible e.g. by alloying aluminium, vanadium, niobium or titanium. For the aforementioned reasons, the nitrogen content is limited to a maximum of 0.1 wt. %, wherein contents <0.05 wt. % are preferably sought to substantially avoid the formation of AlN.

Microalloy elements are generally added only in very small amounts (<0.1 wt. % per element). In contrast to the alloy elements, they mainly act by precipitate formation but can also influence the properties in the dissolved state. Despite the small amounts added, microalloy elements greatly influence the production conditions and the processing properties and final properties.

Typical microalloy 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 act in a grain-refining manner in particular by forming carbides, whereby at the same time the strength, toughness and elongation properties are improved. Contents of more than 1.5 wt. % do not provide any further advantages. Optionally, for vanadium and niobium, a minimum content of greater than or equal to 0.005 wt. % and a maximum content of 0.6 (V) or 0.4 (Nb) wt. % is preferably provided, with which the alloy elements advantageously provide grain refinement. Furthermore, in order to improve the economic feasibility whilst at the same time achieving optimum grain refinement, the contents of V can continue to be restricted to 0.01 wt. % to 0.3 wt. % and the contents of Nb to 0.01 to 0.1 wt. %.

Tantalum Ta: tantalum acts in a similar manner to niobium as a carbide-forming agent in a grain-refining manner and thereby improves the strength, toughness and elongation properties at the same time. Contents over 0.5 wt. % do not provide any further improvement in the properties. Thus, a maximum content of 0.5 wt. % is optionally set. A minimum content of 0.005 wt. % and a maximum content of 0.3 wt. % are preferred, with which the grain refinement can advantageously be produced. In order to improve economic feasibility and to optimise grain refinement, a content of 0.01 wt. % to 0.1 wt. % is particularly preferably sought.

Titanium Ti: acts in a grain-refining manner as a carbide-forming agent, whereby at the same time the strength, toughness and elongation properties are improved, and reduces the inter-crystalline corrosion. Ti contents of more than 1.5 wt. % impair the elongation properties, for which reason a maximum Ti content of 1.5 wt. % is set. Optionally, a minimum content of 0.005 and a maximum content of 0.6 wt. % are set, with which Ti is advantageously precipitated. Preferably, a minimum content of 0.01 wt. % and a maximum content of 0.3 wt. % are provided, which ensures optimum precipitation behaviour with low alloy costs.

Tin Sn: tin increases the strength but, similarly to copper, accumulates beneath the scale layer and at the grain boundaries at higher temperatures. Owing to the penetration into the grain boundaries it leads to the formation of low melting point phases and, associated therewith, to cracks in the microstructure and to solder brittleness, for which reason a maximum content of ≤0.5 wt. % is optionally provided. For the aforementioned reasons, contents of <0.2 wt. % are preferably set. Contents of <0.05 wt. % are particularly advantageously preferred in order to avoid low melting point phases and cracks in the microstructure.

Copper Cu: reduces the rate of corrosion and increases the strength. Contents of over 3 wt. % impair producibility by forming low melting point phases during casting and hot-rolling, for which reason a maximum content of 3 wt. % is set. Optionally, a maximum content of <0.5 wt. % is provided, with which the occurrence of cracks during casting and hot-rolling can be advantageously prevented. Cu contents of <0.1 wt. % have proved to be particularly advantageous in avoiding low melting point phases and in avoiding cracks.

Tungsten W: acts as a carbide-forming agent and increases the strength and heat resistance. W contents of more than 5 wt. % impair the elongation properties, for which reason a maximum content of 5 wt. % is set. Optionally, a maximum content of 3 wt. % and a minimum content of 0.01 wt. % is set, with which the precipitation of carbides advantageously takes place. In particular, a minimum content of 0.2 wt. % and a maximum content of 1.5 wt. % are preferred, which renders possible optimum precipitation behaviour with low alloy costs.

Cobalt Co: increases the strength of the steel, stabilises the austenite and improves the heat resistance. Contents of more than 8 wt. % impair the elongation properties, for which reason a maximum content of 8 wt. % is set. Optionally, a maximum content of ≤5 wt. % and a minimum content of 0.01 wt. % is set which advantageously improve the strength and heat resistance. Preferably, a minimum content of 0.3 wt. % and a maximum content of 2 wt. % are provided which advantageously influences the austenite stability along with the strength properties.

Zirconium Zr: acts as a carbide-forming agent and improves the strength. Zr contents of more than 0.5 wt. % impair the elongation properties, for which reason a maximum content of 0.5 wt. % is set. Optionally, a maximum content of 0.3 wt. % and a minimum content of 0.005 wt. % is set, since in this range carbides are advantageously precipitated. Preferably, a minimum content of 0.01 wt. % and a maximum content of 0.2 wt. % is provided which advantageously render possible optimum carbide precipitation with low alloy costs.

Boron B: delays the austenite conversion, improves the hot-forming properties of steels and increases the strength at ambient temperature. It achieves its effect even with very low alloy contents. Contents above 0.15 wt. % greatly impair the elongation and toughness properties, for which reason the maximum content is set to 0.15 wt. %. Optionally, a minimum content of 0.001 wt. % and a maximum content of 0.08 wt. % are set, with which the strength-increasing effect of boron is advantageously used. A minimum content of 0.002 wt. % and a maximum content of 0.01 wt. % are preferred which render possible optimum use for increasing strength whilst at the same time improving the conversion behaviour.

Tellurium Te: improves the corrosion-resistance and the mechanical properties and machinability. Furthermore, Te increases the solidity of MnS which, as a result, is lengthened to a lesser extent in the rolling direction during hot-rolling and cold-rolling. Contents above 0.5 wt. % impair the elongation and toughness properties, for which reason a maximum content of 0.5 wt. % is set. Optionally, a minimum content of 0.005 wt. % and a maximum content of 0.3 wt. % are set, which advantageously improve the mechanical properties and increase the solidity of MnS present. Furthermore, a minimum content of 0.01 wt. % and a maximum content of 0.1 wt. % are preferred which render possible optimisation of the mechanical properties whilst at the same time reducing alloy costs.

Calcium Ca: is used for modifying non-metallic oxidic inclusions which could otherwise result in the undesired failure of the alloy as a result of inclusions in the microstructure which act as stress concentration points and weaken the metal composite. Furthermore, Ca improves the homogeneity of the alloy in accordance with the invention. In order to achieve a corresponding effect, a minimum content of 0.0005 wt. % may be necessary. Contents above 0.1 wt. % do not provide any further advantage in the modification of inclusions, impair producibility and should be avoided by reason of the high vapour pressure of Ca in steel melts. Therefore, a maximum content of 0.1 wt. % is provided.

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

melting a steel melt containing (in wt. %):

C: 0.0005 to 0.9

Mn: more than 3.0 to 12
with the remainder being iron including unavoidable steel-associated elements, with optional addition by alloying of one or more of the following elements (in wt. %):

Al: to 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: to 0.5 W: to 5 Co: to 8 Zr: to 0.5 Ta: to 0.5 Te: 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

casting the steel melt to form a pre-strip by means of a horizontal or vertical strip casting process approximating the final dimensions or casting the steel melt to form a slab or thin slab by means of a horizontal or vertical slab or thin slab casting process,

re-heating the slab or thin slab to 1050° C. to 1250° C. and then hot-rolling the slab or thin slab to form a hot strip, or re-heating the pre-strip, produced to approximately the final dimensions, to 1000° C. to 1200° C. and then hot-rolling the pre-strip to form a hot strip, or hot-rolling the pre-strip without re-heating from the casting heat to form a hot strip with optional intermediate heating between individual rolling passes of the hot-rolling,

reeling the hot strip at a reeling temperature between 820° C. and ambient temperature,

optionally annealing the hot strip with the following parameters:

annealing temperature: 580 to 820° C., annealing duration: 1 minute to 48 hours,

while avoiding cold-rolling at ambient temperature, rolling the hot strip with a required end thickness of less than 10 mm to a rolled steel strip at a temperature prior to the first pass above 50° C. to 400° C.

optionally annealing the steel strip with the following parameters:

annealing temperature: 580 to 820° C., annealing duration: 1 minute to 48 hours.

optionally acid-cleaning and/or skin-pass rolling the steel strip,

optionally coating the steel strip with a corrosion protection coating.

In relation to further advantages, reference is made to the above statements.

Typical thickness ranges for the pre-strip are 1 mm to 35 mm and for slabs and thin slabs they are 35 mm to 450 mm. Provision is preferably made that the slab or thin slab is hot-rolled to form a hot strip having a thickness of 20 mm to 1.5 mm or the pre-strip, cast to approximately the final dimensions, is hot-rolled to form a hot strip having a thickness of 8 mm to 1 mm. The cold-rolled steel strip produced in accordance with the invention has a thickness of e.g. >0.15 mm to 10 mm.

Re-heating temperatures in the range of 720° C. to 1200° C. are provided for hot-rolling of the pre-strip from the casting heat to form a hot strip with optional intermediate heating between the individual rolling passes of the hot-rolling process. If only a few rolling passes are then necessary, the re-heating temperature can be selected at the lower end of the range.

The hot strip can optionally be subjected to a heat treatment in the temperature range between 580° C. and 820° C. for 1 minute to 48 hours, wherein higher temperatures are associated with shorter treatment times and vice versa. Annealing can take place both in a batch-type annealing process (longer annealing times) and e.g. in a continuous annealing process (shorter annealing times). The optional annealing serves to reduce the strength and/or to increase the residual austenite proportion of the hot strip prior to the cold-rolling process, whereby the deformation properties are advantageously improved for the subsequent process.

After the hot-rolling process, cold-rolling takes place with the hot strip at a temperature raised in accordance with the invention with the aim of setting the thicknesses of ≥0.15 mm to 10 mm for the steel strip as required for the end use. Subsequent thereto, a further annealing process can optionally be performed, if need be coupled with a coating process and finally a skin-pass rolling process, by means of which the surface structure required for the end use is set.

Preferably, the steel strip is galvanised by hot-dipping or electrolytically or is coated metallically, inorganically or organically.

A steel strip produced by the method in accordance with the invention has a tensile strength Rm >800 to 2000 MPa and an elongation at fracture A80 of 3 to 40%, preferably >8 to 40%. In this case, high levels of strength tend to be associated with lower elongations at fracture and vice versa.

The cold-rolled steel strip produced in accordance with the invention can then be processed e.g. as a sheet metal portion, coil or panel by cold-forming at ambient temperature or by warm-forming at temperatures of 60° C. to below the AC3, preferably <450° C., to form a component wherein by means of the considerable residual deformation capability it is possible to dispense with intermediate annealing depending on usage.

In further processing steps, the cold-rolled steel strip produced in accordance with the invention can be processed to form pipes with longitudinal or spiral weld seams, wherein in this case also, by means of the considerable residual deformation capability of the steel strip, it is possible to dispense with intermediate annealing depending on usage. The pipe can thus comprise an outer and/or inner metallic, organic or inorganic coating.

The pipe produced in this way can then be deformed further, e.g. drawn or expanded or deformed using internal high pressure and processed further to form a component.

Areas of usage are thus primarily the automotive or utility vehicle industry and engineering, white goods, construction and uses at temperatures below 0° C. and as ballistic steel. Ballistic steels are used in order to protect vehicles and buildings against shelling and explosions, and have a high level of hardness and toughness.

Trials have been carried out to investigate the mechanical properties of the steel strips produced in accordance with the invention, using e.g. alloys 1 to 4. The alloys 1 to 4 contain the following elements in the stated quantities in wt. %:

Alloy C Mn Al Si Cr Mo 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 the purposes of comparison the steel strips produced from the above-mentioned alloys 1 to 4 were cold-rolled, i.e. at ambient temperature and therefore below 50° C., and also rolled in accordance with the invention at 250° C. The measured rolling forces are given as follows:

Rolling force Rolling force Degree of [kN] [kN] deformation Reduction in cumulative - cumulative - (e = Δd/d0) rolling force Alloy cold-rolling at 250° C. [%] [%] 1 103000 59000 44 ca. 43 2 144000 55000 44 ca. 62 3 161000 63000 44 ca. 60 4 107000 56000 44 ca. 43

Cumulative rolling force is understood to be the adding up of the rolling forces of the individual passes in order to obtain a comparable measure for the expenditure of force. The rolling force was standardised to a band 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 and the initial thickness d0 of the steel strip under investigation. The reduction in rolling force is the calculated decrease in the rolling force at 250° C. compared with the rolling force during cold-rolling.

The elongation at fracture A50 was also evaluated:

Elongation at Elongation at fracture A50 [%] fracture A50 [%] Alloy 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 characteristic values represent the elongation in the rolling direction.

Claims

1.-25. (canceled)

26. A method, comprising:

producing a steel strip from high-strength, manganese-containing steel with TRIP properties, said steel comprising (in wt. %):
C: 0.0005 to 0.9
Mn: more than 3.0 to 12,
with the remainder being iron including unavoidable steel-associated elements; and
cold-rolling the steel strip to a required end thickness at a temperature prior to a first rolling pass above 50° C. to 400° C.

27. The method of claim 26, further comprising adding of one or more of the following alloying elements (in wt. %):

Al: to 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: to 0.5
W: to 5
Co: to 8
Zr: to 0.5
Ta: to 0.5
Te: 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.

28. The method of claim 26, wherein the temperature is 70° C. to 250° C.

29. The method of claim 26, wherein the steel strip is a hot strip or a pre-strip which is heated to a temperature above 50° C. to 400° C., preferably from 70° C. to 250° C., or a hot strip or a pre-strip at a temperature above 50° C. to 400° C., preferably from 70° C. to 250° C., before undergoing cold-rolling to the required end thickness.

30. The method of claim 26, further comprising cooling the steel strip to a temperature of 50° C. to 400° C., in particular to a temperature of 70° C. to 250° C. between rolling passes of the cold-rolling.

31. The method of claim 26, wherein the C content is 0.05 to 0.42.

32. The method of claim 26, wherein the Mn content is >5 to <10.

33. The method of claim 27, wherein the Al content is 0.1 to 5, in particular >0.5 to 3.

34. The method of claim 27, wherein the Si content is 0.05 to 3, in particular >0.1 to 1.5.

35. The method of claim 27, wherein the Cr content is 0.1 to 4, in particular >0.5 to 2.5.

36. The method of claim 27, wherein a sum of Al+Si+Cr is >1.2.

37. The method of claim 27, wherein the Nb content is 0.005 to 0.4, in particular 0.01 to 0.1.

38. The method of claim 27, wherein the V content is 0.005 to 0.6, in particular 0.01 to 0.3.

39. The method of claim 27, wherein the Ti content is 0.005 to 0.6, in particular 0.01 to 0.3.

40. The method of claim 27, wherein the Mo content is 0.005 to 1.5, in particular 0.01 to 0.6.

41. The method of claim 27, wherein the Sn content is <0.2, in particular <0.05.

42. The method of claim 27, wherein the Cu content is <0.5, in particular <0.1.

43. The method of claim 27, wherein the W content is 0.01 to 3, in particular 0.2 to 1.5.

44. The method of claim 27, wherein the Co content is 0.01 to 5, in particular 0.3 to 2.

45. The method of claim 27, wherein the Zr content is 0.005 to 0.3, in particular 0.01 to 0.2.

46. The method of claim 27, wherein the Ta content is 0.005 to 0.3, in particular 0.01 to 0.1.

47. The method of claim 27, wherein the Te content is 0.005 to 0.3, in particular 0.01 to 0.1.

48. The method of claim 27, wherein the B content is 0.001 to 0.08, in particular 0.002 to 0.01.

49. The method of claim 27, wherein the Ca content is 0.005 to 0.1.

50. The method of claim 26, wherein the required end thickness is less than 10 mm, preferably less than 4 mm.

51. The method of claim 26, wherein the steel strip is produced by:

melting a melt of the high-strength, manganese-containing steel;
casting the melt to form a pre-strip through a horizontal or vertical strip casting process approximating a final dimension or casting the steel melt to form a slab or thin slab through a horizontal or vertical slab or thin slab casting process;
re-heating the slab or thin slab to a temperature in a range of 1050° C. to 1250° C. and then hot-rolling the slab or thin slab to form a hot strip, or re-heating the pre-strip to a temperature in a range of 1000° C. to 1200° C. and then hot-rolling the pre-strip to form a hot strip, or hot-rolling the pre-strip without re-heating by using heat generated during casting to form a hot strip with optional intermediate heating between individual rolling passes of the hot-rolling; and
reeling the hot strip at a reeling temperature between 820° C. and ambient temperature.

52. The method of claim 51, further comprising annealing the hot strip at an annealing temperature of 580 to 820° C. and an annealing duration of 1 minute to 48 hours, after the hot strip has been reeled.

53. The method of claim 52, further comprising cold-rolling the hot strip.

54. The method of claim 26, further comprising annealing the steel strip at an annealing temperature of 580 to 820° C. and an annealing duration of 1 minute to 48 hours, after the steel strip has been cold-rolled.

55. The method of claim 26, further comprising acid-cleaning and/or skin-pass rolling the steel strip, after the steel strip has been cold-rolled.

56. The method of claim 26, further comprising coating the steel strip with a metallic, organic or inorganic corrosion protection coating, after the steel strip has been cold-rolled.

57. The method of claim 26, further comprising using the steel strip to produce a component by hot-forming, cold-forming or warm-forming or to produce a pipe with longitudinal or spiral weld seam or to produce a component for automotive and utility vehicle industry and for engineering, white goods and construction, or using the steel strip in a low-temperature range below 0° C. to −273° C., or using the steel strip as a ballistic steel.

Patent History
Publication number: 20190256943
Type: Application
Filed: Jun 8, 2017
Publication Date: Aug 22, 2019
Applicant: SALZGITTER FLACHSTAHL GMBH (38239 Salzgitter)
Inventors: PETER PALZER (Liebenburg), Thomas Evertz (Peine), MARTIN SCHUBERT (Wernigerode)
Application Number: 16/308,319
Classifications
International Classification: C21D 9/52 (20060101); C21D 8/02 (20060101); C21D 6/00 (20060101); C22C 38/38 (20060101); C22C 38/22 (20060101); C22C 38/06 (20060101); C22C 38/02 (20060101);