JP2007131945A - High strength steel wire with excellent ductility and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wire rod excellent in wire drawing workability, and to provide a steel wire made from the wire rod with high yield and low cost under high productivity.
SOLUTION: A hard steel wire having a specified component is heated to a specific temperature range and then subjected to patenting after re-austenite, whereby the area ratio of the pearlite structure is 97% or more and the balance is bainite, pseudo pearlite. Thus, a high carbon steel wire excellent in ductility that has a non-pearlite structure made of pro-eutectoid ferrite and that satisfies the following formulas (1), (2), and (3) at the fracture drawing value RA is obtained.
RA ≧ a−b × pearlite block particle size (μm) (1)
a = −0.0114 × (TS (MPa)) ² + 3.1178 × TS (MPa) −151.32 (2)
b = 0.0003 × TS (MPa) −0.3753 (3)
[Selection] Figure 1

Description

  The present invention relates to a steel wire, a steel wire, and a manufacturing method thereof. More specifically, for example, a steel cord used as a reinforcing material for a radial tire of an automobile, various industrial belts and hoses, a rolled wire suitable for uses such as a sawing wire, a manufacturing method thereof, and the rolled wire described above Relates to steel wires made of steel.

  In general, steel cord steel wires used as reinforcing materials for automobile radial tires, various belts and hoses, or steel wires for sawing wires have a wire diameter (diameter) of 5 to 5 adjusted and cooled after hot rolling. A 6-mm steel wire is subjected to primary wire drawing to a diameter of 3 to 4 mm, followed by a patenting treatment and further subjected to secondary wire drawing to a diameter of 1 to 2 mm. Thereafter, a final patenting process is performed, followed by brass plating, and further a final wet wire drawing to a diameter of 0.15 to 0.40 mm. A steel cord is manufactured by twisting a plurality of the ultrafine steel wires thus obtained into a twisted steel wire by twisting.

  In general, when a wire breakage occurs when a wire rod is processed into a steel wire or a steel wire is twisted, productivity and yield are greatly reduced. Therefore, wire rods and steel wires belonging to the above technical field are strongly required not to be disconnected during wire drawing or twisting. Among the wire drawing processes, in the case of the final wet wire drawing process, the wire diameter of the steel wire to be treated is extremely thin, so that disconnection is particularly likely to occur.

  Furthermore, in recent years, there has been an increasing trend to reduce the weight of steel cords and the like for various purposes. For this reason, high strength is required for the various products described above, and a carbon steel wire having a C content of less than 0.7% by mass cannot obtain a desired high strength. A steel wire having a C content of 75% by mass or more is increasingly used. However, if the C content is increased, the wire drawing workability is lowered, so that the frequency of disconnection is increased. For this reason, the request | requirement with respect to the wire which has high C content, can ensure the high intensity | strength to a steel wire, and was excellent also in wire drawing workability has become very large.

  In response to the above-mentioned demands from the industry in recent years, a technique has been proposed in which segregation and microstructure are controlled, or a specific element is contained to enhance the drawing workability of a high carbon wire.

  For example, Patent Document 1 includes a “high-strength, high-toughness ultrafine steel wire, a high-strength, high-toughness ultrafine steel wire, and the ultrafine steel wire that is made of a steel material having a specific chemical composition and defines the content average area ratio of proeutectoid cementite. A twisted product using a steel wire and a method for producing the ultrafine steel wire "are disclosed. However, since the wire proposed in this document contains one or more of Ni and Co, which are expensive elements, as essential components, the manufacturing cost increases.

  On the other hand, the drawing value of the patenting wire depends on the austenite particle size, and the drawing value is improved by refining the austenite particle size. Therefore, carbides and nitrides such as Nb, Ti, and B are used as pinning particles. Attempts have also been made to refine the austenite grain size. In Patent Document 1, Nb: 0.01 to 0.1% by weight, Zr: 0.05 to 0.1% by weight, and Mo: 0.02 to 0.5% by weight as component elements are included. A technique for further enhancing the toughness of ultra fine steel wire by containing the above is disclosed. Also in patent document 2, refinement | miniaturization of the austenite particle size by NbC is proposed. However, since these additive elements are expensive, cost increases, Nb forms coarse carbides and nitrides, and Ti forms coarse oxides, so that the wire diameter is small, for example, a wire diameter of 0.40 mm or less. When the wire was drawn, it was sometimes broken. Further, according to the verification by the present inventors, in BN pinning, it is difficult to make the austenite grain size fine enough to affect the aperture value.

  Furthermore, as in Patent Document 3, Patent Document 4, and Patent Document 5, a technique for improving the wire drawing workability of a high carbon wire by fixing solute N with Ti and B has also been proposed. However, according to recent reports, the cementite in the wire is decomposed during wire drawing and the amount of solute C increases, so it is difficult to improve wire drawing workability even if solid solution N is fixed before wire drawing. it is conceivable that.

Patent Documents 6 and 7 also propose a technique for suppressing ferrite precipitation by solute B. On the other hand, to coarse cementite whose precipitation is accelerated by solute B, Fe ₂₃ (CB) ₆ . Is not considered, and there is a high possibility of disconnection.

Japanese Patent No. 2609387 JP 2001-131697 A JP 2000-309849 A JP-A-56-44747 JP-A-01-316420 JP 2000-355736 A JP 2004-137597 A

  The present invention has been made in view of the above situation, and its purpose is to obtain a wire material excellent in cold workability such as wire drawing workability suitable for uses such as a steel cord and a sawing wire, and to obtain the wire material described above. It is to provide steel wire as a raw material with high yield and low cost under high productivity.

  The structure of the manufacturing method according to the present invention that has solved the above-mentioned problems is as follows. A steel wire shown in (1) to (3) below, a method of manufacturing a steel wire shown in (4), and a high strength shown in (5) On steel wire.

(1) The area ratio of the pearlite structure after patenting is 97% or more, the balance is a non-pearlite structure composed of bainite, pseudo pearlite, and pro-eutectoid ferrite, and the fracture drawing value RA is expressed by the following formulas (1), (2), (3) A steel wire characterized by a tensile strength TS satisfying the formula (4).
RA ≧ a−b × pearlite block particle size (μm) (1)
a = −0.0114 × (TS (MPa)) ² + 3.1178 × TS (MPa) −151.32 (2)
b = 0.0003 × TS (MPa) −0.3753 (3)
TS ≧ 1000 × C (%) − 10 × wire diameter (mm) +320 MPa (4)
(2) By mass%, C: 0.70 to 1.10%, Si: 0.1 to 1.5%, Mn: 0.1 to 1.0%, Al: 0.01% or less, Ti: 0.01% or less, N: 10 to 60 ppm by mass, B: (0.77 × N (ppm) -17.4) ppm by mass, or 3 ppm by mass, whichever is higher, 52 ppm by mass or less And the remainder consists of Fe and an impurity, The steel wire rod of Claim 1 characterized by the above-mentioned.
(3) Cr: 0.5% or less (not including 0%), Ni: 0.5% or less (not including 0%), Co: 0.5% or less (not including 0%), V : 0.5% or less (not including 0%), Cu: 0.2% or less (not including 0%), Mo: 0.2% or less (not including 0%), W: 0.2% The steel according to claim 2, comprising at least one selected from the group consisting of the following (not including 0%), Nb: not more than 0.1% (not including 0%). wire.
(4) A wire having the chemical composition described in (2) to (3) is heated to a temperature Tmin to 1100 ° C. shown below, and a cooling rate of 800 to 650 ° C. is 50 in an atmosphere of 500 to 650 ° C. The method for producing a steel wire according to (1), wherein the patenting process is performed at a temperature of at least ° C / s.
In the case of B (ppm) −0.77 × N (ppm)> 0.0, the minimum heating temperature Tmin is 850 ° C.
In the case of B (ppm) −0.77 × N (ppm) ≦ 0.0, the minimum heating temperature Tmin is
Tmin = 1000 + 1450 / (B (ppm) −0.77 × N (ppm) −10)
(5) A high-strength steel wire with excellent ductility, which is produced by cold-drawing the steel wire rod according to (1) and has a tensile strength of 2800 MPa or more.

The present inventors have repeatedly investigated and studied the influence of the chemical composition and mechanical properties of the wire on the wire drawing workability, and as a result, have obtained the following knowledge.
(A) In order to increase the tensile strength, the content of alloy elements such as C, Si, Mn, and Cr may be increased. However, an increase in the content of these alloy elements decreases the wire drawing workability, that is, Since the limit working degree at the time of wire drawing is reduced, the frequency of disconnection increases.
(B) The wire drawing workability can be estimated from the tensile strength before the wire drawing, that is, after the heat treatment, and the fracture drawing value. In particular, the wire drawing workability after the final heat treatment shows a good correlation with the tensile strength and drawing value after the final heat treatment, and extremely good wire drawing workability when the drawing value is a certain value or more according to the tensile strength. Is obtained.
(C) B forms a compound with N, and the solid solution B amount is determined by the total B amount, N amount and the heating temperature before pearlite transformation. Solid solution B segregates at the austenite grain boundaries, and during cooling from the austenite temperature during the patenting process, bainite, ferrite, pseudo pearlite, etc. generated from the austenite grain boundaries, the microstructure is coarse and low strength, especially Suppresses the occurrence of bainite. Of these non-pearlite structures, bainite is the structure that most adversely affects the drawability. The proportion of bainite in the non-pearlite structure is 60% or more. When the amount of solute B is small, the above effect is small. When the amount is excessive, coarse Fe ₂₃ (CB) ₆ is precipitated prior to pearlite transformation, and the wire drawing workability is lowered.

  The present invention has been completed based on the above findings.

  Hereinafter, each requirement of the present invention will be described in detail.

Wire structure and mechanical properties:
The drawing value of the patenting wire can be improved by reducing the pearlite block particle size approximately proportional to the austenite particle size to 10 μm or less, and precipitates such as TiN, AlN and NbC contribute to the refinement of the austenite particles. It is known. However, in a steel cord wire, addition of Ti or Al is difficult because a coarse oxide that causes disconnection is formed. Since Nb is also concerned about the generation of coarse NbC, its use is difficult. In order to refine the pearlite block grains without using these precipitates, it is necessary to lower the austenite heating temperature and shorten the heating time. However, it is extremely difficult to stably and finely control the austenite grain size by such a method, and it is difficult in actual operation. On the other hand, in the present invention, the non-pearlite structure consisting of ferrite, pseudo pearlite, and bainite in the wire after patenting is suppressed to 3% or less, so that the block particle size needs to be greatly refined. It is characterized by a higher aperture value of the wire.

According to the study by the inventors, it has been found that the fracture drawing value RA of the steel for wire used so far has a correlation with TS and the particle size of the pearlite block and has the following relationship.
RA <a−b × pearlite block particle size (μm)
a = −0.0114 × (TS (MPa)) ² + 3.1178 × TS (MPa) −151.32 (2)
b = 0.0003 × TS (MPa) −0.3753 (3)

In addition, it is clarified that the crack initiation point in the tensile test is a non-pearlite structure that does not exhibit a regular lamellar structure, such as pro-eutectoid ferrite, bainite or pseudo-pearlite generated at the former γ grain boundary, If this non-pearlite structure ratio can be suppressed to 3% or less, the fracture drawing value can be drastically improved. To reduce the non-pearlite structure, B addition and the heating temperature before patenting treatment should be adjusted according to the amount of added B. Specifically, the heating is performed at the heating lower limit temperature Tmin to 1100 ° C. represented by the following formula, and the patenting process is performed in an atmosphere of 500 to 650 ° C., where the cooling rate of 800 to 650 ° C. is 50 ° C./s or more. I found it effective to do.
In the case of B (ppm) −0.77 × N (ppm)> 0.0, the minimum heating temperature Tmin is 850 ° C.
In the case of B (ppm) −0.77 × N (ppm) ≦ 0.0, the minimum heating temperature Tmin is
Tmin = 1000 + 1450 / (B (ppm) −0.77 × N (ppm) −10)
Thereby, the high intensity | strength wire which has an aperture value more than shown by Formula (1) can be obtained.

Ingredient composition:
C: C is an element effective for increasing the strength of the wire, and when its content is less than 0.70%, it is difficult to stably impart high strength to the final product, The precipitation of proeutectoid ferrite is promoted at the austenite grain boundaries, making it difficult to obtain a uniform pearlite structure. On the other hand, if the content of C is too large, not only is the net-form pro-eutectoid cementite generated at the austenite grain boundaries and breakage is likely to occur during wire drawing, but the toughness and ductility of the ultrafine wire after the final wire drawing is increased. Deteriorate significantly. Therefore, the content of C is set to 0.70 to 1.10% by mass.

  Si: Si is an effective element for increasing the strength. Furthermore, it is an element useful as a deoxidizer, and is also an element necessary when targeting a steel wire containing no Al. If it is less than 0.1% by mass, the deoxidation action is too small. On the other hand, if the amount of Si is too large, precipitation of pro-eutectoid ferrite is promoted even in hypereutectoid steel, and the limit workability in wire drawing decreases. Furthermore, the wire drawing process by mechanical descaling (hereinafter abbreviated as MD) becomes difficult. Therefore, the content of Si is set to 0.1 to 1.5% by mass.

  Mn: Similar to Si, Mn is an element useful as a deoxidizer. It is also effective in improving the hardenability and increasing the strength of the wire. Further, Mn has an action of preventing hot brittleness by fixing S in steel as MnS. If the content is less than 0.1% by mass, it is difficult to obtain the above effect. On the other hand, Mn is an element that easily segregates, and when it exceeds 1.0 mass%, segregation occurs particularly in the center of the wire, and martensite and bainite are generated in the segregated portion, so that the wire drawing workability decreases. Therefore, the Mn content is set to 0.1 to 1.0% by mass.

  Al: 0.01% or less: The content of Al is 0.01% including 0% so that hard non-deformation alumina-based nonmetallic inclusions are generated and the steel wire is not ductile and drawn. It was defined as follows.

  Ti: 0.01% or less: The content of Ti is defined as 0.01% or less including 0% so that a hard non-deformable oxide is generated and ductility deterioration and wire drawing deterioration of the steel wire are not caused. .

  N: 10 to 60 ppm: N produces B and nitride in steel and has an effect of preventing coarsening of the austenite grain size during heating, and the effect is effectively exhibited by containing 10 ppm or more. However, if the content becomes too large, the amount of nitride increases too much, and the amount of dissolved B in austenite is reduced. Furthermore, since there is a possibility that solute N promotes aging during wire drawing, the upper limit was set to 60 ppm.

B: 3 or (0.77 × N (ppm) -17.4) to 50 ppm: When B is present in austenite in a solid solution state, it concentrates at the grain boundary and is non-perlite such as ferrite, pseudo-pearlite, and bainite. Suppresses the formation of precipitation. On the other hand, when B is added too much, precipitation of coarse Fe ₃ (CB) ₆ carbide is promoted in austenite, which adversely affects the drawability. Therefore, the lower limit value of the B content is set to 3 or (0.77 × N (ppm) −17.4), whichever is larger, and the upper limit value is set to 50 mass ppm.

  The impurities P and S are not particularly defined, but are each preferably 0.02% or less from the viewpoint of ensuring ductility as in the case of conventional ultra fine steel wires.

The steel wire used in the present invention has the above-mentioned elements as basic components, but for the purpose of further improving mechanical properties such as strength, toughness, ductility, etc. Two or more kinds may be positively contained.
Cr: 0.5% or less, Ni: 0.5% or less, Co: 0.5% or less, V: 0.5% or less, Cu: 0.2% or less, Mo: 0.2% or less, W: 0.2% or less, Nb: 0.1% or less (all do not include 0%). Hereinafter, each element will be described.

  Cr: 0.5% or less Cr is an element effective for reducing the lamella spacing of pearlite and improving the strength of the wire and the wire drawing workability. Addition of 0.1% or more is preferable for effectively exhibiting such an action. On the other hand, if the amount of Cr is too large, the end time of transformation becomes long, and there is a possibility that a supercooled structure such as martensite or bainite is generated in the hot rolled wire rod. 0.5%.

  Ni: 0.5% or less Ni does not contribute much to the strength increase of the wire, but is an element that increases the toughness of the wire. Addition of 0.1% or more is preferable for effectively exhibiting such action. On the other hand, if Ni is added excessively, the transformation end time becomes longer, so the upper limit was made 0.5%.

  Co: 1% or less Co is an element effective for suppressing precipitation of pro-eutectoid cementite in the rolled material. Addition of 0.1% or more is preferable for effectively exhibiting such an action. On the other hand, even if Co is added excessively, the effect is saturated and economically useless, so the upper limit was set to 0.5%.

  V: 0.5% or less V forms fine carbonitrides in ferrite, thereby preventing coarsening of austenite grains during heating, improving ductility and contributing to an increase in strength after rolling. . Addition of 0.05% or more is preferable for effectively exhibiting such an action. However, if the amount is excessively added, the amount of carbonitride formed becomes excessive and the particle size of the carbonitride increases, so the upper limit was made 0.5%.

  Cu: 0.2% or less Cu has an effect of increasing the corrosion resistance of the ultrafine steel wire. Addition of 0.1% or more is preferable for effectively exhibiting such an action. However, if added excessively, it reacts with S and segregates CuS in the grain boundaries, so that flaws are generated in the steel ingot, wire, etc. during the wire manufacturing process. In order to prevent such adverse effects, the upper limit was made 0.2%.

  Mo: Mo has the effect of increasing the corrosion resistance of the ultrafine steel wire. Addition of 0.1% or more is preferable for effectively exhibiting such an action. On the other hand, if Mo is added excessively, the transformation end time becomes long, so the upper limit was made 0.2%.

  W: W has the effect of increasing the corrosion resistance of the ultrafine steel wire. Addition of 0.1% or more is preferable for effectively exhibiting such an action. On the other hand, if W is added excessively, the transformation end time becomes longer, so the upper limit was made 0.2%.

  Nb: Nb has the effect of increasing the corrosion resistance of the ultrafine steel wire. Addition of 0.05% or more is preferable for effectively exhibiting such an action. On the other hand, if W is added excessively, the transformation end time becomes long, so the upper limit was made 0.1%.

Drawing conditions:
By subjecting the steel wire according to claim 1 to cold drawing, a high-strength steel wire excellent in ductility characterized by a tensile strength of 2800 MPa or more can be obtained. The true strain of cold drawing is 3 or more, preferably 3.5 or more.

  EXAMPLES Next, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples, but may be implemented with appropriate modifications within a scope that can meet the gist of the present invention. Of course, any of these is also included in the technical scope of the present invention.

  After using a hard steel wire having the chemical composition shown in Table 1 and adjusting the wire diameter to 1.2 to 1.6 mm by patenting and wire drawing, a lead furnace (hereinafter referred to as LP) or fluidized bed (hereinafter referred to as FBP) The patenting process was performed.

  In order to measure the non-pearlite volume fraction, the L cross section of the rolled wire rod was embedded with resin, then polished with alumina, corroded with saturated picral, and SEM observation was performed. The SEM observation area is the surface layer, 1 / 4D, 1 / 2D (D is the wire diameter), and in each area, 10 photographs of an area of 50 × 40 μm are arbitrarily taken at a magnification of 3000, and the cementite is granular. The values obtained by image analysis of the area ratio of the pseudo-pearlite part dispersed in the slab, the bainite part in which the plate-like cementite is dispersed at a coarse lamellar spacing of 3 times or more from the surroundings, and the pro-eutectoid ferrite part precipitated along the austenite. The non-pearlite volume fraction was used.

  The pearlite block particle diameter of the patenting wire is calculated by embedding the L cross-section of the wire into a resin and then cutting and polishing it, and analyzing the area surrounded by the interface with a misorientation of 9 ° by EBSP analysis as one block particle. The average particle size determined from

  After removing the scale of the patenting wire rod by pickling, a zinc phosphate coating is applied by a bondage treatment, and a continuous area reduction of 16 to 20% per pass using a die of 10 degrees each approach. Drawing was performed to obtain a high-strength wire drawing material having a diameter of 0.18 to 0.30 mm.

  Table 1 shows the chemical composition of the evaluation material, and Table 2 shows the test conditions, block particle size and mechanical properties.

  In Tables 1 and 2, 1 to 15 and A to D are steels of the present invention, and 16 to 28 are comparative steels. The minimum value of the aperture value represented by Equation (1) is indicated as RAmin.

  Nos. 16 and 22 are examples in which the drawing value was low because the heating temperature before patenting was low, and B nitrides and carbides were precipitated before the patenting treatment, and the amount of solute B could not be secured. Nos. 17 and 23 to 27 are examples in which the aperture value is low because the amount of B is low or no addition. No. 18 is an example in which the amount of B is excessive, and a large amount of B carbide and pro-eutectoid cementite are precipitated at the austenite grain boundaries, resulting in a low drawing value. 19 is an example in which the amount of Si was excessive and precipitation of pro-eutectoid ferrite could not be suppressed. No. 20 is an example in which the amount of C was excessive and the precipitation of proeutectoid cementite could not be suppressed. No. 21 is an example in which the amount of Mn is excessive and the formation of micromartensite could not be suppressed. No. 28 is an example in which the cooling rate at the time of the patenting process was small and the predetermined tensile strength could not be satisfied.

  In addition, when the steel wire for srule cord of φ0.2 mm was prototyped using the developed steels A, B, C, and D in the examples, delamination did not occur when TS was 4053 MPa, 4197 MPa, 4394 MPa, and 4550 MPa, respectively. Steel wire could be made. On the other hand, when a similar prototype was made using comparative steel 23, TS was 4316 MPa and delamination occurred.

  FIG. 1 shows the relationship between the non-pearlite area ratio and the drawing value of the steel of the present invention and the comparative steel. It can be seen that the steel of the present invention having a non-pearlite area ratio of 3% or less tends to have a high aperture value. However, since the aperture value is also affected by the tensile strength as described above, there is data that overlaps.

  FIG. 2 shows the relationship between the block particle size and the drawing value of the steel of the present invention and the comparative steel. It can be seen that the steel of the present invention tends to have a high aperture value. However, since the aperture value is also affected by the tensile strength as described above, there is data that overlaps.

  FIG. 3 shows the relationship between the lower limit value RAmin of the aperture value represented by Expression (1) and the actual aperture value. It can be seen that the aperture value of the developed steel is higher than RAmin.

  1-3, ◆ indicates the steel of the present invention, and □ indicates the comparative steel.

The relationship between a non-pearlite area ratio and an aperture value is shown. The relationship between a pearlite block particle size and an aperture value is shown. The relationship between the lower limit RAmin of the aperture value represented by the equation (1) and the actual aperture value is shown.

Claims (5)

The area ratio of the pearlite structure after patenting is 97% or more, the balance is a non-pearlite structure composed of bainite, pseudo pearlite, and pro-eutectoid ferrite, and the fracture drawing value RA is expressed by the following formulas (1), (2), (3) A steel wire characterized by a tensile strength TS satisfying the formula (4).
RA ≧ a−b × pearlite block particle size (μm) (1)
a = −0.0114 × (TS (MPa)) ² + 3.1178 × TS (MPa) −151.32 (2)
b = 0.0003 × TS (MPa) −0.3753 (3)
TS ≧ 1000 × C (%) − 10 × wire diameter (mm) +320 MPa (4)   In mass%, C: 0.70 to 1.10%, Si: 0.1 to 1.5%, Mn: 0.1 to 1.0%, Al: 0.01% or less, Ti: 0.01 %: N: 10 to 60 ppm by mass, B: (0.77 × N (ppm) -17.4) ppm by mass, or 3 ppm by mass, whichever is higher, 52 ppm by mass or less, and the balance The steel wire rod according to claim 1, which comprises Fe and impurities.   Further, Cr: 0.5% or less (not including 0%), Ni: 0.5% or less (not including 0%), Co: 0.5% or less (not including 0%), V: 0. 5% or less (not including 0%), Cu: 0.2% or less (not including 0%), Mo: 0.2% or less (not including 0%), W: 0.2% or less (0 %)), Nb: 0.1% or less (not including 0%), and at least one selected from the group consisting of the steel wire rod according to claim 2. The wire having the chemical composition according to claim 2 or 3 is heated to a temperature Tmin to 1100 ° C shown below, and a cooling rate of 800 to 650 ° C is 50 ° C / s or more in an atmosphere of 500 to 650 ° C. The method for producing a steel wire according to claim 1, wherein a patenting process is performed.
In the case of B (ppm) −0.77 × N (ppm)> 0.0, the minimum heating temperature Tmin is 850 ° C.
In the case of B (ppm) −0.77 × N (ppm) ≦ 0.0, the minimum heating temperature Tmin is
Tmin = 1000 + 1450 / (B (ppm) −0.77 × N (ppm) −10)   A high-strength steel wire excellent in ductility, wherein the steel wire according to claim 1 is produced by cold-drawing and has a tensile strength of 2800 MPa or more.

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