JPH10183242A - Manufacturing method of high strength steel wire - Google Patents

Abstract

(57) Abstract: Provided is a method for manufacturing a high-strength steel wire having a tensile strength of 4250 MPa or more and having high resistance to the occurrence of delamination. SOLUTION: C: 0.7 to 1.1% by weight, Si:
0.1-1.0%, Mn: 0.2-0.8%, V: 0.
A steel wire processed from a wire having a composition of 0.5 to 0.5%, Cr: 0 to 0.5%, balance Fe and inevitable impurities,
Temperature T1 ° C that satisfies the following formula at a heating rate of 5 ° C / sec or more
Then, the temperature is raised to and maintained for a time t1 second, and then cooled at a cooling rate of 10 ° C./second or more to a temperature in a temperature range of 625 to 550 ° C., and maintained in that temperature range for 5 to 60 seconds. Cold drawing is performed so that the strain ε becomes 3.5 or more. However, 900 ° C
≦ T1 ≦ 1100 ° C., t1 ≦ 300 seconds, and (110
0−T1) ³ × 5.0 × 10 ⁻⁶ +1 second ≦ t1 ≦ (1100
−T1) ³ × 5.1 × 10 ⁻⁵ +15 seconds.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a high-strength steel wire, and more particularly to a method for producing a high-strength steel wire such as a filament for a steel cord wire used as a reinforcing material for an automobile tire.

[0002]

2. Description of the Related Art In recent years, as peripheral technologies have become more sophisticated,
From the viewpoint of economy, steel wires have been required to have higher strength.

For example, there has been a movement to reduce the weight of automobile tires for the purpose of improving the fuel efficiency of automobiles, and there has been an increasing demand for high-strength steel cord wires (hereinafter simply referred to as cord wires) to meet the demand. .

[0004] A cord wire used for an automobile tire is formed by twisting a strand of a steel wire called a filament having a diameter of about 0.2 mm into a strand, and the strength of a filament used in a conventional cord wire is about 3200 MPa. That is, the strength level of the conventional cord wire was around 3200 MPa.

[0005] The above-mentioned conventional type cord wire is usually manufactured by the following method. That is, a steel wire having a diameter of about 1.2 mm is obtained by repeatedly performing lead patenting (hereinafter, referred to as LP) and wire drawing on a wire rod having a diameter of 5.5 mm manufactured by hot rolling, and then forming the steel wire. It was then pickled and plated to draw a wire having a diameter of about 0.2 mm to form a filament, and the filament was twisted to produce a desired cord wire.

That is, a steel wire drawn to a diameter of about 1.2 mm by a usual method is heated to about 900 ° C. in a final LP process, then immersed in a lead bath at about 600 ° C. Becomes a patented steel wire of about 1200 MPa. The patented steel wire is pickled and plated, and then drawn to a diameter of about 0.2 mm to form a filament having a tensile strength of about 3200 MPa. Then, a predetermined number of the filaments are twisted to obtain a cord wire having a desired strength of about 3200 MPa.

[0007] However, as described above, higher strength cord wires have been demanded from the viewpoint of improving fuel efficiency of automobiles. Recently, a cord wire with high strength of 4000MPa or more, especially 4250MP
There is a growing demand for a cord wire having a high strength of a or more.

For this reason, the strength of the filament, that is, the cord wire has been enhanced by increasing the C content of the base steel and further adding an alloying element.

Among the alloying elements, V, Mo and Nb are:
When the structure of the steel material is pearlite, carbide (VC, Mo
₂ C, present as NbC), when the austenite by heating above Ac ₁ point of the steel material, the carbide heating temperature of the steel material, depending on the heating time, go solid solution in the austenite. Therefore, the above V, Mo and Nb are, for example,
JP-A-6-279924 and JP-A-7-90495
In the base metal of the technology proposed in the above-mentioned publication.

[0010] Of these, Japanese Unexamined Patent Publication (Kokai) No. 6-279924 discloses that 0.0 to 0.15% of Mo and 0.05 to 0.3% are used.
A "low-alloy steel wire for high-strength ultrafine wires" containing one or two types of 5% V is disclosed. However, even when the steel described in the above publication is used as the base material steel, the heating of the austenitization for normal patenting is performed only, that is, the heating temperature and the heating time of the austenitization are not properly controlled. Has a tensile strength of 40
It was not that a steel wire having a wire diameter (diameter) of 0.40 mm or less having a pressure of 00 MPa or more could be stably obtained. This is because if the heating temperature and the heating time for austenitizing during patenting are not appropriate, the amount of VC or Mo ₂ C dissolved in austenite becomes unstable, resulting in variations in drawing workability and strength. Is larger. For example, when the amount of solid solution of VC or Mo ₂ C is too large, the tensile strength of the steel wire after patenting becomes high, but the ductility is lowered, so that the cold drawing becomes difficult. On the other hand, if the amount of VC or Mo ₂ C dissolved is small, the tensile strength after patenting is low, so that the tensile strength after cold drawing is also low. Assuming that the wire diameter (diameter) having a tensile strength of 4000 MPa or more is 0.40 m.
m or less, even if a steel wire having a tensile strength of at most 4250 MPa is obtained, even if a steel wire having a tensile strength of at least 4250 MPa is obtained, even if a steel wire having a tensile strength of at most 4250 MPa can be obtained stably. It cannot meet the demands.

Japanese Unexamined Patent Publication (Kokai) No. 7-90495 discloses that a base material steel contains at least one of V and Nb in an amount of 0.02 to 1.0%, and a ferrite having a pearlite structure has a V or Nb content of 0.1 μm or less. 0.05 to 1.0% by volume of carbide
A "high-strength steel wire and a method for producing the same" have been disclosed, in which a high-strength steel wire is precipitated to increase the strength. However, the technique described in the above-mentioned gazette dissolves all carbides such as VC once in austenite during patenting, and after drawing 300 to 500 to 500.
It is intended to secure the strength of the steel wire by heating to ℃ and secondary precipitation. For this reason, as apparent from the description in the examples of the above publication, the tensile strength of the obtained steel wire is as low as about 4000 MPa even at the highest.
Furthermore, the speed of drawing on an industrial scale is 1000 m
/ Min in many cases, the steel wire temperature during drawing is 30
The temperature may reach 0 ° C. or higher, and secondary hardening may occur during drawing, and the steel wire may be broken.

The use of the techniques described in JP-A-6-279924 and JP-A-7-90495 makes it possible to increase the strength of ultrafine steel wires. However, as described above, the product properties vary depending on the patenting conditions, or the tensile strength of the steel wire obtained by drawing is 4250 MPa.
There are still problems such as not reaching. Further, since the product characteristics vary, when the filament is stranded, cracks may occur along the longitudinal direction, so-called "delamination".

[0013]

As described above, when the heating temperature and heating time for austenitizing patenting are not properly controlled by simply adding V, Mo or Nb, the tensile strength after drawing is not sufficient. It was impossible to stably obtain a high-strength steel wire having a strength of 4250 MPa or more. Furthermore, it has been difficult to suppress the occurrence of delamination during stranded wire processing. In view of such a situation, the present invention provides a method for manufacturing a high-strength steel wire having a tensile strength of 4250 MPa or more and having high resistance to the occurrence of delamination which is a problem in the stranded wire process. The purpose is to:

[0014]

The gist of the present invention resides in the following method for producing a high-strength steel wire.

That is, "C: 0.7 to 1.1 by weight%"
%, Si: 0.1 to 1.0%, Mn: 0.2 to 0.8
%, V: 0.05 to 0.5%, Cr: 0 to 0.5%, a steel wire processed from a wire having a composition of the balance of Fe and unavoidable impurities is heated at a heating rate of 5 ° C./sec or more at the following rate. After heating to a temperature T1 ° C. that satisfies the equation and holding the time t1 second,
Cooling to a temperature in the temperature range of 625 to 550 ° C. at a cooling rate of at least ° C./sec, holding at that temperature range for 5 to 60 seconds, and cold drawing in which the true strain ε becomes 3.5 or more. A method for producing a high-strength steel wire. However, 900 ℃ ≦
T1 ≦ 1100 ° C., t1 ≦ 300 seconds, and (1100
−T1) ³ × 5.0 × 10 ⁻⁶ +1 second ≦ t1 ≦ (1100−
T1) ³ × 5.1 × 10 ⁻⁵ +15 seconds ”.

The true strain ε is obtained by the following _equation (A) when A ₀ is the cross-sectional area of the steel wire before drawing and A is the cross-sectional area of the steel wire after drawing.
It is represented by an equation.

Ε = ln (A ₀ / A) (a) In this specification, the term “wire” refers to “a steel material that is hot-rolled into a bar and wound into a coil”. The term includes a so-called “burn-in coil”. Also, "steel wire"
It refers to "a wire that is mainly cold-worked, such as drawn wire, and coiled."

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION First, the present inventors repeated experiments and studies by changing the chemical composition of base steel in various ways. as a result,
The base steel contains V as an essential component, and C and V
Is within a specific range, the steel wire after drawing is 42
It has been found that the steel wire has a large tensile strength of 50 MPa or more, and even if the drawn steel wire is twisted, delamination may not occur.

Therefore, next, 4250M
Chemical composition and patenting conditions of the base steel containing C and V in order to give a large tensile strength of Pa or more and to prevent delamination even if the drawn steel wire is twisted. Experiments and studies were repeated with various changes. As a result, the following findings were obtained.

In order to obtain a tensile strength of 4250 MPa or more by drawing a steel wire, the strength after patenting may be increased. In the case of V-added steel, if the VC is solid-dissolved in austenite by heating for austenitization of the patenting and is precipitated during the pearlite transformation, a large strength can be imparted to the steel wire after the patenting.

In order to improve the drawability of the steel wire after patenting, the austenite grain size during patenting may be made fine. For this purpose, it is important to completely dissolve cementite in austenite, but not to completely dissolve VC.

A high tensile strength is given to the drawn steel wire,
In addition, in order to prevent delamination during stranded wire processing, VC should be used within a range where austenite grains are not coarsened.
Must be dissolved in austenite.

If a fine pearlite structure is formed by patenting, workability during cold drawing is good, high strength is obtained after drawing, and delamination does not occur during stranded wire processing.

By simultaneously satisfying the above conditions, it is possible to obtain a steel wire excellent in cold wire drawing workability and high in tensile strength and excellent in delamination resistance. What is necessary is just to control the heating speed, the heating temperature, and the heating time of the austenitizing of the heating.

In order to obtain a fine pearlite structure by patenting, the cooling rate from the austenite region and the temperature region of the pearlite transformation may be controlled.

When cold drawing with a true strain ε of 3.5 or more is performed, a steel wire can be stably provided with a tensile strength of 4250 MPa or more.

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

Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the component content means “% by weight”.

(A) Chemical Composition The steel wire targeted by the present invention is obtained, for example, by subjecting a raw steel to hot wire rolling by a usual method, and then performing an intermediate drawing process in a cold state and an intermediate heat treatment such as an intermediate LP. Then, after performing patenting as final heat treatment, final cold drawing is performed to form a steel wire having predetermined characteristics (tensile strength of 4250 MPa or more and delamination resistance). The chemical composition of the base steel of the wire used as the material of this steel wire must be as follows.

C: 0.7 to 1.1% C is an effective element for increasing the strength of the steel wire. However, if the content is less than 0.7%, the desired tensile strength of 4250 MPa or more can be obtained even if cold-drawing of 3.5 or more with true strain is performed on the V-containing steel after final patenting. I can't secure it. On the other hand, when the content exceeds 1.1%, proeutectoid cementite is generated, and disconnection frequently occurs during cold drawing. Therefore, the C content is set to 0.7 to 1.1%.

Si: 0.1 to 1.0% Si is an element effective for increasing the strength of the steel wire and an element necessary as a deoxidizing agent. However, if the content is less than 0.1%, the effect of addition is poor. On the other hand, 1.0
%, The wire drawing workability decreases. Therefore, Si
Was set to 0.1 to 1.0%.

Mn: 0.2-0.8% In addition to the effect of increasing the strength of the steel wire, Mn also has the effect of fixing S to prevent hot brittleness. In order to exhibit these effects, it is necessary to contain Mn in an amount of 0.2% or more. On the other hand, if the content of Mn exceeds 0.8%, it tends to segregate in the center of the steel wire, and even if patenting is performed under the conditions of the present invention, martensite or bainite is formed in the center of the steel wire. And the wire drawing workability is reduced. Therefore, the Mn content is set to 0.2 to 0.8%.

V: 0.05 to 0.5% V is an effective element for increasing the strength of the steel wire if it forms a solid solution in austenite during austenitization. on the other hand,
V is an element effective for refining austenite grains and improving wire drawing workability, ductility and strength if it remains in the form of VC during austenitization. In order to exert such an effect, it is necessary that V content is 0.05% or more. On the other hand, when V is contained in excess of 0.5%, coarse VC is generated during steelmaking and remains after hot rolling, so that disconnection frequently occurs during cold drawing. Therefore, the content of V is set to 0.05 to 0.5%.

Cr: 0 to 0.5% Cr need not be added. The amount of C, S
If i, Mn and V are contained as essential components,
), The steel wire has the specified properties (4250M
Tensile strength of Pa or more and delamination resistance) can be imparted. However, Cr has the effect of reducing the lamella spacing of pearlite and increasing the strength of the steel wire. This effect is reliably exhibited when the Cr content is 0.1% or more. However, the content of Cr is 0.5%
If it exceeds 300, the time required for transformation from austenite to pearlite exceeds 60 seconds, and the productivity is reduced.
Therefore, the content of Cr is set to 0 to 0.5%.

It is preferable that P, S, Al, and N among impurities in the base steel of the wire used as the material of the steel wire targeted by the present invention are as follows.

P: not more than 0.03% P segregates at crystal grain boundaries and lowers the drawability. In particular, when the content of P exceeds 0.03%, breakage during wire drawing tends to occur. Therefore, the P content is set to 0.0
It is preferably at most 3%, more preferably at most 0.02%.

S: 0.02% or less S forms an inclusion mainly composed of MnS in steel and causes wire breakage during wire drawing. In particular, when the content of S is 0.02
%, Wire breakage during wire drawing tends to occur. Therefore, the S content is desirably 0.02% or less, and more desirably 0.01% or less.

Al: 0.01% or less Al forms inclusions mainly composed of Al ₂ O ₃ in steel and causes wire breakage during wire drawing. In particular, when the Al content exceeds 0.01%, breakage during wire drawing tends to occur. Therefore, the Al content is preferably 0.01% or less, and more preferably 0.003% or less.

N: not more than 0.01% N causes age hardening when the temperature rises during cold drawing, and lowers drawability. In particular, when the content of N exceeds 0.01%, breakage during wire drawing tends to occur. Therefore, the N content is desirably 0.01% or less, and more desirably 0.007% or less.

(B) Heating In order to obtain a steel wire having excellent cold-drawing workability, high tensile strength, and excellent delamination resistance, heating for austenitizing patenting is required. It is necessary to control the rate, heating temperature and heating time so that VC forms a solid solution in austenite as far as the austenite grains are not coarsened, and that cementite forms a complete solid solution in austenite. To do this, first,
The heating rate for austenitization needs to be 5 ° C./sec or more. If the heating rate for austenitization is less than 5 ° C./sec, VC starts to form a solid solution in the austenite during heating, so that the temperature is raised to a temperature T1 ° C. described later and the time t1
When held for seconds, almost all of the VC is dissolved. For this reason, the drawing after heat treatment (after patenting) becomes low, the wire drawing workability in the cold deteriorates, and the wire breaks during the wire drawing, or even when the wire does not break, delamination occurs at the time of the stranded wire processing Resulting in. Therefore, the heating rate for austenitization was set to 5 ° C./sec or more.

The above "heating rate" refers to "the average heating rate of the surface portion of the steel wire to be heated from room temperature (room temperature) to a predetermined temperature". The upper limit of the "heating rate" does not need to be particularly defined. The upper limit of the heating rate from the viewpoint of equipment is sufficient.
Ultra-rapid heating at 0 ° C./sec may be used.

In order to cause VC to form a solid solution in austenite within a range where the austenite grains are not coarsened, and to make all cementite form a solid solution in austenite, a temperature T1 ° C. which satisfies the following equation at the above heating rate is used. It is necessary to raise the temperature and maintain the time t1 seconds.

900 ° C. ≦ T1 ≦ 1100 ° C., t1 ≦ 300
Seconds and (1100-T1) ³ × 5.0 × 10 ^-6 +1
Second ≦ t1 ≦ (1100−T1) ³ × 5.1 × 10 ⁻⁵ +15
Seconds.

When the temperature T1 is lower than 900 ° C., cementite does not completely dissolve in austenite as shown in the examples described later, so that disconnection frequently occurs during cold drawing. On the other hand, if the temperature T1 exceeds 1100 ° C., the durability of the heat treatment furnace for adjusting the atmosphere is deteriorated and the cost is increased. In the case of a heat treatment furnace that can withstand use at a high temperature, the atmosphere adjustment is difficult. Therefore, decarburization occurs in the steel wire, and a desired steel wire cannot be obtained after cold drawing. If the time t1 is less than ^{(1100-T1) 3 × 5.0} × 10 -6 +1 seconds, for solid solution amount of VC is low, even if the cold drawing of 3.5 or more in true strain 4250MPa The above high tensile strength cannot be obtained. On the other hand, time t1 is (1100-T1) ³
If the time exceeds × 5.1 × 10 ⁻⁵ +15 seconds, the austenite grains become coarse and the drawing after heat treatment (after patenting) becomes low, so that the wire drawing workability in the cold deteriorates and the wire drawing is performed. Even if disconnection occurs during disconnection, or if disconnection does not occur, delamination will occur during stranded wire processing. By the way, the time t1 is (1100-T1) ³ * 5.1 * ^10-5 +
If it is 15 seconds or less, desired characteristics (tensile strength of 4250 MPa or more and delamination resistance) can be obtained, but t1
When the time is long, the industrial productivity is reduced and the cost is increased. For this reason, the upper limit of t1 was set to 300 seconds, and the temperature was raised to the above-mentioned temperature T1 ° C. at the above-mentioned heating rate and maintained for a time t1 second.

FIG. 1 shows the results when the conditions of the present invention in which the heating rate is in the range of 10 to 20 ° C./sec in the embodiment described later in detail, the vertical axis represents the holding time t 1 (second), and the horizontal axis represents the heating temperature. T1
It is the figure which arranged the characteristic of the steel wire after wire drawing in cold as (° C). In the figure, “○” indicates that the tensile strength after cold drawing was 4250 MPa or more, and that no delamination occurred during the twist test. On the other hand, "x"
Indicates that the tensile strength after cold drawing did not reach 4250 MPa, that delamination occurred during the torsion test, or that the wire was broken during cold drawing.

From FIG. 1, it can be seen that heating under the conditions specified in the present invention
It can be seen that when held, a steel wire having excellent cold-drawing workability, high tensile strength, and excellent delamination resistance can be obtained.

(C) Cooling In order to obtain a steel wire having excellent cold-drawing workability, high tensile strength, and excellent delamination resistance, it is necessary to heat austenitizing patenting. The speed, the heating temperature and the heating time are set to the conditions described in the above (B), and at a cooling rate of 10 ° C./sec or more, 625 to
Cool to a temperature in the temperature range of 550 ° C.
It is necessary to hold for ~ 60 seconds.

When the cooling rate for cooling to a temperature in the temperature range of 625 to 550 ° C. is less than 10 ° C./sec,
Since the pearlite transformation starts at a temperature exceeding 5 ° C., the lamella spacing of the pearlite is widened, the “drawing” is reduced, and the wire drawing workability in the cold is deteriorated, and the wire is broken during the cold drawing. Because. Therefore, the cooling rate after heating under the condition (B) was set to 10 ° C./sec or more. The upper limit of the cooling rate does not need to be particularly defined, and the cooling method for obtaining a cooling rate of 10 ° C./sec or more does not need to be particularly limited. Since there is no need to cause supercooling to a temperature range below 550 ° C., a method of immersing or charging in a lead bath or a fluidized bed furnace heated to a predetermined temperature,
A normal cooling method such as a method of cooling by air cooling or mist water cooling may be used.

The above "cooling rate" refers to "the average cooling rate from the austenitizing temperature of the surface portion of the steel wire to be cooled to a predetermined cooling temperature".

The temperature for cooling at the above cooling rate is 625 ° C.
In the case where the temperature exceeds 625 ° C., the pearlite transformation may start at a temperature exceeding 625 ° C. even if the temperature is maintained in a temperature range of 625 ° C. to 550 ° C. The "drawing" is reduced, and the cold-drawability is degraded, resulting in disconnection during cold drawing. On the other hand, the temperature for cooling at the above cooling rate is 550 ° C.
If the temperature is lower than 650 ° C., the transformation may take place at a temperature lower than 550 ° C.
Even when the temperature is maintained in the temperature range of ° C., bainite is generated in addition to pearlite, so that the wire drawing workability in the cold is also deteriorated, and the wire is broken during the cold drawing. Therefore, the temperature for cooling at the above-described cooling rate was set to a temperature in a temperature range of 625 to 550 ° C.

After cooling under the above conditions, 625-55
The reason why the temperature is kept at 0 ° C. is that if the temperature is maintained at a temperature exceeding 625 ° C., the lamella spacing of the pearlite becomes wider, so that the “drawing” decreases and the wire drawing workability in the cold deteriorates. This is because disconnection occurs during wire drawing. On the other hand, when the holding temperature is lower than 550 ° C., bainite is also generated in addition to pearlite, so that the wire drawing workability in the cold is deteriorated and the wire is broken during the cold wire drawing. For the above reasons, the temperature range to be maintained is 625 to 550 ° C.
And

The reason why the holding time in the above temperature range is set to 5 to 60 seconds is that if the holding time is shorter than 5 seconds, bainite or martensite is formed and the drawability in cold is deteriorated. This is because disconnection occurs during cold drawing. On the other hand, if the holding time exceeds 60 seconds, the productivity is reduced and the cost is increased. Therefore, the holding time in the above temperature range is set to 5 to 60 seconds.

In the range of 625 to 550.degree.
The phrase “hold for 60 seconds” may mean “after cooling to a temperature within a temperature range of 625 to 550 ° C., and holding at that constant temperature for 5 to 60 seconds” or “with a temperature range of 625 to 550 ° C.” After cooling to the temperature, for example, the temperature may be continuously or stepwise changed between 625 and 550 ° C., and the total time may be 5 to 60 seconds ”.

(D) Cold drawing In order to impart the desired tensile strength of 4250 MPa or more to the steel wire, the above-mentioned heating and cooling of (B) and (C) are performed, and then the true strain ε is obtained. Must be 3.5 or more. When the true strain ε during cold drawing is less than 3.5, even if the steel wire having the chemical composition described in (A) is treated under the conditions described in (B) and (C), Desired 42
A tensile strength of 50 MPa or more cannot be obtained. Note that the true strain ε
Although the upper limit of is not particularly limited, even when a steel wire having the chemical composition described in (A) is treated under the conditions described in (B) and (C), the true strain ε is generally 4. About 5 becomes the upper limit of cold drawing.

[0055]

EXAMPLES Steel having the chemical composition shown in Table 1 was melted by a conventional method using a 150 kg vacuum furnace. Steels B to E, steels H to J, and steels L to N in Table 1 are steels whose chemical compositions are within the ranges specified in the present invention (steel steels subject to the method of the present invention). Steels A, F, G and K are comparative steels in which any of the components are out of the range of the content specified in the present invention and are not subject to the present invention.

[0056]

[Table 1]

Next, these steels are hot-rolled into a wire having a diameter of 5.5 mm by a usual method, wound up in a coil shape, cooled to room temperature (room temperature), and further cold-drawn by a usual method. It is processed to a steel wire having a diameter of 3.2 mm (true strain ε = 1.
1).

Thereafter, each steel wire having a diameter of 3.2 mm was patented using the apparatus shown in FIG. That is, after straightening a steel wire with a straightening machine, it is charged into a heating furnace,
The temperature was raised to 950 ° C. at a heating rate of / sec, held for 60 seconds, and then immersed in a 575 ° C. lead bath furnace for 30 seconds to perform patenting. The cooling rate from 950 ° C. to 575 ° C. was 20 ° C./sec. 3.2 patented diameter
mm steel wire is then 1.2 mm in diameter (true strain ε = 2.0) or 1.5 mm in diameter (true strain ε = 1.
Cold drawing was performed until 5).

The diameter obtained as described above is 1.2 m.
Using a steel wire of m or 1.5 mm as a test material, heat treatment (patenting) was performed using the apparatus shown in FIG. 2 under the conditions shown in Tables 2 and 3, and a tensile test was performed. Was measured for tensile strength and drawing.

Then, the diameter after patenting was reduced to 1.2 mm by ordinary cold drawing using a wet drawing machine.
Alternatively, a 1.5 mm steel wire to a 0.2 mm diameter steel wire (true strain ε when the diameter after patenting is 1.2 mm)
Was 3.6, and the true strain ε when the diameter after patenting was 1.5 mm was 4.0). Using the thus obtained steel wire having a diameter of 0.2 mm, a tensile test and a twist test were performed. Tables 2 and 3 also show the test results.

[0061]

[Table 2]

[0062]

[Table 3]

From Tables 2 and 3, according to the production method of the present invention, a high strength having a tensile strength of 4250 MPa or more and having a large resistance to the occurrence of delamination which is a problem in the stranded wire process. It is clear that a steel wire is obtained.

On the other hand, in the case of the comparative example in which either the chemical composition of the base steel of the wire material used as the material of the test steel wire or the treatment conditions deviate from the conditions stipulated in the present invention, the cold steel is used. Is inferior in wire drawing work, the wire breaks during wire drawing, the desired strength cannot be obtained when the tensile strength of the steel wire after cold drawing is less than 4250 MPa, or delamination occurs in the twist test. It can be seen that stranded wire processing cannot be performed due to generation of stranded wire.

That is, in the test numbers 14, 15 and 23, the holding time t1 at the described temperature T1 is (1100-T1) ³ ×
Since it exceeded 5.1 × 10 ⁻⁵ +15 seconds, a large amount of VC was dissolved in austenite, resulting in disconnection during cold drawing or delamination during a twist test.

In Test Nos. 28 and 29, the retention time t1 at the described temperature T1 was (1100-T1) ³ × 5.0 × 10 ^-6.
Since it is less than +1 second, the solid solution of VC is insufficient, so that the tensile strength after drawing does not reach 4250 MPa.

Test No. 20 has a lead bath temperature of 550 since the holding time in a predetermined temperature range is less than 5 seconds.
Since the temperature was lower than ° C, low-temperature transformation products such as bainite were generated, and the wires were broken during cold drawing or delamination occurred during a twist test.

In Test No. 18, since the lead bath temperature exceeded 625 ° C., the lamella spacing of pearlite was widened, and the wire was broken during cold drawing.

In Test No. 30, since the heating temperature was lower than 900 ° C., cementite did not completely dissolve in austenite, so that the cold drawability was low and the wire was broken during cold drawing. .

Test No. 1 had a C content of less than 0.7%, and Test No. 7 had a V content of less than 0.05%, so that the tensile strength after cold drawing was 4250 MPa.
a has not been reached.

Test No. 6 has a C content of more than 1.1%, and Test No. 11 has a V content of more than 0.5%. Broken during processing.

[0072]

According to the method for producing a high-strength steel wire of the present invention, a high-strength steel having a tensile strength of 4250 MPa or more and having a large resistance to the occurrence of delamination which is a problem in the stranded wire process. Steel wire is relatively easy to obtain.

[Brief description of the drawings]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows the results when the conditions of the present invention with a heating rate of 10 to 20 ° C./sec are satisfied in the examples, where the vertical axis represents the holding time t1 (second) and the horizontal axis represents the heating temperature T1 (° C.). It is the figure which arranged the characteristic of the steel wire after wire drawing in cold.

FIG. 2 is a diagram showing a steel wire processing apparatus used in an example.

Claims (1)

[Claims] (1) C: 0.7 to 1.1% by weight, Si:
0.1-1.0%, Mn: 0.2-0.8%, V: 0.
A steel wire processed from a wire having a composition of 0.5 to 0.5%, Cr: 0 to 0.5%, balance Fe and inevitable impurities,
Temperature T1 ° C that satisfies the following formula at a heating rate of 5 ° C / sec or more
Then, the temperature is raised to and maintained for a time t1 second, and then cooled at a cooling rate of 10 ° C./second or more to a temperature in a temperature range of 625 to 550 ° C., and maintained in that temperature range for 5 to 60 seconds. A method for producing a high-strength steel wire, comprising performing cold drawing in which the strain ε becomes 3.5 or more. Here, 900 ° C. ≦ T1 ≦ 11
00 ° C., t1 ≦ 300 seconds, and (1100−T1) ³
× 5.0 × 10 ⁻⁶ +1 second ≦ t1 ≦ (1100−T1) ³ ×
5.1 × 10 ⁻⁵ +15 seconds.