EP4660331A1

EP4660331A1 - Wire rod, steel wire, rope and production method of rope

Info

Publication number: EP4660331A1
Application number: EP23919796.5A
Authority: EP
Inventors: Toshihiko TESHIMA; Makoto Okonogi; Manabu Kubota; Nariyasu Muroga
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2023-02-03
Filing date: 2023-02-03
Publication date: 2025-12-10
Also published as: KR20250138760A; WO2024161659A1; JPWO2024161659A1; EP4660331A4; CN120569503A

Abstract

A wire rod having a chemical composition containing, in % by mass, C: from 0.80 to 1.10%, Si: from 0.10 to 1.50%, Mn: from 0.10 to 1.00%, P: 0.030% or less, S: 0.030% or less, N: 0.0120% or less, O: 0.0100% or less, Al: from 0.005 to 0.070%, and Mo: from 0.02 to 0.20%, with a balance being Fe and impurities, in which a total area ratio of a ferrite structure and a martensite structure at a center portion of a cross section is 5% or less, an average Vickers hardness H, a surface layer Vickers hardness Hs, and a center Vickers hardness Hc in the cross section satisfy the relationships of Formulas (1) to (3) ([C] is a content of C contained in the wire rod in % by mass), and a diameter is 7.0 mm or more, and a steel wire that can be produced from the wire rod, are provided. A rope made of bundled steel wires is also provided. 480×C−40<H<480×C+400≤Hs−H<300≤H−Hc<30

Description

Technical Field

The present disclosure relates to a wire rod, a steel wire, a rope, and a method for producing a rope.

Background Art

Steel wires such as bridge cable steel wires and rope steel wires are produced, for example, using a steel wire obtained by subjecting a high carbon wire rod to a patenting treatment to form a pearlite structure and then performing wire drawing and a plating treatment.
For example, a bridge cable is required to have a high strength in order to improve the degree of freedom in cable design, and a steel wire to be used is also required to have a high strength.
For example, as a high-strength steel wire having a tensile strength of 2000 MPa or more, Patent Literature 1 discloses a high-strength PC steel wire excellent in delayed fracture resistance properties, the PC steel wire containing, in % by mass, C: from 0.9 to 1.2%, Si: from 0.01 to 1.5%, Mn: from 0.2 to 1.5%, Al: from 0.001 to 0.05%, and N: from 0.0005 to 0.010% with a balance being Fe and unavoidable impurities, including 90% or more of wire drawn pearlite and 10% or less of ferrite and bainite structure, and having a tensile strength of 2000 MPa or more, in which in a case in which a diameter of the PC steel wire is taken as D, a ratio (Hv surface/Hv inner portion) of a surface layer Hv hardness (Hv surface) in a region (surface layer portion) of 0.1 D from the surface of the PC steel wire and an inner portion Hv hardness (Hv inner portion) in a region (interior) inside the surface layer portion is 1.1 or less.
However, there is a problem in that torsion properties are deteriorated as the strength is increased.
As a steel wire intended to improve torsion properties, for example, Patent Literature 2 discloses a plated steel wire for PWS having excellent torsion properties, in which a steel wire contains, in % by mass, C: from 0.8 to 1.1%, Si: from 0.8 to 1.3%, Mn: from 0.3 to 0.8%, N: from 0.001 to 0.006%, and B: from 0.0004 to 0.0060%, a quantity of solid solute B being 0.0002% or more, and further contains one or two of Al: from 0.005 to 0.1% or Ti: from 0.005 to 0.1%, with the balance being Fe and inevitable impurities, and the steel wire has an area ratio of a non-pearlite structure in a portion from a surface layer to a depth of 50 µm being 10% or less, and a total area ratio of the non-pearlite structure in the entire cross section being 5% or less, and in which a surface of the steel wire is galvanized with a plating quantity of from 300 to 500 g/m².

Patent Literature 1: Japanese Patent Application Laid-Open ( JP-A) No. 2009-280836
Patent Literature 2: WO 2008/093466 A1

SUMMARY OF INVENTION

Technical Problem

As for the steel wire used for a bridge cable or the like, there is a demand for development of a steel wire having a high strength and excellent torsion properties and a wire rod suitable for producing a steel wire having such properties.
Therefore, an object of the disclosure is to provide a wire rod suitable for producing a steel wire having a high strength and excellent torsion properties by wire drawing, a steel wire having a high strength and excellent torsion properties, a rope, and a method for producing a rope.

Solution to Problem

<1> A wire rod comprising a chemical composition containing, in % by mass;
- C: from 0.80 to 1.10%,
- Si: from 0.10 to 1.50%,
- Mn: from 0.10 to 1.00%,
- P: 0.030% or less,
- S: 0.030% or less,
- N: 0.0120% or less,
- O: 0.0100% or less,
- Al: from 0.005 to 0.070%,
- Mo: from 0.02 to 0.20%,
- Cr: from 0 to 1.00%,
- Cu: from 0 to 0.80%,
- Sn: from 0 to 0.50%,
- Ni: from 0 to 0.50%,
- V: from 0 to 0.15%,
- Ti: from 0 to 0.050%,
- Nb: from 0 to 0.050%,
- B: from 0 to 0.0040%,
- REM: from 0 to 0.030%,
- Mg: from 0 to 0.0040%,
- Ca: from 0 to 0.0040%,
- Zr: from 0 to 0.030%,
- W: from 0 to 0.10%,
- Te: from 0 to 0.030%, and
- Sb: from 0 to 0.030%,
- with a balance being Fe and impurities,
- in which:
  - a total area ratio of a ferrite structure and a martensite structure in a center portion within 1.0 mm from a center in a cross section perpendicular to a longitudinal direction is 5% or less with the balance being a mixed structure composed of ferrite and cementite,
  - in a case in which a content of C contained in the wire rod in % by mass is denoted as [C], an average Vickers hardness H in the cross section satisfies Formula (1): $480 \times [C] - 40 < H < 480 \times [C] + 40$
  - a surface layer Vickers hardness Hs at a position of a depth of from 0.5 to 1.0 mm from a surface and the average Vickers hardness H satisfy a relationship of Formula (2): $0 \leq Hs - H < 30$
  - a center Vickers hardness Hc within 1.0 mm from the center and the average Vickers hardness H satisfy a relationship of Formula (3): $0 \leq H - Hc < 30$ and
  - a diameter of the wire rod is 7.0 mm or more.
<2> The wire rod according to <1>, in which the chemical composition contains, in % by mass, at least one or two or more selected from the group consisting of:
- Cr: from 0.03 to 1.00%,
- Cu: from 0.01 to 0.80%,
- Sn: from 0.001 to 0.50%,
- Ni: from 0.01 to 0.50%, and
- V: from 0.01 to 0.15%.
<3> The wire rod according to <1> or <2>, in which the chemical composition contains, in % by mass, one or two or more selected from the group consisting of:
- Ti: from 0.002 to 0.050%,
- Nb: from 0.002 to 0.050%,
- B: from 0.0003 to 0.0040%,
- REM: from 0.002 to 0.030%,
- Mg: from 0.0002 to 0.0040%,
- Ca: from 0.0002 to 0.0040%,
- Zr: from 0.002 to 0.030%,
- W: from 0.02 to 0.10%,
- Te: from 0.001 to 0.030%, and
- Sb: from 0.001 to 0.030%.
<4> A steel wire comprising a chemical composition containing, in % by mass:
- C: from 0.80 to 1.10%,
- Si: from 0.10 to 1.50%,
- Mn: from 0.10 to 1.00%,
- P: 0.030% or less,
- S: 0.030% or less,
- N: 0.0120% or less,
- O: 0.0100% or less,
- Al: from 0.005 to 0.070%,
- Mo: from 0.02 to 0.20%,
- Cr: from 0 to 1.00%,
- Cu: from 0 to 0.80%,
- Sn: from 0 to 0.50%,
- Ni: from 0 to 0.50%,
- V: from 0 to 0.15%,
- Ti: from 0 to 0.050%,
- Nb: from 0 to 0.050%,
- B: from 0 to 0.0040%,
- REM: from 0 to 0.030%,
- Mg: from 0 to 0.0040%,
- Ca: from 0 to 0.0040%,
- Zr: from 0 to 0.030%,
- W: from 0 to 0.10%,
- Te: from 0 to 0.030%, and
- Sb: from 0 to 0.030%,
- with a balance being Fe and impurities,
- in which:
  - a total area ratio of a ferrite structure and a martensite structure in a center portion within 1.0 mm from a center in a cross section parallel to a longitudinal direction and passing through a central axis is 5% or less with the balance being a mixed structure composed of ferrite and cementite,
  - an average Vickers hardness h in a cross section perpendicular to the longitudinal direction is from 450 to 620,
  - a surface layer Vickers hardness hs at a position of a depth of from 0.2 to 0.5 mm from a surface and the average Vickers hardness h satisfy a relationship of Formula (4): $0 \leq hs - h < 30$ and
  - a center Vickers hardness hc within 1.0 mm from the center in the cross section perpendicular to the longitudinal direction and the average Vickers hardness h satisfy a relationship of Formula (5): $0 \leq h - hc < 30$
<5> The steel wire according to <4>, in which the chemical composition contains, in % by mass, at least one or two or more selected from the group consisting of:
- Cr: from 0.03 to 1.00%,
- Cu: from 0.01 to 0.80%,
- Sn: from 0.001 to 0.50%,
- Ni: from 0.01 to 0.50%, and
- V: from 0.01 to 0.15%.
<6> The steel wire according to <4> or <5>, in which the chemical composition contains, in % by mass, one or two or more selected from the group consisting of:
- Ti: from 0.002 to 0.050%,
- Nb: from 0.002 to 0.050%,
- B: from 0.0003 to 0.0040%,
- REM: from 0.002 to 0.030%,
- Mg: from 0.0002 to 0.0040%,
- Ca: from 0.0002 to 0.0040%,
- Zr: from 0.002 to 0.030%,
- W: from 0.02 to 0.10%,
- Te: from 0.001 to 0.030%, and
- Sb: from 0.001 to 0.030%.
<7> The steel wire according to any one of <4> to <6>, which is plated on a surface thereof.
<8> A rope formed by bundling a plurality of the steel wires according to any one of <4> to <7>.
<9> A method for producing a rope, comprising a step of bundling a plurality of the steel wires according to any one of <4> to <7> into a rope.

Advantageous Effects of Invention

According to the disclosure, a wire rod suitable for producing a steel wire having a high strength and excellent torsion properties by wire drawing, a steel wire having a high strength and excellent torsion properties, a rope, and a method for producing a rope are provided.

BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 is a view showing an example of an SEM photograph of each structure of a wire rod.
Fig. 2 is a view showing an example of an SEM photograph of a ferrite structure and a martensite structure of a steel wire.

DESCRIPTION OF EMBODIMENTS

An exemplary embodiment of the disclosure will be described.
In the present specification, a "surface layer" or "surface layer portion" of a wire rod means a range having a depth of from 0.5 to 1.0 mm from the surface (outer peripheral surface) of the wire rod (a region where the depth from the surface is 0.5 mm or more and 1.0 mm or less) in a cross section perpendicular to a longitudinal direction of the wire rod, and a "center portion" means a range within 1.0 mm from a center of the wire rod in the cross section perpendicular to the longitudinal direction of the wire rod.
A "surface layer" of "surface layer portion" of a steel wire means a range having a depth of from 0.2 to 0.5 mm from the surface (outer peripheral surface) of the steel wire (a region where the depth from the surface is 0.2 mm or more and 0.5 mm or less) in a cross section parallel or perpendicular to a longitudinal direction of the steel wire, and a "center portion" means a range within 1.0 mm from a center of the steel wire in the cross section parallel or perpendicular to the longitudinal direction of the steel wire.
A "central axis" means an imaginary line passing through a center point of a cross section perpendicular to an axial direction (longitudinal direction) of the wire rod or the steel wire and extending in the axial direction.
In the present specification, a cross section perpendicular to the longitudinal direction of a wire rod or a steel wire may be referred to as a "transverse cross section", and a cross section parallel to the longitudinal direction and including a central axis may be referred to as a "longitudinal cross section".
In the present specification, a numerical range that has been indicated by use of "to" indicates the range that includes the numerical values which are described before and after "to", as a lower limit value and an upper limit value, respectively. A numerical range in a case in which "more than" or "less than" is attached to the numerical values which are described before and after "to" means a range not including these numerical values as the lower limit value or the upper limit value.
In a numerical range described in a stepwise manner in the present specification, an upper limit value in one stepwise numerical range may be replaced with an upper limit value in another numerical range described in a stepwise manner, or may be replaced with a value shown in Examples.
In a numerical range described in a stepwise manner in the present specification, a lower limit value in one stepwise numerical range may be replaced with a lower limit value in another numerical range described in a stepwise manner, or may be replaced with a value shown in Examples.
The content of the element of the chemical composition may be simply referred to as "amount" (for example, C amount, Si amount, or the like).
As for the content of the element of the chemical composition, "%" means "% by mass".
As for the content of the element of the chemical composition, in a case in which "0 to" is described as the lower limit value, it means that the element is an optional element and is not necessarily contained.
The term "step" includes not only an independent step but also a step by which an intended action of the step is achieved, though the step cannot be clearly distinguished from other steps.

[Wire Rod]

A wire rod according to the disclosure has

a chemical composition containing, in % by mass,
C: from 0.80 to 1.10%,
Si: from 0.10 to 1.50%,
Mn: from 0.10 to 1.00%,
P: 0.030% or less,
S: 0.030% or less,
N: 0.0120% or less,
O: 0.0100% or less,
Al: from 0.005 to 0.070%, and
Mo: from 0.02 to 0.20%, and
optionally containing,
Cr: from 0 to 1.00%,
Cu: from 0 to 0.80%,
Sn: from 0 to 0.50%,
Ni: from 0 to 0.50%,
V: from 0 to 0.15%,
Ti: from 0 to 0.050%,
Nb: from 0 to 0.050%,
B: from 0 to 0.0040%,
REM: from 0 to 0.030%,
Mg: from 0 to 0.0040%,
Ca: from 0 to 0.0040%,
Zr: from 0 to 0.030%,
W: from 0 to 0.10%,
Te: from 0 to 0.030%, and
Sb: from 0 to 0.030%,
with a balance being Fe and impurities.

In the wire rod according to the disclosure, a total area ratio of a ferrite structure and a martensite structure (in the disclosure, referred to as "area ratio of the ferrite structure and the martensite structure" in some cases) in a center portion within 1.0 mm from a center in a cross section perpendicular to a longitudinal direction is 5% or less, with the balance being a mixed structure composed of ferrite and cementite,

in a case in which a content of C contained in the wire rod in % by mass is denoted as [C], an average Vickers hardness (in the disclosure, referred to as "average hardness" in some cases) H in the cross section satisfies Formula (1): $480 \times [C] - 40 < H < 480 \times [C] + 40$
a surface layer Vickers hardness (in the disclosure, referred to as "surface hardness" in some cases) Hs at a position of a depth of from 0.5 to 1.0 mm from a surface and the average Vickers hardness H satisfy the relationship of Formula (2): $0 \leq Hs - H < 30$
a center Vickers hardness (in the disclosure, referred to as "center hardness" in some cases) Hc within 1.0 mm from the center and the average Vickers hardness H satisfy the relationship of Formula (3): $0 \leq H - Hc < 30$ and
a diameter of the wire rod is 7.0 mm or more.

The inventors of the disclosure have found the wire rod and the steel wire according to the disclosure through the following studies.
In order to obtain a predetermined diameter and strength by performing wire drawing and to favorably maintain torsion properties, it is desirable that a mixed structure composed of ferrite and cementite, such as pearlite or bainite, which is excellent in work-hardenability, is used as a main structure. In a case in which a structure containing almost no cementite (a ferrite structure or a martensite structure) is mixed, torsion properties are deteriorated. In the disclosure, pearlite and bainite are not distinguished from each other because in a case in which pearlite and bainite have the same hardness, pearlite and bainite have similar properties after wire drawing, and it is difficult to distinguish pearlite and bainite from a photograph as a structure.
In a steel wire used with a diameter of, for example, from 5.0 mm to 7.0 mm like a bridge cable steel wire, the diameter of the wire rod is often more than 7.0 mm. In the case of a thick wire rod having a diameter of more than 7.0 mm, it is difficult to uniformly form a pearlite structure or a bainite structure in the cross section. A pearlite transformation temperature and a bainite transformation temperature are different between the surface layer having a high cooling rate and the center portion having a low cooling rate. Therefore, the surface layer tends to be harder than the center portion. The hardness distribution formed in the wire rod is carried over until after wire drawing, and the steel wire also has a distribution in which the surface layer is hard. In a torsion test, since strain concentrates on the surface layer, the torsion properties are disadvantageous in a distribution in which the surface layer is hard.
Therefore, the inventors of the disclosure have studied sufficiently delaying the transformation so that the pearlite transformation or the bainite transformation can be performed at a temperature comparable to that of the surface layer even in the center portion where the cooling rate is slow. As a result of repeated studies, the inventors of the disclosure have found that Mo has an effect of delaying the transformation, and that the uniformity of the hardness distribution in the transverse cross section of the wire rod (cross section perpendicular to the longitudinal direction of the wire rod) is improved by the addition of Mo, and have further confirmed that a steel wire containing a predetermined amount of Mo has favorable torsion properties even in a case in which the tensile strength is 2050 MPa or more.

Hereinafter, the chemical composition (content of each element) of the wire rod according to the disclosure will be described.

C: from 0.80 to 1.10%

C is a component necessary for increasing the tensile strength of the wire rod and a steel wire obtained after wire drawing.
In a case in which the content of C is less than 0.80%, the tensile strength is insufficient.
On the other hand, in a case in which the C content of the wire rod is too large, the wire rod becomes hard, and the torsion properties are deteriorated. In a case in which the C content of the wire rod is more than 1.10%, it becomes difficult to suppress the formation of pro-eutectoid cementite, and even in a case in which other requirements are satisfied, the target torsion properties cannot be obtained.
From the viewpoint of tensile strength, the C content is preferably 0.85% or more, and particularly, in the case of 0.90% or more, more favorable properties are exhibited. From the viewpoint of torsion properties, the C content is preferably 1.05% or less, and particularly, in the case of 1.00% or less, more favorable properties are exhibited.

Si: from 0.10 to 1.50%

Si is a component effective for increasing the tensile strength of the wire rod and a steel wire obtained after wire drawing.
In a case in which the Si content of the wire rod is less than 0.10%, the effect obtained by containing Si cannot be sufficiently obtained.
On the other hand, in a case in which the Si content of the wire rod is more than 1.50%, it becomes difficult to suppress the formation of the martensite structure as a hard phase, and even in a case in which other requirements are satisfied, the target torsion properties cannot be obtained.
From the viewpoint of tensile strength, the Si content is preferably 0.50% or more, and particularly, in the case of 0.70% or more, more favorable properties are exhibited. From the viewpoint of torsion properties, the Si content is preferably 1.40% or less, and particularly, in the case of 1.30% or less, more favorable properties are exhibited.

Mn: from 0.10 to 1.00%

Mn is a component effective for increasing the tensile strength of the wire rod and a steel wire obtained after wire drawing. Mn is a component having an action of fixing S in steel as MnS to suppress hot brittleness.
In a case in which the Mn content of the wire rod is less than 0.10%, the effect obtained by containing Mn cannot be sufficiently obtained.
On the other hand, in a case in which Mn is contained in the wire rod in an amount exceeding 1.00%, it becomes difficult to suppress the formation of the martensite structure as a hard phase, and even in a case in which other requirements are satisfied, the target torsion properties cannot be obtained.
From the viewpoint of tensile strength, the Mn content is preferably 0.25% or more. From the viewpoint of torsion properties, the Mn content is preferably 0.80% or less.

P: 0.030% or less

P is an element that segregates at the grain boundary of the wire rod and deteriorates the torsion properties.
In a case in which the P content of the wire rod is 0.030% or less, deterioration of torsion properties is suppressed, and the target torsion properties can be obtained by satisfying other requirements.
The upper limit of the P content is preferably 0.025% and more preferably 0.020% or less. The lower limit of the P content is not limited and may be 0% (that is, P is not contained), but may be more than 0% or 0.001% or more from the viewpoint of reducing the removal cost of P.

S: 0.030% or less

S is an element that deteriorates the torsion properties.
In a case in which the S content of the wire rod is 0.030% or less, the target torsion properties can be obtained by satisfying other requirements.
The preferred upper limit of the S content is 0.020%. The lower limit of the S content is not limited, but may be more than 0% or 0.001% or more from the viewpoint of reducing the removal cost of S.

N: 0.0120% or less

N is an element that deteriorates the torsion properties.
In a case in which the N content of the wire rod is 0.0120% or less, the target torsion properties can be obtained by satisfying other requirements.
The preferred upper limit of the N content is 0.0100% and the more preferred upper limit thereof is 0.0070%. The lower limit of the N content is not limited, but may be more than 0% or 0.0001 % or more from the viewpoint of reducing the refining cost.

O: 0.0100% or less

O is an element that easily forms an oxide-based inclusion in the wire rod.
In a case in which the O content of the wire rod is 0.0100% or less, coarsening of the oxide-based inclusion can be suppressed, and deterioration of torsion properties can be suppressed.
The preferred upper limit of the O content is 0.0070% and the more preferred upper limit thereof is 0.0050%. The lower limit of the O content is not limited, but may be more than 0% or 0.0001 % or more from the viewpoint of reducing the refining cost.

Al: from 0.005% to 0.070%

Al is an element having a deoxidizing action, and is necessary for reducing the amount of oxygen in the wire rod.
In a case in which the Al content of the wire rod is less than 0.005%, it is difficult to obtain the effect obtained by containing Al.
Al is an element that easily forms a hard oxide-based inclusion. In a case in which the Al content of the wire rod is more than 0.070%, a coarse oxide-based inclusion is remarkably easily formed, and the wire drawability is remarkably deteriorated.
From the viewpoint of the effect of deoxidation, the Al content is preferably 0.010% or more and may be 0.020% or more. From the viewpoint of wire drawability, the Al content is preferably 0.050% or less and may be 0.040% or less.

Mo: from 0.02 to 0.20%

Mo is an element that can delay pearlite transformation and bainite transformation in a trace amount, and is an element effective particularly for improving the strength of the center portion of a wire rod having a diameter of 7.0 mm or more. Mo also has an effect of increasing the tensile strength of a steel wire to be obtained after the wire drawing.
In a case in which the Mo content of the wire rod is less than 0.02%, the effect obtained by containing Mo cannot be sufficiently obtained.
On the other hand, in a case in which the Mo content of the wire rod is more than 0.20%, it becomes difficult to suppress the formation of the martensite structure as a hard phase, and even in a case in which other requirements are satisfied, the target torsion properties cannot be obtained.
From the viewpoint of tensile strength, the Mo content is preferably 0.025% or more, and particularly, in the case of 0.03% or more, more favorable properties are exhibited. From the viewpoint of torsion properties, the Mo content is preferably 0.15% or less, and particularly, in the case of 0.12% or less, more favorable properties are exhibited.
The wire rod according to the disclosure may contain, as an optional element, one or two or more of Cr, Cu, Ni, Sn, V, Ti, Nb, B, REM, Mg, Ca, Zr, W, Te, or Sb, instead of a part of Fe. These optional elements are not necessarily contained (that is, 0%), or may be contained within the following ranges. The lower limit value of the content in the case of containing these optional elements may be more than 0%. For example, one or two or more selected from the group consisting of Cr, Cu, Sn, Ni, and V may be contained, or one or two or more selected from the group consisting of Ti, Nb, B, REM, Mg, Ca, Zr, W, Te, and Sb may be contained.

Cr: from 0 to 1.00%

The inclusion of Cr is optional.
Cr has an effect of increasing the tensile strength of the wire rod and a steel wire to be obtained after the wire drawing. In order to stably obtain the effect, the Cr content is preferably 0.03% or more.
In a case in which the Cr content is more than 1.00%, it is difficult to suppress the formation of the martensite structure as a hard phase, and the torsion properties are deteriorated.
Therefore, in a case in which Cr is actively contained in the wire rod, the Cr content is preferably within a range of from 0.03 to 1.00%. The content may be preferably 0.85% or less. The content is more preferably from 0.10 to 0.70%.

Cu: from 0 to 0.80%

The inclusion of Cu is optional.
Cu has an effect of enhancing the corrosion resistance of the wire rod and a steel wire to be obtained after the wire drawing. In order to stably obtain the effect, the Cu content is preferably 0.01% or more.
On the other hand, even in a case in which the Cu content of the wire rod is more than 0.80%, the effect is saturated.
Therefore, in a case in which Cu is actively contained in the wire rod, the Cu content is preferably within a range of from 0.01 to 0.80%. The content is more preferably from 0.05 to 0.60%.

Sn: from 0 to 0.50%

The inclusion of Sn is optional.
Sn has an effect of enhancing the corrosion resistance of the wire rod and a steel wire to be obtained after the wire drawing. In order to stably obtain the effect, the Sn content is preferably 0.005% or more.
On the other hand, even in a case in which the Sn content of the wire rod is more than 0.50%, the effect is saturated.
Therefore, in a case in which Sn is actively contained in the wire rod, the Sn content is preferably within a range of from 0.001 to 0.50%. The content is more preferably from 0.005 to 0.40%.

Ni: from 0 to 0.50%

The inclusion of Ni is optional.
Ni has an effect of enhancing the corrosion resistance of the wire rod and a steel wire to be obtained after the wire drawing. In order to stably obtain the effect, the Ni content is preferably 0.01% or more.
On the other hand, even in a case in which the Ni content of the wire rod is more than 0.50%, the effect is saturated.
Therefore, in a case in which Ni is actively contained in the wire rod, the Ni content is preferably within a range of from 0.01 to 0.50%. The content is more preferably from 0.05 to 0.40%.

V: from 0 to 0.15%

The inclusion of V is optional.
V has an effect of increasing the tensile strength of the wire rod and a steel wire to be obtained after the wire drawing. In order to stably obtain the effect, the V content of the wire rod is preferably 0.01% or more.
On the other hand, in a case in which the V content of the wire rod is more than 0.15%, the torsion properties are deteriorated.
Therefore, in a case in which V is actively contained in the wire rod, the V content of the wire rod is preferably from 0.02 to 0.15%. The content is more preferably from 0.03% to 0.13% and still more preferably from 0.05% to 0.12%.

Ti: from 0 to 0.050%

The inclusion of Ti is optional.
Ti has an effect of forming a carbide or a carbonitride in the wire rod and enhancing the torsion properties. In order to obtain the effect, the Ti content of the wire rod is preferably 0.002% or more.
On the other hand, in a case in which the Ti content of the wire rod is more than 0.050%, coarse carbides or carbonitrides are easily formed, and the torsion properties are deteriorated.
Therefore, in a case in which Ti is actively contained in the wire rod, the Ti content of the wire rod is preferably from 0.002 to 0.050%. The Ti content is more preferably from 0.005 to 0.030%.

Nb: from 0 to 0.050%

The inclusion of Nb is optional.
Nb has an effect of forming a carbide or a carbonitride in the wire rod and enhancing the torsion properties. In order to obtain the effect, the Nb content of the wire rod is preferably 0.002% or more.
On the other hand, in a case in which the Nb content of the wire rod is more than 0.050%, coarse carbides or carbonitrides are easily formed, and the torsion properties are deteriorated.
Therefore, in a case in which Nb is actively contained in the wire rod, the Nb content of the wire rod is preferably from 0.002 to 0.050%. The Nb content is more preferably from 0.005 to 0.030%.

B: from 0 to 0.0040%

The inclusion of B is optional.
B has an effect of suppressing a ferrite structure and enhancing the torsion properties. In order to obtain the effect, the B content of the wire rod is preferably 0.0003% or more.
On the other hand, in a case in which the B content of the wire rod is more than 0.0040%, coarse carbides are easily formed, and the torsion properties are deteriorated.
Therefore, in a case in which B is actively contained in the wire rod, the content of B of the wire rod is preferably from 0.0003 to 0.0040%. The B content is more preferably from 0.0006 to 0.0030%.

REM: from 0 to 0.030%

The inclusion of REM is optional.
In a case in which REM is contained, high torsion properties can be more stably exhibited. In order to obtain the effect, the REM content of the wire rod is preferably 0.002% or more.
On the other hand, in a case in which the REM content of the wire rod is more than 0.030%, the effect is saturated.
Therefore, in a case in which REM is actively contained, the REM content of the wire rod is preferably from 0.002 to 0.030%.
REM refers to Sc, Y, and lanthanoids, 17 elements in total, and in the case of one type of REM, the REM content refers to the content thereof and in the case of two or more types of REM, the REM content refers to the total content thereof.

Mg: from 0 to 0.0040%

The inclusion of Mg is optional.
In a case in which Mg is contained, high torsion properties can be more stably exhibited. In order to obtain the effect, the Mg content of the wire rod is preferably 0.0002% or more.
On the other hand, in a case in which the Mg content of the wire rod is more than 0.0040%, the effect is saturated.
Therefore, in a case in which Mg is actively contained in the wire rod, the Mg content of the wire rod is preferably from 0.0002 to 0.0040%.

Ca: from 0 to 0.0040%

The inclusion of Ca is optional.
In a case in which Ca is contained, high torsion properties can be more stably exhibited. In order to obtain the effect, the Ca content of the wire rod is preferably 0.0002% or more.
On the other hand, in a case in which the Ca content of the wire rod is more than 0.0040%, the effect is saturated.
Therefore, in a case in which Ca is actively contained in the wire rod, the Ca content of the wire rod is preferably from 0.0002 to 0.0040%.

Zr: from 0 to 0.030%

The inclusion of Zr is optional.
In a case in which Zr is contained, high torsion properties can be more stably exhibited. In order to obtain the effect, the Zr content of the wire rod is preferably 0.002% or more.
On the other hand, in a case in which the Zr content of the wire rod is more than 0.030%, coarse carbides or carbonitrides are easily formed, and the torsion properties are deteriorated.
Therefore, in a case in which Zr is actively contained in the wire rod, the Zr content of the wire rod is preferably from 0.002 to 0.030%.

W: from 0 to 0.10%

The inclusion of W is optional.
In a case in which W is contained, high torsion properties can be more stably exhibited. In order to obtain the effect, the W content of the wire rod is preferably 0.02% or more.
On the other hand, in a case in which the W content of the wire rod is more than 0.10%, the effect is saturated.
Therefore, in a case in which W is actively contained in the wire rod, the W content of the wire rod is preferably from 0.02 to 0.10%.

Te: from 0 to 0.030%

The inclusion of Te is optional.
In a case in which Te is contained, high torsion properties can be more stably exhibited. In order to obtain the effect, the Te content of the wire rod is preferably 0.001% or more.
On the other hand, in a case in which the Te content of the wire rod is more than 0.030%, the effect is saturated.
Therefore, in a case in which Te is actively contained in the wire rod, the Te content of the wire rod is preferably from 0.001 to 0.030%.

Sb: from 0 to 0.030%

The inclusion of Sb is optional.
In a case in which Sb is contained, high torsion properties can be more stably exhibited. In order to obtain the effect, the Sb content of the wire rod is preferably 0.001% or more.
On the other hand, in a case in which the Sb content of the wire rod is more than 0.030%, the effect is saturated.
Therefore, in a case in which Sb is actively contained in the wire rod, the Sb content of the wire rod is preferably from 0.001 to 0.030%.

Next, the metallographic structure of the wire rod according to the disclosure will be described.

(Total Area Ratio of Ferrite Structure and Martensite Structure at Center Portion of Cross Section)

The total area ratio of the ferrite structure and the martensite structure at the center portion of the transverse cross section (cross section perpendicular to the longitudinal direction) of the wire rod is 5% or less. These deteriorate the torsion properties of the steel wire after the wire drawing. In a case in which the total area ratio of the ferrite structure and the martensite structure at the center portion of the wire rod is 5% or less, the total area ratio of the ferrite structure and the martensite structure including up to the surface layer is low, and thus the center portion is measured as a representative. The total area ratio of the ferrite structure and the martensite structure at the center portion of the cross section of the wire rod may be 4% or less, 2% or less, or 1% or less. One of the ferrite structure or the martensite structure may be included, or both of them are not necessarily included. The same applies to the structure in the longitudinal cross section of the steel wire described later.

Next, the hardness of the wire rod according to the disclosure will be described.

(Average Vickers Hardness H)

In the wire rod according to the disclosure, the average Vickers hardness H in the transverse cross section satisfies Formula (1). $480 \times [C] - 40 < H < 480 \times [C] + 40$
The average Vickers hardness H in the transverse cross section depends on the amount of carbon [C] contained in the wire rod in % by mass. In a case in which the average value of the Vickers hardness is less than "480 × [C] - 40", it is difficult to stably impart the tensile strength (for example, 2050 MPa or more) required for the steel wire after the wire drawing.
On the other hand, in a case in which the average value of the Vickers hardness in the cross section exceeds "480 × [C] + 40", minute cracks are likely to occur during wire drawing, and even in a case in which other requirements are satisfied, it is not possible to obtain a steel wire having a tensile strength of 2050 MPa or more and a target number of torsions of 12 times or more in a torsion test in Examples described later.
The average Vickers hardness H may be in a range satisfying the following Formula (1A) or in a range satisfying Formula (1B). $480 \times [C] - 30 < H < 480 \times [C] + 30$ $480 \times [C] - 20 < H < 480 \times [C] + 20$

(Relationship between Surface Layer Vickers Hardness Hs and Average Vickers Hardness H)

In the wire rod according to the disclosure, the surface layer Vickers hardness Hs at a position of a depth of from 0.5 to 1.0 mm from the surface and the average Vickers hardness H satisfy the relationship of Formula (2). $0 \leq Hs - H < 30$
In a case in which the surface layer Vickers hardness Hs is larger than the average Vickers hardness H by 30 or more, minute cracks are likely to occur during the torsion test, and even in a case in which other requirements are satisfied, a target torsion test result cannot be obtained.
On the other hand, it is industrially difficult to make the surface layer Vickers hardness Hs lower than the average Vickers hardness H in a normal rolled wire rod.
The relationship between the surface layer Vickers hardness Hs and the average Vickers hardness H may be in a range satisfying the following Formula (2A) or in a range satisfying Formula (2B). $0 \leq Hs - H < 25$ $0 \leq Hs - H < 20$

(Relationship between Center Vickers Hardness Hc and Average Vickers Hardness H)

In the wire rod according to the disclosure, the center Vickers hardness Hc within 1.0 mm from the center in the transverse cross section and the average Vickers hardness H satisfy the relationship of Formula (3). $0 \leq H - Hc < 30$
In a case in which the center Vickers hardness Hc is smaller than the average Vickers hardness H by 30 or more, it is difficult to stably impart the tensile strength (for example, 2050 MPa or more) required for the steel wire after the wire drawing.
It is industrially difficult to make the center Vickers hardness Hc higher than the average Vickers hardness H in a normal rolled wire rod.
The relationship between the center Vickers hardness Hc and the average Vickers hardness H may be in a range satisfying the following Formula (3A) or in a range satisfying Formula (3B). $0 \leq H - Hc < 25$ $0 \leq H - Hc < 20$

The wire rod according to the disclosure has a diameter of 7.0 mm or more. In a case in which the diameter of the wire rod is less than 7.0 mm, the effect of the disclosure cannot be obtained.
The upper limit of the diameter of the wire rod according to the disclosure is not particularly limited, but it is difficult to industrially produce a wire rod having a diameter of more than 20 mm by a normal production process.
The diameter of the wire rod according to the disclosure may be from 7.5 to 18.0 mm or from 8.0 to 16.0 mm.

[Steel Wire]

Next, a steel wire according to the disclosure will be described.
A steel wire according to the disclosure has

In the steel wire according to the disclosure, a total area ratio of a ferrite structure and a martensite structure in a center portion in a cross section parallel to a longitudinal direction and passing through a central axis is 5% or less with the balance being a mixed structure composed of ferrite and cementite,

an average Vickers hardness h in a cross section perpendicular to the longitudinal direction is from 450 to 620,
a surface layer Vickers hardness hs at a position of a depth of from 0.2 to 0.5 mm from a surface and the average Vickers hardness h satisfy the relationship of Formula (4): $0 \leq hs - h < 30$ and
a center Vickers hardness hc within 1.0 mm from the center in the cross section perpendicular to the longitudinal direction and the average Vickers hardness h satisfy the relationship of Formula (5). $0 \leq h - hc < 30$

In a case in which the wire rod according to the disclosure is subjected to wire drawing within a wire drawing strain range of from 1.0 to 2.5 and the average Vickers hardness is controlled to from 450 to 620, each of the difference (hs - h) between the surface layer Vickers hardness and the average Vickers hardness and the difference (h - hc) between the center Vickers hardness and the average Vickers hardness is less than 30. In a case in which the hardness distribution is such that the surface layer is not too hard and the center is not too soft with respect to the average hardness, a steel wire having favorable torsion properties while achieving a high strength can be obtained.
The steel wire according to the disclosure may be subjected to a plating treatment with zinc or a zinc alloy, or may be subjected to heat input by a degreasing treatment. That is, the steel wire according to the disclosure also includes a steel wire with a plated surface (plated wire). In a case in which the steel wire according to the disclosure has a plating layer, the chemical composition, the metallographic structure, and the hardness in the disclosure all mean values of a steel portion.
The steel wire according to the disclosure may contain an optional element similar to the chemical composition of the wire rod described above.
The diameter of the steel wire according to the disclosure is not particularly limited, and is, for example, from 3.0 to 8.0 mm and may be from 4.0 to 7.5 mm.

Next, measurement methods of the area ratio of the metallographic structure and hardness of the wire rod and the steel wire according to the disclosure, and evaluation test methods will be described. Each value in Examples described later is a measurement value by the following measurement methods.
In the following description, the diameter of the wire rod or the steel wire to be measured is defined as D.

(Measurement of Area Ratio of Structure of Wire Rod)

Five images of the center and positions 200 µm away from the center in the vertical and horizontal directions in the transverse cross section are taken in a region of 2.7×10^-3 mm² (length: 0.045 mm, width: 0.060 mm) with a scanning electron microscope (SEM). Before the observation with the SEM, the cross section is mirror-polished, and then the metallographic structure is exposed by using picral for etching.
Figs. 1(A) to 1(D) show examples of a pearlite structure, which is an example of a mixed structure in which ferrite and cementite are mixed, a martensite structure, a ferrite structure (denoted as "ferrite" in Fig. 1(C)), and a mixed structure composed of ferrite and cementite. The ferrite structure and the martensite structure are structures not containing cementite, and the mixed structure composed of ferrite and cementite is pearlite, bainite, or the like, in which cementite is mixed in ferrite. For example, in (C), there is a pearlite structure (an example of a mixed structure of ferrite and cementite), which is a lamellar structure of ferrite (blackish portion) and cementite (whitish portion), but since cementite is not present in the arrow portion and is distinguished from the surrounding pearlite structure, it is determined as a ferrite structure. In (A), the two arrow portions have a structure in which more ferrite is present than cementite but in which divided cementite is present in ferrite, and it is determined that the structure is a mixed structure of ferrite and cementite. In each SEM photograph, the area ratio (%) of the ferrite structure and the martensite structure is measured. The ferrite structure and the martensite structure, which are structures not containing cementite, are not necessarily distinguished from each other. Specifically, after each SEM image is printed on the paper surface, a transparent sheet such as an over head projector (OHP) sheet is overlaid on the paper surface to color the structure (ferrite structure and martensite structure) not containing cementite. Thereafter, the transparent sheet in which the ferrite structure and the martensite structure are colored is analyzed by image analysis to measure the total area ratio of the ferrite structure and the martensite structure. The area per visual field is set to 2.7×10^-3 mm² (length: 0.045 mm, width: 0.060 mm), and image analysis software (for example, LUZEX AP manufactured by NIRECO) is used for image analysis. An average value is calculated from the total area ratio of the ferrite structure and the martensite structure of the five images, and the average value is taken as the total area ratio of the ferrite structure and the martensite structure of the wire rod.

(Measurement of Area Ratio of Structure of Steel Wire)

The center portion in the longitudinal cross section is photographed at intervals of 200 µm by five with a scanning electron microscope (SEM) at 3×10^-4 mm² (length: 0.015 mm, width: 0.02 mm). Before the observation with the SEM, the longitudinal cross section of the steel wire is mirror-polished, and then the metallographic structure is exposed by using picral for etching.
Figs. 2(A) and 2(B) show examples of SEM photographs of the longitudinal cross sections of steel wires each including a ferrite structure or a martensite structure and a mixed structure composed of ferrite and cementite. (a) is an enlarged view of an arrow portion of (A), and (b) is an enlarged view of an arrow portion of (B). The ferrite structure and the martensite structure are structures not containing cementite, and the mixed structure composed of ferrite and cementite is a structure obtained by subjecting pearlite, bainite, or the like to wire drawing, in which cementite is mixed in ferrite. In each SEM photograph, the area ratio (%) of the ferrite structure and the martensite structure is measured. The ferrite structure and the martensite structure, which are structures not containing cementite, are not necessarily distinguished from each other. Specifically, after each SEM image is printed on the paper surface, a transparent sheet such as an over head projector (OHP) sheet is overlaid on the paper surface to color the ferrite structure and the martensite structure. Thereafter, the transparent sheet in which the structures are colored is analyzed by image analysis to measure the total area ratio of the ferrite structure and the martensite structure. The area per visual field is set to 3×10^-4 mm² (length: 0.015 mm, width: 0.02 mm), and image analysis software (for example, LUZEX AP manufactured by NIRECO) is used for image analysis. An average value is calculated from the area ratio of the ferrite structure and the martensite structure of the five images, and the average value is taken as the total area ratio of the ferrite structure and the martensite structure of the steel wire.

(Hardness Measurement of Wire Rod)

An indenter is pushed into the transverse cross section of the wire rod with a load of 1 kgf using a Vickers tester.
Points are formed toward the center at a pitch of 0.5 mm starting from a position of 0.5 mm in depth from the surface (outer peripheral surface) of the wire rod. Points are formed from four directions at intervals of 90°.
A portion within a depth of from 0.5 to 1.0 mm from the surface is taken as the surface layer portion, and an average value of the Vickers hardnesses at eight points in total formed at distances of 0.5 mm and 1.0 mm from the surface is taken as the surface layer Vickers hardness Hs.
A portion within a range of 1.0 mm from the center (the central axis of the wire rod) in the transverse cross section is taken as the center portion, and an average value of the hardnesses at nine points in total formed at the center and at distances of 0.5 mm and 1.0 mm from the center is taken as the center Vickers hardness Hc.
An average value of the Vickers hardnesses measured at the center and at intervals of 0.5 mm from the surface toward the center (excluding a region of less than 0.3 mm from the center) in the entire cross section, that is, the transverse cross section, is taken as the average Vickers hardness. For example, in the case of a wire rod having a diameter of 12 mm, the average value of the Vickers hardnesses measured at 45 locations in total, i.e., one location of the center and 11 locations between the position of 0.5 mm from the surface and the position of 0.5 mm from the center × four directions (= 44 locations), is taken as the average Vickers hardness H. For example, in the case of a wire rod having a diameter of 8.4 mm, the average value of the hardnesses measured at 29 locations in total, i.e., one location of the center and seven locations between the positions of from 0.5 mm to 3.5 mm from the surface (the position of 4.0 mm from the surface is excluded because it is a position of 0.2 mm from the center) × four directions (= 28 locations), is taken as the average Vickers hardness H.

(Hardness Measurement of Steel Wire)

The indenter is pushed into the transverse cross section of the steel wire with a load of 1 kgf using a Vickers tester.
Points are formed toward the center at a pitch of 0.2 mm starting from a position of 0.2 mm in depth from the surface (outer peripheral surface) of the steel wire. Points are formed from four directions at intervals of 90°. In the plated steel wire, a position of 0.2 mm from the surface of the steel portion is set as a starting point.
The average value of the Vickers hardnesses obtained by forming points within a range of from 0.2 to 0.5 mm in depth from the surface of the steel wire is taken as the surface layer Vickers hardness hs. The average value of the hardnesses obtained by forming points within 1 mm from the center is taken as the center Vickers hardness hc.
An average value of the Vickers hardnesses measured at the center and at intervals of 0.2 mm from the surface toward the center (excluding a region of less than 0.2 mm from the center) in the entire cross section, that is, the transverse cross section is taken as the average Vickers hardness. For example, in the case of a steel wire having a diameter of 5.4 mm, the average value of the hardnesses measured at 49 locations in total, i.e., one location of the center and 12 locations between the positions of from 0.2 mm to 2.4 mm from the surface (the position of 2.6 mm from the surface is excluded because it is a position of 0.1 mm from the center) × four directions (= 48 locations), is taken as the average Vickers hardness h.

(Torsion Test of Steel Wire)

Evaluation is performed based on the number of times until the steel wire is broken, and a minimum value is evaluated by performing this for each five wires.
The distance between chucks is 100 × diameter D. The rotation speed is set to 20 rpm.
The breakage of the steel wire described herein includes not only a case in which the entire steel wire is broken but also a case in which a partial crack occurs in the steel wire. That is, in a case in which a partial crack occurs in the steel wire during the test, the evaluation is performed based on the number of twists at that time.

(Tensile Test)

The wire rod to be subjected to a tensile test may be straightened to be a straight line. The length of the wire rod is set to 340 mm, the distance between chucks is set to 200 mm, and the tensile test is performed at a stroke speed of 10 mm/min.
The diameter of the wire rod is measured in two directions orthogonal to each other at the center in the length direction of the wire rod using a caliper, and the average value thereof is taken as the diameter. The tensile strength is the maximum load (N) at the time of the tensile test/the cross-sectional area (mm²) calculated from the diameter of the wire rod.
The tensile test and the measurement of the diameter of the steel wire can be performed in the same manner as in the wire rod. In the case of a plated wire, the plating is peeled off to conduct a tensile test as only a steel wire. The plating peeling is performed by a chemical peeling method such as immersion in hydrochloric acid or a physical peeling method such as polishing.

Methods for producing a wire rod and a steel wire according to the disclosure are not particularly limited, but an example of a suitable production method will be described below.

(Casting of Cast Slab)

A cast slab having the above-described chemical composition is produced. For example, in the case of performing continuous casting, there is a method of obtaining a cast slab by performing melting by a converter, then sufficiently performing electromagnetic stirring of molten steel, and further performing pressing during solidification.

(Rolling of Cast Slab)

The cast slab obtained by casting is heated to from 1200 to 1250°C and then subjected to blooming to obtain a billet.

(Rolling of Wire Rod)

Next, the billet is rolled to obtain a rolled wire rod. The heating temperature of the billet is from 1020°C to 1080°C.
In a case in which the billet is heated at lower than 1020°C, the reaction force increases and rolling is difficult.
On the other hand, in a case in which the heating temperature is higher than 1080°C, the austenite structure becomes coarse, and untransformed austenite may remain when immersed in a molten salt described later, so that there is a possibility that martensite is mixed.
The exit temperature (finish rolling temperature) of the finish rolling is from 900 to 1000°C.
It is industrially difficult to set the finish rolling temperature to lower than 900°C.
On the other hand, in a case in which the finish rolling temperature is higher than 1000°C, the austenite structure becomes coarse, and untransformed austenite may remain when immersed in a molten salt described later, so that there is a possibility that martensite is mixed.

(Cooling before Winding)

After the finish rolling, the cooling to from 800°C to 900°C (temperature before winding) is performed by water cooling or air cooling.
In a case in which the temperature is lower than 800°C, the temperature is excessively lowered until immersion in a molten salt, and the pearlite transformation starts from the surface layer so that the surface layer is softened.
In a case in which the temperature is higher than 900°C, the austenite structure becomes coarse, and untransformed austenite may remain when immersed in a molten salt described later, so that there is a possibility that martensite is mixed.

(Cooling before Molten Salt)

Cooling is performed before immersion in a molten salt.
Before immersion in a molten salt, cooling is performed at 10°C/sec or more to a temperature of from 740°C to 800°C (temperature before the molten salt).
In a case in which the temperature is lower than 740°C, the pearlite transformation starts from the surface layer so that the surface layer is softened. Coarse pearlite degrades torsion properties.
In a case in which the temperature is higher than 800°C, the temperature of the molten salt increases, and pearlite cannot be transformed at a target temperature.
In a case in which the cooling rate is less than 10°C/sec, the torsion properties are deteriorated due to precipitation of pro-eutectoid cementite. The upper limit of the cooling rate is not particularly limited, but may be 50°C/sec or less from a technical viewpoint.

(Immersion in Molten Salt)

After cooling to a temperature of from 740°C to 800°C, immersion in a molten salt induces transformation into pearlite or bainite.
A molten salt tank is set to from 540°C to 570°C, and immersion is performed for from 60 seconds to 120 seconds.
In a case in which the temperature of the molten salt is lower than 540°C, the ferrite structure increases. On the other hand, in a case in which the temperature of the molten salt is higher than 570°C, the cooling rate inside the wire rod is insufficient and the wire rod is softened.
In a case in which the immersion time in the molten salt is less than 60 seconds, there is a possibility that untransformed austenite remains and martensite is mixed. In a case in which the immersion is performed for 120 seconds or more, cementite may be spheroidized after transformation, and the strength may be reduced.

(Wire Drawing)

The wire rod produced through the above steps can provide a steel wire having a high strength and excellent torsion properties by wire drawing.
A wire drawing strain ε due to the wire drawing is expressed by the following formula. $ε = Ln {(D_{0} / D)}^{2}$
D₀ is the diameter before the wire drawing, and D is the diameter after the wire drawing. ε is preferably from 1.0 to 2.5. In a case in which the wire drawing strain is lower than 1.0, the strength is less likely to be obtained, and in a case in which the wire drawing strain is higher than 2.5, the working ratio becomes excessive and the torsion properties are deteriorated. Before the wire drawing, it is preferable to perform a surface lubrication treatment such as a zinc phosphate coating or a borax coating.
A steel wire having a high strength and excellent torsion properties can be produced through the above steps. After the wire drawing, post-treatment such as plating or a degreasing treatment may be performed. The degreasing treatment and the plating treatment with zinc or a zinc alloy are preferably performed with in a range of from 400 to 530°C. In a case in which the temperature is lower than 400°C, the steel material is hardened by age hardening, and the torsion properties are deteriorated. In a case in which the temperature is 530°C or higher, softening due to spheroidization of cementite or the like becomes remarkable, resulting in insufficient strength.

The use application of the steel wire obtained by subjecting the wire rod according to the disclosure to wire drawing is not particularly limited, but it is suitable for various use applications such as a bridge cable steel wire and various ropes, which have a high strength and require torsion properties. The wire rod according to the disclosure is suitable as a material of a steel wire used for these use applications. For example, a rope (strand) can be obtained by bundling a plurality of steel wires according to the disclosure. A plurality of plated wires obtained by plating the steel wire according to the disclosure with zinc may be bundled to form a rope.

Examples

Hereinafter, the wire rod and the steel wire of the disclosure will be described more specifically with reference to Examples. However, these Examples do not limit the wire rod and the steel wire of the disclosure.

A steel material having the chemical composition (unit: mass%) shown in Table 1 was produced, and a wire rod was produced by the method (condition) shown in Table 2. The notation of "-" in Table 1 indicates that the content of the element with the notation is at an impurity level, and it can be determined that the element is not substantially contained. The same applies to "-" in Table 4 described later. The balance of the chemical composition in Tables 1 and 4 is Fe and impurities.

[Table 1]

Steel material	C	Si	Mn	P	S	N	O	Al	Mo	Cr	Cu	Sn	Ni	V	Ti	Nb	B	REM	Mg	Ca	Zr	W	Te	Sb
steel 1	0.92	0.91	0.39	0.007	0.006	0.0033	0.0041	0.031	0.06	0.24	-	-	-	-	0.012	-	0.0012	-	-	-	-	-	-	-

[Table 2]

Condition	Wire rod rolling			Before immersion in molten salt		Molten salt
Condition	Heating temperature °C	Finish rolling temperature °C	Temperature before winding °C	Temperature before molten salt °C	Cooling rate °C/s	Molten salt temperature °C	Immersion time sec
A	1050	920	820	770	15	555	70
B	1030	950	860	750	15	550	65
C	1050	980	830	790	12	565	70
D	1080	920	850	770	15	550	75
E	1100	920	850	760	15	560	70
F	1050	1020	850	760	15	560	70
G	1050	920	780	710	15	560	70
H	1050	920	910	760	15	560	70
I	1050	920	850	720	15	560	70
J	1050	920	850	820	15	560	70
K	1050	920	850	760	8	560	70
L	1050	920	850	760	15	500	70
M	1050	920	850	760	15	580	70
N	1050	920	850	760	15	560	55

For the wire rod having a diameter of from 10.0 to 14.0 mm produced by the above production method, the total area ratio of the ferrite structure and the martensite structure in the center portion, the Vickers hardness, and the tensile strength were each measured by the methods described above.
A steel wire with a diameter of 5.0 mm was produced by subjecting a wire rod to wire drawing, that is, in the case of a wire rod with a diameter of 14.0 mm, a steel wire with a drawing strain of 2.06 was produced, the Vickers hardness and the tensile strength of the steel wire after aging at 450°C for 30 seconds were measured by the methods described above, and the number of torsions was measured by a torsion test.
In a case in which the tensile strength of the steel wire was 2050 MPa or more, it was determined that the tensile strength was favorable.
A case in which the number of times until the steel wire was broken in the torsion test was 12 or more was evaluated as favorable (mark A), and a case in which the number of times was less than 12 was evaluated as poor (mark B).

The measurement results are shown in Tables 3A and 3B.

[Table 3A]

		Wire rod
No.	Steel material	Production method	Area ratio of ferrite structure and martensite structure at center portion (%)	Average hardness H (Hv)	Center hardness He (Hv)	480 × [C] - 40	480 × [C] + 40	Surface layer hardness Hs (Hv)	Hs-H	H-Hc	Diameter (mm)	Tensile strength (MPa)
1	steel 1	A	2	421	409	402	482	434	13	12	14.0	1494
2	steel 1	B	1	424	411	402	482	434	10	13	12.0	1499
3	steel 1	C	1	417	405	402	482	429	12	12	11.0	1477
4	steel 1	D	2	420	408	402	482	433	13	12	13.0	1489
10	steel 1	E	11	427	408	402	482	439	12	19	13.0	1478
11	steel 1	F	22	419	412	402	482	429	10	7	13.0	1498
12	steel 1	G	1	396	393	402	482	404	8	3	12.0	1404
13	steel 1	H	18	419	403	402	482	426	7	16	12.0	1477
14	steel 1	I	1	381	368	402	482	401	20	13	13.0	1366
15	steel 1	J	3	398	381	402	482	421	23	17	12.0	1411
16	steel 1	K	3	411	380	402	482	433	22	31	14.0	1481
17	steel 1	L	14	361	355	402	482	371	10	6	12.0	1318
18	steel 1	M	1	389	378	402	482	401	12	11	12.0	1389
19	steel 1	N	12	423	412	402	482	433	10	11	14.0	1489

[Table 3B]

	Steel wire							Torsion test
No.	Area ratio of ferrite structure and martensite structure at center portion (%)	Tensile strength (MPa)	Average hardness h (Hv)	Surface layer hardness hs (Hv)	Center hardness hc (Hv)	hs-h	h-he	Number of torsions (times)	Pass/fail
1	1	2216	566	581	554	15	12	21	A
2	1	2110	551	562	534	11	17	21	A
3	1	2092	548	561	533	13	15	20	A
4	2	2202	568	583	553	15	15	22	A
10	10	-	-	-	-	-	-	-	-
11	24	-	-	-	-	-	-	-	-
12	1	2024	537	540	527	3	10	21	B
13	18	-	-	-	-	-	-	-	-
14	1	2031	537	539	533	2	4	18	B
15	3	2033	539	549	531	10	8	22	B
16	2	2151	558	589	523	31	35	5	B
17	14	-	-	-	-	-	-	-	-
18	1	1997	536	541	529	5	7	20	B
19	10	-	-	-	-	-	-	-	-

A steel wire having a high strength (2050 MPa or more) and excellent torsion properties was obtained by wire drawing of the wire rods of Nos. 1 to 4.
In Nos. 10 and 11, the ferrite structure or the martensite structure was mixed in the wire rod, the area ratio of the ferrite structure or the martensite structure was high, and wire breakage occurred in the wire drawing.
In No. 12, the surface hardness and the average hardness of the wire rod were small, and the tensile strength of the steel wire after the wire drawing was insufficient.
In No. 13, the ferrite structure or the martensite structure was mixed in the structure of the wire rod, the area ratio of the ferrite structure or the martensite structure was high, and wire breakage occurred in the wire drawing.
In No. 14, the surface hardness and the average hardness of the wire rod were small, and the tensile strength of the steel wire after the wire drawing was insufficient.
In No. 15, the average hardness was small due to the pearlite transformation at a high temperature, and the tensile strength of the steel wire after the wire drawing was insufficient.
In No. 16, the difference between the average hardness and the center portion hardness in the wire rod was large due to the temperature difference between the surface layer and the inside of the wire rod, and the torsion properties of the steel wire after the wire drawing were insufficient.
In No. 17, the ferrite structure or the martensite structure was mixed in the structure of the wire rod, the area ratio of the ferrite structure or the martensite structure was high, and wire breakage occurred in the wire drawing.
In No. 18, the average hardness in the wire rod was small due to the pearlite transformation at a high temperature, and the tensile strength of the steel wire after the wire drawing was insufficient.
In No. 19, the ferrite structure or the martensite structure was mixed in the structure of the wire rod, the area ratio of the ferrite structure or the martensite structure was high, and wire breakage occurred in the wire drawing.

A steel material having the chemical composition (unit: mass%) shown in Table 4 was produced, and a wire rod was produced under the condition of the production method A shown in Table 2.
For the produced wire rod, the total area ratio of the ferrite structure and the martensite structure, the Vickers hardness, and the tensile strength were each measured by the methods described above.
A steel wire having a diameter of 5.0 mm was produced by subjecting the wire rod to wire drawing, the total area ratio of the ferrite structure and the martensite structure in the center portion, the Vickers hardness, and the tensile strength were each measured by the methods described above, and the number of torsions was measured by a torsion test.

The measurement results are shown in Tables 5A and 5B.

[Table 5A]

			Wire rod
No.	Steel material	Production method	Area ratio of ferrite structure and martensite structure at center portion (%)	Average hardness H (Hv)	480 × [C] - 40	480 × [C] + 40	Center hardness Hc (Hv)	Surface layer hardness Hs (Hv)	Hs-H	H-Hc	Diameter (mm)	Tensile strength (MPa)
20	steel 2	A	1	433	402	482	425	442	9	8	12.0	1494
21	steel 3	A	3	413	378	458	406	419	6	7	14.0	1437
22	steel 4	A	1	404	354	434	400	407	3	4	14.0	1411
23	steel 5	A	2	477	474	554	473	482	5	4	11.0	1562
24	steel 5	A	2	482	474	554	479	484	2	3	10.0	1573
25	steel 6	A	2	422	406	486	411	434	12	11	13.0	1502
26	steel 7	A	4	460	450	530	439	479	19	21	12.0	1569
27	steel 8	A	2	436	402	482	428	449	13	8	14.0	1488
28	steel 9	A	2	454	430	510	452	457	3	2	12.0	1554
29	steel 10	A	3	442	387	467	439	444	2	3	13.0	1518
30	steel 11	A	1	441	397	477	439	443	2	2	14.0	1505
31	steel 12	A	2	464	426	506	457	473	9	7	12.5	1580
32	steel 13	A	1	400	378	458	384	414	14	16	13.0	1399
33	steel 14	A	2	434	397	477	427	445	11	7	12.0	1490
34	steel 15	A	3	430	402	482	419	439	9	11	12.0	1481
35	steel 16	A	2	452	430	510	437	467	15	15	12.0	1532
36	steel 17	A	3	401	378	458	384	421	20	17	13.0	1412
40	steel 18	A	2	381	334	414	368	393	12	13	13.0	1343
41	steel 19	A	3	485	498	578	471	496	11	14	12.0	1641
42	steel 20	A	12	431	378	458	418	444	13	13	14.0	1487
43	steel 21	A	13	428	426	506	414	444	16	14	14.0	1479
44	steel 22	A	1	416	397	477	383	447	31	33	12.0	1444
45	steel 23	A	16	439	445	525	435	441	2	4	14.0	1509
46	steel 24	A	2	457	435	515	454	461	4	3	14.0	1560

[Table 5B]

	Steel wire							Torsion test
No.	Area ratio of ferrite structure and martensite structure at center portion (%)	Tensile strength (MPa)	Average hardness h (Hv)	Surface layer hardness hs (Hv)	Center hardness hc (Hv)	hs-h	h-hc	Number of torsions (times)	Pass/fail	Remark
20	1	2113	551	569	537	18	14	22	A	Disclosure Example
21	2	2201	565	571	554	6	11	21	A	Disclosure Example
22	1	2156	559	567	550	8	9	23	A	Disclosure Example
23	1	2191	567	577	559	10	8	21	A	Disclosure Example
24	2	2112	550	561	439	11	111	19	A	Disclosure Example
25	2	2219	571	579	563	8	8	18	A	Disclosure Example
26	2	2226	571	582	557	11	14	17	A	Disclosure Example
27	2	2296	584	601	569	17	15	23	A	Disclosure Example
28	1	2246	574	585	558	11	16	21	A	Disclosure Example
29	2	2232	573	581	565	8	8	22	A	Disclosure Example
30	1	2297	583	598	569	15	14	20	A	Disclosure Example
31	2	2259	577	591	564	14	13	21	A	Disclosure Example
32	1	2059	540	566	512	26	28	22	A	Disclosure Example
33	1	2127	557	566	548	9	9	23	A	Disclosure Example
34	2	2134	554	573	532	19	22	21	A	Disclosure Example
35	2	2216	569	583	558	14	11	22	A	Disclosure Example
36	3	2122	555	571	534	16	21	20	A	Disclosure Example
40	2	1985	511	522	503	11	8	22	B	Comparative Example
41	2	2389	608	621	589	13	19	2	B	Comparative Example
42	11	-	-	-	-	-	-	-	-	Comparative Example
43	13	-	-	-	-	-	-	-	-	Comparative Example
44	2	2084	534	565	502	31	32	4	B	Comparative Example
45	14	-	-	-	-	-	-	-	-	Comparative Example
46	2	-	-	-	-	-	-	-	-	Comparative Example

The wire rods Nos. 20 to 33 satisfied the requirements of the disclosure, and a steel wire having a high strength (2050 MPa or more) and excellent torsion properties was obtained by wire drawing.
In No. 40, the C amount was excessively small, and the tensile strength of the steel wire after the wire drawing was insufficient.
In No. 41, since the C amount was excessively large, the average hardness with respect to the C amount was small, and the torsion properties were insufficient in the steel wire after the wire drawing.
In No. 42, since the Si amount was excessively large, the area ratio of the ferrite and martensite structures in the wire rod was high, and wire breakage occurred in the wire drawing.
In No. 43, since the Mn amount was excessively large, the area ratio of the ferrite and martensite structures in the wire rod was high, and wire breakage occurred in the wire drawing. In No. 44, since the Mo amount was excessively small, the average hardness with respect to the C amount was small, the difference between the average hardness and the center portion hardness and the difference between the surface layer portion hardness and the average hardness were both large, and the tensile strength and the torsion properties of the steel wire after the wire drawing were insufficient.
In No. 45, since the Mo amount was large, martensite was mixed, the area ratio of the martensite structure in the wire rod was high, the average hardness was also small, and wire breakage occurred in the wire drawing.
In No. 46, the Al amount was excessively large, and wire breakage occurred in the wire drawing.
Although the wire rod according to the disclosure and the like have been described above, the wire rod according to the disclosure and the like are not limited to the above-described embodiments and examples. For example, the method for producing a wire rod according to the disclosure is not limited to a method in which immersion in a molten salt is performed after winding. A case in which the wire rod according to the disclosure is produced by lead patenting in which a metallographic structure is formed by heating the wire rod and then immersing the wire rod in a lead bath, the steel wire according to the disclosure is produced by subjecting the wire rod to wire drawing, and a rope (cable) is produced by bundling the steel wires is also included in the scope of the disclosure.

Claims

A wire rod comprising a chemical composition containing, in % by mass:
C: from 0.80 to 1.10%,

Si: from 0.10 to 1.50%,

Mn: from 0.10 to 1.00%,

P: 0.030% or less,

S: 0.030% or less,

N: 0.0120% or less,

O: 0.0100% or less,

Al: from 0.005 to 0.070%,

Mo: from 0.02 to 0.20%,

Cr: from 0 to 1.00%,

Cu: from 0 to 0.80%,

Sn: from 0 to 0.50%,

Ni: from 0 to 0.50%,

V: from 0 to 0.15%,

Ti: from 0 to 0.050%,

Nb: from 0 to 0.050%,

B: from 0 to 0.0040%,

REM: from 0 to 0.030%,

Mg: from 0 to 0.0040%,

Ca: from 0 to 0.0040%,

Zr: from 0 to 0.030%,

W: from 0 to 0.10%,

Te: from 0 to 0.030%, and

Sb: from 0 to 0.030%,

with a balance being Fe and impurities,

wherein:
a total area ratio of a ferrite structure and a martensite structure in a center portion within 1.0 mm from a center in a cross section perpendicular to a longitudinal direction is 5% or less with the balance being a mixed structure composed of ferrite and cementite,

in a case in which a content of C contained in the wire rod in % by mass is denoted as [C], an average Vickers hardness H in the cross section satisfies Formula (1): $480 \times [C] - 40 < H < 480 \times [C] + 40$

a surface layer Vickers hardness Hs at a position of a depth of from 0.5 to 1.0 mm from a surface and the average Vickers hardness H satisfy a relationship of Formula (2): $0 \leq Hs - H < 30$

a center Vickers hardness Hc within 1.0 mm from the center and the average Vickers hardness H satisfy a relationship of Formula (3): $0 \leq H - Hc < 30$ and

a diameter of the wire rod is 7.0 mm or more.
The wire rod according to claim 1, wherein the chemical composition contains, in % by mass, at least one or two or more selected from the group consisting of:
Cr: from 0.03 to 1.00%,

Cu: from 0.01 to 0.80%,

Sn: from 0.001 to 0.50%,

Ni: from 0.01 to 0.50%, and

V: from 0.01 to 0.15%.
The wire rod according to claim 1 or 2, wherein the chemical composition contains, in % by mass, one or two or more selected from the group consisting of:
Ti: from 0.002 to 0.050%,

Nb: from 0.002 to 0.050%,

B: from 0.0003 to 0.0040%,

REM: from 0.002 to 0.030%,

Mg: from 0.0002 to 0.0040%,

Ca: from 0.0002 to 0.0040%,

Zr: from 0.002 to 0.030%,

W: from 0.02 to 0.10%,

Te: from 0.001 to 0.030%, and

Sb: from 0.001 to 0.030%.
A steel wire comprising a chemical composition containing, in % by mass:
C: from 0.80 to 1.10%,

Si: from 0.10 to 1.50%,

Mn: from 0.10 to 1.00%,

P: 0.030% or less,

S: 0.030% or less,

N: 0.0120% or less,

O: 0.0100% or less,

Al: from 0.005 to 0.070%,

Mo: from 0.02 to 0.20%,

Cr: from 0 to 1.00%,

Cu: from 0 to 0.80%,

Sn: from 0 to 0.50%,

Ni: from 0 to 0.50%,

V: from 0 to 0.15%,

Ti: from 0 to 0.050%,

Nb: from 0 to 0.050%,

B: from 0 to 0.0040%,

REM: from 0 to 0.030%,

Mg: from 0 to 0.0040%,

Ca: from 0 to 0.0040%,

Zr: from 0 to 0.030%,

W: from 0 to 0.10%,

Te: from 0 to 0.030%, and

Sb: from 0 to 0.030%,

with a balance being Fe and impurities,

wherein:
a total area ratio of a ferrite structure and a martensite structure in a center portion within 1.0 mm from a center in a cross section parallel to a longitudinal direction and passing through a central axis is 5% or less with the balance being a mixed structure composed of ferrite and cementite,

an average Vickers hardness h in a cross section perpendicular to the longitudinal direction is from 450 to 620,

a surface layer Vickers hardness hs at a position of a depth of from 0.2 to 0.5 mm from a surface and the average Vickers hardness h satisfy a relationship of Formula (4): $0 \leq hs - h < 30$ and

a center Vickers hardness hc within 1.0 mm from the center in the cross section perpendicular to the longitudinal direction and the average Vickers hardness h satisfy a relationship of Formula (5): $0 \leq h - hc < 30$
The steel wire according to claim 4, wherein the chemical composition contains, in % by mass, at least one or two or more selected from the group consisting of:
Cr: from 0.03 to 1.00%,

Cu: from 0.01 to 0.80%,

Sn: from 0.001 to 0.50%,

Ni: from 0.01 to 0.50%, and

V: from 0.01 to 0.15%.
The steel wire according to claim 4 or 5, wherein the chemical composition contains, in % by mass, one or two or more selected from the group consisting of:
Ti: from 0.002 to 0.050%,

Nb: from 0.002 to 0.050%,

B: from 0.0003 to 0.0040%,

REM: from 0.002 to 0.030%,

Mg: from 0.0002 to 0.0040%,

Ca: from 0.0002 to 0.0040%,

Zr: from 0.002 to 0.030%,

W: from 0.02 to 0.10%,

Te: from 0.001 to 0.030%, and

Sb: from 0.001 to 0.030%.
The steel wire according to any one of claims 4 to 6, which is plated on a surface thereof.
A rope formed by bundling a plurality of the steel wires according to any one of claims 4 to 7.
A method for producing a rope, comprising a step of bundling a plurality of the steel wires according to any one of claims 4 to 7 into a rope.