JP7623562B2 - Steel wire for wire drawing - Google Patents

Description

This disclosure relates to steel wire for wire drawing.

There is a strong demand for high strength wire ropes for power transmission cables, suspension bridge cables, etc., to meet the demands for weight reduction, shortening of construction period, etc. Along with the increase in strength of wire ropes, there is also an increasing demand for high strength steel wires used as the material for wire ropes.
Steel wires are generally manufactured by drawing steel wire rods. The steel wires thus obtained are optionally plated and then twisted or bundled in parallel to form a wire rope or the like made of multiple wires.
One of the challenges in industrially and stably producing wire ropes that are manufactured by twisting high-strength steel wires is to prevent cracks and wire breakage during twisting. A typical crack that occurs during twisting is along the longitudinal direction of the wire, and is called delamination or longitudinal crack.

Conventional techniques for preventing delamination (vertical cracks) during twisting include those described in Patent Documents 1 to 3, for example.

Patent Document 1 discloses a wire rod for wire drawing that is eutectoid or hypereutectoid steel, has a microstructure in which pearlite constitutes 80% or more, and has a maximum length of ferrite, which constitutes the second phase, of 10 μm or less, thereby having excellent twisting characteristics.

Patent Document 2 discloses a high carbon steel wire rod that contains C: 0.88% to 1.10%, and further contains one or both of B: 0.0050% or less and Nb: 0.020% or less, and has free N of less than 0.0005%, thereby providing excellent resistance to longitudinal cracking.

Patent Document 3 discloses a wire rod for steel cords that has a specified wire diameter and composition, in which the C content in the surface layer is 40% to 95% of the C content in the center, and the ratio of the thickness of the lamellar cementite at the center of the thickness of the surface layer to the thickness of the lamellar cementite at the center is 95% or less, and that can prevent cracks and the like caused by the delamination phenomenon.

JP 2002-146479 A JP 2005-163082 A International Publication No. 2015/163407

However, it is desirable for steel wire material for producing wire ropes and the like by twisting steel wires with high tensile strength to have superior twisting processability and to more stably suppress cracks and breakages during twisting at an industrial level.

The present disclosure has been made in consideration of the above circumstances, and aims to provide a steel wire material for wire drawing that can more stably produce high-strength steel wire suitable for use as a material for wire ropes and the like by suppressing the occurrence of cracks and breakage during twisting.

Means for solving the above problems include the following aspects.
<1> Chemical composition, in mass%,
C: 0.80-1.10%,
Si: 0.35-2.00%,
Mn: 0.30-1.20%,
Al: 0.005-0.050%,
P: 0.030% or less,
S: 0.015% or less,
N: 0.0100% or less; and O: 0.0020% or less;
The balance is Fe and impurities,
The metal structure has an area ratio of pearlite of 95% or more and an area ratio of pro-eutectoid cementite of 0.50% or less in a cross section perpendicular to the longitudinal direction of the steel wire rod,
A steel wire material for wire drawing that satisfies all of the following formulas (1) to (3), where A is an average Vickers hardness measured in a region within 3r/4 from the center of a cross section perpendicular to the longitudinal direction with a test force of 9.81 N, where r is a radius, and B is an average Vickers hardness measured at a position 40 μm from the outer periphery toward the center with a test force of 0.98 N, and σ is a standard deviation.
360≦A≦480...Formula (1)
30<A-B≦70...Formula (2)
0≦σ≦20...Formula (3)
<2> The chemical composition contains, in mass%, a part of the Fe replaced by
Cr: 0.40% or less,
Cu: 0.80% or less,
Ni: 0.50% or less,
Mo: 0.20% or less; and V: 0.15% or less;
The steel wire material for wire drawing according to claim 1, comprising at least one or more selected from the group consisting of:
<3> The chemical composition contains, in mass%, a part of the Fe replaced by
Ti: 0.050% or less,
Nb: 0.050% or less,
B: 0.0030% or less,
REM: 0.030% or less,
Mg: 0.030% or less,
Ca: 0.030% or less,
The steel wire material for wire drawing according to claim 1 or 2, comprising one or more selected from the group consisting of Zr: 0.020% or less, and W: 0.10% or less.

According to the present disclosure, a steel wire material for wire drawing is provided that can produce high-strength steel wire suitable for use as a material for wire ropes and the like more stably by suppressing the occurrence of cracks and breakage during the wire twisting process.

1 is a SEM image showing an example of a metal structure. FIG. 2 is a diagram illustrating a position for measuring Vickers hardness in an internal region of a C-section of a steel wire. FIG. 2 is a diagram illustrating a position for measuring Vickers hardness near the surface in a C-section of a steel wire. 1 is a diagram for explaining measurement positions of the metal structure on a C-section of a steel wire rod.

Hereinafter, a steel wire material for wire drawing according to one embodiment of the present disclosure (sometimes simply referred to as a "steel wire material" in the present disclosure) will be described in detail.
In the following description, "%" in the chemical composition means mass %.
Unless otherwise specified, a numerical range expressed using "to" means a range that includes the numerical values before and after "to" as the lower and upper limits. In addition, when the numerical values before and after "to" are followed by "more than" or "less than," the numerical range does not include these numerical values as the lower or upper limit.
In the numerical ranges described in stages in this specification, the upper or lower limit of a certain numerical range may be replaced by the upper or lower limit of another numerical range described in stages, or may be replaced by a value shown in the examples.
In addition, the term "process" includes not only an independent process but also a process that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved.

In order to solve the above problems, the present inventors have conducted extensive research and studies on the influence of hardness distribution in steel wire material on the occurrence of cracks and wire breakage during twisting of the steel wire, and have carefully analyzed and examined the results, resulting in the following findings (a) to (d).
(a) The majority of cracks originate from the surface during the stranding process of steel wires. Therefore, the properties near the surface have a large effect. By softening the surface area relatively more than the inside, the deformability near the surface is improved, and the occurrence of cracks and breakage during the stranding process is suppressed.
(b) In order to obtain better wire stranding processability, it is necessary that the variation in hardness in the vicinity of the surface of the steel wire is small.
(c) If the average hardness of the steel wire is too high, it becomes impossible to prevent cracks and breakage during the wire stranding process.
(d) If the amount of structures other than pearlite, particularly pro-eutectoid cementite, is large, cracks and breaks are likely to occur during the stranding process.
The present inventors conducted further detailed experiments and research based on the findings of (a) to (d). As a result, they found that the above-mentioned problems can be solved by adjusting the chemical composition, average hardness, hardness and variation in the vicinity of the surface, the difference between the average hardness and the hardness in the vicinity of the surface, and the area ratio of pearlite and the area ratio of pro-eutectoid cementite in a cross section (sometimes referred to as a "C cross section" in this disclosure) cut perpendicular to the longitudinal direction of the steel wire rod within appropriate ranges, and have made the present disclosure.

The steel wire material for wire drawing according to the present disclosure is a steel wire material for wire drawing that, by undergoing wire drawing, can obtain a steel wire suitable as a material for various wire ropes, such as cables for power transmission lines and cables for suspension bridges.
The tensile strength of the steel wire used as the material for the wire rope varies depending on the diameter of the steel wire, but for example, when the diameter is 1.6 to 2.0 mm, the tensile strength is preferably 2250 MPa or more, more preferably 2350 MPa or more, and even more preferably 2450 MPa or more. Also, when the diameter of the steel wire is 5.0 to 7.0 mm, the tensile strength is preferably 1850 MPa or more.
Hereinafter, the description of numerical values related to tensile strength will be given on the assumption that the diameter of the steel wire is 1.6 to 2.0 mm. However, the steel wire material according to the present disclosure is not limited to steel wire material for producing a steel wire having a diameter of 1.6 to 2.0 mm by wiredrawing, and the diameter of the steel wire material according to the present disclosure and the diameter of the steel wire after wiredrawing may be selected according to the application, etc.
First, the chemical composition of the steel wire material for wire drawing according to the present disclosure will be described in detail.

<Chemical composition>
C (carbon): 0.80-1.10%
C is an effective component for increasing the tensile strength of steel wire rods and steel wires obtained after wire drawing. However, if the C content is less than 0.80%, the tensile strength is insufficient. For this reason, it is difficult to stably impart a high tensile strength of, for example, 2250 MPa or more to the steel wire obtained by wire drawing the steel wire rod. In addition, in order to obtain a steel wire with a tensile strength of 2350 MPa or more, it is desirable to set the C content of the steel wire rod to 0.90% or more.
On the other hand, if the C content of the steel wire is too high, the steel wire becomes hard and the twisting processability decreases. If the C content of the steel wire exceeds 1.10%, it becomes industrially difficult to suppress the formation of pro-eutectoid cementite (cementite precipitated along the prior austenite grain boundaries), and the target torsion test result cannot be obtained even if other requirements are satisfied. Therefore, the C content of the steel wire is set to be within the range of 0.80 to 1.10%. In order to obtain higher tensile strength and torsion properties, the C content of the steel wire is preferably 0.85% or more and 1.05% or less, more preferably 0.90% or more and 1.00% or less.

Si (silicon): 0.35 to 2.00%
Silicon is an effective component for increasing the tensile strength of steel wire rods and steel wires obtained after wire drawing. However, if the Si content of a steel wire rod is less than 0.35%, it becomes difficult to stably impart a high tensile strength of, for example, 1850 MPa or more.
On the other hand, if the Si content of the steel wire rod exceeds 2.00%, the target torsion test result cannot be obtained even if other requirements are satisfied. Therefore, the Si content of the steel wire rod is set to the range of 0.35 to 2.00%. In order to obtain higher tensile strength and torsion properties, the Si content of the steel wire rod is preferably 0.50% to 1.60%, more preferably 0.70% to 1.40%.

Mn (manganese): 0.30 to 1.20%
Mn is an effective component for increasing the tensile strength of steel wire rods and steel wires obtained after wire drawing. Mn also fixes S in steel as MnS, preventing hot brittleness. However, if the Mn content of the steel wire rod is less than 0.30%, the effect of containing Mn cannot be sufficiently obtained.
On the other hand, if the Mn content of the steel wire exceeds 1.20%, the target torsion test result cannot be obtained even if other requirements are satisfied. Therefore, the Mn content of the steel wire is set to the range of 0.30 to 1.20%. In order to obtain higher tensile strength and torsion properties, the Mn content of the steel wire is preferably set to 0.60% or more and 1.00% or less.

Al (aluminum): 0.005% to 0.050%
Al is an element having a deoxidizing effect and is necessary for reducing the amount of oxygen in the steel wire rod. However, if the Al content of the steel wire rod is less than 0.005%, it is difficult to obtain the effect of containing Al.
On the other hand, Al is an element that easily forms hard oxide-based inclusions. If the Al content of the steel wire rod exceeds 0.050%, coarse oxide-based inclusions are significantly more likely to be formed, and wiredrawability is significantly reduced. Therefore, the Al content of the steel wire rod is set to 0.005 to 0.050%. The preferred lower limit of the Al content is 0.010%, and the more preferred lower limit is 0.020%. The preferred upper limit of the Al content is 0.040%, and the more preferred upper limit is 0.030%.

Furthermore, in the steel wire rod according to the present disclosure, in addition to the components described above, one or more of Cr, Cu, Ni, Mo, V, Ti, Nb, B, REM, Mg, Ca, Zr, and W may or may not be contained.
For example, the steel wire rod according to the present disclosure may contain at least one or more elements selected from the group consisting of Cr, Cu, Ni, Mo, V, Ti, Nb, and B in order to obtain the effect of increasing the tensile strength of the steel wire.
Furthermore, the steel wire rod according to the present disclosure may contain at least one or more elements selected from the group consisting of REM, Mg, Ca, Zr, and W in order to obtain the effect of improving the twisting properties of the steel wire.

Cr (chromium): 0.40% or less The inclusion of Cr is optional. Cr has the effect of increasing the tensile strength of the steel wire rod and the steel wire obtained after wire drawing. In order to stably obtain a steel wire with a tensile strength of 1950 MPa or more, it is preferable to make the Cr content 0.10% or more. However, if the Cr content of the steel wire rod exceeds 0.40%, the twisting processability decreases. Therefore, when Cr is intentionally contained in the steel wire rod, the Cr content is preferably within the range of 0.10 to 0.40%. A more preferable Cr content is 0.30% or less.

Cu (copper): 0.80% or less Cu content is optional. Cu has the effect of increasing the corrosion resistance of steel wire rods and steel wires obtained after wire drawing. In order to stably obtain this effect, a Cu content of 0.10% or more is necessary. However, if the Cu content of the steel wire rod exceeds 0.80%, the wire drawing workability decreases. Therefore, when Cu is intentionally contained in the steel wire rod, the Cu content is preferably within the range of 0.10 to 0.80%. A more preferable Cu content is 0.50% or less.

Ni (nickel): 0.50% or less The inclusion of Ni is optional. Ni has the effect of increasing the tensile strength of the steel wire rod and the steel wire obtained after wire drawing. In order to stably obtain a steel wire with a tensile strength of 1950 MPa or more, it is preferable to set the Ni content to 0.10% or more. However, even if the Ni content of the steel wire rod exceeds 0.50%, the effect is saturated. Therefore, when Ni is intentionally contained in the steel wire rod, the Ni content is preferably within the range of 0.10 to 0.50%. A more preferable Ni content is 0.40% or less.

Mo (molybdenum): 0.20% or less The inclusion of Mo is optional. Mo has the effect of increasing the tensile strength of the steel wire rod and the steel wire obtained after wire drawing. In order to stably obtain a steel wire with a tensile strength of 1950 MPa or more, it is preferable to make the Mo content 0.05% or more. However, if the Mo content of the steel wire rod exceeds 0.20%, the twisting processability decreases. Therefore, when Mo is intentionally contained in the steel wire rod, the Mo content is preferably within the range of 0.05 to 0.20%. A more preferable Mo content is 0.15% or less.

V (vanadium): 0.15% or less The inclusion of V is optional. V has the effect of increasing the tensile strength of the steel wire rod and the steel wire obtained after wire drawing. In order to stably obtain a steel wire with a tensile strength of 1950 MPa or more, it is preferable that the V content of the steel wire rod is 0.02% or more. However, if the V content of the steel wire rod exceeds 0.15%, the twisting processability decreases. Therefore, when V is intentionally contained in the steel wire rod, the V content of the steel wire rod is preferably 0.02 to 0.15%. A more preferable V content is 0.08% or less.

Ti (titanium): 0.050% or less The inclusion of Ti is optional. Ti has the effect of forming carbides or carbonitrides in the steel wire rod to improve the twisting workability. To obtain this effect, it is preferable that the Ti content of the steel wire rod is 0.002% or more. From the viewpoint of further improving the twisting characteristics, it is more preferable that the Ti content of the steel wire rod is 0.005% or more. However, if the Ti content of the steel wire rod exceeds 0.050%, coarse carbides or carbonitrides are easily formed, and the wire drawing workability is reduced. Therefore, when Ti is intentionally contained in the steel wire rod, the Ti content of the steel wire rod is preferably 0.002 to 0.050%. A more preferable Ti content is 0.005 to 0.030%.

Nb (niobium): 0.050% or less The inclusion of Nb is optional. Nb has the effect of forming carbides or carbonitrides in the steel wire rod to improve the twisted wire workability. To obtain this effect, it is preferable that the Nb content of the steel wire rod is 0.002% or more. From the viewpoint of further improving the twisted wire workability, it is more preferable that the Nb content of the steel wire rod is 0.005% or more. However, if the Nb content of the steel wire rod exceeds 0.050%, coarse carbides or carbonitrides are easily formed, and the wire drawing workability is reduced. Therefore, when Nb is actively contained in the steel wire rod, the Nb content of the steel wire rod is preferably 0.002 to 0.050%. A more preferable Nb content is 0.005 to 0.030% or less.

B (boron): 0.0030% or less The inclusion of B is optional. B has the effect of improving stranding workability. To obtain this effect, the B content of the steel wire is preferably 0.0003% or more. From the viewpoint of further improving stranding workability, the B content of the steel wire is more preferably 0.0007% or more. However, if the B content of the steel wire exceeds 0.0030%, coarse carbides are likely to be formed, and wire drawing workability is reduced. Therefore, when B is intentionally contained in the steel wire, the B content of the steel wire is preferably 0.0003 to 0.0030%. A more preferable B content is 0.0007 to 0.0020%.

REM (rare earth elements): 0.030% or less The inclusion of REM is optional. If REM is included, high twisting processability can be more stably exhibited. To obtain this effect, it is preferable that the REM content of the steel wire rod is 0.002% or more. However, if the REM content of the steel wire rod exceeds 0.030%, the effect becomes saturated. Therefore, when REM is intentionally included, the REM content of the steel wire rod is preferably 0.002 to 0.030%.
Here, REM refers to a total of 17 elements, namely Sc, Y, and lanthanoids, and the REM content refers to the content of one type of REM when there is one type of REM, and refers to the total content of the REM when there are two or more types of REM.

Mg (magnesium): 0.030% or less The inclusion of Mg is optional. If Mg is included, high twisting processability can be more stably exhibited. To obtain this effect, it is preferable that the Mg content of the steel wire rod is 0.002% or more. However, if the Mg content of the steel wire rod exceeds 0.030%, the effect becomes saturated. Therefore, when Mg is intentionally included in the steel wire rod, the Mg content of the steel wire rod is preferably 0.002 to 0.030%.

Ca (Calcium): 0.030% or less The inclusion of Ca is optional. If Ca is included, high twisting processability can be more stably exhibited. To obtain this effect, it is preferable that the Ca content of the steel wire rod is 0.002% or more. However, if the Ca content of the steel wire rod exceeds 0.030%, the effect is saturated. Therefore, when Ca is intentionally included in the steel wire rod, the Ca content of the steel wire rod is preferably 0.002 to 0.030%.

Zr (zirconium): 0.020% or less The inclusion of Zr is optional. If Zr is included, high twisting workability can be more stably exhibited. To obtain this effect, it is preferable that the Zr content of the steel wire rod is 0.002% or more. However, if the Zr content of the steel wire rod exceeds 0.020%, coarse carbides or carbonitrides are easily formed, and wiredrawing workability is reduced. Therefore, when Zr is intentionally included in the steel wire rod, the Zr content of the steel wire rod is preferably 0.002 to 0.020%.

W (tungsten): 0.10% or less The inclusion of W is optional. If W is included, high twisting processability can be more stably exhibited. To obtain this effect, it is preferable that the W content of the steel wire is 0.02% or more. However, if the W content of the steel wire exceeds 0.10%, the effect is saturated. Therefore, when W is intentionally included in the steel wire, the W content of the steel wire is preferably 0.02 to 0.10%.

The remainder other than the above elements is basically Fe and impurities. In the steel wire material according to the present disclosure, the content of P, S, N, and O contained as impurities is regulated as follows.

P (phosphorus): 0.030% or less P is an element that segregates at the grain boundaries of steel wire rods and reduces wire drawing workability. However, if the P content of the steel wire rod is 0.030% or less, the deterioration of twist workability is suppressed, and the target twist test results can be obtained by satisfying other requirements. Therefore, the P content of the steel wire rod is regulated to 0.030% or less. 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 is preferably 0% (i.e., not contained), but from the viewpoint of reducing the dephosphorization cost, it may be more than 0% or may be 0.001% or more.

S (sulfur): 0.015% or less S is an element that reduces twisting processability. However, if the S content of the steel wire is 0.015% or less, the target twist test result can be obtained while satisfying other requirements. For this reason, the S content of the steel wire is restricted to 0.015% or less. The preferred upper limit of the S content is 0.010%. The lower limit of the S content is not limited, but may be more than 0% or may be 0.001% or more from the viewpoint of reducing the desulfurization cost.

N (nitrogen): 0.0100% or less N is an element that reduces twisting processability. However, if the N content of the steel wire is 0.010% or less, the target twist test result can be obtained by satisfying other requirements. For this reason, the N content of the steel wire is restricted to 0.0100% or less. The preferred upper limit of the N content is 0.0070%. The lower limit of the N content is not limited, but may be more than 0% or may be 0.0001% or more from the viewpoint of reducing refining costs.

O (oxygen): 0.0020% or less O is an element that easily forms oxide-based inclusions in steel wire rod. However, if the O content of the steel wire rod is 0.0020% or less, the coarsening of the oxide-based inclusions is suppressed, and the deterioration of wiredrawability can be suppressed. Therefore, the O content of the steel wire rod is set to 0.0020% or less. A preferred upper limit of the O content is 0.0015%, and a more preferred upper limit is 0.0010%. The lower limit of the O content is not limited, but may be more than 0% or may be 0.0001% or more from the viewpoint of reducing refining costs.

The remainder of the steel wire material according to the present disclosure is Fe and impurity elements. Here, impurity elements are not elements that are intentionally added during the industrial production of steel, but rather elements that are mixed in from the raw materials, such as ore and scrap, or the production environment, and are acceptable within a range that does not adversely affect the effects and characteristics of the steel wire material according to the present disclosure.

Furthermore, the steel wire material according to the present disclosure may contain elements other than the above-mentioned elements and impurity elements, provided that the effect of the present disclosure is not hindered, that is, if a steel wire having high strength suitable as a material for wire ropes and the like can be produced more stably by suppressing the occurrence of cracks and breakage during the twisting process.

<Vickers hardness>
The hardness of the steel wire rod according to the present disclosure in a C-section (cross section perpendicular to the longitudinal direction) satisfies all of the following formulas (1) to (3), where A is the average Vickers hardness measured with a test force of 9.81 N in a region within 3r/4 from the center (hereinafter may be referred to as the "internal region") where r is the radius, B is the average Vickers hardness measured with a test force of 0.98 N at a position 40 μm from the outer periphery toward the center (hereinafter may be referred to as the "surface vicinity"), and σ is the standard deviation.

(1) 360≦A≦480
When the Vickers hardness is measured at the inner region of the steel wire rod under a load of 9.81 N, if the average Vickers hardness A falls below 360, the tensile strength required for the final product (e.g., 2250 MPa or more) cannot be stably imparted. It is difficult to achieve this.
On the other hand, if the hardness of the steel wire is too high, microcracks are likely to occur during the wire drawing process, and when the Vickers hardness is measured at the inner region of the steel wire under a load of 9.81 N, the average Vickers hardness is If the value A exceeds 500, the target torsion test result cannot be obtained even if other requirements are met. Therefore, the average Vickers hardness A of the inner region is set to be within the range of 360 to 480, Preferably, it is 380 to 460.

(2) 30<A-B≦70
When the hardness of the steel wire near the surface is higher than the average hardness of the steel wire, the wire stranding processability of the steel wire is reduced. If the difference between the hardness of the inner region of the steel wire and the hardness near the surface, i.e., "A-B", is more than 30 and 70 or less, and other requirements are satisfied. If the above condition is satisfied, the torsion test result of the steel wire targeted in the present disclosure can be obtained. Preferably, "A-B" is 35 or more and 60 or less.

(3) 0≦σ≦20
When the variation in Vickers hardness increases near the surface of the steel wire, the stranding processability of the steel wire decreases. When the standard deviation σ of the Vickers hardness near the surface satisfies 0 to 20 and other requirements are also satisfied, the torsion test result of the steel wire targeted in this disclosure can be obtained. The standard deviation σ of the Vickers hardness near the surface is preferably 0 to 15, and more preferably 0 to 12.

The Vickers hardness of the inner region and the surface vicinity of the steel wire is measured by taking samples from four locations at 500 mm intervals in the longitudinal direction of the steel wire and measuring the C-section of each sample (the cut surface at one end of each sample). The details of the measurement method will be described in the examples.
For example, even if some samples (C cross sections) taken from four locations on a single steel wire rod do not satisfy the relationship of formulas (1) to (3), the steel wire rod is included in the steel wire rod according to the present disclosure as long as the average value and standard deviation of the Vickers hardness calculated by measuring the four C cross sections as described above satisfy the relationship of formulas (1) to (3).

<Microstructure of steel wire (metal structure)>
Next, the microstructure (metal structure) and characteristics of the steel wire rod according to the present disclosure will be described.

Area ratio of pearlite: 95% or more Area ratio of pro-eutectoid cementite: 0.50% or less In the cross section perpendicular to the longitudinal direction of the steel wire rod, the metal structure of the steel wire rod according to the present disclosure must satisfy an area ratio of pearlite of 95% or more and an area ratio of pro-eutectoid cementite of 0.50% or less. A steel wire rod having such a metal structure has a large work hardening ability, and can be strengthened by a small amount of wire drawing, and can also obtain stable and good twisting workability even if the tensile strength is 2250 MPa or more. From the viewpoint of increasing the strength by wire drawing, the area ratio of pearlite of the steel wire rod according to the present disclosure is preferably 97% or more, and may be 100%.
The metal structure of the steel wire rod according to the present disclosure may contain, as a remaining structure other than pearlite, one or more selected from pro-eutectoid cementite (area fraction 0.50% or less), grain boundary ferrite, bainite, and martensite, but preferably does not contain a martensite structure.

The metal structure is measured by image analysis of a photograph of a C-section of the steel wire rod taken with a field emission scanning electron microscope (FE-SEM, for example, JSM-7100F manufactured by JEOL Ltd.). Details of the measurement method will be described in the examples.
Pearlite is a structure in which ferrite phase (α) and cementite phase (Fe ₃ C) are alternately and continuously laminated in layers. Bainite is a structure that contains ferrite phase (α) and cementite phase (Fe ₃ C) like pearlite, but since bainite is not a structure in which ferrite phase (α) and cementite phase (Fe ₃ C) are alternately laminated in layers, pearlite can be distinguished from these structures by SEM photographs.
Proeutectoid cementite is cementite precipitated at the grain boundaries of prior austenite, and can be distinguished from other structures by SEM photographs. Figure 1 shows an example of an FE-SEM photograph of a field including grain boundary ferrite, martensite, proeutectoid cementite, and bainite, which are structures other than pearlite.

<Method of manufacturing steel wire>
Next, an example of a method for producing a steel wire rod for wire drawing according to the present disclosure will be described. Note that the method for producing a steel wire rod according to the present disclosure is not limited to the method described below.

When manufacturing the steel wire rod according to the present disclosure, the conditions in each manufacturing process are set according to the chemical composition, target performance, wire diameter, etc. so that the chemical composition and hardness conditions can be reliably satisfied.
As an example of the manufacturing method of the steel wire rod according to the present disclosure, a case will be described in which a steel containing C: 0.80 to 1.10%, Si: 0.35 to 2.00%, Mn: 0.30 to 1.20%, Al: 0.005% to 0.050%, and the balance being Fe and impurities, containing as impurities P: 0.030% or less, S: 0.015% or less, N: 0.0100% or less, and O: 0.0020% or less is used.

For example, when performing continuous casting, a method of casting steel involves melting the steel in a converter, thoroughly electromagnetically stirring the molten steel, and then reducing the pressure during solidification to obtain a cast piece.

Next, the cast slab is heated to 1250-1280°C for 6-10 hours and cooled to below 500°C outside the furnace. The slab is then heated to 1200-1250°C and then rolled into a slab to obtain a steel slab.

The surface of the steel slab produced by the above method is ground by 0.2 to 0.5 mm, then heated to 1050 to 1100°C, and hot-rolled at a finishing temperature of 900 to 950°C. The steel wire rod after the finish rolling is cooled to a range of 750 to 790°C at an average cooling rate of 30°C/sec or more by a combination of water cooling and air cooling, and then immersed in a molten salt (salt bath) consisting of two tanks whose temperature can be adjusted within 5 seconds. At this time, the first tank is set to 530 to 550°C, and the second tank is set to 550 to 570°C.
In this disclosure, the temperature of the steel billet in the heating furnace used for hot rolling refers to the surface temperature of the steel billet. In addition, the finish rolling temperature in this disclosure refers to the surface temperature of the steel wire rod immediately after finish rolling. The average cooling rate after finish rolling refers to the surface cooling rate of the steel wire rod after finish rolling. The surface temperatures of the steel billet and the steel wire rod can be measured by a radiation thermometer.

The steel wire material according to the present disclosure, which can be manufactured through the above process, can be drawn until the tensile strength of the steel wire is, for example, 2250 MPa or more, and then twisted, preventing longitudinal cracks and breakage, and enabling stable manufacture of wire ropes and the like.

Next, an example of the present disclosure will be described. The conditions of the example are an example of conditions adopted to confirm the feasibility and effect of manufacturing the steel wire material according to the present disclosure. The steel wire material for wire drawing according to the present disclosure is not limited to this example of conditions. Various conditions may be adopted in the present disclosure as long as they do not deviate from the gist of the present disclosure and the purpose of the present disclosure is achieved.

<Production of Steel Wire Rod>
Steels A1 to F2 having the chemical compositions shown in Table 1 were melted in a converter and then continuously cast. The casting conditions are also shown in Table 1. The chemical composition of each steel was measured according to the method described below.
C JIS G 1211:2011
Si, Mn, P, Cr, Cu, Ni, Mo, V, Ti, Nb, Al, B, REM, Ca, Mg, Zr, W JIS G 1253:2002
S JIS G 1215:2010
N JIS G 1228:1997
O JIS G 1239:2014

Then, after heating under the conditions shown in Table 1, the steel was rolled into 122 mm square pieces, which were then hot rolled under the conditions shown in Table 2 to produce steel wire rod.

For the steel wire rods manufactured under each of the conditions in Tables 1 and 2, the average Vickers hardness, standard deviation of Vickers hardness, area ratio of pearlite, and area ratio of pro-eutectoid cementite were determined by the methods described below and are shown in Table 3. The specific measurement methods were as follows.

<Measurement of Vickers hardness in the internal region>
Samples are taken from four locations at 500 mm intervals in the longitudinal direction of the steel wire, and the C-section (the cut surface at one end of each sample) is mirror-polished, and then the micro Vickers hardness is measured under a load of 9.81 N. As shown in FIG. 2, 21 locations are measured per cross section at intervals of "r/3" (r: radius of the steel wire). The diamond marks (◇) in FIG. 2 indicate the measurement locations. The average value of the Vickers hardness measured at 84 locations (4 cross sections x 21 locations) in this manner is defined as the average Vickers hardness value A. The Vickers hardness measurement is in accordance with the method described in JIS Z 2244 (2009).

<Measurement of Vickers hardness near the surface>
Samples are taken from four locations at 500 mm intervals in the longitudinal direction of the steel wire rod, and the C-sections (cut surfaces at one end of each sample) are mirror-polished, and then the micro Vickers hardness is measured at positions 40 μm from the outer periphery toward the center at 45° intervals in the circumferential direction with a load of 0.98 N as shown in Figure 3. Since eight locations are measured per cross section, the average of the Vickers hardness values at a total of 32 measurement locations (4 cross sections x 8 locations) is taken as the average Vickers hardness value B near the surface. The Vickers hardness measurement method complies with the method described in JIS Z 2244 (2009).

The standard deviation σ of the Vickers hardness near the surface is calculated from the Vickers hardness at these 32 locations by a method that calculates the population standard deviation.

<Metal structure measurement>
A cross section (C section) cut perpendicular to the longitudinal direction of the steel wire rod is mirror-polished, and then immersed in a mixed solution of picric acid and ethanol (4 g of picric acid per 100 ml of alcohol) for 20 to 30 seconds for corrosion. Next, the structure of this corroded surface is photographed at a magnification of 5000 times using a field emission scanning electron microscope (FE-SEM: Field Emission-Scanning Electron Microscope, JSM-7100F manufactured by JEOL Ltd.).
FIG. 4 is a diagram for explaining the measurement positions of the metal structure in the C-section of the steel wire rod. The photographing locations are one at the center A, four at a circle with a distance of r/2 from the surface (arbitrary positions at approximately 90° intervals on the circle C1 shown in FIG. 4) with the radius of the steel wire rod 10 being r, and five at a circle with a distance of r/4 from the surface (arbitrary positions at approximately 70° intervals on the circle C2 shown in FIG. 4). The area per field of view is 3.6×10 ⁻⁴ mm ² (18 μm long, 20 μm wide). Next, the photograph is used to determine the area ratio (%) of parts other than pearlite by image analysis (LUZEX manufactured by Nireco Corporation). The value obtained by subtracting the area ratio of parts other than pearlite from 100 is taken as the area ratio of pearlite. The area ratio of pro-eutectoid cementite was also determined by the same image analysis.
The area ratio of pearlite was calculated for each of the 10 locations in total, and the arithmetic average was taken as the area ratio of pearlite. The area ratio of pro-eutectoid cementite was also calculated by the arithmetic average in the same manner.

<Steel wire manufacturing>
Next, a zinc phosphate coating was formed on the surface of each steel wire. After that, each steel wire coated with the zinc phosphate coating was drawn to the diameter shown in Table 3 according to a pass schedule in which the area reduction rate in each die was 16% on average, to obtain steel wires with test numbers 1 to 50.

<Measurement of Tensile Strength>
The samples used for the tensile test of the steel wire obtained by wire drawing are three pieces of 240 mm length taken every 1 m from about 1/3 of the length from the end of the steel wire. The three pieces are used to perform a tensile test at a crosshead speed of 10 mm/min and a jig distance of 100 mm, and the tensile strength of each sample is measured. The average value of the tensile strengths of the three pieces is taken as the tensile strength of the steel wire.
When the steel wire had a diameter of 1.6 to 2.0 mm, the tensile strength was judged to be good if it was 2250 MPa or more, and when the steel wire had a diameter of 5.0 to 7.0 mm, the tensile strength was judged to be good if it was 1850 MPa or more.

<Torsion test>
The samples used for the tensile test of the steel wire obtained by wire drawing are taken at intervals of 1 m from a point about 1/3 of the length from the end of the steel wire, with a length of diameter x 50 + 100 mm. The five samples were used to carry out a twist test five times each under the following conditions, and if no delamination occurred in any of the five samples, the twisting processability was judged to be good.
Distance between chucks: Wire diameter x 50
Rotation speed: 2 rpm
Axial load: Breaking load in tensile test x 0.01
Judgment method: The twisting torque-twisting angle curve was measured, and if the torque suddenly decreased before the twisting angle reached half the angle at which the specimen broke, it was judged that delamination had occurred.

From Table 3, it is clear that in test numbers outside the conditions specified in this disclosure, at least one of the above-mentioned properties does not reach the target value.
In contrast, it is clear that the test numbers which satisfy all the conditions stipulated in this disclosure have all of the above-mentioned characteristics reaching the target values.

The above describes preferred embodiments and examples of the present disclosure, but these embodiments and examples are merely examples within the scope of the gist of the present disclosure, and additions, omissions, substitutions, and other modifications of the configuration are possible without departing from the scope of the gist of the present disclosure. In other words, the present disclosure is not limited by the above description, but is limited only by the claims, and can of course be modified as appropriate within that scope.

10 Steel wire A Center of steel wire in cross section C r Radius of steel wire C1 Circle at a distance of r/2 from the surface of the steel wire in cross section C2 Circle at a distance of r/4 from the surface of the steel wire in cross section C

Claims (3)

The chemical composition, in mass%, is
C: 0.80-1.10%,
Si: 0.35-2.00%,
Mn: 0.30-1.00%,
Al: 0.008-0.040%,
P: 0.0 25 % or less,
S: 0.015% or less,
N: 0.0100% or less; and O: 0.0020% or less;
The balance is Fe and impurities,
The metal structure has an area ratio of pearlite of 95% or more and an area ratio of pro-eutectoid cementite of 0.36% or less in a cross section perpendicular to the longitudinal direction of the steel wire rod,
A steel wire material for wire drawing that satisfies all of the following formulas (1) to (3), where A is an average Vickers hardness measured in a region within 3r/4 from the center of a cross section perpendicular to the longitudinal direction with a test force of 9.81 N, where r is a radius, and B is an average Vickers hardness measured at a position 40 μm from the outer periphery toward the center with a test force of 0.98 N, and σ is a standard deviation.
360≦A≦480...Formula (1)
30<A-B≦70...Formula (2)
0≦σ≦20...Formula (3) The chemical composition is, in mass %, replacing a part of the Fe,
Cr: 0.40% or less,
Cu: 0.80% or less,
Ni: 0.50% or less,
Mo: 0.20% or less; and V: 0.15% or less;
The steel wire material for wire drawing according to claim 1, comprising at least one or more selected from the group consisting of: The chemical composition is, in mass %, replacing a part of the Fe,
Ti: 0.050% or less,
Nb: 0.050% or less,
B: 0.0030% or less,
REM: 0.030% or less,
Mg: 0.030% or less,
Ca: 0.030% or less,
The steel wire material for wire drawing according to claim 1 or 2, comprising one or more selected from the group consisting of Zr: 0.020% or less, and W: 0.10% or less.