JP7440758B2 - wire rod and steel wire - Google Patents

Description

The present disclosure relates to wire rods and steel wires.

High carbon steel wires used for various purposes such as steel wires for bridge cables, PC (Prestressed Concrete) steel wires, ACSR (Aluminum Conductor Steel-Reinforced) wires, and various ropes have high strength (for example, tensile strength of 2000 MPa or more). is required.
In wire rods used for steel wires that require such high strength, for example, the C content is increased and the Si content is increased by 1. It may be added at a high concentration of about 0% by mass.

Such a high-Si, high-carbon wire rod has an increased coarse ferrite structure compared to a ferrite phase forming a pearlite structure, which may promote strength variations in the cross section and in the longitudinal direction of the wire rod. This strength variation tends to cause vertical cracking (delamination) during subsequent wire drawing and twisting of the steel wire, and an increase in the soft ferrite structure reduces fatigue strength.

As a technique for reducing strength variations in a wire rod, for example, Patent Document 1 discloses a method in which the structure is made uniform by controlling the structure after hot rolling, and specifically, the average value Pave of the particle size number of pearlite nodules and its standard deviation Pσ are set to a predetermined value. It is proposed to control the temperature within the range of .
In addition, in Patent Document 2, for the purpose of improving wire drawability, fatigue resistance, and hydrogen embrittlement resistance, the surface layer contains pearlite of 90 area % or less and bainite and ferrite of 0 to 10 area % in total. A wire rod in which the size of Ti/N-based inclusions is 50 μm or less is disclosed.

Patent No. 6180351 Patent No. 6330920

In the case of high-Si and high-C steel wire rods, many ferrite structures tend to remain, and when the strength is increased, it is difficult to suppress the occurrence of delamination and to suppress the deterioration of fatigue properties.

The present disclosure has been made under these circumstances, and its purpose is to provide a wire rod that can obtain a steel wire with excellent tensile strength, fatigue properties, and twisting properties through wire drawing, and An object of the present invention is to provide a steel wire having excellent twisting properties.

Means for solving the above problems include the following aspects.
<1> In mass%,
C: 0.80% or more and 1.10% or less,
Si: 0.70% or more and 2.00% or less,
Mn: 0.10% or more and 1.00% or less,
P: 0.030% or less,
S: 0.030% or less,
N: 0.0060% or less,
O: 0.0060% or less,
B: 0.0004% or more and 0.0040% or less,
Contains Al: 0.005% or more and 0.070% or less, and Ti: 0.004% or more and 0.025% or less, with the remainder consisting of Fe and impurities,
The content of Si and the content of B satisfy the following formula (1), the content of Ti and the content of N satisfy the following formula (2),
The metal structure includes a pearlite structure and a ferrite structure, and when a cross section of the wire is observed, the area ratio of the pearlite structure is 90% or more in a region where the depth from the outer peripheral surface of the wire is 50 μm to 100 μm. and the area ratio of the ferrite structure is 2.0% or less, and the average value of the individual areas of the ferrite structure is 2.5 μm ² or less.
Formula (1): 0.0025×Si−0.0015<B<0.0025×Si
Formula (2): 2.0<Ti/N<5.5
In the formula (1) and the formula (2), the content (mass %) of each element in the wire is substituted for each element symbol.
<2> Contains Cr: 0.60% or less in mass % in place of a part of the Fe, and the Cr content and the B content satisfy the following formula (3) <1 ＞The wire rod described in ＞.
Formula (3): 0.0048×Cr≦B≦0.013×Cr
In the formula (3), each element symbol is substituted with the content (mass%) of each element in the wire.
<3> In place of a part of the Fe, in mass%,
The wire rod according to <1> or <2>, containing any one or both of Nb: 0.025% or less and V: 0.15% or less.
<4> The wire rod according to any one of <1> to <3>, which contains Mo: 0.20% or less in mass % in place of a part of the Fe.
<5> In place of a part of the Fe, in mass%,
Cu: 0.50% or less,
The wire rod according to any one of <1> to <4>, containing one or more selected from the group consisting of Ni: 0.50% or less and Sn: 0.50% or less.
<6> In place of a part of the Fe, in mass%,
The wire rod according to any one of <1> to <5>, containing one or both of Ca: 0.0040% or less and Mg: 0.0040% or less.
<7> The wire rod according to any one of <1> to <6>, which contains Sb: 0.15% or less in mass % in place of a part of the Fe.
<8> In mass%,
C: 0.80% or more and 1.10% or less,
Si: 0.70% or more and 2.00% or less,
Mn: 0.10% or more and 1.00% or less,
P: 0.030% or less,
S: 0.030% or less,
N: 0.0060% or less,
O: 0.0060% or less,
B: 0.0004% or more and 0.0040% or less,
Contains Al: 0.005% or more and 0.070% or less, and Ti: 0.004% or more and 0.025% or less, with the remainder consisting of Fe and impurities,
The content of Si and the content of B satisfy the following formula (1), the content of Ti and the content of N satisfy the following formula (2),
The metal structure includes cementite and ferrite structure, and when a cross section of the steel wire is observed, the area ratio of the ferrite structure is 2.5 μm in a region where the depth from the outer peripheral surface of the steel wire is 50 μm to 100 μm. 0% or less, and the average value of the individual areas of the ferrite structure is 2.0 μm ^or less,
When a longitudinal section of the steel wire is observed, in a region where the depth from the outer peripheral surface of the steel wire is from 50 μm to 100 μm, the total number of cementites is A steel wire in which the proportion of cementite whose angle with the central axis direction is 30° or less is 60% or more.
Formula (1): 0.0025×Si−0.0015<B<0.0025×Si
Formula (2): 2.0<Ti/N<5.5
In the formula (1) and the formula (2), the content (mass%) of each element in the steel wire is substituted for each element symbol.

According to the present disclosure, there are provided a wire rod from which a steel wire with excellent tensile strength, fatigue properties, and twisting properties can be obtained by wire drawing, and a steel wire with excellent tensile strength, fatigue properties, and twisting properties.

It is a SEM image showing an example of a metal structure including a pearlite structure and a ferrite structure. It is a SEM image showing an example of a bainite structure. FIG. 2 is a schematic diagram illustrating the angle between the long axis direction and the central axis direction of a steel wire for an example of cementite. (A) Field containing bainite, (B) Field containing martensite, (C) Field containing ferrite, (D) Containing pearlite and pearlite with collapsed lamellae (pearlite composed of cementite and ferrite that is not plate-shaped). This is an example of each SEM photograph of the field of view.

In this specification, a numerical range expressed using "~" means a range that includes the numerical values written before and after "~" as lower and upper limits.
In this specification, "%" indicating the content of a component (element) is based on mass.
In the numerical ranges described stepwise in this specification, the upper limit or lower limit of one stepwise numerical range may be replaced with the upper limit or lower limit of another stepwise numerical range. , may also be replaced with the values shown in the examples.

Hereinafter, the wire rod and steel wire according to the present disclosure will be explained in detail. In addition, "cross section" means a cross section perpendicular to the longitudinal direction (direction of the central axis) of a wire rod or steel wire, and "longitudinal cross section" means a cross section parallel to the longitudinal direction (direction of the central axis) of a wire rod or steel wire. , and means a cross section that includes the central axis.

In high C and high Si steel types, the nose of pearlite transformation in the isothermal transformation diagram (TTT diagram) is on the high temperature side, so non-pearlite structures such as ferrite structures tend to increase. The present inventors thought that by lowering the ratio of these non-pearlite structures, twisting characteristics could be improved.

In order to reduce the ferrite structure, it is effective to segregate B at the austenite grain boundaries. However, although B that segregates at grain boundaries has the effect of suppressing the formation of a ferrite structure, even if a large amount of B is added, it precipitates as a B compound, so the effect of B is not clear.
After repeated studies by the present inventors, we found that there is a correlation between the amount of Si and the amount of B. When the amount of Si increases, the nose becomes hotter and a ferrite structure tends to form. However, increasing the amount of B It was found that the formation of ferrite structure can be suppressed. That is, by adjusting the balance between the amount of Si and the amount of B, it is possible to effectively suppress the generation of ferrite structure even in high C and high Si steel types. It was also found that the ratio of the Ti amount to the N amount greatly affects the reduction of the ferrite structure.
Furthermore, the steel wire with less ferrite structure has excellent twisting properties, and by reducing the ferrite structure that tends to cause fracture, the fatigue properties are also improved, and the balance between strength, twisting properties, and fatigue properties is improved.
The wire rod and steel wire according to the present disclosure were derived based on the above findings.

[wire]
The wire according to the present disclosure is expressed in mass%,
C: 0.80% or more and 1.10% or less,
Si: 0.70% or more and 2.00% or less,
Mn: 0.10% or more and 1.00% or less,
P: 0.030% or less,
S: 0.030% or less,
N: 0.0060% or less,
O: 0.0060% or less,
B: 0.0004% or more and 0.0040% or less,
Contains Al: 0.005% or more and 0.070% or less, and Ti: 0.004% or more and 0.025% or less, with the remainder consisting of Fe and impurities,
The content of Si and the content of B satisfy the following formula (1), the content of Ti and the content of N satisfy the following formula (2),
The metal structure includes a pearlite structure and a ferrite structure, and when a cross section of the wire is observed, the area ratio of the pearlite structure is 90% or more in a region where the depth from the outer peripheral surface of the wire is 50 μm to 100 μm. The area ratio of the ferrite structure is 2.0% or less, and the average value of the individual areas of the ferrite structure is 2.5 μm ² or less.
Formula (1): 0.0025×Si−0.0015<B<0.0025×Si
Formula (2): 2.0<Ti/N<5.5
In the formula (1) and the formula (2), the content (mass %) of each element in the wire is substituted for each element symbol.

<Steel composition of wire rod and steel wire>
The steel components (chemical compositions) of the wire rod and steel wire according to the present disclosure will be explained. Note that since the wire rod and the steel wire according to the present disclosure have the same steel components, the steel components of the wire rod and the steel wire will be described together. Hereinafter, when it is written as "steel material", it means both wire rod and steel wire.

C: 0.80% or more and 1.10% or less C contributes to the strength of the steel material. If the C content is less than 0.80%, the tensile strength of the steel material may be insufficient. Therefore, the C content is set to 0.80% or more, preferably 0.84% or more.
On the other hand, when the C content exceeds 1.10%, the cementite fraction increases and contributes to high strength, but on the other hand, the reduction of area (ductility) of the wire decreases, and the fatigue strength of the steel wire after wire drawing decreases. Torsion characteristics deteriorate. Therefore, the C content is 1.10% or less, preferably 1.05% or less, and more preferably 1.00% or less.

Si: 0.70% or more and 2.00% or less Si is a deoxidizing agent, and also contributes to improving the strength of steel materials through solid solution strengthening. Furthermore, when it is heated in a subsequent process, it contributes to suppressing a decrease in strength. In order to obtain these effects, the Si content is set to 0.70% or more, may be 0.80% or more, or may be more than 0.90%.
On the other hand, if the Si content is too high, the drawing area (ductility) of the wire will decrease. Therefore, the Si content is 2.00% or less, may be 1.85% or less, or may be 1.70% or less.
Furthermore, Si reduces the pearlite transformation rate near 550° C. and increases the ferrite structure. Therefore, in steel components with a high Si content, the ferrite structure is suppressed by including B. The wire rod and steel wire according to the present disclosure need to have a suitable B content depending on the Si content and satisfy the following formula (1).
Formula (1): 0.0025×Si−0.0015<B<0.0025×Si
In formula (1), each element symbol (Si and B) is substituted with the content (mass%) of each element (Si and B) in the steel material. Note that the B content will be described later.
The relationship between the Si content and the B content may be expressed by the following formula (1A) or may be expressed by the formula (1B).
Formula (1A): 0.0025×Si-0.0014<B<0.0025×Si-0.0001
Formula (1B): 0.0025×Si-0.0013<B<0.0025×Si-0.0003

Mn: 0.10% or more and 1.00% or less Mn is an element that, in addition to increasing the strength of steel, has the effect of fixing S in steel as MnS and preventing hot embrittlement of steel materials. However, if the Mn content is less than 0.10%, the above effect is insufficient, so the Mn content is set to 0.10% or more, and may be 0.15% or more, or 0.20% or more.
On the other hand, Mn is an element that tends to segregate. When Mn is contained in an amount exceeding 1.00%, Mn becomes concentrated particularly in the center, martensite and bainite are generated in the center, and the drawing of the wire rod decreases. Therefore, the Mn content is 1.00% or less, may be 0.95% or less, or may be 0.85% or less.

P: 0.030% or less P is an element that segregates at the grain boundaries of steel materials and deteriorates the torsional properties of the steel materials. When the P content exceeds 0.030%, the torsional properties deteriorate significantly. Therefore, the P content of the steel material is limited to 0.030% or less, with a preferable upper limit of 0.025%.
The lower the P content is, the more preferable it is, but from the viewpoint of reducing manufacturing costs (dephosphorization costs), the P content may be more than 0%, may be 0.0005% or more, and may be 0.001%. % or more.

S: 0.030% or less S is an element that forms MnS and reduces the drawing area of the wire. When the S content exceeds 0.030%, the reduction in area of the wire becomes significant. From this, the S content of the steel material is limited to 0.030% or less, with a preferable upper limit of 0.015%.
The lower the S content, the more preferable it is, but from the viewpoint of reducing manufacturing costs (desulfurization costs), the S content may be more than 0%, may be 0.002% or more, and may be 0.005%. It may be more than that.

N: 0.0060% or less N is an element that deteriorates twisting properties, and if the N content is high, the amount of B nitrides formed also increases, and the effects of B, which will be described later, are not exhibited. Therefore, the N content is limited to 0.0060% or less, with a preferable upper limit of 0.0050% or 0.0040%.
On the other hand, N increases the strength of the steel wire by fixing to dislocations during cold wire drawing. The lower limit of the N content may be more than 0%, and in order to obtain the above effects, the lower limit of the N content may be 0.0015%, 0.0020%, or 0.0025%.

O: 0.0060% or less O forms oxides in steel, acts as inclusions, and reduces fatigue strength. Therefore, the O content is limited to 0.0060% or less, with a preferable upper limit of 0.0050% or 0.0040%.
The lower limit of the O content may be 0.0015%, 0.0020%, or 0.0025% from the viewpoint of reducing manufacturing costs (deoxidization costs).

B: 0.0004% or more and 0.0040% or less B contributes to reducing the ferrite structure in the steel material. When the B content is less than 0.0040%, the area ratio of the ferrite structure becomes large. Therefore, the B content is set to 0.0004% or more, and may be 0.0005% or more, or 0.0007% or more.
On the other hand, if the B content exceeds 0.0040%, a large amount of B compounds will precipitate, and the effect of reducing the ferrite structure will not be exhibited. Therefore, the B content is set to 0.0040% or less, may be 0.0030% or less, or may be 0.0025% or less.

Al: 0.005% or more and 0.070% or less Al is an element that has a deoxidizing effect, and also forms nitrides in the wire to refine the austenite grain size and improve the twisting characteristics of the steel material. Contribute to In order to obtain these effects, the Al content is set to 0.005% or more, and may be 0.010% or more, or 0.015% or more.
On the other hand, if the Al content is too high, hard oxide-based inclusions are likely to be formed, resulting in poor wire drawability and twisting properties. Therefore, the Al content may be 0.070% or less, may be 0.050% or less, or may be 0.035% or less.

Ti: 0.004% or more and 0.025% or less Ti fixes N by forming a nitride (TiN), which makes it possible to suppress the precipitation of B nitrides and enhance the effect of B. Therefore, the Ti content may be 0.004% or more, may be 0.006% or more, or may be 0.008% or more.
On the other hand, if the Ti content exceeds 0.025%, coarse nitrides will be dispersed and the twisting properties will deteriorate. Therefore, the Ti content is 0.025% or less, may be 0.023% or less, or may be 0.020% or less.

Further, Ti needs to satisfy the following formula (2).
Formula (2): 2.0<Ti/N<5.5
In formula (2), each element symbol (Ti, N) is substituted with the content (mass %) of each element (Ti, N) in the wire.
When Ti/N is less than 2.0, the N content is too large relative to the Ti content, and the B compound precipitates, resulting in no effect of reducing the ferrite structure. On the other hand, when Ti/N is more than 5.5, the N content is insufficient relative to the Ti content, and Ti carbide is formed, resulting in a decrease in twisting properties.
The relationship between the Ti content and the N content may be the relationship expressed by the following formula (2A) or the relationship expressed by the formula (2B).
Formula (2A): 2.1<Ti/N<5.3
Formula (2B): 2.2<Ti/N<5.2

The wire rod and steel wire according to the present disclosure may contain an arbitrary element instead of a part of Fe. The arbitrary elements will be explained below. Note that the lower limit of the content of arbitrary elements described below may be 0% or more than 0%.

Cr: 0.60% or less Cr contributes to the strength of steel materials. Therefore, the Cr content may be greater than 0%, or may be greater than 0.05%.
On the other hand, if the Cr content is too high, the twisting properties will deteriorate. Therefore, the Cr content may be 0.60% or less, or may be 0.50% or less.

It is preferable that Cr is contained at 0.60% or less, and the Cr content and B content satisfy the following formula (3).
Formula (3): 0.0048×Cr≦B≦0.013×Cr
In formula (3), each element symbol (Cr, B) is substituted with the content (mass%) of each element in the wire.
Since Cr forms a solid solution in the B compound (M ₂₃ C ₆ type), it delays the precipitation of the B compound (B carbide). When the Cr content and the B content satisfy the above formula (3), precipitation of B carbides is prevented, and the solid solution B can further reduce the ferrite structure fraction.

Nb: 0.025% or less V: 0.15% or less Nb and V contribute to refinement of the austenite grain size due to the pinning effect, and improve the twisting characteristics of the steel material. In order to obtain this effect, any one or both of Nb and V may be contained in an amount exceeding 0%.
In order to improve the twisting properties of the steel material, the Nb content may be 0.002% or more, or 0.005% or more.
On the other hand, if the Nb content is too high, a large amount of carbides or carbonitrides will be present, which will make the austenite grain size too fine, resulting in poor hardenability and reduced tensile strength. Therefore, the Nb content is 0.025% or less, and may be 0.020% or less.

In order to improve the twisting properties of the steel material, the V content may be 0.008% or more, or 0.010% or more.
On the other hand, if the V content is too high, a large amount of carbides or carbonitrides will be present, which will make the austenite grain size too fine, resulting in poor hardenability and reduced tensile strength. Therefore, the V content is 0.15% or less, and may be 0.08% or less.

In order to improve the twisting characteristics of the steel material, it is preferable to contain one or two of Nb: 0.025% or less and V: 0.15% or less in mass %.

Mo: 0.20% or less Mo contributes to suppressing the ferrite structure and also contributes to increasing hardenability. Therefore, the Mo content may be more than 0%, 0.03% or more, or 0.05% or more.
On the other hand, if the Mo content exceeds 0.20%, the hardenability of the wire may become excessively high. In this case, the pearlite transformation during patenting may be insufficient, the area ratio of the pearlite structure may decrease, and the twisting characteristics after wire drawing may decrease. Therefore, the Mo content is 0.20% or less, and may be 0.15% or less.

Cu: 0.50% or less Cu contributes to improving corrosion resistance. Therefore, the Cu content may be more than 0%, 0.02% or more, 0.05% or more, 0.10% or more, or 0.20% or more.
On the other hand, if the Cu content exceeds 0.50%, the hardenability of the wire may become excessively high. In this case, the pearlite transformation during patenting may be insufficient, the area ratio of the pearlite structure may decrease, and the twisting characteristics after wire drawing may decrease. Therefore, the Cu content may be 0.50% or less, 0.40% or less, or 0.35% or less.

Ni: 0.50% or less Ni contributes to improving corrosion resistance. Therefore, the Ni content may be more than 0%, 0.01% or more, 0.05% or more, or 0.10% or more.
On the other hand, if the Ni content exceeds 0.50%, the hardenability of the wire may become excessively high. In this case, the pearlite transformation during patenting may be insufficient, the area ratio of the pearlite structure may decrease, and the twisting characteristics after wire drawing may decrease. Therefore, the Ni content may be 0.50% or less, 0.40% or less, or 0.30% or less.

Sn: 0.50% or less Sn contributes to improving corrosion resistance. From the viewpoint of the effect of improving corrosion resistance by Sn, the Sn content may be more than 0%, 0.03% or more, 0.05% or more, or 0.10% or more.
On the other hand, if the Sn content exceeds 0.50%, the hardenability of the wire may become excessively high. In this case, the pearlite transformation during patenting may be insufficient, the area ratio of the pearlite structure may decrease, and the twisting characteristics after wire drawing may decrease. Therefore, the Sn content may be 0.50% or less, 0.40% or less, or 0.35% or less.

From the viewpoint of improving corrosion resistance, contains one or more selected from the group consisting of Cu: 0.50% or less, Ni: 0.50% or less, and Sn: 0.50% or less in mass %. It is preferable to do so.

Ca: 0.0040% or less Ca improves ductility by finely dispersing MnS. Ca dissolves in MnS and has the effect of finely dispersing MnS, and by finely dispersing MnS, wire breakage during wire drawing caused by MnS can be suppressed. Therefore, the Ca content may be more than 0%, 0.0002% or more, or 0.0005% or more.
On the other hand, when the Ca content exceeds 0.0040%, the effect is saturated. Furthermore, since oxides are formed, the ductility of the steel material is reduced. Therefore, when Ca is contained, the Ca content is 0.0040% or less, may be 0.0030% or less, or may be 0.0025% or less.

Mg: 0.0040% or less Mg improves ductility by finely dispersing MnS. Mg is a deoxidizing element and generates oxides, but it also generates sulfides, so it is an element that has a mutual relationship with MnS, and has the effect of finely dispersing MnS. This effect makes it possible to suppress wire breakage during wire drawing due to MnS. Therefore, the Mg content may be more than 0%, 0.0002% or more, or 0.0005% or more.
On the other hand, even if the Mg content exceeds 0.0040%, the effect is saturated. Furthermore, since oxides are formed, the ductility of the steel material is reduced. Therefore, the Mg content may be 0.0040% or less, 0.0035% or less, or 0.0030% or less.

From the viewpoint of improving ductility, it is preferable to contain one or two of Ca: 0.0040% or less and Mg: 0.0040% or less in mass %.

Sb: 0.15% or less Sb suppresses decarburization by concentrating in the surface layer and suppresses an increase in the ferrite fraction. Therefore, the Sb content may be more than 0%, 0.01% or more, or 0.03% or more.
On the other hand, if the Sb content is too high, the hardenability of the wire may become excessively high. In this case, the pearlite transformation during patenting may be insufficient, the area ratio of the pearlite structure may decrease, and the twisting characteristics after wire drawing may decrease. Therefore, the Sb content may be 0.15% or less, 0.13% or less, or 0.10% or less.

Remaining portion: Fe and impurities In the chemical composition of the wire rod and steel wire according to the present disclosure, the remaining portion excluding each of the above-mentioned elements is Fe and impurities.
Here, impurities refer to components contained in raw materials or components mixed in during the manufacturing process, and are not intentionally added to steel.

<Metal structure of wire>
The metallographic structure of the wire according to the present disclosure includes a pearlite structure and a ferrite structure, and when a cross section of the wire is observed, the area ratio of the pearlite structure is in a region from 50 μm to 100 μm in depth from the outer peripheral surface of the wire. is 90% or more, the area ratio of the ferrite structure is 2.0% or less, and the average value of the individual areas of the ferrite structure is 2.5 μm ² or less.
Since the variation in the structure is large in the region where the depth from the surface (outer surface) of the wire is 50 μm, in the wire according to the present disclosure, the metal structure is defined in the region where the depth from the outer surface of the wire is 50 μm to 100 μm. do. Hereinafter, a region having a depth of 50 μm to 100 μm from the outer peripheral surface of the wire or steel wire may be referred to as a “surface layer region”.

The metal structure of the wire according to the present disclosure includes a pearlite structure and a ferrite structure. FIG. 1 is a SEM image showing an example of a metal structure including a pearlite structure and a ferrite structure. The pearlite structure P is a structure in which ferrite phases and cementite phases are arranged in alternating layers (lamellar shape). In the pearlite structure P shown in Fig. 1, the whitish layered (linear) portion is cementite, and the dark The layered portion is ferrite. On the other hand, the ferrite structure F is a structure composed of a ferrite phase, and is distinguished from the layered ferrite included in the pearlite structure P.

- Area ratio of pearlite structure in the surface layer region: 90% or more The pearlite structure can effectively increase the strength of the wire rod by wire drawing, and also has good ductility after wire drawing. Note that a region from 50 μm to 100 μm in depth from the outer peripheral surface of the wire reflects the pearlite area ratio of the surface layer, and can be measured with little variation.
If the area ratio of the pearlite structure in the surface layer region is 90% or more, sufficient tensile strength can be obtained even before wire drawing, and sufficient ductility can be obtained even after wire drawing. The area ratio of pearlite structure in the surface layer region is preferably 95% or more, more preferably 97% or more.

- Area ratio of ferrite structure in the surface layer region: 2.0% or less By reducing the ferrite structure fraction, the low-strength structure in the cross section can be reduced, and in particular, the torsional properties are improved. Note that a region with a depth of 50 μm to 100 μm from the outer circumferential surface reflects the ferrite area ratio of the surface layer portion, and can be measured with little variation.
The area ratio of the ferrite structure is affected not only by components (elements) such as C, Si, and B, but also by the molten salt immersion temperature and immersion time in the process of manufacturing a wire rod, which will be described later.
The area ratio of the ferrite structure in the surface layer region is 2.0% or less, preferably 1.8% or less, and more preferably 1.5% or less.

- Average value of the individual areas of ferrite structures in the surface layer region: 2.5 μm ² or less By making the individual ferrite structures finer, the low-strength structures within the cross section can be dispersed, and the fatigue properties are particularly improved. Note that the region from 50 μm to 100 μm in depth from the outer circumferential surface reflects the area of each ferrite in the surface layer, and can be measured with little variation.
Note that the average value of the area of each ferrite structure is influenced by B, Si, Ti/N, and the B concentration present at grain boundaries. Moreover, the maximum value of the area of each ferrite structure is influenced by the austenite grain size and cooling rate (molten salt immersion temperature) in the process of manufacturing a wire rod, which will be described later.
The average value of the individual areas of the ferrite structures in the surface layer region is 2.5 μm ² or less, preferably 2.2 μm ² or less, and more preferably 2.0 μm ² or less.
Note that the lower limit of the average value of the individual areas of the ferrite structures in the surface layer region is not particularly limited, but may be 0.3 μm ² or more from the viewpoint of manufacturing stability.

The metal structure of the wire according to the present disclosure does not need to include pearlite structure and residual structures other than ferrite structure, as long as the area ratio of pearlite structure is 90% or more and the area ratio of ferrite structure is 2% or less. However, it may also be included.
Examples of the residual structure include a bainite structure and a martensitic structure, and it is preferable that the amount of the bainite structure and martensite structure is small because strength variation is promoted.
The area ratio of the residual tissue is less than 10%, preferably 5% or less, more preferably 2% or less, and even more preferably 1% or less.

<Characteristics of wire rod, etc.>
The tensile strength of the wire according to the present disclosure is preferably 1250 MPa or more, more preferably 1300 MPa or more, and particularly preferably 1350 MPa or more.

Further, the diameter D of the wire according to the present disclosure is not particularly limited, but is, for example, 5.0 to 10.0 mm, and may be 10.0 to 16.0 mm.

[Steel wire]
The steel wire according to the present disclosure is, in mass%,
C: 0.80% or more and 1.10% or less,
Si: 0.70% or more and 2.00% or less,
Mn: 0.10% or more and 1.00% or less,
P: 0.030% or less,
S: 0.030% or less,
N: 0.0060% or less,
O: 0.0060% or less,
B: 0.0004% or more and 0.0040% or less,
Contains Al: 0.005% or more and 0.070% or less, and Ti: 0.004% or more and 0.025% or less, with the remainder consisting of Fe and impurities,
The content of Si and the content of B satisfy the following formula (1), the content of Ti and the content of N satisfy the following formula (2),
The metal structure includes cementite and ferrite structure, and when a cross section of the steel wire is observed, the area ratio of the ferrite structure is 2.5 μm in a region where the depth from the outer peripheral surface of the steel wire is 50 μm to 100 μm. 0% or less, and the average value of the individual areas of the ferrite structure is 2.0 μm ^or less,
When a longitudinal section of the steel wire is observed, in a region where the depth from the outer peripheral surface of the steel wire is from 50 μm to 100 μm, the total number of cementites is The proportion of cementite whose angle with the central axis direction is 30° or less is 60% or more.
Formula (1): 0.0025×Si−0.0015<B<0.0025×Si
Formula (2): 2.0<Ti/N<5.5
In the formula (1) and the formula (2), the content (mass%) of each element in the steel wire is substituted for each element symbol.
As mentioned above, the steel composition of the steel wire according to the present disclosure is the same as the steel composition of the wire rod, so a description thereof will be omitted here.

<Metal structure of steel wire>
The metal structure of the steel wire according to the present disclosure includes a cementite and ferrite structure, and when a cross section of the steel wire is observed, the ferrite is present in a region with a depth of 50 μm to 100 μm from the outer peripheral surface of the steel wire. The area ratio of the structure is 2.0% or less, the average value of the individual areas of the ferrite structure is ^2.0μm2 or less, and when a longitudinal section of the steel wire is observed, In a region with a depth of 50 μm to 100 μm, the proportion of cementite with an angle of 30° or less between the long axis direction of the cementite and the central axis direction of the steel wire is 60% or more of the total number of cementites. .

The area ratio of the ferrite structure in the surface layer region of the steel wire according to the present disclosure and the average value of the individual areas of the ferrite structure are the same as those of the above-mentioned wire, so the explanation here will be omitted.

・Existence ratio of cementite in which the angle between the long axis direction of cementite and the central axis direction of the steel wire is 30 degrees or less in the surface layer region: 60% or more The shape is inclined so as to be aligned in the drawing direction of the wire, that is, in the direction of the central axis. Therefore, cementite in which the long axis direction of the cementite in the surface layer region of the steel wire and the direction of the central axis of the steel wire is 30° or less (hereinafter referred to as "cementite with an angle of 30° or less to the central axis") is ) is an index representing the degree of wire drawing. In other words, the higher the proportion of cementite at an angle of 30° or less with respect to the central axis, the greater the wire drawing process. The wire drawing process is performed so that the existence ratio of is 60% or more. The proportion of cementite present at an angle of 30° or less with respect to the central axis is preferably 65% or more, more preferably 70% or more. Note that the upper limit is not particularly limited, but since an increase in the degree of wire drawing is accompanied by an increase in production costs, the proportion of cementite at an angle of 30° or less to the central axis may be 95% or less, or 90% or less. .

<Characteristics of steel wire>
The tensile strength of the steel wire according to the present disclosure is preferably 1800 MPa or more, more preferably 1900 MPa or more, and particularly preferably 2040 MPa or more.

Further, the diameter d of the steel wire according to the present disclosure is not particularly limited, but is, for example, 1.0 to 4.0 mm, and may be 4.0 to 10.0 mm.

<Measurement method of wire structure>
-Measurement of area ratio of pearlite structure The area ratio of pearlite structure can be obtained by marking the part other than pearlite structure, that is, the non-pearlite structure, calculating the area ratio of non-pearlite structure, and subtracting it from 100%. Specifically, the non-pearlite structure is a ferrite structure, a bainite structure, and a martensitic structure. These structures inhibit effective enhancement of strength during wire drawing.
Here, the ferrite structure and martensitic structure are structures in which cementite does not exist in an area of 0.1 μm ^{or more} , and are corroded in 3 to 10 seconds with picric acid saturated alcohol and observed with a scanning electron microscope (SEM). Refers to tissue that sometimes appears black. Note that corrosion is performed within 2 minutes after mirror polishing. This is because an oxide film is formed as time passes after mirror polishing, and the above etching time is insufficient. The part indicated by F in FIG. 1 is a ferrite structure, which is distinguished from a layered ferrite (phase) arranged alternately with layered cementite, like a pearlite structure P.

Moreover, FIG. 2 is a SEM image showing an example of a bainite structure. Bainite structure is similar to pearlite structure in that it is composed of cementite θ (gray part in Fig. 2) and ferrite (dark part in Fig. 2), but like the pearlite structure shown in Fig. 1, cementite and ferrite are Since it is not a structure arranged in alternating layers (lamellae), it can be distinguished from a pearlite structure. In addition, since the bainite structure is a structure that is easily formed in a low temperature range of less than 500°C, it can be considered that it appears when the wire is immersed in a salt bath at less than 500°C. Note that bainite is not limited to the form shown in FIG. 2, and other forms also exist. Furthermore, pearlite is not limited to the lamellar form shown in FIG. 1, but also pearlite with collapsed lamellae. For example, a structure composed of "non-plate-like cementite and ferrite" that is observed when it undergoes pearlite transformation by continuous cooling such as air cooling or natural cooling, or by being immersed in a salt bath at 500° C. or higher is considered to be pearlite.
Figure 4 shows (A) a visual field containing bainite, (B) a visual field containing martensite, (C) a visual field containing ferrite, and (D) pearlite and pearlite with collapsed lamellae (consisting of cementite and ferrite that are not plate-shaped). An example of each SEM photograph of a field of view including pearlite) is shown.

The non-pearlite structure was determined by observing the structure in the surface layer area of the cross section of the wire using SEM, printing it on paper at a magnification of 2000 times, and then overlaying a transparent sheet such as an OHP (Over Head Projector) sheet on the paper. color to the non-pearlite structure. After that, the area ratio of the non-pearlite structure is measured by analyzing the transparent sheet with the non-pearlite structure painted using image analysis.

・Measurement of the area ratio of the ferrite structure and the area of the ferrite structure The ferrite structure was determined by observing the structure in the surface layer area of the cross section of the wire using a SEM, and after printing on paper at a magnification of 2000 times, an OHP sheet was placed on the paper surface. Layer transparent sheets such as and paint the ferrite structure with color. Thereafter, the area of each individual ferrite structure and the total area ratio of all ferrite structures are measured by analyzing the transparent sheet with colored ferrite structures using image analysis.
Furthermore, the area of the ferrite structure is the average value of the areas of the individual ferrite structures obtained.

For each measurement of the pearlite structure and ferrite structure, three samples were taken by cutting the wire at intervals of 400 mm, and the area per field of view for one cross section of each sample was 2.7 × 10 ^-3 . mm ² (length: 0.045 mm, width: 0.060 mm), three visual fields were observed, and the average value of the total of nine visual fields was taken as the area ratio of each tissue. Image analysis software (eg, image-J) is used for image analysis. Note that those with an area of 0.1 μm ² or less are removed as noise.

<Measurement method of steel wire structure>
When a wire rod is drawn into a steel wire, the ferrite structure also expands as the wire diameter becomes thinner, so the observation magnification of the cross section of the steel wire is preferably higher than the observation magnification of the wire rod. Specifically, the observation magnification of the steel wire is preferably 2000 x wire diameter/steel wire diameter. For example, if the wire diameter is 14 mm and the steel wire diameter is 7 mm, the measurement is performed at a magnification of 4000 times. The method for measuring the area ratio of pearlite structure, the area ratio of ferrite structure, and the individual areas of ferrite structure in the surface region of the steel wire is the same as that for the wire rod except for the observation magnification, so the explanation here will be omitted. .

・Measurement of the angle between the longitudinal direction of cementite and the longitudinal direction of steel wire In this disclosure, the long axis direction of cementite is the direction of the line connecting both end points of the long axis of cementite, and the long axis direction of cementite is the direction of the long axis of cementite. The proportion of cementite whose acute angle is 30° or less between the steel wire and the longitudinal direction of the steel wire is determined. FIG. 3 shows an angle α between the long axis direction L of the cementite θ and the central axis direction C of the steel wire for an example of cementite.
Specifically, a vertical cross section of the steel wire is observed using a SEM, and the drawing direction of the steel wire is in the horizontal direction (horizontal direction) of the paper, and the paper is printed at a magnification of 6000 times. A grid is drawn vertically and horizontally on the printed paper at 4 μm intervals, and the longitudinal direction of the cementite and the drawing direction of the steel wire (horizontal direction of the paper) in the area where the lamella spacing closest to the point where the grid overlaps is 0.2 μm or less. Measure the angle formed by If the cementite θ is curved, the long axis direction L of the cementite θ is the direction connecting both end points of the cementite, as shown in FIG. You can also use it as
The angles of 25 or more cementites in a total of 3 fields of view are measured as described above, and the ratio of "number of cementites below 30°/total number of measured cementites x 100" is defined as the proportion of cementites whose angles are below 30°.

[Wire manufacturing method]
Next, a method for manufacturing a wire according to the present disclosure will be described.
Although the method for manufacturing the wire rod according to the present disclosure is not particularly limited, preferred manufacturing methods include, for example, a step of casting a slab having the chemical components; a step of heating the slab; and a step of heating the slab after the heating. a step of hot rolling to obtain a hot-rolled steel billet; a step of cooling the hot-rolled steel billet; a step of heating the hot-rolled steel billet; and a step of hot-rolling the heated hot-rolled billet. obtaining a hot-rolled wire; cooling the hot-rolled wire with water; winding the hot-rolled wire; cooling the coiled hot-rolled wire; patenting the hot-rolled wire. Examples include a method comprising: a step of cooling the hot-rolled wire rod after the patenting to form a wire rod.
Preferred manufacturing conditions for the wire according to the present disclosure will be described in detail below.

(casting)
In the method for manufacturing a wire rod according to the present disclosure, first, steel is melted, and then a slab having the chemical composition of the wire rod according to the present disclosure is manufactured by continuous casting or the like.

(Heating of slab)
Before hot rolling, the slab is preferably heated to a heating temperature at which the average temperature of its surface is within the range of 1220 to 1300°C. If the heating temperature is less than 1220° C., Ti forms carbides and N cannot be fixed efficiently, so BN is formed in the subsequent process and the effect of B cannot be efficiently obtained. On the other hand, if the temperature exceeds 1300°C, decarburization will progress significantly in a short period of time.
Further, the slab is preferably heated within a range of 3 hours to 24 hours. If the heating time is less than 3 hours, the heating of the center will be insufficient and the fixation of N by Ti will be insufficient. On the other hand, if heating is performed for more than 24 hours, decarburization will progress significantly.

(Hot rolling and cooling of slabs)
After heating, the slab is hot-rolled into a steel slab, which is then cooled by air cooling.

(Heating of steel billet)
The steel billet is preferably heated to a heating temperature at which the average temperature of its surface is within the range of 1120 to 1200° C. before hot rolling. When the heating temperature is lower than 1120° C., the B compound formed during the rolling and cooling stages of the slab does not remelt. On the other hand, when heated above 1200° C., TiN precipitated in the slab stage is remelted, and a B compound is more likely to be formed in a subsequent process, resulting in less B in solid solution.

(hot rolling)
After hot rolling, the steel billet is once cooled and heated and held again, and then hot rolled into a wire rod. In hot rolling, it is preferable that the temperature on the entry side of finish rolling is 950°C to 1050°C. If the temperature on the entry side of the finish rolling is 950° C. or higher, it can be considered that the hot rolling step was performed at 950° C. or higher, and B in a solid solution state can remain. If the temperature is lower than 950°C, the B compound may precipitate, and the effect of B cannot be obtained efficiently. On the other hand, if the temperature exceeds 1050°C, there is a possibility that water cooling after finish rolling may not be able to cool the steel to the target temperature.

(Water cooling and winding)
Next, the hot rolled steel after finish rolling is water cooled and wound up. The water cooling stop temperature and winding temperature are preferably 820°C to 865°C. When the water-cooling stop temperature and the winding temperature are lower than 820°C, the deformation resistance increases and winding becomes difficult. When the water cooling stop temperature and the winding temperature exceed 865° C., austenite grains become coarse and a coarse ferrite structure is likely to be formed. Note that water cooling is started immediately after hot rolling is completed.

(cooling)
Next, the wound wire is preferably cooled to a temperature in the range of 730 to 780° C. at a rate of 5 to 15° C./sec between the time of winding and the time of immersion in the molten salt.
After winding, by cooling at a rate of 5 to 15 degrees C/sec to 730 to 780 degrees Celsius (achieved cooling temperature), even if a small amount of B has precipitated on the grain boundaries in the previous process, it can be removed from inside the grains. B can diffuse into the world, making it difficult to create B-deficient regions. Since the ferrite structure is preferentially formed in B-deficient regions, the area ratio of the ferrite structure can be reduced by reducing the number of B-depleted regions.
When the cooling temperature reached is less than 730° C., a ferrite structure is formed, so that coarse ferrite is likely to be formed. On the other hand, if the ultimate cooling temperature exceeds 780°C, the temperature of the wire becomes high when immersed in the molten salt, which may increase the temperature of the molten salt, so it is preferably 780°C or less.
Furthermore, when the cooling rate exceeds 15° C./sec, it becomes difficult for B to diffuse from within the grains, and a depletion layer due to B precipitation is likely to be formed, resulting in coarsening of individual ferrite structures. On the other hand, when the cooling rate is less than 5° C./sec, austenite grains become coarse and individual ferrite structures become coarse.

(patenting)
The cooled wire is preferably immersed in molten salt at 500 to 600°C.
When the molten salt temperature is controlled at 500 to 600°C, it is cooled at a sufficient cooling rate, and the time for B compounds to precipitate is shortened, so that pearlite structure nuclei can be formed without increasing the B-deficient region. , and the area ratio of the ferrite structure can be reduced. When immersed in a molten salt of less than 500°C, bainite transformation progresses and the pearlite structure ratio decreases. When immersing in molten salt at 600° C. or higher, the cooling rate decreases, promoting precipitation of the B compound.
The molten salt immersion time is preferably 30 seconds to 70 seconds. If the immersion time is less than 30 seconds, the pearlite transformation will not end, and the non-pearlite structure fraction may increase. If the immersion time exceeds 70 seconds, the lamellar structure will collapse after pearlite transformation, and the tensile strength will decrease.

Note that patenting may be performed by immersing the substrate in a plurality of molten salts having different temperatures. For example, it is immersed in a primary molten salt bath at 500° C. to 530° C. for 10 seconds to 30 seconds, and then immersed in a secondary molten salt bath at 540° C. to 575° C. for 30 seconds to 60 seconds.

(cooling)
After patenting with molten salt, the wire rod is washed with water to remove the molten salt adhering to the surface, and the wire rod according to the present disclosure can be suitably manufactured through cooling.

[Manufacturing method of steel wire]
Although the method for manufacturing the steel wire according to the present disclosure is not particularly limited, for example, a method of further wire-drawing the wire rod according to the present disclosure manufactured by the above manufacturing method is suitable.

(Wire drawing process)
The wire drawing strain ε due to wire drawing is expressed by the following formula.
ε=Ln(D ₀ /D) ²
D ₀ is the diameter before wire drawing, and D is the diameter after wire drawing. It is preferable that ε is 1.0 or more and 2.5 or less. When ε is less than 1.0, the long axis direction of cementite is not aligned with the long axis direction of the steel wire, so a fiber structure is not formed and the strength cannot be increased. On the other hand, when ε exceeds 2.5, the cementite is forcibly decomposed during processing and the solid solution carbon becomes excessively dissolved in the ferrite, resulting in a decrease in twisting properties.
In addition, it is preferable to perform a surface lubrication treatment such as a zinc phosphate film or a borax film before wire drawing.

The steel wire according to the present disclosure can be manufactured through the above steps. Note that after the wire drawing process, post-treatment such as plating or degreasing treatment may be performed.

The wire rod according to the present disclosure can be subjected to wire drawing to obtain a steel wire with excellent tensile strength, fatigue properties, and twisting properties.
Applications of the steel wire according to the present disclosure are not particularly limited, but applications that require high strength, fatigue properties, and twisting properties, such as steel wire for bridge cables, PC steel wire, ACSR wire, and various ropes, etc. The wire rod according to the present disclosure is suitable for various uses, and is suitable as a material for steel wires used for these uses.

Examples of the present disclosure will be specifically described below with reference to Examples, but the wire rod and steel wire according to the present disclosure are not limited to the following Examples.

[Manufacture of wire rods and steel wires]
<Manufacture of wire>
(No. 1-1 to No. 1-27: Same ingredients)
First, steel type 1 (steel 1) having the chemical composition shown in Table 1 was melted in a converter, and then a 122 mm square billet was obtained by blooming rolling. Note that the content of each element in Table 1 is mass %, the remainder is Fe and impurity elements, and "-" means that the element is not included. Note that the same applies to Table 3, which will be described later.

Next, using steel type 1, hot rolling, winding, cooling, and immersion in molten salt were performed according to manufacturing conditions 1 to 27 shown in Table 2, followed by washing with water and cooling, to form the lines shown in Table 4. A wire rod having a diameter (wire diameter D before wire drawing) and a wire structure was manufactured. In Table 2, "cooling rate" is the cooling rate from the coiling temperature to the temperature at which the sample is immersed in the primary molten salt (primary molten salt immersion temperature).
Note that the underline in Table 2 indicates that the manufacturing conditions are outside the desirable range of the present disclosure. Furthermore, the underline in Table 4 indicates that the value is outside the range defined in the present disclosure.

(No. 2 to No. 31: Same manufacturing conditions except for ingredients)
Steels 2 to 31 having the chemical composition shown in Table 3 were melted in a converter, and then subjected to blooming to obtain a 122 mm square billet. Note that the underline in Table 3 indicates that the content of the element is outside the range specified in the present disclosure.
Next, using steel types 2 to 31, they were hot rolled, coiled, cooled, and immersed in molten salt according to the manufacturing conditions 1 shown in Table 2, and then washed with water and left to cool to form the lines shown in Table 5. A wire rod having a diameter (wire diameter D before wire drawing) was manufactured.

<Manufacture of steel wire>
The wire produced as described above is subjected to wire drawing at the true strain of the "steel wire manufacturing method" shown in Tables 4 and 5, respectively, to produce a steel wire having a wire diameter d after wire drawing. did. In addition, for lubrication during wire drawing, a zinc phosphate film was formed on the surface of the wire and wire drawing was performed.

<Measurement of metal structure>
The wire rods and steel wires produced as described above were cut to take samples, and the metallographic structure was measured by the method described above. Note that, before observing the cross section by SEM, the cross section was mirror polished and corroded with an aqueous picric acid solution. The metal structures are shown in Tables 4 and 5. "Percentage of cementite having a long axis direction angle of 30 degrees or less" means the proportion of cementite in which the angle between the long axis direction of cementite and the central axis direction of the steel wire is 30 degrees or less. Underlining indicates outside the scope of this disclosure. In addition, in cases where the sum of the pearlite area ratio and the ferrite area ratio did not reach 100%, a martensite structure and/or a bainite structure was observed as the remaining structure.
In addition, in the compositions of Tables 4 and 5, when the relational expression of formula (1) or formula (2) is satisfied, it is written as "○", and when it is not satisfied, it is written as "x".

[evaluation]
<Evaluation of mechanical properties>
The mechanical properties of the wire rods and steel wires produced as described above were evaluated by the following method.

(Tensile strength)
After cutting the wire rod or steel wire to a length of 340 mm, it is straightened and made into a straight bar. A tensile test is performed by chucking the length between the chucks (test length) of 70 mm on the top and bottom of 200 mm. The tensile strength (MPa) is calculated by dividing the obtained maximum load by the cross-sectional area.
In the above tensile test, the reduction of area (%) of the wire rod was calculated using the following formula.
Aperture value = (cross-sectional area before tensile test - cross-sectional area of the part broken in tensile test) / cross-sectional area before tensile test x 100
In addition, in the above tensile test, those whose tensile strength of the steel wire was equal to or higher than "2470-70 x wire diameter d (mm)" MPa were considered to have passed. Generally, the thicker the wire diameter, the lower the ductility, so the tensile strength is evaluated taking the wire diameter into account.

(Torsion characteristics)
Cut the steel wire so that the test length is 100 x dmm. A 50 mm end portion is chucked and subjected to a twisting test. The twisting speed is 20 rpm.
Torsion characteristics are evaluated by the presence or absence of delamination and determined by the fracture surface. A torsion test was conducted on 10 pieces of each steel type, and if no delamination occurred, the test was passed (indicated as "none" in Tables 4 and 5), and if even one piece of steel had delamination, the torsional properties were poor (Table 5). 4 and Table 5). In addition, the number of twists at the final break was recorded and the average value (twist value) of each steel wire was calculated, and 18 or more twists were judged to be good.

(Fatigue characteristics)
The fatigue test is a rotary bending fatigue test. In the test, the steel wire is used as it is, without any processing other than straightening. Add a weight to the center 200mm, bend and rotate. Stress due to bending is calculated using curvature. The curvature is calculated using the amount of deflection of the steel wire, assuming that the steel wire is deformed in an arc shape. The Young's modulus used is 200 GPa.
When the value of "fatigue strength/tensile strength" obtained by dividing fatigue strength by tensile strength (denoted as "durability ratio" in Tables 4 and 5) is 0.220 or more, the fatigue properties are evaluated to be good.

Tables 4 and 5 show the tensile strengths of the wire rods and steel wires, as well as the twisting properties and fatigue properties of the steel wires, which were evaluated as described above. An underline indicates that the characteristic is outside the desired range.

<Manufacturing method and metal structure dependence evaluation>
As shown in Table 4, Example No. All of Nos. 1-1 to 1-7 satisfy the requirements of the present disclosure, and steel wires with excellent tensile strength, fatigue properties, and twisting properties are obtained.
On the other hand, No. In Nos. 1-8 to 1-27, the steel wires were either lacking in at least one of tensile strength, fatigue properties, and twisting properties, or a problem arose during production, and production of the steel wires was discontinued. It is estimated as follows.

No. In No. 1-8, the area ratio of the ferrite structure was excessive and the fatigue strength was insufficient. It is thought that the slab heating temperature was too low, so that solid solution N remained and B nitrides precipitated.
No. In No. 1-9, both the average area ratio of the ferrite structure and the average area of the ferrite structure are excessive, and the tensile strength, fatigue strength, and torsional properties are insufficient. It is thought that the heating temperature of the slab was too high, and the ferrite structure increased due to decarburization, and the strength also decreased.
No. In No. 1-10, both the area ratio of the ferrite structure and the average area of the ferrite structure are excessive, and the fatigue strength and torsional properties are insufficient. It is thought that the slab heating time was too short, so that solid solution N remained and B nitrides precipitated.
No. In No. 1-11, both the area ratio of the ferrite structure and the average area of the ferrite structure are excessive, and the tensile strength, fatigue strength, and torsional properties are insufficient. It is thought that the heating time of the slab was too long, and the ferrite structure increased due to decarburization, and the strength also decreased.
No. In No. 1-12, the average area of the ferrite structure is too large and the twisting characteristics are insufficient. It is thought that the heating temperature of the steel billet was too low, resulting in a shortage of B due to undissolved remaining B compounds.
No. In No. 1-13, the area ratio of the ferrite structure is excessive, and the fatigue strength and torsional properties are insufficient. It is thought that the steel billet heating temperature was too high, so that solid solution N remained and B nitrides precipitated.
No. In No. 1-14, both the area ratio of the ferrite structure and the average area of the ferrite structure are excessive, and the fatigue strength and torsional properties are insufficient. It is thought that the finish rolling temperature was too low and B was insufficient due to the precipitation of B nitrides.
No. In No. 1-15, the average area of the ferrite structure was too large and the twisting properties were insufficient. It is thought that coarsening of austenite occurred due to the increase in finish rolling temperature and coiling temperature.
No. 1-16, the work after the winding process was discontinued due to a winding defect. It is thought that the winding failure occurred due to a drop in water cooling temperature before winding.
No. In No. 1-17, the average area of the ferrite structure was too large and the twisting properties were insufficient. It is thought that coarsening of austenite occurred due to the increase in the coiling temperature.
No. In No. 1-18, the average area of the ferrite structure was too large and the torsional properties were insufficient. It is thought that coarsening of austenite occurred due to slow cooling after winding.
No. In No. 1-19, the average area of the ferrite structure was too large and the twisting properties were insufficient. It is thought that a B-depleted layer was generated due to rapid cooling after winding, and a coarse ferrite structure was generated.
No. In No. 1-20, both the area ratio of the ferrite structure and the average area of the ferrite structure are excessive, and the fatigue strength and torsional properties are insufficient. It is thought that the cooling temperature after winding (immersion start temperature) was low and a coarse ferrite structure was formed.
No. In No. 1-21, the average area of the ferrite structure is too large and the twisting characteristics are insufficient. It is thought that the temperature of the molten salt increases and a fine ferrite structure cannot be obtained.
No. In No. 1-22, the area ratio of pearlite is low, the average area of the ferrite structure is excessive, and the tensile strength and twisting properties are insufficient. It is thought that the molten salt temperature was low and excessive bainite structure was formed.
No. In No. 1-23, the average area of the ferrite structure was too large and the twisting characteristics were insufficient. It is thought that the temperature of the molten salt increases and a fine ferrite structure cannot be obtained.
No. No. 1-24 could not be subjected to wire drawing. It is thought that because the immersion time in the primary molten salt was short, the pearlite transformation was not completed and martensite was formed, resulting in a decrease in the area ratio of the pearlite structure.
No. No. 1-25 could not be subjected to wire drawing. It is considered that the area ratio of the pearlite structure of the wire rod was small and the secondary molten salt temperature was too low, so that pearlite transformation was not completed and martensite was formed.
No. In No. 1-26, the average area of the ferrite structure was too large and the twisting properties were insufficient. It is thought that the temperature of the molten salt increases and a fine ferrite structure cannot be obtained.
No. No. 1-27 could not be subjected to wire drawing. It is thought that because the immersion time in the secondary molten salt was short, martensite was formed and the area ratio of the pearlite structure was insufficient.

<Evaluation of component dependence>
As shown in Table 5, Example No. Nos. 2 to 18 all satisfy the requirements of the present disclosure, and steel wires with excellent tensile strength, fatigue properties, and twisting properties are obtained.
On the other hand, No. In Nos. 19 to 31, the steel wires lacked at least one of tensile strength, fatigue properties, and twisting properties, or a problem arose during production, and production of the steel wires was discontinued. It is estimated as follows.

No. No. 19 had insufficient C content and insufficient tensile strength.
No. No. 20 has an excessive C content and is insufficient in fatigue strength and torsional properties.
No. No. 21 has insufficient Si content and insufficient tensile strength.
No. No. 22 could not be subjected to wire drawing. It is considered that the Mn content was excessive and martensite was formed in the segregated areas.
No. No. 23 has excessive Al content and lacks twisting properties.
No. No. 24 has insufficient Ti content and is insufficient in tensile strength, fatigue strength, and twisting properties.
No. In No. 25, the Ti content is excessive, the area ratio of the ferrite structure and the average area of the ferrite structure are both excessive, and the fatigue strength and torsional properties are insufficient.
No. In No. 26, the N content was excessive, the area ratio of the ferrite structure was excessive, and the fatigue strength was insufficient.
No. In No. 27, the B content is insufficient relative to the Si content, and the relationship of formula (1) is not satisfied, and the area ratio of the ferrite structure and the average area of the ferrite structure are both excessive, resulting in poor fatigue strength and torsional properties. It is insufficient.
No. In No. 28, the B content is excessive with respect to the Si content, and the relationship of formula (1) is not satisfied, and the area ratio of the ferrite structure and the average area of the ferrite structure are both excessive, resulting in poor tensile strength, fatigue strength, and torsion. times characteristics are lacking.
No. In No. 29, the Ti content is excessive relative to the N content, and relational expression (2) is not satisfied, and the area ratio of the ferrite structure and the average area of the ferrite structure are both excessive, resulting in insufficient tensile strength, fatigue strength, and torsional properties. are doing.
No. In No. 30, the Ti content was insufficient relative to the N content, and relational expression (2) was not satisfied, and the area ratio of the ferrite structure and the average area of the ferrite structure were both excessive, resulting in insufficient fatigue strength and torsional properties. There is.
No. In No. 31, the B content is insufficient, the area ratio of the ferrite structure and the average area of the ferrite structure are both excessive, and the fatigue strength and torsional properties are insufficient.

C Central axis of steel wire P Pearlite structure F Ferrite structure θ Cementite α Angle between the long axis direction of cementite and the central axis direction of steel wire

Claims (8)

In mass%,
C: 0.80% or more and 1.10% or less,
Si: 0.70% or more and 2.00% or less,
Mn: 0.10% or more and 1.00% or less,
P: 0.030% or less,
S: 0.030% or less,
N: 0.0060% or less,
O: 0.0060% or less,
B: 0.0004% or more and 0.0040% or less,
Contains Al: 0.005% or more and 0.070% or less, and Ti: 0.004% or more and 0.025% or less, with the remainder consisting of Fe and impurities,
The content of Si and the content of B satisfy the following formula (1), the content of Ti and the content of N satisfy the following formula (2),
The metal structure includes a pearlite structure and a ferrite structure, and when a cross section of the wire is observed, the area ratio of the pearlite structure is 90% or more in a region where the depth from the outer peripheral surface of the wire is 50 μm to 100 μm. and the area ratio of the ferrite structure is 2.0% or less, and the average value of the individual areas of the ferrite structure is 2.5 μm ² or less.
Formula (1): 0.0025×Si−0.0015<B<0.0025×Si
Formula (2): 2.0<Ti/N<5.5
In the formula (1) and the formula (2), the content (mass %) of each element in the wire is substituted for each element symbol. 2 . The Cr containing 0.60% or less in mass % in place of a part of the Fe, and the Cr content and the B content satisfy the following formula (3): wire rod.
Formula (3): 0.0048×Cr≦B≦0.013×Cr
In the formula (3), each element symbol is substituted with the content (mass%) of each element in the wire. In place of a part of the Fe, in mass%,
The wire rod according to claim 1 or 2, containing one or both of Nb: 0.025% or less and V: 0.15% or less. The wire rod according to any one of claims 1 to 3, containing Mo: 0.20% or less in mass % in place of a part of the Fe. In place of a part of the Fe, in mass%,
Cu: 0.50% or less,
The wire rod according to any one of claims 1 to 4, containing one or more selected from the group consisting of Ni: 0.50% or less and Sn: 0.50% or less. In place of a part of the Fe, in mass%,
The wire rod according to any one of claims 1 to 5, containing one or both of Ca: 0.0040% or less and Mg: 0.0040% or less. The wire rod according to any one of claims 1 to 6, containing Sb: 0.15% or less in mass % in place of a part of the Fe. In mass%,
C: 0.80% or more and 1.10% or less,
Si: 0.70% or more and 2.00% or less,
Mn: 0.10% or more and 1.00% or less,
P: 0.030% or less,
S: 0.030% or less,
N: 0.0060% or less,
O: 0.0060% or less,
B: 0.0004% or more and 0.0040% or less,
Contains Al: 0.005% or more and 0.070% or less, and Ti: 0.004% or more and 0.025% or less, with the remainder consisting of Fe and impurities,
The content of Si and the content of B satisfy the following formula (1), the content of Ti and the content of N satisfy the following formula (2),
The metal structure includes cementite and ferrite structure, and when a cross section of the steel wire is observed, the area ratio of the ferrite structure is 2.5 μm in a region where the depth from the outer peripheral surface of the steel wire is 50 μm to 100 μm. 0% or less, and the average value of the individual areas of the ferrite structure is 2.0 μm ^or less,
When a longitudinal section of the steel wire is observed, in a region where the depth from the outer peripheral surface of the steel wire is from 50 μm to 100 μm, the total number of cementites is The proportion of cementite whose angle with the central axis direction is 30° or less is 60% or more,
When chucking 50 mm of each end of 10 test pieces with a length of 100 x dmm (wire diameter) and performing a twisting test at a twisting speed of 20 rpm, no delamination occurred in any of the test pieces. And, the steel wire has an average number of twists (twist value) of 18 or more times at final break .
Formula (1): 0.0025×Si−0.0015<B<0.0025×Si
Formula (2): 2.0<Ti/N<5.5
In the formula (1) and the formula (2), the content (mass%) of each element in the steel wire is substituted for each element symbol.