JPWO2015186701A1 - Steel wire rod - Google Patents

Abstract

An object of the present invention is to provide a steel wire having high strength and low electrical resistivity. This steel wire contains C, Si, Mn, Cr, Al as chemical components, restricts N, P, S, and is selectively selected from the group consisting of Mo, V, Ti, Nb, and B 1 or more types, and the balance consists of Fe and impurities. The structure of the steel wire is pearlite with an area ratio of 85% or more, and the average lamellar spacing is 50 nm to 100 nm. Further, in mass%, the Si content is [% Si], the Cr content in the cementite in pearlite is [% Crθ], and the Cr content in the ferrite in pearlite is [% Crα]. The steel wire satisfies ([% Crθ] / [% Crα]) ≧ (2.0 + [% Si] × 10).

Description

The present invention relates to a steel wire.
This application claims priority on June 2, 2014 based on Japanese Patent Application No. 2014-114429 for which it applied to Japan, and uses this content for it here.

As for the aluminum strand used for a power transmission line etc., the strand of the steel wire is used for the inner core material for strength reinforcement. Such a stranded wire is generally referred to as an aluminum conductor STEEL-reinforced cable. Hereinafter, the strands of steel core aluminum are denoted as ACSR.
Steel wires used for the core material of ACSR are often manufactured by cold drawing using a piano wire material such as SWRS72B or SWRS82B of JIS standard as a material. In the production of ACSR, before cold drawing or in the middle of cold drawing, heat treatment called patenting or aluminum plating treatment is performed as necessary.

In ACSR, electricity flows not only through aluminum wires but also through steel wires in steel cores. Therefore, when the electrical resistivity of the steel wire is large, the electrical resistivity of the entire ACSR also increases, and heat generation during power transmission increases. As a result, power transmission efficiency decreases.
Moreover, when the intensity | strength of a steel wire is low, it is necessary to increase the steel wire whose electric resistivity is larger than aluminum, and to reinforce intensity | strength. However, since electricity flows through the steel wire, as a result, the electrical resistivity of the entire ACSR increases.
For these reasons, there is a demand for a steel wire having a low electrical resistivity and a high strength, and a steel wire having a low electrical resistivity and a material thereof.

The electrical resistivity of steel generally increases with increasing element content in the steel. Therefore, in the steel material disclosed in Patent Document 1, the electrical resistivity is reduced by reducing the content of main elements such as C, Mn, and Cr. However, this steel material is intended to reduce electrical resistivity and improve cold forgeability, so that the content of C and Si in the steel is small. Therefore, the tensile strength is insufficient.
Moreover, in the steel plate for springs disclosed in Patent Document 2, the electrical resistivity of the steel material is lowered by making the contents of C, Si, and Mn in the steel less than the value represented by the predetermined formula. . However, in this spring steel plate, since the structure is not optimized and Cr is not contained, the tensile strength cannot be increased. Therefore, the balance between ensuring the strength and reducing the electrical resistivity is insufficient.
Furthermore, in the high strength and high toughness hypereutectoid steel wire disclosed in Patent Document 3, the tensile strength and the degree of wire drawing are ensured by defining the content and structure of C, Si, Mn, etc. in the steel. doing. However, in this high-strength, high-toughness hypereutectoid steel wire, the Si content is 0.5% or more, and the structure is not optimized to reduce the electrical resistivity, so the electrical resistivity is high. .
Patent Document 4 discloses a high carbon steel wire material in which the spheroidizing heat treatment time is shortened by increasing the Cr content in the carbide. However, since the high carbon steel wire has a Cr content in the carbide of 6.0% by mass or more, it is impossible to achieve both reduction in electrical resistivity and increase in tensile strength.

Japanese Laid-Open Patent Publication No. 2003-226938 Japanese Unexamined Patent Publication No. 2004-156120 Japanese Unexamined Patent Publication No. 6-271937 International Publication No. 2012/144630

  An object of this invention is to provide the steel wire with high intensity | strength and low electrical resistivity in view of such a situation.

In the steel wire rod of the present invention, the Si content is particularly limited to increase the electrical resistivity, and further, the Cr content contained in the cementite in the pearlite is increased within a range where both electrical resistivity and tensile strength can be achieved. Furthermore, the increase in electrical resistivity is prevented by reducing the Cr content contained in the ferrite in the pearlite. In addition, the tensile strength is improved by reducing the average lamella spacing of the pearlite, and the steel wire material achieves both reduction of electrical resistivity and improvement of tensile strength.
In addition, the steel wire of this invention is a raw material before performing cold wire drawing, and the steel materials which heat-processed the hot rolling wire and the hot rolling wire are contained.

In order to solve the above problems and obtain a steel wire having high strength and low electrical resistivity, the inventors have conducted detailed investigation and research on the chemical composition, structure, and alloy element distribution state of the steel wire. Piled up. As a result, the following findings (a) to (c) were found.
(A) Since the electrical resistivity at room temperature of the steel wire does not change much depending on the cold drawing, the components of the steel wire, the structure before the cold drawing and the existence state of the alloy elements are cold drawn. It greatly affects the subsequent electrical resistivity and tensile strength.
(B) When the Si content and the Cr content are increased, the strength of the steel wire is increased by solid solution strengthening, but the increase in the Si content significantly increases the electrical resistivity. On the other hand, an increase in the Cr content also increases the electrical resistivity, but the effect is small compared to Si. Further, when Cr dissolves in ferrite, the electrical resistivity is increased. Therefore, when the Cr content in cementite is increased and the Cr content in ferrite is decreased, it is possible to suppress an increase in electrical resistivity. In other words, it is possible to suppress an increase in electrical resistivity by increasing the Cr content contained in the cementite in the pearlite and reducing the Cr content contained in the ferrite in the pearlite.
Also, by controlling the Cr content contained in cementite according to the Si content that contributes to solid solution strengthening, it is possible to control both the improvement of tensile strength and the reduction of electrical resistivity with high efficiency. Can do.
In the following, “Cr content in cementite in pearlite” is simply “Cr content in cementite” and “Cr content in ferrite in pearlite” is simply “Cr content in ferrite”. May be written.
(C) In order to achieve both the improvement of the tensile strength of the steel wire and the reduction of the electrical resistivity, it is effective to make the structure of the steel wire a structure made of pearlite. Furthermore, in the case of pearlite in which ferrite and cementite have a lamellar structure, the tensile strength can be improved by reducing the average lamellar spacing. On the other hand, since the influence of the average lamella spacing in the pearlite on the electrical resistivity is not so great, the average lamella spacing may be reduced in order to achieve both improvement in tensile strength and reduction in electrical resistivity.

This invention is made | formed based on the said knowledge, The summary is as follows. (1) The steel wire according to one embodiment of the present invention has, as chemical components, mass%, C: 0.8% to 1.1%, Si: 0.02% to 0.30%, Mn: 0.00. 1% to 0.6%, Cr: 0.3% to 1.5%, Al: 0.01% to 0.05%, N: 0.008% or less, P: 0.03% or less , S: 0.02% or less, selectively Mo: 0.20% or less, V: 0.15% or less, Ti: 0.050% or less, Nb: 0.050% or less, and B : Containing at least one selected from the group consisting of 0.0030% or less, the balance being Fe and impurities; the structure contains pearlite, and the area ratio of the pearlite is 85% or more; the average of the pearlite The lamellar spacing is 50 nm to 100 nm; the content of the Si is [% Si] in mass%, and the cement in the pearlite is When the content of Cr contained in the tight is [% Crθ] and the content of Cr contained in the ferrite in the pearlite is [% Crα], the [% Si], the [% Crθ], and The [% Crα] satisfies the following formula (a).
([% Crθ] / [% Crα]) ≧ (2.0 + [% Si] × 10) (a)
(2) In the steel wire described in the above (1), the chemical component is, in mass%, Mo: 0.02% to 0.20%, V: 0.02% to 0.15%, Ti: 0. It may contain one or more selected from the group consisting of 0.002% to 0.050%, Nb: 0.002% to 0.050%, and B: 0.0003% to 0.0030%.
(3) In the steel wire described in the above (1) or (2), the tensile strength TS of the steel wire is 1350 MPa or more, and the absolute value of the tensile strength TS of the steel wire is the steel It may be 64 times or more the absolute value of the electrical resistivity ρ represented by the unit μΩ · cm of the wire.

According to each aspect of the above (1) to (3), a steel wire having high strength and low electrical resistivity can be provided. In particular, the steel wire material of the above aspect is suitable as a material for a steel wire used for reinforcing the strength of a power transmission line with little power loss.
Moreover, the steel wire obtained by subjecting the steel wire material of the above aspect to cold drawing and, after cold drawing, aluminum plating treatment, if necessary, has high strength and electrical resistivity. Is low. Therefore, when an ACSR is manufactured using this steel wire, a predetermined strength can be secured and an ASCR with a low electrical resistivity can be obtained. Therefore, the industrial contribution is very remarkable.

The steel wire according to this embodiment will be described.
First, the reason for limiting the chemical components of the steel wire in this embodiment will be described. In addition,% in the following description means the mass%.

C: 0.8% to 1.1%
C is an element effective for increasing the tensile strength by setting the metal structure of the steel wire to pearlite.
When the C content is less than 0.8%, for example, it is difficult to stably impart high strength with a tensile strength of about 1350 MPa to the steel wire. Therefore, the lower limit for the C content is 0.8%. In order to obtain more uniform pearlite and increase the tensile strength, the C content is preferably 0.9% or more, more preferably 1.0% or more.
On the other hand, when there is too much C content, a steel wire will become hard and a drawability will fall. In particular, when the C content exceeds 1.1%, it is difficult to industrially stably suppress the formation of cementite precipitated along the prior austenite grain boundaries, that is, proeutectoid cementite. The wire drawing workability is greatly reduced. Therefore, the upper limit of the C content is 1.1%.

Si: 0.02% to 0.30%
Si is an element effective for increasing the strength of a steel wire by solid solution strengthening, and is also an element necessary as a deoxidizer.
If the Si content is less than 0.02%, these effects are not sufficient. Therefore, the lower limit for the Si content is 0.02%. Moreover, in order to ensure strength by solid solution strengthening and further enjoy the deoxidation effect more stably, the Si content is preferably 0.05% or more.
On the other hand, as the Si content increases, the electrical resistivity increases. In particular, when the Si content exceeds 0.30%, it is impossible to achieve both improvement in tensile strength and reduction in electrical resistivity. Therefore, the upper limit of Si content is set to 0.30%. In order to obtain a lower electrical resistivity, the Si content is preferably 0.20% or less, more preferably 0.10% or less.

Mn: 0.1% to 0.6%
Mn is an element that has the effect of preventing hot brittleness by increasing the strength of the steel wire and fixing S in the steel wire as MnS.
If the Mn content is less than 0.1%, these effects are not sufficient. Therefore, the lower limit of the Mn content is 0.1%. Furthermore, in order to ensure strength and prevent hot brittleness, the Mn content is preferably 0.2% or more, more preferably 0.3% or more.
On the other hand, when the Mn content increases, the electrical resistivity increases. In particular, when the Mn content exceeds 0.6%, it is impossible to achieve both improvement in tensile strength and reduction in electrical resistivity. Therefore, the upper limit of the Mn content is set to 0.6%. In order to obtain a lower electrical resistivity, the Mn content is preferably 0.5% or less, more preferably 0.4% or less.

Cr: 0.3% to 1.5%
Cr has the effect of reducing the average lamellar spacing of pearlite and increasing the tensile strength of the steel wire. Further, when Cr dissolves in ferrite in pearlite, the electrical resistivity increases. Therefore, there is an effect of suppressing an increase in electrical resistivity by increasing the Cr content in cementite and relatively lowering the Cr content in ferrite.
If the Cr content is less than 0.3%, sufficient tensile strength of the steel wire cannot be secured, and the Cr content in cementite cannot be increased. Therefore, in order to achieve both improvement in tensile strength and reduction in electrical resistivity, the Cr content needs to be 0.3% or more. In order to obtain higher tensile strength, the Cr content is preferably 0.4% or more, and more preferably 0.5% or more.
On the other hand, if the Cr content exceeds 1.5%, the wire drawing workability of the steel wire is deteriorated. Therefore, the upper limit of Cr content is 1.5%. In order to further suppress a decrease in wire drawing workability, the Cr content is preferably 1.0% or less, and more preferably 0.8% or less.

Al: 0.01% to 0.05%
Al is an element having a deoxidizing effect and is an element necessary for reducing the amount of oxygen in the steel wire.
If the Al content is less than 0.01%, this effect is insufficient. Therefore, the lower limit of the Al content is 0.01%. In order to obtain a more deoxidizing effect, the Al content is preferably 0.02% or more.
On the other hand, Al is an element that forms hard oxide inclusions and degrades the ductility of the steel wire. In particular, if the Al content exceeds 0.05%, coarse oxide inclusions are likely to be formed, so that the wire drawing workability of the steel wire material is significantly reduced. Therefore, the upper limit of the Al content is set to 0.05%. In order not to lower the wire drawing workability of the steel wire material, the Al content is preferably 0.04% or less, more preferably 0.03% or less.

  In the steel wire material according to the present embodiment, N, P, and S need to be restricted as follows.

N: 0.008% or less N is an element that adheres to dislocations in steel during cold wire drawing and reduces wire drawing workability. In particular, when the N content exceeds 0.008%, the wire drawing workability deteriorates remarkably. Therefore, the N content is limited to 0.008% or less. Preferably it is 0.005% or less, More preferably, it is 0.004% or less.
In addition, the minimum of N content contains 0%. However, considering the current refining technology and manufacturing costs, the lower limit of the N content is preferably 0.0001%.

P: 0.03% or less P is an element that segregates at the grain boundary to lower the wire drawing workability. In particular, when the P content exceeds 0.03%, the wire drawing workability is significantly lowered. Therefore, the P content is limited to 0.03% or less. Preferably it is 0.02% or less, More preferably, it is 0.01% or less.
In addition, the minimum of P content contains 0%. However, considering the current refining technology and manufacturing costs, the lower limit of the P content is preferably 0.001%.

S: 0.02% or less S, like P, is an element that reduces wire drawing workability. In particular, when the S content exceeds 0.02%, the wire drawing workability is significantly lowered. Therefore, the S content is limited to 0.02% or less. Preferably it is 0.01% or less.
In addition, the minimum of S content contains 0%. However, considering the current refining technology and manufacturing costs, the lower limit of the S content is preferably 0.001%.

The above is the basic component composition of the steel wire according to this embodiment, and the balance is iron and impurities. The “impurities” in “the balance is Fe and impurities” refers to what is inevitably mixed from ore as a raw material, scrap, or the manufacturing environment when steel is produced industrially.
However, in the steel wire in this embodiment, in addition to this basic component, instead of a part of the remaining Fe, one or more selected from the group consisting of Mo, V, Ti, Nb and B are selectively used. You may make it contain.

Mo: 0.20% or less The addition of Mo is arbitrary, and the lower limit of its content is 0%.
However, by adding Mo, it is possible to stably enjoy the effect of increasing the balance between the tensile strength and the electrical resistivity of the steel wire. In order to obtain this effect, it is preferable to add 0.02% or more of Mo. More preferably, the Mo content is 0.05% or more.
On the other hand, if the Mo content exceeds 0.20%, a martensitic structure is likely to be generated in the steel, and the wire drawing workability may be reduced. Therefore, the upper limit of the Mo content is preferably 0.20%. More preferably, the upper limit of the Mo content is 0.10%.

V: 0.15% or less The addition of V is arbitrary, and the lower limit of its content is 0%.
However, V has the effect of reducing the pearlite block size by forming carbides or carbonitrides in the steel wire. Therefore, the drawability can be improved by adding V. In order to acquire this effect, it is preferable to add V 0.02% or more. More preferably, the V content is 0.05% or more.
On the other hand, if the V content exceeds 0.15%, coarse carbides or carbonitrides are likely to be formed in the steel wire, and the wire drawing workability may be reduced. Therefore, the upper limit of V content is preferably 0.15%. More preferably, the upper limit of V content is 0.08%.

Ti: 0.050% or less The addition of Ti is arbitrary, and the lower limit of its content is 0%.
However, Ti has the effect of reducing the pearlite block size by forming carbides or carbonitrides in the steel wire. Therefore, the wire drawing workability can be improved by adding Ti. In order to obtain this effect, 0.002% or more of Ti is preferably added. More preferably, the Ti content is 0.005% or more.
On the other hand, if the Ti content exceeds 0.050%, coarse carbides or carbonitrides are likely to be formed in the steel wire, and the wire drawing workability may be reduced. Therefore, the upper limit of the Ti content is preferably 0.050%. More preferably, the upper limit of the Ti content is 0.030%.

Nb: 0.050% or less The addition of Nb is arbitrary, and the lower limit of its content is 0%.
However, Nb has the effect of reducing the pearlite block size by forming carbides or carbonitrides in the steel wire. Therefore, the wire drawing workability can be improved by adding Nb. In order to acquire this effect, it is preferable to add Nb 0.002% or more. More preferably, the Nb content is 0.005% or more.
On the other hand, when the Nb content exceeds 0.050%, coarse carbides or carbonitrides are likely to be formed in the steel wire, and the wire drawing workability may be lowered. Therefore, the upper limit of the Nb content is preferably 0.050%. More preferably, the upper limit of the Nb content is 0.020%.

B: 0.0030% or less The addition of B is arbitrary, and the lower limit of its content is 0%.
However, B combines with N dissolved in the steel wire to form BN, and has the effect of reducing the solid solution N. Therefore, the wire drawing workability can be improved by adding B. In order to acquire this effect, it is preferable to add B 0.0003% or more. More preferably, the B content is 0.0007% or more.
On the other hand, if the B content exceeds 0.0030%, coarse carbides are likely to be formed in the steel wire, and the wire drawing workability may be reduced. Therefore, the upper limit of the B content is preferably 0.0030%. More preferably, the upper limit of the B content is 0.0020%.

  Next, the structure of the steel wire according to this embodiment will be described.

The structure of the steel wire according to the present embodiment includes pearlite in which ferrite and cementite have a layered lamellar structure. The main structure of the steel wire according to this embodiment is pearlite. Here, the “main structure” means a structure that occupies 85% or more in area ratio in the C section perpendicular to the longitudinal direction of the steel wire or in the L section parallel to the longitudinal direction of the steel wire. The area ratio of pearlite can be obtained by subtracting the area ratio of the non-pearlite structure from 100%. The area ratio of pearlite is 85% or more, preferably 90% or more, and more preferably 95% or more. The area ratio of pearlite may be 100%.
The remainder of the structure of the steel wire according to the present embodiment, that is, the structure other than pearlite is composed of a non-pearlite structure such as pro-eutectoid ferrite, bainite, pseudo-pearlite, and pro-eutectoid cementite. When the area ratio of the non-pearlite structure exceeds 15%, the wire drawing workability deteriorates. Therefore, the area ratio of the non-pearlite structure is 15% or less. The area ratio of the non-pearlite structure is preferably 10% or less, more preferably 5% or less. Note that the area ratio of the non-pearlite structure may be 0%.

The area ratio of pearlite can be obtained as follows.
For example, as shown in an example described later, in a steel wire sample, a C section perpendicular to the longitudinal direction of the steel wire is mirror-polished and then the C section is corroded with nital.
Next, with respect to the sample corroded with nital, 10 fields of view are photographed at arbitrary magnifications using a SEM at a magnification of 5000 times. The area per field of view is 3.6 × 10 ⁻⁴ mm ² .
Using the obtained SEM photograph of each field of view, the area ratio of pearlite in each field of view can be determined by a normal image analysis method.
And the area ratio of the pearlite of the steel wire is obtained by averaging the area ratio of the obtained pearlite for 10 visual fields.

Perlite average lamella spacing: 50 nm to 100 nm
The tensile strength of the steel wire can be improved by reducing the average lamella spacing of the pearlite described above. Note that the influence of the average lamella spacing on the electrical resistivity is not so great. Therefore, in order to achieve both improvement in the tensile strength of the steel wire and reduction in electrical resistivity, it is necessary to reduce the average lamella spacing. When the average lamella spacing of pearlite exceeds 100 nm, the effect of improving the tensile strength becomes insufficient. Therefore, in the steel wire according to the present embodiment, the average lamella spacing of pearlite is set to 100 nm or less in order to obtain this effect. The average lamella spacing of pearlite is preferably 75 nm or less.
On the other hand, in order to make the average lamella spacing of pearlite less than 50 nm, it is necessary to transform at a low temperature. However, when transformed at a low temperature, the area ratio of the non-pearlite structure such as bainite exceeds 15%, and the wire drawing workability of the steel wire is deteriorated. Therefore, the average lamella spacing of pearlite is 50 nm or more. The average lamella spacing of pearlite is preferably 55 nm or more.

The average lamella spacing of pearlite can be measured as follows. For example, as shown in the Example mentioned later, after grind | polishing C cross section of the sample of a steel wire rod, this C cross section is etched and pearlite appears. Next, the C cross section in which this pearlite is exposed is photographed with a scanning electron microscope (SEM) in a plurality of fields of view, and a structure photograph of the sample is obtained. The average lamella spacing of pearlite can be measured from the obtained tissue photograph.
Specifically, it can be measured by the following method. First, a 10-view tissue photograph is taken using an SEM. From the photographed tissue photographs of 10 visual fields, a plurality of locations where the lamellas can be measured for 5 intervals are selected in the range where the lamellas are aligned in the visual field. For a plurality of selected locations, a straight line is drawn perpendicular to the lamella to determine the length of the lamella for five intervals. Next, of the selected plurality of locations, two locations are selected from the one having a smaller length for 5 intervals. Then, at the two selected locations, the lamella spacing at each location can be determined by dividing the length of the 5 measured lamella spacing by 5, respectively. That is, two lamella intervals can be obtained in one field of view. The average value of the 10 visual fields thus obtained and the total 20 lamella intervals can be set as the “perlite average lamella interval” of the sample.

  As described above, in order to improve the tensile strength of the steel wire, it is effective to reduce the average lamella spacing of pearlite. Thus, in order to reduce the average lamella spacing of pearlite, it is preferable to set the cooling rate in the cooling step after hot rolling to 50 ° C./second or more and then to perform pearlite transformation at a low temperature of about 600 ° C. .

  Electricity flows mainly in the ferrite part of the pearlite. Cr has the effect of increasing the electrical resistivity when dissolved in ferrite in pearlite. Therefore, if the Cr content contained in the ferrite in the pearlite can be reduced, the electrical resistivity can be reduced. That is, it is possible to suppress an increase in the electrical resistivity of the steel wire by concentrating Cr to cementite in the pearlite and relatively reducing the Cr content contained in the ferrite in the pearlite. Note that Cr is an element that is easily concentrated to cementite in pearlite. Therefore, by controlling the heat treatment conditions, the Cr content contained in the cementite in the pearlite can be increased, and the Cr content contained in the ferrite in the pearlite can be reduced.

Si is an element contributing to solid solution strengthening. Therefore, when the Si content in the steel wire increases, the strength of the steel wire can be improved. However, on the other hand, the electrical resistivity increases as the Si content in the steel wire increases. Therefore, high tensile strength and low electrical resistivity can both be achieved by increasing the Cr content contained in the cementite in the pearlite as the Si content increases.
In the steel wire according to the present embodiment, in order to obtain these effects, the Si content, the Cr content contained in the cementite in the pearlite, and the Cr content contained in the ferrite in the pearlite are expressed by the following formula (%): It is important to satisfy 1). Here, in the following formula (1), by mass%, the Si content is [% Si], the Cr content contained in the cementite in the pearlite is [% Crθ], and is contained in the ferrite in the pearlite. The Cr content is [% Crα].
([% Crθ] / [% Crα]) ≧ (2.0 + [% Si] × 10) (1)

  In the steel wire in the present embodiment, the tensile strength is controlled by controlling the Cr content contained in the cementite in the pearlite according to the Si content contributing to the solid solution strengthening so as to satisfy the above formula (1). It is possible to control so as to achieve both an improvement in thickness and a reduction in electrical resistivity.

The Cr content contained in the cementite in pearlite, that is, [% Crθ] in the above formula (1) can be determined by, for example, chemical analysis of the residue extracted by electrolysis. Specifically, it can be determined by the following method. First, after cutting the steel wire according to the present embodiment to a size suitable for electrolysis, a current density is set to 250 to 350 A / m ² using a 10% AA-based electrolytic solution which is a general condition for electrolytic polishing. Set and electrolyze to extract the solution. Next, the extracted solution is filtered through a filter having a mesh size of 0.2 μm to obtain a residue. The filtrate, that is, the residue can be obtained by performing a general chemical analysis. Here, as a general chemical analysis, for example, there is a method in which a residue is dissolved with an acid solution, and the solution is analyzed by ICP emission spectroscopy. In the steel wire according to the present embodiment, the metal elements contained in the cementite in the pearlite are substantially Fe, Mn, and Cr, and among them, the possibility that Fe and Cr are extracted from other than the cementite is low. Since Mn can form MnS more easily than cementite, the Cr content contained in cementite in pearlite, that is, [% Crθ], can be calculated using the following formula (2). Here, in the following formula (2), the Cr content, the Fe content and the Mn content contained in the residue in mass% are [% residue Cr], [% residue Fe] and [% residue Mn], respectively. Further, the S content contained in the steel wire is defined as [% S].
[% Crθ] = 100 × [% residue Cr] / {[% residue Fe] + [% residue Mn] + [% residue Cr] − [% S] × (55/32)} (2)
In addition, in order to achieve improvement in tensile strength and reduction in electrical resistivity, the Cr content contained in cementite in pearlite is preferably 0.80% to 5.80% in mass%.

The Cr content contained in ferrite in pearlite is based on the premise that C hardly dissolves in ferrite, for example, the Cr content of the entire steel wire, that is, [% Cr], and cementite in pearlite. It can be calculated from the Cr content contained, that is, [% Crθ], and the volume fraction of cementite determined by the C content, ie, [φθ]. Specifically, since C hardly dissolves in ferrite, it is known that the volume fraction of cementite in pearlite is generally obtained by the following formula (3). In the following formula (3), the C content in mass% is [% C], and the volume fraction of cementite in pearlite is [φθ].
The coefficient 0.149 in the following formula (3) can be obtained from 6.69% by mass C of the cementite composition and 7.68 g / cm ³ of the density of cementite.
[Φθ] = [% C] × 0.149 (3)
Next, the volume fraction of ferrite in pearlite, that is, [φα] is obtained by the following equation (4).
[Φα] = 1.0− [φθ] (4)
From the above, the Cr content contained in the ferrite in the pearlite, that is, [% Crα] can be calculated using the following formula (5).
[% Crα] = {[% Cr] − ([% Crθ] × [φθ])} / [φα] (5)

  As described above, in order to suppress an increase in the electrical resistivity of the steel wire, it is effective to concentrate Cr in cementite. Thus, in order to concentrate Cr in cementite, after completing the pearlite transformation from austenite, it is preferable to hold in that temperature range and concentrate Cr to cementite. However, after the completion of the pearlite transformation, if the time for holding in that temperature range becomes long, the cementite in the pearlite may be spheroidized and the strength of the steel wire may be reduced.

  The tensile strength TS of the steel wire according to this embodiment is preferably 1350 MPa or more. Moreover, the absolute value of the tensile strength TS of the steel wire is preferably 64 times or more the absolute value of the electrical resistivity ρ expressed in μΩ · cm.

  When the steel wire according to the present embodiment is applied to the core material of the ACSR, if the strength of the steel wire is low, it may be necessary to increase the steel wire to reinforce the strength. In this case, it is assumed that the electrical resistivity of the entire ACSR increases. Therefore, the tensile strength TS of the steel wire according to this embodiment is preferably 1350 MPa or more, more preferably 1400 MPa or more, and further preferably 1500 MPa or more. In addition, by making the tensile strength TS of the steel wire 1350 MPa or more, for example, in the case of a wire diameter of 11 mm to 5 mm, a true strain of 1.6, which is a general wire drawing amount, The tensile strength of the steel wire can be 1900 MPa or more.

  Moreover, in the steel wire according to the present embodiment, from the viewpoint of achieving both high strength and low electrical resistivity of the steel wire, the electrical resistivity ρ represented by the absolute value and unit of tensile strength TS is μΩ · cm. In the relationship with the absolute value of, it is preferable to set the following numerical range.

Absolute strength of tensile strength TS of wire rods manufactured under general hot rolling conditions using steels having SWRS72B and SWRS82B chemical components that are generally used for core materials of ACSR and specified in JIS G 3502 The value is about 55 times the absolute value of the electrical resistivity ρ. The unit of tensile strength of the wire is MPa, and the unit of electrical resistivity is μΩ · cm.
Therefore, as described above, when the absolute value of the tensile strength TS of the wire is 55 times the absolute value of the electrical resistivity ρ expressed in μΩ · cm, that is, 55 times as a reference. In the steel wire according to the present embodiment, the absolute value of the tensile strength TS is the absolute value of the electrical resistivity ρ expressed in μΩ · cm so that the value is 15% or more of the reference value. It is preferably 64 times or more. Moreover, it is more preferable that the absolute value of the tensile strength TS is 67 times or more the absolute value of the electrical resistivity ρ expressed in μΩ · cm so that the absolute value is 20% or more of the reference value.

  Thus, the tensile strength TS of the steel wire is set to 1350 MPa or more, and the absolute value of the tensile strength TS of the steel wire is 64 times the absolute value of the electrical resistivity ρ expressed in μΩ · cm. By setting it as the above, the intensity | strength of a steel wire can be raised and an electrical resistivity can be reduced. As a result, the number of reinforcements when the steel wire is applied to the core material of ACSR can be reduced. Furthermore, it is possible to suppress an increase in electrical resistivity in the entire ACSR, heat generation during power transmission can be suppressed, and stable power transmission efficiency can be ensured.

In the steel wire according to the present embodiment, the absolute value of the electrical resistivity ρ expressed in μΩ · cm is not particularly limited. That is, in the steel wire according to the present embodiment, the absolute value of the tensile strength TS and the absolute value of the electrical resistivity ρ expressed in μΩ · cm satisfy the following formula (6). preferable.
Absolute value of tensile strength TS ≥ absolute value of electrical resistivity ρ x 64 (6)
By setting it as the steel wire which satisfies the said Formula (6), the tensile strength markedly larger than before can be ensured. As a result, it is possible to reduce the number of reinforcements when such steel wire is reinforced by applying it to the core material of the ACSR, and it is possible to suppress an increase in electrical resistivity in the entire ACSR.
Note that the electrical resistivity ρ of the steel wire in the present embodiment is preferably as low as possible, and the tensile strength TS is as large as possible.

  By satisfying the above-described chemical composition and structure, it is possible to obtain a steel wire material that achieves both improvement in strength and reduction in electrical resistivity. In order to obtain the steel wire described above, the steel wire may be manufactured by a manufacturing method described later. Next, the preferable manufacturing method of the steel wire which concerns on this embodiment is demonstrated.

  The steel wire according to this embodiment can be manufactured as follows. In addition, the manufacturing method of the steel wire material demonstrated below is an example for obtaining the steel wire material which concerns on this embodiment, and is not limited by the following procedures and methods, What is the method which can implement | achieve the structure of this invention. Any method can be employed.

First, after melting steel so that it may become said chemical component, a steel piece is manufactured by continuous casting and hot rolling is performed. In addition, you may perform lump rolling after continuous casting. When the obtained steel slab is hot-rolled, it is heated by a general method so that the center part of the steel slab is 1000 ° C. to 1100 ° C., and the finishing temperature is 900 ° C. to 1000 ° C. To do. After finish rolling, the hot-rolled wire is primarily cooled to 700 ° C. or lower by combining water cooling and air cooling. The average cooling rate in the primary cooling is preferably 50 ° C./second or more. After primary cooling, in order to transform the pearlite, the wire is immersed in a nitrate-based molten salt at 500 ° C. to 530 ° C., and then secondarily cooled to 590 ° C. to 620 ° C. And Cr can be concentrated to cementite by hold | maintaining the wire after secondary cooling to the molten salt whose bath temperature is 550 to 570 degreeC for 30 to 50 seconds. After that, the molten salt is removed with spray water, and after tertiary cooling to room temperature, winding is performed. The winding may be performed immediately after the primary cooling or the secondary cooling.
In the secondary cooling described above, the average cooling rate is preferably 30 ° C./second or more. Moreover, in the holding | maintenance after secondary cooling, it is preferable to hold | maintain for 30 to 50 second in the range from which the temperature of a wire becomes 600 to 550 degreeC. For example, a lead bath or a fluidized bed furnace may be used. When a lead bath is used, it is not necessary to lower the temperature to 700 ° C. during the primary cooling, and the secondary cooling and holding may be performed in the same lead bath. In this case, it is preferable to hold in a lead bath at 550 ° C. to 600 ° C. for 35 to 60 seconds.

As a cooling and holding method after finish rolling, cooling and holding can be performed only with a lead bath. For example, when the temperature of the wire rod after finish rolling is in the range of 900 ° C to 700 ° C, the average cooling rate of the wire rod when immersed in a lead bath having a lead bath temperature of 640 ° C to 500 ° C is 100 ° C / second. ~ 200 ° C / sec.
Moreover, when the temperature of the wire rod after finish rolling is in the range of 700 ° C. to 620 ° C., the average cooling rate of the wire rod is 40 ° C./second to 50 ° C./second when the temperature of the lead bath is 590 ° C. to 600 ° C. Yes, when the temperature of the lead bath is 550 ° C. to 560 ° C., the average cooling rate of the wire is 60 ° C./second to 70 ° C./second, and when the temperature of the lead bath is 490 ° C. to 500 ° C. The average cooling rate is 90 ° C./second to 100 ° C./second.

  In addition, the finishing temperature in the above-mentioned hot rolling refers to the surface temperature of the steel wire immediately after finish rolling. Furthermore, the average cooling rate in the cooling after finish rolling refers to the cooling rate of the surface of the steel wire.

  Hereinafter, the example of the steel wire rod of the present invention will be given, and the effect of the steel wire rod according to this embodiment will be described more specifically. However, the conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is not limited to the following examples. As long as the object of the present invention is achieved without departing from the gist of the present invention, the present invention can be implemented with appropriate modifications within a range that can be adapted to the gist. Therefore, the present invention can employ various conditions, all of which are included in the technical features of the present invention.

Steels A to Y having the chemical compositions shown in Table 1 were melted in a 50 kg vacuum melting furnace and then cast into an ingot. The chemical composition of Steel V satisfied JIS standard SWRS82B.
Each of the ingots was heated at 1250 ° C. for 1 hour, then hot forged to a diameter of 15 mm so that the finishing temperature was 950 ° C. or higher, and then allowed to cool to room temperature to obtain a hot forged material. This hot forged material was cut and cut to obtain a cut material having a diameter of 10 mm and a length of 1000 mm.

Next, each obtained cutting material was heated in a nitrogen atmosphere at 1050 ° C. for 15 minutes, so that the center temperature of the cutting material was 1000 ° C. or higher. Then, the diameter of the cutting material was hot-rolled to 7 mm so that the finishing temperature was in the range of 950 ° C. or higher and 1000 ° C. or lower to obtain a wire. Further, the wire was immersed and held in a lead bath under the conditions shown in Table 2 in a state where the temperature of the wire was 900 ° C. or higher. Thereafter, the wire was taken out of the lead bath and allowed to cool to room temperature to obtain a steel wire.
When the temperature of the wire rod after finish rolling was in the range of 900 ° C to 700 ° C, when the lead bath temperature was 640 ° C to 500 ° C, the average cooling rate of the wire rod was 100 ° C / second to 200 ° C / second.
When the temperature of the wire rod after finish rolling is in the range of 700 ° C to 620 ° C, the average cooling rate of the wire rod is 40 ° C / second to 50 ° C / second when the lead bath temperature is 590 ° C to 600 ° C. When the temperature of 550 ° C. to 560 ° C., the average cooling rate of the wire is 60 ° C./sec to 70 ° C./sec. When the temperature of the lead bath is 490 ° C. to 500 ° C., the average cooling rate of the wire is 90 ° C./sec. It was 100 ° C./second.
For comparison, some of the obtained cutting materials were hot-rolled to a diameter of 7 mm to obtain a wire, and then released in the atmosphere without being immersed in a molten salt or a lead bath. The steel wire was obtained by cooling to room temperature by cooling or cooling with an electric fan. The average cooling rate of the wire when allowed to cool in the atmosphere is 7 ° C./second to 8 ° C./second when the temperature of the wire after finish rolling is 900 ° C. to 700 ° C., and the temperature of the wire after finish rolling However, in the range of 700 ° C. to 620 ° C., it was 4 ° C./second to 5 ° C./second. The average cooling rate of the wire when it is cooled by an electric fan is 12 ° C / second to 14 ° C / second when the temperature of the wire after finish rolling is in the range of 900 ° C to 700 ° C, and the temperature of the wire after finish rolling is In the range of 700 ° C. to 620 ° C., it was 6 ° C./second to 7 ° C./second.

  Each test shown below was implemented and evaluated about the steel wire of the test numbers 1-48 manufactured on said each conditions.

For each steel wire, the C section perpendicular to the longitudinal direction of the steel wire was mirror-polished and then corroded with nital.
In order to obtain the area ratio of pearlite, a sample corroded with nital was photographed with 10 fields of view at any magnification at a magnification of 5000 using an SEM. The area per field of view was 3.6 × 10 ⁻⁴ mm ² .
Next, in the photograph of each field of view, the area ratio of the pearlite portion was determined by a normal image analysis method. The average value of the area ratio of pearlite for 10 fields of view was defined as the area ratio of pearlite of the steel wire.
In addition, in order to obtain the average lamella spacing of pearlite, a sample corroded with nital was photographed at 10 magnifications using a SEM at a magnification of 10,000 times in 10 fields. The area per field of view was 9.0 × 10 ⁻⁵ mm ² .
Next, a range in which the directions of the pearlite lamellas were aligned was selected in each field of view. Next, 5 lamella intervals can be measured, and the length of the 5 lamella intervals is drawn by drawing a straight line perpendicular to the lamella at the location where the lamella interval is the smallest and the location where the lamella interval is the second smallest. Asked. And the length for 5 intervals of the obtained lamella was divided by 5, and the lamella space | interval of the pearlite of each location was calculated | required. The average value of the lamella spacing at 20 locations in total for 10 visual fields thus obtained was defined as the average lamella spacing of the pearlite of the steel wire.

After cutting the steel wire to a diameter of 6 mm by cutting, electrolysis was performed using a 10% AA electrolyte solution, which is a general condition for electrolytic polishing, with a current density set to 250 to 350 A / m ^2. Extracted. The 10% AA electrolyte solution described above was a 10 volume% acetylacetone-1 mass% tetramethylammonium chloride-methanol solution. Next, the extracted solution is filtered through a filter having a mesh size of 0.2 μm to obtain a residue. The residue is dissolved with an acid solution, and the solution is analyzed by ICP emission spectroscopy. The amount [% residue Cr], the Fe content [% residue Fe] and the Mn content [% residue Mn] were obtained. And Cr content contained in the cementite in pearlite, ie, [% Crθ], was calculated using the following formula (A). “The metal element in cementite is substantially composed of Fe, Mn, and Cr”, “Fe and Cr are not extracted from other than cementite” and “When S is contained in steel, Mn is The precondition was that “MnS is formed before cementite”. Here, in the following formula (A), the Cr content, Fe content, and Mn content contained in the residue in mass% are [% residue Cr], [% residue Fe], and [% residue Mn], respectively. In addition, the S content contained in the steel wire rod is defined as [% S].
[% Crθ] = 100 × [% residue Cr] / {[% residue Fe] + [% residue Mn] + [% residue Cr] − [% S] × (55/32)} (A)

The Cr content in the ferrite was calculated as follows. First, after obtaining the volume fraction [φθ] of cementite in pearlite by the following formula (B), the volume fraction [φα] of ferrite in pearlite was obtained by the following formula (C). Thereafter, the Cr content [% Crα] in the ferrite was calculated by the following formula (D).
In addition, in the following formula (B), the C content of the entire steel wire in mass% is [% C], and in the following formula (D), the Cr content of the entire steel wire in [% Cr] is [% Cr]. It was.
[Φθ] = [% C] × 0.153 (B)
[Φα] = 1.0− [φθ] (C)
[% Crα] = {[% Cr] − ([% Crθ] × [φθ])} / [φα] (D)

  By cutting, two tensile test pieces each having a diameter of 3.2 mm (a circle with a radius of 1.6 mm centered on the center of the C cross section) and a length of 18 mm are collected from the center of the C cross section of each steel wire. Then, a tensile test at normal temperature was performed by a method based on JIS Z 2241, and the tensile strength TS was measured. And the measured average value was made into the tensile strength TS of the steel wire. The unit of tensile strength TS is MPa.

  As a test piece for measuring the electrical resistivity ρ, a 3.0 mm × 4.0 mm × 60 mm rectangular parallelepiped test piece is collected from the center of each steel wire, and at a temperature of 20 ° C. by a normal four-terminal method, The electrical resistivity was measured. The unit of the obtained electrical resistivity ρ is μΩ · cm.

The obtained evaluation results are shown in Tables 3 and 4. In addition, [% Crθ] and [% Crα] in Table 3 and Table 4 are mass%, respectively, “Cr content contained in cementite in pearlite”, “Cr content contained in ferrite in pearlite” Is shown.
In Tables 3 and 4, when the above formula (1) was satisfied, it was accepted and indicated by “◯”, and when it was not satisfied, it was rejected and indicated by “x”.

Further, the tensile strength TS of the steel wire expressed in MPa, the absolute value of the electrical resistivity ρ expressed in μΩ · cm, and the value 64 times the absolute value of the electrical resistivity ρ are shown in Table 3. And in Table 4.
In Tables 3 and 4, the case where the absolute value of the tensile strength TS is 64 times or more the absolute value of the electrical resistivity ρ expressed in μΩ · cm is judged as “good” and “good” ", And the case of less than 64 times was judged as" bad "and indicated by" x ".

From Table 3 and Table 4, for test numbers 1, 3, 6, 7, 9-11, 14, 17, 18, 20-22, 24, 26, 27, 30, 33, 34, 36, and 44-47 In the relationship between the chemical composition defined in the present invention, the structure, the average lamella spacing of pearlite, the Si content and the Cr content contained in the cementite in the pearlite and the Cr content contained in the ferrite in the pearlite, at least One technical feature was not met. In Test Nos. 45 and 47, the wire drawing workability was lowered.
On the other hand, test numbers 2, 4, 5, 8, 12, 13, 15, 16, 19, 23, 25, 28, 29, 31, 32, 35, 37-43 and 48 are defined in the present invention. The chemical composition, structure, average lamella spacing of pearlite, Si content, Cr content contained in cementite in pearlite, and Cr content contained in ferrite in pearlite were all satisfied.

  According to the present invention, a steel wire having high strength and low electrical resistivity can be obtained, and the industrial contribution is extremely remarkable.

Claims (3)

As a chemical component,
C: 0.8% to 1.1%
Si: 0.02% to 0.30%,
Mn: 0.1% to 0.6%,
Cr: 0.3% to 1.5%
Al: 0.01% to 0.05%,
Containing
N: 0.008% or less,
P: 0.03% or less,
S: limited to 0.02% or less, selectively,
Mo: 0.20% or less,
V: 0.15% or less,
Ti: 0.050% or less,
Nb: 0.050% or less, and B: containing one or more selected from the group consisting of 0.0030% or less,
The balance consists of Fe and impurities;
The tissue includes pearlite, and the area ratio of the pearlite is 85% or more;
An average lamellar spacing of the pearlite is 50 nm to 100 nm,
In mass%, the Si content is [% Si], the Cr content in the cementite in the pearlite is [% Crθ], and the Cr content in the ferrite in the pearlite is When [% Crα], the [% Si], the [% Crθ] and the [% Crα] satisfy the following formula (1);
A steel wire characterized by this.
([% Crθ] / [% Crα]) ≧ (2.0 + [% Si] × 10) (1) As the chemical component,
Mo: 0.02% to 0.20%
V: 0.02% to 0.15%,
Ti: 0.002% to 0.050%,
Nb: 0.002% to 0.050%, and B: 0.0003% to 0.0030%
The steel wire according to claim 1, comprising at least one selected from the group consisting of:   The tensile strength TS of the steel wire is 1350 MPa or more, and the absolute value of the tensile strength TS of the steel wire is the absolute value of the electrical resistivity ρ represented by the unit μΩ · cm of the steel wire. It is 64 times or more, The steel wire rod of Claim 1 or 2 characterized by the above-mentioned.