EP3988678B1

EP3988678B1 - Wire rod

Info

Publication number: EP3988678B1
Application number: EP20827854.9A
Authority: EP
Inventors: Naoki Matsui; Hiroshi Ooba; Makoto Okonogi; Toshihiko TESHIMA
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2019-06-19
Filing date: 2020-06-19
Publication date: 2023-12-06
Anticipated expiration: 2040-06-19
Also published as: JP7226548B2; WO2020256140A1; CN113748224A; EP3988678A1; CN113748224B; JPWO2020256140A1; EP3988678A4

Description

Technical Field

The present invention relates to a wire rod.

Background Art

In recent years, high-strength steel wires used in the fields of high-strength rope steel wires, bridge cable steel wires, and PC steel wires and the like are required to have ultimate tensile strength of 1700 MPa or more. These high-strength steel wires are manufactured, for example, by subjecting a rolled wire rod having a diameter of from 5.0 to 16.0 mm to a patenting treatment to form a metal structure into a pearlite structure, and then subjecting the wire rod to wire drawing processing.
In a case in which the tensile strength of the steel wire after the wire drawing processing is high, there is a high possibility that the steel wire is affected by strain aging due to processing heat generated in the wire drawing processing, and becomes brittle. In the high-strength steel wire, the strain aging causes a decrease in the number of rotations (torsion value) until breakage in a torsion test. Furthermore, longitudinal cracking called delamination may occur. The occurrence of delamination in the torsion test causes breakage in a stranded wire process for making the steel wire into a product, which causes deteriorated manufacturability. Therefore, the high-strength steel wire desirably achieves both the tensile strength and the torsion properties.
As the strength of the steel wire is higher, the risk of breakage due to the progress of corrosion or hydrogen embrittlement is higher in a case in which the steel wire or stranded wire product is used in a corrosive environment. Therefore, it is desired that the high-strength steel wire used in the above field and the wire rod as a raw material thereof have excellent corrosion resistance and hydrogen embrittlement resistance.
As a technique for improving the torsion properties of the high-strength steel wire, for example, Patent Document 1 proposes a steel wire which contains, in % by mass, C: from 0.75 to 1.10%, Si: from 0.10 to 1.40%, Mn: from 0.10 to 1.0%, Al: from 0 to 0.10%, Ti: from 0 to 0.10%, Cr: from 0 to 0.60%, V: from 0 to 0.10%, Nb: from 0 to 0.10%, Mo: from 0 to 0.20%, W: from 0 to 0.50%, B: from 0 to 0.0030%, and N: 0.006% or less, and is limited to P: 0.03% or less, and S: 0.03% or less, with the balance substantially consisting of Fe. The steel wire includes a metal structure in which a drawn pearlite occupies an area ratio of 90% or more in a region of the axis side with respect to a depth of 100 µm from the surface, and a drawn pearlite occupies an area ratio of 70% or more in a region up to a depth of 100 µm from the surface, on an L cross section along the axial direction including the axis of the steel wire. A diameter (D [mm]) of the steel wire, standard deviation (σHV) of Vickers hardness of the surface of the steel wire, and yield strength (Rp 0.2) of the steel wire satisfy the following Formula (1). The steel wire has high tensile strength of 1770 MPa or more. $σ HV < (- 9500 \times \ln (d) + 30000) \times \exp (- 0.003 \times Rp0 .2)$
As a technique for improving the hydrogen embrittlement resistance of a high-strength steel wire, for example, Patent Document 2 proposes a high-strength steel wire having excellent hydrogen embrittlement resistance. The high-strength steel wire has a chemical composition composed of C: from 0.70 to 1.20%, Si: from 0.10 to 2.00%, Mn: from 0.20 to 1.00%, P: 0.030% or less, S: 0.030% or less, N: from 0.0010 to 0.0100%, Al: from 0 to 0.100%, Cr: from 0 to 2.00%, V: from 0 to 0.30%, B: from 0 to 0.0050%, Ti: from 0 to 0.050%, Nb: from 0 to 0.050%, Zr: from 0 to 0.050%, Ni: from 0 to 2.00%, Cu: from 0 to 1.00%, Sn: from 0 to 0.50%, Mg: from 0 to 0.010%, and Ca: from 0 to 0.010%. The metal structure of the high-strength steel wire has a pearlite structure of 95% by area or more. An average aspect ratio R of a pearlite block, measured at a surface layer in a cross section in the axial direction including the axis of the steel wire, is 2.0 or more. In a case in which the diameter of the steel wire is D, a ratio of an average aspect ratio measured in the surface layer to an average aspect ratio measured at a position of 0.25D is 1.1 or more in the cross section in the axial direction including the axis of the steel wire. The tensile strength of the steel wire is 1,800 MPa or more.
Furthermore, Patent Document 3 proposes a high-strength steel wire having excellent delayed fracture resistance and corrosion resistance. The high-strength steel wire is composed of steel containing C: from 0.5 to 1.0%, and in addition, at least one selected from the group consisting of Cu, Ni, and Ti (with the proviso that Cu and/or Ni are/is contained) and satisfying the following Formula (1). The high-strength steel wire has a pearlite structure being an area ratio of 80% or more, and has strength of 1200 N/mm² or more. $3.1 \geq 3 [Cu] + [Ni] + 6 [Ti] \geq 0.24 (%)$
[Cu], [Ni], and [Ti] represent the contents (% by mass) of Cu, Ni, and Ti, respectively.
Patent Document 4 proposes a rolled material for high-strength spring, The rolled material contains C: from 0.39 to 0.65%, Si: from 1.5 to 2.5%, Mn: from 0.15 to 1.2%, P: from more than 0% to 0.015%, S: from more than 0% to 0.015%, Al: from 0.001 to 0.1%, Cu: from 0.1 to 0.80%, and Ni: from 0.1 to 0.80%, with the balance being iron and inevitable impurities. An amount of nondiffusible hydrogen is 0.40 ppm by mass or less. An area ratio of ferrite represented as a percentage satisfies the following Formula (1), and a total area ratio of bainite and martensite is 2% or less: $Ferrite area ratio < \{(0.77 - [C]) / 0.77 - [C] / 3 + 0.08\} \times 100$
where [name of element] in the above Formula (1) means a content represented in % by mass of each element.
Patent Document 5 proposes a hot-rolled steel bar or wire rod which has a chemical composition containing C: from 0.55 to 0.75%, Si: from 0.1 to 1.0%, Mn: from 0.3 to 1.5%, Cr: from 0.1 to 2.0%, S: from 0.002 to 0.05%, Al: from 0.01 to 0.2%, and N: from 0.002 to 0.01%, with the balance being Fe and impurities, in which P and O satisfy P: 0.025% or less and O: 0.002% or less, respectively, and Fn1 represented by the following Formula [1] is from 2.5 to 4.5, and which has a structure in which a pearlite fraction is 90% or more, an average spacing of pearlite lamellae is from 150 to 300 nm, and the standard deviation of the pearlite lamellar spacing is 25 nm or less. $Fn 1 = 3 Si + Mn + 1.5 Cr$
where the element symbol in Formula [1] means the content (% by mass) of each element.

Prior Art Documents

Patent Documents

Patent Document 1: International Publication No. 2018/012625
Patent Document 2: International Publication No. 2018/021574
Patent Document 3: Japanese Patent No. 4124590
Patent Document 4: Japanese Patent Application Laid-Open ( JP-A) No. 2015-143391
Patent Document 5: Japanese Patent Application Laid-Open ( JP-A) No. 2014-37592 A high-strength PC steel wire is for instance described in EP 3 327 162 A1 .

SUMMARY OF INVENTION

Technical Problem

An object of the present invention is to provide a wire rod which is suitable as a raw material of a high-strength steel wire required to have ultimate tensile strength of 1700 MPa or more, has excellent corrosion resistance and hydrogen embrittlement resistance, is less likely to cause delamination in a steel wire after wire drawing processing, and has excellent torsion properties.

Solution to Problem

Means for solving the above-described problems is a wire rod as defined in the claims.

Advantageous Effects of Invention

According to the present invention, there is provided a wire rod which is suitable as a raw material of a high-strength steel wire required to have ultimate tensile strength of 1700 MPa or more, has excellent corrosion resistance and hydrogen embrittlement resistance, is less likely to cause delamination in a steel wire after wire drawing processing, and has excellent torsion properties.

BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 is a diagram showing the relationship between the tensile strength of a wire rod and an FIP breaking time which is an index of hydrogen embrittlement resistance, obtained in Examples of the present disclosure.
Fig. 2 is a diagram showing the relationship between the tensile strength of a steel wire after wire drawing processing and an FIP breaking time which is an index of hydrogen embrittlement resistance, obtained in Examples of the present disclosure.

DESCRIPTION OF EMBODIMENTS

An exemplary embodiment of the present invention will be described.
In the present disclosure, "%" representing the content of a component (element) means "% by mass".
In the present disclosure, the content of C (carbon) may be expressed as "amount of C". The content of another element may also be expressed in the same manner.
In the present disclosure, the "surface" of a wire rod or a steel wire means an outer peripheral surface. The "surface" of a sample collected by cutting the wire rod or the steel wire also means the outer peripheral surface.
In order to solve the above-described problems, the present inventors have conducted various studies on the influence of elements and metal structures on the corrosion resistance, hydrogen embrittlement resistance, and torsion properties after wire drawing processing of a wire rod suitable as a raw material of a high-strength steel wire required to have ultimate tensile strength of 1700 MPa or more (in the present disclosure, may be referred to as "wire rod for high-strength steel wire"), and have obtained the following findings (a) to (c).

(a) In a high-strength steel wire having tensile strength of 1700 MPa or more, delamination is apt to occur, and breakage due to corrosion or hydrogen embrittlement is apt to be caused. In order to suppress the occurrence of the delamination of the high-strength steel wire, and to prevent the breakage due to corrosion or hydrogen embrittlement, corrosion resistance and hydrogen embrittlement resistance may be improved in consideration of the range of chemical composition of a wire rod serving as a raw material so that the torsion properties are not deteriorated. Cu: from 0.10 to 0.65% and Ni: from 0.05 to less than 0.65% may be contained so as to satisfy [Cu] > [Ni], and Mn, Cr, Cu, and Ni may be contained so that Y1 represented by the following Formula <1> satisfies 1.70 ≤ Y1 ≤ 4.50. $Y 1 = 3 \times [Cr] + 5 \times [Mn] + [Cu] + [Ni]$
Here, [Mn], [Cr], [Cu], and [Ni] in the above Formula represent the contents of the respective elements in % by mass.
(b) The high-strength steel wire is manufactured by wire drawing processing a wire rod. In a high-strength steel wire having tensile strength of 1700 MPa or more, temperature rise and processing heat generation due to frictional heat with a die, or the like in wire drawing processing tend to increase, whereby the high-strength steel wire is apt to be embrittled under the influence of strain aging. In the high-strength steel wire, the strain aging causes longitudinal cracking called delamination to occur in a torsion test, whereby the number of rotations until breakage, that is, a torsion value decreases. Furthermore, in order to prevent the breakage of the high-strength steel wire having tensile strength of 1700 MPa or more due to corrosion or hydrogen embrittlement, and to minimize the influence of the strain aging by wire drawing processing to increase the torsion value after wire drawing processing, C, Si, Ti, and N may be contained so that Y2 represented by the following Formula <2> satisfies Y2 < 1.81. $Y 2 = [C] + [Si] / 10 + A$
Here, A in Formula <2> means A = a in a case in which a value calculated by the following Formula <4> satisfies a ≥ 0, and A = 0 in a case in which the value satisfies a < 0. $a = 350 \times ([N] - 0.29 \times [Ti])$
Here, [C], [Si], [N], and [Ti] in the above Formulae represent the contents of the respective elements in % by mass, and A is a parameter related to a represented by Formula <4>.
(c) In a case in which Mn, Cr, Cu, and Ni are contained in order to improve the corrosion resistance and hydrogen embrittlement resistance of the wire rod, the hardenability of the wire rod is enhanced, which tends to cause increased variation in the hardness of a surface layer of the wire rod depending on the manufacturing conditions of the wire rod. The variation in the hardness of the surface layer of the wire rod causes rapidly deteriorated hydrogen embrittlement resistance in a case in which a portion having locally high hardness of the surface layer is present in the longitudinal direction of the wire rod. The deteriorated hydrogen embrittlement resistance of the wire rod causes deteriorated hydrogen embrittlement resistance of a high-strength steel wire obtained by subjecting the wire rod to wire drawing processing. Therefore, in order to simultaneously improve the corrosion resistance and hydrogen embrittlement resistance of the wire rod, it is necessary to satisfy the range of a specific chemical composition, reduce the variation in hardness of the surface layer of the wire rod, and eliminate the portion having locally high hardness of the surface layer.

A wire rod according to the present invention has been completed based on the above findings. The wire rod contains a chemical composition consisting of, in % by mass,

C: from 0.60 to 1.15%,
Si: from 0.01 to 1.80%,
Mn: from 0.20 to 0.90%,
P: 0.015% or less,
S: 0.015% or less,
Al: from 0.005 to 0.080%,
N: from 0.0015 to 0.0060%,
Cu: from 0.10 to 0.65%,
Ni: from 0.05 to less than 0.65%,
Cr: from 0 to 0.30%,
Mo: from 0 to 0.30%,
Ti: from 0 to 0.100%,
Nb: from 0 to 0.100%,
V: from 0 to 0.20%,
Sn: from 0 to 0.30%,
B: from 0 to 0.0050%,
Ca: from 0 to 0.0050%,
Mg: from 0 to 0.0050%,
Zr: from 0 to 0.100%,
REM: from 0 to 0.0200%; and
the balance: Fe and impurities,
wherein:
in a case in which contents of elements of C, Si, Mn, Cr, Cu, Ni, N, and Ti contained in the wire rod in % by mass are represented by [C], [Si], [Mn], [Cr], [Cu], [Ni], [N], and [Ti], respectively, the wire rod satisfies the following (1) to (3): $[Cu] / [Ni] > 1.00;$
$1.70 \leq Y 1 \leq 4.50,$
$Y 1 = 3 \times [Cr] + 5 \times [Mn] + [Cu] + [Ni];$
and $Y 2 < 1.81,$
$Y 2 = [C] + [Si] / 10 + A;$
A satisfies A = a in a case in which a value of a = 350 × ([N] - 0.29 × [Ti]) satisfies a ≥ 0, and satisfies A = 0 in a case in which the value satisfies a < 0;
a metal structure of the wire rod contains a pearlite structure having an area ratio of 90% or more in a cross section parallel to a longitudinal direction including a central axis of the wire rod (in the present disclosure, may be referred to as "axial cross section"); and
in a case in which a Vickers hardness measured at a position of a depth of 50 µm from a surface (outer peripheral surface) of the wire rod in the axial cross section for each of eight samples si (i is an integer from 1 to 8) collected at arbitrary equal intervals in the longitudinal direction of the wire rod is Hv_si, and an average value and maximum value of Hv_si are respectively Hv_siave and Hv_simax, the wire rod satisfies the following (4): ${Hv}_{simax} - {Hv}_{siave} \leq 50 .$

First, the reason why the ranges of the elements contained in the wire rod for high-strength steel wire according to the present invention are limited will be described.
The chemical composition of the wire rod for high-strength steel wire according to the present invention contain, in % by mass, C: from 0.60 to 1.15%, Si: from 0.01 to 1.80%, Mn: from 0.20 to 0.90%, P: 0.015% or less, S: 0.015% or less, Al: from 0.005 to 0.080%, and N: from 0.0015 to 0.0060%. The chemical composition further contain Cu: from 0.10 to 0.65% and Ni: from 0.05 to less than 0.65% so as to satisfy [Cu] > [Ni]. Furthermore, Cr: from 0 to 0.30%, Mo: from 0 to 0.30%, Ti: from 0 to 0.100%, Nb: from 0 to 0.100%, V: from 0 to 0.20%, Sn: from 0 to 0.30%, B: from 0 to 0.0050%, Ca: from 0 to 0.0050%, Mg: from 0 to 0.0050%, Zr: from 0 to 0.100%, and REM: from 0 to 0.0200% are optionally contained, and the balance consists of Fe and impurities.

C: from 0.60 to 1.15%

C is contained in order to increase the tensile strength of the wire rod. In a case in which the amount of C is less than 0.60%, pro-eutectoid ferrite is generated, whereby tensile strength required for a high-strength steel wire cannot be secured. Therefore, the amount of C is set to 0.60% or more. From the viewpoint of securing excellent hydrogen embrittlement resistance and torsion properties, the amount of C is preferably 0.67% or more, more preferably 0.70% or more, and still more preferably 0.85% or more in order to obtain a high-strength steel wire without excessively increasing a reduction of area by wire drawing. In a case in which the amount of C is more than 1.15%, the amount of pro-eutectoid cementite increases, which causes deteriorated wire drawability. This makes it difficult to obtain a high-strength steel wire, and causes deteriorated torsion properties of the steel wire. Therefore, the amount of C is set to preferably 1.10% or less, and more preferably 1.05% or less.

Si: from 0.01 to 1.80%

Si has an effect of increasing tensile strength by solid solution strengthening, and has an effect of enhancing hydrogen embrittlement resistance. In a case in which the amount of Si is less than 0.01%, these effects cannot be obtained. Therefore, the amount of Si is set to 0.01% or more. In order to reliably obtain these effects, the amount of Si is preferably 0.21% or more, and more preferably 0.70% or more. In a case in which the amount of Si is more than 1.80%, these effects are saturated, and hot ductility is deteriorated, whereby surface defects are apt to occur at the stage of rolling the wire rod, which causes deteriorated manufacturability. The torsion properties of the high-strength steel wire after wire drawing processing are deteriorated. Therefore, the amount of Si is preferably set to 1.49% or less, and more preferably 1.35% or less.

Mn: from 0.20 to 0.90%

Mn has an effect of enhancing the hardenability of steel to increase the tensile strength of the steel after pearlitic transformation. In a case in which the amount of Mn is less than 0.20%, the above effect cannot be sufficiently obtained. Therefore, the amount of Mn is set to 0.20% or more. In order to reliably obtain these effects, the amount of Mn is preferably 0.30% or more, and more preferably 0.35% or more. In a case in which the amount of Mn is more than 0.90%, the hardenability of the steel becomes too high, whereby the above effect is saturated, and the ductility of the wire rod is deteriorated, which causes deteriorated torsion properties of the high-strength steel wire obtained after wire drawing processing. Therefore, the amount of Mn is preferably set to 0.80% or less, and more preferably 0.75% or less.

P: 0.015% or less

P is contained as impurities. P segregates in crystal grain boundaries, which causes deteriorated hydrogen embrittlement resistance and wire drawability, and therefore, the amount of P is desirably lower. Therefore, the upper limit of the amount of P is 0.015%. The amount of P is preferably 0.012% or less, and more preferably 0.010% or less. The lower limit value of the amount of P is not particularly limited, but may be more than 0%, and may be, for example, 0.0001% or more from the viewpoint of reducing a steelmaking cost.

S: 0.015% or less

S is contained as impurities. S segregates in crystal grain boundaries, which causes deteriorated hydrogen embrittlement resistance and wire drawability. This makes it necessary to suppress the amount of S. Therefore, the upper limit of the amount of S is 0.015%. The amount of S is preferably 0.012% or less, and more preferably 0.010% or less. The lower limit value of the amount of S is not particularly limited, but may be more than 0%, and may be, for example, 0.0001% or more from the viewpoint of reducing a desulfurization cost.

Al: from 0.005 to 0.080%

Al is a deoxidizing element, and in a case in which the amount ofAl is less than 0.005%, an oxide becomes coarse, and serves as an origin of cracking due to hydrogen embrittlement, whereby the hydrogen embrittlement resistance of the wire rod is deteriorated. Therefore, the amount of Al is set to 0.005% or more. In order to reliably obtain the above effect, the amount of Al is preferably 0.008% or more, and more preferably 0.010% or more. In a case in which the amount of Al is more than 0.080%, the above effect is saturated, and an oxide and nitride containing Al become coarse, whereby surface flaws occur during rolling. This causes deteriorated manufacturability of the wire rod, and instead causes deteriorated hydrogen embrittlement resistance. Therefore, the amount of Al is preferably set to 0.060% or less, and more preferably 0.050% or less.

N: from 0.0015 to 0.0060%

N reacts with an alloy element such as Ti in steel to form a nitride and a carbonitride, thereby refining crystal grains of the wire rod. Therefore, N has an effect of improving ductility. Therefore, the amount of N is set to 0.0015% or more. In order to reliably obtain the above effect, the amount of N is preferably 0.0021% or more, and more preferably 0.0025% or more. In a case in which a high-strength steel wire is manufactured by wire drawing processing, N dissolved in steel greatly affects strain aging, which causes deteriorated torsion properties. Therefore, attention to the content is required, and the amount of N should be 0.0060% or less. The amount of N is preferably set to 0.0049% or less, and more preferably 0.0040% or less.

Cu: from 0.10 to 0.65%

Cu is an important element having an effect of improving the corrosion resistance and hydrogen embrittlement resistance of the wire rod for high-strength steel wire according to the present invention, and is contained in an amount of 0.10% or more. Cu is present in a state of solid solution in the pearlite structure, and has an effect of improving the corrosion resistance and hydrogen embrittlement resistance of the wire rod. In a case in which the amount of Cu is less than 0.10%, the above effect cannot be obtained, and therefore the amount of Cu is set to 0.10% or more. In order to reliably obtain the above effect, the amount of Cu is preferably 0.15% or more, and more preferably 0.20% or more. In a case in which the amount of Cu is more than 0.65%, the wire rod is embrittled, whereby breakage is apt to occur during wire drawing processing, which instead causes deteriorated hydrogen embrittlement resistance of the wire rod. Therefore, the amount of Cu is set to 0.65% or less, preferably 0.60% or less, and more preferably 0.50% or less.

Ni: from 0.05 to less than 0.65%

Ni is an element essential for suppressing surface flaws during rolling in a case of manufacturing a wire rod containing Cu, and has an effect of improving also the hardenability of the wire rod. The excessive inclusion of Ni induces cracking during wire drawing processing, which causes deteriorated hydrogen embrittlement resistance. In order to obtain the above effect, Ni is contained in an amount of 0.05% or more. In a case in which the amount of Ni is less than 0.05%, surface flaws occur in the surface of the wire rod during rolling, which causes breakage during wire drawing processing, and deteriorated hydrogen embrittlement resistance of the wire rod. In order to obtain the above effect, the amount of Ni is preferably 0.10% or more, and more preferably 0.15% or more. In a case in which the amount of Ni is 0.65% or more, the hardenability excessively increases, which instead causes deteriorated hydrogen embrittlement resistance. Therefore, the amount of Ni is set to be less than 0.65%, preferably 0.60% or less, and more preferably 0.50% or less.
Cu and Ni are contained so as to satisfy [Cu] > [Ni], that is, [Cu]/[Ni] > 1.00, whereby good torsion properties can be secured in the high-strength steel wire after wire drawing processing in the present invention.
In a case in which [Cu]/[Ni] is 1.00 or less, that is, the content of Ni is equal to or greater than the content of Cu, the hardenability of the wire rod for high-strength steel wire according to the present disclosure is too high, whereby sufficient torsion properties cannot be secured in a high-strength steel wire subjected to wire drawing processing. Therefore, Cu and Ni should be contained so as to satisfy [Cu] > [Ni]. In order to stably secure the torsion properties of the steel wire after wire drawing processing, [Cu]/[Ni] is preferably 1.20 or more, and more preferably 1.50 or more. Cu and Ni are only required to satisfy [Cu] > [Ni], and the upper limit of [Cu]/[Ni] is not limited. In a case in which [Cu]/[Ni] is excessively high, surface flaws occur in the hot rolling process of the wire rod, which causes deteriorated manufacturability of the wire rod. Therefore, in consideration of the manufacturability of the wire rod, [Cu]/[Ni] is preferably 5 or less, and more preferably 4 or less.
The wire rod for high-strength steel wire according to the present invention may contain one or two or more of elements of Cr, Mo, Ti, Nb, V, Sn, B, Ca, Mg, Zr, and REM as optional elements. In a case in which these optional elements are contained, one or two or more of Cr: from 0 to 0.30%, Mo: from 0 to 0.30%, Ti: from 0 to 0.100%, Nb: from 0 to 0.100%, V: from 0 to 0.20%, Sn: from 0 to 0.30%, B: from 0 to 0.0050%, Ca: from 0 to 0.0050%, Mg: from 0 to 0.0050%, Zr: from 0 to 0.100%, and REM: from 0 to 0.0200% may be contained in % by mass.

Cr: from 0 to 0.30%

Cr has an effect of enhancing the hardenability of the wire rod to increase the tensile strength of the wire rod after pearlitic transformation, and may be contained in a case in which this effect is desired. In order to obtain this effect, the amount of Cr is preferably 0.01% or more. In order to reliably obtain the above effect, the amount of Cr is preferably 0.05% or more, and more preferably 0.10% or more. In a case in which the amount of Cr is more than 0.30%, a martensite or bainite structure is apt to occur, which causes deteriorated wire drawability and hydrogen embrittlement resistance of the high-strength steel wire after wire drawing processing. Therefore, in a case in which Cr is contained, the amount of Cr is set to 0.30% or less, preferably 0.25% or less, and more preferably 0.20% or less.

Mo: from 0 to 0.30%

Mo has an effect of enhancing the hardenability of the wire rod to increase the tensile strength of the wire rod after pearlitic transformation, and may be contained in a case in which this effect is desired. In order to obtain this effect, the amount of Mo is preferably 0.01% or more. In order to reliably obtain the above effect, the amount of Mo is preferably 0.03% or more, and more preferably 0.05% or more. In a case in which the content of Mo is more than 0.30%, a martensite or bainite structure is apt to occur, which causes deteriorated wire drawability and hydrogen embrittlement resistance of the high-strength steel wire after wire drawing processing. Therefore, in a case in which Mo is contained, the amount of Mo is set to 0.30% or less, preferably 0.20% or less, and more preferably 0.10% or less.

Ti: from 0 to 0.100%

Ti has an effect of precipitating a carbide or a carbonitride by being bonded to C or N, to refine crystal grains, thereby improving the ductility of the wire rod, and has an effect of improving hydrogen embrittlement resistance and torsion properties after wire drawing processing. In addition, inclusion of Ti can reduce solute N, whereby Ti also an effect of suppressing strain aging to improve the torsion properties of the steel wire after wire drawing processing. Ti may be actively contained because the above effects due to inclusion of Ti is effective for obtaining the wire rod for high-strength steel wire according to the present invention. In order to obtain these effects, Ti may be contained in an amount of 0.002% or more. In order to reliably obtain the above effects, the amount of Ti is preferably 0.005% or more, and more preferably 0.008% or more. Even in a case in which Ti is contained in an amount of more than 0.10%, not only the above effects are saturated, but also the strength of the wire rod excessively increases, which instead causes deteriorated hydrogen embrittlement resistance and torsion properties of the steel wire after wire drawing processing. Therefore, in a case in which Ti is contained, the amount of Ti is set to 0.10% or less, preferably 0.050% or less, and more preferably 0.025% or less.

Nb: from 0 to 0.100%

Nb has an effect of precipitating a carbide or a carbonitride to refine crystal grains, thereby improving the ductility of the wire rod, and has an effect of improving hydrogen embrittlement resistance and torsion properties after wire drawing processing. In order to obtain the effects, Nb is preferably contained in an amount of 0.002% or more. In order to reliably obtain the above effecst, the amount of Nb is preferably 0.005% or more, and more preferably 0.008% or more. Even in a case in which Nb is contained in an amount of more than 0.100%, not only the above effect is saturated, but also surface flaws are apt to occur at the stage of obtaining a billet according to bloom rolling or at the stage of rolling a wire rod, which causes deteriorated manufacturability. Therefore, in a case in which Nb is contained, the amount of Nb is 0.100% or less, preferably 0.050% or less, and more preferably 0.025% or less.

V: from 0 to 0.20%

V has an effect of precipitating a carbide VC to increase tensile strength and to improve hydrogen embrittlement resistance, and may be contained in a case in which this effect is desired. In order to obtain this effect, V is preferably contained in an amount of 0.01 % or more. In order to reliably obtain the above effect, the amount of V is preferably 0.03% or more, and more preferably 0.05% or more. Even in a case in which V is contained in an amount of more than 0.20%, not only the above effect is saturated, but also the hydrogen embrittlement resistance and torsion properties of the steel wire after wire drawing processing are deteriorated. Therefore, in a case in which V is contained, the amount of V is set to 0.20% or less, preferably 0.15% or less, and more preferably 0.10% or less.

Sn: from 0 to 0.30%

Sn has an effect of enhancing corrosion resistance and hydrogen embrittlement resistance in a state where Sn is dissolved in a pearlite structure, and may be contained in a case in which this effect is desired. In order to obtain this effect, Sn is preferably contained in an amount of 0.01% or more. In order to reliably obtain the above effect, the amount of Sn is preferably 0.03% or more, and more preferably 0.05% or more. Even in a case in which Sn is contained in an amount of more than 0.30%, not only the above effect is saturated, but also the wire rod is embrittled, whereby surface flaws occur during rolling, which causes deteriorated manufacturability of the rolled material. Therefore, in a case in which Sn is contained, the amount of Sn is preferably from 0.01 to 0.30%. Therefore, in a case in which Sn is contained, the amount of Sn is set to 0.30% or less, preferably 0.20% or less, and more preferably 0.15% or less.

B: from 0 to 0.0050%

B has an effect of increasing a pearlite structure fraction after isothermal transformation to improve the torsion properties of the high-strength steel wire after wire drawing processing, and may be contained in a case in which this effect is desired. In order to obtain this effect, B is preferably contained in an amount of 0.0002% or more. In order to reliably obtain the above effect, the amount of B is preferably 0.0005% or more, and more preferably 0.0007% or more. Even in a case in which B is contained in an amount of more than 0.0050%, not only the above effect is saturated, but also the wire rod is embrittled, whereby surface flaws occur during rolling, which causes deteriorated manufacturability and instead causes deteriorated torsion properties of the high-strength steel wire after wire drawing processing. Therefore, in a case in which B is contained, the amount of B is set to 0.0050% or less, preferably 0.0030% or less, and more preferably 0.0020% or less.

Ca: from 0 to 0.0050%

Ca has an effect of finely dispersing MnS in a state where Ca is dissolved in MnS, and has an effect of improving hydrogen embrittlement resistance, and thus may be contained in a case in which the effects are desired. Ca needs not be contained (Ca: 0%), but in order to obtain an effect of improving hydrogen embrittlement resistance by Ca, Ca may be contained in an amount of 0.0002% or more. Ca may be contained in an amount of 0.0005% or more in a case in which a higher effect is desired. Even in a case in which Ca is contained in an amount of more than 0.0050%, the effect is saturated, and an oxide of Ca generated through reaction with oxygen in the steel becomes coarse, which causes deteriorated torsion properties after wire drawing processing. Therefore, in a case in which Ca is contained, the appropriate amount of Ca is 0.0050% or less. From the viewpoint of improving hydrogen embrittlement resistance and torsion properties, the amount of Ca is preferably 0.0030% or less, and more preferably 0.0025% or less.

Mg: from 0 to 0.0050%

Mg has an effect of finely dispersing MnS in a state where Mg is dissolved in MnS, and has an effect of improving hydrogen embrittlement resistance, and thus may be contained in a case in which the effects are desired. Mg needs not be contained (Mg: 0%), but in order to obtain the effect of improving hydrogen embrittlement resistance by Mg, Mg may be contained in an amount of 0.0002% or more, and Mg may be contained in an amount of 0.0005% or more in a case in which a higher effect is desired. Even in a case in which Mg is contained in an amount of more than 0.0050%, the effect is saturated, and an oxide of Mg generated through reaction with oxygen in the steel becomes coarse, which causes deteriorated torsion properties after wire drawing processing. Therefore, in a case in which Mg is contained, the appropriate amount of Mg is 0.0050% or less. From the viewpoint of improving hydrogen embrittlement resistance and torsion properties, the amount of Mg is preferably 0.0030% or less, and more preferably 0.0025% or less.

Zr: from 0 to 0.100%

Zr reacts with O to generate an oxide, and in a case in which a slight amount of Zr is contained, Zr exhibits an effect of finely dispersing the oxide to suppress hydrogen embrittlement resistance and torsion properties after wire drawing processing, and may be contained in a case in which the effect is desired. In order to obtain the effect, Zr may be contained in an amount of 0.0002% or more, and Zr may be contained in an amount of 0.001% or more in a case in which a higher effect is desired. In a case in which Zr is contained in an amount of more than 0.10%, the effect is saturated, and a coarse nitride or sulfide is generated, which instead causes deteriorated hydrogen embrittlement resistance and torsion properties after wire drawing processing. Therefore, in a case in which Zr is contained, the content of Zr is 0.100% or less. From the viewpoint of reducing inclusions adversely affecting hydrogen embrittlement resistance and torsion properties after wire drawing processing, the content of Zr is preferably 0.080% or less, and more preferably 0.050% or less.

REM: from 0 to 0.0200%

REM is a general term of rare earth elements, and the content of REM is a total content of rare earth elements. REM has an effect of finely dispersing MnS in a state where REM is dissolved in MnS, similarly to Ca and Mg. By finely dispersing MnS, hydrogen embrittlement resistance can be improved, and therefore REM may be contained. REM needs not be contained (REM: 0%), but in order to obtain an effect of improving hydrogen embrittlement resistance by REM, REM may be contained in an amount of 0.0002% or more, and REM may be contained in an amount of 0.0005% or more in a case in which a higher effect is desired. Even in a case in which the amount of REM is more than 0.020%, the effect is saturated, and an oxide of REM generated through reaction with oxygen in the steel becomes coarse, which instead causes deteriorated torsion properties after wire drawing processing. Therefore, in a case in which REM is contained, the appropriate amount of REM is 0.0200% or less. From the viewpoint of improving hydrogen embrittlement resistance and torsion properties, the amount of REM is preferably 0.0100% or less, and more preferably 0.0050% or less.

Balance: Fe and Impurities

The balance is Fe and impurities. The "impurities" are components which are unintentionally contained in the steel material, and refer to components mixed from an ore or a scrap as a raw material, or a manufacturing environment or the like in a case of industrially manufacturing an iron and steel material. Examples of the impurities include, in addition to P, S, and N, elements unintentionally contained in the steel material among the above optional elements, and O (oxygen). For example, in a case in which O (oxygen) is contained in a large amount, an oxide generated in the steel becomes coarse, which causes deteriorated torsion properties after wire drawing processing, and therefore, the amount of O is preferably 0.0030% or less, and more preferably 0.0025% or less.
The wire rod for high-strength steel wire according to the present invention contains the components in the above ranges. In addition, Y1 represented by the following Formula <1> satisfies 1.70 ≤ Y1 ≤ 4.50, and Y2 represented by the following Formula <2> satisfies Y2 < 1.81. $Y 1 = 3 \times [Cr] + 5 \times [Mn] + [Cu] + [Ni]$
$Y$ $2 = [C] + [Si] / 10 + A$
$a = 350 \times ([N] - 0.29 \times [Ti])$
Here, [C], [Si], [Mn], [Cr], [Cu], [Ni], [N], and [Ti] in the above Formulae represent the contents of the respective elements in % by mass. A in Formula <2> is a parameter related to a represented by the above Formula <4>. in a case in which a value calculated by Formula <4> satisfies a ≥ 0, A satisfies A = a, and in a case in which the value satisfies a < 0, A satisfies A= 0.
In the wire rod according to the present invention, Cr and Ti are optional elements, and in a case in which these optional elements are not substantially contained in the wire rod according to the present invention (no addition, that is, in the case of impurity level), the contents of the elements are set to "0" to calculate Y1, Y2, and a.
Y1 is mainly related to the hardenability of the wire rod for high-strength steel wire, and is a parameter necessary for increasing the tensile strength of the wire rod to improve hydrogen embrittlement resistance. Y1 is in the range of 1.70 ≤ Y1 ≤ 4.50, whereby the hydrogen embrittlement resistance of the wire rod for high-strength steel wire can be improved, and the hydrogen embrittlement resistance of the high-strength steel wire after wire drawing processing can be improved.
The high-strength steel wire can be obtained by subjecting a wire rod having an appropriately controlled chemical composition and metal structure to wire drawing processing. It is preferable to form a fine pearlite structure having high uniformity up to a central portion by reheating the wire rod before wire drawing processing to subject the wire rod to a patenting treatment, or directly dipping the wire rod into a salt bath furnace after rolling to subject the wire rod to an isothermal transformation treatment. Y1 is a parameter required for controlling the hardenability of the wire rod to provide strength required by forming a fine pearlite structure having high uniformity from a surface to a central portion, and improving the hydrogen embrittlement resistance of the wire rod. Y1 should be from 1.70 to 4.50. In a case in which Y1 is less than 1.70, hydrogen embrittlement resistance is deteriorated, and sufficient hydrogen embrittlement resistance cannot be obtained even in a steel wire after wire drawing processing. In order to reliably obtain the above effect, Y1 is preferably 2.00 or more, and more preferably 2.50 or more. In a case in which Y1 is more than 4.50, a non-pearlite structure, other than a pearlite structure, such as bainite and martensite is generated after a patenting treatment or an isothermal transformation treatment after rolling, which instead causes deteriorated hydrogen embrittlement resistance of the wire rod. Y1 is set to 4.50 or less, and preferably 4.22 or less. In a case in which it is desired to secure the strength of the wire rod and to more stably secure the hydrogen embrittlement resistance, Y1 may be 4.00 or less, and more preferably 3.75 or less.
Y2 is a parameter mainly affecting the torsion properties of the steel wire after wire drawing processing. The wire rod for high-strength steel wire according to the present invention contains Cu and Ni in order to improve corrosion resistance and hydrogen embrittlement resistance, and has relatively high tensile strength. Therefore, temperature rise caused by frictional heat with a die in wire drawing processing or the like is apt to cause the embrittlement of the wire rod under the influence of strain aging, and cause deteriorated torsion properties of the steel wire after wire drawing processing. In particular, since C, Si, and N dissolved in steel greatly affect strain aging due to wire drawing processing, Y2 can be represented by the following Formula <2>. $Y 2 = [C] + [Si] / 10 + A$
$a = 350 \times ([N] - 0.29 \times [Ti])$
Here, [C], [Si], [N], and [Ti] in the above Formulae represent the contents of the respective elements in % by mass, and A is a parameter related to a represented by Formula <4>. The right side of Formula <4> represents the influence of the solute N. In a case in which the value of the right side is negative, the influence of the solute N on strain aging is eliminated, whereby A satisfies A = 0. In order to minimize the influence of strain aging in wire drawing processing, the value of Y2 is set to less than 1.81. In a case in which Y2 is 1.81 or more, the influence of strain aging due to wire drawing processing causes deteriorated torsion properties after wire drawing processing. In order to improve the torsion properties after wire drawing processing, the value of Y2 is preferably less than 1.70, and more preferably less than 1.50. The value of Y2 may be less than 1.81, and the lower limit value is not particularly limited, but is preferably 0.50 or more, and more preferably 0.80 or more from the viewpoint of securing the tensile strength after wire drawing processing.

In the metal structure of the wire rod for high-strength steel wire according to the present invention, a pearlite structure which is a lamellar structure of ferrite and cementite accounts for 90% or more. In regard to this, ferrite, bainite, or martensite may be generated depending on a chemical composition, a γ particle size before transformation, or a change in a cooling rate at a stage of a patenting treatment or isothermal transformation treatment of a wire rod. These structures cause increased variation in the hardness of the surface layer in the longitudinal direction of the wire rod and deteriorated hydrogen embrittlement resistance of the wire rod. In a case in which the variation in the hardness of the surface layer in the longitudinal direction of the wire rod is large, the hydrogen embrittlement resistance and torsion properties of the steel wire after wire drawing processing are deteriorated. In the metal structure of the wire rod for high-strength steel wire according to the present disclosure, the percentage of the pearlite structure is preferably 92% or more, and more preferably 95% or more.
Examples of the remaining structure (non-pearlite structure) other than the pearlite structure include martensite, bainite, pro-eutectoid ferrite, and pro-eutectoid cementite. The non-pearlite structure is preferably pro-eutectoid ferrite and degenerate pearlite, and more preferably degenerate pearlite from the viewpoint of not extremely deteriorating the torsion properties and hydrogen embrittlement resistance of the steel wire after wire drawing processing. The pearlite structure refers to pearlite which maintains a lamellar structure, and degenerate pearlite in which the lamellar structure is deformed is treated as a non-pearlite structure in the present invention.

(Variation in Hardness of Surface Layer)

The variation in the hardness of the surface layer of the wire rod, which is caused by variation in a chemical composition or a metal structure generated in the longitudinal direction of the wire rod, also affects hydrogen embrittlement resistance, and greatly affects the characteristics of the steel wire after wire drawing processing. In particular, the hydrogen embrittlement resistance is affected by the variation in the hardness of the surface layer of the wire rod. In a case in which a portion having high hardness in the surface layer of the wire rod is present, the portion serves as an origin of hydrogen embrittlement, which causes deteriorated hydrogen embrittlement resistance.
In the wire rod according to the present invention, eight samples are collected at arbitrary equal intervals, and the relationship between the maximum value Hv_simax and average value Hv_siave of Vickers hardness Hv_si measured at a position of a depth of 50 µm (hereinafter, may be referred to as "50 µm depth") from the surface of the wire rod in the axial cross section (cross section parallel to the longitudinal direction including the central axis) of each sample satisfies the following (4): ${Hv}_{simax} - {Hv}_{siave} \leq 50 .$
In the present invention, the "surface layer" of the wire rod is a region from the surface (outer peripheral surface) of the wire rod to a depth of 100 µm, and the hardness is measured at the 50 µm depth which is a middle point of the surface layer.
Samples for measuring the Vickers hardness are collected at arbitrary equal intervals according to the length of the wire rod to be measured. The wire rod is usually manufactured in a state of being wound in a ring shape. Therefore, it is preferable that, in the wire rod having a length equal to or longer than a length corresponding to one ring, eight samples are collected at equal intervals from the length corresponding to one ring, and the Vickers hardness of each sample is measured, to obtain the average value and maximum value of the Vickers hardnesses of the samples. Specifically, for each of eight samples si (i is an integer from 1 to 8) collected at intervals of 600 mm in the longitudinal direction of the wire rod and having a length of 25 mm, in a case in which the average value in each sample of the Vickers hardness at a position of a depth of 50 µm from the surface of the wire rod in the axial cross section (the cross section parallel to the longitudinal direction including the central axis) measured in each sample is Hv_si, and the maximum value of the eight Hv_si (i is an integer from 1 to 8) is Hv_simax, the value of Y3 represented by Formula <3> is set to 50 or less. $Y 3 = {Hv}_{simax} - {Hv}_{siave}$
The right side of Formula <3> represents the difference between the maximum value Hv_simax of Hv_si obtained for the eight samples and the average value Hv_siave (in the present invention, may be referred to as "overall average Vickers hardness") calculated by the following Formula (n = 8) of Hv_si obtained for the eight samples. ${Hv}_{siave} = \frac{1}{n} \sum_{i = 1}^{n} {Hv}_{si}$
A sample in which the axial cross section cut from the wire rod is filled with a resin and mirror-polished is used. The Hv_si of each sample may be determined by measuring 50 positions of a depth of 50 µm from the surface of the wire rod in the axial cross section (that is, a length of 10 mm) at a pitch of 200 µm per sample with a load of 0.98 N using an automatic Vickers hardness meter.
Among the eight Hv_si thus obtained, the maximum value Hv_simax is the Vickers hardness of the sample having the highest hardness at the position of a depth of 50 µm from the surface of the wire rod in the axial cross section among the eight samples having a length of 25 mm and collected at intervals of 600 mm. In other words, the maximum value Hv_simax means the Vickers hardness of a portion where the highest average hardness is measured among variations in the hardness of the surface layer in the longitudinal direction of the wire rod. The hydrogen embrittlement of the wire rod is affected by the variation in hardness of the surface layer of the wire rod, and a portion having locally high hardness of the surface layer of the wire rod serves as an origin of fracture, whereby breakage due to hydrogen embrittlement occurs. In a case in which Hv_simax measured for the eight samples is higher than the overall average Vickers hardness at the position of the 50 µm depth including other samples by more than 50, the possibility of breakage due to hydrogen embrittlement in the portion is increased, which causes deteriorated hydrogen embrittlement resistance of the wire rod. Furthermore, in a steel wire obtained by subjecting a wire rod to wire drawing processing, the variation in the hardness of the surface layer in the longitudinal direction is further increased, which causes more remarkably deteriorated hydrogen embrittlement resistance of the steel wire. In a case in which the value of Y3 is 50 or less, deterioration in the hydrogen embrittlement resistance of the wire rod is suppressed, and deterioration in the hydrogen embrittlement resistance of a steel wire obtained by wire drawing processing is also suppressed. The value of Y3 is preferably smaller from the viewpoint of improving the hydrogen embrittlement resistance. The value of Y3 is preferably 30 or less, and more preferably 25 or less.
The number of samples having a length of 25 mm and collected at intervals of 600 mm in the longitudinal direction of the wire rod is 8. That is, the average value Hv_si of the Vickers hardness in each sample at a position of a depth of 50 µm from the surface of the wire rod in the axial cross section for eight samples is obtained, which makes it possible to know the variation in the hardness of the surface layer of the wire rod. As for the variation in the hardness of the surface layer of the wire rod, it is preferable to investigate the variation in the hardness of the surface layer in a range corresponding to at least one ring of a wire rod coil rolled and wound. This is because a wire rod ring wound up in an austenite region after hot rolling is conveyed on a conveyor in a state of partially overlapping front and rear rings, and thus a portion in contact with or a portion close to the front and rear rings (overlapping portion) and a portion away from the front and rear rings (non-overlapping portion) are present in one ring. For this reason, a difference in cooling rate occurs in one ring of the coil after winding, whereby a difference occurs in a pearlitic transformation temperature between the overlapping portion and the non-overlapping portion. This causes a difference in perlite lamellar spacing between the overlapping portion and the non-overlapping portion, and as a result, large variation in the hardness of the surface layer occurs. Each ring in the wire rod coil is basically wound up under the same condition (constant condition), whereby the variation between the rings is small. Therefore, in a case in which eight samples are collected at intervals of 600 mm, the variation in the hardness of the surface layer is verified in a range of about 4200 mm in the longitudinal direction of the wire rod, whereby the variation in the hardness of the surface layer can be verified by a length corresponding to one ring or more of the wire rod coil. Since the variation between the rings is small as described above, the variation in the wire rod coil can be verified by the sample collection method.
The length of the wire rod according to the present disclosure and the length of one ring during manufacture are not particularly limited, and the interval at which a sample is collected from the wire rod is not limited to 600 mm. The wire rod according to the present disclosure satisfies Hv_simax-Hv_siave ≤ 50 regardless of the length and regardless of the interval of collecting the sample for measuring the Vickers hardness, whereby the wire rod can satisfy the characteristics. In the actual manufacture of the wire rod, variation occurs in the length direction of the wire rod. Therefore, in a case in which the manufacturing method is adjusted if appropriate, a wire rod having reduced variation in the length direction of the wire rod can be obtained. In the case of a wire rod having a length of one ring of about 4200 mm, a Vickers hardness test can be performed at intervals of 600 mm to confirm the characteristics in the length direction of the wire rod coil. In a case in which the length of one ring is not 4200 mm, eight samples are collected at equal intervals from the length corresponding to one ring to measure the Vickers hardness Hv_si of each sample. The difference between the maximum value Hv_simax and the average value Hv_siave is calculated, whereby the Vickers hardness of the wire rod coil in the length direction can be confirmed. It is preferable that in a case in which the length of the wire rod is less than one ring, eight samples are collected at equal intervals from the whole, to measure the Vickers hardness Hv_si of each sample, and the difference between the maximum value Hv_simax and the average value Hv_siave is calculated.

(Tensile Strength)

As the strengths of the wire rod and steel wire are higher, breakage due to progress of corrosion or hydrogen embrittlement is apt to occur. In a case in which a steel wire is manufactured using the wire rod for high-strength steel wire according to the present disclosure, the steel wire has excellent corrosion resistance and hydrogen embrittlement resistance even in a case in which the tensile strength is more than 1700 MPa, whereby breakage is less likely to occur even in a case in which the steel wire or stranded wire product is used in a corrosive environment. Even in a case in which the tensile strength of the wire rod is not high, a high-strength steel wire having tensile strength of more than 1700 MPa can be obtained by increasing the reduction of area by wire drawing. However, in a case in which the reduction of area by wire drawing is excessively increased, the torsion properties are deteriorated, and the hydrogen embrittlement resistance is also deteriorated. Therefore, it is preferable that the tensile strength of the wire rod before wire drawing processing is set to 1000 MPa or more to manufacture a high-strength steel wire without excessively increasing the reduction of area by wire drawing processing. In a case in which the fine pearlite structure is formed to have tensile strength of 1000 MPa or more at the stage of the wire rod, deteriorations in the tensile strength and hydrogen embrittlement resistance of the steel wire after wire drawing processing are suppressed.
In order to improve the torsion properties and hydrogen embrittlement resistance of the steel wire, the tensile strength of the wire rod is more preferably 1200 MPa or more, and still more preferably 1300 MPa or more.
In a case in which the tensile strength of the wire rod is more than 1650 MPa, the ductility of the wire rod is deteriorated, which instead may cause deteriorated torsion properties and hydrogen embrittlement resistance of the steel wire after wire drawing processing. From this viewpoint, the tensile strength of the wire rod is preferably 1600 MPa or less, and more preferably 1550 MPa or less.
The wire rod for high-strength steel wire according to the present invention is subjected to wire drawing processing, whereby a high-strength steel wire having excellent corrosion resistance and hydrogen embrittlement resistance can be obtained even when the high-strength steel wire has high strength of more than 1700 MPa. This is because the segregation of the chemical composition, the metal structure, and the hardness distribution of the surface layer of the wire rod are controlled at the stage of manufacturing the wire rod to improve corrosion resistance and hydrogen embrittlement resistance.
The wire rod according to the present invention has excellent corrosion resistance and hydrogen embrittlement resistance, and provides also excellent torsion properties of a steel wire after wire drawing processing. Therefore, the steel wire having strength increased by wire drawing processing of the wire rod can be used as a high-strength steel wire such as a steel wire for high-strength rope, a steel wire for bridge cable, or a PC steel wire.

The metal structure of the wire rod, the tensile strengths of the wire rod and steel wire, the variation in the hardness of the surface layer of the wire rod, the corrosion resistance of the wire rod, the hydrogen embrittlement resistances of the wire rod and steel wire, and the torsion properties of the steel wire were each investigated by the following methods.

<1> Metal Structure of Wire Rod:

As for the area ratio of the metal structure of the wire rod, a micro sample in which a cross section being parallel to the longitudinal direction of the wire rod and passing through the central axis thereof was filled with a resin was mirror-polished, and the metal structure of the sample was then exposed using a picral solution. Next, in a case in which the diameter of the wire rod was represented by D, ten structure photographs of a metal structure in a portion corresponding to a position of a depth of 0.25D from the surface of the wire rod were taken at 1000x magnification using a scanning electron microscope (SEM). For each of the taken photographs, a portion corresponding to a pearlite structure, which was determined to be a lamellar structure of cementite and ferrite, was filled. An area value was measured by image analysis to calculate a pearlite area ratio, and ten measured values were averaged to determine the pearlite area ratio. A structure other than the pearlite structure is a non-pearlite structure which cannot be discriminated as a pearlite structure being a lamellar structure of cementite and ferrite, such as partially produced martensite, bainite, and pro-eutectoid ferrite.

<2> Tensile Strengths of Wire Rod and Steel Wire:

The tensile test was performed in accordance with JIS Z 2241: 2011 using test pieces having a length of 400 mm and collected at intervals of from 250 to 300 mm in the longitudinal direction of the wire rod and the steel wire after wire drawing processing. The maximum test force until the test piece was broken was divided by the cross-sectional area obtained from the diameter measured before the tensile test to determine the tensile strength. The test was performed using eight test pieces, and the average value thereof was taken as the tensile strengths of the wire rod and the steel wire.

<3> Variation in Hardness of Surface Layer of Wire Rod:

The hardness of the surface layer was measured using test pieces having a length of 25 mm and collected at intervals of 600 mm (an example of equal intervals) in the longitudinal direction of the wire rod. As the test pieces, eight (n = 8) samples were collected. A micro sample obtained by filling a cross section parallel to the length direction of the wire rod with a resin, and then mirror-polishing the cross section was used. The hardness of each micro sample was measured in accordance with JIS Z 2244: 2009 using an automatic Vickers hardness tester. The test load of the hardness measurement was 0.98 N. The Vickers hardness at a position of 50 µm deep from the surface of the wire rod in each micro sample was measured at 50 points to calculate the average value thereof, and Hv_si (i = 1, 2,... 8) of each test piece was obtained. Furthermore, the maximum value Hv_simax of Hv_si measured in each micro sample was obtained, and the overall average Vickers hardness Hv_siave of Hv_si was calculated together, to calculate Y3 represented by the following Formula <3>. $Y 3 = {Hv}_{simax} - {Hv}_{siave}$

<4> Corrosion Resistance of Wire Rod:

The outer peripheral portion of the test piece cut to have a length of 100 mm from the axial direction of the central portion of the wire rod was evenly shaved to cut out two test pieces each machined to have a diameter of 7 mm and a length of 100 mm. The corrosion test was performed with (1) salt water spraying (5% NaCl spray, 35°C, 2 hr), (2) drying (humidity: 20%, 60°C, 4 hr), and (3) wetting (humidity: 95%, 50°C, 2 hr) as one cycle using a dry-wet cyclic corrosion tester capable of spraying salt water. A test period was set to 12 weeks. A volume reduction ratio due to the corrosion of each of the two test pieces was determined, and the average value thereof was used as an evaluation index of the corrosion resistance of each wire rod. The volume reduction ratio (%) due to corrosion was determined by the following Formula. $Corrosion volume reduction ratio (%) = 100 \times (\begin{array}{l} volume of test piece before corrosion \\ test - volume of test piece after corrosion test \end{array}) / volume of test piece before corrosion test$
As for the volume of the test piece before the corrosion test, the average values of the diameters and lengths of the test piece measured at three points while the position was changed were obtained before the test, and the volume of the test piece before the corrosion test was calculated. After the corrosion test, a corrosion product on the surface of the test piece was completely removed using sandblasting. Then, the average values of the diameters and lengths of the test piece measured at three points while the position was changed were then obtained, and the volume of the test piece after the corrosion test was calculated.

<5> Hydrogen Embrittlement resistances of Wire Rod and Steel Wire:

The hydrogen embrittlement resistances of the wire rod and steel wire after wire drawing processing were evaluated by an FIP test standardized by the International Federation of Prestressed Concrete (Federation Internationale de la Precontrainte). The wire rod or the steel wire after wire drawing processing were subjected to a pickling treatment to remove a scale or a lubricating film on the surface, and then subjected to straightening processing to secure straightness, and a sample cut to a length of 700 mm was used as a test piece. Then, the test piece was immersed into an aqueous ammonium thiocyanate (NH₄SCN) solution at 50°C using a solution cell in which a length of 200 mm including the central part of the test piece can be immersed. A constant load of 70% of a breaking load obtained from the tensile test was applied to the test piece, and a time until breakage was measured. The upper limit of the breaking time was set to 200 hours. The test was performed on six test pieces collected from each wire rod or each steel wire, and the average value of the breaking times was calculated to evaluate the hydrogen embrittlement resistances of the wire rod and steel wire.

<6> Torsion Properties of Steel Wire:

As for the torsion properties of the steel wire after wire drawing processing, the steel wire was cut so that a torsion test can be performed in a length of 100 times the diameter of the steel wire. The straightening processing was performed, and the torsion test in which the steel wire was twisted was then performed at a speed of 15 rotations for 1 minute until breakage occurred. As for the occurrence of delamination, a torque curve during twisting was measured, and delamination was determined to occur in a case in which a torque decreased by 20% or more before breakage occurs. In the torsion test, five wires were used for each steel wire, and in a case in which no delamination occurred, the torsion properties were determined to be good.

In a case in which the wire rod for high-strength steel wire according to the present invention satisfies the requirements of the present invention, it is possible to obtain the effect of the steel wire of the present invention without depending on the method of manufacturing the wire rod, but for example, a wire rod may be manufactured through the following manufacturing method, and be used as a raw material to manufacture a high-strength steel wire. The following manufacturing process is an example, and even in a case in which a wire rod having a chemical composition and the other requirements which fall within the range of the present disclosure is obtained by a process other than the following process, the wire rod is included in the wire rod according to the present disclosure.
In the wire rod for high-strength steel wire according to the present invention, it is preferable to control the adjustment of the chemical composition at the stage of smelting steel, and manufacturing conditions such as a heating condition of a cast slab and a heating temperature during rolling, to reduce the segregation of the chemical composition generated in the longitudinal direction of the wire rod, and to control the wire rod so as to have a pearlite structure having high uniformity.
Specifically, a steel ingot or cast slab smelted and cast by a converter or an electric furnace or the like after a chemical composition such as C, Si, Mn, Cu, Ni, or Al are adjusted is subjected to a step of bloom rolling to form a billet being a raw material for product rolling. Before the product rolling, that is, during heating in the bloom rolling or at the stage of the steel ingot or cast slab before that, the steel ingot or the cast slab is subjected to a heating treatment at 1260°C or higher for 12 hr or more. Thereafter, the billet is reheated, subjected to hot-product rolling, and finally finished to be a wire rod having a predetermined diameter.
As described above, the segregation of the chemical composition can be suppressed by applying the heat treatment at a high temperature for a long time before the product rolling to the wire rod, whereby the variation in the hardness of the surface layer in the longitudinal direction of the wire rod after the product rolling can be suppressed. As described above, the variation in the hardness in the longitudinal direction of the wire rod after rolling can be reduced by adding a step of reducing the segregation of the chemical components such as a heat treatment at a high temperature for a long time, which is not performed under normal manufacturing conditions, and control of a cooling rate in a case of solidifying molten steel into a cast slab or a steel ingot.
Then, the billet obtained by the bloom rolling is reheated to 1000°C or higher. The heating temperature at this time may be set to 1150°C or lower, and preferably 1130°C or lower in order to suppress coarsening of austenite grains and duplex grain formation. A holding time after the heating temperature is reached is preferably less than 90 minutes in order to suppress the duplex grain formation of the austenite grains.
The billet heated under the above conditions is subjected to rough rolling, and then subjected to finish rolling to obtain a wire rod having a diameter of from 5.0 to 16.0 mm. At this time, the temperature of the finish rolling is adjusted within a range of from 850°C to 950°C. In a case in which the temperature is lower than 850°C, austenite grains are excessively refined and pearlitic transformation becomes uneven. In a case in which the temperature is higher than 950°C, it is difficult to control the austenite grains in a subsequent cooling process, which causes increased variation in the hardness of the surface layer of the wire rod. Then, the steel material after hot rolling is held at a temperature not lower than 800°C for 15 seconds or more to adjust the austenite grains. Then, the steel material may be directly immersed into a molten salt held at a temperature of from 500 to 580°C to be isothermally transformed into a pearlite structure, and then cooled. Alternatively, the hot-rolled steel material may be cooled to about room temperature by air-blast cooling, then heated at a temperature of an austenite region of an A₃ point or higher, immersed into molten lead held at a temperature of from 500 to 600°C to be isothermally transformed into a pearlite structure, and then cooled.

As an example, the wire rod obtained by the above-described process may be subjected to wire drawing processing to obtain a steel wire having a necessary diameter. The reduction of area by wire drawing may be determined according to the required diameter and strength of the steel wire, but in a case in which the reduction of area by wire drawing is excessively increased, the torsion properties and hydrogen embrittlement resistance of the steel wire after the wire drawing processing are deteriorated. The reduction of area by wire drawing may be from 70 to 92%. In a case in which the reduction of area by wire drawing is less than 70%, it is difficult to obtain required tensile strength. In a case in which the reduction of area by wire drawing is more than 92%, the torsion properties and hydrogen embrittlement resistance of the steel wire are apt to be deteriorated. The method of the wire drawing processing is not particularly limited, but in order to reduce the variation in the hardness of the surface layer of the steel wire, it is preferable to employ a method of suppressing the strain aging of the steel wire due to processing heat generation during the wire drawing processing, such as water cooling of the steel wire after the wire drawing processing.
A step of heating the steel wire, such as hot dip galvanizing, bluing, or heat stretching, may be performed after the wire drawing processing if necessary.

Examples

Hereinafter, a wire rod and a steel wire according to the present invention will be described more specifically with reference to Examples. These Examples do not limit the wire rod and the steel wire according to the present invention.

Specifically, steels having chemical compositions shown in Tables 1 and 2 were smelted to produce wire rods and steel wires through the following method. The notation of "-" in Tables 1 and 2 indicates that the content of the element with the notation is at an impurity level, and it can be determined that the element is not substantially contained. Underlined values in Tables 2 to 5 mean that the values fall outside the range of the present invention or do not satisfy the above-described manufacturing method (manufacturing conditions).

[Table 1]

Steel No.	C	Si	Mn	P	S	Al	N	Cu	Ni	Cr	Ti	Cu/Ni	Y1	Y2	A
A0	0.82	0.22	0.70	0.004	0.005	0.031	0.0027	0.25	0.15	-	-	1.67	3.90	1.79	0.95
A1	0.82	0.21	0.69	0.004	0.005	0.030	0.0026	0.25	0.15	-	-	1.67	3.85	1.75	0.91
B0	0.87	0.85	0.41	0.005	0.005	0.025	0.0038	0.25	0.15	0.03	0.015	1.67	2.54	0.96	0.00
B1	0.87	0.84	0.40	0.004	0.005	0.027	0.0036	0.25	0.15	0.03	0.015	1.67	2.49	0.95	0.00

In order to separately produce wire rods having different variations in tensile strength or hardness of a surface layer in a longitudinal direction using steels having similar chemical composition, the wire rods were rolled under the manufacturing conditions of test Nos. a0, a1, a0-1 to a0-4 and test Nos. b0, b1, and b0-1 to b0-4 shown in Table 3.

[Table 3]

Test No.	Steel No.	Example classification	Manufacturing conditions of wire rod
Test No.	Steel No.	Example classification	Heating temperature of cast slab (°C)	Heating tine of cast slab (hr)	Heating temperature during wire rod rolling (°C)	Heating time during wire rod rolling (mm)	Finish rolling temperature (°C)
a0	A0	Example	1280	24	1105	60	921
a1	A1	Comparative Example	1200	4	1102	60	920
a0-1	A0	Comparative Example	1280	24	1180	60	915
a0-2	A0	Comparative Example	1280	24	1100	180	919
a0-3	A0	Comparative Example	1280	24	1106	so	830
a0-4	A0	Comparative Example	1280	24	1120	60	960
b0	B0	Example	1280	24	1100	60	900
b1	B1	Comparative Example	1200	4	1100	60	898
b0-1	B0	Comparative Example	1280	24	1185	60	905
b0-2	B0	Comparative Example	1280	24	1107	150	895
b0-3	B0	Comparative Example	1280	24	1107	60	820
b0-4	B0	Comparative Example	1280	24	1107	60	965

[Manufacture of Wire Rod and Steel Wire (1)]

Steel Nos. A0, A1, B0, and B1 having chemical compositions shown in Table 1 were smelted. The steel Nos. A0 and A1 and the steel Nos. B0 and B1 had almost the same compositions, but in order to separately produce wire rods having different variations in tensile strength or hardness of a surface layer in a longitudinal direction, for the test Nos. a0 and b0, cast slabs of chemical compositions of the steel Nos. A0 and B0 were respectively subjected to a heating treatment of heating the cast slabs to 1280°C and holding the cast slabs for 24 hours before bloom rolling to billets, and the billets subjected to bloom rolling into 122 mm square were used as raw materials for rolling.
As for the test Nos. a1 and b1, the cast slabs of the steel Nos. A1 and B1 were respectively used. The cast slabs were heated while being held at 1200°C for 4 hours as a normal condition without being heated at a temperature of 1260°C or higher before bloom rolling to billets, and the billets subjected to bloom rolling into 122 mm square were used as raw materials for rolling. Then, each billet was heated at 1100°C for 60 minutes, and then rolled into a wire rod. At this time, the finish rolling temperature was as shown in Table 3, and after the finish rolling, each wire rod was wound up into a wire rod coil. The wound wire rod coil was directly immersed into a molten salt bath held at 550°C to perform an isothermal transformation treatment, and then cooled with water to 300°C or lower to obtain a wire rod.
In the test Nos. a0-1 to a0-4, the cast slab of the steel No. A0 was used, and in the test Nos. b0-1 to b0-4, the cast slab of steel No. B0 was used. Before the cast slabs were subjected to bloom rolling to the billets, the cast slabs were subjected to a heating treatment of heating the cast slabs to 1280°C and holding the cast slabs for 24 hours, and the billets subjected to bloom rolling into 122 mm square were used as raw materials for rolling. In order to separately produce wire rods having different variations in tensile strength or hardness of a surface layer in the longitudinal direction even in steel having the same composition, rolling conditions were changed as shown in Table 3. Specifically, in the test Nos. a0-1 and b0-1, the heating temperature during wire rod rolling was set to 1150°C or higher, and in the test Nos. a0-2 and b0-2, the heating holding time during wire rod rolling was set to 90 minutes or more. In the test Nos. a0-3 and b0-3, the wire rods were rolled at a finish rolling temperature of 850°C or lower, and in the test Nos. a0-4 and b0-4, the wire rods were rolled at a finish rolling temperature of 950°C or higher. Other rolling conditions were set as shown in Table 3, and after finish rolling, each wire rod was wound up into a wire rod coil. The wound wire rod coil was directly immersed into a molten salt bath held at 550°C to perform an isothermal transformation treatment, and then cooled with water to 300°C or lower to obtain a wire rod.
Then, each wire rod was subjected to wire drawing processing to manufacture a steel wire. Specifically, each wire rod was subjected to a pickling treatment to remove a scale, and a zinc phosphate coating was then formed on the surface of the wire rod by a chemical conversion treatment in order to improve lubricity, followed by subjecting the wire rod to wire drawing processing using a cemented carbide die. In the wire drawing processing, the wire drawing processing was performed until a wire diameter reached 5.2 mm according to a pass schedule adjusted so that the reduction of area by wire drawing in each die was around 20% (the wire drawing processing under these conditions may be referred to as "wire drawing processing A"). Then, the steel wire obtained by wire drawing processing was immersed into a lead bath heated to 400°C for 30 seconds, and cooled with water.

[Evaluation (1)]

As for the wire rods and steel wires of the test Nos. a0, a1, and a0-1 to a0-4 and the test Nos. b0, b1, and b0-1 to b0-4 obtained by the above-described method, the metal structure of the wire rod, the tensile strengths of the wire rod and steel wire, the variation in the hardness of the surface layer of the wire rod, the corrosion resistance of the wire rod, the hydrogen embrittlement resistances of the wire rod and steel wire, and the torsion properties of the steel wire were evaluated. The results are shown in Table 4A. As a metal structure other than pearlite, one or both of pro-eutectoid ferrite and degenerate pearlite was observed.

[Table 4A]

Test No.	Steel No.	Example classification	Characteristics of wire rod								Characteristics of steel wire
			Hardness of surface layer			Wire diameter (mm)	Metal structure	Tensile strength (MPa)	Corrosion resistance	Hydrogen embrittlement resistance	Wire diameter (mm)	Reduction of area by wire drawing (%)	Tensile strength (MPa)	Hydrogen embrittlement resistance	Torsion properties
			Hvsiave	Hvsimax	Y3	Wire diameter (mm)	Pearlite area ratio (%)	Tensile strength (MPa)	Corrosion volume reduction ratio (%)	FIP breaking time (hr)	Wire diameter (mm)	Reduction of area by wire drawing (%)	Tensile strength (MPa)	FIP breaking time (hr)	Torsion properties
a0	A0	Example	381	401	20	11.0	95.1	1305	15.9	129	5.2	77.7	1811	65	Good
a1	A1	Comparative Example	385	446	61	11.0	94.8	1310	15.6	51	5.2	77.7	1805	27	Good
a0-1	A0	Comparative Example	395	451	56	11.0	92.5	1330	16.1	55	5.2	77.7	1825	24	Occurrence of delamination
a0-2	A0	Comparative Example	391	445	54	11.0	86.1	1323	15.4	45	5.2	77.7	1840	20	Occurrence of delamination
aO-3	A0	Comparative Example	372	415	43	11.0	84.2	1285	15.4	69	5.2	77.7	1778	24	Occurrence of delamination
a0-4	A0	Comparative Example	396	456	60	11.0	90.4	1335	15.1	41	5.2	77.7	1854	19	Occurrence of delamination
b0	B0	Example	397	420	23	11.0	93.1	1391	18.5	110	5.2	77.7	1875	58	Good
b1	B1	Comparative Example	401	453	52	11.0	91.5	1395	16.4	34	5.2	77.7	1864	21	Good
b0-1	B0	Comparative Example	390	435	45	11.0	88.9	1361	16.7	58	5.2	77.7	1845	23	Occurrence of delamination
b0-2	B0	Comparative Example	380	431	51	11.0	90.6	1340	14.8	42	5.2	77.7	1832	25	Occurrence of delamination
b0-3	B0	Comparative Example	375	426	51	11.0	82.1	1316	15.4	48	5.2	77.7	1791	19	Occurrence of delamination
bO-4	B0	Comparative Example	382	435	53	11.0	90.2	1345	16.7	41	5.2	77.7	1825	24	Occurrence of delamination

As for the wire rods of the test Nos. a0, a0-1, and a0-4, the hardness of the surface layer was measured for each of eight samples collected at intervals of 50 mm in the longitudinal direction of the wire rod at arbitrary positions and having a length of 25 mm. The results are shown in Table 4B. In Table 4B, items other than the hardness of the surface layer of the wire rod are the same as those in Table 4A.

[Table 4B]

Test No.	Steel No.	Example classification	Characteristics of wire rod								Characteristics of steel wire
			Hardness of surface layer			Wire diameter (mm)	Metal structure	Tensile strength (MPa)	Corrosion resistance	Hydrogen embrittlement resistance	Wire diameter (mm)	Reduction of area by wire drawing (%)	Tensile strength (MPa)	Hydrogen embrittlement	Torsion properties
			Hvsiave	Hvsimax	Y3	Wire diameter (mm)	Pearlite area ratio (%)	Tensile strength (MPa)	Corrosion volume reduction ratio (%)	FIP breaking time (hr)	Wire diameter (mm)	Reduction of area by wire drawing (%)	Tensile strength (MPa)	resistance FIP breaking time (hr)	Torsion properties
a0	A0	Example	379	396	17	11.0	95.1	1305	15.9	129	5.2	77.7	1811	65	Good
a0-1	A0	Comparative Example	391	443	52	11.0	92.5	1330	16.1	55	5.2	77.7	1825	24	Occurrence of delamination
a0-4	A0	Comparative Example	399	453	54	11.0	90.4	1335	15.1	41	5.2	77.7	1854	19	Occurrence of delamination

[Manufacture of Wire Rod and Steel Wire (2)]

In order to confirm the effects of the chemical compositions in the present invention, the steel Nos. C1 to C24 and steel Nos. D1 to D22 of the chemical compositions shown in Table 2 were smelted in an electric furnace. The steel Nos. C1 to C24 are Examples satisfying the requirements of the present invention, and the steel Nos. D1 to D22 are Comparative Examples not satisfying any one or more of the requirements of the present invention.
Each of the cast slabs of the steel Nos. C1 to C24 and steel Nos. D1 to D22 was subjected to a heat treatment of heating the cast slab at a target temperature of 1280°C and holding the cast slab for 24 hours before being subjected to bloom rolling to the billets, and subsequently, the billet subjected to bloom rolling into 122 mm square was used as a raw material for rolling.
Then, each of the billets was heated at a target heating temperature of 1080°C for 60 minutes, and then rolled into a wire rod having a wire diameter of from 8.0 to 12.5 mm. At this time, the target finish rolling temperature was 900°C, and the wire rod was wound up into a wire rod coil. The wound wire rod coil was directly immersed into a molten salt bath held at 550°C to perform an isothermal transformation treatment, and cooled with water to 300°C or lower to obtain a wire rod.
Each wire rod was subjected to wire drawing processing to manufacture a steel wire. Specifically, wire drawing processing was performed so as to obtain a steel wire having a wire diameter of from 3.8 to 5.2 mm by the same method as that of the above-described wire drawing processing A. Then, the steel wire obtained by wire drawing processing was immersed into a lead bath heated to 400°C for 30 seconds, and cooled with water.

[Evaluation (2)]

For the wire rods and steel wires of the test Nos. c1 to c24 and test Nos. d1 to d22 obtained by the above-described method, the metal structure of the wire rod, the tensile strengths of the wire rod and steel wire, the variation in the hardness of the surface layer of the wire rod, the corrosion resistance of the wire rod, the hydrogen embrittlement resistance of the wire rod and steel wire, and the torsion properties of the steel wire were evaluated by the above-described methods. The results are shown in Table 5. As a metal structure other than pearlite, martensite, bainite, pro-eutectoid ferrite, and degenerate pearlite were observed.

[Table 5]

Test No.	Steel No.	Example classification	Characteristics of wire rod								Characteristics of steel wire
			Hardness of surface layer			Wire diameter (mm)	Metal structure	Tensile strength (MPa)	Corrosion resistance	Hydrogen embrittlement resistance	Wire diameter (mm)	Reduction of area by wire drawing (%)	Tensile strength (MPa)	Hydrogen embrittlement resistance	Torsion properties
			Hvsiave	Hvsimax	Y3	Wire diameter (mm)	Pearlite area ratio (%)	Tensile strength (MPa)	Corrosion volume reduction ratio (%)	FIP breaking time (hr)	Wire diameter (mm)	Reduction of area by wire drawing (%)	Tensile strength (MPa)	FIP breaking time (hr)	Torsion properties
c1	C1	Example	319	363	44	11.5	91.6	1098	11.1	196	3.8	89.1	1803	64	Good
c2	C2	Example	323	363	40	11.5	92.5	1110	19.0	200	3.8	89.1	1801	66	Good
c3	C3	Example	362	387	25	11.0	90.4	1255	13.8	194	4.8	81.0	1759	58	Good
c4	C4	Example	371	411	40	11.0	94.2	1274	16.9	124	4.8	81.0	1816	59	Good
c5	C5	Example	380	413	33	11.0	93.5	1305	13.2	120	4.8	81.0	1885	46	Good
c6	C6	Example	382	395	13	11.0	93.8	1325	16.4	125	5.2	77.7	1837	45	Good
c7	C7	Example	388	419	31	11.0	95.2	1358	16.9	116	5.2	77.7	1888	48	Good
c8	C8	Example	417	448	31	10.0	96.5	1489	18.5	98	5.2	73.0	2007	39	Good
c9	C9	Example	436	458	22	10.0	97.1	1521	19.0	88	5.2	73.0	1999	37	Good
c10	C10	Example	401	441	40	10.0	92.5	1378	15.4	106	5.2	73.0	1849	53	Good
c11	C11	Example	406	441	35	9.0	91.2	1420	14.3	98	4.8	71.6	1910	40	Good
c12	C12	Example	407	445	38	9.0	90.5	1478	18.5	85	4.8	71.6	1967	36	Good
c13	C13	Example	434	459	25	8.0	91.2	1501	15.9	85	4.2	72.4	1951	45	Good
c14	C14	Example	438	474	36	10.0	91.3	1486	16.4	105	4.8	77.6	1984	38	Good
c15	C15	Example	350	384	34	11.0	92.5	1193	16.9	189	4.8	81.0	1745	70	Good
c16	C16	Example	375	400	25	11.0	92.1	1289	19.5	158	4.8	81.0	1853	58	Good
c17	C17	Example	360	382	22	11.0	93.5	1301	19.0	178	4.8	81.0	1841	52	Good
c18	C18	Example	375	403	28	11.0	92.5	1293	19.5	135	4.8	81.0	1825	60	Good
c19	C19	Example	380	400	20	11.0	92.1	1256	16.4	145	4.8	81.0	1786	65	Good
c20	C20	Example	375	415	40	11.0	94.2	1298	16.9	108	4.8	81.0	1796	61	Good
c21	C21	Example	371	398	27	11.0	91.9	1299	16.4	151	4.8	81.0	1820	54	Good
c22	C22	Example	368	399	31	11.0	92.7	1290	16.4	154	4.8	81.0	1790	69	Good
c23	C23	Example	384	413	29	11.0	93.2	1310	15.4	130	4.8	81.0	1834	51	Good
c24	C24	Example	382	409	27	11.0	92.4	1306	14.8	125	4.8	81.0	1844	58	Good
d1	D1	Comparative Example	395	427	32	11.0	88.1	1325	19.0	119	5.2	77.7	1794	25	Occurrence of delamination
d2	D2	Comparative Example	381	416	35	11.0	90.2	1289	15.9	135	4.8	81.0	1864	28	Occurrence of delamination
d3	D3	Comparative Example	405	449	44	11.0	95.7	1420	17.4	50	5.2	77.7	1951	25	Good
d4	D4	Comparative Example	421	488	67	11.0	95.7	1453	14.0	30	5.2	77.7	1898	9	Occurrence of delamination
d5	D5	Comparative Example	409	451	42	11.0	90.1	1421	15.4	24	5.2	77.7	1951	12	Occurrence of delamination
d6	D6	Comparative Example	376	395	19	11.0	93.8	1311	34.2	80	5.2	77.7	1821	23	Good
d7	D7	Comparative Example	370	400	30	11.0	93.9	1275	31.3	93	4.8	81.0	1867	29	Good
d8	D8	Comparative Example	406	423	17	9.0	91.2	1399	35.1	51	3.8	82.2	2035	15	Good
d9	D9	Comparative Example	399	458	59	11.0	80.6	1373	10.0	41	-	-	-	-	-
d10	D10	Comparative Example	376	445	69	11.0	83.5	1289	11.6	35	5.2	77.7	1795	10	Occurrence of delamination
d11	D11	Comparative Example	411	431	20	11.0	92.5	1384	-	-	-	-	-	-	-
d12	D12	Comparative Example	285	299	14	12.5	82.3	978	17.4	195	3.2	93.4	1869	15	Occurrence of delamination
d13	D13	Comparative Example	446	502	56	11.0	94.7	1558	19.0	15	5.2	77.7	2094	8	Occurrence of delamination
d14	D14	Comparative Example	415	460	45	11.0	83.6	1431	14.0	42	5.2	77.7	1921	20	Occurrence of delamination
d15	D15	Comparative Example	405	489	84	11.0	82.9	1410	19.5	25	5.2	77.7	1854	15	Occurrence of delamination
d16	D16	Comparative Example	369	390	21	11.0	92.5	1284	19.5	43	4.8	81.0	1832	23	Good
d17	D17	Comparative Example	371	393	22	11.0	94.5	1278	21.6	81	4.8	81.0	1840	26	Occurrence of delamination
d18	D18	Comparative Example	386	410	24	11.0	93.8	1315	19.0	75	5.2	77.7	1839	20	Good
d19	D19	Comparative Example	375	431	56	11.0	93.1	1279	19.0	64	4.8	81.0	1821	21	Occurrence of delamination
d20	D20	Comparative Example	399	427	28	9.0	94.5	1370	14.0	127	3.8	82.2	1921	28	Occurrence of delamination
d21	D21	Comparative Example	403	460	57	11.0	91.6	1364	19.5	42	5.2	77.7	1894	24	Occurrence of delamination
d22	D22	Comparative Example	432	496	64	8.0	81.2	1512	15.9	10	-	-	-	-	-

Fig. 1 shows the relationship between the tensile strength of a wire rod and an FIP breaking time which is an index of hydrogen embrittlement resistance, obtained in Examples of the present invention. Fig. 2 shows the relationship between the tensile strength of a steel wire after wire drawing processing and an FIP breaking time which is an index of hydrogen embrittlement resistance, obtained in Examples of the present invention.
From Table 4A and Table 4B, the test Nos. a0 and b0, which are Examples of the present invention, satisfy the chemical composition and other requirements in the present disclosure, and the manufacturing conditions of the wire rod are appropriate. Therefore, wire rods having excellent corrosion resistance and hydrogen embrittlement resistance are obtained. The wire rods have a corrosion volume reduction ratio of less than 25%, which is an evaluation index of the corrosion resistance of the wire rod, and an FIP breaking time of 100 hr or more, which is an index of the hydrogen embrittlement resistance. Furthermore, the steel wires after wire drawing processing obtained in the test Nos. a0 and b0 and having excellent hydrogen embrittlement resistance are obtained. The steel wires have tensile strength of 1700 MPa or more, an FIP breaking time of 30 hr or more, no delamination even in the torsion test.
The Vickers hardness in the surface layer portion is found to be effective in a case in which the relationship of "Hv_simax-Hv_siave ≤ 50" is satisfied in not only the case where samples are collected at intervals of 600 mm assuming a length of one ring and measured, but also the case where the samples are collected at intervals of 50 mm and measured.
In the test No. a1 and the test Nos. a0-1 to a0-4, the wire rod was rolled using the steel A1 having almost the same chemical composition as that of the test No. a0 or the steel No. A0 having the same chemical composition as that of the test No. a0. In the test No. b1 and the test Nos. b0-1 to b0-4, the wire rod was rolled using the steel No. B1 having almost the same chemical composition as that of the test No. b0 or the steel No. B0 having the same chemical composition as that of the test No. b0. The manufacturing conditions of the wire rod were not appropriate, whereby the variation in the hardness of the surface layer or the area ratio of the pearlite structure did not satisfy the requirements of the present invention. Therefore, the hydrogen embrittlement resistance of the wire rod is poor, and the hydrogen embrittlement resistance of the steel wire after wire drawing processing is poor. In the test Nos. a0-1 to a0-4 and the test Nos. b0-1 to b0-4, delamination occurs in the torsion test of the steel wire after wire drawing processing, and the torsion properties are also poor.
From Table 5, all the test Nos. c1 to c24, which are Examples of the present invention, satisfy the chemical composition and the requirements of the present disclosure, and the conditions for manufacturing the steel material are appropriate, whereby the test Nos. c1 to c24 have tensile strength of from 1000 MPa to 1650 MPa and excellent hydrogen embrittlement resistance in a case of being compared in terms of corrosion resistance and equivalent tensile strength.
The test Nos. d1 and d2 satisfy [Cu]/[Ni] < 1.00, and the test No. d2 also satisfies Y2 of 1.81 or more. Delamination occurs in the steel wire after wire drawing processing, and the torsion properties are poor. The hydrogen embrittlement resistance of the steel wire is also poorer than that of the steel wire having the same level of tensile strength in Examples.
The test No. d3 has a value of Y1 of less than 1.70, and the hydrogen embrittlement resistance of the wire rod and the hydrogen embrittlement resistance of the steel wire after wire drawing processing are poor.
In the test No. d4, the value of Y1 is more than 4.50, and the hydrogen embrittlement resistance of the wire rod, and the hydrogen embrittlement resistance and torsion properties of the steel wire after wire drawing processing are poor.
The test No. d5 has a value of Y2 of 1.81 or more, and the hydrogen embrittlement resistance of the wire rod, and the hydrogen embrittlement resistance and torsion properties of the steel wire after wire drawing processing are poor.
As for the test Nos. d6, d7, d8, d10, and d12 to d21, any of the chemical compositions in the present disclosure falls outside the range of the present invention, or the value of Y2 is 1.81 or more. The hydrogen embrittlement resistance of the wire rod and/or the hydrogen embrittlement resistance and torsion properties of the steel wire after wire drawing processing are poor.
In the test Nos. d9 and d22, the chemical compositions fell outside the range of the present invention, and breakage occurred at the stage of wire drawing processing, whereby the tensile strength, hydrogen embrittlement resistance, and torsion properties of the steel wire were not investigated.
In the test No. d11, the chemical compositions fell outside the range of the present invention (Y2 was also 1.81 or more), and large surface flaws appeared at the stage in a case in which the wire rod was rolled, whereby the corrosion resistance and hydrogen embrittlement resistance of the wire rod were not investigated, and wire drawing processing was not performed.
The applications and the like of the wire rod according to the present invention are not limited to the above-described embodiments and Examples. For example, the wire rod according to the present invention is not limited to a raw material of a steel wire having tensile strength of 1700 MPa or more, and may be used as a raw material of a steel wire having required tensile strength of less than 1700 MPa.

Claims

A wire rod comprising a chemical composition consisting of, in % by mass,
C: from 0.60 to 1.15%,

Si: from 0.01 to 1.80%,

Mn: from 0.20 to 0.90%,

P: 0.015% or less,

S: 0.015% or less,

Al: from 0.005 to 0.080%,

N: from 0.0015 to 0.0060%,

Cu: from 0.10 to 0.65%,

Ni: from 0.05 to less than 0.65%,

Cr: from 0 to 0.30%,

Mo: from 0 to 0.30%,

Ti: from 0 to 0.100%,

Nb: from 0 to 0.100%,

V: from 0 to 0.20%,

Sn: from 0 to 0.30%,

B: from 0 to 0.0050%,

Ca: from 0 to 0.0050%,

Mg: from 0 to 0.0050%,

Zr: from 0 to 0.100%,

REM: from 0 to 0.0200%; and

a balance being Fe and impurities,

wherein:
the wire rod satisfies the following (1) to (3): $[Cu] / [Ni] > 1.00;$
$1.70 \leq Y 1 \leq 4.50,$
$Y 1 = 3 \times [Cr] + 5 \times [Mn] + [Cu] + [Ni];$
and $Y 2 < 1.81,$
$Y 2 = [C] + [Si] / 10 + A;$

with the contents of elements of C, Si, Mn, Cr, Cu, Ni, N, and Ti contained in the wire rod in % by mass being represented by [C], [Si], [Mn], [Cr], [Cu], [Ni], [N], and [Ti], respectively,

and A satisfying A = a if a value of a = 350 × ([N] - 0.29 × [Ti]) satisfies a ≥ 0, and A = 0 if the value satisfies a < 0;

a metal structure of the wire rod contains a pearlite structure having an area ratio of 90% or more in a cross section parallel to a longitudinal direction including a central axis of the wire rod; and

the wire rod satisfies the following (4): ${Hv}_{simax} - {Hv}_{siave} \leq 50$
with the Vickers hardness measured at a position of a depth of 50 µm from a surface of the wire rod in the cross section for each of eight samples si (i is an integer from 1 to 8) collected at arbitrary equal intervals in the longitudinal direction of the wire rod being Hv_si, and an average value and maximum value of Hv_si being respectively Hv_siave and Hv_simax,wherein the Vickers hardness is measured as defined in the description.
The wire rod according to claim 1, wherein the arbitrary equal intervals are intervals of 600 mm.
The wire rod according to claim 1 or 2, wherein the chemical composition comprise one or two or more selected from the group consisting of, in % by mass,
Cr: from 0.01 to 0.30%,

Mo: from 0.01 to 0.30%,

Ti: from 0.002 to 0.100%,

Nb: from 0.002 to 0.100%,

V: from 0.01 to 0.20%,

Sn: from 0.01 to 0.30%,

B: from 0.0002 to 0.0050%,

Ca: from 0.0002 to 0.0050%,

Mg: from 0.0002 to 0.0050%,

Zr: from 0.0002 to 0.100%, and

REM: from 0.0002 to 0.0200% in place of a part of the Fe.