JP7226548B2 - wire - Google Patents

Description

The present disclosure relates to wires.

In recent years, a high tensile strength of 1700 MPa or more is required for high-strength steel wires used in fields such as steel wires for high-strength ropes, steel wires for bridge cables, and PC steel wires. These high-strength steel wires are manufactured, for example, by patenting a rolled wire rod having a diameter of 5.0 to 16.0 mm to make the metal structure into a pearlite structure, and then drawing the wire.
When the tensile strength of the steel wire after wire drawing is high, there is a high risk of embrittlement due to strain aging due to heat generated during wire drawing. Due to strain aging, the number of rotations (twisting value) before breaking in a twisting test decreases, and longitudinal cracks called delamination may occur in high-strength steel wires. Occurrence of delamination in the torsion test causes breakage in the wire stranding process for making the steel wire into a product, thus deteriorating productivity. Therefore, it is desirable that high-strength steel wires have both tensile strength and torsional properties.
In addition, the higher the strength of the steel wire, the higher the risk of breakage due to progress of corrosion or hydrogen embrittlement when the steel wire or stranded wire product is used in a corrosive environment. Therefore, high-strength steel wires used in the above fields and wire rods used as raw materials thereof are desired to have excellent corrosion resistance and hydrogen embrittlement resistance.

As a technique for improving the torsional properties of high-strength steel wires, for example, Patent Document 1 discloses, in mass %, C: 0.75 to 1.10%, Si: 0.10 to 1.40%, Mn: 0 .10-1.0%, Al: 0-0.10%, Ti: 0-0.10%, Cr: 0-0.60%, V: 0-0.10%, Nb: 0-0. 10%, Mo: 0-0.20%, W: 0-0.50%, B: 0-0.0030%, N: 0.006% or less, P: 0.03% or less, S: The area ratio of the metal structure is limited to 0.03% or less, the balance is substantially composed of Fe, and the metal structure is in the area on the side of the axis from the depth of 100 μm from the surface of the L cross section along the axial direction including the axis of the steel wire contains 90% or more of drawn pearlite, and in the region from the surface of the L cross section to a depth of 100 μm, the metal structure contains 70% or more of drawn pearlite in terms of area ratio, and the diameter of the steel wire (D [mm] ), the standard deviation (σHV) of the Vickers hardness of the surface of the steel wire, and the yield strength (Rp0.2) of the steel wire satisfy the following formula (1), and the tensile strength is a steel wire having a high strength of 1770 MPa or more is proposed.
σHV<(−9500×ln(d)+30000)×exp(−0.003×Rp0.2) (1)

Further, as a technique for improving the hydrogen embrittlement resistance of high-strength steel wires, for example, Patent Document 2 discloses C: 0.70 to 1.20%, Si: 0.10 to 2.00%, Mn: 0.00%. 20-1.00%, P: 0.030% or less, S: 0.030% or less, N: 0.0010-0.0100%, Al: 0-0.100%, Cr: 0-2.00 %, V: 0-0.30%, B: 0-0.0050%, Ti: 0-0.050%, Nb: 0-0.050%, Zr: 0-0.050%, Ni: 0 ~2.00%, Cu: 0-1.00%, Sn: 0-0.50%, Mg: 0-0.010%, Ca: 0-0.010%, a chemical component consisting of metal The structure has a pearlite structure of 95 area% or more, the average aspect ratio R of the pearlite blocks measured in the surface layer in the axial cross section including the axis of the steel wire is 2.0 or more, and the diameter of the steel wire is D Then, in the cross section in the axial direction including the axis of the steel wire, (average aspect ratio measured at the surface layer) / (average aspect ratio measured at the position of 0.25D) is 1.1 or more, and the tensile strength is A high-strength steel wire having excellent hydrogen embrittlement resistance of 1800 MPa or more has been proposed.

Furthermore, in Patent Document 3, in addition to containing C: 0.5 to 1.0%, one or more selected from the group consisting of Cu, Ni and Ti (however, Cu and / or Ni is contained) It is characterized by being made of steel containing so as to satisfy the following formula (1), having an area ratio of pearlite structure of 80% or more, and having a strength of 1200 N/mm ² or more. A high-strength steel wire with excellent delayed fracture resistance and corrosion resistance has been proposed.
3.1≧3[Cu]+[Ni]+6[Ti]≧0.24(%) (1)
However, [Cu], [Ni] and [Ti] indicate the contents (% by mass) of Cu, Ni and Ti, respectively.
Further, in Patent Document 4, C: 0.39 to 0.65%, Si: 1.5 to 2.5%, Mn: 0.15 to 1.2%, P: more than 0%, 0.015 % or less, S: more than 0%, 0.015% or less, Al: 0.001 to 0.1%, Cu: 0.1 to 0.80%, Ni: 0.1 to 0.80% , The balance is iron and inevitable impurities, the non-diffusible hydrogen content is 0.40 mass ppm or less, and the area ratio of ferrite expressed as a percentage satisfies the following formula (1), and the sum of bainite and martensite A high-strength rolled material for a spring has been proposed, characterized by having an area ratio of 2% or less.
Ferrite area ratio<{(0.77−[C])/0.77−[C]/3+0.08}×100 (1)
However, in the above formula (1), [element name] means the content of each element represented by mass %.
Further, in Patent Document 5, C: 0.55 to 0.75%, Si: 0.1 to 1.0%, Mn: 0.3 to 1.5%, Cr: 0.1 to 2.0% , S: 0.002-0.05%, Al: 0.01-0.2% and N: 0.002-0.01%, the balance being Fe and impurities, P and O has a chemical composition in which P: 0.025% or less and O: 0.002% or less, and Fn1 represented by the following formula [1] is 2.5 to 4.5, and a structure However, a hot-rolled steel bar or wire rod has been proposed, which has a pearlite fraction of 90% or more, an average pearlite lamellar spacing of 150 to 300 nm, and a standard deviation of the pearlite lamellar spacing of 25 nm or less. .
Fn1=3Si+Mn+1.5Cr [1]
However, the element symbol in the formula [1] means the content (% by mass) of each element.

Patent Document 1: International Publication No. 2018/012625 Patent Document 2: International Publication No. 2018/021574 Patent Document 3: Patent No. 4124590 Patent Document 4: JP-A-2015-143391 Patent Document 5: JP-A-2014-37592 publication

The present disclosure is a wire suitable as a material for high-strength steel wires that require a high ultimate tensile strength of 1700 MPa or more, is excellent in corrosion resistance and hydrogen embrittlement resistance, and is steel after wire drawing. An object of the present invention is to provide a wire that is less prone to delamination and has excellent twisting properties.

Means for solving the above problems include the following aspects.
<1> The chemical component is mass %,
C: 0.60 to 1.15%,
Si: 0.01 to 1.80%,
Mn: 0.20-0.90%,
P: 0.015% or less,
S: 0.015% or less,
Al: 0.005 to 0.080%,
N: 0.0015 to 0.0060%,
Cu: 0.10-0.65%,
Ni: less than 0.05 to 0.65%,
Cr: 0 to 0.30%,
Mo: 0-0.30%,
Ti: 0 to 0.100%,
Nb: 0 to 0.100%,
V: 0 to 0.20%,
Sn: 0 to 0.30%,
B: 0 to 0.0050%,
Ca: 0 to 0.0050%,
Mg: 0-0.0050%,
Zr: 0 to 0.100%,
REM: 0 to 0.0200%, and the balance: Fe and impurities,
The content in mass% of each element of C, Si, Mn, Cr, Cu, Ni, N, and Ti contained in the wire is expressed as [C], [Si], [Mn], [Cr], [ Cu], [Ni], [N], and [Ti] satisfy the following (1) to (3),
(1) [Cu]/[Ni]>1.00
(2) 1.70≤Y1≤4.50
Y1 = 3 x [Cr] + 5 x [Mn] + [Cu] + [Ni]
(3) Y<1.81
Y2=[C]+[Si]/10+A
A has a value of a = 350 x ([N] - 0.29 x [Ti])
If a≧0 then A=a
if a<0 then A=0
The metal structure contains a pearlite structure with an area ratio of 90% or more in a cross section parallel to the longitudinal direction including the central axis of the wire,
Eight samples si (i is an integer of 1 to 8) taken at arbitrary equal intervals in the longitudinal direction of the wire were measured at a depth of 50 μm from the surface of the wire in the cross section of each sample. A wire that satisfies the following (4), where Hv _si is the Vickers hardness of each Hv si, Hv _siave is the average value of the Hv _si , and Hv _simax is the maximum value.
(4) _Hvsimax - _Hvsiave≤50
<2> The wire according to <1>, wherein the arbitrary equal interval is an interval of 600 mm.
<3> The chemical component is replaced by a part of the Fe, in mass%,
Cr: 0.01 to 0.30%,
Mo: 0.01-0.30%
Ti: 0.002 to 0.100%,
Nb: 0.002 to 0.100%,
V: 0.01 to 0.20%,
Sn: 0.01 to 0.30%,
B: 0.0002 to 0.0050%
Ca: 0.0002 to 0.0050%,
Mg: 0.0002-0.0050%,
Zr: 0.0002-0.100%, and REM: 0.0002-0.0200%,
The wire according to <1> or <2>, including one or more selected from the group consisting of:

According to the present disclosure, a wire rod suitable as a material for a high-strength steel wire that requires a high tensile strength of 1700 MPa or more, has excellent corrosion resistance and hydrogen embrittlement resistance, and is detrimental to the steel wire after wire drawing. Provided is a wire that is less prone to lamination and has excellent twisting properties.

FIG. 4 is a diagram showing the relationship between the tensile strength of a wire rod and the FIP rupture time, which is an index of hydrogen embrittlement resistance, obtained in an example of the present disclosure. FIG. 2 is a diagram showing the relationship between the tensile strength of a steel wire after wire drawing and the FIP rupture time, which is an index of hydrogen embrittlement resistance, obtained in an example of the present disclosure.

An embodiment that is an example of the present disclosure will be described.
In the present disclosure, a numerical range represented using "to" means a range including the numerical values described before and after "to" as lower and upper limits. However, the numerical range when "greater" or "less than" is attached to the numerical value described before and after "to" means the range that does not include these numerical values as the lower or upper limit.
In addition, in the numerical ranges described step by step in the present disclosure, the upper limit of one step of the numerical range may be replaced with the upper limit of another step of the numerical range. Lower limits of ranges may be replaced with lower limits of other numerical ranges recited. Also, the upper limit value or the lower limit value may be replaced with the values shown in the examples.
In the present disclosure, "%" indicating the content of a component (element) means "% by mass".
In the present disclosure, the content of C (carbon) may be referred to as "C amount". Contents of other elements may also be expressed similarly.
In the present disclosure, the "surface" of the wire rod or steel wire means the outer peripheral surface. The "surface" of a sample obtained by cutting a wire rod or steel wire also means the outer peripheral surface.

In order to solve the above-described problems, the present inventors have developed a wire rod suitable as a material for high-strength steel wires that require a high tensile strength of 1700 MPa or more (referred to as "wire rod for high-strength steel wires" in the present disclosure). We conducted various studies on the effects of elements and metal structures on the corrosion resistance, hydrogen embrittlement resistance, and torsional characteristics after wire drawing, and found the following (a) to (c). got

(a) A high-strength steel wire having a tensile strength of 1700 MPa or more is likely to undergo delamination and breakage due to corrosion or hydrogen embrittlement. In order to suppress the occurrence of delamination of high-strength steel wires and prevent breakage due to corrosion or hydrogen embrittlement, it is necessary to consider the range of chemical composition of the wire material to prevent deterioration of torsional characteristics, corrosion resistance and hydrogen embrittlement resistance. It is sufficient to improve the characteristics, and Cu: 0.10 to 0.65% and Ni: less than 0.05 to 0.65% are contained in a range that satisfies [Cu]>[Ni], and the following formula <1 Mn, Cr, Cu, and Ni may be contained within a range in which Y1 represented by > satisfies 1.70≦Y1≦4.50.
Y1=3×[Cr]+5×[Mn]+[Cu]+[Ni] <1>
Here, [Mn], [Cr], [Cu], and [Ni] in the above formula represent the content of each element in mass %.

(b) A high-strength steel wire is manufactured by drawing a wire rod. A high-strength steel wire with a tensile strength of 1700 MPa or more tends to have a large temperature rise and work heat generation due to frictional heat with a die during wire drawing, and is easily embrittled by strain aging. Due to strain aging, longitudinal cracks called delamination occur in high-strength steel wires in a twisting test, and the number of rotations until breakage, that is, the twisting value, decreases. Furthermore, while preventing breakage due to corrosion or hydrogen embrittlement of high-strength steel wires with a tensile strength of 1700 MPa or more, the effect of strain aging is minimized by wire drawing, and the torsion value after wire drawing is improved. For this purpose, C, Si, Ti, and N should be contained within the range where Y2 represented by the following formula <2> satisfies Y2<1.81.
Y2=[C]+[Si]/10+A <2>
Here, A in the formula <2> is A=a when the value calculated by the following formula <4> is a≧0, and A=0 when a<0.
a=350×([N]−0.29×[Ti]) <4>
Here, [C], [Si], [N], and [Ti] in the above formula represent the content in mass% of each element, and A is related to a represented by formula <4> is a parameter.

(c) When Mn, Cr, Cu, and Ni are contained in order to improve the corrosion resistance and hydrogen embrittlement resistance of the wire, the hardenability of the wire increases, and the hardness variation of the surface layer of the wire increases depending on the manufacturing conditions of the wire. tend to become Due to this variation in the hardness of the surface layer of the wire, if there is a portion with a locally high surface layer hardness in the longitudinal direction of the wire, the hydrogen embrittlement resistance drops sharply. The deterioration of the hydrogen embrittlement resistance of the wire leads to the deterioration of the hydrogen embrittlement resistance of the high-strength steel wire obtained by drawing the wire. Therefore, in order to simultaneously improve the corrosion resistance and hydrogen embrittlement resistance of the wire, it is necessary to satisfy the range of specific chemical components, reduce the hardness variation of the surface layer of the wire, and eliminate the part where the surface layer hardness is locally high. It is necessary.

The wire according to the present disclosure has been completed based on the above knowledge, and the chemical component is, in mass%,
C: 0.60 to 1.15%,
Si: 0.01 to 1.80%,
Mn: 0.20-0.90%,
P: 0.015% or less,
S: 0.015% or less,
Al: 0.005 to 0.080%,
N: 0.0015 to 0.0060%,
Cu: 0.10-0.65%,
Ni: less than 0.05 to 0.65%,
Cr: 0 to 0.30%,
Mo: 0-0.30%,
Ti: 0 to 0.100%,
Nb: 0 to 0.100%,
V: 0 to 0.20%,
Sn: 0 to 0.30%,
B: 0 to 0.0050%,
Ca: 0 to 0.0050%,
Mg: 0-0.0050%,
Zr: 0 to 0.100%,
REM: 0 to 0.0200%, and the balance: Fe and impurities,
The content in mass% of each element of C, Si, Mn, Cr, Cu, Ni, N, and Ti contained in the wire is expressed as [C], [Si], [Mn], [Cr], [ Cu], [Ni], [N], and [Ti] satisfy the following (1) to (3),
(1) [Cu]/[Ni]>1.00
(2) 1.70≤Y1≤4.50
Y1 = 3 x [Cr] + 5 x [Mn] + [Cu] + [Ni]
(3) Y<1.81
Y2=[C]+[Si]/10+A
A has a value of a = 350 x ([N] - 0.29 x [Ti])
If a≧0 then A=a
if a<0 then A=0
The metal structure contains a pearlite structure with an area ratio of 90% or more in a cross section parallel to the longitudinal direction including the central axis of the wire (sometimes referred to as an "axial cross section" in the present disclosure),
For each of eight samples si (i is an integer of 1 to 8) taken at arbitrary equal intervals in the longitudinal direction of the wire, in the axial cross section of each sample, the surface (peripheral surface) of the wire at a depth of 50 μm When the Vickers hardness measured at each position is _Hvsi , the average value of _Hvsi is _Hvsiave , and the maximum value is _Hvsimax , the following (4) is satisfied.
(4) _Hvsimax - _Hvsiave≤50

<Chemical composition>
First, the reason for limiting the range of elements contained in the wire rod for high-strength steel wire according to the present disclosure will be described.
The chemical composition of the wire rod for high-strength steel wire according to the present disclosure is, in mass%, C: 0.60 to 1.15%, Si: 0.01 to 1.80%, Mn: 0.20 to 0.90. %, P: 0.015% or less, S: 0.015% or less, Al: 0.005 to 0.080%, N: 0.0015 to 0.0060%, and [Cu]> Cu: 0.10 to 0.65% and Ni: 0.05 to less than 0.65% within the range satisfying [Ni], and further optionally contained components are Cr: 0 to 0.30 %, Mo: 0-0.30%, Ti: 0-0.100%, Nb: 0-0.100%, V: 0-0.20%, Sn: 0-0.30%, B: 0 ~0.0050%, Ca: 0-0.0050%, Mg: 0-0.0050%, Zr: 0-0.100%, and REM: 0-0.0200%, the balance being Fe and impurities consists of

C: 0.60-1.15%
C is contained in order to increase the tensile strength of the wire. If the amount of C is less than 0.60%, pro-eutectoid ferrite is formed, and the tensile strength required for high-strength steel wires cannot be secured. Therefore, the amount of C is made 0.60% or more. In order to obtain a high-strength steel wire without excessively increasing the reduction in drawing area from the viewpoint of ensuring excellent hydrogen embrittlement resistance and torsional properties, the C content is 0.67% or more. is preferred, 0.70% or more is more preferred, and 0.85% or more is even more preferred. On the other hand, if the amount of C exceeds 1.15%, the amount of proeutectoid cementite increases and wire drawability deteriorates, making it difficult to obtain a high-strength steel wire. to degrade. Therefore, the C content is preferably 1.10% or less, more preferably 1.05% or less.

Si: 0.01-1.80%
Si has the effect of increasing tensile strength through solid solution strengthening, and has the effect of increasing hydrogen embrittlement resistance. If the amount of Si is less than 0.01%, these effects cannot be obtained. Therefore, the amount of Si is made 0.01% or more. In order to reliably obtain these effects, the Si content is preferably 0.21% or more, more preferably 0.70% or more. However, when the amount of Si exceeds 1.80%, these effects are saturated and the hot ductility deteriorates. In addition, the torsional properties of the high-strength steel wire after wire drawing are degraded. Therefore, the Si content is preferably 1.49% or less, more preferably 1.35% or less.

Mn: 0.20-0.90%
Mn has the effect of increasing the hardenability of steel and increasing the tensile strength of steel after pearlitic transformation. If the Mn content is less than 0.20%, the above effect cannot be sufficiently obtained. Therefore, the amount of Mn should be 0.20% or more. In order to reliably obtain these effects, the Mn content is preferably 0.30% or more, more preferably 0.35% or more. On the other hand, if the amount of Mn exceeds 0.90%, the hardenability of the steel becomes too high, the above effects are saturated, and the ductility of the wire is reduced, resulting in the twisting characteristics of the high-strength steel wire obtained after wire drawing. deteriorates. Therefore, the Mn content is preferably 0.80% or less, more preferably 0.75% or less.

P: 0.015% or less P is contained as an impurity. P segregates at grain boundaries to degrade hydrogen embrittlement resistance and wire drawability, so the lower the P content, the better. Therefore, the upper limit of the amount of P is 0.015%. A preferable range of the amount of P is 0.012% or less, more preferably 0.010% or less. Although the lower limit of the amount of P is not particularly limited, it may be more than 0%, and for example, it may be 0.0001% or more from the viewpoint of steelmaking cost reduction.

S: 0.015% or less S is contained as an impurity. Since S segregates at grain boundaries and degrades hydrogen embrittlement resistance and wire drawability, the amount of S must be suppressed. Therefore, the upper limit of the amount of S is 0.015%. A preferable range of the S amount is 0.012% or less, and a more preferable range is 0.010% or less. The lower limit of the amount of S is not particularly limited, but may be more than 0%, for example, from the point of reducing desulfurization cost, it may be 0.0001% or more.

Al: 0.005-0.080%
Al is a deoxidizing element, and if the amount of Al is less than 0.005%, the oxide becomes coarse and becomes the starting point of cracks due to hydrogen embrittlement, which lowers the hydrogen embrittlement resistance of the wire. Therefore, the Al content is set to 0.005% or more. In order to reliably obtain the above effect, the Al content is preferably 0.008% or more, more preferably 0.010% or more. However, when the amount of Al exceeds 0.080%, the above effect is saturated, and the oxides and nitrides containing Al become coarse, and the manufacturability of the wire is reduced, such as the occurrence of surface defects during rolling. Rather, it lowers the hydrogen embrittlement resistance. Therefore, the Al content is preferably 0.060% or less, more preferably 0.050% or less.

N: 0.0015-0.0060%
N reacts with alloying elements such as Ti in steel to form nitrides and carbonitrides to refine the crystal grains of the wire, thus improving ductility. Therefore, the amount of N should be 0.0015% or more. In order to reliably obtain the above effects, the N content is preferably 0.0021% or more, more preferably 0.0025% or more. On the other hand, when manufacturing high-strength steel wire by wire drawing, N dissolved in the steel has a large effect on strain aging and deteriorates torsional characteristics. Must be 0.0060% or less. The amount of N is preferably 0.0049% or less, more preferably 0.0040% or less.

Cu: 0.10-0.65%
Cu is an important element that has the effect of improving the corrosion resistance and hydrogen embrittlement resistance of the high-strength steel wire according to the present disclosure, and is contained in an amount of 0.10% or more. Since Cu exists in a solid solution in the pearlite structure, it has the effect of improving the corrosion resistance and hydrogen embrittlement resistance of the wire. If the Cu content is less than 0.10%, the above effect cannot be obtained, so the Cu content is made 0.10% or more. In order to reliably obtain the above effects, the Cu content is preferably 0.15% or more, more preferably 0.20% or more. On the other hand, when the content exceeds 0.65%, the wire material becomes brittle, so wire breakage is likely to occur during wire drawing, and rather the hydrogen embrittlement resistance of the wire material is deteriorated. Therefore, the Cu content is 0.65% or less, preferably 0.60% or less, and more preferably 0.50% or less.

Ni: 0.05 to less than 0.65% Ni is an essential element for suppressing surface defects during rolling when producing a wire containing Cu, and has the effect of improving the hardenability of the wire. . However, an excessive content induces cracking during wire drawing and degrades hydrogen embrittlement resistance. In order to obtain the above effects, Ni is contained in an amount of 0.05% or more. If the Ni content is less than 0.05%, surface defects occur on the surface of the wire during rolling, causing wire breakage during wire drawing, and also deteriorating the hydrogen embrittlement resistance of the wire. In order to obtain the above effects, the Ni content is preferably 0.10% or more, more preferably 0.15% or more. On the other hand, when the Ni content is 0.65% or more, the hardenability becomes too high, and rather the hydrogen embrittlement resistance is lowered. Therefore, the Ni content is less than 0.65%, preferably 0.60% or less, and more preferably 0.50% or less.

In addition, Cu and Ni are contained in a range that satisfies [Cu]>[Ni], that is, [Cu]/[Ni]>1.00, so that the high-strength steel wire after wire drawing in the present disclosure Good torsional characteristics can be secured.
When [Cu]/[Ni] is 1.00 or less, that is, when the Ni content is equal to or greater than the Cu content, the hardenability of the wire rod for high-strength steel wire according to the present disclosure becomes too high. Sufficient torsional properties cannot be secured with the worked high-strength steel wire. Therefore, Cu and Ni must be contained within a range that satisfies [Cu]>[Ni]. [Cu]/[Ni] is preferably 1.20 or more, more preferably 1.50 or more, in order to stably ensure the twisting properties of the steel wire after wire drawing. Cu and Ni should satisfy [Cu]>[Ni], and the upper limit of [Cu]/[Ni] is not limited. The manufacturability of the wire deteriorates, such as occurrence of Therefore, considering the manufacturability of the wire, [Cu]/[Ni] is preferably 5 or less, more preferably 4 or less.

The wire rod for high-strength steel wire according to the present disclosure contains one or more of the elements Cr, Mo, Ti, Nb, V, Sn, B, Ca, Mg, Zr, and REM as arbitrary elements. may When these optional elements are contained, in mass%, Cr: 0 to 0.30%, Mo: 0 to 0.30%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, V : 0-0.20%, Sn: 0-0.30%, B: 0-0.0050%, Ca: 0-0.0050%, Mg: 0-0.0050%, Zr: 0-0. 100%, and REM: 0 to 0.0200%, one or more of which may be contained.

Cr: 0-0.30%
Cr has the effect of increasing the hardenability of the wire and increasing the tensile strength of the wire after pearlite transformation, and may be contained when this effect is desired. In order to obtain this effect, it is preferable to contain 0.01% or more of Cr. In order to reliably obtain the above effects, the Cr content is preferably 0.05% or more, more preferably 0.10% or more. However, if the Cr content exceeds 0.30%, martensite or bainite structure is likely to occur, degrading the wire drawability and hydrogen embrittlement resistance of the high-strength steel wire after wire drawing. Therefore, when Cr is contained, the amount of Cr is 0.30% or less, preferably 0.25% or less, and more preferably 0.20% or less.

Mo: 0-0.30%
Mo has the effect of increasing the hardenability of the wire and increasing the tensile strength of the wire after pearlite transformation, and may be contained when this effect is desired. In order to obtain this effect, the Mo content is preferably 0.01% or more. In order to reliably obtain the above effects, the Mo content is preferably 0.03% or more, more preferably 0.05% or more. However, if the Mo content exceeds 0.30%, a martensite or bainite structure is likely to occur, degrading the wire drawability and hydrogen embrittlement resistance of the high-strength steel wire after wire drawing. . Therefore, when Mo is contained, the Mo content is 0.30% or less, preferably 0.20% or less, and more preferably 0.10% or less.

Ti: 0-0.100%
Ti combines with C or N to precipitate carbides or carbonitrides, which has the effect of refining crystal grains and improving the ductility of the wire rod. have the effect of improving In addition, since the content of Ti can reduce dissolved N, it also has the effect of suppressing strain aging and improving the torsional properties of the steel wire after wire drawing. Since the above-mentioned effect due to the inclusion of Ti is effective in obtaining the high-strength steel wire rod according to the present disclosure, Ti may be positively included. In order to obtain these effects, Ti should be contained in an amount of 0.002% or more. In order to reliably obtain the above effects, the Ti content is preferably 0.005% or more, more preferably 0.008% or more. However, even if the Ti content exceeds 0.10%, not only does the above effect saturate, but the strength of the wire rod becomes too high, and the hydrogen embrittlement resistance and torsion resistance of the steel wire after wire drawing are rather increased. characteristics deteriorate. Therefore, when Ti is contained, the amount of Ti is 0.10% or less, preferably 0.050% or less, and more preferably 0.025% or less.

Nb: 0-0.100%
Nb precipitates carbides or carbonitrides, refines crystal grains, and has the effect of improving the ductility of the wire, and has the effect of improving hydrogen embrittlement resistance and torsional properties after wire drawing. In order to obtain this effect, it is preferable to contain 0.002% or more of Nb. In order to reliably obtain the above effects, the Nb content is preferably 0.005% or more, more preferably 0.008% or more. However, even if the content of Nb exceeds 0.100%, not only does the above effect saturate, but surface defects are likely to occur at the stage of obtaining a billet by blooming or at the stage of rolling a wire rod. sexuality worsens. Therefore, when 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: 0-0.20%
V precipitates carbide VC, and has the effect of increasing tensile strength and improving hydrogen embrittlement resistance. In order to obtain this effect, the V content is preferably 0.01% or more. In order to reliably obtain the above effect, the V content is preferably 0.03% or more, more preferably 0.05% or more. However, even if the V content exceeds 0.20%, not only the above effect is saturated, but also the hydrogen embrittlement resistance and torsional properties of the steel wire after wire drawing are deteriorated. Therefore, when V is contained, the amount of V is 0.20% or less, preferably 0.15% or less, and more preferably 0.10% or less.

Sn: 0-0.30%
Sn dissolves in the pearlite structure and has the effect of enhancing corrosion resistance and hydrogen embrittlement resistance. 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 effects, the Sn content is preferably 0.03% or more, more preferably 0.05% or more. However, even if the Sn content exceeds 0.30%, not only does the above effect saturate, but also the wire becomes embrittled, surface defects occur during rolling, and the manufacturability of the rolled material deteriorates. Therefore, when Sn is contained, the amount of Sn is preferably 0.01 to 0.30%. Therefore, when Sn is contained, the amount of Sn is 0.30% or less, preferably 0.20% or less, and more preferably 0.15% or less.

B: 0 to 0.0050%
B has the effect of increasing the pearlite structure fraction after isothermal transformation and improving the twisting properties of the high-strength steel wire after wire drawing, and may be contained when 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, more preferably 0.0007% or more. However, even if the content of B exceeds 0.0050%, not only does the above effect saturate, but the wire becomes embrittled, surface defects occur during rolling, productivity deteriorates, and wire drawing Rather, it deteriorates the torsional properties of the high-strength steel wire after processing. Therefore, when B is contained, the amount of B is 0.0050% or less, preferably 0.0030% or less, and more preferably 0.0020% or less.

Ca: 0-0.0050%
Ca forms a solid solution in MnS, has the effect of finely dispersing MnS, and has the effect of improving hydrogen embrittlement resistance. Ca does not have to be contained (Ca: 0%), but in order to obtain the effect of improving hydrogen embrittlement resistance by Ca, Ca should be contained in an amount of 0.0002% or more, and a higher effect can be obtained. If desired, the content should be 0.0005% or more. However, even if the amount of Ca exceeds 0.0050%, the effect is saturated, and the oxides formed by reacting with oxygen in the steel become coarse, resulting in a decrease in torsional characteristics after wire drawing. invite. Therefore, the proper amount of Ca to contain is 0.0050% or less. From the viewpoint of improving hydrogen embrittlement resistance and torsional properties, the Ca content is preferably 0.0030% or less, and more preferably 0.0025% or less.

Mg: 0-0.0050%
Mg dissolves in MnS, has the effect of finely dispersing MnS, and has the effect of improving the resistance to hydrogen embrittlement. Mg does not have to be contained (Mg: 0%), but in order to obtain the effect of improving the hydrogen embrittlement resistance by Mg, it is sufficient to contain 0.0002% or more of Mg, and a higher effect can be obtained. If desired, the content should be 0.0005% or more. However, even if the Mg content exceeds 0.0050%, the effect is saturated, and the oxides formed by reacting with oxygen in the steel become coarse, resulting in a decrease in torsional characteristics after wire drawing. invite. Therefore, the proper amount of Mg to contain is 0.0050% or less. From the viewpoint of improving hydrogen embrittlement resistance and torsional properties, the Mg content is preferably 0.0030% or less, more preferably 0.0025% or less.

Zr: 0-0.100%
Zr reacts with O to form an oxide, and if contained in a trace amount, the oxide is finely dispersed, and has the effect of suppressing hydrogen embrittlement resistance and torsional characteristics after wire drawing. It may be included if you want to obtain In order to obtain this effect, the Zr content should be 0.0002% or more, and if a higher effect is desired, the Zr content should be 0.001% or more. However, when the content of Zr exceeds 0.10%, the effect is saturated, and coarse nitrides or sulfides are formed, so the hydrogen embrittlement resistance after wire drawing is rather reduced. This leads to deterioration of torsional characteristics. Therefore, the content of Zr when it is included is 0.100% or less. From the viewpoint of reducing inclusions that adversely affect hydrogen embrittlement resistance and torsional properties after wire drawing, the Zr content is preferably 0.080% or less, and if it is 0.050% or less More preferred.

REM: 0-0.0200%
REM is a generic term for rare earth elements, and the content of REM is the total content of rare earth elements. Like Ca and Mg, REM dissolves in MnS and has the effect of finely dispersing MnS. By finely dispersing MnS, hydrogen embrittlement resistance can be improved, so it may be contained. REM does not have to be contained (REM: 0%), but in order to obtain the effect of improving hydrogen embrittlement resistance by REM, REM should be contained in an amount of 0.0002% or more, and a higher effect can be obtained. If desired, the content should be 0.0005% or more. However, even if the amount of REM exceeds 0.020%, the effect is saturated, and the oxides formed by reacting with oxygen in the steel become coarse, and the twisting characteristics after wire drawing are rather deteriorated. lead to decline. Therefore, the appropriate amount of REM when contained is 0.0200% or less. From the viewpoint of improving hydrogen embrittlement resistance and torsional properties, the REM content is preferably 0.0100% or less, more preferably 0.0050% or less.

Balance: Fe and Impurities The balance is Fe and impurities. “Impurities” are components that are unintentionally contained in steel materials, and refer to those that are mixed from ores, scraps, or manufacturing environments used as raw materials during the industrial production of steel materials. Impurities include, in addition to P, S, and N, elements that are unintentionally contained in the steel among the arbitrary elements described above, and O (oxygen). For example, O (oxygen) is preferably 0.0030% or less because if it is contained in a large amount, oxides formed in the steel become coarse and lead to deterioration in torsion characteristics after wire drawing. is preferably 0.0025% or less.

The wire rod for high-strength steel wire according to the present disclosure contains each component in the above range, and Y1 represented by the following formula <1> is 1.70 ≤ Y1 ≤ 4.50, and is represented by the formula <2>. , satisfies Y2<1.81.
Y1=3×[Cr]+5×[Mn]+[Cu]+[Ni] <1>
Y2=[C]+[Si]/10+A <2>
a=350×([N]−0.29×[Ti]) <4>
Here, [C], [Si], [Mn], [Cr], [Cu], [Ni], [N], and [Ti] in the above formula represent the content of each element in mass%. where A in formula <2> is a parameter related to a represented by formula <4> above, and when the value calculated by formula <4> is a≧0, A=a and a< If 0, then A=0.
It should be noted that Cr and Ti are optional elements in the wire according to the present disclosure, and if these optional elements are not substantially contained in the wire according to the present disclosure (no addition, that is, the impurity level), the Y1, Y2, and a are calculated assuming that the element content is "0".

Y1 is mainly related to the hardenability of the wire for high-strength steel wire, and is a parameter necessary for increasing the tensile strength of the wire and improving the hydrogen embrittlement resistance. In addition, by setting Y1 within the range of 1.70 ≤ Y1 ≤ 4.50, the hydrogen embrittlement resistance of the wire rod for high-strength steel wire can be improved, and the hydrogen resistance of the high-strength steel wire after wire drawing can be improved. Embrittlement properties can be improved.

A high-strength steel wire can be obtained by drawing a wire whose chemical composition and metallographic structure are appropriately controlled. Prior to wire drawing, the wire is reheated for patenting treatment, or directly immersed in a salt bath furnace for isothermal transformation treatment after rolling. preferably. Y1 is a parameter necessary for controlling the hardenability of the wire, giving the necessary strength as a fine pearlite structure with high uniformity from the surface to the center, and improving the hydrogen embrittlement resistance of the wire. , from 1.70 to 4.50. If Y1 is less than 1.70, the hydrogen embrittlement resistance deteriorates, and the steel wire after wire drawing cannot obtain sufficient hydrogen embrittlement resistance. In order to reliably obtain the above effect, Y1 is preferably 2.00 or more, more preferably 2.50 or more. On the other hand, when Y1 exceeds 4.50, a non-pearlite structure other than a pearlite structure such as bainite and martensite is generated after patenting treatment or isothermal transformation treatment after rolling, and the hydrogen embrittlement resistance of the wire is rather reduced. Let Y1 is 4.50 or less, preferably 4.22 or less. In order to ensure the strength of the wire and more stably ensure hydrogen embrittlement resistance, Y1 may be 4.00 or less, and more preferably 3.75 or less.

Y2 is a parameter that mainly affects the torsional properties of the steel wire after wire drawing. The wire for high-strength steel wire according to the present disclosure contains Cu and Ni in order to improve corrosion resistance and hydrogen embrittlement resistance, and the tensile strength of the wire is relatively high. Due to the temperature rise due to frictional heat, etc., the steel wire tends to become embrittled under the influence of strain aging, and the torsional characteristics of the steel wire after wire drawing tend to deteriorate. In particular, strain aging due to wire drawing is greatly affected by C, Si, and N solid-dissolved in steel, so Y2 can be represented by the following formula <2>.
Y2=[C]+[Si]/10+A <2>
a=350×([N]−0.29×[Ti]) <4>
Here, [C], [Si], [N], and [Ti] in the above formula represent the content in mass% of each element, and A is related to a represented by formula <4> is a parameter. The right side of Equation <4> represents the effect of solute N, and when the value of the right side is negative, A=0 because the effect of solute N on strain aging disappears. The value of Y2 should be less than 1.81 to minimize the effects of strain aging on wire drawing. When Y2 is 1.81 or more, the torsional properties after wire drawing are degraded due to the effect of strain aging due to wire drawing. In order to improve the twisting property after wire drawing, the value of Y2 is preferably less than 1.70, more preferably less than 1.50. The value of Y2 may be less than 1.81, and the lower limit is not particularly limited, but from the viewpoint of ensuring the tensile strength after wire drawing, it is preferably 0.50 or more, It is even better if there is.

<Metal structure>
The metallic structure of the wire rod for high-strength steel wire according to the present disclosure is 90% or more of the pearlite structure, which is a layered structure of ferrite and cementite. Ferrite, bainite, or martensite may be generated due to changes in the chemical composition, the γ grain size before transformation, or the cooling rate in the stage of patenting the wire or the stage of isothermal transformation. increases the variation in surface layer hardness in the longitudinal direction of the wire and reduces the hydrogen embrittlement resistance of the wire. When the variation in surface layer hardness in the longitudinal direction of the wire is large, the hydrogen embrittlement resistance and torsional characteristics of the drawn steel wire are degraded. The metallic structure of the wire rod for high-strength steel wire according to the present disclosure preferably has a pearlite structure of 92% or more, more preferably 95% or more.
In addition, examples of the residual structure (non-pearlite structure) other than the pearlite structure include martensite, bainite, pro-eutectoid ferrite, and pro-eutectoid cementite. As the non-pearlite structure, pro-eutectoid ferrite and quasi-pseudo-pearlite are preferable, and quasi-pseudo-pearlite is more preferable, from the viewpoint that the torsional properties and hydrogen embrittlement resistance of the steel wire after wire drawing are not significantly deteriorated. Note that the pearlite structure refers to pearlite that maintains the lamellar structure, and the quasi-perlite whose lamellar structure has collapsed is treated as a non-pearlite structure in the present disclosure.

<Characteristics>
(Variation in surface layer hardness)
Variations in the hardness of the surface layer of the wire due to variations in the chemical composition or metallographic structure occurring in the longitudinal direction of the wire affect the hydrogen embrittlement resistance and also greatly affect the properties of the steel wire after wire drawing. In particular, the resistance to hydrogen embrittlement is affected by variations in the hardness of the surface layer of the wire, and if there is a portion with high hardness in the surface layer of the wire, it becomes a starting point for hydrogen embrittlement, resulting in a decrease in the resistance to hydrogen embrittlement.
For the wire according to the present disclosure, eight samples were taken at arbitrary equal intervals, and the axial cross section (cross section parallel to the longitudinal direction including the central axis) of each sample was located at a depth of 50 μm from the surface of the wire (hereinafter , and sometimes referred to as “50 μm depth”), the relationship between the maximum value Hv _simax and the average value Hv _siave of the Vickers hardness Hv _si measured at a depth of 50 μm satisfies the following (4).
(4) _Hvsimax - _Hvsiave≤50

In the present disclosure, the “surface layer” of the wire is a region from the surface (peripheral surface) of the wire to a depth of 100 μm, and the hardness is measured at a depth of 50 μm at the midpoint of the surface layer.
Samples for measuring Vickers hardness are taken at arbitrary equal intervals according to the length of the wire to be measured. Wire rods are usually manufactured in a ring-shaped wound state, so if the wire rod has a length equal to or greater than one ring, 8 samples are taken at equal intervals from the length corresponding to one ring. It is preferable to measure the Vickers hardness of each sample by using a measuring tape, and obtain the average value and the maximum value of the Vickers hardness of each sample. Specifically, the axial cross section ( The average value of the Vickers hardness in each sample at a depth of 50 μm from the surface of the wire (cross section parallel to the longitudinal direction including the central axis) is Hv _si , 8 Hv _si (i is an integer from 1 to 8 ) is set to Hv _simax , the value of Y3 represented by the formula <3> is set to 50 or less.

The right side of the formula <3> is the maximum value Hv _simax of Hv _si obtained from 8 samples and the average value Hv _siave of Hv _si obtained from 8 samples calculated by the following formula (n=8). (which may be referred to as “overall average Vickers hardness” in the present disclosure).

Hv _si of each sample was measured by using a sample whose axial cross section was embedded in resin and mirror-polished from the wire, and measured at a depth of 50 μm from the surface of the wire in the axial cross section with an automatic Vickers hardness tester under a load of 0.98 N. It can be obtained by measuring 50 points (that is, 10 mm length) at a pitch of 200 μm for each sample.
Of the eight Hv _si thus obtained, the maximum value, Hv _simax , was found at a depth of 50 μm from the surface of the wire rod in the axial cross-section among eight samples of 25 mm length collected at intervals of 600 mm. In other words, it means the Vickers hardness of the portion where the highest average hardness was measured among the surface layer hardness variations in the longitudinal direction of the wire. Hydrogen embrittlement of a wire is affected by variations in the hardness of the surface layer of the wire, and the portion of the surface of the wire having a locally high hardness serves as the starting point of fracture, resulting in fracture due to hydrogen embrittlement. If Hv _simax measured in eight samples is higher than the overall average Vickers hardness at a depth of 50 μm including other samples by more than 50, the possibility of breakage due to hydrogen embrittlement at that site increases, and the wire rod The hydrogen embrittlement resistance of is lowered. Furthermore, in the steel wire obtained by wire drawing, the variation in surface layer hardness in the longitudinal direction becomes even greater, and the reduction in hydrogen embrittlement resistance in the steel wire becomes more pronounced. If the value of Y3 is 50 or less, the deterioration of the hydrogen embrittlement resistance of the wire is suppressed, and the deterioration of the hydrogen embrittlement resistance of the steel wire obtained by wire drawing is also suppressed. From the viewpoint of improving hydrogen embrittlement resistance, the value of Y3 is preferably as small as possible, preferably 30 or less, and more preferably 25 or less.

The number of samples of 25 mm length taken at intervals of 600 mm from the longitudinal direction of the wire is eight. That is, by finding the average value _Hvsi of the Vickers hardness in each sample at a depth of 50 μm from the surface of the wire in the axial cross section for eight samples, it is possible to know the hardness variation of the surface layer of the wire. It is preferable to investigate the hardness variation of the surface layer of the wire material in a range corresponding to at least one ring of the rolled and wound wire material coil. This is because the wire rod rings wound in the austenite region after hot rolling are transported on the conveyor while partially overlapping the front and rear rings, so the contact area or distance in one ring There is a portion (overlapping portion) where the rings are close to each other and a portion (non-overlapping portion) which is separated from the front and rear rings. For this reason, a difference in cooling rate occurs within one ring of the coil after winding, resulting in a difference in pearlite transformation temperature between the overlapping portion and the non-overlapping portion. This causes a difference in the pearlite lamellar spacing between the overlapping portion and the non-overlapping portion, resulting in a large variation in surface layer hardness. On the other hand, since the rings in the wire coil are basically wound under the same conditions (constant conditions), variations between the rings are small. Therefore, if 8 samples are taken at intervals of 600 mm, the variation in surface layer hardness is verified in a range of approximately 4200 mm in the longitudinal direction of the wire, and the length is equivalent to one or more rings of the wire coil. Variation in surface layer hardness can be verified. Since the variation between each ring is small as described above, the variation within the wire coil can be verified by the sampling method described above.

The length of the wire according to the present disclosure and the length of one ring at the time of manufacture are not particularly limited, and the interval at which samples are taken from the wire is not limited to 600 mm. The wire according to the present disclosure satisfies the characteristics by satisfying Hv _simax −Hv _siave ≦50 regardless of the length and regardless of the interval at which samples for measuring Vickers hardness are taken. can. In addition, in the actual production of the wire rod, variations occur in the length direction of the wire rod. Therefore, if the manufacturing method is appropriately adjusted, a wire with reduced variation in the length direction of the wire can be obtained. If the length of one ring is about 4200 mm, the Vickers hardness test can be performed at intervals of 600 mm to confirm the characteristics in the length direction of the wire coil. In addition, when the length of one ring is not 4200 mm, 8 samples are taken at equal intervals from the length corresponding to one ring, the Vickers hardness Hv _si of each sample is measured, and the maximum value Hv _simax maximum value and the average value Hv _siave , the Vickers hardness in the length direction of the wire coil can be confirmed. In addition, when the length of the wire is less than one ring, 8 samples are taken at equal intervals from the whole, the Vickers hardness Hv _si of each sample is measured, and the maximum value Hv _simax and the average value Hv _siave Preferably the difference is calculated.

(Tensile strength)
The higher the strength of the wire rod and steel wire, the more likely it is that breakage due to progression of corrosion or hydrogen embrittlement will occur. is more than 1700 MPa, the corrosion resistance and hydrogen embrittlement resistance are excellent, so even when the steel wire or stranded wire product is used in a corrosive environment, breakage is less likely to occur. In addition, even if the tensile strength of the wire is not high, it is possible to obtain a high-strength steel wire with a tensile strength exceeding 1700 MPa by increasing the reduction in area during wire drawing. If the area reduction rate is too large, the torsion characteristics will deteriorate and the hydrogen embrittlement resistance will also deteriorate. Therefore, it is preferable to manufacture a high-strength steel wire by setting the tensile strength of the wire before drawing to 1000 MPa or more and without excessively increasing the reduction in area during drawing. If the steel wire has a tensile strength of 1000 MPa or more as a fine pearlite structure at the wire rod stage, the deterioration of the tensile strength and hydrogen embrittlement resistance of the steel wire after wire drawing is suppressed.
In order to improve the torsional properties and hydrogen embrittlement resistance of the steel wire, the tensile strength of the wire is more preferably 1200 MPa or more, more preferably 1300 MPa or more.
On the other hand, if the tensile strength of the wire exceeds 1650 MPa, the ductility of the wire may decrease, and the torsional properties and hydrogen embrittlement resistance of the drawn steel wire may rather decrease. From this point of view, the tensile strength of the wire is preferably 1600 MPa or less, more preferably 1550 MPa or less.

By drawing using the wire rod for high-strength steel wire according to the present disclosure, it is possible to obtain a high-strength steel wire excellent in corrosion resistance and hydrogen embrittlement resistance even at a high strength exceeding 1700 MPa. This is because the segregation of chemical components, the metallographic structure, and the hardness distribution of the surface layer of the wire are controlled at the stage of manufacturing the wire to improve corrosion resistance and hydrogen embrittlement resistance.
The wire rod according to the present disclosure has excellent corrosion resistance and hydrogen embrittlement resistance, and also has excellent twisting properties in the steel wire after wire drawing. It can be used as high-strength steel wires such as wires, steel wires for bridge cables, and PC steel wires.

<Measurement method>
Metal structure of wire, tensile strength of wire and steel wire, dispersion of surface layer hardness of wire, corrosion resistance of wire, resistance to hydrogen embrittlement of wire and steel wire, and twisting property of steel wire are investigated by the following methods. bottom.

<1> Metal structure of wire:
The area ratio of the metal structure of the wire is parallel to the longitudinal direction of the wire. After mirror-polishing a micro sample in which the cross section passing through the central axis is filled with resin, the metal structure is exposed using a picral solution. Next, when the diameter of the wire is D, the metal structure of the portion corresponding to the position of the depth of 0.25D from the surface of the wire was examined using a scanning electron microscope (SEM) at a magnification of 1000 times at 10 locations. A tissue photo was taken. For each photograph taken, the part corresponding to the pearlite structure, which is judged to be a layered structure of cementite and ferrite, is painted over, the area value is measured by image analysis, the pearlite area ratio is calculated, and the measured values at 10 locations are averaged. The pearlite area ratio was obtained by The structure other than the pearlite structure is a non-pearlite structure such as partially formed martensite, bainite, and proeutectoid ferrite, which cannot be distinguished as a pearlite structure that is a layered structure of cementite and ferrite.

<2> Tensile strength of wire and steel wire:
The tensile test was performed in accordance with JIS Z 2241:2011 using test pieces of 400 mm in length, taken at intervals of 250 to 300 mm in the longitudinal direction of the wire rod and steel wire after wire drawing. The tensile strength was determined by dividing the maximum test force until the test piece broke by the cross-sectional area obtained from the diameter measured before the tensile test. The test was measured using eight test pieces, and the average value was taken as the tensile strength of the wire rod and steel wire.

<3> Variation in surface layer hardness of wire:
For the measurement of the surface layer hardness, 25 mm long test pieces were used, which were sampled at intervals of 600 mm (an example of equal intervals) in the longitudinal direction of the wire. Eight (n=8) samples were taken as the test piece, and after filling the cross section parallel to the length direction of the wire with a resin, a mirror-polished micro sample was used. For each micro sample, hardness measurement was performed using an automatic Vickers hardness tester in accordance with JIS Z 2244:2009. The test load for hardness measurement is 0.98 N, the Vickers hardness is measured at 50 locations at a depth of 50 μm from the wire rod surface in each micro sample, the average value is calculated, and the Hv _si (i = 1, 2, . . . 8). Further, the maximum value Hv _simax of Hv _si measured for each micro sample was determined, the overall average Vickers hardness Hv _siave of Hv _si was calculated, and Y3 represented by the following formula <3> was calculated.

<4> Corrosion resistance of wire:
Two test pieces each of φ7×100 mmL were cut out by machining the outer circumference of the test piece with a length of 100 mm from the center axial direction of the wire rod to a diameter of 7 mm by shaving evenly. The corrosion test uses a dry-wet repeated corrosion tester capable of spraying salt water, (1) salt spray (5% NaCl spray, 35 ° C., 2 hr), (2) drying (humidity 20%, 60 ° C., 4 hr), ( 3) A test was conducted in which one cycle was wet (95% humidity, 50°C, 2 hours). The test period was 12 weeks, and the volume reduction rate due to corrosion of each two test pieces was determined, and the average value was used as the corrosion resistance evaluation index of each wire. The volume reduction rate (%) due to corrosion was determined by the following formula.
Corrosion volume reduction rate (%) = 100 × (specimen volume before corrosion test - specimen volume after corrosion test) / specimen volume before corrosion test The average value of the diameter and length of the test piece measured at three different points was obtained, and the volume of the test piece before the corrosion test was calculated. The test piece volume after the corrosion test is the average value of the diameter and length of the test piece measured at three different positions after completely removing corrosion products from the surface of the test piece using sandblasting after the corrosion test. and the volume of the test piece after the corrosion test was calculated.

<5> Hydrogen embrittlement resistance of wire and steel wire:
The resistance to hydrogen embrittlement of the wire rod and the steel wire after wire drawing was evaluated by the FIP test standardized by the International Federation of Prestressed Concrete (Federation Internationale de la Precontraint). A wire rod or steel wire after wire drawing is pickled to remove surface scale or lubricating film, then straightened to ensure straightness, and a sample cut to a length of 700 mmL is used as a test piece. board. Next, using a solution cell in which a 200 mm length including the center of the test piece can be immersed, the test piece was immersed in an aqueous ammonium thiocyanate (NH ₄ SCN) solution at 50 ° C., and the breaking load obtained from the tensile test was 70. % constant load was applied to the test piece, and the time until breakage was measured. The upper limit of the breaking time was 200 hours. The test was performed on six test pieces taken from each wire rod or each steel wire, and the average value of the rupture time was calculated to evaluate the hydrogen embrittlement resistance of the wire rod and steel wire.

<6> Twisting properties of steel wire:
The torsion characteristics of the steel wire after wire drawing are determined by cutting the steel wire so that the torsion test can be performed at a length 100 times the diameter of the steel wire, straightening it, and then rotating it 15 times per minute. A twisting test was performed in which the steel wire was twisted until breakage occurred at a speed of . The occurrence of delamination was determined by measuring the torque curve during twisting and judging that delamination had occurred when the torque decreased by 20% or more before breakage occurred. The twisting test was performed on five steel wires for each wire, and the twisting characteristics were judged to be good when delamination did not occur in even one wire.

<Manufacturing method of wire>
The wire for high-strength steel wire according to the present disclosure can obtain the effects of the steel wire of the present disclosure regardless of the wire manufacturing method as long as the requirements of the present disclosure are satisfied. , a wire rod is manufactured, and a high-strength steel wire is manufactured using it as a material. The following manufacturing process is an example, and even if a wire whose chemical composition and other requirements are within the scope of the present disclosure is obtained by a process other than the following, the wire is included in the wire according to the present disclosure. be

In the wire rod for high-strength steel wire according to the present disclosure, the chemical composition is adjusted at the stage of melting the steel, and the manufacturing conditions such as the heating conditions of the slab and the heating temperature during rolling are controlled, and the It is preferable to reduce the segregation of chemical components or to control the pearlite structure to have a high uniformity.
Specifically, a steel ingot or slab obtained by adjusting the chemical components such as C, Si, Mn, Cu, Ni, Al, etc., and melting and casting in a converter or an electric furnace is subjected to a blooming process. After passing through, it is made into a billet that will be used as a raw material for product rolling. Heat treatment is performed at 1260° C. or higher for 12 hours or longer before product rolling, that is, at the time of heating for blooming, or at the stage of steel ingots or slabs prior to that. After that, the steel slab is reheated and hot-rolled into a product, and finally finished as a wire rod having a predetermined diameter.
By applying a high-temperature, long-time heat treatment to the wire rod before it is rolled into a product, segregation of the chemical components can be suppressed, so variations in surface layer hardness in the longitudinal direction of the wire rod after product rolling can be suppressed. be able to. In addition, in order to reduce the segregation of chemical components, such as controlling the cooling rate when solidifying molten steel into cast slabs or steel ingots, such as heat treatment at high temperatures and long hours that are not performed under normal manufacturing conditions. By adding the step of (2), it is possible to reduce variations in hardness in the longitudinal direction of the wire rod after rolling.

Next, the steel slab obtained by blooming is reheated to 1000° C. or higher. The heating at this time should be 1150° C. or less, preferably 1130° C. or less, in order to suppress coarsening and mixed grains of austenite grains. In addition, the holding time after reaching the heating temperature is preferably less than 90 minutes in order to suppress the mixing of austenite grains.

The steel slab heated under the above conditions is subjected to rough rolling and then finish rolling to obtain a wire rod having a diameter of 5.0 to 16.0 mm. At this time, the temperature of finish rolling is adjusted within the range of 850°C to 950°C. If the temperature is below 850°C, the austenite grains become too fine and the pearlite transformation becomes uneven. Thereafter, the hot-rolled steel material is held at a temperature not lower than 800° C. for 15 seconds or more to adjust the austenite grains. Next, the steel is directly immersed in molten salt maintained at a temperature of 500 to 580° C. to isothermally transform into a pearlite structure, and then cooled. Alternatively, after cooling the hot-rolled steel material to about room temperature by air blow cooling, it is heated at a temperature in the austenite region of _A3 point or higher, and immersed in molten lead maintained at a temperature of 500 to 600 ° C to form a pearlite structure. After the isothermal transformation, cooling may be performed.

<Manufacturing method of steel wire>
As an example, the wire obtained by the above-described process may be drawn into a steel wire having a required diameter. The area reduction rate for wire drawing may be determined according to the required diameter and strength of the steel wire. The torsional properties and hydrogen embrittlement resistance of the steel wire are degraded. The area reduction rate of wire drawing is preferably 70 to 92%. If the reduction in area by working is less than 70%, it is difficult to obtain the required tensile strength. On the other hand, if the reduction in area by working exceeds 92%, the torsional properties and hydrogen embrittlement resistance of the steel wire tend to deteriorate. The wire drawing method is not particularly limited, but in order to reduce the hardness variation of the surface layer of the steel wire, the steel wire is water-cooled after the wire drawing process to suppress the strain aging of the steel wire due to the heat generated during the wire drawing process. It is preferable to use the method of
In addition, a step of heating the steel wire, such as hot dip galvanizing, bluing, or heat stretching treatment, may be carried out after wire drawing, if necessary.

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

Specifically, steels having chemical compositions shown in Tables 1 and 2 were melted, and wire rods and steel wires were produced by the following methods. The notation of "-" in Tables 1 and 2 indicates that the content of the element is at the level of impurities, and it can be determined that the element is not substantially contained. In addition, the underlined values in Tables 2 to 5 mean that they are outside the scope of the present disclosure or do not satisfy the manufacturing method (manufacturing conditions) described above.

Test Nos. shown in Table 3 were used to produce wire rods with different variations in tensile strength or surface layer hardness in the longitudinal direction using steels with the same chemical composition. a0, a1, a0-1 to a0-4, Test No. The wire was rolled under manufacturing conditions b0, b1, b0-1 to b0-4.

[Manufacture of wire rods and steel wires (1)]
Steel No. of the chemical composition shown in Table 1. A0, A1, B0 and B1 were melted. Steel no. A0 and A1, steel no. B0 and B1 have substantially the same composition, but in order to produce different wires with different variations in tensile strength or surface layer hardness in the longitudinal direction, Test No. For a0 and b0, steel No. Using a slab with chemical compositions of A0 and B0, before blooming into a steel slab, the slab was heated to 1280° C., subjected to heat treatment for 24 hours, and bloomed into a 122 mm square. was used as a material for rolling.
Test no. For a1 and b1, steel No. Using slabs A1 and B1, before blooming into steel slabs, heating was performed at 1200° C. under normal conditions for 4 hours without heating at a temperature of 1260° C. or higher, and the slabs were bloomed into 122 mm squares. The rolled steel slab was used as a material for rolling. Each billet was then 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, the wire rod was coiled. The wound wire coil was directly immersed in a molten salt bath maintained at 550° C. for isothermal transformation treatment, and then water-cooled to 300° C. or less to obtain a wire.

Test no. Steel Nos. a0-1 to a0-4. A0 slabs were subjected to test no. b0-1 to b0-4 are steel Nos. B0 slabs are used, and before blooming into steel slabs, the slabs are heated to 1280 ° C., subjected to heat treatment for 24 hours, and bloomed into 122 mm squares. Using. The rolling conditions were changed as shown in Table 3 in order to produce different wire rods with different tensile strength or surface layer hardness variations in the longitudinal direction, even if the steels had the same composition. Specifically, Test No. For a0-1 and b0-1, the heating temperature during wire rod rolling was set to 1150° C. or higher, and Test No. For a0-2 and b0-2, the heating holding time during wire rolling was set to 90 minutes or longer. Also, test no. a0-3 and b0-3 were rolled at a finish rolling temperature of wire rod rolling of 850° C. or less. A0-4 and b0-4 were rolled at a finish rolling temperature of 950° C. or higher in wire rod rolling. Other than that, the rolling conditions were as shown in Table 3, and after the finish rolling, the wire rod was wound into a coil. The wound wire coil was directly immersed in a molten salt bath maintained at 550° C. for isothermal transformation treatment, and then water-cooled to 300° C. or less to obtain a wire.

Then, each wire was drawn to produce a steel wire. Specifically, after each wire is pickled to remove scale, a zinc phosphate film is formed on the surface by chemical conversion treatment to improve lubricity, and wire drawing is performed using a carbide die. gone. The wire drawing process is performed until the wire diameter is 5.2 mm with a pass schedule adjusted so that the reduction in area for each die is about 20% (the wire drawing process under these conditions is called "wire drawing process A". ) was performed. Next, the drawn steel wire was immersed in a lead bath heated to 400° C. for 30 seconds and cooled with water.

[Evaluation (1)]
Test No. obtained by the method described above. a0, a1, a0-1 to a0-4, Test No. Regarding the wires and steel wires of b0, b1, b0-1 to b0-4, the metallographic structure of the wires, the tensile strength of the wires and steel wires, the variation in the surface layer hardness of the wires, the corrosion resistance of the wires, and the hydrogen resistance of the wires and steel wires The embrittlement properties and torsional properties of the steel wire were evaluated. The results are shown in Table 4A. One or both of pro-eutectoid ferrite and pseudo-pearlite were observed as the metallographic structure other than pearlite.

Test no. For the wires of a0, a0-1, and a0-4, the surface layer hardness was measured for each of eight samples of 25 mm length taken at arbitrary positions at intervals of 50 mm in the longitudinal direction of the wire. Results are shown in Table 4B. In Table 4B, the items other than the surface layer hardness of the wire are the same as in Table 4A.

[Manufacture of wire rods and steel wires (2)]
In order to confirm the effect of the chemical composition in the present disclosure, steel No. 2 having the chemical composition shown in Table 2 was tested. C1-C24, and Steel No. D1 to D22 were melted in an electric furnace. Steel no. C1-C24 are examples satisfying the requirements of the present disclosure, Steel Nos. D1-D22 are comparative steels that do not satisfy any one or more of the requirements of the present disclosure.
Before blooming into billets, steel no. C1-C24, and Steel No. The slabs of D1 to D22 were each heated at a target of 1280° C., heat-treated for 24 hours, and then bloomed into 122 mm squares to obtain rolling stock.
Next, the steel slabs were each heated at a heating temperature of 1080° C. for 60 minutes, and then rolled into wire rods having a wire diameter of 8.0 to 12.5 mm. At this time, the finish rolling temperature was aimed at 900° C., and the wire rod was coiled. The wound wire coil was directly immersed in a molten salt bath maintained at 550° C. for isothermal transformation treatment, and water-cooled to 300° C. or lower to obtain a wire.

Each wire rod was drawn to produce a steel wire. Specifically, wire drawing was performed by the same method as wire drawing A described above so as to obtain a steel wire with a wire diameter of 3.8 to 5.2 mm. Next, the drawn steel wire was immersed in a lead bath heated to 400° C. for 30 seconds and cooled with water.

[Evaluation (2)]
Test No. obtained by the method described above. c1 to c24, and Test No. For the wire rods and steel wires of d1 to d22, the metallographic structure of the wire rod, the tensile strength of the wire rod and the steel wire, the variation in surface layer hardness of the wire rod, the corrosion resistance of the wire rod, and the hydrogen embrittlement resistance of the wire rod and steel wire were measured by the method described above. The bending properties and the twisting properties of the steel wire were evaluated respectively. Table 5 shows the results. As metal structures other than pearlite, martensite, bainite, proeutectoid ferrite and pseudo-pearlite were observed.

FIG. 1 shows the relationship between the tensile strength of the wire and the FIP rupture time, which is an index of hydrogen embrittlement resistance, obtained in the examples of the present disclosure.
FIG. 2 shows the relationship between the tensile strength of the steel wire after wire drawing and the FIP rupture time, which is an index of hydrogen embrittlement resistance, obtained in the examples of the present disclosure.

From Tables 4A and 4B, test no. Since a0 and b0 satisfy the chemical composition and other requirements in the present disclosure and the manufacturing conditions of the wire are appropriate, the corrosion volume reduction rate, which is an evaluation index of the corrosion resistance of the wire, is less than 25%. The FIP rupture time, which is an index of hydrogen embrittlement properties, is 100 hours or longer, and a wire rod excellent in corrosion resistance and hydrogen embrittlement resistance is obtained. Furthermore, test no. Even in the steel wires after wire drawing obtained in a0 and b0, the tensile strength was 1700 MPa or more, the FIP breaking time was 30 hr or more, no delamination occurred in the torsion test, and hydrogen embrittlement resistance was observed. A steel wire with excellent hardening properties is obtained.
In addition, the Vickers hardness of the surface layer is not limited to the case where samples are taken and measured at intervals of 600 mm assuming the length of one ring, and even when samples are taken and measured at intervals of 50 mm, "Hv _simax - Hv _siave ≤ 50", it is effective.

On the other hand, Test No. a1 and test no. a0-1 to a0-4 are test numbers. Steel A1, which has almost the same chemical composition as a0, or Steel No., which has the same chemical composition. A0 was used, and test no. b1 and test no. b0-1 to b0-4 are test Nos. Steel No. which has almost the same chemical composition as b0. B1 or steel no. A wire rod was rolled using B0, but the manufacturing conditions for the wire rod were not appropriate, so the variation in surface layer hardness or the area ratio of the pearlite structure did not satisfy the requirements of the present disclosure. Therefore, the hydrogen embrittlement resistance of the wire is inferior, and the hydrogen embrittlement resistance of the steel wire after wire drawing is inferior. Also, test no. a0-1 to a0-4, Test No. In b0-1 to b0-4, delamination occurs in the twisting test of the steel wire after wire drawing, and the twisting characteristics are also inferior.

From Table 5, test no. All of c1 to c24 satisfy the chemical composition and the requirements of the present disclosure, and the steel manufacturing conditions are appropriate. Excellent resistance to hydrogen embrittlement when compared in terms of strength.

Test no. [Cu]/[Ni]<1.00 for d1 and d2, and Y2 for d2 is also 1.81 or more. bad. Also, the hydrogen embrittlement resistance of the steel wire is inferior to that of the steel wires of the examples having the same level of tensile strength.
Test no. In d3, the value of Y1 is less than 1.70, and the hydrogen embrittlement resistance of the wire and the hydrogen embrittlement resistance of the steel wire after wire drawing are inferior.
Test no. In d4, the value of Y1 exceeds 4.50, and the hydrogen embrittlement resistance of the wire and the hydrogen embrittlement resistance and torsional characteristics of the steel wire after wire drawing are inferior.
Test no. In d5, the value of Y2 is 1.81 or more, and the hydrogen embrittlement resistance of the wire rod and the hydrogen embrittlement resistance and torsional characteristics of the steel wire after wire drawing are inferior.
Test no. For d6, d7, d8, d10, d12 to d21, any of the chemical components in the present disclosure is outside the scope of the present disclosure, or the value of Y2 is 1.81 or more, and the hydrogen embrittlement resistance of the wire and / Or the steel wire after wire drawing is inferior in hydrogen embrittlement resistance and twisting characteristics.
Test no. The chemical components of d9 and d22 were outside the scope of the present disclosure, and wire breakage occurred during the wire drawing process. Therefore, the tensile strength, hydrogen embrittlement resistance, and twisting properties of the steel wire were not investigated.
Test no. The chemical composition of d11 is outside the range of the present disclosure (Y2 is also 1.81 or more), and large surface defects appeared at the stage of rolling the wire, so the corrosion resistance and hydrogen embrittlement resistance of the wire should be investigated. No further wire drawing was performed.

Applications and the like of the wire according to the present disclosure are not limited to the embodiments and examples described above. For example, the wire according to the present disclosure is not limited to a steel wire material with a tensile strength of 1700 MPa or more, and may be used as a steel wire material with a required tensile strength of less than 1700 MPa.

The disclosure of Japanese Patent Application 2019-113720 filed on June 19, 2019 is incorporated herein by reference in its entirety. All publications, patent applications and technical standards mentioned herein are to the same extent as if each individual publication, patent application and technical standard were specifically and individually noted to be incorporated by reference. incorporated herein by reference.

Claims (3)

The chemical composition, in mass %,
C: 0.60 to 1.15%,
Si: 0.01 to 1.80%,
Mn: 0.20-0.90%,
P: 0.015% or less,
S: 0.015% or less,
Al: 0.005 to 0.080%,
N: 0.0015 to 0.0060%,
Cu: 0.10-0.65%,
Ni: less than 0.05 to 0.65%,
Cr: 0 to 0.30%,
Mo: 0-0.30%,
Ti: 0 to 0.100%,
Nb: 0 to 0.100%,
V: 0 to 0.20%,
Sn: 0 to 0.30%,
B: 0 to 0.0050%,
Ca: 0 to 0.0050%,
Mg: 0-0.0050%,
Zr: 0 to 0.100%,
REM: 0 to 0.0200%, and the balance: Fe and impurities,
The content in mass% of each element of C, Si, Mn, Cr, Cu, Ni, N, and Ti contained in the wire is expressed as [C], [Si], [Mn], [Cr], [ Cu], [Ni], [N], and [Ti] satisfy the following (1) to (3),
(1) [Cu]/[Ni]>1.00
(2) 1.70≤Y1≤4.50
Y1 = 3 x [Cr] + 5 x [Mn] + [Cu] + [Ni]
(3) Y<1.81
Y2=[C]+[Si]/10+A
A has a value of a = 350 x ([N] - 0.29 x [Ti])
If a≧0 then A=a
if a<0 then A=0
The metal structure contains a pearlite structure with an area ratio of 90% or more in a cross section parallel to the longitudinal direction including the central axis of the wire,
Eight samples si (i is an integer of 1 to 8) taken at intervals of 50 mm in the longitudinal direction of the wire are measured at a depth of 50 μm from the surface of the wire in the cross section of each sample. A wire that satisfies the following (4), where Hv _si is the Vickers hardness, Hv _siave is the average value of Hv _si , and Hv _simax is the maximum value.
(4) _Hvsimax - _Hvsiave≤50 The chemical composition, in mass %,
C: 0.60 to 1.15%,
Si: 0.01 to 1.80%,
Mn: 0.20-0.90%,
P: 0.015% or less,
S: 0.015% or less,
Al: 0.005 to 0.080%,
N: 0.0015 to 0.0060%,
Cu: 0.10-0.65%,
Ni: less than 0.05 to 0.65%,
Cr: 0 to 0.30%,
Mo: 0-0.30%,
Ti: 0 to 0.100%,
Nb: 0 to 0.100%,
V: 0 to 0.20%,
Sn: 0 to 0.30%,
B: 0 to 0.0050%,
Ca: 0 to 0.0050%,
Mg: 0-0.0050%,
Zr: 0 to 0.100%,
REM: 0 to 0.0200%, and
Balance: Fe and impurities,
The content in mass% of each element of C, Si, Mn, Cr, Cu, Ni, N, and Ti contained in the wire is expressed as [C], [Si], [Mn], [Cr], [ Cu], [Ni], [N], and [Ti] satisfy the following (1) to (3),
(1) [Cu]/[Ni]>1.00
(2) 1.70≤Y1≤4.50
Y1 = 3 x [Cr] + 5 x [Mn] + [Cu] + [Ni]
(3) Y<1.81
Y2=[C]+[Si]/10+A
A has a value of a = 350 x ([N] - 0.29 x [Ti])
If a≧0 then A=a
if a<0 then A=0
The metal structure contains a pearlite structure with an area ratio of 90% or more in a cross section parallel to the longitudinal direction including the central axis of the wire,
Eight samples si (i is an integer of 1 to 8) taken at intervals of 600 mm in the longitudinal direction of the wire are measured at a depth of 50 μm from the surface of the wire in the cross section of each sample . A wire that satisfies the following (4), where Hv _si is the Vickers hardness , Hv _siave is the average value of Hv _si , and Hv _simax is the maximum value .
( 4) _Hvsimax - _Hvsiave≤50 The chemical component is replaced by a part of the Fe, in % by mass,
Cr: 0.01 to 0.30%,
Mo: 0.01-0.30%
Ti: 0.002 to 0.100%,
Nb: 0.002 to 0.100%,
V: 0.01 to 0.20%,
Sn: 0.01 to 0.30%,
B: 0.0002 to 0.0050%
Ca: 0.0002 to 0.0050%,
Mg: 0.0002-0.0050%,
Zr: 0.0002-0.100%, and REM: 0.0002-0.0200%,
The wire according to claim 1 or 2, comprising one or more selected from the group consisting of:

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