WO2025220750A1 - Plated steel wire, rope, and method for producing rope - Google Patents

Abstract

Provided is a plated steel wire in which: a steel portion has a prescribed steel composition; in a central part within 1.0 mm from a center axis in a cross-section that is parallel to a longitudinal direction and passes through the center axis, the area ratio of a mixed structure of ferrite and cementite phases is 95.0% or greater; the diameter of the steel portion is 5.0 mm or greater; the surface of the steel portion is covered, at an amount of at least 100 g/m², by a Zn-based plating layer containing Zn as the main component; the tensile strength TS is 1900-2250 MPa; and the ratio YS/TS of the yield strength (0.2% proof stress) YS to the tensile strength TS is 0.87 or greater. Also provided is a rope obtained by bundling a plurality of the plated steel wires.

Description

Plated steel wire, rope, and rope manufacturing method

This disclosure relates to plated steel wire, ropes, and methods for manufacturing ropes.

BACKGROUND ART Steel wires for bridge cables, ropes, and the like are usually made by subjecting high carbon steel wire rods to a patenting treatment to form a pearlite structure, and then subjecting the wire to wire drawing.
In recent years, there has been a demand for high-strength steel wires in order to reduce construction costs and weight of structures.
However, when high-strength steel wires are used in environments where hydrogen penetrates or in corrosive environments, the possibility of fracture increases due to the progression of hydrogen embrittlement and corrosion. Therefore, high-strength steel wires are required to have excellent resistance to hydrogen embrittlement and corrosion resistance.
High-strength plated steel bars and plated steel wires that are high in strength and have excellent resistance to hydrogen embrittlement have been proposed (see, for example, Patent Documents 1 and 2).

Patent Document 1: JP 2003-328077 A Patent Document 2: JP 2021-183709 A

Steel wires for bridges and other structures are plated to ensure corrosion resistance, but if the plating peels off over many years of use, corrosion will progress.
Recently, steel wire for bridges has become stronger, and the possibility of delayed fracture increases if the plating peels off.
In steel wires that have been drawn from wire rod, a large amount of dislocations is introduced by the drawing process, which is thought to increase the amount of hydrogen that penetrates. If the amount of hydrogen that penetrates is large, there is a possibility of delayed fracture, so it is necessary to suppress hydrogen penetration.

Furthermore, increasing the strength reduces the delayed fracture resistance. In particular, the possibility of delayed fracture increases when the tensile strength exceeds 1900 MPa. One way to prevent delayed fracture is to suppress hydrogen penetration.
Furthermore, plated steel wires for bridges and other structures require torsional properties as ductility.

In light of the above circumstances, the present disclosure aims to provide a plated steel wire, rope, and rope manufacturing method that is high-strength and combines excellent torsional properties with hydrogen penetration resistance.

The means for solving the above problems include the following aspects.
<1> The steel portion is, in mass%,
C: 0.80-1.10%,
Si: 0.10 to 1.50%,
Mn: 0.10-1.00%,
P: 0.050% or less,
S: 0.050% or less,
N: 0.0120% or less,
O: 0.0100% or less,
Al: 0.005-0.070%,
Cu: 0.04-0.80%,
Mo: 0 to 0.20%,
Cr: 0-1.00%,
V: 0 to 0.15%,
Sn: 0 to 0.50%,
Ni: 0 to 0.80%,
Ti: 0 to 0.050%,
Nb: 0 to 0.050%,
B: 0 to 0.0040%,
REM: 0-0.030%,
Bi: 0 to 0.020%,
Mg: 0 to 0.0040%,
Ca: 0-0.0040%,
Zr: 0.030%,
W: 0-0.10%,
Te: 0 to 0.030%,
Sb: 0 to 0.030%, and the balance being Fe and impurities,
an area ratio of a mixed structure consisting of a ferrite phase and a cementite phase of 95.0% or more at a center portion within 1.0 mm from the central axis of a cross section parallel to the longitudinal direction and passing through the central axis;
The diameter of the steel portion is 5.0 mm or more,
The surface of the steel part is coated with a Zn-based plating layer containing Zn as a main component in an amount of 100 g/m2 ^{or more} ,
The tensile strength TS is 1900 MPa or more and 2250 MPa or less,
A plated steel wire having a ratio YS/TS of a yield strength (0.2% proof stress) YS to a tensile strength TS of 0.87 or more.
<2> The plated steel wire according to <1>, wherein the steel portion contains, by mass %, one or more elements selected from the group consisting of the following groups A to C:
(Group A)
Mo: 0.20% or less,
Cr: 1.00% or less, and V: 0.15% or less,
One or more selected from the group consisting of (Group B)
Sn: 0.50% or less, and Ni: 0.80% or less,
One or two selected from the group consisting of (Group C)
Ti: 0.050% or less,
Nb: 0.050% or less,
B: 0.0040% or less,
REM: 0.030% or less,
Bi: 0.020% or less,
Mg: 0.0040% or less,
Ca: 0.0040% or less,
Zr: 0.030% or less,
W: 0.10% or less,
Te: 0.030% or less, and Sb: 0.030% or less,
<3> The plated steel wire according to <2>, wherein the steel composition includes Group A.
<4> The plated steel wire according to <2> or <3>, wherein the steel composition includes the B group.
<5> The plated steel wire according to any one of <2> to <4>, wherein the steel composition includes the Group C.
<6> A rope formed by bundling a plurality of plated steel wires according to any one of <1> to <5>.
<7> A method for producing a rope, comprising a step of bundling a plurality of plated steel wires according to any one of <1> to <5> to form a rope.

This disclosure provides a plated steel wire, rope, and rope manufacturing method that combine high strength with excellent torsional properties and hydrogen penetration resistance.

1 is an example of an SEM image of a longitudinal section of a steel wire containing a mixed structure of ferrite and cementite phases and a non-mixed structure. 1B is a magnified image of a portion of FIG. 1A containing unmixed tissue. 10 is another example of an SEM image of a longitudinal section of a steel wire including a mixed structure of a ferrite phase and a cementite phase and an unmixed structure. 2B is a magnified image of a portion of FIG. 2A containing unmixed tissue.

An embodiment that is an example of the present disclosure will be described.
In this disclosure, a numerical range expressed using "to" means a range that includes the numerical values written before and after "to" as the lower and upper limits. However, when the numerical values written before and after "to" are followed by "greater than" or "less than," the numerical range does not include these numerical values as the lower or upper limit.
The content of an element in a chemical composition may be expressed by adding "amount" to the element symbol (for example, C amount, Si amount, etc.).
With respect to the content of elements in the chemical composition, "%" means "mass %."
When the content of an element in a chemical composition is described as "0 or more," this means that the element does not necessarily need to be contained.
The term "process" includes not only an independent process but also a process that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved.
The "surface" of a steel wire means the "outer surface."
The "central axis" of a steel wire means an imaginary line that passes through the center point of a cross section perpendicular to the longitudinal direction of the steel wire and extends in the longitudinal direction (axial direction).

As a result of repeated experiments and studies, the present inventors have come to the following findings.
(a) Plated steel wire has many dislocations and a large amount of hydrogen penetrates. If the plating peels off, a large amount of hydrogen penetrates into the steel wire (base steel), which may lead to delayed fracture.
(b) Addition of Cu is effective in suppressing the amount of hydrogen penetration.
(c) When the wire is plated after drawing, it is heated to about 450°C for several tens of seconds, and carbon adheres to the dislocation core, preventing hydrogen from adhering to the dislocation periphery and reducing the amount of diffusible hydrogen that contributes to delayed fracture. However, if the plating peels off, a large amount of diffusible hydrogen still penetrates.
(d) To further reduce the amount of diffusible hydrogen, it is effective to heat the plate to 450°C or higher. However, heating above 450°C is not applicable because the plating will re-melt.
(e) Therefore, by simultaneously performing aging and constant load application (heat stretching) after plating, carbon can be efficiently fixed to the dislocation core, further reducing the amount of diffusible hydrogen (in terms of material properties, the 0.2% yield strength increases).
(f) By combining the addition of Cu with heat stretching after plating, it is possible to obtain a plated steel wire that can effectively suppress hydrogen penetration even if the plating peels off, thereby reducing the possibility of delayed fracture.
(g) Furthermore, if plating is not performed, the numerous dislocations introduced into the lamellar ferrite by the wire drawing process cannot be recovered or recrystallized, resulting in a decrease in torsional properties and making it impossible to achieve both delayed fracture properties.
(h) When bundling multiple plated steel wires to manufacture ropes for bridge applications, etc., if the diameter of the plated steel wires is small, the rigidity as a structure will be insufficient. Therefore, the diameter (wire diameter) of the steel wire before plating used for ropes for bridges, etc., usually needs to be 5.0 mm or more. On the other hand, if the area reduction rate during wire drawing is reduced to increase the diameter of the steel wire, the tensile strength will be insufficient. Therefore, the diameter of high-strength steel wire for manufacturing plated steel wires for bridge applications, etc., is preferably 7.5 mm or less.

The plated steel wire, rope, and rope manufacturing method disclosed herein were discovered based on these findings.

[Plated steel wire]
The plated steel wire according to the present disclosure comprises, in mass %,
C: 0.80-1.10%,
Si: 0.10 to 1.50%,
Mn: 0.10-1.00%,
P: 0.050% or less,
S: 0.050% or less,
N: 0.0120% or less,
O: 0.0100% or less,
Al: 0.005 to 0.070%, and Cu: 0.04 to 0.80%,
and the balance being Fe and impurities, or Fe, optional elements, and impurities.
Furthermore, the steel portion of the plated steel wire according to the present disclosure has an area ratio of a mixed structure consisting of a ferrite phase and a cementite phase (sometimes referred to as a "mixed structure of a ferrite phase and a cementite phase" or simply as a "mixed structure" in the present disclosure) of 95.0% or more in a central portion parallel to the longitudinal direction and within 1.0 mm from the central axis in a cross section passing through the central axis.
Furthermore, the plated steel wire according to the present disclosure has a steel portion (i.e., steel wire portion) with a diameter of 5.0 mm or more, and the surface is coated with a Zn-based plating layer containing Zn as a main component at a coating weight of 100 g/m2 ^{or more} .
The plated steel wire according to the present disclosure has a tensile strength of 1900 MPa or more and 2250 MPa or less,
The ratio YS/TS of the yield strength (0.2% proof stress) YS to the tensile strength TS is 0.87 or more.

(steel composition)
The steel composition means the chemical composition of the steel wire portion coated with the Zn-based plating layer in the plated steel wire according to the present disclosure. Hereinafter, the contents of each element in the steel composition will be described.

C: 0.80-1.10%
C is a component necessary for increasing the tensile strength of the steel wire.
If the C content is less than 0.80%, the tensile strength is insufficient.
On the other hand, if the C content of the steel wire is too high, the steel wire becomes hard and the delayed fracture properties deteriorate. If the C content of the steel wire exceeds 1.10%, it becomes difficult to suppress the formation of pro-eutectoid cementite, and the torsional properties deteriorate. Therefore, the C content is set to 0.80 to 1.10%.
From the viewpoint of tensile strength, the C content is preferably 0.85% or more, and particularly 0.90% or more to exhibit better properties. On the other hand, from the viewpoint of compatibility with torsional properties, the C content is preferably 1.05% or less, and particularly 1.00% or less to exhibit better properties.

Si: 0.10~1.50%
Si is an effective component for increasing the tensile strength of the steel wire.
If the Si content of the steel wire is less than 0.10%, the effect of containing Si cannot be sufficiently obtained.
On the other hand, if the Si content of the steel wire exceeds 1.50%, it becomes difficult to suppress the formation of a martensite structure, which is a hard layer, and wire drawability deteriorates.
Therefore, the Si content is set to 0.10 to 1.50%, preferably 0.50 to 1.40%, and more preferably 0.70 to 1.30%.

Mn: 0.10-1.00%
Mn is an effective component for increasing the tensile strength of steel wire, and also has the effect of fixing S in steel as MnS, thereby suppressing hot embrittlement.
If the Mn content of the steel wire is less than 0.10%, the effect of containing Mn cannot be sufficiently obtained.
On the other hand, if the Mn content in the steel wire exceeds 1.00%, it becomes difficult to suppress the formation of a martensite structure, which is a hard layer, and the wire drawability deteriorates.
Therefore, the Mn content is set to 0.10 to 1.00%, and preferably 0.25 to 0.80%.

P: 0.050% or less P is an element that segregates at the grain boundaries of a steel wire and reduces the delayed fracture properties.
If the P content of the steel wire is 0.050% or less, the deterioration of delayed fracture properties is suppressed, and by satisfying other requirements, the targeted delayed fracture properties can be obtained.
The upper limit of the P content is preferably 0.030%, more preferably 0.020% or less. The lower limit of the P content is not limited and is preferably 0% (i.e., no P is contained), but from the viewpoint of reducing the dephosphorization cost, it may be more than 0% or may be 0.001% or more.

S: 0.050% or less S is an element that reduces delayed fracture resistance.
If the S content of the steel wire is 0.050% or less, the target delayed fracture properties can be obtained by satisfying other requirements.
The upper limit of the S content is preferably 0.030%. The lower limit of the S content is not limited, but may be more than 0% or may be 0.001% or more from the viewpoint of reducing the desulfurization cost.

N: 0.0120% or less N is an element that deteriorates the torsional properties.
If the N content of the steel wire is 0.0120% or less, the target delayed fracture properties can be obtained by satisfying other requirements.
The upper limit of the N content is preferably 0.0100%, and more preferably 0.0070%. The lower limit of the N content is not limited, but may be more than 0% or may be 0.0001% or more from the viewpoint of reducing refining costs.

O: 0.0100% or less O is an element that easily forms oxide-based inclusions in the steel wire.
If the O content of the steel wire is 0.0100% or less, the coarsening of oxide-based inclusions is suppressed, and deterioration of torsional properties and delayed fracture properties can be suppressed.
The upper limit of the O content is preferably 0.0070%, and more preferably 0.0050%. The lower limit of the O content is not limited, but may be more than 0% or may be 0.0001% or more from the viewpoint of reducing refining costs.

Al: 0.005-0.070%
Al is an element having a deoxidizing effect and is necessary for reducing the amount of oxygen in the steel wire.
If the Al content of the steel wire is less than 0.005%, it is difficult to obtain the effect of containing Al.
On the other hand, Al is an element that easily forms hard oxide-based inclusions. If the Al content of the steel wire exceeds 0.070%, coarse oxide-based inclusions are significantly more likely to be formed, resulting in a significant decrease in wire drawability.
Therefore, the Al content is set to 0.005 to 0.070%, preferably 0.010 to 0.050%, and more preferably 0.020 to 0.040%.

Cu: 0.04-0.80%
Cu has the effect of suppressing hydrogen penetration and is necessary for improving delayed fracture resistance. If the Cu content of the steel wire is less than 0.04%, the effect of containing Cu cannot be sufficiently obtained.
On the other hand, if the Cu content in the steel wire exceeds 0.80%, it becomes difficult to suppress the formation of a martensite structure, which is a hard layer, and the wire drawability deteriorates.
Therefore, the Cu content is set to 0.04 to 0.80%, and preferably 0.10 to 0.60%.

The plated steel wire according to the present disclosure may contain elements (optional elements) other than those described above, provided that the elements do not impair the delayed fracture properties. The optional elements that may be contained in the steel portion (steel wire) of the plated steel wire according to the present disclosure are described below. The optional elements that may be contained in the steel portion of the plated steel wire according to the present disclosure are divided into the following groups A to C in terms of their effects.
(Group A)
Mo: 0.20% or less,
Cr: 1.00% or less, and V: 0.15% or less,
One or more selected from the group consisting of (Group B)
Sn: 0.50% or less, and Ni: 0.80% or less,
One or two selected from the group consisting of (Group C)
Ti: 0.050% or less,
Nb: 0.050% or less,
B: 0.0040% or less,
REM: 0.030% or less,
Bi: 0.020% or less,
Mg: 0.0040% or less,
Ca: 0.0040% or less,
Zr: 0.030% or less,
W: 0.10% or less,
Te: 0.030% or less, and Sb: 0.030% or less,
Each optional element is explained below.

Mo: 0-0.20%
The inclusion of Mo is optional.
Mo has the effect of increasing the tensile strength of steel wire. In order to stably obtain this effect, the Mo content is preferably 0.01% or more.
On the other hand, even if the Mo content of the steel wire exceeds 0.20%, the effect is saturated.
Therefore, when Mo is intentionally contained in the steel wire, the Mo content is preferably within the range of 0.01 to 0.20%, more preferably 0.02 to 0.10%.

Cr: 0-1.00%
The inclusion of Cr is optional.
Cr has the effect of increasing the tensile strength of steel wire. In order to stably obtain this effect, the Cr content is preferably 0.03% or more.
If the Cr content exceeds 1.00%, it becomes difficult to suppress the formation of a martensite structure, which is a hard layer, and wire drawability deteriorates.
Therefore, when Cr is intentionally contained in the steel wire, the Cr content is preferably in the range of 0.03 to 1.00%. The Cr content may be preferably 0.85% or less, and more preferably 0.10 to 0.70%.

V: 0 to 0.15%
The inclusion of V is optional.
V has the effect of increasing the tensile strength of the steel wire. In order to stably obtain this effect, it is preferable that the V content of the steel wire be 0.01% or more.
On the other hand, if the V content of the steel wire exceeds 0.15%, the wire drawability decreases.
Therefore, when V is intentionally contained in the steel wire, the V content of the steel wire is preferably 0.02 to 0.15%, more preferably 0.03 to 0.13%, and even more preferably 0.05 to 0.12%.

Sn: 0-0.50%
The inclusion of Sn is optional.
Sn has the effect of increasing the corrosion resistance of the steel wire. In order to stably obtain this effect, the Sn content is preferably 0.005% or more.
On the other hand, even if the Sn content of the steel wire exceeds 0.50%, the effect is saturated.
Therefore, when Sn is intentionally contained in the steel wire, the Sn content is preferably within a range of 0.001 to 0.50%, more preferably 0.005 to 0.40%.

Ni: 0-0.80%
The inclusion of Ni is optional.
Ni has the effect of increasing the corrosion resistance of steel wire. In order to stably obtain this effect, the Ni content is preferably 0.01% or more.
On the other hand, even if the Ni content of the steel wire exceeds 0.80%, the effect is saturated.
Therefore, when Ni is intentionally contained in the steel wire, the Ni content is preferably within a range of 0.01 to 0.80%, more preferably 0.05 to 0.60%.

Ti: 0~0.050%
The inclusion of Ti is optional.
Ti forms carbides or carbonitrides in the steel wire and has the effect of improving delayed fracture resistance. To obtain this effect, the Ti content of the steel wire is preferably 0.002% or more.
On the other hand, if the Ti content of the steel wire exceeds 0.050%, coarse carbides or carbonitrides are likely to be formed, and the wire drawability deteriorates.
Therefore, when Ti is intentionally contained in the steel wire, the Ti content of the steel wire is preferably 0.002 to 0.050%, more preferably 0.005 to 0.030%, and may be 0.025% or less.

Nb: 0-0.050%
The inclusion of Nb is optional.
Nb forms carbides or carbonitrides in the steel wire and acts to improve delayed fracture resistance. To obtain this effect, the Nb content of the steel wire is preferably 0.002% or more.
On the other hand, if the Nb content of the steel wire exceeds 0.050%, coarse carbides or carbonitrides are likely to be formed, and the wire drawability deteriorates.
Therefore, when Nb is intentionally contained in the steel wire, the Nb content of the steel wire is preferably 0.002 to 0.050%, more preferably 0.005 to 0.030%, and may be 0.025% or less.

B: 0-0.0040%
The inclusion of B is optional.
B has the effect of suppressing the formation of ferrite structure and improving delayed fracture resistance. To obtain this effect, the B content of the steel wire is preferably 0.0003% or more.
On the other hand, if the B content of the steel wire exceeds 0.0040%, coarse carbides are likely to be formed, and the wire drawability deteriorates.
Therefore, when B is intentionally contained in the steel wire, the B content of the steel wire is preferably 0.0003 to 0.0040%, more preferably 0.0006 to 0.0030%.

REM: 0-0.030%
The inclusion of REM is optional.
If REM is contained, high delayed fracture resistance can be exhibited more stably. To obtain this effect, the REM content of the steel wire is preferably 0.002% or more.
On the other hand, if the REM content of the steel wire exceeds 0.030%, the effect saturates.
Therefore, when REM is intentionally contained, the REM content of the steel wire is preferably 0.002 to 0.030%, and the REM content may be 0.020% or less.
REM refers to a total of 17 elements, including Sc, Y, and lanthanoids, and the REM content refers to the content of one type of REM when there is one type of REM, and refers to the total content of these when there are two or more types of REM.

Bi: 0~0.020%
The inclusion of Bi is optional.
If Bi is contained, high delayed fracture resistance can be exhibited more stably. To obtain this effect, the Bi content of the steel wire is preferably 0.001% or more.
On the other hand, if the Bi content of the steel wire exceeds 0.020%, the effect is saturated.
Therefore, when Bi is intentionally contained in the steel wire, the Bi content of the steel wire is preferably 0.001 to 0.020%. The Bi content may be 0.015% or less.

Mg: 0-0.0040%
The inclusion of Mg is optional.
If Mg is contained, high delayed fracture resistance can be exhibited more stably. To obtain this effect, the Mg content of the steel wire is preferably set to 0.0002% or more.
On the other hand, if the Mg content of the steel wire exceeds 0.0040%, the effect is saturated.
Therefore, when Mg is intentionally contained in the steel wire, the Mg content of the steel wire is preferably 0.0002 to 0.0040%, and may be 0.0030% or less.

Ca: 0-0.0040%
The inclusion of Ca is optional.
If Ca is contained, high delayed fracture resistance can be exhibited more stably. To obtain this effect, the Ca content of the steel wire is preferably 0.0002% or more.
On the other hand, if the Ca content of the steel wire exceeds 0.0040%, the effect is saturated.
Therefore, when Ca is intentionally contained in the steel wire, the Ca content of the steel wire is preferably 0.0002 to 0.0040%. The Ca content may be 0.0030% or less.

Zr: 0-0.030%
The inclusion of Zr is optional.
If Zr is contained, high delayed fracture resistance can be exhibited more stably. To obtain this effect, the Zr content of the steel wire is preferably 0.002% or more.
On the other hand, if the Zr content of the steel wire exceeds 0.030%, coarse carbides or carbonitrides are likely to be formed, and the wire drawability deteriorates.
Therefore, when Zr is intentionally contained in the steel wire, the Zr content of the steel wire is preferably 0.002 to 0.030%, and may be 0.025% or less.

W: 0 to 0.10%
The inclusion of W is optional.
If W is contained, high delayed fracture resistance can be exhibited more stably. To obtain this effect, the W content of the steel wire is preferably 0.02% or more.
On the other hand, when the W content of the steel wire exceeds 0.10%, the effect is saturated.
Therefore, when W is intentionally contained in the steel wire, the W content of the steel wire is preferably 0.02 to 0.10%. The W content may be 0.08% or less.

Te: 0 to 0.030%,
The inclusion of Te is optional.
If the steel wire contains Te, it can exhibit high delayed fracture resistance more stably. To obtain this effect, the Te content of the steel wire is preferably 0.001% or more.
On the other hand, if the Te content of the steel wire exceeds 0.030%, the effect is saturated.
Therefore, when Te is intentionally contained in the steel wire, the Te content of the steel wire is preferably 0.001 to 0.030%, and the Te content may be 0.020% or less.

Sb: 0-0.030%
The inclusion of Sb is optional.
If Sb is contained, high delayed fracture resistance can be more stably exhibited. To obtain this effect, the Sb content of the steel wire is preferably 0.001% or more.
On the other hand, if the Sb content of the steel wire exceeds 0.030%, the effect is saturated.
Therefore, when Sb is intentionally contained in the steel wire, the Sb content of the steel wire is preferably 0.001 to 0.030%, and may be 0.020% or less.

(Metal structure)
The steel portion (steel wire) of the plated steel wire according to the present disclosure has an area ratio of a mixed structure of ferrite phase and cementite phase of 95.0% or more in a center portion parallel to the longitudinal direction of the steel wire and within 1.0 mm from the central axis of a cross section (longitudinal cross section) passing through the central axis.
If the area ratio of the mixed structure of ferrite phase and cementite phase at the center of the longitudinal cross section (cross section parallel to the longitudinal direction of the steel wire) of the steel wire is 95.0% or more, the delayed fracture properties of the steel wire after wiredrawing will be good. If the area ratio of the mixed structure of ferrite phase and cementite phase at the center of the steel wire is 95.0% or more, and if the area ratio of the mixed structure of ferrite phase and cementite phase including the surface layer is 95.0% or more, the total area ratio of the remainder will be low, so the center is measured as a representative. The area ratio of the mixed structure of ferrite phase and cementite phase at the center of the cross section of the steel wire may be 96.0% or more, 97.0% or more, 98.0% or more, 99.0% or more, or 100.0%.

-Measuring the area ratio of metal structure-
The central part of the longitudinal cross section of the steel wire is photographed with a scanning electron microscope (SEM) at 3×10 ⁻⁴ mm ² (length 0.015 mm, width 0.02 mm) and five images are taken every 200 μm. Picral is used for etching.
FIG. 1A shows an example of an SEM image of a longitudinal cross section of a steel wire including a mixed structure of a ferrite phase and a cementite phase and a portion that is not a mixed structure of a ferrite phase and a cementite phase (sometimes referred to as a "non-mixed structure" in the present disclosure). The portion M indicated by the arrow is the non-mixed structure. FIG. 1B is an enlarged image of the portion including the non-mixed structure M in FIG. 1A. FIG. 2B is another example of an SEM image of a longitudinal cross section of a steel wire including a mixed structure of a ferrite phase and a cementite phase and a non-mixed structure, and FIG. 2B is an enlarged image of the portion including the non-mixed structure M in FIG. 2A. The region M surrounded by a black dotted line in each of FIGS. 1B and 2B is the non-mixed structure.
The mixed structure of ferrite and cementite phases is obtained by wiredrawing pearlite, bainite, etc., and the cementite phase is mixed in the ferrite phase. The area ratio (%) of each SEM image is measured. Specifically, each SEM image is printed on paper, and then a transparent sheet such as an OHP (Overhead Projector) sheet is placed on top of the paper to color the non-mixed structure. The colored transparent sheet is then analyzed by image analysis to measure the total area ratio of the non-mixed structure. The area per field of view is set to 3 x 10 ^-4 mm ² (length 0.015 mm, width 0.02 mm), and image analysis software (e.g., Luzex AP manufactured by Nireco Corporation) is used for image analysis. The average value (%) of the area ratios of the non-mixed structure of the five images is calculated, and the value obtained by subtracting the average value from 100.0 is used as the area ratio of the mixed structure of the ferrite phase and cementite phase of the steel wire.

(diameter of steel part)
The diameter of the steel portion (steel wire) of the plated steel wire according to the present disclosure is 5.0 mm or more. The diameter of the steel portion may be 5.1 mm or more, 5.3 mm or more, 5.5 mm or more, or 6.0 mm or more. There is no particular upper limit to the diameter of the steel portion, but for bridge applications, it may be 10.0 mm or less, 8.0 mm or less, or 7.0 mm or less.

(Tensile strength)
The plated steel wire according to the present disclosure has a tensile strength TS of 1900 MPa or more and 2250 MPa or less.
When the tensile strength of a plated steel wire is 1900 MPa or more, the delayed fracture resistance deteriorates. This is because the amount of hydrogen penetration increases and the cracking susceptibility due to hydrogen increases. Even if the plated steel wire according to the present disclosure has a tensile strength of 1900 MPa, the delayed fracture resistance can be improved by satisfying other requirements.
If the tensile strength of the steel wire exceeds 2250 MPa, the twisting characteristics deteriorate, so the upper limit of the tensile strength is set to 2250 MPa.

(yield ratio)
The plated steel wire according to the present disclosure has a ratio of yield strength (0.2% proof stress) YS to tensile strength TS (yield ratio: YS/TS) of 0.87 or more.
It is presumed that when the ratio of the yield strength YS to the tensile strength TS of a steel wire (YS/TS) is 0.87 or more, carbon is fixed at the dislocation core, and hydrogen fixation around the dislocation is suppressed, thereby suppressing hydrogen penetration. The yield ratio (YS/TS) of the steel wire is more preferably 0.90 or more, and even more preferably 0.93 or more.

-Measurement of tensile strength and yield strength-
The plated steel wire to be subjected to the tensile test may be straightened by straightening. However, since strict straightening may change the YS, if the wire is bent and cannot be chucked, multi-point bending may be performed.
The tensile test is performed with a plated steel wire length of 340 mm, a chuck distance of 200 mm, and a stroke speed of 10 mm/min.
The diameter D of the plated steel wire is measured in two perpendicular directions at the center of the steel wire length using a vernier caliper, and the average value is used. The tensile strength is the cross-sectional area (mm ² ) calculated from the "maximum load (N) during the tensile test / diameter of the plated steel wire".
The yield strength is calculated as the strength at 0.2% plastic deformation (0.2% proof stress) using the linear relationship between stress and strain for tensile strength of 0.2 to 0.4.

(Zn-based plating layer)
In the plated steel wire according to the present disclosure, the surface of the steel wire (base steel, i.e., steel portion) is coated with a Zn-based plating layer containing Zn as the main component at 100 g/ ^m2 . The Zn-based plating layer containing Zn as the main component is a plating layer having the highest Zn content (mass %) and may be a plating layer consisting of Zn alone or a plating layer consisting of an alloy of Zn and other elements. Examples of elements that may be contained in the Zn-based plating layer other than Zn include one or more selected from the group consisting of Al, Cu, Sn, Mg, and Si.
By covering the surface of the steel wire (base steel) with a Zn-based plating layer at 100 g/m ² or more, hydrogen penetration can be effectively suppressed.

Here, we will explain how to measure the torsion characteristics, plating coverage, and diffusible hydrogen content in the examples described below.

(Torsion characteristics)
The evaluation is based on the number of times until the plated steel wire breaks, and five wires are tested for each test, and the minimum value is evaluated.
The distance between the chucks is 100 x wire diameter D. The rotation speed is 20 rpm.
The breakage of a plated steel wire includes not only the breakage of the entire plated steel wire but also the occurrence of a partial crack in the plated steel wire. In other words, if a partial crack occurs in the plated steel wire during the test, the number of twists at that point is used for evaluation. The occurrence of a crack during rotation can be determined by a sudden drop in torque.

(plating coverage)
The coating weight is calculated from the weight difference when the plating is peeled and the peeled area. After cutting the plated steel wire into 100 mm pieces, the wire is immersed in a chemical solution containing hydrochloric acid and an inhibitor to chemically peel off the plating layer. The coating weight is calculated by dividing the weight difference before and after peeling the plating layer by the surface area calculated from the wire diameter and length after peeling.

(Diffusible hydrogen content)
In accordance with JASO M610-92 (Automotive Parts Visual Corrosion Test Method), the plating-removed steel wire is subjected to one cycle: 2 hours of salt spraying with a 5% sodium chloride solution at 35°C, 4 hours of drying at 60°C and 30% RH, and 2 hours of wetting at 50°C and 95% RH. After 168 cycles, the wire is recovered, rust is removed by sandblasting, and then a temperature-programmed hydrogen analysis is performed using a gas chromatograph. The amount of released hydrogen below 200°C is measured for two samples, and the average value is taken as the amount of diffusible hydrogen (HE) absorbed in the operating environment.

[Method of manufacturing plated steel wire]
The method for producing the plated steel wire according to the present disclosure is not particularly limited, but an example of a suitable production method will be described below.

(Wire rod manufacturing)
A wire rod having the above-mentioned chemical composition (steel composition) is used. In order to adjust the structure of the wire rod to be suitable for wire drawing, it is preferable to heat the wire rod to 950 to 1100°C and then immerse it in a molten salt or lead bath at 550 to 650°C to form a pearlite structure.

(Wire drawing process)
The wire is drawn to produce steel wire.
The wire drawing strain ε due to wire drawing is expressed by the following formula:
ε=Ln(D ₀ /D) ²
_D0 is the diameter before wiredrawing, and D is the diameter after wiredrawing. The wiredrawing strain ε is preferably 1.00 or more and 2.50 or less. If the wiredrawing strain ε is lower than 1.00, it is difficult to obtain strength, and if it is higher than 2.50, the strength becomes excessive and the torsional properties deteriorate. It is preferable to perform a surface lubrication treatment such as a zinc phosphate coating or a borax coating before wiredrawing.
As described above, the diameter D of the steel wire is 5.0 mm or more. Although there is no particular upper limit to the diameter D of the steel wire, if the diameter D exceeds 7.5 mm, it becomes difficult to obtain the target strength, so the diameter of the steel wire is set to be in the range of 5.0 mm to 7.5 mm.

(hot-dip galvanizing)
After wire drawing, the wire is degreased and then plated with zinc or an alloy containing zinc as the main component. The molten zinc temperature is preferably within the range of 400 to 530°C. If the molten zinc temperature is less than 400°C, the steel material will harden due to age hardening and the torsional properties will deteriorate. On the other hand, if the molten zinc temperature is above 530°C, softening due to spheroidization of cementite will become significant, resulting in insufficient strength. Therefore, it is preferable to perform the plating process at a molten zinc temperature within the range of 400 to 530°C.

(heat stretch)
To increase the ratio of yield strength YS to tensile strength TS (YS/TS), it is preferable to apply a load of approximately 0.35 to 0.55 times the tensile strength TS of the plated steel wire at 300 to 400°C after the hot-dip galvanizing for approximately 10 to 45 seconds. If the temperature during such heat stretching is less than 300°C, the strength of the plated steel wire increases and the torsional properties deteriorate. On the other hand, if the temperature exceeds 400°C, an improvement in the durability ratio cannot be expected, and delayed fracture properties do not improve. Furthermore, if the temperature is 420°C or higher, there is a possibility that the plating will remelt. Therefore, it is preferable to perform heat stretching at 300 to 400°C.

The plated steel wire according to the present disclosure can be manufactured through the above steps. Note that the above method is one example of a preferred method for manufacturing the plated steel wire according to the present disclosure, and the method for manufacturing the plated steel wire according to the present disclosure is not limited to the above method.

(Application)
The applications of the plated steel wire according to the present disclosure are not particularly limited, but it is suitable for a variety of applications requiring high strength, delayed fracture properties, and torsional properties, such as steel wires for bridge cables and various ropes, etc. For example, a rope (strand) obtained by bundling a plurality of plated steel wires according to the present disclosure can be used for a bridge cable.

The plated steel wire according to the present disclosure will be explained in more detail below using examples. However, these examples do not limit the plated steel wire, rope, or rope manufacturing method according to the present disclosure.

Example 1
A steel material (steel 1) having the chemical composition (unit: mass %) shown in Table 1 was prepared, and a wire material was produced by the method (conditions) shown in Table 2. Plated steel wires were then produced through wire drawing, hot-dip galvanizing, and heat stretching. The notation "-" in Table 1 indicates that the content of the element in question is at the impurity level and it can be determined that the element is not substantially contained. The same applies to the notation "-" in Table 4 described below. The remainder of the chemical compositions in Tables 1 and 4 is Fe and impurities. The notation "-" in Table 2 indicates that plating was not performed.
In each table, underlines indicate that the items are outside the scope of the present disclosure.

 

For the plated steel wires manufactured by the above-mentioned manufacturing method, the area ratio of the mixed structure of ferrite and cementite phases in the center of the longitudinal cross section of the steel wire portion (base steel), tensile strength, yield strength (0.2% proof stress) YS, coating weight, torsion value, and diffusible hydrogen content HE were measured by the above-mentioned methods. Note that the structure other than the mixed structure of ferrite and cementite phases (non-mixed structure) was mainly a ferrite structure and a martensite structure.
The following cases were evaluated as good.
Torsion value: 10 or more Diffusible hydrogen content HE: 0.100 ppm or less The measurement results are shown in Table 3. Torsion value and diffusible hydrogen content HE that did not meet the above criteria for "good" are underlined.
In Comparative Example 9, the measured values were obtained for a steel wire that had not been plated and had been drawn.

 

The plated steel wires of Examples 1 to 4 had high strength of 1900 MPa or more and 2250 MPa or less, and were excellent in both torsional properties and hydrogen penetration resistance.
In Comparative Example 1, the heating temperature during wire production was high, resulting in coarse grains, and pearlite transformation was not completed during immersion in molten salt, resulting in the formation of martensite structures, which reduced workability and caused cracks during wire drawing.
In Comparative Example 2, the wire drawing strain was high, resulting in excessive strength, and therefore the twisting properties were insufficient.
In Comparative Example 3, the heating temperature in the heat stretching was low, so the strength was too high and the twisting properties were insufficient.
In all of Comparative Examples 4 to 8, the yield strength ratio was insufficient and the amount of diffusible hydrogen HE was large.
Comparative Example 9 had insufficient twisting properties, which is thought to be because there was no heat input during plating and ductility was reduced by heat stretching.

Example 2
Steel materials (steels 2 to 25) having the chemical compositions (unit: mass %) shown in Table 4 were prepared, and plated steel wires were produced under the conditions of manufacturing method A shown in Table 2.

 

The total area ratio of the ferrite and martensite structures, tensile strength, yield strength (0.2% proof stress) YS, coating weight, torsion value, and diffusible hydrogen content HE of the produced plated steel wire were measured by the methods described above. Note that the structure other than the mixed structure consisting of ferrite and cementite was mainly ferrite and martensite.
The measurement results are shown in Table 5.

 

The plated steel wires of Examples 11 to 24 have high strength of 1900 MPa or more and 2250 MPa or less, and are excellent in both torsional properties and hydrogen penetration resistance.
In Comparative Example 11, the amount of C was insufficient, and the strength was insufficient, being less than 1900 MPa.
In Comparative Example 12, the C amount was excessive, the strength was too high, and the torsional properties were insufficient.
In Comparative Example 13, the Si amount was excessive, martensite was formed, and workability was reduced.
In Comparative Example 14, the Mn amount was excessive, martensite was formed, and workability was reduced.
In Comparative Example 15, the amount of N was excessive, and the twisting properties were insufficient.
In Comparative Example 16, the amount of Al was excessive, and the workability was reduced.
In Comparative Example 17, the Cu amount was excessive, martensite was formed, and workability was reduced.
In Comparative Example 18, the Cu amount was less than the lower limit, and the diffusible hydrogen amount HE was large.
In Comparative Example 19, the amount of Mo was excessive, martensite was formed, and workability was reduced.
In Comparative Example 20, the Cr amount was excessive, and the workability was reduced.

The above describes the plated steel wire, etc., according to the present disclosure, but the plated steel wire, etc., according to the present disclosure is not limited to the above embodiments and examples. For example, the method for manufacturing the plated steel wire according to the present disclosure may involve winding a wire rod, immersing it in molten salt to produce a wire rod with the desired metal structure, drawing the wire rod to produce a steel wire, and then galvanizing and heat stretching the wire to produce a plated steel wire. Furthermore, the scope of the present disclosure also includes the case where the plated steel wire is bundled to produce a rope (cable).

(Additional Note)
The present disclosure includes the following aspects.
<1> The steel portion is, in mass%,
C: 0.80-1.10%,
Si: 0.10 to 1.50%,
Mn: 0.10-1.00%,
P: 0.050% or less,
S: 0.050% or less,
N: 0.0120% or less,
O: 0.0100% or less,
Al: 0.005 to 0.070%, and Cu: 0.04 to 0.80%,
The balance is Fe and impurities,
a cross section parallel to the longitudinal direction, the cross section passing through the central axis having a central portion within 1.0 mm from the center, in which the total area ratio of the ferrite structure and the martensite structure is 5.0% or less, and the remainder is a mixed structure consisting of ferrite and cementite,
the surface of the steel portion is coated with a Zn-based plating layer containing Zn as a main component in an amount of 100 g/m or more;
The tensile strength TS is 1900 MPa or more and 2250 MPa or less,
A plated steel wire having a ratio YS/TS of a yield strength (0.2% proof stress) YS to a tensile strength TS of 0.87 or more.
<2> The steel portion is, in mass%,
C: 0.80-1.10%,
Si: 0.10 to 1.50%,
Mn: 0.10-1.00%,
P: 0.050% or less,
S: 0.050% or less,
N: 0.0120% or less,
O: 0.0100% or less,
Al: 0.005-0.070%,
Cu: 0.04 to 0.80%, and further containing one or more elements selected from the group consisting of the following Groups A to C, with the balance being steel components that are Fe and impurities:
(Group A)
Mo: 0.20% or less,
Cr: 1.00% or less, and V: 0.15% or less,
One or more selected from the group consisting of (Group B)
Sn: 0.50% or less, and Ni: 0.80% or less,
One or two selected from the group consisting of (Group C)
Ti: 0.050% or less,
Nb: 0.050% or less,
B: 0.0040% or less,
REM: 0.030% or less,
Bi: 0.020% or less,
Mg: 0.0040% or less,
Ca: 0.0040% or less,
Zr: 0.030% or less,
W: 0.10% or less,
Te: 0.030% or less, and Sb: 0.030% or less,
one or more selected from the group consisting of: a structure parallel to the longitudinal direction, in which the total area ratio of the ferrite structure and the martensite structure in the central portion within 1.0 mm from the center in a cross section passing through the central axis is 5.0% or less, and the remainder is a mixed structure consisting of ferrite and cementite,
the surface of the steel portion is coated with a Zn-based plating layer containing Zn as a main component in an amount of 100 g/m or more;
The tensile strength TS is 1900 MPa or more and 2250 MPa or less,
A plated steel wire having a ratio YS/TS of a yield strength (0.2% proof stress) YS to a tensile strength TS of 0.87 or more.
<3> The plated steel wire according to <2>, wherein the steel components include the Group A.
<4> The plated steel wire according to <2> or <3>, wherein the steel components include the B group.
<5> The plated steel wire according to any one of <2> to <4>, wherein the steel components include the C group.
<6> A rope comprising a plurality of plated steel wires according to any one of <1> to <5> bundled together. <7> A method for manufacturing a rope, comprising a step of bundling a plurality of plated steel wires according to any one of <1> to <6> into a rope.

The disclosure of Japanese Patent Application No. 2024-067775, filed April 18, 2024, is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards mentioned herein are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard was specifically and individually indicated.

Claims (7)

The steel portion is, by mass%,
C: 0.80-1.10%,
Si: 0.10 to 1.50%,
Mn: 0.10-1.00%,
P: 0.050% or less,
S: 0.050% or less,
N: 0.0120% or less,
O: 0.0100% or less,
Al: 0.005-0.070%,
Cu: 0.04-0.80%,
Mo: 0 to 0.20%,
Cr: 0-1.00%,
V: 0 to 0.15%,
Sn: 0 to 0.50%,
Ni: 0 to 0.80%,
Ti: 0 to 0.050%,
Nb: 0 to 0.050%,
B: 0 to 0.0040%,
REM: 0-0.030%,
Bi: 0 to 0.020%,
Mg: 0 to 0.0040%,
Ca: 0-0.0040%,
Zr: 0.030%,
W: 0-0.10%,
Te: 0 to 0.030%,
Sb: 0 to 0.030%, and the balance being Fe and impurities,
an area ratio of a mixed structure consisting of a ferrite phase and a cementite phase of 95.0% or more at a center portion within 1.0 mm from the central axis of a cross section parallel to the longitudinal direction and passing through the central axis;
The diameter of the steel portion is 5.0 mm or more,
The surface of the steel part is coated with a Zn-based plating layer containing Zn as a main component in an amount of 100 g/m2 ^{or more} ,
The tensile strength TS is 1900 MPa or more and 2250 MPa or less,
A plated steel wire having a ratio YS/TS of a yield strength (0.2% proof stress) YS to a tensile strength TS of 0.87 or more. 2. The plated steel wire according to claim 1, wherein the steel portion contains, in mass %, one or more elements selected from the group consisting of the following groups A to C:
(Group A)
Mo: 0.20% or less,
Cr: 1.00% or less, and V: 0.15% or less,
One or more selected from the group consisting of (Group B)
Sn: 0.50% or less, and Ni: 0.80% or less,
One or two selected from the group consisting of (Group C)
Ti: 0.050% or less,
Nb: 0.050% or less,
B: 0.0040% or less,
REM: 0.030% or less,
Bi: 0.020% or less,
Mg: 0.0040% or less,
Ca: 0.0040% or less,
Zr: 0.030% or less,
W: 0.10% or less,
Te: 0.030% or less, and Sb: 0.030% or less,
One or more selected from the group consisting of The plated steel wire according to claim 2, wherein the steel composition includes Group A. The plated steel wire according to claim 2 or 3, wherein the steel composition includes Group B. The plated steel wire according to any one of claims 2 to 4, wherein the steel composition includes the C group. A rope comprising a plurality of bundled plated steel wires according to any one of claims 1 to 5. A method for manufacturing a rope, comprising the step of bundling a plurality of plated steel wires according to any one of claims 1 to 5 into a rope.