HIGH-STRENGTH STEEL WIRE

Abstract

 The present invention provides a high strength drawn steel wire having a predetermined chemical composition, in which, in a cross section which includes the central axis of the drawn steel wire and which is in parallel with the central axis, the area ratio of a pearlite structure in the interior of the drawn steel wire is 90% or more, and the area ratio of the pearlite structure in a surface layer portion of the drawn steel wire is 80% or more; in which, in the entire structure of the drawn steel wire, the area ratio of a lamellar pearlite structure in which cementite has an average length of 1.0 µm or more is from 30% to 65%, and the area ratio of a fragmented pearlite structure in which cementite has an average length of 0.30 µm or less is from 20% to 50%; and in which the drawn steel wire has a tensile strength of 1,960 MPa or more.

Description

Technical Field

[0001] The present disclosure relates to a high strength drawn steel wire.

Background Art

[0002] High strength drawn steel wires, such as drawn steel wires for ropes, drawn steel wires for bridge cables, and PC drawn steel wires are produced using drawn steel wires obtained by subjecting a high carbon steel wire rod to a patenting treatment so as to allow the formation of a pearlite structure, followed by wire drawing and an aging treatment. In recent years, high strength drawn steel wires having a tensile strength of 1,960 MPa or more are demanded, for the purpose of reducing construction cost or reducing the weight of building structures. However, high strength drawn steel wires exhibit a low number of rotations until fracture (value of torsion) in a torsion test, and further, there is a case in which a longitudinal crack referred to as delamination may occur. Therefore, achieving both favorable torsion characteristics and a high strength has been a problem to be solved.

[0003] Patent Document 1 proposes a high strength drawn steel wire whose hardness in a region 0.1 d (d represents a diameter of the drawn steel wire) from a surface layer, in a transverse cross section of the drawn steel wire, is controlled, in order to improve the torsion characteristics of the high strength drawn steel wire.

[0004] Patent Document 2 proposes a high strength zinc-coated drawn steel wire having a helical structure, which has been processed by rotating the drawn steel wire two or more times in the same direction per length of 100 d (d: wire diameter).

[0005] Patent Document 3 proposes a coated drawn steel wire, in which an area ratio of a non-pearlite structure in a portion extending from the surface layer to a depth of 50 µm is 10% or less, in which the area ratio of the non-pearlite structure in the entire area of a cross section is 5% or less, and whose surface is coated with zinc to a coating deposition amount of from 300 to 500g/m².

[0006] Patent Document 4 proposes a method of producing a drawn steel wire, in which a drawn steel wire after being subjected to wire drawing is allowed to pass through a plurality of rolls that have been arranged to provide a certain bending angle, while applying a tensile force to the wire.

[0007] Patent Document 5 proposes a method of producing a zinc-coated drawn steel wire, in which a drawn steel wire after drawing and before being coated with zinc is subjected to a blueing treatment at a temperature of 430°C or higher, so as to satisfy a relationship: T(20 + log t) ≥ 12,700 (T: blueing temperature shown in absolute temperature, t: blueing time shown in hours).

[0008] 

Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2000-336459

Patent Document 2: Japanese Patent Publication (JP-B) No. 3130445

Patent Document 3: JP-B No. 5169839

Patent Document 4: JP-B No. 3725576

Patent Document 5: JP-B No. 2553612

SUMMARY OF INVENTION

Technical Problem

[0009] However, in conventional high strength drawn steel wires, an increase in tensile strength results in unstable torsion characteristics, and therefore, it has been unable to sufficiently prevent the occurrence of delamination by improving the value of torsion. After all, achieving both favorable torsion characteristics and a high strength still remains a problem to be solved, in the present circumstances.

[0010] Patent Document 5 describes that it is possible to improve the torsion characteristics of a drawn steel wire, by subjecting the drawn steel wire after wire drawing to a predetermined blueing treatment. However, in Patent Document 5, a steel wire rod obtained by hot rolling and cooling by ordinary methods is subjected to reheating in an ordinary atmosphere (namely, in an air atmosphere), immersion in a molten lead bath, cooling, and wire drawing to obtain a drawn steel wire, and the thus obtained drawn steel wire is then subjected to a predetermined blueing treatment. Accordingly, decarbonization of a surface layer portion during production process causes a decrease in the area ratio of a pearlite structure in the surface layer portion of the drawn steel wire, leaving much room for improvement in torsion characteristics.

[0011] An object of one embodiment of the present disclosure is to provide a high strength drawn steel wire having a high strength and excellent torsion characteristics.

Solution to Problem

[0012] The above-mentioned problems are solved by the following means.

<1> A high strength drawn steel wire having a chemical composition including, by mass,
from 0.85 to 1.20% of C,
from 0.10 to 2.00% of Si,
from 0.20 to 1.00% of Mn,
0.030% or less of P,
0.030% or less of S,
from 0.0010 to 0.0080% of N,
from 0 to 0.0050% of B,
from 0 to 0.100% of Al,
from 0 to 0.050% of Ti,
from 0 to 0.60% of Cr,
from 0 to 0.10% of V,
from 0 to 0.050% of Nb,
from 0 to 0.050% of Zr, and
from 0 to 1.00% of Ni,
with a balance consisting of Fe and impurities,
wherein, in a cross section that includes a central axis of the drawn steel wire and is parallel with the central axis, the area ratio of a pearlite structure in an interior of the drawn steel wire is 90% or more, and the area ratio of the pearlite structure in a surface layer portion of the drawn steel wire is 80% or more,
wherein, in the entire structure of the drawn steel wire, the area ratio of a lamellar pearlite structure in which cementite has an average length of 1.0 µm or more is from 30% to 65%, and the area ratio of a fragmented pearlite structure in which cementite has an average length of 0.30 µm or less is from 20% to 50%, and
wherein the drawn steel wire has a tensile strength of 1,960 MPa or more.

<2> The high strength drawn steel wire according to <1>, wherein the chemical composition of the drawn steel wire includes, by mass, one or more of from 0.0001 to 0.0050% of B, from 0.001 to 0.100% of Al, or from 0.001 to 0.050% Ti.

<3> The high strength drawn steel wire according to <1> or <2>, wherein the chemical composition of the drawn steel wire includes, by mass, one or more of from 0.01 to 0.60% of Cr, from 0.01 to 0.10% of V, from 0.001 to 0.050% of Nb, from 0.001 to 0.050% of Zr, or from 0.01 to 1.00% of Ni.

<4> The high strength drawn steel wire according to any one of <1> to <3>, wherein the drawn steel wire has a diameter of from 1.5 to 8.0 mm.

<5> The high strength drawn steel wire according to any one of <1> to <4>, wherein a coating layer including one of a Zn layer or a Zn alloy layer is coated on the surface of the drawn steel wire.

Advantageous Effects of Invention

[0013] One embodiment of the present disclosure provides a high strength drawn steel wire having a high strength and excellent torsion characteristics.

BRIEF DESCRIPTION OF DRAWINGS

[0014] 

FIG. 1 is a schematic diagram for describing observation regions for measuring the area ratios of a pearlite structure in the interior and in a surface layer portion of a drawn steel wire.

FIG. 2 is a schematic diagram for describing observation regions for measuring the area ratio of a lamellar pearlite structure and the area ratio of a fragmented pearlite structure.

DESCRIPTION OF EMBODIMENTS

[0015] An embodiment which is one example of the present disclosure will now be described.

[0016] In the present specification, any numerical range indicated using the expression "from * to" represents a range in which numerical values described before and after the "to" are included in the range as a lower limit value and an upper limit value.

[0017] Further, any numerical range in which the expression "more than" or "less than" is added to any of the numerical values described before and after the "to" represents a range in which any of these numerical values is/are not included in the range as a lower limit value or a upper limit value.

[0018] The content of each element included in a chemical composition is referred to as the "amount of element" (for example, the amount of C, the amount of Si or the like).

[0019] Further, the symbol "%" used to describe the content of each element included in the chemical composition represents "% by mass".

[0020] The definition of the term "step" includes not only an independent step, but also a step which is not clearly distinguishable from another step, as long as an intended purpose of the step is achieved.

[0021] The expression "cross section that includes a central axis of the drawn steel wire and is parallel with the central axis" refers to a cross section that includes the central axis of the drawn steel wire, and is obtained by cutting the drawn steel wire along a longitudinal direction (namely, the direction of drawing) of the wire, so as to be parallel with the central axis.

[0022] The expression "central axis" refers to a virtual line which passes through a central point of the cross section that is orthogonal to an axial direction (longitudinal direction) of the drawn steel wire, and which extends in the axial direction.

[0023] The expression "length of cementite (or an expression equivalent thereto" refers to the length of a long axis of a piece of cementite present in a pearlite structure, when observed in a cross section that includes a central axis of the drawn steel wire and is parallel with the central axis.

[0024] The expression "interior of the drawn steel wire" refers to a region at a depth of more than 100 µm from the surface of the drawn steel wire toward the central axis (in a radially inward direction).

[0025] The expression "surface layer portion of the drawn steel wire" refers to a region up to a depth of 100 µm from the surface of the drawn steel wire toward the central axis (in a radially inward direction).

[0026] The description "XD" (X represents a numerical value) refers, when the diameter of the drawn steel wire is defined as D, to a location at a depth of X times the diameter D from the surface of the drawn steel wire toward the central axis (in a radially inward direction). For example, the description "0.25 D" refers to a location at a depth of 0.25 times the diameter D.

[0027] A high strength drawn steel wire according to the present embodiment is a high strength drawn steel wire which has a predetermined chemical composition, which has a microstructure satisfying the following (1) and (2), and which has a tensile strength of 1,960 MPa or more.

(1) In a cross section that includes a central axis of the drawn steel wire and is parallel with the central axis, the area ratio of a pearlite structure in the interior of the drawn steel wire is 90% or more, and the area ratio of the pearlite structure in the surface layer portion of the drawn steel wire is 80% or more.

(2) In the entire structure of the drawn steel wire, the area ratio of a lamellar pearlite structure in which cementite has an average length of 1.0 µm or more is from 30% to 65%, and the area ratio of a fragmented pearlite structure in which cementite has an average length of 0.30 µm or less is from 20% to 50%.

[0028] By adopting the above described constitution, the high strength drawn steel wire according to the present embodiment will be a drawn steel wire having a high strength and excellent torsion characteristics. The high strength drawn steel wire according to the present embodiment has been found based on the following findings.

[0029] To begin with, in order to improve the torsion characteristics of a high strength drawn steel wire having a tensile strength of 1,960 MPa or more, it is effective to allow the drawn steel wire to have a pearlite structure as the microstructure, and to allow the pearlite structure to be a mixed structure of a lamellar pearlite structure including cementite having a long length, and a fragmented pearlite structure including cementite having a short length as a result of fragmentation. The pearlite structure has a layered structure composed of a cementite phase and a ferrite phase.

[0030] In a case in which a steel wire rod having a pearlite structure is subjected to wire drawing, the microstructure of the drawn steel wire after drawing has an uneven and complicated structure in which a pearlite structure having a fine interlayer spacing, a pearlite structure including irregularly bent layers, a pearlite structure including layers which are locally deformed by shear, and the like, coexist in a mixed state. When the drawn steel wire in this state is subjected to an ordinary molten zinc coating treatment, in which the drawn steel wire is immersed in a molten zinc bath at a temperature of from 440 to 460°C for about 30 seconds, the resulting drawn steel wire will have microscopically uneven mechanical properties. Such a drawn steel wire having microscopically uneven mechanical properties deforms locally when subjected to torsional deformation, as a result of which the value of torsion is reduced.

[0031] In contrast, in the case of a drawn steel wire having microscopically even mechanical properties, the drawn steel wire deforms uniformly under torsional deformation, as a result of which the value of torsion is improved.

[0032] Accordingly, the present inventors have investigated the effects of the chemical composition and the microstructure of a drawn steel wire on the torsion characteristics. As a result, the present inventors have obtained the following findings. Specifically, it is possible to improve the torsion characteristics of a high strength drawn steel wire, even in a case in which the drawn steel wire has a tensile strength of 1,960 MPa or more, by: adjusting the chemical composition of the drawn steel wire; reducing the area ratio of the non-pearlite structure in the drawn steel wire (namely, increasing the area ratio of the pearlite structure in the drawn steel wire); and allowing the pearlite structure to be a structure in which a lamellar pearlite structure including cementite having a long length, and a fragmented pearlite structure including cementite having a short length, coexist in a mixed state.

[0033] In other words, it is possible to obtain a drawn steel wire which exhibits high torsion characteristics even in a case in which the drawn steel wire has a strength of 1,960 MPa or more, when the microstructure of the drawn steel wire satisfies the above described (1) and (2). As described above, an improvement in the structure of a high strength drawn steel wire has made it possible to improve the torsion characteristics of the drawn steel wire.

[0034] In the above described manner, it has been found out that the high strength drawn steel wire according to the present embodiment is a drawn steel wire having a high strength and excellent torsion characteristics.

[0035] The high strength drawn steel wire according to the present embodiment is a drawn steel wire having excellent torsion characteristics and a tensile strength of 1,960 MPa or more, and can be used, for example, as a drawn steel wire for a rope, a drawn steel wire for a bridge cable, a PC drawn steel wire, or the like. Therefore, the high strength drawn steel wire according to the present embodiment contributes, for example, to a reduction in the weight of civil engineering structures and building structures as well as a reduction in construction cost, and thus is extremely useful in industrial applications.

(Chemical composition)

[0036] The high strength drawn steel wire has a chemical composition including, by mass, from 0.85 to 1.20% of C, from 0.10 to 2.00% of Si, from 0.20 to 1.00% of Mn, 0.030% or less of P, 0.030% or less of S, from 0.0010 to 0.0080% of N, from 0 to 0.0050% of B, from 0 to 0.100% of Al, from 0 to 0.050% of Ti, from 0 to 0.60% of Cr, from 0 to 0.10% of V, from 0 to 0.050% of Nb, from 0 to 0.050% of Zr, and from 0 to 1.00% of Ni, with the balance consisting of Fe and impurities.

[0037] It is noted, however, B, Al, Ti, Cr, V, Nb, Zr, and Ni are optional elements. In other words, the high strength drawn steel wire does not necessarily contain these elements.

[0038] A description will now be given of reasons for restricting the ranges of the amounts of the respective elements contained in the high strength drawn steel wire.

[0039] C is added for the purpose of ensuring the tensile strength of the drawn steel wire. When the amount of C is less than 0.85%, proeutectoid ferrite is formed, making it difficult to ensure a predetermined tensile strength. When the amount of C is more than 1.20%, on the other hand, the amount of proeutectoid cementite is increased, resulting in the deterioration of wire drawability. Accordingly, the amount of C is set within the range of from 0.85 to 1.20%. A preferred lower limit of the amount of C for achieving both a high strength and wire drawability is 0.90%. A preferred upper limit of the amount of C for achieving both a high strength and wire drawability is 1.10%.

[0040] Si has the effect of increasing relaxation characteristics, and the effect of increasing the tensile strength as a result of solid solution strengthening. When the amount of Si is less than 0.10%, these effects cannot be obtained sufficiently. When the amount of Si is more than 2.00%, these effects are saturated and hot ductility is deteriorated, resulting in a reduced productivity. Accordingly, the amount of Si is set within the range of from 0.10 to 2.00%. A preferred lower limit of the amount of Si is 0.50%. More preferably, the lower limit of the amount of Si may be 1.00%. On the other hand, a preferred upper limit of the amount of Si is 1.80%. A more preferred upper limit of the amount of Si is 1.50%.

[0041] Mn has the effect of increasing the tensile strength of steel after pearlite transformation. When the amount of Mn is less than 0.20%, the above described effect cannot be obtained sufficiently. When the amount of Mn is more than 1.00%, the effect is saturated. Accordingly, the amount of Mn is set within the range of from 0.20 to 1.00%. A preferred lower limit of the amount of Mn is 0.30%. A preferred upper limit of the amount of Mn is 0.90%.

[0042] P and S are contained in the drawn steel wire as impurities. It is required that the amounts of P and S are reduced, since these elements cause the deterioration of the ductility of the resulting drawn steel wire. Accordingly, the upper limits of both the amount of P and the amount of S are set to 0.030%. Preferred upper limits of the amount of P and the amount of S are 0.020%. More preferred upper limits of the amount of P and the amount of S are 0.015% or less. The lower limits of the amount of P and the amount of S are preferably 0% (namely, P and S are preferably not contained). From the viewpoint of reducing the costs of removing P and S (namely, costs of dephosphorization and desulphurization), the lower limits are preferably more than 0% (or 0.0001% or more).

[0043] N has the effect of forming a nitride with Al, Ti, Nb, V, or the like, and refining crystal grain size to improve ductility. When the amount of N is less than 0.0010%, such an effect cannot be obtained. When the amount of N is more than 0.0080%, it results in the deterioration of the wire drawability and the ductility. Accordingly, the amount of N is set within the range of from 0.0010 to 0.0080%. A preferred lower limit of the amount of N is 0.0020%. A preferred upper limit of the amount of N is 0.0060%. A more preferred upper limit of the amount of N is 0.0050%.

[0044] The drawn steel wire according to the present embodiment may further contain, by mass, one or more of from 0.0001 to 0.0050% of B, from 0.001 to 0.100% of Al, or from 0.001 to 0.050% Ti, for the purpose of reducing the area ratio of the non-pearlite structure in the surface layer portion of the drawn steel wire.

[0045] B has the effect of preventing the formation of the non-pearlite structure by segregating to grain boundaries, as a solid solution of B, thereby improving the torsion characteristics and the wire drawability. When the amount of B is more than 0.0050%, carbides may be formed at grain boundaries, resulting in the deterioration of the wire drawability. Accordingly, it is required that the amount of B is set within the range of from 0.0001 to 0.0050%. A preferred lower limit of the amount of B is 0.0005%. On the other hand, a preferred upper limit of the amount of B is 0.0030%. A more preferred upper limit of the amount of B is 0.0020%.

[0046] Al functions as a deoxidizing element. Further, Al has the effect of forming AlN and refining crystal grains to improve the ductility; the effect of reducing the amount of solid solution of N to improve the ductility; the effect of facilitating the formation of a solid solution of B to prevent the formation of the non-pearlite structure, thereby improving the torsion characteristics and the wire drawability; and the like. When the amount of Al is more than 0.100%, it may result in the saturation of these effects and a reduction in productivity. Accordingly, it is required that the amount of Al is set within the range of from 0.001 to 0.100%. A preferred lower limit of the amount of Al is 0.010%. A more preferred lower limit of the amount of Al is 0.020%. On the other hand, a preferred upper limit of the amount of Al is 0.080%. A more preferred upper limit of the amount of Al is 0.070%.

[0047] Ti functions as a deoxidizing element. Further, Ti has the effect of precipitating carbides and nitrides to increase the tensile strength; the effect of refining crystal grains to improve the ductility; the effect of reducing the amount of solid solution of N to improve the wire drawability; the effect of facilitating the formation of a solid solution of B to prevent the formation of the non-pearlite structure, thereby improving the torsion characteristics and the wire drawability; and the like. When the amount of Ti is more than 0.050%, it may lead to the saturation of these effects and the formation of coarse oxides or nitrides, possibly resulting in the deterioration of the wire drawability. Accordingly, it is required that the amount of Ti is set within the range of from 0.001 to 0.050%. A preferred lower limit of the amount of Ti is 0.010%. On the other hand, a preferred upper limit of the amount of Ti is 0.030%. A more preferred upper limit of the amount of Ti is 0.025%.

[0048] The high strength drawn steel wire according to the present embodiment may contain one or more of from 0.01 to 0.60% of Cr, from 0.01 to 0.10% of V, from 0.001 to 0.050% of Nb, from 0.001 to 0.050% of Zr, or from 0.01 to 1.00% of Ni, for the purpose of improving the properties to be described below.

[0049] Cr has the effect of increasing the tensile strength of steel after pearlite transformation. When the amount of Cr is more than 0.60%, the formation of a martensite structure may be facilitated, possibly resulting in the deterioration of the wire drawability and the torsion characteristics. Accordingly, it is required that the amount of Cr is set within the range of from 0.01 to 0.60%. A preferred upper limit of the amount of Cr is 0.50%. A more preferred upper limit of the amount of Cr is 0.40%.

[0050] V has the effect of precipitating a carbide, VC, to increase the tensile strength. When the amount of V is more than 0.10%, it may lead to an increase in alloy cost and the deterioration of the torsion characteristics. Accordingly, it is required that the amount of V is set within the range of from 0.01 to 0.10%. A preferred upper limit of the amount of V is 0.08%. A more preferred upper limit of the amount of V is 0.07%.

[0051] Nb has the effect of precipitating carbides and nitrides to increase the tensile strength; the effect of refining crystal grains to improve the ductility; the effect of reducing the amount of solid solution of N to improve the wire drawability; and the like. When the amount of Nb is more than 0.050%, it may result in the saturation of these effects and the deterioration of the torsion characteristics. Accordingly, it is required that the amount of Nb is set within the range of from 0.001 to 0.050%. A preferred upper limit of the amount of Nb is 0.030%. A more preferred upper limit of the amount of Nb is 0.020%.

[0052] Zr functions as a deoxidizing element. Further, Zr has the effect of reducing the amount of solid solution of S by forming sulfides, thereby improving the ductility. When the amount of Zr is more than 0.050%, it may lead to the saturation of such an effect and the formation of coarse oxides, possibly resulting in the deterioration of the wire drawability. Accordingly, it is required that the amount of Zr is set within the range of from 0.001 to 0.050%. A preferred upper limit of the amount of Zr is 0.030%. A more preferred upper limit of the amount of Zr is 0.020%.

[0053] Ni has the effect of preventing hydrogen penetration to achieve an improvement in hydrogen embrittlement resistance. When the amount of Ni is more than 1.00%, the alloy cost may be increased, and the formation of the martensite structure may be facilitated to result in the deterioration of the wire drawability. Accordingly, it is required that the amount of Ni is preferably set within the range of from 0.01 to 1.00%. A preferred upper limit of the amount of Ni is 0.50%. A more preferred upper limit of the amount of Ni is 0.30%.

[0054] In the chemical composition of the high strength drawn steel wire according to the present embodiment, the balance consists of Fe and impurities.

[0055] The term "impurities" as used herein refers to components which are included in raw materials, or components which are mixed during the production process and which are not intentionally incorporated. Further, the definition of the "impurities" also includes components which are intentionally incorporated, but contained in amounts within the ranges which do not affect the performance of the resulting drawn steel wire.

[0056] Examples of the impurities include O. O is contained unavoidably in the drawn steel wire and is present therein in the form of oxides of Al, Ti etc. A large amount of O leads to the formation of coarse oxides, and causes wire breakage during wire drawing. Accordingly, the amount of O is preferably reduced to 0.010% or less.

(Microstructure)

[0057] Next, a description will be given of the reasons for restricting the microstructure of the high strength drawn steel wire according to the present embodiment.

[0058] The high strength drawn steel wire has such a microstructure that the area ratio of the pearlite structure in the interior of the drawn steel wire is 90% or more, and the area ratio of the pearlite structure in the surface layer portion of the drawn steel wire is 80% or more.

[0059] The area ratio of the pearlite structure as used herein refers to the area ratio thereof in a cross section that includes a central axis of the drawn steel wire and is parallel with the central axis.

[0060] When the area ratio of the pearlite structure, in the microstructure of the interior of the drawn steel wire, is less than 90%, the drawn steel wire has a reduced strength or deteriorated torsion characteristics. Accordingly, the lower limit of the area ratio of the pearlite structure is set to 90%. A preferred lower limit of the area ratio of the pearlite structure is 95%. A more preferred lower limit of the area ratio of the pearlite structure is 97%. The upper limit of the area ratio of the pearlite structure may be 100%, and may be 99%.

[0061] Examples of a balance structure other than the pearlite structure (namely, the non-pearlite structure), in the interior of the drawn steel wire, include structures of ferrite, bainite, tempered bainite, martensite, tempered martensite, and proeutectoid cementite.

[0062] When the area ratio of the pearlite structure in the surface layer portion of the drawn steel wire is less than 80%, it leads to the deterioration of the torsion characteristics or wire drawability. Accordingly, the lower limit of the area ratio of the pearlite structure in the surface layer portion is set to 80%. A preferred lower limit of the area ratio of the pearlite structure is 85%. A more preferred lower limit of the area ratio of the pearlite structure is 90%. The upper limit of the area ratio of the pearlite structure may be 95%, and may be 99%. Further, the area ratio of the pearlite structure may be 100%.

[0063] The area ratio of the pearlite structure in the surface layer portion of the drawn steel wire can be adjusted to 80% or more, for example, by: a method of adjusting the chemical composition of the drawn steel wire to contain B, and to further contain at least one of Al or Ti; or a method of controlling the cooling rate of the steel wire rod after hot rolling. The area ratio of the pearlite structure in the surface layer portion of the drawn steel wire can be increased, by carrying out either or both of these methods.

[0064] Examples of the balance structure other than the pearlite structure (namely, the non-pearlite structure), in the surface layer portion of the drawn steel wire, include structures of ferrite, bainite, tempered bainite, martensite, tempered martensite, and proeutectoid cementite.

[0065] In order to impart high torsion characteristics to a high strength drawn steel wire having a tensile strength of 1,960 MPa or more, it is effective to allow the drawn steel wire to have a pearlite structure in which a lamellar pearlite structure including cementite having a long length, and a fragmented pearlite structure including cementite having a short length, coexist in a mixed state, at an appropriate ratio. After being subjected to wire drawing and before being subjected to an aging treatment, the drawn steel wire in the present embodiment has an uneven and complicated structure which contains transpositions introduced by wire drawing. In a case in which a drawn steel wire having an uneven pearlite structure is subjected to an ordinary molten zinc coating treatment (or an aging treatment by a heat treatment under equivalent conditions), the drawn steel wire after the coating treatment (or after the aging treatment) has microscopically uneven mechanical properties. Such a drawn steel wire deforms locally when subjected to torsional deformation, and thus has a low value of torsion. However, by subjecting the drawn steel wire in this state to an appropriate aging treatment (or a coating treatment under appropriate conditions), it is possible to reduce the unevenness in the microscopic mechanical properties and to improve the torsion characteristics.

[0066] The term "lamellar pearlite structure" as used herein refers to a pearlite structure in which cementite has a long length with an average length of 1.0 µm or more. The lamellar pearlite structure is the portion of the pearlite which has been relatively less affected by the aging treatment, of the entire pearlite which had been present until the aging treatment. An area ratio of the lamellar pearlite structure of less than 30% results in a reduced strength (namely, it becomes difficult to obtain a strength of 1,960 MPa or more), whereas an area ratio of more than 65% results in the deterioration of the torsion characteristics.

[0067] Accordingly, the area ratio of the lamellar pearlite structure is set within the range of from 30% to 65%. A preferred lower limit of the area ratio of the lamellar pearlite structure is 40%, and more preferably 50%. A preferred upper limit of the area ratio of the lamellar pearlite structure is 60%.

[0068] The term "fragmented pearlite structure" as used herein refers to a pearlite structure in which cementite has a short length with an average length of 0.30 µm or less. The fragmented pearlite structure is the portion of the pearlite which has been formed as a result of the fragmentation of the cementite in the pearlite due to distortion introduced by wire drawing and the effect of the aging treatment, of the entire pearlite which had been present until the aging treatment. An area ratio of the fragmented pearlite structure of less than 20% results in the deterioration of the torsion characteristics, whereas an area ratio of more than 50% results in a reduced strength.

[0069] Accordingly, the area ratio of the fragmented pearlite structure is set within the range of from 20% to 50%. A preferred lower limit of the area ratio of the fragmented pearlite structure is 25%, and a more preferred lower limit is 30%. On the other hand, a preferred upper limit of the area ratio of the fragmented pearlite structure is 45%, and more preferably 40%.

[0070] The area ratio of the fragmented pearlite structure in the drawn steel wire can be adjusted within the range of from 20% to 50%, for example, by: a method in which a drawn steel wire whose area ratio of the pearlite structure in the surface layer portion is 80% or more, after being subjected to wire drawing to a total area reduction of from 65 to 95%, is maintained at a temperature of from 500 to 600°C for a period of from one second to 20 seconds; or a method in which the drawn steel wire is maintained at a temperature of from 420 to 480°C for a period of from 60 seconds to 600 seconds.

[0071] The measurements of the respective structures are carried out as follows.

[0072] The area ratio of the pearlite structure in the interior of the drawn steel wire is determined according to the following procedure.

[0073] First, a cross section that includes a central axis of the drawn steel wire and is parallel with the central axis, "hereinafter, also referred to as "L cross section") is etched with picral, so as to expose the microstructure. Subsequently, the regions of the microstructure each having a size of 50 µm in the radial direction of the drawn steel wire and 60 µm in the longitudinal direction of the drawn steel wire are photographed by SEM (scanning electron microscopy), at a magnification of 2,000-fold. SEM photographs of the microstructure are taken at three locations at intervals of 5 mm in the longitudinal direction of the drawn steel wire, at each of the following depths. Specifically, the SEM photographs are taken, when the diameter of the drawn steel wire is defined as D, at the three locations at a depth of 0.25 D from the surface (namely, an outer peripheral surface) of the drawn steel wire in the radial direction of the drawn steel wire; and at the three locations at a depth of 0.5 D from the surface of the drawn steel wire in the radial direction of the drawn steel wire. In this manner, the SEM photographs are taken at a total of six locations (see FIG. 1). The descriptions "OA1" in FIG. 1 indicate the regions at which the SEM photographs of the microstructure in the interior of the drawn steel wire are taken.

[0074] The non-pearlite structure (including respective structures of ferrite, bainite, tempered bainite, martensite, tempered martensite, and proeutectoid cementite) in the thus captured SEM photographs of the microstructure is visually marked, and the area ratio thereof is determined by an image analysis. The area ratio of the pearlite structure is determined by subtracting the area of the non-pearlite structure from the area of the entire visual filed observed. The above described measurement is carried out for two samples, and the mean value of the values measured at a total of twelve locations is determined as the area ratio of the pearlite structure in the interior of the drawn steel wire.

[0075] Next, the area ratio of the pearlite structure in the surface layer portion of the drawn steel wire is determined according to the following procedure.

[0076] First, in the same manner as described above, the L cross section of the drawn steel wire is etched with picral, so as to expose the microstructure. Subsequently, the regions of the microstructure each including the surface of the drawn steel wire and having a size of 50 µm in the depth direction (the radial direction of the drawn steel wire) from the surface and 60 µm in the longitudinal direction of the drawn steel wire are photographed by SEM, at a magnification of 2,000-fold. SEM photographs of the microstructure are taken at six locations at intervals of 5 mm in the longitudinal direction of the drawn steel wire (see FIG. 1). The descriptions "OA2" in FIG. 1 indicate the regions at which the SEM photographs of the microstructure in the surface layer portion of the drawn steel wire are taken.

[0077] The non-pearlite structure (including respective structures of ferrite, bainite, tempered bainite, martensite, tempered martensite, and proeutectoid cementite) in the thus captured SEM photographs of the microstructure was visually marked, and the area ratio thereof was determined by an image analysis. The area ratio of the pearlite structure is determined by subtracting the area of the non-pearlite structure from the area of the entire visual filed observed. The above described measurement is carried out for two samples, and the mean value of the values measured at a total of twelve locations is determined as the area ratio of the pearlite structure in the surface layer portion of the drawn steel wire.

[0078] Next, the area ratio of the lamellar pearlite structure and the area ratio of the fragmented pearlite structure are determined according to the following procedure.

[0079] First, in the same manner as described above, the L cross section of the drawn steel wire is etched with picral, so as to expose the microstructure. Subsequently, the regions of the microstructure each having a size of 8 µm in the radial direction of the drawn steel wire and 12 µm in the longitudinal direction of the drawn steel wire are photographed by SEM, at a magnification of 10,000-fold. SEM photographs of the microstructure are taken at three locations at intervals of 5 mm in the direction parallel to the longitudinal direction of the drawn steel wire, at each of the following depths. Specifically, the SEM photographs are taken, when the diameter of the drawn steel wire is defined as D, at the three locations at a depth of 50 µm from the surface of the drawn steel wire in the radial direction; at the three locations at a depth of 0.25 D from the surface of the drawn steel wire in the radial direction of the drawn steel wire; and at the three locations at a depth of 0.5 D from the surface of the drawn steel wire in the radial direction of the drawn steel wire. In this manner, the SEM photographs are taken at a total of nine locations (see FIG. 2). The descriptions "OA" in FIG. 2 indicate the regions at which the SEM photographs are taken.

[0080] On the thus captured images of the SEM photographs of the microstructure, a set of straight lines are drawn at intervals of 2 µm, so as to be in parallel with each other in the longitudinal direction of the drawn steel wire. Further another set of straight lines are drawn at intervals of 2 µm so as to intersect the set of lines first drawn as described above. Then the structure at each of the respective intersections of the two sets of straight lines is observed by the following method. For three pieces of cementite that are present in the vicinity of each intersection at which the pearlite structure exists, the lengths of the long axes of the pieces of cementite are measured by an image analysis, and the mean value of the measured values is determined as the mean value of the lengths of the long axes of the pieces of cementite (namely, the average length of cementite). In a case in which the pieces of cementite are too small to be distinguished in the SEM photographs taken at a magnification of 10,000-fold, SEM photographs may be taken at a larger magnification. Thereafter, the number of intersections at each of which the mean value of the lengths of the long axes of the three pieces of cementite present in the vicinity of the intersection is 1.0 µm or more, is determined; the thus determined number is divided by the number of the total intersections including the intersections at which the pearlite structure does not exist (namely, the number of all the intersections of the above drawn two sets of straight lines); and the resulting quotient is expressed in percentage. In other words, a value obtained by (the number of intersections at each of which the mean value of the lengths of the long axes of the three pieces of cementite present in the vicinity of the intersection is 1.0 µm or more)/(the number of the total intersections) × 100 is determined as the area ratio of the lamellar pearlite structure.

[0081] The area ratio of the fragmented pearlite structure can be determined, in the same manner as described above, by: taking the SEM photographs of the microstructure; measuring the lengths of the long axes of three pieces of cementite present in the vicinity of each intersection at which the pearlite structure exists, by an image analysis; determining the mean value of the lengths of the long axes of the pieces of cementite (namely, the average length of cementite). Thereafter, the number of intersections at each of which the mean value of the lengths of the long axes of the three pieces of cementite present in the vicinity of the intersection is 0.30 µm or less, is determined; the thus determined number is divided by the number of the total intersections including the intersections at which the pearlite structure does not exist; and the resulting quotient is expressed in percentage. The thus calculated value is determined as the area ratio of the fragmented pearlite structure.

(Properties of High Strength drawn Steel Wire)

[0082] The tensile strength of the high strength drawn steel wire according to the present embodiment will now be described.

[0083] When the drawn steel wire has a tensile strength of less than 1,960 MPa, and in cases where the drawn steel wire is used in a civil engineering or building structure application, for example, the effect of the drawn steel wire to reduce the construction cost and the weight of the structures is decreased. Accordingly, the lower limit of the tensile strength of the drawn steel wire is set to 1,960 MPa.

[0084] The upper limit of the tensile strength of the drawn steel wire is not particularly limited. However, too high a tensile strength may lead to a reduced ductility, possibly resulting in the occurrence of cracks at the time of performing wire drawing. In this regard, it is required that the upper limit of the tensile strength of the drawn steel wire is 3,000 MPa (preferably 2,800 MPa, and more preferably 2500 MPa).

[0085] Next, the wire diameter of the high strength drawn steel wire according to the present embodiment will be described.

[0086] The high strength drawn steel wire according to the present embodiment is suitably used as a high strength drawn steel wire for use as a drawn steel wire for a rope, a drawn steel wire for a bridge cable, a PC drawn steel wire, or the like. Therefore, when the drawn steel wire has a wire diameter (diameter) of less than 1.5 mm, it leads to an increase in the cost of producing such a commodity; whereas when the drawn steel wire has a wire diameter of more than 8.0 mm, the strength and the torsion characteristics are more likely to be deteriorated. Accordingly, it is required that the drawn steel wire has a wire diameter (diameter) of from 1.5 mm to 8.0 mm. The wire diameter (diameter) of the drawn steel wire is more preferably within the range of from 3.0 mm to 7.5 mm.

[0087] In the high strength drawn steel wire according to the present embodiment, a coating layer including one of a Zn layer or a Zn alloy layer may be coated on the surface of the drawn steel wire. Examples of the Zn alloy layer include a Zn-Al layer and a Zn-Al-Mg alloy layer.

[0088] There is a case in which a drawn steel wire whose surface has been coated with plating is used as a high strength drawn steel wire to be used as a drawn steel wire for a rope, a drawn steel wire for a bridge cable, or the like. The high strength drawn steel wire according to the present embodiment exhibits a high strength and excellent torsion characteristics, even in a case in which the surface of the drawn steel wire is coated with plating.

[0089] Further, the surface of the high strength drawn steel wire according to the present embodiment, or the surface of the drawn steel wire which has been coated with plating, may be coated with a resin coating layer (such as an epoxy resin layer).

[0090] (Method of Producing High Strength drawn Steel Wire)

[0091] One example of a method of producing the high strength drawn steel wire according to the present embodiment will now be described.

[0092] The method of producing the high strength drawn steel wire according to the present embodiment include the step of obtaining a steel wire rod, by heating a steel billet having the chemical composition of the high strength drawn steel wire according to the present embodiment to a temperature within the range of from 1,000 to 1,150°C, and hot rolling the heated steel billet at a finish rolling temperature within the range of from 850 to 1,000°C.

[0093] Examples of the method of producing the high strength drawn steel wire according to the present embodiment include Embodiments (1) to (6), which include the steps as described below, as post steps of the step of obtaining a steel wire rod.

- Embodiment (1) -

[0094] A method of producing the high strength drawn steel wire, the method including the steps of:

cooling the steel wire rod after the hot rolling, whose temperature is from 850 to 1,000°C, to a temperature of from 500 to 600°C, at such a rate that an average cooling rate within the temperature range of from 800°C to 600°C is from 30 to 80°C/s;

maintaining the steel wire rod which has been cooled to a temperature of from 500 to 600°C, at a temperature of from 500 to 600°C for 50 seconds or more, to carry out a pearlite transformation treatment; and

cooling the steel wire rod after the pearlite transformation treatment to room temperature, and then drawing the cooled steel wire rod to a total area reduction of from 65 to 95%, followed by maintaining the resultant at a temperature of from 500 to 600°C for a period of from one second to 20 seconds, to obtain a drawn steel wire.

- Embodiment (2) -

[0095] A method of producing the high strength drawn steel wire, the method including the steps of:

cooling the steel wire rod after the hot rolling, whose temperature is from 850 to 1,000°C, to a temperature of from 500 to 600°C, at such a rate that the average cooling rate within the temperature range of from 800°C to 600°C is from 30 to 80°C/s;

maintaining the steel wire rod which has been cooled to a temperature of from 500 to 600°C, at a temperature of from 500 to 600°C for 50 seconds or more, to carry out a pearlite transformation treatment; and

cooling the steel wire rod after the pearlite transformation treatment to room temperature, and then drawing the cooled steel wire rod to a total area reduction of from 65 to 95%, followed by maintaining the resultant at a temperature of from 420 to 480°C for a period of from 60 seconds to 600 seconds, to obtain a drawn steel wire.

- Embodiment (3) -

[0096] A method of producing the high strength drawn steel wire, the method including the steps of:

cooling the steel wire rod after the hot rolling, whose temperature is from 850 to 1,000°C, at such a rate that the average cooling rate within the temperature range of from 700°C to 550°C is from 1.0 to 5.0°C/s; and

drawing the steel wire rod which has been cooled to room temperature to a total area reduction of from 65 to 95%, followed by maintaining the resultant at a temperature of from 500 to 600°C for a period of from one second to 20 seconds, to obtain a drawn steel wire.

- Embodiment (4) -

[0097] A method of producing the high strength drawn steel wire, the method including the steps of:

cooling the steel wire rod after the hot rolling, whose temperature is from 850 to 1,000°C, at such a rate that the average cooling rate within the temperature range of from 700°C to 550°C is from 1.0 to 5.0°C/s; and

drawing the steel wire rod which has been cooled to room temperature to a total area reduction of from 65 to 95%, followed by maintaining the resultant at a temperature of from 420 to 480°C for a period of from 60 seconds to 600 seconds, to obtain a drawn steel wire.

- Embodiment (5) -

[0098] A method of producing the high strength drawn steel wire, the method including the steps of:

reheating the steel wire rod which has been cooled after the hot rolling to a temperature of from 800 to 1,050°C, maintaining the steel wire rod at a temperature of from 480 to 600°C for 20 seconds or more, followed by cooling; and

drawing the steel wire rod which has been cooled to room temperature to a total area reduction of from 65 to 95%, followed by maintaining the resultant at a temperature of from 500 to 600°C for a period of from one second to 20 seconds, to obtain a drawn steel wire.

- Embodiment (6) -

[0099] A method of producing the high strength drawn steel wire, the method including the steps of:

reheating the steel wire rod which has been cooled after the hot rolling to a temperature of from 800 to 1,050°C, maintaining the steel wire rod at a temperature of from 480 to 600°C for 20 seconds or more, followed by cooling; and

drawing the steel wire rod which has been cooled to room temperature to a total area reduction of from 65 to 95%, followed by maintaining the resultant at a temperature of from 420 to 480°C for a period of from 60 seconds to 600 seconds, to obtain a drawn steel wire.

[0100] Specific details of the method of producing the high strength drawn steel wire according to the present embodiment will be described below.

[0101] In the method of producing the high strength drawn steel wire according to the present embodiment, a steel billet having the chemical composition of the high strength drawn steel wire according to the present embodiment is first heated to a temperature of from 1,000 to 1,150°C.

[0102] When the heating temperature is less than 1,000°C, deformation resistance during the hot rolling is increased, resulting in an increase in rolling cost. When the heating temperature is more than 1,150°C, the area ratio of the non-pearlite structure in the surface layer portion is increased, resulting in the deterioration of the wire drawability and the torsion characteristics. The lower limit of the range of the heating temperature is preferably 1,050°C. The upper limit of the range of the heating temperature is preferably 1,100°C.

[0103] Subsequently, the heated steel billet is hot rolled at a finish rolling temperature of from 850 to 1,000°C, to obtain a steel wire rod.

[0104] When the finish rolling temperature is less than 850°C, the deformation resistance during the hot rolling is increased, resulting in an increase in the rolling cost. When the finish rolling temperature is more than 1,000°C, the microstructure is coarsened, resulting in the deterioration of the wire drawability. The lower limit of the range of the finish rolling temperature is preferably 870°C. The upper limit of the range of the finish rolling temperature is preferably 980°C.

[0105] The finish rolling temperature as used herein refers to a surface temperature of the steel wire rod after the finish rolling.

[0106] Thereafter, the steel wire rod after the hot rolling (specifically, the steel wire rod after the finish rolling), whose temperature is from 850 to 1,000°C, is cooled to a temperature of from 500 to 600°C, at such a rate that the average cooling rate within the temperature range of from 800°C to 600°C is from 30 to 80°C/s.

[0107] When the average cooling rate is less than 30°C/s, the area ratio of the non-pearlite structure in the surface layer portion is increased, resulting in the deterioration of the wire drawability and the torsion characteristics. Achieving an average cooling rate of 80°C/ sec or more entails an increase in the production cost. The lower limit of the range of the average cooling rate is preferably 40°C/s. The upper limit of the range of the average cooling rate is preferably 75°C/s. The average cooling rate as used herein refers to the average of a surface cooling rate of the steel wire rod.

[0108] When the cooling temperature is less than 500°C, the area ratio of the pearlite structure is reduced, resulting in the deterioration of the torsion characteristics. When the cooling temperature is more than 600°C, it leads to a reduction in strength. The lower limit of the range of the cooling temperature is preferably 530°C. The upper limit of the range of the cooling temperature is preferably 580°C.

[0109] Subsequently, the steel wire rod which has been cooled to a temperature of from 500 to 600°C is maintained at a temperature of from 500 to 600°C for 50 seconds or more, to carry out a pearlite transformation treatment.

[0110] When the retention temperature is less than 500°C, the area ratio of the pearlite structure is reduced, resulting in the deterioration of the torsion characteristics. When the retention temperature is more than 600°C, it leads to a reduction in strength. The lower limit of the range of the retention temperature is preferably 530°C. The upper limit of the range of the retention temperature is preferably 580°C.

[0111] When the retention time is less than 50 seconds, it leads to an incomplete pearlite transformation and the formation of martensite, resulting in the deterioration of the wire drawability and the torsion characteristics. It is noted that, however, the upper limit of the retention time is required to be 150 seconds, from the viewpoint of the production cost. The lower limit of the range of the retention time is preferably 60 seconds. The upper limit of the range of the retention time is preferably 120 seconds. The maintenance of the temperature within the range of from 500 to 600°C is carried out, for example, using a molten salt bath tank.

[0112] At this time, instead of carrying out the cooling and the pearlite transformation treatment described above, the steel wire rod after the hot rolling, whose temperature is from 850 to 1,000°C, may be cooled at such a rate that the average cooling rate within the temperature range of from 700°C to 550°C is from 1.0 to 5.0°C/s. The cooling may be carried out, for example, using an air blast cooling equipment, such as Stelmor.

[0113] When the average cooling rate is less than 1.0°C/s, it leads to a reduction in strength. When the average cooling rate is more than 5.0°C/s, variations in the microscopic strength and the microstructure are increased, resulting in the deterioration of the torsion characteristics. The lower limit of the range of the average cooling rate is preferably 1.2°C/s. The upper limit of the range of the average cooling rate is preferably 3.0°C/s.

[0114] Further, instead of carrying out the cooling treatment and the pearlite transformation treatment described above, the steel wire rod after the hot rolling which has been cooled to room temperature (for example, to 25°C) may be reheated to a temperature of from 800 to 1,050°C, and then maintained at a temperature of from 480 to 600°C for 20 seconds or more, followed by cooling.

[0115] When the reheating temperature is less than 800°C, it causes insufficient austenitization and thus leads to a failure to obtain a uniform pearlite structure, resulting in a reduction in strength and the deterioration of the wire drawability. When the reheating temperature is more than 1,050°C, the area ratio of the non-pearlite structure in the surface layer portion is increased, resulting in the deterioration of the wire drawability and the torsion characteristics. The lower limit of the range of the reheating temperature is preferably 940°C. The upper limit of the range of the reheating temperature is preferably 1020°C.

[0116] When the retention temperature is less than 480°C, the area ratio of the pearlite structure is reduced, resulting in the deterioration of e torsion characteristics. When the retention temperature is more than 600°C, it causes an increase in intervals between the lamellae of the pearlite structure, resulting in a reduced strength. The lower limit of the range of the retention temperature is preferably 520°C. The upper limit of the range of the retention temperature is preferably 590°C.

[0117] When the retention time is less than 20 seconds, it leads to an incomplete pearlite transformation and the formation of martensite, resulting in the deterioration of the wire drawability and the torsion characteristics. It is noted that, however, the upper limit of the retention time is required to be 120 seconds, from the viewpoint of the production cost. The lower limit of the range of the retention time is preferably 30 seconds. The upper limit of the range of the retention time is preferably 80 seconds.

[0118] In a case in which the reheating treatment is carried out in an oxidizing atmosphere, the area ratio of the pearlite structure in the surface layer portion of the drawn steel wire may be decreased, possibly resulting in the deterioration of the wire drawability and the torsion characteristics. Therefore, the reheating heat treatment is carried out, for example, in an inert gas (such as Ar gas) atmosphere, a neutral gas (such as nitrogen gas) atmosphere, or a modified endothermic gas atmosphere. Further, the reheating treatment may be a short-term heating, such as induction heating.

[0119] The maintenance of the temperature within the range of from 480 to 600°C is carried out, for example, using a molten lead bath. Instead of the molten lead bath, a molten salt bath, a fluidized bed or the like may also be used.

[0120] Thereafter, the steel wire rod after the pearlite transformation treatment or the cooling (specifically, the steel wire rod which has been cooled to room temperature (for example, to 25°C)) is drawn to a total area reduction of from 65 to 95%, and maintained at a temperature of from 500 to 600°C for a period of from one second to 20 seconds, to obtain a drawn steel wire. By maintaining the steel wire rod at a temperature of from 500 to 600°C for a period of from one second to 20 seconds, the torsion characteristics of the resulting drawn steel wire is improved. The heat treatment carried out after the wire drawing is also referred to as "aging treatment".

[0121] When the total area reduction is less than 65%, it leads to a reduction in strength. When the total area reduction is more than 95%, the ductility of the resulting drawn steel wire is reduced, resulting in the deterioration of the wire drawability and the torsion characteristics. The total area reduction is preferably within the range of from 70 to 90%. The total area reduction as used herein refers to a value calculated by Equation: (difference between the cross-sectional area of a steel wire rod before wire drawing (the area of a surface vertical to the longitudinal direction of the steel wire rod) and the cross-sectional area of the drawn steel wire after wire drawing / the cross-sectional area of the steel wire rod before wire drawing) × 100.

[0122] When the retention temperature is less than 500°C, the effect of improving the torsion characteristics cannot be obtained. When the retention temperature is more than 600°C, it leads to a reduction in strength. The retention temperature is preferably within the range of from 510 to 550°C.

[0123] When the retention time is less than one second, the effect of improving the torsion characteristics cannot be obtained. When the retention time is more than 20 seconds, it leads to a reduction in strength. The retention temperature is preferably within the range of from 2 to 15 seconds.

[0124] At this time, instead of maintaining the drawn steel wire after wire drawing at a temperature of from 500 to 600°C for a period of from one second to 20 seconds, the drawn steel wire may be maintained at a temperature of from 420 to 480°C for a period of from 60 seconds to 600 seconds.

[0125] When the retention temperature is less than 420°C, the torsion characteristics of the resulting drawn steel wire are reduced. When the retention temperature is more than 480°C, it leads to a reduction in strength. The retention temperature is preferably within the range of from 430 to 470°C.

[0126] When the retention time is less than 60 seconds, the torsion characteristics of the resulting drawn steel wire is reduced. When the retention time is more than 600 seconds, it leads to an increase in the production cost. The retention temperature is preferably within the range of from 100 to 500 seconds.

[0127] The high strength drawn steel wire according to the present embodiment can be obtained through the above described steps.

[0128] The method of producing the high strength drawn steel wire according to the present embodiment may include, after the aging treatment described above, the step of carrying out a coating treatment, in which a coating layer including one of a Zn layer or a Zn alloy layer is coated on the drawn steel wire at a temperature of from 420 to 480°C.

[0129] Further, the method of producing the high strength drawn steel wire according to the present embodiment may include, instead of the aging treatment descried above, the step of carrying out a coating treatment, in which a coating layer including one of a Zn layer or a Zn alloy layer is coated on the surface of the drawn steel wire, under the conditions of a temperature of from 420 to 480°C for a period of from 60 seconds to 600 seconds, or under conditions of a temperature of from 500 to 600°C for a period of from one second to 20 seconds. Even in this case, the same structure is formed in the drawn steel wire, due to changes in the temperature of the drawn steel wire associated with the coating treatment.

[0130] In other words, by carrying out the coating treatment on the surface of the drawn steel wire under the conditions of the temperature and time corresponding to the aging treatment, it is possible to obtain a high strength drawn steel wire which has a structural state of the drawn steel wire according to the present embodiment, and which is coated with a coating layer including one of a Zn layer or a Zn alloy layer.

[0131] Further, the method of producing the high strength drawn steel wire according to the present embodiment may further include the step of coating a resin coating layer (such as an epoxy resin layer) on the surface of the drawn steel wire or on the surface of the drawn steel wire which has been coated with plating. Even in a case in which the resin coating layer is present, the drawn steel wire can exhibit an excellent strength and torsion characteristics, as long as the drawn steel wire present inside the resin coating layer has a structural state of the drawn steel wire according to the present embodiment.

EXAMPLES

[0132] The present invention will now be described more specifically, with reference to Examples. It is noted, however, the invention is in no way limited by these Examples.

[0133] Drawn steel wires were produced as follows, using steel billets of steel types A to S having the chemical compositions shown in Table 1, and under the conditions shown in Table 2 to Table 6.

[0134] Specifically, drawn steel wires of Test Nos. 1 to 30, which are shown in Table 2, were produced as follows.

[0135] First, each of the steel billets was heated and then subjected to hot rolling, and the resulting steel wire rod was wound in the form of a ring, followed by cooling to a temperature of from 500 to 600°C. Subsequently, each resulting steel wire rod was immersed in a molten salt bath at a downstream of a hot rolling line, to carry out a patenting treatment (pearlite transformation treatment). Thereafter, each steel wire rod which had been cooled to room temperature (25°C) was drawn to the wire diameter shown in Table 2 (described in the Table as "Wire diameter after drawing), and heated after the wire drawing to carry out an aging treatment. Through the above described steps, the drawn steel wires of Test Nos. 1 to 30 were produced.

[0136] Drawn steel wires of Test Nos. 31 to 34 shown in Table 3 were produced as follows.

[0137] First, each of the steel billets was heated and then subjected to hot rolling, and the resulting steel wire rod was wound in the form of a ring, followed by air blast cooling. Thereafter, each steel wire rod which had been cooled to room temperature (25°C) was drawn to the wire diameter shown in Table 3, and heated after the wire drawing to carry out an aging treatment. Through the above described steps, the drawn steel wires of Test Nos. 31 to 34 were produced.

[0138] Drawn steel wires of Test Nos. 35 to 40 shown in Table 4 were produced as follows.

[0139] Each of the steel billets was heated and then subjected to hot rolling, and the resulting steel wire rod was wound in the form of a ring, followed by cooling at an average cooling rate of 2.0°C/s. Subsequently, each steel wire rod which had been cooled to room temperature (25°C) was reheated under a predetermined atmosphere, and immersed in a molten lead bath. Thereafter, each steel wire rod which had been cooled to room temperature (25°C) was drawn to the wire diameter shown in Table 4, and heated after the wire drawing to carry out an aging treatment. Through the above described steps, the drawn steel wires of Test Nos. 35 to 40 were produced.

[0140] A drawn steel wire of Test No. 41 shown in Table 5 was produced as follows.

[0141] First, the steel billet was heated and then subjected to hot rolling, and the resulting steel wire rod was wound in the form of a ring, followed by cooling to a temperature of from 500 to 600°C. Subsequently, the resulting steel wire rod was immersed in a molten salt bath at the downstream of the hot rolling line, to carry out a patenting treatment. Thereafter, the steel wire rod which had been cooled to room temperature (25°C) was drawn to the wire diameter shown in Table 5, and heated after the wire drawing to carry out an aging treatment. The resulting drawn steel wire was then subjected to a molten zinc coating treatment. Through the above described steps, the drawn steel wire of Test No. 41 was produced.

[0142] A drawn steel wire of Test No. 42 shown in Table 6 was produced in the same manner as the drawn steel wire of Test No. 22, except that the order of carrying out the wire drawing and the aging treatment were reversed.

[0143] The observation of the microstructure, a tensile test, and a torsion test were carried out for each of the thus produced drawn steel wires.

[0144] The area ratio of the pearlite structure in the interior of the drawn steel wire, the area ratio of the pearlite structure in the surface layer portion of the drawn steel wire, the area ratio of the lamellar pearlite structure (a lamellar pearlite structure in which cementite has an average length of 1.0 µm or more), and the area ratio of the fragmented pearlite structure (a fragmented pearlite structure in which cementite has an average length of 0.30 µm or less) were measured in accordance with the methods previously described. The results are shown in Table 2 to Table 5.

[0145] The tensile test was carried out in accordance with JIS Z 2241 (2011) using No. 9A test pieces. Three pieces of each of the drawn steel wires were evaluated and the mean value of the measured tensile strengths was determined. The results are shown in Table 2 to Table 5.

[0146] The torsion test was carried out in accordance with the torsion test method defined in JIS G 3521 (1991) for ten pieces of each of the drawn steel wires, and the presence or absence of the delamination and the value of torsion were evaluated. In a case in which the occurrence of delamination was observed in at least one of the ten pieces, it was determined that "delamination is present" (described in the Tables as "Yes") in the corresponding drawn steel wire. In a case in which the value of torsion was less than 12 times, the torsion characteristics of the corresponding drawn steel wire was determined as "poor". In a case in which the occurrence of delamination was not observed, and the value of torsion was 12 times or more, the torsion characteristics of the corresponding drawn steel wire was determined as "favorable". The results are shown in Table 2 to Table 6.

[Table 1]

Steel Type	C	Si	Mn	P	S	N	Al	Cr	V	B	Ti	Nb	Zr	Ni
A	0.88	1.05	0.77	0.011	0.008	0.0041	0.035	-	-	0.0015	0.017	-	-	-
B	0.90	0.94	0.73	0.008	0.004	0.0030	0.041	-	-	0.0010	0.021	-	-	-
C	0.93	0.96	0.48	0.009	0.012	0.0039	0.025	-	-	-	0.012	-	0.009	0.08
D	0.96	0.22	0.77	0.010	0.005	0.0031	-	-	-	-	0.024	0.008	0.015	-
E	0.97	1.31	0.39	0.007	0.006	0.0028	0.039	-	-	-	-	0.014	-	-
F	0.99	1.23	0.41	0.008	0.003	0.0037	0.037	0.25	-	0.0010	0.013	-	-	-
G	0.99	0.92	0.40	0.008	0.005	0.0031	0.036	0.21	0.04	0.0012	-	-	-	0.11
H	1.00	1.23	0.42	0.009	0.003	0.0029	0.037	0.23	-	0.0011	0.012	-	-	-
I	1.01	0.55	0.52	0.008	0.009	0.0035	0.032	0.21	-	-	-	0.010	-	-
J	1.08	1.46	0.37	0.013	0.004	0.0042	0.036	0.22	-	-	-	0.009	0.008	-
K	1.12	0.57	0.76	0.007	0.006	0.0038	0.062	-	-	0.0009	-	-	-	-
L	0.80	1.19	0.51	0.009	0.007	0.0040	0.028	-	-	-	-	-	-	-
M	1.26	0.57	0.71	0.008	0.009	0.0037	0.033	-	-	-	-	-	-	-
N	1.02	1.22	0.45	0.007	0.008	0.0035	-	-	-	-	-	-	-	-
O	1.02	1.04	0.42	0.009	0.008	0.0039	0.033	-	-	-	-	-	-	-
P	0.92	1.01	0.25	0.012	0.010	0.0044	0.029	0.47	-	-	-	-	-	-
Q	0.96	0.98	0.90	0.013	0.011	0.0037	-	-	-	0.0015	0.014	-	-	-
R	0.94	0.93	0.42	0.008	0.009	0.0031	-	-	-	0.0012	0.041	0.007	-	-
S	0.96	1.20	0.41	0.009	0.007	0.0072	0.042	0.18	-	-	-	-	-	-

[0147] It can be seen from the above described results that the drawn steel wires of Test Nos. 1 to 11, 21 to 25, 30 to 32, 35 to 38, and 41, which satisfy all of the requirements defined in the present disclosure, have a tensile strength of 1,960 MPa or more, and favorable torsion characteristics.

[0148] In contrast, in the drawn steel wire of Test No. 12, the area ratio of the pearlite structure is less than the lower limit defined in the present disclosure.

[0149] In each of the drawn steel wires of Test Nos. 13, 17, 27, 39, and 42, the area ratio of the fragmented pearlite structure falls outside the range defined in the present disclosure.

[0150] In each of the drawn steel wires of Test Nos. 18 and 40, the area ratio of the pearlite structure in the surface layer portion is below the lower limit defined in the present disclosure. It is noted that the drawn steel wire of Test No. 40 is an example corresponding to the drawn steel wire disclosed in Patent Document 5.

[0151] In each of the drawn steel wires of Test Nos. 28 and 33, the area ratio of the lamellar pearlite structure is more than the upper limit defined in the present disclosure.

[0152] In each of the drawn steel wires of Test Nos. 14, 16, 26, and 29, the area ratio of the lamellar pearlite structure and the area ratio of the fragmented pearlite structure fall outside the ranges defined in the present disclosure.

[0153] In each of the drawn steel wires of Test Nos. 15 and 34, all of the area ratio of the pearlite structure in the interior of the drawn steel wire, the area ratio of the pearlite structure in the surface layer portion of the drawn steel wire, the area ratio of the lamellar pearlite structure, and the area ratio of the fragmented pearlite structure fall outside the ranges defined in the present disclosure.

[0154] In each of the drawn steel wires of Test Nos. 19 and 20, the amount of C falls outside the range defined in the present disclosure.

[0155] The drawn steel wires in which any of the above described properties fall outside the ranges defined in the present disclosure have poor torsion characteristics or an insufficient tensile strength.

[0156] The disclosure of Japanese Patent Application No. 2017-128871 is incorporated herein by reference in their entirety. All publications, patent applications, and technical standards mentioned in the present specification are incorporated herein by reference to the same extent as if such individual publication, patent application, or technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. A high strength drawn steel wire having a chemical composition comprising, by mass,
from 0.85 to 1.20% of C,
from 0.10 to 2.00% of Si,
from 0.20 to 1.00% of Mn,
0.030% or less of P,
0.030% or less of S,
from 0.0010 to 0.0080% of N,
from 0 to 0.0050% of B,
from 0 to 0.100% of Al,
from 0 to 0.050% of Ti,
from 0 to 0.60% of Cr,
from 0 to 0.10% of V,
from 0 to 0.050% of Nb,
from 0 to 0.050% of Zr, and
from 0 to 1.00% of Ni,
with a balance consisting of Fe and impurities,
wherein, in a cross section that includes a central axis of the drawn steel wire and is parallel with the central axis, an area ratio of a pearlite structure in an interior of the drawn steel wire is 90% or more, and an area ratio of the pearlite structure in a surface layer portion of the drawn steel wire is 80% or more,
wherein, in an entire structure of the drawn steel wire, an area ratio of a lamellar pearlite structure in which cementite has an average length of 1.0 µm or more is from 30% to 65%, and an area ratio of a fragmented pearlite structure in which cementite has an average length of 0.30 µm or less is from 20% to 50%, and
wherein the drawn steel wire has a tensile strength of 1,960 MPa or more.

2. The high strength drawn steel wire according to claim 1, wherein the chemical composition of the drawn steel wire comprises, by mass, one or more of from 0.0001 to 0.0050% of B, from 0.001 to 0.100% of Al, or from 0.001 to 0.050% Ti.

3. The high strength drawn steel wire according to claim 1 or 2, wherein the chemical composition of the drawn steel wire comprises, by mass, one or more of from 0.01 to 0.60% of Cr, from 0.01 to 0.10% of V, from 0.001 to 0.050% of Nb, from 0.001 to 0.050% of Zr, or from 0.01 to 1.00% of Ni.

4. The high strength drawn steel wire according to any one of claims 1 to 3, wherein the drawn steel wire has a diameter of from 1.5 to 8.0 mm.

5. The high strength drawn steel wire according to any one of claims 1 to 4, wherein a coating layer including one of a Zn layer or a Zn alloy layer is coated on a surface of the drawn steel wire.

Drawing