Wire rod

Abstract

To provide a wire rod which has sufficient drawability and serves as a raw material to fabricate a steel wire having a superior endurance ratio.SOLUTION: A wire rod according to the present disclosure comprises, in mass%, C: 0.70% to less than 1.10%, Si: 0.10% to 1.00%, Mn: 0.20% to 1.00%, V: 0.10% to 0.40%, P: 0.020% or less, S: 0.020% or less, Al: 0.080% or less, N: 0.0100% or less, and O: 0.0050% or less. The balance consists of Fe and impurities. An area ratio of pearlite is 90% or more. When a C concentration by mass% determined by point analysis of a region including lamellar cementite in the pearlite using an SEM-EDX device is defined as CC(%), and a V concentration by mass% determined by the point analysis is defined as VC(%), Formula (1) is satisfied, 0.15≤VC / (CC×V)≤1.00 (1).SELECTED DRAWING: None

Description

Technical Field

[0001] The present disclosure relates to wire rods, and more particularly to wire rods suitable as materials for steel wires used in beads for automobile tires, steel cords, wire ropes for bridges, wire ropes for elevators, PC steel wires, transmission wires, and the like.

Background Art

[0002] Steel wires are used in beads for automobile tires, steel cords, wire ropes for bridges, wire ropes for elevators, PC steel wires, transmission wires, and the like. For example, steel wires are manufactured by performing wire drawing on a wire rod and then performing a blueing treatment.

[0003] In recent years, high tensile strength has been required for steel wires. On the other hand, for steel wires used in the above applications, since repeated loads are applied under the use conditions, excellent endurance ratio (= fatigue strength / tensile strength) is also required. Furthermore, when manufacturing steel wires, wire drawing may be performed on the wire rod, which is the material of the steel wire, a plurality of times. Therefore, sufficient wire drawing workability is also required for the wire rod.

[0004] Techniques related to improving the wire drawing workability of wire rods and improving the endurance ratio of steel wires manufactured using the wire rods as materials are proposed in Japanese Patent Application Laid-Open No. 2011-225990 (Patent Document 1).

[0005] The wire rod disclosed in Patent Document 1 contains, by mass%, C: 0.70 to 1.2%, Si: 0.1 to 1.5%, Mn: 0.1 to 1.5%, P: 0.015% or less (excluding 0%), S: 0.015% or less (excluding 0%), Al: 0.005% or less (excluding 0%), B: 0.0005 to 0.010%, N: 0.002 to 0.005%, the solid solution N is 0.0015% or less (including 0%), the balance is composed of iron and inevitable impurities, the area ratio of the pearlite structure is 90% or more, and the pearlite structure is 2000 μm 2in which the number of BN-based compounds with an equivalent circle diameter of 100 nm or more and less than 1000 nm is 100 or less (including 0), and the number of BN-based compounds with an equivalent circle diameter of 1000 nm or more is 10 or less (including 0). In Patent Document 1, by controlling the number of BN-based compounds, the wire drawing processability and the durability ratio are improved.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] In the wire rod proposed in Patent Document 1, the BN-based compounds in steel are controlled to improve the wire drawing processability and the durability ratio. However, these properties may also be enhanced by other means.

[0008] An object of the present disclosure is to provide a wire rod that has sufficient wire drawing processability and provides an excellent durability ratio in a steel wire used as a material and manufactured.

Means for Solving the Problems

[0009] The wire rod of the present disclosure is The chemical composition is, in mass%, C: 0.70 to less than 1.10%, Si: 0.10 to 1.00%, Mn: 0.20 to 1.00%, V: 0.10 to 0.40%, P: 0.020% or less, S: 0.020% or less, Al: 0.080% or less, N: 0.0100% or less, and O: 0.0050% or less, and contains the balance consists of Fe and impurities, In the microstructure, the area ratio of pearlite is 90% or more, The C concentration in mass% obtained by point-analyzing the region containing lamellar cementite of the pearlite using an SEM-EDX apparatus is defined as C C (%). When the V concentration in mass% obtained by the point analysis is defined as V C (%), Equation (1) is satisfied. 0.15 ≤ V C / (C C × V) ≤ 1.00 (1) Here, in Equation (1), the V content in mass% in the chemical composition is substituted for V.

[0010] The wire rod of the present disclosure has a chemical composition in mass% of C: 0.70 to less than 1.10%, Si: 0.10 to 1.00%, Mn: 0.20 to 1.00%, V: 0.10 to 0.40%, P: 0.020% or less, S: 0.020% or less, Al: 0.080% or less, N: 0.0100% or less, and O: 0.0050% or less, contains further contains one or more selected from the group consisting of Group 1 and Group 2, with the balance being composed of Fe and impurities, in the microstructure, the area ratio of pearlite is 90% or more, The C concentration in mass% obtained by point-analyzing the region containing lamellar cementite of the pearlite using an SEM-EDX apparatus is defined as C C (%). When the V concentration in mass% obtained by the point analysis is defined as V C (%), Equation (1) is satisfied. [Group 1] Cr: 0.50% or less, Cu: 0.50% or less, Ni: below 0.50%, Sn: below 0.100%, Mo: below 0.20%, Nb: below 0.100%, Ti: below 0.100%, B: below 0.0050%, Co: below 0.50%, and, W: below 0.20%, one or more selected from the group consisting of [Group 2] Ca: below 0.0050%, Mg: below 0.0050%, Zr: below 0.010%, and, rare earth elements: below 0.005%, one or more selected from the group consisting of 0.15 ≦ V C / (C C × V) ≦ 1.00 (1) Here, in formula (1), the V content in mass % in the chemical composition is substituted for V.

Advantages of the Invention

[0011] The wire of the present disclosure has sufficient drawability, and in the steel wire used and manufactured as a material, an excellent endurance ratio can be obtained.

Modes for Carrying Out the Invention

[0012] First, the inventors examined wire rods from the viewpoints of chemical composition and microstructure, which exhibit sufficient drawability and excellent endurance ratio when used as materials for steel wires. As a result, when the chemical composition is, in mass%, C: 0.70 to less than 1.10%, Si: 0.10 to 1.00%, Mn: 0.20 to 1.00%, P: 0.020% or less, S: 0.020% or less, Al: 0.080% or less, N: 0.0100% or less, O: 0.0050% or less, Cr: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Sn: 0 to 0.100%, Mo: 0 to 0.20%, Nb: 0 to 0.100%, Ti: 0 to 0.100%, B: 0 to 0.0050%, Co: 0 to 0.50%, W: 0 to 0.20%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Zr: 0 to 0.010%, and rare earth elements: 0 to 0.005%, with the balance being composed of Fe and impurities and being substantially pearlite structure, sufficient drawability can be obtained, and an excellent endurance ratio can be obtained in steel wires manufactured using the wire rod as a material.

[0013] However, even in the case of wire rods having the above chemical composition and microstructure, excellent endurance ratio may not be obtained when made into steel wires. Therefore, the inventors examined the cause. As a result, the following matters were found.

[0014] In steel wires manufactured using wire rods having the above chemical composition and microstructure as materials, a large number of dislocations are introduced by wire drawing. As a result, high strength is obtained in the manufactured steel wires. However, when the steel wire is repeatedly fatigued, the dislocations in the steel wire may move within the crystal grains and cancel each other out. The increase in such cancellation reduces the fatigue strength of the steel wire. As a result, the endurance ratio of the steel wire decreases. That is, in order to obtain a steel wire with an excellent endurance ratio, it is only necessary to suppress the cancellation of dislocations in the steel wire in which a large number of dislocations are introduced.

[0015] In order to suppress the cancellation of such dislocations, it is effective to suppress the movement of dislocations when the steel wire is repeatedly fatigued. Therefore, the present inventors have studied means for suppressing the movement of dislocations.

[0016] The structure of the wire rod mainly consists of a pearlite structure. The pearlite structure has a lamellar structure in which ferrite and cementite are arranged in layers. Here, the ferrite in pearlite is referred to as lamellar ferrite, and the cementite in pearlite is referred to as lamellar cementite. Ferrite is softer than cementite. Therefore, the dislocations introduced by wire drawing of a wire rod composed of a pearlite structure are mainly accumulated in the lamellar ferrite. Further, when a repeated load such as a fatigue phenomenon is applied to the steel wire manufactured by the wire drawing process, the dislocations in the lamellar ferrite move and disappear. Therefore, it is considered that if the movement of dislocations in the lamellar ferrite can be suppressed, the endurance ratio of the steel wire can be increased.

[0017] Fine precipitates function as obstacles when dislocations move. Therefore, by generating fine precipitates in the lamellar ferrite, the fine precipitates become obstacles to the movement of dislocations in the lamellar ferrite. Therefore, there is a possibility of suppressing the movement of dislocations during repeated fatigue.

[0018] Therefore, the present inventors have studied the fine precipitates generated in the lamellar ferrite. As a result, it was considered to further contain V in the above chemical composition to generate V-based precipitates in the lamellar ferrite. V-based precipitates are precipitates containing V, such as V carbide and V carbonitride. In the wire rod having the above chemical composition and microstructure, V-based precipitates are generated in the lamellar ferrite with a fine size. Therefore, if the V-based precipitates are finely dispersed in the lamellar ferrite, the movement of dislocations due to fatigue is suppressed by the V-based precipitates. In this way, it is considered that a steel wire having an excellent endurance ratio can be obtained.

[0019] However, it has been found that if the amount of V-based precipitates in lamellar ferrite is excessive, the wire drawing processability of the wire rod will rather deteriorate. Therefore, in order to have sufficient wire drawing processability and obtain an excellent endurance ratio in the steel wire used and manufactured as a material, it is necessary to adjust the amount of V-based precipitates in the lamellar ferrite of the wire rod to an appropriate range.

[0020] As described above, since the V-based precipitates are fine, it is extremely difficult to measure the amount of V-based precipitates in lamellar ferrite by microstructural observation. Therefore, it seemed difficult to quantify the amount of V-based precipitates.

[0021] However, it is possible to quantify the amount of V-based precipitates in lamellar ferrite by the following means. In the wire rod having the above chemical composition and microstructure, V is contained not only in lamellar ferrite but also in lamellar cementite. Most of the V in lamellar ferrite exists in the form of precipitates (V-based precipitates). On the other hand, the V in lamellar cementite exists in a solid solution state (solid solution V). Therefore, by quantifying the V content in the chemical composition of the entire wire rod and the V concentration in lamellar cementite, the amount of V-based precipitates in lamellar ferrite can be predicted.

[0022] Under the above assumptions, further examination was carried out on the V content in the chemical composition of the wire rod and the V concentration in lamellar cementite when the amount of V-based precipitates in lamellar ferrite is in an appropriate range. As a result, in a wire rod containing V: 0.10 to 0.40% in addition to the above chemical composition, the V content in mass% in the chemical composition of the wire rod and the V concentration V in mass% obtained by point analysis of the region containing lamellar cementite using an SEM-EDX apparatus C (%), and further the C concentration C in mass% obtained by the point analysis C (%), are used to represent V C / (C CIf (V / (C × V)) is in the range of 0.15 to 1.00, the amount of V-based precipitates in the lamellar ferrite is within an appropriate range, so that sufficient drawability can be obtained, and an excellent endurance ratio can be obtained in a steel wire manufactured using the wire as a raw material. The inventors have found this.

[0023] The wire of this embodiment is completed based on the above technical idea and has the following configuration.

[0024] The wire of the first configuration is The chemical composition is, by mass percentage, C: 0.70 to less than 1.10%, Si: 0.10 to 1.00%, Mn: 0.20 to 1.00%, V: 0.10 to 0.40%, P: 0.020% or less, S: 0.020% or less, Al: 0.080% or less, N: 0.0100% or less, and O: 0.0050% or less, and contains The balance consists of Fe and impurities, In the microstructure, the area ratio of pearlite is 90% or more, When the C concentration in mass percentage obtained by point-analyzing the region containing the lamellar cementite of the pearlite using a SEM-EDX apparatus is defined as C C (%), and When the V concentration in mass percentage obtained by the point analysis is defined as V C (%), Equation (1) is satisfied. 0.15 ≤ V C / (C C × V) ≤ 1.00 (1) Here, the V in Equation (1) is substituted with the V content in mass percentage in the chemical composition.

[0025] The wire of the second configuration is The chemical composition is, by mass percentage, C: 0.70 to less than 1.10%, Si: 0.10 to 1.00%, Mn: 0.20 to 1.00%, V: 0.10 to 0.40%, P: 0.020% or less, S: 0.020% or less, Al: 0.080% or less, N: 0.0100% or less, and O: 0.0050% or less, and contains further contains one or more selected from the group consisting of Group 1 and Group 2, the balance consists of Fe and impurities, in the microstructure, the area ratio of pearlite is 90% or more, The C concentration in mass % obtained by point analysis of the region containing lamellar cementite of the pearlite using an SEM-EDX apparatus is defined as C C (%), The V concentration in mass % obtained by the point analysis is defined as V C (%), when Equation (1) is satisfied. [Group 1] Cr: 0.50% or less, Cu: 0.50% or less, Ni: 0.50% or less, Sn: 0.100% or less, Mo: 0.20% or less, Nb: 0.100% or less, Ti: 0.100% or less, B: 0.0050% or less, Co: 0.50% or less, and W: 0.20% or less, one or more selected from the group consisting of [Group 2] Ca: 0.0050% or less, Mg: 0.0050% or less, Zr: 0.010% or less, and rare earth elements: 0.005% or less, one or more selected from the group consisting of 0.15 ≤ V C / (C C × V) ≤ 1.00 (1) Here, in Equation (1), the V content in mass % in the chemical composition is substituted for V.

[0026] The wire rod of the third configuration is a wire rod of the second configuration, and contains the first group.

[0027] The wire rod of the fourth configuration is a wire rod of the second or third configuration, and contains the second group.

[0028] Hereinafter, the wire rod according to this embodiment will be described in detail. Note that, unless otherwise specified, "%" regarding an element means mass%.

[0029] [Features of the Wire Rod of the Present Embodiment] The wire rod of the present embodiment includes the following features. (Feature 1) The chemical composition is, in mass%, C: 0.70 to less than 1.10%, Si: 0.10 to 1.00%, Mn: 0.20 to 1.00%, V: 0.10 to 0.40%, P: 0.020% or less, S: 0.020% or less, Al: 0.080% or less, N: 0.0100% or less, O: 0.0050% or less, Cr: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Sn: 0 to 0.100%, Mo: 0 to 0.20%, Nb: 0 to 0.100%, Ti: 0 to 0.100%, B: 0 to 0.0050%, Co: 0 to 0.50%, W: 0 to 0.20%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Zr: 0 to 0.010%, and rare earth elements: 0 to 0.005%, and the balance consists of Fe and impurities. (Feature 2) When the C concentration in mass% obtained by point-analyzing a region containing lamellar cementite using a SEM-EDX apparatus is defined as C C (%), and the V concentration in mass% obtained by the point analysis is defined as V C (%), the formula (1) is satisfied. 0.15 ≦ V C / (C C × V) ≦ 1.00 (1) Here, in V in formula (1), the V content in mass% in the chemical composition is substituted. The following describes Feature 1 and Feature 2.

[0030] [(Feature 1) Regarding the chemical composition] The chemical composition of the wire rod of this embodiment contains the following elements.

[0031] C: 0.70 to less than 1.10% Carbon (C) increases the strength of the wire rod and the steel wire manufactured using the wire rod as a raw material. When the C content is less than 0.70%, even if the contents of other elements are within the range of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the C content is 1.10% or more, primary cementite is excessively generated. Therefore, even if the contents of other elements are within the range of this embodiment, the drawability of the wire rod decreases. Furthermore, the toughness and ductility of the steel wire after wire drawing decrease. Therefore, the C content is 0.70 to less than 1.10%. The preferable lower limit of the C content is 0.74%, more preferably 0.78%, and even more preferably 0.80%. The preferable upper limit of the C content is 1.09%, more preferably 1.05%, and even more preferably 1.03%.

[0032] Si: 0.10 to 1.00% Silicon (Si) increases the strength of the wire rod. Si further deoxidizes the steel in the steelmaking process during the manufacturing process of the wire rod. If the Si content is less than 0.10%, even if the contents of other elements are within the range of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Si content exceeds 1.00%, coarse Si oxides are generated. Therefore, even if the contents of other elements are within the range of this embodiment, the drawability of the wire rod decreases. Therefore, the Si content is 0.10 to 1.00%. The preferable lower limit of the Si content is 0.13%, more preferably 0.15%, and even more preferably 0.18%. The preferable upper limit of the Si content is 0.95%, more preferably 0.90%, and even more preferably 0.85%.

[0033] Mn: 0.20 - 1.00% Manganese (Mn) enhances the hardenability of the wire rod and increases the strength of the steel wire manufactured using the wire rod as a raw material. If the Mn content is less than 0.20%, even if the contents of other elements are within the range of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Mn content exceeds 1.00%, Mn segregates in the wire rod. In this case, bainite or martensite is generated in the region where Mn segregates. Therefore, even if the contents of other elements are within the range of this embodiment, the drawability of the wire rod deteriorates. Therefore, the Mn content is 0.20 - 1.00%. The preferable lower limit of the Mn content is 0.25%, more preferably 0.30%, and even more preferably 0.40%. The preferable upper limit of the Mn content is 0.95%, more preferably 0.90%, and even more preferably 0.80%.

[0034] V: 0.10 - 0.40% Vanadium (V) forms V-based precipitates and increases the strength of the wire rod by precipitation strengthening. Furthermore, the V-based precipitates inhibit the movement of dislocations introduced into the steel wire after wire drawing during repeated loading. Therefore, the endurance ratio of the steel wire manufactured using the wire rod as a raw material increases. If the V content is less than 0.10%, even if the contents of other elements are within the range of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the V content exceeds 0.40%, excessive V-based precipitates are formed. In this case, even if the contents of other elements are within the range of this embodiment, the drawability of the wire rod deteriorates. Therefore, the V content is 0.10 - 0.40%. The preferable lower limit of the V content is 0.11%, more preferably 0.13%. The preferable upper limit of the V content is 0.38%, more preferably 0.35%, and even more preferably 0.30%.

[0035] P: Below 0.020% Phosphorus (P) is an impurity. That is, the lower limit of the P content is over 0%. If the P content exceeds 0.020%, P segregates at grain boundaries and the grain boundaries become brittle. Therefore, even if the contents of other elements are within the range of this embodiment, the strength of the steel wire manufactured using the wire rod as a raw material decreases. If the P content exceeds 0.020%, furthermore, the wire drawing processability of the wire rod decreases. Therefore, the P content is 0.020% or less. It is preferable that the P content be as low as possible. However, excessive reduction of the P content increases the manufacturing cost. Therefore, considering normal industrial production, the preferable lower limit of the P content is 0.001%, more preferably 0.003%, and even more preferably 0.004%. The preferable upper limit of the P content is 0.018%, more preferably 0.015%, and even more preferably 0.010%.

[0036] S: Below 0.020% Sulfur (S) is an impurity. That is, the lower limit of the S content is over 0%. If the S content exceeds 0.020%, S segregates at grain boundaries and the grain boundaries become brittle. Therefore, even if the contents of other elements are within the range of this embodiment, the strength of the steel wire manufactured using the wire rod as a raw material decreases. If the S content exceeds 0.020%, furthermore, the wire drawing processability of the wire rod decreases. Therefore, the S content is 0.020% or less. It is preferable that the S content be as low as possible. However, excessive reduction of the S content increases the manufacturing cost. Therefore, considering normal industrial production, the preferable lower limit of the S content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferable upper limit of the S content is 0.018%, more preferably 0.015%, and even more preferably 0.010%.

[0037] Al: Below 0.080% Aluminum (Al) is an impurity. That is, the Al content is more than 0%. If the Al content exceeds 0.080%, excessive Al oxides will be generated in the wire rod. Therefore, even if the contents of other elements are within the range of this embodiment, the drawability of the wire rod will deteriorate. Therefore, the Al content is 0.080% or less. It is preferable that the Al content is as low as possible. However, excessive reduction of the Al content will increase the manufacturing cost. Therefore, considering normal industrial production, the preferable lower limit of the Al content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferable upper limit of the Al content is 0.079%, more preferably 0.070%.

[0038] N: 0.0100% or less Nitrogen (N) is an impurity. That is, the lower limit of the N content is more than 0%. If the N content exceeds 0.0100%, when the wire rod is drawn, the dissolved N promotes age hardening. Therefore, even if the contents of other elements are within the range of this embodiment, the drawability of the wire rod will deteriorate. Therefore, the N content is 0.0100% or less. It is preferable that the N content is as low as possible. However, excessive reduction of the N content will increase the manufacturing cost. Therefore, considering normal industrial production, the preferable lower limit of the N content is 0.0001%, more preferably 0.0005%, even more preferably 0.0010%, and even more preferably 0.0030%. The preferable upper limit of the N content is 0.0090%, more preferably 0.0070%, and even more preferably 0.0050%.

[0039] O: 0.0050% or less Oxygen (O) is an impurity. That is, the O content is more than 0%. If the O content exceeds 0.0050%, coarse oxides will be generated in the wire rod. Therefore, even if the contents of other elements are within the range of this embodiment, the drawability of the wire rod will deteriorate. Therefore, the O content is 0.0050% or less. Preferably, the O content is as low as possible. However, excessive reduction of the O content increases the manufacturing cost. Therefore, considering normal industrial production, the preferable lower limit of the O content is 0.0001%, more preferably 0.0003%. The preferable upper limit of the O content is 0.0045%, more preferably 0.0040%.

[0040] The balance of the chemical composition of the wire rod according to the present embodiment consists of Fe and impurities. Here, the impurities in the chemical composition mean those mixed from ores, scraps, or manufacturing environments as raw materials during industrial production of the wire rod, and are allowed within a range that does not adversely affect the wire rod according to the present embodiment.

[0041] [Regarding Optional Elements] The chemical composition of the wire rod of the present embodiment may further contain one or more selected from the group consisting of Group 1 and Group 2 in place of a part of Fe. [Group 1] Cr: 0.50% or less, Cu: 0.50% or less, Ni: 0.50% or less, Sn: 0.100% or less, Mo: 0.20% or less, Nb: 0.100% or less, Ti: 0.100% or less, B: 0.0050% or less, Co: 0.50% or less, and, W: 0.20% or less, one or more selected from the group consisting of [Group 2] Ca: 0.0050% or less, Mg: 0.0050% or less, Zr: 0.010% or less, and, Rare earth elements: 0.005% or less, one or more selected from the group consisting of Hereinafter, these optional elements will be described.

[0042] [Group 1: Cr, Cu, Ni, Sn, Mo, Nb, Ti, B, Co, and W] The chemical composition of the wire rod of the present embodiment may further contain the above-mentioned Group 1 in place of a part of Fe. These elements are optional elements, and all of them increase the strength of the steel wire produced using the wire rod as a raw material. Hereinafter, each element of Group 1 will be described.

[0043] Cr: 0.50% or less Chromium (Cr) is an optional element and may not be contained. That is, the Cr content may be 0%. When contained, that is, when the Cr content exceeds 0%, Cr increases the hardenability of the wire rod and increases the strength of the steel wire produced using the wire rod as a raw material. If even a small amount of Cr is contained, the above effects can be obtained to some extent. On the other hand, if the Cr content exceeds 0.50%, Cr segregates in the wire rod. In this case, bainite or martensite is generated in the region where Cr segregates. Therefore, even if the other element contents are within the range of the present embodiment, the drawability of the wire rod decreases. Therefore, the Cr content is 0 to 0.50%, and when contained, it is 0.50% or less. The preferable lower limit of the Cr content is 0.01%, more preferably 0.05%, and even more preferably 0.08%. The preferable upper limit of the Cr content is 0.45%, more preferably 0.40%, and even more preferably 0.30%.

[0044] Cu: 0.50% or less Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When contained, that is, when the Cu content exceeds 0%, Cu increases the hardenability of the wire rod and increases the strength of the steel wire produced using the wire rod as a raw material. If even a small amount of Cu is contained, the above effects can be obtained to some extent. On the other hand, if the Cu content exceeds 0.50%, the hardness of the wire rod increases excessively. Therefore, even if the other element contents are within the range of the present embodiment, the drawability of the wire rod decreases. Therefore, the Cu content is 0 to 0.50%, and when contained, it is 0.50% or less. The preferable lower limit of the Cu content is 0.01%, more preferably 0.05%, and even more preferably 0.08%. The preferable upper limit of the Cu content is 0.45%, more preferably 0.40%, and even more preferably 0.30%.

[0045] Ni: 0.50% or less Nickel (Ni) is an optional element and may not be contained. That is, the Ni content may be 0%. When contained, that is, when the Ni content exceeds 0%, Ni delays the transformation of primary cementite and grain boundary ferrite during cooling from the austenite temperature range in the manufacturing process of the wire rod. As a result, a fine lamellar structure of pearlite is formed. As a result, while sufficiently maintaining the drawability of the wire rod, the strength of the steel wire manufactured using the wire rod as a material is increased. If even a small amount of Ni is contained, the above effects can be obtained to a certain extent. On the other hand, if the Ni content exceeds 0.50%, the hardenability of the wire rod becomes excessively high. In this case, bainite is generated in the wire rod. Therefore, even if the contents of other elements are within the range of this embodiment, the drawability of the wire rod decreases. Therefore, the Ni content is 0 to 0.50%, and when contained, it is 0.50% or less. The preferable lower limit of the Ni content is 0.01%, more preferably 0.03%, and even more preferably 0.10%. The preferable upper limit of the Ni content is 0.48%, more preferably 0.45%, and even more preferably 0.40%.

[0046] Sn: 0.100% or less Tin (Sn) is an optional element and may not be contained. That is, the Sn content may be 0%. When contained, that is, when the Sn content exceeds 0%, Sn suppresses ferrite decarburization occurring during hot working and suppresses a decrease in the strength of the wire rod. As a result, the strength of the steel wire manufactured using the wire rod as a raw material is increased. If even a small amount of Sn is contained, the above effects can be obtained to a certain extent. On the other hand, if the Sn content exceeds 0.100%, Sn segregates at the grain boundaries and red hot embrittlement occurs. Therefore, even if the contents of other elements are within the range of this embodiment, the hot workability of the wire rod decreases. Therefore, the Sn content is 0 to 0.100%, and when contained, it is 0.100% or less. The preferable lower limit of the Sn content is 0.001%, more preferably 0.010%, and even more preferably 0.020%. The preferable upper limit of the Sn content is 0.090%, more preferably 0.080%, and even more preferably 0.070%.

[0047] Mo: 0.20% or less Molybdenum (Mo) is an optional element and may not be contained. That is, the Mo content may be 0%. When contained, that is, when the Mo content exceeds 0%, Mo increases the hardenability of the wire rod and increases the strength of the steel wire manufactured using the wire rod as a raw material. If even a small amount of Mo is contained, the above effects can be obtained to a certain extent. On the other hand, if the Mo content exceeds 0.20%, the hardenability of the wire rod becomes excessively high. In this case, bainite is generated in the wire rod. Therefore, even if the contents of other elements are within the range of this embodiment, the drawability of the wire rod decreases. Therefore, the Mo content is 0 to 0.20%, and when contained, it is 0.20% or less. The preferable lower limit of the Mo content is 0.01%, more preferably 0.03%, and even more preferably 0.05%. The preferable upper limit of the Mo content is 0.18%, more preferably 0.15%, and even more preferably 0.10%.

[0048] Nb: 0.100% or less Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When it is contained, that is, when the Nb content exceeds 0%, Nb forms precipitates that are carbides and / or carbonitrides, increasing the strength of the wire rod and the steel wire manufactured from the wire rod. Even if it is contained in a small amount, the above effect can be obtained to a certain extent. On the other hand, if the Nb content exceeds 0.100%, excessive precipitates are formed. In this case, even if the contents of other elements are within the range of this embodiment, the drawability of the wire rod decreases. Therefore, the Nb content is 0 to 0.100%, and when it is contained, it is 0.100% or less. The preferable lower limit of the Nb content is 0.001%, more preferably 0.010%, and even more preferably 0.020%. The preferable upper limit of the Nb content is 0.090%, more preferably 0.080%, and even more preferably 0.070%.

[0049] Ti: 0.100% or less Titanium (Ti) is an optional element and may not be contained. That is, the Ti content may be 0%. When it is contained, that is, when the Ti content exceeds 0%, Ti forms precipitates that are carbides / or nitrides, increasing the strength of the wire rod and the steel wire manufactured from the wire rod. Even if it is contained in a small amount, the above effect can be obtained to a certain extent. On the other hand, if the Ti content exceeds 0.100%, excessive precipitates are formed. In this case, even if the contents of other elements are within the range of this embodiment, the drawability of the wire rod decreases. Therefore, the Ti content is 0 to 0.100%, and when it is contained, it is 0.100% or less. The preferable lower limit of the Ti content is 0.001%, more preferably 0.010%. The preferable upper limit of the Ti content is 0.090%, more preferably 0.080%.

[0050] B: 0.0050% or less Boron (B) is an optional element and may not be contained. That is, the B content may be 0%. When it is contained, that is, when the B content exceeds 0%, B enhances the hardenability of the wire rod and increases the strength of the wire rod. As a result, the strength of the steel wire manufactured using the wire rod as a raw material increases. Even if a small amount of B is contained, the above effects can be obtained to a certain extent. On the other hand, if the B content exceeds 0.0050%, even if the contents of other elements are within the range of this embodiment, the hot workability of the wire rod decreases. Therefore, the B content is 0 to 0.0050%, and when it is contained, it is 0.0050% or less. The preferable lower limit of the B content is 0.0001%, and more preferably 0.0010%. The preferable upper limit of the B content is 0.0040%, and more preferably 0.0030%.

[0051] Co: 0.50% or less Cobalt (Co) is an optional element and may not be contained. That is, the Co content may be 0%. When it is contained, that is, when the Co content exceeds 0%, Co enhances the hardenability of the wire rod and increases the strength of the wire rod. As a result, the strength of the steel wire manufactured using the wire rod as a raw material increases. Even if a small amount of Co is contained, the above effects can be obtained to a certain extent. On the other hand, if the Co content exceeds 0.50%, the hardness of the wire rod becomes excessively hard. Therefore, even if the contents of other elements are within the range of this embodiment, the drawing workability of the wire rod decreases. Therefore, the Co content is 0 to 0.50%, and when it is contained, it is 0.50% or less. The preferable lower limit of the Co content is 0.01%, more preferably 0.05%, and even more preferably 0.08%. The preferable upper limit of the Co content is 0.45%, more preferably 0.40%, and even more preferably 0.35%.

[0052] W: 0.20% or less Tungsten (W) is an optional element and may not be contained. That is, the W content may be 0%. When contained, that is, when the W content exceeds 0%, W enhances the hardenability of the wire rod and increases the strength of the wire rod. As a result, the strength of the steel wire produced using the wire rod as a raw material increases. Even if a small amount of W is contained, the above effects can be obtained to a certain extent. On the other hand, if the W content exceeds 0.20%, even if the contents of other elements are within the range of this embodiment, the hot workability of the wire rod decreases. Therefore, the W content is 0 to 0.20%, and when contained, it is 0.20% or less. The preferable lower limit of the W content is 0.01%, more preferably 0.03%, and even more preferably 0.05%. The preferable upper limit of the W content is 0.18%, more preferably 0.16%, and even more preferably 0.14%.

[0053] [Group 2: Ca, Mg, Zr, and rare earth elements (REM)] The chemical composition of the wire rod of this embodiment may further contain the above Group 2 in place of a part of Fe. These elements are optional elements, and all enhance the drawability of the wire rod.

[0054] Ca: 0.0050% or less Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When contained, that is, when the Ca content exceeds 0%, Ca reduces hard alumina-based inclusions and enhances the drawability of the wire rod. Even if a small amount of Ca is contained, the above effects can be obtained to a certain extent. On the other hand, if the Ca content exceeds 0.0050%, oxides are excessively generated. In this case, even if the contents of other elements are within the range of this embodiment, the drawability of the wire rod decreases. Therefore, the Ca content is 0 to 0.0050%, and when contained, it is 0.0050% or less. The preferable lower limit of the Ca content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%. The preferable upper limit of the Ca content is 0.0040%, more preferably 0.0030%, and even more preferably 0.0020%.

[0055] Mg: 0.0050% or less Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%. When it is contained, that is, when the Mg content exceeds 0%, Mg forms fine oxides. The fine oxides refine the microstructure of the wire rod and improve the drawability of the wire rod. If even a small amount of Mg is contained, the above effects can be obtained to some extent. On the other hand, if the Mg content exceeds 0.0050%, excessive oxides are generated. In this case, even if the contents of other elements are within the range of this embodiment, the drawability of the wire rod decreases. Therefore, the Mg content is 0 to 0.0050%, and when it is contained, it is 0.0050% or less. The preferable lower limit of the Mg content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%. The preferable upper limit of the Mg content is 0.0040%, more preferably 0.0030%, and even more preferably 0.0020%.

[0056] Zr: 0.010% or less Zirconium (Zr) is an optional element and may not be contained. That is, the Zr content may be 0%. When it is contained, that is, when the Zr content exceeds 0%, Zr forms fine oxides. The fine oxides refine the microstructure of the wire rod and improve the drawability of the wire rod. If even a small amount of Zr is contained, the above effects can be obtained to some extent. On the other hand, if the Zr content exceeds 0.010%, excessive oxides are generated. In this case, even if the contents of other elements are within the range of this embodiment, the drawability of the wire rod decreases. Therefore, the Zr content is 0 to 0.010%, and when contained, it is 0.010% or less. The preferable lower limit of the Zr content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferable upper limit of the Zr content is 0.009%, more preferably 0.008%, and even more preferably 0.007%.

[0057] Rare earth elements: 0.005% or less The rare earth element (REM) is an optional element and may not be contained. That is, the REM content may be 0%. When contained, that is, when the REM content exceeds 0%, REM fixes S. Therefore, the drawability of the wire rod is improved. If even a small amount of REM is contained, the above effect can be obtained to a certain extent. On the other hand, if the REM content exceeds 0.005%, excessive oxides are generated. In this case, even if the contents of other elements are within the range of this embodiment, the drawability of the wire rod decreases. Therefore, the REM content is 0 to 0.005%, and when contained, it is 0.005% or less. The preferable lower limit of the REM content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferable upper limit of the REM content is 0.004%, more preferably 0.003%.

[0058] In addition, REM in this specification is one or more elements selected from the group consisting of scandium (Sc) with an atomic number of 21, yttrium (Y) with an atomic number of 39, and lanthanum (La) with an atomic number of 57 to lutetium (Lu) with an atomic number of 71 which are lanthanoids. Further, the REM content in this specification is the total content of these elements.

[0059] [(Feature 2) Regarding Formula (1)] The C concentration in mass% obtained by point-analyzing the region containing lamellar cementite using a SEM-EDX apparatus is CC (%) is defined, and the V concentration in mass % obtained by the point analysis is defined as V C (%). In this case, the wire rod of the present embodiment further satisfies the formula (1). 0.15 ≦ V C / (C C ×V) ≦ 1.00 (1) Here, the V in the formula (1) is substituted with the V content in mass % in the chemical composition.

[0060] Define F1 as follows. F1 = V C / (C C ×V) F1 is an index representing the amount of V-based precipitates in lamellar ferrite. As described above, V exists not only as V-based precipitates in lamellar ferrite but also in a solid solution state in lamellar cementite. The higher F1 is, the higher the V concentration in lamellar cementite is with respect to the V content of the entire wire rod. At this time, the amount of V-based precipitates present in lamellar ferrite is small. Also, the lower F1 is, the lower the V concentration in lamellar cementite is with respect to the V content of the entire wire rod. At this time, the amount of V-based precipitates present in lamellar ferrite is large.

[0061] C C and V C are obtained by performing point analysis on a region containing lamellar cementite using a SEM (Scanning Electron Microscope)-EDX (Energy Dispersive X-ray Spectrometry) apparatus. In the point analysis, the region irradiated with the electron beam of the SEM-EDX apparatus becomes the measurement region. On the other hand, the thickness of the lamellar cementite in the wire rod of the present embodiment may be smaller than the electron beam diameter of the SEM-EDX apparatus. In this case, the measurement region of the point analysis includes not only lamellar cementite but also adjacent lamellar ferrite. It can be assumed that the C concentration in lamellar cementite is uniform regardless of the chemical composition of the wire rod. Therefore, the V concentration V C obtained by point analysis of the region containing lamellar cementite is the C concentration C obtained by the point analysisC By removing it, it is possible to reduce the error caused by the area ratio of lamellar ferrite included in the measurement region of the point analysis.

[0062] If F1 is less than 0.15, the V concentration in lamellar cementite is too low with respect to the V content of the entire wire rod. That is, the amount of V-based precipitates present in lamellar ferrite is too large. As a result, the drawability of the wire rod deteriorates. If F1 exceeds 1.00, the V concentration in lamellar cementite is too high with respect to the V content of the entire wire rod. That is, the amount of V-based precipitates present in lamellar ferrite is too small. Therefore, the movement of dislocations due to fatigue is not sufficiently suppressed. As a result, in the steel wire manufactured using the wire rod as a raw material, an excellent endurance ratio cannot be obtained.

[0063] If F1 is 0.15 to 1.00, the V concentration in lamellar cementite is appropriate with respect to the V content of the entire wire rod. That is, the amount of V-based precipitates present in lamellar ferrite is also appropriate. As a result, assuming that the wire rod satisfies Feature 1, sufficient drawability is obtained, and in the steel wire manufactured using the wire rod as a raw material, an excellent endurance ratio is obtained. Therefore, F1 is 0.15 to 1.00. The preferable lower limit of F1 is 0.16, more preferably 0.17, and even more preferably 0.18. The preferable upper limit of F1 is 0.90, more preferably 0.80, and even more preferably 0.70. Note that F1 is a value up to the second decimal place obtained by rounding the third decimal place of the obtained numerical value.

[0064] [C C and V C measurement method] C C and V C are specifically obtained by the following method. A test piece is sampled from the wire rod. The size of the test piece is not particularly limited. Among the surfaces of the test piece, a cross-section perpendicular to the axis of the wire rod is defined as the observation surface. The observation surface is mirror-polished. Among the observation surfaces after mirror polishing, four observation fields centered at the D / 4 position are observed at a magnification of 10,000 times using a scanning electron microscope equipped with an elemental analysis function (SEM-EDX apparatus). Here, the D / 4 position means the central part of the radius (that is, the part at a depth of D / 4 in the radial direction from the surface of the wire rod) when the diameter passing through the center of the circular observation surface is D. The size of the observation field is not particularly limited, but for example, it is 10 μm × 10 μm.

[0065] Based on the contrast obtained by SEM observation of each observation field, the lamellar cementite region is specified. Targeting the specified lamellar cementite region, point analysis is performed using EDX (energy dispersive X-ray analyzer) attached to the SEM. In the EDX analysis, the acceleration voltage is set to 15 kV and the analysis time is 30 seconds.

[0066] An arithmetic mean value is calculated from the C concentrations in mass% obtained by point analysis in the four observation fields. The obtained arithmetic mean value is taken as C C (%). Similarly, an arithmetic mean value is calculated from the V concentrations in mass% obtained by point analysis in the four observation fields. The obtained arithmetic mean value is taken as V C (%). When the C concentration in mass% obtained by point analysis in a certain observation field is less than 1.45%, it is determined that the area ratio of lamellar cementite in the measurement region of the point analysis is too low. In this case, a different lamellar cementite region is specified again and point analysis is performed in the same manner. This operation is repeated until the C concentration obtained by point analysis becomes 1.45% or more. The C concentration obtained by point analysis when the C concentration in mass% becomes 1.45% or more is taken as the C concentration obtained by point analysis in the observation field. The V concentration obtained by point analysis when the C concentration in mass% becomes 1.45% or more is taken as the V concentration obtained by point analysis in the observation field.

[0067] [Effect of the wire rod of the present embodiment] The wire rod of this embodiment satisfies Feature 1 and Feature 2. Therefore, in the wire rod of this embodiment, sufficient wire drawing processability can be obtained, and in the steel wire used and manufactured as a material, an excellent endurance ratio can be obtained.

[0068] [Microstructure of the wire rod of this embodiment] The microstructure of the wire rod of this embodiment consists substantially of a pearlite structure. Here, consisting substantially of a pearlite structure means that the area ratio of pearlite is 90% or more.

[0069] [Method for observing the microstructure of the wire rod] The microstructure observation and the area ratio of pearlite of the wire rod of this embodiment are measured by the following method.

[0070] A test piece is taken from the wire rod. Among the surfaces of the test piece, a cross section perpendicular to the axis of the wire rod is taken as the observation surface. The observation surface is mirror polished. The observation surface after mirror polishing is etched with nital to reveal the structure. For four visual fields centered on the D / 4 position among the observation surfaces after etching, observation is carried out at a magnification of 200 times using an optical microscope. A photographic image of each visual field is generated. The size of each visual field is, for example, 0.60 mm × 0.48 mm.

[0071] Based on the contrast of the photographic image of each visual field, each structure (ferrite, primary cementite, pearlite, bainite, and martensite) is specified. The total area (μm 2 ) of the pearlite specified in the four visual fields is obtained. Based on the total area of the four visual fields and the total area of the pearlite, the area ratio (%) of the pearlite is obtained.

[0072] Note that it is already known that the microstructure of steel wires for uses such as beads for automobile tires, steel cords, wire ropes for bridge applications, wire ropes for elevator applications, PC steel wires, and transmission wires, and the wire rods used as materials for such steel wires is substantially a pearlite structure.

[0073] [Tensile strength of the wire rod of this embodiment] The tensile strength of the wire rod of this embodiment is 1100 MPa or more. The tensile strength is obtained by conducting a tensile test in accordance with JIS Z 2241:2011 at room temperature in the air.

[0074] [Applications applicable to the wire rod of this embodiment] The wire rod of this embodiment is widely applicable, for example, as a material for steel wires manufactured by wire drawing. The steel wires are applicable, for example, to beads for automobile tires, steel cords, wire ropes for bridge applications, wire ropes for elevator applications, PC steel wires, and wires for power transmission.

[0075] [An example of the manufacturing method of the wire rod of this embodiment] An example of the manufacturing method of the wire rod of this embodiment will be described. The manufacturing method of the wire rod described hereinafter is an example for manufacturing the wire rod of this embodiment. Therefore, the wire rod having the above-described configuration may be manufactured by other manufacturing methods other than the manufacturing method described hereinafter. However, the manufacturing method described hereinafter is a preferred example of the manufacturing method of the wire rod of this embodiment.

[0076] An example of the manufacturing method of the wire rod of this embodiment includes the following steps. (Step 1) Stock preparation step (Step 2) Block rolling step (Step 3) Finish rolling step Hereinafter, each step will be described.

[0077] [(Step 1) Stock preparation step] In the stock preparation step, the stock of the wire rod of this embodiment is prepared. Specifically, molten steel whose chemical composition satisfies Feature 1 is manufactured. The refining method is not particularly limited, and a well-known method may be used. For example, refining (primary refining) in a converter is performed on hot metal manufactured by a well-known method. Secondary refining known to the public is performed on the molten steel tapped from the converter. Through the above steps, molten steel having a chemical composition that satisfies Feature 1 is manufactured.

[0078] Using the produced molten steel, a material is manufactured by a well-known casting method. For example, an ingot may be manufactured by an ingot-making method using the molten steel. Alternatively, a bloom may be manufactured by a continuous casting method using the molten steel. By the above methods, a material (ingot or bloom) is manufactured.

[0079] [(Step 2) Ingot Block Rolling Process] In the ingot block rolling process, ingot block rolling is performed on the material (ingot or bloom) prepared in the material preparation process to manufacture billets. In the ingot block rolling process, first, the material is heated using a heating furnace by a well-known method. The heating temperature is not particularly limited. A well-known temperature is sufficient for the heating temperature. The heating temperature is, for example, 1000 to 1200 °C.

[0080] The heated material is rolled using an ingot block rolling mill, or an ingot block rolling mill and a continuous rolling mill, to manufacture billets. Specifically, the heated material is reversely rolled using an ingot block rolling mill to manufacture billets. When a well-known continuous rolling mill is arranged downstream of the ingot block rolling mill, tandem rolling may be further performed on the billets after ingot block rolling using the continuous rolling mill to manufacture even smaller-sized billets. The manufactured billets are air-cooled to room temperature before the finish rolling process.

[0081] [(Step 3) Finish Rolling Process] In the finish rolling process, finish rolling is performed on the billets manufactured in the ingot block rolling process to manufacture wire rods. The finish rolling process includes the following steps. (Step 31) Heating Process (Step 32) Rolling Process (Step 33) Cooling Process Hereinafter, the heating process, rolling process, and cooling process in the finish rolling process will be described.

[0082] [(Step 31) Heating Process] In the heating process, first, the billet is heated to 950 °C (the first heating temperature) using a heating furnace. After holding at 950 °C for 10 minutes, the billet is further heated to the second heating temperature T2 (°C) defined by formula (A). Then, the billet is held at a temperature equal to or higher than the second heating temperature T2 (°C). T2 = 66.6 × C + 487.5 × V + 878.8 (A) Here, the content (mass %) of the corresponding element is substituted for each element symbol in formula (A).

[0083] In the heating process, the following conditions are satisfied. (Condition 1) The holding time t2 (minutes) at a temperature equal to or higher than the second heating temperature T2 (°C) is 10 minutes or more.

[0084] The V-based precipitates in the billet dissolve into the matrix phase at a temperature equal to or higher than the second heating temperature T2 (°C). At this time, if the holding time t2 at a temperature equal to or higher than the second heating temperature T2 (°C) is less than 10 minutes, the dissolution of the V-based precipitates in the heating process is insufficient. Therefore, the enrichment of V in the lamellar cementite is suppressed, and the amount of V-based precipitates in the lamellar ferrite becomes excessive. In this case, F1 becomes low, and the wire rod does not satisfy Feature 2. Therefore, the holding time t2 is 10 minutes or more. Note that the upper limit of the holding time t2 is not particularly limited, but for example, it is 60 minutes.

[0085] [(Process 32) Rolling Process] In the rolling process, finish rolling (continuous rolling) using a continuous rolling mill is performed on the billet after the heating process to produce a wire rod. The continuous rolling mill includes a plurality of rolling stands arranged in a row from upstream to downstream. Each rolling stand includes a pair of work rolls. A caliber is formed on each work roll, and a pass is formed by the calibers of the pair of work rolls.

[0086] [(Process 33) Cooling Process] In the cooling process, the wire rod after the rolling process is cooled. The wire rod is coiled when its temperature is between 750 and 900 °C. The average cooling rate from 700 °C to 600 °C after coiling is designated as v1 (°C / second), and the average cooling rate below 600 °C is 10 °C / second or more.

[0087] In the cooling process, the following conditions are satisfied. (Condition 2) The average cooling rate v1 (°C / second) from 700 °C to 600 °C satisfies Equation (B). -0.40×V + 0.50 < v1 < -1.30×V + 1.24 (B) Here, in Equation (B), the V content in mass% in the chemical composition of the wire rod is substituted for V.

[0088] Lower limit average cooling rate v L (°C / second), and the upper limit average cooling rate v U (°C / second) are defined as follows. v L = -0.40×V + 0.50 v U = -1.30×V + 1.24 During the cooling from 700 °C to 600 °C, the microstructure of the wire rod transforms from austenite structure to pearlite structure. Furthermore, due to the interphase precipitation occurring simultaneously with the transformation to pearlite structure, V-based precipitates are formed in the lamellar ferrite. If the average cooling rate v1 from 700 °C to 600 °C is below the lower limit average cooling rate v L the amount of V-based precipitates formed in the lamellar ferrite becomes excessive. In this case, F1 becomes low, and the wire rod does not satisfy Feature 2. On the other hand, if the average cooling rate v1 from 700 °C to 600 °C is above the upper limit average cooling rate v U the transformation to pearlite structure is insufficient, and the amount of V-based precipitates formed in the lamellar ferrite also becomes excessively small. In this case, F1 becomes high, and the wire rod does not satisfy Feature 2. Therefore, the average cooling rate v1 from 700 °C to 600 °C is within the range that satisfies Equation (B).

[0089] The average cooling rate v1 (°C / second) from 700°C to 600°C is obtained by the following method. A conveying table for conveying the wire rod is arranged downstream of the continuous rolling mill that performs finish rolling. A plurality of thermometers are arranged on the conveying table from upstream to downstream. Each thermometer measures the wire rod temperature at the point where the thermometer is arranged. Based on the wire rod temperature measured by each thermometer and the conveying speed of the wire rod, the time (seconds) until the wire rod temperature drops from 700°C to 600°C is obtained. Based on the obtained time (seconds), the average cooling rate v1 (°C / second) is obtained.

[0090] In addition, for a wire rod whose chemical composition satisfies Feature 1, by cooling at the above cooling rate, the microstructure substantially becomes a pearlite structure.

[0091] Through the above steps, the wire rod of the present embodiment is manufactured.

[0092] [Manufacturing method of steel wire] The manufacturing method of the steel wire using the wire rod of the present embodiment is a well-known manufacturing method. The manufacturing method of the steel wire using the wire rod of the present embodiment is, for example, as follows.

[0093] Wire drawing is performed on the wire rod to manufacture a steel wire. The wire drawing may be performed only once or multiple times. When performing wire drawing multiple times, after wire drawing, before performing the next wire drawing, a well-known patenting treatment may be performed. In addition, before wire drawing, the oxide scale of the wire rod may be removed in advance. Also, a lubrication treatment may be performed on the wire rod before wire drawing.

[0094] A well-known low-temperature heat treatment is performed on the steel wire after wire drawing. The heat treatment may be, for example, a bluing treatment, or a heat diffusion treatment of electroplating such as molten plating or brass plating. The heating temperature in the low-temperature heat treatment is, for example, 300 to 500°C. Through the above manufacturing steps, a steel wire is manufactured.

Example

[0095] The effects of the wire rod of the present embodiment will be described more specifically by way of examples. The conditions in the following examples are one example of the conditions adopted to confirm the feasibility and effects of the wire rod of the present embodiment. Therefore, the wire rod of the present embodiment is not limited to this one example of the conditions.

[0096] [Material Preparation Step] Wire rods having the chemical compositions shown in Table 1A and Table 1B were manufactured by the following method.

[0097] [Table 1A]

[0098] [Table 1B]

[0099] The bloom thus manufactured was subjected to a cogging rolling process to produce billets. Specifically, the bloom was heated to 1100 °C using a heating furnace. The heated bloom was rolled using a cogging rolling mill and a continuous rolling mill to produce billets. The billets produced in the cogging rolling process were allowed to cool to room temperature.

[0100] The billets thus manufactured were subjected to a finishing rolling process to produce wire rods of each test number. In the finishing rolling process, first, a heating process was carried out on the billets produced in the cogging rolling process. Specifically, each billet of each test number was heated to 950 °C using a heating furnace. After holding at 950 °C for 10 minutes, each billet of each test number was further heated to a second heating temperature T2 (°C). Further, it was heated from the second heating temperature T2 (°C) to the maximum reaching temperature, and then held at the maximum reaching temperature. The second heating temperature T2 (°C), the maximum reaching temperature (°C) in the heating process, and the holding time t2 (minutes) at a temperature of the second heating temperature T2 (°C) or higher for each test number are shown in Table 2.

[0101] [Table 2]

[0102] The rolling process was carried out on the billet after the heating process. In the rolling process, finish rolling (continuous rolling) was carried out using a continuous rolling mill to produce wire rods.

[0103] The cooling process was carried out on the wire rods after the rolling process. Specifically, the rolled wire rods were wound at a winding temperature of 750 to 900 °C. The wound wire rods were cooled to room temperature. The average cooling rate v1 (°C / second) from 700 °C to 600 °C, the lower limit average cooling rate v L (°C / second), and the upper limit average cooling rate v U (°C / second) were as shown in Table 2. The average cooling rate below 600 °C was 10 °C / second. Through the above manufacturing process, wire rods with a wire diameter of 5.5 mm were produced.

[0104] [Regarding the evaluation test] The following evaluation tests were carried out on the wire rods of each test number produced. (Test 1) Measurement test of C C and V C (Test 2) Measurement test of pearlite area ratio (Test 3) Drawing workability evaluation test (Test 4) Endurance ratio evaluation test The following is an explanation of each test.

[0105] [(Test 1) Measurement test of C C and V C Based on the above [measurement method of C C and V C , C C and V C in the wire rods of each test number were obtained. Based on the obtained C C (%) and V C (%), F1 was calculated. The calculated F1 is shown in Table 2.

[0106] [(Test 2) Measurement test of pearlite area ratio] ​​Based on the above [method for observing the microstructure of wire rods], the area ratio (%) of pearlite in the wire rods of each test number was determined. The obtained results are shown in Table 2.

[0107] [(Test 3) Drawing processability evaluation test] The drawing processability of the wire rods of each test number was evaluated by the following method. Drawing was performed on the wire rods of each test number to produce intermediate steel wires of each test number. Specifically, for a wire rod with a wire diameter of 5.5 mm, 11 passes of drawing were performed to produce an intermediate steel wire with a wire diameter of 1.58 mm. Here, "pass" refers to the number of times the wire rod passes through the die. The maximum area reduction rate in one pass of drawing was set to 20%. The drawing speed was 10 m / min, and the die angle was 5° (half angle). As the lubricant, "Cosin S80" manufactured by Kyoeisha Chemical Co., Ltd. was used. In each of the manufactured intermediate steel wires of each test number, the occurrence or non-occurrence of wire breakage during drawing was investigated.

[0108] When no wire breakage occurred, the evaluation was "E (Excellent)", and it was judged that sufficient drawing processability was obtained (denoted as "E" in the "Drawing processability" column in Table 2). On the other hand, when wire breakage occurred, the evaluation was "B (Bad)", and it was judged that sufficient drawing processability was not obtained (denoted as "B" in the "Drawing processability" column in Table 2). In addition, for the test numbers where wire breakage occurred, Test 4 described below was not performed (denoted as "-" in the "Endurance ratio" column in Table 2).

[0109] [(Test 4) Endurance ratio evaluation test] The endurance ratio of the steel wires manufactured using the wire rods of each test number as raw materials was determined by the following method. First, a known bluing treatment was performed on the intermediate steel wires manufactured in Test 3 to produce steel wires. The heat treatment temperature of the bluing treatment was 400°C.

[0110] Using the above-mentioned steel wires manufactured using the wire rods of each test number as raw materials, the tensile strength and fatigue strength were determined by the following method. [(Test 4A) Tensile test] [(Test 4B) Fatigue strength evaluation test] The following describes Test 4A and Test 4B.

[0111] [(Test 4A) Tensile Test] Two tensile test pieces were taken from an arbitrary position of the steel wire of each test number. The wire diameter of the tensile test piece was 1.58 mm and the length was 250 mm. A tensile test conforming to JIS Z 2241:2011 was carried out on the tensile test piece at room temperature in the air, and the tensile strength (MPa) was obtained. In the tensile test, the crosshead speed was 10 mm / min, the chuck distance was 150 mm, and the distance between extensometers was 100 mm. The arithmetic mean value of the tensile strengths obtained from the two tensile test pieces was taken as the tensile strength (MPa) of the steel wire of each test number.

[0112] [(Test 4B) Fatigue Strength Evaluation Test] A test piece with a wire diameter of 1.58 mm and a length of 400 mm was taken from an arbitrary position of the steel wire of each test number. Using a Nakamura type rotating bending fatigue testing machine, the fatigue strength (MPa) was determined. The span distance (length of the evaluation part) was 100 mm, the rotation speed was 3000 rpm, and the maximum number of repetitions was 1.0×10 7 cycles. Among the test pieces of each test number that endured up to the maximum number of repetitions, the maximum stress was taken as the fatigue strength (MPa) of the steel wire manufactured from the wire rod of each test number.

[0113] Based on the tensile strength (MPa) of the steel wire of each test number obtained in Test 4A and the fatigue strength (MPa) of the steel wire of each test number obtained in Test 4B, the endurance ratio of the steel wire of each test number was determined by the following formula. Endurance ratio = Fatigue strength (MPa) / Tensile strength (MPa)

[0114] When the endurance ratio was 0.33 or more, the evaluation was "E (Excellent)", and it was judged that an excellent endurance ratio was obtained (denoted by "E" in the "Endurance Ratio" column in Table 2). On the other hand, when the endurance ratio was less than 0.33, the evaluation was "B (Bad)", and it was judged that an excellent endurance ratio was not obtained (denoted by "B" in the "Endurance Ratio" column in Table 2).

[0115] [Test Results] Referring to Table 1A, Table 1B, and Table 2, the wire rods with test numbers 1 to 21 satisfied Feature 1 and Feature 2. Therefore, sufficient wire drawing workability was obtained. Furthermore, in the steel wires manufactured using the wire rods as raw materials, excellent endurance ratios were obtained.

[0116] On the other hand, in Test No. 22, the V content in the chemical composition of the wire rod was too low. As a result, excellent endurance ratios were not obtained in the steel wires manufactured using the wire rods as raw materials.

[0117] In Test No. 23, the V content in the chemical composition of the wire rod was too high. As a result, sufficient wire drawing workability was not obtained.

[0118] In Test Nos. 24 and 25, the holding time t2 (minutes) at a temperature of 2 or higher than the second heating temperature T2 (°C) was less than 10 minutes. Therefore, F1 was too low. As a result, sufficient wire drawing workability was not obtained.

[0119] In Test Nos. 26 and 27, the average cooling rate v1 from 700 °C to 600 °C was equal to or lower than the lower limit average cooling rate v L (°C / second). Therefore, F1 was too low. As a result, sufficient wire drawing workability was not obtained.

[0120] In Test Nos. 28 and 29, the average cooling rate v1 from 700 °C to 600 °C was equal to or higher than the upper limit average cooling rate v U (°C / second). Therefore, F1 was too high. Furthermore, the area ratio of pearlite was too low. As a result, excellent endurance ratios were not obtained in the steel wires manufactured using the wire rods as raw materials.

[0121] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and implemented without departing from the spirit thereof.

Claims

1. The chemical composition is by mass percentage: C: 0.70 to less than 1.10%, Si: 0.10 to 1.00%, Mn: 0.20 to 1.00%, V: 0.10 to 0.40%, P: 0.020% or less, S: 0.020% or less, Al: 0.080% or less, N: 0.0100% or less, and O: 0.0050% or less, and contains, the balance consists of Fe and impurities, in the microstructure, the area ratio of pearlite is 90% or more, The C concentration in mass % obtained by point analysis of the region containing lamellar cementite of the perlite using an SEM-EDX device is defined as C Ｃ (%) and When the V concentration in mass % obtained by the above point analysis is defined as V Ｃ (%), satisfies formula (1), a wire rod. 0.15 ≤ V Ｃ / (C Ｃ × V) ≤ 1.00 (1) Here, in V in formula (1), the V content by mass percentage in the chemical composition is substituted.

2. The chemical composition is by mass percentage: C: 0.70 to less than 1.10%, Si: 0.10 to 1.00%, Mn: 0.20 to 1.00%, V: 0.10 to 0.40%, P: 0.020% or less, S: 0.020% or less, Al: 0.080% or less, N: 0.0100% or less, and O: 0.0050% or less, and contains, further contains one or more selected from the group consisting of a first group and a second group, the balance consists of Fe and impurities, in the microstructure, the area ratio of pearlite is 90% or more, The C concentration in mass % obtained by point analysis of the region containing lamellar cementite of the perlite using an SEM-EDX device is defined as C Ｃ (%) and When the V concentration in mass % obtained by the point analysis is defined as V Ｃ (%), satisfies formula (1), a wire rod. [First group] Cr: 0.50% or less, Cu: 0.50% or less, Ni: 0.50% or less, Sn: 0.100% or less, Mo: 0.20% or less, Nb: 0.100% or less, Ti: 0.100% or less, B: 0.0050% or less, Co: 0.50% or less, and W: 0.20% or less, one or more selected from the group consisting of [Second group] Ca: 0.0050% or less, Mg: 0.0050% or less, Zr: 0.010% or less, and rare earth elements: 0.005% or less, one or more selected from the group consisting of 0.15 ≤ V Ｃ / (C Ｃ × V) ≤ 1.00 (1) Here, in V in formula (1), the V content by mass percentage in the chemical composition is substituted.

3. The wire rod according to claim 2, containing the first group, a wire rod.

4. The wire rod according to claim 2, containing the second group, a wire rod.

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