Steel wire

Abstract

To provide a steel wire having sufficient ductility and a superior endurance ratio.SOLUTION: A steel wire according to the present disclosure has a chemical composition comprising, 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. When a C concentration by mass% determined by point analysis of a region including lamellar cementite in a pearlite structure using a TEM-EDX device is defined as CC(%), and a V concentration by mass% determined by point analysis is defined as VC(%), Formula (1) is satisfied, and a microstrain ε is 0.0040 or more. 0.15≤VC / (CC×V)≤1.00 (1).SELECTED DRAWING: None

Description

Technical Field

[0001] The present disclosure relates to steel wires, and more particularly to steel wires suitable for beads for automotive tires, wire ropes for bridges, wire ropes for elevators, PC steel wires, transmission wires, and the like.

Background Art

[0002] Steel wires are used for beads for automotive tires, 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 wire rods and then performing a bluing 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-mentioned applications, since repeated loads are applied under the use conditions, excellent endurance ratio (= fatigue strength / tensile strength) is also required. Furthermore, when manufacturing beads and the like from steel wires, the steel wires are twisted together to form twisted steel wires. Therefore, sufficient ductility is also required for steel wires.

[0004] A technique for improving the ductility and endurance ratio of steel wires has been proposed in Japanese Patent Application Laid-Open No. 06-049592 (Patent Document 1).

[0005] The steel wire disclosed in Patent Document 1 contains, by mass%, C: 0.80 to 1.10%, Si ≤ 0.25%, Mn ≤ 0.45%, P ≤ 0.010%, S ≤ 0.010%, Cr: 0.15 to 0.35%, N ≤ 0.0040%, B: 0.0005 to 0.01 × Cr [%] + 0.001%, and the balance is Fe and inevitable impurities. In Patent Document 1, B having a content corresponding to the Cr content is added to promote the growth of cementite in pearlite, thereby improving the ductility and endurance ratio.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Non-Patent Literature

[0007]

Non-Patent Literature 1

Summary of the Invention

Problems to be Solved by the Invention

[0008] However, the ductility and durability ratio of the steel wire may be increased by means other than those proposed in Patent Document 1.

[0009] An object of the present disclosure is to provide a steel wire that exhibits sufficient ductility and an excellent durability ratio.

Means for Solving the Problems

[0010] The steel wire 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, and contains the balance being composed of Fe and impurities, defining the C concentration in mass% obtained by point analyzing a region containing lamellar cementite in the pearlite structure using a TEM-EDX apparatus as C C (%), defining the V concentration in mass% obtained by the point analysis as V CWhen defined as (%) satisfies formula (1), the microstrain ε is 0.0040 or more. 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.

[0011] The steel wire 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, and contains further contains one or more selected from the group consisting of Group 1 and Group 2, the balance being composed of Fe and impurities, The C concentration in mass % obtained by point-analyzing a region containing lamellar cementite in the pearlite structure using a TEM-EDX apparatus is defined as C C (%), the V concentration in mass % obtained by the point analysis is defined as V C (%), when satisfies formula (1), the microstrain ε is 0.0040 or more. [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: less than 0.50%, and one or more selected from the group consisting of W: less than 0.20% [Group 2] Ca: less than 0.0050% Mg: less than 0.0050% Zr: less than 0.010%, and one or more selected from the group consisting of rare earth elements: less than 0.005% 0.15 ≦ V C / (C C × V) ≦ 1.00 (1) Here, the V in formula (1) is substituted with the V content in mass% in the chemical composition.

Advantages of the Invention

[0012] In the steel wire of the present disclosure, sufficient ductility and excellent durability ratio can be obtained.

Modes for Carrying Out the Invention

[0013] The inventors first examined a steel wire capable of obtaining sufficient ductility and excellent durability ratio from the viewpoint of chemical composition. As a result, if the chemical composition contains, 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%, and the balance consists of Fe and impurities, it was considered that sufficient ductility and excellent durability ratio could be obtained.

[0014] However, even in the case of a steel wire having the above chemical composition, an excellent durability ratio may not be obtained. Therefore, the inventors examined the cause. As a result, the following matters were found.

[0015] In the steel wire having the above chemical composition, a large number of dislocations are introduced by wire drawing. As a result, high strength is obtained. 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. As such cancellation increases, the fatigue strength of the steel wire decreases. As a result, the endurance ratio of the steel wire decreases. That is, in order to obtain a steel wire having 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.

[0016] In order to suppress such cancellation of dislocations, it is effective to suppress the movement of dislocations when the steel wire is repeatedly fatigued. Therefore, the present inventors examined means for suppressing the movement of dislocations.

[0017] The structure of the steel wire and the wire rod that is the material of the steel wire 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 a wire rod having a pearlite structure mainly accumulate 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, 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.

[0018] 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.

[0019] Therefore, the present inventors examined the fine precipitates formed in the lamellar ferrite. As a result, it was considered that by further containing V in the above chemical composition, V-based precipitates would be formed in the lamellar ferrite. V-based precipitates are precipitates containing V, such as V carbides and V carbonitrides. In the steel wire having the above chemical composition, the V-based precipitates are formed 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.

[0020] However, it was found that if the V-based precipitates in the lamellar ferrite are excessive, the ductility of the steel wire will rather decrease. Therefore, in order to have sufficient ductility and obtain an excellent endurance ratio, it is necessary to adjust the amount of V-based precipitates in the lamellar ferrite of the steel wire to an appropriate range.

[0021] As described above, since the V-based precipitates are fine, it is extremely difficult to measure the amount of V-based precipitates in the lamellar ferrite by microstructural observation. Therefore, it seemed difficult to quantify the amount of V-based precipitates.

[0022] However, it is possible to quantify the amount of V-based precipitates in the lamellar ferrite by the following means. In the steel wire having the above chemical composition, V is contained not only in the lamellar ferrite but also in the lamellar cementite. Most of the V in the lamellar ferrite exists in the form of precipitates (V-based precipitates). On the other hand, the V in the lamellar cementite exists in a solid solution state (solid solution V). Therefore, by quantifying the V content in the chemical composition of the entire steel wire and the V concentration in the lamellar cementite, the amount of V-based precipitates in the lamellar ferrite can be predicted.

[0023] Under the above assumptions, further examination was carried out on the V content in the chemical composition of the steel wire and the V concentration in the lamellar cementite when the amount of V-based precipitates in the lamellar ferrite was within an appropriate range. As a result, in addition to the above chemical composition, the steel wire containing V: 0.10 to 0.40% and having a microstrain ε representing the amount of dislocations in the steel wire of 0.0040 or more, the V content in mass% in the chemical composition of the steel wire and the V concentration V in mass% obtained by point analysis of the region containing lamellar cementite using a TEM-EDX apparatus C (%), and further the C concentration C in mass% obtained by the point analysis C (%) are used, and V C / (C C ×V) is in the range of 0.15 to 1.00, the inventors have found that the amount of V-based precipitates in the lamellar ferrite is in an appropriate range, so sufficient ductility and excellent endurance ratio can be obtained.

[0024] The steel wire of this embodiment is completed based on the above technical idea and has the following configuration.

[0025] The steel wire of the first configuration 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, When the C concentration in mass% obtained by point analysis of the region containing lamellar cementite in the pearlite structure using a TEM-EDX apparatus is defined as C C (%), and the V concentration in mass% obtained by the point analysis is defined as V C (%), when Equation (1) is satisfied, The microstrain ε is 0.0040 or more. 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.

[0026] The steel wire of the second configuration 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 further contains one or more selected from the group consisting of the first group and the second group, the balance consists of Fe and impurities, When the C concentration in mass % obtained by point-analyzing the region containing lamellar cementite in the pearlite structure using a TEM-EDX apparatus is defined as C C (%), and the V concentration in mass % obtained by the point analysis is defined as V C (%), formula (1) is satisfied, and the microstrain ε is 0.0040 or more. [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 One or more selected from the group consisting of 0.20% or less of W [Group 2] 0.0050% or less of Ca 0.0050% or less of Mg 0.010% or less of Zr, and One or more selected from the group consisting of 0.005% or less of rare earth elements 0.15 ≦ V C / (C C × V) ≦ 1.00 (1) Here, in formula (1), the content of V in mass % in the chemical composition is substituted for V

[0027] The steel wire of the third configuration is The steel wire of the second configuration, Containing the first group

[0028] The steel wire of the fourth configuration is The steel wire of the second or third configuration, Containing the second group

[0029] Hereinafter, the steel wire according to the present embodiment will be described in detail. Note that “%” regarding elements means mass % unless otherwise specified

[0030] [Features of the steel wire of the present embodiment] The steel wire of the present embodiment includes the following features (Feature 1) The chemical composition, in mass %, is as follows: 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 the region containing lamellar cementite using a TEM-EDX apparatus is defined as C C (%), and the V concentration in mass % obtained by the point analysis is defined as V C (%), the following formula (1) is satisfied. 0.15 ≤ V C / (C C × V) ≤ 1.00 (1) Here, the V in formula (1) is substituted with the V content in mass % in the chemical composition. (Feature 3) The microstrain ε in the steel wire is 0.0040 or more. Hereinafter, Features 1 to 3 will be described.

[0031] [(Feature 1) Regarding the chemical composition] The chemical composition of the steel wire of this embodiment contains the following elements.

[0032] C: 0.70 to less than 1.10% Carbon (C) increases the strength of the steel wire. 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 obtained sufficiently. On the other hand, if the C content is 1.10% or more, primary cementite is generated excessively. Therefore, even if the contents of other elements are within the range of this embodiment, the ductility of the steel wire decreases. Therefore, the C content is less than 0.70 to 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%.

[0033] Si: 0.10 to 1.00% Silicon (Si) increases the strength of the steel wire. Si further deoxidizes the steel in the steelmaking process during the manufacturing process of the steel wire. 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 ductility of the steel wire 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 0.80%.

[0034] Mn: 0.20 to 1.00% Manganese (Mn) increases the hardenability of the wire rod used as the material of the steel wire and increases the strength of the steel wire. 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 steel wire. 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 ductility of the steel wire decreases. Therefore, the Mn content is 0.20 to 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%.

[0035] V: 0.10 - 0.40% Vanadium (V) forms V-based precipitates and increases the strength of the steel wire 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 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 ductility of the steel wire decreases. 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%.

[0036] P: 0.020% or less Phosphorus (P) is an impurity. That is, the lower limit of the P content is more than 0%. If the P content exceeds 0.020%, P segregates at the 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 decreases. Moreover, if the P content exceeds 0.020%, the ductility of the steel wire further decreases. Therefore, the P content is 0.020% or less. It is preferable that the P content is 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%.

[0037] S: Below 0.020% Sulfur (S) is an impurity. That is, the lower limit of the S content is more than 0%. If the S content exceeds 0.020%, S segregates at the grain boundaries, embrittling the grain boundaries. Therefore, even if the contents of other elements are within the range of this embodiment, the strength of the steel wire decreases. If the S content exceeds 0.020%, furthermore, the ductility of the steel wire 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%.

[0038] Al: 0.080% or less Aluminum (Al) is an impurity. That is, the lower limit of the Al content is more than 0%. If the Al content exceeds 0.080%, excessive Al oxides are formed in the steel wire. Therefore, even if the contents of other elements are within the range of this embodiment, the ductility of the steel wire decreases. Therefore, the Al content is 0.080% or less. It is preferable that the Al content be as low as possible. However, excessive reduction of the Al content increases 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%.

[0039] 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%, the dissolved N promotes age hardening. Therefore, even if the contents of other elements are within the range of this embodiment, the ductility of the steel wire decreases. Therefore, the N content is 0.0100% or less. The N content is preferably as low as possible. However, excessive reduction of the N content increases the manufacturing cost. Therefore, considering normal industrial production, the preferable lower limit of the N content is 0.0001%, more preferably 0.0005%, still 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 still more preferably 0.0050%.

[0040] O: 0.0050% or less Oxygen (O) is an impurity. That is, the lower limit of the O content is more than 0%. If the O content exceeds 0.0050%, coarse oxides are formed in the steel wire. Therefore, even if the contents of other elements are within the range of this embodiment, the ductility of the steel wire decreases. Therefore, the O content is 0.0050% or less. The O content is preferably 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%.

[0041] The balance of the chemical composition of the steel wire according to this 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 when the steel wire is industrially manufactured, and are allowed within a range that does not adversely affect the steel wire according to this embodiment.

[0042] [Regarding Optional Elements] The chemical composition of the steel wire of this 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: Below 0.50%, Cu: Below 0.50%, 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 Hereinafter, these optional elements will be described.

[0043] [Group 1: Cr, Cu, Ni, Sn, Mo, Nb, Ti, B, Co and W] The chemical composition of the steel wire of this embodiment may further contain the above 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. Hereinafter, each element of Group 1 will be described.

[0044] 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 enhances the hardenability of the wire rod that is the material of the steel wire and increases the strength of the steel wire. Even if a little 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 steel wire. In this case, bainite or martensite is generated in the region where Cr segregates. Therefore, even if the contents of other elements are within the range of this embodiment, the ductility of the steel wire 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%.

[0045] 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 enhances the hardenability of the wire rod that is the material of the steel wire and increases the strength of the steel wire. Even if a small amount of Cu is contained, the above effects can be obtained to a certain extent. On the other hand, if the Cu content exceeds 0.50%, the hardness of the steel wire excessively increases. Therefore, even if the contents of other elements are within the range of this embodiment, the ductility of the steel wire 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%.

[0046] 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 steel wire. As a result, a fine lamellar structure of pearlite is formed. Consequently, the strength of the steel wire is increased. Even if a small amount of Ni is contained, the above effects can be obtained to a certain extent. On the one hand, if the Ni content exceeds 0.50%, the hardenability of the wire rod used as the material of the steel wire becomes excessively high. In this case, bainite is generated in the steel wire. Therefore, even if the contents of other elements are within the range of the present embodiment, the ductility of the steel wire 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%.

[0047] 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 steel wire. As a result, the strength of the steel wire is increased. If even a small amount of Sn is contained, the above effects can be obtained to some 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 the present embodiment, the ductility of the steel wire 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%.

[0048] 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 enhances the hardenability of the wire rod that is the material of the steel wire and increases the strength of the steel wire. Even if 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 that is the material of the steel wire becomes excessively high. In this case, bainite is generated in the steel wire. Therefore, even if the contents of other elements are within the range of this embodiment, the ductility of the steel wire 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%.

[0049] 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 contained, that is, when the Nb content exceeds 0%, Nb forms precipitates that are carbides and / or carbonitrides and increases the strength of the steel wire. Even if a small amount is contained, the above effects can be obtained to a certain extent. On the other hand, if the Nb content exceeds 0.100%, excessive precipitates are generated. In this case, even if the contents of other elements are within the range of this embodiment, the ductility of the steel wire decreases. Therefore, the Nb content is 0 to 0.100%, and when 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%.

[0050] 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 and / or nitrides, increasing the strength of the steel wire. 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 ductility of the steel wire 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%, and more preferably 0.010%. The preferable upper limit of the Ti content is 0.090%, and more preferably 0.080%.

[0051] 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 that is the material of the steel wire and increases the strength of the steel wire. Even if B is contained in a small amount, the above effect 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 ductility of the steel wire 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%.

[0052] 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 contained, that is, when the Co content is more than 0%, Co enhances the hardenability of the wire rod that is the material of the steel wire and increases the strength of the steel wire. 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 steel wire becomes excessively hard. Therefore, even if the contents of other elements are within the range of this embodiment, the ductility of the steel wire decreases. Therefore, the Co content is 0 to 0.50%, and when 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%.

[0053] 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 is more than 0%, W enhances the hardenability of the wire rod that is the material of the steel wire and increases the strength of the steel wire. 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%, the hot workability of the steel wire decreases even if the contents of other elements are within the range of this embodiment. 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%.

[0054] [Group 2: Ca, Mg, Zr, and rare earth elements (REM)] The chemical composition of the steel wire of this embodiment may further contain the above-mentioned Group 2 in place of a part of Fe. These elements are optional elements, and all of them enhance the ductility of the steel wire.

[0055] Ca: Below 0.0050% 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 ductility of the steel wire. The above effects can be obtained to a certain extent if Ca is contained even slightly. On the other hand, if the Ca 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 ductility of the steel wire 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%.

[0056] 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 contained, that is, when the Mg content exceeds 0%, Mg forms fine oxides. The fine oxides refine the microstructure of the steel wire and enhance the ductility of the steel wire. The above effects can be obtained to a certain extent if Mg is contained even slightly. 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 ductility of the steel wire decreases. Therefore, the Mg content is 0 to 0.0050%, and when 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%.

[0057] 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 contained, that is, when the Zr content exceeds 0%, Zr forms fine oxides. The fine oxides refine the microstructure of the steel wire and enhance the ductility of the steel wire. Even if a small amount of Zr is contained, the above effects can be obtained to a certain 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 scope of this embodiment, the ductility of the steel wire 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%.

[0058] Rare earth element: 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 ductility of the steel wire increases. Even if a small amount of REM is contained, the above effects 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 scope of this embodiment, the ductility of the steel wire 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%.

[0059] In addition, in this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, which are lanthanoids. Further, the REM content in this specification refers to the total content of these elements.

[0060] [(Characteristic 2) Regarding formula (1)] The steel wire of this embodiment further defines the C concentration in mass% obtained by point-analyzing the region containing lamellar cementite using a TEM-EDX apparatus as C C (%), and defines the V concentration in mass% obtained by the point analysis as V C (%). In this case, the steel wire of this embodiment further satisfies formula (1). 0.15 ≦ V C / (C C × V) ≦ 1.00 (1) Here, the V in formula (1) is substituted with the V content in mass% in the chemical composition.

[0061] 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 steel wire. At this time, the amount of V-based precipitates existing 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 steel wire. At this time, the amount of V-based precipitates existing in lamellar ferrite is large.

[0062] C C and V C are obtained by point-analyzing a region containing lamellar cementite using a TEM (Transmission Electron Microscope)-EDX (Energy Dispersive X-ray Spectrometry) apparatus. In this point analysis, the region irradiated with the electron beam of the TEM-EDX apparatus becomes the measurement region. On the other hand, the thickness of the lamellar cementite in the steel wire of the present embodiment may be smaller than the electron beam diameter of the TEM-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 the lamellar cementite is uniform regardless of the chemical composition of the steel wire. Therefore, the V content V C obtained by point analysis of the region containing lamellar cementite is divided by the C concentration C C obtained by the point analysis, whereby it is possible to mitigate the error caused by the area ratio of the lamellar ferrite contained in the measurement region of the point analysis.

[0063] If F1 is less than 0.15, the V concentration in the lamellar cementite is too low with respect to the V content of the entire steel wire. That is, the amount of V-based precipitates present in the lamellar ferrite is too large. As a result, the ductility of the steel wire decreases. If F1 exceeds 1.00, the V concentration in the lamellar cementite is too high with respect to the V content of the entire steel wire. That is, there are too few V-based precipitates present in the lamellar ferrite. Therefore, the movement of dislocations due to fatigue is not sufficiently suppressed. As a result, an excellent endurance ratio cannot be obtained.

[0064] If F1 is between 0.15 and 1.00, the V concentration in the lamellar cementite is appropriate with respect to the V content of the entire steel wire. That is, the amount of V-based precipitates present in the lamellar ferrite is also appropriate. As a result, assuming that the steel wire satisfies Feature 1 and Feature 3, sufficient ductility and an excellent endurance ratio can be obtained. Therefore, F1 is between 0.15 and 1.00. The preferred lower limit of F1 is 0.16, more preferably 0.17, and even more preferably 0.18. The preferred upper limit of F1 is 0.90, more preferably 0.80, and even more preferably 0.70. Note that F1 is the value up to the second decimal place obtained by rounding the third decimal place of the obtained numerical value.

[0065] [C C and V C measurement method] C C and V C are specifically determined by the following method. Perform FIB (Focused Ion Beam) processing on the steel wire to obtain a thin film test piece with a cross-section parallel to the axial direction of the steel wire as the observation surface. The thickness of the thin film test piece is 100 nm. The observation surface includes the central axis of the steel wire. The size of the observation surface is not particularly limited. Four observation fields are determined within a strip-shaped region with a width of 20 μm centered on the central axis of the steel wire in the observation surface. The size of each observation field is 0.25 μm × 0.25 μm. Also, the observation fields do not overlap with each other. Observe the determined four observation fields at a magnification of 100,000 times using a transmission electron microscope (TEM-EDX device) equipped with an elemental analysis function.

[0066] Based on the contrast obtained from the TEM observation of each observation field, identify the lamellar cementite region. Aim at the identified lamellar cementite region and perform point analysis using EDX (Energy Dispersive X-ray Analyzer) attached to the TEM. In the EDX analysis, the acceleration voltage is 200 kV and the analysis time is 30 seconds.

[0067] Calculate the arithmetic mean value from the C concentration in mass% obtained by point analysis in the four observation fields. The obtained arithmetic mean value is C C (%). Similarly, calculate the arithmetic mean value from the V concentration in mass% obtained by point analysis in the four observation fields. The obtained arithmetic mean value is V CLet it be (%) . 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, different lamellar cementite regions are identified 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 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.

[0068] [(Feature 3) Regarding microstrain]

[0069] The steel wire of this embodiment further has a microstrain ε in the steel wire of 0.0040 or more.

[0070] Microstrain ε is an index representing the amount of dislocations in the steel wire. As described above, in the steel wire, large dislocations are introduced by wire drawing, and high strength is obtained. If the microstrain ε in the steel wire is less than 0.0040, the amount of dislocations in the steel wire is too small. As a result, even if the steel wire satisfies Features 1 and 2, an excellent durability ratio cannot be obtained.

[0071] If the microstrain ε in the steel wire is 0.0040 or more, the amount of dislocations in the steel wire is sufficiently large. As a result, on the premise that the steel wire satisfies Features 1 and 2, an excellent durability ratio can be obtained. Therefore, the microstrain ε in the steel wire is 0.0040 or more. The preferable lower limit of microstrain ε is 0.0045, and more preferably 0.0050. The upper limit of microstrain ε is not particularly limited. Considering normal industrial production, for example, it is 0.0085, and more preferably 0.0080.

[0072] [Measurement method of microstrain ε] The microstrain ε in the steel wire is determined by the following method.

[0073] A test piece including a cross section parallel to the axial direction of the steel wire is taken from the steel wire, with the center of the observation surface located on the central axis of the steel wire. After wet polishing the observation surface, chemical polishing is carried out using a mixed solution of hydrogen peroxide and hydrofluoric acid. Among the observation surfaces after chemical polishing, the X-ray diffraction intensity is measured using an X-ray diffractometer at an arbitrary measurement point on the central axis of the steel wire. The radiation source of the X-ray diffractometer is Cu—Kα, the accelerating voltage is 50 kV, and the accelerating current is 300 mA. Based on the obtained diffraction intensity profile, the microstrain is determined by the DF method (Direct fitting method) described in Non-Patent Document 1.

[0074] The method for calculating the microstrain by the DF method is as follows. Analyze the diffraction intensity profile obtained by X-ray diffraction measurement to obtain the diffraction angle 2θ and the half-width β (rad) of the (111), (200), (211), (220), (310), and (222) diffraction planes. From the diffraction angle 2θ and the half-width β (rad), K and ΔK of each diffraction plane defined by the following equations are obtained. K = 2×sinθ / λ (a) ΔK = β×cosθ / λ (b) In equations (a) and (b), λ is the wavelength λ (nm) of the X-ray used in the X-ray diffraction measurement. Regarding the relationship between K and ΔK, the Williamson-Hall equation shown in equation (c) is known. ΔK = α + ε×K (c) Here, α is a value depending on the crystal grain size, and ε represents the microstrain.

[0075] Equation (c) indicates that the relationship between K and ΔK (hereinafter referred to as the WH plot) is linear. However, in reality, due to the influence of elastic anisotropy for each crystal plane, the linearity of the WH plot decreases and it becomes irregularly distributed. Therefore, in the DF method, ω is used as an index of elastic anisotropy, and the Williamson-Hall equation of equation (c) is corrected by the following equation. ΔK = α + ε×(K / ω) (d) Specifically, an appropriate value is introduced for ω in formula (d), and the fitting index of a plot with the horizontal axis being K / ω and the vertical axis being ΔK (hereinafter referred to as the corrected WH plot) is obtained. Here, the fitting index is an index indicating the accuracy of the linear relationship between K / ω and ΔK. The relationship between the value of ω and the fitting index is sorted out, and ω at which the fitting index takes the maximum value (the best linear relationship is obtained) is determined. Based on the corrected WH plot when ω at which the best linear relationship is obtained is substituted, the microstrain ε is obtained. Note that the microstrain ε is a value up to the fourth decimal place obtained by rounding the fifth decimal place.

[0076] [Effect of the steel wire of the present embodiment] The steel wire of the present embodiment satisfies features 1 to 3. Therefore, with the steel wire of the present embodiment, sufficient ductility and excellent durability ratio can be obtained.

[0077] [Microstructure of the steel wire of the present embodiment] The microstructure of the steel wire of the present embodiment substantially consists of a pearlite structure. Note that it is already known that the microstructure of steel wires for uses such as beads for automobile tires, wire ropes for bridges, wire ropes for elevators, PC steel wires, and wires for power transmission is substantially a pearlite structure.

[0078] [Tensile strength of the steel wire of the present embodiment] The tensile strength of the steel wire of the present embodiment is 1800 MPa or more. Note that the tensile strength is obtained by conducting a tensile test in accordance with JIS Z 2241:2011 at normal temperature in the atmosphere.

[0079] [Applications applicable to the steel wire of the present embodiment] The steel wire of the present embodiment is applicable to, for example, beads for automobile tire applications, wire ropes for bridge applications, wire ropes for elevator applications, PC steel wires, and wires for power transmission applications.

[0080] [An example of the manufacturing method of the steel wire of the present embodiment] An example of the method for manufacturing the steel wire of the present embodiment will be described. The method for manufacturing the steel wire to be described hereinafter is an example for manufacturing the steel wire of the present embodiment. Therefore, the steel wire having the above-described configuration may be manufactured by other manufacturing methods other than the manufacturing method to be described hereinafter. However, the manufacturing method to be described hereinafter is a preferable example of the method for manufacturing the steel wire of the present embodiment.

[0081] An example of the method for manufacturing the steel wire of the present embodiment includes the following steps. (Step 1) Material preparation step (Step 2) Block rolling step (Step 3) Finish rolling step (Step 4) Wire drawing process (Step 5) Low-temperature heat treatment step By Steps 1 to 3, a wire rod that is a material for the steel wire is manufactured. By Steps 4 and 5, a steel wire is manufactured using the wire rod as a material. Hereinafter, each step will be described.

[0082] [(Step 1) Material preparation step] In the material preparation step, the material for the steel wire of the present 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, which is well known, is performed on the molten steel tapped from the converter. By the above steps, molten steel having a chemical composition that satisfies Feature 1 is manufactured.

[0083] Using the manufactured 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 molten steel. Alternatively, a bloom may be manufactured by a continuous casting method using molten steel. By the above methods, a material (ingot or bloom) is manufactured.

[0084] [(Step 2) Block rolling step] In the block rolling step, block rolling is performed on the material (ingot or bloom) prepared in the material preparation step to manufacture a billet. In the 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.

[0085] The heated material is rolled using a block rolling mill, or a block rolling mill and a continuous rolling mill, to produce a billet. Specifically, the heated material is reversely rolled using a block rolling mill to produce a billet. When a well-known continuous rolling mill is arranged downstream of the block rolling mill, tandem rolling may be further performed on the billet after block rolling using the continuous rolling mill to produce a smaller-sized billet. The produced billet is air-cooled to room temperature before the finish rolling process.

[0086] [(Process 3) Finish rolling process] In the finish rolling process, finish rolling is performed on the billet produced in the block rolling process to produce a wire rod. The finish rolling process includes the following processes. (Process 31) Heating process (Process 32) Rolling process (Process 33) Cooling process Hereinafter, the heating process, rolling process, and cooling process in the finish rolling process will be described.

[0087] [(Process 31) Heating process] In the heating process, first, the billet is heated to 950 °C (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 of T2 (°C) or higher. 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).

[0088] In the heating process, the following conditions are satisfied. (Condition 1) The holding time t2 (minutes) at a temperature of T2 (°C) or higher is 10 minutes or more.

[0089] The V-based precipitates in the billet dissolve into the matrix phase at a temperature of the second heating temperature T2 (°C) or higher. At this time, if the holding time t2 at a temperature of the second heating temperature T2 (°C) or higher is less than 10 minutes, the dissolution of the V-based precipitates in the heating process is insufficient. Therefore, the enrichment of V into 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 steel wire 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. Considering normal industrial production, the upper limit of the holding time t2 is, for example, 60 minutes.

[0090] [(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.

[0091] [(Process 33) Cooling Process] In the cooling process, the wire rod after the rolling process is cooled. The wire rod is coiled at a temperature of 750 to 900 °C. The cooling rate after coiling is such that the average cooling rate from 700 °C to 600 °C is v1 (°C / second), and the average cooling rate at 600 °C or lower is 10 °C / second or more.

[0092] 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, the V in Equation (B) is substituted with the V content in mass % in the chemical composition of the steel wire.

[0093] Lower limit average cooling rate v L (°C / second), and upper limit average cooling rate v U(°C / second) is 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. Further, due to the interfacial 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 as follows, the V-based precipitates formed in the lamellar ferrite become excessive. In this case, F1 becomes low and the steel wire 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 as follows, the transformation to pearlite structure is insufficient and the V-based precipitates formed in the lamellar ferrite also become excessively small. In this case, F1 becomes high and the steel wire does not satisfy Feature 2. Therefore, the average cooling rate v1 from 700°C to 600°C shall be in the range that satisfies Equation (B).

[0094] Note that 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 the upstream to the downstream of the conveying table. 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.

[0095] Note that if the chemical composition satisfies Feature 1, by cooling at the above cooling rate, the microstructure of the wire rod becomes substantially pearlite structure. Therefore, the microstructure of the steel wire manufactured by the following Process 4 and Process 5 using the wire rod as a material also becomes substantially pearlite structure.

[0096] [(Process 4) Wire Drawing Process] In the wire drawing process, wire drawing is performed on the manufactured wire rod. The wire drawing may be only primary wire drawing, or multiple wire drawing processes such as secondary wire drawing may be performed. When performing multiple wire drawing processes, after the wire drawing process, before performing the next wire drawing process, a well-known patenting process may be performed.

[0097] [(Process 5) Low-Temperature Heat Treatment Process] In the low-temperature heat treatment process, the steel wire after the wire drawing process is held at a predetermined heating temperature for a certain period of time. The low-temperature heat treatment may be, for example, a bluing process, or a heat diffusion process of electroplating such as electroplating or brass plating. The heating temperature in the low-temperature heat treatment is, for example, 300 to 500 °C. In the heat treatment process, unnecessary residual stress that adversely affects the endurance ratio of the steel wire is removed.

[0098] In the low-temperature heat treatment process, the following conditions are satisfied. (Condition 3) The heating temperature T3 (°C) of the steel wire in the low-temperature heat treatment process and the holding time t3 (seconds) at the heating temperature T3 (°C) satisfy Equation (C). (T3 + 273) × log(t3) ≤ 1200 (C) Here, log in Equation (C) means the common logarithm.

[0099] Define the low-temperature heat treatment parameter H as follows. H = (T3 + 273) × log(t3) The low-temperature heat treatment parameter H is an index of the amount of heat applied to the steel wire in the low-temperature heat treatment process. As the heating temperature T3 in the low-temperature heat treatment process increases, the low-temperature heat treatment parameter H also increases. Also, as the holding time t3 in the low-temperature heat treatment process increases, the low-temperature heat treatment parameter H also increases. In the low-temperature heat treatment process, the dislocations introduced in the wire drawing process move and cancel each other out. As a result, the microstrain ε in the steel wire decreases. The larger the low-temperature heat treatment parameter H, the more the movement of dislocations in the low-temperature heat treatment process is promoted. If the low-temperature heat treatment parameter H is within an appropriate range and there are sufficiently many V-based precipitates in the steel wire, the movement of dislocations in the low-temperature heat treatment process can be suppressed.

[0100] If the low-temperature heat treatment parameter H exceeds 1200, even for a steel wire satisfying Feature 1 and Feature 2, the amount of dislocation annihilation in the low-temperature heat treatment process is too large. Therefore, the microstrain ε in the steel wire becomes low, and the steel wire does not satisfy Feature 3. Therefore, the low-temperature heat treatment parameter H is 1200 or less. Note that the lower limit of the low-temperature heat treatment parameter H is not particularly limited. Considering normal industrial production, the lower limit of the low-temperature heat treatment parameter H is, for example, 450.

[0101] Through the above steps, the steel wire of this embodiment is manufactured.

Example

[0102] The effects of the steel wire of this embodiment will be further specifically described by examples. The conditions in the following examples are one set of conditions adopted to confirm the feasibility and effects of the steel wire of this embodiment. Therefore, the steel wire of this embodiment is not limited to this one set of conditions.

[0103] [Material preparation process] Steel wires having the chemical compositions shown in Table 1A and Table 1B were manufactured by the following method.

[0104]

Table 1A

[0105]

Table 1B

[0106] A blooming mill was subjected to a block rolling process to produce billets. Specifically, the blooming mill was heated to 1100°C using a heating furnace. The heated blooming mill was rolled using a block rolling mill and a continuous rolling mill to produce billets. The billets produced in the block rolling process were cooled to room temperature.

[0107] A finishing rolling process was carried out on the manufactured billets to produce wire rods of each test number. In the finishing rolling process, first, a heating process was carried out on the billets manufactured in the block rolling process. Specifically, using a heating furnace, the billets of each test number were heated to 950 °C. After holding at 950 °C for 10 minutes, the billets of each test number were further heated to a second heating temperature T2 (°C). Further, the billets were 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 T2 (°C) or higher for each test number are shown in Table 2.

[0108]

Table 2

[0109] A rolling process was carried out on the billets after the heating process. In the rolling process, using a continuous rolling mill, finishing rolling (continuous rolling) was carried out to produce wire rods.

[0110] A 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 - 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) for each test number 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 manufactured.

[0111] A wire drawing process was carried out on the manufactured wire rods. In the wire drawing process, multiple wire drawing processes were carried out to produce steel wires with a wire diameter of 1.5 mm.

[0112] A low-temperature heat treatment process was carried out on the steel wire after wire drawing. Specifically, bluing treatment was carried out on the steel wire. The heating temperature T3 (°C), holding time t3 (seconds), and low-temperature heat treatment parameter H in the bluing treatment for each test number were as shown in Table 2. The steel wires of each test number were manufactured through the above manufacturing process.

[0113] [Regarding the evaluation test] The following evaluation tests were carried out on the manufactured steel wires of each test number. (Test 1) Measurement test of C C and V C (Test 2) Measurement test of the microstrain ε in the steel wire (Test 3) Ductility evaluation test (Test 4) Endurance ratio evaluation test The following describes each test.

[0114] [(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 steel wires 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 3.

[0115]

Table 3

[0116] [(Test 2) Measurement test of the microstrain ε in the steel wire] Based on the above [measurement method of the microstrain ε], the microstrain ε in the steel wires of each test number was obtained. The obtained results are shown in Table 3.

[0117] [(Test 3) Ductility evaluation test] The ductility of the steel wires of each test number was evaluated by the following method. ​Two tensile test specimens were taken from an arbitrary position of the steel wire. The wire diameter of the tensile test specimen was 1.5 mm and the length was 250 mm. A tensile test conforming to JIS Z 2241:2011 was carried out on the tensile test specimen at normal temperature in the atmosphere, and the reduction of area and tensile strength (MPa) were obtained. In the tensile test, the crosshead speed was set to 10 mm / min, the chuck distance was set to 150 mm, and the extensometer interval was set to 100 mm. The arithmetic mean value of the reduction of area obtained from the two tensile test specimens was taken as the reduction of area (%) of the steel wire for each test number. The arithmetic mean value of the tensile strength obtained from the two tensile test specimens was taken as the tensile strength (MPa) of the steel wire for each test number. Note that the tensile strength of the steel wire for each test number is used in Test 4 described later.

[0118] When the reduction of area was 10% or more, the evaluation was "E (Excellent)", and it was judged that sufficient ductility was obtained (denoted by "E" in the "Ductility" column in Table 3). On the other hand, when the reduction of area was less than 10%, the evaluation was "B (Bad)", and it was judged that sufficient ductility was not obtained (denoted by "B" in the "Ductility" column in Table 3).

[0119] [(Test 4) Endurance ratio evaluation test] The endurance ratio of the steel wire for each test number was determined by the following method. Test specimens with a wire diameter of 1.5 mm and a length of 400 mm were taken from an arbitrary position of the steel wire for 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 section) was 100 mm, the rotation speed was 3000 rpm, and the maximum number of repetitions was 1.0×10 7 cycles. Among the test specimens for 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 for each test number.

[0120] Based on the fatigue strength (MPa) of the steel wire for each test number obtained and the tensile strength (MPa) of the steel wire for each test number obtained in Test 3, the endurance ratio of the steel wire for each test number was determined by the following formula. Endurance ratio = Fatigue strength (MPa) / Tensile strength (MPa)

[0121] When the durability ratio is 0.38 or more, the evaluation is "E (Excellent)", and it is judged that an excellent durability ratio has been obtained (indicated by "E" in the "Durability Ratio" column in Table 3). On the other hand, when the durability ratio is less than 0.38, the evaluation is "B (Bad)", and it is judged that an excellent durability ratio has not been obtained (indicated by "B" in the "Durability Ratio" column in Table 3).

[0122] [Test Results] Referring to Table 1A, Table 1B, Table 2, and Table 3, the steel wires of Test Nos. 1 to 21 satisfied Features 1 to 3. Therefore, sufficient ductility and an excellent durability ratio were obtained.

[0123] On the other hand, in Test No. 22, the V content in the chemical composition of the steel wire was too low. Therefore, the microstrain ε was too low. As a result, an excellent durability ratio was not obtained.

[0124] In Test No. 23, the V content in the chemical composition of the steel wire was too high. As a result, sufficient ductility was not obtained.

[0125] In Test Nos. 24 and 25, the holding time t2 (minutes) at a temperature of 2 or more of the second heating temperature T2 (°C) was less than 10 minutes. Therefore, F1 was too low. As a result, sufficient ductility was not obtained.

[0126] 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 ductility was not obtained.

[0127] 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 microstrain ε in the steel wire was too low. As a result, an excellent durability ratio was not obtained.

[0128] In Test Nos. 30 and 31, the low-temperature heat treatment parameter H was too high. Therefore, the microstrain ε in the steel wire was too low. As a result, an excellent durability ratio was not obtained.

[0129] 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%, 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, the balance consisting of Fe and impurities, The C concentration in mass % obtained by point analysis of the region containing lamellar cementite in the pearlite structure using a TEM-EDX apparatus 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), the microstrain ε is 0.0040 or more, steel wire. 0.15 ≤ V Ｃ / (C Ｃ × V) ≤ 1.00 (1) Here, the V in formula (1) is substituted with the V content in mass% in the chemical composition.

2. The chemical composition is by 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, contains, further contains one or more selected from the group consisting of Group 1 and Group 2, the balance consisting of Fe and impurities, The C concentration in mass% obtained by point analysis of the region containing lamellar cementite in the pearlite structure using a TEM-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), the microstrain ε is 0.0040 or more, steel wire. [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, the V in formula (1) is substituted with the V content in mass% in the chemical composition.

3. The steel wire according to Claim 2, containing the above Group 1, steel wire.

4. The steel wire according to Claim 2, containing the above Group 2, steel wire.

Images