WO2020230880A1 - 鋼線、及び熱間圧延線材 - Google Patents

鋼線、及び熱間圧延線材 Download PDF

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WO2020230880A1
WO2020230880A1 PCT/JP2020/019393 JP2020019393W WO2020230880A1 WO 2020230880 A1 WO2020230880 A1 WO 2020230880A1 JP 2020019393 W JP2020019393 W JP 2020019393W WO 2020230880 A1 WO2020230880 A1 WO 2020230880A1
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Prior art keywords
steel wire
less
wire
amount
ferrite
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PCT/JP2020/019393
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English (en)
French (fr)
Japanese (ja)
Inventor
真 小此木
直樹 松井
山▲崎▼　浩一
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日本製鉄株式会社
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Priority to KR1020217030894A priority Critical patent/KR102588222B1/ko
Priority to CN202080026373.XA priority patent/CN113710821B/zh
Priority to JP2021519499A priority patent/JP7151885B2/ja
Publication of WO2020230880A1 publication Critical patent/WO2020230880A1/ja

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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
    • C21D8/065—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

  • the present disclosure relates to steel wire and hot rolled wire.
  • the present application claims priority based on Japanese Patent Application No. 2019-092640 filed in Japan on May 16, 2019, the contents of which are incorporated herein by reference.
  • cold forging is excellent in dimensional accuracy and productivity of molded products, it is possible to switch from conventional hot forging to cold forging when molding mechanical parts such as steel bolts, screws, and nuts. It is expanding. Further, parts such as bolts and nuts are often used for structural purposes, and for this reason, alloying elements such as C, Mn, and Cr are added to impart strength.
  • the shape of parts has become complicated for the purpose of reducing parts manufacturing costs and improving the functionality of parts. Therefore, the steel material used for cold forging is required to be soft and have extremely high ductility. Therefore, conventionally, the hot-rolled material has been softened by heat treatment such as spheroidizing annealing to improve workability.
  • the workability of cold forging steel includes deformation resistance that affects the mold load and ductility that affects the occurrence of machining cracks. Although the required characteristics differ depending on the use of the steel material, it is usual that both deformation resistance and ductility, or one of them, is required.
  • Patent Document 1 a region in which the average particle size of ferrite grains is 2 to 5.5 ⁇ m, the major axis is 3 ⁇ m or less, and the ratio of cementite having an aspect ratio of 3 or less is 70% or more with respect to all cementite is defined from the surface. It is disclosed that the cold workability is improved by setting the wire diameter to 10% or more.
  • Patent Document 2 the standard deviation of the cementite distance divided by the average value of the cementite distance is set to 0.50 or less, that is, by making the distance between cementites substantially uniform, during cold forging.
  • a steel wire having a reduced deformation resistance and a reduced cracking has been disclosed.
  • an average particle size of the ferrite grains is not less than 15 [mu] m, 0.8 [mu] m or less average grain size of globular carbides, the maximum particle size 4.0 ⁇ m or less, 1 mm 2 per 0.5 ⁇ 10 6 ⁇ the number of and C% ⁇ 5.0 ⁇ 10 6 ⁇ C% number particle size is cold forgeability that excellent disclosed by the maximum distance between 0.1 ⁇ m or more globular carbides and 10 ⁇ m or less.
  • Patent Document 1 The method disclosed in Patent Document 1 is effective for processing in which the crack generation position is near the surface of the rolled wire, but is workable for processing in which the crack generation position is inside the rolled wire.
  • the improvement effect is small.
  • the rolled wire is cut and then cold forged. Therefore, in many cases, the vicinity of the surface of the rolled wire does not become the crack generation position, and the effect is limited.
  • Patent Document 3 has a problem that the Cr content is 0.20% or less, the hardenability is low, and the strength of the parts after quenching and tempering becomes unstable as the wire diameter becomes large. ..
  • the means for solving the above problems include the following aspects. ⁇ 1> Ingredient composition is mass%, C: 0.10 to 0.60%, Si: 0.01-0.50%, Mn: 0.20 to 1.00%, P: 0.030% or less, S: 0.050% or less, Cr: 0.85 to 1.50%, Al: 0.001 to 0.080%, N: 0.0010 to 0.0200%, and the balance: Fe and impurity elements.
  • Ingredient composition is mass%, C: 0.10 to 0.60%, Si: 0.01-0.50%, Mn: 0.20 to 1.00%, P: 0.030% or less, S: 0.050% or less, Cr: 0.85 to 1.50%, Al: 0.001 to 0.080%, N: 0.0010 to 0.0200%, and the balance: Fe and impurity elements.
  • a cross section that includes the central axis of the steel wire and is parallel to the central axis. More than 95 area% of the metallographic structure is composed of ferrite and spherical carbides.
  • the spherical carbide has an average aspect ratio of 2.5 or less for the spherical carbide having a diameter equivalent to a circle of 0.1 ⁇ m or more, and the content (mass%) of C contained in the steel wire is represented by [C].
  • a circle number of equivalent diameter 0.1 ⁇ m or more of the globular carbides is 1.5 ⁇ 10 6 ⁇ [C] ⁇ 7.0 ⁇ 10 6 ⁇ [C] number / mm 2, the steel wire. ⁇ 2> The above.
  • the average particle size of the spherical carbide having a diameter equivalent to a circle of 0.1 ⁇ m or more is 0.50 ⁇ m or less, and the maximum particle size of the spherical carbide is 3.00 ⁇ m or less.
  • Steel wire
  • a steel wire containing an alloying element and having excellent cold forging property, and a hot-rolled wire rod for manufacturing the steel wire are provided.
  • the numerical range represented by using “-” means a range including the numerical values before and after “-” as the lower limit value and the upper limit value.
  • the numerical range when "greater than” or “less than” is added to the numerical values before and after “to” means a range in which these numerical values are not included as the lower limit value or the upper limit value.
  • the upper limit value or the lower limit value of the numerical range described stepwise may be replaced with the upper limit value or the lower limit value of the numerical range described stepwise. , Or you may replace it with the value shown in the examples.
  • the content of elements in the component composition may be expressed as an elemental amount (for example, C amount, Si amount, etc.).
  • “%” means “mass%”.
  • the term “process” is included in this term not only as an independent process but also as long as the intended purpose of the process is achieved even when it cannot be clearly distinguished from other processes.
  • the "cross section including the central axis of the steel wire and parallel to the central axis” includes the central axis of the steel wire and is cut along the longitudinal direction (that is, the drawing direction) of the steel wire. A cross section parallel to the axial direction (also referred to as an L cross section) is shown.
  • the "central axis” indicates a virtual line extending in the axial direction through the center point of the cross section orthogonal to the axial direction (longitudinal direction) of the steel wire.
  • the "surface layer portion of the steel wire” indicates a region having a depth of up to 500 ⁇ m from the surface (outer peripheral surface) of the steel wire toward the central axis (in the radial direction).
  • the notation “numerical value XD” is a position at a depth X times the diameter D from the surface of the steel wire toward the central axis (in the radial direction), where D is the diameter of the steel wire. Is shown. For example, "0.25D” indicates a position at a depth of 0.25 times the diameter D.
  • the steel wire according to the present disclosure is a steel wire having a predetermined component composition and having a metal structure satisfying the following (1) and (2).
  • (2) The ferrite has an average particle size of 10.0 ⁇ m or more, and the spherical carbide has an average aspect ratio of a spherical carbide having a circular equivalent diameter of 0.1 ⁇ m or more (hereinafter, simply referred to as “average aspect ratio of spherical carbide”).
  • the number (which may be referred to) is 2.5 or less, and the number of pieces per 1 mm 2 in the L cross section is 1.5 ⁇ 10 6 ⁇ [C] to 7.0 ⁇ 10 6 ⁇ [C] ([[].
  • C] represents the content (mass%) of carbon (C) contained in the steel wire).
  • the steel wire according to the present disclosure is a steel wire having excellent cold forging properties due to the above configuration.
  • the steel wire according to the present disclosure was found based on the following findings.
  • the present inventors can achieve both coarse graining of ferrite grains and miniaturization of carbides even in steel containing 0.85% or more of Cr, and reduce deformation resistance. And succeeded in achieving improvement in ductility at the same time.
  • the structure of the hot-rolled material shall be a structure mainly composed of bainite having a small proeutectoid ferrite fraction.
  • strain is applied to the steel wire by wire drawing with a total surface reduction rate of 20% or more.
  • C Performing spheroidizing annealing at a temperature of Ac 1 or less, Found to be important.
  • the reason why the cold forging property of the steel wire having a structure composed of coarse ferrite grains and fine spherical carbides is excellent is that the coarse carbides that are likely to generate molding cracks and the spherical carbide grains having a large aspect ratio. It is considered that the occurrence of cracks can be suppressed by making the diameter finer, and the strength is lowered and the deformation resistance is reduced by making the ferrite grain size coarse.
  • the steel wire according to the present disclosure can be formed into a complicated shape part by cold forging, and the product yield can be obtained. And productivity are improved. Further, with the steel wire according to the present disclosure, it is possible to integrally mold a complex-shaped part having high strength, which has been difficult in the past. That is, the steel wire according to the present disclosure can be suitably used for machine structural steel used as a material for machine parts such as bolts, screws, and nuts.
  • the steel wire according to the present disclosure can suppress molding cracks, it contributes to high functionality by complicating the shape of parts and improvement of productivity of mechanical parts, and is extremely useful in industry.
  • the composition and metallographic structure of the steel wire according to the present disclosure will be specifically described.
  • the composition of the steel wire according to the present disclosure is, in mass%, C: 0.10 to 0.60%, Si: 0.01 to 0.50%, Mn: 0.20 to 1.00%, P: 0.030% or less, S: 0.050% or less, Cr: 0.85 to 1.50%, Al: 0.001 to 0.080%, N: 0.0010 to 0.0200%, and the balance: It consists of Fe and impurity elements.
  • the steel wire according to the present disclosure may contain an element other than the above instead of a part of Fe, and the component composition is mass%, for example, Ti: 0 to 0.050%, B: 0 to 0.
  • Mo 0 to 0.50%
  • Ni 0 to 1.00%
  • Cu 0 to 0.50%
  • V 0 to 0.50%
  • Nb 0 to 0.050%
  • Ca One or more of 0 to 0.0050%, Mg: 0 to 0.0050%, and Zr: 0 to 0.0050% may be satisfied.
  • Ti, B, Mo, Ni, Cu, V, Nb, Ca, Mg, and Zr are arbitrary elements. That is, these elements do not have to be contained in the steel wire.
  • the reason for limiting the range of the amount of each element contained in the steel wire will be described.
  • C 0.10 to 0.60% C is contained in order to secure the strength as a mechanical part. If the amount of C is less than 0.10%, it is difficult to secure the required strength as a mechanical part. On the other hand, when the amount of C exceeds 0.60%, ductility, toughness, and cold forging property deteriorate. Therefore, the amount of C was set to 0.10 to 0.60%.
  • the amount of C may be 0.15% or more, 0.20% or more, or 0.25% or more.
  • the amount of C may be 0.55% or less, 0.50% or more, or 0.40% or less.
  • Si 0.01 to 0.50%
  • Si is an element that functions as a deoxidizing element, imparts hardenability, improves temper softening resistance, and imparts the strength required for mechanical parts. If the amount of Si is less than 0.01%, these effects are insufficient. When the amount of Si exceeds 0.50%, the ductility and toughness of the mechanical parts are deteriorated, and the deformation resistance of the steel wire is increased to deteriorate the cold forging property. Therefore, the amount of Si was set to 0.01 to 0.50%.
  • the amount of Si may be 0.03% or more, 0.05% or more, or 0.10% or more.
  • the amount of Si may be 0.35% or less, 0.30% or less, or 0.25% or less.
  • Mn 0.20 to 1.00% Mn is an element necessary for imparting hardenability and imparting strength required for mechanical parts. If the amount of Mn is less than 0.20%, the effect is insufficient. If the amount of Mn exceeds 1.00%, the toughness of the mechanical parts deteriorates, the deformation resistance of the steel wire increases, and the cold forging property deteriorates. Therefore, the amount of Mn was set to 0.20 to 1.00%.
  • the amount of Mn may be 0.25% or more, 0.30% or more, or 0.35% or more.
  • the Mn content may be 0.90% or less, 0.85% or less, or 0.80% or less.
  • P 0.030% or less
  • P is contained in the steel wire as an impurity. It is desirable to reduce P because it segregates at the grain boundaries of mechanical parts after quenching and tempering and deteriorates toughness. Therefore, the upper limit of the amount of P is 0.030%.
  • the upper limit of the preferable amount of P is 0.020%.
  • the upper limit of the more preferable amount of P is 0.015% or less, or 0.012% or less.
  • the lower limit of the amount of P is preferably 0% (that is, it is preferable not to include it), but it exceeds 0% (or 0.0001% or more or 0.005% or more) from the viewpoint of reducing the de-P cost. There should be.
  • S (S: 0.050% or less) S is contained in the steel wire as a sulfide such as MnS. These sulfides improve the machinability of steel wire.
  • the upper limit of the amount of S is set to 0.050%.
  • the upper limit of the preferable amount of S is 0.030%.
  • a more preferable upper limit of the amount of S is 0.015% or 0.010%.
  • the lower limit of the amount of S is preferably 0% (that is, it is preferable not to include it), but it exceeds 0% (or 0.0001% or more or 0.005% or more) from the viewpoint of reducing the cost of removing S. There should be.
  • Cr 0.85 to 1.50%
  • Cr is an element required to improve hardenability and impart the required strength to mechanical parts. Further, by containing Cr, the shape of the carbide after annealing becomes spherical, and the cold workability is improved. If the amount of Cr is less than 0.85%, the effect is insufficient. When the amount of Cr exceeds 1.50%, the spheroidizing time becomes long, the manufacturing cost increases, the deformation resistance of the steel wire increases, and the cold forging property deteriorates. Therefore, the amount of Cr was set to 0.85 to 1.50%.
  • the amount of Cr may be 0.87% or more, 0.90% or more, or 0.95% or more.
  • the amount of Cr may be 1.40% or less, 1.30% or less, or 1.20% or less.
  • Al has the effect of functioning as a deoxidizing element and forming AlN to refine austenite crystal grains and improve the toughness of mechanical parts. Further, Al has an effect of fixing the solid solution N, suppressing dynamic strain aging, and reducing deformation resistance. If the amount of Al is less than 0.001%, these effects are insufficient. If the amount of Al exceeds 0.080%, the effect may be saturated and the manufacturability may be lowered. Therefore, the amount of Al was set to 0.001 to 0.080%.
  • the amount of Al may be 0.010% or more, 0.020% or more, or 0.025% or more.
  • the amount of Al may be 0.060% or less, 0.050% or less, or 0.040% or less.
  • N (N: 0.0010 to 0.0200%) N has the effect of forming a nitride with Al, Ti, Nb, V and the like, making austenite crystal grains finer, and improving the toughness of mechanical parts. If the amount of N is less than 0.0010%, the amount of nitride precipitated is insufficient and no effect can be obtained. When the amount of N exceeds 0.0200%, the deformation resistance of the steel wire becomes high due to the dynamic strain aging due to the solid solution N, and the workability deteriorates. Therefore, the amount of N was set to 0.0010 to 0.0200%. The range of the amount of N may be 0.0020% or more, 0.0025% or more, or 0.0030% or more. The amount of N may be 0.0080% or less, less than 0.0050%, or 0.0040% or less.
  • the steel wire according to the present disclosure has Ti: 0 to 0.050%, B: 0 to 0.0050%, Mo: 0 to 0.50%, Ni: 0 to 0 to improve the characteristics described below.
  • 1.00%, Cu: 0 to 0.50%, V: 0 to 0.50%, Nb: 0 to 0.050%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, And Zr: 0 to 0.0050% may contain one or more.
  • the steel wire according to the present disclosure can solve the problem without containing these elements. Therefore, the lower limit of the content of these arbitrary elements is 0%.
  • Ti functions as a deoxidizing element. Furthermore, Ti has the effect of forming nitrides and carbides to refine austenite crystal grains to improve the toughness of mechanical parts, the effect of promoting the formation of solid solution B, and the effect of enhancing hardenability, and solid solution N. It has the effect of fixing and suppressing dynamic strain aging and reducing deformation resistance. If the amount of Ti exceeds 0.050%, these effects may be saturated and coarse oxides or nitrides may be generated to deteriorate the fatigue strength of mechanical parts. Therefore, the amount of Ti may be contained in the range of more than 0 to 0.050%. The amount of Ti may be 0.005% or more, or 0.010% or more. The amount of Ti may be 0.030% or less, or 0.025% or less.
  • B has the effect of segregating into grain boundaries as a solid solution B to improve hardenability and impart the required strength to mechanical parts.
  • the amount of B may be contained in the range of more than 0 to 0.0050%.
  • the amount of B may be 0.0003% or more, or 0.0005% or more.
  • the amount of B may be 0.0030% or less, or 0.0020% or less.
  • Mo 0 to 0.50%
  • Mo has the effect of improving hardenability and imparting the required strength to mechanical parts.
  • the amount of Mo may be contained in the range of more than 0 to 0.50%.
  • the amount of Mo may be 0.10% or more, or 0.15% or more.
  • the amount of Mo may be 0.40% or less, or 0.30% or less.
  • Ni 0 to 1.00%
  • Ni has the effect of improving hardenability and imparting the required strength to mechanical parts.
  • the amount of Ni may be contained in the range of more than 0 to 1.00%.
  • the amount of Ni may be 0.02% or more, or 0.10% or more.
  • the amount of Ni may be 0.50% or less, or 0.30% or less.
  • Cu has the effect of improving hardenability, imparting the necessary strength to mechanical parts, and improving corrosion resistance.
  • the amount of Cu may be contained in the range of more than 0 to 0.50%.
  • the amount of Cu may be 0.02% or more, or 0.10% or more.
  • the amount of Cu may be 0.40% or less, or 0.35% or less.
  • V has the effect of precipitating carbide VC and increasing the strength of mechanical parts.
  • the amount of V may be contained in the range of more than 0 to 0.50%.
  • the amount of V may be 0.01% or more, or 0.05% or more.
  • the amount of V may be 0.20% or less, or 0.15% or less.
  • Nb has the effect of precipitating carbides and nitrides to increase the strength of mechanical parts, the effect of finely granulating austenite grains to improve toughness, the effect of reducing solid solution N, and the effect of reducing deformation resistance. is there.
  • the amount of Nb may be contained in the range of more than 0 to 0.050%.
  • the amount of Nb may be 0.001% or more, or 0.005% or more.
  • the amount of Nb may be 0.030% or less, or 0.020% or less.
  • Ca, Mg and Zr may be used for the purpose of deoxidation. These elements have the effect of making oxides finer and improving fatigue strength. On the other hand, if the content of each of these elements exceeds 0.050%, the effect is saturated and coarse oxides are generated, which may deteriorate the fatigue characteristics. Therefore, the Ca amount, the Mg amount, and the Zr amount may each be contained in the range of more than 0 to 0.050%.
  • the amount of Ca, the amount of Mg, and the amount of Zr may be 0.0001% or more and 0.0005% or more, respectively.
  • the amount of Ca, the amount of Mg, and the amount of Zr may be 0.030% or less, or 0.020% or less, respectively.
  • the balance is Fe and impurity elements.
  • the impurity element refers to, for example, a component contained in a raw material or a component that is unintentionally mixed in a manufacturing process and is not intentionally contained. Further, the impurity element includes a component contained in an amount within a range that does not affect the performance of the steel wire even if the component is intentionally contained.
  • the impurity element examples include O and the like.
  • O exists in the steel wire as an oxide such as Al and Ti. If the amount of O is high, coarse oxides are formed, which causes a decrease in fatigue strength of mechanical parts. Therefore, the amount of O is preferably suppressed to 0.01% or less.
  • the metal structure of the steel wire according to the present disclosure is composed of ferrite and spherical carbide (spherical cementite) in a cross section (L cross section) parallel to the central axis and including the central axis of the steel wire. Will be done. If the metal structure contains a martensite structure, a bainite structure, a pearlite structure, or the like, the deformation resistance increases, the ductility decreases, and the cold forging property deteriorates. Therefore, it is preferable that these structures are not included.
  • the cross section including the central axis of the steel wire and parallel to the central axis, 97 area% or more, 98 area% or more, or 99 area% or more of the metal structure.
  • it may be composed of ferrite and spherical carbide (spherical cementite).
  • the fact that 95 area% or more of the metal structure is composed of ferrite and spherical carbide in the L cross section means that 95 area% or more of the metal structure is formed of ferrite and spherical carbide when the L cross section is observed. means.
  • the area% of ferrite and spherical carbide is calculated by the following procedure. After mirror-polishing the cross section (L cross section) including the central axis of the steel wire and parallel to the central axis, the sample is immersed in nital (5% nitric acid + 95% ethanol solution) at room temperature for 20 seconds to reveal the metal structure. Using this sample, a scanning electron microscope (SEM) was used to measure a depth of 250 ⁇ m (the central part of the surface layer in the depth direction) and a depth of 0.25D (of the steel wire) from the surface (outer peripheral surface of the steel wire).
  • SEM scanning electron microscope
  • the area% of ferrite and spherical carbide can be obtained by subtracting the total area of martensite, bainite and pearlite from the entire imaging field as the area of ferrite and spherical carbide, and dividing this value by the area of the imaging field. it can.
  • the pearlite structure was a structure in which carbides having an aspect ratio (major axis / minor axis) of more than 5.0 were present in layers.
  • the lower limit of the average particle size of the ferrite grains is preferably 10.0 ⁇ m.
  • the preferable lower limit of the average particle size of the ferrite grains is 11.5 ⁇ m.
  • a more preferable lower limit of the average particle size of the ferrite grains is 13.0 ⁇ m.
  • the upper limit of the average particle size of the ferrite grains was set to 30.0 ⁇ m.
  • the upper limit of the average particle size of the preferred ferrite grains is 20.0 ⁇ m.
  • the average particle size of ferrite grains can be measured by an electron backscattering diffraction (EBSD) method. Specifically, as shown in FIG. 1, a depth portion 250 ⁇ m from the surface of a cross section (L cross section) including the central axis C of the steel wire 10 and parallel to the central axis C (central portion in the depth direction of the surface layer portion).
  • EBSD electron backscattering diffraction
  • the measurement step is 1.0 ⁇ m in a region of 500 ⁇ m in the depth direction (diametrical direction) and 500 ⁇ m in the central axis direction, that is, in a region of 500 ⁇ m square shown by A1, A2, and A3 in FIG.
  • the crystal orientation of bcc-Fe at each measurement point in the region is measured.
  • a boundary having an orientation difference of 15 degrees or more is defined as a ferrite grain boundary.
  • a region of 5 pixels or more surrounded by the ferrite grain boundaries is defined as a ferrite grain.
  • Johnson-Saltykov's measurement method (“Measurement Morphology", Uchida Otsuru Farm Shinsha, S47.7.30), which is a method for determining the average particle size of a grain population on the premise of mixing ferrite grains. , Original work: RT DeHoff. F.N.Rbiness. P189). This is performed for two samples, and the average value of the average particle diameters measured in a total of six measurement regions is taken as the average particle diameter of the ferrite grains.
  • the spherical carbide means cementite having an aspect ratio of 5.0 or less represented by the major axis / minor axis of the carbide.
  • the aspect ratio (major axis / minor axis) of the spherical carbide becomes large, cracks are generated from around the strained carbide and it becomes easy to crack.
  • the average aspect ratio of the spherical carbide having a circle equivalent diameter of 0.1 ⁇ m or more exceeds 2.5, the ductility is lowered and processing cracks are likely to occur.
  • the upper limit of the average aspect ratio of spherical carbide having a circle equivalent diameter of 0.1 ⁇ m or more is set to 2.5.
  • the preferable upper limit of the average aspect ratio of the spherical carbide having a circle equivalent diameter of 0.1 ⁇ m or more is 2.0.
  • a more preferable upper limit of the average aspect ratio of the spherical carbide having a circle equivalent diameter of 0.1 ⁇ m or more is 1.8.
  • the particle size of the spherical carbide is not particularly specified.
  • the maximum particle size of the spherical carbide affects the occurrence of molding cracks. When the maximum particle size is reduced, it is possible to prevent cracks from being generated around the strained carbide, and it is possible to more effectively prevent cracks in the steel wire.
  • the upper limit of the maximum particle size of the spherical carbide may be 3.00 ⁇ m.
  • the preferable upper limit of the maximum particle size of the spherical carbide is 2.00 ⁇ m.
  • a more preferable upper limit of the maximum particle size of the spherical carbide is 1.50 ⁇ m.
  • the upper limit of the average particle size of the spherical carbide may be 0.50 ⁇ m.
  • the preferable upper limit of the average particle size of the spherical carbide is 0.40 ⁇ m.
  • a more preferable upper limit of the average particle size of the spherical carbide is 0.32 ⁇ m.
  • the average particle size of the spherical carbide means the average number of circle-equivalent diameters of the spherical carbide. The number average is calculated after excluding spherical carbides with a circle equivalent diameter of less than 0.1 ⁇ m.
  • [C] indicates the C content in the steel wire represented by mass%.
  • Circle preferable lower limit of the number of 1 mm 2 per equivalent diameter 0.1 ⁇ m or more globular carbide, 3.0 ⁇ 10 6 ⁇ [C ] number, or 3.5 ⁇ 10 is a 6 ⁇ [C] number.
  • Circle equivalent diameter preferred upper limit is 6.5 ⁇ 10 6 ⁇ the number of 1 mm 2 per 0.1 ⁇ m or more globular carbides [C] number, or 6.0 ⁇ 10 6 ⁇ [C] number.
  • the maximum particle size of the spherical carbide, the average particle size of the spherical carbide, the aspect ratio of the spherical carbide, and the number density of the spherical carbide can be obtained by image analysis of a scanning electron microscope (SEM) photograph.
  • the sample is placed in picral (5% picric acid + 95% ethanol solution) at room temperature for 50 seconds. Immerse to reveal the metallographic structure.
  • the 250 ⁇ m depth part (the central part in the depth direction of the surface layer part), the 0.25D depth part, and the 0.5D depth part from the surface of the steel wire are set to be the center of the measurement field of view.
  • a metal structure of a total of 15 fields of view is photographed at a magnification of 5000 times in a region of 20 ⁇ m in the depth direction and 25 ⁇ m in the central axis direction.
  • image analysis software name: Nireco's small general-purpose image processing analysis system LUZEX_AP
  • the average number of circle-equivalent diameters of spherical carbides of 0.1 ⁇ m or more is defined as the average particle size of spherical carbides, and the maximum particle size in the measurement field is defined as the maximum particle size of spherical carbides.
  • the circle-equivalent diameter of the spherical carbide is the diameter of a circle having an area equal to the area of the spherical carbide.
  • the aspect ratio of the spherical carbide of 0.1 ⁇ m or more is obtained by the length of the major axis / the length of the minor axis.
  • the number density of spherical carbides is obtained by dividing the number of spherical carbides having a circle-equivalent diameter of 0.1 ⁇ m or more by the area of the measurement field of view.
  • Step wire manufacturing method> An example of a method for manufacturing a steel wire according to the present disclosure will be described. However, the method for manufacturing a steel wire described below does not limit the steel wire according to the present disclosure. That is, regardless of the manufacturing method, the steel wire satisfying the above requirements is the steel wire according to the present disclosure.
  • An example of the method for manufacturing a steel wire according to the present disclosure is The process of drawing a wire mainly composed of bainite with a total surface reduction rate of 20 to 50%, The process of annealing the wire drawn wire by holding it at 650 ° C or higher and Ac 1 temperature (° C) or lower for 3 hours or longer and cooling it. To be equipped.
  • a step of holding the cooled wire rod in a temperature range of 400 ° C. to less than 500 ° C. for 20 seconds or more (first holding step) and Further, a wire rod mainly composed of bainite may be produced by a step of holding the wire rod that has undergone the first holding step within a temperature range of 500 ° C. to 600 ° C. for 30 seconds or longer (second holding step).
  • first holding step a step of holding the wire rod that has undergone the first holding step within a temperature range of 500 ° C. to 600 ° C. for 30 seconds or longer.
  • Heating process In the heating step, the steel piece having the component composition of the steel wire according to the present disclosure is heated to 950 to 1150 ° C. If the heating temperature is less than 950 ° C., the deformation resistance during hot rolling increases and the rolling cost increases. When the heating temperature exceeds 1150 ° C., decarburization of the surface becomes remarkable, and the surface hardness of the final product decreases.
  • the heated steel pieces are hot rolled at a finish rolling temperature of 850 to 1000 ° C. If the finish rolling temperature is less than 850 ° C., the ferrite grains become finer and a structure having an average grain size of 10.0 to 30.0 ⁇ m cannot be obtained after the annealing step. When the finish rolling temperature exceeds 1000 ° C., the transformation completion time in the first holding step becomes long, and the manufacturing cost increases.
  • the finish rolling temperature refers to the surface temperature of the wire rod immediately after finish rolling.
  • the wire rod at 850 to 1000 ° C. is cooled from 850 ° C. to 550 ° C. at an average cooling rate of 30 to 250 ° C./s to less than 400 to 500 ° C.
  • the wire rod after hot rolling may be wound into a ring shape and immersed in a molten salt tank so as to have the above average cooling rate. If the average cooling rate is less than 30 ° C./s, the area ratio of ferrite and spherical carbides tends to decrease after the annealing step, and the number density of spherical carbides tends to decrease.
  • the cooling rate refers to the surface cooling rate of the wire rod.
  • First holding step In the first holding step, the cooled wire rod is held at 400 ° C. to less than 500 ° C. for 20 seconds or more. If the holding temperature is less than 400 ° C., the strength after the annealing step becomes high and the cold forging property deteriorates. When the holding temperature is 500 ° C. or higher, the transformation completion time in the first holding step becomes remarkably long, and the untransformed portion remains after the first holding step and the second holding step. The untransformed portion causes disconnection in the wire drawing process and deteriorates the cold forging property after the annealing process.
  • the holding time in the first holding step is less than 20 seconds, untransformed portions remain after the first holding step and after the second holding step, which causes disconnection in the wire drawing process and cold after the annealing step. Deteriorates forgeability. From the viewpoint of manufacturing cost, the upper limit of the holding time is preferably 120 seconds.
  • the first holding step is carried out, for example, by immersing the wire rod in the molten salt tank.
  • the wire rod that has undergone the first holding step is held at 500 ° C. to 600 ° C. for 30 seconds or longer. If the holding temperature is less than 500 ° C., the strength of the wire rod is high, which causes disconnection in the wire drawing process. If the holding temperature is 600 ° C. or higher, the manufacturing cost increases. From the viewpoint of manufacturing cost, the upper limit of the holding time is preferably 150 seconds.
  • the second holding step is carried out, for example, by immersing in a molten salt tank.
  • the wire rod cooled to room temperature suppresses proeutectoid ferrite and pearlite, and has a structure mainly composed of bainite.
  • the area ratio of bainite measured in the C cross section is 50% or more, and the area ratio of martensite is 0% or more.
  • the area ratio of martensite may be 0%, preferably more than 0%.
  • the structure of the wire rod produced by hot rolling and cooling by a normal method using subeutectoid steel with a carbon content of 0.50% or less is a mixed structure of ferrite and pearlite.
  • carbon in the steel is unevenly distributed in the pearlite portion. Therefore, after spheroidizing annealing, the carbides are unevenly distributed in the portion that was pearlite before annealing, and the ductility is lowered.
  • the structure of the wire is a bainite structure or martensite structure in which ferrite is suppressed, carbon in the steel is uniformly distributed, so that carbides are uniformly dispersed after spheroidizing annealing, and ductility is improved.
  • Martensite is effective in improving ductility because it makes carbides after spheroidizing annealing finer, but on the other hand, it increases deformation resistance because it makes the ferrite grain size after annealing finer. Therefore, in order to improve the ductility of the steel wire after spheroidizing annealing and reduce the deformation resistance, it is effective to make the structure of the wire rod mainly composed of bainite.
  • bainite in the present disclosure includes as with pearlitic ferrite phase (alpha) and cementite phase (Fe 3 C) are.
  • pearlite is a structure in which ferrite phases and cementite phases are alternately and continuously laminated in layers.
  • bainite is a structure in which lath (needle-shaped lower structure) is contained in grains and granular or needle-shaped carbides are dispersed. In this respect, pearlite and bainite are distinguished.
  • the area ratio (area%) of bainite, ferrite, and martensite of the wire is determined by the following procedure.
  • the C cross section of the wire rod to be measured (hereinafter sometimes referred to as "object") is mirror-polished, and then the object is immersed in picral (5% picric acid + 95% ethanol solution) at room temperature for 50 seconds. To reveal the organization.
  • the diameter of the object is referred to as D.
  • D the diameter of the object.
  • the visual field shape is a rectangle having a length of 80 ⁇ m in the depth direction and a length of 120 ⁇ m in the circumferential direction, and the center of the visual field is aligned with the above-mentioned measurement position.
  • the visual field shape is a rectangle having a length of 80 ⁇ m in the depth direction and a length of 120 ⁇ m in the circumferential direction, and the center of the visual field is aligned with the above-mentioned measurement position.
  • One point on the central axis One point on the central axis that overlaps with the central axis (depth position with a depth of 0.5D from the surface) is defined as the measurement point on the central axis.
  • the visual field shape is a square with a length and width of 80 ⁇ m centered on the central axis.
  • cementite and ferrite having (length of major axis) / (length of minor axis) of 5.0 or more are alternately and continuously laminated in layers, and these layers are laminated.
  • a tissue containing no granular or acicular cementite between the two was designated as pearlite.
  • pearlite includes pseudo pearlite.
  • the pseudo-pearlite was a structure in which the divided cementites were arranged in a row, did not contain granular or needle-like carbides between the rows, and did not contain a lath (needle-shaped substructure) in the grains.
  • Bainite had a structure in which laths were contained in the grains and granular or needle-like carbides were dispersed between the laths and in the laths.
  • each structure of bainite, ferrite, martensite, pearlite, austenite, and proeutectoid cementite in the photographed microstructure is visually marked. Then, the area of the region of each tissue is obtained by image analysis (software name: Nireco's small general-purpose image processing analysis system LUZEX_AP). In addition, the above-mentioned series of operations is performed on at least two samples, the area ratio of the structure in these samples is measured and calculated, the average value thereof is obtained, and the average value is the area% of each structure of the wire rod in the present disclosure. And.
  • the region that is corroded by nital but weakly corroded by piclar is determined to be martensite, and the region where both nital and picral are weakly corroded is determined to be ferrite. Then, the region of each tissue is visually marked by the above method, and the area% of each tissue is obtained by image analysis.
  • the C cross section of the wire rod or steel wire to be measured is mirror-polished.
  • the tissue is then exposed by immersion in Nital at room temperature for 20 seconds.
  • a histological photograph of the entire area in the nine regions in the C cross section is taken using SEM at a magnification of 5000 times.
  • a structure in which laths (needle-shaped substructures) were present in the grains and granules or needle-like carbides were present was determined to be bainite.
  • the "nine regions in the C cross section" are (1) four locations on the surface layer portion, (2) four locations at a depth position of 0.25D, and (3) one location on the central axis. Is.
  • wire drawing process In the wire drawing process, after the second holding step, the wire rod cooled to room temperature is drawn with a total surface reduction rate of 20 to 50%.
  • a total surface reduction rate of 20 to 50%.
  • spheroidization of carbides is promoted during the annealing step, and growth of ferrite grains is promoted. If the total surface reduction rate during wire drawing is less than 20%, these effects are insufficient and the cold forging property deteriorates. Even if the total surface reduction rate exceeds 50%, the effect may be saturated and the steel wire diameter may become smaller, which may limit the application.
  • the steel wire diameter (diameter) according to the present disclosure is not particularly limited and may be determined according to the intended use. However, when used as a material for mechanical parts such as bolts, screws and nuts, for example, the diameter is Wire drawing is performed so that the steel wire has a diameter of 3.5 to 16.0 mm.
  • the steel wire obtained by wire drawing is held at 650 ° C. or higher and Ac 1 temperature (° C.) or lower for 3 hours or longer to cool.
  • Ac 1 723-10.7 ⁇ [Mn] + 29.1 ⁇ [Si] + 16.9 ⁇ [Cr].
  • the annealing temperature is less than 650 ° C.
  • the average particle size of the ferrite grains is less than 10 ⁇ m, and the cold forging property deteriorates.
  • the annealing temperature exceeds Ac 1 , the average particle size of the ferrite grains becomes less than 10 ⁇ m, the number of carbides decreases, and the cold forging property may deteriorate.
  • the holding time is less than 3 hours, the average particle size of the ferrite grains is less than 10 ⁇ m, and the cold forging property deteriorates.
  • the steel wire according to the present disclosure can be suitably manufactured.
  • the method for producing the steel wire according to the present disclosure is not particularly limited.
  • the method for producing a steel wire composed of the above steps is only a preferable example for obtaining the steel wire according to the present disclosure.
  • the wire rod according to the present disclosure is a hot-rolled wire rod for manufacturing the steel wire according to the present disclosure.
  • the wire rod according to the present disclosure inevitably has substantially the same chemical composition as the steel wire according to the present disclosure.
  • the metal structure of the wire rod according to the present disclosure and various aspects of spherical carbide are not particularly limited.
  • a preferable example of the metal structure of the wire rod is that the area ratio of bainite is 50% or more and the area ratio of martensite is 0% or more in the C cross section.
  • the present disclosure relates to a wire rod having such a metal structure by applying strain to the steel wire by wire drawing with a total surface reduction rate of 20% or more, and further performing spheroidizing annealing at a temperature of Ac 1 or less. Steel wire can be obtained.
  • the steel wires of test numbers 1 to 16, 32 to 36, and 41 shown in Tables 2-1 to 2-4 were manufactured as follows.
  • the steel pieces were heated and then hot-rolled, and the obtained wire rod was wound into a ring shape and immersed in a molten salt tank installed behind the hot-rolling line and cooled to 470 to 520 ° C. ..
  • the wire rod immersed in the molten salt tank was first held and second held in the molten salt bath of two tanks. Then, the wire rod cooled to room temperature (25 ° C.) was wire-drawn at the total surface reduction rate shown in Table 2-1 and Table 2-2, and after the wire was drawn, it was heated and annealed.
  • the annealing treatment of the steel wires of test numbers 1 to 12, 15, 32 and 35 was held at 710 ° C. for 5 hours and then air-cooled, and the annealing treatment of the steel wire of test number 16 was held at 760 ° C. for 5 hours. After air cooling, the annealing treatment of the steel wire of test number 33 was held at 740 ° C.
  • the annealing treatment of the steel wire of test number 34 was held at 695 ° C. for 5 hours and then air cooled.
  • the annealing treatment of the steel wire of 36 was held at 730 ° C. for 5 hours and then air-cooled, and the annealing treatment of the steel wire of Test No. 41 was carried out after holding at 735 ° C. for 5 hours and then air-cooled.
  • the steel wire of test number 31 was manufactured as follows. First, the steel pieces were heated and then hot-rolled, and the obtained wire rod was wound into a ring shape and cooled to 470 ° C. by impulse cooling. Then, the obtained wire rod was immersed in two molten salt tanks for first holding and second holding. Then, the wire rod cooled to room temperature (25 ° C.) was drawn at the total surface reduction rate shown in Table 2-2, held at 710 ° C. for 5 hours after the wire drawing, and then air-cooled.
  • the steel wires of test numbers 17 to 28 and 37 to 40 shown in Tables 2-1 to 2-4 were manufactured as follows. First, the steel pieces were heated and then hot-rolled, and the obtained wire rod was wound into a ring shape and cooled by an impulse. Then, the wire rod cooled to room temperature (25 ° C.) was wire-drawn at the total surface reduction rate shown in Table 2-1 and Table 2-2, and after the wire was drawn, it was heated and annealed. The steel wires of test numbers 17 to 28 and 37 to 40 were annealed at 760 ° C. for 5 hours, cooled to 660 ° C. at a cooling rate of 15 ° C./h, and then air-cooled. Through these steps, the steel wires shown in test numbers 17 to 28 and 37 to 40 were manufactured.
  • the steel wire of test number 29 shown in Table 3-1 and Table 3-2 was manufactured as follows. After heating the steel pieces, they were hot-rolled, and the obtained wire rod was wound into a ring shape and cooled by an impulse. Then, the wire rod cooled to room temperature (25 ° C.) was heated to 850 ° C. and quenched, and then heated to 650 ° C. and tempered. Then, the wire was drawn at the total surface reduction rate shown in Table 3-1 and then heated and annealed after the wire was drawn.
  • the steel wire of test number 30 shown in Table 4-1 and Table 4-2 was manufactured as follows. After heating the steel pieces, they were hot-rolled, and the obtained wire rod was wound into a ring shape and cooled by an impulse. Then, the wire rod cooled to room temperature (25 ° C.) was heated and subjected to the first annealing treatment. Then, the wire rod cooled to room temperature (25 ° C.) was wire-drawn at the total surface reduction rate shown in Table 4-1 and heated after the wire drawing to be subjected to the second annealing treatment. The first annealing treatment and the second annealing treatment were carried out at 760 ° C. for 5 hours, then cooled to 660 at a cooling rate of 15 ° C./h, and then air-cooled.
  • the particle size (described as “average particle size” in the table) and the maximum particle size of spherical carbide having a circle-equivalent diameter of 0.1 ⁇ m or more are the methods described above. Measured according to. The results are shown in the table.
  • number / C% is the number of spherical carbides with a circular equivalent diameter of 0.1 ⁇ m or more observed per 1 mm 2 L cross section of each steel wire, and the C content (C content) contained in the steel wire. %) Divided by.
  • the deformation resistance and limit compressibility of the steel wire were measured by a compression test.
  • the annealed steel wire was drawn with a surface reduction rate of 8% to prepare a columnar test piece having a diameter D and a height of 1.5 D from the drawn steel wire.
  • the compression test method is a compression test in which the end face is constrained by a die with concentric grooves based on the standards of the Cold Forging Subcommittee of the Japan Society for Plasticity Processing (Plasticity and Processing, vol.22, No.211, 1981, p139). Was done.
  • the deformation resistance was the equivalent stress when processed at an equivalent strain of 1.6 and a compressibility of 73.6% according to Kosakada's method (K. Osakada: Ann.
  • the critical compressibility is a curvature of 0.15 mm, a depth of 0.8 mm, and an angle in the circumferential axial direction of a cylindrical test piece having a diameter of 5.0 mm and a height of 7.5 mm manufactured from the drawn steel wire by machining.
  • a compression test was performed using a test piece having a 30 ° notch. When cracks with a length of 0.5 mm or more were observed, it was determined that cracks had occurred, and the maximum compressibility at which cracks did not occur was defined as the limit compressibility.
  • the table shows the measurement results of deformation resistance and critical compressibility, and also shows the comparison results with ordinary steel wires (test numbers 17 to 28 and 37 to 40).
  • Deformation resistance and / or the steel wire with the test number described as "equivalent” to the limit compressibility has a deformation resistance within ⁇ 20 MPa and less than ⁇ 20 MPa as compared with the normal steel wire (test numbers 17 to 28 and 37 to 40).
  • the critical compressibility is within ⁇ 2%.
  • the steel wire with the test number described as "good” had better characteristics than the normal steel wire, and the steel wire with the test number described as "poor” was inferior to the normal steel wire.
  • the steel wires of test numbers 1 to 12, 33 to 36, and 41 that satisfy all the requirements specified in the present disclosure are compared with the normal steel wires (test numbers 17 to 28 and 37 to 40) in terms of deformation resistance.
  • the steel wires of test numbers 1 to 12, 33 to 36, and 41 that satisfy all the requirements specified in the present disclosure are superior in the critical compressibility to the normal steel wire.
  • the structure before wire drawing that is, the structure of the wire rod
  • it had a structure mainly composed of bainite (see Table 5 described later).
  • the ordinary steel wires 17 to 28 and 37 to 40 are manufactured under manufacturing conditions in which it is presumed that the structure before wire drawing does not become a structure mainly composed of bainite.
  • the structure before wire drawing that is, the structure of the wire rod
  • it was not mainly bainite see Table 5 described later.
  • the steel wire could not be manufactured because the wire was broken during the wire drawing. It is presumed that this is because the holding temperature in the first holding step was too high and the hardness of the wire before wire drawing became excessive.
  • the steel wire 14 the steel wire could not be manufactured because the wire was broken during the drawing.
  • the metallographic structure of the wire rod as the raw material was also evaluated.
  • the evaluation method was as described above in the present specification.
  • the evaluation results are shown in Table 5.
  • the wire rods which are the materials of the steel wires of test numbers 1 to 12, 33 to 36, and 41 that satisfy all the requirements specified in the present disclosure, have a bainite area ratio of 50% or more in the C cross section before wire drawing. Therefore, the area ratio of martensite was 0% or more.
  • the wire rods of test numbers 13 and 14 in which the wire was broken during the wire drawing process the amount of bainite was insufficient and the amount of martensite was large.
  • the wire rod of Test No. 19 for steel wire in which the average aspect ratio of the spherical carbide exceeds the upper limit of the present disclosure and the number density of the spherical carbide is less than the lower limit of the present disclosure includes both bainite and martensite. There wasn't.
  • the wire rods of test numbers 22 and 24 on the steel wire having an average ferrite grain size below the lower limit of the present disclosure contained both bainite and martensite, but the amount was insufficient.

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