EP3165625A1 - Drahtmaterial für stahldraht sowie stahldraht - Google Patents

Drahtmaterial für stahldraht sowie stahldraht Download PDF

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Publication number
EP3165625A1
EP3165625A1 EP15814624.1A EP15814624A EP3165625A1 EP 3165625 A1 EP3165625 A1 EP 3165625A1 EP 15814624 A EP15814624 A EP 15814624A EP 3165625 A1 EP3165625 A1 EP 3165625A1
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EP
European Patent Office
Prior art keywords
less
wire
proeutectoid
amount
steel wire
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EP15814624.1A
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English (en)
French (fr)
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EP3165625A4 (de
Inventor
Tomonobu Ishida
Tomokazu Masuda
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Kobe Steel Ltd
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Kobe Steel Ltd
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Priority to EP18171309.0A priority Critical patent/EP3378964A1/de
Publication of EP3165625A1 publication Critical patent/EP3165625A1/de
Publication of EP3165625A4 publication Critical patent/EP3165625A4/de
Withdrawn legal-status Critical Current

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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
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • 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
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • 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
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • 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
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • 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
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • 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
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • 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
    • C22C38/10—Ferrous alloys, e.g. steel alloys containing cobalt
    • C22C38/105—Ferrous alloys, e.g. steel alloys containing cobalt containing Co and Ni
    • 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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • 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
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing 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
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium 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
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/009—Pearlite
    • 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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length

Definitions

  • the present invention relates to a wire rod for a steel wire, which is a material for a high-strength steel wire used for a wire rope, a PC steel wire, and the like, and to such a steel wire.
  • the bending fatigue characteristics of an element wire are important factors to determine the design strength and life of a rope.
  • needs for reduction in weight of a rope are increasing along with the acceleration of elevators and the reduction in size of cranes, and a high-strength wire rod for a steel wire is demanded, which satisfies the needs and is excellent in the bending fatigue characteristics.
  • the high-strength wire rod for a steel wire which is excellent in the bending fatigue characteristics, is also useful as a material for a PC (Prestressed Concrete) steel wire. Specifically, such a wire rod for a steel wire is required not to generate low cycle fatigue that occurs in 10 4 to 10 5 repeating cycles.
  • Patent Literature 1 discloses a technique of improving the fatigue strength by finely precipitating a BN-based inclusion in steel.
  • Patent Literature 2 discloses a technique of obtaining a high-strength wire rod by directly performing a molten salt patenting treatment after hot rolling so that the structure of the wire rod is controlled to a pearlite structure having an area ratio of proeutectoid ferrite of 3% or less.
  • Patent Literature 3 discloses a technique of obtaining a high-ductility wire rod by making the metallographic structure of the wire rod a 95% or more pearlite structure and controlling to predetermined ranges the maximum value and the average value of pearlite block grain sizes of the pearlite in a core part of a cross-section perpendicular to the axis direction of the wire rod. This technique also discloses that it is useful to adjust the volume ratio of proeutectoid ferrite to 2% or less for excellent wire drawability.
  • Patent Literature 1 The characteristic to be an issue in the technique of Patent Literature 1 is high cycle fatigue that occurs near the fatigue limit of 10 7 repeating cycles, and the mechanism of the high cycle fatigue is different from that of the above-mentioned low cycle fatigue.
  • products that are exposed to outside air over a long period of time, such as a wire rope a crack easily occurs on a surface layer part by the influence such as oxidation and intrusion of hydrogen on the surface layer part, and friction between element wires, and there is a possibility that breaking of a wire occurs much earlier than its intrinsic fatigue limit, resulting in a short life. Therefore, it is necessary to take measures to suppress development of a crack.
  • Patent Literature 2 special equipment for performing the direct patenting treatment after hot rolling is required for acquisition of a high-strength wire rod, causing equipment investment to increase. Such equipment also has a drawback of being inferior in productivity and maintainability to so-called Stelmor cooling equipment that cools a wire rod while conveying it on a conveyor. In addition, a sufficient effect of improving the low cycle fatigue characteristics cannot be exhibited only with reduction in the area ratio of proeutectoid ferrite in a wire rod.
  • the present invention has been made under the circumstances as described above, and an object of the present invention is to provide a wire rod for a steel wire, which is excellent in low cycle fatigue characteristics and is useful as a material for a high-strength steel wire such as a wire rope or a PC steel wire, and to provide a steel wire that can exhibit such characteristics.
  • a wire rod for a steel wire of the present invention which can solve the above problems, comprises by mass: C: 0.70 to 1.3%; Si: 0.1 to 1.5%; Mn: 0.1 to 1.5%; N: 0.001 to 0.006%; Al: 0.001 to 0.10%; Ti: 0.02 to 0.20%; B: 0.0005 to 0.010%; P: 0% or more and 0.030% or less; and S: 0% or more and 0.030% or less, with the balance being iron and inevitable impurities, wherein, the wire rod has pearlite as a main phase, an area ratio of proeutectoid ferrite is 1.0% or less, and an average thickness of the proeutectoid ferrite is 5 ⁇ m or less.
  • the phrase "having pearlite as a main phase" means that 95 area% or more of the metallographic structure is a pearlite structure.
  • the average thickness of proeutectoid ferrite means an average value of the thickness in the width direction of the proeutectoid ferrite when the proeutectoid ferrite is observed by an optical microscope.
  • the wire rod for a steel wire of the present invention further comprises by mass:
  • a content of a solid solution B is preferred to be 0.0003% by mass or more.
  • the present invention also encompasses a steel wire comprising the above-mentioned chemical composition, wherein 100000 cycle fatigue strength ⁇ satisfies a relationship of formula (1) below with tensile strength TS: ⁇ > 0.45 TS
  • the area ratio of proeutectoid ferrite of a steel wire rod before a wire drawing process is reduced and the thickness of the proeutectoid ferrite is decreased to improve the bending fatigue strength of a steel wire after cold working (a wire drawing process), allowing the steel wire to exhibit excellent fatigue characteristics.
  • the steel wire exhibits excellent characteristics against low cycle fatigue caused by repetitive stress load of about 10 4 to 10 5 cycles.
  • proeutectoid ferrite (hereinafter, may be abbreviated as "proeutectoid ⁇ ) slightly precipitated in a pearlite structure accelerates development of a fatigue crack.
  • proeutectoid ⁇ proeutectoid ferrite
  • plate-shaped proeutectoid ⁇ is precipitated in a prior austenite grain boundary as shown in Fig.
  • the steel wire rod having a pearlite structure as a main phase a void is generated in an interface between the proeutectoid ⁇ and the pearlite to accelerate the development of a fatigue crack. Therefore, it is important to minimize the area ratio of the proeutectoid ⁇ as much as possible so that the amount of the interface is reduced. In addition, the reduction in the area ratio of the proeutectoid ⁇ also gives an effect of suppressing a longitudinal crack during a torsion test. When a longitudinal crack is generated, the steel wire cannot endure a wire stranding process, and therefore the steel wire having a longitudinal crack is determined to be defective.
  • the area ratio of the proeutectoid ⁇ is preferably 0.8% or less, more preferably 0.6% or less.
  • B exhibits the effect of reducing the area ratio of the proeutectoid ⁇ when B exists as a solid solution B, and with regard to the portion that B is precipitated as a compound such as BN, the effect is lost. Therefore, it is necessary in the steel wire rod of the present invention to control the amounts of N and B in appropriate ranges, and it is preferred to produce the steel wire rod under production conditions in which BN is hardly precipitated.
  • an increase in thickness of the proeutectoid ⁇ causes concentration of stress to a void generated in an interface to enlarge the void, so that the development of a fatigue crack is encouraged to deteriorate the fatigue strength.
  • the proeutectoid ⁇ small in thickness is deformed by a wire drawing process to be rendered harmless, while the proeutectoid ⁇ large in thickness remains even after a wire drawing process to deteriorate bending fatigue strength (hereinafter, may be simply referred to as "fatigue strength").
  • the average thickness of the proeutectoid ⁇ is necessary to be 5 ⁇ m or less.
  • the average thickness of the proeutectoid ⁇ is preferably 4 ⁇ m or less, more preferably 3 ⁇ m or less.
  • the chemical composition of the steel wire rod so as to exhibit the basic characteristics of the steel wire rod when the steel wire rod is applied.
  • the chemical composition including the amounts of the above-mentioned B, N and Ti is as follows. It is to be noted that “%” in the chemical composition is “% by mass” in any case.
  • the amount of C is an element effective for enhancing the strength, and a wire rod (steel wire rod) before cold working and a steel wire after cold working are improved in the strength with the increasing amount of C.
  • the amount of C also affects the precipitation amount of the proeutectoid ⁇ , and when the amount of C is small, the precipitation of the proeutectoid ⁇ cannot be sufficiently suppressed. Therefore, the amount of C is determined to be 0.70% or more.
  • the amount of C is preferably 0.74% or more, more preferably 0.78% or more.
  • proeutectoid ⁇ An excessive amount of C, however, causes precipitation of proeutectoid cementite (hereinafter, may be abbreviated as "proeutectoid ⁇ ") to lead to breaking of a wire during a wire drawing process. Therefore, the amount of C is determined to be 1.3% or less.
  • the amount of C is preferably 1.2% or less, more preferably 1.1% or less.
  • Si has an action as a deoxidizing agent, and also has an action of improving the strength of a wire rod.
  • the amount of Si is determined to be 0.1% or more.
  • the amount of Si is preferably 0.15% or more, more preferably 0.20% or more.
  • an excessive amount of Si deteriorates cold wire drawability to cause the increase in the ratio of breaking of a wire. Therefore, the amount of Si is determined to be 1.5% or less.
  • the amount of Si is preferably 1.4% or less, more preferably 1.3% or less.
  • Mn particularly has, in addition to a deoxidizing action as with Si, an action of fixing S in steel as MnS to enhance toughness and ductility of steel.
  • the amount of Mn is set to 0.1% or more.
  • the amount of Mn is preferably 0.15% or more, more preferably 0.20% or more.
  • Mn is, however, an element that is easily segregated, and excessive addition thereof may possibly cause excessive increase of hardenability at a Mn segregated portion to produce a supercooled structure such martensite. Therefore, the amount of Mn is determined to be 1.5% or less.
  • the amount of Mn is preferably 1.4% or less, more preferably 1.3% or less.
  • N chemically combines with B in steel to form BN, so that the effect of B is lost. Further, N in a solid solution state causes deterioration of torsion characteristics due to strain aging during wire drawing, and causes a longitudinal crack in significant cases.
  • the amount of N is set to 0.006% or less.
  • the amount of N is preferably 0.005% or less, more preferably 0.004% or less.
  • a small amount of N gives an effect of refining crystal grains by a nitride such as TiN or AlN to enhance ductility of a wire rod.
  • the amount of N is set to 0.001% or more.
  • the amount of N is preferably 0.0015% or more, more preferably 0.0020% or more.
  • Al is an effective deoxidizing element. Al also has an effect of forming a nitride such as AlN to refine crystal grains. For effective exhibition of such an effect, the amount of Al is set to 0.001% or more. The amount of Al is preferably 0.002% or more, more preferably 0.003% or more. On the other hand, excessive addition of Al forms an oxide such as Al 2 O 3 to increase breaking of a wire during wire drawing. From the viewpoint described above, the amount of Al is set to 0.10% or less. The amount of Al is preferably 0.09% or less, more preferably 0.08% or less.
  • Ti has an action of forming a carbide such as TiC to reduce the grain size (thickness) of the proeutectoid ⁇ . Further, Ti also has an action of chemically combining with N in steel to form a nitride such as TiN, so that Ti prevents the deterioration of torsion characteristics caused by N. For effective exhibition of those effects, the amount of Ti is set to 0.02% or more. The amount of Ti is preferably 0.03% or more, more preferably 0.04% or more. On the other hand, an excessive amount of Ti causes precipitation of large amounts of Ti inclusions such as TiC and TiN to increase breaking of a wire during wire drawing. Therefore, the amount of Ti is set to 0.20% or less. The amount of Ti is preferably 0.15% or less, more preferably 0.10% or less.
  • B has an action of preventing production of the proeutectoid ⁇ so that the area ratio of the proeutectoid ⁇ is reduced.
  • the lower limit of the amount of B is preferably 0.0007% or more, more preferably 0.001% or more.
  • an excessive amount of B causes precipitation of a Fe-B compound as a compound of B and Fe, e.g., FeB 2 to cause a crack during hot rolling. Therefore, it is necessary to set the amount of B to 0.010% or less.
  • the amount of B is preferably 0.008% or less, more preferably 0.006% or less.
  • B is contained as a solid solution B in steel in an amount of preferably 0.0003% or more, more preferably 0.0005% or more.
  • the amount of P is set to 0.030% or less.
  • the amount of P is preferably 0.025% or less, more preferably 0.020% or less.
  • the amount of P may be 0%, but P is generally contained in an amount of 0.001% or more.
  • the amount of S is set to 0.030% or less.
  • the amount of S is preferably 0.025% or less, more preferably 0.020% or less.
  • the amount of S may be 0%, but S is generally contained in an amount of 0.001% or more.
  • the basic components of the wire rod of the present invention are as described above, and the balance is substantially iron. However, inclusion of inevitable impurities in steel, which are brought depending on the state of a raw material, a material, manufacturing equipment or the like, is naturally accepted.
  • the wire rod of the present invention further contain, as necessary, for further improvement of characteristics such as strength, toughness and ductility:
  • Cr and V are elements useful for enhancing the strength (tensile strength) of a wire rod, and one or two in combination of these elements may be contained.
  • Cr has an action of reducing the lamellar spacing of pearlite to enhance the strength and toughness of a wire rod.
  • the amount of Cr is preferably 0.05% or more.
  • the amount of Cr is more preferably 0.10% or more, further preferably 0.15% or more.
  • an excessive amount of Cr increases hardenability to increase a risk of generating a supercooled structure during hot rolling. Therefore, the amount of Cr is preferably set to 1.0% or less.
  • the amount of Cr is more preferably 0.8% or less, further preferably 0.6% or less.
  • V has an effect of forming a carbonitride to improve the strength of a wire rod.
  • V not only forms a nitride with an excessive solid solution N after precipitation of AlN as with Nb to contribute to refining of crystal grains, but also has an effect of suppressing aging embrittlement by fixation of the solid solution N.
  • the amount of V is preferably 0.01% or more, more preferably 0.02% or more, further preferably 0.03% or more.
  • V is, however, an expensive element, and even excessive addition of V only causes saturation of the effect of V, leading to economical waste. Therefore, the amount of V is preferably 0.5% or less, more preferably 0.4% or less, further preferably 0.2% or less.
  • Ni and Nb are elements useful for enhancing the toughness of a steel wire, and one or two in combination of these elements may be contained.
  • Ni is an element that enhances the toughness of a steel wire after wire drawing.
  • the amount of Ni is preferably 0.05% or more, more preferably 0.1% or more, further preferably 0.2% or more.
  • the amount of Ni is preferably 0.5% or less, more preferably 0.4% or less, further preferably 0.3% or less.
  • Nb not only forms a nitride as with Ti and Al to contribute to refining of crystal grains for improvement of the toughness of a steel wire, but also has an effect of suppressing aging embrittlement by fixation of a solid solution N.
  • the amount of Nb is preferably 0.01% or more, more preferably 0.03% or more, further preferably 0.05% or more.
  • Nb is, however, an expensive element, and even excessive addition of Nb only causes saturation of the effect of Nb, leading to economical waste. Therefore, the amount of Nb is preferably 0.5% or less, more preferably 0.4% or less, further preferably 0.3% or less.
  • Co has an action of reducing production of proeutectoid cementite to make the structure a uniform pearlite structure, particularly when the amount of C is high.
  • the amount of Co is preferably 0.05% or more, more preferably 0.1% or more, further preferably 0.2% or more.
  • the amount of Co is preferably 1.0% or less, more preferably 0.8% or less, further preferably 0.6% or less.
  • Mo is an element that improves corrosion resistance of a steel wire.
  • the amount of Mo is preferably 0.05% or more, more preferably 0.1% or more, further preferably 0.2% or more.
  • An excessive amount of Mo however, easily causes generation of a supercooled structure during hot rolling and also deteriorates the ductility. Therefore, the amount of Mo is preferably 0.5% or less, more preferably 0.4% or less, further preferably 0.3% or less.
  • Cu is an element that improves corrosion resistance of a steel wire as with Mo.
  • the amount of Cu is preferably 0.05% or more, more preferably 0.08% or more, further preferably 0.10% or more.
  • An excessive amount of Cu causes reaction of Cu with S to segregate CuS in a grain boundary portion, so that a flaw is generated during a manufacturing process of a wire rod.
  • the amount of Cu is preferably 0.5% or less, more preferably 0.4% or less, further preferably 0.3% or less.
  • One or two in combination of Mo and Cu may be contained.
  • a wire rod before cold wire drawing is generally manufactured by melting, bloom rolling, and hot rolling of steel obtained by appropriately controlling the chemical components thereof, further followed by a patenting treatment as necessary. It is important to appropriately control the precipitation behavior of TiC or BN in addition to appropriate control of the content of Ti, B and N to the above-mentioned ranges, for manufacturing the wire rod of the present invention while satisfying the requirements (metallographic structure, area ratio of proeutectoid ⁇ , and average thickness of proeutectoid ⁇ ) defined in the present invention.
  • a slab cast is heated to 1200°C or higher in bloom rolling to decompose coarse TiC precipitated during casting.
  • a heating temperature lower than 1200°C causes coarse TiC to remain in a wire rod so that the thickness of proeutectoid ⁇ cannot be sufficiently reduced, resulting in deterioration of the fatigue strength.
  • the heating temperature is more preferably 1250°C or higher, further preferably 1300°C or higher.
  • An excessively high heating temperature causes melting of a wire rod, and therefore the heating temperature is generally set to up to about 1400°C.
  • the billet be heated to a temperature range of 1000°C or higher to sufficiently decompose coarse BN in the billet followed by sufficient cooling with water after the rolling, and the placing temperature of the rolled material (wire rod) is controlled to 800 to 1000°C at a laying head.
  • a placing temperature exceeding 1000°C causes precipitation of a large amount of BN in a wire rod during cooling on a conveyor after the placing so that a solid solution B may not possibly be sufficiently secured.
  • the placing temperature is more preferably 980°C or lower, further preferably 950°C or lower.
  • a placing temperature lower than 800°C causes increase in deformation resistance of a wire rod so that a placing defect in a laying head, e.g., unavailable coiling and the like, may occur. Therefore, the placing temperature is preferably 800°C or higher.
  • the placing temperature is more preferably 820°C or higher, further preferably 850°C or higher.
  • the strain rate at final 4 passes of the rolling is set to 0.5 sec -1 or more, and crystal grains are refined by dynamic recrystallization, so that fine TiC is precipitated.
  • the strain rate of less than 0.5 sec -1 does not allow sufficient refining of TiC so that the average thickness of proeutectoid ⁇ cannot be sufficiently reduced.
  • the strain rate at this time is more preferably 0.8 sec -1 or more, further preferably 1.0 sec -1 or more. However, in terms of equipment load, generally, the strain rate is preferably set to 5 sec -1 or less.
  • the wire rod After the placing, the wire rod is cooled on a cooling conveyor to cause pearlite transformation during the cooling. It is preferred that the wire rod is rapidly cooled by setting the average cooling rate before the start of the pearlite transformation to 5°C/sec or more. A low average cooling rate at this time easily causes precipitation of proeutectoid ⁇ at a high temperature to coarsen the proeutectoid ⁇ , so that the thickness of the proeutectoid ⁇ may not possibly be sufficiently reduced. Further, in some cases, an average cooling rate of less than 5°C/sec causes local precipitation of a structure in which the lamellar spacing is extremely coarse, which is so-called coarse pearlite, so that the wire drawability is deteriorated.
  • the start of the pearlite transformation may be obtained by measuring the temperature of the wire rod and determining a changing point (inflection point) caused by transformation heating in a cooling curve.
  • the average cooling rate is more preferably 10°C/sec or more, further preferably 15°C/sec or more.
  • the upper limit of the average cooling rate is preferably 100°C/sec or less, more preferably 50°C/sec or less.
  • the wire rod obtained as described above can be used as a steel wire after directly performing a wire drawing process (cold working) on the wire rod, while a patenting treatment may be performed on the wire rod before the wire drawing process.
  • a patenting treatment before the wire drawing process can enhance the strength of the wire rod and reduce the variation in strength.
  • the heating temperature during the patenting treatment (hereinafter, this temperature may be referred to as a "reheating temperature”) is preferably about 900 to 1000°C, more preferably 920°C or higher and 980°C or lower.
  • the reheating temperature is preferably 900°C or higher from the viewpoints of preventing a carbide from remaining in a non-solid solution state and completely austenitizing the structure, while an excessively high reheating temperature causes, in some cases, coarsening of TiC, and reduction of a solid solution B caused by reaction thereof with N, so that a predetermined area ratio and a predetermined average thickness of proeutectoid ⁇ cannot be obtained.
  • the holding temperature at the patenting treatment is preferably about 530 to 600°C, more preferably 550°C or higher and 580°C or lower.
  • the amount of proeutectoid ⁇ that causes generation and development of a fatigue crack is sufficiently reduced, and the thickness of the proeutectoid ⁇ is controlled to be small. Therefore, a steel wire obtained by performing cold working on the wire rod, and products such as a wire rope and a PC steel wire, which partially or wholly include the steel wire, are excellent in fatigue characteristics compared to a general product.
  • the tensile strength and the fatigue strength are in a proportional relationship
  • a steel wire manufactured from the wire rod of the present invention is characterized by that 100000 cycle fatigue strength ⁇ satisfies the relationship of the following formula (1) with tensile strength TS, and the present invention encompasses such a steel wire.
  • the present invention encompasses a product such as a wire rope that is manufactured by using such a steel wire for a partial or whole part of the wire rope. ⁇ > 0.45 TS
  • a sample obtained from each of the wire rods before finish wire drawing was used for a tensile test, evaluation of the metallographic structure (area ratio of proeutectoid ⁇ , area ratio of pearlite, and average thickness of proeutectoid ⁇ ), and measurement of the amount of a solid solution B which were conducted by the following methods.
  • the tensile strength TS of the obtained sample was measured in accordance with JIS Z 2241 (2011). The results are shown in Table 4 below.
  • the obtained sample was embedded in a resin or the like, subjected to mirror polishing, and observed by an optical microscope with a mixed liquid of trinitrophenol and ethanol used as a corrosive liquid, to measure the area ratio of proeutectoid ⁇ by image analysis.
  • a part that emerges white by the corrosive liquid is proeutectoid ⁇ .
  • D the diameter of a wire rod
  • a D/4 part on a transverse section of the wire rod was regarded as a representative structure, and the structure was imaged with a magnification of 400 to evaluate total 5 visual fields.
  • the "area ratio of proeutectoid ⁇ " shown in Table 4 below shows the average value of the visual fields.
  • the transverse section herein indicates a surface perpendicular to the longitudinal direction of the wire rod.
  • the area ratio of pearlite was also measured by this method. It means that one indicated by "P” in the item of the metallographic structure of Table 4 below has a pearlite structure of 95 area% or more, i.e., pearlite as a main phase. While, it means that one indicated by "P + ⁇ " or "P + ⁇ ” in the item of the metallographic structure has a pearlite structure of less than 95 area%, and has a mixed structure of pearlite with ferrite ( ⁇ ) or cementite ( ⁇ ).
  • a specimen subjected to mirror polishing in the same manner as in the above sample was structurally observed by a SEM (Scanning Electron Microscope), the thickness of 10 proeutectoid ⁇ grains observed was measured, and the average value of the thickness of the grains was obtained to calculate the thickness per grain.
  • the measurement was conducted at a D/4 part on a transverse section in the same manner as in the above sample. The results are shown in Table 4 below.
  • the amount of a solid solution B was evaluated by an electrolytic extraction residue measurement.
  • the electrolytic extraction residue measurement was conducted by using a 10% acetylacetone solution, and the amount of a compound type B in a residue was measured with a mesh having an opening of 0.1 ⁇ m by a bromoester method.
  • the amount of a solid solution B was obtained by subtracting the amount of the compound type B from the whole amount of B in steel. The results are shown in Table 4 below.
  • the specimen used for the bromoester method was 3 g. Since the amount of a solid solution B does not change without a heat history of 900°C or higher, the examination may be conducted with a steel wire after cold working.
  • the tensile strength TS and yield point YP of the steel wires were measured in accordance with JIS Z 2241 (2011). The results are shown in Table 5 below. A value obtained by multiplying the tensile strength TS by 0.45 is also shown in Table 5 below.
  • the torsion characteristics were evaluated on the basis of a torsion value required before breaking (number of times of torsion before breaking) by conducting a torsion test.
  • the torsion rate was set to 52 times/min, and the tensile force was set to 500 gf (4.9N).
  • the torsion value was normalized by converting the distance between chucks (length of the sample wire) to 100 times of the wire diameter d (100d).
  • a longitudinal crack was discriminated from a normal fracture surface by fracture surface observation, and one having even one longitudinal crack in 5 wires was described as "yes" in the item of the longitudinal crack in the following Table 5.
  • a numeral 1 is a test piece (wire rod), a numeral 2 is a direction to which a repetitive stress is applied, and a circle represents a supporting point.
  • the maximum stress amplitude of the specimen determined to be acceptable was defined as 100000 cycle fatigue strength ⁇ .
  • the 100000 cycle fatigue strength ⁇ is shown in Table 5 below.
  • the stress wave form was a sine wave, and the frequency was set to 10 Hz.
  • Test No. Steel type Steel wire rod Average thickness of proeutectoid ⁇ ( ⁇ m) Area ratio of proeutectoid ⁇ (%) Solid solution B (ppm by mass)
  • Metallographic structure TS (MPa) 1 A 1 0.3 7 P 1205 2 B 2 0.4 5 P 1212 3 C 4 0.3 10 P 1252 4 C 6 0.8 8 P 1252 5 C 3 1.4 2 P 1237 6 C 10 0.6 3 P 1267 7 C - 8 C 8 0.7 5 P 1260 9 C 7 0.7 6 P 1240 10 D 5 0.5 5 P 1271 11 E 4 0.6 3 P 1241 12 F 2 0.5 6 P 1383 13 G 4 0.4 5 P 1277 14 H 5 0.2 4 P 1321 15 I 4 0.1 5 P 1259 16 J 1 0.2 5 P 1225 17 K 2 0.1 6 P 1279 18 L 4 0.3 6 P 13
  • both the chemical composition and the metallographic structure were within the ranges defined in the present invention, and therefore a steel wire (wire) was obtained, which had a tensile strength exceeding the tensile strength of the "piano wire B" described in JIS G 3522 (1991) (in the specification, 1620 to 1770 MPa in a wire having a wire diameter of 7.0 mm, for example), while achieving a fatigue strength satisfying the relationship of the above formula (1).
  • Test Nos. 4 to 9 and 22 to 27 are examples that did not satisfy any of the requirements of the present invention. Among them, Test No. 4 was low in the heating temperature during bloom rolling as shown in Table 2, causing precipitation of coarse TiC to increase the average thickness of the proeutectoid ⁇ as shown in Table 4 so that the fatigue strength was deteriorated.
  • Test No. 5 was low in the heating temperature during hot rolling as shown in Table 2, causing the increase in the area ratio of the proeutectoid ⁇ as shown in Table 4 while the solid solution B was reduced, so that the fatigue strength was deteriorated.
  • Test No. 6 was low in the strain rate during finish rolling as shown in Table 2, causing precipitation of coarse TiC to increase the average thickness of the proeutectoid ⁇ as shown in Table 4 so that the fatigue strength was deteriorated.
  • Test No. 7 was low in the placing temperature after hot rolling as shown in Table 2, causing a placing defect so that a specimen could not be obtained.
  • Test No. 8 was high in the placing temperature after hot rolling as shown in Table 2, causing coarsening of TiC to increase the average thickness of the proeutectoid ⁇ as shown in Table 4 so that the fatigue strength was deteriorated.
  • Test No. 9 was low in the average cooling rate after placing as shown in Table 2, causing the increase in the average thickness of the proeutectoid ⁇ as shown in Table 4 so that the fatigue strength was deteriorated.
  • Test No. 22 is an example of use of a steel type P containing a small amount of C, in which both the area ratio and average thickness of the proeutectoid ⁇ increased as shown in Table 4 so that the torsion characteristics and the fatigue strength were deteriorated.
  • Test No. 23 is an example of use of a steel type Q containing a large amount of C, in which large amounts of proeutectoid cementite was precipitated to cause breaking of a wire during wire drawing.
  • Test No. 24 is an example of use of a steel type R containing a small amount of Ti, in which, due to small amounts of TiC, the average thickness of the proeutectoid ⁇ increased so that the fatigue strength was deteriorated.
  • Test No. 25 is an example of use of a steel type S containing a large amount of Ti, in which large amounts of Ti-based inclusions were precipitated to cause breaking of a wire during wire drawing.
  • Test No. 26 is an example of use of a steel type T containing a large amount of B, in which breaking of a wire occurred during hot rolling so that a specimen could not be obtained.
  • Test No. 27 is an example of use of a steel type U containing a small amount of B, in which the area ratio of the proeutectoid ⁇ increased so that the torsion characteristics and the fatigue strength were deteriorated.
  • Fig. 2 is a drawing-replacing micrograph showing an example of the proeutectoid ⁇ observed in Test No. 3 as an example.
  • An oval 3 shown in Fig. 2 indicates a precipitation position of the proeutectoid ⁇ . From Fig. 2 , it is understood that the proeutectoid ⁇ is precipitated in a plate shape and the "width direction" and "length direction" of grains can be easily discriminated.

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JP5162875B2 (ja) * 2005-10-12 2013-03-13 新日鐵住金株式会社 伸線特性に優れた高強度線材およびその製造方法
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