US20170058376A1 - Rolled material for high strength spring, and wire for high strength spring - Google Patents

Rolled material for high strength spring, and wire for high strength spring Download PDF

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US20170058376A1
US20170058376A1 US15/120,168 US201515120168A US2017058376A1 US 20170058376 A1 US20170058376 A1 US 20170058376A1 US 201515120168 A US201515120168 A US 201515120168A US 2017058376 A1 US2017058376 A1 US 2017058376A1
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amount
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Atsuhiko Takeda
Tomokazu MASUDA
Sho TAKAYAMA
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Kobe Steel Ltd
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Kobe Steel Ltd
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    • C—CHEMISTRY; METALLURGY
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    • 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
    • C21D9/525—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length for wire, for rods
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/02—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for springs
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    • 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
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
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    • 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
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    • 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
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    • 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
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    • 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
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    • 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
    • C21D2211/004—Dispersions; Precipitations
    • 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
    • C21D2211/008—Martensite
    • 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
    • C21D3/00—Diffusion processes for extraction of non-metals; Furnaces therefor
    • C21D3/02—Extraction of non-metals
    • C21D3/06—Extraction of hydrogen
    • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/06—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires

Definitions

  • the present invention relates to a rolled material for high strength spring, and a wire for high strength spring using the same. More particularly, the present invention relates to a rolled material and a wire for high strength spring, which are useful as raw materials of high strength springs that are used in a state of being subjected to refining, namely, quenching and tempering, particularly a rolled material having excellent in corrosion fatigue properties after quench and temper, which are excellent in corrosion fatigue properties even though a tensile strength is a high strength in a range of 1,900 MPa or more after wire drawing.
  • Coil springs used in automobiles for example, a valve spring and a suspension spring used in the engine, suspension, and the like are required to reduce the weight and to increase the strength so as to achieve exhaust gas reduction and improvement in fuel economy.
  • the spring imparted with high strength is likely to cause hydrogen brittleness because of its poor toughness and ductility, leading to degradation of corrosion fatigue properties. Therefore, the steel wire (hereinafter, the steel wire is sometimes referred to as a wire) for high strength spring used in the manufacture of a spring is required to have excellent corrosion fatigue properties. Hydrogen generated by corrosion enters into a steel and may lead to embrittlement of a steel material, thus causing corrosion fatigue fracture, so that there is a need to improve corrosion resistance and hydrogen embrittlement resistance of the steel material so as to improve corrosion fatigue properties.
  • Patent Document 1 discloses technology in which a wire material is cold-drawn and then the structure is adjusted by quenching and tempering through high frequency induction heating.
  • a structural fraction of pearlite is set at 30% or less and a structural fraction composed of martensite and bainite is set at 70% or more and then cold drawing is performed at a predetermined area reduction rate, followed by quenching and tempering to thereby reduce the insoluble carbides, leading to an improvement in delayed fracture properties.
  • Patent Document 2 a rolled wire material is subjected to wire drawing, followed by a quenching and tempering treatment through high frequency induction heating in Examples. This technology focuses primarily on achievement of the reconciliation of high strength and coiling properties, and gives no consideration to corrosion fatigue properties.
  • Patent Document 3 proposes a hot rolled wire material having excellent wire drawability under high degree wire drawing conditions.
  • Patent Document 3 focuses only on wire drawability during special processing such as high degree wire drawing, and also gives no consideration to corrosion fatigue properties after quenching and tempering, which becomes most important in a suspension spring.
  • the present invention has been made, and it is an object thereof is to provide a rolled material, which is a material for high strength spring of hot coiling and cold coiling, and also can exhibit excellent corrosion fatigue properties after quenching and tempering even when suppressing the addition amount of an alloying element; and a wire for high strength spring obtained from such a rolled material.
  • the present invention that can solve the foregoing problems provides a rolled material for high strength spring, including, in % by mass:
  • the number of oxide inclusions having an average diameter of 25 ⁇ m or more is 30 or less per 100 g of a steel material, and an amount of nondiffusible hydrogen is 0.40 ppm by mass or less.
  • a major axis and a minor axis of an oxide inclusion are respectively measured by observing using an electron probe micro analyzer (EPMA), and an average of the major axis and the minor axis of the oxide inclusion, namely, a value obtained by dividing the sum of the major axis and the minor axis by 2 is regarded as an average diameter.
  • Inclusions exhibiting this average of 25 ⁇ m or more are objects that are subjected to measurement of the number in the present invention.
  • the rolled material for high strength spring of the present invention further includes, in % by mass, at least one of following (a) to (d):
  • the present invention also include a wire for high strength spring, including any of chemical components of the steel mentioned above, wherein an area ratio of tempered martensite is 80% or more, and a tensile strength is 1,900 MPa or more.
  • oxide inclusions in the rolled material are reduced and the amount of nondiffusible hydrogen is suppressed without adding a large amount of an alloying element, thus making it possible to exhibit excellent corrosion fatigue properties after quenching and tempering.
  • a rolled material it is possible to improve corrosion fatigue properties of the wire even when suppressing the cost of steel materials, thus making it possible to supply a high strength spring which is very unlikely to cause corrosion fatigue fracture, for example, a coil spring such as a suspension spring that is one of automobile components, at a cheap price.
  • FIG. 1 is a graph showing an influence of the number of inclusions and an amount of nondiffusible hydrogen in a rolled material on corrosion fatigue properties.
  • the number of oxide inclusions having an average diameter of 25 ⁇ m or more is set at 30 or less per 100 g of a steel material (hereinafter sometimes referred to as “30 or less per 100 g).
  • the number of oxide inclusions is preferably 20 or less per 100 g, and more preferably 10 or less per 100 g. To improve corrosion fatigue properties, there is no need to set the lower limit of the number of oxide inclusions.
  • the lower limit is preferably 2 or more per 100 g in view of industrial production.
  • Oxide inclusions having an average diameter of 25 ⁇ m or more serve as fracture starting point that is a stress concentration source, thus degrading corrosion fatigue properties, whereas, oxide inclusions having an average diameter of less than 25 ⁇ m do not exert an adverse influence on corrosion fatigue properties.
  • the amount of nondiffusible hydrogen is set at 0.40 ppm by mass or less. If a large amount of nondiffusible hydrogen exists in the rolled material, the amount of nondiffusible hydrogen also increases in the wire after quenching and tempering. If a large amount of nondiffusible hydrogen exists in the wire, a permissible amount of hydrogen, which further enters until the steel material embrittles, decreases. Therefore, even though a small amount of hydrogen entered during use as a spring, embrittlement of the steel material occurs and early fracture is likely to occur, resulting in degraded hydrogen embrittlement resistance.
  • the amount of nondiffusible hydrogen is preferably 0.35 ppm by mass or less, and more preferably 0.30 ppm by mass or less. The less the amount of nondiffusible hydrogen, the better. However, it is difficult to set at 0 ppm by mass and the lower limit is about 0.01 ppm by mass.
  • the amount of nondiffusible hydrogen is an amount of hydrogen measured by the method mentioned in Examples below, and specifically means the total amount of hydrogen released at 300 to 600° C. when the temperature of a steel material is raised at 100° C./hour.
  • the rolled material for high strength spring according to the present invention is a low alloy steel in which the content of an alloying element is suppressed, and the chemical composition is as follows.
  • the present invention also includes a wire obtained by wire-drawing the above-mentioned rolled material, followed by quenching and tempering, and the chemical composition is the same as that of the rolled material.
  • chemical composition means % by mass.
  • C is an element that is required to ensure the strength of a wire for spring, and is also required to generate fine carbides that serve as hydrogen trapping sites. From such a viewpoint, the amount of C was determined in a range of 0.39% or more.
  • the lower limit of the amount of C is preferably 0.45% or more, and more preferably 0.50% or more. Excessive C amount, however, might generate coarse residual austenite and non-solid soluted carbides after quenching and tempering, which further degrades hydrogen embrittlement resistance.
  • C is an element that degrades corrosion resistance, so that there is a need to suppress the amount of C so as to enhance corrosion fatigue properties of a spring product such as a suspension spring which is a final product. From such a viewpoint, the amount of C was determined in a range of 0.65% or less.
  • the upper limit of the amount of C is preferably 0.62% or less, and more preferably 0.60% or less.
  • Si is an element that is required to ensure the strength of a wire for spring, and also exhibits the effect of refining carbides. To effectively exhibit these effects, the amount of Si was determined in a range of 1.5% or more.
  • the lower limit of the amount of Si is preferably 1.7% or more, and more preferably 1.9% or more.
  • Si is also an element that accelerates decarburization
  • excessive Si amount accelerates formation of a decarburized layer on a surface of a wire rod, thus requiring the peeling step for removal of the decarburized layer, resulting in increased manufacturing costs.
  • Non-solid solution carbides also increase, thus degrading hydrogen embrittlement resistance. From such a viewpoint, the amount of Si was determined in a range of 2.5% or less.
  • the upper limit of the amount of Si is preferably 2.3% or less, more preferably 2.2% or less, and still more preferably 2.1% or less.
  • Mn is an element that is employed as a deoxidizing element and reacts with S, which is a harmful element in a steel, to form MnS, and is useful for detoxication of S. Mn is also an element that contributes to an improvement in strength. To effectively exhibit these effects, the amount of Mn was determined in a range of 0.15% or more.
  • the lower limit of the amount of Mn is preferably 0.2% or more, and more preferably 0.3% or more. Excessive Mn amount, however, degrades toughness, thus causing embrittlement of a steel material. From such a viewpoint, the amount of Mn was determined in a range of 1.2% or less.
  • the upper limit of the amount of Mn is preferably 1.0% or less, and more preferably 0.85% or less.
  • P is a harmful element that degrades ductility such as coiling properties of a rolled material such as a wire rod, and the amount thereof is preferably as small as possible. is likely to segregate in grain boundaries to cause grain boundary embrittlement, and hydrogen is likely to cause fracture of grain boundaries, thus exerting an adverse influence on hydrogen embrittlement resistance. From such a viewpoint, the amount of P was determined in a range of 0.015% or less. The upper limit of the amount of P is preferably 0.010% or less, and more preferably 0.008% or less. The amount of P is preferably as small as possible, and is contained usually about 0.001%.
  • S is a harmful element that degrades ductility such as coiling properties of a rolled material, and the amount thereof is preferably as small as possible. S is likely to segregate in grain boundaries to cause grain boundary embrittlement, and hydrogen is likely to cause fracture of grain boundaries, thus exerting an adverse influence on hydrogen embrittlement resistance. From such a viewpoint, the amount of S was determined in a range of 0.015% or less. The upper limit of the amount of S is preferably 0.010% or less, and more preferably 0.008% or less. The amount of S is preferably as small as possible, and is usually contained about 0.001%.
  • Al is mainly added as a deoxidizing element. This element reacts with N to form AlN to thereby detoxicate solid-soluted N, and also contributes to refining of the structure. To adequately exhibit these effects, the amount of Al was determined in a range of 0.001% or more.
  • the lower limit of the amount of Al is preferably 0.002% or more, and more preferably 0.005% or more.
  • Al is an element that accelerates decarburization, like Si, there is a need to suppress the amount of Al in a steel for spring, which includes a large amount of Si. Therefore, in the present invention, the amount of Al was determined in a range of 0.1% or less.
  • the upper limit of the amount of Al is preferably 0.07% or less, more preferably 0.030% or less, and particularly preferably 0.020% or less.
  • Cu is an element that is effective in suppressing surface decarburization and improving corrosion resistance. Therefore, the amount of Cu was determined in a range of 0.10% or more.
  • the lower limit of the amount of Cu is preferably 0.15% or more, and more preferably 0.20% or more. Excessive Cu amount, however, causes cracks during hot working and increases costs. Therefore, the amount of Cu was determined in a range of 0.80% or less.
  • the upper limit of the amount of Cu is preferably 0.70% or less, and more preferably 0.60% or less.
  • the amount of Cu is preferably 0.48% or less, 0.35% or less, and 0.30% or less.
  • Ni is an element that is effective in suppressing surface decarburization and improving corrosion resistance. Therefore, the amount of Ni was determined in a range of 0.10% or more.
  • the lower limit of the amount of Ni is preferably 0.15% or more, and more preferably 0.20% or more. Excessive Ni amount, however, increases costs. Therefore, the amount of Ni was determined in a range of 0.80% or less.
  • the upper limit of the amount of Ni is preferably 0.70% or less, and more preferably 0.60% or less.
  • the amount of Ni is preferably 0.48% or less, 0.35% or less, and 0.30% or less.
  • oxide inclusions such as Al 2 O 3 , SiO 2 , CaO, MgO and TiO 2 are formed. Oxide inclusions are hard, and thus strain is generated around oxide inclusions due to a difference in hardness with a material around oxide inclusions. Hydrogen accumulated on the strain causes embrittlement of grain boundaries around the oxide inclusions. Therefore, reducing the amount of oxygen is important to improve corrosion fatigue properties. Therefore, the upper limit of the amount of O was set at 0.0010% or less. The upper limit is preferably 0.0008% or less, and more preferably 0.0006% or less. Whereas, the lower limit of the amount of O is generally 0.0002% or more on industrial production.
  • the rolled material for spring of the present invention has the chemical composition mentioned above and can achieve excellent coiling properties and hydrogen embrittlement resistance while having high strength. Elements mentioned below may be further included for the purpose of improving corrosion resistance according to application.
  • Cr is an element that is effective in improving corrosion resistance. To effectively exhibit these effects, the amount of Cr is preferably 0.05% or more, more preferably 0.08% or more, and still more preferably 0.10% or more. However, Cr is an element that has a strong tendency to form carbides, and forms peculiar carbides in a steel material and is likely to be dissolved in cementite in a high concentration. It is effective to include a small amount of Cr, however, the heating time of the quenching step decreases in high frequency induction heating, leading to insufficient austenitizing of dissolving carbide, cementite, and the like into a matrix.
  • the amount of Cr is preferably 1.2% or less, more preferably 0.8% or less, and still more preferably 0.6% or less.
  • Ti is an element that is useful to react with S to form sulfide to thereby detoxicate S. Ti also has the effect of refining the structure by forming carbonitride. To effectively exhibit these effects, the amount of Ti is preferably 0.02% or more, more preferably 0.05% or more, and still more preferably 0.06% or more. Excessive Ti amount, however, may form coarse Ti sulfide, thus degrading ductility. Therefore, the amount of Ti is preferably 0.13% or less. From a viewpoint of cost reduction, the amount of Ti is preferably 0.10% or less, and more preferably 0.09% or less.
  • B is an element that improve hardenability and strengthens prior austenite crystal grain boundaries, and also contributes to suppression of fracture.
  • the amount of B is preferably 0.0005% or more, and more preferably 0.0010% or more. Excessive B amount, however, causes saturation of the above effects, so that the amount of B is preferably 0.01% or less, more preferably 0.0050% or less, and still more preferably 0.0040% or less.
  • Nb is an element that forms carbonitride together with C and N, and mainly contributes to refining of the structure.
  • the amount of Nb is preferably 0.003% or more, more preferably 0.005% or more, and still more preferably 0.01% or more. Excessive Nb amount, however, form coarse carbonitride, thus degrading ductility of a steel material. Therefore, the amount of Nb is preferably 0.1% or less. From a viewpoint of cost reduction, the amount is preferably set at 0.07% or less.
  • Mo is also an element that forms carbonitride together with C and N, and contributes to refining of the structure.
  • Mo is an element that is also effective in ensuring the strength after tempering.
  • the amount of Mo is preferably 0.15% or more, more preferably 0.20% or more, and still more preferably 0.25% or more. Excessive Mo amount, however, form coarse carbonitride, thus degrading ductility, for example, coiling properties of a steel material. Therefore, the amount of Mo is preferably 0.5% or less, and more preferably 0.4% or less.
  • Nb and Mo may be included individually, or both of them may be included in combination.
  • the rolled material of the present invention includes N as inevitable impurities, and the amount of it is preferably adjusted in a range mentioned below.
  • N is an element included in inevitable impurities. As the amount of N increases, it forms coarse nitride together with Ti and Al, thus exerting an adverse influence on fatigue properties. Therefore, the amount of N is preferably as small as possible. The amount of N for example, 0.007% or less, and more preferably 0.005% or less. Meanwhile, if the amount of N is too reduced, productivity is drastically degraded. N forms nitride together with Al to thereby contribute to refining of crystal grains. From such a viewpoint, the amount of N is preferably 0.001% or more, more preferably 0.002% or more, and still more preferably 0.003% or more.
  • a method for producing a rolled material of the present invention will be described below.
  • it is possible to control the amount of nondiffusible hydrogen of the rolled material by adjusting at least one of (A) the amount of hydrogen in a molten steel stage, (B) the homogenizing treatment temperature and time before blooming, and (C) the cooling rate in a range of 400 to 100° C. after hot rolling.
  • the method of reducing the amount of hydrogen in the steel include a method of adjusting in a molten steel stage, a method of adjusting in a stage of a continuously cast material at 1,000° C. or higher after solidification, a method of adjusting in a heating stage before hot rolling, a method of adjusting in a heated stage during hot rolling, and a method of adjusting in a cooling stage after rolling. It is particularly preferred to perform at least one of treatments for reducing nondiffusible hydrogen (A) to (C) mentioned below.
  • a degassing treatment is performed in a melting steel process to thereby adjust the amount of hydrogen in a molten steel at 2.5 ppm by mass or less.
  • This method is excellent in hydrogen removing capability and reduction in inclusion.
  • the amount of hydrogen in the molten steel is preferably 2.0 ppm by mass or less, more preferably 1.5 ppm by mass or less, and particularly preferably 1.0 ppm by mass or less.
  • a homogenizing treatment (heating) before blooming is performed at 1,100° C. or higher, and preferably 1,200° C. or higher for 10 hours or more.
  • An average cooling rate in a range of 400 to 100° C. after hot rolling is set at 0.5° C./second or less, and preferably 0.3° C./second or less.
  • coiling temperature TL after hot rolling there is no particular limitation on the coiling temperature TL after hot rolling, and cooling conditions beyond a temperature range of 400 to 100° C. after coiling.
  • the coiling temperature TL can be set, for example, in a range of 900° C. or higher and 1,000° C. or lower, and is preferably 910° C. or higher, and more preferably 930° C. or higher.
  • An average cooling rate at the coiling temperature of TL to 650° C. can be set in a range of 2° C./second or more and 5° C./second or less.
  • the lower limit of the average cooling rate at the coiling temperature of TL to 650° C. is preferably 2.3° C./second or more, and more preferably 2.5° C./second or more.
  • the average cooling rate at 650 to 400° C. can be set at 2° C./second or less.
  • the average cooling rate at 650 to 400° C. is preferably 1.5° C./second or less, and more preferably 1° C./second or less.
  • a wire is manufactured by wire processing, namely, wire drawing of the rolled material mentioned above.
  • wire processing namely, wire drawing of the rolled material mentioned above.
  • quenching and tempering by high frequency induction heating are performed after wire drawing, and such a wire is also included in the present invention.
  • a high strength wire having a tensile strength of 1,900 MPa or more can be obtained by subjecting the rolled material to wire working, namely, wire drawing, followed by quenching and tempering by high frequency induction heating.
  • the rolled material is subjected to wire drawing at an area reduction rate of about 5 to 35%, followed by quenching at about 900 to 1,000° C. and further tempering at about 300 to 520° C.
  • the quenching temperature is preferably 900° C. or higher so as to sufficiently perform austenitizing, and preferably 1,000° C. or lower so as to prevent grain coarsening.
  • the heating temperature for tempering may be set at an appropriate temperature in a range of 300 to 520° C. according to a target value of a wire strength. When quenching and tempering are performed by high frequency induction heating, quenching and tempering times are respectively in a range of about 10 to 60 seconds.
  • the tempered martensite structure has 80 area or more. As a result of increasing the proportions of non-solid-soluted ferrite and residual austenite, the strength decreases.
  • the tempered martensite structure preferably has 88 area % or more. To set the proportion of the tempered martensite structure at 80 area % or more, it is preferable that the material is heated to 900° C. or higher when heating before quenching, followed by sufficient austenitizing and further cooling to 100° C. or lower by water cooling or oil cooling.
  • the thus obtained wire of the present invention can realize a high tensile strength in a range of 1,900 MPa or more.
  • the tensile strength is usually selected in a range of 1,900 MPa to 2,200 MPa.
  • the upper limit is about 2,500 MPa.
  • the wire of the present invention can exhibit corrosion fatigue properties even at a high strength in a range of 1,900 MPa or more because of use of the rolled material of the present invention.
  • Each of steel materials having chemical compositions shown in Tables 1 to 3 was melted by melting in a converter and then subjecting to continuous casting and a homogenizing treatment at 1,100° C. or higher. After the homogenizing treatment, blooming was performed, followed by heating at 1,000 to 1,280° C. and further hot rolling to obtain a rolled material having a diameter of 14.3 mm, namely, a wire rod. It is as shown in Tables 4 to 6 below whether or not a degassing treatment of a molten steel by the above-mentioned material is implemented, and whether or not cooling is implemented after coiling, namely, whether or not cooling at an average cooling rate of 0.5° C./second or less is implemented at 400 to 100° C. after rolling. The amount of 0 in the molten steel shown in Tables 4 to 6 was adjusted by controlling the degree of deoxidizing with aluminum and silicon.
  • the coiling temperature TL after hot rolling was set at 950° C., and other cooling after coiling was performed at an average cooling rate of 4° C./second at a temperature in a range of TL to 650° C., and performed at an average cooling rate of 1° C./second at a temperature in a range of 650 to 400° C.
  • the homogenizing treatment is performed at 1,100° C. for 10 hours or more.
  • the time of the homogenizing treatment at 1,100° C. is less than 10 hours.
  • a specimen measuring 20 mm in width ⁇ 40 mm in length was cut out from the rolled material, namely, wire rod. After raising the temperature of the specimen at a temperature rise rate of 100° C./hour, a hydrogen release amount at 300 to 600° C. was measured using a gas chromatogram, and the hydrogen release amount was regarded as the amount of nondiffusible hydrogen.
  • the number of oxide inclusions was determined by the following procedure: an average of the results of examination of six rolled material samples each having 50 g in weight was determined, followed by conversion into the number per 100 g. The number of inclusions was examined by an acid dissolution method. Each sample (50 g) was dissolved with an acid and inclusions, remaining without being dissolved, was allowed to remain on a filter paper. Inclusions having an average diameter of 25 ⁇ m or more were sorted by EPMA, analyzed by energy dispersive X-ray spectrometry (EDX), and then oxide inclusions were sorted.
  • EDX energy dispersive X-ray spectrometry
  • the number of oxide inclusions having an average diameter of 25 ⁇ m or more was measured and an average thereof was determined, followed by conversion into the number per 100 g of a steel material.
  • nitric acid adjusted so as not to dissolve oxide inclusions was used for dissolution with an acid.
  • An average diameter of oxide inclusions means an average of a major axis and a minor axis, namely, the value obtained by dividing the sum of the major axis and the minor axis by 2.
  • oxygen was removed by sufficiently perform vacuum degassing when melting in a converter.
  • wire drawing namely, cold drawing to thereby reduce to a diameter of 12.5 mm
  • quenching and tempering An area reduction rate of wire drawing is about 23.6%, and the conditions of quenching and tempering are as follows.
  • Heating rate 200° C./second
  • Tempering each temperature in a range of 300 to 520° C., 20 seconds, water cooling
  • a wire was cut into a predetermined length and a tensile test was performed at a distance between chucks of 200 mm and a tensile speed of 5 mm/minute in accordance with JIS Z2241 (2011).
  • Corrosion fatigue properties were evaluated by fracture life after subjecting to a corrosion treatment and performing the Ono-type rotating-bending fatigue test.
  • Each wire subjected to quenching and tempering was cut to fabricate a No. 1 test specimen (JIS Z 2274 (1978)).
  • the parallel part of this test specimen was polished using a sand paper of No. 800.
  • a test was carried out without shot peening of a surface. First, the test specimen thus processed was subjected to a corrosion treatment under the following conditions.
  • test specimen was subjected to a corrosion treatment by repeating 10 cycles in total. After the corrosion treatment, the test specimen was subjected to the rotating-bending test and then corrosion fatigue properties were evaluated. Using ten test specimens for each test, load stress was set at 500 MPa and the Ono-type rotating-bending fatigue test was carried out. Measurement was made of fatigue life until fracture of each test specimen occurs. An average of each fatigue life of ten test specimens was measured. The case where the average of fatigue life is 100,000 times or more was rated as excellent corrosion fatigue life.
  • the samples of test Nos. 17 to 31 shown in Table 5 are inferior in corrosion fatigue properties because at least any one of requirements of chemical composition of a steel material, the number of oxide inclusions and the amount of nondiffusible hydrogen defined in the present invention is invalid.
  • test Nos. 17 and 18 are examples using steels Nos. 17 ad 18 that contains neither Cu nor Ni added therein, or do not meet the defined lower limit, and thus corrosion fatigue properties were degraded.
  • insufficient deoxidizing treatment leads to excess amount of O in the steel, so that the number of oxide inclusions in the rolled material increased and corrosion fatigue properties were degraded.
  • FIG. 1 an influence of the number of oxide inclusions and the amount of nondiffusible hydrogen in the rolled material on corrosion fatigue properties is shown in FIG. 1 .
  • invented examples denote samples of test Nos. 1 to 16 in Table 4 and comparative examples (expressed by the symbol “x” (cross)) denote samples of test Nos. 19 to 31 in Table 5, and the number of oxide inclusions in the rolled material was mentioned as “Number of Inclusions”.
  • the rolled material and the wire of the present invention are industrially useful since they can be suitably used for coil springs that are used in automobiles, for example, a valve spring, a suspension spring, and the like that are used in the engine, suspension, and the like.

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