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US20160319393A1 - Rolled material for high strength spring, and wire for high strength spring using the same - Google Patents

Rolled material for high strength spring, and wire for high strength spring using the same Download PDF

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US20160319393A1
US20160319393A1 US15/107,994 US201415107994A US2016319393A1 US 20160319393 A1 US20160319393 A1 US 20160319393A1 US 201415107994 A US201415107994 A US 201415107994A US 2016319393 A1 US2016319393 A1 US 2016319393A1
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amount
wire
rolled material
high strength
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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
    • 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
    • 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
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES, PROFILES OR LIKE SEMI-MANUFACTURED PRODUCTS OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
    • B21C1/00—Manufacture of metal sheets, wire, rods, tubes or like semi-manufactured products by drawing
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES, PROFILES OR LIKE SEMI-MANUFACTURED PRODUCTS OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
    • B21C1/00—Manufacture of metal sheets, wire, rods, tubes or like semi-manufactured products by drawing
    • B21C1/003—Drawing materials of special alloys so far as the composition of the alloy requires or permits special drawing methods or sequences
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
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    • 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
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    • 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
    • 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
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
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    • 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
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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    • 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
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • 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
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
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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
    • C22C38/42—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
    • 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
    • C21D2211/002—Bainite
    • 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
    • C21D2211/005—Ferrite
    • 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
    • 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
    • C21D2261/00—Machining or cutting being involved

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, which are useful as raw materials of high strength springs that are used in a state of being subjected to heat treatment, namely, quenching and tempering, particularly a rolled material having excellent wire drawability, and a wire for high strength spring, 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 efficiency.
  • wire drawing is applied for the purpose of achieving improvement in dimensional accuracy of a wire diameter and uniformization of a structure due to plastic working before a heat treatment of quenching and tempering.
  • a wire drawing reduction rate is sometimes increased so as to further uniformize the structure in the high strength wire, and a rolled material is required to have satisfactory wire drawability.
  • 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.
  • the steel wire (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 rod 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 unsolved carbides, leading to an improvement in delayed fracture properties.
  • Patent Document 2 a rolled wire rod is subjected to wire drawing, followed by a quenching and tempering treatment through high frequency induction heating. This technology focuses primarily on achievement of the reconciliation of high strength and formability (coiling properties), and gives no consideration to hydrogen embrittlement resistance.
  • Patent Document 3 proposes a hot rolled wire rod having excellent wire drawability under severe wire drawing conditions.
  • Patent Document 3 focuses only on wire drawability during special processing such as severe wire drawing, and also gives no consideration to hydrogen embrittlement resistance 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 for high strength spring, which is a material for high strength spring including both materials for hot coiling and cold coiling, and which has excellent wire drawability even when suppressing the addition amount of an alloying element, and also can exhibit corrosion fatigue properties after quenching and tempering.
  • the present invention that can solve the foregoing problems provides a rolled material for high strength spring, including, in % by mass:
  • an amount of nondiffusible hydrogen is 0.40 ppm by mass or less
  • [name of element] in the above inequality expression (1) means a content expressed in % by mass of each element.
  • the rolled material for high strength spring of the present invention preferably includes, in % by mass, at least one belonging to any one of the following (a), (b), (c) and (d):
  • an ideal critical diameter D i is preferably in a range of 65 to 140 mm, and is calculated using an equation (2) below when B is not included or using an equation (3) below when B is included. If some elements are not included in the rolled material of the present invention among elements mentioned in the equations, calculation is made under the condition that the content of the elements is 0%.
  • the present invention also includes a wire for high strength spring, having a tensile strength of 1,900 MPa or more, obtained by wire-drawing anyone of the rolled materials for high strength spring mentioned above, followed by a quenching and tempering treatment.
  • the rolled material since the amount of nondiffusible hydrogen in a rolled material is suppressed and formation of supercooled structures such as bainite and martensite is suppressed, the rolled material exhibits excellent wire drawability without adding a large amount of an alloying element.
  • an area ratio of ferrite is appropriately adjusted according to the concentration of C, specifically, the area ratio of ferrite decreases as the concentration of C increases, so that a wire obtained by wire-drawing this rolled material, followed by quenching and tempering is excellent in corrosion fatigue properties even though the strength is a high strength in a range of 1,900 MPa or more.
  • FIG. 1 is a graph showing an influence of an amount of C and a ferrite area ratio on hydrogen embrittlement resistance.
  • Wire drawability of a rolled material is usually influenced by ductility of the rolled material. Poor ductility of a basis material or degradation of ductility due to the presence of a supercooled structure may lead to fracture during wire drawing, resulting in drastic degradation of manufacturability. Therefore, wire drawability can be improved by enhancing ductility of the rolled material.
  • corrosion fatigue fracture occurs with these corrosion pits, wall thickness reduction sections, and embrittled sections of the steel material as starting points. Therefore, corrosion fatigue fracture can be improved by improving hydrogen embrittlement resistance and corrosion resistance of the steel material.
  • the inventors of the present invention have made a study of factors that exert an influence on ductility, hydrogen embrittlement resistance and corrosion resistance of a steel material from various viewpoints. As a result, they have found that proper control of both a ferrite area ratio of a rolled material and the amount of hydrogen in a steel expressed particularly by the amount of nondiffusible hydrogen enables an improvement in ductility of the rolled material and significant improvement in hydrogen embrittlement resistance when the rolled material is subjected to wire drawing, followed by quenching and tempering. They have also found that corrosion resistance can also be improved by appropriately adjusting the chemical composition, leading to significant improvement in corrosion fatigue properties, thus completing the present invention.
  • the structure, the amount of hydrogen in steel, and the chemical composition of the rolled material of the present invention will be sequentially described below.
  • the ferrite structure is likely to form a carbide depleted region after quenching and tempering, and formation of the carbide depleted region serves as a fracture starting point, as a strength lowering section. While carbides are capable of detoxicating hydrogen by trapping hydrogen, the carbide depleted region becomes an area lacking such a capability, so that hydrogen embrittlement is likely to occur, leading to fracture. In order to suppress formation of the carbide depleted region after a quenching and tempering treatment to thereby uniformly disperse carbides, there is a need to form a structure in which carbides are uniformly dispersed in a stage of a rolled material before quenching and tempering.
  • a ratio of a pearlite structure which is a structure that ferrite and carbides form layers, is increased to thereby decrease a ratio of a ferrite structure.
  • the inventors of the present invention have found that it is important to make an area ratio of the ferrite structure smaller than that of the ferrite structure obtained by allowing to cool after rolling, so as to improve hydrogen embrittlement resistance, and that the ferrite structure obtained by allowing to cool after rolling has a close relation with the amount of C.
  • the rolled material of the present invention is characterized by controlling the ratio of the ferrite structure so as to satisfy the inequality expression (1) below.
  • the [name of element] in the inequality expression (1) below means a content expressed in % by mass of each element.
  • the ferrite area ratio means a ratio expressed as a percentage.
  • FIG. 1 is a graph showing an influence of an amount of C and a ferrite area ratio on hydrogen embrittlement resistance on the basis of Example data mentioned later.
  • the rolled material of the present invention is significantly characterized by decreasing an area ratio of ferrite as the amount of C increases.
  • the steel material including a large amount of C is required to reduce the ratio of the ferrite structure from a viewpoint that a martensite structure is likely to embrittle, particularly. The less an area ratio of ferrite, the better, and the area ratio of ferrite may be 0%.
  • the ratio of the ferrite structure is preferably reduced by at least 10% lower than that of the ferrite structure obtained by allowing to cool after rolling, namely, the ratio of the ferrite structure preferably satisfies an inequality expression (1-2) below.
  • the area percentage is 2 percentage or less, preferably 1 percentage or less, most preferably 0 percentage or less.
  • an amount of nondiffusible hydrogen is set at 0.40 ppm by mass or less. If a large amount of nondiffusible hydrogen exists, hydrogen is accumulated around inclusions and segregating zones in the rolled material to thereby generate microcracks, resulting in degraded wire drawability of the rolled material. If a large amount of nondiffusible hydrogen exists, 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.
  • Carbon 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 is 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 unsolved 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 is 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, and also exhibits the effect of refining carbides. To effectively exhibit these effects, the amount of Si is 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 steel material, thus requiring the peeling step for removal of the decarburized layer, resulting in increased manufacturing costs. Unsolved carbides also increase, thus degrading hydrogen embrittlement resistance. From such a viewpoint, the amount of Si is 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 is 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 is determined in a range of 1.2% or less.
  • the upper limit of the amount of Mn is preferably 1.0% or less, more preferably 0.85% or less, and still more preferably 0.70% or less.
  • P is a harmful element that degrades ductility such as coiling properties of a rolled material, namely, a wire rod, and the amount thereof is preferably as small as possible. P 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 is 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 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 is 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 about 0.001%.
  • Al is mainly added as a deoxidizing element. This element reacts with N to form AlN to thereby detoxicate solid-saluted N, and also contributes to refining of the structure. To adequately exhibit these effects, the amount of Al is 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 is 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 is determined in a range of 0.1% or more.
  • the lower limit of the amount of Cu is preferably 0.15% or more, more preferably 0.20% or more, and still more preferably 0.25% or more. Excessive Cu amount, however, causes cracks during hot working and increases costs. Therefore, the amount of Cu is determined in a range of 0.80% or less.
  • the upper limit of the amount of Cu is preferably 0.70% or less, more preferably 0.60% or less, still more preferably 0.48% or less, particularly preferably 0.35% or less, and most preferably 0.30% or less.
  • Ni is an element that is effective in suppressing surface decarburization and improving corrosion resistance. Therefore, the amount of Ni is determined in a range of 0.1% or more.
  • the lower limit of the amount of Ni is preferably 0.15% or more, more preferably 0.20% or more, and still more preferably 0.35% or more, and most preferably 0.45% or more. Excessive Ni amount, however, increases costs. Therefore, the amount of Ni is determined in a range of 0.80% or less.
  • the upper limit of the amount of Ni is preferably 0.70% or less, more preferably 0.60% or less, still more preferably 0.55% or less, and yet preferably 0.48% or less, 0.35% or less, and 0.30% or less.
  • the rolled material for spring of the present invention has the chemical composition mentioned above even when suppressing an alloying element such as Cu, 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 0.01% or more, more preferably 0.05% 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 base material.
  • 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 such as coiling properties of a steel material. Therefore, the amount of Mo is preferably 0.5% or less, and more preferably 0.4% or less.
  • V exceeding 0% and 0.4% or less
  • V is an element that contributes to an improvement in strength and refining of crystal grains. To effectively exhibit these effects, the amount of V is preferably 0.1% or more, more preferably 0.15% or more, and still more preferably 0.20% or more. Excessive V amount, however, increases costs. Therefore, the amount of V is preferably 0.4% or less, and more preferably 0.3% or less.
  • Nb, Mo and V may be included individually, or two or more kinds of them may be included in combination.
  • the rolled material of the present invention includes 0 and N as inevitable impurities, and the amount of them is preferably adjusted in a range mentioned below.
  • the upper limit of the amount of O is preferably 0.002% or less, more preferably 0.0015% or less, and still more preferably 0.0013% or less.
  • the lower limit of the amount of O is generally 0.0002% or more (preferably 0.0004% or more) from an industrial viewpoint.
  • the amount of N is preferably as small as possible, 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.
  • an ideal critical diameter D i represented by the equation (2) or (3) below is preferably in a range from 65 to 140 mm.
  • the ideal critical diameter D i is large, hardenability is enhanced and supercooled structures are likely to be generated, so that the upper limit of the ideal critical diameter D i is preferably 140 mm or less.
  • the upper limit of the ideal critical diameter D i is more preferably 135 mm or less, still more preferably 130 mm or less, and particularly preferably 120 mm or less.
  • the lower limit of the ideal critical diameter D i is preferably 65 mm or more, more preferably 70 mm or more, and still more preferably 80 mm 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 average 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 heating stage during 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 by a molten steel treatment to thereby adjust the amount of hydrogen in a molten steel at 2.5 ppm by mass or less.
  • a vacuum tank equipped with two immersion tubes is mounted in a ladle in a secondary refining step and then an Ar gas is blown from the side of one immersion tube, followed by vacuum degassing that enables circulation of a molten steel to the vacuum tank utilizing the buoyancy.
  • 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 1.8 ppm by mass or less, still more preferably 1.5 ppm by mass or less, and particularly preferably 1.0 ppm by mass or less.
  • a homogenizing treatment 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 rolling is set at 0.5° C./second or less, and preferably 0.3° C./second or less.
  • TL is more preferably 910° C. or higher, and still more preferably 930° C. or higher.
  • the upper limit of TL is not particularly limited and is about 1,000° C., although it depends on a finish rolling temperature.
  • an average cooling rate in a range of TL to 650° C. is preferably 2° C./second or more, more preferably 2.3° C./second or more, and still more preferably 2.5° C./second or more. If the cooling rate in a range of TL to 650° C. is excessively increased, supercooled structures such as martensite and bainite are likely to be formed. Therefore, the cooling rate at TL to 650° C. is preferably 5° C./second or less, more preferably 4.5° C./second or less, and still more preferably 4° C./second or less.
  • a cooling rate in a range of 650 to 400° C., at which formation of supercooled structures is initiated, is preferably decreased.
  • An average cooling rate in a range of 650 to 400° C. is preferably 2° C./second or less, more preferably 1.5° C./second or less, and still more preferably 1° C./second or less.
  • the lower limit of the average cooling rate is not particularly limited and is, for example, about 0.3° C./second.
  • a wire is manufactured by wire processing of the rolled material mentioned above, namely, wire drawing.
  • quenching and tempering such as high frequency induction heating are performed after wire drawing, and such a wire is also included in the present invention.
  • 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.
  • quenching and tempering times are respectively in a range of about 10 to 60 seconds.
  • 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 preferably 1,950 MPa or more, and more preferably 2,000 MPa or more.
  • the upper limit of the tensile strength is not particularly limited and 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 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 1,100 to 1,280° C. and further hot rolling to obtain a wire rod having a diameter of 14.3 mm, namely, a rolled material. Whether or not a degassing treatment of a molten steel is implemented, coiling temperature TL after hot rolling, and cooling conditions after cooling are as shown in Tables 4 to 6. In test examples in which “Implementation” is written in the column of the homogenizing treatment, the homogenizing treatment is performed at 1,100° C. for 10 hours or more. In test examples in which the mark “-” is written, the time of the homogenizing treatment at 1,100° C. is less than 10 hours.
  • the structure was identified by the procedure below, and the amount of nondiffusible hydrogen was measured and also wire drawability was measured.
  • bainite and martensite structures are collectively referred to as supercooled structures.
  • the measurement was performed at the position of 1 mm deep from a surface.
  • the observation field has a size of 400 ⁇ m ⁇ 300 ⁇ m and the measurement was performed with respect to five visual fields, and the average was regarded as a ratio of each structure.
  • the ratio of the pearlite structure was determined by subtracting the ratios of ferrite and supercooled structures from 100%.
  • a specimen measuring 20 mm in width ⁇ 40 mm in length was cut out from the rolled material. 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.
  • Wire drawability was evaluated by reduction of area of a tensile test.
  • a JIS No. 14 specimen was cut out from the rolled material and a tensile test was performed under the conditions of a crosshead speed of 10 mm/minute in accordance with JIS 22241 (2011) using a universal tester, and then reduction of area RA was measured
  • the rolled material was subjected to wire drawing, namely, cold drawing to obtain a wire having a diameter of 12.5 mm, followed by quenching and tempering.
  • An area reduction rate of the drawn wire mentioned above is about 23.6% and the conditions of quenching and tempering are as follows.
  • a specimen measuring 10 mm in width ⁇ 1.5 mm in thickness ⁇ 65 mm in length was cut out from the wire after quenching and tempering.
  • stress of 1,400 MPa is applied to the specimen by four-point bending
  • the specimen was immersed in a mixed solution of 0.5 mol/L of sulfuric acid and 0.01 mol/L of potassium thiocyanate.
  • a voltage of ⁇ 700 mV which is less nobler than that of a saturated calomel electrode (SCE) was applied and the fracture time required for the occurrence of cracking was measured.
  • SCE saturated calomel electrode
  • a specimen measuring 10 mm in diameter ⁇ 100 mm in length was cut out from the wire after quenching and tempering by cutting.
  • the specimen was subjected to a salt spray test with an aqueous 5% NaCl solution for 8 hours and then held in a wet atmosphere at 35° C. and a relative humidity of 60% for 16 hours. After repeating this cycle seven times in total, a difference in weight before and after the test was measured and the thus obtained difference was regarded as a corrosion weight loss.
  • Samples of test Nos. 1 to 4, 7 to 11, 15 to 18, 21 to 25, 33, 34, 37 to 40, 45 to 47, 49 to 53, 55 to 60, and 65 to 81 are manufactured from a steel having appropriately adjusted chemical composition under preferred manufacturing conditions mentioned above, so that the amount of nondiffusible hydrogen, and the area ratio of ferrite and supercooled structures satisfy the requirements of the present invention. Therefore, the rolled material exhibits a reduction of area RA of 30% or more in the tensile test and is excellent in wire drawability, and the wire obtained by wire drawing of the rolled material, followed by quenching and tempering has an excellent tensile strength in a range of 1,900 MPa or more.
  • the wire obtained after quenching and tempering exhibits a fracture time of 1,000 seconds or more in an evaluation test of hydrogen embrittlement resistance and a corrosion weight loss of 5.0 g or less in an evaluation test of corrosion resistance, so that the wire is excellent in both hydrogen embrittlement resistance and corrosion resistance.
  • “reduction rate” in Tables 4 to 6 is a value in which a ratio of a difference between a value of right side of the inequality expression (1) and an actual value of a ferrite area ratio to a value of right side of the inequality expression (1) is expressed as percentage.
  • At least any one of the requirements including the chemical composition of a steel, the amount of nondiffusible hydrogen, the ferrite area ratio, and the supercooled structure area ratio does not satisfy the requirements of the present invention, leading to the result that at least any one property of wire drawability of a rolled material, tensile strength, hydrogen embrittlement resistance, and corrosion resistance of a wire is inferior.
  • test Nos. 5, 6, 19 and 20 are not subjected to the above-mentioned treatment for reduction of nondiffusible hydrogen, so that the amount of nondiffusible hydrogen in the rolled material increased, thus degrading wire drawability.
  • 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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