US20190376167A1 - Steel Material and Method for Producing Steel Material - Google Patents

Steel Material and Method for Producing Steel Material Download PDF

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US20190376167A1
US20190376167A1 US16/476,704 US201816476704A US2019376167A1 US 20190376167 A1 US20190376167 A1 US 20190376167A1 US 201816476704 A US201816476704 A US 201816476704A US 2019376167 A1 US2019376167 A1 US 2019376167A1
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steel material
chemical composition
content
steel
material according
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Shinji Yoshida
Yuji Arai
Atsushi Soma
Hiroki KAMITANI
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Nippon Steel Corp
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Nippon Steel Corp
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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
    • C22C38/40—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
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • 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
    • 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
    • C21D8/005—Modifying the physical properties by deformation combined with, or followed by, heat treatment 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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • 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
    • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
    • 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
    • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
    • C21D8/105—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies 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
    • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
    • C21D9/085—Cooling or quenching
    • 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
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    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • 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
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/28—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
    • C22C38/32—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
    • C22C38/18—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
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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
    • 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
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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
    • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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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
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron

Definitions

  • the present invention relates to a steel material and a method for producing the steel material, and more particularly relates to a steel material suitable for use in a sour environment, and a method for producing the steel material.
  • oil wells and gas wells Due to the deepening of oil wells and gas wells (hereunder, oil wells and gas wells are collectively referred to as “oil wells”), there is a demand to enhance the strength of oil-well steel pipes.
  • 80 ksi grade (yield strength is 80 to 95 ksi, that is, 551 to 655 MPa) and 95 ksi grade (yield strength is 95 to 110 ksi, that is, 655 to 758 MPa) oil-well steel pipes are being widely utilized, and recently requests are also starting to be made for 110 ksi grade (yield strength is 110 to 125 ksi, that is, 758 to 862 MPa), 125 ksi grade (yield strength is 125 ksi to 140 ksi, that is, 862 to 965 MPa) and 140 ksi grade (yield strength is 140 ksi to 155 ksi, that is, 965 to 1069 MPa)
  • SSC resistance sulfide stress cracking resistance
  • Patent Literature 1 Japanese Patent Application Publication No. 62-253720
  • Patent Literature 2 Japanese Patent Application Publication No. 59-232220
  • Patent Literature 3 Japanese Patent Application Publication No. 6-322478
  • Patent Literature 4 Japanese Patent Application Publication No. 8-311551
  • Patent Literature 5 Japanese Patent Application Publication No. 2000-256783
  • Patent Literature 6 Japanese Patent Application Publication No. 2005-350754
  • Patent Literature 7 National Publication of International Patent Application No. 2012-519238 (Patent Literature 8) and Japanese Patent Application Publication No. 2012-26030
  • Patent Literature 9 Japanese Patent Application Publication No.
  • Patent Literature 1 proposes a method for improving the SSC resistance of steel for oil wells by reducing impurities such as Mn and P.
  • Patent Literature 2 proposes a method for improving the SSC resistance of steel by performing quenching twice to refine the grains.
  • Patent Literature 3 proposes a method for improving the SSC resistance of a 125 ksi grade steel material by refining the steel microstructure by a heat treatment using induction heating.
  • Patent Literature 4 proposes a method for improving the SSC resistance of steel pipes of 110 to 140 ksi grade by enhancing the hardenability of the steel by utilizing a direct quenching process and also increasing the tempering temperature.
  • Patent Literature 5 and Patent Literature 6 each propose a method for improving the SSC resistance of a steel for low-alloy oil country tubular goods of 110 to 140 ksi grade by controlling the shapes of carbides.
  • Patent Literature 7 proposes a method for improving the SSC resistance of steel material of 125 ksi (862 MPa) grade or higher by controlling the dislocation density and the hydrogen diffusion coefficient to desired values.
  • Patent Literature 8 proposes a method for improving the SSC resistance of steel of 125 ksi (862 MPa) grade by subjecting a low-alloy steel containing 0.3 to 0.5% of C to quenching multiple times.
  • Patent Literature 9 proposes a method for controlling the shapes or number of carbides by employing a tempering process composed of a two-stage heat treatment. More specifically, in Patent Literature 9, a method is proposed that enhances the SSC resistance of 125 ksi (862 MPa) grade steel by suppressing the number density of large M 3 C particles or M 2 C particles.
  • Patent Literature 1 Japanese Patent Application Publication No. 62-253720
  • Patent Literature 2 Japanese Patent Application Publication No. 59-232220
  • Patent Literature 3 Japanese Patent Application Publication No. 6-322478
  • Patent Literature 4 Japanese Patent Application Publication No. 8-311551
  • Patent Literature 5 Japanese Patent Application Publication No. 2000-256783
  • Patent Literature 6 Japanese Patent Application Publication No. 2000-297344
  • Patent Literature 7 Japanese Patent Application Publication No. 2005-350754
  • Patent Literature 8 National Publication of International Patent Application No. 2012-519238
  • Patent Literature 9 Japanese Patent Application Publication No. 2012-26030
  • Patent Literatures 1 to 9 even if the techniques disclosed in the aforementioned Patent Literatures 1 to 9 are applied, in the case of steel material (for example, oil-well steel pipes) having a yield strength of 140 ksi grade (965 to 1069 MPa), excellent SSC resistance cannot be stably obtained in some cases.
  • An objective of the present disclosure is to provide a steel material that has a yield strength within a range of 965 to 1069 MPa (140 to 155 ksi; 140 ksi grade) and that also has excellent SSC resistance.
  • a steel material according to the present disclosure contains a chemical composition consisting of, in mass %, C: more than 0.50 to 0.80%, Si: 0.05 to 1.00%, Mn: 0.05 to 1.00%, P: 0.025% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, Cr: 0.20 to 1.50%, Mo: 0.25 to 1.50%, Ti: 0.002 to 0.050%, B: 0.0001 to 0.0050%, N: 0.002 to 0.010%, O: 0.0100% or less, V: 0 to 0.30%, Nb: 0 to 0.100%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Co: 0 to 0.50%, W: 0 to 0.50%, Ni: 0 to 0.50% and Cu: 0 to 0.50%, with the balance being Fe and impurities.
  • the steel material according to the present disclosure also contains an amount of dissolved C within a range of 0.010 to 0.060 mass %.
  • the steel material according to the present disclosure also has a yield strength within a range of 965 to 1069 MPa, and a yield ratio of the steel material is 90% or more.
  • a method for producing a steel material according to the present disclosure includes a preparation process, a quenching process and a tempering process.
  • the preparation process an intermediate steel material containing the aforementioned chemical composition is prepared.
  • the quenching process after the preparation process, the intermediate steel material that is at a temperature in a range of 800 to 1000° C. is cooled at a cooling rate of 50° C./min or more.
  • the tempering process the intermediate steel material after the quenching is held for 10 to 90 minutes at a temperature in a range of 660° C. to an A c1 point, and thereafter is cooled from 600° C. to 200° C. at an average cooling rate of 5 to 300° C./sec.
  • the steel material according to the present disclosure has a yield strength within a range of 965 to 1069 MPa (140 ksi grade), and also has excellent SSC resistance.
  • FIG. 1 is a view illustrating the relation between the amount of dissolved C and a fracture toughness value K 1SSC .
  • FIG. 2A shows a side view and a cross-sectional view of a DCB test specimen that is used in a DCB test in the examples.
  • FIG. 2B is a perspective view of a wedge that is used in the DCB test in the examples.
  • the present inventors conducted investigations and studies regarding a method for obtaining both a yield strength in a range of 965 to 1069 MPa (140 ksi grade) and SSC resistance in a steel material that it is assumed will be used in a sour environment, and obtained the following findings.
  • the yield strength of the steel material will increase.
  • dislocations will occlude hydrogen. Therefore, if the dislocation density of the steel material increases, there is a possibility that the amount of hydrogen that the steel material occludes will also increase.
  • the hydrogen concentration in the steel material increases as a result of increasing the dislocation density, even if high strength is obtained, the SSC resistance of the steel material will decrease. Accordingly, at first glance it seems that, in order to obtain both a high strength of 140 ksi grade (965 to 1069 MPa) and SSC resistance, utilizing the dislocation density to enhance the strength is not preferable.
  • the present inventors discovered that by adjusting the amount of dissolved C in a steel material, excellent SSC resistance can also be obtained while at the same time raising the yield strength to 140 ksi grade (965 to 1069 MPa) by utilizing the dislocation density.
  • the reason is not certain, it is considered that the reason may be as follows.
  • Dislocations include mobile dislocations and sessile dislocations, and it is considered that dissolved C in a steel material immobilizes mobile dislocations to thereby form sessile dislocations.
  • dissolved C in a steel material immobilizes mobile dislocations to thereby form sessile dislocations.
  • the disappearance of dislocations can be inhibited, and thus a decrease in the dislocation density can be suppressed. In this case, the yield strength of the steel material can be maintained.
  • the sessile dislocations that are formed by dissolved C reduce the amount of hydrogen that is occluded in the steel material more than mobile dislocations. Therefore, it is considered that by increasing the density of sessile dislocations that are formed by dissolved C, the amount of hydrogen that is occluded in the steel material is reduced. As a result, the SSC resistance of the steel material can be increased. It is considered that because of this mechanism, excellent SSC resistance is obtained even when the steel material has high strength of 140 ksi grade.
  • the present inventors considered that by appropriately adjusting the amount of dissolved C in a steel material, the SSC resistance of the steel material can be increased while maintaining a yield strength of 140 ksi grade. Therefore, using a steel material containing chemical composition consisting of, in mass %, C: more than 0.50 to 0.80%, Si: 0.05 to 1.00%, Mn: 0.05 to 1.00%, P: 0.025% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, Cr: 0.20 to 1.50%, Mo: 0.25 to 1.50%, Ti: 0.002 to 0.050%, B: 0.0001 to 0.0050%, N: 0.002 to 0.010%, 0: 0.0100% or less, V: 0 to 0.30%, Nb: 0 to 0.100%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Co: 0 to 0.50%, W: 0 to 0.50%, Ni:
  • FIG. 1 is a view illustrating the relation between the amount of dissolved C and a fracture toughness value K 1SSC for respective test numbers of the examples.
  • FIG. 1 was obtained by the following method.
  • FIG. 1 was created using the amount of dissolved C (mass %) and the fracture toughness value K 1SSC (MPa ⁇ m) obtained with respect to steel materials for which, among the steel materials of the examples that are described later, conditions other than the amount of dissolved C satisfied the range of the present embodiment.
  • the yield strength YS of each of the steel materials shown in FIG. 1 was within the range of 965 to 1069 MPa (140 ksi grade). Adjustment of the yield strength YS was performed by adjusting the tempering temperature. Further, with respect to the SSC resistance, if the fracture toughness value K 1SSC that is an index of SSC resistance was 30.0 MPa ⁇ m or more, it was determined that the SSC resistance was good.
  • the fracture toughness value K 1SSC becomes 30.0 MPa ⁇ m or more and excellent SSC resistance can be obtained.
  • the amount of dissolved C of the steel material is set within the range of 0.010 to 0.060 mass %.
  • the microstructure of the steel is made a microstructure that is principally composed of tempered martensite and tempered bainite.
  • the term “principally composed of tempered martensite and tempered bainite” means that the total volume ratio of tempered martensite and tempered bainite is 90% or more.
  • a steel material according to the present embodiment that was completed based on the above findings contains a chemical composition consisting of, in mass %, C: more than 0.50 to 0.80%, Si: 0.05 to 1.00%, Mn: 0.05 to 1.00%, P: 0.025% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, Cr: 0.20 to 1.50%, Mo: 0.25 to 1.50%, Ti: 0.002 to 0.050%, B: 0.0001 to 0.0050%, N: 0.002 to 0.010%, 0: 0.0100% or less, V: 0 to 0.30%, Nb: 0 to 0.100%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Co: 0 to 0.50%, W: 0 to 0.50%, Ni: 0 to 0.50% and Cu: 0 to 0.50%, with the balance being Fe and impurities.
  • the steel material according to the present embodiment further contains an amount of dissolved C within a range of 0.010 to 0.060 mass %. Further, in the steel material according to the present embodiment, the yield strength is within a range of 965 to 1069 MPa, and the yield ratio is 90% or more.
  • the steel material is, for example, a steel pipe or a steel plate.
  • the aforementioned chemical composition may contain one or more types of element selected from the group consisting of V: 0.01 to 0.30% and Nb: 0.002 to 0.100%.
  • the aforementioned chemical composition may contain one or more types of element selected from the group consisting of Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100% and Zr: 0.0001 to 0.0100%.
  • the aforementioned chemical composition may contain one or more types of element selected from the group consisting of Co: 0.02 to 0.50% and W: 0.02 to 0.50%.
  • the aforementioned chemical composition may contain one or more types of element selected from the group consisting of Ni: 0.02 to 0.50% and Cu: 0.01 to 0.50%.
  • the aforementioned steel material may be an oil-well steel pipe.
  • the oil-well steel pipe may be a steel pipe that is used for a line pipe or may be a steel pipe used for oil country tubular goods (OCTG).
  • the oil-well steel pipe may be a seamless steel pipe, or may be a welded steel pipe.
  • the oil country tubular goods are, for example, steel pipes that are used as casing pipes or tubing pipes.
  • an oil-well steel pipe according to the present embodiment is a seamless steel pipe. If the oil-well steel pipe according to the present embodiment is a seamless steel pipe, even if the wall thickness is 15 mm or more, the oil-well steel pipe will have a yield strength within a range of 965 to 1069 MPa (140 ksi grade) and will also have excellent SSC resistance.
  • excellent SSC resistance means, specifically, that a value of K 1SSC (MPa ⁇ m) is 30.0 MPa ⁇ m or more in a DCB test performed in accordance with “Method D” described in NACE TM0177-2005 using an autoclave in which a solution obtained by mixing a degassed 5% saline solution and 4 g/L of sodium acetate and adjusting to pH 3.5 using hydrochloric acid, and a gaseous mixture consisting of 10% H 2 S gas and 90% CO 2 gas at a total pressure of 1 atm were sealed.
  • the term “amount of dissolved C” mentioned above means the difference between the amount of C (mass %) in carbides in the steel material and the C content of the chemical composition of the steel material.
  • the amount of C in carbides in the steel material is determined by Formula (1) to Formula (5) using an Fe concentration ⁇ Fe>a, a Cr concentration ⁇ Cr>a, an Mn concentration ⁇ Mn>a, an Mo concentration ⁇ Mo>a, a V concentration ⁇ V>a and an Nb concentration ⁇ Nb>a in carbides (cementite and MC-type carbides) obtained as residue when extraction residue analysis is performed on the steel material, and an Fe concentration ⁇ Fe>b, a Cr concentration ⁇ Cr>b, an Mn concentration ⁇ Mn>b and an Mo concentration ⁇ Mo>b in cementite obtained by performing point analysis by EDS with respect to cementite identified by performing TEM observation of a replica film obtained by an extraction replica method.
  • ⁇ Mo> c ( ⁇ Fe> a + ⁇ Cr> a + ⁇ Mn> a ) x ⁇ Mo> b /( ⁇ Fe> b + ⁇ Cr> b + ⁇ Mn> b ) (1)
  • ⁇ C> a ( ⁇ Fe> a/ 55.85+ ⁇ Cr> a/ 52+ ⁇ Mn> a/ 53.94+ ⁇ Mo> c/ 95.9)/3 ⁇ 12 (3)
  • ⁇ C> b ( ⁇ V> a/ 50.94+ ⁇ Mo> d/ 95.9+ ⁇ Nb> a/ 92.9) ⁇ 12 (4)
  • cementite means carbides containing an Fe content of 50 mass % or more.
  • a method for producing a steel material according to the present embodiment includes a preparation process, a quenching process and a tempering process.
  • the preparation process an intermediate steel material containing the aforementioned chemical composition is prepared.
  • the quenching process after the preparation process, the intermediate steel material that is at a temperature in a range of 800 to 1000° C. is cooled at a cooling rate of 50° C./min or more.
  • the tempering process the intermediate steel material after quenching is held at a temperature in a range of 660° C. to the A c1 point for 10 to 90 minutes, and thereafter the intermediate steel material is cooled at an average cooling rate of 5 to 300° C./sec with respect to cooling from 600° C. to 200° C.
  • intermediate steel material refers to a hollow shell in a case where the end product is a steel pipe, and refers to a plate-shaped steel material in a case where the end product is a steel plate.
  • the preparation process of the aforementioned production method may include a starting material preparation process of preparing a starting material containing the aforementioned chemical composition, and a hot working process of subjecting the starting material to hot working to produce an intermediate steel material.
  • the chemical composition of the steel material according to the present embodiment contains the following elements.
  • Carbon (C) enhances the hardenability and increases the strength of the steel material. C also promotes spheroidization of carbides during tempering in the production process, and increases the SSC resistance of the steel material. If the carbides are dispersed, the strength of the steel material increases further. These effects will not be obtained if the C content is too low. On the other hand, if the C content is too high, the toughness of the steel material will decrease and quench cracking is liable to occur. Therefore, the C content is within the range of more than 0.50 to 0.80%. A preferable lower limit of the C content is 0.51%. A preferable upper limit of the C content is 0.70%, and more preferably is 0.62%.
  • Si deoxidizes the steel. If the Si content is too low, this effect is not obtained. On the other hand, if the Si content is too high, the SSC resistance of the steel material decreases. Therefore, the Si content is within the range of 0.05 to 1.00%. A preferable lower limit of the Si content is 0.15%, and more preferably is 0.20%. A preferable upper limit of the Si content is 0.85%, and more preferably is 0.50%.
  • Mn Manganese deoxidizes the steel material. Mn also enhances the hardenability. If the Mn content is too low, these effects are not obtained. On the other hand, if the Mn content is too high, Mn segregates at grain boundaries together with impurities such as P and S. In such a case, the SSC resistance of the steel material will decrease. Therefore, the Mn content is within a range of 0.05 to 1.00%. A preferable lower limit of the Mn content is 0.25%, and more preferably is 0.30%. A preferable upper limit of the Mn content is 0.90%, and more preferably is 0.80%.
  • Phosphorous (P) is an impurity.
  • the P content is more than 0%. P segregates at the grain boundaries and decreases the SSC resistance of the steel material. Therefore, the P content is 0.025% or less.
  • a preferable upper limit of the P content is 0.020%, and more preferably is 0.015%.
  • the P content is as low as possible. However, if the P content is excessively reduced, the production cost increases significantly. Therefore, when taking industrial production into consideration, a preferable lower limit of the P content is 0.0001%, more preferably is 0.0003%, and further preferably is 0.001%.
  • S Sulfur
  • the S content is more than 0%. S segregates at the grain boundaries and decreases the SSC resistance of the steel material. Therefore, the S content is 0.0100% or less.
  • a preferable upper limit of the S content is 0.0050%, and more preferably is 0.0030%.
  • the S content is as low as possible. However, if the S content is excessively reduced, the production cost increases significantly. Therefore, when taking industrial production into consideration, a preferable lower limit of the S content is 0.0001%, more preferably is 0.0002%, and further preferably is 0.0003%.
  • Chromium (Cr) enhances the hardenability of the steel material. Cr also increases temper softening resistance of the steel material and enables high-temperature tempering. As a result, the SSC resistance of the steel material increases. If the Cr content is too low, aforementioned effects are not obtained. On the other hand, if the Cr content is too high, the toughness and SSC resistance of the steel material decreases. Therefore, the Cr content is within a range of 0.20 to 1.50%. A preferable lower limit of the Cr content is 0.25%, and more preferably is 0.30%. A preferable upper limit of the Cr content is 1.30%.
  • Molybdenum (Mo) enhances the hardenability of the steel material. Mo also forms fine carbides and increases the temper softening resistance of the steel material. As a result, Mo increases the SSC resistance of the steel material by high temperature tempering. If the Mo content is too low, these effects are not obtained. On the other hand, if the Mo content is too high, the aforementioned effects are saturated. Therefore, the Mo content is within a range of 0.25 to 1.50%. A preferable lower limit of the Mo content is 0.50%, and more preferably is 0.65%. A preferable upper limit of the Mo content is 1.20%, and more preferably is 1.00%.
  • Titanium (Ti) forms nitrides, and refines crystal grains by the pinning effect. As a result, the strength of the steel material increases. If the Ti content is too low, this effect is not obtained. On the other hand, if the Ti content is too high, Ti nitrides coarsen and the SSC resistance of the steel material decreases. Therefore, the Ti content is within a range of 0.002 to 0.050%. A preferable lower limit of the Ti content is 0.003%, and more preferably is 0.005%. A preferable upper limit of the Ti content is 0.030%, and more preferably is 0.020%.
  • B Boron
  • a preferable lower limit of the B content is 0.0003%, and more preferably is 0.0007%.
  • a preferable upper limit of the B content is 0.0035%, and more preferably is 0.0025%.
  • N Nitrogen
  • N is unavoidably contained. N combines with Ti to form fine nitrides and thereby refines the grains. On the other hand, if the N content is too high, N will form coarse nitrides and the SSC resistance of the steel material will decrease. Therefore, the N content is within the range of 0.002 to 0.010%. A preferable upper limit of the N content is 0.005%, and more preferably is 0.004%.
  • Oxygen (O) is an impurity.
  • the O content is more than 0%.
  • O forms coarse oxides and reduces the corrosion resistance of the steel material. Therefore, the O content is 0.0100% or less.
  • a preferable upper limit of the O content is 0.0030%, and more preferably is 0.0020%.
  • the O content is as low as possible. However, if the O content is excessively reduced, the production cost increases significantly. Therefore, when taking industrial production into consideration, a preferable lower limit of the O content is 0.0001%, more preferably is 0.0002%, and further preferably is 0.0003%.
  • the balance of the chemical composition of the steel material according to the present embodiment is Fe and impurities.
  • impurities refers to elements which, during industrial production of the steel material, are mixed in from ore or scrap that is used as a raw material of the steel material, or from the production environment or the like, and which are allowed within a range that does not adversely affect the steel material according to the present embodiment.
  • the chemical composition of the steel material described above may further contain one or more types of element selected from the group consisting of V and Nb in lieu of a part of Fe. Each of these elements is an optional element, and increases the SSC resistance of the steel material.
  • Vanadium (V) is an optional element, and need not be contained. In other words, the V content may be 0%. If contained, V combines with C or N to form carbides, nitrides or carbo-nitrides and the like (hereinafter, referred to as “carbo-nitrides and the like”). These carbo-nitrides and the like refine the substructure of the steel material by the pinning effect, and improve the SSC resistance of the steel. V also forms fine carbides during tempering. The fine carbides increase the temper softening resistance of the steel material, and increase the strength of the steel material.
  • V also forms spherical MC-type carbides
  • V suppresses the formation of acicular M 2 C-type carbides and thereby increases the SSC resistance of the steel material. If even a small amount of V is contained, aforementioned effects are obtained to a certain extent. However, if the V content is too high, the toughness of the steel material decreases. Therefore, the V content is within the range of 0 to 0.30%.
  • a preferable lower limit of the V content is more than 0%, more preferably is 0.01%, and further preferably is 0.02%.
  • a preferable upper limit of the V content is 0.20%, more preferably is 0.15%, and further preferably is 0.12%.
  • Niobium (Nb) is an optional element, and need not be contained. In other words, the Nb content may be 0%. If contained, Nb forms carbo-nitrides and the like. These carbo-nitrides and the like refine the substructure of the steel material by the pinning effect, and increase the SSC resistance of the steel material. In addition, because Nb also forms spherical MC-type carbides, Nb suppresses the formation of acicular M 2 C-type carbides and thereby increases the SSC resistance of the steel material. If even a small amount of Nb is contained, aforementioned effects are obtained to a certain extent.
  • the Nb content is within the range of 0 to 0.100%.
  • a preferable lower limit of the Nb content is more than 0%, more preferably is 0.002%, further preferably is 0.003%, and further preferably is 0.007%.
  • a preferable upper limit of the Nb content is 0.025%, and more preferably is 0.020%.
  • a total of the contents of the aforementioned V and Nb is preferably 0.30% or less, and further preferably is 0.20% or less.
  • the chemical composition of the steel material described above may further contain one or more types of element selected from the group consisting of Ca, Mg and Zr in lieu of a part of Fe. Each of these elements is an optional element, and increases the SSC resistance of the steel material.
  • Ca Calcium
  • the Ca content may be 0%. If contained, Ca refines sulfides in the steel material and increases the SSC resistance of the steel material. If even a small amount of Ca is contained, aforementioned effect is obtained to a certain extent. However, if the Ca content is too high, oxides in the steel material coarsen and the SSC resistance of the steel material decreases. Therefore, the Ca content is within the range of 0 to 0.0100%.
  • a preferable lower limit of the Ca content is more than 0%, more preferably is 0.0001%, further preferably is 0.0003%, and further preferably is 0.0006%.
  • a preferable upper limit of the Ca content is 0.0025%, and more preferably is 0.0020%.
  • Magnesium (Mg) is an optional element, and need not be contained.
  • the Mg content may be 0%. If contained, Mg renders S in the steel material harmless by forming sulfides, and increases the SSC resistance of the steel material. If even a small amount of Mg is contained, aforementioned effect is obtained to a certain extent. However, if the Mg content is too high, oxides in the steel material coarsen and decrease the SSC resistance of the steel material. Therefore, the Mg content is within the range of 0 to 0.0100%.
  • a preferable lower limit of the Mg content is more than 0%, more preferably is 0.0001%, further preferably is 0.0003%, and further preferably is 0.0006%, and even further preferably is 0.0010%.
  • a preferable upper limit of the Mg content is 0.0025%, and more preferably is 0.0020%.
  • Zirconium (Zr) is an optional element, and need not be contained.
  • the Zr content may be 0%. If contained, Zr refines sulfides in the steel material and increases the SSC resistance of the steel material. If even a small amount of Zr is contained, aforementioned effect is obtained to a certain extent. However, if the Zr content is too high, oxides in the steel material coarsen. Therefore, the Zr content is within the range of 0 to 0.0100%.
  • a preferable lower limit of the Zr content is more than 0%, more preferably is 0.0001%, further preferably is 0.0003%, and further preferably is 0.0006%.
  • a preferable upper limit of the Zr content is 0.0025%, and more preferably is 0.0020%.
  • the total of the contents of these elements is preferably 0.0100% or less, and more preferably is 0.0050% or less.
  • the chemical composition of the steel material described above may further contain one or more types of element selected from the group consisting of Co and W in lieu of a part of Fe.
  • element selected from the group consisting of Co and W in lieu of a part of Fe.
  • Each of these elements is an optional element that forms a protective corrosion coating in a hydrogen sulfide environment and suppresses hydrogen penetration. By this means, each of these elements increases the SSC resistance of the steel material.
  • Co Co
  • the Co content may be 0%. If contained, Co forms a protective corrosion coating in a hydrogen sulfide environment and suppresses hydrogen penetration. By this means, Co increases the SSC resistance of the steel material. If even a small amount of Co is contained, aforementioned effect is obtained to a certain extent. However, if the Co content is too high, the hardenability of the steel material will decrease, and the steel material strength will decrease. Therefore, the Co content is within the range of 0 to 0.50%.
  • a preferable lower limit of the Co content is more than 0%, more preferably is 0.02%, and further preferably is 0.05%.
  • a preferable upper limit of the Co content is 0.45%, and more preferably is 0.40%.
  • Tungsten (W) is an optional element, and need not be contained.
  • the W content may be 0%. If contained, W forms a protective corrosion coating in a hydrogen sulfide environment and suppresses hydrogen penetration. By this means, W increases the SSC resistance of the steel material. If even a small amount of W is contained, aforementioned effect is obtained to a certain extent. However, if the W content is too high, coarse carbides form in the steel material and the SSC resistance of the steel material decreases. Therefore, the W content is within the range of 0 to 0.50%.
  • a preferable lower limit of the W content is more than 0%, more preferably is 0.02%, and further preferably is 0.05%.
  • a preferable upper limit of the W content is 0.45%, and more preferably is 0.40%.
  • the chemical composition of the steel material described above may further contain one or more types of element selected from the group consisting of Ni and Cu in lieu of a part of Fe. Each of these elements is an optional element, and increases the hardenability of the steel.
  • Nickel (Ni) is an optional element, and need not be contained. In other words, the Ni content may be 0%. If contained, Ni enhances the hardenability of the steel material and increases the steel material strength. If even a small amount of Ni is contained, aforementioned effect is obtained to a certain extent. However, if the Ni content is too high, the Ni will promote local corrosion, and the SSC resistance of the steel material will decrease. Therefore, the Ni content is within the range of 0 to 0.50%. A preferable lower limit of the Ni content is more than 0%, more preferably is 0.02%, and further preferably is 0.05%. A preferable upper limit of the Ni content is 0.35%, and more preferably is 0.25%.
  • Copper (Cu) is an optional element, and need not be contained.
  • the Cu content may be 0%. If contained, Cu enhances the hardenability of the steel material and increases the steel material strength. If even a small amount of Cu is contained, aforementioned effect is obtained to a certain extent. However, if the Cu content is too high, the hardenability of the steel material will be too high, and the SSC resistance of the steel material will decrease. Therefore, the Cu content is within the range of 0 to 0.50%.
  • a preferable lower limit of the Cu content is more than 0%, more preferably is 0.01%, further preferably is 0.02%, and further preferably is 0.05%.
  • a preferable upper limit of the Cu content is 0.35%, and more preferably is 0.25%.
  • the steel material according to the present embodiment contains an amount of dissolved C which is within the range of 0.010 to 0.060 mass %. If the amount of dissolved C is less than 0.010 mass %, the immobilization of dislocations in the steel material will be insufficient and excellent SSC resistance of the steel material will not be obtained. On the other hand, if the amount of dissolved C is more than 0.060 mass %, conversely, the SSC resistance of the steel material will decrease. Therefore, the amount of dissolved C is within the range of 0.010 to 0.060 mass %. A preferable lower limit of the amount of dissolved C is 0.020 mass % and more preferably is 0.030 mass %
  • An amount of dissolved C within the aforementioned range is obtained by, for example, controlling the holding time for tempering and controlling the cooling rate in the tempering process. The reason is as described hereinafter.
  • the amount of dissolved C is highest immediately after quenching. Immediately after quenching, C is dissolved except for a small amount thereof that precipitated as carbides during quenching. In the tempering process thereafter, some of the C precipitates as carbides as a result of being held for tempering. As a result, the amount of dissolved C decreases toward the thermal equilibrium concentration with respect to the tempering temperature. If the holding time for tempering is too short, this effect will not be obtained and the amount of dissolved C will be too high. On the other hand, if the holding time for tempering is too long, the amount of dissolved C will approach the aforementioned thermal equilibrium concentration, and will hardly change. Therefore, in the present embodiment the holding time for tempering is set within the range of 10 to 90 minutes.
  • cooling rate for cooling after tempering is slow, dissolved C will reprecipitate while the temperature is decreasing.
  • the cooling rate has been slow. Consequently, the amount of dissolved C has been almost 0 mass %. Therefore, in the present embodiment, the cooling rate after tempering is raised, and a dissolved C amount in the range of 0.010 to 0.060 mass % is obtained.
  • the cooling method is, for example, a method that performs forced cooling of the steel material continuously from the tempering temperature to thereby continuously decrease the surface temperature of the steel material.
  • Examples of this kind of continuous cooling treatment include a method that cools the steel material by immersion in a water bath, and a method that cools the steel material in an accelerated manner by shower water cooling, mist cooling or forced air cooling.
  • the cooling rate after tempering is measured at a region that is most slowly cooled within a cross-section of the steel material that is tempered (for example, in the case of forcedly cooling both surfaces, the cooling rate is measured at the center portion of the steel material thickness).
  • the cooling rate after tempering can be measured by inserting a sheath-type thermocouple into the center portion of the thickness of the steel plate and measuring the temperature.
  • the cooling rate after tempering can be measured by inserting a sheath-type thermocouple into the center portion of the wall thickness of the steel pipe and measuring the temperature.
  • the surface temperature on the non-forcedly cooled side of the steel material can be measured by means of a non-contact type infrared thermometer.
  • the temperature region from 600° C. to 200° C. is a temperature region in which diffusion of C is comparatively fast. Therefore, in the present embodiment, the average cooling rate in the temperature region from 600° C. to 200° C. is made 5° C./sec or more.
  • the cooling rate after tempering is made 300° C./sec or less.
  • the amount of dissolved C can be made to fall within the range of 0.010 to 0.060 mass %.
  • the amount of dissolved C in the steel material may be adjusted to within a range of 0.010 to 0.060 mass % by another method.
  • the term “amount of dissolved C” means the difference between the amount of C (mass %) in carbides in the steel material and the C content of the chemical composition of the steel material.
  • the amount of C in carbides in the steel material is determined by Formula (1) to Formula (5) using an Fe concentration ⁇ Fe>a, a Cr concentration ⁇ Cr>a, an Mn concentration ⁇ Mn>a, an Mo concentration ⁇ Mo>a, a V concentration ⁇ V>a and an Nb concentration ⁇ Nb>a in carbides (cementite and MC-type carbides) obtained as residue when extraction residue analysis is performed on the steel material, and an Fe concentration ⁇ Fe>b, a Cr concentration ⁇ Cr>b, an Mn concentration ⁇ Mn>b and an Mo concentration ⁇ Mo>b in cementite obtained by performing point analysis by EDS with respect to cementite identified by performing TEM observation of a replica film obtained by an extraction replica method.
  • ⁇ Mo> c ( ⁇ Fe> a + ⁇ Cr> a + ⁇ Mn> a ) x ⁇ Mo> b /( ⁇ Fe> b + ⁇ Cr> b + ⁇ Mn> b ) (1)
  • ⁇ C> a ( ⁇ Fe> a/ 55.85+ ⁇ Cr> a/ 52+ ⁇ Mn> a/ 53.94+ ⁇ Mo> c/ 95.9)/3 ⁇ 12 (3)
  • ⁇ C> b ( ⁇ V> a/ 50.94+ ⁇ Mo> d/ 95.9+ ⁇ Nb> a/ 92.9) ⁇ 12 (4)
  • cementite means carbides containing an Fe content of 50 mass % or more.
  • the method for calculating the amount of dissolved C is described in detail.
  • an analysis sample having the shape of a machined chip is taken from a center portion of the thickness
  • an analysis sample having the shape of a machined chip is taken from a center portion of the wall thickness.
  • the C content (mass %) is analyzed by an oxygen-stream combustion-infrared absorption method. The resulting value was taken to be the C content ( ⁇ C>) of the steel material.
  • the precipitated C amount is calculated by the following procedures 1 to 4. Specifically, in procedure 1 an extraction residue analysis is performed. In procedure 2, an extraction replica method using a transmission electron microscope (hereunder, referred to as “TEM”), and an element concentration analysis (hereunder, referred to as “EDS analysis”) of elements in cementite is performed by energy dispersive X-ray spectrometry (hereunder, referred to as “EDS”). In procedure 3, the Mo content is adjusted. In procedure 4, the precipitated C amount is calculated.
  • TEM transmission electron microscope
  • EDS analysis element concentration analysis
  • EDS energy dispersive X-ray spectrometry
  • carbides in the steel material are captured as residue, and the contents of Fe, Cr, Mn, Mo, V and Nb in the residue are determined.
  • carbides is a generic term for cementite (M 3 C-type carbides) and MC-type carbides.
  • the specific procedure is as follows. In a case where the steel material is a plate material, a cylindrical test specimen having a diameter of 6 mm and a length of 50 mm is extracted from a center portion of the thickness.
  • a cylindrical test specimen having a diameter of 6 mm and a length of 50 mm is extracted from a center portion of the wall thickness of the steel pipe in a manner so that the center of the wall thickness becomes the center of the cross-section.
  • the surface of the extracted test specimen is polished to remove about 50 ⁇ m by preliminary electropolishing to obtain a newly formed surface.
  • the electropolished test specimen is subjected to electrolysis in an electrolyte solution of 10% acetylacetone+1% tetra-ammonium+methanol.
  • the electrolyte solution after electrolysis is passed through a 0.2- ⁇ m filter to capture residue.
  • the obtained residue is subjected to acid decomposition, and the concentrations of Fe, Cr, Mn, Mo, V and Nb are determined in units of mass percent by ICP (inductively coupled plasma) optical emission spectrometry.
  • concentrations are defined as ⁇ Fe>a, ⁇ Cr>a, ⁇ Mn>a, ⁇ Mo>a, ⁇ V>a and ⁇ Nb>a, respectively.
  • procedure 2 the content of each of Fe, Cr, Mn and Mo in cementite is determined.
  • the specific procedure is as follows. A micro test specimen is cut out from a center portion of the thickness in a case where the steel material is a plate material, and is cut out from a center portion of the wall thickness in a case where the steel material is a steel pipe, and the surface of the micro test specimen is finished by mirror polishing. The test specimen is immersed for 10 minutes in a 3% nital etching reagent to etch the surface. The surface thereof is covered with a carbon deposited film. The test specimen whose surface is covered with the deposited film is immersed in a 5% nital etching reagent, and held therein for 20 minutes to cause the deposited film to peel off.
  • the deposited film that peeled off is cleaned with ethanol, and thereafter is scooped up with a sheet mesh and dried.
  • the deposited film (replica film) is observed using a TEM, and point analysis by EDS is performed with respect to 20 particles of cementite.
  • the concentration of each of Fe, Cr, Mn and Mo is determined in units of mass percent when taking the total of the alloying elements excluding carbon in the cementite as 100%.
  • the concentrations are determined for 20 particles of cementite, and the arithmetic average values for the respective elements are defined as ⁇ Fe>b, ⁇ Cr>b, ⁇ Mn>b and ⁇ Mo>b.
  • the Mo concentration in the carbides is determined.
  • Fe, Cr, Mn and Mo concentrate in cementite.
  • V, Nb and Mo concentrate in MC-type carbides.
  • Mo is caused to concentrate in both cementite and MC-type carbides by tempering. Therefore, the Mo amount is calculated separately for cementite and for MC-type carbides.
  • a part of V also concentrates in cementite.
  • the amount of V that concentrates in cementite is negligibly small in comparison to the amount of V that concentrates in MC-type carbides. Therefore, when determining the amount of dissolved C, V is regarded as concentrating only in MC-type carbides.
  • the amount of Mo precipitating as cementite ( ⁇ Mo>c) is calculated by Formula (1).
  • ⁇ Mo> c ( ⁇ Fe> a + ⁇ Cr> a + ⁇ Mn> a ) x ⁇ Mo> b /( ⁇ Fe> b + ⁇ Cr> b + ⁇ Mn> b ) (1)
  • the precipitated C amount is calculated as the total of the C amount precipitating as cementite ( ⁇ C>a) and the C amount precipitating as MC-type carbides ( ⁇ C>b).
  • ⁇ C>a and ⁇ C>b are calculated in units of mass percent by Formula (3) and Formula (4), respectively.
  • Formula (3) is a formula that is derived from the fact that the structure of cementite is a M 3 C type structure (M include Fe, Cr, Mn and Mo).
  • ⁇ C> a ( ⁇ Fe> a/ 55.85+ ⁇ Cr> a/ 52+ ⁇ Mn> a/ 53.94+ ⁇ Mo> c/ 95.9)/3 ⁇ 12 (3)
  • ⁇ C> b ( ⁇ V> a/ 50.94+ ⁇ Mo> d/ 95.9+ ⁇ Nb> a/ 92.9) ⁇ 12 (4)
  • the precipitated C amount is ⁇ C>a+ ⁇ C>b.
  • the amount of dissolved C (hereunder, also referred to as “ ⁇ C>c”) is calculated in units of mass percent by Formula (5) as a difference between the C content ( ⁇ C>) and the precipitated C amount of the steel material.
  • the microstructure of the steel material according to the present embodiment is principally composed of tempered martensite and tempered bainite. More specifically, the volume ratio of tempered martensite and/or tempered bainite in the microstructure is 90% or more. In other words, the total of the volume ratios of tempered martensite and tempered bainite in the microstructure is 90% or more. The balance of the microstructure is, for example, retained austenite or the like. If the microstructure of the steel material containing the aforementioned chemical composition contains tempered martensite and tempered bainite in an amount equivalent to a total volume ratio of 90% or more, the yield strength will be within the range of 965 to 1069 MPa (140 ksi grade), and the yield ratio will be 90% or more.
  • the microstructure is composed of only tempered martensite and/or tempered bainite.
  • the following method can be adopted in the case of determining the total of the volume ratios of tempered martensite and tempered bainite by observation.
  • the steel material is a plate material
  • a small piece having an observation surface with dimensions of 10 mm in the rolling direction and 10 mm in the plate width direction is cut out from a center portion of the thickness.
  • the steel material is a steel pipe
  • a small piece having an observation surface with dimensions of 10 mm in the pipe axis direction and 10 mm in the pipe circumferential direction is cut out from a center portion of the wall thickness.
  • the small piece After polishing the observation surface to obtain a mirror surface, the small piece is immersed for about 10 seconds in a nital etching reagent, to reveal the microstructure by etching.
  • the etched observation surface is observed by means of a secondary electron image obtained using a scanning electron microscope (SEM), and observation is performed for 10 visual fields.
  • the area of each visual field is 400 m 2 (magnification of ⁇ 5000).
  • tempered martensite and tempered bainite are identified based on the contrast.
  • the total of the area fractions of tempered martensite and tempered bainite that are identified is determined.
  • the arithmetic average value of the totals of the area fractions of tempered martensite and tempered bainite determined in all visual fields is taken as the volume ratio of tempered martensite and tempered bainite.
  • the shape of the steel material according to the present embodiment is not particularly limited.
  • the steel material is, for example, a steel pipe or a steel plate.
  • the steel material is a seamless steel pipe.
  • a preferable wall thickness is 9 to 60 mm.
  • the steel material according to the present embodiment is, in particular, suitable for use as a heavy-wall oil-well steel pipe. More specifically, even if the steel material according to the present embodiment is an oil-well steel pipe having a thick wall of 15 mm or more or, furthermore, 20 mm or more, the steel material exhibits excellent strength and SSC resistance.
  • the yield strength YS of the steel material according to the present embodiment is within a range of 965 to 1069 MPa (140 ksi grade), and the yield ratio YR of the steel material is 90% or more.
  • yield strength YS means the stress when elongation of 0.65% is obtained in a tensile test.
  • the strength of the steel material according to the present embodiment is of 140 ksi grade. Even though the steel material according to the present embodiment has such high strength, the steel material also has excellent SSC resistance by satisfying the conditions regarding the chemical composition, amount of dissolved C and microstructure, which are described above.
  • the SSC resistance of the steel material according to the present embodiment can be evaluated by a DCB test performed in accordance with “Method D” described in NACE TM0177-2005.
  • the liquid solution used is obtained by mixing a degassed 5% saline solution and 4 g/L of sodium acetate and adjusting to pH 3.5 using hydrochloric acid.
  • the gas charged inside the autoclave is a gaseous mixture of 10% H 2 S gas and 90% CO 2 gas at a total pressure of 1 atm.
  • a DCB test specimen into which a wedge was driven is enclosed inside the vessel, and is held for three weeks at 24° C. while agitating the liquid solution and also continuously blowing in the aforementioned gaseous mixture.
  • the K 1SSC (MPa ⁇ m) value of the steel material according to the present embodiment determined under the foregoing conditions is 30.0 MPa ⁇ m or more.
  • the method for producing a steel material according to the present embodiment includes a preparation process, a quenching process and a tempering process.
  • the preparation process may include a starting material preparation process and a hot working process.
  • a method for producing an oil-well steel pipe will be described as one example of a method for producing a steel material.
  • the method for producing an oil-well steel pipe includes a process of preparing a hollow shell (preparation process), and a process of subjecting the hollow shell to quenching and tempering to obtain an oil-well steel pipe (quenching process and tempering process). Each of these processes is described in detail hereunder.
  • an intermediate steel material containing the aforementioned chemical composition is prepared.
  • the method for producing the intermediate steel material is not particularly limited as long as the intermediate steel material contains the aforementioned chemical composition.
  • the term “intermediate steel material” refers to a plate-shaped steel material in a case where the end product is a steel plate, and refers to a hollow shell in a case where the end product is a steel pipe.
  • the preparation process may preferably include a process in which a starting material is prepared (starting material preparation process), and a process in which the starting material is subjected to hot working to produce an intermediate steel material (hot working process).
  • starting material preparation process a process in which a starting material is prepared
  • hot working process a process in which the starting material is subjected to hot working to produce an intermediate steel material
  • a starting material is produced using molten steel containing the aforementioned chemical composition.
  • a cast piece (a slab, bloom or billet) is produced by a continuous casting process using the molten steel.
  • An ingot may also be produced by an ingot-making process using the molten steel.
  • the slab, bloom or ingot may be subjected to blooming to produce a billet.
  • the starting material (a slab, bloom or billet) is produced by the above described process.
  • the starting material that was prepared is subjected to hot working to produce an intermediate steel material.
  • the intermediate steel material corresponds to a hollow shell.
  • the billet is heated in a heating furnace.
  • the heating temperature is not particularly limited, for example, the heating temperature is within a range of 1100 to 1300° C.
  • the billet that is extracted from the heating furnace is subjected to hot working to produce a hollow shell (seamless steel pipe).
  • the Mannesmann process is performed as the hot working to produce the hollow shell. In this case, a round billet is piercing-rolled using a piercing machine.
  • the piercing ratio is, for example, within a range of 1.0 to 4.0.
  • the round billet that underwent piercing-rolling is further hot-rolled to form a hollow shell using a mandrel mill, a reducer, a sizing mill or the like.
  • the cumulative reduction of area in the hot working process is, for example, 20 to 70%.
  • a hollow shell may also be produced from the billet by another hot working method.
  • a hollow shell may be produced by forging such as Ehrhardt process.
  • a hollow shell is produced by the above process.
  • the wall thickness of the hollow shell is, for example, 9 to 60 mm.
  • the hollow shell produced by hot working may be air-cooled (as-rolled).
  • the hollow shell produced by hot working may be subjected to direct quenching after hot rolling without being cooled to normal temperature, or may be subjected to quenching after undergoing supplementary heating (reheating) after hot rolling.
  • reheating supplementary heating
  • a stress relief treatment may be performed at a time that is after quenching and before the heat treatment of the next process.
  • an intermediate steel material is prepared in the preparation process.
  • the intermediate steel material may be produced by the aforementioned preferable process, or may be an intermediate steel material that was produced by a third party, or an intermediate steel material that was produced in another factory other than the factory in which a quenching process and a tempering process that are described later are performed, or at a different works.
  • the quenching process is described in detail hereunder.
  • the intermediate steel material (hollow shell) that was prepared is subjected to quenching.
  • quenching means rapidly cooling the intermediate steel material that is at a temperature not less than the A 3 point.
  • a preferable quenching temperature is 800 to 1000° C.
  • the quenching temperature corresponds to the surface temperature of the intermediate steel material that is measured by a thermometer placed on the exit side of the apparatus that performs the final hot working.
  • the quenching temperature corresponds to the temperature of the furnace that performs the supplementary heating.
  • the quenching method for example, continuously cools the hollow shell from the quenching starting temperature, and continuously decreases the surface temperature of the hollow shell.
  • the method of performing the continuous cooling treatment is not particularly limited.
  • the method of performing the continuous cooling treatment is, for example, a method that cools the hollow shell by immersing the hollow shell in a water bath, or a method that cools the hollow shell in an accelerated manner by shower water cooling or mist cooling.
  • the intermediate steel material is rapidly cooled during quenching.
  • the average cooling rate when the surface temperature of the intermediate steel material (hollow shell) is within the range of 800 to 500° C. during quenching is defined as a cooling rate during quenching CR 800-500 .
  • the cooling rate during quenching CR 800-500 is 50° C./min or higher.
  • a preferable lower limit of the cooling rate during quenching CR 800-500 is 100° C./min, and more preferably is 250° C./min.
  • an upper limit of the cooling rate during quenching CR 800-500 is not particularly defined, for example, the upper limit is 60000° C./min.
  • quenching is performed after performing heating of the hollow shell in the austenite zone a plurality of times.
  • the SSC resistance of the steel material increases because austenite grains are refined prior to quenching.
  • Heating in the austenite zone may be repeated a plurality of times by performing quenching a plurality of times, or heating in the austenite zone may be repeated a plurality of times by performing normalizing and quenching.
  • the tempering process will be described in detail.
  • Tempering is performed after performing the aforementioned quenching.
  • the tempering temperature is appropriately adjusted in accordance with the chemical composition of the steel material and the yield strength YS, which is to be obtained.
  • the tempering temperature is adjusted so as to adjust the yield strength YS of the steel material to within a range of 965 to 1069 MPa (140 ksi grade) and to make the YR of the steel material 90% or more.
  • a preferable tempering temperature is in a range from 660° C. to the A ct point. If the tempering temperature is 660° C. or more, carbides are sufficiently spheroidized and the SSC resistance is further increased.
  • the tempering time is set within a range of 10 to 90 minutes.
  • a preferable lower limit of the tempering time is 15 minutes.
  • a preferable upper limit of the tempering time is 70 minutes, and more preferably is 60 minutes. Note that, in a case where the steel material is a steel pipe, in comparison to other shapes, temperature variations with respect to the steel pipe are liable to occur during holding for tempering.
  • the tempering time is preferably set within a range of 15 to 90 minutes.
  • a person skilled in the art will be sufficiently capable of making the yield strength YS of the steel material containing the chemical composition of the present embodiment fall within the range of 965 to less than 1069 MPa by appropriately adjusting the aforementioned holding time at the aforementioned tempering temperature.
  • the average cooling rate when the surface temperature of the intermediate steel material (hollow shell) is within the range of 600 to 200° C. after tempering is defined as a cooling rate after tempering CR 600-200 .
  • the cooling rate after tempering CR 600-200 is 5° C./sec or higher.
  • the temperature region from 600° C. to 200° C. is a temperature region in which diffusion of C is comparatively fast.
  • the cooling rate after tempering is too fast, in some cases very little of the C that had dissolved will precipitate, and the amount of dissolved C will be excessive. In such a case, the SSC resistance of the steel material decreases. Furthermore, in such a case, the low-temperature toughness of the steel material may decrease.
  • the cooling rate after tempering CR 600-200 is within the range of 5 to 300° C./sec.
  • a preferable lower limit of the cooling rate after tempering CR 600-200 is 10° C./sec, and more preferably is 15° C./sec.
  • a preferable upper limit of the cooling rate after tempering CR 600-200 is 100° C./sec, and more preferably is 50° C./sec.
  • a method for cooling so that the cooling rate after tempering CR 600-200 is within the range of 5 to 300° C./sec is not particularly limited, and a well-known method can be used.
  • the cooling method for example, is a method that performs forced cooling of a hollow shell continuously from the tempering temperature to thereby continuously decrease the surface temperature of the hollow shell. Examples of this kind of continuous cooling treatment include a method that cools the hollow shell by immersion in a water bath, and a method that cools the hollow shell in an accelerated manner by shower water cooling, mist cooling or forced air cooling.
  • the cooling rate after tempering CR 600-200 is measured at a region that is most slowly cooled within a cross-section of the intermediate steel material that is tempered (for example, in the case of forcedly cooling both surfaces, the cooling rate is measured at the center portion of the thickness of the intermediate steel material).
  • a method for producing a steel pipe has been described as one example of the aforementioned production method.
  • the steel material according to the present embodiment may be a steel plate or another shape.
  • An example of a method for producing a steel plate or a steel material of another shape also includes, for example, a preparation process, a quenching process and a tempering process, similarly to the production method described above.
  • the aforementioned production method is one example, and the steel material according to the present embodiment may be produced by another production method.
  • Ingots were produced using the aforementioned molten steels. The ingots were hot rolled to produce steel plates having a thickness of 20 mm.
  • the steel plates of each test number were reheated to bring the steel plate temperature to the quenching temperature (920° C., which is in the austenite single-phase zone), and were held for 20 minutes. After being held, the steel plates were immersed in a water bath and quenched. At this time, the cooling rate during quenching (CR 800-500 ) was 400° C./min. With respect to Test Number 23, after holding at the quenching temperature, the steel plate was cooled by immersion in an oil bath. At this time, the average cooling rate from 800° C. to 500° C. was 40° C./min.
  • the steel plates of each test number were subjected to tempering.
  • the tempering temperature was adjusted so that the steel plates became 140 ksi grade as specified in the API standards (yield strength of 965 to 1069 MPa).
  • the steel plates were cooled.
  • controlled cooling by mist water cooling from both sides of the steel plate was performed. Note that, a type K thermocouple of a sheath type was inserted into a center portion of the thickness of the steel plate in advance, and the temperature was measured with respect to tempering and the cooling thereafter.
  • the tempering temperature (° C.) and tempering time (min) and the cooling rate (CR 600-200 ) (° C./sec) after tempering are shown in Table 2. Note that, the A c1 point of the steel material in each of Test Number 1 to Test Number 25 was 750° C., and the tempering temperature was set so as to be lower than the A c1 point.
  • a tensile test was performed in accordance with ASTM E8. Round bar tensile test specimens having a diameter of 6.35 mm and a parallel portion length of 35 mm were prepared from the center parts of the thickness of the steel plates of each test number after the quenching and tempering described above. The axial direction of each of the tensile test specimens was parallel to the rolling direction of the steel plate. A tensile test was performed in the atmosphere at normal temperature (25° C.) using each round bar test specimen, and the yield strength YS (MPa) and tensile strength TS (MPa) were obtained. Note that, in the present examples, the stress at the time of 0.65% elongation obtained in the tensile test defined as the YS for each test number. Further, the largest stress during uniform elongation was taken as the TS. A ratio between the YS and the TS was adopted as the yield ratio YR (%).
  • the amount of dissolved C was measured and calculated by the measurement method described above.
  • the TEM used was JEM-2010 manufactured by JEOL Ltd.
  • the acceleration voltage was set to 200 kV
  • the irradiation current was 2.56 nA
  • measurement was performed for 60 seconds at each point.
  • the observation regions for the TEM observation were 8 ⁇ m ⁇ 8 ⁇ m, and observation was performed with respect to an arbitrary 10 visual fields.
  • the residual amounts of each element and the concentrations of each element in cementite that were used to calculate the amount of dissolved C were as listed in Table 3.
  • a DCB test was conducted in accordance with “Method D” of NACE TM0177-2005, and the SSC resistance was evaluated. Specifically, three of the DCB test specimen illustrated in FIG. 2A were taken from a center portion of the thickness of each steel plate. The DCB test specimens were taken in a manner such that the longitudinal direction of each DCB test specimen was parallel with the rolling direction. A wedge illustrated in FIG. 2B was further prepared from each steel plate. A thickness t of the wedge was 3.10 mm.
  • the wedge was driven in between the arms of the DCB test specimen. Thereafter, the DCB test specimen into which the wedge was driven was enclosed in a vessel. A liquid solution obtained by mixing a degassed 5% saline solution and 4 g/L of sodium acetate and adjusting to pH 3.5 with hydrochloric acid was poured into the vessel so that a gas portion remained in the vessel. Thereafter, a gaseous mixture consisting of 10% H 2 S gas and 90% CO 2 gas was charged at a total pressure of 1 atm inside the autoclave to stir the liquid phase, and the gaseous mixture was saturated in the liquid solution.
  • the vessel After sealing the vessel that had undergone the above described process, the vessel was held for three weeks at 24° C. while stirring the liquid solution and also continuously blowing in the aforementioned gaseous mixture. Thereafter, the DCB test specimens were taken out from inside the vessel.
  • a pin was inserted into a hole formed in the tip of the arms of each DCB test specimen that was taken out and a notch portion was opened with a tensile testing machine, and a wedge releasing stress P was measured.
  • the notch in the DCB test specimen was released in liquid nitrogen, and a crack propagation length “a” with respect to crack propagation that occurred during immersion was measured.
  • the crack propagation length “a” was measured visually using vernier calipers.
  • a fracture toughness value K 1SSC (MPa ⁇ m) was determined using Formula (6) based on the obtained wedge releasing stress P and the crack propagation length “a”.
  • the fracture toughness value K 1SSC (MPa ⁇ m) of the three DCB test specimens was determined.
  • the arithmetic average of the three fracture toughness values was defined as the fracture toughness value K 1SSC (MPa ⁇ m) of the relevant steel.
  • K 1 ⁇ SSC Pa ⁇ ( 2 ⁇ 3 + 2.38 h / a ) ⁇ ( B / Bn ) 1 / 3 Bh 3 / 2 ( 6 )
  • h represents the height (mm) of each arm of the DCB test specimen
  • B represents the thickness (mm) of the DCB test specimen
  • Bn represents the web thickness (mm) of the DCB test specimen.
  • the obtained fracture toughness values K 1SSC are shown in Table 2. If the fracture toughness value K 1SSC that was defined as described above was 30.0 MPa ⁇ m or more, it was determined that the SSC resistance was good. Note that, the clearance between the arms when the wedge is driven in prior to immersion in the test bath influences the K 1SSC value. Accordingly, actual measurement of the clearance between the arms was performed in advance using a micrometer, and it was also confirmed that the clearance was within the range in the API standards.
  • the chemical compositions of the steel plates of Test Numbers 1 to 13 were appropriate, the YS was in the range of 965 to 1069 MPa (140 ksi grade), and the YR was 90% or more. In addition, the amount of dissolved C was in the range of 0.010 to 0.060 mass %. As a result, the K 1SSC value was 30.0 MPa ⁇ m or more and excellent SSC resistance was exhibited.
  • the tempering time was too short. Consequently, the amount of dissolved C was more than 0.060 mass %. As a result, the fracture toughness value K 1SSC was less than 30.0 MPa ⁇ m and excellent SSC resistance was not exhibited.
  • the Mo content was too low.
  • the fracture toughness value K 1SSC was less than 30.0 MPa ⁇ m and excellent SSC resistance was not exhibited.
  • the N content was too high.
  • the fracture toughness value K 1SSC was less than 30.0 MPa ⁇ m and excellent SSC resistance was not exhibited.
  • the Si content was too high.
  • the fracture toughness value K 1SSC was less than 30.0 MPa ⁇ m and excellent SSC resistance was not exhibited.
  • the YR was less than 90%.
  • the fracture toughness value K 1SSC was less than 30.0 MPa ⁇ m and excellent SSC resistance was not exhibited. It is considered that the reason was that ferrite mixed into the microstructure because the cooling rate during quenching was slow.
  • the cooling rate after tempering was too slow. Consequently, the amount of dissolved C was less than 0.010 mass %. As a result, the fracture toughness value K 1SSC was less than 30.0 MPa ⁇ m and excellent SSC resistance was not exhibited.
  • the tempering temperature was too low. Consequently, the YS was more than 1069 MPa. As a result, the fracture toughness value K 1SSC was less than 30.0 MPa ⁇ m and excellent SSC resistance was not exhibited.
  • the steel material according to the present invention is widely applicable to steel materials to be utilized in a sour environment, and preferably can be utilized as a steel material for oil wells that is utilized in an oil well environment, and further preferably can be utilized as oil-well steel pipes, such as casing, tubing and line pipes.

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