EP4481062A1 - Matériau en acier approprié pour une utilisation en environnements acides - Google Patents

Matériau en acier approprié pour une utilisation en environnements acides Download PDF

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EP4481062A1
EP4481062A1 EP23756422.4A EP23756422A EP4481062A1 EP 4481062 A1 EP4481062 A1 EP 4481062A1 EP 23756422 A EP23756422 A EP 23756422A EP 4481062 A1 EP4481062 A1 EP 4481062A1
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Prior art keywords
steel material
content
steel
test
oxides
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German (de)
English (en)
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EP4481062A4 (fr
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Hiroki KAMITANI
Yuji Arai
Jin KAWASAKI
Keiichi Kondo
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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
    • C22C38/32—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
    • C22C38/54—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
    • 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
    • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
    • 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
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/10—Modifying the physical properties of ferrous metals or ferrous alloys 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
    • 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
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/22—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
    • 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
    • C22C38/26—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
    • C22C38/40—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

Definitions

  • the present disclosure relates to a steel material, and more particularly relates to a steel material suitable for use in a sour environment.
  • oil wells and gas wells are collectively referred to as simply "oil wells"
  • oil-well steel pipes of 80 ksi grade yield strength is 80 to less than 95 ksi, that is, 552 to less than 655 MPa
  • 95 ksi grade yield strength is 95 to less than 110 ksi, that is, 655 to less than 758 MPa
  • yield strength is 758 MPa or more
  • sour environment means an acidified environment containing hydrogen sulfide.
  • a sour environment may also contain carbon dioxide.
  • Oil-well steel pipes for use in such sour environments are required to have not only high strength, but to also have sulfide stress cracking resistance (hereunder, referred to as "SSC resistance").
  • SSC resistance sulfide stress cracking resistance
  • Patent Literature 1 Japanese Patent Application Publication No. 2000-297344
  • Patent Literature 2 Japanese Patent Application Publication No. 2001-271134
  • Patent Literature 3 International Application Publication No. WO2008/123422
  • Patent Literature 1 discloses a steel for oil wells containing, in mass%, C: 0.15 to 0.3%, Cr: 0.2 to 1.5%, Mo: 0.1 to 1%, V: 0.05 to 0.3%, and Nb: 0.003 to 0.1%.
  • the total amount of precipitating carbides is within the range of 1.5 to 4% by mass
  • the proportion that MC-type carbides occupy among the total amount of carbides is within the range of 5 to 45% by mass
  • the wall thickness of the product is taken as t (mm)
  • the proportion of M 23 C 6 -type carbides is (200/t) or less in percent by mass. It is described in Patent Literature 1 that this steel for oil wells is excellent in SSC resistance.
  • Patent Literature 2 discloses a low-alloy steel material that consists of, in mass%, C: 0.2 to 0.35%, Si: 0.05 to 0.5%, Mn: 0.1 to 1%, P: 0.025% or less, S: 0.01% or less, Cr: 0.1 to 1.2%, Mo: 0.1 to 1%, B: 0.0001 to 0.005%, Al: 0.005 to 0.1%, N: 0.01% or less, V: 0.05 to 0.5%, Ni: 0.1% or less, W: 1.0% or less, and O: 0.01% or less, with the balance being Fe and impurities, and satisfies the formula (0.03 ⁇ Mo ⁇ V ⁇ 0.3) and the formula (0.5 ⁇ Mo-V+GS/10 ⁇ 1) and has a yield strength of 1060 MPa or more.
  • GS in the formula represents the ASTM grain size number of prior-austenite grains. It is described in Patent Literature 2 that this low-alloy steel material is excellent in SSC resistance.
  • the low-alloy steel contains P: 0.025% or less, S: 0.010% or less, N: 0.007% or less, and B: less than 0.0003%.
  • the number density of M 23 C 6 -type precipitates having a grain size of 1 ⁇ m or more is 0.1 /mm 2 or less. It is described in Patent Literature 3 that in this low-alloy steel, the SSC resistance is enhanced.
  • An objective of the present disclosure is to provide a steel material that has high strength and has excellent SSC resistance in a sour environment.
  • a steel material according to the present disclosure consists of, in mass%,
  • the steel material according to the present disclosure has high strength and has excellent SSC resistance in a sour environment.
  • the present inventors conducted studies regarding obtaining a steel material having a yield strength of 125 ksi (862 MPa) or more as high strength. That is, the present inventors conducted investigations and studies regarding a method for obtaining a yield strength of 125 ksi or more and excellent SSC resistance in a sour environment in a steel material for which use in a sour environment is assumed. As a result, the present inventors obtained the following findings.
  • the present inventors conducted studies regarding obtaining a steel material having a yield strength of 125 ksi or more and excellent SSC resistance in a sour environment. As a result, the present inventors considered that by decreasing the content of manganese (Mn) to 0.30% or less, there is a possibility of increasing the SSC resistance of a steel material while maintaining the strength of the steel material. Mn combines with sulfur (S) in a steel material to form Mn sulfides. Mn sulfides are easily elongated by rolling, and tend to become inclusions that have a long major axis.
  • Mn sulfides that have a long major axis are liable to become starting points of fractures in a sour environment. Therefore, there is a possibility that, by reducing the content of Mn to 0.30% or less, formation of Mn sulfides will be suppressed and the SSC resistance of the steel material will be increased.
  • a steel material consists of, in mass%, C: 0.15 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.05 to 0.30%, P: 0.030% or less, S: 0.0050% or less, Al: 0.005 to 0.100%, Cr: 0.30 to 1.10%, Mo: 0.40 to 2.00%, Ti: 0.002 to 0.020%, Nb: 0.002 to 0.100%, B: 0.0005 to 0.0040%, N: 0.0100% or less, O: less than 0.0040%, V: 0 to 0.30%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, W: 0 to 1.50%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, and rare earth metal: 0 to 0.0100%, with the balance being Fe and impurities, there is a possibility that the steel material will have a yield strength of 125 ks
  • Si oxides in which, in mass%, the content of Si is 20% or more and the content of O is 10% or more, and which have a major axis of 5.0 ⁇ m or more are also referred to as "coarse Si oxides".
  • the present inventors also conducted detailed studies regarding the cause of a decrease in SSC resistance for each yield strength with respect to steel materials having the chemical composition described above. As a result, the present inventors obtained the following findings.
  • FIG. 1 is a view illustrating the relation between the number density (/100 mm 2 ) of coarse Si oxides (Si oxides having a major axis of 5.0 ⁇ m or more) and the number of specimens in which SSC occurred (specimens) in an SSC resistance test with respect to steel materials having a yield strength of 125 to less than 135 ksi among the present examples.
  • FIG. 1 is a view illustrating the relation between the number density (/100 mm 2 ) of coarse Si oxides (Si oxides having a major axis of 5.0 ⁇ m or more) and the number of specimens in which SSC occurred (specimens) in an SSC resistance test with respect to steel materials having a yield strength of 125 to less than 135 ksi among the present examples.
  • FIG. 1 is a view illustrating the relation between the number density (/100 mm 2 ) of coarse Si oxides (Si oxides having a major axis of 5.0 ⁇ m or more) and the number
  • a steel material has the chemical composition described above, has a yield strength of 125 to less than 135 ksi, and the number density of coarse Si oxides in the steel material is made 5 /100 mm 2 or less. As a result, a yield strength of 125 ksi or more and excellent SSC resistance can both be achieved.
  • FIG. 2 is a view illustrating the relation between the number density (/200 mm 2 ) of coarse Si oxides (Si oxides having a major axis of 5.0 ⁇ m or more) and the number of specimens in which SSC occurred (specimens) in an SSC resistance test with respect to steel materials having a yield strength of 135 ksi or more among the present examples.
  • FIG. 2 was prepared using a number density of coarse Si oxides determined by a method to be described later, and the number of specimens in which SSC occurred (specimens) as the result of an SSC resistance test conducted by a method to be described later with respect to steel materials which, among examples to be described later, had the chemical composition described above and had a yield strength of 135 ksi or more.
  • a steel material has the chemical composition described above, has a yield strength of 135 ksi or more, and the number density of coarse Si oxides in the steel material is made 5 /200 mm 2 or less. As a result, a yield strength of 135 ksi or more and excellent SSC resistance can both be achieved.
  • a steel material according to the present embodiment has the chemical composition described above, has a yield strength of 862 MPa or more, and the number density of coarse Si oxides in the steel material is 5 /100 mm 2 or less, and in a case where the yield strength is 931 MPa or more, the number density of coarse Si oxides in the steel material is 5 /200 mm 2 or less.
  • the steel material according to the present embodiment can achieve both a yield strength of 125 ksi or more and excellent SSC resistance.
  • the gist of the steel material according to the present embodiment which has been completed based on the findings described above, is as follows.
  • the oil-well steel pipe may be a steel pipe used for oil country tubular goods.
  • 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 for casing pipes or tubing pipes.
  • the 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 thereof is 15 mm or more, the oil-well steel pipe has a yield strength of 125 ksi or more and has excellent SSC resistance in a sour environment.
  • the shape of the steel material according to the present embodiment is not particularly limited. That is, the steel material according to the present embodiment may be a steel pipe, may be a round steel bar (solid material), or may be a steel plate. Note that, the term "round steel bar” refers to a steel bar in which a cross section in a direction perpendicular to the axial direction is a circular shape. Further, the steel pipe may be a seamless steel pipe or may be a welded steel pipe.
  • the chemical composition of the steel material according to the present embodiment contains the following elements.
  • the content of C is to be 0.15 to 0.45%.
  • a preferable lower limit of the content of C is 0.18%, more preferably is 0.20%, further preferably is 0.22%, and further preferably is 0.25%.
  • a preferable upper limit of the content of C is 0.40%, more preferably is 0.38%, and further preferably is 0.35%.
  • Silicon (Si) deoxidizes the steel. If the content of Si is too low, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Si is too high, even if the contents of other elements are within the range of the present embodiment, the SSC resistance of the steel material will decrease. Therefore, the content of Si is to be 0.05 to 1.00%.
  • a preferable lower limit of the content of Si is 0.10%, more preferably is 0.15%, and further preferably is 0.20%.
  • a preferable upper limit of the content of Si is 0.85%, more preferably is 0.75%, further preferably is 0.60%, further preferably is 0.50%, and further preferably is 0.40%.
  • Manganese (Mn) deoxidizes the steel. Mn also increases hardenability of the steel material. If the content of Mn is too low, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Mn is too high, even if the contents of other elements are within the range of the present embodiment, coarse sulfide-based inclusions will form and the SSC resistance of the steel material will decrease. Therefore, the content of Mn is to be 0.05 to 0.30%. A preferable lower limit of the content of Mn is 0.06%, more preferably is 0.08%, and further preferably is 0.10%. A preferable upper limit of the content of Mn is 0.28%, more preferably is 0.25%, and further preferably is 0.20%.
  • Phosphorus (P) is an impurity. That is, the lower limit of the content of P is more than 0%. If the content of P is too high, even if the contents of other elements are within the range of the present embodiment, P will segregate to grain boundaries and the SSC resistance of the steel material will decrease. Therefore, the content of P is to be 0.030% or less.
  • a preferable upper limit of the content of P is 0.025%, more preferably is 0.020%, and further preferably is 0.015%.
  • the content of P is preferably as low as possible. However, extremely reducing the content of P will greatly increase the production cost. Therefore, when taking industrial production into consideration, a preferable lower limit of the content of P is 0.001%, more preferably is 0.002%, and further preferably is 0.003%.
  • Aluminum (Al) deoxidizes the steel. If the content of Al is too low, even if the contents of other elements are within the range of the present embodiment, the aforementioned advantageous effect will not be sufficiently obtained and the SSC resistance of the steel material will decrease. On the other hand, if the content of Al is too high, even if the contents of other elements are within the range of the present embodiment, coarse oxide-based inclusions will form and the SSC resistance of the steel material will decrease. Therefore, the content of Al is to be 0.005 to 0.100%. A preferable lower limit of the content of Al is 0.010%, more preferably is 0.015%, and further preferably is 0.020%.
  • a preferable upper limit of the content of Al is 0.080%, more preferably is 0.060%, and further preferably is 0.040%.
  • the term content of "Al” means the content of "acid-soluble Al", that is, “sol. Al”.
  • Chromium (Cr) increases hardenability of the steel material. Cr also increases temper softening resistance of the steel material and thereby enables high-temperature tempering. As a result, the SSC resistance of the steel material increases. If the content of Cr is too low, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Cr is too high, the SSC resistance of the steel material will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of Cr is to be 0.30 to 1.10%. A preferable lower limit of the content of Cr is 0.35%, more preferably is 0.40%, and further preferably is 0.50%. A preferable upper limit of the content of Cr is 1.00%, more preferably is 0.90%, and further preferably is 0.80%.
  • Molybdenum (Mo) increases hardenability of the steel material. Mo also increases temper softening resistance of the steel material and thereby enables high-temperature tempering. As a result, the SSC resistance of the steel material increases. If the content of Mo is too low, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Mo is too high, the aforementioned advantageous effects will be saturated. Therefore, the content of Mo is to be 0.40 to 2.00%. A preferable lower limit of the content of Mo is 0.45%, more preferably is 0.50%, and further preferably is 0.60%. A preferable upper limit of the content of Mo is 1.80%, more preferably is 1.60%, and further preferably is 1.40%.
  • Titanium (Ti) combines with N to form nitrides, and refines the grains of the steel material by the pinning effect. As a result, strength of the steel material increases. If the content of Ti is too low, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Ti is too high, even if the contents of other elements are within the range of the present embodiment, Ti nitrides will coarsen and the SSC resistance of the steel material will decrease. Therefore, the content of Ti is to be 0.002 to 0.020%. A preferable lower limit of the content of Ti is 0.003%, and more preferably is 0.004%. A preferable upper limit of the content of Ti is 0.018%, further preferably is 0.015%, and further preferably is 0.010%.
  • Niobium combines with C and/or N to form carbides, nitrides, or carbo-nitrides (hereunder, also referred to as "carbo-nitrides and the like").
  • the carbo-nitrides and the like refine the grains of the steel material by the pinning effect, and thereby increase low-temperature toughness and the SSC resistance of the steel material.
  • Nb also forms fine carbides during tempering and thereby increases temper softening resistance of the steel material and increases strength of the steel material. If the content of Nb is too low, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.
  • the content of Nb is to be 0.002 to 0.100%.
  • a preferable lower limit of the content of Nb is 0.005%, more preferably is 0.010%, further preferably is 0.015%, and further preferably is 0.020%.
  • a preferable upper limit of the content of Nb is 0.080%, more preferably is 0.060%, and further preferably is 0.040%.
  • B Boron
  • B Boron
  • the content of B is to be 0.0005 to 0.0040%.
  • a preferable lower limit of the content of B is 0.0006%, and more preferably is 0.0008%.
  • a preferable upper limit of the content of B is 0.0035%, more preferably is 0.0030%, further preferably is 0.0025%, and further preferably is 0.0020%.
  • N Nitrogen
  • the lower limit of the content of N is more than 0%.
  • N combines with Ti to form nitrides, thereby refining the grains of the steel material by the pinning effect. As a result, strength of the steel material increases.
  • the content of N is to be 0.0100% or less.
  • a preferable upper limit of the content of N is 0.0080%, more preferably is 0.0060%, and further preferably is 0.0040%.
  • a preferable lower limit of the content of N for more effectively obtaining the aforementioned advantageous effect is 0.0005%, more preferably is 0.0010%, further preferably is 0.0015%, and further preferably is 0.0020%.
  • Oxygen (O) is an impurity. That is, the lower limit of the content of O is more than 0%. If the content of O is too high, even if the contents of other elements are within the range of the present embodiment, coarse oxides will form and the SSC resistance of the steel material will decrease. Therefore, the content of O is to be less than 0.0040%.
  • a preferable upper limit of the content of O is 0.0035%, more preferably is 0.0030%, further preferably is 0.0025%, and further preferably is 0.0020%.
  • the content of O is preferably as low as possible. However, extremely reducing the content of O will greatly increase the production cost. Therefore, when taking industrial production into consideration, a preferable lower limit of the content of O 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 substances which, when industrially producing the steel material, are mixed in from ore or scrap that is used as the raw 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 V in lieu of a part of Fe.
  • Vanadium (V) is an optional element, and does not have to be contained. That is, the content of V may be 0%. When contained, V forms carbo-nitrides and the like. The carbo-nitrides and the like refine the grains of the steel material by the pinning effect, and thereby increase the SSC resistance of the steel material. V also forms fine carbides during tempering and thereby increases temper softening resistance of the steel material and increases strength of the steel material. If even a small amount of V is contained, the aforementioned advantageous effects will be obtained to a certain extent.
  • the content of V is to be 0 to 0.30%.
  • a preferable lower limit of the content of V is more than 0%, more preferably is 0.01%, further preferably is 0.02%, further preferably is 0.05%, and further preferably is 0.07%.
  • a preferable upper limit of the content of V is 0.25%, more preferably is 0.20%, and further preferably is 0.15%.
  • the chemical composition of the steel material described above may further contain one or more elements selected from the group consisting of Cu and Ni in lieu of a part of Fe. Each of these elements is an optional element, and increases hardenability of the steel material.
  • Copper (Cu) is an optional element, and does not have to be contained. That is, the content of Cu may be 0%. When contained, Cu increases hardenability of the steel material and increases strength of the steel material. If even a small amount of Cu is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Cu is too high, even if the contents of other elements are within the range of the present embodiment, hardenability of the steel material will be too high and the SSC resistance of the steel material will decrease. Therefore, the content of Cu is to be 0 to 0.50%.
  • a preferable lower limit of the content of Cu 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 content of Cu is 0.35%, more preferably is 0.25%, further preferably is 0.15%, and further preferably is 0.10%.
  • Nickel (Ni) is an optional element, and does not have to be contained. That is, the content of Ni may be 0%. When contained, Ni increases hardenability of the steel material and increases strength of the steel material. Ni also dissolves in the steel and increases low-temperature toughness of the steel material. If even a small amount of Ni is contained, these advantageous effects will be obtained to a certain extent. However, if the content of Ni is too high, even if the contents of other elements are within the range of the present embodiment, local corrosion will be promoted and the SSC resistance of the steel material will decrease. Therefore, the content of Ni is to be 0 to 0.50%. A preferable lower limit of the content of Ni is more than 0%, more preferably is 0.01%, and further preferably is 0.02%. A preferable upper limit of the content of Ni is 0.30%, more preferably is 0.20%, and further preferably is 0.10%.
  • Tungsten (W) is an optional element, and does not have to be contained. That is, the content of W may be 0%.
  • W forms a protective corrosion coating and suppresses penetration of hydrogen into the steel material.
  • the SSC resistance of the steel material is increased. If even a small amount of W is contained, the aforementioned advantageous effect will be obtained to a certain extent.
  • the content of W is to be 0 to 1.50%.
  • a preferable lower limit of the content of W is more than 0%, more preferably is 0.01%, further preferably is 0.03%, and further preferably is 0.05%.
  • a preferable upper limit of the content of W is 1.30%, and more preferably is 1.10%.
  • the chemical composition of the steel material described above may further contain one or more elements selected from the group consisting of Ca, Mg, Zr and rare earth metal in lieu of a part of Fe.
  • Each of these elements is an optional element, and each element renders S in the steel material harmless by forming sulfides. As a result, these elements increase the SSC resistance of the steel material.
  • Ca is an optional element, and does not have to be contained. That is, the content of Ca may be 0%. When contained, Ca renders S in the steel material harmless by forming sulfides, and thereby increases the SSC resistance of the steel material. If even a small amount of Ca is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Ca is too high, even if the contents of other elements are within the range of the present embodiment, oxides in the steel material will coarsen and the SSC resistance of the steel material will decrease. Therefore, the content of Ca is to be 0 to 0.0100%.
  • a preferable lower limit of the content of Ca 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 content of Ca is 0.0040%, more preferably is 0.0025%, and further preferably is 0.0020%.
  • Magnesium (Mg) is an optional element, and does not have to be contained. That is, the content of Mg may be 0%. When contained, Mg renders S in the steel material harmless by forming sulfides, and thereby increases the SSC resistance of the steel material. If even a small amount of Mg is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Mg is too high, even if the contents of other elements are within the range of the present embodiment, oxides in the steel material will coarsen and the SSC resistance of the steel material will decrease. Therefore, the content of Mg is to be 0 to 0.0100%.
  • a preferable lower limit of the content of Mg 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 content of Mg is 0.0040%, more preferably is 0.0025%, and further preferably is 0.0020%.
  • Zirconium (Zr) is an optional element, and does not have to be contained. That is, the content of Zr may be 0%. When contained, Zr renders S in the steel material harmless by forming sulfides, and thereby increases the SSC resistance of the steel material. If even a small amount of Zr is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Zr is too high, even if the contents of other elements are within the range of the present embodiment, oxides in the steel material will coarsen and the SSC resistance of the steel material will decrease. Therefore, the content of Zr is to be 0 to 0.0100%.
  • a preferable lower limit of the content of Zr 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 content of Zr is 0.0040%, more preferably is 0.0025%, and further preferably is 0.0020%.
  • Rare earth metal is an optional element, and does not have to be contained. That is, the content of REM may be 0%. When contained, REM renders S in the steel material harmless by forming sulfides, and thereby increases the SSC resistance of the steel material. REM also combines with P in the steel material and thereby suppresses segregation of P to the grain boundaries. Therefore, a decrease in the SSC resistance of the steel material attributable to segregation of P is suppressed. If even a small amount of REM is contained, the aforementioned advantageous effects will be obtained to a certain extent even if the contents of other elements are within the range of the present embodiment.
  • REM Rare earth metal
  • the content of REM is to be 0 to 0.0100%.
  • a preferable lower limit of the content of REM 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 content of REM is 0.0040%, more preferably is 0.0025%, and further preferably is 0.0020%.
  • REM means one or more types of element selected from the group consisting of scandium (Sc) which is the element with atomic number 21, yttrium (Y) which is the element with atomic number 39, and the elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71 that are lanthanoids.
  • Sc scandium
  • Y yttrium
  • Lu lutetium
  • REM means one or more types of element selected from the group consisting of scandium (Sc) which is the element with atomic number 21, yttrium (Y) which is the element with atomic number 39, and the elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71 that are lanthanoids.
  • content of REM refers to the total content of these elements.
  • the yield strength of the steel material according to the present embodiment is 862 MPa or more.
  • yield strength means the stress at a time of 0.65% elongation (0.65% proof stress) obtained in a tensile test.
  • the steel material according to the present embodiment has excellent SSC resistance in a sour environment even when the yield strength of the steel material is 862 MPa or more.
  • the upper limit of the yield strength of the steel material is, for example, 1069 MPa (155 ksi), and preferably is 1034 MPa (150 ksi).
  • the yield strength of the steel material according to the present embodiment can be determined by the following method. Specifically, a tensile test is carried out by a method in accordance with ASTM E8/E8M (2021). First, a round bar specimen is prepared from the steel material according to the present embodiment. If the steel material is a steel plate, the round bar specimen is prepared from the center portion of the thickness. In this case, the axial direction of the round bar specimen is to be made a direction that is parallel to the rolling elongation direction of the steel plate. If the steel material is a steel pipe, the round bar specimen is prepared from the center portion of the wall thickness. In this case, the axial direction of the round bar specimen is to be made a direction that is parallel to the axial direction of the steel pipe.
  • the round bar specimen is prepared from an R/2 position.
  • R/2 position means the center position of a radius R in a cross section perpendicular to the axial direction of the round steel bar.
  • the axial direction of the round bar specimen is to be made a direction that is parallel to the axial direction of the round steel bar.
  • the round bar specimen has a parallel portion diameter of 8.9 mm and a gage length of 35.6 mm.
  • a tensile test is carried out in the atmosphere at normal temperature (25°C) using the round bar specimen, and the obtained stress at a time of 0.65% elongation (0.65% proof stress) is defined as the yield strength (MPa). Note that, a value obtained by rounding off decimals of the obtained numerical value is adopted as the yield strength (MPa) in the present embodiment.
  • Si oxides in which, in mass%, the content of Si is 20% or more and the content of O is 10% or more, and which have a major axis of 5.0 ⁇ m or more are also referred to as "coarse Si oxides".
  • the steel material according to the present embodiment has the chemical composition described above and the yield strength described above, and in addition, in the steel material, the number density of Si oxides in which, in mass%, the content of Si is 20% or more and the content of O is 10% or more, and which have a major axis of 5.0 ⁇ m or more (coarse Si oxides) is 5 /100 mm 2 or less. Furthermore, in a case where the yield strength of the steel material according to the present embodiment is 931 MPa or more, the number density of the coarse Si oxides in the steel material is 5 /200 mm 2 or less.
  • the steel material according to the present embodiment in a case where the yield strength is 862 to less than 931 MPa, the number density of coarse Si oxides is 5 /100 mm 2 or less (that is, 10/200 mm 2 or less), and in a case where the yield strength is 931 MPa or more, the number density of coarse Si oxides is 5 /200 mm 2 or less.
  • the steel material according to the present embodiment can achieve both a yield strength of 125 ksi or more and excellent SSC resistance.
  • a preferable upper limit of the number density of coarse Si oxides is 4 /100 mm 2 , and more preferably is 3 /100 mm 2 .
  • a preferable upper limit of the number density of coarse Si oxides is 4 /200 mm 2 , and more preferably is 3 /200 mm 2 .
  • the lower limit of the number density of coarse Si oxides is not particularly limited, and may be 0 /100 mm 2 , that is, 0 /200 mm 2 .
  • the number density of coarse Si oxides in the steel material can be determined by the following method.
  • a test specimen in which a face including the rolling elongation direction and the rolling reduction direction is adopted as an observation surface is prepared from the steel material according to the present embodiment.
  • the steel material is a steel plate
  • a test specimen in which a face including the rolling elongation direction and the thickness direction is adopted as the observation surface is prepared from a center portion of the thickness.
  • the steel material is a steel pipe
  • a test specimen in which a face including the pipe axis direction and the pipe radius direction is adopted as the observation surface is prepared from a center portion of the wall thickness.
  • the steel material is a round steel bar, a test specimen which includes an R/2 position at the center thereof and in which a face including the axial direction and the radial direction is adopted as the observation surface is prepared.
  • the area of the observation surface is not limited, for example, the area is set to a size of 300 mm 2 (20 mm ⁇ 15 mm).
  • the number of Si oxides having a major axis of 5.0 ⁇ m or more is determined.
  • the EDS analysis is conducted with an accelerating voltage of 20 kV for quantification of N, O, Mg, Al, Si, P, S, Ca, Ti, Cr, Mn, Fe, Cu, Zr, and Nb as elements to be analyzed. Based on the EDS analysis result for each particle, particles in which, in mass%, the content of Si is 20% or more, and the content of O is 10% or more are identified as "Si oxides".
  • Si oxides having a major axis of 5.0 ⁇ m or more are identified, and the total number of the coarse Si oxides is determined.
  • the major axis of the Si oxides can be determined by a well-known method.
  • the term "major axis" of the Si oxides means, at the observation surface, the longest line segment ( ⁇ m) among line segments linking an arbitrary two points on the outer circumference of the each of the Si oxides.
  • the number density of coarse Si oxides (/100 mm 2 or /200 mm 2 ) is determined based on the total number of the coarse Si oxides and the gross area of the observation surface.
  • a number obtained by rounding off decimals of the obtained numerical value is adopted as the number density of coarse Si oxides (/100 mm 2 or /200 mm 2 ). Further, measurement of the number density of coarse Si oxides can be performed using an apparatus in which a scanning electron microscope is provided with a composition analysis function (SEM-EDS apparatus). For example, an automatic analyzer having the trade name "Metals Quality Analyzer” manufactured by FEI (ASPEX) Company can be used as the SEM-EDS apparatus.
  • the SSC resistance of the steel material according to the present embodiment can be evaluated by an SSC resistance test conducted by a method carried out in accordance with NACE TM0177-2016 Method A. Specifically, the SSC resistance can be evaluated by the following method.
  • a mixed aqueous solution containing 5.0% by mass of sodium chloride and 0.4% by mass of sodium acetate that is adjusted to pH 3.5 with acetic acid (NACE solution B) is employed as the test solution.
  • a round bar specimen is prepared from the steel material according to the present embodiment. If the steel material is a steel plate, the round bar specimen is prepared from the center portion of the thickness. In this case, the axial direction of the round bar specimen is to be made a direction that is parallel to the rolling elongation direction of the steel plate. If the steel material is a steel pipe, the round bar specimen is prepared from the center portion of the wall thickness. In this case, the axial direction of the round bar specimen is to be made a direction that is parallel to the axial direction of the steel pipe.
  • the round bar specimen is prepared from an R/2 position.
  • the axial direction of the round bar specimen is to be made a direction that is parallel to the axial direction of the round steel bar.
  • the size of the round bar specimen for example, the round bar specimen has a diameter of 6.35 mm, and the length of a parallel portion is 25.4 mm. Note that, the axial direction of the round bar specimen is parallel to the rolling elongation direction of the steel material.
  • the yield strength of the steel material is less than 931 MPa
  • a stress equivalent to 90% of the actual yield stress is applied to the prepared round bar specimen.
  • the test solution at 24°C is poured into a test vessel in a manner so that the round bar specimen to which the stress has been applied is immersed therein, and this is adopted as a test bath. After degassing the test bath, a gaseous mixture of H 2 S gas at 0.1 atm and CO 2 gas at 0.9 atm is blown into the test bath to saturate the test bath. The test bath saturated with the gaseous mixture is held at 24°C for 1440 hours.
  • the steel material according to the present embodiment has a yield strength of less than 931 MPa
  • in an SSC resistance test conducted under the above conditions cracking is not confirmed after 1440 hours elapses.
  • the yield strength of the steel material is 931 MPa or more
  • a stress equivalent to 90% of the actual yield stress is applied to the prepared round bar specimen.
  • the test solution at 24°C is poured into a test vessel in a manner so that the round bar specimen to which the stress has been applied is immersed therein, and this is adopted as a test bath. After degassing the test bath, a gaseous mixture of H 2 S gas at 0.01 atm and CO 2 gas at 0.99 atm is blown into the test bath to saturate the test bath. The test bath saturated with the gaseous mixture is held at 24°C for 1440 hours.
  • the steel material according to the present embodiment has a yield strength of 931 MPa or more
  • an SSC resistance test conducted under the above conditions cracking is not confirmed after 1440 hours elapses.
  • the total of the volume ratios of tempered martensite and tempered bainite is 90% or more.
  • the balance of the microstructure is, for example, ferrite or pearlite. If the total of the volume ratios of tempered martensite and tempered bainite contained in the microstructure of a steel material having the chemical composition described above is 90% or more, on the condition that the other requirements of the present embodiment are satisfied, the steel material will exhibit excellent SSC resistance in a sour environment. That is, in the present embodiment, if the steel material has excellent SSC resistance, it is determined that the total of the volume ratios of tempered martensite and tempered bainite contained in the microstructure is 90% or more.
  • the volume ratios can be determined by the following method.
  • a test specimen having an observation surface is prepared from the steel material according to the present embodiment. If the steel material is a steel plate, a test specimen in which a face including the rolling elongation direction and the thickness direction is adopted as the observation surface is prepared from a center portion of the thickness. If the steel material is a steel pipe, a test specimen in which a face including the pipe axis direction and the pipe radius direction is adopted as the observation surface is prepared from a center portion of the wall thickness. If the steel material is a round steel bar, a test specimen which includes an R/2 position at the center thereof, and in which a face including the axial direction and the radial direction is adopted as the observation surface is prepared.
  • the test specimen After polishing the observation surface of the test specimen to obtain a mirror surface, the test specimen 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 the observation is performed in 10 visual fields.
  • the area of each visual field is, for example, 0.01 mm 2 (magnification of 1000 ⁇ ).
  • tempered martensite and tempered bainite are identified based on contrast.
  • the area fractions of the identified tempered martensite and tempered bainite are determined.
  • the method for determining the area fractions is not particularly limited, and a well-known method can be used.
  • the area fractions of tempered martensite and tempered bainite can be determined by image analysis.
  • an arithmetic average value of the area fractions of tempered martensite and tempered bainite determined in all of the visual fields is defined 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, a steel plate, or a round steel bar.
  • a preferable wall thickness is 9 to 60 mm.
  • the steel material according to the present embodiment is a seamless steel pipe.
  • the steel material according to the present embodiment is a seamless steel pipe, even when the steel material is a heavy-wall seamless steel pipe with a wall thickness of 15 mm or more, the steel material has a yield strength of 125 ksi or more and has excellent SSC resistance in a sour environment.
  • the method for producing a seamless steel pipe includes a process of preparing a starting material (steelmaking process), a process of subjecting the starting material to hot working to produce a hollow shell (hot working process), and a process of subjecting the hollow shell to quenching and tempering to make a seamless steel pipe (quenching process and tempering process).
  • a production method according to the present embodiment is not limited to the production method described below. Each process is described in detail hereunder.
  • molten iron produced by a well-known method is subjected to refining using a converter (primary refining).
  • the molten steel that has undergone the primary refining is then subjected to secondary refining.
  • alloy elements that were subjected to composition adjustment are added to the molten steel to thereby produce a molten steel that satisfies the chemical composition described above.
  • an RH (Ruhrstahl-Hausen) vacuum degassing treatment is performed. Thereafter, final adjustment of the alloy elements is performed.
  • composite refining may be performed.
  • a refining treatment that uses an LF (ladle furnace) or VAD (vacuum arc degassing) is performed prior to the RH vacuum degassing treatment.
  • a starting material is produced using the molten steel that underwent the secondary refining.
  • a cast piece (a slab, a bloom, or a billet) is produced by a continuous casting process using the molten steel subjected to the secondary refining.
  • the continuous casting process first, molten steel is poured from a ladle into a tundish. At such time, in order to seal the nozzle of the ladle, usually the nozzle is filled with sand. Therefore, in some cases, the sand for filling may get mixed in together with the molten steel poured from the ladle to the tundish.
  • Si oxides may be used as the sand for filling. In such a case, there is a concern that Si oxides will be introduced into the produced starting material.
  • the molten steel and the Si oxides are separated.
  • the method for separating Si oxides from the molten steel is not particularly limited, for example the following method can be used.
  • An inclined metal plate is placed at a position that is below the nozzle of the ladle and is above the opening of the tundish.
  • Si oxides are discharged from the nozzle, and next molten steel is discharged.
  • the Si oxides are light in comparison to the molten steel.
  • the Si oxides discharged from the nozzle are guided to outside of the opening of the tundish along the inclination of the metal plate.
  • the inclination of the metal plate may be provided, for example, by arranging a metal plate machined into a conical shape without a bottom surface in a manner so that the apex of the conically shape metal plate is directly below the nozzle of the ladle, or may be provided by the other methods. Further, one metal plate may be used, or a plurality of metal plates may be stacked on each other and used. In addition, although not particularly limited, the thickness of the metal plate is, for example, about 1 to 10 mm.
  • the metal plate in the present embodiment is preferably a metal plate composed of an alloying element contained in the molten steel.
  • an aluminum plate can be used as a metal plate composed of an alloying element contained in the molten steel.
  • the term "aluminum plate” means a metal plate which is formed of aluminum and the balance of impurities.
  • the metal plate is removed from below the nozzle before discharging the molten steel.
  • Si oxides that adhered to the metal plate can be prevented from becoming mixed in with the molten steel.
  • the number density of coarse Si oxides can be reduced to 5 /200 mm 2 or less in some cases. Therefore, in the present embodiment, it is preferable to remove the metal plate from below the nozzle at a timing that is after the Si oxides have been discharged from the nozzle and is before the molten steel is discharged.
  • a method for removing the metal plate from below the nozzle is not particularly limited, and for example a hole may be formed in advance in one part of the metal plate, and the metal plate may be removed using a rod that has a hook formed at the front end thereof.
  • the metal plate can be removed by hooking the hook that is formed at the front end of the rod into the hole in the metal plate and then pulling the rod.
  • Si oxides can be separated from the molten steel, and the molten steel can be introduced into the tundish. Note that, a method for separating the Si oxides from the molten steel is not limited to the method described above.
  • the molten steel is cast and a starting material is produced by the above method.
  • the starting material is a billet having a circular cross section (a round billet).
  • a method for producing the starting material is not particularly limited.
  • the molten steel may be cast into a round billet by a continuous casting process.
  • the molten steel may be cast to produce a billet having a rectangular cross section, or to produce a bloom. In these cases, it is preferable to perform blooming to produce a round billet from the billet having a rectangular cross section or the bloom.
  • the prepared starting material is subjected to hot working to produce an intermediate steel material.
  • the steel material is a seamless steel pipe
  • the intermediate steel material corresponds to a hollow shell.
  • a billet is heated in a heating furnace.
  • the heating temperature is, for example, 1100 to 1300°C.
  • the billet is subjected to hot working to produce a hollow shell (seamless steel pipe).
  • the method of hot working is not particularly limited, and it suffices to use a well-known method.
  • the Mannesmann process may be performed as hot working to produce a hollow shell.
  • a round billet is subjected to piercing-rolling using a piercing machine.
  • the piercing ratio is 1.0 to 4.0.
  • the round billet subjected to piercing-rolling is further subjected to hot rolling with a mandrel mill, a reducer, a sizing mill or the like to produce a hollow shell.
  • the cumulative reduction of area in the hot working process is, for example, 20 to 70%.
  • a hollow shell may be produced from the billet by the other hot working methods.
  • a hollow shell may be produced by forging by the Ehrhardt process or the like.
  • 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 the hot working without being cooled to normal temperature, or may be subjected to quenching after undergoing supplementary heating (reheating) after the hot working.
  • cooling may be stopped midway through the quenching process or slow cooling may be performed. In this case, the occurrence of quench cracking in the hollow shell can be suppressed.
  • stress relief annealing SR may be performed at a time that is after quenching and before the heat treatment of the next process. In this case, residual stress of the hollow shell is eliminated.
  • the starting material is heated in a heating furnace.
  • the heating temperature is, for example, 1100 to 1300°C.
  • the starting material extracted from the heating furnace is subjected to hot working to produce an intermediate steel material in which a cross section perpendicular to the axial direction is a circular shape.
  • the hot working is, for example, blooming that is performed using a blooming mill or hot rolling that is performed using a continuous mill.
  • a continuous mill a horizontal stand having a pair of grooved rolls arranged one on the other in the vertical direction, and a vertical stand having a pair of grooved rolls arranged side by side in the horizontal direction are alternately arranged.
  • the starting material is heated in a heating furnace.
  • the heating temperature is, for example, 1100 to 1300°C.
  • the starting material extracted from the heating furnace is subjected to hot rolling using a blooming mill and a continuous mill to produce an intermediate steel material having a steel plate shape.
  • the prepared starting material is subjected to hot working to produce an intermediate steel material.
  • the quenching process is described in detail.
  • the prepared intermediate steel material (hollow shell) is subjected to quenching.
  • quenching means rapidly cooling the intermediate steel material which is at a temperature not lower than the A 3 point.
  • a preferable quenching temperature is 800 to 1000°C. If the quenching temperature is too high, in some cases prior- ⁇ grains will become coarse and the SSC resistance of the steel material will decrease. Therefore, a quenching temperature in the range of 800 to 1000°C is preferable.
  • 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.
  • quenching temperature corresponds to the temperature of the furnace that performs the supplementary heating or reheating.
  • the quenching method is a method that, for example, continuously cools the intermediate steel material (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, and a well-known method can be used.
  • 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 microstructure will not become a microstructure that is principally composed of martensite and bainite, and the mechanical property defined in the present embodiment (yield strength of 125 ksi or more) will not be obtained. In such case, in addition, excellent low-temperature toughness and SSC resistance will not be obtained.
  • 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 "cooling rate during quenching CR 800-500 ".
  • the cooling rate during quenching CR 800-500 is determined based on a temperature measured at a region that is most slowly cooled within a cross-section of the intermediate steel material that is being quenched (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 preferable cooling rate during quenching CR 800-500 is 300°C/min or more.
  • a more preferable lower limit of the cooling rate during quenching CR 800-500 is 450°C/min, and further preferably is 600°C/min.
  • an upper limit of the cooling rate during quenching CR 800-500 is not particularly defined, the upper limit is, for example, 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.
  • quenching and tempering that is described later may be performed in combination a plurality of times. That is, both quenching and tempering may be performed a plurality of times. In such case, the SSC resistance of the steel material increases further.
  • the tempering process is described in detail hereunder.
  • the hollow shell on which the aforementioned quenching was performed is subjected to tempering.
  • tempering means reheating the intermediate steel material after quenching to a temperature that is less than the A c1 point and holding the intermediate steel material at that temperature.
  • tempering temperature corresponds to the temperature of the furnace when the intermediate steel material after quenching is heated and held at the relevant temperature.
  • tempering time means the period of time from when the temperature of the intermediate steel material reaches a predetermined tempering temperature until the steel material is extracted from the heat treatment furnace.
  • the tempering temperature is appropriately adjusted in accordance with the chemical composition of the seamless steel pipe and the yield strength to be obtained. That is, for a hollow shell having the chemical composition of the present embodiment, the tempering temperature is adjusted so as to adjust the yield strength of the seamless steel pipe to 862 MPa or more. Note that, a person skilled in the art is fully capable of adjusting the tempering temperature so as to adjust the yield strength of the seamless steel pipe to 862 MPa or more, and to 931 MPa or more. Specifically, in the tempering process according to the present embodiment, a preferable tempering temperature is 650 to 690°C. A more preferable lower limit of the tempering temperature is 655°C. A more preferable upper limit of the tempering temperature is 685°C.
  • the tempering time is set within a range of 10 to 90 minutes.
  • a more preferable lower limit of the tempering time is 15 minutes.
  • a more preferable upper limit of the tempering time is 80 minutes.
  • the steel material according to the present embodiment can be produced by the production method described above. Note that, in the foregoing description of the production method, a method for producing a seamless steel pipe has been described as one example. However, the steel material according to the present embodiment may also be a steel plate or the other shapes. A method for producing a steel plate or a steel material of the other shapes also include, for example, a preparation process, a quenching process, and a tempering process, similarly to the production method described above. Further, the production method described above is an example, and the steel material may also be produced by the other production methods.
  • Example 1 steel materials having a yield strength of 862 to less than 931 MPa were evaluated. Specifically, first, molten steels having the chemical compositions shown in Table 1-1 and Table 1-2 were produced. Note that, the symbol “-" in Table 1-2 means that the content of the relevant element was at the level of an impurity. Specifically, “-” means that the content of V, the content of Cu, the content of Ni, and the content of W of steel A were each 0% when rounded off to second decimal places. In addition, “-” means that the content of Ca, the content of Mg, the content of Zr, and the content of rare earth metal (REM) of steel A were each 0% when rounded off to fourth decimal places.
  • REM rare earth metal
  • Round billets were produced by a continuous casting process using the molten steels described above.
  • a metal plate which had been machined into a conical shape without a bottom surface was arranged above the opening of the tundish in a manner so that the apex of the conically shape metal plate was directly below the nozzle of the ladle.
  • Table 2 Whether or not a metal plate having the aforementioned shape was arranged above the opening of the tundish is indicated in Table 2. Specifically, a case where a metal plate having the aforementioned shape was arranged above the opening of the tundish is indicated by "A" in the column "Metal Plate” in Table 2.
  • a case where a metal plate having the aforementioned shape was not arranged above the opening of the tundish is indicated by "B" in the column “Metal Plate” in Table 2.
  • metal plates were used as metal plates having the aforementioned shape arranged above the opening of the tundish. Specifically, three aluminum plates each having a thickness of 2 mm were stacked on top of each other and used. Further, in Test Nos. 3, 8 to 10, and 12, at a timing that was after Si oxides were discharged from the nozzle and was before the molten steel was discharged, the metal plates were removed from below the nozzle using a rod that had a hook formed at the front end thereof.
  • the produced round billets of Test Nos. 1 to 19 were held at 1250°C for one hour, and thereafter were subjected to hot rolling by the Mannesmann-mandrel process to produce hollow shells (seamless steel pipes) of Test Nos. 1 to 19.
  • the obtained hollow shells of Test Nos. 1 to 19 were subjected to quenching.
  • the hollow shells of Test Nos. 1 to 19 were held at quenching temperatures (°C) for quenching times (mins) which are each described in the column "Quenching" in Table 2, and thereafter were subjected to quenching by shower water cooling. Note that, the cooling rates during quenching CR 800-500 were within the range of 480 to 30000°C/min for Test Nos. 1 to 19.
  • the temperatures (°C) of the heat treatment furnace that heated the hollow shells were adopted as the quenching temperatures (°C) described in Table 2. Further, the times (mins) for which the hollow shells were held at the quenching temperatures were adopted as the quenching time (mins) described in Table 2.
  • the obtained hollow shells of Test Nos. 1 to 19 were subjected to tempering. Specifically, tempering of the hollow shells of Test Nos. 1 to 19 was carried out by holding the hollow shells at tempering temperatures (°C) for tempering times (mins) which are each described in the column "Tempering" in Table 2. Here, the temperatures (°C) of the tempering furnace that heated the hollow shells were adopted as the tempering temperature (°C) described in Table 2. Further, the times (mins) for which the hollow shells were held at the tempering temperature were adopted as the tempering time (mins) described in Table 2. Seamless steel pipes of Test Nos. 1 to 19 were obtained by the production process described above.
  • the seamless steel pipes of Test Nos. 1 to 19 after the tempering described above were subjected to a tensile test, a test to measure the number density of coarse Si oxides, and an SSC resistance test.
  • the seamless steel pipes of Test Nos. 1 to 19 were subjected to a tensile test, and the yield strength was determined.
  • the tensile test was carried out in accordance with ASTM E8/E8M (2021). Round bar specimens having a parallel portion diameter of 8.9 mm and a gage length of 35.6 mm were prepared from the center portion of the wall thickness of the seamless steel pipes of Test Nos. 1 to 19. The axial direction of the round bar specimen was parallel to the axial direction of the seamless steel pipe.
  • the tensile tests were carried out in the atmosphere at normal temperature (25°C) using the prepared round bar specimens, and the yield strength (MPa) of the seamless steel pipes of Test Nos. 1 to 19 were determined.
  • the stress at a time of 0.65% elongation (0.65% proof stress) obtained in the tensile test was defined as the yield strength.
  • the obtained yield strength (MPa) is shown in Table 2 as "YS (MPa)".
  • Tests to measure the number density of coarse Si oxides were carried out on the seamless steel pipes of Test Nos. 1 to 19, and the number density of Si oxides having a major axis of 5.0 ⁇ m or more (coarse Si oxides) was determined.
  • the number density of coarse Si oxides was determined by the method described above using test specimens prepared from the center portion of the wall thickness of the seamless steel pipes of Test Nos. 1 to 19. For each of Test Nos. 1 to 19, the obtained number density of coarse Si oxides (/100 mm 2 ) is shown in the column "Coarse Si Oxides (/100 mm 2 )" in Table 2.
  • the seamless steel pipes of Test Nos. 1 to 19 were subjected to an SSC resistance test conducted by a method carried out in accordance with NACE TM0177-2016 Method A, and the SSC resistance was evaluated. Specifically, round bar specimens of 6.35 mm in diameter in which the length of a parallel portion was 25.4 mm were prepared from the center portion of the wall thickness of the seamless steel pipes of Test Nos. 1 to 19. The SSC resistance test was performed on three test specimens among the prepared test specimens. Note that the axial direction of each test specimen was parallel to the pipe axis direction.
  • Tensile stress was applied in the axial direction of the round bar specimens of Test Nos. 1 to 19. At this time, the applied stress was adjusted so as to be 90% of the actual yield stress of each seamless steel pipe.
  • a mixed aqueous solution containing 5.0% by mass of sodium chloride and 0.4% by mass of sodium acetate that was adjusted to pH 3.5 with acetic acid (NACE solution B) was used as the test solution.
  • the test solution at 24°C was poured into three test vessels, and these were adopted as test baths. Three round bar specimens to which the stress was applied were immersed individually in mutually different test vessels as the test baths.
  • test bath After each test bath was degassed, a gaseous mixture of H 2 S gas at 0.1 atm and CO 2 gas at 0.9 atm was blown into each test bath to saturate the test baths.
  • the test baths saturated with the gaseous mixture of H 2 S gas at 0.1 atm and CO 2 gas at 0.9 atm were held at 24°C for 1440 hours.
  • the round bar specimens of Test Nos. 1 to 19 were observed to determine whether or not sulfide stress cracking (SSC) had occurred. Specifically, after being held for 1440 hours, the round bar specimens were observed with the naked eye. For Test Nos. 1 to 19, the number of round bar specimens in which SSC had occurred among the three round bar specimens is shown in the column "Number of Specimens in which SSC Occurred (Specimens)" in Table 2.
  • the chemical compositions of the seamless steel pipes of Test Nos. 1 to 12 were appropriate, and the production methods of Test Nos. 1 to 12 also satisfied the preferable conditions described above.
  • the yield strength was 862 to less than 931 MPa
  • the number density of coarse Si oxides was 5 /100 mm 2 or less.
  • SSC did not occur in the SSC resistance test. That is, the seamless steel pipes of Test Nos. 1 to 12 had a yield strength of 862 to less than 931 MPa and had excellent SSC resistance.
  • Example 2 steel materials having a yield strength of 931 MPa or more were evaluated. Specifically, first, molten steels having the chemical compositions shown in Table 3-1 and Table 3-2 were produced. Note that, the symbol “-" in Table 3-2 means that the content of the relevant element was at the level of an impurity. Specifically, “-” means that the content of Cu, the content of Ni, and the content of W of steel R were each 0% when rounded off to second decimal places. In addition, “-” means that the content of Ca, the content of Mg, the content of Zr, and the content of rare earth metal (REM) of steel R were each 0% when rounded off to fourth decimal places.
  • REM rare earth metal
  • Round billets were produced by a continuous casting process using the molten steels described above.
  • a metal plate which had been machined into a conical shape without a bottom surface was arranged above the opening of the tundish in a manner so that the apex of the conically shape metal plate was directly below the nozzle of the ladle.
  • Table 4 Whether or not a metal plate having the aforementioned shape was arranged above the opening of the tundish is indicated in Table 4. Specifically, a case where a metal plate having the aforementioned shape was arranged above the opening of the tundish is indicated by "A" in the column "Metal Plate” in Table 4.
  • the produced round billets of Test Nos. 20 to 40 were held at 1250°C for one hour, and thereafter were subjected to hot rolling by the Mannesmann-mandrel process to produce hollow shells (seamless steel pipes) of Test Nos. 20 to 40.
  • the obtained hollow shells of Test Nos. 20 to 40 were subjected to quenching.
  • the hollow shells of Test Nos. 20 to 40 were held at quenching temperatures (°C) for quenching times (mins) which are each described in the column "Quenching" in Table 4, and thereafter were subjected to quenching by shower water cooling. Note that, in each of Test Nos.
  • the cooling rate during quenching CR 800-500 was within the range of 480 to 30000°C/min.
  • the temperatures (°C) of the heat treatment furnace that heated the hollow shells were adopted as the quenching temperatures (°C) described in Table 4.
  • the times (mins) for which the hollow shells were held at the quenching temperatures were adopted as the quenching times (mins) described in Table 4.
  • the hollow shell of Test No. 22 was subjected to a second quenching. Specifically, the hollow shell of Test No. 22 was held at 900°C in the heat treatment furnace for 10 minutes, and thereafter was subjected to quenching by shower water cooling. Note that, in the second quenching performed on the hollow shell of Test No. 22 also, the cooling rate during quenching CR 800-500 was within the range of 480 to 30000°C/min.
  • the obtained hollow shells of Test Nos. 20 to 40 were subjected to tempering. Specifically, tempering of the hollow shells of Test Nos. 20 to 40 was carried out by holding the hollow shells at the tempering temperatures (°C) for the tempering times (mins) which are each described in the column "Tempering" in Table 4. Here, the temperatures (°C) of the tempering furnace that heated the hollow shells were adopted as the tempering temperatures (°C) described in Table 4. Further, the times (mins) for which the hollow shells were held at the tempering temperatures were adopted as the tempering times (mins) described in Table 4. Seamless steel pipes of Test Nos. 20 to 40 were obtained by the production process described above.
  • the seamless steel pipes of Test Nos. 20 to 40 after the tempering described above were subjected to a tensile test, a test to measure the number density of coarse Si oxides, and an SSC resistance test which are described hereunder.
  • the seamless steel pipes of Test Nos. 20 to 40 were subjected to a tensile test, and the yield strength was determined.
  • the tensile test was carried out in accordance with ASTM E8/E8M (2021). Round bar specimens having a parallel portion diameter of 8.9 mm and a gage length of 35.6 mm were prepared from the center portion of the wall thickness of the seamless steel pipes of Test Nos. 20 to 40. The axial direction of the round bar specimen was parallel to the axial direction of the seamless steel pipe.
  • the tensile tests were carried out in the atmosphere at normal temperature (25°C) using the prepared round bar specimens, and the yield strength (MPa) of the seamless steel pipes of Test Nos. 20 to 40 were determined.
  • the stress at a time of 0.65% elongation (0.65% proof stress) obtained in the tensile test was defined as the yield strength.
  • the obtained yield strength (MPa) is shown in Table 4 as "YS (MPa)".
  • Tests to measure the number density of coarse Si oxides were carried out on the seamless steel pipes of Test Nos. 20 to 40, and the number density of Si oxides having a major axis of 5.0 ⁇ m or more (coarse Si oxides) was determined.
  • the number density of coarse Si oxides was determined by the method described above using test specimens prepared from the center portion of the wall thickness of the seamless steel pipes of Test Nos. 20 to 40. For each of Test Nos. 20 to 40, the obtained number density of coarse Si oxides (/200 mm 2 ) is shown in the column "Coarse Si Oxides (/200 mm 2 )" in Table 4.
  • the seamless steel pipes of Test Nos. 20 to 40 were subjected to an SSC resistance test conducted by a method carried out in accordance with NACE TM0177-2016 Method A, and the SSC resistance was evaluated. Specifically, round bar specimens of 6.35 mm in diameter in which the length of a parallel portion was 25.4 mm were prepared from the center portion of the wall thickness of the seamless steel pipes of Test Nos. 20 to 40. The SSC resistance test was performed on three test specimens among the prepared test specimens. Note that the axial direction of each test specimen was parallel to the pipe axis direction.
  • Tensile stress was applied in the axial direction of the round bar specimens of Test Nos. 20 to 40. At this time, the applied stress was adjusted so as to be 90% of the actual yield stress of each steel plate.
  • a mixed aqueous solution containing 5.0% by mass of sodium chloride and 0.4% by mass of sodium acetate that was adjusted to pH 3.5 with acetic acid (NACE solution B) was used as the test solution.
  • the test solution at 24°C was poured into three test vessels, and these were adopted as test baths. Three round bar specimens to which the stress was applied were immersed individually in mutually different test vessels as the test baths.
  • test bath After each test bath was degassed, a gaseous mixture of H 2 S gas at 0.01 atm and CO 2 gas at 0.99 atm was blown into each test bath to saturate the test baths.
  • the test baths saturated with the gaseous mixture of H 2 S gas at 0.01 atm and CO 2 gas at 0.99 atm were held at 24°C for 1440 hours.
  • the round bar specimens of Test Nos. 20 to 40 were observed to determine whether or not sulfide stress cracking (SSC) had occurred. Specifically, after being held for 1440 hours, the round bar specimens were observed with the naked eye. For Test Nos. 20 to 40, the number of round bar specimens in which SSC had occurred among the three round bar specimens is shown in the column "Number of Specimens in which SSC Occurred (Specimens)" in Table 4.
  • the chemical compositions of the seamless steel pipes of Test Nos. 20 to 32 were appropriate, and the production methods of Test Nos. 20 to 32 also satisfied the preferable conditions described above.
  • the yield strength was 931 MPa or more, and the number density of coarse Si oxides was 5 /200 mm 2 or less.
  • SSC did not occur in the SSC resistance test. That is, the seamless steel pipes of Test Nos. 20 to 32 had a yield strength of 931 MPa or more and had excellent SSC resistance.

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EP23756422.4A 2022-02-17 2023-02-16 Matériau en acier approprié pour une utilisation en environnements acides Pending EP4481062A4 (fr)

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