EP4286542A1 - Martensitisches edelstahlmaterial - Google Patents

Martensitisches edelstahlmaterial Download PDF

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Publication number
EP4286542A1
EP4286542A1 EP22775695.4A EP22775695A EP4286542A1 EP 4286542 A1 EP4286542 A1 EP 4286542A1 EP 22775695 A EP22775695 A EP 22775695A EP 4286542 A1 EP4286542 A1 EP 4286542A1
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Prior art keywords
steel material
content
less
yield strength
present
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English (en)
French (fr)
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EP4286542A4 (de
Inventor
Kyohei KANKI
Hideki Takabe
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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
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • 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
    • C21D6/004—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
    • C21D6/007—Heat treatment of ferrous alloys containing Co
    • 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
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips 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
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the 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
    • C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
    • C21D8/065—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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
    • C21D9/0075—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for rods of limited length
    • 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
    • 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
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • 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
    • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
    • 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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/52—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
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present disclosure relates to a steel material, and more particularly relates to a martensitic stainless steel material.
  • oil wells and gas wells include environments which contain large amounts of corrosive substances.
  • the corrosive substances are, for example, corrosive gases such as hydrogen sulfide (H 2 S) gas and carbon dioxide (CO 2 ) gas. It is known that chromium (Cr) is effective for improving the carbonic-acid gas corrosion resistance of a steel material.
  • martensitic stainless steel materials containing about 13% by mass of Cr that are typified by API L80 13Cr steel material (normal 13Cr steel material) and Super 13Cr steel material in which the content of C is reduced are used according to the partial pressure and temperature of the carbon dioxide gas.
  • an environment containing hydrogen sulfide and carbon dioxide gas is referred to as a "sour environment”.
  • Steel materials for oil wells to be used in a sour environment are required to have sulfide stress cracking resistance (hereunder, referred to as "SSC resistance”). That is, in recent years, steel materials for oil wells are required to have both high strength and excellent SSC resistance.
  • Patent Literature 1 Japanese Patent Application Publication No. 2000-192196 (Patent Literature 1), Japanese Patent Application Publication No. 2012-136742 (Patent Literature 2), and International Application Publication No. WO2008/023702 (Patent Literature 3) each proposes a steel material that has high strength and excellent SSC resistance.
  • the steel material proposed in Patent Literature 1 is a martensitic stainless steel for oil wells consisting of, in weight%, C: 0.001 to 0.05%, Si: 0.05 to 1%, Mn: 0.05 to 2%, P: 0.025% or less, S: 0.01% or less, Cr: 9 to 14%, Mo: 3.1 to 7%, Ni: 1 to 8%, Co: 0.5 to 7%, sol.
  • a steel material contains Mo, the Ms point decreases.
  • this steel material contains Co as well as Mo, a decrease in the Ms point is suppressed, and the microstructure is made a martensitic single-phase structure. It is described in Patent Literature 1 that, as a result, in this steel material, the SSC resistance can be increased while maintaining the strength at 80 ksi or more (552 MPa or more).
  • the steel material proposed in Patent Literature 2 is a martensitic stainless steel seamless pipe consisting of, in mass%, C: 0.01% or less, Si: 0.5% or less, Mn: 0.1 to 2.0%, P: 0.03% or less, S: 0.005% or less, Cr: 14.0 to 15.5%, Ni: 5.5 to 7.0%, Mo: 2.0 to 3.5%, Cu: 0.3 to 3.5%, V: 0.20% or less, Al: 0.05% or less, and N: 0.06% or less, with the balance being Fe and unavoidable impurities.
  • the steel material has a yield strength of 655 to 862 MPa, and a yield ratio of 0.90 or more.
  • Patent Literature 2 It is described in Patent Literature 2 that by setting the content of C to 0.01% or less, adjusting Cr, Ni and Mo to within a preferable range, and also containing suitable amounts of Cu and V or a suitable amount of W, excellent SSC resistance is obtained while also having a strength of 655 MPa or more.
  • the steel material proposed in Patent Literature 3 is a martensitic stainless steel that consists of, in mass%, C: 0.010 to 0.030%, Mn: 0.30 to 0.60%, P: 0.040% or less, S: 0.0100% or less, Cr: 10.00 to 15.00%, Ni: 2.50 to 8.00%, Mo: 1.00 to 5.00%, Ti: 0.050 to 0.250%, V: 0.25% or less, N: 0.07% or less, and one or more kinds of element among Si: 0.50% or less and Al: 0.10% or less, with the balance being Fe and impurities, and that satisfies the formula (6.0 ⁇ Ti/C ⁇ 10.1).
  • the yield strength is 758 to 862 MPa.
  • Patent Literatures 1 to 3 propose techniques for increasing the yield strength and improving the SSC resistance of a steel material.
  • a martensitic stainless steel material that has excellent SSC resistance while also increasing the yield strength may be obtained by a technique other than the techniques proposed in the aforementioned Patent Literatures 1 to 3.
  • An objective of the present disclosure is to provide a martensitic stainless steel material that can achieve both a high yield strength, and excellent SSC resistance in a sour environment with a pH of 3.0.
  • a martensitic stainless steel material according to the present disclosure consists of, in mass%,
  • the martensitic stainless steel material according to the present disclosure can achieve both a high yield strength, and excellent SSC resistance in a sour environment with a pH of 3.0.
  • the present inventors conducted studies from the viewpoint of the chemical composition with respect to a martensitic stainless steel material that can achieve both a high yield strength, and excellent SSC resistance in a sour environment with a pH of 3.0.
  • the present inventors considered that if a martensitic stainless steel material contains, in mass%, C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.0050% or less, Cu: 0.01 to 3.50%, Cr: 10.00 to 14.00%, Ni: 4.50 to 7.50%, Mo: 1.00 to 4.00%, Ti: 0.050 to 0.300%, V: 0.01 to 1.00%, Al: 0.001 to 0.100%, Co: 0.010 to 0.500%, Ca: 0.0005 to 0.0050%, N: 0.0010 to 0.0500%, O: 0.050% or less, W: 0 to 0.50%, and Nb: 0 to 0.500%, there
  • the present inventors conducted detailed studies regarding contents of Sn, As and Sb that can sufficiently increase the SSC resistance of a martensitic stainless steel material. As a result, it has been clarified that, in addition to the contents of elements described above, by the martensitic stainless steel material according to the present embodiment also containing Sn in an amount of 0.0005 to 0.0500%, As in an amount of 0 to 0.0100%, and Sb in an amount of 0 to 0.0100%, the SSC resistance of the steel material can be increased.
  • a martensitic stainless steel material consists of, in mass%, C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.0050% or less, Cu: 0.01 to 3.50%, Cr: 10.00 to 14.00%, Ni: 4.50 to 7.50%, Mo: 1.00 to 4.00%, Ti: 0.050 to 0.300%, V: 0.01 to 1.00%, Al: 0.001 to 0.100%, Co: 0.010 to 0.500%, Ca: 0.0005 to 0.0050%, Sn: 0.0005 to 0.0500%, N: 0.0010 to 0.0500%, O: 0.050% or less, W: 0 to 0.50%, Nb: 0 to 0.500%, As: 0 to 0.0100%, and Sb: 0 to 0.0100%, with the balance being Fe and impurities, there is a possibility that both a yield strength of 758 MPa or more, and excellent S
  • the present inventors have found that even in the case of a martensitic stainless steel material having the aforementioned chemical composition, when the martensitic stainless steel material has a yield strength of 758 MPa or more, there are some cases where the SSC resistance is not stably increased in a sour environment with a pH of 3.0. Therefore, with respect to a martensitic stainless steel material having the aforementioned chemical composition, the present inventors conducted detailed studies regarding means for increasing the SSC resistance in a sour environment with a pH of 3.0 while maintaining a yield strength of 758 MPa or more. As a result, the present inventors obtained the following findings.
  • F1 (Sn+As+Sb)/ ⁇ (Cu+Ni)/YS ⁇ .
  • As and Sb assist the effect of increasing the SSC resistance of the steel material produced by Sn.
  • the SSC resistance of the steel material markedly increases by making a ratio of the contents of Sn, As and Sb to the contents of Cu and Ni fall within a certain range.
  • F1 is an index of an increase in the SSC resistance in a sour environment with a pH of 3.0 obtained by a synergetic effect between Sn, As and Sb, and Cu and Ni that is adjusted according to the yield strength.
  • the relation between F1 and the SSC resistance in a sour environment with a pH of 3.0 is described specifically hereunder using the accompanying drawing.
  • FIG. 1 is a view illustrating the relation between F1 and SSC resistance in the present Examples.
  • FIG. 1 was created using F1 and the number of specimens in which pitting occurred (specimens) that is an index of SSC resistance, with respect to Examples which had the aforementioned chemical composition and in which the yield strength was 758 MPa or more among Examples that are described later. Note that, the number of specimens in which pitting occurred was obtained by performing an SSC resistance evaluation test that assumed a sour environment with a pH of 3.0, which will be described later.
  • the martensitic stainless steel material according to the present embodiment has the aforementioned chemical composition, has a yield strength of 758 MPa or more, and furthermore, within the ranges of the contents of the elements and the yield strength, the contents of the elements and the yield strength satisfy Formula (1).
  • the martensitic stainless steel material according to the present embodiment can achieve both a high yield strength of 758 MPa or more, and excellent SSC resistance in a sour environment with a pH of 3.0.
  • the gist of the martensitic stainless steel material according to the present embodiment which has been completed based on the above findings, is as follows.
  • the shape of the martensitic stainless steel material according to the present embodiment is not particularly limited.
  • the martensitic stainless steel material according to the present embodiment may be a pipe, may be a round steel bar (solid material), or may be a steel plate.
  • 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.
  • the pipe may be a seamless pipe or may be a welded pipe.
  • martensitic stainless steel material according to the present embodiment is described in detail.
  • the symbol “%” in relation to an element means mass percent unless otherwise stated. Further, in the following description, the martensitic stainless steel material is also referred to as simply "steel material”.
  • the martensitic stainless steel material according to the present embodiment contains the following elements.
  • Carbon (C) is unavoidably contained. That is, the lower limit of the content of C is more than 0%. C increases hardenability of the steel material and increases strength of the steel material. On the other hand, if the content of C is too high, even if the contents of other elements are within the range of the present embodiment, strength of the steel material will be too high. As a result, the SSC resistance of the steel material will decrease. Therefore, the content of C is to be 0.030% or less.
  • a preferable upper limit of the content of C is 0.028%, more preferably is 0.025%, further preferably is 0.020%, and further preferably is 0.018%.
  • the content of C is preferably as low as possible. However, extremely reducing the content of C will increase the production cost. Therefore, when taking industrial production into consideration, a preferable lower limit of the content of C is 0.001%, more preferably is 0.003%, and further preferably is 0.005%.
  • Silicon (Si) is unavoidably contained. That is, the lower limit of the content of Si is more than 0%. Si deoxidizes the steel. 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, hot workability of the steel material will decrease. Therefore, the content of Si is to be 1.00% or less.
  • a preferable lower limit of the content of Si for effectively obtaining the aforementioned advantageous effect is 0.01%, more preferably is 0.05%, further preferably is 0.10%, and further preferably is 0.15%.
  • a preferable upper limit of the content of Si is 0.80%, more preferably is 0.60%, further preferably is 0.50%, and further preferably is 0.45%.
  • Manganese (Mn) is unavoidably contained. That is, the lower limit of the content of Mn is more than 0%. Mn increases hardenability of the steel material and increases strength of the steel material. 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, in some cases Mn will segregate at grain boundaries together with impurity elements such as P and S. In such a case, the SSC resistance of the steel material will decrease. Therefore, the content of Mn is to be 1.00% or less.
  • a preferable lower limit of the content of Mn for effectively obtaining the aforementioned advantageous effect is 0.01%, more preferably is 0.05%, further preferably is 0.10%, and further preferably is 0.15%.
  • a preferable upper limit of the content of Mn is 0.80%, more preferably is 0.70%, further preferably is 0.60%, and further preferably is 0.50%.
  • Phosphorus (P) is an impurity that is unavoidably contained. That is, the lower limit of the content of P is more than 0%. P segregates at grain boundaries and facilitates the occurrence of SSC. Therefore, if the content of P 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 markedly 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 0.018%.
  • the content of P is preferably as low as possible. However, extremely reducing the content of P will raise the production cost. Accordingly, 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%.
  • S is an impurity that is unavoidably contained. That is, the lower limit of the content of S is more than 0%. Similarly to P, S segregates at grain boundaries and facilitates the occurrence of SSC. Therefore, if the content of S 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 markedly decrease. Therefore, the content of S is to be 0.0050% or less.
  • a preferable upper limit of the content of S is 0.0040%, more preferably is 0.0030%, further preferably is 0.0025%, and further preferably is 0.0020%.
  • the content of S is preferably as low as possible. However, extremely reducing the content of S will raise the production cost. Accordingly, when taking industrial production into consideration, a preferable lower limit of the content of S is 0.0001%, more preferably is 0.0002%, and further preferably is 0.0003%.
  • Copper (Cu) is an austenite forming element and causes the microstructure after quenching to become martensitic. Cu also increases the SSC resistance of the steel material in a sour environment with a pH of 3.0, by a synergetic effect with Sn, As and Sb. If the content of Cu is too low, even if the contents of other elements are within the range of the present embodiment, the aforementioned advantageous effects will not be sufficiently obtained. On the other hand, if the content of Cu is too high, even if the contents of other elements are within the range of the present embodiment, the aforementioned advantageous effects will be saturated, and furthermore, hot workability of the steel material will markedly decrease. In this case, in addition, the production cost will rise.
  • the content of Cu is to be 0.01 to 3.50%.
  • a preferable lower limit of the content of Cu is 0.02%, more preferably is 0.03%, and further preferably is 0.05%.
  • a preferable upper limit of the content of Cu is 3.30%, more preferably is 3.10%, and further preferably is 2.90%.
  • Chromium (Cr) forms a passive film on the surface of the steel material and thereby increases the SSC resistance of the steel material. If the content of Cr 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. On the other hand, if the content of Cr is too high, even if the contents of other elements are within the range of the present embodiment, in some cases ferrite will be included in the microstructure, and it will be difficult to secure sufficient strength. If the content of Cr is too high, in addition, even if the contents of other elements are within the range of the present embodiment, intermetallic compounds or Cr carbo-nitrides will easily form in the steel material. As a result, the SSC resistance of the steel material will decrease.
  • the content of Cr is to be 10.00 to 14.00%.
  • a preferable lower limit of the content of Cr is 10.30%, more preferably is 10.50%, and further preferably is 11.00%.
  • a preferable upper limit of the content of Cr is 13.80%, more preferably is 13.60%, further preferably is 13.50%, further preferably is 13.45%, further preferably is 13.40%, and further preferably is 13.35%.
  • Nickel (Ni) is an austenite forming element and causes the microstructure after quenching to become martensitic. Ni also forms sulfides on the passive film in a sour environment. The Ni sulfides inhibit chloride ions (Cl - ) and hydrogen sulfide ions (HS - ) from coming into contact with the passive film, thereby suppressing destruction of the passive film by chloride ions and hydrogen sulfide ions. As a result, the SSC resistance of the steel material increases. Furthermore, Ni increases the SSC resistance of the steel material in a sour environment with a pH of 3.0 by a synergetic effect with Sn, As and Sb.
  • the content of Ni is to be 4.50 to 7.50%.
  • a preferable lower limit of the content of Ni is 4.80%, more preferably is 5.00%, and further preferably is 5.50%.
  • a preferable upper limit of the content of Ni is 7.30%, more preferably is 7.00%, and further preferably is 6.50%.
  • Molybdenum (Mo) forms sulfides on the passive film in a sour environment.
  • the Mo sulfides inhibit chloride ions (Cl - ) and hydrogen sulfide ions (HS - ) from coming into contact with the passive film, thereby suppressing destruction of the passive film by chloride ions and hydrogen sulfide ions.
  • the SSC resistance of the steel material increases.
  • Mo also dissolves in the steel material, and thereby increases strength of the steel material. If the content of Mo is too low, even if the contents of other elements are within the range of the present embodiment, the aforementioned advantageous effects will not be sufficiently obtained.
  • the content of Mo is to be 1.00 to 4.00%.
  • a preferable lower limit of the content of Mo is 1.30%, more preferably is 1.50%, and further preferably is 1.80%.
  • a preferable upper limit of the content of Mo is 3.80%, more preferably is 3.60%, and further preferably is 3.40%.
  • Titanium (Ti) combines with C and/or N to form carbides or nitrides. In such case, coarsening of grains is suppressed by the pinning effect, and the yield strength of the steel material increases. If the content of Ti 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. 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, strength of the steel material will be too high, and the SSC resistance of the steel material will decrease. Therefore, the content of Ti is to be 0.050 to 0.300%. A preferable lower limit of the content of Ti is 0.060%, and more preferably is 0.080%. A preferable upper limit of the content of Ti is 0.250%, more preferably is 0.200%, and further preferably is 0.180%.
  • Vanadium (V) increases hardenability of the steel material and raises the yield strength of the steel material. If the content of V 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. On the other hand, if the content of V is too high, even if the contents of other elements are within the range of the present embodiment, strength of the steel material will be too high and the SSC resistance of the steel material will decrease. Therefore, the content of V is to be 0.01 to 1.00%. A preferable lower limit of the content of V is 0.02%, and more preferably is 0.03%. A preferable upper limit of the content of V is 0.80%, more preferably is 0.60%, and further preferably is 0.50%.
  • 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. 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 oxides will form and the SSC resistance of the steel material will decrease. Therefore, the content of Al is to be 0.001 to 0.100%. A preferable lower limit of the content of Al is 0.005%, more preferably is 0.010%, and further preferably is 0.015%. A preferable upper limit of the content of Al is 0.080%, more preferably is 0.060%, further preferably is 0.055%, and further preferably is 0.050%. As used in the present description, the term "content of Al" means the content of sol. Al (acid-soluble Al).
  • Co Cobalt
  • the Co sulfides inhibit chloride ions (Cl - ) and hydrogen sulfide ions (HS - ) from coming into contact with the passive film, thereby suppressing destruction of the passive film by chloride ions and hydrogen sulfide ions.
  • the SSC resistance of the steel material increases.
  • Co also increases hardenability of the steel material, and particularly during industrial production, ensures consistent high strength of the steel material. Specifically, Co suppresses the formation of retained austenite, and suppresses the occurrence of variations in strength of the steel material.
  • the content of Co is to be 0.010 to 0.500%.
  • a preferable lower limit of the content of Co is 0.015%, more preferably is 0.020%, further preferably is 0.030%, further preferably is 0.050%, and further preferably is 0.100%.
  • a preferable upper limit of the content of Co is 0.450%, more preferably is 0.400%, and further preferably is 0.350%.
  • the content of Ca immobilizes S in the steel material as a sulfide to make it harmless, and thereby improves hot workability of the steel material. If the content of Ca 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. On the other hand, if the content of Ca is too high, even if the contents of other elements are within the range of the present embodiment, coarse inclusions will be formed in the steel material and the SSC resistance of the steel material will decrease. Therefore, the content of Ca is to be 0.0005 to 0.0050%. A preferable lower limit of the content of Ca is 0.0006%, more preferably is 0.0008%, and further preferably is 0.0010%. A preferable upper limit of the content of Ca is 0.0045%, more preferably is 0.0040%, and further preferably is 0.0035%.
  • Tin (Sn) increases the SSC resistance of the steel material in a sour environment with a pH of 3.0. If the content of Sn 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. On the other hand, if the content of Sn is too high, even if the contents of other elements are within the range of the present embodiment, Sn will segregate at grain boundaries and, on the contrary, the SSC resistance of the steel material will decrease. Therefore, the content of Sn is to be 0.0005 to 0.0500%. A preferable lower limit of the content of Sn is 0.0008%, more preferably is 0.0010%, and further preferably is 0.0015%. A preferable upper limit of the content of Sn is 0.0400%, more preferably is 0.0300%, further preferably is 0.0200%, further preferably is 0.0100%, and further preferably is 0.0080%.
  • Nitrogen (N) combines with Ti to form fine Ti nitrides.
  • the fine TiN suppresses coarsening of grains by the pinning effect. As a result, the yield strength of the steel material increases. If the content of N 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. On the other hand, if the content of N is too high, even if the contents of other elements are within the range of the present embodiment, coarse nitrides will be formed and the SSC resistance of the steel material will decrease. Therefore, the content of N is to be 0.0010 to 0.0500%.
  • a preferable lower limit of the content of N is 0.0015%, more preferably is 0.0020%, further preferably is 0.0030%, and further preferably is 0.0040%.
  • a preferable upper limit of the content of N is 0.0450%, more preferably is 0.0400%, further preferably is 0.0350%, and further preferably is 0.0300%.
  • Oxygen (O) is an impurity that is unavoidably contained. That is, the lower limit of the content of O is more than 0%. O forms oxides and reduces the SSC resistance of the steel material. Accordingly, if the content of O 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 markedly decrease. Therefore, the content of O is to be 0.050% or less.
  • a preferable upper limit of the content of O is 0.040%, more preferably is 0.030%, and further preferably is 0.020%.
  • the content of O is preferably as low as possible. However, extremely reducing the content of O will raise the production cost. Therefore, when taking industrial production into consideration, a preferable lower limit of the content of O is 0.0005%, more preferably is 0.001%, and further preferably is 0.002%.
  • the balance in the martensitic stainless steel material according to the present embodiment is Fe and impurities.
  • impurities refers to elements 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 not intentionally contained but are allowed within a range that does not adversely affect the martensitic stainless steel material according to the present embodiment.
  • the martensitic stainless steel material according to the present embodiment may also contain W in lieu of a part of Fe.
  • Tungsten (W) is an optional element and need not be contained. That is, the content of W may be 0%. When contained, W stabilizes the passive film in a sour environment, and suppresses destruction of the passive film by chloride ions and hydrogen sulfide ions. As a result, the SSC resistance of the steel material increases. If even a small amount of W is contained, the aforementioned effect will be obtained to a certain extent. On the other hand, if the content of W is too high, W will combine with C and form coarse carbides. In such case, 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 W is to be 0 to 0.50%. A preferable lower limit of the content of W is 0.01%, more preferably is 0.03%, and further preferably is 0.05%. A preferable upper limit of the content of W is 0.45%, more preferably is 0.40%, and further preferably is 0.35%.
  • W markedly increases the SSC resistance when the content of Cu is high.
  • the content of Cu is 0.50% or more, it is preferable to set the content of W to 0.10% or more.
  • a more preferable lower limit of the content of W is 0.12%, and further preferably is 0.15%.
  • the martensitic stainless steel material according to the present embodiment may also contain Nb in lieu of a part of Fe.
  • Niobium (Nb) is an optional element, and need not be contained. That is, the content of Nb may be 0%. When contained, Nb combines with C and/or N to form Nb carbides and Nb carbo-nitrides. In such case, coarsening of grains is suppressed by the pinning effect, and the yield strength of the steel material increases. If even a small amount of Nb is contained, the aforementioned effect will be obtained to a certain extent. On the other hand, if the content of Nb is too high, Nb carbides and/or Nb carbo-nitrides will excessively form even if the contents of other elements are within the range of the present embodiment. As a result, the SSC resistance of the steel material will decrease.
  • the content of Nb is to be 0 to 0.500%.
  • a preferable lower limit of the content of Nb is 0.001%, more preferably is 0.002%, and further preferably is 0.003%.
  • a preferable upper limit of the content of Nb is 0.450%, more preferably is 0.400%, and further preferably is 0.350%.
  • the martensitic stainless steel material according to the present embodiment may further contain one or more elements selected from the group consisting of As and Sb in lieu of a part of Fe. Each of these elements assists the effect of increasing the SSC resistance of the steel material produced by Sn.
  • Arsenic (As) is an optional element, and need not be contained. That is, the content of As may be 0%. When contained, As assists the effect of increasing the SSC resistance of the steel material produced by Sn. If even a small amount of As is contained, the aforementioned effect will be obtained to a certain extent. On the other hand, if the content of As is too high, even if the contents of other elements are within the range of the present embodiment, As will segregate at grain boundaries and the SSC resistance of the steel material will decrease. Therefore, the content of As is to be 0 to 0.0100%. A preferable lower limit of the content of As is 0.0001%, more preferably is 0.0005%, and further preferably is 0.0010%. A preferable upper limit of the content of As is 0.0090%, and more preferably is 0.0080%.
  • Antimony (Sb) is an optional element, and need not be contained. That is, the content of Sb may be 0%. When contained, Sb assists the effect of increasing the SSC resistance of the steel material produced by Sn. If even a small amount of Sb is contained, the aforementioned effect will be obtained to a certain extent. On the other hand, if the content of Sb is too high, even if the contents of other elements are within the range of the present embodiment, Sb will segregate at grain boundaries and the SSC resistance of the steel material will decrease. Therefore, the content of Sb is to be 0 to 0.0100%. A preferable lower limit of the content of Sb is 0.0001%, more preferably is 0.0005%, and further preferably is 0.0010%. A preferable upper limit of the content of Sb is 0.0090%, and more preferably is 0.0080%.
  • the yield strength of the martensitic stainless steel material according to the present embodiment is 758 MPa (110 ksi) or more, and more preferably is 862 MPa (125 ksi) or more.
  • the upper limit of the yield strength is not particularly limited, the upper limit of the yield strength of the steel material of the present embodiment is, for example, 1034 MPa (150 ksi).
  • a more preferable upper limit of the yield strength of the steel material is 1000 MPa (145 ksi).
  • yield strength means 0.2% offset yield stress (MPa) obtained by a tensile test at normal temperature (24 ⁇ 3°C) conducted in conformity with ASTM E8/E8M (2013).
  • the yield strength is determined by the following method.
  • a tensile test specimen is prepared from the martensitic stainless steel material according to the present embodiment. If the steel material is a pipe, the tensile test specimen is prepared from a central position of the wall thickness. If the steel material is a round steel bar, the tensile test specimen is prepared from an R/2 position. Note that, in the present description, the term "R/2 position" of a round steel bar means the center position of a radius R in a cross section perpendicular to the axial direction of the round steel bar. If the steel material is a steel plate, the tensile test specimen is prepared from a central position of the thickness.
  • the size of the tensile test specimen is not particularly limited.
  • a round bar tensile test specimen in which the diameter of the parallel portion is 8.9 mm and the gage length is 35.6 mm is used as the tensile test specimen.
  • the longitudinal direction of the parallel portion of the tensile test specimen is to be made parallel to the rolling direction and/or axial direction of the steel material.
  • a tensile test is conducted at normal temperature (24 ⁇ 3°C) in accordance with ASTM E8/E8M (2013) using the prepared tensile test specimen, and the 0.2% offset yield stress (MPa) is determined.
  • the determined 0.2% offset yield stress is defined as the yield strength (MPa).
  • the yield strength of the martensitic stainless steel material according to the present embodiment is 758 MPa or more, and preferably is 862 MPa or more.
  • the martensitic stainless steel material consists of, in mass%, C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.0050% or less, Cu: 0.01 to 1.00%, Cr: 10.00 to 14.00%, Ni: 4.50 to 6.50%, Mo: 1.00 to 3.00%, Ti: 0.050 to 0.300%, V: 0.01 to 1.00%, Al: 0.001 to 0.100%, Co: 0.010 to 0.500%, Ca: 0.0005 to 0.0050%, Sn: 0.0005 to 0.0500%, N: 0.0010 to 0.0500%, O: 0.050% or less,
  • the martensitic stainless steel material when the content of Cu is 1.00% or less and attempting to obtain a yield strength of 862 MPa or more, preferably the martensitic stainless steel material consists of, in mass%, C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.0050% or less, Cu: 0.01 to 1.00%, Cr: 10.00 to 14.00%, Ni: 5.00 to 7.50%, Mo: 2.00 to 4.00%, Ti: 0.050 to 0.300%, V: 0.01 to 1.00%, Al: 0.001 to 0.100%, Co: 0.010 to 0.500%, Ca: 0.0005 to 0.0050%, Sn: 0.0005 to 0.0500%, N: 0.0010 to 0.0500%, O: 0.050% or less, W: 0 to 0.50%, Nb: 0 to 0.500%, As: 0 to 0.0100%, and Sb: 0 to 0.0100
  • the martensitic stainless steel material when the content of Cu is 0.50% or more and attempting to obtain a yield strength of 862 MPa or more, preferably the martensitic stainless steel material consists of, in mass%, C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.0050% or less, Cu: 0.50 to 3.50%, Cr: 10.00 to 14.00%, Ni: 5.00 to 7.50%, Mo: 2.00 to 4.00%, Ti: 0.050 to 0.300%, V: 0.01 to 1.00%, Al: 0.001 to 0.100%, Co: 0.010 to 0.500%, Ca: 0.0005 to 0.0050%, Sn: 0.0005 to 0.0500%, N: 0.0010 to 0.0500%, O: 0.050% or less, W: 0.10 to 0.50%, Nb: 0 to 0.500%, As: 0 to 0.0100%, and Sb: 0 to 0.0100
  • the martensitic stainless steel material according to the present embodiment within the range of the contents of elements described above and a yield strength of 758 MPa or more, the contents of elements and the yield strength satisfy Formula (1). As a result, on the condition that the other requirements of the present embodiment are satisfied, the martensitic stainless steel material according to the present embodiment has excellent SSC resistance in a sour environment with a pH of 3.0. 0.15 ⁇ Sn + As + Sb / Cu + Ni / YS ⁇ 1.00
  • F1 (Sn+As+Sb)/ ⁇ (Cu+Ni)/YS ⁇ ) is an index of an increase in the SSC resistance in a sour environment with a pH of 3.0 obtained by a synergetic effect between Sn, As and Sb, and Cu and Ni that is adjusted according to the yield strength.
  • F1 is too low, the steel material will not exhibit excellent SSC resistance in a sour environment with a pH of 3.0.
  • F1 is too high, the steel material will not exhibit excellent SSC resistance in a sour environment with a pH of 3.0.
  • F1 is in the range of 0.15 to 1.00, the steel material will exhibit excellent SSC resistance in a sour environment with a pH of 3.0.
  • F1 is to be within the range of 0.15 to 1.00.
  • a preferable lower limit of F1 is 0.16, and more preferably is 0.18.
  • a preferable upper limit of F1 is 0.95, and more preferably is 0.90.
  • the microstructure of the martensitic stainless steel material according to the present embodiment is mainly composed of martensite.
  • the phrase "mainly composed of martensite” means that the microstructure is composed of, in volume ratio, 0 to 5.0% of retained austenite and 0 to 5.0% of ferrite, and the balance is martensite.
  • the phrase "composed of retained austenite, ferrite, and tempered martensite” means that the amount of any phase other than retained austenite, ferrite, and tempered martensite is negligible.
  • the volume ratios of precipitates and inclusions are negligible as compared with the volume ratios of retained austenite, ferrite, and tempered martensite. That is, the microstructure of the martensitic stainless steel material according to the present embodiment may contain minute amounts of precipitates, inclusions and the like, in addition to retained austenite, ferrite, and tempered martensite.
  • the term "martensite” includes not only fresh martensite but also tempered martensite.
  • the lower limit of the volume ratio of martensite in the microstructure of the martensitic stainless steel material according to the present embodiment is 90.0%, and more preferably is 95.0%. Further preferably, the microstructure of the steel material is composed of single-phase martensite.
  • the volume ratio of retained austenite is to be 0 to 5.0%. From the viewpoint of ensuring strength, a preferable upper limit of the volume ratio of retained austenite is 4.0%, and more preferably is 3.0%. The volume ratio of retained austenite may be 0%.
  • the volume ratio of retained austenite is to be more than 0 to 5.0%, more preferably is to be more than 0 to 4.0%, and further preferably is to be more than 0 to 3.0%.
  • the volume ratio of ferrite is to be 0 to 5.0%.
  • a preferable upper limit of the volume ratio of ferrite is 3.0%, more preferably is 2.0%, and further preferably is 1.0%.
  • the volume ratio of ferrite may be 0%.
  • the volume ratio of ferrite is to be more than 0 to 5.0%, more preferably more than 0 to 3.0%, further preferably more than 0 to 2.0%, and further preferably more than 0 to 1.0%.
  • the volume ratio (%) of martensite in the microstructure of the steel material is determined by subtracting a volume ratio (%) of retained austenite determined by a method described hereunder and a volume ratio (%) of ferrite determined by a method described hereunder from 100%.
  • the volume ratio of retained austenite in the microstructure of the steel material is determined by an X-ray diffraction method. Specifically, a test specimen for measuring the volume ratio of retained austenite is prepared from the steel material according to the present embodiment. If the steel material is a pipe, the test specimen is taken from a central position of the wall thickness. If the steel material is a round steel bar, the test specimen is taken from an R/2 position. If the steel material is a steel plate, the test specimen is taken from a central position of the thickness. The size of the test specimen is not particularly limited. The test specimen is for example, 15 mm ⁇ 15 mm ⁇ a thickness of 2 mm. If the steel material is a pipe, the thickness direction of the test specimen is the pipe diameter direction.
  • the thickness direction of the test specimen is the radial direction. If the steel material is a steel plate, the thickness direction of the test specimen is the plate thickness direction.
  • the X-ray diffraction intensity of each of the (110) plane of ⁇ phase (martensite), the (200) plane of ⁇ phase, the (211) plane of ⁇ phase, the (111) plane of y phase (retained austenite), the (200) plane of y phase, and the (220) plane of y phase are measured, and an integrated intensity of the respective planes is calculated.
  • the target of the X-ray diffraction apparatus is Co (Co K ⁇ radiation), and the output is set to 30 kV - 100 mA.
  • the measurement angle (2 ⁇ ) is set to 45 to 105°.
  • V ⁇ 100 / 1 + I ⁇ ⁇ R ⁇ / I ⁇ ⁇ R ⁇
  • I ⁇ is the integrated intensity of ⁇ phase.
  • R ⁇ is a crystallographic theoretical calculation value of ⁇ phase.
  • Iy is the integrated intensity of y phase.
  • Ry is a crystallographic theoretical calculation value of y phase.
  • Values incorporated into a retained y quantitative analysis system belonging to RINT-TTR (product name) manufactured by Rigaku Corporation can be used for the values of R ⁇ and R ⁇ for each plane. Note that, a value obtained by rounding off the second decimal place of the obtained numerical value is adopted as the volume ratio of retained austenite.
  • the volume ratio of ferrite in the microstructure of the steel material is determined by a point counting method. Specifically, a test specimen for measuring the volume ratio of ferrite is prepared from the steel material according to the present embodiment. If the steel material is a pipe, the test specimen is taken from a central position of the wall thickness. If the steel material is a round steel bar, the test specimen is taken from an R/2 position. If the steel material is a steel plate, the test specimen is taken from a central position of the thickness.
  • the test specimen is not particularly limited as long as the test specimen has a face that is parallel to the rolling direction as an observation surface. For example, if the steel material is a pipe, the observation surface of the test specimen is parallel to the axial direction of the pipe.
  • the observation surface is mechanically polished, the observation surface is subjected to electrolytic etching to reveal the microstructure.
  • the electrolytic etching is performed using a 30% sodium hydroxide aqueous solution as the electrolyte, at a current density of 1 A/cm 2 , for an electrolysis time of 1 minute.
  • each observation visual field is set as a rectangle of 250 ⁇ m ⁇ 250 ⁇ m. Note that, the observation magnification is ⁇ 400.
  • Those skilled in the art can distinguish ferrite from other phases (retained austenite or tempered martensite) based on contrast in each observation visual field. Therefore, ferrite in each observation visual field is identified based on contrast. The area fraction of the identified ferrite is determined by a point counting method conforming to ASTM E562 (2019).
  • each observation visual field 20 vertical lines are drawn at regular intervals from the top end to the bottom end of the observation visual field. That is, the observation visual field is divided into 21 regions in the left-right direction by the 20 vertical lines.
  • 20 horizontal lines are also drawn at regular intervals from the left end to the right end of the observation visual field. That is, the observation visual field is divided into 21 regions in the vertical direction by the 20 horizontal lines.
  • intersections between the vertical lines and the horizontal lines are called lattice points. That is, in each observation visual field, 400 lattice points are arranged at regular intervals. In accordance with ASTM E562 (2019), the lattice points that overlap with ferrite in the observation visual field are counted.
  • the area fraction of ferrite determined by the above method is defined as the volume ratio (%) of ferrite. Note that, a value obtained by rounding off the second decimal place of the obtained numerical value is adopted as the volume ratio of ferrite.
  • volume ratio (%) of martensite in the microstructure of the steel material is determined by the following formula.
  • Volume ratio % of martensite 100 .0 ⁇ volume ratio % of retained austenite + volume ratio % of ferrite
  • the martensitic stainless steel material according to the present embodiment has a high yield strength of 758 MPa or more, the martensitic stainless steel material has excellent SSC resistance in a sour environment with a pH of 3.0.
  • the SSC resistance of the martensitic stainless steel material according to the present embodiment can be evaluated by an SSC resistance evaluation test at normal temperature.
  • the SSC resistance evaluation test is performed by a method in conformity with NACE TM0177-2016 Method A.
  • a round bar specimen is prepared from the steel material according to the present embodiment. If the steel material is a pipe, the round bar specimen is prepared from a central position of the wall thickness. If the steel material is a round steel bar, the round bar specimen is prepared from an R/2 portion. If the steel material is a steel plate, the round bar specimen is prepared from a central position of the thickness.
  • the size of the round bar specimen is not particularly limited.
  • the round bar specimen for example, is a specimen in which the diameter of the parallel portion is 6.35 mm and the length of the parallel portion is 25.4 mm. Note that, the axial direction of the round bar specimen is to be made parallel to the rolling direction and/or axial direction of the steel material.
  • test solution An aqueous solution containing 0.17% sodium chloride by mass and having a pH of 3.0 is used as the test solution.
  • the test solution is prepared by adding acetic acid to an aqueous solution containing 0.17% sodium chloride by mass and 0.41 g/L of sodium acetate to adjust the pH to 3.0.
  • a stress equivalent to 90% of the actual yield stress is applied to the round bar specimen prepared as described above.
  • the test solution at 24°C is poured into a test vessel so that the round bar specimen to which the stress has been applied is immersed therein to form a test bath. After degassing the test bath, H 2 S gas at 0.03 bar and CO 2 gas at 0.97 bar are blown into the test bath to saturate the test bath with H 2 S gas.
  • the test bath in which the H 2 S gas is saturated is held at 24°C for 720 hours. After the test specimen has been held for 720 hours, the surface of the parallel portion of the test specimen is observed with a magnifying glass having a magnification of ⁇ 10 to confirm the presence or absence of pitting. In the martensitic stainless steel material according to the present embodiment, the presence of pitting is not confirmed after 720 hours elapse in an SSC resistance evaluation test conducted by the method described above.
  • the shape of the martensitic stainless steel material according to the present embodiment is not particularly limited.
  • the martensitic stainless steel material according to the present embodiment may be a pipe, may be a round steel bar (solid material), or may be a steel plate.
  • the pipe may be a seamless pipe or may be a welded pipe.
  • the pipe is, for example, a pipe for oil country tubular goods.
  • the term "pipe for oil country tubular goods" means a pipe that is to be used in oil country tubular goods.
  • Oil country tubular goods are, for example, casing pipes, tubing pipes, and drilling pipes which are used for drilling an oil well or a gas well, collecting crude oil or natural gas, and the like.
  • the steel material of the present embodiment is a seamless pipe for oil country tubular goods.
  • the content of each element is within the range of the present embodiment, the yield strength is 758 MPa or more, and within the range of the contents of elements described above and the yield strength of 758 MPa or more, a condition that F1 is 0.15 to 1.00 is satisfied.
  • the steel material according to the present embodiment has both a high yield strength and excellent SSC resistance in a sour environment with a pH of 3.0.
  • the production method described hereunder is an example, and a method for producing the martensitic stainless steel material according to the present embodiment is not limited to the production method described hereunder. That is, as long as the martensitic stainless steel material according to the present embodiment that is composed as described above can be produced, a method for producing the martensitic stainless steel material is not limited to the production method described hereunder. However, the production method described hereunder is a favorable method for producing the martensitic stainless steel material according to the present embodiment.
  • One example of a method for producing the martensitic stainless steel material according to the present embodiment includes a process of preparing an intermediate steel material (preparation process), and a process of subjecting the intermediate steel material to quenching and tempering (heat treatment process). Each of these processes is described in detail hereunder.
  • an intermediate steel material having the aforementioned chemical composition is prepared.
  • the method for preparing the intermediate steel material is not particularly limited as long as the intermediate steel material has 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 or a welded pipe, and refers to a hollow shell in a case where the end product is a seamless pipe.
  • the preparation process may include a process of preparing a starting material (starting material preparation process), and a process of subjecting the starting material to hot working to produce an intermediate steel material (hot working process).
  • starting material preparation process a process of preparing a starting material
  • hot working process a process of subjecting the starting material to hot working to produce an intermediate steel material
  • a starting material is produced using a molten steel having the aforementioned chemical composition.
  • the method for producing the starting material is not particularly limited, and it suffices to use a well-known method.
  • a cast piece (a slab, a bloom, or a billet) may be 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.
  • a starting material (a slab, a bloom, or a billet) is produced by the above-described process.
  • the prepared starting material is subjected to hot working to produce an intermediate steel material.
  • the steel material is a seamless pipe
  • the intermediate steel material corresponds to a hollow shell.
  • a billet is heated in a reheating furnace.
  • the heating temperature is, for example, 1100 to 1300°C.
  • the billet is subjected to hot working to produce a hollow shell (seamless 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 performing another hot working method.
  • 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 starting material is heated in a reheating furnace.
  • the heating temperature is, for example, 1100 to 1300°C.
  • the starting material extracted from the reheating 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 performed using a blooming mill or hot rolling 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 reheating furnace.
  • the heating temperature is, for example, 1100 to 1300°C.
  • the starting material extracted from the reheating 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 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 working without being cooled to normal temperature, or may be subjected to quenching after undergoing supplementary heating (reheating) after 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.
  • 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 produced by a third party, or an intermediate steel material produced in another factory other than the factory in which a quenching process and a tempering process to be described later are performed, or at a different place of business may be prepared.
  • the heat treatment process is described in detail hereunder.
  • the heat treatment process includes a quenching process and a tempering process.
  • the intermediate steel material produced in the hot working process is subjected to quenching (quenching process).
  • quenching is performed by a well-known method. Specifically, the intermediate steel material after the hot working process is charged into a heat treatment furnace and held at a quenching temperature.
  • the quenching temperature is equal to or higher than the Acs transformation point, and for example is 900 to 1000°C. After being held at the quenching temperature, the intermediate steel material is rapidly cooled (quenched).
  • the holding time at the quenching temperature is, for example, 10 to 60 minutes.
  • the quenching method is, for example, water cooling.
  • the quenching method is not particularly limited.
  • the hollow shell may be rapidly cooled by immersing the hollow shell in a water bath or an oil bath, or the hollow shell may be rapidly cooled by pouring or jetting cooling water onto the outer surface and/or inner surface of the hollow shell by shower cooling or mist cooling.
  • quenching may be performed immediately after the hot working, without cooling the intermediate steel material to normal temperature, or quenching may be performed after the intermediate steel material has been held at the quenching temperature after being charged into a holding furnace before the temperature of the hollow shell decreased after the hot working.
  • the intermediate steel material after quenching is also subjected to a tempering process.
  • the yield strength of the steel material is adjusted.
  • the tempering temperature is to be set in the range of 540 to 620°C.
  • the holding time at the tempering temperature is, for example, 10 to 180 minutes. It is well-known by those skilled in the art that the yield strength of a steel material can be adjusted by appropriately adjusting the tempering temperature according to the chemical composition. Therefore, the tempering conditions are adjusted so that the yield strength of the steel material becomes 758 MPa or more.
  • the martensitic stainless steel material according to the present embodiment can be produced by the process described above.
  • a method for producing the martensitic stainless steel material according to the present embodiment is not limited to the above production method.
  • the production method of the present embodiment is not limited to the production method described above.
  • the martensitic stainless steel material according to the present embodiment is described more specifically by way of examples.
  • Molten steels having the chemical compositions shown in Table 1 were produced.
  • the symbol "-" in Table 1 means that the content of the corresponding element was at an impurity level. For example, it means that the content of W of Test No. 1 was 0% when rounded off to two decimal places. It means that the content of Nb of Test No. 1 was 0% when rounded off to three decimal places. It means that the content of AS and the content of Sb of Test No. 1 were each 0% when rounded off to four decimal places.
  • the respective molten steels described above were melted in a 180-kg vacuum furnace, and ingots were produced by an ingot-making process.
  • the ingots were heated for three hours at 1250°C.
  • the ingot after heating was subjected to hot forging to produce a block.
  • a block after hot forging was heated for three hours at 1230°C, and thereafter the block was subjected to hot rolling. In this way, a steel material (a steel plate) having a thickness of 13 mm was produced.
  • the produced steel material of each test number was subjected to quenching. Specifically, the steel plate of each test number was heated so as to reach a quenching temperature (°C) described in Table 2. The steel plate of each test number was held at the quenching temperature for 15 minutes, and thereafter subjected to water cooling. The steel material of each test number after quenching was subjected to tempering in which the steel material was held at a tempering temperature (°C) described in Table 2 for 30 minutes.
  • the steel plate of each test number was produced by the above production process.
  • the produced steel plate of each test number was subjected to a microstructure observation test, a tensile test, and a SSC resistance evaluation test.
  • the steel plate of each test number was subjected to a microstructure observation test.
  • the volume ratio (%) of retained austenite in the steel plate of each test number was determined by the aforementioned X-ray diffraction method. Note that, in the measurement of the X-ray diffraction intensity, an apparatus with the product name "RINT-TTR" manufactured by Rigaku Corporation was used as the X-ray diffraction apparatus. Measurement was conducted using Co K ⁇ radiation as the radiation source, with the output set to 30 kV - 100 mA, and the measurement angle (2 ⁇ ) set to 45 to 105°. The obtained volume ratio (%) of retained austenite in the steel plate of each test number is shown in the column "Retained y (%)" in Table 2.
  • the volume ratio (%) of ferrite was determined by the aforementioned point counting method. Specifically, a test specimen was prepared from a central position of the thickness of the steel plate of each test number. For each test specimen, a surface parallel to the rolling direction was adopted as the observation surface. Note that, in the present Examples, an area fraction of ferrite determined by a method conforming to ASTM E562 (2019) described above was defined as the volume ratio (%) of ferrite. The obtained volume ratio of ferrite in the steel plate of each test number is shown in the column "Ferrite (%)" in Table 2.
  • volume ratio (%) of martensite was determined by the following formula using the volume ratio (%) of retained austenite and the volume ratio (%) of ferrite.
  • Volume ratio % of martensite 100 ⁇ volume ratio % of retained austenite + volume ratio % of ferrite
  • the steel plate of each test number was subjected to a tensile test in accordance with ASTM E8/E8M (2013). Specifically, from a central position of the thickness of the steel plate of each test number, a round bar tensile test specimen in which the diameter of the parallel portion was made 8.9 mm, and the gage length was made 35.6 mm was prepared. The longitudinal direction of the round bar tensile test specimen was parallel to the rolling direction of the steel plate. A tensile test was performed at normal temperature (24 ⁇ 3°C) in the atmosphere using the round bar tensile test specimen of each test number, and the 0.2% offset yield stress (MPa) was determined. The determined 0.2% offset yield stress was defined as the yield strength (MPa).
  • the obtained yield strength of each test number is shown in the column “YS (MPa)" in Table 2.
  • F1 was determined using the chemical composition, the yield strength, and Formula (1).
  • the obtained value of F1 for each test number is shown in the column “F1" in Table 2.
  • the steel plate of each test number was subjected to a SSC resistance evaluation test. Specifically, round bar specimens having a diameter of 6.35 mm and in which the length of the parallel portion was 25.4 mm were prepared from a central position of the thickness of the steel plate of each test number. Three of the prepared round bar specimens were subjected to a SSC resistance evaluation test conducted in conformity with NACE TM0177-2016 Method A. Note that, the axial direction of each round bar specimen was parallel to the rolling direction.
  • test solution An aqueous solution containing 0.17% sodium chloride by mass and having a pH of 3.0 was used as the test solution.
  • the test solution was prepared by adding acetic acid to an aqueous solution containing 0.17% sodium chloride by mass and 0.41 g/L of sodium acetate to adjust the pH to 3.0.
  • a stress equivalent to 90% of the actual yield stress was applied to the round bar specimen of each test number.
  • the test solution at 24°C was poured into a test vessel so that the round bar specimen to which the stress was applied was immersed therein to form a test bath. After degassing the test bath, H 2 S gas at 0.03 bar and CO 2 gas at 0.97 bar were blown into the test bath to saturate the test bath with H 2 S gas.
  • the test bath in which the H 2 S gas was saturated was held at 24°C for 720 hours.
  • each of these steel plates had a microstructure composed of 0 to 5.0 vol% of retained austenite and 0 to 5.0 vol% of ferrite, with the balance being martensite.
  • each of these steel plates had a high strength which was a yield strength of 758 MPa or more.
  • the condition that F1 was within the range of 0.15 to 1.00 was satisfied.
  • the steel plate of Test No. 31 did not contain Sn. As a result, with respect to this steel plate, in the sour environment with a pH of 3.0, pitting occurred in one specimen, and hence the steel plate did not have excellent SSC resistance.
  • the steel plates of Test Nos. 32 to 34 did not contain Sn. In addition, F1 was too low in these steel plates. As a result, with respect to each of these steel plates, in the sour environment with a pH of 3.0, pitting occurred in at least one of the specimens, and hence these steel plates did not have excellent SSC resistance.
  • the steel plate of Test No. 35 did not contain Sn. In addition, F1 was too high in this steel plate. As a result, with respect to this steel plate, in the sour environment with a pH of 3.0, pitting occurred in three specimens, and hence the steel plate did not have excellent SSC resistance.

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