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  • 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
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  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • 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
  • C21D1/26—Methods of annealing
• • 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
  • C21D1/78—Combined heat-treatments not provided for above
• • 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
  • C22C38/18—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
  • C22C38/30—Ferrous alloys, e.g. steel alloys containing chromium with cobalt
• • C—CHEMISTRY; METALLURGY
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/32—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
  • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—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.
• 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 also to have sulfide stress cracking resistance (hereunder, referred to as "SSC resistance").
• An oil-well steel disclosed in Patent Literature 1 contains, 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 the aforementioned oil-well steel is excellent in SSC resistance.
• a low-alloy steel material disclosed in Patent Literature 2 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 the aforementioned 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 which has high yield strength, and also has excellent fracture toughness in a low-temperature sour environment.
• the steel material according to the present disclosure has high yield strength, and has excellent fracture toughness in a low-temperature sour environment.
• the present inventors carried out investigations and studies regarding methods for obtaining a steel material which has a high yield strength and also has excellent fracture toughness in a low-temperature sour environment, and obtained the following findings.
• the present inventors attempted to obtain a steel material having a yield strength of 758 MPa or more (110 ksi or more) as a high yield strength. Therefore, first, the present inventors conducted studies from the viewpoint of the chemical composition with respect to a steel material having a yield strength of 110 ksi or more and having excellent fracture toughness in a low-temperature sour environment.
• a steel material consists of, in mass%, C: more than 0.20 to 0.60%, Si: 0.05 to 2.00%, Mn: 0.02 to 0.60%, P: 0.025% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, Cr: 0.20 to 1.50%, Mo: 0.35 to 1.50%, V: 0.01 to 0.60%, Ti: 0.002 to 0.050%, B: 0.0001 to 0.0050%, N: 0.0100% or less, O: 0.0100% or less, Nb: 0 to 0.030%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, rare earth metal: 0 to 0.0100%, Co: 0 to 0.50%, W: 0 to 0.50%, Ni: 0 to 0.20%, and Cu: 0 to 0.50%, with the balance being Fe and impurities, there is a possibility of obtaining
• prior-austenite grains are also referred to as "prior-y grains".
• Mo molybdenum
• the present inventors focused their attention on molybdenum (Mo) as an element that contributes to strengthening of prior-y grain boundaries. If Mo concentrates at prior-y grain boundaries, the prior-y grain boundaries can be strengthened and there is a possibility that the propagation of cracks can be suppressed even in a low-temperature sour environment. As a result, there is a possibility that excellent fracture toughness will be obtained in a low-temperature sour environment.
• the steel material according to the present embodiment has the chemical composition described above and a grain boundary Mo amount ⁇ Mo of 5.0% by mass or more, and when the yield strength ⁇ YS is 758 to less than 862 MPa, Fn1 is -300 or more.
• the steel material according to the present embodiment has the chemical composition described above and a grain boundary Mo amount ⁇ Mo of 5.0% by mass or more, and when the yield strength ⁇ YS is 862 MPa or more, Fn1 is -520 or more.
• the dislocation density ⁇ contributes to increasing the yield strength ⁇ YS .
• the fracture toughness in a low-temperature sour environment can be increased by controlling the relation between the yield strength ⁇ YS , the grain boundary Mo amount ⁇ Mo , and the dislocation density ⁇ , rather than simply lowering the dislocation density ⁇ .
• the present inventors surmise that in the steel material according to the present embodiment, high strength as well as excellent fracture toughness in a low-temperature sour environment are obtained by the mechanism described above. Note that, it is also possible that by adjusting Fn1 in a steel material having the chemical composition, the yield strength ⁇ YS , and the grain boundary Mo amount ⁇ Mo which are described above, the fracture toughness in a low-temperature sour environment can be increased by a mechanism other than the mechanism described above.
• 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 shape of the steel material according to the present embodiment is not particularly limited.
• 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.
• 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 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 symbol "%" in relation to an element means mass percent unless otherwise stated.
• Carbon (C) increases hardenability of the steel material and increases the strength of the steel material. C also promotes spheroidization of carbides during tempering in the production process, and thereby enhances the SSC resistance of the steel material. If carbides are dispersed, the strength of the steel material will increase further. If the content of C 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 C is too high, even if the contents of other elements are within the range of the present embodiment, there will be too many carbides produced and the toughness of the steel material will decrease.
• the content of C is to be more than 0.20 to 0.60%.
• a preferable lower limit of the content of C is 0.22%, more preferably is 0.24%, and further preferably is 0.25%.
• a preferable upper limit of the content of C is 0.55%, more preferably is 0.50%, and further preferably is 0.45%.
• Silicon (Si) deoxidizes the steel. If the Si content 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 2.00%. A preferable lower limit of the content of Si is 0.15%, and more preferably is 0.20%. A preferable upper limit of the content of Si is 1.80%, more preferably is 1.60%, further preferably is 1.50%, and further preferably is 1.40%.
• Manganese (Mn) deoxidizes the steel. Mn also increases hardenability of the steel material and increases the strength of the steel material. If the content of Mn 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 Mn is too high, even if the contents of other elements are within the range of the present embodiment, Mn will segregate to grain boundaries together with impurities such as P and S, and the fracture toughness of the steel material in a low-temperature sour environment will decrease. Therefore, the content of Mn is to be 0.02 to 0.60%.
• a preferable lower limit of the content of Mn is 0.03%, more preferably is 0.04%, and further preferably is 0.06%.
• a preferable upper limit of the content of Mn is 0.55%, more preferably is 0.50%, and further preferably is 0.45%.
• 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 fracture toughness of the steel material in a low-temperature sour environment will decrease. Therefore, the content of P is to be 0.025% or less. A preferable upper limit of the content of P is 0.020%, and more 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%.
• S Sulfur
• the lower limit of the content of S is more than 0%. If the content of S is too high, even if the contents of other elements are within the range of the present embodiment, S will segregate to grain boundaries and the SSC resistance of the steel material will decrease. Therefore, the content of S is to be 0.0100% or less.
• a preferable upper limit of the content of S is 0.0075%, more preferably is 0.0050%, and further preferably is 0.0030%.
• the content of S is preferably as low as possible. However, extremely reducing the content of S will greatly increase the production cost. Therefore, 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%.
• Aluminum (Al) deoxidizes the steel. If the content of Al 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. As a result, 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 0.015%, and more preferably is 0.020%. A preferable upper limit of the content of Al is 0.080%, and more preferably is 0.060%. As used in the present description, the content of "Al” means the content of "acid-soluble Al", that is, "sol. Al".
• Chromium (Cr) increases hardenability of the steel material and increases the strength of the steel material. Cr also increases the temper softening resistance of the steel material and thereby enables high-temperature tempering. As a result, the fracture toughness of the steel material in a low-temperature sour environment 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, 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 Cr is to be 0.20 to 1.50%.
• a preferable lower limit of the content of Cr is 0.25%, more preferably is 0.30%, further preferably is 0.35%, and further preferably is 0.40%.
• a preferable upper limit of the content of Cr is 1.40%, and more preferably is 1.30%.
• Molybdenum (Mo) increases hardenability of the steel material and increases the strength of the steel material. Mo also increases the grain boundary Mo amount ⁇ Mo , and increases the fracture toughness of the steel material in a low-temperature sour environment. 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 0.35 to 1.50%. A preferable lower limit of the content of Mo is 0.40%, more preferably is 0.45%, and further preferably is 0.50%. A preferable upper limit of the content of Mo is 1.40%, more preferably is 1.30%, and further preferably is 1.25%.
• Vanadium (V) combines with C or N to form carbides, nitrides, or carbo-nitrides (hereunder, also referred to as "carbo-nitrides and the like"), and refines the grains of the steel material by the pinning effect.
• carbides nitrides, or carbo-nitrides (hereunder, also referred to as "carbo-nitrides and the like")
• carbo-nitrides hereunder, also referred to as "carbo-nitrides and the like”
• V also forms fine carbides during tempering and thereby increases the temper softening resistance of the steel material and increases the strength of the steel material. If the content of V 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 V is to be 0.01 to 0.60%.
• a preferable lower limit of the content of V is 0.02%, more preferably is 0.04%, and further preferably is 0.06%.
• a preferable upper limit of the content of V is 0.40%, more preferably is 0.30%, further preferably is 0.25%, and further preferably is 0.20%.
• Titanium (Ti) combines with N to form nitrides, and refines the grains of the steel material by the pinning effect. As a result, the fracture toughness of the steel material in a low-temperature sour environment 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 0.050%. A preferable lower limit of the content of Ti is 0.003%, and more preferably is 0.005%. A preferable upper limit of the content of Ti is 0.030%, and more preferably is 0.020%.
• B Boron
• B Boron
• a preferable lower limit of the content of B is 0.0003%, and more preferably is 0.0007%.
• a preferable upper limit of the content of B is 0.0030%, more preferably is 0.0025%, further preferably is 0.0020%, and further preferably is 0.0015%.
• N Nitrogen
• the lower limit of the content of N is more than 0%.
• N combines with Ti to form nitrides, and refines grains of the steel material by the pinning effect.
• the fracture toughness of the steel material in a low-temperature sour environment increases.
• 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 fracture toughness of the steel material in a low-temperature sour environment will, on the contrary, decrease. Therefore, the content of N is to be 0.0100% or less.
• a preferable upper limit of the content of N is 0.0060%, more preferably is 0.0050%, and further preferably is 0.0045%.
• 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 low-temperature toughness and SSC resistance of the steel material will decrease. Therefore, the content of O is to be 0.0100% or less.
• a preferable upper limit of the content of O is 0.0050%, more preferably is 0.0030%, 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 Nb in lieu of a part of Fe.
• Niobium (Nb) is an optional element, and does not have to be contained. That is, the content of Nb may be 0%. When contained, Nb forms carbo-nitrides and the like, and refines the grains of the steel material by the pinning effect. As a result, the fracture toughness of the steel material in a low-temperature sour environment increases. Nb also forms fine carbides during tempering and thereby increases the temper softening resistance of the steel material and increases the strength of the steel material. If even a small amount of Nb is contained, the aforementioned advantageous effects will be obtained to a certain extent.
• the content of Nb is to be 0 to 0.030%.
• a preferable lower limit of the content of Nb is more than 0%, more preferably is 0.001%, further preferably is 0.002%, further preferably is 0.003%, further preferably is 0.005%, and further preferably is 0.007%.
• a preferable upper limit of the content of Nb is 0.025%, and more preferably is 0.020%.
• 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 render 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 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, on the contrary, 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%, further preferably is 0.0006%, and further preferably is 0.0010%.
• a preferable upper limit of the content of Ca is 0.0040%, more preferably is 0.0030%, further 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 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%, further preferably is 0.0006%, and further preferably is 0.0010%.
• a preferable upper limit of the content of Mg is 0.0040%, more preferably is 0.0030%, further 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 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%, further preferably is 0.0006%, and further preferably is 0.0010%.
• a preferable upper limit of the content of Zr is 0.0040%, more preferably is 0.0030%, further 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 0.0030%, and further preferably is 0.0025%.
• the term "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.
• the term “content of REM” refers to the total content of these elements.
• the chemical composition of the steel material described above may further contain one or more elements selected from the group consisting of Co and W in lieu of a part of Fe.
• Each of these elements is an optional element that forms a protective corrosion coating in a sour environment and suppresses penetration of hydrogen into the steel material. By this means, each of these elements increases the SSC resistance of the steel material.
• Co Co
• the content of Co may be 0%.
• Co forms a protective corrosion coating in a sour environment and suppresses penetration of hydrogen into the steel material.
• the SSC resistance of the steel material increases. If even a small amount of Co is contained, the aforementioned effect will be obtained to a certain extent.
• the content of Co is too high, even if the contents of other elements are within the range of the present embodiment, hardenability of the steel material will decrease, and the strength of the steel material will decrease. Therefore, the content of Co is to be 0 to 0.50%.
• a preferable lower limit of the content of Co is more than 0%, more preferably is 0.01%, further preferably is 0.02%, further preferably is 0.03%, and further preferably is 0.05%.
• a preferable upper limit of the content of Co is 0.45%, and more preferably is 0.40%.
• Tungsten (W) is an optional element, and does not have to be contained. That is, the content of W may be 0%. When contained, W forms a protective corrosion coating in a sour environment and suppresses penetration of hydrogen into the steel material. 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. However, if the content of W is too high, even if the contents of other elements are within the range of the present embodiment, coarse carbides will form in the steel material, and the low-temperature toughness and SSC resistance of the steel material will decrease. Therefore, the content of W is to be 0 to 0.50%.
• a preferable lower limit of the content of W is more than 0%, more preferably is 0.01%, further preferably is 0.02%, further preferably is 0.03%, and further preferably is 0.05%.
• a preferable upper limit of the content of W is 0.45%, and more preferably is 0.40%.
• the chemical composition of the steel material described above may further contain one or more types of element selected from the group consisting of Ni and Cu in lieu of a part of Fe.
• element selected from the group consisting of Ni and Cu in lieu of a part of Fe.
• Each of these elements is an optional element, and each element increases hardenability of the steel material.
• 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 the strength of the steel material. Ni also dissolves in the steel and increases the 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 0.20%. 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 0.15%, more preferably is 0.10%, further preferably is 0.09%, further preferably is 0.08%, and further preferably is 0.06%.
• a yield strength ⁇ YS of the steel material according to the present embodiment is 758 MPa or more (110 ksi or more).
• yield strength means 0.2% offset proof stress obtained in a tensile test carried out in conformity with ASTM E8/E8M (2021).
• an upper limit of the yield strength of the steel material according to the present embodiment is not particularly limited. However, it has been demonstrated by Examples that are described later that at least when the yield strength is within a range of 758 to 965 MPa, the steel material according to the present embodiment has excellent fracture toughness in a low-temperature sour environment.
• the yield strength of the steel material according to the present embodiment can be determined by the following method. Specifically, a tensile test is performed in conformity with ASTM E8/E8M (2021).
• 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.
• a grain boundary Mo amount ⁇ Mo of the steel material according to the present embodiment is 5.0% by mass or more.
• the term "grain boundary Mo amount ⁇ Mo" means the content of Mo near prior-austenite grain boundaries.
• the content of Mo near prior-austenite grain boundaries ⁇ Mo is defined as follows.
• the grain boundary Mo amount ⁇ Mo can be determined by the following method.
• a test specimen for measuring the grain boundary Mo amount ⁇ Mo is prepared from the steel material according to the present embodiment. Specifically, a test specimen is prepared in which a surface perpendicular to a C direction of the steel material is made the observation surface. That is, the observation surface of the test specimen includes an L direction and a T direction.
• the steel material is a steel plate
• the rolling elongation direction is defined as "L direction”
• the thickness direction is defined as "T direction”
• the plate width direction is defined as "C direction”.
• the pipe axis direction is defined as "L direction”
• the pipe radius direction is defined as “T direction”
• the direction that is orthogonal to the L direction and the T direction is defined as "C direction”.
• the axial direction is defined as “L direction”
• the radial direction is defined as “T direction”
• the direction that is orthogonal to the L direction and the T direction is defined as "C direction”.
• the phrase "specified position of the steel material” means a position at which it is possible to stably measure the grain boundary Mo amount ⁇ Mo .
• the term "thickness t/4 position” means the center position between the center portion of the thickness and the surface of the steel plate in the thickness direction.
• the steel material is a steel pipe, the center portion of the wall thickness is defined as the specified position.
• an R/2 position is defined as the specified position.
• the size of the test specimen is not particularly limited, and for example the test specimen may have a size that is 10 mm in the L direction ⁇ 5 mm in the C direction ⁇ 8 mm in the T direction.
• FIG. 3 is a schematic diagram of an observation surface of a test specimen polished in order to determine the grain boundary Mo amount.
• the L direction and the T direction in FIG. 3 are defined as described above.
• an observation target region 100 of the observation surface of the test specimen after polishing is observed using an optical microscope at a magnification of 200 ⁇ , and an optical micrograph is obtained. Further, as described above, the observation target region 100 includes the aforementioned specified position.
• an arbitrary rectangular measurement region 10 of 100 ⁇ m ⁇ 100 ⁇ m is selected.
• the measurement region 10 is selected so as to include the aforementioned specified position.
• Marks 20 are attached at a plurality of corners of the measurement region 10 so that the location of the selected measurement region 10 can be identified.
• the mark 20 may be, for example, an indentation made by a micro Vickers hardness tester.
• Electron backscatter diffraction (EBSD) analysis is performed with respect to the selected measurement region 10 to obtain crystal orientation information with respect to the martensite phase.
• the step size is set to 0.1 ⁇ m.
• Grain boundaries (prior-y grain boundaries) of prior-y grains (prior-austenite grains) are identified based on the obtained crystal orientation information and the Kurdjumov-Sachs relationship.
• FIG. 4 is a schematic diagram of the measurement region 10 in which prior-y grain boundaries have been identified.
• a prior-y grain boundary GB in which the crystal orientation difference between adjacent prior-y grains is 18° or more is selected.
• a thin film sample TP that is orthogonal to the selected prior-y grain boundary GB is then prepared.
• FIG. 5 is a schematic diagram illustrating the manner in which the thin film sample TP is prepared from the measurement region 10 illustrated in FIG. 4 .
• a plate-shaped thin film sample TP having a top face orthogonal to the selected prior-y grain boundary GB is extracted from the measurement region 10 by focused ion beam (FIB) processing.
• FIB focused ion beam
• a machine with the trade name "SMI 3050SE” manufactured by Hitachi High-Tech Science Corporation can be used as the FIB processing apparatus.
• FIG. 6 is a perspective view of the thin film sample TP illustrated in FIG. 5 .
• a surface 10 corresponds to one part of the measurement region 10.
• a surface 30 is a surface which is orthogonal to the surface 10.
• the selected prior-y grain boundary GB at the surface 10 is also observed at the surface 30.
• a thickness T10 of the thin film sample TP is set to 100 nm.
• the prepared thin film sample TP is observed using a transmission electron microscope (TEM).
• TEM transmission electron microscope
• EDS analyzer an elemental analyzer (EDS analyzer) that uses energy dispersive X-ray spectrometry (EDS)
• a transmission electron microscope with a spherical aberration corrector (trade name: NEOARM) manufactured by JEOL Ltd.
• NEOARM a transmission electron microscope with a spherical aberration corrector
• an EDS analyzer (trade name: JED-2300T) manufactured by JEOL Ltd. can be used as the EDS analyzer attached to the TEM.
• a line segment SL which, as seen from the incident direction of the electron beam EB, has an overall length of 10 nm and which is centered on the prior-y grain boundary GB of the surface 30 and is orthogonal to the prior-y grain boundary GB is specified.
• one endpoint of the line segment SL is defined as an endpoint E1
• the other endpoint as seen from the incident direction of the electron beam EB is defined as an endpoint E2.
• the distance between the endpoint E1 and the endpoint E2 will be taken as 10 nm.
• FIG. 8 is a schematic diagram illustrating the relation between the irradiation direction of the electron beam EB when performing TEM observation and elemental analysis by EDS, the prior-y grain boundary GB of the surface 10 of the thin film sample TP, and the element concentration profile.
• the peak of an element concentration profile PR on the line segment SL obtained by elemental analysis by EDS is broad.
• the thin film sample TP is tilted such that the electron beam EB becomes parallel to the prior-y grain boundary GB of the surface 10 of the thin film sample TP.
• FIG. 9 is a view illustrating one example of the content of Mo on the line segment SL which is centered on the prior-y grain boundary GB and which is orthogonal to the prior-y grain boundary GB. Referring to FIG. 9 , the respective items shown in FIG. 9 are defined as follows.
• ⁇ Mo1 (mass%) in the first Mo intra-grain region BM1 and ⁇ Mo2 (mass%) in the second Mo intra-grain region BM2 are regarded as the content of Mo within adjacent prior-y grains that sandwich the prior-y grain boundary GB.
• ⁇ Mo1 (mass%) and ⁇ Mo2 (mass%) will be approximately equal, and in some cases, as in the example in FIG. 9 , they will be different concentrations to each other. Therefore, the average of the contents of Mo ( ⁇ Mo1 , ⁇ Mo2 ) inside the prior-y grains in each of the first Mo intra-grain region BM1 and the second Mo intra-grain region BM2 is determined.
• the grain boundary Mo amount ⁇ M0 on the line segment SL is defined by Formula (2) using the content of Mo at each measurement point on the line segment SL, and ⁇ Mo1 and ⁇ Mo2 .
• ⁇ Mo ⁇ (sum total of contents of Mo at all measurement points on line segment SL ⁇ 0.2) - ⁇ Mo1 ⁇ (distance between endpoint E1 and Mo peak measurement point P) - ⁇ Mo2 ⁇ (distance between endpoint E2 and Mo peak measurement point P) ⁇ /0.8 + ( ⁇ Mo1 + ⁇ Mo2 )/2
• Formula (2) is divided as follows.
• A (sum total of contents of Mo at all measurement points on line segment SL ⁇ 0.2)
• B ⁇ Mo 1 ⁇ distance between endpoint E 1 and Mo peak measurement point
• P C ⁇ Mo 2 ⁇ distance between endpoint E 2 and Mo peak measurement point
• P D ⁇ Mo 1 + ⁇ Mo 2 / 2
• A corresponds to the total area of the Mo content distribution in FIG. 9 , that is, the total amount of the content of Mo on the line segment SL.
• B corresponds to the total amount of the content of Mo in a region BM10 that, in the line segment SL, is between the endpoint E1 and the Mo peak measurement point P.
• ⁇ Mo1 (mass%) is used as the average content of Mo in the region BM10.
• the region BM10 corresponds to an ideal intra-grain region of a prior-y grain in a case where the Mo peak measurement point P is assumed to be an ideal prior-y grain boundary.
• FIG. 10 is a schematic diagram for describing Formula (2). As illustrated in FIG.
• (A - B - C) among Formula (2) is a value obtained by subtracting the total amount of the contents of Mo in the adjacent ideal intra-grain regions BM10 and BM20 that sandwich the ideal prior-y grain boundary GB from the total amount of the content of Mo on the line segment SL. That is, (A - B - C) in Formula (2) corresponds to a differential amount ⁇ Mo of the content of Mo in the Mo concentrated region GB0 in FIG. 10 .
• the prior-y grain boundary GB that is orthogonal to the line segment SL does not exist as a wide area (line) like the Mo concentrated region GB0, and instead exists in an extremely narrow area that is centered on the Mo peak measurement point P on the line segment SL. Therefore, in the present description, in the line segment SL, a region with a width of 0.8 nm that is centered on the Mo peak measurement point P is assumed as the region of the prior-y grain boundary GB.
• the ideal prior-y grain boundary GB is located in a region with a width of 0.8 nm that is centered on the Mo peak measurement point P, and that as described above, the ideal intra-grain regions BM10 and BM20 are present in regions other than the aforementioned region with a width of 0.8 nm.
• the ideal distribution of the content of Mo on the line segment SL is one in which the content of Mo becomes a largest value at the ideal prior-y grain boundary GB, and the content of Mo becomes constant at the average content of Mo in the ideal intra-grain regions BM10 and BM20.
• the aforementioned D corresponds to the average content of Mo in the ideal intra-grain regions.
• the grain boundary Mo amount ⁇ Mo defined by Formula (2) which is determined by the method described above is an index of the content of Mo near the prior-y grain boundary GB. Note that, a value obtained by rounding off to the first decimal place of the obtained numerical value is adopted as the grain boundary Mo amount (mass%) in the present embodiment.
• the grain boundary Mo amount defined as described above is 5.0% by mass or more. If the grain boundary Mo amount is too low, the prior-y grain boundaries cannot be strengthened sufficiently, and in a steel material having the chemical composition described above and the yield strength ⁇ YS , the fracture toughness in a low-temperature sour environment cannot be sufficiently increased. On the other hand, if the grain boundary Mo amount is 5.0% by mass or more, on the condition that the other requirements of the present embodiment are satisfied, the fracture toughness in a low-temperature sour environment can be increased.
• the grain boundary Mo amount is to be 5.0% by mass or more.
• a preferable lower limit of the grain boundary Mo amount is 5.5% by mass, more preferably is 6.0% by mass, further preferably is 6.5% by mass, further preferably is 7.0% by mass, further preferably is 7.3% by mass, further preferably is 7.5% by mass, and further preferably is 7.7% by mass.
• the upper limit of the grain boundary Mo amount is not particularly limited, the upper limit of the grain boundary Mo amount may be 30.0% by mass, may be 25.0% by mass, or may be 20.0% by mass.
• Fn1 defined by Formula (1) satisfies -300 or more
• Fn1 defined by Formula (1) satisfies -520 or more.
• Fn 1 83 ⁇ ⁇ Mo ⁇ 10 ⁇ 7 ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ YS
• the yield strength ⁇ YS In a steel material having the chemical composition, the yield strength ⁇ YS , and the grain boundary Mo amount ⁇ Mo which are described above, in a case where the yield strength ⁇ YS is less than 862 MPa, if Fn1 satisfies - 300 or more, the steel material will have excellent fracture toughness even in a low-temperature sour environment.
• the yield strength ⁇ YS In a steel material having the chemical composition, the yield strength ⁇ YS , and the grain boundary Mo amount ⁇ Mo which are described above, in addition, in a case where the yield strength ⁇ YS is 862 MPa or more, if Fn1 satisfies -520 or more, the steel material will have excellent fracture toughness even in a low-temperature sour environment.
• a preferable lower limit of Fn1 is -290, more preferably is -280, and further preferably is -270.
• the upper limit of Fn1 is, for example, 1606.
• a preferable lower limit of Fn1 is -515, more preferably is -510, further preferably is -505, and further preferably is -500.
• the upper limit of Fn1 is, for example, 1408.
• the dislocation density ⁇ in Fn1 is not particularly limited.
• the dislocation density ⁇ is, for example, 2.3 ⁇ 10 14 m -2 to less than 7.0 ⁇ 10 14 m -2 .
• a preferable upper limit of the dislocation density ⁇ is 6.9 ⁇ 10 14 m -2 , and more preferably is 6.8 ⁇ 10 14 m -2 .
• a preferable lower limit of the dislocation density ⁇ is more than 2.5 ⁇ 10 14 m -2 , more preferably is more than 2.7 ⁇ 10 14 m -2 , further preferably is 2.8 ⁇ 10 14 m -2 , and further preferably is 3.0 ⁇ 10 14 m -2 .
• the dislocation density ⁇ is, for example, 7.0 ⁇ 10 14 m -2 to less than 12.0 ⁇ 10 14 m -2 .
• a preferable upper limit of the dislocation density ⁇ is 11.9 ⁇ 10 14 m -2 , more preferably is 11.5 ⁇ 10 14 m -2 , further preferably is 11.2 ⁇ 10 14 m -2 , and further preferably is 11.0 ⁇ 10 14 m -2 .
• the dislocation density ⁇ of the steel material according to the present embodiment can be determined by the Williamson-Hall method. Specifically, a test specimen for dislocation density measurement is prepared from the steel material according to the present embodiment. If the steel material is a steel plate, the test specimen is prepared from the center portion of the thickness. If the steel material is a steel pipe, the test specimen is prepared from the center portion of the wall thickness. If the steel material is a round steel bar, the test specimen is prepared from an R/2 position. The size of the test specimen is, for example, 20 mm in width ⁇ 20 mm in length ⁇ 2 mm in thickness. The thickness direction of the test specimen is the thickness direction of the steel material (T direction: thickness direction, pipe radius direction, or radial direction).
• the observation surface of the test specimen is a surface with dimensions of 20 mm in width ⁇ 20 mm in length.
• the observation surface of the test specimen is mirror-polished, and furthermore is subjected to electropolishing using a 10 vol% perchloric acid (acetic acid solvent) solution to remove strain from the outer layer.
• the observation surface after the electropolishing is subjected to X-ray diffraction (XRD) to determine the half-value width ⁇ K of the peaks of the (110), (211), and (220) planes of the body-centered cubic structure (iron).
• XRD X-ray diffraction
• measurement of the half-value width ⁇ K is performed by employing Co K ⁇ radiation as the radiation source, 30 kV as the tube voltage, and 100 mA as the tube current.
• LaB 6 (lanthanum hexaboride) powder is used in order to measure a half-value width originating from the X-ray diffractometer.
• the heterogeneous strain ⁇ of the test specimen is determined based on the half-value width ⁇ K determined by the aforementioned method and the Williamson-Hall equation (Formula (3)).
• ⁇ K ⁇ cos ⁇ / ⁇ 0.9 / D + 2 ⁇ ⁇ sin ⁇ / ⁇
• ⁇ represents the diffraction angle
• ⁇ represents the wavelength of the X-ray
• D represents the crystallite diameter
• dislocation density ⁇ (m -2 ) can be determined using the obtained heterogeneous strain ⁇ and Formula (4).
• ⁇ 14.4 ⁇ ⁇ 2 / b 2
• the steel material according to the present embodiment has the chemical composition described above, and in the steel material, the yield strength ⁇ YS is 758 MPa or more, the grain boundary Mo amount ⁇ Mo is 5.0% by mass or more, and in a case where the yield strength ⁇ YS is less than 862 MPa, Fn1 satisfies -300 or more, and in a case where the yield strength ⁇ YS is 862 MPa or more, Fn1 satisfies -520 or more.
• the steel material according to the present embodiment has a yield strength of 110 ksi or more and has excellent fracture toughness in a low-temperature sour environment.
• a DCB test specimen illustrated in FIG. 11 and a wedge illustrated in FIG. 12 are prepared from the steel material according to the present embodiment. If the steel material is a steel plate, the DCB test specimen and the wedge are prepared from a center portion of the thickness. If the steel material is a steel pipe, the DCB test specimen and the wedge are prepared from a center portion of the wall thickness. If the steel material is a round steel bar, the DCB test specimen and the wedge are prepared from an R/2 position.
• the longitudinal direction (transverse direction in the drawing) of the DCB test specimen is to be parallel to the L direction (rolling elongation direction, pipe axis direction, or axial direction) of the steel material.
• a thickness t1 of the wedge is to be 3.10 mm.
• the prepared wedge is driven in between the arms of the DCB test specimen.
• the DCB test specimen into which the wedge has been driven is then enclosed in a test vessel.
• a test solution is poured into the test vessel so as to leave a vapor phase portion, and this is adopted as a test bath.
• the test solution is 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 using acetic acid (NACE solution B).
• the amount of the test bath is to be 1 L per test specimen.
• N 2 gas is blown into the test bath for three hours to degas the test bath until the dissolved oxygen in the test bath becomes 20 ppb or less.
• a gaseous mixture consisting of 1% H 2 S gas and 99% CO 2 gas is blown as a total pressure of 1 atm into the degassed test bath to make the test bath a corrosive environment.
• the inside of the test vessel is held at 4°C for 14 days (336 hours) while stirring the test bath. After being held for 14 days, the DCB test specimen is taken out from the test vessel.
• a pin is inserted into a hole formed in the tip of the arms of the DCB test specimen that is taken out from the test vessel, a notch portion is opened with a tensile testing machine, and a wedge releasing stress P is measured.
• the notch in the DCB test specimen is released in liquid nitrogen, and a crack propagation length "a" of the DCB test specimen with respect to a crack propagation which occurred during immersion in the test bath is measured.
• the crack propagation length "a” can be measured visually using vernier calipers.
• a fracture toughness value K 1SSC (MPa ⁇ m) is determined using Formula (5) based on the measured wedge releasing stress P and the crack propagation length "a".
• h (mm) represents the height of each arm of the DCB test specimen
• B (mm) represents the thickness of the DCB test specimen
• Bn (mm) represents the web thickness of the DCB test specimen.
• the yield strength ⁇ YS of the steel material in a case where the yield strength ⁇ YS of the steel material is less than 862 MPa, if the K 1SSC value obtained as a result of a DCB test carried out under the conditions described above is 29.5 MPa ⁇ m or more, it is determined that the steel material has excellent fracture toughness in a low-temperature sour environment.
• the yield strength ⁇ YS of the steel material in a case where the yield strength ⁇ YS of the steel material is 862 MPa or more, if the K 1SSC value obtained as a result of a DCB test carried out under the conditions described above is 24.1 MPa ⁇ m or more, it is determined that the steel material has excellent fracture toughness in a low-temperature sour environment.
• 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 microstructure of a steel material having the chemical composition described above contains tempered martensite and tempered bainite in an amount equivalent to a total volume ratio of 90% or more, on the condition that the other requirements of the present embodiment are satisfied, the yield strength will be 758 MPa (110 ksi) or more, and the steel material will exhibit excellent fracture toughness in a low-temperature sour environment.
• the steel material has a yield strength of 758 MPa (110 ksi) or more and has excellent fracture toughness in a low-temperature sour environment, it can be determined that the total of the volume ratios of tempered martensite and tempered bainite in the microstructure is 90% or more.
• 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 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 the 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 110 ksi or more, and excellent fracture toughness in a low-temperature sour environment.
• the method for producing a seamless steel pipe includes a process of preparing a hollow shell (preparation process), and a process of subjecting the hollow shell to quenching and tempering to form 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 hereunder. Each process is described in detail hereunder.
• an intermediate steel material having the chemical composition described above is prepared.
• a method for producing the intermediate steel material is not particularly limited as long as the intermediate steel material has the chemical composition described above.
• the term "intermediate steel material” refers to a plate-shaped steel material in a case where the end product is a steel plate, refers to a hollow shell in a case where the end product is a steel pipe, and refers to steel material in which a cross section perpendicular to the axial direction is a circular shape in a case where the end product is a round steel bar.
• 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 chemical composition described above.
• 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 process.
• 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 performing 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 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 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 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 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.
• reheating supplementary heating
• 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 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 may be prepared that was produced in another factory other than the factory in which a quenching process and a tempering process to be described later are performed or that was produced at different works.
• 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-y 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 tempered martensite and tempered bainite.
• the mechanical property defined in the present embodiment (a yield strength of 125 ksi or more) will not be obtained.
• excellent low-temperature toughness and excellent 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 a "cooling rate during quenching CR 800-500 ".
• the cooling rate during quenching CR 800-500 is determined based on a temperature that is 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 to be 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 a case, the SSC resistance of the steel material increases further.
• the tempering process is described in detail hereunder.
• 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.
• holding temperature corresponds to the temperature of the furnace when the intermediate steel material after quenching is heated and held at the relevant temperature.
• holding time means the period of time from the temperature of the intermediate steel material reaching a predetermined holding temperature until the steel material is extracted from the heat treatment furnace.
• the holding temperature has been set within a range of 630 to 730°C for the purpose of increasing the SSC resistance and the like.
• Mo can be selectively concentrated at prior-y grain boundaries by performing tempering at a temperature within a range of 450 to 600°C, which is lower than the temperature at which tempering is generally performed. Therefore, in the tempering process according to the present embodiment, tempering is performed in the range of 450 to 600°C.
• tempering is merely performed at a temperature in the range of 450 to 600°C, in some cases the dislocation density ⁇ becomes too high and Fn1 decreases too much.
• a diffusion length ⁇ Dt during tempering is defined by the following Formula (6).
• s 2 in Formula (6) is defined by the following Formula (7).
• FORMULA 3 s 2 ⁇ 1.20745921 ⁇ 10 ⁇ 10 ⁇ T + 273.15 4 + 4.13480963 ⁇ 10 ⁇ 7 ⁇ T + 273.15 3 ⁇ 5.33104487 ⁇ 10 ⁇ 4 ⁇ T + 273.15 2 + 0.305049675 ⁇ T + 273.15 ⁇ 64.4602206
• the holding temperature during tempering is substituted in °C for T in Formulae (6) and (7), and the holding time during tempering is substituted in seconds for t in Formula (6).
• a diffusion length during tempering in the range of 450 to 600°C is defined as "diffusion length ⁇ Dt L ".
• the diffusion length ⁇ Dt L during tempering in the range of 450 to 600°C is 1.3 nm or more, and a diffusion length ⁇ Dt To in the overall tempering is 60.0 nm or more.
• the process for performing tempering is not particularly limited as long as the diffusion lengths ⁇ Dt L and ⁇ Dt To satisfy the aforementioned ranges. For example, so-called “tempering in two stages” in which tempering is first performed at a high temperature and thereafter tempering is performed at a temperature in the range of 450 to 600°C may be performed. Further, for example, only tempering at a temperature in the range of 450 to 600°C may be performed.
• a preferable holding temperature is 640 to 740°C.
• a more preferable lower limit of the holding temperature in the high-temperature tempering process is 645°C, and further preferably is 650°C.
• a more preferable upper limit of the holding temperature in the high-temperature tempering process is 730°C.
• a preferable holding time is 5 to 120 minutes.
• a more preferable upper limit of the holding time in the high-temperature tempering process is 100 minutes.
• a more preferable lower limit of the holding time in the high-temperature tempering process is 10 minutes, and further preferably is 20 minutes.
• a diffusion length in the high-temperature tempering process is defined as " ⁇ Dt H ".
• ⁇ Dt H can be determined using the holding temperature (°C) and holding time (seconds) in the high-temperature tempering process, and the aforementioned Formulae (6) and (7).
• the diffusion length ⁇ Dt H in the high-temperature tempering process is not particularly limited.
• the intermediate steel material (hollow shell) is held at 450 to 600°C.
• holding the intermediate steel material (hollow shell) at 450 to 600°C causes Mo to concentrate at prior-y grain boundaries.
• a preferable holding time is 100 minutes or more. If the holding time in the intermediate-temperature tempering process is too short, in some cases the grain boundary Mo amount will not be sufficiently increased in the produced steel material.
• an upper limit of the holding time may be, for example, 8,333 hours.
• the diffusion length ⁇ Dt L it is preferable to make the diffusion length ⁇ Dt L a length of 1.3 nm or more. If the diffusion length ⁇ Dt L is too small, in some cases the grain boundary Mo amount will not be sufficiently increased in the produced steel material. Therefore, in the intermediate-temperature tempering process according to the present embodiment, a preferable lower limit of the diffusion length ⁇ Dt L is 1.3 nm or more. Note that, although not particularly limited, an upper limit of the diffusion length ⁇ Dt L is, for example, 250.0 nm.
• the diffusion length ⁇ Dt To in the overall tempering can be determined as the square root of sum of squares of the diffusion length ⁇ Dt H in the high-temperature tempering process and the diffusion length ⁇ Dt L in the intermediate-temperature tempering process.
• the tempering process according to the present embodiment may be carried out by performing only the intermediate-temperature tempering process.
• the diffusion length ⁇ Dt To in the overall tempering is the same as the diffusion length ⁇ Dt L in the intermediate-temperature tempering process.
• the tempering process according to the present embodiment may be carried out by performing tempering in three stages or more. Even in such a case, it is preferable to make the diffusion length ⁇ Dt L in the intermediate-temperature tempering process 1.3 nm or more, and to make the diffusion length ⁇ Dt To in overall tempering 60.0 nm or more.
• the holding temperature and holding time are appropriately adjusted to obtain a steel material having a yield strength of 758 MPa or more.
• those skilled in the art are fully capable of subjecting an intermediate steel material (hollow shell) having the chemical composition of the present embodiment to tempering in which the aforementioned holding temperature and aforementioned holding time have been appropriately adjusted to make the yield strength 758 MPa or more.
• the steel material according to the present embodiment can be produced according to the production method described above.
• a method for producing a seamless steel pipe as one example of the steel material according to the present embodiment has been described.
• the steel material according to the present embodiment may also be a steel plate or in another shape.
• a method for producing a steel plate or a steel material in another shape also includes, for example, a preparation process, a quenching process, and a tempering process, similarly to the production method described above.
• the production method described above is an example, and the steel material according to the present embodiment may also be produced by the other production methods.
• the present disclosure is described more specifically by way of examples.
• Example 1 steel materials having a yield strength of less than 862 MPa were investigated. Specifically, molten steels which each had a weight of 180 kg and had the chemical compositions shown in Table 1-1 and Table 1-2 were produced. Note that, the symbol “-" in Table 1-2 means that contents of the respective elements are at the level of an impurity. Specifically, “-” means that the content of Co, the content of W, the content of Ni, and the content of Cu of Test No. 1-1 were each 0% when rounded off to second decimal place. In addition, “-” means that the content of Nb of Test No. 1-1 was 0% when rounded off to third decimal place. Further, “-” means that the content of Ca, the content of Mg, the content of Zr, and the content of REM of Test No. 1-1 were each 0% when rounded off to fourth decimal place.
• Ingots were produced using the molten steels of Test Nos. 1-1 to 1-26.
• the produced ingots were subjected to hot rolling to produce steel plates having a thickness of 15 mm.
• the steel plates of Test Nos. 1-1 to 1-26 after hot rolling were allowed to cool to bring the temperature of the steel plate to normal temperature (25°C).
• the steel plates of Test Nos. 1-1 to 1-26 were held for 20 minutes at the quenching temperature (920°C), and thereafter the steel plates were immersed in a water bath to be quenched.
• the cooling rate during quenching (CR 800-500 ) was 600°C/min for each test number.
• a type K thermocouple of a sheath type was inserted into a center portion of the thickness of the steel plate in advance, and the quenching temperature and cooling rate during quenching CR 800-500 were measured using the type K thermocouple.
• tempering at a high temperature is also referred to as “first tempering”
• tempering at an intermediate temperature is also referred to as “second tempering”.
• the holding temperature (°C), holding time (minutes), and diffusion length ( ⁇ Dt H (nm) or ⁇ Dt L (nm)) for the first tempering and the second tempering are shown in Table 2.
• the diffusion length ⁇ Dt To (nm) in the overall tempering as determined based on the diffusion lengths ⁇ Dt H and ⁇ Dt L and Formula (8) is shown in Table 2. Note that, in Table 2, the symbol “-” in the column “First Tempering” means that the first tempering was not performed. Similarly, in Table 2, the symbol “-” in the column “Second Tempering” means that the second tempering was not performed.
• the temperature of the heat treatment furnace where tempering was performed was adopted as the holding temperature during tempering in the present example.
• the holding time during tempering in the present example was taken as the time from when the temperature of the steel plate of each test number reached a predetermined tempering temperature until the steel plate was extracted from the heat treatment furnace.
• the steel plates of Test Nos. 1-1 to 1-26 on which tempering had been performed were subjected to a tensile test, a dislocation density measurement test, a grain boundary Mo amount measurement test, and a DCB test that are described hereunder.
• the steel plates of Test Nos. 1-1 to 1-26 were subjected to a tensile test by the method described above. Specifically, a round bar tensile test specimen having a parallel portion diameter of 4 mm and a gage length of 16 mm was prepared from a center portion of the thickness of each steel plate of Test Nos. 1-1 to 1-26. The axial direction of the round bar tensile test specimen was parallel to the rolling elongation direction of the steel plate. Using the round bar specimens of Test Nos. 1-1 to 1-26, a tensile test in accordance with ASTM E8/E8M (2021) was carried out at normal temperature (25°C) in the atmosphere, and the yield strength (MPa) of each steel plate of Test Nos. 1-1 to 1-26 was determined. The obtained yield strength is shown as "Yield Strength ⁇ YS (MPa)" in Table 3.
• the steel plates of Test Nos. 1-1 to 1-26 were subjected to a dislocation density measurement test by the method described above. Specifically, a test specimen having dimensions of 20 mm in width ⁇ 20 mm in length ⁇ 2 mm in thickness was prepared from a center portion of the thickness of each steel plate of Test Nos. 1-1 to 1-26. The half-value width ⁇ K of the peaks of the (110), (211), and (220) planes was determined using X-ray diffraction according to the method described above. The determined half-value widths ⁇ K were used to determine the dislocation density ⁇ (m -2 ) by the method described above. The obtained dislocation density is shown as "Dislocation Density ⁇ (10 14 m -2 )" in Table 3.
• the steel plates of Test Nos. 1-1 to 1-26 were subjected to a grain boundary Mo amount measurement test by the method described above. Specifically, for each of the steel plates of Test Nos. 1-1 to 1-26, a test specimen was prepared which included a thickness t/4 position at the center thereof and which had a size having a length of 10 mm in the rolling elongation direction ⁇ 5 mm in the width direction ⁇ 8 mm in the thickness direction. The prepared test specimen was used to determine the grain boundary Mo amount (mass%) by the method described above. The obtained grain boundary Mo amount is shown as "Grain Boundary Mo Amount ⁇ Mo (mass%)" in Table 3. In addition, Fnl was obtained using the obtained yield strength ⁇ YS (MPa), dislocation density ⁇ (m -2 ), and grain boundary Mo amount ⁇ Mo (mass%), and Formula (1). The obtained Fn1 is shown in Table 3.
• the steel plates of Test Nos. 1-1 to 1-26 were subjected to a DCB test by the method described above. Specifically, a DCB test specimen and a wedge that are described above were prepared from a center portion of the thickness of each of the steel plates of Test Nos. 1-1 to 1-26. A DCB test in accordance with NACE TM0177-2016 Method D was performed under the conditions described above using the prepared test specimen and wedge. The fracture toughness value K 1SSC obtained by the DCB test carried out according to the method described above is shown as "K 1SSC (MPa ⁇ m)" in Table 3.
• Example 2 steel materials having a yield strength of 862 MPa or more were investigated. Specifically, molten steels having a weight of 180 kg and the chemical compositions shown in Table 4-1 and Table 4-2 were produced. Note that, the symbol “-” in Table 4-2 means that the contents of the respective elements are at the level of an impurity. Specifically, “-” means that the content of Co, the content of W, the content of Ni, and the content of Cu of Test No. 2-1 were each 0% when rounded off to second decimal place. In addition, “-” means that the content of Nb of Test No. 2-1 was 0% when rounded off to third decimal place. Further, “-” means that the content of Ca, the content of Mg, the content of Zr, and the content of REM of Test No. 2-1 were each 0% when rounded off to fourth decimal place.
• Ingots were produced using the molten steels of Test Nos. 2-1 to 2-26.
• the produced ingots were subjected to hot rolling to produce steel plates having a thickness of 15 mm.
• the steel plates of Test Nos. 2-1 to 2-26 after hot rolling were each allowed to cool to bring the temperature of the steel plate to normal temperature (25°C).
• the steel plates of Test Nos. 2-1 to 2-26 were held for 20 minutes at the quenching temperature (920°C), and thereafter the steel plates were immersed in a water bath to be quenched.
• the cooling rate during quenching (CR 800-500 ) was 600°C/min for each test number.
• a type K thermocouple of a sheath type was inserted into a center portion of the thickness of the steel plate in advance, and the quenching temperature and cooling rate during quenching CR 800-500 were measured using the type K thermocouple.
• tempering at a high temperature is also referred to as “first tempering”
• tempering at an intermediate temperature is also referred to as “second tempering”.
• the holding temperature (°C), holding time (minutes), and diffusion length ( ⁇ Dt H (nm) or ⁇ Dt L (nm)) for the first tempering and the second tempering are shown in Table 5.
• the diffusion length ⁇ Dt To (nm) in the overall tempering as determined based on the diffusion lengths ⁇ Dt H and ⁇ Dt L and Formula (8) is shown in Table 5. Note that, in Table 5, the symbol “-" in the column “First Tempering” means that the first tempering was not performed. Similarly, in Table 5, the symbol “-” in the column “Second Tempering” means that the second tempering was not performed.
• the temperature of the heat treatment furnace where tempering was performed was adopted as the holding temperature during tempering in the present example.
• the holding time during tempering in the present example was taken as the time from when the temperature of the steel plate of each test number reached a predetermined tempering temperature until the steel plate was extracted from the heat treatment furnace.
• the steel plates of Test Nos. 2-1 to 2-26 on which tempering had been performed were subjected to a tensile test, a dislocation density measurement test, a grain boundary Mo amount measurement test, and a DCB test that are described hereunder.
• the steel plates of Test Nos. 2-1 to 2-26 were subjected to a tensile test by the method described above. Specifically, a round bar tensile test specimen having a parallel portion diameter of 4 mm and a gage length of 16 mm was prepared from a center portion of the thickness of each steel plate of Test Nos. 2-1 to 2-26. The axial direction of the round bar tensile test specimen was parallel to the rolling elongation direction of the steel plate. Using the round bar specimens of Test Nos. 2-1 to 2-26, a tensile test in accordance with ASTM E8/E8M (2021) was carried out at normal temperature (25°C) in the atmosphere, and the yield strength (MPa) of each steel plate of Test Nos. 2-1 to 2-26 was determined. The obtained yield strength is shown as "Yield Strength ⁇ YS (MPa)" in Table 6.
• the steel plates of Test Nos. 2-1 to 2-26 were subjected to a dislocation density measurement test by the method described above. Specifically, a test specimen of 20 mm in width ⁇ 20 mm in length ⁇ 2 mm in thickness was prepared from a center portion of the thickness of each steel plate of Test Nos. 2-1 to 2-26. The half-value width ⁇ K of the peaks of the (110), (211), and (220) planes was determined using X-ray diffraction according to the method described above. The determined half-value widths ⁇ K were used to determine the dislocation density ⁇ (m -2 ) by the method described above. The obtained dislocation density is shown as "Dislocation Density ⁇ (10 14 m -2 )" in Table 6.
• the steel plates of Test Nos. 2-1 to 2-26 were subjected to a grain boundary Mo amount measurement test by the method described above. Specifically, for each of the steel plates of Test Nos. 2-1 to 2-26, a test specimen was prepared which included a thickness t/4 position at the center thereof and which had a size having a length of 10 mm in the rolling elongation direction ⁇ 5 mm in the width direction ⁇ 8 mm in the thickness direction. The prepared test specimen was used to obtain the grain boundary Mo amount (mass%) by the method described above. The obtained grain boundary Mo amount is shown as "Grain Boundary Mo Amount ⁇ Mo (mass%)" in Table 6. In addition, Fnl was obtained using the obtained yield strength ⁇ YS (MPa), dislocation density ⁇ (m -2 ), and grain boundary Mo amount ⁇ Mo (mass%), and Formula (1). The obtained Fn1 is shown in Table 6.
• the steel plates of Test Nos. 2-1 to 2-26 were subjected to a DCB test by the method described above. Specifically, a DCB test specimen and a wedge that are described above were prepared from a center portion of the thickness of each of the steel plates of Test Nos. 2-1 to 2-26. A DCB test in accordance with NACE TM0177-2016 Method D was performed under the conditions described above using the prepared test specimen and wedge. The fracture toughness value K 1SSC obtained by the DCB test performed by the method described above is shown as "K 1SSC (MPa ⁇ m)" in Table 6.

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