EP4512921A1 - Stahlmaterial - Google Patents

Stahlmaterial Download PDF

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Publication number
EP4512921A1
EP4512921A1 EP23791942.8A EP23791942A EP4512921A1 EP 4512921 A1 EP4512921 A1 EP 4512921A1 EP 23791942 A EP23791942 A EP 23791942A EP 4512921 A1 EP4512921 A1 EP 4512921A1
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EP
European Patent Office
Prior art keywords
steel material
test
ssc resistance
content
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Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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EP23791942.8A
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English (en)
French (fr)
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EP4512921A4 (de
Inventor
Hiroyuki Fuji
Shinji Yoshida
Yuji Arai
Keiichi Kondo
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Nippon Steel Corp
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Nippon Steel Corp
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Priority claimed from PCT/JP2023/015878 external-priority patent/WO2023204294A1/ja
Publication of EP4512921A1 publication Critical patent/EP4512921A1/de
Publication of EP4512921A4 publication Critical patent/EP4512921A4/de
Pending legal-status Critical Current

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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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
    • 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
    • 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
    • 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
    • C22C38/02—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
    • C22C38/22—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
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present disclosure relates to a steel material, and more particularly relates to a steel material that is suitable for use in a sour environment.
  • oil wells and gas wells are collectively referred to as simply "oil wells"
  • oil wells and gas wells are collectively referred to as simply "oil wells”
  • oil-well steel materials of 80 ksi grade yield strength is 80 to less than 95 ksi, that is, 552 to less than 655 MPa
  • 95 ksi grade yield strength is 95 to less than 110 ksi, that is, 655 to less than 758 MPa
  • sour environment means an acidified environment containing hydrogen sulfide.
  • a sour environment may also contain carbon dioxide.
  • Steel materials to be used 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").
  • Patent Literature 1 Japanese Patent Application Publication No. 2000-297344
  • Patent Literature 2 International Application Publication No. WO2008/123422
  • Patent Literature 1 discloses a steel material containing, in mass%, C: 0.15 to 0.3%, Cr: 0.2 to 1.5%, Mo: 0.1 to 1%, V: 0.05 to 0.3%, and Nb: 0.003 to 0.1%.
  • the total amount of precipitating carbides is within the range of 1.5 to 4% by mass
  • the proportion that MC-type carbides occupy among the total amount of carbides is within the range of 5 to 45% by mass
  • the wall thickness of the product is defined as t (mm)
  • the proportion of M 23 C 6 -type carbides is (200/t) or less in percent by mass.
  • SSC resistance is increased by suppressing the proportion of M 23 C 6 -type carbides.
  • the number density of M 23 C 6 -type precipitates having a grain size of 1 ⁇ m or more is 0.1 pieces/mm 2 or less.
  • SSC resistance is increased by suppressing the proportion of M 23 C 6 -type carbides.
  • Patent Literatures 1 and 2 As described above, recently, as the severity of oil well environments increases, there is an increasing demand for steel materials capable of achieving both a high strength of 110 ksi grade or more (758 MPa or more) and excellent SSC resistance. In the aforementioned Patent Literatures 1 and 2, it is attempted to achieve both high strength and SSC resistance by suppressing precipitates. However, a steel material that can achieve both high strength and SSC resistance may also be obtained by means other than the means disclosed in Patent Literatures 1 and 2.
  • An objective of the present disclosure is to provide a steel material that has both high strength and excellent SSC resistance in a sour environment.
  • a steel material according to the present disclosure is as follows.
  • the steel material according to the present disclosure has high strength and has excellent SSC resistance in a sour environment.
  • the present inventors carried out investigations and studies pertaining to a steel material that has high strength and has excellent SSC resistance in a sour environment. As a result, the present inventors obtained the following findings.
  • the present inventors conducted studies from the viewpoint of the chemical composition with regard to a steel material that has high strength and has excellent SSC resistance in a sour environment. As a result, the present inventors considered that if the chemical composition of a steel material satisfies the following Feature 1, there is a possibility that a high strength of 110 ksi grade (758 to less than 862 MPa) to 125 ksi grade (862 to less than 965 MPa), and excellent SSC resistance in a sour environment will be obtained.
  • the chemical composition consists of, in mass%, C: 0.20 to 0.35%, Si: 0.60 to 1.30%, Mn: 0.05 to 0.25%, P: 0.050% or less, S: 0.0100% or less, Al: 0.010 to 0.100%, N: 0.0100% or less, Cr: 0.20 to 1.00%, Mo: 0.10 to 1.00%, Ti: 0.003 to 0.030%, O: 0.0050% or less, Zr: 0 to 0.0040%, Sb: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Co: 0 to 0.50%, Ca: 0 to 0.0040%, Mg: 0 to 0.0040%, rare earth metal: 0 to 0.0040%, Nb: 0 to 0.150%, V: 0 to 0.500%, and B: 0 to 0.0030%, with the balance being Fe and impurities.
  • FIG. 1 is a schematic diagram for describing a process in which hydrogen penetrates into the interior of a steel material from the surface of the steel material.
  • a steel material 1 corrodes in a sour environment
  • the surface of the steel material 1 enters an electrochemically active state.
  • Fe in the steel material 1 becomes Fe 2+ and dissolves in the environment.
  • electrons e - are released to the outside of the steel material 1 (A: occurrence of corrosion).
  • Hydrogen ions H + that are present in the environment receive electrons e - released from the steel material 1 and are reduced, and are then adsorbed on the surface of the steel material 1 as adsorbed hydrogen atoms H ad (B: adsorption reaction of hydrogen ions). Due to the above mechanism, a plurality of adsorbed hydrogen atoms H ad are present on the surface of the steel material 1. Almost all of these adsorbed hydrogen atoms H ad combine together to form hydrogen gas Hz, and are released from the surface of the steel material 1 into the environment.
  • the present inventors considered that in order to suppress the occurrence and propagation of SSC and obtain excellent SSC resistance, it is effective to suppress the penetration of hydrogen from the steel material surface. Therefore, the present inventors conducted studies regarding means for suppressing the penetration of hydrogen from the steel material surface.
  • the present inventors carried out investigations and studies regarding elements that affect the penetration of hydrogen from the surface of steel materials. As a result, the present inventors obtained the following finding regarding steel materials that have a chemical composition which satisfies Feature 1.
  • Si, Cr, Mo, Zr, Sb, Cu, Ni, and Co suppress electrochemical activity on a steel material surface in a sour environment. As a result, these elements suppress penetration of hydrogen from the steel material surface.
  • C and Mn promote electrochemical activity on a steel material surface in a sour environment. As a result, these elements promote the penetration of hydrogen from the steel material surface.
  • the present inventors considered that if the content of these hydrogen penetration suppressing elements (Si, Cr, Mo, Zr, Sb, Cu, Ni, and Co) and the content of these hydrogen penetration promoting elements (C and Mn) are appropriately adjusted, the penetration of hydrogen from the steel material surface can be suppressed by electrochemical action. Therefore, the present inventors studied the relation between hydrogen penetration suppressing elements, hydrogen penetration promoting elements, and SSC resistance in steel materials having a chemical composition satisfying Feature 1.
  • EE -0.25C + 2Si - 5.8Mn + 2.1Cr + Mo + 4.1Zr + 2.6Sb + 0.3Cu + 0.4Ni + 1.5Co where, a content in percent by mass of a corresponding element is substituted for each symbol of an element in Formula (1).
  • the present inventors considered that the occurrence and propagation of SSC in a steel material whose chemical composition satisfies Feature 1 are influenced not only by the aforementioned electrochemical elements, but also by physical elements that depends on the microstructure. Therefore, the present inventors also conducted studies regarding means for increasing the SSC resistance of a steel material from the viewpoint of physical elements, and not just from the viewpoint of electrochemical elements. As a result, the present inventors discovered that the average equivalent circular diameter ( ⁇ m) of prior-austenite grains in a steel material acts synergistically with the aforementioned electrochemical elements and markedly influences the SSC resistance of the steel material.
  • the present inventors also investigated the relation among the aforementioned electrochemical elements (hydrogen penetration suppressing elements and hydrogen penetration promoting elements), the physical element (average equivalent circular diameter of prior-austenite grains), and SSC resistance.
  • FN is an index which indicates the degree to which the electrochemical elements (hydrogen penetration suppressing elements and hydrogen penetration promoting elements) and the physical element (average equivalent circular diameter of prior-austenite grains) influence SSC resistance.
  • EE is to be made 2.75 or more
  • FN is to be made 0.185 or more.
  • EE is to be made 3.00 or more
  • FN is to be made 0.200 or more. In such a case, excellent SSC resistance is obtained even when the steel material has a high strength of 110 ksi grade to 125 ksi grade. This point is described hereunder.
  • FIG. 2A is a view illustrating the relation between FN and a fracture toughness value Kissc (MPa ⁇ m) obtained in a DCB test performed on steel materials having a chemical composition satisfying Feature 1 and having a yield strength of 110 ksi grade (758 to less than 862 MPa) and in which EE was 2.75 or more.
  • FIG. 2A was prepared based on data obtained in Example 1 to be described later.
  • the fracture toughness value Kissc is a high value of 25.0 MPa ⁇ m or more, and excellent SSC resistance is obtained.
  • the fracture toughness value Kissc markedly decreases to less than 25.0 MPa ⁇ m. Accordingly, by making FN 0.185 or more, excellent SSC resistance is obtained in a steel material of 110 ksi grade (758 to less than 862 MPa).
  • FIG. 2B is a view illustrating the relation between FN and the fracture toughness value Kissc (MPa ⁇ m) obtained in a DCB test performed on steel materials having a chemical composition satisfying Feature 1 and having a yield strength of 125 ksi grade (862 to less than 965 MPa) and in which EE was 3.00 or more.
  • FIG. 2B was prepared based on data obtained in Example 2 to be described later.
  • the fracture toughness value Kissc is a high value of 24.0 MPa ⁇ m or more, and excellent SSC resistance is obtained.
  • the fracture toughness value Kissc markedly decreases to less than 24.0 MPa ⁇ m. Accordingly, by making FN 0.200 or more, excellent SSC resistance is obtained in a steel material of 125 ksi grade (862 to less than 965 MPa).
  • the relation between the electrochemical elements and the physical element and the SSC resistance described above is a relation that is surmised by the present inventors, and there is also a possibility that excellent SSC resistance is obtained by a mechanism that is different from the mechanism described above.
  • excellent SSC resistance is obtained even when the steel material has a high strength of 110 ksi grade (758 to less than 862 MPa) to 125 ksi grade (862 to less than 965 MPa) if EE is 2.75 or more and FN is 0.185 or more in a case where the yield strength is 110 ksi grade, and if EE is 3.00 or more and FN is 0.200 or more in a case where the yield strength is 125 ksi grade has been proven by examples that are described later.
  • a steel material according to the present embodiment which has been completed based on the findings described above, is as follows.
  • the steel material of the present embodiment satisfies the following Feature 1 to Feature 3.
  • the chemical composition consists of, in mass%, C: 0.20 to 0.35%, Si: 0.60 to 1.30%, Mn: 0.05 to 0.25%, P: 0.050% or less, S: 0.0100% or less, Al: 0.010 to 0.100%, N: 0.0100% or less, Cr: 0.20 to 1.00%, Mo: 0.10 to 1.00%, Ti: 0.003 to 0.030%, O: 0.0050% or less, Zr: 0 to 0.0040%, Sb: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Co: 0 to 0.50%, Ca: 0 to 0.0040%, Mg: 0 to 0.0040%, rare earth metal: 0 to 0.0040%, Nb: 0 to 0.150%, V: 0 to 0.500%, and B: 0 to 0.0030%, with the balance being Fe and impurities.
  • the yield strength is 758 to less than 965 MPa.
  • the chemical composition of the steel material of the present embodiment contains the following elements.
  • Carbon (C) increases the strength of the steel material by increasing hardenability of the steel material and forming carbides. C also promotes spheroidization of carbides during tempering in the production process, and thereby increases the SSC resistance of the steel material. If the content of C is less than 0.20%, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.
  • C is a hydrogen penetration promoting element. Therefore, even if the contents of other elements are within the range of the present embodiment, the SSC resistance of the steel material will decrease.
  • the content of C is 0.20 to 0.35%.
  • a preferable lower limit of the content of C is 0.22%, more preferably is 0.23%, further preferably is 0.24%, and further preferably is 0.25%.
  • a preferable upper limit of the content of C is 0.32%, more preferably is 0.30%, further preferably is 0.28%, and further preferably is 0.27%.
  • Silicon (Si) is a hydrogen penetration suppressing element, and suppresses penetration of hydrogen from the steel material surface. As a result, the SSC resistance of the steel material increases. If the content of Si is less than 0.60%, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.
  • the content of Si is 0.60 to 1.30%.
  • a preferable lower limit of the content of Si is 0.62%, more preferably is 0.65%, further preferably is 0.70%, further preferably is 0.72%, further preferably is 0.75%, and further preferably is 0.80%.
  • a preferable upper limit of the content of Si is 1.28%, more preferably is 1.25%, and further preferably is 1.20%.
  • Manganese (Mn) deoxidizes the steel. Mn also increases hardenability of the steel material. If the content of Mn is less than 0.05%, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.
  • Mn is a hydrogen penetration promoting element. If the content of Mn is more than 0.25%, Mn sulfides will excessively form. The Mn sulfides will act as starting points for pitting. Therefore, if Mn sulfides excessively form, the corrosion rate will increase and penetration of hydrogen into the steel material will be promoted. As a result, even if the contents of other elements are within the range of the present embodiment, the SSC resistance of the steel material will decrease.
  • the content of Mn is 0.05 to 0.25%.
  • a preferable lower limit of the content of Mn is 0.06%, more preferably is 0.07%, further preferably is 0.08%, and further preferably is 0.10%.
  • a preferable upper limit of the content of Mn is 0.24%, more preferably is 0.23%, further preferably is 0.22%, further preferably is 0.20%, and further preferably is 0.18%.
  • Phosphorus (P) is an impurity. That is, the content of P is more than 0%. If the content of P is more than 0.050%, even if the contents of other elements are within the range of the present embodiment, P will segregate to grain boundaries and the SSC resistance of the steel material will decrease.
  • the content of P is 0.050% or less.
  • the content of P is preferably as low as possible. However, excessively reducing the content of P will significantly increase the production cost. Therefore, when taking industrial production into consideration, a preferable lower limit of the content of P is 0.001%, and more preferably is 0.003%.
  • a preferable upper limit of the content of P is 0.030%, more preferably is 0.025%, further preferably is 0.020%, and further preferably is 0.015%.
  • S is an impurity. That is, the content of S is more than 0%. If the content of S is more than 0.0100%, 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.
  • the content of S is 0.0100% or less.
  • the content of S is preferably as low as possible. However, excessively reducing the content of S will significantly 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%.
  • a preferable upper limit of the content of S is 0.0070%, more preferably is 0.0050%, further preferably is 0.0030%, further preferably is 0.0025%, further preferably is 0.0020%, and further preferably is 0.0015%.
  • Aluminum (Al) deoxidizes the steel. If the content of Al is less than 0.010%, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.
  • the content of Al is 0.010 to 0.100%.
  • a preferable lower limit of the content of Al is 0.012%, more preferably is 0.015%, further preferably is 0.020%, and further preferably is 0.025%.
  • a preferable upper limit of the content of Al is 0.080%, more preferably is 0.070%, and further preferably is 0.060%.
  • the content of "Al” means the content of "acid-soluble Al", that is, “sol. Al”.
  • N Nitrogen
  • a lower limit of the content of N is more than 0%.
  • N combines with Ti to form nitrides, and refines the grains of the steel material by a pinning effect. As a result, the strength of the steel material increases.
  • the content of N is 0.0100% or less.
  • a preferable lower limit of the content of N is 0.0001%, more preferably is 0.0005%, further preferably is 0.0010%, further preferably is 0.0015%, and further preferably is 0.0020%.
  • a preferable upper limit of the content of N is 0.0070%, more preferably is 0.0060%, further preferably is 0.0050%, further preferably is 0.0045%, and further preferably is 0.0040%.
  • Chromium (Cr) increases hardenability of the steel material. Cr also functions as a hydrogen penetration suppressing element. Specifically, in a sour environment, Cr stabilizes a corrosion product film that is formed on the steel material surface and thereby suppresses penetration of hydrogen into the steel material. As a result, the SSC resistance of the steel material increases. If the content of Cr is less than 0.20%, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.
  • the content of Cr is 0.20 to 1.00%.
  • a preferable lower limit of the content of Cr is 0.25%, more preferably is 0.30%, further preferably is 0.35%, further preferably is 0.40%, further preferably is 0.45%, further preferably is 0.50%, further preferably is 0.55%, and further preferably is 0.60%.
  • a preferable upper limit of the content of Cr is 0.98%, more preferably is 0.95%, and further preferably is 0.90%.
  • Molybdenum (Mo) functions as a hydrogen penetration suppressing element. Specifically, in a sour environment, Mo stabilizes a corrosion product film that is formed on the steel material surface and thereby suppresses penetration of hydrogen into the steel material. As a result, the SSC resistance of the steel material increases. Mo also increases hardenability of the steel material. In addition, Mo increases the temper softening resistance of the steel material and thereby enables high-temperature tempering. As a result, the SSC resistance of the steel material increases. If the content of Mo is less than 0.10%, 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 Mo is 0.10 to 1.00%.
  • a preferable lower limit of the content of Mo is 0.20%, more preferably is 0.25%, further preferably is 0.30%, and further preferably is 0.35%.
  • a preferable upper limit of the content of Mo is 0.95%, more preferably is 0.90%, further preferably is 0.85%, further preferably is 0.80%, and further preferably is 0.70%.
  • Titanium (Ti) combines with N to form nitrides, and refines the grains of the steel material by a pinning effect. As a result, the strength of the steel material increases. If the content of Ti is less than 0.003%, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.
  • the content of Ti is 0.003 to 0.030%.
  • a preferable lower limit of the content of Ti is 0.004%, and more preferably is 0.005%.
  • a preferable upper limit of the content of Ti is 0.028%, more preferably is 0.025%, further preferably is 0.022%, and further preferably is 0.020%.
  • the content of O is 0.0050% or less.
  • the balance of the chemical composition of the steel material according to the present embodiment is Fe and impurities.
  • impurities in the chemical composition means substances which are mixed in from ore and scrap used as the raw material or from the production environment or the like when industrially producing the steel material, and which are not intentionally contained but are permitted within a range that does not adversely affect the steel material according to the present embodiment.
  • the chemical composition of the steel material according to the present embodiment may further contain one or more elements selected from the group consisting of Zr, Sb, Cu, Ni, Co, Ca, Mg, and rare earth metal (REM) in lieu of a part of Fe.
  • Each of these elements is an optional element, and increases the SSC resistance of the steel material.
  • Zirconium (Zr) is an optional element, and does not have to be contained. That is, the content of Zr may be 0%.
  • Zr When Zr is contained, that is, when the content of Zr is more than 0%, Zr functions as a hydrogen penetration suppressing element. Specifically, in a sour environment, Zr stabilizes a corrosion product film formed on the steel material surface and thereby 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 Zr is contained, the aforementioned advantageous effect will be obtained to a certain extent.
  • the content of Zr is 0 to 0.0040%.
  • a preferable lower limit of the content of Zr is 0.0001%, more preferably is 0.0003%, further preferably is 0.0006%, further preferably is 0.0010%, and further preferably is 0.0015%.
  • a preferable upper limit of the content of Zr is 0.0038%, more preferably is 0.0035%, and further preferably is 0.0032%.
  • Antimony (Sb) is an optional element, and does not have to be contained. That is, the content of Sb may be 0%.
  • Sb When Sb is contained, that is, when the content of Sb is more than 0%, Sb functions as a hydrogen penetration suppressing element. Specifically, under a sour environment, Sb 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 Sb is contained, the aforementioned advantageous effect will be obtained to a certain extent.
  • the content of Sb is 0 to 0.50%.
  • a preferable lower limit of the content of Sb is 0.01%, more preferably is 0.03%, further preferably is 0.05%, and further preferably is 0.08%.
  • a preferable upper limit of the content of Sb is 0.40%, more preferably is 0.35%, further preferably is 0.30%, and further preferably is 0.25%.
  • Copper (Cu) is an optional element, and does not have to be contained. That is, the content of Cu may be 0%.
  • Cu When Cu is contained, that is, when the content of Cu is more than 0%, Cu functions as a hydrogen penetration suppressing element. Specifically, under a sour environment, Cu concentrates at the interface between the corrosion product film and the base metal. As a result, surface activity of the base metal is suppressed, and penetration of hydrogen into the steel material is suppressed. Consequently, the SSC resistance of the steel material increases. Cu also dissolves in the steel material and increases hardenability of the steel material and increases the strength of the steel material. If even a small amount of Cu is contained, the aforementioned advantageous effects will be obtained to a certain extent.
  • the SSC resistance of the steel material will decrease.
  • the content of Cu is 0 to 0.50%.
  • a preferable lower limit of the content of Cu is 0.01%, more preferably is 0.02%, and further preferably is 0.05%.
  • a preferable upper limit of the content of Cu is 0.40%, more preferably is 0.38%, further preferably is 0.35%, and further preferably is 0.30%.
  • Nickel (Ni) is an optional element, and does not have to be contained. That is, the content of Ni may be 0%.
  • Ni When Ni is contained, that is, when the content of Ni is more than 0%, Ni functions as a hydrogen penetration suppressing element. Specifically, under a sour environment, Ni concentrates at the interface between the corrosion product film and the base metal. As a result, surface activity of the base metal is suppressed, and penetration of hydrogen into the steel material is suppressed. Consequently, the SSC resistance of the steel material increases. If even a small amount of Ni is contained, the aforementioned advantageous effect will be obtained to a certain extent.
  • the content of Ni is more than 0.50%, even if the contents of other elements are within the range of the present embodiment, it will become easy for local corrosion to proceed, and a hydrogen embrittlement resistance characteristic of the steel material will decrease.
  • the content of Ni is 0 to 0.50%.
  • a preferable lower limit of the content of Ni is 0.01%, more preferably is 0.05%, and further preferably is 0.07%.
  • a preferable upper limit of the content of Ni is 0.45%, more preferably is 0.40%, further preferably is 0.35%, and further preferably is 0.32%.
  • Co Co is an optional element, and does not have to be contained. That is, the content of Co may be 0%.
  • Co When Co is contained, that is, when the content of Co is more than 0%, Co functions as a hydrogen penetration suppressing element. Specifically, under a sour environment, Co concentrates at the interface between the corrosion product film and the base metal. As a result, surface activity of the base metal is suppressed, and penetration of hydrogen into the steel material is suppressed. Consequently, the SSC resistance of the steel material increases. If even a small amount of Co is contained, the aforementioned advantageous effect will be obtained to a certain extent.
  • the content of Co is more than 0.50%, 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.
  • the content of Co is 0 to 0.50%.
  • a preferable lower limit of the content of Co is 0.01%, more preferably is 0.02%, further preferably is 0.03%, further preferably is 0.05%, and further preferably is 0.08%.
  • a preferable upper limit of the content of Co is 0.40%, more preferably is 0.30%, further preferably is 0.20%, and further preferably is 0.15%.
  • Calcium (Ca) is an optional element, and does not have to be contained. That is, the content of Ca may be 0%.
  • Ca When Ca is contained, that is, when the content of Ca is more than 0%, Ca renders S in the steel material harmless by forming sulfides, and thereby increases the SSC resistance of the steel material. If even a small amount of Ca is contained, the aforementioned advantageous effect will be obtained to a certain extent.
  • the content of Ca is 0 to 0.0040%.
  • a preferable lower limit of the content of Ca is 0.0001%, and more preferably is 0.0003%.
  • a preferable upper limit of the content of Ca is 0.0030%, more preferably is 0.0020%, further preferably is 0.0015%, and further preferably is 0.0012%.
  • Mg When Mg is contained, that is, when the content of Mg is more than 0%, Mg renders S in the steel material harmless by forming sulfides, and thereby increases the SSC resistance of the steel material. If even a small amount of Mg is contained, the aforementioned advantageous effect will be obtained to a certain extent.
  • the content of Mg is 0 to 0.0040%.
  • a preferable lower limit of the content of Mg is 0.0001%, and more preferably is 0.0003%.
  • a preferable upper limit of the content of Mg is 0.0030%, more preferably is 0.0025%, further preferably is 0.0020%, and further preferably is 0.0015%.
  • Rare earth metal 0 to 0.0040%
  • Rare earth metal is an optional element, and does not have to be contained. That is, the content of REM may be 0%.
  • REM When REM is contained, that is, when the content of REM is more than 0%, 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 grain boundaries. Therefore, a decrease in the SSC resistance of the steel material that is 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.
  • the content of REM is 0 to 0.0040%.
  • a preferable lower limit of the content of REM is 0.0001%, more preferably is 0.0003%, and further preferably is 0.0005%.
  • a preferable upper limit of the content of REM is 0.0035%, more preferably is 0.0030%, and further preferably is 0.0025%.
  • 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.
  • scandium Sc
  • Y yttrium
  • Li lutetium
  • content of REM means the total content of these elements.
  • the chemical composition of the steel material according to the present embodiment may further contain one or more elements selected from the group consisting of Nb, V, and B in lieu of a part of Fe. Each of these elements is an optional element, and increases the strength of the steel material.
  • Niobium (Nb) is an optional element, and does not have to be contained. That is, the content of Nb may be 0%.
  • Nb When Nb is contained, Nb combines with C and/or N to form Nb carbo-nitrides and the like. These Nb carbo-nitrides and the like refine the grains of the steel material by a pinning effect. As a result, the strength of the steel material increases. If even a small amount of Nb is contained, the aforementioned advantageous effect will be obtained to a certain extent.
  • the content of Nb is 0 to 0.150%.
  • a preferable lower limit of the content of Nb is 0.001%, more preferably is 0.003%, further preferably is 0.005%, further preferably is 0.008%, and further preferably is 0.012%.
  • a preferable upper limit of the content of Nb is 0.100%, more preferably is 0.050%, further preferably is 0.030%, further preferably is 0.025%, and further preferably is 0.020%.
  • Vanadium (V) is an optional element, and does not have to be contained. That is, the content of V may be 0%.
  • V When V is contained, V combines with C and/or N to form V carbo-nitrides and the like. These V carbo-nitrides and the like refine the grains of the steel material by a pinning effect. As a result, the strength of the steel material increases. If even a small amount of V is contained, the aforementioned advantageous effect will be obtained to a certain extent.
  • the toughness of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.
  • the content of V is 0 to 0.500%.
  • a preferable lower limit of the content of V is 0.001%, more preferably is 0.005%, and further preferably is 0.010%.
  • a preferable upper limit of the content of V is 0.300%, more preferably is 0.250%, further preferably is 0.200%, further preferably is 0.150%, further preferably is 0.120%, and further preferably is 0.100%.
  • Boron (B) is an optional element, and does not have to be contained. That is, the content of B may be 0%.
  • B dissolves in the steel material and increases hardenability of the steel material and increases the strength of the steel material. If even a small amount of B is contained, the aforementioned advantageous effect will be obtained to a certain extent.
  • the content of B is 0 to 0.0030%.
  • a preferable lower limit of the content of B is 0.0001%, more preferably is 0.0005%, and further preferably is 0.0008%.
  • a preferable upper limit of the content of B is 0.0028%, more preferably is 0.0025%, and further preferably is 0.0023%.
  • the yield strength of the steel material according to the present embodiment is 758 to less than 965 MPa (110 ksi grade to 125 ksi grade). In a case where the steel material of the present embodiment satisfies Feature 1 and Feature 3, the steel material has excellent SSC resistance even though the yield strength is a high strength of 758 to less than 965 MPa.
  • the yield strength is measured by the following method.
  • a tensile test is carried out in accordance with ASTM E8/E8M (2021). Specifically, a tensile test specimen is taken from the steel material.
  • the size of the tensile test specimen is not particularly limited. For example, a round bar tensile test specimen in which the diameter of the parallel portion is 6.0 mm and the gage length is 30.0 mm is used as the tensile test specimen.
  • the tensile test specimen is to be taken from a central portion of the wall thickness. In this case, the longitudinal direction of the tensile test specimen is to be made parallel to the axial direction of the steel pipe. If the steel material is a steel plate, the tensile test specimen is to be taken from a central portion of the thickness. In this case, the longitudinal direction of the tensile test specimen is to be made parallel to the rolling direction of the steel plate. If the steel material is a round steel bar, the tensile test specimen is to be taken from an R/2 portion. In the present description, the term "round steel bar" means a steel bar in which a cross section perpendicular to the axial direction is a circular shape.
  • R/2 portion means the central portion of a radius R in a cross section perpendicular to the axial direction (rolling direction) of the round steel bar.
  • the longitudinal direction of the tensile test specimen is to be made parallel to the axial direction of the round steel bar.
  • a tensile test is carried out in air at normal temperature (24 ⁇ 3°C) using the taken tensile test specimen.
  • the obtained 0.2% offset yield stress (MPa) is defined as the yield strength (MPa).
  • the total area fraction of tempered martensite and tempered bainite is 90% or more.
  • the balance of the microstructure is, for example, ferrite and/or pearlite.
  • the total area fraction of tempered martensite and tempered bainite will be 90% or more. Accordingly, if the chemical composition of a steel material satisfies Feature 1 and the yield strength of the steel material satisfies Feature 2, it can be regarded that the total area fraction of tempered martensite and tempered bainite in the microstructure of the relevant steel material is 90% or more.
  • the total area fraction of tempered martensite and tempered bainite can be determined by the following method. First, a test specimen is taken from the steel material.
  • a test specimen having an observation surface with dimensions of 10 mm in the pipe axis direction and 10 mm in the pipe radius direction is to be taken from a central portion of the wall thickness.
  • a test specimen having an observation surface with dimensions of 10 mm in the pipe axis direction and the wall thickness of the steel pipe in the pipe radius direction is to be taken.
  • the steel material is a steel plate
  • a test specimen having an observation surface with dimensions of 10 mm in the rolling direction and 10 mm in the thickness direction is to be taken from a central portion of the thickness.
  • a test specimen having an observation surface with dimensions of 10 mm in the rolling direction and the thickness of the steel plate in the thickness direction is to be taken.
  • a test specimen is to be taken from a cross section parallel to the axial direction (rolling direction) of the round steel bar. Specifically, a test specimen having an observation surface with dimensions of 10 mm in the axial direction and 10 mm in the radial direction of the aforementioned cross section and which includes the R/2 portion at the center thereof is to be taken. In a case where the diameter of the cross section is less than 10 mm, a test specimen having an observation surface with dimensions of 10 mm in the axial direction and the diameter of the round steel bar in the radial direction of the cross section and which includes the R/2 portion is to be taken.
  • the observation surface of the test specimen is polished to obtain a mirror surface.
  • the polished observation surface is then immersed for about 10 seconds in a nital etching reagent to perform etching of the observation surface.
  • the etched observation surface is observed in 10 visual fields by means of a secondary electron image using a scanning electron microscope (SEM).
  • the visual field area is, for example, 10,000 ⁇ m 2 (magnification of ⁇ 1000).
  • tempered martensite and tempered bainite are identified based on the contrast.
  • tempered martensite and tempered bainite can be distinguished from the other structures (ferrite, pearlite and the like) based on the morphology.
  • a structure that has a lamellar structure can be identified as pearlite.
  • Structures including a lath-shaped structure or a lens-shaped structure can be identified as tempered martensite and tempered bainite.
  • a structure without a substructure in the grain can be identified as ferrite.
  • the total area fraction of the identified tempered martensite and tempered bainite is determined.
  • the method for determining the total area fraction is not particularly limited, and a well-known method can be used.
  • the total area fraction of tempered martensite and tempered bainite can be determined by image analysis.
  • the arithmetic average value of the total area fractions of tempered martensite and tempered bainite determined in all of the visual fields (10 visual fields) is defined as the total area fraction (%) of tempered martensite and tempered bainite.
  • the elements Si, Cr, Mo, Zr, Sb, Cu, Ni, and Co are elements that suppress penetration of hydrogen from the steel material surface (hydrogen penetration suppressing elements).
  • the elements C and Mn are elements that promote penetration of hydrogen from the steel material surface (hydrogen penetration promoting elements).
  • EE is an index of an electrochemical hydrogen penetration suppression effect in the steel material.
  • a preferable lower limit of EE in a case where the yield strength is 110 ksi grade is 2.78, more preferably is 2.80, further preferably is 2.85, further preferably is 2.90, further preferably is 2.95, and further preferably is 3.00.
  • a preferable upper limit of EE is 6.60, more preferably is 6.00, further preferably is 5.80, and further preferably is 5.50.
  • the yield strength of the steel material is 125 ksi grade (862 to less than 965 MPa)
  • the strength is higher than 110 ksi grade
  • EE is 3.00 or more
  • the penetration of hydrogen from the steel material surface will be electrochemically suppressed.
  • FN is 0.200 or more
  • a preferable lower limit of EE in a case where the yield strength is 125 ksi grade is 3.10, more preferably is 3.15, further preferably is 3.20, further preferably is 3.25, further preferably is 3.30, and further preferably is 3.35.
  • a preferable upper limit of EE is 6.60, more preferably is 6.00, further preferably is 5.80, and further preferably is 5.50.
  • a synergistic effect between electrochemical elements EE defined by Formula (1) and a physical element is effective for suppressing penetration of hydrogen from the steel material surface.
  • a preferable lower limit of FN in a case where the yield strength is 110 ksi grade is 0.187, more preferably is 0.190, further preferably is 0.192, and further preferably is 0.195.
  • the upper limit of FN is not particularly limited, a preferable upper limit of FN is 0.580, more preferably is 0.550, further preferably is 0.500, and further preferably is 0.450.
  • a preferable lower limit of FN in a case where the yield strength is 125 ksi grade is 0.205, more preferably is 0.210, further preferably is 0.215, and further preferably is 0.220.
  • the upper limit of FN is not particularly limited, a preferable upper limit of FN is 0.580, more preferably is 0.550, further preferably is 0.500, and further preferably is 0.450.
  • the average equivalent circular diameter D ( ⁇ m) of prior-austenite grains of the steel material according to the present embodiment is determined by the following method. First, a test specimen is taken from the steel material.
  • a test specimen having an observation surface with dimensions of 10 mm in the pipe axis direction and 10 mm in the pipe radius direction is to be taken from a central portion of the wall thickness.
  • a test specimen having an observation surface with dimensions of 10 mm in the pipe axis direction and the wall thickness of the steel pipe in the pipe radius direction is to be taken.
  • the steel material is a steel plate
  • a test specimen having an observation surface with dimensions of 10 mm in the rolling direction and 10 mm in the thickness direction is to be taken from a central portion of the thickness.
  • a test specimen having an observation surface with dimensions of 10 mm in the rolling direction and the thickness of the steel plate in the thickness direction is to be taken.
  • a test specimen is to be taken from a cross section parallel to the axial direction (rolling direction) of the round steel bar. Specifically, a test specimen having an observation surface with dimensions of 10 mm in the axial direction and 10 mm in the radial direction of the aforementioned cross section and which includes the R/2 portion at the center thereof is to be taken. In a case where the diameter of the cross section is less than 10 mm, a test specimen having an observation surface with dimensions of 10 mm in the axial direction and the diameter of the round steel bar in the radial direction of the cross section and which includes the R/2 portion is to be taken.
  • the test specimen is embedded in resin, and the observation surface is polished to obtain a mirror surface.
  • the test specimen after polishing is immersed for about 60 seconds in an aqueous solution saturated with picric acid.
  • the observation surface is etched and prior-austenite grain boundaries are revealed on the observation surface.
  • Ten visual fields on the etched observation surface are observed at a magnification of ⁇ 420 using an optical microscope.
  • the visual field area of each visual field is set to a rectangle of 450 ⁇ m ⁇ 450 ⁇ m.
  • the grain size No. of each prior-austenite grain in each visual field is determined by an intercept method in accordance with JIS G 0551 (2020). At such time, the number of grid points, which are points of intersection between grid lines, is set to 16.
  • the arithmetic average value of the grain size Nos. of the prior-austenite grains determined in the 10 visual fields is determined.
  • the average area of the prior-austenite grains is calculated based on the arithmetic average value of the grain size Nos. of the prior-austenite grains.
  • the equivalent circular diameter is calculated based on the calculated average area of the prior-austenite grains.
  • the term "equivalent circular diameter” refers to the diameter of a circle having the same area as the average area of the prior-austenite grains.
  • the calculated equivalent circular diameter is defined as the average equivalent circular diameter D ( ⁇ m) of the prior-austenite grains.
  • the average equivalent circular diameter D is to be made an integer obtained by rounding off decimals of the calculated value.
  • a preferable upper limit of the average equivalent circular diameter of the prior-austenite grains is 40 ⁇ m, more preferably is 35 ⁇ m, further preferably is 30 ⁇ m, and further preferably is 25 ⁇ m.
  • a preferable lower limit of the average equivalent circular diameter of the prior-austenite grains is 10 ⁇ m, more preferably is 15 ⁇ m, and further preferably is 17 ⁇ m.
  • 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.
  • the steel material of the present embodiment is an oil-well steel pipe.
  • the oil-well steel pipe is, for example, a casing pipe, a tubing pipe, a drilling pipe or the like which are used for drilling an oil well or a gas well, extracting crude oil or natural gas, and the like.
  • the wall thickness is, for example, 9 to 60 mm.
  • the steel material of the present embodiment satisfies the aforementioned Feature 1 to Feature 3. Therefore, even though the steel material of the present embodiment has a high strength of 110 ksi grade (758 to less than 862 MPa) to 125 ksi grade (862 to less than 965 MPa), excellent SSC resistance is obtained.
  • the SSC resistance is evaluated by a normal-temperature SSC resistance evaluation test and a low-temperature SSC resistance evaluation test conducted in accordance with NACE TM0177-2016 Method A, and a DCB test conducted in accordance with NACE TM0177-2016 Method D.
  • NACE solution A a mixed aqueous solution containing 5.0% by mass of sodium chloride and 0.5% by mass of acetic acid
  • the round bar specimen is to be taken from a central portion of the wall thickness.
  • the longitudinal direction of the round bar specimen is to be made parallel to the axial direction of the steel pipe.
  • the steel material is a steel plate
  • the round bar specimen is to be taken from a central portion of the thickness.
  • the longitudinal direction of the round bar specimen is to be made parallel to the rolling direction of the steel plate.
  • the steel material is a round steel bar
  • the round bar specimen is to be taken from an R/2 portion.
  • the longitudinal direction of the round bar specimen is to be made parallel to the axial direction of the round steel bar.
  • the round bar specimen has a diameter of 6.35 mm and a parallel portion length of 25.4 mm. Three round bar specimens are taken from the steel material.
  • a stress equivalent to 90% of the actual yield stress is applied to each round bar specimen.
  • 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, and this is adopted as the test bath.
  • H 2 S gas is blown into the test bath and is caused to saturate in the test bath. Specifically, H 2 S gas at 1 atm is blown into the test bath.
  • the test bath into which the H 2 S gas has been blown is held at 24°C for 720 hours.
  • NACE solution A is adopted as the test solution.
  • three round bar specimens are taken from the steel material.
  • the round bar specimen has a diameter of 6.35 mm and a parallel portion length of 25.4 mm.
  • the longitudinal direction of the round bar specimen is to be made the same direction as in the case of the normal-temperature SSC resistance evaluation test.
  • a stress equivalent to 85% of the actual yield stress is applied to each round bar specimen.
  • the test solution at 4°C is poured into a test vessel so that the round bar specimen to which the stress has been applied is immersed therein, and this is adopted as the test bath.
  • H 2 S gas is blown into the test bath and is caused to saturate in the test bath. Specifically, H 2 S gas at 1 atm is blown into the test bath.
  • the test bath into which the H 2 S gas has been blown is held at 4°C for 720 hours.
  • each round bar specimen is observed to confirm whether or not sulfide stress cracking (SSC) occurred. Specifically, after being held for 720 hours, each round bar specimen is observed with the naked eye and using a projector with a magnification of ⁇ 10.
  • the DCB test is carried out by the following method.
  • An aqueous solution containing 5.0% by mass of sodium chloride is adopted as a test solution.
  • a DCB test specimen illustrated in FIG. 3A is taken from the steel material.
  • the DCB test specimen is to be taken from a central portion of the wall thickness.
  • the longitudinal direction of the DCB test specimen is to be made parallel to the axial direction of the steel pipe.
  • the steel material is a steel plate
  • the DCB test specimen is to be taken from a central portion of the thickness.
  • the longitudinal direction of the DCB test specimen is to be made parallel to the rolling direction of the steel plate.
  • the steel material is a round steel bar
  • the DCB test specimen is to be taken from an R/2 portion.
  • the longitudinal direction of the DCB test specimen is to be made parallel to the axial direction of the round steel bar.
  • a wedge illustrated in FIG. 3B is taken from the steel material.
  • a thickness t of the wedge is to be 3.10 (mm).
  • the aforementioned 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. Thereafter, 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 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.
  • H 2 S gas is blown into the degassed test bath to make the test bath a corrosive environment. Specifically, H 2 S gas at 5 atm (0.5 MPa) is blown into the test bath. The pH of the test bath is adjusted so as to be within the range of 3.5 to 4.0 throughout the immersion period. The inside of the test vessel is held at 24 ⁇ 3°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 during immersion in the test bath is measured.
  • the crack propagation length "a” is measured visually using a vernier calipers.
  • a fracture toughness value Kissc (MPa ⁇ m) is determined using the following formula based on the measured wedge releasing stress P and the crack propagation length "a".
  • K 1SSC Pa 2 3 + 2.38 h a B Bn 1 3 Bh 3 2
  • 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 fracture toughness value Kissc determined in the DCB test in a case where the steel material has a yield strength of 110 ksi grade (758 to less than 862 MPa), if the fracture toughness value Kissc determined in the DCB test is 25.0 MPa ⁇ m or more, it is determined that excellent fracture toughness was obtained in the DCB test. In a case where the steel material has a yield strength of 125 ksi grade (862 to less than 965 MPa), if the fracture toughness value Kissc determined in the DCB test is 24.0 MPa ⁇ m or more, it is determined that excellent fracture toughness was obtained in the DCB test.
  • the phrase "excellent SSC resistance is obtained" in a case where the steel material has a yield strength of 110 ksi grade (758 to less than 862 MPa) means that a crack is not confirmed in the normal-temperature SSC resistance evaluation test at 24°C and in the low-temperature SSC resistance evaluation test at 4°C which are conducted in accordance with NACE TM0177-2016 Method A, and also that a fracture toughness value Kissc obtained in the DCB test conducted in accordance with NACE TM0177-2016 Method D is 25.0 MPa ⁇ m or more.
  • the phrase "excellent SSC resistance is obtained" in a case where the steel material has a yield strength of 125 ksi grade (862 to less than 965 MPa) means that a crack is not confirmed in the normal-temperature SSC resistance evaluation test at 24°C and in the low-temperature SSC resistance evaluation test at 4°C which are conducted in accordance with NACE TM0177-2016 Method A, and also that a fracture toughness value Kissc obtained in the DCB test conducted in accordance with NACE TM0177-2016 Method D is 24.0 MPa ⁇ m or more.
  • a method for producing a seamless steel pipe will now be described as an example of a method for producing the steel material according to the present embodiment.
  • the method for producing the steel material described hereunder is one example for producing the steel material of the present embodiment. Accordingly, a steel material composed as described above may also be produced by a production method other than the production method described hereinafter. However, the production method described hereunder is a preferred example of the method for producing the steel material of the present embodiment.
  • One example of the method for producing a seamless steel pipe of the present embodiment includes the following processes.
  • the principal production condition in the above production method is as follows.
  • a starting material is produced using a molten steel having a chemical composition satisfying Feature 1.
  • 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.
  • 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 method of hot working for producing the intermediate steel material is not particularly limited.
  • the hot working may be hot forging, may be hot extrusion, or may be hot rolling.
  • the hot working process is, for example, as follows.
  • the starting material is subjected to blooming using a blooming mill to produce a billet.
  • the heating temperature before blooming is, for example, 1100 to 1350°C.
  • Piercing-rolling according to the Mannesmann process is performed using a billet produced by blooming or using a billet produced by the continuous casting process in the starting material preparation process.
  • the billet is heated in a heating furnace.
  • the heating temperature is, for example, 1100 to 1350°C.
  • the billet is subjected to piercing-rolling to produce an intermediate steel material (hollow shell).
  • the piercing ratio is, for example, 1.0 to 4.0.
  • the billet after piercing-rolling is subjected to elongating using a mandrel mill.
  • the billet after elongating is subjected to diameter adjusting rolling using a stretch reducing mill or a sizing mill.
  • An intermediate steel material (hollow shell) is produced by the above processes.
  • the cumulative reduction of area in the hot working process is, for example, 20 to 70%.
  • the billet may be subjected to the Ugine-Sejournet process or the Ehrhardt push bench process (that is, hot extrusion) to produce an intermediate steel material (hollow shell).
  • the hot working process is, for example, as follows.
  • a slab is subjected to rough rolling using a reverse rolling mill to produce a sheet bar.
  • the heating temperature before rough rolling is, for example, 1100 to 1350°C.
  • the sheet bar is subjected to finish rolling using a tandem rolling mill to produce an intermediate steel material (steel plate).
  • the hot working process is, for example, as follows.
  • the starting material is subjected to blooming using a blooming mill to produce a billet.
  • the heating temperature before blooming is, for example, 1100 to 1350°C.
  • the billet produced by blooming or the billet produced by a continuous casting process in the starting material preparation process is heated.
  • the heating temperature is, for example, 1100 to 1350°C.
  • the heated billet is subjected to finish rolling using a continuous mill to produce an intermediate steel material (round steel bar).
  • a continuous mill In 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 arranged in an alternating manner.
  • the intermediate steel material produced by the above hot working may be air-cooled.
  • the intermediate steel material produced by the hot working may also 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
  • stress relief annealing SR treatment
  • the intermediate steel material produced by the hot working process is subjected to quenching.
  • the 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 to be equal to or higher than the A c3 transformation point. However, if the quenching temperature is too high, the prior-austenite grains may become coarse. Therefore, the quenching temperature is, for example, 800 to 950°C.
  • 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 or oil cooling.
  • the quenching method is not particularly limited.
  • the intermediate steel material may be rapidly cooled by immersing the intermediate steel material in a water bath or an oil bath. If the intermediate steel material is a steel pipe, the steel pipe may be rapidly cooled by pouring or jetting cooling water onto the outer surface and/or the inner surface of the steel pipe by shower cooling or mist cooling.
  • quenching may be performed immediately after the hot working, without cooling the hollow shell to normal temperature after the hot working process. Further, quenching may be performed after the hollow shell 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 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. Further, in a case where quenching is performed after supplementary heating or reheating is performed after hot working, the quenching temperature corresponds to the temperature of the furnace that performs the supplementary heating or the reheating.
  • the intermediate steel material after quenching is further subjected to tempering.
  • the yield strength of the steel material can be adjusted by appropriately adjusting the tempering temperature according to the chemical composition. Specifically, the tempering conditions are adjusted so that the yield strength of the steel material becomes 110 ksi grade (758 to less than 862 MPa) to 125 ksi grade (862 to less than 965 MPa).
  • a tempering temperature T is to be set to 660 to 740°C, and a holding time t at the tempering temperature T is to be set to 20 to 180 minutes.
  • FA defined by Formula (A) is adjusted according to the strength. Specifically, if the yield strength of the steel material to be produced is 110 ksi grade, FA is to be 2500 or less. If the yield strength of the steel material to be produced is 125 ksi grade, FA is to be 2400 or less.
  • the steel material of the present embodiment can be produced by the processes described above.
  • the production method described above is a description of one example of the method for producing the steel material according to the present embodiment.
  • the steel material according to the present embodiment may also be produced by a production method other than the production method described above. Even in such a case, as long as the steel material satisfies Feature 1 to Feature 3, a high strength of 110 ksi grade (758 to less than 862 MPa) to 125 ksi grade (862 to less than 965 MPa) and excellent SSC resistance will be obtained.
  • the advantageous effect of the steel material of the present embodiment is described more specifically by way of examples.
  • the conditions adopted in the following examples are one example of conditions adopted for confirming the feasibility and advantageous effect of the steel material of the present embodiment. Accordingly, the steel material of the present embodiment is not limited to this one example of conditions.
  • Example 1 the SSC resistance of steel materials having a yield strength of 110 ksi grade (758 to less than 862 MPa) was investigated. Specifically, steel materials (seamless steel pipes) having the chemical compositions shown in Table 1-1 and Table 1-2 were produced.
  • Blooms were produced by a continuous casting process using molten steels. Thereafter, each bloom was subjected to blooming to produce a round billet with a diameter of 310 mm. The heating temperature before blooming was 1100 to 1350°C.
  • the round billets produced by the blooming were subjected to hot working. Specifically, the round billets were charged into a heating furnace, and heated at 1100 to 1350°C. After being taken out from the heating furnace, the round billets were subjected to hot rolling (hot working) by the Mannesmann process to thereby produce hollow shells (seamless steel pipes) of each test number. At such time, the piercing ratio was within the range of 1.0 to 4.0, and the cumulative reduction of area in the hot working was within the range of 20 to 70%.
  • the quenching temperature (°C) in the quenching is shown in the column “Quenching temperature (°C)" of the “Quenching condition” column in Table 2.
  • the holding time at the quenching temperature was set to 15 minutes.
  • the hollow shell after quenching was subjected to tempering.
  • the tempering temperature T (°C) in the tempering is shown in the column “Tempering temperature T (°C)” of the “Tempering conditions” column in Table 2.
  • the holding time t (min) at the tempering temperature T is shown in the column “Holding time t (min)” of the "Tempering conditions” column in Table 2.
  • FAin the tempering is shown in the column “FA” in Table 2.
  • Steel materials (seamless steel pipes) were produced by the above production process.
  • the steel material (seamless steel pipe) of each test number was subjected to the following evaluation tests.
  • the total area fraction (%) of tempered martensite and tempered bainite of the steel material of each test number was determined by the following method.
  • a test specimen having an observation surface with dimensions of 10 mm in the pipe axis direction and 10 mm in the pipe radius direction was taken from a central portion of the wall thickness of the steel material (seamless steel pipe) of each test number. Note that, in a case where the steel material was a steel pipe whose wall thickness was less than 10 mm, a test specimen having an observation surface with dimensions of 10 mm in the pipe axis direction and the wall thickness of the steel pipe in the pipe radius direction was taken.
  • the total area fraction (%) of tempered martensite and tempered bainite was determined by the method described above in the section [Microstructure observation method]. As a result, it was determined that the total area fraction of tempered martensite and tempered bainite in each test number was 90% or more.
  • the average equivalent circular diameter D ( ⁇ m) of prior-austenite grains of the steel material of each test number was determined by the following method.
  • a test specimen having an observation surface with dimensions of 10 mm in the pipe axis direction and 10 mm in the pipe radius direction was taken from a central portion of the wall thickness of the steel material (seamless steel pipe) of each test number. Note that, in a case where the steel material was a steel pipe whose wall thickness was less than 10 mm, a test specimen having an observation surface with dimensions of 10 mm in the pipe axis direction and the wall thickness of the steel pipe in the pipe radius direction was taken.
  • the average equivalent circular diameter D ( ⁇ m) of prior-austenite grains was determined by the method described above in the section [Method for determining average equivalent circular diameter D of prior-austenite grains].
  • the determined average equivalent circular diameter D of prior-austenite grains is shown in the column "D ( ⁇ m)" in Table 2.
  • the yield strength (MPa) of the steel material of each test number was determined by the following method.
  • a round bar tensile test specimen was taken from a central portion of the wall thickness of the steel material (seamless steel pipe) of each test number.
  • the diameter of the parallel portion was 6.0 mm and the gage length was 30.0 mm.
  • the longitudinal direction of the round bar tensile test specimen was parallel to the pipe axis direction of the steel material (seamless steel pipe).
  • the taken round bar tensile test specimen was used to determine the yield strength (MPa) by the method described above in the section [Method for measuring yield strength].
  • the determined yield strength is shown in the column “YS (MPa)" in Table 2.
  • the steel material of each test number was subjected to an SSC resistance evaluation test in accordance with NACE TM0177-2016 Method A by the following method.
  • the SSC resistance at 24°C was evaluated by the method described above in the section [Normal-temperature SSC resistance evaluation test]. Note that, round bar specimens were taken from the central portion of the wall thickness of the steel material (seamless steel pipe) of each test number. Regarding the size of the round bar specimen, the diameter was 6.35 mm and the length of the parallel portion was 25.4 mm. The longitudinal direction of the round bar specimen was parallel to the pipe axis direction of the steel material (seamless steel pipe). H 2 S gas at 1 atm was blown into the test bath.
  • the SSC resistance at 4°C was evaluated by the method described above in the section [Low-temperature SSC resistance evaluation test].
  • round bar specimens were taken from the central portion of the wall thickness of the steel material (seamless steel pipe) of each test number.
  • the diameter was 6.35 mm and the length of the parallel portion was 25.4 mm.
  • the longitudinal direction of the round bar specimen was parallel to the pipe axis direction of the steel material (seamless steel pipe). H 2 S gas at 1 atm was blown into the test bath.
  • the fracture toughness value Kissc (MPa ⁇ m) of the steel material of each test number was determined by the method described above in the section [DCB test]. The obtained fracture toughness value Kissc (MPa ⁇ m) is shown in the column “Kissc (MPa ⁇ m)" in Table 2.
  • a DCB test specimen illustrated in FIG. 3A was taken from the central portion of the wall thickness of the steel material (seamless steel pipe) of each test number.
  • the longitudinal direction of the DCB test specimen was parallel to the pipe axis direction of the steel material (seamless steel pipe).
  • a wedge illustrated in FIG. 3B was taken from the steel material.
  • a thickness t of the wedge was 3.10 mm.
  • H 2 S gas at 5 atm (0.5 MPa) was blown into the test bath.
  • Example 2 the SSC resistance of steel materials having a yield strength of 125 ksi grade (862 to less than 965 MPa) was investigated.
  • the round billets produced by the blooming were subjected to hot working. Specifically, the round billets were charged into a heating furnace, and heated at 1100 to 1350°C. After being taken out from the heating furnace, the round billets were subjected to hot rolling (hot working) by the Mannesmann process to thereby produce hollow shells (seamless steel pipes) of each test number. At such time, the piercing ratio was within the range of 1.0 to 4.0, and the cumulative reduction of area in the hot working was within the range of 20 to 70%.
  • the quenching temperature (°C) in the quenching is shown in the column “Quenching temperature (°C)" of the “Quenching condition” column in Table 3.
  • the holding time at the quenching temperature was set to 15 minutes.
  • the hollow shell after quenching was subjected to tempering.
  • the tempering temperature T (°C) in the tempering is shown in the column “Tempering temperature T (°C)” of the “Tempering conditions” column in Table 3.
  • the holding time t (min) at the tempering temperature T is shown in the column “Holding time t (min)” of the “Tempering conditions” column in Table 3.
  • FA in the tempering is shown in the column “FA” in Table 3.
  • Steel materials (seamless steel pipes) were produced by the above production process. Note that, EE of each test number is shown in the column “EE” in Table 3.
  • Example 2 the steel material (seamless steel pipe) of each test number was subjected to the following evaluation tests.
  • the total area fraction (%) of tempered martensite and tempered bainite of the steel material of each test number was determined. As a result, it was determined that the total area fraction of tempered martensite and tempered bainite in each test number was 90% or more.
  • the steel material of each test number was subjected to an SSC resistance evaluation test in accordance with NACE TM0177-2016 Method A by the following method.
  • the SSC resistance at 24°C was evaluated by the method described above in the section [Normal-temperature SSC resistance evaluation test]. Note that, round bar specimens were taken from the central portion of the wall thickness of the steel material (seamless steel pipe) of each test number. Regarding the size of the round bar specimen, the diameter was 6.35 mm and the length of the parallel portion was 25.4 mm. The longitudinal direction of the round bar specimen was parallel to the pipe axis direction of the steel material (seamless steel pipe). H 2 S gas at 1 atm was blown into the test bath.
  • the SSC resistance at 4°C was evaluated by the method described above in the section [Low-temperature SSC resistance evaluation test].
  • round bar specimens were taken from the central portion of the wall thickness of the steel material (seamless steel pipe) of each test number.
  • the diameter was 6.35 mm and the length of the parallel portion was 25.4 mm.
  • the longitudinal direction of the round bar specimen was parallel to the pipe axis direction of the steel material (seamless steel pipe). H 2 S gas at 1 atm was blown into the test bath.
  • the fracture toughness value Kissc (MPa ⁇ m) of the steel material of each test number was determined by the method described above in the section [DCB test]. The obtained fracture toughness value Kissc (MPa ⁇ m) is shown in the column "Kissc (MPa ⁇ m)" in Table 3.
  • a DCB test specimen illustrated in FIG. 3A was taken from the central portion of the wall thickness of the steel material (seamless steel pipe) of each test number.
  • the longitudinal direction of the DCB test specimen was parallel to the pipe axis direction of the steel material (seamless steel pipe).
  • a wedge illustrated in FIG. 3B was taken from the steel material.
  • a thickness t of the wedge was 3.10 mm.
  • H 2 S gas at 5 atm (0.5 MPa) was blown into the test bath.

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EP23791942.8A 2022-04-22 2023-04-21 Stahlmaterial Pending EP4512921A4 (de)

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