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  • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
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  • C21D1/18—Hardening; Quenching with or without subsequent tempering
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/26—Methods of annealing
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  • C21D6/00—Heat treatment of ferrous alloys
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
  • C21D8/105—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies of ferrous alloys
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  • 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
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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
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
  • C21D9/085—Cooling or quenching
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  • C22C—ALLOYS
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  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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  • 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
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  • 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
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  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
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  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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  • 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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  • 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
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—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
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
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  • 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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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
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  • 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
  • C21D2211/001—Austenite
• • C—CHEMISTRY; METALLURGY
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  • 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
  • C21D2211/005—Ferrite
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  • 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 invention relates to a stainless steel seamless pipe suitable for use in oil wells and gas wells (hereinafter, simply referred to as oil wells).
• the present invention relates particularly to a stainless steel seamless pipe having improved corrosion resistance under severely corrosive high-temperature environments containing carbon dioxide (CO 2 ) or chlorine ion (Cl - ), environments containing hydrogen sulfide (H 2 S), and the like and having an improved strength at high temperatures.
• 13 Cr martensitic stainless steel pipes have been ordinarily used as steel pipes for an oil well that are used for mining in oil fields and gas fields under environments containing CO 2 , Cl - , and the like.
• 13 Cr martensitic stainless steel pipes lack in corrosion resistance.
• steel pipes for an oil well having high corrosion resistance that can be used even under such environments.
• Patent Literature 1 describes a stainless steel for an oil well having a composition containing, in terms of mass%, C: 0.05% or less, Si: 1. 0 % or less, Mn: 0.01 to 1.0 %, P: 0.05% or less, S: less than 0.002%, Cr: 16 to 18%, Mo: 1.8 to 3%, Cu: 1.0 to 3.5%, Ni: 3.0 to 5.5%, Co: 0.01 to 1.0 %, Al: 0.001 to 0.1%, O: 0.05% or less, and N: 0.05% or less, in which Cr, Ni, Mo, and Cu satisfy a specific relationship.
• Patent Literature 2 describes a high strength stainless steel seamless pipe for an oil well having a composition containing, in terms of mass%, C: 0.05% or less, Si: 1.0% or less, Mn: 0.1 to 0.5%, P: 0.05% or less, S: less than 0.005%, Cr: more than 15.0% and 19.0% or less, Mo: more than 2.0% and 3.0% or less, Cu: 0.3 to 3.5%, Ni: 3.0% or more and less than 5.0%, W: 0.1 to 3.0%, Nb: 0.07 to 0.5%, V: 0.01 to 0.5%, Al: 0.001 to 0.1%, N : 0.010 to 0.100%, and O: 0.01% or less, in which Nb, Ta, C, N, and Cu satisfy a specific relationship, and having a microstructure including, in terms of volume fraction, 45% or more of a tempered martensitic phase, 20 to 40% of a ferritic phase, and more than 10% and 25% or less of a retained austenitic phase.
• Patent Literature 3 describes a high strength stainless steel seamless pipe for an oil well having a composition containing, in terms of mass%, C: 0.005 to 0.05%, Si: 0.05 to 0.50%, Mn: 0.20 to 1.80%, P: 0.030% or less, S: 0.005% or less, Cr: 12.0 to 17.0%, Ni: 4.0 to 7.0%, Mo: 0.5 to 3.0%, Al: 0.005 to 0.10%, V: 0.005 to 0.20%, Co: 0.01 to 1.0%, N: 0.005 to 0.15%, and O: 0.010% or less, in which Cr, Ni, Mo, Cu, C, Si, Mn, and N satisfy a specific relationship.
• Patent Literature 4 describes a high strength stainless steel seamless pipe for an oil well having a composition containing, in terms of mass%, C: 0.05% or less, Si: 0.5% or less, Mn: 0.15 to 1.0%, P: 0.030% or less, S: 0.005% or less, Cr: 14.5 to 17.5%, Ni: 3.0 to 6.0%, Mo: 2.7 to 5.0%, Cu: 0.3 to 4.0%, W: 0.1 to 2.5%, V: 0.02 to 0.20%, Al: 0.10% or less, and N: 0.15% or less, in which C, Si, Mn, Cr, Ni, Mo, Cu, N, and W satisfy a specific relationship, and having a microstructure containing, in terms of volume fraction, more than 45% of a martensitic phase as the main phase, 10 to 45% of a ferritic phase as the second phase, and 30% or less of a retained austenitic phase.
• Patent Literature 5 describes a high strength stainless steel seamless pipe for an oil well having a composition containing, in terms of mass%, C: 0.05% or less, Si: 0.5% or less, Mn: 0.15 to 1.0%, P: 0.030% or less, S: 0.005% or less, Cr: 14.5 to 17.5%, Ni: 3.0 to 6.0%, Mo: 2.7 to 5.0%, Cu: 0.3 to 4.0%, W: 0.1 to 2.5%, V: 0.02 to 0.20%, Al: 0.10% or less, N: 0.15% or less, and B: 0.0005 to 0.0100%, in which C, Si, Mn, Cr, Ni, Mo, Cu, N, and W satisfy a specific relationship, and having a microstructure containing, in terms of volume fraction, more than 45% of a martensitic phase as the main phase, 10 to 45% of a ferritic phase as the second phase, and 30% or less of a retained austenitic phase.
• Patent Literature 1 to Patent Literature 5 disclose stainless steels having improved corrosion resistance, but there is a case where the stainless steels are not satisfactory in terms of having all of corrosion resistance at high temperatures, high sulfide stress cracking resistance, and a high elevated temperature strength.
• aspects of the present invention intend to solve such a problem of the related art, and an object according to aspects of the present invention is to provide a stainless steel seamless pipe having a high strength of 758 MPa (110 ksi) or higher in terms of yield strength, excellent corrosion resistance, and an excellent elevated temperature strength and a method for manufacturing the same.
• excellent corrosion resistance refers to a case where the stainless steel seamless pipe has “excellent carbon dioxide corrosion resistance” and “excellent sulfide stress cracking resistance”.
• excellent carbon dioxide corrosion resistance refers to a case where the corrosion rate is 0.127 mm/y or slower in a test in which a test piece is immersed in a test solution held in an autoclave: 20 mass% NaCl aqueous solution (liquid temperature: 200° C., CO 2 gas atmosphere of 30 atm) for an immersion time set to 336 hours.
• excellent sulfide stress cracking resistance refers to a case where a test piece does not break or crack after a test that is carried out by immersing the test piece in an aqueous solution having a pH adjusted to 3.0 by adding acetic acid and sodium acetate to a test solution held in an autoclave: 0.165 mass% NaCl aqueous solution (liquid temperature: 25° C., CO 2 gas of 0.99 atm, H 2 S atmosphere of 0.01 atm) and exposing the test piece for 720 hours in a state in which 90% of the yield stress is applied to the test piece.
• excellent elevated temperature strength refers to a case where the ratio of the yield stress (0.2% proof stress) at 200° C. to the yield stress (0.2% proof stress) at room temperature is 0.85 or more after carrying out a tensile test based on the specification of JIS Z 2241 and an elevated temperature tensile test based on the specification of JIS G 0567.
• the present inventors carried out intensive studies regarding a variety of factors affecting the elevated temperature strength and corrosion resistance of stainless steel. As a result, it was possible to obtain an excellent elevated temperature strength by containing a predetermined amount or more of V. In addition, it was possible to obtain excellent corrosion resistance (excellent carbon dioxide corrosion resistance and excellent sulfide stress cracking resistance) by containing a predetermined amount or more of Cr, Mo, and Cu and, additionally, setting the Mn content to a certain amount or less.
• each of C, Si, Mn, Cr, Ni, Mo, Cu, and N is the content (mass%) of each element and is regarded as zero in a case of being not contained.
• the stainless steel seamless pipe according to [1] in which the volume fraction of the martensitic phase is 40% or more, the volume fraction of the retained austenitic phase is 30% or less, and the yield strength is 862 MPa or higher.
• the stainless steel seamless pipe according to [1] or [2], further contains, in addition to the component composition, in terms of mass%, one group or two or more groups selected from Group A to Group D below:
• a stainless steel seamless pipe has a component composition containing, in terms of mass%, C: 0.06% or less, XSi: 1.0% or less, Mn: 0.01% or more and 0.90% or less, P: 0.05% or less, S: 0.005% or less, Cr: 15.70% or more and 18.00% or less, Mo: 1.60% or more and 3.80% or less, Cu: 1.10% or more and 4.00% or less, Ni: 3.0% or more and 6.0% or less, Al: 0.10% or less, N: 0.10% or less, O: 0.010% or less, V: 0.120% or more and 1.000% or less, C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfying Formula (1) below, a remainder consisting of Fe and unavoidable impurities, in which the stainless steel seamless pipe has a microstructure including, in terms of volume fraction, 30% or more of a martensitic phase, 60% or less of a ferritic phase, and 40% or less of a
• each of C, Si, Mn, Cr, Ni, Mo, Cu, and N is the content (mass%) of each element and is regarded as zero in a case of being not contained.
• C is an element that is unavoidably contained in steelmaking processes.
• the C content is set to 0.06% or less.
• the C content is preferably 0.05% or less, more preferably 0.04% or less, and still more preferably 0.03% or less.
• the lower limit of the C content is preferably 0.002% and more preferably 0.003% or more.
• Si is an element that acts as a deoxidizing agent. However, when more than 1.0% of Si is contained, the hot workability and the corrosion resistance deteriorate. Therefore, the Si content is set to 1.0% or less.
• the Si content is preferably 0.7% or less, more preferably 0.5% or less, and still more preferably 0.4% or less.
• the lower limit is not particularly provided as long as a deoxidizing effect can be obtained. However, for the purpose of obtaining a sufficient deoxidizing effect, the Si content is preferably 0.03% or more and more preferably 0.05% or more.
• Mn 0.01% or More and 0.90% or Less
• Mn is an element that acts as a deoxidizing material and a desulfurizing material and improves hot workability.
• the Mn content is set to 0.01% or more.
• the Mn content is set to 0.01% or more and 0.90% or less.
• the Mn content is preferably 0.03% or more and more preferably 0.05% or more.
• the Mn content is preferably 0.7% or less, more preferably 0.5% or less, and still more preferably 0.4% or less.
• P is an element that degrades the carbon dioxide corrosion resistance and the sulfide stress cracking resistance and is preferably reduced as much as possible in accordance with aspects of the present invention, but 0.05% or less of P is acceptable. Therefore, the P content is set to 0.05% or less.
• the P content is preferably 0.04% or less, more preferably 0.03% or less, and still more preferably 0.02% or less.
• S is an element that significantly degrades the hot workability and impairs stable operation in hot pipe making processes.
• S is present as a sulfide-based inclusion in steel and degrades the sulfide stress cracking resistance. Therefore, S is preferably reduced as much as possible, but 0.005% or less of S is acceptable. Therefore, the S content is set to 0.005% or less.
• the S content is preferably 0.004% or less, more preferably 0.003% or less, and still more preferably 0.002% or less.
• Cr is an element that forms a protective film on the surface of the steel pipe and contributes to improvement in the corrosion resistance.
• the Cr content is set to 15.70% or more and 18.00% or less.
• the Cr content is preferably 16.00% or more and more preferably 16.30% or more.
• the Cr content is preferably 17.50% or less and more preferably 17.00% or less.
• Mo stabilizes the protective film on the surface of the steel pipe, increases resistance to pitting corrosion caused by Cl - or a low pH, and enhances the carbon dioxide corrosion resistance and the sulfide stress cracking resistance.
• it is necessary to contain 1.60% or more of Mo.
• the Mo content is set to 1.60% or more and 3.80% or less.
• the Mo content is preferably 1.80% or more and more preferably 2.00% or more.
• the Mo content is preferably 3.5% or less, more preferably 3.0% or less, and still more preferably 2.8% or less.
• Cu has effects of strengthening the protective film on the surface of the steel pipe and enhancing the carbon dioxide corrosion resistance and the sulfide stress cracking resistance.
• the Cu content is set to 4.00% or less. Therefore, the Cu content is set to 1.10% or more and 4.00% or less.
• the Cu content is preferably 1.80% or more and more preferably 2.00% or more.
• the Cu content is preferably 3.20% or less, more preferably 3.00% or less, and still more preferably 2.7% or less.
• Ni increases the strength of steel by solid solution strengthening and improves the toughness of steel.
• it is necessary to contain 3. 0% or more of Ni.
• the Ni content is set to 3.0% or more and 6.0% or less.
• the Ni content is preferably 3.5% or more, more preferably 4.0% or more, and still more preferably 4.5% or more.
• the Ni content is preferably 5.5% or less and more preferably 5.2% or less.
• Al is an element that acts as a deoxidizing agent. However, when more than 0.10% of Al is contained, the corrosion resistance deteriorates. Therefore, the Al content is set to 0.10% or less.
• the Al content is preferably 0.07% or less, more preferably 0.05% or less, and still more preferably 0.04% or less.
• the lower limit is not particularly provided as long as a deoxidizing effect can be obtained. However, for the purpose of obtaining a sufficient deoxidizing effect, the Al content is preferably 0.005% or more and more preferably 0.01% or more.
• N is an element that is unavoidably contained in steelmaking processes, but also an element that increases the strength of steel. However, when more than 0.10% of N is contained, a nitride is formed to degrade the corrosion resistance. Therefore, the N content is set to 0.10% or less.
• the N content is preferably 0.08% or less, more preferably 0.05% or less, and still more preferably 0.03% or less.
• the lower limit value of the N content is not particularly provided, but the excessive reduction of the N content causes an increase in the steelmaking cost. Therefore, the N content is preferably 0.002% or more and more preferably 0.003% or more.
• Oxygen (O) is present as an oxide in steel and thus adversely affects a variety of characteristics. Therefore, in accordance with aspects of the present invention, oxygen is desirably reduced as much as possible. In particular, when O content is more than 0.010%, the hot workability and the corrosion resistance deteriorate. Therefore, the O content is set to 0.010% or less. The O content is preferably 0.005% or less.
• V 0.120% or More and 1.000% or Less
• V is an important element in accordance with aspects of the present invention that improves the elevated temperature strength.
• V forms a carbonitride and enables a high strength to be obtained not only at room temperature but also at high temperatures by precipitation strengthening.
• 0.120% or more of V is contained.
• the V content is set to 0.120% or more and 1.000% or less.
• the V content is preferably 0.180% or more, more preferably 0.250% or more, and still more preferably 0.300% or more.
• the V content is preferably 0.500% or less, more preferably 0.400% or less, and still more preferably 0.300% or less.
• the stainless steel seamless pipe contains the elements such that the above-described component composition is satisfied and, furthermore, C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfy Formula (1) below.
• each of C, Si, Mn, Cr, Ni, Mo, Cu, and N is the content (mass%) of each element and is regarded as zero in a case of being not contained.
• the value of the central polynomial expression of Formula (1) is less than 13.0, a ferritic phase decreases in a hot working temperature range, and the yield at the time of manufacturing the stainless steel seamless pipe is decreased.
• the value of the central polynomial expression of Formula (1) exceeds 50.0, the ferritic phase exceeds 60% in terms of the volume fraction, and it becomes impossible to secure a desired strength. Therefore, in Formula (1) specified in accordance with aspects of the present invention, the value of the left side, which serves as the lower limit, is set to 13.0, and the value of the right side, which serves as the upper limit, is set to 50.0.
• the value of the left side, which serves as the lower limit, in Formula (1) specified in accordance with aspects of the present invention is preferably 15.0 and more preferably 20.0.
• the value of the right side is preferably 45.0 and more preferably 40.0. That is, the value of the central polynomial expression in Formula (1) is set to 13.0 or more and set to 50.0 or less.
• the value is preferably set to 15.0 or more and set to 45.0 or less.
• the value is more preferably set to 20.0 or more and set to 40.0 or less.
• the remainder other than the above-described component composition includes Fe and unavoidable impurities.
• the above-described essential elements make it possible for the stainless steel seamless pipe according to aspects of the present invention to obtain target characteristics.
• one or more of the following selective elements W, Nb, B, Ta, Co, Ti, Zr, Ca, REM, Mg, Sn, and Sb may be contained as necessary.
• Nb less than 0.10%.
• the present invention it is possible to contain, in addition to the above-described component composition, one or more selected from B: 0.010% or less, Ta: 0.3% or less, Co: 1.5% or less, Ti: 0.3% or less, and Zr: 0.3% or less.
• W is an element that contributes to improvement in the strength of steel and is capable of enhancing the carbon dioxide corrosion resistance and the sulfide stress cracking resistance by stabilizing the protective film on the surface of the steel pipe.
• W significantly improves, particularly, the corrosion resistance when contained in combination with Mo.
• W can be contained as necessary in order to obtain the above-described effects.
• the W content is preferably set to 3.0% or less.
• the W content is more preferably less than 1.5% and still more preferably 1.0% or less.
• the W content is more preferably 0.05% or more and still more preferably 0.10% or more.
• Nb is an element that increases the strength and also an element that improves the corrosion resistance and can be contained as necessary.
• the Nb content is preferably set to less than 0.10%.
• the Nb content is more preferably 0.05% or less and still more preferably 0.03% or less.
• the Nb content is more preferably 0.005% or more and still more preferably 0.010% or more.
• B is an element that increases the strength and can be contained as necessary.
• B also contributes to improvement in the hot workability and also has an effect of suppressing the occurrence of fissuring or cracking in pipe making processes.
• the B content is preferably set to 0.010% or less.
• the B content is more preferably 0.008% or less and still more preferably 0.007% or less.
• the B content is more preferably 0.0005% or more and still more preferably 0.0010% or more.
• Ta is an element that increases the strength and also an element that improves the corrosion resistance and can be contained as necessary. In order to obtain such an effect, 0.001% or more of Ta is preferably contained. On the other hand, even when more than 0.3% of Ta is contained, the effect is saturated. Therefore, in the case of containing Ta, the Ta content is preferably limited to 0.3% or less.
• the Ta content is more preferably 0.25% or less, still more preferably 0.06% or less, more preferably 0.050% or less, and still more preferably 0.025% or less.
• the Ta content is more preferably 0.005% or more.
• Co is an element that increases the strength and can be contained as necessary. In addition to the above-described effect, Co also has an effect of improving the corrosion resistance. In order to obtain such an effect, 0.0005% or more of Co is preferably contained.
• the Co content is more preferably 0.005% or more and still more preferably 0.010% or more. On the other hand, even when more than 1.5% of Co is contained, the effect is saturated. Therefore, in the case of containing Co, the Co content is preferably limited to 1.5% or less. The Co content is more preferably less than 0.150%.
• Ti is an element that increases the strength and can be contained as necessary. In order to obtain such an effect, 0.0005% or more of Ti is preferably contained. On the other hand, when more than 0.3% of Ti is contained, the toughness deteriorates. Therefore, in the case of containing Ti, the Ti content is preferably limited to 0.3% or less.
• Zr is an element that increases the strength and can be contained as necessary.
• Zr also has an effect of improving the sulfide stress cracking resistance.
• 0.0005% or more of Zr is preferably contained.
• the Zr content is preferably limited to 0.3% or less.
• Ca is an element that contributes to improvement in the sulfide stress cracking resistance by the control of the form of a sulfide and can be contained as necessary. In order to obtain such an effect, 0.0005% or more of Ca is preferably contained. On the other hand, even when more than 0.01% of Ca is contained, the effect is saturated, and it becomes impossible to expect an effect corresponding to the content. Therefore, in the case of containing Ca, the Ca content is preferably limited to 0.01% or less.
• REM rare earth metal
• REM is an element that contributes to improvement in the sulfide stress cracking resistance by the control of the form of a sulfide and can be contained as necessary. In order to obtain such an effect, 0.0005% or more of REM is preferably contained. On the other hand, even when more than 0.3% of REM is contained, the effect is saturated, and it becomes impossible to expect an effect corresponding to the content. Therefore, in the case of containing REM, the REM content is preferably limited to 0.3% or less.
• the REM mentioned in accordance with aspects of the present invention refers to lanthanoid elements of scandium (Sc) (atomic number: 21) and yttrium (Y) (atomic number: 39), and lanthanum (La) (atomic number: 57) to lutetium (Lu) (atomic number: 71).
• the REM concentration in accordance with aspects of the present invention is the total content of one or more elements selected from the above-described REMs.
• Mg is an element that improves the corrosion resistance and can be contained as necessary. In order to obtain such an effect, 0.0005% or more of Mg is preferably contained. On the other hand, even when more than 0.01% of Mg is contained, the effect is saturated, and it becomes impossible to expect an effect corresponding to the content. Therefore, in the case of containing Mg, the Mg content is preferably limited to 0.01% or less.
• Sn is an element that improves the corrosion resistance and can be contained as necessary. In order to obtain such an effect, 0.001% or more of Sn is preferably contained. On the other hand, even when more than 1.0% of Sn is contained, the effect is saturated, and it becomes impossible to expect an effect corresponding to the content. Therefore, in the case of containing Sn, the Sn content is preferably limited to 1.0% or less.
• Sb is an element that improves the corrosion resistance and can be contained as necessary. In order to obtain such an effect, 0.001% or more of Sb is preferably contained. On the other hand, even when more than 1.0% of Sb is contained, the effect is saturated, and it becomes impossible to expect an effect corresponding to the content. Therefore, in the case of containing Sb, the Sb content is preferably limited to 1.0% or less.
• the stainless steel seamless pipe according to aspects of the present invention has the above-described component composition and has a microstructure including, in terms of volume fraction, 30% or more of a martensitic phase, 60% or less of a ferritic phase, and 40% or less of a retained austenitic phase.
• a martensitic phase is set to 30% or more in terms of the volume fraction.
• the martensitic phase is preferably set to 40% or more.
• the martensitic phase is preferably set to 70% or less and more preferably set to 65% or less.
• the volume fraction of ferritic phase included is 60% or less.
• a ferritic phase is included, it is possible to suppress the propagation of sulfide stress cracking, and excellent corrosion resistance can be obtained.
• the volume fraction exceeds 60%, and a large amount of a ferritic phase is precipitated, there is a case where it becomes impossible to secure a desired strength.
• the ferritic phase is preferably 5% or more in terms of the volume fraction.
• the ferritic phase is more preferably 10% or more.
• the ferritic phase is preferably 50% or less in terms of the volume fraction.
• the ferritic phase is more preferably 45% or less.
• the retained austenitic phase in addition to the martensitic phase and the ferritic phase, 40% or less of an austenitic phase (retained austenitic phase) is included in terms of the volume fraction.
• the presence of the retained austenitic phase improves the ductility and the toughness.
• the retained austenitic phase is set to 40% or less in terms of the volume fraction.
• the retained austenitic phase is preferably 5% or more in terms of the volume fraction.
• the retained austenitic phase is 35% or less in terms of the volume fraction.
• the retained austenitic phase is more preferably 30% or less in terms of the volume fraction.
• the above-described microstructure of the stainless steel seamless pipe can be measured by the following method.
• a test piece for microstructure observation is corroded with a Vilella’s reagent (a reagent obtained by mixing picric acid, hydrochloric acid, and ethanol in fractions of 2 g, 10 ml, and 100 ml, respectively), an image of the microstructure is captured with a scanning electron microscope (magnification: 1000 times), and the microstructural fraction (area ratio (%)) of the ferritic phase is calculated using an image analyzer. This area ratio is defined as the volume fraction (%) of the ferritic phase.
• a test piece for X-ray diffraction is ground and polished such that a cross section orthogonal to the pipe axial direction (C cross section) becomes a measurement surface, and the structural fraction of the retained austenitic ( ⁇ ) phase is measured using the X-ray diffraction method.
• the microstructural fraction of the retained austenitic phase is obtained by measuring the diffraction X-ray integrated intensities of the (220) plane of ⁇ and the (211) plane of ⁇ (ferrite) and converting the diffraction X-ray integrated intensities using the following equation.
• volume fraction 100 / 1 + I ⁇ R ⁇ / I ⁇ R ⁇
• I ⁇ indicates the integrated intensity of ⁇
• R ⁇ indicates the crystallographic theoretical calculation value of ⁇
• I ⁇ indicates the integrated intensity of ⁇
• R ⁇ indicates the crystallographic theoretical calculation value of ⁇ .
• the remainder other than the ferritic phase and the retained ⁇ phase obtained by the above-described measurement method is defined as the fraction of the martensitic phase.
• molten steel having the above-described component composition is melted by a common melting method such as a converter and made into a steel pipe material such as a billet by an ordinary method such as a continuous casting method or an ingot-blooming method.
• the heating temperature of the steel pipe material before hot working is preferably 1100° C. to 1350° C. In such a case, it is possible to satisfy both hot workability during pipe making and the low-temperature toughness of a final product.
• the obtained steel pipe material is made into a pipe by hot working using a Mannesmann plug mill-type or Mannesmann mandrel mill-type step, which is an ordinary well-known pipe-making method, thereby producing a seamless steel pipe having desired dimensions and the above-described composition.
• a cooling treatment may be carried out. There is no particular need to limit this cooling treatment (cooling step).
• a heat treatment including a quenching treatment and a tempering treatment is further carried out on the obtained seamless steel pipe.
• the quenching treatment is a treatment in which the seamless steel pipe is reheated to a temperature within a heating temperature range of 850° C. to 1150° C. and then cooled at a cooling rate for air cooling or faster.
• the cooling stop temperature at this time is 50° C. or lower in terms of the surface temperature of the seamless steel pipe.
• the heating temperature of the quenching treatment is set to a temperature within a range of 850° C. to 1150° C.
• the heating temperature of the quenching treatment is preferably 900° C. or higher.
• the heating temperature of the quenching treatment is preferably 1100° C. or lower.
• the cooling stop temperature during the cooling in the quenching treatment is set to 50° C. or lower.
• the expression “cooling rate for air cooling or faster” is 0.01° C./s or faster.
• the soaking time is preferably set to 5 to 30 minutes in order to uniform the temperature in the wall thickness direction and prevent variations in the quality of the material.
• the tempering treatment is a treatment in which the seamless steel pipe that has undergone the quenching treatment is heated to a tempering temperature of 500° C. to 650° C. In addition, after this heating, it is possible to cool the seamless steel pipe in the air.
• the tempering temperature is set to a temperature within a range of 500° C. to 650° C.
• the tempering temperature is preferably 520° C. or higher.
• the tempering temperature is preferably 630° C. or lower.
• the holding time (soaking holding time) is preferably set to 5 to 90 minutes in order to uniform the temperature in the wall thickness direction and prevent variations in the quality of the material.
• the microstructure of the seamless steel pipe becomes a microstructure including a martensitic phase, a ferritic phase, and a retained austenitic phase that are specified by predetermined volume fractions. This makes it possible to obtain a stainless steel seamless pipe having a desired strength and excellent corrosion resistance.
• stainless steel seamless pipes that are obtained in accordance with aspects of the present invention are high-strength steel pipes having a yield strength of 758 MPa or higher and have excellent corrosion resistance and an elevated temperature strength.
• the yield strength is preferably 862 MPa or higher.
• the yield strength is preferably 1034 MPa or lower.
• the stainless steel seamless pipe according to aspects of the present invention can be made into a stainless steel seamless pipe for an oil well (high strength stainless steel seamless pipe for an oil well).
• Steel pipe materials were cast using molten steels having a component composition shown in Table 1-1 and Table 1-2. After that, the steel pipe materials were heated and made into pipes by hot working using a model seamless rolling mill, thereby producing seamless steel pipes having an outer diameter of 83.8 mm and a wall thickness of 12.7 mm. The seamless steel pipes were air-cooled. At this time, the heating temperature of the steel pipe materials before the hot working was set to 1250° C.
• Test piece materials were cut out from the obtained seamless steel pipes, and a quenching treatment was carried out by reheating the test piece materials to a heating temperature of 960° C., holding the test piece materials for a soaking holding time of 20 minutes, and cooling (water-cooling) the test piece materials to a cooling stop temperature of 30° C.
• a tempering treatment was carried out by holding the test piece materials at a heating temperature (tempering temperature) of 575° C. for a soaking holding time of 20 minutes, at a heating temperature (tempering temperature) of 525° C. for a soaking holding time of 20 minutes, or at a heating temperature (tempering temperature) of 620° C.
• Blank cells in Table 1-1 and Table 1-2 indicate that the corresponding element is intentionally not added, which includes not only a case where the element is not contained (0%) but also a case where the element is unavoidably contained.
• Test pieces were collected from the obtained heat treatment-finished test piece materials (seamless steel pipes), and microstructure observation, tensile tests, elevated temperature tensile tests, and corrosion resistance tests were carried out.
• the test methods were as described below.
• a test piece for microstructure observation was collected such that a cross section orthogonal to the pipe axial direction became an observation surface.
• the obtained test piece for microstructure observation was corroded with a Vilella’s reagent (a reagent obtained by mixing picric acid, hydrochloric acid, and ethanol in fractions of 2 g, 10 ml, and 100 ml, respectively), an image of the microstructure was captured with a scanning electron microscope (magnification: 1000 times), and the microstructural fraction (area ratio (%)) of the ferritic phase was calculated using an image analyzer. This area ratio was defined as the volume fraction (%) of the ferritic phase.
• a test piece for X-ray diffraction was collected from the obtained heat treatment-finished test material, ground and polished such that a cross section orthogonal to the pipe axial direction (C cross section) became a measurement surface, and the microstructural fraction of a retained austenite ( ⁇ ) phase was measured using the X-ray diffraction method.
• the microstructural fraction of the retained austenitic phase was obtained by measuring the diffraction X-ray integrated intensities of the (220) plane of ⁇ and the (211) plane of ⁇ (ferrite) and converting the diffraction X-ray integrated intensities using the following equation.
• volume fraction 100 / 1 + I ⁇ R ⁇ / I ⁇ R ⁇
• I ⁇ indicates the integrated intensity of ⁇
• R ⁇ indicates the crystallographic theoretical calculation value of ⁇
• I ⁇ indicates the integrated intensity of ⁇
• R ⁇ indicates the crystallographic theoretical calculation value of ⁇ .
• the fraction of a martensitic phase is the remainder other than the ferritic phase and the retained ⁇ phase.
• test pieces having a yield strength (YS) of 758 MPa or higher were regarded as having a high strength and determined as pass, and test pieces having a yield strength of lower than 758 MPa were determined as fail.
• corrosion test pieces having a thickness of 3 mm, a width of 30 mm, and a length of 40 mm were produced by machining. Corrosion tests were carried out using the corrosion test pieces, and the carbon dioxide corrosion resistance was evaluated.
• the corrosion test for evaluating the carbon dioxide corrosion resistance was carried out by immersing the corrosion test piece in a test solution held in an autoclave: 20 mass% NaCl aqueous solution (liquid temperature: 200° C., CO 2 gas atmosphere of 30 atm) for an immersion period set to 14 days (336 hours). The weight of the test piece after the test was measured, and the corrosion rate calculated from the weight reduced before and after the corrosion test was obtained. Test pieces having a corrosion rate of 0.127 mm/y or slower were determined as pass, and test pieces exceeding 0.127 mm/y were determined as fail.
• round rod-shaped test pieces (diameter: 3.81 mm) were prepared from the obtained test piece materials by machining, and sulfide stress cracking resistance tests (SSC resistance tests) were carried out.
• test pieces were immersed in an aqueous solution having a pH adjusted to 3.0 by adding acetic acid and sodium acetate to a test solution held in an autoclave: 0.165 mass% NaCl aqueous solution (liquid temperature: 25° C., CO 2 gas of 0.99 atm, H 2 S atmosphere of 0.01 atm) and exposed for 720 hours in a state in which 90% of the yield stress was applied to the test pieces, and whether or not the test pieces after the test broke or cracked was observed.
• Test pieces that neither broke nor cracked were determined as pass (indicated by a symbol “0” in Table 2-1 and Table 2-2), and test pieces that broke or cracked were determined as fail (indicated by a symbol “X” in Table 2-1 and Table 2-2).
• the elevated temperature strength refers to the ratio of the yield stress (0.2% proof stress) at 200° C. to the yield stress (0.2% proof stress) at room temperature.
• the elevated temperature strength refers to the ratio of the yield stress (0.2% proof stress) at 200° C. to the yield stress (0.2% proof stress) at room temperature.
• the present invention examples were all stainless steel seamless pipes having a high strength of 758 MPa or higher in terms of the yield strength of YS, excellent corrosion resistance (carbon dioxide corrosion resistance) under highly corrosive high-temperature (200° C.) environments containing CO 2 and Cl - , excellent sulfide stress cracking resistance, and an excellent elevated temperature strength.

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