Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
  • C21D1/22—Martempering
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
  • C21D8/105—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies of ferrous alloys
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/08—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
• • 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
• • 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
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• the present invention relates to a martensitic stainless steel seamless pipe for oil country tubular goods for use in crude oil well and natural gas well applications (hereinafter, referred to simply as “oil country tubular goods”), and to a method for manufacturing such a martensitic stainless steel seamless pipe.
• the invention relates to a seamless steel pipe for oil country tubular goods having a yield stress YS of 758 MPa or more, and excellent sulfide stress corrosion cracking resistance (SSC resistance) in a hydrogen sulfide (H 2 S)-containing environment, and to a method for manufacturing such a seamless steel pipe.
• Oil country tubular goods used for mining of oil fields and gas fields of an environment containing carbon dioxide gas, chlorine ions, and the like typically use 13% Cr martensitic stainless steel pipes.
• PTL 1 describes a 13% Cr-base martensitic stainless steel pipe of a composition containing carbon in an ultra low content of 0.015% or less, and 0.03% or more of Ti. It is stated in PTL 1 that this stainless steel pipe has high strength with a yield stress on the order of 95 ksi, low hardness with an HRC of less than 27, and excellent SSC resistance.
• PTL 2 describes a martensitic stainless steel that satisfies 6.0 ⁇ Ti/C ⁇ 10.1, based on the finding that Ti/C has a correlation with a value obtained by subtracting a yield stress from a tensile stress. It is stated in PTL 2 that this technique, with a value of 20.7 MPa or more yielded as the difference between tensile stress and yield stress, can reduce hardness variation that impairs SSC resistance.
• PTL 3 describes a martensitic stainless steel containing Mo in a limited content of Mo ⁇ 2.3 ⁇ 0.89Si+32.2C, and having a metal microstructure composed mainly of tempered martensite, carbides that have precipitated during tempering, and intermetallic compounds such as a Laves phase and a ⁇ phase formed as fine precipitates during tempering. It is stated in PTL 3 that the steel produced by this technique has high strength with a 0.2% proof stress of 860 MPa or more, and excellent carbon dioxide corrosion resistance and sulfide stress corrosion cracking resistance.
• PTL 1 states that sulfide stress cracking resistance can be maintained under an applied stress of 655 MPa in an atmosphere of a 5% NaCl aqueous solution (H 2 S: 0.10 bar) having an adjusted pH of 3.5.
• the steel described in PTL 2 has sulfide stress cracking resistance in an atmosphere of a 20% NaCl aqueous solution (H 2 S: 0.03 bar, CO 2 bal.) having an adjusted pH of 4.5.
• the steel described in PTL 3 has sulfide stress cracking resistance in an atmosphere of a 25% NaCl aqueous solution (H 2 S: 0.03 bar, CO 2 bal.) having an adjusted pH of 4.0.
• the invention is also intended to provide a method for manufacturing such a martensitic stainless steel seamless pipe.
• excellent sulfide stress corrosion cracking resistance means that a test piece dipped in a test solution (a 0.165 mass % NaCl aqueous solution; liquid temperature: 25° C.; H 2 S: 1 bar; CO 2 bal.) having an adjusted pH of 3.5 with addition of sodium acetate and hydrochloric acid, and in a test solution (a 20 mass % NaCl aqueous solution; liquid temperature: 25° C.; H 2 S: 0.1 bar; CO 2 bal.) having an adjusted pH of 5.0 with addition of 0.82 g/L of sodium acetate and acetic acid does not crack even after 720 hours under an applied stress equal to 90% of the yield stress.
• a test solution a 0.165 mass % NaCl aqueous solution; liquid temperature: 25° C.; H 2 S: 1 bar; CO 2 bal.
• a test solution a 20 mass % NaCl aqueous solution; liquid temperature: 25° C.; H 2 S: 0.1 bar; CO 2 bal.
• the present inventors conducted intensive studies of the effects of various alloy elements on sulfide stress corrosion cracking resistance (SSC resistance) in a CO 2 , Cl ⁇ -, and H 2 S-containing corrosive environment, using a 13% Cr-base stainless steel pipe as a basic composition.
• SSC resistance sulfide stress corrosion cracking resistance
• the present invention is based on this finding, and was completed after further studies. Specifically, the gist of the exemplary embodiments of the present invention is as follows.
• the martensitic stainless steel seamless pipe comprising, in mass %, C: 0.010% or more, Si: 0.5% or less, Mn: 0.05 to 0.24%, P: 0.030% or less, S: 0.005% or less, Ni: 4.6 to 8.0%, Cr: 10.0 to 14.0%, Mo: 1.0 to 2.7%, Al: 0.1% or less, V: 0.005 to 0.2%, N: 0.1% or less, Ti: 0.06 to 0.25%, Cu: 0.01 to 1.0%, and Co: 0.01 to 1.0%, in which the values of the following formulae (1), (2), and (3) satisfy the formulae (4) below, and the balance is Fe and incidental impurities.
• C, Mn, Cr, Cu, Ni, Mo, W, Nb, N, and Ti represent the content of each element in mass %, and the content is 0 (zero) for elements that are not contained.
• composition further comprises, in mass %, one or more selected from Ca: 0.010% or less, REM: 0.010% or less, Mg: 0.010% or less, and B: 0.010% or less.
• the exemplary embodiments of present invention has enabled production of a martensitic stainless steel seamless pipe for oil country tubular goods having excellent sulfide stress corrosion cracking resistance (SSC resistance) in a CO 2 , Cl ⁇ -, and H 2 S-containing corrosive environment, and high strength with a yield stress YS of 758 MPa (110 ksi) or more.
• SSC resistance sulfide stress corrosion cracking resistance
• C is an important element involved in the strength of the martensitic stainless steel, and is effective at improving strength.
• the C content is limited to 0.010% or more to provide the desired strength.
• C is contained in an amount of desirably 0.040% or less.
• the preferred C content is therefore 0.010 to 0.040%.
• Si acts as a deoxidizing agent, and is contained in an amount of desirably 0.05% or more.
• a Si content of more than 0.5% impairs carbon dioxide corrosion resistance and hot workability. For this reason, the Si content is limited to 0.5% or less. From the viewpoint of stably providing strength, the Si content is preferably 0.10 to 0.30%.
• Mn is an element that improves hot workability and strength. Mn is contained in an amount of desirably 0.05% or more to provide the necessary strength. When contained in an amount of more than 0.24%, Mn generates large amounts of MnS as inclusions. This becomes an initiation point of pitting corrosion, and impairs the sulfide stress corrosion cracking resistance. For this reason, the Mn content is limited to 0.05 to 0.24%.
• P is an element that impairs carbon dioxide corrosion resistance, pitting corrosion resistance, and sulfide stress corrosion cracking resistance, and should desirably be contained in as small an amount as possible in the present invention.
• an excessively small P content increases the manufacturing cost.
• the P content is limited to 0.030% or less, which is a content range that does not cause a severe impairment of characteristics, and that is economically practical in industrial applications.
• the P content is 0.015% or less.
• S is an element that seriously impairs hot workability, and should desirably be contained in as small an amount as possible.
• a reduced S content of 0.005% or less enables pipe production using an ordinary process, and the S content is limited to 0.005% or less in an embodiment of the present invention.
• the S content is 0.002% or less.
• Ni is an element that increases the strength of the protective coating, and improves the corrosion resistance. Ni also increases steel strength by forming a solid solution. Ni needs to be contained in an amount of 4.6% or more to obtain these effects. With a Ni content of more than 8.0%, the martensite phase becomes less stable, and the strength decreases. For this reason, the Ni content is limited to 4.6 to 8.0%.
• Cr is an element that forms a protective coating, and improves the corrosion resistance.
• the required corrosion resistance for oil country tubular goods can be provided when Cr is contained in an amount of 10.0% or more.
• a Cr content of more than 14.0% facilitates ferrite generation, and a stable martensite phase cannot be provided. For this reason, the Cr content is limited to 10.0 to 14.0%.
• the Cr content is 11.0 to 13.5%.
• Mo is an element that improves the resistance against pitting corrosion by Cl ⁇ . Mo needs to be contained in an amount of 1.0% or more to obtain the corrosion resistance necessary for a severe corrosive environment. Mo is also an expensive element, and a Mo content of more than 2.7% increases the manufacturing cost. For this reason, the Mo content is limited to 1.0 to 2.7%. Preferably, the Mo content is 1.5 to 2.5%.
• Al acts as a deoxidizing agent, and an Al content of 0.01% or more is needed to obtain this effect.
• Al has an adverse effect on toughness when contained in an amount of more than 0.1%.
• the Al content is limited to 0.1% or less in an embodiment of the present invention.
• the Al content is 0.01 to 0.03%.
• V needs to be contained in an amount of 0.005% or more to improve steel strength through precipitation hardening, and to improve sulfide stress corrosion cracking resistance. Because a V content of more than 0.2% impairs toughness, the V content is limited to 0.005 to 0.2% in an embodiment of the present invention.
• N acts to improve pitting corrosion resistance, and to increase strength by forming a solid solution in the steel.
• N forms various nitride inclusions in large amounts, and impairs pitting corrosion resistance when contained in an amount of more than 0.1%.
• the N content is limited to 0.1% or less in an embodiment of the present invention.
• the N content is 0.010% or less.
• Ti When contained in an amount of 0.06% or more, Ti forms carbides, and can reduce hardness by reducing solid-solution carbon.
• the Ti content is limited to 0.06 to 0.25% because, when contained in an amount of more than 0.25%, Ti generates TiN as inclusions, and impairs the sulfide stress corrosion cracking resistance as it becomes an initiation point of pitting corrosion.
• the Ti content is preferably 0.08 to 0.15%.
• Cu is contained in an amount of 0.01% or more to increase the strength of the protective coating, and improve the sulfide stress corrosion cracking resistance. However, when contained in an amount of more than 1.0%, Cu precipitates into CuS, and impairs hot workability. For this reason, the Cu content is limited to 0.01 to 1.0%.
• Co is an element that reduces hardness, and improves pitting corrosion resistance by raising the Ms point, and promoting a transformation. Co needs to be contained in an amount of 0.01% or more to obtain these effects. When contained in excessively large amounts, Co may impair toughness, and increases the material cost. For this reason, the Co content is limited to 0.01 to 1.0% in an embodiment of the present invention. The Co content is more preferably 0.03 to 0.6%.
• C, Mn, Cr, Cu, Ni, Mo, W, Nb, N, and Ti are contained so that the values of the following formulae (1), (2), and (3) satisfy the formulae (4) below.
• Formula (1) correlates these elements with an amount of retained ⁇ . By making the value of formula (1) smaller, the retained austenite occurs in smaller amounts, the hardness decreases, and the sulfide stress corrosion cracking resistance improves.
• Formula (2) correlates the elements with repassivation potential.
• Formula (3) correlates the elements with pitting corrosion potential.
• C, Mn, Cr, Cu, Ni, Mo, W, Nb, N, and Ti represent the content of each element in mass %, and the content is 0 (zero) for elements that are not contained.
• At least one selected from Nb: 0.1% or less, and W: 1.0% or less may be contained as optional elements, as needed.
• Nb forms carbides, and can reduce hardness by reducing solid-solution carbon.
• Nb may impair toughness when contained in an excessively large amount.
• W is an element that improves pitting corrosion resistance.
• W may impair toughness, and increases the material cost when contained in an excessively large amount. For this reason, Nb and W, when contained, are contained in limited amounts of Nb: 0.1% or less, and W: 1.0% or less.
• One or more selected from Ca: 0.010% or less, REM: 0.010% or less, Mg: 0.010% or less, and B: 0.010% or less may be contained as optional elements, as needed.
• Ca, REM, Mg, and B are elements that improve corrosion resistance by controlling the form of inclusions.
• the desired contents for providing this effect are Ca: 0.0005% or more, REM: 0.0005% or more, Mg: 0.0005% or more, and B: 0.0005% or more.
• Ca, REM, Mg, and B impair toughness and carbon dioxide corrosion resistance when contained in amounts of more than Ca: 0.010%, REM: 0.010%, Mg: 0.010%, and B: 0.010%.
• the contents of Ca, REM, Mg, and B, when contained, are limited to Ca: 0.010% or less, REM: 0.010% or less, Mg: 0.010% or less, and B: 0.010% or less.
• the balance is Fe and incidental impurities in the composition.
• the following describes a preferred method for manufacturing a stainless steel seamless pipe for oil country tubular goods of the present invention.
• a steel pipe material of the foregoing composition is used.
• the method of production of a stainless steel seamless pipe used as a steel pipe material is not particularly limited, and any known seamless pipe manufacturing method may be used.
• a molten steel of the foregoing composition is made into steel using an ordinary steel making process such as by using a converter, and formed into a steel pipe material, for example, a billet, using a method such as continuous casting, or ingot casting-blooming.
• the steel pipe material is then heated, and hot worked into a pipe using a known pipe manufacturing process, for example, the Mannesmann-plug mill process, or the Mannesmann-mandrel mill process to produce a seamless steel pipe of the foregoing composition.
• the process after the production of the steel pipe from the steel pipe material is not particularly limited.
• the steel pipe is subjected to quenching in which the steel pipe is heated to a temperature equal to or greater than an Ac 3 transformation point, and cooled to a cooling stop temperature of 100° C. or less, followed by tempering at a temperature equal to or less than an Ac 1 transformation point.
• the steel pipe is subjected to quenching, in which the steel pipe is reheated to a temperature equal to or greater than an Ac 3 transformation point, held for preferably at least 5 min, and cooled to a cooling stop temperature of 100° C. or less.
• quenching heating temperature is less than an Ac 3 transformation point, the microstructure does not occur in the austenite single-phase region, and a sufficient martensite microstructure does not occur in the subsequent cooling, with the result that the desired high strength cannot be obtained.
• the quenching heating temperature is limited to a temperature equal to or greater than an Ac 3 transformation point.
• the cooling method is not limited.
• the steel pipe is air cooled (at a cooling rate of 0.05° C./s or more and 20° C./s or less) or water cooled (at a cooling rate of 5° C./s or more and 100° C./s or less), and the cooling rate conditions are not limited either.
• the quenched steel pipe is tempered.
• the tempering is a process in which the steel pipe is heated to a temperature equal to or less than an Ac 1 transformation point, held for preferably at least 10 min, and air cooled.
• the tempering temperature is higher than an Ac 1 transformation point, the martensite phase precipitates after the tempering, and the desired high toughness and excellent corrosion resistance cannot be provided. For this reason, the tempering temperature is limited to a temperature equal to or less than an Ac 1 transformation point.
• the Ac 3 transformation point (° C.) and the Ac 1 transformation point (° C.) can be determined by giving a heating and cooling temperature history to a test piece, and finding a transformation point from a microdisplacement due to expansion and contraction in a Formaster test.
• Molten steels containing the components shown in Table 1 were made into steel with a converter, and cast into billets (steel pipe material) by continuous casting.
• the billet was hot worked into a pipe with a model seamless rolling mill, and cooled by air cooling or water cooling to produce a seamless steel pipe measuring 83.8 mm in outer diameter and 12.7 mm in wall thickness.
• Each seamless steel pipe was cut to obtain a test material, which was then subjected to quenching and tempering under the conditions shown in Table 2.
• a test piece for microstructure observation was taken from the quenched and tempered test material. After polishing, the amount of retained austenite ( ⁇ ) was measured by X-ray diffractometry.
• the amount of retained austenite was found by measuring the diffraction X-ray integral intensities of the ⁇ (220) plane and the ⁇ (211) plane. The results were then converted using the following equation.
• I ⁇ represents the integral intensity of ⁇
• R ⁇ represents a crystallographic theoretical calculation value for ⁇
• I ⁇ represents the integral intensity of ⁇
• R ⁇ represents a crystallographic theoretical calculation value for ⁇
• the SSC test was conducted according to NACE TM0177, Method A.
• a test environment was created by adjusting the pH of a test solution (a 0.165 mass % NaCl aqueous solution; liquid temperature: 25° C.; H 2 S: 1 bar; CO 2 bal.) to 3.5 with addition of sodium acetate and acetic acid (test environment 1), and by adjusting the pH of a test solution (a 20 mass % NaCl aqueous solution; liquid temperature: 25° C.; H 2 S: 0.1 bar; CO 2 bal.) to 5.0 with addition of 0.82 g/L of sodium acetate and acetic acid (test environment 2), and a stress 90% of the yield stress was applied for 720 hours in the solutions. Samples were determined as being acceptable when there was no crack in the test piece after the test, and unacceptable when the test piece had a crack after the test.
• the steel pipes of the present examples all had high strength with a yield stress of 758 MPa or more, demonstrating that the steel pipes were martensitic stainless steel seamless pipes having excellent SSC resistance that do not crack even when placed under a stress in a H 2 S-containing environment.
• the steel pipes did not have desirable SSC resistance, even though the desired high strength was obtained.

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