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  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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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
  • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
  • C21D1/22—Martempering
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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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  • 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
  • C21D9/085—Cooling or quenching
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  • 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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  • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
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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
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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
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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
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/004—Dispersions; Precipitations
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• the present invention relates to a seamless stainless steel pipe that is favorably used in an oil well or a gas well of crude oil or natural gas (which may be hereinafter referred simply to as an oil well) and the like.
• the present invention relates to a seamless stainless steel pipe that is excellent in carbon dioxide gas corrosion resistance under an extremely severe corrosive environment containing carbon dioxide gas (CO 2 ) and chloride ion (Cl - ) at a high temperature of 150°C or more and in SSC resistance under a low temperature environment.
• PTL 1 describes a martensitic stainless steel containing, in terms of % by mass, C: 0.010-0.030%, Mn: 0.30-0.60%, P: 0.040% or less, S: 0.0100% or less, Cr: 10.00-15.00%, Ni: 2.50-8.00%, Mo: 1.00-5.00%, Ti: 0.050-0.250%, V: 0.25% or less, N: 0.07% or less, and at least one of Si: 0.50% or less and Al: 0.10% or less, the balance being Fe and impurities, satisfying the expression (1), 6.0 ⁇ Ti/C ⁇ 10.1, and having a yield strength of 758 to 862 MPa.
• PTL 2 describes a production method of a martensitic seamless stainless steel pipe, including subjecting a martensitic stainless steel having a composition containing, in terms of % by mass, C: ⁇ 0.050, Si: ⁇ 0.5, Mn: ⁇ 1.5, P: ⁇ 0.03, S: ⁇ 0.005, Cr: 11.0-14.0, Ni: 4.0-7.0, Mo: 1.0-2.5, Cu: 1.0-2.5, Al: ⁇ 0.05, and N: 0.01-0.10, the balance being Fe and unavoidable impurities to hot working, then to a heat treatment including cooling to a temperature of the Ms point or less, then heating to a temperature T of 550°C or more and Ac 1 or less at an average heating rate of from 500 to T°C of 1.0°C/sec or more, and then cooling to a temperature of the Ms point or less.
• PTL 3 describes a high-strength martensitic stainless steel excellent in stress corrosion cracking resistance containing C: 0.06% or less, Cr: 12-16%, Si: 1.0% or less, Mn: 2.0% or less, Ni: 0.5-8.0%, Mo: 0.1-2.5%, Cu: 0.3-4.0%, and N: 0.05% or less, having an area ratio of a ⁇ -ferrite phase of 10% or less, and having a fine precipitate of Cu dispersed in the matrix.
• the steel pipe for oil well is being demanded to have both high strength and excellent carbon dioxide gas corrosion resistance under a severe corrosive environment at a high temperature of 150°C or more containing CO 2 and Cl - .
• the steel pipe is demanded to have excellent SSC resistance under a low temperature environment, such as deep sea.
• an object of the present invention is to solve the problems of the conventional techniques and to provide a high-strength seamless stainless steel pipe for oil well that has high strength, is excellent in hot workability, has excellent carbon dioxide gas corrosion resistance, and is excellent in SSC resistance under a low temperature environment.
• the "high-strength" referred herein means that a yield strength YS of 95 ksi (655 MPa) or more is achieved.
• the excellent carbon dioxide gas corrosion resistance means that the corrosion rate is 0.125 mm/y or less in the case where a test specimen is immersed in a test solution, i.e., a 20% by mass NaCl aqueous solution (solution temperature: 150°C, CO 2 gas atmosphere of 10 atm), retained in an autoclave, for an immersion time of 14 days.
• a test solution i.e., a 20% by mass NaCl aqueous solution (solution temperature: 150°C, CO 2 gas atmosphere of 10 atm)
• the "excellent SSC resistance under a low temperature environment” means that no crack is formed in a test specimen after subjecting to the test of immersing the test specimen in an aqueous solution having pH of 4.5 which is obtained by adding Na acetate and hydrochloric acid to a test solution, i.e., a 25% by mass NaCl aqueous solution (liquid temperature: 4°C, H 2 S: 0.1 bar, CO 2 : 0.9 bar) for an immersion time of 720 hours, while applying thereto a load stress of 90% of the yield stress.
• the present inventors have investigated an influence on the low temperature SSC resistance of stainless steel pipes of various compositions. As a result, it has been found that all SSC in the stainless steels occurs from pitting corrosion as the initiation site. As a result of the investigation on the occurrence of the pitting corrosion, it has been found that an oxide or a sulfide containing Al, Ca, Mg, and the like as a major component among various inclusions most tends to be the initiation site of the pitting corrosion under a low temperature environment. Accordingly, for enhancing the SSC resistance under a low temperature environment, it is important to decrease as far as possible the oxide based or sulfide based inclusion containing Al, Ca, Mg, and the like as a major component.
• the oxide based inclusion and the sulfide based inclusion are formed with oxygen and sulfur contained as impurities in the steel, and therefore are impossible to be avoided completely industrially.
• an idea of detoxification of the oxide based inclusion and the sulfide based inclusion through structural change thereof. Specifically, it has been found that the SSC resistance under a low temperature environment can be enhanced by covering the inclusion, which tends to be pitting corrosion, with TiN to prevent the inclusion from becoming the initiation site of pitting corrosion.
• the present inventors have also investigated the influence of the microstructure of steel on the low temperature SSC resistance. As a result, it has been found that under a low temperature environment, the growth of pitting corrosion and the occurrence of cracks can be suppressed to enhance the SSC resistance, by decreasing the prior austenite grain diameter. It is considered that the phenomenon is caused in such a mechanism that P and S segregated on the prior austenite grain boundary (1) accelerate the selective dissolution of the austenite grain boundary in the growth of pitting corrosion, and (2) accelerate the embrittlement of the grain boundary in the invasion of hydrogen into the steel.
• the grain boundary area per unit volume of steel becomes larger, and thus the concentration of P and S segregated on the prior austenite grain boundary is decreased to enhance the SSC resistance.
• the mechanism of the conspicuous influence of the prior austenite grain boundary on the SSC resistance under a low temperature environment is considered that the solubility of hydrogen sulfide, which accelerates the invasion of hydrogen to the steel, in the test solution is increased, and the gasification of hydrogen is suppressed by the decrease of the temperature.
• the present invention has been completed based on the findings and the further investigations made thereon.
• the substance of the present invention is as follows.
• a high-strength seamless stainless steel pipe for oil well that is excellent in hot workability, has excellent carbon dioxide gas corrosion resistance, is excellent in SSC resistance under a low temperature environment, and has high strength with a yield strength YS of 655 MPa or more can be obtained.
• C is an important element that increases the strength of the martensitic stainless steel.
• 0.002% or more of C is necessarily contained for securing the target strength.
• the C content is 0.002-0.05%.
• the C content is preferably 0.03% or less.
• the C content is more preferably 0.002% or more, more preferably 0.015% or less, further preferably 0.002% or more, and further preferably 0.010% or less.
• Si is an element that functions as a deoxidizing agent. The effect thereof can be obtained with a Si content of 0.05% or more. With a Si content exceeding 0.50%, the hot workability is deteriorated, and the carbon dioxide gas corrosion resistance is also deteriorated. Accordingly, the Si content is 0.05 to 0.50%.
• the Si content is preferably 0.10% or more, preferably 0.40% or less, more preferably 0.10% or more, and more preferably 0.30% or less.
• Mn is an element that suppresses the formation of ⁇ -ferrite in hot working and enhances the hot workability, and in the present invention, 0.04% or more of Mn is necessarily contained. With an excessive content thereof, the toughness and the SSC resistance under a low temperature environment are adversely affected. Accordingly, the Mn content is in a range of 0.04 to 1.80%.
• the Mn content is preferably 0.04% or more, preferably 0.80% or less, more preferably 0.05% or more, more preferably 0.50% or less, further preferably 0.05% or more, and further preferably 0.26% or less.
• the P is an element that lowers all the carbon dioxide gas corrosion resistance, pitting corrosion resistance, and the SSC resistance, and the content thereof is desirably decreased as far as possible in the present invention, but excessive decrease thereof may bring about rise in production cost. Accordingly, the P content is 0.030% or less, which is a range that can be practiced industrially inexpensively without significant deterioration of the characteristics. The P content is preferably 0.020% or less.
• S considerably deteriorates the hot workability, and also deteriorates the SSC resistance under a low temperature environment through the segregation of S on the prior austenite grain boundary and the formation of a Ca based inclusion, and therefore the content of S is desirably decreased as far as possible.
• a S content of 0.002% or less the number density of the Ca based inclusions can be decreased, and the segregation of S on the prior austenite grain boundary can be suppressed, resulting in achieving the target SSC resistance.
• the S content is 0.002% or less.
• the S content is preferably 0.0015% or less.
• Cr is an element that forms a protective film to contribute to the enhancement of the corrosion resistance, and 11.0% or more of Cr is necessarily contained in the present invention for securing the corrosion resistance at a high temperature.
• the Cr content is 11.0 to 14.0%.
• the Cr content is preferably 11.5% or more, preferably 13.5% or less, more preferably 12.0% or more, and more preferably 13.0% or less.
• Ni is an element that has a function reinforcing the protective film to enhance the corrosion resistance. Ni also increases the strength of the steel through the formation of solid solution therewith. These effects can be obtained with a Ni content of 3.0% or more. With a Ni content exceeding 6.5%, the stability of the martensitic phase is deteriorated since residual austenite tends to occur without the martensitic transformation, resulting in decrease the strength. Accordingly, the Ni content is 3.0 to 6.5%. The Ni content is preferably 5.0% or more, and preferably 6.0% or less.
• Mo is an element that increases the resistance against pitting corrosion by Cl - and low pH, and 0.5% or more of Mo is necessarily contained in the present invention.
• Mo content With a Mo content of less than 0.5%, the corrosion resistance under a severe corrosive environment is deteriorated. With a Mo content exceeding 3.0%, the hot workability and the corrosion resistance are deteriorated through the formation of ⁇ -ferrite. Accordingly, the Mo content is 0.5 to 3.0%.
• the Mo content is preferably 0.5% or more, preferably 2.5% or less, more preferably 1.5% or more, and more preferably 2.3% or less.
• Al is an element that functions as a deoxidizing agent. The effect thereof can be obtained with an Al content of 0.005% or more. With an Al content exceeding 0.10%, the toughness is adversely affected since the oxide amount becomes too large. Accordingly, the Al content is 0.005 to 0.10%.
• the Al content is preferably 0.01% or more, and preferably 0.03% or less.
• V is an element that enhances the strength of the steel through precipitation strengthening. The effect thereof can be obtained with a V content of 0.005% or more. With a V content exceeding 0.20%, the low temperature roughness is deteriorated. Accordingly, the V content is 0.005 to 0.20%.
• the V content is preferably 0.03% or more, and preferably 0.08% or less.
• Ti is an element that forms TiN, and enhances the SSC resistance under a low temperature environment by covering the oxide based or sulfide based inclusion with the TiN.
• the Ti content is necessarily 0.01% or more. With a Ti content exceeding 0.20%, the effect is saturated. Accordingly, the Ti content is 0.01 to 0.20%.
• the Ti content is preferably 0.03% or more, preferably 0.20% or less, more preferably 0.05% or more, and more preferably 0.15% or less.
• Co is an element that decreases the residual austenite fraction through increase of the Ms point, and enhances the strength and the SSC resistance. The effect can be obtained with a Co content of 0.01% or more. With a Co content exceeding 1.0%, the hot workability is deteriorated. Accordingly, the Co content is 0.01 to 1.0%.
• the Co content is preferably 0.05% or more, and preferably 0.15% or less.
• the Co content is more preferably 0.05% or more, and more preferably 0.09% or less.
• N is an element that significantly enhances the pitting corrosion resistance. The effect can be obtained with a N content of 0.002% or more. With a N content exceeding 0.15%, the low temperature toughness is deteriorated. Accordingly, the N content is 0.002 to 0.15%.
• the N content is preferably 0.002% or more, and preferably 0.015% or less.
• the N content is more preferably 0.003% or more, and more preferably 0.008% or less.
• O oxygen
• oxide exists in the form of oxides in the steel, and adversely affects the various characteristics. Accordingly, O is desirably decreased as far as possible.
• O content is 0.010% or less.
• the O content is preferably 0.006% or less.
• the O content is more preferably 0.004% or less.
• Cr, Ni, Mo, Cu, and C are contained in the aforementioned ranges, and simultaneously contained to satisfy the following expression (1).
• Cr, Ni, Mo, Cu, and C represent the contents (% by mass) of the elements, and an element that is not contained is designated as 0.
• the carbon dioxide gas corrosion resistance under a high temperature corrosive environment at a high temperature of 150°C or more containing CO 2 and Cl - is deteriorated. Accordingly, in the present invention, Cr, Ni, Mo, Cu, and C are contained to satisfy the expression (1).
• the number density of an inclusion having a major axis of 5 ⁇ m or more and 0.5 ⁇ Ti/(Ti+Al+Mg+Ca) ⁇ 1.0 is 0.5 per mm 2 or more and 3 per mm 2 or less.
• the number density of an inclusion having a major axis of 5 ⁇ m or more and 0.5 ⁇ Ti/(Ti+Al+Mg+Ca) ⁇ 1.0 is less than 0.5 per mm 2 , the amount of the inclusion that is not covered with TiN is increased to become pitting corrosion as the initiation site of SSC, and thus the target SSC resistance under a low temperature environment cannot be obtained.
• the inclusion that has a major axis of 5 ⁇ m or more is targeted since the inclusion that has a major axis of 5 ⁇ m or more tends to become the initiation site of pitting corrosion.
• the balance except for the aforementioned components is Fe and unavoidable impurities.
• one kind or two kinds selected from Cu: 0.05-3.0% and W: 0.05-3.0% may be contained as an optional element depending on necessity in addition to the basic composition.
• one kind or two or more kinds selected from Nb: 0.01-0.20%, Zr: 0.01-0.20%, B: 0.0005-0.01%, REM: 0.0005-0.01%, Ca: 0.0005-0.0025%, Sn: 0.02-0.20%, Ta: 0.01-0.1%, and Mg: 0.002-0.01% may also be contained.
• Cu is an element that reinforces the protective film to enhance the corrosion resistance, and may be contained depending on necessity. The effect can be obtained with a Cu content of 0.05% or more. With a Cu content exceeding 3.0%, the grain boundary precipitation of CuS occurs to deteriorate the hot workability. Accordingly, in the case where Cu is contained, the Cu content is 0.05 to 3.0%.
• the Cu content is preferably 0.5% or more, and preferably 2.5% or less.
• the Cu content is more preferably 0.5% or more, and more preferably 1.1% or less.
• W is an element that contributes to the enhancement of the strength, and may be contained depending on necessity.
• the effect can be obtained with a W content of 0.05% or more. With a W content exceeding 3.0%, the effect is saturated. Accordingly, in the case where W is contained, the W content is 0.05 to 3.0%.
• the W content is preferably 0.5% or more, and preferably 1.5% or less.
• Nb is an element that enhances the strength, and may be contained depending on necessity.
• the effect can be obtained with a Nb content of 0.01% or more. With a Nb content exceeding 0.20%, the effect is saturated. Accordingly, in the case where Nb is contained, the Nb content is 0.01 to 0.20%.
• the Nb content is preferably 0.05% or more, and preferably 0.15% or less.
• the Nb content is more preferably 0.07% or more, and more preferably 0.13% or less.
• Zr is an element that contributes to the enhancement of the strength, and may be contained depending on necessity.
• the effect can be obtained with a Zr content of 0.01% or more. With a Zr content exceeding 0.20%, the effect is saturated. Accordingly, in the case where Zr is contained, the Zr content is 0.01 to 0.20%.
• B is an element that contributes to the enhancement of the strength, and may be contained depending on necessity. The effect can be obtained with a B content of 0.0005% or more. With a B content exceeding 0.01%, the hot workability is deteriorated. Accordingly, in the case where B is contained, the B content is 0.0005 to 0.01%.
• a REM is an element that contributes to the improvement of the corrosion resistance, and may be contained depending on necessity.
• the effect can be obtained with a REM content of 0.0005% or more. With a REM content exceeding 0.01%, the effect is saturated, and the effect corresponding to the content cannot be expected, which is economically disadvantageous. Accordingly, in the case where a REM is contained, the REM content is 0.0005 to 0.01%.
• Ca is an element that contributes to the improvement of the hot workability, and may be contained depending on necessity. The effect can be obtained with a Ca content of 0.0005% or more. With a Ca content exceeding 0.0025%, the number density of coarse Ca based inclusions is increased, and the target SSC resistance under a low temperature environment cannot be obtained. Accordingly, in the case where Ca is contained, the Ca content is 0.0005 to 0.0025%.
• Sn is an element that contributes to the improvement of the corrosion resistance, and may be contained depending on necessity.
• the effect can be obtained with a Sn content of 0.02% or more. With a Sn content exceeding 0.20%, the effect is saturated, and the effect corresponding to the content cannot be expected, which is economically disadvantageous. Accordingly, in the case where Sn is contained, the Sn content is 0.02 to 0.20%.
• Ta is an element that enhances the strength, and also has an effect of improving the sulfide stress cracking resistance.
• Ta is an element that exerts the similar effect as Nb, and a part of Nb may be replaced by Ta.
• the effect can be obtained with a Ta content of 0.01% or more. With a Ta content exceeding 0.1%, the toughness is deteriorated. Accordingly, in the case where Ta is contained, the Ta content is 0.01 to 0.1%.
• Mg is an element that enhances the corrosion resistance, and may be contained depending on necessity.
• the effect can be obtained with a Mg content of 0.002% or more. With a Mg content exceeding 0.01%, the effect is saturated, and the effect corresponding to the content cannot be expected. Accordingly, in the case where Mg is contained, the Mg content is 0.002 to 0.01%.
• the high-strength seamless stainless steel pipe for oil well of the present invention has a martensitic phase (tempered martensitic phase) as a main phase for securing the target strength.
• the balance except for the main phase contains at least one kind of a residual austenitic phase and a ferrite phase.
• the main phase referred herein means a phase having a volume ratio (area ratio) of 45% or more.
• the average prior austenite grain diameter is preferably 40 ⁇ m or less from the standpoint of the achievement of the target SSC resistance under a low temperature environment.
• the number density of an inclusion having a major axis of 5 ⁇ m or more and 0.5 ⁇ Ti/ (Ti+Al+Mg+Ca) ⁇ 1.0 and the prior austenite grain diameter may be measured by the methods shown in the examples described later.
• a material for steel pipe having the aforementioned composition is used as a starting material.
• the production method of the material for steel pipe as the starting material is not particularly limited, and a generally known production method of a seamless steel pipe may be applied. It is preferred that a molten steel having the composition is manufactured by an ordinary manufacturing method, such as a converter furnace, and formed into a material for steel pipe, such as a billet, by an ordinary method, such as a continuous casting method or an ingot-bloom rolling method.
• the number density of an inclusion having 0.5 ⁇ Ti/ (Ti+Al+Mg+Ca) ⁇ 1.0 can be controlled to the target value, for example, by measuring the oxygen amount, and changing the amounts of Ti and N added corresponding to the measured value, in the steel manufacturing process.
• the material for steel pipe is heated and formed into a pipe through hot working using a pipe production process by the Mannesmann plug mill process or the Mannesmann mandrel mill process, which is a generally known pipe production method, so as to provide a seamless steel pipe having the aforementioned composition with a target dimension.
• a seamless steel pipe may also be produced by hot extrusion by a pressing process.
• the seamless steel pipe after production is preferably cooled to room temperature at a cooling rate of air cooling or larger. According to the procedure, a steel pipe structure having a martensitic phase as a main phase can be secured.
• the pipe is preferably produced under condition with a ratio (cross sectional area of steel pipe after production) / (cross sectional area of material for steel pipe) of 0.20 or less.
• the pipe is also preferably produced under condition with a ratio (cross sectional area of steel pipe after production)/(cross sectional area of steel pipe after piercing) of 0.40 or less.
• the steel pipe is subjected to a quenching treatment by reheating to the Ac 3 transformation point or more, preferably a temperature of 800°C or more, preferably retaining for 5 minutes or more, and then cooling to a temperature of 100°C or less at a cooling rate of air cooling or larger.
• a fine martensitic phase and high strength can be achieved.
• the heating temperature in the quenching treatment is preferably 800 to 950°C from the standpoint of the prevention of coarsening of the structure.
• the "cooling rate of air cooling or larger” herein means 0.01°C/s or more.
• the steel pipe subjected to the quenching treatment is then subjected to a tempering treatment.
• the tempering treatment may be a treatment of heating to a temperature (tempering temperature) of 500°C or more and less than the Ac 1 transformation point, retaining for a prescribed period of time, preferably 10 minutes or more, and then air cooling.
• a temperature such as the Ac 1 transformation point or more
• the tempering temperature is more preferably 500°C or more and less than the Ac 1 transformation point.
• the structure becomes a structure having a tempered martensitic phase as a main phase, and a seamless steel pipe having the target strength and the target corrosion resistance can be obtained.
• the operation of the quenching and the tempering is preferably repeated twice or more from the standpoint of the reduction of the average prior austenite grain diameter.
• the Ac 3 transformation point and the Ac 1 transformation point above are actual measured values read from the change in expansion coefficient (linear expansion coefficient) in heating and cooling a test specimen (3 mm in diameter ⁇ 10 mm in length) at a rate of 15°C/min.
• An electric resistance welded steel pipe or an UOE steel pipe may be produced according to the ordinary process by using the material for steel pipe having the aforementioned composition, so as to provide a steel pipe for oil well.
• the molten steels having the compositions shown in Table 1 each were manufactured, cast into a material for steel pipe, and formed into a pipe through hot working using a model seamless mill, and the manufactured pipe was cooled to provide a seamless steel pipe having an outer diameter of 83.8 mm and a thickness of 12.7 mm.
• the oxygen amount was measured online in the steel manufacturing process, and the amounts of Ti and N added were changed corresponding to the measured value, so as to control the number thereof.
• the oxygen amount was measured online in the steel manufacturing process, and the amounts of Ti and N added were changed corresponding to the measured value, so as to control the number thereof.
• a material test specimen was cut out from the resulting seamless steel pipe, and subjected to a quenching treatment by heating to a heating temperature (reheating temperature) for a soaking time shown in Table 2, and then air cooling to a cooling termination temperature shown in Table 2. Furthermore, the material test specimen was subjected to a tempering treatment by heating to a tempering temperature for a soaking time shown in Table 2, and then air cooling.
• An API American Petroleum Institute arc-shaped tensile test specimen was cut out from the material test specimen having been subjected to the quenching treatment and the tempering treatment, and subjected to a tensile test according to the regulation of API, so as to measure the tensile characteristics (i.e., the yield strength YS and the tensile strength TS).
• a corrosion test specimen of 3 mm in thickness ⁇ 30 mm in width ⁇ 40 mm in length was produced through machining from the material test specimen having been subjected to the quenching treatment and the tempering treatment, and subjected to a corrosion test.
• the corrosion test was performed by immersing the test specimen in a test solution, i.e., a 20% by mass NaCl aqueous solution (solution temperature: 150°C, CO 2 gas atmosphere of 10 atm), retained in an autoclave, for an immersion time of 14 days.
• a test solution i.e., a 20% by mass NaCl aqueous solution (solution temperature: 150°C, CO 2 gas atmosphere of 10 atm)
• solution temperature 150°C, CO 2 gas atmosphere of 10 atm
• the test specimen after the corrosion test was observed with a magnifying glass having a magnification of 10 for the presence or absence of occurrence of pitting corrosion on the surface of the test specimen.
• Pitting corrosion with a diameter of 0.2 mm or more was designated as presence.
• a specimen that had no occurrence of pitting corrosion was designated as pass, and a specimen that had the presence of pitting corrosion was designated as fail.
• An SSC test was performed according to NACE TM0177Method A.
• the test environment used was an aqueous solution obtained by adding Na acetate and hydrochloric acid to a 25% by mass NaCl aqueous solution (liquid temperature: 4°C, H 2 S: 0.1 bar, CO 2 : 0.9 bar) to provide pH 4.5.
• the test was performed with an immersion time of 720 hours and a load stress of 90% of the yield stress.
• a test specimen after the test that had no crack occurring was designated as pass (no in Table 3), and a test specimen that had a crack occurring was designated as fail (yes in Table 3).
• a smooth test specimen in a round bar shape having a diameter of parallel portion of 10 mm was used, and with a Greeble tester, was heated to 1,250°C, and after retaining for 100 seconds, cooled to 1,000°C at 1°C/sec, and after retaining for 10 seconds, strained until fracture, and the decrease rate of cross section was measured.
• the case where the decrease rate of cross section was 70% or more was assumed to achieve excellent hot workability and designated as pass.
• the case where the decrease rate of cross section was less than 70% was designated as fail.
• the inclusion For the number of the inclusions, as a specimen at an arbitrary one position in the circumferential direction at the end of the steel pipe for observing the cross section perpendicular to the pipe thickness direction with a scanning electron microscope (SEM), a region of 500 mm 2 was collected at each of the position at 1/4 of the thickness and the position at 3/4 of the thickness from the outer surface of the pipe. For each of the specimens thus collected, the inclusion was identified by SEM observation, and analyzed for the chemical composition thereof by the characteristic X-ray analyzer attached to the SEM. The inclusion having 0.5 ⁇ Ti/(Ti+Al+Mg+Ca) ⁇ 1.0 was calculated, and the number of the inclusion per unit area was calculated.
• SEM scanning electron microscope
• the inclusion having a major axis of 5 ⁇ m or more was discriminated by defining the outer periphery of the inclusion through binarization of the contrast of the backscattered electron image by the scanning electron microscope, and measuring the major axis from the outer periphery of the inclusion.
• a specimen for measuring the average prior austenite grain diameter was collected from an arbitrary one position in the circumferential direction at the end of the steel pipe from the cross section perpendicular to the longitudinal direction of the pipe at the position at 1/2 of the thickness from the outer surface of the pipe.
• the specimen thus collected was observed by EBSD, and then the prior austenite grains were reconstructed from the EBSD observation data by using a reverse analysis software for prior austenite grans.
• three straight lines of 300 ⁇ m were drawn in the circumferential direction of the pipe with a distance of 500 ⁇ m, and the average prior austenite grain diameter was measured by the section method.
• All the examples of the invention were excellent in hot workability with a yield strength YS of 655 MPa or more, were excellent in corrosion resistance under a corrosive environment at a high temperature of 150°C or more containing CO 2 and Cl - (carbon dioxide gas corrosion resistance), were excellent in SSC resistance under a low temperature environment, and had a decrease rate of cross section of 70% or more.
• the intended value was not obtained in at least one of the yield strength YS, the hot workability, the SSC resistance under a low temperature environment, the corrosion rate, and the decrease rate of cross section.

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