High strength stainless steel seamless pipe with excellent corrosion resistance for oil well and method of manufacturing the same

Abstract

A pipe having chemical composition contains, by mass %, C: 0.05% or less, Si: 0.5% or less, Mn: 0.15% or more and 1.0% or less, Cr: 13.5% or more and 15.4% or less, Ni: 3.5% or more and 6.0% or less, Mo: 1.5% or more and 5.0% or less, Cu: 3.5% or less, W: 2.5% or less, and N: 0.15% or less so that the relationship −5.9×(7.82+27C−0.91 Si+0.21Mn−0.9Cr+Ni−1.1Mo−0.55W+0.2Cu+11N)≧13.0 is satisfied.

Claims

1. A high strength stainless steel seamless pipe for an oil well, the pipe having a chemical composition containing, by mass %, C: 0.05% or less, Si: 0.5% or less, Mn: 0.15% or more and 1.0% or less, P: 0.030% or less, S: 0.005% or less, Cr: 13.5% or more and 15.4% or less, Ni: 3.5% or more and 6.0% or less, Mo: 1.5% or more and 5.0% or less, Cu: 3.5% or less, W: 2.5% or less, N: 0.15% or less, Sn: 0.05% or more and 0.20% or less, and the balance being Fe and inevitable impurities so that formula (1) is satisfied by C, Si, Mn, Cr, Ni, Mo, W, Cu, and N: formula (1) is
−5.9×(7.82+27C−0.91Si+0.21Mn−0.9Cr+Ni−1.1Mo−0.55W+0.2Cu+11N)≧13.0  (1), where C, Si, Mn, Cr, Ni, Mo, W, Cu, and N respectively denote the contents (mass %) of corresponding chemical elements.

2. The high strength stainless steel seamless pipe according to claim 1, wherein the pipe has a chemical composition further containing, by mass %, V: 0.02% or more and 0.12% or less.

3. The high strength stainless steel seamless pipe according to claim 1, wherein the pipe has a chemical composition further containing, by mass %, Al: 0.10% or less.

4. The high strength stainless steel seamless pipe according to claim 1, wherein the pipe has a chemical composition further containing, by mass %, one or more selected from among Nb: 0.02% or more and 0.50% or less, Ti: 0.02% or more and 0.16% or less, Zr: 0.50% or less, and B: 0.0030% or less.

5. The high strength stainless steel seamless pipe according to claim 1, wherein the pipe has a chemical composition further containing, by mass %, one or more selected from among REM: 0.005% or less and Ca: 0.005% or less.

6. The high strength stainless steel seamless pipe according to claim 1, wherein the pipe further has a microstructure including a martensite as a base phase and 10% or more and 60% or less, in terms of volume fraction, of a ferrite phase as a second phase.

7. The high strength stainless steel seamless pipe according to claim 6, wherein the pipe has a microstructure further including, in terms of volume fraction, 30% or less of a retained austenite phase.

8. The high strength stainless steel seamless pipe according to claim 2, wherein the pipe has a chemical composition further containing, by mass %, Al: 0.10% or less.

9. The high strength stainless steel seamless pipe according to claim 2, wherein the pipe has a chemical composition further containing, by mass %, one or more selected from among Nb: 0.02% or more and 0.50% or less, Ti: 0.02% or more and 0.16% or less, Zr: 0.50% or less, and B: 0.0030% or less.

10. The high strength stainless steel seamless pipe according to claim 3, wherein the pipe has a chemical composition further containing, by mass %, one or more selected from among Nb: 0.02% or more and 0.50% or less, Ti: 0.02% or more and 0.16% or less, Zr: 0.50% or less, and B: 0.0030% or less.

11. The high strength stainless steel seamless pipe according to claim 2, wherein the pipe has a chemical composition further containing, by mass %, one or more selected from among REM: 0.005% or less and Ca: 0.005% or less.

12. The high strength stainless steel seamless pipe according to claim 3, wherein the pipe has a chemical composition further containing, by mass %, one or more selected from among REM: 0.005% or less and Ca: 0.005% or less.

13. The high strength stainless steel seamless pipe according to claim 4, wherein the pipe has a chemical composition further containing, by mass %, one or more selected from among REM: 0.005% or less and Ca: 0.005% or less.

14. A method of manufacturing a high strength stainless steel seamless pipe comprising performing a quenching treatment and a tempering treatment on a stainless steel seamless pipe having a chemical composition containing, by mass %, C: 0.05% or less, Si: 0.5% or less, Mn: 0.15% or more and 1.0% or less, P: 0.030% or less, S: 0.005% or less, Cr: 13.5% or more and 15.4% or less, Ni: 3.5% or more and 6.0% or less, Mo: 1.5% or more and 5.0% or less, Cu: 3.5% or less, W: 2.5% or less, N: 0.15% or less, Sn: 0.05% or more and 0.20% or less, and the balance being Fe and inevitable impurities so that formula (1) is satisfied by C, Si, Mn, Cr, Ni, Mo, W, Cu, and N: formula (1) is
−5.9×(7.82+27C−0.91Si+0.21Mn−0.9Cr+Ni−1.1Mo−0.55W+0.2Cu+11N)≧13.0  (1), where C, Si, Mn, Cr, Ni, Mo, W, Cu, and N respectively denote the contents (mass %) of corresponding chemical elements, the quenching treatment including heating the pipe up to a temperature of 850° C. or higher and cooling the heated pipe at a cooling rate equal to or more than that of air cooling to a temperature of 50° C. or lower, the tempering treatment including heating the treated pipe up to a temperature equal to or lower than the A.sub.c1 transformation point and cooling the heated pipe.

15. The method according to claim 14, wherein the pipe has a chemical composition further containing, by mass %, V: 0.02% or more and 0.12% or less.

16. The method according to claim 14, wherein the pipe has a chemical composition further containing, by mass %, Al: 0.10% or less.

17. The method according to claim 14, wherein the pipe has a chemical composition further containing, by mass %, one or more selected from among Nb: 0.02% or more and 0.50% or less, Ti: 0.02% or more and 0.16% or less, Zr: 0.50% or less, and B: 0.0030% or less.

18. The method according to claim 14, wherein the pipe has a chemical composition further containing, by mass %, one or more selected from among REM: 0.005% or less and Ca: 0.005% or less.

19. The method according to claim 15, wherein the pipe has a chemical composition further containing, by mass %, Al: 0.10% or less.

20. The method according to claim 15, wherein the pipe has a chemical composition further containing, by mass %, one or more selected from among Nb: 0.02% or more and 0.50% or less, Ti: 0.02% or more and 0.16% or less, Zr: 0.50% or less, and B: 0.0030% or less.

Description

DETAILED DESCRIPTION

(1) We conducted investigations in the case of a stainless pipe having a chemical composition having a comparatively low Cr content of about 15 mass %, regarding various factors having influences on corrosion resistance in a corrosive environment in which CO.sub.2 and Cl.sup.− are present and the temperature is as high as 200° C. and corrosion resistance in an environment in which H.sub.2S is present and, as a result, found that excellent resistance to carbon dioxide corrosion can be achieved even in an environment in which CO.sub.2 and Cl.sup.− are present and the temperature is as high as 200° C. and that resistance to sulfide stress corrosion cracking equivalent to that of 17Cr steel can be achieved even in a corrosive environment in which H.sub.2S is present, by controlling a microstructure to be a compound microstructure including a martensite phase as a main phase and 10% to 60%, in terms of volume fraction, of a ferrite phase as a second phase, or further, 30% or less, in terms of volume fraction, of a retained austenite phase.

(2) Then, we found that to control the microstructure having a comparatively low Cr content of about 15 mass % to be the specified compound microstructure, it is important to control the contents of C, Si, Mn, Cr, Ni, Mo, W, Cu, and N so that formula (1) below is satisfied:
−5.9×(7.82+27C−0.91Si+0.21Mn−0.9Cr+Ni−1.1Mo−0.55W+0.2Cu+11N)≧13.0  (1),
(where C, Si, Mn, Cr, Ni, Mo, W, Cu, and N respectively denote the contents (mass %) of corresponding chemical elements). The left-hand side of formula (1) was experimentally derived as an indicator of a tendency for a ferrite phase to be formed and we found that it is important in achieving the required compound microstructure to control the contents and kinds of the alloy elements so that formula (1) is satisfied.

(3) We believe that the reason why resistance to sulfide stress cracking equivalent to that of steel containing 17% of Cr can be achieved by forming a compound microstructure including at least a ferrite phase in addition to a martensite phase is as follows.

(4) Since a ferrite phase is a phase which has good pitting resistance (pitting corrosion resistance) and is stable in a temperature range from high to low, a ferrite phase is precipitated in a form of a layer in the rolling direction, that is, in the axis direction of a pipe. Therefore, it is presumed that, since the layered microstructure is parallel to the direction of loaded stress in a sulfide stress cracking test, which means the direction of loaded stress is at a right angle to the direction in which a crack (SSC) easily propagates when a sulfide stress cracking (SSC) test is performed, the propagation of a crack (SSC) is suppressed, which results in an improvement in corrosion resistance (resistance to SSC).

(5) The high strength stainless steel seamless pipe for an oil well has a chemical composition containing, by mass %, C: 0.05% or less, Si: 0.5% or less, Mn: 0.15% or more and 1.0% or less, P: 0.030% or less, S: 0.005% or less, Cr: 13.5% or more and 15.4% or less, Ni: 3.5% or more and 6.0% or less, Mo: 1.5% or more and 5.0% or less, Cu: 3.5% or less, W: 2.5% or less, N: 0.15% or less, and the balance being Fe and inevitable impurities so that formula (1) below is satisfied by C, Si, Mn, Cr, Ni, Mo, W, Cu, and N:
−5.9×(7.82+27C−0.91Si+0.21Mn−0.9Cr+Ni−1.1Mo−0.55W+0.2Cu+11N)≧13.0  (1),
(where C, Si, Mn, Cr, Ni, Mo, W, Cu, and N respectively denote the contents (mass %) of corresponding chemical elements).

(6) First, the reason for the limitations on the chemical composition of the pipe will be described. Hereinafter, mass % shall be denoted simply by %, unless otherwise noted. C: 0.05% or less

(7) Although C is an important chemical element which increases the strength of martensitic stainless steel and it is preferable that C content be 0.01% or more to achieve the required strength, there is a deterioration in resistance to sulfide stress cracking when the C content is more than 0.05%. Therefore, the C content is limited to 0.05% or less, preferably 0.02% or more and 0.04% or less.

(8) Si: 0.5% or Less

(9) Si is a chemical element effective as a deoxidizing agent, and it is preferable that Si content be 0.1% or more to realize this effect. On the other hand, there is a deterioration in hot workability when the Si content is more than 0.5%. Therefore, the Si content is limited to 0.5% or less, preferably 0.2% or more and 0.3% or less.

(10) Mn: 0.15% or More and 1.0% or Less

(11) Mn is a chemical element which increases the strength of steel, and it is necessary that Mn content be 0.15% or more to achieve the required strength. On the other hand, there is a deterioration in toughness when the Mn content is more than 1.0%. Therefore, the Mn content is limited to 0.15% or more and 1.0% or less, preferably 0.2% or more and 0.5% or less.

(12) P: 0.030% or Less

(13) Although, since P deteriorates corrosion resistance such as resistance to carbon dioxide corrosion, pitting corrosion resistance, and resistance to sulfide stress cracking, it is preferable that P content be as small as possible, it is acceptable if the P content is 0.030% or less. Therefore, the P content is limited to 0.030% or less, preferably 0.020% or less.

(14) S: 0.005% or Less

(15) Although, since S is a chemical element having a negative effect on stable operation of a pipe manufacturing process as a result of decreasing hot workability, it is preferable that S content be as small as possible, pipe manufacturing through use of a normal process is possible when the S content is 0.005% or less. Therefore, the S content is limited to 0.005% or less, preferably 0.002% or less.

(16) Cr: 13.5% or More and 15.4% or Less

(17) Cr is a chemical element which contributes to an improvement in corrosion resistance as a result of forming a protective film, and it is necessary that Cr content be 13.5% or more. On the other hand, the required strength cannot be achieved due to an increase in the phase fraction of a ferrite phase when the Cr content is more than 15.4%. Therefore, the Cr content is limited to 13.5% or more and 15.4% or less, preferably 14.0% or more and 15.0% or less.

(18) Ni: 3.5% or More and 6.0% or Less

(19) Ni is a chemical element which improves corrosion resistance as a result of strengthening a protective film. In addition, Ni increases the strength of steel through solid solution strengthening. These effects become noticeable when Ni content is 3.5% or more. On the other hand, there is a decrease in strength due to a deterioration in the stability of a martensite phase when the Ni content is more than 6.0%. Therefore, the Ni content is limited to 3.5% or more and 6.0% or less, preferably 3.5% or more and 5.0% or less.

(20) Mo: 1.5% or More and 5.0% or Less

(21) Mo is a chemical element which improves resistance to pitting corrosion caused by Cl.sup.− and low pH, and it is necessary that Mo content be 1.5% or more. It cannot be said that sufficient corrosion resistance can be achieved in a severe corrosive environment when the Mo content is less than 1.5%. On the other hand, when the Mo is contained in a large amount of more than 5.0%, there is a sharp rise in manufacturing cost because Mo is an expensive chemical element, and there is a deterioration in toughness and corrosion resistance due to the precipitation of a χ phase. Therefore, the Mo content is limited to 1.5% or more and 5.0% or less, preferably 3.0% or more and 5.0% or less.

(22) Cu: 3.5% or Less

(23) Cu is a chemical element which improves resistance to sulfide stress cracking by suppressing hydrogen penetration into steel as a result of strengthening a protective film. It is preferable that Cu content be 0.3% or more to realize this effect. On the other hand, there is a deterioration in hot workability as a result of causing the intergranular precipitation of CuS when the Cu content is more than 3.5%. Therefore, the Cu content is limited to 3.5% or less, preferably 0.5% or more and 2.0% or less.

(24) W: 2.5% or Less

(25) W contributes to an increase in the strength of steel and improves resistance to sulfide stress cracking. It is preferable that W content be 0.5% or more to realize these effects. On the other hand, there is a deterioration in toughness and corrosion resistance due to the precipitation of a χ phase when the W is contained in a large amount of more than 2.5%. Therefore, the W content is limited to 2.5% or less, preferably 0.8% or more and 1.2% or less.

(26) N: 0.15% or Less

(27) N is a chemical element which significantly improves pitting resistance. This effect becomes noticeable when N content is 0.01% or more. On the other hand, various kinds of nitrides are formed when the N content is more than 0.15%, which results in a deterioration in toughness. Therefore, the N content is limited to 0.15% or less, preferably 0.01% or more and 0.07% or less.

(28) The pipe has a chemical composition containing the chemical elements described above in amounts in the ranges described above, while formula (1) is satisfied by C, Si, Mn, Cr, Ni, Mo, W, Cu, and N.
−5.9×(7.82+27C−0.91Si+0.21Mn−0.9Cr+Ni−1.1Mo−0.55W+0.2Cu+11N)≧13.0  (1)

(29) The left-hand side of formula (1) was derived as an indicator of a tendency for a ferrite phase to be formed, and the dual phase microstructure of martensite and ferrite phases can be stably achieved as the microstructure of a product when the contents of the alloy elements represented in formula (1) are controlled so that formula (1) is satisfied. Therefore, the contents of the alloy elements are controlled so that formula (1) is satisfied.

(30) The chemical composition described above is the basic chemical composition and, in addition to the basic chemical composition, the chemical composition may further contain V: 0.02% or more and 0.12% or less and/or Al: 0.10% or less and/or one or more selected from among Nb: 0.02% or more and 0.50% or less, Ti: 0.02% or more and 0.16% or less, Zr: 0.50% or less, and B: 0.0030% or less and/or one or more selected from among REM: 0.005% or less, Ca: 0.005% or less, and Sn: 0.20% or less as selective chemical elements, as needed.

(31) V: 0.02% or More and 0.12% or Less

(32) V is a chemical element which increases the strength of steel through precipitation strengthening and resistance to sulfide stress cracking and may be contained as needed. It is preferable that V content be 0.02% or more to realize these effects. On the other hand, there is a deterioration in toughness in the case where the V content is more than 0.12%. Therefore, it is preferable that the V content be limited to 0.02% or more and 0.12% or less, more preferably 0.04% or more and 0.08% or less.

(33) Al: 0.10% or Less

(34) Al is a chemical element effective as a deoxidization agent and may be contained as needed. It is preferable that Al content be 0.01% or more to realize this effect. On the other hand, there is a negative effect on toughness due to the amount of oxides being excessive when Al is contained in a large amount of more than 0.10%. Therefore, it is preferable that the Al content be 0.10% or less, more preferably 0.02% or more and 0.06% or less.

(35) One or more selected from among Nb: 0.02% or more and 0.50% or less, Ti: 0.02% or more and 0.16% or less, Zr: 0.50% or less, and B: 0.0030% or less

(36) Nb, Ti, Zr, and B are all chemical elements which contribute to an increase in strength and may be contained as needed.

(37) Nb contributes not only to an increase in strength as described above but also to an improvement in toughness. It is preferable that Nb content be 0.02% or more to realize these effects. On the other hand, there is a deterioration in toughness when the Nb content is more than 0.50%. Therefore, when Nb is contained, the Nb content is set to be 0.02% or more and 0.50% or less.

(38) Ti contributes not only to an increase in strength as described above but also to an improvement in resistance to sulfide stress cracking. It is preferable that Ti content be 0.02% or more to realize these effects. On the other hand, there is a deterioration in toughness and resistance to sulfide stress cracking due to formation of precipitates of a large size when the Ti content is more than 0.16%. Therefore, when Ti is contained, it is preferable that the Ti content be limited to 0.02% or more and 0.16% or less.

(39) Zr contributes not only to an increase in strength as described above but also to an improvement in resistance to sulfide stress cracking. It is preferable that Zr content be 0.02% or more to realize these effects. On the other hand, there is a deterioration in toughness when the Zr content is more than 0.50%. Therefore, in the case where Zr is contained, it is preferable that the Zr content be limited to 0.50% or less.

(40) B contributes not only to an increase in strength as described above but also to an improvement in resistance to sulfide stress cracking and hot workability. It is preferable that B content be 0.0005% or more to realize these effects. On the other hand, there is a deterioration in toughness and hot workability when the B content is more than 0.0030%. Therefore, it is preferable that the B content be limited to 0.0005% or more and 0.0030% or less.

(41) One or more selected from among REM: 0.005% or less, Ca: 0.005% or less, and Sn: 0.20% or less

(42) REM, Ca, and Sn are all chemical elements which contribute to an improvement in resistance to sulfide stress cracking, and one or more selected from among these may be contained as needed. It is preferable that REM content be 0.001% or more, Ca content be 0.001% or more, and Sn content be 0.05% or more to realize these effects. On the other hand, there is an economic disadvantage when the REM content is more than 0.005%, the Ca content is more than 0.005%, and the Sn content is more than 0.20% because effects corresponding to the contents cannot be expected due to the saturation of the effects. Therefore, when REM, Ca, and Sn are contained, it is preferable that the REM content be limited to 0.005% or less, the Ca content be limited to 0.005% or less, and the Sn content be limited to 0.20% or less.

(43) The remainder of the chemical composition other than chemical elements described above consists of Fe and inevitable impurities.

(44) Second, the reason for limitations on the microstructure of the high strength stainless steel seamless pipe for an oil well will be described.

(45) The high strength stainless steel seamless pipe for an oil well has a chemical composition described above and a microstructure including a martensite phase as a base phase and 10% or more and 60% or less, in terms of volume fraction, of a ferrite phase as a second phase, or further, 30% or less, in terms of volume fraction, of a retained austenite phase.

(46) The base phase of the seamless pipe is a martensite phase to achieve a required high strength. In addition, the microstructure of the seamless pipe is a dual (compound) phase microstructure of martensite and ferrite phases at least by precipitating 10% or more and 60% or less, in terms of volume fraction, of a ferrite phase as a second phase to achieve resistance to sulfide stress cracking equivalent to that of steel containing 17% of Cr. Since a layered microstructure is formed in the axis direction of a pipe by this method, propagation of cracks is suppressed, which results in an improvement in resistance to sulfide stress cracking. The required corrosion resistance cannot be achieved when the phase fraction of a ferrite phase is less than 10% because the layered microstructure is not formed. On the other hand, the required strength cannot be achieved when a ferrite phase is precipitated in a large amount of more than 60%. Therefore, the volume fraction of a ferrite phase as a second phase is set to be 10% or more and 60% or less, preferably 20% or more and 50% or less.

(47) In addition to a ferrite phase as a second phase, a retained austenite phase may be precipitated in an amount of 30% or less in terms of volume fraction. There is an improvement in toughness and ductility due to the presence of a retained austenite phase. These effects can be achieved when the volume fraction of a retained austenite phase is 30% or less. The required strength cannot be achieved when there is a retained austenite phase in a large amount of more than 30% in terms of volume fraction. Therefore, it is preferable that the volume fraction of a retained austenite phase as a second phase be 30% or less.

(48) Third, a preferable method of manufacturing the high strength stainless steel seamless pipe for an oil well will be described.

(49) A stainless steel seamless pipe having the chemical composition described above is a starting material. There is no particular limitation on a method of manufacturing the stainless steel seamless pipe as a starting material, and any of commonly well-known manufacturing methods may be applied.

(50) For example, it is preferable that molten steel having the chemical composition described above be refined by a common refining method such as one using a converter furnace and that a material for a pipe such as a billet be made by a common method such as a continuous casting method or an ingot-casting and slabbing-rolling method. Subsequently, this material for a pipe is heated and subjected to pipe-rolling using a commonly well-known pipe-rolling process such as a Mannesmann plug mill process or a Mannesmann mandrel mill process and made into a seamless pipe having a required size and the chemical composition described above.

(51) It is preferable that the seamless pipe be cooled to room temperature at a cooling rate equal to or more than that of air cooling (about more than 0.3° C./sec.) after pipe-rolling has been performed. A microstructure having a martensite phase as a base phase can be achieved by this method. Note that, a seamless pipe may be made by a hot extrusion method of a pressing method.

(52) Following the cooling process in which the seamless pipe is cooled to room temperature at a cooling rate equal to or more than that of air cooling, a quenching treatment, in which the pipe is further heated up to a temperature of 850° C. or higher and then cooled to a temperature of 50° C. or lower at a cooling rate equal to or more than that of air cooling (about more than 0.3° C./sec.), is performed. A seamless pipe having a martensite phase as a base phase and an appropriate amount of a ferrite phase is made by this method. The required strength cannot be achieved when the heating temperature is lower than 850° C. Note that, it is preferable that the heating temperature for a quenching treatment be 960° C. to 1100° C.

(53) The seamless pipe subjected to a quenching treatment is subjected to a tempering treatment in which the pipe is heated up to a temperature equal to or lower than the A.sub.cl transformation temperature and then cooled with air.

(54) The microstructure of the pipe becomes a microstructure including a tempered martensite phase, a ferrite phase, and a small amount of a retained austenite phase (retained γ phase) by performing a tempering treatment in which the pipe is heated up to a temperature equal to or lower than the A.sub.cl transformation temperature, preferably 700° C. or lower and 520° C. or higher. A seamless pipe having the required high strength, high toughness and excellent resistance to sulfide stress cracking is made by this method. The required high strength, high toughness, and excellent resistance to sulfide stress cracking cannot be achieved when the tempering temperature is higher than the A.sub.cl transformation temperature because a as-quenched martensite phase is formed. Note that, the tempering treatment described above may be performed without performing a quenching treatment.

(55) Our pipes and methods will be further described on the basis of the examples hereafter.

EXAMPLES

(56) Molten steel having a chemical composition given in Table 1 was refined using a converter furnace and cast into a billet (steel material for pipes) using a continuous casting method. The billet was subjected to pipe-rolling using a model seamless pipe rolling mill, cooled with air after pipe-rolling had been performed and made into a seamless pipe having an outer diameter of 83.8 mm and a wall thickness of 12.7 mm.

(57) A test piece material was cut out of the obtained seamless pipe and subjected to a quenching treatment in which the material was heated and cooled under the conditions given in Table 2. Subsequently, the test piece material was further subjected to a tempering treatment in which the material was heated and cooled with air under the conditions given in Table 2.

(58) The photograph of the microstructure of a test piece to be used for observation of microstructure, which was cut out of the test piece material which had been subjected to a quenching-tempering treatment and etched with a Vilella reagent, was taken using a scanning electron microscope (at a magnification of 1000 times) and the phase fraction (volume %) of a ferrite phase was calculated using an image analysis apparatus.

(59) In addition, the phase fraction of a retained austenite phase was observed using X-ray diffractometry. The integrated intensities of diffracted X-rays of the (220) plane of a γ phase and the (211) plane of an a phase of a test piece to be used for measurement, which was cut out of the test piece material which had been subjected to a quenching-tempering treatment, were measured using X-ray diffraction and the volume fraction of a γ phase was derived through conversion using the following equation:
γ(volume fraction)=100/(1+(IαRγ/IγRα)),

(60) where Iα: integrated intensity of a α phase Rα: theoretically calculated value of a on the basis of crystallography

(61) Iγ: integrated intensity of a γ phase Rγ: theoretically calculated value of γ on the basis of crystallography. In addition, the volume fraction of a martensite phase was derived as the remainder other than these phases.

(62) In addition, a tensile test was carried out in accordance with the API standards using an strip tensile test piece specified in the API standards, which was cut out of the test piece material which had been subjected to a quenching-tempering treatment, and tensile properties (yield strength YS and tensile strength TS) were obtained.

(63) In addition, a Charpy impact test was carried out in accordance with JIS Z 2242 using a test piece having a V notch (10 mm in thickness), which was cut out of the test piece material which had been subjected to a quenching-tempering treatment, and an absorbed energy .sub.vE.sub.−10 (J) at a temperature of −10° C. was obtained, through which toughness was evaluated.

(64) In addition, a corrosion test was carried out using a corrosion test piece having a thickness of 3 mm, a width of 30 mm, and a length of 40 mm, which was made, by performing machining, of the test piece material which had been subjected to a quenching-tempering treatment.

(65) The corrosion test was carried out under conditions in which the test piece was immersed in a testing solution, which was an aqueous solution containing 20% of NaCl (solution temperature was 200° C., in a CO.sub.2 atmosphere under a pressure of 30 atmospheres) held in an autoclave, for a duration of 14 days. The weight of the test piece was measured after the test had been carried out, and a corrosion rate was calculated from a decrease in weight between before and after the corrosion test. In addition, the surface of the test piece was observed using a loupe at a magnification of 10 times after the corrosion test had been carried out in order to find out whether or not pitting corrosion occurred. When the diameter of pits was 0.2 mm or more this is referred to pitting corrosion has occurred.

(66) Moreover, a SSC resistance test was carried out in accordance with NACE TM0177 Method A using a test piece having a round bar shape (6.4 mmφ in diameter), which was made, by performing machining, of the test piece material which had been subjected to a quenching-tempering treatment.

(67) The SSC resistance test was carried out under conditions in which the test piece was immersed in a testing solution, in which an aqueous solution containing 20% of NaCl (solution temperature was 25° C., in an atmosphere containing 0.1 atmospheres of H.sub.2S and 0.9 atmospheres of CO.sub.2) was mixed with acetic acid and sodium acetate so that the pH value of the testing solution was 3.5, for a duration of 720 hours with a loading stress being 90% of a yield stress. The test piece was observed after the test had been carried out to find out whether or not a crack occurred.

(68) The obtained results are given in Table 2.

(69) TABLE-US-00001 TABLE 1 Steel Chemical Composition (mass %) Code C Si Mn P S Cr Ni Mo Cu V W N Al A 0.03 0.28 0.31 0.018 0.0007 14.4 3.83 4.56 1.01 0.056 0.91 0.0556 0.035 B 0.01 0.22 0.30 0.019 0.0007 15.1 5.20 2.46 1.99 0.057 0.99 0.0092 0.021 C 0.03 0.28 0.31 0.019 0.0007 14.8 3.96 4.47 1.01 0.054 0.88 0.0535 0.036 D 0.03 0.28 0.31 0.017 0.0007 14.1 3.93 4.54 1.01 0.055 0.93 0.0534 0.036 E 0.03 0.29 0.31 0.018 0.0007 14.4 3.83 4.49 1.03 0.057 0.88 0.0548 0.034 F 0.03 0.28 0.31 0.018 0.0007 14.1 4.00 4.56 1.02 0.057 0.89 0.0563 0.035 G 0.03 0.29 0.32 0.018 0.0007 14.0 3.72 4.43 1.00 0.056 0.92 0.0573 0.035 H 0.01 0.36 0.44 0.009 0.0008 12.6 6.45 2.42 0.03 — — 0.0085 0.021 I 0.01 0.22 0.29 0.019 0.0007 14.4 5.01 2.55 2.01 0.060 0.97 0.0096 — J 0.01 0.23 0.31 0.018 0.0007 14.7 5.22 2.46 1.97 — 0.94 0.0098 — K 0.01 0.23 0.30 0.019 0.0007 14.5 4.99 2.44 1.92 — 1.03 0.0089 0.021 L 0.03 0.29 0.31 0.018 0.0007 15.4 3.99 4.60 0.98 — 0.92 0.0526 0.036 M 0.03 0.29 0.31 0.018 0.0007 14.1 3.68 4.32 1.02 — 0.85 0.0541 0.034 Value of Judgment Chemical Composition (mass %) Left-hand of Confor- Steel Nb, Ti, REM, Side of For- mity to Code Zr, B, Ca, Sn mula (1)* Formula (1) Note A Nb: 0.091 — 31.8 ∘ Example B — — 18.8 ∘ Example C Nb: 0.093, — 32.6 ∘ Example Ti: 0.090 D Nb: 0.094, — 29.7 ∘ Example B: 0.0012 E Nb: 0.092 REM: 0.001 31.3 ∘ Example F Nb: 0.094 Ca: 0.0020 29.1 ∘ Example G Nb: 0.093 Sn: 0.10 29.5 ∘ Example H Ti: 0.097 — −2.4 x Comparative Example I — — 16.7 ∘ Example J — — 16.6 ∘ Example K — — 16.9 ∘ Example L Nb: 0.095, — 36.9 ∘ Example Ti: 0.094, Zr: 0.05, B: 0.0012 M — REM: 0.001, 29.0 ∘ Example Ca: 0.0021, Sn: 0.10 *−5.9 × (7.82 + 27C − 0.91Si + 0.21Mn − 0.9Cr + Ni − 1.1Mo − 0.55W + 0.2Cu + 11N) ≧ 13.0 (1)

(70) TABLE-US-00002 TABLE 2 Quenching Treatment Microstructure Heating Hold- Cooling Cooling Tempering Treatment F Retained Temper- ing Rate for Stop Tem- Heating Holding Phase γ Phase Pipe Steel ature time Quenching* perature Temper- Time Fraction fraction No. Code (° C.) (min) (° C./sec.) (° C.) ature (° C.) (min) Class** (%) (%) 1 A 1030 20 0.5 25 600 30 M + 30 15 F + γ 2 A 840 20 0.5 25 600 30 M + 25 15 F + γ 3 A 1030 20 0.5 65 600 30 M + 30 20 F + γ 4 A 1030 20 0.5 25 675 30 M + 30 30 F + γ 5 B 960 15 25 25 615 30 M + 20 5 F + γ 6 C 1030 20 0.5 25 600 30 M + 30 15 F + γ 7 D 1030 20 0.5 25 600 30 M + 30 15 F + γ 8 E 1030 20 0.5 25 600 30 M + 30 15 F + γ 9 F 1030 20 0.5 25 600 30 M + 30 15 F + γ 10 G 1030 20 0.5 25 600 30 M + 30 15 F + γ 11 H 920 15 26 25 525 30 M + γ — 15 12 I 960 15 25 25 615 30 M + 20 5 F + γ 13 J 960 15 25 25 615 30 M + 15 5 F + γ 14 K 960 15 25 25 615 30 M + 20 5 F + γ 15 L 1030 20 0.5 25 600 30 M + 30 15 F + γ 16 M 1030 20 0.5 25 600 30 M + 30 15 F + γ Corrosion Test Tensile Properties Weight loss SSC Yield Tensile Toughness corrosion resistance Pipe Strength Strength vE.sub.−10 rate Pitting test No. (MPa) (MPa) (J) (mm/y) Corrosion Crack Note 1 919 1112 224 0.08 Not Not Example Occurred Occurred 2 878 1125 114 0.08 Not Occurred Comparative Occurred Example 3 759 1149 236 0.08 Not Occurred Comparative Occurred Example 4 661 994 183 0.08 Not Occurred Comparative Occurred Example 5 892 956 241 0.03 Not Not Example Occurred Occurred 6 935 1108 218 0.05 Not Not Example Occurred Occurred 7 924 1119 234 0.10 Not Not Example Occurred Occurred 8 915 1069 236 0.09 Not Not Example Occurred Occurred 9 905 1147 228 0.10 Not Not Example Occurred Occurred 10 954 1142 209 0.12 Not Not Example Occurred Occurred 11 919 1107 271 0.26 Occurred Occurred Comparative Example 12 803 945 244 0.07 Not Not Example Occurred Occurred 13 796 911 243 0.07 Not Not Example Occurred Occurred 14 844 927 238 0.03 Not Not Example Occurred Occurred 15 915 1143 218 0.05 Not Not Example Occurred Occurred 16 862 1018 242 0.09 Not Not Example Occurred Occurred *Mean Cooling Rate from 800° C. to 500° C. **M: Martensite. F: Ferrite, γ: Retained Austenite

(71) The examples are all seamless pipes having a yield strength of 758 MPa or more, a toughness of an absorbed energy .sub.vE.sub.10 of 40 J or more at a temperature of −10° C., excellent corrosion resistance (resistance to carbon dioxide corrosion) in a corrosive environment of a high temperature in which CO.sub.2 and Cl.sup.− are present and resistance to sulfide stress cracking so excellent that a crack does not occur in an environment in which H.sub.2S is present. On the other hand, the comparative examples out of our range had strength lower than was required, deteriorated corrosion resistance, or deteriorated resistance to sulfide stress cracking.