MARTENSITIC STAINLESS STEEL SEAMLESS PIPE FOR OIL COUNTRY TUBULAR GOODS, AND METHOD FOR MANUFACTURING SAME

Abstract

The invention provides a martensitic stainless steel seamless pipe for oil country tubular goods having a yield stress of 758 MPa (110 ksi) or more, and excellent sulfide stress corrosion cracking resistance and a method for manufacturing the same. The martensitic stainless steel seamless pipe for oil country tubular goods having a yield stress of 758 MPa or more has a composition that contains, 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 C, Mn, Cr, Cu, Ni, Mo, W, Nb, N, and Ti satisfy the predetermined relations, and the balance is Fe and incidental impurities.

Claims

1. A martensitic stainless steel seamless pipe for oil country tubular goods having a yield stress of 758 MPa or more, the martensitic stainless steel seamless pipe having a composition that comprises, 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:
109.37C+7.307Mn+6.399Cr+6.329Cu+11.343Ni13.529Mo+1.276W+2.925Nb+196.775N2.621Ti120.307,Formula (1)
0.0278Mn+0.0892Cr+0.00567Ni+0.153Mo0.0219W1.984N+0.208Ti1.83,Formula (2)
1.324C+0.0533Mn+0.0268Cr+0.0893Cu+0.00526Ni+0.0222Mo0.0132W0.473N0.5Ti0.514,Formula (3) wherein 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,
35.0value of formula (1)45.0,
0.600value of formula (2)0.250, and
0.400value of formula (3)0.100.Formulae (4)

2. The martensitic stainless steel seamless pipe for oil country tubular goods according to claim 1, wherein the composition further comprises, in mass %, one or two selected from Nb: 0.1% or less, and W: 1.0% or less.

3. The martensitic stainless steel seamless pipe for oil country tubular goods according to claim 1, wherein the composition further comprises, in mass %, one, two 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.

4. A method for manufacturing a martensitic stainless steel seamless pipe for oil country tubular goods, the method comprising: forming a steel pipe from a steel pipe material of the composition of claim 1; quenching the steel pipe by heating the steel pipe to a temperature equal to or greater than an Ac.sub.3 transformation point, and cooling the steel pipe to a cooling stop temperature of 100 C. or less; and tempering the steel pipe at a temperature equal to or less than an Ac.sub.1 transformation point.

5. The martensitic stainless steel seamless pipe for oil country tubular goods according to claim 2, wherein the composition further comprises, in mass %, one, two 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.

6. A method for manufacturing a martensitic stainless steel seamless pipe for oil country tubular goods, the method comprising: forming a steel pipe from a steel pipe material of the composition of claim 2; quenching the steel pipe by heating the steel pipe to a temperature equal to or greater than an Ac.sub.3 transformation point, and cooling the steel pipe to a cooling stop temperature of 100 C. or less; and tempering the steel pipe at a temperature equal to or less than an Ac.sub.1 transformation point.

7. A method for manufacturing a martensitic stainless steel seamless pipe for oil country tubular goods, the method comprising: forming a steel pipe from a steel pipe material of the composition of claim 3; quenching the steel pipe by heating the steel pipe to a temperature equal to or greater than an Ac.sub.3 transformation point, and cooling the steel pipe to a cooling stop temperature of 100 C. or less; and tempering the steel pipe at a temperature equal to or less than an Ac.sub.1 transformation point.

8. A method for manufacturing a martensitic stainless steel seamless pipe for oil country tubular goods, the method comprising: forming a steel pipe from a steel pipe material of the composition of claim 5; quenching the steel pipe by heating the steel pipe to a temperature equal to or greater than an Ac.sub.3 transformation point, and cooling the steel pipe to a cooling stop temperature of 100 C. or less; and tempering the steel pipe at a temperature equal to or less than an Ac.sub.1 transformation point.

Description

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0027] The following describes the reasons for specifying the composition of a steel pipe of the present invention. In the following, % means percent by mass, unless otherwise specifically stated.

C: 0.010% or More

[0028] C is an important element involved in the strength of the martensitic stainless steel, and is effective at improving strength. In an embodiment of the present invention, the C content is limited to 0.010% or more to provide the desired strength. When contained in excessively large amounts, C increases hardness, and susceptibility to sulfide stress corrosion cracking increases. For this reason, C is contained in an amount of desirably 0.040% or less. The preferred C content is therefore 0.010 to 0.040%.

Si: 0.5% or Less

[0029] 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: 0.05 to 0.24%

[0030] 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: 0.030% or Less

[0031] 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. However, an excessively small P content increases the manufacturing cost. For this reason, 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. Preferably, the P content is 0.015% or less.

S: 0.005% or Less

[0032] 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. Preferably, the S content is 0.002% or less.

Ni: 4.6 to 8.0%

[0033] 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: 10.0 to 14.0%

[0034] 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%. Preferably, the Cr content is 11.0 to 13.5%.

Mo: 1.0 to 2.7%

[0035] Mo is an element that improves the resistance against pitting corrosion by Cl.sup.. 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: 0.1% or Less

[0036] Al acts as a deoxidizing agent, and an Al content of 0.01% or more is needed to obtain this effect. However, Al has an adverse effect on toughness when contained in an amount of more than 0.1%. For this reason, the Al content is limited to 0.1% or less in an embodiment of the present invention. Preferably, the Al content is 0.01 to 0.03%.

V: 0.005 to 0.2%

[0037] 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: 0.1% or Less

[0038] N acts to improve pitting corrosion resistance, and to increase strength by forming a solid solution in the steel. However, N forms various nitride inclusions in large amounts, and impairs pitting corrosion resistance when contained in an amount of more than 0.1%. For this reason, the N content is limited to 0.1% or less in an embodiment of the present invention. Preferably, the N content is 0.010% or less.

Ti: 0.06 to 0.25%

[0039] When contained in an amount of 0.06% or more, Ti forms carbides, and can reduce hardness by reducing solid-solution carbon. However, 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: 0.01 to 1.0%

[0040] 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: 0.01 to 1.0%

[0041] 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%.

[0042] In an embodiment of the present invention, 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. When C, Mn, Cr, Cu, Ni, Mo, W, Nb, N, and Ti are contained so that the value of formula (1) satisfies the range of formula (4), and Mn, Cr, Ni, Mo, W, N, and Ti are contained so that the value of formula (2) satisfies the range of formula (4), the passivation film can more easily regenerate, and the repassivation improves. Formula (3) correlates the elements with pitting corrosion potential. When C, Mn, Cr, Cu, Ni, Mo, W, Nb, N, and Ti are contained so that the value of formula (1) satisfies the range of formula (4), and C, Mn, Cr, Cu, Ni, Mo, W, N, and Ti are contained so that the value of formula (3) satisfies the range of formula (4), generation of pitting corrosion, which becomes an initiation point of sulfide stress corrosion cracking, can be reduced, and the sulfide stress corrosion cracking resistance greatly improves. When the value of formula (1) satisfies the range of formula (4), the hardness increases when the value of formula (1) is 10 or more. However, it is still possible to regenerate a passivation film, and to effectively reduce generation of pitting corrosion and improve sulfide stress corrosion cracking resistance when the value of formula (2) or (3) satisfies the range of formula (4).

109.37C+7.307Mn+6.399Cr+6.329Cu+11.343Ni13.529Mo+1.276W+2.925Nb+196.775N2.621Ti120.307Formula (1)

0.0278Mn+0.0892Cr+0.00567Ni+0.153Mo0.0219W1.984N+0.208Ti1.83Formula (2)

1.324C+0.0533Mn+0.0268Cr+0.0893Cu+0.00526Ni+0.0222Mo0.0132W0.473N0.5Ti0.514Formula (3)

[0043] In the formulae, 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.

[0044] Formulae (4)

35.0value of formula (1)45.0,

0.600value of formula (2)0.250, and

0.400value of formula (3)0.100

[0045] At least one selected from Nb: 0.1% or less, and W: 1.0% or less may be contained as optional elements, as needed.

[0046] Nb forms carbides, and can reduce hardness by reducing solid-solution carbon. However, Nb may impair toughness when contained in an excessively large amount. W is an element that improves pitting corrosion resistance. However, 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.

[0047] 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.

[0048] 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%. For this reason, 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.

[0049] The balance is Fe and incidental impurities in the composition.

[0050] The following describes a preferred method for manufacturing a stainless steel seamless pipe for oil country tubular goods of the present invention.

[0051] In the present invention, a steel pipe material of the foregoing composition is used. However, 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.

[0052] Preferably, 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.

[0053] The process after the production of the steel pipe from the steel pipe material is not particularly limited. Preferably, the steel pipe is subjected to quenching in which the steel pipe is heated to a temperature equal to or greater than an Ac.sub.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.sub.1 transformation point.

Quenching

[0054] In an embodiment of the present invention, the steel pipe is subjected to quenching, in which the steel pipe is reheated to a temperature equal to or greater than an Ac.sub.3 transformation point, held for preferably at least 5 min, and cooled to a cooling stop temperature of 100 C. or less. This makes it possible to produce a refined, tough martensite phase. When the quenching heating temperature is less than an Ac.sub.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. For this reason, the quenching heating temperature is limited to a temperature equal to or greater than an Ac.sub.3 transformation point. The cooling method is not limited. Typically, 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.

Tempering

[0055] 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.sub.1 transformation point, held for preferably at least 10 min, and air cooled. When the tempering temperature is higher than an Ac.sub.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.sub.1 transformation point. The Ac.sub.3 transformation point ( C.) and the Ac.sub.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.

EXAMPLES

[0056] The present invention is further described below through Examples.

[0057] 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.

[0058] 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.

[0059] Specifically, 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.

(volume fraction)=100/(1+(I.sub.R.sub./I.sub.R.sub.))

[0060] In the equation, I.sub. represents the integral intensity of , R.sub. represents a crystallographic theoretical calculation value for , I.sub. represents the integral intensity of , and R.sub. represents a crystallographic theoretical calculation value for .

[0061] An arc-shaped tensile test specimen specified by API standard was taken from the quenched and tempered test material, and the tensile properties (yield stress, YS; tensile stress, TS) were determined in a tensile test conducted according to the API specification. The Ac.sub.3 point ( C.) and Ac.sub.1 point ( C.) in Table 2 were measured in a Formaster test using a test piece (4 mm10 mm) taken from the quenched test material. Specifically, the test piece was heated to 500 C. at 5 C./s, and to 920 C. at 0.25 C./s. After being held for 10 minutes, and test piece was cooled to room temperature at a rate of 2 C./s. The expansion and contraction of the test piece with this temperature history were then detected to obtain the Ac.sub.3 point ( C.) and Ac.sub.1 point ( C.).

[0062] 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.sub.2S: 1 bar; CO.sub.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.sub.2S: 0.1 bar; CO.sub.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.

[0063] The results are presented in Table 2.

TABLE-US-00001 TABLE 1 Steel Composition (mass %) No. C Si Mn P S Ni Cr Mo Al V N Ti Cu A 0.0106 0.20 0.21 0.015 0.001 5.91 12.0 1.86 0.040 0.044 0.0045 0.092 0.21 B 0.0112 0.19 0.21 0.017 0.001 5.98 12.4 2.20 0.042 0.014 0.0065 0.088 0.07 C 0.0109 0.20 0.19 0.015 0.001 5.84 11.9 2.02 0.044 0.038 0.0078 0.101 0.32 D 0.0116 0.19 0.20 0.015 0.001 5.71 11.9 1.97 0.042 0.044 0.0048 0.106 0.14 E 0.0125 0.21 0.21 0.014 0.001 4.61 12.5 1.84 0.039 0.023 0.0054 0.097 0.31 F 0.0104 0.17 0.21 0.014 0.001 6.12 11.9 2.69 0.039 0.024 0.0152 0.082 0.56 G 0.0154 0.20 0.12 0.015 0.001 7.23 12.7 2.26 0.040 0.014 0.0048 0.152 0.51 H 0.0113 0.19 0.18 0.014 0.001 6.34 12.5 2.18 0.039 0.037 0.0065 0.143 0.34 I 0.0161 0.20 0.07 0.014 0.001 5.23 12.8 1.78 0.041 0.013 0.0074 0.084 0.46 J 0.0103 0.19 0.23 0.015 0.001 6.84 13.1 2.22 0.039 0.044 0.0084 0.113 0.21 K 0.0094 0.19 0.21 0.015 0.001 5.81 11.8 1.92 0.040 0.015 0.0103 0.098 0.34 L 0.0108 0.17 0.25 0.015 0.001 6.08 12.0 2.61 0.041 0.025 0.0143 0.072 0.54 M 0.0131 0.18 0.11 0.014 0.001 4.52 13.9 1.36 0.039 0.028 0.0063 0.121 0.54 N 0.0104 0.19 0.22 0.015 0.001 6.29 12.3 2.81 0.039 0.044 0.0074 0.108 0.18 O 0.0148 0.20 0.08 0.014 0.001 5.27 12.9 1.69 0.041 0.033 0.0038 0.054 0.43 P 0.0115 0.18 0.20 0.015 0.001 5.66 11.8 1.94 0.042 0.028 0.0053 0.108 1.08 Q 0.0109 0.17 0.23 0.014 0.001 6.18 11.7 2.68 0.039 0.015 0.0134 0.065 0.69 R 0.0104 0.19 0.23 0.015 0.001 7.82 13.8 1.23 0.041 0.009 0.0150 0.069 0.93 S 0.0487 0.20 0.07 0.014 0.001 4.78 11.0 2.66 0.042 0.013 0.0038 0.211 0.02 T 0.0479 0.20 0.08 0.015 0.001 7.84 13.8 2.65 0.040 0.014 0.0048 0.210 0.91 U 0.0112 0.19 0.23 0.015 0.001 4.69 11.1 1.21 0.040 0.042 0.0154 0.072 0.03 V 0.0100 0.19 0.24 0.015 0.001 7.42 13.3 2.27 0.040 0.015 0.0023 0.061 2.31 W 0.0101 0.19 0.06 0.013 0.001 4.62 10.7 2.02 0.041 0.042 0.0546 0.405 0.01 Value Value Value of of of formula formula formula Steel Composition (mass %) (1) (2) (3) No. Co Nb, W Ca, REM, Mg, B (*1) (*2) (*3) Remarks A 0.23 0.6 0.438 0.153 Compliant Example B 0.09 1.1 0.354 0.146 Compliant Example C 0.35 1.8 0.426 0.149 Compliant Example D 0.17 Nb: 0.04 4.2 0.428 0.169 Compliant Example E 0.29 W: 0.31 9.6 0.411 0.147 Compliant Example F 0.08 Ca: 0.003 4.4 0.341 0.104 Compliant Example G 0.41 Ca: 0.002, 15.4 0.292 0.132 Compliant Example REM: 0.002 H 0.32 Mg: 0.003 5.2 0.334 0.147 Compliant Example I 0.38 B: 0.002 0.3 0.385 0.126 Compliant Example J 0.21 Nb: 0.02 Ca: 0.002 14.4 0.283 0.121 Compliant Example K 0.33 0.4 0.457 0.149 Comparative Example L 0.12 3.2 0.346 0.098 Comparative Example M 0.41 5.2 0.347 0.114 Comparative Example N 0.16 Nb: 0.02 5.4 0.266 0.132 Comparative Example O 0.38 1.4 0.389 0.109 Comparative Example P 0.17 Nb: 0.04 1.1 0.442 0.090 Comparative Example Q 1.09 4.3 0.361 0.088 Comparative Example R 0.31 Nb: 0.02, 50.0 0.401 0.043 Comparative Example W: 0.56 S 0.24 36.2 0.380 0.301 Comparative Example T 0.41 22.6 0.117 0.129 Comparative Example U 0.15 Nb: 0.04, 7.7 0.669 0.220 Comparative Example W: 0.87 V 0.42 33.8 0.253 0.106 Comparative Example W 0.04 W: 0.91 16.5 0.586 0.408 Comparative Example * Underline means outside the range of the invention * The remainder is Fe and incidental impurities (*1) Formula (1): 109.37C + 7.307Mn + 6.399Cr + 6.329Cu + 11.343Ni 13.529Mo + 1.276W + 2.925Nb + 196.775N 2.621Ti 120.307 (*2) Formula (2): 0.0278Mn + 0.0892Cr + 0.00567Ni + 0.153Mo 0.0219W 1.984N + 0.208Ti 1.83 (*3) Formula (3): 1.324C + 0.0533Mn + 0.0268Cr + 0.0893Cu + 0.00526Ni + 0.0222Mo 0.0132W 0.473N 0.5Ti 0.514

TABLE-US-00002 TABLE 2 Quenching Tempering Steel Ac.sub.3 Heating Holding Ac.sub.1 Heating Holding Pipe Steel point temp. time Cooling stop point temp. time No. No. ( C.) ( C.) (min) Cooling method temp. ( C.) ( C.) ( C.) (min) 1 A 745 920 20 Air cooling 25 640 610 60 2 B 735 920 20 Air cooling 25 650 615 60 3 C 755 920 20 Air cooling 25 645 615 60 4 D 745 920 20 Air cooling 25 645 595 60 5 E 715 810 20 Water cooling 25 655 590 60 6 F 705 810 20 Air cooling 25 640 590 60 7 G 775 920 20 Water cooling 25 665 620 60 8 H 765 920 20 Water cooling 25 660 610 60 9 I 745 900 20 Air cooling 25 655 585 60 10 J 770 920 20 Air cooling 25 660 600 60 11 A 745 730 20 Air cooling 25 640 610 60 12 B 735 920 20 Air cooling 25 650 660 60 13 K 760 920 20 Air cooling 25 640 600 60 14 L 715 920 20 Water cooling 25 645 615 60 15 M 750 810 20 Air cooling 25 650 615 60 16 N 735 900 20 Air cooling 25 665 620 60 17 O 755 920 20 Air cooling 25 645 595 60 18 P 745 810 20 Water cooling 25 640 610 60 19 Q 760 920 20 Air cooling 25 655 590 60 20 R 760 920 20 Air cooling 25 660 585 60 21 S 715 920 20 Air cooling 25 645 600 60 22 T 765 920 20 Water cooling 25 660 600 60 23 U 755 920 20 Air cooling 25 635 585 60 24 V 760 920 20 Air cooling 25 655 610 60 25 W 725 920 20 Air cooling 25 640 585 60 Tensile Micro- properties SSC resistance test structure Yield Tensile Presence or absence of Steel Retained stress stress cracking Pipe (*1) YS TS Test Test No. (volume %) (MPa) (MPa) environment 1 environment 2 Remarks 1 2.9 808 848 Absent Absent Present Example 2 0.5 786 825 Absent Absent Present Example 3 0.5 796 837 Absent Absent Present Example 4 0.3 786 831 Absent Absent Present Example 5 0.2 769 809 Absent Absent Present Example 6 0.4 779 819 Absent Absent Present Example 7 18.4 798 834 Absent Absent Present Example 8 8.1 812 855 Absent Absent Present Example 9 0.5 778 825 Absent Absent Present Example 10 19.3 784 834 Absent Absent Present Example 11 5.9 765 848 Present Present Comparative Example 12 4.7 761 825 Present Present Comparative Example 13 0.6 804 846 Present Present Comparative Example 14 1.2 771 825 Present Present Comparative Example 15 0.3 768 801 Present Present Comparative Example 16 10.4 812 861 Present Present Comparative Example 17 0.4 783 829 Present Present Comparative Example 18 0.6 788 831 Present Present Comparative Example 19 3.5 793 846 Present Present Comparative Example 20 8.6 853 885 Present Present Comparative Example 21 1.3 787 835 Present Present Comparative Example 22 20.4 769 887 Present Present Comparative Example 23 9.3 786 833 Present Present Comparative Example 24 0.5 806 865 Present Present Comparative Example 25 18.7 784 842 Present Present Comparative Example (*1) Retained : Retained austenite * Underline means outside the range of the invention

[0064] 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.sub.2S-containing environment. On the other hand, in Comparative Examples outside the range of the present invention, the steel pipes did not have desirable SSC resistance, even though the desired high strength was obtained.