PATENT DOCUMENT

Abstract:
A steel composition contains: 0.05% or less of C; 0.5% or less of Si; 0.20% to 1.80% of Mn; 0.03% or less of P; 0.005% or less of S; 14.0% to 18.0% of Cr; 5.0% to 8.0% of Ni; 1.5% to 3.5% of Mo; 0.5% to 3.5% of Cu; 0.05% or less of Al; 0.20% or less of V; 0.01% to 0.15% of N; and 0.006% or less of O on a mass basis, and satisfies the following expressions: Cr+0.65Ni+0.6Mo+0.55Cu−20C≧18.5 and Cr+Mo+0.3Si−43.5C−0.4Mn−Ni−0.3Cu−9N≦11 (where Cr, Ni, Mo, Cu, C, Si, Mn, and N represent their respective contents (mass %)). After such a steel pipe material is formed into a steel pipe, the steel pipe is quenched by cooling after heating to a temperature of the A C3  transformation point or more and tempered at a temperature of the A C1  transformation point or less.

Full Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 10/488,980, filed Mar. 10, 2004, now abandoned, which is a §371 of International Application No. PCT/JP2003/007709, with an international filing date of Jun. 18, 2003 (WO 2004/001082 A1, published Dec. 31, 2003), which is based on Japanese Patent Application Nos. 2002-178974, filed Jun. 19, 2002, 2003-114775, filed Apr. 18, 2003, and 2003-156234, filed Jun. 2, 2003. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to steel pipes for oil country tubular goods used in crude oil wells and natural gas wells. In particular, this disclosure relates to an improvement of corrosion resistance to extremely severe, corrosive environment in which carbon dioxide gas (CO 2 ), chloride ions (Cl − ), and the like are present. 
     BACKGROUND 
     Deep oil wells, which have not conventionally been regarded at all, and corrosive sour gas wells, the development of which was abandoned for a time, have recently been developed increasingly on a world scale to cope with increase of crude oil price and anticipated oil resource depletion in the near future. These oil wells and gas wells generally lie at great depths in a severe, corrosive environment of a high-temperature atmosphere containing corrosive substances, such as CO 2  and Cl − . Accordingly, steel pipes for oil country tubular goods used for digging such an oil or gas well have to be highly strong and corrosion-resistant. 
     In general, highly CO 2  corrosion-resistant 13%-Cr martensitic stainless steel pipes are used in oil wells and gas wells whose atmospheres contain CO 2 , Cl − , or the like. However, conventional martensitic stainless steels cannot wear in environments at high temperatures of more than 100° C. containing a large amount of Cl − . Accordingly, two-phase stainless steel pipes are used in oil wells requiring corrosion resistance. Unfortunately, the two-phase stainless steel pipes contain large amounts of alloying elements to reduce the hot workability. Consequently, they must be manufactured only by special heat treatment due to their reduced hot workability, and besides, they are disadvantageously expensive. Accordingly, an inexpensive 13%-Cr martensitic stainless steel-based pipe for oil country tubular goods having a superior hot workability and CO 2  corrosion resistance has been strongly desired. On the other hand, oil well development in cold districts has recently become active, and, accordingly, superior toughness at low temperatures is often required in addition to high strength. 
     To these demands, improved martensitic stainless steels (or steel pipes) based on a 13%-Cr martensitic stainless steel (or steel pipe), having an enhanced corrosion resistance have been proposed in, for example, Japanese Unexamined Patent Application Publication Nos. 8-120345, 9-268349, and 10-1755 and Japanese Patent Nos. 2814528 and 3251648. 
     Japanese Unexamined Patent Application Publication No. 8-120345 has disclosed a method for manufacturing a seamless martensitic stainless steel pipe having a superior corrosion resistance. For a steel composition of a 13%-Cr martensitic stainless steel pipe, the C content is limited to the range of 0.005% to 0.05%, 2.4% to 6% of Ni and 0.2% to 4% of Cu are added in combination, and 0.5% to 3% of Mo is further added. Furthermore, Ni eq  is set at 10.5 or more. This steel material is subjected to hot working, subsequently cooled at air-cooling speed or more, and then tempered. Alternatively, after being cooled, the steel material is further heated to a temperature between A C3  transformation point+10° C. and A C3  transformation point+200° C., or a temperature between A C1  transformation point and A C3  transformation point, subsequently cooled to room temperature at air-cooling speed or more, and then tempered. According to this method, a seamless martensitic stainless steel pipe is achieved which has a high strength of the grade API-C95 or grater, corrosion resistance in environments at 180° C. or more containing CO 2 , and SCC resistance. 
     Japanese Unexamined Patent Application Publication No. 9-268349 has disclosed a method for manufacturing a martensitic stainless steel having a superior stress-corrosion cracking resistance to sulfides. In this method, a steel composition of a 13%-Cr martensitic stainless steel contains 0.005% to 0.05% of C, 0.005% to 0.1% of N, 3.0% to 6.0% of Ni, 0.5% to 3% of Cu, and 0.5% to 3% of Mo. After hot working and being left to cool down to room temperature, this steel material is heated to a temperature between (A C1  point+10° C.) and (A C1  point+40° C.) for 30 to 60 minutes, then cooled to a temperature of Ms point or less, and tempered at a temperature of A C1  point or less. Thus, the resulting steel has a structure in which tempered martensite and 20 percent by volume or more of γ phase are mixed. According to this method, the sulfide stress-corrosion cracking resistance is remarkably enhanced by forming a martensitic structure containing 20 percent by volume or more of γ phase. 
     Japanese Unexamined Patent Application Publication No. 10-1755 has disclosed a martensitic stainless steel containing 10% to 15% of Cr, having a superior corrosion resistance and sulfide stress-corrosion cracking resistance. This martensitic stainless steel has a composition in which the Cr content is set at 10% to 15%; the C content is limited to the range of 0.005% to 0.05%; 4.0% or more of Ni and 0.5% to 3% of Cu are added in combination; and 1.0% to 3.0% of Mo is further added. Furthermore, Ni eq  of the composition is set at −10 or more. The structure of the martensitic stainless steel contains a tempered martensitic phase, a martensitic phase, and a residual austenitic phase. The total percentage of the tempered martensitic phase and the martensitic phase is set in the range of 60% to 90%. According to this disclosure, corrosion resistance and sulfide stress-corrosion cracking resistance in environments where wet carbon dioxide gas or wet hydrogen sulfide is present are enhanced. 
     Japanese Patent No. 2814528 relates to an oil well martensitic stainless steel product having a superior sulfide stress-corrosion cracking resistance. This steel product has a steel composition containing more than 15% and 19% or less of Cr, 0.05% or less of C, 0.1% or less of N, 3.5% to 8.0% of Ni, and 0.1% to 4.0% of Mo, and simultaneously satisfying the relationships: 30Cr+36Mo+14Si−28Ni≦455(%); and 21Cr+25Mo+17Si+35Ni≦731(%). According to this disclosure, the resulting steel product exhibits a superior corrosion resistance in severe environments in oil wells where chloride ions, carbon dioxide gas, and a small amount of hydrogen sulfide gas are present. 
     Japanese Patent No. 3251648 relates to a precipitation hardening martensitic stainless steel having superior strength and toughness. This martensitic stainless steel has a steel composition containing 10.0% to 17% of Cr, 0.08% or less of C, 0.015% or less of N, 6.0% to 10.0% of Ni, 0.5% to 2.0% of Cu, and 0.5% to 3.0% of Mo. The structure of the steel is formed by 35% or more cold working and annealing and it has a mean crystal grain size of 25 μm or less and precipitates with a particle size of 5×10 −2  μm or more in the matrix. The number of the precipitates is limited to 6×10 6  per square millimeter or less. According to this disclosure, a high-strength precipitation hardening martensitic stainless steel in which toughness degradation does not occur can be achieved by forming a structure containing fine crystal grains and less precipitation. 
     However, improved 13%-Cr martensitic stainless steel pipes manufactured by the techniques of Japanese Unexamined Patent Application Publication Nos. 8-120345, 9-268349, and 10-1755 and Japanese Patent Nos. 2814528 and 3251648 do not stably exhibit desired corrosion resistance in severe, corrosive environments at temperatures of more than 180° C. containing CO 2 , Cl − , or the like. 
     In view of the circumstances of the known arts stated above, this disclosure has been achieved. The object of this disclosure is to provide an inexpensive, corrosion-resistant stainless steel pipe for oil country tubular goods, preferably a high-strength stainless steel pipe for oil country tubular goods, having a superior hot workability and exhibiting a superior CO 2  corrosion resistance even in severe, corrosive environments at temperatures of more than 180° C. containing CO 2 , Cl − , or the like. 
     SUMMARY 
     We provide:
         (1) A corrosion-resistant stainless steel pipe for oil country tubular goods having a steel composition comprising, on a mass basis, 0.05% or less of C; 0.50% or less of Si; 0.20% to 1.80% of Mn; 0.03 or less of P; 0.005% or less of S; 14.0% to 18.0% of Cr; 5.0% to 8.0% of Ni; 1.5% to 3.5% of Mo; 0.5% to 3.5% of Cu; 0.05% or less of Al; 0.20% or less of V; 0.01% to 0.15% of N; 0.006% or less of O and the balance being Fe and incidental impurities. The composition satisfies expressions (1) and (2):
 
Cr+0.65Ni+0.6Mo+0.55Cu+20C≧18.5  (1)
 
Cr+Mo+0.3Si−43.5C−0.4Mn−Ni−0.3Cu−9N≦11  (2)
   where Cr, Ni, Mo, Cu, C, Si, Mn, and N represent their respective contents on a mass % basis.   (2) A corrosion-resistant stainless steel pipe for oil country tubular goods according to (1) in which the composition further contains at least one element of 0.20% or less of Nb and 0.30% or less of Ti on a mass basis.   (3) A corrosion-resistant stainless steel pipe for oil country tubular goods according to (1) or (2) in which the composition further contains at least one element selected from the group consisting of 0.20% or less of Zr, 0.01% or less of B, and 3.0% or less of W on a mass basis.   (4) A corrosion-resistant stainless steel pipe for oil country tubular goods according to any one of (1) to (3) in which the composition further contains 0.0005% to 0.01% of Ca on a mass basis.   (5) A stainless steel pipe for oil country tubular goods according to any one of (1) to (4) and whose structure includes 5 to 25 percent by volume of a residual austenitic phase and the balance being a martensitic phase.   (6) A corrosion-resistant stainless steel pipe for oil country tubular goods according to any one of (1) to (4) and whose structure includes 5 to 25 percent by volume of a residual austenitic phase, 5 percent by volume or less of a ferrite phase, and the balance being a martensitic phase.   (7) A method for manufacturing a corrosion-resistant stainless steel pipe for oil country tubular goods including the steps of: forming a steel pipe from a steel pipe material having a composition; quenching the steel pipe by heating the steel pipe to a temperature of the A C3  transformation point thereof or more and subsequently cooling to room temperature at air-cooling speed or more; and then tempering the steel pipe at a temperature of the A C1  transformation point thereof or less. The composition contains, on a mass basis, 0.05% or less of C; 0.50% or less of Si; 0.20% to 1.80% of Mn; 0.03 or less of P; 0.005% or less of S; 14.0% to 18.0% of Cr; 5.0% to 8.0% of Ni; 1.5% to 3.5% of Mo; 0.5% to 3.5% of Cu; 0.05% or less of Al; 0.20% or less of V; 0.01% to 0.15% of N; 0.006% or less of O, and the balance being Fe and incidental impurities, and the composition satisfies expressions (1) and (2):
 
Cr+0.65Ni+0.6Mo+0.55Cu+20C≧18.5  (1)
 
Cr+Mo+0.3Si−43.5C−0.4Mn−Ni−0.3Cu−9N≦11  (2)
   where Cr, Ni, Mo, Cu, C, Si, Mn, and N represent their respective contents.   (8) A method for manufacturing a stainless steel pipe for oil country tubular goods according to (7) in which the composition further contains at least one element of 0.20% or less of Nb and 0.30% or less of Ti on a mass basis.   (9) A method for manufacturing a stainless steel pipe for oil country tubular goods according to (8) in which the quenching includes heating to a temperature in the range of 800 to 1100° C. and cooling to room temperature at air-cooling speed or more, and the tempering is performed at a temperature in the range of 500 to 630° C.   (10) A method for manufacturing a stainless steel pipe for oil country tubular goods according to any one of (7) to (9) in which the composition further contains at least one element selected from the group consisting of 0.20% or less of Zr, 0.01% or less of B, and 3.0% or less of W on a mass basis.   (11) A method for manufacturing a stainless steel pipe for oil country tubular goods according to any one of (7) to (10) in which the composition further contains 0.0005% to 0.01% of Ca on a mass basis.   (12) A method for manufacturing a corrosion-resistant seamless stainless steel pipe for oil country tubular goods, including the steps of: forming a steel pipe from a steel pipe material having a composition by hot working; cooling the steel pipe to room temperature at air-cooling speed or more, or quenching the steel pipe by further heating to a temperature of the A C3  transformation point thereof or more and cooling to room temperature at air-cooking speed or more; and then tempering the steel pipe at a temperature of the A C1  transformation point thereof or less. The composition contains, on a mass basis, 0.05% or less of C; 0.50% or less of Si; 0.20% to 1.80% of Mn; 0.03 or less of P; 0.005% or less of S; 14.0% to 18.0% of Cr; 5.0% to 8.0% of Ni; 1.5% to 3.5% of Mo; 0.5% to 3.5% of Cu; 0.05% or less of Al; 0.20% or less of V; 0.01% to 0.15% of N; 0.006% or less of O, and the balance being Fe and incidental impurities, and the composition satisfies expressions (1) and (2):
 
Cr+0.65Ni+0.6Mo+0.55Cu+20C≧18.5  (1)
 
Cr+Mo+0.3Si−43.5C−0.4Mn−Ni−0.3Cu−9N≦11  (2)
   where Cr, Ni, Mo, Cu, C, Si, Mn, and N represent their respective contents on a mass % basis.   (13) A method for manufacturing a seamless stainless steel pipe for oil country tubular goods according to (12) in which the composition further contains at least one element of 0.20% or less of Nb and 0.30% or less of Ti on a mass basis.   (14) A method for manufacturing a seamless stainless steel pipe for oil country tubular goods according to (13) in which the quenching includes heating to a temperature in the range of 800 to 1100° C. and cooling to room temperature at air-cooling speed or more, and the tempering is performed at a temperature in the range of 500 to 630° C.   (15) A method for manufacturing a seamless stainless steel pipe for oil country tubular goods according to any one of (12) to (14) in which the composition further contains at least one element selected from the group consisting of 0.20% or less of Zr, 0.01% or less of B, and 3.0% or less of W on a mass basis.   (16) A method for manufacturing a seamless stainless steel pipe for oil country tubular goods according to any one of (12) to (15) in which the composition further contains 0.0005% to 0.01% of Ca on a mass basis.       

    
    
     DETAILED DESCRIPTION 
     “High strength” refers to a strength (yield strength: 550 MPa or more) that conventional 13%-Cr martensitic stainless steel pipes for oil country tubular goods have, and preferably to a yield strength of 654 MPa or more. 
     To accomplish the above-described objects, we have conducted intensive research on the effects of alloying element contents to corrosion resistance in corrosive environments at high temperatures in the range of more than 180° C. to 230° C. containing CO 2 , Cl − , or the like, based on the compositions of the improved 13%-Cr martensitic stainless steel pipes. 
     As a result, it has been found that both of a favorable hot workability and a superior corrosion resistance in severe, corrosive environments can be ensured by reducing the C content to be lower than that of the known 13%-Cr martensitic stainless steels and adding suitable amounts of Ni, Mo, and Cu to adjust alloying element contents, so as to satisfy following expressions (1) and (2):
 
Cr+0.65Ni+0.6Mo+0.55Cu−20C≧18.5  (1)
 
Cr+Mo+0.3Si−43.5C−0.4Mn−Ni−0.3Cu−9N≦11  (2)
 
wherein Cr, Ni, Mo, Cu, C, Si, Mn, and N represent their respective contents (mass %). Furthermore, it has been found that a high strength of 654 MPa or more in terms of yield strength can be ensured.
 
     This disclosure has been completed based on these findings. 
     The reason why the steel compositions are controlled will now be explained. Hereinafter, mass percent is expressed by simply %. 
     C: 0.05% or Less 
     C is an essential element relating to the strength of martensitic stainless steel, but a C content of more than 0.05% promotes sensitization at the stage of tempering due to the presence of Ni. To prevent the sensitization at the stage of tempering, the C content is limited to 0.05% or less. In view of corrosion resistance, it is preferable that the C content be set as lower as possible. Preferably, it is 0.03% or less. More preferably, it is set in the range of 0.01% to 0.03%. 
     Si: 0.50% or Less 
     The element Si serves as a deoxidizer, and, preferably, its content is 0.05% or more. However, a content of more than 0.50% reduces the CO 2  corrosion resistance and further reduces the hot workability. Accordingly, the Si content is limited to 0.50% or less. Preferably, it is set in the range of 0.10% to 0.30%. 
     Mn: 0.20% to 1.80% 
     The element Mn enhances steel strength. To ensure a strength desired, the Mn content has to be 0.20% or more. However, a content of more than 1.80% negatively affects the toughness. Accordingly, the Mn content is limited to the range of 0.20% to 1.80%. Preferably, it is set in the range of 0.20% to 1.00%. More preferably, it is set in the range of 0.20% to 0.80%. 
     P: 0.03% or Less 
     The element P negatively affects the CO 2  corrosion resistance, CO 2  stress-corrosion cracking resistance, pitting corrosion resistance, and sulfide stress-corrosion cracking resistance, and it is preferable that the P content be reduced as low as possible. However, an excessive reduction of P content increases cost. Accordingly, the P content is limited to 0.03% or less so as to allow industrial production at a low cost and prevent the degradation of CO 2  corrosion resistance, CO 2  stress-corrosion cracking resistance, pitting corrosion resistance, and sulfide stress-corrosion resistance. Preferably, it is set at 0.02% or less. 
     S: 0.005% or Less 
     The element S seriously reduces hot workability in manufacture of pipes, and the S content is, preferably, as low as possible. A S content of 0.005% or less makes it possible to manufacture pipes through a common process, and, therefore, the S content is limited to 0.005% or less. Preferably, it is set at 0.003% or less. 
     Cr: 14.0% to 18.0% 
     The element Cr forms a protective film on the surface of steel to increase the corrosion resistance, and particularly to increase the CO 2  corrosion resistance and CO 2  stress-corrosion cracking resistance. A Cr content of 14.0% or more is necessary from the viewpoint of increasing the corrosion resistance at high temperatures. However, a content of more than 18.0% reduces the hot workability. Accordingly, the Cr content is limited to the range of 14.0% to 18.0%. Preferably, it is set in the range of 14.5% to 17.5%. 
     Ni: 5.0% to 8.0% 
     The element Ni strengthens the protective film on the surface of steel to enhance the CO 2  corrosion resistance and CO 2  stress-corrosion cracking resistance, pitting corrosion resistance, and sulfide stress-corrosion cracking resistance. Furthermore, it has the effect of a solid solution strengthening and, accordingly, increases steel strength. These effects are exhibited when the Ni content is 5.0% or more. However, a content of more than 8.0% reduces the stability of the martensitic structure to decrease the strength. Accordingly, the Ni content is limited to the range of 5.0% to 8.0%. Preferably, it is set in the range of 5.5% to 7.0%. 
     Mo: 1.5% to 3.5% 
     The element Mo enhances the resistance to pitting by Cl − , and a content of 1.5% or more is necessary. While a content of less than 1.5% does not efficiently achieve the corrosion resistance in severe, corrosive environments at high temperatures, a content of more than 3.5% causes the formation of 6-ferrite to reduce the hot workability, CO 2  corrosion resistance, and CO 2  stress-corrosion cracking resistance and increases cost. Accordingly, the Mo content is limited to the range of 1.5% to 3.5%. Preferably, it is set in the range of 1.5% to 2.5%. 
     Cu: 0.5% to 3.5% 
     The element Cu strengthens the protective film on the surface of the steel to prevent from hydrogen-penetration into the steel, thereby enhancing the sulfide stress-corrosion cracking resistance. This effect is achieved when the Cu content is 0.5% or more. However, a content of more than 3.5% allows CuS to precipitate in grain boundaries to reduce the hot workability. Accordingly, the Cu content is limited to the range of 0.5% to 3.5%. Preferably, it is set in the range of 0.5% to 2.5%. 
     Al: 0.05% or Less 
     The element Al has a strong effect of deoxidation, but a content of more than 0.05% negatively affects the toughness of the steel. Accordingly, the Al content is limited to 0.05% or less. Preferably, it is set in the range of 0.01% to 0.03%. 
     V: 0.20% or Less 
     The element V enhances the strength of steel and also has the effect of improving the stress-corrosion cracking resistance. These effects are noticeably exhibited when the V content is 0.03% or more. However, a content of more than 0.20% reduces the toughness. Accordingly, the V content is limited to 0.20% or less. Preferably, it is set in the range of 0.03% to 0.08%. 
     N: 0.01% to 0.15% 
     The element N extremely enhances the pitting corrosion resistance. This effect is exhibited when the N content is 0.01% or more. However, a content of more than 0.15% allows the formation of various nitrides to reduce the toughness. Accordingly, the N content is limited to the range of 0.01% to 0.15%. Preferably, it is set in the range of 0.03% to 0.15%, and more preferably in the range of 0.03% to 0.08%. 
     O: 0.006% or Less 
     The element O is present in oxide forms in steel and negatively affects various characteristics. It is, therefore, preferable to be reduced as low as possible. In particular, an O content of more than 0.006% seriously reduces the hot workability, CO 2  stress-corrosion cracking resistance, pitting corrosion resistance, sulfide stress-corrosion cracking resistance, and toughness. Accordingly, the O content is limited to 0.006% or less. 
     The above-described basic composition may further contain at least either 0.20% or less of Nb or 0.30% or less of Ti. 
     Both the elements Nb and Ti enhance the strength and the toughness, and particularly increase the strength remarkably by tempering at a relatively low temperature in the range of 500 to 630° C. This effect is noticeably exhibited when the Nb and Ti contents are 0.02% or more and 0.01% or more, respectively. On the other hand, a Nb content of more than 0.20% and a Ti content of more than 0.30% reduce the toughness. In addition, Ti has the effect of improving the stress-corrosion cracking resistance. Accordingly, the Nb content is preferably limited to 0.20% or less, and the Ti content, 0.30% or less. 
     The above-described composition may further contain at least one element selected from the group consisting of 0.20% or less of Zr, 0.01% or less of B, and 3.0% or less of W. 
     Zr, B, and W each increases the strength, and at least one of them may be added if necessary. In addition to the effect of increasing the strength, Zr, B, and W can improve the stress-corrosion cracking resistance. These effects are noticeably exhibited when the composition contains 0.01% or more of Zr, 0.0005% or more of B, or 0.1% or more of W. On the other hand, if the composition contains more than 0.20% of Zr, more than 0.01% of B, or more than 3.0% of W, the toughness is reduced. Accordingly, the Zr content is preferably limited to 0.20% or less; the B content, 0.01% or less; and the W content, 3.0% or less. 
     The composition may further contain 0.0005% to 0.01% of Ca. 
     The element Ca forms CaS to fix the element S and, thus, to spheroidize sulfide inclusions, thereby reducing lattice distortion of the matrix in the vicinity of the inclusions to reduce the capability of trapping hydrogen of the inclusions advantageously. This effect is achieved when the Ca content is 0.0005% or more. However, a content of more than 0.01% increases CaO, and reduces the CO 2  corrosion resistance and pitting resistance. Accordingly, the Ca content is preferably limited to the range of 0.0005% to 0.01%. 
     In addition to the above-described requirements, the each element content have to satisfy following expressions (1) and (2):
 
Cr+0.65Ni+0.6Mo+0.55Cu−20C≧18.5  (1)
 
Cr+Mo+0.3Si−43.5C−0.4Mn−Ni−0.3Cu−9N≦11  (2)
 
wherein Cr, Ni, Mo, Cu, C, Si, Mn, and N represent their respective contents.
 
     By adjusting the Cr, Ni, Mo, Cu, and C contents so as to satisfy expression (1), the corrosion resistance in environments at high temperatures up to 230° C. including CO 2  or Cl −  is remarkably increased. Also, by adjusting the Cr, Mo, Si, C, Mn, Ni, Cu, and N contents so as to satisfy expression (2), the hot workability is enhanced. P, S, and O contents are significantly reduced to enhance the hot workability. However, reducing the P, S, and O contents is not enough to ensure a hot workability sufficient to produce seamless martensitic stainless steel pipes. To ensure a hot workability sufficient to make seamless martensitic stainless steel pipes, it is important to extremely reduce the P, S, and O contents, and besides to adjust the Cr, Mo, Si, C, Mn, Ni, Cu, and N contents so as to satisfy expression (2). 
     The balance of the foregoing elements is Fe and incidental impurities. 
     Preferably, the steel pipe has a structure comprising 5% to 25% of residual austenite phase on a volume basis and the balance being a martensite phase. Alternatively, the steel pipe has a structure comprising 5% to 25% of residual austenite phase, 5% or less of ferrite phase, and the balance being a martensite phase on a volume basis. 
     Although the structure of the steel pipe is essentially composed of the martensite phase, the martensite phase, preferably, contains 5% to 25% of a residual austenite phase, or further contains 5% or less of a ferrite phase, on a volume basis. 
     By allowing 5 percent by volume or more of residual austenite phase to be present, a high toughness can be achieved. However, more than 25 percent by volume of residual austenite phase reduces the strength. Accordingly, it is preferable that the percentage of the residual austenite phase is set in the range of 5 to 25 percent by volume. In addition, to enhance the corrosion resistance, it is preferable that 5 percent by volume or less of ferrite phase is allowed to be present. However, more than 5 percent by volume of ferrite phase remarkably reduces the hot workability. Accordingly, it is preferable that the percentage of the ferrite phase is set at 5 percent by volume or less. 
     A method for manufacturing the steel pipe will now be described taking a seamless steel pipe as an example. 
     First, it is preferable that a molten steel having the above-described composition be melted by a conventional steel making process using a converter, an electric furnace, a vacuum melting furnace, or the like, and then formed into a steel pipe material, such as, a billet by a conventional method, such as continuous casting or ingot making-slabbing. Then, the steel pipe material is heated and subjected to hot working to make a pipe by a common manufacturing process, such as that of Mannesmann-plug mill or Mannesmann-mandrel mill. Thus a seamless steel pipe with a desired size is yielded. After pipe making, the resulting seamless steel pipe is preferably cooled to room temperature at air-cooling speed or more. 
     The seamless steel pipe having the above-described steel composition can be given a structure mainly composed of a martensite phase by cooling at air-cooling speed or more after hot working. After the cooling at air-cooling speed or more, preferably, quenching is performed in which the steel pipe is heated again to a temperature of the A C3  transformation point or more and cooled to room temperature at air-cooling speed or more. Thus, the martensitic structure can be refined and the toughness of the steel can be increased. 
     Preferably, the quenched seamless steel pipe is subjected to tempering by being heated to a temperature of the A C1  transformation point or less. By heating to a temperature of the A C1  transformation point or less, preferably to 400° C. or more, for tempering, the resultant structure comprises a tempered martensite phase, further comprises a residual austenite phase, or still further comprises a small amount of ferrite phase in some cases. Thus, the resulting seamless steel pipe exhibits a desired strength, a desired toughness, and a desired, superior corrosion resistance. 
     Only tempering may be performed without quenching. 
     The description above illustrates a steel pipe taking the seamless steel pipe as an example, but it is not limited to this form. A steel pipe material having the composition within the scope may result in an electric welded steel pipe or a UOE steel pipe used as a steel pipe for oil country tubular goods through a conventional process. However, for the electric welded steel tube and UOE steel pipe, it is preferable that, after pipe making, the pipe is quenched by heating the pipe again to a temperature of the A C3  transformation point or more and cooling to room temperature at air-cooling speed or more, and is subsequently tempered at a temperature of the A C1  transformation point or less. 
     In the case of a steel pipe having a composition containing at least one element of Nb and Ti, quenching includes heating to a temperature of 800 to 1100° C., and cooling to room temperature at air-cooling speed or more. Also, tempering is preferably performed at a temperature in the range of 500 to 630° C. By subjecting the steel pipe having the composition containing at least one element of Nb and Ti to these quenching and tempering, a sufficient amount of fine precipitates can occur to achieve a high strength of 654 MPa or more in terms of yield strength. 
     A quenching temperature of less than 800° C. does not sufficiently achieve the effect of tempering to provide a desired strength. On the other hand, a quenching temperature of more than 1100° C. coarsens the crystal grains to reduce the toughness of the steel. While a tempering temperature of less than 500° C. does not precipitate a sufficient amount of precipitations, a tempering temperature of more than 630° C. remarkably reduces the strength of the steel. 
     EXAMPLES 
     This disclosure will be further described in detail with reference to Examples. 
     Example 1 
     After degassing, each molten steel having a composition shown in Table 1 was cast into a steel ingot of 100 kgf (980 N). The ingot was subjected to hot working to make a pipe with a model seamless rolling mill, followed by air cooling to yield a seamless steel pipe with an outer diameter of 3.3 in. by a thickness of 0.5 in. 
     The hot workability was evaluated by visually observing the presence of cracks in the internal and external surfaces of the resulting seamless steel pipe as air-cooled after pipe making. 
     The seamless steel pipe was cut into a test piece. The test piece was heated at 920° C. for 1 hour and then water-cooled. The test piece was further subjected to tempering at 600° C. for 30 minutes. It was ensured that quenching was performed on each sample at a temperature of its A C3  transformation point or more, and that tempering was performed at a temperature of its A C1  transformation point or less. The quench-tempered test piece was machined into a corrosion-test piece of 3 mm in thickness by 30 mm in width by 40 mm in length, followed by a corrosion test. Some of the steel pipe samples were subjected to only tempering without quenching. 
     In the corrosion test, the test piece was immersed in a test solution being 20% NaCl aqueous solution placed in an autoclave (solution temperature: 230° C., CO 2  gas atmosphere at a pressure of 100 atmospheres) and was allowed to keep for 2 weeks. 
     The test piece after the corrosion test was weighed, and the corrosion rate was obtained from the difference between the weight of the test piece before the test and that after the test. The surface of the corrosion test piece after the test was observed to check for the occurrence of pitting with a loupe of a magnification of 10 times. 
     The results are shown in Table 2. 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                 Steel 
                 Chemical compositions (mass %) 
               
             
          
           
               
                 No. 
                 C 
                 Si 
                 Mn 
                 P 
                 S 
                 Al 
                 Cr 
                 Ni 
                 Mo 
                 Cu 
                 V 
               
               
                   
               
               
                 A 
                 0.019 
                 0.19 
                 0.48 
                 0.02 
                 0.001 
                 0.01 
                 14.8 
                 5.19 
                 1.60 
                 0.65 
                 0.049 
               
               
                 B 
                 0.024 
                 0.20 
                 0.44 
                 0.02 
                 0.001 
                 0.01 
                 14.9 
                 5.50 
                 1.50 
                 0.59 
                 0.051 
               
               
                 C 
                 0.015 
                 0.24 
                 0.46 
                 0.03 
                 0.001 
                 0.02 
                 15.3 
                 6.12 
                 2.04 
                 1.05 
                 0.059 
               
               
                 D 
                 0.025 
                 0.22 
                 0.50 
                 0.01 
                 0.002 
                 0.01 
                 15.1 
                 5.59 
                 2.49 
                 1.63 
                 0.048 
               
               
                 E 
                 0.027 
                 0.20 
                 0.42 
                 0.01 
                 0.001 
                 0.02 
                 15.5 
                 6.27 
                 1.75 
                 0.77 
                 0.040 
               
               
                 F 
                 0.024 
                 0.21 
                 0.40 
                 0.02 
                 0.001 
                 0.01 
                 16.2 
                 5.93 
                 1.66 
                 1.14 
                 0.041 
               
               
                 G 
                 0.020 
                 0.26 
                 0.41 
                 0.01 
                 0.001 
                 0.01 
                 16.5 
                 6.05 
                 2.17 
                 0.88 
                 0.030 
               
               
                 H 
                 0.016 
                 0.33 
                 0.40 
                 0.01 
                 0.001 
                 0.02 
                 16.9 
                 5.99 
                 1.52 
                 1.02 
                 0.052 
               
               
                 I 
                 0.026 
                 0.28 
                 0.48 
                 0.01 
                 0.001 
                 0.01 
                 17.3 
                 6.54 
                 1.69 
                 0.64 
                 0.049 
               
               
                 J 
                 0.017 
                 0.27 
                 0.49 
                 0.01 
                 0.001 
                 0.01 
                 17.7 
                 7.05 
                 1.53 
                 0.85 
                 0.042 
               
               
                 K 
                 0.034 
                 0.27 
                 0.50 
                 0.02 
                 0.001 
                 0.02 
                 17.4 
                 5.58 
                 2.87 
                 0.67 
                 0.046 
               
               
                 L 
                 0.022 
                 0.26 
                 0.45 
                 0.02 
                 0.001 
                 0.01 
                 
                   13.8 
                 
                 6.19 
                 1.68 
                 0.71 
                 0.055 
               
               
                 M 
                 0.045 
                 0.31 
                 0.49 
                 0.01 
                 0.002 
                 0.01 
                 14.6 
                 5.11 
                 1.55 
                 0.59 
                 0.048 
               
               
                 N 
                 0.020 
                 0.26 
                 0.42 
                 0.03 
                 0.002 
                 0.02 
                 14.7 
                 
                   4.55 
                 
                 1.53 
                 0.69 
                 0.063 
               
               
                 O 
                 0.016 
                 0.33 
                 0.44 
                 0.01 
                 0.001 
                 0.01 
                 14.8 
                 5.27 
                 
                   0.56 
                 
                 0.73 
                 0.065 
               
               
                 P 
                 0.021 
                 0.21 
                 0.44 
                 0.02 
                 0.001 
                 0.02 
                 17.1 
                 5.15 
                 1.96 
                 0.57 
                 0.056 
               
               
                 Q 
                 0.026 
                 0.35 
                 0.39 
                 0.02 
                 0.001 
                 0.02 
                 14.6 
                 5.19 
                 1.64 
                 
                   0.26 
                 
                 0.045 
               
               
                   
               
             
          
           
               
                 Steel 
                 Chemical compositions (mass %) 
                 Expression 
                 Expression 
                   
               
             
          
           
               
                 No. 
                 N 
                 O 
                 Other 
                 (1)* 
                 (2)** 
                 Remarks 
               
               
                   
               
               
                 A 
                 0.059 
                 0.0019 
                   
                 19.11 
                 9.5225 
                 Example 
               
               
                 B 
                 0.062 
                 0.0025 
                 Nb: 0.026 
                 19.22 
                 9.005  
                 Example 
               
               
                 C 
                 0.043 
                 0.0037 
                 Zr: 0.017 
                 20.78 
                 9.7535 
                 Example 
               
               
                 D 
                 0.072 
                 0.0021 
                 Ti: 0.034 
                 20.62 
                 9.6415 
                 Example 
               
               
                 E 
                 0.033 
                 0.0018 
                   
                 20.51 
                 9.1695 
                 Example 
               
               
                 F 
                 0.039 
                 0.0019 
                 Ti: 0.021, 
                 21.20 
                 10.096  
                 Example 
               
               
                   
                   
                   
                 B: 0.001 
               
               
                 G 
                 0.054 
                 0.0026 
                 Ca: 0.002 
                 21.82 
                 10.914  
                 Example 
               
               
                 H 
                 0.095 
                 0.0036 
                 Nb: 0.019, 
                 21.95 
                 10.512  
                 Example 
               
               
                   
                   
                   
                 Ca: 0.001 
               
               
                 I 
                 0.066 
                 0.0030 
                 W: 0.270 
                 22.40 
                 10.425  
                 Example 
               
               
                 J 
                 0.069 
                 0.0016 
                 B: 0.001 
                 23.33 
                 10.4495  
                 Example 
               
               
                 K 
                 0.056 
                 0.0028 
                   
                 22.44 
                   12.387   
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 L 
                 0.106 
                 0.0017 
                   
                 18.78 
                 7.064  
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 M 
                 0.042 
                 0.0024 
                 Ti: 0.024 
                 
                   18.28 
                 
                 8.4245 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 N 
                 0.059 
                 0.0026 
                   
                 18.56 
                 9.982  
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 O 
                 0.058 
                 0.0034 
                   
                 18.64 
                 8.576  
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 P 
                 0.062 
                 0.0028 
                 Nb: 0.033 
                 21.52 
                   12.1545   
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 Q 
                 0.038 
                 0.0019 
                 — 
                 19.10 
                 9.45  
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                   
               
               
                 *Expression (1) = (Cr) + 0.65 (Ni) + 0.6 (Mo) + 0.55 (Cu) − 20 (C) 
               
               
                 **Expression (2) = (Cr) + (Mo) + 0.3 (Si) − 43.5 (C) − 0.4 (Mn) − (Ni) − 0.3 (Cu) − 9 (N) 
               
             
          
         
       
     
     
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                   
                   
                 Cooling 
                   
                   
                 Hot 
                 Corrosion 
                   
               
               
                 Steel 
                   
                 after 
                 Quenching 
                 Tempering 
                 work- 
                 resistance 
               
             
          
           
               
                 pipe 
                 Steel 
                 pipe- 
                 Temp 
                 Cool- 
                 Temp 
                 Cool- 
                 ability 
                 Corrosion 
                   
                   
               
               
                 No. 
                 No. 
                 making 
                 (° C.) 
                 ing 
                 (° C.) 
                 ing 
                 Crack 
                 rate (mm/yr) 
                 Pitting 
                 Remarks 
               
               
                   
               
             
          
           
               
                 1 
                 A 
                 Air 
                 920 
                 Air 
                 600 
                 Air 
                 Good 
                 0.113 
                 Good 
                 Example 
               
               
                 2 
                 B 
                 Air 
                 920 
                 Air 
                 600 
                 Air 
                 Good 
                 0.102 
                 Good 
                 Example 
               
               
                 3 
                 C 
                 Air 
                 920 
                 Air 
                 600 
                 Air 
                 Good 
                 0.091 
                 Good 
                 Example 
               
               
                 4 
                 D 
                 Air 
                 920 
                 Air 
                 600 
                 Air 
                 Good 
                 0.092 
                 Good 
                 Example 
               
               
                 5 
                 E 
                 Air 
                 920 
                 Air 
                 600 
                 Air 
                 Good 
                 0.091 
                 Good 
                 Example 
               
               
                 6 
                 F 
                 Air 
                 920 
                 Air 
                 600 
                 Air 
                 Good 
                 0.063 
                 Good 
                 Example 
               
               
                 7 
                 G 
                 Air 
                 920 
                 Air 
                 600 
                 Air 
                 Good 
                 0.061 
                 Good 
                 Example 
               
               
                 8 
                 H 
                 Air 
                 920 
                 Air 
                 600 
                 Air 
                 Good 
                 0.045 
                 Good 
                 Example 
               
               
                 9 
                 I 
                 Air 
                 920 
                 Air 
                 600 
                 Air 
                 Good 
                 0.036 
                 Good 
                 Example 
               
               
                 10 
                 J 
                 Air 
                 920 
                 Air 
                 600 
                 Air 
                 Good 
                 0.044 
                 Good 
                 Example 
               
               
                 11 
                 K 
                 Air 
                 920 
                 Air 
                 600 
                 Air 
                 Bad 
                 0.036 
                 Good 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 12 
                 L 
                 Air 
                 920 
                 Air 
                 600 
                 Air 
                 Good 
                 0.149 
                 Good 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 13 
                 M 
                 Air 
                 920 
                 Air 
                 600 
                 Air 
                 Good 
                 0.162 
                 Good 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 14 
                 N 
                 Air 
                 920 
                 Air 
                 600 
                 Air 
                 Good 
                 0.132 
                 Good 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 15 
                 O 
                 Air 
                 920 
                 Air 
                 600 
                 Air 
                 Bad 
                 0.179 
                 Bad 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 16 
                 P 
                 Air 
                 920 
                 Air 
                 600 
                 Air 
                 Good 
                 0.078 
                 Good 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 17 
                 Q 
                 Air 
                 920 
                 Air 
                 600 
                 Air 
                 Good 
                 0.119 
                 Bad 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 18 
                 A 
                 Air 
                 — 
                 — 
                 600 
                 Air 
                 Good 
                 0.107 
                 Good 
                 Example 
               
               
                   
               
             
          
         
       
     
     Each example exhibited no occurrence of cracks in the steel pipe surfaces, a low corrosion rate, and no occurrence of pitting. Hence, it has been shown that the steel pipes of these examples have a superior hot workability and a superior corrosion resistance in a severe, corrosive environment at a high temperature of 230° C. containing CO 2 . In contrast, comparative examples outside the scope of this disclosure exhibited occurrence of cracks, thus showing a reduced hot workability, or exhibited a high corrosion rate, thus showing a reduced corrosion resistance. In particular, there were surface flaws in the steel pipes of comparative examples not satisfying expression (2) due to a reduced hot workability. 
     Example 2 
     After sufficient degassing, each molten steel having a composition shown in Table 3 was cast into a steel ingot of 100 kgf (980 N). The ingot was formed into a seamless steel pipe with an outer diameter of 3.3 in. by a thickness of 0.5 in. with a model seamless rolling mill. 
     After the pipe making, the hot workability was evaluated by visually observing the presence of cracks in the internal and external surfaces of the resulting seamless steel pipe. 
     The seamless steel pipe was cut into a test piece. The test piece was subjected to quenching and tempering under the conditions shown in Table 4. An ark-shaped API tensile test piece was taken from the quench-tempered test piece and subjected to a tensile test for the tensile properties (yield strength YS, tensile strength TS). Also, a corrosion-test piece of 3 mm in thickness by 30 nm in width by 40 mm in length was taken from the foregoing quench-tempered test piece by machining, and was subjected to a corrosion test. 
     In the corrosion test, the test piece was immersed in a test solution being 20% NaCl aqueous solution placed in an autoclave (solution temperature: 230° C., CO 2  gas atmosphere at a pressure of 30 atmospheres) and was allowed to keep for 2 weeks. 
     The test piece after the corrosion test was weighed, and the corrosion rate was obtained from the difference between the weight of the corrosion test piece before the test and that after the test. The surface of the corrosion test piece after the test was observed to check for the occurrence of pitting with a loupe of a magnification of 10 times. The results are shown in Table 4. 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
             
             
               
                 Steel 
                 Chemical compositions (mass %) 
               
             
          
           
               
                 No. 
                 C 
                 Si 
                 Mn 
                 P 
                 S 
                 Al 
                 Cr 
                 Ni 
                 Mo 
                 Cu 
                 V 
                 N 
               
               
                   
               
               
                 2A 
                 0.025 
                 0.19 
                 0.34 
                 0.02 
                 0.001 
                 0.01 
                 14.7 
                 6.20 
                 1.90 
                 0.65 
                 0.044 
                 0.059 
               
               
                 2B 
                 0.022 
                 0.29 
                 0.49 
                 0.02 
                 0.001 
                 0.01 
                 14.9 
                 5.85 
                 1.94 
                 0.65 
                 0.049 
                 0.078 
               
               
                 2C 
                 0.034 
                 0.18 
                 0.56 
                 0.02 
                 0.001 
                 0.02 
                 14.9 
                 6.13 
                 2.06 
                 0.71 
                 0.035 
                 0.045 
               
               
                 2D 
                 0.028 
                 0.31 
                 0.41 
                 0.01 
                 0.002 
                 0.01 
                 15.1 
                 7.03 
                 1.63 
                 0.58 
                 0.059 
                 0.052 
               
               
                 2E 
                 0.015 
                 0.17 
                 0.36 
                 0.02 
                 0.001 
                 0.02 
                 15.4 
                 6.17 
                 2.34 
                 1.24 
                 0.064 
                 0.042 
               
               
                 2F 
                 0.027 
                 0.30 
                 0.35 
                 0.02 
                 0.001 
                 0.01 
                 16.8 
                 7.06 
                 1.71 
                 0.62 
                 0.080 
                 0.320 
               
               
                 2G 
                 0.017 
                 0.25 
                 0.44 
                 0.01 
                 0.001 
                 0.01 
                 16.7 
                 6.29 
                 1.77 
                 0.91 
                 0.040 
                 0.062 
               
               
                 2H 
                 0.028 
                 0.24 
                 0.39 
                 0.01 
                 0.001 
                 0.02 
                 17.2 
                 6.34 
                 1.59 
                 0.74 
                 0.037 
                 0.099 
               
               
                 2I 
                 0.035 
                 0.35 
                 0.39 
                 0.02 
                 0.001 
                 0.01 
                 17.1 
                 5.96 
                 2.81 
                 0.63 
                 0.049 
                 0.029 
               
               
                 2J 
                 0.046 
                 0.30 
                 0.40 
                 0.02 
                 0.001 
                 0.01 
                 
                   13.4 
                 
                 5.30 
                 2.57 
                 2.48 
                 0.062 
                 0.053 
               
               
                 2K 
                 0.023 
                 0.25 
                 0.36 
                 0.01 
                 0.002 
                 0.01 
                 14.3 
                 5.05 
                 1.55 
                 0.59 
                 0.056 
                 0.059 
               
               
                 2L 
                 0.035 
                 0.26 
                 0.45 
                 0.02 
                 0.002 
                 0.02 
                 15.4 
                 
                   4.06 
                 
                 1.63 
                 0.53 
                 0.051 
                 0.071 
               
               
                   
               
             
          
           
               
                 Steel 
                 Chemical compositions (mass %) 
                 Expression 
                 Expression 
                   
               
             
          
           
               
                 No. 
                 O 
                 Nb 
                 Ti 
                 Other 
                 (1)* 
                 (2)** 
                 Remarks 
               
               
                   
               
               
                 2A 
                 0.0019 
                 0.074 
                 — 
                 — 
                 19.84 
                 8.45 
                 Example 
               
               
                 2B 
                 0.0015 
                 — 
                 0.077 
                 — 
                 19.78 
                 9.03 
                 Example 
               
               
                 2C 
                 0.0021 
                 0.049 
                 0.072 
                 — 
                 19.83 
                 8.56 
                 Example 
               
               
                 2D 
                 0.0027 
                 0.087 
                 — 
                 Zr: 0.061 
                 20.41 
                 7.77 
                 Example 
               
               
                 2E 
                 0.0047 
                 0.038 
                 0.075 
                 B: 0.001 
                 21.20 
                 10.07  
                 Example 
               
               
                 2F 
                 0.0026 
                 0.089 
                 0.036 
                 Ca: 0.003 
                 22.22 
                 7.16 
                 Example 
               
               
                 2G 
                 0.0017 
                 0.087 
                 0.042 
                 W: 0.220 
                 22.01 
                 10.51  
                 Example 
               
               
                 2H 
                 0.0028 
                 — 
                 0.150 
                 Zn: 0.083 
                 22.12 
                 10.04  
                 Example 
               
               
                   
                   
                   
                   
                 Ca: 0.001 
               
               
                 2I 
                 0.0051 
                 0.078 
                 — 
                 — 
                 22.31 
                   11.93   
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 2J 
                 0.0026 
                 — 
                 0.062 
                 — 
                 18.83 
                 7.38 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 2K 
                 0.0022 
                 0.073 
                 0.047 
                 Zr: 0.024 
                 
                   18.38 
                 
                 9.02 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 2L 
                 0.0019 
                 0.049 
                 0.023 
                 Ca: 0.003 
                 18.61 
                 10.55  
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                   
               
               
                 *Expression (1) = (Cr) + 0.65 (Ni) + 0.6 (Mo) + 0.55 (Cu) − 20 (C) 
               
               
                 **Expression (2) = (Cr) + (Mo) + 0.3 (Si) − 43.5 (C) − 0.4 (Mn) − (Ni) − 0.3 (Cu) − 9 (N) 
               
             
          
         
       
     
     
       
         
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                   
                   
                 Cooling 
                   
                   
                 Tensile 
                 Hot 
                 Corrision 
                   
               
               
                 Steel 
                   
                 after 
                 Quenching 
                 Tempering 
                 properties 
                 work- 
                 resistance 
               
             
          
           
               
                 pipe 
                 Steel 
                 pipe- 
                 Temp 
                   
                 Temp 
                   
                 YS 
                 TS 
                 ability 
                 Corrosion 
                   
                   
               
               
                 No. 
                 No. 
                 making 
                 (° C.) 
                 Cooling 
                 (° C.) 
                 Cooling 
                 MPa 
                 MPa 
                 Crack 
                 rate (mm/yr) 
                 Pitting 
                 Remarks 
               
               
                   
               
             
          
           
               
                 21 
                 2A 
                 Air 
                 890 
                 Air 
                 530 
                 Air 
                 910 
                 1138 
                 Good 
                 0.115 
                 Good 
                 Example 
               
               
                 22 
                 2A 
                 Air 
                 890 
                 Air 
                 610 
                 Air 
                 874 
                 1110 
                 Good 
                 0.112 
                 Good 
                 Example 
               
               
                 23 
                 2B 
                 Air 
                 890 
                 Air 
                 530 
                 Air 
                 926 
                 1123 
                 Good 
                 0.109 
                 Good 
                 Example 
               
               
                 24 
                 2B 
                 Air 
                 890 
                 Air 
                 610 
                 Air 
                 891 
                 1049 
                 Good 
                 0.118 
                 Good 
                 Example 
               
               
                 25 
                 2C 
                 Air 
                 890 
                 Air 
                 580 
                 Air 
                 892 
                 1032 
                 Good 
                 0.104 
                 Good 
                 Example 
               
               
                   
                 2D 
                 Air 
                 890 
                 Air 
                 580 
                 Air 
                 821 
                 1004 
                 Good 
                 0.065 
                 Good 
                 Example 
               
               
                 27 
                 2E 
                 Air 
                 890 
                 Air 
                 580 
                 Air 
                 836 
                 966 
                 Good 
                 0.071 
                 Good 
                 Example 
               
               
                 28 
                 2F 
                 Air 
                 890 
                 Air 
                 580 
                 Air 
                 715 
                 884 
                 Good 
                 0.053 
                 Good 
                 Example 
               
               
                 29 
                 2G 
                 Air 
                 890 
                 Air 
                 580 
                 Air 
                 723 
                 901 
                 Good 
                 0.049 
                 Good 
                 Example 
               
               
                 30 
                 2H 
                 Air 
                 890 
                 Air 
                 580 
                 Air 
                 720 
                 877 
                 Good 
                 0.051 
                 Good 
                 Example 
               
               
                 31 
                 2I 
                 Air 
                 890 
                 Air 
                 580 
                 Air 
                 713 
                 864 
                 Bad 
                 0.056 
                 Good 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 32 
                 2J 
                 Air 
                 890 
                 Air 
                 580 
                 Air 
                 908 
                 1073 
                 Good 
                 0.172 
                 Good 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 33 
                 2K 
                 Air 
                 890 
                 Air 
                 580 
                 Air 
                 875 
                 943 
                 Good 
                 0.148 
                 Good 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 34 
                 2L 
                 Air 
                 890 
                 Air 
                 580 
                 Air 
                 892 
                 968 
                 Good 
                 0.162 
                 Good 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 35 
                 2A 
                 Air 
                 780 
                 Air 
                 600 
                 Air 
                 469 
                 934 
                 Good 
                 0.109 
                 Good 
                 Example 
               
               
                 36 
                 2B 
                 Air 
                 760 
                 Air 
                 600 
                 Air 
                 492 
                 972 
                 Good 
                 0.113 
                 Good 
                 Example 
               
               
                 37 
                 2G 
                 Air 
                 890 
                 Air 
                 650 
                 Air 
                 603 
                 783 
                 Good 
                 0.044 
                 Good 
                 Example 
               
               
                 38 
                 2H 
                 Air 
                 910 
                 Air 
                 640 
                 Air 
                 613 
                 768 
                 Good 
                 0.046 
                 Good 
                 Example 
               
               
                   
               
             
          
         
       
     
     Each example exhibited no occurrence of cracks in the steel pipe surfaces, a low corrosion rate, and no occurrence of pitting. Hence, it was shown that the steel pipes of these examples had a superior hot workability and a superior corrosion resistance in a severe, corrosive environment at a high temperature of 230° C. containing CO 2 . In contrast, comparative examples outside the scope of this disclosure exhibited occurrence of cracks, thus showing a reduced hot workability, or exhibited a high corrosion rate, thus showing a reduced corrosion resistance. When the manufacture conditions were outside the preferred ranges as set forth, the strength was reduced and, accordingly, a high yield strength of 654 MPa or more was not achieved. 
     Example 3 
     After sufficient degassing, each molten steel having a composition shown in Table 5 was cast into a steel ingot of 100 kgf (980 N). The ingot was formed into a seamless steel pipe with an outer diameter of 3.3 in. by a thickness of 0.5 in. with a model seamless rolling mill. 
     The hot workability was evaluated by visually observing the presence of cracks in the internal and external surfaces of the resulting seamless steel pipe, as in Example 1. 
     The seamless steel pipe was cut into a test piece. The test piece was subjected to quenching and tempering under the conditions shown in Table 6. It was ensured that quenching was performed on each sample at a temperature of its A C3  transformation point or more, and that tempering was performed at a temperature of its A C1  transformation point or less. A structure observation test piece was taken from the quench-tempered test piece. The structure observation test piece was etched by aqua regia. The resulting structure was observed with a scanning electron microscope (1000 times), and the percentage of the ferrite phase (percent by volume) was computed with an image analysis system. The percentage of the residual austenite phase was determined by X-ray diffraction. 
     An ark-shaped API tensile test piece was taken from the quench-tempered test piece and subjected to a tensile test for the tensile properties (yield strength YS, tensile strength TS), as in Example 1. Also, a V-notch test piece (thickness: 5 mm) was taken from the quench-tempered test piece, in accordance with JIS Z 2202, and the Charpy impact test was performed on the V-notch test piece to determine the absorption energy vE −40  (J) at −40° C. in accordance with JIS Z 2242. 
     Furthermore, a corrosion-test piece of 3 mm in thickness by 30 mm in width by 40 mm in length was taken from the foregoing quench-tempered test piece by machining, and was subjected to a corrosion test, as in Example 2. 
     In the corrosion test, the test piece was immersed in a test solution being 20% NaCl aqueous solution placed in an autoclave (solution temperature: 230° C., CO 2  gas atmosphere at a pressure of 30 atmospheres) and was allowed to keep for 2 weeks. 
     The test piece after the corrosion test was weighed, and the corrosion rate was obtained from the difference between the weight of the test piece before the test and that after the test. The surface of the corrosion test piece after the test was observed to check for the occurrence of pitting with a loupe of a magnification of 10 times. 
     The results are shown in Table 6. 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
             
             
               
                 Steel 
                 Chemical compositions (mass %) 
               
             
          
           
               
                 No. 
                 C 
                 Si 
                 Mn 
                 P 
                 S 
                 Al 
                 Cr 
                 Ni 
                 Mo 
                 Cu 
                 V 
               
               
                   
               
               
                 3A 
                 0.027 
                 0.24 
                 0.31 
                 0.02 
                 0.001 
                 0.01 
                 15.2 
                 6.14 
                 1.60 
                 0.82 
                 0.039 
               
               
                 3B 
                 0.024 
                 0.21 
                 0.34 
                 0.02 
                 0.001 
                 0.01 
                 14.9 
                 5.50 
                 1.50 
                 1.22 
                 0.051 
               
               
                 3C 
                 0.018 
                 0.23 
                 0.36 
                 0.01 
                 0.002 
                 0.01 
                 16.1 
                 6.22 
                 1.62 
                 1.09 
                 0.059 
               
               
                 3D 
                 0.028 
                 0.20 
                 0.41 
                 0.02 
                 0.001 
                 0.02 
                 15.1 
                 5.59 
                 2.49 
                 1.63 
                 0.048 
               
               
                 3E 
                 0.017 
                 0.25 
                 0.29 
                 0.02 
                 0.001 
                 0.01 
                 16.8 
                 6.26 
                 1.57 
                 0.85 
                 0.042 
               
               
                 3G 
                 0.032 
                 0.26 
                 0.33 
                 0.02 
                 0.001 
                 0.01 
                 
                   13.7 
                 
                 6.19 
                 1.97 
                 0.71 
                 0.055 
               
               
                 3H 
                 0.035 
                 0.31 
                 0.29 
                 0.02 
                 0.002 
                 0.01 
                 14.5 
                 5.11 
                 1.55 
                 0.59 
                 0.048 
               
               
                   
               
             
          
           
               
                 Steel 
                 Chemical compositions (mass %) 
                 Expression 
                 Expression 
                   
               
             
          
           
               
                 No. 
                 N 
                 O 
                 Other 
                 (1)* 
                 (2)** 
                 Remarks 
               
               
                   
               
               
                 3A 
                 0.049 
                 0.0021 
                 — 
                 20.06 
                 8.75 
                 Example 
               
               
                 3B 
                 0.062 
                 0.0025 
                 Nb: 0.077 
                 19.57 
                 8.86 
                 Example 
               
               
                 3C 
                 0.043 
                 0.0037 
                 Zr: 0.017, 
                 21.35 
                 9.93 
                 Example 
               
               
                   
                   
                   
                 Ca: 0.002 
               
               
                 3D 
                 0.072 
                 0.0021 
                 Ti: 0.034, 
                 20.56 
                 9.54 
                 Example 
               
               
                   
                   
                   
                 Nb: 0.058 
               
               
                 3E 
                 0.069 
                 0.0016 
                 B: 0.001, 
                 21.94 
                 10.45 
                 Example 
               
               
                   
                   
                   
                 W: 0.19 
               
               
                 3G 
                 0.106 
                 0.0017 
                 — 
                 18.66 
                 6.87 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 3H 
                 0.042 
                 0.0024 
                 Ti: 0.024 
                 
                   18.38 
                 
                 8.94 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                   
               
               
                 *Expression (1) = (Cr) + 0.65 (Ni) + 0.6 (Mo) + 0.55 (Cu) − 20 (C) 
               
               
                 **Expression (2) = (Cr) + (Mo) + 0.3 (Si) − 43.5 (C) − 0.4 (Mn) − (Ni) − 0.3 (Cu) − 9 (N) 
               
             
          
         
       
     
     
       
         
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 6 
               
             
             
               
                   
                   
               
               
                   
                 Corrosion 
                   
               
               
                   
                 resistance 
                   
               
             
          
           
               
                   
                 Cooling 
                   
                   
                   
                   
                 Structure 
                 Tensile 
                 Impact 
                 Hot 
                 Corro- 
                   
                   
               
             
          
           
               
                 Steel 
                   
                 after 
                 Quenching 
                 Tempering 
                 γ 
                 α 
                 properties 
                 property 
                 work- 
                 sion 
                   
                   
               
             
          
           
               
                 pipe 
                 Steel 
                 Pipe- 
                 Temp 
                 Cool- 
                 Temp 
                 Cool- 
                 quantity 
                 quantity 
                 YS 
                 TS 
                 Absorbed 
                 ability 
                 rate 
                   
                   
               
               
                 No. 
                 No. 
                 making 
                 (° C.) 
                 ing 
                 (° C.) 
                 ing 
                 vol % 
                 vol % 
                 MPa 
                 MPa 
                 energy E 40 J 
                 Crack 
                 (mm/yr) 
                 Pitting 
                 Remarks 
               
               
                   
               
             
          
           
               
                 A1 
                 3A 
                 Air 
                 890 
                 Air 
                 550 
                 Air 
                 7.1 
                 — 
                 868 
                 1021 
                 80.2 
                 Good 
                 0.109 
                 Good 
                 Example 
               
               
                 A2 
                 3A 
                 Air 
                 890 
                 Air 
                 600 
                 Air 
                 10.9 
                 — 
                 792 
                 1047 
                 86.1 
                 Good 
                 0.107 
                 Good 
                 Example 
               
               
                 A3 
                 3B 
                 Air 
                 890 
                 Air 
                 500 
                 Air 
                 6.3 
                 0.3 
                 889 
                 1061 
                 83.4 
                 Good 
                 0.111 
                 Good 
                 Example 
               
               
                 A4 
                 3B 
                 Air 
                 890 
                 Air 
                 600 
                 Air 
                 11.2 
                 0.7 
                 847 
                 1030 
                 85.7 
                 Good 
                 0.112 
                 Good 
                 Example 
               
               
                 A5 
                 3C 
                 Air 
                 890 
                 Air 
                 550 
                 Air 
                 12.5 
                 1.5 
                 820 
                 1035 
                 91.2 
                 Good 
                 0.058 
                 Good 
                 Example 
               
               
                 A6 
                 3D 
                 Air 
                 890 
                 Air 
                 550 
                 Air 
                 16.3 
                 1.9 
                 771 
                 974 
                 95.4 
                 Good 
                 0.102 
                 Good 
                 Example 
               
               
                 A7 
                 3E 
                 Air 
                 890 
                 Air 
                 550 
                 Air 
                 22.7 
                 3.8 
                 723 
                 982 
                 95.9 
                 Good 
                 0.039 
                 Good 
                 Example 
               
               
                 A8 
                 3D 
                 Air 
                 890 
                 Air 
                 650 
                 Air 
                 26.3 
                 1.7 
                 634 
                 915 
                 104.3 
                 Good 
                 0.105 
                 Good 
                 Example 
               
               
                 A9 
                 3E 
                 Air 
                 890 
                 Air 
                 650 
                 Air 
                 29.6 
                 4.0 
                 599 
                 907 
                 107.6 
                 Good 
                 0.037 
                 Good 
                 Example 
               
               
                 A10 
                 3F 
                 Air 
                 890 
                 Air 
                 500 
                 Air 
                 3.2 
                 5.4 
                 999 
                 1149 
                 42.3 
                 Bad 
                 0.096 
                 Good 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 A11 
                 3G 
                 Air 
                 890 
                 Air 
                 550 
                 Air 
                 6.1 
                 — 
                 875 
                 1095 
                 79.3 
                 Good 
                 0.179 
                 Bad 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 A12 
                 3H 
                 Air 
                 890 
                 Air 
                 540 
                 Air 
                 7.3 
                 2.7 
                 827 
                 1046 
                 77.0 
                 Good 
                 0.150 
                 Good 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 A13 
                 3A 
                 Air 
                 890 
                 Air 
                 450 
                 Air 
                 — 
                 — 
                 949 
                 1018 
                 37.5 
                 Good 
                 0.124 
                 Good 
                 Example 
               
               
                   
               
               
                 γ: residual austenite, 
               
               
                 α: ferrite (δ) 
               
             
          
         
       
     
     Each example exhibited no occurrence of cracks in the steel pipe surfaces, a low corrosion rate, and no occurrence of pitting; hence it was shown that steel pipes of these examples had a superior hot workability. In addition, their structure containing 5 to 25 percent by volume of residual austenite phase, or further containing 5 percent by volume or less of ferrite phase leads to a superior corrosion resistance in a severe, corrosive environment at a high temperature of 230° C. containing CO 2 . Furthermore, the strength is as high as 654 MPa or more in terms of yield strength YS and the toughness is as high as 60 J or more in terms of absorbed energy at −40° C. 
     In contrast, comparative examples outside the scope of this disclosure exhibited occurrence of cracks, thus showing a reduced hot workability, or exhibited a high corrosion rate, thus showing a reduced corrosion resistance. When the manufacture conditions were outside the preferred ranges, the strength was decreased and, accordingly, a high yield strength of 654 MPa or more was not achieved. 
     INDUSTRIAL APPLICABILITY 
     A high-strength martensitic stainless steel pipe for oil country tubular goods can be manufactured at a low cost with stability which has a sufficient corrosion resistance in severe, corrosive environments at high temperatures containing CO 2  or Cl −  or which has a high toughness in addition to such a sufficient corrosion resistance, thus producing particularly advantageous industrial effects.