Patent Publication Number: US-2023137295-A1

Title: Stainless steel seamless pipe and method for manufacturing stainless steel seamless pipe

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is the U.S. National Phase application of PCT/JP2021/009891, filed Mar. 11, 2021 which claims priority to Japanese Patent Application No. 2020-048549, filed Mar. 19, 2020, the disclosures of these applications being incorporated herein by reference in their entireties for all purposes. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a stainless steel seamless pipe suited for oil country tubular goods for oil wells and gas wells (hereinafter, referred to simply as “oil wells”). Particularly, the invention relates to a stainless steel seamless pipe having improved corrosion resistance in various corrosive environments, particularly, severe high-temperature corrosive environments containing carbon dioxide (CO 2 ) and chlorine ions (Cl − ). 
     BACKGROUND OF THE INVENTION 
     An expected shortage of energy resources in the near future has prompted active development of oil wells that were unthinkable in the past, for example, such as those in deep oil fields, a carbon dioxide gas-containing environment, and a hydrogen sulfide-containing environment, or a sour environment as it is also called. The steel pipes for oil country tubular goods intended for these environments require high strength and high corrosion resistance. 
     Oil country tubular goods used for mining of oil fields and gas fields in environments containing CO 2 , Cl − , and the like typically use 13Cr martensitic stainless steel pipes. There has also been development of oil wells at higher temperatures (a temperature as high as 200° C.). However, the corrosion resistance of 13Cr martensitic stainless steel pipes is not always sufficient for such applications. Accordingly, there is a need for a steel pipe for oil country tubular goods that shows high corrosion resistance even when used in such environments. 
     In connection with such a demand, for example, PTL 1 describes a stainless steel for oil country tubular goods having a composition that contains, in mass %, C: 0.05% or less, Si: 1.0% or less, Mn: 0.01 to 1.0%, P: 0.05% or less, S: less than 0.002%, Cr: 16 to 18%, Mo: 1.8 to 3%, Cu: 1.0 to 3.5%, Ni: 3.0 to 5.5%, Co: 0.01 to 1.0%, Al: 0.001 to 0.1%, O: 0.05% or less, and N: 0.05% or less, and in which Cr, Ni, Mo, and Cu satisfy specific relationships. 
     PTL 2 describes a high-strength stainless steel seamless pipe for oil country tubular goods having a composition that contains, in mass %, C: 0.05% or less, Si: 1.0% or less, Mn: 0.1 to 0.5%, P: 0.05% or less, S: less than 0.005%, Cr: more than 15.0% and 19.0% or less, Mo: more than 2.0% and 3.0% or less, Cu: 0.3 to 3.5%, Ni: 3.0% or more and less than 5.0%, W: 0.1 to 3.0%, Nb: 0.07 to 0.5%, V: 0.01 to 0.5%, Al: 0.001 to 0.1%, N: 0.010 to 0.100%, and O: 0.01% or less, and in which Nb, Ta, C, N, and Cu satisfy a specific relationship, and having a microstructure that contains 45% or more tempered martensitic phase, 20 to 40% ferrite phase, and more than 10% and 25% or less retained austenite phase, by volume. It is stated in this related art document that such a high-strength stainless steel seamless pipe for oil country tubular goods can have strength with a yield strength, YS, of 862 MPa or more, and shows sufficient corrosion resistance also in severe high-temperature corrosive environments containing CO 2 , Cl − , and H 2 S. 
     PTL 3 describes a high-strength stainless steel seamless pipe for oil country tubular goods having a composition that contains, in mass %, C: 0.005 to 0.05%, Si: 0.05 to 0.50%, Mn: 0.20 to 1.80%, P: 0.030% or less, S: 0.005% or less, Cr: 14.0 to 17.0%, Ni: 4.0 to 7.0%, Mo: 0.5 to 3.0%, Al: 0.005 to 0.10%, V: 0.005 to 0.20%, Co: 0.01 to 1.0%, N: 0.005 to 0.15%, and O: 0.010% or less, and in which Cr, Ni, Mo, Cu, C, Si, Mn, and N satisfy specific relationships. 
     PTL 4 describes a high-strength stainless steel seamless pipe for oil country tubular goods having a composition that contains, in mass %, C: 0.05% or less, Si: 0.5% or less, Mn: 0.15 to 1.0%, P: 0.030% or less, S: 0.005% or less, Cr: 14.5 to 17.5%, Ni: 3.0 to 6.0%, Mo: 2.7 to 5.0%, Cu: 0.3 to 4.0%, W: 0.1 to 2.5%, V: 0.02 to 0.20%, Al: 0.10% or less, and N: 0.15% or less, and in which C, Si, Mn, Cr, Ni, Mo, Cu, N, and W satisfy specific relationships, and having a microstructure that contains more than 45% martensitic phase as a primary phase, and 10 to 45% ferrite phase and 30% or less retained austenite phase as secondary phases, by volume. It is stated in this related art document that such a high-strength stainless steel seamless pipe for oil country tubular goods can have strength with a yield strength, YS, of 862 MPa or more, and shows sufficient corrosion resistance also in severe high-temperature corrosive environments containing CO 2 , Cl − , and H 2 S. 
     PTL 5 describes a high-strength stainless steel seamless pipe for oil country tubular goods having a composition that contains, in mass %, C: 0.05% or less, Si: 0.5% or less, Mn: 0.15 to 1.0%, P: 0.030% or less, S: 0.005% or less, Cr: 14.5 to 17.5%, Ni: 3.0 to 6.0%, Mo: 2.7 to 5.0%, Cu: 0.3 to 4.0%, W: 0.1 to 2.5%, V: 0.02 to 0.20%, Al: 0.10% or less, and N: 0.15% or less, and in which C, Si, Mn, Cr, Ni, Mo, Cu, N, and W satisfy specific relationships, and having a microstructure that contains more than 45% martensitic phase as a primary phase, and 10 to 45% ferrite phase and 30% or less retained austenite phase as secondary phases, by volume. It is stated in this related art document that such a high-strength stainless steel seamless pipe for oil country tubular goods can have strength with a yield strength, YS, of 862 MPa or more, and shows sufficient corrosion resistance also in severe high-temperature corrosive environments containing CO 2 , Cl − , and H 2 S. 
     PATENT LITERATURE 
     
         
         PTL 1: WO2013/146046 
         PTL 2: WO2017/138050 
         PTL 3: WO2017/168874 
         PTL 4: WO2018/020886 
         PTL 5: WO2018/155041 
       
    
     SUMMARY OF THE INVENTION 
     As discussed above, the development of oil wells in increasingly higher temperature environments has created a demand for high corrosion resistance in steel pipes to be used in such oil wells. A measure of evaluation of corrosion resistance required for steel pipes for oil country tubular goods to be used in oil wells as high as 200° C. is a corrosion rate of 0.127 mm/y or less, measured by immersing a test specimen in a 20 mass % NaCl aqueous solution (solution temperature: 200° C., an atmosphere of 30 atm CO 2  gas) for 336 hours. 
     Aside from the issues discussed above, enough oil may not be produced when petroleum reservoirs located for extraction of petroleum is of poor quality (most notably, permeability). Oil production also falls below the expected volume in case of accidents such as clogging in the reservoir. Acidizing is a technique that pumps acids such as hydrochloric acid into the reservoir to enhance production. Steel pipes for oil country tubular goods used in such wells need to have desirable corrosion resistance against such acid environments. 
     When steel pipes for oil country tubular goods are to be used in cold climates, desirable low-temperature toughness needs to be satisfied. A measure of evaluation of desirable low-temperature toughness is an absorption energy vE −40  of 200 J or more, measured in a Charpy impact test conducted at −40° C. 
     PTL 1 to PTL 5 disclose stainless steels having improved corrosion resistance. However, the stainless steels disclosed in these related art documents are not necessarily satisfactory in terms of high-temperature corrosion resistance, acid-environment corrosion resistance, and low-temperature toughness. 
     Aspects of the present invention are intended to provide a solution to the problems of the related art, and it is an object according to aspects of the present invention to provide a stainless steel seamless pipe having excellent corrosion resistance and desirable low-temperature toughness while satisfying high strength with a yield strength of 758 MPa (110 ksi) or more. 
     As used herein, “excellent corrosion resistance” means “excellent carbon dioxide gas corrosion resistance” and “excellent acid-environment corrosion resistance” 
     As used herein, “excellent carbon dioxide gas corrosion resistance” means that a test specimen immersed in a test solution (a 20 mass % NaCl aqueous solution; a liquid temperature of 200° C.; an atmosphere of 30 atm CO 2  gas) kept in an autoclave has a corrosion rate of 0.127 mm/y or less after 336 hours in the solution. 
     As used herein, “excellent acid-environment corrosion resistance” means a corrosion rate of 600 mm/y or less, as measured when a test specimen is immersed in an 80° C. 15 mass % hydrochloric acid solution for 40 minutes. 
     As used herein, “desirable low-temperature toughness” means an absorption energy vE −40  of 200 J or more, as measured at −40° C. in a Charpy impact test conducted for a V-notch test specimen (10-mm thick) taken from a steel pipe in such an orientation that the longitudinal axis of the test specimen is along the pipe axis, in compliance with the JIS Z 2242 (2018) specifications. 
     In order to achieve the foregoing objects, the present inventors conducted intensive investigations of various factors that affect the corrosion resistance of stainless steel, particularly the acid-environment corrosion resistance of stainless steel. The studies found that excellent carbon dioxide gas corrosion resistance and excellent acid-environment corrosion resistance can be obtained by adding a predetermined amount or more of Sn, in addition to Cr, Mo, and Cu. It was also found that, in addition to excellent corrosion resistance, desirable low-temperature toughness can be achieved by adding a predetermined amount or more of Ni, and by restraining from excessive addition of Mo. 
     Aspects of the present invention were completed after further studies based on these findings. Specifically, the gist of aspects of the present invention is as follows. 
     [1] A stainless steel seamless pipe having a composition that contains, in mass %, C: 0.06% or less, Si: 1.0% or less, Mn: 0.01% or more and 1.0% or less, P: 0.05% or less, S: 0.005% or less, Cr: 15.2% or more and 18.5% or less, Mo: 1.5% or more and 4.3% or less, Cu: 1.1% or more and 3.5% or less, Ni: 3.0% or more and 6.5% or less, Al: 0.10% or less, N: 0.10% or less, O: 0.010% or less, and Sn: 0.001% or more and 1.000% or less, and in which C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfy the following formula (1), and the balance is Fe and incidental impurities, 
     the stainless steel seamless pipe having a microstructure containing 30% or more martensitic phase, 65% or less ferrite phase, and 40% or less retained austenite phase by volume, 
     the stainless steel seamless pipe having a yield strength of 758 MPa or more, 
       13.0≤−5.9×(7.82+27C−0.91Si+0.21Mn−0.9Cr+Ni−1.1Mo+0.2Cu+11N)≤55.0  (1),
 
     wherein C, Si, Mn, Cr, Ni, Mo, Cu, and N represent the content of each element in mass %, and the content is zero for elements that are not contained. 
     [2] A stainless steel seamless pipe having a composition that contains, in mass %, C: 0.06% or less, Si: 1.0% or less, Mn: 0.01% or more and 1.0% or less, P: 0.05% or less, S: 0.005% or less, Cr: 15.2% or more and 18.5% or less, Mo: 1.5% or more and 4.3% or less, Cu: 1.1% or more and 3.5% or less, Ni: 3.0% or more and 6.5% or less, Al: 0.10% or less, N: 0.10% or less, O: 0.010% or less, and Sn: 0.001% or more and 1.000% or less, and in which C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfy the following formula (1), and the balance is Fe and incidental impurities, 
     the stainless steel seamless pipe having a microstructure containing 40% or more martensitic phase, 60% or less ferrite phase, and 30% or less retained austenite phase by volume, 
     the stainless steel seamless pipe having a yield strength of 862 MPa or more, 
       13.0≤−5.9×(7.82+27C−0.91Si+0.21Mn−0.9Cr+Ni−1.1Mo+0.2Cu+11N)≤55.0  (1),
 
     wherein C, Si, Mn, Cr, Ni, Mo, Cu, and N represent the content of each element in mass %, and the content is zero for elements that are not contained. 
     [3] The stainless steel seamless pipe according to [2], wherein the Cr content is 15.2% or more and 18.0% or less, and the Ni content is 3.0% or more and 6.0% or less, and the composition satisfies the following formula (1)′, instead of the formula (1), 
       13.0≤−5.9×(7.82+27C−0.91Si+0.21Mn−0.9Cr+Ni−1.1Mo+0.2Cu+11N)≤50.0  (1)′,
 
     wherein C, Si, Mn, Cr, Ni, Mo, Cu, and N represent the content of each element in mass %, and the content is zero for elements that are not contained. 
     [4] The stainless steel seamless pipe according to anyone of [1] to [3], wherein the composition further contains, in mass %, one or two or more groups selected from the following groups A to E, 
     Group A: V: 1.0% or less 
     Group B: W: 0.8% or less 
     Group C: one or two selected from Nb: 0.30% or less, and B: 0.01% or less 
     Group D: one or two or more selected from Ta: 0.3% or less, Co: 1.5% or less, Ti: 0.3% or less, and Zr: 0.3% or less 
     Group E: one or two or more selected from Ca: 0.01% or less, REM: 0.3% or less, Mg: 0.01% or less, and Sb: 1.0% or less. 
     [5] A method for manufacturing a stainless steel seamless pipe of any one of [1] to [4], 
     the method including: 
     hot working a steel pipe material of said composition into a seamless steel pipe; 
     quenching that reheats the seamless steel pipe to a temperature of 850 to 1, 150° C., and cools the seamless steel pipe at a cooling rate of air cooling or faster until a pipe surface reaches a cooling stop temperature of 50° C. or less; and 
     tempering that heats the quenched seamless steel pipe to a temperature of 500 to 650° C. 
     Aspects of the present invention can provide a stainless steel seamless pipe having high strength with a yield strength of 758 MPa (110 ksi) or more, having excellent corrosion resistance and desirable low-temperature toughness. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     Embodiments of the present invention are described below in detail. 
     A stainless steel seamless pipe according to aspects of the present invention has a composition that contains, in mass %, C: 0.06% or less, Si: 1.0% or less, Mn: 0.01% or more and 1.0% or less, P: 0.05% or less, S: 0.005% or less, Cr: 15.2% or more and 18.5% or less, Mo: 1.5% or more and 4.3% or less, Cu: 1.1% or more and 3.5% or less, Ni: 3.0% or more and 6.5% or less, Al: 0.10% or less, N: 0.10% or less, O: 0.010% or less, and Sn: 0.001% or more and 1.000% or less, and in which C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfy the following formula (1), and the balance is Fe and incidental impurities, 
     the stainless steel seamless pipe having a microstructure containing 30% or more martensitic phase, 65% or less ferrite phase, and 40% or less retained austenite phase by volume, 
     the stainless steel seamless pipe having a yield strength of 758 MPa or more, 
       13.0≤−5.9×(7.82+27C−0.91Si+0.21Mn−0.9Cr+Ni−1.1Mo+0.2Cu+11N)≤55.0  (1),
 
     wherein C, Si, Mn, Cr, Ni, Mo, Cu, and N represent the content of each element in mass %, and the content is zero for elements that are not contained. 
     The following describes the reasons for specifying the composition of a stainless steel seamless pipe according to aspects of the present invention. In the following, “%” means percent by mass, unless otherwise specifically stated. 
     C: 0.06% or Less 
     C is an element that becomes incidentally included in the process of steelmaking. Corrosion resistance decreases when C is contained in an amount of more than 0.06%. For this reason, the C content is 0.06% or less. The C content is preferably 0.05% or less, more preferably 0.04% or less, even more preferably 0.03% or less. Considering the decarburization cost, the lower limit of C content is preferably 0.002%, more preferably 0.003% or more, even more preferably 0.005% or more. 
     Si: 1.0% or Less 
     Si is an element that acts as a deoxidizing agent. However, hot workability and corrosion resistance decrease when Si is contained in an amount of more than 1.0%. For this reason, the Si content is 1.0% or less. The Si content is preferably 0.7% or less, more preferably 0.5% or less, even more preferably 0.4% or less. It is not particularly required to set a lower limit, as long as the deoxidizing effect is obtained. However, in order to obtain a sufficient deoxidizing effect, the Si content is preferably 0.03% or more, more preferably 0.05% or more, even more preferably 0.1% or more. 
     Mn: 0.01% or More and 1.0% or Less 
     Mn is an element that acts as a deoxidizing agent and a desulfurizing agent, and that improves hot workability. Mn is contained in an amount of 0.01% or more to obtain the deoxidizing and desulfurizing effects, and to improve strength. The effects become saturated with a Mn content of more than 1.0%. For this reason, the Mn content is 0.01% or more and 1.0% or less. The Mn content is preferably 0.03% or more, more preferably 0.05% or more, even more preferably 0.1% or more. The Mn content is preferably 0.8% or less, more preferably 0.6% or less, even more preferably 0.4% or less. 
     P: 0.05% or Less 
     P is an element that impairs carbon dioxide gas corrosion resistance and acid-environment corrosion resistance. P is therefore contained preferably in as small an amount as possible in accordance with aspects of the present invention. However, a P content of 0.05% or less is acceptable. For this reason, the P content is 0.05% or less. The P content is preferably 0.04% or less, more preferably 0.03% or less. 
     S: 0.005% or Less 
     S is an element that seriously impairs hot workability, and interferes with stable operations of hot working in the pipe manufacturing process. S exists as sulfide inclusions in steel, and impairs corrosion resistance. S should therefore be contained preferably in as small an amount as possible. However, a S content of 0.005% or less is acceptable. For this reason, the S content is 0.005% or less. The S content is preferably 0.004% or less, more preferably 0.003% or less, even more preferably 0.002% or less. 
     Cr: 15.2% or More and 18.5% or Less 
     Cr is an element that forms a protective coating on steel pipe surface, and contributes to improving corrosion resistance. The desired carbon dioxide gas corrosion resistance and the desired acid-environment corrosion resistance cannot be obtained when the Cr content is less than 15.2%. For this reason, Cr needs to be contained in an amount of 15.2% or more. With a Cr content of more than 18.5%, the ferrite fraction overly increases, and the desired strength cannot be provided. For this reason, the Cr content is 15.2% or more and 18.5% or less. The Cr content is preferably 15.5% or more, more preferably 16.0% or more, even more preferably 16.30% or more, yet more preferably 16.40% or more. The Cr content is preferably 18.0% or less, more preferably 17.5% or less, even more preferably 17.0% or less. 
     Mo: 1.5% or More and 4.3% or Less 
     By stabilizing the protective coating on steel pipe surface, Mo increases the resistance against pitting corrosion due to Cl −  and low pH, and increases carbon dioxide gas corrosion resistance and acid-environment corrosion resistance. Mo needs to be contained in an amount of 1.5% or more to obtain the desired corrosion resistance. The toughness (low-temperature toughness) decreases with a Mo content of more than 4.3%. For this reason, the Mo content is 1.5% or more and 4.3% or less. The Mo content is preferably 1.8% or more, more preferably 2.0% or more, even more preferably 2.3% or more. The Mo content is preferably 4.0% or less, more preferably 3.5% or less, even more preferably 3.0% or less. 
     Cu: 1.1% or More and 3.5% or Less 
     Cu has the effect to strengthen the protective coating on steel pipe surface, and improve carbon dioxide gas corrosion resistance and acid-environment corrosion resistance. Cu needs to be contained in an amount of 1.1% or more to obtain the desired strength and corrosion resistance. An excessively high Cu content results in decrease of hot workability of steel, and the Cu content is 3.5% or less. For this reason, the Cu content is 1.1% or more and 3.5% or less. The Cu content is preferably 1.8% or more, more preferably 2.0% or more, even more preferably 2.3% or more. The Cu content is preferably 3.2% or less, more preferably 3.0% or less, even more preferably 2.7% or less. 
     Ni: 3.0% or More and 6.5% or Less 
     Ni strengthens the strength of steel by solid solution strengthening, and improves the toughness (low-temperature toughness) of steel. A Ni content of 3.0% or more is needed to obtain the desired toughness (low-temperature toughness). A Ni content of more than 6.5% results in stability of martensitic phase decrease, and the strength decreases. For this reason, the Ni content is 3.0% or more and 6.5% or less. The Ni content is preferably 3.8% or more, more preferably 4.0% or more, even more preferably 4.5% or more. The Ni content is preferably 6.0% or less, more preferably 5.5% or less, even more preferably 5.2% or less. 
     Al: 0.10% or Less 
     Al is an element that acts as a deoxidizing agent. However, corrosion resistance decreases when Al is contained in an amount of more than 0.10%. For this reason, the Al content is 0.10% or less. The Al content is preferably 0.07% or less, more preferably 0.05% or less. It is not particularly required to set a lower limit, as long as the deoxidizing effect is obtained. However, in order to obtain a sufficient deoxidizing effect, the Al content is preferably 0.005% or more, more preferably 0.01% or more, even more preferably 0.015% or more. 
     N: 0.10% or Less 
     N is an element that becomes incidentally included in the process of steelmaking. N is also an element that increases the steel strength. However, when contained in an amount of more than 0.10%, N forms nitrides, and decreases the corrosion resistance. For this reason, the N content is 0.10% or less. The N content is preferably 0.08% or less, more preferably 0.07% or less, even more preferably 0.05% or less. The N content does not have a specific lower limit. However, an excessively low N content leads to increased steelmaking costs. For this reason, the N content is preferably 0.002% or more, more preferably 0.003% or more, even more preferably 0.005% or more. 
     O: 0.010% or Less 
     O (oxygen) exists as an oxide in steel, and causes adverse effects on various properties. For this reason, 0 is contained preferably in as small an amount as possible in accordance with aspects of the present invention. An 0 content of more than 0.010% results in decrease of hot workability and corrosion resistance. For this reason, the 0 content is 0.010% or less. 
     Sn: 0.001% or More and 1.000% or Less 
     Sn improves corrosion resistance, particularly acid-environment corrosion resistance. This makes Sn an important element in accordance with aspects of the present invention. Sn is contained in an amount of 0.001% or more to obtain the desired corrosion resistance. The effect becomes saturated with a Sn content of more than 1.000%. For this reason, the Sn content is 0.001% or more and 1.000% or less in accordance with aspects of the present invention. The Sn content is preferably 0.005% or more, more preferably 0.01% or more, even more preferably 0.02% or more. The Sn content is preferably 0.5% or less, more preferably 0.3% or less, even more preferably 0.1% or less. 
     In accordance with aspects of the present invention, C, Si, Mn, Cr, Ni, Mo, Cu, and N are contained so as to satisfy the following formula (1), in addition to satisfying the foregoing composition. 
       13.0≤−5.9×(7.82+27C−0.91Si+0.21Mn−0.9Cr+Ni−1.1Mo+0.2Cu+11N)≤55.0  (1)
 
     In the formula, C, Si, Mn, Cr, Ni, Mo, Cu, and N represent the content of each element in mass %, and the content is 0 (zero) for elements that are not contained. 
     In formula (1), the expression −5.9×(7.82+27C−0.91Si+0.21Mn−0.9Cr+Ni−1.1Mo+0.2Cu+11N) (hereinafter, referred to also as “middle polynomial of formula (1)”, or, simply, “middle value”) is determined as an index that indicates the likelihood of ferrite phase formation. With the alloy elements indicated in formula (1) contained in and adjusted so as to satisfy formula (1), it is possible to stably produce a composite microstructure of martensitic phase and ferrite phase, or a composite microstructure of martensitic phase, ferrite phase, and retained austenite phase. When any of the alloy elements occurring in formula (1) is not contained, the value of the middle polynomial of formula (1) is calculated by regarding the content of such an element as zero percent. 
     When the value of the middle polynomial of formula (1) is less than 13.0, the ferrite phase decreases, and the manufacturing yield decreases. On the other hand, when the value of the middle polynomial of formula (1) is more than 55.0, the ferrite phase becomes more than 65% by volume, and the desired strength cannot be provided. For this reason, the formula (1) specified in accordance with aspects of the present invention sets a left-hand value of 13.0 as the lower limit, and a right-hand value of 55.0 as the upper limit. 
     The lower-limit left-hand value of the formula (1) specified in accordance with aspects of the present invention is preferably 15.0, more preferably 20.0, even more preferably 23.0. The right-hand value is preferably 50.0, more preferably 45.0, even more preferably 40.0. 
     That is, the middle polynomial of formula (1) has a value of 13.0 or more and 55.0 or less. Preferably, the middle polynomial has a value of 13.0 or more and 50.0 or less, as represented by the formula (1)′ below. The value of middle polynomial is more preferably 15.0 or more and 45.0 or less, even more preferably 20.0 or more and 40.0 or less, yet more preferably 23.0 or more and 40.0 or less. 
       13.0≤−5.9×(7.82+27C−0.91Si+0.21Mn−0.9Cr+Ni−1.1Mo+0.2Cu+11N)≤50.0  (1)′
 
     In the formula, C, Si, Mn, Cr, Ni, Mo, Cu, and N represent the content of each element in mass %, and the content is 0 (zero) for elements that are not contained. 
     In accordance with aspects of the present invention, the balance in the composition above is Fe and incidental impurities. 
     A stainless steel seamless pipe according to aspects of the present invention can provide the desired characteristics by containing the essential elements described above. In accordance with aspects of the present invention, for further improvement of characteristics, the composition may further contain one or two or more optional elements (V, W, Nb, B, Ta, Co, Ti, Zr, Ca, REM, Mg, and Sb), as required, in addition to the foregoing basic components, as follows. 
     Specifically, in accordance with aspects of the present invention, the composition may additionally contain V: 1.0% or less. 
     In accordance with aspects of the present invention, the composition may additionally contain W: 0.8% or less. 
     In accordance with aspects of the present invention, the composition may additionally contain one or two selected from Nb: 0.30% or less, and B: 0.01% or less. 
     In accordance with aspects of the present invention, the composition may additionally contain one or two or more selected from Ta: 0.3% or less, Co: 1.5% or less, Ti: 0.3% or less, and Zr: 0.3% or less. 
     In accordance with aspects of the present invention, the composition may additionally contain one or two or more selected from Ca: 0.01% or less, REM: 0.3% or less, Mg: 0.01% or less, and Sb: 1.0% or less. 
     V: 1.0% or Less 
     V, an optional element, is an element that increases strength. The effect becomes saturated with a V content of more than 1.0%. For this reason, V, when contained, is contained in an amount of preferably 1.0% or less. The V content is more preferably 0.5% or less, even more preferably 0.3% or less. The V content is more preferably 0.01% or more, even more preferably 0.03% or more. 
     W: 0.8% or Less 
     W is an element that contributes to improving steel strength, and that can increase carbon dioxide gas corrosion resistance and acid-environment corrosion resistance by stabilizing the protective coating on steel pipe surface. W greatly improves corrosion resistance when contained with Mo. As an optional element, W may be contained to obtain these effects. The toughness (low-temperature toughness) decreases when the W content is more than 0.8%. For this reason, W, when contained, is contained in an amount of preferably 0.8% or less. The W content is more preferably 0.50% or less, even more preferably 0.3% or less. When W is contained, the W content is more preferably 0.05% or more, even more preferably 0.10% or more. 
     Nb: 0.30% or Less 
     Nb, an optional element, is an element that increases steel strength, and that improves corrosion resistance. The effects become saturated with a Nb content of more than 0.30%. For this reason, Nb, when contained, is contained in an amount of preferably 0.30% or less. The Nb content is more preferably 0.20% or less, even more preferably 0.15% or less. The Nb content is more preferably 0.03% or more, even more preferably 0.05% or more. 
     B: 0.01% or Less 
     B, an optional element, is an element that increases strength. B also contributes to improving hot workability, and has the effect to reduce fracture and cracking during the pipe making process. On the other hand, a B content of more than 0.01% produces hardly any hot workability improving effect, and results in decrease of low-temperature toughness. For this reason, B, when contained, is contained in an amount of preferably 0.01% or less. The B content is more preferably 0.008% or less, even more preferably 0.007% or less. The B content is more preferably 0.0005% or more, even more preferably 0.001% or more. 
     Ta: 0.3% or Less 
     Ta, an optional element, is an element that increases strength, and that improves corrosion resistance. Ta is contained in an amount of preferably 0.001% or more to obtain these effects. With a Ta content of more than 0.3%, the effects become essentially saturated, and the low-temperature toughness decreases. For this reason, Ta, when contained, is contained in a limited amount of preferably 0.3% or less. The Ta content is more preferably 0.1% or less, even more preferably 0.040% or less. 
     Co: 1.5% or Less 
     Co, an optional element, is an element that increases strength. Co also has the effect to improve corrosion resistance, particularly acid-environment corrosion resistance, in addition to increasing strength. Co is contained in an amount of preferably 0.0005% or more to obtain these effects. The Co content is more preferably 0.005% or more, even more preferably 0.010% or more. The effects become saturated with a Co content of more than 1.5%. For this reason, Co, when contained, is contained in a limited amount of preferably 1.5% or less. The Co content is more preferably 1.0% or less. 
     Ti: 0.3% or Less 
     Ti, an optional element, is an element that increases strength. Ti is contained in an amount of preferably 0.0005% or more to obtain this effect. The toughness (low-temperature toughness) decreases when the Ti content is more than 0.3%. For this reason, Ti, when contained, is contained in a limited amount of 0.3% or less. The Ti content is more preferably 0.1% or less, and is more preferably 0.001% or more. 
     Zr: 0.3% or Less 
     Zr, an optional element, is an element that increases strength. In order to obtain this effect, Zr is contained in an amount of preferably 0.0005% or more. The effect becomes saturated with a Zr content of more than 0.3%. For this reason, Zr, when contained, is contained in a limited amount of preferably 0.3% or less. 
     Ca: 0.01% or Less 
     Ca, an optional element, is an element that contributes to improving corrosion resistance by controlling the shape of sulfide. In order to obtain this effect, Ca is contained in an amount of preferably 0.0005% or more. When Ca is contained in an amount of more than 0.01%, the effect becomes saturated, and Ca cannot produce the effect expected from the increased content. For this reason, Ca, when contained, is contained in a limited amount of preferably 0.01% or less. The Ca content is more preferably 0.007% or less, and is more preferably 0.005% or more. 
     REM: 0.3% or Less 
     REM (rare-earth metal), an optional element, is an element that contributes to improving corrosion resistance by controlling the shape of sulfide. In order to obtain this effect, REM is contained in an amount of preferably 0.0005% or more. When REM is contained in an amount of more than 0.3%, the effect becomes saturated, and REM cannot produce the effect expected from the increased content, and actually causes decrease of low-temperature toughness. For this reason, REM, when contained, is contained in a limited amount of preferably 0.3% or less. The REM content is more preferably 0.130% or less, even more preferably 0.1% or less. 
     As used herein, “REM” means scandium (Sc; atomic number 21) and yttrium (Y; atomic number 39), as well as lanthanoids from lanthanum (La; atomic number 57) to lutetium (Lu; atomic number 71). As used herein, “REM concentration” means the total content of one or two or more elements selected from the foregoing REM elements. 
     Mg: 0.01% or Less 
     Mg, an optional element, is an element that improves corrosion resistance. In order to obtain this effect, Mg is contained in an amount of preferably 0.0005% or more. When Mg is contained in an amount of more than 0.01%, the effect becomes saturated, and Mg cannot produce the effect expected from the increased content. For this reason, Mg, when contained, is contained in a limited amount of preferably 0.01% or less. 
     Sb: 1.0% or Less 
     Sb, an optional element, is an element that improves corrosion resistance. In order to obtain this effect, Sb is contained in an amount of preferably 0.001% or more. When Sb is contained in an amount of more than 1.0%, the effect becomes saturated, and Sb cannot produce the effect expected from the increased content. For this reason, Sb, when contained, is contained in a limited amount of preferably 1.0% or less. 
     The following describes the reason for limiting the microstructure in the stainless steel seamless pipe according to aspects of the present invention. 
     In addition to having the foregoing composition, the stainless steel seamless pipe according to aspects of the present invention has a microstructure that contains 30% or more martensitic phase, 65% or less ferrite phase, and 40% or less retained austenite phase by volume. 
     In order to provide the desired strength, the stainless steel seamless pipe according to aspects of the present invention contains 30% or more martensitic phase by volume. The martensitic phase is preferably 40% or more, more preferably 45% or more. The martensitic phase is preferably 70% or less, more preferably 65% or less. 
     The stainless steel seamless pipe according to aspects of the present invention contains 65% or less ferrite phase by volume. With the ferrite phase, propagation of sulfide stress corrosion cracking and sulfide stress cracking can be reduced, and excellent corrosion resistance can be obtained. If the ferrite phase is more than 65% by volume, and precipitates in large amounts, it might not be possible to provide the desired strength. The ferrite phase is preferably 5% or more by volume, more preferably 10% or more, even more preferably 20% or more. The ferrite phase is preferably 60% or less by volume, more preferably 50% or less, even more preferably 45% or less. 
     The stainless steel seamless pipe according to aspects of the present invention contains 40% or less austenitic phase (retained austenite phase) by volume, in addition to the martensitic phase and the ferrite phase. Ductility and toughness (low-temperature toughness) improve by the presence of the retained austenite phase. If the austenitic phase is more than 40% by volume, and precipitates in large amounts, it is not possible to provide the desired strength. For this reason, the retained austenite phase is 40% or less by volume. The retained austenite phase is preferably 5% or more by volume. The retained austenite phase is preferably 30% or less by volume. The retained austenite phase is more preferably 10% or more, and is more preferably 25% or less. 
     The microstructure of the stainless steel seamless pipe according to aspects of the present invention can be measured as follows. First, a test specimen for microstructure observation is corroded with a Vilella&#39;s reagent (a mixed reagent containing at a rate of 2 g of picric acid, 10 ml of hydrochloric acid, and 100 ml of ethanol), and the structure is imaged with a scanning electron microscope (1,000 times magnification). The fraction of the ferrite phase microstructure (area ratio (%)) is then calculated with an image analyzer. The area ratio is defined as the volume ratio (%) of the ferrite phase. 
     Separately, an X-ray diffraction test specimen is ground and polished to have a measurement cross section (C cross section) orthogonal to the axial direction of pipe, and the fraction of the retained austenite (γ) phase microstructure is measured by an X-ray diffraction method. The fraction of the retained austenite phase microstructure is determined by measuring X-ray diffraction integral intensity for the (220) plane of the austenite phase (γ), and the (211) plane of the ferrite phase (a), and converting the calculated values using the following formula. 
       γ(volume ratio)=100/(1+( IαRγ/IγR α)),
 
     wherein Iα is the integral intensity of α, Rα is the crystallographic theoretical value for α, Iγ is the integral intensity of γ, and Rγ is the crystallographic theoretical value for γ. 
     The fraction of the martensitic phase is the remainder other than the fractions of the ferrite phase and retained γ phase determined by the foregoing measurement method. The method of microstructure observation will also be described in detail in the Examples section below. 
     The following describes a preferred method for manufacturing a stainless steel seamless pipe according to aspects of the present invention. 
     Preferably, a molten steel of the foregoing composition is made using a common steelmaking process such as by using a converter, and formed into a steel pipe material, for example, a billet, using an ordinary method such as continuous casting, or ingot casting-billeting. The steel pipe material before hot working is heated at a temperature of preferably 1, 100 to 1, 350° C. In this way, the final product can satisfy the desired low-temperature toughness while ensuring hot workability in pipe making. The steel pipe material is then hot worked into a pipe using a known pipe manufacturing process, for example, the Mannesmann-plug mill process or the Mannesmann-mandrel mill process, to produce a seamless steel pipe of desired dimensions having the foregoing composition. The hot working may be followed by cooling. The cooling process (cooling step) is not particularly limited. After the hot working, the pipe is cooled to preferably room temperature at a cooling rate about the same as air cooling, provided that the composition falls in the range according to aspects of the present invention. 
     In accordance with aspects of the present invention, the seamless steel pipe so obtained is subjected to a heat treatment that includes quenching and tempering. 
     In quenching, the steel pipe is reheated to a temperature of 850 to 1,150° C., and cooled at a cooling rate of air cooling or faster. The cooling stop temperature is 50° C. or less in terms of a surface temperature of the seamless steel pipe. 
     When the heating temperature (quenching temperature) is less than 850° C., a reverse transformation from martensite to austenite does not occur, and the austenite does not transform into martensite during cooling, with the result that the desired strength cannot be provided. On the other hand, the crystal grains coarsen when the heating temperature (quenching temperature) exceeds 1,150° C. For this reason, the heating temperature of quenching is 850 to 1,150° C. The heating temperature of quenching is preferably 900° C. or more. The heating temperature of quenching is preferably 1,100° C. or less. When the cooling stop temperature is more than 50° C., the austenite does not sufficiently transform into martensite, and the fraction of retained austenite becomes overly high. For this reason, the cooling stop temperature of the cooling in quenching is 50° C. or less in accordance with aspects of the present invention. Here, “cooling rate of air cooling or faster” means 0.01° C./s or more. 
     In quenching, the soaking time (quenching time) is preferably 5 to 30 minutes, in order to achieve a uniform temperature along a wall thickness direction, and prevent variation in the material. 
     In tempering, the quenched seamless steel pipe is heated to a tempering temperature of 500 to 650° C. The heating may be followed by natural cooling. 
     A tempering temperature of less than 500° C. is too low to produce the desired tempering effect as intended. When the tempering temperature is higher than 650° C., precipitation of intermetallic compounds occurs, and it is not possible to obtain desirable low-temperature toughness. For this reason, the tempering temperature is 500 to 650° C. The tempering temperature is preferably 520° C. or more. The tempering temperature is preferably 630° C. or less. 
     In tempering, the retention time (tempering time) is preferably 5 to 90 minutes, in order to achieve a uniform temperature along a wall thickness direction, and prevent variation in the material properties. 
     After the heat treatment (quenching and tempering), the seamless steel pipe has a microstructure in which the martensitic phase, the ferrite phase, and the retained austenite phase are contained in a specific predetermined volume ratio. In this way, the stainless steel seamless pipe can have the desired strength and excellent corrosion resistance. 
     The stainless steel seamless pipe obtained in accordance with aspects of the present invention in the manner described above is a high-strength steel pipe having a yield strength of 758 MPa or more, and has excellent corrosion resistance. Preferably, the yield strength is 862 MPa (125 ksi) or more. Preferably, the yield strength is 1,034 MPa or less. The stainless steel seamless pipe according to aspects of the present invention can be used as a stainless steel seamless pipe for oil country tubular goods (a high-strength stainless steel seamless pipe for oil country tubular goods). 
     EXAMPLES 
     Aspects of the present invention are further described below in detail through Examples. It is to be noted that the present invention is not limited by the following Examples. 
     Molten steels of the compositions shown in Table 1-1 and Table 1-2 were cast into steel pipe materials. The steel pipe material was heated, and hot worked into a seamless steel pipe measuring 83.8 mm in outer diameter and 12.7 mm in wall thickness, using a model seamless rolling mill. The seamless steel pipe was then cooled by air cooling. The heating of the steel pipe material before hot working was carried out at a heating temperature of 1,250° C. 
     Each seamless steel pipe was cut into a test specimen material, which was then subjected to quenching that included reheating to the quenching temperatures shown in Table 2-1 to Table 2-3, and cooling (water cooling) to a cooling stop temperature of 30° C. after the quenching retention time shown in Table 2-1 to Table 2-3. This was followed by tempering that included heating to the tempering temperatures shown in Table 2-1 to Table 2-3, and air cooling after the tempering retention time shown in Table 2-1 to Table 2-3. In quenching, the water cooling was carried out at a cooling rate of 11° C./s. The air cooling (natural cooling) in tempering was carried out at a cooling rate of 0.04° C./s. In Table 1-1 and Table 1-2, the blank cells indicate that elements were not intentionally added, meaning that elements may be absent (0%), or may be incidentally present. 
     
       
         
           
               
               
             
               
                 TABLE 1-1 
               
               
                   
               
             
            
               
                 Steel 
                 Composition (mass %) 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 No. 
                 C 
                 Si 
                 Mn 
                 P 
                 S 
                 Cr 
                 Mo 
                 Cu 
                 Ni 
                 Al 
                 N 
               
               
                   
               
               
                 A 
                 0.014 
                 0.33 
                 0.252 
                 0.014 
                 0.0009 
                 17.00 
                 2.78 
                 2.14 
                 4.85 
                 0.024 
                 0.018 
               
               
                 B 
                 0.013 
                 0.27 
                 0.322 
                 0.018 
                 0.0012 
                 17.01 
                 2.46 
                 2.76 
                 4.15 
                 0.024 
                 0.017 
               
               
                 C 
                 0.013 
                 0.27 
                 0.366 
                 0.018 
                 0.0011 
                 17.20 
                 2.75 
                 3.25 
                 4.97 
                 0.027 
                 0.015 
               
               
                 D 
                 0.057 
                 0.33 
                 0.262 
                 0.016 
                 0.0012 
                 17.40 
                 2.41 
                 2.41 
                 5.28 
                 0.026 
                 0.018 
               
               
                 E 
                 0.015 
                 0.93 
                 0.194 
                 0.015 
                 0.0013 
                 16.49 
                 2.57 
                 2.81 
                 5.13 
                 0.026 
                 0.014 
               
               
                 F 
                 0.012 
                 0.30 
                 0.910 
                 0.017 
                 0.0011 
                 16.93 
                 2.39 
                 1.83 
                 4.17 
                 0.025 
                 0.014 
               
               
                 G 
                 0.010 
                 0.26 
                 0.050 
                 0.016 
                 0.0013 
                 16.59 
                 2.61 
                 3.24 
                 4.24 
                 0.027 
                 0.013 
               
               
                 H 
                 0.010 
                 0.30 
                 0.175 
                 0.047 
                 0.0012 
                 16.74 
                 2.36 
                 3.02 
                 4.48 
                 0.025 
                 0.014 
               
               
                 I 
                 0.010 
                 0.29 
                 0.208 
                 0.017 
                 0.0043 
                 16.77 
                 2.48 
                 1.82 
                 4.95 
                 0.027 
                 0.020 
               
               
                 J 
                 0.011 
                 0.27 
                 0.292 
                 0.016 
                 0.0012 
                 17.92 
                 2.55 
                 2.83 
                 4.77 
                 0.025 
                 0.015 
               
               
                 K 
                 0.011 
                 0.29 
                 0.319 
                 0.015 
                 0.0011 
                 15.32 
                 2.57 
                 1.88 
                 5.40 
                 0.028 
                 0.018 
               
               
                 L 
                 0.011 
                 0.26 
                 0.254 
                 0.015 
                 0.0011 
                 16.43 
                 2.41 
                 2.42 
                 4.53 
                 0.025 
                 0.015 
               
               
                 M 
                 0.011 
                 0.30 
                 0.185 
                 0.015 
                 0.0011 
                 17.01 
                 4.17 
                 3.21 
                 4.82 
                 0.027 
                 0.013 
               
               
                 N 
                 0.011 
                 0.29 
                 0.165 
                 0.016 
                 0.0009 
                 17.29 
                 1.63 
                 2.65 
                 4.51 
                 0.028 
                 0.020 
               
               
                 O 
                 0.012 
                 0.28 
                 0.207 
                 0.015 
                 0.0013 
                 16.64 
                 2.90 
                 3.36 
                 4.17 
                 0.026 
                 0.014 
               
               
                 P 
                 0.014 
                 0.24 
                 0.374 
                 0.014 
                 0.0012 
                 16.80 
                 2.35 
                 1.30 
                 4.30 
                 0.027 
                 0.016 
               
               
                 Q 
                 0.012 
                 0.33 
                 0.153 
                 0.017 
                 0.0012 
                 17.02 
                 2.53 
                 2.76 
                 5.86 
                 0.025 
                 0.014 
               
               
                 R 
                 0.011 
                 0.29 
                 0.279 
                 0.017 
                 0.0012 
                 16.88 
                 3.24 
                 2.24 
                 3.10 
                 0.028 
                 0.018 
               
               
                 S 
                 0.017 
                 0.30 
                 0.287 
                 0.015 
                 0.0010 
                 16.78 
                 2.46 
                 2.48 
                 3.89 
                 0.033 
                 0.019 
               
               
                 T 
                 0.012 
                 0.28 
                 0.374 
                 0.015 
                 0.0013 
                 16.60 
                 2.98 
                 2.75 
                 4.51 
                 0.087 
                 0.017 
               
               
                 U 
                 0.015 
                 0.32 
                 0.250 
                 0.017 
                 0.0012 
                 16.74 
                 2.35 
                 2.81 
                 5.12 
                 0.025 
                 0.079 
               
               
                 V 
                 0.012 
                 0.28 
                 0.215 
                 0.014 
                 0.0010 
                 16.90 
                 2.51 
                 3.17 
                 5.41 
                 0.025 
                 0.017 
               
               
                 W 
                 0.014 
                 0.24 
                 0.342 
                 0.016 
                 0.0011 
                 16.82 
                 2.26 
                 2.08 
                 5.51 
                 0.024 
                 0.014 
               
               
                 X 
                 0.011 
                 0.33 
                 0.249 
                 0.016 
                 0.0010 
                 16.74 
                 2.34 
                 1.89 
                 5.48 
                 0.025 
                 0.017 
               
               
                 Y 
                 0.005 
                 0.88 
                 0.294 
                 0.015 
                 0.0011 
                 17.50 
                 4.01 
                 1.82 
                 4.31 
                 0.019 
                 0.006 
               
               
                 Z 
                 0.031 
                 0.05 
                 0.453 
                 0.013 
                 0.0009 
                 16.44 
                 1.82 
                 2.50 
                 5.20 
                 0.025 
                 0.009 
               
               
                 AA 
                 0.008 
                 0.27 
                 0.398 
                 0.017 
                 0.0010 
                 16.60 
                 2.56 
                 2.87 
                 5.20 
                 0.027 
                 0.017 
               
               
                 AB 
                 0.008 
                 0.32 
                 0.271 
                 0.016 
                 0.0012 
                 16.52 
                 2.53 
                 1.82 
                 4.46 
                 0.024 
                 0.013 
               
               
                 AC 
                 0.012 
                 0.24 
                 0.249 
                 0.015 
                 0.0012 
                 16.73 
                 2.50 
                 3.02 
                 4.09 
                 0.026 
                 0.018 
               
               
                 AD 
                 0.012 
                 0.33 
                 0.261 
                 0.016 
                 0.0012 
                 17.08 
                 2.82 
                 1.87 
                 5.49 
                 0.025 
                 0.016 
               
               
                 AE 
                 0.022 
                 0.18 
                 0.235 
                 0.013 
                 0.0009 
                 16.84 
                 2.18 
                 2.31 
                 4.77 
                 0.022 
                 0.016 
               
               
                 AF 
                 0.015 
                 0.26 
                 0.392 
                 0.014 
                 0.0010 
                 17.35 
                 3.21 
                 3.04 
                 4.23 
                 0.027 
                 0.017 
               
               
                 AG 
                 0.013 
                 0.31 
                 0.160 
                 0.016 
                 0.0010 
                 17.01 
                 2.59 
                 3.18 
                 4.54 
                 0.027 
                 0.015 
               
               
                 AH 
                 0.011 
                 0.33 
                 0.372 
                 0.015 
                 0.0013 
                 16.90 
                 2.36 
                 2.39 
                 4.15 
                 0.027 
                 0.016 
               
               
                 AI 
                 0.009 
                 0.27 
                 0.264 
                 0.016 
                 0.0012 
                 16.47 
                 2.84 
                 2.80 
                 4.83 
                 0.026 
                 0.018 
               
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Formula (1) (*3) 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Steel 
                 Composition (mass %) 
                 Middle 
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 No. 
                 O 
                 Sn 
                 Other 
                 value 
                 Result 
                 Remarks 
               
               
                   
                   
               
               
                   
                 A 
                 0.002 
                 0.0173 
                   
                 29.1 
                 ∘ 
                 Present Steel 
               
               
                   
                 B 
                 0.002 
                 0.0986 
                   
                 30.3 
                 ∘ 
                 Present Steel 
               
               
                   
                 C 
                 0.002 
                 0.0636 
                   
                 27.8 
                 ∘ 
                 Present Steel 
               
               
                   
                 D 
                 0.002 
                 0.0646 
                   
                 19.1 
                 ∘ 
                 Present Steel 
               
               
                   
                 E 
                 0.002 
                 0.0874 
                   
                 26.0 
                 ∘ 
                 Present Steel 
               
               
                   
                 F 
                 0.002 
                 0.0336 
                   
                 30.2 
                 ∘ 
                 Present Steel 
               
               
                   
                 G 
                 0.002 
                 0.0626 
                   
                 29.0 
                 ∘ 
                 Present Steel 
               
               
                   
                 H 
                 0.002 
                 0.0650 
                   
                 27.0 
                 ∘ 
                 Present Steel 
               
               
                   
                 I 
                 0.003 
                 0.0894 
                   
                 26.1 
                 ∘ 
                 Present Steel 
               
               
                   
                 J 
                 0.003 
                 0.0086 
                   
                 32.4 
                 ∘ 
                 Present Steel 
               
               
                   
                 K 
                 0.003 
                 0.0926 
                   
                 16.1 
                 ∘ 
                 Present Steel 
               
               
                   
                 L 
                 0.003 
                 0.0311 
                   
                 25.5 
                 ∘ 
                 Present Steel 
               
               
                   
                 M 
                 0.003 
                 0.0672 
                   
                 37.8 
                 ∘ 
                 Present Steel 
               
               
                   
                 N 
                 0.003 
                 0.0589 
                   
                 24.8 
                 ∘ 
                 Present Steel 
               
               
                   
                 O 
                 0.002 
                 0.0973 
                   
                 30.9 
                 ∘ 
                 Present Steel 
               
               
                   
                 P 
                 0.003 
                 0.0500 
                   
                 29.0 
                 ∘ 
                 Present Steel 
               
               
                   
                 Q 
                 0.003 
                 0.0181 
                   
                 21.6 
                 ∘ 
                 Present Steel 
               
               
                   
                 R 
                 0.002 
                 0.0476 
                   
                 41.9 
                 ∘ 
                 Present Steel 
               
               
                   
                 S 
                 0.002 
                 0.0028 
                   
                 30.4 
                 ∘ 
                 Present Steel 
               
               
                   
                 T 
                 0.003 
                 0.1155 
                   
                 29.5 
                 ∘ 
                 Present Steel 
               
               
                   
                 U 
                 0.002 
                 0.0628 
                   
                 18.4 
                 ∘ 
                 Present Steel 
               
               
                   
                 V 
                 0.009 
                 0.1090 
                   
                 22.5 
                 ∘ 
                 Present Steel 
               
               
                   
                 W 
                 0.002 
                 0.9090 
                   
                 20.6 
                 ∘ 
                 Present Steel 
               
               
                   
                 X 
                 0.002 
                 0.0019 
                   
                 22.0 
                 ∘ 
                 Present Steel 
               
               
                   
                 Y 
                 0.002 
                 0.0515 
                   
                 48.4 
                 ∘ 
                 Present Steel 
               
               
                   
                 Z 
                 0.002 
                 0.0336 
                   
                 13.5 
                 ∘ 
                 Present Steel 
               
               
                   
                 AA 
                 0.002 
                 0.0137 
                 V: 0.05 
                 23.1 
                 ∘ 
                 Present Steel 
               
               
                   
                 AB 
                 0.002 
                 0.0348 
                 W: 0.23 
                 28.8 
                 ∘ 
                 Present Steel 
               
               
                   
                 AC 
                 0.002 
                 0.0712 
                 Nb: 0.12, B: 0.004 
                 29.1 
                 ∘ 
                 Present Steel 
               
               
                   
                 AD 
                 0.003 
                 0.0730 
                 Ta: 0.26, Co: 1.27, 
                 26.8 
                 ∘ 
                 Present Steel 
               
               
                   
                   
                   
                   
                 Ti: 0.25, Zr: 0.28 
               
               
                   
                 AE 
                 0.002 
                 0.0301 
                 Ta: 0.037, Co: 1.20, 
                 22.7 
                 ∘ 
                 Present Steel 
               
               
                   
                   
                   
                   
                 Ti: 0.23, Zr: 0.25 
               
               
                   
                 AF 
                 0.003 
                 0.1073 
                 Ca: 0.007, REM: 0.125, 
                 35.7 
                 ∘ 
                 Present Steel 
               
               
                   
                   
                   
                   
                 Mg: 0.007, Sb: 0.96 
               
               
                   
                 AG 
                 0.002 
                 0.1104 
                 V: 0.08, W: 0.48 
                 28.9 
                 ∘ 
                 Present Steel 
               
               
                   
                 AH 
                 0.002 
                 0.0665 
                 V: 0.10, B: 0.005 
                 30.1 
                 ∘ 
                 Present Steel 
               
               
                   
                 AI 
                 0.002 
                 0.0147 
                 V: 0.23, Ta: 0.23 
                 26.5 
                 ∘ 
                 Present Steel 
               
               
                   
                   
               
               
                   
                 (*1) The balance is Fe and incidental impurities 
               
               
                   
                 (*2) Underline means outside of the range of the present invention 
               
               
                   
                 (*3) Formula (1): 13.0 ≤ −5.9 × (7.82 + 27C − 0.91Si + 0.21Mn − 0.9Cr + Ni − 1.1Mo + 0.2Cu + 11N) ≤ 55.0 
               
            
           
         
       
     
     
       
         
           
               
               
             
               
                 TABLE 1-2 
               
               
                   
               
             
            
               
                 Steel 
                 Composition (mass %) 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 No. 
                 C 
                 Si 
                 Mn 
                 P 
                 S 
                 Cr 
                 Mo 
                 Cu 
                 Ni 
                 Al 
               
               
                   
               
               
                 AJ 
                 0.023 
                 0.40 
                 0.149 
                 0.020 
                 0.0017 
                 17.31 
                 2.77 
                 2.69 
                 4.29 
                 0.021 
               
               
                 AK 
                 0.008 
                 0.33 
                 0.168 
                 0.017 
                 0.0009 
                 16.55 
                 3.22 
                 3.20 
                 4.83 
                 0.027 
               
               
                 AL 
                 0.012 
                 0.28 
                 0.283 
                 0.015 
                 0.0010 
                 16.96 
                 3.11 
                 2.79 
                 4.53 
                 0.025 
               
               
                 AM 
                 0.008 
                 0.31 
                 0.247 
                 0.018 
                 0.0010 
                 16.64 
                 2.48 
                 3.11 
                 5.31 
                 0.026 
               
               
                 AN 
                 0.009 
                 0.25 
                 0.193 
                 0.016 
                 0.0012 
                 17.32 
                 2.54 
                 2.64 
                 4.78 
                 0.028 
               
               
                 AO 
                 0.018 
                 0.56 
                 0.391 
                 0.011 
                 0.0016 
                 17.33 
                 2.48 
                 2.43 
                 4.51 
                 0.022 
               
               
                 AP 
                 0.011 
                 0.24 
                 0.399 
                 0.015 
                 0.0010 
                 16.85 
                 2.46 
                 2.47 
                 4.34 
                 0.026 
               
               
                 AQ 
                 
                   0.065 
                 
                 0.33 
                 0.266 
                 0.017 
                 0.0013 
                 17.10 
                 2.99 
                 2.67 
                 4.96 
                 0.025 
               
               
                 AR 
                 0.008 
                 
                   1.28 
                 
                 0.320 
                 0.014 
                 0.0009 
                 16.54 
                 3.12 
                 2.01 
                 4.32 
                 0.025 
               
               
                 AS 
                 0.013 
                 0.27 
                 0.271 
                 
                   0.055 
                 
                 0.0013 
                 16.59 
                 2.76 
                 1.85 
                 5.17 
                 0.025 
               
               
                 AT 
                 0.014 
                 0.31 
                 0.339 
                 0.018 
                 
                   0.0058 
                 
                 16.60 
                 2.67 
                 2.55 
                 4.62 
                 0.028 
               
               
                 AU 
                 0.014 
                 0.26 
                 0.393 
                 0.018 
                 0.0011 
                 18.13 
                 3.21 
                 3.10 
                 4.66 
                 0.027 
               
               
                 AV 
                 0.010 
                 0.25 
                 0.267 
                 0.018 
                 0.0010 
                 
                   15.02 
                 
                 2.69 
                 2.05 
                 4.79 
                 0.025 
               
               
                 AW 
                 0.015 
                 0.27 
                 0.253 
                 0.017 
                 0.0010 
                 17.27 
                 
                   4.41 
                 
                 2.52 
                 4.27 
                 0.025 
               
               
                 AX 
                 0.009 
                 0.24 
                 0.355 
                 0.015 
                 0.0012 
                 16.59 
                 
                   1.37 
                 
                 1.96 
                 5.20 
                 0.026 
               
               
                 AY 
                 0.011 
                 0.33 
                 0.357 
                 0.015 
                 0.0012 
                 16.88 
                 2.99 
                 
                   1.03 
                 
                 5.38 
                 0.027 
               
               
                 AZ 
                 0.013 
                 0.29 
                 0.324 
                 0.017 
                 0.0010 
                 17.36 
                 2.68 
                 3.17 
                 6.17 
                 0.026 
               
               
                 BA 
                 0.014 
                 0.32 
                 0.360 
                 0.016 
                 0.0010 
                 17.19 
                 2.58 
                 2.09 
                 
                   2.88 
                 
                 0.025 
               
               
                 BB 
                 0.008 
                 0.27 
                 0.370 
                 0.015 
                 0.0010 
                 16.61 
                 2.54 
                 3.09 
                 5.15 
                 
                   0.131 
                 
               
               
                 BC 
                 0.013 
                 0.28 
                 0.271 
                 0.016 
                 0.0011 
                 17.10 
                 2.76 
                 1.81 
                 4.42 
                 0.026 
               
               
                 BD 
                 0.015 
                 0.33 
                 0.283 
                 0.015 
                 0.0011 
                 16.69 
                 2.42 
                 2.45 
                 4.60 
                 0.026 
               
               
                 BE 
                 0.008 
                 0.33 
                 0.350 
                 0.015 
                 0.0010 
                 16.71 
                 2.26 
                 3.04 
                 4.65 
                 0.027 
               
               
                 BF 
                 0.005 
                 0.89 
                 0.290 
                 0.015 
                 0.0011 
                 17.70 
                 4.01 
                 1.82 
                 4.12 
                 0.020 
               
               
                 BG 
                 0.012 
                 0.26 
                 0.320 
                 0.013 
                 0.0016 
                 16.85 
                 2.48 
                 2.45 
                 4.51 
                 0.031 
               
               
                 BH 
                 0.010 
                 0.30 
                 0.281 
                 0.011 
                 0.0013 
                 16.63 
                 2.33 
                 2.67 
                 4.83 
                 0.019 
               
               
                 BI 
                 0.011 
                 0.24 
                 0.263 
                 0.008 
                 0.0007 
                 17.21 
                 2.73 
                 2.12 
                 4.99 
                 0.040 
               
               
                 BJ 
                 0.014 
                 0.35 
                 0.273 
                 0.019 
                 0.0013 
                 
                   18.61 
                 
                 2.83 
                 2.04 
                 4.31 
                 0.034 
               
               
                 BK 
                 0.018 
                 0.38 
                 0.315 
                 0.020 
                 0.0010 
                 17.39 
                 2.19 
                 2.73 
                 
                   6.70 
                 
                 0.027 
               
               
                 BL 
                 0.005 
                 0.89 
                 0.290 
                 0.015 
                 0.0011 
                 17.89 
                 4.20 
                 1.38 
                 3.53 
                 0.020 
               
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Formula (1) (*3) 
                   
               
            
           
           
               
               
               
               
            
               
                 Steel 
                 Composition (mass %) 
                 Middle 
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 No. 
                 N 
                 O 
                 Sn 
                 Other 
                 value 
                 Result 
                 Remarks 
               
               
                   
               
               
                 AJ 
                 0.030 
                 0.003 
                 0.0792 
                 V: 0.23, Ta: 0.037 
                 31.6 
                 ∘ 
                 Present Steel 
               
               
                 AK 
                 0.013 
                 0.002 
                 0.0954 
                 V: 0.92, Ca: 0.008 
                 29.8 
                 ∘ 
                 Present Steel 
               
               
                 AL 
                 0.015 
                 0.003 
                 0.0870 
                 W: 0.12, Nb: 0.09 
                 32.4 
                 ∘ 
                 Present Steel 
               
               
                 AM 
                 0.017 
                 0.002 
                 0.0233 
                 W: 0.33, Co: 1.38 
                 22.3 
                 ∘ 
                 Present Steel 
               
               
                 AN 
                 0.019 
                 0.002 
                 0.0502 
                 W: 0.44, REM: 0.24 
                 29.4 
                 ∘ 
                 Present Steel 
               
               
                 AO 
                 0.016 
                 0.002 
                 0.0123 
                 W: 0.12, REM: 0.118 
                 31.1 
                 ∘ 
                 Present Steel 
               
               
                 AP 
                 0.016 
                 0.003 
                 0.0775 
                 V: 0.35, W: 0.36, 
                 28.8 
                 ∘ 
                 Present Steel 
               
               
                   
                   
                   
                   
                 B: 0.007, Ti: 0.21, 
               
               
                   
                   
                   
                   
                 Ca: 0.007 
               
               
                 AQ 
                 0.014 
                 0.003 
                 0.0199 
                   
                 21.8 
                 ∘ 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Steel 
               
               
                 AR 
                 0.018 
                 0.003 
                 0.0090 
                   
                 38.1 
                 ∘ 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Steel 
               
               
                 AS 
                 0.018 
                 0.002 
                 0.0424 
                   
                 25.1 
                 ∘ 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Steel 
               
               
                 AT 
                 0.017 
                 0.002 
                 0.0963 
                   
                 27.0 
                 ∘ 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Steel 
               
               
                 AU 
                 0.016 
                 0.002 
                 0.0493 
                   
                 37.5 
                 ∘ 
                 Present Steel 
               
               
                 AV 
                 0.014 
                 0.002 
                 0.0959 
                   
                 18.9 
                 ∘ 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Steel 
               
               
                 AW 
                 0.017 
                 0.002 
                 0.0220 
                   
                 43.7 
                 ∘ 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Steel 
               
               
                 AX 
                 0.014 
                 0.002 
                 0.0640 
                   
                 16.4 
                 ∘ 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Steel 
               
               
                 AY 
                 0.017 
                 0.002 
                 0.0669 
                   
                 28.4 
                 ∘ 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Steel 
               
               
                 AZ 
                 0.016 
                 0.002 
                 0.0994 
                   
                 21.3 
                 ∘ 
                 Present Steel 
               
               
                 BA 
                 0.019 
                 0.003 
                 0.0100 
                   
                 40.2 
                 ∘ 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Steel 
               
               
                 BB 
                 0.017 
                 0.002 
                 0.0994 
                   
                 23.1 
                 ∘ 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Steel 
               
               
                 BC 
                 
                   0.115 
                 
                 0.002 
                 0.0449 
                   
                 26.0 
                 ∘ 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Steel 
               
               
                 BD 
                 0.016 
                 
                   0.013 
                 
                 0.0087 
                   
                 26.2 
                 ∘ 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Steel 
               
               
                 BE 
                 0.016 
                 0.003 
                 
                   0.0004 
                 
                   
                 25.3 
                 ∘ 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Steel 
               
               
                 BF 
                 0.010 
                 0.002 
                 0.0125 
                   
                 50.4 
                 ∘ 
                 Present Steel 
               
               
                 BG 
                 0.015 
                 0.002 
                 0.0137 
                 Ca: 0.0028 
                 28.0 
                 ∘ 
                 Present Steel 
               
               
                 BH 
                 0.013 
                 0.003 
                 0.0235 
                 Ca: 0.0079 
                 24.5 
                 ∘ 
                 Present Steel 
               
               
                 BI 
                 0.018 
                 0.002 
                 0.0073 
                 Ca: 0.0043 
                 29.1 
                 ∘ 
                 Present Steel 
               
               
                 BJ 
                 0.020 
                 0.003 
                 0.0183 
                   
                 41.2 
                 ∘ 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Steel 
               
               
                 BK 
                 0.011 
                 0.002 
                 0.0292 
                   
                 15.7 
                 ∘ 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Steel 
               
               
                 BL 
                 0.010 
                 0.002 
                 0.0125 
                   
                 
                   56.6 
                 
                 
                   x 
                 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Steel 
               
               
                   
               
               
                 (*1) The balance is Fe and incidental impurities 
               
               
                 (*2) Underline means outside of the range of the present invention 
               
               
                 (*3) Formula (1): 13.0 ≤ −5.9 × (7.82 + 27C − 0.91Si + 0.21Mn − 0.9Cr + Ni − 1.1Mo + 0.2Cu + 11N) ≤ 55.0 
               
            
           
         
       
     
     A test specimen was taken from the heat-treated test specimen material (seamless steel pipe), and was subjected to microstructure observation, a tensile test, a corrosion resistance test, and a Charpy impact test. The test methods are as follows. 
     (1) Microstructure Observation 
     A test specimen for microstructure observation was taken from the heat-treated test material in such an orientation that an axial plane section was exposed for observation. The test specimen for microstructure observation was corroded with a Vilella&#39;s reagent (a mixed reagent containing at a rate of 2 g of picric acid, 10 ml of hydrochloric acid, and 100 ml of ethanol), and the structure was imaged with a scanning electron microscope (1,000 times magnification). The fraction (area ratio (%)) of the ferrite phase microstructure was then calculated with an image analyzer. Here, the area ratio was calculated as the volume ratio (%) of the ferrite phase. 
     Separately, an X-ray diffraction test specimen was taken from the heat-treated test material. The test specimen was ground and polished to have a measurement cross section (C cross section) orthogonal to the axial direction of pipe, and the fraction of the retained austenite (γ) phase microstructure was measured by an X-ray diffraction method. The fraction of the retained austenite phase microstructure was determined by measuring X-ray diffraction integral intensity for the (220) plane of the austenite phase (γ), and the (211) plane of the ferrite phase (a), and converting the calculated values using the following formula. 
       γ(volume ratio)=100/(1+( IαRγ/IγR α)),
 
     wherein Iα is the integral intensity of α, Rα is the crystallographic theoretical value for α, Iγ is the integral intensity of γ, and Rγ is the crystallographic theoretical value for γ. The fraction of the martensitic phase is the remainder other than the fractions of the ferrite phase and retained γ phase. 
     (2) Tensile Test 
     An API (American Petroleum Institute) arc-shaped tensile test specimen was taken from the heat-treated test material in such an orientation that the test specimen had a tensile direction along the pipe axis direction. The tensile test was conducted according to the API specifications to determine tensile properties (yield strength YS). Here, the steel was determined as being high strength and acceptable when it had a yield strength YS of 758 MPa or more, and unacceptable when it had a yield strength YS of less than 758 MPa. 
     (3) Corrosion Resistance Test (Carbon Dioxide Gas Corrosion Resistance Test and Acid-Environment Corrosion Resistance Test) 
     A corrosion test specimen measuring 3 mm in thickness, 30 mm in width, and 40 mm in length was prepared from the heat-treated test material by machining, and was subjected to a corrosion test to evaluate carbon dioxide gas corrosion resistance and acid-environment corrosion resistance. 
     The corrosion test for evaluation of carbon dioxide gas corrosion resistance was conducted by immersing the corrosion test specimen for 14 days (336 hours) in a test solution (a 20 mass % NaCl aqueous solution; liquid temperature: 200° C.; an atmosphere of 30 atm CO 2  gas) kept in an autoclave. The corrosion rate was determined from the calculated reduction in the weight of the tested specimen measured before and after the corrosion test. Here, the steel was determined as being acceptable when it had a corrosion rate of 0.127 mm/y or less, and unacceptable when it had a corrosion rate of more than 0.127 mm/y. 
     The corrosion test for evaluation of acid-environment corrosion resistance was conducted by immersing the test specimen in an 80° C. 15 mass % hydrochloric acid solution for 40 minutes. The corrosion rate was determined from the calculated reduction in the weight of the tested specimen measured before and after the corrosion test. Here, the steel was determined as being acceptable when it had a corrosion rate of 600 mm/y or less, and unacceptable when it had a corrosion rate of more than 600 mm/y. 
     (4) Charpy Impact Test 
     A Charpy impact test was conducted for a V-notch test specimen (10-mm thick) taken from the steel pipe in such an orientation that the longitudinal axis of the test specimen was along the pipe axis, in compliance with the JIS Z 2242 specifications. Here, the steel was determined as being acceptable when it had an absorption energy vE −40  at −40° C. (test temperature) of 200 J or more. 
     The results are presented in Table 2-1 to Table 2-3. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 2-1 
               
             
            
               
                   
                   
               
               
                   
                 Quenching 
                 Tempering 
                 Microstructure 
                   
                 Corrosion 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Steel 
                 Quenching 
                   
                 Tempering 
                   
                 (volume %) 
                 Yield 
                   
                 Corrosion 
                 rate in acid 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Steel 
                 Pipe 
                 temp. 
                 Quenching 
                 temp. 
                 Tempering 
                 M 
                 F 
                 A 
                 strength 
                 vE −40   
                 rate 
                 environment 
                   
               
               
                 No. 
                 No. 
                 (° C.) 
                 time (min) 
                 (° C.) 
                 time (min) 
                 (*1) 
                 (*1) 
                 (*1) 
                 YS (MPa) 
                 (J) 
                 (mm/y) 
                 (mm/y) 
                 Remarks 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 A 
                 1 
                 960 
                 20 
                 575 
                 20 
                 54 
                 32 
                 14 
                 925 
                 215 
                 0.025 
                 566.8 
                 Present Example 
               
               
                 B 
                 2 
                 960 
                 20 
                 575 
                 20 
                 58 
                 32 
                 10 
                 944 
                 211 
                 0.027 
                 567.6 
                 Present Example 
               
               
                 C 
                 3 
                 960 
                 20 
                 575 
                 20 
                 51 
                 31 
                 18 
                 911 
                 219 
                 0.026 
                 570.0 
                 Present Example 
               
               
                 D 
                 4 
                 960 
                 20 
                 575 
                 20 
                 45 
                 27 
                 28 
                 883 
                 230 
                 0.103 
                 594.3 
                 Present Example 
               
               
                 E 
                 5 
                 960 
                 20 
                 575 
                 20 
                 57 
                 30 
                 13 
                 939 
                 214 
                 0.058 
                 591.2 
                 Present Example 
               
               
                 F 
                 6 
                 960 
                 20 
                 575 
                 20 
                 61 
                 32 
                 7 
                 958 
                 208 
                 0.026 
                 566.8 
                 Present Example 
               
               
                 G 
                 7 
                 960 
                 20 
                 575 
                 20 
                 59 
                 32 
                 9 
                 949 
                 210 
                 0.025 
                 562.0 
                 Present Example 
               
               
                 H 
                 8 
                 960 
                 20 
                 575 
                 20 
                 59 
                 31 
                 10 
                 949 
                 211 
                 0.082 
                 596.1 
                 Present Example 
               
               
                 I 
                 9 
                 960 
                 20 
                 575 
                 20 
                 60 
                 30 
                 10 
                 953 
                 211 
                 0.026 
                 593.8 
                 Present Example 
               
               
                 J 
                 10 
                 960 
                 20 
                 575 
                 20 
                 48 
                 33 
                 19 
                 897 
                 220 
                 0.020 
                 568.3 
                 Present Example 
               
               
                 K 
                 11 
                 960 
                 20 
                 575 
                 20 
                 69 
                 26 
                 5 
                 995 
                 205 
                 0.097 
                 594.0 
                 Present Example 
               
               
                 L 
                 12 
                 960 
                 20 
                 575 
                 20 
                 60 
                 27 
                 13 
                 968 
                 208 
                 0.073 
                 591.8 
                 Present Example 
               
               
                 M 
                 13 
                 960 
                 20 
                 575 
                 20 
                 42 
                 36 
                 22 
                 870 
                 203 
                 0.020 
                 575.4 
                 Present Example 
               
               
                 N 
                 14 
                 960 
                 20 
                 575 
                 20 
                 61 
                 30 
                 9 
                 958 
                 228 
                 0.077 
                 595.5 
                 Present Example 
               
               
                 O 
                 15 
                 960 
                 20 
                 575 
                 20 
                 56 
                 33 
                 11 
                 935 
                 212 
                 0.019 
                 561.6 
                 Present Example 
               
               
                 P 
                 16 
                 960 
                 20 
                 575 
                 20 
                 62 
                 32 
                 6 
                 937 
                 206 
                 0.035 
                 587.4 
                 Present Example 
               
               
                 Q 
                 17 
                 960 
                 20 
                 575 
                 20 
                 52 
                 29 
                 19 
                 916 
                 237 
                 0.024 
                 578.4 
                 Present Example 
               
               
                 R 
                 18 
                 960 
                 20 
                 575 
                 20 
                 54 
                 35 
                 11 
                 925 
                 205 
                 0.031 
                 588.7 
                 Present Example 
               
               
                 S 
                 19 
                 960 
                 20 
                 575 
                 20 
                 56 
                 34 
                 10 
                 918 
                 209 
                 0.033 
                 584.3 
                 Present Example 
               
               
                 T 
                 20 
                 960 
                 20 
                 575 
                 20 
                 56 
                 32 
                 12 
                 935 
                 213 
                 0.059 
                 595.2 
                 Present Example 
               
               
                 U 
                 21 
                 960 
                 20 
                 575 
                 20 
                 54 
                 27 
                 19 
                 925 
                 220 
                 0.066 
                 595.9 
                 Present Example 
               
               
                 V 
                 22 
                 960 
                 20 
                 575 
                 20 
                 54 
                 29 
                 17 
                 925 
                 218 
                 0.071 
                 596.6 
                 Present Example 
               
               
                 W 
                 23 
                 960 
                 20 
                 575 
                 20 
                 58 
                 28 
                 14 
                 944 
                 215 
                 0.025 
                 578.0 
                 Present Example 
               
               
                 X 
                 24 
                 960 
                 20 
                 575 
                 20 
                 59 
                 29 
                 12 
                 949 
                 213 
                 0.035 
                 596.2 
                 Present Example 
               
               
                 Y 
                 25 
                 960 
                 20 
                 575 
                 20 
                 40 
                 49 
                 11 
                 879 
                 212 
                 0.027 
                 587.2 
                 Present Example 
               
               
                 Z 
                 26 
                 960 
                 20 
                 575 
                 20 
                 68 
                 25 
                 7 
                 991 
                 208 
                 0.026 
                 562.8 
                 Present Example 
               
               
                 AA 
                 27 
                 960 
                 20 
                 575 
                 20 
                 58 
                 29 
                 13 
                 944 
                 214 
                 0.025 
                 590.4 
                 Present Example 
               
               
                 AB 
                 28 
                 960 
                 20 
                 575 
                 20 
                 62 
                 32 
                 6 
                 963 
                 206 
                 0.028 
                 564.8 
                 Present Example 
               
               
                 AC 
                 29 
                 960 
                 20 
                 575 
                 20 
                 59 
                 32 
                 9 
                 949 
                 210 
                 0.028 
                 568.8 
                 Present Example 
               
               
                 AD 
                 30 
                 960 
                 20 
                 575 
                 20 
                 52 
                 31 
                 17 
                 916 
                 218 
                 0.025 
                 571.2 
                 Present Example 
               
               
                 AE 
                 31 
                 960 
                 20 
                 575 
                 20 
                 65 
                 22 
                 13 
                 931 
                 205 
                 0.051 
                 585.6 
                 Present Example 
               
               
                 AF 
                 32 
                 960 
                 20 
                 575 
                 20 
                 48 
                 35 
                 17 
                 897 
                 218 
                 0.026 
                 599.6 
                 Present Example 
               
               
                 AG 
                 33 
                 960 
                 20 
                 575 
                 20 
                 54 
                 32 
                 14 
                 925 
                 205 
                 0.025 
                 599.2 
                 Present Example 
               
               
                 AH 
                 34 
                 960 
                 20 
                 575 
                 20 
                 60 
                 32 
                 8 
                 953 
                 209 
                 0.027 
                 562.4 
                 Present Example 
               
               
                 AI 
                 35 
                 960 
                 20 
                 575 
                 20 
                 57 
                 31 
                 12 
                 939 
                 213 
                 0.024 
                 593.6 
                 Present Example 
               
               
                   
               
               
                 Underline means outside of the range of the present invention 
               
               
                 (*1) M: Martensitic phase, F: Ferrite phase, A: Retained austenite phase 
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 2-2 
               
             
            
               
                   
                   
               
               
                   
                 Quenching 
                 Tempering 
                 Microstructure 
                   
                 Corrosion 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Steel 
                 Quenching 
                   
                 Tempering 
                   
                 (volume %) 
                 Yield 
                   
                 Corrosion 
                 rate in acid 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Steel 
                 Pipe 
                 temp. 
                 Quenching 
                 temp. 
                 Tempering 
                 M 
                 F 
                 A 
                 strength 
                 vE −40   
                 rate 
                 environment 
                   
               
               
                 No. 
                 No. 
                 (° C.) 
                 time (min) 
                 (° C.) 
                 time (min) 
                 (*1) 
                 (*1) 
                 (*1) 
                 YS (MPa) 
                 (J) 
                 (mm/y) 
                 (mm/y) 
                 Remarks 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 AJ 
                 36 
                 960 
                 20 
                 575 
                 20 
                 53 
                 28 
                 19 
                 897 
                 218 
                 0.040 
                 587.2 
                 Present Example 
               
               
                 AK 
                 37 
                 960 
                 20 
                 575 
                 20 
                 54 
                 32 
                 14 
                 925 
                 215 
                 0.024 
                 585.2 
                 Present Example 
               
               
                 AL 
                 38 
                 960 
                 20 
                 575 
                 20 
                 52 
                 33 
                 15 
                 916 
                 204 
                 0.026 
                 581.6 
                 Present Example 
               
               
                 AM 
                 39 
                 960 
                 20 
                 575 
                 20 
                 57 
                 29 
                 14 
                 939 
                 205 
                 0.027 
                 567.6 
                 Present Example 
               
               
                 AN 
                 40 
                 960 
                 20 
                 575 
                 20 
                 53 
                 32 
                 15 
                 921 
                 204 
                 0.025 
                 568.0 
                 Present Example 
               
               
                 AO 
                 41 
                 960 
                 20 
                 575 
                 20 
                 53 
                 30 
                 17 
                 903 
                 214 
                 0.039 
                 578.5 
                 Present Example 
               
               
                 AP 
                 42 
                 960 
                 20 
                 575 
                 20 
                 59 
                 32 
                  9 
                 949 
                 203 
                 0.027 
                 599.2 
                 Present Example 
               
               
                 AQ 
                 43 
                 960 
                 20 
                 575 
                 20 
                 42 
                 29 
                 29 
                 870 
                 231 
                 0.135 
                 608.9 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 AR 
                 44 
                 960 
                 20 
                 575 
                 20 
                 55 
                 36 
                  9 
                 930 
                 210 
                 0.141 
                 605.1 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 AS 
                 45 
                 960 
                 20 
                 575 
                 20 
                 58 
                 30 
                 12 
                 944 
                 213 
                 0.138 
                 602.8 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 AT 
                 46 
                 960 
                 20 
                 575 
                 20 
                 58 
                 31 
                 11 
                 944 
                 212 
                 0.154 
                 607.8 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 AU 
                 47 
                 960 
                 20 
                 575 
                 20 
                 41 
                 35 
                 24 
                 843 
                 226 
                 0.018 
                 575.3 
                 Present Example 
               
               
                 AV 
                 48 
                 960 
                 20 
                 575 
                 20 
                 70 
                 27 
                  3 
                 1000  
                 203 
                 0.150 
                 604.3 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 AW 
                 49 
                 960 
                 20 
                 575 
                 20 
                 41 
                 38 
                 21 
                 865 
                 178 
                 0.027 
                 581.6 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 AX 
                 50 
                 960 
                 20 
                 575 
                 20 
                 69 
                 26 
                  5 
                 995 
                 205 
                 0.139 
                 604.6 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 AY 
                 51 
                 960 
                 20 
                 575 
                 20 
                 55 
                 31 
                 14 
                 867 
                 215 
                 0.153 
                 606.8 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 AZ 
                 52 
                 960 
                 20 
                 575 
                 20 
                 39 
                 29 
                 32 
                 841 
                 239 
                 0.028 
                 560.8 
                 Present Example 
               
               
                 BA 
                 53 
                 960 
                 20 
                 575 
                 20 
                 57 
                 34 
                  9 
                 939 
                 180 
                 0.025 
                 576.4 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 BB 
                 54 
                 960 
                 20 
                 575 
                 20 
                 58 
                 29 
                 13 
                 944 
                 214 
                 0.145 
                 605.0 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 BC 
                 55 
                 960 
                 20 
                 575 
                 20 
                 50 
                 30 
                 20 
                 907 
                 221 
                 0.160 
                 607.9 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 BD 
                 56 
                 960 
                 20 
                 575 
                 20 
                 59 
                 31 
                 10 
                 949 
                 211 
                 0.152 
                 610.3 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 BE 
                 57 
                 960 
                 20 
                 575 
                 20 
                 60 
                 30 
                 10 
                 953 
                 211 
                 0.139 
                 606.1 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 BF 
                 58 
                 960 
                 20 
                 575 
                 20 
                 30 
                 61 
                  9 
                 834 
                 217 
                 0.027 
                 567.6 
                 Present Example 
               
               
                 A 
                 59 
                 960 
                 20 
                 620 
                 20 
                 38 
                 31 
                 31 
                 837 
                 229 
                 0.023 
                 565.3 
                 Present Example 
               
               
                 B 
                 60 
                 960 
                 20 
                 620 
                 20 
                 38 
                 30 
                 32 
                 830 
                 227 
                 0.025 
                 568.9 
                 Present Example 
               
               
                 C 
                 61 
                 960 
                 20 
                 620 
                 20 
                 37 
                 30 
                 33 
                 817 
                 231 
                 0.027 
                 565.7 
                 Present Example 
               
               
                 BG 
                 62 
                 960 
                 20 
                 575 
                 20 
                 58 
                 29 
                 13 
                 950 
                 209 
                 0.030 
                 567.9 
                 Present Example 
               
               
                 BH 
                 63 
                 960 
                 20 
                 575 
                 20 
                 56 
                 33 
                 11 
                 922 
                 220 
                 0.027 
                 572.1 
                 Present Example 
               
               
                 BI 
                 64 
                 960 
                 20 
                 575 
                 20 
                 55 
                 30 
                 15 
                 917 
                 217 
                 0.022 
                 573.5 
                 Present Example 
               
               
                 BG 
                 65 
                 960 
                 20 
                 620 
                 20 
                 42 
                 27 
                 31 
                 849 
                 225 
                 0.029 
                 573.9 
                 Present Example 
               
               
                 BH 
                 66 
                 960 
                 20 
                 620 
                 20 
                 39 
                 31 
                 30 
                 826 
                 232 
                 0.030 
                 570.1 
                 Present Example 
               
               
                 BI 
                 67 
                 960 
                 20 
                 620 
                 20 
                 39 
                 28 
                 33 
                 832 
                 233 
                 0.025 
                 576.4 
                 Present Example 
               
               
                 BJ 
                 68 
                 960 
                 20 
                 575 
                 20 
                 
                   29 
                 
                 48 
                 23 
                 
                   729 
                 
                 203 
                 0.019 
                 537.2 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 BK 
                 69 
                 960 
                 20 
                 575 
                 20 
                 
                   28 
                 
                 27 
                 
                   45 
                 
                 
                   700 
                 
                 231 
                 0.030 
                 578.1 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 BL 
                 70 
                 960 
                 20 
                 575 
                 20 
                   8   
                 
                   66 
                 
                 26 
                 
                   615 
                 
                 202 
                 0.020 
                 576.2 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                   
               
               
                 Underline means outside of the range of the present invention 
               
               
                 (*1) M: Martensitic phase, F: Ferrite phase, A: Retained austenite phase 
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 2-3 
               
             
            
               
                   
                   
               
               
                   
                 Quenching 
                 Tempering 
                 Microstructure 
                   
                 Corrosion 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Steel 
                 Quenching 
                   
                 Tempering 
                   
                 (volume %) 
                 Yield 
                   
                 Corrosion 
                 rate in acid 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Steel 
                 Pipe 
                 temp. 
                 Quenching 
                 temp. 
                 Tempering 
                 M 
                 F 
                 A 
                 strength 
                 vE −40   
                 rate 
                 environment 
                   
               
               
                 No. 
                 No. 
                 (° C.) 
                 time (min) 
                 (° C.) 
                 time (min) 
                 (*1) 
                 (*1) 
                 (*1) 
                 YS (MPa) 
                 (J) 
                 (mm/y) 
                 (mm/y) 
                 Remarks 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 A 
                 71 
                 
                   800 
                 
                 20 
                 575 
                 20 
                 36 
                 21 
                 
                   43 
                 
                 
                   607 
                 
                 201 
                 0.029 
                 570.3 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 A 
                 72 
                   1200   
                 20 
                 575 
                 20 
                 
                   27 
                 
                 63 
                 10 
                 
                   671 
                 
                   15   
                 0.027 
                 572.1 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 A 
                 73 
                 960 
                 10 
                 575 
                 20 
                 55 
                 30 
                 15 
                 898 
                 213 
                 0.028 
                 569.8 
                 Present Example 
               
               
                 A 
                 74 
                 960 
                 30 
                 575 
                 20 
                 54 
                 32 
                 14 
                 917 
                 218 
                 0.027 
                 567.8 
                 Present Example 
               
               
                 A 
                 75 
                 960 
                 20 
                 575 
                 10 
                 58 
                 31 
                 11 
                 973 
                 208 
                 0.030 
                 573.8 
                 Present Example 
               
               
                 A 
                 76 
                 960 
                 20 
                 575 
                 80 
                 54 
                 28 
                 18 
                 867 
                 231 
                 0.029 
                 566.6 
                 Present Example 
               
               
                   
               
               
                 Underline means outside of the range of the present invention 
               
               
                 (*1) M: Martensitic phase, F: Ferrite phase, A: Retained austenite phase 
               
            
           
         
       
     
     As shown in Table 2-1 to Table 2-3, the stainless steel seamless pipes of the present examples all had high strength with a yield strength, YS, of 758 MPa or more. The stainless steel seamless pipes of the present examples also had excellent corrosion resistance (carbon dioxide gas corrosion resistance) in a CO 2 - and Cl − -containing high-temperature corrosive environment of 200° C., and excellent acid-environment corrosion resistance. The low-temperature toughness was also desirable.