Method for producing seamless steel pipe

1. A method for producing a seamless steel pipe having a yield strength of not less than 759 MPa, austenite grains with a grain size number regulated in ASTM E 112 of not less than 7.2 and a critical stress of 90% or more of the actual yield strength in accordance with the SSC resistance test by the NACE-TM-0177-A-96 method, which comprises the steps of making a pipe by heating a steel billet, which has a chemical composition on the mass percent basis, C: 0.15 to 0.20%, Si: not less than 0.01% to less than 0.15%, Mn: 0.05 to 1.0%, Cr: 0.05 to 1.5%, Mo: 0.05 to 1.0%, Al: not more than 0.10%, V: 0.01 to 0.2%, Ti: 0.002 to 0.03%, B: 0.0003 to 0.005% and N: 0.002 to 0.01%, under the provision that the following formulas (1) and (2) are satisfied, with the balance being Fe and impurities, wherein the content of P is not more than 0.025%, the content of S is not more than 0.010% and the content of Nb is less than 0.0004% among the impurities, to a temperature of 1000 to 1250° C. followed by pipe-making rolling at a final rolling temperature adjusted to 900 to 1050° C., and then quenching the resulting steel pipe directly from a temperature not lower than the Ar 3 transformation point followed by tempering at a temperature range from 600° C. to the Ac 1 transformation point, or instead of the above after the said pipe-making rolling, complementarily heating the resulting steel pipe in a temperature range from the Ac 3 transformation point to 1000° C. in-line and then quenching it from a temperature not lower than the Ar 3 transformation point followed by tempering at a temperature range from 600° C. to the Ac 1 transformation point:

C+(Mn/6)+(Cr/5)+(Mo/3)≧0.43  (1),

Ti×N<0.0002−0.0006×Si  (2),

wherein C, Mn, Cr, Mo, Ti, N and Si in the above formulas (1) and (2) represent the mass percent of the respective elements.

2. A method for producing a seamless steel pipe having a yield strength of not less than 759 MPa, austenite grains with a grain size number regulated in ASTM E 112 of not less than 7.2 and a critical stress of 90% or more of the actual yield strength in accordance with the SSC resistance test by the NACE-TM-0177-A-96 method, which comprises the steps of making a pipe by heating a steel billet, which has a chemical composition on the mass percent basis, C: 0.15 to 0.20%, Si: not less than 0.01% to less than 0.15%, Mn: 0.05 to 1.0%, Cr: 0.05 to 1.5%, Mo: 0.05 to 1.0%, Al: not more than 0.10%, V: 0.01 to 0.2%, Ti: 0.002 to 0.03%, B: 0.0003 to 0.005% and N: 0.002 to 0.01% and, further, one or more elements selected from among Ca: 0.0003 to 0.01%, Mg: 0.0003 to 0.01% and REM: 0.0003 to 0.01%, under the provision that the following formulas (1) and (2) are satisfied, with the balance being Fe and impurities, wherein the content of P is not more than 0.025%, the content of S is not more than 0.010% and the content of Nb is less than 0.0004% among the impurities, to a temperature of 1000 to 1250° C. followed by pipe-making rolling at a final rolling temperature adjusted to 900 to 1050° C., and then quenching the resulting steel pipe directly from a temperature not lower than the Ar 3 transformation point followed by tempering at a temperature range from 600° C. to the Ac 1 transformation point, or instead of the above after the said pipe-making rolling, complementarily heating the resulting steel pipe in a temperature range from the Ac 3 transformation point to 1000° C. in-line and then quenching it from a temperature not lower than the Ar 3 transformation point followed by tempering at a temperature range from 600° C. to the Ac 1 transformation point:

C+(Mn/6)+(Cr/5)+(Mo/3)≧0.43  (1),

Ti×N<0.0002−0.0006×Si  (2),

wherein C, Mn, Cr, Mo, Ti, N and Si in the above formulas (1) and (2) represent the mass percent of the respective elements.