Patent ID: 7815755

Claim:
A quenched and tempered seamless steel pipe with a wall thickness of not less than 25 mm having a chemical composition, comprising, by mass %, C: 0.03 to 0.08%, Mn: 0.3 to 2.5%, Al 0.001 to 0.10%, Cr: 0.02 to 1.0%, Ni: 0.02 to 1.0%, Mo: 0.02 to 0.8%, Ti: 0.004 to 0.010%, N: 0.002 to 0.008%, Ca: 0.0005 to 0.005%, and the balance being Fe and impurities, with less than 0.10% of Si, not more than 0.05% of P, not more than 0.005% of S, less than 0.005% of Nb, and less than 0.0003% of B as the impurities, wherein the steel pipe before tempering has a microstructure consisting of not more than 12 volume % of polygonal ferrite, not more than 3.6 volume % of a mixed microstructure of martensite and retained austenite, and the balance being bainite, and the steel pipe after being tempered has a yield strength of not less than 565 MPa and an energy transition temperature of not higher than −60° C.