Seamless steel pipe and manufacturing method thereof
View Patent ↗The present invention relates to the following seamless steel pipes excellent in strength, toughness and weldability, particularly suitable for submarine flow lines, and a manufacturing method thereof. An as-quenched seamless steel pipe having a chemical composition consisting of, 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 Fe and impurities, with not more than 0.25% of Si, not more than 0.05% of P, not more than 0.005% of S, less than 0.005% Nb, and less than 0.0003% of B as the impurities, and having a microstructure consisting of not more than 20 volume% of polygonal ferrite, not more than 10 volume% of a mixed microstructure of martensite and retained austenite, and balance bainite. B can be 0.0003 to 0.001%. Mg and/or REM can be contained. The manufacturing method is characterized by the cooling rate during quenching.
1. 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.
2. A quenched and tempered seamless steel pipe according to claim 1 , further containing, instead of a part of Fe, by mass %, one or more elements selected from V: not more than 0.08%, Cu: not more than 1.0%, Mg:
not more than 0.005%, REM: not more than 0.005%.
3. 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%, B: 0.0003 to 0.01%, 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, and less than 0.005% of Nb 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.
4. A quenched and tempered seamless steel pipe according to claim 3 , further containing, instead of a part of Fe, by mass %, one or more elements selected from V: not more than 0.08%, Cu: not more than 1.0%, Mg: not more than 0.005% and REM: not more than 0.005%.