Method for manufacturing seamless steel pipe for line pipe and seamless steel pipe for line pipe

A method for manufacturing a seamless steel pipe for line pipe, comprising the steps of:

• heating a round billet having a chemical composition comprising, by mass percent, C:
• 0.02 to 0.15%, Si:
• at most 0.5%, Mn:
• 0.5 to 2.5%, and Cu:
• 0.05 to 1.5% the balance being Fe and impurities;
• producing a hollow shell by piercing-rolling the heated round billet;
• producing a seamless steel pipe by elongation rolling and sizing the hollow shell;
• water cooling the seamless steel pipe at a cooling rate of at least 10° C./sec and stopping water cooling when the temperature of the seamless steel pipe reaches lower than 400° C. to cause bainitic transformation;
• heating and soaking the water-cooled seamless steel pipe to cause reverse transformation from a bainite to an austenitic structure;
• water cooling the heated and soaked seamless steel pipe;
• and
• tempering the heated, soaked, and water-cooled seamless steel pipe.