Seamless steel pipe for line pipe and method for manufacturing the same

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

• heating a steel material having a chemical composition comprising, by mass percent, C:
• 0.02 to 0.10%, Si:
• at most 0.5%, Mn:
• 0.5 to 2.0%, Al:
• 0.01 to 0.1%, P:
• at most 0.03%, S:
• at most 0.005%, Ca:
• at most 0.005%, and N:
• at most 0.007%, and further comprising at least one selected from a group consisting of Ti:
• at most 0.008%, V:
• less than 0.06%, and Nb:
• at most 0.05%, the balance being Fe and impurities, the carbon equivalent Ceq defined by Formula (1) being at least 0.38, and content Ti, V and Nb satisfying Formula (2);
• producing a hollow shell by piercing the heated steel material;
• producing a seamless steel pipe by rolling the hollow shell;
• acceleratedly cooling the rolled seamless steel pipe to at most the Ar1 point at a cooling rate of at least 100° C/min;
• quenching the acceleratedly-cooled seamless steel pipe after temperature of the seamless steel pipe reaches at least the Ac3 point by heating the seamless steel pipe at a heating rate of at least 3° C/min at the time when temperature of the seamless steel pipe is 600 to 900° C.;
• and
• tempering the quenched seamless steel pipe at a temperature of at most the Ac1 point:
• Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15 (1)
• Ti+V+Nb<0.06 (2)
• where, into each of the symbols of elements in Formulas (1) and (2), the content (mass percent) of each element is substituted, and in the case where an element corresponding to the symbol of element in Formulas (1) and (2) is not contained, “0” is substituted into the corresponding symbol of the element in Formulas (1) and (2).