Manufacturing method of high strength, high toughness seamless steel pipe for linepipe

Abstract

A manufacturing method of high-strength, high-toughness seamless steel pipe for linepipe comprises: (1) hot piercing a billet, in which the carbon equivalent value Ceq defined by expression (a): Ceq=C+Mn/6+(Cu+Ni)/5+(Cr+Mo+V)/5[%] . . . (a) is not greater than 0.60 wt %, and then subjecting the pierced billet to hot rolling at a temperature of 950° C. or higher to obtain a seamless steel pipe; (2) putting the seamless steel pipe immediately, without cooling to Ar3 point or lower, into a furnace kept at Ar3 point+50° C. to 1100° C. to carry out heating; (3) cooling the seamless steel pipe at a cooling rate of 5° C./sec or higher; and (4) tempering the seamless steel pipe at temperature of 550° C. to Ac1 point. A high-strength and high-toughness seamless steel pipe with little strength unevenness can be manufactured, which can be effectively employed for linepipes in cold areas.

Dependencies are inferred from the claim text.