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.

1 . A manufacturing method of high-strength, high-toughness seamless steel pipe for linepipe comprising the steps of: 
 (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.    
   
   
       2 . A manufacturing method of high-strength, high-toughness seamless steel pipe for linepipe according to  claim 1 , wherein the billet comprises, by weight percent, 0.02 to 0.15% of C, not greater than 1.0% of Si, 0.3 to 2.5% of Mn, not greater than 0.015% of P, not greater than 0.010% of S, 0 to 0.10% of Ti, 0 to 0.05% of Nb, 0.01 to 0.10% of V and not great than 0.006% of N.  
   
   
       3 . A manufacturing method of high-strength, high-toughness seamless steel pipe for linepipe according to  claim 1 , wherein the billet comprises, by weigh percent, one or more of elements selected from not greater than 1.0% of Cr, not greater than 1.0% of Mo, not greater than 1.0% of Cu, not greater than 1.0% of Ni, not greater than 0.003% of B and not greater than 0.005% of Ca.  
   
   
       4 . A manufacturing method of high-strength, high-toughness seamless steel pipe for linepipe according to  claim 2 , wherein the billet comprises, by weigh percent, one or more of elements selected from not greater than 1.0% of Cr, not greater than 1.0% of Mo, not greater than 1.0% of Cu, not greater than 1.0% of Ni, not greater than 0.003% of B and not greater than 0.005% of Ca.