Seamless steel pipe and method for producing same

Abstract

A seamless steel pipe has a carbon equivalent Ceq of 0.50% to 0.58%, and contains specified carbides containing Mo at a ratio of 50 mass % or more, V, and at least one selected from the group consisting of Ti and Nb, and having a size of 20 nm or more.

1 - 7 . (canceled) 
     
     
         8 . A method for producing a seamless steel pipe comprising:
 heating a steel material including, as a chemical composition, by mass %,
 C: 0.02% to 0.10%, 
 Si: 0.05% to 0.5%, 
 Mn: 1.0% to 2.0%, 
 Mo: 0.5% to 1.0%, 
 Cr: 0.1% to 1.0%, 
 Al: 0.01% to 0.10%, 
 P: 0.03% or less, 
 S: 0.005% or less, 
 Ca: 0.0005% to 0.005%, 
 V: 0.010% to 0.040%, 
 N: 0.002% to 0.007%, 
 at least one selected from the group consisting of Ti: 0.008% or less and Nb: 0.02% to 0.05%, and 
 a balance consisting of Fe and impurities and having a carbon equivalent Ceq defined by the following Formula (2) of 0.50% to 0.58%; 
   producing a raw pipe by piercing-rolling the heated steel material;   producing a seamless steel pipe by rolling the raw pipe;   quenching the seamless steel pipe at a quenching temperature of an A c3  point or higher; and   tempering the seamless steel pipe after the quenching at a tempering temperature of 660° C. to 700° C.
     Ceq =C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15   (2)
 
   here, into each of the symbols of elements in the formula (2), the amount (mass %) of the corresponding element is substituted, and in the case where an element corresponding to the symbol of the element is not contained, “0” is substituted into the corresponding symbol of the element.   
     
     
         9 . The method for producing a seamless steel pipe according to  claim 8 , further comprising
 acceleratedly cooling the seamless steel pipe at a cooling rate of 100° C./min or higher until a temperature of the seamless steel pipe reaches a temperature of an A r1  point or lower between the producing of the seamless steel pipe and the quenching of the seamless steel pipe,   wherein the acceleratedly-cooled seamless steel pipe is quenched in the quenching of the seamless steel pipe.   
     
     
         10 . The method for producing a seamless steel pipe according to  claim 8 ,
 wherein the seamless steel pipe further includes, as the chemical composition, at least one selected from the group consisting of Cu: 1.0% or less and Ni: 1.0% or less in place of some of Fe.   
     
     
         11 . The method for producing a seamless steel pipe according to  claim 9 ,
 wherein the seamless steel pipe further includes, as the chemical composition, at least one selected from the group consisting of Cu: 1.0% or less and Ni: 1.0% or less in place of some of Fe.