High strength seamless steel pipe and its manufacturing method

Abstract

Anon-heat treated seamless steel pipe comprising, by weight, C: 0.10 to 0.25%, Si: 0.05 to 1.00%, Mn: 0.50 to 2.50%, P: not more than 0.03%, S: not more than 0.05%, Cr: 0.40 to 1.50%, Mo: 0.05 to 1.50%, V: 0.02 to 0.30%, Al: 0.003 to 0.10%, B: 0.0003 to 0.01%, and N: 0.001 to 0.02%, in which the balance comprises Fe and impurities. According to this non-heat treated seamless steel pipe, a ferrite area ratio is not more than 10%, and a carbon equivalent Ceq. (%) defined by the following equation is 0.45 to 0.85%. Ceq.=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15

1 . A high strength non-heat treated seamless steel pipe comprising, by weight, 
 C: 0.10 to 0.25%,    Si: 0.05 to 1.00%,    Mn: 0.50 to 2.50%,    P: not more than 0.03%,    S: not more than 0.05%,    Cr: 0.40 to 1.50%,    Mo: 0.05 to 1.50%,    V: 0.02 to 0.30%,    Al: 0.003 to 0.10%,    B: 0.0003 to 0.01%, and    N: 0.001 to 0.02%,    wherein the balance comprises Fe and impurities,    a ferrite area ratio is not more than 10%, and    a carbon equivalent Ceq. (%) defined by the following equation is 0.45 to 0.85%.      Ceq.=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15    
     
     
         2 . The high strength non-heat treated seamless steel pipe according to  claim 1 , wherein a part of Fe is replaced with at least one kind selected from a group consisting of 
 Ti: 0.005 to 0.2%,    Cu: 0.05 to 1.5%,    Ni: 0.05 to 1.5%, and    Nb: 0.005 to 0.2%.    
     
     
         3 . A method of manufacturing the high strength non-heat treated seamless steel pipe according to  claim 1 , wherein a temperature of finish rolling at a manufacturing step is not less than 900° C. and the following relational expression is satisfied.  
         Ceq. (%)×finish rolling temperature (° C.)≧450  
     
     
         4 . The method of the high strength non-heat treated seamless steel pipe according to  claim 3 , wherein a part of Fe is replaced with at least one kind selected from a group consisting of 
 Ti: 0.005 to 0.2%,    Cu: 0.05 to 1.5%,    Ni: 0.05 to 1.5%, and    Nb: 0.005 to 0.2%.