Seamless steel pipe and method of manufacturing the same

Abstract

A seamless steel pipe contains, (mass %), C: 0.02 to 0.15%; Si: 0.05 to 0.5%; Mn: 0.30 to 2.5%; Al: 0.01 to 0.10%; Ti: 0.001 to 0.010%; N: up to 0.007%; Cr: 0.05 to 1.0%; Mo: not less than 0.02% and less than 0.5%; Ni: 0.03 to 1.0%; Cu: 0.02 to 1.0%; V: 0.020 to 0.20%; Ca: 0.0005 to 0.005%; and Nb: 0 to 0.05%, where carbon equivalent is not less than 0.430% and less than 0.500%, the microstructure main phase from the surface to an in-the-wall portion is tempered martensite or tempered bainite, prior austenite grain size is lower than 6.0, a portion between 1 mm from the inner surface and 1 mm from the outer surface has Vickers hardness of 250 Hv or lower, and yield strength is 555 MPa or higher.

1 . A seamless steel pipe having a chemical composition of, in mass %,
 C: 0.02 to 0.15%;   Si: 0.05 to 0.5%;   Mn: 0.30 to 2.5%;   P: up to 0.03%;   S: up to 0.006%;   O: up to 0.004%;   Al: 0.01 to 0.10%;   Ti: 0.001 to 0.010%;   N: up to 0.007%;   Cr: 0.05 to 1.0%;   Mo: not less than 0.02% and less than 0.5%;   Ni: 0.03 to 1.0%;   Cu: 0.02 to 1.0%;   V: 0.020 to 0.20%;   Ca: 0.0005 to 0.005%; and   Nb: 0 to 0.05%,   the balance being Fe and impurities,   where a carbon equivalent Ceq as defined by equation (1) below is not less than 0.430% and less than 0.500%,   a main phase of a microstructure from a surface layer to an in-the-wall portion is tempered martensite or tempered bainite,   a size of prior austenite grains in the microstructure is lower than 6.0 in crystal grain size number according to ASTM E112-10,   a portion between a position at 1 mm from an inner surface and a position at 1 mm from an outer surface has a Vickers hardness of 250 Hv or lower, and   a yield strength is 555 MPa or higher,
   Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15  (1),
 
   where a symbol of each element in equation (1) is substituted by a content of this element in mass %.   
     
     
         2 . The seamless steel pipe according to  claim 1 , wherein the chemical composition contains, in mass %:
 Nb: 0.010 to 0.05%.   
     
     
         3 . The seamless steel pipe according to  claim 1 , wherein a difference between a Vickers hardness of a portion at 1 mm from the inner surface and that of a portion in a middle in a wall thickness, a difference between a Vickers hardness of a portion at 1 mm from the outer surface and that of a portion in the middle in the wall thickness, and a difference between a Vickers hardness of a portion at 1 mm from the inner surface and that of a portion at 1 mm from the outer surface are each 25 Hv or lower. 
     
     
         4 . The seamless steel pipe according to  claim 1 , wherein:
 the seamless steel pipe is produced by quenching and tempering, and   a Larson-Miller parameter PL as defined by equation (2) below is 18800 or higher:
   PL=(T+273)×(20+log( t ))  (2),
 
   in equation (2), T is a tempering temperature and t is a holding time for that temperature, T is in ° C., and t is in hours.   
     
     
         5 . A method of manufacturing a seamless steel pipe, comprising:
 preparing a raw material having a chemical composition of, in mass %, C: 0.02 to 0.15%; Si: 0.05 to 0.5%; Mn: 0.30 to 2.5%; P: up to 0.03%; S: up to 0.006%; O: up to 0.004%; Al: 0.01 to 0.10%; Ti: 0.001 to 0.010%; N: up to 0.007%; Cr: 0.05 to 1.0%; Mo: not less than 0.02% and less than 0.5%; Ni: 0.03 to 1.0%; Cu: 0.02 to 1.0%; V: 0.020 to 0.20%; Ca: 0.0005 to 0.005%; and Nb: 0 to 0.05%, the balance being Fe and impurities;   hot working the raw material to produce a hollow shell;   quenching the hollow shell by direct quenching or in-line quenching; and   tempering the quenched hollow shell,   no reheating-and-quenching is performed between the quenching and tempering,   a carbon equivalent Ceq as defined by equation (3) below is not less than 0.430% and less than 0.500%,   a Larson-Miller parameter PL as defined by equation (4) below is not less than 18800,
   Ceq=C+Mn/6+(Cr+Mo+V)/5±(Ni+Cu)/15  (3), and
 
   PL=(T+273)×(20+log( t ))  (4),
 
   a symbol of each element in equation (3) is substituted by a content of this element in mass %, and in equation (4), T is a tempering temperature, and t is a holding period for this temperature, and T is in ° C., and t is in hours.   
     
     
         6 . The seamless steel pipe according to  claim 2 , wherein a difference between a Vickers hardness of a portion at 1 mm from the inner surface and that of a portion in a middle in a wall thickness, a difference between a Vickers hardness of a portion at 1 mm from the outer surface and that of a portion in the middle in the wall thickness, and a difference between a Vickers hardness of a portion at 1 mm from the inner surface and that of a portion at 1 mm from the outer surface are each 25 Hv or lower. 
     
     
         7 . The seamless steel pipe according to  claim 2 , wherein:
 the seamless steel pipe is produced by quenching and tempering, and   a Larson-Miller parameter PL as defined by equation (2) below is 18800 or higher:
   PL=(T+273)×(20+log( t ))  (2),
 
   in equation (2), T is a tempering temperature and t is a holding time for that temperature, T is in ° C., and t is in hours.   
     
     
         8 . The seamless steel pipe according to  claim 3 , wherein:
 the seamless steel pipe is produced by quenching and tempering, and   a Larson-Miller parameter PL as defined by equation (2) below is 18800 or higher:
   PL=(T+273)×(20+log( t ))  (2),
 
   in equation (2), T is a tempering temperature and t is a holding time for that temperature, T is in ° C., and t is in hours.   
     
     
         9 . The seamless steel pipe according to  claim 6 , wherein:
 the seamless steel pipe is produced by quenching and tempering, and   a Larson-Miller parameter PL as defined by equation (2) below is 18800 or higher:
   PL=(T+273)×(20+log( t ))  (2),
 
   in equation (2), T is a tempering temperature and t is a holding time for that temperature, T is in ° C., and t is in hours.