Low alloy steel

Abstract

A low alloy steel for a heat-resisting structural member being improved in long time creep ductility at high temperatures and temper softening resistance, characterized by comprising, by mass percent, C: 0.03 to 0.10%, Si: not more than 0.30%, Mn: not more than 1.0%, Cr: more than 1.5% to not more than 2.5%, Mo: 0.01 to 1.0%, V: 0.04 to 0.30%, Nb: 0.001 to 0.10%, Ti: 0.001 to 0.020%, B: 0.0001 to 0.020%, Al: 0.001 to 0.01% and Nd: 0.0001 to 0.050%, with the balance being Fe and impurities, wherein the content of P is not more than 0.020%, the content of S is not more than 0.003%, the content of N is less than 0.0050% and the content of O (oxygen) is not more than 0.0050% among the impurities, in which the value of BSO represented by the following formula (1) is 0.0001 to 0.010: BSO=B−(11/14)N−(11/32)S−(11/16)O  (1), wherein each element symbol in the formula ( 1 ) represents the content (by mass %) of the element concerned. The steel may further contain one or more element selected from among W, Cu, Ni, Co, Mg, Ca, La, Ce, Y, Sm and Pr.

1 . A low alloy steel, which comprises by mass percent, C, 0.03 to 0.10%, Si: not more than 0.30%, Mn: not more than 1.0%, Cr: more than 1.5% to not more than 2.5%, Mo: 0.01 to 1.0%, V: 0.04 to 0.30%, Nb: 0.001 to 0.10%, Ti: 0.001 to 0.020%, B: 0.0001 to 0.020%, Al: 0.001 to 0.01% and Nd: 0.0001 to 0.050%, with the balance being Fe and impurities, wherein the content of P is not more than 0.020%, the content of S is not more than 0.003%, the content of N is less than 0.0050% and the content of 0 (oxygen) is not more than 0.0050% among the impurities, in which the value of BSO represented by the following formula (I) is 0.0001 to 0.010:  
         BSO=B−(11/14)N−(11/32)S−(11/16)O  (1),  
       wherein each element symbol in the formula (I) represents the content (by mass %) of the element concerned.  
     
     
         2 . A low alloy steel according to  claim 1 , which further contains W: 
 not more than 2.0% by mass in lieu of part of Fe.    
     
     
         3 . A low alloy steel according to  claim 1 , which further contains one or more elements selected from among Cu, Ni and Co each at a level not more than 0.50% by mass in lieu of part of Fe.  
     
     
         4 . A low alloy steel according to  claim 2 , which further contains one or more elements selected from among Cu, Ni and Co each at a level not more than 0.50% by mass in lieu of part of Fe.  
     
     
         5 . A low alloy steel according to  claim 1 , which further contains one or more elements selected from among Mg: not more than 0.005% by mass, Ca: not more than 0.005% by mass, La: not more than 0.02% by mass, Ce: not more than 0.02% by mass, Y: not more than 0.05% by mass, Sm: not more than 0.05% by mass and Pr: not more than 0.05% by mass.  
     
     
         6 . A low alloy steel according to  claim 2 , which further contains one or more elements selected from among Mg: not more than 0.005% by mass, Ca: not more than 0.005% by mass, La: not more than 0.02% by mass, Ce: not more than 0.02% by mass, Y: not more than 0.05% by mass, Sm: not more than 0.05% by mass and Pr: not more than 0.05% by mass.  
     
     
         7 . A low alloy steel according to  claim 3 , which further contains one or more elements selected from among Mg: not more than 0.005% by mass, Ca: not more than 0.005% by mass, La: not more than 0.02% by mass, Ce: not more than 0.02% by mass, Y: not more than 0.05% by mass, Sm: not more than 0.05% by mass and Pr: not more than 0.05% by mass.  
     
     
         8 . A low alloy steel according to  claim 4 , which further contains one or more elements selected from among Mg: not more than 0.005% by mass, Ca: not more than 0.005% by mass, La: not more than 0.02% by mass, Ce: not more than 0.02% by mass, Y: not more than 0.05% by mass, Sm: not more than 0.05% by mass and Pr: not more than 0.05% by mass.