Low alloy steel

Abstract

A low alloy steel subjected to post weld heat treatment, containing, by mass percent, of C: 0.01 to 0.15%, Si: 3% or less, Mn: 3% or less, and Al: 0.08% or less, one or more kinds of elements selected from Ti, V and Nb: the range satisfying Formula (1), and the balance being Fe and impurities, wherein in the impurities, N: 0.01% or less, P: 0.05% or less, S: 0.03% or less, and O: 0.03% or less: 0.1×[C(%)]≦[Ti(%)]+[V(%)]+0.5×[Nb(%)]≦0.2  (1) where, the symbol of element in the formula represents the content (mass %) of each element. In the alloy steel, a HAZ subjected to PWHT, especially short-time PWHT, has excellent hydrogen embrittlement resistance in wet hydrogen sulfide environments or the like.

1 . A low alloy steel subjected to post weld heat treatment, containing, by mass percent, of
 C: 0.01 to 0.15%,   Si: 3% or less,   Mn: 3% or less, and   Al: 0.08% or less,   one or more kinds of elements selected from Ti, V and Nb: the range satisfying Formula (1), and   the balance being Fe and impurities,   wherein in the impurities,   N: 0.01% or less,   P: 0.05% or less,   S: 0.03% or less, and   O: 0.03% or less:
   0.1×[C(%)]≦[Ti(%)]+[V(%)]+0.5×[Nb(%)]≦0.2  (1)
 
   where, the symbol of element in the formula represents the content (mass %) of each element.   
     
     
         2 . A low alloy steel subjected to post weld heat treatment, containing, by mass percent, of
 C: 0.01 to 0.15%,   Si: 3% or less,   Mn: 3% or less, and   Al: 0.08% or less,   one or more kinds of elements selected from Ti, V and Nb: the range satisfying Formula (1),   one or more kinds of elements selected from the elements described in (A) to (D) and   the balance being Fe and impurities,
 wherein in the impurities, 
 N: 0.01% or less, 
 P: 0.05% or less, 
 S: 0.03% or less, and 
 O: 0.03% or less: 
   (A) Cr and/or Mo: 1.5% or less in total   (B) Ni and/or Cu: 0.8% or less in total   (C) Ca and/or Mg: 0.05% or less in total   (D) B: the range satisfying Formula (2):
   0.1×[C(%)]≦[Ti(%)]+[V(%)]+0.5×[Nb(%)]≦0.2  (1)
 
   [B(%)]<0.1×[C(%)]  (2)
 
   where, the symbol of element in the formula represents the content (mass %) of each element.   
     
     
         3 . The low alloy steel according to  claim 1 , wherein post weld heat treatment is performed under the condition satisfying Formula (3):
   8000 ≦T×{ 20+log( t/ 3600)}≦15000  (3)
   where, T is treatment temperature (° C.) of post weld heat treatment, and t is treatment time (sec) of post weld heat treatment.   
     
     
         4 . The low alloy steel according to  claim 1 , wherein the low alloy steel contains, by mass percent, Ni and/or Cu: 0.8% or less in total in lieu of a part of Fe. 
     
     
         5 . The low alloy steel according to  claim 2 , wherein the low alloy steel contains, by mass percent, Ni and/or Cu: 0.8% or less in total in lieu of a part of Fe. 
     
     
         6 . The low alloy steel according to  claim 1 , wherein the low alloy steel contains, by mass percent, Ca and/or Mg: 0.05% or less in total in lieu of a part of Fe. 
     
     
         7 . The low alloy steel according to  claim 2 , wherein the low alloy steel contains, by mass percent, Ca and/or Mg: 0.05% or less in total in lieu of a part of Fe. 
     
     
         8 . The low alloy steel according to  claim 4 , wherein the low alloy steel contains, by mass percent, Ca and/or Mg: 0.05% or less in total in lieu of a part of Fe. 
     
     
         9 . The low alloy steel according to  claim 5 , wherein the low alloy steel contains, by mass percent, Ca and/or Mg: 0.05% or less in total in lieu of a part of Fe. 
     
     
         10 . The low alloy steel according to  claim 1 , wherein the low alloy steel contains, by mass percent, B: the range satisfying Formula (2) in lieu of a part of Fe:
   [B(%)]<0.1×[C(%)]  (2)
   where, the symbol of element in the formula represents the content (mass %) of each element.   
     
     
         11 . The low alloy steel according to  claim 2 , wherein the low alloy steel contains, by mass percent, B: the range satisfying Formula (2) in lieu of a part of Fe:
   [B(%)]<0.1×[C(%)]  (2)
   where, the symbol of element in the formula represents the content (mass %) of each element.   
     
     
         12 . The low alloy steel according to  claim 4 , wherein the low alloy steel contains, by mass percent, B: the range satisfying Formula (2) in lieu of a part of Fe:
   [B(%)]<0.1×[C(%)]  (2)
   where, the symbol of element in the formula represents the content (mass %) of each element.   
     
     
         13 . The low alloy steel according to  claim 5 , wherein the low alloy steel contains, by mass percent, B: the range satisfying Formula (2) in lieu of a part of Fe:
   [B(%)]<0.1×[C(%)]  (2)
   where, the symbol of element in the formula represents the content (mass %) of each element.   
     
     
         14 . The low alloy steel according to  claim 6 , wherein the low alloy steel contains, by mass percent, B: the range satisfying Formula (2) in lieu of a part of Fe:
   [B(%)]<0.1×[C(%)]  (2)
   where, the symbol of element in the formula represents the content (mass %) of each element.   
     
     
         15 . The low alloy steel according to  claim 7 , wherein the low alloy steel contains, by mass percent, B: the range satisfying Formula (2) in lieu of a part of Fe:
   [B(%)]<0.1×[C(%)]  (2)
   where, the symbol of element in the formula represents the content (mass %) of each element.   
     
     
         16 . The low alloy steel according to  claim 8 , wherein the low alloy steel contains, by mass percent, B: the range satisfying Formula (2) in lieu of a part of Fe:
   [B(%)]<0.1×[C(%)]  (2)
   where, the symbol of element in the formula represents the content (mass %) of each element.   
     
     
         17 . The low alloy steel according to  claim 9 , wherein the low alloy steel contains, by mass percent, B: the range satisfying Formula (2) in lieu of a part of Fe:
   [B(%)]<0.1×[C(%)]  (2)
   where, the symbol of element in the formula represents the content (mass %) of each element.