High strength bolt having excellent hydrogen embrittlement resistance

Abstract

The present invention provides a high strength bolt that has excellent hydrogen embrittlement resistance. The high strength bolt having excellent hydrogen embrittlement resistance which bolt comprises 0.20 to 0.60% of C, 1.0 to 3.0% of Si, 1.0 to 3.5% of Mn, higher than 0% and not higher than 1.5% of Al, 0.15% or less P, 0.02% or less S, and balance of iron and inevitable impurities, and the structure includes: 1% or more residual austenite; 80% or more in total content of bainitic ferrite and martensite; and 10% or less (may be 0%) in total content of ferrite and pearlite in the proportion of area to the entire structure, and also the mean axis ratio (major axis/minor axis) of the residual austenite grains is 5 or higher and the bolt has tensile strength of 1180 MPa or higher.

1 . A high strength bolt having excellent hydrogen embrittlement resistance, which comprises: 
 0.20 to 0.60% of C,    1.0 to 3.0% of Si,    1.0 to 3.5% of Mn,    higher than 0% and not higher than 1.5% of Al,    0.15% or less P,    0.02% or less S,    and balance of iron and inevitable impurities, wherein the structure includes:    1% or more of residual austenite;    80% or more in total content of bainitic ferrite and martensite; and    10% or less (may be 0%) in total content of ferrite and pearlite in the proportion of area to the entire structure, and wherein the mean axis ratio (major axis/minor axis) of the residual austenite grains is 5 or higher and the steel has tensile strength is 1180 MPa or higher.    
   
   
       2 . A high strength bolt having excellent hydrogen embrittlement resistance, which comprises: 
 0.20 to 0.60% of C,    1.0 to 3.0% of Si,    1.0 to 3.5% of Mn,    higher than 0% and not higher than 0.5% of Al,    0.15% or less P,    0.02% or less S,    and balance of iron and inevitable impurities, wherein the structure includes:    1% or more residual austenite;    80% or more in total content of bainitic ferrite and martensite; and    10% or less (may be 0%) in total content of ferrite and pearlite in the proportion of area to the entire structure, and wherein the mean axis ratio (major axis/minor axis) of the residual austenite grains is 5 or higher and the steel has tensile strength is 1180 MPa or higher.    
   
   
       3 . A high strength bolt having excellent hydrogen embrittlement resistance according to the  claim 1 , which further comprises: 
 higher than 0% and not higher than 0.1% of Nb and/or higher than 0% and not higher than 1.0% of Mo.    
   
   
       4 . A high strength bolt having excellent hydrogen embrittlement resistance according to the  claim 2 , which further comprises: 
 higher than 0% and not higher than 0.1% of Nb and/or higher than 0% and not higher than 1.0% of Mo.    
   
   
       5 . A high strength bolt having excellent hydrogen embrittlement resistance according to the  claim 1 , which further comprises: 
 higher than 0% and not higher than 2% of Cu and/or higher than 0% and not higher than 5% of Ni.    
   
   
       6 . A high strength bolt having excellent hydrogen embrittlement resistance according to the  claim 2 , which further comprises: 
 higher than 0% and not higher than 2% of Cu and/or higher than 0% and not higher than 5% of Ni.    
   
   
       7 . A high strength bolt having excellent hydrogen embrittlement resistance according to the  claim 3 , which further comprises: 
 higher than 0% and not higher than 2% of Cu and/or higher than 0% and not higher than 5% of Ni.    
   
   
       8 . A high strength bolt having excellent hydrogen embrittlement resistance according to the  claim 4 , which further comprises: 
 higher than 0% and not higher than 2% of Cu and/or higher than 0% and not higher than 5% of Ni.