High strength thin steel sheet having high hydrogen embrittlement resisting property

Abstract

The purpose of the present invention is to provide a high strength thin steel sheet that has high hydrogen embrittlement resisting property. In order to achieve the above purpose, a high strength thin steel sheet having high hydrogen embrittlement resisting property comprises: C: 0.10 to 0.25%; Si: 1.0 to 3.0%; Mn: 1.0 to 3.5%; P: 0.15% or less; S: 0.02% or less; and Al: 1.5% or less (higher than 0%) in terms of percentage by weight, with balance of iron and inevitable impurities; and the metal structure comprises: residual austenite; 1% by area or more in proportion to the entire structure; bainitic ferrite and martensite: 80% or more in total; and ferrite and pearlite: 9% or less (may be 0%) in total, while the mean axis ratio (major axis/minor axis) of said residual austenite grains is 5 or higher, and the steel has tensile strength of 1180 MPa or higher.

1 . A high strength thin steel sheet having high hydrogen embrittlement resisting property, which comprises: 
 C: 0.10 to 0.25%;    Si: 1.0 to 3.0%;    Mn: 1.0 to 3.5%;    P: 0.15% or less;    S: 0.02% or less; and    Al: 1.5% or less (higher than 0%) in terms of percentage by weight, with balance of iron and inevitable impurities; wherein the metal structure comprises:    residual austenite; 1% by area or more in proportion to the entire structure;    bainitic ferrite and martensite: 80% or more in total; and ferrite and pearlite: 9% or less (may be 0%) in total, while the mean axis ratio (major axis/minor axis) of said residual austenite grains is 5 or higher, and the steel has tensile strength of 1180 MPa or higher.    
   
   
       2 . The high strength thin steel sheet according to  claim 1 , wherein the metal structure further satisfies the requirements that: 
 mean length of minor axes of said residual austenite grains is 1 μm or less; and    minimum distance between said residual austenite grains is 1 μm or less.    
   
   
       3 . The high strength thin steel sheet according to  claim 1 , wherein 0.5% or less (higher than 0%) by weight of Al is contained.  
   
   
       4 . The high strength thin steel sheet according to  claim 1 , wherein 
 0.003 to 0.5% of Cu and/or    0.003 to 1.0% of Ni    in terms of percentage by weight are further contained.    
   
   
       5 . The high strength thin steel sheet according to  claim 1 , wherein 
 0.003 to 1.0% of Ti and/or V    in terms of percentage by weight are further contained.    
   
   
       6 . The high strength thin steel sheet according to  claim 1 , wherein 
 1.0% or less (higher than 0%) of Mo and    0.1% or less (higher than 0%) of Nb    in terms of percentage by weight are further contained.    
   
   
       7 . The high strength thin steel sheet according to  claim 1 , wherein 0.0002 to 0.01% of B in terms of percentage by weight is further contained.  
   
   
       8 . The high strength thin steel sheet according to  claim 1 , wherein at least one element selected from the group consisting of: 
 0.0005 to 0.005% of Ca;    0.0005 to 0.01% of Mg; and    0.0005 to 0.01% of REM    in terms of percentage by weight is further contained.    
   
   
       9 . A high strength thin steel sheet having high hydrogen embrittlement resisting property, which comprises: 
 C: 0.10 to 0.25%;    Si: 1.0 to 3.0%;    Mn: 1.0 to 3.5%;    P: 0.15% or less;    S: 0.02% or less; and    Al: 1.5% or less (higher than 0%) in terms of percentage by weight,    with balance of iron and inevitable impurities, 
 wherein the metal structure comprises: residual austenite of 1% by area or more in proportion to the entire structure,  
 while the mean axis ratio (major axis/minor axis) of said residual austenite grains is 5 or higher;  
   mean length of minor axes of said residual austenite grains is 1 μm or less; and    minimum distance between said residual austenite grains is 1 μm or less; and    the steel has tensile strength of 1180 MPa or higher.    
   
   
       10 . The high strength thin steel sheet according to  claim 9 , wherein 0.5% or less (higher than 0%) by weight of Al is contained.  
   
   
       11 . The high strength thin steel sheet according to  claim 9 , wherein 
 0.003 to 0.5% of Cu and/or    0.003 to 1.0% of Ni    in terms of percentage by weight are further contained.    
   
   
       12 . The high strength thin steel sheet according to  claim 9 , wherein 
 0.003 to 1.0% of Ti and/or V    in terms of percentage by weight are further contained.    
   
   
       13 . The high strength thin steel sheet according to  claim 9 , wherein 
 1.0% or less (higher than 0%) of Mo and    0.1% or less (higher than 0%) of Nb    in terms of percentage by weight are further contained.    
   
   
       14 . The high strength thin steel sheet according to  claim 9 , wherein 0.0002 to 0.01% of B in terms of percentage by weight is further contained.  
   
   
       15 . The high strength thin steel sheet according to  claim 9 , wherein at least one element selected from the group consisting of: 
 0.0005 to 0.005% of Ca;    0.0005 to 0.01% of Mg; and    0.0005 to 0.01% of REM    in terms of percentage by weight is further contained.