Ultra-high strength steel sheet having excellent hydrogen embrittlement resistance, and method for manufacturing the same

Abstract

The present invention provides an ultra-high strength steel sheet having excellent hydrogen embrittlement resistance, which includes: 0.06 to 0.6% of C; 0.5 to 3% of Si+Al; 0.5 to 3% of Mn; 0.15% or lower of P; and 0.02% or lower of S in terms of mass percentage, and also includes 3% or higher of residual austenite structure, 30% or higher of bainitic ferrite structure, and preferably 50% or lower of polygonal ferrite in terms of an areal ratio to the entire structure, wherein a mean grain size of bainite blocks is smaller than 20 μm as determined by comparing observations of the same region of the bainitic ferrite structure by EBSP (electron back scatter diffraction pattern) and SEM.

1 . An ultra-high strength steel sheet having excellent hydrogen embrittlement resistance, which includes: 
 0.06 to 0.6% of C;    0.5 to 3% of Si+Al;    0.5 to 3% of Mn;    0.15% or lower of P; and    0.02% or lower of S in terms of mass percentage,    and also includes 3% or higher of residual austenite structure and    30% or higher of bainitic ferrite structure in terms of an areal ratio to the entire structure,    wherein a mean grain size of bainite blocks is smaller than 20 μm as determined by comparing observations of the same region of the bainitic ferrite structure by EBSP (electron back scatter diffraction pattern) and SEM.    
     
     
         2 . The ultra-high strength steel sheet according to  claim 1 , wherein 5% to 50% of polygonal ferrite structure is included in terms of an areal ratio to the entire structure.  
     
     
         3 . The ultra-high strength steel sheet according to  claim 1 , wherein 40% or higher of bainitic ferrite structure is included in terms of an areal ratio to the entire structure.  
     
     
         4 . The ultra-high strength steel sheet according to  claim 1 , wherein 50% or higher of bainitic ferrite structure is included in terms of an areal ratio to the entire structure.  
     
     
         5 . The ultra-high strength steel sheet according to  claim 1 , further comprising at least one of: 
 1% or lower (higher than 0%) of Mo;    0.5% or lower (higher than 0%) of Ni;    0.5% or lower (higher than 0%) of Cu; and    1% or lower (higher than 0%) of Cr in mass percentage.    
     
     
         6 . The ultra-high strength steel sheet according to  claim 1 , further comprising at least one of: 
 0.1% or lower (higher than 0%) of Ti;    0.1% or lower (higher than 0%) of Nb; and    0.1% or lower (higher than 0%) of V in mass percentage.    
     
     
         7 . The ultra-high strength steel sheet according to  claim 1 , further comprising: 
 0.003% or lower (higher than 0%) of Ca; and/or    0.003% or lower (higher than 0%) of REM in mass percentage.    
     
     
         8 . The ultra-high strength steel sheet according to  claim 1 , that has a strength of 1180 MPa class or higher.  
     
     
         9 . A method for manufacturing the ultra-high strength steel sheet according to  claim 1 , which comprises a heat treatment process of: 
 heating a steel that contains the components described in  claim 1  at a temperature in a range from A3 point to (A3 point+20° C.) for 10 to 600 seconds after rolling the steel, then cooling the steel at a mean cooling rate of 3° C./s or more to a temperature not lower than Ms point and not higher than Bs point, and keeping the steel in this temperature range for 1 to 1800 seconds.    
     
     
         10 . The method according to  claim 6 , wherein the heat treatment process is included in a molten zinc plating process.