High strength steel sheet and method for manufacturing the same

Abstract

A high strength steel sheet includes a specific microstructure having a specific chemical composition and satisfying the formulas (1) and (2) defined below: KAM ⁡ ( S ) / KAM ⁡ ( C ) < 1. ( 1 ) wherein KAM (S) is a KAM (Kernel average misorientation) value of a superficial portion of the steel sheet, and KAM (C) is a KAM value of a central portion of the steel sheet, Hv ⁡ ( Q ) - Hv ⁡ ( S ) ≥ 8 ( 2 ) wherein Hv (Q) indicates the hardness of a portion at ¼ sheet thickness and Hv (S) indicates the hardness of a superficial portion of the steel sheet.

1 . A high strength steel sheet comprising a microstructure having a chemical composition comprising, by mass %:
 C: 0.15% or more and 0.45% or less,   Si: 0.10% or more and 2.00% or less,   Mn: 0.5% or more and 3.5% or less,   P: 0.100% or less,   S: 0.0200% or less,   Al: 0.010% or more and 1.000% or less,   N: 0.0100% or less, and   H: 0.0020% or less,   the balance being Fe and incidental impurities;   the microstructure being such that:   the area fraction of tempered martensite is 85% or more,   the volume fraction of retained austenite is less than 5%, and   the area fraction of the total of ferrite and bainitic ferrite is 10% or less;   the microstructure satisfying formulas (1) and (2) defined below:   
       
         
           
             
               
                 
                   
                     
                       
                         KAM 
                         ⁡ 
                         ( 
                         S 
                         ) 
                       
                       / 
                       
                         KAM 
                         ⁡ 
                         ( 
                         C 
                         ) 
                       
                     
                     < 
                     1. 
                   
                 
                 
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
         wherein KAM (S) is a KAM (Kernel average misorientation) value of a superficial portion of the steel sheet, and KAM (C) is a KAM value of a central portion of the steel sheet, 
       
       
         
           
             
               
                 
                   
                     
                       
                         Hv 
                         ⁡ 
                         ( 
                         Q 
                         ) 
                       
                       - 
                       
                         Hv 
                         ⁡ 
                         ( 
                         S 
                         ) 
                       
                     
                     ≥ 
                     8 
                   
                 
                 
                   
                     ( 
                     2 
                     ) 
                   
                 
               
             
           
         
         wherein Hv (Q) indicates the hardness of a portion at ¼ sheet thickness and Hv (S) indicates the hardness of a superficial portion of the steel sheet. 
       
     
     
         2 . The high strength steel sheet according to  claim 1 , wherein the chemical composition further comprises one, or two or more elements selected from, by mass %:
 Ti: 0.100% or less,   B: 0.0100% or less,   Nb: 0.100% or less,   Cu: 1.00% or less,   Cr: 1.00% or less,   V: 0.100% or less,   Mo: 0.500% or less,   Ni: 0.50% or less,   Sb: 0.200% or less,   Sn: 0.200% or less,   As: 0.100% or less,   Ta: 0.100% or less,   Ca: 0.0200% or less,   Mg: 0.0200% or less,   Zn: 0.020% or less,   Co: 0.020% or less,   Zr: 0.020% or less, and   REM: 0.0200% or less.   
     
     
         3 . The high strength steel sheet according to  claim 1 , which has a coated layer on a surface of the steel sheet. 
     
     
         4 . The high strength steel sheet according to  claim 2 , which has a coated layer on a surface of the steel sheet. 
     
     
         5 . A method for manufacturing a high strength steel sheet described in  claim 1 , the method comprising:
 providing a cold rolled steel sheet produced by subjecting a steel slab to hot rolling, pickling, and cold rolling;   annealing the steel sheet under conditions where:   a temperature T1 is 850° C. or above and 1000° C. or below and   a holding time t1 at T1 is 10 seconds or more and 1000 seconds or less;   cooling the steel sheet to 100° C. or below;   starting working at an elapsed time t2 of 1000 seconds or less from the time when the temperature reaches 100° C.,   the working being performed under conditions where:   a working start temperature T2 is 80° C. or below and   an equivalent plastic strain is 0.10% or more and 5.00% or less;   tempering the steel sheet under conditions where:   a temperature T3 is 100° C. or above and 400° C. or below and   a holding time t3 at T3 is 1.0 second or more and 1000.0 seconds or less; and   cooling the steel sheet under conditions where a cooling rate 01 from T3 to 80° C. is 100° C./sec or less.   
     
     
         6 . A method for manufacturing a high strength steel sheet described in  claim 2 , the method comprising:
 providing a cold rolled steel sheet produced by subjecting a steel slab to hot rolling, pickling, and cold rolling;   annealing the steel sheet under conditions where:   a temperature T1 is 850° C. or above and 1000° C. or below and   a holding time t1 at T1 is 10 seconds or more and 1000 seconds or less;   cooling the steel sheet to 100° C. or below;   starting working at an elapsed time t2 of 1000 seconds or less from the time when the temperature reaches 100° C.,   the working being performed under conditions where:   a working start temperature T2 is 80° C. or below and   an equivalent plastic strain is 0.10% or more and 5.00% or less;   tempering the steel sheet under conditions where:   a temperature T3 is 100° C. or above and 400° C. or below and   a holding time t3 at T3 is 1.0 second or more and 1000.0 seconds or less; and   cooling the steel sheet under conditions where a cooling rate 01 from T3 to 80° C. is 100° C./sec or less.   
     
     
         7 . The method for manufacturing a high strength steel sheet according to  claim 5 , wherein the working before the tempering is performed under conditions where strain is applied by two or more separate working operations, and the total of the equivalent plastic strains applied in the working operations is 0.10% or more. 
     
     
         8 . The method for manufacturing a high strength steel sheet according to  claim 6 , wherein the working before the tempering is performed under conditions where strain is applied by two or more separate working operations, and the total of the equivalent plastic strains applied in the working operations is 0.10% or more. 
     
     
         9 . The method for manufacturing a high strength steel sheet according to  claim 5 , further comprising performing coating treatment during the annealing or after the annealing. 
     
     
         10 . The method for manufacturing a high strength steel sheet according to  claim 6 , further comprising performing coating treatment during the annealing or after the annealing. 
     
     
         11 . The method for manufacturing a high strength steel sheet according to  claim 7 , further comprising performing coating treatment during the annealing or after the annealing. 
     
     
         12 . The method for manufacturing a high strength steel sheet according to  claim 8 , further comprising performing coating treatment during the annealing or after the annealing.