High strength steel sheet, method for producing same, member, and method for producing same

Abstract

A high strength steel sheet having a yield strength of not less than 800 MPa is disclosed. The amount of diffusible hydrogen in steel is not more than 0.50 mass ppm. The chemical composition includes, by mass, C: 0.150 to 0.500%, Si: 0.01 to 3.00%, Mn: 1.50 to 4.00%, P: not more than 0.100%, S: not more than 0.0200%, Al: not more than 0.100%, N: not more than 0.0100%, and O: not more than 0.0100%, with a balance being Fe and inevitable impurities. The total area fraction of tempered martensite and bainite is 55 to 95%. The presence ratio A/B of a structure A to a structure B is 0.8 to 2.5, the structure A having a nanohardness of 7 GPa or more, and the structure B having a nanohardness of 6 GPa or less. The concentration of solid solution carbon in retained austenite is 0.50 to 0.90 mass %.

1 . A high strength steel sheet comprising a steel sheet,
 wherein an amount of diffusible hydrogen in steel of the steel sheet is not more than 0.50 mass ppm,   the steel sheet has chemical composition and microstructure,   the chemical composition including, by mass,   C in an amount of 0.150 to 0.500%,   Si in an amount of 0.01 to 3.00%,   Mn in an amount of 1.50 to 4.00%,   P in an amount of not more than 0.100%,   S in an amount of not more than 0.0200%,   Al in an amount of not more than 0.100%,   N in an amount of not more than 0.0100%, and   O in an amount of not more than 0.0100%, with a balance being Fe and inevitable impurities,   in the microstructure,   a total area fraction of tempered martensite and bainite is 55 to 95%,   a presence ratio A/B of a structure A to a structure B is 0.8 to 2.5, the structure A having a nanohardness of 7 GPa or more, and the structure B having a nanohardness of 6 GPa or less, and   a concentration of solid solution carbon in retained austenite is 0.50 to 0.90 mass %.   
     
     
         2 . The high strength steel sheet according to  claim 1 ,
 wherein the chemical composition further includes at least one element selected from the group consisting of, by mass,   B in an amount of not more than 0.0100%,   Ti in an amount of not more than 0.200%,   Nb in an amount of not more than 0.200%,   V in an amount of not more than 0.200%,   W in an amount of not more than 0.100%,   Mo in an amount of not more than 1.000%,   Cr in an amount of not more than 1.000%,   Sb in an amount of not more than 0.200%,   Sn in an amount of not more than 0.200%,   Zr in an amount of not more than 0.1000%,   Te in an amount of not more than 0.100%,   Cu in an amount of not more than 1.000%,   Ni in an amount of not more than 1.000%,   Ca in an amount of not more than 0.0100%,   Mg in an amount of not more than 0.0100%,   REM in an amount of not more than 0.0100%,   Co in an amount of not more than 0.010%,   Ta in an amount of not more than 0.10%,   Hf in an amount of not more than 0.10%, and   Bi: in an amount of not more than 0.200%.   
     
     
         3 . The high strength steel sheet according to  claim 1 ,
 wherein a plating layer is further provided on a surface of a steel sheet.   
     
     
         4 . The high strength steel sheet according to  claim 3 ,
 wherein the plating layer is a galvanizing layer, a galvannealing layer, or an electrogalvanizing layer.   
     
     
         5 . A method of producing the high strength steel sheet according to  claim 1 , the method comprising:
 subjecting a steel slab having the chemical composition to hot rolling to obtain a hot rolled steel sheet;   subjecting the hot rolled steel sheet to cold rolling to obtain a cold rolled steel sheet; and   heating the cold rolled steel sheet at a heating temperature T1 of 750° C. to 950° C. for 10 s to 500 s, cooling the heated cold rolled steel sheet to a cooling stop temperature T2 of not lower than 120° C. and lower than 280° C., re-heating the cooled cold rolled steel sheet to a re-heating temperature T3, re-cooling the re-heated cold rolled steel sheet without retaining the re-heated cold rolled steel sheet at the re-heating temperature T3, and retaining the re-cooled cold rolled steel sheet at a temperature T4 lower than the re-heating temperature T3 for 1 s or more,   wherein a heat-input effect index J from the cooling stop temperature T2 to the re-heating temperature T3, as expressed by Formula (1) below, is 1500 to 4000, and   the cold rolled steel sheet retained at the temperature T4 is subjected to temper rolling using a roll having a surface roughness of 1.5 to 5.0 μm,   
       
         
           
             
               
                 
                   
                     J 
                     = 
                     
                       
                         ( 
                         
                           
                             T 
                             ⁢ 
                             3 
                           
                           - 
                           
                             T 
                             ⁢ 
                             2 
                           
                         
                         ) 
                       
                       ⁢ 
                          
                       
                         ( 
                         
                           
                             log 
                             ⁡ 
                             ( 
                             
                               9 
                               ⁢ 
                               t 
                             
                             ) 
                           
                             
                           + 
                             
                           20 
                         
                         ) 
                       
                     
                   
                 
                 
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
         in Formula (1), t is a heating time from the cooling stop temperature T2 to the re-heating temperature T3° C., and a unit of the heating time is s. 
       
     
     
         6 . A method of producing the high strength steel sheet according to  claim 2 , the method comprising:
 subjecting a steel slab having the chemical composition to hot rolling to obtain a hot rolled steel sheet;   subjecting the hot rolled steel sheet to cold rolling to obtain a cold rolled steel sheet; and   heating the cold rolled steel sheet at a heating temperature T1 of 750° C. to 950° C. for 10 s to 500 s, cooling the heated cold rolled steel sheet to a cooling stop temperature T2 of not lower than 120° C. and lower than 280° C., re-heating the cooled cold rolled steel sheet to a re-heating temperature T3, re-cooling the re-heated cold rolled steel sheet without retaining the re-heated cold rolled steel sheet at the re-heating temperature T3, and retaining the re-cooled cold rolled steel sheet at a temperature T4 lower than the re-heating temperature T3 for 1 s or more,   wherein a heat-input effect index J from the cooling stop temperature T2 to the re-heating temperature T3, as expressed by Formula (1) below, is 1500 to 4000, and   the cold rolled steel sheet retained at the temperature T4 is subjected to temper rolling using a roll having a surface roughness of 1.5 to 5.0 μm,   
       
         
           
             
               
                 
                   
                     J 
                     = 
                     
                       
                         ( 
                         
                           
                             T 
                             ⁢ 
                             3 
                           
                           - 
                           
                             T 
                             ⁢ 
                             2 
                           
                         
                         ) 
                       
                       ⁢ 
                          
                       
                         ( 
                         
                           
                             log 
                             ⁡ 
                             ( 
                             
                               9 
                               ⁢ 
                               t 
                             
                             ) 
                           
                             
                           + 
                             
                           20 
                         
                         ) 
                       
                     
                   
                 
                 
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
         in Formula (1), t is a heating time from the cooling stop temperature T2 to the re-heating temperature T3° C., and a unit of the heating time is s. 
       
     
     
         7 . The method of producing the high strength steel sheet according to  claim 5 ,
 wherein the cold rolled steel sheet is subjected to a plating treatment.   
     
     
         8 . The method of producing the high strength steel sheet according to  claim 7 ,
 wherein the plating treatment is a galvanizing treatment, a galvannealing treatment, or an electrogalvanizing treatment.   
     
     
         9 . A member obtained by using the high strength steel sheet according to  claim 1 . 
     
     
         10 . A method of producing a member, the method comprising subjecting the high strength steel sheet according to  claim 1  to at least one of a forming process and a joining process to obtain a member. 
     
     
         11 . The high strength steel sheet according to  claim 2 ,
 wherein a plating layer is further provided on a surface of a steel sheet.   
     
     
         12 . The high strength steel sheet according to  claim 11 ,
 wherein the plating layer is a galvanizing layer, a galvannealing layer, or an electrogalvanizing layer.   
     
     
         13 . The method of producing the high strength steel sheet according to  claim 6 ,
 wherein the cold rolled steel sheet is subjected to a plating treatment.   
     
     
         14 . The method of producing the high strength steel sheet according to  claim 13 ,
 wherein the plating treatment is a galvanizing treatment, a galvannealing treatment, or an electrogalvanizing treatment.   
     
     
         15 . A member obtained by using the high strength steel sheet according to  claim 2 . 
     
     
         16 . A member obtained by using the high strength steel sheet according to  claim 3 . 
     
     
         17 . A member obtained by using the high strength steel sheet according to  claim 4 . 
     
     
         18 . A member obtained by using the high strength steel sheet according to  claim 11 . 
     
     
         19 . A member obtained by using the high strength steel sheet according to  claim 12 . 
     
     
         20 . A method of producing a member, the method comprising subjecting the high strength steel sheet according to  claim 2  to at least one of a forming process and a joining process to obtain a member.