High strength steel sheet, high strength coated or plated steel sheet, methods of producing these, and member

Abstract

The steel sheet comprising a chemical composition containing C, Si, Mn, P, S, Al, N, Ti, Nb, and B, with the balance being Fe and inevitable impurities, and satisfying predetermined formula, in which the total area ratio of martensite and bainite is 95% or more, the average grain size of prior austenite grains is 10 μm or less, the B concentration at a prior austenite grain boundary is 0.10% or more in mass %, a C-concentrated region is provided along a martensitic grain boundary, and the C-concentrated region has a C concentration of 4.0 times or more than the C content in the steel, and the C-concentrated region has a concentration width of 3 nm or more and 100 nm or less in a direction perpendicular to the martensitic grain boundary and a length of 100 nm or more in a direction parallel to the martensitic grain boundary.

1 . A high strength steel sheet comprising a chemical composition containing, in mass %:
 C: 0.10% or more and 0.30% or less;   Si: 0.20% or more and 1.20% or less;   Mn: 2.5% or more and 4.0% or less;   P: 0.050% or less;   S: 0.020% or less;   Al: 0.10% or less;   N: 0.01% or less;   Ti: 0.100% or less;   Nb: 0.002% or more and 0.050% or less; and   B: 0.0005% or more and 0.0050% or less,   
       with the balance being Fe and inevitable impurities, and satisfying the following formula (1), wherein
 the total area ratio of martensite and bainite is 95% or more, 
 the average grain size of prior austenite grains is 10 μm or less, 
 the B concentration at a prior austenite grain boundary is 0.10% or more in mass %, 
 a C-concentrated region is provided along a martensitic grain boundary, 
 the C concentration in the C-concentrated region is 4.0 times or more than the C content in the steel, and 
 the C-concentrated region has a concentration width of 3 nm or more and 100 nm or less in a direction perpendicular to the martensitic grain boundary and a length of 100 nm or more in a direction parallel to the martensitic grain boundary: 
 
       
         
           
             
               
                 
                   
                     
                       
                         
                           ( 
                           
                             
                               [ 
                               
                                 % 
                                 ⁢ 
                                     
                                 N 
                               
                               ] 
                             
                             / 
                             14 
                           
                           ) 
                         
                         / 
                         
                           ( 
                           
                             
                               [ 
                               
                                 % 
                                 ⁢ 
                                     
                                 Ti 
                               
                               ] 
                             
                             / 
                             47.9 
                           
                           ) 
                         
                       
                       < 
                       1. 
                     
                       
                     , 
                   
                 
                 
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
       
       in the formula (1), [% N] and [% Ti] indicate the N content and the Ti content in the steel in mass %, respectively. 
     
     
         2 . The high strength steel sheet according to  claim 1 , wherein the chemical composition further contains at least one element selected from, in mass %:
 V: 0.100% or less;   Mo: 0.500% or less;   Cr: 1.00% or less;   Cu: 1.00% or less;   Ni: 0.50% or less;   Sb: 0.200% or less;   Sn: 0.200% or less;   Ta: 0.200% or less;   W: 0.400% or less;   Zr: 0.0200% or less;   Ca: 0.0200% or less;   Mg: 0.0200% or less;   Co: 0.020% or less;   REM: 0.0200% or less;   Te: 0.020% or less;   Hf: 0.10% or less; or   Bi: 0.200% or less.   
     
     
         3 . A high strength coated or plated steel sheet having a coated or plated layer on at least one surface of the high strength steel sheet according to  claim 1 . 
     
     
         4 . A method of producing a high strength steel sheet, comprising:
 hot rolling a steel slab having the chemical composition according to  claim 1  to form a hot-rolled sheet;   cold rolling the hot-rolled sheet to form a cold-rolled sheet;   performing an annealing process in which the cold-rolled sheet is heated to a first heating temperature of 850° C. or higher and 920° C. or lower and held for 10 seconds or longer, the temperature is then raised to a second heating temperature of 1000° C. or higher and 1200° C. or lower at an average heating rate of 50° C./s or more, and the sheet is cooled to 500° C. or lower at an average cooling rate of 50° C./s or more within 5 seconds after reaching the second heating temperature,   after the annealing process, performing a rolling process in which the cold-rolled sheet is rolled at an elongation rate of 0.5% or more to obtain a second cold-rolled sheet, and   after the rolling process, performing a reheating process in which the second cold-rolled sheet is held at a reheating temperature of 70° C. or higher and 200° C. or lower for 600 seconds or longer to obtain a high strength steel sheet.   
     
     
         5 . A method of producing a high strength coated or plated steel sheet, comprising a coating or plating process in which, after the annealing process according to  claim 4  and before the reheating process, at least one surface of the high strength steel sheet is subjected to coating or plating treatment to obtain a high strength coated or plated steel sheet. 
     
     
         6 . A member formed using the high strength steel sheet according to  claim 1  for at least a portion thereof. 
     
     
         7 . A member formed using the high strength coated or plated steel sheet according to  claim 3  for at least a portion thereof. 
     
     
         8 . A high strength coated or plated steel sheet having a coated or plated layer on at least one surface of the high strength steel sheet according to  claim 2 . 
     
     
         9 . A method of producing a high strength steel sheet, comprising:
 hot rolling a steel slab having the chemical composition according to  claim 2  to form a hot-rolled sheet;   cold rolling the hot-rolled sheet to form a cold-rolled sheet;   performing an annealing process in which the cold-rolled sheet is heated to a first heating temperature of 850° C. or higher and 920° C. or lower and held for 10 seconds or longer, the temperature is then raised to a second heating temperature of 1000° C. or higher and 1200° C. or lower at an average heating rate of 50° C./s or more, and the sheet is cooled to 500° C. or lower at an average cooling rate of 50° C./s or more within 5 seconds after reaching the second heating temperature,   after the annealing process, performing a rolling process in which the cold-rolled sheet is rolled at an elongation rate of 0.5% or more to obtain a second cold-rolled sheet, and   after the rolling process, performing a reheating process in which the second cold-rolled sheet is held at a reheating temperature of 70° C. or higher and 200° C. or lower for 600 seconds or longer to obtain a high strength steel sheet.   
     
     
         10 . A method of producing a high strength coated or plated steel sheet, comprising a coating or plating process in which, after the annealing process according to  claim 9  and before the reheating process, at least one surface of the high strength steel sheet is subjected to coating or plating treatment to obtain a high strength coated or plated steel sheet. 
     
     
         11 . A member formed using the high strength steel sheet according to  claim 2  for at least a portion thereof. 
     
     
         12 . A member formed using the high strength coated or plated steel sheet according to  claim 8  for at least a portion thereof.