Steel sheet and method of producing same

Abstract

A steel sheet has a chemical composition containing a predetermined amount of C, Si, Mn, Cu, P, S, Al, and N, and optionally a predetermined amount of one or more selected from the group consisting of Ti, B, Nb, Cr, V, Mo, Ni, As, Sb, Sn, Ta, Ca, Mg, Zn, Co, Zr, and REM, with the balance being Fe and inevitable impurities; a microstructure comprising, in volume fraction, tempered martensite: 90% or more, retained austenite: 1% to 7%, one or both of bainitic ferrite and fresh martensite: 3% to 9% in total, and ferrite: 0% to 5%, where the retained austenite has a carbon concentration of 0.35% or more; a tensile strength TS of 1470 MPa to 1650 MPa, and a yield strength YS of 1100 MPa or more.

What is claimed is: 
     
         1 . A steel sheet comprising:
 a chemical composition containing, in mass %,
 C: 0.24% to 0.28%, 
 Si: 0.40% to 0.80%, 
 Mn: 2.30% to 2.70%, 
 Cu: 0.010% to 1.000%, 
 P: 0.001% to 0.100%, 
 S: 0.0001% to 0.0200%, 
 Al: 0.010% to 0.050%, and 
 N: 0.0010% to 0.0100%, and 
 optionally at least one selected from the group consisting of
 Ti: 0.1000% or less, 
 B: 0.01000% or less, 
 Nb: 0.1000% or less, 
 Cr: 1.00% or less, 
 V: 0.100% or less, 
 Mo: 0.500% or less, 
 Ni: 0.500% or less, 
 As: 0.500% or less, 
 Sb: 0.200% or less, 
 Sn: 0.200% or less, 
 Ta: 0.100% or less, 
 Ca: 0.0200% or less, 
 Mg: 0.0200% or less, 
 Zn: 0.0200% or less, 
 Co: 0.0200% or less, 
 Zr: 0.0200% or less, and 
 REM: 0.0200% or less, 
 
 with the balance being Fe and inevitable impurities; 
   a microstructure comprising, in volume fraction,
 tempered martensite: 90% or more, 
 retained austenite: 1% to 7%, 
 one or both of bainitic ferrite and fresh martensite: 3% to 9% in total, and 
 ferrite: 0% to 5% 
 where the retained austenite has a carbon concentration of 0.35% or more; 
   a tensile strength TS of 1470 MPa to 1650 MPa; and   a yield strength YS of 1100 MPa or more.   
     
     
         2 . The steel sheet according to  claim 1 , wherein the yield strength YS is 1200 MPa or more. 
     
     
         3 . A method of producing a steel sheet, comprising:
 preparing an uncoated steel sheet having the chemical composition as recited in  claim 1 ;   heating the steel sheet to a heating temperature T1 of 850° C. or higher;   holding the steel sheet at the heating temperature T1 for 10 seconds to 1000 seconds;   continuously cooling the steel sheet from the heating temperature T1 to a cooling stop temperature T2 of 130° C. to 170° C. under a set of conditions including:
 (i) an average cooling rate in a temperature range from the heating temperature T1 to 550° C. being 16° C./s or higher; and 
 (ii) an average cooling rate in a temperature range from 550° C. to the cooling stop temperature T2 being 150° C./s or lower; 
   holding the steel sheet at the cooling stop temperature T2 for 1.0 seconds to 200.0 seconds;   heating the steel sheet from the cooling stop temperature T2 to a tempering temperature T3 of 280° C. to 350° C. at an average heating rate of 10° C./s or higher;   holding the steel sheet at the tempering temperature T3 for 10 seconds to 1000 seconds; and   cooling the steel sheet to 50° C. or lower,   
       to thereby produce the steel sheet as recited in  claim 1 . 
     
     
         4 . The method of producing a steel sheet according to  claim 3 , further comprising: after the cooling to 50° C. or lower, subjecting the steel sheet to temper rolling with an elongation rate of 0.1% to 1.0%.