Cold-rolled steel sheet and method of manufacturing cold-rolled steel sheet

Abstract

A cold-rolled steel sheet having a chemical composition of, in mass %: 0.10 to 0.30% C; up to 2.50% Si; 0.50 to 3.50% Mn; up to 0.100% P; up to 0.020% S; 0.010 to 0.100% Al; up to 0.0100% N; 0 to 0.100% Ti; 0 to 0.100% Nb; 0 to 0.50% V; 0 to 0.50 % Mo; 0 to 0.50% W; 0 to 0.0050% B; 0 to 0.0100% Ca; 0 to 0.0100% Mg; 0 to 0.0100% REM; and balance Fe and impurities, the microstructure containing tempered martensite and bainite in not less than 85 volume % in total, with other structure(s) being present in not more than 5 volume %, the regions where the Mn concentration in the microstructure is not lower than 8.0 mass % being present in 0.05 to 1.5 area %.

1 . A cold-rolled steel sheet having a chemical composition of, in mass %:
 0.10 to 0.30% C;   up to 2.50% Si;   0.50 to 3.50% Mn;   up to 0.100% P;   up to 0.020% S;   0.010 to 0.100% Al;   up to 0.0100% N;   0 to 0.100% Ti;   0 to 0.100% Nb;   0 to 0.50% V;   0 to 0.50% Mo;   0 to 0.50% W;   0 to 0.0050% B;   0 to 0.0100% Ca;   0 to 0.0100% Mg;   0 to 0.0100% REM; and   balance Fe and impurities,   a microstructure containing tempered martensite and bainite in not less than 85 volume % in total, a structure other than the tempered martensite, bainite and ferrite being present in not more than 5 volume %,   regions where an Mn concentration in the microstructure is not lower than 8.0 mass % being present in 0.05 to 1.5 area %.   
     
     
         2 . The cold-rolled steel sheet according to  claim 1 , wherein:
 the microstructure further contains ferrite in not less than 5 volume %; and   an average crystal grain size of the ferrite is not larger than 10.0 μm.   
     
     
         3 . The cold-rolled steel sheet according to  claim 1 , wherein the chemical composition includes one or more selected from the group consisting of, in mass %:
 0.005 to 0.100% Ti;   0.005 to 0.100% Nb;   0.01 to 0.50% V;   0.01 to 0.50% Mo; and   0.01 to 0.50% W.   
     
     
         4 . The cold-rolled steel sheet according to  claim 1 , wherein the chemical composition includes, in mass %:
 0.0003 to 0.0050% B.   
     
     
         5 . The cold-rolled steel sheet according to  claim 1 , wherein the chemical composition includes one or more selected from the group consisting of, in mass %:
 0.0001 to 0.0100% Ca;   0.0001 to 0.0100% Mg; and   0.0001 to 0.0100% REM.   
     
     
         6 . The cold-rolled steel sheet according to  claim 1 , wherein the tempered martensite and bainite of the microstructure contains cementite with an average particle size larger than 30 nm. 
     
     
         7 . A method of manufacturing the cold-rolled steel sheet according to  claim 1 , comprising:
 performing a heat treatment by holding a hot-rolled steel sheet in a temperature range from 620 to 700° C. for not shorter than 10 minutes;   cold rolling the heat-treated steel sheet;   holding the cold-rolled steel sheet in a temperature range from 880 to 1050° C. for not shorter than 10 seconds and then cooling the steel sheet to a temperature not higher than 450° C. at a cooling rate not lower than 20° C./s; and,   after cooling the steel sheet to a temperature not higher than 450° C. in the cooling step, performing one of the following sub-processes, (A) and (B):   (A) a sub-process for cooling to a temperature not higher than 350° C. and not higher than a martensite transformation start temperature (Ms point) and then reheating and holding in a temperature range from 300 to 450° C.; and   (B) a sub-process for holding in a temperature range from 300 to 450° C. during the cooling without performing reheating.