Cold-rolled steel sheet and method for manufacturing same

Abstract

Provided is a cold-rolled steel sheet consisting of carbon (C): 0.23 wt % to 0.35 wt %, silicon (Si): 0.05 wt % to 0.5 wt %, manganese (Mn): 0.3 wt % to 2.3 wt %, phosphorus (P): more than 0 wt % and not more than 0.02 wt %, sulfur (S): more than 0 wt % and not more than 0.005 wt %, aluminum (Al): 0.01 wt % to 0.05 wt %, chromium (Cr): more than 0 wt % and not more than 0.8 wt %, molybdenum (Mo): more than 0 wt % and not more than 0.4 wt %, titanium (Ti): 0.01 wt % to 0.1 wt %, boron (B): 0.001 wt % to 0.005 wt %, a balance of iron (Fe), and unavoidable impurities, wherein a final microstructure of the cold-rolled steel sheet includes cementite, a transition carbide, and a fine precipitate, the transition carbide including ε-carbide having an atomic ratio of a substitutional element selected from Fe, Mn, Cr, and Mo, to C of 2.5:1, or η-carbide having an atomic ratio of the substitutional element to C of 2:1, and the fine precipitate having an atomic ratio of an alloying element selected from Mo and Ti, to C of 1:1.

1 . A cold-rolled steel sheet consisting of carbon (C): 0.23 wt % to 0.35 wt %, silicon (Si): 0.05 wt % to 0.5 wt %, manganese (Mn): 0.3 wt % to 2.3 wt %, phosphorus (P): more than 0 wt % and not more than 0.02 wt %, sulfur (S): more than 0 wt % and not more than 0.005 wt %, aluminum (Al): 0.01 wt % to 0.05 wt %, chromium (Cr): more than 0 wt % and not more than 0.8 wt %, molybdenum (Mo): more than 0 wt % and not more than 0.4 wt %, titanium (Ti): 0.01 wt % to 0.1 wt %, boron (B): 0.001 wt % to 0.005 wt %, a balance of iron (Fe), and unavoidable impurities,
 wherein a final microstructure of the cold-rolled steel sheet comprises cementite, a transition carbide, and a fine precipitate, the transition carbide comprising ε-carbide having an atomic ratio of a substitutional element selected from Fe, Mn, Cr, and Mo, to C of 2.5:1, or η-carbide having an atomic ratio of the substitutional element to C of 2:1, and the fine precipitate having an atomic ratio of an alloying element selected from Mo and Ti, to C of 1:1, and   wherein the cold-rolled steel sheet has a yield point (YP) of 1170 MPa or more, a tensile strength (TS) of 1400 MPa or more, an elongation (El) of 3.0% or more, a yield ratio of 70% or more, and a bendability (R/t) of 4.0 or less.   
     
     
         2 . The cold-rolled steel sheet of  claim 1 , wherein the cementite, the transition carbide, and the fine precipitate each has an average size of 50 nm or less and an average aspect ratio of 4.0 or less. 
     
     
         3 . The cold-rolled steel sheet of  claim 1 , wherein the cementite, the transition carbide, and the fine precipitate each has an area fraction of more than 0% and not more than 5%. 
     
     
         4 . The cold-rolled steel sheet of  claim 1 , wherein the final microstructure consists of only tempered martensite. 
     
     
         5 . The cold-rolled steel sheet of  claim 1 , wherein the final microstructure consists of tempered martensite, ferrite, and bainite, the tempered martensite having an area fraction of 80% or more and less than 100%, and the ferrite and bainite having an area fraction of more than 0% and not more than 20%. 
     
     
         6 . A method of manufacturing a cold-rolled steel sheet, the method comprising
 (a) hot-rolling a steel material consisting of carbon (C): 0.23 wt % to 0.35 wt %, silicon (Si): 0.05 wt % to 0.5 wt %, manganese (Mn): 0.3 wt % to 2.3 wt %, phosphorus (P): more than 0 wt % and not more than 0.02 wt %, sulfur (S): more than 0 wt % and not more than 0.005 wt %, aluminum (Al): 0.01 wt % to 0.05 wt %, chromium (Cr): more than 0 wt % and not more than 0.8 wt %, molybdenum (Mo): more than 0 wt % and not more than 0.4 wt %, titanium (Ti): 0.01 wt % to 0.1 wt %, boron (B): 0.001 wt % to 0.005 wt %, and a balance of iron (Fe);   (b) cold-rolling the hot-rolled steel material; and   (c) sequentially performing annealing, first heat treatment, and second heat treatment processes on the cold-rolled steel material,   wherein a final microstructure of the cold-rolled steel sheet obtained by performing steps (a) to (c) comprises cementite, a transition carbide, and a fine precipitate, the transition carbide comprising ε-carbide having an atomic ratio of a substitutional element selected from Fe, Mn, Cr, and Mo, to C of 2.5:1, or η-carbide having an atomic ratio of the substitutional element to C of 2:1, and the fine precipitate having an atomic ratio of an alloying element selected from Mo and Ti, to C of 1:1, and   wherein the cementite is formed during the first heat treatment process, the transition carbide is formed during the second heat treatment process, and the fine precipitate is formed during the hot-rolling.   
     
     
         7 . The method of  claim 6 , wherein step (a) is performed under conditions of a reheating temperature of 1150° C. to 1300° C., a finishing delivery temperature of 800° C. to 1000° C., and a coiling temperature of 500° C. to 650° C., and
 wherein step (c) is performed under conditions of an annealing temperature of 800° C. to 900° C., a first heat treatment temperature of 100° C. to 300° C., and a second heat treatment temperature of 100° C. to 210° C. 
 
     
     
         8 . The method of  claim 6 , wherein step (a) is performed under conditions of a reheating temperature of 1150° C. to 1300° C., a finishing delivery temperature of 800° C. to 1000° C., and a coiling temperature of 500° C. to 650° C., and
 wherein step (c) comprises performing plating and is performed under conditions of an annealing temperature of 800° C. to 900° C., a first heat treatment temperature of 450° C. to 600° C., and a second heat treatment temperature of 100° C. to 210° C. 
 
     
     
         9 . The method of  claim 6 , wherein, in step (c), cooling is performed to a first heat treatment temperature after the annealing process, and then the first heat treatment process is performed. 
     
     
         10 . The method of  claim 6 , wherein, in step (c), cooling is performed to room temperature after the first heat treatment process, and then the second heat treatment process is performed by performing heating. 
     
     
         11 . The method of  claim 7 , wherein the second heat treatment process comprises a process of maintaining the second heat treatment temperature for 3 hours to 20 hours. 
     
     
         12 . The method of  claim 8 , wherein the second heat treatment process comprises a process of maintaining the second heat treatment temperature for 3 hours to 20 hours.