Ultrahigh-strength steel sheet and manufacturing method therefor

Abstract

Provided is an ultra-high-strength steel sheet including carbon (C): 0.1 wt % to 0.30 wt %, silicon (Si): 1.0 wt % to 2.0 wt %, manganese (Mn): 1.5 wt % to 3.0 wt %, aluminum (Al): more than 0 wt % and not more than 0.05 wt %, a sum of one or more elements selected from niobium (Nb), titanium (Ti), and vanadium (V): more than 0 wt % and not more than 0.05 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 %, nitrogen (N): more than 0 wt % and not more than 0.006 wt %, and a balance of iron (Fe) and other unavoidable impurities, wherein a final microstructure of the ultra-high-strength steel sheet consists of tempered martensite, martensite, ferrite, and retained austenite, and wherein the ultra-high-strength steel sheet has a yield strength (YP) of 850 MPa or more, a tensile strength (TS) of 1180 MPa or more, and an elongation (El) of 14% or more.

1 . An ultra-high-strength steel sheet comprising:
 carbon (C): 0.1 wt % to 0.30 wt %, silicon (Si): 1.0 wt % to 2.0 wt %, manganese (Mn): 1.5 wt % to 3.0 wt %, aluminum (Al): more than 0 wt % and not more than 0.05 wt %, a sum of one or more elements selected from niobium (Nb), titanium (Ti), and vanadium (V): more than 0 wt % and not more than 0.05 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 %, nitrogen (N): more than 0 wt % and not more than 0.006 wt %, and a balance of iron (Fe) and other unavoidable impurities,   wherein a final microstructure of the ultra-high-strength steel sheet consists of tempered martensite, martensite, ferrite, and retained austenite, and   wherein the ultra-high-strength steel sheet has a yield strength (YP) of 850 MPa or more, a tensile strength (TS) of 1180 MPa or more, and an elongation (El) of 14% or more.   
     
     
         2 . The ultra-high-strength steel sheet of  claim 1 , wherein the final microstructure has a martensite matrix. 
     
     
         3 . The ultra-high-strength steel sheet of  claim 1 , wherein the ferrite occupies an area fraction of 10% to 20%,
 wherein the retained austenite occupies an area fraction of 10% to 20%, and   wherein the tempered martensite and martensite occupy a remaining area fraction.   
     
     
         4 . The ultra-high-strength steel sheet of  claim 1 , wherein the ferrite comprises carbides, and
 wherein a density of the carbides with an average size of 50 nm or more is 20 particles/μm 2  or less.   
     
     
         5 . The ultra-high-strength steel sheet of  claim 1 , wherein a product of TS and El is no less than 15,000 MPa·%. 
     
     
         6 . A method of manufacturing an ultra-high-strength steel sheet, the method comprising:
 producing a hot-rolled steel sheet comprising carbon (C): 0.1 wt % to 0.30 wt %, silicon (Si): 1.0 wt % to 2.0 wt %, manganese (Mn): 1.5 wt % to 3.0 wt %, aluminum (Al): more than 0 wt % and not more than 0.05 wt %, a sum of one or more elements selected from niobium (Nb), titanium (Ti), and vanadium (V): more than 0 wt % and not more than 0.05 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 %, nitrogen (N): more than 0 wt % and not more than 0.006 wt %, and a balance of iron (Fe) and other unavoidable impurities;   softening the hot-rolled steel sheet by maintaining a temperature of 500° C. to 650° C.;   producing a cold-rolled steel sheet by cold rolling the softened steel sheet;   annealing the cold-rolled steel sheet at a temperature of 820° C. to 860° C.;   multi-stage cooling the cold-rolled steel sheet to a temperature of 200° C. to 260° C.; and   partitioning the cooled cold-rolled steel sheet at a temperature of 370° C. to 460° C.   
     
     
         7 . The method of  claim 6 , wherein the producing of the hot-rolled steel sheet comprises:
 preparing a steel slab having the above alloy composition;   reheating the steel slab at 1150° C. to 1250° C.;   producing a hot-rolled steel sheet by hot rolling the reheated steel slab at a finishing delivery temperature (FDT) of 850° C. to 1000° C.; and   coiling the hot-rolled steel sheet at 500° C. to 700° C.   
     
     
         8 . The method of  claim 6 , wherein the annealing of the cold-rolled steel sheet is performed by heating the cold-rolled steel sheet at a heating rate of 1° C./s to 10° C./s and maintaining a temperature of 820° C. to 860° C. for 40 sec. to 120 sec. 
     
     
         9 . The method of  claim 6 , wherein the multi-stage cooling of the cold-rolled steel sheet comprises:
 a primary cooling step for slowly cooling the cold-rolled steel sheet at a cooling rate of 1° C./s to 10° C./s to an end temperature of 550° C. to 750° C.; and   a secondary cooling step for rapidly cooling the cold-rolled steel sheet at a cooling rate of 50° C./s or more to an end temperature of 200° C. to 260° C.   
     
     
         10 . The method of  claim 6 , wherein the partitioning is performed by heating the cooled cold-rolled steel sheet at a heating rate of 20° C./s or more and maintaining a temperature of 370° C. to 460° C. for 10 sec. to 240 sec.