Ultra-high tensile cold-rolled steel sheet and method for manufacturing same

Abstract

Provided is an ultra-high-strength cold-rolled steel sheet with a balanced improvement in strength and ductility, and a method of manufacturing the same. According to an embodiment of the present disclosure, the ultra-high-strength cold-rolled steel sheet includes carbon (C): 0.1 wt % to 0.3 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 up to 0.05 wt %, a combination of one or more selected from titanium (Ti), niobium (Nb), and vanadium (V): more than 0 wt % and up to 0.05 wt %, phosphorus (P): more than 0 wt % and up to 0.02 wt %, sulfur (S): more than 0 wt % and up to 0.005 wt %, nitrogen (N): more than 0 wt % and up to 0.006 wt %, and a balance of iron (Fe) and other unavoidable impurities, wherein the ultra-high-strength cold-rolled steel sheet meets a yield strength (YS): 850 MPa or more, a tensile strength (TS): 1180 MPa or more, an elongation (EL): 14% or more, a hole expansion ratio (HER): 25% or more, and TS×EL×HER/1000: 500 or more.

1 . An ultra-high-strength cold-rolled steel sheet comprising carbon (C): 0.1 wt % to 0.3 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 up to 0.05 wt %, a combination of one or more selected from titanium (Ti), niobium (Nb), and vanadium (V): more than 0 wt % and up to 0.05 wt %, phosphorus (P): more than 0 wt % and up to 0.02 wt %, sulfur (S): more than 0 wt % and up to 0.005 wt %, nitrogen (N): more than 0 wt % and up to 0.006 wt %, and a balance of iron (Fe) and other unavoidable impurities,
 wherein the ultra-high-strength cold-rolled steel sheet meets a yield strength (YS): 850 MPa or more, a tensile strength (TS): 1180 MPa or more, an elongation (EL): 14% or more, a hole expansion ratio (HER): 25% or more, and TS×EL×HER/1000: 500 or more.   
     
     
         2 . The ultra-high-strength cold-rolled steel sheet of  claim 1 , wherein the ultra-high-strength cold-rolled steel sheet has a mixed structure of ferrite, retained austenite, bainite, fresh martensite, and tempered martensite,
 wherein an area fraction of ferrite ranges from 10% to 20%,   wherein an area fraction of retained austenite ranges from 5% to 20%,   wherein an area fraction of bainite ranges from 5% to 20%, and   wherein a sum of area fractions of fresh martensite and tempered martensite is a remaining are fraction.   
     
     
         3 . The ultra-high-strength cold-rolled steel sheet of  claim 2 , wherein a ratio (FM/TM) of fresh martensite (FM) to tempered martensite (TM) is 0.1 to 0.6. 
     
     
         4 . The ultra-high-strength cold-rolled steel sheet of  claim 2 , wherein a density of iron carbide particles in tempered martensite is 1.0×10 6  particles/mm 2  or more. 
     
     
         5 . The ultra-high-strength cold-rolled steel sheet of  claim 2 , wherein a grain size of tempered martensite is 5 μm or less. 
     
     
         6 . The ultra-high-strength cold-rolled steel sheet of  claim 1 , further comprising a combination of chromium (Cr) and molybdenum (Mo): more than 0 wt % and up to 1.0 wt %. 
     
     
         7 . A method of manufacturing an ultra-high-strength cold-rolled steel sheet, the method comprising:
 producing a hot-rolled steel sheet with an alloy composition of carbon (C): 0.1 wt % to 0.3 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 up to 0.05 wt %, a combination of one or more selected from titanium (Ti), niobium (Nb), and vanadium (V): more than 0 wt % and up to 0.05 wt %, phosphorus (P): more than 0 wt % and up to 0.02 wt %, sulfur (S): more than 0 wt % and up to 0.005 wt %, nitrogen (N): more than 0 wt % and up to 0.006 wt %, and a balance of iron (Fe) and other unavoidable impurities;   producing a cold-rolled steel sheet by cold rolling the hot-rolled steel sheet;   primarily soaking the cold-rolled steel sheet at a primary soaking temperature of Ac3-30° C. to 900° C. for 30 sec. to 200 sec.;   primarily cooling the primarily soaked cold-rolled steel sheet at a cooling rate of 5° C./s to 15° C./s to a primary cooling temperature of 620° C. to 720° C.;   secondarily cooling the primarily cooled cold-rolled steel sheet at a cooling rate of 15° C./s to 100° C./s to a secondary cooling temperature of 250° C. to 480° C.;   secondarily soaking the secondarily cooled cold-rolled steel sheet at a secondary soaking temperature of 250° C. to 480° C. for 50 sec. to 300 sec.;   tertiarily cooling the secondarily soaked cold-rolled steel sheet to a tertiary cooling temperature of 150° C. or lower; and   tertiarily soaking the tertiarily cooled cold-rolled steel sheet at a tertiary soaking temperature of 150° C. to 300° C. for 100 sec. to 30000 sec.   
     
     
         8 . The method of  claim 7 , wherein the producing of the hot-rolled steel sheet comprises:
 reheating a steel material with the alloy composition at a slab reheating temperature of 1,150° C. to 1,250° C.;   hot rolling the reheated steel material;   cooling the hot-rolled steel material at a cooling rate of 10° C./s to 50° C./s; and   coiling the cooled steel material at a coiling temperature of 500° C. to 700° C., and   wherein the hot rolling comprises:   a rough rolling process performed at 1,000° C. to 1,150° C. with a reduction ratio of 40% to 50% in a last pass; and   a finishing rolling process performed at a finishing delivery temperature of 880° C. to 980° C., with rolling through a final 3-high stand performed at a temperature of 1020° C. or lower and a total reduction ratio of 40% or more, and a reduction ratio of 40% to 60% in a 1 st  pass.   
     
     
         9 . The method of  claim 8 , wherein a time taken for the steel sheet to pass through the final 3-high stand during the finishing rolling process is no longer than 2.0 sec. (and longer than 0 sec.). 
     
     
         10 . The method of  claim 8 , wherein a time taken from an end of the finishing rolling process to a start of cooling of the hot-rolled steel material is no longer than 1.5 sec. 
     
     
         11 . The method of  claim 7 , further comprising softening the hot-rolled steel sheet at a temperature ranging from 500° C. to 650° C., after the hot-rolled steel sheet is produced. 
     
     
         12 . The method of  claim 7 , further comprising hot-dip galvanizing the cold-rolled steel sheet after the cold-rolled steel sheet is secondarily soaked. 
     
     
         13 . The method of  claim 12 , wherein the secondary soaking is hot-dip galvanizing the cold-rolled steel sheet. 
     
     
         14 . The method of  claim 12 , further comprising alloying the cold-rolled steel sheet after the cold-rolled steel sheet is hot-dip galvanized.