Ultra-high strength cold-rolled steel sheet and manufacturing method therefor

Abstract

Provided are an ultra-high strength cold-rolled steel sheet whose microstructure has been controlled to have high strength and elongation, and a manufacturing method thereof. In accordance with an embodiment of the present disclosure, the ultra-high strength cold-rolled steel sheet includes: carbon (C): 0.28% to 0.45%; silicon (Si): 1.0% to 2.5%; manganese (Mn): 1.5% to 3.0%; aluminum (Al): 0.01% to 0.05%; chromium (Cr): greater than 0% and 1.0% or less; molybdenum (Mo): greater than 0% and 0.5% or less; a total of niobium (Nb), titanium (Ti) and vanadium (V): greater than 0% and 0.1% or less; phosphorus (P): greater than 0% and 0.03% or less; sulfur (S): greater than 0% and 0.03% or less; and nitrogen (N): greater than 0% and 0.01% or less, based on % by weight; and the remainder being Fe and other unavoidable impurities, wherein the ultra-high strength cold-rolled steel sheet satisfies a yield strength (YP) of 1180 MPa or more, a tensile strength (TS) of 1470 MPa or more, an elongation (El) of 15% or more, a yield ratio (YR) of 75% or more, and a bendability (R/t) of 3.0 or less.

1 . An ultra-high strength cold-rolled steel sheet, comprising: carbon (C): 0.28% to 0.45%; silicon (Si): 1.0% to 2.5%; manganese (Mn): 1.5% to 3.0%; aluminum (Al): 0.01% to 0.05%; chromium (Cr): greater than 0% and 1.0% or less; molybdenum (Mo): greater than 0% and 0.5% or less; a total of niobium (Nb), titanium (Ti) and vanadium (V): greater than 0% and 0.1% or less; phosphorus (P): greater than 0% and 0.03% or less; sulfur (S): greater than 0% and 0.03% or less; and nitrogen (N): greater than 0% and 0.01% or less, based on % by weight; and a remainder being Fe and other unavoidable impurities,
 wherein, when observing an area of 100 μm 2  or more in a width direction of the steel sheet in a region between a surface portion and center portion of the cold-rolled steel sheet, a ratio (B/A) of an area of grain (B) having a carbon content of 0.5% or less in austenite to an area (A) of austenite is smaller than 0.1, and   the ultra-high strength cold-rolled steel sheet satisfies a yield strength (YP) of 1180 MPa or more, a tensile strength (TS) of 1470 MPa or more, an elongation (El) of 15% or more, a yield ratio (YR) of 75% or more, and a bendability (R/t) of 3.0 or less.   
     
     
         2 . The ultra-high strength cold-rolled steel sheet according to  claim 1 , wherein a ratio (C/A) of an area (C) of martensite-austenite grain to the area (A) of austenite is smaller than 0.5. 
     
     
         3 . The ultra-high strength cold-rolled steel sheet according to  claim 1 , wherein, when observing remaining austenite grains in a width direction of the steel sheet using electron backscatter diffraction (EBSD) analysis in an area (t/4 thickness) between the surface portion and center portion of the cold-rolled steel sheet and, based on one arbitrary region in the remaining austenite grain, calculating the distribution of average values of the crystal orientation differences in the remaining austenite grain by corresponding an average value (K) of crystal orientation differences, obtained by averaging differences in crystal orientations between comparison regions adjacent to the region, to the region, a maximum value (Kmax), minimum value (Kmin), and average value (Kavg) of the crystal orientation differences whose average value is in a region distribution of 0° to 3° satisfy a relationship of (Kmax−Kavg)/(Kmax−Kmin)>0.4. 
     
     
         4 . The ultra-high strength cold-rolled steel sheet according to  claim 3 , wherein, when observing remaining austenite grains in a width direction of the steel sheet using electron backscatter diffraction (EBSD) analysis in an area (t/4 thickness) between the surface portion and center portion of the cold-rolled steel sheet, comparison regions adjacent to the region comprise first comparison regions positioned in contact with the region, second comparison regions positioned further apart from the region than the first comparison regions based on the region and positioned in contact with the first comparison regions, and third comparison regions positioned further apart from the region than the second comparison regions based on the region and positioned in contact with the second comparison regions, and
 an average crystal orientation difference value obtained by, based on one arbitrary region in the remaining austenite grain, averaging differences in crystal orientations between comparison regions adjacent to the region is an average crystal orientation difference value obtained by averaging differences between the third comparison regions and crystal orientations based on the arbitrary region.   
     
     
         5 . The ultra-high strength cold-rolled steel sheet according to  claim 1 , wherein the ultra-high strength cold-rolled steel sheet comprises a mixed structure in which ferrite, tempered martensite, martensite, residual austenite, upper bainite, and lower bainite are mixed. 
     
     
         6 . The ultra-high strength cold-rolled steel sheet according to  claim 5 , wherein a fraction of the ferrite is in a range of greater than 0% and 5% or less,
 a fraction of the martensite is in a range of greater than 0% and 20% or less,   a fraction of the residual austenite is in a range of 10% to 30%,   a fraction of the upper bainite is in a range of greater than 0% and 30% or less,   a fraction of the lower bainite is in a range of greater than 0% and 30% or less, and   a fraction of the tempered martensite is a remaining fraction.   
     
     
         7 . The ultra-high strength cold-rolled steel sheet according to  claim 6 , wherein a minimum value of a sum of the fraction of the upper bainite and the fraction of the lower bainite is 10%. 
     
     
         8 . The ultra-high strength cold-rolled steel sheet according to  claim 1 , wherein the ultra-high strength cold-rolled steel sheet comprises a mixed structure in which tempered martensite, martensite, residual austenite, upper bainite, and lower bainite are mixed,
 wherein a fraction of the martensite is in a range of greater than 0% and 20% or less,   a fraction of the residual austenite is in a range of 10% to 30%,   a fraction of the upper bainite is in a range of greater than 0% and 30% or less,   a fraction of the lower bainite is in a range of greater than 0% and 30% or less, and   a fraction of the tempered martensite is a remaining fraction.   
     
     
         9 . The ultra-high strength cold-rolled steel sheet according to  claim 5 , wherein the residual austenite has an average diameter of 1.0 μm or less. 
     
     
         10 . The ultra-high strength cold-rolled steel sheet according to  claim 8 , wherein the residual austenite has an average diameter of 1.0 μm or less. 
     
     
         11 . The ultra-high strength cold-rolled steel sheet according to  claim 8 , wherein the residual austenite has an average diameter of 1.0 μm or less.