Ultra high strength hot-rolled steel sheet having excellent ductility and method for manufacturing same

A method of fabricating a ultra high strength hot-rolled steel sheet having excellent ductility, comprising:

• heating a slab at a temperature in a range between 1200° C. and 1350° C., wherein the slab includes 0.15% to 0.25% carbon (C), 0.6% to 2.0% silicon (Si), 1.5% to 3.0% manganese (Mn), 0.01% to 0.1% aluminum (Al), 0.01% to 0.5% chrome (Cr), 0.005% to 0.2% molybdenum (Mo), 0.001% to 0.05% phosphorus (P), 0.001% to 0.05% sulfur (S), 0.001% to 0.01% nitrogen (N), 0.003% to 0.1% niobium (Nb), 0.003% to 0.1% titanium (Ti), 0.003% to 0.1% vanadium (V), 0.0005% to 0.005% boron (B), a balance of iron (Fe) and unavoidable impurities, by weight percentage, and satisfies the following relational expressions 1 and 2;
• forming a hot-rolled steel sheet by hot rolling the heated slab at a temperature in a range between 850° C. and 1150° C.;
• cooling the hot-rolled steel sheet to a cooling end temperature in a range between 200° C. and 400° C. at an average cooling rate of 50 to 100° C./s and coiling the cooled hot-rolled steel sheet;
• and
• loading the coiled hot-rolled steel sheet into a heating furnace to be heat-insulated or heated to a temperature in a range between 200° C. to 400° C. so as to satisfy the following relational expression 3,
• 4.5≤[Mn]+12[sol.C]+2.5[Mo]+2[Cr]+300[B]+[V]≤5.3 [Relational Expression 1]
• [sol.C]=[C]−(0.25[Ti]+0.13[Nb]+0.125[Mo]), 0.17≤[sol.C]≤0.22, [Relational Expression 2]
• T=Temp(25[sol.C]+Log(Time)), 2000≤T≤2500, [Relational Expression 3]
• wherein each element symbol represents a content (% by weight) of the element in Relational Expression land Relational Expression 2, and
• the Temp and the Time represent a temperature (° C.) or time (minutes) of the heating furnace after coiling in Relational Expression 3.