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

1. An ultra high strength hot-rolled steel sheet having excellent ductility, comprising:

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-′soluble aluminum (sol. Al), 0.01% to 0.5% chromium (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 0.01% vanadium (V), 0.0005% to 0.005% boron (B), and a balance of iron (Fe) and unavoidable impurities, by weight percentage, and satisfying the following Relational Expressions 1 and 2:

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]

where each element symbol represents a content, by weight percentage, of each element; and

a microstructure including 85% or more martensite, 3% to 15% retained austenite, and unavoidable phases, by area fraction,

wherein the hot-rolled steel sheet has a tensile strength of 1200 MPa or greater, and an elongation rate of 10% or greater.

2. The ultra high strength hot-rolled steel sheet of claim 1 , wherein the unavoidable phases includes ferrite and bainite, and a sum of the ferrite and the bainite is lower than 5% by area fraction.

3. The ultra high strength hot-rolled steel sheet of claim 1 , further comprising: a galvanized layer formed on a surface thereof.

4. A method of fabricating the ultra high strength hot-rolled steel sheet having excellent ductility according to claim 1 , 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% soluble aluminum (sol. Al), 0.01% to 0.5% chromium (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 0.01% 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° C./s 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. and 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 percentage, of each element in Relational Expression 1 and Relational Expression 2, and

the Temp and the Time respectively represent a temperature, in ° C., and time, in minutes, of the heating furnace after coiling in Relational Expression 3.

5. The method of claim 4 , further comprising pickling and oiling the hot-rolled steel sheet after heat-insulating or heating.

6. The method of claim 4 , further comprising heating the hot-rolled steel sheet until a temperature of the hot-rolled steel is in a range between 450° C. and 480° C. and hot-dip galvanizing the heated hot-rolled steel sheet, after pickling.

7. The method of claim 4 , wherein the slab is fabricated in a process in which a continuous casting process and a hot-rolling process are directly combined.