Ferritic lightweight high-strength steel sheet having excellent stiffness and ductility, and method of manufacturing the same

1. A ferritic lightweight high-strength steel sheet comprising:

0.02 wt % to 0.1 wt % of carbon (C);

4 wt % to 15 wt % of manganese (Mn);

4 wt % to 10 wt % of aluminum (Al);

2.0 wt % or less (excluding 0) of silicon (Si);

0.01 wt % to 0.3 wt % of titanium (Ti);

0.005 wt % to 0.2 wt % of antimony (Sb); and

iron (Fe) as well as unavoidable impurities as a remainder,

wherein a value of 0.25×Ti/C is in a range of 0.17 to 1.0 and a value of Mn/Al×Log(C×Ti×10000) is in a range of 1.0 to 10.

2. The ferritic lightweight high-strength steel sheet of claim 1 , wherein the steel sheet comprises one or more precipitates selected from the group consisting of TiC, (Fe,Mn) 3 AlC, (Fe,Al) 3 C, and (Fe,Mn) 3 C.

3. The ferritic lightweight high-strength steel sheet of claim 1 , wherein an average diameter of grains in the steel sheet is 10 μm or less.

4. The ferritic lightweight high-strength steel sheet of claim 1 , wherein the steel sheet has a specific gravity of 7.2 g/cc or less, a tensile strength of 540 MPa or more, a yield ratio of 0.8 or more, a value of tensile strength×elongation (TS×El) of 17,000 MPa % or more, and an planar anisotropy of 0.3 or less.

5. The ferritic lightweight high-strength steel sheet of claim 1 , wherein the steel sheet is any one of a hot-rolled steel sheet, a cold-rolled steel sheet, and a plated steel sheet.

6. The ferritic lightweight high-strength steel sheet of claim 5 , wherein the plated steel sheet comprises a plating layer selected from the group consisting of a zinc (Zn) based, a Zn—Fe based, a Zn—Al based, a Zn-magnesium (Mg) based, a Zn—Al—Mg based, an Al—Si based, and an Al—Mg—Si based.

7. The ferritic lightweight high-strength steel sheet of claim 5 , wherein the plated steel sheet comprises a plating layer having an average thickness ranging from 10 μm to 200 μm.

8. A method of manufacturing a ferritic lightweight high-strength steel sheet, the method comprising:

reheating a slab including 0.02 wt % to 0.1 wt % of C, 4 wt % to 15 wt % of Mn, 4 wt % to 10 wt % of Al, 2.0 wt % or less (excluding 0) of Si, 0.01 wt % to 0.3 wt % of Ti, 0.005 wt % to 0.2 wt % of Sb, and Fe as well as unavoidable impurities as a remainder, and having a value of 0.25×Ti/C ranging from 0.17 to 1.0 and a value of Mn/Al×Log(C×Ti×10000) ranging from 1.0 to 10, and then hot rolling the slab at a temperature of 850° C. or more to obtain a hot-rolled steel sheet; and

coiling the hot-rolled steel sheet within a temperature range of 500° C. to 700° C.

9. The method of claim 8 , further comprising:

cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet, after the coiling of the hot-rolled steel sheet;

heating the cold-rolled steel sheet to a temperature ranging from Tnr to 900° C. at a rate ranging from 1° C./s to 20° C./s, where Tnr is a recrystallization temperature; and

annealing the heated cold-rolled steel sheet for 10 seconds to 180 seconds.

10. The method of claim 9 , wherein the cold rolling is performed at a reduction rate ranging from 40% to 90%.

11. The method of claim 9 , further comprising performing an isothermal heat treatment after cooling the cold-rolled steel sheet to a temperature ranging from 200° C. to 500° C. at a rate ranging from 1° C./s to 100° C./s, after the annealing.

12. The method of claim 9 , further comprising plating the cold-rolled steel sheet, after the annealing.