High strength steel sheet having excellent cryogenic temperature toughness and low yield ratio properties, and method for manufacturing same

1. A high strength steel sheet comprising: 0.02 to 0.12 wt % of carbon (C), 0.5 to 2.0 wt % of manganese (Mn), 0.05 to 0.5 wt % of silicon (Si), 0.05 to 1.0 wt % of nickel (Ni), 0.005 to 0.1 wt % of titanium (Ti), 0.005 to 0.5 wt % of aluminum (Al), 0.015 wt % or less of phosphorus (P), 0.015 wt % or less of sulfur (S), and the balance of Fe and other inevitable impurities,

wherein a microstructure formed throughout an entire thickness thereof consists of: 70% to 90% of ultrafine ferrite having 10 μm or less of a crystal grain size and 10% to 30% of MA (martensite/austenite) structure by area fraction, and a yield ratio (YS/TS) thereof is 0.8 or less.

2. The high strength steel sheet of claim 1 , wherein the steel sheet further includes one or two or more selected from the group consisting of 0.01 to 0.5 wt % of copper (Cu), 0.005 to 0.1 wt % of niobium (Nb), and 0.005 to 0.5 wt % of molybdenum (Mo).

3. The high strength steel sheet of claim 1 , wherein the MA (martensite/austenite) structure has 5 μm or less of an average grain size.

4. The high strength steel sheet of claim 1 , wherein the steel sheet has 150 J or more of impact toughness at −75° C. and 530 MPa or more of tensile strength.

5. A method of manufacturing a high strength steel sheet, the method comprising:

heating a slab including 0.02 to 0.12 wt % of carbon (C), 0.5 to 2.0 wt % of manganese (Mn), 0.05 to 0.5 wt % of silicon (Si), 0.05 to 1.0 wt % of nickel (Ni), 0.005 to 0.1 wt % of titanium (Ti), 0.005 to 0.5 wt % of aluminum (Al), 0.015 wt % or less of phosphorus (P), 0.015 wt % or less of sulfur (S), and a balance of Fe and other inevitable impurities;

rough-rolling the heated slab to form a rough-rolled slab having an average crystal grain size of austenite of 40 m or less;

finish-rolling the rough-rolled slab to form a finish-rolled steel sheet having a matrix structure, the matrix structure being formed of ferrite having an average crystal grain size of 10 μm or less;

after the finish-rolling, maintaining the finish-rolled steel sheet for 30 to 90 seconds to form a maintained steel sheet; and

cooling the maintained steel sheet to form a steel sheet having a microstructure consisting of: 70% to 90% of ultrafine ferrite having 10 m or less of a crystal grain size and 10% to 30% of MA (martensite/austenite) structure by area fraction throughout an entire thickness of the finish-rolled steel sheet, and a yield ratio (YS/TS) thereof is 0.8 or less.

6. The method of claim 5 , wherein the slab further includes one or two or more selected from the group consisting of 0.01 to 0.5 wt % of copper (Cu), 0.005 to 0.1 wt % of niobium (Nb), and 0.005 to 0.5 wt % of molybdenum (Mo).

7. The method of claim 5 , wherein the heating is performed at 1000° C. to 1200° C.

8. The method of claim 5 , wherein the rough-rolling is performed at 1200° C. to austenite recrystallization temperature (Tnr).

9. The method of claim 5 , wherein the rough-rolling is performed at 15% or more of a reduction ratio per pass and 30% or more of an accumulated reduction ratio.

10. The method of claim 5 , wherein the finish-rolling is performed at (Ar3+30° C.) to (Ar3+100° C.).

11. The method of claim 5 , wherein the finish-rolling is performed at 10% or more of a reduction ratio per pass and 60% or more of an accumulated reduction ratio.

12. The method of claim 5 , wherein the cooling is performed at 300° C. to 500° C. with a cooling rate of 10° C./s or more.

13. The method of claim 5 , wherein the MA (martensite/austenite) structure has an average grain size of 5 μm or less.

14. The method of claim 5 , wherein the steel sheet has 150 J or more of impact toughness and 530 MPa or more of tensile strength.