Dual-phase steel sheet with excellent formability and manufacturing method therefor

1. A steel sheet, having excellent comprising:

by wt %, 0.01% to 0.08% of carbon (C), 1.5% to 2.5% of manganese (Mn), 1.0% or less, excluding 0%, of chromium (Cr), 1.0% or less excluding 0%, of silicon (Si), 0.1% or less, excluding 0%, of phosphorus (P), 0.01% or less excluding 0%, of sulfur (S), 0.01% or less, excluding 0%, of nitrogen (N), 0.02% to 0.1% of acid soluble aluminum (sol. Al), 0.1% or less, excluding 0%, of molybdenum (Mo), 0.003% or less excluding 0%, of boron (B), and a balance of iron (Fe) and inevitable impurities, the sum (Mn+Cr) of wt % of manganese (Mn) and chromium (Cr) satisfying 1.5% to 3.5%, wherein the steel sheet includes ferrite as a main phase, a fraction of fine martensite at a ¼t point, based on a total thickness (t) of the steel sheet is from 1% to 8%, an occupancy ratio (M %) of martensite having an average particle diameter of less than 1 μm and present in grain boundaries of the ferrite defined as the following Formula 1, is 90% or higher, and an area ratio (B %) of bainite of an overall secondary phase microstructure, defined as the following Formula 2, is 3% or lower (including 0%),

M (%)={ M gb /( M gb +M in )}×100,  Formula 1

where M gb refers to the amount of martensite present in the grain boundaries of the ferrite, and M in refers to the amount of martensite present in crystal grains of the ferrite, and

B (%)={ BA /( MA+BA )}×100,  Formula 2

where BA refers to a bainite area, and MA refers to a martensite area,

wherein a fraction of the martensite of the overall microstructure is 1% to 8%.

2. The steel sheet of claim 1 , wherein a yield ratio (YR) is 0.45 to 0.6.

3. The steel sheet of claim 1 , wherein a yield ratio (YR) is greater than 0.6 and less than or equal to 0.8.

4. A method of manufacturing a steel sheet, the method comprising:

reheating a steel slab, including, by wt %, 0.01% to 0.08% of carbon (C), 1.5% to 2.5% of manganese (Mn), 1.0% or less excluding 0%, of chromium (Cr), 1.0% or less excluding 0%, of silicon (Si), 0.1% or less excluding 0%, of phosphorus (P), 0.01% or less excluding 0%, of sulfur (S), 0.01% or less excluding 0%, of nitrogen (N), 0.02% to 0.1% of acid soluble aluminum (sol. Al), 0.1% or less excluding 0%, of molybdenum (Mo), 0.003% or less excluding 0%, of boron (B), and a balance of iron (Fe) and inevitable impurities, the sum (Mn+Cr) of wt % of manganese (Mn) and chromium (Cr) satisfying 1.5% to 3.5%;

manufacturing a hot-rolled steel sheet by finish hot rolling the reheated steel slab at a transformation point Ar3 or higher;

coiling the hot-rolled steel sheet at 450° C. to 700° C.;

manufacturing a cold-rolled steel sheet by cold rolling the coiled hot-rolled steel at a reduction ratio of 40% to 80%; and

annealing the cold-rolled steel sheet in a continuous annealing furnace or a continuous galvannealing furnace in a temperature range of 760° C. to 850° C.,

wherein the annealed steel sheet includes ferrite as a main phase, a fraction of fine martensite at a ¼t point, based on a total thickness (t) of the annealed steel sheet, is from 1% to 8%, an occupancy ratio (M %) of martensite, having an average particle diameter of less than 1 μm and present in grain boundaries of the ferrite defined as the following Formula 1, is 90% or higher, and an area ratio (B %) of bainite of an overall secondary phase microstructure, defined as the following Formula 2, is 3% or lower (including 0%),

M (%)={ M gb /( M gb +M in )}×100,  Formula 1

where Mgb refers to the amount of martensite present in the grain boundaries of the ferrite, and Min refers to the amount of martensite present in crystal grains of the ferrite, and

B (%)={ BA /( MA+BA )}×100,  Formula 2

where BA refers to a bainite area, and MA refers to a martensite area,

wherein a fraction of the martensite of the overall microstructure is 1% to 8%.

5. The method of claim 4 , further comprising skin pass rolling the annealed steel sheet after the annealing.

6. The method of claim 5 , wherein, when the reduction ratio during the skin pass rolling is 0. 85% or lower, excluding 0%, a value calculated by the following Formula 3 satisfies a range of 0.45 to 0.6, and

Calculated value=(0.1699 ×x )+0.4545,  Formula 3

where x refers to skin pass reduction ratio(%).

7. The method of claim 5 , wherein, when the reduction ratio during the skin pass rolling is 0.86% to 2.0%, a value calculated by Formula 3, above, satisfies a range of greater than 0.6 and less than or equal to 0.8.