Ultra-high-strength steel sheet having excellent hole expandability and yield ratio, and method of manufacturing the same

A hot-rolled steel sheet comprising:

• in terms of wt %:
• 0.05-0.2% of carbon (C);
• 2.0% or less of silicon (Si);
• 4.1-9.0% of manganese (Mn);
• 0.05% or less (excluding 0%) of phosphorus (P);
• 0.02% or less (excluding 0%) of sulfur (S);
• 0.5% or less (excluding 0%) of aluminum (Al);
• 0.02% or less (excluding 0%) of nitrogen (N);
• at least one selected among 0.1% or less (excluding 0%) of titanium (Ti), 0.1% or less (excluding 0%) of niobium (Nb), 0.2% or less (excluding 0%) of vanadium (V), and 0.5% or less (excluding 0%) of molybdenum (Mo);
• and a balance of iron (Fe) and other inevitable impurities, and satisfying the following Equation 1,
• Equation 1:
• C/12+Ti/48+Nb/93+V/51+Mo/96 >0.015 where each element symbol represents a value of content of each element in wt %;
• a microstructure including, in volume percentage, 10-30% of retained austenite, 50% or more of annealed martensite, and 20% or less of other phases including alpha martensite and epsilon martensite;
• precipitates having a size of 30 nm or less in an amount of 1013 ea·/m2, wherein the precipitates are carbides including at least one among titanium (Ti), niobium (Nb), vanadium (V) and molybdenum (Mo), nitrides including at least one among titanium (Ti), niobium (Nb), vanadium (V) and molybdenum (Mo), or complex carbonitrides including at least one among titanium (Ti), niobium (Nb), vanadium (V) and molybdenum (Mo);
• and
• a hole expansion ratio of 15% or more, a yield ratio of 0.65 or more, and a tensile strength of 900 MPa or more, wherein a product of the tensile strength and the elongation ratio is 23,000 MPa % or more.