Abstract: Provided is an ultra-high-strength steel sheet having an excellent hole expandability and yield ratio, including, 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); and a balance of iron (Fe) and other inevitable impurities, wherein the following Equation 1 is satisfied, and wherein microstructures includes, in volume percentage, 10-30% or retained austenite, 50% or more of annealed martensite, and 20% or less of other phases including alpha martensite and epsilon martensite, Equation 1: C/12+Ti/48+Nb/93+V/51+Mo/96?0.015.

Abstract: Provided is an ultrahigh-strength steel sheet for a vehicle and, more specifically, to an ultrahigh-strength and high-ductility steel sheet having excellent yield ratio and a manufacturing method therefor. Provided is an ultrahigh-strength and high-ductility steel sheet for cold press forming and a manufacturing method therefor, the steel sheet ensuring ultrahigh strength and high ductility since an alloy component of steel and manufacturing conditions are controlled and, simultaneously, having excellent impact characteristics due to a high yield strength ratio (yield ratio). Provided is the steel sheet capable of satisfying formability and impact stability, which are required for a vehicle steel sheet for cold forming, and replacing a conventional steel sheet for hot press forming, thereby reducing manufacturing costs.

Abstract: Provided is a high-strength steel sheet for use in lightening materials for electronic products and materials for vehicles including automobiles, trains and ships and, more particularly, to a high-strength cold rolled steel sheet, a coated steel plate, and a method for manufacturing the same, wherein yield strength and ductility are improved by controlling the internal oxidation depth and the amount of residual austenite after first annealing operation, and the high-strength cold rolled steel sheet and the coated steel plate is stably manufactured and provided without any dent defects during the manufacturing thereof.

Abstract: Provided is an ultra high-strength steel sheet having excellent hole expandability. The ultra high-strength steel sheet includes: 0.15 to 0.30% of C, 1.0 to 3.0% of Si, 3.0 to 5.0% of Mn, 0.020% or less of P, 0.010% or less of S, 0.01 to 3.0% of Al, and 0.020% or less of N (excluding 0%) by weight, with the remainder consisting of Fe and other inevitable impurities, wherein a microstructure contains 5% to 20% by area fraction of retained austenite with the remainder including ferrite, bainite and fresh martensite.