STEEL PLATE HAVING LOW YIELD RATIO, HIGH STRENGTH AND HIGH UNIFORM ELONGATION AND METHOD FOR PRODUCING SAME

Provided is a low yield ratio, high strength and high uniform elongation steel plate having excellent strain ageing resistance equivalent to API 5L X70 Grade or lower and a method for manufacturing the same. In particular, the steel plate contains 0.06% to 0.12% C, 0.01% to 1.0% Si, 1.2% to 3.0% Mn, 0.015% or less P, 0.005% or less S, 0.08% or less Al, 0.005% to 0.07% Nb, 0.005% to 0.025% Ti, 0.010% or less N, and 0.005% or less O on a mass basis, the remainder being Fe and unavoidable impurities. The low yield ratio, high strength and high uniform elongation steel plate has a metallographic microstructure that is a two-phase microstructure consisting of bainite and M-A constituent, the area fraction of the M-A constituent being 3% to 20%, the equivalent circle diameter of the M-A constituent being 3.0 µm or less. The low yield ratio, high strength and high uniform elongation steel plate has a uniform elongation of 7% or more and a yield ratio of 85% or less after being subjected to strain ageing treatment at a temperature of 250°C or lower for 30 minutes or less.