High-yield-ratio ultra-high-strength steel sheet having excellent thermal stability, and manufacturing method therefor

Abstract

Provided is an ultra-high-strength steel sheet having excellent thermal stability and a high yield ratio, and ultra-high strength even after heat treatment at a relatively low temperature, and a manufacturing method therefor. The steel sheet includes, by wt %, 0.05 to 0.13% of C, 0.01 to 0.5% of Si, 0.8 to 2.0% of Mn, 0.005 to 1.2% of Cr, 0.001 to 0.5 of Mo, 0.001 to 0.02% of P, 0.001 to 0.01% of S, 0.01 to 0.1% of Al, 0.001 to 0.01% of N, 0.01 to 0.05% of Ti, 0.001 to 0.03% of Nb, 0.001 to 0.2% of V, 0.0003 to 0.003% of B, and a balance of Fe and unavoidable impurities, and a microstructure including, by area %, 60 to 90% of martensite, including tempered martensite, 10 to 40% of bainite, and 5% or less of ferrite.