TEMPERED MARTENSITIC STEEL HAVING LOW YIELD RATIO AND EXCELLENT UNIFORM ELONGATION PROPERTY AND METHOD FOR MANUFACTURING THE SAME

摘要

An aspect of the present invention relates to a steel sheet comprising, by weight, 0.2 to 0.6% of C, 0.01 to 2.0% of Si, 0.5 to 3.0% of Mn, 0.015% or less of P, 0.005% or less of S, The steel sheet contains 0.01 to 0.1% of Ti, 0.05 to 0.5% of Cr, 0.0005 to 0.005% of B, 0.05 to 0.5% of Mo, 0.01% of N or less and Fe and unavoidable impurities. A yield ratio of 0.4 to 0.6, and a product of a tensile strength and a uniform elongation (TS * U-El) of 10000 MPa% or more, The microstructure relates to a tempered martensite steel having an area fraction of 90% or more of tempered martensite, 5% or less of ferrite, a low yield ratio including the remaining bainite and an excellent uniform elongation.