Patent ID: 8702880

Claim:
A high strength and low yield ratio steel, comprising, by weight percent: C: 0.02 to 0.12%, Si: 0.01 to 0.8%, Mn: 0.3 to 2.5%, P: 0.02% or less, S: 0.01% or less, Al: 0.005 to 0.5%, Nb: 0.06 to 0.10%, B: 3 to 50 ppm, Ti: 0.005 to 0.1%, N: 15 to 150 ppm, Ca: 60 ppm or less, and the balance of Fe and inevitable impurities, and having a tensile strength of 600 MPa or more and a yield ratio of 80% or less, wherein the steel comprises 1 to 5% by volume percent of an MA (martensite/austenite) structure having an average particle size of 5 μm or less and at least 95% by volume percent of granular bainite and bainitic ferrite.