Patent ID: 7413617

Claim:
A composite structure cold-rolled sheet steel with excellent elongation and stretch flange formability comprising 0.02 to 0.12% C, 0.5 to 2.0% Si+Al and 1.0 to 2.0% Mn by mass percentage, with the remainder being Fe and unavoidable impurities, the sheet steel having a composite structure comprising 80% or more polygonal ferrite (steel structure space factor) and 1 to 7% retained austenite (volume fraction measured by the saturation magnetization method), with the remainder being bainite and martensite, wherein the polygonal ferrite is defined as a main-phase structure and martensite and retained austenite is defined as a second-phase structure, and within the second-phase structure, not more than 15 massive second-phase structures with an aspect ratio of 1:3 or less and a mean grain size of 0.5 μm or more are contained per 750 μm 2 as observed under a scanning electron microscope at 4000×.