Patent ID: 8349471

Claim:
A cold-rolled steel sheet: comprising, as the steel components, C: 0.10 to 0.20 mass %, Si: 0.5 to 2.5 mass %, Mn: 0.5 to 2.5 mass %, and Al: 0.01 to 0.10 mass %, iron and unavoidable impurities; having a structure comprising a mother phase structure of ferrite and a second phase structure of retained austenite and optionally, of martensite; and satisfying the following expressions (1) and (2) where the volume fraction of the ferrite in the whole structure is represented by Vf (%), the volume fraction of the retained austenite in the whole structure is represented by Vγ (%), the carbon content in the retained austenite is represented by Cγ (mass %), the shortest distance between the second phase structures is represented by dis (μm), and the average grain size of the second phase structures is represented by dia (μm), ( Vf×Vγ×C γ×dis)/dia≧300 (1), dis≧1.0 μm (2); wherein the tensile strength (TS (MPa))×total elongation (EL (%)) value (TS×EL) of the cold-rolled steel sheet is 20,000 or more; and the mother phase structure of ferrite contains 70 volume % or more polygonal ferrite.