Patent ID: 8986468
Filing Date: 2015-03-24
Classification: C21D,C22C

Abstract:
1. A cold rolled steel sheet wherein said cold rolled steel sheet has a chemical composition comprising, on the basis of percent by mass: C ranging from 0.06 to 0.6%; Si ranging from 0.1 to 2%; Al ranging from 0.07 to 3%; Si+Al ranging from 1 to 4%; Mn ranging from 1 to 6%; S of 0.02% or less; N of 0.01% or less; O of 0.01% or less; and the remainder being Fe and unavoidable impurities; wherein: Si/Mn≦0.40; said cold rolled steel sheet has, in the surface of said steel sheet, 10 or more pieces/100 μm said cold rolled steel sheet comprises, on the basis of the total volume fraction of the steel: 40% or over bainitic ferrite; from 1 to 50% polygonal ferrite; and from 3% to 25% retained austenite, wherein a combined fraction of bainitic ferrite and polygonal ferrite is 75% or more and the tensile strength of the steel sheet is 780 MPa or higher.