Patent ID: 8932414
Filing Date: 2015-01-13
Classification: C21D,C22C

Abstract:
1. A steel sheet, wherein the steel sheet has a chemical composition, on the percent by mass basis (hereinafter the same is applied to contents in the chemical composition), comprising: carbon (C) in a content of from 0.05% to 0.4%; silicon (Si) and aluminum (Al), wherein a total content of Si and Al, [Si+Al] is from 0.5% to 3%; manganese (Mn) in a content of from 0.5% to 3%; phosphorus (P) in a content of 0.15% or less (excluding 0%); sulfur (S) in a content of 0.02% or less (including 0%); and iron and impurities, wherein the steel sheet has a tensile strength of 879 MPa or more and a structure comprising: at least one of martensite and bainitic ferrite in a total amount of 45 to 80 percent by area relative to a total structure of the steel sheet; polygonal ferrite in an amount of 15.7 to 40 percent by area relative to the total structure; retained austenite in an amount of 5 to 20 percent by area relative to the total structure, wherein a carbon concentration (C optionally bainite in the range of 5% by area or less.