Patent ID: 7455736

Claim:
A high tensile strength steel sheet comprising a matrix and a second phase, wherein the matrix comprises at least tempered martensite or tempered bainite, and optional ferrite, and the second phase comprises retained austenite, wherein the retained austenite comprises lath-like retained austenite having a long axis/short axis ratio of 3 or larger at 60% or larger by area relative to total retained austenite, and wherein (1) the steel comprises C: 0.10 to 0.6 mass %, Si: 1.0 mass % or smaller, Mn: 1.0 to 3 mass %, Al: 0.3 to 2.0 mass %, P: 0.02 mass % or smaller, and S: 0.03 mass % or smaller, (2) a volume rate of retained austenite, obtained by a saturated magnetization measuring method, is 10 to 40% by volume, and (3) a relationship of a carbon amount in mass % in the steel, a volume rate (fγR) of retained austenite and a carbon concentration (CγR) of the retained austenite satisfies the following equation (I): ( fγR×CγR )/ C> 50 (I).