US Patent 11104974 High yield ratio type high-strength cold-rolled steel sheet and manufacturing method thereof

Provided is a cold-rolled steel sheet manufactured by a cold-rolled steel sheet manufacturing method comprising a continuous annealing step, which has a composition comprising, by weight %: C: 0.1-0.15%; Si: 0.2% or less (including 0%); Mn: 2.3-3.0%; P: 0.001-0.10%; S: 0.010% or less (including 0%); Sol.Al: 0.01-0.10%; N: 0.010% or less (excluding 0%); Cr: 0.3-0.9%; B: 0.0010-0.0030%; Ti: 0.01-0.03%; Nb:0.01-0.03%; the balance being Fe and other impurities, and satisfies following relationship 1. [relationship 1] 1650≤5541.4C+239Mn+169.1Cr+0.74SS-1.36RCS≤1688, in which microstructure comprises, in area %, at least 90% of martensite and tempered martensite; and 10% or less of ferrite and bainite, in which the fraction of the tempered martensite in the martensite and the tempered martensite is 90% or more, in area %, and the ratio (b/a) of the C+Mn concentration (a) in the martensite to the C+Mn concentration (b) in the ferrite and the bainite is 0.65 or more.