POSCO Holdings Inc.
Cold-rolled steel sheet with excellent formability, galvanized steel sheet, and manufacturing method thereof

Abstract:

Provided is a cold-rolled steel sheet, a galvanized steel sheet and a galvannealed steel sheet, comprising, by wt %, 0.06-0.15% of C, 1.2% or less (excluding 0%) of Si, 1.7-2.7% of Mn, 0.15% or less (excluding 0%) of Mo, 1.0% or less (excluding 0%) of Cr, 0.1% or less (excluding 0%) of P, 0.01% or less (excluding 0%) of S, 0.001-0.04% of Ti, 0.001-0.04% of Nb, 0.01% or less (excluding 0%) of N, 0.01% or less (excluding 0%) of B, and a remainder of Fe and unavoidable impurities, wherein the Si, C, Mn, Mo and Cr contents satisfy Relationship 1, and wherein a microstructure comprising, in area %, 10-70% of ferrite, 10-50% of the sum of bainite and retained austenite, and a remainder of fresh martensite, wherein a ratio (Mb/Mt) is 60% or more, and methods of manufacturing these steel sheets. ([Si]+[C].times.3)/([Mn]+[Mo]+[Cr]).gtoreq.0.18. [Relationship 1]