Patent ID: 11634802

Abstract:
The cold-rolled steel sheet having a high bake hardening amount and excellent bendability after bake hardening according to the present invention has a predetermined composition, and contains 20% or more and 70% or less of ferrite and 30% or more of tempered martensite in terms of area ratio, in which a sum of ferrite and tempered martensite is 90% or more, and in a case where a microstructure image of 30 μm×30 μm obtained by photographing a structure at a magnification of 2,000-fold is disposed in an xy coordinate system having a sheet thickness direction as an x-axis and a rolling direction as a y-axis, the microstructure image is divided into 1024 pieces in an x-axis direction and 1024 pieces in a y-axis direction to form 1024×1024 divided regions, and a two-dimensional image is created by performing double gradation by assuming a value of “1” in each of the divided regions in one case where the structure is ferrite and assuming a value of “0” in the other cases, a heterogeneity α when two-dimensional discrete Fourier transform is performed on the two-dimensional image is 1.20 or less.