Patent attributes
A cold-rolled steel sheet of carbon steel or low-alloy steel comprising C: 0.01 to 0.25% by mass with a ferrite main phase is has an average ferrite crystal grain diameter D (μm) at the depth of ¼ of the sheet thickness from the steel sheet surface satisfying formulas (5,6) and the increase rate X (μm/min) in average ferrite crystal grain diameter at 700° C. at the depth of ¼ of the sheet thickness from the steel sheet surface and D (μm) satisfying formula (3):1.2≦D≦9.3 (5)D≦5.0−2.0·Cr+5000/(5+350·C+40·Mn)2 (6)D·X≦0.1 (3)and, at said ¼ inch depth, the area percentage of ferrite crystal grains the crystal grain diameter d (μm) of which satisfying formula (4) is at least 80%:D/3≦d≦3D (4)wherein C, Cr and Mn represent mass % of the respective elements in the steel.
