| 摘要 |
<P>PROBLEM TO BE SOLVED: To provide a cold rolled steel sheet and a plated steel sheet which have a composite structure providing satisfactory surface properties, causing no generation of pinholes in a product after being coated and having excellent bake hardenability, natural aging resistance and a tensile strength of ≥340 MPa. <P>SOLUTION: The cold rolled steel sheet for coating is provided with structure in which a main phase is a ferritic phase and a second phase is a low-temperature transformation forming phase including a martensitic phase, wherein a surface defect has iron oxide therein and has rugged shape when observed from the cross section of the steel sheet, and wherein the size of the surface defect observed from the surface of the steel sheet is 25 μm, the size representing the maximum distance between two parallel straight lines when the surface defect is interposed between the two straight lines. The cold rolled steel sheet for coating preferably has chemical composition containing by mass%, 0.0025 to <0.04% C, ≤0.5% Si, 0.5-3.0% Mn, ≤0.05% P, ≤0.01% S, ≤0.15% sol.Al, <0.008% N, 0.02-2.0% Cr and the balance Fe with impurities. <P>COPYRIGHT: (C)2012,JPO&INPIT |
