| 摘要 |
<P>PROBLEM TO BE SOLVED: To provide a hot dip galvannealed steel strip which has high strength satisfying the tensile strength of ≥590 MPa, has excellent ductility and bendability that n value is ≥0.15 and a 180 degree critical bend radius is twice or less of the sheet thickness, and further has satisfactory surface properties that the area ratio of an island scale pattern is <10%. <P>SOLUTION: The steel sheet base material has a chemical composition including, by mass, 0.02 to 0.30% C, 0.05 to 2.0% Si, 0.5 to 3.0% Mn, ≤0.03% P, ≤0.02% S, 0.001 to 2.0% sol.Al and ≤0.015% N, a steel structure including retained austenite by ≥2.0 area%, and a hardness distribution in the sheet thickness direction satisfying Hb/Hs≥1.1 (Hb denotes the Vickers hardness in the 1/4 depth position of the sheet thickness from the surface of the steel sheet and Hs denotes the Vickers hardness in the depth position of 10 μm from the surface of the steel sheet). In the hot dip galvannealing layer, the concentration of Fe is 3 to 20 mass%, and, in the hot dip galvannealed steel sheet, the area ratio of an island scale pattern is <10%. <P>COPYRIGHT: (C)2013,JPO&INPIT |
