| 摘要 |
<p>In a high-strength hot-dip galvanized cold-rolled steel sheet which is excellent in ductility, work hardenability, and stretch flangeability and which has a tensile strength of 750 MPa or higher, a cold-rolled steel sheet as the base metal has: a chemical composition consisting, in mass percent, of C: more than 0.10% and less than 0.25%, Si: more than 0.50% and less than 2.0%, and Mn: more than 1.50% and 3.0% or less, and optionally containing one or more types of Ti, Nb, V, Cr, Mo, B, Ca, Mg, REM, and Bi, P: less than 0.050%, S: 0.010% or less, sol. Al: 0.50% or less, and N: 0.010% or less; and a metallurgical structure in which a main phase is a low-temperature transformation product and a second phase contains retained austenite. The retained austenite has a volume fraction of more than 4.0% and less than 25.0% with respect to the whole structure, and an average grain size of less than 0.80 µm, and in the retained austenite, a number density of retained austenite grains having a grain size of 1.2 µm or more is 3.0 × 10 -2 /µm 2 or less.</p> |
