HIGH-STRENGTH GALVANIZED STEEL SHEET HAVING EXELLENT DUCTILITY AND MANUFACTURING METHOD THEREOF

摘要

A steel plate having a composition in mass %: C: 0.05 to 0.20 %, Si: 0.3 to 1.8 %, Mn: 1.0 to 3.0 % and balance: Fe and inevitable impurities is subject ed to a primary step comprising a primary heat treating and then quenching to a temperature of the Ms point thereof or lower, a secondary step comprising a secondary heat treating and then quenching and a third step comprising hot-d ip zinc plating and then quenching, to thereby a hot-dip zinc-coated steel plat e comprising a steel plate having a composite structure comprising 20 vol % or more of tempered martensite, 2 vol % or more of retained austenite or ferrit e and a low temperature transformation phase and, formed thereon, a hot-dip zi nc plating layer. The zinc plated steel plate is preferably cooled at a speed o f 5 ~C/sec or more to 300 ~C after plating and may be optionally subjected to an alloying treatment after plating.