| 摘要 |
PROBLEM TO BE SOLVED: To provide a method for manufacturing a high tensile hot-dip galvanized steel sheet having sufficient ductility required for a stock for automotive parts by utilizing a continuous hot-dip galvanizing line. SOLUTION: A steel sheet having a composition consisting of, by mass, 0.05-0.20% C, 0.3-1.8% Si, 1.0-3.0% Mn and the balance Fe with inevitable impurities is subjected to: a primary step where the steel sheet is rapidly cooled to a temperature not higher than the MS point after primary heat treatment; a secondary step where the steel sheet is rapidly cooled to 470-350 deg.C after secondary heat treatment and held at a temperature ranging from cooling stop temperature to 350 deg.C for 10-500 s to undergo holding treatment; and a tertiary step where the steel sheet is subjected to hot-dip galvanizing treatment and rapid cooling. In this way, a composite structure consisting of tempered martensite, >=3% retained austenite, ferrite, and low-temperature transformation phase is formed, and also a hot-dip galvanizing layer is formed on the surface of the steel sheet. Moreover, alloying treatment can be applied after the hot-dip galvanizing treatment.
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