| 摘要 |
PROBLEM TO BE SOLVED: To produce a high tensile strength hot dip galvanized steel sheet having sufficient ductility as the stock for automotive parts by utilizing a continuous hot dip galvanizing line, and to provide a method for producing the same. SOLUTION: A steel sheet having a compsn. contg., by mass, 0.05 to 0.20% C, 0.3 to 1.8% Si and 1.0 to 3.0% Mn, and the balance Fe with inevitable impurities is subjected to a primary stage in which it is rapidly cooled to a temp. equal to or below the Ms point after primary heat treatment and a secondary stage in which rapid cooling is executed after secondary heat treatment and is thereafter subjected to a third stage in which it is subjected to hot dip galvanizing treatment and is rapidly cooled to obtain a composite structure consisting of tempered martensite of >=20% by volume, retained austenite of >=2%, ferrite and a low temp. transformed phase, and a hot dip galvanizing layer is formed on the surface of the steel sheet. Moreover, it is preferably cooled to 300 deg.C at a cooling rate of >=5 deg.C/sec after the hot dip galvanizing treatment, and furthermore, alloying treatment is optionally executed after the hot dip galvanizing treatment.
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