| 摘要 |
PROBLEM TO BE SOLVED: To produce a high tensile strength hot dip galvanized steel sheet good in plating properties and excellent in workability. SOLUTION: A high tensile strength hot rolled steel sheet having a compsn. contg., by weight, 0.01 to 0.3% C, <=1.0% Si, <=3.0% Mn, <=0.5% P and 0.03 to 0.3% Ti, in which the average crystal grain size of ferrite as the main phase is <=3.5μm is prepd., which is subjected to annealing treatment of executing heating to the temp. range of the Ac1 transformation point to (the Ac1 transformation point +80 deg.C) and is thereafter applied with hot dip galvanizing. In this way, a high tensile strength hot dip galvanized steel sheet in which the average crystal grain size of ferrite as the main phase is <=3.5μm, the average grain size of a 2nd phase is <=3.5μm, and the 2nd phase has martensite of >=70% volume ratio and austenite of >=2% volume ratio is produced. After the hot dip galvanizing, heating alloying treatment may be executed.
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