| 摘要 |
PROBLEM TO BE SOLVED: To provide a high tensile strength hot dip plated steel sheet stably exhibiting a satisfactory strength-ductility balance, and further having a low yield ratio and excellent plating adhesion on an ordinary continuous annealing-hot dip plating line. SOLUTION: An Fe based plating layer is formed on a steel sheet having a composition comprising, by mass, 0.03 to 0.18% C, 0.1 to 0.5% Si, 1.0 to 2.5% Mn,≤0.03% P and≤0.005% S, and the balance Fe with inevitable impurities, thereafter, the steel sheet is annealed in a ferrite+austenite two phase region, so as to control the ratio of the ferrite phase to 20 to 50%, is subsequently cooled at the average cooling rate of 5 to 20°C/s to 650 to 500°C as primary cooling, is thereafter cooled to a galvanizing bath temperature at the average cooling rate of≤5°C/s as second cooling, so as to control the ratio of the ferrite phase after the cooling to 50 to 70%, and is subsequently subjected to hot dip galvanizing and alloying treatment, thus the steel sheet having a composite structure in which the ratio of the ferrite phase is 60 to 80%, the ratio of a low temperature transformation phase is 10 to 30% and the ratio of a pearlite phase is≤10% is obtained. COPYRIGHT: (C)2007,JPO&INPIT
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