| 摘要 |
PROBLEM TO BE SOLVED: To provide a method for manufacturing a galvannealed steel sheet capable of controlling the temperature raising rate for alloying and the cooling rate after reaching the alloying temperature in order to suppress dispersion of non-plating or alloying, and defective appearance attributable thereto which occur in a hot-dip galvanizing equipment of an oxidation-free furnace and a reduction furnace type. SOLUTION: In the method for manufacturing the galvannealed steel sheet, after annealing the steel sheet by using a hot-dip galvanizing equipment of an oxidation-free furnace and a reduction furnace type, the steel sheet is passed through a molten metal mainly consisting of the hot-dip galvanizing without bringing the steel sheet into contact with the atmosphere, and then, heat-alloyed. The oxygen concentration in the oxidation-free furnace is set to be≤1% by volume, the oxygen concentration in the reduction furnace is set to be≤100 ppm by volume, and the hydrogen concentration in the reduction furnace is set to be≤5% by volume. The heat-alloying after the hot-dip galvanizing is performed by an induction heating device at the sheet temperature raising rate of≤70°C/s, and the gas cooling is performed at the cooling rate of 5°C/s to 30°C/s from the heat-alloying temperature without maintaining the temperature before the sheet temperature reaches≤400°C. COPYRIGHT: (C)2007,JPO&INPIT
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