| 摘要 |
PROBLEM TO BE SOLVED: To obtain a plated steel sheet having decreased surface roughness and excellent surface smoothness without changing the concentration of Al in a plating bath in a manufacturing method of a galvannealed steel sheet, in which a steel sheet is subjected to alloying heat treatment after immersion plating in a hot-dip galvanizing bath. SOLUTION: Temperature-rise rate V( deg.C/s) and holding temperature T( deg.C) at the alloying heat treatment satisfy the following relations, according to the Al concentration [Al](mass%) in the plating bath: T>=530-4V, in the case of [Al]<=0.115%; T>=430+V, in the case of 0.115%<[Al]<=0.135%; T<=490 or T>=610-2V, in the case of [Al]>0.135%.
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