| 摘要 |
PROBLEM TO BE SOLVED: To manufacture a galvannealed steel sheet excellent in external surface appearance without decreasing productivity nor requiring large-scale alterations on the existing manufacturing equipment. SOLUTION: After a steel sheet is continuously passed through a molten zinc bath in a plating tank and immersed in it to undergo hot dipping, the steel sheet is heated to alloy the resultant plating layer, by which the galvannealed steel sheet is manufactured. In this manufacturing method, when T( deg.C) and Cz represent the temperature of the molten zinc bath and the boundary Al concentration defined by equation Cz=-0.0015×T+0.76 (wt.%), respectively, the molten zinc bath temperature T( deg.C) is regulated to 435-500 deg.C and the concentration of Al in the bath is held within the range of Cz±0.01 wt.%. Dross can be refined in the course of the repetition of phase transformation betweenξ-phase andδ1 -phase.
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