| 摘要 |
PROBLEM TO BE SOLVED: To simply and surely obtain a target spangle grain diameter by adjusting the temp. of molten coating metal and the dew point at a cooling zone or a snout, etc., according to the deviation between the spangle grain diameter and the target value on molten Zn-Al base coating film having specific composition composed of Al, Si and Zn. SOLUTION: A steel sheet 12 heated in an annealing furnace 11 is dipped into the molten Zn-Al base coating metal 15 held in the plating vessel without contacting with the air in the snout 13 and pulled up after founding around an immersion roll 16. This molten coating metal 15 has the composition, by weight, 0.05-70% Al, 0-7.0% Si and the balance Zn with inevitable impurities. The plated steel sheet 12' obtd. in this process, is cooled in the cooling zone 17. The spangle grain diameter S1 generated on the coating film surface in the process is measured by an optical picture processing method, etc., desirably, as quick as possible after cooling. The S1 is compared with the target value S with a computing element 18 to obtain the deviation|S-S1|. The temp. of the molten coating metal, the dew point at the cooling zone or the snout, cooling speed, etc., are changed so that this deviation becomes <= a prescribed value K1 >0.
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