Process for hot dip-coating steel material with molten aluminum alloy by one-stage coating method using flux and bath of molten aluminum alloy metal

A process for hot dip-coating a steel material with a molten aluminum alloy according to a one-stage metal alloy coating method using a flux, wherein said method consists essentially of:

• removing an oxide layer which is present on a steel material surface,
• adjusting an average roughness of the steel material surface to the range of 2.5 to 7.5 .mu.mRa,
• conducting activating treatment for the steel material surface,
• (1) coating the steel material surface with a chloride flux solution consisting essentially of (a) at least one chloride selected from the group consisting of calcium chloride and magnesium chloride and (b) at least one chloride selected form the group consisting of potassium chloride, lithium chloride and sodium chloride and then drying the steel material surface, thereby forming a coating film of a dried chloride flux in an adhered amount of 1.5 g/m.sup.2 to 30 g/m.sup.2 on the steel material, wherein the content of (i) (a) chloride flux component selected from the group consisting of calcium chloride and magnesium chloride, based on total weight of the chloride flux solution, satisfies the following range:
• 5% by weight.ltoreq.content (a)/{content (a)+content (b)}.times.100.ltoreq.70% by weight
• (ii) then, dipping the thus treated steel material in a molten aluminum alloy coating bath containing a floating, molten fluoride-containing flux consisting essentially of (c) a fluoride containing aluminum, (d) at least one chloride selected from the group consisting of calcium chloride and magnesium chloride and (e) at least one chloride selected from the group consisting of potassium chloride, lithium chloride and sodium chloride, wherein an iron component of 0.25 to 1.5% by weight to the total weight of the molten alloy metal except the flux has been added to the coating bath and the content of (ii) (c) fluoride-containing aluminum, based on total weight of the fluoride-containing flux, satisfies the following range:
• 15% by weight.ltoreq.content (c)/{content (c)+content (d)+content (a)}.times.100.ltoreq.60% by weight
• and then, conducting treatment for control of coating weight, thereby coating the steel material surface with the molten aluminum alloy to adhere an amount thereof to the steel material surface.