The present invention relates to the hot dip coating of a continuous metal sheet, and more specifically to a method for preventing the deposition of a metal oxide on such a sheet.
In certain continuous processes in which hot metal sheets are coated by dipping in a molten metal bath, of a different metal, problems can arise because of the migration of the other metal as a vapor migrating into the furnace in which the metal strip is heated. Both the temperature and the atmosphere in the furnace must be controlled in order to prevent deposition of the metal vapor as an oxide on the sheet. Such oxidized deposits can produce imperfections in the coating of the final product.
Galvanizing of steel sheets is a particular type of hot dip coating and the resulting steel sheet has found many useful applications because of its resistance to corrosion. The method of hot dip coating is by far the most widely used method of producing galvanized steel sheets. In particular, the problem which has plagued those in the galvanizing industry is the migration of zinc vapor from the zinc coating bath into the furnace which results in the accumulation of a zinc oxide dust throughout the furnace. If this zinc oxide dust is present on the continuous steel sheet prior to its being dipped in the zinc bath, an acceptable galvanizing coating cannot be deposited onto the sheet. This problem has required those in the galvanizing industry to periodically shut down the furnace and clean out the zinc oxide dust when coating defects have reached an intolerable level. Such a shutdown is time consuming and costly.
It is therefore an object of the present invention to reduce the migration of metal vapor from the bath, i.e., the hot dip pot surface, into the furnace.
Another object of this invention is to insure that the furnace atmosphere is not oxidizing to the metal vapor.