Gas-jet wiping of continuous galvanized coatings

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In order to protect sheet steels from corrosion in automotive applications, a thin zinc coating is often applied via the continuous galvanizing process. The coating thickness (usually measured in terms of the coating weight in g/m2) is conventionally controlled using a planar slot-jet device commonly referred to as an air knife. Using this device, a high pressure gas stream is applied to the liquid zinc coating, resulting in a shear force being applied to the coating and the excess liquid zinc returning to the galvanizing bath under the action of gravity. However, the single slot-jet air knife has now reached its limit in terms of being able to produce the very thin coatings being demanded by the automotive industry at productivity levels consistent with the desired profitability outcomes of the steel industry. Furthermore, it has been established that the jet flow from the single slot jet geometry interacts with itself, resulting in the coating thickness not being uniform and which sometimes leads to internal or customer rejections. The current project proposes to develop an alternative, multi-slot air knife geometry which will stabilize the jet flow and thereby increase coating uniformity. It is also intended to use this alternative geometry to increase the jet pressure beyond the currently used process parameters in order to achieve lighter coating weights at acceptable productivity levels. The question of noise - an industrial hygiene issue - will also be addressed as part of the development of the multi-slot air knife geometry. In addition, computational tools will be developed to aid in the design of future generations of air knives and to improve the coating weight control models currently in existence. In partnership with our industrial partner - ArcelorMittal (Dofasco) - measurements of pressure uniformity along the length of their current single-slot production air-knives will be carried out with the objective of increasing its uniformity and eliminating coating uniformity rejections.