The steel industry places a high value on running galvanizing lines in a continuous manner. Significant losses (energy, capacity, productivity, etc.) are associated with down time in galvanizing zinc lines. Attack by molten zinc and the adherence of zinc dross limit the time hardware can be submerged in a zinc pot. Thermal spray coatings are used to coat submerged rolls in an effort to extend the time between maintenance shut-downs.
Galvanized steel sheets, including zinc-aluminum hot dipped steel sheets, are used as outer body panels for vehicles, corrosion resistant material for buildings and the like, and are manufactured by conventional galvanization processes.
In a typical galvanization process, a steel sheet is first annealed in a continuous annealing furnace, and then, the steel sheet, guided by a turn down roll, is introduced into a galvanizing bath, where the steel sheet is galvanized while passing along a sink roll, a front support roll and a back support roll. Thereafter, the galvanized steel sheet is passed through wiping nozzles, a touch roll and a top roll to adjust the thickness of the resulting galvanized layer.
In general, the rolls that are immersed in the galvanizing bath or are in contact with the high temperature galvanized steel sheet desirably satisfy the following conditions: the rolls are subject to only minimal erosion due to molten metal; the rolls are subject to only minimal abrasion by contact with the passing steel sheet; when the rolls are taken out of the galvanizing bath for maintenance and inspection, zinc easily peels off of the surface of the rolls; the rolls can be used over a long period of time; and the cost of the rolls is low.
U.S. Pat. No. 5,316,859 discloses a roll for continuous galvanization. The surface of the roll has a spray coated layer made from a cermet spraying material consisting essentially of WC—Co. The spray-coated layer consists of WC, at least one specified intermetallic compound and at least one amorphous W—C—Co compound and free C, but contains no free W and free Co.
Molten zinc resistant steels are basically iron base alloys and do not have enough resistance to molten zinc attack. The cost of those alloys are much higher than normal structural steels. Coatings such as self fluxing alloys and WC—Co are used as thermally sprayed coatings to protect substrates from attack by molten zinc, but sufficient resistance has not been achieved due to the permeation of molten zinc through interconnected porosity and selective attack on the metal binders.
There continues to be a need in the art for rolls that can be submerged in molten metal baths for long periods of time and thereby extend the time between maintenance shut-downs in, for example, galvanization processes. There also continues to be a need for rolls that have improved resistance to molten metal attack (such as molten zinc) and to adherence of dross.