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Hot-Dip Process

Typically, the process begins with an as-cold-reduced or pre-annealed steel sheet especially selected for its final attributes (strength and formability) after processing through the hot-dip coating line.

As the sheet is uncoiled, it is cleaned, because a clean surface is essential to develop a good bond between the steel and the coating.  On many lines, a U. S. Steel-developed electrolytic cleaning unit subjects the sheet to a combination of fast-flowing alkaline solution and electrical current for efficient removal of oils, lubricants and other surface contaminants.  Brushes remove any vestige of remaining soil as the sheet passes through scrubber units.  After hot water rinsing, the sheet is completely dried by passing through rubber squeegee rolls and a hot air dryer.

The cleaned sheet then goes through a continuous-annealing furnace where it is given either a high-temperature or pre-heat anneal to prepare it for coating.  When a significant change in mechanical properties is desired, a high-temperature cycle is used.  In this cycle, the steel is heat-treated in a precision-controlled, oxygen-free atmosphere to provide the different strength and formability levels required by customer applications.  The furnace is divided into zones to allow exact control of temperatures for heating, holding and cooling the sheet to attain the desired mechanical properties.  When either minimal or no changes in mechanical properties are desired, the steel is subjected to only a pre-heating cycle.  When the sheet leaves the continuous annealing furnace, the surface is fully prepared to accept a smooth, even coating.

As the steel exits the coating bath, a molten layer of the coating metal adheres to both sides of the sheet.  The sheet passes between a set of air knives, located on opposite sides of the strip, where air is directed under pressure against the strip to regulate the thickness of the molten coating and achieve a closely controlled, uniform coating thickness on each side of the sheet. In effect, the air knives wipe off excess metal leaving a controlled amount of molten coating on each side. 

The molten coating solidifies as the sheet moves vertically through the cooling tower. The solidification process may be controlled to impart selective surface finishes, such as minimized spangle, to the coating.

After the coating has solidified and the sheet has cooled, it is often passed through a "temper mill" to achieve a uniform smooth finish on the surface of the coating. This processing step is most often applied when the coated sheet product requires a high quality finish after painting. After temper rolling, the sheet passes through a leveler which is used to improve the flatness of the sheet.

Prior to being recoiled for shipment, a surface treatment is often applied to protect the coating during storage and shipment to the customer. This surface treatment may be an inorganic chemical solution, a thin organic film that serves as a lubricant plus a corrosion-inhibitor, or a rust-preventative oil. These thin protective films resist storage stain that might occur if the coating gets wet during storage or shipment.

Also, ahead of the recoiler, the product is inspected to assure that the surface quality meets the requirements of the intended application.

As noted above, U. S. Steel Terne Sheet and NI-TERNE Sheet are produced by a modified hot dip process. This process eliminates passage through a continuous-annealing furnace but includes cleaning, pickling and applying a flux to the surface of the steel prior to immersion in the coating bath.

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