Advanced Control of Coating Weight in a Hot-Dip Galvanizing Line
An adaptive time delay compensation strategy
S. Sarawgi (TU Delft - Mechanical Engineering)
Riccardo Ferrari – Mentor (TU Delft - Team Jan-Willem van Wingerden)
Cor Jan Boeder – Graduation committee member (Tata Steel Europe Limited)
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Abstract
Flat steel strip processing is carried out through a sequence of continuous methods, commonly recognized as hot rolling, pickling, cold rolling, annealing, and hot-dip galvanizing. Among all these processes, hot-dip galvanizing is a process that has been popularly used to produce high quality galvanized cold-rolled sheets that are extensively used in the manufacture of automobile and domestic electrical appliances. In particular, the main objective of galvanizing is to protect the steel strip from corrosion by applying a suitable coating of zinc-based alloy.
During hot-dip galvanization, the steel strip after leaving the annealing furnace is dipped into a bath of molten zinc, enabling the formation of zinc coating. The amount of this coating is controlled by the widely used air-knife wiping system installed just above the zinc bath. The coating thickness obtained after the wiping process depends mainly on the strip speed, wiping gas pressure, and strip-to-knife distance. After a significant distance from the air knives, the cold coating gauge is located to measure the thickness of the zinc coating.
The quality of the product depends on the amount of zinc-based alloy deposited at the strip's surface. The over-deposition of zinc at the strip surface results in excessive use of zinc, which is expensive, and under-coated strip results in a product of poor quality. The control of the coating deposition is based on the air-knife wiping pressure, and therefore, the challenge is to determine and control the pressure, given the operating conditions, which are the strip speed, strip-to-knife distance, and the target coating thickness.
One of the main concerns in the closed-loop control of coating thickness is the need to account for the measurement delay arising from the time-varying strip speed across the gap between the air knives and the cold coating gauge. In this thesis, different modeling and control strategies have been studied to improve the quality of the galvanized sheet produced at Hot-dip Galvanizing Line 1 (HDGL1) of Tata Steel in IJmuiden.