The surface of hot rolled steel sheet oxidizes and forms iron oxide. It is necessary to remove the iron oxide (scale) prior to cold rolling the sheet. Pickling process is used in steel plants for oxide removal. A typical pickling line has 3-4 tanks filled with acid at various concentrations. Typical length of a pickling tank is 15-20 meters, width is ~ 2.5 meters and depth is 1.5 to 2 feet. Steel sheets are passed through the acid bath at speeds varying from 70 to 220 m/min. The acid tanks do not have rolls to support the steel sheet while it passes through the tank. This leads to sagging of the steel sheet (due to its own weight) in the acid tanks. The gap between the bottom of the steel sheet and the bottom of picking tank is called ‘Catenary’ (refer figure). Improper (or Zero gap) catenary results in defects on steel sheet leading to about 2-3% of the total defects attributed to pickling process.
Since the pickling tank is filled with acid and contains lot of fumes, the conventional type of physical sensors for measuring Catenary are not feasible. Presently, catenary is calculated theoretically, and correction is done using trial and error method. Catenary correction is done through a set of rolls which are used to maintain / alter the line tension.
Solution is sought for real-time measurement of sheet catenary inside the tank to help control the catenary.