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Scale Monitoring System Tested at Ruukki Metals Plate Mill in Raahe

Some surface defects on hot rolled steels arise due to poor descaling. Generally reheated slabs are efficiently descaled but, now and then, residual scale remains on the slab after primary descaling. Commonly it is difficult to trace the few poorly descaled slabs since there often is no scale monitoring system. In the work presented here an image based primary scale monitoring-and evaluation system is presented. It is a stand-alone online vision system consisting of a camera connected to a computer. The camera is located directly after the descaling unit and the computer in the control room. The program automatically locates the boundaries of the top surface of a slab and calculates the quantity of residual scale on the top surface. The quantity of residual scale is logged and can afterwards be evaluated together with process parameters and downstream surface defects to identify problem areas. The system has been tested in industrial scale in the Ruukki plate mill. This work is a cooperation between Swerea MEFOS and Ruukki. The result of the test demonstrated the feasibility of using vision system for detection of residual scale. Some problems were identified and suggestions for solving them are reported. If the suggested modifications are carried out, it is believed that the vision system can be an on-line tool for the operator and also be an off-line resource for the ongoing quality work at the hot rolling mill.