Two different torsional vibration modes growing under operation were observed. One is the in-phase mode of top and bottom rolls normally found at plate impact, and the other is an out-of-phase mode, not found in hot rolling mills. To identify the cause of vibration, field measurement and vibration analysis were performed. As a result, the vibratory divergence was found to depend on self-excited vibration and the high gain of speed control was found to show a strong possibility for this phenomenon. From evaluation results, the verification test was carried out to validate a vibration cause and a stable operation was realized.
