STRIP QUALITY OF HIGHLY ALLOYED METALS BY TWIN ROLL CASTING

The commercial success of twin roll casting, like of other strip castings, depends largely upon the quality of its cast products. For quality improvement, various aspects of twin roll casting have been investigated. An on-line measurement of flow rate and strip thickness has proved that the change of strip thickness at width center is proportional to the change of flow rate. A TV monitor system has shown that the pool surface on the cooling rolls repeats fluctuation of +/- 4 mm due to melt feeding and roll rotation, causing the strip thickness variation of +/- 7%. Casting with different roll-sleeve thicknesses has shown that thinner sleeves produce convex strip and thicker sleeves produce concave strip. Strip temperature observation at the coiling stage has indicated that precipitation of sigma phase makes austenitic stainless steels quite brittle. A sliding gate nozzle has been introduced to control the flow rate; cooling rolls with complicated sleeve profiles have been devised to control the roll crown; a secondary cooling with water jets has been applied to prevent sigma phase precipitation. Consequently, stainless steels or Ni base alloys are being cast into commercial grades of overlay welding hoops.