THIN STRIP CASTING OF NI BASE ALLOYS BY TWIN-ROLL PROCESS

The rapid solidification process, which omits the hot rolling stage in the production of thin strip, was used to produce materials which typically have poor workability. Ni base alloys were cast into a thin strip less than 1 mm in thickness by the twin roll process. A cast strip was continuously coiled with low tension control and secondary cooling. Coils of these alloys are being delivered as overlay welding hoops after appropriate post treatment. For quality improvement, various aspects of twin roll casting have been investigated. Melt level control by side-darns and roll profile control have decreased the strip thickness deviation to within +/-7%. Internal cavities and localized shrinkage were often detected in strips of these alloys made with flat rolls, while they are rarely detected in the strips produced with grooved roll. This difference was studied in a two dimensional calculation of solidification. it has been considered that these alloys, if cast with flat rolls, have a longer liquid phase in the strip than the strip cast with grooved rolls, when gas or scale is trapped between the roll and the melt during casting, and that the elongated liquid phase forms internal cavities due to the difficulty of melt flow to the final solidification point.