An Investigation of Some Mold Powder Related Startup Problems

This paper examines two mold powder related process problems experienced at BHP's Whyalla operations. Bands of deep transverse depressions extending about the perimeter of the strand in the first meter of cast product were identified as a potential cause of breakouts. These were associated with level drops in the mold during startup that allow sintered material to accumulate against the mold wall creating a template for depression formation. Extended delays in achieving steady state heat transfer conditions were observed on grades employing low basicity operating powders, and are ascribed to the retention of high basicity slag in the mold. Laboratory studies have identified a possible difficulty in replacing high basicity slags from the mold using low basicity slags due to their greater adhesion with the mold wall (as indicated by lower interfacial heat transfer resistance measurements). Mathematical modelling simulations indicated that higher slag consumption and a static solid slag layer both decrease mold temperature. Steady state conditions were achieved more rapidly by reducing the amount of mixing between startup and operating powders in the mold by delaying the addition of operating powder. This practice also reduced the severity of transverse depressions during startup.