A molten steel level control method for continuous casting based on the disturbance observer theory

Kazuya Asano, Visiting Associate in ChE, Caltech

This talk will describe a new control method for stabilizing the molten steel level in the molding process of a continuous casting mill in a steelmaking factory. The continuous casting process allows molten steel to be continuously cast a strand, which is cut by torches into slabs with a size of about 250 X 2000 X 10000 mm. Securing the stability of the molten steel level will ensure the surface quality of cast slabs by stabilizing the solidification process and preventing surface defects such as pinholes and enclosures. The molten steel level control has generally been carried out by applying a PID control system; however, it is difficult to obtain satisfactory control performance by the conventional control method when various disturbances, such as breakaway deposits in the immersion nozzle and unsteady bulgings, break the equilibrium of massflow in the mold. The proposed method is based on estimation and cancellation of a disturbance flow, which is defined as the difference between the ingoing and outgoing flow caused by the above disturbances. A disturbance observer is employed to estimate the disturbance flow. Then, the manipulating value which counterbalance the effect of the estimated disturbance flow is calculated and added to the output of the PI controller. This control scheme enables a straightforward and intuitively comprehensible way of designing and tuning controllers, in addition to high control performance. The process contains a lot of uncertainties, which can make the system unstable. In the case of the proposed controller, the trade-off between the robust stability and the control performance can be easily incorporated into the design by adjusting a few parameters of the controller. This is helpful for tuning in actual plants. The control method has been successfully applied to the molten steel level control systems in several continuous casting mills in my company (Kawasaki Steel Corporation in Japan), and those systems effectively operate to suppress the effects of the disturbances and to improve the surface quality of the slabs.