Robust Control Method of Melt Level in the Twin Roll Strip Caster

Abstract

The present invention provides a robust control method for maintaining a melt level at a constant value in a mold in a twin roll strip casting process. In the twin roll strip casting process, whether using a stopper system ( 3 ), which is provided in a tundish ( 2 ) to supply melt into the mold, or using a camera system, which measures the melt level in the mold, the robust control method of the present invention controls the melt level both using an advanced controller, which controls, at an initial stage of the casting process or when a disturbance arises, the target value of the melt level such that the target level corresponds to the performance characteristics of the melt level control system, and using a feedback controller, which maintains the melt level at a constant value under normal casting conditions.

1 . A robust control method for a melt level in a twin roll strip casting process both using a stopper system, which is provided in a tundish to supply melt into a mold, and using a camera system, which measures the melt level in the mold, the method comprising: controlling the melt level both using an advanced controller, which controls, at an initial stage of the casting process or when a disturbance arises, a target value of the melt level such that the target level corresponds to a performance characteristic of a melt level control system, and using a feedback controller, which maintains the melt level at a constant value under normal casting conditions.  
     
     
         2 . The robust control method of the melt level in the twin roll strip casting process according to  claim 1 , wherein a process of designing both the advanced controller and the feedback controller comprises the steps of: obtaining discharge coefficients of a stopper and an immersion nozzle, used in the twin roll strip casting process, a melt feed time delay and a strip manufacturing speed from a stored casting data and determining variable ranges thereof; setting the target value of the melt level to be controlled by the melt level control system of a twin roll strip caster within the determined variable ranges; and designing a robust controller such that the melt level satisfies specifications required in the twin roll strip casting process, despite an outlet closing/opening event resulting in a rapid change in the discharge coefficients of the stopper and the immersion nozzle, thus realizing the target value of the melt level.  
     
     
         3 . The robust control method of the melt level in the twin roll strip casting process according to  claim 1 , wherein application of the advanced controller and the feedback controller comprises the steps of: starting the advanced controller at the initial stage of the casting process or when a disturbance arises; applying the advanced controller until the melt level reaches a normal value; and applying the feedback controller when the melt level reaches the normal value.  
     
     
         4 . The robust control method of the melt level in the twin roll strip casting process according to  claim 1 , wherein the advanced controller is set such that a time ranging from 8 seconds to 26 seconds is required until the melt level reaches the target value at the initial stage of the casting process, and such that a maximum overshoot occurring at the initial stage is within 1% of the target value.  
     
     
         5 . The robust control method of the melt level in the twin roll strip casting process according to  claim 1 , wherein the feedback controller is set such that, when the discharge coefficient of the stopper system is abruptly changed from 0.4 to 0.8 or the discharge coefficient of the immersion nozzle is abruptly changed from 0.45 to 0.85 so that the outlet, which has been slightly closed, becomes normally open, the melt level is within 3% of the target value.  
     
     
         6 . The robust control method of the melt level in the twin roll strip casting process according to  claim 1 , wherein the feedback controller is set such that, even if a time required to feed the melt into the mold through both the stopper system and the immersion nozzle is delayed for 0.5 seconds to 0.7 seconds, the maximum overshoot at the initial stage of the casting process is within 1% of the target value, and such that, even if the discharge coefficient of the stopper or the immersion nozzle is abruptly changed from 0.4 to 0.8 or from 0.45 to 0.85, the melt level is within 3% of the target value.