Scheduling and optimisation for heating of steel soaking pits: case study for hybrid systems

A heat scheduling and optimisation problem, which consists of the decisions on the optimal number of soaking pits to be used, the optimal soaking time, and the optimal temperature set-points of individual soaking pits for a practical soaking pit/rolling mill complex, is addressed. First, a hybrid system model based on extended coloured Petri nets for the soaking pit process is given. Then, the optimal temperature set-points for a soaking pit for different soaking times are obtained by a simulation-based offline heuristic optimisation strategy. Algorithms to determine the optimal number of soaking pits to be used and the optimal soaking time have also been developed for the desired optimal rolling rate, which can be obtained via simulation. Thus, the overall heat scheduling and optimisation problem of soaking pits can be divided into three subproblems: determine the optimal number of soaking pits, determine the optimal soaking time, and determine the optimal temperature set-points for the soaking pits. Finally, a practical example is given to illustrate the method developed in the paper