Research On Some Optimization Problems In Steelmaking-Continuous Casting-Hot Rolling Production

With the development of the industrial technology and the fierce market competition, the resources and energy issues have been received more attentions. How to potentially improve the product quality, reduce the energy consumption, decrease the production cost are the favorable strategies for the most steel enterprises. The integrated process of steelmaking-continuous casting-hot rolling is complex, energy extensive, and three processes are tightly connected, therefore, the planning and scheduling problems for the integrated process become more important and urgent for domestic and foreign iron-steel plants.The background of this dissertation is the integrated process of steelmaking-continuous casting-hot rolling for the iron-steel enterprises. Based on analyzing the research status of the related scheduling problems, some optimization problems existed in the integrated production process are studied. The major research results of this dissertation falls on five aspects as follows:(1) A scheduling problem for steelmaking-refining-continous casting with controllable processing time is studied. In view of the processing time of the molten steel varied in a certain domain, a hybrid two phase algorithm combining scatter search (SS) and mathematical programming method is proposed. In the first phase, the SS algorithm is used to assign the charges to the devices and sequence them based on the normal processing time. The temporal constraint network graph is applied in the second phase, and a mathematical programming model with controllable processing time, determined processing path and sequence is established. The objective is to minimize the machine waiting time, the heat waiting time and the maximum completion time. By comparison and analysis with other algorithms, computational results show that the mathematical model and the proposed algorithm are effective.(2) A scheduling problem for steelmaking-refining-continuous casting with considering variable electricity price is studied. Based on the decomposition strategy, the problem is divided into the scheduling problem of the single cast and all casts, and the solution methodology combining the mathematical programming method and the heuristics algorithm is developed. According to the technological programs of steelmaking, a mixed integer programming model with objective of minimizing makespan for charges in the same cast is established. With considering time dependent electricity price, the mathematical model with partial precedence between casts and resource-constrained for all casts scheduling problem is presented, the objective of which is to minimize the electricity cost. A heuristic algorithm based on the constraint propagation and multiple rules is developed to solve the second problem. Computational results demonstrate that the models and the proposed algorithm are very efficient and effective in providing high quality scheduling plans, and the electricity cost can be effectively reduced for the iron-steel plants.(3) A ladle scheduling problem is studied. According to the process programs of ladles servicing the molten steel and the time window constraint, a mathematical model with the objective of minimizing the number of ladles is established, and the SS algorithm is proposed. Further, according to the controllable processing time of the molten steel and the soft time window constraint, a ladle scheduling problem with minimizing the occupancy rate of ladles and maximizing the servicing satisfaction level is formulated. A nonlinear mathematical model with multiple objectives is presented, and the SS algorithm is proposed. Computational results show that the mathematical models and the SS algorithms are effective for solving the ladle scheduling problem. The scheduling plans can effectively reduce the number and occupancy rate of ladles, and improve the satisfaction level.(4) A scheduling problem of reheating furnace is studied. According to the process programs of the reheating furnace and hot rolling, a nonlinear mathematical model is proposed for the reheating furnace scheduling, and a SS algorithm is presented. For improving the search effectiveness, two combination operators, which are random variable sequence based voting combination operator and one point crossover combination operator, are proposed. In order to test the performance of the proposed algorithm, the genetic local search (GLS) algorithm is presented. In experiments, we analyze the impact of the different sizes of the reference set, the effectiveness and efficiency of the two algorithms. Computational results show that the model and the SS algorithm are effective for solving the reheating furnace scheduling problem.(5) An integrated scheduling problem for the continuous casting-hot rolling processes is studied. Taking into account the technological and practical constraints, a mathematical model to maximize the number of slabs processed in hot charge rolling (HCR) or direct hot charge rolling (DHCR) mode is established. In a natural way using the special features of the integrated process, the original problem is divided into a master problem and a constraint programming (CP) subproblem. The start time of the casts and the rolling units are performed in the master problem, while the CP subproblem is used to check the feasibility of the obtained solutions, a cut generation method based on the heuristic rules is presented. Computational results show that the hybrid algorithm is efficient and effective for the integrated scheduling problem. The HCR and DHCR ratio is improved, and more energy is saved.