Minimizing order earliness and lateness supports good customer delivery performance, but may hurt tundish utilization and increase operating costs. Maximizing tundish length reduces operating costs, but may result in stock production and over-grading. Maximizing hot charge rolling may reduce tundish utilization, etc.
The complexity of balancing above-mentioned goals taking into account specific technological constraints and rules prevented for a long time companies from achievement of efficient results in the area of melt shop and caster scheduling. Existing methods usually targeted only one of the goals. There was no method available for complex and efficient balancing of all the goals. The necessity for development of such a method was obvious for specialists at the Czech steel maker.
The key provisions of a new method
The complexity of melt shop and caster scheduling problem defines necessity to apply a decomposition method to it. Using problem structure this scientific method allows decomposing original problem into set of small interrelated and more simple tasks.
The main idea of the new method is as follows: a) proper decomposition of the problem into layers; b) application of the most useful optimization techniques for each layer; and c) rapid iteration between layers until the model converges on a solution that respects the planner’s business goal priorities.
The proposed method solves the problem using three layers of optimization (see Figure 3).