Process optimization is largely motivated by the economic incentive. Because of the large volumes involved in production, even small improvements can result in large productivity gains. Under competition, continuous improvement is also necessary to maintain and to improve a market position.
Artificial neural networks have been applied in monitoring and control of industrial processes in [1], [2], [4], [7], [10], [11], [18], [23], [24], [25], [26], [28], [29], [32], [33], [37], [38], [40], [41], [43].
Rautaruukki produces steel coils to be used, for instance, by the construction industry. The process consists of rolling operations and an annealing process. The steel rolls may also be coated with plastic or zinc.
Figure 5.1: An oversimplified model of the process (black box approach)
The products leave the production line one by one. Each of these products has attributes associated with it. These attributes are the raw material charcteristics in the form of element concentrations, the process parameter settings during the processing and the end product characteristics in the form of the quality parameters. The quality parameters reflect the mechanical properties of the steel. The quality parameters are determined through standard procedures.
After steel strips have been hot rolled the strips are pickled. The strip goes through a pickling process. A pickling process is an acid bath, which removes an oxide layer from the surface of the strip. After pickling, the strip are rolled in four consecutive roll stands to make the steel strip thinner.
After rolling, the strip is coiled and warmed up to a hot temperature in a bell-shaped oven. The roll is warmed and then let to cool up during a long period of time. This part of the process is called annealing. Annealing process improves the microstructure of the steel: on a microlevel, the steel particles become ordered. After annealing, the coils are rolled with a temper mill to improve formability.
The material chain in the process is illustrated in the Figure 5.2. The incoming raw material consists of the hot rolled coils from suppliers.
Figure 5.2: The material flow in the process
After the treatments described above, the coils are ready to be packed and shipped to the customer. The coils may also be coated with zinc or plastic. The coating process was not a part of this analysis.