CONTINUOUS-CASTING OF STEEL - A DOSE CONNECTION BETWEEN SOLIDIFICATION STUDIES AND INDUSTRIAL-PROCESS DEVELOPMENT

Today, the continuous casting process has to face two main challenges, which are apparently contradictory: the permanent demand for higher q uality steels and the need for an increase in productivity, so as to r educe production costs. To meet the customers' demands, the two main c haracteristics required for cast products are a surface free from defe cts or a minimum macrosegregation in the center, depending on the fina l use of the end-products. To achieve these goals, process development has to be supported by basic research activities in various scientifi c fields connected with solidification phenomena: heat transfer, fluid now, mechanics, structure growth, etc. Detrimental surface and subsur face defects are mainly cracks and inclusions entrapped in the thin so lidified shell formed just below the free surface in the mold. Their o rigin is the poor operation of the complex system composed of the mold , the liquid steel to be solidified and the lubricant used. In fact, o ne of the main phenomena to consider is the formation of oscillations marks which control the geometry and the structure of the solidified s hell. From trials performed on industrial casters, the effects of heat transfer through the mold wall and of the chemical composition of the cast steel were clearly evidenced. Laboratory studies are in progress to obtain the precise basic knowledge necessary to understand these o bservations and to define the most effective industrial tools for good quality cast products. The main fields investigated are heat transfer at the interface between liquid steel and cooled substrates, the meta llurgy of the thin solidified shell and its mechanical behavior. Macro segregation takes place during the final stage of the solidification o f the cast product. Its intensity mainly depends on the mechanical beh avior of the solid shell in the lower part of the casters. Several tec hniques have been developed recently to control these deformations in industrial conditions and to keep the core of the product under compre ssion. Their efficiency has been proved through on-line industrial tri als. However, to adapt these techniques to various steel grades and ca sting conditions, the development of a comprehensive model of the form ation of macrosegregation, including the theology of the solid shell a nd of the mushy zone, has to be achieved.