Continuous casting of steel: a close connection between solidification studies and industrial process development

Abstract

Today, the continuous casting process has to face two main challenges, which are apparently contradictory: the permanent demand for higher quality steels and the need for an increase in productivity, so as to reduce production costs. To meet the customers' demands, the two main characteristics required for cast products are a surface free from defects or a minimum macrosegregation in the center, depending on the final use of the end-products. To achieve these goals, process development has to be supported by basic research activities in various scientific fields connected with solidification phenomena: heat transfer, fluid flow, mechanics, structure growth, etc. Detrimental surface and subsurface defects are mainly cracks and inclusions entrapped in the thin solidified shell formed just below the free surface in the mold. Their origin is the poor operation of the complex system composed of the mold, the liquid steel to be solidified and the lubricant used. In fact, one of the main phenomena to consider is the formation of oscillations marks which control the geometry and the structure of the solidified shell. 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 observations 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 metallurgy of the thin solidified shell and its mechanical behavior. Macrosegregation takes place during the final stage of the solidification of the cast product. Its intensity mainly depends on the mechanical behavior of the solid shell in the lower part of the casters. Several techniques have been developed recently to control these deformations in industrial conditions and to keep the core of the product under compression. Their efficiency has been proved through on-line industrial trials. However, to adapt these techniques to various steel grades and casting conditions, the development of a comprehensive model of the formation of macrosegregation, including the rheology of the solid shell and of the mushy zone, has to be achieved.