Effect of casting speed on solidification structure and central macrosegregation during continuous casting of high-carbon rectangular billet

IntroductIon Solidification structure and macrosegregation have great influence on the quality of high carbon steels. Center segregation in high carbon steel results in a high concentrations of chemical components especially carbon element and has a great influence on the quality of wire rod [1-3]. Wire rod with center cementite network resulting from severe segregation not only shows low ductility in the center portion, but also exhibits cup fracture during drawing [4]. A fundamental task of high carbon steel continuous casting is to optimize the processing parameters to minimize the formation of carbon macrosegregation. The optimization of the process variables may provide a simple low cost solution to controlling macrosegregation [5]. Product quality and production efficiency depend essentially on the appropriate casting speed. The effect of casting speed on the middle size of 180 mm×240 mm rectangular billets in continuous casting has not been researched before. As is known, different billet sizes require different casting speeds in order to reduce the center macrosegregation and increase the production efficiency. Generally, the billet size less than 160 mm ×160 mm needs high casting speeds to increases the production efficiency by using intensive cooling intensity in secondary cooling zone. Intensive cooling intensity can achieve a rapid temperature drop at the surface corresponding to the temperature drop at the center when the latter solidifies and, consequently decreases the grain size and prevents V-segregation formation [6]. The blooms generally need small casting speeds in continuous casting by using softer cooling intensity. It is known that “soft” cooling intensity can minimizes alloy components segregation intensity and increases equiaxed grain ratio [4]. So, to the middle size rectangular billet, it is of great importance to study the casting speed on macrostructure and center segregation. The solidification structures including the total grain number and the dendrite morphology can affect the center macrosegregation. The grain number can be used to evaluate the mean grain size in the whole cross section and large grain number in the cross section means that the mean grain size is small. The liquid flow comes from interdendritic areas and therefore, is highly enriched, which causes the central macrosegregation. This liquid flow from the interdendritic region can be effectively influenced by the grain size [7]. Besides, center macrosegregation is closely related Effect of casting speed on solidification structure and central macrosegregation during continuous casting of high-carbon rectangular billet