Modeling the formation of longitudinal facial cracks during continuous casting of hypoperitectic steel

The mechanism of formation of longitudinal facial cracks during continuous casting of peritectic steels has been studied using a process modeling approach. First, the delta-to-gamma transformation was modeled, assuming carbon diffusion control. The problem of the moving boundary (delta/gamma interface) was solved by employing a one-dimensional finite-difference method. Second, a heat-transfer model for continuous casting of steel was developed and combined with the phase transformation model. Three heat-flux conditions (i.e., low, medium, and high) were obtained from literature data and assumed as the thermal boundary condition. The delta-to-gamma transformation rate was evaluated as a function of the heat-flux conditions. Finally, the results of the coupled model were adopted to calculate the stresses in the solid shell applying the commercial finite-element program, ABAQUS. The results showed that the transformation from delta to gamma is comparatively rapid due to the high diffusivity of carbon in this temperature range. The variation of the heat flux at the meniscus results in large changes of the transformation rate in the meniscus region. Based on the results of the stress calculations, it was concluded that, in order to generate a longitudinal crack on the solid shell surface, not only the tensile stress caused by rapid transformation (i.e., rapid cooling) but also the presence of hot spots is required. A threshold value of approximately 16 pct was obtained for the retardation of shell growth required to generate longitudinal cracks.