Modelling of the formation of longitudinal facial cracks in the continuous casting of steel slabs
Creators: name: Konishi, Junpei
Publisher: name: University of British Columbia
Year: 2009

description: Longitudinal facial cracks are a serious quality problem in the continuous casting of steel slabs. Although numerous investigations have been conducted to eliminate this kind of surface defect, and significant improvement has been achieved, there is still a problem especially operating at high casting speeds. Thus, to improve both productivity and quality, additional research is required. The purpose of this study is to understand the mechanism of formation of longitudinal facial cracks in the continuous casting process of peritectic steels and to propose methods to eliminate the formation of these defects. To achieve this objective, process modeling approach was applied. Firstly, the delta-to-gamma transformation was modeled numerically assuming carbon diffusion control. The moving boundary (delta/gamma interface) problem was solved by employing a one-dimensional finite-difference method. The result of this calculation shows considerably rapid transformation from delta to gamma due to the high diffusivity of carbon in this temperature range. Secondly, a heat transfer model of continuous casting of steel was developed and was combined with the phase transformation model. Three heat flux conditions (i.e., low, medium, and high) were obtained from literature data and applied as the thermal boundary condition. Differences in the delta-to-gamma transformation rate were compared for the heat flux conditions investigated. The results of the coupled model indicated that the difference in the heat flux at the meniscus results in large variations of the transformation rate in the meniscus region. The results of the coupled model were transformed to fictitious temperature by using an steel shrinkage model and adopted to calculate the stresses in the solid shell applying the commercial finite-element program, ABAQUS. Based on the results of the 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. The threshold values for the retardation of both heat removal at the meniscus and shell growth required to generate longitudinal cracks were obtained; in the present work, the values were approximately 10% and 16%, respectively. Based on the findings of this study, uniform heat removal in the meniscus region is of utmost important to eliminate the longitudinal cracks. If uniformity in the heat extraction is achieved, even under high heat flux condition, the tensile stress at the shell surface does not exceed the UTS of the shell surface and cracking will not occur. However, as heat flux increases (i.e., cooling rate increases), the maximum temperature fluctuation permissible before cracking occurs decreases. Thus, ironically, the practical way to eliminate longitudinal cracks when casting at high speeds is to reduce the heat flux in the meniscus region in conjunction with the elimination of non-uniform heat extraction.