Abstract: A two-dimensional mathematical model of solidification and heat transfer for round blooms was established with ANSYS software, and it was verified by nail-shooting experiment and surface temperature testing. The solidified shell thickness distribution at any fixed location in the casting direction and the position of the solidification end point can be accurately obtained by using this model. Combining the model results with the macrographs of samples, it is found that the shell thickness and the square root of solidification time show a linear relationship in the columnar crystal zone, which fits a solidification square root law. The law is corrected, and the correction term is relevant to the superheat and solidification velocity. The relationship between the shell thickness and the square root of solidification time in the equiaxed crystal zone is not fit for the solidification square root law. It is indirect argument that heat transfer during columnar crystal solidification is unidirectional, but heat transfer during equiaxed crystal solidification is in multiple directions.