This study investigates the evolution of initial solidification during the continuous casting of 230 mm × 1255 mm slabs by establishing a heat transfer inverse calculation model and a slab solidification heat transfer model, with the results of the former used as boundary conditions for the latter. The results show that at the thermocouple measurement positions, the average heat flux in the outer arc direction is greater than that in the inner arc direction, which causes the flow field inside the slab to deflect. Meanwhile, with increasing distance, the transverse distributions of temperature and solidified shell thickness become uneven. The lowest temperature occurs within 100 mm of the slab center, and the maximum difference in solidified shell thickness is 4.47 mm. A “reheat” phenomenon occurs in the shell at the mold outlet. Increasing casting speed and decreasing superheat both alleviate the flow deflection, but they do not reduce the uneven distribution. Specifically, for every 0.1 m/min increase in average casting speed, the solidified shell thickness decreases by 0.74 mm, and for every 5 °C increase in superheat, the solidified shell thickness decreases by 0.07 mm. Superheat has almost no effect on the growth of the solidified shell. However, when the slab enters the secondary cooling section, the solidified shell growth rate accelerates.