There are still internal defects such as triangular zone cracks, centerline cracks, and intermediate cracks in 65Mn-Cr steel during the production process, which mostly occur in the initial solidification. In order to explore the evolution of intermediate cracks during the initial solidification process of 230 mm × 1255 mm slab 65Mn-Cr steel, this study was based on a combination of numerical simulation and experiment, using COMSOL numerical simulation software to establish a flow and heat transfer coupling model and stress model, and carried out simulation research. The results show that the solidification speed of slab 65Mn-Cr steel is different at different positions from the meniscus. At the position where the reheating occurs, the heat transfer speed from the solidification front to the surface of the slab slows down, but the solidification speed varies in different areas of the section. At the same time, the flow field, temperature field, and cross-sectional stress and strain field are all non-uniformly distributed, and the maximum plastic strain value exceeds the critical strain 0.004. The experimental results show that internal cracks occur within the range of 9–35 mm below the surface. This shows that the intermediate crack defects of 65Mn-Cr steel are easily caused by stress and strain. Adjusting the spray distribution and cooling intensity of the spray water in the secondary cooling section can be a feasible solution to reduce the occurrence of internal cracks.

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.