How to coordinate the contradiction between lubrication and heat transfer in the peritectic steel casting process is the key technical difficulty in preparing mold fluxes. The mold fluxes that are required for casting are subjected to the shear stress generated by mold oscillation and slab movement, which affects the crystallization performance of slags. The quantitative effect of slags’ crystallization performance by shear stress is studied to develop a low-basicity and high-crystallization mold flux to solve the above problem. The results show that the crystallization kinetic condition is promoted, and the crystallization activation energy is reduced by the shear stress, which leads to an increase in the crystallization temperature. Concurrently, the crystal size is reduced. However, the shear stress has no effect on the crystalline phase. The influence of different shear stresses on the crystallization ability of molten slags is related to the crystal nucleation and growth mechanisms. The crystalline fraction of the slag films at 300 rpm (69 s−1) is 44.7%, which is an increase of 17.7% compared with the crystalline fraction of the slag films at 200 rpm (46 s−1). Moreover, the shear stress has little effect on the lubricating properties of the mold fluxes, although the crystallization ability is promoted by the agitation.

Surface longitudinal cracks are a serious problem and particularly prevalent in the casting of peritectic steel (carbon content between 0.10%C and 0.18%C, non-alloyed). It is usually alleviated by controlling the horizontal heat transfer from the steel shell to the mold through increasing the crystallization performance of slags. In the actual continuous casting process, a large number of bubbles are formed in the molten slags, and the crystallization properties of the mold fluxes are affected by bubbles. Therefore, an investigation of the influence of bubbles on the crystallization performance of mold fluxes was carried out by applying the hot thermocouple technique and it is hoped that surface longitudinal cracks can be solved in this way in the peritectic steel casting process. The continuous cooling transformation (CCT) diagrams and time–temperature transformation (TTT) diagrams were constructed for an analysis of the crystallization kinetics. The results showed that the crystallization ability of mold fluxes was enhanced by adding bubbles through shortening the incubation time of crystallization, increasing the critical cooling rate, and decreasing the activation energy of crystallization. As a result, the crystalline fraction, slag film thickness, and surface roughness of the slag films were improved, but the crystalline phase was not affected by bubbles. With an increase of the bubble content remaining in the molten slag, the growth mechanism of the cuspidine crystal phase changed from a low dimension to a high dimension, and the content of the molten slag’s structure unit (Q1) needed for cuspidine in the molten slag was markedly increased.

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