A transverse crack on the slab corner became a severe defect for Nb-containing steel due to precipitation of NbN particles along the prior austenite grain boundary. By adding Ti, the TiN particles were in priority precipitation than NbN which kept more Nb as solution condition in steel. In the present research, the formation mechanism of TiN was investigated by thermodynamics calculation and experiments. It came to the following conclusions. With an increase in the cooling rate of molten steel, the precipitation location of TiN particles moves from the γ grains boundary into the matrix. Based on the Ohnaka micro-segregation model, the precipitation behaviour of [Ti] and [N] was investigated under various solid fractions during the solidification process. As the solid fraction is larger than 0.95, the [Ti][N] value was higher than equilibrium value. Furthermore, inclusion of a high melting material such as Al₂O₃ played an important role in decreasing the nucleation potential barrier of TiN, which enhanced the formation of TiN particles in the molten steel. For micro-segregation models, according to the segregation degree at the solidification front, they can be sorted in the following sequence, Scheil > Ohnaka(2α) > B–F > C–K > Ohnaka(4α) > V–B > Lever.

The influence of Ca treatment, Mg treatment and Ca–Mg combined treatment on the inclusions in 45MnVS non-quenched and tempered steel were studied in the present work. After the melting experiment, a hot rolling test was carried out on the steel ingot. Additionally, the composition, quantity and morphology of inclusions in the test steel samples were analyzed by automatic scanning electron microscopy (ASPEX) and an energy dispersive X-ray spectrometer connected to scanning electron microscope (SEM-EDS). The results indicated that the inclusions in 45MnVS steel mainly consisted of a large amount of sulfides and a small amount of oxides. Sulfide inclusions could be nucleated and precipitated using oxides as a core during solidification. The proportion of spindle-shaped inclusions in sulfide with smaller sizes was higher. The sizes of MnS–oxide inclusions were larger than those of MnS. After hot rolling, the proportion of spindle-shaped MnS and complex sulfides with oxide cores in the samples was increased significantly. Compared with Ca treatment and Ca–Mg treatment, more oxides were formed in the steel with Mg treatment, which can in turn become the cores for sulfide nucleation and precipitation. Thus, the proportion of MnS–oxide inclusions in steel increased. Compared with Ca treatment and Mg treatment, steel with Ca–Mg treatment was more conducive to the formation of complex sulfides, and increased the proportion of spindle-shaped sulfides in 45MnVS steel. After Ca treatment, Mg treatment and Ca–Mg combined treatment, the proportions of spindle-shaped sulfides in steel were 23.31%, 19.39% and 43.24%, respectively.