<p>The effects of various cooling rates on non-metallic inclusions were studied by using hot-formed steel with a large reduction produced by twin-roll thin strip continuous casting. The characteristics of inclusions and the composition of molten steel were, respectively, detected by SEM-EDS and ICP-OES, and the CSLM equipment was employed to prepare the 50 K/s and 5 K/s cooling rate samples. The results show that the cooling rate affects the morphology, type proportion, size distribution, and composition of inclusions. There are MnS-type and Oxide-MnS-type inclusions at lower cooling rates in large inclusions (&gt;&#xa0;1&#xa0;µm). The average size and number density of inclusions in lower cooling rate samples are larger than those in 1000 K/s cooling rate samples. Besides, the contents of SiO₂, MnO, and Al₂O₃ in the lower cooling rates were higher than that of sub-rapid solidification, thus reducing the plasticity of the inclusions. Furthermore, the formation mechanism of the SiO₂–MnO–Al₂O₃–CaO–MgO liquid inclusions was analyzed by thermodynamic calculation based on the first-order and second-order interaction coefficients. The behavior of Ca and Mg elements in modified SiO₂–MnO–Al₂O₃ inclusions was elucidated. The numerical statistics of large inclusions and fine inclusions in three cooling rates were carried out by the tracking method. The results indicated that the increments of SiO₂, MnO, and Al₂O₃ contents at lower cooling rates were derived from the aggregation of fine inclusions and the reoxidation of Si, Mn, and Al during solidification. Finally, the nucleation and growth models of MnS precipitates were constructed, and the size of the MnS precipitates kinetic model was compared with the experiments, indicating that the model can well predict the maximum size of MnS precipitates at different cooling rates.</p>