Controlling nonmetallic inclusions in steels is critical during the steelmaking process. Temperature affects the chemical equilibrium between steel and the inclusions, the composition of the inclusions changes with changes in temperature. During the solidification and cooling processes, the cooling rate is a significant factor affecting the temperature. Therefore, the composition of nonmetallic inclusions transforms during the solidification and cooling of steels. To study the evolution of the inclusion composition in pipeline steel at cooling rates of 800, 600, 400, 200, 100, and 5^oC/min, high-temperature confocal scanning laser microscopy was employed to accurately control the temperature during the cooling process. The thermochemical software FactSage was employed to reveal the theoretical basis of the transformation of the inclusion composition. A kinetic model for the evolution of the inclusion composition in pipeline steel during the cooling process was established, and the effect of inclusion diameter and cooling rate on the transformation was analyzed. The results revealed that with the decrease in the cooling rate, the Al₂O₃ content in the inclusions increased from 66.33% to 75.06%, the CaS content increased from 1.07% to 10.55%, and the CaO content decreased from 28.27% to 11.24%. Further, the MgO content decreased from 4.33% to 3.15% during the cooling process. The number densities of the inclusions were 76.15 and 15.28 mm^-2 at cooling rates of 800 and 5^oC/min, respectively. As the cooling rate decreased, the average diameter of the inclusions first decreased from 2.09 to 1.62 μm and subsequently increased. The thermodynamic equilibrium composition of the inclusions in the molten steel was 41.71%CaO-50.76%Al₂O₃-6.50%MgO-1.03%SiO₂. With a decrease in temperature, inclusions transformed from Al₂O₃-CaO-MgO to CaS-Al₂O₃-MgO-(CaO). The cooling rate had little effect on the MgO and Al₂O₃ contents in the inclusions. The inclusion diameter and cooling rate had an apparent influence on the CaO and CaS contents in the inclusions. The critical cooling rate at which the CaS content became greater than the CaO content was impacted by the inclusions' diameter. The critical cooling rates for inclusions with diameters of 1 and 2 μm were approximately 400 and 100^oC/min, respectively, whereas the rates were much smaller than 1^oC/min for inclusions with diameters larger than 5 μm.