利用高温共聚焦扫描激光显微镜精准控制冷却速率，研究了冷却速率分别为800、600、400、200、100和5℃/min条件下管线钢中非金属夹杂物成分的演变，然后计算分析了夹杂物成分转变过程的热力学机理，最后建立了冷却过程夹杂物成分演变的动力学模型并自主编程进行求解，讨论了冷却速率和夹杂物直径对钢凝固和冷却过程中夹杂物成分演变的影响。结果表明，随着冷却速率由800℃/min降低到5℃/min，夹杂物中Al₂O₃含量由66.33%增至75.06%，CaS含量由1.07%增至10.55%，CaO含量由28.27%降至11.24%，MgO含量由4.33%降至3.15%。夹杂物数密度由76.15 mm^-2降至15.28 mm^-2，夹杂物平均直径先由2.09 μm缓慢降至1.62 μm，后又逐渐增大至2.65 μm。高温钢液中夹杂物的热力学平衡成分主要为41.71%CaO-50.76%Al₂O₃-6.50%MgO-1.03%SiO₂，随着温度的降低，夹杂物逐渐由Al₂O₃-CaO-MgO转变为CaS-Al₂O₃-MgO-(CaO)。冷却速率对夹杂物中MgO和Al₂O₃含量的影响较小。夹杂物直径和冷却速率对夹杂物中CaO和CaS含量有显著影响，在钢的凝固冷却过程，夹杂物中CaS含量超过CaO含量的临界冷却速率与夹杂物直径存在直接关系，夹杂物直径为1和2 μm时，这一临界冷却速率分别为400和100℃/min，而当夹杂物直径大于5 μm时，这一转折冷却速率则远小于1℃/min。

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.