非金属夹杂物是影响石油套管钢性能的重要因素之一，稀土(RE)能有效变质钢中的非金属夹杂物。本工作开展了套管钢精炼、连铸和热轧过程中稀土合金化改性夹杂物的工业实验研究，利用SEM-EDS、FactSage 8.3热力学计算软件及全自动夹杂物分析，对钢中夹杂物形貌、数量、尺寸等的演变行为进行了研究。结果表明，钢中稀土含量为4 × 10^-6时，石油套管钢液全氧含量可降低到7 × 10^-6，同时，钢中的Ca-Al-O类夹杂物转变为Ca-RE-Al-O夹杂物。稀土改性后铸坯样中夹杂物数量密度由44 mm^-2增加至46 mm^-2，尺寸为0~2 μm的夹杂物数量密度由13 mm^-2增加到18 mm^-2，而尺寸为5~30 μm的夹杂物数量密度由7 mm^-2降低至5 mm^-2，这表明稀土合金化工艺使钢液夹杂物尺寸减小。热力学计算和实验结果表明，CaS夹杂物在凝固阶段形成，其包裹在Ca-Al-O/Ca-RE-Al-O类夹杂物外围，夹杂物可能起到了异质形核的作用。在热轧过程中，未添加稀土的钢中出现条串状分布的夹杂物，而稀土合金化改性后夹杂物变形能力提高，未出现夹杂物破碎引起的条串状夹杂物。钢液/精炼渣反应、稀土类夹杂物的上浮去除等，是造成稀土收得率低的主要原因。

Oil casing steel plays a critical role in the oil and natural gas industry, and its performance is significantly influenced by non-metallic inclusions. Rare earth (RE) elements can effectively modify these inclusions. In this study, industrial experiments were conducted to investigate the effects of rare earth metal alloying on inclusion characteristics during the refining, continuous casting, and hot-rolling processes. The evolution of inclusion morphology, quantity, and size was analyzed using SEM-EDS, FactSage 8.3 thermodynamic software, and an OTS One Bond inclusion analysis system. Results show that when the rare earth content was 4 × 10^-6, the total oxygen content decreased to 7 × 10^-6. In addition, rare earth microalloying transformed Ca-Al-O inclusions into Ca-RE-Al-O inclusions. Following alloying, the inclusion number density in continuous casting billet samples increased from 44 mm^-2 to 46 mm^-2, with the number density of 0-2 μm inclusions rising from 13 mm^-2 to 18 mm^-2, while the number density of 5-30 μm inclusions fell from 7 mm^-2 to 5 mm^-2. Overall, the average inclusion size decreased after rare earth metal addition. XRD and XRF analyses revealed the formation of rare earth phases in the refined slag after vacuum degassing rare earth alloying. Thermodynamic calculations indicate that at 1600 ^oC, the Gibbs formation energies of CaO and CeAlO₃ in steel were -357088.82 and -86892.89 J/mol, respectively, supporting the formation of these inclusions upon rare earth addition. In RE-free furnaces, both thermodynamic calculations and experimental results showed that CaS inclusions formed during solidification, with CaS precipitating around the edges of Ca-Al-O/Ca-RE-Al-O inclusions. In RE-containing furnaces, the addition of rare earth reduced the precipitation of calcium aluminate inclusions, leading instead to the formation of CaO·REAlO₃ inclusions, which likely serve as nucleation sites for CaS precipitation during solidification. During hot-rolling, long strip inclusions were observed in steel without rare earth; however, rare earth alloying improved the deformation ability of inclusions, preventing the formation of long strips due to inclusion crushing. Notably, the modifications induced by rare earth were independent of the inclusions’ Ca content. In hot rolled tube samples without rare earth, only inclusions with moderate Ca content exhibited good deformability. The low yield of rare earth metal was primarily attributed to reactions between the molten steel and refining slag, as well as the removal of rare earth inclusions from the molten steel to the slag.