An Industrial Practice on the Effect of Deoxidation Process on Inclusions in U75V Heavy Rail Steel

This paper investigates the effects of different deoxidation processes on non-metallic inclusions in U75V heavy rail steel. Based on the production process of U75V heavy rail steel and the content of impurity elements in the alloy, three experimental schemes were designed. The results show that during LF (Ladle Furnace) and VD (Vacuum Degassing) refining, the oxygen and nitrogen contents in all schemes were significantly reduced, with the total oxygen content in the casting billet being less than 10 ppm. At the end of LF alloying, the aluminum and calcium contents in all three schemes reached their maximum values. The evolution of inclusions was analyzed according to typical smelting processes. At the beginning of LF refining, the main inclusions in all schemes were MnO-SiO2, with small amounts of Al2O3 and CaO. After slagging, the inclusions in the steel were primarily CaO-SiO2-Al2O3, with a small amount of MnO-SiO2-Al2O3. The contents of CaO and Al2O3 in the inclusions increased significantly, while the contents of SiO2 and MnO decreased. At the end of LF refining, the inclusions were still dominated by the CaO-SiO2-Al2O3 type, with the Al2O3 content significantly higher than after slagging. After vacuum breaking, prolonged stirring under vacuum conditions promoted chemical reactions between the steel, slag, inclusions, and refractory materials, resulting in a sharp increase in CaO and MgO contents in the inclusions. Ultimately, the inclusions were completely transformed into CaO-SiO2-Al2O3-MgO composite inclusions. Analyzing the number density and area fraction of inclusions throughout the smelting process shows that both decreased significantly during LF and VD refining, indicating effective inclusion removal. By replacing standard ferrosilicon with low-aluminum ferrosilicon and calcium carbide with silicon carbide, and adjusting the silicon-calcium-barium alloy addition from 2 to 1 kg/t, the number density of large inclusions (> 10 mu m) in the casting billet decreased from 0.33 to 0.08/mm(2), and the maximum inclusion size decreased from 81.24 to 27.87 mu m, significantly reducing both the number density and maximum size of the inclusions.