Abstract:  In order to study the change law of typical inclusions in the smelting process of quenching rails and further improve the control level of non-metallic inclusions in steel, the quenching rail produced by the "150 t converter steelmaking →150 t LF refining →VD degassing → 280 mm×380 mm billet continuous casting" process of a domestic steel mill was taken as the research object, and the smelting process system was sampled, combined with oxygen and nitrogen analysis, molten steel composition analysis and non-metallic inclusion analysis in steel, and its change law was studied from the aspects of inclusion composition, numerical density and size. The results show that the oxygen and nitrogen content and the density of inclusions in the quenching rail smelting process continue to decrease, and the effect of inclusion removal is obvious. The main component of the typical inclusions in LF inlet steel is SiO₂-MnO-Al₂O₃, which is mainly the deoxidation product produced by the converter using silicon manganese deoxidation alloying. The main component of the typical inclusions after LF alloying is CaO-SiO₂-Al₂O₃-MgO, the content of CaO and Al₂O₃ in the inclusions is significantly increased, and the content of SiO₂ and MnO is reduced, mainly due to the reaction of Ca and Al_s brought into the alloy accessories with dissolved oxygen or oxide inclusion in the steel, and the increase in MgO content is related to the erosion of the ladle refractory material. The main component of the typical inclusions in LF outbound steel is CaO-SiO₂-Al₂O₃-MgO, and the content of CaO and MgO in the inclusions is increased, and the content of  Al₂O₃ is decreased. It is mainly the result of the reaction of steel-slag and the transfer to molten steel after the reaction of molten steel and refractory. The typical inclusion core at inner arc 1/4 of the casting blank is Al₂O₃-MgO, and the outer layer is Al₂O_3-SiO₂-CaO. Affected by the addition of alloy accessories and the generation of inclusions and the aggregation and floating of inclusions, the size of inclusions fluctuates periodically over time within a certain range. In the LF and VD stages, due to the soft blowing, the inclusions are reduced in density and the size is increased. The size of the inclusions in the tundish is relatively stable, and the size of the inclusions in the casting blank is significantly increased due to the collision and aggregation of the inclusions in the continuous casting process. Some spinel-like inclusions during the rolling process are brittle, resulting in a decrease in the average size of the inclusions and a slight increase in the number. Therefore, strict controlling the content of Al_s in ferrosilicon and calcium barium silicon, as well as other impurities in accessories material, plays an important role in improving the cleanliness of the quenching rail.

Key words: quenched rail, deoxidation product, typical inclusions, refractory material, cleanliness