The inclusions in cast steel have an important impact on the properties of cast steel products. Domestic and foreign scientific and technological workers have carried out a lot of research work in this regard. The results show that the addition of rare earth elements can control the composition, morphology and distribution of cast steel inclusions, and can play the role of modifying cast steel inclusions. In recent years, the research direction mainly focuses on the research of rare earth addition method and the optimal addition amount of rare earth. An appropriate amount of rare earth remains in the cast steel, so that the [re]/[s] ratio in the molten steel of the cast steel can reach the optimal value, so as to achieve the purpose of controlling the morphology and distribution of inclusions in the cast steel.
Huowenxia found that the inclusions in cast steel without rare earth are mainly calcium aluminate composite inclusions with high melting point and hardness, and the size is about 4 ~ 5 μ M, which may produce stress concentration and constitute the source of microcracks. After the addition of rare earth, the inclusions in cast steel are mainly the composite inclusions of rare earth sulfur oxide and rare earth aluminate with a certain amount of calcium dissolved. These inclusions are irregular ellipsoidal, with an average size of about 3 μ m. Dispersed in molten steel. Rare earth and sulfides in cast steel form spherical rare earth sulfides, which replace the original strip MNS inclusions and control the morphology of inclusions in cast steel, thus improving the toughness and plasticity of castings, especially the impact toughness and fatigue resistance. However, with the further increase of the amount of rare earth, rare earth sulfide appears segregation, and the average size increases to about 5 μ m. It is in sharp angle shape, which is easy to cause stress concentration and fracture. Li Shiyan found in his experimental study on reducing inclusions in wear-resistant cast steel that rare earth elements first combine with oxygen to form rare earth oxides, which float up from the molten steel and discharge into the slag; When the oxygen content in the steel continues to decrease, the rare earth oxide is taken as the core and continuously adsorbed to form rare earth oxygen sulfide. Due to the high melting point of the rare earth compound, the rare earth inclusions continue to float up and remove from the liquid steel, which greatly reduces the inclusions in the cast steel.