Effect of rare earth on as cast microstructure of cast steel

Many researchers have found that rare earths have the effect of refining grains. The reason is that on the one hand, the atomic radius of rare earths is large, and the amount of solid solution in cast steel is small. When the grains grow, they are enriched on the crystal interface, which reduces the interface energy and hinders the grain growth; At the same time, the segregation of harmful elements at the grain boundary is prevented, and the austenite grains are fully refined. On the other hand, the low melting point of rare earth can significantly reduce the liquidus temperature of steel, form large component supercooling, make the crystal branch form a thin necking and easy to fuse, and promote the dissociation of crystal and the increase of crystal nucleus; At the same time, rare earth has a strong affinity with oxygen, sulfur and other elements in molten steel. Small rare earth compounds can be used as heterogeneous nucleation centers, which improves the nucleation rate.

After adding rare earth into ultra-low carbon cast steel, Meizhi found that a large number of high melting point rare earth compounds were formed in the molten steel, the nucleation sites were increased, the as cast grains were significantly refined, and the yield strength of the castings was significantly improved. Jiexiaohua, Chen Fangming and others have shown that the addition of rare earth in hot forging die steel can significantly refine the grain size, and the thermal fatigue resistance, plasticity and impact properties of the material have been significantly improved. Dong Li found that the grain size of low alloy high strength and toughness cast steel treated with rare earth is grade 8, and that without rare earth is about grade 6. In addition, Jin Zehong and other researchers found that after the addition of rare earth elements, the number of ferrite in the as cast structure increased, the number of pearlite decreased, and the sheet spacing decreased. Zheng kaihong found that the addition of rare earth elements can reduce the number of twin martensite, increase the number of lath martensite, and refine lath martensite.

With the continuous optimization of the production process of cast steel, the purity of molten steel of cast steel is continuously improved, and the microalloying effect of rare earth elements in cast steel is becoming increasingly prominent. Yue Lijie’s research shows that rare earth atoms are larger than iron atoms. Trace rare earth dissolved in cast steel will distort the lattice of iron matrix, produce distortion energy, and strengthen the solid solution. At the same time, rare earth elements are enriched at the grain boundary, which reduces the segregation of impurity elements, purifies the grain boundary and improves the grain boundary strength due to the interaction. In addition, rare earth elements can also reduce the activity of carbon and nitrogen elements in the molten steel of cast steel, reduce their desolvation, so as to reduce the resistance of dislocation movement, and significantly improve the plasticity and impact toughness of cast steel.

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