Effect of cerium addition on inclusion evolution, phase transformation and microstructure during solidification in Al-killed high-strength steel for construction machinery

Different contents of Ce–Fe alloy were added to Al-killed high-strength steel. Through analysis of ingot structure, characterization of inclusions, and in situ observation of solidification process, combined with thermodynamic and dynamic analyses, the effects of cerium addition on inclusion evolution, solidification phase transformation, and solidification microstructure were investigated. The results demonstrated that as the addition of Ce–Fe alloy increased, the size of inclusions progressively decreased. The composition of inclusions gradually evolved from Al 2 O 3 and MnS to CeAlO 3 , Ce 2 O 3 , and Ce 2 O 2 S, with the proportion of Ce 2 O 2 S continuously increasing. The solid solution Ce in 0.0035% Ce steel suppressed initial nucleation and phase transformation during solidification, resulting in a long solidification time and coarse grains in the equiaxed crystal zone. The disregistry values between Ce 2 O 3 and Ce 2 O 2 S formed in 0.0070% and 0.0085% Ce-containing steel and δ-Fe were 6.16% and 3.49%, respectively. These inclusions provided more heterogeneous nucleation sites for δ-Fe, promoting initial nucleation. Simultaneously, the accelerated solid-liquid phase transformation shortened the solidification time. Consequently, the equiaxed crystal zone expanded, and grain size was significantly refined. However, when the Ce content reached 0.0085%, massive grain merger and growth led to an increase in grain size, thereby weakening the effect of Ce in refining the solidification microstructure.