Effect of La content on inclusions, grain refinement, and impact toughness in low alloy wear-resistant steel: in-situ tensile testing and EBSD orientation analysis of inclusions

Abstract

In the current study, the effects of varying La concentrations—specifically 0, 25, 40, and 96 ppm—on the feature of inclusions, the alteration of prior austenite grain size, and the impact toughness of the low alloy wear-resistant steel were systematically investigated. As the La content in the steel increased, the impact toughness first rose and then decreased, reaching a maximum of 76.83 J cm⁻² at 40 ppm La, with an increase of 19.6 %. The improvement in impact toughness primarily depended on the beneficial effects of La on inclusions and grain refinement. Adding La reduced the quantity of large non-metallic inclusions and the proportion of multiphase inclusions, which significantly improved the impact toughness. However, at 96 ppm La, the occurrence of large-sized La-O-P-As compromised impact toughness. With the increase in La content from 0 ppm to 96 ppm, the prior austenite grain size decreased, achieving the optimal refinement effect (from 14.18 μm to 9.94 μm) at 96 ppm. This is because the number of fine-dispersed inclusions (LaAlO₃ and La₂O₂S) that could act as nucleation sites increased significantly. EBSD showed that fine prior austenite grains were only present around the La₂O₂S (100), regardless of single-phase or multiphase inclusions. This indicated that if inclusions had planes with suitable planar disregistry matching the γ-Fe, they could serve as cores for heterogeneous nucleation. However, the ability of single-phase inclusions for heterogeneous nucleation was much higher than that of multiphase inclusions. The optimal performance was observed at 40 ppm La due to the ideal balance of inclusion type, size, and prior austenite grain refinement.