Abstract: Erosions of O'-Sialon/BN in molten Steel and O'-Sialon/ZrO₂ in protective slag of continuous casting were investigated. According to eperimental results and theoretical analysis, erosion kinetics of the O'-Sialon/BN in molten steel was controlled by tWo steps:the first one is controlled by a chemical reaction taking place on the interface; the second one is controlled by diffusion through boundary layers. The O'- Sialon/ZrO₂ composites had good erosion resistance to continuous casting protective slag because the solubility of ZrO₂ in silicate melt is limited and the increment of ZrO₂ content will prevent O'-Sialon from reacting with dissolved CO and other components in slag.