Al2O3-SiC castables, for the relining of BF hearths in mid-campaign repair, were sampled and investigated after two years’ service. The chemical and mineralogical characteristics of the residual castables were analyzed by X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectrometry to study the erosion mechanism. The results show that as gaseous K diffuses in the castables, leucite (KAlSi2O8) and multiple cracks are formed. Molten iron and slag penetrate through the cracks to form anorthite ([Ca,Na][Al,Si]4O8) and iron, which will not form an embrittlement layer similar to carbon bricks. The entire repair of the hearth with Al2O3-SiC castables combined with the design of thin-wall lining hearth eliminates “elephant foot shaped” erosion, greatly prolonging the service life of the hearth lining.