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The formation and evolution of centerline carbides in a martensitic stainless steel slab are investigated using various characterization techniques. The intensive formation of carbides was observed close to the shrinkage cavity at the centerline of the slab. Carbide structures include a complex eutectic-like mixture of δ-ferrite and M7C3 phases with M6C carbides at the δ/M7C3 interface. Nano-sized M7C3 particles also precipitate in the δ holding the orientation relationship. This complex structure was surrounded by the retained γ. The decomposition of the coarse M7C3 was investigated after the reheating treatment to 1200, 1250, and 1300 °C. It was found that by increasing the reheating temperature, coarse M7C3 carbide completely decomposed, whereas δ-ferrite islands enlarged due to the thermodynamic stability of δ-ferrite at high temperatures. At the same time, the retained austenite reduces because of homogenizing of the segregated solutes. The new formation of M23C6 and the growth of M6C were identified after reheating to 1200 °C, while all carbides were fully decomposed at 1250 and 1300 °C. The effect of reheating temperature on the decomposition process of coarse carbide was further discussed.