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The solidified iron layer in blast furnace (BF) hearth was estimated based on the blast furnace damage. The phase compositions of the solidified iron layer were studied using scanning electron microscope and energy dispersive spectrometer. The temperature and proportions of graphite precipitation were calculated by using the Thermol-calc software and the TCFE8 database. Finally, the formation of the solidified iron layer was examined. The results suggest that the solidified iron comprises iron and graphite. The temperature of the graphite precipitation is affected by the composition of the hot metal, and it is much higher than the solidification temperature of the hot metal. The proportions of precipitated graphite are affected by the C and Si in the hot metal, whereas the precipitated graphite increases the viscosity of the hot metal by 11.9%. The graphite precipitates at the interface with the Fe-refractory at lower temperature than that of the graphite saturation, which allows the C migration from the hot metal to the refractory interface.