To investigate the nucleation behavior during the single-phased metallic solidification process, the commercial ultrapure ferritic stainless steels with no (Initial steel) and various melt treatments (R1, MR1, Y2, MY1, and M1 steels) were used to carry out the differential scanning colorimetry (DSC) and high-temperature confocal laser scanning microscope (HT-CLSM) experiments. Based on the results of DSC experiments, the equilibrium solidification process as well as the relationship among the critical undercooling degree ( \({\Delta{{T}}}_{\text{c}}^{\text{DSC}}\) ), latent heat of fusion/crystallization ( \({\Delta{{H}}}_{\text{f}}\) / \({\Delta{{H}}}_{\text{c}}\) ), equiaxed grain ratio (ER), and average grain size ( \({{D}}_{\text{ave.}}^{\text{ingot}}\) ) was revealed. ER is increased with the decreasing \({\Delta{{T}}}_{\text{c}}^{\text{DSC}}\) and increasing \({\Delta{{H}}}_{\text{f}}\) / \({\Delta{{H}}}_{\text{c}}\) ; however, \({{D}}_{\text{ave.}}^{\text{ingot}}\) is decreased with them. Referring to the results of HT-CLSM experiments, the average sizes of micro-/macrostructures ( \({{d}}_{\text{ave.}}\) / \({{D}}_{\text{ave.}}\) ) are decreased with the increasing cooling rate, as well as the difference between \({\Delta{{T}}}_{\text{c}}^{\text{DSC}}\) and apparent critical undercooling degree ( \({\Delta{{T}}}_{\text{c}}^{\text{CLSM}}\) ) was revealed. The heterogeneous nucleation of the crystal nuclei occurs only if \({\Delta{{T}}}_{\text{c}}^{\text{CLSM}}\) > \({\Delta{{T}}}_{\text{c}}^{\text{DSC}}\) . Combining with the interfacial wetting-lattice mismatch heterogeneous nucleation model, the dynamic mechanism of the metallic solidification was revealed. The as-cast grains of the melt-treated samples were obviously refined, owing to the much higher actual heterogeneous nucleation rates ( \({{I}}_{\text{heter., }{{i}}}\) ) obtained through melt treatments, and the heterogeneous nucleation rates ( \({{I}}_{\text{heter., }{{ij}}}\) ) for all samples are increased with the cooling rates, firmly confirming that the as-cast grains of each sample could be refined by the increasing cooling rates.
