Evolution of nonmetallic inclusions in 80-t 9CrMoCoB large-scale ingots during electroslag remelting process

In combination with theoretical calculations, experiments were conducted to investigate the evolution behavior of nonmetallic inclusions (NMIs) during the manufacture of large-scale heat-resistant steel ingots using 9CrMoCoB heat-resistant steel and CaF₂–CaO–Al₂O₃–SiO₂–B₂O₃ electroslag remelting (ESR)-type slag in an 80-t industrial ESR furnace. The main types of NMI in the consumable electrode comprised pure alumina, a multiphase oxide consisting of an Al₂O₃ core and liquid CaO–Al₂O₃–SiO₂–MnO shell, and M₂₃C₆ carbides with an MnS core. The Al₂O₃ and MnS inclusions had higher precipitation temperatures than the M₂₃C₆-type carbide under equilibrium and nonequilibrium solidification processes. Therefore, inclusions can act as nucleation sites for carbide layer precipitation. The ESR process completely removed the liquid CaO–Al₂O₃–SiO₂–MnO oxide and MnS inclusion with a carbide shell, and only the Al₂O₃ inclusions and Al₂O₃ core with a carbide shell occupied the remelted ingot. The M₂₃C₆-type carbides in steel were determined as Cr₂₃C₆ based on the analysis of transmission electron microscopy results. The substitution of Cr with W, Fe, or/and Mo in the Cr₂₃C₆ lattice caused slight changes in the lattice parameter of the Cr₂₃C₆ carbide. Therefore, Cr_21.34Fe_1.66C₆, (Cr₁₉W₄)C₆, Cr_18.4Mo_4.6C₆, and Cr₁₆Fe₅Mo₂C₆ can match the fraction pattern of Cr₂₃C₆ carbide. The Al₂O₃ inclusions in the remelted ingot formed due to the reduction of CaO, SiO₂, and MnO components in the liquid inclusion. The increased Al content in liquid steel or the higher supersaturation degree of Al₂O₃ precipitation in the remelted ingot than that in the electrode can be attributed to the evaporation of CaF₂ and the increase in CaO content in the ESR-type slag.