Evolution of non-metallic inclusions in 80-tonne 9CrMoCoB large-scale ingots during electroslag remelting (ESR) process

Experiments to investigate the evolution behavior of non-metallic inclusion (NMI) during the manufacturing of large-scale heat-resistant steel ingot were conducted using 9CrMoCoB heat-resistant steel and CaF2-CaO-Al2O3-SiO2-B2O3 ESR type slag in an 80-tonne industrial electroslag remelting (ESR) furnace in combination with the theoretical calculations. The main types of NMI in the consumable electrode are pure alumina, a multiphase oxide consisting of an Al2O3 core and liquid CaO-Al2O3-SiO2-MnO shell, and M23C6 carbides with MnS core. The precipitation temperatures of the Al2O3 and MnS inclusions was higher than that of M23C6-type carbide under the equilibrium and nonequilibrium solidification process, therefore the inclusions can act as nucleation sites for the precipitation of the carbide layer. The liquid CaO-Al2O3-SiO2-MnO oxide and MnS inclusion with carbide shell were completely removed during the ESR process, only Al2O3 inclusions and Al2O3-core with a carbide shell can be observed in the remelted ingot. The M23C6-type carbides in the steel were determined as Cr23C6 based on the analysis of TEM results, the substitution of W, Fe, or/and Mo for Cr in the Cr23C6 lattice can slightly change the lattice parameter of Cr23C6 carbide, therefore Cr21.34Fe1.66C6, (Cr19W4)C6, Cr18.4Mo4.6C6, and Cr16Fe5Mo2C6 can match the fraction pattern of the Cr23C6 carbide. The formation of Al2O3 inclusions in the remelted ingot could result from the reduction of CaO, SiO2, and MnO components in the liquid inclusion the increased Al content in the liquid steel or the supersaturation degree for the Al2O3 precipitation in the remelted ingot was higher than that in the electrode, the rationale behind the fact is that the evaporation of CaF2 and increase of CaO content in the ESR type slag.