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

Abstract

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 Cr(23)C6 carbide. Therefore, Cr21.34Fe1.66C6, (Cr19W4)C-6, Cr18.4Mo4.6C6, and Cr16Fe5Mo2C6 can match the fraction pattern of Cr23C6 carbide. The Al(2)O(3 )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.