Multiphysics Simulation of Slag-Skin Evolution and Process Parameter Effects During Electroslag Remelting of X2CrNiMo18.12 (Nitrogen-Controlled) Steel

X2CrNiMo18.12 (nitrogen-controlled) stainless steel is prone to slag-shell erosion during electroslag remelting (ESR), which may lead to steel breakout and mold leakage. To investigate the evolution behavior of the slag shell and molten pool, a transient multiphysics model coupling electromagnetic, flow, and thermal fields was developed based on the volume of fluid (VOF) method and dynamic mesh technique. The distributions of the coupled physical fields and the effects of electrical parameters and cooling intensity on slag-shell stability and molten-pool morphology were systematically analyzed. The results show that a pronounced edge effect exists at the lower electrode corner, resulting in concentrated current density, Joule heating, and Lorentz force. Under the combined effects of electromagnetic force and thermal buoyancy, a dominant circulation vortex is formed in the slag pool, which governs heat transfer and slag-shell evolution. The slag-shell thickness is determined by the competition among vortex-induced erosion near the mold wall, erosion by molten steel at the slag–metal interface, and mold cooling. Among these factors, erosion by molten steel is the primary cause of steel breakout and mold leakage. Increasing the current from 2.5 to 3.5 kA significantly increases the melt superheat, transforms the molten pool from a shallow U-shape to a deep V-shape, and reduces the slag-shell thickness to approximately 0.95 mm, leading to leakage failure. In contrast, a moderate current of 2.5–3.0 kA combined with a cooling intensity above 2400 W·m−2·K−1 maintains the slag-shell thickness at approximately 2 mm and effectively suppresses steel breakout and mold leakage. These findings provide guidance for process optimization and operational safety in the ESR of X2CrNiMo18.12 steel.