The mass transfer kinetics of niobium solution into liquid steel

Abstract

Niobium cylinders were immersed into liquid steel and their mass-transfer rates measured under dynamic conditions. The cylinders were suspended from a load cell, and the apparent weight of the cylinder as well as temperatures at various locations in the immersed specimens were measured continuously during immersion and recorded with a data acquisition system. From the weight measurements, the mass-transfer rate was deduced. A steel shell period and free dissolution period were identified. During the steel shell period, a shell of frozen steel wraps the cylinder following its initial immersion. When niobium cylinders were immersed into liquid steel with low superheat, a reaction was detected at the steel shell/niobium interface. This reaction took place during the later stages of the steel shell period. The intermetallic compounds Fe₂Nb and Fe₂Nb₃ were identified as reaction products. The mass transfer which takes place during the free dissolution period was found to be exothermic, and the rate-controlling step was found to be liquid phase diffusion through a mass-boundary layer. The apparent activation energy was estimated to be 172 (±15) kJ/mol. This uncommonly high value of apparent activation energy is best explained on the basis of the macroexothermic reaction which takes place as niobium dissolves into liquid steel.