Dissolution of FeNb in liquid steel and best practices to increase niobium recovery during ladle refining

This paper presents a microstructural characterization of standard ferroniobium ( FeNb) and a study of its dissolution in steel. Interrupted dissolution trials at different temperatures were carried out in a laboratory induction furnace. The microstructure at the interface between solid FeNb and frozen steel was analyzed and compared with the original FeNb particles. The results allowed clarifying both the FeNb dissolution mechanism and its incorporation kinetics at steelmaking temperatures. Experimental results from the dissolution of standard FeNb in liquid steel proved that the dissolution rates of FeNb particles were higher than those reported in the pertinent literature,where models assume that FeNb dissolves only by the diffusion of solid particles due to its high melting temperatures ( over 1 600℃ ). Most recent FeNb phase diagrams and DTA analyses have shown that standard FeNb begins to melt down below 1 600℃. The experimental evidences in this work have led to the adoption of a new description of the dissolution mechanism whereby standard FeNb dissolves predominantly by the melting of its eutectic zones at 1 500℃ to 1 510℃ and,later on,by the melting of the primary laths between 1 530℃ and 1 575℃ ,i. e. ,below the steelmaking temperatures of around 1 600℃. Based on these results and on previous melting-shop experience,this paper recommends the best practices for the addition of FeNb during the microalloyed steels secondary refining phase.