CHARACTERIZATION OF FERRONIOBIUM AND THE THERMODYNAMICS AND KINETICS OF DISSOLUTION OF NIOBIUM COMPOUNDS IN LIQUID IRON

Abstract

<p>Solidification of Nb-microalloyed HSLA steels may result in the precipitation of niobium carbonitrides, which is hardly surprising in view of their extreme thermodynamic stability. Recently, it was proposed in literature that coarse Nb-rich particles found along the centerline of continuously cast HSLA steels originated from ferroniobium additions during ladle metallurgy. In particular, it was hypothesized that thermally stable phases formed during manufacturing of ferroniobium were released into the melt once the ferroniobium had partially fused. In this contribution, Scheil–Gulliver formalism is employed to predict the phase portrait of ferroniobium in an attempt to simulate the manufacturing process of ferroniobium.</p> <p>To corroborate the predictions, the microstructure of ferroniobium is characterized to determine if thermally stable particles exist in ferroniobium. Further, a model is developed to predict the dissolution rate of thermally stable phases that were observed in ferroniobium as well as in the centerline region of as-cast HSLA steel. Finally, a sample near the centreline region of a Nb-microalloyed HSLA steel is characterized and centreline compositions are measured. Based on experimental evidences, an alternative explanation to the origin of thermally stable particles found near the centreline of HSLA steels is proposed.</p>