The Effect of NbC Precipitates and Nb in Solid-Solution on the Phase Transformation Kinetics in High-Purity Fe-C-Mn-Nb Alloys

Abstract

The precipitation of NbC in austenite is an important mechanism for improving the strength of steel because NbC-precipitates are known to decrease the ferrite grain size during the subsequent phase transformations upon cooling. The effect of the interaction between niobium (Nb) in solid solution and NbC-precipitates on the austenite-to-ferrite phase-transformation kinetics is not entirely clear. We study a high-purity Fe-C-Mn-Nb alloy cooled at different rates. Different annealing times at 850°C were applied to create different number densities and sizes of the NbC-precipitates in order to study the effect of NbC precipitation on the transformation kinetics. The alloy that is used in this study has an atomic ratio of Nb:C=1.3:1. The fraction of ferrite is measured as a function of temperature during cooling by means of dilatometry. The ferrite grain size is measured by means of optical microscopy. The results are interpreted with thermodynamic and kinetic models.

Effects of Solute Nb Atoms and Nb Precipitates on Isothermal Transformation Kinetics from Austenite to Ferrite

“…Nb has a strong tendency to form a carbo-nitride, and thus there are two typical forms of Nb present in steels during steel manufacturing, which are solute Nb atoms and Nb carbo-nitride precipitates [1][2][3][4][5]. Nb addition in steels is beneficial for mechanical properties by grain size refinement and precipitation strengthening.…”

“…Nb addition in steels is beneficial for mechanical properties by grain size refinement and precipitation strengthening. Nb can also be used to control the transformation kinetics, and thus hardenability is improved by the presence of solute Nb [1][2][3][4][5][6][7].…”