Influence of V Precipitates on Acicular Ferrite Transformation Part 2: Transformation Kinetics

Abstract

A combination of thermodynamic models as well as some physical metallurgical principles has been used to analyse the influence that V(C, N) precipitates have on the transformation kinetics of acicular ferrite by isothermal decomposition of austenite. Those precipitates were found absolutely necessary for the nucleation of acicular ferrite as reported in Part 1 of this work, now in Part 2, through the proper design of a heat treatment route we have studied the effect that those precipitates have on the kinetics of acicular ferrite transformation.

Structure and Strength of Isothermally Heat-Treated Medium Carbon Ti-V Microalloyed Steel

“…Results of previous research indicate that the fine interlocked structure of acicular ferrite has a beneficial effect on the toughness of the medium carbon microalloyed steels [9][10][11]. Vanadium-nitride particles, in addition to their role in precipitation hardening, are found to be highly effective as a preferential place for intragranular nucleation of acicular ferrite [12][13][14][15]. In this way, the addition of vanadium together with high nitrogen levels in the steel favors the formation of acicular ferrite at the expense of bainite [15] and thus acts in the direction of fracture toughness improvement.…”

“…Vanadium-nitride particles, in addition to their role in precipitation hardening, are found to be highly effective as a preferential place for intragranular nucleation of acicular ferrite [12][13][14][15]. In this way, the addition of vanadium together with high nitrogen levels in the steel favors the formation of acicular ferrite at the expense of bainite [15] and thus acts in the direction of fracture toughness improvement. However, there are questions about the strength of the steels with bainitic or acicular ferrite structure, which seems to be relatively low compared with the medium carbon microalloyed steels with ferrite-pearlite structure [7].…”