Modelling the Kinetics of Austenite Decomposition in Nb-Ti-V Microalloyed Steel after Hot Strip Rolling

摘要

In hot strip rolling of Nb-microalloyed steels the austenite transformation usually starts in the hot run-out table (HRT) under a high cooling rate and finishes while the strip is coiled, when the cooling rate is much slower. Thus the transformation takes place during a two-step-cooling regime. This paper presents a mathematical model able to predict such transformation. The experimental alloy used here was a. Nb-V-Ti microalloyed steel. The model employs an Avrami type equation associated with the additivity rule. The time constant b in the Avrami equation was expressed as a function of parameters A and B whereas the time exponent n was best modelled as a constant of value 1. Experiments have shown that the constants A and B are linearly dependent. An artificial neural network (ANN) model was used to predict B. The ANN model takes into account the following process variables: austenite deformation applied above the non-recrystallization temperature (Tnr), deformation applied below Tnr, cooling rate in the HRT, and coiling temperature. From the results it was proposed that the transformation is governed by early site saturation, diffusion being the only operating mechanism. This conclusion was based on the experiment value found for n and on the need of a single temperature dependent parameter, either A or B, to predict the course of the overall austenite transformation.