The study of austenite grain growth in Nb-Ti-V microalloyed steels after reheating during and after rough hot rolling

Abstract

The effect of thermomechanical controlled processing (TMCP) parameters and chemistry on the grain growth constants n, Q and A in the grain growth equation was studied on microalloyed steels with varying compositions of Nb, Ti and V. The two sets of steels used in the study consisted of industrially produced steels with varying additions of Nb, Ti and or V and laboratory produced steels with systematically varied compositions of Nb content. The tests conducted included reheating of steels at austenitizing conditions and deformation on the Bähr dilatometer to simulate the effect of deformation conditions on grain growth. In order to find a reasonable heating system, the precipitation and solubility behaviour of the microalloyed steels was simulated on ThermocalcTM for temperatures between 800 oC and 1500 oC. Experimentally measured data were obtained through TMCP by considering parameters such as austenitizing temperature, austenitizing time, delay time in between roughing passes and deformation temperatures. The data obtained from the experimentally measured austenite grain sizes in the heat treatment and the deformation processes were analysed and used in the determination of the grain growth constants n, Q and A which were then analyzed quantitatively as a function of the Nb content to develop constitutive equation for austenite grain growth prediction in the microalloyed steels. The mutual effects of particle pinning by Nb (C,N) and Ti (C,N) on grain growth kinetics were studied through simulation of the solubility of the precipitates using the thermodynamic software, ThermocalcTM and thermodynamic calculations. The particle dissolution, the undissolved particle coarsening and the changes in Nb solute in solution during reheating at isothermal heat treatment processes were taken into account in the constitutive equations through the experimentally measured grain sizes. The systematically varied Nb content in the second set of steels was used to study the effect of increasing microalloying element on grain growth in the steels. Comparative analysis of the results show that the constants generated under deformation conditions are slightly higher than those generated from reheating conditions. The activation energy for grain boundary migration, Q was found to be in the range of 254 and 572 kJ/mol, the grain growth exponent n ranged from 2.4 to 6.5 and the material and processing condition s constant A was found to range from 1.5 x 1011 to 4.96 x 1028. Constitutive grain growth equations that incorporate the initial grain size Do as well as the microalloying element (Nb) compositions in the prediction of austenite grain growth in microalloyed steels has been developed. Analysis of the influence of the initial grain size Do showed that any contribution of Do can be neglected unless it is about seventy percent (70%) or higher of the size of the measured austenite grain size D. A logical degree of precision in predicting austenite grain growth in microalloyed steels has been achieved in the current work from the comparison of experimentally measured grain sizes with predicted grain sizes using the constitutive grain growth equations developed in this study.