Abstracts Category : Other

A New View of the Grain-Coarsening Behavior of Austenite in Ti-Microalloyed Low-Carbon Steels

by Victor Blancas-Garcia

The role of Microalloying Element (MAE) additions to high strength low-allow (HSLA) steels has been well-documented since their commercial introduction in the late 1950s. Because of the benefits of MAE in terms of grain size control, among other metallurgical reactions, it has been important to study, understand and predict the role of MAE content on the grain coarsening behavior under different thermal cycles. Titanium content has shown beneficial effects in terms of austenite grain size control by the grain boundary pinning effect of TiN particles due to the low solubility of these precipitates at high temperatures. The thermodynamical stability of TiN is mainly influenced by the contents of both Ti and N which leads to hypo-stoichiometric and hyper-stoichiometric conditions for TiN particles nucleation, coarsening, and dissolution. Prior Austenite Grain Size studies and precipitation analysis have shown that these stoichiometric conditions play a role on the kinetics of the grain coarsening behavior under different thermal cycles by means of the available solute concentration in the matrix as predicted by particle coarsening theories and grain growth models. A systematic analysis of the grain boundaries character distribution (GBCD) by Electron Backscattered Diffraction and Electron Microscopy during the dissolution process of TiN precipitates has been conducted. These observations provided a better understanding of the role of GBCD in the presence or absence of TiN particles located at the matrix and along the austenite grain boundaries. The results obtained from this study gave a strong correlation between the high angle boundaries with misorientations between 20 and 45 and the estimated pinning force exerted by TiN particles, providing therefore a new view for studying the kinetics of austenite grain coarsening behavior for different MAE contents.