Precipitation of Niobium Carbonitrides: Chemical Composition Measurements and Modeling M. Perez 1,a , E. Courtois 1,b , D. Acevedo 1,c , T. Epicier 1,d and P. Maugis 2,e 1 GEMPPM, UMR CNRS 5510, 25 av. Capelle, 69 621 Villeurbanne Cedex, France 2 ARCELOR Research SA, Voie Romaine, BP 30320, 57283 MaiziLres-lLs-Metz, France (present address: CIRIMAT-CNRS-INP, 118 route de Narbonne, 31077 Toulouse, France) a Michel.Perez@insa-lyon.fr, b Eglantine.Courtois@insa-lyon.fr, c Daniel.Acevedo@insa-lyon.fr, d Thierry.Epicier@insa-lyon.fr, e Philippe.Maugis@ensiacet.fr Keywords: Carbonitrides, Micro-alloyed steels, Precipitation, EELS, HRTEM. Abstract. High Resolution Transmission Electron Microscope and Electron Energy Loss Spectroscopy and have been used to characterize the structure and chemical composition of niobium carbonitrides in the ferrite of a Fe-Nb-C-N model alloy at different precipitation stages. Experiments seem to indicate the coexistence of two types of precipitates: pure niobium nitrides and mixed sub-stoichiometric niobium carbonitrides. In order to predict the chemical composition of these precipitates, a thermodynamical formalism has been developed to evaluate (i) the nucleation and growth rates (classical nucleation theory) and (ii) the chemical composition of nuclei and existing precipitates. A model based on the numerical resolution of former equations, is used to compute precipitates size distribution evolution at a given temperature. The predicted compositions are in very good agreement with experimental results. Introduction Microalloyed steels have received considerable interest over many years and continue to gain wider industrial applications. A small addition of niobium to steel is known to yield significant improvements in mechanical properties [1, 2, 3]. At high temperatures (1000C-1300C) niobium in solid solution retards austenite recrystallisation and grain growth. At lower temperatures niobium combines with free carbon and nitrogen to form a fine dispersion of niobium carbide or carbonitride precipitates in order to further inhibit austenite recovery and recrystallisation prior to the け/g transformation. The final effect is to increase the density of ferrite nucleation sites and thus reduce the final ferrite grain size. However, the detailed evolution of the precipitation in High-Strength Low Alloy (HSLA) steels is still only partly understood. For this reason, models have been developed in order to predict the influence of the process parameters on the state of precipitation. Precipitation models are generally based on the classical nucleation theory (see the review of Kampmann [4]), and treat simultaneously the nucleation, growth and ripening phenomena [5, 6, 7, 8, 9]. Output of these models are usually: the particle size distribution, their number and volume fraction. However, very few models predict the chemical composition of precipitates. Nevertheless, this theoretical approach needs experimental data to compare with. Several TEM works have already been published on such so-called High-Strength Low Alloys (HSLA) [10, 11, 3, 12, 13]. However, and owing to the previously mentioned difficulties, there is still a lack of comprehensive study on the evolution of the precipitation state as a function of the annealing time and temperature. In this paper, we will present quantitative measurements of precipitates chemical composition and structure that experimental results provide. A new thermodynamic formalism will be presented and included in a precipitation model in order to give a better understanding of the complex precipitation sequence experimentally observed. Materials Science Forum Vols. 539-543 (2007) pp 4196-4201 Online available since 2007/Mar/15 at www.scientific.net © (2007) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.539-543.4196 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 160.36.178.25, University of Tennessee, Knoxville, USA-01/09/14,07:33:43)