Simulation of intercritical annealing in low-alloy TRIP steels

Abstract

A coupled thermodynamic/kinetic calculation of austenite formation during intercritical annealing of low‐alloy TRIP steels is presented. The simulation was performed with the use of Dictra computational kinetics software, which employs a procedure for the numerical solution of the coupled diffusion equations involved, as well as mobility databases for the retrieval of the appropriate kinetic data. Calculated results are compared with available experimental data, in order to evaluate the model. Simulation results, regarding the amount and composition of austenite, the rate of transformation and the effect of annealing temperature and heating conditions, are presented and discussed. It is concluded that the simulation can assist the design of the intercritical annealing in these steels.

Fundamentals and application of solid-state phase transformations for advanced high strength steels containing metastable retained austenite

“…Austenite formation process is complicated and consists of three more or less consecutive steps [66,67]: (i) the very rapid transformation of pearlite into a carbon enriched austenite of more or less the same dimensions as the pearlite colony, due to the high C activity of the cementite and the small ferrite-cementite spacings, (ii) the fast growth of this austenite into the surrounding ferrite matrix with kinetics determined by carbon diffusion with only a spike of M at the interface and (iii) the very slow austenite growth dictated by the diffusion of M. As the stage (iii) is extremely slow, and thus the transition from stage (ii) to (iii) is of practical interest. The transition between stage (ii) and (iii) during the austenite formation starting from a ferrite-pearlite microstructure has also been extensively modelled using a multi-component diffusional approach [54,68] and the evolution of the critical concentrations are illustrated in Fig. 6.…”

Prediction and Validation of the Austenite Phase Fraction upon Intercritical Annealing of Medium Mn Steels

“…The models should be capable to handle different initial microstructures corresponding to different process routes and to capture the effects of key alloying elements such as C, Si, and Mn. [9,10] It has been shown that in steels with a low concentration of Mn, the Mn enrichment at the moving austeniteferrite interface is of crucial importance in determining the transformation kinetics. [11][12][13][14][15][16][17] Three main concepts have been formulated to describe the transformation between austenite and ferrite in the two-phase region: (i) Full equilibrium (FE) [18] in which all alloying elements redistribute till equilibrium is reached everywhere in system.…”

Present Status and Future Prospects of Simulation Models for Predicting the Microstructure of Cold-rolled Steel Sheets

“…Detailed studies on the reaustenitization behavior of steels have already been carried out, 18,20, and a number of predictive models have been developed. [28][29][30][31][32][33][34][43][44][45][46][47][48][49][50][51][52][53][54] In most of these cases, the microstructure before annealing consisted of ferrite-pearlite or ferrite-cementite. Speich et al 28) observed the evolution of the microstructure of steel with a ferrite + pearlite microstructure and classified the austenite transformation behavior into three stages: (1) very rapid growth of austenite into pearlite leading to complete dissolution of pearlite, (2) slower growth of austenite into ferrite that was primarily controlled by carbon diffusion in the austenite, and (3) very slow growth of austenite controlled by the diffusion of substitutional alloying elements in austenite.…”