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Modeling the austenite decomposition into ferrite and bainite Fazeli, Fateh


Novel advanced high-strength steels such as dual-phase (DP) and transformation induced plasticity (TRIP) steels, are considered as promising materials for new generation of lightweight vehicles. The superior mechanical properties of these steels, compared to classical high strength steels, are associated with their complex micro structures. The desired phase configuration and morphology can only be achieved through well-controlled processing paths with rather tight processing windows. To implement such challenging processing stages into the current industrial facilities a significant amount of development efforts, in terms of mill trials, have to be performed. Alternatively, process models as predictive tools can be employed to aid the process development' and also to design new steel grades. Knowledge-based process models are developed by virtue of the underlying physical phenomena occurring during the industrial processing and are validated with experimental data. The goal of the present work is to develop an integrated microstructure model to adequately describe the kinetics of austenite decomposition into polygonal ferrite and bainite, such that for complex thermal paths simulating those of industrial practice, the final microstructure in advanced high strength steels can reasonably be predicted. This is in particular relevant to hot-rolled DP and TRIP steels, where the intercritical ferrite evolution due to its crucial influence on the onset and kinetics of the subsequent bainite formation, has to be quantified precisely. The calculated fraction, size and spatial carbon distribution of the intercritical austenite are employed as input to characterize adequately the kinetic of the bainite reaction Pertinent to ferrite formation, a phenomenological, physically-based model was developed on the ground of the mixed-mode approach. The model deals with the growth stage since nucleation site saturation at prior austenite grain boundaries is likely to be attained during the industrial treatments. The thermodynamic boundary conditions for the kinetic model were assessed with respect to paraequilibrium. The potential interaction between the alloying atoms and the moving ferrite-austenite interface, referred to as solute drag effect, was accounted for rigorously in the model. To quantify the solute drag pressure the Purdy- Brechet approach was modified prior to its implementation into the model. The integrated model employs three main parameters, the intrinsic mobility o f the ferrite-austenite interface, the binding energy of the segregating solute to the interface and its diffusivity across the transformation interface. These parameters are clearly defined in terms of their physical meaning and the potential ranges of their values are well known. However, no direct characterization techniques are currently available to precisely measure them hence they are treated as adjustable parameters in the model. The model predicts successfully the overall kinetics of ferrite formation in a number of advanced steels. The bainite evolution in different TRIP steels was analyzed using three available approaches, i.e. Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation, the Zener-Hillert formulation for diffusional growth and the displacive definition proposed by Bhadeshia. Overall, it turned out that the predictive capability of the three methodologies is similar. Further, some of the ensuing model parameters pertinent to each approach are difficult to interpret in terms of the underlying physics, which implies that all three models are employed in a semi-empirical manner. Assuming diffusional transformation mechanism for bainite, the isothermal incubation time and the onset of bainite formation during continuous cooling treatments were described adequately. Consistently, for the purpose of process modeling, the diffusional description of bainite growth can potentially be employed. However, from an academic point of interest a more precise quantification for the nucleation part is still missing.

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