New insights in understanding the interaction between recrystallization and phase transformation during intercritical annealing in Dual Phase steels
Type d'événement
Soutenance de thèse
Thèse soutenue par Clélia COUCHET
The formation of microstructures is a crucial step for steelmakers. In the case of DP steels, used for automotive construction, this formation takes place during intercritical annealing after cold-rolling. During this thermal treatment, after the heating step, the microstructure is made of recrystallized ferrite and austenite. During cooling, the austenite partially transforms into ferrite and then into martensite to reach the expected final ferrite/martensite microstructure. The austenitization step is therefore crucial for the manufacturers of these steels, to control the final phase fractions and sizes and, consequently, their mechanical properties. Numerous studies show that the heating rate controls the transformation kinetics and the morphology of the austenite ("necklace" or "banded"), but the underlying mechanisms remains a bone of contention. The overlap between ferrite recrystallization and austenite formation is often made responsible for these effects, through different mechanisms.
Using recent advances in in situ experiments on synchrotron beamlines, this PhD proposes a new insight in the understanding of the interactions between ferrite recrystallization and austenite formation and develops a predictive model for the austenite formation kinetics.
The main experimental development of this thesis is a new coupled time-resolved analysis technique, based on in situ High-Energy X-Ray Diffraction to track recrystallization and phase transformations during the annealing phase, including at high heating speeds. Our new method, called Isolated Diffraction Spot Tracking (IDST), is first validated to study recrystallization on model ferritic steels. These in situ measurements are supplemented by observations of microstructures after interrupted treatments in microscopy (optical, Scanning Electron Microscopy and Transmission Electron Microscopy), and from local chemistry measurements (Energy-Dispersive X-ray Spectroscopy and Wavelength Dispersion Spectroscopy).
We first reproduce experiments to study the influence of the heating rate on the studied steel during the intercritical annealing. In such experiments, the overlap between ferrite recrystallization and austenite formation is governed by the heating rate. To go further, we designed experiments to decorrelate the effect of the heating rate and this overlap. During these, the heating rate is fixed to maintain the same conditions for thermo-activated mechanisms, but the niobium microalloying and lower cold-rolling ratio are used to delay ferrite recrystallization. These experiments show unambiguously that austenite transformation kinetics is not controlled by the recrystallization, but by the sole thermodynamic condition of interfaces and maybe by the diffusion distance in the microstructures.
Finally, we propose a detailed thermo-kinetics analysis of the mechanisms of austenite formation during the intercritical annealing based on DICTRA/Thermo-Calc simulations and on our experimental work. The effect of minor alloying elements on the austenite growth kinetics is investigated. This work finally proposes new predictive models for austenite formation during the intercritical annealing.
This PhD work finally shows no significant effect of the concomitance of the two studied mechanisms on the austenite formation kinetics along the heating stage. We demonstrate that the austenite formation kinetics is diffusion-controlled. The difference in austenite formation kinetics along the holding stage is explained by microstructural considerations, affecting the diffusion distances.
Date
Date de fin
Lieu
Nancy, Campus ARTEM, Ecole des Mines, Salle A006