Characterizing and modeling on microstructural evolution during intercritical annealing of high performance medium Mn steel

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The medium Mn TRIP steels have received increasing global attention due to their excellent combination of the tensile strength (1-1.5GP) and elongation (30-60%), the latter results from larger fraction of retained austenite with good stability than the classical TRIP steels. The key production process for this type of steels is the intercritical annealing (IA), during which part of the martensite or ferrite will be reversely transformed to austenite and the solute elements be partitioned between ferrite and austenite. This transformation is crucial as it determines both the fraction and composition of austenite grains formed, i.e. the amount and mechanic stability of retained austenite grains available for TRIP effect during deformation. Therefore, a deep insight into the microstructural evolution during IA should be necessary for accurately tailoring the fraction and stability of austenite grains retained for improved tensile properties. However, the preliminary researches have revealed many discrepancies between experimental measurements and the theoretic prediction by the classical diffusive transformation theory. (a) Mn atoms could diffuse and partition much faster than that expected by the classical diffusive transformation theory; (b) The theory also predicts a sharp Mn concentration gradient in austenite，i.e., Mn spike, near the austenite/ferrite phase interface after a short period of reverse transformation, which have not been experimentally confirmed until now. Therefore, such great discrepancies form a big challenge for physical metallurgist. In order to solve this riddle, we plan to study the reverse transformation from the two aspects. One is the precise and reliable experimental measurements on the microstructural change during transformation. In this case, high-resolution transmission electron microscopy (HR-TEM) and atom probe tomography (APT) down to atomic level should be used to accurately measure the Mn concentration profile near the phase interface; moreover, an in-situ measurement by high-energy synchrotron X-ray diffraction (SYXRD) on the transformation kinetics will be much more reliable than the ex-situ one. The other is for us to set up a new theory which can not only elucidate the mechanism but also quantitatively model the rapid partition of Mn atoms during the reverse transformation. Finally, the output of this research shall greatly help to design the composition and the IA process of medium Mn steels for better properties.