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

中Mn TRIP钢由于其优异的抗拉强度（1-1.5GP）和伸长率（30-60%）的组合而受到全球越来越多的关注，后者是由于比传统TRIP钢更多的残余奥氏体，具有良好的稳定性。这类钢的关键生产工艺是临界间退火（IA），在此过程中，部分马氏体或铁素体向奥氏体反向转变，溶质元素在铁素体和奥氏体之间分配。这种转变至关重要，因为它决定了形成的奥氏体晶粒的比例和组成，即变形过程中可用于TRIP效应的残余奥氏体晶粒的数量和力学稳定性。因此，深入了解IA过程中的微观组织演变对于精确调整奥氏体晶粒的比例和稳定性以提高拉伸性能是必要的。然而，初步的研究表明，实验测量结果与经典扩散变换理论预测存在许多差异。(a) Mn原子的扩散和分拆速度远快于经典扩散变换理论；(b)该理论还预测了奥氏体中Mn浓度的急剧梯度，即。在奥氏体/铁素体相界面附近，经过短时间的反向转变后产生Mn尖峰，这一现象至今尚未得到实验证实。因此，这种巨大的差异对物理冶金学家构成了巨大的挑战。为了解开这个谜团，我们打算从这两个方面来研究反向变换。一是对相变过程中微观组织变化的精确、可靠的实验测量。在这种情况下，应该使用高分辨率透射电子显微镜（HR-TEM）和原子探针断层扫描（APT）精确测量相界面附近的Mn浓度分布；此外，用高能同步x射线衍射（SYXRD）原位测量相变动力学要比原位测量可靠得多。二是建立一种新的理论，既能阐明机理，又能定量模拟锰原子在逆转化过程中的快速配分。最后，本研究的成果对设计中锰钢的成分和IA工艺以获得更好的性能有很大的帮助。

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.