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

中锰TRIP钢由于其优异的抗拉强度（1-1.5GP）和延伸率（30-60%）的组合而受到越来越多的全球关注，后者是由于比经典TRIP钢具有更大比例的具有良好稳定性的残余奥氏体。这类钢的关键生产工艺是亚温退火（IA），在此过程中，部分马氏体或铁素体将向奥氏体转变，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分数和组成，即变形过程中可用于TRIP效应的残留奥氏体晶粒的数量和机械稳定性。因此，深入了解IA过程中的微观结构演变应该是必要的，以准确定制的分数和稳定性的奥氏体晶粒保留改善拉伸性能。然而，初步的研究表明，实验测量与经典扩散相变理论的理论预测之间存在许多差异。(a)Mn原子扩散和分配的速度比经典扩散转变理论所预期的要快得多;（B）该理论还预测奥氏体中Mn浓度梯度很大，即，Mn尖峰，奥氏体/铁素体相界面附近的一个短暂的时间后，逆相变，这还没有实验证实，直到现在。因此，这种巨大的差异对物理学家形成了巨大的挑战。为了解开这个谜团，我们计划从这两个方面来研究逆变换。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，高分辨率透射电子显微镜（HR-TEM）和原子探针层析成像（APT）下降到原子水平，应使用精确测量的Mn浓度分布附近的相界面;此外，通过高能同步辐射X射线衍射（SYXRD）的相变动力学的原位测量将比非原位的更可靠。二是建立一个新的理论，不仅可以解释这一机制，而且可以定量地模拟Mn原子在逆相变过程中的快速分配。最后，本研究的结果将大大有助于设计中锰钢的成分和IA工艺，以获得更好的性能。

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.