Liquid metal embrittlement by Zn in Mn-containing steels with significant austenite content (T07*)

降低车辆重量和提高乘客安全性的需求推动了汽车工业先进高强度钢（AHSS）的发展。中锰钢和高锰钢是这一领域的一个重要类别。与已建立的钢材类似，用于白色车身的钢种通常镀锌以防止腐蚀。然而，锌活性涂层AHSS钢板和部件的焊接可诱发液态金属脆化（LME），这限制了腐蚀保护和耐撞性。对于极限抗拉强度超过800 MPa的钢尤其如此。因此，LME长期以来的现象在过去二十年中再次成为钢铁供应商的焦点。具有增加的Mn含量的AHSS与已建立的钢种的区别在于大量的奥氏体，通过奥氏体，可成形性以及碰撞性能（碰撞冲击能量的吸收）得以增强。这可能是LME对其中一些AHSS来说是一种严重现象的核心原因，但这一点尚未明确确定。因此，本项目的使命是系统地研究奥氏体在含锰高强度钢中对LME的作用。该项目利用了先前在合作研究中心“Steel - ab initio”内开发的实验和计算技术以及对中锰钢的见解，并将其转移到Zn与这些钢中晶界的相互作用。虽然大规模海洋生态系统将作为一个多尺度现象来处理，但模拟和实验的选择将使原子尺度上与材料有关的过程成为焦点。为此，从头算方法将被用来揭示化学，结构和脱粘晶界之间的相互作用。在实验方面，通过结合原子探针断层扫描与扫描电子显微镜和透射电子显微镜进行结构和化学的相关研究。将奥氏体中的所有研究与铁素体中的类似情况进行比较，以解决该相在LME中的特殊作用。相应地，将由应用伙伴提供、加工和表征各种钢等级，所述钢等级包含不同量的奥氏体和800至1400 MPa之间的拉伸强度。该组合方法旨在确定LME的关键机制，以系统地提高AHSS对该效应的抵抗力。

The requirement of lowering vehicles weights and increasing passengers’ safety has motivated the development of advanced high strength steels (AHSS) for the automotive industry. Medium and high-Mn steels constitute an important class in this field. Similar to established steels, the steel grades that are used in body-in-white are commonly Zn coated to protect against corrosion. Welding of Zn actively coated AHSS steel sheets and parts, however, can induce liquid metal embrittlement (LME), which limits corrosion protection and crashworthiness. This is especially true for steels with an ultimate tensile strength exceeding 800 MPa. Therefore, the long-known phenomenon of LME has again come into the focus of steel suppliers in the last two decades. What distinguishes AHSS with an increased Mn content from established steel grades is the significant amount of austenite, through which formability as well as crash performance (absorption of crash impact energy) is enhanced. This could be a central reason why LME is a severe phenomenon for some of these AHSS, but this was not yet unequivocally established. The mission of the present project is, therefore, to systematically investigate the role of austenite for LME in Mn-containing high-strength steels. The project makes use of experimental and computational techniques that have previously been developed within the collaborative research center “Steel – ab initio” as well as insights into medium-Mn steels achieved therein, and transfers them to the interaction of Zn with grain boundaries in these steels. Though LME will be addressed as a multiscale phenomenon, the selection of simulations and experiments will be such that materials-related processes at the atomic scale will be in the focus. To this end, ab initio methods will be used to reveal the interplay between chemistry, structure and decohesion in grain boundaries. On the experimental side, a correlative study of structure and chemistry is performed by combining atom probe tomography with scanning electron microscopy and transmission electron microscopy. All the investigations in austenite will be compared to similar situations in ferrite in order to resolve the special role of this phase for LME. Correspondingly, a variety of steel grades will be provided, processed and characterized by the application partner that contains different amounts of austenite and a tensile strength between 800 and 1400 MPa. The combined approach aims identifying key mechanisms for LME in order to systematically improve resistance of AHSS to this effect.