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

减轻车辆重量和提高乘员安全性的要求推动了汽车工业用先进高强度钢(AHSS)的发展。中锰钢和高锰钢是这一领域的重要类别。与现有的钢材类似，用于白车身的钢种通常是镀锌以防止腐蚀。然而，镀锌AHSS钢板和零件的焊接会导致液态金属脆化，从而限制了防腐和耐撞性。对于极限抗拉强度超过800兆帕的钢来说尤其如此。因此，LME这一早已为人熟知的现象在过去20年再次成为钢铁供应商关注的焦点。含锰量增加的AHSS与现有钢种的不同之处在于有大量的奥氏体，通过奥氏体可以提高成形性和冲击性能(吸收冲击能量)。这可能是LME对其中一些AHSS来说是一个严重现象的核心原因，但这一点尚未明确确立。因此，本项目的任务是系统地研究LME中奥氏体在含锰高强度钢中的作用。该项目利用了之前在“钢-从头算”合作研究中心内开发的实验和计算技术，以及对其中锰钢的洞察，并将其转移到锌与这些钢中晶界的相互作用。尽管LME将被视为一个多尺度现象，但模拟和实验的选择将是原子尺度上与材料相关的过程将是焦点。为此，将使用从头算方法来揭示化学、结构和晶界退粘之间的相互作用。在实验方面，将原子探针层析成像与扫描电子显微镜和透射电子显微镜相结合，对其结构和化学进行了相关研究。所有关于奥氏体的研究将与铁素体中的类似情况进行比较，以解决这一阶段对LME的特殊作用。相应地，应用合作伙伴将提供、加工和表征各种钢种，这些钢种含有不同数量的奥氏体量，抗拉强度在800至1400兆帕之间。这一综合方法旨在确定LME的关键机制，以便系统地提高AHSS对这一影响的抵抗力。

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.