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

点击翻译按钮获取中文摘要

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.