Oxidation phenomena of medium-Mn steels

中锰钢，也被称为第三代先进高强度钢，具有通过热机械处理对其性能进行应用导向调整的非凡潜力。由于锰含量高，在空气冷却期间形成的马氏体在亚温退火期间转化为奥氏体，其稳定性通过元素再分布（分配）而增加。在最终冷却期间，形成除了高延展性之外还表现出显著的应变硬化潜力的显微组织。然而，相对较高的锰浓度既有利又有害。锰对氧具有高亲和力，这导致在热处理期间甚至在低氧分压下形成氧化物层，这进而严重损害防腐蚀涂层的附着力。这些钢在几分钟的极短退火时间的热机械处理期间的氧化行为的知识主要基于工业经验，而长期氧化行为，特别是发电厂钢，已经被定量研究并且被广泛理解。在本项目范围内，中锰钢在400°C至900°C温度下的早期氧化阶段将通过实验量化，并结合相关的传输和相形成过程进行描述。为了阐明Mn、Si和Al对氧化和相形成的重要性，生产了具有精确控制成分的高纯度实验室熔体，并专门调整了微观结构。氧化动力学的分析、形成的氧化物相及其形态的局部识别以及氧化物和金属中浓度分布的局部和整体确定形成了基于有限差分和CALPHAD方法的数值模型的基础，这些钢的氧化行为应该变得可预测。根据高纯度实验室熔体的结果，选择有限数量的工业熔体，并与通常的伴随元素一起生产，以测试所确定的机制的可转移性。在工业熔体的基础上，通过适当的过程控制，确定中锰钢可以退火到适合热浸镀锌的表面的程度。该项目的结果预计将提供通过中锰钢的微观结构以及通过控制温度和环境气氛来经济有效地控制不同氧化物相的形成的途径。此外，在氧化的早期阶段优先形成合适的氧化物相，氧化物的形成将被最小化，这减少了材料损失并改善了CO2平衡。

Medium Mn-steels, also referred to as 3rd generation Advanced High Strength Steels, have an extraordinary potential for an application-oriented adjustment of their properties via thermomechanical treatment. Martensite, which form during air cooling due to the high content of manganese, is transformed into austenite during intercritical annealing, the stability of which is increased by elemental redistribution (partitioning). During the final cooling, a microstructure is formed which, in addition to high ductility, also exhibits a pronounced strain hardening potential. However, the comparatively high manganese concentration is both a beneficial and detrimental. Manganese has a high affinity for oxygen, which leads to the formation of an oxide layer during heat treatment, even at low oxygen partial pressures, which in turn severly impairs the adhesion of anti-corrosion coatings. Knowledge of the oxidation behavior of these steels during such thermomechanical treatment with comparably short annealing times of a few minutes is largely based on industrial experience, whereas the long-term oxidation behavior, especially of power plant steels, has already been quantitatively investigated and is widely understood. Within the scope of this project, the early stage of oxidation of medium manganese steels at temperatures between 400°C and 900°C is to be experimentally quantified and described with regard to the relevant transport and phase formation processes. To clarify the importance of Mn, Si and Al for oxidation and phase formation, high-purity laboratory melts with a precisely controlled composition are produced and the microstructure is specifically adjusted. The analysis of the oxidation kinetics, the local identification of formed oxide phases and their morphology as well as the local and integral determination of the concentration distribution in the oxide and metal form the basis for a numerical model based on finite differences and the CALPHAD method, with which the oxidation behavior of these Steels should become predictable. Based on the results of the high-purity laboratory melts, a limited number of industrial melts are selected and produced with the usual accompanying elements in order to test the transferability of identified mechanisms. On the basis of industrial melts, the extent to which medium-Mn steels can be annealed with a surface suitable for hot-dip galvanization is determined using suitable process control. The results of the project are expected provide access to cost-effectively controlling the formation of different oxide phases through the microstructure of a medium-Mn steel and by controlling the temperature and the ambient atmosphere. In addition, preferential formation of suitable oxide phases in the early stages of oxidation, the formation of oxide is to be minimized, which reduces material loss and improves the CO2 balance.