Oxidation phenomena of medium-Mn steels

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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.