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High-nitrogen, high-manganese austenitic steels because of their non-magnetic behavior and corrosion resistance in combination with high strength and toughness serve various applications. The Schaeffler diagram is commonly used for predicting the as-cast microstructures but it does not reflect the nature of the austenite associated with the sequence of phase formation during solidification. Thermodynamic simulations are an aid to understanding the metallurgical nature of austenite. In this study, the microstructure was revealed by tint etching with examination using polarized light with the light optical microscope. The fragments of dendritic arms that share a common crystallographic orientation were identified by this technique. Two kinds of austenitic microstructures, both with grains formed by sharply defined primary and secondary dendritic arms and with grains having a blurred dendritic pattern, were observed. Solidification through austenite freezes the chemical inhomogeneity, whereas solidification through δ-ferrite, with subsequent transformation of δ-ferrite to austenite, leads to a “blurring” of the dendritic structure. These two kinds of microstructures were interpreted by a Schaeffler–Spiedel diagram which is modified in this paper. Electrochemical investigations showed that the chemical homogeneity of the austenite obtained by primary δ-ferrite solidification exhibited improved corrosion properties.