Microstructure and tensile properties of chemically heterogeneous steel consisting of martensite and austenite

We made the chemical heterogeneity in steel consisting of mostly martensite with retained austenite by applying intercritical annealing prior to full austenitization. It enables simultaneous improvement in strength and ductility, which have been regarded as mutually exclusive properties. Contribution from two types of chemical heterogeneity could be identified; micro-scale and nano-scale heterogeneity inheriting the chemistry of intercritical austenite and Mn-enriched cementite respectively. The micro-scale chemical heterogeneity creates soft and hard domains in the martensitic microstructure by dissimilar solid solution strengthening, generating strain partition and back stress which contribute to the improvement of strength. On the other hand, the nano-scale chemical heterogeneity from highly Mn-enriched cementite evolved into Mn-enriched austenite in the final microstructure. The exceptional stability of nano-scale Mn-enriched austenite renders persistent strain hardening, contributing the enhanced ductility.