The deformation behavior of duplex low-density steel was analyzed by correlation between macroscale uniaxial tension and nanoscale indentation. A dramatic difference in the tensile behavior was observed between two specimens obtained under specific heat-treatment conditions, despite these specimens having the same chemical composition and similar microstructures. In order to understand this difference, the intrinsic mechanical properties of each phase were analyzed based on nanoindentation results considering the Hall–Petch relationship. In addition, the mechanical stability of retained austenite was investigated by in situ electron backscattered diffraction.