Here, we attempted to improve the toughness of medium-Mn steels through implementation of nano-sandwich microstructure via warm rolling. The hot-rolled and annealed (HRA) specimen exhibits the martensite and austenite with multi-variant laminate morphology. The warm-rolled (WR) specimen shows similar microstructure and phase fraction with HRA, however, it reveals the single directional laminate structure, which is aligned to rolling direction. The resulting microstructure is called nano-sandwich structure. The HRA shows the poor toughness, because the crack propagated rapidly along the PAGBs weakened by Mn segregation. However, WR showed improved toughness relative to HRA, because the cracking is suppressed by breaking of PAGBs during warm deformation. Besides, the crack could propagate in a direction perpendicular to the crack propagation, reducing the propagation energy and increasing the toughness; this phenomenon is called delamination. Therefore, it is expected that warm rolling is suggested as novel route for damage tolerant microstructural design of medium-Mn steel.