Abstract：A novel medium-Mn automotive steel has been designed to achieve unprecedented mechanical properties with tensile yield strength above 1.0 GPa and the product of strength and elongation exceeding 60–75 GPa·%. The tensile behavior is characterized by inhomogeneous deformation, which shows an incipient yield drop effect followed by a large Lüders straining and then several Portevin-Le Chatelier bandings during work-hardening stage. The multiple work- hardening mechanisms behind the unique mechanical properties are revealed by EBSD, TEM, 3D-ATP, and in-situ DIC observations. These results indicate that the inhomogeneous martensitic transformation occurs only within the local deformation zone, and only a small amount of martensite is required for the generation and propagation of Lüders and PLC microbands, which can provide a sustainable transformation-induced plasticity (TRIP) effect prolonging to a larger strain range. Consequently, not only the microband propagation, but also its induced local phase transitions play an important role in the multiplication of mobile dislocations, both of which provide a high density of mobile dislocations to enable strong strain hardening capabilities and the resulting unique mechanical properties.
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