Influence of co-existing medium Mn and dual phase steel microstructures on ductility and Lüders band formation

The mechanical property spectrum of steels can be expanded to new limits through combining the characteristic microstructures of different steel grades in a single steel. Here, we demonstrate this microstructural grading concept in a single steel sheet that possesses microstructures representative of two steel grades, i.e., the duplex medium Mn steel and the dual-phase steel, in the form of a multi-layered structure. This graded steel was produced by applying the surface mechanical attrition treatment to a duplex medium Mn steel with metastable retained austenite. The resulting graded steel collects the respective advantages of the two steels: high ductility of the former, and the Lüders-band-free plastic flow of the latter. By carrying out systematic experiments and finite element simulations, we reveal that the suppression of Lüders band formation in this graded steel is due to the high early-stage strain hardening rate of the dual-phase steel microstructure, which offsets the post-yield strain softening tendency of the medium Mn steel microstructure. The ductility, on the other hand, results from the transformation-induced plasticity effect of the medium Mn steel microstructure, which enhances the overall strain hardening rate and suppresses the necking of the dual-phase steel microstructure at large strain levels.