Optimizing ductility of medium-Mn Q&P steel via heterogeneous microstructure induced by asynchronous recrystallization strategy

Abstract

This study proposes a novel asynchronous recrystallization-controlled strategy via intercritical warm-rolling (IWR) to fabricate high strength medium-Mn quenching-partitioning (Q&P) steel with excellent ductility. Owing to the low strain energy storage induced by the preliminary IWR process, asynchronous recovery and recrystallization between ferrite and martensite are triggered during the subsequent Q&P process. Mn-enriched martensite with high dislocation density preferentially recrystallizes, while Mn-depleted ferrite with low dislocation density undergoes incomplete recrystallization, remaining as banded, elongated grains that reduce effective grain boundary density. This ferrite not only slows austenitic interface migration during subsequent austenitization to suppress excessive grain coarsening but also forms an initial alternating banded structure of Mn-depleted ferrite and Mn-enriched martensite along the rolling direction, establishing Mn distribution heterogeneity. Furthermore, the slower recrystallization of Mn-depleted ferrite ensures the inheritance of this banded Mn heterogeneity. During the partitioning stage, Mn-depleted zones further induce the formation of carbide-free bainite (CFB), ultimately generating a heterogeneous Q&P microstructure composed of Mn-depleted CFB/tempered martensite with low geometrically necessary dislocation (GND) density and Mn-enriched martensite (hard phase with high GND density), alongside retained austenite (RA) in diverse morphologies. This heterogeneous microstructure enables multi-mechanism collaboration: grain refinement, GND heterogeneity and sustained TRIP effect. The intercritical warm-rolling Q&P (WQP) specimen thus achieves tensile strength of 1362 MPa, yield strength of 1129 MPa, total elongation of 22%, and a product of strength and elongation (PSE) of 30 GPa·%. This study provides a novel process approach for the coordinated optimization of strength and ductility in medium-Mn Q&P steel.