Gradient microstructure tailoring and synergistic deformation mechanisms in medium-manganese steel via high-ratio differential speed rolling: experimental and molecular dynamics simulation

Medium-Mn steel (MMS) was processed by high-ratio differential speed rolling (HRDSR) at speed ratios of 1:2, 1:3, and 1:4, producing a dual-phase gradient microstructure. With the ratio increasing from 2 to 4, the edge grain size refined from 1.23 to 0.86 μm and the core from 1.43 to 0.98 μm; microhardness correspondingly rose from 263 to 281 HV (edge) and from 228 to 248 HV (core), forming a V-shaped hardness profile. Yield strength (YS) progressively increased from 467 to 500 MPa; ultimate tensile strength (UTS) first increased from 719 to 787 MPa and then decreased to 770 MPa; total elongation (TE) peaked at 60% at ratio 3, achieving an optimal strength-ductility combination (787 MPa, 60%). During tensile deformation, the gradient microstructure induced spatially heterogeneous TRIP behavior: austenite in the core transformed more extensively and at earlier strain stages than at the edge. The fine-grained edge effectively impeded crack propagation; fracture initiated preferentially in the core. Molecular dynamics (MD) simulations independently validated the heterogeneous phase transformation sequence and crack propagation path.