Medium-Mn steels have attracted immense attention for automotive applications owing to their outstanding combination of high strength and superior ductility. This steel class is generally characterized by an ultrafine-grained duplex microstructure consisting of ferrite and a large amount of austenite. Such a unique microstructure is processed by intercritical annealing, where austenite reversion occurs in a fine martensitic matrix. In the present study, austenite reversion in a medium-Mn alloy was simulated by the multiphase-field approach using the commercial software MICRESS® coupled with the thermodynamic database TCFE8 and the kinetic database MOBFE2. In particular, a faceted anisotropy model was incorporated to replicate the lamellar morphology of reversed austenite. The simulated microstructural morphology and phase transformation kinetics (indicated by the amount of phase) concurred well with experimental observations by scanning electron microscopy and in situ synchrotron high-energy X-ray diffraction, respectively.

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Austenitic high manganese steels exhibit outstanding mechanical properties, such as high energy absorption, owing to various deformation-mechanisms such as dislocation slip, twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP). Here, we show a novel thermomechanical treatment to manufacture a high manganese steel Fe–18Mn-0.3C (wt.-%) with excellent mechanical performance by combining these three deformation-mechanisms. This process of mechanism-controlled rolling resulted in ultra-high tensile strength of the high manganese steel up to 1.6 GPa, simultaneously with uniform elongations up to 15%. A thermomechanical process was developed to establish this combination of properties. Warm rolling was conducted at 200 °C, to suppress TRIP and activate TWIP as deformation mechanism. Thus, a high density of deformation twins and dislocations was introduced to the microstructure, avoiding martensite formation. During a subsequent recovery annealing at 520 °C or 550 °C, the dislocation density was reduced, yet the high density of deformation twins was preserved. The combination of warm rolling and recovery annealing resulted in an ultrafine microstructure with a high density of twins and moderate density of dislocations. The TRIP effect is predominant during plastic deformation at ambient conditions in the highly twinned microstructure. The resulting steel exhibits an ultra-high yield strength and sufficient ductility, favorable properties for lightweight construction in automotive or aerospace industry.

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The combination of different phase constituents to realize a mechanical composite effect for superior strength-ductility synergy has become an important strategy in microstructure design in advanced high-strength steels. Introducing multiple phases in the microstructure essentially produces a large number of phase boundaries. Such hetero-interfaces affect the materials in various aspects such as dislocation activity and damage formation. However, it remains a question whether the characteristics of phase boundaries, such as their chemical decoration states, would also have an impact on the mechanical behavior in multiphase steels. Here we reveal a phase boundary segregation-induced strengthening effect in ultrafine-grained duplex medium-Mn steels. We found that the carbon segregation at ferrite-austenite phase boundaries can be manipulated by adjusting the cooling conditions after intercritical annealing. Such phase boundary segregation in the investigated steels resulted in a yield strength enhancement by 100–120 MPa and simultaneously promoted discontinuous yielding. The sharp carbon segregation at the phase boundaries impeded interfacial dislocation emission, thus increasing the stress required to activate such dislocation nucleation process and initiate plastic deformation. This observation suggests that the enrichment of carbon at the phase boundaries can enhance the energy barrier for dislocation emission, which provides a favorable condition for plastic flow avalanches and thus discontinuous yielding. These findings extend the current understanding of the yielding behavior in medium-Mn steels, and more importantly, shed light on utilizing and manipulating phase boundary segregation to improve the mechanical performance of multiphase metallic materials.

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Medium-Mn steel is the newly developed steel acting as a promising candidate of the 3rd-generation advanced high strength steels. In the present study, the effect of hydrogen on the mechanical behavior and martensite transformation process in a duplex medium-Mn steel is investigated by in-situ nanoindentation test. The mechanical response of individual phase: ferrite, and retained austenite by introducing hydrogen is studied. With the presence of hydrogen, the reduction of activation energy for dislocation nucleation was verified by using a stress-biased, statistical thermal activation model. The stacking fault energy (SFE) was reduced, which was revealed by electron channeling contrast (ECC) technique. Magnetic force microscopy (MFM) revealed a suppression of retained austenite to α′-martensite transformation with the presence of hydrogen, which is related to hydrogen induced SFE reduction and enhanced slip planarity.

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The ultrafine-grained (UFG) duplex microstructure of medium-Mn steel consists of a considerable amount of austenite and ferrite/martensite, achieving an extraordinary balance of mechanical properties and alloying cost. In the present work, two heat treatment routes were performed on a cold-rolled medium-Mn steel Fe-12Mn-3Al-0.05C (wt.%) to achieve comparable mechanical properties with different microstructural morphologies. One heat treatment was merely austenite-reverted-transformation (ART) annealing and the other one was a successive combination of austenitization (AUS) and ART annealing. The distinct responses to hydrogen ingression were characterized and discussed. The UFG martensite colonies produced by the AUS + ART process were found to be detrimental to ductility regardless of the amount of hydrogen, which is likely attributed to the reduced lattice bonding strength according to the H-enhanced decohesion (HEDE) mechanism. With an increase in the hydrogen amount, the mixed microstructure (granular + lamellar) in the ART specimen revealed a clear embrittlement transition with the possible contribution of HEDE and H-enhanced localized plasticity (HELP) mechanisms.@en