| High Mn steels attract many attentions and show great potential as structural components in automotive engineering due to their complex deformation microstructures and exceptional mechanical properties.These microstructures and properties are based on different deformation mechanisms,i.e.transformation-induced plasticity?TRIP effect?and twinning induced plasticity?TWIP effect?,during plastic deformation.These high Mn steels are also named as TRIP steels or TWIP steels.In this paper,the effects of grain size,deformation mode and temperature on deformation mechanism of TRIP/TWIP steels were well-studied using transmission electron microscope?TEM?,X-ray diffraction?XRD?,electron back-scatter diffraction?EBSD?and mechanical tests,i.e.tensile,hardness and Charpy impact tests.Moreover,this paper also investigated the relationship between microstructures and mechanical properties in TRIP/TWIP steels.Finally,the nature of strength and toughness was uncovered.Base on the strengthening-toughening mechanism of TRIP/TWIP steels,the surface of TRIP/TWIP steel samples was strengthened by surface mechanical grinding treatment?SMGT?.Furthermore,this paper studied the thermo-mechanical process of medium-manganese TRIP steel to improve the yielding strength of TRIP steel and alleviate the economic concerns.The main conclusions are drawn.?1?The strain dependence of Hall-Petch relations in Fe-20/25/29Mn-3Al-3Si steels was investigated.The strengthening mechanism of these steels was also revealed,which is that the strengthening mechanism transforms from grain boundary strengthening to intragranular strengthening with increasing the strain,and the latter is attributed to deformation twinning and phase transformation.?2?TEM results showed that high tensity of dislocations and SFs appeared near grain boundaries at the early stage of tensile deformation in Fe-20/25/29Mn-3Al-3Si steels.With the increase of strain,phase transformation or twinning takes place inside grains.The products of phase transformation and deformation twinning provide barriers to dislocation motion and thus gradually decreasing their mean free path.They also afford room for storage of dislocations at the same time.The two respects contribute to high strain hardening rate of three high Mn steels.?3?The defective structures of deformation twins in Fe-29Mn-3Al-3Si TWIP steel were characterized,and the strengthening-toughening mechanism of deformation twins was revealed.Large numbers of SPDs slip parallel to the TB plane,which greatly improve the plasticity of TWIP steels.On the other hand,twin boundaries interact with partial dislocations and store massive dislocations,which sustain pronounced strain hardening.?4?Deformation twins show high thermal stability.TEM observations indicate that there are three detwinning mechanisms during annealing treatments,i.e.the motion of?3{112}ITBs,the dissociation of dislocations and recrystallization.The motion of?3{112}ITBs is most efficient among three detwinning mechanisms.?5?In contrast to the typical fine-grain strengthening behavior,an inverse grain size dependence of impact toughness in Fe-25Mn-3Al-3Si steel was reported.This phenomenon has been attributed to the grain size effect,i.e.increasing the grain size benefits the TRIP/TWIP effect.Base on structural characterizations and thermodynamical calculations,the impact deformation mechanisms of coarse-grained samples were investigated in the temperature range of–190 to 200°C.With increasing temperatures,the main deformation mechanisms during impact deformation have been concluded as:?fcc??hcp??hcp-twin,?fcc??hcp???bcc,?fcc??fcc-twin and dislocation slip,respectively.?6?A gradient nanotwinned layer that contains the nano-grained layer,the nano-twins layer and the coarse-grained matrix was produced on a Fe-20Mn-3Al-3Si TRIP steel by means of SMGT,which illustrates that the steel undergoes a transition from TRIP to TWIP under high-strain-rate deformation.?7?A heterogeneous microstructure with ultrafine grains,Laves phases and hardening nanoprecipitates was fabricated in a Fe-Mn TRIP steel by thermomechanical processing,i.e.combining cold rolling,intercritical annealing and aging.The ultrafine-grained?UFG?steel with a heterogeneous microstructure showed an excellent yield strength?0.9 GPa?,high ultimate strength?1.2 GPa?,good ductility?25.4%?and high thermal stability. |
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