Tailoting Superior Strength-ductility Balance In A Novel Medium Mn Steel With Heterostructured Metastable Austenite Via Cyclic Thermomechanical Process

High-strength medium manganese steel（Mn content in 3.5-12wt%）,typical of the third generation of advanced high-strength steels,usually has an ultra-fine dual-phase structure.It is a meaningful way and research hotspot to achieve automobile lightweight using the phase transition-induced plasticity（TRIP）and twinning-induced plasticity（TWIP）effects induced during deformation.The multi-stage heterogeneous structural design of microstructure can further solve the contradiction between the strength and plasticity of metal materials and provide a new idea for further significantly improving the comprehensive properties of manganese steel,but how to customize the adapted heterogeneous structure and obtaining practical,strong plasticizing effect still lacks regular combing and mechanism interpretation.In this paper,10.86Mn-0.3C-3Al-1.2Si medium manganese steel with austenitic（γ）and ferrite（α）dual-phase structure was proposed for a novel thermomechanical treatment to regulate the microstructure state type.A high-strength medium manganese steel with a unique heterogeneous metastable austenite phase was prepared and combined with SEM/EBSD,TEM,digital image processing（DIC）,nanoindentation,and other characterization methods.The microstructure evolution characteristics and mechanical behavior of manganese steel in this thermomechanical treatment process were systematically explored,the evolution law and interaction of the substructure of the plastic deformation of test steel at different process stages were analyzed,and the correlation between process-microstructure and performance was clarified,and the mechanism of heterogeneous structure to comprehensively improve mechanical properties was revealed.Here are the key results:（1）The Thermo-calc thermodynamic software was used to calculate the phase diagram of the test steel and determined that the reasonable critical annealing temperature was 800°C.Different microstructures were obtained based on the process strategy of small weekly deformation（20%）cold rolling+necessary annealing（800°C）after hot rolling.The microstructure of the microstructure after one cold-rolled annealing（CR1-IA）was mainly composed of coarse-grained austenite and residual martensite;After a cold-rolled annealing（CR2-IA）,the size of the metastable austenite phase showed the characteristics of"bimodal"distribution,that is,it included:recrystallization(γ_C-RX),uncrystallized coarse austenite(γ_C-NRX),recrystallized austenite fine grain(γ_F-NRX),which together with fine grainαform a heterogeneous structure;After three cold rolling annealing（CR3-IA）,an ultra-fine and uniform duplex structure can be obtained.In addition,as the process progresses,the rolling texture in ferrite gradually evolves from random texture to GOSS,and the texture strength increases.（2）The mechanical properties of manganese steel in different thermal process stages were compared and studied.After the circulating heat engine process,CR2-IA and CR3-IA both showed excellent comprehensive mechanical properties,especially the metastable isomeric CR2-IA,its tensile strength was 1250 MPa,the elongation was 70%,and the solid plastic volume was as high as 88 GPa·%.The transition of theα′-martensite phase occurs at all stages of metastable austenite deformation and mainly evolves by the mechanism ofγ→twins→α′,and TWIP and TRIP effects are induced in order.The martensitic phase change kinetic results show that the cyclic heat engine treatment changes the austenite stability,and the intermediate phase change rate is the critical factor in achieving the balance between work hardening,plasticity,and fracture resistance.（3）The mechanical properties of isomeric CR2-IA and finely homogeneous CR3-IA are sensitive to the critical annealing temperature.The coarse-grained austenite(γ_C-RX)in CR2-IA is more unstable than the fine-grained austenite(γ_F-RX),and the nanoindentation of theγ_C-RXgrain shows two stages of pop-in,indicating that it is more susceptible to martensite phase transitions.In addition,the results of EBSD base in-situ tensile results show that challenging phase fine crystalline ferrite andγ_F-RXare distributed on the soft phaseγ_C-RXmatrix in the CR2-IA heterogeneous tissue,that is,heterogeneous deformation induction（HDI）strengthening is induced.The local strain between the heterogeneous phases is incompatible,which improves the phase transition driving force and makes the TRIP effect continue to act,further enhancing the comprehensive mechanical properties.（4）A new process strategy of flash heating based on a two-stage cyclic thermomechanical process was proposed.Medium manganese steel（CR2-FA）with multi-stage isomerization was customized,and its structure was mainly facilitated byγ_C-NRX,γ_F-NRX,fine-grained ferrite,and residual martensite.In particular,compared with the traditional annealed CR2-IA,flash heating increased the intra-phase dislocation density of the size heterogeneous austenite,which increased the yield strength by nearly1.5 times,1.05 GPa.At the same time,the HDI effect is induced under the joint action of size and dislocation density heterogeneity,which ensures the sustainable martensitic phase transition ofγ_C-NRXwith high dislocation density so that CR2-FA still has excellent comprehensive properties and the strong plastic product is above 80 GPa·%（tensile strength 1.38 GPa,elongation～58%）.