Study On The Metastable Austenite Stability And The Springback Behavior Of Medium Manganese Steel With High Strength And High Plasticity

Advanced high strength steels(AHSSs)with high strength and plasticity have been developed and applied to automotive body to meet the requirements of lightweighting strategy and automotive industry.Especially the medium Mn steel,one of the third-generation AHSSs,acquires the excellent mechanical properties with low cost.For example,the product of ultimate tensile strength and uniform elongation of medium Mn steel could reach more than 30 GPa·%.During the development process,metastable austenite and its stability are considered to be the key factors affecting the excellent mechanical properties of medium Mn steel.During the application process,the role of metastable austenite and its stability would result in problems of high springback.Thus,the study of metastable austenite and its stability during application process is needed in medium Mn steel.In this paper,the medium Mn steel with a product of ultimate tensile strength and uniform elongation of 30 GPa%was used.The effects of strain rate and strain path on metastable austenite stability were analyzed.Based on the results of austenite stability,the phase transformation of metastable austenite during bending deformation was investigated.Moreover,the effects of metastable austenite on bending springback were discussed.Finally,a springback prediction model was established based on the results above.The model could predict the springback angle of medium Mn steel accurately.The main work and conclusions of this article are as follows:The effects of strain rates on metastable austenite stability were investigated using high-speed tensile test and digital image correlation(DIC)technique.Results showed that the austenite stability increased with the increase of strain rate in the range of strain rate of 10-3 s-1 to 5×10-1 s-1.EBSD and TEM characterization illustrated that the microstructure evolution at different strain rates was the same basically.Metastable austenite deformed gradually and some of austenite transformed into martensite with the increasing strain.Moreover,stacking faults occurred in the austenite during the deformation process.Furthermore,the density of geometrically necessary dislocations increased significantly and some small-angle grain boundaries could be observed as the strain increased.In addition,the width between the stacking faults increased with increasing strain rate.On the basis of thermodynamic calculations and microstructure evolution analysis,the stacking faults energy of austenite increased from 9.8 mJ/m2 to 18.7 mJ/m2,which suppressed the transformation of austenite.Therefore,the stability of austenite increased with the increasing strain rate.Meanwhile,the required energy for transformation increased and the driving force for transformation decreased that also contributed to the high austenite stability at high strain rate.The effects of strain paths on metastable austenite stability were investigated using sheet forming test and DIC technique.Results showed that the mechanical stability of austenite decreased when strain state changed from simple shear to uniaxial tension,then plane strain and finally equibiaxial stretch.It can be seen from EBSD results that metastable austenite deformed gradually and some of austenite transformed into martensite with the increasing strain.Moreover,the density of geometrically necessary dislocations increased significantly as the strain increased.Based on the texture evolution analysis and the calculation of the Schmid factors and the work done by applied stress,Schmid factors and work done by applied stress increased simultaneously when the strain state changed from uniaxial tension to plane strain to equibiaxial stretch,which resulted in decreasing mechanical stability of austenite.While the work done by applied stress was the lowest when subjected to the simple shear state,which corresponded to the highest mechanical stability.Based on the research of austenite stability at different strain rates and strain paths,the effects of forming process parameters on the springback of medium Mn steel were investigated using the bending springback test.Results showed that the heterogeneous deformation occurred in the thickness direction of medium Mn steel during the deformation process.With the increasing loading velocity,the deformation degree of inner layer region of specimen decreased,which resulted in the decrease in the martensitic transformation of austenite and the geometrically necessary dislocation density.Therefore,the work hardening of specimen reduced,which caused the low springback angle.As the bending angle increased,the deformation degree of inner layer region of specimen increased,which caused the increase in the martensitic transformation of austenite and the geometrically necessary dislocation density.Thus,the work hardening of specimen increased,which leaded to the high springback angle.With increasing horizontal distance between upper die and lower die,the deformation degrees of inner layer region and outer layer region decreased simultaneously,which caused the decrease in the martensitic transformation of austenite and the geometrically necessary dislocation density.On the condition of the same displacement,the elastic deformation stage increased with increasing horizontal distance between upper die and lower die.Therefore,the springback angle of medium Mn steel increased.The effects of austenite volume fraction and texture on the bending springback of medium Mn steel were investigated by changing the intercritical annealing process parameters and rolling types.Results showed that the Young's modulus of medium Mn steel increased with increasing volume fraction of austenite.The different intercritical annealing process parameters resulted in the difference in yield strength and work hardening.Differences in Young's modulus,yield strength,and work hardening contributed the changes in the springback angle.Moreover,the Young's modulus decreased as the intensity of the texture component containing<111>decreased.The work hardening of medium Mn steel increased as the intensity of the texture component with the content of<1-10>and<001>increased.The decrease in Young's modulus and the increase in work hardening were the main reasons for the increase in the springback angle.The finite element simulation model considering the effects of austenite volume fraction and texture on the Young's modulus can more accurately predict the springback angle of medium Mn steel used in this research.