| The rapid development of automobile industry not only brings the convenience of transportation,but also leads to the deterioration of global environment and energy crisis.The key strategy to solve this problem is to enhance the combination of strength and plasticity of steel to reduce vehicle's weight and save energy.Based on phase transformation-induced-plasticity(TRIP)effect,medium Mn steel has excellent comprehensive mechanical properties and relative low alloy cost,so it has received wide attention.In this study,the 15Mn7(Fe-0.15C–7.46Mn–0.20Si,in wt%)medium Mn steel is investigated using the Gleeble-3500 isothermal compression tests simulating the rolling deformation procedure.The work aims to explore the fine and controllable processing window of the steel,which provides a theoretical basis for microstructure control of subsequent deformation and heat treatment.Based on the theoretical basis of recrystallization,the stability of austenite,microstructural morphologies and mechanical properties of 15Mn7 steel are controlled by the coupling technology of different kinds of deformations(hot/warm/cold deformation)and heat treatment processes(deep cryogenic pretreatment(DCT)and intercritical annealing(IA)).With the help of microstructural characterization technologies and deformation analysis means such as scanning electron microscope(SEM),transmission microscope(TEM),electron backscatter diffraction(EBSD),X-ray diffraction(XRD),nanoindentation,uniaxial tension and digital image correlation(DIC)technique,the stability of austenite,deformation mechanism as well as plastic instability behavior and its influencing factors of medium Mn steel are systematically studied,so as to provide a theoretical basis for the process optimization and microstructure-properties control of the steel.The main results and conclusions are as follows:(1)Based on elavated temperature isothermal compression tests,dynamic and static recrystallization behavior as well as microstructure evolution were investigated.Flow stress constitutive relation,static/dynamic recrystallization kinetic model and grain size evolution model were established.The accuracy and reliability of the model were verified by finite element simulation and experiment.The optimum thermal deformation temperature and strain rate were 1000°C-1050°C and 0.1s-1-1s-1,respectively.It is found that the predominant softening mechanism is static recovery,and the dominant mechanism of recrystallized nucleation is the strain-induced grain boundary migration.(2)Based on the influence of deformation process parameters on the initial microstructure,the effect of prior austenite grain(PAG)size and recrystallization degree on microstructure and properties was studied by using the Gleeble-3500 to simulate different hot deformation process.Smaller prior austenite grain sizes lead to higher nucleation rate as well as higher amount and smaller grain size of retained austenite after annealing.A higher degree of recrystallization leads to higher proportion of film microstructures and smaller grain boundary misorientations after annealing.These all eventually lead to the differences in mechanical properties.The submicron and multi-morphological austenite grains are successfully constructed by incomplete recrystallization of‘warm deformation+intercritical annealing',which leads to austenite grains with different levels of stability,thus realizing the gradual TRIP effect and improving the mechanical properties of the steel.Block austenite with uneven Mn concentration was obtained by complete recrystallization after‘cold deformation+IA'.Different regions in the block austenite gradually transformation to martenite(Segmented martensite for short)during deformation,which can also obtain better mechanical properties.(3)In order to further improve the strength and plasticity of 15Mn7 steel,the compound process of‘warm deformation+deep cryogenic treatment+intercritical annealing'was conducted.Based on the high dislocation density and lattice distortion after deep cryogenic treatment,superfine austenite grains with higher chemical gradients of C,Mn element and mixed morphologies were obtained.The coordination of the TRIP effect of and fine-grain strengthening helps to maintain a remarkable work hardening rate at different deformation stages,and enhances the comprehensive mechanical performance of 15Mn7 steel.With the decrease of IA temperature,the improvement effect of deep cryogenic treatment on mechanical properties is more remarkable.And the most significant improvement of mechanical properties can be as high as doubled,that is the condition of IA at 600°C.During deformation,segmented martensite presents in samples annealed at 600°C-630°C.The segmentation of strain induced martensite makes austenite change from blocky to much finer sizes with different morphologies.The TRIP effect in the segmented austenite was greatly induced at subsequent larger plastic deformation stage,thus leading to a great improvement in comprehensive mechanical properties.(4)Based on the influence of deformation and heat treatment process on the strain field during tensile process,generation mechanisms of the PLC band were discussed.It is found that occurance of PLC bands mainly depends on the grain size of retained austenite(RA).When the RA grain size exceeds the critical value,it is possible to generate sufficient dislocation density to motivate PLC effect.In this study the critical RA grain size is 0.34?m.While the type and starting position of PLC band rely on the stability of the RA.When the stability of austenite is low,more strain induced martensite is produced at low strain level,leads to the start of PLC band as well as discontinuous propagation and frequent jump.And when stability of austenite is appropriate,the PLC band starts at relative high strain level and propagates continuously. |
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