?/?' Martensitic Transformation And Mechanical Properties Of High-Mn TRIP Steel

High-Mn steels are characterized by a higher work-hardening capacity that allows superior combination of ductility and strength to be achieved.They are thus potentially attractive for application in the fields of automobile,machinery,construction,etc.According to the contents of Mn and other alloying elements,twinning induced plasticity(TWIP)or twinning induced plasticity(TRIP)effect can take place during mechanlcal loading in high-Mn steels.In order to develop new high-Mn steels with superior performance,a thorough research should be made regarding the influence of element Mn on the work-hardening behavior and mechanical properties related to their processes.In this thesis,the aim of the study is to identify the relationship between ?/?' martensitic transformation and mechanical properties in high-Mn TRIP steel.The main contents and results obtained in this thesis are summarized as follows.(1)To order to study the influence of Mn content on physical performce of TRIP steel,three kinds of high-Mn steels containing?15wt%,?17wt%and?19wt%Mn were developed.Through thermodynamic calculation,the stacking fault energy(SFE)of these three steels are 3.1mJ/m2,4.6mJ/m2 and 6.6mJ/m2,respectively.The Ms,As anUliltransus temperatures of these steels were determined by dilatometry and they decrease with increasing Mn content.(2)The dynamic and static recrystallization kinetics of 17Mn steel in the hot rolling temperature range were investigated by compression tests.The results show that dynamic recrystallization is easy to occur at higher temperature and lower strain rate.?'-martensite tends to produce in deformed austenitic grains.Also,the hot-working equation and static recrystallization model were established.(3)The microstructures and mechanical properties of 17Mn steel were studued at different solid-solution temperatures,cooling rates and amounts of austenite prior deformation.The amount of thermally induced ?-martensite is increased when both solid-solution temperature and cooling rate are high.A good mechanical property of the steel can be obtained when the prior austenite grain is fine and the amount of ?-martensite is large.Thermally induced ?-martensitic transformation is restrained by prior deformation of austenite,but the strength of steel is improved by prior deformation of austenite.(4)The mechanical properties and microstructures of these high-Mn TRIP steels were compared and analyzed,and the influence of thermally and deformation induced ?/?' martensite on work-hardening behavior of high-Mn TRIP steels are clarified.15Mn steel has the lowest SFE and consists of austenite and thermally induced ?/?' martensite.The ?-martensite is easy to transfore to a'-martensite at early stage of deformation and decreases the yield stress of steel.In subsequent deformation,the thermally induced ?'-martensite plates block the phases transformation and improve the work hardening rate and tensile strength of steel.The SFEs of 17Mn and 19Mn steel are higher than that in 15Mn steel,and there are only austenite and s-martensite in the steel after solid-solution treatment.The s-martensite plates are found to be strong barriers to plastic flow,and the deformation-induced transformation of ?-to ?'?martensite favors plasticity of the steel.