Austenite Reversion And Microstructure Control In The Medium Manganese Steels

Medium manganese steel is a promising candidate for the 3rd generation advanced high strength steel.Retained austenite is introduced into the medium manganese steel by the so-called intercritical annealing process.It is the critical phase as of medium manganese steel as its fraction and stability significantly affects the mechanical property of the steel.In this study,an Fe-0.2C-4.5Mn wt.% steel is investigated.The thermodynamics and kinetics of its austenite reversion during intercritical annealing is systematically studied.It is found that the austenite reversion can be divided into three stages,during which the kinetics of austenite reversion is controlled by the diffusion of carbon in the ferrite and austenite,diffusion of manganese in the ferrite and austenite,and diffusion of manganese in the austenite,respectively.The nucleation position of the austenite has a significant influence on the migrating velocity of the ferrite/austenite interface,the solute partitioning in the austenite and ferrite,and the migrating direction of the ferrite/austenite interface at different stages of the transformation.By comparing with the experimental results in the literature,it is indicated that austenite is more likely to nucleate at the ferrite/cementite interface.A novel cyclic austenite reversion treatment is proposed.The microstructure evolution and the mechanical property is carefully studied by a series of comparative experiments with the conventional intercritical annealing treatment in the hot-rolled and cold-rolled steel.A significant enhancement of retained austenite fraction is achieved in a short time by the cyclic austenite reversion.The partitioning of carbon in the martensite is found to play an important role in this process.The mechanical properties of the steel are also improved and the extent of improvement is related to the enhancement of retained austenite fraction.The cyclic austenite reversion is a promising approach of tailoring the microstructure and mechanical property of the medium manganese steel and requires further investigation.