Study On Austenite Adjustment And Cu Precipitation In Medium Mn Steels

As a candidate for 3rd generation advanced high-strength steel(AHSS),the medium Mn steel has received extensive attention in the lightweight design of automobiles to achieve energy saving and emission reduction.By designing the alloy composition and heat treatment process,this paper studies the austenite adjustment and Cu precipitation in the Cu-Ni added medium manganese steel.The paper focuses on the nucleation and alloying element partitioning behaviors of reversed austenite,and the evolution mechanism of Cu precipitation in the ferriteaustenite dual-phase microstructure.Both strength and ductility of medium manganese steels are improved by the TRIP effect and the strengthening effect of Cu precipitation.The prior austenite grain size is related to the microstructure characteristics of quenched martensite.With the increase of the prior austenite grain size,the density of the prior austenite grain boundary of the quenched martensite gradually decreases,and the density of the block boundary gradually increases.In addition,the number of packets and variants in a prior austenite grain gradually increases as the austenite grain size increases.The quenched martensite is composed of coarse blocks with lower KAM value and fine blocks with higher KAM value,the size difference between which gradually increases with the increase of the prior austenitic grain size.During intercritical annealing,the reversed austenite tends to nucleate at the interfaces,i.e.,the original austenite grain boundaries,high-angle and low-angle packet boundaries,block boundaries and sub-block interfaces.The austenite seldom nucleates in regions inside the variant without interfaces.This makes the non-uniform distribution of austenite nucleation,i.e.,less austenite nucleation in the coarse block area,and more austenite nucleation in the fine block area.The nano-scale alloying element enrichment layer was formed near the interface in retained austenite after intercritical annealing and tempering,so that the non-uniform distribution of alloying elements in the austenite lath was achieved.The STEM-EDS results indicated that the distribution of alloying elements in the austenite was relatively uniform after the first-step intercritical annealing at higher temperature.Then the nanoscale enrichment layer of Mn and Ni formed at the austenite interface after the second-step tempering.In this way,the Mn concentration is low in austenite center and high in austenite interface to show a Mn gradient inside the austenite lath.When the annealing temperature is lower,the retained austenite is relatively stable,and the TRIP effect does not occur during the deformation process.Thus,the alloying element enrichment layer has little effect on the mechanical properties of steel.When the annealing temperature is higher,the retained austenite is metastable,and the TRIP effect will continue to occur during the deformation process.The alloying element enrichment layer under this condition can delay the TRIP effect in the initial stage of deformation and significantly improve the stability of retained austenite and then the plasticity of steel.This paper studied the evolution behavior of Cu precipitation in the dual-phase microstructure(austenite and intercritical ferrite)during intercritical annealing.It is found that almost all the Cu precipitates have been formed in the intercritical ferrite during the heating process.During the holding process,part of the Cu precipitates in the intercritical ferrite dissolved,and then those left coarsened.The calculation on phase interface migration and miscibility gap shows that there is no new nucleation of Cu precipitates in the austenite during the holding process,but some Cu precipitates in intercritical ferrite will be encased into austenite with the migration of the ferrite-austenite interface.Once those Cu precipitates was enclosed in austenite,they will begin to dissolve with very slow dissolution rate.The different evolution behaviors of Cu precipitates in austenite and intercritical ferrite lead to an uneven distribution of Cu precipitates in austenite of the specimen annealed at 680℃ for 60 min.The size and volume fraction were greater near theα/γ interface in austenite which are opposite in the center.The strengthening effect of Cu precipitates in intercritical ferrite was significant during deformation,and the Cu precipitates in the austenite delayed the decrease in strength significantly due to slow diffusivity of Cu in austenite.