Study On Microstructure Evolution Behavior Of Nb-containing Manganese TRIP Steel During Critical Annealing

In recent years,the rapid development of the automotive industry has brought about energy and environmental problems.To solve these problems,domestic and foreign researchers have proposed the concept of the third-generation AHSS,which can reduce the weight of the car body while ensuring safety.To achieve the effect of energy saving and emission reduction.Medium manganese TRIP steel is the most representative among them,which has excellent mechanical properties and has been widely concerned.In this paper,hot rolled 0.2C-5Mn medium manganese TRIP steel with different Nb content was used as the experimental material,which was kept in a muffle furnace at 800 ? for 15 min,and then quenched and cooled to obtain a complete martensite structure.In the critical annealing process,the holding time at 625 ? and 675 ? is 120 min,and the holding time at 650 ? is 30 min,60 min,90 min,120 min,180 min,and 240 min.After annealing,air cooling is performed to obtain the final structure morphology.Field emission scanning electron microscope(FE-SEM),electron backscatter diffraction(EBSD)and X-ray diffraction(XRD)were used to study the effect of carbide dissolution behavior on the microstructure of medium manganese TRIP steel.Through the critical annealing heat treatment process,the carbide evolution behavior of two hot-rolled medium-manganese TRIP steels with and without Nb under different annealing regimes,as well as their microstructure,retained austenite volume fraction and medium-manganese TRIP steel were observed.The effect of stability.The test results show that after the critical annealing,the test steel will produce a complex structure of ultrafine grained ferrite and residual austenite.With the increase of annealing temperature,the carbide gradually dissolves,and the carbide completely disappears at 675 ?;the volume fraction of retained austenite first increases and then decreases.The residual austenite content at 650 ? is up to 35.4%;With the extension of annealing time,the carbides gradually dissolved,and the volume fraction of retained austenite gradually increased.After 180 min of annealing,the carbide was completely dissolved,and the volume fraction of retained austenite remained unchanged.Compared with non-Nb-containing steels,Nb-containing steels cannot thermodynamically change the phase composition of the experimental steels,but experiments have found that kinetics can delay the austenite recrystallization time,reduce the residual austenite grain size,and increase the film The austenite-like content improves the stability of residual austenite,slows the dissolution process of carbides,slows the transformation process of austenite,and is beneficial to the control of the residual austenite content.The martensite and retained austenite of medium manganese TRIP steel contain a large amount of carbides and alloying elements,and have a specific influence on the activity coefficient of carbon atoms.Taking into account this characteristic of the medium manganese TRIP steel,we used the "constrained carbon quasi-equilibrium-alloy(CCEA)" model to study the effect of austenitizing temperature on carbon distribution,and the actual measured carbon content and The calculated CCEA results were compared.The results show that the calculation results of the CCEA model are consistent with the trend of the measured carbon content in residual austenite,and can be used to guide the selection of austenitizing temperature during the actual heat treatment process.