Effect Of ART On Microstructure And Properties Of 5% Mn Medium Manganese Steel

The third generation of automobile steel with high strength and high plasticity,is used to improve the lightweight of automobiles and has the advantages of energy saving,environmental protection and cost saving,it has become a research hotspot in the field of steel.This study took 5%Mn medium manganese automobile steel as the trial-manufacture material.With the aid of Olympus GX51F metallographic microscope,FEI QUANTA650scanning electron microscopy,the Netherlands X Pert PRO MPD type X-ray diffractometer,FEI TecnaiG~2F20 transmission electron microscope,SHT-4605 microcomputer control electronic universal testing machine and other equipments,we systemically studied the microstructural evolution and mechanical properties at two-phase region reverse-phase transformation annealing of hot rolled and cold rolled manganese steel,and determined the optimum process.The research results are as follows:Quenching at a high temperature at 930°C with 20min and annealing in different temperature at two-phase region of reverse-phase transformation,the microstructure of hot rolled medium manganese steel was mainly lath martensite.The microstructure of the steel after reverse-phase transformation annealing was ultra-fine grained ferrite,martensite and different the volume fraction of retained austenite.When annealing temperature reached625°C,the main microstructure was lamellar martensite and ultra-fine grained ferrite,the corresponding the volume fraction austenite was 6.83%;At 650°C,the main microstructure was ultra-fine grained ferrite,lath martensite and austenite,the corresponding the volume fraction of austenite increased to 12.63%,the average lath austenite was 0.14μm,the average lath martensite was 0.19μm;At 675°C,the austenite appeared in the ferrite grain boundary and martensite lath boundary in the shape of blocky and lath.The blocky austenite presented austenitic stacking fault and twinning,and the volume fraction of austenite was 22.34%;At 700°C,the volume fraction of austenite decreased to 10.10%,the ultra-fine grained ferrite and lath martensite were dominant.The average martensite lath width was 0.30μm.Quenching at a high temperature at 930°C with 20min and annealing in different temperature at two-phase region of reverse-phase transformation could be found that:the microstructure of the cold rolled medium manganese after quenching was mainly lath martensite and reverse-phase transformation annealing was ultra-fine grained ferrite,lath martensite and different the volume fraction of austenite.When the annealing temperature was 625°C,the microstructure of the steel mainly included lath martensite and ultra-fine grained ferrite as well as the volume fraction of austenite was only 5.09%;At 650°C,the main microstructure was ultra-fine grained ferrite,lath martensite and austenite,the corresponding the volume fraction of austenite increased to 12.63%,the average lath martensite was 0.23μm;At 675°C,the main microstructure was ultra-fine grained ferrite and austenite.The volume fraction of austenite increased to 22.34%,and the average grain size was 0.30μm;At 700°C,the microstructure mainly included lath martensite and ultra-fine grained ferrite,and the width of martensite increased to 0.32μm.What’s more,the density decreased significantly,the volume fraction of austenite decreased to 9.32%.Based on the above research,the optimal reverse phase transformation annealing process and corresponding mechanical properties of 5%Mn medium manganese steel can be obtained as follow:Quenching at 930°C with 20 minutes and reverse phase transformation annealing at 675°C with 30 minutes,the hot rolled medium manganese steel got the best comprehensive mechanical properties:Its tensile strength was 1050MPa,the elongation was 25%and the strength-ductility combination was 26.3GPa%.Quenching at 930°C with 20 minutes and reverse phase transformation annealing at 675°C with 30minutes,the cool rolled manganese steel got the best comprehensive mechanical properties:Its tensile strength was 1100MPa,the elongation was 22%and the strength-ductility combination was 23.1GPa%.