Effect Of Reverse Phase Transformation Annealing On Microstructure And Properties Of The Third Medium Manganese Automotive Steel

Due to high strength and high plasticity characteristics,medium manganese steel meets the development requirements of advanced high-strength automotive steel and has become a research hotspot worldwide.In this subject,with 5%Mn medium manganese steel as the research object,the reverse-phase change annealing treatment is performed on hot-rolled sheet and cold-rolled sheet,respectively.Besides,the organization and mechanical properties are investigated with the help of metallographic microscope,scanning electron microscope,X-ray diffractometer,transmission electron microscope,and other equipment.Moreover,the effects of annealing temperature and time,dislocation density,and TRIP effect on the microstructure and mechanical properties of Medium manganese steel are analyzed and discussed.The results are demonstrated as follows:The hot-rolled medium manganese steel is austenitized and quenched at 930°C,followed by reverse-phase transformation annealing at 675°C for 10 minutes.Afterward,the test steel microstructure is dominated by lath martensite and ultrafine-grained ferrite.The concentration of austenit was gradually increasing with the holding time increased.When the holding time increases to 30 minutes,the microstructure is dominated by ultra-fine-grained ferrite and massive austenite.The hot-rolled medium manganese steel is austenitized and quenched at 930°C,followed by reverse-phase transformation annealing at 675°C for different times,tensile strength and elongation increase,and reach the maximum when the holding time is 30minutes;the yield strength decreases first and then increases,and the yield strength is the largest after holding for 10 minutes.The cold-rolled sheet is water-cooled at 930°C for 20 minutes and then air-cooled at 660°C for 30 minutes.The obtained microstructure is mainly ultra-fine-grained ferrite and lath martensite,with the volume fraction of austenite of5.62%.When the annealing temperature increases to 665?,the microstructure is mainly composed of ultra-fine-grained ferrite,a small amount of unbroken martensite and austenite,with the austenite volume fraction increasing to 24.24%.The microstructure after reverse transformation annealing at 675?mainly consists of ultra-fine-grained ferrite and austenite,with the volume fraction of austenite of22.34%.When the annealing temperature further increases to 685?,the microstructure mainly contains lath martensite and ultra-fine-grained ferrite,and the austenite volume fraction increases to 4.43%.The cold-rolled sheet is water-cooled at930°C for 20 minutes and then air-cooled at different annealed temperatures,the tensile strength increases continuously and reaches the maximum value at 685?;the yield strength first increases and then decreases,and the maximum after annealing at675?;the elongation first increases and then decreases,and reaches the maximum value after annealing at 665?.The cold-rolled sheet is austenitized and quenched at 930°C,and then kept at675°C for 10 minutes.The obtained microstructure is mainly composed of lath martensite,and the volume fraction of retained austenite is 10.56%.When the holding time increases to 20 minutes,the microstructure contains lath martensite,lath austenite and ultra-fine-grained ferrite,with the austenite volume fraction increasing to 19.7%.When the holding time is 40min,the microstructure consists of ultrafine-grained ferrite,lath martensite and austenite,with an austenite volume fraction of 20.53%.When the holding time increases to 60min,the austenite volume fraction decreases to 16.26%.The cold-rolled medium manganese steel is water-cooled at 930°C for 20 minutes and then air-cooled at 675°C for different times,the tensile strength continues to increase;the yield strength first decreases and then increases,and reaches the maximum value at the holding time of 60 minutes;the elongation first increases and then decreases,and reaches the maximum value at the holding time of 30 minutes.The dislocation density of austenite decreases from9.9×10¹⁴m^-2to 2.3×10¹⁴m^-2with the holding time increased.Finally,the heat treatment conditions with the best comprehensive mechanical properties of hot-rolled and cold-rolled sheets were obtained.Specifically,the hot-rolled sheet is austenitized and quenched,and kept at 675°C for 30 minutes.It is obtained that the austenite volume fraction is 29.23%,the tensile strength is 1050MPa,the elongation rate is 25%,and the strength-ductility combination is 26.3GPa·%.The cold-rolled sheet is austenitized and quenched at 930°C,followed by reverse-phase transformation annealing at 665°C for 30 minutes.Then,the tensile strength is980MPa,the elongation is 23.68%,and the strength-ductility combination reaches23.21GPa·%.