Investigation On Hot Deformation Behavior And Heat Treatment Characteristics Of Deformed Medium Manganese Steel

The hardenability of medium manganese steel is very high for that manganese can expand the austenitic phase. Different from austenitic high manganese steel, the microstructure can be controlled by proper heat treatment, so that martensitic organization can be obtained with a larger range of cooling rates in medium manganese steel. Therefore, this thesis focuses on the deformed medium manganese steel(5%-7%Mn), made a calculation about the phase transformation with the thermodynamics and dynamics simulation software and then investigating the hot deformation as well as heat treatment characteristics with a series of tests.The equilibrium pseudobinary phase diagrams and the phase content were calculated by using Thermo-Calc.The CCT and TTT curves were got by using materials properties simulation software-JMatPro. The calculation results of the phase transition properties were obtained by these two calculation software.The hot tensile and hot compressive tests of medium manganese steels have been studied by Gleeble-3500 thermal- mechanical simulation machine. According to the tensile test, the best beginning forging temperature is 1200 ℃ and the best final forging temperature is no less than 950℃. After hot compressive teats, dynamic recrystallization took place on the boundary. 6#(0.25C6Mn) steel formed many small dynamic recrystallization grains under the strain rate of 0.1s-1 at 950℃ while the dynamic recrystallization just happens under the strain rate of 0.5s-1 at 950℃A series of researches on the temperatures of quenching and microstructure transformation about tempering were also investigated. The scanning electron microscope(SEM), transmission electron microscopy(TEM) and X-ray diffraction(XRD) analysis were used to analysis the microstructures. The best quenching temperature was obtained. The martensite transformation and hardness during tempering were studied. Fine dispersion particle which are rich in Cr and Mo precipitated in the boundary of martensite tempered during 650℃. Retained austenite presents good stability, there still exits 2.6% austenite even the tempering temperature is as high as 650℃.Hardness steps down with tempering temperature increasing, no secondary hardening occurs in the tempering process.By the fundamental research work above, the hot deformation and heat treatment characteristics of the deformd medium steel can be understand more clearly. These will be the theoretical basis for the thermal processing of materia ls, so that the excellent performance can be achieved by using reasonable forging heat treatment technology in the engineering practice.