Research On Hot Deformation Behavior And Microstructure Of M50 Steel

M50 steel is a high-temperature bearing steel with excellent comprehensive performance and its service temperature reaches up to 315 ℃. The irregularly distributed chunks of carbides seriously affect the contact fatigue strength and service life of M50 steel. This paper studied the effect of heat treatment and hot deformation on the morphology and distribution of carbides, the high temperature constitutive equation and processing map were established using the true stress-true strain curves. A material model of M50 steel was built to simulate the ring rolling process and then optimized the related parameters of the ring rolling process.During the hot deformation process of M50 steel, the true stress-true strain curves rised rapidly through elastic stage and then gradually leveled off or even declined. As the deformation temperature increased and the strain rate decreased, the percentage of dynamic recrystallization became bigger and the flow stress became smaller. A processing map was established based on Murty instability criterion, and in the true strain of 0.8, the optimum zone for heat processing were respectively 900~950℃, 0.01~0.05s-1 and 1075~1100℃, 0.01~1s-1 and 1100~1150℃, 0.06~1.8s-1.The organizations of M50 steel were martensite, retained austenite and carbides after quenching. The relative amounts of martensite decreased and the relative contents of retained austenite increased and carbides gradually reduced as the temperature rised. The grain size did not grow obviously before 1100℃, while overheat happened at 1150℃ and overburing phenomenon occured at 1200℃In the hot compression process of M50 steel, chunks of cabides in the stagnant zone of specimen were substantially insoluble, carbides in the highly deformed area crushed and dissolved, carbides in the small deformation zone were completely dissolved, while there were many voids remaining in the bulk. As the deformation temperature increased and the strain rate decreased, the volume percentage of recrystallization became greater and the recrystallized grain size became larger.The ring rolling process was simulated at different conditions using DEFORM-3D, and the process parameters were optimized combined with the processing map of M50 steel: speed of the drive roller was 75~90 r/min, the drive roller feed rate was 4~6 mm/s, initial rolling temperature was 1050~1070℃.