Microstructure Formattom Mechamism Of Ultrafine-Grained Low Alloy Steel With High Strength And High Ductility

Ultrafine-grained materials processed via severe plastic deformation usually have high strength but relatively low ductility. The application of ultrafine-grained materials is restricted largely. In this paper, an ultrafine-grained medium carbon low alloy steel produced by multidirectional forging and heat treatment show high strength and good plasticity.A medium carbon low alloy steel was subjected to multidirectional forging and annealing treatment in this work. The micro structure of samples was characterized by optical microscope, scanning electron microscope, transmission electron microscope. Then, the hardness, stress-strain were tested. The internal relation between the microstructure evolution and the change of strength, hardness and ductility was discussed.The subcritical quenching before multidirectional forging was to get the ferrite and martensite dual phase microstructure, then the samples were subjected to multidirectional forging at 600℃ to refine the grains which played a role in strengthening. After annealing, fine ferrite grains and coarse ferrite grains coexist in the sample. Moreover, there are a mass of dispersing ultrafine carbides particles widespread within the grains. These played a role in toughening. Finally, the hardness, tensile strength and elongation of processed samples are 318.9HV,992.8MPa and 13.1%, respectively. The mode of the tensile fracture is dimple fracture.After processed, fine grains and coarse grains coexisting distribution and small carbides particles into UFG ferrite microstructures are formed in samples. Both coarse ferrite grains and dispersed carbides contribute to the improvement of ductility, because of a large number of dislocations resulting from plastic deformation stored in coarse ferrite grains can produce strain hardening effect, and dispersed carbides can improve the work hardening rate of materials. The hardness and strength of sample could be enhanced through fine ferrite grains and dispersed carbides, owing to the fine ferrite grains are propitious to increase the area of grain boundary, and dispersed carbides are advantageous to increase work hardening ability.