Structure and mechanical properties of the low-carbon steel after severe plastic deformation and forging
The effect of the hot forming obtained by multi-directional forging on the structure and mechanical properties of the ferrite-pearlite dual phase steel 09Mg2Si after equal channel angular pressing (ECAP) is studied. Four passes of ECAP were conducted using a route of "Bc" (90-deg rotation after each pass) adjusted to 120° internal angle of channels, 450 °C pressing temperature. Subsequent multi-directional forging (MDF) was performed with accumulated strain of deformation of 60 and 80%. To remove the excess of internal stress after severe plastic deformation (SPD) billets were annealed at 350 °C for 1 hour with following furnace cooling. Thermo mechanical processing by MDF affects not only the shape and structure, but, respectively, the mechanical properties of the billets too, due to the processes of deformation and recrystallization of metal. After ECAP and MDF more fine-grained structure was expected to ensure the best strength characteristics, but there was decrease in values compared to steel, which was processed only by ECAP. Nevertheless its yield point after combined processing by SPD and forging exceeds the initial values by 40-80%, the ultimate tensile strength - by 32-45%. Subsequent MDF also resulted in increase of ductility of the ultrafine grained (UFG) steel 2-3 times in comparison with steel after ECAP. After ECAP and MDF with accumulated strain of deformation 80% ferrite and pearlite grains of the steel were significantly refined to 3.8 and 2,5 μm respectively, that is 2 times less than after ECAP. Research results show the applicability of combined treatment which consists of the ECAP and MDF as manufacturing technology of work pieces with shaped form with sufficient preservation of structural and mechanical conditions obtained after ECAP.