Thermal stability and mechanical properties of ultrafine grained low carbon steel

Ultrafine grained low carbon steel manufactured by equal channel angular pressing was annealed at 753 K, where negligible grain growth occurred, up to 72 h and the microstructural change and the mechanical properties were examined. This investigation was aimed at providing the guiding information for the effective use of ultrafine grained low carbon steel manufactured by severe plastic deformation processes. Under the present annealing conditions, the microstructural change was dominated by recovery. The tensile behavior of annealed ultrafine grained steel was characterized by much higher strength and the absence of strain hardening compared with that of large grained steel. In addition, the present ultrafine grained steel became mechanically stable by 24 h annealing treatment although recovery was in progress. The microstructure of the deformed sample of annealed ultrafine grained steel exhibited the elongated grains and dislocations distributed densely in the vicinity of grain boundaries. This finding indicated that dynamic recovery during deformation was associated with the absorption of dislocation by grain boundaries. The mechanical behavior of the present ultrafine grained low carbon steel was discussed in light of the recent development explaining that of nanocrystalline materials, i.e. the dislocation bow-out mechanism for high strength and the spreading kinetics of trapped lattice dislocation into grain boundary for the absence of strain hardening.