Roles of solute c and grain boundary in strain aging behaviour of fine-grained ultra-low carbon steel sheets

The roles of solute C and the grain boundary in the strain aging phenomenon of polycrystalline ferritic steel were investigated using Nb-bearing ULC steel sheets with a relatively low solute C content of 1-3 mass ppm and ferrite grain sizes of 9.5 μm and 183 μm at aging temperatures from 70 to 400°C. The steels exhibited two definite hardening stages. The first hardening stage appeared in both fine- and coarse-grained specimens, in which the increase in YP (?YP) became saturated at around 30 MPa. From the apparent activation energy and hardening kinetics, the hardening mechanism was assumed to be dislocation pinning by solute C atoms. The second hardening stage, significantly appeared in fine-grained specimens accompanying a large increase in the Hall-Petch coefficient; ?YP was quite large, reaching 90 MPa. Fine precipitates were not detected in aged specimens observed by TEM and 3DAP. Segregation of solute C to the grain boundaries and diffusion of Fe atoms in the grain boundaries were proposed as possible mechanisms of this second hardening. Grain-boundary hardening is assumed to be one of the hardening mechanisms in the strain aging of the polycrystalline ferritic steel.