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 ppm and ferrite grain sizes of 9.5 μ m and 183 μ m at aging temperatures from 70 to 400°C. The steels exhibited two definite hardening stages. The 1st hardening stage appeared in both fine- and coarse-grained specimens, in which the increase in YP (ΔYP) 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 2nd hardening stage, significantly appeared in fine-grained specimens accompanying a large increase in the Hall-Petch coefficient; ΔYP 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 2nd hardening. Grain-boundary hardening was assumed to be one of the hardening mechanisms in the strain aging in polycrystalline ferritic steel.
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