多結晶フェライト鋼の静的ひずみ時効発現機構

The mechanisms of the static strain aging phenomenon of polycrystalline ferritic steels were investigated focusing on segregation of solute C atoms to dislocations and grain boundaries, precipitation of carbides and structural changes of dislocations and grain boundaries, using partially C-stabilized Nb-bearing ULC steel sheets and IF steel sheets with solute C content of 0–4 mass ppm and ferrite grain sizes from 9.5 μm to 183 μm with aging temperatures from 70ºC to 600ºC. The partially C-stabilized steel sheets exhibited two definite hardening stages. The first hardening appeared in the every partially C-stabilized steel sheet accompanied by an increase in the grain-interior strength, σ₀, which saturated at around 30 MPa. During the first hardening stage, linear and strong segregation of solute C atoms was developed, which was observed by 3DAP, and their concentration was in good agreement with Cottrell’s formula. The second hardening significantly appeared in fine-grained steels, reaching 90 MPa and accompanied by a large increase in the critical grain boundary strength, τ_c. The apparent activation energy of the second hardening was 135 kJ/mol. Neither carbide precipitation nor obvious segregation of carbon atoms to grain boundaries was detected during the aging period. Increase in τ_c was exhibited in a quite wide range of aging temperatures from 170ºC to 600ºC and τ_c reached the value close to that of the recrystallized condition. Local recovery or rearrangement of Fe atoms in deformed grain boundaries is proposed as a possible mechanism of the second hardening.