2020 年 106 巻 6 号 p. 391-401
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, τc. 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 τc was exhibited in a quite wide range of aging temperatures from 170ºC to 600ºC and τc 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.
