中炭素鋼の熱間延性に及ぼすC,Sの影響

抄録

The effects of carbon and sulfur contents on the hot-ductility of laboratory melted medium carbon steels were investigated.
At a strain rate of 2 × 10^-3s^-1, 0.16% C-steel showed the ductility trough at 800°C, which was slightly higher than the starting temperature of Ar3 transformation. The microstructure observation revealed that the filmy ferrite covered the austenite grain boundaries. This suggests that the deformation induced transformation occurred. Decreasing C content down to below 0.10% improves the hot-ductility around the temperature range of 750850°C and also raise the temperature of ductility trough. The change in hot-ductility with decrease in C content may be attributed to the change in morphology of filmy ferrite at grain boundaries.
The ductility of 0.16%C-0.008%S-steel around the temperature range of 750850°C was markedly improved by prolonging the holding time for over 20 min at the temperature above Ar3 while ultra low S-steel (0.16%C-0.0003%S) showed the ductility trough without isothermal holding. The recovery of hot-ductility can be attributable to re-precipitation and coarsening of the MnS precipitates in the filmy ferrite at grain boundaries, which decrease the void initiation site in the filmy ferrite.