2013 年 54 巻 9 号 p. 1624-1628
Brittle-to-ductile transitions (BDT) in fine-grained Ti added ultra-low carbon steels were investigated. Temperature dependences of impact absorbed energy were measured using specimens with grain sizes of 0.4, 2, 9 and 70 µm of which the grain sizes were controlled by heat treatments after five cycles of accumulative roll-bonding process. Although the BDT temperatures were decreased with the decrease in the grain size, the shape of BDT curves in fine-grained steels are different from those of coarse grains. Specimens with coarse grains show abrupt increase in the absorbed energy at the BDT temperature while fine-grained steels exhibit slight jump of absorbed energy at the BDT temperature and then the energy gradually increases with temperature, showing the enhancement of toughness at low temperatures in fine-grained steels. In the present paper, the characteristic BDT in fine-grained steels was discussed in terms of the thermally activated process of dislocation glide.
