準安定Fe-Ni-Cオーステナイトの変態誘起塑性におよぼす結晶粒径の影響

抄録

The effect of austenite grain size (within the range 5 to 289 μ) on the TRIP phenomenon in metastable austenitic Fe-30%Ni-0.2%C alloy was examined by the tensile test at various temperatures between +80°C and −196°C. The main results are as follows.
(1) The Ms and Md temperatures are lowered with decreasing grain size of austenite, but the amount of decrease in Md is smaller than that of Ms. Then, the temperature difference between Md and Ms (that is, temperature range where the TRIP phenomenon occurs) becomes wider in the specimen with fine grain. Petch-type relation is satisfied between 0.2% proof stress of stable austenite and its grain size. Elongation of stable austenite is also increased with decreasing grain size.
(2) At the temperatures accompanying the TRIP phenomenon, the features of stress-strain curves vary with grain size of austenite. As the grain size decreases, the region at which the strain-hardening rate is small appears just after yielding in the stress-strain curve.
(3) With decreasing grain size of austenite, the temperature range showing TRIP becomes wider and shifts toward lower temperatures, and the maximum elongation value due to TRIP is increased. It was found that the change in maximum elongation with grain size is mainly caused by the change of elongation of stable austenite itself, to which the contribution of grain size by the TRIP phenomenon itself is not so much.
(4) 0.2% proof stress drops at the temperatures around Ms. The temperature range where the TRIP phenomenon occurs becomes wider and the amount of drop becomes larger with increasing strength due to the decrease in grain size of austenite.