2015 年 55 巻 10 号 p. 2217-2224
Highly refined grain size can be obtained in metastable austenitic stainless steels by martensitic reversion treatment, but the annealing must be controlled due to the high coarsening tendency of ultrafine grains. The effect of Nb microalloying on reversion and retardation of grain growth has been investigated in an austenitic 204Cu stainless steel containing Nb from 0 to 0.45 wt.%. Hot-rolled samples were cold rolled to a 60% reduction, leading to the formation of 47–60% strain-induced martensite, and subsequently annealed at various temperatures between 973 and 1373 K up to 1000 s. Grain size and precipitate structures were examined with optical, scanning and transmission electron microscopes. In the Nb-bearing steels, precipitate particles were found to retard the reversion. The reversion could be completed even at the low temperature of 973 K, but at 973–1073 K the structure remained non-uniform consisting of fine reversed austenite grains and coarser grains with different degree of recrystallization. More uniform grain size in the range of 2–9 µm was obtained by reversion treatment at 1173–1373 K for 1 s, the grain size decreasing with increasing Nb content. Grain growth was effectively retarded by 0.28 wt.% Nb alloying even at 1373 K and by 0.11 wt.% Nb at 1273 K. The activation energy for grain growth increases with increasing Nb content from 363 to 458 kJ/mol. Zener’s model and the flexible boundary model for the driving and pinning forces, respectively, can explain the observed retardation of grain growth.
