Ni-Cr-Mo-V鋼のオーステナイト結晶粒の挙動におよぼすNiの影響

Abstract

In relation to the fact that Ni-Cr-Mo-V Steels containing 3 to 4%Ni have a tendency to form coarse austenite grains, an investigation was made on the effect of Ni on the behavior of austenite grains in this kind of steel.Four materials of a fixed chemical composition except for Ni content which was varied in the range from 0 to 4.5%were prepared. Specimens which have martensitic structure obtained by preliminary quenching from 1250°C were heated to 900°C. Austenite grain size at 900°C and structural changes during heating in these specimens were observed.It was found that austenitie grain size tend to become coarser with increase in Ni content. Further, it was known that acicular austenite grains which were formed in early stage of austenitization became finer and more stable with increase in Ni content.Based on these results, considerations were made on the austenitizing mechanism of steels which have acicular prior structure.It seems reasonable to consider that coarse equiaxed austenite grains are formed as a result of preferential growth of a small number of acicular austenite grains.It is believed that the role of Ni in formation of coarse austenite grain in Ni-Cr-Mo-V steels is to make the lattice orientation of acicular austenite grains uniform, and, consequently to increase the stabilization of acicular grains, this being a necessary condition for preferential growth of a small number of these grains.