鋼のオーステナイト結晶粒度におよぼすNb添加の影響

抄録

The effects of niobium in steels on austenite grain size and its coarsening behaviour were investigated in relation to the precipitation of niobium carbonitride. The main results may be summarized as follows:
1) The grain refining effect of niobium in steels is mainly atributable to the retardening effect of Nb (CN) precipitates on the growth of the initial austenite grains formed just after α-γ, transformation.
2) Onisothermal treatment, the coagulation of Nb (CN) precipitates causes the abnormal grain growth when steels were austenitized from super saturated state of Nb (CN) and some fractions of Nb (CN) precipitated preferably in austenite grain boundaries. On the other hand, when Nb (CN) had precipitated in equilibrium before the austenitization, on subsequent austenitization, the abnormal growth hardly occurs and grains gradually grow holding relatively fine grain sizes in a long period of time. In former case, the starting time of the abnormal growth becomes longer with increase in Nb contents up to a certain contents. When niobium increases over this contents, however, the time becomes shorter again. This may be due to the fact that when niobium increases over a certain extent, the amounts of fine Nb (CN) decrease, which precipitates secondarily during the isothermal treatment and inhibits effectively the abnormal growth.
3) These two types of the grain growth can reasonably be explained from an analysis based on the grain growth theory proposed by HILLE