Effect of niobium addition on austenite grain size in steels

Abstract

The grain refining effect of columbium in steels is mainly attributable to the retarding effect of Cb(CN) precipitates on the growth of the initial austenite grains. When Cb(CN) has precipitated in equilibrium before the austenization, 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 Cb contents up to a certain contents. When columbium increases over this content, however, the time becomes shorter again. This may be dueo the fact that when columbium increases over a certain extent, the amounts of fine Cb(CN) decrease, which preicpitates secondarily during the isothermal treatment and inhibits effectively the abnormal growth. These two types of grain frowth can reasonably be explained from an analysis based on the grain growth theory proposed by M. Hillert.