Characteristics of Precipitates and Austenite Grain Growth in Nb and Ti Bearing High Strength Steels

A relation between austenite grain growth and characteristics of precipitates formed has been studied in the steels containing Nb and Ti. Ti supresses austenite grain growth more remarkably than Nb. It was demonstrated, however, that such a powerful effect of Ti was reduced by simultaneous addition of Nb at the temperatures up to 1 250°C. X-ray analysis revealed that under these situations, two kinds of precipitate compounds were formed at the same time, that is, one is (Ti, Nb)N where Nb is dissolved in TiN, another is (Nb, Ti)C where Ti is dissolved in NbC. Electron micrographs, however, indicated that there existed little fine cubic precipitates characteristic of (Ti, Nb)N, but only large grobular ones. So, it was speculated that the coalescence of (Ti, Nb)N and (Nb, Ti)C took place. Furthermore, the amount of (Ti, Nb)N unexpectedly decreased with decreasing temperature below about 1 250°C. This is owing to the increase of(Nb, Ti)C which causes short of Ti to form (Ti, Nb)N. It has been concluded that larger austenite grains of Nb and Ti bearing steels can be attributed to the reduction of (Ti, Nb)N and the presence of larger grobular precipitates.