INTERACTIONS BETWEEN RECRYSTALLIZATION AND PRECIPITATION IN HOT-DEFORMED MICROALLOYED STEELS

The static recrystallization and precipitation characteristics of low alloy steels containing Nb were studied following high temperature compression in the austenite range. The recrystallization kinetics in the Nb-steels were slower than those in the plain carbon steels by approximately an order of magnitude when Nb was in solution. However, much greater retardation of recrystallization was observed in the Nb-steels when precipitates were formed. The initiation of both recrystallization and precipitation appeared to be localized in the early stage of their nucleation. Recrystallization initiated predominantely at the prior austenite grain boundaries. Boundaries were also the preferential nucleation sites for NbCN precipitation. Hence, the NbCN particles were distributed in a highly localized fashion, sometimes delineating what may have been the prior austenite grain or subgrain boundaries. The heterogeneous nature of the nucleation process for recrystallization and precipitation suggested that the critical factor which determined the retardation of recrystallization was the local pinning effect of precipitation. The local pinning force was estimated from the local distribution of precipitation and was found to be of magnitude comparable to that of the driving force for recrystallization. The precipitation pinning force increased in the beginning of precipitation, showed a peak in the intermediate stage, and finally decreased as particles coarsened and were distributed more uniformly.