Grain Structure and Precipitation of Aluminium Nitride in Aluminium and Calcium Deoxidized Low Carbon Steels

The effects of cold reduction, annealing temperature, heating rate and thermal history before cold reduction on the recrystallization in silicon-deoxidized, aluminium-deoxidized and aluminium-calcium-deoxidized low carbon steels were investigated by means of microscopical observation, hardness test, chemical analysis and low frequency internal friction measurement. The recrystallization behaviour of aluminium-deoxidized and aluminium-calcium-deoxidized steels differed from that of silicon-deoxidized steel, and in the former two steels annealed the structure of elongated grain was obtained as compared with that of equi-axed one in the silicon-deoxidized steel. By means of low frequency internal friction measurement, it was shown that in the steel of elongated grain structure the lowering of dissolved nitrogen occurred during the period from the recovery to an early stage of recrystallization. This would suggest that the nitrogen, at this stage, existed either as preprecipitation clusters with aluminium or as very fine precipitates of AlN. The mechanism of the formation of elongated grain was considered in terms of the effectiveness of AlN precipitation during recrystallization