Graphitization in high carbon sheet steel, which reduces its tensile strength and increases its elongation, is expected to improve both formability and quench hardenability. However, it is examined to shorten annealing period for graphitization in recent investigation with B added steel, clear and accurate mechanism had not been obtained. In this work, the effect of Al, N and microstructure, prior to annealing, on the relationship between graphitization and annealing period was investigated.
0.65%C-0.2%Si-0.15%Mn-0.002%B steels, with Al ranging from 0.01 to 0.10%, and N ranging from 0.0 to 0.09%were provided. These steels were varied its microstructure by normalizing at 900°C prior to cold rolling, then microscopic observation was conducted at various period of annealing at 670°C.
Under the condition, a remarkable graphitization was brought in steel with higher Al content. On the other hand, N was found to be weekly affected to the graphitization. However, steels, normalized prior to cold rolling, graphitization during annealing was promoted, exceeded addtion of Al inhibited its graphitization. BN, which becomes of nucleus of graphite precipitation, was reduced by exceedingly added Al. Therefore, graphitization was thought to be inhibited insome condition of steels with exceedingly high Al and normalizing prior to cold rolling and annealing.