Graphite formation in high-purity cold-rolled carbon steels

Low-carbon (0.06 pet C) and high-carbon (0.50 pet C) Al-killed (AK) steels were prepared by melting high-purity electrolytic iron. They were hot-rolled, cold-rolled, and then batch-annealed for the investigation of microstructure and mechanical properties. It was found that graphitization of cementite took place in steels of extremely low phosphorus and sulfur contents. Graphite was thought to originate in voids formed by cracking of cementite during the cold rolling. It was speculated that in steels of commercial purity, phosphorus and sulfur segregated to the void surface and, thus, suppressed graphitization during the annealing. It was also found that the high-carbon steels became as soft and ductile as low-carbon steels by graphitization.