Assessment of ideal TTT diagram in C-Mn steels

An ideal TTT diagram of a steel with given compositions is unique and independent of cooling path. To clarify the existence of such ideal TTT diagrams in two C-Mn steels, three models were first examined to verify if they can precisely predict the experimental data of TTT and CCT diagrams, then numerical methods were developed to derive ideal TTT diagrams from experimental TTT and CCT diagrams and vice versa. It was found that the ideal TTT diagrams derived from different cooling processes were not identical, but the discrepancy was smaller in the steel possessing higher Mn content. When the ideal TTT diagram was used to predict TTT or CCT diagrams by applying additivity rule, the agreement between prediction and experiment was largely dependent on the cooling method used to derive the ideal TTT diagram, but the influence of cooling path became small in the high hin steel. The results suggested that a unique ideal TTT diagram does not exist in the low Mn steel, but may exist in the high Mn steel.