ABSTRACT
The effects of variation of the intercritical annealing (ICA) temperature and time on the microstructural and mechanical property behaviours of high martensite dual phase (HMDP) steel have been investigated by intermediate quenching of microalloyed steel. The treatment consisted of initial austenitization and rapid quenching of the steel samples to achieve a fully martensitic state followed by annealing in ferrite and austenite (?+?) region between 730oC and 810oC and subsequent quenching to produce dual phase microstructure containing varying proportion of ferrite and martensite constituents. The annealed samples were further subjected to tempering treatment at different tempering temperatures. Microstructural characterization and mechanical properties of these steel samples were studied and compared to those of conventionally quenched and tempered steel specimens. The results show that HMDP steel specimens annealed in the intercritical temperature range of 770oC to 790oC revealed finer martensite and precipitate-free ferrite microstructure. The tensile, hardness and impact properties of the developed HMDP steels increase with increasing intercritical annealing temperatures, with optimum properties obtained at 790oC. At all the ICA temperatures, an increase in the intercritical annealing time from 30 to 90 minutes was found to enhance the tensile, hardness as well as impact properties of the HMDP steels. The tempering of HMDP steel samples significantly increases their ductility and toughness with only slight loss to strength as compared to the normal quenched and tempered steel specimens. Hence, the developed HMDP and tempered HMDP steel samples exhibited enhanced mechanical properties as compared to the conventional quenched and tempered heat-treated ones.