S Hatefi - Postgraduate Students of Advanced Materials Research Center, Faculty of Materials Engineering, NajafabadBranch, Islamic Azad University, Najafabad, Isfahan, Iran
M Davari - Branch, Islamic Azad University, Najafabad, Isfahan, Iran
M Mansouri - Assistant Professor of Advanced Materials Research Center, Faculty of Materials Engineering, Najafabad Branch, Islamic Azad University, Najafabad, Isfahan, Iran`

Nowadays, due to their excellent mechanical properties, there is increasing demand on using Advanced High Strength Steel (AHSS) especially Dual Phase (DP) in automotive industry. The microstructure of DP steels consists of hard martensite phase in soft ferrite matrix and their mechanical properties are under control of ferrite and martensite volume fraction (Vm). In this study the effect of inter-critical temperatures on the Vm and its effect on mechanical properties were examined. Low carbon steel (LCS) samples were annealed at various temperatures of 780, 800 and 840 ˚C in salt bath furnace and the microstructures and corresponding mechanical properties were investigated. Experimental results showed that increase in inter-critical temperature from 780 to 840 ˚C caused to increase in the Vm from 31% to 68% and the ultimate tensile strength from 646 to 947 Mpa respectively. The results indicated that the energy absorption also changed from 15.33kJ to 17.01kJ. The best combination of strength and ductility was obtained at annealing temperature of 840 ˚C.

Heat Treatment, DP Steels, Martensite Volume Fraction, Energy Absorption, Tensile Strength