Investigation of Micromechanical Behavior of Ferrite and Martensite in Dual Phase Steels

The aim of this research was to investigate ferrite and martensite micromechanical behavior in dual phase (DP) steels. For this purpose, a low carbon steel with ferrite-pearlite initial structure was cold-rolled up to 80% and annealed at 600 ºC for 20 min and subsequently water quenched. The final processing step involved heating to the intercritical annealing region and holding for 10 min at 760, 780, 800 and 820 ºC followed by water quenching. DP steels consisting different volume fractions of ferrite and martensite and different ferrite grain size were produced. Scanning electron microscopy was supplemented by energy dispersive spectroscopy (EDS) and nanoindentation test to follow microstructural changes and their correlations to the variation in phase's hardness. Nanoindentation study of ferrite and martensite hardening response in the DP microstructures showed that the average ferrite and martensite nanohardness has significantly increased from about 181 to 281 HV10mN and decreased from about 644 to 337 HV10mN with increasing intercritical annealing temperatures from 760 to 820 ºC, respectively. Higher intercritical annealing temperatures resulted in finer and harder ferrite grains in DP microstructures. Martensite nanohardness variation with intercritical annealing temperatures is related to change in its carbon content. By applying the rule of mixtures, the calculated hardness values meet well with the experimental values.