Nanoindentation study of ferrite–martensite dual phase steels developed by a new thermomechanical processing

Dual phase (DP) steels consisting different volume fractions of ferrite and martensite and different ferrite grain size were produced by a new route utilizing cold-rolling and subsequent intercritical annealing of ferrite/martensite duplex starting structure at 770 °C for different times. Scanning electron microscopy has been supplemented by nanoindentation and tensile test to follow microstructural changes and their correlations to the variation in phase's hardness and mechanical properties. The results showed that longer holding times resulted in coarser and softer ferrite grains in DP microstructures. Martensite nanohardness variation with holding time is related to change in its carbon content. Mechanical properties such as strength, elongation and toughness are well correlated with the martensite/ferrite hardness ratio