Influence of microstructure on tensile properties of dual phase steel [Çi•ft fazli çeli•gi•n çekme özelli•kleri•ne mi•kroyapinin etki•si•]

The influence of volume fraction of martensite and new ferrite on the tensile behaviour of dual-phase steel containing 0,0981% C, 1,653% Mn, 0,54% Si and 0,69% Ni has been studied. After intercritical annealing at 715, 725 and 752 °C, the specimens were cooled in water, oil and furnace. The formation of carbide was not observed in the optical microscope investigation even at furnace cooling rate. The results showed that after annealing at the same intercritical temperature, the highest volume fraction of martensite was obtained by water cooling among all cooling conditions. By increasing the martensite content, both yield and tensile strengths increased while uniform elongation was decreasing. It was observed that at constant volume fraction of martensite, new ferrite content was varied by using controlled cooling from intercritical annealing temperatures. The presence of new ferrite caused a decrease in strength and increase in ductility. Scanning electron microscope investigations of fracture surface showed that the specimens with new ferrite had higher densities of microvoids in the necked region and exhibited completely ductile type of fracture. In contary, it was also observed that the specimens with no new ferrite had lower densities of microvoids and exhibited mixed type of fracture which comprises both cleavage and ductile fracture together