Effect of microstructure on mechanical properties of dual phase steels in the presence of stress concentrators

The study investigated the effect of intercritical heat treatment on mechanical properties of three hot rolled steels with carbon contents ranging from 0.06 to 0.18 wt%. The steels were heat-treated at temperatures ranging from 740 to 820 °C, followed by water quenching (in some cases air cooling). The primary objective of this study was to investigate the relationship between microstructure and the effect of stress concentrator on deformation and fracture behavior of ferrite–martensite dual-phase (DP) steels. Besides the tensile tests with smooth specimens, notched tensile specimens with two notch geometries (flat rectangular specimens with a central hole and circumferentially notched cylindrical specimens) were used in order to modify the stress triaxiality. Moreover, Charpy impact test was carried out on sub-size specimens (10 mm×2.5 mm×55 mm) with notch root radius of 0.25 mm and on pre-cracked specimens. It was found that with increasing martensite volume fraction (MVF) the capacity of the steels to absorb energy (expressed as the product of the ultimate tensile strength and uniform elongation) remains roughly stable or decreases in case of smooth tensile specimens while it always increases in case of specimens with stress concentrator. The experimental results presented in this study clearly showed that DP steels with an excellent combination of strength and uniform or total elongation may have poor energy absorption capacity in the presence of stress concentrators.