Effect of Microalloying Additions on the Microstructure and Mechanical Properties of Low Carbon Steel

Low carbon steels are characterized by good weldability, formability and fracture toughness properties. However, the low strength levels of these steel grades limit their wide applications. On the other hand, increasing the strength by increasing the carbon content and alloying elements deteriorates the other properties. In this study, the microalloying technique was used to examine the possibility of attaining low carbon steels with good combination of strength, ductility and impact properties. A low carbon steel microalloyed with single addition of vanadium and another one microalloyed with combined addition of vanadium and titanium were used in this investigation and their properties were compared with non-microalloyed low carbon steel having the same base composition. Furthermore, other two non-microalloyed and V-microalloyed steels with higher carbon, silicon and manganese contents were also investigated to reveal the effect of base composition. Tensile, hardness, room and zero temperature Charpy V-notch impact tests were conducted to evaluate the variations in the mechanical properties of low carbon hot forged steel containing vanadium and combinations of vanadium and titanium. In addition, the microstructures of the different investigated steels were observed using both optical microscope and scanning electron microscope. Furthermore, the hardness of the ferrite phase was also determined using micro-hardness technique. The results showed improvement of the mechanical properties of the investigated steels by both single V- and combined V + Ti - microadditions. Tensile, hardness and impact tests results indicated that good combinations of strength, ductility and impact properties can be achieved by V-microalloying addition. Steel with combination of V and Ti microaddition has much higher hardness, yield strength, ultimate tensile strength and impact energy at both room and zero temperatures compared with non-microalloyed and single V-microalloyed steels. Higher C, Si and Mn contents result in increasing the strength accompanied with decreasing the impact energy. Scanning electron microscopy and optical microscopy studies revealed grain refinement effect of both V-and V+Ti - microadditions. The micro-hardness measurements of the ferrite phase confirmed the precipitation strengthening effect of microalloying elements.