Effect of cooling rate after controlled forging on properties of low carbon multi-microalloyed steels

Two low carbon steel grades were used in the present investigation. One of them was microalloyed with Ti, V, and Nb. Both steel grades were subjected to a controlled hot forging followed by either cooling in air or quenching water. The microstructures of all TMT conditions are dominated by ferrite phase with different morphologies and grain sizes according to both chemical composition and cooling rate. Polygonal ferrite is considered to be a dominated phase of air cooled microstructures for both steel grades that is responsible for decreasing the hardness, yield, and tensile strength with the attendant increase in ductility. Water quenching leads to a formation of relatively fine polygonal ferrite in low carbon steel or transformation into acicular ferrite in low carbon microalloyed steel. Relatively fine polygonal ferrite and acicular ferrite increase strength but decrease ductility. The cooling rate has a negligible effect on the impact toughness at room temperature.