Microstructures and mechanical properties of low-carbon microalloyed direct-quenched steel

Reheat-quenching (RQ) and direct-quenching (DQ) heat treatment experiments after rolling were carried out on a kind of low carbon Nb-V microalloyed steel in order to study the toughening and strengthening mechanisms for direct-quenching steel to improve its toughess and strength. Effect of heat treatment processes after rolling on microstructures and mechanical properties of the low-carbon Nb-V microalloyed steel was investigated by means of optical microscope, transparent electron microscope, hardmeter, tensile testing machine and impact testing machine. The experimental results show that DQ steel has shorter lath spacing and higher dislocation density than RQ steel, resulting in higher strength and toughness. A large amount of dislocation in martensite matrix of DQ steel promotes the precipitation of fine and dense carbides, leading to significant secondary hardening during tempering. DQ processed steel exhibits higher tempering resistance than RQ steel because a large number of dissolved Nb and V elements retard various changes of martensite during tempering, and fine and dense precipitates prohibit martensite ferrite from recovery and recrystallization.