Effect of heat treatment on microstructure and properties of 300 M ultrahigh strength steel

Effect of different tempering temperature on microstructure and mechanical properties of 300 M ultrahigh strength steel was investigated by using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The results show that the microstructure of 300 M ultrahigh strength steel consists of tempered martensite, lower bainite and residual austenite after quenching at 870°C followed by tempering at 290-320°C. With tempering temperature increasing, the width of lath martensite increases from 260 nm to 437 nm, the density of dislocation decreases and the amount of lower bainite increases. The tensile strength of 300 M steel declines and its impact toughness increases, however, the yield strength, elongation and area reduction are nearly unchanged with the increase of tempering temperature. The optimal mechanical properties of 300 M ultrahigh strength steel can be obtained by tempering at 300°C.