Mechanism exploration of an ultrahigh strength steel by quenching–partitioning–tempering process

The mechanical property of a Nb-microalloyed steel with different heat treatment processes are investigated. The sample with good strengthening-ductility match after quenching–partitioning–tempering (Q–P–T) 2-step process is selected as further microstructure characterization by means of scanning electron microscope (SEM) equipped with electron backscattered diffraction (EBSD) mapping device and transmission electron microscope (TEM). The results indicate that the Q–P–T 2-step sample shows best mechanical property due to its optimized microstructure with multiphase and multiscale, i.e., lath martensite (submicron-scale in thickness), retained austenite (nano-scale in thickness for interlath film-like type and submicron in diameter for island-like type) and carbide precipitates (nano-scale in diameter). This microstructure feature is named as multiphase-metastable-multiscale (3M) configuration. The idea of the 3M microstructure is proposed to explain the reason why Q–P–T steels possess good mechanical properties.

► The Q–P–T 2-step sample exhibits excellent mechanical property.
► Different phases in the Q–P–T 2-step sample are clearly characterized.
► A 3M microstructure is proposed to explain the mechanism of such a good property.