Improvement of strength and toughness of 1 GPa Cu-bearing HSLA steel by direct quenching

A novel process based on recrystallization controlled rolled (RCR) + on-line direct quenching (DQ) and tempering (DQT) was developed to improve both yield strength and low-temperature toughness for 1 GPa Cu-bearing HSLA steel. Reheating quenching (RQ) and tempering (RQT) processes were compared to examine the effects of DQ and DQT on co-precipitation behavior of Cu-rich particles and Mo, V carbides. The evolution of microstructures under various heat treatments was characterized using optical microscope (OM), scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and X-ray diffraction (XRD). Tensile properties and low-temperature impact toughness were also tested. Differential scanning calorimetry (DSC) shows that lower activation energy of Cu precipitation in DQ steel than that in RQ steel. The lower activation energy can be attributed to higher density dislocations and finer martensitic laths, which results in high-density and finer Cu-rich particles formed in DQT steel. Meanwhile, the coarsening of Cu-rich particles in DQT steel is also inhibited resulting in the Cu-rich precipitates with similar to 5.8 nm in DQ steel aging at 550 degrees C for 240 min. Subsequently, the precipitation of Mo, V carbides is promoted at the nucleation sites provided by the pre-precipitated high-density Cu-rich particles, with the rapid dissolution of cementite. Synergistic enhancement of yield strength and low temperature toughness has been achieved with a yield strength of 1056 MPa as well as impact energies of 204 J and 74 J at -40 degrees C and -85 degrees C, respectively.