Effect of Quenching and Partitioning on Microstructure and Mechanical Properties of High-Carbon Nb Microalloyed Steel

In this study, high-carbon Nb microalloyed hot rolled steel plates are subjected to quenching and partitioning (Q&P) treatment at different temperatures and time durations. Dilatometry results show that increasing cooling rate (CR) leads to suppression of transformation start and finish temperatures of high-temperature transformation products like pearlite (P) or bainite (B), whereas martensite (M) transformation triggers with increasing cooling rate from 5 to 50 °C/s. The observations made by optical (OM), scanning (SEM), and transmission electron microscopy (TEM) reveal mixed-phase microstructures consisting of preformed/tempered martensite (PTM), retained austenite (RA), twin martensite (TM), and lower bainite (LB) for the isothermally heat-treated (IHT) samples subjected to Q&P at 200 °C and 180 °C for 30 minutes. The maximum volume percentage of RA ( \({V}_{\gamma }\) $V_{\gamma }$ ) and C content in RA ( \({C}_{\gamma }\) $C_{\gamma }$ ) is witnessed in the IHT samples subjected to Q&P for 30 minutes at 200 °C and 180 °C, respectively, followed by the hot rolled air-cooled (HRAC) sample. The formation of very fine NbC precipitates (~17 to 33 nm) is also evident in HRAC and Q&P at 160 °C for 30 minutes samples, which are expected to contribute significantly to precipitation strengthening. Hardness shows an increasing trend from 36 HRC to 65 HRC with increasing CR from 0.5 to 50 °C/s. The best combinations of the ultimate tensile strength (UTS) and total elongation (TEL) in Q&P 200 samples (36043 MPa pct) followed by Q&P 180 samples (32759 MPa pct) can be correlated with their higher values of \({V}_{\gamma }\) $V_{\gamma }$ and \({C}_{\gamma }\) $C_{\gamma }$ .