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

In this study, high-carbon Nb micro alloyed 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 degrees 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 degrees C and 180 degrees C for 30 minutes. The maximum volume percentage of RA (V-gamma) and C content in RA (C-gamma) are witnessed in the IHT samples subjected to Q&P for 30 minutes at 200 degrees C and 180 degrees C, respectively, followed by the hot rolled air-cooled (HRAC) sample. The formation of very fine NbC precipitates (similar to 17 to 33 nm) is also evident in HRAC and Q&P at 160 degrees C for 30-minute 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 degrees 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 and C-gamma.