Effect of Quenching Temperature on the Strengthening Mechanism of Low-Carbon Microalloyed Quenching and Partitioning Steel

Herein, the microstructures of the low-carbon microalloyed quenching and partitioning (Q-P) steels are multiphase with ferrite (F), martensite (M), and retained austenite (RA) at different quenching temperatures (QTs), i.e., 140, 180, 240, and 300 degrees C. The volume fraction of fresh martensite (FM) increases from 8.6% to 61.4%, but the tempered martensite (TM) decreases from 49.7% to 0% with the QT increase. Meanwhile, the size of the precipitates (Nb, Ti)C tends to increase, whereas the fraction of the precipitates decreases first and then increases. With the increase of the QT, the yield strength (YS) decreases, the tensile strength (TS) increases, and the elongation first increases and then decreases. Additionally, the strengthening mechanism of Q-P steel is quantitatively analyzed in terms of F, M, RA, and precipitation strengthening. This study shows that F makes the same contribution to YS for the tested steels. The major strengthening mechanism of the Q-P steel is contributed by M, which accounts for about 43.7% to 56.0% of total YS. RA greatly influences the plasticity of Q-P steel and provides little to YS. With an increase in QT, the precipitation strengthening values first decrease and then increase.