Effects of cooling after rolling and heat treatment on microstructures and mechanical properties of Mo–Ti microalloyed medium carbon steel

The effects of four different processes, direct quenching + tempering (DQ + T), direct quenching + reheating and quenching + tempering (DQ + RQ + T), forced air cooling + reheating and quenching + tempering (AC + RQ + T), and furnace cooling + reheating and quenching + tempering (FC + RQ + T), on microstructures and precipitates of Mo–Ti microalloyed medium carbon steel were investigated by OM, SEM, TEM, EBSD and mechanical tests. The width of martensite laths (0.22 ± 0.01 μm) produced in DQ + RQ + T was similar to that generated in DQ + T and was narrower than those formed during AC + RQ + T and FC + RQ + T. The effective grain size of the DQ + RQ + T sample was found to be the smallest (0.98 ± 0.38 μm), thus it yielded the finest (Ti, Mo) C precipitates, manifested high strength and excellent plastic toughness, and generated uniform transverse and longitudinal properties. The strength of the DQ + RQ + T sample (longitudinal Rm = 1773 MPa) was slightly lower than that of the DQ + T samples and its elongation (A = 11.0%) and impact energy at −40 °C (Akv = 50 J/cm2) were greater than those of the other three samples. Before the RQ process, the DQ cooling process was employed in order to generate large angle grain boundaries to provide more nucleation sites for austenitization. Moreover, the DQ process inhibited the precipitation of Ti during cooling and then formed fine (Ti, Mo) C precipitates during reheating of RQ to prevent austenite grain growth. Therefore, the DQ + RQ + T process generated fully refined austenite grains, made the martensite structure and (Ti, Mo) C precipitates more uniform, and enhanced the effects of fine grain strengthening and precipitation strengthening. In addition, the plasticity and the toughness of the fine and uniform equiaxed grains were also significantly improved, and the formation of acicular cementites was impeded.