Effect of heat-treatment on-line process temperature on the microstructure and tensile properties of a low carbon Nb-microalloyed steel

The microstructures of a low carbon Nb-microalloyed steel processed with the Heat-treatment On-line Process (HOP) technology, whose highest heating temperature ranges from 560 °C to 720 °C, were characterized. The tensile properties were evaluated from the ThermoMechanical Control Process (TMCP) treated samples. The results indicate that the microstructure is primarily composed of non-equiaxed ferrite grains with martensite/austenite (M/A) constituent dispersed at grain boundaries for the specimens with different HOP temperature. The refinement of niobium precipitate particle and increase of volume fraction of precipitation could induce the enhancement of the yield strength when increasing the HOP temperature. The relationship between the average size and volume fraction of the precipitate particles and the precipitation strengthening part of yield strength follows the Orowan–Ashby equation. Moreover, the yield ratio slightly reduces with the increasing amount of M/A constituent, and a platform of yield ratio emerges when the HOP temperature ranges from 590 °C to 720 °C.