Precipitation Characteristics and Mechanism of Vanadium Carbides in a V-Microalloyed Medium-Carbon Steel

The precipitation characteristics and mechanism of vanadium carbides during isothermal transformation at 650 °C in a V-microalloyed medium-carbon steel were investigated through scanning electron microscopy and transmission electron microscopy as well as dilatometry test. Five morphologies of vanadium carbides were found to precipitate at different nucleation sites during the transformation. Two kinds of interphase precipitation form simultaneously in both pro-eutectoid and pearlitic ferrites. The linear arrays of fine interphase precipitates are parallel to the γ/α interface, and the fine needles of interphase precipitates are perpendicular to the γ/α interface. The vanadium carbides of long or short fibers, coarse particles and fine particles form in both pro-eutectoid and pearlitic ferrites, displaying different precipitation distributions and orientation relationships with ferrite. The precipitation mechanisms of vanadium carbide precipitates with different modes were proposed, and the precipitation sequence of various vanadium carbide precipitates was finally ascertained.