Transformation and Precipitation Behavior in Low-Carbon Microalloyed Steels

摘要

Thermo-mechanical controlled processing (TMCP) is widely applied to manufacture high strength steel plates. In order to fully utilize the benefit of alloying elements, such as Nb, V and Mo that are usually added to TMCP steels, precipitation behavior of those elements in accelerated cooling (AcC) steel is investigated and compared with those of quenching and tempering (Q-T) steels and isothermal heat treated steels after controlled rolling. Cementite morphology that affects toughness of the materials is also investigated. In the AcC steels, complex carbonitride was hardly precipitated. However, fine complex carbonitrides including mainly Nb were precipitated randomly in the bainitic ferrite matrix during tempering treatment in the Q-T steel. Fine complex carbonitrides were also found in the isothermal heat treated steel, same as the Q-T steel, but the morphology of precipitation was different; row precipitation was observed. Cementite becomes finer in the Q-T steel when proper tempering treatment was applied. These results give possibilities of getting materials with higher strength and superior toughness by controlling the amounts and morphology of fine precipitates and cementite.