Role of V and N Addition on the Microstructure and Mechanical Properties of Q690 MPa Medium and Heavy Plates Produced by Thermomechanically Controlled Processing

This research examines the microalloying behaviors of industrial vanadium-nitrogen (V-N) medium and heavy plates rolled by thermomechanically controlled processing. The mechanical properties and microstructures at the quarter and center thicknesses of the medium and heavy steel plates are carefully compared and analyzed. The transmission electron microscopy data show that various locations on the plates see the formation of a considerable number of fine V(C, N) precipitates. The acicular ferrite phase transition is initiated by these V(C, N) precipitates, which inhibit the production of a broad region of pearlite and guarantee the consistency of the mechanical properties of the steel plates. Yield strengths, tensile strengths, percentages of elongation, and yield ratios of 730, 878 MPa, 17.5%, and 0.83 for quarter-thickness and 711, 847 MPa, 18.8%, and 0.84 for midthickness samples are obtained. Additionally, the impact energies at -20 degrees C are 136 and 100 J for the quarter- and midthickness samples. The effect of V(C, N) precipitation on the uniformity of the microstructure and mechanical properties of the Q690 MPa plate is revealed by adding V to the plate for the first time. Q690 MPa medium and heavy plates with this V-N composition system and new production process are successfully industrialized.