Microalloyed V-Nb-Ti and V steels - Part 2 - Precipitation behaviour during processing of structural beams

A comparative evaluation of the precipitation behaviour in V-Nb-Ti and V steels using transmission electron microscopy was undertaken to study the effective cumulative role of the microalloying additions. While the mechanical properties were similar, there were significant differences in the precipitation behaviour of the two steels. The microstructure of the V-Nb-Ti and V steels consisted predominantly of polygonal ferrite and fine pearlite; in addition, the former contained a small amount of bainite. The V-Nb-Ti steel exhibited significantly greater precipitation of carbonitrides compared with the V steel. In the V-Nb-Ti steel the carbonitrides precipitated as compounds (triplex and duplex type) of Ti, Nb, and V, while in the V steel they were V(C,N). They were characterised by cuboid (45-70 nm), spherical/irregular (20-45 nm), and fine/ needleshaped (10-20 nm) morphology. The carbonitride precipitates grew as multimicroalloying compounds, depending on the processing conditions, as the number of microalloying elements increased. Coarse carbonitrides tended to precipitate preferentially along the grain boundaries, whereas fine carbides were dispersed in the matrix. The stoichiometric ratio of triplex carbonitrides in the V-Nb-Ti steel was Ti0.55Nb0.35V0.10, while those of duplex type were Ti0.95V0.05, Nb0.70V0.30, and Ti0.73Nb0.27. Three microalloying elements (Ti, Nb, V) formed a coherent M4C3 type of carbide in the V-Nb-Ti steel and exhibited short range order with the ferrite matrix, displayed by the diffraction pattern as a 'chemically sensitive' or 'superlattice' reflection. However, Nb formed the finest carbides (<2 nm) and exhibited a ring form of selected area diffraction pattern. The carbonitrides that precipitated in the ferrite followed Baker-Nutting orientation relationships, and their partitioning was not observed in the pearlite or bainitic ferrite. The bainite region was characterised by complete precipitation of cementite. In general, the microstructure had a low dislocation density.