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 Ti 0 . 55 Nb 0 . 35 V 0 . 10 , while those of duplex type were Ti 0 . 95 V 0 . 05 , Nb 0 . 70 V 0 . 30 , and Ti 0 . 73 Nb 0 . 27 . Three microalloying elements (Ti, Nb, V) formed a coherent M 4 C 3 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.

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The paper describes the precipitation behaviour in a thermomechanically processed V bearing microalloyed steel containing small additions of Ti and Nb (0?007-0?008 wt-%) using analytical transmission electron microscopy. An intriguing aspect is the significant precipitation of titanium and niobium at these low concentrations, contributing to strength. A high density of multimicroalloyed precipitates of (V,Nb,Ti)(C,N) are observed instead of simple TiN, TiC, and NbC precipitates. They are characterised as cuboidal (45-70 nm), spherical (20-45 nm), irregular (20-45 nm), and fine (10-20 nm). Estimation of solubility products of carbides and nitrides of V, Nb, and Ti implies that the precipitation of titanium occurs primarily in austenite. Interphase precipitation of niobium occurs during austenite to ferrite transformation, while complete precipitation of vanadium takes place in the austenite-ferrite region close to completion of transformation. Substoichiometric concentrations of Ti and Nb, the presence of nitrogen, and the mutual extensive solubility of microalloying carbonitrides explains the formation of core shell (triplex/duplex) precipitates with highly stable nitrides ((Ti,Nb,V)N) in the core and carbides ((Ti,Nb,V)C) in the shell. The qualitative stochiometric ratios of triplex and duplex carbonitrides were Ti 0 . 53 Nb 0 . 35 V 0 . 12 and Ti 0 . 6 V 0 . 4 , Nb 0 . 51 V 0 . 49 and Ti 0 . 64 Nb 0 . 36 . Extensive precipitation of fine carbides on dislocation substructures, and sub-boundaries occurred. They were generally characterised as vanadium carbide precipitates with ordered cubic L1 2 structure and exhibited a Baker-Nutting orientation relationship with the ferrite matrix. M 4 C 3 types of carbides were also observed similar to the steel, having high concentrations of Ti and Nb.