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In the present study, two distinct starting microstructures of Zr-2.5 wt% Nb have been used: (1) single-phase α hcp martensitic structure; and (2) two-phase, 10% bcc β and rest hcp α, Widmanstätten structure. In the second case, two types of α were present-near grain boundary predominantly single-phase α (about 5% of the total α) and α plates in an apparently continuous β matrix. Both (1) and (2) had similar starting crystallographic texture of the hcp α phase and were deformed by unidirectional and cross rolling. In the two-phase structure the changes in the bulk texture on cold rolling was found to be insignificant, while in the single-phase material noticeable textural changes were observed. Taylor type deformation texture models predicted textural changes in single-phase structure but failed to predict the observed lack of textural development in the two-phase material. Microtexture observations showed that α plates remained approximately single crystalline after cold rolling, while the β matrix underwent significant orientational changes. Relative hardening, estimated by X-ray peak broadening, was observed mainly in β phase; while aspect ratio of α plates remained unchanged with cold rolling-indicating absence of effective macroscopic strain in the hcp α plates. Based on microstructural and microtextural observations, a simple model is proposed in which the plastic flow is mainly confined to the β matrix within which the α plates are subjected to 'in-plane rigid body rotation'. The model explains the observed lack of textural developments in the two-phase structure.

Development of austenite grain structures have been compared in two different microalloyed steels (Nb–Ti and Nb–V steels) and one Al killed C–Mn steel, after soaking at 950–1250°C for 1 h. Minimum austenite grain size in Nb–V steel at the lower soaking temperature (<1075°C) can be attributed to the pinning effect from AlN, Nb(C,N) and V(C,N) precipitates. At the intermediate soaking temperatures (1150–1200°C) dissolution of Nb precipitates led to an abnormal austenite grain growth and the formation of bimodal grain size distributions in microalloyed steels. Grain size bimodality was more severe in Nb–V steel as compared to Nb–Ti steel. Complete absence of precipitates allowed the austenite grains to grow freely at higher soaking temperature (>1200°C) in all the steels. Higher stability of TiN precipitate restricted the grain growth in Nb–Ti steel at higher soaking temperature. An effort has been made to predict the austenite grain size considering both solute drag and Zener drag.

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