Precipitate evolution in Ti-Nb and Ti-V microalloyed steels

In order to gain a quantitative understanding of precipitate evolution in Ti-V and Ti-Nb microalloyed steels during processing, precipitation studies were carried out in (i) Ti-V medium carbon microalloyed bar processed from bloom for forging application, and (ii) Ti-Nb low carbon plates processed from slab for line pipe application. Quantitative investigations involved precipitate characterisation using analytical transmission electron microscopy of samples taken at various stages of steel processing. Experimental results of precipitation were obtained in isothermally heat treated samples from Ti-V blooms. Precipitate dissolution temperatures of samples from Ti-Nb slabs were determined. These results were used to validate predictions based on thermodynamic modelling of precipitation behaviour in Ti-V and Ti-Nb microalloyed steels. Thermo-kinetic analysis of precipitate interaction with recrystallisation in hot rolling of plates was carried out for Ti-Nb microalloyed steel. The model predictions were compared with hot torsion results. The degree of heterogeneity in the size and dispersion of precipitates and the microchemistry of meta-stable precipitates in both Ti-V and Ti-Nb systems are found to be strongly influenced by local concentration of substitutional solutes as set out by the degree of inter-dendritic microsegregation rather than by the bulk chemistry. The influence of precipitate evolution on the controlled rolling temperature is elucidated.