Microstructural evaluation in structural steels containing dilute niobium concentrations

The thermomechanical processing of microalloyed steels has been instrumental in the successful development of HSLA steel plate and strip products with enhanced property combinations. Microalloying additions of niobium (Nb) have the most potent effect in retarding the static recrystallisation of austenite during interpass times by solute drag and precipitate pinning mechanism. However, published research on the use of Nb in carbon steel long products (C > 0.20 wt.%) is somewhat limited due to the low solubility of Nb in austenite at such carbon levels. Such steels represent well over 50 % of overall world steel consumption primarily intended for the construction sector and therefore represent an essential area for cost efficiencies.

This research aimed to understand the influence of dilute niobium concentrations (50-200 ppm) on microstructural evaluation in structural steels. The investigation involved three low carbon steels with varying Nb concentration at constant C (0.20%) and N (0.007%) levels. Isothermal double-hit deformation technique led to the determination of T5% and T95% (beginning and end of recrystallisation, respectively) as a function of strain and interpass time.

The results indicate that the T5% increases with increasing Nb supersaturation in austenite at a rate of 40°C per 0.006% Nb supersaturation for a true stain ε=0.40. The Nb supersaturation ratio ≥ 6 resulted in an unrecrystallised microstructure at respective T5% for all tested steels. A high, localised strain-induced precipitation of Nb(CN) was observed at the austenite subgrain boundaries. This translated into higher values for local precipitate pinning force (FPIN), which were significantly higher than one predicted from equilibrium thermodynamics. The critical FPIN for retardation of static recrystallisation was found to be 1.8 MPa at respective T5% for each steel.

The present study has contributed to advancing our knowledge of the interplay between solute supersaturation and volume fraction of Nb(CN) precipitation. Even the lowest addition of 66 ppm Nb could retard the recrystallisation at certain rolling conditions and raised T5% temperature over base steel composition. It is possible to develop a viable TMCP deformation schedule to best benefit from dilute Nb additions. The better control of austenite microstructure will improve the mechanical properties of existing low value-added products through ferrite grain refinement. The dilute Nb additions might permit the reduction in other substitutional alloying elements such as Mn as the Nb provides a more strengthening effect at low cost.