Some Metallurgical Issues Concerning Austenite Conditioning in Nb-Ti and Nb-Mo Microalloyed Steels Processed by Near-Net-Shape Casting and Direct Rolling Technologies

As thin slab direct rolling technologies are moving to the production of higher quality steel grades, chemical compositions based on Nb-Ti and Nb-Mo become a good option. However, with the use of multiple microalloying additions, the as-cast austenite conditioning becomes more complex. This paper analyzes some of the microstructural features that should be taken into account during the as-cast austenite conditioning in Nb-Ti and Nb-Mo microalloyed steel grades. In the case of Nb-Ti grades, it has been observed that the process parameters during solidification and post-solidification steps affect the austenite evolution during hot rolling. This is due to the differences in the size and volume fraction of TiN particles that can be formed. Fine TiN precipitates have been shown to be able to delay recrystallization kinetics. Moreover, the solute drag effect of Ti cannot be ignored in the case of hyperstoichiometric Ti/N ratios. It is observed that Nb-Ti grades tend to have lower non-recrystallization temperatures compared to Nb grades, which means that pancaking of the austenite is more difficult for these steels. The opposite is observed for the Nb-Mo grades, although in both cases the behavior is affected by the nominal content of Nb.