Apart from the above mentioned role, more important role of Nb in HSLA is to provide dispersion strengthening by forming NbC or Nb(C,N) (in case of Nitrogen containing steel), along with other carbide forming element such as Ti, V,Mo etc. The required strength is achieved by modifying the F+P microstructure into one containing F and carbides.

I agree with Mr Nayak. Additionally Nb(C,N) precipitations  decrease the the austenite grain size at high temperature...

In addition, there are also stain induced precipitation, solid solution strengthening , precipitation and dispersion strengthening depending on whether the carbides are coherent or incoherent. However, all these effects are consequence of controlled rolling that are facilitated by Nb.

Besides above mentioned, one of the important purpose of addtion Nb in HSLA steel is to delay the temperature of the recrystal temperature and make the rolling proceed at even more lower temperature.

Nb significantly delays recrystallisation and large deformations at high temperatures are required for complete recrystallisation- This refines austenite grain size via complete recystallization. At Lower rolling temperatures, Nb retards austenite recrystallisation, as a solute and also via precipitation of fine Nb(C,N)- usually as strain induced precipitation, leading to non-recystallized, deformed austenite grains. Also known as "Pankacke grains".

Deformed austenite is in a high energy state and will promote a high nucleation rate of Ferrtie, thus very fine grains of ferrite are formed. This provides excellent strength and toughness (Hall-Petch relationship). As already mentioned, Nb precipitation strenghtening adds further strengthening.

Well, although enough has been said (and correctly so), I will make just a small contribution. Nb, when in solution delays recrystallization, and when exist in the form of Nb(C,N) hinders austenite grain boundary migration, thus both mechanisms help retain the deformed austenite grains (pan-cake morphology) during controlled rolling of HSLA. The effectiveness of Nb addition, however, is another thing. If Ti(C,N) precipitates exist then Nb(C,N) precipitates tend to form epitaxially on them, leading to reduced efficiency of Nb(C,N) precipitates. Please see the attached TEM micrograph from my unpublished work.

I could also add to previous comments, that Nb helps to avoid sensitizing, relevant when welding operations are envisaged.

If you want it from an authoritative source  then I suggest you get hold of "Physical Metallurgy and the design of Steels", by F.B. Pickering. He was one of the most important figures in the design of HSLA steels.

Absolutely so... Pickering is the best source in my opinion too... Many thanks to Prof. Robinson for reminding us.

The effect of Nb on strenghtening HSLA steels is relevant as mentioned above; Nb acts as solute or precipitate to allow fine grained structure; however it is useful  to mention its harmfull effect on the hot ductility behaviour when compared, for instance, with the other alloying elements such as V and/or Ti