Dear Kamalashwaran Sivasubaramanian and Jaime Marian,

I give some preliminary remarks.

*The Hall-Petch relation can be considered as a practical rule of thumb. The question above hints at possible limits of application (think of the grain size approaching zero or infinitely large).

*So within the common range of application of the Hall-Petch relation Jaime Marian's answer is valid.

*Grain refining is commonly the result of appropriate combinations of deformation and recrystallization. 

*Besides, the Fe-C phase diagram offers the possibility of grain refining of common Fe-C steels by phase transformations by passing through the phase diagram lines. This is unique for Fe-C alloys.

*Martensitic transformations are of utmost importance for hardening and tempering of a large range of steels, but martensitic transformations also play a part in the heattreatment of e.g. certain titanium alloys. 

Thanks @Hamed Mirzadeh  and @P.Van Mourik  @Jaime Marian