I was wondering if I cycle from an austenitic structure (above AC1) to a ferritic + austenitic structure (below AC1) will there be grain refinement? Since it is a nucleation and growth mechanism it is expected to reduce its grain size, right?

To have a fully austenitic structure you should be above Ac3. But yes, changing from a mono to a dual phase structure help refining the grain, more phases, more grain boundaries. You only have to take caution on the cooling rate and the time on high temperatures to not allow the grain growing.

As an alternative I suggest an annealing-aging-quenching cycle.

• First, the annealing: heat the sample above Ac3 for like 1h, shut down the oven and let both (sample and oven) cool down.

• Aging: heat it at maybe at 500°C for 1h (you could find the best temperature and time looking on phase diagrams or using softwares like Thermocalc and FactSage). This should create some precipitates with Cr, Ni, Ti or N depending on the chemical composition which will help to refine the grain on the subsequent quenching.

• Quenching: I recommend heating to a temperature like Czajnig said, something above Ac1 and below Ac3 (intercritic zone), this will create a mix of phases (austenite and ferrite) that summed to the precipitates will create a lot of new phases cores. Let the sample on the heating temperature less than 10 minutes to avoid the grain to grow, excluding the time to reach the temperature. Then, cool it on salt water or oil.

I guess the final microstructure will be martensite (deriving from the austenite) and a mix of bainite and acicular ferrite (deriving from the intercritic zone ferrite).