What could be the cause of coarse grain structure in heat treated D2 Steel ?

I've fracture tested a D2 piece and noticed a big too grainy structure after breaking the piece.. It was heat treated @ 1010-1015 degrees C (15 minutes - small piece) and air cooled, double tempered @ 300 degrees C. I'm not sure if the furnace is calculated properly - could this be a cause of too high Heat Treat temperature ? The heat treat resulted in 60HRC... It wasn't easy to break, but it was about .160" thick. Is a coarse grain a normal thing in D2 Steel ?