This article has been made available through the Brunel Open Access Publishing Fund.In a recent work we reported that the combined addition of Nb and B has higher grain refinement effectiveness than the addition of these individual chemical elements. In this work a commercial Al-12Si-0.6Fe-0.5Mn alloy is employed to fully characterise the refining potency of Nb-B inoculation. It is found that the addition of Nb powder and KBF4 flux to Al-Si melt introduces potent nuclei with low lattice mismatch with the primary α-Al dendrites (i.e. low undercooling) and, thus, promote noteworthy grain refinement through heterogeneous nucleation. Nb-B inoculation induces a significant increment of the volumetric number of grains, decreases the undercooling needed for solidification and makes the grain size of the Nb-B inoculated material less sensitive to the cooling rate employed to solidify the material.