Even though microalloying was the most striking advance in the progress of steel technology in the later half of the 20th century, there are still uncertainties as to how each individual microalloying element functions in steel. In the present work, microstructure control by the fine dispersion of TiO and AlN particles has been analyzed from a thermodynamic viewpoint. TiO particles, which act as effective inoculants for the formation of an equiaxed fine grain structure during solidification are categorized as a metallic oxides from the Ti–TiO phase diagram. On the other hand, AlN particles which precipitate anisotropically in the Fe matrix and induce a texture or abnormal grain growth due to their inverse-pinning effect on specified boundaries, are characterized as ionic nitrides. Particle dispersion and the ultra-refinement of grain structure are examined from this viewpoint and a simple model for grain-boundary thickening in Fe alloys due to ultra-refinement is presented.