where NV is the number of oxide inclusions per unit volume in the specimen, in m−3; Na is the number density of oxide inclusions per unit area in the specimen, in m−2;  di is the apparent particle size of the i-th oxide inclusions among n oxides, in m; d is the harmonic mean of oxide inclusions’ particle size, in m; V is the volume fraction of oxide inclusions; [O]ox is the oxygen content in oxide inclusions in the steel;ρox is the density of oxide inclusions, in kg·m−3;ρFe is the density of the steel, in kg·m−3; (O)ox is the oxygen content of oxide inclusions.