Abstract: In the current study, based on the integrated model for the composition transformation of non-metallic inclusions in the steel, the transformation of the composition of inclusions with time and cooling was introduced. The concept of composition transformation fraction of non-metallic inclusions during solidification and cooling process of the molten steel and during heating process of the solid steel was proposed. The isothermal transformation curve (TTT), the continuous cooling transformation curve (CCT), and the isodiametric transformation curve (TDT) of inclusions composition were introduced. The fluid flow, heat transfer, solidification and solute segregation and the thermodynamics and kinetics for the reaction between the steel and the inclusion were coupled in the integrated model. As examples, effects of the cooling rate during the solidification and cooling process of the liquid steel, heating temperature and heating time during the heating of the solid steel, steel composition, and the size of inclusions on the composition transformation of inclusions in the pipeline steel, the heavy rail steel and the bearing steel were discussed. The concepts and characteristic curves could clearly show the composition transformation of inclusions in the steel and extend the control of inclusions from the molten steel to the solid steel.