Abstract: The composition design idea of IF steel is eliminating the atoms of C and N dissolved in steels. On the basis of the thermodynamics analysis and the TEM observation of extraction precipitates, it is shown that sulphur can act as a positive role to fix the interstitial carbon atoms due to the exist of Ti₄C₂S₂. If a proper ratio of sulphur and carbon in Ti-added IF steel is kept, the favorable precipitate will be product.