Abstract: A thermodynamics model for deoxidation in 1Cr21Ni5Ti system with Al as a deoxidizer and a slag designed for this system was developed based on ion and molecule coexistence theory (IMCT)， thermodynamics equilibrium and mass conservation. The equilibrium mass fractions of Si， Al， Mn in steel and the TiO₂ content in slag designed for controlling the ［Ti］ were obtained depending on the increasing Al content. The results showed that the mass fractions of Si， Al， Mn and Al₂O₃ increase with increasing Al content， while the mass fractions of SiO₂， FeO， MnO and TiO₂ content tend to decrease. When Al content increases to 0.2%， the Al mass fraction in steel may increase with little unstable oxide in slag and the deoxidizer Al dissolving into steel. Considering the limitation of Si， Al， Mn mass fraction and little wave of Ti， the amount of Al added in one ton steel should not exceed 0.15% and the TiO₂ content designed for controlling Ti is 4%.

Key words: coexistence theory (IMCT), aluminum and titanium alloy, ESR (electro slag remelting), slag design, deoxidation thermodynamics