Abstract:The inclusions in the Ti-stabilized SUS436L ultra-pure ferritic stainless steel(FSS)plates were researched by means of the metallographic electron microscopy,the scanning electron microscope and the energy spectrum plane scanning. The formation mechanism of inclusions was analyzed based on the thermomechanical analysis. The results showed that the inclusions in the SUS436L stainless steel included pure TiN particles,composite inclusions of TiN contained MgO·Al2O3 spinel and the composite oxides of Al2O3-CaO-TiO2. When the mass fraction of N was 0.007 0% and the molten steel temperature was in the range of 1 600-1 650 ℃,the equilibrium Ti mass fraction was in the range of 0.23%-0.38%. And MgO·Al2O3 spinel was generated when the mass fraction of Mg was more than 0.000 8%,while MgO was generated when the content of Mg was more than 0.004 4% at the temperature of 1 600 ℃ and the Al mass fraction of 0.02%. On this condition,the low melting point 12CaO·7Al2O3 and 3CaO·Al2O3 were generated when the Ca mass fraction was 0.000 14%-0.000 36% and more than 0.000 36%,respectively,after the calcium treatment. These two kinds of inclusions could form the low melting point A12O3-CaO-TiO2 composite oxides after the Ti alloying process.