Abstract: U75V heavy rail steel was used in present work to investigated the transformation of inclusions during solidification and cooling of molten steel.Steel samples with varied chemical compositions were taken from blooms and molten steel of tundish,which was analyzed by the Aspex automatic scanning electron microscope,combined with thermodynamic calculation,the transformation mechanism of inclusions in the solidification and cooling process of heavy rail steel was obtained.The results show that inclusions in molten steel of tundish were mainly CaO-SiO₂-Al₂O₃-MgO type,which was uniform in chemical compositions.During solidification and cooling,not only the composition of inclusions changed,but also the heterogeneity of phase.The inclusions in blooms were CaO-SiO₂-Al₂O₃-MgO-CaS type with the content of CaO reduced in inclusions,while that of CaS increased,and the inclusions that showed a multiphase structure were mainly composed of phases such as CaS,MgO·Al₂O₃ and CaO-SiO₂-Al₂O₃-MgO,in which the MgO·Al₂O₃ phase located inside the CaO-SiO₂-Al₂O₃-MgO phase,and CaS wrapped outside.Thermodynamic calculation results are basically consistent with the experimental ones,and the difference in inclusion compositions may be attributed to the insufficient thermodynamic and kinetic conditions.

Key words: heavy rail steel, inclusions, solidification and cooling, transformation