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To provide insights into deforming Ce-O-S-Al inclusions in steels and improving the mechanical prop-erties,the evolution process of such harmful inclusions in clean steels was investigated by thermody-namic calculation,metallographic examination and first-principles calculation in this paper.For the tested IF steel,the thermodynamic analysis results are consistent with the calculated formation enthalpy.After Ce addition,the inclusions are transformed from Al2O3 and TiN-Al2TiO5-Al2O3 to Ce2O3,Ce2O2S,CeA1O3,TiN-Al2TiC5-Ce2O3 and TiN-Al2TiO5-Ce2O2S composite inclusions,which can be confirmed by metallographic examination.The elastic constants were calculated,and the bulk modulus,Young's modulus,shear modulus and Poisson's ratio were evaluated by the Voigt-Reuss-Hill(VRH)approxima-tion.All inclusions except Ce2O3 show apparent brittleness.TiN,Al2O3,Al2TiO5 and CeAlO3 present much higher hardness than iron matrix,while the hardness of Ce2O3 or Ce2O2S is close to that of iron matrix.The thermal expansion coefficients of Ce2O3 and CeA1O3 are close to that of iron matrix,whereas,Ce2O2S inclusion has largely different thermal expansion coefficient from iron matrix and may deteriorate the steel performance at higher temperatures.The relatively small differences between Ce inclusions and iron matrix in terms of hardness,toughness,brittleness,and thermal expansion coefficient can explain the improvement of the mechanical properties of the tested steel.