Experimental and DFT study on cerium inclusions in clean steels

To provide insights into deforming Ce-O-S-Al inclusions in steels and improving the mechanical properties,the evolution process of such harmful inclusions in clean steels was investigated by thermodynamic calculation,metallographic examination and first-principles calculation in this paper.For the tested IF steel,the the rmodynamic analysis results are consiste nt with the calculated formation enthalpy.After Ce addition,the inclusions are transformed from Al_2 O_3 and TiN-Al_2 TiO_5-Al_2 O_3 to Ce_2 O_3,Ce_2 O_2 S,CeAlO_3,TiN-Al_2 TiO_5-Ce_2 O_3 and TiN-Al_2 TiO_5-Ce_2 O_2 S 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) approximation.All inclusions except Ce_2 O_3 show appare nt brittle ness.TiN,Al_2 O_3,Al_2 TiO_5 and CeAlO_3 present much higher hardness than iron matrix,while the hardness of Ce_2 O_3 or Ce_2 O_2 S is close to that of iron matrix.The thermal expansion coefficients of Ce_2 O_3 and CeAlO_3 are close to that of iron matrix,whereas,Ce_2 O_2 S 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.