<p>An industrial scale test was conducted to investigate the effects of no treatment, Ca treatment, Ce treatment and Ca + Ce composite treatment on the modification of inclusions, microstructure and mechanical properties of Al-killed Q355B steel. Results showed that the stability of the continuous casting process was improved when Ca pretreatment was added before Ce treatment compared to Ce treatment alone. After Ca + Ce composite treatment, the lowest total oxygen content and average diameter, lower number density and the smallest aspect ratio of inclusions were obtained. The thermodynamic calculations showed that the phases stabilized under the Ce-treated and Ca + Ce composite-treated steel compositions were Al₁₁O₁₈Ce + CeAlO₃ and CeAlO₃ + CaO–Al₂O₃ + liquid inclusion, respectively. It was also found that the oxygen content in the steel affects the Ce modification on inclusions and needs to be reduced before modification. The Ca to Ce ratio had a large effect on the composition of inclusions in the case of Ca + Ce composite treatments. Furthermore, adding rare earth Ce could effectively refine the solidification structure, improve micro-segregation, and also obtain finer equiaxed ferrite grains and dense pearlite lamellar structure. Moreover, a small amount of Ce treatment does not have a significant effect on the strength of the steel, but higher cleanliness and finer organization can have a positive effect on the plasticity and toughness of the material.</p>