<p>Rare earth La was introduced into 40Cr steel in industrial experiments to achieve the purpose of modifying inclusions. The impact of La on the inclusion modification was studied, and its influence on the solidification structure was further investigated. With adding 0.0023% La, the Al₂O₃·CaO·CaS inclusions were modified to the LaAlO₃·CaO·CaS inclusions. Additionally, the morphology tended to be more spherical, and the proportion of small-sized inclusions increased significantly from 77.8% to 93.5%. The large-sized inclusions were almost completely eliminated. Based on experimental results, a dynamical model elucidating the process of inclusion modification by La was developed. Furthermore, the ratio of equiaxed zone of the solidification structure increased from 22.9% to 31.0%, and the average primary dendrite arm spacing decreased significantly from 288.4 to 226.2&#xa0;μm. Two-dimensional lattice mismatch analysis results determined that LaAlO₃ can serve as an effective heterogeneous nucleation core, leading to solidification structure refinement. The beneficial transformation of inclusions and refinement of solidification structure are conducive to the cold heading process of 40Cr steel.</p>