20MnSi steel samples with different rare-earth yttrium (Y) contents are prepared in the laboratory. Microstructure analysis is performed using the methods of microscopy and electron probe microanalysis, and the mechanical and physical properties are assessed using the tensile tester and thermal dilatometer. The results show that with increasing Y content, the thickness of the martensite layer on the surface of 20MnSi steel decreases gradually until it almost disappears, which is attributed to Y reducing martensitic transformation temperature. Y refines the size of the pearlitic lamellae, which improves the yield strength. Y-containing composite inclusions (Y₂O₂S) of microsized spheres are easily formed in the steel, which assists the nucleation of ferrite, thereby resulting in an increase in the yield point elongation. In addition, the content of inclusions increases, thereby resulting in a decrease in uniform elongation.