Effects of Rare Earth Yttrium on Inclusions and Microstructure in FH420 Steel

This investigate the effect of rare earth (RE) yttrium (0, 6, 11, and 19 ppm) on non‐metallic inclusions and microstructure in FH420 offshore engineering steel. Inclusions were characterized by scanning electron microscopy, energy‐dispersive spectroscopy, and electron backscatter diffraction, with mechanisms discussed based on thermodynamic calculations and lattice mismatch analysis. With increasing yttrium content, the average inclusion size decreased from 3.44 to 1.96 μm, while the number density increased from 5.51 to 65.97 #·mm −2 . The fraction of inclusions larger than 4 μm dropped from 38.7% to 7.38%, while that smaller than 3 μm increased from 37.6% to 77.9%. Yttrium addition significantly refined the as‐cast grain structure. At 19 ppm Y, the maximum grain size decreased from 605 μm to 304 μm, and the average grain size from 250 to 125 μm. The minimum lattice mismatch between Y 2 Ti 2 O 7 and α ‐Fe was 1.1%, indicating that Y 2 Ti 2 O 7 inclusions can act as potent heterogeneous nucleation sites for α ‐Fe, thereby promoting grain refinement. This study provides a microstructural foundation for applying trace RE yttrium in offshore engineering steel to support future property optimization.