Effect of Magnesium Modification on Inclusions and Carbide in As‐Cast 21‐4N Valve Steel

To investigate the impact of magnesium on the inclusions, carbides, and microstructure of the 21‐4N valve steel, the magnesium modification industrial experiment is carried out. Optical microscope, a scanning electron microscope with an energy‐dispersive spectrometer, thermodynamic calculation, and a 3D etching method are used. In the results, it is shown that the oxide inclusions in steel modifies from Al 2 O 3 –MnO to MgO–MnO–SiO 2 after magnesium modification, which presents a more dispersed size distribution. The primary precipitates in the steel remain as massive carbide M 23 C 6 and lamellar precipitate γ + M 23 C 6 , and the maximum size of large carbides and lamellar precipitates in the magnesium‐modified steel decreases from 15.1 to 13.4 μm and from 98.8 to 68.8 μm, respectively. Furthermore, Mg‐containing inclusions act as more heterogeneous nucleation cores for austenite precipitation, resulting in the refinement of austenite grain and a reduction in the last‐to‐solidify region. The reduction in the last‐to‐solidify region limits the growth space of carbides and contributes to their size refinement.