| Abstract Scope |
The effects of Mg content on inclusion evolution and acicular ferrite (AF) formation in Ce-La-treated laboratory-prepared X80-type steel were investigated by thermodynamic calculation, automated scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), microscopy, and quantitative metallography. As the measured Mg content increased from 0 to 0.0026 wt.%, the dominant inclusion cores changed from Al-O to Mg-Al-O, while rare earth (RE)-Al-O outer regions evolved toward RE-S-O and MnS-containing composite structures. The mean inclusion size decreased from 2.40 to 2.10 μm, the number density increased from 75 to 217 mm-2, and the AF fraction increased from 12% to 43%. At 0.0048 wt.% Mg, MgO- and MgS-rich particles aggregated, MnS precipitation was reduced, and the AF fraction decreased to 28%. SEM-EDS mapping and line-scan results were consistent with heterogeneous nucleation associated with low-misfit composite inclusions and a local decrease in Mn signal near inclusion-matrix interfaces. At an approximately equal total modifier content of 80 ppm, Mg2 (26 ppm Mg + 54 ppm RE) produced smaller, more numerous, and more frequently AF-associated inclusions than Mg4 (80 ppm RE), together with a higher AF fraction (43% against 13%). Among the Mg levels examined, 0.0026 wt.% gave the most favorable inclusion population and AF response under the present laboratory conditions. |