Evolution of inclusions and change of as-cast microstructure with Mg addition in high carbon and high chromium die steel

The effects of Mg on the inclusions and the as-cast microstructure of high carbon and high chromium die steel, a grade of cold working die steel with high C content of 1.4–1.6% and high Cr content of 11.0–13.0%, were systematically investigated. It is found that inclusions vary with the route as Al2O3 (No Mg) to MgO·Al2O3 + Al2O3 (5 ppm Mg), and then to MgO+MgO·Al2O3 (11 and 15 ppm Mg). The average diameter of the inclusions decreased from 1.91 μm (no Mg) to 1.29 μm (15 ppm Mg), while the number density increased from 2.69 × 104 mm− 3 (no Mg) to ∼5.62 × 104 mm− 3 (15 ppm Mg). The changes in the size and the number density were discussed in terms of the effect of inclusions on the nucleation process and the wettability of them with steel melt. The as-cast microstructures were greatly refined with Mg addition that correlated with the evenly dispersed fine Mg containing inclusions.