Effect of Mg treatment on inclusion behaviors in 430 ferritic stainless steel during solidification

Abstract

To understand the effect of Mg treatment on the precipitation, growth, and transformation of inclusions in 430 ferritic stainless steel during solidification, slag refining and Mg treatment experiments with different cooling methods were conducted at 1873 K (1600 °C). Experimental results showed that the oxide inclusions in 430 stainless steel without Mg treatment were mainly Al2O3–SiO2–MnO (slag phase), Al2O3–Cr2O3–MnO (spinel phase), high-Al2O3 (corundum phase) and CaO–SiO2–Al2O3–MnO (slag phase), while all of the oxides except CaO–SiO2–Al2O3–MnO inclusions transformed to MgO–Al2O3–Cr2O3–MnO (spinel phase) after Mg treatment. With water quenching, the average inclusion diameters were about 1.4 μm in both of the Mg-free and Mg-bearing steels, while they increased to approximate 3.1 and 1.9 μm after furnace cooling, separately. With the decrease of cooling rate from 26.2 to 1.24 K s−1, in the Mg-free steel, the average Al2O3 and Cr2O3 contents in the inclusion increased from 19.6 to 42.6 mass% and from 12.22 to 19.44 mass%, respectively, and the average SiO2 content decreased from 43.5 to 17.1 mass%. However, the average composition of inclusions in Mg-bearing steel was relatively stable and independent of cooling rate. A kinetic model was established with the solute micro-segregation model to predict oxide inclusion growth during cooling and solidification. Without and with Mg treatment, under a cooling rate of 1 K s−1, the calculated final sizes of the most stable oxide inclusions were 3.27 and 1.88 μm, respectively. The predicted average compositions and diameters of oxide inclusions were generally consistent with the experimental results.