Table 2.
Comparative characteristics of different non-metallic inclusions in steels.
| Operation | Si-Deoxidation | Al-Deoxidation | Ca-Treatment (Modification of Oxides) | Addition of S | Ca-Treatment (Modification of Sulfides) | REM-Addition (Modification of Sulfides) | Addition of Zr, Ti, V, Nb or B |
|---|---|---|---|---|---|---|---|
| Non-metallic inclusions (NMI) | SiO2, SiO2-MnO-... | Al2O3, Al2O3-MgO | CaO, CaO-Al2O3, CaO-Al2O3-... CaO-SiO2-… | MnS, (Mn,Fe)S | Ca(O,S), CaS, (Ca,Mn)S | REM-Ox, REM(O,S)x, REM-Sx | ZrN, Zr(N,C), TiN, Ti(N,C), BN, B(N,C), BC, etc. |
| Formation of NMI in steel | Partially in liquid steel. Partially during solidification of melt due to high content of soluble O (~60–80 ppm) | In liquid steel | In liquid steel | During solidification of melt (large size sulfides—mostly in final solidified zones) | In liquid steel | In liquid steel | Mostly during solidification of melt. Partially after solidification of melt |
| Size of NMI in cast metal | 1–8 µm | 0.1–8 µm | 1–25 µm | 0.5–30 µm | 1–5 µm | 1–3 µm | 0.01–7 µm |
| Condition/ Morphology * | Liquid or solid/SP and RE | Solid/Mostly RE and IR | Liquid or solid/Mostly SP | Solid/Mostly RE and IR | Solid/Mostly SP | Solid/SP and RE/IR | Solid/Mostly RE and IR |
| Distribution in steel | Mostly homogeneous | Mostly homogeneous | Mostly homogeneous | Mostly in final solidified zones, S inhomogeneity | Mostly homogeneous | Mostly homogeneous | Mostly on grain boundaries |
| Cluster formation | No | Very easy. Size of clusters 10–1000 µm | No | Dendrite or coral shape sulfides (Type II and IV), 10–100 µm | No | REM-oxides. Size of clusters 10–300 µm | TiN-“clusters”. Size of clusters 5–30 µm |
| Hardness of NMI (kg/mm2) | Middle/Low (~1600) | High (~3000) | Middle/Low (850–1200) | Low | Middle/Low | High | High |
| Deformability of NMI | Low at T < 900 °C High at T > 1000 °C | No at T < 1300 °C Low at T ≥ 1500 °C | No at T < 1200 °C High at T ≥ 1300 °C | Very high at T < 1000 °C | No at T < 1200 °C Low at T ≥ 1300 °C | Very low | Very low |
| Thermal expansion, α (×10−6 1/°C) | Very low (0.5–5.0) | Low (8.0–8.6) | Low/middle (for CaO-Al2O3 5.0–10.0) | MnS—high (18.1) | CaS—high (~14.7) | REM-Ox—middle (11.2–13.4) REM-Sx—middle (12.3–13.2) | TiN—low (~9.4) |
Notes: *: SP: spherical shape of inclusions, RE: regular shape of inclusions, IR: irregular shape of inclusions.