Effect of Mn Content on the Reaction between Fe-xMn (x = 5, 10, 15, and 20 Mass Pct) Steel and CaO-SiO2-Al2O3-MgO Slag

Medium- and high-Mn steels have excellent properties but are very difficult to be commercially produced because of the high content of some alloy elements. To enhance the understanding of the reaction between medium/high-Mn steel and refining slag which is significantly different from the conventional steels, steel and slag composition and the inclusions were investigated by equilibrium reaction between Fe-xMn (x = 5, 10, 15, and 20 mass pct) and CaO-SiO2-Al2O3-MgO top slag at 1873 K in the laboratory. Furthermore, the effect of Mn content on inclusion transformation and steel cleanliness was also explored. After slag–steel reaction, both contents of MnO in slag and Si in steel increased. Most MnO inclusions in master steel transformed to MnO-SiO2 and MnO-Al2O3-MgO. With the increase in Mn content, the amount share of MnO type inclusions decreased and that of MnO-Al2O3-MgO type increased. In addition, both the number density of observed inclusions and the calculated oxygen content in inclusions increased. Thermodynamic analysis indicates that the composition change of steel and slag and the transformation of inclusions are mainly the consequence of the reaction between Mn in molten steel and SiO2 and MgO in top slag. The dissolved Mn in medium/high-Mn steel presents a strong reactivity.

Effect of CaO–SiO2–Al2O3–MgO top slag on solute elements and non-metallic inclusions in Fe-xMn(x = 10, 20 mass pct) steel

Medium/high manganese steels have broad application prospects in automotive industry, cryogenic material, etc. because of excellent properties. Precise control on steel composition and improvement of cleanliness are very important for commercial production of these steel grades. In this study, the effect of CaO–SiO2–Al2O3–MgO slag on solute elements and inclusions of Fe-xMn(x = 10, 20 mass pct) steel was studied and discussed. After slag/steel reaction, the concentration of Mn and S in steel reduced, while Si increased. Most MnO type inclusions, which were the main inclusions in master high manganese steel, transformed to MnO–SiO2 type and MnO–Al2O3–MgO type, with MnO–SiO2 sharing the majority. Thermodynamic analysis indicates that the change of solute elements and inclusions was mainly the result of reaction SiO2(s) + 2[Mn] = 2MnO(s) + [Si] between molten steel and top slag as well as slag desulphurization. Increase of oxygen potential of the reaction system would restrain the reaction. Because of the inclusion absorption by top slag, large sized inclusions decreased and steel cleanliness improved greatly after CaO–SiO2–Al2O3–MgO slag was added.

The Research of Low-Oxygen Control and Oxygen Behavior during RH Process in Silicon-Deoxidization Bearing Steel

The variation of total oxygen (T.O) content, characterization of inclusions, slag composition, and off-gas behavior during the smelting process of silicon-deoxidization bearing steel were investigated with industrial experiments. The change of content of combined oxygen during RH (Ruhrstahl–Hereaeus vacuum degassing furnace) process was calculated and compared with T.O content change. It is found that the decrease of oxygen content is mainly caused by the removal of dissolved oxygen rather than the removal of oxides during RH process. Carbon was found to be a strong deoxidizer (stronger than aluminum) in high vacuum degree. Top slag is an oxygen source of the deoxidization process, leading to the re-oxidization of liquid steel, even though the FeO content is low in top slag. During the RH process, the change of oxygen mainly exists in three processes: 1) Deoxidization reaction in vacuum chamber, 2) oxygen mass transfer process between liquid steel out from a vacuum chamber and in ladle, and 3) oxygen mass transfer between ladle slag and liquid steel. It depends mainly on the mass transfer of the oxygen in the liquid steel.

Metallurgical characteristics of refining slag used for high manganese steel

High manganese steel has excellent mechanical properties, which has garnered much attention. Whereas the research on the refining slag used for high Mn steel is very limited. In this study, the metallurgical characteristics of refining slag for high Mn steel were investigated based on thermodynamic calculation with FactSage 6.3 and slag-metal equilibrium reaction in MgO crucible. The calculated liquid zones of T ≤ 1873 K of CaO-SiO2-Al2O3-8%MgO-5%MnO system are located in the middle region of pseudo-ternary CaO-SiO2-Al2O3. For CaO-SiO2-Al2O3-8%MgO-MnO system, the apparent liquid zone at 1873 K enlarges with MnO content in slag increasing, and moves toward the direction of SiO2 and Al2O3 content increasing. For CaO-SiO2-Al2O3-MgO-MnO system, the liquidus zone shrinks with the basicity increasing, and moves toward the direction of Al2O3 content increasing. The measured MnO content in top slag reacted with high Mn steel was much higher than that reacted with conventional steels. In present experiments, the MnO content was around 5% when CaO-SiO2-Al2O3-MgO slag with basicity of 4 was in equilibrium with high Mn steel (Mn = 10, 20%) at 1873 K. The inclusions in master high Mn steel were mainly MnO type. After reaction with top slag, most inclusions transformed to MnO-SiO2 system and MnO-Al2O3-MgO system, in which the MnO content still shared the majority. Thermodynamic calculations show that SiO2 in top slag can be reduced by [Mn] in steel to supply [Si] under present experimental condition, which subsequently reacts with [O] in steel bath to form SiO2.