Thermodynamic analysis formed by non-metallic inclusions in welding gas vessel steel HP295

The composition of inclusions in the process of steelmaking and solidification is predicted by the thermodynamic calculations for HP235 steel. The morphology and composition of inclusions are analyzed by SEM and EDS. The results show that the inclusions in the molten steel mainly are tridymite (SiO₂), spessartine (3MnO· A1₂O₃·3SiO₂), mullite (3A1₂O₃·2SiO₂) and corundum (A1₂O₃) by using silicon -manganese deoxidization. The composition of inclusions change gradually from tridymite mainly to corundum with the content of Al increased, or O reduced. Main inclusions is tridymite when Al < 1×10^-6, and the main is corundum when aO < 3×10^-3. The composition of inclusions precipitated have relationship with the contents of Al and O during solidification. At the temperature of 1550℃, the precipitation is tridymite when aO>115.6×10^-5 and Al < 4.5×10^-6, the precipitation is mullite when aO < 115.6×10^-5 and Al < 10.5×10^-6, and the precipitation is corundum when Al>10.5×10^-6. At the temperature of 1510℃, the precipitation is tridymite when aO>75.2×10^-5 and Al < 3.51×10^-6, the precipitation is mullite when aO < 75.2×10^-5 and Al < 8.18×10^-6, the precipitation is corundum when Al>8.18×10^-6. In addition, when Ca exists in the steel, freezing precipitation of calcium may even leave wollastonite plagioclase. The composition and quantity of precipitates have relationship with the activity of the calcium in the steel. The predicted results by thermodynamic calculations are mainly consistent with the results by SEM and EDS. So, the thermodynamic calculation method is suitable for predicting composition of inclusions in the steel.