Thermodynamic modelling of phase equilibria in oxide systems containing B 2O 3

Microalloying of steel with boron allows both to save alloying components and to obtain high mechanical properties after heat treatment if appropriate smelting technology is used. High hardenability and primary grain refining is achieved if the metal matrix contains 0.002 to 0.008 mass pct. boron being in solid solution, and not bound into oxides, sulfides or nitrides. Thus after introducing to steel boron is to be protected against interaction with oxygen, sulfur and nitrogen. For understanding of complex processes occurring when boron is introduced to steel, a consistent study of its interaction with oxygen, sulfur and nitrogen in the metal containing aluminium, titanium, calcium, magnesium and silicon is needed. In this work a preparation for modelling of thermodynamic interaction of components (Al, Si, Ca, Mg, Mn) with oxygen in liquid steel is performed. Using fundamental concepts of the subregular ionic solution theory phase diagrams of FeO-B 2O 3, B 2O 3-CaO, B 2O 3-MgO, B 2O 3-Al 2O 3, B 2O 3-MnO and B 2O 3-SiO 2 systems are simulated for the temperature range above 1000 °C, the types of ternary phase diagrams FeO-B2O3-MgO, FeO-B 2O 3-Al 2O 3, FeO-B 2O 3-MnO, FeO-B 2O 3-SiO 2 and FeO-B 2O 3-CaO not available in the literature are revealed (for the latter system only an isothermal section at 1600 °C is known). The possibility of drawing component solubility surfaces in the metal containing boron and aluminium is demonstrated. It follows from the diagram that aluminium is effective as a protector against oxidation of boron introduced to steel. Similar information can be obtained for the description of protective effect of other elements too.