Oxygen Sources
During the steelmaking process, molten steel contains a certain amount of oxygen. Oxygen is intentionally supplied during production to facilitate oxidation and the removal of impurities such as phosphorus (P), sulfur (S), silicon (Si), and carbon (C). Despite variations in steelmaking processes, the relationship between carbon (C) and oxygen (O) in the molten steel pool follows a consistent pattern: as the carbon content gradually decreases, the oxygen content correspondingly increases, maintaining a balanced equilibrium.
Oxygen Forms
Oxygen exists in both combined and free states within steel. Generally, the free state is minimal, while the combined state primarily includes compounds such as Fe₂O₃, Fe₃O₄, FeO, metallic oxide inclusions, silicates, aluminates, oxysulfides, and similar non-metallic inclusions.
Oxygen Effects
Like hydrogen, oxygen negatively impacts the mechanical properties of steel. Its influence is not only determined by its concentration but also by the amount, type, and distribution of oxygen-containing inclusions. These inclusions consist of metallic oxides, silicates, aluminates, oxysulfides, and similar compounds.
Deoxidation is essential during steelmaking because, during solidification, oxygen in the molten steel reacts with carbon to form carbon monoxide (CO), potentially leading to gas porosity. Furthermore, during cooling, oxygen can precipitate as FeO, MnO, and other oxidized inclusions, diminishing the steel’s thermal and cold workability, ductility, toughness, fatigue strength, and overall mechanical properties.
Additionally, the presence of oxygen, nitrogen, and carbon can cause aging—a spontaneous increase in hardness at room temperature. For cast iron, oxides formed during solidification may react with carbon, resulting in porosity and embrittlement of the final product.
