Oxygen in Molten Steel: The Basic Forms
Oxygen is one of the most influential elements in steelmaking. Its behavior directly affects the purity, structure, and performance of the final steel. In molten steel, oxygen mainly exists in the following forms:
- As ferrous oxide (FeO)— oxygen combines with iron.
- As ionic complexes— oxygen interacts with FeO clusters.
- As a surface oxide film— when FeO concentration becomes high, oxides migrate to the surface, forming a thin FeO layer.
In general, the solubility of oxygen in both liquid and solid iron is extremely low, and it becomes even lower in solid iron than in liquid steel.
How Temperature Affects Oxygen Solubility
Temperature plays a critical role in determining how much oxygen can dissolve in molten steel.
Under the same conditions:
- Higher temperature → higher oxygen solubility.
- Lower temperature → lower solubility.
Therefore, precise temperature control during steelmaking is essential for achieving the desired oxygen level and effective deoxidation results.
Principle of Oxygen Measurement in Molten Steel
The oxygen content in molten steel is commonly measured using the solid-electrolyte concentration cell method.
This device consists of two “half-cells”:
- One is a reference electrode with a known oxygen partial pressure.
- The other is inserted into the molten steel for oxygen measurement.
An oxygen-ion-conducting electrolyte connects the two half-cells, forming an oxygen concentration cell.
By measuring the electromotive force (EMF) and temperature, the dissolved oxygen content in the molten steel can be accurately determined.
Factors Affecting Oxygen Content in Steel
The oxidizing nature of molten steel—often expressed by its oxygen content—has a major impact on steel quality, alloy absorption, and deoxidation efficiency. The main influencing factors include:
- Carbon content:Follows the carbon–oxygen equilibrium. Higher carbon → lower oxygen; lower carbon → higher oxygen.
- Manganese content:Higher residual Mn lowers the oxygen content, improving steel purity.
- Temperature:Higher temperatures increase oxygen solubility in molten steel.
