Why Deoxidation Is Essential
When steelmaking reactions reduce carbon and manganese to low levels, the molten steel still contains a significant amount of dissolved oxygen (typically 0.02%–0.08%).
If not removed, this oxygen can cause severe quality problems during solidification:
- Formation of FeO and FeO–FeS at grain boundaries, leading to hot brittleness and reduced ductility.
- Generation of CO gas bubbles as dissolved oxygen reacts with carbon during cooling.
- Creation of porous or loose steel ingots, lowering product density and strength.
To prevent these issues, oxygen must be reduced before casting.
Typical targets are:
- Rimmed steel: oxygen < 0.03%;
- Killed steel: oxygen < 0.005%.
What Are Deoxidation and Alloying?
- Deoxidation is the process of adding elements with a stronger affinity for oxygen than iron, forming stable oxides that separate from the molten steel.
- Alloying adjusts the chemical composition of steel by adding ferroalloys or metals to achieve the target grade.
In modern steelmaking, deoxidation and alloying often occur simultaneously.
Common examples:
- Used for both deoxidation and alloying: ferrosilicon (FeSi), ferromanganese (FeMn).
- Used only for deoxidation: calcium–silicon, aluminum, or silicon–aluminum alloys.
- Used mainly for alloying: ferrochrome, ferromolybdenum, ferroniobium, ferrovanadium, and ferrotungsten.
Main Objectives of Deoxidation
- Reduce dissolved oxygen in molten steel to meet steel grade requirements.
- Remove deoxidation products efficiently to ensure steel cleanliness.
- Control inclusion morphology and distribution, improving mechanical properties.
Principle of Deoxidation
Deoxidation relies on the chemical affinity between oxygen and deoxidizing elements.
When strong deoxidizers (Al, Si, Mn, etc.) are added to molten steel, they react with FeO and free oxygen to form oxides, which float to the slag layer.
However, part of these oxides may remain in the steel as nonmetallic inclusions, potentially affecting toughness and fatigue strength if not controlled.
Common Deoxidation Methods
Based on the mechanism of oxygen removal, four main deoxidation methods are used in steelmaking:
- Precipitation Deoxidation
The most widely used method. Deoxidizers react with dissolved oxygen to form oxides that float up or are removed by convection. - Diffusion Deoxidation
Utilizes low-FeO slags that absorb oxygen from molten steel through diffusion. - Vacuum Deoxidation
Conducted under vacuum to reduce CO partial pressure, allowing oxygen and carbon to combine and escape as CO gas.
→ Ideal for high-purity or special steels. - Combined Deoxidation
A hybrid approach that integrates precipitation and diffusion mechanisms to achieve better results and inclusion control.
