Why oxygen is a problem
Liquid steel at 1600 °C can dissolve roughly 0.05–0.08% oxygen, while in solid iron the figure falls below a tenth of that. The difference has to go somewhere during solidification.
- It combines with carbon to form carbon monoxide bubbles: porosity and cavities in the ingot.
- It combines with iron to form FeO: hot shortness at the grain boundaries.
- Oxide inclusions remain: shorter fatigue life, poorer surface quality and machinability.
Oxygen is therefore captured with an element that binds it more strongly than iron does, and the oxide formed is taken into the slag.
Killed, semi-killed and rimmed steel
- Killed steel
- Fully deoxidised; there is no gas evolution during solidification and the surface stays quiet. Structural steels, pressure vessel steels, forging and deep-drawing grades all have to be killed steel.
- Semi-killed steel
- Partial deoxidation; limited gas evolution offsets the shrinkage cavity. An economical middle course for some structural sections.
- Rimmed steel
- No deoxidation; a clean skin forms at the edge while the centre is porous. Used today only in limited applications.
Deoxidisers and their effectiveness
| Material | Active element | Characteristics |
|---|---|---|
| Ferrosilicon (FeSi) | Si | The most widespread deoxidiser. The silica formed passes easily into the slag, and it also adds silicon to the steel. |
| Silicomanganese (SiMn) | Si + Mn | Supplies silicon and manganese in one addition. The manganese silicate formed stays liquid and rises into the slag more easily; very common in structural steel. |
| Ferromanganese (FeMn) | Mn | A weak deoxidiser, but it ties up sulphur and so prevents hot shortness; generally used together with silicon. |
| Aluminium | Al | The strongest practical deoxidiser. It reaches very low oxygen levels and refines the grain, but the alumina formed is solid and can clog the casting nozzle. |
| Calcium silicide (CaSi) | Ca + Si | As well as deoxidation and desulphurisation it modifies inclusions, converting solid alumina into liquid calcium aluminate. |
The alumina problem and the calcium addition
Deoxidation with aluminium gives a very low oxygen level, but it has a side effect: the alumina (Al₂O₃) formed is solid at steel temperature and tends to cluster. In continuous casting these clusters stick to the inner surface of the submerged entry nozzle and restrict the flow — nozzle clogging.
A calcium addition solves this. Calcium converts the alumina into liquid calcium aluminate; the inclusion becomes spherical, is carried along with the flow and does not clog the nozzle. The same treatment also spheroidises sulphides, reducing anisotropy in the rolling direction.
The calcium addition works in a narrow window. Too little calcium does not prevent clogging; too much creates new types of inclusion. The Ca/Al ratio is usually stated in the specification.
Order of addition and yield
- The weak deoxidisers (manganese, silicon) go in first; most of the oxygen in the bath is tied up here, which stops the strong deoxidiser from burning off for nothing.
- Aluminium is added next; the remaining oxygen is brought down to a very low level.
- The slag is adjusted in the ladle furnace and desulphurisation is carried out.
- Calcium goes in last to modify the inclusions; a long hold after this stage is not wanted.
The main factors governing yield are bath temperature, the oxidation state of the slag, the particle size of the added material and the intensity of stirring. Fine material dissolves quickly but burn-off loss is high; very coarse material can remain in the bath without dissolving fully.
Choosing the material
- For ferrosilicon, write the silicon range and the upper limits for aluminium and carbon.
- For silicomanganese, state both the silicon and the manganese range, plus the carbon and phosphorus limits.
- For calcium silicide, settle the calcium content and the form of application (wire, lump, powder).
- Ask for the particle size that suits your addition method; ladle addition and wire feeding do not use the same size.
- Cap the moisture and the proportion of fines: moisture is both a yield loss and a safety risk.
Frequently asked questions
What is deoxidation and why is it done?
Deoxidation is the operation of tying up the oxygen dissolved in molten steel before it solidifies. Without it, oxygen combines with carbon to form gas cavities or stays in the material as oxide inclusions; either way the mechanical properties and the surface quality suffer.
What is the difference between ferrosilicon and silicomanganese?
Ferrosilicon supplies silicon only. Silicomanganese supplies silicon and manganese together, and the manganese silicate formed stays liquid at steel temperature so it rises into the slag more easily. Manganese also ties up sulphur and prevents hot shortness, which is why it is so common in structural steel.
Why is calcium used alongside aluminium deoxidation?
The alumina formed by aluminium is solid at steel temperature, clusters, and can clog the casting nozzle in continuous casting. Calcium converts the alumina into liquid calcium aluminate, spheroidising the inclusion and preventing clogging; it also modifies sulphides and reduces anisotropy in the rolling direction.
What does killed steel mean?
Steel that has been fully deoxidised, with no gas evolution during solidification. Structural steels, pressure vessel steels and deep-drawing grades are produced as killed steel.
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