Liquid steel coming out of a converter or an electric arc furnace is saturated with dissolved oxygen. Left alone, that oxygen comes out of solution during solidification as gas porosity and oxide inclusions. Ferro silicon is the most widely used way of removing it — and recovery varies enormously depending on how you use it.
The problem being solved
At the end of oxygen steelmaking, dissolved oxygen in the bath is typically in the range of 300 to 800 ppm, depending on carbon content. The lower the carbon, the higher the oxygen — the two are inversely coupled.
Oxygen at that level is a problem for two reasons. It comes out of solution as the steel freezes, forming carbon monoxide bubbles that become blowholes, and it forms oxide inclusions that become fatigue initiation sites and surface defects in the rolled product.
Deoxidation means adding an element with a stronger affinity for oxygen than iron has, so the oxygen leaves the melt as an oxide that can be floated off into the slag rather than trapped in the steel.
Why silicon
Ranked by deoxidising power at steelmaking temperature, the usual order is aluminium strongest, then titanium, then silicon, then manganese, then carbon. Aluminium is used where very low oxygen is required. Silicon is used far more widely, for three reasons.
- Cost. Per unit of oxygen removed, silicon is considerably cheaper than aluminium.
- Inclusion behaviour. Alumina inclusions are solid, hard, angular and prone to clustering — and they are what clogs submerged entry nozzles during continuous casting. Silicate inclusions, especially manganese silicates, are liquid at steelmaking temperature and behave far better.
- Silicon is often wanted anyway. Many grades specify silicon as an alloying element for strength and, in electrical steels, for magnetic properties. The deoxidation is then almost a by-product.
Grades and what they mean
| Grade | Silicon | Notes |
|---|---|---|
| FeSi 75 | 74–80% | The most widely traded grade. Most silicon per tonne handled. |
| FeSi 70 | 70–75% | Sometimes better value per silicon unit depending on the market. |
| FeSi 65 | 63–68% | Regional grade; less common in international trade. |
Choosing between grades is a delivered-cost-per-silicon-unit calculation, not a price-per-tonne one, and we work through it in detail in FeSi 75 vs FeSi 70.
One physical detail worth knowing: ferro silicon around 75% silicon has a density close to 3.0 g/cm³, well below liquid steel at roughly 7.0. It floats. That is part of why addition practice matters so much.
How much to add
The stoichiometry is straightforward. Silicon and oxygen react as Si + 2O → SiO₂, so on an atomic weight basis one part silicon consumes roughly 1.14 parts oxygen. Removing 500 ppm of oxygen therefore needs about 440 ppm of silicon actually reacted.
In practice you need considerably more than the stoichiometric amount, because silicon is also lost to slag reactions, to reoxidation from air and refractory, and to the residual silicon you intend to keep in the steel. A typical working figure is 1.5 to 2.5 kg of FeSi 75 per tonne of steel for deoxidation alone, with more added where silicon is an alloying requirement.
Your own figure should come from your own heat records, not from a handbook.
Getting your recovery up
Recovery on ferro silicon commonly runs anywhere from 75% to 95%. That is a very wide band, and the difference is almost entirely practice rather than product. Four things account for most of it.
1. Add at the right moment
Too early and the silicon is consumed reacting with an oxidising slag rather than with the dissolved oxygen in the steel. Too late and it has not dissolved and distributed before the ladle goes to casting.
The usual practice is to add during tapping, after the first third of the stream has run, so the alloy is carried into the bath by the incoming steel and stirred by the filling ladle. Adding into an empty ladle bottom risks the alloy sitting in a cold spot; adding at the very end gives too little time.
2. Keep the slag off it
Carryover slag from the furnace is oxidising, and it will consume silicon enthusiastically. Good slag stopping practice on the tap is one of the most reliable ways to lift recovery, and it costs nothing per tonne.
3. Size it correctly
Undersized fines are lost — they blow away, they float on the slag layer and oxidise before they dissolve. Oversized lumps do not fully dissolve within the tapping window. For ladle additions, 10–50 mm is a common and sensible fraction, and the purchase order should state a maximum fines percentage as well as the size range.
4. Stir
Because ferro silicon floats, it needs to be pulled down into the melt. Tapping turbulence does much of this; argon stirring does it far better, and mills with bottom-stirring capability generally see both higher and more consistent recovery.
Silicon and manganese together
Silicon and manganese deoxidise better in combination than either does alone. The reason is the nature of the product: silicon alone gives solid silica, while silicon with manganese gives manganese silicate, which is liquid at steelmaking temperature. Liquid inclusions coalesce and float out far more readily than solid ones.
This is why silico manganese is so widely used — it delivers both elements in one addition. Where the silicon ceiling of the grade will not allow it, a combination of ferro manganese and ferro silicon achieves the same metallurgy with independent control of the two levels.
Where else ferro silicon goes
Steel deoxidation is the largest use, but not the only one.
- Alloying. Electrical steels rely on silicon — typically 1% to 3.5% — to raise electrical resistivity and cut core losses.
- Foundry inoculation. Ferro silicon based inoculants control graphite structure in cast iron. Here the silicon is a carrier for the nucleating elements rather than the active agent itself.
- Nodularisation. Ferro silicon magnesium delivers magnesium for ductile iron production, using ferro silicon as the vehicle to moderate an otherwise violent reaction.
- Reduction. Silicon is the reductant in the silicothermic production of other alloys, including low carbon ferro chrome.
Common mistakes
- Blaming the alloy for a practice problem. If recovery drops, check slag carryover and addition timing before you question the supplier's analysis.
- Ignoring the size specification. Chemistry on the certificate and sizing in the bag are two different things, and only one of them usually gets checked.
- Adding everything at once. Split additions generally give better distribution and more consistent results than a single dump.
- Storing it badly. Ferro silicon can evolve hydrogen and other gases in damp conditions, particularly when freshly crushed. Store it dry and ventilated, and observe the supplier's handling guidance.
In short
Ferro silicon is the default deoxidiser because it is economical, because the inclusions it forms behave well, and because the silicon is frequently wanted in the steel regardless. The alloy is largely a commodity; the variable that separates a good result from a poor one is your own practice — when you add it, how clean your slag stopping is, how well it is sized, and how well the ladle is stirred.
If your recovery is inconsistent, that list is where the answer is.
We manufacture ferro silicon in FeSi 75 and FeSi 70 at Durgapur, West Bengal. Get in touch for a quotation.
Frequently asked questions
What is the difference between FeSi 70 and FeSi 75?
The silicon content — roughly 70–75% versus 74–80%. FeSi 75 delivers more silicon per tonne handled, which reduces charging time and freight cost per unit of silicon. FeSi 70 can be more economical where the delivered price per silicon unit is lower.
Why is my ferro silicon recovery low?
The usual causes are oversized lumps that do not dissolve before tapping ends, addition into an oxidising slag, or adding too early so the silicon is consumed by slag oxides rather than dissolved oxygen in the steel.