Some elements you want in steel simply will not survive being dropped into the top of a ladle. Calcium boils below steelmaking temperature; magnesium reacts violently. Cored wire solves this by carrying the reagent, inside a steel sheath, to the bottom of the ladle before it is released.
The problem with lump additions
Calcium boils at about 1,480 °C. Liquid steel in a ladle is typically 1,600 °C or above. Throw calcium metal onto the surface and it flashes to vapour on contact, producing a spectacular plume and very little metallurgical effect. Recovery from surface addition is poor and, worse, wildly inconsistent from heat to heat.
The physics that fixes this is straightforward. Feed the calcium down to the bottom of the ladle inside a steel tube, and by the time the sheath melts through, the reagent is under several metres of liquid steel. Ferrostatic pressure at that depth raises the boiling point substantially and holds the calcium in contact with the melt long enough to dissolve and react.
The same logic applies to any addition that is volatile, easily oxidised, or needed in a small and precisely controlled quantity.
How the wire is made
A cored wire is a low carbon steel strip formed into a tube around a powdered core, then closed with a folded or crimped seam. Common diameters are 9, 13 and 16 mm.
Two parameters govern behaviour:
- Sheath thickness — typically 0.2 to 0.4 mm. This sets how long the wire survives before opening, and therefore the depth at which the reagent is released. Thicker sheath, deeper release.
- Powder fill weight — grams per metre of wire. This is what turns "metres fed" into "kilograms added", so consistency here is what makes the addition calculable at all.
Inconsistent fill weight is the most common quality complaint with cored wire, because it directly corrupts the dosing calculation. Ask for the fill weight tolerance, not just the nominal figure.
The main types
| Type | Typical core | Purpose |
|---|---|---|
| Calcium silicide (CaSi) | 28–32% Ca, 55–60% Si | Inclusion modification, castability, desulphurisation. The most widely used type. |
| Solid calcium / calcium iron | Ca metal or CaFe | Calcium treatment where the silicon in CaSi is unwanted |
| Carbon | Graphite or carbon powder | Precise late carbon trimming with high, repeatable recovery |
| Sulphur | S powder | Deliberate resulphurising for free-machining grades |
| Ferro titanium | FeTi powder | Titanium addition without the heavy oxidation loss of lump addition |
| Ferro boron | FeB powder | Boron for hardenability, where quantities are tiny and control is critical |
| Magnesium | Mg or MgFeSi | Ductile iron nodularisation, as a controlled alternative to sandwich treatment |
The choice between calcium silicide and calcium iron is common enough to be worth its own treatment — see CaSi vs CaFe cored wire.
What calcium treatment achieves
Calcium treatment is the reason most mills have a wire feeder at all. It does three things.
Inclusion modification
Aluminium-killed steel contains alumina inclusions: solid, angular, and inclined to cluster. Calcium converts them to calcium aluminates. Get the calcium-to-aluminium ratio into the right window and those aluminates are liquid at steelmaking temperature — globular, easily floated, and far less damaging to fatigue life.
Preventing nozzle clogging
This is the operational payoff. Solid alumina builds up on the walls of the submerged entry nozzle during continuous casting, progressively restricting flow until the sequence has to be cut short. Liquid calcium aluminates do not deposit in the same way. For a caster running long sequences, calcium treatment is often the difference between hitting the sequence length and not.
Sulphide shape control
Manganese sulphide inclusions are plastic at rolling temperature: they elongate into stringers, which is what gives rolled plate poor through-thickness ductility and makes it vulnerable to hydrogen-induced cracking. Calcium forms calcium sulphide instead, which stays globular through rolling. This is standard practice for line pipe and other plate with through-thickness requirements.
Getting the calcium-to-aluminium ratio right
Calcium treatment has a window, and both edges of it are bad.
- Too little calcium and the alumina is only partly modified. The result can be worse than no treatment: partially converted calcium aluminates with high melting points, which clog nozzles just as readily.
- Too much calcium and you begin forming calcium sulphide as a solid inclusion, which brings back the clogging you were trying to eliminate, and wastes reagent besides.
Mills commonly work to a target Ca/Al ratio — frequently cited around 0.09 to 0.14, though the right value depends on your sulphur, oxygen and aluminium levels. This is a parameter to establish for your own steel and then hold, rather than to copy.
Practical feeding notes
- Feed speed matters. Too slow and the wire melts too high in the ladle, losing the pressure benefit. Too fast and it can reach the bottom refractory intact. Typical speeds run from 100 to 250 m/min, tuned to ladle depth and sheath thickness.
- Stir, but gently. Soft argon stirring after feeding helps distribute the calcium and float out the modified inclusions. Hard stirring re-entrains slag and undoes the work.
- Cover the ladle. Calcium reacts with air. An open ladle after treatment loses reagent to reoxidation.
- Treat late. Calcium fades. Treatment should be one of the last operations before casting.
- Keep the wire dry. Moisture in the core means hydrogen in the steel. Store it under cover and rotate stock.
- Calibrate the feeder. Your dosing depends on metres fed times fill weight. If the feeder slips or the fill weight varies, the calculation is fiction. Check both periodically.
Calculating an addition
To add 0.2 kg of calcium per tonne to a 100 tonne heat, using CaSi wire with 30% calcium and a fill weight of 200 g/m, assuming 20% calcium recovery — a realistic figure, as much of the calcium is consumed by oxides and sulphides:
- Calcium required in steel: 100 × 0.2 = 20 kg
- Allowing for 20% recovery: 20 ÷ 0.20 = 100 kg of calcium to feed
- Powder required at 30% Ca: 100 ÷ 0.30 = 333 kg of core powder
- Wire length at 200 g/m: 333 ÷ 0.2 = about 1,665 metres
Note how much the answer depends on the recovery assumption. Calcium recovery is genuinely low and genuinely variable — it is worth measuring on your own practice, because a wrong assumption here is expensive in both directions.
In short
Cored wire exists to deliver reactive elements where lump addition cannot: deep in the ladle, under pressure, in a measurable quantity. For calcium treatment it is effectively the only practical method. The wire itself is simple; what determines results is consistent fill weight from the supplier, a calibrated feeder, the right feed speed for your ladle depth, and a calcium-to-aluminium target established on your own steel.
If you are seeing nozzle clogging despite treating, the ratio is the first thing to check — under-treatment causes exactly the problem it is meant to prevent.
We manufacture cored wire in the diameters and fill weights your feeder requires. Tell us your ladle practice and we will recommend a specification.
Frequently asked questions
Why use cored wire instead of adding lump alloy?
Calcium boils at around 1,480 °C, well below steelmaking temperature. Dropped in as lump it vaporises at the surface and is largely lost. A steel sheath carries it below the surface, where the ferrostatic pressure keeps it in contact with the melt long enough to react.
What does calcium treatment actually do?
It converts solid alumina inclusions into liquid calcium aluminates. These float out more readily and, critically, do not build up on the submerged entry nozzle — which is what causes clogging during continuous casting.