Cored Wire in Steelmaking: Types, Dosing and Recovery

What Cored Wire Is and How It Works in the Ladle

Cored wire, also called filled wire or powder injection wire, is a composite product: a continuous tube of low-carbon steel strip that carries a powdered or granular material inside. The wire is driven by a wire feeder and injected through a refractory-coated lance deep into the molten metal. Once the sheath melts, the core material is released below the bath surface where it reacts with the liquid metal instead of burning in the air above it.

That is the entire point of the product. Many steelmaking additions are more effective the deeper they are introduced and the more precisely their mass can be controlled. Cored wire delivers both: sub-surface injection for high recovery of reactive elements, and metering by wire length so that the addition is accurate to a fraction of a gram per tonne rather than to a shovel full of lumps.

In a steelmaking shop, a cored wire installation normally consists of a coil stand or reel, a variable-speed wire feeder with multi-roll drive, a straightener, an entry guide, and a lance that is lowered through the ladle slag into the metal. The operating variables that determine whether the treatment works are feed rate, wire length consumed, immersion depth, bath temperature, metal mass, and the chemistry of the metal and slag at the time of treatment. Change any one of them and the apparent recovery of the core element changes with it.

Cored Wire Types and Core Materials

The wire family is defined by what is inside the sheath. The table below sets out the wire types commonly used in steelmaking and hot metal treatment, the typical core content, and the duty each one performs.

Wire type	Core material and typical content	Main function	Typical use conditions
Calcium-silicon wire	Ca-Si alloy core, typically about 28-32% Ca and 55-60% Si, balance Fe	Inclusion modification, castability improvement, deoxidation, some desulphurisation	Aluminium-killed steels, continuous casting grades
Calcium-iron wire	Ca-Fe alloy core, typically about 25-30% Ca	Inclusion shape control, calcium treatment	Where lower silicon pick-up is preferred
Pure calcium wire	Calcium metal granules, typically 97% Ca and above, often with a thin passivating film	High-efficiency calcium treatment with minimum alloy carrier	Deep injection, tight control of residual calcium
Ferro-titanium wire	Fe-Ti alloy core, commonly around 68-72% Ti	Titanium microalloying, stabilisation, grain control	Interstitial-free and microalloyed grades
Ferro-boron wire	Fe-B alloy core, typically about 15-20% B	Boron microalloying for hardenability	Boron-treated steels, additions measured in tens of parts per million
Carbon wire	Graphitised or synthetic graphite powder, typically 94-99% C	Recarburisation of low-carbon heats	Where carbon must be trimmed without oxidising the bath
Sulphur wire	Elemental sulphur or sulphur-bearing compound, typically 90-99% S	Resulphurisation for free-machining steels	Free-cutting grades with a specified sulphur range
Aluminium wire	Aluminium granules or aluminium strip core, typically 95% Al and above	Deoxidation and aluminium trimming	Final deoxidation where precise aluminium control is required
Magnesium-based wire	Passivated magnesium granules, typically 90% Mg and above	Desulphurisation of hot metal	External hot metal treatment stations

Two cautions follow from this table. First, the same nominal wire name can hide a wide range of core chemistry, and the chemistry that matters is the analysis of the core, not the label on the coil. Second, the core is not the only active part of the wire: the steel sheath is consumed in the bath too, adds iron to the heat and slightly dilutes the treatment effect per metre, which is why dosing is calculated from the mass of core carried per metre rather than from the mass of the whole wire.

Sheath Steel, Diameter and Filling Ratio: Reading a Specification

A cored wire specification is only complete when four numbers are stated: nominal diameter, sheath strip thickness, wire mass per unit length, and core filling ratio, together with the core chemistry limits. Diameter and sheath thickness set the mechanical strength of the wire and how it survives the feeder rolls and the slag layer. Mass per metre and filling ratio convert wire length into kilograms of active material.

Common commercial cored wires for steelmaking run from roughly 9 mm to 16 mm in nominal diameter, with 9.5 mm, 11 mm, 13 mm and 16 mm being the sizes most often seen. Sheath strip is generally low-carbon steel in the region of 0.3-0.6 mm thick. Total wire mass typically falls in the range of about 200-450 grams per metre depending on diameter and filling, and the core fraction is commonly in the region of 40-60% of total wire mass, with some product families running higher.

Parameter	Typical commercial range	Why it matters
Nominal diameter	about 9-16 mm (9.5, 11, 13, 16 mm common)	Sets feed-roll grip, lance bore and maximum feed rate
Sheath strip thickness	about 0.3-0.6 mm	Strength through the feeder and dissolution time in the bath
Wire mass per metre	about 200-450 g/m	Converts metres consumed into kilograms added
Core filling ratio	about 40-60% of wire mass	Determines how much active material each metre carries
Core particle size	typically a screened cut within roughly 0.1-2 mm	Flowability when filling and release rate in the bath
Core moisture	controlled to a low maximum on the certificate	Prevents porosity defects and erratic reaction
Coil mass	commonly several hundred kilograms per coil	Handling, change-over frequency and end-of-coil waste

A supplier's specification sheet that omits core mass per metre cannot be used for dosing or for comparing one offer with another; two coils of the same diameter can carry very different amounts of active material. The verification that matters is mechanical and simple: cut a measured length of wire, weigh it, strip the sheath from the core, weigh the core, and check the core chemistry on a representative sample. That gives mass per metre, filling ratio and the analytical basis for the dosing calculation. Sampling of the core material for chemical analysis follows the same principles as for any ferroalloy or powder consignment, and particle size is confirmed by sieve analysis against the agreed cut.

Calcium Treatment and Inclusion Modification

The largest single use of cored wire in steelmaking is calcium treatment. Aluminium deoxidation leaves solid, hard alumina inclusions that agglomerate and clog casting nozzles, and they deform poorly in rolling, which harms through-thickness properties. Calcium injected into the bath reacts with those inclusions and converts them into calcium aluminates that are liquid at steelmaking temperatures. Liquid inclusions do not build up at the nozzle, and they deform rather than shatter during rolling, which is why calcium treatment is standard practice for aluminium-killed steels intended for continuous casting and for plate and linepipe products with inclusion shape requirements.

Depth control is not optional because calcium boils at roughly 1484 degrees Celsius, well below normal steelmaking temperatures. If the wire is released at or above the bath surface, calcium vaporises, burns or is lost to the slag, and recovery collapses. Injecting the wire deep into the ladle keeps the vapour bubble surrounded by liquid steel long enough for the reaction to take place, and it is the reason a cored wire lance is immersed to a substantial fraction of the bath depth rather than merely dipped into it.

Recovery must be treated as a plant-specific number, not a catalogue constant. In practice, calcium recovery in steel from cored wire commonly falls in a broad band of roughly 10-25%, driven by immersion depth, feed rate, bath temperature, slag thickness and composition, sulphur content and the timing of the treatment relative to tapping and casting. Sulphur is particularly important: calcium has an affinity for sulphur as well as for oxygen, so a high-sulphur heat consumes calcium to form calcium sulphide and leaves less for inclusion modification. Engineers therefore monitor a calcium-to-sulphur relationship alongside the residual calcium, and they watch for the pitfall of over-treatment, where surplus calcium produces solid calcium sulphide inclusions and reintroduces the very nozzle problems the treatment was meant to cure.

Feeding Parameters and Dosing: From Millimetres to Parts per Million

Dosing a cored wire treatment is an arithmetic exercise in three steps. Step one: establish how much active element must be added per tonne of steel, based on the difference between the target and the present analysis, plus an allowance for recovery. Step two: convert that mass into metres of wire using the core mass per metre and the core analysis. Step three: set the feed rate so the calculated length is delivered within the treatment window without exceeding the rate the bath can absorb.

Worked example: a 13 mm calcium-silicon wire is quoted at 300 g/m total mass with a 50% core filling ratio, so it carries about 150 g of core per metre. If the core analysis is 30% calcium, the wire delivers about 45 g of calcium per metre. A treatment intended to add 0.15 kg of calcium per tonne of steel therefore needs about 3.3 metres of wire per tonne, before any recovery allowance. At a heat size of 100 tonnes, that is roughly 330 metres of wire, delivered at a feed rate matched to the treatment window.

Feed rates for calcium-bearing wires are typically set in the range of about 150-400 metres per minute, with the setting chosen so that injection lasts long enough for good mixing but short enough to finish before the ladle is moved. Higher feed rates deliver more metres per minute but can drive the wire into the slag or produce an unstable reaction zone; lower feed rates increase heat loss from the lance and lengthen treatment time. Immersion depth is normally set to a substantial fraction of the bath depth, and the lance is held at a fixed height relative to the metal surface, not to the ladle rim, because slag thickness varies between heats.

Because reaction time matters, the sequence around the treatment should be fixed and repeatable: correct the slag condition before injection, trim the oxygen level so the metal is properly killed, inject at the specified depth and rate, then allow a short mixing period before sampling and casting. Sampling the heat too soon after injection gives a residual calcium figure that has not stabilised and leads to unnecessary second treatments.

Microalloying, Recarburisation and Desulphurisation Wires

Calcium wire is only the largest member of the family. Titanium wire is used to add titanium precisely to microalloyed and interstitial-free steels, where a fine, uniform titanium addition improves the consistency of the final analysis and avoids the recovery swings of lump additions. Boron wire delivers very small additions, often measured in tens of parts per million, to steels where boron is used to boost hardenability; at those levels the metering accuracy of a cored wire is the difference between a specification heat and a scrapped one.

Carbon wire is used to trim the carbon content of low-carbon heats upward. Because the graphite core is injected below the surface, recovery is high and the carbon does not burn off at the bath surface the way a surface addition would. Sulphur wire is the reverse exercise: it raises sulphur to a target range in free-machining grades, and the same sub-surface injection logic keeps the addition efficient and repeatable. Aluminium wire is used where the final aluminium adjustment has to be tight or where the deoxidation practice calls for a controlled aluminium addition rather than a bulk one. Magnesium-based wire is used mainly for desulphurisation of hot metal at external treatment stations, where the passivated magnesium reacts with dissolved sulphur in the iron.

Each of these wires shares the same design logic: choose a core chemistry that is stable and free-flowing, size the core to the sheath and the feeder, and control moisture so that the reaction in the bath is reproducible. What differs is the recovery pattern and the sensitivity of the finished steel to small dosing errors, which is why the dosing procedure must be written for the specific wire type rather than transferred between products.

Cored Wire Compared with Lump and Shot Additions

The alternative to cored wire is adding the same material as lumps, granules or shot to the ladle, the tapping stream or the mould. Lump additions are simple and cheap per kilogram of material, and for many bulk duties they are perfectly adequate. They lose on four counts where the element is reactive, expensive or required within a narrow range.

First, recovery. A surface addition of a reactive element is exposed to air and slag; a substantial part of it oxidises or vaporises before it dissolves. Sub-surface injection through a cored wire keeps the addition below the slag-metal interface, so more of the active element reaches the steel. Second, accuracy. Wire length is measured and controlled; the mass added is repeatable heat after heat and shift after shift. Third, timing. Wire treatment can be placed at a defined point in the ladle cycle, so the treatment is delivered where the process window wants it rather than where the ladle happens to be. Fourth, health, safety and housekeeping. Powder or shot additions generate dust and fumes at the ladle, while a sealed wire coil is a controlled product that is released under the metal surface.

The trade-off is real, and it is worth stating plainly: cored wire costs more per kilogram of active element than the same material in lump form, and it requires a feeder, a lance, consumable lance refractory and trained operators. The economics only work when the value of higher and more consistent recovery, tighter analysis control and reduced re-treatment outweighs the cost of the wire, the equipment and the lance maintenance. For calcium treatment, boron microalloying, titanium trimming and other narrow-range duties the calculation normally does clear that bar; for adding silicon or manganese in bulk, it usually does not.

Quality Control, Storage and Handling of Cored Wire

Incoming inspection of a cored wire coil should confirm the coil identity, diameter, wire mass per metre, filling ratio, core chemistry and core particle size, and should include a visual check for a crushed or kinked coil, broken wire, damp core and sheath corrosion. A crushed coil will jam the feeder and can be a safety issue when it is unwound under tension. Damp core material produces violent, erratic reactions and a genuine risk of metal splashing at the injection point, so moisture is not a minor defect.

Storage is simple but not optional. Coils should be kept dry, under cover, off the floor and away from standing water, and coils that have been exposed to rain should not be loaded into the feeder until the condition of the core has been assessed. Cores containing aluminium, calcium or magnesium are reactive with moisture, so the same discipline that applies to ferroalloy storage applies to the wire: keep the material sealed, ventilated and dry, and never leave an opened coil exposed overnight in a humid shop.

Operating discipline is the other half of quality control. The feed rate, immersion depth and treatment sequence should be recorded for every heat, and the achieved residual analysis should be fed back into the dosing calculation so that recovery factors are refreshed from real data rather than left at the values written in a start-up manual. A plant that measures its own recovery and updates its own dosing table will consistently outperform one that keeps transferring the same factor between wire types, ladle sizes and steel grades.

FAQ

Q: What is the purpose of cored wire in steelmaking?
Its purpose is precise, sub-surface addition of reactive or narrow-range elements. The wire carries a core material into the liquid metal through an immersed lance so that alloying, inclusion modification, microalloying or recarburisation is delivered with high recovery and repeatable accuracy.

Q: What is inside a cored wire sheath?
A powdered or granular core, chosen for the duty: calcium-silicon or calcium-iron alloys for calcium treatment, titanium or boron alloys for microalloying, graphite for recarburisation, sulphur for free-machining grades, aluminium for trimming, and passivated magnesium for hot metal desulphurisation.

Q: How is the core filling ratio measured and verified?
Cut a measured length of wire, weigh the total, strip the steel sheath from the core, and weigh the core separately. The core mass divided by the total mass gives the filling ratio, and the total mass divided by the cut length gives the wire mass per metre used for dosing.

Q: Why does calcium recovery from cored wire vary so much?
Because calcium boils well below steelmaking temperature, recovery depends on immersion depth, feed rate, bath temperature, slag thickness and composition, sulphur level and the timing of the treatment. Reported recoveries commonly fall in a broad band of roughly 10-25%, and each plant has to establish its own figure.

Q: When is cored wire preferred over lump additions?
It is preferred when the element is reactive, expensive or required within a tight range, and when consistent recovery and controlled timing have more value than the lower material cost of lumps. For bulk additions of silicon or manganese, lump material is usually the economic choice.

Q: What happens if the core material is damp?
Moisture in the core causes violent, erratic reaction at the injection point, raises the risk of metal splashing and gas-related defects, and makes recovery unpredictable. Coils must be stored dry and under cover, and rain-affected coils should be inspected before use.

Q: How is the correct wire length for a treatment calculated?
Multiply the required mass of the active element per tonne by the heat size, then divide by the mass of that element delivered per metre of wire, which is the core mass per metre multiplied by the core analysis. Add the plant recovery allowance to obtain the wire length to be fed.