Ferromanganese as Deoxidiser and Desulphuriser in Steel

Ferromanganese as a Combined Deoxidiser and Desulphuriser

Ferromanganese is the standard manganese carrier for steelmaking because it performs two jobs that cannot be separated in practice. It lowers the oxygen activity of the bath by forming manganese oxide and, more importantly, manganese silicate when silicon is present; and it fixes sulphur as manganese sulphide, removing a large part of that element from solution in the steel. Neither duty is complete on its own. A manganese addition without adequate deoxidation simply partitions into the slag, and a sulphur addition without manganese leaves iron sulphide at grain boundaries, which is the classic cause of hot shortness during rolling.

Consumption is conventionally quoted at about 3-5 kg of manganese per tonne of steel, which corresponds to a ferromanganese charge in the region of 4-7 kg per tonne depending on grade. That figure is an industry planning range rather than a fixed rule: the actual charge depends on the manganese aim, the residual manganese in the scrap, the sulphur load from the charge mix and the sulphur specification of the finished product. This article explains the grade system, the thermodynamics behind the two duties, the addition strategy, the acceptance tests and the failure modes that purchasing and process teams should control.

Grade Classification by Carbon Content

Ferromanganese is classified commercially by carbon content, because carbon is what limits its use in low-carbon steels. The three families are high-carbon ferromanganese, medium-carbon ferromanganese and low-carbon ferromanganese, with the boundaries conventionally placed at 2.0% C between high and medium carbon, and 0.7% C between medium and low carbon. High-carbon ferromanganese typically carries 6.0-8.0% C with 74-78% Mn; medium-carbon grades carry roughly 1.0-2.0% C with 78-82% Mn; low-carbon grades carry 0.5% C or less with 80-85% Mn and a correspondingly higher price, since carbon removal requires oxygen refining of the molten alloy.

GB/T 3795 is the standard governing ferromanganese grades, and GB/T 5682 covers the low-carbon ferromanganese family. Silicomanganese, standardised in GB/T 4008, occupies the adjacent niche: it carries both manganese and silicon, so a single addition provides deoxidation and alloying, and it is often preferred where the steel also needs a silicon addition. The very low carbon end of the range is served by electrolytic manganese metal rather than by carbothermic ferromanganese, because carbon levels below about 0.03% cannot be reached by the smelting route.

Family	Carbon, %	Manganese, %	Silicon, % max	Phosphorus, % max	Typical use
High-carbon ferromanganese	6.0-8.0	74.0-78.0	1.5	0.30	Rimmed and structural steels, EAF charge
Medium-carbon ferromanganese	1.0-2.0	78.0-82.0	1.5	0.20	Structural and pipe steels
Low-carbon ferromanganese	0.5 max	80.0-85.0	1.0	0.15	Low-carbon and drawing steels
Silicomanganese	1.5-2.5	60.0-67.0	17.0-20.0	0.25	Combined Mn and Si addition

Deoxidation Mechanism and Slag Conditions

Manganese deoxidation is thermodynamically weaker than silicon or aluminium deoxidation, which is why ferromanganese is described as a mild deoxidiser. Its real value lies in the reaction product: manganese oxide combines with silica to form manganese silicate, a liquid slag phase at steelmaking temperatures. Liquid inclusions coalesce and float out far more readily than solid silica or alumina particles, so the manganese addition improves the rate at which deoxidation products leave the steel, even though it does not achieve the low dissolved oxygen levels that aluminium does.

The practical rule is sequence: manganese goes in after or together with the silicon addition, never before it. If ferromanganese is added to an oxidised bath, a substantial fraction of the manganese is lost to the slag as MnO, and the slag becomes richer in manganese, which then re-enters the steel unpredictably during later tapping. A related effect is manganese boil-out during the carbon-oxygen blow: manganese has a higher vapour pressure than iron and is oxidised preferentially at the hot spot, so recovery from converter charging is lower than from ladle addition.

Slag basicity controls how much manganese is retained. A highly basic slag holds manganese as MnO and pulls it out of the steel, while a less basic slag favours manganese retention. Operators therefore balance basicity against the sulphur requirement, because sulphur removal needs a basic, reducing slag with high lime content. The compromise is usually to aim for the sulphur specification with a moderately basic slag and to trim manganese in the ladle rather than chase it in the furnace.

Desulphurisation, MnS Formation and Inclusion Shape

Manganese sulphide formation is the second duty. Sulphur in liquid steel is soluble, and on solidification it forms iron sulphide films along grain boundaries, which melt at low temperature and cause cracking during hot rolling. Manganese has a higher affinity for sulphur than iron does, so manganese sulphide forms preferentially and, critically, at higher temperature than iron sulphide. The result is that sulphur is tied up as discrete MnS particles distributed through the matrix rather than as continuous boundary films, and hot ductility improves.

The manganese-to-sulphur ratio is the controlling index. A ratio of at least about 20:1, and preferably 30:1 or more, is generally needed to avoid uncombined sulphur in the finished product. For a steel at 0.020% S, that means a manganese content of at least 0.40%, and preferably 0.60%. If a steel is specified at 0.010% S, the same rule requires roughly 0.20-0.30% Mn, which is often below the manganese grade's natural aim and therefore satisfied without extra charge. Where very low sulphur is required, ferromanganese alone is not sufficient and a ladle desulphurisation step with a basic reducing slag is needed.

Manganese sulphide is soft and deforms plastically during rolling, producing elongated stringers that reduce through-thickness ductility and toughness in plate products. Calcium treatment changes the inclusion to a globular calcium manganese sulphide that resists elongation. The requirement is that calcium addition be judged against the sulphur level: too little calcium leaves stringers, too much produces solid calcium sulphide clusters and nozzle blockage during continuous casting.

Addition Practice and Recovery

Recovery of manganese depends on where it is added. Ladle addition into a deoxidised melt with a fluid slag typically returns 90-95% of the charged manganese. Tap-stream addition runs lower because the stream is exposed to air. Converter charging of high-carbon ferromanganese runs lower still because of oxidation at the hot spot and losses to the slag. When a purchase order is being written, the addition point should be stated, because it determines whether the supplier's quoted contained-manganese weight translates into the aim analysis.

The addition calculation follows the standard pattern: aim manganese minus residual manganese, divided by recovery and by the manganese content of the grade. For a 150 tonne heat aiming at 1.40% Mn with 0.15% residual, 92% recovery and a grade at 80% Mn, the calculation gives approximately 2.54 tonnes of ferromanganese. Silicon and carbon picked up from the same addition must be checked at the same time: high-carbon ferromanganese at 7% C added at 17 kg per tonne contributes roughly 0.12% carbon, which is significant for a low-carbon steel but irrelevant for a structural grade.

Dissolution behaviour is size dependent. Ferromanganese dissolves by melting rather than by simple dissolution, and large blocks can sit in the ladle bottom through the whole tap. Most operators specify 10-50 mm or 10-80 mm lump for ladle addition, and 0-10 mm fines only for sintering or briquetting. Fines charged directly to a ladle tend to float on the slag, oxidise and report to the slag rather than the steel.

Specification, Sampling and Acceptance

Ferromanganese acceptance rests on the certificate and two independent checks. The certificate should state the grade, heat number, manganese, carbon, silicon, phosphorus and sulphur, plus the size band and net weight. Phosphorus deserves particular attention because it is not removed by the ferromanganese route and because manganese ore is the principal phosphorus source in the alloy; a high-phosphorus ferromanganese can add 0.005-0.015% P to the steel at normal addition rates, which matters for steel with a 0.020% P maximum.

Sampling for chemical analysis should follow GB/T 4010, with the sample taken from the crushed and blended lot rather than from individual pieces, since segregation in ferromanganese is pronounced and a single lump may not represent the heat. Size verification follows GB/T 13247. General delivery requirements including packing, marking and the form of the quality certificate are set out in GB/T 3650, which is a useful reference when a purchase order needs a default clause for packaging and documentation.

Inspection item	Method basis	Typical acceptance rule
Manganese content	GB/T 4010 sampling, chemical analysis	Within the grade range, no negative deviation beyond 1.0% absolute
Carbon content	Combustion infrared	At or below the stated grade maximum
Phosphorus content	Inductively coupled plasma	At or below the ordered maximum, reported per heat
Size distribution	GB/T 13247 sieve test	Oversize 5% max, undersize 10% max unless otherwise agreed
Moisture and contamination	Visual and gravimetric	Free of standing water, slag, sand and ferrous scrap

Storage, Handling and Operational Risks

Ferromanganese is supplied in big bags or in bulk and is stored under cover on a dry, hard-standing floor. Wet ferromanganese is a process problem before it is a safety problem: moisture carried into the ladle generates hydrogen and produces steam stirring that disturbs the slag, and damp fines can cause a minor ejection when they hit the bath. Storage in contact with acids should be avoided, because ferromanganese can release phosphine from phosphide impurities under acidic wet conditions, and confined spaces where fines accumulate should be ventilated before entry.

The most common operational error is under-manganese in the finished product combined with over-sulphur, which traces back to a manganese addition made too early in the sequence or to a slag that was too basic for the manganese level being targeted. The second most common error is carbon pickup from high-carbon ferromanganese where a medium-carbon grade should have been used. Both are visible on the heat analysis and both are cheaper to prevent by grade selection than to correct by re-ladle treatment.

FAQ

Q: Why is ferromanganese used as a deoxidiser rather than a stronger deoxidiser?
Manganese is a mild deoxidiser, but its oxide combines with silica to form liquid manganese silicate inclusions that coalesce and float out quickly, so it improves cleanliness even though aluminium or silicon achieves lower dissolved oxygen.

Q: What is the difference between high-carbon, medium-carbon and low-carbon ferromanganese?
They are separated at 2.0% C and 0.7% C: high-carbon carries 6.0-8.0% C, medium-carbon 1.0-2.0% C, and low-carbon 0.5% C or less with a higher manganese content.

Q: What manganese-to-sulphur ratio is needed to avoid hot shortness?
A ratio of at least 20:1, and preferably 30:1 or higher, is the usual rule so that sulphur reports to manganese sulphide rather than to grain-boundary iron sulphide.

Q: How much ferromanganese is consumed per tonne of steel?
About 3-5 kg of manganese per tonne of steel is a common planning range, equivalent to roughly 4-7 kg of ferromanganese depending on grade and recovery.

Q: Why does manganese recovery drop when ferromanganese is charged to the converter?
Manganese is oxidised preferentially at the oxygen blow hot spot and also partitions into the slag as MnO, so converter charging returns less manganese than ladle addition into a deoxidised melt.

Q: Does ferromanganese remove sulphur completely?
No. It fixes sulphur as manganese sulphide and prevents grain-boundary films, but very low sulphur specifications require a separate ladle desulphurisation step with a basic reducing slag.