Silico-manganese (Si-Mn) is a metallic ferro alloy which is being used to add both silicon (Si) and manganese (Mn) as ladle addition during steelmaking. Because of its lower carbon (C) content, it is a preferred ladle addition material during making of low carbon steels.

Si-Mn is a ferroalloy composed principally of Mn, Si, and Fe (iron), and normally contains much smaller proportions of minor elements, such as C, phosphorus (P), and sulphur (S). The ferroalloy is also sometimes referred to as ferro-silicon-manganese.

Both Mn and Si play an important role in the manufacturing of steel as deoxidizing, desulphurizing, and alloying agents. Si is the primary and more powerful deoxidizer. Mn is a milder deoxidizer than Si but enhances the effectiveness of the latter due to the formation of stable manganese silicates and aluminates. It also serves as desulphurizer. Mn is used as an alloying element in almost all types of steel. Of particular interest is its modifying effect on the iron-carbon (Fe-C) system by increasing the hardenability of the steel.

There are two families of Mn alloys one is called Si-Mn while the other is known as ferro-manganese (Fe-Mn). Si-Mn adds additional silicon in liquid steel which is a stronger deoxidizer and which also helps to improve some mechanical properties of steel. In each family, content of C can be controlled and lowered when producing low C grades. Around 93 % of all the Mn produced is in the form of Mn ferroalloys consists of the Fe-Mn grades and the Si-Mn grades.

The Fe-Mn grades are high carbon (HC), medium carbon (MC), low-carbon (LC) and very low carbon (VLC), whereas the Si-Mn grades include medium carbon (MC) and low carbon (LC). The steel industry is the only consumer of these alloys. However as the average consumption of Mn in one ton of steel is around 7 kg, the requirement of these two ferro alloys amounts to considerable tonnages.

To cover the need for Mn and Si, the steelmaker has the choice of a blend of Si-Mn, HC Fe-Mn and Fe-Si governed of by specifications on C, Si, and Mn. Normally earlier a mixture of HC Fe-Mn and Fe-Si was used, but now a trend towards more use of Si-Mn is seen at the expense of the two others. This is primarily for economic reasons.

Si-Mn is produced in a number of grades and sizes and is consumed in bulk form primarily in the production of steel as a source of both Si and Mn, although some Si-Mn is also used as an alloying agent in the production of iron castings. Mn, which is intentionally present in nearly all steels, is used as a steel desulphurizer and deoxidizer. By removing S from steel, Mn prevents the steel from becoming brittle during the hot rolling process. In addition, Mn increases the strength and hardness of steel. Si is a deoxidizer, aiding in making steels of uniform chemistry and mechanical properties. As such, it is not retained in the steel, but forms SiO2, which separates from the steel as a component of the slag.

Effects of the addition of Si-Mn to steel depend on the amount added and the combined effect with other alloying elements. Both Si and Mn have an important influence on the properties of steel since both of them have a strong affinity for oxygen (O), and act as deoxidizers. Deoxidation with Si-Mn results in cleaner steel, as the liquid manganese silicate formed coagulates and separates easier from the melt, compared to solid SiO2 formed during Fe-Si deoxidation. Use of Si-Mn adds less C to steel compared to combination of standard Fe-Si and HC Fe-Mn. Computational fluid dynamics calculations show that the yield of Si from Si-Mn is higher than that of standard Fe-Si.

Si-Mn normally contains around 65 % to 68 % Mn and around 17 % Si. Various national and international standards for Si-Mn, designates different grades, differentiated by their Si and C contents. All standard grades of Si-Mn are generally acceptable for most uses and they are readily interchangeable.

Si-Mn is added to steel in small quantities, hence it accounts for only a small share of the total cost of end use steel mill products. No single product can substitute for Si-Mn. Steelmakers can and do substitute a combination of high C Fe-Mn and Fe-Si for Si-Mn, although not all steelmakers can make this substitution. In small plants, the facilities for storing and handling material are not sufficient to handle a combination of inputs, so only Si-Mn is used these plants efficiently.

Properties of Si-Mn

The chemical name of the ferro-alloy is FeSiMn. Si-Mn generally contains not less than 4 % of Fe, more than 30 % of Mn, more than 8 % of Si and not more than 0.2 % of P. It is properly classifiable under subheading 7202.30.0000 of the Harmonized Tariff Schedule of the United States (HTSUS). The low C Si-Mn is a ferroalloy with the chemical specifications of minimum 55 % of Mn, minimum 27 % of Si, minimum 4 % of Fe, maximum 0.10 % of P, maximum 0.10 % of C, and maximum 0.5 % of S.

Si-Mn is available as metallic lumps with a silver/grey appearance (Fig 1). It is an odorless solid material. It is usually free from extraneous contaminations such as slag and non-metallic inclusions etc. It is normally supplied as crushed and screened material. The normal standard sizes for Si-Mn consist of (i) 20 mm to 80 mm with undersize 10 % maximum, (ii) 10 mm to 50 mm with undersize 10 % maximum, and (iii) 3 mm to 25 mm with undersize 5 % maximum. The oversize limit in all three cases is 10 % maximum. Si-Mn is usually delivered as bulk normally packed in big bags.