Ferrosilicon (FeSi) is an alloy of iron and silicon with a very variable silicon content between 70% and 75%. It is used as a so-called master alloy in steel production, which is added in small amounts in order to adjust the properties of the melt, the cooling process and the finished product.

It is used in the manufacture of silicon, corrosion-resistant and high-temperature resistant ferrous silicon alloys, and silicon steel for electro motors and transformer cores.