Varying amounts of silicon are usually present in most steels, up to as high as 15%. Silicon alloyed with steel gives a metal of great soundness and toughness since it is a powerful deoxidizer and removes gases and oxides from steel. Silicon in the range of 0.2-0.7% experiences increased hardness, while up to 2% silicon results in materials that are internally sound and elastic. With 4% silicon, the steel becomes stiffer, and steels having silicon content of 5% and greater are not forgeable. Silicon steel has a good tensile strength but a rather low resistance to impact. With higher silicon contents, the steel becomes acid-resisting.
Silicon increases the hardness of ferrite, increases oxidation resistance and is an effective deoxidizer. Silicon is less effective than manganese in increasing as-rolled strength and hardness. In low-carbon steels, silicon is generally detrimental to surface quality. Silicon is almost universally present in magnet sheet.
Silicon improves high-temperature oxidation resistance and is intentionally added to heat-resisting Cr-Mo and Cr-Mo-V steels as well as valve and spring steels. Both ferritic and austenitic stainless steels will contain about 1% silicon (or more) for oxidation resistance. Silicon is also the principal alloying element (0.5-5.0%) in electrical steels (e.g., motor laminations, transformers). This promotes large grain sizes, which produces relative high permeability, raising electrical resistivity and lowering hysteresis (core) losses.