Steel that has been treated with an oxidizing agent, in order to bind the oxygen molecules that are present in the molten steel, is called killed steel. Carbon dioxide in molten steel will form bubbles when the steel cools, which has deleterious effects on many of the qualities of steel. By adding an oxidizer, such as aluminum, silicon, or vanadium, oxygen atoms in the molten steel are bound to the oxidizer instead of binding with carbon and producing the bubbles of carbon dioxide. This results in denser steel without bubbles. Almost any type of steel can be killed, but the intended use generally determines whether a particular steel will be subjected to this process.
Deoxidizing a particular steel can improve many of its properties, particularly hardness and chemical homogeneity, which means that killed steel tends to be more consistent, in terms of chemical make up and molecular structure within a particular sample, than a non-killed steel with the same formula. A killed steel will be denser than the same steel that is not killed, due to the absence of gas bubbles. For this reason, many types of steel are treated this way, though not all applications call for killed steel. For instance, some steels used for casting are not killed, as killed steel tends to exhibit a higher degree of shrinkage than non-killed steels, which can cause problems with cast parts.
Steels with a high carbon content, 0.25% or more, are almost always killed, as are steels with a carbon content between 0.15% and 0.25%, that are used as structural steels. Killed steel tends to be stronger as well as harder than non-killed steel, which is why structural steels are deoxidized. Some lower carbon steels are also killed, depending on the intended use.
Stainless steels are almost always killed steels. The presence of free oxygen or carbon dioxide molecules within the steel can lead to oxidation of the steel itself, which is exhibited as rust. Stainless steel, by definition, is resistant to oxidation. While this resistance to oxidation is mainly a function of additives to the steel alloy, the process of deoxidation improves this quality even further.
All steels used for applications in which the steel is forged are generally killed, regardless of their other properties or additives, such as nickel, vanadium, chromium and others. The bubbles that form in non-killed steels can cause weak points and structural defects in steel. Steels that will be heat treated are also usually killed.