Types of Steel according to deoxidation practice

During steel-manufacturing processes, combination of carbon and oxygen takes place to form a gas. If the oxygen is not removed before or during casting (by adding silicon or other deoxidizer) the gaseous products continue to evolve during solidification. The type of steel produced is determined by the control of the amount of gas evolved during solidification. If no gas is evolved, the steel is called "killed" because it lies quietly in moulds. Increasing degree of gas evolution results in semi-killed, capped or rimmed steels.

Some carbon steels and high strength low-alloy steels can be supplied in all four types. Alloy steels and stainless and heat resisting steels are normally manufactured as killed steel.

1) Killed steel:

• It is produced by involving the use of several deoxidizing elements which act with varying intensities.
• The common of these are silicon and aluminum.
•  Killed steel, because of greater uniformity in chemical composition and soundness is used for forging, carburizing , heat treatment and other applications.
• They are strongly deoxidised and are characterized by high composition and property uniformity.
• All forging steels and in general, all steels containing more than 0.25% carbon are killed.
• These types of steels are free from blow holes and segregation.
• Symbol K stands for killed steel.

2) Semi Killed steel:

• It has characteristics intermediate those of killed or rimmed steels.
• Structural steels containing 0.15 to 0.25% carbon are generally semi-killed.
•  In semi-killed steel, the aim is to produce metal free from surface blowhole and pipe.
• The surface should have a sound kin of considerable thickness.
• They are used for general structural applications.
• During solidification of semi-killed steel, gas is evolved in the body of the ingot, tending to compensate in part of entirely for the shrinkage accompanying solidification.
• Since pipe cavities are minimized, semi-killed steels are usually cast in big-end -down molds without hot-tops.
• This type of steel is suitable for drawing operation (except severe drawing).

3) Capped Steel:

• It has characteristics similar to those of rimmed steels but to a degree intermediate between those of rimmed and semi-killed steels.
• A deoxidizer may be added to effect a controlled running action when the steel is cast. the gas entrapped during solidification is in excess of that needed to counteract normal shrinkage, resulting in a tendency for the steel to rise in the mould.
• The capping operation caused the steel to solidify faster, thereby limiting the time of gas evolution, and prevents the formation of an excessive number of gas voids within the ingot.
• Capped steel is generally cast in bottle-top moulds using a heavy metal cap.
•  Capped steel may also be cast in open-top moulds, by adding aluminum or ferro-silicon on the top of molten steel, to cause the steel on the surface to lie quietly and solidify rapidly.

4) Rimmed Steel:

In rimmed steel, the aim is to produce a clean surface low in carbon content. Rimmed steel is also known as drawing quality steel.

• The typical structure results for a marked gas evolution during solidification of outer rim.
• They exhibit greatest difference in chemical composition across sections and from top to bottom of the ingot.
• They have an outer rim that is lower in carbon, phosphorus, and sulphur than the average composition of the whole ingot and an inner portion or core that is higher the average in those elements.
• In rimming, the steel is partially deoxidized. Carbon content is less than 0.25% and manganese content is less than 0.6%.
• They do not retain any significant percentage of highly oxidizable elements such as Aluminum, silicon or titanium.
• A wide variety of steels for deep drawing is made by the rimming process, especially where ease of forming and surface finish are major considerations.
• These steel are, therefore ideal for rolling, large number of applications, and is adapted to cold-bending, cold-forming and cold header applications.