Deoxidized steel

Deoxidized steel is steel that has some or all of the oxygen removed from the melt during the steelmaking process. Liquid steels contain dissolved oxygen after their conversion from molten iron, but the solubility of oxygen in steel decreases with cooling. As steel cools, excess oxygen can cause blowholes or precipitate FeO. Therefore, several strategies have been developed for deoxidation. This may be accomplished by adding metallic deoxidizing agents to the melt either before or after it is tapped, or by vacuum treatment, in which carbon dissolved in the steel is the deoxidizer.

There are four types, ranging from fully deoxidized to slightly deoxidized: killed, semi-killed, rimmed, and capped. Note that none of the various types are better than the other as each is useful in its own regard.

Killed steel is steel that has been completely deoxidized by the addition of an agent before casting, so that there is practically no evolution of gas during solidification. They are characterized by a high degree of chemical homogeneity and freedom from gas porosity. The steel is said to be "killed" because it will quietly solidify in the mould, with no gas bubbling out. It is marked with a "K" for identification purposes.

Common deoxidizing agents include aluminium, ferrosilicon and manganese. Aluminium reacts with the dissolved gas to form aluminium oxide. The aluminum oxide precipitates provide the additional benefit of pinning grain boundaries, thereby preventing grain growth during heat treatments. For steels of the same grade a killed steel will be harder than rimmed steel.

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