Molten steel contains dissolved oxygen and other gases introduced during refining. When molten steel cools, oxygen solubility decreases, leading to a reaction between carbon and oxygen that forms carbon monoxide gas. “Killed steel” is a classification indicating that rigorous quality control has neutralized these dissolved gases before casting. This process ensures the material meets demanding uniformity and structural integrity requirements.
The Deoxidation Process
The term “killed” refers to the complete removal of dissolved oxygen from the molten metal, preventing the vigorous bubbling action that occurs as gas escapes during solidification. This deoxidation is achieved by adding specific metallic elements known as deoxidizers to the liquid steel, typically while it is still in the ladle. Common deoxidizers include aluminum, silicon, and manganese, which have a strong chemical affinity for oxygen.
When added, these elements react with the dissolved oxygen to form stable, solid oxide compounds. These reaction products are non-gaseous and lighter than the molten steel, allowing them to float to the surface and become part of the slag layer, which is skimmed off. This chemical neutralization eliminates the formation of carbon monoxide gas. Because the molten steel is no longer evolving gas, it solidifies quietly and calmly in the mold, giving rise to the term “killed.” The resulting steel typically has an oxygen content reduced to an extremely low level, often between 0.002% and 0.003% by weight.
Structural Uniformity and Internal Quality
The complete deoxidation process translates into a finished steel product with superior internal quality and structural uniformity. By preventing the release of gases during cooling, killed steel avoids the formation of internal gas pockets, known as blowholes or gas porosity. This absence of internal voids results in a material that is extremely dense and structurally sound.
The chemical homogeneity of killed steel is also significantly improved because the uniform solidification minimizes the segregation of alloying elements. Elements like carbon, sulfur, and phosphorus are distributed more evenly, preventing weak spots or areas of inconsistent mechanical properties. The use of aluminum as a deoxidizer helps control grain growth during subsequent heat treatments, ensuring a finer, tougher microstructure.
Differences From Rimmed and Semi-Killed Steel
Killed steel represents the high end of the deoxidation spectrum, where virtually all dissolved oxygen is removed before casting. The stark contrast is rimmed steel, which receives minimal or no deoxidizing agents, allowing the carbon-oxygen reaction to proceed aggressively. This reaction causes the molten metal to “boil” vigorously in the mold. The result is an ingot with a clean, relatively pure outer shell, or “rim,” but a core characterized by significant chemical segregation and internal porosity.
While the surface quality of rimmed steel is excellent for certain applications, its internal non-uniformity makes it unsuitable for uses requiring high integrity. In comparison, semi-killed steel occupies a middle ground, receiving a partial deoxidation treatment that balances cost and quality. Semi-killed steel allows for some gas evolution, which helps counteract the natural shrinkage that occurs as the metal cools, thereby maximizing the usable yield from the ingot. This partial deoxidation means that some porosity and segregation remain.
Unlike the calm solidification of killed steel, semi-killed steel’s moderate gas release helps minimize the deep shrinkage cavity that forms at the top of the ingot. This offers a compromise between internal soundness and production efficiency.
Primary Uses in Manufacturing
The consistent internal quality and lack of structural flaws make killed steel mandatory for applications where material failure is unacceptable. Its superior density and homogeneity are leveraged in components that undergo severe mechanical stress, such as forgings and parts subjected to extensive cold working. Killed steel is widely specified for the construction of high-pressure vessels and pipelines, particularly those transporting natural gas or other volatile substances, where structural reliability is paramount.
It is also the preferred type for virtually all alloy steels, which require precise chemical control to achieve their intended properties. Its ability to resist hydrogen embrittlement, due to its dense, void-free structure, makes it a frequent choice in critical infrastructure projects and the oil and gas industry.