Killed steel, sometimes called deoxidized steel, is steel that has been entirely deoxidized by adding an agent before casting. Thus, there are minimal gas pockets that present during solidification. This article reviews the process of making killed steel, and various types such as aluminum, carbon steel grades, and killed steel valves.
What is Killed Steel
Killed steel is treated with strong deoxidizing agents during the melting process to remove dissolved oxygen. When molten steel contains oxygen, it reacts with carbon to form carbon monoxide gas, which can create internal voids and porosity in the solidified metal. By introducing deoxidizers such as silicon, aluminum, ferrosilicon, titanium, and manganese, steelmakers prevent the formation of gas bubbles. Hence, producing a more uniform and structurally sound material.
The absence of gas pockets makes killed steel highly suitable for continuous casting. Because, it can be poured through small-diameter nozzles without clogging. It is common in applications where structural integrity, reliability, and resistance to hydrogen embrittlement are critical.
How is Killed Steel Produced
The production of killed steel involves adding deoxidizers to the molten steel to remove excess oxygen. The steps are as follows:
- Melting and Refining: Steel is melted in a basic oxygen furnace (BOF) or an electric arc furnace (EAF). Impurities such as sulfur and phosphorus are removed, and alloying elements may be introduced.
- Deoxidation Process: Strong deoxidizers, including silicon, aluminum, manganese, and titanium, are added to react with dissolved oxygen. Thus, forming non-metallic oxides that separate from the liquid steel.
- Casting and Solidification: The steel is then poured into molds or continuous casting machines. Since the oxygen has been removed, the steel solidifies smoothly without gas formation, resulting in a dense, high-quality material.
This process ensures greater consistency, strength, and resistance to defects. Therefore, making it an ideal choice for structural and industrial applications.
Aluminum Killed Steel: The Role of Aluminum in Deoxidation
Among the various deoxidizing agents, aluminum is one of the most effective and is common due to its affordability and efficiency. When added to molten steel, aluminum reacts with dissolved oxygen to form aluminum oxide, a stable compound that gets removed as slag.
Why is Aluminum Killed Steel Preferred?
Eliminates gas porosity for a more uniform metal structure.
Improves machinability and weldability, making it easier to fabricate.
Prevents grain growth during heat treatment, ensuring finer and stronger steel.
Enhances toughness and resistance to cracking in pressure vessels and pipelines.
The amount of aluminum retained in the steel typically falls between 0.005% and 0.020%. Hence, ensuring effective deoxidation while maintaining desirable mechanical properties.
Killed Carbon Steel Grades
These are available in various grades, each suited for specific industrial applications. The four primary grades are:
Rimmed Steel
Minimal deoxidizer additions, leading to a rim of pure iron around the ingot.
High ductility, making it ideal for sheet metal production.
Typically contains less than 0.2% carbon and also less than 0.6% manganese.
Common in automotive panels, appliances, and deep-drawn parts where formability is a priority.
Capped Steel
A variant of rimmed steel where the mold is sealed with a cast-iron cap.
Reduces the gas evolution inside the steel, thus, producing a more uniform grain structure.
Commonly used in wire, strip, and bars for structural applications.
Semi-Killed Steel
Contains enough oxygen to react with carbon and form controlled amounts of carbon monoxide gas.
Provides moderate strength and ductility, making it ideal for structural steel applications.
Typically has 0.15% to 0.30% carbon content.
Common in I-beams, construction materials, and bridge components.
Fully Killed Steel (Dead Steel)
Completely deoxidized with no gas evolution during solidification.
Offers the most uniform structure, eliminating internal cavities as well as defects.
Maximizes oxidation removal, but in some cases, aluminum killing can introduce undesirable residuals that affect processing (e.g., in high-performance rotating shafts).
Common in pressure vessels, high-stress applications, and critical structural components.
Killed Steel in Valve Manufacturing
Why Are They Preferred?
Killed steel is widely used in valve manufacturing because of its homogeneous structure, superior strength, and resistance to hydrogen-related failures. Valves play a critical role in controlling fluid flow in high-pressure and high-temperature systems. Therefore, making material selection essential.
Enhanced Structural Integrity: Valves using this steel have fewer internal cavities, reducing the risk of cracking or failure under pressure.
Hydrogen Blistering Prevention: The dense structure of this steel prevents hydrogen molecules from accumulating in voids. Hence, avoiding blistering and embrittlement.
Improved Performance in Harsh Environments: Ideal for oil & gas, chemical processing, and power generation industries where corrosion resistance and durability are crucial.
Examples
Many industrial valve manufacturers use fully killed steel for critical service applications. For instance:
A216-WCB Cast Carbon Steel Valves – Commonly used in steam, oil, and gas applications.
A105N Forged Steel Valves – Preferred for high-pressure pipelines and refinery operations.
Manufacturers such as Newco and L&T incorporate these materials into their valve designs to ensure high reliability and long service life.
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