Killed, semi-killed, and rimmed are the three principal deoxidation practices applied to liquid carbon steel before ingot casting, and the distinction is set by how much aluminum, silicon, or manganese is added to the melt, per TWI guidance [S1].
That single process decision controls gas evolution during solidification, internal porosity, ingot yield (typically 80% for killed and 90% for semi-killed), chemical homogeneity, and ultimately which specifications a heat can satisfy [S2][S3].
Deoxidation Mechanism and Chemistry
Liquid steel leaving the converter carries dissolved oxygen whose solubility drops sharply as temperature falls, so producers add metallic deoxidizers to bind that oxygen before casting; aluminum, silicon, and manganese are the most widely used agents, reacting to form Al2O3, SiO2, and MnO that report to the slag [S2]. Killed steel is fully deoxidized, semi-killed is only partially deoxidized, and rimmed steel receives little to no deoxidizing addition at all [S1][S5].
Aluminum is normally fed as pellets or wire, while silicon and manganese enter through ferroalloy additions, and aluminum's Al2O3 product additionally pins austenite grain boundaries during subsequent heat treatment, a secondary benefit specific to the killed route [S2]. Vacuum degassing is the alternative to chemical deoxidation, using dissolved carbon as the deoxidizer under low pressure.
Ingot Structure: Pipe, Blowholes, and Porosity
Killed steel solidifies quietly in the mould with practically no gas evolution, which sounds ideal but produces a deep central pipe from shrinkage; the ingot is therefore cast with a hot top in a tall vertical mould to feed that pipe, and about 20% of the ingot ends up discarded, leaving a usable yield of roughly 80% by weight [S2].
Semi-killed steel is mostly deoxidized, but enough dissolved gas remains that carbon monoxide forms distributed blowhole-type porosity throughout the ingot, eliminating the deep pipe and pushing yield up to approximately 90% by weight [S2][S3]. Rimmed steel is the extreme case: with little to no deoxidizer added, CO evolves rapidly during solidification, leaving a clean low-carbon rim and a core of blowholes that are closed during subsequent hot rolling [S3].
Chemical Homogeneity and Segregation
Because killed steel solidifies uniformly with no gas to stir the remaining liquid, it shows high chemical homogeneity throughout the section, which is why killed grades are mandatory for alloy steels, stainless steels, heat-resisting steels, and any carbon steel above 0.25% C [S2]. Semi-killed steel sits between killed and rimmed on the homogeneity scale, with composition uniformity that is intermediate and a pronounced tendency for chemical segregation to occur at the top of the ingot [S1].
Rimmed steel is the least homogeneous: alloying elements are concentrated in the outer rim and depleted in the core, and the practice is generally restricted to plain low-carbon melts (typically below 0.25% C and below 0.6% Mn) with no intentional Al, Si, or Ti additions [S3].
Mechanical Properties and Hardness
For the same nominal grade, killed steel is harder than rimmed steel because the absence of porosity and the grain-pinning effect of Al2O3 give a tighter, more uniform response to heat treatment [S2]. The trade-off is ductility and surface finish: rimmed steel's blowhole layer is welded shut during hot rolling, leaving a dense, clean surface that is preferred for cold-heading and deep-drawing operations, which is why it is also called drawing-quality steel [S3].
Semi-killed steel sits between those extremes, with mechanical properties less uniform than killed but more consistent than rimmed; it is commonly specified for structural plate and sections in the 0.15–0.25% C range, where the rolling reduction is enough to close the internal porosity [S2][S3].
Selection Criteria and Typical Applications
Selection by deoxidation practice is essentially a decision tree driven by carbon content, required mechanical uniformity, surface finish, and yield economics; the four canonical deoxidation states (killed, semi-killed, rimmed, capped) are not ranked because each is optimal for a different use case [S2].
Killed steel is specified for alloy and stainless grades, forgings, structural sections with greater than 0.25% C, and any application demanding through-thickness homogeneity or impact toughness at low temperature. Semi-killed steel is the default for general structural shapes, plate, and merchant bar in the 0.15–0.25% C band where rolling closes the porosity and the higher 90% ingot yield offsets the property scatter [S2][S3]. Rimmed steel is reserved for cold-rolled sheet, wire, and drawn products where surface quality, drawability, and a high ingot-to-billet yield outweigh the homogeneity penalty [S3].
Comparison: Killed vs Semi-Killed vs Rimmed at a Glance
On ingot yield, killed steel sits at approximately 80% because of pipe shrinkage, semi-killed at approximately 90% thanks to distributed porosity, and rimmed typically higher still with no pipe but with a segregated core [S2][S3]. On deoxidizer addition, killed receives full Al/Si/Mn treatment, semi-killed receives a partial addition, and rimmed receives little to none; on porosity, killed is sound, semi-killed contains distributed blowholes, and rimmed carries a porous core beneath a dense rim [S1][S3].
On chemical homogeneity, killed is the most uniform, semi-killed intermediate, and rimmed the least; on hardness for the same nominal grade, killed is hardest and rimmed softest, while semi-killed sits between [S2]. On typical carbon range, killed covers anything above 0.25% C plus alloy and stainless grades, semi-killed anchors the 0.15–0.25% C structural band, and rimmed is used below 0.25% C with Mn under 0.6% [S2][S3].
Identification Marks and Process Notes
Killed steel is marked with a "K" in the mill identification system, and aluminum is the marker element that buyers and QA teams look for in chemistry printouts to confirm a heat was fully killed rather than semi-killed or rimmed [S2]. For semi-killed steel, the ingot shows a slight domed top with internal porosity but no pipe, and rolling or forging reduction is what welds those internal voids shut [S2][S3]. Rimmed ingots, by contrast, display a very clean, blowhole-free rim and a visibly porous core that disappears during hot rolling into sheet or strip [S3].
Specifiers ordering carbon steel plate, structural sections, or forging stock should always confirm the deoxidation practice on the mill test certificate, because the same ASTM grade can be supplied killed, semi-killed, or rimmed with measurably different impact, ductility, and ultrasonic response.
For buyers comparing carbon steel offers against construction machinery and equipment fabrication schedules, the next tracking signals to watch are the steelmaker quarterly practice notes (which often call out shifts between killed and semi-killed production as order mix changes) and the mill test certificate's deoxidation code on each heat; both are verifiable inputs that will tell you whether a quoted price reflects true killed steel or a semi-killed substitute. For a related look at how process equipment is specified alongside material selection, see the construction machinery and equipment reference page.
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