Deoxidized steel (Also known as killed steel) is
steel
Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant ty ...
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
Iron(II) oxide or ferrous oxide is the inorganic compound with the formula FeO. Its mineral form is known as wüstite. One of several iron oxides, it is a black-colored powder that is sometimes confused with rust, the latter of which consists ...
. 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.
Types
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
Killed steel is steel that has been completely deoxidized by the addition of an agent before
casting such that there is practically no evolution of gas during
solidification. It is characterized by a high degree of chemical
homogeneity and freedom from
gas porosities. 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.
For
ingot casting, 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.
The main disadvantage of killed steel is that it suffers from deep
pipe shrinkage
A casting defect is an undesired irregularity in a metal casting process. Some defects can be tolerated while others can be repaired, otherwise they must be eliminated. They are broken down into five main categories: ''gas porosity'', ''shrinkage ...
defects. To minimize the amount of metal that must be discarded because of the shrinkage, a large vertical mold is used with a
hot top
A riser, also known as a feeder, is a reservoir built into a metal casting mold to prevent cavities due to shrinkage. Most metals are less dense as a liquid than as a solid so castings shrink upon cooling, which can leave a void at the last p ...
. Typical killed-steel ingots have a yield of 80% by weight.
Commonly killed steels include
alloy steels,
stainless steel
Stainless steel is an alloy of iron that is resistant to rusting and corrosion. It contains at least 11% chromium and may contain elements such as carbon, other nonmetals and metals to obtain other desired properties. Stainless steel's corros ...
s,
heat resisting steels,
steels with a carbon content greater than 0.25%, steels used for
forging
Forging is a manufacturing process involving the shaping of metal using localized compressive forces. The blows are delivered with a hammer (often a power hammer) or a die. Forging is often classified according to the temperature at which i ...
s,
structural steel
Structural steel is a category of steel used for making construction materials in a variety of shapes. Many structural steel shapes take the form of an elongated beam having a profile of a specific cross section. Structural steel shapes, sizes, ...
s with a carbon content between 0.15 and 0.25%, and some special steels in the lower carbon ranges.
It is also used for any
steel casting
Steel casting is a specialized form of casting involving various types of steel cast to either final/net or near-net shape. Steel castings are used when iron castings cannot deliver enough strength or shock resistance.Oberg, p. 1332
Examples ...
s.
Note that decrease in carbon content increases the problems with non-metallic
inclusions.
Continuous casting and strip-casting technologies have largely superseded ingot casting techniques in recent times. Through these methods, all steel is killed and the resulting yields are close to 96%.
Semi-killed
Semi-killed steel is mostly deoxidized steel, but the carbon monoxide leaves blowhole type porosity distributed throughout the ingot. The porosity eliminates the pipe found in killed steel and increases the yield to approximately 90% by weight. Semi-killed steel is commonly used for structural steel with a carbon content between 0.15 and 0.25% carbon, because it is
rolled, which closes the porosity. It is also used for
drawing
Drawing is a form of visual art in which an artist uses instruments to mark paper or other two-dimensional surface. Drawing instruments include graphite pencils, pen and ink, various kinds of paints, inked brushes, colored pencils, crayons, ...
applications.
[.]
Characteristics of SEMI KILLED 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 skin 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 or 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).
Rimmed
Rimmed steel, also known as ''drawing quality steel'', has little to no deoxidizing agent added to it during casting which causes carbon monoxide to evolve rapidly from the ingot. This causes small blow holes in the surface that are later closed up in the
hot rolling
In metalworking, rolling is a metal forming process in which metal stock is passed through one or more pairs of rolls to reduce the thickness, to make the thickness uniform, and/or to impart a desired mechanical property. The concept is simil ...
process. Another result is the segregation of elements; almost all of the carbon,
phosphorus, and
sulfur
Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
move to the center of the ingot, leaving an almost perfect "rim" of pure iron on the outside of the ingot. This gives the ingot an excellent
surface finish
Surface finish, also known as surface texture or surface topography, is the nature of a surface as defined by the three characteristics of lay, surface roughness, and waviness.. It comprises the small, local deviations of a surface from the perf ...
because of this iron rim, but also form the most segregated composition. Most rimmed steel has a carbon content below 0.25%, a manganese content below 0.6%, and is not alloyed with aluminum, silicon, and
titanium. This type of steel is commonly used for
cold-
bending, cold-forming, cold-
heading and, as the name implies, drawing. Due to the non-uniformity of alloying elements it is not recommended for
hot-working applications.
Capped
Capped steel starts as rimmed steel but part way through the solidification the ingot is capped. This can be done by literally covering the ingot mold or by adding a deoxidizing agent. The top of the ingot then forms into a solid layer of steel, but the rim of the rest of the ingot is thinner than in a rimmed steel. Also there is less segregation of impurities.
The yield of rimmed and capped steel is slightly better than that of semi-killed steel. These types of steels are commonly used for
sheet
Sheet or Sheets may refer to:
* Bed sheet, a rectangular piece of cloth used as bedding
* Sheet of paper, a flat, very thin piece of paper
* Sheet metal, a flat thin piece of metal
* Sheet (sailing), a line, cable or chain used to control the cle ...
and strip metal because of their excellent surface condition.
It is also used in most cold-working applications.
Due to production processes, as the carbon content of rimmed and capped steel increases above 0.08%, the cleanliness decreases.
[.]
See also
*
Decarburized steel
Decarburization (or decarbonization) is the process of decreasing carbon content, which is the opposite of carburization.
The term is typically used in metallurgy, describing the decrease of the content of carbon in metals (usually steel). Decar ...
References
Bibliography
*{{Citation , last = American Institute of Mining and Metallurgical Engineers , title = The solidification of metals and alloys , publisher =
American Institute of Mining and Metallurgical Engineers , year = 1951 , url = https://books.google.com/books?id=4pJTAAAAMAAJ , postscript =.
Steelmaking
Steels