Deoxidized steel
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Deoxidized steel (Also known as killed steel) is steel that has some or all of the
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
removed from the melt during the
steelmaking Steelmaking is the process of producing steel from iron ore and carbon/or scrap. In steelmaking, impurities such as nitrogen, silicon, phosphorus, sulfur and excess carbon (the most important impurity) are removed from the sourced iron, and alloy ...
process. Liquid steels contain dissolved oxygen after their conversion from molten iron, but the
solubility In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solub ...
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.


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 Casting is a manufacturing process in which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a ''casting'', which is ejected ...
such that there is practically no evolution of gas during
solidification Freezing is a phase transition where a liquid turns into a solid when its temperature is lowered below its freezing point. In accordance with the internationally established definition, freezing means the solidification phase change of a liquid ...
. It is characterized by a high degree of chemical
homogeneity Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the Uniformity (chemistry), uniformity of a Chemical substance, substance or organism. A material or image that is homogeneous is uniform in compos ...
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 Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
,
ferrosilicon Ferrosilicon is an alloy of iron and silicon with a typical silicon content by weight of 15–90%. It contains a high proportion of iron silicides. Production and reactions Ferrosilicon is produced by reduction of silica or sand with coke in t ...
and
manganese Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...
. Aluminium reacts with the dissolved gas to form aluminium oxide. The aluminum oxide precipitates provide the additional benefit of pinning
grain boundaries In materials science, a grain boundary is the interface between two grains, or crystallites, in a polycrystalline material. Grain boundaries are two-dimensional defects in the crystal structure, and tend to decrease the electrical and thermal ...
, thereby preventing
grain growth In materials science, grain growth is the increase in size of grains (crystallites) in a material at high temperature. This occurs when recovery and recrystallisation are complete and further reduction in the internal energy can only be achieved ...
during
heat treatment Heat treating (or heat treatment) is a group of industrial process, industrial, thermal and metalworking, metalworking processes used to alter the physical property, physical, and sometimes chemical property, chemical, properties of a material. ...
s. 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 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. Typical killed-steel ingots have a yield of 80% by weight. Commonly killed steels include
alloy steel Alloy steel is steel that is alloyed with a variety of elements in total amounts between 1.0% and 50% by weight to improve its mechanical properties. Alloy steels are broken down into two groups: low alloy steels and high alloy steels. The differe ...
s, stainless steels, heat resisting steels, steels with a carbon content greater than 0.25%, steels used for forgings, structural steels 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 castings. Note that decrease in carbon content increases the problems with non-metallic
inclusion Inclusion or Include may refer to: Sociology * Social inclusion, aims to create an environment that supports equal opportunity for individuals and groups that form a society. ** Inclusion (disability rights), promotion of people with disabiliti ...
s. 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 Rolling is a Motion (physics)#Types of motion, type of motion that combines rotation (commonly, of an Axial symmetry, axially symmetric object) and Translation (geometry), translation of that object with respect to a surface (either one or the ot ...
, which closes the porosity. It is also used for drawing 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 process. Another result is the segregation of elements; almost all of the carbon,
phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...
, and sulfur 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 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 Titanium is a chemical element with the Symbol (chemistry), symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resista ...
. This type of steel is commonly used for
cold Cold is the presence of low temperature, especially in the atmosphere. In common usage, cold is often a subjective perception. A lower bound to temperature is absolute zero, defined as 0.00K on the Kelvin scale, an absolute thermodynamic ...
-
bending In applied mechanics, bending (also known as flexure) characterizes the behavior of a slender structural element subjected to an external load applied perpendicularly to a longitudinal axis of the element. The structural element is assumed to ...
, cold-forming, cold-
heading Heading can refer to: * Heading (metalworking), a process which incorporates the extruding and upsetting processes * Headline, text at the top of a newspaper article * Heading (navigation), the direction a person or vehicle is facing, usually s ...
and, as the name implies, drawing. Due to the non-uniformity of alloying elements it is not recommended for
hot-working In metallurgy, hot working refers to processes where metals are plastically deformed above their recrystallization temperature. Being above the recrystallization temperature allows the material to recrystallize during deformation. This is impo ...
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 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


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 The American Institute of Mining, Metallurgical, and Petroleum Engineers (AIME) is a professional association for mining and metallurgy, with over 145,000 members. It was founded in 1871 by 22 mining engineers in Wilkes-Barre, Pennsylvania, Un ...
, year = 1951 , url = https://books.google.com/books?id=4pJTAAAAMAAJ , postscript =. Steelmaking Steels