Inclusion nomenclature

Inclusion nomenclature

Aluminum killed steel
(Special killed) steel deoxidized with aluminum in order to reduce the oxygen content to a minimum so that no reaction occurs between carbon and oxygen during solidification. A steel where aluminum has been used as a deoxidizing agent.

Argon-oxygen decarburisation (AOD)
A process of further reducing the carbon content of stainless steel during refinement. AOD is closely related to electric arc furnaces (EAF), but has a shorter operating time and requires lower temperatures.

Argon stirring
An inert gas introduced through a nozzle to stir molten steel to promote chemical and temperature homogenization and float out inclusions.

Basic oxygen process (BOP)
A process in which molten steel is produced in a furnace by blowing oxygen into molten iron, scrap and flux materials. The furnace is known as basic oxygen furnace (BOF).

Batch furnace
A furnace (as opposed to a continuous furnace) into which the work pieces are charged singly or in batches, the furnace temperature then being controlled to produce the desired temperature cycle.

Basic steel
Steel melted in a furnace with a basic bottom and lining and under a slag containing an excess of a basic substance such as magnesia or lime.

Bottom pouring
The process of pouring ingots using a refractory runner system to fill them from the bottom of the ingot molds.

Breakout
Liquid steel flowing through the shell of a ladle or EAF.

Charge
To load a furnace with scrap and flux prior to melting, or to load a soaking pit with ingots for reheating.

Classification of non-metallic inclusions
Non-metallic inclusions, the presence of which defines purity of steel, are classified by chemical and mineralogical content, by stability and by origin. By chemical content non-metallic inclusions are divided into the following groups:

Oxides (simple — FeO, MnO, Cr2O3, SiO2, Al2O3, TiO2 and others; compound — FeO x Fe2O3, FeO x Al2O3, FeO x Cr2O3, Mg x Al2O3, 2FeO x SiO2 and others;
Sulfides (simple — FeS, MnS, Al2S3, CaS, MgS, Zr2S3 and others; compound — FeS x FeO, MnS x MnO and others);
Nitrides (simple — ZrN, TiN, AlN, CeN and others; compound — Nb(C,N), V(C,N) and others), which can be found in alloyed steel and has strong nitride-generative elements in its content: titanium, aluminium, vanadium, cerium and others;
Phosphides (Fe3P, Fe2P and others).

The majority of inclusions in metals are oxides and sulfides since the content of phosphorus is very small. Usually nitrides are present in special steels that contain an element with a high affinity to nitrogen.
By mineralogical content, oxygen inclusions divide into the following main groups:

Free oxides — FeO, MnO, Cr2O3, SiO2 (quartz), Al2O3 (corundum) and others
Spinels — compound oxides formed by bi and trivalent elements
Silicates, which are present in steel like a glass formed with pure SiO2 or SiO2 with admixture of iron, manganese, chromium, aluminium and tungsten oxides and also crystalline silicates. Silicates are the biggest group among non-metallic inclusions. In liquid steel non-metallic inclusions are in solid or liquid condition. It depends on the melting temperature.

Classifying non-metallic inclusions according to their size
In general, inclusions are subdivided into macro- (>20 µm), micro- (1-20 µm), and nano-inclusions (<1 µm). Source: Reference 1: Aspex, Analysis of Non-Metallic Inclusions during Steel Manufacturing Using a PICA 1020 Inclusion Analyzer, Mar 14, 2008. Reference 2: Markus Nuspl, Wolfhard Wegscheider, Johann Angeli, Wilhelm Posch and Michael Mayr, Qualitative and quantitative determination of micro-inclusions by automated SEM/EDX analysis, Analytical and Bioanalytical Chemistry Volume 379, Number 4 (2004), 640-645. Reference 3: AFS INCLUSION ATLAS, http://neon.mems.cmu.edu/afs/afs2/window2.html, Reference 4: Koch E et al (1998) BHM, 143Jg, Heft 9, 338–344.

Continuous casting
A method of pouring steel directly from the furnace into a billet, bloom, or slab directly from its molten form. WHY Continuous casting avoids the need for large, expensive mills for rolling ingots into slabs. Continuous cast slabs also solidify in a few minutes versus several hours for an ingot. Because of this, the chemical composition and mechanical properties are more uniform. HOW Steel from the BOF or electric furnace is poured into a tundish (a shallow vessel that looks like a bathtub) atop the continuous caster. As steel carefully flows from the tundish down into the water-cooled copper mold of the caster, it solidifies into a ribbon of red-hot steel. At the bottom of the caster, torches cut the continuously flowing steel to form slabs or blooms."

Continuous caster
A machine used to continuously produce blooms from molten steel with no interruptions or intermediate operations.

Desulphurisation
The removal of sulphur from a ladle full of hot metal via chemical injection before it is charged into the basic Oxygen Furnace. This is done because sulphur reduces welding and forming capabilities.

Degassing
Usually a chemical reaction resulting from a compound added to molten metal to remove gases from the metal. Often inert gases are used in this operation. Lowering the hydrogen and oxygen content of the steel by placing the molten steel in a vacuum.

Dephosphorization
Elimination of phosphorus from molten steel.

Deoxidation
The final operation in the production of "killed" steels, when elements such as silicon or aluminum are added to stabilize the dissolved oxygen. This prevents it reacting with the carbon to form carbon monoxide which would form blowholes during subsequent solidification. Removal of oxygen from steel.

Distribution of non-metallic inclusions
Besides of the shape of non-metallic inclusions their distribution throughout the steel grain structure is very important factor determining mechanical properties of the steel. Homogeneous distribution of small inclusions is the most desirable type of distribution. In some steels microscopic carbides or nitrides homogeneously distributed in the steel are created by purpose in order to increase the steel strength.Location of inclusions along the grain boundaries is undesirable since this type of distribution weakens the metal. Clusters of inclusions are also unfavorable since they may result in local drop of mechanical properties such as toughness and fatigue strength. Distribution of non-metallic inclusions may change as a result of metal forming (eg. Rolling). Ductile inclusions are deformed and elongated in the rolling direction. The less ductile inclusions are breaking forming chains of fragments. Elongated inclusions and chains result anisotropy of mechanical and other properties. Mechanical properties in transverse direction are lower than those parallel to the rolling direction.

Electric arc furnace (EAF)
In an electric arc furnace, steel is melted using the heat generated from the formation of electric arcs. These furnaces are suitable for a batch production process. A steel producing furnace where scrap generally makes up 100% of the charge. Heat is supplied from electricity that arcs from the electrodes to the metal bath. These furnaces may operate on AC or DC.

Electrode A carbon (graphite) rod that carries electricity to melt the scrap in an EAF.

Electric Steel
Steel made by one of the electric processes where the heat required for melting the steel is provided either by an electric are, usually made to pass between the metal itself and a carbon electrode, i.e., the are process, or by eddy currents induced by a high frequency current. In each process the furnace lining may be either acid or basic.

Electro-slag remelting (ESR)
Electroslag remelting is a secondary refining process, where a consumable electrode is melted by immersion in a molten flux (slag) consisting primarily of fluorspar (CaF2). The heating is generated by an electrical current (usually ac) that flows from the base plate through the liquid slag and the electrode.

Exogenous inclusions
Such inclusions are introduced to the steel from external sources. The most common source of exogenous inclusions is particles of refractory material which detach from their parent material and are entrained in the steel. e.g. erosion of refractories.

Ferralloy
A metal product commonly used as a raw material feed in steelmaking, usually containing iron and otheer metals to aid various stages of the steelmaking process such as deoxidation, desulfurization and adding strength. Examples: ferrochrome,ferromanganese and ferrosilicon.

Formation of non-metallic inclusions
There are three stages of inclusions formation:
1. Nucleation
At this stage nuclei of new phase are formed as a result of supersaturation of the solution (liquid or solid steel) with the solutes (eg. Al and O) due to dissolution of the additives (deoxidation or desulfurization reagents) or cooling down of the metal. The nucleation process is determined by surface tension on the boundary inclusion-liquid steel. The less the surface tension, the lower supersaturation is required for formation of the new phase nuclei. The nucleation process is much easier in the presence of other phase (other inclusions) in the melt. In this case the new phase formation is determined by the wetting angle between a nucleus and the substrate inclusion. Wetting condition (low wetting angle) are favorable for the new phase nucleation.
2. Growth
Growth of a separate inclusion continues until the chemical equilibrium is achieved (no supersaturation). Growth of inclusions in solid steel is very slow process therefore a certain level of non-equilibrium supersaturation may be retained (eg. martensite).
3. Coalescence and agglomeration
Motion of the molten steel due to thermal convection or forced stirring causes collisions of the inclusions, which may result in their coalescence (merging of liquid inclusions) or agglomeration (merging of solid inclusions). The coalescence/agglomeration process is driven by the energetic benefit obtained from decrease of the boundary surface between the inclusion and the liquid steel. Inclusions with higher surface energy have higher chance to merge when collide. Large inclusions float up faster than the smaller ones. The floating inclusions are absorbed by the slag. The floating process may be intensified by moderate stirring. Vigorous stirring will result in breaking the large inclusions into small droplets/fragments. Gas bubbles moving up through the molten steel also promote the inclusions floating and absorption by the slag. Blowing inert gas in Ladle Furnace (LF) or Vacuum ladle degassing of steel in result in obtaining cleaner steel.

Heat
An individual batch of metal as it is treated in a furnace.

High frequency induction furnace
This is essentially an air transformer in which the primary is a water-cooled spiral of copper tubing, and the secondary the metal being melted. Currents at a frequency above about 500 cps are used to induce eddy currents in the charge, thereby setting up enough heat in it to cause melting. The electrical forces operating in the high frequency furnace keep the charge moving rapidly in a vertical plane, and the steel is, therefore, effectively stirred. This stirring ensures homogeneity in the melt and prevents gravitational segregation. The slag being a bad conductor of electricity is not heated by induction and, therefore, depends almost entirely upon its intimate contact with the metal for maintenance of its temperature. Partly because of this it is difficult to refine steel as in the arc furnace and the process is, in effect, one of re-melting. In this respect it is analogous to the crucible process, although, by generating the heat in the` charge, the possibility of contamination
of the metal from furnace gases is avoided.

Hot top
An insulated reservoir on top of an ingot mold that retains heat and holds excess molten metal that is drawn into the ingot as it shrinks.

Hot top compound
Insulating material placed on top of the molten metal after the mold is filled.

Inclusion
Particles of nonmetallic impurities that are mechanically held in steel during solidification, usually oxides, sulphides, and silicates.

Ingot
Steel cast in a metal mold ready for rolling or forging. It is distinct from a casting, which is not rolled or forged. Ingots are usually rectangular, called slabs; square, called blooms; polygonal, eight- or 12-sided for forging. Squares and polygonal ingots can be fluted or corrugated to increase the surface area and reduce the tendency to crack while cooling.

Ingot mould
The receptacle into which molten steel is poured to form an ingot. After solidification the steel is suitable for subsequent working, i.e. rolling or forging.

Inclusions
Usually non-metallic particles contained in metal. Non-metallic inclusions are chemical compounds of metals (Fe, Mn, Al, Si, Ca) with non-metals (O, S, C, H, N). Non-metallic inclusions form separate phases. The non-metallic phases containing more than one compound (eg. different oxides, oxide+sulfide) are called complex non-metallic inclusions (spinels, silicates, oxysulfides, carbonitrides).Despite of small content of non-metallic inclusions in steel (0.01-0.02%) they exert significant effect on the steel properties such as: Tensile strength, deformability (ductility), toughness, fatigue strength, corrosion resistance, weldability, polishability and machinability. In steel they consist of sulphides such as manganese sulphide and oxides such as silicates or alumina. By causing local stress concentrations inclusions can cause a reduction in ductility and fatigue strength, or if large, a reduction in tensile strength. Depending on the source, from which non-metallic inclusion are derived, they are subdivided into two groups: indigenous and exogenous inclusions.

Inclusion deformability
Alumina (Al2O3), calcium hexaluminate (CaO x 6Al2O3) are brittle and undeformable at all working temperatures. Spinel (MgAl2O4) is known to be undeformable from room temperature to 1200 C. Silicate phases (silicates of Ca, Mn, Fe, and Al) are brittle at room temperature, but become deformable at higher temperatures. FeO (wustite), MnO and (Fe,Mn)O exhibit plastic deformation at room temperature. MnS, are soft, and they deform and form elongate inclusions in the direction of rolling.

Indigenous inclusions
These inclusions are determined by the chemistry of the steelmaking system. Here, the 'steelmaking system' is defined as the secondary steelmaking ladle and inputs to the ladle. The continuous caster tundish may also be regarded as an extension of the steelmaking system. It should be noted that the chemistry in the ladle is often not in an equilibrium state due to kinetic factors, and as such prediction of the prevailing indigenous inclusion species can prove very difficult. An example of indigenous inclusions are those that are introduced to improve machining.

Inclusion, stringer
An impurity, metallic or non-metallic, which is trapped in the ingot and elongated subsequently in the direction of working. It may be revealed during working or finishing as a narrow streak parallel to the direction of working

Killed steel
The term killed indicates that the steel has been sufficiently deoxidized to quiet the molten metal when poured into the ingot mold. The general practice is to use aluminum ferrosilicon or manganese as deoxidizing agents. A properly killed steel is more uniform as to analysis and is comparatively free from aging. However, for the same carbon and manganese content Killed Steel is harder than Rimmed Steel. In general all steels above 0.25% carbon are killed, also all forging grades, structural steels from 0.15% to 0.25% carbon and some special steels in the low carbon range. Most steels below 0.15% carbon are rimmed steel.

Ladle
A vessel for receiving and handling liquid steel. it is constructed of a refractory-lined steel shell.

Ladle refining system (LRS)
System where alloys are added to meet exact chemistries, gases are removed by vacuum, and argon bubbles stir the molten mixture to remove impurities and mix the alloying elements. arc heating maintains steel temperature.

Ladle metallurgy furnace (LMF)
An intermediate steel processing unit that further refines the chemistry and temperature of molten steel while it is still in the ladle. The ladle metallurgy step comes after the steel is melted and refined in the electric arc or basic oxygen furnace, but before the steel is sent to the continuous caster.

Liquid carburising A widely used method of case-hardening steel that eliminates scaling and the tendency to decarburisation and results in clean components. Sodium cyanide is the common media for this process, usually heated within the range of 900-930oC. It is advisable to pre-heat the components in neutral salts to avoid a temperature drop resulting from immersing cold components into the cyanide. After carburising, either single quench hardening or refining and hardening and tempering is carried out.

Makeup Relining ladles or furnace with refractories.

Melt down
The process in which steel is transferred from the solid state into the liquid state by introducing electrical power to the scrap in the electric furnace.

Mold
An iron-casting container used to hold and cool molten metal as it solidifies.

Mold cluster
A series of 6 or 8 molds positioned in a circular arrangement interconnected by runners.

Mold stool
Metal plate used to support each mold on the stool plate.

Morphology of non-metallic inclusions
Globular shape of inclusions is preferable since their effect on the mechanical properties of steel is moderate. Spherical shape of globular inclusions is a result of their formation in liquid state at low content of aluminum. Examples of globular inclusions are manganese sulfides and oxysulfides formed during solidification in the spaces between the dendrite arms, iron aluminates and silicates.
Platelet shaped inclusions. Steels deoxidized by aluminum contain manganese sulfides and oxysulfides in form of thin films (platelets) located along the steel grain boundaries. Such inclusions are formed as a result of eutectic transformation during solidification. Platelet shaped inclusions are most undesirable. They considerably weaken the grain boundaries and exert adverse effect on the mechanical properties particularly in hot state (hot shortness).
Dendrite shaped inclusions. Excessive amount of strong deoxidizer (aluminum) results in formation of dendrite shaped oxide and sulfide inclusions (separate and aggregated). These inclusions have melting point higher than that of steel. Sharp edges and corners of the dendrite shaped inclusions may cause local concentration of internal stress, which considerably decrease of ductility, toughness and fatigue strength of the steel part.
Polyhedral inclusions. Morphology of dendrite shaped inclusions may be improved by addition (after deep deoxidation by aluminum) of small amounts of rare earth (Ce,La) or alkaline earth (Ca, Mg) elements. Due to their more globular shape polyhedral inclusions exert less effect on the steel properties than dendrite shape inclusions.

Non-metallic inclusions
Are chemical compounds of metals (Fe, Mn, Al, Si, Ca) with non-metals (O, S, C, H, N). Non-metallic inclusions form separate phases. The non-metallic phases containing more than one compound (eg. different oxides, oxide+sulfide) are called complex non-metallic inclusions (spinels, silicates, oxysulfides, carbonitrides).

NMI - Non-metallic inclusions

Open-hearth furnace
A reverberatory melting furnace with a shallow hearth and a low roof. The flame passes over the charge in the hearth, causing the charge to be heated both by direct flame and radiation from the roof and sidewalls of the furnace. In ferrous industry, the furnace is regenerative.

Oxidation
Process in which carbon combines with oxygen to form oxides.

Pinhead
Small roll-mark which causes swelling hole: Deep protrusion caused by material imperfection rust: Red or white surface oxidation scale: Embedded oxide inclusions rolled into material scratches: Small gouges on surface.

Powder injection
The injection of certain materials into the molten bath at the ladle refining station. the material is usually injected through a consumable lance using argon as a carrier gas.

Pouring
Transfer of molten metal from furnace to ladle, ladle to ladle, or ladle into molds.

Reduction
Process in which oxygen in removed from a compound.

Refractory
A refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as "non-metallic materials having those chemical and physical properties that make them applicable for structures, or as components of systems, that are exposed to environments above 538°C. Refractory materials are used in linings for furnaces, kilns, incinerators and reactors. They are also used to make crucibles. Refractory materials must be chemically and physically stable at high temperatures. Depending on the operating environment, they need to be resistant to thermal shock, be chemically inert, and/or have specific ranges of thermal conductivity and of the coefficient of thermal expansion. The oxides of aluminium (alumina), silicon (silica) and magnesium (magnesia) are the most important materials used in the manufacturing of refractories. Another oxide usually found in refractories is the oxide of calcium (lime). Fire clays are also widely used in the manufacture of refractories. Refractories must be chosen according to the conditions they will face. Some applications require special refractory materials. Zirconia is used when the material must withstand extremely high temperatures. Silicon carbide and carbon (graphite) are two other refractory materials used in some very severe temperature conditions, but they cannot be used in contact with oxygen, as they will oxidize and burn.

Scale
The oxidized surface of steel produced during hot-working or by exposure to air or steam at elevated temperature. It consists of partially adherent layers of corrosion products consisting of iron oxides in the form of FeO, Fe2O3, and Fe3O4. The scale becomes cracked and may break off during working, and according to the operation is known as roil-, hammer-, or mill-scale.

SEN (Submerged Entry Nozzle)
Non-metallic precipitates along the inner walls of the SEN are termed clogging.

Segregation
Nonuniform distribution of alloying elements, impurities or phases.

Semikilled steel
Steel that is incompletely deoxidized and contains sufficient dissolved oxygen to react with the carbon to form carbon monoxide and thus offset solidification shrinkag.

Scrap (Ferrous)
Iron-containing material that is normally remelted and recasted into new steel. Home scrap is left over steel generated from edge trimming and rejects within the mill. It is usually sent straight back to the furnace. Prompt or industrial scrap that is trimmed by stampers and auctioned to buyers. Obsolete scrap is iron bearing waste such as old storage bins and junk cars that can be remelted in mini-mills.

Slag
Is a partially vitreous by-product of smelting ore to separate the metal fraction from the unwanted fraction. It can usually be considered to be a mixture of metal oxides and silicon dioxide. However, slags can contain metal sulfides (see also matte) and metal atoms in the elemental form. While slags are generally used as a waste removal mechanism in metal smelting, they can also serve other purposes, such as assisting in the temperature control of the smelting; and also minimizing any re-oxidation of the final liquid metal product before the molten metal is removed from the furnace and used to make solid metal.

Slab
The most common type of semi-finished steel. Traditional slabs measure 10 inches thick and 30-85 inches wide (and average about 20 feet long), while the output of the recently developed "thin slab" casters is approximately two inches thick. Subsequent to casting, slabs are sent to the hot-strip mill to be rolled into coiled sheet and plate products.

Sliver defect
Linear surface defect in low carbon aluminum killed steel strip. Pencil blister (Pencil pipe or blow holes) - internal defect in ultra low and low carbon steel for deep drawing applications produced in curved mold caster nLamination: sub-surface defect caused by re-rolled separation.

Sources of inclusions formation
Non-metallic inclusions arise because of many physical-chemical effects that occur in molten and consolidated metal during production. Non-metallic inclusions that arise because of different reactions during metal production are called natural or indigenous. They include oxides, sulfides, nitrides and phosphides. Apart from natural inclusions there are also parts of slag, refractories, material of a casting mould (the material the metal contacts during production) in the metal. Such non-metallic inclusions are called foreign, accidental or exogenous. Most inclusions in the reduction smelting of metal formed because of admixture dissolubility decreasing during cooling and consolidation. The present-day level of steel production technology allows the elimination of most natural and foreign inclusions from the metal. However its general content in different steels can vary between wide limits and has a big influence on the metal properties.

Tap A drain molten steel from a steelmaking furnace.

Tapping
The act of draining the molten metal from furnace to ladle.

Tundish
The reservoir at the top of the continuous caster into which molten steel is poured.

Unkilled steel
Steel which has been insufficiently deoxidized and evolves gas during solidification, with the formation of blow-holes.

Vacuum degassing
By casting steel in a vacuum it is possible to reduce considerably the gas content, particularly hydrogen. Other advantages are also obtainable, such as a reduction in non-metallic inclusions. In stream degassing, molten metal is poured into an ingot mould or a ladle previously placed within an evacuated chamber. The molten stream is broken up into droplets by gas evolution which takes place very rapidly. In ladle degassing, the ladle of molten metal is placed within a chamber, which is then evacuated; an inert gas such as argon or helium may be used to stir the molten metal and thus assist hydrogen removal. Other methods of ladle degassing can be used in which the liquid steel is siphoned into an evacuated chamber and returned to the ladle. The ladle of degassed metal can be subsequently poured into moulds in air in the normal manner.

Vacuum melting
Two methods are used for steel. Vacuum induction melting uses a high frequency induction furnace contained in a chamber which can be evacuated. Arrangements are made for adding alloys and for casting into moulds without breaking the vacuum. The other method, consumable electrode re-melting, uses an ingot or billet as an electrode from which an arc is struck, melting the steel into a water-cooled copper crucible and building up an ingot which can be subsequently forged or rolled in the normal manner. No alloy additions can be made during this process, so the electrode must approximate to the required composition. With both methods there is a reduction in the content of gas and of some other elements, such as manganese, which are volatile at the low pressures and high temperatures operating in the vacuum chamber. Improved cleanness is also obtained, although in the vacuum induction process the metal may be contaminated to some extent with oxides from the refractory lining of the furnace. In the consumable are process, since only a small pool of metal is molten at any one time, segregation is usually less marked than in the product of other melting processes and the continuous freezing from the bottom gives a very sound ingot.

Vacuum oxygen decarburization (VOD)
Process for further refinement of stainless steel through reduction of carbon content. The amount of carbon in stainless steel must be lower than that in carbon steel or lower alloy steel (i.e., steel with alloying element content below 5%). While electric arc furnaces (EAF) are the conventional means of melting and refining stainless steel, VOD is an economical supplement, as operating time is reduced and temperatures are lower than in EAF steelmaking. Additionally, using VOD for refining stainless steel increases the availability of the EAF for melting purposes. Molten, unrefined steel is transferred from the EAF into a separate vessel, where it is heated and stirred by an electrical current while oxygen enters from the top of the vessel. Substantial quantities of undesirable gases escape from the steel and are drawn off by a vacuum pump. Alloys and other additives are then mixed in to refine the molten steel further."

Vacuum oxygen decarburisation (VOD)
A refinement of stainless steel that reduces carbon content. Molten, unrefined steel is heated and stirred by an electrical current while oxygen enters from the top. Many undesirable gases escape from the steel and are evacuated by a vacuum pump. Alloys and other additives are then mixed in to refine the molten steel further.

Reference: Website Wikipedia, Subtech, 2015.