Inclusions in Continuous Cast Steel and their Detection
Inclusions in Continuous Cast Steel and their Detection
Continuous casting of steel is an important process for the production of steel worldwide, owing to its inherent advantages of energy saving, high yield, flexibility of operation, and competitive quality of the cast product. With the establishment of the continuous casting as the major route of steel production, emphasis is being focused increasingly on the quality improvement and cost reducing aspects of the steel production through continuous casting technology. One of the most stringent quality requirements today is the cleanliness of the steel. The high steel cleanliness demands strict control of non-metallic inclusions or simply inclusions during the continuous casting process. Inclusions remaining in the final product can damage steel properties and degrade its quality.
The removal of inclusions in the mould of continuous casting is difficult since the liquid steel becomes solid and inclusions have less opportunity to float out. The removal of inclusions and the final distribution of inclusions in the steel product are highly dependent on the properties of inclusions, transport of inclusions in the liquid steel, and the interaction between inclusions and solidifying shell. Hence, the understanding of the entrapment of inclusions and their final distribution in the final product are important for the control of cleanliness and the quality of the steel product.
The surface quality problem of hot and / or cold rolled steel is always one of the important concerns since it is relative to the steel quality and price directly. The surface quality of rolled steel is also influenced by the operation of the continuous casting and reheating process, since the inclusions are one of the major causes to generate the surface cracks in the rolled steel. There are attempts to promote the steel surface quality by modifying inclusion composition and morphology based on thermodynamic calculation. But these attempts seem to be still not enough to solve the surface quality problem completely.
The evaluation of inclusions in steel is of great interest and includes (i) exploring the total quantity, morphology, size distribution, and spatial distribution of inclusions, and (ii) identifying their chemical composition.
The ever-increasing demands for high quality steel products have made the steelmaking personnel increasingly aware of the requirement of the cleanliness the steel. Inclusions are an important problem in cast steel which can lead to its excessive repair or rejection. Several defects in the rolled steel product can be related to the inclusions. The mechanical behaviour of the steel is controlled to a large extent by the volume fraction, size, distribution, composition, and morphology of inclusions and precipitates which act as stress raisers. The inclusion size distribution is particularly important, since large macro-inclusions are the most harmful to the mechanical properties. Sometimes a catastrophic defect is caused by just a single large inclusion in a complete steel heat. Though the large inclusions are far outnumbered by the small inclusions, their total volume fraction can be large.
Ductility is appreciably decreased by increasing quantities of either oxide or sulphide inclusions. Also fracture toughness decreases when inclusions are present in high strength lower ductility alloy steels. Similar property degradation from inclusions is observed in tests which reflect slow, rapid, or cyclic strain rates, such as creep, impact, and fatigue tests. Further, inclusions cause voids, which can induce cracks. Large exogenous inclusions can cause trouble in the form of inferior surface, poor polishability, reduced resistance to corrosion, and in exceptional cases, slag lines and laminations.
Inclusions also lower resistance to ‘hydrogen induced cracks’ (HIC). The source of majority of the fatigue problems in steel are hard and brittle oxides, especially large alumina (Al2O3) particles with size of over 30 micrometers. Although the solidification morphology of inclusions is important in cast steels, the morphology of inclusions in wrought steel products is largely controlled by their mechanical behaviour during steel processing, i.e., whether they are ‘hard’ or ‘soft’ relative to the steel matrix. The behaviour of different types of inclusions with their deformation during rolling is schematically shown in Fig 1.
Fig 1 Behaviour of inclusions during deformation
‘Stringer’ formation, type (b) and (c) in Fig 1, increases the directionality of mechanical properties, hence adversely affecting toughness and ductility in particular. The worst inclusions for toughness and ductility, particularly in through thickness direction properties of flat-rolled product, are those deforming with the matrix, like (d) in Fig 1. For avoiding these problems, the size and frequency of detrimental inclusions are to be carefully controlled. Especially no inclusions are to be present in the cast steel above a critical size.
Characterizing of the inclusions is one of the most important aspects for assuring clean steel. Inclusions are a type of defect present in steel which severely affect the properties like polishability, ductility, and fatigue strength of the steel. Hence, inclusions are required to be controlled for the production of high-performance steel. Primary inclusions are formed during steel treatments in the ladle. Most of these are removed to the ladle slag or on the lining. However, the rest of the inclusions still remain to be removed through the successive process stages and additionally new inclusions are formed during casting and solidification.
Because of the decreasing number of inclusions with increasing size, the different size intervals pose different problems. With respect to polishability, the large number of small inclusions is more harmful than the large but simultaneously rarer inclusions simply by being more frequently occurring. On the other hand, at low stress levels, critical cracks, which can lead to failure within the lifetime of a steel product, most likely grow at the very large inclusions. These inclusions are rare and it is difficult to correctly estimate their occurrence density. At intermediate fatigue stress levels, inclusions of middle sizes compete with surface defects as the crack initiation points.
Inclusions in steel can form either endogenously (indigenously) or exogenously. Endogenous inclusions are a result of alloying elements within the steel reacting with dissolved gas (such as oxygen) to form solid inclusions in the cast steel. The inclusion can be formed during deoxidation, reoxidation, or solidification from reduced gas species solubility in the solid state. Exogenous inclusions come from sources outside the liquid steel, such as slag entrainment, or refractory damage.
Endogenous inclusions are deoxidation products or precipitated inclusions during cooling and solidification of the steel.
Deoxidation products – Alumina inclusions in low carbon aluminum killed (LCAK) steel, and silica (SiO2) inclusions in silicon killed steel, which are generated by the reaction between the dissolved oxygen and the added aluminum and silicon deoxidants, are typical deoxidation inclusions. Alumina inclusions are dendritic when formed in a high oxygen environment. Cluster-type alumina inclusions from deoxidation or reoxidation, are typical of aluminum killed steels. Alumina inclusions easily form three dimensional clusters through collision and aggregation due to their high interfacial energy. Individual inclusions in the cluster can be 1 micrometer to 5 micrometers in diameter. Before collision, breakup or aggregation with other particles, they can be in the shape of flower plate, or (aggregated) polyhedral inclusions. Alternatively coral-like alumina inclusions are believed to result from ‘Ostwald-ripening’ of originally dendritic or clustered alumina inclusions. Silica inclusions are normally spherical owing to being in a liquid or glassy state in the liquid steel. Silica can also agglomerate into clusters.
Precipitated inclusions – These inclusions form during cooling and solidification of the steel. During cooling, the concentration of dissolved oxygen / nitrogen /sulphur in the liquid becomes higher while the solubility of those elements decreases. Thus inclusions such as alumina, silica, aluminum nitride (AlN), and sulphide precipitate. Sulphides form inter-dendritically during solidification, and frequently nucleate on oxides already present in the liquid steel. These inclusions are normally small (less than 10 micrometers in size).
Exogenous inclusions arise primarily from the incidental chemical (reoxidation) and mechanical interaction of liquid steel with its surroundings (slag entrainment and erosion of lining refractory). During machining, they produce chatter, causing pits and gouges on the surface of machined sections, frequent breakage, as well as excessive tool wear.
Exogenous inclusions are always practice related and their size and chemical composition frequently lead to the identification of their sources, and their sources are mainly reoxidation, slag entrainment, lining erosion, and chemical reactions. These inclusions have the following characteristics.
Large size – Exogenous inclusions from refractory erosion are normally larger than those from slag entrainment.
Compound composition / multi-phase nature – Exogenous inclusions are caused by the phenomena namely (i) due to the reaction between liquid steel and silica, FeO, and MnO in the slag and lining refractory with the generated alumina inclusions can stay on their surface, (ii) as exogenous inclusions move, due to their large size, they can entrap deoxidation inclusions such as alumina on their surface, (iii) exogenous inclusions act as heterogeneous nucleus sites for precipitation of new inclusions during their motion in liquid steel, and (iv) slag or reoxidation inclusions can react with the lining refractories or dislodged further material into the liquid steel.
Shape – Exogenous inclusions normally have irregular shape, if not spherical from slag entrainment or deoxidation product silica. The spherical exogenous inclusions are normally large (larger than 50 micrometers) and mostly multiphase, but the spherical deoxidation inclusions are normally small and single phase.
Quantity – Exogenous inclusions are small in number compared with small inclusions.
Distribution – Exogenous inclusions have sporadic distribution in the steel and are not well-dispersed as small inclusions. Since they are normally entrapped in steel during teeming and solidification, their incidence is accidental and sporadic. On the other hand, they easily float out, so only concentrate in regions of the steel section which solidify most rapidly or in zones from which their escape by flotation is in some way hampered. Hence, these inclusions are frequently found near the surface.
Effect on steel properties – Exogenous inclusions are more deleterious to steel properties than small inclusions because of their large size.
One issue which overrides the source of the exogenous inclusions is that why such large inclusions do not float out rapidly once they are there in the steel. Possible reasons can be (i) late formation during steelmaking, transfer, or erosion in the metallurgical vessels leaving insufficient time for them to rise before entering the casting machine mould, (ii) lack of sufficient superheat, (iii) fluid flow during solidification induces mould slag entrapment, or (iv) re-entrainment of floated inclusions before they fully enter the slag.
Exogenous inclusions from reoxidation – The most common form of large macro-inclusions from reoxidation is found in the steel is the alumina cluster. Air is the most common source of reoxidation, which can occur (i) liquid steel in the tundish mixes with air from its top surface at the start of pouring due to the strong turbulence and the oxide films on the surface of the flowing liquid are folded into the liquid, forming weak planes of oxide particles, (ii) air is sucked into the liquid steel at the joints between the ladle and the tundish, and between the tundish and the mould, and (iii) air penetrates into the steel from the top surface of the steel in the ladle, tundish, and mould during pouring.
During this kind of reoxidation, deoxidizing elements, like aluminum, calcium, and silicon etc. are preferentially oxidized and their products develop into the inclusions, normally one to two magnitudes larger than deoxidation inclusions. The solution to prevent this kind of reoxidation is to limit the exposure of air to the process of casting. This can be done (i) by shrouding with inert gas curtain utilizing a steel ring manifold or porous refractory ring around the connections between the ladle and the tundish, and between the tundish and the mould, (ii) by purging some argon gas into the tundish before pouring, and into the tundish surface during pouring, and (iii) by controlling argon gas injection in the ladle to avoid eye formation.
Another reoxidation source is in the slags and lining refractories. By this reoxidation mechanism, inclusions within the steel grow as they near the slag or lining interface through the reaction SiO2 / FeO / MnO + [Al] = [Si] / [Fe] / [Mn] + Al2O3. This leads to larger alumina inclusions with variable composition. This phenomenon further affects exogenous inclusions in the different ways namely (i) this reaction can erode and uneven the surface of the lining, which changes the fluid flow pattern near lining walls and can induce further accelerated breakup of the lining, and (ii) a big exogenous inclusion of broken lining or entrained slag can entrap small inclusions, such as deoxidation products, and also act as a heterogeneous nucleus for new precipitates which complicates the composition of exogenous inclusions.
To prevent reoxidation from slag and lining refractory, it is very important to keep a low SiO2, FeO, and MnO content. It has been reported that high alumina or zirconia bricks containing low levels of free silica are more appropriate for use.
Exogenous inclusions from slag entrainment – Any steelmaking operation or transfer of liquid steel involve turbulent mixing of slag and metal, especially during transfer between vessels. This produces slag particles suspended in the steel. Slag inclusions, 10 micrometers to 300 micrometers in size, contain large quantities of CaO(lime) or MgO (magnesia), and are normally liquid at the liquid steel temperature and hence are spherical in shape. Use of an ‘H’ shaped tundish and pouring it through two ladles diminishes slag entrainment during the ladle change period. The causes which affect slag entrainment into the liquid steel during the continuous casting process include (i) during the transfer operations from ladle to tundish and from tundish to mould especially for open pouring, vortexing at the top surface of liquid steel which at the low level of the liquid steel can be avoided in several ways such as shutting off pouring before the onset of vortexing, (ii) emulsification and slag entrainment at the top surface especially under gas stirring above a critical gas flow rate, (iii) turbulence at the meniscus in the mould, and (iv) slag properties such as interfacial tension and slag viscosity.
As an example, mould powder can be entrapped into the liquid steel due to (i) turbulence at the meniscus (Fig 2A), (ii) vortexing (Fig 2C), (iii) emulsification induced by bubbles moving from the steel to the slag [Fig 2B and 2D), (iv) sucking in along the nozzle wall due to pressure difference (2E), (v) high velocity flow which shears slag from the surface (2A), and (vi) level fluctuation (Fig 2B).
Fig 2 Schematic mould powder entrapment
The interfacial tension between the steel and the liquid casting powder determines the height of the steel meniscus, and the ease of flux entrainment. Specifically an interfacial tension of 1.4 newtons per meter (N/m) for a lime-silica-alumina slag in contact with pure iron yields a meniscus height of around 8 mm. The interfacial tension is reduced to a low value by surface-active species such as sulphur or by an interfacial exchange reaction such as the oxidation of aluminum in steel by iron oxide in the slag. The very low interfacial tension associated with a chemical reaction can provide spontaneous turbulence at the interface, through the Marangoni effect. Such turbulence can create an emulsion at the interface, creating undesirable beads of slag in the steel.
Exogenous inclusions from erosion / corrosion of refractory lining – Erosion of refractories, which include well block sand, loose dirt, broken refractory brickwork, and ceramic lining particles, is a very common source of large exogenous inclusions which are typically solid and related to the materials of the ladle and tundish themselves. These are normally large and irregular-shaped materials. Exogenous inclusions can act as sites for heterogeneous nucleation of alumina, or aggregate with other indigenous inclusions. The occurrence of refractory erosion products or mechanically introduced inclusions can completely impair the quality of otherwise very clean steel.
In some studies to investigate the erosion process, it has been reported that ‘glazed refractories’ and the ‘reaction layers at the surface of bricks’ are formed with liquid steel at 1,550 deg C to 1,600 deg C. The clogs of large inclusion on the surface of the lining can also be released into the liquid steel.
Lining erosion normally occurs at areas of turbulent flow, especially when combined with reoxidation, high pouring temperatures, and chemical reactions. The parameters which strongly affect the lining erosion are described below.
Some steel grades are quite corrosive (such as high manganese and grades which are barely killed and have high soluble oxygen contents) and attack lining bricks.
Reoxidation reactions, such as which the dissolved aluminum in the liquid steel reduces silica in the lining refractory, generating iron oxide based inclusions which are very reactive and wet the lining materials, leading to erosion of lining refractory at areas of high fluid turbulence.
Brick composition and quality has a considerable effect on steel quality. At one of the plant, three types of materials (high alumina, Al2O3-SiC-C, and MgO-C with a wear rate of 1 mm / heat, 0.34 mm / heat, 0.16 mm / heat respectively) have been adopted at the slag line, where the refractory tends to be damaged by erosive tundish flux and slag, and the MgO-C brick shows the highest durability among the three. Manganese oxide preferentially attacks the silica containing portions of the refractory. Very high purity alumina and zirconia grains can withstand attack by manganese oxide.
Rapid refractory erosion from high manganese steels can be constrained by (i) use of very high purity alumina or zirconia refractories, (ii) minimizing oxygen by fully killing the steel with a strong deoxidant such as aluminum or calcium, and preventing air absorption. Silica based tundish linings are worse than magnesia-based sprayed linings. High alumina refractories are being suggested as being the most promising. Incorporating calcium oxide into the nozzle refractory can help by liquefying alumina inclusions at the wall, so long as CaO diffusion to the interface is fast enough and nozzle erosion is not a problem. Nozzle erosion can be countered by controlling nozzle refractory composition, (e.g. avoiding sodium, potassium, and silicon impurities), or coating the nozzle walls with pure alumina, boron nitride, or other resistant material. The refractory at the surface of the shroud walls is to be chosen to minimize reactions with the steel which create inclusions and clogging.
Excessive velocity of liquid steel affects lining erosion along the walls in the tundish, such as the inlet zone. A pad can be used to prevent the bottom of the tundish from erosion, as well as controlling the flow pattern. It has been suggested that liquid steel velocities over 1 meter per second are dangerous with regard to erosion.
Excessive contact or filling time and high temperature deteriorates erosion problems. During long holding period in the ladle, the larger inclusions can float out into the ladle slag. However the longer the steel is in contact with the ladle lining, the more is the tendency for the ladle erosion products. Solutions are based upon developing highly stable refractories for a given steel grade, developing dense wear resistant refractory inserts for high flow areas, and preventing reoxidation.
Exogenous inclusions from chemical reactions – Chemical reactions produce oxides from inclusion modification when calcium treatment is improperly performed. Identifying the source of these inclusions is not always easy, as for example, inclusions containing calcium oxide can also originate from entrained slag.
Inclusion agglomeration and clogging – The agglomeration of solid inclusions can occur on any surface aided by surface tension effects, including on refractory and bubble surfaces. The high contact angle of alumina in liquid steel (134 degrees to 146 degrees) encourages an inclusion to attach itself to refractory in order to minimize contact with steel. High temperatures of 1,530 deg C enable sintering of alumina to occur. Large contact angle and larger inclusion size favour the agglomeration of inclusions. Due to collision and agglomeration, inclusions in steel tend to grow with increasing time and temperature. Inclusion growth by collision, agglomeration, and coagulation in ingot has been the subject of various studies, in which the numerical simulation of inclusion nucleation starting from deoxidant addition and growth by collision and diffusion from nano-size to micro-size is reported.
The fundamentals of alumina sintering into clusters need further investigation, though some studies have used fractal theory to describe the cluster morphology (features). The most obvious example of inclusion agglomeration on the surface of lining refractories is nozzle clogging during the continuous casting of liquid steel.
Effect of fluid flow and solidification on inclusions – Inclusion distribution in continuous casting of steel is affected by the fluid flow, heat transfer, and solidification of the liquid steel. A popular index for inclusion entrapment is the critical advancing velocity of the solidification front, which is affected by several parameters such as inclusion shape, density, surface energy, thermal conductivity, cooling rate (solidification rate), and protruding conditions of the solidification front. It has been reported that entrapment is controlled by drag and interfacial forces (Van der Waals force). It has been suggested that the faster is the solidification rate, the higher is the probability of the entrapment. The probability of entrapment decreases with increasing solidification time, less segregation, smaller protrusions on the solidification front. The dendrite arm spacings have a big effect on the entrapment of inclusions and are related to the phenomena of pushing, engulfment; or entrapment.
Continuous casting operations, inclusions, and clean steel
Continuous casting operations control steel cleanliness. A systematic study of inclusion removal has found that ladle treatment lowers the inclusions by around 65 % to 75 %, the tundish removes the inclusions by around 20 % to 25 %, although reoxidation sometimes has occurred, and the mould removes inclusions by around 5 % to 10 % . Tundish operation has a big effect on the steel cleanliness. The important factors in tundish operations having effect on the steel cleanliness are tundish depth and capacity, casting transitions, tundish lining refractory, tundish flux, argon gas stirring, and tundish flow control.
Top slags – The top slags in the ladle and tundish provides several functions such as (i) insulation of the liquid steel both thermally (to prevent excessive heat loss) and chemically (to prevent air entrainment and reoxidation), and (ii) absorption of the inclusions to provide additional steel refining. A common tundish flux is burnt rice husk, which is inexpensive, a good insulator, and provides good coverage without crusting. However, rice husk is high in silica (around 80 % SiO2), which can be reduced to form a source of inclusions. They are also very dusty and with their high carbon content, (around 10 % C), can cause contamination of ultra low carbon steel.
Basic fluxes (CaO-Al2O3-SiO2 based, and silica less than 10 %) are theoretically much better than rice husk during the refining of LCAK steels, and have been correlated with lower oxygen in the tundish. For example, in a study, the total oxygen has decreased from the range of 25 ppm (parts per million) and 50 ppm to the range of 19 ppm and 35 ppm with the basicity of the flux increasing from 0.83 to 11. In one of the steel plant, use of basic fluxes, the total oxygen in mould has been reported to be lower, and steel product defect has decreased. However, more likely, the basic flux is ineffective since it easily forms a crust at the surface, owing to its faster melting rate and high crystallization temperature. This crust results in the evolution of an open slag-free eye around the ladle shroud during teeming, which not only provides an excessive area for reoxidation, but also allows a significant radiative heat loss and discomfort for the operators on the working platform. Also, basic fluxes normally have lower viscosity. Hence, they are more easily entrained. To avoid these issues, one steel plant has used a two layer flux, with a low-melting point basic flux on the bottom to absorb the inclusions, and a top layer of rice husk based flux to provide insulation. This has lowered the total oxygen from 22.5 ppm to 16.5 ppm.
Tundish depth, capacity and flow control devices – The tundish flow pattern is to be designed to increase the liquid steel residence time, prevent ‘short circuiting’ and promote inclusion removal. Tundish flow is controlled by its geometry, level, inlet (shroud) design, and flow control devices such as impact pads, weirs, dams, baffles, and filters. Deep tundish with a large capacity increase the residence time of liquid steel and particles and hence encourages the inclusion removal. Deep tundish also discourages vortex formation, enabling more time for ladle transitions before slag entrainment becomes an issue. Tundish size for LCAK steel has gradually increased worldwide over the past 20 years, typically reaching 60 tons to 80 tons with around 1.8 meter inches depth in case of a slab continuous casting machine.
If properly aligned, and perhaps together with weir(s) and dam(s), a pour pad can improve steel cleanliness, especially during ladle exchanges. As an example, adding the pour pad at one of the steel plant has decreased alumina during ladle transitions from 48 ppm to 15 ppm. In another steel plant, total oxygen has decreased from 26 ppm (with a domed pad) to 22 ppm (with a hubcap pad). At yet another steel plant, steel cleanliness has improved by putting 77 holes in their dam, making it act as a partial filter. At one other steel plant, a similar technique consisting of baffles combined with an initial tundish cover has lowered the average total oxygen in the tundish during steady state casting from 39 +/- 8 ppm to 24 +/- 5 ppm.
Ceramic filters and CaO filter are very effective at removing inclusions. However, their cost and effective operating time before clogging normally make their use prohibitive. Injecting inert gas into the tundish from its bottom improves mixing of the liquid steel, and promotes the collision and removal of inclusions. At one of the steel plants, by applying this technology total oxygen has been successfully lowered to 16 ppm in tundish. However, the danger of this technology is that any inclusions-laden bubbles which escape the tundish and become entrapped in the strand causes severe defects. It has been reported that oxide area fraction (0.001 %) of steel in tundish decreases 25 % by this technique compared with those without this technique.
Casting transitions – Casting transitions occur at the start of a casting sequence, during ladle exchanges and nozzle changes, and at the end of casting. These transitions are responsible for the majority of the cleanliness defects. Inclusions are frequently generated during transitions and can persist for a long time, thus contaminating a lot of steel. The sliver defect index at the beginning of the first heat has been found to be 5 times higher than that at the middle of the first heat and over 15 times that of successive heats. During these unsteady casting periods, slag entrainment and air absorption are more likely to take place, which induce reoxidation problems. A ‘self-opening’ ladle opens on its own without having to lance the nozzle. Lancing needs removing of the shroud and this allows reoxidation to take place, especially during the first 650 mm to 1,200 mm of the casting. Lanced-opened heats have total oxygen levels which are around 10 ppm higher than the self-opened heats. Careful packing of the ladle opening sand helps in achieving the self-opening of the ladle. Ladle sand is also a source of reoxidation because of high silica content.
One improvement during ladle transitions is to stop the flow of liquid into the mould until the tundish is filled and to bubble gas through the stopper to promote inclusion flotation. Another improvement is to open new ladles with submerged shrouding. With this measure, total oxygen has decreased at one of the steel plant from 41 +/- 14 ppm to 31 +/- 16 ppm with more consistent quality throughout the sequence.
As an example, at one of the steel plant, total oxygen in tundish during transitions is 50 ppm to 70 ppm, compared with only 25 ppm to 50 ppm at steady state. At other steel plants, the difference is only 3 ppm. One of the steel plants has reported transitions to have total oxygen only 19.2 ppm relative to 16 ppm at steady state while another steel plant has reported total oxygen of 27 +/- 5 ppm during transitions and 24 +/- 5 ppm during steady casting. At one other steel plant, the nitrogen pickup in tundish is 5 ppm to 12 ppm during the start period of the teeming which decreases to 0 ppm to 2 ppm after around 12 minute of teeming (steady casting state).
Near the end of the teeming of a ladle, ladle slag can enter the tundish, due to the vortex formed in the liquid steel near the ladle exit. This phenomenon needs some steel to be kept in the ladle upon closing (e.g. a four ton of ‘heel’). In addition, the tundish depth drops after ladle close, which disrupts normal tundish flow and can produce slag vortexing, slag entrainment, and increased total oxygen in the mould.
Shrouding, argon protection, and sealing – Steel shrouding from ladle to the mould includes ladle slide gate shrouding, ladle collector nozzle, ladle shroud connection, tundish well block, and top plate of the tundish slide gate. Shroud design variations are of great importance in the operations of the tundish-to-mould transfer of liquid steel. Use of an optimized shrouding system greatly lowers reoxidation during transfer operations. For example, use of a ladle shroud has lowered nitrogen pickup from 24 ppm to 3 ppm relative to open pouring at one of the steel plant. In another steel plant, replacing the tundish pour box with a ladle shroud and dams has lowered nitrogen pickup (ladle to tundish) from 7.5 ppm to 4 ppm, and also has lowered slag entrainment during transitions. At one other steel plant, improving the shroud system from ladle to tundish has lowered the nitrogen pickup from 14 ppm to 3 ppm.
Shrouding the ladle to tundish stream at one of the steel plants has lowered the dissolved aluminum loss from 130 ppm to 70 ppm and has lowered the total oxygen increase by 12 ppm. When pouring without shrouds, which is common in billet casting, the turbulence of the casting stream is very important. A smooth stream entrains much less oxygen than a turbulent or ‘ropy’ stream. For the production of a smooth stream between the tundish and the mould in these operations, the metering nozzle edges are to be maintained and high speed flow in the tundish across the nozzles is to be avoided. A protective tundish cover with carefully sealed edges also helps in lowering total oxygen from 41.5 ppm to 38 ppm.
A variety of inert gas shrouding systems is now available. Total oxygen in the cast product (LCAK steel) can be lowered from 48.5 ppm to 28.5 ppm by shrouding between the ladle and the tundish, and to 23 ppm by this shrouding plus argon sealing. It is very important to carefully seal the joints in the shrouds, both to improve cleanliness and to prevent clogging. Improving the bayonet system between the ladle nozzle and ladle shroud, lowers the nitrogen pickup there from 8 ppm to less than 1 ppm. Stiffening the submerged entry nozzle (SEN) holder and increasing its maintenance has lowered the initial nitrogen pickup from 1.8 ppm to 0.3 ppm in one of the steel plants.
Inert gas can protect the steel from air reoxidation in several ways. To combat air entrainment at the beginning of a cast, the tundish can be purged with inert gas (to displace the air) prior to ladle opening, which lowers both the total oxygen and the nitrogen pickup during startup. Argon injection to pressurize the shrouds can help to prevent the liquid steel from air reoxidation through any joints or leaks. Guidelines for minimum argon gas flow to ensure positive pressure inside the nozzle are to be made. In addition, flooding the joints with argon gas ensures that any leaks aspirate inert gas and not air.
Injecting argon into the tundish stopper rod and improved sealing at one steel plant has decreased nitrogen pickup from tundish to cast product from 5 ppm to 1.8 ppm, has lowered total oxygen in the cast product from 31 ppm to 22 ppm, has decreased the size of alumina clusters in the cast product, and has decreased clogging. Elsewhere, argon injection through the stopper rod lowered the number of inclusions detected by the Mannesmann inclusion detection by analysis surfboards (MIDAS) method by 25 % to 80 %. Injection of argon gas purge through upper plate of the sliding gate has lowered the quantity of 50 micrometers to 100 micrometers sized inclusions from 3 per square centimeter to 0.6 per square centimeter, and lowered 100 micrometers to 200 micrometers macro-inclusions from 1.4 per square centimeter to 0.4 per square centimeter.
Clogging and new techniques at SEN – The nozzle is one of the few control parameters which is relatively inexpensive to change, yet has a profound influence on the flow pattern and hence on the quality of the cast product. Nozzle parameters include bore size, port angle and opening size, nozzle wall thickness, port shape (round, square, or oval), number of ports (bifurcated or multiport), nozzle bottom design (well, flat , or sloped), and submergence depth. Both too large and too small submergence depth increases problems with longitudinal cracks and transverse depressions.
One of the studies has found the occurrence of corundum (Al2O3) covering the bore surface of nozzles used to pour aluminum killed steel ingot early in 1949. Another study has found that nozzle blockage occurred with high levels aluminum (0.0036 %) and that nozzle sectioning revealed dendritic growth of alumina from the nozzle wall onto the bore. Yet another study has observed clogs of aluminum, zircon, titanium, and the rare earths.
Nozzle clogs are caused by reoxidation, or by the accumulation of solid oxides or sulphides, such as alumina and calcium sulphide (CaS) in the steel. In addition to interfering with the production process, tundish nozzle / SEN clogging is detrimental to steel cleanliness for several reasons such as (i) dislodged clogs either become trapped in the steel, or they change the flux composition, leading to defects in either case, (ii) clogs change the nozzle flow pattern and jet characteristics leaving the nozzle, which disrupt flow in the mould, leading to slag entrainment and surface defects, and (iii) clogging interferes with mould level control, as the flow control device (stopper rod or slide gate) tries to compensate for the clog.
The cure for the nozzle clog problem includes improving steel cleanliness by improving ladle practices, implementing smooth and non-reacting refractories, and controlling fluid flow though the nozzle for ensuring a smooth flow pattern. Changing from a three-plate slide gate system to a stopper rod system has reduced clogging at one of the steel plant. Several practices can be used to minimize clogging. In addition to taking general measures to minimize inclusions, clogging through refractory erosion can be countered by controlling nozzle refractory composition, (e.g. avoiding sodium, potassium, and silicon impurities), or coating the nozzle walls with pure alumina, boron nitride, or other resistant material. There are several new techniques at SEN which have reported to improve the fluid flow pattern and inclusion removal, such as (i) swirl-nozzle technique, (ii) step nozzle technique, (iii) multi-ports nozzle, and (iv) oval offset bore throttle plate.
Swirl-nozzle technique – A fixed blade placed at the upstream end of the SEN induces a swirl flow in nozzle. Centrifugal force generated by the swirling flow in the nozzle can distribute the liquid steel equally to its two spouts. Since liquid steel stream with centrifugal force has the maximum velocity in the vicinity of the wall inside the nozzle, it tends to flow out of the upper part of the spout. Hence, the velocity distribution which tends to have higher values toward the lower part of the spout with a conventional nozzle can become uniform. It has been reported that by using this swirl nozzle for the continuous casting, the defect ratio of finish products (coils) has decreases to 25 % of the conventional nozzles, and casting speed has riseby 30 %. Its cost is higher only by 20 % than the cost of the conventional and hence it is cheaper than using an ‘electro-magnetic brake’. This swirl flow pattern can also be generated by the ‘electro-magnetic stirring’ at the nozzle, which can also improve the solidification structure of the cast steel as well.
Step nozzle – The flow pattern at out-ports of conventional SEN is uneven or biased because of the sliding gate of SEN. This biased flow pattern (swirl flow at out-ports of SEN) increases the impingement of the jet, and hence worsens inclusion removal to top surface. By using inner annular steps, the biased flow in mould can be weakened. The calculation suggests that the removal fraction of 50 micrometers inclusions to the top surface of the mould is 2 % with the conventional SEN, but increases to 7 % with the use of the stepped SEN.
Oval offset bore throttle plate – In the conventional system, gate throttling results in a highly skewed and biased flow in the tundish-to-mould flow channel both upstream and downstream of the gate. These effects have considerably diminished the offset bore system. The offset gate design extracts the fluid more centrally from the tundish well nozzle. Hence, the system is less sensitive to any build-up on the walls of the well nozzle, which extends the useful life of the tundish well nozzle and hence, allowing longer tundish sequences. In practice, it has also been found that clogging within the plates of the offset bore gate is considerably reduced as compared to the conventional gate.
Multiple out-ports – It is well known that the surface velocity of the mould has a big effect on slag entrainment and top surface fluctuation. Several defects are related to the surface velocity of the mould. Thus decreasing the surface velocity is very important to improve the steel cleanliness. This task can be targeted by using multiple out-ports at SEN. Addition of a bottom hole at SEN lowers the momentum of the side jets so it is possible to get a good steel flow and meniscus condition even under high throughput which is better stabilized.
Mould and operation of continuous casting machine
The continuous casting mould region is the last refining step where inclusions either are safely removed into the top slag layer or they become entrapped into the solidifying shell to form permanent defects in the steel product. Mcpherson has used the words ‘mould metallurgy’ in 1985 to emphasize the importance of the mould to improve steel cleanliness. The mould flow pattern is very important for avoiding defects since it affects particle transport and removal to the top slag or entrapment by the solidifying shell.
Top surface control – Directing too much flow towards the top surface generates surface defects, due to transients, turbulence at the meniscus, and inclusion problems from slag entrainment. However, decreasing surface flows too much can also generate problems. These include surface defects due to the meniscus region becoming too stagnant, and a higher fraction of incoming inclusion particles being sent deep before they can be removed into the slag. Hence, a balance is to be found in order to optimize the flow parameters to avoid defects.
The most obvious source of surface defects is the capture of foreign particles into the solidifying shell at the meniscus. If the steel jet is directed too deep or has too little superheat, then the liquid surface has very little motion and becomes too cold. This can lead to freezing of the steel meniscus, which aggravates the formation of meniscus hooks. This allows inclusions and bubbles to be captured, the latter forming pinholes just beneath the surface of the cast product. As an example, decreasing surface velocity below 0.4 metre/second (m/s) has been measured to increase surface pinhole defects. For avoiding these problems, the flow pattern is to be designed to exceed a critical minimum velocity across the top surface, which is estimated to be around 0.1 m/s to 0.2 m/s.
Slag entrainment is less likely with deeper nozzle submergence and slower casting speed. For avoiding shearing slag in this manner, the surface velocity is to be kept below a critical value. This critical velocity has been measured in water – oil models as a function of viscosity and other parameters. Entrainment is more difficult for shallower slag layers, higher slag viscosity, and higher slag surface tension.
A maximum limit of the argon gas injection flow rate into the nozzle has been reported as a function of the casting speed, beyond which mould slag entrainment takes place. Increasing casting speed tends to increase transient turbulent fluctuations, and worsens the extent of flow pattern asymmetries. This in turn worsens detrimental surface turbulence and level fluctuations. Improving internal cleanliness frequently needs limiting the maximum casting speed, to avoid pencil pipe defects. Lower casting speed and avoiding variations in casting speed both reduce the rate of slivers. More precisely, it is important to lower the liquid mass flow rate in order to control the jet velocity leaving the nozzle.
Fluid flow pattern – The mould flow pattern is controlled by adjustable parameters such as nozzle geometry nozzle submergence depth, argon gas injection rate, and the application of electro-magnetic forces. It also depends on parameters which normally cannot be adjusted to accommodate the flow pattern, such as the position of the flow control device (slide gate or stopper rod), nozzle clogging, casting speed, strand width, and strand thickness. All of these parameters together form a system which is to be designed to produce an optimal flow pattern for a given operation.
Bubbles, which are injected into the nozzle and the mould, have five effects related to the control of tge steel quality. These effects are (i) helping to reduce nozzle clogging, (ii) helping influence and control the flow pattern in the mould, (iii) generating serious top surface fluctuation even emulsification if gas flow rate is too large, (iv) capturing inclusions as they flow in the liquid steel, and (v) bubbles entrapped solid oxide particles captured by solidified shell eventually lead to surface slivers or internal defects.
Normally, low gas flow tends to double-roll flow pattern, while a high argon flow rate induces single-roll flow. This phenomenon has been studied as early as in 1983. For maintaining a stable double roll flow pattern, which is frequently optimal, the argon is to be kept safely below a critical level. Excessive argon injection can generate transient variation of the jets entering the mould, introduce asymmetry in the mould cavity, and increase surface turbulence. Argon gas bubbles can also be trapped in the solidifying steel shell to form blister defects, such as pencil pipe in the finish product.
It has been observed that inclusion entrapment varies from side to side, which suggests a link with the variations in the transient flow structure of the lower recirculation zone, and the asymmetrical flow pattern (Fig 3), which can be induced by nozzle clogging, by turbulence, and by excessive argon gas injection. It is especially important to keep nearly constant the liquid steel level in the mould, powder feeding rate, casting speed, gas injection rate, slide gate opening, and nozzle position (alignment and submergence).
Electro-magnetic forces can be applied to the liquid steel in a number of ways to alter considerably the flow pattern in the strand. It has been reported that electro-magnetic stirring of outer strands can improve the steel cleanliness, lowering total oxygen in the cast product from 30 ppm to 20 ppm. Another example is the electro-magnetic brake (EMBR), which bends the jet and shortens its impingement depth, to lessen the likelihood of capture by the solidified shell deep in the strand.
Fig 3 Factors affecting inclusions in cast steel
Casting machine curvature – Continuous casting machines with curved mould are known to entrap more particles than straight (vertical) mould casting machines (Fig 3), since the particles gradually move upwards towards the inside radius while they spiral with the liquid in the lower recirculation zone. Majority of the particles are captured 1 m to 3 m below the meniscus, independent of casting speed, which corresponds to a specific distance through the strand thickness. Frequently, inclusions concentrate at surface and one-eighth to one-fourth of the thickness from the top of the inside radius surface. The vertical bending casting machine has fewer inclusions and pinholes, which are distributed deeper, relative to the curved casting machine. Particle entrapment defects such as pencil pipe can be lessened if at least the top 2.5 m section of the casting machine is straight (vertical).
Inclusions detection methods
The quantity, size distribution, shape and composition of inclusions are required to be measured at all stages of the production of steel. Measurement techniques range from direct methods, which are accurate but costly, to indirect methods, which are fast and inexpensive, but are only reliable as relative indicators. The inclusion detection methods are sometimes divided into two categories namely (i) off-line methods, and (ii) online methods.
There are several direct methods to evaluate steel cleanliness. These methods are described below.
Inclusion evaluation of solid steel sections
Several traditional methods directly evaluate inclusions in a two dimensional section through solidified product samples. The last five of the methods described below add the ability to measure the composition of the inclusions.
Metallographic microscope observation (MMO) – MMO method can only reveal the two-dimensional section of an inclusion though the inclusions are three-dimensional in nature.
Image analysis (IA) – This enhancement to MMO improves on eye evaluation by using high speed computer evaluation of video-scanned microscope images to distinguish dark and light regions based on a grey scale cutoff.
Sulphur print – It is a popular and inexpensive macro-graphic method which distinguishes macro-inclusions and cracks by etching sulphur rich areas. It has the same issues as the other two-dimensional methods.
Scanning electron microscopy (SEM) – This method clearly reveals the three-dimensional morphology and the composition of each inclusion. Composition can also be measured with ‘electron probe micro analyzer’ (EPMA).However, extensive sample preparation is needed to find and expose the inclusion(s).
Optical emission spectrometry with pulse discrimination analysis (OES-PDA) – The optical emission spectrometry (OES) method analyzes elements dissolved in liquid steel. Inclusions cause high intensity spark peaks (relative to the background signal from the dissolved elements), which are counted to give the PDA (pulse discrimination analysis) index.
Laser micro-probe mass spectrometry (LAMMS) – In this method, individual particles are irradiated by a pulsed laser beam, and the lowest laser intensity above a threshold value of ionization is selected for its characteristic spectrum patterns due to their chemical states. Peaks in LAMMS spectra are associated with elements, based on comparison with reference sample results.
X-ray photoelectron spectroscopy (XPS) – This method use x-rays to map the chemical state of individual inclusions which greater than 10 micrometers in size.
Auger electron spectroscopy (AES) – This method use electron beams to map the composition of small areas near the surface of flat samples.
Cathodoluminescence microscope – Under microscope, the steel or lining sample section is stimulated by a cathode-ray (energetic electron-beam), to induce cathodoluminescence (CL). The colour of CL depends on the metal ions type, electric field, and stress, allowing inclusions to be detected.
Inclusion evaluation three-dimensional steel matrix
Several methods directly measure inclusions in the three-dimensional steel matrix. The first four of these scan through the sample with ultrasound or x-rays. The last four of these volumetric methods first separate the inclusions from the steel.
Conventional ultrasonic scanning (CUS) – In this method, the transducer (typically a piezoelectric) emits a sound pressure wave which is transferred into the sample with the aid of a coupling gel. The sound waves propagate through the sample, reflect off at the back wall and return to the transducer. The magnitude of the initial input pulse and the reflected signals are compared on an oscilloscope to indicate the internal quality of the sample. Obstructing objects in the path of the sound scatters the wave energy. This non-destructive method detects and counts inclusions larger than 20 micrometers in the solidified steel samples.
Mannesmann inclusion detection by analysis surfboards (MIDAS) – In MIDAS method the steel samples are first rolled to remove the porosity and then ultrasonically scanned to detect both the solid inclusions and compound solid inclusions / gas pores. This method has been now renamed as the ‘liquid sampling hot processing’ (LSHP) method.
Scanning acoustic Microscope (SAM) – In this method, a cone-shaped volume of continuous cast product is scanned with a spiraling detector, such as a solid ultrasonic system, which automatically detects inclusions at every location in the area of the sample surface, including from surface to centre-line of the product.
X-ray detection – By this method, inclusions images are detected by their causing variation in the attenuation of x-rays transmitted through the solid steel. An inclusion distribution can be constructed by dividing a sample into several wafers and subjecting each to conventional x-rays to print penetrameter radiograghs for image analysis.
Chemical dissolution (CD) – In the CD method, acid is used to dissolve the steel and partially extract the inclusions. The inclusion morphology and composition can be detected by another method like SEM, or be fully extracted by dissolving the complete steel sample. The three dimensional nature of inclusions can be revealed by this method. The disadvantage is that the acid dissolves away FeO, MnO, CaO, and MgO in the inclusions. Hence, this method is good to detect only alumina and silica inclusions.
Slime (electrolysis) technique – This method is also called ‘potentiostatic dissolution technique’. A relatively large (200 grams to 2 kilograms) steel sample is dissolved by applying electric current through the steel sample immersed in a ferrous chloride or ferrous sulphate solution. This method is used to reveal the individual, intact inclusions. One disadvantage of this method is the cluster inclusions possibly break into separate particles after extraction from steel.
Electron beam (EB) melting – In this method, a sample of aluminum killed steel is melted by an electron beam under vacuum. Inclusions float to the upper surface and form a raft on top of the liquid sample. The normal EB index is the specific area of the inclusion raft. An enhanced method (EB-EV – ‘extreme value’) has been developed to estimate the inclusion size distribution.
Cold crucible (CC) melting – Inclusions are first concentrated at the surface of the melted sample as in the EB melting. After cooling, the sample surface is then dissolved, and the inclusions are filtered out of the solute. This method improves on EB melting by melting a larger sample and being able to detect silica.
Fractional thermal decomposition (FTD) – When temperature of a steel sample exceeds its melting point, inclusions can be revealed on the surface of the liquid and decomposed. Inclusions of different oxides are selectively reduced at different temperatures, such as alumina based oxides at 1,400 deg C or 1,600 deg C, or refractory inclusions at 1,900 deg C. The total oxygen content is the sum of the oxygen contents measured at each heating step.
Magnetic particle inspection (MPI) – This method also called magnetic leakage field inspection can locate inclusions larger than 30 micro-meters in steel sheet products. The test procedure consists of generating a homogeneous field within the steel sheet which is parallel to the sheet surface. If an inhomogeneity (such as an inclusion or a pore) is present, the difference in magnetic susceptibility forces the magnetic flux field to bend and extend beyond the surface of the sheet. The main disadvantage of this method is poor resolution of inclusions which are close together.
Inclusion size distribution after inclusion extraction
Several methods can find three-dimensional inclusion size distributions after the inclusions are extracted from the steel using a suitable method described earlier.
Coulter counter analysis – in this method, particles which flow into the sensor through its tiny hole are detected because they change the electric conductivity across a gap. The method measures the size distribution of inclusions extracted by slime and suspended in water.
Photo scattering method – Photo-scattering signals of inclusions (which have been extracted from a steel sample using another method such as slime, are analyzed to evaluate the size distribution.
Laser diffraction particle size analyzer (LDPSA) – This laser technique can evaluate the size distribution of inclusions which have been extracted from a steel sample using another method such as slime.
Inclusion evaluation of liquid steel
There are several approaches which can be used to detect the inclusion quantity and the size distribution in the liquid steel.
Ultrasonic techniques for liquid system – This method captures the reflections from ultrasound pulses to detect on-line inclusions in the liquid steel.
Liquid metal cleanliness analyzer (LIMCA) – This on-line sensor uses the principle of the ‘Coulter counter’ to detect inclusions directly in the liquid steel. This method is normally used for aluminum and other metals, and it is still under development for steel.
Confocal scanning laser microscope – This new in-situ method can observe the behaviour of individual inclusions moving on the surface of the liquid steel, including their nucleation, collision, agglomeration, and pushing by interfaces. The detected alumina inclusion clustering process on a liquid surface by this method is shown in Fig 4.
Fig 4 Alumina inclusion clustering process on a liquid steel surface
Electromagnetic visualization (EV) – This Lorentz-force-based detection system is used to accelerate inclusions to the top free surface of the sample of liquid metals and highly conductive opaque fluids. The technique has better resolution than other on-line methods.
Owing to the cost, time requirements, and sampling difficulties of direct inclusion measurements, steel cleanliness is normally measured in the steel plants using total oxygen, nitrogen pickup, and other indirect methods.
Total oxygen measurement – The total oxygen in the steel is the sum of the free oxygen (dissolved oxygen) and the oxygen combined as inclusions. Free oxygen or ‘active’ oxygen can be measured relatively easily using oxygen sensors. It is controlled mainly by equilibrium thermodynamics with deoxidation elements, such as aluminum. Since the free oxygen does not vary much for example, 3 ppm to 5 ppm at 1,600 deg C for aluminum killed steel. The total oxygen is a reasonable indirect measure of the total amount of oxide inclusions in the steel since there is small population of large inclusions in the steel sample. Hence, the total oxygen content really represents the level of small oxide inclusions only. The total oxygen measured from liquid samples roughly correlates with the incidence of slivers in the product. In particular, tundish samples are normally taken to indicate cleanliness for the cast steel dispositioning.
One of the steel plants needs the total oxygen in the tundish samples less than 30 ppm to ensure shipment of cold-rolled sheet without special inspection. The general conclusions drawn from the data of the total oxygen levels in LCAK steel at each processing step at several steel plants are (i) total oxygen in LCAK steel has steadily decreased with passing years, as new technology is implemented, (ii) plants with RH (Rurhstahl Heraeus) degassing unit achieve lower total oxygen (10 ppm to 30 ppm) than plants with ladle gas stirring (35 ppm to 45 ppm), and (iii) the total oxygen normally drops after every processing step such as 40 ppm in the ladle, 25 ppm in the tundish, 20 ppm in the mould, and 15 ppm in the cast product. Fig 5 shows relationship between total oxygen in tundish and sliver defect index.
Fig 5 Relationship between total oxygen in tundish and sliver defect index for product and between nitrogen pickup and total oxygen
Nitrogen pickup – The difference in nitrogen content between steelmaking vessels is an indicator of the air entrained during transfer operations. Hence, nitrogen pickup serves as a crude indirect measure of total oxygen, steel cleanliness, and quality problems from reoxidation inclusions. For example, a steel plant restricts nitrogen pickup from ladle to tundish to less than 10 ppm for critical clean steel applications. The oxygen pickup is always many times higher than the measured nitrogen pickup, because of its faster absorption kinetics at the air steel interface. Fig 3 shows relationship between nitrogen pickup and total oxygen. In addition, nitrogen pickup is faster when the oxygen and sulphur contents are low. Hence, for the reduction of the nitrogen pickup, deoxidation is best carried out after tapping. Plant measurements confirm this, as nitrogen pickup reduced from 10 ppm to 20 ppm for deoxidation during tapping to 5 ppm after tapping.
The general conclusion drawn from the data of minimum nitrogen pickup and nitrogen contents measured in LCAK steel at every processing step (except tundish and mould) for several steel plants is that the nitrogen in LCAK steel cast products is around 30 ppm to 40 ppm at the majority of the steel plants. It is controlled mainly by the steelmaking converter or electric furnace operation, but is also affected by secondary steelmaking and shrouding operations. However, the nitrogen pickup is decreasing with passing years, because of new technologies and improved operations. Nitrogen pickup can be normally controlled at 1 ppm to 3 ppm from ladle to the mould. With optimal transfer operations to lessen air entrainment, this pickup can be lowered during steady state casting to less than 1 ppm.
Concentration measurement – For LCAK steels, the dissolved aluminum loss also indicates that reoxidation has occurred. However, this indicator is a less accurate measure than nitrogen pickup since aluminum can also be reoxidized by the slag. The silicon pickup, manganese pickup can be also used to evaluate the reoxidation process.
Lining refractory observation – Analysis of the lining refractory composition evolution before and after operations can be used to estimate inclusion absorption to the lining and the lining erosion. Also, the origin of a complex oxide inclusion can be traced to lining refractory erosion by matching the mineral and element fractions in the slag with the inclusion composition.
Slag composition measurement – Analysis of the slag composition evolution before and after operations can be interpreted to estimate inclusion absorption to the slag. Also, the origin of a complex oxide inclusion can be traced to slag entrpment by matching the mineral and element fractions in the slag with the inclusion composition. However, these methods are not easy because of the sampling difficulties and since changes in the thermodynamic equilibrium are to be taken into account.
Tracer studies for determining exogenous inclusions from slag and lining erosion – Tracer oxides can be added into slags and linings in ladle, tundish, mould, or ingot trumpet, and top compound. Typical inclusions in the steel are then analyzed by SEM and other methods. If the tracer oxides are found in these inclusions, then the source of these inclusions can be decided.
Submerged entry nozzle (SEN) clogging – Short SEN life due to clogging is sometimes an indicator of poor steel cleanliness. The composition of a typical clog during LCAK steel continuous casting consists of Al2O3- 51.7 %, Fe – 44 %, MnO – 2.3 %, SiO2 – 1.4 %, and CaO – 0.6 % , which shows that nozzle clogs are frequently caused by a simultaneous build-up of small alumina inclusions and frozen steel. Hence, SEN clogging frequency is another crude method to evaluate steel cleanliness.
Final product tests
The ultimate measure of cleanliness is to use destructive mechanical tests to measure formability, deep-drawing, and / or bending properties of the final sheet product, or fatigue life of test samples or product samples. Other steel sheet tests include the HIC test and magnetoscopy. Another example is the inclusion inspection method in ultra-sonic fatigue test. These tests are needed to reveal facts such as the potential benefit of very small inclusions (less than 1 micrometer), which are not to be counted against cleanliness.
It can be seen from the above that there is no single ideal method to evaluate steel cleanliness. Some methods are better for quality monitoring while others are better for problem investigation. Hence, it is necessary to combine several methods together to give a more accurate evaluation of steel cleanliness in a given operation.
Since exogenous inclusions can originate from a combination of several sources, methods for their prevention are not likely to be simple. It is only through the correct combination of all these sources and removal mechanisms that the incidence of large inclusions in the steels can be reduced. For the detection of the exogenous inclusions in steel, the methods which are suitable are ultrasonic scanning, microscopic observation, sulphur print, slime (electrolysis), X-ray, SEM, slag composition analysis, and refractory observation.
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