Inclusions, Inclusion Engineering and Clean Steels
Inclusions, Inclusion Engineering and Clean Steels
Inclusions are non-metallic compounds and precipitates which form in steel during its production and processing and hence are the by-products of steelmaking which arise from different chemistries and processes. Inclusions can vary widely in size and composition, giving rise to a corresponding wide range of effects and mandating sophisticated analytical equipment for characterization.
Inclusions are constituted by glass-ceramic phases embedded in steel metal matrix. Inclusion control is to promote the removal of inclusions from steel and to reduce their harmful effects on the quality and the processing of steel. It is an important aspect of the steelmaking practice. However, the presence of certain inclusion types can also yield beneficial effects in the steel.
The source, the removal, and the mechanical consequences of the inclusions depend on their types and on their engineering. The chemical composition of the inclusions and their volume fraction are determined by the management of the different steps involved in the production process such as the melting, refining and casting operations. Hence, the inclusion population depends on the relation existing between the applied operative parameters and the features of the steel grades to be produced.
The evolution of an inclusion population in the ladle is influenced by several factors, such as the type and size distribution of the parent inclusions in the steelmaking furnace at tapping, the level of oxygen at tapping, the amount of slag carryover from the steelmaking furnace, the type, quantity and timing of synthetic slag additions to the ladle, the type and timing of deoxidant additions and the timing and the intensity of stirring in the ladle to name a few. The shape of inclusions can be globular, platelet, dendritic, or polyhedral.
Globular shape is desirable. Certain inclusions like MnS (manganese sulphide), oxy-sulphides formed during solidification in the spaces between the dendrite arms, iron aluminates and silicates are globular. Platelet shape is undesirable. Aluminum deoxidized steels contain MnS in the form of thin films located along the grain boundaries. Polyhedral inclusions are not very harmful. The inclusions are having different shapes which are described below. Globular shape of inclusions is most desired since their effect on the mechanical properties of steel is moderate. Spherical shape of globular inclusions is as a result of their formation in liquid state at low content of aluminum.
Platelet shaped inclusions are there in steels which are deoxidized by aluminum. These inclusions contain MnS and oxy-sulphides 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 not desirable. They considerably weaken the grain boundaries and cause adverse effects on the mechanical properties particularly in hot condition (hot shortness).
Dendrite shaped inclusions are due to the use of excessive amount of strong deoxidizer (aluminum). This results in formation of dendrite shaped oxide and sulphide inclusions (separate and aggregated). These inclusions have melting point higher than that of steel. Sharp edges and corners of the dendrite shaped inclusions can cause local concentration of internal stress, which considerably decrease the ductility, toughness, and fatigue strength of the steel.
Polyhedral inclusions are formed when the morphology of dendrite shaped inclusions is improved by addition (after deep deoxidation by aluminum) of small amounts of rare earth (cerium, lanthanum) or alkaline earth (calcium, magnesium) elements. Since their shape nearing the globular shape, polyhedral inclusions cause less effect on the steel properties than dendrite shape inclusions.
There are micro inclusions (size 1 micrometer to 100 micrometers) and macro-inclusions (size larger than 100 micrometers). Macro inclusions are harmful. Micro inclusions are beneficial as they restrict grain growth, increase yield strength and hardness. Micro- inclusions act as nuclei for precipitation of carbides and nitrides. Macro- inclusions are to be removed. Micro inclusions can be used to improve strengthening by dispersing them uniformly in the matrix.
Harmful effects of inclusions are highly dependent on their chemical compositions, volume fractions, dispersions, and morphologies. Normally large and unbreakable inclusions with high melting points are the most unwanted ones. However compared to these inclusions, small and breakable ones or those with lower melting points are more preferred. The reason for these preferences is that inclusions which have lower melting points or are breakable are likely to be deformed, crushed to smaller inclusions, or disappeared in following hot or cold forming processes (effect of forming process and reduction ratio) or heat treatments which the steels undergo after casting and solidification process.
The mechanical behaviour of steel is controlled to a large degree 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 whole steel heat. Though the large inclusions are far outnumbered by the small ones, their total volume fraction can be larger.
Clean steels are those steels which contain limited inclusions in terms of size, shape, composition, distribution and frequency. As a result, clean steels are capable of outperforming other materials and excel in applied high stress states, such as those used in transportation equipment and other applications.
Steel cleanliness is an important factor of steel quality and the demand for cleaner steels increases every year. However, the term ‘clean steel’ is used with caution by the metallurgists. This is because of (i) the varying cleanliness demands for steels for different applications, (ii) varying cleanliness in steels produced in different operations, and (iii) the normal understanding of the term ‘clean steel’, which some literally interpret as meaning the absence of inclusions in the steel. Steel cleanliness has implications from both operational and product performance points of view.
The ever-increasing demands for high quality have made the steel producers to pay high attention to the ‘cleanliness’ requirements of the steel products being produced by them. Different steel grades are being produced by the steel producer for meeting various requirements expected from the steel products. The cleanliness level of the steel for each requirement depends on the inclusions number, morphology, composition, and size distribution of each steel grade. For example, in free machining or resulphurized steel, the idea is not to completely remove the inclusions but to modify them to improve machinability. Hence, a balanced opinion regarding permissible level of the inclusions or cleanliness for each steel grade is really of great technical and economic importance both to the steel producer and the steel user. To a large extent, the term ‘clean steel’ is to be emphasized for meeting the customer’s specifications and requirements for an application with regard to non-metallic inclusion characteristics.
With cleanliness requirements more stringent and the development of new steelmaking grades, understanding the inclusion formation and evolution process and developing methods for improving their removal from the liquid steel is important. Inclusion removal is favoured not only by a large inclusion size but also a high interfacial energy between the inclusion and the steel and large contact angles between the inclusion and steel in a steel-inclusion-gas system.
Clean steel inclusion requirements vary depending on the steel grade and application and the objective of inclusion engineering is to reduce the inclusions which are harmful and promote the formation of those inclusions which have beneficial effects.
Advances in steelmaking during the last several decades have resulted in steel grades with very low level of impurities. In recent years, new ‘clean’ and ‘ultra-clean’ steels have been developed and commercialized by steel producers around the world, thereby responding to the current and future market demands of steel having considerably improved mechanical properties (e.g. fatigue strength and impact toughness) and an improved corrosion resistance. These steels can have an extremely low content of oxygen (less than 10 ppm) and sulphur (less than 10 ppm). The driving force behind these advances has been to development of new steels which can tolerate highly demanding applications e.g. transmission components for the automotive industry, and construction parts and tubes for aggressive and corrosive environments.
Although today’s high-cleanliness steels have excellent mechanical properties and / or corrosion resistance, these advances in functional properties have come at the expense of more difficult chip breaking and in some cases a considerably reduced tool life in machining operations.
Machining of steels with high-cleanliness is, in general, associated with high energy consumption, an increased cutting tool wear, and high production costs. It has been estimated that more than 40 % of the total production cost to produce an automotive component comes from different machining operations. Hence, the main issue is assessed as to optimize today’s steel grades with respect to the combined machinability and performance requirements. Hence the inclusions are to some extent necessary for a proper machinability performance. However, the content and the characteristics of the inclusions are still to ensure that high performance properties of the steel can be obtained.
Important features of the secondary steelmaking process are production process, liquid steel, ladle refractories, additives, slag, temperature, and the time and method of treatment. These are crucial factors which have effects on various characteristics of inclusions as shown in Fig 1.
Fig 1 Factors affecting various characteristics of inclusions during steelmaking
There are three stages in the process of inclusions formation. These stages are (i) nucleation, (ii) growth, and (iii) coalescence and agglomeration. At the stage of nucleation, the nuclei of new phase are formed as a result of super saturation of the solution (liquid or solid steel) with the solutes (e.g. aluminum and oxygen) due to the dissolution of the additives (deoxidation or desulphurization agents) or cooling down of the steel. The nucleation process is determined by surface tension on the boundary inclusion-liquid steel. The less the surface tension, the lower super saturation is needed for formation of the new phase nuclei. The nucleation process is much easier in the presence of other phase (other inclusions) in the liquid steel. 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) is favourable for the new phase nucleation.
In the growth stage, the growth of the nuclei takes place. Growth of a new inclusion continues until the chemical equilibrium is achieved (no super saturation). Growth of inclusions in solid steel is very slow process hence a certain level of non-equilibrium super saturation can be retained.
Coalescence and agglomeration takes place because of the motion of the liquid due to thermal convection or forced stirring causes collisions of the inclusions, which can result in their coalescence (merging of liquid inclusions) or agglomeration (merging of solid inclusions). The coalescence / agglomeration process is driven by the energy advantage obtained due to the decrease of the boundary surface between the inclusion and the liquid steel. Inclusions with higher surface energy have higher chance to merge when collide.
Inclusion removal from the liquid steel involves its flotation to the steel-slag interface, separation from the steel and subsequent absorption into the slag. The fundamental mechanism of inclusion flotation in steel is by stokes law of flotation. Using this equation and for a 20 micrometers size spherical alumina inclusion, the estimated time to float a distance of 2 meters is around 120 minutes. This flotation time is reduced as the inclusion size is increased and it is further improved by argon stirring and the subsequent attachment of the inclusions to the argon gas bubbles. As an example, 100 micrometer size alumina inclusions float out in 5 minutes. Argon stirring also promotes inclusion growth by collision and subsequent agglomeration / coalescence
Large inclusions float up faster than the smaller ones. Large inclusions are normally buoyant, and as a result, they float readily from the steel into the slag phase. Smaller inclusions which are not as buoyant take a longer time to float from the steel. The floating inclusions are absorbed by the slag. The floating process can be intensified by moderate stirring. Vigorous stirring results into breaking the larger inclusions into smaller sized inclusions. Gas bubbles moving up through the liquid steel also promote the inclusions floating and absorption by the slag.
Classification of inclusions
The inclusions are produced in liquid steel during refining at high temperatures and or from precipitation during solidification. Inclusions which are produced during steel refining at high temperatures are known as primary inclusions and inclusions which are produced during solidification are known as secondary inclusions. Once inclusions are formed in steel, the characteristics of the inclusions such as size, quantity, composition, and morphology remain the same or change / evolve due to physico-chemical reactions in the liquid steel, between the liquid steel and surrounding slag and ladle refractory, and from deformation. Depending on their final characteristics, they can be harmful to the casting process, reduce the steel mechanical properties, and decrease the surface and overall quality of the steel product. Inclusions, the presence of which defines purity of steel, are classified by chemical and mineralogical content, by stability, and by origin.
According to a traditional classification, the inclusions can be distinguished in two main classes as a function of their origin. These classes are (i) endogenous inclusions, and (ii) exogenous inclusions. The endogenous inclusions form by precipitation within the liquid phase due to the decrease of the solubility of the chemical species contained in the steels. This class of non-metallic inclusions cannot be completely eliminated from the steel but the decreasing of their volume fraction and of the average size have to be taken under strict control in order to avoid the activation of the damaging phenomena.
On the contrary, the exogenous inclusions are the consequence of trapping of non-metallic materials coming from slag, refractory fragments, or from rising and covering powders used for protecting the steel and avoiding sticking during the steel casting. The inclusions belonging to this class can be featured by large sizes and their origin cannot be immediately recognizable, although their presence can strongly compromise the micro-structural soundness of the steels and the associated mechanical reliability. Since the exogenous inclusions are always process-related, they can be eliminated by implementing suitable processing procedures.
Endogenous inclusions – Endogenous inclusions (also being known as indigenous inclusions) occur within the liquid steel, precipitating out during cooling and solidification. The inclusions belonging to this class result from additives to the steel. They are deoxidation products or precipitated inclusions during cooling and solidification of steel. Alumina (Al2O3) inclusions in LCAK (low carbon aluminum killed) steel, and silica (SiO2) inclusions in silicon-killed steel 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 (Fig 2). Cluster-type alumina inclusions from deoxidation or reoxidation (Fig 2) 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, break-up 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.
Fig 2 Types of inclusions
Precipitated inclusions form during cooling and solidification of the steel. During cooling, the concentration of dissolved oxygen / nitrogen / sulphur in the liquid becomes larger while the solubility of those elements decreases. Thus inclusions such as alumina, silica, aluminum nitride, 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).
Endogenous inclusions are typically more uniformly distributed than exogenous inclusions, which are entrapments of materials from refractory interfaces, slag, or other materials in contact with the liquid steel. The endogenous inclusions are naturally occurring and hence can only be minimized and cannot be completely eliminated. Primary and endogenous oxides such as alumina and magnesium-spinel clog submerged entry nozzles and their irregular shapes act as stress risers during the deformation and decrease the steel mechanical strength.
Exogenous inclusions – Exogenous inclusions arise from unintentional chemical and mechanical interaction of liquid steel with its surroundings. They are normally harmful to fracture sensitive mechanical properties because of their large size and location near the surface. The majority of these inclusions are formed by reoxidation in which liquid steel, having ‘free’ deoxidants (aluminum, silicon, manganese, or calcium) dissolved in the liquid steel, picks up oxygen from contact with the air during pouring and transportation through the gating system. In addition, inclusions can be formed by reaction of the liquid steel with gases or water vapourizing from the improperly preheated transfer vessels. Exogenous inclusions are normally entrapped accidentally during tapping, pouring and solidification resulting in a random distribution throughout the cast steel products. These inclusions act as heterogeneous nucleation sites for precipitation of new inclusions during their motion in the liquid steel.
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). In machining, they produce chatter, causing pits and gouges on the surface of machined sections, frequent breakages, as well as excessive tool wear. Exogenous inclusions have the following common characteristics.
- Large size with inclusions from refractory erosion is normally larger than those from slag entrainment.
- Compound composition / multiphase is caused by the phenomena (i) due to the reaction between the 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 the liquid steel, and (iv) slag or reoxidation inclusions can react with the lining refractories or dislodged further material into steel.
- 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.
- Small number compared with small inclusions
- Sporadic distribution in the steel and 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. As a result, they are frequently found near the surface.
- More harmful to steel properties than small inclusions because of their large size.
One issue which overrides the source of these inclusions is why such large inclusions do not float out rapidly once they are formed. Possible reasons are (i) late formation during steelmaking, transfer, or erosion in the metallurgical vessels leaving insufficient time for them to rise before entering the casting, (ii) lack of sufficient superheat, and (iii) fluid flow during solidification induces mould slag entrapment, or re-entrainment of floated inclusions before they fully enter the slag.
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.
The most common form of large macro-inclusions from reoxidation found in steel such as alumina cluster are shown in Fig 2. Air is the most common source of reoxidation, which can occur in several ways such as (i) liquid steel in the tundish mixes with air from its top surface at the start of pouring due to the strong turbulence and 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 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 casting process by (i) shrouding by 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) purging some gas into the tundish before pouring, and into the tundish surface during pouring, and (iii) controlling gas injection in the ladle to avoid eye formation.
Another reoxidation source is silica, manganese oxide, and FeO 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 / MnO / FeO+[Al] = [Si] / [Mn] / [Fe]+Al2O3. This reaction leads to larger alumina inclusions with variable composition. This phenomenon further affects exogenous inclusions in two 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 break-up of the lining, (ii) a large 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. This complicates the composition of exogenous inclusions.
To prevent reoxidation from slag and lining refractory, keeping a low SiO2, MnO, and FeO content is very important. It has been reported that high alumina or zirconia bricks containing low levels of free silica are more suitable.
The steelmaking or transfer operations involve turbulent mixing of slag and metal, especially during transfer between vessels and hence, produce slag particles suspended in the steel. Slag inclusions, (10 micrometers to 300 micrometers in size, contain large amounts of CaO or MgO, and are normally liquid at the temperature of liquid steel, so are spherical in shape. Using a ‘H-shaped’ tundish and pouring it through two ladles diminishes slag entrainment during the ladle change period. The factors which affect slag entrainment into the liquid steel during the continuous casting process are (i) transfer operations from ladle to tundish and from tundish to mould especially for open pouring, (ii) vortexing at the top surface of the liquid steel with the vortex when liquid steel is at low level can be avoided in several ways such as shutting off pouring before the onset of vortexing, (iii) emulsification and slag entrainment at the top surface especially under gas stirring above a critical gas flow rate, (iv) turbulence at the meniscus in the mould, and (v) slag properties such as interfacial tension and slag viscosity. As an example, mould slag can be entrained into liquid steel due to (i) turbulence at the meniscus, (ii) vortexing, (iii) emulsification induced by bubbles moving from the steel to the slag, (iv) sucking in along the nozzle wall due to the pressure difference, (v) high velocity flow which shears slag from the surface, and (vi) level fluctuation.
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 N/m for a lime-silica-alumina slag in contact with pure iron yields a meniscus height of about 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.
Erosion of refractories, including 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. They are normally large and irregular-shaped. Exogenous inclusions can act as sites for heterogeneous nucleation of alumina and can include the central particle pictured, 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.
Lining erosion normally occurs at areas of turbulent flow, especially when combined with reoxidation, high pouring temperatures, and chemical reactions. The parameters strongly affecting lining erosion are given 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 that the dissolved aluminum in the liquid steel reduce SiO2 in the lining refractory, generating FeO based inclusions which are very reactive and wet the lining materials, leads to erosion of lining refractory at areas of high fluid turbulence. The extent of this reaction can be quantified by monitoring the silicon content of the liquid steel. This oxygen can also come from carbon monoxide, when carbon in the refractory reacts with binders and impurities.
- Brick composition and quality has a significant effect on steel quality. A steel plant has adapted three types of materials (high Al2O3, Al2O3-SiC-C, and MgO-C with a wear rate of 1.0, 0.34, 0.16 mm/heat respectively) 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) using very high purity (expensive) alumina or zirconia refractories, and (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 have been 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 calcium oxide 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 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 m/s are dangerous with regard to erosion.
- Excessive contact or filling time and high temperature worsen 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 tendency is there 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.
Chemical reactions produce oxides from inclusion modification when calcium treatment is improperly performed. Identifying the source is not always easy, as for example, inclusions containing calcium oxide can also originate from entrained slag.
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 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 the collision and agglomeration, inclusions in steel tend to grow with increasing time and temperature. 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, needs further investigation, though some studies have used fractal theory to describe the cluster morphology (features).
Another classification of the inclusions can be based on their chemical composition. The inclusions can be synthetically classified as (i) sulphides, (ii) aluminates, (iii) silicates, (iv) oxides, (v) nitrides, and (vi) complex combinations of two or more of these inclusion types. The majority of the inclusions in steels are oxides and sulphides since the content of phosphorus is very small. Silicates are very detrimental to steels, especially if it has to undergo heat treatment at a later stage. Normally nitrides are present in special steels which contain an element with a high affinity to nitrogen.
Sulphides inclusions are FeS, MnS, Al2S3, CaS, MgS, Zr2S, and others. The sulphides are frequently the consequence of the calcium treatment applied in order to modify the oxide inclusions, but the little and finely dispersed CaS highly refractory inclusions can be detrimental for the casting procedure (nozzle clogging) and for the damaging effect on steel. On the contrary the MnS non-metallic inclusions (frequently modified by the combination with CaS) are exploited for improving the cutting tool workability. In this case the MnS non-metallic inclusions are intentionally formed within the metal matrix in order to make the chipping brittle) during the tool working. This role implies that the volume fraction of the inclusions has to be significant and this aspect is the reason that excludes the application of EN10247:2003 for the estimation of the cleanliness of such a class of steels.
Aluminates inclusions normally consist of calcium aluminates obtained after the calcium treatment of the liquid steel. Calcium aluminates are 12CaO.7Al2O3 (C12A7), 3CaO.Al2O3 (C3A), and CaO.Al2O3 (CA) exist in the liquid state, whereas CaO.2Al2O3 (CA2) and CaO.6Al2O3 (CA6) are solid at steelmaking temperatures.
Silicates 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.
Oxides inclusions in liquid steel are mostly produced during steel deoxidation but can also result from reoxidation and slag or refractory entrainment in the steel. These inclusions can have, single or multiple phases and compositions, spherical or irregular shape, and are either solid or liquid in the steel depending on their melting temperature.
Oxides inclusions can nucleate homogeneously or heterogeneously. Homogeneous nucleation occurs without the presence of foreign surfaces in the steel while heterogeneous nucleation occurs on foreign surfaces. Sources of foreign surfaces in liquid steel can be entrained materials, the surrounding ladle refractory, pre-existing inclusions, and or undissolved alloys. For the formation of a stable oxide, the absolute contribution of the bulk free energy to the overall energy is to be greater than the interfacial energy and this occurs at a critical oxide size. Inclusions less than this critical size are unstable and re-dissolve into the liquid steel while those which are larger than this size grow. For the heterogeneous nucleation, the presence of an existing surface reduces the critical oxide size and hence, reduces the overall free energy needed for a stable oxide to be produced. Heterogeneous nucleation is more favoured compared to homogeneous nucleation.
Important inclusion characteristics are their size, amount, composition, and morphology. After a stable oxide is produced in the steel, the inclusions grow and can also change their composition due to reactions within the steel and between the steel and surrounding slag and ladle refractory. Oxides inclusion growth occurs by diffusion of oxygen and deoxidant to the inclusion, and by agglomeration and coalescing after collision.
Oxides inclusions are FeO, MnO, Cr2O3, SiO2, Al2O3, TiO2 and others. By mineralogical content, oxide inclusions divide into two main groups namely (i) free oxides such as FeO, MnO, Cr2O3, SiO2 (quartz), Al2O3 (corundum) and others, and (ii) spinels which are compound oxides formed by bivalent and trivalent elements. Ferrites, chromites and aluminates are in this group. The fundamental tool for the description of the chemical composition of the oxide inclusions is the ternary phase diagram (CaO-SiO2-Al2O3), since this is the main system ruling the formation of these non-metallic compounds. This class of non-metallic compounds are formed by the deoxidizing elements added to the liquid steel for removing the oxygen content.
Composition, size, and distribution of precipitated oxides are greatly influenced by the deoxidants, conditions of the liquid steel, and the solidification process. Aluminum is widely accepted as deoxidant in steelmaking process. Its addition is very convenient and it effectively reduces oxygen content in liquid steel to low levels. However, the most of the steel problems can be traced to alumina or Aluminum rich oxides. Solid alumina inclusions in the liquid steel tend to rapid clustering due to their dendritic morphology. The alumina clusters hardly float to the top of the liquid steel because of their high apparent density in view of oxide clusters plus engulfed liquid steel. They are detrimental to the castability and quality of continuously cast steel.
The onset of clogging during the casting process starts when an alumina inclusion attaches to the nozzle wall. Certain types of refractories, especially the graphite-stabilized magnesia refractories, have been reported to promote agglomeration of alumina inclusions. The high contact angle between the alumina inclusions and the steel further promotes the tendency of the inclusions to agglomerate on refractories. In addition, the presence of significant amounts of alumina and MnS inclusions negatively impacts the performance of steel products. In general, oxide inclusions can cause lamellar tearing and degrade the toughness, bendability and ductility of steels.
When aluminum is added to liquid steel for deoxidation, the aluminum reacts with the oxygen to form dendritic alumina inclusions (alumina galaxy). Depending on size, the alumina inclusions formed as a result of deoxidation can be divided into macro-inclusions and micro-inclusions. Partial and complete substitution of titanium, zirconium, and / or rare earth metals for aluminum is increasingly pursued. This is done to improve the castability and the quality of the continuously cast steel through generation of finely dispersed oxides which effectively serve as heterogeneous nucleation sites for transformation and precipitation. Hence, control of the amount, size, composition and distribution of inclusions in steel is of importance.
Nitrides inclusions are ZrN, TiN, AlN, CeN and others which can be found in alloyed steel and has strong nitride generative elements in its content. The nitride generative elements are titanium, aluminum, vanadium, cerium and others. The nitride inclusions are normally formed by titanium nitride (TiN) and perform a detrimental effect worsened by the peculiar edged shape which increases the amplification of the stresses which are developed at the interface between the inclusion and the metal matrix. When TiN is present in large numbers, homogeneously distributed, and in relatively small sizes, they promote the formation of equi-axed grains which improve the mechanical strength of the cast steel. Also, the presence of a specific CaO∙Al2O3∙2SiO2 oxide (Anorthite) in stainless steel 316L has been found to improve the machining tool life. These inclusions when present, act as a lubricant by coating the machining tool tip. They also promote the breaking of machining chips.
Examples of complex combinations of two or more of these inclusion types are FeO·Fe2O3, FeO·Al2O3, FeO·Cr2O3, MgO·Al2O3, 2FeO·SiO2, FeS·FeO, MnS·MnO, Nb(C, N), V(C, N) and others.
Three main mechanisms have been recognized at the origin of the inclusions which are related to the damaging effects played by these non-metallic phases against the metal matrix. These mechanisms consider the inclusions as (i) notching elements which amplify the stress field around the inclusions, (ii) pressurized tanks of gas which progressively migrates into the inclusions generating a stress field around the inclusions, (iii) non-metallic phases which generate a residual stress due to the different thermal expansion coefficient associated to the metal phase and the glassy-ceramic ones.
The first mechanism is associated to a ductile process of crack formation which develops starting from the interface between the inclusions and the steel. The voids are the precursor of cracks and on a macroscopic level the cooperative detrimental effect related to the voids formed by a large number of inclusions produces a decrease of the ultimate tensile strain value. This relation points out that the factors detrimentally influencing the toughness and the macroscopic ductility of the steels are (i) the increase of the volume fraction, (ii) the decrease of the curvature radius, and (iii) the fracture of the non-metallic inclusions.
The coalescence among the nucleated voids is very dangerous since the voids of adjacent inclusions can coalesce to form a large crack, so the formation of elongated strips of inclusions represents an extreme situation. Hence, the inclusions constituted by the brittle ceramic phases which can form elongated fractured strips have to be carefully avoided. It is worth noting that the just described mechanism is featured by a ductile process on microscopic scale, but its effect on a macroscopic level turns out as a decrease of the toughness and of the ductility.
The second mechanism is related to the highest solubility shown for hydrogen by the sulphides. Hence, the inclusions become pressurized tanks pulling on the metal matrix and giving rise to a stress field which can be summed to the one formed by the external force applied during the service of the steel.
The third mechanism takes place as a consequence of the different thermal expansion coefficient featuring the steel and the glassy and / or ceramic structures characterizing the inclusions. The silicates, the aluminates, and normally all the oxides (except CaO and MgO) have a thermal expansion coefficient lower than one of the steel metal matrix, while the sulphides are featured by a contrary behaviour. The detrimental action is due to the residual stress generated on the interface between the inclusions and the metal matrix.
The higher the size of the inclusion the larger is the detrimental effect, so in order to prevent this mechanism the limitation of the size of the inclusion is a fundamental aspect while the overall volume fraction of the inclusion population does not play a significant role in this mechanism.
By stability, non-metallic inclusions are either stable or unstable. Unstable inclusions are those which dissolve in dilute acids (less than 10 % concentration). Unstable inclusions are iron and manganese sulphides and also some free oxides.
The formation and the control of the chemical composition of the inclusions involve the different steps of the production processes and the industrial systems through which they are performed. The production process has to be carefully implemented in each step in order to avoid problems related to (i) difficulties during the casting operation associated with the nozzle clogging between the tundish and the mould (continuous casting process) and between the ladle and the casting column (ingot casting), and (ii) detrimental effect on the mechanical properties of the steel.
There are four main treatment mechanisms for the removal of inclusions from the liquid steel. The first mechanism is the flotation of the inclusions. As per the Stokes law, because of the differences between densities of non-metallic inclusions and liquid steel, flotation leads to the removal of the inclusions. It is possible to calculate theoretically the rate of inclusion removal due to flotation. The second mechanism is the use of the magnetic stirring and argon gas injection. These two techniques assist the removal of non-metallic inclusions. Rate of inclusions entrapment by means of argon gas injection can be calculated.
The third mechanism is the calcium treatment. Calcium treatment is an effective way which can facilitate the removal of inclusions from the liquid steel. By adding calcium to the liquid steel (mostly in form of calcium silicide), it is possible to modify unmelted aluminum-magnesium rich inclusions (spinels) to large, isotropic, and spherical calcium aluminates and calcium sulphides with low melting points. This assists the removal of liquid inclusions. However, it can become a problem if for any reasons some of these large calcium aluminates remain or get trapped in the liquid steel.
The fourth mechanism is to optimize the properties of the top slag. Optimized properties of the top slag can enhance the inclusions removal in the ladle furnace. The three mechanisms mentioned above facilitate the inclusions movement from the middle or bottom parts of the liquid steel bring the inclusions to the ladle top. However without a proper top slag, it is highly probable that these inclusions cannot be removed efficiently. Hence, in order to ensure a very effective entrapment and absorption of non-metallic inclusions by means of top slag, it is necessary to have an optimized liquid top slag with high absorbing capacity for inclusions, proper wetting properties, and viscosity.
Solid-phase inclusions can cluster together to clog nozzles and other flow control systems which mediate the flow of liquid steel, posing a threat to the process operations. Some inclusion chemistries reduce ductility, resistance to fatigue, or overall toughness in steels. The absence of inclusions poses issues as well because the ‘clean steels’ can be harder to machine, decreasing the lifespan of cutting tools, and require higher power consumption for machining. Understanding their nature is of critical importance in steelmaking operations, and ‘inclusion engineering’ is needed to be an operational focus during the process of steelmaking.
The term ‘inclusions engineering’ means the design of the inclusions so as to alleviate their harmful effects on the product properties. Inclusion engineering does not refer to removal of inclusions but it refers to modify them either in terms of chemical composition or shape so that harmful effects of the inclusions can be converted to improve the steel properties. Inclusion engineering also involves distribution of inclusion uniformly in the matrix, so that composite properties can be generated in the product. In some cases, deliberate attempts are made to form very fine inclusions (e.g. nitrides, and carbo-nitrides inclusions in hardening steel). Such inclusion can form by reaction between tungsten, titanium, aluminum with oxygen, nitrogen, sulphur, or carbon.
The approach for reducing the harmful effect of inclusions is to tailor the steelmaking process to avoid the presence of macro-inclusions while controlling the population, size, distribution, and morphology of the residual micro-inclusions in the steel. The application of new technology and the knowledge gained from end users on the performance of steel products are valuable information for use in the design of a clean steel strategy. The science of inclusion modification and shape control stems from the need to change the chemistry of the inclusions to enhance the performance of products in the field and ensure the castability during continuous casting. However, macro-size inclusions are required to be removed. In all other cases, depending on applications, inclusion can be modified to minimize their harmful effects.
As far as inclusion modification and shape control are concerned, the inclusions of interest are the endogenous type, particularly the inclusions which result from the process of deoxidation and sulphide-type inclusions. Oxides and sulphides are the two predominant inclusions in steel. The sources of oxides and sulphides are inherent to the steelmaking process. Oxygen is employed to react with the impurity elements (e.g. silicon, manganese) and carbon to generate chemical energy for the melting process. However, a significant amount of the oxygen ends up being dissolved in the liquid steel. The dissolved oxygen is required to be removed during the refining stage because of its harmful effect on the structural integrity of the finished product. Strong deoxidants, like aluminum and silicon, are normally used to scavenge oxygen from the steel. However, aluminum-killed steels routinely clog tundish well nozzles and submerged entry nozzles during continuous casting due to the residual alumina inclusions which remain in the steel.
The element which is to be added to modify the inclusions is to meet three requirements namely (i) it is to have high chemical affinity for the inclusion, (ii) it is to be able to modify the composition so that it becomes liquid, and (iii) it is to be able to modify the shape i.e. sharp edges and corner of inclusions to spherical.
The formation of the non-metallic phases is ruled by the thermodynamic relations. The oxide system represents the most difficult one to be studied because of the presence of different oxide species.
Moreover, the insertion of calcium aiming at the modification of the inclusions makes even more difficult the understanding of the interaction taking place in the steel bath. A good procedure for the engineering of the inclusions is aimed at developing low melting non-metallic oxides in order to avoid the nozzle clogging and at maintaining a prevalently glassy structure of the inclusions during the steel cooling and the successive heating imposed to perform the plastic deformation in order to avoid the formation of ceramic brittle phases. The need to stabilize the glassy structure makes interesting the formation of silicate system based on the presence of anorthite and pseudo-wollastonite which appears to be particularly favourable.
The prediction and the engineering of the oxide inclusions can be based on a powerful and simple thermodynamic model and can be divided into three main steps namely (i) computation of the oxygen potential associated with the slag, (ii) evaluation of the possibility of the development of the reactions to create some pure non metallic compounds, and (iii) definition of a hierarchy of the different reactions as a function of the associated oxygen potentials on the basis of the chemical composition of the steel.
Ductility is appreciably decreased by increasing amounts of either oxides or sulphides. Fracture toughness decreases when inclusions are present in higher-strength lower-ductility alloys. Similar property degradation from inclusions is observed in tests which reflect slow, rapid, or cyclic strain rates, such as creep, impact, and fatigue testing. 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 HIC (hydrogen induced cracks). The source of most fatigue problems in steel are hard and brittle oxides, especially large alumina particles over 30 micrometers. Lowering the amount of large inclusions by lowering the oxygen content to 3 ppm to 6 ppm has extended the life of steel part such as bearing by almost 30 times in comparison with steels with 20 ppm oxygen. To avoid these problems, the size and frequency of detrimental inclusions are to be carefully controlled. Especially there is to be no inclusions in the casting above a critical size.
Although the solidification morphology of inclusions is important in steel castings, the morphology of inclusions in wrought 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 during deformation is schematically illustrated in Fig 3. ‘Stringer’ formation, type (b) and (c), increases the directionality of mechanical properties, adversely affecting the 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 3.
Fig 3 Behaviour of inclusions during deformation
There is a lot of information available on the effect of inclusions on product performance and on the kinetic and thermodynamic phenomena associated with inclusion evolution and formation. With a careful analysis of the available information, it is possible to develop a good practice at each stage of the steelmaking process for clean steel production. However, it is not possible or even necessary to eliminate all inclusions, as certain inclusions which are detrimental to steels for one application can be entirely harmless when present for another application. Hence, steels are expected to have varying degrees of cleanliness depending on their application.
A classification for what has to be considered a macro-inclusion has not been defined in any standard. On the other hand this information can be extremely difficult to be provided, since for a round shape inclusion a diameter of 14 micrometers to 20 micrometers can be dangerous, but for edged inclusions (i.e. TiN) the dangerous size can be stated even at a lower level (2 micrometers to 4 micrometers) as a consequence of the higher stress amplification associated to the edged shape. The treatment of this aspect is further complicated by the fact that the danger level can be strongly affected by the configuration of the non-metallic system which is ruled by the chemical composition of the participating phases. Actually, a correct engineering of the inclusions can permit to realize a sulphide crown precipitated on an oxide core and this system configuration mutually compensates the expansion coefficient of the non- metallic phases, approximating the one of the steel metal matrix.
Calcium treatment – The process of reducing the harmful effect of micro-inclusions by controlling their size, shape, and properties is known as inclusion modification. A common approach to modifying oxide and sulphide inclusions to prevent clogging and minimize any negative effects on the structural integrity of steel is through calcium injection during secondary refining of the steel. Fig 4 gives schematic of inclusion modification with calcium treatment of steel.
Fig 4 Schematic of inclusion modification with calcium treatment of steel
Calcium has a strong affinity for oxygen and can therefore be used as deoxidizers. However, the use of calcium as deoxidizer is challenged by its low boiling point of 1,439 deg C, limited solubility of 0.032 % of calcium in steel at 1,600 deg C, and a high vapour pressure of 1.81 atmospheres at 1,600 deg C. These properties make it difficult and non-economical to use calcium as deoxidizers. However, combinations of calcium and aluminum or manganese / silicon deoxidation form modified primary inclusions with lower activity and melting temperatures. For this reason, in steelmaking, calcium is added to steel more as an inclusion modifier rather than deoxidizer. Most steel grades are treated with calcium using either Ca-Si alloy or Ca-Fe(Ni) mixture depending on the alloy specification. Normally this treatment is effectively done after trim additions and argon rinsing.
The extent of inclusion modification in steel is an essential feature in secondary steel refining by calcium treatment. Portion of the calcium added to the melt undergoes reaction and remain in the melt as dissolved calcium in form of inclusions or go to the slag as slag constituent. The rest escape the system in form of vapour. It is vital that the calcium added is consumed by the liquid steel to the maximum extent to make the calcium injection efficient and cost effective.
The general effect of calcium treatment on inclusions modifications are (i) manganese sulphides are reduced in number and size, and they are transformed to calcium-manganese sulphides with varying properties, (ii) aluminum oxides, which are normally hard, angular and frequently appears in clusters are reduced in number or completely eliminated and replaced with complex CaO-Al2O3 or CaO-Al2O3-SiO2 inclusions, (iii) silicates are eliminated and replaced by CaO-Al2O3-SiO2 inclusions, and (iv) complex globular CaO-Al2O3-SiO2 inclusions are formed, frequently surrounded by sulphide rim.
Calcium is being frequently employed to treat aluminum killed steels to avoid the formation of solid alumina. Calcium treatment effectively improves the castability and the quality of the continuously cast steel, but is limited for all steel products which need either high fatigue resistance in service or high cold formability in very thin gauges. This is because of the presence of the globular calcium-aluminum oxides. The aluminum, calcium, and calcium-aluminum oxides are normally several to tens of micrometers in diameter.
Rare earth metals like cerium, and lanthanum etc., have also been used to modify inclusions, but they are not as efficient as calcium due to the slow flotation (due to their weight) of the modified inclusions. In addition, lanthanum and cerium readily corrode the ladle refractories. When calcium treatment is efficiently performed, the following two primary objectives are achieved.
- The alumina and silica inclusions are converted to liquid calcium aluminate and calcium silicate, which are globular in shape because of a surface tension effect. This change in inclusion composition and shape is known as inclusion morphology control.
- The calcium aluminate inclusions retained in liquid steel suppress the formation of MnS stringers during solidification of steel. This change in the composition and mode of the precipitation of sulphide inclusions during solidification of steel is known as sulphide morphology or sulphide shape control.
The conversion of inclusions to a globular shape plays a significant role on the separation rate of inclusions. For example, it has been observed that the alumina inclusions are non-wetting in liquid steel and tend to have a higher separation rate compared to CaO-SiO2-Al2O3. This implies that, by modifying the alumina inclusions with calcium, their ability to cluster is impeded as the liquid globular inclusions formed, and as a result are wetted by liquid steel. However, the high vapour pressure of calcium with the associated intense bath stirring promotes collision and coalescence of the alumina inclusions in the liquid steel. With the aid of calcium vapour and the resulting coalescence of the alumina inclusions through collision, their removal from the steel is enhanced compared to the small non-buoyant alumina inclusions which are to first cluster on their own (without forced convection) before they are able to separate from the liquid steel. This is why unmodified small alumina inclusions separate from the liquid steel and get attached to the refractory in the tundish only well after refining is complete in the ladle.
After effective calcium treatment all oxide inclusions normally contain some amount of calcium. Effective modification of oxide inclusions in steel depends on the dissolved aluminium and oxygen content of the steel before calcium treatment. For an essential inclusion modification, a calcium lower limit of 15 ppm to 20 ppm is needed. With a CaO-Al2O3 ratio of 12:7, low melting points of 1,455 deg C of calcium aluminate inclusions are formed. These inclusions exist in the liquid state at steelmaking temperatures.
Agglomeration of alumina, calcium aluminate and CaS inclusions on tundish nozzle refractories during continuous casting can result in a premature termination of casting due to a completely clogged nozzle. Depending on the population of the inclusions in the steel, complete clogging of the nozzle can occur within minutes of the start of casting. Analysis of clogged material in the tundish nozzle typically shows the presence of solid calcium aluminate inclusions with composition rich in either Al2O3 or CaO. For avoiding clogging during continuous casting, it is important to ensure low oxygen potential is achieved during refining prior to calcium treatment. The castability of steel has been shown to be directly related to its oxygen content.
When the calcium treatment is effective, alumina inclusions are converted to molten calcium aluminates which are globular in shape. The calcium aluminate inclusions retained in the steel suppress the formation of harmful MnS inclusions during the solidification of steel by modifying MnS inclusions to spherical CaS inclusions. When alumina is modified to calcium aluminate, the reaction sequence with additional calcium additions is Al2O3 to CA6 to CA2 to CA to C12A7. The presence of liquid calcium aluminates, CA2, CA, C12A7 at steelmaking temperatures (around 1,600 deg C) results in inclusions which are much easier to float than the solid alumina inclusions and also reduce the tendency of blocking ladle and casting nozzles.
The practice is to introduce calcium-bearing agents (CaSi, CaFe, CaAl, CaC, etc.) into the steel at the end of the steel refining in the form of powder or wire injection through hollow metallic tubes. Irrespective of the calcium bearing agent employed, the quantity of calcium required for treatment in a given weight of steel depends on the alumina content, and the oxygen and sulphur levels of the steel. A sufficient amount of calcium is required to be added to react with the alumina inclusions to form calcium aluminate compounds which are liquid at steelmaking temperatures. For completely modified inclusions, the equilibrium reactions are (i) [Ca] + [O] = (CaO), (ii) [Ca] + [S] = (CaS), (iii) 7(Al2O3) + 12[Ca] + 12[O] = 12CaO·7Al2O3, and (iv) [MnS] + 2[O] + CaSi = (CaS) + (SiO2) + [Mn].
The reaction in equation (iv) for the precipitation of MnS in the bulk of the liquid steel is possible in steel containing a high sulphur level. Fig 5 shows the binary phase diagram of CaO-Al2O3. The highlighted region in the figure shows the desirable composition of the calcium aluminate inclusions. Outside the highlighted region, the phases are solid at steelmaking temperatures. These phases can be the prominent constituents when there is an over- injection or under-injection of calcium. While MnS inclusions are undesirable in the steel, the formation of solid CaS inclusions is equally undesirable. In terms of clogging, solid calcium aluminate or pure CaS inclusions are just as detrimental as the alumina inclusions. They also sinter and agglomerate on nozzle refractories.
Fig 5 Binary phase diagram of CaO and Al2O3
The efficiency of calcium treatment is dependent on a number of factors, including the type, the amount and the injection rate of the calcium-bearing agent used for the treatment. Overall, by classifying the alumina and MnS inclusions according to their compositions and shapes, the efficiency of calcium treatment can be evaluated as given below.
- Class A inclusions are present when high levels of calcium have been added to the liquid steel and are liquid throughout processing. The intermingled sulphide and aluminate phases of these inclusions indicate that both phases solidified at about the same time. The sulphide phase tends to be a CaS composition. The calcium aluminate phase is either CaO·Al2O3 or 12CaO·7Al2O3. This indicates the presence of calcium aluminates with the lowest melting points and with high levels of calcium.
- Class B inclusions are the ‘bulls-eye’ type most prevalent in calcium-treated steels. The central, dark aluminate phase has solidified first, and then the outer sulphide phase precipitated onto it. In this instance, the sulphide phase tends to be (Ca, Mn)S. The calcium aluminate is of the CaO·Al2O3 or CaO·2Al2O3 composition.
- Class C inclusions are indicative of incomplete calcium treatment. These inclusions have an unmodified MnS phase, which is deformable during hot rolling. The central, dark calcium aluminate tends to be of the CaO·6Al2O3 composition, which has the lowest calcium content and remains undeformed during hot rolling.
- Class D inclusions are alumina-like oxide inclusion clusters which can have some calcium associated with them. However, there is not enough calcium present to result in complete fluxing of the alumina galaxy.
- Class E inclusions are MnS inclusions which are present when sulphur has not been completely tied up by calcium.
- Class F inclusions are inter-dendritic MnS inclusions which are present when sulphur is not completely tied up by calcium and the oxygen potential of the steel is high.
The end results of an optimized calcium treatment are: (a) the alumina is modified to form liquid calcium aluminate, and sulphur is tied up as CaS, which precipitates on the calcium aluminate inclusions, and (b) flotation of the inclusions is improved through the formation and agglomeration of spherical oxide and sulphide inclusions.
Several studies have attempted to determine the required amount of calcium addition for optimal cleanliness. For example, Ca/S ratios have been correlated to reduction of area in the Z direction and impact properties of steel. This approach cannot be generalized to all levels of sulphur. The acceptable level of Ca/S ratio in steels containing low sulphur levels can be several times higher than in steels containing higher sulphur levels, although the absolute amounts of calcium additions in the low-sulphur-containing steels are less than those of the steels containing higher sulphur levels. A good refining practice in the ladle and an efficient calcium treatment results in the majority of the alumina inclusions being converted to liquid calcium aluminate while most of the sulphur is tied up as CaS. The CaS precipitates on the calcium aluminate to produce the desirable bulls-eye shape.
Improvements of steel properties have been reported for calcium treated steel. These include (i) improvement of mechanical properties especially in transversal and through thickness direction by modifying MnS to undeformed globular (Ca-Mn)S or CaS, (ii) improvement of steel machinability at high cutting by forming protective film on the tool surface that prolongs the life of the carbide tool, (iii) improvement of surface quality and polishability, (iv) minimizing lamellar tearing in large restrained welded structures and the susceptibility of steel to reheat cracking as in the heat affected zones (HAZ) of welds, and (v) improvement of steel castability by preventing or minimizing nozzle clogging.
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