Rob Dekkers, Ph.D. Thesis, Katholieke Universiteit Leuven, Leuven, Belgium (2002) 7

Chapter 2

NON-METALLIC INCLUSIONS IN STEEL: A LITERATURE REVIEW

A review of research on non-metallic inclusions is given, emphasising inclusions formed in aluminium killed steels. Both steel and steelmaking evolved markedly over the last hundred years. Some evolutions like continuous casting had strong effect on the inclusion content and introduced the issue of inclusion control.

2.1. The early days of aluminium deoxidation Ever since the mid of the 19th century it was recognised that the addition of aluminium may prevent some types of porosity in the cast steel, which are due to the formation of carbon oxide gas during solidification. However, addition of aluminium also resulted in significant loss of steel ductility. It was already well known that the loss of ductility was not caused by aluminium oxide particles. In 1938 the research of Sims and Dahle [1] focussed on the effect of aluminium on the properties of medium carbon (about 0.25 wt%) cast steel. They were able to prove that aluminium affects the nature of the manganese sulphides, which on turn had a profound effect on the ductility. They found that manganese sulphides with a globular shape (type I) are characteristic for steel containing relatively high oxygen contents (fig. 2.1a). Addition of small amounts of aluminium, such that the steel is partially deoxidised, gives rise to films or envelopes of manganese sulphides (type II) at the austenite grain boundaries (fig. 2.1b). Films or envelopes decrease the ductility of the cast steel significantly.

a b c Fig. 2.1 Morphology of manganese sulphides depends on steel composition: (a) globular (type I), (b) enveloped (type II), and (c) faceted (type III) (taken from [6]).

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Adding excess amounts of strong deoxidisers, e.g. aluminium, zirconium or titanium, such that the liquid steel is completely deoxidised, resulted in large faceted octahedral inclusions (type III) (fig. 2.1c). In fact, the oxide inclusions formed during deoxidation act as nucleus sites for manganese sulphide. Since the oxide particles are randomly distributed the ductility is less affected. The shape of manganese sulphide inclusions has been the subject of many studies [1-6]. For instance Marich and Player [2] discussed the role of dissolved oxygen and the availability of γ–Al2O3 as nucleation sites and Oikawa and co-authors [6] explain the manganese sulphide shapes by considering stable and metastable phase diagrams. The different sulphide shapes only occur in steels with sulphur contents of minimal 0.025 wt%. Though sulphur contents of modern steels are lower, interest in manganese sulphide inclusions is still related to their effect on the mechanical properties of the solidified steel, such as machinability. Furthermore, it has been shown that oxide inclusions may affect precipitation. 2.2. Development of aluminium deoxidation In the early fifties aluminium was known to be the strongest deoxidiser commonly used in steelmaking, but its effectiveness to bind oxygen was still debated. Sloman and Evans [7] concluded that hercynite (FeAl2O4) only forms if the amount of aluminium is insufficient for deoxidation of the steel and not due to a reaction between aluminium oxide and iron oxide formed during solidification. The FeO-Al2O3 phase diagram had not been completed at that time. Neither the congruent melting point of hercynite nor the eutectic reaction between hercynite and aluminium oxide were known (fig. 2.2).

Fig. 2.2 The FeO-Al2O3 system: in 1950 Sloman and Evan [7] had not the disposal of the present-day diagram (left). A few years later, all data had been obtained to complete the diagram (right) [8]. In order to get more knowledge of the aluminium-oxygen equilibrium a number of studies were carried out [9-15]. An example is the experimental study of Gokcen and Chipman [9]. They concluded that the activity of oxygen is strongly reduced by the presence of

Non-metallic inclusions in steel: a literature review 9

aluminium and reciprocally, that oxygen reduces the activity of aluminium. Repetylo et al [16] explained the difference of the equilibrium constants for the formation of aluminium oxide between different studies by coalescence and upward movement of inclusions. Since flotation by Stokes Law is very slow for small inclusions, the time necessary for obtaining equilibrium is significant. The presence of a suspension of inclusions in the liquid iron causes erroneous measurements. Furthermore, they found that an argon atmosphere containing oxygen may account for an aluminium loss in the order of 200-500% of the quantity necessary for the elimination of the oxygen initially present. Repetylo et al also discussed the role of interfacial energies on the removal of inclusions from the melt, i.e. a high interfacial energy favours separation of inclusions at the steel surface. That the interfacial energy plays a major role in inclusion removal was already pointed out by Plöckinger [17] and by Chipman [18]. Plöckinger discussed in 1963 [18] the influence of deoxidation practice on the steel cleanliness. Especially the vacuum treatment and the use of aluminium deoxidation are described. The advantages of aluminium deoxidation are that very low dissolved oxygen contents (a few ppm) are obtained, which prevents carbon monoxide formation during solidification. The large interfacial energy of aluminium oxide in contact with the melt enhances inclusion removal. Interesting was the observation that aluminium oxide inclusions separated much faster than silicate inclusions and that the upward drifting is independent of the inclusion size or density. No information about the ladle slag composition is given, though it may have a significant effect on inclusion separation (chapter 8). The amount of inclusions was found to increase with holding time of the ladle, which was due to accumulation of inclusions in the upper region of the ladle. This opinion was strengthened by the fact that the last ingots contained high densities of inclusions. At this time, inert gas stirring was not common use. Stirring homogenises the steel bath with respect to composition, temperature and inclusions distribution. Contents of 0.005-0.01 wt% inclusions in steels subjected to orthodox deoxidation were considered as extremely small (0.002 wt% for vacuum melted steels). Plöckinger notes further, ‘Since it is impossible to produce absolutely clean steels and since even very small amounts of inclusions may give rise to dangerous accumulations of the latter in certain places of the structure during freezing, the necessity arises to influence their composition in such a manner that the technical properties of the steel do not deteriorate’. In fact, he proposed the use of silicates containing calcium such that liquid calcium silicates are formed that deform plastically during rolling. 2.3. Micro-analysis of non-metallic inclusions So far analyses of inclusions, slag and refractory particles in the steel had been carried out by selective dissolution techniques. The residues were analysed by spectrochemical, chemical and X-ray diffraction techniques. The shapes of inclusions were studied by optical microscopy. In the early sixties, Kiessling and co-authors applied electron probe analysis to

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study non-metallic inclusions in steel samples from a six ton ingot of silicon killed carbon steel with about 0.7 wt% manganese [19-21]. The advantage of electron probe analysis is the possibility to analyse recognisable phases with a high degree of accuracy, while the previous methods were very appropriate to measure the average bulk composition of samples. Kiessling et al were able to prove that the refractory material could effect the inclusion content. Significant values of aluminium oxide were detected in manganese silicate inclusions due to dissolution of the fire clay refractory. Slag droplets of different sizes, consisting of manganese aluminosilicates, were engulfed in the steel stream and increased in size owing to addition of manganese silicate deoxidation products during cooling. Growth was stronger in small inclusions than in large particles, because of the surface-volume ratio. Sometimes manganese silicate inclusions were observed with manganese poor rims giving strength to the assumption that manganese oxide is reduced by silicon.

Fig. 2.3 Luyckx [22] observed three zones with non-metallic inclusions when aluminium is added before or with manganese in an unkilled steel: large hercynite particles (left), aluminium oxide dendrites (middle) and galaxite films (right). Luyckx [22] correlates surface pencil lines in aluminium killed steel products to incomplete mixing of deoxidation alloys in the liquid steel. Due to incomplete mixing, so called occlusions hold manganese-rich but aluminium-poor steel. During rolling the occlusions are elongated and when they emerge at the steel surface the manganese in it oxidises, creating so a black line. In 1962 Chipman [23] already discussed the possibility, that the deoxidation process might be retarded by the formation of a continuous alumina film around liquid aluminium droplets, preventing diffusion of aluminium and oxygen and resulting in two immiscible liquids. Liquid manganese silicates in manganese-silicon deoxidised steels prevents the formation of continuous films due to coagulation of the liquid inclusions. This results in cleaner steel. Luyckx showed that these occlusions can be as large as 500 µm and that they are surrounded by aluminates and aluminium oxides. When aluminium and ferro-manganese were added simultaneously, three zones with non-metallic inclusions were observed, i.e. a zone containing large hercynite (FeAl2O4) particles, a zone with aluminium oxide dendrites and a continuous galaxite (MnAl2O4) film surrounding the ferro-manganese alloy (fig. 2.3). The hercynite is formed in steel with high dissolved oxygen content and low dissolved aluminium and manganese contents. The

Non-metallic inclusions in steel: a literature review 11

aluminium oxide dendrites indicate that the oxygen content of the steel is very low. The galaxite film was supposed to be formed due to partial reduction of the ferro-manganese oxide that enveloped the alloy prior to addition into the steel. From this study it was concluded that physical and chemical conditions of deoxidiser and alloy are very important. McLean [24] pointed out that the presence of aluminates in fully killed steels may be due to oxygen rich regions that exist as a result of inadequate mixing. That turbulent mixing during tapping, with additions of deoxidisers, enhances removal of inclusions was already well known. To promote continued mixing after tapping argon stirring became widespread. 2.4. Continuous casting Argon stirring enabled control of temperature within very close limits and gives rise to a homogeneous steel bath, which are necessary for continuous casting. The evolution of continuous casting in the sixties [25] had a profound effect on steel cleanliness. To prevent the formation of blowholes during solidification steel has to be fully deoxidised. Reoxidation of the steel during continuous casting was minimised by using tundish nozzles and later submerged entry nozzles. Aluminium had proved already to be a very efficient deoxidiser, but could result in deposits of aluminium oxide in the nozzles and even blockage during continuous casting [26-32]. The aluminium oxide deposits consist mainly of clusters. Silicon deoxidised steels are easier to cast, probably due to the presence of liquid manganese silicates, but give rise to coarse grained steels. 2.5. Inclusion control by the addition of rare earth elements and calcium Meanwhile investigations were carried out to modify the morphology of manganese sulphide precipitates. After rolling manganese sulphide precipitates are elongated, which gives rise to directionality. This affects the mechanical properties of the cast steel such as toughness and bend formability. Luyckx et al [33] proved that non-deformable globular sulphides may be formed by the addition of rare earth elements, especially cerium. Since rare earth elements are also strong deoxidisers, the dissolved oxygen content must be minimised before their addition, in order to create cerium sulphide instead of cerium oxide. In the middle seventies Hilty and Farrell [34,35] showed by experiments that modification of angular and cluster-type aluminium oxide inclusions into globular low melting inclusions was more efficient when calcium was added instead of rare earth elements. This led to the modification of aluminium oxide inclusions into liquid calcium aluminates [36-39] (chapter 8). Calcium is added by wire injection, e.g. as ferro-calcium. The use of calcium treatment increased the research and application of wire injection systems that were not only used for calcium addition, but also for alloying and fine tuning of the deoxidised steel.

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2.6. Morphology of aluminium oxide inclusions In the seventies a number of articles were published describing the relationship between the morphology and the activities of dissolved oxygen and aluminium. Scanning electron microscopy (SEM) became popular for observing the morphology of inclusions after that Rege et al. [40] had described in 1970 its use to study the three-dimensional shape of aluminium oxide clusters and dendrites in aluminium killed steel. Okohira et al. [41] in particular investigated the morphology of dendrites in low carbon steels. They found that the formation of dendrites is enhanced at high aluminium concentrations, i.e. 0.04-0.06 wt% compared to 0.01 wt% aluminium. Furthermore, the amount of dendrites was found to decrease drastically during ladle metallurgy. Steinmetz et al. demonstrated that the morphology not only depends on the activity of oxygen and aluminium, but also on the relative contents of reaction elements (fig. 2.4) [42-45]. Despite of the fact that this was already well known for sulphide inclusions, aluminium oxide inclusions had never been studied in detail. If the deoxidation metal is aluminium, increasing aluminium activity and simultaneous decreasing oxygen activity results in the formation of liquid iron aluminates, aluminium oxide dendrites and aluminium oxide polyhedra. Although fig. 2.4 is an oversimplification, Steinmetz et al. were one of the first that focussed on the physical chemical conditions of inclusion growth and tried to explain the morphology of aluminium oxide inclusions. Later, they also carried out reoxidation experiments [46,47] and found similar relationships between oxygen fluxes and the aluminium oxide shapes.

Fig. 2.4 Oxide growth shapes as a function of the local activities of oxygen and deoxidation metal (from [43]). The dotted line and the bold line refer to the aluminium activity and the oxygen activity, respectively. At high oxygen activities hercynite is formed. At this period a number of investigations were carried out on samples taken from laboratory melts [48-50]. In agreement with the work of Luyckx [22] and Straube and co-authors [51] it was observed that liquid iron-manganese oxides, which are present prior to deoxidation with aluminium, become more aluminium rich and transform into solid hercynite or galaxite particles after deoxidation, dependent on the manganese content of the steel. With

Non-metallic inclusions in steel: a literature review 13

increasing aluminium content during deoxidation aluminium oxide is precipitated onto the hercynite-galaxite inclusions. The iron-manganese oxide of these hercynite/galaxite inclusions is reduced by aluminium resulting eventually, after 10 to 30 minutes, in pure aluminium oxide inclusions. Robinson et al [49] noticed that the shape of aluminium oxide inclusions is controlled largely by the supersaturation. From sampling of the liquid iron with high oxygen content, it was found that first liquid iron-aluminium oxides are formed. With increasing aluminium content hercynite and aluminium oxide were observed around the liquid iron-aluminium oxides. Next hercynite precipitated from the liquid iron-aluminium oxides and subsequently transformation to alumina occurs. Mixing of oxygen rich regions and oxygen poor regions during sampling resulted in the formation of alumina dendrites and aggregates from the melt. The aggregates are polycrystalline, which proofs the agglomerated nature. Thus, reduction of prior phases takes place by aluminium and at the same time aluminium oxide is precipitated from the melt. It was found that branched dendrites are formed in oxygen rich and aluminium rich melts, while at low oxygen contents the dendrites spheroidise resulting in almost completely spherical inclusions (fig. 2.5).

Fig. 2.5 Spheroidisation of dendrites in a sample taken a few minutes after deoxidation. In 1979 Braun et al. [50] carried out deoxidation experiments in a vacuum induction furnace by adding 0.1 % aluminium in iron-10% nickel melts. The effects of the time interval between aluminium addition and solidification (holding time), the rate of induction stirring, the rate of mechanical stirring, the time of stirring, and the initial oxygen concentration were investigated. Dependent on the holding times different aluminium oxide morphologies were observed. So far, mainly dendrites and clusters had been described, but Braun et al. observed faceted, plate-like and small spherical aluminium oxides as well.

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Further they proved that stirring enhances cluster formation and that the shapes of particles in the clusters depend on the initial oxygen content of the steel. With the development of techniques to take small liquid steel samples, the formation of inclusions during ladle treatment could be studied. Tiekink et al [52,53] added aluminium by wire in one batch, in two batches, and during tapping. In all cases aluminium oxide clusters were formed. Stirring enhances flotation of the particles and within four minutes about 75 to 85% of the inclusions were removed from the melt. Faceted and plate-like inclusions were observed as well. The number of large plate-like inclusions and the sizes of large single particles present at the end of the ladle treatments increase from addition of aluminium in one batch, in two batches to addition during tapping. Similar sampling of ultra low carbon steel showed needle-like aluminium oxide inclusions, after additional aluminium addition. Surprisingly, recent publications concerning formation, growth and evolution of inclusions are rare, though much effort is taken in assessing steel cleanliness (chapter 3). The field of research seems to be shifted to the modelling of inclusion removal in ladle and tundish [54-56]. 2.7. Conclusions The methodology to study non-metallic inclusions in steel has changed due to new steelmaking technologies and due to the development of (new) research equipment. In the 20th century the major revolution in steelmaking has been the development and industrial application of continuous casting. Continuous casting requires the steel to be completely deoxidised, which is achieved by adding strong deoxidisers, in particular aluminium. The deoxidation practice results in large amounts of non-metallic inclusions that are largely removed into the ladle slag. Yet a fraction of the inclusions stays in the steel melt and can have a detrimental effect on the productivity and on steel quality, due to nozzle clogging and (surface) defect formation. Different aluminium oxide shapes can be recognised. In industrial samples mainly clusters and dendrites are observed. Detailed laboratory studies show spherical, plate-like and faceted inclusions as well. It has been found that the shapes of inclusions depend on dissolved oxygen and aluminium activities, stirring conditions and holding time. Oxygen contents in steels studied in the end sixties and seventies could be fairly high. Although inclusion removal in the ladle and in the tundish can be modelled reasonably well, the mechanism of deoxidation is poorly understood and the various aluminium oxide shapes have not been properly explained. The lack of understanding of the mechanisms of formation and evolution of non-metallic inclusions in liquid steel is probably the cause that no attempts to control the aluminium oxide shapes have been described.

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Rob Dekkers, Ph.D. Thesis, Katholieke Universiteit Leuven, Leuven, Belgium (2002) 17