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Metall. Res. Technol. 112, 410 (2015)c© EDP Sciences, 2015DOI: 10.1051/metal/2015028www.metallurgical-research.org

Metallurgical Research&Technology

A review: influence of refractories on steelquality

Jacques Poirier

CEMHTI, CNRS UPR3079/Université d’Orléans, 1D avenue de la Recherche Scientifique, 45071 OrléansCedex 2, Francee-mail: [email protected]

Key words:Refractories; steel quality; oxidecleanliness; desulphurisation;Ca treatments; clogging ofsubmerged nozzles; oxygen pick up

Received 8 July 2015Accepted 4 August 2015

Abstract – Chemistry and inclusion control are two of the main keys to the productionof quality steel products. The refractory materials have a direct influence on the qualityof elaborate grades at different levels: (i) The control of solute elements such as carbon,sulphur, nitrogen, hydrogen, oxygen. (ii) The prevention of non-metallic inclusions. A goodknowledge of the metal-slag-refractory products interactions is consequently necessary inorder to have a better control of elaboration procedures. Among all the points brought up,we could mention all the developments that will limit the contribution of refractory prod-ucts in clogging phenomena, carbon pick up and atmospheric re-oxidation, in conjunctionwith efforts of the metallurgists to produce clean steels.

U nder the pressure from users andfaced with competition from othermaterials, steel makers have to pro-

pose steel grades with narrower composi-tion ranges, lower guaranteed contents ofcertain residuals and controlled inclusionsize distributions to obtain reproducible ser-vice properties. These results can only bereached by a strict control of processes andalso of products used during steel mak-ing [1]. In particular, steel cleanliness andpurity requirements make the selection of re-fractory products more and more important.

Certain metallic residuals or non metal-lic impurities have a marked influence on thephysical and mechanical properties of steels.Figure 1 summarizes the role that non metal-lic elements could have on various proper-ties of the metal.

Consequently, the steel maker must con-ceive more and more complex elaborationmodes to eliminate these elements and limitpollution risks.

Significant progress has been made latelyon the control of elements C, H, N, O, P, Sfor which contents from a few ppm to sev-eral tens of ppm are currently obtained onthe most sensitive grades, whenever neces-sary. For example, after vacuum treatment

Table 1. Lower limits of residual elements insteel making elaboration.

Elements P C S N H Oppm 10 5 5 10 <1 5

in industrial conditions, a liquid steel with Ccontent <20 ppm is presently possible. In thesame way, the sulphur levels for HIC steelsor oxygen levels for bearing grades can belowered to values of a few ppm, which, inthis case, makes it possible to increase dra-matically the operating life of bearings. Ta-ble 1 shows typical limits of these elementsfor current steel making technologies.

In this context, the impact of refractoryproducts on the metal may be assessed atthree levels:

– the possibility to keep the chemical com-position of the liquid steel within thespecified range for a given process;

– the achievement of the required metalcleanliness, i.e. the amount and natureof non-metallic inclusions;

– the prevention of defects concerning thesteel surface.

Article published by EDP Sciences

http://dx.doi.org/10.1051/metal/2015028

http://www.metallurgical-research.org

http://www.edpsciences.org

J. Poirier: Metall. Res. Technol. 112, 410 (2015)

Hydrogen

Carbon

Nitrogen

Oxygen Control of inclusions

Phosphorus

Sulfur Control of inclusions

Non metallic elements

Electromagnetic properties

Deep drawing

Weldability

Weldability

Toughness

Surface defects

Anisotropy

Fatigue

Bending

Internal soundness

Fig. 1. Influence of non metallic elements on steel properties.

1 Influence of refractorieson steel qualityand inclusionary cleanliness

Adjustment of this high level of quality andof steel cleanliness within the steel plant pro-duction cycle is accomplished during sec-ondary metallurgy and is maintained duringcontinuous casting.

Taking metallurgical aspects more andmore into account imposes a new approachof interactions between metal, slag, atmo-sphere and refractory products [2]. Theseinteractions are generally controlled by themicrostructures, the phases distribution, thechemical and mineral composition of the re-fractory products.

Figure 2 summarises the main classes ofrefractory in relation with the quality andmetal cleanliness.

The refractory parts and products mostinvolved in the problems of metal qualityare:

– The steel ladle and the degassing deviceswhich may be a source of pollution even

if subsequent floatation is still possible.For example:

– corrosion of the magnesia refractorylining by the slag and cracking of thealumina refractory wall have an im-pact on the composition of the desul-phurisation slag;

– carbon pick up of 5 to 10 ppm bythe steel may result from magnesia-carbon refractories pollution.

– The tundish lining, which can have a pol-luting action (exchange of oxygen, hy-drogen, silicon and magnesium betweenthe magnesia refractory and the steel).

– The stopper which may be a source ofreoxidation.

– The submerged nozzle materials withtheir direct and indirect role on clog-ging and unclogging, leading to metalcontamination by alumina particles orclusters.

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Steel ladle

Tundish Sprayed magnesia

Plate Al2O3 - C

Magnesia graphite / magnesia chrome Dolomite, High alumina Alumina- spinel

Al2O3-C Stopper St dl Ladle Al2O3 - C

Shroud

Submerged nozzle Al2O3 - C and

ZrO2-C insert

Magnesia-chrome

Degassing device

Secondary metallurgy

Continuous casting

Fig. 2. Main classes of refractories in relation with the steel quality and inclusionary cleanliness.

2 Interactions of refractoriesand steel during the processesof secondary metallurgy

With the development of secondary metal-lurgy, the role of the steel ladle has changed.It has become a metallurgical reactor in thesteel making process.

In a steel ladle or in a degassing device,many reactions between refractories, steeland slag can contribute to degrading thesteel quality:

– direct dissolution of the refractory withor without precipitation;

– dissociation, volatilisation;

– oxido-reduction reactions, between anoxide and a metallic element;

– combination of the refractory and a non-dissolved element present in the steel (in-clusion).

2.1 Reactions between refractories,steel and slag [3,4]

2.1.1 Dissolution

Among the refractory compounds used insteel ladles, only carbon can be affected bya dissolution in steel. For example carbon

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Slag Refractory Boundary

layer CArefractory

CAsat

CAslag

Initial interface

Fig. 3. Dissolution of the refractory by slag: gradient composition at the interface slag/refractory.

(residual carbon from the binders or addi-tional carbon in MgO-C bricks) can be dis-solved directly into the liquid steel.

The corrosion between refractory liningand slag is often controlled by a directdissolution, chemical exchanges are con-trolled by a boundary layer at the liq-uid/refractory interface [5]. The gradient ofchemical potential, which is usually assimi-lated to a composition gradient, is the driv-ing force of the corrosion process (Fig. 3).Two elementary steps govern the dissolu-tion mechanism:

– a thermo chemical reaction at thesolid/liquid interface;

– a diffusion of species.

The dissolution wear rate is expressed byNersnt’s equation:

d[CA]/dt = h(CAs − CA) with h = D/e

– CAs : saturation solubility of A in the liquid

phase– CA: concentration of A in the liquid phase

outside the boundary layer– h: mass transport constant, D: diffusion

coefficient, e: thickness of the boundarylayer.

The dissolution rate decreases with decreas-ing D or increasing e. To minimise the cor-rosion rate, it is recommended to minimise(CA

s –CA).

The saturation solubility of the refractoryoxides can be determined using thermody-namic calculations or phase diagrams.

Consider the dissolution of a MgO-Crefractory by a CaO-SiO2 slag (CaO/SiO2

weight ratio of 0.9 and T = 1630 ◦C) [6]:if the liquid slag is saturated with magnesia(≈19%wt of MgO) then d[CMgO]/dt = 0 andit cannot dissolve MgO (Fig. 4).

The dissolution mechanism can be het-erogeneous with the precipitation of newphases in the interface layer [7]. In this case,the wear rate will decrease.

The examination of the microstructuresof refractories after laboratory corrosiontests is extremely useful to determine themechanisms of chemical attack.

Laboratory corrosion tests, based on thestatic crucible method, were performed witha bauxite brick and an Al2O3-CaO slag(weight ratio of Al2O3/CaO = 1). Typicalcharacteristics of the bauxite refractory arelisted in Table 2.

For corrosion testing, the crucible wasfilled with 40 g of slag and heat treated inair at 1600 ◦C using electric furnace. Heatingrate was 15 ◦C/min up to 900 ◦C and 9 ◦C/minup to 1600 ◦C. After 6 h firing, the cruciblewas quenched in cold water (T ≈ 8 ◦C) inorder to avoid partial crystallisation of theliquid phase during cooling.

Figure 5 shows a microstructure of theprecipitation zones of the bauxite refractoryafter corrosion.

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MgO in slag %

4

9

14

19

24

0 50 100 150 Time (min)

slag CaO-SiO2 with

SiO2/CaO = 0.9

Saturation solubility of MgO

T = 1630°C

Fig. 4. Dissolution of magnesia in MgO-C refractory by CaO-SiO2 slag for different times, at 1630 ◦C.

Table 2. Typical chemical composition, bulk density and apparent porosity of bauxite bricks.

Composition (wt%) Mineral phases Bulk density (g/cm3) Apparentporosity (%)Al2O3 SiO2 Fe2O3 TiO2 other

79.5 14 1.6 2.9 2 A***, M**, TiO2*, (Al,Fe)2 3.24 16.17TiO5* vitreous phase*

A: corundum, M: mullite. (***: major; **: mean; *: minor).

Corundum layer

CA2 layer

CA6 layer

200 µm

Fig. 5. Microstructure of bauxite bricks corroded by Al2O3-CaO slag (weight ratio of Al2O3/CaO= 1, lab crucible test); Backscattered electrons S.E.M micrographs on polished sections (Transitionbetween the three mono-mineral layers of the precipitation zone).

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Table 3. Mechanism of submerged nozzles clogging by alumina build-up.

Outside Refractory sidewall of the nozzle Hot face

Starting conditions

Air Permeable refractory made of

oxides + carbon + porosity

Molten steel containing Al, O,C

General mechanisms

O2 leakage − Temperature generates direct reduction of some oxides by the carbon and direct oxidation of carbon

− Negative presure generates gas transfer

− Reoxydation of some sub-oxides

− Oxidation of aluminium

Various mechanisms

Dissolution of the refractory carbon

Vapor phases transfer

Temperature generates

− Na2O + C 2 Na(g) +CO(g)

− K2O + C 2 K(g) +CO(g)

− SiO2 + C SiO(g) +CO(g)

− SiO2 + 2 C Si(g) + 2CO(g)

At the interface

− CO C + O

− Reoxidation oxidation Al

Na, K, SiO,Si

− Formation of: o a high silica viscous slag

of composition close to 6 SiO2-Al2O3-Na2O

o Al2O3

O2 pick up Transfert of O2 and oxidation of carbon

C + ½ O2 CO(g)

At the interface

CO(g) C + O

3/2O2 +2 Al Al2O3

The alteration of the refractorymicrostructure can be explained bydissolution-precipitation processes in-side a liquid phase. Several mineral layerscan be observed. The texture of the layers, aswell as the shape and habit of the crystals,clearly indicates that they are precipitatedslowly from a liquid phase and do not resultfrom a fast crystallisation during cooling.Corundum from the first layer showswell-formed crystals that differ clearly fromthose of the transformed bauxite. The CA6layer is still present, but in the case of animportant corrosion of the refractory, whenthe remaining slag composition shows asignificant change: the CA2 layer is absent.

2.1.2 Dissociation-volatilization

Under normal pressure conditions and inthe range of steel treatment temperatures,the oxides composing the refractories cannot dissociate. This type of reaction can oc-cur when vacuum degassing occurs (RH,RH/OB, DH, . . . ).

For example, let us consider thechromium volatilisation of the magnesite-chrome lining in RH/OB vacuum de-gassers [8].

While dissociation or volatilization ofchromium oxides are of no consequencewhen used at atmospheric pressure, it doesbecome a problem under even a vacuum of10−3 atmosphere.

The observations (see Fig. 6) have clearlyshown different factors of corrosion, namely:– the presence of iron oxides and attack by

slag;– the influence of vacuum and atmosphere;– the impregnation-spalling process.

The microstructure of a used brick (Fig. 7) il-lustrates the process of destruction. The mi-crograph shows a shiny corroded area with athickness of 1–2 mm. The periclase grains aresaturated with iron oxide, associated with aswelling effect with dislocations and the for-mation of compounds with a melting pointlower than the treatment temperature. Therefractory, being infiltrated, is worn out by adensification-spalling mechanism.

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Fig. 6. Corrosion by iron oxides of a bottom in a vacuum degasser (RH/OB) and structural spallingof a brick after use.

100 µm

Hot face

Slag

Fig. 7. Microstructure of the hot face of a magnesia-chrome refractory after use in a RH/OB vacuumdegasser.

By comparing X-ray diffraction patternsobtained on samples taken respectively fromnear the hot face, behind the used bricks, andfrom new bricks, the following evolution isalso shown:

– An evolution of the spinel compositionbetween a solid solution of the (Mg Fe)(Cr Al)2O4 type for the new brick and asolution of the type Mg (Al Fe)2O4 for anarea located near the corrosion front (seeFig. 8).

– This evolution results in a net disappear-ance of the chromium.

The chromium oxide volatilisation is a com-plex phenomenon:

– the Cr2O3 oxide dissociates with forma-tion of several possible oxide species;

– it depends upon the nature of the gaseousatmosphere;

– the partial pressure of oxygen plays animportant role; volatility increases withan increase in P(O2); this is due to the fact

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At 1mm from the hot face

Cold face of the brick

Fig. 8. Evolution of the X-ray diffraction patterns of magne-site chrome refractories after use in a RH/OB vacuum degasser.The spinels of general formulation A2+B3+

2O2−

4crystallise in the

cubic crystal system, with the oxide anions arranged in a cubicclose-packed lattice and the cations A and B occupying someor all of the octahedral and tetrahedral sites in the lattice. At1 mm from the hot face of the refractory, the XRD powder pat-tern can be indexed with Mg(AlFe)2O4 spinel. At the cold faceof the brick, the XRD pattern is clearly different. While a goodmatch is obtained with the Mg(AlFe)2O4 spinel at 1 mm fromthe hot face, a better indexation of the pattern can be achievedby adjusting the cubic lattice parameters of the spinel phase.

that CrO3 and CrO2, as gaseous species,are richer in oxygen than Cr2O3.

Under such conditions, the chromium oxidecan decompose and release pure chromium.

2.1.3 Oxido-reduction reactions(between an oxide and a metallicelement)

These metal-refractory reactions promotethe reduction and the progressive disappear-

ance of the refractory oxides. A re-oxidationof the liquid steel occurs which gives rise tothe formation of solid inclusions.

– Al2O3, MgO and CaO are stable whenfaced with Mn, Al, Si and C, while this isnot true for Al2O3 and MgO with regardto Ca. A calcium treated steel can, there-fore, attack an aluminous lining, or reactpartially with a magnesia or dolomiticrefractory;

– SiO2 and Cr2O3 turn out to be especiallyreactive with all the elements found insteel. In particular, an aluminium deox-idized steel can react with a refractorycontaining silica or chromium oxide.

For example, consider the reduction of thesilica by the dissolved manganese:

2Mn + SiO2 → 2MnO + Si

This reaction develops rapidly and is notcurbed by the growth of a protective layerbecause the MnO formed combines withthe silica of the lining to form low fusionpoint phases (1250 ◦C) which are quicklyeliminated from the refractory. Corrosion re-actions can develop easily, and except forhigh silicon steels, the inclusions formedare always liquid. In fact, their composi-tion obtained by microanalysis is generallyvery near that of the rhodonite MnSiO3 (seeFig. 9).

2.1.4 Carbo-reduction

At high temperature, a great number of re-dox or carbo-reduction reactions may occursimultaneously, resulting from the reduc-tion as:

– secondary phases present in the refracto-ries;

– carbonaceous raw materials (graphite,amorphous carbon);

– metallic elements.

As an example, consider the possible carbo-reduction reactions in the case of magnesiacarbon refractories in contact with slag insteel ladles.

Magnesia can be reduced to form mag-nesium as a gas and CO. At equilibrium andat 1600 ◦C, the partial pressure of magne-sium is as high as 1 × 10−2 at. But several

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Fire clay refractory

Silicate of manganese crystals

Fig. 9. Reduction of silica from clay refractory by manganese dissolved in steel; formation ofsilicate-manganese crystals (whose composition is close to MnSiO3) at the interface clay refrac-tory/steel.

other reduction reactions can occur simul-taneously, resulting from the reduction ofother oxides to be found in the brick, or fromthe reduction of the periclase grains by themetallic additions or the carbides which mayform, when antioxidants are used. All thesereactions lead to the formation of gaseousspecies, either Mg(g), SiO(g) or Si(g), whichmay diffuse to the exterior of the brick.

Near the hot face, where more oxidisingconditions prevail (PO2 = 10−8 at), the pre-vious gaseous species Mg, SiO and/or Si re-act with either CO2 or O2to revert back toan oxide form. This leads to the well-knownphenomenon of magnesia transport to com-pensate for carbon oxidation on the hot face.

MgO is reduced within the brick. BothMg(g) and CO diffuse away towards the hotface; in the decarburised zone resulting fromthe oxidation of carbon at 1600 ◦C whenPO2 is >10−16 at, the Mg gas can recombineto form condensed MgO (Fig. 10). Such nettransfer of MgO has seldom been observedunder practical steel making conditions

2.2 Metallurgical consequences

Potential consequences of steel ladles refrac-tory behaviour with respect to liquid metal

and/or slags are examined using a few met-allurgical examples concerning:

– control of oxide cleanliness;– steel desulphurisation;– Ca treatments of alumina de-oxidation

inclusions;– elaboration of ultra low carbon steels.

2.2.1 Control of oxide cleanliness

Oxide cleanliness is measured by the to-tal mass of oxide inclusions that can beformed in the liquid steel. It is determinedby thermodynamics, the treatment tempera-ture and time.

Clean steel elaboration is dependentupon the four fundamental steps necessaryto remove oxide inclusions from steel:

– generation of the inclusion;– transport of the inclusion to the slag/steel

interface;– separation of the inclusion to the inter-

face;– removal of the inclusion from the inter-

face.

Aluminium (or silicon) additions to steelhave been used to transform soluble oxygen

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100 µm _______

Slag

MgO dense layer

MgO-C refractory

Fe particles

Fig. 10. Carbo-reduction of magnesia in a MgO-C refractory at high temperature; microstructure ofa MgO dense layer at the hot face of a magnesia-carbon (steel ladle slag line).

into alumina (or silica). Thus after alu-minium addition at a ladle metallurgy fa-cility, total oxygen minus soluble oxygen isa measure of the mass of alumina in the steeland total oxygen levels are used as a measureof steel cleanliness.

Today, total oxygen contents less than20 ppm are currently obtained for alu-minium killed steels and even in the case ofspecialty steels with medium or high carboncontents (bearing steels, for example), forwhich utmost precautions are taken to reachlevels lower than 5 ppm in the product [9].

However, various mechanical propertiescan be affected by the presence, even invery small numbers, of exogenous, generallylarge inclusions resulting, in particular, fromthe mechanical or chemical deterioration ofrefractory products (erosion of particles af-ter dissolution by the steel of phases serv-ing as a binder). As corrosion and erosionproblems are time and temperature depen-dent, these degradations often occur duringextended ladle treatment times and long se-quence casting. They also tends to increasewith certain steel grades (such as high man-ganese or Ca treated steels).

The choice of refractory products formetal cleanness must also take into accountthe metal-slag-refractory reactions whichmay occur during elaboration. It is in-deed not rare to find often undesirable el-ements in the liquid steel or in the inclu-sions coming from impurities or even madeof refractory products: Ti from bauxite, Crfrom magnesia-chrome, C from alumina-graphite. The calculation models make itpossible to determine potential transferof elements between refractories, slag andmetal [10]. They can be used as a guide to se-lect refractory product qualities best adaptedto the elaboration of a given grade. It shouldbe recalled that the mineralogical composi-tion of the products used has an influenceon the kinetics of these reactions. Solutionsshould be based upon developing highly sta-ble refractories for given steel grades.

2.2.2 Steel desulphurisation

Desulphurisation of liquid steel is obtainedby metal-slag stirring in secondary met-allurgy. The lime in the ladle slag reacts

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Fig. 11. Sulfur partition coefficient at equilibrium between liquid slags of the CaO-Al2O3-SiO2-MgO system and steel with a(Al) = 0.03, at 1625 ◦C.

with the sulphur dissolved in the steel andwith a deoxidation element, forming cal-cium sulphide and the oxide of the reductioncompound.

CaO + S = CaS +O and O + Xi = XiO

→ CaO + S + Xi = CaS + XiO

For aluminium killed steels, rates of desul-phurisation higher than 90% (with final sul-phur contents less than 10 ppm and even5 ppm) can be obtained. Desulphurisationrequires working with a liquid slag, closeto lime saturation, and at low contents inoxides easily reducible by the aluminiumfrom the metal. This is shown by the par-tition coefficient curves between slag andliquid metal in Figure 11 [11]. To obtain re-producible results in industrial conditions,it is, consequently, essential to control wellthe slag composition. Now, considering thenarrowness of the composition domain offavourable slag, any refractory dissolutionadding silica or alumina will cause the prop-erties of desulphurisation of slag to deteri-orate (Fig. 11). The behaviour of ladle re-fractories to slag corrosion was the objectof extensive research [12]. The oxygen po-tential of the refractory products must be aslow as possible. In laboratory tests, dolomiteexhibits the best results, magnesite is some-what poorer and magnesite/chromium aresignificantly worse. The results are degraded

when bauxite refractories with a high oxy-gen potential are used. In industrial condi-tions, advanced desulphurisation can onlybe reached reliably and reproducibly in la-dles with a basic lining. A deterioration ofdesulphurisation results may also be dueto a bad control of the physical quality ofslag (partially solid slag over-saturated inlime, for example). In a study on vacuumtreatments in dolomite ladles, Bergmannet al. [13] have shown (Fig. 12) that the op-timal slag compositions, concerning desul-phurisation rates and the wear of the mag-nesia refractories, corresponded to a narrowdomain of composition located around limesaturation.

2.2.3 Ca treatments of aluminadeoxidation inclusions

One of the goals of these treatments, on alu-minium killed steels, is to improve the casta-bility of these grades by transforming thealumina deoxidation inclusions into liquidlime aluminate inclusions. These liquid in-clusions, contrary to alumina, do not stick tothe nozzle refractories, which they even tendto dissolve when they are too rich in lime.

During treatment, calcium, having ahigher affinity for oxygen than most metallicelements used in iron and steel making, canreduce, at least partially, some constituents

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Fig. 12. Effect of degree of lime saturation of the slag on desulphurisation and refractory wear [13].The two lines for each parameter (MgO content and desulphurization index) represent the dis-persion zone of the measurements. The orange coloured region represent the theoretical target(optimal slag for desulphurisation and magnesia refractories). Lime saturation index lower 1 cor-respond to liquid slags. In practise, beyond lime saturation (index >1) solids are in suspension inthe slag. Reproducible chemical and physical behaviour of slag cannot be expected.

of the refractories (SiO2, Cr2O3, Al2O3,. . . ).A notable improvement in the efficiency ofa calcium addition was, for example, notedwhen high alumina ladle refractories werereplaced by dolomite or magnesia refracto-ries, more stable with respect to alkaline-earth elements. This transformation made itpossible to increase drastically the percent-age of ladles cast in billets without cloggingof the calibrated nozzle [14].

However, even with the use of basic re-fractories, it must not be forgotten that anoxide such as magnesia is, from a thermo-dynamic point of view, less stable than limeand can be reduced by calcium, which leadsto a transfer of magnesia towards the inclu-sions whose MgO content increases at theexpense of Al2O3.

As an example, Figure 13 shows the aver-age composition of inclusions obtained fol-lowing too large an addition of SiCa to steelin a dolomite ladle. These inclusions, havea final composition of 55% MgO-35% CaO-10% Al2O3 after following the path shown onthe figure during treatment. They are solidat casting temperature (Tliq > 2400 ◦C) and,like most solid inclusions, may stick to therefractory walls and especially participate innozzle clogging.

The reliability of calcium treatment thusrequires not only an optimisation of addedquantities, but also an adequate selection ofthe refractory in contact with the metal.

2.2.4 Elaboration of ultra-lowcarbon steels

Ultra-low carbon steels, such as interstitialfree steels (IFS), require a high oxygen con-tent during decarburization (CO degassing)and the slag line of the steel ladle has longlasting contacts with iron oxide rich slag.

Carbon pick up strongly varies with thecomposition of ladle slag after deoxidation(Fig. 14).

The presence, in a limited amount, ofthese iron oxides in the slag can have a ben-eficial effect on the corrosion of magnesiacarbon bricks used in the ladles. Indeed, incontact with FeO, a protective MgO denselayer [16, 17] can be formed on the hot faceof the MgO-C refractories.

Inside the magnesia carbon brick, the fol-lowing reaction occurs:

MgO(s) + C(s)→Mg(g) + CO(g)

At high temperature, magnesia is reducedby the carbon to form Mg. Mg vapor is

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Fig. 13. Formation of inclusions in Al killed steels created by reaction of the dolomitic liningwith calcium addition in excess [15]. The reduction by calcium lead to refractory inclusions. Aftercomplete solidification at equilibrium, compositions marked as red points, correspond to a mixtureCaO, 3CaO.Al2O3, and MgO.

0

2

4

6

8

10

12

14

16

0 2 4 6

[Fe] (%) in slag

Car

bon

pic

k up

(p

pm)

in

stee

l ( a

fter

kil

led

wit

h A

l)

At the interface , condensation with slag Mg(g) =FeO MgO + Fe

Oxido reduction and vaporisation of magnesia in MgO-C refractory

Mg

0.2 MgO

Fig. 14. Relationship between carbon pick up and iron content in slag for a ultra low carbon steel(killed aluminium) [6].

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transported to the hot face where it is oxi-dized to a secondary MgO dense layer byreduction of iron oxides and precipitation ofiron [6].

Mg (g) + FeO (l)→MgO (s) + Fe (s)

A careful control of service conditions suchas the level of iron oxidation (FeO, Fe2O3)and the composition of the slag is requiredto trigger the formation of this secondaryMgO dense layer [2, 17, 18].

3 Interactions of refractorymaterials and steel duringcontinuous casting

The role of shaped refractory parts used incontinuous steel casting is to guide and pro-tect liquid metal from the ladle to the mouldwhere steel is solidified.

In continuous casting, it may be consid-ered that the most relevant refractory partsand products in the problems of metal clean-liness are:

– First, the submerged nozzle materialswith their direct and indirect role in clog-ging and unclogging, leading to metalcontamination by alumina particles orclusters.

– Then, the tundish lining which can havea purifying or polluting action.

– Finally, the ladle shroud tube where re-actions similar to the ones met in sub-merged nozzles can take place and thewhole sliding gate system where the stateof the plates after service indicates pollu-tion risks.

3.1 Submerged nozzles

Submerged nozzles are, for the most part,alumina-graphite products. Clogging ofsubmerged nozzles by alumina build-up(Fig. 15) constitutes one of the major sourcesof dysfunction of aluminum-killed steel con-tinuous casting [19]. This detrimental build-up degrades the quality of the steel pro-duced, reduces the casting sequences, andthus limits the productivity of the steelmaker. Although this phenomenon has beenstudied for the last twenty years, it is not

Fig. 15. Alumina build up clogging in a sub-merged nozzle.

very well understood yet. Build-up is knownto be affected by parameters such as steelgrade, steel cleanliness, flow conditions inthe casting channel, beat flow control, refrac-tory composition, and air leakage, but corre-lation to an exact cause is illusive. Severalmechanisms of build-up are mentioned inthe literature. They include: oxide precipita-tion and deposition on the nozzle bore due toa high thermal conductivity of the refractory,non uniform fluid flow within the nozzle re-sulting in dead spots of liquid steel, chemi-cal wetting of the liner by the steel facilitat-ing oxide deposition, air leakage through therefractory oxidizing aluminium-killed steel,refractory surface roughness enhancing ox-ide deposition, and the redox reactions sup-plying oxygen for dissolved aluminum ox-idation and deposition. Although build-upmay be affected by one or several of thesemechanisms simultaneously, this paper fo-cuses on the influence of the refractory com-position on buildup.

3.1.1 Clogging mechanism [20]

The mechanism described herein focuses onthe deposition of Al2O3 as a result of the

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thermo-chemical reduction of nozzle con-stituents coupled with the oxidation of thealuminium in the steel. In this scenario, thedeposit builds up in the three followingsteps:

– dissolution of the carbon of the refractoryinto the steel;

– build-up of a first layer of deposit madeup of Al2O3 and a vitreous phase byvolatilization and oxidation reactions;

– oxidation of aluminum by carbonmonoxide (CO).

3.1.2 Carbon dissolution

Superficial carbon dissolution from the re-fractory, occurring at the very beginning ofthe casting, results in a localized modifica-tion of the refractory/steel interface. Here,the activity of the aluminium in the steel in-creases as carbon activity increases. Over-all, the steel chemistry is such that dissolvedoxygen in the steel O is in equilibrium withthe dissolved aluminium Al. A localized in-crease in aluminium activity leads to the pre-cipitation of alumina, forming a fledglinglayer of alumina on the refractory surfacevia:

2Al + 3O→ Al2O3(s)

It is the decarburization of the refractory bythe steel which triggers this reaction.

3.1.3 Build- up of a first layer ofdeposit by volatilisation andoxidation reactions

After the carbon dissolution, a layer com-posed predominantly of alumina particlesand, to a lesser extent, a vitreous phase con-sisting of alumina, silica and alkalis is ob-served on the refractory. The origin of thesespecies forming this vitreous phase is be-lieved to be the refractory. Alumina graphiterefractories contain secondary phases andimpurities such as SiO2, Na2O, K2O whichcan be reduced by the refractory carbonat steel-making temperatures, and generategaseous species by the following reactions:

SiO2(s) + C(s)→ SiO(g) + CO(g)

Na2O(s) + C(s)→ 2Na(g) + CO(g)

K2O(s) + C(s)→ 2K(g) + CO(g)

With the flow of molten steel inside thenozzle, a negative pressure is present fromthe outside inward. This “vacuum” tendsto drive these gases from the refractory tothe molten metal. At the steel/refractory in-terface, the partial oxygen pressure is inthe order of 10−13 at. This oxygen poten-tial is enough to re-oxidize and condensethe gaseous species into a low melting pointphase. This phase, at the operating tempera-ture, dissolves either refractory or steel alu-mina to form a very viscous liquid that mayconstitute a sort of glue, whose compositionis close to that of albite (Na2O-Al2O3-6SiO2):

2Na(g) + 6SiO(g) +Al2O3(refractory) + 7O

→ Na2O-Al2O3-6SiO2

2Na(g) + 6SiO(g) + 2Al + 10 O

→ Na2O-A12O3-6SiO2

Here again, it is the carbon in the refractorywhich initiates the gas transfer of refractorysub-oxides, leading to the build-up on thenozzle wall. On the other hand, the mag-nitude of this mechanism’s contribution toalumina build-up is unclear.

3.1.4 Alumina formation throughoxidation of the aluminiumby carbon monoxide

The predominant amount of alumina de-posit occurs outside the thin layers describedabove. Its thickness tends to vary from a fewmillimetres to a few centimetres, and it isphysically an heterogeneous composite ofalumina and metallic nodules at room tem-perature. The particles of alumina take ona plate-like shape and their size does notexceed 20 µm (Fig. 16). The source of thisdeposition is suggested to be oxidation ofaluminium in the steel by oxygen due to therefractory (i.e. air permeation, redox equilib-rium,. . . ).

The sequence of relationships is thus:

CO(refractory) → C+O

↓2Al + 3O→ Al2O3

According to this reaction, if the refrac-tory imposes a partial pressure of CO (PCO)greater than the one already in equilibrium

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Fig. 16. Scanning electron microscopy microstructure of a alumina deposit in a submergednozzle [20].

in the steel, then the reaction will proceedto the right and alumina will precipitate.The carbon content of the steel grade playsan important role in the decomposition ofthe refractory CO(g) and thus the forma-tion of alumina. It is well known that alu-mina build-up occurs predominantly withlow-carbon steel grades. This is because theCO(g) has an opportunity to be dissolvedinto the steel, providing oxygen to the alu-minium. Also illustrated by this reaction isthe increase in the carbon content in the steelC, resulting in the localized increased activ-ity of aluminium, further driving this reac-tion to the right. Once again, it is the carbonin the refractory which triggers the forma-tion of alumina.

It becomes clear that the carbon in therefractory may be responsible for the depo-sition of alumina at the interface between thesteel and the refractory. The carbon acts:

– to increase the aluminium activity;– as a redox agent that carries the oxygen

from the refractory to the steel and inall cases it leads to the precipitation ofalumina causing build-up. Under theseconditions, removal of the carbon fromthe refractory should eliminate some ofthese alumina deposition mechanisms.

The clogging mechanism involves the fol-lowing consequences:

– clogging takes place by in situ nucleationof alumina from the oxidation of alu-

minium dissolved in the steel at the in-terface between the steel and the aluminagraphite refractory. As a result, even ifthe steel is perfectly clean, clogging willstill occur, suggesting that steel born in-clusions are not the primary source ofblockage;

– the alumina build up is caused by thegaseous transfer through the refractorysidewall. The permeability of the refrac-tory and the air tightness of the assemblytherefore play an essential part;

– the clogging phenomenon will be greaterif the content of impurities and sec-ondary phases (silica, alkalis) in the rawmaterial from the refractory is higher.Improvements can be sought by usinghighly pure A12O3-C mixtures, with aslittle silica and alkaline impurities as pos-sible;

– carbon from the refractory is an increas-ing factor for the clogging mechanism.This questions the current use of carbonrefractories in continuous casting andjustifies a change to carbon-free refrac-tory with little permeability and as inertas possible for the steel.

3.1.5 Carbon-free refractories [21,22]

The absence of refractory carbon duringsteel casting would be beneficial to preventalumina build-up. Several approaches have

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been made to produce such materials. Theproperties of such refractories were targetedas follows:

– not permeable to gaseous exchange;– chemically inert with steel;– thermal shock resistant;– mechanically resistant to steel flow.

Among the countermeasures, carbon-freeinside liner nozzles and annular step nozzleshave been developed with different tech-nologies by suppliers and come to be widelyused by many customers. The carbon-freematerials do not supply SiO(g) and CO(g)which react with any dissolved Al in themolten steel to form network alumina. Inaddition, their stable surface condition dueto the lack of decarburized refractory carbonis advantageous in retaining surface flatnessand wettability.

3.2 Tundish lining [23,24]

The tundish refractory is usually made ofmagnesia and forsterite (2MgO-SiO2) rawmaterials. Its lining thus may easily reactwith it, if the conditions, especially compo-sition, allow it. This reaction will be madeeasier by the great porosity, thus the activesurface, of the lining. These reactions can bepositive (purifying role) or negative (pollut-ing role): as a source of oxygen and silicon.

Due to the high potential of some oxidesin the tundish lining, especially the silica andthe iron oxides, reduction according to thefollowing reactions are possible with oxygenpick up by the steel.

3(SiO2)refract. + 4[Al]steel → 3[Si]steel

+ 2(Al2O3)inclusion

3(FeO)refract. + 2[Al]steel → 3[Fe]steel

+ (Al2O3)inclusion

Laboratory tests [25–27], had shown a re-lationship between the FeO content of thetundish refractory and the oxygen pick upby steel (see Fig. 17).

Plant trials as well as the laboratory ex-periments [23] demonstrate also a chemi-cal transformation of the forsterite into the

% FeQ

uant

ity

of o

xyge

n (g

)

0

0,2

0,4

0,6

0,8

1

0 2 4 6 8

Preheating at 1200°C

Preheatingat 180°C

% FeQ

uant

ity

of o

xyge

n (g

)

0

0,2

0,4

0,6

0,8

1

0 2 4 6 8

Preheating at 1200°C

Preheatingat 180°C

O

Fig. 17. Relationship between oxygen (caught byaluminium) and the FeO content of the tundishrefractory (laboratory trials) [25, 27].

MgO-Al2O3 spinel according the reaction:

3(2MgO-SiO2)refract. + 4[Al]steel

→ 2(MgO-Al2O3)refract. + 4(MgO)refract.

+ 3[Si]steel

At the interface steel/refractory lining, alayer composed of MgO-Al2O3 spinel is ob-served (Fig. 18). No oxides are formed in thesteel or migrate to the steel due to this min-eralogical transformation, which has no in-fluence on steel cleanliness. Only a change ofdensity of the lining resulting from the spinelformation could be observed. Spalling due tothe different properties between the spinellayer and the MgO-forsterite refractory lin-ing can lead to MgO-Al2O3 inclusions in thesteel.

Potential hydrogen sources are alsopresent in the tundish during casting. Sub-stantial diffusion of water occurs when ba-sic refractory tundish spray linings are used.Complete expulsion of the moisture can-not always be guaranteed even when thetundish is well pre-heated.

Figure 19 shows the evolution of the hy-drogen content in steel during a sequence ofthree ladles. The initial hydrogen contentswere 1.5 ppm. The hydrogen contents mea-sured in the tundish using Heraeus Electro-Nite technique, indicates hydrogen pick upduring casting, particularly in the first heatof the sequence. In this context, to limit hy-drogen pick up in the steel, it is important toimprove the refractory composition and thepreheating procedures of the tundish.

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Magnesia – olivine based refractory

MgO-Al2O3 spinels

Fig. 18. Observation of spinel crystals at thesteel/tundish lining-laboratory trials (SEM mi-crograph).

3.3 Protection between ladleand tundish

3.3.1 The ladle shroud [28]

Most of the time, this ladle shroud is made ofalumina-graphite. The same reactions as inthe submerged nozzle can take place. How-ever, a few specificities may be noted:

– it is often reused;– it is connected by a collecting nozzle to

the ladle closing system using a highspeed connection system. Consequently,manipulations can lead to a deteriora-tion of its outside enamel and can in-crease its permeability. The connection isnot always perfect and deteriorates dur-ing successive uses, favouring air intake.Consequences will be accelerated wearin the materials and re-oxidation of themetal, and thus its pollution.

3.3.2 Sliding gate system [29]

The sliding gate system for steel ladles con-sists of a mechanical assembly containingthe refractory plates. Plates are differentshapes: rectangular or circular.

The basic function of the sliding gate sys-tem is the control of metal flow-rate duringteeming. This requires:

– reliable and robust mechanics;– operating regularity;– easy dismounting;– easy replacement of worn parts;– fast, easy upkeep.

The systems were then basically defined bymechanical engineers.

The second function is ensuring qualityof the metal.

The plates of the sliding gate systemare subjected to severe thermo-mechanicalstresses which systematically lead to thecracking of the refractory in service. Suchcracks are the cause of air leakage throughthe plates with adverse effects on the clean-liness of the steel and the wear of the refrac-tory by corrosion.

Taking into consideration the complex-ity of thermo-mechanical conditions insideslide gate systems, especially inside refrac-tory parts, is difficult and evolutions areslow. Today the design of most of the existingslide gates only takes little or no account ofsteel quality exigency, even if improvementshave been made, generally based on certainempiricism. Radial and lengthways cracks,due to a concentration of stresses near thehole and a non-symmetry of the design af-fect the behaviour of plates (Fig. 20a). In con-sequence, relatively badly controlled plateswear is encountered due to variable air in-take and an acceleration of wear betweenfirst and last heat because of the deteriora-tion of refractory permeability. This deterio-ration has repercussions on metal cleanliness(oxygen pick up, inclusions).

In an optimised slide gate design(Fig. 20b), it would be better to take intoaccount the thermo- mechanical stresses, towhich the parts will be subjected, to de-fine the refractory plates, their frame andtheir geometry in order to reduce and evensuppress their in-operation cracking whichmay lead to re-oxidation of the metal by airintake.

4 Conclusion

The secondary steel making and casting isthe key to the production of clean steels, with

410-page 18


0 0,5

1 1,5

2 2,5

3 3,5

4

0 1 2 3 4 Number of casting during a sequence

Hydrogen [ppm]

Fig. 19. Measurement of the hydrogen content in steel during a sequence of 3 ladles.

(a)

(b)

Fig. 20. Design of two plates of sliding gate sys-tem [29]. (a) Cracks in a slide gate→ air leakage.(b) Optimised design→ no.

a low content of residuals P,C,O,S, . . . and alow frequency of inclusions.

In this context, refractory products arenot only strategic for the production of steel,but they also have a direct influence on thequality of elaborate grades. The future evo-

lutions of the refractory products, in thisfield, will depend on such considerations. Agood knowledge of the metal-slag-refractoryproduct reactions is, consequently, neces-sary in order to better control steel making.Among all the points raised, we could men-tion all the developments that will limit thecontribution of refractory products to clog-ging and carbon pick up, in conjunction withefforts of the metallurgists to produce cleansteels.

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[15] Slag Atlas, 2nd edition, edited by VDEh,Verlag Stahleisen GmbH, 1995

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[21] O. Fumihiko, Y. Noriaki, Y. Kazuhiro, T.Shigeaki, The evaluation of anti-cloggingmaterials for submerged entry nozzles, inProceedings of UNITECR’2005, Orlando,USA, 2005, pp. 776-780

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[24] L. Dong-ha, L. Je-ha, C. Yong-Moon, U.Chang-Jung, Refinable lining material forclean steel in tundish, in Proceedins ofthe UNITECR’2007, Berlin, Germany, 2007,pp. 354-357

[25] J. Lehmann, M. Boher, H. Gaye, M.C. Kaerle,An experimental study of the interactionsbetween liquid steel and a MgO-basedtundish refractory, 2nd Inter. Symp. on ad-vances in refractories for the metallurgy in-dustry, Montréal, Québec, 1996, pp. 151-165

[26] M. Boher, J. Lehmann, C. Gatellier, M.C.Kaerle, Transfert de Mg des réfractairesde répartiteur vers un acier désoxydé Si-Mn, Les propriétés d’usage des réfrac-taires, Journées de rencontres industries-universités, Nancy, 1999

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