30 49. Internationales Feuerfest-Kolloquium 2006

ANALYSIS OF THE WEAR MECHANISM OF MgO-C SLAG LINE BRICKS FOR STEEL LADLES

Silvia Camelli*, Instituto Argentino de Siderurgia, San Nicolas, ArgentinaMarcelo Labadie, Siderar, San Nicolas, Argentina

ABSTRACTThe main wear mechanisms of the refractory ladle slag

line are chemical corrosion and mechanical erosion due to stirring of the steel bath. The chemical potential difference between the refractory and the slag under high temperature conditions is the driving force for the chemical wear mechanism.

The objective of this work was to analyse the wear mechanism of MgO-C brick at the steel ladle slag line.

A post mortem study was performed through optical and electronic microscopy (SEM) and EDS analysis. By using these techniques glaze characteristics and the slag-refractory interaction were determined. Also,microstructural changes in the bricks were evaluated.

The identified attack mechanisms were: graphiteoxidation, slag penetration into the matrix and around the MgO grains, magnesia-wüstite formation, loosening ofMgO grains and finally dissolution of MgO grains into the slag.

INTRODUCTIONThe main wear mechanisms of the refractory ladle slag line are chemical corrosion and mechanical erosion due to stirring of the steel bath [1]. The corrosion process is a function of many variables including temperature,refractory composition, slag viscosity, slag compositionand degree of agitation.

The working lining in Siderar ladles is composed of two different types of bricks: MgO-C in the slag line and Al2O3-MgO-C in the walls and bottom. The average lifespan of brick slag line is 95 heats.

The objective of this work was to analyse the wear mechanism of MgO-C brick at the steel ladle slag line.

A post mortem study was performed through optical and electronic microscopy (SEM) and EDS analysis. By using these techniques glaze characteristics and the slag-refractory interaction were determined. Also,microstructural changes inside the brick were evaluated.

EXPERIMENTALThe properties of slag line ladle MgO-C bricks as

received are presented in table 1 [2].The post mortem aspects of the MgO-C brick together

with the analyzed sample for microstructural study areshown in figure 1.

The post mortem study was carried out by optical and electronic microscopy (SEM) and EDS analysis. Thisstudy allowed us to determine the slag characteristics on the hot face brick, the slag and the refractory interaction and brick microstructure changes.

SlagThe slag thickness varies between 4 and 6 mm (figure

2). The slag layer shows pores, steel drops and two different layers. These layers contain distinctive

microstructural aspects and phases of different chemical composition.-Layer I, with a thickness of 2,2 mm, is formed bymagnesium oxide dendrite crystal, iron and calciumaluminate needles and calcium aluminium silicate crystals. All of these are immersed in a calcium aluminate matrix (figure 3).-Layer II. Its thickness is 3 mm and is composed ofmagnesium altered crystals by iron and manganese,different content of calcium, iron and titaniumaluminosilicate crystals, calcium-magnesium-ironaluminates and calcium silicate crystals. These phases are immersed in a calcium aluminate matrix (figure 4).

Tab. 1 Properties of MgO-C bricks used in Siderar slag line ladleApparent density (g/cm3) 2.93 ± 0.01Apparent porosity (%) 4.8 ± 0.1Apparent density after heating at 1150°C in reduction atmosphere (g/cm3)

2.86 ± 0.05

Apparent porosity after heating at 1150°C in reduction atmosphere (%)

11.1 ± 0.1

Decarburized thickness (mm) 4Cold crushing strength (MPa) 28 ± 4

Periclase (MgO) �

Graphite (C) �Crystalline phases Aluminium (Al) �

Fig.1. Post mortem MgO-C slag line brick used in steel ladle.

Fig.2. Slag layer and MgO-C brick interface.

49. Internationales Feuerfest-Kolloquium 2006 31

Mg

Al Ca

Fig.3. Elements distribution in slag layer I.

Mg

Fe Ca

Fig. 4. Elements distribution in slag layer II.

Sequential EDS analysis of the different elements in two slag layers is presented in figure 5.

0

5

1 0

1 5

2 0

2 5

3 0

3 5

4 0

4 5

5 0

0 1 2 3 4 5 6

Distance from brick hot face (mm)

Co

mp

on

ents

(%)

MgOAl2O3

SiO2CaOFeO

MnO

Fig. 5. Elements evolution in the both slag layers.

Slag – MgO grains interactionNear the interface between slag and MgO-C brick,

MgO grains loosening from the brick and immersed in slag were identified. Elements distribution in this area is shown in figure 6. In that area it is possible to observe thecorrosion of the grains due to their interaction with iron oxide and manganese oxide. The slag penetrates into the intragranular silicate bond of the sintered and fused MgO

grains. This interaction promotes low temperature phases formation and facilitates the crystal loosening.

Mg

Fe Ca

Fig.6. MgO grains indirect corrosion.

Slag - MgO-C brick matrixThe microstructural analysis permitted observed the

decarburized MgO-C brick matrix in contact with slag (figure 7). The interface between the slag and the brick is shown in figure 8. Spinel crystals were formed in theMgO-C brick matrix. Also, calcium silicate crystals were identified in the brick matrix.

Fig. 7. Decarburized matrix and slag interface

Al Mg

Fig. 8. Elements distribution in the MgO-C brick hot face.

DISCUSIÓNThe first slag layer is composed of a calcium aluminate

matrix with 1400 °C melting point (phase equilibrium diagram for Al2O3 – CaO –FeO system) and different crystals:

32 49. Internationales Feuerfest-Kolloquium 2006

* iron and calcium aluminate, with softening temperature of 1336°C (phase equilibrium diagram for Al2O3 – CaO –FeO system)* calcium and aluminum silicate (gehlenite), withsoftening temperature of 1380°C (phase equilibriumdiagram for Al2O3 – CaO –SiO2 sytem).* dendrite MgO

Chemical composition of the first slag layer ispresented in figure 8. This diagram shows that the slag is completely liquid at 1600°C, and therefore it is notsaturated with periclase.

In table 2, slag ladle furnace chemical composition is presented. This slag sample was taken previous to the end of the ladle campaign. Such slag, like in the first slag layer on MgO-C brick, is liquid and it is not saturated with MgO at 1600°C.

Fig. 8. Phase equilibrium diagram for Al2O3 – CaO –SiO2

10%MgO system [3].

Tab. 2. One slag furnace ladle chemical compositionSiO2

(%)MnO(%)

FeO(%)

MgO(%)

CaO(%)

Al2O3

(%)LF slag 4,5 1,1 1,0 9,3 54,0 32,7

The second slag layer adhered to MgO-C brickcontains a higher level of iron oxide than the first slag layer. Also, the second slag layer has lower calcium oxide content than the slag layer as is shown in figure 5.

The second slag layer is composed of calciumaluminate as a matrix and different types of crystals:*calcium, titanium, iron and aluminium silicates with different contents of these elements with 1400 – 1500 °C melting point*calcium and aluminum silicate with 1450°C melting point(phase equilibrium diagram for Al2O3 – CaO –SiO2

system)* different content of calcium, magnesium and ironaluminates*attacked MgO crystal by iron oxide in the first place and by manganese oxide and calcium oxide afterwards (figure 9). This mechanism is the result of Fe, Mn and Ca being the slag elements that diffuse more rapidly into the

refractory material and form the corresponding reaction products [4].

0

10

20

30

40

50

60

70

80

90

100

0 0,5 1 1,5 2 2,5 3 3,5 4 4,5 5 5,5

Distance from brick hot face (mm)

Co

mp

on

ents

(%)

MgOCaOFeO

MnO

Fig. 9. EDS analysis in MgO grains in slag thickness.

Slag chemical composition of the second layer ispresented in figure 10. This slag is saturated withmagnesium oxide at 1600°C.

Table 3 shows the chemical comp osition of ladle slagbefore the treatment in the ladle furnace.

Fig. 10. Phase equilibrium diagram for CaO-MgO-SiO2 -35% Al2O3 system [3].

Tab. 3. Chemical composition of slag ladle before the treatment in the ladle furnace.

SiO2

(%)MnO(%)

FeO(%)

MgO(%)

CaO(%)

Al2O3

(%)Slag 4,3 2,7 2,6 9,5 44,8 31,1

.Both slag layers identified on slag line post mortem

MgO-C brick are aggressive to the refractory material lining.* The first slag layer, corresponding to the slag of the end of the treatment in the ladle furnace (fig. 8), is not saturated with magnesium oxide and is completely liquid at the process temperature. It is widely known that these conditions are the main causes of corrosion and lining wear [5, 6].

34 49. Internationales Feuerfest-Kolloquium 2006

* The second slag layer, corresponding to the ladle slag previous to ladle furnace treatment (figure 10), is saturatedwith periclase but contains a higher level of iron oxide. This oxide increases slag fluidity – break down the slag silicate network – and promotes the brick matrixdecarburizing, as shown:

FeO + C Fe + CO (I)The graphite oxidation increases the brick porosity,

allowing slag penetration to the system, followed by the dispersion of periclase grain in the slag.

Moreover, the iron diffuses into MgO grain andpromotes magnesia – wüstite formation. This new phase decreases MgO grain refractoriness and also delays the wear speed of the refractory material. This mechanism is known as MgO grain indirect dissolution. Such passive corrosion is the typical mechanism in BOF MgO-C brick,where iron oxide in slag steelmaking is superior to 18% [7,8].

Previous research has shown that the common indirect corrosion mechanism in slag line MgO-C bricks results in spinel (Al2O3.MgO) formation at slag-refractory interface.This new phase is a barrier to aggressive agents, puttingrefractory dissolution off [9].

Different components were identified in the MgO-Cbrick: graphite fibres, fused MgO, sintered MgO and resultant products of aluminium oxidation (antioxideagent). The latter forms aluminum carbide, aluminumoxide and Al2O3.MgO spinel, according to oxygen partial pressure and temperature inside the brick [10].

CONCLUSIONSThe post mortem study was carried out through optical

and electronic microscopy (SEM) and EDS analysis,allowed identification of slag - brick adherencecharacteristics, MgO-C refractory wear mechanism and the microstructural changes inside the brick.

Slag on the brick hot face is composed of two different layers. These layers contain distinctive microstructuralaspects and different chemical composition phases. Both layers are aggressive to the refractory material lining because:*the first slag layer is not saturated with MgO and it is completely liquid at the process temperature. This slag corresponds to the slag at the end of the treatment in the furnace ladle. * the second slag layer is saturated with MgO, but contains a high level of iron oxide content. This slag corresponds to the ladle slag previous to furnace ladle treatment.

The identified attack mechanisms were: graphiteoxidation, slag penetration into the matrix and around the MgO grains, magnesia-wüstite formation, loosening ofMgO grains and finally dissolution of MgO grains into the slag.

The indirect corrosion process with magnesia-wüstiteformation is not a typical damage mechanism in ladle slagline MgO-C brick. This mechanism is a common corrosion process in BOF and electric arc furnace MgO-C brick,where slag oxidation is higher.

Al2O3. MgO spinel in the slag - MgO-C brick interface was not identified.

Magnesia-wüstite formation at MgO-C brick hot face suggests that slag during the laddle campaign contains high iron oxide levels that allowed a new phase formation.

Iron oxide sources result from BOF slag or recycled slag used in the ladle.

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