4. Impact of refractories corrosion on Industrial processes FIRE COURSE – Unitecr’2001, October 30th, 2011 Kyoto, Japan 4. Impact of refractories corrosion on Industrial processes 4.1. STEEL MAKING J. Poirier CNRS-CEMHTI, University of Orleans

4. 1 STEEL MAKING - CONTENTS OF THE PRESENTATION FIRE COURSE – Unitecr’2001, October 30th, 2011 Kyoto, Japan Introduction Part I (4.1.1) : Flow control and interactions of refractories and steel during continuous casting Protection between ladle and tundish Tundish lining Submerged nozzles Part II (4.1.2) : Corrosion, cleanliness and steel quality Reactions between refractories, steel and slag Metallurgical consequences Control of oxide cleanliness, Steel desulphuration, Ca treatments of inclusions, Elaboration of ULC steels Conclusion

Surface micrograph showing fine particles at grain boundaries INTRODUCTION Surface micrograph showing fine particles at grain boundaries

Steel-maker’s challenge To propose steel grades with : narrower composition ranges lower guaranteed contents of residuals controlled inclusion size distributions To obtain reproducible service properties TRIP 800 Introduction Steel challenge Cleanliness /chemistry Non metallic elements Impact of refractories

Chemistry and inclusion control Two main keys to the production of quality steel products Chemistry and inclusion control These results can only be reached by a strict control of process In particular, steel cleanliness and purity requirements make the selection of refractory products more and more important Introduction Steel challenge Cleanliness /chemistry Non metallic elements Impact of refractories

Influence of non metallic elements on steel properties Hydrogen Carbon Nitrogen Oxygen Control of inclusions Phosphorus Sulfur Non metallic elements Electromagnetic properties Deep drawing Weldability Toughness Internal soundness Surface defects Anisotropy Fatigue Bending Introduction Steel challenge Cleanliness /chemistry Non metallic elements Impact of refractories

Element P C S N H O ppm 10 5 <1 More and more complex elaboration to eliminate non metallic elements Vacuum treatment Desulphuration treatment  C content < 15 ppm is possible ! S content~ a few ppm  Element P C S N H O ppm 10 5 <1 Lower limits of residual elements in steel making elaboration Introduction Steel challenge Cleanliness /chemistry Non metallic elements Impact of refractories

Impact of refractories The impact of refractory products on the quality of the metal 3 aspects The possibility to keep the chemical composition of the liquid steel for a given process Introduction Steel challenge Cleanliness /chemistry Non metallic elements Impact of refractories

Impact of refractories The impact of refractory products on the quality of the metal 2. The achievement of the required metal cleanliness : the amount and the nature of non metallic inclusions Introduction Steel challenge Cleanliness /chemistry Non metallic elements Impact of refractories

3. The prevention of defects concerning the steel surface The impact of refractory products on the quality of the metal 3. The prevention of defects concerning the steel surface Introduction Steel challenge Cleanliness /chemistry Non metallic elements Impact of refractories

Main classes of refractories in relation with the quality and metal cleanliness  Secondary metallurgy : for steel ladle Fired and unfired bricks Unshaped high alumina or High alumina spinel content products Magnesia graphite Magnesia chrome Dolomite High alumina, mainly bauxite products Alumina - spinel Introduction Steel challenge Cleanliness /chemistry Non metallic elements Impact of refractories

Main classes of refractories in relation with the quality and metal cleanliness  Secondary metallurgy : for degassing devices RH/OB Magnesia-chrome and alumina unshaped products (containing or not spinel MgO-Al2O3) Introduction Steel challenge Cleanliness /chemistry Non metallic elements Impact of refractories

Main classes of refractories in relation with the quality and metal cleanliness  Tundish lining and continuous casting Steel ladle Al2O3-C  Stopper Plate  Al2O3 - C Ladle  Al2O3 - C Shroud Tundish  Sprayed magnesia Submerged nozzle  Al2O3 - C and ZrO2-C insert Introduction Steel challenge Cleanliness /chemistry Non metallic elements Impact of refractories

Interactions Materials and assembly of refractories Summary of different defect types in steel in relation with the refractory products Steel Interactions Mastery of argon injection Pollution Steel/slag/refractory Erosion of refractories Reoxydation Al2O3 clogging Thermal transfert Materials and assembly of refractories Corrosion of slag line Air leakage Inclusions and defects - exogenous inclusions - endogenous inclusions TiN, Al2O3-MgO, MnO-SiO2, Al2O3, SiO2 - splitting decohesion (inclusions + gaz) Steel purity Carbon pick up Sulphide cleanliness N and H pick up Longitudinal cracks Heterogeneity of solidification Spalling of wall Reactivity Steel refractory Al2O3 build up

INTERACTIONS OF REFRACTORIES AND STEEL DURING CONTINUOUS CASTING PART 1. (4.1.1) FLOW CONTROL INTERACTIONS OF REFRACTORIES AND STEEL DURING CONTINUOUS CASTING Sliding gate system Protection between ladle and tundish Tundish lining Submerged nozzles

The basic function : the control of metal flow rate Sliding gate system consists of a mechanical assembly containing the refractory plates The basic function : the control of metal flow rate Sliding gate Stopper Tundish lining Submerged nozzle Part 1. Continous casting

The plates of the sliding gate system Subjected to severe thermo-mechanical stress  Lead to the cracking of the refractory in use Al2O3 /SiC / C refractory Cause of air leakage with effects on the cleanliness and the wear Sliding gate Stopper Tundish lining Submerged nozzle Part 1. Continous casting

Shape of plates Length of cracks N pick up Effect of the plate cracks on the nitrogen pick up Shape of plates 2 points of blockage 3 points of blockage Length of cracks  121 mm 76 mm  N pick up  1.96 ppm 0.58 ppm Sliding gate Stopper Tundish lining Submerged nozzle Part 1. Continous casting

Design of the plates of the sliding gate system cracks in a slide gate  air leakage (b) optimised design  no crack In order to reduce cracking and to limit the re oxidation of the steel Sliding gate Stopper Tundish lining Submerged nozzle Part 1. Continous casting

The function : the control of metal flow rate The stopper Al2O3/graphite products The function : the control of metal flow rate Sliding gate Tundish lining Submerged nozzle Part 1. Continous casting Stopper

The stopper may be a source of reoxidation Injection of argon The stopper may be a source of reoxidation Air leakage due to : an imperfect airtightness of argon injection connection the permeability of refractory pieces Sliding gate Tundish lining Submerged nozzle Part 1. Continous casting Stopper

A argon injection system in the stopper in order to limit air leakage Graphite compressed joints Air tightness of the stopper : measurement of leakage in use ( at high temperature) Preheating of tundish Casting Leakage (l/mm) Design to limit air leakage Time in mn Sliding gate Tundish lining Submerged nozzle Part 1. Continous casting Stopper

Made of magnesia and forsterite (2MgO-SiO2) monolithic The tundish lining Made of magnesia and forsterite (2MgO-SiO2) monolithic Sliding gate Tundish lining Submerged nozzle Part 1. Continous casting Stopper

The tundish lining Preheating Lining after use In use The close contact between steel and the refractory lining allows a pollution action ( exchange of oxigen, hydrogen, magnesium, silicium) Preheating Lining with a great porosity active surface Lining after use In use Sliding gate Tundish lining Submerged nozzle Part 1. Continous casting Stopper

Reduction of silica and iron oxydes present in refractories with oxygen pick up in steel 3 (SiO2)refract. + 4 [Al]steel  3 [Si]steel + 2(Al2O3) 3 (FeO)refract. + 2 [Al]steel  3 [Fe]steel + 2(Al2O3) Refractory Steel Relationship between oxygen (caught by aluminium) and the FeO content of the tundish refractory (laboratory trials) Lehmann and Al. 2nd Intern. Symp. On advances in refractories for the metallurgy industry, 1996 Sliding gate Tundish lining Submerged nozzle Part 1. Continous casting Stopper

Observation of spinel crystals at the interface steel/refractory Transfer of magnesium and formation of MgO-Al2O3 spinels Plant trials as well as the laboratory experiments demonstrate also a chemical transformation of the forsterite into the MgO-Al2O3 spinel 3(2MgO-SiO2) refr. + 4 [Al]steel  2(MgO-Al2O3)refr. + 4 (MgO)refr. +3 [Si]steel % spinel Observation of spinel crystals at the interface steel/refractory laboratory trials The quantity of spinels is in relation to the magnesia content in the refractory lining Spalling of the MgO-SiO2 lining can lead to MgO-Al2O3 inclusions in steel Sliding gate Tundish lining Submerged nozzle Part 1. Continous casting Stopper

The tundish lining : hydrogen pick up Diffusion of water from sray lining occurs and complete expulsion of the moisture cannot be guaranted even when the tundish is well prea-heated 0,5 1 1,5 2 2,5 3 3,5 4 Number of casting during a sequence Hydrogen [ppm] Hydrogen pick up at the beginning of the casting Measurement of the hydrogen content in steel during a sequence of 3 ladles To limit hydrogen pick up in the steel, it is important to improve the refractory composition and the preheating procedures of the tundish Sliding gate Tundish lining Submerged nozzle Part 1 Continous casting Stopper

Submerged nozzle materials Al2O3/graphite products One of the main problem : alumina clogging for Al killed steels ! Clogging and unclogging lead to metal contamination by alumina particules or clusters Alumina deposits in a submerged nozzle Sliding gate Tundish lining Submerged nozzle Part 1. Continous casting Stopper

What caused clogging ? Hydrodynamic factors : metal flow velocities, turbulence zones associated with dead zones, shape of submerged nozzles Metallurgical factors: steel grades, cleanliness and deoxidation Thermal factors: steel temperature, heterogeneous bath, insufficient preaheating of nozzles Interactions Al2O3-C refractories / steel and refractory factors choice and assembly of refractory materials Sliding gate Tundish lining Submerged nozzle Part 1. Continous casting Stopper

Morphology of deposits in submerged nozzles : 3 zones A decarburized zone 1 2 3 Refractory On the hot face plate like Al2O3 particles Alumina particles + vitreous phase

Dissolution of the carbon of the Al2O3-C refractory into the steel Interactions Al2O3-C refractary/steel : deposit build up mechanism Dissolution of the carbon of the Al2O3-C refractory into the steel Build up of a first layer of deposit by volatilization and oxidation reactions PO2 = 10-17 atm PO2 = 10-11 atm Refractory Steel Mechanism of condensation Sliding gate Tundish lining Submerged nozzle Part 1 Continous casting Stopper

Dissolution of the carbon of the Al2O3-C refractory into the steel Interactions Al2O3-C refractary/steel : deposit build up mechanism Dissolution of the carbon of the Al2O3-C refractory into the steel Build up of a first layer of deposit by volatilization and oxidation reactions Alumina formation through oxidation of aluminium by Carbon monoxide CO (ref) [C]Fe + [O]Fe CO(g) forms in the refractory Aluminium oxidation 2[Al]Fe + [O]Fe Al2O3 Deposit formation Even if the steel is perfectly clean, the clogging will still occur ! Sliding gate Tundish lining Submerged nozzle Part 1. Continous casting Stopper

Interactions Al2O3-C refractary/steel : deposit build up mechanism Consequences The alumina deposit increases with the content of oxide phases in the Al2O3-C refractories (silica, alkalines) that are likely to be reduced by carbon  Alumina clogging does not occur with high carbon content steel Sliding gate Tundish lining Submerged nozzle Part 1. Continous casting Stopper

Many sources of reoxydation Oxygen pick up and permeability of refractory products Oxygen plays a fundamental role in the build up of deposits in submerged nozzles oxydation of dissolved Al in steel condensation of the Na,K, Si, SiO gaz compounds into a oxyde vitreous phase Many sources of reoxydation permeability of the refractory products reduction of oxides by C ( SiO2, K2O, Na2O, B2O3) imperfect assembly seal of the refractory parts Sliding gate Tundish lining Submerged nozzle Part 1. Continous casting Stopper

Prevention of alumina build up in submerged nozzles The alumina build up is caused by a gaseous transfert of oxygen The permeability of the refractory and the air tightness of the assembly play an important part Sliding gate Tundish lining Submerged nozzle Part 1. Continous casting Stopper

Oxygen pick up and behaviour of submerged nozzle for Al killed steels Alumina build up Build up Beyond a certain air leakage, the quantity of oxygen affect is so large that it doesn’t affect the Al in steel Steel oxydation rate Oxidation of liquid steel (Fe-C) and corrosion of refractory by iron oxydes and/or oxygen Oxidation of dissolved Al Wear The steel ther the carbon of the nozzle are oxidized which cause erosion Sliding gate Tundish lining Submerged nozzle Part 1 Continous casting Stopper

Oxydation of steel and wear of the submerged nozzle The oxydation of steel causes the oxydation of the carbon of the submerged nozzle We observe a significant erosion by disintegration of the bonding phase. The alumina particles are thus drawn into the metal  This is a new source of contamination by alumina of refractory origin ! Sliding gate Tundish lining Submerged nozzle Part 1. Continous casting Stopper

Exemple of a catastrophic wear In extreme situation, the permeability of the refractory system becomes very important and the submerged nozzle is damaged Sliding gate Tundish lining Submerged nozzle Part 1. Continous casting Stopper

Erosion of submerged nozzle / effect of the Al2O3-C refractory Material with silica Pure material without silica High erosion no erosion Sliding gate Tundish lining Submerged nozzle Part 1. Continous casting Stopper

Effect of steel grades on the behavior of the submerged nozzles Clogging Corrosion decarburising Mechanisms Al killed High None Moderate Decarburation, oxidation of aluminium , sticking of Al2O3 IFS Interstitial free steel Erratic Weak Formation of Al2TiO5 Clogging/unclogging Steel with SiCa treatment Dissolution of alumina aggregates and formation of a low melting phase High Manganese Corrosion of alumina aggregates with formation of MnAl2O3 High Phosphorus Corrosion of alumina aggregates with formation of aluminate of phosphate carbon Sticking of Al2O3 or Fe2+ (Fe3+,Al 3+) 2 O 4 Sliding gate Tundish lining Submerged nozzle Part 1. Continous casting Stopper

Prevention of alumina build up in submerged nozzles 1. Refractory solutions improve the purity of Al2O3-C refractories with as little silica and impurities as possible reduce the permeability of the products use internal layers to limit the clogging Not permeable to gaseous exchange Chemically inert with steel Thermal shock resistant Mechanically resistant to steel flow A submerged nozzle with a carbon free liner Sliding gate Tundish lining Submerged nozzle Part 1. Continous casting Stopper

Prevention of alumina build up in submerged nozzles 2. Process and metallurgical solutions To ensure perfect steel cleanliness in the tundish To avoid steel reoxidation between the sliding gate of the steel ladle and the mould Sliding gate Tundish lining Submerged nozzle Part 1. Continous casting Stopper

PART II. (4.1.2) Corrosion, cleanliness and steel quality INTERACTIONS OF REFRACTORIES AND STEEL DURING THE PROCESS OF SECONDARY METALLURGY I.1. Reactions between refractories, steel and slag Dissolution Dissociation/volatilization Oxydo-reduction / carbo reduction Formation of new compounds Combination of the refractory and a non-dissolved element in steel I.2. Metallurgical consequences Inclusionnary cleanliness Efficiency of Ca treatments of steel desulfurization Carbon pick up Steel cord Defects on the surface

The refractory- slag – steel system in secondary metallurgy Steel ladle Corrosion by slag :Dissolution and erosion of refractory Dissociation and dissolution Reactive slag Slag line MgO-C Direct transfert Ref steel Wall Al2O3 Spalling Deposit of slag at the end of the previous casting Pollution of the slag Pollution of the steel  Metallurgical consequences Part 2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact

Some considerations about the slag chemistry and mineralogy The slag behavior is very important in determining the steel quality Study of phase assemblage with temperature mineralogical path microstructural changes Exemple : basic oxygen furnace (BOF) slag SiO2 TiO2 Al2O3 FeO MnO MgO CaO P2O5 LOI 1000°C wt % 12.8 0.7 1.4 18.4 2.9 5.2 52.4 2.3 0.3 Slag / MgO-C microstructure Part 2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact

Decrease of the temperature Basic oxygen furnace (BOF) slag  Thermodynamic prediction Fe(s) MgO CaO Ca 3 Ti 2 O 7 SLAG T(C) weight % 0900 1100 1300 1500 1700 1900 10 20 30 40 50 60 70 80 90 100 Ca3SiO5 Ca2SiO4 MnO Ca2Fe2O5 Ca3MgAl4O10 1650°C : Slag + CaO(s) Calcium silicates Ca3SiO5 (C3S) Ca2Si04 (C2S) + CaO Calcium ferrite Ca2Fe2O5 MgO Minor phases Decrease of the temperature Part 2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact

Effect of thermal conditions on the kinetics of cristallisation 10°C/h Slow cooling ~ 72h Rapid cooling ~ 3-5s Industrial cooling ~ 24 -48h Small dendritic crystals 20-80 µm Heterogeneous crystals. 50-150 µm Homogeneous crystals 180-250 µm Size of crystals differs significantly depending on the cooling time: a slow cooling promotes the growth of crystals M. Gauthieu, J. Poirier, F Bodenan, G Franchescini, Wascon 2009 Introduction Par 2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact Conclusion

An industrial example of interaction refractory/ slag corrosion of MgO-C in steel ladles Wear of the slag line Dissolution/corrosion of MgO-C Part 2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact

Correlations between metal cleanliness, corrosion mechanisms of MgO-C in steel ladle and critical slag parameters Steel types Important wear mechanism of MgO-C Critical slag parameters Al deoxidized steels Dissolution of magnesia in CaO-Al2O3 slag [CaO]/[Al2O3] Initial MgO Si deoxidized steels Dissolution of magnesia in CaO-SiO2-Al2O3 slag [SiO2]/[CaO] [Al2O3] Slag T°C Ultra low [C] steels Oxidation of carbon by the slag iron oxide [FeO] Part2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact

Example : case of deoxidation with Al Influence of the [CaO]/[Al2O3] ratio on the MgO saturation of CaO-Al2O3 slags at 1600°C and on the corrosion of MgO-C slag line the variation of [CaO]/[Al2O3] has an important effect on wear In the same time, the solubility of magnesia in the slag increases strongly P Blumenfeld and Al. Effect of service conditions on wear mechanisms of steel ladle refractories Unitecr’97 New Orleans Part2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact

An industrial example of interaction refractory/ steel spalling of bauxite walls Observation of steel ladle lining degradations in service 16 heats : small crack in the lining 24 heats : great evolution of the defect Part 2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact

Identification of the reactional mechanisms Steel ladle Several zones of attack with different textures Impregnation zone Slag Precipitation zone Refractory Part2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact

Evolution of the liquid composition at high temperature (1600°C) Slag Precipitation zone Refractory Impregnation 90 Slag Precipitation zone Impregnation Refractory 80 Mineral phases Hexa-aluminate of lime Corundum Mullite Mullite 70 SiO2 60 Oxide content (wt %) 50 Initial interface 40 Profil of composition of liquid phase 30 Al2O3 20 10 CaO Distance (mm) -2 2 4 6 8 10 Part2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact

Reactions which contribute to degrading the steel quality Dissolution and precipitation Dissolution Dissociation Interactions Steel /slag /refractory Formation of new compounds Volatilisation Oxido reduction Carbo reduction Combination of the refractory and a non-dissolved element in steel Part 2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact

at the liquid/refractory interface Direct dissolution Chemical exchanges are controlled by a boundary layer at the liquid/refractory interface The gradient of composition is the driving force of the corrosion process 2 elementary steps : a thermochemical reaction at the solid/liquid interface and a diffusion of species CArefractory Slag Refractory Boundary layer CAslag Initial interface Part 2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact

Study of dissolution in laboratory Dissolution of MgO in MgO-C refractory for different times by CaO-SiO2 slag [MgO] = f(t) Steel Saturation solubility of MgO T = 1630°C Slag MgO Slag/MgO interface 500 m Slag Part 2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact

Dissolution with precipitation of new compounds Heterogeneous mechanism with the precipitation of new phases  Decrease of the wear rate Initial interface CBrefractory CAslag CAAB2/B CBslag CBAB/AB2 CAAB/AB2 CBAB2/B CArefractory Slag Boundary layer Refractory F. Qafssaoui, J. Poirier, J.P. Ildefonse, P. Hubert :Influence of liquid phase on corrosion behaviour of andalusite-based refractories. Refractories Applications Transactions, 1 (2005) , 2-8 Part 2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact

Transition between the different monomineral layers : in bauxite and andalusite refractories Corundum layer CA2 layer CA6 layer 200 m Bauxite brick 100 m Andalusite brick CA2 : CaO-2Al2O3 CA6 : CaO-6Al2O3 Corrosion of high alumina refractories by Al2O3-CaO slag, T=1600°C Dissolution – precipitation processes inside a liquid phase A slow precicipation from the a liquid phase Part 2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact

Dissociation, volatilization Vacuum = 10-3 atm Example : chromium volatilization of the magnesite-chrome lining in RH/OB vacuum degazer Overview of the brickwork of a vacuum degasser (RH/OB) Vacuum = 10-3 atm D. Brachet, F. Masse, J. Poirier, G. Provost : Refractories behaviour in the Sollac Dunkirk RH/OB steel degasser, Journal of the Canadian Ceramic Society, 58 (1989), 61-66 Part 2 Dissolution Volatilization Oxydo-reduct. Carbo-reduct. New compounds Metallurgical impact

20 and 100 ppm of ΔCr in steel in correlation with oxygen blowing Chrome pick up in steel 20 and 100 ppm of ΔCr in steel in correlation with oxygen blowing Part 2 Dissolution Volatilization Oxydo-reduct. Carbo-reduct. New compounds Metallurgical impact

Oxido-reduction Ex. SiO2 + Al => Al2O3 + Si The reduction of oxides by the desoxidation metals occurs in the steel Ex. SiO2 + Al => Al2O3 + Si This table indicates the oxides which are reduced by desoxidation metals Standard reference: activity = 1 Part 2 Dissolution Volatilization Carbo-reduct. New compounds Metallurgical impact Oxydo-reduction.

Example of oxido-reduction reaction Submerged nozzle in fused silica The fracture of the tube occurs after one hour. Silica was reduced by desoxidation elements (Al,Mn,Ca) presents in liquid steel Part 2 Dissolution Volatilization Carbo-reduct. New compounds Metallurgical impact Oxydo-reduction.

Other exemple of oxydo-reduction Mechanisms Driving force Key parameters Oxydo reduction ∂aO2 / ∂V SiO2 dense layer Coefficients of diffusion SiO2 SiC Slag CaO, MgO K2O, Na2O FeSi ΔG0 (T) : 3SiC + 2FeO  2 FeSi +SiO2 + 3C Oxydation SiC Réduction FeO Slag SiO2 SiC 100μm Part 2 Dissolution Volatilization Carbo-reduct. New compounds Metallurgical impact Oxydo-reduction.

Carbo reduction Ex. SiO2 + C  SiO (gas) + CO (gas) at 1550°C At high temperature, carbo reduction reactions occur in the oxide-carbon refractories Ex. SiO2 + C  SiO (gas) + CO (gas) at 1550°C SiO2 + C  Si (gas) + 2 CO (gas) at 1550°C Disappearance of fused SiO2 aggregates Microstructure of Al2O3-C refractory used in continuous casting 100 m C. Taffin, J. Poirier :The behaviour of metal additives in MgO-C and Al2O3-C refractories. Interceram International, 43 (1994), 356-358 Part 2 Dissolution Volatilization Carbo-reduction New compounds Metallurgical impact Oxydo-reduct

Formation of new compounds Exemple : Al2O3-MgO in situ spinel castables Impact pad Slag Impregnation zone Multicomponent and heterogeneous ceramic - Microscopic observations at room temperature Al2O3-MgO castable corroded by a lime rich slag in a steel ladle Part 2 Dissolution Volatilization Carbo-reduct New compounds Metallurgical impact Oxydo-reduct

Corrosion of MgO-Al2O3 castable by a lime rich slag spinels with the matrix : spinels (Mg,Fe,Mn)O(Fe2Al2)O3 Part 2 Dissolution Volatilization Carbo-reduct New compounds Metallurgical impact Oxydo-reduct

Interaction between slag and matrix Composition and rate of slag and spinel (wt%) (Mg,Fe,Mn)O(Fe2Al2)O3 SEM observation Glassy phase P = 1 at. T= 1600°C Part 2 Dissolution Volatilization Carbo-reduct New compounds Metallurgical impact Oxydo-reduct

Interaction between slag and matrix Rate of oxides in slag phase (wt%) P = 1 at. T= 1600°C Weight% of FeO, Al2O3, MgO and MnO in the liquide state Part 2 Dissolution Volatilization Carbo-reduct New compounds Metallurgical impact Oxydo-reduct

Combination of the refractory and a non-dissolved element in steel Far exemple, consider the reduction of the silica of the refractory by the dissolved manganese in steel 2 Mn + SiO2  2 MnO + Si MnO + SiO2  MnSiO3 Reoxydation of the steel with the formation of solid inclusions + glass Quickly drawn into steel Formation of MnSiO3 crystals at the interface clay refractory / steel Part 2 Dissolution Volatilization Carbo-reduct New compounds Metallurgical impact Oxydo-reduct

PART II. (4.1.2) Corrosion, cleanliness and steel quality INTERACTIONS OF REFRACTORIES AND STEEL DURING THE PROCESS OF SECONDARY METALLURGY I.1. Reactions between refractories, steel and slag Dissolution Dissociation/volatilization Oxydo-reduction Carbo reduction Formation of new compounds I.2. Metallurgical consequences Inclusionnary cleanliness Efficiency of Ca treatments of steel desulfurization Carbon pick up Inclusions of oxydes Part 2 Metallurgical impact cleanliness Ca treatment Desulfurization Carbon pick up O2 content

Metallurgical consequences : inclusionnary cleanliness Oxide cleanliness is measured by the total mass of oxide inclusions formed in the liquid steel Aluminum or silicon additions are used to transform soluble oxygen into alumina (or silica) Total dissolved oxygen contents : Less than 20 ppm for Al killed steels lower than 5 ppm for specialty steels Inclusions of alumina Structural steel Part 2 Metallurgical impact cleanliness Ca treatment Desulfurization Carbon pick up O2 content

The dissolved oxygen content is directly converted to a oxygen partial pressure Part 2 Metallurgical impact cleanliness Ca treatment Desulfurization Carbon pick up O2 content

must be lower than that of the steel What consequences does this low oxygen partial pressure have for the selection of refractories ? To limit the possibility of oxygen pick up, the refractory ’s oxygen potential must be lower than that of the steel 1600°C PO2 = 10-15at PO2 < 10-15at Refractories Al2O3 MgO CaO TiO2 PO2 > 10-15at Cr2O3 SiO2 2 zones

Influence of the refractory material on the oxygen contents Ar atmosphere 50 Kg induction furnace and 3t ladle furnace The refractory material has a significant influence on the oxygen content of steel Al Killed steel at 1600°C Index of oxygen potential (in Kcal/mol O2)

Precipitation of MgO-Al2O3 oxydes Metal/Slag / Refractory reactions : spalling of Al2O3 refractory lining and cleanliness of Si killed steels (steel cords) Liquid silicates + MgO.Al2O3 % MgO (slag) % Al2O3 (slag) Corrosion of slag line MgO Spalling of walls Al2O3 Precipitation of MgO-Al2O3 oxydes  Hard inclusions Oxide cleanliness can be affected by exogenous inclusions from corrosion or erosion of refractories

Case of deoxidation with Si Influence of CaO-SiO2-Al2O3 slag composition on the corrosion of MgO-C with a temperature between 1600 and 1650°C The situation is complex with 3 cases Solid in suspension in Al2O3 poor slags slow corrosion Solids precipitated which MgO saturated in contact with the refractory slow corrosion Totally liquid slag  rapid corrosion

These liquid inclusions do not stick to the nozzle refractories Metallurgical consequences : efficiency of Ca treatments of steel Purpose improving the castability of aluminum killed steels by transforming the alumina deoxidation inclusions into liquid lime aluminate inclusions Advantage These liquid inclusions do not stick to the nozzle refractories After Ca treatment MnS sulphur Alumina Silicoaluminates Al2O3/SiO2/MnO Al2O3 CaO CaS Globular calcic inclusion Before Ca treatment Part 2 Metallurgical impact cleanliness Ca treatment Desulfurization Carbon pick up O2 content

Impact refractories in the efficiency of Ca treatments of steel Ca has a high affinity for oxygen Possibility to reduce some constituents of the refractories SiO2, Cr2O3, Al2O3, ….. Improvement in the efficency of a calcium tretment when high alumina ladle refractories are replaced by dolomite or magnesia refractories Even with the use of basic refractories, possibility to a transfer of magnesia towards the inclusions Part 2 Metallurgical impact cleanliness Ca treatment Desulfurization Carbon pick up O2 content

Composition of inclusions obtained by an too large addition of SiCa to steel in a dolomite ladle Transformation path Initial composition of liquid inclusions Final composition of inclusions 55%MgO-35%CaO- 10%Al2O3 Solid at casting temperature Participate in nozzle clogging Formation of spinel inclusions in Al killed steels created by reaction of the dolomitic lining with calcium addition in excess. Part 2 Metallurgical impact cleanliness Ca treatment Desulfurization Carbon pick up O2 content

Obtained by metal – slag stirring in secondary metallurgy Metallurgical consequences : desulphurization Obtained by metal – slag stirring in secondary metallurgy Porus blocs in a steel ladle Reaction of desulphurization : CaO + S = CaS + O liquid slag close to lime saturation Low oxygen content in steel Requirements For aluminum killed steels the final sulphur contents is less than 10 and even 5 ppm ! Part 2 Metallurgical impact cleanliness Ca treatment Desulfurization Carbon pick up O2 content

Sulfur partition coefficient at equilibrium between liquid slag of the CaO-Al2O3-SiO2-MgO system and steel a (Al) = 0.03 1625°C + 10% Al2O3 in slag Final S 2 or 3 To obtain reproducible results in industrial conditions, it is necessary to control well the slag composition Part 2 Metallurgical impact cleanliness Ca treatment Desulfurization Carbon pick up O2 content

Effect of alumina and dolomite refractories on desulphurisation Consequences : advanced desulphurization can only be reached reliably and reproducibly in ladles with a basic lining Alumina Alumina Dolomite Richter and Wolf Plannenzustellung beim TN-Verfahren Document VDEh 1985 Part 2 Metallurgical impact cleanliness Ca treatment Desulfurization Carbon pick up O2 content

Bannenberg and Al. 6 Int. Iron and Steel congress, 1990, Nagoya Effect of degree of lime saturation of the slag on desulphurisation and refractory wear Consequences : advanced desulphurization can only be reached reliably and reproducibly in ladles with a basic lining Best S conditions Refractory wear Desulphurization index Lime saturation indexes smaller than 1 correspond to liquid slag Bannenberg and Al. 6 Int. Iron and Steel congress, 1990, Nagoya Part 2 Metallurgical impact cleanliness Ca treatment Desulfurization Carbon pick up O2 content

Industrial applications: S vacuum treatment in basic ladles Sur saturation in CaO Slag line Refractory wear / S treatment  [MgO]% Desulphurization index Is = [CaO]/[CaO]s at the end of the treatment Correlation between : the optimal desulfuration rate the slag composition the corrosion of the magnesia refractories Part 2 Metallurgical impact cleanliness Ca treatment Desulfurization Carbon pick up O2 content

Metallurgical consequences : carbon pick up of ULC steel Ultra-low carbon steel, such as intertitial free steel are elaborated by metal-gas reaction under vacuum in oxidizing conditions C Mn P S N Si Al Ti 3 150 7 20 60 Typical chemical composition of a Ti-containing IF steel for drawing applications (concentration in 10-3 % ) Part 2 Metallurgical impact cleanliness Ca treatment Desulfurization Carbon pick up O2 content

Mechanism of carbon transfert from MgO-C refractory to IF steel Carbon pick up strongly varies with the composition of the slag and the importance of argon stirring Slag line Steel ladle ULC steel Relationship between carbon pick up and iron content in slag for a ultra low carbon steel (killed Aluminium) Part 2 Metallurgical impact cleanliness Ca treatment Desulfurization Carbon pick up O2 content

Mechanism of carbon transfert from MgO-C refractory to steel Carbon pick up rises sharply when the slag is strongly deoxidized and contains less than 2% of iron oxide + 10 ppm ΔC ULC steel Relationship between carbon pick up and iron content in slag for a ultra low carbon steel (killed Aluminium) Part 2 Metallurgical impact cleanliness Ca treatment Desulfurization Carbon pick up O2 content

Evolution of the carbon pick up of ULC steel Strong correlation between carbon pick up of ULC steels and MgO-C refractory wear rate of the ladle slag line 2 4 6 8 10 12 14 16 18 1 3 afiter deoxidation (ppm) Carbon pick up Mean wear rate of MgO-C slag line (mm/heat)  The wear of MgO-C slag line by the deoxidized slag plays an important role in the transfert of carbon to steel Part 2 Metallurgical impact cleanliness Ca treatment Desulfurization Carbon pick up O2 content

Presence of iron oxydes in slag Mechanism of carbon transfert from MgO-C refractory to steel Oxido reduction and vaporisation of magnesium Mg 0.2 mm C MgO At the interface , condensation of Mg(g) Mg(g) + FeO  MgO + Fe Presence of iron oxydes in slag Formation of a dense MgO layer with a positive effect on the corrosion Limitation of carbon pick up Part 2 Metallurgical impact cleanliness Ca treatment Desulfurization Carbon pick up O2 content

CONCLUSION

The refractory products are strategic for the production of steel They have a direct role on the quality of elaborated grades  chemical composition of the liquid steel  cleanliness : the amount and the nature of non metallic inclusions  The prevention of defects concerning the steel surface

Prospects The future evolutions of the refractory products should be made by taking into account the interactions : steel quality / refractory reactivity In conjunction with metallurgists efforts to elaborate clean steels, this improvement combines simultaneous control of refractory composition Porosity Permeability And reactivity

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