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 FIRE COURSE – Unitecr’2001, October 30th, 2011 Kyoto, Japan 4. 1 STEEL MAKING - CONTENTS OF THE PRESENTATION • Introduction •Part I (4.1.1) : Flow control and interactions of refractories and steel during continuous casting o Protection between ladle and tundish o Tundish lining o Submerged nozzles •Part II (4.1.2) : Corrosion, cleanliness and steel quality o Reactions between refractories, steel and slag o Metallurgical consequences Control of oxide cleanliness, Steel desulphuration, Ca treatments of inclusions, Elaboration of ULC steels • Conclusion 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 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 Non metallic elements Internal soundness Hydrogen Electromagnetic properties Carbon Deep drawing Nitrogen Surface defects Toughness Oxygen Control of inclusions Weldability Phosphorus Weldability Sulfur Control of inclusions Fatigue Anisotropy Bending Introduction Steel challenge Cleanliness /chemistry Non metallic elements Impact of refractories 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 5 10 <1 5 Lower limits of residual elements in steel making elaboration Introduction Steel challenge Cleanliness /chemistry Non metallic elements Impact of refractories The impact of refractory products on the quality of the metal 3 aspects 1. 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 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 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 Introduction Steel challenge Magnesia graphite Magnesia chrome Dolomite High alumina, mainly bauxite products Alumina - spinel 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 Tundish Al2O3 - C Ladle Al2O3 - C Shroud Sprayed magnesia Submerged nozzle Introduction Steel challenge Al2O3 - C and ZrO2-C insert Cleanliness /chemistry Non metallic elements Impact of refractories Summary of different defect types in steel in relation with the refractory products Steel Interactions Steel purity - Carbon pick up - Sulphide cleanliness - N and H pick up Inclusions and defects - exogenous inclusions - endogenous inclusions TiN, Al2O3-MgO, MnOSiO2, Al2O3, SiO2 - splitting decohesion (inclusions + gaz) Longitudinal cracks Heterogeneity of solidification Pollution Steel/slag/refractory Materials and assembly of refractories Corrosion of slag line Spalling of wall Erosion of refractories Reoxydation Air leakage Reactivity Steel refractory Mastery of argon injection Al2O3 build up Al2O3 clogging Thermal transfert Air leakage 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 Sliding gate system consists of a mechanical assembly containing the refractory plates The basic function : the control of metal flow rate Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle 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 Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle 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 Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle Design of the plates of the sliding gate system (Pa) (a) 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 Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle The stopper Al2O3/graphite products The function : the control of metal flow rate Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle The stopper 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 Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle A argon injection system in the stopper in order to limit air leakage Graphite compressed joints Air tightness of quenouilles the stopper : measurement of Etanchéité - Mesures à chaud leakage in use ( at high temperature) 4 3,5 Design to limit air leakage D f uit e ( l/min.) 3 Preheating Préchauffage of tundish 2,5 Coulée Casting 2 1,5 1 0,5 0 0 20 40 60 80 100 120 140 160 180 200 220 Temps (min.) Time in mn Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle The tundish lining Made of magnesia and forsterite (2MgO-SiO2) monolithic Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle The tundish lining 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 Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle 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) Steel 1 Relationship between oxygen (caught by aluminium) and the FeO content of the tundish refractory (laboratory trials) Preheating at 180°C 0,8 Quantity of oxygen (g) Refractory 0,6 0,4 Preheating at 1200°C 0,2 0 0 2 4 6 8 % FeO Lehmann and Al. 2nd Intern. Symp. On advances in refractories for the metallurgy industry, 1996 Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle 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 % surfacique de spinelle % spinel 25 20 15 10 5 0 40 Observation of spinel crystals at the interface steel/refractory laboratory trials 60 % MgO 80 100 du réfrac 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 Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle The tundish lining : hydrogen pick up Hydrogen [ppm] Diffusion of water from sray lining occurs and complete expulsion of the moisture cannot be guaranted even when the tundish is well prea-heated 4 3,5 3 2,5 2 1,5 1 0,5 0 Hydrogen pick up at the beginning of the casting 0 1 2 3 4 Number of casting during a sequence 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 Part 1 Continous casting Sliding gate Stopper Tundish lining Submerged nozzle 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 Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle 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 Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle Morphology of deposits in submerged nozzles : 3 zones A decarburized zone Refractory 1 2 3 On the hot face plate like Al2O3 particles Alumina particles + vitreous phase 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 Refractory PO2 = 10-17 atm Steel PO2 = 10-11 atm Mechanism of condensation Part 1 Continous casting Sliding gate Stopper Tundish lining Submerged nozzle 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 ! Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle 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 Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle 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 Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle 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 Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle Oxygen pick up and behaviour of submerged nozzle for Al killed steels Build up Alumina 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 dissolved Al Oxidation of liquid steel (Fe-C) and corrosion of refractory by iron oxydes and/or oxygen Wear The steel ther the carbon of the nozzle are oxidized which cause erosion Part 1 Continous casting Sliding gate Stopper Tundish lining Submerged nozzle 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 ! Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle Exemple of a catastrophic wear In extreme situation, the permeability of the refractory system becomes very important and the submerged nozzle is damaged Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle Erosion of submerged nozzle / effect of the Al2O3-C refractory no erosion High erosion Pure material without silica Material with silica Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle Effect of steel grades on the behavior of the submerged nozzles Steel grades Clogging Corrosion decarburising Mechanisms Al killed High None Moderate Decarburation, oxidation of aluminium , sticking of Al2O3 IFS Erratic Weak High Formation of Al2TiO5 Clogging/unclogging Steel with SiCa treatment None High Moderate Dissolution of alumina aggregates and formation of a low melting phase High Manganese None High Moderate Corrosion of alumina aggregates with formation of MnAl2O3 High Phosphorus None High Moderate Corrosion of alumina aggregates with formation of aluminate of phosphate High carbon Weak None Weak Sticking of Al2O3 or Fe2+ (Fe3+,Al 3+) 2 O 4 Interstitial free steel Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle 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 o Not permeable to gaseous exchange o Chemically inert with steel o Thermal shock resistant o Mechanically resistant to steel flow A submerged nozzle with a carbon free liner Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle 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 Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle 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 o Dissolution o Dissociation/volatilization o Oxydo-reduction / carbo reduction o Formation of new compounds o Combination of the refractory and a nondissolved element in steel I.2. Metallurgical consequences o Inclusionnary cleanliness o Efficiency of Ca treatments of steel o desulfurization o Carbon pick up Steel cord Defects on the surface The refractory- slag – steel system in secondary metallurgy Corrosion by slag :Dissolution and erosion of refractory Steel ladle Slag line MgO-C Reactive Wall Al2O3 Direct transfert Ref steel Dissociation and dissolution slag 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 wt % SiO2 TiO2 Al2O3 FeO MnO MgO CaO P2O5 LOI 1000°C 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 Basic oxygen furnace (BOF) slag Thermodynamic prediction 100 • 1650°C : Slag + CaO(s) SLAG 80 • Calcium silicates Ca3SiO5 (C3S) Ca2Si04 (C2S) + CaO • Calcium ferrite Ca2Fe2O5 • MgO weight % 70 60 50 Ca2SiO4 40 Ca3SiO5 30 20 Ca2Fe2O5 10 MnO MgO Ca3MgAl4O1 Fe(s) Ca3Ti2O7 0 0 0900 • Minor phases CaO 1100 1300 1500 1700 Decrease of the temperature 90 1900 T(C) Part 2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact Effect of thermal conditions on the kinetics of cristallisation 1600°C 10°C/h Rapid cooling ~ 3-5s Small dendritic crystals 20-80 µm Industrial cooling ~ 24 -48h Heterogeneous crystals. 50-150 µm Slow cooling ~ 72h 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 Par 2 Dissolution Introduction 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 Part 2 Dissolution 24 heats : great evolution of the defect Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact Identification of the reactional mechanisms Steel ladle Slag Chemical Structural penetration dissolution spalling Several zones of attack with different textures Slag Part2 Dissolution Precipitation zone Impregnation zone Refractory Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact Evolution of the liquid composition at high temperature (1600°C) Slag Precipitation zone Impregnation Refractory 90 Slag Precipitation zone Hexaaluminate of lime 70 Corundum Impregnation Refractory Mullite Mullite 50 40 30 Mineral phases SiO2 60 Initial interface Oxide content (wt %) 80 Al2O3 Distance (mm) CaO Profil of composition of liquid phase 20 10 0 -2 Part2 Dissolution 0 2 4 6 8 10 Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact Reactions which contribute to degrading the steel quality Dissolution Volatilisation Dissolution and precipitation Interactions Steel /slag /refractory Oxido reduction Dissociation Formation of new compounds 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 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 Slag Boundary layer Refractory CArefractory 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) Slag Steel Slag MgO MgO % in slag 24 Saturation solubility of MgO 19 T = 1630°C 14 slag CaO-SiO2 with SiO2/CaO = 0.9 9 4 0 500 m 50 100 150 Time ( mn) Slag/MgO interface 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 CA2 layer CA2 : CaO-2Al2O3 CA6 : CaO-6Al2O3 CA6 layer Corundum layer 200 m Bauxite brick 100 m Andalusite brick 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 Example : chromium volatilization of the magnesite-chrome lining in RH/OB vacuum degazer Vacuum = 10-3 atm Overview of the brickwork of a vacuum degasser (RH/OB) 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 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 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 DissolutionVolatilization Oxydo-reduction. Carbo-reduct. New compounds Metallurgical impact 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 DissolutionVolatilization Oxydo-reduction. Carbo-reduct. New compounds Metallurgical impact Other exemple of oxydo-reduction Mechanisms Driving force Oxydo reduction ∂aO2 / ∂V Slag Key parameters SiO2 dense layer Coefficients of diffusion SiO2 SiC CaO, MgO K2O, Na2O FeSi ΔG0 (T) : 3SiC + 2FeO 2 FeSi +SiO2 + 3C Slag SiO100μm 2 SiC Oxydation SiC Réduction FeO Part 2 DissolutionVolatilization Oxydo-reduction. Carbo-reduct. New compounds Metallurgical impact Carbo reduction 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 Oxydo-reduct Carbo-reduction New compounds Metallurgical impact Formation of new compounds Exemple : Al2O3-MgO in situ spinel castables Impregnation zone Slag Impact pad - 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 Oxydo-reduct Carbo-reduct New compounds Metallurgical impact 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 Oxydo-reduct Carbo-reduct New compounds Metallurgical impact Interaction between slag and matrix (Mg,Fe,Mn)O(Fe2Al2)O3 Glassy phase SEM observation and rate of slag and spinel (wt%) Composition composition and rate of slag and spinel (wt. %) 1 slag P = 1 at. 0,8 T= 1600°C Al O (slag) 2 0,6 3 0,4 spinel CaO(slag) MgAl O (sp) 2 4 Al O (sp) 8 0,2 MnO(slag) Fe O (slag) 2 3 FeO(slag) MgO(slag) 0 0 0,2 Part 2 Dissolution Volatilization Oxydo-reduct Carbo-reduct 12 0,4 <A> <A> 0,6 New compounds 0,8 MnAl O (sp) 2 4 1 FeAl O (sp) 2 4 Metallurgical impact Interaction between slag and matrix 1 FeO P = 1 at. Al O 2 T= 1600°C Rate of oxides in slag phase rate of oxides in slag phase (wt.%) (wt%) 0,8 0,6 3 MgO MnO 0,4 0,2 0 0 0,2 0,4 <A> <A> 0,6 0,8 1 Weight% of FeO, Al2O3, MgO and MnO in the liquide state Part 2 Dissolution Volatilization Oxydo-reduct Carbo-reduct New compounds Metallurgical impact 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 Oxydo-reduct Carbo-reduct New compounds Metallurgical impact 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 o Dissolution o Dissociation/volatilization o Oxydo-reduction o Carbo reduction o Formation of new compounds I.2. Metallurgical consequences o Inclusionnary cleanliness o Efficiency of Ca treatments of steel o desulfurization o Carbon pick up Part 2 Metallurgical impact cleanliness O2 content Inclusions of oxydes Ca treatment Desulfurization Carbon pick up 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 O2 content Ca treatment Desulfurization Carbon pick up The dissolved oxygen content is directly converted to a oxygen partial pressure Part 2 Metallurgical impact cleanliness O2 content Ca treatment Desulfurization Carbon pick up 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 PO2 > 10-15at Refractories Cr2O3 SiO2 2 zones PO2 = 10-15at PO2 < 10-15at Refractories Al2O3 MgO CaO TiO2 1600°C 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) Metal/Slag / Refractory reactions : spalling of Al2O3 refractory lining and cleanliness of Si killed steels (steel cords) Corrosion of slag line % MgO (slag) MgO Liquid silicates + MgO.Al2O3 Spalling of walls Al2O3 Precipitation of MgO-Al2O3 oxydes Hard inclusions Liquid silicates % Al2O3 (slag) 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 1. Solid in suspension in Al2O3 poor slags slow corrosion 2. Solids precipitated which MgO saturated in contact with the refractory slow corrosion 3. Totally liquid slag rapid corrosion 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 Before Ca treatment After Ca treatment Alumina MnS sulphur Silicoaluminates Al2O3/SiO2/MnO CaS Al2O3 CaO Globular calcic inclusion Part 2 Metallurgical impact cleanliness O2 content Ca treatment Desulfurization Carbon pick up 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 O2 content Ca treatment Desulfurization Carbon pick up 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 O2 content Ca treatment Desulfurization Carbon pick up Metallurgical consequences : desulphurization Obtained by metal – slag stirring in secondary metallurgy Reaction of desulphurization Requirements Porus blocs in a steel ladle : CaO + S = CaS + O liquid slag close to lime saturation Low oxygen content in steel For aluminum killed steels the final sulphur contents is less than 10 and even 5 ppm ! Part 2 Metallurgical impact cleanliness O2 content Ca treatment Desulfurization Carbon pick up 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 O2 content Ca treatment Desulfurization Carbon pick up 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 O2 content Ca treatment Desulfurization Carbon pick up 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 Desulphurization index Best S conditions Refractory wear 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 O2 content Ca treatment Desulfurization Carbon pick up Industrial applications: S vacuum treatment in basic ladles Slag line Refractory wear / S treatment [MgO]% Sur saturation in CaO 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 O2 content Ca treatment Desulfurization Carbon pick up 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 7 3 7 20 60 Typical chemical composition of a Ti-containing IF steel for drawing applications (concentration in 10-3 % ) Part 2 Metallurgical impact cleanliness O2 content Ca treatment Desulfurization Carbon pick up Mechanism of carbon transfert from MgO-C refractory to IF steel ULC steel Carbon pick up (ppm) in steel ( after killed with Al) Carbon pick up strongly varies with the composition of the slag and the importance of argon stirring Steel ladle 16 14 Slag line 12 10 8 6 4 2 0 0 2 4 6 [Fe] (%) in slag Relationship between carbon pick up and iron content in slag for a ultra low carbon steel (killed Aluminium) Part 2 Metallurgical impact cleanliness O2 content Ca treatment Desulfurization Carbon pick up Mechanism of carbon transfert from MgO-C refractory to steel ULC steel Carbon pick up (ppm) in steel ( after killed with Al) Carbon pick up rises sharply when the slag is strongly deoxidized and contains less than 2% of iron oxide 16 14 12 + 10 ppm ΔC 10 8 6 4 2 0 0 2 4 6 [Fe] (%) in slag Relationship between carbon pick up and iron content in slag for a ultra low carbon steel (killed Aluminium) Part 2 Metallurgical impact cleanliness O2 content Ca treatment Desulfurization Carbon pick up Evolution of the carbon pick up of ULC steel afiter deoxidation (ppm) Carbon pick up Strong correlation between carbon pick up of ULC steels and MgO-C refractory wear rate of the ladle slag line 18 16 14 12 10 8 6 4 2 0 0 1 2 3 Mean wear rate of MgO-C slag line (mm/heat) 4 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 O2 content Ca treatment Desulfurization Carbon pick up Mechanism of carbon transfert from MgO-C refractory to steel Oxido reduction and vaporisation of magnesium C At the interface , condensation of Mg(g) Mg(g) + FeO MgO + Fe Mg Carbon pick up (ppm) in steel ( after killed with Al) MgO 0.2 mm Presence of iron oxydes in slag 16 14 12 Formation of a dense MgO layer with a positive effect on the corrosion 10 8 6 4 2 Limitation of carbon pick up 0 0 2 4 6 [Fe] (%) in slag Part 2 Metallurgical impact cleanliness O2 content Ca treatment Desulfurization Carbon pick up 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 Thank you for your attention