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4. Impact of refractories corrosion on Industrial processes FIRE COURSE – Unitecr2001, October 30th, 2011 Kyoto, Japan 4.1. STEEL MAKING J. Poirier CNRS-CEMHTI,

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Presentation on theme: "4. Impact of refractories corrosion on Industrial processes FIRE COURSE – Unitecr2001, October 30th, 2011 Kyoto, Japan 4.1. STEEL MAKING J. Poirier CNRS-CEMHTI,"— Presentation transcript:

1 4. Impact of refractories corrosion on Industrial processes FIRE COURSE – Unitecr2001, October 30th, 2011 Kyoto, Japan 4.1. STEEL MAKING J. Poirier CNRS-CEMHTI, University of Orleans

2 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 FIRE COURSE – Unitecr2001, October 30th, 2011 Kyoto, Japan

3 INTRODUCTION Surface micrograph showing fine particles at grain boundaries

4 Steel-makers 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

5 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

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

7 More and more complex elaboration to eliminate non metallic elements Vacuum treatment C content < 15 ppm is possible ! Desulphuration treatment S content~ a few ppm ElementPCSNHO ppm1055 <15 Lower limits of residual elements in steel making elaboration Introduction Steel challenge Cleanliness /chemistry Non metallic elements Impact of refractories

8 The impact of refractory products on the quality of the metal 1.The possibility to keep the chemical composition of the liquid steel for a given process 3 aspects Introduction Steel challenge Cleanliness /chemistry Non metallic elements Impact of refractories

9 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

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

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

12 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-Al 2 O 3 ) Introduction Steel challenge Cleanliness /chemistry Non metallic elements Impact of refractories

13 Main classes of refractories in relation with the quality and metal cleanliness Steel ladle Tundish Sprayed magnesia Plate Al 2 O 3 - CAl 2 O 3 -C Stopper Ladle Al 2 O 3 - C Shroud Al 2 O 3 - C and ZrO 2 -C insert Submerged nozzle Tundish lining and continuous casting Introduction Steel challenge Cleanliness /chemistry Non metallic elements Impact of refractories

14 Summary of different defect types in steel in relation with the refractory products Steel Spalling of wall Reactivity Steel refractory Al 2 O 3 build up Materials and assembly of refractories Corrosion of slag line Air leakage Interactions Mastery of argon injection Pollution Steel/slag/refractory Erosion of refractories Reoxydation Al 2 O 3 clogging Thermal transfert 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

15 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

16 Sliding gate system consists of a mechanical assembly containing the refractory plates The basic function : the control of metal flow rate Sliding gate StopperTundish liningSubmerged nozzle Part 1. Continous casting

17 The plates of the sliding gate system Subjected to severe thermo-mechanical stress Lead to the cracking of the refractory in use Cause of air leakage with effects on the cleanliness and the wear Al 2 O 3 /SiC / C refractory Sliding gate StopperTundish liningSubmerged nozzle Part 1. Continous casting

18 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 StopperTundish liningSubmerged nozzle Part 1. Continous casting

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

20 The stopper The function : the control of metal flow rate Al 2 O 3 /graphite products Sliding gate Tundish liningSubmerged nozzle Part 1. Continous castingStopper

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

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

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

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

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

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

27 Measurement of the hydrogen content in steel during a sequence of 3 ladles 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 Hydrogen pick up at the beginning of the casting 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

28 Submerged nozzle materials Al 2 O 3 /graphite products Alumina deposits in a submerged nozzle Clogging and unclogging lead to metal contamination by alumina particules or clusters One of the main problem : alumina clogging for Al killed steels ! Sliding gateTundish lining Submerged nozzlePart 1. Continous casting Stopper

29 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 Al 2 O 3 -C refractories / steel and refractory factors choice and assembly of refractory materials What caused clogging ? Sliding gateTundish lining Submerged nozzlePart 1. Continous casting Stopper

30 Morphology of deposits in submerged nozzles : 3 zones 12 3 Refractory A decarburized zone Alumina particles + vitreous phase On the hot face plate like Al 2 O 3 particles

31 Interactions Al 2 O 3 -C refractary/steel : deposit build up mechanism Dissolution of the carbon of the Al 2 O 3 -C refractory into the steel Build up of a first layer of deposit by volatilization and oxidation reactions PO 2 = atmPO 2 = atm Refractory Steel Mechanism of condensation Sliding gateTundish lining Submerged nozzlePart 1 Continous casting Stopper

32 Interactions Al 2 O 3 -C refractary/steel : deposit build up mechanism Dissolution of the carbon of the Al 2 O 3 -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 Al 2 O 3 Deposit formation Sliding gateTundish lining Submerged nozzlePart 1. Continous casting Stopper Even if the steel is perfectly clean, the clogging will still occur !

33 Interactions Al 2 O 3 -C refractary/steel : deposit build up mechanism Consequences The alumina deposit increases with the content of oxide phases in the Al 2 O 3 -C refractories (silica, alkalines) that are likely to be reduced by carbon Alumina clogging does not occur with high carbon content steel Sliding gateTundish lining Submerged nozzlePart 1. Continous casting Stopper

34 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 ( SiO 2, K 2 O, Na 2 O, B 2 O 3 ) imperfect assembly seal of the refractory parts Sliding gateTundish lining Submerged nozzlePart 1. Continous casting Stopper

35 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 gateTundish lining Submerged nozzlePart 1. Continous casting Stopper

36 Oxygen pick up and behaviour of submerged nozzle for Al killed steels Oxidation of liquid steel (Fe-C) and corrosion of refractory by iron oxydes and/or oxygen Wear Oxidation of dissolved Al Steel oxydation rate Alumina build up Build up Beyond a certain air leakage, the quantity of oxygen affect is so large that it doesnt affect the Al in steel The steel ther the carbon of the nozzle are oxidized which cause erosion Sliding gateTundish lining Submerged nozzlePart 1 Continous casting Stopper

37 Oxydation of steel and wear of the submerged nozzle The oxydation of steel causes the oxydation of the carbon of the submerged nozzle Sliding gateTundish lining Submerged nozzlePart 1. Continous casting Stopper 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 !

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

39 Erosion of submerged nozzle / effect of the Al 2 O 3 -C refractory Material with silica Pure material without silica High erosion no erosion Sliding gateTundish lining Submerged nozzlePart 1. Continous casting Stopper

40 Effect of steel grades on the behavior of the submerged nozzles Steel gradesCloggingCorrosiondecarburisingMechanisms Al killedHigh NoneModerate Decarburation, oxidation of aluminium, sticking of Al 2 O 3 IFS Interstitial free steel Erratic WeakHigh Formation of Al 2 TiO 5 Clogging/unclogging Steel with SiCa treatment None HighModerate Dissolution of alumina aggregates and formation of a low melting phase High Manganese None HighModerate Corrosion of alumina aggregates with formation of MnAl 2 O 3 High Phosphorus None HighModerate Corrosion of alumina aggregates with formation of aluminate of phosphate High carbon Weak NoneWeak Sticking of Al 2 O 3 or Fe 2+ (Fe 3+,Al 3+ ) 2 O 4 Sliding gateTundish lining Submerged nozzlePart 1. Continous casting Stopper

41 Prevention of alumina build up in submerged nozzles 1. Refractory solutions Sliding gateTundish lining Submerged nozzlePart 1. Continous casting Stopper improve the purity of Al 2 O 3 -C refractories with as little silica and impurities as possible reduce the permeability of the products use internal layers to limit the clogging oNot permeable to gaseous exchange oChemically inert with steel oThermal shock resistant oMechanically resistant to steel flow A submerged nozzle with a carbon free liner

42 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 gateTundish lining Submerged nozzlePart 1. Continous casting Stopper

43 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 non- dissolved 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

44 The refractory- slag – steel system in secondary metallurgy Spalling Pollution of the slagPollution of the steel Slag line MgO-C Wall Al 2 O 3 Deposit of slag at the end of the previous casting Reactive slag Direct transfert Ref steel Dissociation and dissolution Corrosion by slag :Dissolution and erosion of refractory Steel ladle Metallurgical consequences Part 2 DissolutionVolatilizationOxydo-reduct.Carbo-reductionNew compoundsMetallurgical impact

45 Exemple : basic oxygen furnace (BOF) slag SiO 2 TiO 2 Al 2 O 3 FeOMnOMgOCaOP2O5P2O5 LOI 1000°C wt % Study of phase assemblage with temperature - mineralogical path - microstructural changes Slag / MgO-C microstructure Some considerations about the slag chemistry and mineralogy The slag behavior is very important in determining the steel quality Part 2 DissolutionVolatilizationOxydo-reduct.Carbo-reductionNew compoundsMetallurgical impact

46 Thermodynamic prediction Decrease of the temperature Basic oxygen furnace (BOF) slag 1650°C : Slag + CaO(s) Calcium silicates Ca 3 SiO 5 (C3S) Ca 2 Si0 4 (C2S) + CaO Calcium ferrite Ca 2 Fe 2 O 5 MgO Minor phases Part 2 DissolutionVolatilizationOxydo-reduct.Carbo-reductionNew compoundsMetallurgical impact

47 Small dendritic crystals µm Heterogeneous crystals µm Homogeneous crystals µm Introduction Conclusion Industrial cooling ~ h 1600°C Rapid cooling ~ 3-5s 10°C/h Slow cooling ~ 72h Effect of thermal conditions on the kinetics of cristallisation 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 DissolutionVolatilizationOxydo-reduct.Carbo-reductionNew compoundsMetallurgical impact

48 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 DissolutionVolatilizationOxydo-reduct.Carbo-reductionNew compoundsMetallurgical impact

49 Correlations between metal cleanliness, corrosion mechanisms of MgO-C in steel ladle and critical slag parameters Steel typesImportant wear mechanism of MgO-C Critical slag parameters Al deoxidized steelsDissolution of magnesia in CaO-Al 2 O 3 slag [CaO]/[Al 2 O 3 ] Initial MgO Si deoxidized steelsDissolution of magnesia in CaO-SiO 2 -Al 2 O 3 slag [SiO 2 ]/[CaO] [Al 2 O 3 ] Slag T°C Ultra low [C] steelsOxidation of carbon by the slag iron oxide [FeO] Part2 DissolutionVolatilizationOxydo-reduct.Carbo-reductionNew compoundsMetallurgical impact

50 Example : case of deoxidation with Al Influence of the [CaO]/[Al 2 O 3 ] ratio on the MgO saturation of CaO-Al 2 O 3 slags at 1600°C and on the corrosion of MgO-C slag line the variation of [CaO]/[Al 2 O 3 ] 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 Unitecr97 New Orleans Part2 DissolutionVolatilizationOxydo-reduct.Carbo-reductionNew compoundsMetallurgical impact

51 An industrial example of interaction refractory/ steel spalling of bauxite walls 16 heats : small crack in the lining 24 heats : great evolution of the defect Observation of steel ladle lining degradations in service Part 2 DissolutionVolatilizationOxydo-reduct.Carbo-reductionNew compoundsMetallurgical impact

52 Slag Precipitation zone Refractory Steel ladle Impregnation zone Several zones of attack with different textures Identification of the reactional mechanisms Part2 DissolutionVolatilizationOxydo-reduct.Carbo-reductionNew compoundsMetallurgical impact

53 Oxide content (wt %) Initial interface Distance (mm) Precipitation zone CorundumMullite SiO 2 CaO Al 2 O 3 Refractory Impregnation Hexa- aluminate of lime Mullite Mineral phases Profil of composition of liquid phase Slag Precipitation zone Refractory Impregnation Slag Evolution of the liquid composition at high temperature (1600°C) Part2 DissolutionVolatilizationOxydo-reduct.Carbo-reductionNew compoundsMetallurgical impact

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

55 Direct dissolution The gradient of composition is the driving force of the corrosion process C A refractory Slag R efractory Boundary layer C A slag Initial interface 2 elementary steps : a thermochemical reaction at the solid/liquid interface and a diffusion of species Part 2 Dissolution VolatilizationOxydo-reduct.Carbo-reductionNew compoundsMetallurgical impact Chemical exchanges are controlled by a boundary layer at the liquid/refractory interface

56 Study of dissolution in laboratory Slag [MgO] = f(t) Steel Saturation solubility of MgO T = 1630°C Dissolution of MgO in MgO-C refractory for different times by CaO-SiO 2 slag MgO Slag/MgO interface 500 m Slag Part 2 Dissolution VolatilizationOxydo-reduct.Carbo-reductionNew compoundsMetallurgical impact

57 Dissolution with precipitation of new compounds Heterogeneous mechanism with the precipitation of new phases Decrease of the wear rate SlagRefractory C B refractory C B slag Initial interface C A refractory C B AB2/B C B AB/AB2 C A AB/AB2 C A AB2/B C A slag Boundary layer 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 VolatilizationOxydo-reduct.Carbo-reductionNew compoundsMetallurgical impact

58 Transition between the different monomineral layers : in bauxite and andalusite refractories Corundum layer CA 2 layer CA 6 layer 200 m Bauxite brick 100 m Andalusite brick Corrosion of high alumina refractories by Al 2 O 3 -CaO slag, T=1600°C Dissolution – precipitation processes inside a liquid phase A slow precicipation from the a liquid phase CA 2 : CaO-2Al 2 O 3 CA 6 : CaO-6Al 2 O 3 Part 2 Dissolution VolatilizationOxydo-reduct.Carbo-reductionNew compoundsMetallurgical impact

59 Dissociation, volatilization Overview of the brickwork of a vacuum degasser (RH/OB) Vacuum = atm Example : chromium volatilization of the magnesite-chrome lining in RH/OB vacuum degazer 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), Part 2 Dissolution Volatilization Oxydo-reduct.Carbo-reduct. New compoundsMetallurgical impact

60 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 compoundsMetallurgical impact

61 Ex. SiO 2 + Al => Al 2 O 3 + Si Standard reference: activity = 1 Oxido-reduction The reduction of oxides by the desoxidation metals occurs in the steel This table indicates the oxides which are reduced by desoxidation metals Part 2 Dissolution Volatilization Carbo-reduct. New compoundsMetallurgical impact Oxydo-reduction.

62 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 compoundsMetallurgical impact Oxydo-reduction.

63 Other exemple of oxydo-reduction Oxydo reduction SiO 2 dense layer Coefficients of diffusion Oxydation SiC Réduction FeO ΔG 0 (T) : 3SiC + 2FeO 2 FeSi +SiO2 + 3C a O2 / V 100μm SiCSiO 2 Slag SiO 2 SiC Slag CaO, MgO K 2 O, Na 2 O Mechanisms Driving force Key parameters FeSi Part 2 Dissolution Volatilization Carbo-reduct. New compoundsMetallurgical impact Oxydo-reduction.

64 Carbo reduction At high temperature, carbo reduction reactions occur in the oxide-carbon refractories Ex. SiO 2 + C SiO (gas) + CO (gas) at 1550°C SiO 2 + C Si (gas) + 2 CO (gas) at 1550°C 100 m Disappearance of fused SiO 2 aggregates Microstructure of Al 2 O 3 -C refractory used in continuous casting C. Taffin, J. Poirier :The behaviour of metal additives in MgO-C and Al 2 O 3 -C refractories. Interceram International, 43 (1994), Part 2 DissolutionVolatilizationCarbo-reductionNew compoundsMetallurgical impactOxydo-reduct

65 Formation of new compounds Exemple : Al 2 O 3 -MgO in situ spinel castables - Multicomponent and heterogeneous ceramic - Microscopic observations at room temperature Al 2 O 3 -MgO castable corroded by a lime rich slag in a steel ladle Slag Impact pad Impregnation zone Part 2 DissolutionVolatilizationCarbo-reduct New compounds Metallurgical impactOxydo-reduct

66 Corrosion of MgO-Al 2 O 3 castable by a lime rich slag with the matrix : spinels (Mg,Fe,Mn)O(Fe 2 Al 2 )O 3 spinels Part 2 DissolutionVolatilizationCarbo-reduct New compounds Metallurgical impactOxydo-reduct

67 Interaction between slag and matrix Composition and rate of slag and spinel (wt%) (Mg,Fe,Mn)O(Fe 2 Al 2 )O 3 SEM observation Glassy phase P = 1 at. T= 1600°C Part 2 DissolutionVolatilizationCarbo-reduct New compounds Metallurgical impactOxydo-reduct

68 Interaction between slag and matrix Weight% of FeO, Al 2 O 3, MgO and MnO in the liquide state Rate of oxides in slag phase (wt%) P = 1 at. T= 1600°C Part 2 DissolutionVolatilizationCarbo-reduct New compounds Metallurgical impactOxydo-reduct

69 Combination of the refractory and a non-dissolved element in steel Formation of MnSiO 3 crystals at the interface clay refractory / steel Reoxydation of the steel with the formation of solid inclusions + glass Far exemple, consider the reduction of the silica of the refractory by the dissolved manganese in steel 2 Mn + SiO 2 2 MnO + Si MnO + SiO 2 MnSiO 3 Quickly drawn into steel Part 2 DissolutionVolatilizationCarbo-reduct New compounds Metallurgical impactOxydo-reduct

70 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 Inclusions of oxydes Part 2 Metallurgical impact cleanliness Ca treatmentDesulfurizationCarbon pick up O2 content PART II. (4.1.2) Corrosion, cleanliness and steel quality INTERACTIONS OF REFRACTORIES AND STEEL DURING THE PROCESS OF SECONDARY METALLURGY

71 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 treatmentDesulfurizationCarbon pick up O2 content

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

73 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 PO 2 = at PO 2 < at Refractories Al 2 O 3 MgO CaO TiO 2 PO 2 > at Refractories Cr 2 O 3 SiO 2 2 zones

74 Index of oxygen potential (in Kcal/mol O 2 ) Influence of the refractory material on the oxygen contents Al Killed steel at 1600°C Ar atmosphere 50 Kg induction furnace and 3t ladle furnace The refractory material has a significant influence on the oxygen content of steel

75 Metal/Slag / Refractory reactions : spalling of Al 2 O 3 refractory lining and cleanliness of Si killed steels (steel cords) Liquid silicates + MgO.Al 2 O 3 % MgO (slag) % Al 2 O 3 (slag) Corrosion of slag line MgO Spalling of walls Al 2 O 3 Precipitation of MgO-Al 2 O 3 oxydes Hard inclusions Oxide cleanliness can be affected by exogenous inclusions from corrosion or erosion of refractories

76 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

77 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 Metallurgical consequences : efficiency of Ca treatments of steel Part 2 Metallurgical impact cleanliness Ca treatment DesulfurizationCarbon pick up O2 content Before Ca treatment After Ca treatment MnS sulphur Alumina Silicoaluminates Al 2 O 3 /SiO 2 /MnO Al 2 O 3 CaO CaS Globular calcic inclusion

78 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 SiO 2, Cr 2 O 3, Al 2 O 3, ….. 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 DesulfurizationCarbon pick up O2 content

79 Formation of spinel inclusions in Al killed steels created by reaction of the dolomitic lining with calcium addition in excess. Composition of inclusions obtained by an too large addition of SiCa to steel in a dolomite ladle Initial composition of liquid inclusions Final composition of inclusions 55%MgO-35%CaO- 10%Al 2 O 3 Transformation path Solid at casting temperature Participate in nozzle clogging Part 2 Metallurgical impact cleanliness Ca treatment DesulfurizationCarbon pick up O2 content

80 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 treatmentDesulfurizationCarbon pick up O2 content

81 Sulfur partition coefficient at equilibrium between liquid slag of the CaO-Al 2 O 3 -SiO 2 -MgO system and steel a (Al) = °C + 10% Al 2 O 3 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 treatmentDesulfurizationCarbon pick up O2 content

82 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 Dolomite Richter and Wolf Plannenzustellung beim TN-Verfahren Document VDEh 1985 Part 2 Metallurgical impact cleanliness Ca treatmentDesulfurizationCarbon pick up O2 content

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

84 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 treatmentDesulfurizationCarbon pick up O2 content

85 CMnPSNSiAlTi 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 Typical chemical composition of a Ti-containing IF steel for drawing applications (concentration in % ) Part 2 Metallurgical impact cleanliness Ca treatmentDesulfurizationCarbon pick up O2 content

86 Relationship between carbon pick up and iron content in slag for a ultra low carbon steel (killed Aluminium) 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 Part 2 Metallurgical impact cleanliness Ca treatmentDesulfurizationCarbon pick up O2 content

87 Relationship between carbon pick up and iron content in slag for a ultra low carbon steel (killed Aluminium) 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 Part 2 Metallurgical impact cleanliness Ca treatmentDesulfurizationCarbon pick up O2 content

88 Carbon pick up afiter deoxidation (ppm) Mean wear rate of MgO-C slag line (mm/heat) 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 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 treatmentDesulfurizationCarbon pick up O2 content

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

90 CONCLUSION

91 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

92 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

93 Thank you for your attention


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