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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.

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4. Impact of refractories corrosion on Industrial processes

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  1. 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

  2. 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

  3. INTRODUCTION Surface micrograph showing fine particles at grain boundaries

  4. TRIP 800 Introduction Steel challenge Cleanliness /chemistry Non metallic elements Impact of refractories 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

  5. 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

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

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

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

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

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

  11. Main classes of refractories in relation with the quality and metal cleanliness Introduction Steel challenge Cleanliness /chemistry Non metallic elements Impact of refractories  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

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

  13. Main classes of refractories in relation with the quality and metal cleanliness Introduction Steel challenge Cleanliness /chemistry Non metallic elements Impact of refractories  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

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

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

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

  18. 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 Effect of the plate cracks on the nitrogen pick up

  19. Design of the plates of the sliding gate system Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle • 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

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

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

  22. 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 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

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

  24. The tundish lining Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle 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

  25. Reduction of silica and iron oxydes present in refractories with oxygen pick up in steel Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle 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

  26. Transfer of magnesium and formation of MgO-Al2O3 spinels % spinel Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle 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 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

  27. The tundish lining : hydrogen pick up 4 3,5 3 2,5 Hydrogen [ppm] 2 1,5 1 0,5 0 0 1 2 3 4 Number of casting during a sequence Part 1 Continous casting Sliding gate Stopper Tundish lining Submerged nozzle 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 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

  28. Submerged nozzle materials Al2O3/graphite products Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle 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

  29. 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

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

  31. Interactions Al2O3-C refractary/steel : deposit build up mechanism Steel Refractory PO2 = 10-17 atm PO2 = 10-11 atm Part 1 Continous casting Sliding gate Stopper Tundish lining Submerged nozzle • 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 Mechanism of condensation

  32. Interactions Al2O3-C refractary/steel : deposit build up mechanism Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle • 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 refractoryAluminium oxidation 2[Al]Fe + [O]Fe Al2O3 Deposit formation Even if the steel is perfectly clean, the clogging will still occur !

  33. Interactions Al2O3-C refractary/steel : deposit build up mechanism Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle 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

  34. Oxygen pick up and permeability of refractory products Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle • 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

  35. 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

  36. Oxygen pick up and behaviour of submerged nozzle for Al killed steels Part 1 Continous casting Sliding gate Stopper Tundish lining Submerged nozzle 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

  37. 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 !

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

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

  40. Part 1. Continous casting Sliding gate Stopper Tundish lining Submerged nozzle Effect of steel grades on the behavior of the submerged nozzles

  41. 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 • 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

  42. 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 Prevention of alumina build up in submerged nozzles 2. Process and metallurgical solutions

  43. PART II. (4.1.2) Corrosion, cleanliness and steel quality INTERACTIONS OF REFRACTORIES AND STEEL DURING THE PROCESS OF SECONDARY METALLURGY Steel cord Defects on the surface • 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

  44. Part 2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact 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

  45. 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 Slag / MgO-C microstructure

  46. Basic oxygen furnace (BOF) slag 100 90 SLAG 80 70 60 weight % 50 Ca2SiO4 40 Ca3SiO5 30 20 CaO Ca2Fe2O5 MnO MgO 10 Ca3MgAl4O10 Fe(s) Ca Ti O 3 2 7 0 0900 1100 1300 1500 1700 1900 T(C) Part 2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact  Thermodynamic prediction • 1650°C : Slag + CaO(s) • Calcium silicates • Ca3SiO5 (C3S) • Ca2Si04 (C2S) + CaO • Calcium ferrite • Ca2Fe2O5 • MgO • Minor phases Decrease of the temperature

  47. Effect of thermal conditions on the kinetics of cristallisation Par 2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact 1600°C 10°C/h Slow cooling ~ 72h Rapid cooling ~ 3-5s Industrial cooling ~ 24 -48h 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 Conclusion

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

  49. Correlations between metal cleanliness, corrosion mechanisms of MgO-C in steel ladle and critical slag parameters Part2 Dissolution Volatilization Oxydo-reduct. Carbo-reduction New compounds Metallurgical impact

  50. 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

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