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Introduction to Steel Coating Technologies_BCDeCooman_2016
- 1. 1 Coating Technologies B.C. DeCooman MDL GIFT, POSTECH
- 2. 2 Much of thesuccess of coated sheet steel is due to a switch from uncoated to coated sheet in the automotive industry and its increased use in the building industry and household appliance manufacturing. By coating steel via galvanizing or other coating technologies, the steel gains superior corrosion resistance at a low cost without impacting the availability or recycle-ability of the product. This has lead to its widespread use in manufacturing. The session includes cosmetic and perforation corrosion protection of sheet steel, the different types of metallic and organic coatings for hot and cold rolled strip products, and the essentials of the different coating technologies. Also included is the origin of the coating microstructure and the importance of metallic and organic coatings on hot and cold-rolled strip products. Coating Technologies Introduction to Coating Technologies for CRS
- 3. 3 Widespread use inmanufacturing Superior corrosion resistance Low cost Availability Recycle-ability Diversity of coating types Favorable application characteristics: Excellent coating adhesion Formability Weldability Phosphatability. Much of the success of coated sheet steel is due to a switch from uncoated to coated sheet in the automotive industry and its increased use in the building industry and household appliance manufacturing Coated Sheet Steel
- 4. 4 Cosmetic corrosion onCRS Filiform corrosion Perforation corrosion on CRS Corrosion of Uncoated Steel
- 5. 5 CRS Red rust Paint undercreep HotDip Galvanised No red rust «white» rust Zn-Fe Galvannealed Low corrosion rate Excellent paint adhesion , Corrosion of Coated Steel
- 6. 6 1. Cathodic disbondment: Cathodicreduction of dissolved oxygen: O2 + 2H2O + 4e- → 4OH- Anodic reaction occurs at a coating defect: Fe → Fe2+ +2e- 2. Oxide lifting: when anodic corrosion products accumulate under the coating. The cathodic reaction during anodic lifting: O2 + 2H2O + 4e- → 4OH- At the metal surface in the scribe or possibly at the outer magnetite interface: 8FeOOH + Fe2+ + 2e- → 3Fe3O4 + 4H2O Uncoated steel Corrosion of Coated Steel
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- 8. 8 Zn Electro-coated coatingthicknesses: 2.5 - 15µm/side or 20-105g/m2 The density of the Zn-coating is 7.14 g/cm3. HDG Zn coatings: Constructional steels: Important: long term corrosion resistance requirements Coating thickness (can be) high: 700 g/m2 or 49 µm/side. Automotive steels: Important: cosmetic and perforation resistance GI Coating thickness: 6 - 20µm/side (40-140g/m2) GA Coating thickness: 6 - 11µm/side (40-80g/m2) Corrosion of Coated Steel
- 9. 9 Coating Weight, g/m2 Timeto5%RedRustFormation,hours 500 400 300 200 100 0 020 40 60 80 100 Electro-galvanized Hot Dip Galvanized Zn-Ni Fe Zn Zn Galvanic corrosion protection Low atmospheric corrosion rate of Zn pH 0 2 4 6 8 10 12 14 Corrosionrate,cm/y 0.5 0.4 0.3 0.2 0.1 0 Corrosion of Coated Steel
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- 11. 11 Sheet Conductor rolls Electrolyte: sulphate Cell volume:~8m3 Current density: ~150A/dm2 Strip velocity: ~200m/min Insoluble Ti-IrO2 anodes + - + Zn, Zn-alloy layer Electrolytic Zn Coatings of CRS
- 12. 12 pure Zn (ZE)Zn-Ni (ZNE) Zn-Fe (ZFE) Electrolytic Zn, Zn-Alloy Coatings on CRS
- 13. 13 Weight-% Zn Temperature,°C 500 800 700 600 300 400 200 60 7080 90 Zn Weight-% Zn 60 70 80 90 Zn5040 Al Zn 95% 419°C 660°C liquid Weight-% Si 10 20 30 40Al Al 660°C 11.7% Si 660°C liquid 665°C 530°C 550°C 425°C Galvalume Galfan Galvanized Aluminized Galvannealed Galvanized Metallic Alloy Coatings on CRS
- 14. 14 Hot Dip GalvanizedZn and Zn-alloy Coatings General Layout Continuous Hot Dip Galvanizing Line Pay-off reels Tension reels Welder Pre-cleaning section Cleaning section Entry looper Post-treatments section Delivery looper Tension leveller Skinpass mill Cooling tower Zn pot GA furnace Pre-heating Heating Soaking Gas-jet cooling Low-T holding
- 15. 15 RTF DFF Slow Cooling section Fast Cooling section Overaging section Snout Zn Pot Cooling Water Quench HDGline with over-aging section Strip Width: 0.7-1.6m Strip thickness: 0.25-2.5mm Production: 350.000mtpy (~80tph) T max: 820°C HDG Line with Direct Fired Furnace + Radiant Tube Furnace Hot Dip Galvanized Zn and Zn-alloy Coatings Cooling tower
- 16. 16 Direct Fired Furnace HotDip Galvanized Zn and Zn-alloy Coatings Impingement burner panel of a Direct Fired Furnace. Direct Fired Furnace characteristics: 1. Controlled air/gas ratio, with ~1000ppm of excess 02 2. Rapid and homogenous strip heating to 700ºC-750ºC 3. Strip surface cleaned by contact with combustion gas 4. Oxidizes the strip surface 5. Oxygen diffuses into the sheet
- 17. 17 Radiant tube furnace Cooling section Direct FiredFurnace + Radiant Tube Furnace Hot Dip Galvanized Zn and Zn-alloy Coatings Maximum strip temperature 850ºC Cooling after Zn pot 220°C (to 45°C)
- 18. 18 Cooling section Gas-jet cooling Hot DipGalvanized Zn and Zn-alloy Coatings Hot Cooling gas out Cold Cooling gas out
- 19. 19 DFF+RTF Annealing FurnaceEquipment Hot Dip Galvanized Zn and Zn-alloy Coatings Burners RTF with N2+5%H2 reducing gas atmosphereDFF Pyrometer Recuperative Pre-heater Heating zone Flue gas heats incoming strip Pyrometer Heating zone Soaking zone Post-combustion chamber Pyrometer Air inlet Gas fired radiant tubes Transfer section with air-lock Maximum strip temperature 850ºC Preheating strip temperature 320ºC DFF strip temperature 750ºC To the cooling sections…
- 20. 20 Furnace Cooling SectionEquipment Hot Dip Galvanized Zn and Zn-alloy Coatings RTFDFF Slow cooling Fast gas-jet cooling up to 100ºC/s, 1mm strip To the Zn pot… Snout dipped in Zn pot Slow to fast cooling transfer zone with hot bridle to increase the strip tension before rapid cooling Exit section equipment: Small over-aging section Post heating booster to reheat the strip for cooling below Zn pot temperature Hot bridle to increase the strip tension (e.g. 10MPa to 25MPa)
- 21. 21 Hot gauges Galvannealing furnace Annealingfurnace Galvannealing furnace Gas wipers Front view Sideview Cross-sectional view Zn pot Cooling section Zn Pot Equipment Hot Dip Galvanized Zn and Zn-alloy Coatings Snout Zn pot
- 22. 22 Hot Dip GalvanizedZn and Zn-alloy Coatings During annealing the selective oxidation of alloying elements such as Mn, Si, … takes place at the steel surface and in the sub-surface Dew Point Furnace Gas Atmosphere Weigth-%SiinSteel T: 850ºC N2+H2 -27ºC +14ºC External SiO2 Internal SiO2 0% Si 1% Si 2% Si 3% Si Me-oxide Me-oxide Internal oxidation External oxidation
- 23. 23 Hot Dip GalvanizedZn and Zn-alloy Coatings During annealing the selective oxidation of alloying elements such as Mn, Si, … takes place at the steel surface and in the sub-surface Example for Fe-0.16%C-2%Mn-1%Al TRIP Steel Surface Intercritical Annealing temperature: 827ºC Furnace gas compositions N2+10%H2 Atmosphere dew point: -30ºC Oxide particles Oxide film Oxides730nm Nitrides900nm 500nm
- 24. 24 “Heat-to-coat”: No Rex-annealing PicklingFurnace Rinsing, Drying Cooling Molten Zn Cooling 600oC 60s Dipping Time Temperature Pickling Induction furnace Zn pot Hot Dip Galvanized Zn and Zn-alloy Coatings Hot dip Galvanizing of Hot Rolled Strip
- 25. 25 Fe-oxide reduction Selective oxidation Snout: exposure Zn metalvapor Zn-pot: 1. Strip surface dissolution 2. Inhibition layer formation Galvannealing: 1. Inhibition layer breakdown 2. Fe-Zn reaction Gas wipers: 1. Final wetting 2. Solidification Annealing furnace Hot Dip Galvanized Zn and Zn-alloy Coatings
- 26. 26 10µm STEEL Galvanized Fe2Al5-xZnx inhibition layer (<0.5mm) Zn solidsolution 0.3-0.5 m-% Al 0.05 m-% Fe Al2O3-rich surface layer mass-% Al 30-50 0.1-0.2 Zn Bath Management: Inhibition layer formation Hot Dip Galvanized Zn and Zn-alloy Coatings
- 27. 27 • Zn bathmanagement involves the control of the effective Al content in the bath, the timing of the Al additions to the Zn bath, the solute Fe content, the control of the bath hardware. FeZn13, FeZn7 « Top Dross » Fe2Al5-xZnx solute Fe Aleff « Bottom Dross » Annealing Furnace Stabilizing Rolls Air-knives Touch Rolls GA- Furnace Snout 50mm Fe2Al5 Top dross Bottom dross Zn Hot Dip Galvanized Zn and Zn-alloy Coatings Zn Bath Management
- 28. 28 Zn-Fe-Al diagram at465°C Liquid + Liquid + Liquid + Fe2Al5 Liquid Zn + Fe, Al in solution Liquid+ + Fe2Al5 Al content, mass-% Fecontent,mass-% 0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 Liquid+ + 50mm Fe2Al5 Top dross Bottom dross Zn Hot Dip Galvanized Zn and Zn-alloy Coatings Zn Bath Management
- 29. 29 Liquid + Liquid + Liquid + Fe2Al5 Liquid Zn + Fe, Alin solution Al content, mass-% Fecontent,mass-% 0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 Increase of the Al content: Lowers the Fe solubility Changes of compound from to to Fe2Al5 Changes the GA kinetics Hot Dip Galvanized Zn and Zn-alloy Coatings Zn Bath Management
- 30. 30 Al content control: 1.Low, i.e. 0.12 mass-%, for galvannealing (weak inhibition Fe-Zn reactions) 2. High, i.e. 0.2 mass-%, for galvanizing (strong inhibition Fe-Zn reactions by Fe2Al5 formation FeZn13, FeZn7 « Top Dross » Fe2Al5-xZnx solute Fe Aleff « Bottom Dross » Annealing Furnace Stabilizing Rolls Air-knives Touch Rolls GA- Furnace Snout Liquid + Liquid + Liquid + Fe2Al5 Liquid Zn + Fe, Al in solution Liquid+ + Fe2Al5 Al content, mass-% Fecontent,mass-% 0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 Liquid+ + Al addition: 1. Pre-melting in separate induction melting unit 2. Direct addition of small Zn-Al alloy bars Hot Dip Galvanized Zn and Zn-alloy Coatings Zn Bath Management
- 31. 31 Al content distribution: 1.The Al content is homogeneous, due to the efficient mixing by the strip motion and the turbulence caused by the induction heating. 2. The temperature distribution is the most important factor; it is also homogeneous. 451°C 450°C 447°C 0.13% 0.126% 0.14% Temperature distribution Solute Al distribution Hot Dip Galvanized Zn and Zn-alloy Coatings Zn Bath Management
- 32. 32 Liquid + Liquid + Liquid + Fe2Al5 Liquid Zn + Fe,Al in solution Al content, mass-% Fecontent,mass-% 0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 Bath-T: 461°C Surface sampling Bottom sampling Solute Al content : The solute Al content, Al effective, is not the same as the total Al content Determination of the effective Al content requires an accurate temperature control Hot Dip Galvanized Zn and Zn-alloy Coatings Zn Bath Management
- 33. 33 Position Znthickness Air-knive <10 to >100mm Znthickness control Air Knife Hot Dip Galvanized Zn and Zn-alloy Coatings Zn Pot Equipment: the air knife
- 34. 34 0 100 200300 400 0 100 200 300 400 500 600 700 800 Temperature,°C Time, s Soaking Hot dipping in Zn bath Galvannealing Hot Dip Galvanized Zn and Zn-alloy Coatings The Thermal Cycle
- 35. 35 Hot Dip GalvanizedZn and Zn-alloy Coatings The Thermal Cycle: CA vs HDG Line Cycles 0 200 400 600 800 0 100 200 300 400 500 600 700 800 CA Line with overaging Temperature,C Time, s 0 100 200 300 400 500 Time, s 0 100 200 300 400 500 Time, s 0 100 200 300 400 500 Time, s HDG Line no overaging Advanced HDG Line HDG Line with overaging Al-killed Low C grades Interstitial-free grades AHSS grades AKLC and IF grades
- 36. 36 Zn-pot Time Temperature Ac3 Ac1 Ms TRIP Grades DPGrades Q&P Grades Hot Dip Galvanized Zn and Zn-alloy Coatings New Thermal Cycles for Galvanized AHSS DP, TRIP and Q&P Grades Strip reheating (inductors)
- 37. 37 0 100 200300 400 500 600 0 200 400 600 800 280C 350C GA 510C -0.4 C/s DP steel processing Line speed : 100mpm -3 C/s -7.6 C/s 650C Temperature(C) Time (sec) 3 C/s 790C -6.5 C/s GI: 460C 560C 0 100 200 300 400 500 600 700 800 900 1000 0 200 400 600 800 DP Steel processing Line speed : 150mpm -2.5 C/s 270C -19C/s Temperature(C) Time (sec) 2.5 C/s 790C -3 C/s 650C 0 100 200 300 400 500 0 200 400 600 800 280C 350C GA 510C DP Steel processing Line speed: 100mpm -3 C/s -9.2 C/s 650C Temperature(C) Time (sec) 3 C/s 790C -6.5 C/s GI: 460C 560C 10 vol-% g a‘ Ms Mf ~400s ~150s Examples of Processing of DP in HDG Lines
- 38. 38 STEEL Galvannealed -phase FeZn3 -phase FeZn8 -phase FeZn13 5.7-6.3 7.9-10.9 20-28 STEEL Galvanized Fe2Al5-xZnx inhibition layer (<0.5mm) Zn solid solution 0.3-0.5 m-% Al 0.05 m-% Fe Al2O3-rich surface layer mass-% Al30-50 0.1-0.2 mass-% Fe Hot Dip Galvanized Zn and Zn-alloy Coatings
- 39. 39 outbursts 10 µm HotDip Galvanized Zn and Zn-alloy Coatings The Galvannealing Reaction
- 40. 40 Hot Dip GalvanizedZn and Zn-alloy Coatings The Galvannealing Reaction 0 2 4 6 8 10 12 0 2 4 6 8 10 120 2 4 6 8 10 12 0 10 20 30 40 50 60 70 80 90 100 Fe in coating, m-% 0 2 4 6 8 10 12 % : Zn : FeZn13 : FeZn7 : FeZn3 ~10% Fe
- 41. 41 Fe in coating,m-% 0 2 4 6 8 10 12 14 Powdering,g/m2 0 2 4 6 8 10 12 14 16 18 20 Hot Dip Galvanized Zn and Zn-alloy Coatings Powdering of Galvannealed Coatings ~10% Fe
- 42. 42 10 µm 10µm Steel Coating Zn-5%Al Galfan coating. The matrix is primary Zn and the darker areas are Al-rich. Note that the eutectic solidification leads to a lamellar or a rod-like microstructure. Hot Dip Galvanized Zn-5% Al Alloy Coatings Zn-Al eutectic solidification
- 43. 43 Hot Dip GalvanizedAl-10% Si Alloy Coatings
- 44. 44 Sheet steel substrate Metalliccoating (20-120 g/m2 Zn, Zn-alloy, …) Cr-free pre-treatment 3-35 mm Organic primer 15-200mm Organic topcoat Organic backcoat Organic primer Pre-treatment Metallic coating Organic Coatings
- 45. 45 Capacity: 192,000 t/a OrganicCoatings
- 46. 46 20 µm Zn particle OrganicCoatings New Coatings
- 47. 47 Phosphate layer ongalvanised, galvannealed and electroplated pure Zn coatings Phosphate crystal types on cold rolled steel and pure Zn electroplated coatings. Substrate Phosphate crystals CRS Electroplated Zn (ZE) Zn2(Fe,Mn)(PO4)2.4H2O: phosphophyllite Zn3(PO4)2.4H2O: hopeite Mn2Zn(PO4)2.4H2O Zn3(PO4)2.4H2O: hopeite Mn2Zn(PO4)2.4H2O Phosphating
- 48. 48 • The cosmeticand perforation corrosion protection of sheet steel, the different types metallic and organic coatings for (hot and) cold rolled strip products, and the essentials of the different coating technologies, were presented. • The origin of the coating microstructure, in particular the structure of galvanized and galvannealed coatings, was discussed. • The importance of metallic and organic coatings on the continue use of (hot and) cold rolled strip products was emphasized. Session Conclusions