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Hot Dip Galvanizing of TWIP Steels

Presentation for 702 Seminar I. Hot Dip Galvanizing of TWIP Steels. Sahar Ghafurian Supervisor: Dr. J.R. McDermid. April 2012. Outline. Introduction. TWIP Steels: high manganese (15-30wt%) fully austenitic AHSS. Hot Dip Galvanizing.

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Hot Dip Galvanizing of TWIP Steels

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  1. Presentation for 702 Seminar I Hot Dip Galvanizing of TWIP Steels Sahar Ghafurian Supervisor: Dr. J.R. McDermid April 2012

  2. Outline

  3. Introduction • TWIP Steels: high manganese (15-30wt%) fully austenitic AHSS

  4. Hot Dip Galvanizing • Coating the steel strip by immersing it in a molten zinc bath • Barrier Protection • Galvanic Protection • Selective alloying element oxides (here Mn) created during annealing can adversely affect the wetting of the substrate by the molten zinc bath EMF: Anode(Corrosion) Zinc Aluminum Steel … Copper Cathode (Protection) Higher tendency for oxidation JORDAN & MARDER, MET&MAT. TRANS. A, VOL. 28A (1997) 2683 http://www.britannica.com

  5. Background • Purpose of annealing for single phase steels • Reduction of iron oxides • Recrystallize microstructure • Annealing furnace conditions: • N2/5-20%H2+ controlled water vapour • Times of 60 to 120 seconds • Temperatures of 550-850oC http://www.salzgitter-flachstahl.de Cooling Section • Reduction of iron oxides • N2/5-20%H2 • Intercritical annealing/ recrystallization Annealing Furnace  Alkaline/electrolytic cleaning section • 0.14-0.2%Al • 4-6 seconds Zinc Pot

  6. Background • Define the partial pressure of water vapour: • Dew Point: The temperature at which For this fixed pressure of water vapour gas state liquid state Steel Strip Annealing Furnace O H H N2/5%H2

  7. Background Temperature, pH2 and pH2O are fixed (N2-5%H2) DP=-30oC FeO Fe Fe3O4 Fe ZnO MnO Zn SiO2 Mn Si Al2O3 Al Fixed pO2

  8. Selective Oxidation: PROBLEM • Reactive Wetting • Relative surface tensions between interfaces: wetting angle • Reactive wetting: If a reaction product is formed, the surface tension between liquid and solid can decrease vapour(V) Liquid(L) Solid(S) • Fe- Al Interfacial Layer: Intermetallic compound (η-Fe2Al5Znx) enhances reactive wetting • Selective oxides can result in spots over which this layer is not created, and consequently adversely affect reactive wetting γLV θ γSL γSV

  9. Selective Oxidation: PROBLEM TRIP Steel +5oC DP- N2/10H2 870oC 0.11 % C, 1.53 % Mn 1.46 % Si Gong et al, ISIJ International, vol. 49, pp. 557-563, 2009

  10. Selective Oxidation: PROBLEM • Oxidation Mode • Above a critical amount of alloying element M, oxidation mode changes from internal to external Oxygen M M Oxygen Oxygen M Oxygen M Oxygen M Oxygen Oxygen M Oxygen M Oxygen Oxygen M M Oxygen

  11. Selective Oxidation: PROBLEM DP = -30oC DP = +5oC DP = -50oC Fe/FeO At 700oC

  12. Selective Oxidation: PROBLEM • For most of the cases an external layer of MnO is created on the surface: • The Aluminothermic Reduction of MnO layer has been shown by Kavitha and McDermid to take place for high Mn Steels* Y. F. Gong et al., Materials Science Forum Vols. 654-656(2010) * Kavitha and McDermid, Galvatech, Houston, Genova(Italy), 2011

  13. Selective Oxidation: PROBLEM T=770°C T=600s * Kavitha and McDermid, Galvatech, Houston, Genova(Italy), 2011

  14. Objective • Successfully galvanizing two grades of TWIP steels under CGL conditions • Find annealing time and temperatures to achieve a fully recrystallized microstructure via a minimum energy route • Investigating the effect of selected time, temperature and process dew points (pO2) on the selective oxidation • Evaluating the interaction of the selective oxides on the surface with the molten metal for reactive wetting • Defining the proper amount of bath Al, immersion times and bath temperatures, to obtain a well developed interfacial layer and a high quality galvanized coating

  15. Experimental Procedure • Alloy composition: PAT 10oC/s 5oC/s Holding+ Immersion 20oC/s 20oC/s E.M. Bellhouse, PhD thesis, October 2010

  16. Experimental Procedure • The Recrystallization Experiments: • To define the times and temperatures needed for recrystallization • Fraction Recrystallized was assessed using microhardness • Full Recrystallization was obtained • ~700oC + 60 seconds • ~675oC + 120 seconds

  17. Experimental Procedure • The Selective Oxidation Experiments: • The Reactive Wetting Experiments

  18. Results and Discussion • The recrystallization experiments: Bracke et al., ActaMaterialia, vol. 57, pp. 1512-1524, 2009.

  19. Conclusions • A recrystallized microstructure of the 22%Mn-0.6%C was obtained at ~700oC for 60 seconds and ~675oC for 120 seconds • Based on the results of recrystallization experiments, the matrix for oxidation experiments for this alloy was constructed • The combination of bath dissolved Al, immersion time and bath temperature was designed to investigate reactive wetting

  20. Future Work • Carry out oxidation experiments to investigate the effect of several annealing conditions on oxide morphology, thickness, and composition • Select a series of annealing conditions to investigate the reactive wetting • Testing of selected mechanical properties; namely tensile tests and cup tests to evaluate delayed hydrogen cracking

  21. Acknowledgment • My Supervisor: Dr. McDermid • My Supervisor Committee: Dr. Kish and Dr Zurob • John Thomson • Mariana Budiman • All my friends in CAMC (Centre for Automotive Materials and Corrosion) and Steel Research Centre • Doug Colley • Ed McCaffery • CCEM Staff • Feihong Nan

  22. Thanks for your time

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