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Hot Dip Galvanizing of TWIP Steels

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Presentation on theme: "Hot Dip Galvanizing of TWIP Steels"— Presentation transcript:

1 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

2 Outline Introduction Background Objectives Experimental Procedure
Results and Discussion Conclusions and Future Work 4/13/2012

3 Introduction TWIP Steels: high manganese (15-30wt%) fully austenitic AHSS Pros Light weight body parts High stretch forming Cons Expensive Delayed Hydrogen Cracking Needs Protection against corrosion Characteristics Tensile Strengths as high as 1300MPa Elongations of 60-75% High energy absorption Crash management applications in automotive Structures 4/13/2012

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) JORDAN & MARDER, MET&MAT. TRANS. A, VOL. 28A (1997) 2683 Higher tendency for oxidation 4/13/2012

5 Background Purpose of annealing for single phase steels
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 oC Cooling Section  Alkaline/electrolytic cleaning section Annealing Furnace Zinc Pot Reduction of iron oxides N2/5-20%H2 Intercritical annealing/ recrystallization %Al 4-6 seconds 4/13/2012

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 H O Steel Strip Annealing Furnace N2/5%H2 4/13/2012

7 Temperature, pH2 and pH2O are fixed
Background Temperature, pH2 and pH2O are fixed (N2-5%H2) Fe3O4 Fe FeO ZnO Zn MnO Mn SiO2 Si Al2O3 Al DP=-30oC Fixed pO2 4/13/2012

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) γSV γSL γLV θ 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 4/13/2012

9 Selective Oxidation: PROBLEM
Morphology Chemistry Mode TRIP Steel +5oC DP- N2/10H2 870oC 0.11 % C, 1.53 % Mn 1.46 % Si Gong et al, ISIJ International, vol. 49, pp , 2009 4/13/2012

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 4/13/2012

11 Selective Oxidation: PROBLEM
Fe/FeO DP = -30oC DP = +5oC At 700oC DP = -50oC 4/13/2012

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 (2010) 4/13/2012 * Kavitha and McDermid, Galvatech, Houston, Genova(Italy), 2011

13 Selective Oxidation: PROBLEM
T=770°C T=600s 4/13/2012 * 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 4/13/2012

15 Experimental Procedure
Alloy composition: E.M. Bellhouse, PhD thesis, October 2010 22%Mn-0.6%C 12%Mn+0.7%C+1.5%Cu+1%Al+0.25%Si PAT 20oC/s 5oC/s 10oC/s Holding+ Immersion 4/13/2012

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 seconds 4/13/2012

17 Experimental Procedure
The Selective Oxidation Experiments: The Reactive Wetting Experiments Alloy PAT (oC) pO2 (atm) Dew Point (oC) Annealing time (sec) 22%Mn-0.6%C 700 E-27 -50 60,120 E-25 -30 E-23 +5 12%Mn+0.7%C+1.5%Cu To obtain after Recrystallization experiments -50, -30, +5 Bath temperature (oC) Bath dissolved Al content (%) Immersion time (sec) 460 0.20, 0.30 4,6 470 4/13/2012

18 Results and Discussion
The recrystallization experiments: Bracke et al., Acta Materialia, vol. 57, pp , 2009. 4/13/2012

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 4/13/2012

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 4/13/2012

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 4/13/2012

22 Thanks for your time

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