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

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Presentation on theme: "Hot Dip Galvanizing of TWIP Steels Sahar Ghafurian Supervisor: Dr. J.R. McDermid April 2012 Presentation for 702 Seminar I."— Presentation transcript:

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

2 Outline IntroductionBackgroundObjectivesExperimental ProcedureResults and DiscussionConclusions and Future Work 4/13/20122

3 Introduction TWIP Steels: high manganese (15-30wt%) fully austenitic AHSS High energy absorption Crash management applications in automotive Structures Characteristics Tensile Strengths as high as 1300MPa Elongations of 60-75% Light weight body parts High stretch forming Pros Expensive Delayed Hydrogen Cracking Needs Protection against corrosion Cons 4/13/20123

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 JORDAN & MARDER, MET&MAT. TRANS. A, VOL. 28A (1997) 2683 4/13/20124 EMF: Anode(Corrosion) Zinc Aluminum Steel … Copper Cathode (Protection) Higher tendency for oxidation

5 Background Purpose of annealing for single phase steels – Reduction of iron oxides – Recrystallize microstructure Annealing furnace conditions: – N 2 /5-20%H 2 + controlled water vapour – Times of 60 to 120 seconds – Temperatures of 550-850 o C Annealing Furnace Zinc Pot Reduction of iron oxides N 2 /5-20%H 2 Intercritical annealing/ recrystallization 0.14-0.2%Al 4-6 seconds Cooling Section Alkaline/electrolytic cleaning section 4/13/20125

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 H H H O O Annealing Furnace Annealing Furnace N 2 /5%H 2 4/13/20126

7 Fixed pO 2 Background Temperature, pH 2 and pH 2 O are fixed (N 2 -5%H 2 ) Temperature, pH 2 and pH 2 O are fixed (N 2 -5%H 2 ) Fe 3 O 4 Fe FeO Fe ZnO Zn MnO Mn SiO 2 Si Al 2 O 3 Al DP=-30 o C 4/13/20127

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 4/13/20128 Liquid(L)Liquid(L) γ SV γ SL γ LV θ Fe- Al Interfacial Layer: Intermetallic compound (η-Fe 2 Al 5 Zn x ) enhances reactive wetting Selective oxides can result in spots over which this layer is not created, and consequently adversely affect reactive wetting

9 Selective Oxidation: PROBLEM Morphology Chemistry Mode Gong et al, ISIJ International, vol. 49, pp. 557-563, 2009 TRIP Steel +5oC DP- N2/10H2 870 o C 0.11 % C, 1.53 % Mn 1.46 % Si 4/13/20129

10 Selective Oxidation: PROBLEM Oxidation Mode Above a critical amount of alloying element M, oxidation mode changes from internal to external M M M M M M M M M M Oxygen M M M M M M M M 4/13/201210

11 Selective Oxidation: PROBLEM Fe/FeO DP = -30 o C DP = +5 o C At 700 o C DP = -50 o C 4/13/201211

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* 4/13/201212 Y. F. Gong et al., Materials Science Forum Vols. 654-656(2010) * Kavitha and McDermid, Galvatech, Houston, Genova(Italy), 2011

13 Selective Oxidation: PROBLEM 4/13/201213 * Kavitha and McDermid, Galvatech, Houston, Genova(Italy), 2011 T=770°C T=600s

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 (pO 2 ) 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 4/13/201214

15 Experimental Procedure Alloy composition: PAT 20 o C/s 5 o C/s 10 o C/s 20 o C/s Holding+ Immersion E.M. Bellhouse, PhD thesis, October 2010 4/13/201215

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 – ~700 o C + 60 seconds – ~675 o C + 120 seconds 4/13/201216

17 Experimental Procedure The Selective Oxidation Experiments: The Reactive Wetting Experiments 4/13/201217

18 Results and Discussion The recrystallization experiments: 4/13/201218 Bracke et al., Acta Materialia, vol. 57, pp. 1512-1524, 2009.

19 Conclusions – A recrystallized microstructure of the 22%Mn- 0.6%C was obtained at ~700 o C for 60 seconds and ~675 o C 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 4/13/201219

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 4/13/201220

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 4/13/201221


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