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
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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
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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
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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
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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
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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
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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
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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
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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
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11. Selective Oxidation: PROBLEM
Fe/FeO
DP = -30oC
DP = +5oC
At 700oC
DP = -50oC
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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)
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* Kavitha and McDermid, Galvatech, Houston, Genova(Italy), 2011

13. Selective Oxidation: PROBLEM
T=770°C
T=600s
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* 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
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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
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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
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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
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18. Results and Discussion
The recrystallization experiments:
Bracke et al., Acta Materialia, vol. 57, pp , 2009.
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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
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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
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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
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22. Thanks for your time