Hot Dip Galvanizing of TWIP Steels Sahar Ghafurian Supervisor: Dr. J.R. McDermid April 2012...
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Transcript of Hot Dip Galvanizing of TWIP Steels Sahar Ghafurian Supervisor: Dr. J.R. McDermid April 2012...
Hot Dip Galvanizing of TWIP Steels
Sahar Ghafurian
Supervisor: Dr. J.R. McDermid
April 2012
Presentation for 702 Seminar I
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
High energy absorption
Crash management applications in automotive Structures
CharacteristicsTensile 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
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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
http
://w
ww
.bri
tann
ica.
comJORDAN &
MARDER, MET&MAT. TRANS. A, VOL. 28A (1997) 2683
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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:– N2/5-20%H2+ controlled water vapour
– Times of 60 to 120 seconds– Temperatures of 550-850oC
Annealing Furnace
Zinc Pot
• Reduction of iron oxides• N2/5-20%H2
• Intercritical annealing/ recrystallization
• 0.14-0.2%Al• 4-6 seconds
Cooling Section
Alkaline/electrolytic cleaning section
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http://www.salzgitter-flachstahl.de
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
HHO
Annealing Furnace
N2/5%H2
Steel Strip
2 (l) 2 (g)H O =H O
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Fixed pO2
BackgroundTemperature, pH2 and
pH2O are fixed (N2-5%H2)
Fe 3O 4
Fe
FeO
Fe
ZnO
Zn MnO
Mn SiO 2
Si
Al 2O 3
Al
DP=-30oC
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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
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vapour(V)
Liquid(L)
Solid(S)
LV
SLV
γ
γγcosθ
S
γSV γSL
γLV θ
Fe- Al Interfacial Layer: Intermetallic compound (η-Fe2Al5Znx) enhances reactive wettingSelective oxides can result in spots over which this layer is not created, and consequently adversely affect reactive wetting
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Selective Oxidation: PROBLEM
Morphology Chemistry Mode
Gong et al, ISIJ International, vol. 49, pp. 557-563, 2009
TRIP Steel+5oC DP- N2/10H2870oC0.11 % C, 1.53 % Mn1.46 % Si
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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
Oxygen
Oxygen
Oxygen
Oxygen
M
M
M
M
Oxygen
Oxygen
Oxygen
Oxygen
Oxygen
Oxygen
Oxygen
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Selective Oxidation: PROBLEMF
e/FeO
DP =
-30oC
DP =
+5
oC
At 700oCD
P = -50
oC
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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*
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Y. F. Gong et al., Materials Science Forum Vols. 654-656(2010)
* Kavitha and McDermid, Galvatech, Houston, Genova(Italy), 2011
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Selective Oxidation: PROBLEM
4/13/2012* Kavitha and McDermid, Galvatech, Houston, Genova(Italy), 2011
0 2 4 6 8 10 12 14 16 18 20
-250
-200
-150
-100
-50t
MnO
(nm
)
immersion time (s)
r2 = 0.97
T=770°CT=600s
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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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Experimental Procedure
• Alloy composition:
PAT
20oC/s
5oC/s 10oC/s
20oC/s
Holding+ Immersion
E.M. Bellhouse, PhD thesis, October 2010
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22%Mn-0.6%C
12%Mn+0.7%C+1.5%Cu+1%Al+0.25%Si
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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
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Experimental Procedure
• The Selective Oxidation Experiments:
• The Reactive Wetting Experiments
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Alloy PAT (oC) pO2 (atm) Dew Point (oC)Annealing time
(sec)
22%Mn-0.6%C 7002.44486E-27 -50 60,1201.50981E-25 -30 60,1204.29035E-23 +5 60,120
12%Mn+0.7%C+1.5%CuTo obtain after
Recrystallization experiments-50, -30, +5 60,120
Bath temperature (oC)
Bath dissolved Al content (%)
Immersion time (sec)
460 0.20, 0.30 4,6470 0.20, 0.30 4,6
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Results and Discussion
• The recrystallization experiments:
4/13/2012Bracke et al., Acta Materialia, vol. 57, pp. 1512-1524, 2009.
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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
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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
4/13/2012
Thanks for your time