Transcript Hot Dip Galvanizing of high manganese TWIP steels

Presentation for 702 Seminar I
Hot Dip Galvanizing of TWIP
Steels
Sahar Ghafurian
Supervisor: Dr. J.R. McDermid
April 2012
Outline
Introduction
Background
Objectives
Experimental Procedure
Results and Discussion
Conclusions and Future Work
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Introduction
• TWIP Steels: high manganese (15-30wt%)
• Light weight
fully austenitic AHSS
body parts
Pros • High stretch
Characteristics
Tensile Strengths
as high as
1300MPa
forming
Elongations of
60-75%
Cons
High energy absorption
• Expensive
• Delayed
Hydrogen
Cracking
• Needs
Protection
against
corrosion
Crash management applications in
automotive Structures
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Hot Dip Galvanizing
– Coating the steel strip by immersing it in a molten
zinc bath
EMF:
• Barrier Protection
• Galvanic Protection
Zinc
Higher
tendency for
oxidation
Aluminum
Steel
…
Copper
http://www.britannica.com
JORDAN &
MARDER,
MET&MAT.
TRANS. A,
VOL. 28A
(1997) 2683
Anode(Corrosion)
Cathode (Protection)
– Selective alloying element oxides (here Mn)
created during annealing can adversely affect the
wetting of the substrate by the molten zinc bath
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Background
• Purpose
phase
• Reduction
of ironof annealing for single Cooling
Section
oxides
– ReductionAnnealing
of iron oxides
• N2/5-20%H2
Furnace
• Intercritical
– Recrystallize
microstructure
annealing/
recrystallization
• Annealing furnace conditions:
steels
– N /5-20%H2+ controlled water vapour
• 0.14-0.2%Al
Zinc
Pot
60 to 120 seconds
• 4-6 seconds
– Temperatures of 550-850oC
2
Alkaline/electrolytic
cleaning –
section
Times of
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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
H2O(l) =H2O(g)
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O
H
H
Annealing
Furnace
N2/5%H2
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Background
Temperature, pH2 and
pH2O are fixed
(N2-5%H2)
DP=-30oC
Fixed pO2
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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
vapour(V)
γLV
γSV
θ Liquid(L)
γSL
Solid(S)
cosθ 
γ SV  γSL
γ LV
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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Selective Oxidation: PROBLEM
TRIP Steel
+5oC DPN2/10H2
870oC
0.11 % C,
1.53 % Mn
1.46 % Si
Gong et al, ISIJ International, vol. 49, pp. 557-563, 2009
Morphology
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Chemistry
Mode
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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
M
Oxygen
Oxygen
Oxygen
M
Oxygen
M
Oxygen
M
Oxygen
Oxygen
M
M
Oxygen
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Selective Oxidation: PROBLEM
Fe/FeO
DP = +5oC
DP = -30oC
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DP = -50oC
At 700oC
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Selective Oxidation: PROBLEM
• For most of the cases an external layer of MnO
is created on the surface:
Y. F. Gong et al., Materials Science Forum Vols. 654-656(2010)
• The Aluminothermic Reduction of MnO layer has
been shown by Kavitha and McDermid to take place
for high Mn Steels*
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* Kavitha and McDermid, Galvatech, Houston, Genova(Italy), 2011
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Selective Oxidation: PROBLEM
t MnO (nm)
-50
T=770°C
T=600s
-100
-150
2
r = 0.97
-200
-250
0
2
4
6
8 10 12 14 16 18 20
immersion time (s)
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* Kavitha and McDermid, Galvatech, Houston, Genova(Italy), 2011
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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:
22%Mn-0.6%C
12%Mn+0.7%C+1.5%Cu+1%Al
+0.25%Si
PAT
5oC/s
10oC/s
Holding+ Immersion
20oC/s
20oC/s
E.M. Bellhouse, PhD thesis, October
2010
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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:
Alloy
PAT (oC)
pO2 (atm)
2.44486E-27
22%Mn-0.6%C
700
1.50981E-25
4.29035E-23
To obtain after
12%Mn+0.7%C+1.5%Cu
Recrystallization experiments
-50
-30
+5
Annealing time
(sec)
60,120
60,120
60,120
-50, -30, +5
60,120
Dew Point (oC)
• The Reactive Wetting Experiments
Bath temperature Bath dissolved Al content Immersion time
(oC)
(%)
(sec)
460
0.20, 0.30
4,6
470
0.20, 0.30
4,6
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Results and Discussion
• The recrystallization experiments:
Bracke et al., Acta Materialia, vol. 57, pp. 1512-1524, 2009.
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Conclusions
– A recrystallized microstructure of the 22%Mn0.6%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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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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Acknowledgment
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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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