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

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Introduction

• TWIP Steels: high manganese (15-30wt%) fully austenitic AHSS

High energy absorptionCrash 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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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

.brit

anni

ca.c

omJORDAN & 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

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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

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Background

• Define the partial pressure of water vapour:

• Dew Point:The temperature at whichFor 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

FeOFe

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

γγγcosθ

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: PROBLEMFe/FeO

DP = -30

oC

DP = +5

oC

At 700oCD

P = -50oC

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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%Cu To 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

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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– 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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Thanks for your time