Effect of Annealing Atmosphere on the GI Behaviour of a DP Steel

122 Pages·2017·12.45 MB·English

Effect of Annealing Atmosphere on the GI
Behaviour of a DP Steel
•''.
McMaster
University~~
Effect of Annealing Atmosphere on the
Galvanizing Behaviour of a Dual Phase Steel
By
Rubaiyat Khondker
(M.Sc. in Metallurgical Engineering)
A Thesis
Submitted to the School of Graduate Studies
in Partial Fulfillment of the Requirements
for the Degree of
Master of Applied Science
McMaster University
©Copyright by Rubaiyat Khondker, July 2006.
R. Khondker-MA.Sc. Thesis-Dept. ofM aterials Science and Engineering, McMaster University (2006).
MASTER OF APPLIED SCIENCE (2006) McMaster University
(Materials Science & Engineering) Hamilton, Ontario
TITLE Effect of Annealing Atmosphere on the Galvanizing
Behaviour of a Dual Phase Steel
AUTHOR Rubaiyat Khondker.
B.Sc. in Metallurgical Engineering, (BUET, Dhaka,
Bangladesh), 1991.
M.Sc. in Metallurgical Engineering, (BUET, Dhaka,
Bangladesh), 1995.
SUPERVISOR Dr. Joseph R McDermid
NUMBER OF PAGES: xiv, 107
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R. Khondker-MA.Sc. Thesis-Dept. ofM aterials Science and Engineering, McMaster University (2006).
ABSTRACT
The selective surface oxidation of alloying elements such as Mn can cause dual
phase (DP) steel wettability problems by liquid Zn during continuous galvanizing. It is
well known that process parameters, such as the annealing atmosphere %H2 and dew
point, can affect surface and subsurface oxidation. The purpose of this research was to
study the effect of the annealing atmosphere to determine the optimum DP steel surface
that would result in better reactive wetting by zinc. In particular, the evolution of the
surface phases and structures during the continuous galvanizing annealing cycle were
studied. It was shown that the internal I external oxidation behavior of the alloying
elements of DP steel (e.g. Mn and Mo) at the surface and subsurface can be controlled by
changing process parameters (dew point and H2/N2 ratio) and that some segregation of
elements is unavoidable but can result in good reactive wetting by liquid galvanizing
alloys. A transition from external to internal oxidation was observed when the oxidation
potential (pH20ipH2) of the annealing atmosphere was increased from 0.00844 to
0.03451. Despite the presence of 9-19 wt% Mn as MnO in the pre-dipped steel surface,
the coatings exhibited good adhesion and a well developed Fe2Als inhibition layer at the
coating I substrate interface for all experimental annealing atmospheres as a result of
reactive wetting. This is attributed to aluminothermic reduction of manganese oxide by
aluminum present in the liquid galvanizing alloy.
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R. Khondker-M.A.Sc. Thesis-Dept. ofM aterials Science and Engineering, McMaster University (2006).
ACKNOWLEDGEMENT
The author wishes to thank Stelco Inc., the Natural Sciences and Engineering
Research Council of Canada (NSERC) and the members of the McMaster Steel Research
Centre for their financial support of this research through their funding of the
Stelco/NSERC Industrial Research Chair in Steel Product Application. The author would
also like to thank Mr. Jason Lavallee (galvanizing simulator), Mr. John Rodda, Mr. Doug
Culley, Mr. Rob Lemon, Mr. Steve Koprich and Mr. Fred Pearson for their technical
support, Dr. Li Sun (Dofasco Inc.) for her assistance with the XPS analysis and Dr.
Carmen Andrei for her assistance with the HRTEM analysis. Author wishes to thank Dr.
M.H. Kaye (RMC, Kingston, ON) for providing the results of thermodynamic
calculations regarding AI activity in the Zn bath. Author wishes to thank all members of
the research group, specifically, Dr. A.I.M. Mertens and Erika Bellhouse for their
valuable comments and co-operation. Above all, the author wishes to express heartiest
gratitude to his supervisor Dr. Joseph R. McDermid for his all kind of support,
encouragement and direction to complete this research work.
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R. Khondker-MA.Sc. Thesis-Dept. ofM aterials Science and Engineering, McMaster University (2006).
TABLE OF CONTENTS
ABSTRACT. ..................................................................................................................... .iii
ACKNOWLEDGEMENTS ....................................................................... .iv
TABLE OF CONTENTS ........................................................................... v
LIST OF FIGURES ................................................................................. .ix
LIST OF TABLES ................................................................................. xiv
CHAPTER 1 INTRODUCTION ............................................................................. l
........................................................ 3
CHAPT~R2 ~ITE~T~ ~'VIE~
2.1 Dual Phase steel. .................................................................................4
2.1.1 The potential uses ofDP steels in the automotive industry .........................4
2.1.2 Thermal cycle and alloying for DP microstructure .................................. 6
2.2 Continuous Galvanizing Line (CGL) ......................................................... 8
2.2.1 Continuous galvanizing process ......................................................... 8
2.2.2 Role of AI in the zinc bath .............................................................. 11
2.2.3 Structure and properties ofhigh quality galvanizing coatings ..................... 13
2.3 Oxidation of metal and alloys ................................................................. 17
2.3 .1 External oxidation of simple binary alloys-Wagner's law ......................... 17
2.3.2 Internal oxidation ..........................................................................2 0
2.4 Galvanizing ofDP steel. .......................................................................2 4
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R. Khondker-MA.Sc. Thesi~ Dept. ofM aterials Science and Engineering, McMaster University (2006).
2.4.1 Selective oxidation of alloying elements in DP steel.. ..............................2 5
2.4.2 Effect of surface oxides on DP steel wetting .........................................2 7
2.4.3 Promoting internal oxidation by increasing dew point ...............................2 8
2.4.4. Risk of manganese oxide formation during CGL. ...................................3 0
2.5 Reactive wetting ................................................................................3 2
2.6 Objective of the research .......................................................................3 3
CHAPTER 3 EXPERIMENTAL PROCEDURE ............................................ 35
3.1 Experimental Dual Phase Steels ..............................................................3 5
3 .1.1 Chemistry of as received experimental steel. ........................................3 5
3.1.2 Microstructure of as received DP600 steel. ......................................... .36
3.1.3 Phase diagram of the experimental DP600 steel. ................................... 38
3.2 Galvanizing Simulator Trials ..................................................................3 9
3.2.1 Description ofMcMaster galvanizing simulator .................................... .39
3.2.2 Sample geometry ........................................................................4 1
3.2.3 Sample cleaning ..........................................................................4 2
3.2.4 Annealing cycle, atmosphere and dipping parameters ............................. .43
3.3 Sample Characterization .......................................................................4 7
3.3.1 Metallographic sample preparation .................................................. .47
3.3 .2 Chemical removal of the zinc coating layer ........................................ .48
3.3.3 Microscopy .............................................................................. 48
3.3.3.1 Optical Microscopy ...........................................................4 8
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R. Khondker-MA.Sc. Thesis-Dept. ofM aterials Science and Engineering, McMaster University (2006).
3.3.3.2 Scanning Electron Microscopy. ............................................. .48
3.3.3.3 Focused ion beam (FIB) sample preparation technique .................. .49
3.3.3.4 Transmission Electron Microscopy. .........................................4 9
3.3.4 Glow Discharge Optical Emission spectrometry. ....................................5 0
3.3.5 X-ray Photoelectron Spectroscopy. ...................................................5 2
5"
..........................................................................
<:~~1r~lt'i ~s;(J~Jrs;
4.1 Microstructure and Mechanical properties obtained after annealing. ...................5 4
4.2 Evolution of surfaces during annealing ......................................................5 6
4.2.1 XPS analysis of elemental depth profiles as a function of
annealing atmosphere ..................................................................... 56
4.2.2 GDOES observations of elemental distribution as a function of
atmosphere and time in the thermal cycle .............................................5 8
4.2.3 XPS observation ofthe oxidation state ofthe surface before dipping ............ 66
4.3 Microstructure ofthe surfaces before dipping in the Zn bath .............................7 0
4.4 Coating Characterization ......................................................................7 2
4.4.1 General Observation of coating ........................................................7 2
4.4.2 Coating Adherence .......................................................................7 3
4.4.3 GDOES observation of the coated samples .............................................7 5
4.5 Structure of the coating/substrate interface .................................................7 6
4.5.1 SEM observation of the coating/substrate interface .................................7 6
4.5.2 GDOES observation of the coating/substrate interface ............................. 77
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R. Khondker-MA.Sc. Thesis-Dept. ofM aterials Science and Engineering, McMaster University (2006).
4.5.3 TEM observation at the coating/substrate interface ................................. 79
CHAPTER 5 DISCUSSION ..................................................................... 85
5.1 Oxidation behaviour of steel alloying elements during annealing ........................ 85
5.2 Coating Characterization ......................................................................9 3
5.3 Reactive wetting and aluminothermic reaction ofMn oxides ............................9 4
CHAPTER 6 CONCLUSIONS AND RECOMMENDATIONS
FOR FUTURE WORK. ....................................................................................9 8
6.1 Conclusions ...................................................................................... 98
6.2 Recommendations for future work ...........................................................9 9
REFERENCES ......................................................................................1 00
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R. Khondker-M.A.Sc. Thesis-Dept. ofM aterials Science and Engineering, McMaster University (2006).
LIST OF FIGURES Page No.
Figure 2.1 Elongation as a function of yield strength for various steel 5
grades [2].
Figure 2.2 (a) Schematic diagram of the continuous annealing process 6
(b) obtained DP steel microstructure [18].
Figure 2.3 Schematic diagram of a typical Sendzimir type continuous 9
hot dip galvanizing line [5].
Figure 2.4 (a) A schematic representation of Fe-Zn phase layer 12
formation in a 0.00 wt% Al-Zn galvanizing bath [22], (b)
A schematic representation ofFe-Zn phase layer formation
in a 0.20 wt% Al-Zn galvanizing bath [23].
Figure 2.5 Morphology of the Al-rich inhibition layer observed in a 15
Zn bath with 0.18 wt% AI and immersion time of3.5 s: (a)
small equiaxed crystal (b) coarse elongated crystal [29].
Figure 2.6 Metastable Zn-rich Zn-Al-Fe phase diagrams [31 ]. 16
Figure 2.7 Diffusion process during oxidation of A-B alloys: 19
(a) Exclusive A oxide formation (bulk alloy
concentration N~ ), (b) selective oxidation of B oxide (bulk
alloy concentration N;) and (c) simultaneous formation of
A oxide and B oxide [7, 32]. N is the equilibrium
8
concentration of B in the three phase mixture (alloy+ A
oxide + B oxide).
Figure 2.8 Schematic concentration profiles for internal oxidation (a) 21
Movement of the oxidation front is based on the diffusion
of oxygen in the base metal only and (b) Outward diffusion
of the solute as well as inward diffusion of the oxygen is
important in determining the oxidation kinetics [33].
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