Post on 22-Apr-2018
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Control of Microstructure during Solidification & Homogenization of Thin-Slab Cast Direct-Rolling
(TSCDR) Microalloyed Steels
Tihe (Tom) Zhou
Supervisors: Dr. Hatem. S. Zurob, Dr. Nikolas. Provatas
February 27, 2007702 (Part 1)
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Outline• Introduction • Objectives• Literature Review• Solidification• δ To γ Phase Transformation• Austenite γ Coarsening Process4. Experiment Approach5. Preliminary Results • Heat Transfer Model• Effect of Cooling Rate• Effect of Dipping Time • Verification of the simulation process6. Summary and Future work
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Introduction
Holding furnace Rolling Stands
caster
Layout of the most common SMS-Demag Compact Strip Production Mill
C. Klinkenberg et al, Mater. Sci. Forum, 2005, 500-501, 235 J. Muller et al, 33rd McMaster Symposium on Iron & Steelmaking, 2005, 240
Thin Slab Casting Direct Rolling Process (TSCDR)
Started In 1989 NucorThird Revolution of Production of Steel Slab thickness from 50mm to 70mm50 Installations, 55 million tons, 14% of the world output
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Low Capital Costs: Shot length, Less high and less space for casting and rolling
equipment, eliminating rough rolling mills
Energy Savings: No re-heating stage, short production time, economically
variable at low capacities, more flexible than CCS, products can change easier and quicker
Environmental Advantages: Electric arc furnace using scraps, reduce energy consumption
D. Shi et al, 33rd McMaster Symposium on Iron & Steelmaking, 2005, 59G. Megahed et al, 33rd McMaster Symposium on Iron & Steelmaking, 2005, 292
Benefits of TSCDR Process:Introduction
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Comply with American Petroleum Institute (API) standardsMicroalloy steel used for oil and gas pipelines must meet the requirements:High strength, High toughness, Low ductile-to-brittle transition temperature, Good weldability, Corrosion resistance.
TSCDR Process for Microalloyed Steel
Examples of plants Developing Nb-(Ti)-API tube grades
C. Klinkenberg et al, Mater. Sci. Forum, 2005, 500-501, 235
Introduction
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Challenges of TSCDR Microalloyed Steels- Non-uniform as Cast Microstructure/Grains
Dendrite morphology and SDAS (Second Dendrite Arm Spacing ) of cast structure
Wang et al, Mater. Sci. Forum, 2005, 500-501, 29
400 um
Introduction
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Challenges of TSCDR Microalloyed Steels- Non-uniform as Cast Austenite Grains
Wang et al, Mater. Sci. Forum, 2005, 500-501, 29
Austenite grains in the as cast slab
Introduction
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Microstructure evolution during the solidification and subsequent cooling
N. S. Pottore et al, Metall. Trans A, 1991,vol. 22A, pp, 1871-1879
Challenges of TSCDR Microalloyed Steels- Rapid Coarsening of Austenite Grains
1520ºC
1380ºC1430ºC1460ºC
1480ºC1500ºC
γ
γγ γγγ
Introduction
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Challenges with TSCDR Microalloyed Steel- Persistence of Large Austenite Grains
P. Uranga et al., Mater. Sci, Forum, 2005, 500-501, 245.
not eliminate the non-uniformity large grains still exist refinement limited by number of passes
Average grain size reduced by thermo-mechanical processes
Introduction
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Research ObjectivesObtain finer & more uniform microstructure
Simulation of initial solidification process Study the mechanism of coarsening in the solid state
- delta dendrite coarsening - delta grain growth
- gamma phase transformation- austenite grain coarsening
Find methods to refine microstructure and prevent grain coarsening
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1. Liquid 2. Delta dendrites
3. Delta grains 4. Austenite Grains 5. Alpha ferrite grains
(1) Liquid/Solid, (2) δ-ferrite/γ-austenite, (3) γ-austenite/α-ferrite
C%
Literature Review
Microstructure Evolution
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Solidification
Theories and Models- Heat flow
- Mass flow
- Solute redistribution
- Liquid-solid interface
- Processing parameters and microstructure parameters
Literature Review
M. C. Flemings, Solidification Processing, McGraw-Hill Inc., 1974W. Kurz and D. J. Fisher, Trans Tech Publication Ltd., Switzerland, 1998Bruce Chalmers, John Wiley & Sons, Inc., 1964W. C. Winegard, Institute of Metals, London, 1964
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)( TTck
tT ∇⋅∇=
∇⋅∇=∂∂ α
ρ
- Merton C. Flemings Model- First finite difference Model- Incorporated two-phase changes- 3-D Mode: steady and unsteady state- Thin slab casting process Model
α: thermal diffusivity cm²/sK: thermal conductivity cal/(cm-ºC-s)ρ: density g/cm³c: specific heat, cal/(g-ºC)
Solidification
M. C. Flemings, Solidification Processing, McGraw-Hill Inc., 1974.B. G. Thomas, Metall. Mater. Trans B, 2002, Vol. 33B, No. 12, 795.M. Gonzalez et al, Metall. Mater. Trans B, 2003, Vol.34B, No. 8, 455S. Louhenkilpi, Mater. Sci. Eng. A 413–414, 2005, 135J.E. Camporredondo S et al Metall. Mater. Trans B, 2004, Vol.35B, No. 6, 541.
Heat Transfer Model
Literature Review
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Solidification- Affects influence As-cast Microstructure
• Control Processing Parameters- Increase cooling rate (thinner slab 20mm)
2. Addition of inoculation substances- Particles tend to accumulate at liquid steel
3. Electromagnetic stirring (EMS) fields- Need special set-up
4. In TSCDR process core reduction- Break the dendrite arms - Homogenize the as cast microstructure
Literature Review
Microstructure evolution in the solid state: Coarsening, transformation
From liquid to solid
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Which stage dominate the coarsening process?Dendrite arm spacing, delta grains, delta to austenite phase transformation, austenite coarsening before and inside the soaking furnace
Grain Growth Model
( )dtCrt
∫ +=ℜ
0 20
4αλ
γ
γ: surface energy
r0 :initial grain size
C: solute concentration
t: time
λ and α: constant
ℜ :dimensionless grain size
Time (S)
Gra
in s
ize
(μm
)
H. S. Zurob et al, Acta Meter. 2002, 50 ,3075
Literature Review
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Delta δ to γ Austenite Phase Transformation- Nucleation of γ-grains
H. Yin et al Acta mater. 1999, Vol. 47, No. 5,1523
At the Triples Points of δ GBs Along the δ GBs
Literature Review
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Austenite γ-grains refinement methods
• Thermomechanical treatment during δ to γ austenite phase transformation
F. Zarandi and S. Yue, Mater. Sci. Forum, 2006, 500-501, 115.
100μm 400μm
Literature Review
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2. Various oxide particles act as heterogeneous nucleation sites for austenite.
H. Suito et al, ISIJ Int., 2006, Vol. 46, 840
Austenite γ-grains refinement methodLiterature Review
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Austenite γ-grains refinement method
3. Precipitates pin grain boundaries
Literature Review
C. J. Tweed et al, Acta Metall., 1407
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Literature Summary
From the literature review, the best chance to produce fine and uniform initial microstructure is in the solid state.
Research approach:
• Step 1: Produce as cast microstructure which is resemble the Thin Slab Cast slab in the industry
• Step 2: Examine the coarsening process of delta grains, delta to austenite phase transformation and austenite grains.
Literature Review
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γ-austenite
Liquid
δ-ferrite
α-ferrite
Tem
pera
ture
(o C)
Time (hr)
References Method Current Method
Real time simulation of the TSCDR Process in the industries is impossible
15300C
15000C11500C
Experimental approach Experiment Process
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Equipment
Furnace Chamber
Pressure Gauge
Gas Inlet
Gas Outlet
Diffusion Pump
Mechanical Pump
ADL Model-MP Crystal Growing Furnace Sketch of ADL Model-MP Furnace
Experimental approach
Courtesy Mr. John Thomson Courtesy Dr. Dmitri Malakhov
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Dipping bar
Graphite
Crucible
Thermocouples
Experiment Set-up
TransverseLongitudinal
Thermocouples
Doll Pin
Dipping Block
Experimental approach
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qq q
h
h
h
Preliminary Results
K: conduction C: convection, R: radiation
Complicated solution- geometries- variation of the properties with temperature- unstable S-L interfaces
- Heat Transfer Model
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Heat Transfer Model (cont.)
Temperature Measurement
Preliminary result
0
200
400
600
800
1000
70 80 90 100 110 120Time(s)
Tem
pera
ture
( C)
top bottom center
0
Dipping Bar
Thermocouple
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)( TTck
tT ∇⋅∇=
∇⋅∇=∂
∂ αρ
2
2
zT
tT
∂∂=
∂∂ α
)2(
0
0
txerf
TTTT
i α=
−−
Boundary conductions: z=0, T = T0
z=S, T=TM
tS sαγ2=
Heat Transfer Model (cont.)
α: thermal diffusivity cm²/sK: thermal conductivity cal/(cm-ºC-s)ρ: density g/cm³c: specific heat, cal/(g-ºC)γ: depends on the properties of mold and solid
Chill Solid Liquid
Preliminary result
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Pre-Heat Treatment: 970°C, 30 mins; 3%Picral, 1 min
Etching Solution & Pre-heat Treatment( Ø0.75˝×3˝ (Cu) ,t= 6s )
100μm
100μm100μm
100μm
Preliminary result
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Processing and Microstructural Parameters
nmVGA −−= 11λ
G: Temperature gradient, V: Solidification rate
Effect of Cooling Rate
nVGB −= )(12λ
B1 and n are constants
R. W. Cahn et al, Physical Metallurgy, 1983, 478
Using the heat transfer model: Control the processing parameters, the microstructural parameters can be predicted
Preliminary result
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Ø0.75˝×3˝ (Cu), 6s
Ø0.5˝×3˝ (Cu), Al2O3 coating 6sØ0.5˝×3˝ (Fe-pipe), 6s
Ø0.5˝×3˝ (Cu), 6s
Effect of Cooling Rate
100μm 100μm
100μm100μm
Preliminary result
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Ø0.75˝×3˝ (Cu), t= 4s, 7s, 10s, 15s
Effect of Dipping Time
10S100μm 15S100μm
7S100μm4S100μm
Preliminary result
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Preliminary Results Ø1˝- Ø0.5×3˝(Steel) , t= 7s
100μm
100μm
100μm
100μm
Preliminary result
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Wang et al, Mat. Sci. Forum, 2005, 500-501, 29
1. Microstructures are the same as the references 2. Primary arm spacing has the same scale as the references:
100~150um: In all the samples
TSCDR: 120~180um: H. J. Diepers, Mater. Sci. Forum 2006, Vol. 508, 145
3. Solidification rate is approximately 1 mm/s
TSCDR is1mm/s. J. E. Camporredondo, Metall. and Mater. Trans B, 2004, vol.35B, 541.
A. H. Castillejos et al, 33rd McMaster Symposium on Iron &Steelmaking 2005, P,47
This set-up successfully simulates TSCDR Process.
Verification of the solidification simulation process
100μm 100μm
Preliminary result
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Summary
2. Unique etching technique was established to reveal the
dendrite microstructure of the as-cast microalloyed steel
3. Secondary dendrites arms were generated during the
simulation process
4. 1-D heat transfer model can predict the relationship
between the processing and microstructural parameters
5. Experimental set-up successfully simulated the initial
solidification stage in the thin slab casting process
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2. Build a set-up to study the kinetics of grain growth and its contribution to the final coarsening- δ-ferrite growth- δ-ferrite to γ-austenite phase transformation - Simulate the coarsening process of γ-austenite grains
2. Refine δ-ferrite and γ-austenite grains
Future Work
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Thermocouples
Heating elements
Solidified shell
Graphite crucible
Alumina crucible
Dipping barThermocouples
Experimental Set-upFuture work
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Acknowledgements
Dr. Hatem. S. Zurob Dr. Sumanth. Shankar Dr. Nikolas. Provatas Dr. Mani. SubramanianMs. Connie Barry Mr. Jim GarrettMr. Martin Vanooste Mr. John Thomson
Mr. Doug CulleyMr. John Roddaand Mr. Ed McCaffery
Financial support
Steel Research Center, McMaster University
Courtesy John Thomson
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Thank you
Questions??