Source and Transport Mechanism of Exogenous Inclusions in ...
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Source and Transport Mechanism of ExogenousInclusions in Vacuum Poured Ingots: Part I -Identification of Sources and Fluid Flow in PonyLadle (Tundish)
IMF Meeting
April 14, 2015
T. Bhattacharya, P. Kaushik: ArcelorMittal Global R&D – East Chicago
J. Fehr, M. Wooddell, B. Nester, C. Chappell: ArcelorMittal Steelton
Outline
• Background and problem identification
• Laboratory investigations
a. Analysis of potential sources
b. Laboratory investigations
c. Inclusion analysis
• Numerical modeling of fluid flow in pony ladle
a. Effect of stopper rod design
b. Effect of bath height and stopper rod movement
c. Effect of impact pad
• Summary and conclusions
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Background/Problem description
The Steelton steelmaking shop isequipped with a 150 MT DCelectric arc furnace (EAF) which ischarged completely with scrap,carbon and oxygen injection. Thetapped steel from the EAF isrefined at a ladle furnace (LMF).Desulfurization, alloy additionsand temperature adjustments aremade at the LMF, followed byhydrogen removal in a vacuumtank degasser (VD) unit. Ladleslags are not removed betweenthe LMF and VD.
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Steelmaking process flow at AM Steelton
Set up of vacuum stream pouring process at ArcelorMittalSteelton: position of pony ladle over vacuum dome
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Ultrasonic (UT) indications on the breech end of theforging (bottom) and location across the radius in a disc(top) sectioned along dotted green line.
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The steel grade of interest is a Si-killed steel with 0.2% C, 0.2% Mn,0.08% Si, 3% Ni, 1.8% Cr, 0.45%Mo, ~ 30 ppm S and ~ 75 ppm N.
The macro-inclusions areconcentrated at a location about athird of the way from the bottom ofthe ingot, and mostly in a arc inthe cross-section
Microstructure and composition of macro-inclusionsrevealed complex oxide phases
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A D
BC
EF
Oxide M1 M2
A B C D E F
Fe 100
SiO2 50 100 48 45
MnO 34 48
Cr2O3 58 2
Al2O3 8 17
MgO 14 55
CaO 22
M1
M2
The macro-inclusions M1 and M2 werecomposed of 4 and 2 phases, respectivelyand were different from previously identifiedsilica-alumina particles arising due tobreakdown of ladle lining refractory, andalumina-silica-magnesia particles fromentrapment of hot topping compounds.
Research motivation: this work was initiated to eliminateinclusionary defects M1 and M2.
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•Which exogenous sources are responsible for creating these macro-inclusions in as-cast ingots?
What experimental techniques can be used to understand thesephenomena at the laboratory scale and do they represent the industrialsituation? Does the composition of exogenous source change in steel withtime?
•If the sources are from steelmaking, what cause these inclusions totransport in the ingot during teeming?
•What steps can be taken to eliminate the conditions that cause thesemacro-inclusions to become entrapped in the ingots?
•What causes these macro-inclusions to concentrate at a certain distancefrom the bottom of the ingot?
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Identification of Sources
Chemical composition of potential sources
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Material Location Fe Al2O3 CaO MgO MnO SiO2 Cr2O3 K2O
Pony ladle
cover splash
Steel splash attached to
bottom of pony cover
71 1 1 3
Pony ladle
scum
Steel-slag mixture floating
at the top of pony ladle
during teeming
23 7 15 19 26
Pony ladle wall
scum
Steel-slag attached to walls
of the pony ladle
15 8 20 25 27
Slag wool Hot top compound added
on top of ingot mold after
completion of teeming
10 31 10 44
Vermiculite Insulating compound
added on top of ingot mold
after completion of
teeming
4 13 1 24 41 6
Ladle slag Ladle slag after end of
degassing from VD
15 59 6 16
Ramming
material
Material used in mold 45 47
SEM-EDX analysis of pony scum material and slag wool
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MnO-SiO2MnO-Cr2O3 SiO2
Ca-Mg-Si-Al
The SEM examination of pony ladle scummaterial showed phases that bearresemblance to the features found in complexoxide macro-inclusions M1. The slag woolappeared to be the closest match to macro-inclusions M2.
What is pony scum and how it is formed?
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Inclusion composition at the endof degassing in steelmaking
MnO SiO2 Cr2O3
Liquid steel (end-VD) 19 74 9
Pony ladle scum 54 43 3
Due to steel reoxidation,% MnO ofindigenous inclusions increased inthe pony scum at the expense ofSiO2 and Cr2O3. Thus, pony scumwas an agglomerated mass ofreoxidized steel and indigenousinclusions (or a reoxidation product).Reoxidation occurs during steeltransport from the teeming ladle tothe pony ladle as the steel isexposed to atmosphere and no fluxcover is used (no submerged entry)
Research motivation: this work was initiated to eliminateinclusionary defects M1 and M2.
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√ Which exogenous sources are responsible for creating these macro-inclusions in as-cast ingots?
The pony scum material and slag wool beads appear to be the closestmatch in micro-structure and chemistry to the macro-inclusions M1 and M2,respectively.
•Does the composition of exogenous source change in steel with time?•If the sources are from steelmaking, what cause these inclusions to transport in the ingot during teeming?•What experimental techniques can be used to understand these phenomena at the laboratory scale and dothey represent the industrial situation?•What steps can be taken to eliminate the conditions that cause these macro-inclusions to becomeentrapped in the ingots?•What causes these macro-inclusions to concentrate at a certain distance from the bottom of the ingot?
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Laboratory investigations to understand thebehavior of exogenous sources
Laboratory heats
• In a vacuum induction furnace at R&D, about 45 Kg of pure electrolyticFe was melted and the heat was deoxidized using Si-Mn alloy (Si ~30%). Alloys were added after 10 minutes of deoxidation and afteranother 15 minutes, about 0.5 lbs of pony scum material sized in 2” wasadded. The heat was stirred and tapped after 1.5 minutes in a 2.5” thickslab mold.
• The slab was subsequently rolled into a 10 ft long x 8” wide x 0.5” thickplate (hot top was intentionally included for rolling to detect the ponyscum).
• The plate was sectioned into 3 smaller manageable plates and UTinspected. In addition, the smaller plates were mapped to identify theprecise location of the macro-inclusions for subsequent cross-sectioningand SEM examination.
• The UT indicative sections of the plate sample showed multi-phasemacro inclusions.
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Location and size of defects in a UT inspected plate
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Defect North West Size Defect North West Size
A 3” 7 ¾” 3/8”x1” F 5 ½” 20 1/8” ¼” Dia.
B 4” 9” 3/8” x ¾” G 4 7/8” 21” ½” x 3 ½”
C 3” 10” 1” x 7” H 2 ¼” 24 ½” 3/8” x 2 ¾”
D 6” 16 ¼” ½” x 4 ½” I 4 1/2” 26 ½” 3/8” x 7/8”
E 4” 19 5/8” ¼” Dia. J 3” 29” ½” x 1”
Examination of macro-inclusions in plates
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Optical
SEM-EDX
The structure and phases inside the macro-inclusions found in the rolled plates wereessentially identical to those noticed in thepony scum particularly the SiO2-MnO phasethat formed the base matrix of macro-inclusion M1 and that of the pony scum.Some contamination from Zr is thought tocome through the zirconia liner used formold release in the laboratory.
Melting behavior of slag wool compound wasdetermined using confocal laser scanningmicroscopy at R&D
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390 °C 1060 °C
1270 °C 1280 °C
1288 °C
1297 °C
1310 °C
1250 °C
The addition practice of slag wool is usually around 30 minutes after completion of teeming forindustrial ingots. In order for slag wool beads to entrain inside the ingot, the temperature of theingot top has to be a minimum of the softening temperature of the bead. Thermodynamicallypredicted melting point of oxide phase in macro-inclusion M2 was 1350 0C which was closethe melting point of slag wool bead at 1330 0C. The CSLM experiment showed that the beadstarted to soften around 1060 0C and was completely molten at 1310 0C.
Research motivation: this work was initiated to eliminateinclusionary defects M1 and M2.
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√Which exogenous sources are responsible for creating these macro-inclusions in as-cast ingots?
√ What experimental techniques can be used to understand these phenomena at the laboratory scale and do they represent the industrialsituation? Does the composition of exogenous source change in steel withtime?
• Laboratory experiments showed:• Pony scum did not change composition in molten steel or during
solidficiation• Phases inside the pony scum are similar to those observed in macro-
inclusion M1.• CSLM experiment helped to understand the softening behavior of slag
wool bead which can partially melt under temperature fluctuations atthe top of the ingot.
•If the sources are from steelmaking, what cause these inclusions to transport in the ingot during teeming?•What steps can be taken to eliminate the conditions that cause these macro-inclusions to become entrappedin the ingots?•What causes these macro-inclusions to concentrate at a certain distance from the bottom of the ingot?
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Numerical modeling of fluid flow (CFD) inpony ladle
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Composition of Macro-Inclusions in Ingot& Its Possible Source
Re-oxidized material on bath free surface (“pony-scum”) closely resembles the complex oxide foundin rejected ingots and is identified as one of the potential sources
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Focus of Investigation
Since pony scum has been suspected to be the source of exogenousinclusions found in the forged ingot, this CFD study has been aimed atinvestigating how fluid flow may cause scum material to entrain and dislodgefrom pony walls/floors In order to characterize the fluid flow and discriminate between differentconfigurations/cases, velocity field, vortex core regions and wall shear stresseshave been monitored The stopper rod movement (as a result of periodic shut-offs) could promoteentrainment of re-oxidized materials, so fluid flows corresponding to differentstopper rod positions have been analyzedWall shear stress distribution due to fluid flow is a good indicator of theprobability of pony scum getting “abraded” from pony wall/floor – this aspect hasbeen investigated for various configurations Effect of introducing a turbulence-inhibiting impact pad has been examined indetails to investigate its performance in terms of entrainment and erosion Possible solutions have been proposed based on the results of the mathematicalmodeling
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Part 1: Modeling of old-style stopper rod vs. new-stylestopper rod
Part 2: Modeling of nozzle well vs. no nozzle well
Part 3: Modeling of critical liquid level during pouring
Part 4: Modeling of impact pad
Objective of CFD Modeling:Determine fluid flow conditions that may lead to ponyscum getting into the mold & recommend ways(or evaluate process changes) that may prevent it
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Part 1: Old Stopper Rodvs.
New Stopper Rod
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Simulation Setup -- Geometry
Only the fluid volume has been modeled
nozzle
stopper rod
nozzle well(funnel)
(SR+funnel+nozzle)
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Typical Mesh
About 0.5 million elements(near wall regions have ~10 inflation layers with 2 mm starting element size to capture
boundary layer or near wall phenomena accurately)
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Simulation Conditions
*isothermal (well-mixed) condition is a good assumption because the flow is dominated byinertia; natural convection in pony ladle is not important as the ratio Gr/Re2 << 1 (Gr: GrashofNo., Re: Reynolds No.) : buoyancy effect is low as compared to inertia; this is not the case in
the ingot mold post-teeming – turbulent natural convection dominates
Steady stateLiquid steel flow-rate: 11,500 lbs/minIsothermal at ~1500 OC*Turbulent flow (k-epsilon turbulence model with scalable wall functionused, rough walls)Top surface is assumed to be a wall with free shear
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Geometry: Old SR vs. New SR
Previous SR design had a wider tip and body
Old SR
New SR
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Free Surface Velocity: Old SR vs. New SR
(6.86 in.) (9 in.)
(6.86 in.) (9 in.)
Old SR
New SR
The range of free surface flow activity is comparable between old and new SR designs; location ofhigh velocity regions differs slightly
A
AV
~ 0.23 – 0.25 m/s
area avg. vel.
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Velocity Vectors at SR Mid Plane
Two counter-current rolls could be see on a vertical plane
vectors shown onthis plane
SR Position 1 SR Position 3
SR Position 4 SR Position 6
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Velocity Field at SR Mid Plane
Low velocity zones around stopper tip may cause build-up whichcould be flushed out during periodic SR shut-offs (more on this later)
vertical slice plane
bi-secting SR
SR Position 16.86 in.
SR Position 39 in.
SR Position 410 in.
SR Position 610.8 in.
low velocity zone
low velocity zone
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Velocity Vectors Around SR Tip: New
Two recirculation zones for new SR for full open position; the velocityfield looks balanced/streamlined on both sides of SR
recirculationzones
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Velocity Vectors Around SR Tip: Old
One strong recirculation zone inside funnel around SR tip for old SR(perhaps with slightly larger area); the velocity field looks “twisted”
recirculationzone
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Wall Shear Stress: Full Open (Old SR)
(different views forSR Position 1: fullopen case)
Maximum wall shear stress occurs within 1/3rd heightHigher wall shear would signify higher erosion due to the abrasive effect of fluid flow
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Wall Shear Stress: Full Open (New SR)
(different views forSR Position 1: fullopen case)
Areas under high wall shear stress remain nearly the same for new SR (theseareas need special attention during cleaning: SOP has been modified toinclude cleaning of the lower portion of the walls before re-using a pony)
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How Strong is 20 Pa Wall Shear StressIn Terms of Its Erosion Capability ?
It is equivalent to the “adhesive” force requiredto stop a 2 kg refractory block (1 m X 1 m area)
from sliding down a vertical wall
2 kg
adhesive patch(bond strength > 20 Pa)
1 m
1 m
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Vortexing: Background
Vortex: a region within a fluid where theflow is mostly a spinning motion aboutan imaginary axis (localized tornadoes)
They are a major components ofturbulent flow
Core has highest “vorticity” or“swirling strength”
Fluid velocity is highest at core (v ~ 1/r)and pressure is lowest there (p ~ pinf -1/r2)– promotes sucking action from surfaceto core (entrainment)
Vortices can move, stretch, twist,merge and interact in complex ways; itcan carry mass, momentum and energyfrom one spot to other
(animation courtesy: wikipedia.com)
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Vortex Core Regions: New SR (Full Open)
SR Position 1~ full open (6.86 in.)
Vortex core regions based on “swirling strength”: responsible forentrainment (larger core regions not favorable)
Eigen analysis of local velocitygradient tensor:
The tensor has one realeigenvalue & and a pair ofconjugate complexeigenvalues, depending uponthe discriminant
is called Swirling Strength,and represents the strengthof the local swirling motion
(real) (complex)
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Vortex Core Regions: old SR (Full Open)
SR Position 1~ full open (6.86 in.)
A dramatic change in vortexcore region found: old SR has
almost three times more vortexcore area for the same“swirling strength” ascompared to new SR
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Vortex Core Regions: Old SR
SR Position 11.5 in.
(full open)
SR Position 25.5 in.
SR Position 48 in.
SR Position 69.5 in.
(almost closed)
Old SR case has more vortex core areas for all SR positions(almost same for all SR positions)
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Vortex Core Regions: New SR
SR Position 16.86 in.
SR Position 39 in.
SR Position 410 in.
SR Position 610.8 in.
New SR case has less vortex core areas for all SR positions(about 3 times lesser than old SR)
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Part 2: Effect of RemovingNozzle Well Funnel
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Geometry: With Funnel vs. No Funnel
New SR New SR
Funnel No Funnel
Nozzlepiece
Nozzlepiece
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Velocity Vectors Around SR Tip
As expected, no stagnation zone around SR tip
SR Position 1(1 in.)
SR Position 2(2 in.)
SR Position 3(2.5 in.)
SR Position 4(2.8 in., almost closed)
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Wall Shear Stress: With Funnel
(different views forSR Position 1: fullopen case)
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Wall Shear Stress: No Funnel
(different views forSR Position 1)
No dramatic change in wall shear stress if funnel is removed
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Vortex Core Regions: With Funnel
SR Position 16.86 in.
SR Position 39 in.
SR Position 410 in.
SR Position 610.8 in.
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Vortex Core Regions: No Funnel
SR Position 16.86 in.
SR Position 39 in.
SR Position 410 in.
SR Position 610.8 in.
No dramatic change in vortex core regions if the funnel is eliminated
SR Position 11 in.
SR Position 22 in.
SR Position 32.5 in.
SR Position 42.8 in.
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Part 3: Critical Liquid LevelDuring Pouring to Minimize
Vortexing & Entrainment
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Bath Heights Considered: New SR
SR is 100% open (SR tip 6.86 in. into funnel, 4 in. stroke)
Full bath (~ 30 in.)
20 in. bath
Half bath (~ 15 in.)10 in. bath
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Vortex Core Regions: New SR
(SR is 100% open, flow-rate is 1/3rd of 11,500 lb/min)
1/3rd bath height shows a continuous central vortex core area whichwould increase the risk of entrainment from free surface
Full bath (~ 30 in.) 20 in. bath
Half bath (~ 15 in.) 10 in. bath
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Vortex Core Regions: New SR
(SR is 100% open, flow-rate is 1/3rd of 11,500 lb/min)
1/3rd bath height shows a continuous central vortex core area whichwould increase the risk of entrainment from free surface
Full bath (~ 30 in.) 20 in. bath
Half bath (~ 15 in.) 10 in. bath
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Vortex Core Area per Unit Bath Volume
0
2
4
6
8
10
12
Full 20 in. Half 10 in.
+ 0 %
+ 14 %
+ 31 %
+ 47 %
Vo
rte
xC
ore
Are
a/
Vo
lum
e(m
2/m
3)
For 1/3rd bath height, the specific vortex core area could be ~ 50%higher than a full bath
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Vortexing in Pony Ladle: Effect of SR Movement
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Vortexing in Pony Ladle: Effect of SR Movement
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Vortexing in Pony Ladle: Effect of SR Movement
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Part 4: Modeling the Effectof an Impact Pad
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Turbulence Suppressing Impact Pad
A well designed impact pad would inhibit turbulence & vortexing, provide a quieter flowfield, increase residence time for inclusion flotation, reduce wall shear, etc.
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Free Surface Velocity Field
An impact could reduce the free surface flow activity by up to 35%
no impact padflow activity: 0.236 m/s
central impact pad0.159 m/s
6 in. off-centric0.154 m/s
12 in. off-centric0.163 m/s
(all cases correspond to SR Position 1)
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Velocity on a Vertical Plane
Flow activity could be ~ 28% less on a vertical plane
no impact pad central impact pad
6 in. off-centric 12 in. off-centric
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Velocity Vectors Around SR Tip
The local recirculation zone disappears around the SR tip evenwithout changing the floor or funnel taper
no impact pad central impact pad
6 in. off-centric 12 in. off-centric
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Streamlines
Streamline: A mass less particle will essentially follow these lines (spaghettis) – a twisted line on free surface creates aswirling flow (vortex) and increases the risk of entrainment of another phase (e.g., re-oxidized scum on free surface);
possible solution: use a shroud to protect the jet and guide the flow
(different views forSR Position 1 case,new SR)
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Streamlines
Quieter flow (less red spaghettis – higher residence time for inclusion flotation), lessswirl at inlet and less risk of entrainment if an impact pad is used
no impact pad 6 in. off-centricimpact pad
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Vortex Core Regions
Swirling flow area could be 26% lower with impact pad
no impact pad central impact pad
6 in. off-centric 12 in. off-centric
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Wall Shear Stress: Erosion Risk
Wall shear stress is reduced dramatically – less risk of solidified ponyscum layers being scrapped off due to fluid flow
no impact pad central impact pad
6 in. off-centric 12 in. off-centric
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Summary of Results
Part 1: Modeling of old-style stopper rod vs. new-stylestopper rod Higher vortexing (and hence higher risk of entrainment)
found for old-style stopper rod
Part 2: Modeling of nozzle well vs. no nozzle well No significant change in fluid flow, except vanishing
stagnant zone around SR tip
Part 3: Modeling of critical liquid level during pouring 10 -15 in. is the lowest safe level to avoid entrainment due to
vortexing; avoid periodic shut-offs/SR movement
Part 4: Modeling of impact pad Use of a turbulence-inhibiting impact pad improves all
aspects of the fluid flow in pony ladle
Research motivation: this work was initiated to eliminateinclusionary defects M1 and M2.
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√ Which exogenous sources are responsible for creating these macro-inclusions in as-cast ingots?
√ What experimental techniques can be used to understand these phenomena at the laboratory scale and do they represent the industrial situation? Does the composition of exogenous source change in steel with time?
√ If the sources are from steelmaking, what cause these inclusions to transport in the ingot during teeming?
Multiple possible reasons for entrapment of pony scum from the pony ladleto the mold were found by performing CFD analysis of fluid flow conditionsin the pony ladle. The major contributors are: the design of stopper rod(SR), throttling of SR towards the end of teeming, funnel design at thebottom of the pony ladle and residual bath height in the pony ladle towardscompletion of teeming.
•What steps can be taken to eliminate the conditions that cause these macro-inclusions to becomeentrapped in the ingots?
•What causes these macro-inclusions to concentrate at a certain distance from the bottom of the ingot?
Research motivation: this work was initiated to eliminateinclusionary defects M1 and M2.
67
√ Which exogenous sources are responsible for creating these macro-inclusions in as-cast ingots?
√ What experimental techniques can be used to understand these phenomena at the laboratory scale and do they represent the industrial situation? Does the composition of exogenous source change in steel with time?
√ If the sources are from steelmaking, what cause these inclusions to transport in the ingot during teeming?
√ What steps can be taken to eliminate the conditions that cause these macro-inclusions to become entrapped in the ingots?
Increasing rinse time at VD towards the end of treatment, use of new designof SR, reduction in throttling of SR towards end of teeming, revisitingcleaning practices of the pony ladle, and leaving additional bath height inthe pony ladle towards the completion of teeming have shown improvementand elimination of macro-inclusion M1 from this product line. In future,impact pads will be trialed to evaluate their influence on pony ladle fluid flowand quality of steel.
•What causes these macro-inclusions to concentrate at a certain distance from the bottom of the ingot?
68
Summary and Conclusions