source and transport mechanism of exogenous inclusions in

007/07/2015 Confidential 0

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

1

2

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.

3

Steelmaking process flow at AM Steelton

Set up of vacuum stream pouring process at ArcelorMittalSteelton: position of pony ladle over vacuum dome

4

Ultrasonic (UT) indications on the breech end of theforging (bottom) and location across the radius in a disc(top) sectioned along dotted green line.

5

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

6

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?

8

Identification of Sources

Chemical composition of potential sources

9

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

10

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?

11

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)


12

√ 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?

13

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

16

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.


18

√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?

19

Numerical modeling of fluid flow (CFD) inpony ladle

20

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

21

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

22

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

23

Part 1: Old Stopper Rodvs.

New Stopper Rod

24

Simulation Setup -- Geometry

Only the fluid volume has been modeled

nozzle

stopper rod

nozzle well(funnel)

(SR+funnel+nozzle)

25

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)

26

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

27

Geometry: Old SR vs. New SR

Previous SR design had a wider tip and body

Old SR

New SR

28

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.

29

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

30

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.


low velocity zone

low velocity zone

31

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

32

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

33

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

34

Wall Shear Stress: Full Open (New SR)


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)

35

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

36

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)

37

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)

38

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

39

Vortex Core Regions: Old SR


(full open)


SR Position 48 in.


(almost closed)

Old SR case has more vortex core areas for all SR positions(almost same for all SR positions)

40

Vortex Core Regions: New SR


SR Position 39 in.

SR Position 410 in.


New SR case has less vortex core areas for all SR positions(about 3 times lesser than old SR)

41

Part 2: Effect of RemovingNozzle Well Funnel

42

Geometry: With Funnel vs. No Funnel

New SR New SR

Funnel No Funnel

Nozzlepiece

Nozzlepiece

43

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)

44

Wall Shear Stress: With Funnel


45

Wall Shear Stress: No Funnel

(different views forSR Position 1)

No dramatic change in wall shear stress if funnel is removed

46

Vortex Core Regions: With Funnel


SR Position 39 in.

SR Position 410 in.


47

Vortex Core Regions: No Funnel


SR Position 39 in.

SR Position 410 in.


No dramatic change in vortex core regions if the funnel is eliminated

SR Position 11 in.

SR Position 22 in.



48

Part 3: Critical Liquid LevelDuring Pouring to Minimize

Vortexing & Entrainment

49

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

50


(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

51


(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

52

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

53

Vortexing in Pony Ladle: Effect of SR Movement

54


55


56

Part 4: Modeling the Effectof an Impact Pad

57

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.

58

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)

59

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

60

Velocity Vectors Around SR Tip

The local recirculation zone disappears around the SR tip evenwithout changing the floor or funnel taper



61

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)

62

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

63

Vortex Core Regions

Swirling flow area could be 26% lower with impact pad



64

Wall Shear Stress: Erosion Risk

Wall shear stress is reduced dramatically – less risk of solidified ponyscum layers being scrapped off due to fluid flow



65

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


66


√ 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?


67


√ 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

source and transport mechanism of exogenous inclusions in

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