Hyoung -Jun Lee, Seong -Mook Cho, Seon -Hyo Kim

Thermal Stress Cracking of Sliding Gate Plates

ANNUAL REPORT 2011UIUC, August 18, 2011

Pohang University of Science and Technology Materials Science and Engineering Hyoung-Jun Lee 1

Hyoung -Jun Lee, Seong -Mook Cho, Seon -Hyo Kim

Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang, Kyungbuk 790-784, South Korea

Brian G. Thomas

Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, 1206 W.Green St., Urbana, IL, USA, 61801

Sang-Woo Han, Tae-In Jung, Joo Choi

POSCO Technical Research Laboratories, POSCO, Pohang, Kyungbuk 790-785, South Korea

- Evaluate possible mechanisms for crack formation

- Explore thermal and mechanical stress in a sliding gate plate during preheating and

Objectives

Pohang University of Science and Technology Materials Science and Engineering Hyoung-Jun Lee 2

sliding gate plate during preheating and casting induced by thermal expansion and/or mechanical movement

- Predict crack formation

Research Background

UTNMetal Band

Plate

Ar Injection

Insulation

Bolt

Pohang University of Science and Technology Materials Science and Engineering Hyoung-Jun Lee 3

Cassette

SEN

Insulation

Line Contact

� Why are cracks in sliding gate a concern?- Safety problem (Steel leakage)- Clogging (Air penetration)- Productivity Issue

Schematic of Sliding Nozzle System

Type of Cracks

Bottom view of used plate Photo of common

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Bottom view of used plate

Cross section view of common crack

In Out

Photo of common through-thickness crack

Schematic of rare cracks locations

* Refractory component: 85% Alumina, 7% Zirconia, 8% Graphite

� Through-thickness cracks commonly form during the C.C. process

�If these cracks are serious, air penetration can oc cur: White area on the crack surface indicates that grap hite in

Understanding of cracking

Pohang University of Science and Technology Materials Science and Engineering Hyoung-Jun Lee 5

sliding plate is oxidized�This suggests that steel is being oxidized

� Sometimes, different type of cracks can form, likely caused by abnormal non-uniform cassette pressure on middle of plate (not always through-thickness)

Possible Mechanisms of Common Crack Formation

� Thermal stress induced by temperature distribution of sliding plate with pre-heating and molten steel temperature

� High surface pressure from cassette bolted to the 3 sliding gate plates

Pohang University of Science and Technology Materials Science and Engineering Hyoung-Jun Lee 6

3 sliding gate plates

� Friction force caused by mechanical movements for stabilizing the mold meniscus level

� Ferro-static pressure due to height difference between tundish free surface and sliding gate locat ion

Parameters Considered for Modeling

▽

h

Tundish

UTN

Tundish Free Surface

Schematic of sliding plate front view

Metal Band

Plate

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Ferrostatic Pressure

Thermal Expansion

Comparing with thermal expansion pressure, ferrostatic pressure is negligible

MPa 540

C1000C 10367.8Pa 1065 169

−≈°×°×××−=

∆⋅⋅−=

−−

TEA

F α

MPa 124.0

m 8.1m/s 81.9kg/m 7020

23

≈××=

= ghh ργ UTN

SEN

� Pre-heatingSliding gate is heated from room temperature to pre-determined temperature

LNG (Liquefied Natural Gas) composition [1]

Internal Gas Temperature Prediction

Pohang University of Science and Technology Materials Science and Engineering Hyoung-Jun Lee 8

- Methane(CH4): 88%- Ethane(C2H6): 5%- Propane(C3H8):5%- Butane(C4H6): 2%

Internal gas temp. is lower than flame temp. ���� 750 °°°°C is assumed

Singh VBA model [2] is used Flame Temperature: 1518.3 °°°°C

Free convection : Churchill and Chu equation[3] is used

2

278169

61

])Pr492.0(1[

387.0825.0

++= Ra

Nu ���� hhhho,preheato,preheato,preheato,preheat = = = = 7 W/m7 W/m7 W/m7 W/m2222·K·K·K·Kr

kNuh

2

⋅=

Heat Transfer Coefficient of Preheating

Pohang University of Science and Technology Materials Science and Engineering Hyoung-Jun Lee 9

Forced convection : Petukhov, Kirillov and Popov equation[3] is used

)]1(Pr)8(7.1207.1[

Pr]Re)8[(3221 −+

=f

fNu D

r

kNuh

2

⋅=���� hhhhi,preheati,preheati,preheati,preheat = = = = 65 W/m65 W/m65 W/m65 W/m2222·K·K·K·K

�Molten steel

Molten steel flows through sliding plate bore (1550 ºC)

Sleicher and Rouse Equation[3] is used to calculate forced heat transfer coefficient

Forced Heat Transfer Coefficient of Molten Steel

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forced heat transfer coefficient

�The forced heat transfer coefficient from molten st eel flow was calculated to be 28.7 x 103 W/mW/mW/mW/m2222·K·K·K·K

υur2

Re =αυ=Pr

Pr4

24.088.0

+−=a Pr)6.0(5.0

3

1 −+= Expb

baNu PrRe015.05 += r

kNuh

2

⋅=

200

300

400

Tem

pera

ture

(D

egre

e C

)

20 Elements-Internal Temp. 20 Elements-External Temp. 40 Elements-Internal Temp. 40 Elements-External Temp. Every 1 Sec.-Internal Temp.

Test Problem of Transient Heat ConductionAbaqus Model Validation with Singh VBA Model

ho = 20 W/m2KTo = 20 °C

t = 120 min.Tinitial = 20 °C

Schematic of test problem

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0 20 40 60 80 100 1200

100

Tem

pera

ture

(D

egre

e C

)

Time (min.)

Every 1 Sec.-Internal Temp. Every 1 Sec.-External Temp. Every 100 Sec.-Internal Temp. Every 100 Sec.-External Temp. Singh VBA-Internal Temp. Singh VBA-External Temp.

k = 20 W/mKρ = 2460 kg/m3

Cp = 1500 J/kgK

hi = 70 W/m2KTi = 600 °C

*Other surfaces are insulated

- 20 and 40 elements results match- Time step of every 1 sec. and 100 sec. models matc h�The 100 sec. time step is used to tundish sliding ga te model- Abaqus model matchs Singh VBA model

Model Validation Problem – Ladle Plate Heating

①②③④①②③④①②③④①②③④

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Schematic of casting equipment [4] Location of thermocouples in plate[5]

- Ladle plate temperature was measured during top teeming of molten steel into mold

- 4 Thermocouples were installed on upper plate- Plates were subjected to heat treatment at 170 °°°°C [5]

Heat Transfer Model Validation:Ladle-Plate Heating Problem

h i,preheat = 65 W/m2K, Ti,preheat = 25 ºC h i,steel = 29 x 103 W/m2K , Ti,steel = 1590 ºCε = 0

Insulatedh = 0ε = 0

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Run transient 3-D heat conduction simulation of upp er ladle plate, to compare with measurements

ho,preheat = 7 W/m2K, To,preheat = 25 ºC ho,steel = 7 W/m2K, To,steel = 150 ºC ε = 0.92

Boundary conditionsFinite element mesh

Part Elements

Ladle Plate (Wedge) 92,378

Symbol Value Units

Initial Nozzle Temperature Tinitial 25 °°°°C

Preheating

Internal Gas Temperature Ti,preheat 750 °°°°C

Internal Convection Heat Transfer Coefficient (Forced) h i,preheat 65.24 W/m2·K

External Ambient Temperature To,preheat 25 °°°°C

External Convection Heat Transfer Coefficient (Free) ho,preheat 7 W/m2·K

Molten Steel Temperature Ti,steel 1590 °°°°C

Properties for Ladle Plate Model Validation Problem

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Casting

Ti,steel 1590 C

Internal Convection Heat Transfer Coefficient (Forced) h i,steel 28719.63 W/m2·K

External Ambient Temperature To,steel 150 °°°°C

External Convection Heat Transfer Coefficient (Free) ho,steel 7 W/m2·K

Density [6] ρ 3200 kg/m 3

Thermal Conductivity [6] k 8.26 W/m·K

Specific Heat [6] Cp 1004.64 J/kg· °°°°C

Stefan-Boltzmann Const. σ 5.669 x 10-8 W/m2·K4

Emissivity [7] ε 0.92 -

Comparison of Measured and Predicted Temperature in Ladle Plate

600

800 Measured 1-a Measured 1-b Measured 2-a Measured 2-b

Tem

pera

ture

(D

egre

e C

)Temperature contour, 90 min

Temp ( °°°°C)

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0 20 40 60 80 1000

200

400

600 Measured 2-b Measured 3 Measured 4-a Measured 4-b Predicted 1 Predicted 2-a Predicted 2-b Predicted 3 Predicted 4-a Predicted 4-bT

empe

ratu

re (

Deg

ree

C)

Time (min.)

Preheating Molten steel pouring

� Preheating is calculated until #1 T/C is reached to 150 °°°°C for 50 min.

� During 40 min. casting, experimental and predicted results are well-matched in heat transfer model of ladle plate

Model Validation Summary

Pohang University of Science and Technology Materials Science and Engineering Hyoung-Jun Lee 16

transfer model of ladle plate

� Assumed internal gas temperature (750 °°°°C) is reasonable to be applied to tundish sliding gate nozzle model for preheating stage

� The heat transfer coefficients are used to tundish sliding gate nozzle model

½ Symmetric Model

Tundish Sliding Gate NozzleComponent & Finite Element Mesh

Pohang University of Science and Technology Materials Science and Engineering Hyoung-Jun Lee 17

Part Elements

Upper Plate (Wedge) 15,133

Middle Plate (Wedge) 18,008

Lower Plate (Wedge) 18,782

Upper Band (Hex) 512

Middle Band (Hex) 512

Lower Band (Hex) 384

Total 52,871

Initial ConditionRoom Temp.

Pre-heating (750 °°°°C)Opening Ratio 100%

12600 sec

Mechanical Movement100% � 0%

Vel. : 25 mm/sec

Molten Steel Pouring (1550°°°°C) Mechanical Movement

Molten Steel Pouring (1550°°°°C)

Modeling Approach

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(1550°°°°C)60%

12600 sec

Mechanical Movement0% � 60%

Vel. : 25 mm/sec

(1550°°°°C)0%

750 sec

Mechanical Movement60% � 0%

Vel. : 25 mm/sec

Preheating :3 hr 30 minC.C : 3 hr 30 min

Casting Finished

Casting Started

Symbol Value Units

Initial Nozzle Temperature Tinitial 25 °°°°C

Preheating

Internal Gas Temperature (Preheating) Ti,preheat 750 °°°°C

Internal Convection Heat Transfer Coefficient (Forced) h i,preheat 65.24 W/m2·K

External Ambient Temperature (Preheating) To,preheat 25 °°°°C

External Convection Heat Transfer Coefficient (Free) ho,preheat 7 W/m2·K

Molten Steel Temperature Ti,steel 1550 °°°°C

Internal Convection Heat Transfer Coefficient h 28719.63 W/m2·K

Properties for Tundish Sliding GateHeat Transfer Model

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Casting

Internal Convection Heat Transfer Coefficient (Forced) h i,steel 28719.63 W/m2·K

External Ambient Temperature To,steel 150 °°°°C

External Convection Heat Transfer Coefficient (Free) ho,steel 7 W/m2·K

Refractory [6]

Density ρref 3200 kg/m 3

Thermal Conductivity kref 8.26 W/m·K

Specific Heat Cp,ref 1004.64 J/kg· °°°°C

Steel [6]

Density ρsteel 7860 kg/m 3

Thermal Conductivity ksteel 48.6 W/m·K

Specific Heat Cp,steel 418.6 J/kg· °°°°C

Stefan-Boltzmann Const. σ 5.669 x 10-8 W/m2·K4

Emissivity [7]εref 0.92 -

εsteel 0.75 -

Boundary Conditions of Preheating Step

Insulatedh = 0, ε = 0

h = 65 W/m2K Y Symmetry

Touching with nozzle

*Gap conductance= 7 W/m·K

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h i = 65 W/m2KTi = 750 °°°°Cε = 0

ho = 7 W/m2KTo = 25 °°°°Cε = 0.92

h = 7 W/m2KT = 25 °°°°Cε = 0.75

[Schematics of heat transfer model] [Schematics of s tress model]

Y SymmetryUZ=0

Touching with cassetteUX = 0, UY = 0

nozzleUX = 0, UY = 0

*Gap conductance= 0 W/m·K

Boundary Conditions of Molten Steel Pouring Step

Insulatedh = 0, ε = 0

h = 29 x 103 W/m2K Y Symmetry

Touching with nozzleUX = 0, UY = 0

*Gap conductance= 7 W/m·K

Pohang University of Science and Technology Materials Science and Engineering Hyoung-Jun Lee 21

h i = 29 x 103 W/m2KTi = 1550 °°°°Cε = 0

ho = 7 W/m2KTo = 150 °°°°Cε = 0.92

h = 7 W/m2KT = 150 °°°°Cε = 0.75

[Schematics of heat transfer model] [Schematics of stress model]

Y SymmetryUZ=0

Touching with cassetteUX = 0, UY = 0

*Gap conductance= 0 W/m·K

Boundary Conditions of Continuous Casting Step

Insulatedh = 0, ε = 0

h = 29 x 103 W/m2K Y Symmetry

Touching with nozzleUX = 0, UY = 0

*Gap conductance= 7 W/m·K

Pohang University of Science and Technology Materials Science and Engineering Hyoung-Jun Lee 22

h i = 29 x 103 W/m2KTi = 1550 °°°°Cε = 0

ho = 7 W/m2KTo = 150 °°°°Cε = 0.92

h = 7 W/m2KT = 150 °°°°Cε = 0.75

[Schematics of heat transfer model]

Y SymmetryUZ=0

Touching with cassetteUX = 0, UY = 0

[Schematics of stress model]

*Gap conductance= 0 W/m·K

Thermo & Mechanical Behaviorof Preheating Step

Temperature ( °°°°C) Stress (Pa)

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Thermo & Mechanical Behaviorof Molten Steel Pouring Step

Temperature ( °°°°C) Stress (Pa)

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Thermo & Mechanical Behaviorof Continuous Casting Step

Temperature ( °°°°C) Stress (Pa)

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Common Through-Thickness Crack Formation Mechanism – Tensile Stress

Middle section of middle plate, Pre-heating (750 °°°°C), 502 sec

Stress Distribution(Direction &Magnitude)

X

Y

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Magnitude)

Photographof used plate Crack

Crack direction

Compressive strength ≈ 245 MpaTensileStrength≈ 12 Mpa

Middle section of middle plate, Pre-heating (750 °°°°C), 12600 sec

Stress Distribution(Direction &Magnitude)

Common Through-Thickness Crack Formation Mechanism – Compressive Stress

X

Y

Pohang University of Science and Technology Materials Science and Engineering Hyoung-Jun Lee 27

Crack

Magnitude)

Photographof used plate

Compressive strength ≈ 245 MPa

Crack direction

Conclusion

� During preheating step, tensile and compressive stresses exceed tensile and compressive strength���� Crack location matches prediction

Pohang University of Science and Technology Materials Science and Engineering Hyoung-Jun Lee 28

���� Crack location matches prediction

� If cracks are already formed in preheating step, different stress and temperature distributions will result

Future Work

� Creep and residual stress behavior is needed to better investigate cracks in sliding gate

� Thermal-stress simulation should extend after preheating

Pohang University of Science and Technology Materials Science and Engineering Hyoung-Jun Lee 29

after preheating

� Cassette design is needed for exact deformation and pressure to plates

� UTN and SEN design is needed for knowing contact region

References

[1] Korea Gas Safety Corporation[2] Varun Kumar Singh, B.G. Thomas, CCC Annual report 2010, UIUC, August 12, 2010[3] Incropera, F.P., and Dewitte, P.D., 2002, Fundamentals of Heat and Mass Transfer, John Wiley and Sons, New York[4] “New Generation Ladle Slide Gate System for Performance

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[4] “New Generation Ladle Slide Gate System for Performance Improvement”, J. Chaudhuri, 2007[5] “Thermal Loading of Periclase Plates in Sliding Gates of Steel-teeming Ladles”, K. V. Simonov, 1980[6] Chosun Refractories Co. Ltd. Research Center[7] Monarch Instrument, “Table of Total Emissivity”

Acknowledgements

• Continuous Casting Consortium Members(ABB, Arcelor-Mittal, Baosteel, Tata Steel, Magnesita Refractories, Nucor Steel, Nippon Steel, Postech, POSCO, SSAB, ANSYS-Fluent)

• National Center for Supercomputing Applications (NC SA)

Pohang University of Science and Technology Materials Science and Engineering Hyoung-Jun Lee 31

• National Center for Supercomputing Applications (NC SA) at UIUC – “Tungsten” cluster

• POSCO – Sung-Kwang Kim, Kwon-Myung Lee

• UIUC – Lance C. Hibbeler