Research Background: Transient Surface Flow Problems

Transcription

1 CCC Annual Report UIUC, August 14, 2013 EMBr Effect on Mold Level Fluctuations POSTECH: Seong-Mook Cho, Seon-Hyo Kim UIUC: Brian G. Thomas POSCO: Yong-Jin Kim Research Background: Transient Surface Flow Problems Soild flux SEN (Submerged Entry Nozzle) layer Liquid flux layer (1) High surface flow velocity (3) (1) (2) Mold flux entrainment by increasing the instability between mold flux layer Argon and molten steel layer bubbles Jet flow (2) Level fluctuation Mold flux entrapment by time-variations Solidifying steel shell in the interface level between mold flux layer and molten steel layer Molten steel < Flow phenomena in the mold of steel continuous caster > Transient surface flow problems (3) Vortex formation Mold flux entrainment by pulling a funnel of mold flux into the mold Slab surface defects Pohang University of Science and Technology Department of Materials Science and Engineering Seong-Mook Cho 2/28

2 Braking Research Background: Electromagnetic Systems - Local Electro-Magnetic Brake(EMBr): Locally braking using Direct Current (DC) - Double-ruler EMBr FC (Flow Control): Linear braking using DC - EMLS (Electro-Magnetic Level Stabilizer): Moving braking using Alternating Current (AC) Accelerating - EMLA (Electro-Magnetic Level Accelerater): Moving accelerating using AC Rotating - EMRS (Electro-Magnetic Rotate Stirrier): Moving rolating using AC Pohang University of Science and Technology Department of Materials Science and Engineering Seong-Mook Cho 3/28 Research Scope Objectives: - To gain insight of double-ruler EMBr (FC) effect on transient surface flow pattern and surface level fluctuation Methodologies: - Computational modeling for understanding nozzle and mold flow pattern without and with EMBr - Nail board dipping tests & eddy current sensor measurements for visualizing surface flow pattern, level and quantifying surface velocity, level fluctuation Pohang University of Science and Technology Department of Materials Science and Engineering Seong-Mook Cho 4/28

3 Process Conditions Steel flow rate Casting speed Argon gas injection rate Flow control system Bottom type Port angle LPM (3.9 ton/min) 1.70 m/min 9.2 SLPM (1atm and 273K); 33.0 LPM (1.87 atm, 1827 K) & 5.6 % (hot) Slide-gate Well bottom (depth: 19 mm) 35 degree angle at both top and bottom Nozzle Port area Bore diameter (inner/outer) 80mm (width) x 85mm (height) 90 mm (at UTN top) to 80 mm (at bottom well) / 160 mm (at UTN top) to 140 mm (at SEN bottom) Width Mold Thickness EMBr Current 1300 mm 250 mm Upper: 300A, Lower: 300A Pohang University of Science and Technology Department of Materials Science and Engineering Seong-Mook Cho 5/28 Computational Modeling - LES coupled with Lagrangian DPM - Standard k ε model coupled with MHD - ~1.8 million cells Pohang University of Science and Technology Department of Materials Science and Engineering Seong-Mook Cho 6/28

4 Nail Board Dipping Test 148 mm 63 mm 50 mm SEN STS nail 650 mm OR 70 mm 110 mm 63 mm N F 250 mm 80 mm Al nail IR 70 mm STS steel bent rods STS steel bent rods Wood nail board Fixed outer cover Mold top Solid mold flux Sintered mold flux Liquid mold flux Molten steel Aluminum nail STS nail Inner cover region Mold 250 mm OR IR <Photos & schematics of nail board in the mold> Pohang University of Science and Technology Department of Materials Science and Engineering Seong-Mook Cho 7/28 Eddy Current Sensor Measurements Tundish UTN Left Upper plate Middle plate Lower plate SEN Argon bubbles Molten steel Slide-Gate Eddy current sensor Jet flow Mold top Solidifying steel shell Controler Solid mold flux Sintered mold flux Liquid mold flux Right - Position: Quarter point located midway between the SEN and the - 1 sec time-averaging for controlling the surface level sec recording Mold < Position of eddy current sensor > Pohang University of Science and Technology Department of Materials Science and Engineering Seong-Mook Cho 8/28

5 Transient Nozzle Flow Towards IR Towards OR Time-averaged 4.8 sec 13.8 sec 15.0 sec - Clockwise rotating flow pattern in the nozzle well - When clockwise rotating flow becomes weak, small counterclockwise rotating flow is also induced in the nozzle well Pohang University of Science and Technology Department of Materials Science and Engineering Seong-Mook Cho 9/28 Transient Mold Flow Velocity magnitude (m/sec) Time-averaged 4.8 sec 18.6 sec 19.8 sec - Up-and-down wobbling of the jet flow induces variations of velocity magnitude and direction at the surface and changes the jet flow impingement point on the Pohang University of Science and Technology Department of Materials Science and Engineering Seong-Mook Cho 10/28

6 Argon Gas Distribution Argon gas volume fraction sec 19.2 sec 19.8 sec - The jet wobbling also influences argon gas distribution with time in the mold Pohang University of Science and Technology Department of Materials Science and Engineering Seong-Mook Cho 11/28 Surface Flow Pattern 0.2 m/sec OR Match SEN Time-averaged 4.8 sec IR Average of measurements during 9 min 18.6 sec 19.8 sec - Surface flow mostly goes towards to the SEN - Transient asymmetric flow between the IR and the OR mainly goes towards to the IR at the region near the OR and shows random variation in the region near the IR Pohang University of Science and Technology Department of Materials Science and Engineering Seong-Mook Cho 12/28

7 Model Validation Surface level height (mm) sec (prediction) 12.6 sec (prediction) 13.2 sec (prediction) 13.8 sec (prediction) 14.4 sec (prediction) 15.0 sec (prediction) Nail board (measurement) Surface velocity magnitude (m/sec) sec (prediction) 12.6 sec (prediction) 13.2 sec (prediction) 13.8 sec (prediction) 14.4 sec (prediction) 15.0 sec (prediction) Nail board (measurement) - LES coupled with Lagrangian DPM shows a very good quantitative match with the average surface profile and velocities - The model under-predicts the magnitude of the measured variations of both level and velocity, likely due to the short modeling time, which is insufficient to capture the important low-frequency fluctuations Pohang University of Science and Technology Department of Materials Science and Engineering Seong-Mook Cho 13/28 Applied Magnetic Field by Double-ruler EMBr Center 350mm 700mm Distance from mold top (mm) mm Measured Extrapolated Center: 350mm: 700mm: Magnetic field strength (Tesla) Narrow Face () Weaker Magnitude of external magnetic field (Tesla) Pohang University of Science and Technology Department of Materials Science and Engineering Seong-Mook Cho 14/28

8 Electromagnetic Force - Much bigger in the nozzle regions - Two regions in the mold; smaller near the <Nozzle> <Mold> Pohang University of Science and Technology Department of Materials Science and Engineering Seong-Mook Cho 15/28 EMBr Effect on Nozzle Flow: Velocity Magnitude - Not effective to break the velocity in the nozzle Pohang University of Science and Technology Department of Materials Science and Engineering Seong-Mook Cho 16/28

9 EMBr Effect on Nozzle Flow: Turbulent Kinetic Energy (TKE) - With EMBr, TKE is decreased in the nozzle well region, where rotating swirl flow Pohang University of Science and Technology Department of Materials Science and Engineering Seong-Mook Cho 17/28 EMBr Effect on Mold Flow Velocity magnitude (m/sec) Turbulent kinetic energy (m 2 /sec 2 ) No EMBr EMBr No EMBr EMBr - Jet flow is deflected downward, resulting in slower surface flow - TKE is reduced at the surface, but increased deep into the mold Pohang University of Science and Technology Department of Materials Science and Engineering Seong-Mook Cho 18/28

10 Surface Velocity 2010 Trial Surface velocity magnitude (m/sec) EMBr off: EMBr on: IR OR 0.00 Stdev of surface velocity by time (m/sec) IR OR 0.02 EMBr off: 0.01 EMBr on: 0.00 Surface velocity Surface velocity fluctuation No EMBr EMBr Velocity 0.22 m/sec 0.18 m/sec ~20% Velocity fluctuation 0.12 m/sec 0.07 m/sec ~40% Pohang University of Science and Technology Department of Materials Science and Engineering Seong-Mook Cho 19/28 Distance from mold top(mm) Surface Level Variation Measured Avg level:~103 mm EMBr off: EMBr on: Time(sec) Surface level variation by time by Eddy-Current Sensor Power spectrum(mm 2 ) EMBr off: EMBr on: E Frequency(Hz) 2010 Trial ~0.03 Hz (35 sec) Power spectrum of level variation Surface level fluctuation No EMBr EMBr 0.6 mm 0.4 mm 33 % Pohang University of Science and Technology Department of Materials Science and Engineering Seong-Mook Cho 20/28

11 Distance from mold top (mm) Surface Level : Nail Board Tests EMBr off: EMBr on: Steel Slag 115 Average of sensor measurements EMBr off 120 EMBr on Stdev of surface level by time (mm) Stdev of sensor measurements EMBr off EMBr on 2010 Trial EMBr off: EMBr on: Steel Slag No EMBr EMBr Surface level fluctuation Sensor 0.6 mm 0.4 mm 33 % Nail board 4 mm 3 mm 25 % Pohang University of Science and Technology Department of Materials Science and Engineering Seong-Mook Cho 21/28 Surface Level Profiles (2008 nailboard trial) Distance from mold top (mm) : Average of sensor measurements (EMBr off) 100 Time: minute without EMBr Distance from mold top (mm) : Average of sensor measurements (EMBr on) 100 Time: minute With EMBr No EMBr EMBr Level Profile variation 12 mm 20 mm Level fluctuation 25 mm 15 mm ~40% Pohang University of Science and Technology Department of Materials Science and Engineering Seong-Mook Cho 22/28

12 Surface Level Variations (2008 trial) EMBr off Stdev of surface level by time (mm) EMBr off EMBr on S E N Eddy current sensor EMBr on S E N Pohang University of Science and Technology Department of Materials Science and Engineering Seong-Mook Cho 23/28 Level height of liquid mold flux (mm) OR 4 SEN IR -2-4 Region 1: h slag = h Fe Region 2: -6 h slag = h Fe Region 3: h slag = h Fe Level height of molten steel (mm) No EMBr 135 mm 235 mm Slag Motion 485 mm 585 mm Region 1: h slag= h Fe Region 2: h slag = h Fe Region 3: h slag = h Fe Level height of molten steel (mm) Pohang University of Science and Technology Department of Materials Science and Engineering Seong-Mook Cho 24/28 Level height of liquid mold flux (mm) OR SEN IR EMBr 2010 Trial 135 mm Motion of the steel-slag interface level mainly causes lifting of the slag layers near the SEN. Elsewhere, the slag layers are partially displaced by the steel near the, especially with EMBr - Slag pool is slightly thicker with EMBr 235 mm 485 mm 585 mm

13 Summary & Conclusions: Transient Two-phase Flow - Nail board dipping tests and eddy current sensor measurements together reveal level, velocity, & variations at the surface during nominally steady-state casting - LES coupled with Lagrangian DPM agrees quantitatively with level & vel. measurements, and trends of fluctuations. - Asymmetric slide-gate opening causes clockwise rotating swirl in the nozzle well leading to surface cross flow - Both with and without EMBr, surface level has large (~8mm) sloshing waves with low frequency ~0.03 Hz (~35 sec) - Surface level fluctuations measured by an eddy-current sensor are much smaller (<1mm) than those by nail board tests, (3-4mm), (due to sensor location and time filtering). - Slag layer is mainly lifted (vs. displaced) by steel motion Pohang University of Science and Technology Department of Materials Science and Engineering Seong-Mook Cho 25/28 Summary & Conclusions: EMBr Effect on Flow - Double-ruler FC-Mold EMBr creates two regions of equalstrength magnetic fields, that decrease greatly towards - EMBr causes: - Lower turbulent kinetic energy in nozzle well - jet deflected downward - flatter surface level with less fluctuations near SEN - 20% slower surface velocity with 40% less variations - Slightly thicker slag pool - EMBr may help to reduce defects caused by surface instability if used properly Pohang University of Science and Technology Department of Materials Science and Engineering Seong-Mook Cho 26/28

14 References - S-M. Cho, G-G. Lee, S-H. Kim, R. Chaudhary, O-D. Kwon, and B. Thomas: Proc. of TMS 2010, TMS, Warrendale, PA, USA, 2010, 71 - S-M. Cho, S-H. Kim, R. Chaudhary, B. G. Thomas, H-J. Shin, W-R. Choi, S-K. Kim: Iron and Steel Technology, Vol.9 (2012), 85 - S. Cho, H. Lee, S. Kim, R. Chaudhary, B.G. Thomas, D. Lee, Y. Kim, W. Choi, S. Kim, and H. Kim: Proc. of TMS2011, TMS, Warrendale, PA, USA, 2011, 59 - H. Shin, S. Kim, B. G. Thomas, G. Lee, J. Park, and J. Sengupta: ISIJ Int., Vol. 46 (2006), K. Cukierski and B. G. Thomas: Metallurgical and Materials Transaction B, Vol. 39B (2008), 94 - K. Timmel, S. Eckert, G. Gerbeth, F. Stefani, and T. Wondrak: ISIJ Int., Vol. 50 (2010), R. Chaudhary, B. G. Thomas, and S.P. Vanka: Metallurgical and Materials Transaction B, Vol. 43B (2012), R. Singh, B.G. Thomas, and S.P. Vanka: Metallurgical and Materials Transaction B, Online (2012) - S. Cho, H. Lee, S. Kim, R. Chaudhary, B.G. Thomas, D. Lee, Y. Kim, W. Choi, S. Kim, and H. Kim: Proc. of TMS2011, TMS, Warrendale, PA, USA, 2011, 59 Pohang University of Science and Technology Department of Materials Science and Engineering Seong-Mook Cho 27/28 Acknowledgments Continuous Casting Consortium Members (ABB, ArcelorMittal, Baosteel, Magnesita Refractories, Nippon Steel and Sumitomo Metal Corp., Nucor Steel, POSTECH/ POSCO, Severstal, SSAB, Tata Steel, ANSYS/ Fluent) POSCO (Grant No ) National Science Foundation Grant CMMI Pohang University of Science and Technology Department of Materials Science and Engineering Seong-Mook Cho 28/28