Investigation of Oscillation Marks and Hook Formation in ULC Steels using Metallurgical Analysis and Models

Transcription

1 Investigation of Oscillation Marks and Hook Formation in ULC Steels using Metallurgical Analysis and Models Ho Jung Shin Department of Mechanical & Industrial Engineering University of Illinois at Urbana-Champaign May 10, 2004 University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 1

2 Acknowledgement Professor Brian G. Thomas Professor Seon-Hyo Kim Dr. Ya Meng Mr. Go-Gi Lee, POSTECH Continuous Casting Consortium POSCO University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 2

3 Introduction Initial solidification during continuous casting of steel slabs greatly affects surface quality of slabs Molten steel flow Superheat Casting speed Heat flux to gap Mold level fluctuation Mold oscillation Initial solidification Mold powder properties and consumption rate Other process parameters Most operation parameters have strong interdependencies This makes prediction and interpretation of initial solidification phenomena difficult University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 3

4 Importance of Initial Solidification Initial solidification behavior affects the formation of oscillation marks (OM) and subsurface hooks Initial solidification features related to surface quality problems - Deep oscillation mark : Transverse crack formation - Deep hook : Easy Entrapment of mold flux and inclusion-laden gas bubbles < Transverse Crack at valley of OM> < Bubble captured by hook > University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 4

5 Modeling analysis Objectives Plant production Metallurgical analysis Gathering plant operation data during production of slabs Gathering samples after production of slabs Model validation Prediction of the phenomena in the mold for different conditions Analysis of the OM profiles Depth, width, pitch Etching analysis of hook characteristics Shape, size, primary dendrite arm spacing Interpretation & Understanding of Initial solidification behavior Investigate effect of casting speed, EMS and superheat Suggest solutions to achieve small hook on slabs University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 5

6 Test Conditions (2003) - Steel grade : Ultra low carbon steel ( C 0.005% ) - Slab thickness : 230 mm - Nozzle submergence depth : ~ 160 mm - Oscillation asymmetry : 59 % 160 mm 104 mm 35 mm 80 mm Test 1 Test 2 Test 3 Test 4 Test 5 Casting speed (m/min) Pour Temperature ( ) Electromagnetic current (A) Slab width (mm) Working mold length (mm) Oscillation stroke (mm) Oscillation frequency (mm) University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 6

7 Mold Powder Properties Melting Characteristics Physical Properties Density (g/ ml ) 2.7 Crystal Temp. ( ) Softening, Melting, Flowing point ( ) 1, Crystal Ratio (%) 6.80 Viscosity (poise) ~ 1000 SF-Tension (d/cm) Solid Temp ( ) 1,101 Shape Granule µ = ' TS T µ s T T fsol fsol n Mold flux n : 1.15 n : 1.75 µ s ' T S : viscosity at 1300 : 1300 n : Exponent for temperature dependency of viscosity Viscosity (poise) ?? 0 {( T s ' - T fsol ) / ( T - T fsol )} n Solid Temperautre = 1101 ( o C) 0 n = 1.15 was chosen for this modeling Temperature ( o C) University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 7

8 Mold Calibration (Heat Flux Profile) Differences of cooling water temperature and mean heat flux Measured ΔT ( ) Measured mean heat flux ( kw/m 2 ) Calculated ΔT ( ) Calculated mean heat flux ( kw/m 2 ) Test Test Test Test Test Distance of thermocouple location below meniscus Test 1 Test 2 Test 3 Test 4 Test 5 Mean temperature from hot thermocouple Test 1 Test 2 Test 3 Test 4 Test 5 1 st (mm) st ( ) nd (mm) nd ( ) rd (mm) rd ( ) University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 8

9 Calibration with measured temperatures of thermocouples Mold 40 mm Hot- Thermocouple Contact resistance 21 mm Measured Temperature (T 1 ) Cooling water Mold 22 mm Predicted Temperature by CON 1D (T 2 ) Cooling water channel Mold 22 mm (T 2 ) (T 1 ) Cooling water Mold? mm (T 1 ) Predicted Temperature by CON 1D (T 2 ) < Fig 1. Thermocouple location> < Fig 2. Predicted temperature > by CON 1D < Fig 3. Temperature gradient between > water channel and thermocouple < Fig 4. Concept of Offset> University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 9

10 Matching model with mold temperature Predicted Thermocouple Temperature ( 7mm offset) Measuered Thermocouple Temperature 180 Predicted Thermocouple Temperature ( 7mm offset) Measuered Thermocouple Temperature 180 Predicted Thermocouple Temperature ( 7mm offset) Measuered Thermocouple Temperature 180 Temperature ( o C) Temperature ( o C) Temperature ( o C) Distance below meniscus (mm) < Test 1 > Distance below meniscus (mm) < Test 2 > Distance below meniscus (mm) < Test 3 > Predicted Thermocouple Temperature ( 7mm offset) Measuered Thermocouple Temperature Predicted Thermocouple Temperature ( 7mm offset) Measuered Thermocouple Temperature Temperature ( o C) Temperature ( o C) Distance below meniscus (mm) < Test 4 > Distance below meniscus (mm) < Test 5 > University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 10

11 Heat Flux Comparison Heat Flux (W/m 2 ) Test 1 (Vc = 1.75 m/min, Hf = 1.71 MW/m 2 ) Test 2 (Vc = 1.42 m/min, Hf = 1.43 MW/m 2 ) Test 3 (Vc = 1.22 m/min, Hf = 1.42 MW/m 2 ) Test 4 (Vc = 1.47 m/min, Hf = 1.45 MW/m 2 ) Test 5 (Vc = 1.47 m/min, Hf = 1.54 MW/m 2 ) Distance below meniscus (mm) University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 11

12 Mold Temperature Comparison Temperature of hot face mold ( o C) Test 1 (Vc = 1.75 m/min, Hf = 1.71 MW/m 2 ) Test 2 (Vc = 1.42 m/min, Hf = 1.43 MW/m 2 ) Test 3 (Vc = 1.22 m/min, Hf = 1.42 MW/m 2 ) Test 4 (Vc = 1.47 m/min, Hf = 1.45 MW/m 2 ) Test 5 (Vc = 1.47 m/min, Hf = 1.54 MW/m 2 ) Distance below meniscus (mm) University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 12

13 Mold Temperature Comparison Temperature of cold face mold ( o C) Test 1 (Vc = 1.75 m/min, Hf = 1.71 MW/m 2 ) Test 2 (Vc = 1.42 m/min, Hf = 1.43 MW/m 2 ) Test 3 (Vc = 1.22 m/min, Hf = 1.42 MW/m 2 ) Test 4 (Vc = 1.47 m/min, Hf = 1.45 MW/m 2 ) Test 5 (Vc = 1.47 m/min, Hf = 1.54 MW/m 2 ) Distance below meniscus (mm) University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 13

14 Shell Temperature Comparison Temperature of shell ( o C) Test 1 (Vc = 1.75 m/min, Hf = 1.71 MW/m 2 ) Test 2 (Vc = 1.42 m/min, Hf = 1.43 MW/m 2 ) Test 3 (Vc = 1.22 m/min, Hf = 1.42 MW/m 2 ) Test 4 (Vc = 1.47 m/min, Hf = 1.45 MW/m 2 ) Test 5 (Vc = 1.47 m/min, Hf = 1.54 MW/m 2 ) Distance below meniscus (mm) University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 14

15 Mold Powder Layer Thickness in Gap Thickness (mm) Test 1 (Solid layer), (Liquid layer) Test 2 (Solid layer), (Liquid layer) Test 3 (Solid layer), (Liquid layer) Test 4 (Solid layer), (Liquid layer) Test 5 (Solid layer), (Liquid layer) Distance below meniscus (mm) University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 15

16 Shell Thickness Comparison Shell Thickness (mm) Test 1 (Vc = 1.75 m/min, Hf = 1.71 MW/m 2 ) Test 2 (Vc = 1.42 m/min, Hf = 1.43 MW/m 2 ) 4 Test 3 (Vc = 1.22 m/min, Hf = 1.42 MW/m 2 ) Test 4 (Vc = 1.47 m/min, Hf = 1.45 MW/m 2 ) 2 Test 5 (Vc = 1.47 m/min, Hf = 1.54 MW/m 2 ) Distance below meniscus (mm) University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 16

17 Measurement of OM Profile X axis Method Y axis Starting point Center of narrow slab face 115 Casting Direction Profile of oscillation mark (mm) Y = 0 mm, Y = 10 mm, Y = 20 mm, Y = 30 mm, Y = 40 mm, Y = 50 mm X axis : 0 ~ 80 mm Y axis : 0 ~ 200 mm (10 mm Interval) X Axis (mm) < Example of Oscillation Mark Profiles > University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 17

18 Average Results of OM Profiles OM Pitch (mm) Calculated Measured OM Depth (mm) OM Width (mm) Oscillation stroke (mm) Oscillation frequency (cycle / minute) Test Test Test Test Test Oscillation mark depth (mm) Averaged OM depth of of Test 1 Averaged OM depth of of Test 2 Averaged OM depth of of Test 3 Averaged OM depth of of Test 4 Averaged OM depth of of Test Casting speed (m/min) Stoke Set Point = A S + B S V C Frequency Set Point = A F + B F V C A F, B F, A S, B S : Coefficients, F: Frequency, S: Stroke, V C : Casting speed Same tendency with stroke and frequency of mold oscillation University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 18

19 16 14 Test 1 (Vc : 1.75 m / min, cating width : 1300 mm) Mean pitch of oscillation mark Calculated oscillation mark pitch (= Vc / Frequency ) Results of Mean Pitch of OM around Strand Perimeter Test Test 2 (Vc (Vc: : m // min, min, cating cating width width :: mm) mm) Mean Mean pitch pitch of of oscillation oscillation mark mark Calculated Calculated oscillation oscillation mark mark pitch pitch (= (= Vc Vc // Frequency Frequency )) Test Test 3 (Vc (Vc: : m // min, min, cating cating width width :: mm) mm) Mean Mean pitch pitch of of oscillation oscillation mark mark Calculated Calculated oscillation oscillation mark mark pitch pitch (= (= Vc Vc // Frequency Frequency )) Mean Pitch of OM (mm) Mean Pitch of of OM (mm) Mean Pitch of of OM (mm) Distance from center of narrow slab face (mm) < Test 1 - EMS 313 A > Distance from center of of narrow slab face (mm) Distance from center of of narrow slab face (mm) < Test 2 - EMS 234 A > < Test 3 - EMS 0 A > Test 4 (Vc : 1.47 m / min, cating width : 1570 mm) Mean pitch of oscillation mark Calculated oscillation mark pitch (= Vc / Frequency ) Test 5 (Vc : 1.46 m / min, cating width : 1570 mm) Mean pitch of oscillation mark Calculated oscillation mark pitch (= Vc / Frequency ) Mean Pitch of OM (mm) Mean Pitch of OM (mm) Distance from center of narrow slab face (mm) Distance from center of narrow slab face (mm) < Test 4 - EMS 277 A > < Test 5 - EMS 0 A > Higher EMS causes greater pitch variations University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 19

20 Relation between Level Fluctuation and OM Pitch Measured Level fluctuation with Eddy Current Sensor (mm) 3 Test 1 Test 2 Test 3 Test 4 Test Time (sec) Standard deviation of OM pitch difference (mm) Test 1 ( Vc= 1.75 m/min) Test 2 ( Vc= 1.42 m/min) 0.5 Test 3 ( Vc= 1.22 m/min) Test 4 ( Vc= 1.47 m/min) Test 5 ( Vc= 1.47 m/min) Standard deviation of level fluctuation (mm) - Standard deviation of level fluctuation : average value during 45 seconds while the samples were produced - OM pitch difference = ( Measured OM Pitch ) ( Calculated OM Pitch ) * Calculated OM Pitch = casting speed / Oscillating frequency University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 20

21 Relation OM Pitch and Depth OM Depth (mm) Test 1 Test 2 Test 3 Test 4 Test 5 Mean depth of OM (mm) Test 1 (( Vc= 1.75 m/min) Test 2 (( Vc= 1.42 m/min) Test 3 (( Vc= 1.22 m/min) Test 4 (( Vc= 1.47 m/min) Test 5 (( Vc= 1.47 m/min) OM pitch difference (mm) Standard deviation of OM pitch difference (mm) OM pitch difference = ( Measured OM Pitch ) ( Calculated OM Pitch ) * Calculated OM Pitch = casting speed / Oscillating frequency University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 21

22 Definition of Hook Characteristics Hook angle Effective Hook Depth (EHD) Hook length Hook Shell thickness Oscillation mark depth University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 22

23 Measured Hook Characteristics Effective hook depth (mm) Hook Length (mm) Hook Angle (degree) Hook Shell thickness (mm) OM Depth (mm) Test Test Test Test Test Effective hook depth (mm) Average EHD of of Test 1 Average EHD of of Test 2 Average EHD of of Test 3 Average EHD of of Test 4 Average EHD of of Test Casting speed ( m/min ) Hook length (mm) Average Hook Length of of Test 1 Average Hook Length of of Test 2 Average Hook Length of of Test 3 Average Hook Length of of Test 4 Average Hook Length of of Test Casting speed ( m/min ) University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 23

24 CON 1D Predictions at Meniscus Region Shell Temperature ( o C ) Test 1 ( Vc = 1.75 m/min ) Test 2 ( Vc = 1.42 m/min ) Test 3 ( Vc = 1.22 m/min ) Test 4 ( Vc = 1.47 m/min ) Test 5 ( Vc = 1.47 m/min ) Shell Thickness (mm) Test 1 (( Vc Vc = 1.75 m/min )) Test 2 (( Vc Vc = 1.42 m/min )) Test 3 (( Vc Vc = 1.22 m/min )) Test 4 (( Vc Vc = 1.47 m/min )) Test 5 (( Vc Vc = 1.47 m/min )) Distance below meniscus (mm) Distance below meniscus (mm) - Ignoring local effect of oscillation mark University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 24

25 Comparison hook shell thickness with modeling prediction 3 CON1D: Solidus and liquidus locations in mold Starting point of solidification 1 mm Location (mm) Starting point of solidification test1 Solidus test1 Liquidus 1 mm mm 1 mm Distance below meniscus (mm) Time (sec) CON 1D Predition Measured data mm Shell thickness (mm) mm < Test 1 > Distance below meniscus (mm) University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 25

26 Typical Hook Shape Matching to Model Prediction Time (sec) CON 1D Prediction Measured data T p? 0.5 mm Melting? Hook thickness (mm) Melted? T p = sec Distance below meniscus (mm) 1 mm < Test 3 > University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 26

27 Comparison of Shell Thickness Measured shell Thickness just above OM (mm) Average Test 1 Test 2 Test 3 Test 4 Test Predicted shell thickness at T p (mm) - Scatter due to local variations in undisturbed hook solidification time or level fluctuation University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 27

28 Relation between EHD and shell thickness 2.5 Effective Hook Depth (mm) Test 1 ( Vc = 1.75 m/min ) Test 2 ( Vc = 1.42 m/min ) Test 3 ( Vc = 1.22 m/min ) Test 4 ( Vc = 1.47 m/min ) Test 5 ( Vc = 1.47 m/min ) Measured Hook Shell Thickness (mm) University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 28

29 Relation between Hook and Superheat 6.5 Length of solid shell left 6.0 Test 1 Test 2 Test 3 Test 4 Test 5 Superheat = Tundish Temperature Liquidus Temperature Length of solid shell during T p (mm) Length of left solid shell (mm) Remelted length of solid shell (mm) Average 3.5 Test 1 Test 2 Test Test 4 Test Tundish temperature ( ) 1560 Liquidus temperature ( ) - CON 1D University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 29 Melted length of solid shell (mm) Tundish Temperature - Liquidus Temperature ( o C) (Superheat) Occurrence hook rate (%)

30 Occurrence rate of Hook with Superheat Hook rate Segregation rate Occurence rate of hook (%) Occurence rate of segregation (%) (T Tundish - T Liquidus ) ( o C) University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 30

31 Relation between Melted Shell and Superheat Melted length of solid shell (mm) Test 1 Test 2 Test 3 Test 4 Test Tundish Temperature - Liquidus Temperature ( o C) (Superheat) - Assumption : no hook occurrence case and segregation hook type are all melted - EMS off : little effect to melt shell - EMS on : longer shell melted with increasing superheat University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 31

32 Conclusions Modeling analysis of shell formation focusing on initial hook formation was conducted with CON1D and parameters of real plant operation Measurement of OM depth, pitch and hook shape were made from samples of continuous casting slabs for different casting speeds, EMS power and superheat OM pitch is directly related to mold level fluctuations and deviations indicate the stability of level during production Increasing OM pitch deviation is correlated with deeper oscillation mark depth Hook shell thickness can be predicted using CON1D and the hook shell thickness at top of OM correlates with positive strip time Effective hook depths increase in proportion to hook shell thickness Hook length correlates with size and shape : longer hooks are deeper EHD Increasing superheat decreases hook length, likely due to melting. Increasing EMS correlates with more OM pitch deviation, level fluctuation and deeper hooks Increasing casting speed effect is complex because it increases both level fluctuation and superheat. University of Illinois at Urbana-Champaign Metals Processing Simulation Lab Ho-Jung Shin 32