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1
Characterization of Inclusions in IF Steelsfrom RH-OB Degasser to Mold
Tsai Hwan-Tang
蔡 煥 堂
2
Purpose of this study
• Characterize the in-process steel cleanliness to develop countermeasures to improve nozzle clogging and steel surface quality.
3
Different mechanisms of nozzle clogging have been proposed.
• Prior formation and transport
– Inclusion formation by deoxidation or reoxidation
– Transport of oxides to nozzle
– Adherence of oxides to nozzle and to existing build-up
• In-situ formation due to cooling
4
Steel grades studied
Grade C Mn P Ti Nb N
A ULC Added Added Added
B ELC Added Added
C ULC Added Added
D ULC Added Added Added Added
5
Steel and Slag Sampling Locations
Ladle
xx
WellBox
Moldx
Well
Mold
x
RHOB
Ladle
Strand 1
First three heats of a sequence Good- and bad-plugging casts
1,2,3,4,5,6,7,8Minutes after Kill
Start Middle
End
Start Middle
End
Start Middle
End
After Cast
6
Outline
• Indication of origin of plugging inclusion from
– Cr2O3 pick-up in tundish slag
– Variation of total oxygen content
– Shape and distribution of inclusions
• Electrochemical method• Remelt button
– Shapes– Changes during processing– In Nozzle clogs
• Trials with ladle sand with less reducible oxides
7
Tundish slag picked up chrome oxide.
• Pouring box
– ~5% Cr2O3
• Above nozzle well
– up to 9% Cr2O3
80 10 20 30 40
0
10
20
30
40
Otot, Avg. Each Heat in Tundish Pouring Box, ppm
Otot, Avg. Each Heat Last 2 RH Samples, ppm
Total oxygen decreased from ladle at the RH-OB to the tundish pouring box.
Higher in Tundish Pour Box
Higher in Ladle
90 10 20 30 40
0
10
20
30
40
Otot, Avg. Each Heat in Tundish Well, ppm
Otot, Avg. Each Heat in Tundish Pouring Box, ppm
In contrast, total oxygen increased from the tundish pouring box to the tundish well.
Higher in Tundish Well
Higher in Tundish Pour Box
10
Total oxygen in the tundish well also increased with increasing residence time in the tundish.
0 10 20 30 405
6
7
8
9
10
11
Otot, Avg. Each Heat in Tundish Well, ppm
Heat Avg. Mean Residence Time, min
11
The increase in total oxygen was much greater than the increase in nitrogen.
O:N for Air
-20 -10 0 10 20-20
-10
0
10
20
Ntot, Heat Average in Well - Pouring Box , ppm
Otot, Heat Average in Well - Pouring Box, ppm
12
Total oxygen
Increased from the tundish pouring box to the tundish well. Increased more with increasing residence time in the tundish.Increase was greater than nitrogen increase.
Total oxygen results suggested oxygen pickup in the tundish by reaction with tundish slag or ladle sand.
13
Alumina inclusion shape - Electrochemical method
0
20
40
60
80
100
.1 - 1.0 1.0 - 5.0 5.0 - 10 10 - 50 >50
Size (um)
%
AgglomerationFlakeGranular
14
Alumina inclusion size - Electrochemical method
1
10
100
1000
10000
< 0.1 .1 -1.0
1.0 -5.0
5.0 -10
10 -50
>50
Size (um)
Number
Number
15
Alumina mass by inclusion size - Electrochemical method
02468
101214
< 0.1 .1 -1.0
1.0 -5.0
5.0 -10
10 -50
>50
Size (um)
Mass, ppm
Al2O3 (ppm)
16
Inclusion Classification
High Surface-Area Faceted Spherical Agglomeration
17
Literature review – Nippon Steel
M. Akiyoshi et al. Nippon Steel Oita R&D (1991)
18
Literature review – Hoogovens (Corus)
Tiekink et al. Hoogovens Ijmuiden (1994)
19
The literature indicates that different alumina inclusions for from different conditions.
High Surface-Area– High super-saturation of O and/or Al– i.e. initial deoxidation or re-oxidation
Faceted– Formation or growth at lower super-saturation– i.e. later deoxidation or cooling
Spherical– 'Ripening' of dendrites– Compaction of agglomerated small inclusions– Local chemical variations in steel
Agglomeration– Collection of inclusions by stirring or bubbling
20
SEM Analysis of Inclusions on Remelt Sample
21
High Surface Area Inclusions
DendriticStarfish
Gingerbread
O, Al
O, Al
O, Al
22
Faceted Inclusions
Faceted, < 2 um Flat, Faceted
Globular, Faceted> 5 um
O, Al O, Al
O, Al
23
Spherical Inclusions
Smooth BallsGlobular, Non-Faceted > 5 um
O, AlO, Al, Mg
24
Agglomeration Inclusions
Coral > 25 um Fine Coral
Lace Balloon
O, Al O, Al
O, Al
25
Inclusions - Steel grade & process location
• There were no definite differences between grades in inclusion shape or size distribution.
• But, there was a remarkable variation of shape and size distributions from ladle to mold.
26
The frequency of small, faceted inclusions (<2 um) was highest at the end of RH-OB treatment.
0
2000
4000
No. of Inclusions
per Six-Photo Strip
Faceted <2 um
RH Pour Well MoldBox
27
The frequency of high surface-area and coral inclusions were highest at RH-OB.
0
5
10N
o.
of
Inc
lus
ion
s p
er
Six
-P
ho
to S
trip
High Surface Area
Spheres > 5um
Coral > 25 um
RH Pour Well MoldBox
28
The frequency of dendritic inclusions at the end of RH-OB treatment increased with decreasing aO at kill.
0 5 10200
250
300
Avg. No. of Inc's with Sec. Arms in Last 2 RH Samples
RHOB aO deox1, ppm
29
The frequency of dendritic inclusions increased with increasing oxygen activity in the tundish slag.
0 1 2 30
2
4
6
8
No. of Inc's with Secondary Arms in Well
%MnO in Well Chamber
30
The frequency of larger globular, faceted inclusions (> 5um) was highest in the tundish.
0
50
No. of Inclusions
per Six-Photo Strip
Globular, Faceted >5um
High Surface Area
Spheres > 5um
Coral > 25 um
RH Pour Well MoldBox
31
The frequency of globular, faceted inclusions >5 um decreased from pouring box to well in the tundish.
0 20 40 60 80 1000
20
40
60
80
100
No. of Globular, Faceted Inc's >5 um in Well
No. of Globular, Faceted Inc's >5 um in Pour Box
More in Tundish Well
More in Tundish Pour Box
32
The frequency of globular, faceted inclusions >5 um decreased from the tundish to the mold.
0 20 40 60 80 1000
20
40
60
80
100
No. of Globular, Faceted Inc's >5 um in Mold
No. of Globular, Faceted Inc's >5 um in Well
More in Mold
More in Tundish Pour Well
33
The number of globular faceted inclusions (>5um) increased as tundish superheat decreased.
0 20 40 60 80 10020
25
30
35
40
45
No. of Globular, Faceted Inc's >5 um in Pour Box
Tundish Superheat, C
34
The size of globular faceted Inclusions increased during casting.
Globular, Faceted Inclusions in the Pouring Box
2 3 4 5 6 7 8 9 10 >100
10
20
30
40
50
microns
Percent
Ladle Start
Ladle End
35
The results indicate that globular faceted inclusions grew in the ladle by cooling and were removed in the tundish.
• Globular faceted inclusions > 5um
– were not present in the ladle immediately after killing.
– decreased from pour box to well to mold.
– Increased during casting.
– increased with decreasing superheat.
• Globular faceted inclusions got bigger during casting.
36
Analysis of Well Nozzle Plugs
Grade A
37
Analysis of Well Nozzle Plugs – Loose powder
38
Analysis of Well Nozzle Plugs - Boundary between plugged material and steel
39
Analysis of Well Nozzle Plugs - Remelt sample from boundary region
40
The distributions of inclusion types were similar in the tundish well, well nozzle and mold.
CoralSphere
High SurfaceGlobular Faceted > 5
Faceted <2 um0.00001
0.0001
0.001
0.01
0.1
1
10
No. of Inclusions per Six-Photo StripThousands
Well Nozzle Mold
41
Relationship of Inclusion Morphology to Clogging
• The distribution of inclusion types is similar in the steel and the plugs.
– Indicating that plugging comes from inclusions formed by deoxidation or reoxidation before the steel gets to the nozzle.
42
Overall, the results pointed to the reducible ladle sand as a cause of clogging.
• Reduction of chrome oxide
– Chrome oxide in slag
– Chromium Pick-up in steel
• Total oxygen
– Increased from the pouring box to the well
– Increased with longer time in the tundish
– Lack of N Pick-up
• Dendritic Inclusions
– Increased with oxygen activity in the tundish slag
43
Nozzle Clogging Factor (NCF), derived from the slide-gate position, is used to quantify plugging.
50
60
70
80
90
100
110
0 10 20 30 40 50 60
Slab Number in Cast String
NCF, Standardized
to 95% on 1st Heat
Plugging
Higher is better
44
Ladle sand chemistry
Old New
Cr2O3 33.1% 16.7%
SiO2 29.0% 30.9%
Fe2O3 18.7% 9.4%
Al2O3 11.0% 5.9%
MgO 7.2% 3.6%
CaO - -
ZrO2 - 33.3%
C 0 0
45
Trial ladle sands with a lower percentage of reducible oxides resulted in less nozzle clogging.
Nozzle Clogging Factor
LCAK ULC
(Fe, Cr, Si) O 94% 87%
(Cr, Si, Zr) O 96% 91%
46
Conclusions
• Inclusion morphology in IF steels ranges from dendritic to globular depending on the degree of super-saturation of Al and O.
• Inclusion morphology is similar between grades, but changes significantly from ladle to tundish.
• Globular faceted inclusions are the most frequent in the tundish, nozzle clog, and mold.
• At all locations, many inclusion forms coexist in the steel.
47
Conclusions
• All forms of alumina inclusions clog nozzles.
• The presence of dendritic inclusions in the tundish indicates either insufficient rinsing or reoxidation.
• Increase of total oxygen as tundish residence time increases and as the steel flows from pouring box to well indicated that the tundish design is less optimal and needs improvement.
• Ladle sand is a significant factor in nozzle clogging.
48
Acknowledgements
• Co-authors: Dr. Howard Pielet of R & D and Mr. Richard Gass of Operating Technology.
• Members of the “Inclusion Characterization Team”.
• The chemical analysis laboratories of Quality Department.
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