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

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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

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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

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Agglomeration Inclusions

Coral > 25 um Fine Coral

Lace Balloon

O, Al O, Al

O, Al

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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.

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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.