ISIJ International, Vol. 34 (1 994), No. 11, pp. 868-875

Technology for Cleaning of Molten Steel in Tundish

Hiroyuki TANAKA.Ryoji NISHIHARA.Ryusuke MIURA.1) Ryoji TSUJINO.Takeshi KIMURA.1)Takashi NISH11)and Tatuo IMOTO1)

YawataR&DLaboratories, Nippon Steel Corporation, Tobihata, Tobata-ku, Kitakyushu. Fukuoka-ken, 804 Japan.1)YawataWorks, Nippon Steel Corporation. Tobihata. Tobata-ku. Kitakyushu, Fukuoka-ken. 804 Japan.

(Received on June 30. l994 accepted in final form on September l6. 1994)

Sheet and coated steel have been demandedto meet quality requirements of ever increasing severity in

recent years. Reduction in nonmetallic inclusions is an important challenge in this respect.Protection on the moiten steel in the tundish against contamination is of special importance in not only

improving steel product quality, but also avoiding immersion nozzle clogging and increasing the numberof sequence-cast heats. To supply molten stee] with high cleanliness in a stable manner, the cleaningbehavior of the molten steel in the tundish was investigated and mechanismwhereby the molten steel is

kept clean in the tundish wasquantified. Basedon the results achieved, technology wasstudied for keepingthe molten steel clean in the tundish.

KEYWORDS:continuous casting; steelmaking; casting; inclusion; clean steel; air oxidation.

1. Introduction

Ensuring the cleanliness of molten steel in the tundishis very important in not only producing clean steel

products, but also preventing immersion nozzle blockage.The tundish has the functions of distributing the moltensteel to multiple strands and cleaning the molten steel

by facilitating the flotation and removal of nonmetallicinclusions.1 ~4) The tundish is also knownto cause thecontamination of the molten steel, however.5~9)Technology for improving the fiotation and removal ofinclusions in the tundishl0-13) and technology forpreventing the contamination of molten steel in thetundishl4-21) have been traditionally studied as pre-requisites for the manufacture of clean steel products.

To produce clean steel stably by making the most of theinclusion fiotation and removal function of the tundish,it is essential identify factors responsible in combinationfor the contamination of the molten steel in the tundish,to determine the contribution of the specific factors, andto take appropriate measures for preventing the con-tamination of the molten steel in the tundish.

This study investigated inclusions in the molten steel

in the tundish of an actual continuous caster andclarified

the causes of contamination of molten steel in the tun-dish. Based on the findings thus obtained, technologyfor preventing the air oxidation of molten steel andtechnology for using harmless ladle well-packing strands

were investigated for their effectiveness in cleaning themolten steel in the tundish.

2. Results of Analysis of Behavior of Inclusions in

Tundish

Three heats of aluminum-killed steel with analuminum

C 1994 ISIJ 868

content of about 0.05 o/o were cast on No. I continuouscasting machine(with two strands and a 60-ton tundish)at No. 3steelmaking plant. YawataWorks, Nippon Steel

Corporation. Samplesof molten steel in the tlmdish weretaken with a rapid cooling sampler at the long nozzleposition (tundish inlet) and the immersion nozzle position(tundish outlet). The samples were examinedby opticalmicroscopy to count 10 ,lm or larger inclusions by shape(cluster, Iump or globular) and size. The compositionsof the inclusions were investigated by 'an electron-probemicroanalyzer (EPMA). '

Before casting the first test heat, the sliding nozzle ofthe ladle was opened above a slag pot outside of thetundish and the ladle well-packing sand wasdischargedoutside of the tundish. At the end of the first heat, about20 tons of molten steel was left in the ladle to preventthe carryover of the ladle slag into the tundish by theladle-to-tundish pouring stream. The second and thirdheats were sequencecast as normally done.

Figure I shows the change in the numberof aluminaand slag inclusions counted in the molten steel in thetundish. The numberof inclusions in the tundish inlet

increased with increasing number of heats. The slaginclusion count almost similarly changed in the tundishinlet and the tundish outlet, but wasslightly lower in thetundish outlet than in the tundish inlet. This meansthat

someof the slag inclusions introduced into the tundish

were floated and removed. The numberof alumina in-

clusions increased in the tundish, probably because ofreduction of the ladle well-packing material by alumi-

numand oxidation of molten steel by air.

The theoretical cleanliness of molten steel in thetundish outlet as determined on the assumption that themolten steel is not contaminated in the tundish [Eq. (1)]

andthe observedcleanliness of molten steel as determined

ISIJ International, Vol. 34 (1 994), No. 11

c!)

~~~:;~",

* *D ~*1'*

'e.~.S~~ 12 tlst, heat 2ntL heat~~lO t '

th ,~

s~o~ 8 ~\ ' \ 1'~~(\~;~ 6o- V'~S,oF:*,

4~~~~~ 2o~p~~ Oo

40 80

Time

Fig.

3r~L heat~,

,~/ \•*~4f

120 160 200 O 40 80

(min) Time (min)l. Changein inclusion density (> lOum) in tundish.

~lh+e 5~o_,e ~~*oQ "4\1

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.L

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+ 3*d' h**t,,+Jj+'~~~:~~" ~~l'~""ti"I

~~!{~:N~~~~\~+

+_

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~ +~~\

O 80 120 160 20040

Time (min)Fig. 2. Experimental analysis of steel cleanliness in tundish

during the casting.

~~v~O~:!

~f:j

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~ee

*~O

100

80

60

40

20

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i : """"'\'~"

l;::;f\lT~'lA11f

SlagCOntamI nat lon

from samples taken at the tundish outlet [Eq. (2)] arecomparatively shownin Fig. 2. Thenumberof inclusions

countered by optical microscopy is multiplied by thedensity ratio of the inclusions to the molten steel

(pi*c/pFe)' converted into the corresponding weightfraction of the inclusions (Wi..), 22) andpresented as indexof steel cleanliness in Fig. 2.

Theoretical cleanliness of molten steel in tundish outlet

=~[1/4l~Ni(Dpi)2/S(1-

oci)] ..,..............(1)

Observedcleanliness of molten steel in tundish outlet

=~[1/4 ILNi(Dpi)2/S(1-

oci)] +Np .............(2)

Wi*.=A~**xp. /p ..........(3)

These differences indicate the amount of contami-nation caused in the tundish. Since the atmosphereconditions in the tundish inlet are the samefor the threeheats, poor theoretical steel cleanliness meansthat the

molten steel is badly contaminated by the slag or well-

packing sand carried from the ladle into the tundish.

The theoretical steel cleanliness of the first heat cast bypreventing the carryover of the ladle slag and well-

packing sands is almost constant throughout and is

better than that of the second and third heats. Theamountof contamination increased in the second heatwhere the well-packing sand was allowed to flow into

the tundish. In the third heat, the accumulation of theladle slag and well-packing sand in the tundish addedtothe contarnination level of the second heat and the steel

cleanliness changedpractically at the samelevel as in thesecond heat. The difference between the observed steel

cleanliness and the theoretical steel cleanliness denotes

O 40 80 120 160 200

Time (min )Fig. 3. Influence of oxidation by air and slag contamination

on contamination index during the casting.

the contamination of the molten steel. in the inclusionflotation region betweenthe tundish inlet andoutlet. Thebehavior of nitrogen separately investigated showedthatair oxidation is a factor responsible for the contaminationof the molten steel in the tundish. Figure 3shows the

contributio~ of air oxidation and slag inclusions to the

contamination of the molten steel in the tundish. Airoxidation has a great effect from the first to the secondheat and accounts for 70 o/o or moreof the contamina-tion of the molten steel leaving the tundish between thefirst and second heats cast by reducing the carryoverof the ladle slag and well-packing material into the tun-dish. The amountof contamination by slag inclusions

increased with increasing nuinber of heats, but the

amountof contamination by air oxidation decreasedwithincreasing numberof heats. This is probably becausethe

amountof slag accumulated in the tundish increased withincreasing numberof heats, but the bath surface wasincreasingly covered with the slag to reduce the air

oxidation of the bulk of the molten steel.

The above discussion indicates that the molten steel

in the tundish can be kept clean by: (1) protecting it

against air oxidation; (2) rendering the ladle well-packingsand harmless to it; and (3) suppressing ladle slag

carryover into the tundish. Thetechniques developed for

the first and second items are discussed here. Techniquesfor preventing the carryover of ladle slag into the tundish

are presently applied with good results.23)

3. Technology for Cleaning Molten Steel in Tundish

3.1. Protection of Molten Steel against Air Oxidation

Air in the tundish has been traditionally replaced by

869 C 1994 ISIJ

ISIJ International, Vol.

Table l. Composition offluxes used for sealing molten steel

surface.

Flux

2345

Composition ("/o)

50Ca0-50Al20347.5Ca0~~7.5Al203-5Si0245CaO~}5Al203-10Si0240Ca0~~OAl203-20Si02100MgO(conventional heat insulating material)

argon to protect the molten steel in the tundish againstair oxidation. This technique has not been perfect dueto the difficulty of completely shrouding the tundish anddistortion of the tundish cover. The results discussed in

the preceding chapter indicate that the air oxidation ofthe molten steel can be increasingly prevented by in-

creasing the amount of slag accumulated on the bathsurface in the tundish. Covering the bath surface with alow-melting fusible fiux was investigated as one methodfor protecting the molten steel in the tundish against air

oxidation.

3.1.1. Methodsof Laboratory ExperimentThe optimum composition of fiux to seal the bath

surface in the tundish was studied by laboratoryexperiment, Using a 100-kg high frequency meltingfurnace, 70kg of steel was melted and adjusted toaluminum-killed steel composition (0.038 o/o C, 0.012 o/o

Si, 0.18 o/o Mn, and 0.042 o/o Al) with the mouthof thefurnace sealed with argon. Then, 2.5 kg of fiux wasaddedto the bath surface, the argon seal was stopped, andaluminum was added to achieve the aim aluminumcontent of about 0.05 o/o. Themolten steel wassampledfor 16min at 2-min intervals and analyzed. The com-positions of fluxes used in the laboratory experimentare given in Table I .

First, the bath surface wascoveredwith each flux to investigate its effect in protecting themolten steel against air oxidation. Second, to investigatethe effect on moiten steel oxidation of silica (Si02) withsmaller affinity for oxygen than aluminumhas, the silica

content of each flux waschanged. Third, a conventional98 o/o MgOmaterial was used to thermally insulate the

molten steel in the tundish.

3. I .2. Results of Laboratory ExperimentFigures 4 to 6 show the changes in the nitrogen,

aluminum, and silicon contents of the molten steel in

the tundish, respectively. Whenthe conventional heatinsulating material (5 in Table l) wasused, the moltensteel picked up somenitrogen. Whena flux fusible onthe molten steel surface (1, 2, 3, or 4 in Table l) wasused, the molten steel changed little in the nitrogencontent. The aluminum content of the molten steel

somewhatdeclined with all fluxes. Theconventional heatinsulating, material (5 in Table l) and the new fluxes

containing 100/0 or more Si02 (3 and 4 in Table l)

considerably decreased the aluminum content andsignificantly increased the air oxidation of the moltensteel in the tundish. Thechange in the silicon content in

Fig. 6 indicates that the flux containing 100/0 or moreSi02 (3 and 4 in Table l) raised the silicon content ofthe molten steel.

C 1994 ISIJ 870

34 (1994), No.

~O'~

r~~~

LJ~

~~:+

~r~c~,

~l

~

~~~

nCQ

LJ~

20

15

lO

5

O

-5

11

key FluxO lA 2D 3V 4

5

"v" :•

O 2 4 6 8 10 12 14 16

Elapsed time (min)

Fig. 4. Changein [N] in the molten steel,

o.oo 5

- O005

-0.010

-OO15

- OD2-0.025

key FIux

o lA 2O 3v 4~ 5

5o5

o5

o

Fig. 5.

On14

0.0 12

0.010

O.008

O~06O~04

0.002

O-0.002

o

Fig.

2 6 8 10 12 14 164Elapsed time (min)

Changein A[Al] in the molten steel.

key FluxOACl

V~ 5

.~~.

2 4 6 8 10 12 14 16Elapsed time (min)

6. Changein A[Si] in the molten steel.

3.1.3. Discussion3,1.3.1. Prevention of Air Oxidation by Fusible Flux

Thechangein the nitrogen content in Fig. 4indicates

that the conventional heat insulating material aloneincreased the nitrogen content of the molten steel. This

meansthat the conventional heat insulating material is

effective little in shielding the bath surface from air. Sincethe conventional heat insulating material contains 98 olo

of MgO,a main componentused in refractories as well,

it becomespumiceousor sintered on the bath surface in

the tundish and is considered incapable of providing full

ISIJ International, Vol.

protection against air infiltration.

The nitrogen pickup can be stably prevented bycovering the bath surface with a flux fusible at the

temperature of the molten steel. A Iiquid flux layer

formed between the molten steel and the outside air is

believed to reduce the speed at which the oxygen in theair enters the molten steel.

3.1.3.2. Effect of Si02 on Oxidation of Molten Steel

According to the change in the aluminum contentshownin Fig. 5, the air oxidation of the molten steel canbe controlled by a fusible fiux, but the aluminumcontentof the molten steel is reduced if the flux contains 10'/~

or more Si02 (3 and 4 in Table 1). The change in thesilicon content in Fig. 6indicates that whenit containslO "/• or moreSi02, the flux raises 'the silicon content ofthe molten steel. According to these results, a fusible fiux

can protect the molten steel against air oxidation, butwhen it contains Si02 and other oxides with smalleraffinity for oxygenthan the aluminumin the molten steel,

the Si02 in the flux reacts with the aluminum in the

molten steel to oxidize the molten steel. The flux doesnot cause the oxidation of the molten steel whenits Si02contents is up to 5o/o (2 in Table 1). Whenthe fiux

contains lO'/, or more Si02 (3 and 4 in Table l), the

oxidation of the molten steel increases with increasing

Si02 content. The probable reason for this situation is

the difference in the rate of reaction due to the difference

in the activity of Si02' The above results indicate that

the air oxidation of the molten steel and the secondaryoxidation of the molten steel by the flux can be preventedby using such a flux that is fusible at the temperature ofthe molten steel and contains no or negligible amountsof componentshaving smaller affinity for oxygen than

Weir

LadIen07,zle

Lad I e

mlet

Sampling position

oc\]

Dam

outlet

950nwn

SubmergednozzIe

Frg. 7. Schematicdrawing ofYawataWorksNo. I slab castertundish.

l~~~~fLOU~I

10

864

2O

type

34 (1 994). No. 11

the aluminum in the molten steel does.

3.1.4. Results of Plant ExperimentAccording to the results of the laboratory experiment

discussed in the previous section, a fusible flux wasapplied to a tundish in a plant experiment. The plantexperiment involved sequencecasting of three heats ofaluminum-killed steel (aluminum content of about0.05 o/o) by using the tundish illustrated in Fig. 7. Afusible

flux was added to the impact region between the weirs

at the center of the tundish and to the first strand side

outside the left-hand weir. Theconventional 98 olo MgOheat insulation material wasadded to the second strandside. The molten steel was sampled at the tundish in-

let and outlet at the same timing. A fusible fiux ofthe CaO-Al203 composition was used according tothe results of the laboratory experirnent. A 48Ca0-37Al203-15MgOflux composition (with a melting pointof 1530'C) wherein MgOis saturated at the moltensteel temperature of 1550'C in the tundish was evalu-

ated for its effectiveness in protecting the molten steel

against air oxidation. Figure 8shows the change in thetotal oxygen content of the molten steel in the tundish.

The total oxygen content was 0.6ppm for the fusible

flux and 5.9ppm for the nonfusible fiux. This finding

confirmed the effectiveness of the fusible flux in re-

ducing the formation of inclusion in the tundish.

3.2. Useof Harmless Well-packing Sands

The ladle well-packing material is required to havedifferent characteristics in the contact area with themolten steel (top layer) and the sliding plate area ornon-contact area with the molten steel (bottom layer).

The top layer sand must be highly sinterable so that it

can prevent the penetration and solidification of the

molten steel into its grains. Thebottom-1ayer sand mustnot be readily sinterable so that it can easily drain withthe molten steel whenthe sliding nozzle is openedandmustalso be small in the coefficient of thermal expansion

so that it cannot push up the contact surface with the

molten steel. Asand, mainly composedof Si02, has beentraditionally employedas a suitable material to meet thecontradicting characteristics. Figure 9 illustrates thetechnique24) applied in recent years for placing different

sands in top and bottom layers to improve the free

opening performance of ladles and for adding R20(R=Na, K) to the top sand for better sintering per-formance. Few studies have been conducted on thebasic well-packing sand composition as required to

prevent the contamination by the well-packing sand ofthe molten steel in the tundish.

Top sand

Bottoms a nd

Fig. 8. Improvementof AT[O] by newtype tundish flux.

871

Single layer Double layer

Fig. 9. Filler packing methods.

C 1994 ISIJ

ISIJ International, Vol. 34 (1994), No. 11

Nozzle filler Table 3. Chemical composition and grain size ofthe nozzlefiller.

SampleNo. I 2 3 4 5

Material Lowpure Pure MgO Spinel A1203Si02 Si02

TChemical composition (olo)

Cruc ib IeFig. 10. Thermai expansion evaluation method.

Si02 74Al203 13

MgOMa20+K20 11

98 370 9928

l 96

Table 2. Sandsinterability evaluation criteria.

Rating Sintered condition

Grain size (o/o)

+2mm21l~).5

0.5-

55341

l

l45 59 59 5055 39 40 49

l I lS-OS- lS-2S-3

S-4S-5

Not sintered

Slightly sintered and easily coiiapsed by finger

Sintered and easily collapsed by finger

Sintered and barely collapsed by finger

Sintered with surface grains removeby finger

Completely sintered and vitrified

To prevent the contamination of molten steel by theladle well-packing material, this study extended theabove-mentioned double-layer sand practice and inves-tigated the conditions of the sands capable of preventingthe contamination of the molten steel while improvingthe free opening performance of ladles.

3.2.1. Methodsof Laboratory ExperimentThe laboratory experiment first examinedthe physical

properties of well-packing sands. Analumina crucible,

measuring57mmin inside diameter and65mmin height,

was packed with a sand sample to the full height asshownin Fig. 10. The sand sample was heated for 2hat I OOO,1300, or 1500'C to simulate the temperature dis-

tribution in the sliding nozzle and was then evaluatedfor sinterability and thermal expansion. Whenthe heatedsand samplecooled down, its top was touched by finger

and its sinterability was rated according to the criteria

shownin Table 2. After the sand samplehadbeenheatedat 1500'C for 2h, the crucible was cut. The maximumrise (+) or depression (-) of the sanrple from the topof the crucible was measuredon the cut section. Thethermal expansion of the sand samplewascalculated bythe following equation:

Thermal expansion = t/Tx 100 (o/o) ...........(4)

where t=maximumrise or depression from the top ofthe crucible (mm)and T=height of the crucible (mm).

Five types of well-packing sands were investigated in

the laboratory experiment: (1) Si02 based sand used in

the top layer of the conventional double-1ayer sandsystem and containing R20with improved sinterability;

(2) high-purity Si02 basedsand used in the bottom layerof the conventional double-1ayer sand system; (3) MgObased sand; (4) Al203-MgOspinel based sand; andA1203 based sand. The sand materials were selected

according to the criteria that they should react little withthe aluminumin the molten steel and should be available

Nozzlc filler

C~>

Heat

Reactionzone

r

Crucible Heat in electric furnace15001Cx2hrs

Fig. Il. Double-layer sand property evaluation method.

on an industrial basi.s. CaOwas considered unsuitable

as a well-packing sand and excluded from amongthe

evaluation samples because it is likely to react with theladle slag or the aluminum in the molten steel to formlow-melting compounds,is sinterable, and is feared tohave the problems of water absorption and sublimationduring storage. Thetest materials weresized so that morethan 90 "/o of grains were I to 2mmin size. Thecrucible

wasmadeof alumina as a material close to that of theladle sliding nozzle. Thechemical composition and grainsize distribution of the test materials are presented in

Table 3.

The well-packing sands were then evaluated for their

characteristics whenapplied in double layers. The Si02content was changed in the conventional single-1ayer

sand practice, but no composition wasfound that couldsatisfy the sintering requirement of the top layer andthe non-sintering requirement of the bottom layer at the

sametime. An attempt was madeto reduce the Si02proportion by such a double-layer sand design that usesthe conventional Si02 sand in the top layer to providedesired sinterability and a new Si02-free sand in the

bottom layer. The characteristics of the double-1ayerwell-packing sands were evaluated by using Si02 sand(1 in Table 3) in the top layer to improve sinterability

and Si02-free sand (3, 4, or 5in Table 3) in the bottomlayer. As shown in Fig. 11, an alumina crucible waspackedwith the top andbottom sands, heated at 1500'Cfor 2h in an electric furnace, and investigated for:

(1) Penetration of molten componentsfrom the toplayer into the bottom layer and thickness of the reaction

C 1994 ISIJ 872

ISIJ International, Vol. 34 (1 994). No. 11

Table 4. Experimental results of single course nozzle filler.

SampleNo. l 2 3 4 5

Material Lowpure Pure MgO Spinel Al203Si02 Si02

Sinterability

1OOO"Cx 2hl 300'C x 2hl 500'C x 2h

S- I S-O S-O S-O S-OS-5 S-O S-O S-O S-O

S-5 S-3 S-2 S-O S-O

Thermal expansion ("/.)

l 500'C x 2h 25 30 o o o

Table 5. Experimental results ofdouble course nozzle filler.

Top sand Lowpure Si02Bottom sand Si02 MgO Spinel Al203

Thickness ofreaction zone (mm) lO 40 lO 5

Sinterability ofbottom sand S-3 S-5 s-o S-o

zone between the top and bottom layers; and(2) Sintering or not of the bottom sand

After the heated crucible had cooled down, it was cut,

and the thickness of the reaction zone wasmeasuredonthe cut section. Thesinterability of the bottom sand wasevaluated according to the criteria described in Table 2.

3.2.2. Results of Laboratory ExperimentTable 4 shows the evaluation results of single-layer

well-packing sands. SampleNo. l, or a sand containing

R20and Al203 to improve sinterability, was mostlymolten whenheated to more than 1300'C and formed

a glassy sintered layer. The formation of the sintered

layer is considered to prevent the penetration of the

molten steel into the well-packing sand and the so-lidification of the molten steel in the well-packing sand.

SampleNo. I also decreased in volume because of its

sintering and melting characteristics. Whenheated to

l 500'C, sampleNo. 2, or a sand composedof relatively

pure Si02, exhibited the sinterability rating S-3 andincreased in thermal expansion. SamplesNo, 3to 5, orsands containing little or no Si02, did not sinter at all

and had small thermal expansion. These results indi-

cate that low-Si02 materials cannot provide desiredsinterability and cannot be used signly as well-packingsands.

Table 5 shows the thickness of the reaction zonebetween the top and bottom layers and the sinterability

of the bottom sand as investigated whena high-Si02sand with good sinterability and a low Si02 sand wereplaced in the top and bottom courses, respectively. Withthe conventional double-layer sandpractice, the top sand

was fully sintered, the bottom sand wassemi-sintered to

the rating S-3, anda5mmthick reaction zonewasformedbetweenthe top andbottom layers. WhenMgOwasusedin the bottom layer, the top and bottom sands fully

reacted, melted, and sintered. The thickness of the

reaction zone was 40mm,and the bottom sand was

873

completely sintered to the rating S-5. Fromthese results,

it is predicted that the ladle will not open freely if its

well is packed with the Si02 sands in the top layer andthe MgOsands in the bottom layer. Whenthe A1203orspinel sand was used in the bottom layer, the reaction

zone formed between the top and bottom layers was5to 10mmthick, and the bottom sand wasnon-sintered

to the rating S-O. These results indicate that the A1203or spinel sand can be used in the bottom layer if the toplayer is that of the Si02 sandswith improved sinterability.

3.2.3. Discussion

Whendifferent well-packing sands are used in doublelayers, the formation of a thick reaction zone betweenthe top and bottom layers or the sintering of the bottomsand reduces the draining performance of the well-

packing sands through the sliding nozzle whenanattemptis madeto open the sliding nozzle, resulting in the failure

to open freely. The reaction zone must be madeas thin

as possible to preclude this situation. The results of the

laboratory experiment suggest that the Al203 or spinel

sand maybe used in the top layer if the Si02 sands is

used in the bottom layer. The thickness of the reaction

zone dependson the composition of the bottom sand.

Themechanismwhereby the reaction zone is formed is

discussed below.The low-purity Si02 Sandused in the top layer of the

conventional double-layer sand design has a liquidus

temperature of about I OOO'C,25)melts whenaluminum-killed steel with a temperature of over 1500'C is received

into the ladle, and is considered to penetrate into andreact with the bottom sand. Thehighly-purity Si02 sandused in the bottom layer has a melting point of overl 600'C. Whenthe penetrating top-sand componentsreact with the bottom sand, they rise in the Si02 contentand melting point. Thepenetration of the top sand into

the bottom sand is stopped when the reaction zone is

formed to somedegree. Whenthe Si02 sand and MgOsand are used in the top and bottom layers, iespectively,

the top MgOsand reacts with the bottom Si02 sand asthe sliding nozzle and well-packing material are heatedby the molten steel in the ladle. The top sand thus falls

in the melting point and acceleratedly penetrates into the

bottom sand. If the Al203 or spinel sand is used in the

bottom layer, the bottom sand rises in the Al203 contentandmelting point whenthe top sandmelts andpenetratesinto the bottom sand. Thepenetration of the molten topsand into the bottom sand is arrested accordingly. Theabove results indicate that the optimumcombination of

top and bottom sands for the double-layer sand practiceis such that high-melting and high-viscosity compoundsshould be formed whenthe top sand is melted in contactwith the molten steel, penetrates into the bottom sand,

and reacts with the bottom sand.

3.2.4. Result of Plant ExperimentLadle well-packing sand were applied in a plant

experiment, based on the results of the laboratoryexperiment. Aluminum-killed steel with an aluminumcontent of about 0.05 ~/* was cast on No. I continuouscasting machine at No. 3 steel making plant, Yawataworks, as shown in Fig. 7. The free opening perfor-

C 1994 ISIJ

100

99

98

ISIJ International, Vol. 34 (1994), No. 11

~~

~o~Ft

f~

~

97

96

95Jun. Aug. Sep Oct.Nov' Dec.

1991Fig. 12. Changein free opening performance with Si02-free

sand.

8 50~~~~ 9

It -~ Non-si~ ica type~ 40 Il

: t--o-- Silica type

::~ 30 IoF:

q> 20 j __JO--_c~ I --*~---

10 ~h1's 6~1

~ O:s lOO 20 30~ 40 50

Time from ladle start (min)

Fig. 13. Reduction in amountof alumina inclusions by Si02-free sand.

manceof ladles and the prevention of contamination ofmolten steel in the tundish were investigated whenonewell-packing sand wasreplaced by another. Toeliminatethe effects of air oxidation whenthe molten steel waspoured from the ladle into the tundish at the beginningof a cast and of contamination by carryover of the ladleslag into the tundish at the end of a heat, two heats weresequencecast and 20 tons of molten steel was left in theladle at the end of the first heat so that the ladle slag

would not be carried over into the tundish.

The second heat was sampled at a depth of 400mmbelow the bath surface in the tundish stopper positionwith a rapid cooling sampler at interval of I to 10 min.

Aspecimenwascut at 100mmfrom the bottom of eachsample, polished, and examined by optical microscopyto count lO,xm or larger inclusions by shape (cluster,

lump, or globular) and size.

Figure 12 shows the change in the free openingperformance of ladles with the type of sand. The free

opening performance with the Si02-free sand is as highas with the conventional Si02 based sand. The changein the amountof alumina inclusions in the tundish withthe type of sand from the start of ladle pouring is shownin Fig. 13. Theconventional Si02 based sand increasedthe amountof alumina inclusions at 5min after the startof ladle pouring. This increase in the amountof aluminainclusions at the tundish was successfully prevented bythe newSi02-free sand.

4. Conclusions

Factors responsible for the contamination of molten

C 1994 ISIJ 874

steel in the tundish were quantified by investigating thecleanliness of molten steel in the tundish for the purposeof manufacturing molten steel with high cleanliness in

a stable manner. Various techniques were studied toimprove the factors identified by the investigation. Thefollowing finding were obtained:

(1) The molten steel in the tundish is mostlycontaminated by air oxidation and ladle well-packingmaterial.

(2) As the numberof sequencecast heats increases,the effect of air oxidation decreases, but contaminationby slag inclusions mainly arising from the ladle well-packing material increases.

(3) The molten stee] in the tundish can be stablyprotected against air oxidation if the bath surface is

sealed with a flux that is fusible at the molten steel tem-perature and that contains no components to reactwith the aluminum in the molten steel.

(4) The contamination of the molten steel in thetundish can be prevented while keeping the free openingperformance high by packing the ladle well with sandsin double layers or top and bottom layers and using aSi02-free sand in the bottom layer.

(5) The bottom sand for the double-layer sandpractice must not be sintered at the temperature of thesliding nozzle and must be of such a composition thathigh-melting and high-viscosity compoundsare formedwhenthe top sand melts in contact with the molten steel,

penetrates into the bottom sand, and reacts with thebottom sand. If Si02 sand is used in the top layer, Al203or Al203MgOspinel sand can be used in the bottomlayer.

NomenclatureAi.. : Area fraction of inclusions (-)DFi : Particle size of inclusions (,Im)

Ni : Numberof inclusions in every size (L)Np: Amountof contamination in molten steel in

tundish (-)S: Area inspected by optical microscopy (m2.)

V~~*: Volumefraction of inclusions (-)Wi.. : Weight fraction of inclusions (-)

c( : Volumeratio ofalumina in aluminaclusters (-)oci : Flotation ratio of inclusions by size (-)

pp. : Density of molten steel (=7OOO)(kg/m3)Pi.. : Density of inclusions (=3600) (kg/m3)

l)

2)

3)

4)

5)

6)

7)

8)

REFERENCESH, Ishikawa. I. Shibuya, J. Tanakaand S. Nakamura:CAMP-ISIJ, 66 (1980), 866.T. Komai, H. Miyamura, M. Ohsaki, A. Kusano. T. Yamada,I. Shimazu, N. Sato and T. Yamamoto:Telsu-to-Hagan~. 67(1981), I152.

M. Nakada,S. Fukuda, K. MurakamiandK. Yano: CAMP-ISIJ,69 (1983), 212.

K, NakajimaandM. Kawasaki: Tetsu-to-Ifaganb, 73(1987), 852.Y. Habu, H. Kitaoka, H. Yoshii, T. Emi, Y. Iida and N. Ueda:Tetsu-to-Haganb, 62 (1976), 180.

Y. Iida, M. Kodama,Y. Ohnishi, K. Ohsugi, M. Ozawaand T.Nozaki: CAMP-ISIJ. 64 (1978), 148.S. Yamahara, I. Sugawara, K. Taguchi, S. Uchida and T.Masaoka: CAMP-ISIJ, 65 (1979), 226.K. Murakami, Y. Sasajima, K. Yano, M. Tate, M. Ishiguro and

9)

lO)

1l)

12)

13)

l4)

l5)

16)

17)

ISIJ International, Vol. 34

Y. Ogura: CAMP-ISIJ, 66 (1980), 865.

O. Naka, N. Katsuyama,M. Waki, T. Komiya, H. Misumi and 18)

S. Iguchi: CAMP-ISIJ, 68 (1982), 1006.

H. Tomono,T. Ura, H. Sakamotoand K. Iwata: CAMP-ISIJ, 19)

69 (1983), 210,

K. Yamanaka,S. Terashima, K. Nakada,T. Koshikawa, N. Ueda 20)

and H. Yoshii: CAMP-ISIJ, 69 (1983), 213.S. Imamura, A. Imamura, H. Ikezaki. A. Kusamo,H. Kuwatori 21)

and H. Miyamura: CAMP-ISIJ, 73 (1987), 281.

K. NakajimaandM.Kawasaki: Tetsu-to-Hagan~, 73 (1987), 860. 22)

M. Yoshida. S. Ishitobi, J. Wakidaand S. Mizoguchi: CAMP-ISIJ, 66 (1980), 863. 23)J. Wakida, S. Mizoguchi, Y. Yoshida and S. Ishitobi: CAMP-ISIJ, 66 (1980), 864.

M. Ohnishi. Y. Iwamoto, S. Hiwasa, Y. Kato and H. Daizu: 24)

CAMP-ISIJ, 69 (1983), 21 I. 25)

T. Ito. T. Koshikywa. A. Takahashi and T. Imai: CAMP-ISIJ,

(1 994). No. 11

67 (1981), 847.S. Miyahara, I. Sugawara, K. Taguchi. S, Uchida and T.Masaoka:CAMP-ISIJ, 65 (1979), 228.

H. Tomono, K. Ozaki, T. Ura, K. Iwata and T. Suzuki:CAMP-ISIJ, 69 (1983), 914.

N. Morioka, K. Washio, K. Hamaguchi,S. Ogura, H, Nishikawaand R. Asaho: CAMP-ISIJ, 73 (1987), 280.

N. Kasai. O. Miyazaki, H. Yamazoe,M. Yoshii andT. Shinozuka:CAMP-ISIJ,7(1994), 325.

H. Tanaka, Y. Nishihara. I. Kitagawa and R. Tsujino: ISIJ Int.,

33 (1993), 1238.

H. Tanaka. R. Tsujino, R. Nishihara, I. Kitamura. H. Nomoto,R. Miura, Y. Takasaki andT. Imoto: Tetsu-to-Hagan~, 78 (1992),

T201.

T. Suruga: Refractories, 39 (1987), 298.

Handbookof Physico~ChemicalProperties at High Temperature,ed. by ISIJ. Tokyo.

875 C 1994 ISIJ