ISIJ International. Vol. 38 (1998). No 5, pp. 461-468

Control of Early Solidification

Osci IIation in Synchronization

in Continuous Casting by Horizontal

with Vertical Oscillation of the Mold

Seiji ITOYAMA.Hirokazu TOZAWA.Tetsuo MOCHIDA.Toshitane MATSUKAWA1)and Kenichi SORIMACHI

Katsumi KUROKAWA.1)

Technical Research Laboratories, Kawasaki Steel Corporation, Kawasaki-cho, Chuo-ku, Chiba, Chiba-ken, 260-0835Japan.1)Chiba Works, Kawasaki Steel Corporation, Kawasaki-cho, Chuo-ku, Chiba, Chiba-ken, 260-0835 Japan.

(Received on December10. 1997; accepted in final form on February 5. 1998)

Anewmethodof mold oscillation for continuous casting termed "horizontal oscillation" wasdevelopedto reduce the depth of oscillation marks without increasing the possibility of breakout, In this method, the

wide faces of the mold oscillate horizontally in synchronization with conventional vertical oscillation,

Oscillation marks on slabs of SUS304cast with a piiot continuous caster were examined under variousconditions of mold oscillation. Horizontal osciliation successfully reduced the depth of oscillation marksand surface segregation at oscillation marks, and wasextremely effective whenthe wide face of the moldwasexpandedagainst the solidified shell during positive strip time. Experiments with an actual machinereproduced these effects of horizontal osci!lation. A numerical analysis of the deformation of the solidified

shell during one cycle of vertical oscillation wasperformed to study the effect of horizontal oscillation onthe control of early solidification near the meniscus, Friction force acting on the shell near meniscus wasalso discussed to explain the reduction in the surface segregation.

KEYWORDS:oscillation mark depth; surface segregation; mold oscillation; horizontal oscillation; shell

deformation; mold friction.

1. Introductron

Cracks,1) surface segregation,2) jnclusions,3) and otherdefects of continuously cast slabs which have their origin

in oscillation marks (OSM)can be decreased in numberby reducing the depth of OSM.High cycle mold oscil-

lation has long beenproposed4) as a methodof reducingthe depth of OSM.This method, however, reduces the

consumption of mold flux,5) which deterlorates the

lubrication between the mold and shel] and therebyincreases the frequency of sticker breakout.6) For this

reason, too high cycle mold oscillation has rarely beenadopted in industrial casting operations.7) Against this

background, there have beennumerousreports in recent

years of improvement in slab surface quality by tech-

nologies which do not dependon mold oscillation, such

as molten steel flow control in the mold3) and electro-

magnetic force imposed on the meniscus.8,9)

On the other hand, in order to develop a new tech-

nology for mold oscillation that will reduce the depthof OSMwhile also ensuring stable casting, the authorsproposed a method which is termed "horizontal oscil-

lation,"lo) jn which the wide faces of the mold areretracted and then advancedhorizontally relative to thesolidified shell and in synchronization with the vertical

oscillation of the mold. Thls report describes the effect

of horizontal oscillation on the depth of OSMandsegregation at OSM,and the results achieved with this

method.

2. Experimental Procedure

2.1. Concept of Horizontal Oscillation

The depth of OSMis thought to be affected by the

behavior of pressure generated by mold oscil]ation in the

channel of mold flux betweenthe mold and the solidifiedshell.1 1) Specifically. OSMare formed whenthe shell atthe meniscus is pushedand bent toward the molten steel

side by the positive pressure generated in the channel bymold oscillation during negative strip time.1 1) However,the decisive period for the formation of concave area in

OSMis considered to be positive strip time.12) TakingOSMformation mechanism into consideration, the

authors considered that it would be possible to controlthe depth of OSMduring either positive or negativestrip time by oscillating the pair of wide mold faces sym-metrically in the horizontal direction in synchronizationwith vertical oscillation. In practical terms, this meansthat the wide faces are forcib]y pulled away frorn theshell in order to Increase the thickness of the fiux film

and thus relieve the pressure in the film, and are thenreturned to their original position.

2.2. Horizontal Oscillation MethodFigure I is a schematic representation of a mold with

the horizontal oscillation function in a pilot continuous

461 O 1998 ISIJ

ISIJ International, Vol. 38 (1998). No. 5

Fig, l. Schematic plan view of experimental mold withmechanism for horizontal oscillation synchronizedwith vertical oscillation,

Table l. Experimental conditlons.

ftem sLaboratory No. 4casterscale caster at Chiba Works

Steel grade SUS304Slab size

Thlckness (mm) l lOWidth (mm) 400

Casting speed, Vc (m/min) O8-1.2

Mold frequency. f (Hz) 2.5 - 5Vertical stroke of mold, S(mm) 2.0 - 5.3Horizontal stroke ofmold, ~h(mm) 0.3

Maximumhorizontal speedof wide face of mold (mm/s) 50

Superheat in tundlsh (~C) 32 - 60Mold flux

Vlscosity at 1300~C(pa.s) 0.09Melting temperature (~O 960

Slab iength cast with horizontal 3osclllation (m /condition)

SUS304

200l270- 1280lO3,l

3.0

OI-0,27

2540 - 45

O. 141140

l0-20

Ah[~}•- ~~ Displacement of shell e

Open/'5~+

CloseSolidified shell

(Norinal position ):~:

E~loc,$

(Upward)Mold

o"o~~ Time

-~cgc.~; ,,,,•ll~lll

'- a)t;a)> Slab

> : Ne~~~'gativesthp time, t

ModeA : OpenF:

o Closed : Ah~:::::,s ModeBoc,, oo "~ff~~

Open: : Closed :

o ModeC :opet~Q

open j'F:

o:E Closed Closed

Displacement of shell edge

Negative strip time, tn

TlmeFig. 2. Three modes of horizontal mold oscillation syn-

chronized with vertical oscillation.

caster. Here, the copper plates of the wide faces areoscillated in the horizontal direction in synchronizationwith vertical oscillation by a single oil hydraulic cyiindermountedon the fixed frame at the rear of the back upframe, following groove-type linear guides. In an actualcaster, four oil hydraulic cylinders were used at each ofthe wide faces. Drive position control of the wide facesduring horizontal oscillatlon is performed by a sequencerandmicro computerat an operating time lag of less than5ms, using an oil hydraulic servo system to control theposition of the wide faces, as measuredby displacementsensors mountedon the cylinders, to the specified values.As shown in Fig. 2, horizontal oscillation has a tri-

angular waveform, and is based on a method(modeB)of forcibly expanding the distance. Ah, betweenthe moldand shell during positive strip time, tp, in synchroniza-tion with the vertical oscillation of either a sinusoidal ora non-sinusoidal wave (distortion rati0=46•/o),13) andthen returning to the original position during negativestrip time, t~. Accuracy of control of horizontal strokeis within ~0.01 mm.Rapeseedoil is supplied from the

Segregation Thickness of Depth of oscillation marksegregation

Frg. 3. Definition of thickness of segregation and depth ofoscillation mark.

'(c) 1998 ISIJ 462

mold corners at the meniscus to prevent the penetrationof mold flux and molten steel into the gap formed bythe horizontal oscillation. In order to investigate theeffect of the timing of horizontal oscillation on the OSMdepth, other two methods of expanding Ah during t~

(modeA) and part of both t~ and tp (modeC) were alsostudied.

2.3. Experimental Conditions and Methodof Investiga-tion

Themain experimental conditions are shownin Table1. Castings were performed first with a pilot casterlo)

using SUS304as the object steel grade, because surfacesegregation in this steel results in the surface defect incold rolled sheets. During the experiments, the moldoscillation conditlons were changed 3-4 times in each3msection of the strand in the samecharge (total slablength, ca. 13m). The solidified microstructure in thecross-section perpendicular to the unrolled wide faces ofthe slabs and longitudinally parallel to the castingdirection were examined at 30 to 120 OSMfor eachoscillation condition. Yokoi reagent, which compoundsof CuC12: I lOg, HCl: 40cc, solution of water saturatedwith picric acid: 50cc, and distilled water: 100cc, wasused as the etchant. As shownin Fig. 3, the OSMdepth,d2 (=dl +d~.g), was defined as the depth of the con-cavity from the surface of the slab to the bottom of thesegregated zone at the OSM.Theeffect of the horizontaloscillation on the average value, ~~, of d2, the averagesegregation thickness, d**g, the frequency of segregation,f,.g(y.) [=100 x (number of OSMwith segregation/numberof OSMobserved)], and other conditions relat-

ed to the segregation was studied.Moreover, a certain amount of surface segregation

ISIJ lnternational, Vol.

mayremain on the product even after descaling in the

processes from soaking furnace before hot rol]ing

through annealing/pickling after hot rolling, and is anindicator of the surface defect in final rolled products.ldeally, it should be possible to remove all the surface

segregation by descaling. Therefore, the total thicknessof the scale removedIn the processes, converted to slab

thickness, approximately 0.32mmin this work, wassubtracted from d~*g, and the frequency of segregationunder this condition, ,f=*g (d~*g>0.32), was also inves-

tigated to determine the effectiveness of horizontal os-ci]lation in producing defect-free final products.

Casting tests were also performed with an actual

machinebasedon the knowledgeobtained with the pilot

continuous caster. Theexperimental conditions at ChibaWorksNo. 4continuous caster are shown in Table l.

The tota] Iength of the slabs received horizontal oscil-

lation was 10-20mper one osclllation condition. Be-

cause the OSMwere reduced by the rolls, making it

impossible to measured2, d~*g Wasinvestigated in theseslabs. The stroke of horizontal oscillation was O.ll-0.3 mm.

3. Experimental Results

3.1. Results with Pilot Continuous Caster

3,1.1. Effect of Horizontal Oscillation on Depth ofOscillation Marks

The relationship between t~ and d2 is shownin Fig.

4(a). Whenhorizontal oscillation is applied, d2 is

reduced, even at the samet*. This effect is remarkablein the high t~ region, which corresponds to lower oscll-

lation frequencies. However, in horizontal oscillation

~~c~l~

O.5

0.4

0,3

O.2

0.1

O

0.6Os Casting speed= 0.8- I.2 m/min

,5 Os+A S=2-5.3mmf 255HzAs+B

.4 Is+C O _ConventionalDns o oscillatione.3

ns + AA ns+B

,2 o.1

D With horizontaloscillation(Modes B, C) (a)

o

~:'

o~s+A$:~

o' s+C~ce

o s+B~

38 (1998), No. 5

modeA, d2 waslarge in the low t~ region, and this effect

was therefore inconsistent. In the case of t =0,136s(f=2.5 Hz, V.= 1.2m/min, S=5.3 mm),d2 in modesBandCwasreduced by approximately half, in comparisonwith values whenhorizontal oscillation wasnot applied[Fig. 4(b)]

.

3.1.2. Effect of Horizontal Oscillation on SurfaceSegregation

(1) Segregation Thickness, d*,g

Theeffect of horizontal oscillation on the relationship

between t~ and d~*g is shownin Fig. 5. Although the effect

here is similar to the effect of horizontal oscillation onthe relationship between d2 and t. in Fig. 4(a), no seg-regation was observed in mode C in the case oft.=0.136s. The value of d~,g becamegreater as d2increased. However, effect of horizontal oscillation onthe relationship between d**g and d2 at the sameOSMcould not be recognized,15)

(2) Frequency of Segregation, f;*g

As shownin Fig. 6(a), f*.g decreased whenhorizontaloscillation was applied at the samet*, and the effect oft* was also reduced. In Fig. 6(b), which shows the re-lationship between t. and f~.g (d,,g >O.32), the effect of

Fig. 5.

~~e')~)

l~:'v)

0.3

Os0.2 Os+ A

As+ BI s+ C

O. 1 Dnse ns+ A

A A ns+BoO 0.05 O, 10 O, 15

tn (s)

Effect of horizontal oscillation

between t~ and ~**g.

80

0.20

modes on relation

0.05 O, IOtn (s)

O. 15 0.20

Fig. 4.

60~~:s)

eJ) 404)

~~20

o30

Convenuonal D."'O

.

*":

~' ..""'

O'

9 (a)O""

" ModeA

n =0,136sVc = 1.2 m/minf = 2.5 HzS= 5.3 mm

(b)

O O.2 O.3 O.4 0.5O. 1d2 (mm)

Effect of horizontal oscillation modes on relation

between negative strip time, l., and oscillation markdepth, d2, and on d2 at constant t*. (symbols, s: si-

nusoidal wave, ns: non-sinusoidal wave, A: ModeA,B: ModeB, C: ModeC)

463

ModesBandC/L/

~~:o

~C~]cr)

OAbJO4)

~o~eJO

~t~4

Fig. 6.

20

lO

OsOs+ AJL s+ BI s+ cDnso ns+AA ns+B

Conventionaloscillation .O

..•••••'

."tl

p

(b)

O 0.20O 0.05 O, IO O. 15tn (sec)

Effect of horizontal oscillation modes on relation

between negative strip time, !~, and segregation fre-

quency, f;.g. [(a): cl,,g >0mm,(b): d,.g> 0,3_ 2mm]

C,) 1998 ISIJ

ISIJ International, Vol,

horizontal oscillation is moreevident. Figure 7showstheeffect of the modeof horizontal oscillation on .f,.g

(d,,g>0.32) at a constant casting condition. The fre-

quency of segregation improved by the application ofhorizontal oscillation. The lowest value of ,f~.g (d~,g>0.32) was observed wlth sinusoidal +horizontal oscilla-

tion in modeC.

(3) Morphology of SegregationAs shownin Fig. 8, the segregation observed at OSM

is categorized into three types, 1) without a hook (typel), 2) with overflow onto the hook (type 2), and 3) brokenhook (type 3). Figure 9shows the percentage of f~,g bytype of segregation for al] the investigated samples. Type1represented the overwhelming majority, the respectivevalues being type I,

70 "/o, type 2, 15 o/*, and type 3, 15"/..

It should be noted that whenhorizontal oscillation modesBandCwere applied in the high t. region, a remarkabledecrease wasobserved in type l.

3.2. Results with Actual Caster

Figure 10 showsthe effect of the respective horizontaloscillation modeson d*.g. This result is substantially the

sameas those with the pilot caster (Figs. 4, 5) in thatd~.g was smaller with modesBand C than with either

modeA or vertica] osclllation oniy. In particular, thefact that the maximumvalue of d~.g Wasless than thetotal descaled value indlcates that it is possible to preventthe surface defect in hot-rolled coils by applying hori-

zontal oscillation. Further, d**g decreased as the strokeof horizontal oscillation, A/1, increased. Andin particular,

both the average value and the maximumvalue of d*.g

were reduced by half whenAh was 0.27mm(Fig. 11).

There was initially someconcern that segregation mightdiffer across the width of the slab due to bulging of the

~:'::'o~)

s8r: s+Aoc:1 s+Bo~ s+CO

O 10 20 30f (ds >0 32) ((~70)seg eg '

Fig. 7. Effect ofhorizontal oscillation modeon frequency ofsegregation, J;*g (d**g > 0,32).

38 (1998), No. 5

shell during horizontal oscillation, considering the factthat the width of slabs produced by the actual machineis approximately three times greater than at the pilotcaster. However, no such tendency was observed. Thisis considered to meanthat amountof the bulging inducedby transient creep in the shell at the meniscus is negligib]ysmall In comparison with the stroke of horizontaloscillation.

4. Discussions

4.1. Shell Deformation Behavior in Early Solidificationduring Horizontal Oscillation

To evaluate the mechanismof the decrease in OSMdepth when the horizontal oscillation was applied, amathematical simulation model wasdeveloped for shel]

deformatlon at the meniscus. Based on the analyticalmodel proposed by Anzai et al.,Is) a two-dimensionalcoupled model, 16) which calculates the pressure generatedin the mold flux channel between the mold and shell atthe meniscus by vertical oscillation of the mold and theelasto-plastic deformatlon of the shell caused by that

pressure, was expandedby the authorsl7) to be able toestimate the shel] deformation behavior during thehorizontal oscillation. The pressure behavior in thechannel during horizontal oscil]ation was calculatedunder the assumption that the value of /1 in Reynolds'lubrication Eq. (1) was the flux channel thickness, or in

other words, the distance between the mold and shell.

In the conditions of this calculation, the fluid in thechannel shows laminar flow, and a phase differencebetween the pressure and the displacement of mold canbe negligibly small even If the fiuid possessesa viscoelasticproperty.18) Therefore, the calculation does not lackappropriateness even if a Reynolds' fluid is assumed.Although the propagation of pressure is a non-steadyphenomenon,it can be considered as a quasi-steadycondition if Eq. (2) can be satisfied.19) Conditions in this

analysis satisfy Eq. (2) [/7(a)p/n)0.5=0.09], so that thepropagation of pressure wascalculated as a steady stateduring an each time interval. The main constants usedas conditions of these calcLrlations are shown in Tab]e2. Other conditions can be referred in the previous re-port. 17)

*'() 1998 ISIJ

Type1Without hook

Fig. 8. Morphology of segregation at oscillarion mark.

464

ISIJ International, Vol, 38 (1998). No. 5

~~:s)

~eJ)e' 40~H

a) Type l

OOse s+ AAs+ BI s+ Cr] ns

C] OoOe ns+ A

A ns+ B

80

60

40

20

o

20

o

20

o

oA

e tb) Type2

[] 0~c)T e3

o []

AO80

o 0,05 O. 10 O. If

Table 2. Main constants used in numerical analysis of shell

deformation at meniscus.

For solidified shell

Poisson's ratio

Young's modulusYield stress

Workhardening coefficientFor mold flux

Thickness of liquid filmbetweenmoldand shell

Viscosity at 1300~~Thickness of molten slag

layer on molten steel

Density of molten slag

0.278 (-)

50 (GPa)1 (MPa)

700 (MPa)

O. 15O, 14lO

2450

(mm)(Pa,s)

(mm)(kg/m3 )

~;~s)

~~o~LH

~~s)

~eJ)q)"~

O 0.05 O. IO O. 15tn (s)

Segregation frequency, J:;*g, of three types ofsegregation in Fig. 8. [a) Type 1, without hook; b)

Type2, with overflow on hook; c) Type3,with brokenhook]

~EeJ)(L)(1,

~:,

o.5

0.4

O.3

0.2

O. l

o

max.

av .

min.

s+B

Fig. 9.

~~eJ)(L)

~5~:l

0.5

0.4

0.3

0.2

O. l

Fig. lO.

Descaled thickness from soaking fumaceto annealing and pickling

.5

.4

.3

.2

o

max.

~•-ave.

min.

S s+A j s+B i s+C

Efrlect ofmachine.

Oscillation modehori7_ontai oscillation on

(t~ =0.099 s, Ah=0.22 mm)c!**g in actual

V(ph3VP/ 12n) =V[ph( U+ V.)/2

+p2113Fll2n] +a(ph)/at ...........(])

wherep, h, and nare the density, thickness, and viscosity

of the molten flux in the channel, respectively, P is the

pressure in the channel, and U, V*, and Fare the speedof movementof the mold and shell, and external volu-metric force (gravity), respectively.

05h(cop/n) ' ..........(2)

where a) is 2~f~, and ./1 is the frequency of horizontaloscillation of the wide faces of the mold.

It was difficult to makethe numerical computationconverge in short time because of quick change in

horizontal speed of the wide face of moldwith a triangle

wave form. Therefore, ca]culations were carried out bysubstituting the triangular wave form of horizonta]osci]1ation with sinusoidal in conslderation of stability

and convergence of the numerical computation. Figure12 showsthe changesover tlme in the maximumpressure

(a)

(b)

Fig. 11.

O O. I 0.2 0.3

Ah (mm)Effect of horizontal stroke, Ah, on d**g.

~c~

~i o 0.30

~go~o o 0,15Ncg'~

7~-~

o* O~~ ;~3

40

o~ 30:'

oo*~s::~- 20

~~~)~~~~: lO

'~q::c:$

~;.F:: e~ O0.03

,4H~~~ 0.02F~~o- OOl(C) ~~ '

_ OcsQ)o~:::": o o.O1~~::E,,

1~ -0.021:'~} :

Time (s)

~ _0.031:'

o O 0.1 0.2 O.3 0.4

Fig. 12. Behavior ofshell edge deformation at meniscus andmaximumpressure in flux channel betweenshell andmold for three modesof horizontal oscillation during

l cycle of vertical oscillation. (calculation conditions:V. =0.8 m/min, ,f= 3. IHz. S=3mm)

'ri;*~~} *~

II-INt

,,,

tn 25-50%tp,~1 A

~i

,,

,:

If I

~t

ModeBj

Conventional;#MOdec

ModeA

in the flux channel and the displacement of the upperedge of the shell in the mold thickness direction. Theseresults are from the second cycle of the iterative cal-

culation cycle, 1/f when the deformation behavior ofthe shell during one cycle of vertical oscillation becomessubstantially stable. In conventional oscillation, the

upper edge of the shell is subjected to pushing-bendingandpull-back deformation, corresponding to the changesin pressure. This behavior is also simi]ar whenhorizontal

465 (c~~=, 1998 ISIJ

ISIJ International, Vol. 38

oscillation is applied; the pressure decreases when the

mold is pulled awayfrom the shell, and increases as the

mold approaches the shell, and the upper edge of theshell is deformed accordingly. Thus, the change in the

amountof shell deformation is large during one cycle ofoscillation. Therefore, if the OSMdepth is considered tobe represented by the amountof deformation of the shell

at a certain timing during one cycle, the evaluation ofthe effect of horizontal oscillation on d2 will vary,depending on ho.w the tlming is selected in that cycle. Inthis connection, direct observations of OSMformationby Tadaet al.20) and Itoh et al.21) with Pb-Bi alloy andCualloy have confirmed that the formation of OSMin

the shell, occurs within roughly the first 50 "/* of the tp

period. Accordingly, the following discussions will givespecial attention to the amountof shell deformation inthis period.

4.2. Mechanism of Reduction in OSMDepth byHorizontal Oscillation

The results of the analysis with respect to the amountof shell deformation in the 25-500/* tp Period, whichroughly corresponds to the actual perlod of OSMformation, substantially explain the experimental results[Fig. 4(a)] that the OSMdepth decreases in the order ofconventional oscillation or modeA, modeB, and modeC. Fromthe foregoing, this pattern in the reduction of

OSMdepth by horizontal oscillation can be consideredas follows.

(1) ModeA: In the t~ period, because the pressureon the shell is reduced as the thickness of the channelincreases, the amountof pushing-bending deformationof the shell toward the molten steel side also decreases.Although the pressure is temporarily reduced, it increasesagain in the following tp Period, and the shell againundergoes pushing-bending deformation toward themolten steel side. As a result, if the amountof pushing-bending deformation during tp is large, the possibility

that the OSMdepth may increase is conceivable. InFig. 4(a), the fact that d2 increases when horizontaloscillation is applied in the low t. region is presumablydue to this cause.

(2) ModeB: The shell, which has been pushed andbent toward the molten steel side during t~, is then pulledback toward the mold side due to the reduction in the

pressure in the channel durlng the following tp, reducingthe OSMdepth.

(3) ModeC: The amoLmtof pushing-bending de-formation of the shel] toward the molten steel side is

reduced within the t~ period. In addition, although theshell has been deformed toward the molten steel side, it

is pulled back toward the mold within the fol]owing tp

period. This combination of factors is moreeffective in

reducing the OSMdepth than the simple pull-back effect

in modeB.

Fromthe above, it can be inferred that the reason forthe reduction in the OSMdepth by horizonta] oscillationis the fact that the shell is pulled back toward the moldside during the 25-500/, tp Period, which basically cor-responds to the period whenOSMforms, more than in

the case of conventional vertical oscillation.

~.', 1998 ISIJ 466

(1998). No. 5

4.3. Mechanismof Reduction in Surface Segregationby Horizontal Oscillation

4.3,1. Reasonfor Reduction in Thickness of Segrega-tion

If type 2segregation is not considered, the mechanismsof the generation of surface segregation which have beenproposed to date can be broadly classified as "penetra-tion"2,11) and "bleeding. "21,22) Hook-less segregation(type l), which is reduced remarkably by horizontaloscillation, wasseemedto be origlnally attributed to the

pressure changein the flux channel, i.e., the driving forceof penetration. Consequently, it is natural to makeanassurnption that the pressure change causes the changein the amountof enriched molten steel penetrated fromthe interdendrites and results in a change of d..g. How-ever, d,.g wasdetermined almost entirely by d2, even if

the oscillation conditions were changedvariously in or-der to induce a pressure change in the channel.14) There-fore, it is difficult to expiain this phenomenonbythe penetration. In order to understand this result, theauthors refer to a phenomenonthat Miyazaki et al.24)

found. They found that the deeper the OSMbecomes,the thicker the negative segregation zone beneath the

OSMwill be, and concluded from solidification analysesthat the semi-solidified zone expands because the so-lidification rate decreases as the OSMbecomesdeeper.Fromthe aboveconsideration, it is estimated that volumeof enriched molten steel, which is penetrated, dependson the OSMdepth. Thephenomenonthat d,,g dependson d2 might be understood from this explanation.

Fromabove, the reduction in the thickness of seg-regation which is realized with horizontal oscillation

can be considered a result of the reduction in OSMdepth.

4.3.2. Reason for Reduction In Frequency of Seg-regation

Based on the finding that when type 3 segregationshowsa high rate of occurrence, type I segregation alsotends to occur frequently (Fig. 13), the reason for themarked reduction in .f;*g with horizontal osclllation

modesBand Cis considered to be as follows.

The maximumtensile force, F., acting on the shell atthe meniscus during tp, whenhorizontal oscillation is

applied, can be expressed by the fol]owing equation:

F*=ryclu/d(h~+hp-8)......... .........

(3)

where u is the relative speed of movementof the mold

60~~~q:)

~)~ 40hh

~::

~'~ 20~)c,)

~*

oO 5 10 15 20

f segwith Type3("/~)

Flg 13 Relatlon between Type 3/;~g and Type I J;,g-

ISIJ International, Vol. 38 (1 998), No. 5

~~S)

~le~o

~~H

-O. I -0.2 -0.3 -0.4 -0.5-0.6

d2 (mm)Fig. 14. Relation between d2 and f**g.

lOO OsO s+A80

o,

JL s+B ~)

I s+C a60Vc=I .

2m/mintn=0, 136s

Oll l A40 C;

620

oo o. 1 0.2 0•3 0•4 0•5 >0•6

0.4

~'~] 03~ '

:i~~~eJ)

0.2~':

~r~~ 0.1

O

oscillation modes s+Bs+C s s+A-~~~e, .:6::"I-" '

"""""""",ef':e,t~""""""""

'----e

O 10 20 30 40 50Time(min)

Fig. 15. Changeofaverage tension,"*, acting onthe solidified

shell in mold (S=3.0mm,.f=3,1 Hz, V.= I.Om/min,All =0.22 mm)

and shell, /1~ is the thlckness of the molten flux channelbetween the mold and shell during conventional oscil-

lation, hp is the horizontal travel of the wide faces of the

mold during tp, and ~is the distance that the upper edgeof the shell is pulled back toward the mold side during tp.

F* depending on oscillation modeis expressed by Eqs.(4)-(7).

Conventional oscillation, F*= F* +=ndu/dh~........

(4)

ModeA, F*=F.A=ndu/d(h.+~A).....................

(5)

ModeB, F, =F.I} =ndu/cl(h. +A/7 -

~1;)"""""-"

(6)

ModeC, F* =F,c =ndu/d(h. +Ah-~c)

"-""-"-(7)

where 8 ~ and ~c are relative values of ~in the caseA, B,of modesA, B, and C, respectively, based on the valueof 8in the conventional oscillation.

In the present analyses (Fig. 12), the average relatlve

value of ~at the 25-500/0 tp decreases in the order of8A, 8B, and 8c' Consequently, F* becomessmaller in the

order of F F F and F*c For this reason it can*.~,. *.A, *.B,

.. ,

be inferred that rupture of the shell at the meniscus is

more difficult in the latter three modes. In this case, if

the degree of rupture of the shell is minimal, there is apossibility that distinct areas of bleeding, such as thoseIn type 3segregation, becomemoredifficult to observe,

andsomeof type 3are dlstinguished as type I segregation

on the basis of the solldified structure. In fact, there havebeen cases in which segregation lines were found whentype I segregation was subjected to solute elementsanalysis by EPMA,as seen in other research.23) In other

words, it can be thought that type I segregation is aminor variant of type 3segregation, and for this reason,not only the .f=*g of type 3, but also that of type I, wasreduced by horizontal oscillation modesBand C, whichhave smali estimated values of F.. The fact that the ,f;.g

of types I and 3segregation tends to showprogressivelydecreases in the order of modeA, modeB, and modeC(Fig. 14) at the samed2, supports this thinking.

It should be noted that the total tensile friction betweenthe mold and strand, as obtained by the methodof oil

hydraulic pressure measurementin the mold oscillationdrive system,13) reduced whenhorizontal oscillation wasapplied (Fig. 15). Generally, it is considered that thetensile friction mainly consists of the liquid friction.25)

Therefore, it maybe inferred, at least qualitatively, thatthe liquid friction plays the samerole at the meniscus.Fromabove mentioned, it suggests that horizontal os-cillation also reduces a tensile friction at the meniscus.

To summarize this discussion briefly, the reason forthe decrease in ,f;*g by horizonta] oscillation is estimatedto be a reduction of tensile force acting on the shell atthe meniscus.

5. Conclusions

Anewly-deve]oped methodof mold oscillation for thecontinuous caster wasproposed, in which the wide faces

of the mold are successively retracted and advanced in

the horizontal direction relative to the solidified shell in

synchronization with vertical oscillatlon. The effect ofthe method, termed "horizontal oscillation," on thedepth of oscillation marks and the thickness of segrega-tion at marks on the surface of continuously cast s]absof SUS304was investigated In castlng experiments with

a pilot continuous caster and an actual machine. Theresults obtained were summarizedas follows.

(]) The horizontal oscillation method, in which the

mold plates are pulied back from the shell during posi-tive strip time, has the effect of reducing the depth ofoscillation marks, the thickness of segregation, and thefrequency of segregation.

(2) The reduction in mark depth was attributed tothe fact that horizontai oscillation increases the amountof bending back deformatlon of the shell at the meniscustoward the moldside during the first half of positive strip

time.

(3) The reduction in surface segregation was at-

trlbuted to the decrease in the mark depth and the ten-slle force between the mold and shell acting on theshell at the meniscus.

(4) The effectiveness of the horizontal oscillation

methodwasconfirmed with an actual continuous caster.

1)

2)

3)

4)

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