Technical Field
[0001] This invention relates to rnold flux for continuous-casting steel which can
prevent longitudinal cracks from fonning on a snrface of a slab especially when
hypo-peritectic steel whose C concentration (carbon concentration. Hereinafter the
same will be referred to.) is 0.08 to 0.18 masso% is continuous-cast.
Background Art
[0002] In continuous casting of hypo-peritectic steel whose C concentration is 0.08
to 0.18 masso/0, a solidified shell that is formed from soliclification of molten steel in a
mold tends to be unequal in thickness, which causes longitudinal cracks to easily forrn
on a surface of a slab.
[0003] Upon continuous casting, it is effective to mildly cooling an edge portion of
a solidified shell (hereinafter referred to as "mild cooling") in order to make the
solidified shell in the mold equal in thickness. It is relatively easy therefor to use rnold
flux.
[0004] Molcl flux is supplied to the surface of molten steel that is poured iirto a
mold, melts with heat supplied from the molten steel, and flows along the mold, to
infiltrate into a gap between the rnold and the solidified shell, and to form a film. Just
after casting is stafted, this film is cooled by the mold, to solidify like glass. Crystals
are educed from the glass as time passes. If crystallization of this fihn is promoted, the
15
20
L
10
roughness of the surface of the film in the mold side increases, which causes the
interfacial thermal resistance between the mold and the fihn to increase. In addition,
radiative heat transfer in the film is suppressed. Thus, these effects allow the molten
steel and the solidified shell that touch the film to be rnildly cooled down.
[0005] It is cuspidine (CaaSi2OzFz) that is common composition of crystals educed
from the film.
[0006] The following urethods have been worked out Llpon prornoting
crystallization of the film:
First, if fluid physical properties of mold flux are controlled, it is an effective
rnethod for prornoting crystallization to raise the solidification temperature. Patent
Literature 1 discloses that the crystaìlinity is improved by raising the solidification
temperature to the range of 1 150 to 1250'C.
[0007] Patent Literature I describes that if the solidification temperature of the
mold flux is raised more than 1250"C, the lubricity between the mold and the solidified
shell is disturbed and breakout (the solidified shell breaks and the molten steel is
leaked) cannot be prevented.
[0008] When chemical components in mold flux are controlled, it is effective to
increase the mass concentration ratio of CaO to SiOz (hereinafter referred to as
"basicity"). [t is also effective to reduce the MgO concentration.
[0009] For example, Patent Literature 2 describes it is effective for crystallization
of a film that the basicity is specified by 1.2 to 1.6 and then the MgO concentration is
specifìed by no more than 1.5 masso/0. However, since the highest temperature where
crystals form in Examples of the mold flux described in Patent Literature 2 is 1145'C
only an effect of rnild cooling corresponding to this is obtained.
15
o J
10
f
[0010] On the other hand, Patent Literature 3 discloses a rnethod for suppressing
radiative heat transfer in a film by adding an iron oxide and/or a transition metal oxide
to mold flux.
[0011] However, CaO, SiOz and CaF2 in the mold flux are diluted by the addition of
any of these oxides. Specifically, in the invention of Patent Literature 3, no less than
10 mass% of an iron oxide and/or a transition metal oxide in total has to be added as
shown in its Examples in order to obtain a suffrcient effect of suppressing radiative heat
transfer. In this case, cuspidine is difficult to be educed when the basicity of the
composition is about 1.0, which is shown in the Examples, and the solidification
ternperature of the mold flr-rx drops.
[0012] The soliclification ternperature in Examples described in Patent Literature 3
is about 1050"C, which is lower than that of the mold flux for hypo-peritectic steel by
no less than 100'C, considering that the solidification temperature of the mold flux for
hypo-peritectic steel is about 1150 to 1250'C as described above. Therefore, as a
result, crystallization of the f,rhn is blocked. Thus, an effect of mild cooling according
to crystallization, that is, increase of interfacial thermal resistance and the like, is
marred.
[0013] Patent Literature 4, which was formerly proposed by this inventor, discloses
a range of the cornposition of rnold flux where cuspidine is easily educed, in the
quaternary system of CaO-SiOz-CaFz-NaF. This range of the composition is
substantially same as a primary crystallization freld of cuspidine accorcling to the report
thereafter (IS IJ Internat ional, 42 (2002), p. aB 9).
[0014] This inventor also proposes, in Patent Literature 5, the method for adding a
transition rnetal oxide to the basic composition prepared within the range of the
15
4
10
f
invention of Patent Literature 4, to drop the solidifìcation temperature without marring
an effect of mild cooling.
[0015] The invention proposed in Patent Literature 5 is to obtain an effect of mild
cooling that mold flux whose solidification temperature is 1250'C or more has, which
has been conventionally considered to be difficult to be used because the lubricity is
disturbed, from the solidification temperature of the general range, 1209 to 1239'C as
illr"lstrated in its Examples, for example.
[0016] However, in casting of hypo-peritectic steel as well, longitudinal cracks
more easily form when the degree of superheat (which means "difference between the
temperature of the molten steel and liquidus temperature". Hereinafter the same will
be referred to.) of molten steel is high, as the case when a grade of steel containing an
alloying element such as Cu, Ni, Cr, Mo, Nb, V Ti and B.
[0017] When the degree of superheat of rnolten steel is high upon casting
hypo-peritectic steel as the above, an effect enough for preventing or suppressing
longitudinal cracks rnight not be obtained even with the inventions of Patent Literatures
4 and 5 proposed by this inventor. That is, a sufficient effect might not be obtained
even frort rnold flux where cuspidine is easy to be educed by adding a transition metal
oxide to its composition range, when hypo-peritectic steel containing an alloying
element such as Cu, Ni, Cr, Mo, Nb, V Ti and B is continuous-cast.
Citation List
Patent Literature
[00181 Patent Literature 1: JP H8-1912\4A
Patent Literature 2: JP H8-141713A
15
20
5
Patent Literatr.rre 3: JP H7-1857554
Patent Literature 4: JP200 1 - 1 7 9408A
Patent Literature 5 : 1P2006-2893 834
5 Summary of Invention
Technical Problem
[0019] An object of this invention is to provide mold flux for continuous-casting
steel which can prevent longitudinal cracks from fonning on the surface of a slab when
hypo-peritectic steel whose C concentration is 0.08 to 0.18 masso/o is continuous-cast
10 even if the degree of superheat of molten steel is high as a grade of steei containing an
alloying element such as Cu, Ni, Cr, Mo, Nb, V Ti and B.
Solution to Problem
t0020] This invention is Mold flux used for continuous-casting hypo-peritectic steel
15 whose C concentration is 0.08 to 0.18 massoá, wherein
CaO, SiO2, an alkali metal oxide and a fluorine compound are contained,
the following fonnulas (1), (2) and (3) are satisfied, and
a solidiflrcation temperature is no less than 1300'C and viscosity at 1450'C is
no more than 0.1 Pa's:
20 r.1 s f(1) < r.9 ... (i)
0. r0 I f(2) 50.40 ... (2)
0 s f(3) r 0.l0 ... (3)
wherein f(1) : (CaO)6/(SiO2)¡ ... (a)
f(2) : (CaF2)¡/((CaO)¡ + (SiOz)¡ + (CaF2)¡) ... (b)
a
10
/"
f(3): (alkali metal fluoricle)¡/((CaO)¡ + (SiO2)¡ + (alkali metal fluoride)r,) ...
(c)
(CaO)¡ : (Wcuo - (CaF2)¡ x 0.718) ... (A)
(SiO2)h: Wsioz (B)
(CaF2)¡ : (W¡ - tùy'rizo x 1.27 - WNuzo x 0.613 - Wrzo x 0.403) x 2.05 .., (C)
(alkali metalfluoride)r,:WLizo x 1.14+ WNozo x 1.35 *Wrzo x 1.23 (D)
wherein W¡ is a rnass concentration (rnass%) of a component i in tl-re mold flux.
[0021] In the above invention, preferably, 0.1-10 mass% of MnO is fufther
contained.
10022) In this invention, the solidification temperature of mold flux is raised to
1300'C or lrìore, which is the range having been seldom considered conventionally, to
promote further crystallization and to obtain an effect of rniid cooling. At the same
time, the lubricity in a mold is also kept by decreasing the viscosity at 1450'C enough,
to 0.1 Pa's or less. Whereby, longitudinal cracks can be prevented from forming on the
surface of a slab even if irypo-peritectic steel containing an alloying eiement sllch as Cu,
Ni, Cr, Mo, Nb, V, Ti and B is continuolts-cast.
In addition, crystallizatior, of cuspidine is difücult to be blocked by containing
0.1 to 10 masso/o of MnO. Thus, longitudinal cracks are easy to be prevented from
forming on the surface of a slab even if hypo-peritectic steel containing an alloying
element such as Cu, Ni, Cr, Mo, Nb, V Ti and B is continuous-cast.
Advantageous Effects of lnvention
[0023] In this inventiou, further crystallization of a forming fihn is prornoted and an
effect of rnild cooling is obtained, and at the same time, the lubricity in a rnoid can be
15
+
10
,ar
also kept. Thus, longitLrdinalcracks can be prevented from fonning on the surface of a
slab even if hypo-peritectic steel containing an alloying element such as Cu, Ni, Cr, Mo,
Nb, V Ti and B is continuous-cast.
Brief Description of Drawings
100241 [Fig. 1] is a view showing the mold flux for continuous-casting steel of this
invention.
Description of Embodiments
[0025] In this invention, an object of preventing longitudinal cracks fi'om forming
on the surface of a slab when hypo-peritectic steel containing an alloying element such
as Cu, Ni, Cr, Mo, Nb, V, Ti arrd B is continuous-cast is realized by: preparing CaO,
SiO2, an alkali metal oxide and a fluorine compound within the optimr-un range, to keep
the composition within the primary crystallization field of cuspidine; and setting the
solidification temperature in 1300'C or more and the viscosity af.1450'C in 0.1 Pa.s or
less. Preferably, the rnold flux for continuous-casting steel according to this invention
fufther contains 0.1 to 10 masso/o of MnO.
[0026] Hereinafter the mold flux for continuous-casting steel of this invention (may
be siniply referred to as "the mold flux of this invention" below) will be explained.
Fig. 1 is a view showing the mold flux for continuous-casting steel of this invention.
As shown in Fig. 1, mold flux 1 of this invention is supplied to the surface of molten
steel 4 that was poured into a rnold 3 via an immersion nozzle 2. The mold flux 1 of
this invention supplied in this way melts with heat supplied from the molten steel 4.
After that, the mold flux i flows along the mold 3, ancl infrltrates into a gap between the
l5
I
10
I
mold 3 and a solidified shell 5, to fonn a film. The solidified shell 5, which is formed
by cooling from tl-re side of the rnold 3 that is cooled by cooling lneans not shown, is
withdrawn toward a lower part of the rnold 3 with rolls 6, and is cooled by cooling
water 7.
100211 Patent Literature 1 forrnerly explained describes that if the solidification
tetnperature is highel than 1250'C, the lubricity is disturbed and breakout cannot be
prevented. V/hile the proper viscosity of the mold flux is specified as 0.6 to 2.5 poises
(:0.06 to 0.25 Pa's) at 1300'C in the invention of Patent Literature 1, the viscosity in
most of Fxarnples illustrated in Patent Literature 1 is no less than 1 poise (:0.1 Pa.s).
[0028] It is necessary to lessen the resistance when the solidified shell is withdrawn
toward a lower part of the mold (fi'ictional force in the rnold) for keeping the lubricity in
the mold upon continuous casting. Because the mold flux exists between the inner
wall of the rnold and the solidified shell, the above frictional force can be iessened by
decreasing its viscosity.
[0029] There is a problern that when the viscosity of the mold flux is decreased, the
mold flux tends to be involved in the molten steel in the mold, and droplets of the
involved mold flux becomes non metallic inclusions in the vicinity of the surface of a
slab, to rnake the cleanness deteriorate.
[0030] "Tetsu-to-Hagané", Vol. 93 (2006), No. 5, page 362 describes that the
involvement can be prevented by keeping the composition of mold flux having the
basicity even if the viscosity at 1300'C is as low as 1 poise (:0.1 Pa's) or less.
[003 1] The present invention was made about mold flux used for
continuous-casting hypo-peritectic steel whose C concentration is 0.08 to 0.18 masso/o
based on the above approaches.
l5
I
10
I
[0032] ln this invention, the basic components are CaO, SiOz and a fluorine
cornpound, which are components of cuspidine. In addition, an alkali metai oxide is
added, so that the solidification temperature can be easy to be controlled comparably.
Here, "basic components" are the components of cuspidine, which mean that the sum
total of the concentrations of CaO, SiOz and fluorine in a fluorine compound is no less
than 60 masso/0.
[0033] Each concentration of CaO, SiOz, a fluorine compound and an alkali metal
oxide is adjusted so as to satisfy the following formulas (1), (2) and (3), which represent
the conditions under which cuspidine is easy to be crystallized:
[0034] 1.1 < f(1) < I .e ... (1)
0.r0 < rQ)<0.40 ... (2)
0 < f(3) < 0.10 ... (3)
where
f(1) : (Cao)¡/(SiOz)¡ ... (a)
f(2) : (CaF2)r,/((CaO)n + (SiOz)¡ + (CaF2)¡) ... (b)
f(3) : (alkali metal fluoride)¡/((CaO)¡ + (SiOz)r, + (alkali metal fluoride)¡) ... (c)
(CaO)¡ : (Wcuo - (CaF2)¡ x 0.718) (A)
(SiO2)h : Vy'sioz ... (B)
(CaFz)n: (We - Wrizo x 1.27 - Wu¿o x 0.613 - Wrczo x 0.403) x2.05... (C)
(alkali metal fluorid.)n : Wrizox 1.74 +WNuzóx 1.35 *Wrzo x 1.23 . (D)
where Wi is a firass concentration (rnass%) of a component i in the mold flux.
[0035] The composition of the mold flux can be kept within the prirnary
crystallization field of cLrspidine by adjusting each concentration of Cao, Sio2, a
fluorine compound and an alkali metal oxide so as to satisfy the conditions represented
15
ttr
by the above formulas (1), (2) and (3).
[0036] Here, when f(l) does not reach 1.1 or when f(i) is over 1.9, the composition
of the mold flux deviates from cuspidine. Thus, crystallization enough for rnild
cooling cannot be obtained. Therefore, in this invention, f(1) is 1.1 to 1.9. Preferably,
f(1) is no less than L2 and especially preferably no less than 1.3 because it rnakes the
composition of tire rnold flux more similar to cuspidine, which brings about the
embodiment where effective crystallization is easy to be achieved. Further, in the
same point of view, f(1) is preferably no more than 1.8, and especially preferably no
rrore than 1.7. In this invention, the preferable range of f(1) is 1.2 to 1.8, and
especially preferably 1.3 to i.7.
When f(2) does not reach 0.l0 or when f(2) is over 0.40, the composition of the
rnold flux deviates from cuspidine. Thus, crystallization enough for mild cooling
cannot be obtained. Therefore, in this invention, f(2) is 0.10 to 0.40. Preferably, f(2)
is no less fhan 0.72 and especially preferably no less than 0.15 because it makes the
composition of the rnold flux more similar to cuspidine, which brings about the
embodiment where effective crystallization is easy to be achieved. Further, in the
same point of view, f(2) is preferably no more than 0.35, and especially pteferably no
more than 0.30. In this invention, the preferable range of f(2) is 0.12 to 0.35, and
especially preferably 0.15 to 0.30.
When f(3) does not reach 0 or when f(3) is over 0.10, the composition of the
mold flux deviates from cuspidine. Thus, crystallization enough for mild cooling
cannot be obtained. Therefore, in this invention, f(3) is 0 to 0.10. Preferably, f(3) is
no more than 0.08 because it makes the composition of the mold flux more similar to
cuspidine, which brings about the embodiment where effective crystallization is easy to
10
15
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f
be achieved.
[0037] Generally, the viscosity of mold flux at 1300"C is used as the basis.
However, in this invention where the composition is kept within the prirnary
crystallization field of cuspidine whose melting point is l4l0 to 1420"C, the mold flux
is ah'eady solidified at 1300'C by crystallization, ancl thus, the value at i300'C cannot
be obtained. Thus, in this invention, the viscosity at 1450'C is clefined as 0.1 Pa's or
below.
[0038] The lubricify can be kept with this viscosity even in the state where the
solidification temperature is raised to 1300'C or more, which has been conventionally
said to be difficult. If the solidification temperature rises, an effect of mild cooling in
the mold is irnproved according thereto. [n this inventiorr, it is irnpossible to raise the
solidification temperature of the mold flux to or over the above described melting point
of cuspidine.
[0039] Not a little Mn is added to hypo-peritectic steel to be practically used in
order to improve the strength when used as a steel material. Thus, MnO generated by
oxidation of Mn in the molten steel migrates to the mold flux during casting.
[0040] This MnO is a component that blocks crystallization of cuspidine. Thus, in
this inver-rtion, preferably MnO was mixed in advance by amount coresponding to the
MnO concentration which is increased by MnO rnigrating from the molten steel to the
mold flux. According to this inventor's examination, the concentration of the
contained MnO is no less than 0.1 rnasso/o in view of acliieving an embodiment where
an effect by the addition is easily obtained. In addition, the concentration of the
contained MnO is no more than 10 masso% in view of preventing such a case from
happening that the solidification temperature of the mold flux is too much low not to
l5
20
IL
10
obtain crystallization, which is necessary for mild cooling. in this invention, it is
proper that the MnO concentration in the mold flux is set according to the Mn
concentration in the molten steel.
[0041] In some cases, MgO, AlzO:, BaO, BzO: and the like rnay be added in order
to control physicalproperties of the mold flux such as the solidification temperature, the
viscosity and the sllrface tension. However, it is better that their concentrations are
low in order to promote crystallization of cuspidine. Their concentratious are
desirably not beyond J mass%o in total. When nonnal raw materials are nsed, the total
concentration of them that are inevitably contained is about 2 to 5 masso%, and can be no
more than 2 to 5 rnasso/o by using artificial raw materials like a pre-melted base material.
Examples
10042] The results of the experiments done for confirming effects of this invention
will be described.
[0043] Exarnple I
Mold flux of this invention of each Example of this invention, Reference
Exarnple and Cornparative Example showr-r in Tables 1 and 2 was made. Here, f(1),
f(2) and f(3) shown in Table 2 below were indexes calculated from the above fonnulas
(a), (b) and (c). Crystallization of cuspidine in the rnold flux was able to be promoted
by adjusting these indexes within the ranges of the above formulas (1), (2) and (3).
Conceming the components whose lrass concentrations (rnass%) are not shown in Table
I and2, f(1), f(2) and f(3) were calculated assuming that the mass 0% was zero. It is
diffjcult in a general chernical analysis to evaluate the concentration of no more than 0.1
masso/o with high accuracy. Thus, in Tables I and 2, "< 0.I" is shown if the
15
t3
concentration of a component was less than 0. 1 masso/0.
100441 fTable I ]
Category Type
Concentration of Component (rnass%)
SiOz CaO AlzO: Mgo NazO MnO F
Exarnple of
This
Invention
A 30.6 55.1 4.2 0.1 <0.1 9.5
B 3r.2 56.2 2.4 0.1 <0.1 9.5
C 30.1 51.2 4.0 0.6 3.5 10.5
D 3 0.3 54.6 2.4 0.7 <0. I 2.5 9.5
E t').1 53.9 2.3 0.1 <0.1 9.5
F 35.7 s6.3 2.1 0.6 <0.1 5.3
G 34.1 54.9 2.5 1.0 1.0 6.5
H 3 0.5 5 i.5 2.0 <0.1 4.5 I 1.5
I 30.6 55.1 3.2 0.7 <0.1 1.0 9.5
J )).I s3.9 2.0 0.5 <0.1 0.5 9.5
K 3 0.5 51.5 1.9 <0.1 1.0 0.1 1 1.5
Comparative
Example
a 28.8 48.9 3.3 0.5 6.0 1.5 1 1.0
b 21.0 48.7 3.8 0.6 8.0 12.0
c JI.I 28.1 8.0 0.1 16.8 9.5
[004s] lTable 2l
Category Type
Index of This Invention Solidification
Temperature
('c)
Viscosity
f,(1) f(2) f(3 ) (Pa's)
Example of
This
Invention
A r.34 0.21 0 l3l 0.07
B 1.35 0.21 0 I 35s 0.01
C t.29 0.20 0.06 t302 0.0s
D 1.34 0.22 0 1334 0.06
E 1.19 0.21 0 1301 0.07
F 1.36 0.1 1 0 1329 0.08
G 1.35 0.14 0.02 ]JJ¿ 0.01
H 1.27 0.21 0.08 1302 0.06
1.34 0.21 0 1 306 0.06
J 1.19 0.21 0 I 303 0,06
K 1.21 0.21 0.08 1301 0.01
Comparative d 1.33 0.18 0.1 1 1255. 0.04
Ì ¿{-
10
yr
Example b t.41 0.18 0.14 1236 0.03
c 0.8- 0* 0.25. I 060. 0.12
10046] All the compositions of the mold flux of Exarnples of this invention Ato K
satisfied the above fonnulas (1) to (3). Their solidification temperatures were no less
than i300'C, and the viscosities at 1450'C were no more than 0.1 Pa's. In contrast,
concerning the cornpositions of the mold flux of Comparative Examples a to c, the
compositions of this mold flux did not satisfy any of the above formulas (l) to (3). As
a result, at least one of the solidification temperature and the viscosity at 1450'C of each
thereof was out of the range of this invention. Values with marks of asterisks in Tables
1 and2 represent being out of the range of this invention.
[0047] The mold flux of each Example of this invention A to K and Comparative
Example a to c was used for continuous casting of hypo-peritectic steel having the
composition shown in Table 3 below, containing Nb and Ti, and whose degree of
superheat in a state of molten steel is high, and the results were compared. Here, a slab
of 500 mm in width and 85 mm in thickness was produced with a vertical bending type
continuous casting machine, using 2.5 tons of the rnolten steel, under the conditions
where the casting speed was 1.0 m/min and the specific water flow of the secondary
cooling water was 1.1 litter/kg.
[0048] fTable 3]
Concentration of Component (mass%)
C Si Mn P S Nb Ti AI Remainder
0,09 0.1s t.40 0.010 0.005 0.0014 0.01 0 0.03 Fe and hnpurities
[0049] The rnold flux added to the inside of the mold was selectively used, and as
to an effect of mild cooling, a solidification coefficient was obtained by calculating
20 local heat flux in the mold, the surface temperature of the slab and the thickness of the
solidified shell and its growth rate, to compare the obtained results. As to the lubricity,
15
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10
v
the frictional force in the mold was obtained, to compare the obtained results. The
results are shown in Table 4.
[0050] The local heat flux in the mold was obtained from temperature lneasured
with a thermoconple that was embedded at the center of the width of the longer side
surface 35 mn under the meniscns, to be evaluated. The frictional force in the rnold
was obtained from difference in pressure of a lrydraulic cylinder used for oscillation of
the mold. The surface temperature of the slab was u'ìeasllred at the center of the width
in the side of the inward curve of roll segments just before the curving point of the
vertical bending type contirruous casting machine. The thickness of the solidified shell
and its growth rate were evaluated by such a method that: a FeS alloy was added to the
molten steel in the mold at the time point just before the casting was ended, a porlion of
the slab where the alloy was added was cut in the casting direction, and the
concentration distribution ofS on the cut surface was transferred to photographic paper.
[00s 1] fTable 4]
Category Type
Local
Heat Flux
in Mold
(MV//rn2)
Frictional
Force in
Mold
(N/rnm2)
Surface Temperature
of Slab (Center of
Width in Side of
Inward Curve) ('C)
Solidification
Coeffrcient
(mm/mino5)
Length and
Number of
LongitLrdinal
Cracks
Exarnple
of This
Invention
A 1.32 1.12x10-2 1 1s0 t12 None
B 1.26 2.09x10-2 1 140 12.3 None
C t.41 1.14x10-2 1110 t3.4 None
D 1.29 1 .1 9x l0-2 II2O 1 1.9 None
E 1.42 1.13 x 10-2 I 105 13.8 None
F 1.39 1.53 x 10-2 1 109 13.5 None
G 1.34 1.4'/xl0-2 1118 12.9 None
H t.41 1.06x 10-2 t101 13.1 None
t.34 I .10x 10-2 I 150 1 1.3 None
J t.44 1.09x10-2 I 100 14.0 None
K t.42 1.04x 10-2 I 105 15.¿ None
tb
,6
Comparative
Example
a 1.48 1.26x10-2 I 100 18.1 None
b 1.11 1.04x 10-2 1020 20.6 None
c 1.93 0.87x 10-2 950 24.8 100mmx2cracks
10
[0052] From the results of the evaluation of the local heat flux in the rnold, while
the local heat flux was no less than 1.48 MW/m' in
"u"ry
Comparative Example, it
decreased to no more than 1 .44ly'rWlm2 in every Example of this invention, from which
an effect of mild cooling was confirmed.
t00531 The frictional force in the mold was no more than 2.09 x 10-2 1N/rnm2¡ in
every Example of this invention and Comparative Example. Thus, no problems occur
to the lubricity, and normal oscillation marks were formed on the surface of the slab at
regular intervals.
[0054] The results of tire measurement of the surface temperature of the slab were
as follows: the temperature in cases where the mold flux of Examples of this invention
was used was equal to or higher than that in cases where the mold flux of Cornparative
Examples was used, from which an effect of mild cooling was confirmed.
[0055] From the lesults of the evaluation of the thickness of the solidified shell and
its growth rate, whiie the sotidification coefficient was 18.1 to 24.Smmlninot ir'r
"u.ry
Comparative Example, ìt decreased Ío 11.2 to 14.0 mm/minOs in every Exarnple of this
invention, from which an efflect of mild cooling was clearly confirmed on the growth of
the solidified shell.
[0056] In every Exarnple of this invention, the obtained slab had excellent surface
properties and condition. No surface defects such as longitudinal cracks or
depressions appeared. In contrast, in Comparative Example c, two longitudinal cracks
of about 100 mm in length appeared at the center of the width.
[0057]
15
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yf
The rrold flux of each Example of this invention A and Comparative Example
awas used out of the mold flux tested in Example 1, and larger-scale continuous casting
than Example 1 was carried out.
[0058] The molten steel of 300 tons, having the composition shown in Table 5, was
5 supplied for casting r-rsing the mold flux of each Example, to cast twelve slabs of about
2300 rnm in width,300 mm in thickness and 6 m in length atthe speed of 0.70 m/min.
The results of the surfaces of the obtained slabs were as follows:
[00s9] [Table 5]
[0060] From Example of this invention A, twelve slabs of excellent surfaces
without any longitudinal cracks were obtained. These slabs were able to be supplied
for a rolling process as they were.
[0061] In contrast, in Comparative Exarnple a, longitudìnal cracks formed on the
surface of fours slabs, that is, the first, second, eleventh and twelfth slabs after castino
was started. AIl the slabs where longitudinal clacks formed were necessary to be
repaired (scarfing with a scarfer).
[0062] It is needless to say that this invention is not limited to the above described
examples, and the embodiments may be properly modified if such rnodificatior-r is
within the scope of the technical concepts recited in the claims.
Reference Signs List
10063] 1 ...mold flux for continuous-casting steel
2 ... immersion nozzle
10
l5
IB
Concentration of Cornponent (rnass%)
C Si Mn P S Cr Mo AI Remainder
0.1 I 0.30 0.90 < 0.025 < 0.005 0.50 0.40 0.02 Fe and Lnpurities
3 ... mold
4 ... rnolten steel

We claim:
[Claim 1] Mold flux used for continuous-casting hypo-peritectic steel whose C
concentration is 0.08 to 0.18 masso/0, wherein
CaO, SiOz, an alkali metal oxide and a fluorine compound are contained,
the following formulas (1), (2) and (3) are satisfìed, and
a solidification temperature is no less than 1300'C and viscosity at 1450'C is no
more than 0.1 Pa's:
1.1 r f(1) r 1.e ... (1)
0.10 < f(2) s 0.40 ... (2)
0 < ft3) < 0.10 ... (3)
wherein f(1) : (CaO)r'l(SiOz)n- ... (a)
f\2) : (CaFz)r'l((CaO)r' + (SiOz)r' + (CaFz)r') ... (b)
f(3) : (alkali metal fluoride)r'l((CaO)r.' + (SiOz)r' + (alkali metal fluoride)r') ... (c)
(CaO)r, : (Wcuo - (CaFz)r, x 0.718) ... (A)
(SiOz)r': Wsioz .., (B)
(CaFz)r, : (We - Wr¡zo x 1.21 - Wruuzo x 0.613 - Wrczo x 0.403) x 2.05 ... (C)
(alkali rnetalfluoride)r':tùy'rizo x 1.74 *\ùy'nuzo x 1.35 *Wrzo x 1.23...(D)
wherein Wi is a mass concentration (mass%) of a component i in the mold flux.
fClaim 2] The mold flux for continuous-casting hypo-peritectic steel according to
claim l, wherein 0.1-10 masso% of MnO is further contained.