This invention relates to mold flux that is used for continuous-casting
Ti-containing hypo-peritectic steel, and a method for continuous-casting hypo-peritectic
steel containing 0.1 to 1 mass% of Ti, using the mold flux.
10
Background Art
[0002] When hypo-peritectic steel containing 0.08 to 0.18 mass% of C is
continuous-cast, a solidified shell that is formed by solidification of molten steel in a
mold tends to be unequal in thickness, which causes longitudinal cracks to easily form
15 on a surface of a slab.
[0003] It is effective to mildly cooling the solidified shell (hereinafter, may be
referred to as "mild cooling") in order to make the solidified shell in the mold equal in
thickness. It is relatively easy to use mold flux for mild cooling.
[0004] The mold flux is supplied on molten steel in the mold, and melts with heat
20 supplied from the molten steel. The mold flux in a melting state flows along the mold,
to come into a gap between the mold and the solidified shell, and to form a mold flux
film (hereinafter may be referred to as "film"). Just after the casting starts, this film is
cooled by the mold, to solidify like glass. Crystals are educed from the glass as time
passes. When crystallization of this film is promoted, the roughness of the surface of
the film in the mold side increases, which causes the thermal resistance at the interface
between the mold and the film (hereinafter may be referred to as "interfacial thermal
resistance") to increase. Radiative heat transfer in the film is also suppressed. These
effects allows the molten steel and the solidified shell touching the film to be mildly
5 cooled down.
[0005] It is cuspidine (Ca4Si207F2) that is common composition of crystals educed
from the film.
[0006] The following methods have been worked out as means for promoting
crystallization of films:
10 [0007] A method for controlling fluid physical properties of mold flux, specifically,
a method for raising the solidification point is effective for promoting crystallization of
the film. About this method, Patent Literature 1 describes that crystallinity of a film is
improved by raising the solidification point of mold flux to the range of 11 50 to 1250°C.
However, it is said there is a problem that if the solidification point of the mold flux is
15 raised to 1250°C or over, the lubricity is disturbed and breakout cannot be prevented.
[0008] A method for controlling components in mold flux, specifically, a method
for increasing the ratio of the contents of CaO to Si02 (hereinafter may be referred to as
"basicity") is also effective for promoting crystallization of the film. A method for
reducing the MgO content in mold flux is also effective for promoting crystallization of
20 the film. Concerning these methods, Patent Literature 2 discloses it is effective for
crystallization of a film that in mold flux, the basicity is specified by 1.2 to 1.6 and the
MgO content is specified by no more than 1.5 mass%. However, the highest
temperature where the mold flux forms crystals disclosed in Patent Literature 2 is about
1150°C at most, and only an effect of mild cooling corresponding to this is obtained.
That is, the effect of mild cooling is insuflicient.
[0009] On the other hand, Patent Literature 3 discloses a method for suppressing
radiative heat transfer in a film by adding an iron oxide or a transition metal oxide to
mold flux.
5 [OOlO] However, CaO, Si02, and CaF2 in the mold flux are diluted by the addition
of any of these oxides. Specifically, in Patent Literature 3, no less than 10 mass% of
an iron oxide or a transition metal oxide in total has to be added as shown in its
Examples in order to obtain a sufficient effect of suppressing radiative heat transfer. In
this case, cuspidine is difficult to be educed when the composition has about 1.0 of the
10 basicity shown in Examples of Patent Literature 3, and the solidification point of the
mold flux drops. The solidification point shown in Examples of Patent Literature 3 is
about 105O0C, which is lower than that in Patent Literature 1 by no less than 100°C
considering that the solidification point of the mold flux for hypo-peritectic steel
suggested in Patent Literature 1 is about 11 50 to 125O0C. As a result, crystallization of
15 the film is blocked. Thus, with the art of Patent Literature 3, an effect of mild cooling
obtained from increase of interfacial thermal resistance and the like according to the
crystallization is marred.
[0011] Patent Literature 4 discloses a range of the composition of mold flux of the
quaternary system of CaO-Si02-CaF2-NaF where cuspidine is easily educed. The
20 range of the composition is substantially same as a primary crystallization field of
cuspidine as published in Non-Patent Literature 1 thereafter. According to the mold
flux of Patent Literature 4 as described above, longitudinal cracks do not form on a
surface of a slab when hypo-peritectic steel is rapidly cast, which makes it possible to
obtain the slab which has a good surface quality.
[0012] Patent Literature 5 discloses a method for adding a transition metal oxide to
the basic composition prepared within the range of Patent Literature 4, to drop the
solidification point without marring an effect of mild cooling. Patent Literature 5 is
aimed at the problem that when the Mn content in molten steel is high, crystallization of
5 cuspidine is blocked because the MnO content in the film increases due to oxidation
reaction of Mn, and thus the effect of mild cooling cannot be sufficiently obtained. For
this problem, a necessary content of MnO is contained in advance, to suppress oxidation
reaction of Mn, and then the solidification point is raised to a desired level. Whereby,
it is possible to prevent longitudinal cracks on high-strength steel of the high Mn
10 content from forming, according to Patent Literature 5.
Citation List
Patent Literature
[OO 131 Patent Literature 1 : JP H8- 1972 14A
15 Patent Literature 2: JP H8-141713A
Patent Literature 3: JP H7-185755A
Patent Literature 4: JP200 1 - 179408A
Patent Literature 5: JP2006-289383A
Non-Patent Literature
20 [0014] Non-Patent Literature 1: ISIJ International, Vol. 42 (2002), pp. 489 to 497
Summary of Invention
Technical Problem
[00 151 As described above, in continuous casting of hypo-peritectic steel,
longitudinal cracks easily form on a surface of a slab. It is effective for preventing the
formation of longitudinal cracks to carry out mild cooling on the solidified shell, and
mold flux can be used for this mild cooling.
[0016] However, the mold flux of Patent Literatures 1 to 3 as described above has
5 the problems that the lubricity is disturbed and breakout cannot be prevented, and that
the effect of mild cooling is insufficient.
[0017] On the other hand, according to the mold flux of Patent Literature 4,
longitudinal cracks do not form on a surface of a slab when hypo-peritectic steel is
rapidly cast, which makes it possible to obtain the slab which has a good surface quality.
10 According to the mold flux of Patent Literature 5, it is possible to prevent longitudinal
cracks on high-strength steel of the high Mn content from forming.
[0018] One of steel grades of hypo-peritectic steel is of no less than 0.1 mass% of
the Ti content. In casting of this Ti-containing hypo-peritectic steel, Ti02 forms in
mold flux in a melting state through the influence of oxidation reaction of Ti in molten
15 steel. This TiOz not only dilutes cuspidine in the solidified film, but also forms
another new crystal phase, perovskite (CaTi03). Therefor, this perovskite grows up in
the film unilaterally, and a glass phase (cuspidine) necessary for lubrication is marred.
As a result, stable casting gets difficult, and the problem of forming longitudinal cracks
on a surface of a slab rises.
20 [0019] Therefore, there is a case in the casting of Ti-containing hypo-peritectic steel
that longitudinal cracks form on a surface of a slab through the influence of Ti02
forming in the mold flux even if the mold flux of any of Patent Literatures 4 and 5 is
used.
[0020] This invention was made in view of these problems. An object of this
invention is, in continuous casting of Ti-containing hypo-peritectic steel, to provide
mold flux that can prevent longitudinal cracks from forming on a surface of a slab and
to provide a method for continuous-casting hypo-peritectic steel containing 0.1 to 1
mass% of Ti, using this mold flux.
5
Solution to Problem ~
[002 11 The inventors of this invention found that in continuous casting of
Ti-containing hypo-peritectic steel, the composition of mold flux in a melting state
changes according to oxidation reaction of Ti in molten steel. Specifically, they found
10 that the MnO and TiOz contents of the mold flux increase in its melting state from less
than 0.1 mass% in its initial composition.
Further, they found that even if f(l), f(2) and f(3) described below, which are
calculated from the initial composition of the mold flux, satisfy the formulas (I), (2) and
(3) described below as well, composition changes in the mold flux in the melting state
15 become large when the Ti02 content of the mold flux in the melting state during casting
exceeds 20 mass%. If the composition changes in the mold flux in the melting state
become large, the ratio of the first peak height of perovskite to the first peak height of
cuspidine (hereinafter may be merely referred to as "strength") which is obtained from
X-ray diffraction analysis of powder obtained by pulverizing the film of the mold flux
20 in a solidifying state after the casting takes a value more than 1.0, formation of
cuspidine is blocked, and evaluation of continuous casting and longitudinal cracks
becomes "failure". Thus, it is important for preventing longitudinal cracks on a
surface of a slab from forming in continuous casting of Ti-containing hypo-peritectic
steel that the Ti02 content of mold flux in the melting state during the casting is less
than 20 mass% and the strength ratio is no more than 1.0. This invention was made
based on these findings. The gist of this invention is as follows.
LO0221 A first aspect of this invention is mold flux for continuous-casting
Ti-containing hypo-peritectic steel, wherein in continuous casting of Ti-containing
5 hypo-peritectic steel, the mold flux contains CaO, Si02, an alkali metal oxide and a
fluorine compound as major components, chemical composition of the mold flux before
the mold flux is put into a mold satisfies the formulas (I), (2) and (3), a TiOz content of
the mold flux in a melting state during the casting is no more than 20 mass%, and a
strength ratio of a film of the mold flux in a solidifying state after the casting is no more
10 than 1.0:
1.1 -0.5 xT 5 f(1) 5 1.9-0.5 x T ... (1)
0.05 2 f(2) 5 0.40 . . . (2)
0 2 f(3) I 0.40 . . . (3),
wherein in the formulas (1) to (3),
f( 1 ) = (CaO)hl(Si02)h . . . (A)
f(2) = (CaF2)h/{(Cao), + (SiO2)h + (CaF2)h) . . . (B)
f(3) = {(alkali metal fluoride)h)/{(CaO)h + (SiOZ)h + (alkali metal fl~oride)~))
(C),
wherein in the formulas (A) to (C),
20 (CaO)h = Wcao - (CaF2), x 0.7 1 8 . . . (D)
(SiO2)h = Wsi02 . . . (E)
(CaF2)h = (WF- W ~ i X2 1~.2 7 - WNno x 0.613 - WK~XO 0 .403) x 2.05 . . . (F)
(alkali metal fluoride)h = WLi~xo 1.74 + WNao x 1.35 + WK20x 1.23 . . . (G)
wherein T is a Ti content of molten steel (mass%), Wcao is a CaO content of
the mold flux (mass%), Wsi02 is a Si02 content of the mold flux (mass%), WF is a F
content of the mold flux (mass%), and WLi10 ,WNaO and WKZo are contents of Li20,
NazO and KzO respectively, which are alkali metal oxides, of the mold flux (mass%),
and
5 wherein the strength ratio of the film means a ratio of a first peak height of
perovskite (strength of an angle (33.2"), which is twice as wide as a Bragg angle when
Co was a source, X2) to a first peak height of cuspidine (strength of an angle (29.2"),
which is twice as wide as a Bragg angle when Co was a source, XI), the ratio (X2/X1)
being obtained from X-ray diffraction analysis of powder obtained by pulverizing the
10 film of the mold flux.
[0023] A second aspect of this invention is a method for continuous-casting Ticontaining
hypo-peritectic steel, the method comprising: continuous-casting
hypo-peritectic steel containing 0.1 to 1 mass% of Ti, using the mold flux of the above
first aspect of this invention.
15 [0024] "... contains CaO, SO2, an alkali metal oxide and a fluorine compound as
major components" in this invention means that the content of each object component is
no less than 5 mass%, and the total content thereof is no less than 70 mass%.
Advantageous Effects of Invention
20 [0025] Each of the indexes (f(l), f(2) and f(3)) of the mold flux for
continuous-casting Ti-containing hypo-peritectic steel of this invention (hereinafter may
be referred to as "mold flux of this invention") is prepared within a predetermined
range; these indexes are calculated from the chemical composition before the mold flux
is supplied into a mold (hereinafter may be referred to as "initial chemical
composition"). Moreover, the TiOz content in the melting state during the casting is no
more than 20 mass% and the strength ratio of the film in the solidifying state after the
casting is no more than 1.0. Whereby, even if the composition of the mold flux in the
melting state changes according to oxidation reaction of Ti in molten steel, cuspidine
5 stabilizes in a crystal phase in the film, and a state where cuspidine is dominant over
perovskite can be kept. As a result, effects of lubricity and mild cooling in the mold
are stable, to prevent longitudinal cracks on a surface of a slab from forming.
COO261 The method for continuous-casting Ti-containing hypo-peritectic steel of
this invention (hereinafter may be referred to as "continuous casting method of this
10 invention") uses the above described mold flux of this invention. Whereby, cuspidine
stabilizes in a crystal phase in the film that is formed in the mold, and the state where
cuspidine is dominant over perovskite can be kept. As a result, the effects of lubricity
and mild cooling in the mold are stable, to prevent longitudinal cracks on a surface of a
slab from forming.
15
Brief Description of Drawings
[0027] [Fig. 11 is a view showing the mold flux and the continuous casting method
of this invention.
[Fig. 21 is a cross-sectional view showing partially enlarged Fig. 1.
20
Description of Embodiments
[0028] Fig. 1 is a view showing this invention. Fig. 2 is a cross-sectional view
showing a part of Fig. 1 surrounded by a dashed line enlarged. This invention will be
described below with reference to Figs. 1 and 2 when demanded. "X to Y" means "no
less than X and no more than Y" unless there is any special mention.
As shown in Fig. 1, mold flux 1 of this invention is supplied on the surface of
molten steel 4 that is poured into a mold 3 via a submerged nozzle 2. The mold flux 1
of this invention supplied in this way melts with heat supplied from the molten steel 4.
5 After that, as shown in Fig. 2, the mold flux 1 flows along the mold 3, and comes into a
gap between the mold 3 and a solidified shell 5, to form a film 8. The solidified shell 5,
which is formed by cooling from the side of the mold 3 that is cooled by cooling means
not shown, is withdrawn toward a lower part of the mold 3 with rolls 6, and is cooled by
cooling water 7. In the continuous casting method of this invention, hypo-peritectic
10 steel containing 0.1 to 1 mass% of Ti is continuous-cast in this way.
[0029] The reasons why the mold flux and continuous casting method of this
invention are specified like the above, and preferred embodiments thereof will be
described below.
[0030] The mold flux of this invention contains CaO, SO2, an alkali metal oxide
15 and a fluorine compound as major components. CaO, Si02 and a fluorine compound
are contained as essential components for cuspidine that bears crystallization. An
alkali metal oxide is contained as a component for controlling the solidification point of
the flux relatively easily.
[0031] As described above, in 'the continuous casting of hypo-peritectic steel
20 containing 0.1 to 1 mass% of Ti, the chemical composition of the mold flux changes
according to oxidation reaction of Ti in the molten steel in the mold. Thus, each of the
indexes (f(l), f(2) and f(3), hereinafter the same will be referred to) of the mold flux of
this invention, which is calculated from 'the initial chemical composition, is prepared
within a predetermined range. Here, "initial chemical composition" means the
composition before the supply into the mold for continuous casting. The intention is to
exclude the composition changes in the mold flux according to oxidation reaction of Ti
in the molten steel.
[0032] The preparation of the indexes makes cuspidine stabilize in a crystal phase
5 in the film even if the composition of the mold flux in the melting state (hereinafter may
be referred to as "melting mold flux") changes according to oxidation reaction of Ti in
the molten steel. Thus, a state where cuspidine is dominant over perovskite is easily
kept. As a result, the effects of lubricity and mild cooling in the mold can be stable,
and longitudinal cracks on a surface of a slab can be prevented from forming.
10 [0033] Specifically, the initial chemical composition satisfies the following
formulas (I), (2) and (3). That is, the indexes (f(l), f(2) and f(3)), which are calculated
from the initial chemical composition using the following formulas (A) to (H), satisfy
the following formulas (1), (2) and (3), respectively.
[0034] 1.1 - 0.5 x T < f(1) 5 1.9 -0.5 x T ... (1)
15 0.05 (_ f(2) 5 0.40 ... (2)
0 S f(3) I 0.40 . , . (3)
[0035] The indexes f(1) to f(3) are specified by the following formulas (A) to (G).
f(1) = (CaO)h/(Si02)h . . . (A)
f(2) = (CaF2)h/{(Cao)h + (Si02)h + (CaF2)h) . . . (B)
20 f(3) = {(alkali metal fl~oride)~}l{(CaO+) ~(S i02)h + (alkali metal fl~oride)~))
. . . (C)
(CaO)h = Wcao - (CaF2)h x 0.71 8 . . . (D)
(SiO2)h = Wsi02 . . . (E)
(CaF2)h = (WF- WLizO X 1.27 - WNao X 0.613 - WKZoX 0.403) x 2.05 .. . (F)
(alkali metal fl~oride=)~ W LiSOx 1.74 + WNdo X 1.35 + WKZoX 1.23 . . . (G)
[0036] Here T is the Ti content in the molten steel (mass%), Wc,o is the CaO
content in the mold flux (mass%), WSi02 is the Si02 content in the mold flux (mass%),
WFi s the F content in the mold flux (mass%), and W ~ i 2,W~N ~~anOd WK20 are the
5 contents of Li20, Na20 and K20 respectively, which are alkali metal oxides, in the mold
flux (mass%).
[0037] The index f(l), which is calculated using the formula (A), is a ratio of the
CaO content to the Si02 content in view of CaF2, and is an important index to promote
crystallization of cuspidine.
10 [0038] Here, in the case of hypo-peritectic steel whose Ti content is less than 0.1
mass%, the value of f(1) has to take 1.1 to 1.9 in order to keep the composition of the
melting mold flux within the range of the composition of a primary crystal of cuspidine.
[0039] In the case of hypo-peritectic steel containing 0.1 to 1 mass% of Ti, SiOz in
the melting mold flux is reduced by reaction with Ti in the molten steel in the mold.
15 Therefore, such a situation arises that even if f(1) from the initial chemical component is
within the above described range (1.1 to 1.9), the value of f(1) from the composition of
the melting mold flux is far beyond the preferred state. Thus, f(1) from the initial
chemical composition is prepared so as to be low according to the Ti content of the
molten steel, and thereby the value of f(1) from the composition of the melting mold
20 flux is set within the above described range (1.1 to 1.9). As a result, the value of f(1)
from the composition of the melting mold flux increases because of the reaction in the
mold, and the composition of the melting mold flux can be kept within the range of the
composition of a primary crystal of cuspidine.
[0040] Specifically, f(1) of the mold flux of this invention has to be (1.1 - 0.5 x T)
to (1.9 - 0.5 x T). In view of more stable crystallization of cuspidine, the upper limit
of f(1) is preferably (1.7 - 0.5 x T), and more preferably (1.5 - 0.5 x T). In the same
view, the lower limit of f(1) is preferably (1.2 - 0.5 x T), and more preferably (1.3 - 0.5
x T).
5 [0041] The index f(2) calculated using the formula (B) indicates a proportion of
CaF2 for the total content of CaO, Si02 and CaF2, and is an important index to promote
crystallization of cuspidine. Setting f(2) in 0.05 to 0.40 makes it possible to keep the
composition of the melting mold flux within the range of the composition of a primary
crystal of cuspidine. In view of more stable crystallization of cuspidine, the upper
10 limit of f(2) is preferably 0.3, and more preferably 0.25. In the same view, the lower
limit of f(2) is preferably 0.1, and more preferably 0.15.
[0042] The index f(3) calculated using the formula (C) indicates a proportion of a
component that plays a role like a solvent for cuspidine. Setting f(3) in no more than
0.4 makes it possible to keep crystallization of cuspidine. The lower limit of f(3) is 0
15 according to the definition of the formula (C). In view of more stable crystallization of
cuspidine, the upper limit of f(3) is preferably 0.20, and more preferably 0.15. In the
same view, the lower limit of f(3) is preferably 0.05, and more preferably 0.10.
[0043] The f(l), f(2) and (3) of the mold flux of this invention from the initial
chemical composition satisfy the formulas (I), (2) and (3), respectively. Whereby,
20 even if the composition changes according to reaction with the molten steel, cuspidine
stabilizes in a crystal phase in the film, and the state where cuspidine is dominant over
perovskite can be kept.
[0044] The Ti02 content of the mold flux of this invention in the melting state in
the casting is no more than 20 mass%, and the strength ratio of the film of the mold flux
of this invention in the solidifying state after the casting is no more than 1 .O. No more
than 20 mass% of the Ti02 content of the melting mold flux makes it possible to
suppress composition changes in the melting mold flux. Thus, cuspidine stabilizes in a
crystal phase in the film, and the state where cuspidine is dominant over perovskite can
5 be kept. No more than 1.0 of the strength ratio of the film of the mold flux in the
solidifying state after the casting makes it possible not to block formation of cuspidine.
No more than 20 mass% of the Ti02 content of the melting mold flux and no more than
1.0 of the strength ratio of the film of the mold flux in the solidifying state after the
casting in addition to the satisfaction of the formulas (I), (2) and (3) makes it possible to
10 prevent longitudinal cracks from forming on a surface of a slab.
[0045] The solidification point of the mold flux is preferably 1150 to 1400°C. If
the solidification point is under 1 15O0C, crystallization of cuspidine might be poor. It
is technically difficult to make the solidification point over 1400°C. The solidification
point of 1150 to 1400°C improves the effect of mild cooling by film. Thus,
15 longitudinal cracks can be surely prevented from forming.
[0046] The viscosity of the mold flux is preferably no more than 2 poises (= 0.2
Pas) at 1300°C. If the viscosity is over 2 poises, the crystallization rate might be
down. If the viscosity is no more than 2 poises, the effect of mild cooling by film is
improved and longitudinal cracks can be surely prevented from forming. On the other
20 hand, concerning the lower limit of the viscosity, there arises no problem due to low
viscosity. However, it is difficult to make the viscosity of generally used mold flux
less than 0.1 poise (= 0.01 Paas). Thus, no less than 0.1 poise is preferable.
[0047] When the Ti content of hypo-peritectic steel is no less than 0.1 mass%, the
problem is outstanding that longitudinal cracks form on a surface of a slab through the
influence of oxidation reaction of Ti in the steel. In contrast, when the Ti content of
hypo-peritectic steel exceeds 1 mass%, composition changes in the melting mold flux in
the mold through the influence of oxidation reaction of Ti in the molten steel become
large. As a result, it becomes difficult to keep the composition of the melting mold
5 flux within the range of the composition of a primary crystal of cuspidine. Therefore,
the hypo-peritectic steel continuous-cast using the mold flux of this invention is
specified as hypo-peritectic steel containing 0.1 to 1 mass% of Ti.
[0048] In this invention, for example, at least one of Li20, Na20 and KzO can be
used as an alkali metal oxide. For example, fluorite that contains CaFz as a major
10 component, or NaF can be used as a fluorine compound.
[0049] In addition, A1203 may be contained in the mold flux of this invention in
order to adjust physical properties such as the solidification point and the viscosity.
A1203 has the fhnctions of dropping the solidification point and increasing the viscosity.
However, the A1203 content is preferably low in order to promote crystallization of
15 cuspidine. The A1203 content is preferably no more than 5 mass%. In contrast, when
general raw materials for mold flux are used, about 0.5 mass% or more of A1203 is
inevitably contained therein. While the A1203 content can be less than 0.5 mass% by
using artificial raw materials like a pre-melting base material, it might be accompanied
by a rise in raw material costs. Therefore, the AI2O3 content is preferably no less than
20 0.5 mass%.
[OOSO] The continuous casting method of this invention is directed to
hypo-peritectic steel containing 0.1 to 1 mass% of Ti. The method uses the above
described mold flux of this invention as mold flux. Whereby, the composition of a
crystal phase in the film formed in the mold is maintained during the casting. That is,
the state where cuspidine is dominant over perovskite in a crystal phase in the film can
be kept during the casting. Thus, the effects of lubricity and mild cooling in the mold
can be stable, and longitudinal cracks on a surface of a slab can be prevented.
[005 11 The continuous casting method of this invention has no specific limitation to
5 the casting conditions other than the mold flux. That is, the casting conditions can be
properly set as well as a conventional continuous casting method.
Examples
[0052] For confirming effects of the mold flux and continuous casting method of
10 this invention, continuous casting tests were carried out and results thereof were
evaluated.
[0053] In these tests, a slab was continuous-cast from 2.5 ton of molten steel while
the mold flux was supplied onto the molten steel in a mold. At this time, the
withdrawal rate was 1 .O mlmin, and the size of the slab was: 500 mm in width, 84 mm
15 in thickness and 7000 mm in length.
[0054] Table 1 shows grades (symbol), initial chemical composition (mass%),
basicities, solidification points ('C) and viscosities (poise) at 1300°C of the mold flux
used for the tests. Table 2 shows the chemical composition (mass%) of the molten
steel used for the tests.
20 [0055] [Tablel]
Mold Flux
Grade
(Symbol)
A l
A2
R1
Initial Chemical Composition (mass%) Basicity
(-1
1.7
1.4
1.2
SiOz
29.2
33.5
33.8
Solidification
Point ("C)
1245
1228
1210
CaO
49.7
46.8
39.5
Viscosity
(poise)
0.5
0.7
2.3
A1203
3.9
2.5
7.2
MgO
0.6
0.5
0.6
Na20
5.8
6.1
10.8
MnO
< 0.1
c0.1
c0.1
Ti02
<0.1
<0.1
c0.1
F
10.8
10.6
8.1
COO561 [Table 21
[0057] Test numbers 1 to 7 were set in the tests. The grade of the mold flux and
the chemical composition of the molten steel were changed in each test. Table 3 shows
grades of the mold flux, the Ti contents in the molten steel (mass%), values of f(l), f(2)
5 and f(3) calculated using the initial chemical composition (hereinafter may be referred
to as "initial composition"), and test categories used in the tests.
Chemical Composition of Molten Steel (unit: mass%) Remainder: Fe and Impurities
[0058]
[Table 31
C
0.09-0.1 1
[0059] In each test, the mold flux in the melting state was taken out of the mold
10 during the casting, and its components were analyzed. Table 4 shows the chemical
composition of the mold flux in the melting state, and values of f(l), f(2) and f(3)
S i
0.10-0.20
Test
Number
1
2
3
4
5
6
7
Mn
1.30-1.40
Ti Content in
Molten Steel
(mass%)
0.19
0.42
0.4 1
1.12
0.20
0.18
0.19
P
0.01 0-0.01 5
Test
Category
Ex. of This
Invention
Ex. of This
Invention
Ex. of This
lnvention
Comp. Ex.
Comp. Ex.
Comp. Ex.
Comp. Ex.
Mold Flux
S
0.002-0.005
Ti
0.1 - 1.2
Grade
A l
A 1
A 2
A 1
R 1
R2
R3
A1
0.01-0.04
f( 1 ) from
Initial
Composition
1.34
1.34
1.10
1.34
1.1 1
0.99
0.70
f(2) from
Initial
Composition
0.18
0.18
0.06
0.18
0.04
0
0
f(3) from
Initial
Composition
0.10
0.10
0.10
0.10
0.17
0.19
0.28
calculated using the composition in the melting state.
[0060] [Table 41
[0061] The film in a solidifying state was taken out of the mold when the. casting
was ended, and pulverizing was carried out on the film to obtain powder. The obtained
Test
Number
1
5 powder underwent X-ray diffraction analysis. From the results of the diffraction
analysis, the strength of cuspidine and the strength of perovskite were obtained, to
calculate the ratio (X2lX1) of the strength of perovskite (X2) to the strength of
cuspidine (XI). At this time, the strength of cuspidine was the first peak height,
specifically,.the strength of an angle (29.2"), which was twice as wide as a Bragg angle
10 when. Co was a source. The strength of perovskite was the first peak height,
specifically, the strength of an angle (33.2"), which was twice as wide as a Bragg angle
when Co was a source.
[0062] Longitudinal cracks on a surface of a slab was checked. In this check, a
surface of a cast slab was observed visually, and the length of an observed crack in a
15 longitudinal direction was measured. At this time, if a crack of no less than 10 mm
was detected, it was determined to fonn longitudinal cracks. In addition, temperature
of a copper plate of the mold was measured upon continuous casting, and its
1
Mold Flux
Grade
A1
f ( l ) in
Melting
State
1.5
f(2) in
Melting
State
0.18
Chemical Composition in Melting State (mass%) f(3) in
Melting
State -
0.1 1
Si02
24.1
CaO
47.1
AI203
5.4
MgO
0.5
NazO
5.6
MnO
1.3
Ti02
5.8
F
10.2
temperature change was observed. From them, continuous casting and longitudinal
cracks were evaluated for each test.
[0063] The symbols in the "Evaluation of Continuous Casting and Longitudinal
Cracks" column in Table 5 represent the following:
5 0: represents that the temperature of a copper plate of the mold was stable upon
continuous casting, the continuous casting was able to be completed, and no
longitudinal crack formed on a surface of the cast slab; that is, "excellent".
A : represents that while the temperature of a copper plate of the mold changed
upon continuous casting, the continuous casting was able to be completed, and
10 longitudinal crack formed on a surface of the cast slab; that is, "failure".
x: represents that the temperature of a copper plate of the mold considerably
changed upon continuous casting, and the continuous casting was stopped in the middle;
that is, "failure".
LO0641 Table 5 shows test numbers, grades of the mold flux, the Ti contents
15 (mass%) in the molten steel, the ratio of the strength of perovskite to the strength of
cuspidine (strength ratio) and evaluations of continuous casting and longitudinal cracks.
[0065] [Table 51
Test
Number
1
2
3
4
5
6
7
Grade of
Mold Flux
A1
A1
A2
A1
R1
R2
R3
Ti Content in
Molten Steel
(mass%)
0.19
0.42
0.4 1
1.12
0.20
0.18
0.19
Strength Ratio
0.6
0.8
0.6
1.5
2.1
pppp
1.6
1.2
Evaluation of
Continuous Casting and
Longitudinal Cracks
o
o
o
x
x
A
A
[0066] As seen from Tables 1 to 5, each MnO and Ti02 content of the mold flux of
all the test numbers 1 to 7 was less than 0.1 mass% in the initial composition. On the
other hand, in the melting state, each MnO and TiOz content increased. From these
results, it was confirmed that in the continuous casting of Ti-containing hypo-peritectic
5 steel, the composition of the mold flux in the melting state changed according to
oxidation reaction of Ti in the molten steel.
[0067] The index f(2) of the mold flux used in the test number 5, which was
calculated from the initial composition, did not satisfy the formula (2). The indexes
f(1) and f(2) of the mold flux used in the test numbers 6 to 7, which were calculated
10 from the initial chemical composition, did not satisfy the formulas (1) and (2),
respectively. As a result, in each test number 5 to 7, the strength ratio of the film took
a value larger than 1.0, that is, formation of cuspidine was blocked. Therefore, the
evaluation of continuous casting and longitudinal cracks was "failure".
[0068] In contrast, f(l), f(2) and f(3)of the mold flux used in each test number 1 to
15 3, which were calculated from the initial composition, satisfied the formulas (I), (2) and
(3), respectively. In addition, the TiOz content of the melting mold flux was no more
than 20 mass%, and the strength ratio of the film was less than 1 .O. As a result, a state
where cuspidine was dominant over perovskite was kept during the casting in each test
number 1 to 3. Therefore, the evaluation of continuous casting and longitudinal cracks
20 was "excellent".
[0069] The indexes f(l), f(2) and f(3) of the mold flux used in the test number 4,
which were calculated from the initial composition, satisfied the formulas (I), (2) and
(3), respectively. However, in the test number 4, the Ti content of the molten steel was
over 1.0 mass%, and thus the Ti02 content of the melting mold flux was over 20 mass%.
Thus, the composition change in the melting mold flux was large. As a result, the
strength ratio of the film took a value larger than 1 .O, that is, formation of cuspidine was
blocked. Therefore, the evaluation of continuous casting and longitudinal cracks was
"failure".
5 [0070] From these results, it was made clear that according to the mold flux and the
continuous casting method of this invention, the state where cuspidine was dominant
over perovskite was able to be kept in a crystal phase of the film, and longitudinal
cracks on a surface of a slab was able to be prevented.
[0071] While this invention has been described concerning the embodiments that
10 are considered to be the most practical and preferable at present, this invention is not
limited to the embodiments disclosed in this description, and can be properly modified
within the scope not contrary to the gist and ideas of this invention readable from the
claims and whole of the description, and it should be understood that mold flux for
continuous-casting Ti-containing hypo-peritectic steel and a continuous casting method
15 with such modification are also encompassed within the technical range of this
invention.
Industrial Applicability
[0072] According to the mold flux and the continuous casting method of this
20 invention, the effect of lubricity and mild cooling in the mold is stable, and longitudinal
cracks on a surface of a slab can be prevented from forming. Thus, they can be
effectively used in continuous casting of hypo-peritectic steel containing 0.1 to 1 mass%
of Ti.
Reference Signs List
[0073] 1 ... mold flux for continuous-casting Ti-containing hypo-peritectic steel
2 . . . submerged nozzle
3 . . . mold
5 4 . . . molten steel
5 . . . solidified shell
6 ... rolls
7 . . . cooling water
8 ... film

We claim:
[Claim 11 Mold flux for continuous-casting Ti-containing hypo-peritectic steel,
wherein in continuous casting of Ti-containing hypo-peritectic steel,
the mold flux contains CaO, Si02, an alkali metal oxide and a fluorine
5 compound as major components,
chemical composition of the mold flux before the mold flux is put into a mold
satisfies the formulas (I), (2) and (3),
a Ti02 content of the mold flux in a melting state during the casting is no more
than 20 mass%, and
10 a strength ratio of a film of the mold flux in a solidifying state after the casting
is no more than 1 .O:
1.1 -0.5 xT 5 f(1) 5 1.9-0.5 x T ... (1)
0.05 i f(2) 5 0.40 . . . (2)
0 (_ f(3) I 0.40 ... (3),
15 wherein in the formulas (I) to (3),
f(1) = (CaO)h/(siOz)h . . . (A)
f(2) = (CaF2)h/{(Cao)l1 + (SiOz)h + (CaF2)h) . . . (B)
f(3) = {(alkali metal fl~oride)~)/{(CaO+) ~(S i02)h+ (alkali metal flu0ride)h))
-. . (C),
2 0 wherein in the formulas (A) to (C),
(CaO)h = Wcao - (CaF2)h x 0.7 I 8 . . . (D)
(SiO2)h = Wsio2 . . . (E)
(CaF2)h = (WF - WLiSOX I .27 - WNa20X 0.6 13 - WKZOX 0.403) x 2.05 .. . (F)
(a1 kali metal fl~oride=)~ W LiZOx 1.74 + WNao x 1.35 + WKZox 1.23 .. . (G)
wherein -1' is a Ti content of molten steel (mass%), Wcao is a CaO content of
the mold flux (mass%), Wsioz is a Si02 content of the mold flux (mass%), WF is a F
content of the mold flux (mass%), and WLi~o,W N*~an d WKZoa re contents of Li20,
Na20 and K20 respectively, which are alkali metal oxides, of the mold flux (mass%),
and
wherein the strength ratio of the film means a ratio of a first peak height of
perovskite to a first peak height of cuspidine, the ratio being obtained from X-ray
diffraction analysis of powder obtained by pulverizing the film of the mold flux.
[Claim 21 A method for continuous-casting Ti- containing hypo-peritectic steel,
the method comprising: continuous-casting hypo-peritectic steel containing 0.1 to 1
mass% of Ti, using the mold flux according to claim 1.