Note: Descriptions are shown in the official language in which they were submitted.
CA 02539701 2006-03-20
SPECIFICATION
Powdered Molding Powder for Continuous Casting of Steel
Technical Field
0001 The present invention relates to a powdered molding powder for
continuous casting of steel that is provided onto a molten steel surface in a
mold during continuous casting of steel.
Related Art
0002 In the continuous casting of steel, a molding powder is added onto a
molten steel surface in the mold, is melted and slagged by heat derived
from the molten steel to form a molten slag layer, and progressively flows
into a gap between the mold and a cast slab, to be consumed. Examples of
primary functions of the molding powder during this time include: (1)
lubrication between the mold and the cast slab; (2) absorption of inclusions
which float from the molten steel; (3) prevention of reoxidation of the
molten steel, and heat insulation thereof; and (4) control of the rate of heat
transfer from the solidifying shell to the mold.
0003 A chemical composition of the molding powder usually contains
Si02 and Ca0 as a main (base) component, and is further constituted from
elements such as A1203, MgO, BaO, SrO, Li20, Na20, F, MnO, B203,
carbonaceous raw material and the like. Raw materials comprise a base raw
material, silica raw material, flux raw material, carbonaceous raw material
and the like. For example, portland cement, synthesized calcium silicate,
wollastnite, blast furnace slag, yellow phosphorous slag, dicalcium silicate
(2CaO.Si02) and the like may be used as the main raw material. Also, the
silica raw material is added in order to adjust the powder properties such as
basicity (CaOlSi02 mass ratio), bulk density and the like, and for example,
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glass, silica rock, diatomaceous earth and the like may be illustrated. The
flux raw material functions as an adjusting agent for melting properties
such as softening point, viscosity and the like and for example, fluorides
such as fluorite, cryolite, borax, sodium fluoride, magnesium fluoride, etc.,
carbonates such as sodium carbonate, lithium carbonate, manganese
carbonate, barium carbonate, etc., and the like may be employed. Further,
the carbonaceous raw material functions as a melting speed adjusting agent
and exothermic agent, and carbon black, coke, graphite, acid-treated
graphite and the like may be used.
0004 Also, the shape of the molding powder for continuous casting of
steel is classified to powdered or granular form. An advantage of the
granular molding powder is the low dusting amount. On the other hand, in
order to improve operating stability of the continuous casting and quality of
the obtained steel, a powdered molding powder is suitable, since the
powdered molding powder has superior heat retaining properties for molten
steel, and melting properties.
0005 Conventionally, in order to prevent sintering and to adjust the
melting speed of the powdered molding powder, not less than 1 percent by
mass (weight) of carbon black is added to the molding powder. However, as
described hereinafter, the addition of carbon black is not preferable with
regard to the operating conditions and work environment. Therefore, a
molding powder wherein the amount of carbon black added is remarkably
lowered, has been developed. For example, Patent document 1 discloses a
colored powder for continuous casting characterized in that not more than
0.7 percent by mass (weight) of carbon black and not less than 0.5 percent
by mass (weight) of a pigment other than carbon black are compounded to a
base raw material, and hollow and spherical form powder particles are
possible (claim 1 ); and a colored powder for COI1t111uo11S CaStlIlg
characterized in that not more than 0.7 percent by mass (weight) of carbon
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black and not less than 0.5 percent by mass (weight) of a pigment other than
carbon black are compounded to a base raw material and the sum of sodium
fluoride (NaF), cryolite (3NaF-A1F3) and soda ash (Na2C03) in the base raw
material is not more than 5 percent by mass (claim 2).
0006 Further, Patent document 2 discloses a powdered flux for casting
iron metal characterized in that said flux comprises ( 1 ) a sintered or
molten
composition containing a fire resistant metal oxide and a solvent; and (2) a
cellulosic material.
0007 Further, Patent document 3 discloses a molding powder for
continuous casting of steel characterized in that from 0.5 to 5 percent by
weight (percent by mass) of carbonaceous powder of under 100 mesh and
from 0.1 to 4 percent by weight (percent by mass) of organic fibrous
material of under 100 mesh as a melting speed adjusting agent are
compounded to a base material for the molding powder that is constituted
mainly by metal oxides and contains one or more components selected from
a group consisting of alkali metal or alkali-earth metal fluorides, alkali
metal oxides or carbonates.
0008 Also, Non-Patent document 1 reports a molding powder wherein
cellulose is added to the powder thereby speeding melting speed in order to
make the thickness of a molten layer in a mold uniform.
0009 Patent document 1: Japanese Patent Laid-Open No. 8-25007;
Claims
Patent document 2: Japanese Patent Laid-Open No. 56-89372;
Claims
Patent document 3: Japanese Patent Laid-Open No. 59-27759;
Claims
Non-Patent document 1: Nippon Steel Technical Report, Vol.
377 (2002), pp55-58 [Development of
Technology to Improve Surface Layer
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Quality of Continuous Cast 304
Austenitic Stainless Steel Ingots]
Disclosure of the Invention
Problems to be Solved by the Invention
0010 In conventional powdered molding powders, in order to control
melting speeds and to maintain melting properties, not less than 1 percent
by mass (weight) of carbon black is usually added. The reasons why carbon
black has remarkable effects in preventing sintering and in adjusting
melting speed are as follows. Surfaces of all raw material particles that
constitute the powdered molding powder are completely covered by carbon
black, since carbon black has strongly adhesive properties and dispersibility.
Also, since carbon black has wetting resistance to slag, from the time the
powdered molding powder is heated until it reaches a softening and melting
temperature, fusion between raw material particles can be inhibited and the
sintering and melting thereof can be delayed by carbon black which
prevents direct contact between the raw material particles.
0011 Where there is excess carbon black in present the powdered molding
powder, when the molten metal has a smooth surface, the molding powder
melts upward smoothly progressively from the bottom, but when the molten
metal has a disturbed surface such as when there is a lot of gas floating up
from the molten metal, the unmelted molding powder turns red from heat
and flows. Such a phenomenon is referred to as red heat. The red heat
phenomenon takes place because carbon black excessively covers surfaces
of raw material particles of the molding powder, and thus, fusion between
the raw material particles does not occur at all even when the raw material
particles are heated to a softening and melting temperature, and the raw
material particles are liable to flow. When the red heat phe110Il1eIlo11 takes
place, flowing molding powder serves as a heating medmm to enhance heat
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dissipation from the molten metal surface and to cool the molten metal
surface in which heat insulation is essentially important, thereby
developing problems of deteriorated quality of the cast slab and causing
breakout in some cases. Further, when the red heat phenomenon takes place,
the molding powder is also cooled to delay the melting and it becomes
difficult to maintain an appropriate thickness of the molten layer. Then,
supply of slag flowing between the mold and the solidifying shell stops, so
that there is also the problem of breakout occurring easily. Thus, an excess
amount of carbon black drastically changes melting properties due to the
active state of the molten metal surface, resulting in disadvantages such as
the occurrence of product defects and deterioration of operating stability.
0012 On the other hand, in the presence of a small amount of carbon
black in the molding powder, the raw material particles are in direct contact
with each other and are fused from a relatively low temperature equal to or
lower than the melting point, thereby developing problems of causing
sintering easily and too rapid melting. The molding powder is sintered to
form a lump of a sintered product referred to as a slag bear on a periphery
of the mold, to inhibit flow of a powder slag between the mold and the
solidifying shell, and to cause failure in lubrication of the solidifying
shell,
thereby causing breakout in some cases. Too rapid melting of the molding
powder disturbs a balance in the flow rate of the molten powder slag
between the mold and the solidifying shell, thereby causing a problem of
forming too thick a molten slag layer. Too thick a molten slag layer
adversely affects the operation stability and product quality to a great
extent due to deterioration of melting properties, deterioration of heat
insulation properties, increase in fluctuation of a molding powder
composition, and the like.
0013 As described above, the amount of carbon black added remarkably
depends on the melting properties of the powdered molding powder, and at
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the present time, it is difficult to improve adaptability to changes in
operating conditions in a conventional powdered molding powder.
0014 Further, the melting properties of the powdered molding powder
vary remarkably according to the mixing conditions at the time of
production even if the amount of carbon black added is the same. That is,
since the carbon black does not be uniformly coat to the surface of raw
material particles, if the mixing conditions are bad, nonuniform melting can
be generated and the melting speed may be faster than the expected speed.
On the other hand, if the mixing conditions are too good, it becomes easy
for a red heat phenomenon to occur. As described above, in the powdered
molding powder with added carbon black, there is a problem in that setting
production conditions becomes very difficult.
0015 Nevertheless, usually, more than 1.0 percent by mass (weight) of
carbon black is added to most all conventional powdered molding powders
in order to control the melting speed and maintain the melting properties,
accordingly, the color of the powdered molding powder is naturally black.
The soiling from dusting by black color powdered molding powder is
readily apparent, and when the powder adheres to equipment, clothes, skin
and the like, there is a problem in that the soil cannot be easily removed.
Also, since most all powdered molding powders are black, there is no
difference of color for each product (quality), accordingly, distinction of
products (qualities) cannot be carried out by appearance alone. Therefore, if
an error of addition of powdered molding pov~~der is caused by an operator,
the operator cannot recognize the error and operational trouble may occur.
However, it is felt that such molding powder adding errors may become
more difficult to occur, if the color for each product (quality) differs with
each other.
0016 Next, the colored powder for continuous casting described in Patent
document 1 prevents degradation of the melting property by being limited
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form to hollow granules, and limiting the total amount of sodium fluoride,
cryolite and sodium carbonate that are low melting point raw materials, if
the molding powder is in powdered form. However, in Patent document 1,
since carbon black is only compounded at an amount of not more than 0.7
percent by mass (weight), so it is difficult to control slagging and melting
speed just by limiting the total amount of sodium fluoride, cryolite and
sodium carbonate, and further the degradation of melting properties is
difficult to prevent, so said colored powder is imperfect.
0017 Further, Patent document 2 discloses the addition of a cellulosic
material to powdered flux (mold powder) for casting of iron metal in order
to inhibit the formation of clinker. However, since no elementary carbon is
contained, combustion of the cellulose material occurs, and oxidation of the
carbide that is formed by the combustion is too fast. There is therefore a
problem that the generation of sintering material becomes facilitated.
0018 Also, even though Patent document 3 discloses a molding powder to
which is added carbonaceous powder and organic fibrous material, an
object of the combination of carbonaceous powder with organic fibrous
material is to maintain a melted slag layer, thereby preventing carburizing.
Further, there is no concrete description in Patent document 3 concerning
the compounding amount of carbon black and in the working examples, 2.0
percent by mass (weight) of carbon black was added to the powdered
molding powder. However, red phenomena can easily occur in this molding
powder and the color of the mold powder is black. Further, without carbon
black, this is only granular molding powder.
0019 Also, in the molding powder used in Non-Patent document 1, even if
cellulose is employed, it is impossible for the melting speed to be
accelerated.
0020 Therefore, an object of the present invention is to provide a
powdered molding powder for continuous casting of steel which has
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melting properties allowing: maintenance of an appropriate melting rate
without formation of a slag bear or a sintered product; prevention of a red
heat phenomenon even when a molten steel surface is active; prevention of
contamination due to dust formation by having a color tone besides black;
and easy distinction of products by being colored any color except black.
Means for Solving the Problem
0021 Accordingly, the present invention provides a powdered molding
powder for continuous casting of steel characterized in that the powdered
molding powder contains not more than 0.5 percent by mass (weight)
(including zero) of carbon black, from 0.5 to 20 percent by mass (weight) of
carbon powders other than carbon black and from 0.1 to 7.0 percent by mass
(weight) of carbohydrate powder as a carbonaceous raw material.
0022 Also, the powdered molding powder for continuous casting of steel
according to the present invention is characterized in that the powder
contains from 0.1 to 8.0 percent of mass (weight) of acid-treated graphite as
a part or all of the carbon powders other than carbon black.
0023 Further, the powdered molding powder for continuous casting of
steel according to the present invention is characterized in that the powder
contains from 0.3 to 7.0 percent by mass of pigment.
Effect of the Invention
0024 The powdered molding powder for continuous casting of steel
according to the present invention exerts the effects of: contamination due
to dust formation being inconspicuous to improve the operating
environment since the molding powder has a color tone other than black;
and smooth melting without occurrence of a red heat phenomenon even
when the molten steel surface is active in terms of melting.
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Best Mode for Carrying Out the Invention
0025 When the amount of carbon black added is reduced to eliminate
problems due to excess carbon black in the molding powder for continuous
casting of steel, a substance for preventing fusion between molding powder
particles may be added after the molding powder is exposed to heat. The
present inventors have conducted various studies and have found that
carbohydrates can be used as a raw material alternative to carbon black.
Carbohydrate powder that is dispersed in per se the molding powder for
continuous casting disperses among the raw material particles of the
molding powder to prevent fusion therebetween, and is carbonized by heat
in use to coat the surfaces of the raw material particles of the molding
powder, thereby preventing fusion between the raw material particles of the
molding powder.
0026 However, if in the absence of carbon powder, carbohydrate powder
is used alone, it is easy for the carbohydrate powder to combust completely,
making it difficult for carbide to form, so that the fusion between raw
material particles of the molding powder together cannot be prevented and
the melting speed of the powdered molding powder cannot be delayed.
Hence, by adding carbohydrate powder and carbon powder simultaneously,
carbohydrate powder is subjected to incomplete combustion to form carbide
and coat the raw material particles of the mold powder, so that sintering can
be prevented and the melting speed can be controlled. Further, even among
carbon powders, the addition of acid-treated graphite is preferable, since
when this graphite is heated, the graphite itself expands to prevent the
fusion of raw material particles to each other in the mold powder, thereby
increasing the sintering preventive effects.
0027 In the powdered molding powder for continuous casting of steel
according to the present invention, carbohydrate powder that is added as
raw material alternative to carbon black is not limited in particular as long
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as it contains carbohydrate as a main component, and powders that are
obtained by grinding cereals or root crops such as rice, wheat, soybean,
corn, corm and the like; manufactured goods such as starch powder,
cellulose and the like that are produced from cereals or root crops; waste
paper powder and wood chip grinding powder may be used. Among
carbohydrate powders, it is preferable that wheat powder, dogtooth violet
starch powder, starch powder and carboxymethylcellulose (CMC) are easily
available and have good particle size.
0028 The compounding amount of the carbohydrate powder ranges from
0.1 to 7.0 percent by mass (weight), preferably from 0.3 to 4.0 percent by
mass (weight). The carbohydrate powder functions to disperse between the
raw material particles of the molding powder, thereby preventing fusion of
said particles as well as to carbonize when the carbohydrate powder is
heated during the operation, to coat the particle surface of the raw material
of the molding powder, thereby preventing fusion between the raw material
particles of the molding powder. Accordingly, if the compounding amount
of the carbohydrate powder is less than 0.1 percent by mass (weight), the
melting speed of the molding powder for continuous casting of steel cannot
be controlled and sintering can not be prevented, which is not preferable.
Further, if this amount is more than 7 percent by mass (weight), although
the melting property is good, it is not preferable as the melting is too late.
Smaller particle size carbohydrate powder is preferable as it tends to easily
disperse, so that the coating of the carbohydrate powder to the raw material
particles for the molding powder is remarkably high and the fusion together
of the raw material particles for the molding powder can be prevented. That
is, the particle size of the carbohydrate powder ranges preferably from
under 100 mesh, more preferably under 150 mesh.
0029 In the powdered molding powder for continuous casting of steel, the
amount of carbon black added is preferably not more than 0.5 percent by
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mass (weight), more preferably not more than 0.3 percent by mass (weight),
much more preferably 0.1 percent by mass (weight) with no carbon black
being most preferable. If a large amount of carbon black is added, it is easy
for the red heat phenomenon to occur when the molten steel surface in the
mold is active, so it is not preferable. Also, if carbon black and
carbohydrate powder are added simultaneously, irregular melting speeds in
the molding powder are generated locally, accordingly, deterioration of the
melting properties can be recognized. Since the dispersibility of
carbohydrate powder is degraded by carbon black, it is preferably that
smaller amounts of carbon black are added. Further, when the amount of
carbon black added is great, the color of the molding powder becomes black,
which is not preferable. It is not preferable that the amount of carbon black
added be more than 0.5 percent by mass (weight), because it causes red heat
phenomenon, the melting properties degrade and the color tone becomes
black.
0030 To the powdered molding powder for continuous casting of steel
according to the present invention, carbon powders other than carbon black
are added. For example, one or more of coke, graphite, acid-treated graphite
and the like can be used as the carbon powders other than carbon black. The
total amount of carbon powders other than carbon black added ranges from
0.5 to 20 percent by mass (weight), preferably from 1.0 to 10 percent by
mass (weight). If the total amount of carbon powders other than carbon
black added is less than 0.5 percent by mass (weight), since combustion
speed of the carbohydrate is too fast, it is not preferable as the melting
speed of the molding powder is too fast and sintering tends to be caused
easily. Also, if the amount of carbon powders other than carbon black added
is more than 20 percent by mass (weight), it is not preferable as the melting
speed of the powdered molding powder is too slow. Further, it is not
preferable as the color tone of the obtained powdered molding powder is
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black.
0031 In the powdered molding powder for continuous casting of steel
according to the present invention, the acid-treated graphite can be
preferably used as a part or all of the carbon powders other than carbon
black added. The amount of acid-treated graphite added ranges preferably
from 0.1 to 8.0 percent by mass (weight), more preferably from 0.3 to 3.0
percent by mass (weight). If the amount of acid-treated graphite is less than
0.1 percent by mass (weight), the sintering inhibiting effect of the
powdered molding powder is small and no adding effect is found. Also, if
the amount of acid-treated graphite is more than 8.0 percent by mass
(weight), it is not preferable as the dusting from expansion is remarkably
increased. Further, in order to obtain a certain amount of expansion, an
exorbitantly small particle size of acid-treated graphite is not preferable,
so
acid-treated graphite having from 325 to 10 mesh particle size range for
example, may be preferable. Also, since coke or graphite with a smaller
particle size has greater adding effects, those having under 100 mesh, for
example, preferably under 150 mesh of particle size may be preferable.
0032 Bulk density of the powdered molding powder for continuous
casting of steel according to the present invention is less than 1.0,
preferably less than 0.9. If the bulk density of the powdered molding
powder is greater than 1.0, it is not preferable as good melting properties
cannot be maintained, since raw material particles easily contact each other.
0033 In order to obtain a desired color tone, a pigment may suitably be
added to the powdered molding powder for continuous casting of steel
according to the present invention. A compounding amount of the pigment
is not less than 0.3 percent by mass (weight). If the amount is less than 0.3
percent by mass (weight), it is not preferable as there is no effect fr0111
adding the pigment. Also, the upper limit of said amount differs depending
on the kind of pigment used, but it is approximately 7 percent by mass
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(weight). If this amount is greater than 7 percent by mass (weight), it is not
preferable as other components that constitute the powdered molding
powder for continuous casting of steel may be affected. By suitably
selecting the pigment, the powdered molding powder for continuous casting
of steel can be pigmented to a desired color tone such as red, blue, yellow,
green, purple and the like. Also, an organic or inorganic pigment may be
used, but an organic pigment having a strong tinting strength in small
amounts is more preferable. Further, for example, red iron oxide, umber,
yellow ocher and the like may be illustrated as inorganic pigments, and
alizarin lake, phthalocyanine green, phthalocyanine blue, greenish yellow
and the like may be illustrated as organic pigments.
0034 Further, raw materials other than those described above that
constitute the powdered molding powder for continuous casting of steel
according to the present invention are not particularly limited, and can be
constituted from conventional main raw materials, silica raw materials, flux
raw materials and the like. For example, portland cement, synthesized
calcium silicate, wollastnite, blast furnace slag, yellow phosphorous slag,
dicalcium silicate (2CaO.Si02) and the like may be illustrated as the main
raw material. Also, the silica raw material is added in order to adjust the
powder properties such as basicity (Ca0/Si02 mass ratio), bulk density and
the like. The flux raw material functions as an adjusting agent for the
melting properties such as softening point, viscosity and the like and for
example, fluorides such as fluorite, cryolite, borax, magnesium fluoride,
etc., carbonates such as sodium carbonate, lithium carbonate, manganese
carbonate, barium carbonate, etc., and the like can be employed. Further,
the amount of these raw materials added is not particularly limited, and
ranges from 98 to 40 percent by mass (weight), preferably from 80 to 50
percent by mass (weight) of the main raw material, from 0 to 40 percent by
mass (weight), preferably 3 to 20 percent by mass (weight) of silica raw
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material and from 0 to 40 percent by mass (weight), preferably from 3 to 30
percent by mass (weight) of the flux raw material, for example.
0035 Also, the basicity [Ca0/Si02 mass (weight) ratio] of the powdered
molding powder for continuous casting of steel according to the present
invention is not particularly limited, the present invention can be adapted to
a powdered molding powder having from 0.3 to 2.5 basicity.
Examples
0036 Hereinafter, the molding powder for continuous casting of steel
according to the present invention will be further described based on
examples. Powdered molding powder for continuous casting as inventive
products and as comparative products shown in Table 1 were prepared by
using as a base a molding powder for continuous casting having properties
of a Ca0/Si02 mass ratio of 0.88 and molten slag viscosity of 4 poise at
1300°C. A base raw material other than carbohydrate powder, carbon and
pigments included: 69 percent by mass (weight) of synthetic calcium
silicate as a main raw material; 9 percent by mass (weight) of a silica raw
material; and 22 percent by mass (weight) in total of sodium fluoride,
calcium fluoride, sodium carbonate, lithium carbonate, alumina and
magnesia as flux raw material. Bulk specific gravity of the obtained
molding powder fell within a range of 0.71 to 0.73.
An actual casting test was carried out using the obtained powdered
molding powder. Casting conditions were: low carbon steel; a mold size of
210mm x 1,SOOmm; and a casting speed of 1.4m/minute. Table 1 shows the
results of the casting test.
Further, the melting properties were evaluated comprehensively
from visual observation and bear formation. The melting speed was
determined by measuring the thickness of the molten layer to evaluate
whether a stable constant value was secured. The operating environment
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CA 02539701 2006-03-20
was evaluated from a level of contamination due to adhered molding
powder. The product quality was evaluated from cracks in the obtained cast
slab and inclusions.
Table 1 shows the obtained results.
. ' CA 02539701 2006-03-20
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0038 Also, in Table 1, carbohydrate, coke and graphite powders having
under 150 mesh of particle size were used. Further, acid-treated graphite
having under 65 mesh of particle size was used.
17
