DESCRIPTION
IMMERSION NOZZLE AND CONTINUOUS CASTING METHOD
Technical Field
The present invention relates to an immersion nozzle used in a continuous casting method of molten metal and to a continuous casting method including a preheating step of preheating this immersion nozzle.
Background Art
In the past, a continuous casting method continuously cooling and solidifying molten metal to form a predetermined shape of a cast slab has been known. In this continuous casting method, a casting step is performed feeding molten metal through an immersion nozzle from a tundish into a mold (water-cooled casting mold).
The immersion nozzle is attached to the bottom of the tundish and is designed to eject molten metal inside the tundish from a discharge port at a bottom end of the nozzle bottom end into the mold. This immersion nozzle is used in a state with its bottom end side immersed inside the molten metal in the mold. Due to this, splattering of the fed molten metal is prevented and contact of the fed molten metal with the atmosphere is prevented so as to suppress oxidation. Further, the immersion nozzle enables feed in a smoothed flow state, so prevents the slag, non-metal inclusions, and other impurities floating in the molten metal from being entrained in the molten metal. As a result, the cast slab quality can be improved and the operational stability can be secured.
Such an immersion nozzle is generally formed from Al2O3-SiO2-C (carbon) refractories or Al2O3-C refractories. Immersion nozzles made from these Al2O3-C-containing refractories are currently being most widely used in continuous casting of molten metal since Al2O3 is
excellent in refractory properties and resistance to corrosion by the molten metal and C is resistant to wetting of the inclusions (slag ingredients), low in expansion, and excellent in heat conductivity.
Usually, in an immersion nozzle, since the inside and outside layers are exposed to different environments, the inside and outside layers are made using separate types of refractories. Further, for protecting the powder line part from the slag floating on the surface of the molten metal in the mold, the powder line part is made using another type of refractories.
However, an immersion nozzle made from Al2O3-C-containing refractories has the property of susceptibility to deposition of precipitates at the nozzle inside circumference through which the molten metal flows. The precipitates particularly deposit in large amounts at the part of the nozzle inside circumference with a large temperature gradient at the nonimmersed part and the part near the discharge holes where the flow rate of the molten metal falls. The' deposits make the casting work difficult in some cases. Further, the work of removing deposits becomes necessary during casting. The deposits removed here are entrained in the cast slab to form large inclusions which become causes of deterioration of the cast slab quality. The deposited precipitates are mainly comprised of aAl2O3. It is believed that the Al2O3 contained in the molten metal as a product of the deoxidation process precipitates and deposits at the nozzle inside circumference. The deposition of precipitates at the nozzle inside circumference is particularly remarkably observed in continuous casting of aluminum killed steel.
To deal with this problem, in the past, a nozzle with an inside circumference formed by CaO-MgO-graphite-containing refractories with CaO of 20 mass% or more, with graphite of 30 mass% or less, with a maximum grain size of the component grains of 0.5 mm or less has been
disclosed, for example, in Japanese Patent Publication (A) No. 2005-60128. However, with such refractories containing CaO-MgO-graphite, when the Al2O3 contained in the molten metal deposits at the nozzle inside circumference, the CaO in the refractories and the deposited Al2O3 react to form low melting point substances. Due to this, the Al2O3 will not deposit at the nozzle inside circumference, but will be successively washed away by the molten metal whereby deposition of precipitates at the nozzle inside circumference can be prevented.
Further, to deal with the above problem, a nozzle with an inside circumference formed by spinel-periclase-graphite-based refractories containing spinel (MgO-Al2O3): 50 to 95 mass%, periclase (MgO): 0 to 20 mass%, graphite: 5 to 30 mass%, and unavoidable impurities: 3 mass% or less is disclosed in, for example, Japanese Patent Publication (A) No. 11-320047, but with such spinel-periclase-graphite-based refractories, the molten metal flows and the refractories are exposed to a high temperature environment, whereby the Mg ingredient in the refractories reacts with the 0 ingredient or CO ingredient and a dense MgO layer is formed at the surface of the refractories. This MgO layer has an extremely dense structure with a porosity extremely close to zero, so the Al2O3 inclusions in the molten metal will seldom deposit on the MgO layer. Due to this, deposition of precipitates at the nozzle inside circumference can be prevented.
In this regard, in the above casting step, when the temperature of the immersion nozzle is low, when starting to feed the molten metal, sometimes the immersion nozzle will crack or clog. For this reason, it has been considered to preheat the immersion nozzle so as to reduce the temperature difference occurring at the immersion nozzle at the time of starting feed of the molten metal and thereby prevent the above trouble from
occurring.
As such a preheating method, for example, as shown in FIG. 4, it may be considered to use a burner 100 to spray combustion gas.
Further, the method of surrounding the outer circumference of a immersion nozzle by an electrical heater and heating it by heat conduction or radiant heat has been proposed (for example, see Japanese Patent Publication (A) No. 10-118746).
However, the refractories containing CaO-MgO-graphite described in the above Japanese Patent Publication (A) No. 2005-60128 and the spinel-periclase-graphite-based refractories described in Japanese Patent Publication (A) No. 11-320047 are all high heat expanding materials. For this reason, when preheating an immersion nozzle using these refractories, then performing the casting step, there are the following problems (A) and (B) .
(A) When using a burner 100 as shown in FIG. 4 for
preheating, the burner 100 is inserted from the top end
of the nozzle and combustion gas is blown inside it and
discharged from discharge holes at the bottom end side.
For this reason, it is difficult to uniformly heat the
nozzle as a whole. Cracking due to the thermal stress
caused by this temperature difference and heat expansion
difference between materials ends up occurring.
Further, when using a burner for preheating, the preheating requires a long time and the oxidizing atmosphere arising due to the combustion gas causes the C ingredient in the refractories to oxidize and form CO gas or C02 gas and thereby end up being consumed. For this reason, the refractories are formed with large pores into which mold powder easily enters in the casting step thereby having the problem of easier progression of wear.
(B) When using the electric heater described in Japanese
Patent Publication (A) No. 11-320047 for preheating, the
consumption of the C ingredient can be prevented, but since heat conduction and radiant heat are used to heat the nozzle, parts will reach 1400°C, but uniformly heating the entirety is again difficult.
Disclosure of Invention
The object of the present invention is to provide an immersion nozzle able to be improved in durability and a continuous casting method including a preheating step of preheating this immersion nozzle.
The present invention was made based on the discovery that to uniformly heat an immersion nozzle, use of high frequency induction heating is good. The gist of the present invention is as follows: (1) The immersion nozzle according to the present invention is an immersion nozzle used in a continuous casting method of molten metal, characterized in that it is formed by refractories comprised of either magnesia, spinel, dolomite clinker, a mixture of magnesia and spinel, or a mixture of magnesia and dolomite clinker and of free carbon and in that high frequency induction heating is used for preheating.
The immersion nozzle of the present invention is preferably a structure forming only the nozzle inside circumference by the above refractories, a structure forming all of the nozzle by the above refractories, or another structure forming at least the nozzle inside circumference by the above refractories. Further, the temperature at the time of ending of the preheating is at the least 1100°C or more.
For the magnesia, spinel, and dolomite clinker, usually rock like forms which can be added are used as materials.
The "magnesia" means a material containing, as main ingredients, MgO in 90 mass% or more and unavoidable impurities in 10 mass% or less, more preferably a material containing MgO in 95 mass% or more and
unavoidable impurities in 5 mass% or less.
The "spinel" means a material containing Al2O3•MgO in 90 mass% or more and unavoidable impurities in 10 mass% or less, more preferably Al2O3•MgO in 95 mass% or more and unavoidable impurities in 5 mass% or less.
The "dolomite clinker" means a material obtained by firing natural dolomite at a high temperature to obtain a sintered body of MgO and CaO as main ingredients.
The "free carbon", for example, usually includes graphite flakes, electrode dust, anthracite, amorphous graphite, and other added graphite and also carbon remaining when the binder is fired.
Such an immersion nozzle is, for example, formed by mixing powders of various types of inorganic matter, graphite flakes, and a binder such as phenol resin, using the CIP method etc. to form the mixture into a predetermined shape, and firing this in a reducing atmosphere. Note that a combination of spinel and dolomite clinker is unsuitable since the Al2O3 in the spinel and the CaO in the dolomite clinker form low melting point substances.
According to this invention, due to the presence of the free carbon in the refractories, it is possible to selectively heat the above free carbon by high frequency induction heating and thereby uniformly preheat the immersion nozzle compared with the case of preheating an immersion nozzle by the conventional heating methods such as shown in FIG. 4 or the above Japanese Patent Publication (A) No. 10-118746. For this reason, when starting to feed the molten metal in the casting step, it is possible to ease the thermal shock received by the immersion nozzle from the molten metal and prevent the occurrence of cracking and other trouble.
Further, in high frequency induction heating, combustion gas is not used like in the past and the preheating can be completed in a short time, so the consumption of free carbon in the refractories will
become lower and the speed of wear of the nozzle inside circumference due to the molten metal can be reduced.
Further, magnesia, spinel, and dolomite clinker all contain MgO ingredients, so in the casting step, it is possible to form a dense MgO layer at the nozzle inside circumference and prevent deposition of Al2O3 inclusions at the nozzle inside circumference.
Further, when using dolomite clinker to form the nozzle, the CaO in the dolomite clinker and the Al2O3 contained in the molten metal form low melting point substances, so it is possible to prevent Al2O3 inclusions from depositing at the nozzle inside circumference. Furthermore, in this case, after the above low melting point compounds are formed in the refractories, the MgO grains remaining in the refractories merge and coarse to form reaction products with relatively high melting points. Due to this, wear of the nozzle inside circumference can be prevented.
Further, when using a mixture of magnesia and spinel or of magnesia and dolomite clinker, by adjusting the percentage of magnesia, it is possible to freely set the wear speed. Therefore, it is possible to improve the durability of the immersion nozzle. (2) The immersion nozzle according to the present invention is an immersion nozzle as set forth in the above (1) wherein the nozzle is comprised of a two-layer structure of an inside layer through which the molten steel flows and forming the nozzle inside circumference and an outside layer formed laminated in a state covering the outer side of this inside layer, the inside layer is formed by refractories comprised of any one of magnesia, spinel, dolomite clinker, a mixture of magnesia and spinel, and a mixture of magnesia and dolomite clinker and of free carbon, and the outside layer is formed by refractories different in either composition or formulation from the inside layer and comprised of one to three of alumina, mullite, silica, zirconia, Ca0-Zr02
clinker, spinel, magnesia, zirconia mullite, and silicon carbide and of free carbon or refractories comprised of one type or two types of dolomite clinker, zirconia, CaO-ZrO2 clinker, and magnesia and of free carbon.
These materials are all materials obtained naturally or synthesized. The alumina includes Al2O3 as its main ingredient. The mullite includes 3Al2O3-2SiO2 as its main ingredient. The silica includes SiO2 as its main ingredient. The zirconia includes ZrO2 as its main ingredient.
The CaO-ZrO2 clinker includes a sintered body of CaO and ZrO2 as its main ingredient.
The zirconia mullite includes ZrO2: 32 to 42 mass%, Al2O3: 40 to 50 mass%, and SiO2: 13 to 23 mass% as its main ingredients. The silicon carbide includes SiC as its main ingredient. Note that, the magnesia, spinel, and dolomite clinker are as explained above.
These materials all contain their main ingredients in 90 mass% or more and unavoidable impurities in 10 mass% or less, more preferably contain their main ingredients in 95 mass% or more and unavoidable impurities in 5 mass% or less.
According to this invention, free carbon is present in each of the refractories forming the inside layer and outside layer, so high frequency induction heating can be used to selectively heat the free carbon and to uniformly preheat the immersion nozzle. For this reason, at the start of feeding the molten metal in the casting step, cracking and other trouble can be prevented from occurring.
Further, the outside layer uses refractories of a composition different from the inside layer or refractories of the same materials used in the inside layer, but of different ratios of mixture, so the inside layer and outside layer may be given different functions.
That is, due to the function of the inside layer, in the same way as with the immersion nozzle described in
the above (1) , Al2O3 inclusions in the molten metal can be prevented from depositing on the nozzle inside circumference and wear of the inside layer due to the molten metal can be suppressed.
Further, for example, when including magnesia, spinel, or dolomite clinker in the refractories forming the outside layer, the inside layer and outside layer become substantially equal in coefficients of heat expansion, so stress cracking due to a difference in heat expansion can be prevented.
Further, for example, when including zirconia in the refractories forming the outside layer, the corrosion resistance with respect to the slag floating on the surface of the molten metal in the mold can be improved and the wear of the outside layer due to the slag can be suppressed.
Furthermore, for example, when using alumina, silica, mullite, CaO-ZrO2 clinker, silicon carbide, or zirconia mullite for the refractories forming the outside layer, it is possible to improve the thermal shock resistance of the nozzle as a structural member compared with even magnesia etc.
(3) The immersion nozzle according to the present invention is an immersion nozzle as set forth in the above (1) or (2), characterized in that at least the nozzle inside circumference through which the molten metal flows is covered by an antioxidizing material including silica.
In general, an antioxidizing material is provided for the purpose of preventing oxidation of the nozzle inside circumference by the molten metal. Such an antioxidizing material is, for example, comprised of silica powder: 60 to 100 mass%. If the silica powder is less than 100 mass%, as the balance, Al2O3 powder is mixed in with a binder to form a paste which is then coated and fired over the nozzle inside circumference. Note that, this antioxidizing material may also be provided in a
state covering the entire exposed surface of the nozzle including the nozzle inside circumference.
Further, in the past, when preheating an immersion nozzle covered at its inside circumference with such an antioxidizing agent using the burner 100 as shown in FIG. 4, the problem arose of the molten metal causing tremendous wear at the nozzle inside circumference. That is, with the heating method using a burner 100, the preheating time is long and the heat from the burner is hard to conduct through the antioxidizing material to the nozzle inside circumference side, so the antioxidizing material ends up becoming higher in temperature than the nozzle inside circumference. For this reason, the SiO2 in the antioxidizing material ends up diffusing into the nozzle inside circumference resulting in the formation of low melting point substances in the nozzle inside circumference. Due to this, the molten metal ends up causing tremendous wear at the nozzle inside circumference.
On this point, according to the present invention, high frequency induction heating is used to selectively heat the free carbon in the refractories, so the refractories themselves can be heated without going through the antioxidizing material and the preheating time can also be kept short. For this reason, the SiO2 in the antioxidizing material will not diffuse into the nozzle inside circumference and wear of the nozzle inside circumference due to molten metal flowing through the inside can be prevented. Therefore, the function of the antioxidizing material can be secured and the wear of the nozzle inside circumference can be prevented, it is possible to further improve the durability of the immersion nozzle.
(4) The continuous casting method according to the present invention is characterized by being provided with a preheating step of preheating an immersion nozzle as set forth in any one of (1) to (3) by high frequency
induction heating and a casting step of pouring molten metal through the immersion nozzle preheated at the preheating step from a tundish to a mold.
According to this invention, it is possible to obtain the advantageous effects described in any of the above (1) to (3). Therefore, it is possible to improve the durability of the immersion nozzle.
Brief Description of Drawings
FIG. 1 shows the schematic configuration of a continuous casting machine in one embodiment of the present invention.
FIG. 2 is a side cross-sectional view showing an immersion nozzle according to the embodiment of FIG. 1.
FIG. 3 is a side cross-sectional view showing a preheating device in a state mounting the immersion nozzle in the embodiment of FIG. 1.
FIG. 4 is a side cross-sectional view showing the state using a conventional heating method using a burner to preheat an immersion nozzle.
Best Mode for Carrying Out the Invention
Below, an embodiment of the present invention will be explained based on the drawings.
(Schematic Configuration of Continuous Casting Machine)
FIG. 1 shows the schematic configuration of a continuous casting machine in the present embodiment. In FIG. 1, 1 is a continuous casting machine. This continuous casting machine 1 continuously cools and solidifies molten steel to form a predetermined shape of a steel ingot. Such a continuous casting machine 1 is provided with a ladle 2, a long nozzle 3, a tundish 4, a plurality of immersion nozzles 5, and a plurality of molds 6. Note that, in FIG. 1, just one each of the immersion nozzles 5 and molds 6 are shown.
The ladle 2 is a heat resistant container into which
the molten steel is first introduced in continuous casting. Its bottom part is provided with a feed hole 21.
The long nozzle 3 is attached to the feed hole 21 of the ladle 2 and is designed so that molten steel stored inside the ladle 2 is discharged from a nozzle bottom end opening 31 into the tundish 4.
The tundish 4 is a heat resistant container laid underneath the long nozzle 3 and storing molten steel fed from the ladle 2 through the long nozzle 3. This tundish 4 is formed at its bottom part with a plurality of feed holes 41 corresponding to the different molds 6. Inside of each feed hole 41, a flow regulator (not shown) is provided for regulating the flow rate of the molten steel flowing out from the feed hole 41. Using such a tundish 4, the molten steel from the ladle 2 is smoothed in flow and the molten steel is distributed to the molds 6 in predetermined amounts.
Each immersion nozzle 5, while explained later more specifically, is attached to the bottom of a feed hole 41 of the tundish 4. The molten steel in the tundish 4 is fed through this nozzle into the mold 6.
Each mold 6 is a water-cooled type casting mold provided below a corresponding immersion nozzle 5. Inside this mold 6, molten steel from the tundish 4 is continuously fed through the immersion nozzle 5. Due to such a mold 6, the molten steel inside the mold 6 is cooled whereby a solidified shell is formed and grows from the inside circumference side of the mold 6 and therefore solidified steel is formed.
Further, below each mold 6, a roller apron and withdrawal rolls are provided for continuously pulling out the steel formed inside the mold 6 downward from the bottom opening of the mold 6 (not shown). Further, at the downstream side of the withdrawal rolls, mechanical shears (not shown) are provided for cutting the steel in the state pulled out by the withdrawal rolls and continuously extending from the inside of the mold 6 into

predetermined length dimensions. By the mechanical shears cutting the steel, for example, plate shape, bar shape, or other predetermined shapes of steel ingots are formed.
(Configuration of Immersion Nozzle)
Next, the configuration of the immersion nozzle 5 will be explained based on FIG. 2. FIG. 2 is a cross-sectional view showing an immersion nozzle according to the present embodiment.
In FIG. 2, the immersion nozzle 5 is provided with a nozzle body 51 and a holder 52 attached to the bottom of the feed hole 41 and holding the top end of the nozzle body 51. This immersion nozzle 5 is used after being preheated in the later explained preheating step by high frequency induction heating.
The nozzle body 51 is formed into a substantially cylindrical shape and is provided with a bottom part 511 closing the bottom end. Near the bottom part 511 at the side parts of this nozzle body 51, a pair of discharge holes 512 are provided in a state facing each other. Further, the nozzle body 51 is used in a state with its bottom end side immersed in the molten steel in the mold 6. By such a nozzle body 51, molten steel flowing into the nozzle body 51 from the top end is discharged through the pair of discharge holes 512 to the inside of the mold 6.
Such a nozzle body 51, as shown in FIG. 2, therefore has a two-layer structure of an inside layer 513 through which the molten steel flows and forming the nozzle inside circumference and an outside layer 514 formed laminated in a state covering the outer side of this inside layer 513.
The inside layer 513 is formed by refractories comprised of any of the following aggregates and free carbon.
Single-type aggregate Magnesia Spinel
Dolomite clinker Two-type aggregate
Magnesia and spinel
Magnesia and dolomite clinker The outside layer 514 is formed from refractories different in either composition or formulation from the inside layer 513 and, as explained above, refractories comprised of one to three of alumina, mullite, silica, zirconia, CaO-ZrO2 clinker, spinel, magnesia, zirconia mullite, and silicon carbide and of free carbon or refractories comprised of one type or two types of dolomite clinker, zirconia, CaO-ZrO2 clinker, and magnesia and of free carbon. Among these, for example, refractories comprised of any of the following aggregates and free carbon are often used: Single-type aggregate
Alumina
Zirconia
CaO-ZrO2 clinker
Spinel
Magnesia Two-type aggregate
Alumina and silica
Alumina and zirconia mullite
Alumina and mullite
Alumina and spinel
Spinel and silica
Magnesia and spinel
Zirconia and CaO-ZrO2 clinker
Dolomite clinker and zirconia
Dolomite clinker and magnesia Three-type aggregate
Alumina, silica, and zirconia mullite
Alumina, silica, and zirconia
Alumina, mullite, and silica
Alumina, spinel, and silica
Alumina, silica, and silicon carbide
Alumina, zirconia mullite, and silicon carbide Alumina, mullite, and silicon carbide Magnesia, spinel, and silica Dolomite clinker, zirconia, and magnesia Alumina, mullite, and zirconia Note that, the nozzle body 51 need not be the above two-layer structure and may also be formed as a single piece by refractories comprising any of the following aggregates and free carbon. One-type aggregate Magnesia Spinel
Dolomite clinker Two-type aggregate
Magnesia and spinel Magnesia and dolomite clinker Further, in the nozzle body 51, the entire exposed surface of the nozzle body 51, including the nozzle inside circumference through which the molten steel flows, is covered by an antioxidizing material including silica. Due to this, oxidation of the nozzle body 51 by the molten steel is prevented.
(Configuration of Preheating Device) Next, a preheating device for preheating the above configured immersion nozzle 5 will be explained based on FIG. 3. FIG. 3 is a side cross-sectional view showing the preheating device in the state with the immersion nozzle mounted.
In FIG. 3, 7 is the preheating device. This preheating device 7 preheats the immersion nozzle 5 by high frequency induction heating. Such a preheating device 7 is comprised of a heat resistant container 71, outside coil 72, inside coil 73, and induction current application device (not shown).
The outside coil 72 is an induction heating coil used inside of the heat resistant container 71 and is configured to be able to house at its inside
circumference the nozzle body 51 from the bottom end to above the intermediate part.
The inside coil 73 is an induction heating coil similar to the outside coil 72 and is configured to enable insertion of the nozzle body 51 from its top opening.
The induction current application device is a device for applying high frequency induction current to the outside coil 72 and inside coil 73.
(Continuous Casting Method)
The continuous casting method according to the present embodiment will be explained with reference to the example of using the above configurations of a continuous casting machine 1 and preheating device 7.
The continuous casting method of the present embodiment is comprised of a preheating step, a casting step, a withdrawal step, and a steel ingot forming step.
In the preheating step, the preheating device shown in FIG. 3 is used to preheat the immersion nozzle 5 by high frequency induction. Specifically, first, the preheating device 7 is set at the immersion nozzle 5 in a state detached from the tundish 4. In this set state, the nozzle body 51 is housed inside the outside coil 72 and the inside coil 73 is inserted inside the nozzle body 51 from the top opening. Further, an induction current application device is used to apply an induction current to the outside coil 72 and inside coil 73. Due to this, near the free carbon contained in the nozzle body 51, high density eddy currents are formed and large Joule's heat is generated so the nozzle body 51 as a whole is uniformly heated.
Using this high frequency induction heating, in a heating time of for example 0.5 to 2 hours, the temperature of the nozzle body 51 reaches 1000°C or more. Further, for example, when heating the nozzle body 51 to 1100°C or more, if like in the past using a burner 100
(see FIG. 4) for heating, as much as a 500°C to 600°C temperature difference occurs, but with high frequency induction heating, only a maximum of about a 300°C temperature difference occurs between parts.
Further, with high frequency induction heating, no combustion gas is used like in the past and the preheating is completed in a short time, so consumption of C in the nozzle body 51 is made more difficult and enlargement of pores in the nozzle body 51 is prevented. Further, the SiO2 in the antioxidizing material will not diffuse to the nozzle inside circumference and low melting point substances will not be formed in the nozzle inside circumference. For this reason, it is possible to protect the nozzle inside circumference from wear due to the molten steel flowing through it in the later explained casting step.
In the casting step, the continuous casting machine 1 shown in FIG. 1 is used for casting the molten steel. First, the immersion nozzle 5 preheated at the preheating step is attached to the feed hole 41 of the tundish 4, then molten steel is introduced inside the ladle 2. This molten steel flows through the long nozzle 3 from the ladle 2 to the inside of the tundish 4. Inside the tundish 4, the flow is smoothed out. After this, while regulating the outflow rate by a flow regulator (illustrated), the molten steel smoothed out in flow is fed through the immersion nozzle 5 into a mold 6 to maintain a constant level of molten metal in the mold 6.
In this casting step, when starting the feed of the molten steel, since the nozzle body 51 is uniformly preheated at the preheating step, the thermal shock received by the immersion nozzle 5 from the molten steel is eased and cracking and other trouble can be prevented from occurring. Further, the magnesia in the nozzle inside circumference, needless to say, and the spinel and dolomite all contain MgO, while the dolomite contains CaO, so the Al2O3 inclusions in the molten metal can
prevent deposition at the nozzle inside circumference. Therefore, the durability of the immersion nozzle 5 is improved.
In the withdrawal step, the steel cooled and solidified in the mold 6 is continuously withdrawn downward by a not shown roller apron and withdrawal rolls.
In the steel ingot forming step, the steel withdrawn by the withdrawal rolls is cut by mechanical shears into predetermined length dimensions to continuous form predetermined shapes of cast slabs.
Note that, in the preheating step, in addition to the immersion nozzle 5, the long nozzle 3 and tundish 4 are also preheated. Further, in the preheating step, the immersion nozzle 5 was preheated in the state not attached to the tundish 4, but it is also possible to preheat the immersion nozzle 5 in the state attached to the tundish 4.
Examples
Examples for confirming the advantageous effects of the present embodiment described below will be explained next.
(Test Samples)
In the test, the following immersion nozzles (Examples 1 to 14 and Comparative Examples 1 to 3) were prepared. These immersion nozzles were similar in structure to the immersion nozzle 5 of the above embodiment shown in FIG. 2. The maximum outside diameter dimension of the nozzle body 51 was Φ140 mm, the inside diameter dimension was Φ80 mm, and the length dimension was 700 mm. Further, the nozzle body 51 in each sample was formed by mixing powders of various types of inorganic matter and free carbon constituted by graphite flakes together with a phenol resin, using the CIP method to shape it, and firing the result in a reducing atmosphere. Below the refractory compositions of the
samples will be shown.
Further, in all nozzles, the nozzle inside circumference was covered by an antioxidizing material. The antioxidizing material used was a mixture of SiO2 of 80 mass% and Al2O3 of 20 mass% to which sodium silicate was applied in 30 mass% (SiO2: 35 mass%, Na2O: 18 mass%, balance of water) and mixed, that is, as the antioxidizing material, one including SiO2: 78 mass%, Al2O3: 16 mass%, and Na2O: 6 mass% was employed.
As the method of coating this antioxidizing material, the method of spray coating the nozzle inside circumference, then drying it was used. (Two-Layer Structure)
(Inside layer) dolomite clinker 79 mass%, graphite 18 mass%, binder 3 mass%
(Outside layer) alumina 66 mass%, silica 4 mass%, zirconia 5 mass%, graphite 23 mass%, binder 2 mass% (Two-Layer Structure)
(Inside layer) magnesia 17 mass%, dolomite clinker 62 mass%, graphite 18 mass%, binder 3 mass%
(Outside layer) alumina 66 mass%, silica 4 mass%, zirconia 5 mass%, graphite 23 mass%, binder 2 mass% (Two-Layer Structure)
(Inside layer) magnesia 17 mass%, dolomite clinker 62 mass%, graphite 18 mass%, binder 3 mass%
(Outside layer) magnesia 70 mass%, graphite 28 mass%, binder 2 mass% (Single Piece)
Magnesia 70 mass%, graphite 28 mass%, binder 2 mass% (Two-Layer Structure)
(Inside layer) magnesia 17 mass%, dolomite clinker 62 mass%, graphite 18 mass%, binder 3 mass%
(Outside layer) alumina 80 mass%, graphite 17 mass%, binder 3 mass% (Two-Layer Structure)
(Inside layer) magnesia 17 mass%, dolomite clinker 62 mass%, graphite 18 mass%, binder 3 mass%
(Outside layer) CaO-ZrO2 clinker 80 mass%, graphite 17 mass%, binder 3 mass% (Two-Layer Structure)
(Inside layer) magnesia 17 mass%, dolomite clinker 62 mass%, graphite 18 mass%, binder 3 mass%
(Outside layer) alumina 75 massl, silica 5 mass%, graphite 17 mass%, binder 3 mass% (Two-Layer Structure)
(Inside layer) magnesia 17 mass%, dolomite clinker 62 mass%, graphite 18 mass%, binder 3 mass%
(Outside layer) magnesia 30 massl, spinel 50 mass%, graphite 17 mass%, binder 3 mass% (Two-Layer Structure)
(Inside layer) magnesia 17 mass%, dolomite clinker 62 mass%, graphite 18 mass%, binder 3 mass%
(Outside layer) alumina 73 mass%, silica 3 mass%, zirconia mullite 4 massl, graphite 17 mass%, binder 3 mass% (Two-Layer Structure)
(Inside layer) magnesia 17 mass%, dolomite clinker 62 massl, graphite 18 massl, binder 3 mass%
(Outside layer) alumina 74 massl, silica 3 mass%, silicon carbide 3 mass%, graphite 17 massl, binder 3 mass% (Two-Layer Structure)
(Inside layer) magnesia 17 mass%, dolomite clinker 62 massl, graphite 18 massl, binder 3 massl
(Outside layer) alumina 70 massl, mullite 7 massl, zirconia 3 massl, graphite 17 massl, binder 3 massl (Two-Layer Structure)
(Inside layer) magnesia 17 massl, dolomite clinker 62 massl, graphite 18 massl, binder 3 massl
(Outside layer) alumina 74 massl, silica 3 massl, zirconia 3 massl, graphite 17 massl, binder 3 massl (Two-Layer Structure)
(Inside layer) magnesia 17 massl, dolomite clinker 62 massl, graphite 18 massl, binder 3 massl
(Outside layer) magnesia 50 mass%, spinel 25 mass%, silica 5 mass%, graphite 17 mass%, binder 3 mass% (Two-Layer Structure)
(Inside layer) magnesia 17 mass%, dolomite clinker 62 mass%, graphite 18 mass%, binder 3 mass%
(Outside layer) magnesia 14 mass%, dolomite clinker 65 mass%, graphite 17 mass%, binder 3 mass% (Single Piece)
Corundum 66 mass%, silica 4 mass%, zirconia 5 mass%, graphite 23 mass%, binder 2 mass% (Two-Layer Structure)
(Inside layer) dolomite 79 mass%, graphite 18 mass%, binder 3 mass%
(Outside layer) corundum 66 mass%, silica 4 mass%, zirconia 5 mass%, graphite 23 mass%, binder 2 mass% (Two-Layer Structure)
(Inside layer) magnesia 17 mass%, dolomite clinker 62 mass%, graphite 18 mass%, binder 3 mass%
(Outside layer) magnesia 70 mass%, graphite 28 mass%, binder 2 mass%
(Preheating by High Frequency Induction Heating)
Preheating coverage: Examples 1 to 14
Preheating device: Similar to preheating device 7 shown in FIG. 3. For outside coil 72, one of a diameter dimension of Φ200 mm and a length dimension of 500 mm was used, while for the inside coil 73, one of a diameter dimension of Φ70 mm and a length dimension of 300 mm was used.
Induction current: The outside coil 72 was run through by an induction current of a frequency 30 kHz, current 200A, and power 15 kW. The inside coil 73 was run through by an induction current of a frequency 37 kHz, current 200A, and power 12 kW.
Preheating time: 40 minutes
(Preheating by Burner Heating)
Preheating coverage: Comparative Examples 1 to 3
Preheating device: The burner 100 shown in FIG. 4
was used for preheating. In FIG. 4, the immersion nozzle 5 was placed in a heat resistant container 101 and in that state a burner 100 was inserted from the top end opening of the immersion nozzle 5 to the inside and used to blow combustion gas.
Combustion gas: COG (coke oven gas)
Air ratio: 1.2
Preheating time: 90 minutes
(Casting Test)
Test coverage: Examples 1 to 14 and Comparative Examples 1 to 3
Continuous casting machine: One similar to the continuous casting machine 1 of the above embodiment shown in FIG. 1 was used (8 charges).
Casting method: Similar to the casting step in the above embodiment. Specifically, each immersion nozzle 5 was preheated alone, then the nozzles were attached to the tundish 4. After 5 minutes from the end of preheating, casting was started.
Steel type: Low carbon steel
Mold powder basicity: 1.0
Operating time: Total 360 minutes
(Test Results)
The results of the above casting test for the immersion nozzles 5 of Examples 1 to 14 and Comparative Examples 1 to 3 (alumina deposition index, wear speed index, and trouble index) are shown together with the composition and component minerals of the refractories in the following Tables 1 to 3.
Alumina deposition index: Amounts of deposition of alumina for Examples 1 to 14 and Comparative Examples 2 and 3 indexed to the amount of deposition of alumina for Comparative Example 1 (maximum thickness dimension of alumina layer deposited on nozzle inside circumference after casting divided by the operating time) as 100.
Wear speed index: The wear speeds of Examples 1 to 14 and Comparative Examples 1 and 3 indexed to the wear
speed for Comparative Example 2 (amount of wear of nozzle inside circumference after casting divided by the operating time) as 100.
Trouble index: The rate of occurrence of trouble for Examples 1 to 14 and Comparative Examples 1 and 3 indexed to the rate of occurrence of trouble for Comparative Example 2 (ratio of times of casting and times of occurrence of breakage, cracking, or other trouble) as 100.
(Table 1 to 3 Removed )
(Study 1: Regarding Alumina Deposition Index)
The nozzle inside circumference of Example 1 is comprised of refractories made from dolomite clinker, graphite, etc., while the nozzle inside circumferences of Examples 2 to 14 are comprised of refractories made from magnesia, dolomite clinker, graphite, etc. Further, the nozzle inside circumference of Comparative Example 1 is comprised of refractories made from alumina, silica, zirconia, graphite, etc. and does not contain magnesia or dolomite clinker.
From the alumina deposition indexes of Tables 1 to 3, it is learned that the nozzle inside circumference of Comparative Example 1 had alumina deposited on it, while the nozzle inside circumferences of Examples 1 to 14 did not have alumina deposited on them. Note that, while not shown in Table 1 to 3, in the same way as the case where the nozzle inside circumference contains spinel, the nozzle inside circumference did not have alumina deposited on it.
Due to this, it was learned that by including at least MgO at the nozzle inside circumference, the resistance to deposition of alumina can be improved.
(Study 2: Regarding Wear Speed Index)
Example 1 and Comparative Example 2 are identical in the refractories forming the inside layers and outside layers. They differ in the point that the preheating method of Example 1 is high frequency induction heating (IH), while in Comparative Example 2 it is using burners for preheating. In Table 1, looking at the wear speed indexes for these, the wear speed index of Example 1 is a value 20% lower than Comparative Example 2. Due to this, it is learned that by just using high frequency induction heating for preheating, the wear due to the molten steel can be suppressed.
Further, in Example 1, the aggregate of the inside layer includes the single type of the dolomite clinker, while in Examples 2 and 3, the aggregates of the inside
layers include the two types of dolomite clinker and magnesia. In Table 1, if viewing the wear speed indexes for these, the wear speed indexes of Examples 2 and 3 are values 12.5% lower than Example 1. Due to this, it is learned that by making the inside layer a mixture of magnesia and dolomite clinker, it is possible to further suppress the wear due to molten steel.
Further, in Example 4, the aggregate is comprised of only magnesia. If looking at the wear speed indexes in Table 1, the wear speed index of Example 4 is the same value as Example 1. Due to this, it is learned that if making the aggregate of the nozzle inside circumference only dolomite or only magnesia, it is possible to keep down the wear to the same extent. Note that, while not shown in Table 1, even when making the aggregate of the nozzle inside circumference only spinel, the wear could be similarly suppressed.
(Study 3: Regarding Trouble Index)
Example 1 and Comparative Example 2 are identical in the refractories forming the inside layers and outside layers. They differ in the point that the preheating method of Example 1 is high frequency induction heating (IH), while in Comparative Example 2 it is using burners for preheating. In Table 1, looking at the trouble occurrence indexes for these, the trouble occurrence index of Example 1 is a value 80% lower than Comparative Example 2. Due to this, it is learned that by just using high frequency induction heating for preheating, the frequency of occurrence of cracking and other trouble when starting the feed of molten steel in the casting step can be remarkably reduced.
Note that, the present invention is not limited to the above-mentioned examples. Modifications, improvements, etc. of a range able to realize the object of the present invention are included in the present invention. For example, the composition and component minerals of the refractories are not limited to those of

Examples 1 to 14. That is, any nozzle where at least the inside circumference contains at least one of magnesia, spinel, and dolomite clinker is included in the present invention.
Industrial Applicability
According to the present invention, due to the presence of the free carbon in the refractories, high frequency induction heating can be used to selectively heat the free carbon and to uniformly preheat the immersion nozzle. For this reason, at the start of casting after preheating, the immersion nozzle can be protected from cracking and other trouble and the wear of the nozzle inside circumference due to the molten metal can be suppressed. Further, spinel and dolomite clinker all contain MgO and the dolomite clinker contains CaO. Refractories containing these minerals form the immersion nozzle, so the Al2O3 inclusions in the molten metal can be prevented from depositing at the nozzle inside circumference. Therefore, the durability of the immersion nozzle can be improved.

CLAIMS
1. An immersion nozzle used in a continuous
casting method of molten metal, said immersion nozzle
characterized in that
it is formed by refractories comprised of either magnesia, spinel, dolomite clinker, a mixture of magnesia and spinel, or a mixture of magnesia and dolomite clinker and of free carbon and in that high frequency induction heating is used for preheating.
2. An immersion nozzle as set forth in claim 1,
wherein the nozzle is comprised of a two-layer structure
of an inside layer through which the molten steel flows
and forming the nozzle inside circumference and an
outside layer formed laminated in a state covering the
oute-r side of this inside layer, said inside layer is
formed by refractories comprised of any one of magnesia,
spinel, dolomite clinker, a mixture of magnesia and
spinel, and a mixture of magnesia and dolomite clinker
and of free carbon, and said outside layer is formed by
refractories different in either composition or
formulation from said inside layer and comprised of one
to three of alumina, mullite, silica, zirconia, CaO-ZrO2
clinker, spinel, magnesia, zirconia mullite, and silicon
carbide and of free carbon or refractories comprised of
one type or two types of dolomite clinker, zirconia, CaO-
Zr02 clinker, and magnesia and of free carbon.
3. An immersion nozzle as set forth in claim 1 or
2, characterized in that at least the nozzle inside
circumference through which the molten metal flows is
covered by an antioxidizing material including silica.
4. A continuous casting method characterized by being provided with a preheating step of preheating an immersion nozzle as set forth in any one of claims 1 to 3 by high frequency induction heating and a casting step of pouring molten metal through said immersion nozzle preheated at said preheating step from a tundish to a mold.