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 of 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-contai.ning 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.
Here, at the time of continuous casting of molten metal, low basicity and strongly corrosive slag called "mold powder" floats on the melt surface of the molten steel in. the mold. This mold powder generally contains Cao, SiO2, CaF2, Na2O, and C. The basicity is about 1, so this ends up causing remarkable wear at refractories containing Al2O3 and SiG2. For this reason, with conventional Al2C3-C-containing refractories, there was the problem of large wear at locations of the outer-circumference of the immersion nozzle contiguous with the mold powder (hereinafter, called the "powder line part") and the inability to withstand long term use.
To deal with this problem, in the past, an immersion nozzle using ZrO2-C-based refractories at the powder line parr was known, for example, in Japanese Patent Publication (A) No. 11-302073.
ZrO2-C-based refractories have the feature of a combination of the excellent corrosion resistance of ZrO2 against mold powder and the thermal shock resistance of C. By using such ZrO2-C-based refractories for the powder line part, the durability of the immersion nozzle can be improved.
In such ZrC2-C-based refractories, to better improve the corrosion resistance, it is effective to reduce the amount of mixture of C and increase the amount of mixture of ZrO2. However, increasing the amount of ZrO2 causes a drop in the thermal shock resistance and causes the problems of cracking and breakage at the time of use. Cn the other hand, to improve the thermal shock resistance, it is effective to increase the amount of mixture of C so as to reduce the amount of mixture of ZrO1, but the corrosion resistance falls.
In this way, to raise the corrosion resistance and
high durability, the amounts of mixture of ZrO2 and C have to be optimized. With the constitution described in the above Japanese Patent Publication (A) No. 11-302073, the amount of mixture of ZrO2 is made 70 to 95 mass% and the amount of mixture of C is made 5 to 30 mass% to optimise the amounts.
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, the slag will not sufficient rise up on the molten metal and the quality of the cast, slab will end up falling, or other trouble will occur. 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. 5, 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 in, for example, Japanese Patent Publication (A) No. 10-118746.
However, when preheating an immersion nozzle using ZrO2-C-based refractories for the powder line part as described in the above Japanese Patent Publication (A) No. 11-302073, then performing the casting step, since ZrO2-C-based refractories are high heat expanding materials, there are the following problems (A) and (B). (A) When using a burner 100 as shown in FIG. 5 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. Stress cracking etc. end up occurring
due to the difference in thermal expansion of the ZrO2 accompanying this temperature difference.
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 ZrO2-C-based refractories to oxidize and form CO gas or CO2 gas and thereby end up being consumed. For this reason, ZrO2-C-based refractories are formed with large pores into which mold powder easily enters thereby having the problem of aggravating the wear due to the mold powder.
(B) When using the electric heater described in the above Japanese Patent Publication (A) No. 10-118746 for preheating, consumption of the C ingredient can be prevented, but since heat conduction and radiant heat are used to heat the nozzle, parts will reach 14 00°C, but uniformly heating the entirety is again difficult.
The present invention provides an immersion nozzle able to be improved in durability and a continuous casting method including a preheating step of preheating this immersion nozzle.
Disclosure of Invention
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 -he present invention is an immersion nozzle used in a continuous casting method of molten metal, characterized in that at least a part of an outer circumference contacting slag is formed by refractories comprised of ZrO2: 70 mass% or more and FC (free carbon): 30 mass% or less and in that high frequency induction heating is used for preheating.
More preferably, it is characterized in that ZrO2 is 8 0 mass% or more and the FC is 20 mass% or less.
Here, when the amount of mixture of ZrO2 is lower
than 70 mass% and when the amount of mixture of FC is higher than 30 mass%, sufficient corrosion resistance against mold powder cannot be obtained.
Such an immersion nozzle is, for example, formed by mixing powders of various types of inorganic materials 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. ZrO2 of a crystal grain size .of several pm to 2 mm. or so is used. Further, the FC includes, for example, usually graphite flakes, electrode dust, anthracite, amorphous graphite, and other added graphite and also carbon remaining when the binder is fired.
According to the present invention, due to the presence of the FC in the refractories, it is possible to selectively heat the FC 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. 5 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. In particular, it is possible to uniformly preheat the nozzle, so even if lowering the amount of mixture of FC superior in thermal shock resistance as well to 20 mass% or less, cracking and other trouble will not occur. Due to this, the amount of mixture of ZrO2 can. be further increased, so the speed of wear due to slag can be reduced.
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 FC in the refractories will become lower and the speed of wear due to slag can be reduced.
Therefore, the durability of the immersion nozzle can be improved.
(2) The immersion nozzle according to the present
invention also stands as the following in addition to the
immersion nozzle described in the above (1). That is, the
immersion nozzle according to the present invention is an
immersion nozzle used in a continuous casting method of
molten metal, characterized in that at least a part of an
outer circumference contacting slag is formed by
refractories comprised of ZrO2: 70 massl or more, FC (free
carbon): 20 mass% or less, and a balance including a ZrO2
stabilizing material 10 mass% or less and in that high
frequency induction heating is used for preheating.
According to the aspect of the invention of this (2), advantageous effects similar to the present invention of the above (1) can be exhibited. In addition to this, by addition of a stabilizing material, it is possible to fix the ZrQ3 in a stable state in the refractory structure and possible to prevent ZrO2 crystal grains from falling into the slag. Due to this, the part contacting the slag can be protected from wear due to the slag. Therefore, the durability of the immersion nozzle can be further improved.
(3) The immersion nozzle according to the present
invention preferably comprises the immersion nozzle as
described in the above (2) wherein the stabilizing
material includes at least one type of material of CaO,
MgO, and Y2O3.
Here, if the amount of mixture of ZrO2 is lower than 7 0 mass% and the total of the amounts of mixture of the FC and the balance is higher than 30 mass%, sufficient corrosion resistance against mold powder cannot be obtained.
(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 the above (1) to (3) by high
frequency induction heating and a casting step of feeding molten metal through the immersion nozzle preheated at. the preheating step from a tundish to a mold.
According to the aspect of the invention of this (4), the advantageous effects described in any of the above (1) to {3) can be exhibited. Therefore, the durability of the immersion nozzle can be further improved.
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 longitudinal section showing an immersion nozzle according to the embodiment of FIG. 1.
FIG, 3 is a view showing the amounts of mixture of the ZrO2 and FC in the refractories used for the powder line part of the immersion nozzle in the embodiment of FIG. 1.
FIG. 4 is a side cross-sectional view showing a preheating device in a state mounting the immersion nozzle in the embodiment of FIG. 1.
FIG. 5 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 mclten 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 firsr 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.
Each mold 6 is a water-cooled type casting mold provided below a corresponding immersion nozzle 5. The inside of the mold 6 has a predetermined cross-sectional shape. Inside this mold 6, molten steel from the tundish 4 is continuously fed through the immersion noz2le 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 inner 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, cutting machine (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 FIGS. 2 and 3. FIG. 2 is a cross-sectional view showing an immersion nozzle according to the present embodiment. FIG. 3 is a view showing the amounts of mixture of ZrO2 and FC of the refractories used for the powder line part of the immersion nozzle.
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. 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.
Further, the nozzle body-51 is used in a state with that bottom end side immersed in the molten steel in the mold. 6. Here, the long dashed double-short dashed line in FIG. 2 shows the slag line S. In the state with the nozzle body 51 immersed in the molten steel, the part of the outer circumference of the nozzle body 51 below the
slag line S contacts the mold powder (powder thickness is about 10 mm). The part below the mold powder is immersed in the molten steel. In -he case of poor preheating, the part above the powder line S sometimes cracks.
Such a nozzle body 51 therefore has a two-layer structure of a powder line part 512 above the discharge holes 512 at the outer circumference and other parts -formed by respectively different refractories.
The refractories forming the powder line part 513, as shown by the region A and region B in FIG. 3, are comprised of ZrO2: 70 mass% or more and FC (free carbon): 3.0 mass% or less. Further, the refractories forming the powder line part 513, as shown by the region A in FIG. 3, may be comprised of ZrO2: 70 mass% or more, FC containing graphite: 20 mass? or less, and a balance, including stabilizing materials for stabilizing the ZrO2, of 10 mass% or less.
The upper limit of the ZrO2 content is not particularly defined. It is sufficient that it be less than 100 mass%. Further, the lower limit of the FC (free carbon) content is also not particularly defined. It is sufficient that it be over 0 mass%. Further, the lower limit of the balance including the stabilizing material is also not particularly defined. It is sufficient that it be over 0 mass%.
The parts of the nozzle body 51 other than the powder line part 513 are, for example, formed by A12C3-SiO2-C,. Al2O3-C, cr other refractories. Note that, the refractories used for the parts other than the powder line part 513 are not limited to these. Any material giving a superior refractory property and low melt wettability with respect to the molten steel flowing through the inside of the nozzle body 51 may be employed.
(Configuration of Preheating Device)
Next, a preheating device for preheating the above configured immersion nozzle 5 will be explained based on FIG. 4. FIG. 4 is a side cross-sectional view showing the
preheating device in the state with the immersion nozzle mounted.
In FIG. 4, 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 inner 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. 4 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 inducticn current application device is used to apply an induction current
to the outside coil 72 and inside coil 73. Due to this, near the FC 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 no22le 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. 5) 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 at the powder line part 513 is made more difficult and enlargement of pores in the refractories is prevented.
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 5.
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 powder line part 513 is formed by refractories containing ZrO2, FC, etc. in the above ranges, so has high corrosion resistance against mold powder and can suppress wear due to mold powder. Further, the pores in the powder line part 513 are not enlarged due to the preheating in the preheating step, so it is possible to prevent mold powder from corroding the insides of the pores and crystal grains in the refractories from falling into the mold powder. 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)
Immersion nozzle: A plurality of nozzles similar tc the immersion nozzle 5 of the above embodiment shown in FIG. 2 were prepared.
Nozzle dimensions: The maximum outside diameter
dimension of the nozzle body 51 was made Φ140 mm, the inside diameter dimension was made Φ80 mm, and the length dimension was made 700 mm.
Refractory compositions: The compositions of the refractories forming the powder line parts 513 include those shown in the following Table 1 and include the compositions shown at the plots in FIG. 3.
Method of formation: Refractory aggregate and graphite flakes were mixed together with a binder, then the mixture was poured into a nozzle shaped rubber mold, when pouring different materials, they were poured while inserting a partition in the rubber mold so that they did not mix. After that, the wet type GIF shaping method was used and the mixture hardened while applying a high pressure (50 to 100 MPa] of water pressure. The shaped article was taken out from the mold, then fired in a reducing atmosphere at a 1000°C or higher high temperature. After cooling, the article was worked to the necessary dimensions, coated with antioxidizing materials, then used in an actual machine.
Table. 1
(Table Removed )
(Preheating by High Frequency Induction Heating;
Preheating coverage: Examples 1 to 6
Preheating device: Similar to preheating device 7 shown in FIG. 4. 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 Φ7 0 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: 4 0 minutes
(Preheating by Burner Heating)
Preheating coverage: Comparative Examples 1 and 2
Preheating device: The burner 100 shown in FIG. 5 was used for preheating. In FIG. 5, 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 6 and Comparative Examples 1 and 2
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 tundisb 4. After 5 minutes from the end of preheating, casting was started.
Steel type: Low carbon steel (carbon content of 0.06 mass!)
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 6 and Comparative Examples 1 and 2 (wear speed index and trouble index) are shown together in Table 1.
Wear speed index: The wear speeds of Examples 1 to 6 and Comparative Example 2 indexed to the wear speed for Comparative Example 1 (amount of wear of powder line part 513 due to casting divided by the operating time) as 100.
Trouble index: The rate of occurrence of trouble for Example 1 indexed to the rate of occurrence of trouble for Comparative Example 1 (ratio of times of casting and times of occurrence of breakage, cracking, or other trouble) as 100.
(Discovery 1: Regarding Advantageous Effects of High Frequency Induction Heating)
As shown in Table 1, Example 1 and Comparative Example 1 are identical in compositions of the powder line parts 513 (ZrO2: FC:CaO=75:20:5) , while Example 2 and Comparative Example 2 are identical in compositions of the powder line parts 513 (ZrO2: FC : CaO=82 :13 : 5) . Further, Examples 1 and 2 are preheated by the method of high frequency induction heating (IH), while Comparative Examples 1 and 2 differ in the point of using burners for heating.
In the results of Table 1, if comparing the wear speed indexes, Example 1 has a value 10% lower than Comparative Example 1, further, Example 2 has a value about 9.5% lower than Comparative Example 2. This is believed to be because when using high frequency induction heating for preheating, unlike the case of using a burner for preheating, no combustion, gas is used and the preheating is completed in a short time, so consumption of C at the powder line part 513 is prevented.
Further, in the results of Table 1, if comparing the trouble indexes, Example 1 has a value 85% lower than Comparative Example 1. This is believed to be because when using high frequency induction heating for preheating, compared with the case of using a burner for preheating, all the parts of the nozzle body 51 are uniformly preheated.
Due to the above, it was learned that by using high frequency induction heating to preheat the immersion nozzle 5, there is greater resistance to wear due to the mold powder and the frequency of cracking and other trouble occurring at the time of start of casting can be remarkably reduced. That is, it was learned that the durability of the immersion nozzle 5 can be improved.
(Discovery 2: Regarding Ratios of Mixture of ZrO2 and FC)
As shown in Table 1, if comparing Examples 1 to 3 and 6, in each case, the powder line part 513 contained CaO in about 5 mass%. The amounts of mixture of ZrO2 were 75 mass% (Example 1), 82 mass% (Example 2), 88 mass! (Example 3), and 70 mass% (Example 6). Example 3 was the highest, while Example 6 was the lowest. Corresponding to this, the amounts of mixture of FC were 20 mass% (Example 1), 13 mass% (Example 2), 8 mass% (Example 3), and 26 mass% (Example 6). Example 3 was the lowest, while Example 6 was the highest.
Further, in the results of Table 1, if comparing the
wear speed indexes, Example 1 has a value about 5% lower than Example 6, Example 2 has a value about 9% lower than Example 6, and Example 3 has a value about 13% lower than Example 6. This is believed to be because the amount of mixture of ZrO2 superior in corrosion resistance increased, so the corrosion resistance of the powder line part 513 against mold powder was improved.
In Examples 1 to 3 and 6, at the time of start of feeding of molten steel, almost no cracking etc. occurred. In each case, excellent thermal shock resistance was exhibited. This is believed to be because the amount of mixture of FC superior in thermal shock resistance was sufficient and the high frequency induction heating enabled the nozzle body 51. to be uniformly heated -
Note that, while not shown in Table 1, when the amount of mixture of ZrO2 was lower than 70 mass% and. the amount of mixture of FC was greater than 30 mass%, the amount of mixture of ZrO2 was not sufficient and sufficient corrosion resistance against the mold powder could not be obtained.
From the above, it was learned that by making the amount of mixture of ZrO2 7 0 .mass% or more, sufficient corrosion resistance against mold powder can be obtained, while by making the amount of mixture of ZrO2 90 massl or more, that corrosion resistance can be further improved.
Further, it was learned that by making the amount of mixture of FC 30 mass% or less, a high thermal shock resistance of the powder line part 513 can be obtained. Furthermore, it was learned that even if making the amount of mixture of FC 20 mass% or less, a good, thermal shock resistance of the powder line part 513 can be maintained.
(Discovery 3: Regarding Advantageous Effects Due To Addition Of Stabilizing Material)
As shown in Table 1, if comparing Examples 3 and 4 and Example 5, in each case the amount of ZrO2 contained
at the powder line part 513 is about the same, that is, 88 mass% (Example 3), 86 raass% (Example 4), and 85 mass% (Example 5). Further, Examples 3 and 4 contain stabilizing materials comprised of CaO and MgO in 4%, while Example 5 has no stabilizing material added to it.
Further, in the results of Table 1, if comparing the wear speed indexes, Example 3 has a value about 5% lower than Example 5 while Example 4 has a value about 7% lower than Example 5. This is believed because the addition of the stabilizing material causes ZrO2 crystal grains to become more resistant to detachment from inside the refractory structure.
Further, even when the balance including the-stabilizing material is greater than 10 mass%, while that advantageous effect is exhibited, the ratio of mixture of ZrO2 is relatively small and sufficient corrosion resistance against the mold powder becomes hard to obtain, so the ratio is preferably made 10 mass% or less.
Note that, while not shown in Table 1, similar results were obtained when adding Y2O3 as the stabilizing material.
From the above, it was learned that by adding a stabilizing material in 10 mass% or less, the speed of wear of the powder line part 513 can be 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 of the powder line part 513 is not limited to the compositions of Examples 1 to 6. Any composition falling with the regions A and E of FIG. 3 is included in the present invention.
Industrial Applicability
According to the present invention, due to the presence of the FC in the refractories, high frequency induction heating can be used to selectively heat the FC
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 due to slag at the part contacting the slag at the time of the casting step can be suppressed. 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 at least a part of an outer
circumference contacting slag is formed by refractories
comprised of ZrO2: 70 ma.ss% or more and FC (free carbon) :
30 ruass% or less and. in that high frequency induction
heating is used for preheating.
2. An immersion nozzle used in a continuous
casting method of molten metal, said immersion nozzle
characterized in that at least a part o£ an outer
circumference contacting slag is formed by refractories
comprised of ZrO2: 7 0 mass% or more, FC (free carbon): 20
mass% or less, and a balance including a ZrO2 stabilizing
material 10 mass% or less and in that high frequency
induction heating is used for preheating.
3. An immersion nozzle as set forth in claim. 2,
said immersion nozzle characterized in that said
stabilizing material includes at least one type of
material of CaO, MgO, and Y2C3.
4. A continuous casting method characterized by
being provided with a preheating srep 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. tundrsh to a
mold.