JP2002194517A - Hot-dip metal plating tank and induction heating device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot-dip metal plating tank which can improve the durability through reducing erosion caused by molten metal, and an induction heating device. SOLUTION: The hot-dip metal plating tank with a container made of steel shell, inside of which is covered with refractory, is constituted by arranging a metal plate inside of the refractory or between the refractory and the steel shell.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a molten metal plating bath and an induction heating device used for heating and melting a molten metal plating bath.

[0002]

2. Description of the Related Art Hot-dip galvanizing is performed on steel sheets and steel pipes for the purpose of preventing corrosion. This is a continuous hot-dip galvanizing method that heats and holds hot-dip zinc in a hot-dip galvanizing bath, immerses steel sheets and steel pipes in this hot-dip galvanizing bath, and continuously coats the surface of steel sheets and steel pipes with zinc. It is done by.

[0003] Plating products manufactured by the continuous hot-dip galvanizing method are widely used in automobiles, home electric appliances, building materials and the like. Recently, the requirements for corrosion resistance of plated products have become strict, and in order to respond to these requirements, hot-dip zinc
Plating products coated with aluminum alloy are increasing.

[0004] In a hot-dip galvanizing bath in which a continuous hot-dip galvanizing method is performed, the steel in the inner surface of the bath is heated by an electric method or a gas combustion method to indirectly heat the zinc in the bath. There are two types: a type and a type called a ceramic pot. here,
With a ceramic pot, the inner surface of a welded steel plate container is lined with fire-resistant insulating bricks, and an induction heating device is provided in at least one place around a plating bath tub configured by applying a castable lining to the inner surface of the fire-resistant insulating bricks. This is a method in which zinc provided in the plating bath is induction-heated by an induction heating device and convection is generated in the zinc bath by electromagnetic force generated from the inductor, thereby directly heating zinc. .

In the hot-dip galvanizing baths described above, in which zinc is indirectly heated, when aluminum is mixed with zinc to improve the corrosion resistance of the galvanized product, the aluminum concentration in the bath increases, Accordingly, the temperature in the bathtub must be increased, so that the corrosion of the plating bathtub progresses quickly and the life of the plating bathtub is shortened. In addition, as the temperature in the bath increases, dross is generated due to the elution of iron from the steel sheet into the molten zinc-aluminum bath, and dross is generated. By adhering to the surface, the surface appearance of the plated product deteriorates. Therefore, a hot-dip zinc-aluminum plating bath by electromagnetic induction heating using a ceramic pot which does not cause such a problem has been generally used.

In other words, in the method using a ceramic pot, since zinc-aluminum itself generates heat, there is no need to use a heating element such as a steel plate material which comes into direct contact with the molten zinc-aluminum. Refractory materials such as fire-resistant heat-insulating bricks with good heat insulation and high corrosion resistance can be used for the inner wall of the interior, so that heat loss can be reduced and dross generation can be suppressed. Surface appearance quality can be improved.

The induction heating apparatus used in the method using the ceramic pot generally has a W-shaped or U-shaped flow path formed on the outer wall of a bathtub, and an induction heating coil is wound around a part of the flow path. , An iron core wound with an induction heating coil is provided, and the molten zinc-aluminum is heated in a W or U-shaped flow path, and convection is generated in the molten zinc-aluminum by an inductor. Then, the molten zinc-aluminum is caused to flow between the molten zinc-aluminum bath. For example, when the aluminum concentration in the plating bath is 50% or more, the temperature of the molten zinc-aluminum of about 600 to 650 ° C. is set to 700 in the W or U-shaped flow path.
While heating and raising the temperature to about 800 ° C., natural convection is generated in the molten zinc-aluminum by the electromagnetic force generated from the inductor, and the molten zinc-aluminum is caused to flow between the molten zinc-aluminum plating bath.

Incidentally, the following measures have been taken to improve the durability of a hot-dip zinc-aluminum plating bath and an induction heating device.

(A) A portion of the W or U-shaped channel in a hot-dip zinc-aluminum plating bath or an induction heating device, which is in contact with the hot-dip zinc-aluminum, has good corrosion resistance to the hot-dip zinc-aluminum and has a small magnetic field loss. Material, for example, high alumina castable, 3-6
No. 299, an inorganic material mainly composed of graphite, aluminum titanate disclosed in JP-A-5-212266, cordierite disclosed in JP-A-7-41922, and the like. Use a castable refractory or fire-resistant insulation brick.

(B) By changing the shape of the molten zinc-aluminum flow path in the induction heating apparatus, local heating in the flow path due to the convection of the molten zinc-aluminum is prevented, and castable refractories, fire-resistant insulating bricks, etc. To prevent damage.

(C) A temperature sensor is provided on the entire surface of the steel refractory of the steel plate hot-dip bath which is in contact with the molten zinc, and the molten zinc penetrates into cracks generated in the refractory, and the steel When the temperature rises, the temperature sensor detects the rise in the temperature of the steel shell, thereby cooling the portion where the temperature has risen and solidifying the molten zinc in contact with the steel shell (Japanese Patent No. 2825136).

[0012]

However, in the measures described in the above (a), the flow path of the ceramic hot-dip zinc-aluminum plating bath and the induction heating device using castable refractory or refractory insulating brick is: In terms of durability, it is relatively effective when the hot-dip galvanized metal is only zinc, but not so effective for hot-dip zinc-aluminum. In particular, in recent production of zinc-aluminum plated products, in order to further improve the corrosion resistance of the produced plated products, the content of aluminum is set to 50% by weight or more, and the operating temperature is set to 600 to 600%.
Since the temperature is as high as 650 ° C., the permeability of molten zinc-aluminum to castable refractories and refractory insulating bricks is increased, and castable refractories and refractory insulating bricks are exposed to extremely severe conditions.

In addition, in the flow path of the induction heating device, molten zinc-aluminum penetrates into castable refractories or refractory insulating bricks used in the flow path due to the reducing action of aluminum to form a deteriorated layer. The spalling causes cracks in castable refractories and refractory insulation bricks. Molten zinc-aluminum penetrates into this crack,
When the steel comes into contact with the steel shell of the plating bath made of steel plate, the steel shell is eroded and a hole is formed, and leakage of molten zinc-aluminum may occur. Therefore, the life of the induction heating device is about one to one and a half years.

Further, in the measures described in the above (b), the molten zinc of castable refractories and refractory insulating bricks is
The corrosion resistance to aluminum is basically the same as the above (a)
Issues similar to the issues in the measures described in Section 2 arise.

Further, in the measure described in the above (c),
Although it has a primary effect on preventing corrosion and leakage of steel in a hot-dip zinc-aluminum plating bath made of steel plate, deterioration of castable refractories and fire-resistant insulation bricks also occurs in areas other than those where temperature sensors are installed. Therefore, it is not possible to perform cooling by specifying an accurate position of the iron shell in contact with the molten zinc-aluminum. Therefore, in order to completely solidify the molten zinc-aluminum in contact with the steel shell, the cooling device becomes complicated, and the running cost of the cooling medium is increased.

On the other hand, in the hot-dip zinc-aluminum plating bath and the induction heating device, the fact that the hot-dip zinc-aluminum penetrated into the castable refractory or the refractory insulating brick and contacted the steel shell was detected by an electric signal or a temperature signal. In this case, the service life is unpredictable.In this case, it is necessary to stop the hot-dip zinc-aluminum plating equipment suddenly and repair or replace the hot-dip zinc-aluminum plating bath or induction heating equipment. There is.

The present invention has been made in view of the above-mentioned conventional problems, and does not corrode a steel shell even when a castable refractory or a fire-resistant insulating brick is eroded by a molten metal, and has excellent durability. It is an object to provide a hot-dip metal plating bath and an induction heating device.

[0018]

In order to achieve the above-mentioned object, the following inventions are provided. (1) A hot-dip metal plating bath in which the inside of a steel shell formed in a container shape is covered with a refractory, wherein a metal plate is arranged inside the refractory or between the refractory and the steel. A hot metal plating bath characterized by the following. (2) The molten metal plating bath according to (1), further comprising a temperature detecting unit for detecting a temperature change of the metal plate or an electric signal detecting unit for detecting an electric signal. (3) The molten metal plating bath according to (1) or (2), wherein the refractory comprises a refractory insulating brick and a castable refractory. (4) The molten metal plating bath according to any one of (1) to (3), wherein the metal plate is subjected to nitriding treatment. (5) The molten metal plating bath according to any one of (1) to (3), wherein the metal plate is covered with a metal film by thermal spraying. (6) An induction heating device in which the outer periphery of a flow path through which a molten metal flows is covered with a refractory material and the outer periphery of the refractory material is covered with an iron shell, wherein the inside of the refractory or the refractory and the iron shell are covered. An induction heating device, wherein a metal plate is arranged between the induction heating device and the heating device. (7) The induction heating apparatus according to (6), further comprising a temperature detecting unit that detects a temperature change of the metal plate or an electric signal detecting unit that detects an electric signal. (8) The induction heating apparatus according to (6) or (7), wherein the refractory comprises a refractory heat insulating brick and a castable refractory. (9) The induction heating device according to any one of (6) to (8), wherein the metal plate is subjected to a nitriding treatment. (10) The induction heating device according to any one of (6) to (8), wherein the metal plate is covered with a metal coating by thermal spraying.

According to the invention described above, even if the molten metal enters a crack or the like generated in a refractory such as a castable refractory or a refractory insulating brick, the metal plate prevents the molten metal from coming into contact with the steel. Holes can be prevented, and the durability of the hot-dip metal bath and the induction heating device can be improved.

The metal plate may be placed in a hot-dip plating bath or an induction heating device so as to prevent contact between the molten metal and the steel. For example, the metal plate may be placed between the refractory and the steel. Or inside a refractory. In addition, the refractory has a two-layer structure composed of fire-resistant insulation brick and castable refractory, and a metal plate is disposed between the fire-resistant insulation brick layer and the castable refractory layer, or inside the fire-resistant insulation brick layer or inside the castable refractory layer. They may be arranged. Further, the metal plates may be arranged at a plurality of positions among the positions where the metal plates are arranged as described above.

Further, by providing a temperature detecting means or an electric signal detecting means on the metal plate so as to detect the contact of the molten metal with the metal plate, it is possible to detect the refractory before the molten metal comes into contact with the steel shell. You can know the damage. More specifically, a temperature sensor (temperature detecting means) is provided at one or more locations on the surface of the metal plate, and the temperature measurement result from the temperature sensor is displayed on a display device, so that the molten metal is applied to the metal plate. Monitor for contact, that is, for refractory damage. If the damage of the refractory can be known at an early stage, the service life of the hot-dip plating bath and the induction heating device can be predicted at an early stage. it can.

Further, in the case where molten zinc-aluminum is used as the molten metal, if the metal plate is nitrided, when the molten zinc-aluminum comes into contact with the metal plate,
When aluminum reacts with nitrogen, a ceramic coating of aluminum nitride is formed on the surface of the metal plate, and this ceramic coating can also prevent molten zinc-aluminum from coming into contact with the steel shell. For this reason, the durability of the molten metal plating bath and the induction heating device can be further improved.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is an external perspective view of a plating pot device provided with a hot-dip zinc-aluminum plating bath and an induction heating device. A plurality of induction heating devices 2 are provided on the outer periphery of the hot-dip zinc-aluminum plating bath 1.

(First Embodiment) A hot-dip zinc-aluminum plating bath according to a first embodiment of the present invention will be described. FIG. 2 is a partial cross-sectional view of the hot-dip zinc-aluminum plating bath in this embodiment.

The inner surface of the exterior part forming the exterior of the hot-dip zinc-aluminum plating bath 1 is covered with an iron shell 3.
The inner surface of the steel shell 3 is made of fire-insulated brick 4 (Shinagawa ALT-P8
5. Shinagawa White Brick Co., Ltd.), and the inner surface of the refractory brick 4 is a metal plate 5 (SUS 316
Or SUS 316L). Further, the inner surface of the metal plate 5 is covered with a castable refractory 6 (Shinagawa CAST-A63, manufactured by Shinagawa White Brick Co., Ltd.).

Here, the steel shell 3 and the refractory insulating brick 4 are fixed by a refractory mortar such as a non-asbestos mill board. Similarly, the metal plate 5 is also fixed to each of the refractory heat-insulating bricks 4 and the castable refractories 6 with refractory mortar.

When the metal plate 5 subjected to the nitriding treatment is used, when a mixture of zinc and aluminum is put into the bath of the hot-dip zinc-aluminum plating bath 1 and heated,
Even if the aluminum erodes the castable refractory 6, the aluminum of the molten zinc-aluminum reacts with nitrogen by contact with the metal plate 5, and a ceramic coating of aluminum nitride is formed on the surface of the metal plate 5. The aluminum nitride ceramic coating and the metal plate 5 can prevent molten zinc-aluminum from eroding the refractory insulating bricks 4 and the steel shell 3.
The durability of the aluminum plating bath 1 can be improved. Here, a method of manufacturing the metal plate 5 subjected to the nitriding treatment is described in Japanese Patent Application Laid-Open No.
The plating bath equipment and manufacturing method disclosed in Japanese Patent Publication No. 23 can be used.

In this embodiment, the metal plate 5 is made of SU
Although S 316 or SUS 316L is used, the invention is not limited to this, and various metal materials can be used as appropriate.

Further, in this embodiment, the metal plate 5 is disposed between the refractory insulating brick 4 and the castable refractory 6, but the metal plate 5 is arranged so as to prevent the contact between the molten zinc-aluminum and the steel shell 3. The plate 5 may be arranged, for example, the metal plate 5 may be arranged on the inner surface of the steel shell 3, or the metal plate 5 may be arranged inside the refractory insulating brick 4 or the castable refractory 6. In addition, the metal plate 5 is placed at a plurality of locations, specifically, at least on the inner surface of the steel shell 3, between the refractory heat insulating brick 4 and the castable refractory 6, and inside the refractory heat insulating brick 4 and the castable refractory 6. It may be provided at two places. By arranging a plurality of metal plates in this way, the durability of the molten zinc-aluminum bath can be further improved.

If the metal plate 5 arranged as described above is provided with a temperature sensor, when the molten zinc-aluminum comes into contact with the metal plate 5, the temperature sensor detects an increase in the temperature of the metal plate 5. Thus, before the molten zinc-aluminum comes into contact with the steel shell 3 (before the hot-dip zinc-aluminum plating bath 1 is found to have a life), the fire-resistant insulating brick 4
Alternatively, it is possible to know that the castable refractory 6 is broken such as a crack.

Further, it is possible to detect that the molten zinc-aluminum has come into contact with the metal plate 5 also by detecting the electric signal. Specifically, as shown in FIG.
Is the one detection end, and the other is connected to the ground. Here, since the molten zinc-aluminum in the bath falls to the ground, a direct current is applied between the metal plate 5 and the ground, and when the molten zinc-aluminum comes into contact with the metal plate 5, a short circuit occurs, and the current flows. As a result, the circuit breaker in the circuit operates, the indicator light turns on, and an alarm is issued. This indicates that the molten zinc-aluminum has come into contact with the metal plate 5, and before the molten zinc-aluminum comes into contact with the steel shell 3, breakage such as a crack has occurred in the refractory insulating brick 4 or the castable refractory 6. You can know that.

As described above, if the damage of the fire-resistant insulating brick 4 or the castable refractory 6 can be known at an early stage, the replacement or repair of the fire-resistant insulating brick 4 or the castable refractory 6 can be planned. A sudden stop of the equipment can be avoided. In addition, since the operation of the equipment can be performed systematically, the delivery date of the plated product can be secured,
A plated product can be manufactured efficiently.

In the present embodiment, the metal plate 5 subjected to the nitriding treatment is used. However, a metal plate which has not been subjected to the nitriding treatment or a metal plate covered with a metal coating may be used.
Here, a metal coating can be formed on the surface of the metal plate by thermal spraying the metal plate. As the metal coating, a self-fluxing alloy, WC, ceramic made of ZrO2-Al2O3, WC-CO cermet material, SiO2 -MgO
A composite ceramic or the like can be used. Even when using a metal plate that has not been subjected to nitriding or a metal plate covered with a metal coating, these metal plates can prevent erosion of molten zinc-aluminum and prevent holes in the steel shell. Therefore, the durability of the hot-dip zinc-aluminum plating bath can be improved.

(Second Embodiment) An induction heating apparatus according to a second embodiment of the present invention will be described below. FIG. 4 is a longitudinal sectional view of the induction heating device 2 in the present embodiment, and FIG.
Is a cross-sectional view of the induction heating device 2 in the present embodiment, and FIG. 6 is a partial cross-sectional view of the induction heating device 2 in the present embodiment.

A portion of the induction heating device 2 around the flow path 12 which comes into contact with the molten zinc-aluminum is covered with a castable refractory 13 (Shinagawa CAST-JAMCO-Y2, manufactured by Shinagawa White Brick Co., Ltd.). The refractory 13 is covered with a metal plate 15 (SUS 316 or SUS 316L) that has been subjected to a nitriding treatment. Further, the metal plate 15 is made of a fire-resistant and insulating brick 14 (Shinagawa ALT)
-P85, manufactured by Shinagawa White Brick Co., Ltd.). Further, the steel shell 11 is a casing 1 constituting an exterior of the induction heating device 2.
8 covered.

On the other hand, at the center of the induction heating device 2, FIG.
As shown in FIG. 2, an iron core 17 wound with an induction heating coil 17 'for heating molten zinc-aluminum is provided. The iron core 17 is covered with a bushing on which a copper pipe 20 for flowing cooling water to a copper plate 19 is attached. Further, on the surface of the copper plate 19 opposite to the surface on which the copper pipe 20 is disposed, a metal plate 15 that has been subjected to nitriding is disposed, and the metal plate 15 is formed of the castable refractory 1.
Covered with 3.

When the metal plate 15 subjected to the nitriding treatment is used, even if the molten zinc-aluminum erodes the castable refractory 13, the molten zinc-aluminum contacts the metal plate 15 to form
Of the aluminum, aluminum reacts with nitrogen to form a ceramic coating of aluminum nitride on the surface of the metal plate 15. The aluminum nitride ceramic coating and the metal plate 15 can prevent the molten zinc-aluminum from eroding the refractory and heat-insulating bricks 14, the steel shell 11 or the copper plate 19, and improve the durability of the induction heating device. it can.

In particular, in the induction heating apparatus 2 of the present embodiment, the temperature of the flow path 12 in contact with the molten zinc-aluminum is 700 to 800 ° C., which is higher than the temperature in the molten zinc-aluminum plating bath, and Since 13 is easily damaged, there is a high need to improve the durability of the induction heating device 2.

In this embodiment, the metal plate 15
SUS 316 or SUS 316L is used as the material, but the present invention is not limited to this, and various metal materials can be used as appropriate.

In the present embodiment, the metal plate 15
Is disposed between the refractory insulating brick 14 and the castable refractory 13, but the metal plate 15 is disposed between the iron shell 11 and the refractory insulating brick 14, or the refractory insulating brick 14 or the castable refractory 13 It may be arranged inside. In addition, the metal plate 15 is provided at a plurality of locations, specifically, at least on the inner surface of the steel shell 11, between the refractory insulating brick 14 and the castable refractory 13, and inside the refractory insulating brick 14 and the castable refractory 13. It may be provided at two places. FIG. 7 shows a longitudinal sectional view of an induction heating device in which a metal plate 15 is disposed between the steel shell 11 and the refractory heat-insulating brick 14 and inside the castable refractory 13. Thus, by arranging a plurality of metal plates, the durability of the induction heating device can be further improved.

Further, in this embodiment, the metal plate 1
5 is disposed between the copper plate 19 and the castable refractory 13, the metal plate 15 may be disposed inside the castable refractory 13, or the metal plate 15 may be disposed between the copper plate 19 and the castable refractory 13. It may be arranged in the space and inside the castable refractory 13 respectively.

Then, similarly to the first embodiment, the metal plate 1
5 is provided with a temperature sensor, or the metal plate 15 and the molten zinc-aluminum bath are connected by wiring to detect the contact of the molten zinc-aluminum with the metal plate 15 so that the refractory insulating brick 14 or the castable The damage of the refractory 13 can be known at an early stage.

On the other hand, in the present embodiment, the metal plate 15 subjected to the nitriding treatment is used. However, a metal plate not subjected to the nitriding treatment or a metal plate covered with a thermal spray coating may be used. Here, by spraying the metal plate, a thermal spray coating can be formed on the surface of the metal plate, and a self-fluxing alloy, WC, or the like can be used as the metal coating.

Even when a metal plate that has not been subjected to nitriding treatment or a metal plate whose surface has been covered with a coating by thermal spraying is used, the hot-dip zinc-aluminum-plated metal comes into contact with the iron shell 11 due to these metal plates. Since it is possible to prevent holes from being formed in the iron shell 11, it is possible to improve the durability of the induction heating device.

In the above-described embodiment, zinc-aluminum is used as the hot-dip metal, but the present invention is not limited to this. The present invention can be applied to other hot-dip metals as well. The same effect as the effect obtained can be obtained.

[0046]

According to the present invention, even if a molten metal enters a crack formed in a refractory such as a castable refractory or a refractory insulating brick, the metal plate prevents the molten metal from coming into contact with the steel shell. It can prevent the skin from piercing,
The durability of the hot-dip metal bath and the induction heating device can be improved.

Further, by providing a temperature detecting means or an electric signal detecting means on the metal plate to detect the contact of the molten metal with the metal plate, it is possible to detect the refractory before the molten metal comes into contact with the steel shell. You can know the damage. If it is possible to know the damage of refractory material at an early stage, it is possible to predict the life of a hot-dip plating bath and an induction heating device at an early stage.
Repair of a hot-dip plating bath and an induction heating device can be performed systematically. In addition, since the operation of the hot-dip metal plating facility can be performed systematically, the delivery date of the plated product can be ensured, and the production of the plated product can be performed efficiently.

Further, in the case where molten zinc-aluminum is used as the molten metal, if the metal plate is nitrided, when the molten zinc-aluminum comes into contact with the metal plate,
When aluminum reacts with nitrogen, a ceramic coating of aluminum nitride is formed on the surface of the metal plate, and this ceramic coating can also prevent molten zinc-aluminum from coming into contact with the steel shell. For this reason, the durability of the molten metal plating bath and the induction heating device can be further improved.

[Brief description of the drawings]

FIG. 1 is an external perspective view of a plating pot provided with a hot-dip zinc-aluminum plating bath and an induction heating device.

FIG. 2 is a partial cross-sectional view of a hot-dip zinc-aluminum plating bath according to the first embodiment.

FIG. 3 is a partial sectional view of a hot-dip zinc-aluminum plating bath according to the first embodiment.

FIG. 4 is a longitudinal sectional view of an induction heating device according to a second embodiment.

FIG. 5 is a cross-sectional view of an induction heating device according to a second embodiment.

FIG. 6 is a partial cross-sectional view of an induction heating device according to a second embodiment.

FIG. 7 is a longitudinal sectional view of an induction heating device provided with two metal plates 15.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Hot-dip zinc-aluminum plating bath 2 Induction heating device 3 Iron shell 4 Fireproof and insulating brick 5 Metal plate 6 Castable refractory 11 Iron shell 12 Channel 13 Castable refractory 14 Fireproof and insulating brick 15 Metal plate 17 Iron core 18 Casing 19 Copper plate 20 Copper pipe

Continuing from the front page (72) Inventor Masaki Takagusa 1-1-1 Nishiura, Funabashi-shi, Chiba F-term in the Funabashi Works of Taiyo Steel Corporation (reference) 3K059 AA08 AB16 AB26 AB27 AB28 AD03 AD30 AD34 BD19 CD63 4K027 AA02 AA22 AD04 AD25 AD29 4K056 AA05 BA02 BB07 CA04 FA13 FA19 4K063 FA35 FA38

Claims (10)

[Claims] 1. A hot-dip metal plating bath in which a shell formed in a container shape is covered with a refractory, wherein a metal plate is disposed inside the refractory or between the refractory and the steel. A bath for hot-dip metal plating characterized by the following. 2. The bath according to claim 1, further comprising a temperature detecting means for detecting a temperature change of the metal plate or an electric signal detecting means for detecting an electric signal. 3. The molten metal plating bath according to claim 1, wherein the refractory comprises a refractory heat insulating brick and a castable refractory. 4. The bath according to claim 1, wherein the metal plate is subjected to a nitriding treatment. 5. The bath according to claim 1, wherein the metal plate is covered with a metal film by thermal spraying. 6. An induction heating apparatus comprising: an outer periphery of a flow path through which a molten metal flows; and a refractory covering an outer periphery of the refractory with an iron shell. An induction heating device characterized in that a metal plate is arranged between a steel shell. 7. The induction heating apparatus according to claim 6, further comprising a temperature detecting means for detecting a temperature change of the metal plate or an electric signal detecting means for detecting an electric signal. 8. The induction heating apparatus according to claim 6, wherein the refractory comprises a refractory insulating brick and a castable refractory. 9. The induction heating apparatus according to claim 6, wherein the metal plate is subjected to a nitriding treatment. 10. The induction heating apparatus according to claim 6, wherein the metal plate is covered with a metal coating by thermal spraying.