JP2009266428A - Plasma heating device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a plasma heating device in which the lifetime of an insulating film to cover the external exposed face of a plasma torch is improved greatly without using a special material. <P>SOLUTION: Nozzle cylinders 12, 13 have nozzle outer cylinders 14, 15, nozzle inner cylinders 18, 19, and nozzle tips 20, 21 which close tips of spaces 32, 33 formed between the nozzle outer cylinders 14, 15 and the nozzle inner cylinders 18, 19, and insulating films 30, 31 for preventing a side arc in which a current flows directly between an anode plasma torch 10 and a cathode plasma torch 11 are formed on external exposed faces of the nozzle cylinders 12, 13. The insulating film 30 has an interlayer 30b formed of a mixture of a lowermost layer 30a and an uppermost layer 30c between the lowermost layer 30a composed of a bonding coat material such as Ni-Al and Ni-Cr and the uppermost layer 30a composed of alumina or spinel or mullite. The mixture has a ratio of the bonding coat material in a range of 1/3-2/3 in mass. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

The present invention relates to a twin torch type plasma heating apparatus for heating molten metal in a tundish by a plasma arc.

The molten metal in the tundish radiates heat constantly. When the capacity of the ladle is large and the casting time is long, or when the heating temperature of the molten metal is limited to be low depending on the steel type, the temperature of the molten metal in the tundish becomes the standard temperature or less from the middle of casting. Due to this temperature decrease, the immersion nozzle installed at the bottom of the tundish is clogged, or the separation of impurities (inclusions) is hindered. For this reason, the quality of the slab is impaired, and when the temperature of the molten metal is extremely lowered, the casting operation itself may be interrupted. For this reason, there is a method in which an anode plasma torch and a cathode plasma torch are arranged above the surface of the molten metal in the tundish, and a plasma arc is generated between each plasma torch and the molten metal to heat the molten metal (patent) (Refer to [Prior Art] in Document 1).

In the twin torch type plasma heating apparatus, as shown in FIG. 4, in order to prevent a side arc E in which a direct current flows between a pair of plasma torches 40 and 41, a nozzle outer cylinder constituting the outermost surface of the plasma torch is provided. Must be electrically insulated.
The nozzle outer cylinder is made of a metal such as stainless steel or copper, and the outer surface is usually sprayed with alumina Al ₂ O ₃ as an electrical insulating material (coating material). Although the nozzle outer cylinder is water-cooled from the inside, the coating material is worn out because it receives the radiant heat H generated by the plasma arc P generated from the other plasma torch at a temperature of 10000 ° C. or more (see FIG. 5). When the covering material is worn and a side arc E is generated between the pair of plasma torches 40 and 41, a high current flows through the nozzle outer cylinder, leading to melting of the nozzle outer cylinder and water leakage due to Joule heat generation. Therefore, conventionally, as a solution, Ni-Al, Ni-Cr, or Ni-Al-Cr is bonded to the nozzle outer cylinder, and alumina is sprayed thereon to increase the adhesion strength of the alumina.

JP 2003-266178 A

However, since the alumina of the coating material has a small thermal expansion coefficient of about 1/2 compared with the base material forming the nozzle outer cylinder and the bonding coat material covering the base material surface, a plasma torch is used at a high current of 5000 A or more. In such a case, there is a problem that the coating material falls off due to a large thermal stress acting on the coating material due to the difference in thermal expansion coefficient between the base material, the bonding coat material, and alumina. Then, due to the removal of the covering material, the electrical insulation between the pair of plasma torches was reduced, a side arc was generated, and water leakage occurred in the plasma torch.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a plasma heating apparatus that significantly improves the life of an insulating coating that covers the outer exposed surface of a plasma torch without using a special material. And

In order to achieve the above object, the present invention has an anode plasma torch and a cathode plasma torch each having an outer exposed surface coated with an insulating film, and current flows from the anode plasma torch to the cathode plasma torch via a molten metal. In the plasma heating apparatus that heats the molten metal by flowing a gas, the insulating coating is formed between the lowermost layer and the uppermost layer between a lowermost layer made of a bond coat material and an uppermost layer made of alumina, spinel, or mullite. An intermediate layer made of a mixture is formed.

The present invention relates to a thermal stress caused by a difference in thermal expansion coefficient between a base material that forms a nozzle outer cylinder of a plasma torch and a bonding coating material that covers the base material surface and alumina, spinel, or mullite that is a coating material. Therefore, an intermediate layer having a thermal expansion coefficient intermediate between the lowermost layer and the uppermost layer is formed between the lowermost layer and the uppermost layer.

Specifically, an intermediate layer having a thermal expansion coefficient between the lowermost layer and the uppermost layer is formed of a mixture of the lowermost layer and the uppermost layer. At that time, the ratio of the lowermost layer (bonding coating material) is 1/3 to 2 / in mass so that the thermal expansion coefficient of the mixture is approximately the median of the thermal expansion coefficient of the lowermost layer and the uppermost layer. A range of 3 is preferable, and 1/2 is more preferable.

In the plasma heating apparatus according to the present invention, it is preferable that the thickness of the intermediate layer is not less than the thickness of the lowermost layer and not more than the thickness of the uppermost layer.
Extending the life of the insulation coating by decreasing the coefficient of thermal expansion in the order of bottom layer> intermediate layer> top layer, and increasing the thickness of each layer in the order of bottom layer <intermediate layer <top layer It turned out that the effect of.

In the plasma heating apparatus according to the present invention, the lowermost layer, the intermediate layer, and the uppermost layer are preferably formed by thermal spraying. In the case of thermal spraying, there is an advantage that the application range of the workpiece and the spraying material is wide, and deformation and distortion due to thermal effects do not occur in the workpiece.

In the plasma heating apparatus according to the present invention, the insulating film covering the outer exposed surface of the plasma torch has a lowermost layer and an uppermost layer between the lowermost layer made of a bond coat material and the uppermost layer made of alumina, spinel, or mullite. Therefore, the thermal expansion coefficient of the base material that forms the nozzle outer cylinder of the outermost surface of the plasma torch and the lowermost layer that covers the surface of the base material and the uppermost layer that is the coating material The thermal stress resulting from the difference is reduced, and the covering material does not fall off. As a result, the life of the insulating coating can be greatly improved without using a special material.

Next, embodiments of the present invention will be described with reference to the accompanying drawings for understanding of the present invention. In the following description, for the part of the plasma torch, the side facing the molten metal is referred to as the “front” side for convenience.

FIG. 1 is a longitudinal sectional view of an anode plasma torch used in a plasma heating apparatus according to an embodiment of the present invention, and FIG. 2 is a longitudinal sectional view of a cathode plasma torch.
An anode plasma torch 10 and a cathode plasma torch 11 of a plasma heating apparatus according to an embodiment of the present invention are a bottomed cylindrical torch electrodes 22 and 23 and a cylindrical nozzle cylinder surrounding the torch electrodes 22 and 23, respectively. 12 and 13. Gaps provided between the torch electrodes 22, 23 and the nozzle cylinders 12, 13 serve as argon gas supply paths 26, 27.

The tip portions of the torch electrodes 22 and 23 of the anode plasma torch 10 and the cathode plasma torch 11 are discharge electrode portions 28 and 29, respectively, and generate a plasma arc with the molten metal. The shapes of the discharge electrode portions 28 and 29 are different between the anode plasma torch 10 and the cathode plasma torch 11, the discharge electrode portion 28 of the anode plasma torch 10 is a flat surface, and the discharge electrode portion 29 of the cathode plasma torch 11 is convex. It has a shape. In addition, partition tubes 24 and 25 are inserted inside the torch electrodes 22 and 23 so that the cooling water circulates.

As a material used for the discharge electrode portions 28 and 29, it is preferable to use copper Cu or a composition obtained by adding one or more of tungsten W, chromium Cr, nickel Ni, zircon Zr, cobalt Co, etc. to copper Cu. it can.

The nozzle cylinders 12 and 13 include nozzle outer cylinders 14 and 15, nozzle inner cylinders 18 and 19, and tip portions of space portions 32 and 33 formed between the nozzle outer cylinders 14 and 15 and the nozzle inner cylinders 18 and 19. It has a double tube structure having nozzle tip portions 20 and 21 for closing the nozzle. In the space portions 32 and 33, partition tubes 16 and 17 are inserted, and cooling water is circulated. As the material of the nozzle cylinders 12 and 13, stainless steel such as SUS304, SUS304-L, SUS316, and SUS316-L, copper, or the like can be used.

In addition, current is directly applied between the anode plasma torch 10 and the cathode plasma torch 11 on the outer exposed surfaces of the nozzle cylinders 12 and 13 (the outer peripheral surfaces of the nozzle outer cylinders 14 and 15 and the tip surfaces of the nozzle tip portions 20 and 21). Insulating coatings 30 and 31 are formed to prevent side arcs flowing through.

Although the cross section of the insulating coating 30 is shown in FIG. 3, the structure of the insulating coating 31 is also the same.
The insulating coating 30 has a lowermost layer 30a and an uppermost layer between a lowermost layer 30a made of a bonding coat material and an uppermost layer 30c made of alumina Al ₂ O _3, spinel MgAl ₂ O _4, or mullite 3Al ₂ O ₃ .2SiO _2. It has the intermediate | middle layer 30b which consists of a mixture with 30c. In the mixture, the ratio of the bonding coat material is in the range of 1/3 to 2/3 by mass.
As the bonding coat material, conventionally used Ni—Al, Ni—Cr, or Ni—Al—Cr can be used.

The intermediate layer 30b reduces thermal stress caused by the difference in thermal expansion coefficient between the base material forming the nozzle outer cylinders 14 and 15, the lowermost layer 30a covering the base material surface, and the uppermost layer 30c as the covering material. By using a mixture of the lowermost layer 30a and the uppermost layer 30c, the intermediate layer 30b having a thermal expansion coefficient intermediate between the lowermost layer 30a and the uppermost layer 30c is formed.

Incidentally, the coefficient of thermal expansion [unit: × 10 ⁻⁶ / ° C.] of the material used for the plasma torch is as follows.
Stainless steel: 16 to 17, copper: 16.8, Ni—Al: 13 to 15, Ni—Cr: 15.5 to 16.5, Ni—Al—Cr: 15 to 16, alumina: 6.5 to 7. 5, spinel: 6-7, mullite: 5.5-6

The lowermost layer 30a, the intermediate layer 30b, and the uppermost layer 30c are preferably applied by a thermal spraying method such as a plasma spraying method. In particular, in the case of the plasma spraying method, in addition to being able to perform thermal spraying in the atmosphere, the formed film has a high density and high adhesion to the base material.

Although one embodiment of the present invention has been described above, the present invention is not limited to the configuration described in the above-described embodiment, and is within the scope of matters described in the claims. Other possible embodiments and modifications are also included. For example, in the above embodiment, the intermediate layer is one layer, but a plurality of intermediate layers having different ratios of the bonding coat material may be provided.

A plurality of plasma torches used in the plasma heating apparatus according to the present invention and a plasma torch having a conventional coating were manufactured, and a plasma heating test was conducted. The specifications of the two types of specimens used in the test are shown below.
(1) Plasma torch nozzle outer cylinder used in the plasma heating apparatus according to the present invention: SUS304-L, thickness 3 mm
-Bottom layer: Ni-Al, thickness 0.1 mm
Intermediate layer: Alumina and Ni-Al mixture, 0.2 mm
-Top layer: Alumina, thickness 0.5mm

(2) Plasma torch nozzle outer cylinder having conventional coating: SUS304-L, thickness 3 mm
Bonding coat: Ni-Al, thickness 0.1mm
・ Coating material: Alumina, thickness 0.5mm

The molten steel in the tundish was heated by supplying a current of 5000 to 7000 A to each of the two types of test bodies. As a result, the plasma torch having the conventional coating caused water leakage of the nozzle in 25 to 55 hours, but the plasma torch used in the plasma heating apparatus according to the present invention leaked the nozzle in 85 to 115 hours. It was confirmed that it has a life of 2 to 3 times that of a plasma torch having a conventional coating.

It is a longitudinal cross-sectional view of the anode plasma torch used for the plasma heating apparatus which concerns on one embodiment of this invention. It is a longitudinal cross-sectional view of the cathode plasma torch used for the same plasma heating apparatus. It is a partial longitudinal cross-sectional view of the plasma torch outer cylinder part of the plasma heating apparatus. It is explanatory drawing of the side arc which generate | occur | produces between an anode plasma torch and a cathode plasma torch. It is explanatory drawing of the radiant heat received from the other plasma torch.

Explanation of symbols

10: Anode plasma torch, 11: Cathode plasma torch, 12, 13: Nozzle cylinder, 14, 15: Nozzle outer cylinder, 16, 17: Partition cylinder, 18, 19: Nozzle inner cylinder, 20, 21: Nozzle tip, 22, 23: Torch electrode, 24, 25: Partition tube, 26, 27: Supply path, 28, 29: Discharge electrode part, 30, 31: Insulating coating, 30a: Lowermost layer, 30b: Intermediate layer, 30c: Uppermost layer 32, 33: Space part, 40, 41: Plasma torch

Claims (4)

Plasma heating that has an anode plasma torch and a cathode plasma torch each having an outer exposed surface coated with an insulating coating, and heats the molten metal by passing a current from the anode plasma torch to the cathode plasma torch via the molten metal In the device
The insulating coating is characterized in that an intermediate layer made of a mixture of the lowermost layer and the uppermost layer is formed between the lowermost layer made of a bonding coating material and the uppermost layer made of alumina, spinel or mullite. Plasma heating device. 2. The plasma heating apparatus according to claim 1, wherein the mixture has a mass ratio of the bonding coating material of 1/3 to 2/3. 3. 3. The plasma heating apparatus according to claim 1, wherein the thickness of the intermediate layer is greater than or equal to the thickness of the lowermost layer and less than or equal to the thickness of the uppermost layer. The plasma heating apparatus according to any one of claims 1 to 3, wherein the lowermost layer, the intermediate layer, and the uppermost layer are formed by thermal spraying.

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