JP7492430B2 - Swinging duct and its manufacturing method - Google Patents

Description

The present invention relates to a swing duct and a manufacturing method thereof, and more specifically to a swing duct for sending exhaust gas from a steelmaking furnace to a dust collector, and a manufacturing method thereof.

Exhaust gas discharged from steelmaking furnaces such as electric furnaces is sucked in by a dust collector, where harmful gases and dust are removed before being released into the atmosphere. The steelmaking furnace and the dust collector are connected by a duct, which is often installed horizontally. As the dust contained in the exhaust gas discharged from the steelmaking furnace is often coarse, coarse dust tends to accumulate in horizontally installed ducts. When dust accumulates in the duct, the cross-sectional area inside the duct decreases, and the exhaust efficiency decreases. For this reason, hatches are installed in the duct, and after the steelmaking furnace is shut down, the dust accumulated in the duct is periodically removed using a scraper. However, this method results in longer shutdown times for the steelmaking furnace, which reduces operating efficiency.

In order to solve this problem, various proposals have been made in the past.
For example, Patent Document 1 discloses a dust accumulation prevention device for an electric furnace smoke collection duct, in which a horizontally extending smoke collection duct is constructed as a separate body from other parts, the smoke collection duct is supported so that it can rotate about its axis, and a turning device is further provided which rotates the smoke collection duct alternately left and right about its axis.

The same document states:
(A) There is a bias in the flow of exhaust gas inside the smoke collection duct, and the flow velocity of the exhaust gas is faster at the top of the smoke collection duct than at the bottom. Therefore, when the smoke collection duct is rotated, the dust that has settled on the bottom of the smoke collection duct moves upward and is pushed by the strong exhaust gas flow and sent downstream.
(B) As the smoke collection duct is rotated, the dust that has settled on the bottom of the duct moves upward together with the bottom, and when it is raised to a certain height, the dust slides down toward the bottom due to its own weight. At this time, the dust is pushed by the flow of exhaust gas and sent downstream; and
(C) It is described that this prevents dust from accumulating on the bottom surface of the smoke collection duct.

If a steelmaking furnace and a dust collector are connected using a duct that can rotate left and right around an axis (hereinafter, such a duct will be referred to as a "swinging duct"), the accumulation of dust in the duct can be suppressed. Since a swinging duct can send even large dust particles downstream, it is suitable as a duct for connecting a large steelmaking furnace and a large dust collector.
When a large steelmaking furnace and a large dust collector are connected by a swing duct, the swing duct also has a large diameter. In this case, since it is physically difficult to connect the steelmaking furnace and the dust collector with a single swing duct, it is usually practiced to connect multiple swing ducts that are relatively short in length. Hereinafter, a duct in which multiple swing ducts are connected is also referred to as a "dust collection connection duct."

However, since the exhaust gas discharged from the steelmaking furnace is at a high temperature, the load on the abrasion of the oscillating duct is large. In particular, the end face of the oscillating duct (i.e., the connecting portion of the oscillating duct) is easily abraded at a very high rate by the hot dust.
Measures to extend the lifespan of ducts generally involve increasing the thickness of the end panels or changing the material or shape, but these measures are insufficient in terms of workability and durability.
Alternatively, it is possible to form a coating made of a heat-resistant and abrasion-resistant material by using various methods on the portion where abrasion resistance is required. However, it is generally not easy to form such a coating on the end face of a large-diameter swing duct.

Japanese Patent Application Laid-Open No. 06-194069

The problem to be solved by the present invention is to provide a swing duct having a highly wear-resistant end surface and a manufacturing method thereof.
Another problem to be solved by the present invention is to provide a swing duct having a large diameter and an end face with high wear resistance, and a manufacturing method thereof.

In order to solve the above problems, the swing duct according to the present invention has the following configuration.
(1) The swing duct is
A cylindrical body having a water-cooling structure;
A cooling pipe formed on an end surface of the main body;
and a laser clad layer formed on the surface of the cooling pipe.
(2) The body is made of general structural steel (A),
The cooling pipe is made of general structural steel (B) or carbon steel,
The laser clad layer is made of a Co-based alloy or a Ni-based alloy and contains 0 mass % to 60 mass % of dispersed metal carbides.

The manufacturing method of the swing duct according to the present invention includes the steps of:
A first step of preparing a cylindrical body having a water-cooling structure and a cooling pipe;
a second step of forming a laser clad layer on a surface of the cooling pipe by using a laser clad method;
and a third step of joining the cooling pipe to the end face of the main body to obtain the swing duct according to the present invention.

When forming a coating made of a Co-based alloy or Ni-based alloy on the surface of a cooling tube, using the laser clad method as the coating formation method can obtain a coating (laser clad layer) with high adhesion and minimize the heat-affected zone near the laser clad layer. As a result, even if the main body of the oscillating duct has a large diameter, the cooling tube on which the laser clad layer is formed can be joined to the end face of the main body. This also improves the wear resistance against high-temperature dust. Furthermore, the improved wear resistance of the oscillating duct reduces the frequency of repairs of the oscillating duct, and the maintenance costs of the oscillating duct can be reduced.

This is a front view (right figure) of the oscillating duct of the present invention and its cross-sectional view (left figure) along line A-A'. FIG. 1 is a schematic diagram of a laser cladding method. FIG. 1 is a schematic diagram of a connected dust collection duct in which multiple swing ducts are connected.

An embodiment of the present invention will be described in detail below.
[1. Swinging duct]
FIG. 1 shows a front view (right) of a swing duct according to the present invention and a cross-sectional view (left) taken along line AA′ thereof. In FIG. 1, the swing duct 10 is
A cylindrical main body 20 equipped with a water-cooling structure;
A cooling pipe 30 formed on an end surface of the main body 20;
A laser clad layer 32 is formed on the surface of the cooling pipe 30.

[1.1. Main body]
[1.1.1. material]
In the present invention, the main body 20 is made of general structural steel (A) (SS). SS is:
(a) Excellent can forming properties (bending, welding),
(b) It is widely available in the market and inexpensive;
For these reasons, SS is suitable as the material for the main body 20.

[1.1.2. Water-cooled structure]
High-temperature exhaust gas discharged from a steelmaking furnace flows inside the main body 20. Therefore, the main body 20 needs to have a water-cooling structure. The water-cooling structure is not particularly limited, and may be any structure depending on the purpose. In FIG. 1, the inside of the side wall of the main body 20 is hollow, and cooling water can be flowed into the hollow. Exhaust gas and dust 50 flow inside.

In other words, in FIG. 1, the main body 20 comprises an inner cylinder 22 and an outer cylinder 24. A spiral rib 22a is formed on the outer surface of the inner cylinder 22 (the surface facing the outer cylinder 24). The space between adjacent ribs 22a, 22a is a cooling water passage 22b for flowing cooling water. An inlet 24a for introducing cooling water from a cooling water supply source (not shown) into the cooling water passage 22b and an outlet 24b for discharging cooling water from the cooling water passage 22b to the outside are provided on the outer surface of the outer cylinder 24 (the surface not facing the inner cylinder 22).

Inner diameter
In the present invention, the inner diameter of the main body 20 is not particularly limited. In general, the larger the inner diameter of the duct, the larger the size of the cooling pipe formed on the end face. If the size of the cooling pipe is large, the construction for forming a film on the surface of the cooling pipe may become difficult. In addition, if the size of the cooling pipe is large, the radius of curvature also becomes large accordingly, and the cooling pipe may be significantly deformed during the film formation process, making it difficult to join the main body and the cooling pipe.
In contrast, in the present invention, the cooling pipe 30 is easily joined after the film is formed, even if the main body 20 has a large inner diameter, because the deformation of the cooling pipe 30 during the film formation process is small. By using the method described later, even if the main body 20 has an inner diameter of 1000 mm or more, or 1500 mm or more, a swing duct 10 can be manufactured.

[1.2. Cooling pipe]
[1.2.1. material]
The cooling pipe 30 is for cooling the end surface of the main body 20 and is formed on the end surface of the main body 20. A part of the cooling water flowing inside the main body 20 is distributed inside the cooling pipe 30. It looks like this.
The material of the cooling pipe 30 is not particularly limited, and an optimum material can be selected depending on the purpose. Examples of the material of the cooling pipe 30 include general structural steel (B), carbon steel ( For example, there are carbon steel pipes for pressure piping STPG370 and carbon steel pipes for high temperature piping STPT370. When the cooling pipe 30 is made of general structural steel (B), the general structural steel (B) is used to form the main body 20. The material may have the same composition as the general structural steel (A) to be used, or may have a different composition.

[1.2.2. shape]
In FIG. 1, the cooling pipe 30 has a shape of a hollow ring cut in half, but this is merely an example, and the cross-sectional shape of the cooling pipe 30 is not particularly limited.
The cooling pipe 30 may be an integral body, or may be made up of a plurality of arc-shaped segments arranged in the circumferential direction and joined together.

[1.3. Laser clad layer]
[1.3.1. material]
A laser clad layer 32 is formed on the surface of the cooling pipe 30. In the present invention, the laser clad layer 32 is made of a Co-based alloy or a Ni-based alloy and contains 0 mass% to 60 mass% of dispersed metal carbides. Both the Co-based alloy and the Ni-based alloy have higher high-temperature hardness and higher wear resistance against high-temperature dust than general structural steel (B) or carbon steel, and are therefore suitable as materials for forming the laser clad layer 32 that covers the surface of the cooling pipe 30.

Here, the term "Co-based alloy" refers to an alloy in which the metallic element that accounts for the largest proportion by mass is Co.
The term "Ni-base alloy" refers to an alloy in which the metallic element that accounts for the largest proportion by mass is Ni.

The laser clad layer 32 may or may not contain a metal carbide, but the wear resistance is further improved when the laser clad layer 32 contains a metal carbide. Examples of the metal carbide include tungsten carbide, molybdenum carbide, chromium carbide, titanium carbide, niobium carbide, tantalum carbide, hafnium carbide, vanadium carbide, and zirconium carbide.
When the laser clad layer 32 contains a metal carbide, if the content of the metal carbide is too small, the effect of improving the wear resistance is difficult to be achieved. Therefore, the content of the metal carbide is preferably 1 mass% or more. The content of the metal carbide is preferably 5 mass% or more.
On the other hand, if the content of the metal carbide is excessive, cracks are likely to occur due to internal stress. Therefore, the content of the metal carbide is 60 mass% or less. The content of the metal carbide is preferably 30 mass% or less.

Examples of Co-based alloys include Stellite 1, Stellite 6, Stellite 12, Stellite 21, Stellite F, Stellite X-40, Tribaloy T-400, and Tribaloy T-800.
Examples of Ni-based alloys include Hastelloy B, Hastelloy C-276, Hastelloy W, Hastelloy X, Inconel X, Inconel 625, Inconel 700, and Inconel 718.

1.2.2. Thickness
It is preferable to select an optimum thickness for the laser clad layer 32 depending on the purpose. In general, if the thickness of the laser clad layer 32 is too thin, the durability decreases. Therefore, the thickness of the laser clad layer 32 is preferably 0.5 mm or more. The thickness is preferably 1.0 mm or more.
On the other hand, if the thickness of the laser clad layer 32 is made thicker than necessary, cracks are likely to occur due to internal stress. Therefore, the thickness of the laser clad layer 32 is preferably 3.0 mm or less. The thickness is preferably 2.0 mm or less.

[2. Manufacturing method of swing duct]
The manufacturing method of the swing duct according to the present invention includes the steps of:
A first step of preparing a cylindrical body having a water-cooling structure and a cooling pipe;
a second step of forming a laser clad layer on a surface of the cooling pipe by using a laser clad method;
and a third step of joining the cooling pipe to the end face of the main body to obtain the swing duct according to the present invention.

[2.1. First step]
First, there are prepared the cylindrical main body 20 having a water-cooling structure, and the cooling pipe 30. Details of the main body 20 and the cooling pipe 30 are as described above, and therefore will not be described here.

[2.2. Second step]
Next, a laser clad layer 32 is formed on the surface of the cooling pipe 30 by using a laser clad method.
FIG. 2 shows a schematic diagram of the laser cladding method. In the laser cladding method, a nozzle 40 having a double-tube structure is used. The nozzle 40 has an outer tube 42 and a nozzle hole 43 disposed inside the outer tube 42. The apparatus includes an inner tube 44 and a condenser lens 46 disposed within the inner tube 44. An inert gas can be supplied into the inner tube 44 as a shielding gas to prevent oxidation. In addition, the gap between the outer tube 42 and the inner tube 44 is capable of supplying the powdered material 48 and a carrier gas for transporting the powdered material 48 .

Specifically, the formation of the laser clad layer 32 using the laser clad method is performed as follows. That is, first, a nozzle 40 is placed above the surface of the cooling tube 30. Next, material powder 48 and carrier gas are supplied into the gap between the outer tube 42 and the inner tube 44, and shielding gas is supplied into the inner tube 44. In this state, when a laser beam is irradiated toward the condenser lens 46, the material powder 48 and the surface portion of the cooling tube 30 melt and become one with each other near the focus of the laser beam. When the nozzle 40 moves, the melted and integrated liquid solidifies to become the laser clad layer 32.

At this time, by adjusting the laser output, the distance between the cooling tube 30 and the tip of the nozzle 40, the supply amount of material powder 48, and/or the movement speed of the nozzle 40, it is possible to selectively melt only the material powder 48 and the surface portion of the cooling tube 30 while minimizing the thermal effects on the cooling tube 30, and further to adjust the thickness of the laser clad layer 32.

[2.3. Third step]
Next, the cooling pipe 30 is joined to the end surface of the main body 20 (third step). In this way, the swing duct 10 according to the present invention is obtained. The method of joining the main body 20 and the cooling pipe 30 is not particularly limited. Rather, the most suitable method can be selected according to the purpose.

[3. Action]
The swing duct for connecting a large steelmaking furnace and a large dust collector is usually large in diameter. In this case, connecting the steelmaking furnace and the dust collector with an integrated swing duct is physically difficult. Since it is difficult to connect the oscillating ducts, it is common to connect a plurality of oscillating ducts each having a relatively short length.

Figure 3 shows a schematic diagram of a connected dust collection duct in which multiple oscillating ducts are connected. As shown in Figure 3, a connected dust collection duct 1 is placed between a large steelmaking furnace (not shown) and a dust collector (not shown). The connected dust collection duct 1 consists of multiple oscillating ducts 10, 10... each having a predetermined length arranged in the horizontal direction. Each of the oscillating ducts 10, 10... can be oscillated around an axis using a drive device (not shown).

Adjacent swing ducts 10, 10 are fastened together using various fastening means (not shown). Examples of fastening methods include:
(a) A method of providing a flange at each end of the main body 20 of each of the swing ducts 10, 10, ... and fastening adjacent flanges together with bolts and nuts;
(b) There is a method of wrapping a belt around the gap between adjacent main bodies 20.

When exhaust gas is passed through the connected oscillating ducts 10, 10... while they are oscillating around their axis, the large, high-temperature dust 50, 50... can be sent downstream (from the steelmaking furnace to the dust collector). However, since it is physically difficult to completely eliminate the gaps at the connection points of the large-diameter oscillating ducts 10, 10..., the high-temperature dust 50, 50... may enter the gaps between the oscillating ducts 10, 10 during use. Alternatively, the high-temperature dust 50, 50... may collide with the end faces of the oscillating ducts 10, 10.... Therefore, the surface of the cooling pipe 30 formed on the end faces of the oscillating ducts 10, 10... is particularly prone to being worn down very quickly by the high-temperature dust 50, 50....

In order to solve this problem, it is conceivable to spray a heat-resistant and wear-resistant material onto the surface of the cooling pipe 30. However, a sprayed film generally has low adhesion and therefore poor durability.
On the other hand, when a self-fluxing alloy spraying method is used in which a self-fluxing alloy powder is sprayed and then fusing is performed, a coating with high adhesion can be formed on the surface of the cooling pipe 30. However, when the main body 20 of the oscillating duct 10 has a large diameter and the cooling pipe 30 has a large size, it may be physically difficult to perform heat treatment for fusing. Even if heat treatment is possible, the larger the radius of curvature of the cooling pipe 30, the greater the deformation after heat treatment, so that it may be difficult to join the main body 20 and the cooling pipe 30. In particular, when the cooling pipe 30 is divided into a plurality of segments, if a large deformation occurs after heat treatment, it may be physically difficult to join the segments to be integrated.
This problem can also occur when a coating is formed by a weld overlay method such as Tig welding or PTA.

In contrast, when a coating made of a Co-based alloy or Ni-based alloy is formed on the surface of the cooling pipe 30, if the laser clad method is used as the coating formation method, a coating (laser clad layer 32) with high adhesion can be obtained, and the heat-affected zone near the laser clad layer 32 can be minimized. As a result, even if the main body 20 of the oscillating duct 10 has a large diameter, the cooling pipe 30 on which the laser clad layer 32 is formed can be joined to the end face of the main body 20. This also improves the wear resistance against high-temperature dust. Furthermore, the improved wear resistance of the oscillating duct 10 reduces the frequency of repairs to the oscillating duct 10, and the maintenance costs of the oscillating duct 10 can be reduced.

(Examples 1 to 13, Comparative Examples 1 and 2)
1. Preparation of Samples
A coating was formed on the surface of the substrate using various coating materials. General structural steel (SS400) was used as the substrate. Co-based alloys (Examples 1-3) and Ni-based alloys (Examples 4-13, Comparative Example 2) were used as the coating materials. Tungsten carbide was used as the metal carbide (Examples 3, 6-13, Comparative Example 2). The coating was formed by a laser cladding method. In addition, a flat plate of general structural steel (SS400) was prepared as a comparative material (Comparative Example 1).

2. Test Method
[2.1. Film Thickness]
After coating, the samples were cut and the thickness of the laser clad layer was measured using an optical microscope.
[2.2. hardness]
The Vickers hardness of the laser clad layer was measured at room temperature.

[2.3. Presence or absence of cracks]
After the film was formed, the presence or absence of cracks on the surface of the film was confirmed by visual inspection. The meanings of the evaluation indexes are as follows:
A: No cracks were observed.
△: Several cracks were present.

2.4. Corrosion resistance
A salt spray test was conducted to visually confirm the occurrence of rust on the coating surface. The test was a neutral salt spray test in accordance with JIS standard Z2371:2015. The test time was 300 hours. The meanings of the evaluation indexes for corrosion resistance are as follows:
○: No rust was observed.
△: Rust was partially observed.
x: Rust was observed all over the surface.

2.5. Abrasion test
A Suga abrasion test was conducted in accordance with JIS standard H8503:1989 to evaluate the abrasion resistance of the coating. #320SiC was used as the abrasive paper. The load was 3.25 kg. The number of reciprocations was 2400. After the test, the mass of abrasion powder was measured.

[3. result]
[3.1. Film thickness, hardness, crack generation, corrosion resistance, and abrasion resistance]
The results are shown in Table 1. From Table 1, the following can be seen.
(1) Examples 1 to 13 were superior to Comparative Example 1 in both corrosion resistance and wear resistance.
(2) Examples 3, and 6 to 13 were particularly excellent in wear resistance. This is believed to be due to the presence of an appropriate amount of metal carbide dispersed in the coating.
(3) In Examples 1 to 8 and 10, the occurrence of cracks was particularly small. This is because the influence of internal stress was suppressed due to the thin film thickness of the coating or the low content of metal carbide. it is conceivable that.
(4) Examples 1, 4, and 10 had better corrosion resistance than Examples 11 and 12. This is because Examples 1, 4, and 10, unlike Examples 11 and 12, had less cracking. This is probably because there was no
(5) Comparative Example 1 was inferior to Examples 1 to 13 in both corrosion resistance and wear resistance.
(6) In Comparative Example 2, a coating was formed under conditions in which the content of metal carbide was 70 mass% of the entire coating. However, numerous cracks were generated and a part of the coating peeled off from the substrate. The reason for the cracks is believed to be that the content of metal carbides was too high, which caused the internal stress to become too large.

[3.2. Actual device verification]
An actual machine verification was carried out with the specifications of Example 4 and Example 10. As a result, almost no wear was observed at the time of the service life of 12 months for SS400 (corresponding to Comparative Example 1), and the effect of improving wear resistance was confirmed. When Example 4 and Example 10 were compared, some wear was observed in Example 4 by visual observation, but no wear was observed at all in Example 10.

Although the embodiment of the present invention has been described in detail above, the present invention is not limited to the above embodiment, and various modifications are possible without departing from the gist of the present invention.

The oscillating duct of the present invention can be used as a duct connecting various steelmaking furnaces and dust collectors.

Claims (5)

A swinging duct having the following configuration:
(1) The swing duct is
A cylindrical body having a water-cooling structure;
A cooling pipe formed on an end surface of the main body;
and a laser clad layer formed on the surface of the cooling pipe.
(2) The body is made of general structural steel (A),
The cooling pipe is made of general structural steel (B) or carbon steel,
The laser clad layer is made of a Co-based alloy or a Ni-based alloy and contains 0 mass % to 60 mass % of dispersed metal carbides. The oscillating duct according to claim 1, wherein the content of the metal carbide is 5 mass% or more and 30 mass% or less. The oscillating duct according to claim 1 or 2, wherein the thickness of the laser clad layer is 0.5 mm or more and 3.0 mm or less. The oscillating duct according to any one of claims 1 to 3, wherein the inner diameter of the main body is 1000 mm or more. A first step of preparing a cylindrical body having a water-cooling structure and a cooling pipe;
a second step of forming a laser clad layer on a surface of the cooling pipe by using a laser clad method;
and a third step of joining the cooling pipe to the end face of the main body to obtain the oscillating duct according to any one of claims 1 to 4.

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