KR20020051089A - Thermal Spray Method of Oxide Coatings for Rolls used in Molten Zinc Pot - Google Patents

Abstract

PURPOSE: A method for coating oxides on a roll is provided to extend a life cycle of the coating layer by forming an appropriate under coating layer in applying thermal spray coating of oxides with strong corrosion resistance against molten zinc onto the matrix of the roll, thereby preventing defoliation of a coating layer. CONSTITUTION: In a method for coating oxides on a roll dipped in a hot dip galvanizing pot of a hot dip galvanizing plant, the method for coating oxides on the roll used in the hot dip galvanizing pot is characterized in that a WC-Co thermal spray coating layer(21) is formed between the matrix(1) of the roll and an oxide coating layer(4) as an under coating layer, wherein one or more boron compounds comprising WB(tungsten boride), CrB(chromium boride) and TiB(titanium boride) are added to coating constituents forming the WC-Co thermal spray coating layer(21) that is the under coating layer, and wherein W, Cr, or a mixture thereof is added to Co acting as a binder in the WC-Co thermal spray coating layer(21) that is the under coating layer.

Description

Thermal Spray Method of Oxide Coatings for Rolls used in Molten Zinc Pot}

The present invention relates to an oxide coating method of a roll used in a hot dip galvanizing bath, and more particularly, to a method of applying an oxide coating to various rolls used in a hot dip galvanizing bath in a hot dip galvanizing plant. The present invention relates to a method for coating an oxide of a roll to prevent the peeling of the coating layer to increase the service life of the coating by forming an appropriate undercoat layer in applying a spray coating of an oxide having strong corrosion resistance to zinc.

In the hot dip galvanizing plant, a hot dip zinc bath is installed to plate zinc on the surface of the steel sheet so that the steel sheet passes therethrough. In the molten zinc bath, three rolls are usually installed to guide the steel sheet passing through the zinc bath. These rolls are usually made of stainless steel, which is eroded by the reaction of molten zinc, causing defects on the surface of the plated product. Accordingly, in order to keep the surface of the roll clean and to extend the life of the roll, the surface of the roll is generally spray coated.

The most commonly used thermal spray coating is WC-Co-based coating material, because cobalt (Co) contained in the coating component is eroded by zinc (Zn), thereby improving the corrosion resistance of the coating layer and also In order to improve the characteristics, the following techniques have been proposed as prior art.

That is, a method of mixing WB with a coating of WC (JP948114), and coating a material such as WB or MoB using physical vapor deposition (PVD), chemical vapor deposition (CVD) or thermal spraying (JP94299310), MSi ₂ (JP94228723) and the like using a compound such as (M = Cr, Mo, Ta, Nb, W, Zr, Ti, V) are known as prior arts.

However, the physical vapor deposition method or the chemical vapor deposition method of the above method has a problem that it is difficult to apply realistically because the size of the roll is too large when applied to the roll of the hot dip galvanizing bath, for this reason it can be said that the spray coating method is the most suitable coating method. .

The coating material is ideally a material that is not eroded or reacted by molten zinc. In general, oxides based on alumina, zirconia, etc. do not react with zinc, so they are good materials, and a technique of combining various kinds of oxides is also known. (JP94-330278)

However, since oxides generally have different coefficients of thermal expansion from the roll metals, nickel-based alloys (eg Ni-Cr alloys) or cobalt-based alloys (eg For example, Co-Cr alloy) may be used as the undercoat. In addition, in order to further improve resistance to thermal shock, a coating layer 3 containing a mixture of metal and oxide may be formed between the metal undercoat 2 and the oxide coating layer 4 as shown in FIG. 1. This is because when the coating layer is formed in this manner, the thermal shock applied to the oxide coating layer 4 can be alleviated, thereby improving the thermal shock resistance of the coating layer.

By the way, the following problems occurred when applying the coating layer as described above.

The coating layer having the structure as shown in FIG. 1 was applied to a sink roll, which is one of the rolls immersed in zinc, and after 10 days of use, peeling occurred in the coating layer, thereby preventing its use. Figure 2 is a photograph of the surface of the coating layer peeled off after applying the coating layer of Figure 1 to the sink roll. As shown in the photograph, it was observed that the coating layer was peeled off at various places on the roll surface.

In the case of sink rolls, shallow grooves are provided between the steel sheets and the rolls so that the molten zinc can escape well. This is called a groove. Investigation of the cause of the peeling of the coating layer revealed that the adhesion of the coating layer was not due to the thermal shock. In particular, referring to FIG. 1, the site where peeling occurred was determined to be an interface between the oxide coating layer 4 and the mixed coating layer 3 of the oxide and the metal. Such peeling occurred due to the frictional force applied between the steel sheet and the roll, and particularly, peeling started because the frictional force of the groove could not be tolerated.

In particular, in the case of forming the coating layer as described above, once the peeling occurs, the mixed coating layer 3 of oxide and metal, which is less corrosion resistant to zinc than the oxide layer, is exposed to zinc, so that the oxide coating layer 4 and the oxide and metal are mixed. As the zinc continues to erode along the interface of the coating layer 3, a problem arises in that the exfoliation site becomes wider.

The present invention has been made to solve the above problems, the object of which is to apply an oxide coating to various rolls used in the molten zinc bath of the hot dip galvanizing plant, the corrosion resistance to the molten zinc in the roll base material When applying the thermal spray coating of the oxide to form an appropriate undercoat layer in the middle, to provide an oxide coating method of the roll to prevent the peeling of the oxide coating layer to increase the life of the coating.

1 is a schematic cross-sectional view showing the structure of a typical oxide coating layer,

Figure 2 is a photograph of the surface of the coating layer peeled off after applying the coating layer of Figure 1 to the sink roll,

Figure 3 is a schematic cross-sectional view showing the structure of the coating layer by the oxide coating method of the present invention,

Figure 4a is a photograph showing the shape after the thermal shock test of the coating layer specimen having the structure of the general oxide coating layer of Figure 1,

Figure 4b is a photograph showing the shape after the thermal shock test of the coating layer specimen to which the oxide coating method of the present invention of Figure 3 is applied.

<Description of Symbols for Main Parts of Drawings>

1: Base material 2: Metallic undercoat

3: oxide-metal mixed coating layer 4: oxide coating layer

11: peeling part of coating layer 21: WC-Co undercoat layer

The present invention for achieving the above object, in the method of coating the oxide on the roll immersed in the molten zinc bath of the hot-dip galvanizing plant, in the middle between the base material (1) and the oxide coating layer (4) of the roll It characterized in that the coating to form a WC-Co thermal spray coating layer (21).

In addition, the present invention, in the coating component for forming the WC-Co thermal spray coating layer 21 which is an undercoat, among the boron compounds containing WB (tungsten boride), CrB (chromium boride), TiB (titanium boride) 1 It is characterized by adding a species or two or more compounds.

In addition, the present invention is characterized by adding one or two of W or Cr to Co acting as a binder in the WC-Co spray coating layer 21 which is undercoating.

EMBODIMENT OF THE INVENTION Hereinafter, the structure of this invention is described in detail with reference to drawings.

In general, there are two major requirements for the undercoat of the oxide thermal spray coating layer. First, it should be excellent in corrosion resistance for the environment used, and then it should be able to mitigate thermal shock by having a coefficient of thermal expansion intermediate between the base material and the oxide coating layer. WC-Co-based coatings, which are currently used as coatings for rolls, can meet both of these requirements.

However, as mentioned above, a problem when the oxide coating layer is actually applied was a low adhesion of the oxide thermal spray coating layer. Therefore, in the present invention, in order to improve the adhesion of the oxide thermal spray coating layer, instead of forming a coating layer by a conventional method as shown in Figure 1 to form a WC-Co thermal spray coating layer by undercoat as shown in Figure 3 and thereon The method of forming an oxide layer was used immediately.

There are three characteristics required for undercoating: corrosion resistance, intermediate coefficient of thermal expansion between the base material and the oxide layer, and increased adhesion of the oxide. WC-12% Co can satisfy all of these conditions. Meanwhile, the Co content in the WC-Co may be variously changed according to the type of the base material and the oxide to be used.

In the present invention, in order to improve the corrosion resistance of WC-Co, a coating in which one or two or more kinds of boron compounds such as WB (tungsten boride), CrB (chromium boride) and TiB (titanium boride) is added is used. It may be. It is also possible to use a coating in which Cr and W are added in the binder phase Co.

Oxides that can be used as coatings in rolls of molten zinc baths include alumina, zirconia, titania, magnesia. Calcium oxide, etc. One or more of these components may be used in combination. In addition, yttria, hafnium oxide, cerium oxide, or the like may be added and used to stabilize the phase formed after the thermal spraying.

In general, the method of coating the oxide uses a plasma spray method, which forms a lot of pores. Therefore, when the zinc reaches the base metal along the pores, the coating layer is peeled off, so that the thickness of the oxide coating layer is generally 0.2 to 1.0 mm. However, when the coating is so thick, the residual stress in the coating layer increases, so that the coating layer is easily cracked or peeled off, thereby shortening the life of the coating layer.

However, when the WC-Co coating layer is used as the undercoat layer as in the present invention, the thickness of the oxide coating layer can be reduced to 0.2 mm or less. Thus, even though zinc (Zn) penetrates through the pores of the oxide, the erosion is not reached upon reaching the WC-Co layer. Due to the very slow progression, the service life is much longer than that of the conventional coating layer.

In particular, in the case of coating the oxide by an explosion spray method instead of the conventionally used plasma spray method, the coating layer is much more dense, so that even if the oxide coating layer is formed as thin as 0.02 to 0.1 mm, sufficient corrosion resistance can be exhibited.

By forming the WC-Co undercoating layer in the above manner, the adhesion of the oxide coating layer can be improved and the undercoating layer having improved corrosion resistance is used to prevent the oxide coating layer of the roll from being peeled off even if the steel sheet is rubbed with the steel sheet in the molten zinc bath. You can do it.

Hereinafter, embodiments of the present invention will be described in detail.

EXAMPLE

In the present embodiment, WC-12% Co was used as the coating material of the zinc bath immersion roll as the undercoat layer. When forming the WC-12% Co coating layer was used a high-speed spraying method (HVOF: High Velocity Oxygen Fuel (HVOF) having a better adhesion than the plasma spray and a dense coating layer).

The thermal shock test of the spray coating layer to which the conventional method and the method by this invention were applied was performed. In the conventional method, a cobalt-based alloy was used as the metal-based undercoat material, and the oxide was an alumina-zirconia alloy. The coating layer according to the present invention was coated with an undercoat layer of WC-12% Co having a thickness of 0.1 mm using a high speed spraying method, and an oxide layer of the same component was formed thereon with a thickness of 0.1 mm using a plasma spraying method. As a test specimen for thermal shock, a sink roll-like groove was formed in a cylindrical stainless steel tube having a diameter of 50 mm.

The thermal shock test method was carried out by repeating the process of rapidly quenching 20 times after holding in a furnace at 500 ° C. which is higher than the temperature of the zinc bath actually used for 20 minutes. This method is the most used method to test the thermal shock resistance of the oxide coating layer.

4a and 4b is a photograph of the appearance of the coated specimen prepared by the conventional method and the method after the thermal shock test is finished. In both cases, no cracking due to thermal shock or peeling of the coating layer occurred.

In order to compare the adhesion of the coating layer was used the method of ASTM C633. When the adhesion of the coating layer was measured by a conventional method, a numerical value of 80 to 150 Kg / cm ² was obtained. However, when the WC-12% Co coating layer was undercoated by the method of the present invention and the oxide layer was formed by the plasma spray method. The adhesion of 250 to 380 Kg / cm ² was obtained, and the adhesion of the oxide layer was improved by 3 to 4 times compared with the conventional method. If the oxide layer was formed by the explosion spray instead of the plasma spray, the adhesion increased more than 500 ~ 650 Kg / cm ² .

As such, when the WC-Co is used as an undercoating, it is confirmed that there is no problem in the thermal shock resistance and the adhesion is greatly improved.

Based on these results, the present invention was actually applied to the sink roll of the hot dip galvanizing plant. The undercoating layer was formed using the WC-12% Co coating layer to a thickness of 0.1 mm using a high speed spraying method, and an alumina-zirconia oxide coating layer was formed thereon to a thickness of 0.15 mm using a plasma spraying method. After using the roll coated in this manner in a molten zinc bath for 35 days, the appearance was examined, and a good result was obtained without cracking or peeling of the coating layer on the surface.

The oxide coating method of the roll used in the hot-dip galvanizing bath of the present invention is to use the WC-Co as the undercoat of the oxide coating to obtain an oxide coating layer having significantly improved adhesion without cracking or peeling due to thermal shock. In addition, since the undercoating layer is excellent in corrosion resistance to molten zinc, even if zinc penetrates through the oxide coating layer, it provides an effect that the roll can be used for a much longer time than the coating by the conventional method.

Claims (3)

In the method of coating the oxide on the roll immersed in the molten zinc bath of the hot dip galvanizing plant, An oxide coating method for a roll used in a hot dip galvanizing bath, characterized in that a WC-Co thermal spray coating layer (21) is formed between the base material (1) of the roll and the oxide coating layer (4) by undercoating. The boron compound according to claim 1, wherein WB (tungsten boride), CrB (chromium boride), and TiB (titanium boride) are included among the coating components forming the WC-Co thermal spray coating layer 21, which is an undercoat. An oxide coating method of a roll used in a hot dip galvanizing bath characterized by adding one or two or more compounds. The roll used in the hot dip galvanizing bath according to claim 1, wherein one or two kinds of W or Cr are added to Co acting as a binder in the WC-Co thermal spray coating layer 21, which is an undercoat. Oxide coating method.