JPH0255502B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to members for molten metal plating baths, and particularly to members for molten metal plating baths that exhibit excellent corrosion resistance against molten aluminum, molten zinc, or their molten alloys. be. [Prior Art] Molten aluminum and hot-dip galvanized steel sheets are used as heat-resistant and corrosion-resistant members for automobiles and construction materials, but they are mainly manufactured by continuous plating. The continuous plating processing equipment includes a dipping roll immersed in the molten metal, a plating roll installed near the surface of the molten metal, and a guide roll that guides the plated steel sheet after passing through these rolls. is arranged. Since these rolls are immersed in molten metal or come into contact with a high-temperature steel plate coated with molten metal, they are required to have the following performance. (1) It is difficult to cause uneven corrosion due to molten metal, (2) It is difficult to wear even if it comes into contact with the steel plate being passed, (3) It is easy to peel off adhered molten metal and maintain and inspect, (4) The roll (5) Low cost. Conventional technology aimed at providing rolls that meet these requirements, that is, members for plating baths, has the following properties: (5) low cost; Those sprayed with a self-fluxing alloy, those coated with a ceramic coating layer consisting of ZrO ₂ and Al ₂ O ₃ as disclosed in JP-A-61-117260, and those as disclosed in JP-B No. 58-37386. Rolls have been proposed in which a surface coating layer of 0.1 to 2.4 mm thick is formed of one or more of WC, CrC, and TiC, with the remainder being a hot corrosion-resistant metal or its oxide. [Problems to be Solved by the Invention] Regarding each of the above-mentioned conventional technologies, for example, although the lifespan is longer than that of previous untreated rolls, self-fluxing alloy The film or ceramic film peels off locally, and this is transferred to the surface of the plated steel plate as a discolored pattern, resulting in a significant reduction in commercial value. Also, in the case of WC, CrC,
Although carbides such as TiC exhibit excellent corrosion resistance that meets the requirements, carbides alone cannot form a coating layer, so depending on the type of metal coexisting with the carbide, the performance may deteriorate significantly, making it unsuitable for practical use. There were flaws. That is, in this prior art, if Ni or Si is mixed with carbide and used, even if a coating layer is formed by thermal spraying, it has no effect on the molten metal. However, it has been reported that cermet materials containing Co in carbides can withstand molten metal relatively well, but still require a film thickness of 0.1 mm or more, and that coatings are ineffective below this. There is. However, since WC, Co, etc. are very expensive materials, forming such a thick film inevitably increases costs. Moreover, this type of cermet material made of metal carbide has a different expansion coefficient from the base steel material, so in high-temperature molten metal, it may peel off from the base material due to internal stress due to the difference in thermal expansion between the two. This is a concern, and from this point of view as well, the thick film used in the prior art is disadvantageous, and there is a strong demand for the development of a thin coated member that is highly resistant to molten metal erosion. SUMMARY OF THE INVENTION The object of the present invention is to overcome the above-mentioned drawbacks of the prior art of spray coating plating bath components with cermet materials, particularly WC-Co cermet materials. In other words, the special public official court in 1982-
The technology disclosed in Publication No. 37386 proposes a molten metal plating bath immersion member whose surface is thermally sprayed with a coating material consisting of carbide and metal to a thickness of 0.1 to 2.4 mm. This is an attempt to solve the problem of the fact that the coating is not effective unless the coating is thicker than 0.1 mm. In short, for thick coatings such as 0.1 to 2.4 mm, due to thermal changes when immersed in a hot molten metal bath and pulled up, the difference in expansion coefficient between the coating layer and the base material In particular, if this value becomes greater than the adhesion between the coating and the base material, the coating will peel off. Generally, it is well known that as the thickness of the coating layer increases, the expansion difference between them increases and the adhesion to the base material tends to decrease. [Means for Solving the Problems] The present invention uses the following means to solve the drawbacks of the prior art described above. That is, when a cermet material made of WC as a carbide and Co as a metal to be contained is coated on the surface of a plating bath member by a thermal spraying method, the porosity of the sprayed coating layer is 1.8%. Make the coating layer as shown below. Such a coating layer (hereinafter referred to as "film") is a thin film of 0.1 mm or less, which was considered impossible with conventional technology, but according to the findings of the present inventors, it has excellent molten metal resistance. It was found that it exhibited. In addition, in the present invention, it is preferable to control the thickness of the above-mentioned film within a range of 0.040 to less than 0.10, and the thickness of the Co
The amount of is preferably 5 to 28 wt%. [Function] In the present invention, the WC-Co cermet material thermal spray coating formed on the surface of the member that is immersed in the molten metal plating bath and comes into contact with the molten or softened metal has a hardness Hv of about 1100 to 1300 and is wear resistant. Excellent,
It will not wear out even if it comes into contact with hot-dip galvanized steel sheets. This kind of cermet thermal spray material is
It is a mixture of highly hard WC particles and metal (Co) and its reaction product, and the film obtained by plasma spraying this is a structure in which WC particles are surrounded by particles (Co) with a high metal content. have. but,
Since WC particles are hard and do not undergo plastic deformation, they can be destroyed by collisions during thermal spraying, and the energy of the collisions can cause cracks in the already formed film, resulting in the formation of pores in the thermally sprayed coating. On the other hand, if pores exist in the thermal spray coating, molten metal enters the interior through the pores, causes a metallurgical reaction with the base steel material, expands the volume, and causes the thermal spray coating to peel off from the bottom. Further, even if the pores do not penetrate through the pores, molten metal may accumulate inside the pores, and the thermal sprayed coating may be destroyed by repeatedly expanding and contracting as the temperature changes in the environment. Therefore, the present inventors first investigated the thermal spraying conditions to obtain a dense thermal sprayed coating, and found that (1) the thermal spray particles become softer due to an increase in the ambient temperature, and (2) the collision energy on the surface of the object to be thermally sprayed increases. , we focused on However, in the method (1), the high temperature causes WC to change into an oxide, making it impossible to utilize the high hardness of a carbide. Therefore, the present inventors
We further investigated this method. Specifically, by using oxygen-hydrogen gas or oxygen-hydrocarbon as a heat source and increasing the supply pressure to the thermal spray gun, the injection velocity of combustion gas (fuel gas supply pressure was used as a variable factor. In this case, the higher the supply pressure, the faster the combustion gas injection speed,
Along with this, we aimed to reduce the porosity of the sprayed coating. )
was increased to increase the collision energy of sprayed particles. FIG. 1 shows the relationship between the supply pressure of oxygen-hydrogen gas to the thermal spray gun and the porosity of the obtained thermal spray coating. As is clear from the results shown in FIG. 1, it was found that the higher the fuel gas supply pressure, the more a sprayed coating with a lower porosity could be obtained. Therefore, by adjusting the supply pressure of the fuel gas, we have created a material with a thickness of 0.094 to 0.098 with various porosity.
mm (average 0.096mm) and 1.80-1.90 (average 1.85mm)
Diameter with WC-Co cermet spray coating formed
A cylindrical test piece made of mild steel with a diameter of 3.0 mm and a length of 100 mm was prepared, and one step involved immersing it in a 480°C molten zinc bath for 1 hour, then pulling it out and cooling it to room temperature, and this process was repeated. The state of destruction of the sprayed coating was then observed. Figure 2 shows the relationship between the porosity of the WC-Co cermet spray coating and the number of immersions at which the coating was destroyed. As is clear from these results, it was confirmed that even if the thermal sprayed coating is thick but has a high porosity, it will break after a small number of immersions, and conversely, even if the coating is thin but has a low porosity, it can withstand many immersions. . Moreover, as shown in Figure 2,
It is clear that excellent corrosion resistance is exhibited when the porosity of the coating is at a limit of 1.8%, and for this reason, in the present invention, the porosity of the thermal sprayed coating was set to 1.8% or less. With such a porosity, good resistance to molten zinc is exhibited. The thermal spray material used in this test had a weight ratio of WC (88%) - Co (12%). After coloring, the area occupied by the colored portion was determined using an image analysis device. Next, in the present invention, the amount of Co contained in the cermet material to be thermally sprayed on the surface of the member is 5 to 5.
The reason for this is that if it is less than 5wt%, the bonding force between the WC particles that make up the coating will decrease, and the adhesion between the coating and the object to be thermally sprayed will be weakened, whereas if it exceeds 28wt% and Co, which is resistant to molten metals.
However, it is more easily eroded compared to WC, so
This is because the molten metal resistance of the film decreases. Furthermore, in the present invention, the thickness of the thermal spray coating is
It is desirable that the thickness be less than 0.04 to 0.10 mm, because if it is less than 0.040 mm, the porosity in the coating will be high and the molten metal resistance will be poor, whereas if the thickness is 0.10 mm or more, the thermal spray coating will This is because the internal stress increases, making it easy to peel off, and it is not economically advisable. [Example] Example 1 In this example, the immersion roll, plating roll, and guide roll (each material is JIS G3445 (1983) STKM13A) of a continuous hot-melting machine are equipped with WC as shown in the margin of Table 1. −Co cermet material,
Thickness 0.096~0.098mm, porosity 0.6, 1.0, 1.8, 2.2,
2.8% thermal spray coating, and using this thermal spray coated roll, a steel plate with a width of 900 mm and a thickness of 0.35 mm is
The sheets were passed through a hot-dip zinc (JISH2107 (1957) distilled zinc special grade equivalent) plating bath maintained at ℃ for continuous hot-dip plating treatment, and the properties of the sprayed coating were investigated. Table 1 shows the observation results of the thermal spray coatings treated on the surfaces of each roll after 20 consecutive days of hot-dip Zn plating treatment. As is clear from the results shown in Table 1, the thermal sprayed coating according to the present invention has a porosity of 1.8% or less regardless of the Co content and even if it is a thin film of less than 0.1 mm. It was confirmed that if controlled, it exhibits excellent corrosion resistance against molten zinc over a long period of time. On the other hand, the comparative examples with high porosity (2.2%, 2.8%) have thermal sprayed coatings.
More than 30% of the zinc has fallen off, and it is clear that the resistance to molten zinc is poor.

[Table] Example 2 A steel plate (width 900 mm x thickness 0.22 mm) was prepared using a roll with the same thermal spray material, thermal spray coating thickness, and porosity as in Example-1.
Continuous plating treatment was performed in a molten aluminum plating bath maintained at 700-710°C. JIS H2102 (1968) Aluminum 1 for hot-dip galvanizing metals.
seeds were used. Table 2 shows the results of the visual inspection of the thermal spray rolls after continuous hot-melt Al plating for 15 days. From these results, the thermal sprayed roll of the present invention maintained all of the dipped roll, plated roll, and guide roll in a healthy state, but in the roll of the comparative example, most of the thermal sprayed coating peeled off on the dipped roll, and on the plated roll. More than 50% peeling was observed, and more than 35% peeling was observed on the guide roll.

[Table] Example 3 In this example, when manufacturing a thermal spray roll according to the procedure of Example-1, the thickness of the thermal spray coating was set to 0.020 to 0.020.
The roll was varied within a range of 0.100 mm to test its properties as a dip roll in molten zinc and molten aluminum plating equipment. Hot dip galvanizing temperature is
After continuous plating treatment at 470-480°C for 20 days and molten aluminum plating temperature at 700-710°C for 15 days, the immersion roll was pulled up and inspected. Table 3 shows the results of the immersion rolls used in the molten zinc bath, and it can be seen that the lower the porosity of the sprayed coating, the better the erosion resistance. Minimum 0.040mm
It has been found that a coating thickness of 100 mL is sufficient to withstand daily operations. On the other hand, all the films of Comparative Examples were peeled off regardless of the film thickness.

[Table] 〓Remarks〓 ○ Sound △ Peeling area 5% or less ×
Peeling area: 5% or more Table 4 shows the results for immersion rolls used in a molten aluminum bath. Even in this case, the smaller the porosity of the sprayed coating, the better the erosion resistance. It was confirmed that if the film thickness is 1.8% or less and the film thickness is 0.040mm or more, it can withstand use for 15 days. In addition, all the films of the comparative examples were peeled off, and no corrosion resistance was observed at all. Note that when the WC-Co cermet thermal spray material of the present invention is manufactured by the sintering method, it undergoes thermal history, and a part of WC changes to W ₂ C, W ₆ C _{2.54 ,} or WC. and Co react to form intermetallic compounds such as Co ₃ W ₃ C. It is also clear that exposing these materials to the high temperatures of a thermal spray environment accelerates the decomposition and reactions described above. Therefore, it was found that the WC-Co cermet thermal spray material and thermal spray coating of the present invention contain not only a mixture of WC and Co, but also a decomposition component of WC and a reaction product of both components.

[Table] 〓Remarks〓 ○ Sound △ Peeling area 5% or less ×
Peeling area of 5% or more [Effect of the invention] As explained above, the present invention provides a thin film with a porosity of 1.8% of less than 0.04 to 0.10 mm on the surface of a plating bath member.
As a result of forming the following WC-Co cermet spray coating, it is possible to provide an inexpensive plating bath member that exhibits excellent corrosion resistance against molten zinc, molten aluminum, and the like. Moreover, by using such a member, stable hot-dip galvanizing work, high productivity, and improvement in the quality of plated steel sheets can be ensured.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between the supply pressure of fuel gas for thermal spray heat source to the spray gun and the porosity in the coating formed by the combustion gas, and Figure 2 is the graph showing the porosity in the sprayed coating. It is a graph showing the relationship between the number of peelings of the film and the number of times the film is peeled off by repeated immersion in a molten zinc bath, pulling up, and cooling.

Claims (1)

[Claims] 1. In a member having a cermet coating layer on the surface, the coating layer is made of WC-containing 5 to 28 wt% of Co.
A member for a molten metal plating bath, characterized by having a thermally sprayed coating layer made of Co-based cermet material and having a porosity of 1.8% or less. 2. The molten metal plating bath member according to claim 1, wherein the thermal spray coating layer has a thickness of 0.040 to less than 0.10 mm.