JPS6350428B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to a hearth roll for high-temperature heat treatment furnaces, and particularly improves the durability of its ceramic spray coating.
It proposes the development results to be achieved under the effective prevention of pick-up. In a furnace that heat-treats metal strips such as silicon steel strips, foreign substances such as metal powder and metal oxides adhere to the surface of hearth rolls (hereinafter referred to as rolls) that support the strips, causing pick-ups on the strips. Therefore, it is necessary to thermally spray the roll surface with a substance that prevents the foreign matter from adhering to the roll surface to prevent the above troubles. Conventionally, hearth rolls made of heat-resistant alloys have been used in furnaces that heat-treat steel strips at high temperatures of 750°C or higher; When annealing is performed, iron powder and iron oxide carried by the steel strip significantly adhere to and stack on the roll surface. A method to prevent this phenomenon is to thermally spray ceramics or metal cermets that have low wettability with iron and iron oxide onto the roll surface, and it is well known that ZrO ₂ is extremely superior as a thermal spraying material. That's right. When a sprayed coating is obtained using only ZrO ₂ , it forms a tetragonal crystal due to the high temperature during spraying, and as it is rapidly cooled on the roll surface, the coating remains as a tetragonal crystal. When the temperature of the furnace is raised and lowered, it is slowly heated and cooled, so the tetragonal crystal transforms into the orthorhombic crystal in the low temperature range, and this is accompanied by a large volume increase, which causes peeling due to the difference in expansion coefficient with the roll metal.
Two methods have been used to prevent this phenomenon. One is to add 5 to 10% by weight (hereinafter expressed as %) of one or more of MgO, CaO, TiO ₂ , Fe ₂ O ₃ and Al ₂ O ₃ as additives to prevent volume changes in ZrO ₂ . The second method is to stabilize ZrO ₂ by adding metals such as Ni and Co.
This is a method to buffer the volume change during ZrO ₂ transformation. However, in either method, components other than ZrO ₂ induce and promote the adhesion of iron powder and iron oxide to the coating surface.
The problem was that it was not possible to take advantage of the effects of ZrO ₂ . This invention suppresses harmful components that reduce the wettability of iron and iron oxides to a certain amount or less, and also suppresses ZrO ₂
Its origins lie in the discovery of the usefulness of silicon oxide, particularly SiO ₂ , as a substance that buffers changes in volume. SiO ₂ is used as hearth roll material for silicon steel.
A fused silica roll containing 80% or more is effective, but cannot be sprayed alone. Therefore, by further combining this as a thermal spraying component, the above-mentioned drawbacks of the prior art have been advantageously solved. Now, when ZrO ₂ is sprayed, it forms a tetragonal and some cubic crystal structure depending on the conditions, but when the film is slowly cooled,
There is a risk of delamination due to the difference in thermal expansion coefficient between the material and the roll material, and delamination is even more likely when orthorhombic crystals are formed in a low temperature range. In this invention, iron and iron oxide have low wettability.
Volumetric expansion is buffered by blending SiO ₂ .
This SiO ₂ has an amorphous molten structure and is characterized by not having rapid and large expansion changes. The reasons for limiting the composition of the ceramic coating material are as follows. SiO ₂ increases the bonding force between particles, and it is preferable that SiO ₂ exists in a large amount. Figure 1 shows the relationship between the bonding strength between particles and the amount of SiO ₂ at 1000°C in a thermally sprayed coating layer _of SiO 2 -ZrO ₂ based ceramic. The bonding force is SiO ₂
A strong bond is shown when the amount is 25% by weight or more (hereinafter simply expressed as %). Therefore, in order to obtain high interparticle bonding strength,
The SiO ₂ content shall be 25% or more. On the other hand, ZrO ₂ is an important component as it is a material that has sufficient heat resistance even at the high ambient temperature of 900 to 1300℃ where this hearth roll is installed.
The total amount of SiO ₂ as a binder that buffers the thermal expansion of ZrO ₂ and prevents peeling is 85
% or more, that is, ZrO ₂ +SiO ₂ needs to be blended so that it is 85% or more. Conventionally, _MgO , CaO, and
Stabilizers made of metals such as TiO ₂ , Fe ₂ O ₃ and Al ₂ O ₃ as well as Ni and Co have high wettability with iron and iron oxide, and therefore are not preferred even if they stabilize ZrO ₂ . It is necessary to make the composition less than 5% so as to achieve the purpose of stabilization and not cause the above-mentioned adverse effects due to wettability. The ZrO ₂ -SiO ₂ based thermal spray coating according to this invention is SiO ₂
Since the large expansion coefficient difference of ZrO ₂ can be buffered by ZrO 2, a coating thickness of 1.0 mm is sufficient for practical use;
If it is less than mm, it will be too thin and the object of the invention will not be achieved, so it should be in the range of 0.1 mm to 1.0 mm. If the SiO ₂ content is less than 25%, the effect of buffering the volume change of ZrO ₂ will not be sufficiently produced, and the coating may peel off. Therefore, the lower limit of the SiO ₂ content is 25%. In addition, the hearth roll of this invention is mainly
It consists of ZrO ₂ and SiO ₂ , and since it is necessary to reduce the wettability to prevent the pick-up phenomenon, ZrO ₂ +
SiO ₂ should be 85% or more, and if the total amount is less than 85%, the wettability of iron and iron oxides is high and harmful.
MgO, CaO, _TiO2 and _Fe2O3 and Ni, _Co
If the amount of stabilizers such as In addition, the stabilizer should be less than 5%. The second comparison graph shows the relationship between the amount of MgO added to ZrO ₂ -SiO ₂ ceramic at 1000°C and the pick-up index due to the reaction with iron oxide, with the occurrence of pick-up when adding 10% MgO as an index of 10.
Shown in the figure. Thermal sprayed coatings naturally wear out with use, so if the coating is less than 0.1 mm, it is not suitable for practical use due to its lifespan, while if the coating is thicker than 1.0mm, the expansion force will be greater than the bonding force between the sprayed coating and the roll surface. There is a risk of peeling due to aging, so it is practically required that the thickness be in the range of 0.1 to 1.0 mm. Figure 3 shows the relationship between coating layer thickness and thermal shock resistance (the number of thermal shocks until peeling occurs when the sprayed coating layer is repeatedly subjected to water cooling from 1000°C to 25°C). From this, the upper limit of the coating layer thickness is preferably about 1.0
It turns out that it should be mm. The surface of the thermal spray coating layer exhibits the surface roughness of R _nax 50S, and if left as is, it may cause scratches on the material to be heat treated, so it is preferable to polish the surface to make it smooth before use. The material of the roll to which the ceramic coating is applied is
Stainless steel such as 25Cr-20Ni steel and 25Cr-12Ni steel, heat-resistant cast steel, etc. are commonly used, and by providing the ceramic coating layer on the surface of these base materials, the service life of the roll can be significantly increased. The coating stability can be further improved by providing one or more intermediate layers having a thermal expansion coefficient intermediate between the base material and the ceramic layer. Next, a coating layer is formed on the roll surface using the ceramic material having the above composition using a gas or plasma spraying device. There are also water plasma spraying equipment and oxygen-acetylene flame spraying equipment, but in water plasma spraying, the plasma jet temperature is over 30,000K, which is too high and SiO ₂ in the component composition decreases due to phase separation and evaporation. It does not form the desired coating layer. In addition, in the oxygen-acetylene flame spraying method, the combustion flame temperature is around 3000K, so the sprayed material is not completely molten, and the bonding strength of the coating layer is weak, making it impractical. Further, the ceramic powder to be coated is preferably 125 μm or less. This is because the plasma jet temperature obtained by gas or plasma spraying is about 10,000 K, so the particle size is such that complete melting is achieved at that temperature and no phase separation or evaporation of SiO ₂ occurs. Example 1 25% Cr-20 in a continuous annealing furnace for silicon steel strip
%Ni heat-resistant alloy steel rolls were coated with a coating material having the following ceramic composition, adjusted to a grain size of 125μ or less, and then placed in the same area with the thermally sprayed coating coated using a gas or plasma spraying device for a period of 5 months. At the same time, a comparison test was conducted. As shown in Table 1, the 65% ZrO ₂ -30% SiO ₂ film was good, but when the amount of SiO ₂ was small, there was a risk of peeling, and the 95% ZrO ₂ + 5% CaO film was stabilized. The ZrO ₂ coating was defective due to the adhesion of metal grains and powder in the form of flakes due to the additives. It was also found that using Al ₂ O ₃ or Al ₂ O ₃ and TiO ₂ instead of ZrO ₂ was also bad.

[Table] Example 2 A comparative test was conducted under the same conditions as in Example 1, but as shown in Table 2, CaO exceeded the limit.
It can be seen that MgO and Fe ₂ O ₃ are the causes of adhesion.

[Table] Example 3 Using SCH13 as a base material, the same ceramic as in Example 1 was applied on the surface to a particle size of 125μ or less, and then sprayed with a gas mainly composed of nitrogen gas and a plasma spraying device to a thickness of 0.4 mm. When a roll with a coating layer formed and polished was installed in a continuous heat treatment furnace for silicon steel strips, there was no build-up at all even after 5 months, and there was no peeling or cracking of the coating layer. Conventional uncoated
The service life was more than twice that of the SCH13 roll. Example 4 SCH22 with a thermal expansion coefficient of 15×10 ^-6 /°C as a base material
The thermal ^expansion coefficient of the roll body using
A 0.2 mm base sprayed layer made of 80% Ni-20% Cr metal at ℃ is applied, and a sprayed coating of the same ceramic (thermal expansion coefficient 8 x 10 ^-6 /℃) as in Example-1 is formed on top of that. After adjusting the material to a particle size of 125μ or less,
A coating layer was formed to a thickness of 0.6 mm using a gas and plasma spraying device mainly composed of nitrogen gas, and the polished roll was installed in a continuous heat treatment furnace for silicon steel strips and no build-up was observed after 8 months. There was no cracking or peeling of the coating layer, and the service life was more than 1.5 times that of the comparative ZrO ₂ coated roll. Example-5 SCH22 with a thermal expansion coefficient of 15×10 ^-6 /℃ as the base material
80%Ni-20%Cr metal underlayer (thermal expansion coefficient 13× ^10-6 /℃) and 80%Ni-20%Cr alloy.
A cermet undercoat layer (thermal expansion coefficient 9×10 ^-6 /°C) having a composition of 1:1 by weight with ZrSiO ₄ was added to each layer.
After spraying 0.2 mm each and applying the same coating material as in Example 1 on the surface to a particle size of 125μ or less,
A ceramic coating layer was formed to a thickness of 0.4 mm using a nitrogen gas or plasma spraying device, and the polished roll was installed in a continuous heat treatment furnace for silicon steel strips and no build-up occurred after 15 months. There was no cracking or peeling of the coating layer, and the service life was more than four times longer than that of the conventional SCH22 roll. As described above, according to this invention, the adhesion of metal powder and metal oxide carried by the steel strip to the roll surface is significantly reduced, so the continuous use period of the roll is significantly extended, and the operation is stabilized. In addition, it has great effects such as greatly contributing to lowering the cost of regrinding rolls.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between the SiO ₂ content and interparticle bonding strength of a SiO ₂ -ZrO ₂ based ceramic spray coating layer, Figure 2 is a graph showing the relationship between the amount of MgO added and the pick-up index, and Figure 3 is a graph showing the relationship between the amount of MgO added and the pick-up index. 2 is a graph showing the relationship between the layer thickness and thermal shock resistance of a SiO ₂ −ZrO ₂ based ceramic spray coating layer.

Claims (1)

[Scope of Claims] 1. A hearth roll for a high-temperature heat treatment furnace for a steel plate in which a ceramic sprayed coating mainly composed of zirconium oxide is formed on the circumferential surface of a roll material made of a heat-resistant alloy, in which the sprayed coating has a stable zirconium oxide coating. The composition is characterized by having a composition containing less than 5% by weight of silicon oxide and at least 25% by weight in terms of SiO ₂ and 85% by weight or more in total with zirconium oxide in terms of ZrO ₂ . Hearth roll for high-temperature heat treatment furnaces. 2 The thermal spray coating has a thickness in the range of 0.1 to 1.0 mm1
Hearth roll as described. 3. The hearth roll according to 1 or 2, wherein the thermal spray coating is provided with a base thermal spray coating made of a heat-resistant alloy layer having a thermal expansion coefficient larger than that of the roll material and smaller than that of the thermal spray coating.