Hearth rolls and how to use them

Abstract

The objective is to provide hearth rolls with excellent build-up resistance and crack resistance. [Solution] A hearth roll in which the roll surface in contact with a steel plate is covered with a thermal spray coating, wherein the thermal spray coating comprises a top coat layer having build-up resistance (provided that the chromium oxide content is 0% by mass) and an undercoat layer consisting of a cermet thermal spray coating containing 20% ​​to 30% by mass of chromium, and when the linear expansion coefficient of the roll substrate is A, the linear expansion coefficient of the top coat layer is B, and the linear expansion coefficient of the undercoat layer is C, B / A is 0.35 or more and 0.5 or less, and C / A is 0.55 or more.

Description

Technical Field

[0001] The present invention relates to hearth rolls used in continuous annealing furnaces and the like.

Background Art

[0002] In a heat treatment furnace for heat-treating metal plates such as steel plates, conveying rolls (hereinafter referred to as "hearth rolls") for loading, transferring, and unloading the steel plates are installed. When the hearth rolls are rotated at high speed and the steel plate is passed through, phenomena such as slip, meandering, dust adhesion to the surface of the conveying rolls, and build-up occur on the steel plate. In particular, the hearth rolls arranged in a continuous annealing furnace carry the steel plate in a high-temperature state, so build-up is likely to occur on the surface of the hearth rolls.

[0003] Build-up is a phenomenon in which iron, iron oxides, etc. present on the surface of the steel plate adhere to and accumulate on the surface of the hearth roll. When build-up occurs, the shape of the deposit derived from the build-up is transferred to the surface of the steel plate, deteriorating the surface quality, not only deteriorating the grade of the steel plate, but also requiring maintenance to remove foreign substances adhering to the surface of the hearth roll during regular repairs, which is one of the causes of productivity decline.

[0004] In addition, for hearth rolls used in a continuous annealing furnace where the furnace temperature is 850 °C or higher and the inside of the furnace is in an acidic atmosphere (for example, an atmosphere with a trace amount of oxygen in a nitrogen atmosphere or an atmosphere with carbon dioxide), in addition to the build-up resistance, durability against a high-temperature oxidation atmosphere is also required.

[0005] Patent Document 1 discloses a hearth roll for a continuous heat treatment furnace aimed at improving build-up resistance, in which a nickel-chromium alloy is sprayed on the surface of the hearth roll, and chromium carbide is sprayed on this outer layer.

[0006] Patent Document 2 discloses a hearth roll for a heat treatment furnace aimed at improving build-up resistance, wherein the hearth roll surface is sprayed with a Ni-based alloy (Ni: 70-80 mass%, Cr: 3-15 mass%, Al: 3-15 mass%, Mo: 3-10 mass%, Fe: 3-10 mass%) as an undercoat, a low-carbon Cr-Fe alloy as an intermediate coat, and Cr3C2 as a topcoat.

[0007] Patent Document 3 discloses a roll for a heat treatment furnace aimed at improving build-up resistance, wear resistance, and slip resistance, characterized in that a ceramic spray coating layer made of zirconia ceramics containing 1 to 10% by weight of chromium carbide is formed on the surface of a roll made of heat-resistant steel.

[0008] Patent Document 4 describes a furnace roll for the purpose of improving build-up resistance and oxidation resistance, comprising ceramic powder (Cr 23 A furnace roll is disclosed, in which a cermet powder consisting of a mixture of C6:10-20 mass%, Y2O3:10-20 mass%, and heat-resistant alloy powder (Al:4-6 mass%, Cr:12-16 mass%, Y:1 mass%, or less, with the remainder being Co and / or Ni) is sprayed onto the roll surface.

[0009] Patent Document 5 discloses a hearth roll for the purpose of improving build-up resistance and thermal shock resistance, wherein a thermal spray coating is formed on the roll surface using a thermal spray powder that contains 30 to 50% by mass of chromium carbide, with the remainder being an alloy containing at least one of cobalt and nickel, chromium, aluminum and yttrium, and having an average particle size of 20 to 60 μm.

[0010] Patent Document 6 discloses a hearth roll for a continuous annealing furnace, intended to prevent slippage, meandering, debris adhesion to the surface of the conveyor roll, buildup, etc. of steel plates, characterized in that it has a cermet coating on its surface made of ceramics (containing 50 to 90 vol% of Cr3C2, 1 to 40 vol% of Al2O3, 0 to 3 vol% of Y2O3, and 0 to 40 vol% of ZrB2, with the remainder being unavoidable impurities and pores) and a heat-resistant alloy (containing 5 to 20 mass% of Cr, 5 to 20 mass% of Al, and 0.1 to 6 mass% of one or two of Y and Si, with the remainder being one or two of Co and Ni and unavoidable impurities), wherein 50 to 90 vol% of the cermet coating is the ceramics and the remainder is the heat-resistant alloy.

[0011] Patent Document 7 discloses a hearth roll for a continuous annealing furnace, the outer surface of which is covered with a ceramic thermal spray coating, the ceramic thermal spray coating mainly composed of chromium carbide, containing dispersed chromium oxide, and containing 5 to 35 wt% oxygen.

[0012] Patent Document 8 discloses a hearth roll for a heat treatment furnace, comprising a roll body having a flat section and a tapered section, wherein the roll body has a flat section for 20-60% of its total roll length, and the flat section is coated with a thermal spray coating of ZrO2, Cr2O3, or Al2O3, either individually or in combination, oxide ceramics, for continuous annealing of steel sheets. [Prior art documents] [Patent Documents]

[0013] [Patent Document 1] Japanese Patent Application Publication No. 56-069321 [Patent Document 2] Japanese Patent Application Laid-Open No. 62-103315 [Patent Document 3] Japanese Patent Application Publication No. 07-011323 [Patent Document 4] Patent No. 4009255 [Patent Document 5] Patent No. 5058645 [Patent Document 6] Patent No. 5306227 [Patent Document 7] Patent No. 7027624 [Patent Document 8] Japanese Patent Application Publication No. 7-173524 [Overview of the project] [Problems that the invention aims to solve]

[0014] The hearth rolls disclosed in Patent Documents 1 to 7 all lacked sufficient buildup resistance. Furthermore, in thermal spray coatings containing chromium carbides, the generated chromium oxide evaporates when the furnace temperature exceeds 850°C, leading to deterioration of the coating. Furthermore, Patent Document 8 states that when subjected to furnace heat, the difference in thermal expansion between the ceramic thermal spray coating and the heat-resistant steel of the roll material is large, which may cause the ceramic thermal spray coating to crack and peel off. Moreover, once the ceramic thermal spray coating cracks, chromium and other elements in the roll material are oxidized, generating metal oxides, which makes the ceramic thermal spray coating even more prone to peeling. Similar problems exist with cermet thermal spray coatings.

[0015] The present invention aims to provide hearth rolls with excellent build-up resistance and crack resistance. [Means for solving the problem]

[0016] To solve the above problems, the hearth roll according to the present invention is (1) a hearth roll in which the roll surface in contact with a steel plate is covered with a thermal spray coating, wherein the thermal spray coating comprises a top coat layer having build-up resistance (provided that the chromium oxide content is 0% by mass) and an undercoat layer consisting of a cermet thermal spray coating containing 20% ​​by mass or more and 30% by mass or less of chromium, and when the linear expansion coefficient of the roll substrate is A, the linear expansion coefficient of the top coat layer is B, and the linear expansion coefficient of the undercoat layer is C, then B / A is 0.35 or more and 0.5 or less, and C / A is 0.55 or more. It is characterized by the following:

[0017] (2) The top coat layer is composed of a ceramic thermal sprayed coating of an oxide system, and the hearth roll according to (1) above is characterized thereby.

[0018] (3) The top coat layer contains at least one of aluminum oxide and titanium oxide, and the content of the oxide is 85% by mass or more. The hearth roll according to (2) above is characterized thereby.

[0019] (4) The cermet thermal sprayed coating contains a heat-resistant alloy and a ceramic of an oxide system or a carbide system. The hearth roll according to (1) or (2) above is characterized thereby.

[0020] (5) The cermet thermal sprayed coating contains a heat-resistant alloy and a ceramic of an oxide system or a carbide system. The hearth roll according to (3) above is characterized thereby.

[0021] (6) The ceramic of the oxide system is a complex oxide of a rare earth element and aluminum or a transition metal (excluding rare earth elements). The hearth roll according to (4) above is characterized thereby.

[0022] <好 (7) The transition metal is any one of Ta, Zr, and Cr. The hearth roll according to (6) above is characterized thereby.

[0023] (8) The ceramic of the carbide system contains at least one of chromium carbide and titanium carbide. The hearth roll according to (4) above is characterized thereby.

[0024] (9) The heat-resistant alloy contains at least one of cobalt and nickel and chromium. The hearth roll according to (4) above is characterized thereby.

[0025] (10) A method of using the hearth roll described in (1) above, characterized in that, after the top coat layer has disappeared due to friction with the steel sheet, the steel sheet is transported while in contact with the iron-chromium complex oxide layer that has formed at the interface between the top coat layer and the undercoat layer. [Effects of the Invention]

[0026] According to the present invention, it is possible to provide hearth rolls with excellent build-up resistance and crack resistance. [Brief explanation of the drawing]

[0027] [Figure 1] This is a schematic diagram of a continuous annealing furnace. [Figure 2] A perspective view of a hearth roll (immediately after the thermal spray coating is formed). [Modes for carrying out the invention]

[0028] (First Embodiment) An embodiment of the present invention is a hearth roll installed in a continuous annealing furnace, and has a thermal spray coating on its surface. Figure 1 is a schematic diagram of a continuous annealing furnace. Figure 2 is a perspective view of the hearth roll immediately after the thermal spray coating has been formed, and shows an enlarged cross-section of a part of the roll surface.

[0029] Referring to Figure 1, the continuous annealing furnace 1 of this embodiment comprises a furnace body 2 and a plurality of gas supply nozzles 3 and 4 for supplying atmospheric gas to the inside of the furnace body 2. The furnace body 2 continuously anneals steel plates 10 in the atmospheric gas. An acidic atmospheric gas can be used as the atmospheric gas. The acidic atmospheric gas mainly consists of nitrogen and contains trace amounts of oxygen and carbon dioxide. Note that the non-reducing gas of this embodiment does not contain a mixture of nitrogen and hydrogen. However, the hearth roll of the present invention may be used under such a mixed gas atmosphere.

[0030] The furnace temperature of the furnace body 2 is preferably 850°C or higher, and more preferably 1000°C to 1300°C.

[0031] A hearth roll 20 is installed in the furnace body 2. The hearth roll 20 can convey the steel plate 10 inside the furnace body 2 by rotating around its longitudinal axis. The hearth roll 20 includes a roll base material 21 and a thermal spray coating 22 formed on the surface of the roll base material 21. The thermal spray coating 22 includes a top coat layer 22a and an undercoat layer 22b.

[0032] The topcoat layer 22a is a thermal spray coating with buildup resistance. By forming a thermal spray coating with buildup resistance on the surface of the roll substrate 21, it is possible to prevent foreign matter on the steel plate 10 from adhering to and accumulating on the hearth roll 20. The undercoat layer 22b contains at least chromium. The inclusion of chromium in the undercoat layer 22b allows for the formation of an iron-chromium complex oxide with excellent buildup resistance at the interface between the topcoat layer 22a and the undercoat layer 22b when the hearth roll 20 is in use.

[0033] In order to produce iron-chromium complex oxide, the coefficients of thermal expansion of the roll substrate 21, the topcoat layer 22a, and the undercoat layer 22b must be adjusted to a predetermined relationship. Specifically, when the relationship is expressed as a relative value, if the coefficient of thermal expansion of the roll substrate 21 is A, the coefficient of thermal expansion of the topcoat layer 22a is B, and the coefficient of thermal expansion of the undercoat layer 22b is C, then B / A is between 0.35 and 0.5, and C / A is 0.55 or greater. For the roll substrate 21, a heat-resistant alloy with a heat resistance temperature of 850°C or higher can be used. For example, heat-resistant alloys commonly known as hearth roll substrates, such as heat-resistant steel castings (SCH21, SCH24) and NA22H, can be used. These heat-resistant alloys have small differences in their coefficients of thermal expansion.

[0034] Because the furnace of the hearth roll 20 is at a high temperature, the roll substrate 21 and the topcoat layer 22a expand thermally according to their respective coefficients of thermal expansion. As described above, if B / A is set to 0.5 or less, the roll substrate 21 expands relatively large thermally, which can generate tortoise-shell-like cracks throughout the topcoat layer 22a. These cracks include those that reach the undercoat layer 22b. Furthermore, if B / A is less than 0.35, not only will cracks be generated, but delamination will occur immediately, so this case was excluded from the present invention. When the hearth roll 20 is used in the furnace, iron oxide powder adhering to the steel plate 10 penetrates cracks in the topcoat layer 22a, and an iron-chromium complex oxide layer is formed at the interface between the topcoat layer 22a and the undercoat layer 22b. The mechanism of iron-chromium complex oxide layer formation will be described in detail later.

[0035] In the above configuration, during the initial use of the hearth roll 20, the steel plate 10 and the topcoat layer 22a are in contact, and the topcoat layer 22a prevents buildup. The topcoat layer 22a wears down and thins with use of the hearth roll 20, but in parallel with the wear and deterioration, an iron-chromium complex oxide layer grows at the interface. At least until the topcoat layer 22a disappears, the iron-chromium complex oxide layer has grown sufficiently, and after the topcoat layer 22a disappears, the surface of the undercoat layer 22b is covered with the iron-chromium complex oxide layer. Since this iron-chromium complex oxide layer has buildup resistance, even if the topcoat layer 22a disappears, the undercoat layer 22b can prevent the adhesion of foreign matter.

[0036] Furthermore, if the coefficient of thermal expansion of the undercoat layer 22b is similar to that of the topcoat layer 22a, cracks will form in the undercoat layer 22b, preventing the formation of a useful iron-chromium complex oxide layer at the interface between the topcoat layer 22a and the undercoat layer 22b, which can lead to intralayer oxidation of the undercoat layer 22b. Therefore, as described above, the coefficient of thermal expansion of the undercoat layer 22b must be set to a higher value than that of the topcoat layer 22a (C / A: 0.55 or higher).

[0037] Next, the component composition of the thermal spray coating 22 will be described in detail. The units for the elemental content in the component composition are all "mass%", but unless otherwise specified, they will be simply referred to as "%" below. (Regarding top coat layer 22a) A ceramic thermal spray coating with build-up resistance can be used for the top coat layer 22a. The ceramic thermal spray coating is not particularly limited as long as it satisfies the above-mentioned relationship of thermal expansion coefficients, but it is preferably oxide-based. The oxide-based ceramics preferably contain at least one of aluminum oxide and titanium oxide, and more preferably aluminum oxide. The chromium oxide contained in the oxide-based ceramics is 0% by mass. Chromium oxide evaporates when it receives heat in the furnace (furnace temperature: 850°C or higher), but the high evaporation pressure during evaporation degrades the build-up resistance of the topcoat layer 22a. Note that the chromium oxide derived from unavoidable impurities that may be contained in the topcoat layer 22a is in trace amounts and is therefore considered to be 0% by mass. Ceramic thermal spray coatings are known to have excellent build-up resistance. Therefore, the topcoat layer 22a can prevent the adhesion of foreign matter until the topcoat layer 22a wears away.

[0038] Here, if the main component of the ceramic thermal spray coating is an oxide, the oxide concentration is preferably 85% or more, and more preferably 90% or more. If the oxide content is 85% or more, the ceramic thermal spray coating can satisfy the desired coefficient of thermal expansion and improve build-up resistance. If the ceramic thermal spray coating contains aluminum oxide, the aluminum oxide content is preferably 70% or more. The remainder of the ceramic thermal spray coating after removing the oxide is not particularly limited. This remainder may contain unavoidable impurities. For example, these may enter as contaminants from the mixing container during the thermal spray coating manufacturing process, carbon may enter from kerosene during high-velocity gas spraying, or enter through the decarburization reaction by plasma spraying. Unavoidable impurities include at least one of oxygen, boron, carbon, silicon, phosphorus, iron, and manganese.

[0039] (Regarding undercoat layer 22b) The undercoat layer 22b can be a cermet thermal spray coating containing 20% ​​to 30% chromium. By including 20% ​​or more chromium in the undercoat layer 22b, it becomes easier to generate iron-chromium complex oxide at the interface between the topcoat layer 22a and the undercoat layer 22b. Furthermore, by limiting the chromium content to 30% or less, phenomena such as the undercoat layer 22b becoming hard and brittle are suppressed, making the undercoat layer 22b less prone to cracking. For these reasons, the chromium content must be set to 30% or less.

[0040] For example, a cermet spray coating made of ceramics and a heat-resistant alloy can be suitably used. The ceramics are preferably oxide-based or carbide-based. The oxide-based ceramics are preferably one or more of aluminum oxide, yttrium oxide, and chromium oxide, and more preferably a complex oxide. The complex oxide is preferably a complex oxide of a rare earth element and aluminum or a transition metal (excluding the rare earth element). The rare earth element is preferably La and Nd, and more preferably Y. The transition metal is preferably Ta and Zr, and more preferably Cr.

[0041] The carbide-based ceramic is preferably at least one of titanium carbide and chromium carbide, and more preferably chromium carbide.

[0042] The heat-resistant alloy preferably contains chromium as an essential component, but is not limited to other elements. These other elements may include at least one of cobalt and nickel, and may also include one or more of aluminum, yttrium, and silicon. For example, CoNiCrAlY, CoCrAlY, NiCrAlY, CoNiCrAlSiY, etc., can be used. These heat-resistant alloys have small differences in their coefficients of thermal expansion.

[0043] The undercoat layer 22b is formed by spraying thermal spray particles onto the surface of the roll substrate 21. For example, granulated sintered powder can be used as the thermal spray particles. Thermal spray particles can be produced by a granulation sintering method in which secondary particles of 5 to 60 μm are granulated from primary particles of a cermet material adjusted to the above-mentioned component composition and sintered in a non-oxidizing atmosphere at 1000 to 1300°C. The roll substrate 21 may also be subjected to blast treatment beforehand. By performing blast treatment, the adhesion between the roll substrate 21 and the undercoat layer 22b can be improved.

[0044] The topcoat layer 22a is formed by spraying thermal spray particles onto the undercoat layer 22b. For example, molten pulverized powder or granulated sintered powder can be used as the thermal spray particles. Thermal spray particles of 5 to 80 μm can be produced by sintering ceramic raw materials adjusted to the above-mentioned component composition to create ceramic lumps, and then pulverizing them. The thickness of the topcoat layer 22a is preferably 10 μm to 50 μm. If it is less than 10 μm, the topcoat layer 22a may disappear before the iron-chromium complex oxide is formed. If it is greater than 50 μm, the thermal expansion may cause the thermal spray coating to partially peel off, resulting in unevenness on the roll surface and potentially degrading the quality of the sheet. The thickness of the undercoat layer 22b is preferably 50 μm to 200 μm. If it is less than 50 μm, sufficient iron-chromium complex oxide will not be formed, and if it is greater than 200 μm, there is a risk of peeling.

[0045] For example, a high-velocity gas spraying method can be used for thermal spraying. For example, kerosene, C3H8, C2H2, or C3H6 can be used as the fuel gas in the high-velocity gas spraying method. For example, the gas pressure of the fuel gas may be 0.1 to 1 MPa, the flow rate of the fuel gas may be 10 to 500 l / min, the pressure of the oxygen gas may be 0.1 to 1 MPa, and the flow rate of the oxygen gas may be 100 to 1200 l / min.

[0046] A hearth roll 20 with a thermal spray coating 22 is placed in the furnace body 21, and the steel plate 10 is subjected to continuous annealing. As described above, the environment in which the hearth roll 20 is used is a high-temperature environment of 850°C or higher. The topcoat layer 22a of the hearth roll 20 used in such a high-temperature environment has a build-up prevention function, which prevents foreign matter from adhering to the hearth roll 20. As the hearth roll 20 is used further, the topcoat layer 22a gradually thins due to wear, and tortoise-shell-like cracks develop throughout the topcoat layer 22a. These cracks include cracks that reach the undercoat layer 22b.

[0047] Then, the iron oxide powder and furnace atmosphere gas adhering to the steel plate 10 penetrate through cracks formed in the topcoat layer 22a and reach the interface between the topcoat layer 22a and the undercoat layer 22b. The furnace atmosphere gas that reaches the interface reacts with the chromium contained in the undercoat layer 22b, causing external oxidation of the chromium. The chromium oxide produced by this external oxidation reacts with the iron oxide powder to form an iron-chromium complex oxide layer with excellent build-up resistance at the interface between the topcoat layer 22a and the undercoat layer 22b. This iron-chromium complex oxide layer gradually grows as the topcoat layer 22a thins. When the topcoat layer 22a disappears, the iron-chromium complex oxide layer is exposed on the roll surface, allowing the steel sheet 10 to be conveyed while in frictional contact with the iron-chromium complex oxide layer. Therefore, the build-up resistance of the hearth roll 20 can be maintained even after the topcoat layer 22a has disappeared.

[0048] Here, we will explain the behavior of the thermal spray coating when a hearth roll covered only by the undercoat layer 22b, in other words, a hearth roll without the topcoat layer 22a (hereinafter also referred to as the "comparative example hearth roll") is used in a furnace. Since the undercoat layer 22b is exposed on the roll surface of the comparative example hearth roll, the chromium contained in the undercoat layer 22b reacts with oxygen in the furnace atmosphere to produce chromium oxide. When exposed to high temperatures (850°C or higher, especially 1000°C or higher), this chromium oxide evaporates, and because of the high evaporation pressure, the thermal spray coating may deteriorate and its build-up resistance may decrease, and build-up may occur in the initial stages of roll use. In contrast, in this embodiment, the chromium oxide in the undercoat layer 22b is consumed in the formation of the iron-chromium complex oxide layer without evaporating, thus preventing deterioration of the thermal spray coating 22 due to the evaporation of chromium oxide.

[0049] Next, the present invention will be specifically described with reference to examples. We conducted build-up resistance tests and crack resistance tests by varying the composition of the topcoat layer (ceramic thermal spray coating) and the undercoat layer (chromium-containing cermet thermal spray coating). (Build-up resistance test) The substrate surface was sprayed with two layers of thermal spray material using the high-velocity flame (HVOF) method. The thickness of the topcoat layer was approximately 30 μm. The thickness of the undercoat layer was approximately 100 μm. Table 1 shows the film components of the topcoat layer, and Table 2 shows the film components of the undercoat layer. [Table 1] [Table 2]

[0050] The thermal spraying conditions were set as follows: Combustion energy source: Kerosene, oxygen Kerosene amount: 16~24L / Hour Oxygen flow rate: 780-980 L / min Thermal spray distance: 200~450mm

[0051] Iron oxide powder was sprinkled on the surface of the topcoat layer as a build-up source, and a crescent-shaped weight was placed on top of it. The weight was then slid while heating to 1050°C in a nitrogen gas atmosphere (with trace amounts of oxygen). This sliding treatment was carried out for a total of 12 hours, with additional build-up source being added each time. After cooling, the amount of iron oxide adhering to the surface of the topcoat layer was measured using X-ray fluorescence (previous build-up resistance test). Samples with a total iron oxide content of 15% to 30% were evaluated as having excellent build-up resistance and rated A. Samples with a total iron oxide content of less than 15% were evaluated as having very good build-up resistance and rated AA. Samples with a total iron oxide content of more than 30% were evaluated as having poor build-up resistance and rated B.

[0052] After conducting the build-up resistance test in the first phase, the surface was polished until the top coat was almost completely removed. Then, under the build-up resistance test conditions described above, the surface was heated in an atmosphere only without sliding for more than 168 hours. The build-up resistance test was then resumed, and the surface was slid for a total of 12 hours. The amount of iron oxide adhering to the undercoat layer surface was measured using X-ray fluorescence (late-stage build-up resistance test). Samples with a total iron oxide content of 15% to 30% were evaluated as having excellent build-up resistance and rated A; samples with a total iron oxide content of less than 15% were evaluated as having very good build-up resistance and rated AA; and samples with a total iron oxide content of more than 30% were evaluated as having poor build-up resistance and rated B.

[0053] (Crack resistance test) Crack resistance was evaluated using a Vickers hardness tester. After the initial build-up resistance test was completed, the cross-section of the sample was mirror-polished, and a roughly square Vickers indentation was formed on the cross-section of the thermal spray coating under a load of 5 kg. The crack resistance of the thermal spray coating was evaluated based on the cracks in the diagonal direction of the Vickers indentation. If no cracks occurred in the diagonal direction, the material was evaluated as having excellent crack resistance and received a rating of A. If clear cracks occurred in the diagonal direction, the material was evaluated as having poor crack resistance and received a rating of B.

[0054] The test results are shown in Tables 3 and 4. [Table 3] [Table 4] Furthermore, the inventors also conducted build-up resistance tests on samples in which only sample B2 was thermal sprayed onto the substrate (i.e., single-layer thermal spraying). The build-up resistance evaluation after the previous build-up resistance test was B. [Explanation of symbols]

[0055] 1. Continuous annealing furnace 2 Furnace body 3,4 Gas supply nozzles 10 steel plate

Claims

1. A hearth roll in which the roll surface that comes into contact with the steel plate is covered with a thermal spray coating, The aforementioned thermal spray coating is A topcoat layer with build-up resistance (however, the chromium oxide content shall be 0% by mass), An undercoat layer consisting of a cermet thermal spray coating containing 20% ​​to 30% by mass of chromium, It has, When the linear expansion coefficient of the roll substrate is A, the linear expansion coefficient of the topcoat layer is B, and the linear expansion coefficient of the undercoat layer is C, then B / A is 0.35 or more and 0.5 or less, and C / A is 0.55 or more. A hearth roll characterized by the following features.

2. The hearth roll according to claim 1, characterized in that the top coat layer consists of an oxide-based ceramic thermal spray coating.

3. The top coat layer contains at least one of aluminum oxide and titanium oxide, and the oxide content is 85% by mass or more. The hearth roll according to feature 2.

4. The cermet spray coating comprises a heat-resistant alloy and an oxide-based or carbide-based ceramic. The hearth roll according to claim 1 or 2.

5. The cermet spray coating comprises a heat-resistant alloy and an oxide-based or carbide-based ceramic. The hearth roll according to claim 3.

6. The aforementioned oxide-based ceramics are complex oxides of rare earth elements and aluminum or transition metals (excluding rare earth elements). The hearth roll according to feature 4.

7. The transition metal is one of Ta, Zr, and Cr. The hearth roll described in claim 6.

8. The carbide-based ceramics include at least one of chromium carbide and titanium carbide. The hearth roll according to feature 4.

9. The heat-resistant alloy comprises at least one of cobalt and nickel, and chromium. The hearth roll according to feature 4.

10. A method for using a hearth roll according to claim 1, After the topcoat layer is removed by friction with the steel plate, the steel plate is transported while in contact with the iron-chromium complex oxide layer that has formed at the interface between the topcoat layer and the undercoat layer. A method of using hearth rolls characterized by the following.

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