Hearth roll and method for manufacturing hearth roll

Abstract

To provide a hearth roll including a spray coating excellent in build-up resistance, crack resistance, thermal shock resistance and peeling resistance.SOLUTION: A hearth roll has a surface covered with a spray coating and contacting a steel sheet. The spray coating includes: a topcoat layer consisting of a nickel group alloy spray coating containing aluminum of, by mass%, 3% or more 15% or less and chromium of 25% or less and 10% or more; and an under coat layer consisting of a nickel group alloy spray coating containing molybdenum of, by mass%, 5% or more and 15% or less.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a hearth roll and the like used in a continuous annealing furnace. [Background technology]

[0002] Heat treatment furnaces for heat treating metal sheets such as steel sheets are equipped with transport rolls (hereinafter referred to as "hearth rolls") for loading, transporting, and unloading the steel sheets. When the hearth rolls are rotated at high speed to pass the steel sheets through them, phenomena such as slippage, meandering, adhesion of dirt to the transport roll surfaces, and build-up of the steel sheets occur. In particular, the hearth rolls arranged in continuous annealing furnaces transport the steel sheets at high temperatures, and therefore build-up is likely to occur on the hearth roll surfaces.

[0003] Build-up is a phenomenon in which iron, iron oxide, etc. present on the surface of a steel sheet adheres to and accumulates on the surface of a hearth roll. When build-up occurs, the shape of the deposits resulting from the build-up is transferred to the surface of the steel sheet, impairing the surface quality and not only degrading the grade of the steel sheet, but also requiring maintenance to remove the foreign matter adhering to the hearth roll surface during regular maintenance, which is one of the causes of reduced productivity.

[0004] Furthermore, hearth rolls used in continuous annealing furnaces, in which the furnace temperature is 850° C. or higher and the interior of the furnace is in a non-reducing atmosphere (e.g., a nitrogen atmosphere), are required to have durability against a high-temperature nitrogen atmosphere in addition to build-up resistance.

[0005] Patent Document 1 discloses a hearth roll for a continuous heat treatment furnace, which is intended to improve build-up resistance, and in which a nickel-chromium alloy is sprayed onto the surface of the hearth roll and chromium carbide is sprayed onto the outer layer.

[0006] Patent Document 2 discloses a hearth roll for a heat treatment furnace intended to improve build-up resistance, in which a Ni-based alloy (Ni: 70-80 mass%, Cr: 3-15 mass%, Al: 3-15 mass%, Mo: 3-10 mass%, Fe: 3-10 mass%) is sprayed onto the surface of the hearth roll as an undercoat, a low-carbon Cr-Fe alloy is sprayed as an intermediate coat, and Cr3C2 is sprayed as a top coat.

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

[0008] Patent Document 4 describes a furnace roll for improving build-up resistance and oxidation resistance, which is made of ceramic powder (Cr 23 This paper discloses an in-furnace roll, the surface of which is thermally sprayed with 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, the remainder being Co and / or Ni).

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

[0010] Patent Document 6 discloses a hearth roll for a continuous annealing furnace intended to prevent slippage, meandering, adhesion of dirt to the surface of a conveying roll, build-up, and the like of a steel sheet, the hearth roll having a cermet coating on its surface which is made of ceramics (containing more than 50 to 90 vol% Cr3C2, 1 to 40 vol% Al2O3, 0 to 3 vol% Y2O3, 0 to 40 vol% ZrB2, the balance being unavoidable impurities and pores) and a heat-resistant alloy (containing 5 to 20 mass% Cr, 5 to 20 mass% Al, and 0.1 to 6 mass% of one or both of Y and Si, the balance being one or both of Co and Ni and unavoidable impurities), in which 50 to 90 vol% of the cermet coating is the ceramics and the balance 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 spray coating, the ceramic spray coating containing chromium carbide as a main component, dispersed chromium oxide, and containing 5 to 35 wt% oxygen.

[0012] Patent Document 8 discloses a hearth roll for continuous annealing of steel sheets in a heat treatment furnace, which is composed of a roll body having a flat portion and a tapered portion, in which 20 to 60% of the total roll length of the roll body is the flat portion, and the flat portion is provided with a ceramic thermal spray coating of a single or composite oxide-based material such as ZrO2, Cr2O3 or Al2O3. [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 Summary of the Invention [Problem to be solved by the invention]

[0014] None of the hearth rolls disclosed in Patent Documents 1 to 7 have sufficient build-up resistance.

[0015] In addition, in Patent Document 8, when exposed to furnace heat, there is a large difference in thermal expansion between the ceramic spray coating and the heat-resistant steel of the roll material, so that the ceramic spray coating may crack or peel off. In addition, once the ceramic spray coating cracks, chromium and other components of the roll material are oxidized to generate metal oxides, which makes the ceramic spray coating more likely to peel off. The same problem exists with cermet spray coatings.

[0016] Furthermore, when aluminum contained in the thermal spray coating reacts with nitrogen that has penetrated from the roll substrate to form an aluminum nitride layer at the interface between the thermal spray coating and the roll substrate, the thermal spray coating becomes prone to peeling.

[0017] An object of the present invention is to provide a hearth roll having a thermal spray coating that is excellent in build-up resistance, crack resistance, thermal shock resistance and peeling resistance. [Means for solving the problem]

[0018] In order to solve the above problems, the hearth roll of the present invention is (1) a hearth roll in which the roll surface that comes into contact with a steel sheet is covered with a thermal spray coating, the thermal spray coating comprising: a topcoat layer made of a nickel-based alloy thermal spray coating containing, by mass%, 3% to 15% aluminum and 10% to 25% chromium; and an undercoat layer made of a nickel-based alloy thermal spray coating containing, by mass%, 5% to 15% molybdenum.

[0019] (2) The hearth roll according to (1) above, wherein the nickel-based alloy thermal spray coating of the top coat layer contains niobium and / or tungsten as optional components.

[0020] (3) The hearth roll according to (2) above, characterized in that the content of the optional components contained in the top coat layer is 8% by mass or less.

[0021] (4) A hearth roll according to any one of (1) to (3) above, characterized in that the nickel-based alloy thermal spray coating of the undercoat layer contains titanium and / or niobium as an optional component.

[0022] (5) The hearth roll according to (4) above, characterized in that the content of the optional components in the undercoat layer is 8% by mass or less.

[0023] In order to solve the above problems, the manufacturing method of the hearth roll according to the present invention includes: (6) a first step of forming an undercoat layer by thermally spraying a thermal spray powder made of a nickel-based alloy containing, by mass%, 5% to 15% of molybdenum on a roll surface of the hearth roll; and a second step of forming a topcoat layer on the undercoat layer by spraying a thermal spray powder made of a nickel-based alloy containing, by mass%, 3% to 15% aluminum and 10% to 25% chromium.

[0024] (7) The method for producing a hearth roll according to (6) above, wherein the nickel-based alloy sprayed in the second step contains niobium and / or tungsten as optional components.

[0025] (8) The method for producing a hearth roll according to (7) above, characterized in that the content of the optional components contained in the nickel-based alloy sprayed in the second step is 8% or less, by mass percent.

[0026] (9) The method for manufacturing a hearth roll according to any one of (6) to (8) above, wherein the nickel-based alloy sprayed in the first step contains titanium and / or niobium as optional components.

[0027] (10) The method for producing a hearth roll according to (9) above, characterized in that the content of the optional components contained in the nickel-based alloy sprayed in the first step is 8% or less, by mass percent. Effect of the Invention

[0028] According to the present invention, it is possible to provide a hearth roll having a thermal spray coating which is excellent in build-up resistance, crack resistance, thermal shock resistance and peeling resistance. [Brief description of the drawings]

[0029] [Figure 1] FIG. 1 is a schematic diagram of a continuous annealing furnace. [Diagram 2] FIG. 2 is a perspective view of a hearth roll (immediately after a thermal spray coating has been formed). [Diagram 3] FIG. 2 is a perspective view of a hearth roll (after being used in a continuous annealing furnace). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] A hearth roll according to an embodiment of the present invention is a transport roll disposed in a continuous annealing furnace, and has a thermal spray coating on the roll surface. Fig. 1 is a schematic diagram of a continuous annealing furnace. Fig. 2 is a perspective view of the hearth roll immediately after a nickel-based alloy thermal spray coating is formed, and shows an enlarged cross section of a part of the roll surface. Fig. 3 is a perspective view of a hearth roll used in a continuous annealing furnace, and shows an enlarged cross section of a part of the roll surface.

[0031] With reference to FIG. 1, a continuous annealing furnace 1 of this embodiment includes a furnace body 2 and a plurality of gas supply nozzles 3, 4 for supplying an atmospheric gas into the furnace body 2. The furnace body 2 continuously anneals a steel sheet 10 in the atmospheric gas. A non-reducing atmospheric gas can be used as the atmospheric gas. The non-reducing atmospheric gas contains nitrogen as a main component and a trace amount of oxygen. The non-reducing gas does not include a mixed gas of nitrogen and hydrogen. The hearth roll of the present invention may be used in such a mixed gas atmosphere. The furnace temperature of the furnace body 2 is preferably 850°C or higher, and more preferably 1000°C or higher and 1200°C or lower.

[0032] A hearth roll 20 is disposed in the furnace body 2. The hearth roll 20 rotates about its longitudinal axis, thereby transporting the steel sheet 10 inside the furnace body 2. As described above, the thermal spray coating 22 is formed on the surface of the roll base material 21 in the hearth roll 20. For the roll base material 21, heat-resistant steel (e.g., SUS310S, SCH22) that is generally used as the base material for the hearth roll 20 can be used. The following describes the reasons for limiting the composition of the thermal spray coating 22. The unit of the content of elements in the composition is always "mass %", but hereinafter, unless otherwise specified, it will be simply represented as "%". The thermal spray coating 22 includes a topcoat layer 22a and an undercoat layer 22b. The topcoat layer 22a is a nickel-based alloy sprayed coating containing aluminum and chromium as essential elements, and the undercoat layer 22b is a nickel-based alloy sprayed coating containing molybdenum as an essential element.

[0033] (Regarding aluminum of the topcoat layer 22a) When the topcoat layer 22a is taken as 100%, the aluminum content is 3% or more, and preferably 6% or more. Aluminum is an element necessary for generating an alumina layer 22a1 on the surface of the topcoat layer 22a. The alumina layer 22a1 appears on the topcoat layer 22a by using the hearth roll 20, and the details will be described later.

[0034] By containing 3% or more of aluminum, a dense alumina layer 22a1 can be continuously formed on the surface of the top coat layer 22a. Since the alumina layer 22a1 is poorly reactive to foreign matter that causes build-up, the build-up resistance of the hearth roll 20 is improved by continuously forming the dense alumina layer 22a1 on the surface of the top coat layer 22a. Furthermore, the aluminum contained in the topcoat layer 22a is consumed in the production of the alumina layer 22a1, which can suppress the reaction between the nitrogen contained in the atmospheric gas and the aluminum to produce aluminum nitride. By suppressing the production of aluminum nitride, peeling of the topcoat layer 22a can be prevented. However, if the aluminum content is excessively increased, the topcoat layer 22a becomes weak and cracks occur, so the aluminum content must be set to 15% or less.

[0035] (Chromium in the topcoat layer 22a) When the topcoat layer 22a is taken as 100%, the chromium content is 10% or more, and preferably 15% or more. Chromium is an element necessary for making the alumina layer 22a1 dense. By forming the alumina layer 22a1 densely, the build-up resistance of the topcoat layer 22a can be improved. However, if an excessive amount of chromium is contained, the topcoat layer 22a becomes more susceptible to cracking, so the chromium content must be set to 25% or less. Chromium is less susceptible to oxidation than aluminum, so chromium oxide is less likely to be generated when the hearth roll 20 is used in a high-temperature environment. In other words, the generation of alumina is prioritized, so the generation of chromium oxide is suppressed. Furthermore, the formation of the alumina layer 22a1 makes it difficult for chromium to come into contact with oxygen in the atmosphere, so that chromium oxide is less likely to be generated even after the formation of the alumina layer 22a1.

[0036] (Nickel in the topcoat layer 22a) The difference in thermal expansion coefficient between the nickel-based alloy and the roll substrate 21 is smaller than the difference in thermal expansion coefficient between the ceramic sprayed coating and the roll substrate 21. The difference in thermal expansion coefficient between the nickel-based alloy and the roll substrate 21 is smaller than the difference in thermal expansion coefficient between the cermet sprayed coating and the roll substrate 21. Therefore, by forming the top coat layer 22a from a nickel-based alloy instead of a ceramic sprayed coating or a cermet sprayed coating, it is possible to improve the thermal shock resistance and peeling resistance of the top coat layer 22a.

[0037] Nickel-based alloys may contain unavoidable impurities. For example, unavoidable impurities may flow in as contaminants from a mixing vessel during the manufacturing process of the thermal spray coating, carbon may flow in from kerosene during high-velocity gas spraying, or the impurities may flow in as a result of a decarburization reaction caused by plasma spraying. The unavoidable impurities may include at least one of oxygen, boron, carbon, silicon, phosphorus, iron, and manganese. It is desirable to keep the content of unavoidable impurities below 2%.

[0038] The nickel-based alloy of the topcoat layer 22a may contain niobium and / or tungsten as optional components. The inclusion of such optional components in the nickel-based alloy can increase the high-temperature hardness of the topcoat layer 22a. If the optional components are excessively increased, the ratio of the basic components decreases, and there is a risk that the effects of the basic components described above will not be achieved. Therefore, it is desirable to limit the optional components to 0% or more and 8% or less.

[0039] (Molybdenum in the undercoat layer 22b) When the undercoat layer 22b is taken as 100%, the molybdenum content is 5% or more, and preferably 7% or more. The undercoat layer 22b of this embodiment functions as a barrier layer (diffusion barrier layer) that suppresses the generation of aluminum nitride caused by the reaction between the aluminum contained in the topcoat layer 22a and the nitrogen contained in the roll substrate 21. To realize this function, molybdenum is an essential element in the undercoat layer 22b. Note that nitrogen contained in the roll substrate 21 is an inevitable impurity. Furthermore, because the undercoat layer 22b suppresses the formation of aluminum nitride, the aluminum contained in the topcoat layer 22a can be directed to the formation of the alumina layer 22a1, thereby preventing the thermal spray coating 22 from peeling off.

[0040] However, if the molybdenum content is excessively increased, the undercoat layer 22b becomes hard and brittle, which may cause cracks to occur in the undercoat layer 22b. For the above reasons, the molybdenum content must be set to 15% or less.

[0041] (Nickel in the undercoat layer 22b) The difference in thermal expansion coefficient between the nickel-based alloy and the roll substrate 21 is smaller than the difference in thermal expansion coefficient between the ceramic spray coating and the roll substrate 21. The difference in thermal expansion coefficient between the nickel-based alloy and the roll substrate 21 is smaller than the difference in thermal expansion coefficient between the cermet spray coating and the roll substrate 21. Therefore, by forming the undercoat layer 22b from a nickel-based alloy rather than a ceramic spray coating or a cermet spray coating, the thermal shock resistance and peeling resistance of the undercoat layer 22b can be improved. In other words, by forming the spray coating 22 (topcoat layer 22a and undercoat layer 22b) from a nickel-based alloy, the thermal shock resistance and peeling resistance of the spray coating 22 can be improved.

[0042] The nickel-based alloy of the undercoat layer 22b may also contain unavoidable impurities. The unavoidable impurities may, for example, flow in as contaminants from a mixing vessel during the manufacturing process of the thermal spray coating, flow in as carbon from kerosene during high-velocity gas spraying, or flow in as a result of a decarburization reaction during plasma spraying. The unavoidable impurities include at least one of oxygen, boron, carbon, silicon, phosphorus, iron, and manganese. It is desirable to keep the content of unavoidable impurities below 2%.

[0043] The nickel-based alloy of the undercoat layer 22b may contain titanium and / or niobium as optional components. The inclusion of optional components in the nickel-based alloy can increase the hardness of the topcoat layer 22a. In addition, when titanium is added, the nitrogen that has entered from the roll substrate 21 reacts with the titanium to generate titanium nitride (dispersed particles), so that the nitrogen can be stabilized. If the optional components are excessively increased, the ratio of the basic components decreases, and there is a risk that the effects of the basic components described above will not be achieved. Therefore, it is preferable to limit the optional components to 0% or more and 8% or less.

[0044] The undercoat layer 22b is formed by spraying spray particles onto the surface of the hearth roll 20. For example, atomized powder can be used as the spray particles. A molten metal made of a nickel-based alloy adjusted to the above-mentioned composition is prepared, and the spray particles can be produced from the molten metal by an atomization method. For example, a high-velocity gas spraying method can be used as the spraying method. The topcoat layer 22a is formed by spraying spray particles onto the undercoat layer 22b. For example, atomized powder can be used as the spray particles. A molten metal made of a nickel-based alloy adjusted to the above-mentioned composition is prepared, and spray particles can be produced from the molten metal by an atomization method. For example, a high-velocity gas spraying method can be used as the spraying method.

[0045] The hearth roll 20 on which the thermal spray coating 22 is formed is placed in the furnace body 2, and the steel sheet 10 is subjected to continuous annealing treatment. As described above, the environment in which the hearth roll 20 is used is a high-temperature environment of 850°C or higher. By using the hearth roll 20 in such a high-temperature environment, an alumina layer 22a1 is formed on the surface layer of the topcoat layer 22a. The alumina layer 22a1 enhances the build-up resistance of the hearth roll 20. The coating body 22a2 of the topcoat layer 22a has a reduced aluminum concentration because aluminum is consumed in the production of alumina. In other words, the coating body 22a2 can be said to be a nickel-based alloy sprayed layer with a reduced aluminum concentration compared to the topcoat layer 22a before the hearth roll is used. The effect of the alumina layer 22a1 has been described above, so a description thereof will not be repeated.

[0046] Here, when a ceramic material or a cermet material containing chromium carbide is sprayed onto the topcoat layer 22a, the chromium in the chromium carbide is oxidized to generate chromium oxide. This chromium oxide evaporates when exposed to high temperatures (850°C or higher, particularly 1000°C or higher), and the evaporation pressure is high, so there is a risk that the sprayed coating will deteriorate and the build-up resistance will decrease. As described above, the topcoat layer 22a of this embodiment has a composition that makes it difficult for chromium oxide to be generated, and therefore deterioration of the sprayed coating due to evaporation of chromium oxide can be suppressed.

[0047] Next, the present invention will be specifically described with reference to examples. [Table 1] (Build-up resistance test) Samples were prepared by spraying the surface of a SUS304 substrate with various compositions shown in Table 1 using a high velocity flame spraying (HVOF) method to form a sprayed coating with a thickness of 100 μm. Comparative Example 7 used a cermet sprayed coating containing 55 mass% Cr2C3 and 45 mass% CoNiAlCr alloy. The spraying conditions were set as follows: Combustion energy source: kerosene, oxygen Kerosene amount: 16~24L / Hour Oxygen volume: 780~980L / min Spray distance: 200~450mm Fe powder was scattered on the sample surface as a build-up source, and a crescent-shaped weight was placed on top of it and slid while heating to 1050°C in a nitrogen gas atmosphere (containing a trace amount of oxygen). After cooling, the amount of Fe attached to the sample surface was measured using fluorescent X-rays. Samples with a total Fe content of 15% to 30% by mass were evaluated as having excellent build-up resistance and rated A, samples with a total Fe content of less than 15% by mass were evaluated as having very excellent build-up resistance and rated AA, and samples with a total Fe content of more than 30% by mass were evaluated as having poor build-up resistance and rated B.

[0048] The build-up resistance of Examples 1-2 and Examples 6-10 was rated as "AA." The build-up resistance of Examples 3-5 was lower than that of Example 1 etc. because the aluminum and / or chromium contained in the top coat layer was slightly low. However, the build-up resistance was rated as "A." The build-up resistance of Comparative Examples 1-2 was rated as "B" because the aluminum or chromium contained in the top coat layer was too low.

[0049] (peeling resistance) After heating the sample in air, it was immediately placed in room temperature water to cool. The heating conditions were a heating temperature of 1050°C and a heating time of 168 hours. After cooling, the sample was cut and the cross section was observed. Peel resistance was evaluated based on the occurrence of cracks at the interface between the topcoat layer and the undercoat layer, and at the interface between the undercoat layer and the substrate. When no cracks occurred at the interface, the peel resistance was deemed very excellent and rated as AA. When only minor cracks occurred at the interface, the peel resistance was deemed excellent and rated as A. When clear cracks occurred at the interface, the peel resistance was deemed poor and rated as B. The peel resistance of all of Examples 1-2, 4, 6-7, and 9-10 was rated "AA." In Examples 3, 5, and 8, the aluminum contained in the topcoat layer or the molybdenum contained in the undercoat layer was slightly less, so the peeling resistance was lower than that of Example 1, etc. However, the evaluation was "A". In Comparative Examples 1 and 6, the aluminum contained in the topcoat layer or the molybdenum contained in the undercoat layer was too low, so aluminum nitride was formed at the interface between the topcoat layer and the undercoat layer, and the peeling resistance was rated as "B". In Comparative Example 7, the topcoat layer was formed of a cermet spray coating, and therefore the difference in thermal expansion coefficient between the topcoat layer and the substrate was large, and the peeling resistance was evaluated as "B."

[0050] (Crack resistance) The crack resistance was evaluated using a Vickers hardness tester. The cross section of the sample on which the thermal spray coating was formed was mirror-polished, and a roughly square Vickers indentation was made on the cross section of the thermal spray coating with 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. When no cracks occurred in the diagonal direction, the crack resistance was deemed very excellent and rated as AA. When the cracks in the diagonal direction were slight, the crack resistance was deemed excellent and rated as A. When clear cracks occurred in the diagonal direction, the crack resistance was deemed poor and rated as B. In Comparative Examples 3 and 4, the aluminum or chromium contained in the topcoat layer was excessive, which caused clear cracks in the topcoat layer, and the evaluation was "B." In Comparative Example 5, the molybdenum contained in the undercoat layer was excessive, which caused clear cracks in the undercoat layer, and the evaluation was "B."

[0051] (Thermal shock resistance) The sample was heated at 1000°C for 20 minutes and then immediately cooled in room temperature water. After repeating this heating and cooling cycle 20 times, the topcoat layer was visually inspected for peeling to evaluate thermal shock resistance. When the topcoat layer did not peel off at all, it was deemed to have excellent thermal shock resistance and was rated as AA. 2If the thermal shock resistance is suppressed to below 1 mm, it is considered to have excellent thermal shock resistance and is rated as A. 2 When the above peeling was confirmed, the thermal shock resistance was deemed poor and the evaluation was given as B. In all of Examples 1 to 10, the topcoat layer and undercoat layer were composed of nickel-based alloy sprayed coatings, so the difference in thermal expansion coefficient between these sprayed coatings and the substrate was small, and the thermal shock resistance was rated "A." In Comparative Example 7, since the topcoat layer was composed of a cermet spray coating, the difference in thermal expansion coefficient between the topcoat layer and the substrate was excessively large, and the thermal shock resistance and peeling resistance were evaluated as "B." For Comparative Example 7, the evaluation tests for build-up resistance and cracking resistance were not performed. [Explanation of symbols]

[0052] 1 Continuous annealing furnace 2. Furnace body 3,4 Gas supply nozzle 20 Hearth Roll 21 Roll substrate 22 Thermal spray coating 22a Topcoat layer 22a1 Alumina layer 22a2 Membrane body 22b Undercoat layer

Claims

1. A hearth roll in which the roll surface that comes into contact with a steel sheet is covered with a thermal spray coating, The thermal spray coating is a topcoat layer made of a nickel-based alloy thermal spray coating containing, by mass%, 3% or more and 15% or less of aluminum and 10% or more and 25% or less of chromium; an undercoat layer made of a nickel-based alloy thermal spray coating containing, by mass%, 5% to 15% of molybdenum; A hearth roll comprising:

2. The nickel-based alloy thermal spray coating of the topcoat layer contains niobium and / or tungsten as an optional component.

2. The hearth roll according to claim 1 .

3. 3. The hearth roll according to claim 2, wherein the content of the optional components contained in the top coat layer is 8% by mass or less.

4. The nickel-based alloy thermal spray coating of the undercoat layer contains titanium and / or niobium as an optional component.

4. The hearth roll according to claim 1, wherein the hearth roll is a flat roll.

5. 5. The hearth roll according to claim 4, wherein the content of the optional components contained in the undercoat layer is 8% by mass or less.

6. A method for manufacturing a hearth roll, comprising the steps of: a first step of forming an undercoat layer by spraying a thermal spray powder made of a nickel-based alloy containing, by mass%, 5% to 15% of molybdenum on a roll surface of the hearth roll; a second step of forming a topcoat layer on the undercoat layer by spraying a thermal spray powder made of a nickel-based alloy containing, by mass%, 3% to 15% aluminum and 10% to 25% chromium; A method for producing a hearth roll, comprising the steps of:

7. The nickel-based alloy sprayed in the second step optionally contains niobium and / or tungsten. The method for producing a hearth roll according to claim 6 .

8. The method for manufacturing a hearth roll according to claim 7, characterized in that the content of the optional components contained in the nickel-based alloy sprayed in the second step is 8% or less by mass %.

9. The nickel-based alloy sprayed in the first step contains titanium and / or niobium as optional components.

9. The method for manufacturing a hearth roll according to claim 6, wherein the hearth roll is made of a carbide material.

10. The method for manufacturing a hearth roll according to claim 9, wherein the content of the optional components contained in the nickel-based alloy sprayed in the first step is 8% or less by mass %.

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