Hearth roll and manufacturing method of hearth roll

Abstract

To provide a spray coating film with an excellent build-up resistance, a peeling resistance, a crack resistance, and a hardness, or a hearth roll having a welding film.SOLUTION: A hearth roll has a roll surface that contacts with a steel plate and is covered by a spray coating film. The spray coating film is a nickel-based alloy spray coating film containing 3 mass% or more and 15 mass% or less of aluminum, 1 mass% or more and 5 mass% or less of titanium, 5 mass% or more and 15 mass% or less of molybdenum, 10 mass% or more and 25 mass% or less of chromium. When the hearth roll is used under a high temperature environment in a continuous annealing furnace, a dense alumina layer is formed on a surface of the spray coating film and a build-up is suppressed. A peeling of the spray coating film is prevented because nitrogen entering from a roll base material is trapped with titanium and the generation of aluminum nitride is suppressed.SELECTED DRAWING: Figure 2

Description

[Technical Field]

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

[0002] In a heat treatment furnace for heat-treating a metal sheet such as a steel sheet, transport rolls (hereinafter referred to as "hearth rolls") are installed to load, transport, and unload the steel sheet. When the hearth rolls are rotated at high speed to pass the steel sheet through them, phenomena such as slippage, meandering, adhesion of dirt to the transport roll surface, and buildup occur. In particular, since the hearth rolls arranged in a continuous annealing furnace transport the steel sheet at a high temperature, buildup is likely to occur on the hearth roll surface.

[0003] Build-up is a phenomenon in which iron, iron oxides, etc. present on the surface of a steel sheet adhere to and accumulate 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 steel sheet surface, impairing 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 periodic maintenance, which is one of the causes of reduced productivity.

[0004] Furthermore, hearth rolls used in continuous annealing furnaces, which have furnace temperatures of 850°C or higher and are filled with 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 wt % 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 mixed powder 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, 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 the conveying roll, build-up, and the like of steel sheets, characterized in that the roll has a cermet coating on its surface that is 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 balance 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 both of Y and Si, with the balance being one or both of Co and Ni and unavoidable impurities), with 50 to 90 vol% of the cermet coating being the ceramics and the balance being 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, which is used in a heat treatment furnace and is composed of a roll body having a flat portion and a tapered portion, and in which the roll body has a flat portion that accounts for 20 to 60% of the total roll length, and the flat portion is provided with a ceramic thermal spray coating of a single or composite oxide system of 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. In addition, in Patent Document 8, when exposed to furnace heat, there is a large difference in thermal expansion between the ceramic sprayed coating and the heat-resistant steel of the roll material, so the ceramic sprayed coating may crack or peel off. Furthermore, once the ceramic sprayed coating cracks, chromium and other components of the roll material are oxidized to produce metal oxides, making the ceramic sprayed coating even more susceptible to peeling. The same problem exists with cermet sprayed coatings. Furthermore, if the aluminum contained in the thermal spray coating reacts with nitrogen that has penetrated from the roll substrate, and an aluminum nitride layer is formed at the interface between the thermal spray coating and the roll substrate, the thermal spray coating becomes prone to peeling. Furthermore, the coating of the hearth roll is required to have crack resistance and hardness.

[0015] An object of the present invention is to provide a hearth roll having a thermal sprayed coating or a welded coating that is excellent in build-up resistance, peeling resistance, cracking resistance, and hardness. [Means for solving the problem]

[0016] In order to solve the above problems, the method for manufacturing a hearth roll according to the present invention is characterized in that (1) a nickel-based alloy coating is formed on the roll surface of the hearth roll by spraying a thermal spray powder made of a nickel-based alloy containing, by mass%, 3% to 15% aluminum, 1% to 5% titanium, 5% to 15% molybdenum, and 10% to 25% chromium, or by welding a welding material made of the nickel-based alloy.

[0017] (2) The method for producing a hearth roll according to (1) above, wherein the nickel-based alloy coating contains niobium and / or tungsten as optional components.

[0018] (3) The method for producing a hearth roll according to (2) above, characterized in that the content of the optional components contained in the nickel-based alloy coating is 8% or less by mass.

[0019] (4) A hearth roll in which the roll surface that comes into contact with the steel sheet is covered with a thermal spray coating, characterized in that the thermal spray coating is a nickel-based alloy thermal spray coating containing, by mass%, 3% to 15% of aluminum, 1% to 5% of titanium, 5% to 15% of molybdenum, and 10% to 25% of chromium.

[0020] (5) The hearth roll according to (4) above, wherein the nickel-based alloy thermal spray coating contains niobium and / or tungsten as optional components.

[0021] (6) The hearth roll according to (5) above, wherein the content of the optional components contained in the nickel-based alloy thermal spray coating is 8% or less by mass.

[0022] (7) A hearth roll in which the roll surface that comes into contact with the steel sheet is covered with a weld coating, characterized in that the weld coating is a nickel-based alloy weld coating containing, by mass%, 2% to 13% aluminum, 0.3% to 4.5% titanium, 4% to 13.5% molybdenum, 9% to 23% chromium, and 2% to 15% iron.

[0023] (8) The hearth roll according to (7) above, wherein the nickel-based alloy weld coating contains niobium and / or tungsten as optional components.

[0024] (9) The hearth roll according to (8) above, characterized in that the content of the optional components contained in the nickel-based alloy weld coating is 8% or less by mass. [Effects of the Invention]

[0025] According to the present invention, it is possible to provide a hearth roll having a thermal spray coating or a welded coating that is excellent in build-up resistance, peeling resistance, cracking resistance, and hardness. [Brief explanation of the drawings]

[0026] [Figure 1] FIG. 1 is a schematic diagram of a continuous annealing furnace. [Figure 2] FIG. 2 is a perspective view of a hearth roll (immediately after a thermal spray coating is formed). [Figure 3] FIG. 1 is a perspective view of a hearth roll (after use in a continuous annealing furnace). [Figure 4] FIG. 2 is an image diagram of the cross-sectional structure of a thermal spray coating. DETAILED DESCRIPTION OF THE INVENTION

[0027] (First embodiment) A hearth roll according to an embodiment of the present invention is a transport roll disposed in a continuous annealing furnace and has a thermally sprayed coating on its surface. Fig. 1 is a schematic diagram of a continuous annealing furnace. Fig. 2 is a perspective view of the hearth roll immediately after the nickel-based alloy thermally sprayed coating is formed, showing an enlarged cross section of a portion of the roll surface. Fig. 3 is a perspective view of a hearth roll used in a continuous annealing furnace, showing an enlarged cross section of a portion of the roll surface.

[0028] Referring to FIG. 1 , a continuous annealing furnace 1 of this embodiment includes a furnace body 2 and a plurality of gas supply nozzles 3 and 4 that supply 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 its main component and a trace amount of oxygen. Note that the non-reducing gas does not include a mixed gas of nitrogen and hydrogen. Note that 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.

[0029] 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 substrate 21 in the hearth roll 20. The roll substrate 21 can be made of heat-resistant steel (e.g., SUS310S, SCH22) that is generally used as the substrate for the hearth roll 20. The reasons for limiting the composition of the thermal spray coating 22 are explained below. The unit of elemental content in the composition is "mass %," but hereinafter, unless otherwise specified, it will be simply expressed as "%." The thermal spray coating is a nickel-based alloy thermal spray coating containing aluminum, titanium, molybdenum, and chromium as essential elements.

[0030] (About aluminum) The aluminum content is 3% or more, and preferably 6% or more. Aluminum is an element necessary for producing an alumina layer 22a on the surface of the thermal spray coating 22. The alumina layer 22a appears on the thermal spray coating 22 by using the hearth roll 20, and details will be described later.

[0031] By including 3% or more aluminum, a dense alumina layer 22a can be continuously formed on the surface of the thermal spray coating 22. Since the alumina layer 22a is poorly reactive to foreign matter that causes buildup, the buildup resistance of the hearth roll 20 is improved by continuously forming the dense alumina layer 22a on the surface of the thermal spray coating 22.

[0032] Furthermore, the aluminum contained in the thermal spray coating 22 is consumed in the formation of the alumina layer 22a, which prevents aluminum from reacting with nitrogen contained in the atmospheric gas to form aluminum nitride. By preventing the formation of aluminum nitride, peeling of the thermal spray coating 22 can be prevented.

[0033] However, if the aluminum content increases excessively, the thermal spray coating 22 becomes brittle and cracks occur, so the aluminum content must be set to 15% or less.

[0034] (About titanium) The titanium content is 1% or more, and preferably 3% or more. Titanium is an element necessary for generating titanium nitride (in other words, for trapping nitrogen) in the thermal spray coating 22. Titanium nitride is precipitated in the thermal spray coating 22 by using the hearth roll 20.

[0035] The roll substrate 21 of the hearth roll 20 contains nitrogen as an unavoidable impurity. When nitrogen penetrates from the roll substrate 21 and reacts with aluminum contained in the thermal spray coating 22, aluminum nitride is produced. This aluminum nitride appears in a layer at the interface between the roll substrate 21 and the thermal spray coating 22, and once aluminum nitride is produced, the thermal spray coating 22 becomes prone to peeling. Nitrogen reacts more readily with titanium than aluminum, so by including titanium in the thermal spray coating 22, the production of titanium nitride is prioritized and the production of aluminum nitride is suppressed. Titanium nitride in the thermal spray coating 22 precipitates and disperses in a granular form, so it does not cause peeling of the thermal spray coating 22. Figure 4 is an image of the thermal spray coating 22 created based on a photograph of the thermal spray coating observed with an optical microscope. Titanium nitride precipitated in a granular form in the thermal spray coating is indicated by diamonds.

[0036] Furthermore, the titanium nitride precipitated in the form of particles is dispersed in the thermal spray coating 22, thereby increasing the hardness of the thermal spray coating 22. Titanium nitride is derived from nitrogen contained in the roll substrate 21, and is more likely to be produced in regions closer to the roll substrate 21. Therefore, when the thermal spray coating 22 is divided into upper and lower layers in the thickness direction, the concentration of titanium nitride contained in the lower layer is higher than the concentration of titanium nitride contained in the upper layer. The lower layer is the layer in contact with the roll substrate 21, and the upper layer is the layer in contact with the steel sheet 10. For the above reasons, the titanium content must be set to 1% or more.

[0037] However, if the titanium content is excessively high, the thermal spray coating becomes brittle and prone to cracking in high-temperature environments. In other words, the thermal shock resistance of the thermal spray coating decreases. For these reasons, the titanium content must be set to 5% or less.

[0038] (About molybdenum) The molybdenum content is 5% or more, and preferably 8% or more. The inclusion of molybdenum inhibits the solid diffusion of aluminum and titanium in the nickel-based alloy thermal spray coating, and promotes the formation of the titanium nitride described above. However, if the molybdenum content is excessively high, the thermal spray coating becomes hard and brittle, and is prone to cracking. For these reasons, the molybdenum content must be set to 15% or less.

[0039] (About Chromium) The chromium content is 10% or more, and preferably 15% or more. Chromium is an element necessary for making the alumina layer 22a have a dense structure. Forming the alumina layer 22a densely can improve the build-up resistance of the thermal spray coating 22. For these reasons, the chromium content needs to be set to 10% or more. However, if too much chromium is contained, the thermal spray coating 22 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 produced when the hearth roll 20 is used in a high-temperature environment. In other words, the production of alumina takes precedence, so the production of chromium oxide can be suppressed.

[0040] (About Nickel) 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 sprayed coating 22 from a nickel-based alloy, the thermal shock resistance and peeling resistance of the sprayed coating 22 can be improved.

[0041] Nickel-based alloys may contain unavoidable impurities. These impurities may be introduced, for example, as contaminants from a mixing vessel during the thermal spray coating manufacturing process, carbon from kerosene during high-velocity gas spraying, or as a result of decarburization reactions during plasma spraying. These impurities include at least one of oxygen, boron, carbon, silicon, phosphorus, iron, and manganese. It is desirable to keep the content of unavoidable impurities to 2% or less.

[0042] The nickel-based alloy may contain niobium and / or tungsten as optional components. The inclusion of these optional components in the nickel-based alloy can increase the high-temperature hardness of the thermal spray coating 22. If the optional components are excessively increased, the proportion of the basic components will decrease, and the effects of the basic components described above may not be achieved. Therefore, it is desirable to keep the optional components to 8% or less.

[0043] The thermal spray coating 22 is formed by spraying thermal spray particles onto the surface of the hearth roll 20. For example, atomized powder can be used as the thermal spray particles. A molten metal made of a nickel-based alloy adjusted to the above-mentioned composition is prepared, and the thermal spray particles can be produced from this molten metal by an atomization method. For example, a high-velocity gas thermal spraying method can be used as the thermal spraying method.

[0044] The hearth roll 20 on which the thermal sprayed coating 22 is formed is placed in the furnace body 2, and the steel sheet 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. By using the hearth roll 20 in such a high-temperature environment, an alumina layer 22a is formed on the surface of the thermal sprayed coating 22 (see FIG. 3). This alumina layer 22a improves the build-up resistance of the hearth roll 20. The coating body 22b of the thermal sprayed coating 22 has a reduced aluminum concentration because aluminum is consumed in the production of alumina. Furthermore, granular titanium nitride is dispersed and precipitated in the coating body 22b. In other words, compared to the thermal sprayed coating 22 before use of the hearth roll, the coating body 22b can be said to be a nickel-based alloy sprayed layer in which the aluminum concentration is reduced and precipitated titanium nitride is dispersed. The effects of the alumina layer 22a and granular titanium nitride have been described above, so a detailed description will not be repeated.

[0045] Here, when a ceramic material or cermet material containing chromium carbide is sprayed onto the roll surface of the hearth roll 20, the chromium in the chromium carbide oxidizes to form chromium oxide. This chromium oxide evaporates when exposed to high temperatures (850°C or higher, particularly 1000°C or higher), and because the evaporation pressure is high, the thermal sprayed coating may deteriorate and its build-up resistance may decrease. As described above, the thermal sprayed coating 22 of this embodiment has a composition that makes it difficult for chromium oxide to be formed, and therefore, deterioration of the thermal sprayed coating due to evaporation of chromium oxide can be suppressed.

[0046] (Second embodiment) The surface of the hearth roll of this embodiment is covered not with a thermal spray coating 22 but with a welded coating (hereinafter also referred to as the welded coating 22). The welding material can be a nickel-based alloy containing 3% to 15% aluminum, 1% to 5% titanium, 5% to 15% molybdenum, and 10% to 25% chromium, similar to the thermal spray material of the first embodiment. The reasons for limiting the components are the same as those for the thermal spray material of the first embodiment, so a detailed explanation will be omitted.

[0047] The welding method may be plasma welding or the like. When the weld material is welded to the surface of the hearth roll 20, iron is dissolved from the base material by the heat generated during welding and is contained in the weld coating 22. Therefore, as will be explained below, the weld coating 22 has a slightly different composition from the thermal spray coating of the first embodiment. That is, while the thermal spray coating has a composition substantially identical to that of the thermal spray material, the weld coating has a slightly different composition from that of the weld material because iron is dissolved from the base material during welding.

[0048] The present inventors have separately confirmed that by welding a welding material having the above-mentioned chemical composition, the composition of the weld coating becomes a nickel-based alloy containing, by mass%, 2% to 13% aluminum, 0.3% to 4.5% titanium, 4% to 13.5% molybdenum, 9% to 23% chromium, and 2% to 15% iron.

[0049] Next, the present invention will be specifically described with reference to examples. (First Example) The first example corresponds to the first embodiment. [Table 1] (Build-up resistance test) Samples were prepared by spraying the surface of a SUS304 substrate with a thermal spray material having the various compositions shown in Table 1 using a high-velocity oxygen flame (HVOF) method to form a thermal spray coating with a thickness of 100 μm. Comparative Example 9 used a cermet thermal spray coating containing 55 mass% of Cr2C3 and 45 mass% of a CoNiAlCr alloy. The thermal spraying conditions were set as follows: Combustion energy source: kerosene, oxygen Spray gun chamber pressure: 6.8~7.2bar Spraying distance: 350mm Fe powder was scattered on the sample surface as a build-up source, and a crescent-shaped weight was placed on top of it. The weight was slid while heating to 1,050°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% by mass or more and 30% by mass or less were deemed to have excellent build-up resistance and were rated A. Samples with a total Fe content of less than 15% by mass were deemed to have very excellent build-up resistance and were rated AA. Samples with a total Fe content of more than 30% by mass were deemed to have poor build-up resistance and were rated B.

[0050] The build-up resistance of Examples 1 and 2, 4, 6, 8 and 9 was rated "AA." The build-up resistance of Examples 3, 5, 7 and 10 was lower than that of Example 1, etc., because the aluminum and / or chromium content was slightly low. However, the build-up resistance was rated "A." The build-up resistance of Comparative Examples 1 and 4 was rated "B" because the aluminum or chromium content was too low.

[0051] (peeling resistance) The sample was heated in the atmosphere and then 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 thermal spray coating and the substrate. If no cracks occurred at the interface, the peel resistance was deemed very excellent and rated AA. If only minor cracks occurred at the interface, the peel resistance was deemed excellent and rated A. If clear cracks occurred at the interface, the peel resistance was deemed poor and rated B. In all of Examples 1 to 2 and Examples 7 to 9, the peel resistance was evaluated as "AA." In Examples 3 to 6 and 10, the content of at least one of aluminum, titanium, and molybdenum was slightly low, and therefore the peel resistance was lower than in Example 1, etc. However, the evaluation was "A." In Comparative Examples 1 to 3, the amount of at least one of aluminum, titanium, and molybdenum was too small, and therefore the evaluation of peeling resistance was "B." In Comparative Example 9, the thermal spray coating was formed of a cermet thermal spray coating, and therefore the difference in thermal expansion coefficient with the substrate was large, and the evaluation of peeling resistance was "B."

[0052] (crack resistance) 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. If no cracks occurred in the diagonal direction, the crack resistance was deemed very excellent and rated as AA. If the cracks in the diagonal direction were slight, the crack resistance was deemed excellent and rated as A. If clear cracks occurred in the diagonal direction, the crack resistance was deemed poor and rated as B. In Comparative Examples 5 and 7 to 8, the thermal spray coating contained an excess of one of aluminum, molybdenum, and chromium, which caused clear cracks in the thermal spray coating, resulting in an evaluation of "B."

[0053] (thermal shock resistance) The sample was heated at 1000°C for 20 minutes and then immediately immersed in room temperature water to cool. After repeating this heating and cooling cycle 20 times, the thermal spray coating was visually inspected for peeling to evaluate its thermal shock resistance. When there is absolutely no peeling of the thermal spray coating, it is considered to have excellent thermal shock resistance and is rated as AA. 2 If the thermal shock resistance is suppressed to 1 mm or less, it is considered to have excellent thermal shock resistance and is rated as A. 2 When peeling of more than 100% was confirmed, the thermal shock resistance was deemed to be poor and the result was evaluated as B. Since all of Examples 1 to 10 were formed with a nickel-based alloy thermal spray coating, the difference in thermal expansion coefficient with the substrate was small, and the thermal shock resistance was evaluated as "A." In Comparative Example 6, the titanium content was excessive, and therefore the thermal shock resistance was evaluated as "B." In Comparative Example 9, the thermal spray coating was composed of a cermet thermal spray coating, and therefore the difference in thermal expansion coefficient with the substrate was excessively large, and therefore the thermal shock resistance was evaluated as "B." Note that, for Comparative Example 9, evaluation tests for build-up resistance, cracking resistance, and hardness were not performed.

[0054] (hardness) The hardness of the thermal spray coating was evaluated using a micro Vickers hardness measuring device. The sample was cut and the cross-sectional hardness of the thermal spray coating was measured under a load of 300 g. When the cross-sectional hardness was 400 Hv or more, the hardness was evaluated as very excellent and rated as "AAA." When the cross-sectional hardness was 350 Hv or more but less than 400 Hv, the hardness was evaluated as excellent and rated as "AA." When the cross-sectional hardness was 300 Hv or more but less than 350 Hv, the hardness was evaluated as generally excellent and rated as "A." When the cross-sectional hardness was less than 300 Hv, the hardness was evaluated as poor and rated as "B." In Examples 8 and 9, tungsten or niobium was added in addition to titanium and molybdenum, and therefore the hardness was rated as "AAA." In Comparative Example 2, the hardness was evaluated as "B" because there was no titanium. In Comparative Example 3, the hardness was evaluated as "B" because there was too little molybdenum.

[0055] (Second Example) The second example corresponds to the second embodiment. Nickel-based welding materials with various compositions shown in Table 1 were applied to the surface of the SUS substrate using the powder plasma welding (PTA) method, and a weld coating with a thickness of 1 mm or more was formed on each sample. The unavoidable impurities were 2% or less in all samples. The welding conditions were set as follows: Welding current: 250~350A Welding voltage: 200V Plasma gas volume: 3 to 7 L / min Welding speed: 50~140mm / min

[0056] The evaluation results were the same as those of the first example. [Explanation of symbols]

[0057] 1 Continuous annealing furnace 2 Furnace body 3,4 Gas supply nozzle 20 Hearth Roll 21 Roll substrate 22 Thermal spray coating, welding coating 22a Alumina layer 22b Membrane body

Claims

1. A method for manufacturing a hearth roll, A nickel-based alloy coating is formed on the roll surface of the hearth roll by spraying a thermal spray powder made of a nickel-based alloy containing, by mass%, 3% to 15% of aluminum, 1% to 5% of titanium, 5% to 15% of molybdenum, and 10% to 25% of chromium, or by welding a welding material made of the nickel-based alloy. A method for manufacturing a hearth roll, comprising:

2. The nickel-based alloy coating contains niobium and / or tungsten as optional components. The method for manufacturing a hearth roll according to claim 1 .

3. The content of the optional components contained in the nickel-based alloy coating is 8% or less by mass%, The method for manufacturing a hearth roll according to claim 2 .

4. A hearth roll in which the roll surface that comes into contact with the steel sheet is covered with a thermal spray coating, The thermal spray coating is a nickel-based alloy thermal spray coating containing, by mass%, 3% to 15% of aluminum, 1% to 5% of titanium, 5% to 15% of molybdenum, and 10% to 25% of chromium. A hearth roll characterized by:

5. The nickel-based alloy thermal spray coating contains niobium and / or tungsten as optional components.

5. The hearth roll according to claim 4.

6. The content of the optional components contained in the nickel-based alloy thermal spray coating is 8% or less by mass%, 6. The hearth roll according to claim 5.

7. A hearth roll in which the roll surface that comes into contact with the steel sheet is covered with a welded coating, The weld coating is a nickel-based alloy weld coating containing, by mass%, 2% to 13% aluminum, 0.3% to 4.5% titanium, 4% to 13.5% molybdenum, 9% to 23% chromium, and 2% to 15% iron. A hearth roll characterized by:

8. The nickel-based alloy weld coating optionally contains niobium and / or tungsten.

8. The hearth roll according to claim 7.

9. The content of the optional components contained in the nickel-based alloy weld coating is 8% or less by mass%, 9. The hearth roll according to claim 8.

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