Atomized powder, thermal spray coating, hearth roll, and method for manufacturing hearth roll

Abstract

An atomized powder which has a heat-resistant alloy phase and Cr7C3 phases that are dispersed in the heat-resistant alloy phase, and which contains, on a mass basis, 20-46% of Ni, 22-43% of Cr, 4-13% of Al, 0.1-1.0% of Y and 0.3-4.2% of C, with the balance being made up of Co and unavoidable impurities.

Description

[Technical Field]

[0001] The present invention relates to an atomized powder, a thermal spray coating, a hearth roll, and a method for manufacturing a hearth roll. This application claims priority based on Japanese Application No. 2021-122668, filed on July 27, 2021, and incorporates by reference all of the contents of the above-mentioned international patent application. [Background technology]

[0002] In heat treatment furnaces such as continuous annealing furnaces for steel sheets, rolls called hearth rolls are placed to transport the steel sheets. The steel sheets are heat treated in the furnace, and during this process, deposits called build-up may form on the surface of the hearth rolls due to reactions with the steel sheets. When build-up is formed, it causes indentations and the like on the surface of the steel sheet transported on the hearth roll, resulting in poor quality of the steel sheet. Therefore, when build-up occurs, it is necessary to immediately stop operation and clean the roll surface, which significantly reduces production efficiency. Therefore, the occurrence of build-up is prevented by providing a thermal spray coating on the hearth roll surface (see, for example, Patent Documents 1 to 3). The thermal spray powders used in Patent Documents 1 to 3 contain chromium carbide and a heat-resistant metal.

[0003] Furthermore, Patent Document 4 proposes a chromium carbide-based thermal spray powder as a thermal spray powder for forming a thermal spray coating that is highly hard even at high temperatures. The thermal spray powder is composed of chromium carbide and a metal phase, and the metal phase is formed by dispersing and strengthening the metal or alloy matrix with fine ceramic hard particles in a volume ratio of 0.5 to 15% relative to the metal phase. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 59-126772 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-240072 [Patent Document 3] International Publication No. 2009 / 069829 [Patent Document 4] Japanese Patent Application Publication No. 62-099449 Summary of the Invention [Problem to be solved by the invention]

[0005] The thermal spray powders proposed in Patent Documents 1 to 4 are all produced by a granulation sintering method. Thermal spray powders produced by a granulation sintering method have small primary particles of chromium carbide and heat-resistant alloy powder, are porous, and have a large specific surface area. Therefore, thermal spray coatings formed using this thermal spray powder are prone to oxidation. When this thermal spray powder is used to form a thermal spray coating on the surface of a hearth roll, the roll surface is oxidized early, which creates the problem of buildup, in which foreign matter adheres to and grows on the roll surface, and pickup, in which foreign matter becomes trapped on the roll surface. Furthermore, the thermal spray coatings proposed in Patent Documents 1 to 4 have uneven distribution and size of chromium carbides, resulting in areas with low hardness, which also makes them prone to pickup and build-up. [Means for solving the problem]

[0006] The present inventors have conducted extensive research to solve these problems and have discovered that the above problems can be solved by using an atomized powder having a specific composition as a thermal spray powder, thereby completing the present invention.

[0007] (1) The atomized powder according to one aspect of the present invention is An atomized powder having a heat-resistant alloy phase and a Cr7C3 phase dispersed in the heat-resistant alloy phase, It contains, by mass, 20 to 46% Ni, 22 to 43% Cr, 4 to 13% Al, 0.1 to 1.0% Y, 0.3 to 4.2% C, and the remainder being Co and unavoidable impurities.

[0008] By forming a thermal spray coating using this atomized powder, a coating having excellent high-temperature hardness, oxidation resistance, toughness, and thermal shock resistance can be obtained.

[0009] (2) In the atomized powder of (1) above, the heat-resistant alloy phase is preferably a Co-based alloy phase. (3) In the atomized powder of (1) or (2) above, preferably, part of the Cr7C3 phase has an acicular structure. (4) In any of the atomized powders (1) to (3) above, the proportion of the Cr7C3 phase in the cross-sectional structure of each powder is preferably 50% by area or less. Atomized powders having one or more of the above structures (2) to (4) are suitable as thermal spray powders for forming thermal spray coatings that have high hardness and little variation in hardness from area to area.

[0010] (5) A thermal spray coating according to one aspect of the present invention is a thermal spray coating formed using the atomized powder according to any one of (1) to (4) above. (6) Another thermal spray coating according to one embodiment of the present invention is a thermal spray coating comprising a heat-resistant alloy phase and a Cr7C3 phase dispersed in the heat-resistant alloy phase, the heat-resistant alloy phase consisting of Co, Ni, Cr, Al, Y and unavoidable impurities, and containing 5% by mass or more and less than 30% by mass of Cr7C3 as a whole. (7) In the thermal spray coating of (6) above, the heat-resistant alloy phase is preferably a Co-based alloy phase. (8) In the thermal spray coating of (6) or (7) above, preferably, part of the Cr7C3 phase has an acicular structure. These thermal spray coatings have excellent high-temperature hardness, oxidation resistance, toughness, and thermal shock resistance.

[0011] (9) A hearth roll according to one embodiment of the present invention comprises a roll body and a thermal sprayed coating, the thermal sprayed coating being provided on the surface thereof, and the thermal sprayed coating being any one of the thermal sprayed coatings described above in (5) to (8). The hearth roll has a thermal spray coating on the surface that is excellent in high-temperature hardness, oxidation resistance, toughness, and thermal shock resistance. Therefore, it is possible to suppress the occurrence of pickup and build-up on the surface of the hearth roll (the surface in contact with the steel sheet).

[0012] (10) A method for manufacturing a hearth roll according to one aspect of the present invention is a method for manufacturing a hearth roll comprising a roll body and a thermal spray coating, the thermal spray coating being provided on a surface thereof, the method comprising: This is a manufacturing method in which a thermal spray coating is formed on the surface of the roll body using the atomized powder of any one of (1) to (4) above, and then the thermal spray coating is hardened by heat treatment. According to this method for producing a hearth roll, the formed thermal spray coating is subjected to a heat treatment, which causes finer chromium carbides to precipitate in the heat-resistant alloy phase, thereby further improving the coating hardness and wear resistance of the produced hearth roll.

[0013] (11) In the method for producing the hearth roll, after the heat treatment, it is preferable to further irradiate the heat-treated thermal sprayed coating with a laser beam to melt and solidify the surface of the heat-treated thermal sprayed coating, thereby reducing the hardness of the heat-treated thermal sprayed coating. In this case, the laser treatment makes the surface of the thermal spray coating smooth and gives the surface layer a dense structure, which further reduces the possibility of pickup and build-up occurring. Furthermore, when a thermal spray coating formed using the above-mentioned atomized powder is heat-treated and then the surface is laser-treated, the fine chromium carbide structure in the surface layer disappears, the hardness of the thermal spray coating decreases, the toughness of the thermal spray coating improves, and the coating structure has excellent thermal shock resistance. Furthermore, the above laser treatment melts the heat-resistant alloy phase and chromium carbide, giving the surface layer of the thermal spray coating a uniform composition, eliminating unevenness in oxidation resistance and improving the oxidation resistance of the thermal spray coating. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a thermal spray coating that is excellent in high-temperature hardness, oxidation resistance, toughness, and thermal shock resistance, a thermal spray powder for obtaining such a thermal spray coating, and a hearth roll equipped with the above-mentioned thermal spray coating. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a cross-sectional SEM photograph of an example of atomized powder according to an embodiment of the present invention. [Figure 2] FIG. 2 is a cross-sectional SEM photograph of an example of the granulated and sintered powder. [Figure 3] FIG. 3 is a diagram showing an example of a hearth roll according to an embodiment of the present invention. [Figure 4] FIG. 4 is a diagram showing the observation results of a cross section of the atomized powder of Example 1. [Figure 5] 5(a) to 5(c) are diagrams showing the results of XRD measurements of the atomized powder and thermal spray coating produced in Example 1. FIG. [Figure 6] FIG. 6 is a graph showing the results of evaluation (1) of the hardness of the thermal spray coatings in the examples. [Figure 7] FIG. 7 is a graph showing the results of evaluation (2) of the hardness of the thermal spray coatings in the examples. [Figure 8] FIG. 8 shows SEM-BEI images of the cross sections of the thermal spray coatings of Example 1 and Comparative Example 1. [Figure 9] FIG. 9 is a graph showing the results of evaluation (3) of the hardness of the thermal spray coatings in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. <Atomized powder> The atomized powder according to the embodiment of the present invention is an atomized powder having a heat-resistant alloy phase and a Cr7C3 phase. The heat-resistant alloy phase is preferably a Co-based alloy phase. The atomized powder is produced using an atomization method, and has a structure in which fine chromium carbides are uniformly dispersed and precipitated in the heat-resistant alloy phase.

[0017] Unlike powders produced by granulating and sintering chromium carbide and heat-resistant alloy, the atomized powder has a structure in which fine chromium carbide (Cr7C3) is uniformly dispersed and precipitated within the heat-resistant alloy phase. Therefore, by forming a thermal spray coating using the atomized powder as the thermal spray powder, the resulting thermal spray coating has high hardness throughout the coating and little variation in hardness from area to area.

[0018] Unlike powders produced by a granulation and sintering method (hereinafter also referred to as granulated sintered powders), the atomized powders are solid powders and have a small specific surface area. Therefore, the atomized powders have higher oxidation resistance than the granulated sintered powders. Furthermore, thermal spray coatings formed using the atomized powders have better oxidation resistance than thermal spray coatings formed using the granulated sintered powders.

[0019] In the atomized powder, the chromium carbide is Cr7C3. Cr7C3 is more stable in high-temperature environments than Cr3C2, so a thermal spray coating containing Cr7C3 formed using the atomized powder is less likely to change over time even when exposed to a high-temperature environment.

[0020] When a thermal spray coating formed using a thermal spray powder containing Cr3C2 and a heat-resistant alloy is exposed to a high-temperature environment, the Cr3C2 changes to Cr7C3. This change causes a deficiency of Cr in the heat-resistant alloy phase, resulting in a decrease in the oxidation resistance of the thermal spray coating. Furthermore, when Cr3C2 changes to Cr7C3 in the thermal spray coating, the dispersion of chromium carbide is impaired, which can result in variations in the hardness of the thermal spray coating. In contrast, a thermal spray coating containing Cr7C3 formed using the atomized powder maintains a uniformly dispersed state of the chromium carbide phase (Cr7C3 phase) even in a high-temperature environment. Therefore, even if the chromium carbide content in the thermal spray coating is less than 30 mass%, the hardness can be maintained. Furthermore, since the Cr in the heat-resistant alloy phase is not reduced, the thermal spray coating does not lose its oxidation resistance and toughness. Furthermore, since the thermal spray coating can contain a sufficient amount of the heat-resistant alloy phase (e.g., 70 mass% or more), it can exhibit excellent thermal shock resistance.

[0021] The atomized powder contains, by mass, 20 to 46% Ni, 22 to 43% Cr, 4 to 13% Al, 0.1 to 1.0% Y, 0.3 to 4.2% C, and the remainder Co and unavoidable impurities. The atomized powder having such a composition is designed to contain 5% by mass or more and less than 30% by mass of Cr7C3 in the powder.

[0022] If the content of Cr7C3 in the atomized powder is less than 5% by mass, the amount of chromium carbide precipitated will be insufficient, and sufficient hardness may not be obtained. On the other hand, if the Cr7C3 content in the atomized powder is 30% by mass or more, the content of the heat-resistant alloy phase in the atomized powder will be reduced, and the thermal shock resistance of the thermal spray coating formed using the atomized powder may be insufficient. Also, it is difficult to produce powder having a heat-resistant alloy phase and a Cr7C3 phase and a Cr7C3 content of 30% by mass or more by atomization.

[0023] Examples of the heat-resistant alloy include those containing Co, Ni, Cr, Al, and Y. Specific examples include CoNiCrAlY alloys and NiCoCrAlY alloys.

[0024] The atomized powder contains, as constituent elements, predetermined amounts of Ni, Cr, Al, Y, and C. The reason for this is as follows. Ni: 20~46% by mass In the atomized powder, Ni is a basic constituent element along with Co, and is contained to impart heat resistance and oxidation resistance. If the Ni content is less than 20 mass%, the toughness decreases and the oxidation resistance is poor, whereas if the Ni content exceeds 46 mass%, the Cr and Al contents decrease, resulting in poor oxidation resistance.

[0025] Cr:22~43% by mass In the atomized powder, Cr is contained in order to precipitate chromium carbide and to form an oxide film with excellent protective properties. If the Cr content is less than 22 mass %, the amount of chromium carbide precipitated will be insufficient, resulting in insufficient film hardness and poor oxidation resistance. On the other hand, if the Cr content exceeds 43 mass %, the toughness is impaired, and nozzle clogging occurs easily during production by the atomization method, making production difficult.

[0026] Al: 4~13% by mass In the atomized powder, A1 is contained to form an oxide film with excellent protection properties. If the Al content is less than 4 mass%, it is difficult to form a dense Al2O3 layer on the coating surface, whereas if the Al content exceeds 13 mass%, the coating becomes brittle and its thermal shock resistance deteriorates.

[0027] Y:0.1~1.0% by mass In the atomized powder, Y is contained to stably form an oxide film with excellent protection and to prevent peeling. If the Y content is less than 0.1 mass %, the effect of adding it is not apparent, whereas if the Y content exceeds 1.0 mass %, the coating becomes embrittled and the oxidation resistance deteriorates.

[0028] C: 0.3~4.2% by mass In the atomized powder, C is contained to precipitate chromium carbide. If the C content is less than 0.3 mass %, the amount of chromium carbide precipitated will be insufficient, and sufficient coating hardness will not be obtained. On the other hand, if the C content exceeds 4.2 mass %, the toughness is impaired, and nozzle clogging occurs easily during production by the atomization method, making production difficult.

[0029] Unlike granulated and sintered powder, the atomized powder has chromium carbide dispersed uniformly throughout the powder. FIG. 1 is a cross-sectional SEM photograph of an example of atomized powder according to an embodiment of the present invention (20 wt% Cr7C3-CoNiCrAlY / see Example 1). FIG. 2 is a cross-sectional SEM photograph of the granulated and sintered powder (40 wt % Cr3C2-CoNiCrAlY / see Comparative Example 1).

[0030] As is clear from the cross-sectional SEM image shown in Figure 1, in each of the atomized powders, fine Cr7C3 phases are dispersed as chromium carbide phases in the heat-resistant alloy phase (Co-based alloy phase). On the other hand, as shown in Figure 2, in the granulated and sintered powder, coarse chromium carbides (Cr3C2 particles) separated from the heat-resistant alloy phase are observed, and the fine Cr7C3 phase dispersed in the heat-resistant alloy phase as shown in Figure 1 is not observed.

[0031] In the atomized powder, a portion of the Cr7C3 phase dispersed in the heat-resistant alloy phase (Co-based alloy phase) observed with an SEM preferably has an acicular structure. The atomized powder preferably contains a large number of acicular structures having an aspect ratio of 2 or more and 100 or less. Here, the aspect ratio of the acicular structures is the ratio of the length of the major axis to the length of the minor axis of each acicular structure. Atomized powder in which acicular structures of the Cr7C3 phase having the above aspect ratio are dispersed throughout the powder is suitable as a thermal spray powder for forming a thermal spray coating with high hardness and little variation in hardness from area to area.

[0032] The length of the minor axis of the acicular structure of the Cr7C3 phase is, for example, 8 μm or less, preferably 3 μm or less. On the other hand, the length of the minor axis of the acicular structure is, for example, 0.1 μm or more. In this case, the Cr7C3 phase can be said to be a fine structure. The distance between adjacent acicular structures of the Cr7C3 phase (the distance between the closest parts) is, for example, 5 μm or less. On the other hand, the distance between adjacent acicular structures of the Cr7C3 phase (the distance between the closest parts) is, for example, 0.1 μm or more. In this case, it can be said that the Cr7C3 phase has excellent uniform dispersion.

[0033] In the atomized powder, the proportion of the Cr7C3 phase in the cross-sectional structure of each powder is preferably 50 area % or less. If it exceeds 50 area %, the uniform dispersion of Cr7C3 is poor. On the other hand, the proportion of the Cr7C3 phase in the cross-sectional structure of each powder is preferably 20 area % or more. If it is less than 20 area %, it is difficult to obtain sufficient coating hardness. The proportion of the Cr7C3 phase in the cross-sectional structure of each powder can be calculated from the cross-sectional SEM image of the atomized powder.

[0034] Examples of the atomization method include gas atomization, water atomization, and disk atomization. In the atomization method, the produced powder may be classified with a sieve to adjust the particle size.

[0035] In the gas atomization method, the raw metal is heated and melted to produce molten metal. This molten metal flows out of a nozzle, and gas (argon gas, nitrogen gas, etc.) is sprayed onto this molten metal. The energy of this gas breaks the molten metal into droplets, which cool as they fall. These droplets solidify and form particles.

[0036] Water atomization is a method in which water is sprayed instead of gas in gas atomization. In disk atomization, raw metal is heated and melted to produce a molten metal. This molten metal flows out of a nozzle and is then dropped onto a rapidly rotating disk. The molten metal is then rapidly cooled and solidified to produce a powder.

[0037] As the atomization method, gas atomization is preferred. In gas atomization, the molten metal instantly turns into droplets and is cooled at the same time, resulting in a uniform microstructure. Moreover, because the droplets are formed continuously, the compositional difference between particles is extremely small. This is one of the reasons why gas atomization is preferred.

[0038] <Thermal spray coating> A thermal spray coating according to an embodiment of the present invention includes a heat-resistant alloy phase and a Cr7C3 phase dispersed in the heat-resistant alloy phase, the heat-resistant alloy phase being composed of Co, Ni, Cr, Al, Y and unavoidable impurities, and containing 5% by mass or more and less than 30% by mass of Cr7C3 as a whole. Cr7C3 is more stable in high-temperature environments than Cr3C2, so the thermal spray coating is less likely to change over time even when exposed to high-temperature environments.

[0039] Furthermore, a thermal spray coating containing Cr7C3 as the chromium carbide maintains a uniformly dispersed state of the chromium carbide phase even in a high-temperature environment. Therefore, the thermal spray coating can maintain hardness even when the chromium carbide content in the coating is less than 30 mass%. Furthermore, the thermal spray coating does not lose Cr in the heat-resistant alloy phase even in a high-temperature environment, so its oxidation resistance and toughness are not impaired. Furthermore, the thermal spray coating can contain 70 mass% or more of the heat-resistant alloy phase, so it has excellent thermal shock resistance.

[0040] The thermal spray coating contains 5% by mass or more and less than 30% by mass of Cr7C3 based on the total mass. If the content of Cr7C3 is less than 5 mass %, the hardness and heat resistance of the thermal spray coating may be insufficient. On the other hand, if the Cr7C3 content is 30 mass % or more, the amount of the heat-resistant alloy phase in the thermal spray coating decreases, and the thermal shock resistance of the thermal spray coating may become insufficient.

[0041] The composition of the thermal spray coating preferably contains, by mass, 20 to 46% Ni, 22 to 43% Cr, 4 to 13% Al, 0.1 to 1.0% Y, 0.3 to 4.2% C, with the remainder being Co and unavoidable impurities. Such a composition is suitable for producing a thermal spray coating that is excellent in high-temperature hardness, oxidation resistance, toughness, and thermal shock resistance.

[0042] The thermal spray coating can be produced by thermal spraying using the atomized powder as the thermal spray powder. A thermal spray coating formed using the above-described atomized powder is also an aspect of the present invention.

[0043] The thermal spraying method for forming the thermal spray coating is not particularly limited, and for example, high velocity oxygen flame spraying (HVOF) or the like can be used.

[0044] In the above HVOF, the fuel gas is usually kerosene, C3H8, C2H2, or C3H6. The fuel gas pressure should be 0.1 to 1 MPa, and the flow rate of the fuel gas should be 10 to 500 L / min. The oxygen gas pressure should be 0.1 to 1 MPa, and the flow rate of the oxygen gas should be 100 to 1200 L / min.

[0045] <Hearth roll and manufacturing method thereof> FIG. 3 is a diagram showing an example of a hearth roll according to an embodiment of the present invention. As shown in FIG. 3, a hearth roll 10 according to an embodiment of the present invention includes a roll body 11 and a thermal spray coating 14.

[0046] The roll body 11 is composed of a roll shaft 12 and a roll substrate 13 attached to the roll shaft 12 . The thermal spray coating 14 is provided on the surface (peripheral surface) of the roll substrate 13 . The hearth roll 10 functions as a steel sheet transport roll that transports the steel sheet. The hearth roll 10 transports the steel sheet by bringing the peripheral surface of the hearth roll 10 (the surface of the thermal spray coating 14) into contact with the steel sheet while rotating around a roll axis 12.

[0047] The roll substrate 13 is made of a metal such as steel. As the metal, for example, stainless steel-based heat-resistant cast steel or the like is used.

[0048] The thermal spray coating 14 is the thermal spray coating according to the embodiment of the present invention described above. Therefore, pickup and build-up are unlikely to occur on the circumferential surface of the hearth roll 10. The thickness of the thermal spray coating 14 is preferably 20 μm or more and 300 μm or less. If the thickness of the thermal spray coating 14 is less than 20 μm, the effect of providing the thermal spray coating (suppression of build-up and pickup) cannot be fully obtained. On the other hand, if the thickness of the thermal spray coating 14 exceeds 300 μm, the thermal spray coating becomes prone to cracking due to the difference in thermal expansion with the roll substrate.

[0049] As a method for manufacturing the hearth roll 10, for example, a manufacturing method in which a thermal spray coating is formed on the surface of the roll body 11 using the atomized powder can be adopted. In the method for manufacturing the hearth roll 10, it is preferable to harden the thermal sprayed coating by heat treatment after forming the thermal sprayed coating. By heat treating the thermal sprayed coating, it is possible to increase the hardness compared to the thermal sprayed coating before the heat treatment.

[0050] The heat treatment may be carried out, for example, at a temperature of 300° C. to 600° C. for 1 hour to 10 hours, more preferably at a heating temperature of 400° C. or higher. This heat treatment may be performed in an oxidizing atmosphere (e.g., air) or a non-oxidizing atmosphere. As a treatment method in a non-oxidizing atmosphere, for example, a method in which the roll body on which the thermal spray coating is formed is placed in a heating furnace in an inert gas atmosphere such as nitrogen or argon can be mentioned.

[0051] By carrying out the heat treatment, finer chromium carbide (Cr7C3) is precipitated in the heat-resistant alloy phase of the thermal spray coating, which results in further improved hardness and wear resistance of the thermal spray coating.

[0052] In the method for producing the hearth roll 10, it is preferable that after the heat treatment, the thermal spray coating is further irradiated with a laser beam to melt and solidify the surface of the heat-treated thermal spray coating, thereby reducing the hardness of the heat-treated thermal spray coating. When the laser treatment is performed, the fine chromium carbide (Cr7C3) structure in the surface layer of the thermal spray coating disappears, reducing the hardness of the thermal spray coating. On the other hand, the laser-treated thermal spray coating improves its toughness and becomes a coating with excellent thermal shock resistance.

[0053] Furthermore, the laser treatment makes the surface of the thermal spray coating smooth and gives the surface layer a dense structure, making pickup and build-up even less likely to occur. Furthermore, the laser treatment melts and solidifies the heat-resistant alloy phase and chromium carbide, so that the melted portion of the thermal spray coating has a uniform composition, thereby improving the oxidation resistance of the thermal spray coating.

[0054] The conditions for the laser treatment are not particularly limited, and the treatment may be carried out under conditions that allow a portion of the thermal spray coating to melt. Examples of the laser used in the laser treatment include a fiber laser, an Nd / YAG laser, and a disk laser. The depth of the thermal spray coating melted by the laser treatment may be, for example, 5 μm or more and 20 μm or less.

[0055] The embodiments disclosed herein are illustrative in all respects and are not restrictive. The technical scope of the present invention is not limited to the above-described embodiments, and includes all modifications within the scope of equivalents to the configurations described in the claims. [Example]

[0056] Hereinafter, the embodiments of the present invention will be described in more detail with reference to examples, but the embodiments of the present invention are not limited to the following examples. Here, a test specimen was prepared by forming a thermal spray coating on the surface of a plate-shaped substrate (made of austenitic stainless steel (SUS304), 50 mm long x 50 mm wide x 5 mm thick) using a high-velocity oxygen flame (HVOF) thermal spraying method. The obtained test specimen was then subjected to heat treatment and laser treatment.

[0057] Example 1 1. Preparation of thermal spray powder (atomized powder) The atomized powder was 20 wt% Cr7C3-CoNiCrAlY (particle size -38 / +10 μm) powder. The raw materials were weighed to the prescribed composition shown in Table 1 below and induction melted in an argon atmosphere in a refractory crucible. Argon gas was sprayed onto the molten metal flowing out of the nozzle at the bottom of the crucible. The molten metal was rapidly cooled and solidified, yielding a gas-atomized powder. This gas-atomized powder was classified to obtain a thermal spray powder.

[0058] [Table 1]

[0059] 2. Observation of carbide precipitates in atomized powder The cross-section of the atomized powder was observed by SEM-BEI. The observed images were then subjected to image processing (binarization) to calculate the area ratio of precipitated carbides to the entire atomized powder. FIG. 4 shows a cross-sectional SEM-BEI observation image and a binarized image of the atomized powder, as well as the calculation results of the area ratio of precipitated carbides.

[0060] 3. Thermal spray coating The atomized powder was used as a thermal spray powder to form a thermal spray coating on the surface of the substrate by high velocity flame spraying to obtain a test piece. The high-velocity flame spraying device used was JP-5000 (Praxair / TAFA).

[0061] The details of the thermal spraying conditions in this example were as follows. Oxygen: 896 L / min Kerosene: 0.32L / min Spraying distance: 380mm Material supply amount: 50g / min

[0062] 4.Heat treatment of thermal spray coating The test piece prepared in 3 above was placed in a heating furnace and subjected to heat treatment in the atmosphere. Here, the heat treatment temperature was 400° C., 500° C., or 600° C., and the heat treatment time was 6 hours.

[0063] 5. Laser Treatment Of the test pieces that had been heat-treated in 4 above, the test pieces that had been heat-treated at 500°C for 6 hours were further subjected to laser treatment.

[0064] The laser treatment was carried out under the following conditions. Laser type: Yb-based fiber laser Laser wavelength: 1070nm Power density: 1.4 x 10 6 W / cm 2 Output: 1000W Laser spot diameter: 300 μm

[0065] (Comparative Example 1) 1. Thermal spray powder 40wt% Cr3C2-CoNiCrAlY (particle size -53 / +20μm) powder was prepared by granulation sintering.

[0066] 2. Thermal spray coating The powder produced by the above granulation and sintering method was used as a thermal spraying powder, and a thermal spray coating was formed on the surface of the above substrate by high velocity flame thermal spraying under the same conditions as in Example 1 to obtain a test piece.

[0067] 3.Heat treatment of thermal spray coating The test piece was subjected to heat treatment under the same conditions as in Example 1.

[0068] 4. Laser Treatment Of the test pieces that had been heat-treated in 3 above, the test piece that had been heat-treated at 500°C for 6 hours was subjected to laser treatment under the same conditions as in Example 1.

[0069] <Physical property evaluation> 1. Confirmation of constituent phases by XRD analysis The atomized powder produced in Example 1, the thermal spray coating (before heat treatment), and the thermal spray coating (after laser treatment) were measured by XRD. The results are shown in Figures 5(a) to 5(c). Figure 5(a) shows the measurement results for the atomized powder. Figure 5(b) shows the measurement results for the thermal spray coating (before heat treatment). Figure 5(c) shows the measurement results for the thermal spray coating (after laser treatment). In Figures 5(a) to 5(c), the downward arrows indicate the peak positions of Cr3C2. As shown in Figures 5(a) to 5(c), the atomized powder and thermal spray coating in Example 1 contained Cr7C3 as chromium carbide, and no Cr3C2 was detected.

[0070] 2. Evaluation of hardness of thermal spray coating (1) The Vickers hardness of the surface layer of the thermal spray coating was measured for each of the test pieces (i) after the formation of the thermal spray coating and (ii) after the formation of the thermal spray coating and subsequent heat treatment at a predetermined temperature (400°C, 500°C, 600°C) for 6 hours in Example 1 and Comparative Example 1. Measurements were made at 10 locations, and the average value was taken as the coating hardness. The results are shown in Figure 6. The Vickers hardness was measured using a micro Vickers hardness tester under a load of 25 g. As shown in FIG. 6, in both Example 1 (indicated as gas atomized powder in the figure, the same applies to FIGS. 7 to 9) and Comparative Example 1 (indicated as granulated sintered powder in the figure, the same applies to FIGS. 7 to 9) the hardness of the coating increased when heat treatment was performed after the formation of the thermal spray coating. Furthermore, in Example 1, the variation in hardness of the thermal spray coating was smaller than in Comparative Example 1 at any stage.

[0071] 3. Evaluation of hardness of thermal spray coating (2) The Vickers hardness of the surface layer of the thermal spray coating was measured for each of the test pieces in Example 1 and Comparative Example 1 (i) after the formation of the thermal spray coating, (ii) after the formation of the thermal spray coating followed by heat treatment (500°C x 6 hours), and (iii) after the heat treatment followed by laser treatment. Measurements were made at 10 locations, and the average value was taken as the coating hardness. The results are shown in Figure 7. The Vickers hardness was measured using a micro Vickers hardness tester under a load of 25 g. 7, the hardness of the thermal sprayed coating in Example 1 increased once after heat treatment, but then decreased after laser treatment. In contrast, the hardness of the thermal sprayed coating in Comparative Example 1 increased after heat treatment, and then increased further after laser treatment. Furthermore, in Example 1, the variation in hardness of the thermal spray coating was smaller than in Comparative Example 1 at any stage.

[0072] 4. Observation of sprayed coating The cross sections of the thermal spray coatings produced in Example 1 and Comparative Example 1 were observed with a scanning electron microscope (SEM). Fig. 8 shows cross-sectional SEM-BEI images at various stages of the thermal spray coatings produced in Example 1 and Comparative Example 1. Fig. 8 shows observation images of the thermal spray coatings produced in Example 1 and Comparative Example 1 (i) after formation of the thermal spray coating, (ii) after formation of the thermal spray coating and heat treatment (500°C x 6 hours), and (iii) after the heat treatment and laser treatment. From the observation images shown in FIG. 8, it can be seen that at each stage, the thermal sprayed coating of Example 1 maintained fine chromium carbides and had fewer pores compared to the thermal sprayed coating of Comparative Example 1. In Example 1, the surface layer of the thermal spray coating was melted and solidified by the laser treatment, resulting in a densified and smooth surface. On the other hand, in Comparative Example 1, although the surface of the thermal spray coating was smooth after the laser treatment, the surface layer did not have a uniform structure.

[0073] 5. Evaluation of hardness of thermal spray coating (3) For each of the test pieces that had been subjected to heat treatment and laser treatment in Example 1 and Comparative Example 1, the Vickers hardness of the center of the thermal spray coating (the portion that was not melted during laser treatment) was measured at room temperature and at high temperature. Specifically, measurements were taken in an Ar atmosphere at temperatures of 23°C, 400°C, 600°C, and 800°C. Measurements were taken at five locations, and the average value was taken as the Vickers hardness at room temperature or high temperature. The results are shown in Figure 9. Measurements of the Vickers hardness at room temperature and high temperature were taken using a high-temperature microhardness tester under a load of 200 g. As shown in FIG. 9, there was not much difference in hardness between the thermal spray coating produced in Example 1 and the thermal spray coating produced in Comparative Example 1, and both maintained a constant hardness in a high-temperature environment. [Explanation of symbols]

[0074] 10 Hearth Roll 11 Roll body 12 Roll axis 13 Roll substrate 14 Thermal spray coating

Claims

1. A heat-resistant alloy phase which is a Co-based alloy phase and Cr dispersed in the heat-resistant alloy phase. ７ C ３ and a phase, An atomized powder containing, by mass, 20 to 46% Ni, 22 to 43% Cr, 4 to 13% Al, 0.1 to 1.0% Y, 0.3 to 4.2% C, and the remainder being Co and unavoidable impurities.

2. The atomized powder according to claim 1, containing 5 mass % or more and less than 30 mass % of Cr 7 C 3 .

3. The Cr ７ C ３ 2. The atomized powder according to claim 1, wherein a portion of the phase has an acicular structure.

4. Cr content in the cross-sectional structure of each powder ７ C ３ 2. The atomized powder according to claim 1, wherein the ratio of the phase is 20 area % or more and 50 area % or less.

5. The atomized powder according to claim 1, wherein no Cr 3 C 2 peak is detected in XRD measurement.

6. A thermal spray coating formed using the atomized powder according to any one of claims 1 to 5.

7. A heat-resistant alloy phase and Cr dispersed in the heat-resistant alloy phase. ７ C ３ and a phase, wherein the heat-resistant alloy phase is composed of Co, Ni, Cr, Al, Y and unavoidable impurities, and Cr ７ C ３ Contains 5% by mass or more and less than 30% by mass of the total, A thermal spray coating containing, by mass, 20 to 46% Ni, 22 to 43% Cr, 4 to 13% Al, 0.1 to 1.0% Y, 0.3 to 4.2% C, and the remainder being Co and unavoidable impurities.

8. The thermal spray coating according to claim 7, wherein the heat-resistant alloy phase is a Co-based alloy phase.

9. The Cr ７ C ３ The thermal spray coating according to claim 7, wherein a portion of the phase has an acicular structure.

10. The thermal spray coating according to claim 7, wherein no Cr 3 C 2 peak is detected in XRD measurement.

11. A hearth roll comprising a roll body and a thermal sprayed coating, the thermal sprayed coating being provided on a surface thereof, wherein the thermal sprayed coating is the thermal sprayed coating according to any one of claims 7 to 10.

12. A method for manufacturing a hearth roll comprising a roll body and a thermal sprayed coating, the thermal sprayed coating being provided on a surface thereof, the method comprising forming the thermal sprayed coating on the surface of the roll body using the atomized powder according to any one of claims 1 to 5, and then heat treating the thermal sprayed coating to harden it.

13. 13. The method for manufacturing a hearth roll according to claim 12, wherein after the heat treatment, a laser beam is further irradiated to melt and solidify the surface of the heat-treated thermal sprayed coating, thereby reducing the hardness of the heat-treated thermal sprayed coating.

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