Low thermal expansion alloy

Abstract

[Problem] To obtain Alloy with low thermal expansion properties in the vicinity of room temperature to 800°C. [Solution] The low thermal expansion alloy of the present invention comprises, by mass%, Fe:20 to 60%, Ni:20 to 35 %, Cr:0 to 30%, and the balance of Co and impurities.

Description

[Technical Field]

[0001] The present invention relates to a low thermal expansion alloy, in particular to an alloy having low thermal expansion characteristics in the vicinity of 400 to 600°C or in the vicinity of room temperature to 800°C, and suitable, for example, for an interconnector of a solid oxide electrolyte fuel cell, a component of a gas / steam turbine, a component of an internal combustion engine, a mold material for forming glass, and a material for a heat sink used in a high-temperature environment.[Background Art]

[0002] For solid oxide electrolyte fuel cells, ceramics such as stabilized zirconia are used as an electrolyte, and in recent years, applications for operating at medium and high temperatures of 700 to 800°C have also been developed. An interconnector of the solid oxide electrolyte fuel cell is a conductive plate electrically connected in series to form a cell stack, and is an interconnector plate separating a fuel gas and an oxidizing gas, and supports three layers of an electrolyte, a fuel electrode, and an air electrode, and has a function of forming a gas flow path and flowing a current.

[0003] Therefore, the interconnector is required to have excellent electrical conductivity at medium and high temperatures, oxidation resistance, small difference in thermal expansion from the electrolyte, low cost, ease of processing, and the like. Various alloys suitable as interconnectors have been developed.

[0004] Patent Literature 1 discloses a high-strength low-thermal expansion cast alloy for high temperature wherein the alloy comprises in mass%, C:0.02 to 0.06%, Si:0.2 to 0.6%, Mn:0.3 to 1.5%, Ni:24.0 to 29.5%, Co: 17.5 to 25.5%, and the balance of Fe and unavoidable impurities, Ni equivalent is in the range of 40.5 to 44.5%, the value of A defined by A=30 [C]-1.5×[Si]+0.5×([Mn]-55 × [S] / 32)+[Ni]+0.05×[Co]+0.1 is in the range of 27.5 to 29.5, and an area ratio of martensite phase in the microstructure is 30 to 90%, whereby the alloy can be used for producing a complicated shape components and large components used in precision equipment without the need for special equipment and materials, and the alloy has high strength and low thermal expansion at temperatures up to 600 °C.

[0005] Patent Literature 2 discloses an alloy having high strength and low coefficient of thermal expansion which is designed for a gas turbine engine, wherein the alloy comprises 7-9 wt.% chromium, 21-24 wt.% molybdenum, more than 5 wt.% tungsten, up to 3 wt.% iron, the balance being nickel and impurities, and the R-value defined by R=2.66A1+0.19Co+0.84Cr-0.16Cu+0.39Fe+0.60Mn+Mo+0.69Nb+2.16Si+0.47Ta+1.36Ti+1.07V+0.40W satisfies 31.95<R<33.45.

[0006] Patent Literature 3 discloses an alloy comprising C:≤0.15%, Si:≤1%, Mn:≤1%, Cr: less than 5-20%, Mo+1 / 2 (W+ Re): less than 5-17%, W:≤10%, Al:0.1 to 2.5%, Ti:0.10 to 0.95%, Nb+1 / 2Ta:≤1.5%, B:0.001 to 0.02%, Zr:0.001 to 0.2%, Fe:≤4.0%, Al+Ti+Nb+Ta:2.0 to 6.5% (atomic%) balance alloy consisting of unavoidable impurities, as a low thermal expansion Ni base superalloy of γ' precipitation hardening type which has a coefficient of thermal expansion of the same degree as the ferritic 12 Cr steel, excellent high temperature strength, corrosion resistance, oxidation resistance, good hot workability, and excellent.

[0007] Patent Literature 4 discloses a steel for a solid oxide fuel cell separator which comprises, by mass%, C:0.2% or less, Si:1.0% or less, Ni:2% or less, Cr:15 to 30%, Al:1% or less, one or more selected from Y:0.5% or less, rare earth element: 0.2% or less, and Zr:1% or less, and the balance consisting essentially of Fe and comprising S:0.015% or less, O:0.010% or less, N:0.050% or less, B:0.0030% or less as an unavoidable impurities, satisfies formula 1, and has hardness of 280 HV or less and an average ferrite granularity of ASTM2 or more, as a solid oxide fuel cell separator which forms an oxide film having good electric conductivity at about 700 to 950°C, has good oxidation resistance and peeling resistance in use over a long period of time, has excellent impact characteristics at room temperature, and has a small difference in thermal expansion with an electrolyte.[Citation List][Patent Literature]

[0008] [PTL 1] WO 2017 / 006659 A1 [PTL 2] JP 2014-501845 A [PTL 3] JP 2011-231410 A [PTL 4] JP 2003-173795 A [Summary of Invention][Technical Problem]

[0009] Conventional low thermal expansion alloys for high temperature environments have low thermal expansion properties of about 11.0 to 14.0 × 10 -6< / °C at 400 to 600°C and about 12.0 to 14.0×10 -6< / °C and at room temperature to 800°C. However, alloys with smaller coefficient of thermal expansion in high temperature environments have not been obtained.

[0010] The object of the present invention is to obtain an alloy having low thermal expansion characteristics, i.e., the absolute value of the coefficient of thermal expansion is small, in the vicinity of 600 to 800° C.[Solution to Problem]

[0011] The inventors have studied alloys having low thermal expansion characteristics at high temperatures. As a result, they achieved an alloy having low thermal expansion properties at high temperatures by controlling the chemical composition of Fe-Co-Ni alloy.

[0012] The present invention was achieved as a result of further studies, and includes the following embodiments. (1) A low thermal expansion alloy comprising, by mass%, Fe:20 to 60%, Ni:20 to 35 %, Cr:0 to 30%, and the balance of Co and impurities. (2) The low thermal expansion alloy according to (1), wherein the microstructure of the low thermal expansion alloy contains an austenite phase in an area ratio of 90% or more. (3) The low thermal expansion alloy according to (1) or (2), wherein the low thermal expansion alloy contains Co of 20 mass% or more. (4) The low thermal expansion alloy according to any one of (1) to (3), wherein the low thermal expansion alloy contains Ni of 25 mass% or more. [Advantageous Effects of Invention]

[0013] According to the present invention, an alloy having low thermal expansion characteristics in the vicinity of 600 to 800°C can be obtained.[Brief Description of Drawings]

[0014] Fig. 1 is a diagram showing an example of a thermal expansion curve of the alloy produced in the examples.[Description of Embodiments]

[0015] Hereinafter, the present invention will be described in detail.

[0016] First, the chemical composition of the low thermal expansion alloy of the present invention will be described.

[0017] The low thermal expansion alloy of the present invention disclosure is an alloy containing Fe, Ni based on Co and further containing Cr as needed.

[0018] Alloys comprising Fe, Ni and Co form a ferrite phase having a body-centered cubic crystal structure or an austenite phase having a face-centered cubic crystal structure, depending on chemical composition. In the alloy of which chemical composition is near the ferrite-austenite boundary and which comprises mainly austenite phase, volume shrinkage due to a decrease in magnetostriction occurs in the temperature range below the Curie temperature. This volume shrinkage cancels the natural thermal expansion, whereby low thermal expansion characteristics can be obtained. Since the Curie temperature can be changed to 200 to 800°C when the content of Co is increased, it is possible to obtain low thermal expansion properties in the high temperature range. In the low thermal expansion alloy of the present invention, the austenite phase preferably has an area ratio of 90% or more, more preferably 92% or more, 94% or more, 96% or more, or 98% or more, and even more preferably 100%.

[0019] In the low thermal expansion alloy, the content of Fe is 20 to 60 mass%, the content of Ni is 20 to 35 mass%, and the balance is Co. The content of Co is 5 to 60 mass%. The content of Fe may be 22 mass% or more, 25 mass% or more, 30 mass% or more, 35 mass% or more, or 40 mass% or more. The content of Fe may be 58 mass% or less, 55 mass% or less, 52 mass% or less, 50 mass% or less, or 48 mass% or less. The content of Ni may be 21 mass% or more, 23 mass% or more, 25 mass% or more, 27 mass% or more, or 30 mass% or more. The content of Ni may be 55 mass% or less, 50 mass% or less, 45 mass% or less, 40 mass% or less, or 35 mass% or less. The content of Co may be 15 mass% or more, 20 mass% or more, 22 mass% or more, 24 mass% or more, 26 mass% or more, 28 mass% or more, or 30 mass% or more. The content of Co may be 58 mass% or less, 55 mass% or less, 50 mass% or less, 45 mass% or less, or 40 mass% or less.

[0020] Ni is effective in stabilizing the austenitic phase. Co is effective in raising the Curie temperature. By adjusting the content of Ni and Co in the above range, the temperature range in which low thermal expansion properties occur in a high temperature environment can be controlled. Further, since Ni is effective with respect to changing the chemical composition areas in which the austenitic phase is stable, the low thermal expansion properties in a high temperature environment can be controlled by way of Ni being contained an proper amount.

[0021] Further, it is preferable to adjust [Co] and [Ni], which represent the content of Ni and Co (mass%), respectively, to satisfy the following formula, whereby it is possible to obtain a microstructure containing 98% or more of the austenitic phase. Co ≥ 77.5 − 2.25 × Ni

[0022] In addition to the above elements, impurities may be included within a range which does not affect the effects of the present invention. The impurities includes C, S, P, and Cu which are elements not intentionally added in the manufacturing process (unavoidable impurities), and Si, Al, and Mn which are added for deoxidation, etc. The contents of the impurities are not limited as long as they do not affect the effect of the present invention. The content of the impurities may be, in total, 0.30 mass% or less, 0.25 mass% or less, 0.20 mass% or less, 0.15 mass% or less, 0.10 mass% or less, 0.05 mass% or less, or 0 mass%.

[0023] The low thermal expansion alloy of the present invention may contain Cr instead of a part of Co, since Cr is effective in preventing high-temperature oxidation and corrosion. Cr is not an essential element for obtaining the alloy having low thermal expansion properties in the vicinity of 600 to 800°C, and the lower limit of the content in the present invention is 0. The effect of Cr content can be obtained, even with only a slight amount. However, in order to effectively prevent high-temperature oxidation and corrosion, the content is preferably 5 mass% or more, and more preferably 10 mass% or more. Since Cr is also an element for increasing the coefficient of thermal expansion, the content is 30 mass% or less. The content of Cr may be 1 mass% or more, 3 mass% or more, 5 mass% or more, 7 mass% or more, or 10 mass% or more. The content of Cr may be 28 mass% or less, 26 mass% or less, 24 mass% or less, 22 mass% or less, or 20 mass% or less.

[0024] Next, a method for producing the low thermal expansion alloy of the present invention will be described.

[0025] The low thermal expansion alloy of the present invention can be obtained by casting. A mold used for casting, an apparatus for injecting molten steel into the mold, and an injection method are not limited. Known apparatuses and methods may be used.

[0026] The as-cast alloy having the chemical composition described above has a low coefficient of thermal expansion at high temperature, i.e., an absolute value of the coefficient of thermal expansion is small.

[0027] For the purpose of forming the as-cast alloy, hot forging may be performed at a temperature of 1050 to 1250° C. In this case, the forging ratio is preferably 3 or more. Low thermal expansion characteristics are maintained even when hot forging is applied. Further, it is also possible to process in the thickness 0.1 to 10mm by hot rolling and cold rolling. Even in this case, the low thermal expansion characteristics are maintained.

[0028] The alloy may be as casting, forging, and rolling. However, in order for 90% or more of the austenitic phase to be contained in stable form, it is preferable to heat the cast steel, forged steel, or rolled steel as described above, hold 0.5 to 5hr, and then cool in a furnace. The cooling rate is preferably 10 to 100°C / hr, since the lower the cooling rate, the higher amount of the ordered phase.

[0029] Further, even in the rapidly cooled alloy, the ordered phase is formed by heating to a temperature of 300 to 700°C and holding for a certain period of time. After the heat treatment is performed at a temperature of 800 to 1100°C, the ordered phase can be formed by heating and holding at a temperature of 300 to 700°C using a salt bath for a certain period of time.

[0030] More specifically, the low thermal expansion alloy of the present invention has an average coefficient of thermal expansion at room temperature to 800°C of 11.0×10 -6< / °C or less, preferably 10.8×10 -6< / °C or less, 10.5×10 -6< / °C or less, or 10.2×10 -6< / °C or less, more preferably 10.0×10 -6< / °C or less. Further, an average coefficient of thermal expansion at 400 to 600°C is 14.0×10 -6< / °C or less, preferably 13.0×10 -6< / °C or less, 12.0×10 -6< / °C or less, or 11.0×10 -6< / °C or less, more preferably 10.0×10 -6< / °C or less.[Examples](Example 1)

[0031] Molten metal prepared to have the chemical composition described in Table 1 was poured into the mold, whereby an alloy is produced. In Nos. 1 to 39 in Table 1, the cast alloy was hot forged at 1100°C, then heated at 1100°C for 2 hours, and then furnace-cooled in a 100°C / hr. From the alloy, the thermal expansion test piece (φ5×20L) was collected, and the coefficient of thermal expansion from room temperature to 1000 °C was measured by differential expansion method at a temperature increase rate of 5°C / min, using a NETZSCH thermal expansion measuring machine, using quartz as a reference sample. Then, the mean coefficient of thermal expansion from 400°C to 600°C and room temperature to 800°C was calculated. The results are shown in Table 1. [Table 1]No.CoFeNiCrOthersaverage coefficient of thermal expansionaverage coefficient of thermal expansionat 400 to 600°C (10 -6< / °C)at room temperature to 800°C (10 -6< / °C)14238209.210.024037239.19.934038229.09.844039218.99.753939228.59.963940218.49.873941208.29.783839238.210.093840228.09.8103841217.99.7113740237.99.9123741227.89.8133742217.79.7143641237.79.9153642227.59.8163643217.49.7173541246.39.8183542236.19.7inventive example193543227.39.6203342256.69.8213343246.59.7223344236.49.6233044266.89.8243045256.69.7253046246.59.6262546298.29.7272547288.19.6282548278.09.5292049319.49.7302050309.39.6312051299.29.53215533213.09.63315543112.89.53415553012.79.43514.352.328.45.013.39.93618.045.926.110.09.810.03721.340.822.915.08.610.23824.036.020.020.07.310.43918.735.321.025.08.510.55155321311.012.0comparative example5219bal.280.05C:0.0413.812.1Si:0.43Mn:0.3553--bal.7Mo:2113.113.5W:16543.20.2bal.16C:0.0212.012.5Si:0.05Mn:0.05Mo:14W:2Nb:0.3Al:1.4Ti:0.9Zr:0.01B:0.00355-bal.0.422C:0.0211.713.0Si:0.43Mn:0.48Zr:0.16

[0032] According to the present invention, an alloy having low thermal expansion characteristics in the vicinity of 400 to 600°C and in the vicinity of room temperature to 800°C can be obtained.

[0033] FIG. 1 shows examples of the temperature dependence of the coefficient of thermal expansion from room temperature to 1000°C of the alloys produced in the Examples. It was confirmed that the alloys of inventive examples have lower thermal expansion in all temperature ranges than SUS304, which is a typical austenitic alloy, and has particularly low thermal expansion in a temperature range below the Curie temperature. In 55Co-32Fe-13Ni alloy of the comparative example, although the Curie temperature is high as 750°C, the coefficient of thermal expansion in the respective temperature regions was not as reduced as the alloy of the inventive example.(Example 2)

[0034] Molten metal prepared to have the chemical composition described in Table 2 was poured into the mold, whereby an alloy is produced. From the alloy, oxidation resistance test piece (φ8×25L) was collected. To the test piece, heat treatment was performed at a temperature of 800°C, and the mass increase by oxide formation was measured every 24 hours. The results are shown in Table 2. As shown in Table 2, it was confirmed that the low thermal expansion alloy of the present invention can improve the oxidation resistance at high temperature (800°C) by including Cr. [Table 2]No.CoFeNiCroxide increases after each heat treatment time (g)24Hr48Hr72Hr96Hr1354223-0.04730.06350.07810.0902inventive example214.352.328.45.00.02830.04230.05920.0666318.045.926.110.00.02860.04050.05810.0638421.340.822.915.00.00130.00320.00650.0078524.036.020.020.00.00110.00170.00210.0023626.332.316.425.00.00020.00080.00140.0001511-83-1700.00050.0010.0005comparative example

Claims

1. A low thermal expansion alloy comprising, by mass%, Fe:20 to 60%, Ni:20 to 35 %, Cr:0 to 30%, and the balance of Co and impurities.

2. The low thermal expansion alloy according to claim 1, wherein the microstructure of the low thermal expansion alloy contains an austenite phase in an area ratio of 90% or more.

3. The low thermal expansion alloy according to claim 1 or 2, wherein the low thermal expansion alloy contains Co of 20 mass% or more.

4. The low thermal expansion alloy according to claim 1 or 2, wherein the low thermal expansion alloy contains Ni of 25 mass% or more.

5. The low thermal expansion alloy according to claim 3, wherein the low thermal expansion alloy contains Ni of 25 mass% or more.

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