Controlled Thermal Expansion Alloy

Abstract

The problem of the present invention is to obtain an alloy having low thermal expansion characteristics or negative thermal expansion characteristics in the vicinity of 600 to 800˚C. This thermal expansion-controlled alloy contains, by mass%, 20 to 50% of Fe, 0 to 25% of Ni, 0 to 30% of Cr, and the balance Co and impurities.

Description

[Technical Field]

[0001] The present invention relates to a thermal expansion controlled alloy, and in particular to an alloy that has low or negative thermal expansion characteristics at temperatures in the range of 600 to 800°C, and is suitable for use, for example, in interconnectors for solid oxide fuel cells, gas and steam turbine parts, internal combustion engine parts, glass molding die materials, and heat sink materials used in high-temperature environments. [Background technology]

[0002] Solid oxide fuel cells use ceramics such as stabilized zirconia as the electrolyte, and in recent years applications have been developed for operation at medium to high temperatures of 700 to 800°C. The interconnector of a solid oxide fuel cell is a conductive plate that electrically connects cells in series to form a cell stack, and is also an interconnector plate that separates the fuel gas and oxidant gas, supporting the three layers of electrolyte, anode, and cathode, forming gas flow paths, and conducting current.

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

[0004] Patent Document 1 discloses a high-strength, low-thermal-expansion casting alloy for high temperatures, which can be used to manufacture complex-shaped parts and large parts used in precision devices without requiring special equipment or materials, and which has high strength and low thermal expansion at temperatures up to 600°C. The alloy contains, by mass, 0.02-0.06% C, 0.2-0.6% Si, 0.3-1.5% Mn, 24.0-29.5% Ni, and 17.5-25.5% Co, with an Ni equivalent in the range of 40.5-44.5%, and a value of A, expressed as A = 30[C] - 1.5 × [Si] + 0.5 × ([Mn] - 55 × [S] / 32) + [Ni] + 0.05 × [Co] + 0.1, in the range of 27.5-29.5, with the remainder being Fe and unavoidable impurities, and the alloy has a martensite phase area fraction of 30-90% in the microstructure.

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

[0006] Patent Document 3 discloses a γ'-precipitation hardening type low thermal expansion Ni-based superalloy having a coefficient of thermal expansion equivalent to that of ferritic 12Cr steel, excellent high-temperature strength, corrosion and oxidation resistance, good hot workability, and excellent weldability, with C: ≤ 0.15%, Si: ≤ 1%, Mn: ≤ 1%, Cr: 5-20% (less than), Mo + 1 / 2(W + Re): 5-17% (less than), W: ≤ 10%, Al: 0.1-2.5%, Ti: 0.10-0.95%, Nb + 1 / 2Ta: ≤ 1.5%, B: 0.001-0.02%, Zr: 0.001-0.2%, Fe: ≤ 4.0%, Al + Ti + Nb + Ta: 2.0-6.5% (atomic %), and the balance being inevitable impurities and Ni.

[0007] Patent Document 4 describes an inexpensive steel for solid oxide fuel cell separators that forms an oxide film with good electrical conductivity at around 700 to 950°C, has good oxidation resistance, particularly peeling resistance, even after long-term use, has excellent impact properties at room temperature, and has a small thermal expansion difference with the electrolyte. The steel contains, in mass%, C: 0.2% or less, Si: 1.0% or less, Mn: 1.0% or less, Ni: 2% or less, Cr: 15 to 30%, Al: 1% or less, and (Y: 0. The present invention discloses a steel for solid oxide fuel cell separators, which contains one or more elements selected from the group consisting of: rare earth elements: 0.5% or less, rare earth elements: 0.2% or less, and Zr: 1% or less, with the balance consisting essentially of Fe, and contains unavoidable impurities of S: 0.015% or less, O: 0.010% or less, N: 0.050% or less, and B: 0.0030% or less, and which satisfies formula (1), has a hardness of 280 HV or less, and is fine-grained with an average ferrite grain size of ASTM 2 or more. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 2017 / 006659 [Patent Document 2] Special Publication No. 2014-501845 [Patent Document 3] Japanese Patent Application Laid-Open No. 2011-231410 [Patent Document 4] Japanese Patent Application Laid-Open No. 2003-173795 Summary of the Invention [Problem to be solved by the invention]

[0009] Conventional low thermal expansion alloys for high temperature environments have a thermal expansion coefficient of 10.0 to 16.0 × 10 at 600 to 800°C. -6 / °C, it is possible to obtain low thermal expansion properties, but no alloys with a smaller thermal expansion coefficient in high temperature environments or with a negative thermal expansion coefficient have been obtained.

[0010] Therefore, the present invention aims to obtain an alloy that has low thermal expansion characteristics (small absolute value of thermal expansion coefficient) or negative thermal expansion characteristics (negative value of thermal expansion coefficient) in the vicinity of 600 to 800°C. [Means for solving the problem]

[0011] The present inventors have conducted extensive research into alloys with low thermal expansion at high temperatures, and have found that by controlling the components of an Fe-Co-Ni alloy, it is possible to obtain an alloy with low or negative thermal expansion at high temperatures.

[0012] The present invention was made as a result of further investigation, and the gist of the present invention is as follows.

[0013] (1) A thermal expansion controlled alloy characterized by containing, by mass%, 20 to 50% Fe, 0 to 25% Ni, and 0 to 30% Cr, with the remainder being Co and impurities.

[0014] (2) The controlled thermal expansion alloy according to (1) above, characterized by containing 3 to 25% Ni.

[0015] (3) A controlled thermal expansion alloy according to (1) or (2) above, characterized by containing 5 to 30% Cr.

[0016] (4) A controlled thermal expansion alloy that satisfies any one of the components (1) to (3) above and contains 5% or more of a region in which the crystal structure is an ordered phase.

[0017] (5) An interconnector for a solid oxide electrolyte fuel cell, comprising the controlled thermal expansion alloy according to any one of (1) to (4) above.

[0018] (6) A gas turbine or steam turbine part made of a controlled thermal expansion alloy according to any one of (1) to (4) above.

[0019] (7) A glass molding die made of a thermal expansion controlled alloy according to any one of (1) to (4) above.

[0020] (8) A heat sink made of a controlled thermal expansion alloy according to any one of (1) to (4). [Effects of the Invention]

[0021] According to the present invention, an alloy having low or negative thermal expansion characteristics at temperatures in the vicinity of 600 to 800°C can be obtained. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 is a diagram showing an example of a thermal expansion curve of an alloy produced in an example. DETAILED DESCRIPTION OF THE INVENTION

[0023] The present invention will be described in detail below.

[0024] First, the chemical components of the thermal expansion controlled alloy of the present invention will be explained.

[0025] The controlled thermal expansion alloy of the present invention is an alloy based on Co, containing Fe and Ni, and optionally further containing Cr.

[0026] In alloys containing Co and Fe, when the Co:Fe ratio is in the range of 4:1 to 1:10, some of the crystals will have a structure called an ordered phase at room temperature. When the temperature rises above 550-650°C, the ordered phase will change to a crystal structure called a disordered phase, resulting in volumetric shrinkage. This volumetric shrinkage counteracts the natural thermal expansion, resulting in low or negative thermal expansion characteristics at high temperatures. Furthermore, within the above composition range, the alloy will have a body-centered cubic crystal structure, resulting in lower thermal expansion than alloys with a face-centered cubic crystal structure, even at temperatures below 550-650°C.

[0027] The thermal expansion controlled alloy of the present invention contains 20 to 50% by mass of Fe. The Fe content is preferably 20.0% by mass or more, more preferably 25.0% by mass or more, and even more preferably 28.0% by mass or more. Also, the Fe content is preferably 50.0% by mass or less, more preferably 46.0% by mass or less, and even more preferably 42.0% by mass or less.

[0028] The thermal expansion controlled alloy of the present invention contains 0 to 25 mass % Ni. Ni has the effect of lowering the temperature at which the ordered phase begins to become disordered. Ni is not essential and the content may be 0, but by adjusting the content, it becomes possible to control the temperature range in which low or negative thermal expansion characteristics occur in high-temperature environments. Furthermore, since Ni has the effect of increasing the abundance ratio of the ordered phase, adding an appropriate amount makes it possible to control low or negative thermal expansion characteristics in high-temperature environments.

[0029] The Ni content is preferably 3.0% by mass or more, more preferably 4.0% by mass or more, and even more preferably 6.0% by mass or more, and is preferably 25.0% by mass or less, more preferably 22.0% by mass or less, and even more preferably 20.0% by mass or less.

[0030] In addition to the above elements, impurities may be contained to the extent that they do not affect the effects of the present invention. Examples of impurities include 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 the purpose of deoxidation, etc.

[0031] The controlled thermal expansion alloy of the present invention may contain Cr instead of part of the Fe. Cr has the effect of preventing high-temperature oxidation and corrosion. Cr is not an essential element for obtaining an alloy with low or negative thermal expansion characteristics at temperatures around 600 to 800°C, and the lower limit of its content in the present invention is 0. Although the effect of adding Cr can be obtained even with a small amount, in order to effectively prevent high-temperature oxidation and corrosion, a content of 5 mass% or more is preferred, and 10 mass% or more is more preferred. Since Cr is also an element that increases the thermal expansion coefficient, the content is set to 30 mass% or less.

[0032] The Cr content is preferably 5.0 mass% or more, more preferably 10.0 mass% or more, and even more preferably 15.0 mass% or more, and is preferably 30.0 mass% or less, more preferably 25.0 mass% or less, and even more preferably 20.0 mass% or less.

[0033] The structure of the thermal expansion controlled alloy of the present invention preferably contains 5% or more of a region in which the crystalline structure is the above-mentioned ordered phase. The presence of 5% or more of the crystalline structure can be confirmed by determining the lattice constant by measuring the X-ray diffraction spectrum of the thermal expansion controlled alloy. The lattice constant of the thermal expansion controlled alloy of the present invention varies depending on the chemical composition. If the composition is constant, the abundance ratio of the ordered phase and the disordered phase can be determined by proportionally allocating the lattice constants of each phase in terms of abundance ratio. Specifically, the lattice constants of the ordered phase and the disordered phase are respectively expressed as A O Å, A D Å, the lattice constant is 0.05Å O +0.95A D If the thickness is .ANG. or more, it is determined that the region that becomes an ordered phase accounts for 5% or more. The proportion of the region that becomes an ordered phase is preferably 10% or more, more preferably 15% or more, and even more preferably 20% or more.

[0034] Next, a method for producing a thermal expansion controlled alloy of the present invention will be described.

[0035] The thermal expansion controlled alloy of the present invention can be obtained by casting. There are no particular limitations on the mold used for casting, or the device and method for pouring molten steel into the mold, and any known device and method may be used.

[0036] As-cast alloys having the above-mentioned chemical compositions have low coefficients of thermal expansion at high temperatures, i.e., small absolute values ​​or even negative values ​​of the coefficient of thermal expansion.

[0037] For the purpose of forming, the as-cast alloy may be hot forged at a temperature of 1050 to 1250°C. In this case, the forging ratio is preferably 3 or more. Even when hot forging is performed, the low or negative thermal expansion properties are maintained. It is also possible to process the alloy to a thickness of 0.1 to 10 mm by hot rolling and cold rolling. In this case, the low or negative thermal expansion properties are maintained.

[0038] Although alloys containing 5% or more of ordered phase can be obtained simply by casting, forging, or rolling, to ensure a stable content of 5% or more of ordered phase, it is preferable to heat the above-mentioned cast steel, forged steel, or rolled steel to a temperature of 900 to 1100°C, hold for 0.5 to 5 hours, and then cool in a furnace. A slower cooling rate increases the amount of ordered phase, so a cooling rate of 10 to 100°C / hr is preferable. If the cooling rate is too fast, the ordered phase may not be contained in an amount of 5% or more.

[0039] Furthermore, even in a rapidly cooled alloy, an ordered phase can be formed if it is heated to a temperature of 300 to 700°C and held for a certain period of time. It is also possible to form an ordered phase by carrying out a heat treatment at a temperature of 800 to 1100°C, followed by heating and holding at a temperature of 300 to 700°C in a salt bath for a certain period of time.

[0040] More specifically, the controlled thermal expansion alloy of the present invention has an average thermal expansion coefficient of 9.0 × 10 at 600 to 800 °C. -6 / ℃ or less, preferably 8.0 × 10 -6 / °C or less, more preferably 7.5 × 10 -6 / ℃ or less. [Example]

[0041] Example 1 Molten metal prepared to have the composition listed in Table 1 was poured into a mold to produce alloys. For Nos. 21 to 27 in Table 1, the cast alloys were hot forged at 1100°C, then heated at 1100°C for 2 hours, and then furnace-cooled at 100°C / hr. Thermal expansion test pieces (φ5×20L) were taken from the produced alloys, and the thermal expansion coefficients were measured from room temperature to 1000°C at a heating rate of 5°C / min using a NETZSCH dilatometer with quartz as the standard specimen, by the differential expansion method. The average thermal expansion coefficient from 600°C to 800°C was calculated. The results are shown in Table 1.

[0042] [Table 1]

[0043] According to the present invention, an alloy having low or negative thermal expansion characteristics at temperatures in the vicinity of 600 to 800°C can be obtained.

[0044] Figure 1 shows an example of the thermal expansion curve of the alloy produced in the example from room temperature to 1000°C. It was confirmed that the alloy of the example had lower thermal expansion than the austenitic alloy over the entire temperature range, and that a low thermal expansion region and a negative thermal expansion region appeared at temperatures between 600 and 800°C. The ferritic alloy of the comparative example had a thermal expansion curve similar to that of the alloy of the example up to 600°C, but did not exhibit low thermal expansion and negative thermal expansion characteristics.

[0045] It was confirmed that the structure of alloys Nos. 1 to 27 contained 5% or more of regions in which the crystal structure was an ordered phase.

[0046] Example 2 Molten metal adjusted to have the composition shown in Table 2 was poured into a mold to produce an alloy. Test pieces (φ8 × 25L) for evaluating oxidation resistance were taken from the produced alloy. The taken test pieces were heat treated at a temperature of 800°C, and the mass increase due to oxide formation was measured every 24 hours. The results are shown in Table 2. As shown in Table 2, it was confirmed that the oxidation resistance of the thermal expansion controlled alloy of the present invention at high temperatures (800°C) can be improved by adding Cr.

[0047] [Table 2]

Claims

1. In mass%, Fe: 20-50%, Ni: 2.9-25%, Cr: 0 to 5.0%, and Co: 49.2% or more, and the remainder being unavoidable impurities, Contains 5% or more of an area in which the crystal structure is an ordered phase A controlled thermal expansion alloy. However, the total content of the components including the above balance is 100%.

2. In mass%, Fe: 20-50%, Ni: 7.7-25%, Cr: 10-30%, Co: 50.3% or more and the remainder being unavoidable impurities, Contains 5% or more of an area in which the crystal structure is an ordered phase A controlled thermal expansion alloy. However, the total content of the components including the above balance is 100%.

3. 3. An interconnector for a solid oxide electrolyte fuel cell, comprising the controlled thermal expansion alloy according to claim 1.

4. A gas turbine or steam turbine part made of the thermal expansion controlled alloy according to claim 1 or 2.

5. A glass molding die made of the thermal expansion controlled alloy according to claim 1 or 2.

6. A heat sink made of the controlled thermal expansion alloy according to claim 1 or 2.

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