Fe-Ni-Cr ALLOY, PRODUCING METHOD THEREOF, AND COMPONENT

Abstract

To provide an Fe-Ni-Cr alloy that can suppress intergranular cracking (nitriding resistance) and also inhibit the formation of red scale (oxidation resistance) even in a gas atmosphere containing nitrogen and water vapor, such as ammonia combustion gas, at a medium-high temperature range of 500 to 700°C.SOLUTION: The present invention relates to an Fe-Ni-Cr alloy that has a predetermined component composition, a nitriding resistance index of 81 or higher, and 3.0 or more of fine crystalline grains with a grain size of 0.5 μm or less present in the plate thickness direction in the alloy surface layer. The Fe-Ni-Cr alloy is obtained by introducing strain (transformation) on the surface of the alloy material by polishing or the like, then performing heat treatment, and generating fine crystalline grains using the introduced strain (transformation) as a driving force. Nitriding resistance index: Cr+3Ni-10Mo≥81.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an alloy material made of an Fe-Ni-Cr alloy (simply referred to as an Fe-Ni-Cr alloy; the form of the alloy material is not particularly limited and includes any shape such as a plate, a rod, or a wire), a method for producing the same, and a part using the Fe-Ni-Cr alloy. [Background technology]

[0002] Global warming has become an international environmental issue, and active technological development is underway to achieve a decarbonized society, including carbon-zero and carbon-neutral goals. Amid this trend, ammonia has attracted attention as an alternative to carbon fuels. The combustion reaction of ammonia is 4NH3 + 3O2 → 2N2 + 6H2O. It produces water and nitrogen, resulting in a low environmental impact and is therefore a promising recyclable fuel. Ammonia's combustion temperature, an adiabatic flame temperature of 1750°C, is lower than that of hydrogen (2120°C), methane (1970°C), and gasoline (approximately 2000°C). The combustion temperature in actual engines and gas turbines is also lower than that of existing fuels. Therefore, when ammonia is used as a fuel, the exhaust gas temperature is lower, approximately 500–700°C, compared to existing fuels. This 500–700°C temperature range is where steel, used in exhaust pipes and other applications, is prone to oxidation and the formation of so-called red scale.

[0003] Patent Document 1 proposes an austenitic stainless steel that has good corrosion resistance even in environments containing high sulfur (S) and chlorine (Cl), such as those used in boiler superheater tubes, waste incinerators, and ammonia synthesis plants.

[0004] Patent Document 2 proposes an austenitic stainless steel that has good corrosion resistance in an ammonia atmosphere without adding chromic acid, for use in an ammonia-water absorption heat exchanger or the like. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 3-126842 [Patent Document 2] Japanese Patent Application Publication No. 10-280100 [Patent Document 3] JP 2019-035122 A Summary of the Invention [Problem to be solved by the invention]

[0006] Development is underway to utilize ammonia as a fuel not only by burning it alone, but also by co-firing it with other fuels (heavy oil, light oil, hydrogen, etc.). However, even in co-firing, ammonia, which has a lower combustion temperature than existing fuels, is added and burned, so the combustion exhaust gas temperature is lower than that of existing fuels, at around 500-700°C. Furthermore, ammonia combustion gas contains large amounts of nitrogen and water vapor.

[0007] The inclusion of water vapor makes steam oxidation and red scale more likely to occur. Furthermore, the combustion gas temperature is around 500 to 700°C, which is also a temperature range in which red scale is likely to occur. For this reason, steel materials used in ammonia combustion gas systems, etc., are required to have heat resistance and oxidation resistance (red scale resistance).

[0008] Furthermore, the large amount of nitrogen contained in ammonia combustion exhaust gas causes nitrogen to penetrate (nitriding) the surface of the steel, which leads to embrittlement caused by intergranular cracking. Therefore, steel for use in ammonia combustion gas systems must also be resistant to nitriding (intergranular cracking).

[0009] Although Patent Document 1 describes the stainless steel as being applicable to ammonia synthesis equipment, it is not intended for use with ammonia combustion gas (combustion exhaust gas), and no consideration is given to measures against grain boundary cracking due to nitriding or red scale (oxidation) at 500 to 700°C.

[0010] The stainless steel of Patent Document 2 is intended for use in an ammonia-water absorption heat exchanger, i.e., is intended to come into contact with ammonia gas or ammonia solution, and does not take into consideration measures against grain boundary cracking due to nitriding or red scale (oxidation) at 500 to 700°C.

[0011] Furthermore, if intergranular cracking due to nitriding occurs, external factors such as vibration can cause cracks to propagate from the intergranular cracking, potentially resulting in deterioration of creep properties. Austenitic Fe-Ni-Cr alloys are known as alloys with corrosion resistance and high-temperature creep properties. For example, Patent Document 3 proposes an Fe-Ni-Cr-Mo alloy with enhanced resistance to general corrosion, pitting corrosion, crevice corrosion, and stress corrosion cracking by increasing the Ni, Cr, and Mo concentrations. However, even with such heat-resistant austenitic Fe-Ni-Cr-Mo alloys, although they have improved resistance to stress corrosion cracking, further improvements in steam oxidation resistance and nitriding resistance are required for application to ammonia combustion exhaust gas.

[0012] The present invention aims to provide an alloy that has red scale resistance (steam oxidation resistance) and intergranular cracking resistance (nitriding resistance) even in gases containing large amounts of nitrogen and water (water vapor) at temperatures of about 500 to 700°C, such as ammonia combustion exhaust gas. [Means for solving the problem]

[0013] In order to achieve the above object, the present inventors have conducted extensive research and have obtained the following findings.

[0014] (a) From the viewpoint of heat resistance and corrosion resistance, we focused on the surface oxide film of an austenitic Fe-Ni-Cr alloy. For example, Fe-Ni-Cr alloys such as those described in Patent Document 3 have high Cr and Mo concentrations, which tend to form intermetallic compounds such as the σ phase, which can cause stress corrosion cracking. Therefore, they are subjected to high-temperature annealing to dissolve the σ phase. We found that this high-temperature annealing increases the crystal grain size and reduces the grain boundaries, thereby restricting the diffusion paths of Cr and Si in the steel sheet, preventing the formation of strong Cr- and Si-based oxide films on the surface.

[0015] (b) The inventors of the present invention conceived the idea of ​​creating fine crystal grains on the alloy surface to ensure a diffusion path for Cr and Si to the surface, and proceeded with development. As a result, they discovered that creating fine crystal grains with a grain size of 0.5 μm or less just below the alloy surface (surface layer) allows a strong oxide film to form on the alloy surface.

[0016] (c) Furthermore, from the viewpoint of suppressing nitriding, the alloy was designed by optimizing the elements. As a result, it was found that not only Ni but also Cr is effective in nitriding resistance, but on the other hand, it was thought that optimizing the content of Mo, an element that promotes nitriding, would be effective. As a result, a nitriding resistance index, which indicates the degree of nitriding resistance of an alloy, was derived, and it was found that it would be effective to set this index at 81 or higher. Nitriding resistance index: Cr+3Ni―10Mo≧81

[0017] The present invention was made based on these findings, and the gist of the present invention is as follows.

[0018] [1] In mass%, C: 0~0.150%, Si: 0.10 to 1.50% Mn: 0.10 to 2.00%, P: 0.050% or less, S: 0.0050% or less, Ni: 18.00~46.00%, Cr: 18.00~26.00%, Al: 0.002 to 1.000%, Ti: 0.01 to 1.00% N: 0~0.250%, Mo: 0-3.50% Cu: 0-3.00% V: 0~1.00%, Nb: 0 to 1.00% B: 0~0.0100%, Ca: 0 to 0.0150% Zr: 0-0.60% Co: 0-1.50% W: 0~2.00%, Sn: 0 to 1.000%, Mg: 0 to 0.0050%, Sb: 0 to 0.60% Hf: 0~0.60%, Ta: 0 to 1.00%, Ga: 0 to 0.50% REM: 0~0.20%, including Equation 1 is satisfied, The balance consists of Fe and impurities, The following formula 1 is satisfied: An Fe-Ni-Cr alloy material characterized in that, in a cross section perpendicular to the surface, there are 3.0 or more fine crystal grains with a crystal grain size (area-equivalent circular diameter) of 0.5 μm or less in the thickness direction within a range from the surface to a depth of 2.0 μm. Cr+3Ni―10Mo≧81 (Formula 1) However, the element symbol in formula 1 indicates the content (mass%) of the element, and 0 is substituted if the element is not contained. [2] The Fe-Ni-Cr alloy material according to the above [1], wherein in a cross section perpendicular to the surface of the alloy material, the number of grain boundaries of crystal grains (alloy base crystal grains) other than the fine crystal grains in contact with the surface is 3 to 10 within 500 μm in a direction parallel to the surface. [3] The Fe-Ni-Cr alloy material according to [1] or [2], wherein in a cross section perpendicular to the surface of the alloy material, the total length of intergranular cracks in any three fields of view, with a 50 μm square area adjacent to the surface being taken as one field of view, is 15 μm or less. [4] The Fe—Ni—Cr alloy material according to any one of [1] to [3], wherein the grain size number of the alloy material according to JIS G 0551 is 6.0 or less. [5] In mass%, C: 0~0.150%, Si: 0.10 to 1.50% Mn: 0.10 to 2.00%, P: 0.050% or less, S: 0.0050% or less, Ni: 18.00~46.00%, Cr: 18.0~26.0%, Al: 0.002 to 1.000%, Ti: 0.01 to 1.00% N: 0~0.250%, Mo: 0-3.50% Cu: 0-3.00% V: 0~1.00%, Nb: 0 to 1.00% B: 0~0.0100%, Ca: 0 to 0.0150% Zr: 0-0.60% Co: 0-1.50% W: 0~2.00%, Sn: 0 to 1.000%, Mg: 0 to 0.0050%, Sb: 0 to 0.60% Hf: 0~0.60%, Ta: 0 to 1.00%, Ga: 0 to 0.50% REM: 0~0.20%, including Equation 1 is satisfied, The balance consists of Fe and impurities, An alloy material that satisfies the following formula 1, An Fe-Ni-Cr alloy material characterized in that, after the alloy material is held at 800°C for 30 seconds in an air atmosphere and then cooled, in a vertical cross section of the surface of the alloy material, there are 3.0 or more crystal grains with a crystal grain size (area-equivalent circular diameter) of 0.5 μm or less in the thickness direction within a range from the surface to a depth of 2.0 μm. Cr+3Ni―10Mo≧81 (Formula 1) However, the element symbol in formula 1 indicates the content (mass%) of the element, and 0 is substituted if the element is not contained. [6] The Fe—Ni—Cr alloy material according to [5], wherein the grain size number of the alloy material based on JIS G 0551 is 6.0 or less. [7] The Fe—Ni—Cr alloy material according to any one of [1] to [4] above, which is for an ammonia burning appliance. [8] A part at least partially comprising the Fe—Ni—Cr alloy material according to any one of [1] to [4] above. [9] The part according to [8] above, which is a part for an ammonia combustion device.

[10] A method for producing an Fe-Ni-Cr alloy material according to any one of [1] to [4] above, comprising the steps of: heating and holding an Fe-Ni-Cr alloy material having the composition according to [1] above at 1100 to 1250°C after final cold rolling, and then cooling to a temperature of 50°C or less; then immersing the material in a pickling solution containing 2.0% or less hydrofluoric acid and 6 to 15% nitric acid at a temperature of 50 to 70°C for 60 to 90 seconds to pickle the material; then polishing, shot blasting or shot peening at least a part of the surface of the Fe-Ni-Cr alloy material; and then holding the material at a temperature of 750 to 800°C for 30 to 120 seconds and cooling the material. [Effects of the Invention]

[0019] The Fe-Ni-Cr alloy of the present invention can provide an alloy with excellent corrosion resistance and wear resistance, even when it comes into contact with gases containing large amounts of nitrogen and water (water vapor), such as ammonia combustion exhaust gas at temperatures of approximately 500 to 700°C. Furthermore, the Fe-Ni-Cr alloy of the present invention has a smaller thermal expansion coefficient than ferritic stainless steels, so it is less susceptible to thermal fatigue, and it also has excellent high-temperature strength and high-temperature corrosion resistance. Therefore, the Fe-Ni-Cr alloy of the present invention is an extremely effective material in environments requiring high-temperature corrosion resistance and strength, such as ammonia combustion exhaust gas, which contains large amounts of nitrogen and water vapor. DETAILED DESCRIPTION OF THE INVENTION

[0020] An embodiment of the present invention (hereinafter simply referred to as the present invention) will be described below. Unless otherwise specified, "%" for components indicates mass% in the steel. When no lower limit is specified or when the lower limit is 0%, this also includes the case where the component is not contained (0%).

[0021] <About steel composition> C: 0 to 0.150% Since C is an element that reduces formability (r-value), the lower the content, the better, with the upper limit set at 0.150%. From the viewpoint of formability, 0.140% or less, 0.120% or less, or 0.100% or less is preferred. There is no particular lower limit, but since an excessive reduction leads to an increase in refining costs, 0.001% or more is preferred, and 0.002% or more is more preferred.

[0022] Si: 0.10 to 1.50% Since Si is effective in suppressing oxidation, particularly steam oxidation, and is also effective in suppressing nitriding, the content is set to 0.05% or more. The lower limit of Si is preferably 0.10%, 0.12%, 0.14%, 0.16%, 0.18%, 0.20%, 0.25%, 0.30%, 0.35%, 0.40%, 0.45%, or 0.50%. On the other hand, since a high Si content deteriorates workability and weldability, the upper limit is set to 1.50%. The upper limit of Si is preferably 1.30%, 1.10%, or 1.00%.

[0023] Mn: 0.10 to 2.00% Mn, like Si, is an element effective in improving oxidation resistance, so it is recommended that it be contained in an amount of 0.10% or more. The lower limit of Mn is preferably 0.15%, 0.20%, 0.25%, or 0.30%. On the other hand, a high Mn content deteriorates workability, so it is recommended that it be contained in an amount of 2.00% or less. The upper limit of Mn is preferably 1.80%, 1.60%, 1.50%, or 1.40%.

[0024] P:0.050% or less Since P is harmful to stainless steel, reducing toughness, hot workability, and corrosion resistance, the less P there is, the better, and it should be 0.050% or less, and preferably 0.040% or less. However, an excessive reduction in P content increases the load during refining or requires the use of expensive raw materials, so in reality, a content of 0.001% or more is acceptable.

[0025] S: 0.0050% or less Since S is harmful to stainless steel, reducing toughness, hot workability, and corrosion resistance, the less S it contains, the better, and the upper limit should be set to 0.0050% or less, preferably 0.0030% or less. However, an excessive reduction increases the load during refining or requires the use of expensive raw materials, so in reality, a content of 0.0001% or more is acceptable.

[0026] Ni: 18.00~46.00% Ni is an element that stabilizes the austenite phase and has the effect of improving corrosion resistance to various acids and low-temperature toughness, so it is recommended that Ni be contained in an amount of 18.00% or more, and preferably 19.00% or more, 20.00% or more, 22.00% or more, 24.00% or more, or 25.00% or more. On the other hand, since Ni is an expensive element, even if it is contained in a large amount, the effect is not commensurate with the increase in alloy cost, so it is recommended that Ni be contained in an amount of 46.00% or less, and preferably 44.00% or less, 42.00% or less, 40.00% or less, 38.00% or less, or 35.00% or less.

[0027] Cr: 18.0 to 26.0% Cr is an important element that provides corrosion resistance to stainless steel, and should be contained at 18.0% or more, preferably 19.0% or more, 20.0% or more, or 21.0% or more. On the other hand, since a large content leads to a decrease in workability, it should be contained at 26.0% or less, preferably 25.5% or less, 25.0% or less, 25.5% or less, or 24.0% or less.

[0028] Al: 0.002 to 1.000% Al is an element that bonds with N to form AlN and promotes nitriding, and since excessive addition reduces workability, the Al content should be 1.000% or less, and preferably 0.900% or less, 0.800% or less, 0.700% or less, 0.600% or less, 0.500% or less, 0.400% or less, 0.300% or less, or 0.200% or less. On the other hand, because Al has the effect of desulfurizing and improving corrosion resistance, the Al content should be 0.002% or more, and preferably 0.004% or more, 0.006% or more, or 0.008% or more.

[0029] Ti: 0.01 to 1.00% Ti ensures corrosion resistance by stabilizing C and N, so the Ti content should be 0.01% or more, and preferably 0.03% or more, 0.05% or more, 0.07% or more, 0.09% or more, or 0.10% or more. On the other hand, Ti is an element that promotes nitriding, and if added in excess, TiN is significantly formed, leading to nozzle clogging during manufacturing and surface defects in the product, so the Ti content should be 1.00% or less, and preferably 0.80% or less, 0.60% or less, or 0.30% or less.

[0030] N: 0 to 0.250% From the viewpoint of suppressing intergranular cracking due to surface N, it is preferable that the N content of the steel material is small. Furthermore, since N reduces workability and reduces corrosion resistance by combining with Cr, the smaller the N content, the better, and it is good to set it to 0.250% or less, and preferably 0.200% or less, 0.180% or less, 0.160% or less, 0.140% or less, 0.120% or less, or 0.100% or less. On the other hand, since an excessive reduction in N places a large burden on the refining process, it may be preferably contained at 0.001% or more, 0.005% or more, or 0.010% or more.

[0031] Nb: 0 to 1.00% Nb may be included because it has the effect of improving formability and corrosion resistance. On the other hand, if added in excess of 1.00%, recrystallization becomes difficult and the structure becomes coarse, so it is best to keep it at 1.00% or less, preferably 0.90% or less, 0.80% or less, or 0.70% or less. There is no particular lower limit for the Nb content, but to ensure the desired effect, it is preferable to include 0.01% or more.

[0032] Mo: 0-3.50% Mo may be added because it has the effect of further enhancing the high corrosion resistance. However, it is an element that promotes nitriding and also forms a brittle sigma phase (σ phase) with high Cr content, which leads to embrittlement and a decrease in corrosion resistance. Therefore, it is recommended that the Mo content be 3.50% or less, and preferably 3.00% or less, or 2.50% or less. There is no particular lower limit for the Mo content, but to ensure the effect of corrosion resistance, it is preferable to add 0.01% or more.

[0033] Cu: 0-3.00% Cu may be added because its addition further enhances the high corrosion resistance. However, excessive addition does not improve performance enough to justify the manufacturing costs, so the content should be 3.00% or less, preferably 2.80% or less, 2.50% or less, or 2.00% or less. There is no particular lower limit for the Cu content, but to ensure the desired effect, it is preferable to add 0.01% or more, 0.03% or more, or 0.05% or more.

[0034] V: 0 to 1.00% V may be added because it has the effect of further enhancing the high corrosion resistance. On the other hand, if it is added at a high concentration, it will cause a decrease in toughness, so the upper limit should be set to 1.00%, and preferably 0.90% or less, 0.70% or less, or 0.50% or less. There is no particular lower limit for the V content, but to ensure the effect, it is preferably added at 0.01% or more, or 0.05% or more.

[0035] B: 0 to 0.0100% B is an element that increases the strength of grain boundaries and contributes to improving workability, so it may be included. On the other hand, excessive addition of B can actually reduce elongation and workability, so the content should be 0.0100% or less, and preferably 0.0090% or less, 0.0070% or less, or 0.0050% or less. There is no particular lower limit for the B content, but to ensure the desired effect, it is preferably 0.0001% or more, or 0.0005% or more.

[0036] Ca: 0 to 0.0150% If the Ca content is too high, the concentration in the oxides that promote TiN formation increases, resulting in a loss of this ability. Therefore, the Ca content should be 0.0150% or less, and preferably 0.0120% or less, 0.0090% or less, 0.0070% or less, or 0.0050% or less. There is no particular lower limit, but Ca is the main component of slag, and some inclusion in it is unavoidable. Furthermore, complete removal is difficult, and excessive reduction increases the load during refining, so in practical operation, the Ca content may be 0.0001% or more, or 0.0002% or more.

[0037] Sn: 0 to 1.000% Sn may be added because it has the effect of further enhancing the high corrosion resistance. However, excessive addition leads to a decrease in workability, so the content should be 1.000% or less, and preferably 0.700% or less, 0.500% or less, or 0.300% or less. There is no particular lower limit for the Sn content, but to ensure the desired effect, it is preferable to add 0.001% or more, or 0.002% or more.

[0038] In addition, the steel may contain, in mass %, Hf: 0-0.60%, Zr: 0-0.60%, Sb: 0-0.60%, Co: 0-1.50%, W: 0-2.00%, Ta: 0-1.00%, Ga: 0-0.50%, Mg: 0-0.0050%, and REM: 0-0.20%. The addition of these elements enhances corrosion resistance. However, because these elements are expensive, excessive inclusion of these elements does not provide benefits commensurate with the increased cost, so upper limits have been set. While there are no particular lower limits for the content of these elements, to ensure the desired effects, it is preferable to include at least 0.0003% Mg and at least 0.001% or 0.01% of each element other than Mg.

[0039] The balance of the above steel components is Fe and impurities. Here, the term "impurities" refers to components that are mixed in during industrial steel production due to various factors in the manufacturing process, including raw materials such as ore and scrap, and are acceptable within a range that does not adversely affect the present invention.

[0040] <Nitriding resistance index> The alloy composition was optimized to suppress nitridation of the alloy, and the relationship between the content of elements that affect nitridation was considered. Ni is known to have high nitridation resistance. On the other hand, Mo is known to promote nitridation in high-Ni, high-Cr Fe-Ni-Cr alloys, but a certain amount may be included to ensure functions such as corrosion resistance. Furthermore, the inventors discovered that the presence of a strong Cr-based oxide film on the surface also has the effect of suppressing nitridation. Therefore, they envisioned a balanced combination of Mo, an element that promotes nitridation, with Ni and Cr, which are effective in suppressing nitridation, and found that the nitridation resistance of an Fe-Ni-Cr alloy can be evaluated using the nitridation resistance index shown in Equation 1 below. From the perspective of ensuring nitridation resistance, they found that a nitridation resistance index of 81 or greater is desirable.

[0041] The nitriding resistance index is, in short, an index of the nitriding suppression effect, and a higher value is preferable for nitriding. Therefore, the nitriding resistance index should be 81 or more, and preferably 85 or more, 90 or more, 95 or more, 100 or more, 105 or more, 110 or more, 115 or more, or 120 or more. Nitriding resistance index: Cr+3Ni―10Mo≧81 ...(Formula 1) However, the element symbol in formula 1 indicates the content (mass%) of the element, and 0 is substituted if the element is not contained.

[0042] <Surface fine crystal grains> The presence of fine crystals in the alloy surface layer (just below the surface) creates grain boundaries around them. Grain boundaries act as diffusion paths for Cr, Si, and Al in the alloy, so an increase in the number of grain boundaries allows these elements to diffuse outward toward the alloy surface, making it easier to form a strong oxide film (Cr-based oxide film, Si-based oxide film, Al-based oxide film, or a composite oxide film that is a mixture of these) on the alloy surface. The formation of a strong oxide film on the surface prevents nitrogen from penetrating from the outside, suppressing nitridation of the alloy.

[0043] In the present invention, fine crystal grains refer to crystal grains having a crystal grain size (diameter equivalent to an area circle, also called the diameter equivalent to an area circle or the diameter equivalent to a projected area circle) of 0.5 μm or less. In the direction perpendicular to the alloy surface (hereinafter referred to as the depth direction, but for example, in the case of a plate-like alloy, this refers to the thickness direction, and in the case of a rod-like alloy, the central axis direction), it is desirable that 3.0 or more fine crystal grains are present in the thickness direction from the surface to a depth of 2.0 μm (hereinafter referred to as the surface layer). From the viewpoint of ensuring diffusion paths for Cr and Si, it is desirable that there are 3.5 or more, 4.0 or more, 4.5 or more, or 5.0 or more fine crystal grains. There is no particular upper limit, but since too many fine crystal grains in the surface layer will make the surface brittle, it is desirable that the number be 100.0 or less, and preferably 90.0 or less, 80.0 or less, 70.0 or less, 60.0 or less, or 50.0 grains / μm. 2 It would be better to do the following:

[0044] Furthermore, it is preferable that the fine crystal grains be in contact with the alloy surface, so that Cr, Si, and Al in the alloy can be diffused to the alloy surface. Furthermore, it is preferable that the fine crystal grains be arranged adjacent to each other in the depth direction from the alloy surface, so that Cr and other elements can be diffused from deep within the alloy.

[0045] The method for measuring fine crystal grains is explained below. A vertical cross section of the surface of the alloy is observed using a scanning electron microscope (SEM). The observation plane is set at 10 μm parallel to the surface and 4.0 μm from the surface in the thickness direction, and three lines perpendicular to the surface (perpendicular lines to the surface) are drawn at random for each observation plane, and the number of fine crystal grains crossed by each line is counted. This is performed for at least three observation planes, and the arithmetic average (total number of fine crystals / number of perpendicular lines) of the number of fine crystals on each perpendicular line to the surface is taken as the number of fine crystals in the thickness direction.

[0046] <Number of grain boundaries of alloy base material crystal grains in contact with the surface> It is preferable to ensure that the crystal interfaces of the crystal grains in the alloy other than the fine crystal grains (hereinafter referred to as alloy base crystal grains) are in contact with the alloy surface in order to ensure a diffusion path for elements such as Cr in the alloy. The alloy base crystal grains are particularly effective in ensuring a diffusion path from the depths of the alloy to the surface. The number of grain boundaries of these alloy base crystal grains that contact (intersect with) the alloy surface within a 500 μm range in a direction parallel to the surface in a cross section perpendicular to the surface of the alloy (hereinafter referred to as a surface-normal cross section) is preferably 3 or more, more preferably 4 or more, 5 or more, 6 or more, or 7 or more. While there is no upper limit to the number of grain boundaries that contact the surface, increasing the number of grain boundaries means that the alloy base crystal grains become finer, which may deteriorate the inherent creep resistance of the alloy. Therefore, the number of grain boundaries of the alloy base crystal grains that contact the surface is preferably 20 or less, 18 or less, 16 or less, 14 or less, 13 or less, 12 or less, 11 or less, or 10 or less.

[0047] The method for observing the grain boundaries of alloy base material crystal grains is explained below. A vertical cross section of the alloy surface is observed using an optical microscope. The observation plane is a plane 500 μm parallel to the surface and 250 μm in the depth direction, and at least three observation planes are selected. The number of grain boundaries of alloy base material crystal grains that contact the surface on each observation plane is counted, and the arithmetic average of these (total number of grain boundaries that contact the surface / number of observation planes) is taken as the number of grain boundaries of crystal grains (alloy base material crystal grains) other than fine crystal grains that contact the surface of the alloy.

[0048] <Grain size of alloy base material crystal grains> The grain size of the alloy matrix is ​​important in determining the alloy's properties. Heat-resistant Fe-Ni-Cr alloys contain high concentrations of Cr and Mo, which can easily form intermetallic compounds such as the σ phase, leading to poor creep resistance. Therefore, high-temperature annealing is performed to dissolve the σ phase and suppress its formation, thereby improving creep resistance. This high-temperature annealing coarsens the grains in the alloy (the alloy matrix grains). Typically, in Fe-Ni-Cr alloys, the alloy matrix grains have a grain size number of 6.0 or less, as measured by the comparison method based on JIS G 0551. Preferably, it is 5.5 or less, or even 5.0 or less. On the other hand, a smaller grain size number (i.e., coarser grains) leads to softening of the alloy. Therefore, the grain size number should be 3.0 or more, preferably 3.5 or more, or even 4.0 or more.

[0049] <Intergranular crack length> As a result of suppressing nitriding of the alloy surface, intergranular cracking due to the penetration of nitrogen (N) is suppressed. The length of intergranular cracking can be measured by observing the crystal grain boundaries. Three 50 μm square areas are selected at any location in the surface layer of a vertical cross section of the alloy surface, and the total intergranular crack length at these locations is preferably 15 μm or less. If the total intergranular crack length at the three observation locations is 10 μm or less, embrittlement of the alloy surface can be suppressed, and alloy strength can be ensured in the temperature range of 500 to 700°C. The shorter the total intergranular crack length, the better, and it is more preferably 9 μm or less, 8 μm or less, 7 μm or less, 6 μm or less, or 5 μm or less.

[0050] The length of intergranular cracks in the surface layer of an alloy can be measured as follows: A vertical cross section of the surface of the sample alloy is observed under an optical microscope with a 50 μm square field of view, and the length of the intergranular cracks is measured. At this time, since the area closer to the alloy surface is more susceptible to the influence of nitrogen, it is advisable to set the observation field to the area directly below the alloy surface. It is preferable to perform the measurement using image processing. For example, the intergranular crack area can be marked on the measurement image, and its length can be measured using image processing.

[0051] <Nitriding depth> The alloy according to the present invention has its composition adjusted to suppress nitriding and has an oxide film such as a Cr-based oxide on its surface, so that the nitriding depth is shallow on average. In particular, the larger the nitriding resistance index value, the shallower the nitriding depth tends to be. The nitriding depth varies slightly depending on the nitrogen (N) content of the contact gas, but it has been confirmed that surface embrittlement is suppressed if the nitriding depth is generally 100 μm or less. The nitriding depth is preferably 95 μm or less, 90 μm or less, 85 μm or less, or 80 μm or less.

[0052] <Manufacturing method> Next, a manufacturing method will be described. The manufacturing method described below is one embodiment for obtaining the alloy according to the present invention, and the manufacturing method is not limited to this. As long as the alloy according to the present invention can be obtained, the manufacturing method is not limited.

[0053] In one embodiment of the method for producing an alloy according to the present invention, an alloy is produced by conventional methods, and then the Cr oxide layer and Fe oxide layer formed on the surface during the production process are etched away by acid washing. The surface is then subjected to polishing, shot blasting, shot peening, or the like. This is intended to introduce strain into the alloy surface and also to remove the Cr-depleted region formed near the surface by final annealing. The inventors have found that subsequent heat treatment drives the formation of fine crystal grains, driven by the strain introduced into the surface. The grain boundaries of the fine crystal grains in the surface layer thus formed serve as diffusion paths for elements such as Cr. Below, an example will be described in which the alloy is in the form of a plate (alloy plate), and polishing is used as a method for introducing strain into the surface.

[0054] The alloy before final annealing may be produced by a conventional production method, for example, by the steps of melting, hot rolling, melting, hot rolling and annealing, or melting, hot rolling, pickling and cold rolling.

[0055] However, in the melting process, a suitable method is to melt an alloy containing the components adjusted to the above-described composition in a vacuum melting furnace or the like, followed by secondary refining. The alloy thus adjusted to the desired composition is formed into a slab by a known casting method (for example, continuous casting or ingot casting). The slab is heated to a predetermined temperature and hot-rolled to a desired thickness. After hot rolling, the slab may be cold-rolled (cold-rolled) if necessary. Cold rolling may also be performed by a conventional method.

[0056] The conditions in the manufacturing process may be selected as appropriate. For example, the slab thickness, hot-rolled sheet thickness, etc. may be set as appropriate. After coiling, the hot-rolled sheet may be immersed in a water-cooled pool. There are no particular restrictions on the pickling process after hot rolling or after hot-rolling annealing, and the mechanical descaling method, such as shot blasting, bending, or brushing, may be selected as appropriate. There are also no particular restrictions on the pickling solution after hot rolling, and existing conditions, such as sulfuric acid or nitric hydrofluoric acid, may be used. Furthermore, the coil surface may be subsequently ground.

[0057] The hot-rolled alloy, hot-rolled annealed alloy, and cold-rolled alloy thus obtained are subjected to final annealing. The annealing atmosphere is not particularly limited, and air atmosphere is also acceptable. Annealing is preferably performed in a temperature range of 1100 to 1250°C. The holding time is not particularly limited, but is preferably 30 seconds to 5 minutes.

[0058] After final annealing, the alloy is cooled to below 60°C and pickled to etch away the upper Cr oxide and Fe oxide layers. The pickling solution used should contain 3.0% or less hydrofluoric acid (HF) and 6-15% nitric acid, and should be adjusted to a temperature of 50-60°C for an immersion time of 50-80 seconds. This allows the alloy surface to be free of Fe oxide and Cr oxide that formed on the surface during the manufacturing process.

[0059] The surface of the pickled alloy sheet is polished to introduce dislocations (strain) into the surface and harden it. The thickness of the surface-hardened layer can be adjusted by adjusting the polishing conditions (such as polishing time and pressure). The polishing conditions are not particularly limited, but dry polishing or wet polishing can be used. For example, in dry polishing, the alloy surface can be polished using a #220 abrasive, followed by a #400 to #600 abrasive. For example, the alloy surface can be polished using a #220 abrasive, followed by a #400 and / or #600 abrasive. Polishing in order from coarse to fine abrasives reduces the surface roughness of the alloy and eliminates defects. The combination of abrasives is not particularly limited. The polishing amount is also not particularly limited, but is preferably about 10 μm to 50 μm in actual production. Since the conditions vary depending on the steel material, it can be appropriately determined by polishing in advance to check the surface hardness and surface defects.

[0060] In wet polishing, oil mixed with abrasives is applied to the alloy surface, and the alloy surface is polished while rotating the polishing cloth. In this case, too, it is recommended to polish the alloy surface in order from coarse to fine abrasives.

[0061] After polishing, the surface may be washed with water or alkaline to remove any abrasive remaining on the alloy surface.

[0062] When shot blasting or shot peening is employed as a method for imparting dislocations (strain) to the surface, the conditions for shot blasting or shot peening are not particularly limited, and may be appropriately determined by checking the surface hardness and surface defects through prior testing.

[0063] Next, the surface is polished, and the alloy plate with the introduced strain (dislocation) is heat-treated to generate fine crystal grains in the surface layer just below the surface. The heat treatment conditions are preferably a temperature of 750 to 800°C, with a holding time in this temperature range of 30 seconds or more and 120 seconds or less. The heat treatment atmosphere is not particularly limited, and may be air, Ar, hydrogen, nitrogen, a hydrogen-nitrogen mixture, or other atmosphere. Preferably, the N (nitrogen) content in the atmospheric gas is 5% or less, more preferably 2% or less, or 1% or less. Furthermore, when the heat treatment is performed in an atmosphere other than air, the dew point is preferably -30°C or less, and even more preferably -40°C or less.

[0064] <Application> Even when used in a gas environment with a high nitrogen (N) content, the alloy according to the present invention exhibits excellent nitriding resistance, resulting in little nitrogen penetration into the alloy surface layer and suppression of grain boundary cracking. Furthermore, it also has steam oxidation resistance, which is effective in preventing the red scale generation problematic with conventional Fe-Ni-Cr alloys, particularly in the medium- to high-temperature range of about 500 to 700°C. For this reason, it can be used, for example, in ammonia-burning appliances with a high nitrogen content and gas temperatures in the medium- to high-temperature range of 500 to 700°C. It can be used in exhaust components of ammonia-burning appliances, among other applications. Of course, due to its resistance to nitridation and steam oxidation, even when used in containers or piping parts that come into direct contact with ammonia or urea, nitrogen ions in the solution and nitrogen in the evaporated gas from ammonia or urea are prevented from penetrating the alloy surface, thereby suppressing grain boundary cracking. In addition, when the Fe-Ni-Cr alloy according to the present invention is applied to parts that require resistance to nitriding and steam oxidation, the effects can be utilized efficiently. [Example]

[0065] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0066] An alloy having the composition shown in Table 1 was melted in a vacuum melting furnace and cast into a 30 kg slab. The slab was heated to 1230°C and then hot-rolled to obtain a hot-rolled alloy sheet with a thickness of 4.5 mm. The hot-rolled alloy sheet was then annealed at a temperature of 1100 to 1250°C depending on the alloy composition, with a 1-minute soaking time to obtain a hot-rolled annealed alloy sheet. The obtained alloy sheet was pickled in a nitric acid solution (3% hydrofluoric acid, 10% nitric acid, balance water) at 60°C to remove oxide scale. The alloy sheet was then cold-rolled to obtain a cold-rolled alloy sheet with a thickness of 2.0 mm. The obtained cold-rolled alloy sheet was annealed at a temperature of 1100 to 1250°C (final annealing), then immersed in a pickling solution (2% hydrofluoric acid + 10% nitric acid + water) at 50 to 60°C for 50 to 80 seconds (final pickling), and then rinsed with water. The surface of the alloy sheet after the final pickling was brushed to finish it. Brushing was performed using a brush with SiC abrasive grains at a load current of 90 to 120 A, a rotation speed of 1100 to 1300 rpm, and a reduction of 1.0 to 2.0 mm. This introduced strain (dislocation) into the surface of the alloy sheet.

[0067] The alloy plate with the surface strain (dislocation) introduced was then heat-treated by holding the plate in a 100% H2 atmosphere at a temperature range of 750 to 800°C for 30 to 120 seconds, and then cooling (natural cooling) to room temperature in the air.

[0068] Four 20mm x 25mm test pieces were cut out from the obtained alloy plate, and one of them was used to measure the number of microcrystal grains in the vertical cross section of the alloy surface and the number of alloy base material crystal grain boundaries in contact with the surface, while the remaining three were evaluated by nitriding and oxidation treatment simulating ammonia combustion gas.

[0069] For evaluation of the alloy surface perpendicular cross section, the test piece was cut perpendicular to the surface and etched with a solution of hydrofluoric acid: nitric acid: glycerin = 3:1:1, and the metal structure of the cross section was observed using an optical microscope. For observation, the surface of the test piece was used as the reference point, and the observation field was set to a range of 500 μm parallel to the surface and 250 μm in the depth direction, and three observation planes were selected arbitrarily within the surface perpendicular cross section of the test piece, and the number of fine crystal grains and the number of grain boundaries of the alloy base material crystal grains in contact with the surface were measured.

[0070] For the nitriding and oxidation treatment, 100 vol% ammonia gas was introduced into the annealing furnace at a rate of 100 ml / min. The test specimens were placed in the furnace, heated to 600°C, and held there for 50 hours. They were then cooled and removed, and the intergranular crack length and nitriding depth were measured.

[0071] The length of intergranular cracking was measured by cutting the test piece after nitriding and oxidation treatment so that the cross section in the thickness direction could be observed, and observing the cross section of the test piece using an optical microscope. The observation was performed with a field of view of 50 μm x 50 μm just below the alloy surface, and three randomly selected locations on the sample cross section were observed to measure the length of intergranular cracking. A satisfactory result was achieved if the total intergranular crack length at the three observation points was 10 μm or less.

[0072] The nitriding depth was measured by cutting the test piece after nitriding and oxidation treatment, electrolytically etching it with a 10% oxalic acid solution at a voltage of 6 V for 5 seconds, and observing it with an optical microscope. The nitriding depth was measured using photographs. The nitriding depth was measured by defining the area with a nitrogen concentration of 5 mass% or more using EPMA as the nitriding area, and the thickness (depth) of this area in the depth direction. A nitriding depth of 50 μm or less from the surface in the depth direction was considered satisfactory.

[0073] The red scale was visually inspected, and samples with no red scale were rated as "pass" (◯), while samples with even a small amount of red scale were rated as "fail" (×). The results of these measurements are shown in Table 2. The data in Table 2 show that the alloys according to the present invention have reduced intergranular crack length.

[0074] Furthermore, the creep properties of one of the nitriding and oxidation treated specimens were evaluated. The specimen was placed in a high-temperature tensile testing furnace and subjected to a tensile stress of 300 MPa in an atmosphere at 600°C, and the time until the specimen broke was measured. Those with a creep rupture time of 200 hours or more were rated as passing (◯).The data in Table 2 show that the alloy according to the present invention also has good creep properties.

[0075] [Table 1]

[0076] [Table 2] [Industrial Applicability]

[0077] The present invention can be used in a variety of industries, including the automobile industry and general machinery industry.

Claims

1. In mass%, C: 0 to 0.150%, Si: 0.10 to 1.50%, Mn: 0.10-2.00%, P: 0.050% or less, S: 0.0050% or less, Ni: 18.00-46.00%, Cr: 18.0-26.0%, Al: 0.002-1.000%, Ti: 0.01~1.00% N: 0 to 0.250%, Mo: 0-3.50% Cu: 0-3.00% V: 0 to 1.00%, Nb: 0-1.00% B: 0 to 0.0100%, Ca: 0-0.0150% Zr: 0 to 0.60%, Co: 0 to 1.50%, W: 0-2.00%, Sn: 0-1.000%, Mg: 0 to 0.0050%, Sb: 0 to 0.60%, Hf: 0-0.60%, Ta: 0 to 1.00%, Ga: 0-0.50%, REM: 0 to 0.20%; Equation 1 is satisfied, The balance is Fe and impurities, The following formula 1 is satisfied: An Fe-Ni-Cr alloy material characterized in that, in a cross section perpendicular to the surface, in a range from the surface to a depth of 2.0 μm, there are 3.0 or more fine crystal grains having a crystal grain size (area equivalent circular diameter) of 0.5 μm or less in the plate thickness direction. Cr+3Ni-10Mo≧81 (Formula 1) In the formula 1, the element symbol indicates the content (mass%) of the element, and 0 is substituted when the element is not contained.

2. 2. The Fe-Ni-Cr alloy material according to claim 1, wherein the number of grain boundaries of crystal grains other than the fine crystal grains in contact with the surface is 3 or more within 500 μm in a direction parallel to the surface in a cross section perpendicular to the surface of the alloy material.

3. The Fe-Ni-Cr alloy material according to claim 1 or 2, wherein in a cross section perpendicular to the surface of the alloy material, a 50 μm square area in contact with the surface is defined as one field of view, and the total length of intergranular cracks in any three fields of view is 15 μm or less.

4. 3. The Fe-Ni-Cr alloy material according to claim 1, wherein the grain size number of the alloy material according to JIS G 0551 is 6.0 or less.

5. In mass%, C: 0 to 0.150%, Si: 0.10 to 1.50%, Mn: 0.10-2.00%, P: 0.050% or less, S: 0.0050% or less, Ni: 18.00-46.00%, Cr: 18.0-26.0%, Al: 0.002-1.000%, Ti: 0.01~1.00% N: 0 to 0.250%, Mo: 0-3.50% Cu: 0-3.00% V: 0 to 1.00%, Nb: 0-1.00% B: 0 to 0.0100%, Ca: 0-0.0150% Zr: 0 to 0.60%, Co: 0 to 1.50%, W: 0-2.00%, Sn: 0-1.000%, Mg: 0 to 0.0050%, Sb: 0 to 0.60%, Hf: 0-0.60%, Ta: 0 to 1.00%, Ga: 0-0.50%, REM: 0 to 0.20%; Equation 1 is satisfied, The balance is Fe and impurities, An alloy material that satisfies the following formula 1: The alloy material is maintained at 800°C for 30 seconds in an air atmosphere and then cooled, and thereafter, in a cross section perpendicular to the surface of the alloy material, there are present 3.0 or more crystal grains in the thickness direction, each having a crystal grain size (circular diameter equivalent to area) of 0.5 μm or less, within a range from the surface to a depth of 2.0 μm. Cr+3Ni-10Mo≧81 (Formula 1) In the formula 1, the element symbol indicates the content (mass%) of the element, and 0 is substituted when the element is not contained.

6. 6. The Fe-Ni-Cr alloy material according to claim 5, wherein the grain size number of the alloy material according to JIS G 0551 is 6.0 or less.

7. 3. The Fe-Ni-Cr alloy material according to claim 1, which is used for an ammonia burning appliance.

8. A part having at least a part made of the Fe-Ni-Cr alloy material according to claim 1 or 2.

9. 9. The component of claim 8, which is an ammonia-burning appliance component.

10. 3. A method for producing an Fe—Ni—Cr alloy material according to claim 1, comprising the steps of: heating and holding an Fe—Ni—Cr alloy material having the composition according to claim 1 at 1100 to 1250°C after final cold rolling, and then cooling to a temperature of 50°C or less; then immersing the Fe—Ni—Cr alloy material in a pickling solution containing 2.0% or less hydrofluoric acid and 6 to 15% nitric acid at a temperature of 50 to 70°C for 60 to 90 seconds to pickle the material; then polishing, shot blasting, or shot peening at least a part of the surface of the Fe—Ni—Cr alloy material; and then holding the material at a temperature of 750 to 800°C for 30 to 120 seconds and cooling the material.

Images