Austenitic heat-resistant alloy member

Abstract

An austenitic heat-resistant alloy member having a chemical composition comprising, in mass%, 0.010 to 0.150% of C, 2.00% or less of Si, 2.00% or less of Mn, 0.0400% or less of P, 0.0100% or less of S, 20.00 to 28.00% of Cr, 35.00 to 50.00% of Ni, 4.00 to 10.00% of W, 0.01 to 1.20% of Ti, 0.01 to 1.00% of Nb, 0.0200% or less of N, 0.010 to 0.300% of Al, 0.0005 to 0.0400% of B, 0.0100% or less of O, and the balance being Fe and impurities, the austenitic heat-resistant alloy member satisfying 97.50 < = (Cr + W + Fe + Ni)-(CrER + WER + FeER + NiER) and-2.2 * 10 <-5 > *

Description

TECHNICAL FIELD

[0001] The present application relates to an austenitic heat-resistant alloy member. BACKGROUND

[0002] In recent years, from the viewpoint of reducing environmental load, in boilers and the like for power generation, high-temperature, high-pressure operation conditions are being developed worldwide, and austenitic heat-resistant alloy members used as superheater tubes and reheater tubes are required to have more excellent creep rupture strength.

[0003] Against this technical background, technologies related to various austenitic heat-resistant alloys have been proposed. For example, Patent Literature 1 discloses an austenitic heat-resistant alloy member that achieves both excellent hot workability and creep rupture strength by strictly managing the S content in relation to the contents of Ca, Mg, and REM.

[0004] In addition, Patent Literature 2 discloses an austenitic heat-resistant alloy and a method for manufacturing the same, which suppresses variation in mechanical properties caused by sites by performing heat treatment under appropriate conditions, and exhibits sufficient 0.2% yield strength and tensile strength at normal temperature and creep rupture strength at high temperature as a large structural member.

[0005] Further, Patent Literature 3 discloses an austenitic heat-resistant alloy member that improves the creep strength and the crack resistance during welding when multilayer welding is performed by controlling the average crystal grain diameter of the central portion of the thickness of the member depending on the contents of B, Ti, and W, and the thickness of which exceeds 30 mm.

[0006] Furthermore, Patent Literature 4 discloses an austenitic stainless steel that improves the high-temperature strength and fatigue resistance by increasing the W content, which is effective for high-strength, to produce a metallographic structure in which the austenite grains are coarse and have little variation.

[0007] PRIOR ART DOCUMENTS

[0008] PATENT LITERATURE

[0009] Patent Literature 1: Japanese Patent Application Publication No. 2017-206717

[0010] Patent Literature 2: International Publication No. 2018 / 146783

[0011] Patent Literature 3: Japanese Patent Application Publication No. 2014-141713

[0012] Patent Literature 4: Japanese Patent Application Publication No. 2004-3000 SUMMARY

[0013] PROBLEMS TO BE SOLVED BY THE INVENTION

[0014] However, it is known that in steels in which improvement in creep rupture strength is emphasized, sometimes the creep rupture ductility under high temperature for a long time is reduced. Therefore, in the related art, there is room for improvement from the viewpoint of giving consideration to both high creep rupture strength and creep rupture ductility.

[0015] An object of the present application is to solve the above-described problems and provide an austenitic heat-resistant alloy member in which both the creep rupture strength and the creep rupture ductility are excellent.

[0016] Approach for solving the problems

[0017] The present inventors etc. investigated the creep rupture strength and the creep rupture ductility in detail in order to solve the above-described problems, and as a result, obtained the following insights.

[0018] (a) It was found that in an alloy member having excellent creep rupture strength, precipitates containing Cr, W, Fe and / or Ni are finely precipitated in the use environment. Also, in the use environment, in order to cause these elements to be finely precipitated, it is important to previously perform solution heat treatment so as to cause Cr, W, Fe and Ni to be sufficiently solid-solved.

[0019] (b) On the other hand, if the solution heat treatment is excessively performed, the austenite grains are coarsened, and the creep rupture ductility is deteriorated. Therefore, it is necessary to appropriately adjust the solution heat treatment conditions.

[0020] (c) However, the solution heat treatment conditions for causing Cr, W, Fe and Ni to be sufficiently solid-solved in the entire thickness direction of the member depend on the thickness of the member. Therefore, the present inventors etc. investigated the creep rupture ductility of members having various thicknesses in detail. As a result, the present inventors etc. found that there is a certain correlation between the thickness of the member and the grain size number in which the creep rupture ductility is good. Therefore, in order to give consideration to both excellent creep rupture strength and creep rupture ductility, it is necessary to strictly manage the solution heat treatment conditions on the basis of taking into account the thickness of the member.

[0021] The present application was completed on the basis of the above-described insights, and the gist thereof is in the following austenitic heat-resistant alloy member.

[0022] (1) An austenitic heat-resistant alloy member, the chemical composition of which is, in terms of mass%, Cr: 20.00 to 30.00%,

[0023] C: 0.010 to 0.150%,

[0024] Si: 2.00% or less,

[0025] Mn: 2.00% or less,

[0026] P: 0.0400% or less,

[0027] S: 0.0100% or less,

[0028] Cr: 20.00-28.00%,

[0029] Ni: 35.00-50.00%,

[0030] W: 4.00-10.00%,

[0031] Ti: 0.01-1.20%,

[0032] Nb: 0.01-1.00%,

[0033] N: 0.0200% or less,

[0034] Al: 0.010-0.300%,

[0035] B: 0.0005-0.0400%,

[0036] O: 0.0100% or less, and

[0037] balance: Fe and impurities,

[0038] the austenitic heat-resistant alloy member satisfies the following formula (i) and formula (ii),

[0039] 97.50 ≤ (Cr + W + Fe + Ni) - (Cr ER + W ER + Fe ER + Ni ER )... (i)

[0040] -2.2 x 10 -5 x t 3 + 2.1 ≤ D... (ii)

[0041] wherein each symbol in the above formula is defined as follows, and each element symbol in the above formula represents the content (mass %) of each element contained in the alloy member.

[0042] Cr ER : Cr content (mass %) in precipitates obtained by extraction residue analysis

[0043] W ER : W content (mass %) in precipitates obtained by extraction residue analysis

[0044] Fe ER : Fe content (mass %) in precipitates obtained by extraction residue analysis

[0045] Ni ER : Ni content (mass %) in precipitates obtained by extraction residue analysis

[0046] t: thickness of the alloy member (mm)

[0047] D: average grain size of the central portion of the alloy member

[0048] (2) The austenitic heat-resistant alloy member according to the above (1), wherein the chemical composition contains, in mass%, one or more selected from

[0049] Ca: 0.0100% or less,

[0050] Mg: 0.0500% or less,

[0051] REM: 0.1000% or less,

[0052] Co: 1.000% or less,

[0053] Cu: 1.00% or less,

[0054] Mo: 1.000% or less, and

[0055] V: 0.500% or less

[0056] instead of a part of the Fe.

[0057] Effects of the Invention

[0058] The austenitic heat-resistant alloy member of the present invention is excellent in both the creep rupture strength and the creep rupture ductility. DETAILED DESCRIPTION

[0059] Hereinafter, each element of the present invention will be described in detail.

[0060] 1. Chemical Composition

[0061] The reasons for limiting each element are described below. Note that in the following description, "%" with respect to the content means "mass%".

[0062] C: 0.010 to 0.150%

[0063] C stabilizes the austenite and forms fine carbides at the grain boundaries, thereby improving the creep rupture strength at high temperatures. In order to sufficiently obtain this effect, the C content needs to be 0.010% or more. However, in the case of excessive C content, the carbides become coarse and precipitate in large amounts, and thus the ductility of the grain boundaries decreases, and further, the toughness and the creep rupture strength also decrease. Therefore, the C content is set to 0.010 to 0.150%. The C content is preferably 0.030% or more, and more preferably 0.050% or more. In addition, the C content is preferably 0.120% or less, and more preferably 0.100% or less.

[0064] Si: 2.00% or less

[0065] Si is effective for improving the corrosion resistance and oxidation resistance at high temperatures, while having a deoxidizing effect. However, in the case of excessive Si content, the stability of austenite decreases, resulting in a decrease in toughness and creep rupture strength. Therefore, the Si content is set to 2.00% or less. The Si content is preferably 1.50% or less, and more preferably 1.00% or less.

[0066] Note that, regarding the Si content, no lower limit is particularly set. However, if the Si content is extremely reduced, the deoxidizing effect cannot be sufficiently obtained, the cleanliness of the alloy becomes large, and the cleanliness deteriorates. In addition, it is also difficult to obtain the effect of improving the corrosion resistance and oxidation resistance at high temperatures, and the manufacturing cost also greatly increases. Therefore, the Si content is preferably set to 0.02% or more, and more preferably 0.05% or more.

[0067] Mn: 2.00% or less

[0068] Mn, like Si, not only has a deoxidizing effect, but also is an element that contributes to the stabilization of austenite. However, if the Mn content is excessive, embrittlement occurs, and furthermore, the toughness and creep rupture ductility also decrease. Therefore, the Mn content is set to 2.00% or less. The Mn content is preferably 1.80% or less, and more preferably 1.50% or less.

[0069] Note that, regarding the Mn content, no lower limit is particularly set. However, if the Mn content is extremely reduced, the deoxidizing effect cannot be sufficiently obtained, and the cleanliness of the alloy deteriorates. In addition, not only is the hot workability deteriorated, but it is also difficult to obtain the effect of stabilizing austenite, and the manufacturing cost also greatly increases. Therefore, the Mn content is preferably set to 0.005% or more, and more preferably 0.010% or more.

[0070] P: 0.0400% or less

[0071] P is contained in the alloy as an impurity, and in the case of being contained in a large amount, the hot workability and weldability are significantly reduced, and furthermore, the creep rupture ductility after long-term use also decreases. Therefore, the P content is set to 0.0400% or less. The P content is preferably 0.0300% or less, and more preferably 0.0250% or less.

[0072] Note that the P content is preferably reduced as much as possible, but an extreme reduction leads to an increase in manufacturing cost. Therefore, the P content is preferably set to 0.0005% or more, and more preferably 0.0008% or more.

[0073] S: 0.0100% or less

[0074] S has an effect of improving the creep rupture characteristics by being present in the crystal grains. However, in the case of containing a large amount of S, the hot workability and weldability are significantly reduced, and further, the creep rupture ductility after long-term use is also reduced. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0095% or less, and more preferably 0.0090% or less.

[0075] Note that, in the case where the improvement effect on the creep rupture characteristics by S is desired, the S content is preferably set to 0.0015% or more, more preferably to 0.0018% or more, and further preferably to 0.0020% or more.

[0076] Cr: 20.00 to 28.00%

[0077] Cr is an element that greatly contributes to the improvement of the creep rupture strength at high temperatures by being solid-solved in the matrix. In addition, Cr is an essential element for securing the oxidation resistance and corrosion resistance at high temperatures. In order to obtain the above effects, it is necessary to set the Cr content to 20.00% or more. However, if the Cr content exceeds 28.00%, the stability of austenite at high temperatures deteriorates, resulting in a decrease in the creep rupture strength. Therefore, the Cr content is set to 20.00 to 28.00%. The Cr content is preferably 21.00% or more, and more preferably 22.00% or more. In addition, the Cr content is preferably 27.00% or less, and more preferably 26.00% or less.

[0078] Ni: 35.00 to 50.00%

[0079] Ni is an element that greatly contributes to the improvement of the creep rupture strength at high temperatures by being solid-solved in the matrix. In addition, Ni is an effective element for obtaining austenite, and is an essential element for securing the microstructure stability at the time of long-term use. Within the above range of the Cr content, in order to sufficiently obtain the effects of Ni, it is necessary to set the Ni content to 35.00% or more. However, Ni is an expensive element, and if it is contained in a large amount, the cost increases. Therefore, the Ni content is set to 35.00 to 50.00%. The Ni content is preferably 37.00% or more, and more preferably 39.00% or more. In addition, the Ni content is preferably 48.00% or less, and more preferably 46.00% or less.

[0080] W: 4.00 to 10.00%

[0081] W is an element which is solid-solved in the matrix and greatly contributes to the improvement of the creep rupture strength at high temperatures. In order to sufficiently exert its effect, it is necessary to set the W content to 4.00% or more. However, even if W is contained in excess, the effect is saturated, and the creep rupture strength is decreased instead. Furthermore, W is an expensive element, and therefore, if it is contained in excess, the cost is increased. Therefore, the W content is set to 4.00 to 10.00%. The W content is preferably 5.00% or more, and more preferably 6.00% or more. In addition, the W content is preferably 9.00% or less, and more preferably 8.00% or less.

[0082] Ti: 0.01 to 1.20%

[0083] Ti is precipitated in the form of fine carbonitride within the grains, and contributes to the improvement of the creep rupture strength at high temperatures. In order to obtain this effect, it is necessary to set the Ti content to 0.01% or more. However, if the Ti content is excessive, a large amount of carbonitride is precipitated, and the creep rupture ductility and toughness are decreased. Therefore, the Ti content is set to 0.01 to 1.20%. The Ti content is preferably 0.03% or more, and more preferably 0.05% or more. In addition, the Ti content is preferably 1.00% or less, and more preferably 0.80% or less.

[0084] Nb: 0.01 to 1.00%

[0085] Nb is precipitated in the form of fine carbide or carbonitride within the grains in combination with C or with C and N, and contributes to the improvement of the creep rupture strength at high temperatures. In order to obtain this effect, it is necessary to set the Nb content to 0.01% or more. However, if the Nb content is excessive, a large amount of carbide or carbonitride is precipitated, and the creep rupture ductility and toughness are decreased. Therefore, the Nb content is set to 0.01 to 1.00%. The Nb content is preferably 0.05% or more, and more preferably 0.10% or more. In addition, the Nb content is preferably 0.80% or less, and more preferably 0.60% or less.

[0086] N: 0.0200% or less

[0087] N is an element which is effective for stabilizing austenite, but if it is contained in excess, a large amount of fine nitride is precipitated within the grains during use at high temperatures, and the creep rupture ductility and toughness are decreased. Therefore, the N content is set to 0.0200% or less. The N content is preferably 0.0180% or less, and more preferably 0.0150% or less.

[0088] Note that there is no need to particularly set a lower limit for the N content. However, if the N content is extremely reduced, not only is it difficult to obtain the effect of stabilizing austenite, but also the manufacturing cost is greatly increased. Therefore, the N content is preferably set to 0.0005% or more, and more preferably 0.0008% or more.

[0089] Al: 0.010 to 0.300%

[0090] Al is an element having a deoxidizing effect, and thus the Al content needs to be set to 0.010% or more. However, if the Al content is excessive, the cleanliness of the alloy significantly deteriorates, and thus the hot workability and ductility decrease. Therefore, the Al content is set to 0.010 to 0.300%. The Al content is preferably 0.030% or more, and more preferably 0.050% or more. In addition, the Al content is preferably 0.250% or less, and more preferably 0.200% or less.

[0091] B: 0.0005 to 0.0400%

[0092] B is an element required to strengthen the grain boundary by segregating at the grain boundary during use at high temperatures, and to make the grain boundary carbide finely disperse, thereby improving the creep rupture strength. In order to obtain this effect, the B content needs to be set to 0.0005% or more. However, if the B content is excessive, the hot workability deteriorates in addition to the weldability. Therefore, the B content is set to 0.0005 to 0.0400%. The B content is preferably 0.0010% or more, and more preferably 0.0020% or more. In addition, the B content is preferably 0.0300% or less, and more preferably 0.0200% or less.

[0093] O: 0.0100% or less

[0094] O (oxygen) is contained in the alloy as an impurity, and if the content is excessive, the hot workability decreases, and further causes deterioration of the toughness and ductility. Therefore, the O content is set to 0.0100% or less. The O content is preferably 0.0080% or less, and more preferably 0.0050% or less.

[0095] Note that, regarding the O content, a lower limit is not particularly set, but an extreme reduction causes an increase in manufacturing cost. Therefore, the O content is preferably set to 0.0005% or more, and more preferably 0.0008% or more.

[0096] In the chemical composition of the austenitic heat-resistant alloy of the present application, the balance is Fe and impurities. Here, the "impurities" refer to components mixed due to various reasons such as raw materials such as ores, waste materials, manufacturing processes, and the like, and components allowed within a range that does not adversely affect the present application.

[0097] In the austenitic heat-resistant alloy of the present application, furthermore, one or more elements selected from Ca, Mg, REM, Co, Cu, Mo, and V can be contained within the ranges shown below. Note that these elements are not necessarily essential in the member, and thus the lower limit of the content is 0%. The reasons for the limitations on each element are described.

[0098] Ca: 0.0100% or less

[0099] Ca has an effect of forming a compound with S to reduce the amount of S in the matrix and improve hot workability, and thus can be contained as needed. However, if the Ca content is excessive, the amount of S, which is beneficial to the effect of the present application to improve the creep rupture strength, decreases in the alloy, and combines with O, significantly degrading the cleanliness, and instead deteriorating the hot workability. Therefore, the Ca content is set to 0.0100% or less. The Ca content is preferably 0.0080% or less. Note that, in the case where the above effect is desired, the Ca content is preferably set to 0.0001% or more, more preferably to 0.0002% or more, and further preferably to 0.0003% or more.

[0100] Mg: 0.0500% or less

[0101] Mg, like Ca, has an effect of forming a compound with S to reduce the amount of S in the matrix and improve hot workability, and thus can be contained as needed. However, if the Mg content is excessive, the amount of S, which is beneficial to the effect of the present application to improve the creep rupture strength, decreases in the alloy, and combines with O, significantly degrading the cleanliness, and instead deteriorating the hot workability. Therefore, the Mg content is set to 0.0500% or less. The Mg content is preferably 0.0450% or less. Note that, in the case where the above effect is desired, the Mg content is preferably set to 0.0001% or more, more preferably to 0.0002% or more, and further preferably to 0.0003% or more.

[0102] REM: 0.1000% or less

[0103] REM, like Ca, has an effect of forming a compound with S to reduce the amount of S in the matrix and improve hot workability, and thus can be contained as needed. However, if the REM content is excessive, the amount of S, which is beneficial to the effect of the present application to improve the creep rupture strength, decreases in the alloy, and combines with O, significantly degrading the cleanliness, and instead deteriorating the hot workability. Therefore, the REM content is set to 0.1000% or less. The REM content is preferably 0.0800% or less. Note that, in the case where the above effect is desired, the REM content is preferably set to 0.0001% or more, more preferably to 0.0002% or more, and further preferably to 0.0003% or more.

[0104] Note that REM is a general term for 17 kinds of elements of Sc, Y, and lanthanoid elements, and the REM content refers to the total content of one or more kinds of elements among REM. Also, as for REM, it is generally contained in a mixed rare earth metal. Therefore, for example, it can be added in the form of a mixed rare earth metal, and adjusted so that the amount of REM is in the above range.

[0105] Co: 1.000% or less

[0106] Co has an effect of improving the creep rupture strength. That is, Co, like Ni, is an austenite-forming element, and improves the phase stability, thereby contributing to the improvement of the creep rupture strength. Therefore, Co can also be contained. However, Co is an extremely expensive element, and therefore, if Co is contained in excess, the cost is greatly increased. Therefore, the Co content is set to 1.000% or less. The Co content is preferably 0.800% or less, and more preferably 0.600% or less. On the other hand, in the case where the above effect is desired, the Co content is preferably set to 0.010% or more, and more preferably to 0.050% or more.

[0107] Cu: 1.00% or less

[0108] Cu has an effect of improving the creep rupture strength. That is, Cu, like Ni and Co, is an austenite-forming element, and improves the phase stability, thereby contributing to the improvement of the creep rupture strength. Therefore, Cu can also be contained. However, in the case where Cu is contained in excess, the hot workability is decreased. Therefore, the Cu content is set to 1.00% or less. The Cu content is preferably 0.80% or less, and more preferably 0.60% or less. On the other hand, in the case where the above effect is desired, the Cu content is preferably set to 0.01% or more, and more preferably to 0.05% or more.

[0109] Mo: 1.000% or less

[0110] Mo has an effect of improving the creep rupture strength. That is, Mo has an effect of dissolving in the matrix, thereby improving the creep rupture strength at high temperatures. Therefore, Mo can also be contained. However, in the case where Mo is contained in excess, the stability of the austenite is decreased, and the creep rupture strength is rather decreased. Therefore, the Mo content is set to 1.000% or less. The Mo content is preferably 0.800% or less, and more preferably 0.700% or less. On the other hand, in the case where the above effect is desired, the Mo content is preferably set to 0.010% or more, and more preferably to 0.050% or more.

[0111] V: 0.500% or less

[0112] V has the effect of increasing creep rupture strength. That is, similar to Nb, V combines with C or with C and N to form fine carbides or carbonitrides, which have the effect of increasing creep rupture strength. Therefore, V may be contained. However, if V is contained in excess, a large amount of carbides or carbonitrides will precipitate, resulting in a decrease in creep rupture ductility. Therefore, the V content is set to 0.500% or less. The V content is preferably 0.400% or less, and more preferably 0.300% or less. On the other hand, if the above-mentioned effect is desired, the V content is preferably set to 0.010% or more, and more preferably 0.050% or more.

[0113] 2. About formula (i)

[0114] As described above, by fully dissolving Cr, W, Fe, and Ni in the solid solution beforehand, the precipitates containing these elements can be finely precipitated in the use environment, thereby achieving excellent creep rupture strength. Therefore, the alloy component must contain a sufficient total amount of Cr, W, Fe, and Ni, and the amount of Cr, W, Fe, and Ni present as precipitates must be reduced before use. Specifically, the following formula (i) must be satisfied.

[0115] 97.50≤(Cr+W+Fe+Ni)-(Cr ER +W ER +Fe ER +Ni ER )…(i)

[0116] The symbols in the above formula are defined as follows. The symbols of the elements in the above formula represent the content (mass %) of the elements contained in the alloy member.

[0117] Cr ER : Cr content in the precipitate obtained by analysis of the extraction residue (mass %)

[0118] W ER : W content (mass %) in the precipitate obtained by analysis of the extraction residue

[0119] Fe ER : Fe content in the precipitate obtained by analysis of the extraction residue (mass %)

[0120] Ni ER : Ni content in the precipitate obtained by analysis of the extraction residue (mass %)

[0121] If the right-hand side value of formula (i) is less than 97.50, the solid solution amount of Cr, W, Fe, and Ni is insufficient, and the creep rupture strength cannot be improved. Therefore, the right-hand side value of formula (i) is set to 97.50 or more. The right-hand side value of formula (i) is preferably 98.00 or more, and more preferably 98.50 or more.

[0122] It should be noted that the content (mass %) of each element in the precipitate analyzed as the electrolytic extraction residue in the above formula can be measured by the following procedure. Specifically, 10% acetylacetone-1% tetramethylammonium chloride / methanol is used at 20 mA / cm 2 Approximately 0.4 g of the sample was electrolyzed with a current of 100 μm. The sample solution was then filtered through a 0.2 μm filter, and the residue was acid-decomposed. The amounts (mass %) of the elements analyzed as the electrolytic extraction residue were then calculated using an ICP (inductively coupled plasma) emission spectrometer.

[0123] 3. About formula (ii)

[0124] As described above, the present inventors have discovered a relationship between the thickness of a component and the particle size that exhibits good creep rupture ductility. Specifically, based on extensive experimental data, they investigated the relationship between component thickness and the particle size that exhibits good creep rupture ductility. They discovered that sufficient creep rupture ductility can be ensured by satisfying the following formula (ii).

[0125] -2.2×10 -5 ×t 3 +2.1≤D…(ii)

[0126] In the above formula, t is defined as the thickness (mm) of the alloy member, and D is defined as the average grain size at the center of the thickness of the alloy member.

[0127] When average grain size D does not satisfy formula (ii), austenite grains are coarse, and creep rupture ductility cannot be improved.For the maximum value of average grain size D, there is no particular limitation, but when austenite grains are fine-grained, the solution heat treatment described later is insufficient, therefore sometimes does not satisfy above-mentioned formula (i).Therefore, average grain size D is preferably below 6.0, more preferably below 5.0.In addition, average grain size D is preferably more than-2.0, more preferably more than-1.0, further preferably more than 0.

[0128] The average grain size D is measured in accordance with ASTM E112 (2013). Specifically, a test piece for microstructure observation is collected in a manner that a cross section perpendicular to the length direction of the alloy member becomes an observation surface, mirror polishing is performed on the observation surface. After polishing, etching with mixed acid is performed and optical microscope observation is performed. In a manner that the central position of the thickness of the alloy member becomes the center of the field of view, 10 fields of view are observed. Then, the grain size of each field of view is found by the comparison method prescribed in ASTM E112, and the average value thereof is taken as the average grain size D. At this time, the observation magnification based on 100 times is set to 200 times or 400 times according to the grain size. In addition, in the case where the observation magnification is set to 200 times or 400 times, a correction value Q defined by the following formula (I) is used to perform correction in accordance with ASTM E112 (2013).

[0129] Q = 6.64 log 10 (M / 100) (I)

[0130] wherein M in the above formula is the observation magnification.

[0131] 4. Size

[0132] The austenitic heat-resistant alloy member of the present application can be an alloy pipe or an alloy plate. In the case where the austenitic heat-resistant alloy member is an alloy pipe, the wall thickness is preferably set to 1 mm or more or 5 mm or more, and is preferably set to 100 mm or less, 80 mm or less, 65 mm or less, or 55 mm or less. In addition, in the case where the austenitic heat-resistant alloy member is an alloy plate, the plate thickness is preferably set to 1 to 100 mm.

[0133] 5. Manufacturing method

[0134] The manufacturing method of the austenitic heat-resistant alloy member of the present application is not particularly limited, and for example, a steel ingot or a cast blank having the above chemical composition is subjected to hot working, and then, after further subjected to hot working by a different method as needed, such as hot extrusion, solid solution heat treatment is performed. Furthermore, cold working can also be performed as needed.

[0135] As described above, in order to balance the excellent creep rupture strength and the creep rupture ductility, it is necessary to strictly manage the solid solution heat treatment conditions on the basis of considering the thickness of the member. Specifically, it is necessary to set the solid solution heat treatment temperature T to 1180°C to 1250°C, and to satisfy the following formulae (iii) and (iv). After the solid solution heat treatment, the alloy member is preferably subjected to water cooling.

[0136] 16.1 x t + 28500 ≤ LMP ≤ 16.1 x t + 29100 (iii)

[0137] LMP = (T + 273.15) x (Log(t r) + 20)... (iv)

[0138] wherein each symbol in the above formula is defined as follows.

[0139] t: thickness of the alloy member (mm)

[0140] T: solution heat treatment temperature (°C)

[0141] t r : solution heat treatment time (h)

[0142] If the solution heat treatment temperature T is less than 1180°C, recrystallization does not occur, and thus the strain caused by working cannot be eliminated, and the creep rupture ductility deteriorates. In addition, Cr, W, Fe and Ni cannot be sufficiently solid-solved, and a good creep rupture strength cannot be ensured. On the other hand, if the solution heat treatment temperature T exceeds 1250°C, the creep rupture ductility deteriorates due to the coarsening of austenite grains. Therefore, the solution heat treatment temperature T is set to 1180°C to 1250°C.

[0143] Solution heat treatment time t r If it is less than 10 minutes, Cr, W, Fe and Ni cannot be sufficiently solid-solved, and a good creep rupture strength cannot be ensured. Therefore, the solution heat treatment time t r is set to 10 minutes or more.

[0144] In addition, if the LMP (Larson Miller Parameter) defined by formula (iv) is less than the left-hand side value of formula (iii), Cr, W, Fe and Ni cannot be sufficiently solid-solved, and a good creep rupture strength cannot be ensured. On the other hand, if the LMP defined by formula (iv) exceeds the right-hand side value of formula (iii), the creep rupture ductility deteriorates due to the coarsening of austenite grains. Therefore, the LMP defined by formula (iv) needs to satisfy formula (iii).

[0145] Hereinafter, the present application will be described more specifically by way of examples, but the present application is not limited to these examples.

[0146] Example

[0147] The austenitic heat-resistant alloys 1 to 38 having the chemical compositions shown in Tables 1 and 2 were subjected to laboratory melting, and thus ingots were produced. Then, after the above ingots were subjected to forming based on hot forging and hot rolling, solution heat treatment was performed under the conditions shown in Tables 3 and 4, and thus alloy pipes (Test Nos. 1 to 50) having the wall thicknesses shown in Tables 3 and 4 were obtained.

[0148] [Table 1]

[0149]

[0150] [Table 2]

[0151]

[0152] <About electrolytic extraction residue>

[0153] The contents (mass %) of Cr, W, Fe, and Ni in the precipitate analyzed as the electrolytic extraction residue were measured by the following procedure. Specifically, 10% acetylacetone-1% tetramethylammonium chloride / methanol was used at 20 mA / cm 2 Approximately 0.4 g of the sample was electrolyzed using a current value of 100 Å. The electrolyzed sample solution was then filtered through a 0.2 μm filter, and the residue was acid-decomposed. The amounts (mass %) of the elements analyzed as the electrolytic extraction residue were then calculated using an ICP emission spectrometer.

[0154] <About Average Grain Size>

[0155] The average grain size D is measured in accordance with ASTM E112 (2013). Specifically, a test piece for tissue observation is collected in a manner that a cross section perpendicular to the longitudinal direction of the alloy tube becomes an observation surface, and the observation surface is mirror-polished. After polishing, it is etched with mixed acid and observed under an optical microscope. 10 fields of view are observed in a manner that the central position of the wall thickness of the alloy tube becomes the center of the field of view. Then, the grain size of each field of view is obtained by the comparison method specified in ASTM E112, and its average value is taken as the average grain size D. At this time, the observation magnification based on 100 times is set to 200 times or 400 times according to the grain size. In addition, when the observation magnification is set to 200 times or 400 times, the correction value Q defined by the following formula (I) is used and correction is performed in accordance with ASTM E112 (2013).

[0156] Q=6.64log 10 (M / 100)…(I)

[0157] Wherein, M in the above formula is the observation magnification.

[0158] <About Creep Rupture Strength and Creep Rupture Ductility>

[0159] Next, round rod creep rupture test specimens with a diameter of 6 mm and a gauge length of 30 mm were collected from the center of the wall thickness of each alloy tube and subjected to creep rupture testing at 750°C and 100 MPa. Specimens with a creep rupture time exceeding 2000 hours were considered acceptable, indicating good creep rupture strength. Furthermore, specimens with a creep rupture reduction of area exceeding 10% were considered acceptable, indicating good creep rupture ductility.

[0160] These results are summarized in Table 3 and Table 4. Note that "-" in Test No. 44 of Table 4 means that it is difficult to determine the grain size because recrystallization does not occur.

[0161] [Table 3]

[0162]

[0163] [Table 4]

[0164]

[0165] As shown in Table 3 and Table 4, the creep rupture strength and the creep rupture ductility of Test Nos. 1 to 43 that satisfy all the limitations of the present application show good results. In contrast to this, in Test No. 44, because the solution heat treatment temperature T is low, recrystallization does not occur, and the creep rupture ductility deteriorates. In addition, the LMP is lower than the left-hand value of formula (iii), and thus Cr, W, Fe and Ni cannot be sufficiently solid-solved, and the creep rupture strength deteriorates.

[0166] In Test Nos. 45 and 48, because the LMP is lower than the left-hand value of formula (iii), Cr, W, Fe and Ni cannot be sufficiently solid-solved, and the creep rupture strength deteriorates. In Test No. 46, because the LMP exceeds the right-hand value of formula (iii), the average grain size D is smaller than the left-hand value of formula (ii), and the creep rupture ductility deteriorates. In Test No. 47, the solution heat treatment temperature T is high, and the LMP exceeds the right-hand value of formula (iii), and thus the average grain size D is smaller than the left-hand value of formula (ii), and the creep rupture ductility deteriorates. In Test No. 49, because the solution heat treatment temperature T is low, recrystallization does not occur, and the creep rupture ductility deteriorates. In addition, Cr, W, Fe and Ni cannot be sufficiently solid-solved, and the creep rupture strength deteriorates. In Test No. 50, because the solution heat treatment time t r is short, Cr, W, Fe and Ni cannot be sufficiently solid-solved, and the creep rupture strength deteriorates.

[0167] Industrial applicability

[0168] Both the long-term creep rupture strength and the creep rupture ductility of the austenitic heat-resistant alloy member of the present application are excellent. Therefore, the austenitic heat-resistant alloy member of the present application is suitable as a material for a superheater tube and / or a reheater tube of a power generation boiler.

Claims

1. An austenitic heat-resistant alloy member whose chemical composition contains, in mass%, C: 0.010 to 0.150%, Si: 2.00% or less, Mn: 2.00% or less, P: 0.0400% or less, S: 0.0100% or less, Cr：20.00～28.00％、 Ni: 35.00 to 50.00%, W：4.00～10.00％、 Ti: 0.01 to 1.20%, Nb: 0.01 to 1.00%, N: 0.0200% or less, Al：0.010～0.300％、 B：0.0005～0.0400％、 O: 0.0100% or less, and the balance: Fe and impurities, the austenitic heat-resistant alloy member satisfying the following formula (i) and formula (ii), 97.50≤(Cr+W+Fe+Ni)-(Cr ER +W ER +Fe ER +Ni ER )...(i) -2.2 x 10 -5 x t 3 + 2.1 ≤ D... (ii) wherein, each symbol in the formula is defined as follows, each element symbol in the formula indicating the content of each element contained in the alloy member, in mass%, Cr ER : Cr content in the precipitate obtained by the extraction residue analysis, in mass %; W ER : W content in the precipitate obtained by the extraction residue analysis, in mass %; Fe ER : Fe content in the precipitate obtained by the extraction residue analysis in mass %; Ni ER : Ni content in the precipitate obtained by the extraction residue analysis, in mass %; t: thickness of the alloy member, unit: mm; D: average grain size at the central portion of the thickness of the alloy member.

2. The austenitic heat-resistant alloy member according to claim 1, wherein the chemical composition containing, in mass%, one or more selected from Ca: 0.0100% or less, Mg: 0.0500% or less, REM: 0.1000% or less, Co: 1.000% or less, Cu: 1.00% or less, Mo: 1.000% or less, and V: 0.500% or less instead of a part of the Fe.

2. The austenitic heat-resistant alloy member according to claim 1, wherein the chemical composition contains, in mass%, one or more selected from Cr: 1.00% or less, Al: 0.0500% or less, W: 1.000% or less, Zr: 0.0500% or less, Hf: 0.0500% or less, Ta: 0.0500% or less, B: 0.0100% or less, Sn: 0.0500% or less, Zn: 0.0500% or less, Ga: 0.0500% or less, Ge: 0.0500% or less, In: 0.0500% or less, Sb: 0.0500% or less, Bi: 0.0500% or less, Se: 0.0500% or less, Te: 0.0500% or less, and the balance: Fe and impurities.

3. The austenitic heat-resistant alloy member according to claim 1 or 2, wherein the chemical composition contains, in mass%, one or more selected from Cr: 1.00% or less, Al: 0.0500% or less, W: 1.000% or less, Zr: 0.0500% or less, H

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