EP4678773A1 - Austenite-based heat-resistant alloy member - Google Patents

Abstract

An austenitic heat resistant alloy member having a chemical composition including, 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 to 28.00%, Ni: 35.00 to 50.00%, W: 4.00 to 10.00%, Ti: 0.01 to 1.20%, Nb: 0.01 to 1.00%, N: 0.0200% or less, Al: 0.010 to 0.300%, B: 0.0005 to 0.0400%, O: 0.0100% or less, and the balance: Fe and impurities, and satisfies 97.50 ≤ (Cr+W+Fe+Ni) - (Cr_ER+W_ER+Fe_ER+Ni_ER) and -2.2×10^-5×t³+2.1≤D.

Description

TECHNICAL FIELD
• The present invention relates to an austenitic heat resistant alloy member.
BACKGROUND ART
• Recently, from the viewpoint of reducing environmental burden, power generation boilers and the like are increasingly operated under conditions of higher temperatures and pressures on a global scale. Austenitic heat resistant alloy members used as materials for superheater tubes and reheater tubes are required to have more excellent creep rupture strength.
• Given such a technical background, there have been proposed techniques related to various austenitic heat resistant alloys. For example, Patent Document 1 discloses an austenitic heat resistant alloy member that has both excellent hot workability and creep rupture strength, which are achieved by strictly controlling the content of S relative to the contents of Ca, Mg, and REM.
• In addition, Patent Document 2 discloses an austenitic heat resistant alloy, and a production method of the same, in which a 0.2% yield stress and a tensile strength sufficient for large structural members at normal temperature and a creep rupture strength at high temperature become evident by carrying out a heat treatment under suitable conditions to reduce variation in mechanical properties depending on locations.
• Furthermore, Patent Document 3 discloses an austenitic heat resistant alloy member that has a thickness exceeding 30 mm and that has an improved creep strength and crack resistance during welding when being subjected to multi-layer welding, which are achieved by controlling an average grain diameter at a thickness-center portion of a member depending on the contents of B, Ti, and W.
• Still further, Patent Document 4 discloses an austenitic stainless steel that has an improved high-temperature strength and fatigue resistance, which are achieved by increasing the content of W, which is effective for increasing the strength, to generate a microstructure that has coarse austenite grains and less variation.
LIST OF PRIOR ART DOCUMENTS PATENT DOCUMENT
• Patent Document 1: JP2017-206717A
• Patent Document 2: WO 2018/146783
• Patent Document 3: JP2014-141713A
• Patent Document 4: JP2004-3000A
SUMMARY OF INVENTION TECHNICAL PROBLEM
• However, in the case of a steel with emphasis on the improvement of the creep rupture strength, it has been revealed that the creep rupture ductility for long-term at high temperature may be slightly lower in some cases. Accordingly, in the prior arts, further improvement is still necessary from the viewpoint of achieving both a high creep rupture strength and a high creep rupture ductility.
• An objective of the present invention is to solve the above-described problems and provide an austenitic heat resistant alloy member that is excellent in both the creep rupture strength and the creep rupture ductility.
SOLUTION TO PROBLEM
• To solve the above-described problems, the inventors have conducted detailed studies on the creep rupture strength and the creep rupture ductility, and as a result, obtained the following findings.
1. (a) It has been found that in an alloy member that has an excellent creep rupture strength, precipitates that contain Cr, W, Fe, and/or Ni have finely precipitated in a usage environment. Then, for causing these elements to finely precipitate in a usage environment, it is important to perform a solution heat treatment in advance such that Cr, W, Fe, and Ni are sufficiently dissolved.
2. (b) On the other hand, an excessive solution heat treatment leads to coarsening of austenite grains, so that the creep rupture ductility degrades. Accordingly, it is necessary to appropriately adjust the solution heat treatment conditions.
3. (c) However, the solution heat treatment conditions for sufficiently dissolving Cr, W, Fe, and Ni along the thickness direction of the member depend on the thickness of the member. The inventors have then conducted detailed studies on the creep rupture ductility for members that have various thicknesses. As a result, the inventors have found that there is a certain relationship between the thickness of a member and the grain size number that leads to a better creep rupture ductility. Accordingly, to achieve both an excellent creep rupture strength and creep rupture ductility, it is necessary to strictly control the solution heat treatment conditions in consideration of the thickness of the member.
• The gist of the present invention, which has been completed based on the above-described findings, is an austenitic heat resistant alloy member described below.
1. (1) An austenitic heat resistant alloy member having a chemical composition including, 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 to 28.00%,
   • Ni: 35.00 to 50.00%,
   • W: 4.00 to 10.00%,
   • Ti: 0.01 to 1.20%,
   • Nb: 0.01 to 1.00%,
   • N: 0.0200% or less,
   • Al: 0.010 to 0.300%,
   • B: 0.0005 to 0.0400%,
   • O: 0.0100% or less, and
   • the balance: Fe and impurities, and
   • satisfies following Formulas (i) and (ii): $$97.50\le \left(\mathrm{Cr}+\mathrm{W}+\mathrm{Fe}+\mathrm{Ni}\right)-\left({\mathrm{Cr}}_{\mathrm{ER}}+{\mathrm{W}}_{\mathrm{ER}}+{\mathrm{Fe}}_{\mathrm{ER}}+{\mathrm{Ni}}_{\mathrm{ER}}\right)$$ $$-2.2\times {10}^{-5}\times {\mathrm{t}}^3+2.1\le \mathrm{D}$$
   • where each symbol in the formulas is defined as below, and each element symbol in the formulas represents a content of each element (mass%) contained in the alloy member,
   • Cr_ER: a content of Cr (mass%) in precipitates obtained by extracted residue analysis
   • W_ER: a content of W (mass%) in precipitates obtained by extracted residue analysis
   • Fe_ER: a content of Fe (mass%) in precipitates obtained by extracted residue analysis
   • Ni_ER: a content of Ni (mass%) in precipitates obtained by extracted residue analysis
   • t: a thickness of the alloy member (mm)
   • D: an average grain size at a thickness-center portion of the alloy member.
2. (2) The austenitic heat resistant alloy member according to the above (1), wherein in lieu of a part of the Fe, the chemical composition contains one or more elements, in mass%, 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.
ADVANTAGEOUS EFFECTS OF INVENTION
• The austenitic heat resistant alloy member of the present invention has both an excellent creep rupture strength and creep rupture ductility.
DESCRIPTION OF EMBODIMENTS
• The requirements for the present invention will now be described in detail.
1. Chemical Composition
• The reason for limitation for each element is as described below. Note that in the description below, "%" for the content refers to "mass%".
C: 0.010 to 0.150%
• C (carbon) stabilizes the austenite and forms fine carbide in a grain boundary, leading to the improvement of the creep rupture strength at high temperature. To sufficiently obtain the effect, the content of C needs to be 0.010% or more. However, when C is excessively contained, the carbide will be coarsened and precipitate in large amounts, leading to the degradation of ductility of the grain boundary and also the degradation of toughness and the creep rupture strength. Accordingly, the content of C is 0.010 to 0.150%. The content of C is preferably 0.030% or more, and more preferably 0.050% or more. Furthermore, the content of C is preferably 0.120% or less, and more preferably 0.100% or less.
Si: 2.00% or less
• Si (silicon) has a deoxidation function and is an element that is effective for improving corrosion resistance and oxidation resistance at high temperature. However, when Si is excessively contained, the stability of the austenite degrades, leading to the degradation of toughness and the creep rupture strength. Accordingly, the content of Si is 2.00% or less. The content of Si is preferably 1.50% or less, and more preferably 1.00% or less.
• It is not particularly necessary to provide a lower limit for the content of Si. However, excessive reduction of the content of Si leads to an insufficient deoxidation effect, and the level of cleanliness of alloy increases, leading to the degradation of cleanliness. Furthermore, it is difficult to produce the effect of improving corrosion resistance and oxidation resistance at high temperature, and production costs will significantly increase. Accordingly, the content of Si is preferably 0.02% or more, and more preferably 0.05% or more.
Mn: 2.00% or less
• As in Si, Mn (manganese) has not only a deoxidation function, but is also an element that contributes to stabilizing the austenite. However, an excessive content of Mn leads to embrittlement, and also the degradation of toughness and the creep rupture ductility. Accordingly, the content of Mn is 2.00% or less. The content of Mn is preferably 1.80% or less, and more preferably 1.50% or less.
• It is not particularly necessary to provide a lower limit also for the content of Mn. However, excessive reduction of the content of Mn leads to an insufficient deoxidation effect, leading to the degradation of cleanliness of alloy. In addition, not only does hot workability degrade, but it is also difficult to produce the effect of stabilizing the austenite, and production costs will significantly increase. Accordingly, the content of Mn is preferably 0.005% or more, and more preferably 0.010% or more.
P: 0.0400% or less
• P (phosphorus) is contained as impurities in the alloy, and when contained in large amounts, leads to a significant degradation of hot workability and weldability, and also the degradation of the creep rupture ductility after being used for a long time. Accordingly, the content of P is 0.0400% or less. The content of P is preferably 0.0300% or less, and more preferably 0.0250% or less.
• While it is preferable that the content of P is lowered as much as possible, excessive reduction leads to an increase in production costs. Accordingly, the content of P is preferably 0.0005% or more, and more preferably 0.0008% or more.
S: 0.0100% or less
• S (sulfur) produces an effect of improving creep rupture characteristics by being present in grains. However, when a large amount of S is contained, hot workability and weldability significantly degrade, and further the creep rupture ductility after being used for a long time degrades. Accordingly, the content of S is 0.0100% or less. The content of S is preferably 0.0095% or less, and more preferably 0.0090% or less.
• When it is desirable to obtain an effect of improving the creep rupture characteristics by S, the content of S is preferably 0.0015% or more, more preferably 0.0018% or more, and further preferably 0.0020% or more.
Cr: 20.00 to 28.00%
• Cr (chromium) is dissolved in a matrix and is an element that significantly contributes to the improvement of the creep rupture strength at high temperature. Furthermore, Cr is an essential element for securing oxidation resistance and corrosion resistance at high temperature. To obtain the above-described effects, the content of Cr needs to be 20.00% or more. However, when the content of Cr exceeds 28.00%, the stability of the austenite at high temperature degrades, leading to the degradation of the creep rupture strength. Accordingly, the content of Cr is 20.00 to 28.00%. The content of Cr is preferably 21.00% or more, and more preferably 22.00% or more. Furthermore, the content of Cr is preferably 27.00% or less, and more preferably 26.00% or less.
Ni: 35.00 to 50.00%
• Ni (nickel) is dissolved in a matrix and is an element that significantly contributes to the improvement of the creep rupture strength at high temperature. Furthermore, Ni is an element that is effective for obtaining the austenite and is an essential element for securing the stability of the microstructure when being used for a long time. To sufficiently obtain the effect of Ni as described above to the extent of the above-described content of Cr, the content of Ni needs to be 35.00% or more. However, Ni is an expensive element, and when contained in large amounts, leads to an increase in costs. Accordingly, the content of Ni is 35.00 to 50.00%. The content of Ni is preferably 37.00% or more, and more preferably 39.00% or more. Furthermore, the content of Ni is preferably 48.00% or less, and more preferably 46.00% or less.
W: 4.00 to 10.00%
• W (tungsten) is dissolved in a matrix and is an element that significantly contributes to the improvement of the creep rupture strength at high temperature. To sufficiently produce the effect, the content of W needs to be 4.00% or more. However, excessively contained W leads only to the saturation of the effect, and what is worse, the creep rupture strength degrades. Furthermore, since W is an expensive element, costs will increase when excessively contained. Accordingly, the content of W is 4.00 to 10.00%. The content of W is preferably 5.00% or more, and more preferably 6.00% or more. Furthermore, the content of W is preferably 9.00% or less, and more preferably 8.00% or less.
Ti: 0.01 to 1.20%
• Ti (titanium) precipitates in grains as fine carbo-nitride and contributes to the improvement of the creep rupture strength at high temperature. To obtain the effects, the content of Ti needs to be 0.01% or more. However, an excessive content of Ti leads to the precipitation of a large amount of carbo-nitride and the degradation of the creep rupture ductility and toughness. Accordingly, the content of Ti is 0.01 to 1.20%. The content of Ti is preferably 0.03% or more, and more preferably 0.05% or more. Furthermore, the content of Ti is preferably 1.00% or less, and more preferably 0.80% or less.
Nb: 0.01 to 1.00%
• Nb (niobium) is combined with C, or C and N, precipitates in grains as fine carbide or carbo-nitride, and contributes to the improvement of the creep rupture strength at high temperature. To obtain the effects, the content of Nb needs to be 0.01% or more. However, an excessive content of Nb leads to the precipitation of a large amount of carbide carbo-nitride, and the degradation of the creep rupture ductility and toughness. Accordingly, the content of Nb is 0.01 to 1.00%. The content of Nb is preferably 0.05% or more, and more preferably 0.10% or more. Furthermore, the content of Nb is preferably 0.80% or less, and more preferably 0.60% or less.
N: 0.0200% or less
• N (nitrogen) is an element that is effective for stabilizing the austenite, whereas when excessively contained, a large amount of fine nitride precipitates in grains during the use at high temperature, leading to the degradation of the creep rupture ductility and toughness. Accordingly, the content of N is 0.0200% or less. The content of N is preferably 0.0180% or less, and more preferably 0.0150% or less.
• It is not particularly necessary to provide a lower limit for the content of N. However, excessive reduction of the content of N leads to not only difficulty in obtaining an effect of stabilizing the austenite, but also a significant increase in production costs. Accordingly, the content of N is preferably 0.0005% or more, and more preferably 0.0008% or more.
Al: 0.010 to 0.300%
• Al (aluminum) is an element that has a deoxidation function, and therefore, the content of Al needs to be 0.010% or more. However, an excessive content of Al leads to a significant degradation of cleanliness of alloy, and the degradation of hot workability and ductility. Accordingly, the content of Al is 0.010 to 0.300%. The content of Al is preferably 0.030% or more, and more preferably 0.050% or more. Furthermore, the content of Al is preferably 0.250% or less, and more preferably 0.200% or less.
B: 0.0005 to 0.0400%
• B (boron) is an element that is necessary to improve the creep rupture strength by segregating in a grain boundary during the use at high temperature to strengthen the grain boundary and finely dispersing grain boundary carbide. To obtain the effects, the content of B needs to be 0.0005% or more. However, an excessive content of B leads to the degradation of weldability and the degradation of hot workability. Accordingly, the content of B is 0.0005 to 0.0400%. The content of B is preferably 0.0010% or more, and more preferably 0.0020% or more. Furthermore, the content of B is preferably 0.0300% or less, and more preferably 0.0200% or less.
O: 0.0100% or less
• O (oxygen) is contained in alloy as impurities, and an excessive content of O leads to the degradation of hot workability, and further the degradation of toughness and ductility. Accordingly, the content of O is 0.0100% or less. The content of O is preferably 0.0080% or less, and more preferably 0.0050% or less.
• It is not particularly necessary to provide a lower limit for the content of O, whereas excessive reduction leads to an increase in production costs. Accordingly, the content of O is preferably 0.0005% or more, and more preferably 0.0008% or more.
• In the chemical composition of the austenitic heat resistant alloy of the present invention, the balance is Fe and impurities. Here, "impurities" refer to components that are introduced due to various factors in raw materials such as ore and scrap and production processes when the alloy is industrially produced and that are acceptable to the extent that they do not adversely affect the present invention.
• The austenitic heat resistant alloy of the present invention may further contain one or more elements selected from Ca, Mg, REM, Co, Cu, Mo, and V to the extent indicated below. Note that since these elements are not essential for the member, the lower limit value of the content is 0%. The reason for limitation for each element will be described.
Ca: 0.0100% or less
• Ca (calcium) forms a compound with S to reduce the amount of S in a matrix, producing an effect of improving hot workability, and therefore, Ca may be contained as necessary. However, an excessive content of Ca leads to a reduction in the amount of S in alloy that contributes to the improvement of the creep rupture strength, which is the advantageous effect of the present invention, and due to being combined with O, leads to a significant degradation of cleanliness, and what is worse, the degradation of hot workability. Accordingly, the content of Ca is 0.0100% or less. The content of Ca is preferably 0.0080% or less. When it is desirable to obtain the above-described effects, the content of Ca is preferably 0.0001% or more, more preferably 0.0002% or more, and further preferably 0.0003% or more.
Mg: 0.0500% or less
• As in Ca, Mg (magnesium) forms a compound with S to reduce the amount of S in a matrix, producing an effect of improving hot workability, and therefore, Mg may be contained as necessary. However, an excessive content of Mg leads to a reduction in the amount of S in alloy that contributes to the improvement of the creep rupture strength, which is the advantageous effect of the present invention, and due to being combined with O, leads to a significant degradation of cleanliness, and what is worse, the degradation of hot workability. Accordingly, the content of Mg is 0.0500% or less. The content of Mg is preferably 0.0450% or less. When it is desirable to obtain the above-described effects, the content of Mg is preferably 0.0001% or more, more preferably 0.0002% or more, and further preferably 0.0003% or more.
REM: 0.1000% or less
• As in Ca, REM (rare earth metal) forms a compound with S to reduce the amount of S in a matrix, producing an effect of improving hot workability, and therefore, REM may be contained as necessary. However, an excessive content of REM leads to a reduction in the amount of S in alloy that contributes to the improvement of the creep rupture strength, which is the advantageous effect of the present invention, and due to being combined with O, leads to a significant degradation of cleanliness, and what is worse, the degradation of hot workability. Accordingly, the content of REM is 0.1000% or less. The content of REM is preferably 0.0800% or less. When it is desirable to obtain the above-described effects, the content of REM is preferably 0.0001% or more, more preferably 0.0002% or more, and further preferably 0.0003% or more.
• REM is a collective term for a total of 17 elements of Sc, Y, and lanthanoid, and the content of REM refers to a total content of one or more elements of the REM. Furthermore, REM is generally contained in misch metal. Accordingly, for example, the amount of REM may be adjusted into the above-described range by adding the REM in the form of misch metal.
Co: 1.000% or less
• Co (cobalt) has a function of improving the creep rupture strength. That is, as in Ni, Co is an austenite forming element, increases phase stability and contributes to the improvement of the creep rupture strength. Accordingly, Co may be contained. However, Co is a highly expensive element, and therefore, excessively contained Co leads to a significant increase in costs. Accordingly, the content of Co is 1.000% or less. The content of Co is preferably 0.800% or less, and more preferably 0.600% or less. On the other hand, when it is desirable to obtain the above-described effects, the content of Co is preferably 0.010% or more, and more preferably 0.050% or more.
Cu: 1.00% or less
• Cu (copper) has a function of improving the creep rupture strength. That is, as in Ni and Co, Cu is an austenite forming element, increases phase stability and contributes to the improvement of the creep rupture strength. Accordingly, Cu may be contained. However, when Cu is excessively contained, hot workability degrades. Accordingly, the content of Cu is 1.00% or less. The content of Cu is preferably 0.80% or less, and more preferably 0.60% or less. On the other hand, when it is desirable to obtain the above-described effects, the content of Cu is preferably 0.01% or more, and more preferably 0.05% or more.
Mo: 1.000% or less
• Mo (molybdenum) has a function of improving the creep rupture strength. That is, Mo is dissolved in a matrix and has a function of improving the creep rupture strength at high temperature. Accordingly, Mo may be contained. However, when Mo is excessively contained, the stability of austenite degrades, and what is worse, the creep rupture strength degrades. Accordingly, the content of Mo is 1.000% or less. The content of Mo is preferably 0.800% or less, and more preferably 0.700% or less. On the other hand, when it is desirable to obtain the above-described effects, the content of Mo is preferably 0.010% or more, and more preferably 0.050% or more.
V: 0.500% or less
• V (vanadium) has a function of improving the creep rupture strength. That is, as in Nb, V is combined with C, or C and N, forms fine carbide or carbo-nitride, and has a function of improving the creep rupture strength. Accordingly, V may be contained. However, when V is excessively contained, it precipitates in large amounts as carbide or carbo-nitride, and the creep rupture ductility degrades. Accordingly, the content of V is 0.500% or less. The content of V is preferably 0.400% or less, and more preferably 0.300% or less. On the other hand, when it is desirable to obtain the above-described effects, the content of V is preferably 0.010% or more, and more preferably 0.050% or more.
2. Formula (i)
• As described above, with Cr, W, Fe, and Ni being sufficiently dissolved, it is possible to allow precipitates that contain these elements to finely precipitate in a usage environment, so that an excellent creep rupture strength can be obtained. Accordingly, it is necessary to ensure that, in the alloy member, Cr, W, Fe, and Ni are sufficiently contained in total whereas the amount of Cr, W, Fe, and Ni that are present as precipitates is lowered in advance before being used, and specifically, it is necessary to satisfy the following Formula (i):
  $$97.50\le \left(\mathrm{Cr}+\mathrm{W}+\mathrm{Fe}+\mathrm{Ni}\right)-\left({\mathrm{Cr}}_{\mathrm{ER}}+{\mathrm{W}}_{\mathrm{ER}}+{\mathrm{Fe}}_{\mathrm{ER}}+{\mathrm{Ni}}_{\mathrm{ER}}\right)$$
• where each symbol in the formulas is defined as below, and each element symbol in the formulas represents a content of each element (mass%) contained in the alloy member,
• Cr_ER: the content of Cr (mass%) in precipitates obtained by extracted residue analysis
• W_ER: the content of W (mass%) in precipitates obtained by extracted residue analysis
• Fe_ER: the content of Fe (mass%) in precipitates obtained by extracted residue analysis
• Ni_ER: the content of Ni (mass%) in precipitates obtained by extracted residue analysis.
• When the right value of Formula (i) is less than 97.50, the amount of solid solution of Cr, W, Fe, and Ni is insufficient, and therefore, the creep rupture strength cannot be improved. Accordingly, the right value of Formula (i) is 97.50 or more. The right value of Formula (i) is preferably 98.00 or more, and more preferably 98.50 or more.
• The content of each element (mass%) in precipitates to be analyzed as extraction residues in the above formula can be measured according to the procedure as described below. Specifically, by using 10% acetylacetone - 1% tetramethylammonium chloride / methanol, a sample of about 0.4 g is electrolyzed at a current value of 20 mA/cm². Thereafter, a solution of the electrolyzed sample is filtered through a 0.2 µm filter, followed by acid decomposition of the residues. Then, an ICP (high frequency inductively coupled plasma) optical emission spectrometer is used to calculate a quantity (mass%) analyzed as extraction residues for the above-described elements.
3. Formula (ii)
• As described above, the inventors have found that there is a certain relationship between the thickness of a member and the grain diameter that leads to a better creep rupture ductility. Specifically, as a result of studies conducted on the relationship between the thickness of a member and the grain diameter that leads to a better creep rupture ductility based on a large amount of experimental data, it has been found that a sufficient creep rupture ductility can be secured by satisfying the following Formula (ii):
  $$-2.2\times {10}^{-5}\times {\mathrm{t}}^3+2.1\le \mathrm{D}$$ where t in the formulas is defined as the thickness of alloy member (mm), and D as the average grain size at a thickness-center portion of the alloy member.
• When the average grain size D does not satisfy Formula (ii), austenite grains are coarse, so that the creep rupture ductility cannot be improved. The maximum value of the average grain size D is not particularly limited, whereas when austenite grains are fine, there may be a case in which the above Formula (i) is not satisfied due to an insufficient solution heat treatment, which will be described later. Accordingly, the average grain size D is preferably 6.0 or less, and more preferably 5.0 or less. Furthermore, the average grain size D is preferably -2.0 or more, more preferably -1.0 or more, and further preferably 0 or more.
• The average grain size D is measured in compliance with ASTM E112 (2013). Specifically, specimens for microstructural observation are collected such that a section that is perpendicular to the longitudinal direction of the alloy member is an observation surface, and then the observation surface is subjected to mirror polishing. After polishing, etching in mixed acid is carried out, followed by observation with an optical microscope. Observations are made at 10 visual fields such that the thickness-center position of the alloy member is brought into the center of the visual field. Then, the grain size of each visual field is determined according to the comparison method defined in ASTM E112, and the average value of them is determined as average grain size D. At this time, 100× is taken as a reference observation magnification, and 200× or 400× is selected depending on the grain size. When 200× or 400× is selected as the observation magnification, a correction is made in compliance with ASTM E112 (2013) by using a correction value Q defined by the following formula (I): $$\mathrm{Q}=6.64{\log }_{10}\left(\mathrm{M}/100\right)$$ where M in the above formula is an observation magnification.
4. Dimension
• For example, the austenitic heat resistant alloy member of the present invention may be an alloy tube or an alloy plate. When the austenitic heat resistant alloy member is an alloy tube, the wall thickness is preferably 1 mm or more, or 5 mm or more, and preferably 100 mm or less, 80 mm or less, 65 mm or less, or 55 mm or less. Furthermore, when the austenitic heat resistant alloy member is an alloy plate, the plate thickness is preferably 1 to 100 mm.
5. Production Method
• While no particular limitation is placed on the production method for the austenitic heat resistant alloy member of the present invention, for example, an ingot or a cast piece that has the above-described chemical composition is subjected to hot working, followed by different types of hot working such as hot extrusion as necessary, and thereafter, a solution heat treatment is carried out. Furthermore, cold working may be carried out as necessary.
• As described above, to achieve both excellent creep rupture strength and creep rupture ductility, it is necessary to strictly control the solution heat treatment conditions in consideration of the thickness of the member. Specifically, it is necessary that the solution heat treatment temperature T is 1180 to 1250°C, and the following Formulas (iii) and (iv) are satisfied. After the solution heat treatment, the alloy member is preferably water-cooled. $$16.1\times \mathrm{t}+28500\le \mathrm{LMP}\le 16.1\times \mathrm{t}+29100$$ $$\mathrm{LMP}=\left(\mathrm{T}+273.15\right)\times \left(\mathrm{Log}\left({\mathrm{t}}_{\mathrm{r}}\right)+20\right)$$ where each symbol in the formulas is defined as below,
• t: the thickness of alloy member (mm)
• T: solution heat treatment temperature (°C)
• t_r: solution heat treatment time (h).
• When the solution heat treatment temperature T is less than 1180°C, recrystallization does not occur, so that strain caused by working cannot be eliminated, leading to the degradation of the creep rupture ductility. In addition, Cr, W, Fe, and Ni cannot sufficiently be dissolved, and therefore, a better creep rupture strength cannot be secured. On the other hand, when the solution heat treatment temperature T is more than 1250°C, the creep rupture ductility degrades due to coarsening of austenite grains. Accordingly, the solution heat treatment temperature T is 1180 to 1250°C.
• When the solution heat treatment time t_r is less than 10 min, Cr, W, Fe, and Ni cannot sufficiently be dissolved, and therefore, a better creep rupture strength cannot be secured. Accordingly, the solution heat treatment time t_r is 10 min or more.
• Furthermore, when LMP (Larson Miller Parameter) defined by Formula (iv) is less than the left value of Formula (iii), Cr, W, Fe, and Ni cannot sufficiently be dissolved, and therefore, a better creep rupture strength cannot be secured. On the other hand, the LMP defined by Formula (iv) is more than the right value of Formula (iii), the creep rupture ductility degrades due to coarsening of austenite grains. Accordingly, it is necessary that the LMP defined by Formula (iv) satisfies Formula (iii).
• Hereinunder, the present invention will more specifically be described with reference to examples, whereas the present invention is not limited to the examples.
EXAMPLE
• Austenitic heat resistant alloys 1 to 38, each of which had the chemical composition indicated in Table 1 and Table 2, were melted in a laboratory to fabricate ingots. The ingots were then shaped through hot forging and hot rolling, followed by solution heat treatments under conditions indicated in Table 3 and Table 4, so that alloy tubes (Test Nos. 1 to 50), each of which had a wall thickness as indicated in Table 3 and Table 4, were obtained.
<Extraction Residues>
• The contents (mass%) of Cr, W, Fe, and Ni in precipitates analyzed as extraction residues were measured according to the procedure as described below. Specifically, by using 10% acetylacetone - 1% tetramethylammonium chloride / methanol, a sample of about 0.4 g was electrolyzed at a current value of 20 mA/cm². Thereafter, a solution of the electrolyzed sample was filtered through a 0.2 µm filter, followed by acid decomposition of the residues. Then, an ICP optical emission spectrometer was used to calculate a quantity (mass%) analyzed as extraction residues for the above-described elements.
<Average Grain Size>
• The average grain size D was measured in compliance with ASTM E112 (2013). Specifically, specimens for microstructural observation were collected such that a section that was perpendicular to the longitudinal direction of the alloy tube was an observation surface, and then the observation surface was subjected to mirror polishing. After polishing, etching in mixed acid was carried out, followed by observation with an optical microscope. Observations were made at 10 visual fields in such a way that the wall-thickness-center position of the alloy tube was brought into the center of the visual field. Then, the grain size of each visual field was determined according to the comparison method defined in ASTM E112, and the average value of them was determined as average grain size D. At this time, 100× was taken as a reference observation magnification, and 200× or 400× was selected depending on the grain size. When 200× or 400× was selected as the observation magnification, a correction was made in compliance with ASTM E112 (2013) by using a correction value Q defined by the following formula (I): $$\mathrm{Q}=6.64{\log }_{10}\left(\mathrm{M}/100\right)$$ where M in the above formula is an observation magnification.
<Creep Rupture Strength and Creep Rupture Ductility>
• Next, creep rupture specimens, each of which was a round bar having a diameter of 6 mm and a gage length of 30 mm, were collected from a wall-thickness-center portion of each alloy tube, and creep rupture tests were conducted under conditions of 750°C and 100 MPa. Then, those that exceeded 2000 hours in the creep rupture time were determined to have passed and have a better creep rupture strength. Furthermore, those that exceeded 10% in the reduction of area after creep rupture were determined to have passed and have a better creep rupture ductility.
• The results are collectively shown in Table 3 and Table 4. Note that "-" of Test No. 44 in Table 4 means that determination of the grain size was difficult because recrystallization did not occur.
[Table 3]
• Table 3

  Test No.	Alloy	Wall thickness t (mm)	Solution heat treatment temperature T(°C)	Solution heat treatment time tr (min)	Left value of Formula (iii)	LMP	Right value of Formula (iii)	Cr+W+ Fe+Ni (mass%)	CrER+WER+ FeER+NiER (mass%)	Right value of Formula (i)	Left value of Formula (ii)	Average grain size D	Creep rupture time (h)	Creep rupture reduction (%)
  1	1	8	1210	30	28629	29217	29229	98.29	0.26	98.03	2.1	3.2	3277	34.5
  2	2	8	1220	10	28629	28701	29229	98.24	0.16	98.08	2.1	3.5	3201	29.6
  3	3	15	1230	10	28742	28893	29342	98.18	0.06	98.12	2.0	4.1	3509	23.1
  4	4	19	1210	20	28806	28955	29406	97.63	0.12	97.51	2.0	2.2	2135	31.4
  5	5	11	1230	15	28677	29158	29277	98.02	0.27	97.75	2.1	3.3	3441	25.9
  6	6	11	1210	20	28677	28955	29277	98.00	0.35	97.65	2.1	3.4	2038	34.7
  7	7	38	1240	20	29112	29541	29712	97.78	0.12	97.66	0.9	2.4	2451	19.3
  8	8	48	1250	10	29273	29278	29873	97.72	0.09	97.63	-0.3	0.5	2437	15.2
  9	9	48	1220	30	29273	29414	29873	97.82	0.26	97.56	-0.3	0.7	2169	35.8
  10	10	38	1220	30	29112	29414	29712	97.72	0.21	97.51	0.9	2.2	2471	37.3
  11	11	38	1210	30	29112	29217	29712	97.64	0.13	97.51	0.9	1.9	2387	30.9
  12	12	48	1220	30	29273	29414	29873	97.79	0.16	97.63	-0.3	0.6	2469	28.1	Inventive example
  13	13	48	1230	30	29273	29611	29873	98.10	0.06	98.04	-0.3	1.1	2355	22.1
  14	14	26	1210	20	28919	28955	29519	97.98	0.38	97.60	1.7	3.4	2471	33.4
  15	15	26	1180	60	28919	29063	29519	99.79	0.29	98.50	1.7	2.2	3822	50.7
  16	16	8	1210	20	28629	28955	29229	97.84	0.20	97.64	2.1	3.0	2246	37.4
  17	17	8	1240	10	28629	29086	29229	98.11	0.30	97.81	2.1	3.9	2437	21.3
  18	18	5	1240	10	28581	29086	29181	98.06	0.24	97.82	2.1	4.1	3098	13.7
  19	19	5	1220	10	28581	28701	29181	97.79	0.23	97.56	2.1	4.0	2213	34.1
  20	20	6	1220	20	28597	29151	29197	98.03	0.13	97.90	2.1	3.8	2349	22.8
  21	21	6	1230	10	28597	28893	29197	97.73	0.21	97.52	2.1	2.8	2471	26.4
  22	22	19	1240	10	28806	29086	29406	97.65	0.13	97.52	2.0	3.3	2219	22.0
  23	23	8	1230	10	28629	28893	29229	97.81	0.28	97.53	2.1	4.7	2391	23.7
  24	24	8	1230	10	28629	28893	29229	97.91	0.20	97.71	2.1	4.6	2861	29.1

  97.50≦(Cr+W+Fe+Ni)-(CrER+WER+FeER+NiER) ...(i) -2.2×10-5×t3+2.1≦D ...(ii) 16.1×t+28500≤LMP≦16.1×t+29100 ...(iii) LMP=(T+273.15)×((Log(tr)+20) ...(iv)

[Table 4]
• Table 4

  Test No.	Alloy	Wall thickness t(mm)	Solution heat treatment temperature T(°C)	Solution heat treatment time tr (min)	Left value of Formula (iii)	LMP	Right value of Formula (iii)	Cr+W+ Fe+Ni (mass%)	CrER+WER+ FeER+NiER (mass%)	Right value of Formula (i)	Left value of Formula (ii)	Average grain size D	Creep rupture time (h)	Creep rupture reduction (%)
  25	25	8	1220	10	28629	28701	29229	97.62	0.10	97.52	2.1	4.6	2285	31.7
  26	26	8	1220	10	28629	28701	29229	97.96	0.37	97.59	2.1	4.1	2793	25.5
  27	27	35	1230	30	29064	29611	29664	97.76	0.21	97.55	1.2	1.9	2277	19.1
  28	28	80	1250	30	29788	30004	30388	98.56	0.26	98.30	-9.2	1.1	2116	16.7
  29	29	60	1250	30	29466	30004	30066	98.19	0.39	97.80	-2.7	0.8	2357	15.5
  30	30	8	1200	30	28629	29020	29229	98.13	0.31	97.82	2.1	4.7	2339	37.1
  31	31	8	1200	30	28629	29020	29229	97.95	0.35	97.60	2.1	4.8	2581	31.6
  32	32	19	1210	20	28806	28955	29406	98.19	0.41	97.78	2.0	3.9	2506	29.8
  33	33	19	1210	20	28806	28955	29406	97.96	0.18	97.78	2.0	2.8	2360	33.3	Inventive example
  34	34	23	1230	20	28870	29346	29470	97.78	0.13	97.65	1.8	4.3	2497	38.1
  35	35	35	1230	20	29064	29346	29664	98.13	0.22	97.91	1.2	4.1	2991	34.2
  36	36	35	1220	20	29064	29151	29664	98.19	0.27	97.92	1.2	3.4	2334	23.4
  37	37	48	1220	30	29273	29414	29873	97.74	0.15	97.59	-0.3	2.4	2401	24.5
  38	38	48	1220	30	29273	29414	29873	97.73	0.16	97.57	-0.3	2.6	2346	21.1
  39	1	10	1230	10	28661	28893	29261	98.29	0.26	98.03	2.1	4.7	2213	30.8
  40	2	20	1240	10	28822	29086	29422	98.24	0.23	98.01	1.9	4.6	2267	28.0
  41	3	30	1240	20	28983	29541	29583	98.18	0.16	98.02	1.5	4.4	3071	40.8
  42	4	40	1240	20	29144	29541	29744	97.63	0.09	97.54	0.7	1.8	3220	16.8
  43	5	50	1250	20	29305	29736	29905	98.02	0.11	97.91	-0.7	0.8	3001	13.7
  44	1	8	1160	10	28629	27548	29229	98.29	1.07	97.22	2.1	-	1873	7.9
  45	4	19	1210	10	28806	28509	29406	97.63	0.54	97.09	2.0	4.5	1921	20.1
  46	6	11	1240	30	28677	29807	29277	98.00	0.11	97.89	2.1	-0.5	2551	6.6	Comparative example
  47	7	38	1270	20	29112	30127	29712	97.78	0.26	97.52	0.9	-2.0	3811	4.1
  48	10	35	1200	10	29064	28317	29664	97.72	0.49	97.23	1.2	6.7	1628	24.1
  49	8	48	1100	30	29273	27050	29873	97.72	1.25	96.47	-0.3	7.2	1540	8.7
  50	3	15	1200	5	28742	27873	29342	98.18	1.44	96.74	2.0	5.7	1891	22.7

  97.50≦(Cr+W+Fe+Ni)-(CrER+WER+FeER+NiER) ...(i) 16.1×t+28500≦LMP≦16.1×t+29100 ...(iii) -2.2×10-5×t3+2.1≦D ...(ii) LMP=(T+273.15)×((Log(tr)+20) ...(iv) The underline indicates that the value fell out of the requirements of the present invention.

• As shown in Table 3 and Table 4, Test Nos. 1 to 43, which satisfied all the provisions of the present invention, produced better results for both the creep rupture strength and the creep rupture ductility. In contrast, in Test No. 44, the creep rupture ductility degraded because the solution heat treatment temperature T was low, and therefore, recrystallization did not occur. In addition, LMP was less than the left value of Formula (iii), and therefore, Cr, W, Fe, and Ni could not sufficiently be dissolved, leading to the degradation of the creep rupture strength.
• In Test Nos. 45 and 48, LMP was less than the left value of Formula (iii), and therefore, Cr, W, Fe, and Ni could not sufficiently be dissolved, leading to the degradation of the creep rupture strength. In Test No. 46, LMP was more than the right value of Formula (iii), and therefore, the average grain size D was lower than the left value of Formula (ii), leading to the degradation of the creep rupture ductility. In Test No. 47, the solution heat treatment temperature T was high and LMP was more than the right value of Formula (iii), and therefore, the average grain size D was lower than the left value of Formula (ii), leading to the degradation of the creep rupture ductility. In Test No. 49, the solution heat treatment temperature T was low, and therefore, recrystallization did not occur, leading to the degradation of the creep rupture ductility. In addition, Cr, W, Fe, and Ni could not sufficiently be dissolved, leading to the degradation of the creep rupture strength. In Test No. 50, the solution heat treatment time t_r was short, and therefore, Cr, W, Fe, and Ni could not sufficiently be dissolved, leading to the degradation of the creep rupture strength.
INDUSTRIAL APPLICABILITY
• The austenitic heat resistant alloy member of the present invention is excellent in both the creep rupture strength and the creep rupture ductility for a long time. Accordingly, the austenitic heat resistant alloy member of the present invention is suitably used as a material for superheater tubes or reheater tubes of power generation boilers.

Claims (2)

1. An austenitic heat resistant alloy member having a chemical composition comprising, 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 to 28.00%,

   Ni: 35.00 to 50.00%,

   W: 4.00 to 10.00%,

   Ti: 0.01 to 1.20%,

   Nb: 0.01 to 1.00%,

   N: 0.0200% or less,

   Al: 0.010 to 0.300%,

   B: 0.0005 to 0.0400%,

   O: 0.0100% or less, and

   the balance: Fe and impurities, and

   satisfies following Formulas (i) and (ii): $$97.50\le \left(\mathrm{Cr}+\mathrm{W}+\mathrm{Fe}+\mathrm{Ni}\right)-\left({\mathrm{Cr}}_{\mathrm{ER}}+{\mathrm{W}}_{\mathrm{ER}}+{\mathrm{Fe}}_{\mathrm{ER}}+{\mathrm{Ni}}_{\mathrm{ER}}\right)$$ $$-2.2\times {10}^{-5}\times {\mathrm{t}}^3+2.1\le \mathrm{D}$$

   where each symbol in the formulas is defined as below, and each element symbol in the formulas represents a content of each element (mass%) contained in the alloy member,

   Cr_ER: a content of Cr (mass%) in precipitates obtained by extracted residue analysis

   W_ER: a content of W (mass%) in precipitates obtained by extracted residue analysis

   Fe_ER: a content of Fe (mass%) in precipitates obtained by extracted residue analysis

   Ni_ER: a content of Ni (mass%) in precipitates obtained by extracted residue analysis

   t: a thickness of the alloy member (mm)

   D: an average grain size at a thickness-center portion of the alloy member.
2. The austenitic heat resistant alloy member according to claim 1, wherein in lieu of a part of the Fe, the chemical composition contains one or more elements, in mass%, 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.