[0001]The present invention, austenitic heat resistant alloy, and a welded structure comprising the alloy.
Background technique
[0002]
　Recently, from the viewpoint of environmental load reduction, high temperature and high pressure operating conditions such as power boiler is underway on a global scale. The materials used in the superheater tubes and reheater tubes, and more excellent high temperature strength and corrosion resistance are required.
[0003]
　As a material satisfying such requirements, various austenitic heat resistant alloy is disclosed which contains a large amount of nitrogen.
[0004]
　For example, JP-A-2004-250783 discloses, 0.1 to 0.35% of N, defining the metal structure as well as 22 percent less than 30% of Cr, austenitic stainless steels excellent in high temperature strength and corrosion resistance There has been proposed.
[0005]
　JP-A-2009-084606 discloses, N and 0.1 to 0.35%, was defined impurity element is a 22% greater than 30% of Cr, been proposed austenitic stainless steel excellent in high temperature strength corrosion resistance ing.
[0006]
　JP-A-2012-1749 discloses includes N 0.09 ~ 0.30%, austenitic heat-resisting steel of Mo and W are excellent in large amount composite addition with high temperature strength and hot workability is disclosed.
[0007]
　WO 2009/044796, the N 0.03 ~ 0.35%, and Nb, V, and including one or more of Ti, high strength austenitic stainless steel is disclosed ing.
Disclosure of the Invention
[0008]
　These austenitic heat resistant alloy is generally after assembly by welding, are provided for use at high temperatures. However, when a welded structure used for a long time at high temperature using austenitic heat resistant alloy containing a high N, SIPH: to cracking called (Strain Induced Precipitation Hardening strain-induced precipitation hardening) cracks occur in the welding heat affected zone there is.
[0009]
　WO 2009/044796, supra, describes the the element to strengthen the element and grain to embrittle the grain boundary by defining a predetermined range, it is possible to prevent the cracking that occurs during long use ing. In certainly certain conditions, it is possible to prevent cracking by these materials. However, in recent years, large amounts of W, the addition of Mo or the like, an austenitic heat resistant alloy, measured to further improve performance such as high temperature strength are used. These austenitic heat resistant alloy, the welding conditions, the shape of the structure, the dimensions and the like, it may be impossible to prevent a stable crack. Specifically, or by increasing the heat input, or by increasing the thickness and or use at high temperatures exceeding 650 ° C., it may not be possible to prevent a stable crack.
[0010]
　An object of the present invention is that excellent cracking resistance and high temperature strength to provide a stable obtained austenitic heat resistant alloy.
[0011]
　Austenitic heat resistant alloy according to an embodiment of the present invention, the chemical composition, in mass%, C: 0.04 ~ 0.14%, Si: 0.05 ~ 1%, Mn: 0.5 ~ 2.5 %, P: 0.03% or less, S: less than 0.001%, Ni: 23 ~ 32%, Cr: 20 ~ 25%, W: 1 ~ 5%, Nb: 0.1 ~ 0.6%, V: 0.1 ~ 0.6%, N: 0.1 ~ 0.3%, B: 0.0005 ~ 0.01%, Sn: 0.001 ~ 0.02%, Al: 0.03% hereinafter, O: 0.02% or less, Ti: 0 ~ 0.5%, Co: 0 ~ 2%, Cu: 0 ~ 4%, Mo: 0 ~ 4%, Ca: 0 ~ 0.02%, Mg : 0 ~ 0.02%, REM: 0 ~ 0.2%, the balance is Fe and impurities, grain size 7.0 No. less than 2.0 number than the crystal grain size number defined in ASTM E112 have a certain organization That.
[0012]
　According to the present invention, excellent cracking resistance and high-temperature strength is obtained austenitic heat resistant alloy which is obtained stably.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013]
[1] Figure 1 is a sectional view showing an open succeeding shape of the plate produced in Example.
DESCRIPTION OF THE INVENTION
[0014]
　The present inventors have conducted detailed study to solve the above problems. As a result, it was revealed following knowledge.
[0015]
　In the welded joint using austenitic heat resistant alloy containing a high N, it was investigated in detail SIPH cracks that occur during use. As a result, (1) cracking occurred at the grain boundaries of the coarse-grained HAZ of fusion line vicinity, (2) from its cracking on the fracture surface, a clear enrichment of S is detected. Further, (3) the crack vicinity of the grains, nitride or carbo-nitride had a large amount of precipitation. In particular, it was remarkable when containing a large amount of Nb. In addition, (4) as the initial grain size of the austenitic heat resistant alloy used is large, crystal grain size of the weld heat affected zone is also increased, cracks were found to be likely to occur.
[0016]
　From these, SIPH cracks, due to a large amount of nitrides or carbonitrides transgranular during use at high temperatures are precipitated, that the grains are less likely to deform, concentrated creep deformation in the grain boundary It was considered to have led to the opening as a result of. S segregates to grain boundaries, or during use in welding, lowering the bonding force of the grain boundaries. Moreover, as the crystal grain size is large, the area of ​​grain boundaries per unit volume decreases. Crystal grain boundaries function as nucleation sites nitrides or carbonitrides. Therefore, when the crystal grain boundary is reduced, nitrides and carbo-nitrides is easily precipitated in larger quantities in the grains. In addition, external forces experienced during use, for example due to creep deformation caused by the residual stress, etc., tends to concentrate the specific particle surface. Therefore, as the initial crystal grain size of the base material is large, cracks were considered likely to occur. At high temperatures, especially in excess of 650 ° C., in addition to the precipitate is precipitated in a short time, since the grain boundary segregation occurs early, the problem was considered likely to become apparent.
[0017]
　In order to prevent this cracking, the enhanced precipitation strengthening or solid solution is effective to reduce the element to increase the deformation resistance of the grains. However, these elements are an essential element in terms of creep strength ensured at high temperatures. Therefore, the creep strength ensured in prevention and high temperature cracking have a trade-off relationship, it is difficult to achieve both of these.
[0018]
　A result of extensive investigations, C: 0.04 ~ 0.14%, Si: 0.05 ~ 1%, Mn: 0.5 ~ 2.5%, P: 0.03% or less, Ni: 23 ~ 32 %, Cr: 20 ~ 25%, W: 1 ~ 5%, N: 0.1 ~ 0.3%, B: 0.0005 ~ 0.01%, Al: 0.03% or less, and O: 0 in austenitic heat resistant alloy containing .02% or less, in order to prevent the SIPH cracking, respectively Nb and S content, with strictly managed than 0.1 to 0.6 percent, and 0.001%, the initial particle size of the base material ASTM: revealed the crystal grain size number as defined in (American Society for testing and material American Society for testing and materials) be 2.0 number higher to be effective. However, with finely than necessary grain size, to limit the Nb content, creep strength of the base material does not satisfy a predetermined value. Therefore, the crystal grain size was found to be required to be less than 7.0 number in grain size number. In addition, found that the V low precipitation strengthening ability than Nb be 0.1 to 0.6% without impairing the anti SIPH cracking resistance, it is required to satisfy the predetermined creep strength did.
[0019]
　By the way, although it is SIPH cracking in these measures it can certainly prevent was able to confirm, that there is a possibility that another problem arises in to continue the investigation it was found.
[0020]
　As mentioned above, the austenitic heat resistant alloy is often assembled by welding. When welding them, usually using a filler material. However, a small and thin components, in the first layer welding or tack welded in thick parts, which may be gas-shielded arc welding without using a filler material. At this time, the penetration depth is insufficient, the remaining abutting surfaces unmelted as a defect, not necessary strength is obtained at the welded joint. S, while lowering the resistance to SIPH cracking resistance, has the effect of increasing the depth penetration. Therefore, strictly manage the S content to less than 0.001% from the viewpoint of resistance to SIPH cracking resistance, problems of insufficient penetration was found to be easily actualized.
[0021]
　To prevent insufficient penetration may be simpler to increase the weld heat input. However, increasing the heat input, coarsening of the weld heat affected zone is promoted, it becomes impossible to prevent SIPH cracking even when more than 2.0 number of initial grain size of the base material in grain size number.
[0022]
　Study result is that desired to prevent failure stably penetration was found that it is effective to contain in the range of 0.001 to 0.02% of Sn. This, Sn is liable to evaporate from the molten pool surface in the weld, by ionized in the arc, was considered to be due to increasing the current density of the arc contribute to the formation of the current-carrying path.
[0023]
　Based on the above findings, the present invention has been completed. Hereinafter, it details the austenitic heat resistant alloy according to an embodiment of the present invention.
[0024]
　[Chemical composition]
　austenitic heat resistant alloy according to the present embodiment has a chemical composition described below. In the following description, "%" of the content of the element means mass%.
[0025]
　C: 0.04 ~ 0.14%
　carbon (C) is configured to stabilize the austenitic structure, improve the creep strength in high-temperature use to form a fine carbide. In order to obtain this effect sufficiently, it is necessary to contain 0.04% or more. However, if excessively contained C, carbides large amount deposited, anti SIPH cracking resistance decreases. Therefore, the upper limit is made 0.14%. The lower limit of the C content is preferably 0.05%, more preferably from 0.06%. The upper limit of the C content is preferably 0.13%, more preferably 0.12%.
[0026]
　Si: 0.05 ~ 1%
　silicon (Si) has a deoxidizing effect and is an element effective for improving the corrosion resistance and oxidation resistance at high temperatures. In order to obtain this effect sufficiently, it is necessary to contain 0.05% or more. However, if excessively contained Si, it decreases the stability of the tissue, leading to reduction in toughness and creep strength. Therefore, the upper limit is made 1%. The lower limit of the Si content is preferably 0.08%, more preferably 0.1%. The upper limit of the Si content is preferably 0.6%, more preferably 0.5%.
[0027]
　Mn: 0.5 ~ 2.5%
　manganese (Mn), like Si, has a deoxidizing effect. Mn also contributes to the stabilization of austenite structure. In order to obtain this effect sufficiently, it is necessary to contain 0.5% or more. However, if excessively contained Mn, lead to embrittlement of the alloy, further, creep ductility is decreased. Therefore, the upper limit is made 2.5%. The lower limit of the Mn content is preferably 0.6%, more preferably from 0.7%. The upper limit of the Mn content is preferably 2%, more preferably from 1.5%.
[0028]
　P: 0.03% or less
　Phosphorus (P) is contained in the alloy as an impurity, segregates in the grain boundary of the weld heat affected zone during welding enhance liquation cracking susceptibility. P further, reduce the long-term creep ductility after use. Therefore, the P content is set to 0.03% or less an upper limit. The upper limit of the P content is preferably 0.028%, more preferably 0.025%. P content is preferably reduced as much as possible, extreme reduction causes an increase in steelmaking cost. Therefore, the lower limit of the P content is preferably 0.0005%, more preferably 0.0008%.
[0029]
　S: less than 0.001%
　sulfur (S) is included in the alloy as an impurity like the P, segregates at grain boundaries of the weld heat affected zone during welding enhance liquation cracking susceptibility. S further lead to embrittlement and segregation at grain boundaries for a long time during use, it is an element which greatly reduces the resistance to SIPH cracking. To prevent these in the range of the chemical composition of the present embodiment, it is necessary that the S content to less than 0.001%. The upper limit of the S content is preferably 0.0008%, more preferably from 0.0005%. It is preferable S content be reduced as much as possible, extreme reduction causes an increase in steelmaking cost. Therefore, the lower limit of the S content is preferably 0.0001%, more preferably 0.0002%.
[0030]
　Ni: 23 ~ 32%
　nickel (Ni) is an essential element for ensuring the stability of long-time use at the time of the austenite phase. To obtain this effect sufficiently in Cr, the range of W content in the present embodiment, it is necessary to contain Ni 23% or more. However, Ni is an expensive element, a large amount of content leads to increase in cost. For this reason, the upper limit is 32%. The lower limit of Ni content is preferably 25%, still more preferably 25.5%. The upper limit of the Ni content is preferably 31.5%, more preferably 31%.
[0031]
　Cr: 20 ~ 25%
　chromium (Cr) is an essential element for ensuring the oxidation resistance and corrosion resistance at high temperatures. Cr also contributes to ensuring the creep strength by forming a fine carbide. To obtain this effect sufficiently in the range of Ni content of the present embodiment, it is necessary to contain Cr 20% or more. However, if excessively contained Cr, creep strength decreases deteriorated structural stability austenite phase at high temperatures. For this reason, the upper limit is 25%. The lower limit of the Cr content is preferably 20.5%, more preferably 21%. The upper limit of the Cr content is preferably 24.5%, more preferably 24%.
[0032]
　W: 1 ~ 5%
　of tungsten (W) is a solid solution in the matrix, or forms a fine intermetallic compound, greatly contributes to the improvement of creep strength and tensile strength at elevated temperatures. In order to obtain this effect sufficiently, it is necessary to contain 1% or more. However, if contained excessively W, together with resistance to SIPH cracking resistance decreases becomes high deformation resistance in the grains, there is a case where the creep strength decreases. Further, W is an expensive element, a large amount of content leads to increase in cost. For this reason, the upper limit is 5%. The lower limit of the W content is preferably 1.2%, more preferably from 1.5%. The upper limit of the W content is preferably 4.5%, more preferably from 4%.
[0033]
　Nb: 0.1 ~ 0.6%
　niobium (Nb) is added to precipitate as fine MX type carbonitrides, precipitated in grains as Z-phase (CrNbN), tensile and creep strength at high temperatures greatly contributes to the improvement of strength. In order to obtain this effect sufficiently, it is necessary to contain 0.1% or more. However, if contained excessively Nb, too large enhancements ability of these precipitates, together with resistance to SIPH cracking resistance decreases, causing a decrease in creep ductility and toughness. Therefore, the upper limit is made 0.6%. The lower limit of the Nb content is preferably 0.12%, more preferably 0.15%. The upper limit of the Nb content is preferably 0.55%, more preferably 0.5%.
[0034]
　V: 0.1 ~ 0.6%
　vanadium (V) is precipitated in the grains as fine MX type carbonitrides, it contributes to the improvement of creep strength and tensile strength at elevated temperatures. In order to obtain this effect sufficiently, it is necessary to contain 0.1% or more. However, if excessively contained V, along with resistance to SIPH cracking resistance decreases carbonitride and a large amount of precipitation, lead to a decrease in creep ductility and toughness. Therefore, the upper limit is made 0.6%. The lower limit of the V content is preferably 0.12%, more preferably 0.15%. The upper limit of the V content is preferably 0.55%, more preferably 0.5%.
[0035]
　N: 0.1 ~ 0.3%
　of nitrogen (N), together to stabilize the austenite structure, and dissolves in the matrix or precipitates as nitrides and contributes to improvement of high temperature strength. In order to obtain this effect sufficiently, it is necessary to contain 0.1% or more. However, if excessively contained N, by solid solution in a short time when using, by the large amount of fine nitrides are precipitated in the grains for long in use, intragranular deformation resistance is increased, resistance to SIPH cracking resistance descend. Furthermore, the creep ductility and toughness is lowered. Therefore, the upper limit is made 0.3%. The lower limit of the N content is preferably 0.12%, more preferably from 0.14%. The upper limit of the N content is preferably 0.28%, more preferably 0.26%.
[0036]
　B: 0.0005 ~ 0.01%
　boron (B), along with improving the creep strength by dispersing the grain boundary carbides finely to enhance grain boundary segregated at the grain boundaries. In order to obtain this effect sufficiently, it is necessary to contain 0.0005% or more. However, when the content of B in excess, segregated in a large amount is B in the weld heat-affected zone in the vicinity of the molten boundary reduces the melting point of the grain boundary by the welding heat cycle during welding, liquation cracking susceptibility increases. Therefore, the upper limit is made 0.01%. The lower limit of the B content is preferably 0.0008, more preferably 0.001%. The upper limit of the B content is preferably 0.008%, and more preferably 0.006% or.
[0037]
　Sn: 0.001 ~ 0.02%
　tin (Sn), by increasing the current density of the arc evaporates from the molten pool, has the effect of increasing the penetration depth during welding. In order to obtain this effect sufficiently, it is necessary to contain 0.001% or more. However, if excessively contained Sn, SIPH cracking susceptibility of the weld heat affected zone liquation cracking susceptibility and in use during welding is increased. Therefore, the upper limit is made 0.02%. The lower limit of the Sn content is preferably 0.0015%, more preferably 0.002%. The upper limit of the Sn content is preferably 0.018%, more preferably 0.015%.
[0038]
　Al: 0.03% or less
　Aluminum (Al) has a deoxidizing action. However, if excessively contained Al, the hot workability is deteriorated cleanliness of the alloy is deteriorated. Therefore, the upper limit is made 0.03%. The upper limit of the Al content is preferably 0.025%, still more preferably 0.02%. The lower limit is not necessary to particularly provide, but extreme reduction causes an increase in steelmaking cost. Therefore, the lower limit of the Al content is preferably 0.0005%, more preferably 0.001%. In the present invention, Al means acid soluble Al (sol. Al).
[0039]
　O: 0.02% or less
　oxygen (O) is contained in the alloy as an impurity, it has the effect of increasing the penetration depth of the weld. However, if excessively contained O, with hot workability is deteriorated, toughness and ductility are deteriorated. Therefore, the upper limit is made 0.02%. The upper limit of the O content is preferably 0.018%, more preferably 0.015%. The lower limit is not necessary to particularly provide, but extreme reduction causes an increase in steelmaking cost. Therefore, the lower limit of the O content is preferably 0.0005%, more preferably 0.0008%.
[0040]
　The remainder of the chemical composition of the austenitic heat resistant alloy according to the present embodiment is Fe and impurities. The impurities here means when the industrial production of heat-resistant alloy, elements mixed from ore and scrap to be used as a raw material, or an element which is mixed from the environment or the like of the manufacturing process.
[0041]
　The chemical composition of the austenitic heat resistant alloy according to the present embodiment, instead of a part of the above Fe, contain one or more elements selected from any of the group of the third group from the first group of the following it may be. The following elements are all selected elements. That is, the following elements are all may not be contained in the austenitic heat resistant alloy according to the present embodiment. In addition, only a portion may be contained.
[0042]
　More specifically, for example, to select only one group from the group from the first group to the third group may be selected at least one element from the group. In this case, it is not necessary to select all the elements belonging to the group selected. Further, from the first group to select multiple groups from the third group, it may select one or more elements from each group. In this case, it is not necessary to select all the elements belonging to the group selected.
[0043]
　Group 1 Ti: 0 ~ 0.5%
　elements belonging to the first group is Ti. Ti is to improve the creep strength of the alloy by precipitation hardening.
[0044]
　Ti: 0 ~ 0.5%
　of titanium (Ti), as well as Nb and V, to form fine carbides or carbonitrides in combination with carbon or nitrogen, which contributes to the improvement of creep strength. If containing Ti even a little this effect can be obtained. However, if excessively contained Ti, it precipitates resistance SIPH resistance and creep ductility is decreased becomes large quantity. Therefore, the upper limit is made 0.5%. The lower limit of the Ti content is preferably 0.01%, more preferably from 0.03%. The upper limit of the Ti content is preferably 0.45%, further preferably 0.4%.
[0045]
　Group 2 Co: 0 ~ 2%, Cu : 0 ~ 4%, Mo: 0 ~ 4%
　of elements belonging to the second group is a Co, Cu, and Mo. These elements, to improve the creep strength of the alloy.
[0046]
　Co: 0 ~ 2%
　Cobalt (Co) is an austenite forming element like the Ni, contributes to the improvement of creep strength by increasing the stability of the austenitic structure. If the Co-containing even a little, this effect can be obtained. However, Co is a very expensive element, a large amount of content leads to increase in cost. For this reason, the upper limit is 2%. The lower limit of the Co content is preferably 0.01%, more preferably from 0.03%. The upper limit of the Co content is preferably 1.8%, more preferably from 1.5%.
[0047]
　Cu: 0 ~ 4%
　copper (Cu), like Ni or Co, as well as the stability of the austenitic structure, which contributes to the improvement of creep strength by finely precipitated during use. If containing Cu even a little, this effect can be obtained. However, if excessively contained Cu, lowering the hot workability. For this reason, the upper limit is 4%. The lower limit of the Cu content is preferably 0.01%, more preferably from 0.03%. The upper limit of the Cu content is preferably 3.8%, more preferably from 3.5%.
[0048]
　Mo: 0 ~ 4%
　molybdenum (Mo) is, W similar, contributes to the improvement of creep strength and tensile strength at high temperature and solid solution in the matrix. If containing Mo even a little, this effect can be obtained. However, if contained excessively Mo, together with resistance to SIPH cracking resistance decreases becomes high deformation resistance in the grains, there is a case where the creep strength decreases. Further, Mo is an expensive element, a large amount of content leads to increase in cost. For this reason, the upper limit is 4%. The lower limit of the Mo content is preferably 0.01%, more preferably from 0.03%. The upper limit of the Mo content is preferably 3.8%, more preferably from 3.5%.
[0049]
　Group 3 Ca: 0 ~ 0.02%, Mg : 0 ~ 0.02%, REM: 0 ~ 0.2%
　elements belonging to the third group are Ca, Mg, and REM. These elements improve the hot workability of the alloy.
[0050]
　Ca: 0 ~ 0.02%
　of calcium (Ca) improves the hot workability during manufacturing. If containing Ca at all, the effect is obtained. However, if excessively contained Ca, oxygen bond significantly reduces the cleanliness of the alloy and, rather hot workability is deteriorated. Therefore, the upper limit is made 0.02%. The lower limit of the Ca content is preferably 0.0005%, more preferably 0.001%. The upper limit of the Ca content is preferably 0.01%, further preferably 0.005%.
[0051]
　Mg: 0 ~ 0.02%
　magnesium (Mg), as well as Ca, to improve the hot workability during manufacturing. If containing Mg even a little, this effect can be obtained. However, if excessively contained Mg, oxygen bond significantly reduces the cleanliness of the alloy and, rather hot workability is deteriorated. Therefore, the upper limit is made 0.02%. The lower limit of the Mg content is preferably 0.0005%, more preferably 0.001%. The upper limit of the Mg content is preferably 0.01%, further preferably 0.005%.
[0052]
　REM: 0 ~ 0.2%
　rare earth elements (REM), like Ca and Mg, to improve the hot workability during manufacturing. If containing the REM even a little, this effect can be obtained. However, if excessively contained REM, oxygen bond significantly reduces the cleanliness of the alloy and, rather hot workability is deteriorated. Therefore, the upper limit is made 0.2%. The lower limit of the REM content is preferably 0.0005%, more preferably 0.001%. The upper limit of the REM content is preferably 0.15%, more preferably 0.1%.
[0053]
　The "REM" Sc, is a generic name for a total of 17 elements Y and lanthanoid, and the content of REM refers to the total content of one or more elements of the REM. In addition, REM is generally contained in the misch metal. Thus for example, the addition of misch metal alloy, the content of REM can be set to be within the above range.
[0054]
　It should be noted that, among the "REM", Nd strong affinity for S and P, since it has particularly the effect of forming the sulfide and phosphide, decreases the welding liquation cracking susceptibility, it is more preferable to use it.
[0055]
　Organization
　grain size number: less than 7.0 No. 2.0 No. or
　austenitic heat resistant alloy according to the present embodiment, 7.0 No. 2.0 No. or more grain size number of crystal grain size is defined in ASTM E112 having a tissue is less than.
[0056]
　In a welded structure using austenitic heat resistant alloy according to the present embodiment, in order to impart sufficient anti SIPH cracking resistance in the weld heat affected zone, even when subjected to thermal cycle due to welding of the weld heat affected zone grain such that the diameter is not excessively coarse, the grain size of the tissue prior to welding, is required to be 2.0 number higher fines at grain size number defined in ASTM E112. However, when the crystal grain diameter is 7.0 number higher fines, creep strength can not be obtained necessary. Therefore, the 7.0 th less than 2.0 number than the crystal grain size.
[0057]
　Tissue having a crystal grain diameter mentioned above is obtained by heat treating the alloy of the chemical composition under appropriate conditions. This organization for example, after the alloy of the chemical composition was molded into a predetermined shape by hot working and cold working, to implement the solution heat treatment to water-cooling after holding at a temperature of 900 ~ 1250 ℃ 3 ~ 60 minutes It is achieved by. Higher the holding temperature of the solution heat treatment is high, also the longer the retention time, the crystal grain diameter increases (grain size number decreases). Solution heat treatment is more preferable to water cooling after holding at a temperature of 1120 - 1220 ° C. 3 - 45 minutes, 1140 ~ be cooled after and held for 3 to 30 minutes at a temperature of 1210 ° C. Further preferred.
[0058]
　It has been described above austenitic heat resistant alloy according to an embodiment of the present invention. According to this embodiment, excellent cracking resistance and high-temperature strength is obtained austenitic heat resistant alloy which is obtained stably.
Example
[0059]
　The following examples illustrate the present invention more specifically. The present invention is not limited to these examples.
[0060]
　The ingot was cast in a laboratory dissolved material A ~ J having the chemical compositions shown in Table 1, hot rolled forging and heat in a temperature range of 1000 ~ 1150 ° C., and a plate thickness of 20 mm. The plate was further cold rolled to a thickness of 16 mm. The solution heat treatment to water-cooling after holding for a predetermined time in the plate 1200 ° C. was performed. After solution heat treatment, the thickness of 14mm by machining, width 50 mm, was molded into a plate of length 100 mm. Further, this plate separately, and taking a sample for microstructure observation from the solution treatment to plates to measure the crystal grain size of the tissue in compliance with ASTM E112. Note that the material A, the retention time of the solution heat treatment was varied in the range of 3 to 30 minutes, to produce materials having different grain size.
[0061]
[Table 1]

[0062]
　[Weldable]
　along the longitudinal direction of the plate prepared above was subjected to beveling illustrated in FIG. Butt plate together subjected to beveling, the gas tungsten arc welding process, each die marks per each second joint, to produce a welded joint by performing a butt welding. Welding without using a filler material, heat input was 5 kJ / cm.
[0063]
　Among the obtained welded joint, over the entire length of Both joint weld line, what width is formed over the back bead 2 mm, was evaluated as "pass" weldability is as good. Weldable what was the portion back bead is not formed even a part of the second joint is determined to be FuKaoru as "impossible".
[0064]
　[Resistance to weld cracking resistance]
　The above welded joint welded only initial layer, a commercially available steel corresponding SM400B specified in JIS G 3106 (2008) (thickness 30 mm, width 200 mm, length 200 mm) above the, JIS Z detained welded four laps 3224 (2010) using a covered electrode ENi6625 provisions. Then, using the SNi6625 applicable Tiguwaiya specified in JIS Z 3334 (2011), with heat input 10 ~ 15 kJ / cm by performing the lamination welding in GMA by TIG welding, the welded joint by two joints per each cash marks It was produced.
[0065]
　While the weld joint of each die marks, were subjected to an aging heat treatment at 700 ° C. × 500 hours. The welded joint and aging the as welded from the five locations of the welded joint subjected observation surface a sample was taken to be the cross-section of the joint (weld bead section perpendicular). After the collected sample was mirror-polished, corrosion, and microscopic examination by an optical microscope to examine the existence of cracks in the weld heat affected zone. Five of the welded joint cracks in all samples was observed as "good", "good" weld joint cracking was observed in one sample, was judged as acceptable. The weld joint cracking was observed in two or more samples were determined as "impossible".
[0066]
　[Creep rupture strength]
　from the weld joint while welding pass in resistance to weld cracking resistance test, the weld metal were taken round bar creep rupture test specimen such that the center of the parallel portion. 700 ° C. target rupture time of the base material is about 1000 hours for creep rupture test under the conditions of 167 MPa. Broken base material and the rupture time over 90% of the rupture time of the base material (i.e., more than 900 hours) was made as a "pass".
[0067]
　[Evaluation Results]
　shows the performance evaluation results in Table 2. Table 2 also shows the grain size number of each die marks of austenitic heat resistant alloy.
[0068]
[Table 2]

[0069]
　Weld joint that Duif A-1 ~ A-4, B ~ D, and austenitic heat resistant alloy of the I and the base material is a chemical composition suitable, 2 initial grain size of the base material with grain size number. No. 0 or more was less than 7.0 number. These welded joints, the penetration bead in root pass weld is formed over the entire length, it had good weldability. Also, even though the thickness of the base material was relatively thicker and 14 mm, no cracking at weld heat affected zone even when subjected to aging heat treatment, and had excellent resistance to cracking. Furthermore, the creep rupture strength of high temperature was also sufficient.
[0070]
　Welded joint of austenitic heat resistant alloy Duif A-5 as a base material, cracking considered SIPH cracking after aging heat treatment occurs. This crystal grain size of the austenitic heat resistant alloy Duif A-5 is considered to be because the too coarse.
[0071]
　Welded joint a Duif A-6 austenitic heat resistant alloy of the base material, although had excellent cracking resistance, creep rupture time is below the target. This crystal grain size of the austenitic heat resistant alloy Duif A-6 is considered due to too fine.
[0072]
　Welded joint of austenitic heat resistant alloy of Duif E as the base material, in initial layer welding, some back bead is not formed. This is probably because the Sn content of austenitic heat resistant alloy Duif E is too small.
[0073]
　Welded joint of austenitic heat resistant alloy of Duif F as a base material, since containing a large amount of S but containing no Sn, back bead had been sufficiently formed. However, cracks that would SIPH cracking after aging heat treatment occurs.
[0074]
　Welded joint of austenitic heat resistant alloy of Duif G and base material, welding remains and after aging heat treatment, cracking considered respectively liquation cracking and SIPH cracking occurs. This is probably because the Sn content of austenitic heat resistant alloy Duif G is too large.
[0075]
　Welded joint of austenitic heat resistant alloy of Duif H as the base material, although was good welding resistance and weld cracking resistance, was not satisfied creep strength required. This is because the Ni content of the austenitic heat resistant alloy of Duif H was too low, presumably because the phase stability becomes unstable.
[0076]
　The austenitic heat resistant alloy Duif J was not satisfied creep strength required also welded joint to the base material. This is probably because the amount of V contained in the austenitic heat resistant alloy Duif J is below the lower limit.
Industrial Applicability
[0077]
　The present invention, as austenitic heat resistant alloy used as a high-temperature member of the main steam pipe and the high temperature reheat steam pipe and the like of the power boiler, can be suitably used.
The scope of the claims

WE CLAIM

[Claim 1]Chemical composition, in
　mass%, C:
　0.04 ~
　0.14%, Si: 0.05 ~ 1%, Mn: 0.5 ~
　2.5%, P: 0.03% or
　less, S: 0 less than%
　.001,
　Ni:
　23 ~
　32%, Cr: 20 ~ 25%, W: 1 ~ 5%,
　Nb: 0.1 ~ 0.6%, V: 0.1 ~
　0.6%, N:
　~
　0.3% 0.1, B: 0.0005 ~ 0.01%, Sn: 0.001
　~ 0.02%, Al: 0.03% or
　less, O: 0.02% or
　less, Ti: 0 　0.5%
　~,　Co: 　0 ~ 2%, Cu: 0 ~ 4%, Mo: 0 ~ 　4%, Ca: 0 ~ 0.02%, Mg: 0 ~ 　0.02%, REM: 0 ~ 0 .2%, 　the balance is Fe and impurities, 　grain size has a tissue is 7.0 number less than 2.0 number than the crystal grain size number defined in ASTM E112, austenitic heat resistant alloy.

[Claim 2]
　A heat-resistant, austenitic alloy according to claim 1,
　wherein the chemical composition, in mass%, containing one or more elements selected from any of the group of the first group of the following up to the third group , austenitic heat-resistant alloy.
　Group 1 Ti: 0.01 ~ 0.5%
　Group 2 Co: 0.01 ~ 2%, Cu : 0.01 ~ 4%, Mo: 0.01 ~ 4%
　Group 3 Ca: 0.0005 ~ 0.02%, Mg: 0.0005 ~ 0.02%, REM: 0.0005 ~ 0.2%
[Claim 3]
　Claim 1 or 2 comprising austenitic heat resistant alloy according to, welded structures.