0001]The present invention relates to a heat-resistant alloy and a manufacturing method thereof, and more particularly, relates to an austenitic heat resistant alloy and a manufacturing method thereof.
BACKGROUND
[0002]Conventionally, in equipment such as boilers and chemical plants used in high temperature environments, as heat-resistant steel, 18-8 stainless steel is used. 18-8 stainless steel is an austenite stainless steel containing about 18% of Cr and 8% of Ni, for example, SUS304H mentioned JIS standard, a SUS316H, SUS321H, and SUS347H like.
[0003]
　Recently, significantly severer use conditions of equipment in a high temperature environment, has been required a higher creep strength than 18-8 stainless steel. Recently Further, in boilers for thermal power generation, advanced ultra-supercritical pressure power plan to improve the conventional steam temperature was about 600 ° C. above 700 ° C. has been promoted. Also in chemical plants, in order to increase the operating efficiency, increase the operating temperature is planned. The steel used in these high-temperature environment is obtained even excellent corrosion resistance with high creep strength.
[0004]
　Refractory materials with enhanced corrosion resistance, for example, has been proposed Japanese Patent 02-115348 discloses (Patent Document 1) and JP-A-07-316751 (Patent Document 2). Since high Al content in these heat-resistant alloys, in use, at high temperatures, Al to the surface 2 O 3 film is formed. The coating obtained has high corrosion resistance.
[0005]
　However, the heat-resistant alloy disclosed in Patent Documents 1 and 2 described above, it may creep strength is low at 700 ° C. or higher high-temperature environments.
[0006]
　As the heat-resistant material having a high creep strength at 700 ° C. or more high temperature environment containing Ni and Co, gamma 'phase (Ni as strengthening phase 3 heat-resistant alloy containing Al) it has been developed. Such heat-resistant alloy, for example, a Alloy617,263, and 740 like the Ni-based alloy. However, alloy materials of these heat-resistant alloys are expensive. Furthermore, because of low workability, manufacturing cost becomes high.
[0007]
　Accordingly, less expensive than the Ni-based alloy, and excellent heat-resistant alloy in creep strength is proposed in JP-A-2014-43621 (Patent Document 3) and JP 2013-227644 (Patent Document 4) ing.
[0008]
　Patent Document 3 austenitic heat resistant alloys disclosed in, in mass%, C: less than 0.02%, Si: 2% or less, Mn: 2% or less, Cr: 15 ~ 26%, Ni: 20 ~ 35% , Al: 0.3% or less, P: 0.04% or less, S: (including 0%) 3.0% or less,: together with 0.05% or less, Ti: 0.01% or less and N V: 3.0% or less (including 0%), Nb: less than 2.3% (including 0%) and Ta: include one or more selected from 2.0% or less (including 0%) and f1 = 2Ti + 2V + Nb + (1/2) f1 represented by Ta satisfies 1.5-6.0, the balance has a chemical composition consisting of Fe and impurities. The austenitic heat resistant alloy has excellent high-temperature strength and toughness by precipitation hardening of Laves phase and gamma 'phase, and is described in Patent Document 3.
[0009]
　Patent Document 4 disclosed austenitic heat resistant alloy containing, in mass%, C: less than 0.02%, Si: 0.01 ~ 2%, Mn: 2% or less, Cr: 20% or more and less than 28%, Ni : 50% greater than 35% or less, W: 2.0 ~ 7.0%, Mo: (including 0%) less than 2.5%, Nb: less than 2.5% (including 0%), Ti : less than 3.0% (including 0%), Al: 0.3% or less, P: 0.04% or less, S: 0.01% or less and N: 0.05% or less, the balance being Fe and impurities, further, f1 = 1 / 2W + f1 represented by Mo is 1.0-5.0, at f2 = 1 / 2W + Mo + Nb + f2 represented by 2Ti 2.0 to 8.0 and f3 = Nb + 2Ti f3 represented has a chemical composition of 0.5 to 5.0. The austenitic heat resistant alloy has excellent high-temperature strength and toughness by precipitation hardening of Laves phase and gamma 'phase, and is described in Patent Document 4.
CITATION
Patent Document
[0010]
Patent Document 1: JP-A-02-115348 Publication
Patent Document 2: JP-A 07-316751 JP-
Patent Document 3: JP 2014-43621 JP
Patent Document 4: JP 2013-227644 JP
Summary of the Invention
Problems that the Invention is to Solve
[0011]
　However, as the heat-resistant alloy of Patent Documents 3 and 4, when the alloy using strengthening mechanism by Laves phase and gamma 'phase, there is a case where the creep strength and toughness after aging long lowered.
[0012]
　An object of the present invention, even in a high temperature environment, and to provide a austenitic heat resistant alloy having high creep strength and high toughness.
Means for Solving the Problems
[0013]
　Austenitic heat resistant alloy according to the present embodiment, by mass%, C: less than 0.03 ~ 0.25%, Si: 0.01 ~ 2.0%, Mn: 2.0% or less, Cr: 10 ~ 30 % less, Ni: 25 ultra ~ 45%, Al: 2.5 and less than ~ 4.5%, Nb: 0.2 ~ 3.5%, N: 0.025% or less, Ti: 0 ~ 0.2 % less, W: 0 ~ 6%, Mo: 0 ~ 4%, Zr: 0 ~ 0.1%, B: 0 ~ 0.01%, Cu: 0 ~ 5%, rare earth elements: 0-0.1 % Ca: 0% to 0.05%, and, Mg: 0 contains ~ 0.05%, the balance being Fe and impurities, each P and S in the impurities, P: 0.04% or less, and, S: has the following chemical composition 0.01%. In tissue, the total volume ratio of the equivalent circle diameter of 6μm or more precipitates is 5% or less. Here, the precipitates, for example, carbides, nitrides, and NiAl and alpha-Cr.
Effect of the invention
[0014]
　Austenitic heat resistant alloy according to the present embodiment, even in a high temperature environment, has a long high-temperature strength, and excellent toughness.
DESCRIPTION OF THE INVENTION
[0015]
　The present inventors have, 700 ° C. or more high-temperature environment (hereinafter, simply referred to as a high temperature environment) performs research and investigated creep strength and toughness of the austenitic heat resistant alloys in, and obtained the following findings.
[0016]
　As described above, Laves phase and, Ni 3 heat-resistant alloy containing Al or the like of the gamma 'phase has a high creep strength in high temperature environments. However, these precipitation phases, to coarsen when used for a long time under a high temperature environment, creep strength and toughness of the heat-resistant alloy is lowered.
[0017]
　On the other hand, in use heat-resistant alloy in a high temperature environment, carbides, nitrides, NiAl, precipitates if finely dispersed precipitates such as alpha-Cr, can be maintained for a long time high even using creep strength and high toughness. These precipitates, to coat the grain boundaries, increasing the grain boundary strength. Moreover, these precipitates if precipitated in the grains increases the deformation resistance of the heat-resistant alloy, the creep strength is increased.
[0018]
　In order to increase the creep strength and toughness by fine precipitates above, it controls the tissue in front of the heat resistant alloy used as follows.
[0019]
　Circle equivalent diameter in the amount of more precipitate 6μm Limit
　The solidification structure after casting heat-resistant alloy, carbides, nitrides, NiAl, precipitates such as alpha-Cr (hereinafter, referred to simply deposit) is It exists. These precipitates, the solute elements present between the dendrite is generated in the liquid phase was concentrated. These precipitates usually have a coarse shape, unevenly distributed into the tissues. Therefore, toughness of the heat-resistant alloy is lowered.
[0020]
　Furthermore, even when performing these precipitates solution treatment, hardly dissolved, easily remains in coarse state. If these precipitates if remaining coarse in heat resistant alloys, fine precipitates during use at high temperature environment is not easily formed. Therefore, the total volume fraction of coarse precipitates in the heat-resistant alloy is preferably as small as possible.
[0021]
　In tissues of heat resistant alloys, equivalent circle diameter 6μm or more precipitate (hereinafter, referred to as coarse precipitates) if is less than 5% total volume ratio, while using heat-resistant alloy in a high temperature environment, a sufficient amount of can precipitate fine precipitates, it is possible to obtain high creep strength and toughness.
[0022]
　The total volume fraction of coarse precipitates in tissue to below 5%, the C content in the heat-resistant alloy is less than 0.25%. Furthermore, the reduction of area during hot forging is 30% or more. In this case, coarse precipitates are uniformly dispersed by hot forging. Therefore, during the solution treatment in a subsequent step, the precipitates can be dissolved to a total volume fraction of coarse precipitates is 5% or less.
[0023]
　Austenitic heat resistant alloy according to the present embodiment has been completed based on the above findings, by mass%, C: less than 0.03 ~ 0.25%, Si: 0.01 ~ 2.0%, Mn: 2.0 % or less, Cr: less than 10 ~ 30%, Ni: 25 ultra - 45%, Al: 2.5 super ~ less than 4.5%, Nb: 0.2 ~ 3.5%, N: 0.025% or less , Ti: 0 ~ less than 0.2%, W: 0 ~ 6%, Mo: 0 ~ 4%, Zr: 0 ~ 0.1%, B: 0 ~ 0.01%, Cu: 0 ~ 5%, rare earth element: 0 ~ 0.1%, Ca: 0 ~ 0.05%, and, Mg: 0 contains ~ 0.05%, the balance being Fe and impurities, P and S in the impurities are each P: 0.04% or less, and, S: has the following chemical composition 0.01%. In tissue, the total volume ratio of the equivalent circle diameter of 6μm or more precipitates is 5% or less.
[0024]
　The chemical composition, by mass%, Ti: less than 0.005 ~ 0.2%, W: 0.005 ~ 6%, Mo: 0.005 ~ 4%, Zr: 0.0005 ~ 0.1%, and B: it may contain one or more members selected from the group consisting of from 0.0005 to 0.01 percent.
[0025]
　The chemical composition, in mass%, Cu: 0.05 ~ 5%, and rare earth elements: may contain one or more selected from the group consisting of 0.0005 to 0.1 percent.
[0026]
　The chemical composition, by mass%, Ca: from 0.0005 to 0.05 percent, and Mg: may contain one or more selected from the group consisting of from 0.0005 to 0.05 percent.
[0027]
　Production method of the above-mentioned austenitic heat resistant alloy for cast material having the chemical composition described above, the step of performing a hot forging at a cross-section reduction rate of 30% or more, heat to the material after hot forging and a step of manufacturing the intermediate material was carried out while processing, and a step of performing a solution treatment at 1100 ~ 1250 ° C. for the intermediate member.
[0028]
　It described in detail below austenitic heat resistant alloy of the present embodiment. "%" Related elements, unless otherwise specified, it means mass%.
[0029]
　[Chemical composition]
　austenitic heat resistant alloy according to the present embodiment, for example, an alloy tube. The chemical composition of the austenitic heat resistant alloy contains the following elements.
[0030]
　C: less than from 0.03 to 0.25%
　carbon (C) forms a carbide to increase the creep strength. Specifically, C is, in use in a high temperature environment, combined with alloying elements to form fine carbides in the grain boundaries and within the grains. Fine carbides increases the deformation resistance, increasing the creep strength. If the C content is too low, the effect can not be obtained. On the other hand, if the C content is too high, to form a large number of coarse eutectic carbides in the solidified structure during subsequent casting of heat resistant alloys. Eutectic carbides to remain in the tissues remain coarse after solution treatment, to reduce the toughness of the heat-resistant alloy. Furthermore, if coarse eutectic carbides remaining, fine carbides are hardly precipitated during use in a high temperature environment, the creep strength decreases. Therefore, C content is less than 0.03 to 0.25 percent. The preferable lower limit of C content is 0.05%, more preferably 0.08%. The preferable upper limit of C content is 0.23%, and more preferably 0.20%.
[0031]
　Si: 0.01 ~ 2.0%
　silicon (Si) is a deoxidizing the heat-resistant alloy. Si further enhance the corrosion resistance of the heat-resistant alloy (oxidation resistance and steam oxidation resistance). Si is an element which is inevitably contained, if the deoxidation can be carried out sufficiently by other elements, the content of Si may be less as possible. On the other hand, if the Si content is too high, hot workability is deteriorated. Therefore, Si content is 0.01-2.0%. A preferable lower limit of Si content is 0.02%, more preferably from 0.03%. The preferable upper limit of the Si content is 1.0%.
[0032]
　Mn: 2.0% or less
　manganese (Mn) is inevitably contained. Mn forms MnS by combining with S contained in the heat resistant alloy, increasing the hot workability of the heat-resistant alloy. However, if the Mn content is too high, heat resistant alloy is too hard, hot workability and weldability is decreased. Therefore, Mn content is 2.0% or less. The preferable lower limit of the Mn content is 0.1%, more preferably 0.2%. The preferable upper limit of the Mn content is 1.2%.
[0033]
　Cr: less than 10-30%
　chromium (Cr) enhances the corrosion resistance of heat resistant alloys in high temperature environments (oxidation resistance, steam oxidation resistance, etc.). Cr is further during use at a high temperature environment, finely precipitates as alpha-Cr, increasing the creep strength. If the Cr content is too low, these effects can not be obtained. On the other hand, if the Cr content is too high, the creep strength decreases the stability of the tissue is lowered. Therefore, Cr content is less than 10-30%. A preferable lower limit of the Cr content is 11%, more preferably 12%. The preferable upper limit of the Cr content is 28%, even more preferably 26%.
[0034]
　Ni: 25 super 45%
　nickel (Ni) is to stabilize the austenite. Ni further enhance the corrosion resistance of the heat-resistant alloy. If the Ni content is too low, these effects can not be obtained. On the other hand, if the Ni content is too high, not only these effects are saturated, hot workability is deteriorated. Further if the Ni content is too high, the raw material cost becomes high. Therefore, Ni content is 25 super 45%. A preferable lower limit of Ni content is 26%, more preferably 28%. The preferable upper limit of the Ni content is 44%, more preferably 42%.
[0035]
　Al: 2.5 super ~ 4.5% less than
　aluminum (Al), during use at a high temperature environment, combined with Ni to form a fine NiAl, increasing the creep strength. Al further enhance the corrosion resistance at 1000 ° C. or more high temperature environments. If the Al content is too low, these effects can not be obtained. On the other hand, if the Al content is too high, structural stability is lowered, the strength is lowered. Therefore, Al content is less than 2.5 super to 4.5%. A preferable lower limit of the Al content is 2.55%, more preferably 2.6%. The preferable upper limit of Al content is 4.4%, more preferably from 4.2%. In austenitic heat resistant alloy according to the present invention, Al content is meant the total amount of Al contained in the steel material.
[0036]
　Nb: 0.2 ~ 3.5%
　niobium (Nb), the Laves phase and Ni as a precipitation strengthening phase 3 formed of Nb-phase, and precipitation strengthening the grain boundaries and in crystal grains, the creep strength of the heat-resisting alloy increased. If the Nb content is too low, not the effect. On the other hand, if the Nb content is too high, Laves phase and Ni 3 Nb-phase is excessively generated, it decreases toughness and hot workability of the alloy. Further if the Nb content is too high, also decreases long toughness after aging. Therefore, Nb content is 0.2 to 3.5%. The preferable lower limit of Nb content is 0.35%, more preferably 0.5%. The preferable upper limit of the Nb content is less than 3.2%, more preferably from 3.0%.
[0037]
　N: 0.025% or less
　Nitrogen (N) stabilizes the austenite, the conventional melting method is inevitably contained. Further, N represents, in use in a high temperature environment, combined with alloying elements to form fine nitride at grain boundaries and within the grains. Fine nitride enhances the deformation resistance, increasing the creep strength. However, if the N content is too high, it forms coarse nitrides remain without forming a solid solution even after the solution treatment decreases the toughness of the alloy. Therefore, N content is 0.025% or less. The upper limit of the preferred N content is 0.02%, more preferably 0.01%.
[0038]
　P: 0.04% or less
　Phosphorus (P) is an impurity. P decreases the weldability and hot workability of the heat-resistant alloy. Accordingly, P content is 0.04% or less. The preferable upper limit of the P content is 0.03%. P content is preferably as small as possible.
[0039]
　S: 0.01% or less
　of sulfur (S) is an impurity. S decreases weldability and hot workability of the heat-resistant alloy. Thus, S content is 0.01% or less. The preferable upper limit of the S content is 0.008%. S content is preferably as small as possible.
[0040]
　The remainder of the chemical composition of the austenitic heat resistant alloy of the present embodiment is composed of Fe and impurities. Here, the impurity, when the industrial production of austenitic heat resistant alloy, ore as a raw material, there is to be mixed etc. Scrap or manufacturing environment, the allowable range of the present invention does not adversely affect It is the means something.
[0041]
　[For any element]
　The chemical composition of the above-mentioned austenitic heat resistant alloy addition, in place of part of Fe, containing Ti, W, Mo, one or more members selected from the group consisting of Zr and B it may be. All of these elements are optional elements, increasing the creep strength.
[0042]
　Ti: 0 ~ 0.2% less
　titanium (Ti) is optional element and may not be contained. If contained, Laves phase and Ni as a precipitation strengthening phase 3 to form a Ti phase, increasing the creep strength through precipitation strengthening. However, if the Ti content is too high, Laves phase and Ni 3 Ti phase is excessively generated, high temperature ductility and hot workability is deteriorated. Further if the Ti content is too high, the toughness after aging long lowered. Therefore, Ti content is 0 to less than 0.2%. A preferable lower limit of the Ti content is 0.005%, more preferably 0.01%. The preferable upper limit of the Ti content is 0.15%, more preferably 0.1%.
[0043]
　W: 0 ~ 6%
　tungsten (W) are optional elements may not be contained. If contained, as a solid solution in the austenite parent phase (matrix), increase the creep strength through solution hardening. W further to form a crystal grain boundary and crystal grains in the Laves phase, increasing the creep strength through precipitation strengthening. However, if the W content is too high, high temperature ductility Laves phase is excessively generated, lowers hot workability, and the toughness. Therefore, W content is 0-6%. The preferable lower limit of the W content is 0.005%, more preferably 0.01%. Preferred upper limit of the content of W is 5.5%, more preferably 5%.
[0044]
　Mo: 0 ~ 4%
　molybdenum (Mo) is an optional element and may not be contained. If contained, as a solid solution in the austenite parent phase to increase the creep strength through solution hardening. Mo is further to form a crystal grain boundary and crystal grains in the Laves phase, increasing the creep strength through precipitation strengthening. However, if Mo content is too high, the high temperature ductility Laves phase is excessively generated, lowers hot workability, and the toughness. Therefore, Mo content is 0-4%. A preferable lower limit of Mo content is 0.005%, more preferably 0.01%. Preferred upper limit of the content of Mo is 3.5%, more preferably 3%.
[0045]
　Zr: 0 ~ 0.1%
　zirconium (Zr) is optional element and may not be contained. If contained, Zr enhances the creep strength by grain boundary strengthening. However, if the Zr content is too high, weldability and hot workability of the heat-resistant alloy is lowered. Accordingly, Zr content is 0 to 0.1%. A preferred lower limit of Zr is 0.0005%, more preferably 0.001%. The preferable upper limit of the Zr content is 0.06%.
[0046]
　B: 0 ~ 0.01%
　boron (B) is optional element and may not be contained. If contained, improve the creep strength by grain boundary strengthening. However, if the B content is too high, weldability decreases. Therefore, B content is 0 to 0.01%. A preferred lower limit of B is 0.0005%, more preferably 0.001%. The preferable upper limit of B content is 0.005%.
[0047]
　Moreover the chemical composition of the above-mentioned austenitic heat resistant alloy, instead of a part of Fe, may contain one or more selected from the group consisting of Cu and rare earth elements. All of these elements are optional elements, enhances the corrosion resistance of the heat-resistant alloy.
[0048]
　Cu: 0 ~ 5%
　copper (Cu) is an optional element and may not be contained. If contained, Al near the surface 2 O 3 to promote the formation of a film, increase the corrosion resistance of the heat-resistant alloy. However, if the Cu content is too high, the effect is not only saturated, high-temperature ductility is decreased. Therefore, Cu content is 0-5%. The preferable lower limit of Cu content is 0.05%, more preferably 0.1%. The preferable upper limit of Cu content is 4.8%, more preferably from 4.5%.
[0049]
　Rare earth element: 0 to 0.1%
　rare earth element (REM) are optional elements may not be contained. If it contained, fixing S as sulfides, improving the hot workability. REM further to form an oxide, corrosion resistance, creep strength, and improve the creep ductility. However, if REM content is too high, the number of inclusions such as oxides, reduces the hot workability and weldability, manufacturing cost is increased. Therefore, REM content is 0 to 0.1%. The preferable lower limit of the REM content is 0.0005%, more preferably 0.001%. The preferable upper limit of the REM content is 0.09%, more preferably from 0.08%.
[0050]
　In the present specification, REM is, Sc, is a generic name for a total of 17 elements Y and lanthanoids. REM content, if REM is contained in the heat-resistant alloy is one of these elements, means the content of the element. If REM is contained in the heat-resistant alloy is 2 or more, REM content means the total content of these elements. For REM, which are generally contained in the misch metal. Thus, for example, be added in the form of misch metal, REM content may be contained so that the above range.
[0051]
　Moreover the chemical composition of the above-mentioned austenitic heat resistant alloy, instead of a part of Fe, may contain one or more selected from the group consisting of Ca and Mg. All of these elements are optional elements, enhancing the hot workability of heat resistant alloys.
[0052]
　Ca: 0 ~ 0.05%
　of calcium (Ca) is an arbitrary element, it may not be contained. If it contained, fixing S as sulfides, improving the hot workability. On the other hand, if the Ca content is too high, toughness, ductility and detergency is lowered. Therefore, Ca content is from 0 to 0.05%. The preferable lower limit of Ca is 0.0005%. The preferable upper limit of the Ca content is 0.01%.
[0053]
　Mg: 0 ~ 0.05%
　magnesium (Mg) is an arbitrary element, it may not be contained. If it contained, fixing S as sulfides, improving the hot workability of the heat-resistant alloy. On the other hand, if the Ca content is too high, toughness, ductility and detergency is lowered. Therefore, Ca content is from 0 to 0.05%. The preferable lower limit of Ca is 0.0005%. The preferable upper limit of the Ca content is 0.01%.
[0054]
　[Total volume fraction of a circle equivalent diameter of 6μm or more precipitate (coarse precipitates): 5% or less]
　As described above, the austenitic heat resistant alloy of the present embodiment, the fine precipitates during use at high temperature environment It precipitated, increasing the creep strength to maintain the toughness. And it precipitates example carbides, nitrides, and NiAl and alpha-Cr. If precipitates are coarse, creep strength and toughness is reduced. Therefore, during the heat-resistant alloy prior to use, it is preferred coarse precipitates is small. In tissues of heat resistant alloys, if the total volume fraction of 6μm or more precipitates in circle equivalent diameter (coarse precipitates) is 5% or less, fine precipitates during use at high temperature environment is precipitated, creep strength and toughness increases. The preferable upper limit of the total volume fraction of coarse precipitates is 4%, more preferably 3%. Here, the circle equivalent diameter means a diameter ([mu] m) in a case where the area of the precipitate was converted to the area of the circle.
[0055]
　[Measurement method of total volume fraction of coarse precipitates in Organization
　total volume fraction of coarse precipitates in the tissue of the austenitic heat resistant alloy of the present embodiment can be measured by the following method.
[0056]
　Collecting a specimen of a cross section perpendicular from the surface of the heat-resistant alloy material. For example, austenitic heat resistant alloy material may alloy tube, collecting specimens from the thick central portion of the cross section perpendicular to the axial direction.
[0057]
　After polishing the cross section of the collected specimens (observation surface), etching the observation surface a mixed acid solution of hydrochloric acid and nitric acid. By photographing an arbitrary 10 fields of the observation plane with a scanning electron microscope (SEM) to create an SEM image (backscattered electron image). Each field is set to 100 [mu] m × 100 [mu] m.
[0058]
　In SEM images, precipitate and a matrix, contrast respectively different. Seeking area identified precipitates due to the difference in contrast, calculating the circle equivalent diameter of each precipitate. After the calculation, the equivalent circle diameter is a particular 6μm or more precipitates (coarse precipitates).
[0059]
　Determining the total area of ​​the identified coarse precipitates. Of the total area of ​​coarse precipitates, determining the ratio (%) with respect to the viewing area. Since the area ratio of the precipitates is equivalent to the volume ratio, the ratio of the determined coarse precipitates, is defined as the total volume fraction of coarse precipitates (%).
[0060]
　The shape of austenitic heat resistant alloy according to the present embodiment is not particularly limited. Austenitic heat resistant alloy, for example, an alloy tube. Austenitic heat resistant alloy tube is used as a pipe or a chemical plant reaction tube boiler. Austenitic heat resistant alloy, sheet, rod, or may be a wire.
[0061]
　[Manufacturing Method]
　An example of a manufacturing method of austenitic heat resistant alloy of the present embodiment, a method for manufacturing the alloy tube. The manufacturing method of this embodiment includes a step of preparing a raw material of the aforementioned chemical composition (preparation step), the step of the prepared material to hot forging and (hot forging step), with respect to hot forged material comprising the step of producing the intermediate member by carrying out the hot working (hot working step), and a step of performing a solution heat treatment to the intermediate material (solution heat treatment step) Te. Hereinafter, the respective steps will be described.
[0062]
　[Preparation Step]
　to produce a molten steel having the chemical composition described above. Against the molten steel, carrying out the known degassing treatment if necessary. With molten steel, to produce a material by casting. Material may be a ingot by ingot-making method, a slab or bloom by continuous casting, may be a slab of billets or the like.
[0063]
　[Hot forging process]
　to produce a cylindrical material by carrying out hot forging respect manufactured material. In hot forging, the reduction of area defined by formula (1) more than 30%.
　Reduction of area = 100 (cross-sectional area of the material after hot working / cross-sectional area of the hot forging material before) × 100 (%) (1 )
[0064]
　As described above, the tissue material produced by casting, there are precipitates such as eutectic carbide. These precipitates are coarse, which become 6μm or a circle equivalent diameter there are many. Such coarse precipitates hardly dissolved in the solution treatment in a later step.
[0065]
　If reduction of area in the hot forging step is 30% or more, coarse precipitates during hot forging is destroyed, the size is reduced. Therefore, it precipitates easily dissolved in solution heat treatment in a later step. As a result, the volume ratio of the equivalent circle diameter of 6μm or more precipitates is 5% or less.
[0066]
　Preferred reduction of area is 35% or more, more preferably 40% or more. The upper limit of the reduction of area is not particularly limited, in consideration of productivity, it is 90%.
[0067]
　[Hot working step]
　to implement a hot working against hot forged material (cylindrical material), to produce an alloy element tube is an intermediate member. For example, to form a through hole in the cylindrical element center by machining. The hot extrusion was performed on a cylindrical material formed with a through-hole, to produce the alloy raw tube. The cylinder material piercing to be produced alloy base pipe (intermediate member). The cold working may be performed for the intermediate material after hot working. Cold working example, a cold drawing or the like. Through the above steps, to produce an intermediate material.
[0068]
　[Solution heat treatment step]
　implementing the solution heat treatment for the production, intermediate material. The solution heat treatment, a solid solution of the precipitates in the intermediate member.
[0069]
　Heat treatment temperature in the solution heat treatment is 1100 ~ 1250 ° C.. Is less than the heat treatment temperature is 1100 ° C., the precipitate is not sufficiently dissolved, resulting in a volume ratio of coarse precipitates is more than 5%. On the other hand, if the heat treatment temperature is too high, the austenite grains are coarsened, productivity is lowered.
[0070]
　If 1100 ~ 1250 ° C. heat treatment temperature, the precipitate is sufficiently dissolved, the total volume fraction of coarse precipitates is 5% or less.
[0071]
　Solution heat treatment time is not particularly limited. The solution heat treatment time is, for example, 1 minute to 1 hour.
[0072]
　The intermediate material after solution heat treatment, the removal of scale formed on the surface may be subjected to a pickling treatment purposes. The pickling for example, a mixed acid solution of nitric acid and hydrochloric acid. Pickling time, for example, is 30 to 60 minutes.
[0073]
　Moreover, the intermediate material after pickling, the shot material blasted may be carried out using. For example, to implement the blasting treatment the alloy tube surface. In this case, to form a processed layer on the surface, increase the corrosion resistance (oxidation resistance, etc.).
[0074]
　By the above manufacturing method, austenitic heat resistant alloy of the present embodiment is manufactured. In the above described method for manufacturing the alloy tube. However, the same manufacturing method (preparation step, hot forging step, hot working process, solution heat treatment step), the plate material, rod material, may be manufactured wire or the like.
Example
[0075]
　[Production Method]
　The molten steel having the chemical compositions shown in Table 1, were prepared using a vacuum melting furnace.
[0076]
[Table 1]

[0077]
　Using the above molten steel was produced with an outer diameter of 120mm cylindrical ingot (30kg). By carrying out hot forging in cross-section reduction rate shown in Table 2 with respect to the ingot to produce a rectangular-shaped material. By carrying out hot rolling and cold rolling with respect to a rectangular-shaped material to produce a plate-like intermediate member having a thickness of 1.5 mm. The solution treatment for holding 10 minutes at the heat treatment temperature shown in Table 2 were performed on the intermediate member. After holding for 10 minutes, the intermediate material was water-cooled, to produce an alloy sheet.
[0078]
[Table 2]

[0079]
　[Creep rupture test]
　from manufactured alloy sheet, to prepare a test piece. Specimens were parallel to taken in the longitudinal direction (rolling direction) from the thickness center of the alloy sheet. Specimens are round specimens, the diameter of the parallel portion is 6 mm, between the gauge length was 30 mm. By using the test piece was subjected to creep rupture test. Creep rupture test was carried out in an air atmosphere of 700 ~ 800 ℃. Based on the obtained breaking strength, Larson - by Miller parameter method, 1.0 × 10 at 700 ° C. 4 obtained creep strength at hours (MPa).
[0080]
　[Charpy impact test]
　with respect to the manufactured alloy sheet, after performing the aging treatment of holding 8000 hours at 700 ° C., cooled with water. From the thickness direction central portion of the plate after aging treatment were taken V-notch Charpy impact test piece specified in JIS Z2242 (2005). Notch was made parallel to the longitudinal direction of the alloy sheet. Width of the specimen 5 mm, height 10 mm, length is 55 mm, notch depth was 2 mm. At 0 ° C., by carrying out the Charpy impact test according to JIS Z2242 (2005), impact value (J / cm 2 was determined).
[0081]
　[Test Results]
　Table 2 shows the test results.
[0082]
　Referring to Table 2, the chemical composition of Test No. 1 Test No. 11 are suitable, the volume ratio of coarse precipitates is 5% or less. As a result, the creep strength of not less than 140 MPa, showed excellent creep strength. Further, the Charpy impact value 40 J / cm 2 or more, exhibited excellent toughness even after prolonged aging.
[0083]
　On the other hand, in Test No. 12, C-content is too high. Therefore, the volume ratio of coarse precipitates exceeds 5%. As a result, the creep strength is less than 140 MPa, the Charpy impact value 40 J / cm 2 was less than.
[0084]
　In Test No. 13, Al content was too low. Therefore, creep strength was less than 140 MPa. It is considered that the amount of precipitation of NiAl was small.
[0085]
　In Test No. 14, Al content is too high. Therefore, creep strength was less than 140 MPa. Since the Al content is too high, the tissue is not stable, is considered to creep strength was low.
[0086]
　In Test No. 15, Cr content is too low. Therefore, creep strength was less than 140 MPa. It is considered to be because the amount of precipitation of α-Cr was small.
[0087]
　In Test No. 16, Cr content is too high. Therefore, creep strength was less than 140 MPa. Since the Cr content is too high, the tissue is not stable, is considered to creep strength was low.
[0088]
　In Test No. 17, reduction of area during hot forging was less than 30%. Therefore, the total volume fraction of coarse precipitates exceeds 5%. As a result, the creep strength is less than 140 MPa, the Charpy impact value 40 J / cm 2 was less than.
[0089]
　In Test No. 18, solution heat treatment temperature is less than 1100 ° C.. Therefore, the total volume fraction of coarse precipitates exceeds 5%. As a result, the creep rupture strength of less than 140 MPa, the Charpy impact value 40 J / cm 2 was less than.
[0090]
　In Test No. 19, Nb content was too high. Therefore, the Charpy impact value 40 J / cm 2 was less than.
[0091]
　In Test No. 20, Nb content was too low. Therefore, creep strength was less than 140 MPa.
[0092]
　It has been described an embodiment of the present invention. However, the above-described embodiment is merely an example for implementing the present invention. Accordingly, the present invention is not limited to the embodiments described above, it can be implemented by changing the above-described embodiments without departing from the scope and spirit thereof as appropriate.
Industrial Applicability
[0093]
　Austenitic heat resistant alloy of the present invention can be widely used in 700 ° C. or higher high-temperature environments. In particular, it is particularly suitable for use as the alloy tubes in the power boiler exposed to 700 ° C. to temperatures higher than, the chemical industrial plant or the like.

WE CLAIM

By
　mass%, C: less than - 0.25%
　0.03, Si: 0.01
　~ 2.0%, Mn: 2.0% or
　less, Cr: less than ~ 30%
　10, Ni: 25 ultra - 45% ,
　Al: 2.5 and less than 4.5%
　~, Nb: 0.2
　~ 3.5%, N: 0.025% or
　less, Ti: 0 ~ less than% 0.2,
　W: 0 ~ 6%,
　Mo:
　0 ~
　4%, Zr: 0 ~ 0.1%, B: 0 ~
　0.01%, Cu: 0 ~ 5%,
　rare earth
　element: 0 ~ 0.1%, Ca: 0 ~ 0.05%
　and, Mg: 0 contains ~ 0.05%,
　the balance being Fe and impurities,
　P and S in the impurities
　respectively, P: 0.04% or less, and
　S: 0.01% or less of the chemical It has a composition,
　in a tissue, and wherein the total volume fraction of the circle equivalent diameter of 6μm or more of precipitates is 5% or less, austenitic heat resistant alloy.
[Requested item 2]
　A heat-resistant, austenitic alloy according to claim 1,
　wherein the chemical
　composition, Ti: less than ~ 0.2%
　0.005,
　W: 0.005 ~ 6%, Mo: 0.005 ~
　4%, Zr : from 0.0005 to 0.1%,
　and, B: characterized in that it contains one or more members selected from the group consisting of 0.0005 to 0.01%, the austenitic heat resistant alloy.
[Requested item 3]
　A heat-resistant, austenitic alloy according to claim 1 or claim 2,
　wherein the chemical
　composition, Cu: 0.05 ~ 5%, and,
　selected from the group consisting of from .0005 to 0.1%: a rare earth element It characterized in that it contains at least one member, austenitic heat resistant alloy.
[Requested item 4]
　A heat-resistant, austenitic alloy according to any one of claims 1 to 3,
　wherein the chemical
　composition, Ca: 0.0005 - 0.05%,
　and, Mg: 0.0005 ~ 0.05 characterized in that it contains at least one selected from the group consisting of%, austenitic heat resistant alloy.
[Requested item 5]
　Against material having a chemical composition according to any one of claims 1 to 4, the step of performing a hot forging at a cross-section reduction rate of 30% or more,
　with respect to hot forged the material a step of producing an intermediate member by carrying out the hot working, Te
　, characterized in that it comprises a step of performing a solution treatment at 1100 ~ 1250 ° C. relative to the intermediate member, a manufacturing method of austenitic heat resistant alloy .