Technical field
[0001]The present invention relates to an austenitic heat resistant alloy and a manufacturing method thereof.
[0002]Conventionally, in a thermal power boiler and chemical plants or the like is used in a high temperature environment, SUS304H, SUS316H, SUS321H, 18-8 austenitic stainless steels such as SUS347H, have been used as a device material.
[0003]However, in recent years, construction of new ultra-supercritical boiler with improved temperature and pressure of the steam for high efficiency have been developed throughout the world. Such use conditions of the device in a high temperature environment is remarkably severer, to the request performance materials used with it have become stricter. Then, in addition to the corrosion resistance at 18-8 austenitic stainless steels have been used conventionally, high temperature strength, especially creep rupture strength and has a situation where insufficient significantly.
[0004]
　In order to solve the above problem, until now various studies have been made. For example, in Patent Documents 1 to 4, good high corrosion resistant austenitic steels in high-temperature strength is disclosed. Further, Patent Document 5, austenitic stainless steel is disclosed in which excellent high-temperature strength and corrosion resistance. According to Patent Documents 1 to 5, to increase the Cr content to 20% or more, by incorporating the W and / or Mo, thereby improving the high temperature strength.
CITATION
Patent Document
[0005]
Patent Document 1: JP 61-179833 Patent Publication
Patent Document 2: JP 61-179834 Patent Publication
Patent Document 3: JP 61-179835 Patent Publication
Patent Document 4: JP 61-179836 JP
Patent Literature 5: JP 2004-3000 JP
Summary of the Invention
Problems that the Invention is to Solve
[0006]
　Incidentally, since a large structural members, such as a device material such as boilers and chemical plants for thermal power generation, which after hot rolling or hot forging are used to implement a final heat treatment without performing cold working, crystal grain size is relatively large. Therefore, usually, 0.2% proof stress and tensile strength at normal temperature is defined as a specification of the material, there is a problem of lower than those subjected to final heat treatment after cold working.
[0007]
　In addition, the structural members of a large, since the cooling rate in the heat treatment may vary greatly depending on the site, the amount of which contributes solid solution elements to strengthen as a precipitate at the time of use at high temperature varies depending on the site. Due to its, there is also a problem that variations in the creep rupture strength occurs. Therefore, the steel described in Patent Documents 1-5, it is difficult to apply to a structural member of a large.
[0008]
　The present invention solves the above problems, 0.2% proof stress and tensile strength at a sufficient room temperature as a structural member of a large, well, the austenitic heat resistant alloy and a manufacturing method thereof expressing creep rupture strength at high temperatures an object of the present invention is to provide.
Means for Solving the Problems
[0009]
　The present invention has been made to solve the above problems, and the gist of austenitic heat resistant alloy and a manufacturing method thereof below.
[0010]
　(1) the chemical composition of the alloy, in
　weight%, C: 0.02 ~
　0.12%, Si: 2.0% or
　less, Mn: 3.0% or
　less, P: 0.030% or
　less, S: 0.015% or
　less, Cr: less than 20.0% or more Pasento
　28.0, Ni: 55.0 Pasento by more than 35.0% or
　less, Co: 0
　～ 20.0%, W: 4.0 ～ 10
　% .0,
　Ti: 0.01 ~ 0.50%, Nb: 0.01
　~ 1.0%, Mo: less than
　0.50%, Cu: less than
　0.50%, Al: 0.30% or less,
　N: less than%
　0.10,
　Mg:
　0 ~ 0.05%, Ca: 0 ~ 0.05%,
　REM: 0 ~ 0.50%, V: 0 ~
　1.5%, B: 0 ~ 0.
　Pasento 01,
　Zr: 0
　～ 0.10 Pasento, Hf: 0 ～ 1.0 Pasento,
　Ta: 0 ～ 8.0 Pasento, Re: 0 ～ 8.0 Pasento,
　The balance is Fe and impurities,
　in the longitudinal direction perpendicular to the cross section of the alloy, the shortest distance from the center portion to the outer surface portion is not less 40mm or more,
　the austenite grain size number is -2.0 to 4 in the outer surface portion. is 0,
　Cr content present as a precipitate obtained by extraction residue analysis satisfies the following formula (i),
　mechanical properties at normal temperature satisfy the following (ii) expression and (iii) expression,
　austenitic heat-resistant alloy.
　Cr PB / Cr PS ≦ 10.0 · · · (i)
YS S / YS B ≦ 1.5 · · · 　(ii) TS S / TS B ≦ 1.2 · · · (iii) 　where in the formula the meaning of each symbol is as follows. 　Cr PB : center Cr content exists as a precipitate obtained by extraction residue analysis in section 　Cr PS : Cr content present as a precipitate obtained by extraction residue analysis in the outer surface portion

　YS B : 0.2% proof stress at the center
　YS S : 0.2% proof stress on the outer surface portion
　TS B : tensile at the center strength
　TS S : tensile at the outer surface portion Strength
[0011]
　(2) the chemical composition, by
　mass%,
　Mg: 0.0005
　~ 0.05%, Ca: 0.0005 ~ 0.05%, REM: 0.0005 ~
　0.50%, V: 0.02
　1.5%
~,
　0.0005 ~ 0.01%, Zr: 0.005 ~ 0.10%,
　Hf: 0.005 ~ 1.0%, Ta: 0.01 ~ 8.0%,
　and, Re: 0.01 ~ 8.0%,
　containing one or more selected from,
　austenitic heat resistant alloy according to the above (1).
[0012]
　(3) 10,000 h creep rupture strength at 700 ° C. of the longitudinal direction in the center portion is not less than 100 MPa,
　austenitic heat resistant alloy according to (1) or (2).
[0013]
　(4) above (1) or a steel ingot or slab having a chemical composition according to (2), a step of performing hot working,
　then heated to the heat treatment temperature T in the range of 1100 ~ 1250 ° C. (° C.) and, after holding 1000D / T ~ 1400D / T ( min), and a step of performing heat treatment cooled to,
　manufacturing method of austenitic heat resistant alloy.
　However, D is, in the longitudinal direction perpendicular to the cross section of the alloy is the maximum value of the linear distance between any other point on the arbitrary point and the outer edge of a outer edge of the cross section (mm).
[0014]
　(5) In the step of subjecting the hot working, subjecting the longitudinal direction substantially perpendicular to the processing one or more times,
　the manufacturing method of austenitic heat resistant alloy according to (4).
The invention's effect
[0015]
　Austenitic heat resistant alloy of the present invention has less variation in mechanical properties by site, also excellent in creep rupture strength at high temperatures.
DESCRIPTION OF THE INVENTION
[0016]
　It will be described in detail below each requirement of the present invention.
[0017]
　1. Chemical composition
　reasons for limiting each element are as follows. Incidentally, "%" for the content in the following description means "mass%".
[0018]
　C: 0.02 ~ 0.12%
　C, the carbides required high-temperature tensile strength austenitic heat-resistant alloy to form, is an essential element in order to hold the creep rupture strength. Therefore, C content should be 0.02% or more. However, when the content exceeds 0.12%, not only undissolved carbides occurs, increasing carbides Cr ductility deteriorates the mechanical properties and weldability, such as toughness. Therefore, C content is 0.02 to 0.12%. C content is preferably 0.05% or more is preferably not more than 0.10%.
[0019]
　Si: 2.0% or less
　Si is contained as a deoxidizing element. Further, Si is oxidation resistance, an element effective to enhance steam oxidation resistance and the like. Furthermore there is also an element for improving the fluidity in casting material. However, when the Si content exceeds 2.0%, sigma since promote the formation of intermetallic compounds of equality, it leads to a decrease in toughness and ductility deteriorates stability of tissue at high temperatures. In addition, it reduced weldability. Therefore, Si content is 2.0% or less. If the structural stability is important is, Si content is preferably 1.0% or less. Incidentally, if the deoxidation by another element is sufficiently secured, it is not necessary to particularly specify any lower limit for the Si content. However, the action deoxidation, oxidation resistance, when importance is attached to such steam oxidation resistance is, Si content is preferably to 0.05% or more, more preferably 0.10% or more.
[0020]
　Mn: 3.0% or less
　Mn, as well as has a deoxidizing effect similar to the Si, has the effect of improving the ductility at high temperatures to fix the S which is inevitably contained in the alloy as a sulfide. However, Mn content exceeds 3.0%, since promotes the precipitation of intermetallic compounds of σ phase etc., mechanical properties such as structural stability and high-temperature strength is deteriorated. Therefore, Mn content is 3.0% or less. Mn content is preferably 2.0% or less, and more preferably 1.5% or less. Incidentally, it is not necessary to provide a lower limit for the Mn content, when importance is attached to ductility improving effect at high temperature, the Mn content is preferably to 0.10% or more, to 0.20% or more more preferable.
[0021]
　P: 0.030% or less
　P is inevitably mixed into the alloy as an impurity, significantly reduces weldability and ductility at high temperatures. Therefore, the P content to 0.030% or less. P content should be as low as possible, it is preferable to be 0.020% or less, more preferably 0.015% or less.
[0022]
　S: 0.015% or less
　S is unavoidably mixed into the alloy as an impurity in the same manner as P, significantly reduces weldability and ductility at high temperatures. Therefore, the S content to 0.015% or less. When emphasizing hot workability, it is preferable to be S content is 0.010% or less, more preferably to 0.005% or less, further preferably 0.003% or less.
[0023]
　Cr: less 20.0% than 28.0%
　Cr oxidation resistance, steam oxidation resistance, is an important element which exhibits an excellent effect in improving corrosion resistance such as high-temperature corrosion resistance. However, if the content is less than 20.0%, these effects can not be obtained. On the other hand, increasing number Cr content, in particular, becomes 28.0% or more, leading to destabilization of the tissue due to σ phase precipitates, also deteriorates weldability. Therefore, Cr content is less than 28.0% or more 20.0%. Cr content is preferably at 21.0 percent or more, more preferably 22.0% or more. Further, it is preferable Cr content is less 26.0%, and more preferably not more than 25.0%.
[0024]
　Ni: 55.0% or less than 35.0%
　Ni is an element to stabilize the austenite structure is an important element to secure corrosion resistance. From balance with the Cr content of the, Ni content should be greater than 35.0%. Meanwhile, since the cost is increased when the Ni content is excessive, to 55.0% or less. Ni content is preferably 40.0% or more, and more preferably at least 42.0%. Further, it is preferable Ni content is less 50.0%, and more preferably not more than 48.0%.
[0025]
　Co: 0 ~ 20.0%
　but Co is not always necessary to be contained to stabilize the same austenitic organization and Ni, in order to contribute to the creep rupture strength increase may be contained instead of a part and Ni. However, the content thereof is effective exceeds 20.0% drops saturated economy. Therefore, Co content is 0 to 20.0%. Co content is preferably not more than 15.0%. Incidentally, when it is desired to obtain the effects described above, Co content is preferably 0.5% or more.
[0026]
　W: 4.0 ~ 10.0%
　W not only contributes to an improvement in creep rupture strength as a solid solution strengthening element in solid solution in the matrix, Fe 2 W-type Laves phase or Fe 7 W 6 of type It precipitates as μ phase, is an important element to significantly improve the creep rupture strength. However, W content is less than 4.0%, the effects can not be obtained. On the other hand, also contain W in an amount exceeding 10.0%, the strength improvement effect structural stability as well as saturated, also deteriorate ductility at high temperatures. Therefore, W content is 4.0 to 10.0%. W content is preferably 5.0% or more, more preferably 5.5% or more. Further, it is preferable W content is less 9.0%, and more preferably not more than 8.5%.
[0027]
　Ti: 0.01 ~
　0.50% Ti is an element having an effect of improving the creep rupture strength by forming carbo-nitrides. However, Ti content is sufficient effect can not be obtained is less than 0.01%, whereas, reduced ductility at a high temperature exceeding 0.50%. Therefore, Ti content is 0.01 to 0.50 percent. Ti content is preferably to 0.05 or more, more preferably 0.10% or more. Further, Ti content is preferably not more than 0.40%, and more preferably, 0.35% or less.
[0028]
　Nb: 0.01 ~
　1.0% Nb has the effect of improving the creep rupture strength by forming carbo-nitrides. However, Nb content is sufficient effect can not be obtained is less than 0.01%, while when it exceeds 1.0%, decrease the ductility at high temperatures. Therefore, Nb content is 0.01 to 1.0%. Nb content is preferably 0.10% or more. Further, it is preferable Nb content is less 0.90%, and more preferably 0.70%.
[0029]
　Mo: less than 0.50%
　conventional, Mo is a solid solution in the matrix, as an element contributing to the improvement of creep rupture strength as a solid solution strengthening element, has been considered an element having a function similar to that of a W. However, a study of the present inventors, when a Mo alloy including W and Cr in the amount mentioned above is contained in combination, it is possible to σ phase is precipitated when used for a long time, Therefore, creep rupture strength, it may cause a reduction in ductility and toughness were found. Therefore, Mo content is desirably as low as possible, to less than 0.50%. Incidentally, Mo content is preferably limited to less than 0.20%.
[0030]
　Cu: less than 0.50%
　in the present invention Cu lowers the melting point, decreases the hot workability and weldability. Therefore, Cu content is desirably as low as possible, to less than 0.50%. Incidentally, Cu content is preferably limited to less than 0.20%.
[0031]
　Al: 0.30% or less
　Al is an element to be contained as a deoxidizer for molten steel. However, if the Al content exceeds 0.30%, deterioration of the ductility at high temperatures. Therefore, Al content is at most 0.30%. Al content is preferably 0.25% or less, and more preferably not more than 0.20%. Incidentally, when it is desired to obtain the effects described above, Al content is preferably from 0.01% or more, more preferably 0.02% or more.
[0032]
　N: less than 0.10%
　N is an element having an effect of stabilizing the austenitic structure, in the normal melting process is an element unavoidably contained. However, in the present invention that are essential the inclusion of Ti, it is better as low as possible to avoid the consumption of Ti by TiN formation. However, it is difficult to extremely reduce the case of air dissolution, N content is less than 0.10%.
[0033]
　In the chemical composition of the austenitic heat resistant alloys of the present invention, the balance being Fe and impurities. Fe is preferably contained from 0.1 to 40.0%. Further, where the "impurities", when the industrial production of alloys, ores, raw material scraps, a component mixed by various factors of the manufacturing process, to the extent that the present invention does not adversely affect It means what is acceptable.
[0034]
　The austenitic heat resistant alloys of the present invention, further, Mg, Ca, REM, V, B, Zr, Hf, may contain one or more selected from Ta and Re.
[0035]
　Mg, Ca and REM have the effect of both improving the hot ductility by fixing S as sulfides. Therefore, when it is desired to obtain a better high-temperature ductility, one or more of these elements may be positively is contained in the following ranges.
[0036]
　Mg: 0.05% or less
　Mg, so has the effect of improving the hot ductility by fixing S, which inhibits the ductility at high temperatures as a sulfide, may contain Mg in order to obtain this effect. However, if the Mg content exceeds 0.05%, the cleanliness of drops, rather high temperature ductility is impaired. Accordingly, the amount of Mg in the case of containing is 0.05% or less. Mg content is more preferably not more than 0.02%, more preferably not more than 0.01%. On the other hand, in order to ensure the said effects, Mg content is preferably to 0.0005% or more, more preferably 0.001% or more.
[0037]
　Ca: 0.05% or less
　Ca is because it has an effect of improving the hot ductility by fixing S, which inhibits the ductility at high temperatures as a sulfide, may contain Ca in order to obtain this effect. However, when the Ca content exceeds 0.05%, the cleanliness of drops, rather high temperature ductility is impaired. Accordingly, the amount of Ca in the case of containing is 0.05% or less. Ca content is more preferably not more than 0.02%, more preferably not more than 0.01%. On the other hand, in order to ensure the said effects, Ca content is preferably to 0.0005% or more, more preferably 0.001% or more.
[0038]
　REM: 0.50% or less
　REM has the effect of improving the hot ductility by fixing S as sulfides. Further, the REM, Cr of the steel surface 2 O 3 to improve the adhesion of the protective coating, in particular, effect of improving the oxidation resistance at the time of repeated oxidation, and further, contributes to grain boundary strengthening, the creep rupture strength and also it has a function of improving the creep rupture ductility. However, if the REM content exceeds 0.50%, are impaired inclusions is increased workability and weldability, such as oxides. Accordingly, the amount of REM in the case of containing is 0.50% or less. REM content is more preferably not more than 0.30%, further preferably 0.15% or less. On the other hand, in order to ensure the said effects, REM content is preferably to 0.0005% or more, more preferably to 0.001% or more, further that 0.002% or more preferable.
[0039]
　Incidentally, REM is, Sc, refers to a total of 17 elements of Y and lanthanoids, the content of the REM means the total content of these elements.
[0040]
　The above Mg, the total content of Ca and REM may be 0.6% or less, but more preferably more preferably at most 0.4%, 0.2% or less.
[0041]
　V, B, Zr and Hf are all have a function of improving the high temperature strength and creep rupture strength. Therefore, when it is desired to obtain greater high-temperature strength and creep rupture strength, one or more of these elements may be positively is contained in the following ranges.
[0042]
　V: 1.5% or less
　V has the effect of improving the high temperature strength and creep rupture strength by forming carbo-nitrides. Therefore, it may contain a V in order to obtain these effects. However, when the V content exceeds 1.5%, it reduces the high temperature corrosion resistance, leading to further deterioration of ductility and toughness due to precipitation of a brittle phase. Accordingly, the amount of V in the case of containing is 1.5% or less. V content is more preferably 1.0% or less. On the other hand, in order to ensure the said effects, V content is preferably 0.02% or more, more preferably 0.04% or more.
[0043]
　B: 0.01% or less
　B has an effect present in carbide or in a matrix, not only to promote the miniaturization of carbide precipitated, improving the creep rupture strength by strengthening grain boundaries. However, when the B content exceeds 0.01%, the ductility at high temperatures also decreases the melting point decreases. Accordingly, the amount of B in the case of containing the 0.01% or less. B content is more preferably at 0.008% or less, still more preferably 0.006% or less. On the other hand, in order to ensure the said effects, B content is preferably to 0.0005% or more, more preferably to 0.001% or more, still it has to 0.0015% or more preferable.
[0044]
　Zr: 0.10% or less
　Zr, as well to promote the miniaturization of carbonitrides, is an element that improves the creep rupture strength as a grain boundary strengthening element. However, when the Zr content exceeds 0.10%, it decreases the ductility at high temperatures. Accordingly, the amount of Zr in the case of containing the at most 0.10%. Zr content is more preferably not more than 0.06%, more preferably not more than 0.05%. On the other hand, in order to ensure the said effects, Zr content is preferably 0.005% or more, more preferably 0.01% or more.
[0045]
　Hf: 1.0% or less
　and Hf, because it has a function of improving the contribution to the creep rupture strength to precipitation strengthening as a carbonitride may contain Hf, in order to obtain these effects. However, if the Hf content exceeds 1.0%, workability and weldability are impaired. Accordingly, the amount of Hf in the case of incorporating is 1.0% or less. Hf content is more preferably not more than 0.8%, more preferably 0.5% or less. On the other hand, in order to ensure the said effects, Hf content is preferably to 0.005% or more, more preferably from 0.01% or more, further that 0.02% or more preferable.
[0046]
　The above V, B, the total content of Zr and Hf is preferably not more than 2.6%, more preferably at most 1.8%.
[0047]
　Ta and Re have the solid solution strengthening effect by solid solution both a matrix austenite. Therefore, solid solution strengthening effect Therefore, when it is desired to obtain a higher high-temperature strength and creep rupture strength can be positively be contained either or both of these elements in the following ranges.
[0048]
　Ta: 8.0% or less
　Ta has an effect of improving the high temperature strength and creep rupture strength as a solid solution strengthening element to form a carbonitride. Therefore, it may be contained Ta in order to obtain these effects. However, if the Ta content exceeds 8.0%, workability and mechanical properties are impaired. Accordingly, the amount of Ta in the case of incorporating is 8.0% or less. Ta content is more preferably not more than 7.0%, more preferably 6.0% or less. On the other hand, in order to ensure the said effects, Ta content is preferably from 0.01% or more, more preferably to 0.1% or more, further that 0.5% or more preferable.
[0049]
　Re: 8.0% or less
　Re is because it has a function of improving the high temperature strength and creep rupture strength mainly as a solid solution strengthening element, may contain a Re to obtain these effects. However, if the Re content exceeds 8.0%, workability and mechanical properties are impaired. Accordingly, the amount of Re in the case of containing is 8.0% or less. Re content is more preferably not more than 7.0%, more preferably 6.0%. On the other hand, in order to ensure the said effects, Re content is preferably from 0.01% or more, more preferably to 0.1% or more, further that 0.5% or more preferable.
[0050]
　It is preferably the total content of the above-mentioned Ta and Re is less than 14.0%, and more preferably not more than 12.0 percent.
[0051]
　2. Grain size
　austenite grain size number in the outer surface: -2.0 to 4.0
　when the austenite grain size at the outer surface is too coarse, low 0.2% proof stress and tensile strength at room temperature, whereas, if too fine , it is impossible to hold a high creep rupture strength at high temperatures. Thus, the austenite grain size number at the outer surface portion is set to -2.0 to 4.0. In the production process of the Ni-based alloy, the heat treatment temperature and holding time and cooling method after hot working by appropriately adjusting, and the grain size number of the outer surface portion after the final heat treatment can be in the range of the .
[0052]
　3. Dimensions
　shortest distance from the center to the outer surface: 40 mm or more
　as described above, the structural members of a large, in addition to 0.2% proof stress and tensile strength at room temperature is low, variations in the creep rupture strength by site there is also a problem that may occur. However, austenitic heat resistant alloy according to the present invention, 0.2% proof stress and tensile strength at a sufficient room temperature as a structural member of a large, well, express creep rupture strength at high temperatures. That is, the effect of the present invention is remarkably exhibited with respect to members of thick.
[0053]
　Therefore, in the austenitic heat resistant alloys of the present invention, in the longitudinal direction and a section perpendicular to the shortest distance from the center portion to the outer surface portion or more 40 mm. In order to obtain the effect of the present invention more remarkably is preferably the shortest distance to the outer surface portion is 80mm or more from the center, more preferably 100mm or more. Here, the shortest distance from the center portion to the outer surface portion, for example, if the alloy is cylindrical, the cross section of radius (mm), and when a square pillar shape, the half of the short side of the cross-section length (mm) to become.
[0054]
　Incidentally, heat-resistant alloy according to the present invention, as described later, for example, obtained by subjecting a steel ingot or slab obtained by continuous casting or the like, a hot working such as hot forging or hot rolling . And generally the longitudinal direction of the heat-resistant alloy, in the case of using a steel ingot becomes a direction connecting the top portion and the bottom portion of the steel ingot, when using a cast slab, the length direction.
[0055]
　4. Extraction residue is present as a precipitate obtained by analyzing Cr amount
　Cr PB / Cr PS ≦ 10.0 · · · (i)
　where the meaning of each symbol in formula (i) are as follows.
　Cr PB : center Cr content exists as a precipitate obtained by extraction residue analysis in section
　Cr PS : Cr content exists as a precipitate obtained by extraction residue analysis in the outer surface portion
　in the manufacturing process of the alloy, the heat treatment after hot working grain boundaries or in grains precipitate undissolved of Cr having undergone (mainly carbide) occurs. In particular, since the cooling rate is slower than the outer surface portion at the center of the alloy tends to increase the amount of precipitates Cr. Therefore, Cr PB / Cr PS when the value of is more than 10.0, it becomes impossible to maintain a high creep rupture strength at high temperatures. On the other hand, Cr PB / Cr PS need not determine the lower limit of, it is preferable that the central portion is to be 1.0 or more because it tends to increase the amount of precipitates than the outer surface portion.
[0056]
　Incidentally, extraction residue analysis will be made by the following procedure. First, it is taken from the central portion and the outer surface portion in the longitudinal direction perpendicular to the cross section of the alloy samples, test pieces for measuring the Cr precipitates. It said each after having determined the surface area of the test piece of 10% acetylacetone-1% tetramethylammonium chloride - 20 mA / cm in methanol solution 2 completely electrolyzed only the base metal of the alloy sample in the electrolytic conditions. Then, the solution was filtered through a 0.2μm filter after electrolysis to extract precipitates as a residue. Thereafter, the extraction residue from decomposed acid, by analyzing using an inductively coupled plasma emission spectrometer (ICP-AES), the content of Cr contained as a Cr precipitates undissolved (mass%) measured, Cr on the basis of the measured values PB / Cr PS determine the value of.
[0057]
　5. Mechanical Properties
　YS S / YS B ≦ 1.5 · · ·
　(ii) TS S / TS B ≦ 1.2 · · · (iii)
　where the meaning of each symbol in the formula is as follows.
　YS B : 0.2% proof stress at the center
　YS S : 0.2% proof stress on the outer surface portion
　TS B : tensile at the center strength
　TS S : tensile strength at the outer surface portion
　in the structural member of a large, cooling of the heat treatment due to the speed depends on the site, there is a tendency that large variations in mechanical properties of each region. In large structural members, in its central portion and the outer surface portion, the 0.2% proof stress and tensile strength at room temperature is greatly different, a problem that does not meet the specification by site occurs.
[0058]
　Accordingly, the austenitic heat resistant alloy according to the present invention, the mechanical properties at room temperature is assumed to satisfy the above (ii) expression and (iii) expression. Incidentally, it is not necessary to lower limit defined respectively, towards the mechanical properties of the heart from that tends to inferior mechanical properties of the outer surface, and (ii) expression and (iii) 1.0 or more in the expression of both it is preferable.
[0059]
　0.2% yield strength and tensile strength, from the central portion and the outer surface portion of the alloy, parallel to the longitudinal direction, cut out by machining a is 40mm round bar tensile test specimen of length of the parallel portion, a tensile test at room temperature seek by the practice. In addition, tensile test to be carried out in conformity with JIS Z 2241 (2011).
[0060]
　6. Creep rupture strength
　austenitic heat resistant alloy of the present invention, for use in high temperature environments, high temperature strength, in particular, high creep rupture strength is required. Therefore, heat-resistant alloy of the present invention, in its central portion, it is preferable to 10,000 hours creep rupture strength in the longitudinal direction of 700 ° C. is not less than 100 MPa.
[0061]
　Creep rupture strength is determined by the following method. First, cut from the center of the alloy, parallel to the longitudinal direction, the diameter 6mm described JIS Z 2241 (2011), a round bar creep rupture test piece gauge length 30mm by machining. Then, 700 ° C., a creep rupture test conducted at 750 ° C., 800 ° C. in air, 700 ° C. using Larson-Miller parameter method, obtaining the creep rupture strength of 10,000 hours. Furthermore, the creep rupture test shall be done in conformity with JIS Z 2271 (2010).
[0062]
　7. Production method
　austenitic heat resistant alloy of the present invention can be produced in steel ingot or slab having a chemical composition described above, by performing hot working. In the above-described hot working step, the longitudinal direction of the final shape of the alloy, processing is performed to match the longitudinal direction of the steel ingot or slab as a material. Hot working, may be performed only in the longitudinal direction, giving a higher degree of processing, to a more homogeneous structure, with respect to the longitudinal direction substantially perpendicular, one or more times a hot working it may be subjected. Further, after the hot working, it may be further subjected to a hot working different methods such as extrusion hot as necessary.
[0063]
　In the production of austenitic heat resistant alloy of the present invention, in order after the above step, to suppress variations in the metal structure and mechanical properties of each region, to retain the high creep rupture strength, final heat treatment described below the applied.
[0064]
　First, the alloy after hot working, heating to the heat treatment temperature T in the range of 1100 ~ 1250 ℃ (℃), within its scope, 1000D / T ~ 1400D / T (min) holds. Here, D is, for example, if the alloy is cylindrical, the alloy having a diameter (mm), and when a square pillar, a distance of the diagonal (mm). That D is in the longitudinal direction perpendicular to the cross section of the alloy is the maximum value of the linear distance between any other point on the arbitrary point and the outer edge of a outer edge of the cross section (mm).
[0065]
　When the heat treatment temperature is lower than 1100 ° C., creep rupture strength chromium carbides undissolved increases is reduced. On the other hand, if it exceeds 1250 ° C., the ductility is reduced by the grain boundaries or significantly coarsened crystal grains or molten. The heat treatment temperature is more preferably set to a 1150 ° C. or higher, more desirably to 1230 ° C. or less. Further, the holding time is less than 1000D / T (min), undissolved chromium carbide increases Cr central portion PB / Cr PS becomes out of the range defined in the present invention. On the other hand, the crystal grain coarsening of the outer surface portion exceeds 1400D / T (min), the austenite grain size number is outside the range specified in the present invention.
[0066]
　After heating and holding is immediately water-cooled alloy. When the cooling rate is slow, because the particular undissolved Cr precipitate in the grain boundaries or in grains is large amount occurs in the center of the alloy, there may not satisfy the above formula (i).
[0067]
　The following examples illustrate the present invention more specifically, the present invention is not limited to these examples.
Example
[0068]
　An alloy having a chemical composition shown in Table 1 were melted in a high frequency vacuum melting furnace, an outer diameter of 550 mm, the weight is a steel ingot of 3t.
[0069]
[Table 1]

[0070]
　The resulting steel ingot, and processed into a cylindrical shape having an outer diameter of 120 ~ 480 mm by hot forging, the final heat treatment performed under the conditions shown in Table 2, to obtain an alloy member samples. Note that the alloys 1, 2 and 4 after the longitudinal hot forging, prior to the final heat treatment, carried out forging in the longitudinal direction substantially perpendicular went to then further longitudinally final hot forging .
[0071]
[Table 2]

[0072]
　For each sample, we collect specimens for structure observation from the outer surface portion, after polishing a longitudinal section with emery paper and buff, was light microscopy corroded with mixed acid. Grain size number of the observation plane is determined according to the determination method according to the intersection line (grain size) defined in JIS G 0551 (2013).
[0073]
　Then, from the central portion and the outer surface portion in the longitudinal direction perpendicular to the cross section of each sample were taken test specimens for measuring the Cr precipitates. It said each after having determined the surface area of the test piece of 10% acetylacetone-1% tetramethylammonium chloride - 20 mA / cm in methanol solution 2 was completely electrolyte only matrix of the alloy sample in the electrolytic conditions. Then, the solution was filtered through a 0.2μm filter after the electrolytic was extracted precipitates as a residue. Thereafter, the extraction residue from decomposed acid by ICP-AES measurement, the content of Cr contained as a Cr precipitates undissolved (mass%) was measured, Cr on the basis of the measured values PB / Cr PS was determined the value of.
[0074]
　Further, from the central portion and the outer surface portion of each sample, parallel to the longitudinal direction, cut out by machining a tensile test specimen of 40mm length of the parallel portion, and a tensile test at room temperature, 0.2% proof stress and tensile to determine the strength. Furthermore, from the center of each sample, parallel to the longitudinal direction, the length of the parallel portion was cut out by machining the creep rupture test piece 30 mm. Then, 700 ° C., 750 ° C., a creep rupture test conducted at 800 ° C. in air, 700 ° C. using Larson-Miller parameter method to determine the creep rupture strength of 10,000 hours.
[0075]
　The results are summarized in Table 3.
[0076]
[table 3]

[0077]
　Alloy A and B are substantially equal alloy 1 and the chemical composition is obtained by the same final shape by hot forging. However, the retention time of the heat treatment is outside the range of manufacturing conditions defined in the present invention. In the due to its grain size number of the outer surface portion is outside the specified range of the present invention for alloy A, YS S / YS B and TS S / TS B value of has become outside the specified range of the present invention resulted in variation of mechanical properties is increased by site. Furthermore, the creep rupture strength for the alloy B has become outside the specified range of the present invention, it was a significantly lower results as compared with the alloys 1.
[0078]
　Alloy C, D and E are substantially equal alloy 2 and the chemical composition is obtained by the same final shape by hot forging. Alloy C is for less than the specified range of heat treatment temperature present invention, the crystal grain size number and Cr of the outer surface portion PB / Cr PS and values has become out of the range defined in the present invention, as compared with the alloy 2 creep rupture strength has become significantly lower results.
[0079]
　Alloy D is the higher than the specified range of heat treatment temperature present invention, the grain size number of the outer surface portion, YS S / YS B and TS S / TS B is the value of has a specified range of the present invention, creep rupture strength as compared with the alloy 2 becomes significantly lower results.
[0080]
　Further, the alloy E is air-cooled rather than cooling method at the time of the final heat treatment water cooling, due to the cooling rate was significantly slower, Cr PB / Cr PS value of becomes outside a specified range of the present invention, as a result, creep rupture strength as compared with the alloy 2 was significantly lower. Meanwhile, the alloy 1-9 satisfying all the requirements of the present invention, the variation of mechanical properties is small, creep rupture strength was also favorable.
Industrial Applicability
[0081]
　Austenitic heat resistant alloy of the present invention has less variation in mechanical properties by site, also excellent in creep rupture strength at high temperatures. Therefore, the austenitic heat resistant alloy of the present invention can be suitably used as a large structural members such as thermal power generation boilers and chemical plants used in high temperature environments.

The scope of the claims
[Requested item 1]Chemical composition of the alloy, in
　weight%, C: 0.02 ~
　0.12%, Si: 2.0% or
　less, Mn: 3.0% or
　less, P: 0.030% or
　less, S: 0.015 % or
　less, Cr: less than 20.0% or more%
　28.0, Ni: 55.0% by more than 35.0% or
　less, Co:
　0 ～ 20.0%, W: 4.0 ～ 10.0% ,
　Ti:
　0.01 ~ 0.50%, Nb: 0.01
　~ 1.0%, Mo: less than
　0.50%, Cu: less than%
　0.50, Al: 0.30% or
　less, N: 0 less than%
　.10,
　Mg:
　0 ~ 0.05%, Ca: 0 ~
　0.05%, REM: 0 ~ 0.50%,
　V: 0 ~ 1.5%, B: 0 ~ 0.01%,
　Zr:
　0
　～ 0.10 Pasento, Hf: 0 ～ 1.0 Pasento,
　Ta: 0 ～ 8.0 Pasento, Re: 0 ～ 8.0 Pasento,
　The balance is Fe and impurities,
　in the longitudinal direction perpendicular to the cross section of the alloy, the shortest distance from the center portion to the outer surface portion is not less 40mm or more,
　the austenite grain size number is -2.0 to 4 in the outer surface portion. is 0,
　Cr content present as a precipitate obtained by extraction residue analysis satisfies the following formula (i),
　mechanical properties at normal temperature satisfy the following (ii) expression and (iii) expression,
　austenitic heat-resistant alloy.
　Cr PB / Cr PS ≦ 10.0 · · · (i)
YS S / YS B ≦ 1.5 · · · 　(ii) TS S / TS B ≦ 1.2 · · · (iii) 　where in the formula the meaning of each symbol is as follows. 　Cr PB : center Cr content exists as a precipitate obtained by extraction residue analysis in section 　Cr PS : Cr content present as a precipitate obtained by extraction residue analysis in the outer surface portion

　YS B : 0.2% proof stress at the center
　YS S : 0.2% proof stress on the outer surface portion
　TS B : tensile at the center strength
　TS S : tensile at the outer surface portion Strength
[Requested item 2]
　The chemical composition, by
　mass%,
　Mg: 0.0005
　~ 0.05%, Ca: 0.0005 ~ 0.05%, REM: 0.0005
　~ 0.50%, V: 0.02 ~ 1.
　% 5,
　B:
　0.0005 ~ 0.01%, Zr: 0.005 ~ 0.10%,
　Hf: 0.005 ~ 1.0%, Ta: 0.01 ~ 8.0%,
　and, Re : 0.01 to 8.0%,
　containing one or more selected from,
　austenitic heat resistant alloy according to claim 1.
[Requested item 3]
　It said longitudinal 700 10,000 hours creep rupture strength at ℃ is more than 100 MPa, in the center
　austenitic heat resistant alloy according to claim 1 or claim 2.
[Requested item 4]
　A steel ingot or slab having a chemical composition according to claim 1 or claim 2, the step of performing hot working,
　then heated to the heat treatment temperature T in the range of 1100 ~ 1250 ℃ (℃), 1000D / after T ~ 1400D / T (min) retention, and a step of performing heat treatment cooled to,
　manufacturing method of austenitic heat resistant alloy.
　However, D is, in the longitudinal direction perpendicular to the cross section of the alloy is the maximum value of the linear distance between any other point on the arbitrary point and the outer edge of a outer edge of the cross section (mm).
[Requested item 5]
In the step of subjecting the hot working, subjecting the longitudinal direction substantially perpendicular to the processing one or more times,
　the method of manufacturing austenitic heat resistant alloy according to claim 4.