JP5756935B2 - Austenitic stainless steel excellent in intergranular corrosion resistance and stress corrosion cracking resistance and method for producing the same - Google Patents

Description

  The present invention has excellent intergranular corrosion resistance and stress corrosion resistance, for example, in a boiling heat transfer surface corrosion environment of a highly concentrated nitric acid solution containing a highly oxidative metal ion, or in a high temperature and high pressure underwater environment subjected to neutron irradiation. The present invention relates to an austenitic stainless steel exhibiting crackability and a method for producing the same.

  Conventionally, it is known that austenitic stainless steel exhibits corrosion resistance by forming a passive film on the surface in an environment containing a highly oxidizing acid such as nitric acid. For this reason, for example, austenitic stainless steel is widely used as a structural material for nitric acid production plants.

  Specifically, austenitic stainless steel is used to recover nitric acid in a reprocessing plant for spent nuclear fuel by dissolving the spent nuclear fuel with high-concentration nitric acid or by evaporating the solution in the dissolved layer. It is used in acid recovery evaporators.

However, in this case, the environment of the austenitic stainless steel, the spent nuclear fuel or al, ruthenium ion (Ru ^3+), and oxidizing by metal ions such as chromium ions (Cr ⁶⁺⁾ is mixed in nitric acid Becomes even stronger. For this reason, there was a problem of being subject to corrosion accompanied by intergranular corrosion.

  Since an austenitic stainless steel material is used in a high-temperature nitric acid environment containing highly oxidizable metal ions, the following measures are known. First, the carbon content of the austenitic stainless steel is made as low as possible in order to suppress the aggregation of the Cr-deficient layer that is the cause of intergranular corrosion. If necessary, a small amount of Nb is added to the austenitic stainless steel. Further, solution heat treatment is performed on the austenitic stainless steel.

  Moreover, as what further improves the corrosion resistance of austenitic stainless steel, there exists a thing of patent documents 1-7, for example.

  In Patent Document 1, C: 0.005 wt% or less, Si: 0.4 wt% or less, Mn: 0.1 to 12 wt%, P: 0.005 wt% or less, Ni: 7 to 28 wt%, Cr: 15 An austenitic stainless steel containing 30 wt%, N: 0.06 to 0.30 wt%, and the balance being substantially made of Fe is disclosed. This austenitic stainless steel limits the grain boundary segregation of P by limiting the amount of P contained, and improves the intergranular corrosion resistance of the austenitic stainless steel.

  In Patent Document 2, C: 0.015 wt% or less, Si: 0.5 wt% or less, Mn: 2 wt% or less, P: 0.015 wt% or less, Ni: 10-22 wt%, Cr: 15-30 wt% %, Al: 0.01 wt% or less, Ca: 0.002 to 0.010 wt% or less, and the austenitic stainless steel whose balance is substantially made of Fe is disclosed. This austenitic stainless steel exhibits excellent work flow corrosion resistance by limiting the amounts of Si, P, and Al contained and adding an appropriate amount of Ca. It also exhibits excellent hot workability and excellent corrosion resistance in high temperature nitric acid.

  In Patent Document 3, C: 0.02 wt% or less, Si: 0.5 wt% or less, Mn: 0.5 wt% or less, P: 0.03 wt% or less, S: 0.002 wt% or less, Ni: An austenitic stainless steel containing 10 to 16 wt%, Cr: 16 to 20 wt%, Mo: 2.0 to 3.0 wt%, N: 0.06 to 0.15 wt%, and the balance being substantially Fe. It is disclosed. This austenitic stainless steel satisfies Ni (wt%) + 60 N (wt%) − 4 Mo (wt%) ≧ 7, and further contains Ca and / or Ce by weight percent alone or in total of 2 × S (wt% ) -0.03 wt%, it exhibits excellent corrosion resistance against tunnel-like corrosion.

  Patent Document 4 discloses a method for producing an austenitic stainless steel containing an oxidizing metal ion and excellent in high temperature nitric acid corrosion resistance. Specifically, heat treatment is performed by heating at a temperature in the range of 650 ° C. to 950 ° C. for 1 minute or longer. Next, when the temperature of the heat treatment is from 650 ° C. to less than 850 ° C., it is cooled to room temperature by quenching or allowing to cool. On the other hand, when the temperature of the heat treatment is 850 ° C. to 950 ° C., it is cooled to room temperature by rapid cooling. Thereby, this austenitic stainless steel exhibits excellent high-temperature nitric acid corrosion resistance.

  Patent Document 5 discloses an austenitic stainless steel satisfying B (wtppm) × d (μm) ≦ 700 when the B content is 30 wtppm or less and the austenite particle diameter is d, and a method for producing the same. Has been. This austenitic stainless steel has excellent intergranular corrosion resistance and intergranular stress corrosion cracking by heating above a predetermined temperature as a function of B (wtppm) × d (μm) and performing a solution treatment. It exhibits sex.

  In Patent Document 6, C: 0.02 wt% or less, Si: 0.8 wt% or less, Mn: 2.0 wt% or less, P: 0.04 wt% or less, S: 0.03 wt% or less, Ni: Austenitic stainless steel containing 6 to 22 wt%, Cr: 13 to 27 wt%, Al: 0.1 wt% or less, Cu: 0.3 wt% or less, N: 0.1 wt% or less, and the balance being substantially Fe. Steel is disclosed. This austenitic stainless steel satisfies 1.5Ni (wt%) + Mn + 65 (C + N) -5Si (wt%)-2.5 ≦ 52-2.3 (Ni + Mn) -200 (C + N), etc. Is 5 wtppm or less, and the total of one or more elements selected from Ti, Nb, V, Hf, and Ta is 1.0 wt% or less, so that cold processing or deformation Later, it exhibits excellent nitric acid corrosion resistance.

  Patent Document 7 discloses a method for producing austenitic stainless steel, which is to create clean grain boundaries. Specifically, the austenitic stainless steel is cold worked with a workability of 40% or more. Next, the obtained cold-worked material is maintained at a temperature lower than the recrystallization temperature and a temperature at which carbide precipitates, and is recrystallized in a temperature range in which grain boundary segregation such as P does not occur. As a result, this austenitic stainless steel exhibits excellent corrosion resistance even in a corrosive environment of a nitric acid solution containing an oxidizing agent.

JP 59-222563 A Japanese Patent Laid-Open No. 06-306548 Japanese Patent Application Laid-Open No. 07-090497 JP 07-238315 A Japanese Patent Laid-Open No. 07-113146 Japanese Patent Laid-Open No. 08-013095 JP-A-60-1000062

  In addition, when austenitic stainless steel is used in a thermosiphon-type acid recovery evaporator that attempts to recover nitric acid from the solution by heating and boiling the nitric acid solution in the heat transfer tube from the outside of the heat transfer tube, Generation of highly oxidative ions accompanying evaporation and thermal decomposition and dissolution by reduction reaction occur simultaneously. For this reason, the corrosive environment of austenitic stainless steel is boiling heat transfer surface corrosion. As a result, the corrosion rate increases more than in the case of immersion corrosion at the same metal surface temperature, and the corrosion rate shows a gradual increase trend over time. For this reason, the above prior art is not a fundamental solution.

  Specifically, the content of P in the prior art 1 is limited, or the addition of Ca or Ce having a strong binding force with S in the prior art 2 and 3, the formation of MnS is suppressed, and the MnS has progressed in the rolling direction. It is disclosed that it is possible to suppress the occurrence of tunnel-like corrosion. However, since segregation of S to the grain boundary is suppressed, it is only effective for suppressing the intergranular corrosion, and there is no specific description. Prior arts 4 and 5 consider only economic efficiency, and it is unlikely that stable nitric acid corrosion resistance can be obtained with this technique.

  Prior art 6 discloses a knowledge similar to the present invention, in which the B content is 5 wtppm or less. However, the test method is a mild condition of immersion in only 65% boiling nitric acid for 48 hours. In addition, it is not a superiority or inferiority selection based on an evaluation test that simulates a corrosive environment containing highly oxidizing metal ions used in a spent nuclear fuel reprocessing plant. In addition, the amount of B is as low as a normal impurity element, and the amount of B contained in the austenitic stainless steel is the same level in the examples and comparative examples in the embodiment. There is no finding about the need to limit the amount.

  Prior art 7 discloses knowledge similar to the present invention regarding thermomechanical processing, but the amount of C is not sufficiently defined. For this reason, after cold working, Cr-based carbides that cause intergranular corrosion are once dispersed uniformly, but a Cr-deficient layer around Cr-based carbides that precipitates in large amounts causes corrosion promotion. Further, although the heat treatment has no effect on detoxification of grain boundary segregation impurity elements such as P, S, N, O, etc., the amount is not sufficiently defined and no measures are taken. It is unlikely that the desired corrosion resistance can be obtained.

In addition, when austenitic stainless steel is used in a light water reactor core under a high-temperature high-pressure underwater environment subjected to neutron irradiation, sensitivity to intergranular stress corrosion cracking (IGSCC) increases due to long-term irradiation. For example, a solution-treated austenitic stainless steel in a solid solution state has resistance to intergranular stress corrosion cracking outside the core without neutron irradiation, but a high level of irradiation within the core, particularly at a neutron irradiation amount of 1. Such resistance is lost when irradiated with about 0 × 10 ²¹ n / cm ² or more. Such cracks are called irradiation-induced stress corrosion cracking (IASCC) and have recently become a problem in older light water reactors.

  As a technique for solving this problem, for example, Patent Documents 8 and 9 disclose a method of adjusting the content of constituent elements of austenitic stainless steel. Further, in Patent Document 10, C is limited to 0.03 wt% or less in order to suppress precipitation of carbides at grain boundaries that cause grain boundary type stress corrosion cracking, and N having a high solid solubility is set to 0. The chemical composition of Ni-Cr austenitic stainless steel contained in a content of 15 wt% or less is set, and the amount of carbide per unit grain boundary is reduced by heating in the temperature range of 1100 to 1300 ° C in the production of the steel. A steel that reduces the Cr deficiency in the vicinity of the grain boundary and disperses the Cr deficient region and a method for manufacturing the same are disclosed. However, these inventions attempt to prevent intergranular stress corrosion cracking by adjusting the components, but since the impurities that cause intergranular corrosion are not reduced together with the Cr-deficient layer, stress corrosion cracking that occurs in an irradiation environment is not achieved. Cannot be solved essentially.

Patent Document 11 includes C: 0.005 to 0.08 wt% or less, Mn: 0.3 wt% or less, Si + P + S: 0.2 wt% or less, Ni: 25 to 40 wt%, Cr: 25 to 40 wt%, In an austenitic stainless steel having a composition such as Mo + W: 5.0 wt% or less, Nb + Ta: 0.3 wt% or less, Ti: 0.3 wt% or less, B: 0.001 wt% or less, etc., at a temperature range of 1000 to 1150 ° C. Solution treatment, further cold working up to 30%, and then heat treatment up to 100 hours in a temperature range of 600 to 750 ° C. to receive neutron irradiation of at least 1 × 10 ²² n / cm ² . Even after, excellent resistance to stress corrosion cracking in high-temperature high-pressure water or high-temperature high-pressure oxygen-saturated water at 270 to 350 ° C./70 to 160 atmospheres, from room temperature to 400 ° C. The average expansion coefficient of the high Ni austenitic steel having excellent neutron deterioration lies in the range of 15~19 × 10 -6 ^{/ K} techniques are disclosed.

  However, it is preferable that P, S, Si, Nb, Ta, Ti and B are all small, and Nb + Ta and Ti are defined as impurity levels or less when used as a deoxidizer, and are resistant to stress corrosion cracking. It was not actively adjusted for improvement. Further, Mn and B are the minimum values practically possible with the current steelmaking technology, and the amount of B is specified to be 0.001 wt% or less, but the minimum value of the amount of B in the examples leading to the invention is 0.0003 wt%, the amount of B is not sufficiently reduced, and since the amount of C, which is the most important component for degrading the stress corrosion cracking resistance, is not sufficient, the stress corrosion cracking resistance is always good. Cannot be obtained.

  In Patent Document 12, C: 0.05 or less, Si: 1.0 to 4.0 wt%, Mn: 0.3 wt% or less, Ni: 6 to 22 wt%, Cr: 18 to 23 wt%, Cu: 1 to 3 wt%, Mo: 0.3 to 2.0 wt%, N: 0.05 wt% or less, high alloy austenitic stainless steel whose balance is substantially Fe, S content is 0.004 wt% or less And adding 0.05 or 0.005 wt% of trace B, and adding one or two of Ca and Mg to Swt% ≦ Mg + 1/2 and Ca ≦ 0.007 wt%. A technique for significantly improving hot workability without impairing excellent corrosion resistance is disclosed. However, the lower limit of 0.0005 wt% of B addition, which is one of the requirements of the present invention, is from the viewpoint of improving hot workability, while the upper limit of 0.005 to% is a viewpoint that does not cause deterioration of intergranular corrosion. Therefore, it is clear that the corrosion resistance is not positively improved.

  Patent Document 13 discloses a method of eliminating a random grain boundary of austenitic stainless steel by a unidirectional solidification method to form a single crystal. However, there is a limitation on casting conditions, in particular, a drawing speed. In particular, the manufacturing method is difficult, and it is difficult to apply to large-sized members.

  In Patent Document 14, C: 0.02 wt% or less, N: 0.6 wt% or less, Si: 1.0 wt% or less, P: 0.040 wt% or less, S: 0.030 wt% or less, Mn: In an austenitic stainless steel containing 2.0 wt% or less, Mo: 3.0 wt% or less, Ni: 12 to 26 wt%, Cr: 16 to 26 wt%, the austenite phase or ferrite phase is 10 in the austenite matrix at room temperature. The parent phase has sub-crystal grains, and further comprises a single crystal of a grain boundary with a small deviation from the corresponding orientation relationship and a high degree of order, thereby providing corrosion resistance, stress corrosion cracking resistance and mechanical properties. Copper with excellent properties and uses thereof are disclosed. However, specific manufacturing methods include strain annealing method, Tamman method, Bridgeman method, floating zone melting method, unidirectional solidification method, continuous casting method, etc. In order to obtain a relatively large steel, unidirectional solidification method Or, continuous casting is said to be preferable, but specific manufacturing conditions are lacking, and not only the feasibility to obtain the structure of the structure which is a requirement of the invention is questionable, but also the steel components, particularly the Ni content, is in the neutron irradiation environment. It is not considered that sufficient corrosion resistance is obtained because the amount is not sufficient to suppress the swelling at the bottom.

JP 63-303038 A JP 05-059494 A JP-A-8-269550 Japanese Patent Laid-Open No. 09-125205 JP 05-179405 A Japanese Patent Laid-Open No. 03-264651 Japanese Patent Application Laid-Open No. 11-80905

  However, even if the austenitic stainless steel material described in Patent Documents 1 to 7 or the manufacturing method thereof is used, there still remains a problem that severe intergranular corrosion occurs.

  Furthermore, as described above, the austenitic stainless steel materials described in Patent Documents 8 to 14 have a problem that corrosion resistance that can be used in a high-temperature and high-pressure underwater environment that receives neutron irradiation cannot be obtained.

  The present invention has been made in view of the above problems, and its main purpose is in a boiling heat transfer surface corrosion environment of a high concentration nitric acid solution containing highly acidic ions, or in a high temperature and high pressure underwater environment that receives neutron irradiation. An austenitic stainless steel exhibiting excellent corrosion resistance against intergranular corrosion and stress corrosion cracking and a method for producing the same.

Means and effects for solving the problem

  The inventors of the present invention have made extensive studies to solve the above problems. As a result, the boiling heat transfer of a highly concentrated nitric acid solution containing highly oxidizing ions can be reduced as much as possible, preferably completely removed, at the grain boundaries of austenitic stainless steel, which is the starting point of corrosion. It has been found that the corrosion resistance against intergranular corrosion and stress corrosion cracking can be enhanced under surface corrosion environment or high temperature and high pressure underwater environment that receives neutron irradiation like light water reactor core.

Specifically, since the austenitic stainless steel of the present invention achieves its purpose, C: 0.005 wt% or less, Si: 0.5 wt% or less, Mn: 0.5 wt% or less, P: 0.005 wt% or less S: 0.005 wt% or less, Ni: 15.0-40.0 wt%, Cr: 20.0-30.0 wt%, N: 0.01 wt% or less, O: 0.01 wt% or less, balance an austenitic stainless steel consisting of Fe and unavoidable impurities, B included in the inevitable impurities Ri der less 3 wt ppm, boiling heat transfer surface corrosive environment of a high concentration nitric acid solution containing highly acidic ion, or , at high temperature high pressure underwater environment undergoing neutron irradiation, characterized Rukoto that Teisu excellent corrosion resistance against intergranular corrosion and stress corrosion cracking.

  According to the above configuration, by setting the B content to 3 wtppm or less, intergranular corrosion can be reduced and stress corrosion cracking can be completely suppressed.

  Moreover, precipitation of Cr type carbide | carbonized_material can be suppressed by content of C being 0.005 wt% or less. Moreover, deoxidation can be brought about by containing Si 0.5wt% or less. Moreover, the production | generation of a (delta) -ferrite and a process induction transformation can be reduced by containing Mn 0.5 wt% or less. Moreover, deterioration of intergranular corrosion resistance and stress corrosion cracking resistance can be reduced by setting the P content to 0.005 wt% or less. Moreover, deterioration of intergranular corrosion resistance, stress corrosion cracking resistance, and pitting corrosion resistance can be reduced by setting the S content to 0.005 wt% or less.

  Further, by containing Ni in an amount of 15.0 wt% or more, the austenite structure can be stabilized, and intergranular corrosion and stress corrosion cracking can be suppressed. Moreover, cost reduction can be aimed at by content of Ni being 40.0 wt% or less. Further, by setting the Cr content to 20.0 wt% or more, for example, a highly passive corrosive environment in a boiling heat transfer surface corrosive environment of a highly concentrated nitric acid solution containing highly oxidizing ions as in a reprocessing plant. Sufficient corrosion resistance can be ensured under high-temperature and high-pressure underwater environments that receive neutron irradiation, such as under and light water reactor cores. Moreover, precipitation of Cr-rich embrittlement phase can be suppressed by setting the Cr content to 30.0 wt% or less. Moreover, the deterioration of intergranular corrosion resistance and stress corrosion cracking resistance can be reduced by setting the contents of N and O to 0.01 wt% or less, respectively.

  In the austenitic stainless steel of the present invention, the total of wt% of C, P, S, N and O contained may be 0.02 wt% or less.

  According to the said structure, favorable intergranular corrosion resistance and stress corrosion cracking resistance can be obtained because the sum total of wt% of C, P, S, N, and O shall be 0.02 wt% or less.

  In the austenitic stainless steel of the present invention, the Ti content may be stoichiometrically equivalent or more than the total of C, P, S, N and O.

According to the above configuration, the content of Ti is more than stoichiometrically equivalent to the total of C, P, S, N, and O, so that C, which is an impurity element that causes intergranular corrosion, P, S, N, and O TiC, FeTiP, TiS, TiN , and can be rendered harmless by a Ti-based carbonitride and compounds, such as TiO _2.

  Moreover, in order to achieve the objective, the manufacturing method of the austenitic stainless steel of this invention is the heat processing temperature in the 1st temperature range which is 1000 degreeC-1150 degreeC in the manufacturing process of the board | plate material or pipe material of austenitic stainless steel. A heat treatment for 1 minute or longer is followed by a solution heat treatment comprising cooling to room temperature by rapid cooling from the heat treatment temperature within the first temperature range or by allowing to cool.

  According to the above configuration, the austenite phase can be made uniform by solution heat treatment, and the effect of improving the intergranular corrosion resistance and stress corrosion cracking resistance by limiting the chemical composition in the austenitic stainless steel can be exhibited.

  Moreover, in order to achieve the objective, the manufacturing method of the austenitic stainless steel of this invention is the heat processing temperature in the 1st temperature range which is 1000 degreeC-1150 degreeC in the manufacturing process of the board | plate material or pipe material of austenitic stainless steel. After performing the heat treatment for 1 minute or longer, the heat treatment temperature in the second temperature range of 650 ° C. or higher is performed by cooling from the heat treatment temperature in the first temperature range or by cooling by cooling and reheating after cooling. Then, a solution heat treatment is performed, which includes holding by heating so that the heat treatment temperature is within the second temperature range for 10 minutes or more.

  According to the above configuration, the austenite phase can be made uniform by solution heat treatment, and the effect of improving the intergranular corrosion resistance and stress corrosion cracking resistance by limiting the chemical composition in the austenitic stainless steel can be exhibited.

  Moreover, the manufacturing method of the austenitic stainless steel of the present invention is such that after solution heat treatment, cold working is performed at a working degree of 40% or more and less than 75%, and heating is performed within a temperature range of 700 ° C. or more for 10 minutes or more. You may perform the recrystallization process by hold | maintaining heat processing temperature.

  According to the above configuration, by performing cold working, it is possible to sufficiently introduce dislocations as precipitation sites and to prevent strain-induced transformation of the austenite phase into the martensite phase due to excessive working. Thereby, it becomes possible to suppress the difficulty of industrial processing and to obtain a uniform austenite structure in the subsequent recrystallization process. In addition, as a result of obtaining a uniform austenite structure in the recrystallization treatment, excellent intergranular corrosion resistance and stress corrosion cracking resistance can be obtained.

  Moreover, the manufacturing method of the austenitic stainless steel of the present invention maintains the heat treatment temperature within a temperature range of 500 to 650 ° C. by heating for 30 minutes or more after the cold working and before the recrystallization treatment. Strain aging precipitation of precipitates consisting of the above may be performed.

  According to the above configuration, by performing strain aging precipitation of the precipitate after the cold working and before the recrystallization treatment, the carbide and the like can be efficiently and uniformly dispersed.

BEST MODE FOR CARRYING OUT THE INVENTION

  The present embodiment will be described below with reference to FIGS. 1 and 2, including the background to the component design specified in the present invention.

  First, with respect to the formation of a Cr-deficient layer accompanying the precipitation of Cr-based carbides at the grain boundaries, which is the largest cause of intergranular corrosion and stress corrosion cracking, welding is achieved only by reducing the C content, which is one of the conventional measures. It was found that such situations as sensitization by heating and irradiation-induced precipitation in a radiation environment are inevitable.

  For this reason, Cr in the steel is set to 20 wt% or more that can secure about 12 wt% necessary for generating a passive film even after generation of a deficient layer due to carbide precipitation. However, even in these cases, the high-concentration nitric acid solution containing highly oxidizable ions can completely avoid transpassive corrosion in boiling heat transfer surface corrosion environment and grain boundary damage in high temperature and high pressure underwater environment subjected to neutron irradiation. There wasn't.

One of the causes is B, which is an impurity element that segregates at the grain boundaries and lowers the grain boundary binding energy, and its concentration is set to 3 wtppm or less. In addition, the total amount of impurity elements such as C, P, S, N, and O is set to 0.02% or less. Further, if necessary, in order to completely detoxify the influence of these impurity elements, Ti is contained in a stoichiometric equivalent or more with C, P, S, N and O to obtain TiC, FeTiP, TiS, TiN. It was found that it is effective to Ti-based carbonitride and compounds, such as Ti0 _2. By these, intergranular corrosion and stress corrosion cracking could be completely suppressed.

  The greatest effect of these measures is to reduce the B content to 3 wtppm or less as shown in FIG. It is known that the addition of B improves the high temperature ductility of austenitic stainless steel. For example, Japanese Patent Application Laid-Open No. 63-0699947 proposes a technique for improving creep rupture ductility by adding 6 to 25 wtppm of B. Furthermore, the addition of 2 wtppm or more of B improves the hot ductility as described in “IronAge” vol. 179 (1957), p. 95. Thus, B is said to be an effective element for improving high temperature ductility and hot workability. However, on the other hand, it has been reported that the corrosion resistance of austenitic stainless steel deteriorates due to the addition of B.

  “Stainless steel '87”, The Institute of Metals, London, (1987), p. In order to maintain the intergranular corrosion resistance of austenitic stainless steel in H.234, it has been proposed to reduce the amount of B, and when B is added at about 25 wtppm, Cr boride precipitates at the grain boundaries even in the usual solid solution treatment. It has been reported that the intergranular corrosion resistance deteriorates. Furthermore, “Materials and Processes”, Iron and Steel, vol. 6 (1993), p. No. 732 reports that it is necessary to reduce the B content to 9 wtppm or less in order to maintain the intergranular corrosion resistance of austenitic stainless steel in high temperature and high concentration nitric acid at a high level. As described above, it is known that B segregates at the grain boundaries and forms boride rich in Cr to deteriorate the intergranular corrosion resistance. Thus, in steels of conventional impurity levels including prior art 7, the adverse effect is even less, and when the B content exceeds 5 wtppm, deterioration of intergranular corrosion resistance begins to appear, especially when it exceeds 10 wtppm. It will be noticeable.

  Although the problems due to the B content are as described above, the present invention has found that further reduction of the B content itself is important. The reason for this is not clear, but a marked improvement in grain boundary damage is observed at a content of B below the solid solubility limit of B estimated to be about 10 wtppm. From this, it is presumed that the solid solution itself at the grain boundaries has an adverse effect rather than the formation of borides. In addition, as for this invention, the place where the effect of the very small amount of B was able to be found was largely due to the development of analytical equipment / technology and steelmaking technology. In the conventional chemical analysis, the detection limit was about 2 wtppm, but the B content of wtppm or less can be accurately analyzed by the GD-MS analysis method, and trace B amount and intergranular corrosion and stress corrosion cracking The relationship became clear. In addition, it has been unavoidable to mix about 2 to 5 wtppm from raw materials such as alloy iron and scrap in the melting of normal austenitic stainless steel. However, the development of analytical technology enables the selection of raw materials with low B content. Furthermore, austenitic stainless steel with a low B content can be melted by the development of steelmaking techniques such as oxidation refining.

Next, it is also a major component of the present invention that the total amount of impurity elements such as C, P, S, N, and O is 0.02 wt% or less. The reason why the grain boundary damage is remarkably improved when the total amount of these impurity elements is 0.02 wt% or less is not clear. Although the effects of these elements on the grain boundaries and the form in which precipitates are generated are different, current analysis / analysis techniques cannot distinguish the presence of trace elements as in the present invention. Is possible. However, it is presumed that there is no doubt that the impurity elements segregated and dissolved in the crystal grain boundaries have an adverse effect. In order to completely detoxify the effects of these impurity elements, Ti is added to a stoichiometric equivalent or more of C, P, S, N and O, and TiC, FeTiP, TiS, TiN, and TiO ₂ are added. Such Ti-based carbonitrides and compounds are effective.

  A method for melting a high-purity austenitic stainless steel ingot having a total amount of impurity elements of 0.02 wt% or less is not particularly limited, but it is also effective to apply the electron beam melting method in a combination of melting steps It is one of. Contains not only impurity elements such as C, P, S, N, and O that segregate at the austenite grain boundaries by applying the electron beam melting method in the manufacturing process of austenitic stainless steel ingots, but also contains highly volatile alkali-based metals Ultra high cleanliness can be obtained with the amount reduced as much as possible. Note that there is no particular limitation on the prior melting method used as the source electrode for electron beam melting, and an optimal melting method may be selected in accordance with the purity of the primary melting raw material.

Although it is desirable that the number of impurity elements such as C, P, S, N, and O segregated at the grain boundaries is as small as possible, it is difficult to completely remove them by current refining techniques, and it is not economical. In order to reduce the above impurity elements as much as possible, it is effective to add a stabilizing element, but Ti is most desirable in order to make these impurity elements harmless. C, P, S, N, O, etc., which cannot be removed by the electron beam melting method or the like by adding Ti, are made into Ti-based carbonitrides and compounds such as TiC, FeTiP, TiS, TiN, and TiO _2. Thus, segregation as a solid solution element at the grain boundary can be suppressed. In the prior art, Nb or the like is mentioned as a stabilizing element, but even if Nb is added, it is difficult to produce a compound other than NbC within the range of the austenitic stainless steel of the present invention. Therefore, the effect is limited. In addition, the addition amount of Ti needs to be more than the stoichiometric equivalent with C, P, S, N, and O.

  In addition, in order to further exhibit the intergranular corrosion resistance and stress corrosion cracking resistance improvement effects due to the chemical composition limitation in the austenitic stainless steel of the present invention, in the manufacturing process of the plate material or tube material, 1000-1150 ℃ Heat within the temperature range for 1 minute or longer, and further cool from the heat treatment temperature to room temperature by rapid cooling or standing, or heat and hold for 10 minutes or longer in the temperature range of 650 ° C. or higher during cooling or reheating. good. Further, in order to make the Ti addition effect more reliable and further to make the relationship between the generated Ti-based compound distribution state and the existence position of the grain boundary different, it is 1 minute or more within a temperature range of 1000 to 1250 ° C. It is heated and further cooled from the heat treatment temperature to room temperature by rapid cooling or cooling. After performing the solution heat treatment, cold processing is performed at a processing rate of 40% to less than 75%, and then recrystallization is performed by heating and holding in a temperature range of 750 ° C. or higher for 10 minutes or longer. In addition, in a chemical composition with a small amount of impurity elements such as C, P, S, N, and O related to the reaction as in the present invention, the precipitation reaction may not sufficiently proceed due to the reaction rate. After cold working at a processing rate of less than 100%, after performing strain aging precipitation treatment for heating and holding in a temperature range of 500 to 650 ° C. for 30 minutes or more, then heating in a temperature range of 750 ° C. or more for 10 minutes or more -It is also effective to hold.

(Chemical composition of stainless steel)
C: 0.005 wt% or less C precipitates Cr-based carbides at grain boundaries when heat treatment or welding is performed. As a result, a Cr-depleted region is generated in the vicinity of the crystal grain boundary. When placed in a corrosive environment in this state, intergranular corrosion occurs where the area is selectively corroded. Therefore, it becomes a cause of deteriorating nitric acid corrosion resistance and stress corrosion cracking resistance of austenitic stainless steel. In the present embodiment, the addition of Ti and detoxification are attempted by detoxification, but when there is a large amount of C in the austenitic stainless steel, there is a possibility that Cr-based carbides precipitate microscopically, It was 0.005 wt% or less.

Si: 0.5 wt% or less Si is desirably as low as possible from the viewpoint of intergranular corrosion. However, since it is effective as a deoxidizer, the content is set to 0.5 wt% or less.

Mn: 0.5 wt% or less Mn has the effect of increasing the austenite phase stability and preventing the formation of δ-ferrite that is harmful to corrosion resistance and the processing-induced transformation. In addition to being not obtained, Mn in a solid solution state promotes corrosion, so it was made 0.5 wt% or less.

P: 0.005 wt% or less P: P is known to segregate at grain boundaries, and when the content of P is increased, intergranular corrosion resistance and stress corrosion cracking resistance deteriorate. For this reason, the lower content is desirable, and it is set to 0.005 wt% or less.

S: 0.005 wt% or less S: Increase in S promotes the formation of sulfides and degrades intergranular corrosion resistance, stress corrosion cracking resistance, and pitting corrosion resistance by selective corrosion based on them. Let For this reason, the lower content is desirable, and it is set to 0.005 wt% or less.

Ni: 15.0 to 40.0 wt%
Ni: Ni is an element necessary for stabilizing the austenite structure and suppressing intergranular corrosion and stress corrosion cracking. However, if the content is less than 15 wt%, a sufficient austenite structure cannot be secured, and further, it is not possible to obtain swelling resistance under a neutron irradiation environment. On the other hand, since it will become expensive when it exceeds 40 wt%, 15.0-40.0 wt% is desirable.

Cr: 20.0-30.0 wt%
Cr: Cr is an element necessary for forming a passive film and ensuring the corrosion resistance of stainless steel. From the viewpoint of passive film formation, it may be contained in an amount of about 16% as in SUS304 and SUS316 stainless steel, which are typical stainless steels of JIS standards. However, in a high-passage corrosive environment in a boiling heat transfer surface corrosive environment of a high-concentration nitric acid solution containing highly oxidizing ions as in a reprocessing plant, or in a high-temperature and high-pressure water environment that receives neutron irradiation, such as a light water reactor core. In order to ensure sufficient corrosion resistance, 20 wt% is necessary. On the other hand, if it exceeds 30 wt%, a Cr-rich embrittled phase precipitates, so it is necessary to increase the Ni content for avoiding them to make a complete austenite structure, leading to an increase in cost. 0-30.0 wt% is desirable.

B: 3 wtppm or less B: B is the most important factor constituting the present invention. Basically, it is an impurity element, and segregates at the grain boundaries to degrade the intergranular corrosion resistance and stress corrosion cracking resistance. B could not be discriminated for 0.0003 wt% or less by the conventional analysis technique, but in the present invention, the relationship between a trace amount of B and the corrosion resistance was clarified by making use of the latest analysis technique, and as a result, 0.0003 wt% It was found that the grain boundary corrosion and stress corrosion cracking can be completely suppressed by reducing to the following. From this viewpoint, the amount of B is set to 3 wtppm (0.0003 wt%) or less. In addition, More preferably, it is 1.5 wtppm or less.

N: 0.01 wt% or less O: 0.01 wt% or less N, O: N and O all deteriorate the intergranular corrosion resistance and stress corrosion cracking resistance, so the content should be as low as possible. 0.01 wt% is the upper limit.

C + P + S + O + N: 0.02 wt% or less C + P + S + O + N: Even if these impurity elements are individually limited as in the above limiting conditions, if the total exceeds 0.02 wt%, good intergranular corrosion resistance and stress corrosion cracking resistance Since it was not obtained, 0.02% was made the upper limit.

Ti: The content is equal to or more than the stoichiometric amount with respect to the total of C, P, S, N and O Ti: Ti is an important factor constituting the present invention, and causes C that causes intergranular corrosion. Impurity elements such as P, S, N, and O are added to make them completely harmless by using Ti-based carbonitrides and compounds such as TiC, FeTiP, TiS, TiN, and TiO ₂ . In the present invention, these impurity elements are at a very low level in the steel ingot stage by adopting an electron beam melting method or the like, but according to the inventors' investigation, a very small amount of impurities that cannot be removed by electron beam melting. It was found that the elements have an adverse effect on intergranular corrosion. For this reason, Ti is added in order to make these completely harmless. Therefore, the minimum required content is stoichiometric equivalent for all of C, P, S, N, and O to become Ti-based carbonitrides and compounds such as TiC, FeTiP, TiS, TiN, and TiO _2. It is an amount. In particular,
Ti (wt%) = (48/12) C (wt%) + (48/31) P (wt%) + (48/32) S (wt%) + (48/14) N (wt%) + (48/16) x (1/2) O (wt%)
However, if considering the dynamic precipitation reaction of dilute elements, 0.05 wt% or more is desirable. On the other hand, if added in a large amount, the cost increases, so 0.3 wt% or less is desirable.

(Electron beam melting method)
In the present embodiment, the electron beam melting method is adopted in the manufacturing process of the steel ingot. Electron beam melting is basically divided into drip melting and cold hearth melting. The drip melting method is a method in which an electron beam is irradiated to the tip of a raw material electrode, and a generated droplet is dropped directly onto a water-cooled copper mold and laminated and solidified. In the cold hearth melting method, droplets generated at the tip of the raw material are once stored in a water-cooled shallow copper container called cold hearth, and the molten metal overflowed from this is poured into a water-cooled copper mold and placed on a foundation called a starting block. It is a method of laminating and solidifying. In the present embodiment, either dissolution method may be used.

Describes the prescribed conditions of the electron beam melting method. In order to achieve the purification effect by evaporation during dissolution, the degree of vacuum in the chamber needs to be 1 × 10 ⁻² Pa or more. However, if the degree of vacuum is too high, highly volatile elements such as Cr constituting the present invention will evaporate and it will be difficult to adjust the components, and industrial realization will be difficult. ^-4 Pa or less is desirable. In addition, the material used as a raw material electrode may adopt the AOD method (argon oxygen decarburization method) and the VOD method (vacuum oxygen decarburization method) widely known as a melting method of stainless steel, and vacuum as special refining. An induction melting (Vacuum Induction Melting) method, a magnetic levitation induction melting (Cold Crucibee Induction Melting) method, etc. may be adopted.

(Production method)
In the present embodiment, in the manufacturing process of the austenitic stainless steel plate or tube, heat treatment is performed for 1 minute or more at a heat treatment temperature within a temperature range of 1000 ° C. to 1150 ° C. As a subsequent solution treatment, the solution is cooled to normal temperature by rapid cooling or cooling from a heat treatment temperature within a temperature range of 1000 ° C to 1150 ° C. As the solution treatment, the heat treatment temperature is within a temperature range of 650 ° C. or higher by cooling or reheating after cooling, and then the heat treatment temperature is within a temperature range of 650 ° C. or higher by heating for 10 minutes or more. The holding may be performed as described above. Thereby, the austenite phase can be made uniform, and the effect of improving the intergranular corrosion resistance and stress corrosion cracking resistance by limiting the chemical composition in the austenitic stainless steel can be exhibited.

  Further, after the solution treatment, cold working (cold rolling) and recrystallization treatment may be performed. By performing cold working, a large amount of dislocations that become carbide precipitation sites can be introduced. In the recrystallization treatment, the precipitate is uniformly dispersed and precipitated and recrystallized by heat treatment after cold working.

  Specifically, cold working will be described. The workability (working rate) in cold working for sufficiently introducing dislocations as precipitation sites is required to be 40% or more. Even if the degree of processing is increased more than necessary, the introduced dislocation density is saturated, and the austenite phase is strain-induced transformed into the martensite phase due to excessive processing. For this reason, industrial processing becomes difficult, and a uniform austenite structure cannot be obtained in the subsequent recrystallization process, and intergranular corrosion resistance and stress corrosion cracking resistance deteriorate. Therefore, the working degree in cold working is set to 75% or less.

  Next, the recrystallization process will be described. The temperature for recrystallizing the work structure depends on the degree of work of the steel, that is, the dislocation density introduced and the state of carbide dispersion that hinders the movement of dislocations in the recovery / recrystallization process. For this reason, it is necessary to hold | maintain at 700 degreeC or more for 10 minutes or more in the steel composition and structure | tissue state of this invention. On the other hand, the upper limit temperature is not particularly limited.

  In addition, when temperature is too high, intensity | strength falls by coarsening of the obtained recrystallized austenite grain. Further, the precipitates are aggregated and coarsened and distributed at the recrystallized austenite grain boundaries. As a result, intergranular corrosion resistance and stress corrosion cracking resistance deteriorate. Therefore, the recrystallization temperature is desirably 900 ° C. or lower.

  In addition, precipitation treatment may be performed in order to efficiently and uniformly disperse carbides and the like before recrystallization treatment after cold working. At this time, it is desirable that the reaction is heated and maintained for 30 minutes or more in a temperature range of 500 ° C. or more. On the other hand, carbide precipitation occurs in a shorter time as the temperature is higher, but if it is too high, recovery and recrystallization occur prior to carbide precipitation. For this reason, it is impossible to precipitate using the dislocations introduced as the corners as sites. As a result, carbides and the like are preferentially precipitated at the grain boundaries, so that they are not uniformly dispersed and further coarsened, so that excellent intergranular corrosion resistance and stress corrosion cracking resistance cannot be obtained. From the above viewpoint, it is desirable that the carbide precipitation treatment is heated and maintained for 30 minutes or more at a heat treatment temperature within a temperature range of 500 to 650 ° C.

(Experiment 1)
150 kg of austenitic stainless steel having the chemical composition shown in Table 1 was subjected to vacuum induction melting (VIM) and cast into a mold in vacuum to obtain an ingot. Next, the electrode was shaved from the vacuum-melted ingot and subjected to electron beam remelting (EB) to form a cylindrical ingot. Furthermore, after finishing to a 6 mm-thick plate material by forging and hot rolling, a 6 mm plate material subjected to solution treatment under the condition of 1050 ° C. × 1/2 h was obtained. Using these as test materials, Corio corrosion test simulating intergranular corrosion in high-concentration boiling nitric acid solution containing metal ions, and low strain rate tensile test simulating stress corrosion cracking in high-temperature and high-pressure water ( SSRT) and CBB tests were performed. For the low strain rate tensile test and the CBB test, a sensitizing heat treatment at 620 ° C. × 100 hours was performed before these tests in order to simulate the neutron irradiation induced precipitation state and the like.

Table 2 shows the test results after evaluation. The Corio corrosion test uses 500 ml of 8N boiling nitric acid solution with 1.0 g / L of Cr ⁶⁺ ions added, and performs 4 batch immersion tests with 24 batches for 24 hours while renewing the solution, and measuring the corrosion weight loss. Corrosion rate etc. were evaluated. The low strain rate tensile test uses a test piece having a parallel part diameter of 3 mm and a gauge distance of 20 mm, under conditions of high temperature and high pressure water (saturated oxygen concentration 8 wtppm, 70 kgf / cm ² , 290 ° C.), strain rate: 0.5 μm / min. It carried out in.

In the CBB test, a test piece having a thickness of 2 mm, a width of 10 mm, and a length of 50 mm was used, and in an autoclave, 500 in high-temperature high-pressure water (saturated oxygen concentration: 8 wtppm, 70 kgf / cm ² , 290 ° C.) using the jig shown in FIG. It was carried out by soaking for hours. A bolt hole 4 was inserted between the holders 3 together with the graphite fiber wool 2 for providing a gap in the test piece 1, and the holders 3 were rounded and tightened. In the present embodiment, the holder has a portion curved to 100R. After immersion, the test piece was taken out, and the presence or absence of cracking was evaluated from cross-sectional observation of the test piece. Steel numbers A to D and steel numbers K to L are within the scope of the present invention, steel numbers E to G have a B amount exceeding the scope of the present invention, and steel numbers H to J are Cr-Ni. Amounts outside the claimed range of the present invention, steel numbers M to Q are C, P, S, N and O amounts exceeding the claimed range of the present invention, steel numbers R to S are Si or Mn amount Is beyond the scope of the present invention. As can be seen from Table 2, if the chemical composition is within the claimed range of the present invention, good intergranular corrosion resistance and stress corrosion cracking resistance can be obtained.

(Experiment 2)
Using the steel numbers B, K and L in Table 1, 6 mm plate materials were produced under various conditions as shown in Table 3. Production abbreviation 1 is obtained by vacuum induction melting (WM), cast into a mold in a vacuum to obtain an ingot, and others are further subjected to electron beam remelting (EB). The plate material finished by forging and hot rolling is subjected to solution treatment, and then subjected to heat treatment (cold work / recrystallization process / cold work / carbide precipitation process / recrystallization process) (with different cold work rates) Adjusted the plate thickness during solution treatment). By using these, the intergranular corrosion state in a high-concentration boiling nitric acid solution containing highly oxidizable metal ions is simulated to simulate the Corio corrosion test, and the stress corrosion cracking state in high-temperature and high-pressure water is simulated to reduce low strain. A speed tensile test (SSRT) and a CBB test were conducted. For the low strain rate tensile test and the CBB test, a sensitizing heat treatment at 620 ° C. × 100 hours was performed before these tests in order to simulate the neutron irradiation induced precipitation state and the like.

  Table 4 shows the evaluation test results. From the results of Table 4, it can be seen that good intergranular corrosion resistance and stress corrosion cracking resistance can be obtained if the chemical composition and manufacturing process are within the scope of the claims of the present invention.

  Thus, by setting the B content to 3 wtppm or less, intergranular corrosion can be reduced and stress corrosion cracking can be completely suppressed.

  Moreover, precipitation of Cr type carbide | carbonized_material can be suppressed by content of C being 0.005 wt% or less. Moreover, deoxidation can be brought about by containing Si 0.5wt% or less. Moreover, the production | generation of a (delta) -ferrite and a process induction transformation can be reduced by containing Mn 0.5 wt% or less. Moreover, deterioration of intergranular corrosion resistance and stress corrosion cracking resistance can be reduced by setting the P content to 0.005 wt% or less. Moreover, deterioration of intergranular corrosion resistance, stress corrosion cracking resistance, and pitting corrosion resistance can be reduced by setting the S content to 0.005 wt% or less.

  Further, by containing Ni in an amount of 15.0 wt% or more, the austenite structure can be stabilized, and intergranular corrosion and stress corrosion cracking can be suppressed. Moreover, cost reduction can be aimed at by content of Ni being 40.0 wt% or less. Further, by setting the Cr content to 20.0 wt% or more, for example, a highly passive corrosive environment in a boiling heat transfer surface corrosive environment of a highly concentrated nitric acid solution containing highly oxidizing ions as in a reprocessing plant. Sufficient corrosion resistance can be ensured under high-temperature and high-pressure underwater environments that receive neutron irradiation, such as under and light water reactor cores. Moreover, precipitation of Cr-rich embrittlement phase can be suppressed by setting the Cr content to 30.0 wt% or less. Moreover, the deterioration of intergranular corrosion resistance and stress corrosion cracking resistance can be reduced by setting the contents of N and O to 0.01 wt% or less, respectively.

  Moreover, favorable intergranular corrosion resistance and stress corrosion cracking resistance can be obtained by making the total of wt% of C, P, S, N, and O 0.02 wt% or less.

Further, when the Ti content is more than stoichiometrically equivalent to the total of C, P, S, N, and O, C, P, S, which are impurity elements that cause intergranular corrosion. , N, and O can be completely detoxified by using Ti-based carbonitrides and compounds such as TiC, FeTiP, TiS, TiN, and TiO ₂ .

  Further, the austenite phase can be made uniform by solution heat treatment, and the effect of improving the intergranular corrosion resistance and stress corrosion cracking resistance by limiting the chemical composition in the austenitic stainless steel can be exhibited.

  Further, by performing cold working, it is possible to sufficiently introduce dislocations as precipitation sites and to prevent strain-induced transformation of the austenite phase to the martensite phase due to excessive working. Thereby, it becomes possible to suppress the difficulty of industrial processing and to obtain a uniform austenite structure in the subsequent recrystallization process. In addition, as a result of obtaining a uniform austenite structure in the recrystallization treatment, excellent intergranular corrosion resistance and stress corrosion cracking resistance can be obtained.

  Further, by performing strain aging precipitation of the precipitate after the cold working and before the recrystallization treatment, carbides and the like can be efficiently and uniformly dispersed.

(Outline of this embodiment)

  The austenitic stainless steel of this embodiment is C: 0.005 wt% or less, Si: 0.5 wt% or less, Mn: 0.5 wt% or less, P: 0.005 wt% or less, S: 0.005 wt% or less , Ni: 15.0 to 40.0 wt, Cr: 20.0 to 30.0 wt%, N: 0.01 wt% or less, O: 0.01 wt% or less, with the balance being Fe and inevitable impurities It is an austenitic stainless steel, and is configured such that B contained in inevitable impurities is 3 wtppm or less.

  According to the above configuration, by setting the B content to 3 wtppm or less, intergranular corrosion can be reduced and stress corrosion cracking can be completely suppressed.

  Moreover, precipitation of Cr type carbide | carbonized_material can be suppressed by content of C being 0.005 wt% or less. Moreover, deoxidation can be brought about by containing Si 0.5wt% or less. Moreover, the production | generation of a (delta) -ferrite and a process induction transformation can be reduced by containing Mn 0.5 wt% or less. Moreover, deterioration of intergranular corrosion resistance and stress corrosion cracking resistance can be reduced by setting the P content to 0.005 wt% or less. Moreover, deterioration of intergranular corrosion resistance, stress corrosion cracking resistance, and pitting corrosion resistance can be reduced by setting the S content to 0.005 wt% or less.

  Further, by containing Ni in an amount of 15.0 wt% or more, the austenite structure can be stabilized, and intergranular corrosion and stress corrosion cracking can be suppressed. Moreover, cost reduction can be aimed at by content of Ni being 40.0 wt% or less. Further, by setting the Cr content to 20.0 wt% or more, for example, a highly passive corrosive environment in a boiling heat transfer surface corrosive environment of a highly concentrated nitric acid solution containing highly oxidizing ions as in a reprocessing plant. Sufficient corrosion resistance can be ensured under high-temperature and high-pressure underwater environments that receive neutron irradiation, such as under and light water reactor cores. Moreover, precipitation of Cr-rich embrittlement phase can be suppressed by setting the Cr content to 30.0 wt% or less. Moreover, the deterioration of intergranular corrosion resistance and stress corrosion cracking resistance can be reduced by setting the contents of N and O to 0.01 wt% or less, respectively.

  Further, the austenitic stainless steel of the present embodiment is configured such that the total of wt% of C, P, S, N and O contained is 0.02 wt% or less.

  According to the said structure, favorable intergranular corrosion resistance and stress corrosion cracking resistance can be obtained because the sum total of wt% of C, P, S, N, and O shall be 0.02 wt% or less.

  In addition, the austenitic stainless steel of the present embodiment is configured such that the Ti content is more than stoichiometrically equivalent to the total of C, P, S, N, and O.

According to the above configuration, the content of Ti is more than stoichiometrically equivalent to the total of C, P, S, N, and O, so that C, which is an impurity element that causes intergranular corrosion, P, S, N, and O TiC, FeTiP, TiS, TiN , and can be rendered harmless by a Ti-based carbonitride and compounds, such as TiO _2.

  Moreover, the manufacturing method of the austenitic stainless steel of this Embodiment is the manufacturing process of the austenitic stainless steel board | plate material or pipe material, and heats it for 1 minute or more at the heat processing temperature in the 1st temperature range made into 1000 to 1150 degreeC. After the treatment, a solution heat treatment comprising cooling to room temperature by rapid cooling from the heat treatment temperature within the first temperature range or standing to cool is performed.

  According to the above configuration, the austenite phase can be made uniform by solution heat treatment, and the effect of improving the intergranular corrosion resistance and stress corrosion cracking resistance by limiting the chemical composition in the austenitic stainless steel can be exhibited.

  Moreover, the manufacturing method of the austenitic stainless steel of this Embodiment is the manufacturing process of the austenitic stainless steel board | plate material or pipe material, and heats it for 1 minute or more at the heat processing temperature in the 1st temperature range made into 1000 to 1150 degreeC. After performing the treatment, cooling by rapid cooling from the heat treatment temperature in the first temperature range or by allowing to cool, and after reaching the heat treatment temperature in the second temperature range of 650 ° C. or higher by cooling or reheating after cooling. The solution heat treatment is performed by holding by heating so that the heat treatment temperature is within the second temperature range for 10 minutes or more.

  According to the above configuration, the austenite phase can be made uniform by solution heat treatment, and the effect of improving the intergranular corrosion resistance and stress corrosion cracking resistance by limiting the chemical composition in the austenitic stainless steel can be exhibited.

  Moreover, the manufacturing method of the austenitic stainless steel of this Embodiment is a temperature range of 10 minutes or more and 700 degreeC or more by heating after a solution heat treatment with a cold work with a workability of 40% or more and less than 75%. The recrystallization treatment is performed by maintaining the heat treatment temperature inside.

  According to the above configuration, by performing cold working, it is possible to sufficiently introduce dislocations as precipitation sites and to prevent strain-induced transformation of the austenite phase into the martensite phase due to excessive working. Thereby, it becomes possible to suppress the difficulty of industrial processing and to obtain a uniform austenite structure in the subsequent recrystallization process. In addition, as a result of obtaining a uniform austenite structure in the recrystallization treatment, excellent intergranular corrosion resistance and stress corrosion cracking resistance can be obtained.

  Moreover, the manufacturing method of the austenitic stainless steel of this Embodiment is the heat processing temperature within the temperature range of 500 to 650 degreeC by heating for 30 minutes or more after performing a cold working and before a recrystallization process. It is configured to perform strain aging precipitation of the precipitate formed by holding.

  According to the above configuration, by performing strain aging precipitation of the precipitate after the cold working and before the recrystallization treatment, the carbide and the like can be efficiently and uniformly dispersed.

  The embodiments of the present invention have been described above, but only specific examples have been illustrated, and the present invention is not particularly limited. Specific configurations and the like can be appropriately changed in design. Further, the actions and effects described in the embodiments of the invention only list the most preferable actions and effects resulting from the present invention, and the actions and effects according to the present invention are described in the embodiments of the present invention. It is not limited to what was done.

The figure which showed the relationship between the corrosion rate in a Corio corrosion test, the grain boundary test depth, and B amount. The figure which showed the jig | tool used by a CBB test.

Explanation of symbols

1 Test piece 2 Graphite fiber wool 3 Holder 4 Bolt hole

Claims (7)

C: 0.005 wt% or less, Si: 0.5 wt% or less, Mn: 0.5 wt% or less, P: 0.005 wt% or less, S: 0.005 wt% or less, Ni: 15.0 to 40.0 wt% Cr: 20.0-30.0 wt%, N: 0.01 wt% or less, O: 0.01 wt% or less, the balance is austenitic stainless steel consisting of Fe and inevitable impurities,
B in the inevitable impurities is 3 wtppm or less, in a boiling heat transfer surface corrosion environment of a highly concentrated nitric acid solution containing highly acidic ions, or in a high-temperature and high-pressure water environment subjected to neutron irradiation Austenitic stainless steel with excellent corrosion resistance against intergranular corrosion and stress corrosion cracking .   The austenitic system having excellent intergranular corrosion resistance and stress corrosion cracking resistance according to claim 1, wherein the total content of C, P, S, N and O is 0.02 wt% or less. Stainless steel.   The intergranular corrosion resistance and stress corrosion cracking according to claim 2, characterized in that the Ti content is at least stoichiometrically equivalent to the total of C, P, S, N and O. Austenitic stainless steel with excellent properties. A method for producing an austenitic stainless steel having the chemical composition according to claim 1,
In the manufacturing process of the austenitic stainless steel plate or tube, after heat treatment at a heat treatment temperature within a first temperature range of 1000 ° C. to 1150 ° C. for 1 minute or longer, heat treatment within the first temperature range A method for producing an austenitic stainless steel excellent in intergranular corrosion resistance and stress corrosion cracking resistance, characterized by performing a solution heat treatment comprising cooling to room temperature by rapid cooling from temperature or standing to cool. A method for producing an austenitic stainless steel having the chemical composition according to claim 1,
In the manufacturing process of the austenitic stainless steel plate or tube, after heat treatment at a heat treatment temperature within a first temperature range of 1000 ° C. to 1150 ° C. for 1 minute or longer, a heat treatment temperature within the first temperature range After cooling or re-cooling, the heat treatment temperature in the second temperature range of 650 ° C. or higher is reached by the cooling or reheating after the cooling, and the temperature in the second temperature range is 10 minutes or longer. A method for producing an austenitic stainless steel excellent in intergranular corrosion resistance and stress corrosion cracking resistance, characterized by performing solution heat treatment consisting of holding by heating to a heat treatment temperature.   After the solution heat treatment, a recrystallization treatment is performed by performing cold working at a working degree of 40% or more and less than 75% and maintaining a heat treatment temperature within a temperature range of 700 ° C. or more for 10 minutes or more by heating. The method for producing an austenitic stainless steel excellent in intergranular corrosion resistance and stress corrosion cracking resistance according to claim 4 or 5.   Performing strain aging precipitation of the precipitate after holding the heat treatment temperature within a temperature range of 500 to 650 ° C. by heating for 30 minutes or more after the cold working and before the recrystallization treatment The method for producing an austenitic stainless steel excellent in intergranular corrosion resistance and stress corrosion cracking resistance according to claim 6.

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