JP2010275569A - Austenitic stainless steel and method of manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve an IGSCC (Intergranular Stress Corrosion Cracking) resistant characteristic, especially IGSCC resistant progression property to improve durability in an austenitic stainless steel. <P>SOLUTION: In grain-boundary triple-points constituted from three grain-boundaries, frequency of the grain-boundary triple-points (J<SB>2CSL</SB>), that two grain-boundaries are correspondence grain-boundaries and one grain-boundary is a random grain-boundary, is ≥35%. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an austenitic stainless steel excellent in corrosion resistance, particularly intergranular corrosion resistance, applied to nuclear power plants and chemical plants, and a method for producing the same.

Austenitic stainless steel is a material excellent in mechanical properties and corrosion resistance, and is widely used from general structural use to nuclear equipment. However, it is known that stress corrosion cracking (hereinafter referred to as SCC) occurs in a severe corrosive environment. In particular, SCC that propagates along grain boundaries is called intergranular stress corrosion cracking (hereinafter referred to as IGSCC), and is generated in the weld heat affected zone. The welding heat-affected zone is usually heated to a temperature of 600 to 700 ° C. In this temperature range, C in the material exceeds the solid solubility limit, so it combines with Cr and precipitates at grain boundaries as Cr ₂₃ C ₆ . Such a decrease in the Cr concentration in the vicinity of the Cr carbide, that is, the Cr deficiency in the vicinity of the grain boundary, is considered to cause a decrease in the corrosion resistance of the grain boundary and the occurrence of intergranular corrosion cracking.

  With the development of grain boundary engineering, it has become possible to develop materials with higher performance and higher functionality by controlling the crystal orientation and grain boundary character distribution. In particular, research on grain boundary character control utilizing a low energy grain boundary structure of a corresponding grain boundary (Coincidence Site Lattice grain boundary, hereinafter referred to as CSL grain boundary) has attracted attention. A CSL grain boundary means that when one of adjacent crystals sandwiching a crystal grain boundary is rotated around the crystal axis, a part of the lattice point is located at the lattice point of the adjacent crystal grain, A grain boundary that forms a common sublattice. At this time, lattice points that overlap periodically other than the origin are formed by the rotation axis and the rotation angle. This is called a corresponding grid point. The ratio between the unit cell volume of the original crystal lattice and the unit cell volume of the corresponding lattice formed here is called the Σ (sigma) value. According to conventional research data, low ΣCSL grain boundaries (sigma value of 29 or less) with low grain boundary energy and relatively stable lattice structure compared to grain boundaries other than CSL grain boundaries (hereinafter referred to as random grain boundaries) (Corresponding grain boundaries) are recognized to be deficient in Cr, and stress corrosion cracking is unlikely to occur.

  Based on the above theory, in order to improve the corrosion resistance and IGSCC resistance of materials, research and development of technology to improve the low ΣCSL grain boundary frequency by using annealing twins generated in the thermomechanical process has recently been active. . Among them, several material processes for improving the IGSCC resistance of the material by improving the frequency of the low ΣCSL grain boundary have been proposed (Patent Documents 1, 2, and 3).

  In Patent Document 1, in 304 series, 316 series and 347 series stainless steels, a rolling rate of 2 to 30% (the rolling rate is expressed as a percentage or a fraction, and the amount of decrease in the sample cross-sectional area relative to the original cross-sectional area) The low ΣCSL grain boundary frequency with a crystal orientation difference angle of 15 ° or more and a Σ value of 29 or less is set to 65% or more by performing heat treatment at a temperature of 1113K to less than 1173K. Insist that you can. However, in order to achieve a high frequency of 70% or more, there is a problem that a long heat treatment of several tens of hours is required and a great cost is required. Patent Document 1 states that the crystal grain size could be suppressed within 200 μm. Since crystal coarsening occurs due to thermomechanical processing, when used as nuclear material, there is a concern about a decrease in high-temperature strength. In particular, when the grain size grows to nearly ten times the base material, even if the grain size is kept within 200 μm, the reduction in strength cannot be ignored by the Hall-Petch law. Therefore, while increasing the frequency of the low ΣCSL grain boundary by thermomechanical treatment, suppression of crystal coarsening is a problem.

  In Patent Document 2, an austenitic stainless steel having a low ΣCSL grain boundary frequency of 75% or more is recited in the claims, while in the examples, only one steel type of SUS304 is obtained. Austenitic stainless steels defined by JIS standards have a wide range of chemical components, particularly Ni and Cr. According to past research, the difference in stacking fault energy of the material due to the difference in chemical composition affects the grain boundary transition, that is, the growth of low ΣCSL grain boundary. It is not reasonable to assert that it is applicable to steel. Furthermore, in Patent Document 2, after introducing a rolling rate of 2 to 15%, in order to achieve a high low ΣCSL grain boundary frequency of 75% or more, a heat treatment is performed for 5 hours or more at a temperature from 1173K to less than 1273K. There is a problem that the cost for the heat treatment becomes large.

  In Patent Document 3, in an iron-base or nickel-base face-centered cubic alloy containing Cr, after introducing a rolling rate of 5% to 30%, a heat treatment is performed at a temperature of 1173K to 1325K for 2 minutes to 10 minutes, thereby 30 μm. It is stated that the following particle size and low ΣCSL grain boundary frequency of 60% or more are achieved. According to the example, the low ΣCSL grain boundary frequency reaches a maximum of 77.1%, but the cold rolling and heat treatment processes have to be repeated several times until the required rolling rate is achieved. The total time and cost required are not low.

  In addition, several methods for improving IGSCC resistance from a viewpoint other than the low ΣCSL grain boundary using grain boundary engineering have been proposed (Patent Documents 4 and 5). For example, in Patent Document 4, in hot forging or hot rolling, by introducing a rolling rate of 30% or more into a material and then performing heat treatment, the grain boundary energy is the highest among the low ΣCSL grain boundaries. The claim mentions stainless steel having a low twin grain boundary (Σ3 grain boundary) increased to 30% or more and excellent IGSCC resistance. However, according to the example of Patent Document 4, in order to introduce a high rolling rate of 30% or more into the material, a hot forging process or a hot rolling process is required several times, and a great amount of energy is required. The increase in cost becomes a problem. 1 and 2 of Patent Document 4 indicate that the corrosion weight loss could be reduced by about 15% at most by increasing the twin grain boundaries by 30% or more. However, even if the degree of improvement of twin grain boundaries is evaluated with a threshold value of 30%, it is considered insufficient for the IGSCC resistance effect. Further, in Table 3 showing the results of the examples of Patent Document 4, it can be seen that the twins frequency was less than 50% that could be actually realized.

  In Patent Document 5, among the random grain boundaries that have been considered to have low IGSCC resistance compared to the conventional CSL grain boundary, the random grain boundary having a large misorientation at the crystal grain boundary is claimed to be superior in IGSCC resistance. In addition, a method of thermomechanical processing of stainless steel is disclosed in which the random grain boundary with an orientation difference of 50 ° or more is 20% or more. However, Patent Document 5 does not describe a comparison between CSL grain boundaries and random grain boundaries having an orientation difference of 50 ° or more with respect to IGSCC resistance. Further, there is a problem that a high rolling rate or workability of 60% or more is required, high energy is required at the machining stage, and a great deal of cost is required.

JP 2004-339576 A JP 2003-253401 A Japanese Patent Application No. 6-514639 Japanese Patent Laid-Open No. 2005-15896 JP 2005-15899 A

  As described above, any of the conventional grain boundary character control techniques requires a high rolling rate or a long heat treatment time, which is not preferable in terms of energy saving. Furthermore, the conventional technology pays attention only to the suppression effect of IGSCC generation, and the suppression effect of IGSCC progress is not expected. Looking at the results of various materials used in the industry so far, or the data of new materials born from the development of research institutions, there is still no material that can completely eradicate IGSCC. Once IGSCC occurs, the amount of progress until the periodic inspection of the equipment has practically important meaning for safe operation of the actual machine. Therefore, as an anti-IGSCC characteristic, not only suppressing the generation of IGSCC but also suppressing the progress of IGSCC once generated is an important factor.

  Accordingly, an object of the present invention is to provide an austenitic stainless steel excellent in durability by improving the IGSCC resistance characteristics in austenitic stainless steel, particularly the IGSCC progress resistance, and a method for producing the same.

The present inventors have observed that in austenitic stainless steel, two grain boundaries are the corresponding grain boundaries, and one grain boundary is random among the grain boundary triple points composed of three grain boundaries. The frequency of the grain boundary triple point (J _2CSL ), which is a grain boundary, was found to be greatly involved in the progress of IGSCC, and the conditions for controlling the frequency of this grain boundary triple point (J _2CSL ) were found. It came to be completed.

  That is, the present invention includes the following.

(1) Frequency of a grain boundary triple point (J _2CSL ) in which two grain boundaries are corresponding grain boundaries and one grain boundary is a random grain boundary in a grain boundary triple point _{composed of three} grain boundaries. Is an austenitic stainless steel having a content of 35% or more.

  (2) The austenitic stainless steel according to (1), which contains C: 0.001 to 0.100%, Ni: 12 to 30%, and Cr: 15 to 30% as mass%.

  (3) The austenitic stainless steel according to (1), wherein the crystal grain size is 40 to 80 μm.

  (4) Austenitic stainless steel obtained by cold rolling a material, which is a base material containing carbon, nickel and chromium, at a rolling rate of 2 to 5% and then heat-treating it at a temperature higher than the recrystallization temperature.

  (5) Carbon, nickel, and chromium contained in the base material are C: 0.001 to 0.100%, Ni: 12 to 30%, and Cr: 15 to 30% as mass% (4) The austenitic stainless steel described.

  (6) The austenitic stainless steel according to (4), wherein the crystal grain size is 40 to 80 μm.

  (7) A method for producing austenitic stainless steel, in which a raw material containing carbon, nickel and chromium is cold-rolled at a rolling rate of 2 to 5%, and then heat-treated at a temperature equal to or higher than the recrystallization temperature.

  (8) The method for producing an austenitic stainless steel according to (7), wherein the heat treatment temperature is 1300 K or higher.

  (9) The method for producing an austenitic stainless steel according to (7), wherein the heat treatment time is from 30 minutes to 180 minutes.

  The austenitic stainless steel according to the present invention has an excellent IGSCC resistance property by defining the frequency of the grain boundary triple point in which two grain boundaries are corresponding grain boundaries and one grain boundary is a random grain boundary. It will be. For this reason, the austenitic stainless steel according to the present invention can be applied as a structural material used in a stress corrosion environment such as a nuclear power plant or a chemical plant.

  Further, according to the method for producing an austenitic stainless steel according to the present invention, it is possible to control the frequency of grain boundary triple points in which two grain boundaries are corresponding grain boundaries and one grain boundary is a random grain boundary. Austenitic stainless steel having excellent IGSCC resistance can be produced.

It is a graph which shows distribution of the grain boundary triple point on IGSCC. It is a graph which shows the correlation of the low (SIGMA) CSL grain boundary frequency and the IGSCC grain boundary triple point frequency. It is a graph which shows the change of the number of the grain boundaries in which IGSCC generate | occur | produced by the improvement of the IGSCC grain boundary triple point frequency. It is a graph which shows the comparison of the maximum crack length in a CBB test piece. It is a graph which shows the relationship between a rolling rate and IGSCC-resistant grain boundary triple point frequency. It is a graph which shows the relationship between heat processing temperature and IGSCC-resistant grain boundary triple point frequency. It is a conceptual diagram of a boiling water reactor.

  Hereinafter, the present invention will be described in detail by dividing it into chemical components, microscopic structures, mechanical properties, manufacturing methods and practicalities.

1. Chemical Component The austenitic stainless steel of the present invention is composed of a polycrystalline metal material composed of face centered cubic crystals mainly composed of Fe. The composition of the material preferably includes C: 0.001 to 0.100%, Ni: 8 to 30%, and Cr: 15 to 30% as mass%. In addition, elements such as Mn, Mo, and Si may be included as necessary, and the total amount thereof is preferably 7% by mass or less in the material.

C: 0.001 to 0.100%
C is an element effective for obtaining strength. On the other hand, if the content exceeds 0.100%, carbides are likely to be generated at the grain boundaries of the weld heat affected zone, and the IGSCC resistance may be reduced. Therefore, the C content is preferably 0.001% or more and 0.100% or less.

Ni: 12-30%
Ni is an element necessary for maintaining the corrosion resistance of steel. Further, a content of 12% or more is necessary as a stabilizing element for austenite. On the other hand, when the content exceeds 30%, the hot workability is remarkably deteriorated. Therefore, the Ni content is preferably 12% to 30%.

Cr: 15-30%
Cr is an element necessary for maintaining the corrosion resistance of steel. In order to alleviate Cr segregation on the grain boundaries and ensure corrosion resistance, the content needs to be 15% or more. On the other hand, when the content exceeds 30%, the material is easily embrittled and the hot workability is remarkably deteriorated. Therefore, the Cr content is preferably 15% to 30%.

2. Microscopic Structure 2.1 IGSCC Grain Boundary Triple Point Frequency As described above, low ΣCSL grain boundaries have low grain boundary energy and are therefore considered to have better corrosion resistance than random grain boundaries. According to the percolation theory, in an infinite material model, when the low ΣCSL grain boundary frequency exceeds 70%, the generation site of cracks, that is, the cluster of random grain boundaries, is theoretically completely divided. That is, if the low ΣCSL grain boundary frequency exceeds 70%, even if a grain boundary crack occurs at a certain random grain boundary, it becomes difficult for the crack to be transmitted to other random grain boundaries in the measurement region. However, since the actual material has a finite volume and the grain boundary character distribution is not necessarily uniform, even if the low ΣCSL grain boundary frequency exceeds 70%, random grain boundary clusters may be connected to each other. is there. Therefore, when evaluating the IGSCC resistance, particularly the IGSCC resistance, the connectivity of actual random grain boundaries must be considered.

  Therefore, the inventors have introduced the concept of triple joint distribution (TJD). Grain boundary triple points can be divided into four types as shown in Table 1, depending on the nature of the three intersecting grain boundaries.

The grain boundary triple point J _0CSL is composed of three random grain boundaries, and its frequency is f _0CSL .

The grain boundary triple point J _1CSL is _composed of two random grain boundaries and one low ΣCSL grain boundary, and this frequency is _defined as f _1CSL .

The grain boundary triple point J _2CSL is _composed of one random grain boundary and two low ΣCSL grain boundaries, and its frequency is f _2CSL .

The grain boundary triple point J _3CSL is _composed of three low ΣCSL grain boundaries, and its frequency is f _3CSL .

In order to examine the IGSCC growth behavior at each actual grain boundary triple point, the present inventors conducted a constant displacement bending (Crevice Bent Beam, hereinafter referred to as CBB) test using austenitic stainless steel. The results of the investigation of the distribution of all the grain boundary triple points through IGSCC are shown in FIG. In the triple point J _3CSL , IGSCC is not observed at all. However, since there is no connection with random grain boundaries, it is considered that the effect of preventing the progress of IGSCC is not achieved. In the triple point J _2CSL , since the front of the crack is divided by random grain boundaries by two low ΣCSL grain boundaries, the progress is suppressed, and the number of IGSCC paths is very small. On the other hand, since both the triple point J _0CSL and the triple point J _1CSL are connected to at least one random grain boundary in the front, the progress of IGSCC is not completely prevented and continues to progress. From the above consideration, it was verified IGSCC progression suppressing effect by triple point J _2CSL. In some cases, it referred to as a resistance IGSCC grain boundary triple point to distinguish the grain boundary triple point J _2CSL other grain boundary triple points in the following description.

Furthermore, the present inventors have increased the resistance _P2CSL by increasing the IGSCC grain boundary triple point ratio P _2CSL among the grain boundary triple points that are connected to random grain boundaries, that is, the triple points J _0CSL , J _1CSL , J _2CSL. IGSCC progress was imparted to the material. Here, P _2CSL is _referred to as anti-IGSCC grain boundary triple point frequency, and can be defined by equation (1).

  In a measurement region having a sufficient number of grain boundary triple points, Equation (2) is established.

In the formula (2), n represents the number of triple points of each grain boundary. According to the percolation theory calculation based on the grain boundary triple point, when P _2CSL ≧ 35%, the random grain boundary can be completely divided. However, using conventional techniques, it is very difficult to achieve a high IGSCC grain boundary triple point frequency such that P _2CSL ≧ 35%. The present inventors conducted a grain boundary character control test on austenitic stainless steel using a conventional thermomechanical processing technique. The result is shown in FIG. In the figure, even if the low ΣCSL grain boundary frequency exceeds 80%, the IGSCC-resistant grain boundary triple point frequency is only about 30%. This _{suggests that} the frequency of triple point J 3 _CSL is increased by the growth of _twins , but the effect of fragmenting random grain boundaries is not achieved. Moreover, when the low ΣCSL grain boundary frequency approaches 90%, the IGSCC grain boundary triple point frequency reaches 35%, but in order to obtain such a high low ΣCSL grain boundary frequency, the crystal grain size is coarser to 100 μm or more. There is a risk that the strength will decrease. Therefore, in order to suppress crystal coarsening while having high IGSCC resistance, the low ΣCSL grain boundary frequency is preferably set to 75% to 85%. Furthermore, considering the IGSCC progress resistance, the IGSCC grain boundary triple point frequency is preferably 35% or more.

  In order to verify that the IGSCC resistance and IGSCC resistance are improved by improving the IGSCC grain boundary triple point frequency, the present inventors achieved a low ΣCSL grain boundary frequency of 30% or more. A CBB test was performed using the grain boundary character control material of austenitic stainless steel, and the difference from the base material (non-control material) was compared with the IGSCC resistance effect.

  According to the analysis of low ΣCSL grain boundary distribution using EBSD (Electron Backscatter Diffraction), the IGSCC grain boundary triple point frequency of the grain boundary character control material is 30.1%, and the IGSCC grain boundary triple point frequency of the non-control material. Was 17.0%. A 50 mm × 10 mm × 2 mmt CBB test piece was prepared so that the longitudinal direction was perpendicular to the cold rolling direction. In order to promote the generation of IGSCC, V-shaped notches were introduced into four locations at intervals of 10 mm, and four test pieces with V-shaped notches were produced. The notch depth is 0.5 mm, the notch opening angle is 45 °, and the curvature radius of the notch bottom is R = 0.25 mm. The strain at the bottom of the notch is 10%. Both surfaces of the test piece were finished with # 1000 emery paper. A notch CBB test piece and graphite were set in close contact with a test piece fixing jig having a curvature of 100R, and then placed in an autoclave. 288 ° C., DO (dissolved oxygen concentration) 8 ppm, conductivity (inlet) 0.1 μS / It was immersed in high temperature water below cm for 2000 hours.

  In FIG. 3, the average value for every notch of the number of grain boundaries where IGSCC occurred after the test is shown. The number of IGSCC grain boundaries of the grain boundary character control material was 23, which was smaller than 68 of the non-control material and about 1/3. Among the IGSCCs observed at a total of 16 notches in each test piece, those having the maximum crack depth were measured, and the values are shown in FIG. The maximum cracking depth of the grain boundary character controlling material was 225.5 μm, which was smaller than 657.7 μm of the non-controlling material and about 1/3.

  From the above analysis, under the same stress corrosion environment, if the IGSCC grain boundary triple point frequency is further increased to 35%, the maximum depth of stress corrosion cracking and the number of grain boundaries where stress corrosion cracking occurred are Is expected to be reduced to 1/3 or less of the conventional material.

  As described above, significant IGSCC resistance and IGSCC resistance were confirmed by increasing the IGSCC grain boundary triple point frequency from 17% to 30%. That is, resistance to grain boundary corrosion and IGSCC can be improved by controlling the grain boundary character. When such a grain boundary character control material is applied to a plant part where corrosion due to grain boundaries is a problem, such as a nuclear power plant or chemical plant, the deterioration of soundness is suppressed, and conventional materials are used. The life of the plant can be extended compared to Considering the IGSCC grain boundary triple point frequency range and the percolation theory for both test specimens verified in the above experiment, the IGSCC grain boundary triple point frequency is set to 35% or more in order to achieve the IGSCC resistance effect.

2.2 Crystal grain size In the austenitic stainless steel according to the present invention, the low ΣCSL grain boundary frequency, twin (Σ3) grain boundary frequency, and crystal grain size (including twins) were analyzed using EBSD. . The results are shown in Table 2.

  Generally, it is said that coarsening of crystal grains occurs due to heat treatment. The decrease in the grain boundary volume accompanying the coarsening of the particle diameter can bring about a mitigating effect on the intergranular corrosion cracking generated at the grain boundary. Moreover, impurities and precipitates accompanying heat treatment accumulate mainly at the grain boundaries, and reduce the strength of the grain boundaries. When the concentration of the chemical component is constant, the amount of impurities and precipitates accumulated can be suppressed by reducing the grain boundary volume, and the strength of the grain boundary can be prevented from decreasing. Therefore, it is considered that the coarsening of the grain size has an inhibitory effect on the reduction of the grain boundary strength due to the intergranular corrosion cracking and precipitates. The crystal grain size of 40 μm or more is about twice as large as 20 μm of the original material, and it is considered that an effect of suppressing intergranular corrosion cracking superior to that of the original material can be obtained. However, when the particle size is four times or more than the original particle size, the volume of the matrix that is softer than the grain boundary suddenly increases, so the strength of the material may be reduced. Therefore, in the austenitic stainless steel according to the present invention, the crystal grain size after the thermomechanical treatment is preferably 40 to 80 μm.

3. Production Method 3.1 Rolling Ratio In the material of austenitic stainless steel, the present invention provides a solid solution as a technique for obtaining an IGSCC grain boundary triple point frequency of 35% or more, and preferably a crystal grain size of 40 to 80 μm. After the heat treatment, the original plate material is cold-rolled at a rolling rate of 2% to 5% at room temperature, and annealed at a recrystallization temperature or higher, for example, a heat treatment temperature of 1300K or higher.

  In the austenitic stainless steel having the chemical components shown in Table 3, the inventors performed cold rolling at a rolling rate of 1%, 2%, 3%, 5%, and 6%, and then performed a heat treatment temperature of 1300 to 300%. Heat treatment was performed at 1500 K with a heat treatment time of 2 hours.

  The grain boundary character of each test piece after the heat treatment was analyzed by EBSD. FIG. 5 shows the relationship between the rolling rate and the IGSCC grain boundary triple point frequency. As can be seen from FIG. 5, the IGSCC grain boundary triple point frequency of 35% or more was obtained in the test pieces having a rolling rate of 2% to 5%. However, when the rolling rate was less than 1%, the grain boundary migration during the heat treatment was achieved. Was not activated, and the increase in the IGSCC-resistant grain boundary triple point frequency was slight. Moreover, when a rolling rate will be 6% or more, it is estimated that recrystallization is accelerated | stimulated by heat processing and the increase in a significant IGSCC-resistant grain boundary triple point frequency is suppressed. For the above reasons, in the production method of the present invention, a rolling rate of 2% to 5% is an optimum rolling rate range.

3.2 Heat treatment temperature and time The inventors performed cold rolling at a rolling rate of 3% on the austenitic stainless steel having the chemical components shown in Table 3, and the heat treatment time was 2 hours. And 1450K, respectively. The grain boundary character of each test piece after the heat treatment was analyzed by EBSD. Table 4 shows the relationship between the heat treatment temperature and the IGSCC grain boundary triple point frequency. In all the test pieces having a rolling rate of 4%, when the heat treatment temperature was 1350K or higher, the IGSCC grain boundary triple point frequency of 35% or more was achieved by the heat treatment for 2 hours. It took time and it was difficult to improve the low ΣCSL grain boundary frequency in a short time. Therefore, the lower limit of the temperature is set to 1300K. However, in order to obtain good heat treatment efficiency, a temperature range of 1300K to 1500K is preferable.

  In this temperature range, the most efficient heat treatment time for achieving an IGSCC grain boundary triple point frequency of 35% or more is within 30 minutes to 180 minutes.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, the technical scope of this invention is not limited to a following example.

Example 1
Austenitic stainless steels having the chemical components shown in Table 3 were subjected to cold rolling at room temperature at a rolling rate of 3% (test piece A) and 5% (test piece B), and then at a heat treatment temperature of 1450K, respectively. Annealing was performed for 120 minutes. Then, water cooling was implemented. Table 4 shows the results of grain boundary character analysis of these test pieces. Both of the two test pieces achieved an IGSCC grain boundary triple point frequency of 35% or more and a particle size of 40 to 80 μm.

(Example 2)
Austenitic stainless steels having the chemical components shown in Table 5 were cold-rolled at room temperature at a rolling rate of 3% (test piece C) and 5% (test piece D), and then at a heat treatment temperature of 1450K, respectively. Annealing was performed for 120 minutes. Then, water cooling was implemented. Table 6 shows the results of grain boundary character analysis of these test pieces. Both of the two test pieces achieved an IGSCC grain boundary triple point frequency of 35% or more and a particle size of 40 to 80 μm.

(Example 3)
Using the austenitic stainless steel having chemical components shown in Table 3 under the processing conditions of the present invention in which an IGSCC grain boundary triple point frequency of 35% or more is obtained, a boiling water reactor (BWR: Boiling) shown in FIG. Water reactor core shroud is expected to be made.

(Comparative Example 1)
For austenitic stainless steels having the chemical components shown in Table 5, after cold rolling at room temperature at a rolling rate of 6% (test piece E) and 8% (test piece F), each at a heat treatment temperature of 1450K Annealing was performed for 120 minutes. Then, water cooling was implemented. Table 7 shows the results of grain boundary character analysis of these test pieces. Both of the two test pieces have an IGSCC grain boundary triple point frequency of less than 30%, and the effect of dividing random grain boundaries is insufficient.

(Comparative Example 2)
For austenitic stainless steels having the chemical components shown in Table 5, after cold rolling at room temperature at a rolling rate of 3% (test piece G) and 5% (test piece H), Annealing was performed at a heat treatment temperature for 120 minutes or less. Then, water cooling was implemented. Table 8 shows the results of grain boundary character analysis of these test pieces. Since the heat treatment temperature was not higher than the recrystallization temperature, the recrystallization was insufficient for the two test pieces, and thus no significant improvement was obtained in both the low ΣCSL grain boundary frequency and the IGSCC grain boundary triple point frequency.

  The austenitic stainless steel according to the present invention is expected to be widely applied as a structural member used in a stress corrosion environment such as a sheath material in nuclear equipment, a nuclear power plant and a chemical plant.

Claims (9)

The frequency of the grain boundary triple point (J _2CSL ) in which two grain boundaries are corresponding grain boundaries and one grain boundary is a random grain boundary in a grain boundary triple point _{composed of three} grain boundaries is 35%. This is the austenitic stainless steel.   The austenitic stainless steel according to claim 1, comprising, as mass%, C: 0.001 to 0.100%, Ni: 12 to 30%, and Cr: 15 to 30%.   2. The austenitic stainless steel according to claim 1, wherein the crystal grain size is 40 to 80 [mu] m.   An austenitic stainless steel obtained by cold rolling a material, which is a base material containing carbon, nickel and chromium, at a rolling rate of 2 to 5% and then heat-treating it at a temperature higher than the recrystallization temperature.   The austenite according to claim 4, wherein carbon, nickel and chromium contained in the base material are, as mass%, C: 0.001 to 0.100%, Ni: 12 to 30%, and Cr: 15 to 30%. Stainless steel.   The austenitic stainless steel according to claim 4, wherein the crystal grain size is 40 to 80 μm.   A method for producing austenitic stainless steel, in which a raw material containing carbon, nickel and chromium is cold-rolled at a rolling rate of 2 to 5% and then heat-treated at a temperature equal to or higher than a recrystallization temperature.   The method for producing an austenitic stainless steel according to claim 7, wherein the heat treatment temperature is 1300K or higher.   The method for producing an austenitic stainless steel according to claim 7, wherein the time for performing the heat treatment is 30 minutes to 180 minutes.

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