JP2009161802A - Highly corrosion-resistant austenitic stainless steel, nuclear power generation plant constructed by using the stainless steel, weld joint and structural member - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To maintain the soundness of a structure of a nuclear reactor by inhibiting stress corrosion cracking even when a scar due to surface working remains on the surface when a plant has been manufactured. <P>SOLUTION: The austenitic stainless steel includes, by mass%, 17 to 20% Cr, 10 to 13% Ni, 0.001 to 0.020% C, 0.1 to 1.0% Si, 0.1 to 2.0% Mn, 0.035% or less P, 0.015% or less S, 0.01 to 3.0% Mo, 0.001 to 0.08% N, 0.01 to 2.5% Cu and the balance Fe with impurities; and has an M value of 590 to 760, which is calculated according to the expression 1: M=462×(C+N)+9.2×Si+8.1×Mn+13.7×Cr+29×(Ni+Cu)+18.5×Mo, and an S value of 24 to 45, which is calculated according to the expression 2: S=32.7+101×C-13×Si-1.2×Mn-0.9×Cr+2×Ni-5.3×Mo+0.1×Cu-179×N, wherein symbols for elements in the expressions represent mass% of each component. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a highly corrosion-resistant austenitic stainless steel suitable for use as a material for structures exposed to severe corrosive environments where stress corrosion cracking is a problem, particularly in-furnace structures of nuclear power plants, piping for high-temperature water paths, The present invention relates to a highly corrosion-resistant austenitic stainless steel suitable for preventing stress corrosion cracking of control rods, etc., and a nuclear power plant, welded joint, and structural member configured using the stainless steel.

  In recent years, stainless steel (low carbon stainless steel) having a low carbon content and high stress corrosion cracking resistance, such as SUS316L steel, has been used for the in-furnace structure of the boiling water light water reactor and the recirculation cooling water piping.

  However, in those areas where tensile stress remains due to machining of the machined surface, such as in-reactor structures and recirculated cooling water piping, if it is placed in a location where it contacts the reactor water, Damage cases that cause stress corrosion cracking during operation due to the action of the oxidizing environment of reactor water are becoming apparent. Such tensile residual stress is generated on the machined surface by machining such as grinder grinding or cutting at the time of plant production, and may be influenced by itself, or it may be caused by solidification shrinkage of the weld metal part during welding during the manufacturing process. It affects the tensile residual stress generated around the weld.

  By the way, it is said that stress corrosion cracking occurs under the condition where all the factors of material, stress, and environment (water quality) overlap, and measures to alleviate each factor have been studied. As countermeasures against stress, heat treatment and surface treatment have been proposed and partially implemented in order to mitigate tensile stress generated by welding and processing or to create a compressive stress. In addition, as environmental measures, hydrogen and precious metal injections have been proposed and partially implemented in order to reduce impurities in pure water, strictly control the conductivity and dissolved oxygen content, and reduce the corrosion potential. On the other hand, the following material countermeasures have generally been implemented or proposed before the above-mentioned damage examples of the low carbon stainless steel become apparent.

  That is, conventionally, in order to suppress stress corrosion cracking in high-temperature water, methods for suppressing intergranular corrosion, which is one of the elementary processes, have been studied. For example, optimizing the content of elements effective for corrosion resistance such as Cr and Mo, reducing the addition of C to prevent the formation of Cr-deficient regions due to grain boundary precipitation of Cr carbide, or harmful to intergranular corrosion resistance There is a technique for reducing the contents of P and S, which are important elements, and many of these findings have already been implemented. In recent years, for example, as described in Patent Document 1 (Japanese Patent Laid-Open No. 6-122946), intergranular precipitation of Laves phase and χ phase is specified as a factor of intergranular corrosion of low carbon stainless steel, and these There have been proposed chemical components that suppress the precipitation of.

  In recent years, a method for improving the stress corrosion cracking resistance has been proposed focusing on the material structure in steel. Some focus on the grain size and aim at coarsening or fine graining. For example, in Patent Document 2 (Japanese Patent Laid-Open No. 8-246106) and Patent Document 3 (Japanese Patent Laid-Open No. 8-337853), fine graining is used, and in Patent Document 4 (Japanese Patent Laid-Open No. 2005-23343), coarse graining is used. Furthermore, Patent Document 5 (Japanese Patent No. 2897694) shows that single crystallization can suppress stress corrosion cracking and intergranular corrosion which is one of the elementary processes.

  Furthermore, by focusing on the grain boundary structure in steel, the one aiming at suppression of grain boundary corrosion, which is one of the elementary processes of stress corrosion cracking, by increasing the ratio of grain boundaries having a structure effective for corrosion resistance. is there. For example, Patent Document 6 (Japanese Patent No. 2983289), Patent Document 7 (Japanese Patent Laid-Open No. 2003-253401), Patent Document 8 (Japanese Patent Laid-Open No. 2004-339576), Patent Document 9 (Japanese Patent Laid-Open No. 2005-15896). Shows that the intergranular corrosion resistance is improved by increasing the ratio of low energy grain boundaries (special grain boundaries such as twin grain boundaries) effective for corrosion resistance in austenitic stainless steels.

  On the other hand, after the above-mentioned damage examples of low-carbon stainless steel have become apparent, there is a proposal of Patent Document 10 (Japanese Patent Laid-Open No. 2006-291325) as a specific material countermeasure. Patent Document 10 proposes an austenitic stainless steel that suppresses stress corrosion cracking by limiting the chemical composition, assuming the influence of machining.

JP-A-6-122946 JP-A-8-246106 JP-A-8-337853 JP 2005-23343 A Japanese Patent No. 2897694 Japanese Patent No. 2983289 JP2003-253401A JP 2004-339576 A JP-A-2005-15896 JP 2006-291325 A

  The materials having controlled material structures described in Patent Documents 2 to 9 require complicated and advanced control in the material manufacturing process, so that the size that can be manufactured is reduced and the manufacturing cost is often increased. Considering the manufacturing process, there is a fear that the original control structure may not be maintained by molding or welding, and there are many problems to be solved for application to the plant. In other words, a material that can be manufactured without greatly changing the conventional material manufacturing process and that can maintain its characteristics even after the manufacturing process of the plant is easily applied to the plant.

  In addition, stress corrosion cracking of low-carbon stainless steel, which is becoming apparent in recent years, has been pointed out as being related to machining during production as described above. Through-grain type stress corrosion cracking is also observed on the outermost surface that has undergone strong processing, so it is considered that the material that focuses only on intergranular corrosion at least at the stage of stress corrosion cracking is not different from conventional materials.

  Recently, considering the machining during manufacturing as a factor of stress corrosion cracking, the surface processed layer generated by grinder grinding etc. during manufacturing is removed by polishing, and the above stress relaxation or stress compression measures and water quality improvement measures are also combined To reduce the overall risk of stress corrosion cracking.

  Also in materials, measures that can contribute to this risk reduction are proposed, and it is desired to comprehensively suppress stress corrosion cracking.

  From this point of view, Patent Document 10 proposes an austenitic stainless steel that suppresses stress corrosion cracking by limiting the chemical component composition (chemical component and its ratio) assuming the influence of machining.

  More specifically, Patent Document 10 discloses a structural material that is in contact with high-temperature and high-pressure water in a light water reactor, which can reduce the effect on stress corrosion cracking even when subjected to machining on the surface, and has excellent resistance to stress corrosion cracking. It is an object of the present invention to provide a welding material, a welded structure using the same, a nuclear power plant, a high corrosion resistance austenitic stainless steel member having high stress corrosion cracking resistance, and a high corrosion resistance austenitic stainless steel. 0.001 ~ 0.020%, Si0.1 ~ 1.0%, Mn0.2 ~ 2.0%, Cr16 ~ 20%, Ni9 ~ 15%, Mo3% or less, N0.001 ~ 0.12%, the balance from Fe and inevitable impurities Therefore, a welded material is proposed in which Cr and Md30 obtained by Equation 1 satisfy Equation 2.

Md30 = 551-462 (C + N) -9.2Si-8.1Mn-13.7Cr-29Ni-18.5Mo-1.42 (ν-8.0) (Formula 1)
Cr + 0.022Md30 ≧ 14.5 (Formula 2) (ν is crystal grain size number).

  However, the present inventors have found that some of the austenitic stainless steels outside the range specified in Patent Document 10 can suppress stress corrosion cracking due to the influence of machining during production.

  The present invention provides a technique that complements the invention of Patent Document 10, and from the viewpoint different from Patent Document 10, the chemical composition and material structure of austenitic stainless steel used as a material for a nuclear reactor structure. By prescribing, stress corrosion cracking can be suppressed and the soundness of the reactor structure can be maintained even when surface processing traces remain during plant production. The present invention also provides a welded joint and a structural member configured using such austenitic stainless steel.

[1] In order to solve the above-mentioned problems, the present invention has a chemical component composition (chemical component and its ratio) of high corrosion resistance austenitic stainless steel in mass%, Cr is 17 to 20%, Ni is 10 to 15%. C is 0.001 to 0.020%, Si is 0.1 to 1.0%, Mn is 0.1 to 2.0%, P is 0.035% or less, S is 0.015% or less, Containing Mo of 0.01 to 3.0%, N of 0.001 to 0.08%, Cu of 0.01 to 2.5%, the balance consisting of Fe and impurities, and M value in the following formula 1 And when calculating the S value with the following formula 2,
Formula 1:
M = 462 × (C + N) + 9.2 × Si + 8.1 × Mn + 13.7 × Cr + 29 × (Ni + Cu) + 18.5 × Mo
Formula 2:
S = 32.7 + 101 * C-13 * Si-1.2 * Mn-0.9 * Cr + 2 * Ni-5.3 * Mo + 0.1 * Cu-179 * N
Here, the element symbols in formulas 1 and 2 indicate the mass% of each component;
The M value is 590 or more and 760 or less, and the S value is 24 or more and 45 or less.
[2] Preferably, the chemical component composition of the high corrosion resistance austenitic stainless steel has the above-described chemical component composition, and further, in mass%, Mo is 0.5% to 3.0%, and Ti is 0.01. % To 0.4%, Nb from 0.01% to 0.4%, Zr from 0.01% to 1.14%, or Hf from 0.01 to 2.24%, To do.
[3] Preferably, the chemical component composition of the high corrosion resistance austenitic stainless steel has the above chemical component composition, and the material structure has an average crystal grain size of 30 μm or less, desirably 10 μm or less. And
[4] Further, preferably, the chemical component composition of the high corrosion resistance austenitic stainless steel has the above chemical component composition, and the material structure thereof is within a depth range of at least 2 mm from the surface in contact with the corrosive environment. The length direction length of the continuous grain boundary network formed of random grain boundaries excluding the grain boundaries having a Σ value of 29 or less is 1.5 mm or less.
[5] Moreover, in order to solve the said subject, this invention shall manufacture the reactor internal structure, piping, or control rod of a nuclear power plant from said austenitic stainless steel.
[6] Further, in order to solve the above-mentioned problems, the present invention provides a welded joint by welding the above-mentioned austenitic stainless steel whose surface is machined, grinding the surplus portion, polishing, and peening as necessary. Residual stress compression processing shall be performed.
[7] In order to solve the above-mentioned problems, the present invention linings the structural member on the side where the austenitic stainless steel is in contact with the environment where stress corrosion cracking occurs.

  The operation principle of the present invention configured as described above is as follows.

  In the current plant manufacturing process, machining during production is inevitable. The processed surface is considered to be the cause of stress corrosion cracking, and stress corrosion cracking can be achieved by combining surface removal layer removal by polishing, stress relaxation or stress compression measures by heat treatment and peening, and water quality improvement measures. Total risk reduction for The present invention is intended to further reduce the risk by reducing the sensitivity of the material itself to stress corrosion cracking.

  The effects of machining on the surface on stress corrosion cracking include the generation of tensile residual stress, the increase in hardness associated with the formation of the processed structure, and the generation of local corrosion paths. The increase in hardness corresponds to the embrittlement of the processed layer, and promotes the growth of the crack once it has occurred. In addition, since machining on the surface forces a large deformation at a locally high strain rate on the surface, a machining structure consisting of a deformation zone due to giant slip deformation or a shear deformation zone due to deformation twinning or martensitic transformation. Generate. The local deformation zone becomes a priority site for the corrosion phenomenon because the crystallinity is disturbed, and is a starting point and a path for the local corrosion. In addition to these factors, stress corrosion cracking is considered to occur due to the action of tensile residual stress.

  The present invention provides a material that can reduce the effect on stress corrosion cracking even when subjected to machining on the surface. Machining to the surface, such as grinder construction, has large deformations, so the tensile residual stress generated is large enough to cause stress corrosion cracking, and even if the chemical composition of stainless steel is changed, the tensile residual stress is stress corrosion cracking. It cannot be reduced to a level where no occurrence occurs. However, the present inventors have found that stress corrosion cracking can be suppressed by changing the amount of increase in hardness of the processed surface and the processed structure depending on the chemical component composition.

  As for the strongly processed surface of general 316L stainless steel, the outermost surface layer is composed of a fine crystal structure having a particle size of 50 to 200 nm, and in the deep part thereof, a processed layer structure in which a linear shear deformation band continues is exhibited. The linear shear deformation zone is a deformation zone due to giant slip deformation or a shear deformation zone due to deformation twins or ε martensite transformation. Since 304 series and 316 series austenitic stainless steels generally have low stacking fault energy, it is easy to form such a linear shear deformation band. Furthermore, in 304L stainless steel with low austenite stability, a linear shear deformation band is formed by α ′ martensite transformation. Such a linear shear deformation zone is a path of local corrosion, and becomes a factor of intragranular stress corrosion cracking which is one form of stress corrosion cracking. In addition, when the stacking fault energy is low or α ′ martensite transformation occurs, the work hardening rate increases and the hardness increases, which promotes stress corrosion cracking.

  Therefore, by increasing the stacking fault energy and suppressing the α ′ martensite transformation, it is possible to suppress the formation of linear shear deformation bands that become crack paths and reduce the hardness of the hard-worked surface. Stress corrosion cracking can be suppressed.

  The M value shown in Formula 1 is a value related to the α ′ martensite transformation ability. Formula 1 was determined with reference to the chemical composition dependence of the temperature Md30 at which 50% of the volume of the structure transformed into α ′ martensite when 30% tensile deformation was applied. The larger the M value, the less the α ′ martensite transformation occurs and the austenite becomes more stable. An M value of 590 or more is required to suppress stress corrosion cracking due to α ′ martensite. On the other hand, if the austenite becomes too stable, workability and weldability deteriorate, so the M value is limited to 760 or less.

  The S value shown in Equation 2 is a value related to stacking fault energy. Formula 2 was determined from the experimental results of characteristics related to stacking fault energy with reference to the reported chemical composition dependence of stacking fault energy. The larger the S value, the higher the stacking fault energy, and the formation of a linear shear deformation band is suppressed. In order to suppress stress corrosion cracking caused by low stacking fault energy, an S value of 24 or more is required. On the other hand, when the amount of Ni increases, the S value increases. As a result, if the austenite becomes too stable, workability and weldability deteriorate, so the S value is limited to 45 or less.

  On the other hand, by increasing the stacking fault energy and suppressing the α ′ martensite transformation, the work hardening rate is lowered, so that the strength, particularly the tensile strength, may be lowered. For members that require strength, the strength is improved by strengthening the grain refinement. By refining the crystal grain of a material having an average grain size of about 90 μm to an average grain size of 30 μm, the tensile strength can be increased by about 20 Mpa, and when the crystal grain is refined to an average grain size of 10 μm, it is increased by about 40 Mpa. be able to.

  Suppressing intragranular stress corrosion cracking as described above can greatly reduce the stress corrosion cracking susceptibility associated with strong processing. However, in order to ensure complete soundness against stress corrosion cracking, further intergranular stress corrosion cracking is required. Measures against this are also necessary. First, in order to suppress grain boundary sensitization associated with Cr carbide precipitation, Mo, which is a sensitization-inhibiting element, or Ti, Nb, Zr, Hf, etc., which are C-stabilizing elements, are added together with a reduction in the amount of C.

  It is known that the bondability and corrosion resistance of adjacent crystal grains differ depending on the grain boundary character. If the grain boundary network consisting of random grain boundaries with high energy is increased by thermomechanical treatment and the grain boundary network consisting of random grain boundaries with high energy is divided, corrosion and stress corrosion cracking will avoid the corresponding grain boundaries and become random grain boundaries with high energy. Often along the way. Considering that the detection limit of cracks by ultrasonic flaw detection is about 2 mm, if the progress of the generated stress corrosion cracking can be stopped within 2 mm from the surface, stress corrosion cracking will be generated on the inspection. It was suppressed. Further, there is no problem in strength as long as the structure has a sufficient thickness. The depth of the processing-affected layer by the strong surface processing is 300 to 400 μm, and the observed intragranular stress corrosion cracking may reach a depth of 200 to 300 μm. Furthermore, although the intergranular stress corrosion cracking progresses in the deep part, if the progress can be stopped within about 1.5 mm, the total is stopped within the depth of 2 mm. By setting the length in the depth direction of the random grain boundary network in consideration of the three-dimensional distribution to 1.5 mm or less, the grain boundary type stress corrosion cracking along the random grain boundary can be stochastically stopped.

  C is an element necessary for securing the material strength, and its effect can be obtained at 0.001% or more. If it exceeds 0.020%, grain boundary sensitization due to grain boundary precipitation of Cr carbide becomes remarkable, and intergranular corrosion resistance is impaired. Therefore, the range of the C content is set to 0.001 to 0.020%.

  Si is an element necessary as a deoxidizing material in the raw material manufacturing process, and a deoxidizing effect can be obtained at 0.1% or more. If it exceeds 1.0%, the toughness decreases. Therefore, the range of the Si content is set to 0.1 to 1.0%.

  Mn is an element effective for deoxidation in the raw material production process, and a deoxidation effect can be obtained at 0.1% or more. If it exceeds 2.0%, the corrosion resistance decreases. Therefore, the range of the Mn content is set to 0.1 to 2.0%.

  P is an impurity element in steel, which lowers the corrosion resistance of grain boundaries and causes hot cracking during welding. Therefore, it is desirable to reduce the content as much as possible. However, in order to reduce the content, the raw material manufacturing cost becomes high. Therefore, considering the economic efficiency, the current normal process level is set to 0.035% or less.

  S is an impurity element in steel, which lowers the corrosion resistance of grain boundaries and causes hot cracking during welding. Therefore, it is desirable to reduce the content, and the upper limit of the content is set to 0.015%. .

  Cr is an element necessary for improving the corrosion resistance, and an effect can be obtained at 17% or more in a high-temperature water environment. If it exceeds 20%, the hot workability is lowered and the economy is impaired. Therefore, the Cr content range is set to 17 to 20%.

  Ni is an element effective in improving corrosion resistance and forming an austenite phase, and these effects can be obtained at 10% or more. If it exceeds 15%, the stability of the austenite phase is increased, workability and weldability are lowered, and economic efficiency is impaired. Therefore, the range of Ni content is set to 10 to 15%.

  Cu is an element effective in improving corrosion resistance and forming an austenite phase. Since it is a component contained in ordinary stainless steel and has a great influence on the α ′ martensite transformation ability, the upper limit is set to 2.5%.

  Mo has the effect of suppressing grain boundary sensitization, further suppressing irradiation-induced segregation in an irradiation environment, and improving corrosion resistance. The effect can be obtained by adding Mo in an amount of 0.01% or more, more preferably more than 0.4%. However, if it exceeds 3.0%, intermetallic compounds precipitate at the grain boundaries, and intergranular corrosion resistance. Deteriorates. Therefore, the range of the Mo content is set to 3% or less.

  N is an element necessary for securing the material strength, and the effect can be obtained at 0.001% or more. If it exceeds 0.08%, the sensitivity to intragranular stress corrosion cracking increases. Therefore, the range of N content is set to 0.001 to 0.08%.

  Zr and Hf form carbides with C from a small amount, and suppress grain boundary sensitization. Furthermore, it has the effect | action which suppresses irradiation-induced segregation in an irradiation environment and improves corrosion resistance. When improvement in corrosion resistance by containing Zr and Hf is required, the effect can be obtained by containing at least 0.2% of one or more of these elements. Even if it contains more than 1.14% Zr and 2.24% Hf, the effect is saturated. Therefore, the content ranges were set to 0.01% to 1.14% for Zr and 0.01 to 2.24% for Hf.

  By using austenitic stainless steel that satisfies the above conditions, the potential for stress corrosion cracking of the material itself when subjected to machining on the surface can be reduced. Furthermore, by implementing various measures such as removal of surface processed layers by polishing, stress relaxation or stress compression measures by heat treatment and peening, and water quality improvement measures, the overall risk reduction for stress corrosion cracking is achieved, and the reactor structure The health of things can be maintained.

  According to the present invention, it is possible to suppress the occurrence of stress corrosion cracking in a member in which surface processing marks remain at the time of plant manufacture and which is exposed to high-temperature high-pressure water in a nuclear reactor structure. The material shown in the present invention is suitable for application to a welded joint and a structural member disposed in an environment where a nuclear reactor structure or stress corrosion cracking occurs.

<Example (chemical component composition)>
Table 1 is a table showing the chemical components of the test materials used as examples of the present invention and the M and S values calculated from the chemical components in Formulas 1 and 2.

  FIG. 1 shows the range of the material of the present invention in M value and S value. Each sample material was subjected to a sensitizing heat treatment simulating a weld after the solution heat treatment. The surface of the test piece was machined with a grinder or the like, loaded with a constant strain, and then immersed in high pressure high temperature water at 288 ° C. and 8.3 MPa for a maximum of 5000 hours. After immersion, the stress corrosion cracking susceptibility was evaluated by the presence or absence of cracks on the surface of the test piece.

  Table 2 shows the results of stress corrosion cracking tests performed on these test materials in high-pressure and high-temperature water. The stress corrosion cracking test was performed by changing the machining conditions and test water quality conditions and dividing the test materials. In the primary evaluation, the results of tests under different conditions were organized and divided into those that did not crack and those that did not. Secondary evaluation was performed on the test material that was not cracked in the primary evaluation and the test material used as a comparative material. In the secondary evaluation, severe conditions were selected in terms of surface machining, applied strain, and test water quality, and tests were performed under the same conditions. As a result, stress corrosion cracking occurred in any of the test materials, but a significant difference occurred in the number of cracks and crack depth. Compared to the test materials 13, 14, 15, 23, the present invention material had an average number of cracks of about one-third or less. In addition, when the specimen 15 was used as a reference even at the maximum crack depth, the material of the present invention was about one-half or less.

  Table 3 shows the results of measuring the Vickers hardness at a position 20 μm deep from the ground surface with respect to the ground surface. For comparison, the results of Vickers hardness measurement at a position of 2 mm depth not affected by surface processing are also shown.

  Looking at the Vickers hardness measured at three positions, the hardness of the present invention material was lower than that of the comparative material. For the specimen 15, the increase in hardness due to surface processing was low. Although there is no significant difference in hardness increase due to surface processing for the high N material 23, the hardness on the surface is high because the hardness of the material 23 in the unprocessed state is high. It was. The material of the present invention has a chemical component composition that has a high stacking fault energy and a small processing-induced α 'martensite transformation ability. As a result, the work hardening rate is reduced, and the effect of suppressing the increase in hardness due to surface processing is suppressed. is there.

  The material of the present invention has a chemical composition that suppresses coarse sliding, deformation twinning, and processing-induced martensite transformation, and does not deform locally with a linear shear deformation band in the material structure. Since the cross slip of dislocations is relatively easy, it is easily deformed uniformly. In addition to suppressing the increase in hardness due to surface processing, the present invention material suppresses the occurrence of stress corrosion cracking by suppressing the generation of linear shear deformation bands that form the intragranular stress corrosion cracking path in the processed structure. effective.

<Strengthen crystal refinement>
On the other hand, since the work hardening rate is lowered in the material of the present invention, the strength, particularly the tensile strength, tends to be lowered, which is not desirable in a portion where strength is required. For the member used in such a part, a material strengthened by crystal grain refinement is used. In order to show the effect of grain refinement on the strength of the test material 6 which is one of the materials of the present invention in Table 4, the grain size of the test material 6 is refined by processing heat treatment (processing and heat treatment such as rolling and forging). The result of a tensile test in the case of the change is shown. After the normal solution heat treatment, the average crystal grain size was 90 μm, and the average crystal grain size was refined to 25 μm to 9 μm by subjecting it to a heat treatment. As a result, as shown in Table 4, proof stress or tensile strength could be increased.

<Grain boundary character distribution control>
In the secondary evaluation of the stress corrosion cracking test of Table 2, since severe conditions were selected, stress corrosion cracking occurred in all the test pieces. Stress corrosion cracking has shifted to intergranular cracking in the deep part, and measures to suppress intergranular stress corrosion cracking are necessary to suppress crack growth. It was observed that the crack depth was lowered in the test material to which the stabilizing elements Nb to Ti were added and the test material to which the impurity elements were reduced and Hf and Zr were added. In addition to these, focusing on the fact that the bonding strength and corrosion resistance of adjacent crystal grains differ depending on the grain boundary character, suppress the development of intergranular stress corrosion cracking in the material of the present invention that can suppress the occurrence of stress corrosion cracking due to surface hard working Grain boundary character distribution control was examined in terms of adding functions.

  In plant maintenance, damage due to stress corrosion cracking is inspected by visual inspection or ultrasonic inspection during periodic inspection. Considering that the limit of crack detection by ultrasonic flaw detection is about 2 mm, if the progress of the generated stress corrosion cracking can be stopped within a depth of 2 mm from the surface, the occurrence of stress corrosion cracking is suppressed for inspection. That's right. Further, in the case of a thick member that requires inspection by ultrasonic flaw detection, a crack with a depth of about 2 mm has no problem in strength. The processing-affected layer introduced by surface processing is 300 to 400 μm, and the depth of intragranular stress corrosion cracking observed so far is 200 to 300 μm even if it is deep. From this, it is considered that if the intergranular stress corrosion cracking in the deep part is suppressed to about 1.5 mm, the entire depth can be suppressed to within 2 mm.

  FIG. 2 shows an analysis of the normal structure after solution heat treatment of specimen 6 and the structure subjected to grain boundary character distribution control by processing heat treatment (processing and heat treatment such as rolling and forging) by an electron backscatter diffraction method. A grain boundary character distribution map is shown. FIG. 2A shows a normal structure, and FIG. 2B shows a structure subjected to grain boundary character distribution control. Here, the grain boundary character adopts the Σ value determined by the crystal orientation relationship between adjacent crystal grains, the grain boundary having a Σ value of 1 to 29 as the corresponding grain boundary, and the other grain boundaries as random grain boundaries. Treated as. The grain boundaries shown by the black lines in FIG. 2 are random grain boundaries, and the grain boundaries shown in white are corresponding grain boundaries. In the normal structure of FIG. 2A, random grain boundaries are distributed in a network form, but in the grain boundary character distribution control structure of FIG. 2B, the network of random grain boundaries is divided.

  When the vertical direction in FIG. 2 is taken as the depth direction of the member, the length in the depth direction can be obtained for a network in which random grain boundaries are continuous as described in the figure. The maximum depth direction length of each visual field was measured from 10 visual fields, and the mode value of the maximum value distribution was found to be 450 μm. When the stress corrosion cracking test was actually performed on the material of the present invention, it was difficult to obtain a deep crack even under severe test conditions. Therefore, the amount of carbon was increased to 0.06%, and a stress corrosion cracking test was conducted by sensitizing a material which was similarly subjected to grain boundary character distribution control. Table 5 shows a comparison of the results with a normal tissue material. The maximum crack depth of this grain boundary character distribution control material was 870 μm, which was smaller than that of the normal structure material. The stress corrosion cracking generally progressed along random grain boundaries. The maximum value distribution mode of the length direction of the random grain boundary in the grain boundary character distribution control material was 460 μm. In consideration of this, the random grain boundaries are coupled in three dimensions and progress around. However, the depth of this test is about twice as high as the maximum value distribution mode in the test results, and it can be said that it takes a long time for the crack to grow three times as long.

  This indicates that the grain boundary character distribution controlled structure exists at least 2 mm from the surface, and the length in the depth direction of the network in which random grain boundaries are continuous is three times the maximum value distribution mode. If the value of is 1.5 mm or less, it is considered that the probability that the intergranular stress corrosion cracking progresses by 1.5 mm is considerably low.

<Application to reactor structure>
Using the austenitic stainless steel of the present invention, various structural members for a boiling water reactor core shown in FIG. 3 were produced. This reactor is operated at a steam temperature of 286 ° C. and a steam pressure of 70.7 atg, and can generate 500, 800, 1100 MW of power as power generation output.

  The names are as follows.

  Neutron source pipe 51, core support plate 52, neutron instrumentation detection tube 53, control rod 54, core shroud 55, upper lattice plate 56, fuel assembly 57, upper mirror spray nozzle 58, vent nozzle 59, pressure vessel lid 60, Flange 61, measuring nozzle 62, steam separator 63, shroud head 64, feed water inlet nozzle 65, jet pump 66, steam dryer 68, steam outlet nozzle 69, feed water sparger 70, reactor water spray nozzle 71, lower core Lattice 72, recirculation water inlet nozzle 73, baffle plate 74, control rod guide tube 75.

  Using a steel plate and tube formed and solution-treated by hot rolling or hot forging to a predetermined thickness and shape, the core support plate 52, the core shroud 55, and the upper lattice plate 56 are cut, bent, and welded. A jet pump 66, a feed water sparger 70, a reactor water spray nozzle 71, a lower core lattice 72, and a recirculation water inlet nozzle 73 were produced. For the welded portion, after welding, the surplus portion was ground, polished, and subjected to residual stress compression treatment by peening as necessary.

  FIG. 4 shows a perspective view of a control rod made of the austenitic stainless steel of the present invention. Since the control rod is exposed to a high irradiation dose, the present invention material containing 0.8% of Hf was applied to suppress irradiation segregation, and a handle, a sheath, and a blade portion were produced.

  For piping that becomes a pressure boundary, such as recirculation piping, strength is required. In the austenitic stainless steel of the present invention, the carbon and nitrogen contents are suppressed, so that the strength is ensured by refining crystal grains. When the seamless pipe was manufactured, the average crystal grain size was 30 μm or less, and 10 μm or less depending on conditions, by finely pulverizing and drilling in advance. In some of the pipes, the grain boundary character distribution was changed by a cold treatment inside the pipe and subsequent heat treatment. As a result, a grain boundary character distribution control structure layer in which the length in the depth direction of a continuous grain boundary network composed of random grain boundaries was 1.5 mm or less from the inner surface of the pipe to a depth of 2 mm or more could be formed.

  On the other hand, in a large-diameter tube, when the volume is increased, the amount of processing is limited due to the limitation of the press capacity, and it is difficult to make fine particles. Therefore, a clad pipe in which the austenitic stainless steel of the present invention is lined inside a conventional low-carbon stainless steel pipe for nuclear power is used.

  FIG. 5 is a schematic view of a cross section of a welded joint portion of a pipe using the austenitic stainless steel of the present invention described above. FIG. 5 (a) shows a welded joint of a pipe using austenitic stainless steel having the chemical composition of the present invention, which has not been subjected to special processing heat treatment, and FIG. FIG. 5 (c) shows a welded joint of a pipe using the austenitic stainless steel of the present invention subjected to refinement strengthening, and FIG. 5 (c) further shows the austenitic stainless steel of the present invention in which grain boundary character distribution control is applied to the inside of the pipe FIG. 5 (d) shows a pipe welded joint using the austenitic stainless steel of the present invention in which grain boundary character distribution control is applied to the inner diameter matching portion inside the pipe, FIG.5 (e) shows the welded joint of the pipe | tube which gave the lining which consists of an austenitic stainless steel of this invention inside piping. There is a potential for stress corrosion cracking in the vicinity of the welded part inside the pipe due to internal residual machining and tensile residual stress due to welding. can do. Furthermore, the inner and outer surface temperature difference was given by induction heating to compress the internal stress, and a pipe joint having higher reliability against material damage due to stress corrosion cracking was produced.

  Using the austenitic stainless steel of the present invention, various structural members for an improved boiling water reactor core shown in FIG. 6 were produced. This reactor can generate 1350 MW as a power generation output.

  The names are as follows.

  A core support plate 52, a control rod 54, a core shroud 55, an upper lattice plate 56, a fuel assembly 57, a steam separator 63, a steam dryer 68, a steam outlet nozzle 69, and an internal pump 126;

  Using steel plates and tubes formed and solution-treated by hot rolling or hot forging to a predetermined thickness and shape, core support plate 52, control rod 54, core shroud 55, by cutting, bending and welding, An upper lattice plate 56, a water supply sparger 70, a reactor water spray nozzle 71, a lower core lattice 72, and an internal pump 126 were produced. For the welded portion, after welding, the surplus portion was ground, polished, and subjected to residual stress compression treatment by peening as necessary.

  Table 6 compares the specifications of the boiling water reactor and the improved boiling water reactor.

  One of the features of the improved boiling water reactor is that the recirculation system is changed from the external recirculation pump and jet pump of the boiling water reactor to the internal pump, and no recirculation piping is required. One. The nozzle part to which the internal pump is attached does not affect the rotation function of the internal pump even if temperature and pressure changes occur in the reactor pressure vessel, and heat transfer to the electric motor part is reduced. The sleeve shape is optimal. In addition, the internal structure of the furnace reduces the influence on the flow vibration caused by the internal pump.

  The reactor pressure vessel constitutes the pressure boundary of the coolant and has a function of incorporating and holding the core and the pressure vessel internal structure. The boiling water reactor contains 764 fuel assemblies, jet pumps and internal structures and has an inner diameter of about 6.4 m, whereas the improved boiling water reactor has 872 fuel assemblies. In addition, the inner diameter of the internal pump is increased to about 7.1 m by securing a space for handling the internal pump in the furnace. On the other hand, the internal height is about 22m in the boiling water reactor due to the adoption of a high-efficiency steam separator, improved control rod drive mechanism, upper lid / main flange structure change, and lower mirror shape change. The improved boiling water reactor is slightly compact, about 21m. With the adoption of the internal pump, the lower mirror shape was changed from a hemispherical boiling water reactor to a dish type in consideration of the space required for installation of the internal pump under the pressure vessel and the cooling water circulation flow path. . The internal pump was manufactured by integral forging and designed with a small number of weld lines. The support skirt 127 has a conical shape so as to secure a necessary space for handling the internal pump and install the internal pump heat exchanger in the pedestal.

  In boiling water reactors and improved boiling water reactors, each member made of the austenitic stainless steel of the present invention has improved tolerance to material damage due to stress corrosion cracking, and causes of stress corrosion cracking By combining with measures for stress and water quality, it is expected that it can be used without replacement for over 30 years, except for replacement equipment such as control rods and fuel assemblies. Further, after 12 months of operation, periodic inspections are repeatedly performed within 50 days, preferably within 40 days, particularly preferably within 30 days, and the operation rate is 85% or more, preferably 90% or more, particularly preferably. Is operated at 92% and thermal efficiency of 35%.

  Since the austenitic stainless steel of the present invention can reduce the potential for the occurrence of stress corrosion cracking of the material itself when subjected to machining on the surface, it is necessary to take measures against stress corrosion cracking, etc. It is expected to function effectively for the next-generation nuclear reactor and the next generation nuclear reactor under development.

It is a figure which shows the range of this invention material with respect to M value and S value which are calculated from a chemical component. It is a figure which shows the grain-boundary character distribution of the material cross section measured by the electron beam backscattering pattern method, FIG. 2 (a) shows a normal structure | tissue, FIG.2 (b) shows the structure | tissue which performed grain boundary character distribution control. 1 is a partial cross-sectional perspective view showing a boiling water reactor core using the austenitic stainless steel of the present invention. It is a fragmentary sectional perspective view which shows the control rod using the austenitic stainless steel of this invention. FIG. 5 (a) is a schematic cross-sectional view of a welded joint portion of a pipe using the austenitic stainless steel of the present invention, and FIG. 5 (a) is an austenitic stainless steel having the chemical composition of the present invention and subjected to a special thermomechanical treatment. Fig. 5 (b) shows a pipe welded joint using the austenitic stainless steel of the present invention that has been refined and refined, and Fig. 5 (c) Fig. 5 (d) shows the welded joint of a pipe using the austenitic stainless steel of the present invention in which the grain boundary character distribution control is applied to the inside of the pipe. FIG. 5 (e) shows a pipe welded joint with a lining made of the austenitic stainless steel of the present invention on the inner side of the pipe. Each show. It is sectional drawing which shows the improved boiling water reactor core using the austenitic stainless steel of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Hydraulic pump 1a Swash plate 1b Discharge oil path 2-5 Actuator (2: Swing motor)
7a, 7b Directional switching valve 8-10 Directional switching valve 210 Cab

Claims (7)

In mass%, Cr is 17-20%, Ni is 10-15%, C is 0.001-0.020%, Si is 0.1-1.0%, Mn is 0.1-2.0% , P is 0.035% or less, S is 0.015% or less, Mo is 0.01 to 3.0%, N is 0.001 to 0.08%, Cu is 0.01 to 2.5%. When the balance is Fe and impurities, and the M value is calculated by the following formula 1 and the S value is calculated by the following formula 2,
Formula 1:
M = 462 × (C + N) + 9.2 × Si + 8.1 × Mn + 13.7 × Cr + 29 × (Ni + Cu) + 18.5 × Mo
Formula 2:
S = 32.7 + 101 * C-13 * Si-1.2 * Mn-0.9 * Cr + 2 * Ni-5.3 * Mo + 0.1 * Cu-179 * N
Here, the element symbols in formulas 1 and 2 indicate the mass% of each component;
The M value is 590 or more and 760 or less, and the value is 24 or more and 45 or less.   In mass%, Mo is more than 0.4% and not more than 3.0%, Ti is 0.01% to 0.4%, Nb is 0.01% to 0.4%, Zr is 0.01% to 1%. The high corrosion-resistant austenitic stainless steel according to claim 1, containing 0.14% or Hf in an amount of 0.01 to 2.24% or more.   The high-corrosion-resistant austenitic stainless steel according to claim 1 or 2, wherein the average crystal grain size is 30 µm or less.   The depth direction length of the continuous grain boundary network composed of random grain boundaries excluding the grain boundaries with a Σ value of 29 or less within a depth range of at least 2 mm from the surface in contact with the corrosive environment is 1.5 mm or less. The highly corrosion-resistant austenitic stainless steel according to any one of claims 1 to 3.   A nuclear power plant comprising an in-furnace structure, piping, or control rods manufactured from the highly corrosion-resistant austenitic stainless steel according to any one of claims 1 to 4.   After welding the highly corrosion-resistant austenitic stainless steel according to any one of claims 1 to 4 whose surface has been machined, the surplus portion is ground, polished, and subjected to residual stress compression treatment by peening as necessary. A welded joint characterized by   A structural member characterized by lining the highly corrosion-resistant austenitic stainless steel according to any one of claims 1 to 4 on a side in contact with an environment where stress corrosion cracking occurs.

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