JP4503483B2 - Material to be welded, welded structure using the same and high corrosion resistance austenitic stainless steel - Google Patents

Description

  The present invention relates to a welded material having stress corrosion cracking resistance exposed to severe corrosive environments such as a reactor internal structure of a nuclear power plant and piping of a high-temperature water path, and a welded structure using the welded material and a nuclear power plant. The present invention relates to a high corrosion resistance austenitic stainless steel having high stress corrosion cracking property.

  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 these in-reactor structures and recirculated cooling water pipes, especially where the machined surface remains, in locations where tensile residual stress exists on the surface in contact with the reactor water, the reactor water It is necessary to further improve the stress corrosion cracking resistance during operation due to the action of oxidizing environment. Such tensile residual stress is generated around the welded part due to solidification shrinkage of the weld metal part during the manufacturing process, particularly during welding, and may be generated on the work surface by machining.

  It is said that stress corrosion cracking occurs under conditions 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.

  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 are techniques for reducing the contents of P and S, which are various elements, and many of these findings have already been implemented.

  In recent years, for example, as described in Patent Document 1, a chemical component that specifies Laves phase and χ phase grain boundary precipitation as a cause of intergranular corrosion of low carbon stainless steel and suppresses these precipitations has been proposed. Has been.

  Further, a technique for improving the stress corrosion cracking resistance has been proposed by paying attention to the material structure in steel. Some focus on the grain size and aim at coarsening or fine graining. For example, Patent Documents 2 and 3 show that fine graining can be suppressed, and Patent Document 4 shows that stress corrosion cracking and intergranular corrosion, which is one of the elementary processes, can be suppressed by roughening the surface.

  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 5 and Patent Document 6 improve the intergranular corrosion resistance by increasing the ratio of low energy grain boundaries (special grain boundaries such as twin grain boundaries) effective for corrosion resistance in austenitic stainless steel. Has been shown to do.

JP-A-6-122946 JP-A-8-246106 JP-A-8-337853 JP 2005-23343 A JP 2004-339576 A JP-A-2005-15896

  The material having the material structure controlled in Patent Document 1 requires complicated and sophisticated control in the material manufacturing process, so that the size that can be manufactured is reduced, the manufacturing cost is often increased, and the manufacturing process of the plant is considered. Then, there is a fear that the initial 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.

  It has been pointed out that the stress corrosion cracking of low-carbon stainless steel, which is becoming apparent in recent years, is related to machining during production. Since the outermost surface that has undergone strong processing is concerned about through-grain-type stress corrosion cracking, 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.

  In the proposals of Patent Documents 2 to 6, it is desired to propose measures that can contribute to this risk reduction and to comprehensively suppress stress corrosion cracking.

  The object of the present invention is to provide a welded material excellent in stress corrosion cracking resistance and its effect as a structural material in contact with high-temperature 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. It is an object of the present invention to provide a welded structure and a nuclear power plant used, a high corrosion resistance austenitic stainless steel member having high stress corrosion cracking resistance, and a high corrosion resistance austenitic stainless steel.

In the present invention, by mass, C0.001 to 0.020%, Si0.1 to 1.0%, Mn0.2 to 2.0%, Cr16 to 20%, Ni9 to 15%, Mo3% or less, N0. containing 001 to 0.12%, the balance being Fe and inevitable impurities, the Cr and the _{Md 30} obtained by the (equation 1) meets the equation (2), grain size number is 4.0 to 7. The material to be welded is characterized by being zero .

_{Md 30 = 551-462 (C + N} ) -9.2Si-8.1Mn-13.7Cr-29Ni-18.5Mo-1.42 (ν-8.0) ...... ( Equation 1)
Cr + 0.022Md ₃₀ ≧ 14.5 (Formula 2)
(In the formula, each element is% by mass, and ν is the grain size number)
Md ₃₀ is expressed in (° C.) as a unit and is referred to as an austenite stabilization index. The larger the value is on the negative side, the higher the austenite stability. In the present invention, the austenite stability is set according to the relationship with the Cr content.

The inevitable impurities are Cu 0.05% or less, Nb 0.05% or less, P 0.035% or less, and S 0.015% or less, and the Md ₃₀ is obtained by (Equation 3).

Md ₃₀ = 551-462 (C + N) -9.2Si-8.1Mn-13.7Cr-29 (Ni + Cu) -18.5Mo-68Nb-1.42 (ν-8.0) (Formula 3)
(In the formula, each element is% by mass, and ν is the grain size number)
The Mo is 0.001 to 0.2% by mass, or 0.25 to 2.5% by mass, and further, by mass, Zr 0.2 to 1.14% and Hf 0.2 to 2.24%. It is preferable to contain 1 or more types of these.

The present invention, tensile stress - that stress at elongation of 40% in elongation curve is not more than 510 MPa, also has a cold plastic working layer on the surface, the thickness of the cold plastic working layer in 1~500μm Preferably there is. 10-200 micrometers is more preferable.

  The present invention lies in a welded structure in which the welded material is connected by an overlay weld, and the welded material is made of the welded material described above.

A cold plastic working layer is formed on the weld heat affected zone on the surface of the weld weld and the surface of the workpiece, and grinder grinding is performed on the weld heat affected zone on the surface of the weld weld and the workpiece surface. Alternatively, it is preferable to perform flapper wheel grinding and to have a cold plastic working layer having the above-described thickness.

  The present invention resides in a nuclear power plant characterized in that at least one of the in-furnace structure and the recirculation cooling water pipe is formed of the welded structure described above.

In the present invention, by mass, C0.001 to 0.020%, Si0.1 to 1.0%, Mn0.2 to 2.0%, Cr16 to 20%, Ni9 to 15%, Mo3% or less, N0. 001 to 0.12%, the balance is Fe and inevitable impurities, Cr and Md ₃₀ calculated by the above (Formula 1) satisfy the above (Formula 2), and the surface is a cold plastic working layer The inevitable impurities are Cu 0.05% or less, Nb 0.05% or less, P 0.035% or less, and S 0.015% or less, and the Md Is obtained by the above-described (Equation 3).

In the present invention, by mass, C0.001 to 0.020%, Si0.1 to 1.0%, Mn0.2 to 2.0%, Cr16 to 20%, Ni9 to 15%, Mo3% or less, N0. 001-0.12%, the balance is Fe and inevitable impurities, Cr and Md ₃₀ determined by the above (Formula 1) satisfy the above (Formula 2), and the crystal grain size number is 4.0. It is in a high corrosion resistance austenitic stainless steel characterized by being -6.0, the inevitable impurities are the same as described above, and the Md ₃₀ is obtained by the above (Formula 3). The amount of Mo, the amount of Zr and the amount of Hf are the same as described above.

  The present invention, by defining the characteristics and chemical composition of the austenitic stainless steel used as the material for the reactor structure, suppresses stress corrosion cracking even when surface processing traces remain during plant production, It maintains the soundness of the reactor structure.

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 surface machining 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 once the crack is generated, it promotes the propagation of the crack. 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 due to disorder of crystallinity, and becomes a starting point and a path of 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 surface machining by reducing the sensitivity of the material itself to stress corrosion cracking. 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, it has been found that there is a difference in stress corrosion cracking behavior depending on the chemical composition.

  One of the factors of this difference is the Cr concentration related to the ability to form a passive film. Even if the grain boundary or deformation zone becomes a local corrosion site, if the Cr concentration is high, a strong passive film mainly composed of Cr is formed, and even if the film is torn once, the film is quickly regenerated. Therefore, local corrosion can be suppressed under the environment of high-temperature and high-pressure water.

  Another factor is the processing-induced martensitic transformation ability. Martensite is α ′ martensite having a body-centered cubic lattice structure and ε martensite having a hexagonal close-packed lattice structure. As described above, the machined surface is subjected to high-speed and large deformation, and thus has a processed structure having a large number of deformation bands. It has been observed that through-grain type stress corrosion cracking takes the interface of this deformation zone. The reason why the deformation zone becomes a priority site for corrosion is that the crystallinity at the interface is disturbed and the energy is high. Also, some grain boundaries on the machined surface concentrate deformation due to the difference in crystal orientation with the surrounding crystal grains. In such a grain boundary, the crystallinity of the end faces of the adjacent crystal grains via the grain boundary is disturbed, and the energy is higher than that in the unprocessed state, which becomes a priority site for corrosion.

  If the processing-induced martensitic transformation ability is high, it is stable at the atomic level, not the face-centered cubic lattice structure of the parent phase but the body-centered cubic lattice structure or the hexagonal close-packed lattice structure. The rise is small, and the potential for the disordered crystallinity to become the preferred site for corrosion decreases. Further, at the interface of the deformation zone and at the grain boundary where deformation is concentrated, a large strain is inherently present and a large internal stress is present, so that the potential for corrosion and cracking is high. Since the martensitic transformation is accompanied by a volume change, the strain can be reduced and the internal stress can be relaxed by the formation of the martensite phase, thereby lowering the potential for corrosion and cracking.

When the amount of deformation is large, α ′ martensite produces a larger amount of transformation than ε martensite. Therefore, α ′ martensite was selected as a target for controlling the processing-induced martensite transformation ability by chemical components. There are various notation methods for the α ′ martensitic transformation ability, but the temperature M d ₃₀ (° C.) at which 50% martensitic transformation occurs when a true strain of 0.3 is applied is used as an index. M d ₃₀ (° C.) is determined by the chemical component and the crystal grain size number as shown in (Formula 1) and (Formula 3). The higher the M d ₃₀ (° C.), the higher the α ′ martensite transformation ability. When the relationship represented by the Cr concentration and M d ₃₀ shown in mass% _(° C.) for (Equation 2) is satisfied, show excellent stress corrosion cracking resistance in high-temperature high-pressure water.

  Further, in the chemical component within the range represented by (Formula 2), an α ′ martensite phase is generated by deformation-induced transformation during deformation. As described above, the generation of martensite lowers the potential of stress corrosion cracking and cracking by reducing the energy of the deformation concentrated portion and relaxing internal stress. However, depending on the generation amount and distribution in the material structure, the material may be hardened and the potential for stress corrosion cracking may be increased. As the index, the stress value at 40% elongation in the tensile stress-elongation curve is adopted, and if it is 510 MPa or less, the potential for stress corrosion cracking is low. Reduction of the amount of Mo is effective for reducing the potential for the occurrence of stress corrosion cracking from this viewpoint, and is achieved by making Mo 0.1% or less by mass.

  C is an element necessary for securing the material strength, and the effect is obtained at 0.001% or more. When it exceeds 0.020%, grain boundary sensitization due to grain boundary precipitation of Cr carbide becomes remarkable, and intergranular corrosion resistance is impaired. The C content should be 0.001 to 0.020%. Preferably it is 0.001 to 0.016%.

  Si is an element necessary as a deoxidizing material in the material manufacturing process, and a deoxidizing effect can be obtained at 0.1% or more. If it exceeds 1.0%, the toughness decreases. The Si content should be 0.1-1.0%. Preferably, it is 0.2 to 0.6%.

Mn is an element effective for deoxidation in the raw material manufacturing process, and a deoxidation effect can be obtained at 0.2% or more. If it exceeds 2.0%, the corrosion resistance decreases. The Mn content should be 0.2-2.0%. Preferably, it is 0.5 to 1.0%.
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, since it is difficult to manufacture a material to reduce the content, 0.035% or less, which is the current normal process level, is preferable.

  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 preferably 0.015%. .

  Cr is an element necessary for improving the corrosion resistance, and an effect is obtained at 16% or more. If it exceeds 20%, the hot workability is lowered and the production becomes difficult. The Cr content should be 16-20%. Preferably it is 17 to 19%.

  Ni is an element effective in improving corrosion resistance and forming an austenite phase, and these effects can be obtained at 8% or more. If it exceeds 15%, the stability of the austenite phase increases, martensitic transformation is less likely to occur, and economic efficiency is impaired. The Ni content should be 8-15%. Preferably it is 10 to 13.5%.

  Although Mo has the effect | action which suppresses irradiation-induced segregation in an irradiation environment and improves corrosion resistance, it does not need to contain. If it exceeds 3.0%, intermetallic compounds precipitate at the grain boundaries, and the intergranular corrosion resistance decreases. The Mo content is preferably 3% or less. In particular, 0.001-0.2 mass% or 0.25-2.5 mass% is more preferable.

  N is an element necessary for securing the material strength, and the effect can be obtained at 0.001% or more. If it exceeds 0.12%, the toughness decreases. The N content should be 0.001 to 0.12%. Preferably, it is 0.001 to 0.035%.

  Zr and Hf have the action of suppressing irradiation-induced segregation in an irradiation environment and improving 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 Zr1.14% and Hf2.24%, the effect will be saturated. Zr is preferably 0.2% to 1.14%, and Hf is preferably 0.2 to 2.24%.

  Austenitic stainless steel that satisfies the above conditions can reduce the potential for stress corrosion cracking of the material itself when subjected to machining on the surface. 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, as a structural material in contact with high-temperature and high-pressure water in a light water reactor, the influence on stress corrosion cracking can be reduced even when subjected to machining on the surface, and a welded material having excellent stress corrosion cracking resistance and its The welded structure and nuclear power plant used, the high corrosion resistance austenitic stainless steel member having high stress corrosion cracking resistance, and the high corrosion resistance austenitic stainless steel can be provided.

  Table 1 shows the chemical composition (mass%) of the high corrosion resistance austenitic stainless steel specimens according to the present invention and the comparative material, austenite grain size number, Md30 (° C) calculated by (Equation 3) and high-pressure high-temperature water. The test result of the stress corrosion crack occurrence test that was carried out is shown. Each test material is obtained by ingot by vacuum melting, plastic processing by hot forging, then solution heat treatment, and further sensitization by heating at 620 ° C. for 24 hours simulating a weld. A heat treatment was performed. Further, the surplus portion after overlay welding of the surface of each test material was simulated by grinding and polishing using a grinder or the like, and a constant strain having a depth of about 100 μm was loaded, and then 288 ° C., 8. It was immersed in 3 MPa high-pressure high-temperature water for a maximum of 5000 hours, and the stress corrosion cracking susceptibility was evaluated by the presence or absence of cracks on the surface of the test piece. High corrosion resistance austenitic stainless steels according to the present invention are specimen numbers 1 to 7, 9, 14, 20, and 21, and comparative materials are specimen numbers 8, 10 to 13, 15 to 19, and 22 to 29. is there.

図１は、各供試材での割れの有無を、Ｃｒ濃度とＭｄ_３０との関係を示す線図である。Ｍｄ_３０は化学成分と結晶粒度の影響が考慮された（式３）で算出したものである。応力腐食割れの発生は本来、確率論で評価する必要があり、応力腐食割れの発生という事象に対して、割れがあるという実験事実はこれを肯定するものであるが、割れがないという実験事実はこれを必ず否定するものではない。したがって、図１ではＣｒ濃度が低いほど、またＭｄ_３０が低いほど、応力腐食割れの発生が認められるといえる。更に、図１の式２で示される領域では、実験で応力腐食割れの発生が認められておらず、耐応力腐食割れ性に優れているといえる。

FIG. 1 is a diagram showing the relationship between Cr concentration and Md ₃₀ with or without cracks in each test material. Md ₃₀ is calculated by (Equation 3) in consideration of the influence of chemical components and crystal grain size. The occurrence of stress corrosion cracking must be evaluated by probability theory, and the experimental fact that there is a crack against the event of stress corrosion cracking is affirming this, but the experimental fact that there is no cracking Does not necessarily deny this. Therefore, in FIG. 1, it can be said that the lower the Cr concentration and the lower the Md ₃₀ , the more the occurrence of stress corrosion cracking. Furthermore, in the region represented by Equation 2 in FIG. 1, no stress corrosion cracking was observed in the experiment, and it can be said that the stress corrosion cracking resistance is excellent.

As shown by the white marks in FIG. 1, the high corrosion resistance austenitic stainless steel according to the present invention in which the Cr amount of (Equation 2) and 0.022 Md ₃₀ are 14.5 or more is free from stress corrosion cracking. It is. Number 12 shown in the comparative steel is a slightly higher C amount of 0.017%, 15 is slightly higher Mo amount of 2.7%, No. 25 is 16.4% of Cr amount and No. 27 is 16.4% of Cr. No. 27 has a slightly low content, and No. 27 has a slightly high Ni content of 14.7%, so there is no occurrence of stress corrosion cracking, but it has a high potential for its generation. Md ₃₀ is 22.7 ° C. when the Cr content is 15% and 250 ° C. when the Cr content is 20%.

  The Cr concentration is related to the formation of a passive film and greatly affects the corrosion resistance of the material. For local corrosion, which is an elementary process of stress corrosion cracking, the stability of the passive film and the repair ability after the film breakage are related. The higher the Cr concentration, the better. Also, the formation of a Cr-deficient layer due to grain boundary precipitation or grain boundary segregation has a large margin against the Cr concentration where the film becomes unstable if the bulk Cr concentration is high. Therefore, higher Cr concentration is more effective against local corrosion.

Md ₃₀ is related to the processing-induced α ′ martensite transformation ability, and is an index indicating the stability of the body-centered cubic lattice structure from low temperature to near room temperature. Greatly affects performance. In SUS304 and 304L, the processing-induced α ′ martensite transformation ability is high, and the processing-induced α ′ martensite transformation bears a part of the deformation, and a large elongation can be expected by transformation-induced plasticity. On the other hand, SUS316 and 316L have a small processing-induced α ′ martensite transformation ability, and an α ′ martensite phase is hardly detected by X-ray diffraction in deformation at room temperature. However, when microscopically observed, an α ′ martensite phase is present at a site where deformation is concentrated, suggesting that the processing-induced α ′ martensite transformation has affected the deformation behavior and the formation of the processed structure.

  FIG. 2 is a mapping image of the material structure obtained by measuring the cross section of the machined surface described above with respect to the material of specimen number 9 by the electron beam backscattering pattern method. The material of the test material number 9 has a chemical component of SUS316L standard, and is generally a material that hardly causes the processing-induced α ′ martensite transformation. The Image Quality (IQ) value is an index related to the crystallinity of the material defined by the OIM system manufactured by Texemola Laboratories. The IQ value is low in grain boundaries, interfaces, and large strain regions where the crystallinity is disturbed. The IQ value mapping image is darkly displayed.

  In FIG. 2A, since the processing strain on the outermost surface is very large, the IQ value is low and is displayed so dark that the form of the material structure cannot be identified. The effect of surface processing decreases with increasing depth from the outermost surface, and the morphology of the processed structure is revealed. In addition to the grain boundaries, it can be seen that there are also streaky regions with low IQ values in the grains. Further, as shown in FIG. 2 (a), the portion that looks like a streak indicated by a black line is a cold plastic working layer having a thickness of about 100 μm.

  FIG. 2B is an enlarged view of a region indicated by a square in FIG. The streaks in the figure are oriented in substantially the same direction in the same crystal grain, and some of the crystal grains may have streaks in two directions coexisting. When the crystal orientation relation was confirmed, it was found that these streaks are twinned with the austenite matrix and are deformed twins.

  In the phase distribution diagram of FIG. 2C, it is shown that there is a region of a body-centered cubic lattice structure in addition to the deformation twin. The δ ferrite phase generated during solidification and remaining after the heat treatment is clearly present as crystal grains as shown in FIG. On the other hand, the region of the body-centered cubic lattice structure is also distributed in the deformation twins and grain boundaries shown in FIG. 2 (b), which are the crystal orientations of the austenite matrix or twins and Kurdjumv-sachs. A relationship exists and was found to be an α ′ martensite phase. In such a region where deformation is concentrated, α ′ martensite transformation occurs, the deformation is reduced by the volume change accompanying the transformation, and the energy of the deformed portion can be lowered from the state where the transformation does not occur with the same deformation amount. . In addition, it is considered that the body centered cubic lattice structure is stable at the atomic level in a region where crystallinity is disturbed, such as a large deformation region or a grain boundary, and an increase in energy associated with the deformation can be suppressed.

  Regarding the stress generated when a material is processed, there is a concept that a material having a low tensile strength has a lower stress than a material having a high tensile strength. However, when stress corrosion cracking is considered, the stress to be considered for the processed material is not the stress necessary for the above deformation but the residual stress.

  Table 2 shows the tensile strength at room temperature, the 0.2% proof stress at 288 ° C., and the residual stress measurement results of the surface subjected to the above-mentioned grinder application. The material of the present invention has test material numbers 2 to 7 and the comparative material has test material numbers 12, 15, 25, and 27 to 29. The room temperature tensile strength is a value of a material subjected to sensitizing heat treatment after solution heat treatment. Residual stress was measured by X-ray method (parallel tilt method) for stresses parallel and perpendicular to the longitudinal direction of the grinding marks by grinding. There is no correlation between room temperature tensile strength and residual stress, and the residual stress cannot be predicted from the tensile strength. In any of the test materials, the residual stress parallel to the longitudinal direction of the grinding mark exceeds 0.2% proof stress, and according to the conventional knowledge, there is a stress that can cause stress corrosion cracking.

図１においてＣｒ濃度及びＭｄ_３０との関係によって規定した本発明の領域の中で、更に耐応力腐食割れ性に優れた材料の条件を検討した。グラインダ施工表面の応力腐食割れ発生感受性を、４２％塩化マグネシウム沸騰溶液中で評価した。４２％塩化マグネシウム溶液試験では、高圧高温水中の応力腐食割れを正確には再現できないものの、局所的な表面応力分布や腐食優先サイト分布に影響される応力腐食割れ発生初期の挙動を反映した結果が得られると期待される。

In the region of the present invention defined by the relationship between the Cr concentration and Md ₃₀ in FIG. 1, the conditions of materials having further excellent stress corrosion cracking resistance were examined. The stress corrosion cracking susceptibility of the grinder construction surface was evaluated in a 42% magnesium chloride boiling solution. In the 42% magnesium chloride solution test, although stress corrosion cracking in high-pressure and high-temperature water cannot be accurately reproduced, the results reflecting the initial behavior of stress corrosion cracking affected by local surface stress distribution and corrosion priority site distribution Expected to be obtained.

  Table 3 shows the room temperature tensile test results, Mo concentration, and stress corrosion cracking test results in a magnesium chloride boiling solution of the material of the present invention. In the stress corrosion cracking test in the magnesium chloride boiling solution of Test Material Nos. 2 to 7 of the present invention material, the test piece having the grinder construction surface was placed in the magnesium chloride boiling solution for 20 hours without particularly stressing. Soaked. The cross section after immersion was observed, and the maximum crack depth and the average crack depth were determined for cracks with a crack depth of 200 μm or more. Although stress corrosion cracking has occurred in any of the test materials, the crack depth differs depending on the test material.

局所的な表面応力分布や腐食優先サイト分布は変形組織に関連し、変形組織の形成挙動は変形挙動に反映される。一定ひずみの変形を加えるときに必要な応力は、変形組織の形成に影響を与える変形機構に関係するので、変形組織と関連すると考えられる。ここで、変形機構とは、転位によるすべり変形、変形双晶形成、加工誘起マルテンサイト変態、小角粒界導入につづく結晶粒回転（粒界すべり）などを指す。引張試験は、室温でひずみ速度を０．２％耐力まで０．３％／分、耐力後２０％／分で実施した。伸び４０％では、変形組織が十分に形成された状態での変形挙動を示していると考えられる。

Local surface stress distribution and corrosion priority site distribution are related to the deformation structure, and the formation behavior of the deformation structure is reflected in the deformation behavior. The stress required when applying a constant strain deformation is related to the deformation mechanism that affects the formation of the deformed tissue, and is considered to be related to the deformed tissue. Here, the deformation mechanism refers to slip deformation due to dislocation, deformation twin formation, work-induced martensitic transformation, crystal grain rotation (grain boundary slip) following the introduction of small-angle grain boundaries, and the like. The tensile test was conducted at room temperature at a strain rate of 0.3% / min to 0.2% proof stress and 20% / min after proof stress. It is considered that an elongation of 40% indicates a deformation behavior in a state where a deformed structure is sufficiently formed.

  FIG. 3 is a diagram showing the relationship between the crack depth and the stress at an elongation of 40% in the stress corrosion cracking test result in a magnesium chloride boiling solution. In the figure, the white outline represents the maximum crack depth and the black triangle represents the average crack depth. As shown in the figure, the maximum crack depth and the average crack depth are both smaller as the stress at an elongation of 40% decreases, and further the crack depth is below 510 MPa. In particular, the maximum crack depth was reduced to about 1/6 or less. Therefore, a material whose stress value at an elongation of 40% is 510 MPa or less has excellent stress corrosion cracking resistance in terms of local surface stress distribution and corrosion priority site distribution. In particular, it can be seen that the maximum crack depth is 510 MPa or less, the crack depth is remarkably reduced, and the resistance to stress corrosion cracking is improved.

  FIG. 4 is a diagram showing the relationship between the crack depth and the Mo concentration at each stress at an elongation of 40% in the stress corrosion crack test result in a magnesium chloride boiling solution. In the figure, the white outline represents the maximum crack depth and the black triangle represents the average crack depth. As shown in the figure, at any stress, when focusing on the chemical composition of the specimen, the Mo concentration has a correlation with the stress value at 40% elongation. By setting the Mo concentration to 0.2% or less and more than 0.1% by mass%, the stress value at 40% elongation can be 510 MPa or less, and the crack depth can be greatly reduced. That is, it is effective to reduce the Mo concentration to reduce the stress corrosion cracking susceptibility of stainless steel having a machined surface.

  Based on the above test results, lay-up welding was performed using the highly corrosion-resistant austenitic stainless steel of the present invention material shown in Table 1 as a material to be welded, and the surplus portion of the weld and its weld heat affected zone It is obvious that the same result is obtained in a test similar to that described above for a welded structure that has been ground with a grinder. In addition, the welding material of the welded material without Mo has a Ni amount slightly about 1 to 2% higher than the Ni amount of the base material, and the other components are the same as the base material, and have Mo. As the welding material of the workpiece, a material having the same plasticity as that of the base material is used.

  Such a welded structure can be applied to at least one of a reactor internal structure and a recirculation cooling water pipe of a nuclear power plant. As a specific example, it can be applied to manufacture of a shroud as a furnace internal structure and a large diameter pipe having a diameter of 400 to 700 mm as a recirculation cooling water pipe.

  As described above, as shown in this example, as a structural material in contact with high-temperature and high-pressure water in a light water reactor, the influence on stress corrosion cracking can be reduced even when subjected to machining on the surface, and the welded material has excellent stress corrosion cracking resistance. It is possible to obtain a 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.

Is a graph showing the relationship between the Cr concentration and Md ₃₀ of test materials on Stress Corrosion Cracking Test results for high-pressure high-temperature water. It is a figure which shows the cross-sectional mapping image of the machining surface measured by the electron beam backscattering pattern method. It is a diagram which shows the relationship between the crack depth in the stress corrosion cracking test result in a magnesium chloride boiling solution, and the stress in 40% of elongation. It is a diagram which shows the relationship between the crack depth in the stress corrosion cracking test result in a magnesium chloride boiling solution, and Mo density | concentration.

Claims (19)

By mass, C0.001-0.020%, Si0.1-1.0%, Mn0.2-2.0%, Cr16-20%, Ni9-15%, Mo3% or less, N0.001-0. containing 12%, the balance being Fe and unavoidable impurities, and _{Md 30} obtained by the Cr and (equation 1) meets the equation (2), the crystal grain size number is 4.0 to 7.0 Material to be welded characterized by
_{Md 30 = 551-462 (C + N} ) -9.2Si-8.1Mn-13.7Cr-29
Ni-18.5Mo-1.42 (ν-8.0) (Formula 1)
Cr + 0.022Md ₃₀ ≧ 14.5 (Formula 2)
(In the formula, each element is% by mass, and ν is the grain size number) According to claim 1, wherein the inevitable impurities, Cu0.05% or less, Nb0.05% or less, not more than P0.035% or less and S0.015%, said M _{d 30} is determined by the (Equation 3) Characteristic material to be welded.
_{Md 30 = 551-462 (C + N} ) -9.2Si-8.1Mn-13.7Cr-29
(Ni + Cu) -18.5Mo-68Nb-1.42 (ν-8.0) (Formula 3)
(In the formula, each element is% by mass, and ν is the grain size number)   The welded material according to claim 1, wherein the Mo content is 0.001 to 0.2 mass%.   The welded material according to claim 1, wherein the Mo content is 0.25 to 2.5 mass%.   5. The welded material according to claim 1, comprising at least one of Zr 0.2 to 1.14% and Hf 0.2 to 2.24% by mass.   6. The welded material according to claim 1, wherein a stress at an elongation of 40% in the tensile stress-elongation curve is 510 MPa or less. In any of the claims 1-6, having a cold plastic working layer on the surface, the material to be welded, wherein the thickness of the cold plastic worked layer is 1 to 500 [mu] m. A welded structure in which the welded material is connected by a build-up weld, wherein the welded material is the welded material according to any one of claims 1 to 7 . 9. The welded structure according to claim 8 , wherein a cold plastic working layer is formed on a weld heat affected zone on the surface of the build-up weld and the surface of the workpiece. The welded structure according to claim 8 or 9 , wherein grinder grinding or flapper wheel grinding is performed on the weld heat affected zone on the surface of the build-up weld and the surface of the workpiece. Nuclear power plant at least one of the core internals and recirculating cooling water pipe is equal to or formed of welded structures according to any of claims 8-1 0. By mass, C0.001-0.020%, Si0.1-1.0%, Mn0.2-2.0%, Cr16-20%, Ni9-15%, Mo3% or less, N0.001-0. 12% content, the balance being Fe and inevitable impurities, Cr and Md ₃₀ determined by (Formula 1) satisfy (Formula 2), the grain size number is 4.0 to 7.0, A high corrosion resistance austenitic stainless steel member characterized by having a cold plastic working layer.
_{Md 30 = 551-462 (C + N} ) -9.2Si-8.1Mn-13.7Cr-29
Ni-18.5Mo-1.42 (ν-8.0) (Formula 1)
Cr + 0.022Md ₃₀ ≧ 14.5 (Formula 2)
(In the formula, each element is% by mass, and ν is the grain size number) In claim 1 2, wherein the unavoidable impurities, Cu0.05% or less, Nb0.05% or less, not more than P0.035% or less and S0.015%, said _{Md 30} is determined by the (Equation 3) High corrosion resistance austenitic stainless steel member.
_{Md 30 = 551-462 (C + N} ) -9.2Si-8.1Mn-13.7Cr-29
(Ni + Cu) -18.5Mo-68Nb-1.42 (ν-8.0) (Formula 3)
(In the formula, each element is% by mass, and ν is the grain size number) By mass, C0.001-0.020%, Si0.1-1.0%, Mn0.2-2.0%, Cr16-20%, Ni9-15%, Mo3% or less, N0.001-0. 12% content, the balance is made of Fe and inevitable impurities, Cr and Md ₃₀ calculated by (Formula 1) satisfy (Formula 2), and the grain size number is 4.0 to 6.0. High corrosion-resistant austenitic stainless steel.
_{Md 30 = 551-462 (C + N} ) -9.2Si-8.1Mn-13.7Cr-29
Ni-18.5Mo-1.42 (ν-8.0) (Formula 1)
Cr + 0.022Md ₃₀ ≧ 14.5 (Formula 2)
(In the formula, each element is% by mass, and ν is the grain size number) In claims 1-4, wherein the inevitable impurities, Cu0.05% or less, Nb0.05% or less, not more than P0.035% or less and S0.015%, said M _{d 30} is determined by the (Equation 3) High corrosion resistance austenitic stainless steel characterized by
_{Md 30 = 551-462 (C + N} ) -9.2Si-8.1Mn-13.7Cr-29
(Ni + Cu) -18.5Mo-68Nb-1.42 (ν-8.0) (Formula 3)
(In the formula, each element is% by mass, and ν is the grain size number) According to claim 1 4 or 1 5, high corrosion resistance austenitic stainless steel, wherein the Mo is 0.001 to 0.2 wt%. In claims 1-4 or 1-5, the high corrosion-resistant austenitic stainless steel, wherein the Mo is 0.25 to 2.5 mass%. In any of claims 1 4 to 1 7, the mass in, Zr0.2～1.14% and Hf0.2～2.24% highly corrosion-resistant austenitic stainless steel characterized by containing one or more . In any of claims 1 4 to 1 8, tensile stress - highly corrosion-resistant austenitic stainless steel stress at elongation of 40% in elongation curve is equal to or less than 510 MPa.

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