JP5070831B2 - Austenitic stainless steel - Google Patents

Description

  The present invention relates to an austenitic stainless steel having excellent resistance to stress corrosion cracking caused by carbide precipitation at grain boundaries due to sensitization. This steel is suitable for use as a structural member such as a feed water heater of a nuclear power plant or a thermal power plant, such as a plate, a pipe, or a joint.

Conventionally, SUS304 steel, which is an austenitic stainless steel, has been used as a structural member used in a feed water heater of a nuclear power plant or a thermal power plant. However, stress corrosion cracking is one of the aging phenomena of the structural members. This phenomenon is caused by the fact that carbide (Cr ₂₃ C ₆ ) precipitates at the grain boundary when a thermal history heated to the sensitization temperature range by welding or heat treatment, and a Cr-deficient layer is formed near the grain boundary. This phenomenon occurs because the corrosion cracking sensitivity is increased. In order to suppress this carbide precipitation, the method of reducing C content is generally taken. However, when the C content is reduced, there is a problem that the mechanical properties, particularly the strength, of the material is lowered.

  As a method for improving the stress corrosion cracking resistance by mechanical machining of the surface, Patent Document 1 discloses a method of applying compressive residual stress to the surface of a metal material by shot peening. Patent Document 2 discloses a nuclear power generation in which crack growth is suppressed by forming a complex oxide layer on the surface of a metal material and covering the crack surface from the grain boundary that is the starting point of stress corrosion cracking with the complex oxide. An invention of a power plant is disclosed.

  On the other hand, as an improvement of stress corrosion cracking resistance by heat treatment, Patent Document 3 describes that after solution heat treatment, after heating at 550 to 800 ° C. for a specified time or more, further heating at 1000 to 1100 ° C. for a specified time, and then 3 ° C. / Austenite with improved stress corrosion cracking resistance by reducing the Cr concentration at the grain boundary and in the vicinity of the grain boundary to 1.3 times or more of the average value by cooling at a cooling rate of sec or more, thereby suppressing the formation of Cr-deficient layers Stainless steel is disclosed.

  Furthermore, Patent Document 4 discloses that a hardened layer in the vicinity of the surface layer is previously made coarse grains having an average crystal grain size number of 2 or less with a low tensile strength, thereby reducing stress generated when strain is applied and stress corrosion resistance. An austenitic stainless steel with improved crackability is disclosed.

  As an improvement from the viewpoint of materials, Patent Document 5 discloses an austenite system in which carbide grain boundary precipitation is suppressed by adding Nb or Ta, and intergranular corrosion resistance, intergranular stress corrosion cracking resistance and pitting corrosion resistance are improved. Stainless steel is disclosed.

JP-A-7-266230 Japanese Patent Laid-Open No. 2001-91688 JP-A-10-317104 JP 2005-23343 As described above, various countermeasures for preventing stress corrosion cracking have been proposed. However, mechanical shot peening as disclosed in Patent Document 1 and formation of a complex oxide film on the surface disclosed in Patent Document 2 have a problem that the number of man-hours for the processing work is enormous. In addition, in the method in which the heat treatment conditions are defined to increase the Cr concentration at or near the grain boundary, or the surface layer portion is coarsened, there is a problem that the heat treatment step increases and the productivity is lowered. Furthermore, in terms of material improvement, in consideration of intergranular stress corrosion cracking in high-temperature pure water, which is a problem in nuclear water heaters and the like, the development of materials that are less sensitive is desired.

  An object of the present invention is to provide an austenitic stainless steel suitable as a structural member for a feed water heater or the like of a nuclear power plant or a thermal power plant, and to provide a stainless steel remarkably excellent in stress corrosion cracking resistance.

As described above, an austenitic stainless steel such as SUS304 steel has a sensitized structure due to thermal history. The inventor has investigated and studied the generation mechanism in detail. As a result, it is clear that Cr ₂₃ C ₆ carbide is selectively precipitated at the grain boundary by receiving the thermal history, and a Cr-deficient layer is formed in the vicinity of the grain boundary, thereby sensitizing the grain boundary. Became. Therefore, if selective carbide precipitation at the grain boundaries can be suppressed and the depth and width of the Cr-deficient layer near the grain boundaries can be reduced, grain boundary sensitization can be suppressed, and intergranular stress corrosion cracking resistance can also be improved. As a result of further exploration, the following findings were obtained. In the following, “%” for the component content means “mass%”.

  (1) In order to reduce the precipitation of carbides at the grain boundaries, it is necessary to reduce the C content more than before. Specifically, it is necessary to keep the C content below 0.03%.

(2) By setting the Mo content to 0.2 to 1.0%, the formation of a Cr-deficient layer near the grain boundary can be reduced. In ordinary SUS304 steel, when it receives a heat history of sensitization by welding or heat treatment, Cr ₂₃ C ₆ carbide precipitates at the grain boundary, and a Cr-deficient layer is formed in the vicinity of the grain boundary. However, in the steel in which Mo is contained in SUS304 steel, the carbide that precipitates at the grain boundary due to sensitization is (Cr, Mo) ₂₃ C ₆ . (Cr, Mo) ₂₃ C ₆ has a smaller amount of Cr in the carbide than Cr ₂₃ C ₆ , so the width of the Cr-deficient layer can be narrowed, and the Cr amount in the Cr-deficient layer is reduced. Can also be reduced. Furthermore, since Mo diffuses slowly into the grain boundary, (Cr, Mo) ₂₃ C ₆ carbide precipitates finely, so the degree of Cr deficiency near the grain boundary is also reduced.

(3) Furthermore, it is important that (Cr, Mo) ₂₃ C ₆ is precipitated as much as possible at the grain boundaries rather than Cr ₂₃ C ₆ to reduce Cr deficiency near the grain boundaries as much as possible. . Therefore, in the process of sensitization, Mo, which is more likely to precipitate as carbide than Cr, is an amount that can fix at least about half of C in steel as Mo ₂₃ C ₆ , that is, in relation to the Cr content, the following formula (2) By containing Mo in an amount represented by the following, stress corrosion cracking due to sensitization can be further suppressed. In addition, the element symbol in Formula (2) means the content (mass%) of the element.

{Mo / (96 × 23)} / {C / (12 × 6)} ≧ 0.5 (2)
When this formula (2) is arranged, the following formula (1) is obtained.

Mo ≧ (46/3) × C (1)
As described above, the addition of Mo can suppress an increase in the width of the Cr-depleted layer formed near the grain boundary and a decrease in the amount of Cr, so that intergranular stress corrosion cracking due to the formation of the Cr-depleted layer is prevented. Can be prevented. Further, it is more desirable to contain Mo that satisfies the above formula (1) in order to prevent stress corrosion cracking.

  (4) Furthermore, the effect of improving the stress corrosion cracking resistance in high-temperature pure water can be obtained by adding a small amount of 0.05 to 1.0% Cu together with Mo. This is presumably because Cu has the effect of reducing the dissolution rate associated with cracking and suppressing the development of intergranular cracking due to selective corrosion of the Cr-deficient layer.

  (5) By containing at least one of Ti: 0.005-0.1% and V: 0.01-0.2%, the intergranular stress corrosion cracking resistance can be further improved. This is because Ti and V have an action of forming carbides and suppressing carbide precipitation at the grain boundaries.

The present invention is based on the above findings, and the gist thereof is the following austenitic stainless steels (1) and (2) .

(1) By mass%, C: less than 0.03%, Si: less than 1.0%, Mn: 1.2-2.5%, P: 0.04% or less, S: 0.02% or less, Cr: 15-25%, Ni: 6-15 %, Mo: 0.3 to 1.0%, Cu: 0.05 to 1.0% and N: 0.03 to 0.083%, the balance is composed of iron and impurities, and the Mo content satisfies the following formula (1) A feature of austenitic stainless steel.

Mo ≧ (46/3) × C (1)
However, the element symbol in the formula (1) means the content (% by mass) of the element.

(2) An austenitic stainless steel characterized by further containing one or both of Ti: 0.005-0.1% and V: 0.01-0.2% in addition to the components described in (1 ) above.

  In the austenitic stainless steel of the present invention, sensitization is suppressed even when subjected to a thermal history such as welding or heat treatment, and selective carbide precipitation at grain boundaries is suppressed. Therefore, since the depth and width of the Cr-deficient layer in the vicinity of the grain boundary can be reduced, the intergranular stress corrosion cracking resistance is also improved. Therefore, the steel according to the present invention is suitable as a structural member for a feed water heater of a nuclear power plant or a thermal power plant in which carbide precipitation due to sensitization is a problem.

  Hereinafter, the effect of each component of the stainless steel of this invention and the reason for limitation of content are demonstrated.

C: Less than 0.03% C is an element that stabilizes the austenite phase, but if the content is large, carbides precipitate at the grain boundaries. In the present invention, it is prevented that Cr carbide is selectively deposited at the grain boundary due to sensitization due to thermal history such as welding and heat treatment, and a Cr-deficient layer is formed near the grain boundary, thereby reducing the stress corrosion cracking resistance. The purpose is to do. Therefore, the C content is less than 0.03%. The smaller the C content, the better. More desirable is 0.020% or less, and most desirable is 0.015% or less.

Si: Less than 1.0%
Si is optionally used as a deoxidizer. In order to obtain the effect of deoxidation, it is desirable to contain 0.1% or more. On the other hand, when the Si content is 1.0% or more, the toughness deteriorates. More preferred is 0.8% or less. When Si is not used as a deoxidizer, the lower limit of the Si content is an impurity level.

Mn: 1.2-2.5%
Mn is an austenite forming element and fixes S in steel to improve hot workability. Further, in the present invention, the C content is reduced as much as possible in order to suppress carbide precipitation due to sensitization, but in order to compensate for the strength reduction due to this, the content of 1.2% or more is necessary. On the other hand, excessive content of Mn leads to deterioration of ductility and corrosion resistance, so the upper limit is made 2.5%. A more desirable upper limit is 2.0%.

P: 0.04% or less P is an element contained in steel as an inevitable impurity, and its content is 0.04% or less, and the smaller the better. If the content of P is large, it segregates at the grain boundaries and deteriorates the intergranular stress corrosion cracking resistance.

S: 0.02% or less S is an element contained in steel as an inevitable impurity like P. It segregates at the grain boundaries and impairs intergranular stress corrosion cracking resistance, but also deteriorates hot workability. Let Therefore, it should be 0.02% or less and as little as possible. More preferred is 0.01% or less.

Cr: 15-25%
Cr is an essential element for improving the corrosion resistance of steel. The effect is obtained with a content of 15% or more. On the other hand, if it exceeds 25%, ferrite is generated and hot workability deteriorates. A more preferable content of Cr is 17 to 22%.

Ni: 6-15%
Ni is an element that generates an austenite phase and also improves corrosion resistance. The effect can be obtained at 6% or more. On the other hand, if it exceeds 15%, the effect will be saturated and the cost will rise. A more preferable content of Ni is 8 to 12%.

Mo: 0.2-1.0%
Mo is one of the important elements in the steel of the present invention. When Mo is contained, the carbide precipitated at the grain boundary by sensitization is (Cr, Mo) ₂₃ C ₆ . Since (Cr, Mo) ₂₃ C ₆ has a smaller amount of Cr in the carbide than Cr ₂₃ C ₆ , the width of the Cr-deficient layer and the decrease in the Cr amount can be reduced. Further, since Mo diffuses slowly into the grain boundary, the precipitated (Cr, Mo) ₂₃ C ₆ carbide is fine, and therefore the degree of Cr deficiency near the grain boundary is reduced. As described above, the addition of Mo can suppress the increase in the width of the Cr-deficient layer and the decrease in the amount of Cr, thereby obtaining the effect of preventing intergranular stress corrosion cracking due to the formation of the Cr-deficient layer. . The lower limit of the Mo content for obtaining such an effect is 0.2%. On the other hand, even if Mo is excessively contained, the effect is saturated and not only the cost is increased, but also deterioration of hot workability is concerned, so the upper limit is made 1.0%. More preferable Mo content is 0.2 to 0.8%, and still more preferable is 0.3 to 0.6%.

Further, as described above, it is desirable to reduce (Cr, Mo) ₂₃ C ₆ as much as possible rather than Cr ₂₃ C _{6 at} the grain boundaries, thereby reducing Cr deficiency near the grain boundaries as much as possible. Therefore, Mo, which is more likely to precipitate as a carbide than Cr during the sensitization process, is contained in such an amount that at least about half of C in the steel can be fixed as Mo ₂₃ C ₆ . That is, stress corrosion cracking due to sensitization is suppressed by containing Mo in an amount represented by the following formula (2).

{Mo / (96 × 23)} / {C / (12 × 6)} ≧ 0.5 (2)
However, the element symbol in the formula (2) is the content (% by mass) of the element. Then, as described above, this equation (2) can be rearranged into the following equation (1).

Mo ≧ (46/3) × C (1)
Cu: 0.05-1.0%
As described above, the stress corrosion cracking resistance in high-temperature pure water can be improved by adding a small amount of Cu to 0.05 to 1.0% together with Mo. This is presumably because Cu has the effect of reducing the dissolution rate associated with cracking and suppressing the development of intergranular cracking due to selective corrosion of the Cr-deficient layer. In order to obtain the above effects, it is necessary to contain 0.05% or more. On the other hand, when Cu is excessively contained, Cu segregates at the grain boundary and promotes surface cracks and weld cracks during hot working, so the upper limit is made 1.0%. A more preferred upper limit is 0.5%.

N: 0.03-0.3%
N is an austenite-forming element and is also an element necessary for ensuring the strength of steel. N also has the effect of improving the corrosion resistance of steel. In order to obtain these effects, a content of 0.03% or more is necessary. However, the excessive content of N causes a decrease in weldability or induces surface cracking during casting, so the upper limit is made 0.3%. A more desirable upper limit is 0.15%.

At least one of Ti: 0.005-0.1% and V: 0.01-0.2%
Ti and V have the effect of forming carbides and suppressing the precipitation of carbides at the grain boundaries, and have the effect of further improving the intergranular stress corrosion cracking resistance. Therefore, one or both of Ti and V are contained as necessary. . The lower limit for obtaining the effect is 0.005% and 0.01%, respectively. On the other hand, if these are contained excessively, not only the effect is saturated but also the hot workability is deteriorated, so the upper limit is made 0.1% and 0.2%, respectively. More preferable contents are 0.02 to 0.05% for Ti and 0.05 to 0.15% for V.

  Stainless steel having the chemical composition shown in Table 1 was melted, and the ingot was heated to 1200 ° C., and then subjected to hot forging and hot rolling to obtain a plate having a thickness of 5 mm. The plate was heated to 1020 to 160 ° C. and then quenched by water cooling. Next, the plate material was subjected to a sensitizing heat treatment held at 650 ° C. for 10 hours in an Ar gas atmosphere.

  Using the plate material as a sample, a Cr-deficient layer at the grain boundary was confirmed by oxalic acid etching according to the following procedure.

First, the cross-section of the plate material was embedded in a resin as an observation surface, polished, and etched by the 10% oxalic acid etching test method of JIS G0571. Thereafter, the etching state of the grain boundaries was observed with a microscope. The determination of the Cr-deficient layer is made by observing the central part of the plate thickness with a 200x microscope, and when the etched length with respect to the total grain boundary length in the field of view is 10% or more, 5% or more and less than 10% Cases were marked with △ when 3% or more and less than 5%, ◯ when 1% or more and less than 3 %, and ◎ when less than 1%. The results are also shown in Table 1.

As apparent from Table 1, in the steels of the present invention (Nos. 2 and 4 to 7 ), the etching length of the grain boundary after the sensitizing heat treatment is shortened, and the formation of a Cr-deficient layer near the grain boundary is suppressed. ing. That is, the steel of the present invention has improved intergranular stress corrosion cracking resistance. On the other hand, in steels (Nos. 9 to 15) in which the content of any of C, Cr, Mo and Cu is not within the range defined by the present invention, the grain boundaries after the sensitizing heat treatment are significantly etched. This means that the grain-boundary Cr-deficient layer is large, and therefore the intergranular stress corrosion cracking resistance is poor .

  FIG. 1 is a photomicrograph after oxalic acid etching of No. 5, 6 and 15 steels in Table 1. As shown in Table 1, No. 5 is evaluation ◎, No. 6 is evaluation ○, and No. 15 is evaluation ●.

  As described above, even if the austenitic stainless steel of the present invention is subjected to a thermal history such that normal austenitic stainless steel is sensitized, selective carbide precipitation to the grain boundary is suppressed, and the vicinity of the grain boundary is The depth and width of the Cr-deficient layer are small. Therefore, the intergranular stress corrosion cracking resistance is greatly improved. The steel of the present invention is suitable as a plate, pipe, joint, or the like, which is a structural member such as a feed water heater of a nuclear power plant or a thermal power plant in which carbide precipitation due to sensitization is a problem.

It is a reproduction figure of the microscope picture which shows the state of the grain boundary etching of the sample which carried out the sensitizing heat processing.

Claims (2)

In mass%, C: less than 0.03%, Si: less than 1.0%, Mn: 1.2-2.5%, P: 0.04% or less, S: 0.02% or less, Cr: 15-25%, Ni: 6-15%, Mo : 0.3-1.0%, Cu: 0.05-1.0% and N: 0.03-0.083%, the balance is composed of iron and impurities, and the Mo content satisfies the following formula (1) Austenitic stainless steel.
Mo ≧ (46/3) × C (1)
However, the element symbol in the formula (1) means the content (% by mass) of the element. In addition to the components described in claim 1, Ti: 0.005 to 0.1 wt% and V: 0.01 to 0.2% by weight of one or austenitic stainless steel according to claim 1, characterized in that it contains both.

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