KR102223549B1 - Austenitic stainless steel - Google Patents

Abstract

An austenitic stainless steel having excellent polythionic acid SCC resistance and excellent creep ductility is provided. The austenitic stainless steel according to the present embodiment is, by mass, C: 0.030% or less, Si: 0.10 to 1.00%, Mn: 0.20 to 2.00%, P: 0.040% or less, S: 0.010% or less, Cr: 16.0 to 25.0%, Ni: 10.0 to 30.0%, Mo: 0.1 to 5.0%, Nb: 0.20 to 1.00%, N: 0.050 to 0.300%, sol. It contains Al: 0.0005 to 0.100%, and B: 0.0010 to 0.0080%, the balance consists of Fe and impurities, and has a chemical composition that satisfies the formula (1).
B+0.004-0.9C+0.017Mo ² ≥0 (1)
Here, the content (mass %) of the corresponding element is substituted for each element symbol in the formula (1).

Description

Austenitic stainless steel

The present invention relates to stainless steel, and more particularly, to an austenitic stainless steel.

Among the members used in plant facilities such as thermal boilers, petroleum refining and heating furnace pipes of petrochemical plants, it is a high temperature of 600 to 700°C, and a high temperature corrosive environment containing a corrosive fluid containing sulfide and/or chloride There is one used in. When such plant equipment is stopped due to periodic inspection or the like, air, moisture, and sulfide scale react, and polythionic acid is generated on the surface of the member. This polythionic acid causes stress corrosion cracking (hereinafter referred to as polythionic acid SCC) at the grain boundary. Therefore, a member used in the above-described high-temperature corrosion environment is required to have excellent polythionic acid SCC resistance.

Steels having improved polythionic acid SCC resistance are proposed in Japanese Patent Application Laid-Open No. 2003-166039 (Patent Document 1) and International Publication No. 2009/044802 (Patent Document 2). Polythiolic acid SCC is generated when Cr _{precipitates as an M 23} C ₆ type carbide at the grain boundary and a Cr deficient layer is formed in the vicinity of the grain boundary. Therefore, in Patent Document 1 and Patent Document 2, the amount of C is reduced _{to suppress the formation of M 23} C ₆ type carbide and increase the polythionic acid SCC resistance.

Specifically, the austenitic heat-resistant steel disclosed in Patent Literature 1 is, by mass, C: 0.005 to less than 0.03%, Si: 0.05 to 0.4%, Mn: 0.5 to 2%, P: 0.01 to 0.04%, S: 0.0005 to 0.005%, Cr: 18 to 20%, Ni: 7 to 11%, Nb: 0.2 to 0.5%, V: 0.2 to 0.5%, Cu: 2 to 4%, N: 0.10 to 0.30%, B: 0.0005 It contains ~0.0080%, the balance consists of Fe and unavoidable impurities. The sum of the contents of Nb and V is 0.6% or more, and the Nb solid solution in the steel is 0.15% or more. Further, N/14≥Nb/93+V/51, and Cr-16C-0.5Nb-V≥17.5 are satisfied. In Patent Document 1, by reducing the C content and defining the relationship between Cr and C, Nb, and V, polythionic acid SCC resistance is improved.

The austenitic stainless steel disclosed in Patent Document 2 is, by mass, C: less than 0.04%, Si: 1.5% or less, Mn: 2% or less, Cr: 15 to 25%, Ni: 6 to 30%, N: 0.02~0.35%, Sol. Al: 0.03% or less, and Nb: 0.5% or less, Ti: 0.4% or less, V: 0.4% or less, Ta: 0.2% or less, Hf: 0.2% or less, and Zr: 0.2% or less. Or two or more, and the balance consists of Fe and impurities. Among the impurities, P: 0.04% or less, S: 0.03% or less, Sn: 0.1% or less, As: 0.01% or less, Zn: 0.01% or less, Pb: 0.01% or less, and Sb: 0.01% or less. In addition, F1=S+｛(P+Sn)/2｝+｛(As+Zn+Pb+Sb)/5｝≤0.075, and 0.05≤Nb+Ta+Zr+Hf+2Ti+(V/10)≤1.7 -9×F1 is satisfied. In Patent Document 2, polythionic acid SCC resistance is improved by making the C content less than 0.05%. Further, by reducing C immobilization elements such as Nb and Ti, and reducing grain boundary embrittlement elements such as P, S, and Sn in the steel, the embrittlement cracking resistance in the heat affected zone (HAZ) is improved.

Japanese Patent Publication No. 2003-166039 International Publication No. 2009/044802

However, in recent years, high creep ductility is required for members used in the above-described high-temperature corrosive environment. In plant equipment, as described above, equipment may be stopped and periodic inspections may be performed. In the periodic inspection, members that need to be replaced are investigated. At this time, if the creep ductility is high, the degree of deformation of the member can be confirmed at the time of the periodic inspection, and it can be used as a criterion for determining replacement of the member.

In Patent Literature 1 and Patent Literature 2, the aim is to improve the polythionic acid SCC resistance, but not to improve the creep ductility. In the steels proposed in these patent documents, the C content is lowered in order to increase the polythionic acid SCC resistance. In this case, high creep ductility may not be obtained in some cases.

An object of the present invention is to provide an austenitic stainless steel having excellent polythionic acid SCC resistance and excellent creep ductility.

The austenitic stainless steel according to the present invention is, by mass, C: 0.030% or less, Si: 0.10 to 1.00%, Mn: 0.20 to 2.00%, P: 0.040% or less, S: 0.010% or less, Cr: 16.0. ~25.0%, Ni:10.0~30.0%, Mo:0.1~5.0%, Nb:0.20~1.00%, N:0.050~0.300%, sol. Al: 0.0005 to 0.100%, B: 0.0010 to 0.0080%, Cu: 0 to 5.0%, W: 0 to 5.0%, Co: 0 to 1.0%, V: 0 to 1.00%, Ta: 0 to 0.2%, Hf ：0∼0.20%, Ca：0∼0.010%, Mg：0∼0.010%, and rare earth elements：0∼0.10%, the balance consists of Fe and impurities, and satisfies Equation (1) Have a composition.

B+0.004-0.9C+0.017Mo ² ≥0 (1)

Here, the content (mass %) of the corresponding element is substituted for each element symbol in the formula (1).

The austenitic stainless steel according to the present invention is excellent in polythionic acid SCC resistance and also excellent in creep ductility.

The present inventors investigated and examined steels having excellent creep ductility as well as polythionic acid SCC resistance.

When the C content is reduced to 0.030% or less, _{the generation of M 23} C ₆ type carbide is suppressed during use in a high-temperature corrosive environment, and generation of a Cr-deficient layer in the vicinity of the grain boundary is suppressed. In the present invention, further, by containing 0.20 to 1.00% of Nb, C is fixed to Nb, thereby further reducing the amount of solid solution C, which is a cause of formation of the _{M 23} C _{6 type carbide.} In the present invention, Mo is further contained in an amount of 0.1 to 5.0%. Mo suppresses the formation of _{M 23} C _{6 type carbide.} Therefore, the generation of the C-deficient layer is reduced. By the above measures, polythionic acid SCC resistance can be improved.

However, as a result of investigation by the present inventors, it has been found that when the C content is reduced to 0.030% or less, the creep ductility decreases. As a reason, the following can be considered. The precipitate formed in the grain boundary increases the grain boundary strength. The higher the grain boundary strength, the higher the creep ductility. However, when the C content is reduced to 0.030% or less, precipitates (carbide, etc.) generated in the grain boundary are also reduced. As a result, it is considered that it is difficult to obtain grain boundary strength, and creep ductility decreases.

Therefore, the present inventors further studied an austenitic stainless steel that can achieve both excellent polythionic acid SCC resistance and excellent creep ductility. It is conceivable that B (boron) segregates at grain boundaries in the above-described high-temperature corrosive environment of 600 to 700° C. to increase grain boundary strength.

Therefore, the present inventors, by mass%, C: 0.030% or less, Si: 0.10 to 1.00%, Mn: 0.20 to 2.00%, P: 0.040% or less, S: 0.010% or less, Cr: 16.0 to 25.0%, Ni : 10.0 to 30.0%, Mo: 0.1 to 5.0%, Nb: 0.20 to 1.00%, N: 0.050 to 0.300%, sol. Al: 0.0005 to 0.100%, B: 0.0010 to 0.0080%, Cu: 0 to 5.0%, W: 0 to 5.0%, Co: 0 to 1.0%, V: 0 to 1.00%, Ta: 0 to 0.2%, Hf An austenitic stainless steel containing: 0 to 0.20%, Ca: 0 to 0.010%, Mg: 0 to 0.010%, and rare earth elements: 0 to 0.10%, and the balance consisting of Fe and impurities, is excellent in polyolefin resistance. It was thought that the SCC properties of thionic acid and excellent creep ductility could be made compatible.

However, as a result of investigating the polythionic acid SCC resistance and creep ductility of the austenitic stainless steel having the above chemical composition, it was found that although excellent polythiolic acid SCC resistance could be obtained, excellent creep ductility could not necessarily be obtained. Therefore, the inventors further studied. As a result, it turned out that the following mechanism can be considered for creep ductility.

As described above, in this embodiment, in order to increase the polythionic acid SCC resistance, not only the C content is made 0.030% or less, but also 0.20 to 1.00% of Nb is included to fix C to Nb, thereby reducing the solid solution C. do. Specifically, Nb binds to C and precipitates as MX-type carbonitride by solution treatment or aging in a short time. However, in the use environment of the steel material of the present embodiment (a high-temperature corrosion environment of 600 to 700°C), the MX-type carbonitride is metastable. Therefore, when the steel material having the above chemical composition is used for a long time in a high-temperature corrosion environment of 600 to 700°C, the MX-type carbonitride of Nb changes into the _{stable phase Z-phase (CrNbN) and M 23} C _{6 type carbide.} At this time, the B that segregates at the grain boundary, are replaced with a portion of the M ₂₃ C ₆ type carbide C, is absorbed by the M ₂₃ C ₆ type carbides. Therefore, the amount of B segregating at the grain boundary is reduced, and the grain boundary strength is reduced. As a result, it is considered that sufficient creep ductility cannot be obtained.

Therefore, further investigation was conducted on a method of suppressing a reduction in the amount of segregation B at grain boundaries during use in a high-temperature corrosive environment of 600 to 700°C. As a result, it turned out that the following mechanism can be considered.

As described above, Mo suppresses the formation of _{M 23} C _{6 type carbide itself.} Mo is further optionally substituted with a part of M in the M ₂₃ C ₆ type carbide, there is a case that is employed in the M ₂₃ C ₆ type carbides. In this specification, the M ₂₃ C ₆ type carbide in which Mo is solid solution is defined as _{"Mo solid solution M 23} C _{6 type carbide".} Mo solid solution M ₂₃ C ₆ type carbide is difficult to employ B. Therefore, even when the MX-type carbonitride containing Nb is changed to a _{Z-phase and M 23} C _{6 type carbide during use in a high-temperature corrosive environment, if the M 23} C ₆ type carbide is a Mo solid solution M ₂₃ C ₆ type carbide , The solid solution of B into the M ₂₃ C ₆ type carbide can be suppressed, and a reduction in the amount of segregation B at the grain boundary is suppressed. As a result, it is considered that excellent polythionic acid SCC resistance and excellent creep ductility can be made compatible.

Therefore, in the austenitic stainless steel having the above chemical composition, even when the MX-type carbonitride containing Nb is changed _{to the Z-phase and M 23 C 6-} type carbide during use in a high-temperature corrosive environment of 600 to 700°C. , Mo _{solid solution M 23} C ₆ type carbide was further investigated for a chemical composition capable of suppressing a reduction in the amount of segregation B at grain boundaries. As a result, it was found that B in the chemical composition and C and Mo are closely related to suppression of reduction of the amount of segregation B due to the formation of _{Mo solid solution M 23} C _{6 type carbide.} And, in the above chemical composition, if B, C, and Mo satisfy Equation (1), even during use in a high-temperature corrosion environment of 600 to 700°C, excellent polythionic acid SCC resistance and excellent creep ductility can be achieved. I could see that there is.

B+0.004-0.9C+0.017Mo ² ≥0 (1)

Here, the content (mass %) of the corresponding element is substituted for each element symbol in the formula (1).

As a result of further investigation by the present inventors, when the austenitic stainless steel contains Cu, which is an optional element, when Cu is contained in 5.0% or less, excellent creep strength can be obtained and creep ductility can be maintained. It was found that when the upper limit was 1.9% or less, it was possible to maintain higher creep ductility while further increasing the creep strength. As a reason, the following can be considered. During use in a high-temperature corrosive environment, Cu precipitates in the grain to form a Cu phase. The Cu phase increases the creep strength, but sometimes decreases the creep ductility. Therefore, in the austenitic stainless steel which is the above chemical composition and satisfies the formula (1), more preferably, the Cu content is 1.9% or less. When the Cu content is 1.9% or less, excellent creep ductility can be more effectively maintained.

As a result of further investigation by the present inventors, it was found that when the Mo content was 0.5% or more, the creep ductility was further increased. The reason for this is not clear, but you can think of the following: In the above chemical composition (which satisfies the formula (1)), and when the Mo content is 0.5% or more, during use in a high-temperature corrosive environment of 600 to 700°C, Mo further segregates at grain boundaries or intermetallic compounds Is created. The grain boundary strength is further increased by this grain boundary segregation or an intermetallic compound. As a result, the creep ductility becomes higher. Therefore, the lower limit of the preferable Mo content is 0.5%. The lower limit of the preferred Mo content for further increasing the creep ductility is 0.8%, more preferably 1.0%, and still more preferably 2.0%.

Based on the above findings, the austenitic stainless steel according to the present invention, by mass, C: 0.030% or less, Si: 0.10 to 1.00%, Mn: 0.20 to 2.00%, P: 0.040% or less, S: 0.010 % Or less, Cr: 16.0 to 25.0%, Ni: 10.0 to 30.0%, Mo: 0.1 to 5.0%, Nb: 0.20 to 1.00%, N: 0.050 to 0.300%, sol. Al: 0.0005 to 0.1000%, B: 0.0010 to 0.0080%, Cu: 0 to 5.0%, W: 0 to 5.0%, Co: 0 to 1.0%, V: 0 to 1.00%, Ta: 0 to 0.2%, Hf ：0∼0.20%, Ca：0∼0.010%, Mg：0∼0.010%, and rare earth elements：0∼0.10%, the balance consists of Fe and impurities, and satisfies Equation (1) Have a composition.

B+0.004-0.9C+0.017Mo ² ≥0 (1)

Here, the content (mass %) of the corresponding element is substituted for each element symbol in the formula (1).

The said chemical composition may contain 1 type(s) or 2 or more types selected from the group consisting of Cu:0.1-5.0%, W:0.1-5.0%, and Co:0.1-1.0% by mass %.

The chemical composition may contain one or two or more selected from the group consisting of V: 0.1 to 1.00%, Ta: 0.01 to 0.2%, and Hf: 0.01 to 0.20% in mass%.

The chemical composition may contain one or two or more selected from the group consisting of Ca: 0.0005 to 0.010%, Mg: 0.0005 to 0.010%, and rare earth elements: 0.001 to 0.10% by mass.

The said chemical composition may contain Cu: 0-1.9% by mass %.

The chemical composition may contain 0.5% to 5.0% of Mo by mass%.

Hereinafter, the austenitic stainless steel of this embodiment is explained in full detail. "%" with respect to an element means mass% unless otherwise stated.

[Chemical composition]

The chemical composition of the austenitic stainless steel of this embodiment contains the following elements.

C: 0.030% or less

Carbon (C) is inevitably contained. When C is using the austenitic stainless steel of the present embodiment in a high-temperature corrosion environment of 600 to 700°C _{, M 23 C 6} type carbide is generated at the grain boundary, thereby reducing the polythionic acid SCC resistance. Therefore, the C content is not more than 0.030%. The upper limit of the C content is preferably 0.020%, more preferably 0.015%. It is preferable that the C content is as low as possible. However, as described above, since C is inevitably contained, in industrial production, at least 0.0001% of C may be contained. Therefore, the lower limit with a preferable C content is 0.0001%.

Si: 0.10~1.00%

Silicon (Si) deoxidizes steel. Si also increases the oxidation resistance and water vapor oxidation resistance of the steel. If the Si content is too low, the above effect cannot be obtained. On the other hand, when the Si content is too high, a sigma phase (σ phase) is precipitated in the steel, and the toughness of the steel is lowered. Therefore, the Si content is 0.10 to 1.00%. The upper limit of the Si content is preferably 0.75%, more preferably 0.50%.

Mn：0.20~2.00%

Manganese (Mn) deoxidizes steel. Mn also stabilizes austenite and increases creep strength. If the Mn content is too low, the above effect cannot be obtained. On the other hand, when the Mn content is too high, the creep strength of the steel decreases. Therefore, the Mn content is 0.20 to 2.00%. The preferred lower limit of the Mn content is 0.40%, more preferably 0.50%. The upper limit of the Mn content is preferably 1.70%, more preferably 1.50%.

P: 0.040% or less

Phosphorus (P) is an impurity. P deteriorates the hot workability and toughness of the steel. Therefore, the P content is 0.040% or less. The upper limit of the P content is preferably 0.035%, more preferably 0.032%. It is preferable that the P content is as low as possible. However, P is inevitably contained, and in industrial production, at least 0.0001% of P may be contained. Therefore, the lower limit with a preferable P content is 0.0001%.

S: 0.010% or less

Sulfur (S) is an impurity. S deteriorates the hot workability and creep ductility of the steel. Therefore, the S content is 0.010% or less. A preferable upper limit of the S content is 0.005%. It is preferable that the S content is as low as possible. However, S is inevitably contained, and in industrial production, S may contain at least 0.0001%. Therefore, the lower limit with a preferable S content is 0.0001%.

Cr：16.0~25.0%

Chromium (Cr) increases the SCC resistance of polythionic acid in steel. Cr also increases oxidation resistance, water vapor oxidation resistance, high temperature corrosion resistance, and the like. If the Cr content is too low, the above effect cannot be obtained. On the other hand, when the Cr content is too high, the creep strength and toughness of the steel decrease. Therefore, the Cr content is 16.0 to 25.0%. The preferable lower limit of the Cr content is 16.5%, more preferably 17.0%. The upper limit of the Cr content is preferably 24.0%, more preferably 23.0%.

Ni: 10.0~30.0%

Nickel (Ni) stabilizes austenite and increases creep strength. If the Ni content is too low, the above effect cannot be obtained. On the other hand, when the Ni content is too high, the above effect is saturated, and the manufacturing cost is increased. Therefore, the Ni content is 10.0 to 30.0%. The preferred lower limit of the Ni content is 11.0%, more preferably 13.0%. The upper limit of the Ni content is preferably 25.0%, more preferably 22.0%.

Mo: 0.1~5.0%

Molybdenum (Mo) suppresses the formation _{of M 23} C ₆ type carbide at grain boundaries during use in a high-temperature corrosive environment of 600 to 700°C. Mo is also, in the use under a 600 high temperature corrosive environment at ~ 700 ℃, when the MX type carbonitride of Nb can change as M ₂₃ C ₆ type carbides, and inhibit the B is employed in the M ₂₃ C ₆ type carbide , It suppresses reduction of the amount of segregation B of grain boundaries in a high-temperature corrosive environment. Thereby, in a high-temperature corrosive environment, sufficient creep ductility can be obtained. If the Mo content is too low, the above effect cannot be obtained. On the other hand, when the Mo content is too high, the stability of austenite decreases. Therefore, the Mo content is 0.1 to 5.0%. The preferred lower limit of the Mo content is 0.2%, more preferably 0.3%.

When the Mo content is 0.5% or more, Mo segregates at the grain boundary or generates an intermetallic compound to further increase the grain boundary strength. In this case, under a high-temperature corrosive environment, a better creep strength can be obtained. Therefore, a more preferable lower limit of the Mo content is 0.5%, more preferably 0.8%, more preferably 1.0%, more preferably 1.5%, and still more preferably 2.0%. When the Mo content is 1.5% or more, the creep strength is also increased. The upper limit of the Mo content is preferably 4.5%, more preferably 4.0%. When the Mo content is 1.5% or more, the creep strength is also increased.

Nb：0.20~1.00%

Niobium (Nb) combines with C to form MX-type carbonitride during use in a high-temperature corrosive environment of 600 to 700°C, thereby reducing the amount of solid solution C in the steel. Thereby, the polythionic acid SCC resistance of the steel is improved. The produced Nb MX-type carbonitride further increases the creep strength. If the Nb content is too low, the above effect cannot be obtained. On the other hand, when the Nb content is too high, δ ferrite is generated, which lowers the creep strength, toughness, and weldability of the steel for a long time. Therefore, the Nb content is 0.20 to 1.00%. The preferable lower limit of the Nb content is 0.25%. The upper limit of the Nb content is preferably 0.90%, more preferably 0.80%.

N：0.050~0.300%

Nitrogen (N) is dissolved in a matrix (hair phase) to stabilize austenite and increase creep strength. N also forms a fine carbonitride in the grain, thereby increasing the creep strength of the steel. That is, N contributes to the creep strength in both solid solution strengthening and precipitation strengthening. If the N content is too low, the above effect cannot be obtained. On the other hand, when the N content is too high, Cr nitride is formed at the grain boundary, and the polythionic acid SCC resistance in the welding heat affected zone (HAZ) is deteriorated. If the N content is too high, the workability of the steel is further deteriorated. Therefore, the N content is 0.050 to 0.300%. The preferable lower limit of the N content is 0.070%. The upper limit of the N content is preferably 0.250%, more preferably 0.200%.

sol. Al: 0.0005~0.100%

Aluminum (Al) deoxidizes steel. If the Al content is too low, the above effect cannot be obtained. On the other hand, when the Al content is too high, the cleanliness of the steel decreases, and the workability and ductility of the steel decrease. Therefore, the Al content is 0.0005 to 0.100%. The preferred lower limit of the Al content is 0.001%, more preferably 0.002%. The upper limit of the Al content is preferably 0.050%, more preferably 0.030%. In this embodiment, the Al content means the content of Al (sol. Al) for an acid value.

B：0.0010~0.0080%

Boron (B) segregates at grain boundaries during use in a high-temperature corrosive environment at 600 to 700°C, thereby increasing grain boundary strength. As a result, creep ductility is improved. If the B content is too low, the above effect cannot be obtained. On the other hand, when the B content is too high, weldability and hot workability at high temperature are deteriorated. Therefore, the B content is 0.0010 to 0.0080%. The preferable lower limit of the B content is 0.0015%, more preferably 0.0020%. The upper limit of the B content is preferably less than 0.0060%, more preferably 0.0050%.

The balance of the chemical composition of the austenitic stainless steel according to the present embodiment is composed of Fe and impurities. Here, impurities are those that are mixed from ore, scrap, or manufacturing environment as raw materials when industrially manufacturing austenitic stainless steel, and are allowed within the range that does not adversely affect the austenitic stainless steel of the present embodiment. Means being.

[About arbitrary elements]

The austenitic stainless steel according to the present embodiment may further contain one or two or more selected from the group consisting of Cu, W, and Co instead of a part of Fe. All of these elements increase the creep strength of the steel.

Cu: 0~5.0%

Copper (Cu) is an arbitrary element and does not need to be contained. When contained, Cu precipitates as a Cu phase in the grain during use in a high-temperature corrosion environment of 600 to 700°C, and increases the creep strength of the steel by precipitation strengthening. However, if the Cu content is too high, the hot workability and weldability of the steel are deteriorated. Therefore, the Cu content is 0 to 5.0%. The preferred lower limit of the Cu content for increasing the creep strength more effectively is 0.1%, more preferably 2.0%, and still more preferably 2.5%. The upper limit of the Cu content is preferably 4.5%, more preferably 4.0%. On the other hand, the preferable Cu content for maintaining more excellent creep ductility is 0 to 1.9%, and the upper limit of the more preferable Cu content is 1.8%.

W：0~5.0%

Tungsten (W) is an arbitrary element and does not need to be contained. When contained, W is solid solution as a matrix (mother shape), increasing the creep strength of the steel. However, if the W content is too high, the stability of austenite decreases, and the creep strength and toughness of the steel decrease. Therefore, the W content is 0 to 5.0%. The preferred lower limit of the W content is 0.1%, more preferably 0.2%. The upper limit of the W content is preferably 4.5%, more preferably 4.0%.

Co：0~1.0%

Cobalt (Co) is an arbitrary element and does not have to be contained. When contained, Co stabilizes austenite, increasing creep strength. However, if the Co content is too high, the cost of raw materials becomes high. Therefore, the Co content is 0 to 1.0%. The preferred lower limit of the Co content is 0.1%, more preferably 0.2%.

The austenitic stainless steel according to the present embodiment may further contain one or two or more selected from the group consisting of V, Ta, and Hf instead of a part of Fe. All of these elements increase the polythionic acid SCC resistance and creep strength of the steel.

V：0~1.00%

Vanadium (V) is an arbitrary element and does not have to be contained. When contained, V combines with C during use in a high-temperature corrosive environment of 600 to 700°C to form carbonitrides, reduces solid solution C, and increases polythionic acid SCC resistance of the steel. The generated V carbonitride further increases the creep strength. However, if the V content is too high,? Ferrite is generated, and the creep strength, toughness, and weldability of the steel are deteriorated. Therefore, the V content is 0 to 1.00%. The preferred lower limit of the V content for increasing the polythionic acid SCC resistance and creep strength more effectively is 0.10%. The upper limit of the V content is preferably 0.90%, more preferably 0.80%.

Ta：0~0.2%

Tantalum (Ta) is an arbitrary element and does not have to be contained. When contained, Ta combines with C during use in a high-temperature corrosive environment of 600 to 700°C to form carbonitrides, reduces solid solution C, and increases polythionic acid SCC resistance of steel. The produced Ta carbonitride further increases the creep strength. However, when the Ta content is too high,? Ferrite is generated, and the creep strength, toughness, and weldability of the steel are deteriorated. Therefore, the Ta content is 0 to 0.2%. The preferable lower limit of the Ta content for further effectively increasing the polythionic acid SCC resistance and creep strength is 0.01%, more preferably 0.02%.

Hf：0~0.20%

Hafnium (Hf) is an arbitrary element and does not have to be contained. When contained, Hf bonds with C during use in a high-temperature corrosive environment of 600 to 700° C. to form carbonitrides, reduces solid solution C, and increases polythionic acid SCC resistance of the steel. The generated Hf carbonitride further increases the creep strength. However, if the Hf content is too high, δ ferrite is generated, and the creep strength, toughness, and weldability of the steel are deteriorated. Therefore, the Hf content is 0 to 0.20%. The preferred lower limit of the Hf content is 0.01%, more preferably 0.02%.

The austenitic stainless steel according to the present embodiment may further contain one or two or more selected from the group consisting of Ca, Mg, and rare earth elements instead of a part of Fe. All of these elements increase the hot workability and creep ductility of the steel.

Ca: 0~0.010%

Calcium (Ca) is an arbitrary element and does not have to be contained. When contained, Ca fixes O (oxygen) and S (sulfur) as inclusions, thereby enhancing the hot workability and creep ductility of the steel. However, if the Ca content is too high, the hot workability and creep ductility of the steel are deteriorated. Therefore, the Ca content is 0 to 0.010%. The preferable lower limit of the Ca content is 0.0005%, more preferably 0.001%. The upper limit of the Ca content is preferably 0.008%, more preferably 0.006%.

Mg: 0~0.010%

Magnesium (Mg) is an arbitrary element and does not have to be contained. When contained, Mg fixes O (oxygen) and S (sulfur) as inclusions, thereby increasing the hot workability and creep ductility of the steel. However, if the Mg content is too high, the hot workability of the steel and the creep ductility for a long time are deteriorated. Therefore, the Mg content is 0 to 0.010%. The preferred lower limit of the Mg content is 0.0005%, more preferably 0.001%. The upper limit of the Mg content is preferably 0.008%, more preferably 0.006%.

Rare earth elements: 0 to 0.10%

The rare earth element (REM) is an arbitrary element and does not have to be contained. When contained, REM fixes O (oxygen) and S (sulfur) as inclusions, thereby improving hot workability and creep ductility of the steel. However, if the REM content is too high, the hot workability of the steel and the creep ductility for a long time are deteriorated. Therefore, the REM content is 0 to 0.01%. The preferred lower limit of the REM content is 0.001%, more preferably 0.002%. The upper limit of the REM content is preferably 0.08%, more preferably 0.06%.

REM in this specification contains at least one or more of Sc, Y, and lanthanoids (Lu of La to 71 of atomic number 57), and REM content means the total content of these elements.

[About equation (1)]

The chemical composition also satisfies the formula (1).

B+0.004-0.9C+0.017Mo ² ≥0 (1)

For each element symbol in Formula (1), the content (mass%) of the corresponding element is substituted.

As described above, in the present embodiment, in order to increase the polythionic acid SCC resistance, the C content is not only 0.030% or less, but also 0.20 to 1.00% of Nb is contained, and used in a high-temperature corrosion environment of 600 to 700°C. An MX-type carbonitride of Nb is produced in the inside, thereby reducing the amount of solid solution C. However, since the MX-type carbonitride of Nb is metastable, it changes to the _{Z-phase and M 23} C _{6 type carbide during use under the high-temperature use environment.} At this time, B segregating at the grain boundary is _{dissolved in the M 23} C ₆ type carbide, and the amount of B segregation at the grain boundary is reduced. As a result, the creep ductility deteriorates.

However, when Mo is solid solution in M ₂₃ C ₆ type carbide to produce "Mo solid solution M ₂₃ C ₆ type carbide", B is hardly dissolved in Mo solid solution M ₂₃ C _{6 type carbide.} Therefore, the amount of B segregation at the grain boundary is maintained, and not only excellent polythionic acid SCC resistance can be obtained, but also excellent creep ductility can be obtained.

It is defined as F1=B+0.004-0.9C+0.017Mo ^2. F1 is an indicator of the plurality of M ₂₃ C ₆ type carbide during, Mo ratio employed in the M ₂₃ C ₆ type carbide generated in the steel used in a high temperature corrosive environment. When F1 is 0 or more, even if a plurality of M ₂₃ C ₆ type carbides are generated in the _{steel during use in a high temperature corrosive environment, the proportion of Mo solid solution M 23} C ₆ type carbides is high. Therefore, it is difficult for B segregating at the grain boundaries to be dissolved in the M ₂₃ C ₆ type carbide, and the amount of segregating B at the grain boundaries is maintained. Therefore, it is possible to achieve both excellent polythionic acid SCC resistance and excellent creep ductility. Thus, F1 is greater than or equal to 0 (0.00000). Preferably, F1 is 0.00100 or more, more preferably 0.00200 or more, more preferably 0.00400 or more, still more preferably 0.00500, still more preferably 0.00800 or more, and most preferably 0.01000.

Preferably, when the chemical composition of the austenitic stainless steel contains Cu, as described above, the upper limit of the Cu content is 1.9% or less. That is, in consideration of obtaining an excellent creep ductility while increasing the creep strength, the preferable Cu content is 0% to 1.9%. When the Cu content is 1.9% or less, excellent creep ductility can be maintained while obtaining excellent creep strength by precipitation strengthening of the Cu phase.

In the chemical composition of the austenitic stainless steel, the lower limit of the Mo content is preferably 0.5%. In this case, during use in a high-temperature corrosive environment of 600 to 700°C, Mo further segregates at grain boundaries or generates intermetallic compounds. The grain boundary strength is further increased by this grain boundary segregation or an intermetallic compound. As a result, the creep ductility becomes higher. Therefore, the lower limit of the preferable Mo content is 1.0%. In addition, when the lower limit of the Mo content is 1.0% or more, the preferred F1 value is 0.00500 or more, more preferably 0.00800 or more, and still more preferably 0.01000 or more.

[Manufacturing method]

An example of a method for producing an austenitic stainless steel according to the present invention will be described. This manufacturing method includes a preparation step for preparing a material, a hot working step for producing a steel material by hot working on the material, a cold working step for cold working the steel material after the hot working step if necessary, and if necessary. A solution treatment step of performing a solution treatment treatment on a steel material is provided. Hereinafter, the manufacturing method is demonstrated.

[Preparation process]

A molten steel having the above-described chemical composition and satisfying the formula (1) is prepared. For example, the molten steel is manufactured using an electric furnace, Argon Oxygen Decarburization (AOD), or Vacuum Oxygen Decarburization (VOD) furnace. The produced molten steel is subjected to a known degassing treatment as necessary. A material is produced from molten steel subjected to degassing treatment. The manufacturing method of the material is, for example, a continuous casting method. A continuous casting material (material) is manufactured by a continuous casting method. Continuous cast materials are, for example, slabs, blooms and billets. Molten steel may be used as an ingot by the ingot method.

[Hot working process]

The prepared material (continuous casting material or ingot) is hot-worked to produce an austenitic stainless steel material. For example, a material is hot-rolled to manufacture a steel plate, bar, or wire. Further, an austenitic stainless steel pipe is manufactured by hot extrusion or hot drilling rolling. The specific method of hot working is not particularly limited, and hot working may be performed according to the shape of the final product. The processing end temperature of hot working is, for example, 1050°C or higher. The processing end temperature here means the temperature of the steel material immediately after the final hot working is completed.

[Cold working process]

The austenitic stainless steel material after hot working may be subjected to cold working as necessary. When the austenitic stainless steel material is a bar steel, a wire rod, or a steel pipe, the cold working is, for example, cold drawing or cold rolling. When the austenitic stainless steel material is a steel plate, it is cold rolling.

[Solution treatment process]

After hot working or after cold working, you may perform solution treatment as needed. In the solution treatment step, the structure is homogenized and the carbonitride is solidified. The preferred solution treatment temperature is as follows.

Preferred solution treatment temperature: 1000 to 1250°C

When the solution treatment temperature is 1000°C or higher, the carbonitride of Nb is sufficiently dissolved and the creep strength becomes higher. When the heat treatment temperature is 1250°C or less, excessive solid solution of C can be suppressed, and the polythionic acid SCC resistance becomes higher.

The holding time at the solution treatment temperature in the solution treatment is not particularly limited, but is, for example, 2 minutes to 60 minutes.

In addition, instead of the solution treatment described above, rapid cooling may be performed immediately after hot working with respect to the steel material produced by the hot working step. In this case, it is preferable that the processing end temperature of hot working is 1000°C or higher. When the hot working end temperature is 1000°C or higher, the carbonitride of Nb is sufficiently dissolved, and during use in a high-temperature corrosion environment of 600 to 700°C, both excellent polythionic acid SCC resistance and excellent creep ductility are possible. In addition, sufficient creep strength can also be obtained by generation of Nb carbonitride during use in a high-temperature environment.

In addition, the shape of the austenitic stainless steel of this embodiment is not specifically limited. The austenitic stainless steel of this embodiment may be a steel plate, a steel pipe, a bar or a wire, or a shape steel.

Example

A molten steel having the chemical composition shown in Table 1 was prepared.

In the column "F1" in Table 1, the F1 value of the steel of each test number is written. In addition, the element symbol in the column of "other" in the column of "chemical composition" and the numerical value given in front of the symbol refer to an arbitrary element contained and its content (mass%). In the chemical composition of each test number, the remainder other than the elements shown in Table 1 was Fe and impurities.

Using molten steel, an ingot having an outer diameter of 120 mm and 30 kg was manufactured. Hot forging was performed on the ingot to obtain a steel plate having a thickness of 40 mm. Further, hot rolling was performed to obtain a steel plate having a thickness of 15 mm. All of the final working temperatures during hot rolling were 1050°C or higher. The steel sheet after hot rolling was further cold-rolled to produce a steel sheet having a thickness of 10.5 mm, a width of 50 mm, and a length of 100 mm. Each steel sheet after cold rolling was subjected to a solution treatment. The solution treatment temperature of the steel sheets of each test number was 1150°C, and the solution treatment time was 10 minutes. The steel sheet after solution treatment was water-cooled. By the above process, an austenitic stainless steel material was manufactured.

The plate thickness of the manufactured austenitic stainless steel sheet is defined as t (mm), and a known component analysis method (for C and S, combustion-infrared absorption method) using a sample at an arbitrary position t/4 from the surface , For N, a high-temperature exhaust gas analysis method, and for other alloying elements, an ICP analysis method) was performed. As a result, the chemical composition of the austenitic stainless steel sheet of each test number was consistent with Table 1.

[Polythionic acid SCC resistance evaluation test]

For the steel sheets of each test number, aging treatment was performed at 600° C. for 5000 hours, assuming use in a high-temperature environment. From this aging treatment material, a plate-shaped test piece having a thickness of 2 mm, a width of 10 mm, and a length of 75 mm was taken. According to JIS G 0576 (2001) "Stainless Steel Stress Corrosion Crack Test Method", a polythionic acid SCC resistance evaluation test was performed. Specifically, the test piece was bent around a punch having an inner radius of 5 mm to obtain a U-band shape. The U-band type test piece was immersed in a Wackenroder solution (a solution in which a large amount of H _{2 S gas was blown into a saturated aqueous H 2} SO ₃ solution prepared by blowing _{SO 2} gas in distilled water) at room temperature for 100 hours. With respect to the test piece after immersion, the presence or absence of cracking was observed under a microscope at 500 times magnification, and the presence or absence of cracks was confirmed.

When no cracking was observed, it was judged that the polythionic acid SCC resistance was excellent ("E" (Excellent) in the "polythionic acid SCC resistance" column in Table 2). When even one crack was confirmed, it was judged that the polythionic acid SCC resistance was low ("NA" (Not Accepted) in the "polythionic acid SCC resistance" column in Table 2).

[Creep ductility and creep strength evaluation test]

From the steel sheets of each test number, a creep fracture test piece conforming to JIS Z2271 (2010) was produced. The cross section perpendicular to the axial direction of the creep fracture test piece was circular, the outer diameter of the creep fracture test piece was 6 mm, and the parallel portion was 30 mm. The parallel portion was parallel to the rolling direction of the steel sheet. Using the produced creep rupture test piece, a creep rupture test in conformity with JIS Z2271 (2010) was performed. Specifically, after heating the creep rupture test piece at 750°C, a creep rupture test was performed. The test stress was 45 MPa, and the creep rupture time (time) and creep rupture shrinkage (%) were determined.

Regarding the creep strength, when the creep rupture time was 5000 to 10000 h or less, it was judged that the creep strength was excellent (indicated as "G" (Good) in the "creep strength" column in Table 2). When the creep rupture time exceeded 10000 hours, it was judged that the creep strength was remarkably excellent (indicated by "E" (Excellent) in the "creep strength" column in Table 2). When the creep rupture time was less than 5000 hours, it was judged that the creep strength was low (indicated as "NA" (Not Accepted) in the "creep strength" column in Table 2). When the creep rupture time was G or E, it was judged that sufficient creep strength could be obtained.

Regarding the creep ductility, when the creep rupture shrinkage was 20.0% to 30.0% or less, it was judged that the creep ductility was good (indicated by "P" (Passing) in the "creep ductility" column in Table 2). When the creep fracture shrinkage exceeded 30.0% and was 50.0% or less, it was judged that the creep ductility was excellent (indicated as "G" (Good) in the "creep ductility" column in Table 2). In addition, when the creep fracture shrinkage exceeded 50.0%, it was judged that the creep ductility was remarkably excellent (indicated by "E" (Excellent) in the "creep ductility" column in Table 2). When the creep fracture shrinkage was less than 20.0%, it was judged that the creep ductility was low ("NA" (Not Accepted) in the "creep ductility" column in Table 2. When the creep fracture shrinkage was P, G, or E, sufficient creep It was judged that ductility could be obtained.

[Test result]

Table 2 shows the test results.

Referring to Table 1 and Table 2, the content of each element in the chemical composition of the steels of Test Nos. 1 to 16 was appropriate, and F1 also satisfied Formula (1). Therefore, excellent polythionic acid SCC resistance could be obtained in the steel sheets of these test numbers. Moreover, the breaking time was 5000 hours or more, and excellent creep strength could be obtained. Further, the creep fracture shrinkage was 20.0% or more, and excellent creep ductility was obtained. In addition, in Test Nos. 2 to 4, 6 to 12 and 15, since it contains Cu or contains a large amount of Mo, the breaking time in the creep rupture test is longer than that of Test Nos. 1, 5, 13, 14 and 16, and is 10000 hours. It was the above and was able to obtain excellent creep strength.

In addition, in Test Nos. 3 and 4, which contained 1.9% or less of Cu content and contained 0.5% or more of Mo, and Test Nos. 5 to 7, 11, and 12 containing 1.0% or more of Mo even without containing Cu, While sufficient creep strength could be obtained, it was also possible to obtain excellent creep ductility.

On the other hand, in Test Nos. 17 and 18, F1 did not satisfy the formula (1). As a result, the creep fracture shrinkage was less than 20%, and the creep ductility of the steel was low. It is considered that this is because the grain boundary strengthening effect due to the grain boundary segregation of B was not sufficiently obtained. In addition, the creep strength was also low.

In Test No. 19, the C content was too high. As a result, polythionic acid SCC resistance was low.

In Test No. 20, since it contained Cu, although the creep strength was high, F1 did not satisfy the formula (1). As a result, the creep fracture shrinkage was less than 20.0%, and the creep ductility of the steel was low.

In Test No. 21, Mo was not contained. In addition, F1 was less than the lower limit of Formula (1). As a result, the fracture shrinkage was less than 20.0%, and the creep ductility of the steel was low. In addition, the creep strength was also low.

In Test No. 22, the B content was low. As a result, the creep fracture shrinkage was less than 20.0%, and the creep ductility of the steel was low. In addition, the creep strength was also low.

In Test No. 23, it did not contain Nb. As a result, polythionic acid SCC resistance was low. Further, the breaking time was less than 5000 hours, and the creep strength of the steel was low.

In the above, the embodiment of the present invention has been described. However, the above-described embodiment is only an example for carrying out the present invention. Accordingly, the present invention is not limited to the above-described embodiments, and can be implemented by appropriately changing the above-described embodiments within a range not departing from the spirit thereof.

Claims (6)

In% by mass,
C: 0.030% or less,
Si: 0.10 to 1.00%,
Mn: 0.20 to 2.00%,
P: 0.040% or less,
S: 0.010% or less,
Cr: 16.0-25.0%,
Ni: 10.0 to 30.0%,
Mo: 0.1~5.0%,
Nb: 0.20 to 1.00%,
N: 0.050 to 0.300%,
sol. Al: 0.0005~0.100%,
B: 0.0010~0.0080%,
Cu: 0 to 5.0%,
W: 0~5.0%,
Co: 0~1.0%,
V: 0 to 1.00%,
Ta: 0~0.2%,
Hf: 0~0.20%,
Ca: 0 to 0.010%,
Mg: 0 to 0.010%, and,
Rare earth element: contains 0 to 0.10%, the balance consists of Fe and impurities,
An austenitic stainless steel having a chemical composition that satisfies Equation (1).
B+0.004-0.9C+0.017Mo ² ≥0.00200 (1)
Here, the content (mass %) of the corresponding element is substituted for each element symbol in the formula (1). The method according to claim 1,
The chemical composition is,
Cu: 0.1 to 5.0%,
W: 0.1 to 5.0%, and
Co: An austenitic stainless steel containing one or two or more selected from the group consisting of 0.1 to 1.0%. The method according to claim 1,
The chemical composition is,
V: 0.1 to 1.00%,
Ta: 0.01 to 0.2%, and
Hf: An austenitic stainless steel containing one or two or more selected from the group consisting of 0.01% to 0.20%. The method according to claim 2,
The chemical composition is,
V: 0.1 to 1.00%,
Ta: 0.01 to 0.2%, and
Hf: An austenitic stainless steel containing one or two or more selected from the group consisting of 0.01% to 0.20%. The method according to any one of claims 1 to 4,
The chemical composition is,
Ca: 0.0005 to 0.010%,
Mg: 0.0005 to 0.010%, and,
Rare earth elements: austenitic stainless steel containing one or two or more selected from the group consisting of 0.001 to 0.10%. The method according to claim 1,
The chemical composition is,
Cu: an austenitic stainless steel containing 0 to 1.9%.