JP2017066470A - Austenitic stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an austenitic stainless steel having high strength and excellent hydrogen embrittlement resistance even with a Ni content suppressed to less than 10%.SOLUTION: The austenitic stainless steel has a chemical composition which contains, by mass%, C:0.20% or less, Si:1.0% or less, Mn:over 20% to 50%, Cr:10 to 25%, Ni:0.5 to less than 8%, Al:0.10% or less, Mo and/or W:0.1 to 5.0% in total, Cu:0.001 to 3.0%, N:0.05 to 0.40% and rare earth element: 0 to 1.0% and the balance Fe with impurities and satisfies formula (1), the total amount of Cr nitride and a σ phase in the steel being 7 mass% or less. 30≤(-6.5+3.0 Ni+0.5Cr+2.0Mo+1.2 W+0.7Mn+0.2Cu-5.5C-30 N)≤60 (1), where each element symbol in the formula(1) is assigned by content of corresponding element (mass%).SELECTED DRAWING: None

Description

  The present invention relates to an austenitic stainless steel, and more particularly to an austenitic stainless steel for high-pressure hydrogen gas.

  In recent years, attention has been focused on the use of hydrogen as a new energy source. For example, development of a fuel cell vehicle using hydrogen and development of a hydrogen station for supplying the hydrogen are in progress. Equipment that transports, stores and supplies hydrogen is subject to very high pressures. Therefore, materials used in a high-pressure hydrogen environment are required to have excellent hydrogen embrittlement resistance in addition to high strength.

  As a steel excellent in hydrogen embrittlement resistance, SUS316L, which is an austenitic stainless steel defined in JIS standards, is known. However, in a high-pressure hydrogen environment, higher strength than SUS316L is required.

  Generally, if nitrogen (N) is contained, it is known that the strength of steel is increased by solid solution strengthening. However, if the N content is too high, the strength increases, but the hydrogen embrittlement resistance tends to decrease.

  International Publication No. 2012/132992 (Patent Document 1) proposes a stainless steel having high strength and excellent hydrogen embrittlement resistance. The stainless steel for high-pressure hydrogen gas disclosed in this document is, in mass%, C: 0.10% or less, Si: 1.0% or less, Mn: 3% or more and less than 7%, Cr: 15 to 30%, Ni: 10% to less than 17%, Al: 0.10% or less, N: 0.10 to 0.50%, and V: 0.01 to 1.0% and Nb: 0.01 to 0.50% Is contained, and the balance consists of Fe and impurities. This stainless steel has high strength by containing 0.10 to 0.50% N, and contains N by 10% or more and less than 17%. Excellent hydrogen embrittlement resistance.

As another austenitic heat resistant steel excellent in high strength and hydrogen embrittlement resistance, there is AISI Alloy 286. Alloy 286 does not contain N and contains about 25% of Ni. Therefore, Alloy 286 is excellent in hydrogen embrittlement resistance. Furthermore, since γ′-Ni ₃ (Al, Ti) is finely dispersed in the steel, high strength is exhibited.

  However, since the stainless steel and Alloy 286 described in Patent Document 1 contain 10% or more of Ni, the cost increases. Therefore, there is a demand for a steel that suppresses the Ni content to less than 10% and is excellent in strength and hydrogen embrittlement resistance.

  An austenitic stainless steel excellent in hydrogen embrittlement resistance while suppressing the Ni content to less than 10% has been proposed in Japanese Unexamined Patent Application Publication No. 2007-126688 (Patent Document 2).

  The austenitic high Mn stainless steel for high-pressure hydrogen gas disclosed in Patent Document 2 is mass%, C: 0.01 to 0.10%, N: 0.01 to 0.40%, Si: 0.1. -1%, Cr: 10 to 20%, Mn: 6 to 20%, Cu: 2 to 5%, Ni: 1 to 6%, with the balance being Fe and inevitable impurities. In this stainless steel, Md30, which is an index of austenite stability, is set to −120 to 20 ° C. Thus, Patent Document 2 describes that the formation of strain-induced martensite is suppressed and the hydrogen embrittlement susceptibility is higher than that of SUS316L.

International Publication No. 2012/132992 JP 2007-126688 A

  However, although the stainless steel of Patent Document 2 is excellent in the resistance to hydrogen embrittlement, the strength is not disclosed and the strength may be low.

  An object of the present invention is to provide an austenitic stainless steel having high strength and excellent hydrogen embrittlement resistance even when the Ni content is less than 10%.

The austenitic stainless steel according to the present invention is, in mass%, C: 0.20% or less, Si: 1.0% or less, Mn: more than 20% to 50%, Cr: 10 to 25%, Ni: 0.5 ˜less than 8%, Al: 0.10% or less, Mo and / or W: 0.1 to 5.0% in total, Cu: 0.001 to 3.0%, N: 0.05 to 0.40 %, Co: 0 to 3.0%, Ti: 0 to 1.0%, V: 0 to 1.0%, Nb: 0 to 1.0%, Zr: 0 to 1.0%, Hf: 0 -1.0%, B: 0-0.01%, Ca: 0-0.05%, Mg: 0-0.05%, and rare earth elements: 0-1.0%, the balance being Fe And P, S, and O in the impurity are P: 0.050% or less, S: 0.050% or less, and O: 0.020% or less, which satisfies the formula (1) Has chemical composition The total amount of Cr nitrides in the steel and σ-phase is not more than 7 wt%.
30 ≦ (−6.5 + 3.0Ni + 0.5Cr + 2.0Mo + 1.2W + 0.7Mn + 0.2Cu−5.5C-30N) ≦ 60 (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 has excellent strength and hydrogen embrittlement resistance while suppressing the Ni content to less than 10%.

  The present inventors investigated and examined the strength and hydrogen embrittlement resistance of steel, and obtained the following knowledge.

  N increases the strength of the steel by solid solution strengthening. Therefore, the N content is 0.05 to 0.40%. Since N deteriorates the hydrogen embrittlement resistance, even if N is contained, it is necessary to maintain and improve the hydrogen embrittlement resistance. In conventional stainless steel (Patent Document 1), by setting the Ni content to 10% or more, excellent hydrogen embrittlement resistance was obtained while increasing the N content. However, as described above, since Ni is expensive, it is required to be suppressed to less than 10%.

  If Mn, Cr, Mo and / or W and Cu are contained as an alternative to Ni, excellent hydrogen embrittlement resistance can be obtained while suppressing the Ni content to less than 10%. Although the relationship between these elements and hydrogen embrittlement resistance is not necessarily clear, it is considered that these elements increase the stacking fault energy (hereinafter also referred to as SFE). SFE is deeply involved in the structure of metastatic tissue during deformation.

  On the other hand, N significantly reduces SFE, and C also decreases SFE. Therefore, if Mn, Cr, Mo and / or W, and Cu are contained, SFE lowered by N and C can be increased, and the transition structure deformation behavior changes. Thereby, it is considered that the hydrogen embrittlement resistance is improved.

  It is defined as F1 = (− 6.5 + 3.0Ni + 0.5Cr + 2.0Mo + 1.2W + 0.7Mn + 0.2Cu−5.5C-30N). The content (% by mass) of the corresponding element is substituted for each element symbol in F1.

  F1 means the size of SFE and is an index of hydrogen embrittlement resistance. If F1 is too low, the hydrogen embrittlement resistance is low. If F1 is 30 or more, excellent hydrogen embrittlement resistance can be obtained even if the N content is 0.05 to 0.4%.

  In order to make F1 30 or more, the Mn content is made higher than 20%. Mn is inexpensive. Therefore, Mn is used as an alternative to Ni to increase the Mn content. Furthermore, Cr which is an element which raises SFE in F1 is contained 10 to 25%, Mo and / or W is contained 0.1 to 5.0% in total, and F1 value is set to 30 or more.

In order to improve the hydrogen embrittlement resistance, the formation of precipitates is further suppressed. In the case of the chemical composition of the austenitic stainless steel of the present invention, Cr nitride (Cr ₂ N) and σ phase (CrMn ₃ ) are generated as precipitates. Hereinafter, the Cr nitride and the σ phase are also referred to as specific precipitates. When the number of specific precipitates in the steel increases, the specific precipitate becomes a starting point for hydrogen embrittlement, or the specific precipitate decreases SFE, and the hydrogen embrittlement resistance decreases.

  Coarse Cr nitride becomes a starting point of hydrogen embrittlement. Fine Cr nitride does not serve as a starting point for hydrogen embrittlement, but reduces SFE. That is, the hydrogen embrittlement resistance is reduced. In the case of steel with a high Mn content, the σ phase is precipitated during the cooling of the steel. Since the σ phase is usually coarse, it becomes a starting point for hydrogen embrittlement and deteriorates the hydrogen embrittlement resistance. When the total amount of the specific precipitates is 7% by mass or less, a decrease in hydrogen embrittlement resistance is suppressed. The total amount of the specific precipitate can be adjusted by, for example, the treatment temperature and the cooling rate in the solution heat treatment during the production process.

The austenitic stainless steel according to the present invention completed based on the above knowledge is, in mass%, C: 0.20% or less, Si: 1.0% or less, Mn: more than 20% to 50%, Cr: 10 to 10%. 25%, Ni: 0.5 to less than 8%, Al: 0.10% or less, Mo and / or W: 0.1 to 5.0% in total, Cu: 0.001 to 3.0%, N : 0.05 to 0.40%, Co: 0 to 3.0%, Ti: 0 to 1.0%, V: 0 to 1.0%, Nb: 0 to 1.0%, Zr: 0 to 1.0%, Hf: 0 to 1.0%, B: 0 to 0.01%, Ca: 0 to 0.05%, Mg: 0 to 0.05%, and rare earth elements: 0 to 1.0 The balance is composed of Fe and impurities, and P, S, and O in the impurities are P: 0.050% or less, S: 0.050% or less, and O: 0.020% or less. Yes, formula It has a chemical composition that satisfies 1), the total amount is less than 7 wt% of a particular precipitates in steel.
30 ≦ (−6.5 + 3.0Ni + 0.5Cr + 2.0Mo + 1.2W + 0.7Mn + 0.2Cu−5.5C-30N) ≦ 60 (1)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula (1).

The austenitic stainless steel may contain one or more elements selected from the first group to the fourth group in mass%.
First group: Co: 0.1 to 3.0%
Second group: Ti: 0.001 to 1.0%, V: 0.01 to 1.0%, and Nb: 0.01 to 1.0%
Third group: Zr: 0.001 to 1.0%, Hf: 0.001 to 1.0%, and B: 0.0001 to 0.01%
Group 4: Ca: 0.0003 to 0.05%, Mg: 0.0003 to 0.05%, and rare earth elements: 0.05 to 1.0%

  Hereinafter, the austenitic stainless steel of the present invention will be described in detail. “%” Regarding an element means mass% unless otherwise specified.

[Chemical composition]
The chemical composition of the austenitic stainless steel of the present invention contains the following elements.

C: 0.20% or less Carbon (C) increases the strength of steel. However, if the C content is too high, a large amount of carbide precipitates at the grain boundaries, the toughness of the steel decreases, and the hydrogen embrittlement resistance decreases. Therefore, the C content is 0.20% or less. The upper limit with preferable C content is 0.16%, More preferably, it is 0.13%. The C content is preferably as low as possible.

Si: 1.0% or less Silicon (Si) combines with Ni, Cr, etc. to form an intermetallic compound, and deteriorates hot workability. Accordingly, the Si content is 1.0% or less. The upper limit with preferable Si content is 0.5%. The Si content is preferably as low as possible. Considering the refining cost, the preferable lower limit of the Si content is 0.01%.

Mn: more than 20% to 50%
Manganese (Mn) is inexpensive and stabilizes austenite in steel. Mn increases the strength, ductility and toughness of steel by an appropriate combination with Cr, Ni, N, and the like. Mn further enhances the hydrogen embrittlement resistance of the steel. If the Mn content is too low, these effects cannot be obtained. On the other hand, if the Mn content is too high, the hot workability and toughness of the steel decrease. Therefore, the Mn content is more than 20% to 50%. The minimum with preferable Mn content is 22%, More preferably, it is 24%. The upper limit with preferable Mn content is 45%, More preferably, it is 43%.

Cr: 10 to 25%
Chromium (Cr) increases the corrosion resistance of the steel and further improves the hydrogen embrittlement resistance. If the Cr content is too low, this effect cannot be obtained. On the other hand, if the Cr content is too high, precipitates such as Cr nitride are easily generated. Cr nitride reduces the hydrogen embrittlement resistance of the steel. Therefore, the Cr content is 10 to 25%. The minimum with preferable Cr content is 12%, More preferably, it is 14%. The upper limit with preferable Cr content is 24%, More preferably, it is 23%.

Ni: 0.5 to less than 8% Nickel (Ni) stabilizes austenite in steel. As described above, Ni increases the strength, ductility, and toughness of steel by an appropriate combination with Cr, Mn, N, and the like. Ni further enhances the hydrogen embrittlement resistance of the steel. If the Ni content is too low, these effects cannot be obtained. On the other hand, if the Ni content is too high, the raw material cost increases. Accordingly, the Ni content is 0.5 to less than 8%. The minimum with preferable Ni content is 0.6%, More preferably, it is 0.8%. The upper limit with preferable Cr content is 7%, More preferably, it is 6%.

Al: 0.10% or less Aluminum (Al) is inevitably contained. Al deoxidizes steel. However, if the Al content is too high, oxides and intermetallic compounds are likely to be produced in the steel, and the toughness of the steel is reduced. Therefore, the Al content is 0.10% or less. A preferable lower limit of the Al content for more effectively deoxidizing steel is 0.001%. The upper limit with preferable Al content is 0.05%, More preferably, it is 0.03%. In this specification, the Al content is sol. Al (acid-soluble Al) is meant.

Mo and / or W: 0.1 to 5.0% in total
Molybdenum (Mo) and tungsten (W) both increase the strength of the steel by solid solution strengthening. Mo and W further enhance the hydrogen embrittlement resistance of the steel. When the content of either one of these elements, or when both Mo and W are contained, the above effect cannot be obtained if the total content of both is too low. On the other hand, if the content of Mo and / or W is too high, the raw material cost increases. Therefore, the content of Mo and / or W is 0.1 to 5.0%. In this specification, content of Mo and / or W means content of the element, when only one of Mo and W is contained. When both Mo and W are contained, the total content of Mo and W is meant. The minimum with preferable content of Mo and / or W is 0.3%, More preferably, it is 0.5%. The upper limit with preferable content of Mo and / or W is 4.0%, More preferably, it is 3.5%.

N: 0.05 to 0.40%
Nitrogen (N) increases the strength of the steel by solid solution strengthening. If the N content is too low, this effect cannot be obtained. On the other hand, if the N content is too high, Cr nitride which is a precipitate is generated. Cr nitride deteriorates mechanical properties such as toughness of steel and lowers the hydrogen embrittlement resistance of steel. Therefore, the N content is 0.05 to 0.40%. The minimum with preferable N content is 0.08%, More preferably, it is 0.10%. The upper limit with preferable N content is 0.38%, More preferably, it is 0.35%.

Cu: 0.001 to 3.0%
Copper (Cu) stabilizes austenite in the steel. Cu further increases the strength of the steel by solid solution strengthening, and further increases the hydrogen embrittlement resistance of the steel. If the Cu content is too low, these effects cannot be obtained. On the other hand, if the Cu content is too high, the effect is saturated. If the Cu content is too high, the raw material cost is further increased. Therefore, the Cu content is 0.001 to 3.0%. The minimum with preferable Cu content is 0.005%, More preferably, it is 0.01%. The upper limit with preferable Cu content is 2.5%, More preferably, it is 2.0%.

  The balance of the chemical composition of the austenitic stainless steel according to the present invention consists of Fe and impurities. Here, the impurities are those that are mixed from ore, scrap, or the production environment as raw materials when industrially producing austenitic stainless steel, and have an adverse effect on the austenitic stainless steel of the present invention. It means what is allowed in the range not given.

  The contents of P, S, and O in the impurities are as follows.

P: 0.050% or less Phosphorus (P) is an impurity. P decreases the toughness of the steel. Therefore, the P content is 0.050% or less. The upper limit with preferable P content is 0.025%, More preferably, it is 0.018%. The P content is preferably as low as possible.

S: 0.050% or less Sulfur (S) is an impurity. S, like P, lowers the toughness of steel. Accordingly, the S content is 0.050% or less. The upper limit with preferable S content is 0.010%, More preferably, it is 0.005%. The S content is preferably as low as possible.

O: 0.020% or less Oxygen (O) is an impurity. O reduces the hot workability and toughness of steel. Therefore, the O content is 0.020% or less. The upper limit with preferable O content is 0.008%, More preferably, it is 0.006%.

  The austenitic stainless steel described above may further contain one or more elements selected from the following first group to fourth group. Hereinafter, these elements will be described in detail.

[First group]
Co: 0 to 3.0%
Cobalt (Co) is an optional element and may not be contained. When contained, Co stabilizes austenite in the steel. Co further increases the strength of the steel by solid solution strengthening. However, if the Co content is too high, the raw material cost increases. Therefore, the Co content is 0 to 3.0%. A preferable lower limit of the Co content is 0.1%.

[Second group]
Ti: 0 to 1.0%
V: 0 to 1.0%, and
Nb: 0 to 1.0%
Titanium (Ti), vanadium (V), and niobium (Nb) are all optional elements and may not be contained. When contained, all of these elements form carbides to refine crystal grains. This refinement increases the toughness and hydrogen embrittlement resistance of the steel. However, if the content of these elements is too high, the workability of the steel decreases. Therefore, Ti content is 0 to 1.0%, V content is 0 to 1.0%, and Nb content is 0 to 1.0%. The minimum with preferable Ti content is 0.001%, More preferably, it is 0.1%. The minimum with preferable V content and Nb content is 0.01% respectively, More preferably, it is 0.1% respectively. The upper limit with preferable Ti, V, and Nb content is 0.5%, respectively, More preferably, it is 0.3% respectively.

[Group 3]
Zr: 0 to 1.0%,
Hf: 0 to 1.0%, and
B: 0 to 0.01%
Zirconium (Zr), hafnium (Hf), and boron (B) are optional elements and may not be contained. When contained, any of these elements segregates at the grain boundaries to increase the deformation resistance. This contributes to high strength. For this reason, Zr, Hf, and B can be contained as needed. Zr and Hf, like the elements belonging to the second group, form alloy carbides to refine crystal grains. The refined crystal grains are effective in improving toughness and hydrogen embrittlement resistance. On the other hand, when these elements are contained excessively, the workability of steel is lowered. Therefore, when these elements are contained, Zr is 0 to 1.0%, Hf is 0 to 1.0%, and B is 0 to 0.01%. The minimum with preferable Zr content and Hf content is 0.001%, respectively, More preferably, it is 0.1% respectively. The minimum with preferable B content is 0.0001%, More preferably, it is 0.0005%. The upper limit with preferable Zr content and Hf content is 0.5%, respectively, More preferably, it is 0.3% respectively. The upper limit with preferable B content is 0.005%, More preferably, it is 0.003%.

[Group 4]
Ca: 0 to 0.05%,
Mg: 0 to 0.05%, and
Rare earth element (REM): 0 to 1.0%
Calcium (Ca), magnesium (Mg), and rare earth element (REM) are optional elements and may not be contained. When contained, Ca, Mg, and REM suppress solidification cracking during casting. However, if the content of these elements is too high, the hot workability of the steel decreases. Therefore, Ca: 0 to 0.05%, Mg: 0 to 0.05%, and REM: 0 to 1.0%. The minimum with preferable Ca content and Mg content is 0.0003%, respectively, More preferably, it is 0.0005%. The minimum with preferable REM content is 0.05%, More preferably, it is 0.1%. The upper limit with preferable Ca content and Mg content is 0.01%, respectively, More preferably, it is 0.003%. The upper limit with preferable REM content is 0.5%, More preferably, it is 0.3%.

  In this specification, REM contains 1 or more types selected from the group which consists of Sc, Y, and a lanthanoid (La of atomic number 57-Lu of 71). The REM content means the total content of these elements.

[Regarding Formula (1)]
The chemical composition further satisfies formula (1).
30 ≦ (−6.5 + 3.0Ni + 0.5Cr + 2.0Mo + 1.2W + 0.7Mn + 0.2Cu−5.5C-30N) ≦ 60 (1)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula (1).

  It is defined as F1 = (− 6.5 + 3.0Ni + 0.5Cr + 2.0Mo + 1.2W + 0.7Mn + 0.2Cu−5.5C-30N). As described above, the F1 value is an index indicating the degree of hydrogen embrittlement resistance. If the F1 value is 30 or more, sufficient hydrogen embrittlement resistance can be ensured. Although this reason is not certain, there is a possibility that SFE has become a sufficiently high value. On the other hand, if the F1 value exceeds 60, the hydrogen embrittlement resistance deteriorates. Although this reason is not certain, it may be because formation of hydrides or the like was promoted. Therefore, the F1 value is 30-60. A preferred lower limit of the F1 value is 31, more preferably 32. The upper limit with preferable F1 value is 55, More preferably, it is 50.

[Total amount of specific precipitates in steel]
In the austenitic stainless steel of the present invention, the total amount of Cr nitride (CrN ₂ ) and σ phase (CrMn ₃ ) in the steel is 7% by mass or less.

  Specific precipitates (Cr nitride and σ phase) reduce the hydrogen embrittlement resistance of the steel. This is because the specific precipitate becomes a starting point of hydrogen embrittlement, or the specific precipitate decreases SFE.

  In the present invention, the ratio of the total mass of the specific precipitates to the mass of the entire steel, that is, the total amount of the specific precipitates in the steel is 7% by mass or less. In this case, the decrease in hydrogen embrittlement resistance of the steel is suppressed.

  The measuring method of the total amount of the specific precipitate is as follows. A round bar specimen with a diameter of 10 mm and a length of 50 mm is collected from the central part of the austenitic stainless steel (the central part of the thickness and width in the case of an austenitic stainless steel sheet, the central part of the thickness in the case of an austenitic stainless steel pipe). To do. The test piece is subjected to constant current electrolysis using a 10% AA-based electrolytic solution (10 volume% acetylacetone-1 mass% tetramethylammonium chloride-methanol solution), and the precipitate is extracted as a residue. X-ray diffraction is performed on the precipitate, which is a residue, to identify Cr nitride and σ phase. The mass% with respect to the test piece of the total amount of the specified Cr nitride and σ phase is determined.

[Production method]
An example of the manufacturing method of the austenitic stainless steel of the present invention includes a step of preparing a raw material (preparation step), a step of hot working the raw material into a steel material (hot working step), and a steel material after hot working. A solution heat treatment step (solution heat treatment step). Hereinafter, this manufacturing method will be described.

[Preparation process]
A molten steel having the above chemical composition is produced. A well-known degassing process is implemented with respect to the manufactured molten steel as needed. The material is manufactured from the molten steel that has been degassed. The material manufacturing method is, for example, a continuous casting method. A continuous casting material (material) is manufactured by a continuous casting method. The continuous cast material is, for example, a slab, bloom, billet and the like. Molten steel may be made into an ingot by the ingot-making method.

[Hot working process]
A raw material (continuous cast material or ingot) is hot-worked by a known method to obtain an austenitic stainless steel material. Examples of the austenitic stainless steel material include steel pipes, steel plates, steel bars, wire rods, and forged steels. The austenitic stainless steel material may be manufactured, for example, by hot extrusion by the Eugene Sejurne method.

  An austenitic stainless steel material may be produced by a single hot working, or an austenitic stainless steel material may be produced by a plurality of hot workings.

[Solution heat treatment process]
A solution heat treatment is performed on the manufactured austenitic stainless steel material. In the solution heat treatment, the austenitic stainless steel material is soaked at the solution heat treatment temperature and then cooled. The conditions for the solution heat treatment are as follows.

Solution heat treatment temperature: 1000-1150 ° C
If the solution heat treatment temperature is too low, specific precipitates (Cr nitride and σ phase) remain in the steel, and the total amount of the specific precipitates exceeds 7% by mass. On the other hand, if the solution heat treatment temperature is too high, the crystal grains are coarsened, and mechanical properties such as toughness of steel are lowered. Therefore, the solution heat treatment temperature is 1000 to 1150 ° C. The minimum with a preferable solution heat treatment temperature is 1050 degreeC, and a preferable upper limit is 1100 degreeC.

Cooling rate in a steel material temperature range of 1000 to 600 ° C. (hereinafter referred to as a specific temperature range): 100 ° C./min or more.
If the cooling rate in the specific temperature range is too slow, specific precipitates are generated during cooling, and the total amount of the specific precipitates exceeds 7% by mass. Therefore, the cooling rate in the specific temperature range is 100 ° C./min or more. The cooling rate in the specific temperature range means that the average cooling rate is 100 ° C./min or more.

  The austenitic stainless steel of this invention can be manufactured according to the above manufacturing process.

  Molten steel having the chemical composition shown in Table 1 was produced by vacuum melting to produce a 50 kg ingot.

  The ingot was hot forged to produce a block having a thickness of 40 to 60 mm. The block was hot-rolled to produce a hot rolled steel sheet having a plate thickness of 15 mm. A solution heat treatment was performed on the hot-rolled steel sheet under the conditions shown in Table 2. Specifically, in the solution heat treatment, the solution heat treatment temperature (° C.) shown in Table 2 was maintained for 1 hour, and then water-cooled. Table 2 shows the cooling rate in the temperature range (specific temperature range) of 1000 to 600 ° C. during water cooling. By the above manufacturing method, the austenitic stainless steel plate of each test number shown in Table 2 was manufactured. The following test was implemented with respect to the austenitic stainless steel plate of each test number.

[Specific precipitation total amount measurement test]
A test piece for electrolytic extraction having a diameter of 10 mm and a length of 50 mm was collected from the center of each austenitic stainless steel plate having the test number. The test piece was immersed in a 10% AA-based electrolytic solution and subjected to constant current electrolysis, and a residue as a precipitate was extracted. As a result of performing X-ray diffraction on the precipitate, which is a residue, in any test number, the precipitate is Cr nitride (Cr ₂ N) and / or σ phase (CrMn ₃ ). Nothing was observed. The mass% of the residue with respect to the test piece was determined and defined as the total amount (mass%) of the specific precipitate (Cr nitride and σ phase). The total amount of the specific precipitate is shown in Table 2.

[Tensile test]
A round bar tensile specimen was collected from the center of each test number austenitic stainless steel sheet. In the round bar tensile test piece, the diameter of the parallel part was 2.5 mm, and the length of the parallel part was 20 mm. The longitudinal direction of the round bar tensile specimen was the rolling direction of the austenitic stainless steel sheet. A tensile test was performed in a normal temperature (25 ° C.) atmosphere using a round bar tensile test piece to obtain a tensile strength TS (MPa) and a fracture drawing (%). The strain rate during the test was 3 × 10 ⁻⁶ / sec. Table 2 shows the obtained tensile strength TS (MPa).

[Hydrogen embrittlement resistance test]
A round bar tensile test piece similar to the tensile test was newly collected from each test number austenitic stainless steel sheet. Using a low strain rate tester (SSRT), a tensile test was conducted at a strain rate of 3 × 10 ⁻⁶ / sec in a high-pressure hydrogen gas atmosphere at room temperature (25 ° C.) and 85 MPa, and the fracture drawing (%) was Asked. The ratio of the fracture drawing obtained in the hydrogen embrittlement resistance test to the fracture drawing obtained in the tensile test was defined as a relative fracture drawing (%). If the relative fracture drawing was high, it was judged that the hydrogen embrittlement resistance was excellent. The obtained relative fracture drawing (%) is shown in Table 2.

[Test results]
Referring to Table 2, the chemical compositions of Test Nos. 1 to 10 were appropriate and satisfied Formula (1). Furthermore, the manufacturing conditions were also appropriate. As a result, the tensile strength TS of these test numbers was 660 MPa or more, indicating high strength. Furthermore, the relative fracture draws of test numbers 1 to 10 were all 80% or more, and exhibited excellent hydrogen embrittlement resistance.

  On the other hand, in test number 11, the Mn content was too low. As a result, the relative fracture drawing was as low as less than 80%, and the hydrogen embrittlement resistance was low.

  In test number 12, the Cr content was too low. As a result, the relative breaking drawing was as low as less than 80%.

  In test number 13, the Cr content was too high. As a result, the total amount of the specific precipitates exceeded 7% by mass, and the relative fracture drawing was less than 80%. This is probably because the Cr content is high and Cr nitride is excessively precipitated.

  In test number 14, the Ni content was too low. Therefore, the relative fracture drawing was as low as less than 80%.

  In test number 15, the F1 value exceeded the upper limit of formula (1). Therefore, the relative fracture drawing was as low as less than 80%.

  In test number 16, the F1 value was less than the lower limit of formula (1). Therefore, the relative fracture drawing was as low as less than 80%.

  In test number 17, the N content was too low. Therefore, the tensile strength was low.

  In test number 18, the N content was too high and the F1 value was less than the lower limit of formula (1). Therefore, the total amount of the specific precipitates exceeded 7% by mass, and the relative fracture drawing was less than 80%. This is probably because the N content is high and Cr nitride is excessively precipitated.

  In test number 19, although the chemical composition was appropriate, the total amount of the specific precipitates exceeded 7% by mass. This is because the solution heat treatment temperature is too low. As a result, the relative breaking drawing was as low as less than 80%.

  The chemical composition of Test No. 20 did not contain Mo and W. Therefore, the tensile strength TS was as low as less than 660 MPa, and the relative fracture drawing was also less than 80%.

  In Test Nos. 21 and 22, although the chemical composition was appropriate, the cooling rate (average cooling rate) in a specific temperature range in the solution heat treatment was slow. Therefore, the mass ratio of the specific precipitate exceeded 7%, and the relative fracture drawing was as low as less than 80%.

The embodiment of the present invention has been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately changing the above-described embodiment without departing from the spirit thereof.

Claims (2)

% By mass
C: 0.20% or less,
Si: 1.0% or less,
Mn: more than 20% to 50%,
Cr: 10 to 25%,
Ni: Less than 0.5-8%,
Al: 0.10% or less,
Mo and / or W: 0.1 to 5.0% in total,
Cu: 0.001 to 3.0%,
N: 0.05-0.40%
Co: 0 to 3.0%,
Ti: 0 to 1.0%
V: 0 to 1.0%
Nb: 0 to 1.0%,
Zr: 0 to 1.0%,
Hf: 0 to 1.0%,
B: 0 to 0.01%
Ca: 0 to 0.05%,
Mg: 0 to 0.05%, and rare earth element: 0 to 1.0%, the balance consists of Fe and impurities,
P, S, and O in the impurity are
P: 0.050% or less,
S: 0.050% or less, and O: 0.020% or less, having a chemical composition satisfying the formula (1),
An austenitic stainless steel in which the total amount of Cr nitride and σ phase in the steel is 7% by mass or less.
30 ≦ (−6.5 + 3.0Ni + 0.5Cr + 2.0Mo + 1.2W + 0.7Mn + 0.2Cu−5.5C-30N) ≦ 60 (1)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula (1). The austenitic stainless steel of Claim 1 which contains the 1 type, or 2 or more types of element further selected from 1st group-4th group by the mass% further.
First group: Co: 0.1 to 3.0%
Second group: Ti: 0.001 to 1.0%, V: 0.01 to 1.0%, and Nb: 0.01 to 1.0%
Third group: Zr: 0.001 to 1.0%, Hf: 0.001 to 1.0%, and B: 0.0001 to 0.01%
Group 4: Ca: 0.0003 to 0.05%, Mg: 0.0003 to 0.05%, and rare earth elements: 0.05 to 1.0%