JP6801236B2 - Austenitic stainless steel for low temperature hydrogen and its manufacturing method - Google Patents

Description

The present invention relates to austenitic stainless steel for low temperature hydrogen and a method for producing the same.

Research and development of hydrogen-based transportation equipment and practical research of hydrogen stations that supply hydrogen are being actively promoted. In order to put these into practical use, it is urgent to improve the environment for storing and using hydrogen gas at high pressure, and the development and application study of high-strength materials with a tensile strength exceeding 800 MPa are being promoted in parallel.

Against this background, materials used for high-pressure hydrogen include, for example, International Publication No. 2004/083476, International Publication No. 2004/083477, International Publication No. 2004/11695, and International Publication No. 2012 /. High-strength austenitic stainless steel is proposed in No. 132992. In these austenitic stainless steels, the solubility of N is increased by increasing the Mn content. Further, it is disclosed that the strength of austenitic stainless steel is increased by utilizing the solid solution strengthening of N by adding V and Nb, the precipitation strengthening of nitride, and the miniaturization of crystal grains by the pinning. ing.

By the way, expectations for the storage and use of hydrogen using liquid hydrogen are also increasing. By making hydrogen a liquid, the amount of hydrogen that can be stored and transported at one time can be significantly increased as compared with the case of high-pressure hydrogen. Further, when filling a fuel cell vehicle (FCV) or the like, when liquid hydrogen is turned into a gas, the hydrogen gas pressure is automatically increased as the temperature rises. Therefore, it is said that various devices associated with compression, such as those attached to high-pressure hydrogen stations, are not required.

Japanese Unexamined Patent Publication No. 2014-40648 discloses a high-strength thick steel sheet having excellent cryogenic toughness at -196 ° C. or lower. According to International Publication No. 2012/043877, inexpensive stainless steel having excellent hydrogen brittleness even when used in a hydrogen resistant environment with high pressure hydrogen gas exceeding 40 MPa or a hydrogen resistant environment with liquid hydrogen and Inexpensive and high-strength stainless steel is disclosed.

International Publication No. 2004/083476 International Publication No. 2004/083477 International Publication No. 2004/110695 International Publication No. 2012/132992 Japanese Unexamined Patent Publication No. 2014-40648 International Publication No. 2012/043877

Concerns about hydrogen embrittlement of materials are relatively small when hydrogen is stored as a liquid because it is in a temperature range where hydrogen is difficult to diffuse. On the other hand, since it is exposed to hydrogen gas during the pressurization process, there is a great concern about hydrogen embrittlement of the material. Therefore, the material used for low-temperature hydrogen is required to have hydrogen brittleness in the vicinity of −50 ° C. in addition to toughness at extremely low temperatures.

In addition, in order to obtain hydrogen at the target pressure at room temperature, it is necessary to increase the pressure to some extent during liquid hydrogen. Therefore, the material used for low-temperature hydrogen is required to have high strength.

Japanese Unexamined Patent Publication No. 2014-40648 does not describe hydrogen brittleness resistance in the vicinity of -50 ° C. Further, the stainless steel of International Publication No. 2012/043877 is described from the viewpoint of cost reduction, and the tensile strength is less than 800 MPa in the invention example. As described above, there is currently no material that satisfies all the properties of strength, low temperature toughness, and hydrogen brittleness at low temperature.

The present invention has been made in view of the above situation, and an object of the present invention is to provide an austenitic stainless steel having excellent strength, low temperature toughness, and hydrogen brittleness resistance at low temperature.

The austenitic stainless steel for low temperature hydrogen according to one embodiment of the present invention has a chemical composition of C: 0.1% or less, Si: less than 1.0%, Mn: 3.0 to 17.0% in mass%. , Ni: 9.5 to 15%, Cr: 15 to 25%, Mo: 0.1 to 3.0%, N: more than 0.20% and 0.35% or less, Nb: 0 to 0.3% , V: 0 to 0.3%, Cu: 0 to 4.0%, Al: 0.10% or less, P: 0.05% or less, S: 0.05% or less, Co: 0 to 3%, Ti: 0 to 0.3%, B: 0 to 0.01%, Ca: 0 to 0.05%, Mg: 0 to 0.05%, REM: 0 to 0.5%, balance: Fe and impurities The amount of solid dissolved N is 0.20% by mass or more, and the chemical composition satisfies the following formulas (1) and (2).
Nb + V ≧ 0.1 (1)
Ni + 12.93C + 1.11Mn + 0.72Cr + 0.88Mo-0.27Si + 0.19Nb + 0.53Cu + 0.9V + 7.55N ≧ 30.0 (2)
The element symbols of the formulas (1) and (2) are substituted with the content of the corresponding element in% by mass.

The method for producing austenitic stainless steel for low temperature hydrogen according to one embodiment of the present invention has a chemical composition of mass%, C: 0.1% or less, Si: less than 1.0%, Mn: 3.0 to 17. .0%, Ni: 9.5-15%, Cr: 15-25%, Mo: 0.1-3.0%, N: more than 0.20% and 0.35% or less, Nb: 0-0 .3%, V: 0 to 0.3%, Cu: 0 to 4.0%, Al: 0.10% or less, P: 0.05% or less, S: 0.05% or less, Co: 0 to 0 3%, Ti: 0 to 0.3%, B: 0 to 0.01%, Ca: 0 to 0.05%, Mg: 0 to 0.05%, REM: 0 to 0.5%, balance: It includes a step of preparing a steel material which is Fe and impurities and a step of solidifying and heat-treating the steel material at 105 to 1280 ° C., and the chemical composition satisfies the following formulas (3) and (4).
Nb + V ≧ 0.1 (3)
Ni + 12.93C + 1.11Mn + 0.72Cr + 0.88Mo-0.27Si + 0.19Nb + 0.53Cu + 0.9V + 7.55N ≧ 30.0 (4)
The element symbols of the formulas (3) and (4) are substituted with the content of the corresponding element in% by mass.

According to the present invention, an austenitic stainless steel having excellent strength, low temperature toughness, and hydrogen brittleness at low temperature can be obtained.

FIG. 1 is a scatter diagram showing the relationship between the amount of solid solution N and the Charpy impact value (L direction) at -196 ° C. FIG. 2 is a scatter diagram showing the relationship between the amount of solid solution N and the Charpy impact value (T direction) at -196 ° C.

The present inventors have conducted repeated investigations to solve the above problems. Specifically, in terms of mass%, C: 0.1% or less, Si: less than 1.0%, Mn: 3.0 to 17.0%, Ni: 9.5 to 15%, Cr: 15 to 25. %, Mo: 0.1 to 3.0%, Nb: 0 to 0.3%, V: 0 to 0.3%, N: more than 0.20 and 0.35% or less, Cu: 0 to 4. The strength, low temperature toughness, and low temperature hydrogen brittleness of various austenitic stainless steels including 0% were compared. As a result, the following findings were obtained.

(A) Austenitic stainless steel having a Nieq value of 30.0% or more defined by the following formula (A) has high strength and excellent hydrogen brittleness.
Nieq = Ni + 12.93C + 1.11Mn + 0.72Cr + 0.88Mo-0.27Si + 0.19Nb + 0.53Cu + 0.9V + 7.55N (A)
The element symbol of the formula (A) is substituted with the content of the corresponding element in% by mass.

Hydrogen brittleness at low temperatures can be organized from the viewpoint of austenite stability at low temperatures. That is, when the material has a Nieq of less than 30.0%, it is considered that the stability of austenite cannot be ensured at a low temperature and the hydrogen embrittlement sensitivity rapidly increases.

(B) Austenitic stainless steel having a solid solution N content of 0.20% by mass or more in the steel has high strength and excellent low temperature toughness.

In austenitic stainless steel containing the above elements, Nb and V-containing nitrides are precipitated and the crystal grains are refined to increase the strength. On the other hand, it was found that excessive precipitation of Nb and V-containing nitrides adversely affects low temperature toughness and makes it unsuitable as a material for low temperature hydrogen. However, simply limiting the amount of N makes it difficult to maintain strength. Therefore, it was found that both high strength and high toughness can be achieved by strengthening the solid solution with the solid solution N.

(C) By raising the temperature of the heat treatment to a high temperature, the amount of nitride produced can be suppressed and the amount of solid solution N in the steel can be increased.

Specifically, it was found that high strength and high toughness can be achieved at the same time by setting the heat treatment temperature to 1050 to 1280 ° C.

Based on the above findings, the austenitic stainless steel according to the present invention has been completed. Hereinafter, the austenitic stainless steel according to the embodiment of the present invention will be described in detail.

[Chemical composition of steel]
The austenitic stainless steel according to this embodiment has the chemical composition described below. In the following description, "%" of elemental content means mass%.

C: 0.1% or less Carbon (C) is not an element positively added in this embodiment. If the C content exceeds 0.1%, carbides are deposited at the grain boundaries, which adversely affects toughness and the like. Therefore, the C content should be 0.1% or less. The C content is preferably 0.04% or less, and more preferably 0.02% or less. The C content should be as low as possible, but an extreme reduction in the C content causes an increase in refining cost, so it is preferably 0.001% or more in practice.

Si: Less than 1.0% Silicon (Si) deoxidizes steel. However, if a large amount of Si is contained, it may form an intermetallic compound with Ni, Cr, etc., or promote the formation of an intermetallic compound such as a sigma phase, which may significantly reduce hot workability. .. Therefore, the Si content should be less than 1.0%. The Si content is preferably 0.5% or less. The smaller the Si content, the better, but considering the refining cost, it is preferably 0.1% or more.

Mn: 3.0 to 17.0%
Manganese (Mn) is an inexpensive austenite stabilizing element. In the present embodiment, proper combination with Cr, Ni, N, etc. contributes to high strength and improvement of ductility and toughness. Further, in the present embodiment, the solid solution strengthening of N is used, but when the Mn content is small, the dissolved amount of N is small, and the effect of solid solution strengthening of N cannot be sufficiently obtained. Further, in the present embodiment, it is also aimed to make the crystal grains finer by finely precipitating the carbonitrides of Nb and V, but when the amount of dissolved N is small, the formation of the carbonitrides becomes insufficient. Therefore, Mn needs to be contained in an amount of 3.0% or more. On the other hand, if the Mn content exceeds 17.0%, the hot workability and weather resistance may decrease. Therefore, the Mn content is 3.0 to 17.0%. The lower limit of the Mn content is preferably 4.0%. The upper limit of the Mn content is preferably 8.0%, more preferably 7.0%, and even more preferably 6.2%.

Ni: 9.5 to 15%
Nickel (Ni) is added as an austenite-forming element. In the present embodiment, Ni enhances stacking defect energy and hydrogen embrittlement resistance by an appropriate combination with Cr, Mn, N and the like. In order to obtain the effect sufficiently, it is necessary to contain 9.5% or more. On the other hand, since Ni is an expensive element, adding a large amount of Ni causes an increase in cost. Therefore, the Ni content is 9.5 to 15%. The lower limit of the Ni content is preferably 10%, more preferably 11%. The upper limit of the Ni content is preferably 14%.

Cr: 15-25%
Chromium (Cr) is an essential component as an element for ensuring the corrosion resistance of stainless steel. On the other hand, if the content is excessive, a large amount of coarse carbides such as M ₂₃ C ₆ that deteriorate ductility and toughness are likely to be produced. Therefore, the Cr content is 15-25%. The lower limit of the Cr content is preferably 18%. The upper limit of the Cr content is preferably 24%.

Mo: 0.1 to 3.0%
Molybdenum (Mo) is an element effective for improving corrosion resistance and strength in a usage environment. In order to obtain the effect, the content must be 0.1% or more. On the other hand, since Mo is an expensive element, the addition of a large amount causes an increase in cost. Therefore, the Mo content is 0.1 to 3.0%. The lower limit of the Mo content is preferably 0.15%. The upper limit of the Mo content is preferably 2.5%.

N: More than 0.20% and 0.35% or less Nitrogen (N) is the most important solid solution strengthening element, and by forming a nitride, the crystal grains are made finer and contributes to higher strength. .. In order to utilize it for high strength, it is necessary to contain N exceeding 0.20%. However, it is industrially difficult and not preferable to contain N in excess of 0.35%. Therefore, the N content is more than 0.20% and 0.35% or less. The lower limit of the N content is preferably 0.22%, more preferably 0.24%. The upper limit of the N content is preferably 0.34%, more preferably 0.33%.

The austenitic stainless steel according to this embodiment contains at least one of Nb and V. As will be described later, Nb and V may have a total content of 0.1% or more.

Nb: 0-0.3%
Niobium (Nb) is an element that dissolves in a substrate or precipitates as a nitride and is effective for improving the strength. On the other hand, when the Nb content becomes excessive, the nitride is excessively precipitated, which causes a decrease in low temperature toughness. Therefore, the Nb content is 0 to 0.3%. The lower limit of the Nb content is preferably 0.1%. The upper limit of the Nb content is preferably 0.25%.

V: 0-0.3%
Vanadium (V), like Nb, is an effective element for improving strength. On the other hand, when the V content becomes excessive, the nitride is excessively precipitated, which causes a decrease in low temperature toughness. Therefore, the V content is 0 to 0.3%. The lower limit of the V content is preferably 0.1%. The upper limit of the V content is preferably 0.25%.

Al: 0.10% or less Aluminum (Al) deoxidizes steel. On the other hand, when the Al content becomes excessive, the formation of intermetallic compounds such as the sigma phase is promoted. Therefore, the Al content is 0.10% or less. In order to ensure the deoxidizing effect, it is preferable that Al is contained in an amount of 0.001% or more, and more preferably 0.003% or more. The upper limit of the Al content is preferably 0.06%, more preferably 0.04%. In addition, Al in this specification refers to so-called "sol.Al (acid-soluble Al)".

P: 0.05% or less Phosphorus (P) is an impurity and adversely affects the toughness of steel and the like. The P content is 0.05% or less, preferably as low as possible. The P content is preferably 0.025% or less, and more preferably 0.015% or less.

S: 0.05% or less Sulfur (S) is an impurity that adversely affects the toughness of steel. The S content is 0.05% or less, preferably as small as possible. The S content is preferably 0.01% or less, more preferably 0.005% or less.

The rest of the chemical composition of the austenitic stainless steel according to this embodiment consists of Fe and impurities. Here, the impurity means an element mixed from ore or scrap used as a raw material when steel is industrially manufactured, or an element mixed from the environment of the manufacturing process.

The austenitic stainless steel according to the present embodiment may contain one or more elements selected from any of the following groups 1 to 4 in place of a part of Fe described above. The following elements are all selective elements. That is, none of the following elements may be contained in the austenitic stainless steel according to the present embodiment. Moreover, only a part may be contained.

Group 1 Cu: 0-4.0%, Co: 0-3%
The elements belonging to the first group are Cu and Co. These elements stabilize the austenite structure of steel.

Cu: 0-4.0%
Copper (Cu) is an austenite-forming element, which increases the strength of steel by solid solution strengthening. If Cu is contained in an amount of 0.05% or more, the effect of the content becomes remarkable. On the other hand, even if it is contained in an excessive amount, the effect is saturated and the hot workability is lowered. Therefore, the Cu content is 0-4.0%. The lower limit of the Cu content is preferably 0.10%, more preferably 0.15%. The preferable upper limit of the Cu content is 3.0%.

Co: 0-3%
Cobalt (Co) may be contained because it is an element effective for obtaining a stable austenite structure like Cu. This effect can be obtained if even a small amount of Co is contained, but it is more preferable to contain 0.005% or more. However, if it is contained in an excessive amount, the ductility is slightly lowered, and since it is an expensive element, a significant cost increase is caused. Therefore, the Co content is 0 to 3%. The lower limit of the Co content is more preferably 0.008%, still more preferably 0.01%. The upper limit of the Co content is preferably 2.5%, more preferably 2.0%.

Group 2 Ti: 0-0.3%
The element belonging to the second group is Ti. Ti improves the strength of steel.

Ti: 0-0.3%
Titanium (Ti) is an element that is effective in improving the strength by being dissolved in the substrate or precipitated as a carbonitride, and therefore may be contained. This effect can be obtained if even a small amount of Ti is contained, but it is more preferable to contain 0.001% or more. However, if it is contained in an excessive amount, coarse carbonitride is formed during production, which causes a decrease in ductility. Therefore, the Ti content is 0 to 0.3%. The lower limit of the Ti content is more preferably 0.003%, still more preferably 0.005%. The upper limit of the Ti content is preferably 0.25%, more preferably 0.20%.

Group 3 B: 0-0.01%
The element belonging to the third group is B. B suppresses embrittlement of steel in a hydrogen environment.

B: 0-0.01%
Boron (B) segregates at the grain boundaries to increase the grain boundary fixing force, which contributes to the improvement of strength and the ductility. In addition, since it also has an effect of suppressing embrittlement in a hydrogen environment, it may be contained. These effects can be obtained if B is contained even in a small amount, but it is more preferable that B is contained in an amount of 0.0001% or more. However, if it is contained in an excessive amount, the liquefaction cracking sensitivity of the weld heat affected zone is increased. Therefore, the B content is 0 to 0.01%. The lower limit of the B content is more preferably 0.0002%, still more preferably 0.0005%. The upper limit of the B content is preferably 0.008%, more preferably 0.005%.

Group 4 Ca: 0-0.05%, Mg: 0-0.05%, REM: 0-0.5%
The elements belonging to the fourth group are Ca, Mg, and REM. These elements improve the hot workability of steel.

Ca: 0-0.05%
Calcium (Ca) may be contained because it has an effect of improving hot workability. This effect can be obtained if even a small amount of Ca is contained, but it is more preferable to contain 0.0001% or more. However, if it is contained in an excessive amount, it is combined with O to significantly reduce the cleanliness, and on the contrary, the hot workability is deteriorated. Therefore, the Ca content is 0 to 0.05%. The lower limit of the Ca content is more preferably 0.0005%, still more preferably 0.001%. The upper limit of the Ca content is preferably 0.03%, more preferably 0.01%.

Mg: 0-0.05%
Magnesium (Mg) may be contained because it has an effect of improving hot workability like Ca. This effect can be obtained if even a small amount of Mg is contained, but it is more preferable to contain 0.0001% or more. However, if it is contained in an excessive amount, it is combined with O to significantly reduce the cleanliness, and on the contrary, the hot workability is deteriorated. Therefore, the Mg content is 0 to 0.05%. The lower limit of the Mg content is more preferably 0.0005%, still more preferably 0.001%. The upper limit of the Mg content is preferably 0.03%, more preferably 0.01%.

REM: 0-0.5%
Rare earth elements (REM) may be contained because they have a strong affinity for S and have an effect of improving hot workability. This effect can be obtained if even a small amount of REM is contained, but it is more preferable to contain 0.001% or more. However, if it is contained in an excessive amount, it is combined with O to significantly reduce the cleanliness, and on the contrary, the hot workability is deteriorated. Therefore, the REM content is 0 to 0.5%. The lower limit of the REM content is more preferably 0.002%. The upper limit of the REM content is preferably 0.3%, more preferably 0.1%.

In addition, "REM" is a general term for a total of 17 elements of Sc, Y and lanthanoid, and the content of REM refers to the total content of one or more of REM. Further, REM is generally contained in misch metal. Therefore, for example, it may be added in the form of misch metal so that the REM content is within the above range.

[About equation (1)]
The chemical composition of the austenitic stainless steel according to this embodiment further satisfies the following formula (1).
Nb + V ≧ 0.1 (1)
The element symbol of the formula (1) is substituted with the content of the corresponding element in% by mass.

As described above, both Nb and V form nitrides and improve the strength of steel by precipitation strengthening. Further, the Nb nitride and the V nitride are refined by pinning. If the chemical composition does not satisfy the formula (1), even if the N content and the solution treatment temperature described later are adjusted, an appropriate amount of nitride cannot be obtained, and the effect of increasing the strength cannot be obtained.

[About equation (2)]
The chemical composition of the austenitic stainless steel according to this embodiment further satisfies the following formula (2).
Ni + 12.93C + 1.11Mn + 0.72Cr + 0.88Mo-0.27Si + 0.19Nb + 0.53Cu + 0.9V + 7.55N ≧ 30.0 (2)
The element symbol of formula (2) is substituted with the content of the corresponding element in% by mass.

The left side of the equation (2) is the Nieq value. When Nieq is 30.0% or more, excellent hydrogen brittleness can be maintained even at a low temperature. On the other hand, if Nieq is less than 30.0%, the stability of austenite cannot be ensured at a low temperature, and the hydrogen embrittlement sensitivity becomes high. The lower limit of the Nieq value is preferably 32.0%, more preferably 35.0%.

[Solid solution N amount]
The austenitic stainless steel according to this embodiment has a solid solution N content of 0.20% by mass or more in the steel. The amount of solid solution N can be calculated by the following formula (B).
_{[N] SOL = [N]} - ([N used in the Z-phase] + [N used for Cr ₂ N]) (B)
[N] _SOL is the amount of solid solution N, and [N] is the total amount of nitrogen. [N used for Cr ₂ N] is the amount of N calculated from the amount of Cr ₂ N precipitated by the residue extraction method. [N used for the Z phase] is a value obtained by multiplying the precipitation amount of the Z phase, which is also obtained by the residue extraction method, by the amount of substance of N per 1 mol of the precipitate. The Z phase is a Cr (Nb, V) N-type composite nitride. Although C may be mixed in the Z phase, it is negligible in the composition range of the present invention. _[N] SOL, [N], [N used for Z phase, it is either a unit mass% of [N used for Cr ₂ N].

When the chemical composition is within the above range and the amount of solid solution N is 0.20% by mass or more, the strength can be maintained by strengthening the solid solution. As a result, both high strength and low temperature toughness can be achieved at the same time. If even a small amount of nitride is precipitated, the effect of pinning can be expected. Therefore, there is no particular upper limit on the amount of solid solution N, which may be substantially the same as the total amount of nitrogen.

[Tensile strength and low temperature toughness]
The austenitic stainless steel according to this embodiment has a tensile strength of 800 MPa or more at room temperature. Further, the austenitic stainless steel according to the present embodiment has a Charpy impact value of 50 J or more at -196 ° C.

[Production method]
Hereinafter, a method for producing austenitic stainless steel according to an embodiment of the present invention will be described. The austenitic stainless steel according to the present embodiment includes a step of preparing the steel material, a step of hot-working the steel material, and a step of solidifying and heat-treating the hot-worked steel material.

First, steel having the above-mentioned chemical composition (hereinafter referred to as steel material) is prepared. Specifically, for example, steel having the above-mentioned chemical composition is melted and refined.

Hot work steel materials. Hot working is, for example, hot rolling or hot forging.

The hot-worked steel material is subjected to solution heat treatment. Specifically, the steel material is held at a temperature of 1050 to 1280 ° C. (hereinafter referred to as a solution heat treatment temperature) for a predetermined time and then cooled. If the solid solution heat treatment temperature is less than 1050 ° C., the amount of solid solution N cannot be 0.20% by mass or more. On the other hand, it is industrially unfavorable to raise the solution heat treatment temperature to higher than 1280 ° C. The holding time is, for example, 10 to 360 minutes. The steel material is preferably rapidly cooled from the solution heat treatment temperature, and is preferably water-cooled (shower water-cooled or soaked in water).

Note that this solution heat treatment does not have to be an independent process, and the same effect can be obtained by, for example, quenching after hot working. For example, hot extrusion may be performed at around 1150 ° C., and then quenching may be performed.

Alternatively, another heat treatment may be performed between the hot working and the solution heat treatment. That is, in the present embodiment, the temperature of the final heat treatment may be 1050 to 1250 ° C. As a result, the amount of solid solution N of austenitic stainless steel can be increased to 0.20% by mass or more.

Hereinafter, the present invention will be described in more detail with reference to Examples. The present invention is not limited to these examples.

The materials having the chemical compositions shown in Table 1 were melted in the laboratory and hot forged at 1200 ° C. to prepare a steel sheet having a thickness of 40 mm.

Each steel sheet was further hot-rolled at 1200 ° C. to obtain a steel sheet having a thickness of 15 mm. Each hot-rolled steel sheet was held at the temperature shown in Table 2 for 30 minutes and then water-cooled by solution heat treatment. Each steel sheet that had undergone solution heat treatment was machined to a thickness of 15 mm, a width of 100 mm, and a length of 500 mm to prepare a test material. Tensile test pieces, low strain rate tensile test pieces, Charpy test pieces, and extraction residue test pieces were prepared from each test material. The extraction residue test piece was electrolytically extracted in a 10% acetylacetone electrolytic solution, and the obtained extraction residue (precipitate) was subjected to ICP emission spectroscopic analysis.

[Measurement of solid solution N amount]
From the extraction residue, the amount of solid solution N was calculated from the above formula (B).

[Tensile strength measurement]
A round bar tensile test piece having a diameter of 6 mm parallel to the longitudinal direction of the test material was sampled, and a tensile test was conducted in the air at room temperature to measure the tensile strength. Those having a tensile strength of 800 MPa or more were regarded as acceptable.

[Low strain rate tensile test]
Tensile tests were performed at room temperature at a strain rate of 3 × ^10-5 / s in the air and in a high pressure hydrogen environment of 70 MPa. The value obtained by dividing the value of the breaking drawing in the high-pressure hydrogen environment by the value of the breaking drawing in the atmosphere was defined as the relative breaking drawing (%) at room temperature. A similar test was performed at −40 ° C. to determine the relative breaking aperture at −40 ° C. Those with 80% or more in each case were accepted.

[Charpy impact test]
A Charpy impact test was performed at -196 ° C. using a full-size 2 mm V-notch test piece in accordance with JIS Z 2242. The Charpy impact test piece was evaluated in the T direction in which the longitudinal direction is perpendicular to the rolling direction and in the L direction in which the longitudinal direction is parallel to the rolling direction. Those having a Charpy impact value (absorbed energy) of 50 J or more in both the L direction and the T direction were regarded as acceptable.

[Test results]
The solid solution N amount, tensile strength, relative breaking drawing at −40 ° C. and room temperature, and Charpy impact values in the L and T directions at -196 ° C. of each test material are shown in Table 2 above. A "-" in the table indicates that the test has not been performed.

The test materials of Nos. 1-3, 5-8, 17, 20-23, 27, and 28 have a tensile strength of 800 MPa or more, a relative breaking drawing at room temperature and −40 ° C. of 80% or more, and − The Charpy impact value at 196 ° C. was 50 J or more.

The test material of No. 4 had a Charpy impact value of less than 50 J at -196 ° C., although the chemical composition of the steel was the same as that of the test materials of Nos. 1 to 3. It is considered that this is because the amount of solid solution N was less than 0.20% by mass, in other words, the amount of precipitated nitride was too large. It is probable that the amount of solid solution N was small because the solution heat treatment temperature was low.

The test material of No. 9 had a tensile strength of less than 800 MPa. It is considered that this is because the N content of steel F was low.

The test materials of Nos. 10 and 11 had a relative breaking drawing of less than 80% at −40 ° C. and room temperature. It is considered that this is because the Ni contents of the steels G and H were low.

The test material of No. 12 had a tensile strength of less than 800 MPa. It is considered that this is because the Mn content of Steel I was low and therefore the N content was also low.

The test materials of Nos. 13, 14, 24, and 25 had a Charpy impact value of less than 50 J at -196 ° C. It is considered that this is because the amount of solid solution N was less than 0.20% by mass, in other words, the amount of precipitated nitride was too large. It is probable that the solid solution N content was low because the Nb content and V content were too high.

The test materials of Nos. 15 and 16 had a tensile strength of less than 800 MPa. It is considered that this is because the amount of solid solution N was less than 0.20% by mass and the solid solution strengthening was not sufficient. It is considered that this is because the Mn content was low. The test materials of Nos. 15, 16 and 26 also had a relative breaking throttle of less than 80% at −40 ° C. It is probable that this was because Nieq was low.

The test materials of Nos. 18 and 19 had a tensile strength of less than 800 MPa. It is considered that this is because the contents of V and Nb were insufficient and the amount of the nitride precipitated was insufficient.

FIG. 1 is a scatter diagram showing the relationship between the amount of solid solution N and the Charpy impact value (L direction) at -196 ° C. FIG. 2 is a scatter diagram showing the relationship between the amount of solid solution N and the Charpy impact value (T direction) at -196 ° C. As shown in FIGS. 1 and 2, if the amount of solid solution N is 0.20% by mass or more, the Charpy impact value can be 50 J or more in both the L direction and the T direction.

Although the embodiments of the present invention have been described above, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be appropriately modified and implemented within a range that does not deviate from the gist thereof.

The present invention can be suitably used as a material for low temperature hydrogen. The material for low temperature hydrogen is, for example, a material used for a storage container for liquid hydrogen, a pipe, a joint, a valve, a liquid hydrogen booster pump, and the like.

Claims (4)

The chemical composition is mass%,
C: 0.1% or less,
Si: less than 1.0%,
Mn: 3.0 to 17.0%,
Ni: 9.5 to 15%,
Cr: 15-25%,
Mo: 0.1 to 3.0%,
N: More than 0.20% and 0.35% or less,
Nb: 0-0.3%,
V: 0-0.3%,
Cu: 0-4.0%,
Al: 0.003 to 0.10% ,
P: 0.05% or less,
S: 0.05% or less,
Co: 0-3%,
Ti: 0-0.3%,
B: 0-0.01%,
Ca: 0-0.05%,
Mg: 0-0.05%,
REM: 0-0.5%,
Remaining: Fe and impurities,
The amount of solid solution N is 0.20% by mass or more,
The chemical composition satisfies the following formulas (1) and (2).
Charpy impact value at -196 ° C. is Ri der least 52J,
When the chemical composition is mass%,
Ti: 0.001 to 0.3%,
Ca: 0.0001 to 0.05%,
Mg: 0.0001 to 0.05%, and
REM: 0.001-0.5%,
An austenitic stainless steel for low temperature hydrogen containing one or more selected from the group consisting of .
Nb + V ≧ 0.1 (1)
Ni + 12.93C + 1.11Mn + 0.72Cr + 0.88Mo-0.27Si + 0.19Nb + 0.53Cu + 0.9V + 7.55N ≧ 30.0 (2)
The element symbols of the formulas (1) and (2) are substituted with the content of the corresponding element in% by mass. The austenitic stainless steel for low temperature hydrogen according to claim 1.
When the chemical composition is mass%,
Cu: 0.05-4.0%,
Co: 0.005-3%, and
B : 0.0001 to 0.01% ,
One or containing two or more, austenitic stainless steels for low temperature hydrogen is selected either from Ranaru group. The austenitic stainless steel for low temperature hydrogen according to claim 1 or 2.
Austenitic stainless steel for low temperature hydrogen with a tensile strength of 800 MPa or more at room temperature. A method for manufacturing austenitic stainless steel for low-temperature hydrogen.
The chemical composition is mass%, C: 0.1% or less, Si: less than 1.0%, Mn: 3.0 to 17.0%, Ni: 9.5 to 15%, Cr: 15 to 25%. , Mo: 0.1 to 3.0%, N: more than 0.20% and 0.35% or less, Nb: 0 to 0.3%, V: 0 to 0.3%, Cu: 0 to 4. 0%, Al: 0.003 to 0.10% , P: 0.05% or less, S: 0.05% or less, Co: 0 to 3%, Ti: 0 to 0.3%, B: 0 to 0 0.01%, Ca: 0-0.05%, Mg: 0-0.05%, REM: 0-0.5%, balance: Fe and the process of preparing steel as impurities.
The steel material is provided with a step of solution heat treatment at 1100 to 1280 ° C.
The chemical composition satisfies the following formulas (3) and (4).
It said cold hydrogen for austenitic stainless steels is the amount of solute N is 0.20 wt%, the tensile strength at room temperature is at least 800 MPa, Ri der Charpy impact value of more than 52J at -196 ° C.,
When the chemical composition is mass%,
Ti: 0.001 to 0.3%,
Ca: 0.0001 to 0.05%,
Mg: 0.0001 to 0.05%, and
A method for producing an austenitic stainless steel for low temperature hydrogen , which contains one or more selected from the group consisting of REM: 0.001 to 0.5% .
Nb + V ≧ 0.1 (3)
Ni + 12.93C + 1.11Mn + 0.72Cr + 0.88Mo-0.27Si + 0.19Nb + 0.53Cu + 0.9V + 7.55N ≧ 30.0 (4)
The element symbols of the formulas (3) and (4) are substituted with the content of the corresponding element in% by mass.

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