JP2017008413A - Austenite stainless steel for low temperature hydrogen and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an austenite stainless steel excellent in strength, low temperature toughness and hydrogen brittleness resistance at low temperature.SOLUTION: An austenite stainless steel for low temperature hydrogen has a chemical composition containing, by 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:over 0.20% and 0.35% or less, Nb:0 to 0.3%, V:0 to 0.3%, Cu:0 to 4.0% or the like with solid solution N amount of 0.20 mass% or more and satisfying following formulae (1) and (2). Nb+V≥0.1 (1) Ni+12.93C+1.11Mn+0.72Cr+0.88Mo-0.27Si+0.19 Nb+0.53Cu+0.9 V+7.55 N≥30.0 (2) The content of corresponding element in mass% is substituted in each symbol of the elements in the formulae (1) and (2).SELECTED DRAWING: Figure 2

Description

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

  Research and development of transport equipment using hydrogen as a medium and research into practical use of a hydrogen station that supplies hydrogen are being actively pursued. When these are put to practical use, there is an urgent need to improve the environment for storing and using hydrogen gas under high pressure, and development and application studies of high-strength materials having a tensile strength exceeding 800 MPa are in progress.

  Under such circumstances, as materials used for high-pressure hydrogen, for example, International Publication No. 2004/083476, International Publication No. 2004/083477, International Publication No. 2004/110695, and International Publication No. 2012/106 / No. 132992 proposes a high-strength austenitic stainless steel. These austenitic stainless steels increase the solubility of N by increasing the Mn. Further, it is disclosed that the strength of austenitic stainless steel is enhanced by utilizing the solid solution strengthening of N and the precipitation strengthening of nitride by adding V and Nb, and further refinement of crystal grains by the pinning. ing.

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

  Japanese Patent Application Laid-Open No. 2014-40648 discloses a high-strength thick steel plate excellent in cryogenic toughness at −196 ° C. or lower. International Publication No. 2012/043877 includes an inexpensive stainless steel having excellent hydrogen embrittlement resistance even when used in a hydrogen resistant environment with high-pressure hydrogen gas exceeding 40 MPa or a hydrogen resistant environment with liquid hydrogen. 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 JP 2014-40648 A International Publication No. 2012/043877

  Concerns about hydrogen embrittlement of the material are relatively small because hydrogen is a temperature range in which hydrogen is difficult to diffuse when stored as a liquid. 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, materials used for low-temperature hydrogen are required to have hydrogen embrittlement resistance in the vicinity of −50 ° C. in addition to toughness at extremely low temperatures.

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

  Japanese Patent Application Laid-Open No. 2014-40648 does not describe hydrogen embrittlement resistance near -50 ° C. Moreover, the stainless steel of international publication 2012/043877 is described from a viewpoint of cost reduction, and the tensile strength is less than 800 MPa in the example of an invention. As described above, there is no material that satisfies all the properties of strength, low temperature toughness, and resistance to hydrogen embrittlement at low temperatures.

  This invention is made | formed in view of the said present condition, and it aims at providing the austenitic stainless steel excellent in intensity | strength, low-temperature toughness, and hydrogen embrittlement resistance at low temperature.

The austenitic stainless steel for low-temperature hydrogen according to an 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 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 solid solution N amount 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)
In the element symbols of the formulas (1) and (2), the content of the corresponding element is substituted by mass%.

The manufacturing method of the austenitic stainless steel for low-temperature hydrogen according to an 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 0.0%, Ni: 9.5 to 15%, Cr: 15 to 25%, Mo: 0.1 to 3.0%, N: more than 0.20% and not more than 0.35%, 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: It comprises a step of preparing a steel material that is Fe and impurities and a step of subjecting the steel material to a solution heat treatment at 1050 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)
In the element symbols of the formulas (3) and (4), the content of the corresponding element is substituted by mass%.

  According to the present invention, an austenitic stainless steel excellent in strength, low temperature toughness, and resistance to hydrogen embrittlement at a low temperature can be obtained.

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

  The present inventors have repeated investigations to solve the above problems. Specifically, by 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 hydrogen embrittlement resistance at low temperatures of various austenitic stainless steels including 0% were compared. As a result, the following knowledge was obtained.

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

  The hydrogen embrittlement resistance at low temperatures can be arranged from the viewpoint of the stability of austenite at low temperatures. That is, in the case of a material having Nieq of less than 30.0%, it is considered that the stability of austenite cannot be secured at a low temperature, and the hydrogen embrittlement susceptibility rapidly increases.

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

  In the 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 the low temperature toughness, making it unsuitable as a low temperature hydrogen material. However, if the amount of N is simply limited, it becomes difficult to maintain the strength. Therefore, it has been found that by strengthening the solid solution with the solid solution N, both high strength and high toughness can be achieved.

(C) By increasing the temperature of the heat treatment, the amount of nitride produced can be suppressed, and the amount of solute N in the steel can be increased.

  Specifically, it was found that high strength and high toughness can be achieved at a heat treatment temperature of 1050 to 1280 ° C.

  Based on the above knowledge, the austenitic stainless steel according to the present invention was completed. Hereinafter, an austenitic stainless steel according to an embodiment of the present invention will be described in detail.

[Chemical composition of steel]
The austenitic stainless steel according to the present embodiment has a chemical composition described below. In the following description, “%” of the element content means mass%.

C: 0.1% or less Carbon (C) is not an element positively added in the present embodiment. If the C content exceeds 0.1%, carbides precipitate at the grain boundaries and adversely affect toughness and the like. Therefore, the C content is 0.1% or less. The C content is preferably 0.04% or less, and more preferably 0.02% or less. The C content is preferably as low as possible, but an extreme reduction in the C content leads to an increase in refining costs, so it is preferable for practical use to be 0.001% or more.

Si: less than 1.0% Silicon (Si) deoxidizes steel. However, if Si is contained in a large amount, it may form intermetallic compounds with Ni, Cr, etc., or promote the formation of intermetallic compounds such as sigma phase, which may significantly reduce hot workability. . Therefore, the Si content is less than 1.0%. The Si content is preferably 0.5% or less. The lower the Si content, the better. However, 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, an appropriate combination with Cr, Ni, N, etc. contributes to increasing strength and improving ductility and toughness. In this embodiment, solid solution strengthening of N is used. However, when the content of Mn is small, the amount of N dissolved becomes small, and the effect of solid solution strengthening of N cannot be sufficiently obtained. In the present embodiment, the crystal grains are refined by finely precipitating Nb or V carbonitrides. However, when the amount of N dissolved is small, the formation of carbonitrides is insufficient. Therefore, it is necessary to contain 3.0% or more of Mn. On the other hand, when the Mn content exceeds 17.0%, hot workability and weather resistance may be deteriorated. 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 further preferably 6.2%.

Ni: 9.5 to 15%
Nickel (Ni) is added as an austenite forming element. In the present embodiment, Ni increases the stacking fault energy and improves the 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, addition of a large amount 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 that ensures corrosion resistance as stainless steel. On the other hand, when the content is excessive, a large amount of coarse carbides such as M ₂₃ C ₆ that lowers the ductility and toughness is easily generated. Therefore, the Cr content is 15 to 25%. The lower limit of the Cr content is preferably 18%. The upper limit of the Cr content is preferably 24%.

Mo: 0.1-3.0%
Molybdenum (Mo) is an element effective for improving the corrosion resistance under the use environment and increasing the strength. In order to acquire the effect, containing 0.1% or more is required. On the other hand, since Mo is an expensive element, 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 not more than 0.35% Nitrogen (N) is the most important solid solution strengthening element, and also contributes to high strength by making the crystal grains finer by forming nitrides. . In order to utilize it for increasing the strength, it is necessary to contain N exceeding 0.20%. However, it is industrially difficult to contain N exceeding 0.35%, which is not preferable. Therefore, the N content exceeds 0.20% and is 0.35% or less. The lower limit of the N content is preferably 0.22%, and 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 the present 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 to 0.3%
Niobium (Nb) is an element effective for improving the strength by dissolving in a substrate or precipitating as a nitride. On the other hand, when the Nb content is excessive, nitride is excessively precipitated, resulting in 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 to 0.3%
Vanadium (V) is an element effective for improving the strength, like Nb. On the other hand, when the V content is excessive, nitride is excessively precipitated, resulting in 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 is excessive, generation of intermetallic compounds such as a sigma phase is promoted. Therefore, the Al content is 0.10% or less. In order to secure the effect of deoxidation, the Al content is preferably 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 of this specification refers to what is called "sol.Al (acid-soluble Al)".

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

S: 0.05% or less Sulfur (S) is an impurity and adversely affects the toughness of steel. The S content is 0.05% or less and is preferably as small as possible. S content becomes like this. Preferably it is 0.01% or less, More preferably, it is 0.005% or less.

  The balance of the chemical composition of the austenitic stainless steel according to the present embodiment is composed of Fe and impurities. Here, the impurity means an element mixed from ore or scrap used as a raw material when manufacturing steel industrially, 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 one of the following first group to fourth group instead of a part of the above-mentioned Fe. 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.

First group Cu: 0 to 4.0%, Co: 0 to 3%
Elements belonging to the first group are Cu and Co. These elements stabilize the austenitic structure of the steel.

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

Co: 0 to 3%
Cobalt (Co) may be contained because it is an effective element for obtaining a stable austenite structure like Cu. This effect can be obtained as long as Co is contained, but more preferably 0.005% or more. However, if it is contained excessively, the ductility is slightly reduced and it is an expensive element, which causes a significant increase in cost. Therefore, the Co content is 0 to 3%. The lower limit of the Co content is more preferably 0.008%, and still more preferably 0.01%. The upper limit of the Co content is preferably 2.5%, more preferably 2.0%.

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

Ti: 0 to 0.3%
Titanium (Ti) may be contained because it is an element effective for improving the strength by precipitating as a solid solution or carbonitride on the substrate. If Ti is contained even a little, this effect can be obtained, but it is more preferable to contain 0.001% or more. However, if it is contained excessively, coarse carbonitrides are produced during production, resulting in 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%, and further preferably 0.005%. The upper limit of the Ti content is preferably 0.25%, more preferably 0.20%.

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

B: 0 to 0.01%
Boron (B) segregates at the grain boundary to increase the grain boundary fixing force, contributes to the strength improvement, and improves the ductility. In addition, since it also has an effect of suppressing embrittlement in a hydrogen environment, it may be contained. If B is contained even a little, these effects can be obtained, but it is more preferable to contain 0.0001% or more. However, when excessively contained, 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%, and further preferably 0.0005%. The upper limit of the B content is preferably 0.008%, more preferably 0.005%.

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

Ca: 0 to 0.05%
Calcium (Ca) may be included because it has an effect of improving hot workability. This effect can be obtained as long as Ca is contained, but more preferably 0.0001% or more. However, if it is contained excessively, it combines with O to remarkably reduce the cleanliness, and on the contrary, deteriorate the hot workability. Therefore, the Ca content is 0 to 0.05%. The lower limit of the Ca content is more preferably 0.0005%, and further preferably 0.001%. The upper limit of the Ca content is preferably 0.03%, more preferably 0.01%.

Mg: 0 to 0.05%
Magnesium (Mg), like Ca, has the effect of improving hot workability, and therefore may be contained. 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 excessively, it combines with O to remarkably reduce the cleanliness, and on the contrary, deteriorate the hot workability. Therefore, the Mg content is 0 to 0.05%. The lower limit of the Mg content is more preferably 0.0005%, and further preferably 0.001%. The upper limit of the Mg content is preferably 0.03%, and more preferably 0.01%.

REM: 0 to 0.5%
Rare earth elements (REM) may be included because they have a strong affinity for S and have an effect of improving hot workability. If REM is contained even a little, this effect can be obtained, but it is more preferable to contain 0.001% or more. However, if it is contained excessively, it combines with O to remarkably reduce the cleanliness, and on the contrary, deteriorate the hot workability. 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%.

  “REM” is a generic name 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 falls within the above range.

[Regarding Formula (1)]
The chemical composition of the austenitic stainless steel according to the present embodiment further satisfies the following formula (1).
Nb + V ≧ 0.1 (1)
In the element symbol of the formula (1), the content of the corresponding element is substituted by mass%.

  As described above, Nb and V both form nitrides and improve the strength of the steel by precipitation strengthening. Furthermore, Nb nitride and V nitride refine crystal grains 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.

[Regarding Formula (2)]
The chemical composition of the austenitic stainless steel according to the present 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)
In the element symbol of the formula (2), the content of the corresponding element is substituted by mass%.

  The left side of Equation (2) is the Nieq value. If Nieq is 30.0% or more, excellent hydrogen embrittlement resistance can be maintained even at low temperatures. On the other hand, if Nieq is less than 30.0%, the stability of austenite cannot be ensured at low temperatures, and the hydrogen embrittlement susceptibility increases. The lower limit of the Nieq value is preferably 32.0%, more preferably 35.0%.

[Solution N amount]
In the austenitic stainless steel according to this embodiment, the amount of solute N in the steel is 0.20% by mass or more. The amount of solute N can be obtained by the following equation (B).
[N] _SOL = [N] − ([N used for Z phase] + [N used for Cr ₂ N]) (B)
[N] _SOL is the amount of dissolved N, and [N] is the total amount of nitrogen. [N used for Cr ₂ N] is an N amount calculated from the precipitation amount of Cr ₂ N obtained by the residue extraction method. [N used for the Z phase] is a value obtained by multiplying the amount of precipitation of the Z phase similarly obtained by the residue extraction method by the amount of N substance per 1 mol of the precipitate. The Z phase is a Cr (Nb, V) N type composite nitride. Although there is a possibility that C is mixed in the Z phase, it is negligible in the composition range of the present invention. The units of [N] _SOL , [N], [N used for Z phase], and [N used for Cr ₂ N] are all mass%.

  If the chemical composition is within the above-mentioned range and the solid solution N amount is 0.20% by mass or more, the strength can be maintained by solid solution strengthening. Thereby, both high strength and low temperature toughness can be achieved. In addition, if the nitride has precipitated even a little, the effect by pinning can be expected. Therefore, there is no particular upper limit on the amount of solute N, and it may be substantially the same as the total nitrogen amount.

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

[Production method]
Hereinafter, the manufacturing method of the austenitic stainless steel by one Embodiment of this invention is demonstrated. The austenitic stainless steel according to the present embodiment includes a step of preparing a steel material, a step of hot working the steel material, and a step of solution heat treating the hot worked steel material.

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

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

  Hot-worked steel is subjected to solution heat treatment. Specifically, the steel material is cooled to a temperature of 1050 to 1280 ° C. (hereinafter referred to as a solution heat treatment temperature) for a predetermined time, and then cooled. When the solution heat treatment temperature is less than 1050 ° C., the amount of solid solution N cannot be made 0.20% by mass or more. On the other hand, it is industrially unpreferable that the solution heat treatment temperature is 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).

  The solution heat treatment does not have to be an independent process. For example, the same effect can be obtained by performing rapid cooling after hot working. For example, after performing hot extrusion at around 1150 ° C., rapid cooling may be performed.

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

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

  A material having the chemical composition shown in Table 1 was melted in the laboratory and hot forged at 1200 ° C. to produce a steel plate having a thickness of 40 mm.

  Each steel plate was further hot-rolled at 1200 ° C. to obtain a steel plate 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 subjected to a solution heat treatment that was water-cooled. Each steel plate subjected to solution heat treatment was machined to a thickness of 15 mm, a width of 100 mm, and a length of 500 mm to obtain a test material. From each sample material, a tensile test piece, a low strain rate tensile test piece, a Charpy test piece, and an extraction residue test piece were prepared. The extraction residue test piece was subjected to electrolytic extraction in a 10% acetylacetone electrolytic solution, and the obtained extraction residue (precipitate) was subjected to ICP emission spectroscopic analysis.

[Solution N content measurement]
From the extraction residue, the amount of solute N was calculated from the above-described formula (B).

[Tensile strength measurement]
A round bar tensile test piece having a parallel part diameter of 6 mm in the longitudinal direction of the test material was sampled and subjected to a tensile test 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]
A tensile test was performed at a strain rate of 3 × 10 ⁻⁵ / s at room temperature in the atmosphere and in a high-pressure hydrogen environment of 70 MPa. The value obtained by dividing the value of the squeeze squeezing in a high-pressure hydrogen environment by the value of the squeeze squeezing in the atmosphere was taken as the relative squeeze squeezing (%) at room temperature. A similar test was performed at -40 ° C, and the relative fracture drawing at -40 ° C was determined. In any case, 80% or more was regarded as acceptable.

[Charpy impact test]
In accordance with JIS Z 2242, a Charpy impact test was conducted at -196 ° C using a full-size 2 mmV notch test piece. The Charpy impact test piece was evaluated in the T direction in which the longitudinal direction is perpendicular to the rolling direction and the L direction in which the longitudinal direction is parallel to the rolling direction. A Charpy impact value (absorbed energy) in both the L direction and the T direction was 50 J or more.

[Test results]
The amount of solute N, tensile strength, relative fracture drawing at −40 ° C. and room temperature, and Charpy impact values in the L direction and T direction at −196 ° C. are shown in Table 2 above. “-” In the table indicates that the test is not performed.

  The test materials Nos. 1 to 3, 5 to 8, 17, 20 to 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, − The Charpy impact value at 196 ° C. was 50 J or more.

  The specimen No. 4 had a Charpy impact value at −196 ° C. of less than 50 J even though the chemical composition of the steel was the same as the specimens No. 1 to 3. This is presumably because the amount of solid solution N was less than 0.20% by mass, in other words, the amount of deposited nitride was too much. The reason why the amount of solute N was small is considered to be because the solution heat treatment temperature was low.

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

  The specimens Nos. 10 and 11 had a relative fracture drawing of less than 80% at −40 ° C. and room temperature. This is probably because the Ni content of steels G and H was low.

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

  The specimens Nos. 13, 14, 24, and 25 had Charpy impact values at −196 ° C. of less than 50 J. This is presumably because the amount of solid solution N was less than 0.20% by mass, in other words, the amount of deposited nitride was too much. The reason why the amount of solute N was small is considered to be because the Nb content and the V content were too large.

  The test materials of Nos. 15 and 16 had a tensile strength of less than 800 MPa. This is presumably because the amount of solid solution N was less than 0.20% by mass and solid solution strengthening was not sufficient. This is probably because the Mn content was low. The specimens of Nos. 15, 16, and 26 also had a relative fracture drawing at −40 ° C. of less than 80%. This is presumably because Nieq was low.

  The test materials of Nos. 18 and 19 had a tensile strength of less than 800 MPa. This is presumably because the content of V or Nb was insufficient and the amount of nitride deposited was insufficient.

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

  While 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 can be implemented by appropriately modifying the above-described embodiment without departing from the spirit thereof.

  The present invention can be suitably used as a low-temperature hydrogen material. The low-temperature hydrogen material is, for example, a material used for liquid hydrogen storage containers, piping, joints, valves, liquid hydrogen booster pumps, and the like.

Claims (5)

Chemical composition is mass%,
C: 0.1% or less,
Si: less than 1.0%,
Mn: 3.0-17.0%,
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 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: 0-0.5%
Balance: Fe and impurities,
The amount of solute N is 0.20% by mass or more,
An austenitic stainless steel for low-temperature hydrogen, wherein 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)
In the element symbols of the above formulas (1) and (2), the content of the corresponding element is substituted by mass%. The austenitic stainless steel for low-temperature hydrogen according to claim 1,
The chemical composition is mass%,
Cu: 0.05 to 4.0%,
Co: 0.005 to 3%,
Ti: 0.001 to 0.3%,
B: 0.0001 to 0.01%
Ca: 0.0001 to 0.05%,
Mg: 0.0001-0.05%, and REM: 0.001-0.5%,
An austenitic stainless steel for low-temperature hydrogen containing one or more selected from the group consisting of: Austenitic stainless steel for low-temperature hydrogen according to claim 1 or 2,
An austenitic stainless steel for low-temperature hydrogen having a tensile strength at room temperature of 800 MPa or more. Austenitic stainless steel for low-temperature hydrogen according to claim 3,
An austenitic stainless steel for low-temperature hydrogen having a Charpy impact value at −196 ° C. of 50 J or more. 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: 0.10% 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 a step of preparing a steel material as impurities,
A solution heat treatment of the steel material at 1050 to 1280 ° C.,
The manufacturing method of the austenitic stainless steel for low temperature hydrogen whose said chemical composition satisfy | fills following formula (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)
In the element symbols of the above formulas (3) and (4), the content of the corresponding element is substituted by mass%.

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