WO2025177646A1 - Austenitic stainless steel, austenitic stainless steel strip or steel sheet, method for producing same, and apparatus for high-pressure hydrogen gas or apparatus for liquid hydrogen - Google Patents

Abstract

The present invention provides a technology that can stabilize and impart high functionality to a material for use in a high-pressure hydrogen gas environment and a liquid hydrogen environment. This austenitic stainless steel contains, as a component composition, 0.040-0.100% of C, 0.25-1.00% of Si, 13.50-18.50% of Mn, 0.020-0.045% of P, 0.0001-0.0020% of S, 4.00-8.00% of Ni, 16.50-18.00% of Cr, 0.05-2.00% of Mo, 0.05-1.00% of Cu, 0.12-0.45% of N, 0.002-0.020% of Al, 0.002-0.016% of Sn, 0.06-1.00% of Co, and 0.0001-0.0050% of O, the remaining portion being Fe and unavoidable impurities.

Description

Austenitic stainless steel, austenitic stainless steel strip or steel plate, their manufacturing method, and equipment for high-pressure hydrogen gas or liquid hydrogen

This invention relates to austenitic stainless steels used in equipment for high-pressure hydrogen gas or liquid hydrogen. In this specification, "x to y" representing a range of values means "x or more and y or less," and includes the boundary value. The mass unit "t" represents 1,000 kg.

In recent years, the issue of global warming has become more prominent, and as a result of much discussion, there has been strong progress in promoting the use of hydrogen energy. The practical application of equipment for high-pressure hydrogen gas is progressing in earnest, and in this process, the properties of the materials used have been vigorously evaluated, with stainless steel, characterized by its high strength, being proposed.

For example, one example is a project by the New Energy and Industrial Technology Development Organization (NEDO), a national research and development agency, aimed at understanding the properties of existing steels and expanding the environments in which they can be used (NEDO Results Report, FY2013-2017). In this project, SUS316, SUH660, and the overseas standard XM-19 (ASME SA-240/UNS S20910, product name: Nitronic 50 equivalent material) were certified as materials that can be used in high-pressure hydrogen gas environments.

Furthermore, Patent Documents 1 to 4 propose austenitic stainless steels that contain Nb and V, with the nitrogen content increased by adding Mn. All of these achieve high strength in high-pressure hydrogen gas environments by optimizing the chemical composition and controlling the crystal grain size.

In actual structures, manufacturing by welding is essential, and proposals have been made in this area. For example, Patent Document 5 proposes a material that can be gas tungsten welded. In this way, proposals for practical application are also progressing in the materials field, and it is expected that this trend will continue to expand in the future.

WO 2004/083476 International Publication No. 2004/083477 WO 2004/110695 Japanese Patent Application Publication No. 09-137255 JP 2016-074976 A

However, the above-mentioned conventional techniques have the following problems.
That is, typical stainless steel strips and steel plates are obtained by hot-rolling slabs that have been melted, refined, and continuously cast, followed by solution heat treatment and then pickling. In other words, the surface condition is generally used for manufacturing structures after pickling. Therefore, the properties of this surface finish are important. However, in slow strain rate tensile tests (SSRTs) performed to evaluate the properties of high-pressure hydrogen gas environments, evaluations are performed using round bar test specimens or machined plate test specimens. For further functional enhancement and stabilization, evaluation using test specimens with surfaces that have been pickled is effective. However, such studies have not been conducted to date.

Therefore, the objective of this invention is to provide technology that can enhance and stabilize the functionality of materials used in high-pressure hydrogen gas environments and liquid hydrogen environments by suppressing and improving the brittle behavior caused by hydrogen during surface finishing in mass-produced processes.

The austenitic stainless steel of the present invention, which advantageously solves the above problems, contains, by mass, C: 0.040-0.100%, Si: 0.25-1.00%, Mn: 13.50-18.50%, P: 0.020-0.045%, S: 0.0001-0.0020%, Ni: 4.00-8.00%, Cr: 16.50-18.00% It is characterized by its chemical composition consisting of Mo: 0.05-2.00%, Cu: 0.05-1.00%, N: 0.12-0.45%, Al: 0.002-0.020%, Sn: 0.002-0.016%, Co: 0.06-1.00%, and O: 0.0001-0.0050%, with the balance consisting of Fe and unavoidable impurities.

Furthermore, a more preferable means for solving the problems of the austenitic stainless steel according to the present invention is one in which the component composition further includes, on a mass basis, one or two of Nb: 0.05 to 0.30% and V: 0.05 to 0.50%, W: 0.02 to 1.20%, and B: 0.0005 to 0.0050%, and the following relational expressions (1) and (2) are satisfied:
(1) Formula 1.00≦10×C/N≦5.50
(2) Formula 1.5×Sn+B≧0.0060
The element symbols in the above formula indicate the content of each element expressed as a mass percentage.

The austenitic stainless steel strip or steel sheet of the present invention, which advantageously solves the above problem, is a steel strip or steel sheet having any of the above chemical compositions, which has been annealed and pickled, and is characterized in that the ratio of the grain boundary depth to the grain boundary width in the surface layer is 1.5 or less.

The method for producing austenitic stainless steel strip or steel plate according to the present invention, which advantageously solves the above problem, includes the steps of melting an alloy having any of the above-mentioned component compositions and continuously casting it to form a steel billet, hot rolling the steel billet to form a hot-rolled alloy plate, cold rolling the hot-rolled alloy plate to form a cold-rolled alloy plate, and final annealing the cold-rolled alloy plate and pickling the surface, and is characterized in that the average ratio of the grain boundary depth to the grain boundary width in the surface layer is 1.5 or less.

The high-pressure hydrogen gas equipment or liquid hydrogen equipment of the present invention, which advantageously solves the above problems, is characterized by being made of the above-mentioned austenitic stainless steel strip or steel plate.

The austenitic stainless steel of this invention has shallow grain boundaries after pickling, and for materials used in high-pressure hydrogen gas environments or liquid hydrogen environments, surface finishing during mass production can suppress and improve hydrogen-induced embrittlement behavior. Therefore, according to the present invention, it is possible to improve the functionality and stability of austenitic stainless steel.

The following explains how the present invention was developed. The inventors focused primarily on the surface condition formed by annealing and pickling, and conducted extensive research into the relationship between the formation of an oxide film during annealing, descaling behavior, and the corrosion resistance of the material.

First, for SUS304L, plate-shaped test pieces were taken from pickled 2 mm thick plates in a direction perpendicular to the rolling direction and subjected to slow strain rate tensile tests (SSRT). The following two types of test pieces were prepared:
(1) Both sides remain pickled and finished, and the thickness of the plate is machine-finished to a surface roughness of Ra 1.6, and then smoothed with wet abrasive paper #1000.
(2) Both surfaces were machined, the pickled surface was removed, and the entire surface was machined to a surface roughness of Ra 1.6, and then smoothed with wet abrasive paper #1000.

Tests were conducted in both air and 70 MPa high-pressure hydrogen environments at a strain rate of 3 × 10 ^-5 /s, and the elongation behavior of each test piece was compared. As a result, it was found that the reduction in area at break of the material with a pickled surface was significantly reduced and the variation was large. From this, it was considered important to make the surface condition after pickling smoother, and that this would also contribute to stabilization by reducing the variation. Therefore, it was decided to conduct intensive research to make the surface smoother after pickling.

In order to achieve a smoother surface condition after annealing and pickling, the inventors conducted extensive research into the relationship between the chemical composition and roughness after pickling for 6%Ni-17%Cr-16%Mn-1%Mo-0.3%N-0.07%C steel. As a result, they confirmed that several elements have the effect of achieving smoother surfaces. In other words, it is believed that this will improve variation in slow strain rate tensile tests, making it possible to use the steel under more severe conditions.

A notable effect was the effect of Sn and B, which were confirmed to slightly widen the grain boundaries after pickling and improve the shape of the pickled grain boundaries from a notched shape. Adding too much B can lead to a deterioration in hot workability and induce weld cracks, so the amount that can be added is limited. In contrast, Sn can be added in larger amounts, so it is thought to be more effective.

Among other elements, the effects of Cr and Si were also confirmed. These elements are involved in the formation of oxide scale, and are thought to form scale uniformly in the early stages of heating, improving the smoothness of the surface after descaling.

Other elements that were also found to be effective were Ni, Cr, Mo, W, and Co. During the pickling process, the plate surface remains in the acid solution even after the scale has been removed, and some dissolution continues. These elements are thought to inhibit dissolution at this stage. W was particularly effective in ensuring smoothness, and it is believed that this is because it is effectively concentrated in the concentration-modulated area formed directly below the oxide scale, the so-called dechromed layer.

The reasons for limiting the chemical composition of an austenitic stainless steel according to one embodiment of the present invention will be explained below. In the following explanation, unless otherwise specified, "%" representing chemical composition means "% by mass."

C: 0.040-0.100%
C is an effective element for stabilizing the austenite phase and suppressing the precipitation of the σ phase, which is detrimental to corrosion resistance. It is also an important element for ensuring strength and is essential when considering use at low temperatures. Therefore, the addition of at least 0.040% is necessary. However, excessive C content facilitates the precipitation of Cr carbides during welding and cooling during solution heat treatment, degrading corrosion resistance. Therefore, the upper limit is set to 0.100%. The preferred lower limit of the C content is 0.050%, and more preferably 0.055%. The preferred upper limit is 0.090%, and even more preferably 0.080%.

Si: 0.25-1.00%
Si is an important element with deoxidizing properties, contributes to oxidation resistance, and has the effect of smoothing the surface after pickling. Therefore, the addition of at least 0.25% is necessary. However, in austenitic stainless steels containing Mn and N, excessive Si content can cause surface cracking during cold rolling. Furthermore, Si is an element that promotes the precipitation of the σ phase, which deteriorates corrosion resistance. For this reason, the upper limit of the Si content is set at 1.00%. The preferred lower limit is 0.28%, and more preferably 0.30%. The preferred upper limit is 0.80%, and more preferably 0.70%.

Mn: 13.50-18.50%
Mn is an element added as a deoxidizer, stabilizing the austenite phase and increasing the solubility of N. It also suppresses the formation of carbonitrides, ensuring corrosion resistance and contributing to low-temperature strength. For this reason, Mn must be added. However, excessive addition promotes the precipitation of the σ phase, reducing corrosion resistance. It also forms MnS, which acts as a starting point for pitting corrosion and reduces corrosion resistance. Therefore, the Mn content is set to a range of 13.50 to 18.50%. The preferred lower limit of the Mn content is 13.80%, and more preferably 14.00%. The preferred upper limit is 18.00%, and more preferably 17.80%.

P: 0.020-0.045%
P is an element that is inevitably mixed into steel as an impurity. It must be reduced as much as possible because it segregates at grain boundaries and deteriorates hot workability. However, excessive reduction leads to increased costs, so the range is set to 0.020 to 0.045%. The preferred upper limit is 0.042%, and the more preferred upper limit is 0.040%.

S: 0.0001-0.0020%
Sulfur (S) is an impurity element that inevitably gets mixed into steel. It reduces hot workability and forms sulfides that act as starting points for pitting corrosion, adversely affecting corrosion resistance. Therefore, the S content should be as low as possible, with an upper limit of 0.0020%. However, S is also an essential element for welding because it increases the fluidity of the molten metal. To ensure weldability, a S content of 0.0001% or more is preferable. The preferred lower limit is 0.0002%, and a more preferred lower limit is 0.0003%. The preferred upper limit is 0.0015%, and a more preferred upper limit is 0.0010%.

Ni: 4.00-8.00%
Ni is an element that stabilizes the austenite phase, inhibits the precipitation of intermetallic compounds such as the σ phase, and improves pitting corrosion resistance and general corrosion resistance. This makes it an important element for improving surface smoothness after pickling. For this reason, the addition of 4.00% or more is necessary. However, a Ni content exceeding 8.0% leads to increased costs. Therefore, the Ni content is set to a range of 10.00 to 15.00%. The preferred lower limit of the Ni content is 4.05%, and a more preferred lower limit is an addition of more than 4.10%. The preferred upper limit is 7.90%, and a more preferred upper limit is 7.80%.

Cr:16.50~18.00%
Cr not only improves pitting corrosion resistance, crevice corrosion resistance, and intergranular corrosion resistance, but also improves general corrosion resistance and smooths the surface after pickling. Furthermore, it is an essential element for improving surface smoothness after pickling by making oxide scale formation uniform. However, excessive Cr addition promotes the precipitation of the σ phase, which actually deteriorates corrosion resistance. For this reason, the Cr content is set to a range of 16.50 to 18.00%. The preferred lower limit of the Cr content is 16.60%, and more preferably 16.70%. The preferred upper limit is 17.80%, and more preferably 17.50%.

Mo: 0.05-2.00%
Mo not only improves pitting corrosion resistance and crevice corrosion resistance like Cr, but also improves general corrosion resistance and smooths the surface after pickling. For this reason, it is an essential element in this embodiment. However, excessive Mo content significantly promotes precipitation of the σ phase, degrading corrosion resistance. Furthermore, it leads to increased costs. For this reason, the Mo content is set to the range of 0.05 to 2.00%. The preferred lower limit of the Mo content is 0.08%, and more preferably 0.10%. The preferred upper limit is 1.50%, and more preferably 1.30%.

Cu: 0.05-1.00%
Cu is an important element that contributes to structural stability at low temperatures by stabilizing the austenite phase. To achieve this effect, a content of 0.05% or more is necessary. However, excessive addition increases costs and deteriorates hot workability, so the upper limit is set to 1.00%. Therefore, the Cu content is set to the range of 0.05 to 1.00%. The preferred lower limit of the Cu content is 0.07%, and more preferably 0.10%. The preferred upper limit is 0.80%, and more preferably 0.70%.

N: 0.12-0.45%
N is an element that stabilizes the austenite phase and also has the effect of suppressing the precipitation of the σ phase. Like Cr and Mo, it significantly improves pitting corrosion resistance and crevice corrosion resistance, and like C, it is an important element for ensuring strength. Therefore, the addition of at least 0.12% is necessary. However, excessive addition promotes the precipitation of carbonitrides and nitrides, resulting in a decrease in corrosion resistance. Therefore, the N content must not exceed 0.45%. The preferred lower limit of the N content is 0.13%, and more preferably 0.15%. The preferred upper limit is 0.37%, and more preferably 0.34%.

Al: 0.002-0.020%
Al is an important element with a deoxidizing effect. Furthermore, Al promotes desulfurization through deoxidation in the presence of CaO-SiO ₂ -Al ₂ O ₃ -MgO-based slag. Furthermore, Al is an important element for stabilizing the yield of B during refining. However, excessive Al content causes excessive oxide scale, making pickling difficult and promoting defects during welding. Therefore, the Al content is set to a range of 0.002 to 0.020%. The preferred lower limit of the Al content is 0.003%, and more preferably 0.004%. The preferred upper limit is 0.019%, and even more preferably 0.018%.

Sn: 0.002-0.016%
Even a very small amount of Sn is effective in improving corrosion resistance. Furthermore, in this embodiment, Sn is an important element that widens the grain boundaries after pickling and makes the notch shape of the grain boundaries gentler. For this purpose, a content of at least 0.002% is necessary. However, if Sn is contained in an amount greater than a certain amount, it will cause deterioration of hot workability. Therefore, the upper limit is set to 0.016%. The preferred lower limit of the Sn content is 0.006%, and more preferably 0.008%. The preferred upper limit is 0.015%, and more preferably 0.014%.

Co:0.06~1.00%
Co is a useful element that contributes to stabilizing the austenite phase and ensuring strength and toughness at low temperatures. Furthermore, Co also has the effect of smoothing the surface after pickling. To achieve this, at least 0.06% must be added. Conversely, if Co exceeds 1.00%, the cost becomes too high. For this reason, the range is set to 0.06 to 1.00%. The preferred lower limit is 0.08%, and the more preferred lower limit is 0.10%. The more preferred upper limit is 0.80%, and the even more preferred upper limit is 0.60%.

O: 0.0001 to 0.0050%
O is an impurity element that is inevitably mixed into steel, and forms non-metallic inclusions with Si, Mn, and Al, reducing the cleanliness of the steel and causing defects. However, excessive deoxidation increases costs, so the O content is set to a range of 0.0001 to 0.0050%. The preferred upper limit is 0.0040%, and the more preferred upper limit is 0.0030%.

The austenitic stainless steel of this embodiment preferably contains the following optional elements in addition to the essential elements described above, and the above composition satisfies the following relationship formulas. The element symbols in each relationship formula indicate the content of each element expressed as a mass percentage. It is also preferable to add one or two elements selected from Nb and V.

Nb: 0.05-0.50%
Nb is useful because it forms precipitates such as nitrides and carbides and further ensures strength through solid solution strengthening. For this reason, the addition of at least 0.05% is necessary. However, addition of more than 0.50% forms excessive precipitates, which can cause cracks in the weld bead. Therefore, the content is set to the range of 0.05 to 0.50%. The preferred lower limit is 0.08%, and the more preferred lower limit is 0.10%. The preferred upper limit is 0.25%, and the more preferred upper limit is 0.20%.

V: 0.05-0.50%
V, like Nb, is useful because it forms precipitates such as nitrides and carbides and further ensures strength through solid solution strengthening. For this reason, the addition of at least 0.05% is necessary. However, addition of more than 0.50% will form excessive precipitates, causing cracks in the weld bead. Therefore, the content is set to the range of 0.05 to 0.50%. The preferred lower limit is 0.08%, and the more preferred lower limit is 0.10%. The preferred upper limit is 0.28%, and the more preferred upper limit is 0.25%.

W: 0.02~1.20%
Like Cr and Mo, W not only improves pitting corrosion resistance and crevice corrosion resistance, but also improves general corrosion resistance and smooths the surface after pickling. Therefore, its addition is preferred in this embodiment. However, excessive W content may significantly promote σ-phase precipitation, deteriorating corrosion resistance. Furthermore, it may result in increased costs. Therefore, the W content is preferably in the range of 0.02 to 1.2%. The lower limit of the W content is more preferably 0.04%, and even more preferably 0.06%. The upper limit is more preferably 1.00%, and even more preferably 0.8%.

B: 0.0005-0.0050%
B has the effect of improving hot workability even with the addition of a very small amount. Furthermore, in this embodiment, it is an important element that has the effect of widening the grain boundary after pickling and making the notch shape of the grain boundary more gentle. For this purpose, a content of at least 0.0005% or more is necessary. However, excessive B content leads to deterioration of hot workability, and further deterioration of weldability, resulting in cracking of the weld bead. Therefore, the upper limit is set to 0.0050%. The preferred lower limit of the B content is 0.0008%, and more preferably 0.0012%. The preferred upper limit is 0.0045%, and even more preferably 0.0035%.

(1) Formula: 1.00≦10×C/N≦5.50
Both C and N are added to ensure strength at room temperature and low temperatures, and each element forms nitrides, carbides, and carbonitrides. If one element is present in large amounts, the precipitates of that element will dominate, and if you want to control the grain size to a certain size, you will need to change the target heat treatment temperature each time. On the other hand, controlling the heat treatment temperature to satisfy formula (1) can improve the stability of grain size control. Maintaining a constant heat treatment temperature is an important indicator because it leads to stabilizing the surface condition, i.e., smoothness, after pickling, and it is preferable to control it within the range defined by formula (1). A more preferable lower limit of the ratio 10 × C/N is 1.05, and an even more preferable lower limit is 1.10. A more preferable upper limit of the ratio 10 × C/N is 5.00, and an even more preferable upper limit is 4.00.

(2) Formula: 1.5×Sn+B≧0.0060
Formula (2) is an index for optimizing the grain boundaries formed by pickling, and better properties can be obtained by adding a total amount of Sn and B equal to or greater than a certain amount. In other words, the value of the left side of formula (2) is preferably 0.0060 or greater, more preferably 0.0085 or greater, and even more preferably 0.0090 or greater.

The austenitic stainless steel of this embodiment consists of the remainder, other than the above components, consisting of Fe and unavoidable impurities. Here, the unavoidable impurities refer to components that are inevitably mixed in due to various factors during the industrial production of stainless steel, and whose inclusion is permitted to the extent that it does not adversely affect the effects of the present invention.

Next, we will explain a method for producing austenitic stainless steel according to another embodiment of the present invention.

The alloy having the above composition is not particularly limited in terms of melting. The following manufacturing method is preferred. First, raw materials such as stainless steel scrap, Ni alloy scrap, iron scrap, ferrochrome, ferronickel, pure nickel, and metallic chromium are melted in an electric furnace. Subsequently, in an AOD furnace or VOD furnace, oxygen gas and argon gas are blown in to perform decarburization and refining, and quicklime, fluorite, an Al source, a Si source, and the like are added for desulfurization and deoxidation. The slag composition used in this process is preferably adjusted to a CaO-Al ₂ O ₃ -SiO ₂ -MgO-F system. To efficiently promote desulfurization, the slag preferably satisfies the mass ratios CaO/Al ₂ O ₃ ≧2 and CaO/SiO ₂ ≧3. Furthermore, the refractory material used in the AOD furnace or VOD furnace is preferably magnesia-chrome or dolomite. After refining in the AOD furnace or the like, it is preferable to adjust the composition and temperature in the LF process, and then to produce slabs by continuous casting. In the continuous casting process, a vertical type is particularly preferable, in which bending is not performed inside the device until solidification is complete after casting. The reason for this is to make the distribution of precipitates more symmetrical in the plate thickness direction.

In this embodiment, the slab is then hot rolled and, if necessary, cold rolled to form a product. In this manner, thick plates, hot-rolled strips or plates, and cold-rolled strips or plates are produced. The hot-rolled alloy plate produced by hot rolling is preferably subjected to solution heat treatment, followed by cold rolling to form a cold-rolled alloy plate, which is then subjected to final annealing and pickling to form a product. The austenitic stainless steel strips and steel plates according to this embodiment, even after pickling, can suppress and improve the hydrogen-induced embrittlement behavior required for materials used in high-pressure hydrogen gas environments and liquid hydrogen environments. Therefore, they can be used in high-pressure hydrogen gas or liquid hydrogen equipment without smoothing treatment such as polishing.

The present invention will be described in more detail below using examples. However, the present invention is not limited to these examples as long as it does not deviate from the spirit of the invention. First, raw materials such as iron scrap, stainless steel scrap, and ferrochrome were melted in a 60-ton electric furnace. Then, in the AOD process, oxygen and argon were blown into the melt to decarburize and refine it. Then, quicklime, fluorite, an Al source, and a Si source were added to perform desulfurization and deoxidation. The melt was then cast in a vertical continuous casting machine to obtain slabs. The chemical compositions of Samples 1 to 22 are shown in Table 1. Chemical components other than C, S, and N were analyzed by X-ray fluorescence analysis. N was analyzed by inert gas impulse heating and melting, and C and S were analyzed by oxygen flow combustion and infrared absorption spectroscopy. Sn was analyzed by iodide extraction atomic absorption spectroscopy. A "-" in the table indicates that no intentional addition was made.

The slab was then hot-rolled according to conventional methods to obtain a hot-rolled alloy plate with a thickness of 8.0 mm. This hot-rolled alloy plate then underwent solution heat treatment, cold rolling, final annealing, and pickling to obtain a cold-rolled strip with a thickness of 2.0 mm. Final annealing involved holding the plate at 1150°C for 1 minute, followed by water cooling and pickling. Pickling was performed by electrolytic pickling in a sulfuric acid solution, followed by immersion in a mixed solution of nitric acid and hydrofluoric acid.

Subsequently, (1) evaluation of crystal grain size, (2) grain boundary morphology, (3) tensile tests at room temperature, (4) slow strain rate tensile tests at room temperature, and (5) slow strain rate tensile tests at -80°C were performed. The test materials for (3) to (5) were tested on two types of surfaces: (i) as pickled, and (ii) pickled and then mechanically finished to a surface roughness of Ra 1.6 or less, and then smoothed with wet abrasive paper #1000. The side surfaces were treated in the same way as described above.

(1) Grain size An embedded sample was prepared so that a cross section perpendicular to the rolling direction could be observed, and the structure was revealed by etching, and the grain size number (G.S.N.) was determined in accordance with JIS G0551.

(2) Grain boundary morphology In pickled materials, the grain boundaries in the surface layer are preferentially corroded, resulting in notched shapes. To evaluate the morphology of these grain boundaries, the cross-sectional profile of the surface after pickling was measured using a laser microscope. The width and depth of the grain boundaries at 50 locations in each sample were measured, and the ratio of grain boundary depth to grain boundary width was calculated as an evaluation index. The smaller this value, the more improved the notch shape. Evaluation was performed with an average value of (grain boundary depth)/(grain boundary width) of 0.8 or less as ◎, an average value of 1.5 or less as ○, and any other value as ×.

(3) Tensile Test at Room Temperature (RT) Tests were conducted on plate-shaped test specimens taken perpendicular to the rolling direction from a cold-rolled strip with a thickness of 2 mm. The tensile test was conducted in accordance with JIS Z2241, and JIS No. 13B test specimens were used. Tests were conducted for the aforementioned (i) as-pickled and (ii) smoothed steels, with tensile strength variations of less than ±3% being rated as ◎, -3% or more but less than -5% being 〇, -5% or more but less than -7% being △, and -7% or more being ×. However, there was no difference in test data at room temperature between (i) as-pickled and (ii) smoothed steels, with all specimens being rated as ◎. Furthermore, low tensile strengths make the steel unusable for the intended application. The threshold value for tensile strength was set at 600 MPa, with values above this being rated as 〇 and values below this being rated ×. Among these, values above 690 MPa were rated as ◎.

(4) Slow strain rate tensile test at room temperature (RT) Plate-shaped test pieces were taken from a cold-rolled strip with a thickness of 2 mm in a direction perpendicular to the rolling direction, and tested at room temperature in a high-pressure hydrogen environment of 85 MPa. The initial strain rate was 3 × 10 ^-5 /s. Five tests were conducted for the (ii) smoothed steel, and then five tests were conducted for the (i) as-pickled steel. The results of (i) were compared with the results of (ii) and evaluated using the indices summarized in Table 2 to determine their superiority.

(5) Slow strain rate tensile test at -80°C The test was conducted in the same manner as in (4), except that the test temperature was different: the test was conducted in a high-pressure hydrogen environment of 85 MPa at -80°C. The evaluation method was also the same as in Table 2.

The above test results are summarized in Table 3. The test specimens were heat treated at 1150°C for 1 minute. The inventive examples (Nos. 1 to 15) have excellent grain boundary shape, and exhibit excellent low-temperature slow strain rate tensile test results and tensile strength. Looking at the grain size (G.S.N.) in Table 3, those satisfying formulas (1) and (2) generally have a GSN in the range of 6.0 to 7.0. These have smoother surfaces, which contribute to stabilizing SSRT test results.

Thus, the present invention provides an austenitic stainless steel discovered based on the results of the evaluation and the development of an evaluation method that simulates the actual environment and conditions in which it will be used, thereby contributing to improving and stabilizing the performance of material properties in high-pressure hydrogen gas environments, where the trend toward higher pressures is becoming more pronounced. Similarly, it contributes to improving and stabilizing the performance of material properties in liquid hydrogen environments, where mass storage and transportation are being put into practical use. In particular, it contributes to improving and stabilizing the performance of austenitic stainless steel sheets and strips that have undergone annealing and pickling processes. Therefore, it is industrially useful.

Claims (5)

By mass,
C: 0.040-0.100%,
Si: 0.25-1.00%,
Mn: 13.50-18.50%,
P: 0.020-0.045%,
S: 0.0001-0.0020%,
Ni: 4.00-8.00%,
Cr: 16.50-18.00%,
Mo: 0.05-2.00%,
Cu: 0.05-1.00%,
N: 0.12-0.45%,
Al: 0.002-0.020%,
Sn: 0.002 to 0.016%,
Co: 0.06 to 1.00%, and
O: 0.0001 to 0.0050%,
and the balance consisting of Fe and unavoidable impurities. 2. The austenitic stainless steel according to claim 1, wherein the component composition further includes, on a mass basis, one or two of Nb: 0.05 to 0.50% and V: 0.05 to 0.50%, W: 0.02 to 1.20%, and B: 0.0005 to 0.0050%, and satisfies the following relational expressions (1) and (2):
(1) Formula 1.00≦10×C/N≦5.50
(2) Formula 1.5×Sn+B≧0.0060
The element symbols in the above formula indicate the content of each element expressed as a mass percentage. An austenitic stainless steel strip or sheet having the chemical composition described in claim 1 or 2, which has been annealed and pickled, and in which the ratio of the grain boundary depth to the grain boundary width in the surface layer is 1.5 or less. A step of melting an alloy having the component composition according to claim 1 or 2 and continuously casting it into a steel slab;
hot rolling the steel billet to form a hot-rolled alloy plate;
cold-rolling the hot-rolled alloy sheet to obtain a cold-rolled alloy sheet;
The cold-rolled alloy sheet is subjected to final annealing and a surface pickling finish.
A method for manufacturing an austenitic stainless steel strip or steel plate, in which the average ratio of the grain boundary depth to the grain boundary width in the surface layer is 1.5 or less. High-pressure hydrogen gas equipment or liquid hydrogen equipment made from the austenitic stainless steel strip or steel plate described in claim 3.