Austenitic stainless steel, austenitic stainless-steel strip or plate, production methods for these, and apparatus for high-pressure hydrogen gas or apparatus for liquid hydrogen

Abstract

Provided is a technique by which a material to be used in a high-pressure hydrogen gas environment or a liquid-hydrogen environment can be made to have an enhanced functionality and be stabilized. This austenitic stainless steel contains, as constituent components, 0.020-0.055% C, 0.15-0.85% Si, 4.10-8.00% Mn, 0.015-0.030% P, 0.0001-0.0020% S, 10.00-15.00% Ni, 20.00-24.00% Cr, 1.20-3.30% Mo, 0.05-0.25% Cu, 0.12-0.30% Nb, 0.10-0.30% V, 0.0005-0.0050% B, 0.20-0.39% N, 0.002-0.035% Al, 0.002-0.016% Sn, 0.06-1.00% Co, and 0.0001-0.0050% O, the remainder comprising 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

[0001] The present 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 unit of mass "t" represents 1000 kg.

[0002] In recent years, the issue of global warming has become more prominent, and as a result of much discussion, the use of hydrogen energy has been strongly promoted. 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, and stainless steel, which is characterized by its high strength, has been proposed.

[0003] For example, one example is a project by the New Energy and Industrial Technology Development Organization (NEDO) to understand the properties of existing steels and expand their use in different environments (NEDO Report on Results from 2013 to 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.

[0004] 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 stainless steels achieve high strength in a high-pressure hydrogen gas environment by optimizing the chemical composition and controlling the grain size.

[0005] In actual structures, welding is essential for manufacturing, and proposals have been made for this purpose. For example, Patent Document 5 proposes a material that can be gas tungsten welded. As such, proposals for practical application are also progressing in the materials field, and this trend is expected to continue to expand in the future.

[0006] International Publication No. 2004 / 083476 International Publication No. 2004 / 083477 International Publication No. 2004 / 110695 Japanese Patent Application Laid-Open No. 09-137255 Japanese Patent Application Laid-Open No. 2016-074976

[0007] However, the above-mentioned conventional techniques have the following problems. Specifically, typical stainless steel strips and steel plates are produced by hot-rolling slabs that have undergone melting, refining, and continuous casting, and then undergoing solution heat treatment and pickling. In other words, the surface condition of these steels 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) conducted to evaluate the properties of high-pressure hydrogen gas environments, evaluations are performed using round bar test specimens or machined plate test specimens. To further enhance functionality and stability, evaluations using test specimens with surfaces that have been pickled are effective. However, no such studies have been conducted to date.

[0008] Therefore, the present invention aims to provide a 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 when the materials are mass-produced.

[0009] The austenitic stainless steel according to the present invention, which advantageously solves the above problems, contains, by mass, C: 0.020 to 0.055%, Si: 0.15 to 0.85%, Mn: 4.10 to 8.00%, P: 0.015 to 0.030%, S: 0.0001 to 0.0020%, Ni: 10.00 to 15.00%, Cr: 20.00 to 24.00%, Mo: 1.20 to 3.30%, Cu: 0.05 to The alloy is characterized by containing as its component composition the following: 0.25%, Nb: 0.12 to 0.30%, V: 0.10 to 0.30%, B: 0.0005 to 0.0050%, N: 0.20 to 0.39%, Al: 0.002 to 0.035%, Sn: 0.002 to 0.016%, Co: 0.06 to 1.00%, and O: 0.0001 to 0.0050%, with the balance being Fe and unavoidable impurities.

[0010] In addition, a more preferable means for solving the problems of the austenitic stainless steel according to the present invention is one in which the composition further includes, on a mass basis, 0.02 to 1.20% W and satisfies the following relational expressions (1) to (3): (1) Equation 0.80≦Nb / V≦1.20 (2) Equation 1.00≦10×C / N≦1.80 (3) Equation 1.5×Sn+B≧0.0080 The element symbols in the above formulas indicate the content of each element expressed as a mass percentage.

[0011] The austenitic stainless steel strip or steel sheet according to the present invention, which advantageously solves the above-mentioned problems, is a steel strip or steel sheet having any of the above-mentioned 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.

[0012] The method for producing an austenitic stainless steel strip or steel plate according to the present invention, which advantageously solves the above-mentioned problems, includes the steps of melting an alloy having any of the above-mentioned component compositions and continuously casting it to form a steel slab, hot rolling the steel slab 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 finishing the surface with pickling, 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.

[0013] 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 austenitic stainless steel strip or steel plate.

[0014] The austenitic stainless steel of the present invention has shallow grain boundaries after pickling, and when surface finishing is performed in mass production processes for materials to be used in high-pressure hydrogen gas environments or liquid hydrogen environments, it is possible to suppress or improve the brittle behavior caused by hydrogen. Therefore, according to the present invention, it is possible to improve the functionality and stability of austenitic stainless steel.

[0015] The inventors have conducted extensive research, focusing primarily on the surface condition formed by annealing and pickling, and on the relationship between the formation of an oxide film during annealing, descaling behavior, and the corrosion resistance of the material.

[0016] First, for SUS304L, plate-shaped test pieces were taken perpendicular to the rolling direction from pickled 2 mm thick plates and subjected to slow strain rate tensile tests. Two types of test pieces were prepared: (1) one with the pickled finish remaining on both sides, and the thicker part was mechanically finished to a surface roughness of Ra 1.6 and then smoothed with wet abrasive paper #1000; and (2) one with both sides mechanically finished, the pickled surface removed, and the entire surface mechanically finished to a surface roughness of Ra 1.6 and then smoothed with wet abrasive paper #1000.

[0017] In both the atmosphere and the 70 MPa high-pressure hydrogen environment, the strain rate was 3 × 10 －５ The test was conducted at 1000 kJ / s, and the elongation behavior of each test piece was compared. As a result, it was found that the reduction of area at break of the material with a pickled surface was significantly reduced and the variation was large. From this, it was thought that it was 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.

[0018] In order to further smooth the surface condition after annealing and pickling, the inventors conducted extensive research into the relationship between the chemical composition and roughness after pickling for 12%Ni-21%Cr-5%Mn-1%Mo-0.3%N-0.03%C steel. As a result, the effects of several elements on smoothness were confirmed. In other words, it is believed that the variation in the results in slow strain rate tensile tests is improved, making it possible to use the steel under more severe conditions.

[0019] The most notable effect was that 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 state. The amount of B added is limited because adding too much of it deteriorates hot workability and induces weld cracks. In contrast, Sn can be added in larger amounts, so it is considered to be more effective.

[0020] Other elements that have been confirmed to be effective include Cr and Si. These elements are involved in the formation of oxide scale, and are thought to form scale uniformly in the early stages of heating and improve the smoothness of the surface after descaling.

[0021] Other elements that were also found to be effective were Ni, Cr, Mo, W, and Co. In the pickling process, the plate surface remains in the acid solution even after the scale is removed, and some dissolution continues. W is an element that is thought to inhibit dissolution at this stage. W was particularly effective in ensuring smoothness, and we believe this is because it was effectively concentrated in the concentration-modulated area formed directly below the oxide scale, the so-called dechromized layer.

[0022] 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 the chemical composition means "% by mass."

[0023] C: 0.020 to 0.055% 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 using at low temperatures. For this reason, the addition of at least 0.020% 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 0.055%. The preferred lower limit is 0.025%, and more preferably 0.030%. The preferred upper limit is 0.052%, and even more preferably 0.050%.

[0024] Si: 0.15 to 0.85% Si is an important element with deoxidizing properties, contributing to oxidation resistance and smoothing the surface after pickling. For this reason, the addition of at least 0.15% 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 0.85%. The preferred lower limit is 0.20%, and more preferably 0.25%. The preferred upper limit is 0.70%, and even more preferably 0.60%.

[0025] Mn: 4.10 to 8.00% 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 limited to a range of 4.10 to 8.00%. The preferred lower limit is 4.20%, and more preferably 4.40%. The preferred upper limit is 7.70%, and even more preferably 7.40%.

[0026] P: 0.015 to 0.030% 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 0.015 to 0.030%. The preferred upper limit is 0.025%, and the more preferred upper limit is 0.020%.

[0027] S: 0.0001 to 0.0020% 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 content of 0.0001% or more is preferable. The preferred lower limit is 0.0002%, and more preferably 0.0003%. The preferred upper limit is 0.0015%, and even more preferably 0.0010%.

[0028] Ni: 10.00 to 15.00% 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. Therefore, the addition of 10.00% or more is necessary. However, a Ni content exceeding 15.0% leads to increased costs. Therefore, the Ni content is limited to the range of 10.00 to 15.00%. The preferred lower limit of the Ni content is 10.50%, and a more preferred lower limit is an addition of more than 11.00%. The preferred upper limit is 14.50%, and a more preferred upper limit is 14.00%.

[0029] Cr: 20.00 to 24.00% 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, Cr is an essential element for improving surface smoothness after pickling by uniformly regulating the formation of oxide scale. 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 20.00 to 24.00%. The preferred lower limit of the Cr content is 20.20%, and more preferably 20.50%. The preferred upper limit is 23.50%, and even more preferably 23.00%.

[0030] Mo: 1.20 to 3.30% Mo not only improves pitting corrosion resistance and crevice corrosion resistance, similar to Cr and the like, but also improves general corrosion resistance and smooths the surface after pickling. Therefore, it is an essential element in this embodiment. However, excessive Mo content significantly promotes σ-phase precipitation, degrading corrosion resistance. Furthermore, it also increases costs. Therefore, the Mo content is set to a range of 1.20 to 3.30%. The preferred lower limit of the Mo content is 1.40%, and more preferably 1.60%. The preferred upper limit is 3.20%, and more preferably 3.10%.

[0031] Cu: 0.05 to 0.25% Cu is an important element that contributes to structural stability at low temperatures by stabilizing the austenite phase. To obtain 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 0.25%. Therefore, the content is set to the range of 0.05 to 0.25%. The preferred lower limit of the content is 0.06%, and more preferably 0.07%. The preferred upper limit is 0.23%, and more preferably 0.21%.

[0032] Nb: 0.12 to 0.30% 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 0.12% is necessary. However, addition of more than 0.30% forms excessive precipitates, which can cause cracks in the weld bead. Therefore, the range is set to 0.12 to 0.30%. The preferred lower limit is 0.14%, and the more preferred lower limit is 0.16%. The preferred upper limit is 0.27%, and the more preferred upper limit is 0.25%.

[0033] V: 0.10 to 0.30% V, like Nb, forms precipitates such as nitrides and carbides, and is useful for ensuring strength through solid solution strengthening. For this reason, the addition of at least 0.10% is necessary. However, addition of more than 0.30% will result in the formation of excessive precipitates, which will cause cracks in the weld bead. Therefore, the content is set to the range of 0.10 to 0.30%. The preferred lower limit is 0.12%, and more preferably 0.15%. The preferred upper limit is 0.28%, and more preferably 0.25%.

[0034] B: 0.0005 to 0.0050% B has the effect of improving hot workability even with a very small amount of addition. Furthermore, in this embodiment, B is an important element that has the effect of widening the grain boundaries after pickling and making the notch shape of the grain boundaries 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%.

[0035] N: 0.20 to 0.39% 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, N also 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 0.20% 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 0.39%. The preferred lower limit of the N content is 0.24%, and more preferably 0.27%. The preferred upper limit is 0.37%, and more preferably 0.34%.

[0036] Al: 0.002 to 0.035% Al is an important element that has a deoxidizing effect. ２ -Al ２ O ３In the presence of -MgO-based slag, Al promotes desulfurization through deoxidation. 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 the occurrence of defects during welding. Therefore, the Al content is set to a range of 0.002 to 0.035%. The preferred lower limit of the Al content is 0.003%, and the more preferred lower limit is 0.004%. The preferred upper limit is 0.032%, and the more preferred upper limit is 0.029%.

[0037] Sn: 0.002 to 0.016% Even a trace amount of Sn is effective in improving corrosion resistance. Furthermore, in this embodiment, Sn is an important element that has the effect of widening the grain boundaries after pickling and making 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%.

[0038] Co: 0.06 to 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. For this purpose, 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%.

[0039] O: 0.0001 to 0.0050% O is an impurity element that is inevitably mixed into steel. It 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 the range of 0.0001 to 0.0050%. The preferred upper limit is 0.0040%, and the more preferred upper limit is 0.0030%.

[0040] The austenitic stainless steel of this embodiment preferably contains the following optional elements in addition to the above essential elements, and the above composition satisfies the following relational expressions, where the element symbols in each relational expression indicate the content of each element expressed as a mass percentage.

[0041] W: 0.02 to 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%.

[0042] Formula (1): 0.80≦Nb / V≦1.20. Both Nb and V are added to ensure strength at room temperature and low temperatures. However, each element forms nitrides, carbides, carbonitrides, etc. If one element is present in large amounts, the precipitates of that element will dominate. Therefore, if one wishes to control the grain size to a certain size, the target heat treatment temperature must be changed 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 stable surface condition, i.e., smoothness, after pickling. Therefore, it is preferable to control the Nb / V ratio within the range of formula (1). A more preferable lower limit of the Nb / V ratio is 0.83, and an even more preferable lower limit is 0.86. A more preferable upper limit of the Nb / V ratio is 1.17, and an even more preferable upper limit is 1.14.

[0043] Formula (2): 1.00≦10×C / N≦1.80. Both C and N are added to ensure strength at room temperature and low temperatures, but each element forms nitrides, carbides, and carbonitrides. If either 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, satisfying formula (2) can improve the stability of grain size control. Maintaining a constant heat treatment temperature is an important indicator because it leads to stable surface condition, i.e., smoothness, after pickling, and it is preferable to control it within the range of formula (2). 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 1.75, and an even more preferable upper limit is 1.70.

[0044] Formula (3): 1.5 × Sn + B ≧ 0.0080 Formula (3) is an index for optimizing the grain boundaries formed by pickling, and by adding a total amount of Sn and B equal to or greater than a certain amount, better properties can be obtained. In other words, it is preferable that the value of the left side of formula (3) is 0.0080 or greater. More preferably, it is 0.0085 or greater, and even more preferably, it is 0.0090 or greater.

[0045] The austenitic stainless steel of this embodiment is composed of the balance other than the above components, namely, Fe and unavoidable impurities. Here, the unavoidable impurities refer to components that are inevitably mixed in due to various factors during industrial production of stainless steel, and are allowed to be contained to the extent that they do not adversely affect the effects of the present invention.

[0046] Next, a method for producing an austenitic stainless steel according to another embodiment of the present invention will be described.

[0047] There are no particular limitations on the method for producing an alloy having the above-mentioned composition. 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 chrome are melted in an electric furnace. Then, 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 introduced to perform desulfurization and deoxidation treatment. The slag composition in this treatment is CaO-Al. ２ O ３ -SiO ２ At the same time, in order to efficiently advance desulfurization, the slag should be adjusted to a CaO / Al system by mass ratio. ２ O ３ ≧2, CaO / SiO ２ ≧3. Furthermore, the refractories of the AOD furnace and VOD furnace are 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.

[0048] In this embodiment, the slab is then hot rolled, and if necessary, cold rolled to form a product. In this manner, a thick plate, a hot-rolled strip, a plate, or a cold-rolled strip, a plate is 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 strip and steel plate 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 equipment or liquid hydrogen equipment without smoothing treatment such as polishing.

[0049] The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to these examples as long as it does not depart 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 using 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. Note that a "-" in the table indicates that no intentional addition was made.

[0050]

[0051] The slab was then hot-rolled according to a conventional method to obtain a hot-rolled alloy sheet with a thickness of 8.0 mm. Subsequently, this hot-rolled alloy sheet was subjected to a solution heat treatment, followed by cold rolling, final annealing, and pickling to obtain a cold-rolled strip with a thickness of 2.0 mm. Final annealing involved holding at 1150°C for 1 minute, followed by water cooling and pickling. Pickling was performed by electrolytic pickling in a sulfuric acid solution and then immersion in a mixed solution of nitric acid and hydrofluoric acid.

[0052] Thereafter, (1) evaluation of crystal grain size, (2) grain boundary morphology, (3) tensile test at room temperature, (4) slow strain rate tensile test at room temperature, and (5) slow strain rate tensile test at -80°C were performed. The surfaces of the test materials in (3) to (5) were tested on two types: (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 surface treatment was the same as described above.

[0053] (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.

[0054] (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 ×.

[0055] (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 the test data at room temperature between (i) as-pickled and (ii) smoothed steels, and all were rated as ◎. Furthermore, low tensile strengths make the steel unusable for the intended application. The threshold value for tensile strength was set at 690 MPa, with values ​​above this being rated as 〇 and values ​​below this being ×. Of these, values ​​above 800 MPa were rated as ◎.

[0056] (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 the test was carried out at room temperature in a high-pressure hydrogen environment of 85 MPa. The initial strain rate was 3 × 10 －５ The test was carried out five times for the (ii) smoothing treatment, and then five times for the (i) as-pickled treatment. The results of (i) were compared with the results of (ii) and evaluated using the indices summarized in Table 2 to determine their superiority.

[0057]

[0058] (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.

[0059] The 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 shapes and excellent results in low-temperature, slow strain rate tensile tests. 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 7.0 to 8.0. This contributes to the stability of the SSRT test results. Furthermore, those satisfying formula (3) tend to have smoother surfaces and more stable SSRT test results.

[0060]

[0061] 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

1. On a mass basis, C: 0.020 to 0.055%, Si: 0.15 to 0.85%, Mn: 4.10 to 8.00%, P: 0.015 to 0.030%, S: 0.0001 to 0.0020%, Ni: 10.00 to 15.00%, Cr: 20.00-24.00%, Mo: 1.20-3.30%, Cu: 0.05-0.25%, Nb: 0.12-0.30%, V: 0.10-0.30%, B: 0.0005-0.0050%, N: 0.20-0.39%, Al: 0.002 to 0.035%, Sn: 0.002 to 0.016%, An austenitic stainless steel having a chemical composition of Co: 0.06 to 1.00%, and O: 0.0001 to 0.0050%, with the balance being Fe and unavoidable impurities.

2. The austenitic stainless steel according to claim 1, wherein the composition further includes, on a mass basis, 0.02 to 1.20% W, and satisfies the following relationship formulas (1) to (3): (1) Formula 0.80≦Nb / V≦1.20 (2) Formula 1.00≦10×C / N≦1.80 (3) Formula 1.5×Sn+B≧0.0080 The element symbols in the above formulas indicate the content of each element expressed as a mass percentage.

3. Austenitic stainless steel strip or sheet having the chemical composition as defined 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.

4. A method for producing austenitic stainless steel strip or steel plate, comprising the steps of: melting an alloy having the chemical composition defined in claim 1 or 2, 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, wherein the average ratio of the grain boundary depth to the grain boundary width in the surface layer is 1.5 or less.

5. High-pressure hydrogen gas equipment or liquid hydrogen equipment made of the austenitic stainless steel strip or steel plate according to claim 3.

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