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

Abstract

To provide 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 enhanced functionality and be stabilized.SOLUTION: This austenitic stainless steel contains, as constituent components, 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: 0.002-0.016%, Co: 0.06-1.00%, and O: 0.0001-0.0050%, the remainder consisting of Fe and unavoidable impurities.SELECTED DRAWING: None

Description

[Technical Field]

[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. [Background technology]

[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 certain environments (NEDO FY2013-2017 Results Report), in which SUS316, SUH660, and the overseas standard XM-19 (ASME SA-240 / UNS S20910, product name: Nitronic50 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. In all cases, high strength in a high-pressure hydrogen gas environment is achieved by optimizing the chemical composition, controlling the crystal grain size, etc.

[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. In this way, proposals for practical application are also progressing in the materials field, and this trend is expected to continue to expand in the future. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2004 / 083476 [Patent Document 2] International Publication No. 2004 / 083477 [Patent Document 3] International Publication No. 2004 / 110695 [Patent Document 4] Japanese Patent Application Publication No. 09-137255 [Patent Document 5] Japanese Patent Application Laid-Open No. 2016-074976 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the above-mentioned conventional techniques have the following problems. In other words, typical stainless steel strips and plates are produced by hot-rolling strips or plates from melted, refined, and continuously cast slabs, then undergoing solution heat treatment and pickling. In other words, the surface condition is generally used for manufacturing structures in the pickled state. 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 specimens or machined plate specimens. To further improve functionality and stability, evaluations using specimens with pickled surfaces would be 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 in mass-produced processes. [Means for solving the problem]

[0009] The austenitic stainless steel according to the present invention, which advantageously solves the above problems, is characterized by a chemical composition, by mass, of C: 0.040 to 0.100%, Si: 0.25 to 1.00%, Mn: 13.50 to 18.50%, P: 0.020 to 0.045%, S: 0.0001 to 0.0020%, Ni: 4.00 to 8.00%, Cr: 16.50 to 18.00%, Mo: 0.05 to 2.00%, Cu: 0.05 to 1.00%, N: 0.12 to 0.45%, Al: 0.002 to 0.020%, 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] Furthermore, 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, by mass, 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 Equation (2) 1.5×Sn+B≧0.0060 The element symbols in the above formula indicate the content of each element expressed as a mass percentage.

[0011] The austenitic stainless steel strip or steel sheet of 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, characterized in that the ratio of grain boundary depth to grain boundary width in the surface layer after annealing and pickling 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, is characterized by comprising 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 by pickling.

[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. [Effects of the Invention]

[0014] The austenitic stainless steel of this invention has shallow grain boundaries after pickling, and when surface finishing is performed in mass production processes for materials used in high-pressure hydrogen gas environments or liquid hydrogen environments, it is possible to suppress and 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. DETAILED DESCRIPTION OF THE INVENTION

[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 (SSRT). The following two types of test pieces were prepared: (1) Both sides remain pickled, and the thickness of the plate is machine-finished to a surface roughness of Ra1.6, and then smoothed with wet abrasive paper #1000. (2) Both sides are machined, the pickled surface is removed, and the entire surface is machined to a surface roughness of Ra1.6, and then smoothed with wet abrasive paper #1000.

[0017] In both air and 70 MPa high-pressure hydrogen environments, the strain rate was 3×10 -5 / 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 reducing the variation and stabilizing it. Therefore, it was decided to carry out intensive research to make the surface smoother after pickling.

[0018] In order to achieve a smoother surface 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, the effects of several elements on smoothness were confirmed. In other words, it is believed that the variation in results in slow strain rate tensile tests is improved, making it possible to use the steel under more severe conditions.

[0019] The most notable effects were those 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. Adding a large amount of B can lead to a deterioration in hot workability and induce weld cracks, so the amount of B that can be added is limited. In contrast, Sn can be added in larger amounts, so it is thought to be more effective.

[0020] Other elements that were also found to be effective were 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, improving 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. These elements are thought to inhibit dissolution at this stage. W was particularly effective in ensuring smoothness, and this was presumably due to its effective concentration 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: 0.040 to 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. For this reason, the addition of at least 0.040% is necessary. However, excessive C content facilitates the precipitation of Cr carbides during welding and cooling after solution heat treatment, degrading corrosion resistance. Therefore, the upper limit is set at 0.100%. The preferred lower limit is 0.050%, and more preferably 0.055%. The preferred upper limit is 0.090%, and even more preferably 0.080%.

[0024] Si: 0.25 to 1.00% 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 0.25% 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%, more preferably 0.30%. The preferred upper limit is 0.80%, more preferably 0.70%.

[0025] 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 inhibits 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 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%.

[0026] P: 0.020 to 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 of P content is set to 0.020 to 0.045%. The preferred upper limit is 0.042%, and the more preferred upper limit is 0.040%.

[0027] S: 0.0001 to 0.0020% Sulfur (S) is an impurity element that inevitably gets mixed into steel. It reduces hot workability and forms sulfides that act as the starting point 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%.

[0028] Ni: 4.00 to 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. Therefore, 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 limited 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%.

[0029] 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 facilitating the uniform 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 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%.

[0030] Mo: 0.05 to 2.00% Mo, like Cr and the like, not only improves pitting corrosion resistance and crevice corrosion resistance, 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 the precipitation of the σ phase, degrading corrosion resistance and increasing costs. For this reason, the Mo content is set to a 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%.

[0031] Cu: 0.05 to 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 at 1.00%. Therefore, the Cu content is set to the range of 0.05 to 1.00%. The preferred lower limit is 0.07%, and more preferably 0.10%. The preferred upper limit is 0.80%, and more preferably 0.70%.

[0032] N: 0.12 to 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 and crevice corrosion resistance, and like C, it is an important element for ensuring strength. For this reason, 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, it should 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 even more preferably 0.34%.

[0033] Al: 0.002 to 0.020% Al is an important element with deoxidizing properties. Furthermore, Al promotes desulfurization through deoxidation in the presence of CaO-SiO2-Al2O3-MgO slag. Furthermore, Al is an important element for stabilizing the yield of B during refining. However, excessive Al content can cause excessive oxide scale, making pickling difficult and promoting defects during welding. Therefore, the Al content is limited to a range of 0.002 to 0.020%. The preferred lower limit is 0.003%, and more preferably 0.004%. The preferred upper limit is 0.019%, and more preferably 0.018%.

[0034] Sn: 0.002 to 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. To achieve this, 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 a deterioration in 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%.

[0035] 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 has the effect of smoothing the surface after pickling. To achieve this, at least 0.06% of Co 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%.

[0036] O:0001~0.0050% O is an impurity element that is inevitably mixed into steel. It forms non-metallic inclusions with Si, Mn, and Al, which reduce the cleanliness of the steel and cause defects. However, excessive deoxidation increases costs, so the O content is set to a range of 0.0001 to 0.005%. The preferred upper limit is 0.0040%, and the more preferred upper limit is 0.0030%.

[0037] 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. The element symbols in each relational expression indicate the content of each element expressed in mass percentage. It is also preferable to add one or two elements selected from Nb and V.

[0038] Nb: 0.05 to 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%.

[0039] V: 0.05 to 0.50% V, like Nb, forms precipitates such as nitrides and carbides, and is useful for ensuring strength through solid solution strengthening. For this reason, at least 0.05% addition 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 more preferably 0.10%. The preferred upper limit is 0.28%, and more preferably 0.25%.

[0040] 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. For this reason, it is preferable to add W in this embodiment. However, excessive W content may significantly promote the precipitation of the σ phase, which may deteriorate corrosion resistance. Furthermore, it may result in increased costs. For this reason, the W content is preferably set 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%.

[0041] B: 0.0005 to 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 cracks in 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%.

[0042] (1) Formula: 1.00≦10×C / N≦5.50 Both C and N are added to ensure strength at room and low temperatures. Each element forms nitrides, carbides, and carbonitrides. If one element is present in large amounts, the precipitates of that element become dominant. Therefore, if you want to control the grain size to a certain size, you must change the target heat treatment temperature each time. On the other hand, satisfying equation (1) can improve the stability of grain size control. Maintaining a constant heat treatment temperature is an important indicator because it leads to consistent surface condition, or smoothness, after pickling. Therefore, it is preferable to control the heat treatment temperature within the range of equation (1). The preferred lower limit of the ratio 10 × C / N is 1.05, and even more preferred is 1.10. The preferred upper limit of the ratio 10 × C / N is 5.00, and even more preferred is 4.00.

[0043] (2) Formula: 1.5×Sn+B≧0.0060 Equation (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 above a certain level. In other words, the value of the left side of equation (2) is preferably 0.0060 or more, more preferably 0.0085 or more, and even more preferably 0.0090 or more.

[0044] The austenitic stainless steel of this embodiment is composed of the balance other than the above components, including 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 within a range that does not adversely affect the effects of the present invention.

[0045] Next, a method for producing an austenitic stainless steel according to another embodiment of the present invention will be described.

[0046] The alloy having the above composition can be produced by any suitable method. 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 refinement, while quicklime, fluorite, an Al source, a Si source, and other materials are added for desulfurization and deoxidation. The slag composition used in this process is preferably adjusted to a CaO-Al2O3-SiO2-MgO-F system. To efficiently promote desulfurization, the slag preferably has a mass ratio of CaO / Al2O3 ≥ 2 and CaO / SiO2 ≥ 3. The refractories used in the AOD furnace or VOD furnace are preferably magnesia-chrome or dolomite. After refining in the AOD furnace or VOD furnace, the composition and temperature are preferably adjusted in the LF process, followed by continuous casting to produce slabs. In the continuous casting process, a vertical type in which bending is not performed within the apparatus until solidification is complete after casting is particularly preferable, because it allows the distribution of precipitates to be more symmetrical in the thickness direction.

[0047] 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, plates, and cold-rolled strips, plates, and 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 hydrogen-induced embrittlement behavior, which is necessary 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. [Example]

[0048] 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 a slab. 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.

[0049] [Table 1]

[0050] 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. The final annealing was performed by holding at 1150°C for 1 minute, followed by water cooling and pickling. The pickling was performed by electrolytic pickling in a sulfuric acid solution, followed by immersion in a mixed solution of nitric acid and hydrofluoric acid.

[0051] 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 surfaces of the test materials in (3) to (5) were (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 above.

[0052] (1) Crystal grain size Embedded specimens were prepared so that cross sections perpendicular to the rolling direction could be observed, and the structure was revealed by etching. The grain size number (GSN) was determined in accordance with JIS G0551.

[0053] (2) Grain boundary morphology In pickled materials, the grain boundaries in the surface layer are preferentially corroded, resulting in notches. 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 carried out with an average value of (grain boundary depth) / (grain boundary width) of 0.8 or less as ◎, 1.5 or less as ◯, and anything else as ×.

[0054] (3) Tensile test at room temperature (RT) Tests were conducted on plate-shaped test specimens taken perpendicular to the rolling direction from a 2 mm-thick cold-rolled strip. Tensile tests were conducted in accordance with JIS Z2241, using JIS No. 13B test specimens. Tests were conducted for the aforementioned (i) as-pickled and (ii) smoothed steels, with tensile strength variations of less than ±3% rated as ◎, -3% or more but less than -5% rated as 〇, -5% or more but less than -7% rated as △, and -7% or more rated as ×. However, there was no significant difference in test data at room temperature between (i) as-pickled and (ii) smoothed steels, with all specimens rated as ◎. Furthermore, low tensile strengths preclude use for the intended application. The threshold for tensile strength was set at 600 MPa, with values ​​above this rated as 〇 and values ​​below this rated as ×. Of these, values ​​above 690 MPa were rated as ◎.

[0055] (4) Slow strain rate tensile test at room temperature (RT) Plate-shaped test pieces were cut perpendicular to the rolling direction from the cold-rolled strip with a thickness of 2 mm and tested 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 (ii) smoothing treatment, and then five times for (i) as-pickled treatment. The results of (ii) were compared with those of (i) and evaluated using the indices summarized in Table 2 to determine their superiority.

[0056] [Table 2]

[0057] (5) Slow strain rate tensile test at -80℃ 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.

[0058] The test results are summarized in Table 3. The test pieces were heat-treated at 1150°C for 1 minute. The inventive examples (Nos. 1 to 15) had excellent grain boundary morphology, and exhibited excellent low-temperature slow strain rate tensile test results and tensile strength. Looking at the grain size (GSN) in Table 3, those satisfying formulas (1) and (2) generally had a GSN in the range of 6.0 to 7.0. These had smoother surfaces, which contributed to stabilizing the SSRT test results.

[0059] [Table 3] [Industrial Applicability]

[0060] 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. This contributes 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 practical storage and transportation of large quantities of hydrogen is underway. 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. 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. 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.

3. 3. An austenitic stainless steel strip or steel sheet having the chemical composition according to claim 1 or 2, wherein the ratio of the grain boundary depth to the grain boundary width in the surface layer after annealing and pickling is 1.5 or less.

4. 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; A method for producing an austenitic stainless steel strip or steel plate, comprising the steps of final annealing the cold-rolled alloy plate and finishing the surface with pickling.

5. 4. Equipment for high-pressure hydrogen gas or equipment for liquid hydrogen, comprising the austenitic stainless steel strip or steel plate according to claim 3.

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