High-strength austenitic stainless steel having excellent low-temperature toughness

Abstract

This invention relates to a high-strength austenitic stainless steel with excellent low-temperature toughness and a method for manufacturing the same. More specifically, it relates to an austenitic stainless steel and a method for manufacturing the same, wherein the austenitic stainless steel comprises, by weight%,: C: greater than 0% and less than 0.10%, Si: greater than 0% and less than 1.5%, Cr: 17.0% to 23.0%, Ni: 5.5% to 12.0%, Mn: 0.5% to 8.0%, N: 0.10% to 0.30%, Cu: greater than 0% and less than 1.0%, with the balance being Fe and other unavoidable impurities, the content of precipitates being less than 0.001% by weight, and the Ni equivalent value of the following formula (1) being 27 or more. Formula (1): Ni equivalent = Ni + 0.65Cr + 0.98Mo + 1.05Mn + 0.35Si + 12.6C + 33.6N

Description

Technical Field

[0001] This invention relates to a high-strength austenitic stainless steel with excellent low-temperature toughness. Background Technology

[0002] In recent years, research and development to utilize various environmentally friendly energy sources has been increasing from the perspective of global environmental protection. Consequently, the necessity for developing materials that can be used in various industrial sectors, including equipment, containers, and components, to utilize environmentally friendly energy sources has also increased.

[0003] For example, with the increasing demand and market growth for liquefied natural gas (LNG), liquefied petroleum gas (LPG), and liquefied hydrogen, the demand for storage tanks and piping required for the storage and transportation of cryogenic liquefied gases is increasing. Maintaining an extremely low temperature environment is necessary for the transportation and storage of these cryogenic liquefied gases.

[0004] However, the closer the operating temperature is to extremely low temperatures, the more difficult it becomes to manufacture stainless steel that simultaneously possesses excellent corrosion resistance and meets the various physical properties required for different equipment, containers, and components. Therefore, there is increasing attention being paid to stainless steel that not only possesses corrosion resistance but also meets a wide range of physical properties. Summary of the Invention

[0005] (a) Technical problems to be solved In order to solve the problems of the prior art as described above, the object of the present invention is to provide a high-strength austenitic stainless steel with excellent low-temperature toughness that can prevent the reduction of toughness caused by hydrogen and low temperature while improving strength.

[0006] Furthermore, the present invention aims to provide an austenitic stainless steel that exhibits high impact toughness at low temperatures by adjusting the content of precipitates, which has a major influence on low-temperature toughness, and austenite stabilization, which is closely related to martensite formation.

[0007] Furthermore, the present invention aims to provide an austenitic stainless steel, wherein the austenitic strength can be improved by optimizing the content of alloying elements and ensuring the stability of austenite, thereby maximally suppressing the martensitic phase transformation during deformation, thereby ensuring extremely low temperature toughness.

[0008] The technical problems to be solved by the present invention are not limited to those mentioned above. Other technical problems not mentioned can be clearly understood by those skilled in the art from the following description.

[0009] (II) Technical Solution To achieve the stated objective, the present invention comprises austenitic stainless steel, which, by weight percent, comprises: C: greater than 0% and less than 0.10%, Si: greater than 0% and less than 1.5%, Cr: 17.0% to 23.0%, Ni: 5.5% to 12.0%, Mn: 0.5% to 8.0%, N: 0.10% to 0.30%, Cu: greater than 0% and less than 1.0%, the balance being Fe and other unavoidable impurities, the content of precipitates being less than 0.001% by weight, and the Ni equivalent value of the following formula (1) being 27 or more: Equation (1): Ni equivalent = Ni + 0.65Cr + 0.98Mo + 1.05Mn + 0.35Si + 12.6C + 33.6N.

[0010] Furthermore, according to one embodiment of the present invention, the ratio of room temperature tensile strength to yield strength of the stainless steel can be 2.0 or less.

[0011] Furthermore, according to one embodiment of the present invention, the Charpy impact energy value of the stainless steel at -196°C can be above 70J.

[0012] Furthermore, the stainless steel according to one embodiment of the present invention may further contain less than 2.0% of Mo and less than 0.05% of Nb.

[0013] Furthermore, according to one embodiment of the present invention, the room temperature yield strength of the stainless steel can be above 300 MPa.

[0014] Furthermore, according to one embodiment of the present invention, the room temperature tensile strength of the stainless steel can be above 600 MPa.

[0015] Furthermore, according to one embodiment of the present invention, the area fraction of the austenitic phase in the stainless steel can be 90% or more.

[0016] Furthermore, a method for manufacturing the austenitic stainless steel according to one embodiment of the present invention may include the following steps: preparing a slab, which, by weight percent, comprises: C: greater than 0% and less than 0.10%, Si: greater than 0% and less than 1.5%, Cr: 17.0% to 23.0%, Ni: 5.5% to 12.0%, Mn: 0.5% to 8.0%, N: 0.10% to 0.30%, Cu: greater than 0% and less than 1.0%, with the balance being Fe and other unavoidable impurities, the content of precipitates being less than 0.001% by weight, and the Ni equivalent value of the following formula (1) being 27 or more; hot rolling the slab; hot rolling annealing after hot rolling; final cold rolling after hot rolling annealing; and final annealing after cold rolling. Equation (1): Ni equivalent = Ni + 0.65Cr + 0.98Mo + 1.05Mn + 0.35Si + 12.6C + 33.6N.

[0017] Furthermore, according to one embodiment of the present invention, the hot rolling annealing can be carried out at a temperature of 900-1200°C.

[0018] Furthermore, according to one embodiment of the present invention, the final annealing can be performed at a temperature of 900-1200°C.

[0019] Furthermore, the stainless steel according to one embodiment of the present invention may further contain less than 2.0% of Mo and less than 0.05% of Nb.

[0020] Furthermore, according to one embodiment of the present invention, the ratio of tensile strength to yield strength of the stainless steel at room temperature can be 2.0 or less.

[0021] Furthermore, according to one embodiment of the present invention, the Charpy impact energy value of the stainless steel at -196°C can be above 70J.

[0022] Furthermore, according to one embodiment of the present invention, the room temperature yield strength of the stainless steel can be above 300 MPa.

[0023] Furthermore, according to one embodiment of the present invention, the room temperature tensile strength of the stainless steel can be above 600 MPa.

[0024] Furthermore, according to one embodiment of the present invention, the area fraction of the austenitic phase in the stainless steel can be 90% or more.

[0025] (III) Beneficial Effects The high-strength austenitic stainless steel of the present invention, which exhibits excellent low-temperature toughness, can prevent the reduction in toughness caused by hydrogen and low temperature while improving strength by adjusting the precipitates that have a major influence on low-temperature toughness and the austenite stabilization that is closely related to martensite formation.

[0026] Furthermore, it can improve strength by optimizing the content of alloying elements and ensure the stability of austenite, thereby maximally suppressing the martensitic phase transformation during deformation, thus ensuring the effect of extremely low temperature toughness.

[0027] The effects of the present invention are not limited to those mentioned above, and other effects not mentioned can be clearly understood by those skilled in the art from the following description. Detailed Implementation

[0028] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. These embodiments are provided to fully convey the idea of ​​the disclosed invention to those skilled in the art. The disclosed invention is not limited to the embodiments presented herein and may be embodied in other forms. Furthermore, it should be noted that the accompanying drawings are only for ease of understanding of the idea of ​​the invention and should not be construed as limiting the idea of ​​the invention through the drawings.

[0029] Throughout the specification, when a part “contains” or “includes” a constituent element, unless specifically stated otherwise, it does not exclude other constituent elements, but may further include other constituent elements.

[0030] Unless there is an explicit exception in the context, singular expressions include plural expressions.

[0031] Temperature is the primary cause of brittleness in steel materials. Therefore, to test the performance of steel materials in extremely low-temperature environments such as those containing liquefied hydrogen, it is necessary to measure their toughness at these low temperatures.

[0032] Steel exposed to hydrogen environments may be exposed not only to hydrogen but also to a wide range of temperatures. As the temperature decreases, the material's toughness decreases and it becomes more brittle. Therefore, even materials that appear fine at room temperature may experience a decline in material properties as the temperature drops.

[0033] Generally speaking, austenitic structures are considered to be beneficial to low-temperature toughness, while martensite or ferrite structures are considered to be detrimental to low-temperature toughness.

[0034] Therefore, alloys favorable for cryogenic environments such as liquefied hydrogen are represented by 300 series stainless steels with an austenitic structure, currently mainly 304L and 316L. While these commercially available 304L and 316L stainless steels possess relatively excellent low-temperature toughness, their low strength results in increased material thickness when manufacturing cryogenic storage tanks or structures. Therefore, increasing the strength of the material compared to 304L and 316L allows for a reduction in material thickness, which in turn reduces material usage and helps lower the manufacturing cost of cryogenic storage tanks.

[0035] On the other hand, representative methods for improving material strength include cold working and precipitation strengthening methods that utilize precipitates.

[0036] However, cold working can cause a martensitic phase transformation in austenite, which may lead to hydrogen embrittlement or reduced low-temperature toughness due to the transformed martensite.

[0037] Furthermore, the precipitation strengthening method, which utilizes precipitates, is unsuitable for use in extremely low-temperature hydrogen environments because the precipitates reduce the toughness at extremely low temperatures.

[0038] Moreover, the strength improvement achieved by cold working or precipitation strengthening caused by precipitates not only leads to a decrease in the material's physical properties, but also limits its application due to the additional costs of cold working and precipitate precipitation processes.

[0039] Therefore, the strength improvement needs to be achieved not through cold working or precipitation strengthening, but through the control of alloy composition, to develop materials with highly stable austenitic structure and high strength.

[0040] The present invention aims to provide an austenitic stainless steel that simultaneously ensures high strength and low-temperature toughness by optimizing the content of alloying elements to adjust the precipitate content to a level that does not affect the ultra-low temperature toughness, while increasing the Ni equivalent value to ensure the stabilization of the austenitic phase, thereby maximally suppressing the martensitic phase transformation even during deformation, and reducing the tensile strength / yield strength ratio to maximally suppress work hardening, which means the formation of martensite that is detrimental to ultra-low temperature toughness.

[0041] This high-strength austenitic stainless steel of the present invention, with excellent low-temperature toughness, may contain, by weight percent: C: greater than 0% and less than 0.10%, Si: greater than 0% and less than 1.5%, Cr: 17.0% to 23.0%, Ni: 5.5% to 12.0%, Mn: 0.5% to 8.0%, N: 0.10% to 0.30%, Cu: greater than 0% and less than 1.0%, with the balance being Fe and other unavoidable impurities.

[0042] The reasons for specifying the composition of the steel are explained below. Unless otherwise specified, all compositional percentages below are expressed in weight percentages.

[0043] Carbon (C): greater than 0% and less than 0.10% Carbon (C) is an effective element for stabilizing the austenitic phase, suppressing δ-ferrite, and increasing strength through solid solution strengthening. However, when the C content exceeds 0.10%, it readily combines with carbide-forming elements such as Cr, Ti, and Nb, potentially reducing the corrosion resistance, ductility, and toughness of the base material. Therefore, the C content is preferably greater than 0% and less than 0.10%, more preferably 0.01% to 0.08%, and most preferably 0.01% to 0.06%.

[0044] Silicon (Si): greater than 0% and less than 1.5% Si is an effective element for improving corrosion resistance and solid solution strengthening. However, Si is a ferrite stabilizing element, and when the Si content exceeds 1.5%, intermetallic compounds such as the σ phase are formed, which may reduce the ductility and toughness of the base material. Therefore, the Si content is preferably greater than 0% and less than 1.5%, more preferably 0.01% to 1.2%, and most preferably 0.1% to 1.1%.

[0045] Chromium (Cr): 17.0% to 23.0% Cr is an essential element added to stainless steel to improve its corrosion resistance; to ensure this, a content of 17% or more is required. However, when the Cr content exceeds 23%, it promotes excessive δ-ferrite, which may reduce the hot workability of the steel, and the austenite becomes unstable. To maintain phase stability, a large amount of Ni must be included, which may also increase costs. Therefore, the preferred Cr content is 17% to 23%.

[0046] Ni and Mn, together with N, are strong austenitic phase stabilizing elements, especially Mn, which can substitute for high-valence Ni. Furthermore, Ni and Mn are important elements for low-temperature toughness; therefore, Mn and Ni must be added in appropriate proportions to ensure the target low-temperature toughness.

[0047] Manganese (Mn): 0.5% to 8.0% When an excessive amount of Mn is added, the low-temperature toughness of the material may be reduced due to the decrease in stacking fault energy. Therefore, Ni must be added to achieve sufficient low-temperature toughness, but the high cost of Ni makes this costly. Therefore, the Mn content is preferably 0.5% to 8.0%, more preferably 0.8% to 7.8%, and most preferably 1.0% to 7.0%.

[0048] Nickel (Ni): 5.5% to 12.0% Ni is an element that is increasingly beneficial in terms of austenite stabilization and low-temperature toughness, but to suppress the formation of δ-ferrite in the manufacturing process, it is preferable to add 5.5% or more. However, when the Ni content exceeds 12.0%, the probability of surface defects in the manufacturing process increases, leading to a price increase. Therefore, the Ni content is preferably 5.5% to 12.0%, more preferably 5.5% to 11.0%, and most preferably 6.0% to 10.0%.

[0049] Nitrogen (N): 0.10% to 0.30% Nitrogen (N) is an austenite stabilizing element and also an effective element for increasing strength through solid solution strengthening. Therefore, the N content is preferably 0.10% or more. However, when the N content exceeds 0.30%, it may induce a decrease in low-temperature toughness due to reduced productivity and stacking fault energy. Therefore, the N content is preferably 0.10% to 0.30%, more preferably 0.10% to 0.25%, and most preferably 0.15% to 0.21%.

[0050] Copper (Cu): greater than 0% and less than 1.0% Cu is a useful element for stabilizing the austenite phase and can be used as a substitute for the high-valence Ni. It is used to suppress martensite formation during molding and increase austenite stability; however, when the Cu content exceeds 1.0%, a low-melting-point phase is formed, reducing hot workability and thus surface quality. Therefore, the Cu content is preferably greater than 0% and less than 1.0%, more preferably greater than 0.01% and 1.0%, and most preferably 0.1% to 0.9%.

[0051] The stainless steel of the present invention comprising the alloy may further comprise, by weight percent, either less than 2.0% Mo or less than 0.05% Nb.

[0052] Molybdenum (Mo): below 2.0% Mo is an effective element for improving corrosion resistance in stainless steel. However, when the Mo content exceeds 2%, the increased ferrite fraction may induce a decrease in low-temperature toughness and lead to a price increase. Therefore, the Mo content is preferably 2% or less, more preferably 1.6% or less, and most preferably greater than 0.01% and less than 1.0%. In this case, the mechanical properties and corrosion resistance required for application in hydrogen and low-temperature environments can be further improved.

[0053] Niobium (Nb): less than 0.05% Nitrogen (Nb) is an element that contributes to strength improvement by forming precipitates, but Nb precipitates can also act as a major cause of reduced impact toughness. Therefore, the Nb content is preferably 0.05% or less, more preferably 0.08% or less, and most preferably 0.02% or less. In this case, the strength improvement effect caused by precipitate formation can be further enhanced.

[0054] The remaining component of this invention is iron (Fe). However, in the ordinary manufacturing process, unintentional impurities from raw materials or the surrounding environment are inevitably introduced, and therefore these impurities cannot be excluded. Such impurities are well known to those skilled in the art of ordinary manufacturing, and therefore their contents are not specifically mentioned in this specification.

[0055] The austenitic stainless steel of this invention, by appropriately controlling the content of the components as described above, and controlling the precipitate content and Ni equivalent value, can simultaneously satisfy austenite stabilization, low-temperature toughness, and high strength. In particular, by optimizing the content of alloying elements such as Ni, Mn, N, and Cu, which are beneficial to the stabilization of austenite structure, the ultra-low temperature toughness of hydrogen-grade steel is mainly affected. By increasing the Ni equivalent value, which represents the degree of austenite phase stabilization, and controlling the precipitate content to below 0.001% by weight, martensitic phase transformation can be suppressed to the maximum extent even during deformation. This ensures not only strength but also ultra-low temperature toughness.

[0056] Furthermore, the austenitic stainless steel according to one embodiment of the present invention may contain less than 0.001% by weight of precipitates.

[0057] In this invention, precipitates refer to all precipitates that precipitate in steel, and may also include Cr-based, Nb-based, individual or complex carbonitrides and Cu-based metal precipitates.

[0058] While precipitates are highly effective in ensuring strength, they can also become crack initiation or propagation points, potentially reducing the impact toughness of steel. Furthermore, precipitate formation can also affect the extremely low temperature toughness of steel. Therefore, appropriately controlling the precipitate content to simultaneously ensure both strength and extremely low temperature toughness is of paramount importance.

[0059] Therefore, in this invention, an optimal alloy composition is determined that can simultaneously ensure the strength and cryogenic toughness of steel, and by optimally controlling their content, the content of precipitates can be made less than 0.001% by weight. When the content of the precipitates exceeds 0.001% by weight, the impact toughness of the steel is reduced, making it difficult to ensure high-strength steel grades, and the cryogenic toughness is reduced, making it unsuitable for use in cryogenic environments such as high-pressure gas or liquefied hydrogen storage containers and piping.

[0060] Furthermore, in an embodiment of the austenitic stainless steel according to the present invention, the Ni equivalent value of the following formula (1) is adjusted to 27 or more. Equation (1): Ni equivalent = Ni + 0.65Cr + 0.98Mo + 1.05Mn + 0.35Si + 12.6C + 33.6N.

[0061] When the Ni equivalent value is less than 27, martensitic transformation occurs during deformation, which cannot contribute to the stabilization of the austenitic phase. Consequently, it may be difficult to obtain the high strength and low-temperature toughness desired in this invention, especially the room-temperature tensile strength / yield strength ratio below 2.0. Therefore, the Ni equivalent value is preferably 27 or more, more preferably 29 or more, and most preferably 30 or more.

[0062] Furthermore, according to one embodiment of the present invention, the austenitic stainless steel satisfies a Ni equivalent value of 27 or higher, and therefore the ratio of room temperature tensile strength to yield strength can be 2.0 or lower.

[0063] When the ratio of room temperature tensile strength to yield strength exceeds 2.0, it is difficult to suppress work hardening that induces martensitic phase transformation, the austenite stabilization effect is reduced, and it may have an adverse effect on strength and ultra-low temperature toughness.

[0064] A tensile strength / yield strength ratio below 2.0 indicates that the steel grade has a relatively low yield strength but high tensile strength. Generally, in austenitic stainless steels, steel grades with low yield strength and high tensile strength can be obtained through work hardening. However, the more work hardened, the more martensitic phase transformations occur, which are detrimental to cryogenic toughness. In this case, strength increases but cryogenic toughness relatively decreases. However, since martensitic phase transformation occurs with work hardening, a martensitic phase transformation to zero is almost impossible.

[0065] Furthermore, according to one embodiment of the present invention, the austenitic stainless steel can achieve a Charpy impact energy value of 70J or higher at -196°C by controlling the precipitate content to below 0.001% by weight.

[0066] Charpy impact energy is a value obtained through the Charpy impact test. The Charpy impact test involves preparing a plate of material approximately 10 mm thick, making a small notch in the center, mounting the sample on a testing apparatus, and applying impact with a hammer under varying temperatures. When the extremely low temperature impact toughness, i.e., the Charpy impact energy value, is less than 70 J, it is difficult to use in extremely low temperature environments and may not be suitable for applications such as liquefied hydrogen storage containers and piping.

[0067] Therefore, in this invention, by simultaneously controlling the precipitate content and the Ni equivalent value, the austenite stabilization is improved, thereby achieving a room temperature tensile strength / yield strength ratio of 2.0 or less and a Charpy impact energy value of 70 J or more at -196°C. Specifically, in this invention, by adjusting the Ni equivalent value to 27 or more, a low room temperature tensile strength / yield strength ratio of 2.0 or less can be ensured, thereby maximally suppressing work hardening, which implies martensite formation and is detrimental to extremely low temperature toughness. Furthermore, by controlling the precipitate content to less than 0.001%, this invention achieves a Charpy impact energy value of 70 J or more at -196°C, thereby enabling the manufacture of austenitic stainless steel that simultaneously satisfies high strength and low temperature toughness.

[0068] This austenitic stainless steel according to one embodiment of the present invention can meet the requirement of a yield strength of 300 MPa or more at room temperature.

[0069] When an object is stretched by a force greater than a certain magnitude and then the force is released, it cannot return to its original state and instead becomes longer. The maximum force required to return it to its original state is called the yield strength. When the strength of steel is increased, the amount of steel used to manufacture products of the same strength is reduced. Therefore, according to the present invention, stainless steel with excellent strength can be provided, thereby reducing product costs.

[0070] When the yield strength at room temperature is less than 300 MPa, it may be difficult to obtain austenitic stainless steel with excellent low-temperature toughness and high strength. There is no upper limit to the room-temperature yield strength; for example, to meet the mechanical properties, corrosion resistance, strength, etc., required for applications in hydrogen and low-temperature environments, it can be below 700 MPa, below 650 MPa, below 600 MPa, below 550 MPa, below 500 MPa, below 450 MPa, etc.

[0071] Furthermore, the austenitic stainless steel according to one embodiment of the present invention can achieve a room temperature tensile strength of 600 MPa or higher. There is no upper limit to the room temperature tensile strength; for example, it can be below 800 MPa, below 750 MPa, below 700 MPa, below 650 MPa, etc., to meet the mechanical properties, corrosion resistance, strength, etc., required for applications in hydrogen and low-temperature environments.

[0072] Furthermore, a method for manufacturing austenitic stainless steel with excellent low-temperature toughness according to one embodiment of the present invention may include the following steps: preparing a slab, which, by weight percent, comprises: C: greater than 0% and less than 0.10%, Si: greater than 0% and less than 1.5%, Cr: 17.0% to 23.0%, Ni: 5.5% to 12.0%, Mn: 0.5% to 8.0%, N: 0.10% to 0.30%, Cu: greater than 0% and less than 1.0%, the balance being Fe and other unavoidable impurities, the content of precipitates being less than 0.001% by weight, and the Ni equivalent value of the following formula (1) being 27 or more; hot rolling the slab; hot rolling annealing after hot rolling; final cold rolling after hot rolling annealing; and final annealing after cold rolling.

[0073] During the hot rolling annealing process following hot rolling, the annealing temperature has a significant impact on residual stress relief and microstructure. Therefore, the hot rolling annealing is preferably carried out at a temperature of 900-1200℃.

[0074] When the hot rolling annealing temperature is below 900°C, coarse carbides are formed, the microstructure becomes uneven, or Cr forms around the grain boundaries. 23C6 precipitates may cause grain boundary corrosion. When the hot rolling annealing temperature exceeds 1200°C, the grains may become extremely coarse. Therefore, it is preferable to limit the annealing temperature to 900-1200°C, more preferably to 950-1150°C, and most preferably to 1000-1150°C.

[0075] Furthermore, after the hot rolling annealing, a cold rolling followed by a final annealing step can be performed. The cold rolling annealing can be carried out at a temperature of 900-1200°C.

[0076] The austenitic stainless steel of the present invention manufactured by this method has an austenite phase area fraction of 90% or more and a carbide density of 0.001% by weight or less.

[0077] As the development and adoption of fuel cell vehicles that use hydrogen as fuel expand, there is a need to develop containers and components for storing hydrogen.

[0078] Hydrogen storage containers can be categorized into liquefied hydrogen and gaseous hydrogen based on the form of hydrogen. Depending on the form of hydrogen and the operating temperature, liquefied hydrogen exists in an extremely low-temperature environment of -253°C, containing hydrogen produced from the vaporization of liquefied hydrogen within the storage tank. Furthermore, in the device used to vaporize liquefied hydrogen, the steel is exposed to a temperature range from -253°C to room temperature; therefore, hydrogen must not cause any degradation in the steel's physical properties at any temperature. Gaseous hydrogen is generally stored at room temperature, but is pre-cooled to approximately -40°C to -60°C before filling the storage tank. This is to account for the temperature rise during filling; a precooler is used for cooling to prevent excessive temperature increases during filling.

[0079] In particular, liquefied hydrogen storage is more efficient than its gaseous form, and therefore is expected to be used in various fields in the future. It is anticipated that liquefied hydrogen will be used for long-distance transportation of hydrogen from overseas to domestic markets and for large-scale storage of hydrogen at hydrogen refueling stations or hydrogen production plants.

[0080] Therefore, when considering the steel used for hydrogen storage tanks, the reduction in physical properties at extremely low temperatures, in addition to room temperature, can also be a significant determining factor. With this in mind, hydrogen storage tanks and related equipment require protection against the reduction in toughness caused by hydrogen and extremely low temperatures, as well as high mechanical strength and corrosion resistance.

[0081] Currently, the materials commonly used in hydrogen and liquefied hydrogen environments are austenitic stainless steels 304L and 316L. Even materials that appear fine at room temperature may exhibit a decrease in physical properties as the temperature decreases. In particular, reduced toughness is a major problem that occurs with decreasing temperature, which is one of the main reasons for the martensitic phase transformation in the austenitic structure.

[0082] In the austenitic phase, hydrogen diffusion is slow and movement is difficult, thus reducing the likelihood of hydrogen embrittlement. Furthermore, compared to the martensite phase, it is a softer phase, easily maintaining toughness even at extremely low temperatures. Conversely, the martensite phase is a harder phase compared to austenite, prone to brittleness, and its rapid hydrogen diffusion increases the potential for hydrogen embrittlement. Therefore, when exposed to a hydrogen environment, a high martensite fraction allows hydrogen to penetrate the material, leading to a decrease in material properties caused by hydrogen, potentially causing problems in use under liquefied and gaseous hydrogen environments.

[0083] Furthermore, the thickness of materials used in hydrogen storage tanks is determined by their strength; therefore, increasing strength can reduce the amount of material used in hydrogen storage tanks. While precipitates can be used to increase strength, they are a major cause of reduced toughness at low temperatures. Therefore, the precipitate content within the steel must be adjusted for use in hydrogen environments.

[0084] The present invention will now be described in more detail through examples. These examples are for illustrative purposes only and are not intended to limit the scope of protection of the present invention.

[0085] Example 1 After hot rolling, the austenitic slab with the composition shown in Table 1 below is annealed at a temperature of 900-1200°C.

[0086] The alloy compositions of each embodiment and comparative example are shown in Table 1 below.

[0087] [Table 1] To confirm the impact toughness at extremely low temperatures, Charpy impact tests were conducted at -196°C using the experimental steels of the examples and comparative examples in Table 1, and tensile tests were performed in ambient air at room temperature. Yield strength and tensile strength were measured. Charpy impact energy values ​​were determined using ASTM E23A type A specimens at -196°C. Tensile tests were conducted according to ASTM E8 specifications. The precipitate content was measured by quantitative analysis using a residue extraction method.

[0088] The Charpy impact toughness at -196℃, precipitate content (wt%), room temperature yield strength, room temperature tensile strength, tensile strength / yield strength ratio, and Ni equivalent value of the experimental steels according to the embodiments and comparative examples are shown in Table 2 below.

[0089] [Table 2] Observing the results in Table 2, it can be confirmed that Examples 1 to 12 of the present invention satisfy the alloy composition proposed in the present invention, and the Ni equivalent value is 27 or higher, the precipitate content is less than 0.001%, thus satisfying the high value of a room temperature tensile strength / yield strength ratio of 2.0 or lower, and a Charpy impact energy value of 70 J or higher at -196°C. Furthermore, the yield strength at room temperature is 305-405 MPa, the tensile strength is 615-713 MPa, and the tensile strength / yield strength ratio is 2.0 or lower, maximally suppressing martensitic phase transformation during deformation or processing, thus exhibiting excellent low-temperature toughness. Therefore, the austenitic stainless steel of the present invention can be used as a material for liquefied hydrogen storage containers, piping, and other liquefied hydrogen applications. On the other hand, the alloy compositions of Comparative Examples 1 to 3, 5 to 6 and 8 did not meet the requirement of N content. Although the precipitate content was less than 0.001% by weight, the stability of the austenitic phase was low. Due to the martensitic phase transformation caused by work hardening, which is detrimental to the toughness at extremely low temperatures, the Ni equivalent value was less than 27. Based on this, the ratio of room temperature tensile strength to yield strength exceeded 2.0, indicating that it was difficult to use as a material for liquefied hydrogen.

[0090] Furthermore, Comparative Examples 4 and 7 have alloy compositions that satisfy the N content requirement and Ni equivalent value of 27 or more. Therefore, the tensile strength / yield strength ratio is 2.0 or less. Although the austenite is stabilized, the precipitate content exceeds 0.001% by weight. As a result, the Charpy impact energy value at -196°C is a low value of 70J or less. It is evident that these materials are unsuitable for use as liquefied hydrogen materials in extremely low temperature environments.

[0091] The results above show that, according to the present invention, by optimizing the alloy composition and component content of the austenitic stainless steel, the precipitation amount is controlled to be less than 0.001% by weight, and the Ni equivalent value, representing the austenite phase stabilization degree, is controlled to be 27 or more. This satisfies the requirement that the ratio of room temperature tensile strength to yield strength is less than 2.0, and that the Charpy impact energy at -196°C is greater than 70 J. Furthermore, it satisfies the requirement that the room temperature yield strength is greater than 300 MPa and the room temperature tensile strength is greater than 600 MPa, thus providing an austenitic stainless steel with excellent low-temperature toughness and strength.

[0092] Furthermore, it is known that the present invention, through austenite stabilization, can suppress martensitic phase transformation during deformation or processing to the greatest extent, thus satisfying low-temperature toughness, while without the reduction of material properties caused by hydrogen, making it suitable as an austenitic stainless steel for use as a material for liquefied hydrogen storage containers, piping, and other liquefied hydrogen applications.

[0093] The embodiments and accompanying drawings described in this specification are merely illustrative of a portion of the technical ideas contained in this invention. Therefore, the embodiments disclosed in this specification are not intended to limit the technical ideas of this invention, but rather to illustrate them; thus, it is obvious that the scope of the technical ideas of this invention is not limited to these embodiments. Modifications and specific embodiments that can be readily deduced by those skilled in the art within the scope of the technical ideas contained in this specification and accompanying drawings should be interpreted as being included within the scope of the claims of this invention.

Claims

1. An austenitic stainless steel, by weight%, comprising: C: greater than 0% and less than 0.10%, Si: greater than 0% and less than 1.5%, Cr: 17.0% to 23.0%, Ni: 5.5% to 12.0%, Mn: 0.5% to 8.0%, N: 0.10% to 0.30%, Cu: greater than 0% and less than 1.0%, with the balance being Fe and other unavoidable impurities, the content of precipitates being less than 0.001% by weight, and the Ni equivalent value of the following formula (1) being 27 or more. Equation (1): Ni equivalent = Ni + 0.65Cr + 0.98Mo + 1.05Mn + 0.35Si + 12.6C + 33.6N.

2. The austenitic stainless steel according to claim 1, wherein, The ratio of the room temperature tensile strength to the yield strength of the stainless steel is below 2.

0.

3. The austenitic stainless steel according to claim 1, wherein, The Charpy impact energy of the stainless steel at -196°C is above 70J.

4. The austenitic stainless steel according to claim 1, wherein, The stainless steel further comprises less than 2.0% of Mo and less than 0.05% of Nb.

5. The austenitic stainless steel according to claim 1, wherein, The stainless steel has a room temperature yield strength of 300 MPa or higher.

6. The austenitic stainless steel according to claim 1, wherein, The stainless steel has a room temperature tensile strength of 600 MPa or higher.

7. The austenitic stainless steel according to claim 1, wherein, The area fraction of the austenitic phase in the stainless steel is above 90%.

8. A method for manufacturing austenitic stainless steel, comprising the following steps: Prepare a slab, which, by weight%, comprises: C: greater than 0% and less than 0.1%, Si: greater than 0% and less than 1.5%, Cr: 17% to 23%, Ni: 5.5% to 12%, Mn: 0.5% to 8%, N: 0.1% to 0.3%, Cu: greater than 0% and less than 1.0%, the balance being Fe and other unavoidable impurities, the content of precipitates being less than 0.001% by weight, and the Ni equivalent value of the following formula (1) being 27 or more; The slab is hot-rolled; The hot rolling process is followed by hot rolling annealing. The hot-rolled annealing is followed by a final cold rolling; and The cold rolling process is followed by final annealing. Wherein, Equation (1): Ni equivalent = Ni + 0.65Cr + 0.98Mo + 1.05Mn + 0.35Si + 12.6C + 33.6N.

9. The method for manufacturing austenitic stainless steel according to claim 8, wherein, The hot rolling annealing is carried out at a temperature of 900-1200℃.

10. The method for manufacturing austenitic stainless steel according to claim 8, wherein, The final annealing is carried out at a temperature of 900-1200°C.

11. The method for manufacturing austenitic stainless steel according to claim 8, wherein, The stainless steel further comprises less than 2.0% of Mo and less than 0.05% of Nb.

12. The method for manufacturing austenitic stainless steel according to claim 8, wherein, The ratio of the tensile strength to the yield strength of the stainless steel at room temperature is less than 2.

0.

13. The method for manufacturing austenitic stainless steel according to claim 8, wherein, The Charpy impact energy of the stainless steel at -196°C is above 70J.

14. The method for manufacturing austenitic stainless steel according to claim 8, wherein, The stainless steel has a room temperature yield strength of 300 MPa or more and a room temperature tensile strength of 600 MPa or more.

15. The method for manufacturing austenitic stainless steel according to claim 8, wherein, The area fraction of the austenitic phase in the stainless steel is above 90%.

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