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

Abstract

The present invention relates to a high-strength austenitic stainless steel having excellent low-temperature toughness and a manufacturing method therefor and, more specifically, to a high-strength austenitic stainless steel and a manufacturing method therefor, the high-strength austenitic stainless steel comprising, in weight%, C: 0% (exclusive) to 0.10% (inclusive), Si: 0% (exclusive) to 1.5% (inclusive), 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: 0% (exclusive) to 1.0% (inclusive), and the balance being Fe and other inevitable impurities, with a content of precipitate of less than 0.001 wt%, and has a Ni equivalent value of 27 or more as defined by equation (1). Equation (1): Ni equivalent = Ni+0.65Cr+0.98Mo+1.05Mn+0.35Si+12.6C+33.6N

Description

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

[0001] The present invention relates to a high-strength austenitic stainless steel having excellent low-temperature toughness.

[0002] Recently, research and development into utilizing various eco-friendly energies has been increasing from the perspective of protecting the global environment. Accordingly, the need to develop materials that can be used in various industrial fields, including equipment, containers, and components, for the utilization of eco-friendly energy is also increasing.

[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 tanks and piping necessary for the storage and transportation of low-temperature liquefied gases is increasing. Maintaining an ultra-low temperature environment is essential for the transportation and storage of these low-temperature liquefied gases.

[0004] However, as operating temperatures approach cryogenic temperatures, it becomes increasingly challenging to manufacture stainless steel that offers superior corrosion resistance while also meeting the diverse physical properties required for each equipment, container, and component. Consequently, interest in stainless steel that offers both corrosion resistance and a variety of other properties is growing.

[0005] In order to solve the problems of the prior art as described above, the present invention aims to provide a high-strength austenitic stainless steel having excellent low-temperature toughness that can prevent the deterioration of toughness due to hydrogen and low temperature while improving strength.

[0006] In addition, the present invention aims to provide an austenitic stainless steel having high impact toughness at low temperatures by controlling precipitate content, which has a major influence on low-temperature toughness, and austenite stabilization, which is closely related to martensite formation.

[0007] In addition, the present invention aims to provide an austenitic stainless steel capable of improving austenite strength through optimization of alloying element content and ensuring ultra-low temperature toughness by maximally suppressing martensite transformation during deformation by securing the stability of austenite.

[0008] The problems to be solved by the present invention are not limited to the problems mentioned above, and other problems not mentioned will be clearly understood by those skilled in the art from the description below.

[0009] In order to achieve the above object, the present invention includes an austenitic stainless steel which contains, in wt%, C: more than 0% and 0.10% or less, Si: more than 0% and 1.5% or less, 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: more than 0% and 1.0% or less, the remainder Fe and other inevitable impurities, the content of precipitates is less than 0.001 wt%, and the Ni equivalent value of the following formula (1) is 27 or more:

[0010] Equation (1): Ni equivalent = Ni+0.65Cr+0.98Mo+1.05Mn+0.35Si+12.6C+33.6N

[0011] Additionally, the stainless steel according to one embodiment of the present invention may have a room temperature tensile strength / yield strength ratio of 2.0 or less.

[0012] In addition, the stainless steel according to one embodiment of the present invention may have a Charpy impact energy value of 70 J or more at -196°C.

[0013] In addition, the stainless steel according to one embodiment of the present invention may further include one of Mo: 2.0% or less and Nb: 0.05% or less.

[0014] Additionally, the stainless steel according to one embodiment of the present invention may have a room temperature yield strength of 300 MPa or more.

[0015] Additionally, the stainless steel according to one embodiment of the present invention may have a room temperature tensile strength of 600 MPa or more.

[0016] Additionally, the stainless steel according to one embodiment of the present invention may have an austenite phase having an area fraction of 90% or more.

[0017] In addition, the method for manufacturing the austenitic stainless steel according to an embodiment of the present invention may include the steps of: preparing a slab, which contains, in wt%, C: more than 0% and 0.10% or less, Si: more than 0% and 1.5% or less, 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: more than 0% and 1.0% or less, the remainder Fe and other inevitable impurities, and has a precipitate content of less than 0.001 wt%, and an Ni equivalent value of the following formula (1) of 27 or more; hot-rolling the slab; hot-rolling annealing after the hot-rolling; final cold-rolling after the hot-rolling annealing; and final annealing after the cold-rolling.

[0018] Equation (1): Ni equivalent = Ni+0.65Cr+0.98Mo+1.05Mn+0.35Si+12.6C+33.6N

[0019] In addition, the hot rolling annealing according to one embodiment of the present invention can be performed at a temperature of 900 to 1200°C.

[0020] Additionally, the final annealing according to one embodiment of the present invention can be performed at a temperature of 900 to 1,200°C.

[0021] In addition, the stainless steel according to one embodiment of the present invention may further include one of Mo: 2.0% or less and Nb: 0.05% or less.

[0022] In addition, the stainless steel according to one embodiment of the present invention may have a tensile strength / yield strength ratio of 2.0 or less at room temperature.

[0023] In addition, the stainless steel according to one embodiment of the present invention may have a Charpy impact energy value of 70 J or more at -196°C.

[0024] Additionally, the stainless steel according to one embodiment of the present invention may have a room temperature yield strength of 300 MPa or more.

[0025] Additionally, the stainless steel according to one embodiment of the present invention may have a room temperature tensile strength of 600 MPa or more.

[0026] Additionally, the stainless steel according to one embodiment of the present invention may have an austenite phase having an area fraction of 90% or more.

[0027] The high-strength austenitic stainless steel of the present invention, which has excellent low-temperature toughness, has the effect of preventing a decrease in toughness due to hydrogen and low temperature while improving strength by controlling precipitates that have a major effect on low-temperature toughness and austenite stabilization that is closely related to martensite formation.

[0028] Furthermore, by optimizing the content of alloy elements, strength can be improved, and by ensuring the stability of austenite, martensite transformation can be suppressed as much as possible during deformation, thereby securing ultra-low temperature toughness.

[0029] The effects of the present invention are not limited to those mentioned above, and other effects not mentioned will be clearly understood by those skilled in the art from the description below.

[0030] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. The following embodiments are provided to sufficiently convey the spirit 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 attached drawings are merely intended to facilitate understanding of the spirit of the present invention and should not be construed as limiting the spirit of the present invention.

[0031] Throughout the specification, whenever a part is said to "include" a component, this does not mean that it excludes other components, but rather that it may include other components, unless otherwise stated.

[0032] Singular expressions include plural expressions unless the context clearly indicates otherwise.

[0033] Temperature is a major factor in the embrittlement of steel materials. Therefore, to evaluate the use of steel materials in cryogenic environments, such as liquid hydrogen, it is necessary to measure their toughness at cryogenic temperatures.

[0034] Steel exposed to hydrogen environments is likely to be exposed not only to hydrogen but also to a wide range of temperatures. As temperatures decrease, materials tend to exhibit reduced toughness and brittleness, so even materials that appear to be fine at room temperature can exhibit deteriorating properties as the temperature decreases.

[0035] In general, it is known that austenite structure is advantageous for low-temperature toughness, while martensite structure or ferrite structure are known to be disadvantageous for low-temperature toughness.

[0036] Therefore, the alloy that is advantageous for cryogenic environments such as liquefied hydrogen is the 300 series stainless steel with an austenitic structure, and currently, 304L and 316L are mainly used. Although these commercial stainless steels 304L and 316L have relatively excellent low-temperature toughness, their low strength means that the material thickness increases when manufacturing cryogenic tanks or structures. Therefore, if the material strength is increased compared to 304L and 316L, the thickness of the material used can be reduced, which can reduce the amount of material used and also help reduce the cost of manufacturing cryogenic tanks.

[0037] Meanwhile, representative methods for increasing the strength of materials include cold working and precipitation strengthening using precipitates.

[0038] However, the method using cold working has a problem in that transformation of austenite to martensite may occur, and hydrogen embrittlement or a decrease in low-temperature toughness may occur due to the transformed martensite.

[0039] In addition, the method using precipitation strengthening by precipitates is not suitable for use in an ultra-low temperature hydrogen environment because the ultra-low temperature toughness deteriorates due to the precipitates.

[0040] In addition, the method of improving strength by using cold working or precipitation strengthening using precipitates may have limitations in application due to the additional process costs for cold working and precipitation as well as the deterioration of the material properties.

[0041] Therefore, it is necessary to develop a material with high stability and high strength of austenite structure through control of alloy composition rather than improving strength through cold working or precipitation strengthening.

[0042] The present invention aims to provide an austenitic stainless steel capable of simultaneously securing high strength and ultra-low temperature toughness by controlling the precipitate content to not affect ultra-low temperature toughness through optimization of the alloying element content and simultaneously increasing the Ni equivalent value to secure austenite phase stabilization, thereby suppressing martensite transformation as much as possible even during deformation, and further suppressing work hardening, which means the formation of martensite that is disadvantageous to ultra-low temperature toughness, as much as possible by lowering the tensile strength / yield strength ratio.

[0043] The high-strength austenitic stainless steel of the present invention having excellent low-temperature toughness may include, in wt%, C: more than 0% and 0.10% or less, Si: more than 0% and 1.5% or less, 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: more than 0% and 1.0% or less, the remainder being Fe and other unavoidable impurities.

[0044] Below, the reasons for limiting the composition of the above lecture are explained in detail. Unless otherwise specified, the composition of the above lecture lecture refers to weight %.

[0045] Carbon (C): 0% or more and 0.10% or less

[0046] C is an effective element for stabilizing the austenite phase, suppressing delta (δ) ferrite, and increasing strength through solid solution strengthening. However, if the C content exceeds 0.10%, it can easily combine with carbide-forming elements such as Cr, Ti, and Nb, thereby reducing the corrosion resistance, ductility, and toughness of the base material. Therefore, it is preferable that C be included in an amount exceeding 0% and not more than 0.10%, more preferably in an amount of 0.01% to 0.08%, and most preferably in an amount of 0.01% to 0.06%.

[0047] Silicon (Si): 0% or more and 1.5% or less

[0048] Si is an element effective in improving corrosion resistance and strengthening solid solutions. However, Si is a ferrite stabilizing element, and if its content exceeds 1.5%, it may form intermetallic compounds such as sigma phases, thereby reducing the ductility and toughness of the base material. Therefore, it is preferable that Si be included in an amount exceeding 0% and not exceeding 1.5%, more preferably in an amount of 0.01% to 1.2%, and most preferably in an amount of 0.1% to 1.1%.

[0049] Chromium (Cr): 17.0% to 23.0%

[0050] Chromium (Cr) is an essential element for improving corrosion resistance in stainless steel. To ensure this, a Cr content of at least 17% is required. However, if the Cr content exceeds 23%, it can promote excessive delta (δ) ferrite formation, reducing the steel's hot workability. Furthermore, austenite becomes unstable, requiring a large amount of nickel (Ni) to ensure phase stability, potentially increasing costs. Therefore, a Cr content of 17% to 23% is recommended.

[0051] Ni and Mn, along with N, are powerful austenite-stabilizing elements. Mn, in particular, can replace expensive Ni. Furthermore, because Ni and Mn are important elements in terms of low-temperature toughness, the desired low-temperature toughness can only be achieved by adding Mn and Ni in an appropriate ratio.

[0052] Manganese (Mn): 0.5% to 8.0%

[0053] Since excessive addition of Mn can lower the low-temperature toughness of the material due to a decrease in stacking fault energy, additional addition of Ni is necessary to obtain sufficient low-temperature toughness, but this can be disadvantageous in terms of cost due to the addition of expensive Ni. Therefore, Mn is preferably included in an amount of 0.5% to 8.0%, more preferably in an amount of 0.8% to 7.8%, and most preferably in an amount of 1.0% to 7.0%.

[0054] Nickel (Ni): 5.5% to 12.0%

[0055] Ni is an element that is advantageous in terms of austenite stabilization effect and low-temperature toughness, but it is preferable to add 5.5% or more to suppress delta (δ) ferrite formation during the manufacturing process. However, if the Ni content exceeds 12.0%, the probability of surface defects during the manufacturing process increases and may lead to an increase in price. Therefore, the Ni content is preferably included in an amount of 5.5% to 12.0%, more preferably in an amount of 5.5% to 11.0%, and most preferably in an amount of 6.0% to 10.0%.

[0056] Nitrogen (N): 0.10% to 0.30%

[0057] Nitrogen is an austenite stabilizing element and is also effective in increasing strength through solid solution strengthening. Therefore, it is desirable to add Ni in an amount of 0.10% or more. However, if the Ni content exceeds 0.30%, it may cause a decrease in ultra-low temperature toughness due to a decrease in productivity and a decrease in stacking fault energy. Therefore, it is desirable to include Ni in an amount of 0.10% to 0.30%, more preferably 0.10% to 0.25%, and most preferably 0.15% to 0.21%.

[0058] Copper (Cu): 0% or more and 1.0% or less

[0059] Cu is an element useful for stabilizing the austenite phase and can be used as a substitute for expensive nickel. It is an element for suppressing martensite formation during forming and increasing austenite stabilization, but if used in excess of 1.0%, it forms a low-melting-point phase, reducing hot workability and degrading surface quality. Therefore, it is preferable to include Cu in an amount exceeding 0% and not exceeding 1.0%, more preferably exceeding 0.01% and not exceeding 1.0%, and most preferably 0.1% to 0.9%.

[0060] The stainless steel of the present invention including the alloy composition as described above may further include, in wt%, either Mo: 2.0% or less and Nb: 0.05% or less.

[0061] Molybdenum (Mo): 2.0% or less

[0062] Mo is an effective element for improving corrosion resistance in stainless steel. However, if its content exceeds 2%, it can cause a decrease in low-temperature toughness due to an increase in the ferrite fraction, which can lead to a rise in price. Therefore, it is preferable that Mo be contained at 2% or less, more preferably 1.6% or less, and most preferably 0.01% to 1.0%. In this case, the mechanical properties and corrosion resistance required for applications in hydrogen and low-temperature environments can be further improved.

[0063] Niobium (Nb): 0.05% or less

[0064] Niobium (Nb) is an element that helps improve strength by forming precipitates. However, Nb precipitates can be a major factor in reducing 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-enhancing effect due to precipitate formation can be further enhanced.

[0065] The remaining component of the present invention is iron (Fe). However, during the typical manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed in, and thus, this cannot be ruled out. Since these impurities are readily apparent to anyone skilled in the art of typical manufacturing, their full details are not specifically discussed in this specification.

[0066] The austenitic stainless steel of the present invention can simultaneously satisfy austenite stabilization, low-temperature toughness, and high strength by appropriately controlling the content of the components as described above, thereby controlling the precipitate content and Ni equivalent value. In particular, by optimizing the content of alloy elements such as Ni, Mn, N, and Cu, which are advantageous elements for stabilizing the austenite structure, it has a major influence on the ultra-low-temperature toughness of the hydrogen steel, increases the Ni equivalent value representing the austenite phase stabilization, and at the same time, controls the precipitate to 0.001 wt% or less, thereby suppressing martensite transformation as much as possible even during deformation, thereby securing not only strength but also ultra-low-temperature toughness.

[0067] Additionally, the austenitic stainless steel according to one embodiment of the present invention may contain less than 0.001 wt% of precipitates.

[0068] In the present invention, precipitates refer to all precipitates precipitated in steel, and may also include Cr, Nb-based single or composite carbonitrides and metal precipitates such as Cu.

[0069] While precipitates are highly effective in securing strength, they can also act as crack initiation or propagation sites, reducing the steel's impact toughness. Furthermore, precipitate formation can also affect the steel's cryogenic toughness. Therefore, it is crucial to appropriately control the precipitate content to simultaneously secure both strength and cryogenic toughness.

[0070] Therefore, in the present invention, the optimal alloying components capable of simultaneously securing the strength of the steel and the toughness at extremely low temperatures are determined, and by optimally controlling the content thereof, the content of precipitates can be included at less than 0.001 wt%. If the content of the precipitates exceeds 0.001 wt%, the impact toughness of the steel may be reduced, making it difficult to secure a high-strength steel grade, and the toughness at extremely low temperatures may be reduced, making the steel unsuitable for use in extremely low-temperature environments such as high-pressure gas or liquid hydrogen storage containers and piping.

[0071] In addition, the austenitic stainless steel according to one embodiment of the present invention adjusts the Ni equivalent value of the following formula (1) to 27 or more.

[0072] Equation (1): Ni equivalent = Ni+0.65Cr+0.98Mo+1.05Mn+0.35Si+12.6C+33.6N

[0073] When the Ni equivalent value is less than 27, martensite transformation occurs during deformation and cannot contribute to stabilizing the austenite phase. Accordingly, it may be difficult to obtain the high strength and low-temperature toughness desired in the present invention, and in particular, it is impossible to obtain a room-temperature tensile strength / yield strength ratio of 2.0 or less. Therefore, the Ni equivalent value is preferably 27 or more, more preferably 29 or more, and most preferably 30 or more.

[0074] In addition, the austenitic stainless steel according to one embodiment of the present invention may have a Ni equivalent value of 27 or more, and thus may have a room temperature tensile strength / yield strength ratio of 2.0 or less.

[0075] When the ratio of the above room temperature tensile strength / yield strength exceeds 2.0, it is difficult to suppress work hardening that causes martensite transformation, which reduces the austenite stabilization effect and may have a negative effect on strength and ultra-low temperature toughness.

[0076] A tensile strength / yield strength ratio of less than 2.0 refers to a steel grade with relatively low yield strength but high tensile strength. Generally, among austenitic stainless steels, steel grades with low yield strength and high tensile strength can be obtained through work hardening. However, as work hardening increases, martensitic transformation, which is detrimental to ultra-low temperature toughness, occurs more frequently. In this case, strength increases but ultra-low temperature toughness relatively decreases. However, because martensitic transformation occurs as work hardening occurs, it is almost impossible for the martensitic transformation to be 0.

[0077] In addition, the austenitic stainless steel according to one embodiment of the present invention can satisfy a Charpy impact energy value of 70 J or more at -196°C by controlling the precipitate content to 0.001 wt% or less.

[0078] The Charpy impact energy value is a value obtained through a Charpy impact test. The Charpy impact test involves forming a material into a plate approximately 10 mm thick, drilling a small notch in the center, installing the specimen in a testing device, and applying impact with a hammer at different temperatures. If the Charpy impact energy value, which is the cryogenic impact toughness, is less than 70 J, it may be difficult to use in cryogenic environments, making it impossible to apply it as a material for liquefied hydrogen storage containers and piping.

[0079] Therefore, in the present invention, by simultaneously controlling the precipitate content and the Ni equivalent value to increase the austenite stabilization, the room temperature tensile strength / yield strength ratio can be satisfied as 2.0 or less, and the Charpy impact energy value at -196℃ can be satisfied as 70J or more. Specifically, in the present invention, by adjusting the Ni equivalent value to 27 or more, the room temperature tensile strength / yield strength ratio can be secured as a low value of 2.0 or less, thereby suppressing work hardening, which means the formation of martensite, which is detrimental to ultra-low temperature toughness, as much as possible. In addition, the present invention can manufacture an austenitic stainless steel that can satisfy both high strength and low temperature toughness by satisfying the Charpy impact energy value at -196℃ as 70J or more by controlling the precipitate content to less than 0.001%.

[0080] The austenitic stainless steel according to one embodiment of the present invention can satisfy a room temperature yield strength of 300 MPa or more.

[0081] When an object is pulled with a force exceeding a certain amount and the force is released, it cannot return to its original state and instead grows longer. The maximum force required to return to its original state is called the yield strength. Increasing the strength of steel reduces the amount of steel used to manufacture a product with the same strength. Therefore, the present invention provides stainless steel with superior strength, thereby reducing product costs.

[0082] If the yield strength at room temperature is less than 300 MPa, it may be difficult to obtain a high-strength austenitic stainless steel with excellent ultra-low-temperature toughness. The upper limit of the yield strength at room temperature is not limited, but may be, for example, 700 MPa or less, 650 MPa or less, 600 MPa or less, 550 MPa or less, 500 MPa or less, or 450 MPa or less in order to satisfy the mechanical properties, corrosion resistance, strength, etc. required for application to hydrogen and low-temperature environments.

[0083] In addition, the austenitic stainless steel according to one embodiment of the present invention can satisfy a room temperature tensile strength of 600 MPa or more. The upper limit of the room temperature tensile strength is not limited, but may be, for example, 800 MPa or less, 750 MPa or less, 700 MPa or less, 650 MPa or less, etc., in order to satisfy the mechanical properties, corrosion resistance, strength, etc. required for application to hydrogen and low temperature environments.

[0084] In addition, a method for manufacturing an austenitic stainless steel having excellent low-temperature toughness according to an embodiment of the present invention may include the steps of: preparing a slab containing, in wt%, C: more than 0% and 0.10% or less, Si: more than 0% and 1.5% or less, 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: more than 0% and 1.0% or less, the remainder Fe and other inevitable impurities, and having a precipitate content of less than 0.001 wt% and a Ni equivalent value of Formula (1) of 27 or more; hot-rolling the slab; hot-rolling annealing after the hot-rolling; final cold-rolling after the hot-rolling annealing; and final annealing after the cold-rolling.

[0085] During the hot rolling and annealing process following the above hot rolling, the annealing temperature significantly affects the release of residual stress and the microstructure. Therefore, the hot rolling annealing is preferably performed at a temperature of 900 to 1,200°C.

[0086] If the above hot rolling annealing temperature is less than 900℃, coarse carbides are generated, making the structure uneven, or Cr is present around the grain boundaries. 23 Since C6 precipitates may be formed and intergranular corrosion may occur, and if it exceeds 1,200°C, the grains may become extremely coarsened, it is desirable to limit the annealing temperature to 900 to 1,200°C, more preferably 950 to 1,150°C, and most preferably 1,000 to 1,150°C.

[0087] In addition, after the above hot rolling annealing, a step of cold rolling followed by final annealing can be performed. The annealing after the above cold rolling can be performed at a temperature of 900 to 1,200°C.

[0088] The austenitic stainless steel of the present invention manufactured by this method may have an area fraction of 90% or more of an austenite phase and a density of carbides of 0.001 wt% or less.

[0089] As the development and distribution of fuel cell vehicles that use hydrogen as fuel expands, the development of containers and components for storing hydrogen has become necessary.

[0090] Hydrogen storage tanks can be divided into liquid hydrogen and gaseous hydrogen depending on the form of hydrogen. The operating temperature varies depending on the form of hydrogen. Liquid hydrogen is stored in an extremely low temperature environment of -253℃, and inside the liquid hydrogen tank, there is hydrogen that has been vaporized from liquid hydrogen. Furthermore, since steel is exposed to a temperature range from -253℃ to room temperature in the device that vaporizes liquid hydrogen, the steel's physical properties must not deteriorate due to hydrogen at various temperatures. Gaseous hydrogen is generally stored at room temperature, but it is pre-cooled to -40~-60℃ before being filled into the storage tank. This is because the gas temperature rises during filling, and it is cooled through a precooler to prevent excessive temperature rise due to filling.

[0091] In particular, liquefied hydrogen storage is expected to be used in a variety of fields in the future due to its higher storage efficiency compared to gaseous hydrogen. It is anticipated that liquefied hydrogen will be used for long-distance transport of hydrogen from overseas to Korea and for large-scale storage at hydrogen charging stations and hydrogen production plants.

[0092] Therefore, when considering steel for hydrogen storage tanks, the degradation of properties at both room temperature and cryogenic temperatures can be a key factor in determining the steel material. Considering this, hydrogen storage tanks and their peripheral equipment require protection against hydrogen and cryogenic degradation, high mechanical strength, and corrosion resistance.

[0093] Currently, materials commonly used in hydrogen gas and liquid hydrogen environments are austenitic stainless steels, types 304L and 316L. Even materials that appear to be harmless at room temperature often exhibit deteriorating properties as the temperature decreases. In particular, a decrease in toughness is a major problem that occurs with decreasing temperature, and this is one of the main causes of martensitic transformation from austenite to austenite.

[0094] In the austenite phase, the hydrogen diffusion rate is slow, making it difficult for hydrogen to move, making it difficult for hydrogen embrittlement to occur. In addition, compared to the martensite phase, it is a soft phase and can easily secure toughness even at extremely low temperatures, but compared to the austenite phase, the martensite phase is a hard phase and is prone to embrittlement, and the hydrogen diffusion rate is fast, increasing the possibility of hydrogen embrittlement. Therefore, if the martensite fraction is high when exposed to a hydrogen environment, hydrogen may be incorporated into the material, causing a deterioration in the material properties due to hydrogen, which may cause problems when used in liquid hydrogen and gaseous hydrogen environments.

[0095] Furthermore, materials used in hydrogen storage tanks, etc., have design thicknesses determined by strength. Therefore, improving strength can reduce the amount of material used in hydrogen storage tanks. Precipitates can be used to increase strength, but since precipitates are a major cause of low-temperature ductility degradation, controlling the precipitate content within the steel is essential for application in hydrogen environments.

[0096] The present invention will be described in more detail below using examples. These examples are provided for illustrative purposes only and are not intended to limit the scope of protection of the present invention.

[0097] Example 1

[0098] After hot rolling an austenitic slab having the composition shown in Table 1 below, annealing was performed at a temperature of 900 to 1200°C.

[0099] The alloy compositions of each example and comparative example are as shown in Table 1 below.

[0100] Classification CSiMnCrNiMoCuNNbExample 10.020.41.920.49.40.80.60.19-Example 20.020.46.518.15.61.60.90.21-Example 30.020.46.617.95.61.60.90.16-Example 40.020.40.821.510.40.80.60.20-Example 50.020.51.018.38.00.40.40.16-Example 60.020.44.617.97.81.10.90.20-Example 70.020.41.219.69.30 .80.80.15-Example 80.020.44.119.08.50.40.40.20-Example 90.020.43.620.28.40.80.80.19-Example 100.020.47.617.55.50.40.40.21-Example 110.020.45.717.35.60.40.40.18-Example 120.031.10.820.39.8-0.40.160.02Comparative Example 10.020.61.116.110.20.32.10.02-Comparative Example 20.020.41.218.810.20.4- 0.02-Comparative Example 30.050.4118.18.1000.04-Comparative Example 40.020.46.318.36.41.20.50.210.11Comparative Example 50.060.41.118.38000.04-Comparative Example 60.020.41.418.18000.04-Comparative Example 70.020.45.317.65.90.40.50.210.10Comparative Example 80.020.51.316.710.10.32.10.05-

[0101] To confirm the impact toughness value at cryogenic temperatures, a Charpy impact test was conducted at -196℃ using the experimental steels of the examples and comparative examples in Table 1 above, and yield strength and tensile strength were measured by performing a tensile test in the air at room temperature. The Charpy impact energy value was determined by performing an impact test at a temperature of -196℃ using the ASTM E23 type A specimen standard. The tensile test was conducted according to the ASTM E8 standard. The precipitate content was measured through quantitative analysis of the precipitate using the residue extraction method.

[0102] The -196℃ Charpy impact toughness value, precipitate content (weight%), room temperature yield strength, room temperature tensile strength, tensile strength / yield strength ratio, and Ni equivalent value of the experimental steel grades according to the above examples and comparative examples are shown in Table 2 below.

[0103] Classification-196℃Impact energy value (J)Precipitate content (wt%)Yield strength (MPa)Tensile strength (MPa)Tensile strength / yield strength ratioNi Equivalent value Example 1 135 < 0.0013836941.832 Example 2 101 < 0.0013596591.832 Example 3 124 < 0.0013236271.930 Example 4 126 < 0.0013626771.933 Example 5 109 < 0.0013166472.027 Example 6 121 < 0.0013426431.931 Example 7 133 < 0.0013056152.029 Example 8 150 < 0.0013306421.932 Example 9 140 < 0.0013 446521.932 Example 10115<0.0013256622.032 Example 11106<0.0013356842.029 Example 12124<0.0014057131.830 Comparative Example 1164<0.0012385562.325 Comparative Example 2198<0.0012135402.525 Comparative Example 3148<0.0012837122.523 Comparative Example 4690.0253846901.833 Comparative Example 5135<0.0012946772.323 Comparative Example 6143 <0.0012846402.323Comparative Example 7560.0163947251.831Comparative Example 8156<0.0012776152.226

[0104] Looking at the results in Table 2 above, it was confirmed that Examples 1 to 12 according to the present invention satisfied the alloy composition suggested in the present invention, while satisfying the Ni equivalent value of 27 or more and the precipitate content of less than 0.001%, satisfying the room temperature tensile strength / yield strength ratio of 2.0 or less, and the Charpy impact energy value at -196°C was high at 70 J or more. In addition, the yield strength at room temperature is 305 to 405 MPa, the tensile strength is 615 to 713 MPa, and the tensile strength is 615 to 713 MPa, and the tensile strength / yield strength ratio is 2.0 or less, so that martensitic transformation is suppressed as much as possible during deformation or processing, thereby showing excellent ultra-low temperature toughness, and from this, it was found that the austenitic stainless steel of the present invention can be applied as a material for liquefied hydrogen such as liquefied hydrogen storage containers and pipes. Meanwhile, Comparative Examples 1 to 3, 5 to 6, and 8 have alloy compositions that do not satisfy the N content, and the precipitate content is satisfied at 0.001 wt% or less, but the stability of the austenite phase is low, so the Ni equivalent value was less than 27 due to martensitic transformation, which is unfavorable for ultra-low temperature toughness due to work hardening, and as a result, the room temperature tensile strength / yield strength ratio exceeds 2.0, so it was found that it will be difficult to apply it as a material for liquefied hydrogen.

[0105] In addition, Comparative Examples 4 and 7 are alloy compositions that satisfy the N content, and despite the stable austenite stabilization with a tensile strength / yield strength ratio of 2.0 or less due to a Ni equivalent value of 27 or more, the precipitate content exceeded 0.001 wt%, resulting in a low Charpy impact energy value of 70 J or less at -196°C. From this, it was found that it would be difficult to apply them as materials for liquefied hydrogen used in ultra-low temperature environments.

[0106] From the above results, according to the present invention, by optimizing the alloy composition and component content of the austenitic stainless steel, the precipitation amount is controlled to 0.001 wt% or less, and the Ni equivalent value representing the austenite phase stability is controlled to 27 or more, thereby satisfying the room temperature tensile strength / yield strength ratio to 2.0 or less and the Charpy impact energy value at -196℃ to 70 J or more. In addition, it was found that the austenitic stainless steel having excellent ultra-low temperature toughness and strength can be provided by satisfying the room temperature yield strength to 300 MPa or more and the room temperature tensile strength to 600 MPa or more.

[0107] In addition, the present invention was able to obtain an austenitic stainless steel suitable for use as a material for liquefied hydrogen, such as a liquefied hydrogen storage container or pipe, without deterioration of the material's properties due to hydrogen, by maximally suppressing martensite transformation during deformation or processing through austenite stabilization, thereby satisfying low-temperature toughness.

[0108] The embodiments described in this specification and the attached drawings are merely illustrative of some of the technical concepts encompassed by the present invention. Therefore, the embodiments disclosed in this specification are intended to explain, not limit, the technical concepts of the present invention, and it is obvious that the scope of the technical concepts of the present invention is not limited by these embodiments. All modifications and specific embodiments that can be easily inferred by a person of ordinary skill in the art within the scope of the technical concepts contained in the specification and drawings of the present invention should be construed as being included within the scope of the rights of the present invention.

Claims

1. Austenitic stainless steel containing, in wt%, C: more than 0% and 0.10% or less, Si: more than 0% and 1.5% or less, 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: more than 0% and 1.0% or less, the remainder being Fe and other unavoidable impurities, the precipitate content being less than 0.001 wt%, 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. In paragraph 1, The above stainless steel is an austenitic stainless steel having a room temperature tensile strength / yield strength ratio of 2.0 or less.

3. In paragraph 1, The above stainless steel is an austenitic stainless steel having a Charpy impact energy value of 70 J or more at -196°C.

4. In paragraph 1, The above stainless steel is an austenitic stainless steel further containing either Mo: 2.0% or less and Nb: 0.05% or less.

5. In paragraph 1, The above stainless steel is an austenitic stainless steel with a room temperature yield strength of 300 MPa or more.

6. In paragraph 1, The above stainless steel is an austenitic stainless steel with a room temperature tensile strength of 600 MPa or more.

7. In paragraph 1, The above stainless steel is an austenitic stainless steel having an area fraction of 90% or more of the austenite phase.

8. A step for preparing a slab, which contains C: more than 0% and 0.1% or less, Si: more than 0% and 1.5% or less, Cr: 17% to 23%, Ni: 5.5% to 12%, Mn: 0.5% to 8%, N: 0.1% to 0.3%, Cu: more than 0% and 1.0% or less, the remainder being Fe and other unavoidable impurities, and having a precipitate content of less than 0.001% by weight and a Ni equivalent value of the following formula (1) of 27 or more; A step of hot rolling the above slab; A step of hot annealing after the above hot rolling; The step of final cold rolling after the above hot rolling annealing; and The final annealing step after the above cold rolling; A method for manufacturing an austenitic stainless steel comprising: Equation (1): Ni equivalent = Ni+0.65Cr+0.98Mo+1.05Mn+0.35Si+12.6C+33.6N 9. In paragraph 8, The above hot rolling annealing is a method for manufacturing austenitic stainless steel, performed at a temperature of 900 to 1200°C.

10. In paragraph 8, A method for manufacturing austenitic stainless steel, wherein the final annealing is performed at a temperature of 900 to 1,200°C.

11. In paragraph 8, A method for manufacturing an austenitic stainless steel, wherein the stainless steel further contains either Mo: 2.0% or less and Nb: 0.05% or less.

12. In paragraph 8, The above stainless steel is a method for manufacturing austenitic stainless steel having a tensile strength / yield strength ratio of 2.0 or less at room temperature.

13. In paragraph 8, The above stainless steel is a method for manufacturing an austenitic stainless steel having a Charpy impact energy value of 70 J or more at -196°C.

14. In paragraph 8, The above stainless steel has a yield strength of 300 MPa or more at room temperature, A method for manufacturing austenitic stainless steel having a room temperature tensile strength of 600 MPa or more.

15. In paragraph 8, The above stainless steel is a method for manufacturing austenitic stainless steel having an area fraction of 90% or more of the austenite phase.