HIGH-STRENGTH AUSTENITIC STAINLESS STEEL HAVING EXCELLENT LOW-TEMPERATURE TOUGHNESS

Abstract

 The present disclosure relates to a high-strength austenitic stainless steel having excellent low-temperature toughness and a manufacturing method therefor, and more particularly, to an austenitic stainless steel and a manufacturing method therefor, comprising, in percent by weight (wt%), more than 0% and 0.10% or less of carbon (C), more than 0% and 1.5% or less of silicon (Si), 17.0% to 23.0% of chromium (Cr), 5.5% to 12.0% of nickel (Ni), 0.5% to 8.0% of manganese (Mn), 0.10% to 0.30% of nitrogen (N), more than 0% and 1.0% or less of copper (Cu), the remainder of iron (Fe) and inevitable impurities, wherein a content of precipitates is less than 0.001 wt%, and a Ni equivalent value of Formula (1) below is 27 or more.

Description

[Technical Field]

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

[Background Art]

[0002] Recently, research and development for utilizing various eco-friendly energies have been increasing from the perspective of global environmental protection. Accordingly, the necessity for the development of materials that can be used in various industrial fields including facilities, containers, parts, and the like for the use of eco-friendly energy is also increasing.

[0003] For example, demand for tanks and piping required for storage and transportation of low-temperature liquefied gas is increasing in accordance with the increase in demand and market growth of liquefied natural gas (LNG), liquefied petroleum gas (LPG), and liquefied hydrogen. It is necessary to maintain a cryogenic environment for the transportation and storage of these low-temperature liquefied gases.

[0004] However, the closer the temperature of the usage environment is to cryogenic temperatures, the more difficult it is to manufacture a stainless steel that has excellent corrosion resistance while satisfying various physical properties required for respective facilities, containers, parts, and the like. Therefore, interest in stainless steel capable of satisfying not only corrosion resistance but also various physical properties is increasing.

[Disclosure]

[Technical Problem]

[0005] To solve the problems of the prior art as described above, an object of the present disclosure is to provide a high-strength austenitic stainless steel having excellent low-temperature toughness capable of preventing toughness deterioration caused by hydrogen and low temperatures and simultaneously improving strength.

[0006] In addition, an object of the present disclosure is 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, an object of the present disclosure is to provide an austenitic stainless steel capable of improving austenite strength through optimization of alloying element contents and securing cryogenic toughness by securing the stability of austenite to suppress martensite transformation as much as possible during deformation.

[0008] The problems to be solved by the present disclosure 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 following description.

[Technical Solution]

[0009] To achieve the above object, the present disclosure comprises an austenitic stainless steel comprising, in percent by weight (wt%), more than 0% and 0.10% or less of C, more than 0% and 1.5% or less of Si, 17.0% to 23.0% of Cr, 5.5% to 12.0% of Ni, 0.5% to 8.0% of Mn, 0.10% to 0.30% of N, more than 0% and 1.0% or less of Cu, the remainder of Fe and inevitable impurities, wherein a content of precipitates is less than 0.001 wt%, and a Ni equivalent value of Formula (1) below is 27 or more:

[0010] In addition, the stainless steel according to an embodiment of the present disclosure may have a ratio of room temperature tensile strength to yield strength of 2.0 or less.

[0011] In addition, the stainless steel according to an embodiment of the present disclosure may have a Charpy impact energy value at -196°C of 70 J or more.

[0012] In addition, the stainless steel according to an embodiment of the present disclosure may further comprise any one of 2.0% or less of Mo and 0.05% or less of Nb.

[0013] In addition, the stainless steel according to an embodiment of the present disclosure may have a room temperature yield strength of 300 MPa or more.

[0014] In addition, the stainless steel according to an embodiment of the present disclosure may have a room temperature tensile strength of 600 MPa or more.

[0015] In addition, the stainless steel according to an embodiment of the present disclosure may have an austenite phase in an area fraction of 90% or more.

[0016] In addition, a method for manufacturing the austenitic stainless steel according to an embodiment of the present disclosure may comprise: preparing a slab comprising, in percent by weight (wt%), more than 0% and 0.10% or less of C, more than 0% and 1.5% or less of Si, 17.0% to 23.0% of Cr, 5.5% to 12.0% of Ni, 0.5% to 8.0% of Mn, 0.10% to 0.30% of N, more than 0% and 1.0% or less of Cu, the remainder of Fe and inevitable impurities, wherein a content of precipitates is less than 0.001 wt%, and a Ni equivalent value of Formula (1) below is 27 or more; hot rolling the slab; hot-rolled annealing after the hot rolling; final cold rolling after the hot-rolled annealing; and final annealing after the cold rolling.

[0017] 

[0018] In addition, the hot-rolled annealing according to an embodiment of the present disclosure may be performed at a temperature of 900 to 1200°C.

[0019] In addition, the final annealing according to an embodiment of the present disclosure may be performed at a temperature of 900 to 1200°C.

[0020] In addition, the stainless steel according to an embodiment of the present disclosure may further comprise any one of 2.0% or less of Mo and 0.05% or less of Nb.

[0021] In addition, the stainless steel according to an embodiment of the present disclosure may have a ratio of room temperature tensile strength to yield strength of 2.0 or less.

[0022] In addition, the stainless steel according to an embodiment of the present disclosure may have a Charpy impact energy value at -196°C of 70 J or more.

[0023] In addition, the stainless steel according to an embodiment of the present disclosure may have a room temperature yield strength of 300 MPa or more.

[0024] In addition, the stainless steel according to an embodiment of the present disclosure may have a room temperature tensile strength of 600 MPa or more.

[0025] In addition, the stainless steel according to an embodiment of the present disclosure may have an austenite phase in an area fraction of 90% or more.

[Advantageous Effects]

[0026] The high-strength austenitic stainless steel having excellent low-temperature toughness of the present disclosure has the effect of preventing toughness deterioration caused by hydrogen and low temperatures and simultaneously improving strength by regulating precipitates, which have a major influence on low-temperature toughness, and austenite stabilization, which is closely related to martensite formation.

[0027] Furthermore, there is an effect that strength may be improved through optimization of alloying element contents, and cryogenic toughness may be secured by securing the stability of austenite to suppress martensite transformation as much as possible during deformation.

[0028] The effects of the present disclosure are not limited to those mentioned above, and other effects not mentioned will be clearly understood by those skilled in the art from the following description.

[Mode for Invention]

[0029] Hereinafter, preferred embodiments according to the present disclosure will be described in detail with reference to the accompanying drawings. The following embodiments are presented to fully convey the spirit of the disclosed invention to those skilled in the art to which the disclosed invention pertains. The disclosed invention is not limited to the embodiments presented herein and may be embodied in other forms. In addition, it should be noted that the accompanying drawings are only for facilitating the understanding of the spirit of the present disclosure and should not be construed as limiting the spirit of the present disclosure by the accompanying drawings.

[0030] Throughout the specification, where a part is referred to as "comprising" a certain component, it means that it may further comprise other components rather than excluding other components, unless specifically stated to the contrary.

[0031] Expressions of the singular include expressions of the plural unless the context clearly indicates an exception.

[0032] A major cause of brittleness in steel materials is temperature. Therefore, in order to examine the use of steel materials in cryogenic environments such as liquid hydrogen, toughness at cryogenic temperatures must be measured.

[0033] Steel materials exposed to a hydrogen environment are highly likely to be exposed to various temperature ranges as well as the hydrogen environment. Since the toughness of a material tends to decrease and brittleness tends to appear as the temperature decreases, even materials that seem to have no problem at room temperature may show a tendency of material property degradation as the temperature decreases.

[0034] In general, it is known that an austenite structure is advantageous for low-temperature toughness, while a martensite structure or ferrite structure is relatively disadvantageous for low-temperature toughness.

[0035] Therefore, typical alloys advantageous for cryogenic environments such as liquid hydrogen are 300-series stainless steels having an austenite structure, and currently, 304L and 316L are mainly used. Although these commercial stainless steels, 304L and 316L, have relatively excellent low-temperature toughness, they have a disadvantage in that the material thickness increases upon manufacturing tanks or structures for cryogenic use due to low strength. Therefore, in a case where the strength of the material is increased compared to 304L and 316L, the thickness of the used material can be reduced, which reduces the amount of material used and can help reduce the manufacturing cost of tanks for cryogenic use.

[0036] Meanwhile, methods for increasing the strength of a material typically include a method using cold working and a method using precipitation strengthening by precipitates.

[0037] However, the method using cold working has a problem in that transformation from austenite to martensite occurs, and hydrogen embrittlement due to the transformed martensite or degradation of low-temperature toughness may occur.

[0038] In addition, the method using precipitation strengthening by precipitates causes degradation of cryogenic toughness due to precipitates, so it is not suitable for use in a cryogenic hydrogen environment.

[0039] Furthermore, strength improvement using the method by cold working or the method using precipitation strengthening by precipitates may have restrictions in application due to additional process costs for cold working and precipitate precipitation as well as degradation of material properties.

[0040] Therefore, there is a need for the development of a material with high stability of the austenite structure and high strength through control of alloy composition, rather than strength improvement by cold working or precipitation strengthening.

[0041] The present disclosure intends to provide an austenitic stainless steel capable of securing both high strength and cryogenic toughness by controlling precipitate content to a level that does not affect cryogenic toughness through optimization of alloying element contents, and simultaneously increasing a Ni equivalent value to secure austenite phase stabilization, thereby suppressing martensite transformation as much as possible even during deformation, and further lowering the ratio of tensile strength to yield strength to suppress work hardening, which means martensite formation disadvantageous to cryogenic toughness, as much as possible.

[0042] Such a high-strength austenitic stainless steel having excellent low-temperature toughness of the present disclosure may comprise, in percent by weight (wt%), more than 0% and 0.10% or less of C, more than 0% and 1.5% or less of Si, 17.0% to 23.0% of Cr, 5.5% to 12.0% of Ni, 0.5% to 8.0% of Mn, 0.10% to 0.30% of N, more than 0% and 1.0% or less of Cu, and the remainder of Fe and inevitable impurities.

[0043] Hereinafter, the reasons for limiting the component composition of the steel will be described in detail. The following component compositions mean percent by weight (wt%) unless otherwise specified.

Carbon (C): more than 0% and 0.10% or less

[0044] C is an element effective for stabilization of the austenite phase, suppression of delta (δ) ferrite, and strength increase by solid solution strengthening. However, in a case where the content of C exceeds 0.10%, it easily combines with carbide forming elements such as Cr, Ti, and Nb, which may deteriorate the corrosion resistance, ductility, toughness, etc., of the base material. Therefore, C is preferably included in an amount of more than 0% and 0.10% or less, more preferably 0.01% to 0.08%, and most preferably 0.01% to 0.06%.

Silicon (Si): more than 0% and 1.5% or less

[0045] Si is an element effective for corrosion resistance improvement and solid solution strengthening. However, since Si is a ferrite stabilizing element, in a case where its content exceeds 1.5%, intermetallic compounds such as a sigma phase may be formed, deteriorating the ductility and toughness of the base material. Therefore, Si is preferably included in an amount of more than 0% and 1.5% or less, more preferably 0.01% to 1.2%, and most preferably 0.1 to 1.1%.

Chromium (Cr): 17.0% to 23.0%

[0046] Cr is an element that must be added for corrosion resistance improvement in stainless steel, and 17% or more must be added to secure corrosion resistance. However, in a case where the content of Cr exceeds 23%, excessive delta (δ) ferrite may be promoted, deteriorating the hot workability of the steel material, and austenite becomes unstable, requiring a large amount of Ni for phase stability, which may cause a cost increase. Therefore, Cr is preferably included in an amount of 17% to 23%.

[0047] Ni and Mn are strong austenite phase stabilizing elements together with N, and in particular, Mn is an element that can replace expensive Ni. In addition, since Ni and Mn are important elements in terms of low-temperature toughness, desired low-temperature toughness can be secured only upon adding Mn and Ni in an appropriate ratio.

Manganese (Mn): 0.5% to 8.0%

[0048] In a case where the content of Mn is added excessively, the stacking fault energy decreases, which may deteriorate the low-temperature toughness of the material, so sufficient low-temperature toughness can be obtained only if Ni is additionally added, but this may 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 0.8% to 7.8%, and most preferably 1.0 to 7.0%.

Nickel (Ni): 5.5% to 12.0%

[0049] Ni is an element that becomes more advantageous for austenite stabilization effect and low-temperature toughness as it is added, but it is preferable to add 5.5% or more to suppress the formation of delta (δ) ferrite in the manufacturing process. However, in a case where the content of Ni exceeds 12.0%, the probability of surface defects in the manufacturing process increases, and it may cause a price increase. Therefore, the Ni content is preferably included in an amount of 5.5% to 12.0%, more preferably 5.5% to 11.0%, and most preferably 6.0% to 10.0%.

Nitrogen (N): 0.10% to 0.30%

[0050] N is an austenite stabilizing element and is also an element effective for increasing strength through solid solution strengthening. Therefore, the content of N is preferably added in an amount of 0.10% or more. However, in a case where the content of N exceeds 0.30%, it may cause productivity degradation and cryogenic toughness degradation due to a decrease in stacking fault energy. Therefore, N is preferably included in an amount of 0.10% to 0.30%, more preferably 0.10 to 0.25%, and most preferably 0.15% to 0.21%.

Copper (Cu): more than 0% and 1.0% or less

[0051] Cu is an element useful for stabilization of the austenite phase and can be used by substituting for expensive Ni. It is an element for suppressing martensite generation during forming and increasing austenite stabilization degree, but in a case of using in excess of 1.0%, a low melting point phase is formed, reducing hot workability and deteriorating surface quality. Therefore, Cu is preferably included in an amount of more than 0% and 1.0% or less, more preferably more than 0.01% and 1.0%, and most preferably 0.1% to 0.9%.

[0052] The stainless steel of the present disclosure comprising the alloy composition as described above may further comprise any one of 2.0% or less of Mo and 0.05% or less of Nb in percent by weight (wt%).

Molybdenum (Mo): 2.0% or less

[0053] Mo is an element effective for improving corrosion resistance in stainless steel, but in a case where its content exceeds 2%, it may cause degradation of low-temperature toughness due to an increase in ferrite fraction and may cause a price increase. Therefore, Mo is preferably included in an amount of 2% or less, more preferably 1.6% or less, and most preferably more than 0.01% and 1.0% or less. In this case, mechanical properties and corrosion resistance effects required for application to hydrogen and low-temperature environments can be further improved.

Niobium (Nb): 0.05% or less

[0054] Nb is an element that helps improve strength by forming precipitates, but Nb precipitates can act as a major cause of reducing impact toughness. Therefore, Nb is preferably included in an amount of 0.05% or less, and most preferably 0.02% or less. In this case, the strength improvement effect by precipitate formation can be further improved.

[0055] The remainder of the present disclosure is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be introduced in a typical manufacturing process, they cannot be excluded. Since such impurities are known to anyone skilled in the art of a typical manufacturing process, all details thereof are not specifically mentioned in this specification.

[0056] The austenitic stainless steel of the present disclosure can satisfy austenite stabilization, low-temperature toughness, and high strength simultaneously by controlling the precipitate content and Ni equivalent value by appropriately controlling the contents of the above components. In particular, by optimizing the contents of alloying elements such as Ni, Mn, N, and Cu, which are elements advantageous for austenite structure stabilization, the Ni equivalent value, which has a major influence on the cryogenic toughness of steel for hydrogen and represents the degree of austenite phase stabilization, is increased, and at the same time, precipitates are controlled to 0.001 wt% or less to suppress martensite transformation as much as possible even during deformation, thereby securing not only strength but also cryogenic toughness.

[0057] In addition, the austenitic stainless steel according to an embodiment of the present disclosure may comprise less than 0.001 wt% of precipitates.

[0058] In the present disclosure, precipitates mean all precipitates precipitating in steel, and may include Cr-based, Nb-based single or complex carbonitrides, and metal precipitates such as Cu.

[0059] Precipitates are very effective for securing strength, but may become crack initiation sites or crack propagation sites, causing a decrease in impact toughness of steel. In addition, the generation of precipitates can also affect the cryogenic toughness of steel. Accordingly, it is most important to secure not only strength but also cryogenic toughness simultaneously by appropriately controlling the content of precipitates.

[0060] Therefore, in the present disclosure, optimum alloy components capable of simultaneously securing the strength and cryogenic toughness of steel are determined, and by optimally controlling their contents, the content of precipitates can be included at less than 0.001 wt%. In a case where the content of the precipitates exceeds 0.001 wt%, the impact toughness of the steel is deteriorated, making it difficult to secure a high-strength steel grade, and the cryogenic toughness is deteriorated, making it unsuitable for use in cryogenic environments such as high-pressure gas or liquid hydrogen storage containers and piping.

[0061] In addition, the austenitic stainless steel according to an embodiment of the present disclosure controls the Ni equivalent value of Formula (1) below to 27 or more.

[0062] 

[0063] In a case where the Ni equivalent value is less than 27, martensite transformation occurs during deformation, failing to contribute to austenite phase stabilization, and accordingly, it may be difficult to obtain the high strength and low-temperature toughness desired in the present disclosure, and in particular, a ratio of room temperature tensile strength to yield strength of 2.0 or less cannot be obtained. Therefore, the Ni equivalent value is preferably 27 or more, more preferably 29 or more, and most preferably 30 or more.

[0064] In addition, the austenitic stainless steel according to an embodiment of the present disclosure satisfies a Ni equivalent value of 27 or more, so that the ratio of room temperature tensile strength to yield strength may be 2.0 or less.

[0065] In a case where the ratio of room temperature tensile strength to yield strength exceeds 2.0, it is difficult to suppress work hardening that causes martensite transformation, so the austenite stabilization effect is reduced, and strength and cryogenic toughness may be adversely affected.

[0066] A ratio of tensile strength to yield strength of 2.0 or less means a steel grade with relatively low yield strength but high tensile strength. Generally, in austenitic stainless steel, a steel grade with low yield strength and high tensile strength can be obtained through work hardening. However, as work hardening proceeds, martensite transformation, which is disadvantageous for cryogenic toughness, occurs more frequently, and in this case, strength increases but cryogenic toughness relatively decreases. However, since martensite transformation occurs as work hardening proceeds, it is almost impossible for martensite transformation to be zero.

[0067] In addition, the austenitic stainless steel according to an embodiment of the present disclosure can satisfy a Charpy impact energy value at -196°C of 70 J or more by controlling the precipitate content to 0.001 wt% or less.

[0068] The Charpy impact energy value is a value obtainable through a Charpy impact test. The Charpy impact test is a test in which a material is made into a plate with a thickness of about 10 mm, a small notch is dug in the center, the specimen is installed in a test apparatus, and an impact is applied with a hammer while varying the temperature. In a case where the Charpy impact energy value, which is cryogenic impact toughness, is less than 70 J, it is difficult to use in a cryogenic environment, so application as a material for liquid hydrogen such as liquid hydrogen storage containers and piping may be impossible.

[0069] Therefore, in the present disclosure, by simultaneously controlling the precipitate content and the Ni equivalent value to increase the degree of austenite stabilization, the ratio of room temperature tensile strength to yield strength satisfies 2.0 or less, and the Charpy impact energy value at -196°C can satisfy 70 J or more. Specifically, in the present disclosure, by controlling the Ni equivalent value to 27 or more, a ratio of room temperature tensile strength to yield strength can be secured at a low value of 2.0 or less, thereby suppressing work hardening, which means martensite formation disadvantageous for cryogenic toughness, as much as possible. In addition, the present disclosure controls the precipitate content to less than 0.001% to satisfy a Charpy impact energy value at -196°C of 70 J or more, thereby manufacturing an austenitic stainless steel capable of satisfying both high strength and low-temperature toughness.

[0070] The austenitic stainless steel according to an embodiment of the present disclosure may satisfy a room temperature yield strength of 300 MPa or more.

[0071] Upon pulling an object with a force greater than a certain magnitude and releasing the force, it does not return to its original state and becomes longer. At this time, the maximum force at which it can return to its original state is called yield strength. In a case where the strength of the steel is increased, the amount of steel used to manufacture a product of the same strength is reduced, so according to the present disclosure, a stainless steel with excellent strength can be provided, which has the effect of reducing the cost of the product.

[0072] In a case where 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 cryogenic toughness. The upper limit of the room temperature yield strength is not limited, but for example, it may be 700 MPa or less, 650 MPa or less, 600 MPa or less, 550 MPa or less, 500 MPa or less, 450 MPa or less, etc., to satisfy mechanical properties, corrosion resistance, strength, etc., required for application to hydrogen and low-temperature environments.

[0073] In addition, the austenitic stainless steel according to an embodiment of the present disclosure may satisfy a room temperature tensile strength of 600 MPa or more. The upper limit of the room temperature tensile strength is not limited, but for example, it may be 800 MPa or less, 750 MPa or less, 700 MPa or less, 650 MPa or less, etc., to satisfy mechanical properties, corrosion resistance, strength, etc., required for application to hydrogen and low-temperature environments.

[0074] In addition, a method for manufacturing an austenitic stainless steel having excellent low-temperature toughness according to an embodiment of the present disclosure may comprise: preparing a slab comprising, in percent by weight (wt%), more than 0% and 0.10% or less of C, more than 0% and 1.5% or less of Si, 17.0% to 23.0% of Cr, 5.5% to 12.0% of Ni, 0.5% to 8.0% of Mn, 0.10% to 0.30% of N, more than 0% and 1.0% or less of Cu, the remainder of Fe and inevitable impurities, wherein a content of precipitates is less than 0.001 wt%, and a Ni equivalent value of Formula (1) below is 27 or more; hot rolling the slab; hot-rolled annealing after the hot rolling; final cold rolling after the hot-rolled annealing; and final annealing after the cold rolling.

[0075] In the process of hot-rolled annealing after the hot rolling, the annealing temperature greatly affects residual stress relief and microstructure. Therefore, the hot-rolled annealing is preferably performed at a temperature of 900 to 1200°C.

[0076] In a case where the hot-rolled annealing temperature is less than 900°C, coarse carbides may be formed resulting in a non-uniform structure, or Cr₂₃C₆ precipitates may be formed around grain boundaries causing intergranular corrosion, and in a case where it exceeds 1200°C, crystal grains may become extremely coarse, so it is preferable to limit the annealing temperature to 900 to 1200°C, more preferably 950 to 1150°C, and most preferably 1000 to 1150°C.

[0077] In addition, after the hot-rolled annealing, a step of cold rolling and then final annealing may be performed. The annealing after the cold rolling may be performed at a temperature of 900 to 1200°C.

[0078] The austenitic stainless steel of the present disclosure manufactured by this method may have an austenite phase of 90% or more by area fraction, and a precipitate content of 0.001 wt% or less.

[0079] As the development and dissemination of fuel cell vehicles using hydrogen as fuel expands, the development of containers and parts for storing hydrogen has become necessary.

[0080] Hydrogen storage containers can be divided into liquid hydrogen and gas hydrogen according to the form of hydrogen. The operating temperature varies depending on the form of hydrogen, and liquid hydrogen is in a cryogenic environment of -253°C, and hydrogen gasified from liquid hydrogen exists inside the liquid hydrogen tank. In addition, since steel materials are exposed to a temperature range from -253°C to room temperature even in a device for vaporizing liquid hydrogen, there should be no degradation of physical properties of the steel material against hydrogen at various temperatures. Gaseous hydrogen gas is generally stored at room temperature, but is cooled to about -40 to -60°C in advance upon filling a storage tank. The reason is to prevent excessive temperature rise due to filling by cooling through a precooler in consideration of gas temperature rise during filling.

[0081] In particular, the liquid hydrogen storage method has higher storage efficiency than the gas form, so it is expected to be used in various fields in the future. The liquid hydrogen form is expected to be used for long-distance transportation of hydrogen from overseas to domestic, and as a method for storing large-scale hydrogen at hydrogen charging stations or hydrogen production plants.

[0082] Therefore, in consideration of the steel material for hydrogen storage tanks, degradation of physical properties at cryogenic temperatures as well as at room temperature can be an important determinant of steel material. Considering this point, hydrogen storage tanks and peripheral equipment require prevention of toughness degradation due to hydrogen and cryogenic temperatures, high mechanical strength, corrosion resistance, etc.

[0083] Materials generally used under hydrogen gas and liquid hydrogen environments currently are 304L and 316L, which are austenitic stainless steels. Materials that appear to have no problem at room temperature may also show a tendency for physical properties to degrade as the temperature decreases. In particular, toughness degradation is a major problem appearing as temperature decreases, which is one of the main causes of martensite transformation in the austenite structure.

[0084] In the austenite phase, the hydrogen diffusion rate is slow, making it difficult for hydrogen to move, so hydrogen embrittlement is unlikely to occur. Also, compared to the martensite phase, it is a soft phase, making it easy to secure toughness even at cryogenic temperatures, whereas the martensite phase is a hard phase compared to the austenite phase, making brittleness likely to occur and the hydrogen diffusion rate fast, increasing the possibility of hydrogen embrittlement. Therefore, in a case where the martensite fraction is high upon exposure to a hydrogen environment, hydrogen charges into the material, and in a case where degradation of material properties due to hydrogen occurs, problems may occur in use in liquid hydrogen and gas hydrogen environments.

[0085] In addition, since the material applied to hydrogen storage tanks, etc., determines the design material thickness according to strength, there is an advantage that the amount of material used for hydrogen storage tanks can be reduced by improving strength. Precipitates can be used as a method of increasing strength, but since precipitates are one of the main causes of toughness degradation at low temperatures, application in a hydrogen environment is possible only by controlling the precipitate content inside the steel material.

[0086] Hereinafter, the present disclosure will be described in more detail with reference to examples. These examples are for illustrative purposes only and are not intended to limit the scope of protection of the present disclosure.

Example 1

[0087] An austenitic slab having the composition of Table 1 below was hot rolled, and then annealing was performed at a temperature of 900 to 1200°C.

[0088] The alloy compositions of each Example and Comparative Example are shown in Table 1 below.

[Table 1]

Category	C	Si	Mn	Cr	Ni	Mo	Cu	N	Nb
Example 1	0.02	0.4	1.9	20.4	9.4	0.8	0.6	0.19	-
Example 2	0.02	0.4	6.5	18.1	5.6	1.6	0.9	0.21	-
Example 3	0.02	0.4	6.6	17.9	5.6	1.6	0.9	0.16	-
Example 4	0.02	0.4	0.8	21.5	10.4	0.8	0.6	0.20	-
Example 5	0.02	0.5	1.0	18.3	8.0	0.4	0.4	0.16	-
Example 6	0.02	0.4	4.6	17.9	7.8	1.1	0.9	0.20	-
Example 7	0.02	0.4	1.2	19.6	9.3	0.8	0.8	0.15	-
Example 8	0.02	0.4	4.1	19.0	8.5	0.4	0.4	0.20	-
Example 9	0.02	0.4	3.6	20.2	8.4	0.8	0.8	0.19	-
Example 10	0.02	0.4	7.6	17.5	5.5	0.4	0.4	0.21	-
Example 11	0.02	0.4	5.7	17.3	5.6	0.4	0.4	0.18	-
Example 12	0.03	1.1	0.8	20.3	9.8	-	0.4	0.16	0.02
Comparative Example 1	0.02	0.6	1.1	16.1	10.2	0.3	2.1	0.02	-
Comparative Example 2	0.02	0.4	1.2	18.8	10.2	0.4	-	0.02	-
Comparative Example 3	0.05	0.4	1	18.1	8.1	0	0	0.04	-
Comparative Example 4	0.02	0.4	6.3	18.3	6.4	1.2	0.5	0.21	0.11
Comparative Example 5	0.06	0.4	1.1	18.3	8	0	0	0.04	-
Comparative Example 6	0.02	0.4	1.4	18.1	8	0	0	0.04	-
Comparative Example 7	0.02	0.4	5.3	17.6	5.9	0.4	0.5	0.21	0.10
Comparative Example 8	0.02	0.5	1.3	16.7	10.1	0.3	2.1	0.05	-

[0089] Using the experimental steel grades of Examples and Comparative Examples in Table 1, a Charpy impact test was conducted at -196°C to check the impact toughness value at cryogenic temperatures, and a tensile test was conducted in the atmosphere at room temperature to measure yield strength and tensile strength. The Charpy impact energy value was obtained by conducting an impact test at a temperature of -196°C using ASTM E23 type A specimen standards. The tensile test was conducted according to ASTM E8 standards. Precipitate content was measured through quantitative analysis of precipitates using the residue extraction method.

[0090] The -196°C Charpy impact toughness value, precipitate content (wt%), room temperature yield strength, room temperature tensile strength, ratio of tensile strength to yield strength, and Ni equivalent value of the experimental steel grades according to the above Examples and Comparative Examples are shown in Table 2 below.

[Table 2]

Category	-196°C Impact energy value (J)	Precipitate content (wt%)	Yield strength (MPa)	Tensile strength (MPa)	Ratio of Tensile strength to Yield strength	Ni Equivalent value
Example 1	135	<0.001	383	694	1.8	32
Example 2	101	<0.001	359	659	1.8	32
Example 3	124	<0.001	323	627	1.9	30
Example 4	126	<0.001	362	677	1.9	33
Example 5	109	<0.001	316	647	2.0	27
Example 6	121	<0.001	342	643	1.9	31
Example 7	133	<0.001	305	615	2.0	29
Example 8	150	<0.001	330	642	1.9	32
Example 9	140	<0.001	344	652	1.9	32
Example 10	115	<0.001	325	662	2.0	32
Example 11	106	<0.001	335	684	2.0	29
Example 12	124	<0.001	405	713	1.8	30
Comparative Example 1	164	<0.001	238	556	2.3	25
Comparative Example 2	198	<0.001	213	540	2.5	25
Comparative Example 3	148	<0.001	283	712	2.5	23
Comparative Example 4	69	0.025	384	690	1.8	33
Comparative Example 5	135	<0.001	294	677	2.3	23
Comparative Example 6	143	<0.001	284	640	2.3	23
Comparative Example 7	56	0.016	394	725	1.8	31
Comparative Example 8	156	<0.001	277	615	2.2	26

[0091] Referring to the results of Table 2, it was confirmed that Examples 1 to 12 according to the present disclosure satisfied the alloy composition proposed in the present disclosure, had a Ni equivalent value of 27 or more, and satisfied a precipitate content of less than 0.001%, thereby satisfying a ratio of room temperature tensile strength to yield strength of 2.0 or less, and showing a high Charpy impact energy value at -196°C of 70 J or more. In addition, the yield strength at room temperature was 305 to 405 MPa, the tensile strength was 615 to 713 MPa, and the ratio of tensile strength to yield strength was 2.0 or less, suppressing martensite transformation as much as possible during deformation or processing, thereby showing excellent cryogenic toughness, indicating that the austenitic stainless steel of the present disclosure can be applied as a material for liquid hydrogen such as liquid hydrogen storage containers and piping. On the other hand, Comparative Examples 1 to 3, 5 to 6, and 8 were alloy compositions that did not satisfy the content of N, and although the precipitate content was satisfied at 0.001 wt% or less, the stability of the austenite phase was low, resulting in a Ni equivalent value of less than 27 due to martensite transformation disadvantageous for cryogenic toughness according to work hardening, and as a result, the ratio of room temperature tensile strength to yield strength exceeded 2.0, indicating that it would be difficult to apply as a material for liquid hydrogen.

[0092] In addition, Comparative Examples 4 and 7 were alloy compositions satisfying the content of N, and despite stable austenite stabilization with a Ni equivalent value of 27 or more and a tensile strength to yield strength of 2.0 or less, the precipitate content exceeded 0.001 wt%, resulting in a low Charpy impact energy value at -196°C of 70 J or less, indicating that application as a material for liquid hydrogen used in a cryogenic environment would be difficult.

[0093] From the above results, according to the present disclosure, 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 stabilization degree is controlled to 27 or more, thereby satisfying the ratio of room temperature tensile strength to yield strength of 2.0 or less and satisfying the Charpy impact energy value at -196°C of 70 J or more. In addition, it was found that an austenitic stainless steel excellent in cryogenic toughness and strength can be provided by satisfying a room temperature yield strength of 300 MPa or more and a room temperature tensile strength of 600 MPa or more.

[0094] In addition, it was found that the present disclosure can suppress martensite transformation as much as possible during deformation or processing through austenite stabilization, thereby obtaining an austenitic stainless steel suitable for use as a material for liquid hydrogen such as liquid hydrogen storage containers and piping without degradation of physical properties of the material by hydrogen while satisfying low-temperature toughness.

[0095] The embodiments described in this specification and the accompanying drawings are merely illustrative of some of the technical ideas included in the present disclosure. Therefore, the embodiments disclosed in this specification are not intended to limit the technical idea of the present disclosure but to explain it, and it is obvious that the scope of the technical idea of the present disclosure is not limited by these embodiments. All modifications and specific embodiments that can be easily inferred by those skilled in the art within the scope of the technical idea included in the specification and drawings of the present disclosure should be interpreted as being included in the scope of rights of the present disclosure.

Claims

1. An austenitic stainless steel comprising, in percent by weight (wt%), more than 0% and 0.10% or less of carbon (C), more than 0% and 1.5% or less of silicon (Si), 17.0% to 23.0% of chromium (Cr), 5.5% to 12.0% of nickel (Ni), 0.5% to 8.0% of manganese (Mn), 0.10% to 0.30% of nitrogen (N), more than 0% and 1.0% or less of copper (Cu), the remainder of iron (Fe) and inevitable impurities, wherein a content of precipitates is less than 0.001 wt%, and a Ni equivalent value of Formula (1) below is 27 or more:

2. The austenitic stainless steel of claim 1, wherein the austenitic stainless steel has a ratio of room temperature tensile strength to yield strength of 2.0 or less.

3. The austenitic stainless steel of claim 1, wherein the austenitic stainless steel has a Charpy impact energy value at -196°C of 70 J or more.

4. The austenitic stainless steel of claim 1, wherein the austenitic stainless steel further comprises any one of 2.0% or less of molybdenum (Mo) and 0.05% or less of niobium (Nb).

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

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

7. The austenitic stainless steel of claim 1, wherein the austenitic stainless steel has an austenite phase in an area fraction of 90% or more.

8. A method for manufacturing an austenitic stainless steel, comprising:

preparing a slab comprising, in percent by weight (wt%), more than 0% and 0.10% or less of carbon (C), more than 0% and 1.5% or less of silicon (Si), 17.0% to 23.0% of chromium (Cr), 5.5% to 12.0% of nickel (Ni), 0.5% to 8.0% of manganese (Mn), 0.10% to 0.30% of nitrogen (N), more than 0% and 1.0% or less of copper (Cu), the remainder of iron (Fe) and inevitable impurities, wherein a content of precipitates is less than 0.001 wt%, and a Ni equivalent value of Formula (1) below is 27 or more;

hot rolling the slab;

hot-rolled annealing after the hot rolling;

final cold rolling after the hot-rolled annealing; and

final annealing after the cold rolling.

9. The method of claim 8, wherein the hot-rolled annealing is performed at a temperature of 900 to 1200°C.

10. The method of claim 8, wherein the final annealing is performed at a temperature of 900 to 1200°C.

11. The method of claim 8, wherein the austenitic stainless steel further comprises any one of 2.0% or less of molybdenum (Mo) and 0.05% or less of niobium (Nb).

12. The method of claim 8, wherein the austenitic stainless steel has a ratio of room temperature tensile strength to yield strength of 2.0 or less.

13. The method of claim 8, wherein the austenitic stainless steel has a Charpy impact energy value at - 196°C of 70 J or more.

14. The method of claim 8, wherein the austenitic 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 of claim 8, wherein the austenitic stainless steel has an austenite phase in an area fraction of 90% or more.