austenitic stainless steel

Abstract

The present invention relates to an austenitic stainless steel. According to the present invention, because of the improved performance, such as the elevated pitting potential and thereby the excellent corrosion resistance, the price competitiveness compared to the existing 304L STS, and the impact absorption energy at -196℃ of 41J or more, which can be achieved by adding a trace amount of tungsten, silicon, niobium, copper, or boron, compared to the existing austenitic stainless steel.

Description

Technical Field

[0001] This invention relates to an austenitic stainless steel. Background Technology

[0002] Existing 304L STS and 316L STS materials used in liquefied natural gas (LNG) vessels have stabilized the austenite phase below room temperature, but this requires the addition of expensive nickel (Ni), resulting in a cost burden. Therefore, a material with excellent price competitiveness compared to 304L STS and 316L STS needs to be developed. Austenitic materials need to reduce the high-cost nickel content and add low-cost manganese (Mn), carbon (C), and nitrogen (N) elements to achieve austenite phase stabilization at room temperature. Furthermore, according to the International Maritime Organization (IMG) regulations, materials used in LNG vessels should have an impact absorption energy of at least 41 J at -196°C, and a chemical composition for austenitic stainless steel materials with corrosion resistance equivalent to or better than 304L STS and 316L STS needs to be developed.

[0003] Existing technical documents

[0004] Patent documents

[0005] (Patent Document 1) Korean Patent Registration No. 10-1198486 (Published on July 4, 2012) Summary of the Invention

[0006] In a first aspect, an object of the present invention is to provide an austenitic stainless steel whose corrosion resistance and low-temperature impact energy absorption are improved by adding tungsten.

[0007] Another objective of the first aspect of this invention is to provide an austenitic stainless steel containing nickel, manganese, carbon, and nitrogen in a certain proportion for the purpose of improving phase stability, impact energy absorption at extremely low temperatures, processability, and corrosion resistance, as well as a method for manufacturing the same.

[0008] To achieve the objectives described above, the present invention provides an austenitic stainless steel comprising 16 to 20 wt% chromium (Cr), 4.5 to 9 wt% manganese (Mn), 3 to 6 wt% nickel (Ni), 0.001 to 0.3 wt% carbon (C), 0.1 to 0.35 wt% nitrogen (N), more than 0.0 and less than 2.0 wt% tungsten (W), the balance iron (Fe), and other unavoidable impurities.

[0009] According to the present invention, the pitting potential can be increased compared to existing 304L STS by adding trace amounts of tungsten, thereby ensuring excellent corrosion resistance, and it is also price-competitive with existing austenitic stainless steels. Furthermore, the impact absorption energy at -196°C can be above 41 J.

[0010] Furthermore, the austenitic stainless steel according to the present invention is economical because it can reduce or eliminate the content of high-priced nickel and molybdenum. Moreover, the austenitic phase can be stabilized to room temperature by adding low-priced manganese, carbon, and nitrogen. In addition, the processability and corrosion resistance can be improved by adding carbon and nitrogen in a certain proportion. Furthermore, the impact absorption energy at extremely low temperatures can be improved compared with existing high-manganese stainless steels by controlling the composition ratio of nickel, manganese, carbon, and nitrogen.

[0011] In a second aspect, an object of the present invention is to provide an austenitic stainless steel whose corrosion resistance and low-temperature impact energy absorption are improved by adding silicon. Another object of this aspect is to provide an austenitic stainless steel comprising nickel, manganese, carbon, and nitrogen in a certain proportion to improve phase stability, impact energy absorption at extremely low temperatures, processability, and corrosion resistance, and a method thereof.

[0012] The same or similar basic components as those in the first aspect above are used, except that in the second aspect of the invention, a suitable amount of silicon (Si) is used instead of tungsten (W). In the second aspect, silicon (Si) can be used in weight percentages of 0 and less than 0.925%.

[0013] In a third aspect of the invention, an object of the invention is to provide an austenitic stainless steel whose corrosion resistance and low-temperature shock energy absorption are improved by the addition of niobium.

[0014] The same or similar basic components as those in the first aspect above are used, except that in the third aspect of the invention, niobium (Nb) is used instead of tungsten (W) in an appropriate amount. In the third aspect, 0.01 to 0.5 weight percent of niobium (Nb) can be used.

[0015] In a fourth aspect of the invention, an object of the invention is to provide an austenitic stainless steel in which corrosion resistance and low-temperature shock energy absorption are improved by adding copper.

[0016] The same or similar basic components as those in the first aspect above are used, except that in the fourth aspect of the invention, an appropriate amount of copper (Cu) is used instead of tungsten (W). In the fourth aspect, copper (Cu) in a weight percentage greater than 0 and less than 2.0 can be used.

[0017] In a fifth aspect of the invention, an object of the invention is to provide an austenitic stainless steel in which corrosion resistance and low-temperature shock energy absorption are improved by the addition of boron.

[0018] Using the same or similar basic components as in the first aspect above, the difference being that, in the fifth aspect of the invention, a suitable amount of boron (B) is used instead of tungsten (W). In the fifth aspect, boron (B) can be used in a weight percentage greater than 0 and less than 0.055%. Attached Figure Description

[0019] Figure 1 This is a microstructure image of austenitic stainless steel with nickel and tungsten content according to a first aspect embodiment of the present invention.

[0020] Figure 2 This is a graph showing the results of pitting potential evaluation according to a first aspect embodiment of the present invention.

[0021] Figure 3 This is a graph showing the impact absorption energy results according to a first aspect embodiment of the present invention.

[0022] Figure 4 The images are phase maps of austenitic stainless steel according to the composition ratio of the first aspect of the present invention, and images of the austenitic stainless steel corresponding to the phase map taken using an optical microscope, with a scale bar of 75 mm.

[0023] Figure 5 This is an image showing a phase diagram of the composition ratio of austenitic stainless steel according to a first aspect embodiment of the present invention, along with impact absorbed energy.

[0024] Figure 6 The results are from a processability test of a specimen manufactured according to the composition ratio of austenitic stainless steel according to the first aspect of the present invention.

[0025] Figure 7 The results are from an anodic polarization test of a specimen manufactured according to the composition ratio of austenitic stainless steel according to the first aspect of the present invention.

[0026] Figure 8 This is an image illustrating the results of a salt spray test on a specimen manufactured according to the composition ratio of austenitic stainless steel according to the first aspect embodiment of the present invention.

[0027] Figure 9 This is an image illustrating the intergranular corrosion test results of a specimen manufactured according to the composition ratio of austenitic stainless steel according to the first aspect embodiment of the present invention.

[0028] Figure 10This is a microstructure image of austenitic stainless steel with silicon content according to a second aspect embodiment of the present invention.

[0029] Figure 11 This is a graph showing the results of pitting potential evaluation according to a second aspect embodiment of the present invention.

[0030] Figure 12 This is a graph showing the impact absorption energy results according to a second aspect embodiment of the present invention.

[0031] Figure 13 This is a microstructure image of austenitic stainless steel with niobium content according to a third aspect embodiment of the present invention.

[0032] Figure 14 This is a graph showing the results of pitting potential evaluation according to a third aspect embodiment of the present invention.

[0033] Figure 15 This is a graph showing the impact absorption energy results according to a third aspect embodiment of the present invention.

[0034] Figure 16 This is a microstructure image of austenitic stainless steel with copper content according to a fourth aspect embodiment of the present invention.

[0035] Figure 17 This is a graph showing the results of pitting potential evaluation according to a fourth aspect embodiment of the present invention.

[0036] Figure 18 This is a graph showing the impact absorption energy results according to a fourth aspect embodiment of the present invention.

[0037] Figure 19 This is a microstructure image of austenitic stainless steel according to a fifth aspect embodiment of the present invention, based on boron content.

[0038] Figure 20 This is a graph showing the results of pitting potential evaluation according to a fifth aspect embodiment of the present invention.

[0039] Figure 21 This is a graph showing the impact absorption energy results according to a fifth aspect embodiment of the present invention. Detailed Implementation

[0040] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings. The present invention can be modified and has various forms; specific embodiments will be illustrated in the drawings and described in detail in the text. However, this is not intended to limit the present invention to a specific disclosed form, but should be understood to include all modifications, equivalents, and substitutions within the scope of the present invention's concept and technology. Similar reference numerals are used for similar constituent elements in the description of the various drawings. In the drawings, to ensure clarity of the invention, the dimensions of the structures are shown as enlarged compared to actual dimensions.

[0041] In describing various constituent elements, terms such as "first" and "second" may be used, but the constituent elements are not limited to these terms. These terms are merely used to distinguish one constituent element from others. For example, without departing from the scope of this invention, a first constituent element may be named a second constituent element, and similarly, a second constituent element may be named a first constituent element.

[0042] The terminology used in this application is for illustrative purposes only and is not intended to limit the invention. Singular statements also have plural meanings unless the context clearly indicates otherwise. In this application, terms such as "comprising" or "having" are used only to indicate the presence of features, numbers, steps, actions, constituent elements, components, or combinations thereof described in the specification, but should not be construed as excluding the possibility of one or more other features, numbers, steps, actions, constituent elements, components, or combinations thereof being present or added.

[0043] Furthermore, unless otherwise defined, all terms used herein, including technical and scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Terms that are commonly used and defined in dictionaries should be interpreted as having the meaning consistent with their meaning in the context of the relevant art, and should not be interpreted as having an overly idealized or exaggerated form of meaning unless explicitly defined in this application.

[0044] In a first aspect of the invention, the austenitic stainless steel according to an embodiment of the invention may contain 16 to 20 wt% chromium (Cr), 4.5 to 9 wt% manganese (Mn), 3 to 6 wt% nickel (Ni), 0.001 to 0.3 wt% carbon (C), 0.1 to 0.35 wt% nitrogen (N), more than 0.0 and less than 2.0 wt% tungsten (W), the balance iron (Fe), and other unavoidable impurities.

[0045] In one embodiment, the austenitic stainless steel may contain 4 to 9 weight percent of the total content of the nickel and the tungsten.

[0046] In one embodiment, the austenitic stainless steel may contain 4.27 to 8.2 weight percent of the total content of the nickel and the tungsten.

[0047] In one embodiment, the austenitic stainless steel may contain 0.15 to 0.45 weight percent of the total content of the carbon and the nitrogen.

[0048] The total content of carbon and nitrogen is less than 0.33% by weight.

[0049] (First aspect)

[0050] Next, the reasons for limiting the numerical range of the stainless steel component content in the first aspect of the present invention will be explained.

[0051] (1) Chromium (Cr): approximately 16 to 20 percent by weight

[0052] When chromium is added, it imparts corrosion resistance by forming a passivation film on the surface of stainless steel. Therefore, for the corrosion resistance and austenitic phase stabilization of austenitic stainless steel, a chromium content of approximately 16% by weight or more is required. When the chromium content exceeds approximately 20% by weight, a sigma phase may form, leading to weakened corrosion resistance. Therefore, the austenitic stainless steel may contain approximately 16 to 20% by weight of chromium.

[0053] (2) Manganese: approximately 4.5 to 9 percent by weight

[0054] Manganese can stabilize the austenitic phase and increase the solid solubility of nitrogen. The effects described above are achieved when the manganese content reaches approximately 4.5% by weight or more. However, when the manganese content exceeds approximately 9% by weight, manganese sulfides or oxides may form due to combination with impurities, namely sulfur (S) or oxygen (O), leading to a decrease in the corrosion resistance and mechanical properties of the austenitic stainless steel. Therefore, the austenitic stainless steel may contain approximately 4.5 to 9% by weight of manganese.

[0055] (3) Nickel: approximately 3 to 6 percent by weight

[0056] Nickel can stabilize the austenitic phase and increase toughness and formability. Furthermore, it can enhance impact absorption energy at approximately -196°C by increasing pitting potential and minimizing interstitial element content. However, as a high-valence element, adding excessive amounts of nickel leads to increased manufacturing costs. Therefore, the austenitic stainless steel may contain approximately 3 to 6% by weight of nickel.

[0057] (4) Carbon: approximately 0.001 to 0.3% by weight

[0058] Carbon can stabilize the austenitic phase and inhibit martensitic transformation. However, carbon can also lead to the formation of carbides and a decrease in corrosion resistance. Therefore, the austenitic stainless steel may contain approximately 0.001 to 0.3% by weight of carbon. Specifically, it may contain 0.05 to 0.3% by weight of carbon.

[0059] (5) Nitrogen: approximately 0.1 to 0.35 percent by weight

[0060] Nitrogen can stabilize the austenitic phase and replace some of the nickel. However, nitrogen can also lead to the formation of nitrides, resulting in a decrease in corrosion resistance and toughness. Therefore, the austenitic stainless steel may contain approximately 0.1 to 0.35 weight percent nitrogen.

[0061] (6) Tungsten: approximately more than 0.0 and less than 2.0% by weight

[0062] Tungsten, along with nickel, can enhance the corrosion resistance of stainless steel. With increasing tungsten content, a trend of increasing corrosion potential can be observed. When the Ni+2W content reaches 4.7218% or higher by weight, it exhibits a higher pitting potential than 304L STS, while when the Ni+2W content reaches 6.2590% or higher by weight, it exhibits a pitting potential comparable to or higher than 316L STS. Therefore, it can be concluded that both nickel and tungsten contribute to improving the corrosion resistance of stainless steel.

[0063] In one embodiment of the first aspect, the pitting potential of the austenitic stainless steel in a 3.5% NaCl solution at 16–20°C can be 250–500 mV, or 350–500 mV.

[0064] In the first aspect, the austenitic stainless steel may have an impact absorption energy of 41 to 100 J at approximately -196°C, or it may have an impact absorption energy of 60 to 100 J. If the impact absorption energy is less than approximately 41 J, it may not be suitable for liquefied natural gas (LNG) marine applications.

[0065] In the first aspect, when the Ni+2W content of the austenitic stainless steel is in the range of 4.27 to 8.2% by weight, the pitting potential expressed by the following formula (1) can be satisfied:

[0066] Formula 1

[0067] E pit = 0.05263(Ni+2W) + 0.03769

[0068] In Equation 1, Ni and W refer to the weight of each element.

[0069] In one embodiment, the austenitic stainless steel may contain 8 to 15 weight percent or 8.5 to 15 weight percent of the total content of nickel and manganese. Even with a reduction in the amount of nickel added to the austenitic stainless steel, the austenitic phase can be stabilized while satisfying the total content of nickel and manganese and the total content of carbon and nitrogen as described above.

[0070] For example, low-valence manganese is added to stabilize the austenite phase while reducing the amount of nickel added. However, since manganese is about 0.5 times more effective than nickel in stabilizing the austenite phase, carbon and nitrogen are added to reinforce it. Although carbon and nitrogen are about 15 times and 30 times more effective than nickel in stabilizing the austenite phase, respectively, excessive addition of carbon and nitrogen has the disadvantage of reducing impact absorption energy at low temperatures. Therefore, only by meeting the above range can the austenite phase be stabilized and exhibit excellent impact absorption energy at extremely low temperatures.

[0071] Furthermore, in the austenitic stainless steel, the total carbon and nitrogen content can be less than 0.33 wt%, less than 0.3 wt%, less than 0.295 wt%, less than 0.285 wt%, from 0.225 wt% to 0.33 wt%, from 0.255 wt% to 0.3 wt%, or from 0.25 wt% to 0.295 wt%. By satisfying the total carbon and nitrogen content as described above, excellent workability and extremely low temperature shock absorption energy can be exhibited even with a reduction in manganese content.

[0072] Specifically, in the austenitic stainless steel, the weight ratio of carbon [C / (C+N)] relative to the total weight of carbon and nitrogen can be 0.1 to 0.7 or 0.2 to 0.5. By satisfying the weight ratio of carbon relative to the total carbon and nitrogen content as described above, corrosion resistance at the level of existing 304 STS materials can be achieved.

[0073] Furthermore, when the wear of the austenitic stainless steel is measured using an end mill under dry conditions, the machinability of the austenitic stainless steel can be greater than 0 and less than 0.15 mm or greater than 0 and less than 0.125 mm.

[0074] Furthermore, the austenitic stainless steel can satisfy one or more of the following formulas 2 to 5:

[0075] Formula 2

[0076] (Ni+Mn) + 165.94(C+N) ≥ 49.47

[0077]

Formula 3

[0078] (Ni+Mn) - 27.28(C+N) ≥ 2.10

[0079]

Formula 4

[0080] (Ni+Mn) ≥ 12.3

[0081]

Formula 5

[0082] (Ni+Mn) + 13.133(C+N) ≥ 1.73

[0083] In Equations 2 to 5, Ni, Mn, C, and N refer to the weights of nickel, manganese, carbon, and nitrogen, respectively.

[0084] Specifically, when the austenitic stainless steel satisfies one or more of the above formulas 2 to 5, the austenitic phase can be stabilized to room temperature, and it can exhibit excellent impact absorption energy at extremely low temperatures (-196℃).

[0085] Equations 2 and 5 are derived from the phase diagram of austenitic stainless steel and correspond to the stable region of the γ phase, satisfying the region where the austenitic structure is stable at room temperature.

[0086] Equations 2 to 4 are derived based on the phase diagram of austenitic stainless steel and data representing impact absorption energy, and simultaneously satisfy the γ phase stability region and impact absorption energy above 41J.

[0087] (Second aspect)

[0088] In a second aspect, the austenitic stainless steel according to embodiments of the present invention may contain 16 to 20 weight percent of chromium (Cr), 4.5 to 9 weight percent of manganese (Mn), 3 to 6 weight percent of nickel (Ni), 0.001 to 0.3 weight percent of carbon (C), 0.1 to 0.35 weight percent of nitrogen (N), more than 0 and less than 0.925 weight percent of silicon (Si), the balance of iron (Fe), and other unavoidable impurities.

[0089] In a second aspect of the invention, the austenitic stainless steel may contain more than 0.0 and less than 0.925% by weight of the silicon.

[0090] In a second aspect of the invention, the austenitic stainless steel may contain 0.1555 to 0.925 weight percent of the silicon.

[0091] In a second aspect of the invention, the austenitic stainless steel may contain 0.15 to 0.45 weight percent of the total content of the carbon and the nitrogen. The second aspect particularly relates to the following technical solutions:

[0092] Item 1. An austenitic stainless steel comprising 16 to 20 wt% chromium (Cr), 4.5 to 9 wt% manganese (Mn), 3 to 6 wt% nickel (Ni), 0.001 to 0.3 wt% carbon (C), 0.1 to 0.35 wt% nitrogen (N), more than 0 and less than 0.925 wt% silicon (Si), the balance iron (Fe), and other unavoidable impurities.

[0093] 2. The austenitic stainless steel as described in Project 1,

[0094] The austenitic stainless steel contains 0.1555 to 0.925 weight percent of the silicon.

[0095] 3. The austenitic stainless steel as described in Project 1,

[0096] The austenitic stainless steel contains 0.15 to 0.45 weight percent of the total content of the carbon and the nitrogen.

[0097] 4. The austenitic stainless steel as described in Project 1,

[0098] The austenitic stainless steel has a pitting potential of 300 to 800 mV in a 3.5% NaCl solution at 16–20 °C.

[0099] 5. Austenitic stainless steel as described in Project 4.

[0100] The austenitic stainless steel has a pitting potential of 350 to 700 mV at 16–20 °C in a 3.5% NaCl solution.

[0101] 6. The austenitic stainless steel as described in Project 1,

[0102] The austenitic stainless steel absorbs 60 to 100 J of impact energy at -196°C.

[0103] 7. Austenitic stainless steel as described in Project 1,

[0104] The pitting potential of the austenitic stainless steel satisfies the following equation (1):

[0105] Formula 1

[0106] E pit = 0.1576×Si + 0.3426

[0107] In Equation 1, Si represents the weight of silicon.

[0108] 8. Austenitic stainless steel as described in Project 3,

[0109] The austenitic stainless steel contains 8 to 15 weight percent of the total content of nickel and manganese.

[0110] 9. Austenitic stainless steel as described in Project 8.

[0111] The austenitic stainless steel contains 8.5 to 15 weight percent of the total content of nickel and manganese, and 0.225 to 0.33 weight percent of the total content of carbon and nitrogen.

[0112] 10. The austenitic stainless steel as described in Item 9.

[0113] When the wear of the tool is measured using an end mill under dry conditions, the machinability of the austenitic stainless steel is greater than 0 and less than 0.15 mm.

[0114] 11. The austenitic stainless steel as described in Project 1,

[0115] The weight ratio of the carbon to the total weight of the carbon and nitrogen [C / (C+N)] is 0.1 to 0.7.

[0116] 12. Austenitic stainless steel as described in Item 8,

[0117] The following conditions must be met: (Select one or more from Equations 2 to 5)

[0118] Formula 2

[0119] (Ni+Mn) + 165.94(C+N) ≥ 49.47

[0120]

Formula 3

[0121] (Ni+Mn) - 27.28(C+N) ≥ 2.10

[0122]

Formula 4

[0123] (Ni+Mn) ≥ 12.3

[0124]

Formula 5

[0125] (Ni+Mn) + 13.133(C+N) ≥ 1.73

[0126] In Equations 2 to 5, Ni, Mn, C, and N refer to the weights of nickel, manganese, carbon, and nitrogen, respectively.

[0127] 13. The austenitic stainless steel as described in Item 12,

[0128] The conditions of Equations 2 and 5 are satisfied.

[0129] 14. Austenitic stainless steel as described in Item 12,

[0130] The conditions of Equations 2 to 4 are satisfied.

[0131] 15. Austenitic stainless steel as described in Project 3.

[0132] The total content of carbon and nitrogen is less than 0.33% by weight.

[0133] Next, the reasons for limiting the numerical range of the stainless steel component content in the second aspect of the present invention will be explained. For reasons similar to those in the first aspect, the austenitic stainless steel comprises 16 to 20 wt% chromium (Cr), 4.5 to 9 wt% manganese (Mn), 3 to 6 wt% nickel (Ni), 0.001 to 0.3 wt% carbon (C), and 0.1 to 0.35 wt% nitrogen (N). Therefore, the description of the basic components identical to those in the stainless steel of the first aspect is omitted, and only the reasons for the inclusion of Si are described.

[0134] (7) Silicon: approximately more than 0 and less than 0.925 by weight

[0135] Silicon can prevent steel oxidation by forming an oxide film, thereby improving corrosion resistance. With increasing silicon content, a trend of increasing corrosion potential can be observed. When the silicon content exceeds 0.0% by weight, it exhibits a higher pitting potential than 304L STS, while when the silicon content reaches 0.1555% by weight or higher, it exhibits a pitting potential comparable to or higher than 316L STS. Therefore, it can be concluded that silicon contributes to improving the corrosion resistance of stainless steel.

[0136] Conversely, when the silicon content is 1.0% or more by weight, the physical properties of stainless steel may decrease due to the formation of carbides.

[0137] In one embodiment of the second aspect, the pitting potential of the austenitic stainless steel in a 3.5% NaCl solution at 16–20°C can be 300–800 mV, or 350–700 mV.

[0138] In one embodiment of the second aspect, the austenitic stainless steel may have an impact absorption energy of 41 to 100 J at approximately -196°C, or it may have an impact absorption energy of 60 to 100 J. If the impact absorption energy is less than approximately 41 J, it may not be suitable for use in liquefied natural gas (LNG) carriers.

[0139] In one embodiment of the second aspect, when the silicon content of the austenitic stainless steel is in the range of more than 0 and less than 1.0 by weight percentage, the pitting potential expressed by the following formula (1) can be satisfied:

[0140] Formula 1

[0141] E pit = 0.1576×Si + 0.3426

[0142] In Equation 1, Si represents the weight of silicon.

[0143] (Third aspect)

[0144] In a third aspect, the austenitic stainless steel according to embodiments of the present invention may contain 16 to 20 wt% chromium (Cr), 4.5 to 9 wt% manganese (Mn), 3 to 6 wt% nickel (Ni), 0.001 to 0.3 wt% carbon (C), 0.1 to 0.35 wt% nitrogen (N), 0.01 to 0.5 wt% niobium (Nb), the balance iron (Fe), and other unavoidable impurities.

[0145] In one embodiment, the austenitic stainless steel may contain 0.15 to 0.45 weight percent of the total content of the carbon and the nitrogen.

[0146] In one embodiment, the austenitic stainless steel may contain 0.01 to 0.5 weight percent, 0.0289 to 0.5 weight percent, or 0.0289 to 0.05 weight percent of the niobium. The third aspect particularly relates to the following technical solutions:

[0147] Item 1. An austenitic stainless steel comprising 16 to 20 wt% chromium (Cr), 4.5 to 9 wt% manganese (Mn), 3 to 6 wt% nickel (Ni), 0.001 to 0.3 wt% carbon (C), 0.1 to 0.35 wt% nitrogen (N), 0.01 to 0.5 wt% niobium (Nb), the balance iron (Fe), and other unavoidable impurities.

[0148] 2. Austenitic stainless steel as described in Project 1.

[0149] The austenitic stainless steel contains 0.15 to 0.45 weight percent of the total content of the carbon and the nitrogen.

[0150] 3. Austenitic stainless steel as described in Project 1.

[0151] The pitting potential of the austenitic stainless steel is 300 to 400 mV at 16–20 °C and in a 3.5% NaCl solution.

[0152] 4. Austenitic stainless steel as described in Project 3.

[0153] The pitting potential of the austenitic stainless steel is 350 to 400 mV at 16–20 °C and in a 3.5% NaCl solution.

[0154] 5. Austenitic stainless steel as described in Project 1.

[0155] The impact absorption energy of the austenitic stainless steel is 41 to 100 J at -196°C.

[0156] 6. Austenitic stainless steel as described in Project 1.

[0157] The pitting potential of the austenitic stainless steel satisfies the following equation (1):

[0158] Formula 1

[0159] E pit = 0.8833×Nb + 0.3416

[0160] In Equation 1, Nb represents the weight of niobium.

[0161] 7. Austenitic stainless steel as described in Project 2.

[0162] The austenitic stainless steel contains 8 to 15 weight percent of the total content of nickel and manganese.

[0163] 8. Austenitic stainless steel as described in Item 7.

[0164] The austenitic stainless steel contains 8.5 to 15 weight percent of the total content of nickel and manganese, and 0.225 to 0.33 weight percent of the total content of carbon and nitrogen.

[0165] 9. Austenitic stainless steel as described in Item 8.

[0166] When the wear of the tool is measured using an end mill under dry conditions, the machinability of the austenitic stainless steel is greater than 0 and less than 0.15 mm.

[0167] 10. Austenitic stainless steel as described in Project 1.

[0168] The weight ratio of the carbon to the total weight of the carbon and nitrogen [C / (C+N)] is 0.1 to 0.7.

[0169] 11. Austenitic stainless steel as described in Item 7.

[0170] The following conditions must be met: (Select one or more from Equations 2 to 5)

[0171] Formula 2

[0172] (Ni+Mn) + 165.94(C+N) ≥ 49.47

[0173]

Formula 3

[0174] (Ni+Mn) - 27.28(C+N) ≥ 2.10

[0175]

Formula 4

[0176] (Ni+Mn) ≥ 12.3

[0177]

Formula 5

[0178] (Ni+Mn) + 13.133(C+N) ≥ 1.73

[0179] In Equations 2 to 5, Ni, Mn, C, and N refer to the weights of nickel, manganese, carbon, and nitrogen, respectively.

[0180] 12. Austenitic stainless steel as described in Item 11,

[0181] The conditions of Equations 2 and 5 are satisfied.

[0182] 13. Austenitic stainless steel as described in Item 11,

[0183] The conditions of Equations 2 to 4 are satisfied.

[0184] 14. Austenitic stainless steel as described in Project 2.

[0185] The total content of carbon and nitrogen is less than 0.33% by weight.

[0186] Next, the reasons for limiting the numerical range of the stainless steel component content in the third aspect of the present invention will be explained. For reasons similar to those in the first aspect, the austenitic stainless steel comprises 16 to 20 wt% chromium (Cr), 4.5 to 9 wt% manganese (Mn), 3 to 6 wt% nickel (Ni), 0.001 to 0.3 wt% carbon (C), and 0.1 to 0.35 wt% nitrogen (N). Therefore, the description of the basic components of the stainless steel as in the first aspect is omitted, and only the reasons for including niobium are described.

[0187] (8) Niobium: approximately 0.01 to 0.5% by weight

[0188] Niobium can alter the microstructure, increase the stability of the austenitic phase, and improve corrosion resistance. With increasing niobium content, the pitting potential also increases, and at a niobium content of 0.0289% by weight, a pitting potential comparable to or better than that of 316L STS can be ensured. Therefore, it can be concluded that niobium contributes to improving the corrosion resistance of stainless steel.

[0189] However, when the weight percentage of niobium exceeds 0.05, the physical properties of stainless steel may decrease due to the formation of carbides.

[0190] In one embodiment of the third aspect, the pitting potential of the austenitic stainless steel in a 3.5% NaCl solution at 16–20°C can be 300–400 mV, or 350–400 mV.

[0191] In the third aspect, the austenitic stainless steel may have an impact absorption energy of 41 to 100 J, or 41 to 85 J, at approximately -196°C. If the impact absorption energy is less than approximately 41 J, it may not be suitable for liquefied natural gas (LNG) ship applications.

[0192] In the third aspect, when the niobium content of the austenitic stainless steel is in the range of more than 0.01 to 0.5% by weight, it can satisfy the pitting potential expressed by the following formula (1):

[0193] Formula 1

[0194] E pit = 0.8833×Nb + 0.3416

[0195] In Equation 1, Nb refers to the weight of the element.

[0196] (Fourth aspect)

[0197] In the fourth aspect, it relates to austenitic stainless steel which may contain 16 to 20 wt% chromium (Cr), 4.5 to 9 wt% manganese (Mn), 3 to 6 wt% nickel (Ni), 0.001 to 0.3 wt% carbon (C), 0.1 to 0.35 wt% nitrogen (N), more than 0 and less than 2.0 wt% copper (Cu), the balance iron (Fe), and other unavoidable impurities.

[0198] In one embodiment of the fourth aspect, the austenitic stainless steel may contain more than 0 and less than 1.657 weight percent or 0.0222 to 1.5932 weight percent of the copper.

[0199] In one embodiment of the fourth aspect, the austenitic stainless steel may contain 0.15 to 0.45 weight percent of the total content of the carbon and the nitrogen. The fourth aspect particularly relates to the following technical solutions:

[0200] Item 1. An austenitic stainless steel comprising 16 to 20 wt% chromium (Cr), 4.5 to 9 wt% manganese (Mn), 3 to 6 wt% nickel (Ni), 0.001 to 0.3 wt% carbon (C), 0.1 to 0.35 wt% nitrogen (N), more than 0 and less than 2.0 wt% copper (Cu), the balance iron (Fe), and other unavoidable impurities.

[0201] 2. Austenitic stainless steel as described in Project 1.

[0202] The austenitic stainless steel contains more than 0 and less than 1.657, or 0.0222 to 1.5932, or more than 0 and less than 1% of the copper by weight.

[0203] 3. Austenitic stainless steel as described in Project 1.

[0204] The austenitic stainless steel contains 0.15 to 0.45 weight percent of the total content of the carbon and the nitrogen.

[0205] 4. Austenitic stainless steel as described in Project 1.

[0206] The pitting potential of the austenitic stainless steel is 300 to 420 mV at 16–20 °C and in a 3.5% NaCl solution.

[0207] 5. Austenitic stainless steel as described in Project 4.

[0208] The pitting potential of the austenitic stainless steel is 350 to 400 mV at 16–20 °C and in a 3.5% NaCl solution.

[0209] 6. Austenitic stainless steel as described in Project 1.

[0210] The impact absorption energy of the austenitic stainless steel is 80 to 120 J at -196°C.

[0211] 7. Austenitic stainless steel as described in Project 2.

[0212] The pitting potential of the austenitic stainless steel satisfies the following equation (1):

[0213] Formula 1

[0214] E pit = -0.0481×Cu2 + 0.0777×Cu + 0.3654

[0215] In Formula 1, Cu represents the weight of copper.

[0216] 8. Austenitic stainless steel as described in Project 3.

[0217] The austenitic stainless steel contains 8 to 15 weight percent of the total content of nickel and manganese.

[0218] 9. Austenitic stainless steel as described in Item 8.

[0219] The austenitic stainless steel contains 8.5 to 15 weight percent of the total content of nickel and manganese, and 0.225 to 0.33 weight percent of the total content of carbon and nitrogen.

[0220] 10. Austenitic stainless steel as described in Item 9.

[0221] When the wear of the tool is measured using an end mill under dry conditions, the machinability of the austenitic stainless steel is greater than 0 and less than 0.15 mm.

[0222] 11. Austenitic stainless steel as described in Project 1.

[0223] The weight ratio of the carbon to the total weight of the carbon and nitrogen [C / (C+N)] is 0.1 to 0.7.

[0224] 12. Austenitic stainless steel as described in Item 8.

[0225] The following conditions must be met: (Select one or more from Equations 2 to 5)

[0226] Formula 2

[0227] (Ni+Mn) + 165.94(C+N) ≥ 49.47

[0228]

Formula 3

[0229] (Ni+Mn) - 27.28(C+N) ≥ 2.10

[0230]

Formula 4

[0231] (Ni+Mn) ≥ 12.3

[0232]

Formula 5

[0233] (Ni+Mn) + 13.133(C+N) ≥ 1.73

[0234] In Equations 2 to 5, Ni, Mn, C, and N refer to the weights of nickel, manganese, carbon, and nitrogen, respectively.

[0235] 13. Austenitic stainless steel as described in Item 12.

[0236] The conditions of Equations 2 and 5 are satisfied.

[0237] 14. Austenitic stainless steel as described in Item 12.

[0238] The conditions of Equations 2 to 4 are satisfied.

[0239] 15. Austenitic stainless steel as described in Project 3.

[0240] The total content of carbon and nitrogen is less than 0.33% by weight.

[0241] Next, the reasons for limiting the numerical range of the stainless steel component content in the fourth aspect of the present invention will be explained. For reasons similar to those in the first aspect, the austenitic stainless steel comprises 16 to 20 wt% chromium (Cr), 4.5 to 9 wt% manganese (Mn), 3 to 6 wt% nickel (Ni), 0.001 to 0.3 wt% carbon (C), and 0.1 to 0.35 wt% nitrogen (N). Therefore, the description of the basic components identical to those in the stainless steel of the first aspect is omitted, and only the reasons for the inclusion of Cu are described.

[0242] (9) Copper: approximately more than 0 and less than 2.0% by weight

[0243] Copper can contribute to the formation of a highly corrosion-resistant surface by interacting with chromium. The pitting potential gradually increases with increasing copper content, but a decreasing trend is observed when the copper content exceeds 1.0 wt%. Up to 1.657 wt%, the pitting potential is higher than that of 304 STS, and between 0.0222 and 1.5932 wt%, it is higher than that of 316L STS. Therefore, it can be concluded that copper can contribute to improving the corrosion resistance of stainless steel through its interaction with chromium.

[0244] When the weight percentage of copper exceeds 1.657, stainless steel may corrode due to the formation of copper oxides or copper chlorides.

[0245] In the fourth aspect, the pitting potential of the austenitic stainless steel in a 3.5% NaCl solution at 16–20°C can be 300–420 mV, or 350–400 mV.

[0246] In the fourth aspect, the austenitic stainless steel may have an impact absorption energy of 41 to 120 J at approximately -196°C, or it may have an impact absorption energy of 80 to 120 J. If the impact absorption energy is less than approximately 41 J, it may not be suitable for liquefied natural gas (LNG) ship applications.

[0247] In the fourth aspect, when the copper content of the austenitic stainless steel is in the range of more than 0 and less than 1.657 by weight percentage, it can satisfy the pitting potential expressed by the following formula (1):

[0248] Formula 1

[0249] E pit = -0.0481×Cu 2 + 0.0777×Cu + 0.3654

[0250] In Equation 1, Cu refers to the weight of the element.

[0251] (Fifth aspect)

[0252] In the fifth aspect, austenitic stainless steel may contain 16 to 20 wt% chromium (Cr), 4.5 to 9 wt% manganese (Mn), 3 to 6 wt% nickel (Ni), 0.001 to 0.3 wt% carbon (C), 0.1 to 0.35 wt% nitrogen (N), more than 0 and less than 0.055 wt% boron (B), balance iron (Fe), and other unavoidable impurities.

[0253] In one embodiment, the austenitic stainless steel may contain more than 0 and less than 0.0460 or more than 0 and less than 0.0203 by weight of the boron.

[0254] In one embodiment, the austenitic stainless steel may contain 0.15 to 0.45 weight percent of the total content of the carbon and nitrogen. The fifth aspect particularly relates to the following technical solutions:

[0255] Item 1. An austenitic stainless steel comprising 16 to 20 wt% chromium (Cr), 4.5 to 9 wt% manganese (Mn), 3 to 6 wt% nickel (Ni), 0.001 to 0.3 wt% carbon (C), 0.1 to 0.35 wt% nitrogen (N), more than 0 and less than 0.055 wt% boron (B), the balance iron (Fe), and other unavoidable impurities.

[0256] 2. Austenitic stainless steel as described in Project 1.

[0257] The austenitic stainless steel contains boron in a weight percentage of more than 0 and less than 0.0460, or more than 0 and less than 0.0203, or more than 0 and less than 0.028, or more than 0 and less than 0.01.

[0258] 3. Austenitic stainless steel as described in Project 1.

[0259] The austenitic stainless steel contains 0.15 to 0.45 weight percent of the total content of the carbon and the nitrogen.

[0260] 4. Austenitic stainless steel as described in Project 1.

[0261] The pitting potential of the austenitic stainless steel is 220 to 400 mV at 16–20 °C and in a 3.5% NaCl solution.

[0262] 5. Austenitic stainless steel as described in Project 4.

[0263] The pitting potential of the austenitic stainless steel is 300 to 400 mV at 16–20 °C and in a 3.5% NaCl solution.

[0264] 6. Austenitic stainless steel as described in Project 1.

[0265] The impact absorption energy of the austenitic stainless steel is 41 to 100 J at -196°C.

[0266] 7. Austenitic stainless steel as described in Project 1.

[0267] The pitting potential of the austenitic stainless steel satisfies the following equation (1):

[0268] Formula 1

[0269] E pit = -67.85×B 2 + 1.35×B + 0.3676

[0270] In Formula 1, B represents the weight of boron.

[0271] 8. Austenitic stainless steel as described in Project 3.

[0272] The austenitic stainless steel contains 8 to 15 weight percent of the total content of nickel and manganese.

[0273] 9. Austenitic stainless steel as described in Item 8.

[0274] The austenitic stainless steel contains 8.5 to 15 weight percent of the total content of nickel and manganese, and 0.225 to 0.33 weight percent of the total content of carbon and nitrogen.

[0275] 10. Austenitic stainless steel as described in Item 9.

[0276] When the wear of the tool is measured using an end mill under dry conditions, the machinability of the austenitic stainless steel is greater than 0 and less than 0.15 mm.

[0277] 11. Austenitic stainless steel as described in Project 1.

[0278] The weight ratio of the carbon to the total weight of the carbon and nitrogen [C / (C+N)] is 0.1 to 0.7.

[0279] 12. Austenitic stainless steel as described in Item 8.

[0280] The following conditions must be met: (Select one or more from Equations 2 to 5)

[0281] Formula 2

[0282] (Ni+Mn) + 165.94(C+N) ≥ 49.47

[0283]

Formula 3

[0284] (Ni+Mn) - 27.28(C+N) ≥ 2.10

[0285]

Formula 4

[0286] (Ni+Mn) ≥ 12.3

[0287]

Formula 5

[0288] (Ni+Mn) + 13.133(C+N) ≥ 1.73

[0289] In Equations 2 to 5, Ni, Mn, C, and N refer to the weights of nickel, manganese, carbon, and nitrogen, respectively.

[0290] 13. Austenitic stainless steel as described in Item 12.

[0291] The conditions of Equations 2 and 5 are satisfied.

[0292] 14. Austenitic stainless steel as described in Item 12.

[0293] The conditions of Equations 2 to 4 are satisfied.

[0294] 15. Austenitic stainless steel as described in Project 3.

[0295] The total content of carbon and nitrogen is less than 0.33% by weight.

[0296] Next, the reasons for limiting the numerical range of the stainless steel component content in the fifth aspect of the present invention will be explained. Based on reasons similar to those in the first aspect, the austenitic stainless steel comprises 16 to 20 wt% chromium (Cr), 4.5 to 9 wt% manganese (Mn), 3 to 6 wt% nickel (Ni), 0.001 to 0.3 wt% carbon (C), and 0.1 to 0.35 wt% nitrogen (N). Therefore, the description of the basic components identical to those in the stainless steel of the first aspect is omitted, and only the reasons for including component B are described.

[0297] (10) Boron: approximately more than 0 and less than 0.055% by weight

[0298] Boron can react with chromium to form chromium-boron compounds, thereby enhancing corrosion resistance and playing an important role in highly corrosive environments. The pitting potential gradually increases with increasing boron content, but a decreasing trend is observed when the boron content exceeds 0.01 wt%. Up to 0.0460 wt%, the pitting potential is higher than that of 304L STS, and up to 0.0203 wt%, it is higher than that of 316L STS. Therefore, it can be concluded that boron can contribute to improving the corrosion resistance of stainless steel through its interaction with chromium.

[0299] When the boron weight percentage exceeds 0.028, the resulting precipitates may cause a decrease in the physical properties of stainless steel.

[0300] In the fifth aspect, the pitting potential of the austenitic stainless steel in a 3.5% NaCl solution at 16–20°C can be 220–400 mV, or 300–400 mV.

[0301] In the fifth aspect, the austenitic stainless steel may have an impact absorption energy of 41 to 100 J at approximately -196°C, or it may have an impact absorption energy of 60 to 100 J. If the impact absorption energy is less than approximately 41 J, it may not be suitable for liquefied natural gas (LNG) ship applications.

[0302] In the fifth aspect, when the boron content of the austenitic stainless steel is in the range of more than 0 and less than 0.055 by weight percentage, it can satisfy the pitting potential expressed by the following formula (1):

[0303] Formula 1

[0304] E pit = -67.85×B 2 + 1.35×B + 0.3676

[0305] In Equation 1, B refers to the weight of the element.

[0306] Next, to aid in understanding the present invention, detailed descriptions will be provided using examples. However, the following examples are merely illustrative of the content of the present invention, and the scope of the present invention is not limited to the following examples. The examples of the present invention are provided solely to provide a more complete explanation of the present invention to those skilled in the art.

[0307] An embodiment of the first aspect.

[0308] <Example 1.1>

[0309] Nitrogen gas is injected into a vacuum induction melting furnace (VIM), and tungsten is added to steel with a chemical composition of 16–20 wt% Cr, 4.5–9 wt% Mn, 3–6 wt% Ni, 0.05–0.3 wt% C, and 0.1–0.35 wt% N to produce small steel ingots. These ingots are then hot-rolled after being heated to approximately 1200°C to produce sheet metal with a thickness of approximately 11.5 mm. The nitrogen content is adjusted by controlling the amount added to the VIM and the reaction time. The Ni+2W content is adjusted to approximately 4.9 wt%.

[0310] <Example 1.2>

[0311] Except that the Ni+2W content was set to approximately 5.5349 weight percent based on Example 1, small steel ingots were manufactured in the same manner as in Example 1.

[0312] <Example 1.3>

[0313] Except that the Ni+2W content was set to approximately 5.85% by weight based on Example 1, small steel ingots were manufactured in the same manner as in Example 1.

[0314] <Example 1.4>

[0315] Except that the Ni+2W content was set to approximately 7.52% by weight based on Example 1, small steel ingots were manufactured in the same manner as in Example 1.

[0316] <Comparative Example 1.1>

[0317] Small steel ingots were manufactured in the same manner as in Example 1, except that the Ni+2W content was set to approximately 4.27% by weight, or according to the composition not falling within the first aspect in Table 1, or without the addition of tungsten, based on Example 1.

[0318] <Comparative Example 1.2>

[0319] Commercial 304L STS was used.

[0320] <Comparative Example 1.3>

[0321] Commercial 316L STS was used.

[0322] <Experimental Example 1.1>

[0323] Figure 1 These are images showing the results of observing fine tissues according to an embodiment of the present invention, the fine tissues being observed using an optical microscope. See also... Figure 1 It can be confirmed that even with increased tungsten content, only austenite was observed in the microstructure, and no δ-ferrite was observed. Furthermore, annealed twins were observed within the grains, and the grain boundaries exhibited a straight characteristic; no precipitates formed during cooling were observed at the grain boundaries.

[0324] Figure 2 This is a graph showing the pitting potential measured according to an embodiment of the present invention. Figure 2 The figure shows the results of pitting potential measurement for austenitic stainless steel with Ni+2W content changed from 4 wt% to 8.5 wt% (including Examples 1 to 4 and Comparative Examples 1 to 3) under the conditions of 3.5 wt% NaCl, 16–20 °C, 0.2 mV / s (potential sweep rate), and 1500 mesh (grit) SiC (surface finish). See also... Figure 2 A tendency was observed that the pitting potential gradually increased with increasing Ni+2W content. Therefore, it can be confirmed that the embodiments according to the present invention have a pitting potential superior to the existing 304L STS, thus exhibiting excellent corrosion resistance. Specifically, it can be confirmed that when the Ni+2W content is 4.7218 wt% or more, a pitting potential of the same level as 304L STS can be ensured, and when the Ni+2W content is 6.2590 wt% or more, a pitting potential of the same level as 316L STS can be ensured.

[0325] Figure 3This is a graph showing the impact absorption energy results according to an embodiment of the present invention. The impact absorption energy was measured using a Charpy impact tester (Tinius Olsen, IT542E) according to ISO 148-1: Metallic Charpy Impact Test Standard. For liquefied natural gas (LNG) ship applications, the impact absorption energy at approximately -196°C needs to be at least approximately 41 J. It can be confirmed that in Examples 1 to 4, all impact absorption energies reached approximately 60 J or more, and the impact absorption energy at low temperatures increased slightly with increasing Ni+2W content.

[0326] <Example 1.5>

[0327] 1) Manufacturing of small steel ingots

[0328] After the raw materials Fe, Cr, Mn, Ni, and C are added to the crucible, the crucible is placed into the chamber and a vacuum is drawn to remove water vapor, oxygen, and nitrogen present inside the chamber.

[0329] After injecting high-purity Ar and high-purity nitrogen (N) gas under vacuum, the crucible is heated to 1530°C by applying current to an induction furnace. This melts the raw materials, ensuring a uniform distribution of the injected elements. The nitrogen concentration in the molten steel is controlled by adjusting the nitrogen gas pressure. The molten steel is then poured into a mold to produce a cubic steel ingot (44-45mm × 28-30mm × 67-70mm).

[0330] 2) Rolling

[0331] A rolling process is used to shape small steel ingots into plates. After holding the small steel ingots at 1200°C for 2 hours, they are plastically deformed through a rolling process to produce a flat and elongated plate shape. At this point, to process the small steel ingots into plate shapes, heating is used to maintain low processing stress.

[0332] 3) Heat treatment

[0333] The flattened sheet material is subjected to normalizing heat treatment, which involves heating to 1130°C, maintaining the temperature for 1 hour, and then water cooling.

[0334] <Experimental Example 1.2>

[0335] To confirm the stability of the austenitic stainless steel according to the present invention, the phase diagram of the austenitic stainless steel was obtained by observing the austenitic stainless steel manufactured using the various components using an optical microscope. Some exemplary results satisfying the embodiments and comparative examples are as follows: Figure 4And as shown in Table 1. In Table 1, alloy [4] corresponds to tungsten-containing comparative example 1.1 and alloys

[60] and

[69] correspond to tungsten-free comparative example 1.1; alloys whose composition does not meet the basic alloy composition of the present invention or the stainless steel composition of this aspect were prepared according to comparative example 1.1, and alloy samples with a nickel content of less than 3.9% were prepared according to example 1.1, alloy samples with a nickel content of 3.9-4.5% were prepared according to example 1.2, alloy samples with a nickel content of 4.5-5.0% were prepared according to example 1.3, and alloy samples with a nickel content of more than 5.0% were prepared according to example 1.4. In some cases, the content of individual components in the chemical composition slightly exceeds the content range claimed in the present invention, which may still be regarded as falling within the scope of the present invention, or may be regarded as experimental error.

[0336] Table 1

[0337]

[0338]

[0339]

[0340] Figure 4 The phase map is based on the composition ratio of austenitic stainless steel and is generated based on the results in Table 1. Table 1 shows the results of observing the microstructure of each sample according to the composition ratio using an optical microscope.

[0341] See Figure 4 It can be confirmed that when the content of nickel and manganese (Ni+Mn) is 6% by weight or more, and the content of carbon and nitrogen (C+N) is 0.225% by weight, the austenite phase can be stabilized to room temperature. Specifically, based on the phase diagram, mathematical formulas corresponding to the stable region of the γ phase are derived, which can be expressed as the following mathematical formulas 1 and 2. When the following mathematical formulas 1 and 2 are satisfied, the austenite phase can be stabilized to room temperature.

[0342] [Mathematical Expression 1]

[0343] (Ni+Mn) + 165.94(C+N) ≥ 49.47

[0344] [Mathematical Expression 2]

[0345] (Ni+Mn) + 13.133(C+N) ≥ 1.73

[0346] In mathematical formulas 1 and 2, Ni represents the weight percentage of nickel, Mn represents the weight percentage of manganese, C represents the weight percentage of carbon, and N represents the weight percentage of nitrogen.

[0347] <Experimental Example 1.3>

[0348] To confirm the impact absorption properties of the austenitic stainless steel according to the present invention, the impact absorption energy was measured, and based on the impact absorption energy and the phase diagram obtained through Experimental Example 1.2, the composition ratio of austenitic stainless steel with phase stability at room temperature and impact absorption energy above a certain level was confirmed. The results are as follows: Figure 5 And as shown in Table 2.

[0349] The impact absorption energy was determined through the impact experiment described below.

[0350] Test pieces were collected from the intact parts of the plate (where shrinkage cavities are present at the top of the ingot), and standard-sized test pieces (10×10×55mm) were collected along the rolling direction. A notch (2mm V-notch) was machined perpendicular to the rolling surface, and six impact test pieces were machined from each plate.

[0351] The impact test was conducted at -196°C. After immersing the specimen at -196°C for 30 minutes, the test was performed on the impact tester according to ASTM A370-20 standard. Two specimens were used to perform the test at each test temperature.

[0352] Table 2

[0353]

[0354] Figure 5 This is an image displaying the phase diagram and impact absorption energy according to the composition ratio of austenitic stainless steel. See also... Figure 5 It can be confirmed that stability can be improved when the content of nickel and manganese (Ni+Mn) is above 8% by weight and the content of carbon and nitrogen (C+N) is 0.225% by weight.

[0355] In addition, Table 2 shows the results of the impact absorption energy measurement of specimens manufactured according to the composition ratio of austenitic stainless steel.

[0356] The contents of Table 2 are used to confirm the microstructure stability of austenitic stainless steel at room temperature and the impact absorption energy of austenitic stainless steel at low temperature (-196℃) reaching a certain level. It can be confirmed that when the content of nickel and manganese (Ni+Mn) is above 8.5% by weight and the content of carbon and nitrogen (C+N) is above 0.225% by weight, austenite meets the requirements of microstructure stability and impact absorption energy above 41J.

[0357] <Experimental Example 1.4>

[0358] To confirm the machinability of the austenitic stainless steel according to the present invention, a machinability test was performed on specimens manufactured according to the composition ratio of austenitic stainless steel and 316 stainless steel (STS). The results are as follows: Figure 6 As shown.

[0359] The machining tests were conducted by measuring and evaluating tool wear after five passes using an end mill under dry conditions, as shown in Table 3 below. Furthermore, the composition of each test piece is shown in Table 4, with units of weight%.

[0360] Table 3

[0361]

[0362] Table 4

[0363]

[0364] Figure 6 These are the results of a machinability test on specimens manufactured according to the composition ratio of austenitic stainless steel. (See also...) Figure 6 It can be confirmed that its processability is excellent when the total carbon and nitrogen (C+N) is below 0.33% by weight.

[0365] <Experimental Example 1.5>

[0366] To confirm the corrosion resistance of the austenitic stainless steel according to the present invention, anodic polarization tests, salt spray tests, and intergranular corrosion tests were performed on specimens according to the composition ratio of austenitic stainless steel and existing commercial materials, namely 304 stainless steel (STS) and 316 stainless steel (STS). The results are as follows: Figures 7 to 9 And as shown in Table 5.

[0367] The anodic polarization experiment was performed according to ASTM G5, using 3.5% by weight NaCl (seawater) as the solution and maintaining the temperature at 25°C.

[0368] The salt spray test was performed according to KS D 9502:2020, and the experimental facility was KTR (Known Testing Research Institute). The solution used was (50±5) g / L NaCl, the temperature was 35±2℃, and the test was maintained for 500 hours. The sample size was W24×L(40-50)×T(2-3) mm. 3 The experiment was conducted using three samples of each alloy.

[0369] The intergranular corrosion test was performed according to ASTM A262-15 (2021) to assess the degree of intergranular sensitization by immersion in 10% oxalic acid at 25°C followed by application of 1 mA·cm⁻¹. -2 After etching the sample for 90 seconds, the presence or absence of intergranular corrosion was determined by surface observation.

[0370] Table 5

[0371]

[0372] Figure 7 These are the results of anodizing experiments on specimens manufactured according to the composition ratio of austenitic stainless steel. (See also...) Figure 7 As shown in Table 5, it can be confirmed that the pitting potential of the economical austenitic stainless steel according to the present invention ensures a pitting potential that is equal to or greater than that of 304 stainless steel, and has a pitting potential that is almost similar to that of 316 stainless steel.

[0373] Figure 8 This is an image illustrating the results of a salt spray test on specimens manufactured according to the composition ratio of austenitic stainless steel. (See also...) Figure 8 As shown in Table 5, no damage or quality change occurred even after 500 hours of exposure to salt water. In particular, the results of the evaluation and comparison of the corrosion resistance of existing commercial materials, namely STS 304 and STS 316, confirm that austenitic stainless steel ensures corrosion resistance at or above the same level as STS 316.

[0374] Figure 9 This is an image illustrating the intergranular corrosion test results of specimens manufactured according to the composition ratio of austenitic stainless steel. (See also...) Figure 9 It can be confirmed that no intergranular corrosion occurs in the economical austenitic stainless steel according to the present invention.

[0375] This confirms that the austenitic stainless steel of the present invention has strong corrosion resistance.

[0376] An embodiment of the second aspect of the present invention is as follows.

[0377] <Example 2.1>

[0378] Nitrogen gas is injected into a vacuum induction melting furnace (VIM), and silicon is added to steel containing 16–20 wt% Cr, 4.5–9 wt% Mn, 3–6 wt% Ni, 0.05–0.3 wt% C, and 0.1–0.35 wt% N to produce small steel ingots. These ingots are then hot-rolled after being heated to approximately 1200°C to produce sheet metal with a thickness of approximately 11.5 mm. The nitrogen content is adjusted by controlling the amount added to the VIM and the reaction time. The silicon content is adjusted to approximately 0.1555 wt%.

[0379] <Example 2.2>

[0380] Except that the silicon content was set to approximately 0.26% by weight based on Example 1, small steel ingots were manufactured in the same manner as in Example 1.

[0381] <Example 2.3>

[0382] Except that the silicon content was set to approximately 0.46% by weight based on Example 1, small steel ingots were manufactured in the same manner as in Example 1.

[0383] <Example 2.4>

[0384] Except that the silicon content was set to approximately 0.55% by weight based on Example 1, small steel ingots were manufactured in the same manner as in Example 1.

[0385] <Example 2.5>

[0386] Except that the silicon content was set to approximately 0.75% by weight based on Example 1, small steel ingots were manufactured in the same manner as in Example 1.

[0387] <Example 2.6>

[0388] Except that the silicon content was set to approximately 0.925% by weight based on Example 1, small steel ingots were manufactured in the same manner as in Example 1.

[0389] <Example 2.7>

[0390] Except that the silicon content was set to approximately 0.94% by weight based on Example 1, small steel ingots were manufactured in the same manner as in Example 1.

[0391] <Comparative Example 2.1>

[0392] Commercial 304L STS was used.

[0393] <Comparative Example 2.2>

[0394] Commercial 316L STS was used.

[0395] <Experimental Example 2.1>

[0396] Figure 10 These are images showing the results of observing fine tissues according to an embodiment of the present invention, the fine tissues being observed using an optical microscope. See also... Figure 10 It can be confirmed that even with a silicon addition of 0.94 wt%, only austenitic structure was observed, and no δ-ferrite structure was observed. Furthermore, the grain size was uniform at approximately 60 μm, and annealed twins were observed within the grains, with straight grain boundaries. With increasing Si addition, no other precipitates were observed besides Si-rich oxides.

[0397] Figure 11 This is a graph showing the pitting potential measured according to an embodiment of the present invention. Figure 11 The figure shows the results of pitting potential measurement for austenitic stainless steel with silicon content changed from 0 wt% to 0.925 wt% (including Examples 1 to 7, Comparative Example 1 and Comparative Example 2) under the conditions of 3.5 wt% NaCl, 16–20 °C, 0.2 mV / s (potential sweep rate), and 1500 mesh (grit) SiC (surface finish). See also... Figure 11 A tendency was observed that the pitting potential gradually increases with increasing silicon content. Therefore, it can be confirmed that the embodiments according to the present invention have a pitting potential superior to the existing 304L STS, thus exhibiting excellent corrosion resistance. Specifically, it can be confirmed that when the silicon content is 0.1555% by weight or higher, it has a pitting potential superior to the existing 316L STS.

[0398] Figure 12 This is a graph showing the impact absorption energy results according to an embodiment of the present invention. The impact absorption energy was measured using a Charpy impact tester (Tinius Olsen, IT542E) according to ISO 148-1: Metallic Charpy Impact Test Standard. For use in liquefied natural gas (LNG) carriers, the impact absorption energy at approximately -196°C needs to be at least approximately 41 J. In Examples 1 to 7, all impact absorption energies reached at least approximately 60 J. This confirms that the silicon content has a negligible effect on the impact absorption energy at low temperatures.

[0399] <Example 2.8>

[0400] 1) Manufacturing of small steel ingots

[0401] After the raw materials Fe, Cr, Mn, Ni, and C are added to the crucible, the crucible is placed into the chamber and a vacuum is drawn to remove water vapor, oxygen, and nitrogen present inside the chamber.

[0402] After injecting high-purity Ar and high-purity nitrogen (N) gas under vacuum, the crucible is heated to 1530°C by applying current to an induction furnace. This melts the raw materials, ensuring a uniform distribution of the injected elements. The nitrogen concentration in the molten steel is controlled by adjusting the nitrogen gas pressure. The molten steel is then poured into a mold to produce a cubic steel ingot (44-45mm × 28-30mm × 67-70mm).

[0403] 2) Rolling

[0404] A rolling process is used to shape small steel ingots into plates. After holding the small steel ingots at 1200°C for 2 hours, they are plastically deformed through a rolling process to produce a flat and elongated plate shape. At this point, to process the small steel ingots into plate shapes, heating is used to maintain low processing stress.

[0405] 3) Heat treatment

[0406] The flattened sheet material is subjected to normalizing heat treatment, which involves heating to 1130°C, maintaining the temperature for 1 hour, and then water cooling.

[0407] <Experimental Example 2.2>

[0408] To confirm the stability of the austenitic stainless steel according to the present invention, a phase diagram of the austenitic stainless steel was obtained by observing the austenitic stainless steel manufactured using the various components using an optical microscope, and the results were similar to those of the first aspect. Figure 4 And Table 1, thus omitting the relevant descriptions.

[0409] Furthermore, the impact absorption properties of austenitic stainless steel were confirmed and measured according to Experimental Example 1.3 of the first aspect, and the results were similar to those of the first aspect. Figure 5 And Table 2; the processing properties of austenitic stainless steel were confirmed and measured according to Experimental Example 1.4 of the first aspect, and the results were similar to those of the first aspect. Figure 6 The corrosion resistance of austenitic stainless steel was confirmed and measured according to Experimental Example 1.5 of the first aspect, and the results were similar to those of the first aspect. Figure 7-9 And Table 5, the relevant descriptions are omitted here.

[0410] An embodiment of the third aspect of the present invention is as follows.

[0411] <Example 3.1>

[0412] Nitrogen gas is injected into a vacuum induction melting furnace (VIM), and niobium is added to steel containing 16–20 wt% Cr, 4.5–9 wt% Mn, 3–6 wt% Ni, 0.05–0.3 wt% C, and 0.1–0.35 wt% N to produce small steel ingots. These ingots are then hot-rolled after being heated to approximately 1200°C to produce sheet metal with a thickness of approximately 11.5 mm. The nitrogen content is adjusted by controlling the amount added to the VIM and the reaction time. The niobium content is adjusted to 0.01 wt%.

[0413] <Example 3.2>

[0414] Except that the niobium content was set to 0.02% by weight based on Example 1, small steel ingots were manufactured in the same manner as in Example 1.

[0415] <Example 3.3>

[0416] Except that the niobium content was set to 0.048% by weight based on Example 1, small steel ingots were manufactured in the same manner as in Example 1.

[0417] <Example 3.4>

[0418] Except that the niobium content was set to 0.054% by weight based on Example 1, small steel ingots were manufactured in the same manner as in Example 1.

[0419] <Example 3.5>

[0420] Except that the niobium content was set to 0.05% by weight based on Example 1, small steel ingots were manufactured in the same manner as in Example 1.

[0421] <Comparative Example 3.1>

[0422] Except that the niobium content was set to 0.08% by weight based on Example 1, small steel ingots were manufactured in the same manner as in Example 1.

[0423] <Comparative Example 3.2>

[0424] Except that the niobium content was set to 0.1% by weight based on Example 1, small steel ingots were manufactured in the same manner as in Example 1.

[0425] <Comparative Example 3.3>

[0426] Except that the niobium content was set to 0.2% by weight based on Example 1, small steel ingots were manufactured in the same manner as in Example 1.

[0427] <Comparative Example 3.4>

[0428] Except that the niobium content was set to 0.3% by weight based on Example 1, small steel ingots were manufactured in the same manner as in Example 1.

[0429] <Comparative Example 3.5>

[0430] Commercial 304L STS was used.

[0431] <Comparative Example 3.6>

[0432] Commercial 316L STS was used.

[0433] <Experimental Example 3.1>

[0434] Figure 13 These are images showing the results of observing fine tissues according to an embodiment of the present invention, the fine tissues being observed using an optical microscope. See also... Figure 13 It can be confirmed that even with an niobium addition of 0.5% by weight, only austenitic structure was observed, and no δ-ferrite structure was observed. This confirms that the addition of niobium does not affect the microstructure. Furthermore, annealed twins were observed within the grains, and the grain boundaries exhibited a straight characteristic; no precipitates formed during cooling were observed at the grain boundaries.

[0435] Figure 14 This is a graph showing the pitting potential measured according to an embodiment of the present invention. Figure 14 The figure illustrates the results of pitting potential measurements for Examples 1 to 5 and Comparative Examples 1 to 6 under the conditions of 3.5 wt% NaCl, 16–20 °C, 0.2 mV / s (potential sweep rate), and 1500 mesh (grit) SiC (surface finish). See also... Figure 14It can be confirmed that the pitting potential increases with increasing niobium content. Specifically, it can be confirmed that when the niobium content reaches 0.0289% by weight or more, it exhibits a pitting potential comparable to or higher than that of 316L STS.

[0436] Figure 15 This is a graph showing the impact absorption energy results according to an embodiment of the present invention. The impact absorption energy was measured using a Charpy impact tester (Tinius Olsen, IT542E) according to ISO 148-1: Metallic Charpy Impact Test Standard. For liquefied natural gas (LNG) ship applications, the impact absorption energy at approximately -196°C needs to be at least approximately 41 J. In Examples 1 to 5, all impact absorption energies reached at least approximately 41 J, while in contrast, in Comparative Examples 1 to 4, all impact absorption energies were less than 41 J.

[0437] <Example 3.6>

[0438] 1) Manufacturing of small steel ingots

[0439] After the raw materials Fe, Cr, Mn, Ni, and C are added to the crucible, the crucible is placed into the chamber and a vacuum is drawn to remove water vapor, oxygen, and nitrogen present inside the chamber.

[0440] After injecting high-purity Ar and high-purity nitrogen (N) gas under vacuum, the crucible is heated to 1530°C by applying current to an induction furnace. This melts the raw materials, ensuring a uniform distribution of the injected elements. The nitrogen concentration in the molten steel is controlled by adjusting the nitrogen gas pressure. The molten steel is then poured into a mold to produce a cubic steel ingot (44-45mm × 28-30mm × 67-70mm).

[0441] 2) Rolling

[0442] A rolling process is used to shape small steel ingots into plates. After holding the small steel ingots at 1200°C for 2 hours, they are plastically deformed through a rolling process to produce a flat and elongated plate shape. At this point, to process the small steel ingots into plate shapes, heating is used to maintain low processing stress.

[0443] 3) Heat treatment

[0444] The flattened sheet material is subjected to normalizing heat treatment, which involves heating to 1130°C, maintaining the temperature for 1 hour, and then water cooling.

[0445] <Experimental Example 3.2>

[0446] To confirm the stability of the austenitic stainless steel according to the present invention, a phase diagram of the austenitic stainless steel was obtained by observing the austenitic stainless steel manufactured using the various components using an optical microscope, and the results were similar to those of the first aspect. Figure 4 And Table 1, thus omitting the relevant descriptions.

[0447] Furthermore, the impact absorption properties of austenitic stainless steel were confirmed and measured according to Experimental Example 1.3 of the first aspect, and the results were similar to those of the first aspect. Figure 5 And Table 2; the processing properties of austenitic stainless steel were confirmed and measured according to Experimental Example 1.4 of the first aspect, and the results were similar to those of the first aspect. Figure 6 The corrosion resistance of austenitic stainless steel was confirmed and measured according to Experimental Example 1.5 of the first aspect, and the results were similar to those of the first aspect. Figure 7-9 And Table 5, the relevant descriptions are omitted here.

[0448] An embodiment of the fourth aspect of the present invention is as follows.

[0449] <Example 4.1>

[0450] Nitrogen gas is injected into a vacuum induction melting furnace (VIM), and commercially available electrolytic iron, chromium, manganese, nickel, molybdenum, and carbon powder (purity ≥ 95%) are used as a master alloy to melt and produce small steel ingots. These ingots are then heated to approximately 1200°C and hot-rolled to produce sheet metal with a thickness of approximately 11.5 mm. The nitrogen content is adjusted by controlling the amount of nitrogen added to the VIM and the reaction time. The copper content is adjusted to 0.501% by weight.

[0451] <Example 4.2>

[0452] Except that the copper content was set to 0.874% by weight based on Example 1, small steel ingots were manufactured in the same manner as in Example 1.

[0453] <Example 4.3>

[0454] Except that the copper content was set to approximately 1.272% by weight based on Example 1, small steel ingots were manufactured in the same manner as in Example 1.

[0455] <Example 4.4>

[0456] Except that the copper content was set to 1.657% by weight based on Example 1, small steel ingots were manufactured in the same manner as in Example 1.

[0457] <Comparative Example 4.1>

[0458] Commercial 304L STS was used.

[0459] <Comparative Example 4.2>

[0460] Commercial 316L STS was used.

[0461] <Experimental Example 4.1>

[0462] Figure 16 These are images showing the results of observing fine tissues according to an embodiment of the present invention, the fine tissues being observed using an optical microscope. See also... Figure 16 It can be confirmed that even with a copper addition of 1.657 wt%, only austenitic structure was observed, and no δ-ferrite structure was observed. This confirms that the addition of copper does not affect the microstructure. Annealing twins were observed within the grains, and the grain boundaries exhibited a straight characteristic; no precipitates formed during cooling were observed at the grain boundaries.

[0463] Figure 17 This is a graph showing the pitting potential measured according to an embodiment of the present invention. Figure 17 The figure illustrates the results of pitting potential measurements performed on Examples 1 to 4, Comparative Examples 1 and 2, under the conditions of 3.5 wt% NaCl, 16–20 °C, 0.2 mV / s (potential sweep rate), and 1500 mesh (grit) SiC (surface finish). See also... Figure 17 The pitting potential increases with increasing copper content, but a decreasing trend is observed when the addition amount reaches 1.0 wt%. However, it can be confirmed that even with a copper addition of 1.657 wt%, excellent corrosion resistance is exhibited due to its superior pitting potential compared to the existing 304 STS. It can also be confirmed that a higher pitting potential than 316L STS is observed when the copper addition amount is between 0.0222 and 1.5932 wt%.

[0464] Figure 18 This is a graph showing the impact absorption energy results according to an embodiment of the present invention. The impact absorption energy was measured using a Charpy impact tester (Tinius Olsen, IT542E) according to ISO 148-1: Metallic Charpy Impact Test Standard. For liquefied natural gas (LNG) ship applications, the impact absorption energy at approximately -196°C needs to be at least approximately 41 J. In Examples 1 to 4, all impact absorption energies reached at least approximately 80 J.

[0465] <Example 4.5>

[0466] 1) Manufacturing of small steel ingots

[0467] After the raw materials Fe, Cr, Mn, Ni, and C are added to the crucible, the crucible is placed into the chamber and a vacuum is drawn to remove water vapor, oxygen, and nitrogen present inside the chamber.

[0468] After injecting high-purity Ar and high-purity nitrogen (N) gas under vacuum, the crucible is heated to 1530°C by applying current to an induction furnace. This melts the raw materials, ensuring a uniform distribution of the injected elements. The nitrogen concentration in the molten steel is controlled by adjusting the nitrogen gas pressure. The molten steel is then poured into a mold to produce a cubic steel ingot (44-45mm × 28-30mm × 67-70mm).

[0469] 2) Rolling

[0470] A rolling process is used to shape small steel ingots into plates. After holding the small steel ingots at 1200°C for 2 hours, they are plastically deformed through a rolling process to produce a flat and elongated plate shape. At this point, to process the small steel ingots into plate shapes, heating is used to maintain low processing stress.

[0471] 3) Heat treatment

[0472] The flattened sheet material is subjected to normalizing heat treatment, which involves heating to 1130°C, maintaining the temperature for 1 hour, and then water cooling.

[0473] <Experimental Example 4.2>

[0474] To confirm the stability of the austenitic stainless steel according to the present invention, a phase diagram of the austenitic stainless steel was obtained by observing the austenitic stainless steel manufactured using the various components using an optical microscope, and the results were similar to those of the first aspect. Figure 4 And Table 1, thus omitting the relevant descriptions.

[0475] Furthermore, the impact absorption properties of austenitic stainless steel were confirmed and measured according to Experimental Example 1.3 of the first aspect, and the results were similar to those of the first aspect. Figure 5 And Table 2; the processing properties of austenitic stainless steel were confirmed and measured according to Experimental Example 1.4 of the first aspect, and the results were similar to those of the first aspect. Figure 6 The corrosion resistance of austenitic stainless steel was confirmed and measured according to Experimental Example 1.5 of the first aspect, and the results were similar to those of the first aspect. Figure 7-9And Table 5, the relevant descriptions are omitted here.

[0476] An embodiment of the fifth aspect of the present invention is as follows.

[0477] <Example 5.1>

[0478] Nitrogen gas is injected into a vacuum induction melting furnace (VIM), and commercially available electrolytic iron, chromium, manganese, nickel, molybdenum, and carbon powder (purity ≥ 95%) are used as a master alloy to melt and produce small steel ingots. These ingots are then hot-rolled after being heated to approximately 1200°C to produce sheet metal with a thickness of approximately 11.5 mm. The nitrogen content is adjusted by controlling the amount of nitrogen added to the VIM and the reaction time. The boron content is adjusted to approximately 0.0055% by weight.

[0479] <Example 5.2>

[0480] Except that the boron content was set to approximately 0.0124% by weight based on Example 1, small steel ingots were manufactured in the same manner as in Example 1.

[0481] <Example 5.3>

[0482] Except that the boron content was set to approximately 0.028% by weight based on Example 1, small steel ingots were manufactured in the same manner as in Example 1.

[0483] <Comparative Example 5.1>

[0484] Except that the boron content was set to approximately 0.032 weight percent based on Example 1, small steel ingots were manufactured in the same manner as in Example 1.

[0485] <Comparative Example 5.2>

[0486] Except that the boron content was set to approximately 0.055% by weight based on Example 1, small steel ingots were manufactured in the same manner as in Example 1.

[0487] <Comparative Example 5.3>

[0488] Commercial 304L STS was used.

[0489] <Comparative Example 5.4>

[0490] Commercial 316L STS was used.

[0491] <Experimental Example 5.1>

[0492] Figure 19 These are images showing the results of observing fine tissues according to an embodiment of the present invention, the fine tissues being observed using an optical microscope. See also... Figure 19 It can be confirmed that even with a boron addition of 0.055% by weight, only austenitic structure was observed, and no δ-ferrite structure was observed. Furthermore, annealed twins were observed within the grains, and the grain boundaries exhibited a straight characteristic; no precipitates formed during cooling were observed at the grain boundaries.

[0493] Figure 20 This is a graph showing the pitting potential measured according to an embodiment of the present invention. Figure 20 The figure shows the results of pitting potential measurement for austenitic stainless steel with boron content changed from 0 wt% to 0.055 wt% (including Examples 1 to 3 and Comparative Examples 1 to 4) under the conditions of 3.5 wt% NaCl, 16–20 °C, 0.2 mV / s (potential sweep rate), and 1500 mesh (grit) SiC (surface finish). See also... Figure 20 As the boron content increases, the pitting potential gradually increases, but a decreasing trend in pitting potential can be observed when the boron content reaches 0.01 wt% or more. Specifically, when the boron content is 0 to 0.0460 wt%, a higher pitting potential than 304L STS is observed. Furthermore, when the boron content is 0 to 0.0203 wt%, a higher pitting potential than 316L STS is observed. Therefore, it can be confirmed that the embodiments according to the present invention have a pitting potential superior to the existing 304L STS and 316L STS, thus exhibiting excellent corrosion resistance.

[0494] Figure 21 This is a graph showing the impact absorption energy results according to an embodiment of the present invention. The impact absorption energy was measured using a Charpy impact tester (Tinius Olsen, IT542E) according to ISO 148-1: Metallic Charpy Impact Test Standard. For liquefied natural gas (LNG) ship applications, the impact absorption energy at approximately -196°C needs to be at least approximately 41 J. In Examples 1 to 3, all impact absorption energies reached at least approximately 60 J.

[0495] <Example 5.4>

[0496] 1) Manufacturing of small steel ingots

[0497] After the raw materials Fe, Cr, Mn, Ni, and C are added to the crucible, the crucible is placed into the chamber and a vacuum is drawn to remove water vapor, oxygen, and nitrogen present inside the chamber.

[0498] After injecting high-purity Ar and high-purity nitrogen (N) gas under vacuum, the crucible is heated to 1530°C by applying current to an induction furnace. This melts the raw materials, ensuring a uniform distribution of the injected elements. The nitrogen concentration in the molten steel is controlled by adjusting the nitrogen gas pressure. The molten steel is then poured into a mold to produce a cubic steel ingot (44-45mm × 28-30mm × 67-70mm).

[0499] 2) Rolling

[0500] A rolling process is used to shape small steel ingots into plates. After holding the small steel ingots at 1200°C for 2 hours, they are plastically deformed through a rolling process to produce a flat and elongated plate shape. At this point, to process the small steel ingots into plate shapes, heating is used to maintain low processing stress.

[0501] 3) Heat treatment

[0502] The flattened sheet material is subjected to normalizing heat treatment, which involves heating to 1130°C, maintaining the temperature for 1 hour, and then water cooling.

[0503] <Experimental Example 5.2>

[0504] To confirm the stability of the austenitic stainless steel according to the present invention, a phase diagram of the austenitic stainless steel was obtained by observing the austenitic stainless steel manufactured using the various components using an optical microscope, and the results were similar to those of the first aspect. Figure 4 And Table 1, thus omitting the relevant descriptions.

[0505] Furthermore, the impact absorption properties of austenitic stainless steel were confirmed and measured according to Experimental Example 1.3 of the first aspect, and the results were similar to those of the first aspect. Figure 5 And Table 2; the processing properties of austenitic stainless steel were confirmed and measured according to Experimental Example 1.4 of the first aspect, and the results were similar to those of the first aspect. Figure 6 The corrosion resistance of austenitic stainless steel was confirmed and measured according to Experimental Example 1.5 of the first aspect, and the results were similar to those of the first aspect. Figure 7-9 And Table 5, the relevant descriptions are omitted here.

[0506] Furthermore, the impact absorption properties of austenitic stainless steel were confirmed and measured according to Experimental Example 1.3 of the first aspect, and the results were similar to those of the first aspect. Figure 5 And Table 2; the processing properties of austenitic stainless steel were confirmed and measured according to Experimental Example 1.4 of the first aspect, and the results were similar to those of the first aspect. Figure 6 The corrosion resistance of austenitic stainless steel was confirmed and measured according to Experimental Example 1.5 of the first aspect, and the results were similar to those of the first aspect. Figure 7-9 And Table 5, the relevant descriptions are omitted here.

[0507] The preferred embodiments of the present invention have been described above with reference to them. However, those skilled in the art will understand that the present invention can be modified and altered in various ways without departing from the spirit and scope of the invention as set forth in the appended claims.

Claims

1. An austenitic stainless steel comprising 16 to 20 wt% chromium (Cr), 4.5 to 9 wt% manganese (Mn), 3 to 6 wt% nickel (Ni), 0.001 to 0.3 wt% carbon (C), 0.1 to 0.35 wt% nitrogen (N), more than 0.0 and less than 2.0 wt% tungsten (W), the balance iron (Fe), and other unavoidable impurities.

2. The austenitic stainless steel according to claim 1, The austenitic stainless steel contains 4 to 9 weight percent of the total content of the nickel and the tungsten.

3. The austenitic stainless steel according to claim 2, The austenitic stainless steel contains 4.27 to 8.2% by weight of the nickel and the total content of the tungsten.

4. The austenitic stainless steel according to claim 1, The austenitic stainless steel contains 0.15 to 0.45 weight percent of the total content of the carbon and the nitrogen.

5. The austenitic stainless steel according to claim 1, The austenitic stainless steel has a pitting potential of 250 to 500 mV in a 3.5% NaCl solution at 16–20 °C.

6. The austenitic stainless steel according to claim 5, The austenitic stainless steel has a pitting potential of 350 to 500 mV in a 3.5% NaCl solution at 16–20 °C.

7. The austenitic stainless steel according to claim 1, The austenitic stainless steel absorbs 60 to 100 J of impact energy at -196°C.

8. The austenitic stainless steel according to claim 1, The austenitic stainless steel satisfies the pitting potential expressed by the following formula (1): Formula 1 HAVE BEEN pit = 0.05263(Ni+2W) + 0.03769 In Equation 1, Ni and W refer to the weight of each element.

9. The austenitic stainless steel according to claim 4, The austenitic stainless steel contains 8 to 15 weight percent of the total content of nickel and manganese.

10. The austenitic stainless steel according to claim 9, The austenitic stainless steel contains 8.5 to 15 weight percent of the total content of nickel and manganese, and 0.225 to 0.33 weight percent of the total content of carbon and nitrogen.

11. The austenitic stainless steel according to claim 10, When the wear of the tool is measured using an end mill under dry conditions, the machinability of the austenitic stainless steel is greater than 0 and less than 0.15 mm.

12. The austenitic stainless steel according to claim 1, The weight ratio of the carbon to the total weight of the carbon and nitrogen [C / (C+N)] is 0.1 to 0.

7.

13. The austenitic stainless steel according to claim 9, The following conditions must be met: (Select one or more from Equations 2 to 5) Formula 2 (Ni+Mn) + 165.94(C+N) ≥ 49.47 【Formula 3】 (Ni+Mn) - 27.28(C+N) ≥ 2.10 【Formula 4】 (Ni+Mn) ≥ 12.3 【Formula 5】 (Ni+Mn) + 13.133(C+N) ≥ 1.73 In Equations 2 to 5, Ni, Mn, C, and N refer to the weights of nickel, manganese, carbon, and nitrogen, respectively.

14. The austenitic stainless steel according to claim 13, The conditions of Equations 2 and 5 are satisfied.

15. The austenitic stainless steel according to claim 13, The conditions of Equations 2 to 4 are satisfied.

16. The austenitic stainless steel according to claim 4, The total content of carbon and nitrogen is less than 0.33% by weight.

Images