Austenitic stainless steel having improved ultra-low temperature impact toughness and improved strength, and method for manufacturing same

Abstract

An austenitic stainless steel according to an example of the present disclosure, with improved cryogenic impact toughness and strength, comprising, by weight percent, C: more than 0% and 0.03% or less, N: 0.10% to 0.30%, Si: more than 0% and 1.0% or less, Mn: more than 0% and 5.0% or less, Cr: 17.0% to 22.0%, Ni: 5% to 12.0%, Cu: morethan 0% and 1.0% or less, Mo: more than 0% and 2.5% or less, with the remainder being Fe and unavoidable impurities. The steel satisfies the following Formula (1) and has an impact toughness value at a temperature of -253 °C of 70 J to 200J. −0.6≤1+Ni−Mn−2*Mn−2−0.5*Cr−Mo+Si+0.1*Cu≤1.0 (wherein Si, Mn, Cr, Ni, Cu, and Mo represent the content of each element in weight percent)

Description

[Technical Field]

[0001] The present disclosure relates to an austenitic stainless steel, more specifically, to an austenitic stainless steel having improved cryogenic impact toughness and strength.[Background Art]

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

[0003] For example, the increasing demand and market growth for liquefied natural gas (LNG), liquefied petroleum gas (LPG), and liquefied hydrogen, and the like, lead to an increased demand for tanks and pipes, and the like, necessary for storing and transporting low-temperature liquefied gases. For the transport and storage of these low-temperature liquefied gases, it is necessary to maintain a cryogenic environment.

[0004] However, manufacturing stainless steel that exhibits excellent corrosion resistance while also satisfying various physical properties required for respective facilities, containers, and components becomes highly challenging as the use environment temperature approaching cryogenic levels. Therefore, interest is increasing in stainless steel capable of satisfying not only corrosion resistance but also various required physical properties.[Disclosure][Technical Problem]

[0005] An object of the present disclosure for solving the above-mentioned problems is to provide an austenitic stainless steel and a method for manufacturing the same, which provide excellent cryogenic impact toughness and strength while having excellent cost competitiveness.

[0006] The technical problems to be solved by the present disclosure are not limited to the technical problems mentioned above, and other technical problems not mentioned will be clearly understood by those skilled in the art from the following description.[Technical Solution]

[0007] As a means for achieving the above-described object, the austenitic stainless steel according to an example of the present disclosure may be an austenitic stainless steel with improved cryogenic impact toughness and strength, comprising, by weight percent, C: more than 0% and 0.03% or less, N: 0.10% to 0.30%, Si: more than 0% and 1.0% or less, Mn: more than 0% and 5.0% or less, Cr: 17.0% to 22.0%, Ni: 5% to 12.0%, Cu: more than 0% and 1.0% or less, Mo: more than 0% and 2.5% or less, and the remainder being Fe and unavoidable impurities, wherein the stainless steel satisfies the following Formula (1), wherein an impact toughness value at a temperature of -253 °C is 70 J to 200 J. − 0.6 ≤ 1 + Ni − Mn − 2 * Mn − 2 − 0.5 * Cr − Mo + Si + 0.1 * Cu ≤ 1.0 (wherein Si, Mn, Cr, Ni, Cu, and Mo represent the content of each element in weight percent)

[0008] Further, the austenitic stainless steel according to an example of the present disclosure may be an austenitic stainless steel of claim 1, wherein the austenitic stainless steel satisfies Formula (2) below: 12.4 ≤ 4.4 + 23 * C + N + 1.3 * Si + 0.24 * Cr + Ni + Mn ≤ 19.1 (wherein C, N, Si, Mn, Cr, and Ni represent the content of each element in weight percent)

[0009] In addition, the austenitic stainless steel according to an example of the present disclosure may be an austenitic stainless steel having improved cryogenic impact toughness and strength, with a yield strength of 205 MPa to 450 MPa.

[0010] Further, the austenitic stainless steel according to an example of the present disclosure may be an austenitic stainless steel with improved cryogenic impact toughness and strength, having a tensile strength of 515 MPa to 850 MPa.

[0011] Further, the austenitic stainless steel according to an example of the present disclosure may be an austenitic stainless steel having an elongation of 50 % to 80% and improved cryogenic impact toughness and strength.

[0012] Also, a method for manufacturing austenitic stainless steel with improved cryogenic impact toughness and strength, according to an example of the present disclosure, may comprise: manufacturing a slab comprising, by weight percent: C: more than 0% and 0.03% or less, N: 0.10% to 0.30%, Si: more than 0% and 1.0% or less, Mn: more than 0% and 5.0% or less, Cr: 17.0% to 22.0%, Ni: 5% to 12.0%, Cu: more than 0% and 1.0% or less, Mo: more than 0% and 2.5% or less, and the remainder being of Fe and unavoidable impurities, while satisfying Formula (1) below; heating and extracting the said slab; and hot rolling and hot-rolled annealing the said slab to obtain a hot-rolled steel sheet, wherein an impact toughness value at a temperature of -253 °C of the austenitic stainless steel is 70 J to 200 J. − 0.6 ≤ 1 + Ni − Mn − 2 * Mn − 2 − 0.5 * Cr − Mo + Si + 0.1 * Cu ≤ 1.0 (wherein Si, Mn, Cr, Ni, Cu, and Mo represent the content of each element in weight percent)

[0013] In addition, the method for manufacturing austenitic stainless steel according to an example of the present disclosure may be a method for manufacturing an austenitic stainless steel having improved cryogenic impact toughness and strength, wherein said slab satisfies the following Formula (2). 12.4 ≤ 4.4 + 23 * C + N + 1.3 * Si + 0.24 * Cr + Ni + Mn ≤ 19.1 (wherein C, N, Si, Mn, Cr, and Ni represent the content of each element in weight percent)

[0014] Furthermore, a method for manufacturing an austenitic stainless steel according to an example of the present disclosure may be a method for manufacturing an austenitic stainless steel with improved cryogenic impact toughness and strength, wherein the austenitic stainless steel has a yield strength of 205 MPa to 450 MPa.

[0015] Furthermore, the manufacturing method of an austenitic stainless steel according to an example of the present disclosure may be a manufacturing method of an austenitic stainless steel with improved cryogenic impact toughness and strength, wherein heating and extracting the slab is performed at a temperature of 1080 °C to 1280 °C.

[0016] Furthermore, a method for manufacturing an austenitic stainless steel according to an example of the present disclosure may be a method for manufacturing an austenitic stainless steel having improved cryogenic impact toughness and strength, wherein the hot rolling is performed at 800 °C or more with a rolling reduction rate of 70% or more.

[0017] Further, a method for manufacturing an austenitic stainless steel according to an example of the present disclosure, wherein said hot rolling annealing is performed at a temperature of 1000 °C to 1200 °C for more than 0 minutes and 60 minutes or less, may be a method for manufacturing an austenitic stainless steel having improved cryogenic impact toughness and strength.

[0018] Further, a method for manufacturing an austenitic stainless steel according to an example of the present disclosure may further comprise cooling after the hot rolling and before the hot-rolled annealing, wherein the cooling is performed at a cooling rate of 0 °C / s to 50 °C / s for more than 0 minutes and 10 minutes or less for an austenitic stainless steel with improved cryogenic impact toughness and strength.

[0019] Further, a method for manufacturing an austenitic stainless steel according to an example of the present disclosure may be a method for manufacturing an austenitic stainless steel with improved cryogenic impact toughness and strength, wherein the cold rolling is performed at room temperature with a rolling reduction ratio of 50 % or more.

[0020] Further, a method for manufacturing an austenitic stainless steel according to an example of the present disclosure may further comprise cold rolling annealing performed at a temperature of 1000 °C to 1200 °C for more than 0 minutes and 10 minutes or less.[Advantageous Effects]

[0021] According to an example of the present disclosure, an austenitic stainless steel having improved impact toughness at cryogenic temperatures and improved yield strength, and a method for manufacturing the same, can be provided by securing high austenitic phase stability and stacking fault energy, thereby preventing phase transformation due to low temperatures.[Modes of the Invention]

[0022] Hereinafter, preferred embodiments of the present disclosure are described. However, the embodiments of the present disclosure may be modified into various other forms, and the technical spirit of the present disclosure is not limited to the embodiments described below. Further, the embodiments of the present disclosure are provided to more completely explain the present disclosure to those skilled in the art.

[0023] Terms used in this application are used only to describe specific examples. Therefore, for example, a singular expression includes plural expressions unless clearly singular in context. In addition, terms such as "comprise" or "include" used in this application are used to clearly designate the existence of features, steps, functions, components, or combinations thereof described in the specification, and it should be noted that they are not used to preliminarily exclude the existence of other features, steps, functions, components, or combinations thereof.

[0024] Unless otherwise defined, all terms used herein should be construed to have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Accordingly, unless explicitly defined herein, a particular term should not be interpreted with an excessively idealistic or formal meaning. For instance, a singular expression herein includes plural expressions, unless the context clearly dictates otherwise.

[0025] Additionally, as used herein, the terms "about," "substantially," and the like are used in the sense of being at or close to the numerical value to account for manufacturing and material tolerances inherent to the stated meaning, and are used to prevent unscrupulous infringers from unduly exploiting disclosures where precise or absolute numerical values are mentioned to aid in understanding the present disclosure.

[0026] An austenitic stainless steel according to an example of the present disclosure may include, by weight percent, C: more than 0% and 0.03% or less, N: 0.10% to 0.30%, Si: more than 0% and 1.0% or less, Mn: more than 0% and 5.0% or less, Cr: 17.0% to 22.0%, Ni: 5.0% to 12.0%, Cu: more than 0% and 1.0% or less, Mo: more than 0% and 2.5% or less, the remainder including Fe and unavoidable impurities.

[0027] The reasons for limiting the content ranges of respective alloying element are described below. Hereinafter, unless otherwise specified, the unit is weight percent.

[0028] The C content may be more than 0% and 0.03% or less.

[0029] C is an element effective for stabilizing the austenitic phase and may be added to secure the yield strength of an austenitic stainless steel. An excessive content of C may induce grain boundary precipitation of Cr carbides, thereby adversely affecting ductility, toughness, corrosion resistance, and the like. Therefore, it is desirable that the C content is more than 0% and 0.03% or less. Most preferably, the C content may be 0.02% to 0.027%.

[0030] The N content may be 0.10% to 0.30%.

[0031] N is a strong austenite phase-stabilizing element and is effective in improving the yield strength of an austenitic stainless steel. Therefore, it is preferable for the N content is 0.10% or more. An excessive N content may lead to frequent transformation of wavy slip to planar slip due to a decrease in stacking fault energy at cryogenic temperatures, or may cause a decrease in impact toughness due to short-range ordering. Furthermore, excessive N content may cause issues that hinder manufacturability, such as the occurrence of pin holes. Therefore, it is preferable for the N content to be 0.10% to 0.30%. Most preferably, the N content may be 0.14% to 0.29%.

[0032] The Si content may be more than 0% and 1.0% or less.

[0033] Si can be added as an effective element, acting as a deoxidizer during the steelmaking process and simultaneously improving the strength of the material. Additionally, Si can be added as an effective element for improving the stacking fault energy of the material. However, Si, as an effective element for ferrite phase stabilization, excessive addition of Si may impair manufacturability by promoting formation of delta (δ) ferrite within a cast slab. Furthermore, Si may adversely affect the ductility and cryogenic impact properties of the material. Therefore, it is preferable for the Si content to be more than 0% and 1.0% or less. Most preferably, the Si content may be 0.2% to 1%.

[0034] The Mn content may be more than 0% and 5.0% or less.

[0035] Mn is an austenitic phase stabilizing element that can partially replace Ni in the present disclosure and can be added to improve austenite stability. An excessive Mn content may promote formation of S-based inclusions (MnS), which may impair the ductility, toughness, and corrosion resistance of austenitic stainless steel. In addition, an excessive Mn content may generate Mn fumes during the steelmaking process, entailing manufacturing risks, and may induce grain boundary embrittlement, leading to chainreaction material degradation. Furthermore, deviation of the Mn content from a specific range may impair the cryogenic toughness of the material. Therefore, it is preferable that the Mn content is more than 0% and 5.0% or less. Most preferably, the Mn content may be 0.8 to 4.1%.

[0036] The Cr content may be 17.0% to 22.0%.

[0037] Cr is a ferrite phase-stabilizing element but is effective for suppressing the formation of a martensitic phase. In addition, Cr is a basic element for securing the corrosion resistance required for stainless steel, and it is preferable to add 17.0% or more. However, an excessive Cr content may lead to the formation of ferrite, and thus delta ferrite may remain in the slab. Therefore, an excessive Cr content may decrease hot workability. In addition, an excessive Cr content may destabilize austenite, thereby requiring a large amount of Ni for phase stability, which may lead to an increase in cost. Therefore, it is preferable to limit the Cr content to 22% or less. Most preferably, the Cr content may be 17.5 to 21.3%.

[0038] The Ni content may be 5.0% to 12.0%.

[0039] Ni is an austenite phase-stabilizing element. Therefore, from the viewpoint of austenite stabilization effect and low-temperature toughness, Ni is an advantageous element as its content increases. In addition, in order to suppress formation of delta ferrite during the manufacturing process, it is desirable that Ni is added in an amount of 5.0% or more. Conventional products and inventions are designed to prevent a martensitic phase transformation due to temperature or processing by having a content of the expensive Ni element exceeding 12%, thereby enhancing the degree of austenite stabilization. However, Ni is an expensive element with unstable raw material supply and high price volatility. An excessive Ni content may increase cost, and may also increase the likelihood of surface defect occurrence during the manufacturing process. Therefore, it is desirable that the Ni content is 5.0% to 12.0%. Most desirably, the Ni content may be 8.5% to 11.2%.

[0040] The Cu content may be more than 0% and 1.0% or less.

[0041] Cu is an element useful for stabilizing the austenitic phase and may be used to replace expensive Ni. Further, Cu is effective in suppressing martensite formation during forming and increasing austenitic stabilization. However, an excessive Cu content may lead to low-melting-point phases, thereby reducing hot workability and degrading surface quality. Therefore, it is preferable for the Cu content to be 1.0% or less. Most preferably, the Cu content may be 0.4% to 1.0%.

[0042] The Mo content may be more than 0% and 2.5% or less.

[0043] Mo can be added to improve chloride corrosion resistance, considering frequent use of the present disclosure in environments such as seawater. However, Mo is an expensive element, and an excessive Mo content may impair cost competitiveness. Furthermore, Mo is a strong ferrite phase-stabilizing element, and excessive formation of delta (δ) ferrite within a slab may impair hot workability and adversely affect material properities. Therefore, it is preferable to set the Mo content to more than 0% and 2.5% or less. Most preferably, the Mo content may be 0.1% to 2.3%.

[0044] Further, the austenitic stainless steel according to an example of the present disclosure may further include one or more of P: 0% to 0.035% and S: 0% to 0.01%.

[0045] The P content may be 0% to 0.035%.

[0046] P is an unavoidable impurity contained in steel. P causes grain boundary corrosion or hinders hot workability, and therefore it is desirable to control its content to be as low as possible. In the present disclosure, an upper limit of the P content may be controlled to be 0.035% or less.

[0047] The S content may be 0% to 0.01% or less.

[0048] S is an unavoidable impurity contained in steel. Segregation of S at grain boundaries is a primary cause of impaired hot workability; therefore, it is desirable to control its content to be as low as possible. In the present disclosure, the upper limit of the S content may be controlled to be 0.01% or less.

[0049] The remainder is Fe. However, unintended impurities may inevitably be introduced from raw materials or the surrounding environment during a typical manufacturing process, and therefore cannot be excluded. Since such impurities are well known to those skilled in the art, details thereof are not specifically described in this specification.

[0050] An austenitic stainless steel according to an example of the present disclosure may satisfy Formula (1). − 0.6 ≤ 1 + Ni − Mn − 2 * Mn − 2 − 0.5 * Cr − Mo + Si + 0.1 * Cu ≤ 1.0 (wherein Si, Mn, Cr, Ni, Cu, and Mo represent the content of each element in weight percent)

[0051] When a value of Formula (1) less than -0.6, the material has a low stacking fault energy, such that planar slip predominantly occurs during deformation at cryogenic temperatures, which may lead to deterioration of cryogenic impact toughness. In addition, excessive addition of ferrite phase-stabilizing elements may cause formation of delta ferrite due to segregation, and due to the brittle behavior of ferrite at cryogenic temperatures, the cryogenic impact toughness may sharply decrease.

[0052] On the other hand, a value of Formula (1) more than 1.0 may cause a reduction in elongation, potentially degrading cryogenic impact toughness. Furthermore, excessive addition of austenite phase-stabilizing elements may impair cost competitiveness, and during solidification, surface defects and internal cracks may occur due to austenitic solidification behavior, thereby hindering cryogenic toughness.

[0053] An austenitic stainless steel according to an example of the present disclosure may satisfy Formula (2). 12.4 ≤ 4.4 + 23 * C + N + 1.3 * Si + 0.24 * Cr + Ni + Mn ≤ 19.1 (wherein C, N, Si, Mn, Cr, and Ni represent the content of each element in weight percent)

[0054] In the present disclosure, the value of Formula (2) was derived in consideration of improvement of yield strength due to a stress field of a steel material, in order to ensure high yield strength of an austenitic stainless steel.

[0055] A higher value of Formula (2) increases, a stress field between lattices may increase due to defferences in atomic size among alloy elements. Accordingly, a higher value of Formula (2) may increase the resistance to plastic deformation against external stress. When the value of Formula (2) is less than 12.4, it may be difficult to secure the yield strength required in the present disclosure. Accordingly, in order to achieve high strength properties in the present disclosure, the lower limit of the value of Formula (2) may be set to 12.4 and the upper limit may be set to 19.1. Preferably, the lower limit may be set to 16 and the upper limit may be set to 19.1.

[0056] An austenitic stainless steel according to an example of the present disclosure is a suitable material for low-temperature applications, as it can maintain relatively excellent impact toughness when exposed to low temperatures. Due to these advantages, the austenitic stainless steel can be used not only in cryogenic environments such as LNG, liquid ammonia, liquid nitrogen, and liquid carbon dioxide, but also at temperatures of - 253°C or lower, such as liquid hydrogen, making it a suitable material for liquid hydrogen infrastructure in the rapidly growing hydrogen energy era.

[0057] Additionally, an austenitic stainless steel according to an example of the present disclosure possesses excellent corrosion resistance, workability, and elongation, thereby enabling its use in various shapes and environments tailored to diverse customer needs without difficulty. Accordingly, it can be applied to various components, pipes, tanks, equipment, and structural materials that are directly exposed to liquid hydrogen. In particular, the austenitic stainless steel also has an aesthetically pleasing appearance, providing an advantage of superior aesthetics without additional processing.

[0058] Conventional austenitic stainless steel falling outside the scope of the present disclosure may exhibit a rapid degradation of cryogenic impact toughness due to martensitic phase transformation occurring in a metastable state.

[0059] In addition, an austenitic stainless steel according to an example of the present disclosure can achieve a cryogenic impact toughness of 70 J to 200 J, preferably 70 J to 140 J, and most preferably 70.8 J to 93.1 J, while simultaneously maintaining a yield strength of 205 MPa to 450 MPa, preferably 220 MPa to 450 MPa, and most preferably 244 MPa to 388 MPa. As a result, when used in stress-earing environments, the austenitic stainless steel enables the use of increased thickness or provides improved applicability.

[0060] When only austenitic phase stability under theoretical equilibrium conditions is considered, martensite and ferritic phases may not occur. However, in practice, martensitic and ferritic phases frequently exist in some regions due to segregation, thereby causing a redection in cryogenic impact toughness.

[0061] An austenitic stainless steel according to an example of the present disclosure can secure excellent impact toughness in an actual use environment and at cryogenic temperatures, while also achieves cost competitiveness as compared to conventional products.

[0062] According to an example of the present disclosure, an austenitic stainless steel, by controlling the value of Formula (2) to be 12.4 to 19.1, may have a yield strength of 205 MPa to 450 MPa, preferably 220 MPa to 450 MPa, and most preferably 244 MPa to 388 MPa.

[0063] Further, the austenitic stainless steel according to an example of the present disclosure may have a tensile strength of 515 MPa to 850 MPa, preferably 635 MPa to 850 MPa, and most preferably 635 MPa to 728 MPa.

[0064] Further, an austenitic stainless steel according to an example of the present disclosure may have an elongation of 50 % to 80 %, preferably 53 % to 80 %, and most preferably 53 % to 64 %.

[0065] Further, the austenitic stainless steel according to an example of the present disclosure may have an impact toughness at -253 °C of 70 J to 200 J, preferably 70 J to 140 J, and most preferably 70.8 J to 93.1 J, by controlling the value of Formula (1) to be -0.6 to 1.0.

[0066] Hereinafter, a method for manufacturing an austenitic stainless steel according to an example of the present disclosure, having the above-described alloy composition, will be described.

[0067] The austenitic stainless steel of the present disclosure may be manufactured through a process comprising heating and extracting a slab having the above-described alloy composition, followed by hot rolling and hot-rolled annealing of the slab. The process may further comprise cooling after hot rolling and before hot-rolled annealing. In addition, the process may further comprise cold rolling and cold-rolled annealing after the hot-rolled annealing.

[0068] A method for manufacturing an austenitic stainless steel having improved cryogenic impact toughness and strength according to an example of the present disclosure comprise: manufacturing a slab comprising, by weight percent, C: more than 0% to 0.03% or less, N: 0.10% to 0.30%, Si: more than 0% to 1.0% or less, Mn: more than 0% to 5.0% or less, Cr: 17.0% to 22.0%, Ni: 5.0% to 12.0%, Cu: more than 0% to 1.0% or less, Mo: more than 0% to 2.5% or less, the remainder comprising Fe and unavoidable impurities, and satisfying Formula (1); heating and extracting the slab; hot rolling and hot-rolled annealing the slab to obtain a hot-rolled steel sheet; and optionally further comprising cold rolling and cold-rolled annealing the hot-rolled steel sheet after the hot-rolled annealing, wherein an cryogenic impact toughness value at a temperature of -253 °C is 70 J to 200 J, preferably 70 J to 140 J, and most preferably 70.8 J to 93.1 J. − 0.6 ≤ 1 + Ni − Mn − 2 * Mn − 2 − 0.5 * Cr − Mo + Si + 0.1 * Cu ≤ 1.0 (wherein Si, Mn, Cr, Ni, Cu, and Mo represent the content of each element in weight percent)

[0069] Further, a method for manufacturing an austenitic stainless steel according to an example of the present disclosure may be a method for manufacturing an austenitic stainless steel having improved cryogenic impact toughness and strength, wherein the slab satisfies Formula (2) below. 12.4 ≤ 4.4 + 23 * C + N + 1.3 * Si + 0.24 * Cr + Ni + Mn ≤ 19.1 (wherein C, N, Si, Mn, Cr, and Ni represent the content of each element in weight percent)

[0070] Further, the method for manufacturing an austenitic stainless steel according to an example of the present disclosure may be a method for manufacturing an austenitic stainless steel having improved cryogenic impact toughness and strength, wherein the yield strength is 205 MPa to 450 MPa, preferably 220 MPa to 450 MPa, and most preferably 244 MPa to 388MPa.

[0071] After manufacturing a slab having the alloy composition described above, the heating and extracting of the slab may be performed at a temperature of 1080 °C to 1280 °C. Further, the hot rolling may be performed at a temperature of 800 °C or more with a rolling reduction ratio of 70% or more. Further, the hot-rolled annealing may be performed at a temperature of 1000 °C to 1200 °C for more than 0 minutes to 60 minutes or less.

[0072] Further, the method may further comprise cooling after hot rolling and before hot-rolled annealing. The cooling may be performed at a cooling rate of more than 0 °C / s to 50 °C / s or less for a duration of more than 0 minute to 10 minutes or less. Further, the cold rolling may be performed at room temperature with a reduction rate of 50 % or more. Further, the cold-rolled annealing may be performed at a temperature of 1000 °C to 1200 °C for a duration of more than 0 minute to 10 minutes or less. By performing cold rolling and cold-rolled annealing after hot-rolled annealing, additional thickness reduction may be achieved.

[0073] The austenitic stainless steel of the present disclosure, as well as the austenitic stainless steel manufactured by the manufacturing method thereof, can secure costeffective austenitic phase stabilization and high stacking fault energy through the use of Cr, Ni, and Mn.

[0074] Accordingly, the austenitic stainless steel manufactured by the manufacturing method of the present disclosure does not undergo phase transformation due to low temperatures, and thus can secure an impact toughness of 70 J to 200 J even under very low operating temperature conditions, specifically at a temperature of -253 °C, while simultaneously providing a yield strength of 205 MPa to 450 MPa.{Examples}

[0075] Hereinafter, the present disclosure will be described in more detail with reference to examples.

[0076] Slabs having alloy composition as shown in Table 1 below were obtained, and heated at 1200 °C and extracted. Further, hot rolling was performed at a rolling reduction ratio of 70 % at a temperature of 800 °C or more. After cooling at a cooling rate of 30 °C / s, hot-rolled annealing was performed at 1100 °C for 30 minutes.

[0077] Table 1 shows the contents of C, Si, Mn, Ni, Cr, Cu, Mo, and N for Comparative Examples and Examples of austenitic stainless steel, and values of Formula (1) calculated based on the compositional content of each Comparative Example and Example. [Table 1]ClassificationComposition(1)(2)CSiMnNiCrCuMoNComparative Example 10.0200.40.89.321.30.80.60.20-1.91 17.5Comparative Example 20.0250.30.69.019.80.70.50.18-1.99 16.6Comparative Example 30.0290.53.28.018.90.60.10.18-1.43 17.1Comparative Example 40.0240.44.09.318.50.80.10.20-2.57 17.7Comparative Example 50.0240.41.910.520.10.80.40.201.52 17.9Comparative Example 60.0301.01.911.518.50.90.80.113.53 16.6Comparative Example 70.0210.81.712.019.50.80.10.133.94 16.9Comparative Example 80.0210.22.77.017.10.41.60.09-2.40 13.6Comparative Example 90.0230.33.410.322.00.50.70.25-2.01 19.6 Comparative Example 100.0280.40.311.120.70.71.20.23-1.87 18.6Comparative Example 110.0270.65.011.819.50.70.10.27-5.38 20.7 Comparative Example 120.0300.44.57.517.00.92.50.14-8.26 15.8Comparative Example 130.0250.42.29.918.80.80.10.301.84 19.8 Example 10.0240.31.99.320.50.80.60.18-0.1817.1Example 20.0230.80.89.419.81.00.50.26-0.5419.1Example 30.0230.41.79.319.50.80.60.160.3416.4Example 40.0270.42.811.220.80.70.70.170.9317.8Example 50.0210.43.18.518.50.80.10.20-0.5817.2Example 60.0221.04.112.019.30.70.60.14-0.5917.9Example 70.0200.51.39.517.50.62.30.29-0.4819.0Example 80.0250.42.010.421.30.91.20.160.0417.3Example 90.0220.60.98.218.10.40.10.02-0.5212.7Example 100.0210.31.210.216.50.32.10.020.5412.4

[0078] Table 2 shows the yield strength (YS, MPa), yensile strength (TS, MPa), elongation(EL, %), and impact toughness at a temperature of -253 °C. The yield strength(YS, MPa), tensile strength (TS, MPa), and elongation(EL, %) were measured by performing a tensile test in accordance with the ASTM E8 at a crosshead speed in range of 10 mm / min to 20 mm / min., and the results are shown in Table 2.

[0079] The impact toughness at a temperature of -253 °C was measured by a drop weight method using an R&B impact tester under cryogenic conditions. The results are shown in Table 2. [Table 2]ClassificationYield Strength (MPa)Tensile Strength (MPa)Elongation (%)Impact toughness at -253°C (J)Comparative Example 13516985538.1 Comparative Example 23116315439.5 Comparative Example 33336735645.3 Comparative Example 43586815931.5 Comparative Example 53656785165.0 Comparative Example 632164548 30.5 Comparative Example 733067149 35.3 Comparative Example 82816346132.4 Comparative Example 93797105143.5 Comparative Example 103606775855.0 Comparative Example 113907285525.1 Comparative Example 123036316123.5 Comparative Example 133707055468.5 Example 13506985890.1Example 23887285484.1Example 33206555888.8Example 43527015380.0Example 53456906077.9Example 63407125589.5Example 73797215670.8Example 83486895981.7Example 92576656383.9Example 102446356493.1

[0080] Referring to Tables 1 and 2, it was confirmed that Examples 1 to 10 represent austenitic stainless steels that exhibit both excellent mechanical properties and cost competitiveness. Specifically, each of Examples 1 to 10 satisfies a value of Formula (1) in the range of -0.6 to 1.0 and a value of Formula (2) in the range of 12.4 to 19.1. These examples further demonstrate a yield strength of 244 MPa to 388 MPa, an impact toughness at -253 °C of 70.8 J to 93.1 J, a tensile strength of 635 MPa to 728 MPa, and an elongation of 53 % to 64 %. In contrast, Comparative Examples 1 to 4 and 8 to 12, which have Formula (1) values ranging from -5.38 to -1.43 (less than -0.6), exhibited inferior impact toughness at -253 °C, ranging from 23.5 J to 55.0 J. It was observed that in Comparative Examples 1 to 4 and 8 to 12, the presence of excessive internal delta-ferrite, combined with planar slip acting as the primary deformation mechanism at cryogenic temperatures, hindered the attainment of the target impact toughness at -253 °C as contemplated by the present disclosure.

[0081] Additionally, the values of Formula (1) in Comparative Examples 5, 6, and 13 were 1.52, 3.53, and 1.84, respectively. This exceeding of 1.0 resulted in poor product quality or restricted deformation behavior. Furthermore, the impact toughness values at - 253 °C of these examples were 65.0 J, 30.5 J, and 23.5 J, respectively, confirming an inability to secure the toughness required by the present disclosure.

[0082] It was also observed that comparative examples 6 to 7, having elongation values of 48 % to 54 %, did not satisfy the elongation values required by the present disclosure.

[0083] Although exemplary examples of the present disclosure have been described above, the present disclosure is not limited thereto, and one of ordinary skill in the art will understand that various changes and modifications are possible within the concept and scope of the following claims.

Claims

1. An austenitic stainless steel with improved cryogenic impact toughness and strength, comprising, by weight percent, C: more than 0% and 0.03% or less, N: 0.10% to 0.30%, Si: more than 0% and 1.0% or less, Mn: more than 0% and 5.0% or less, Cr: 17.0% to 22.0%, Ni: 5% to 12.0%, Cu: more than 0% and 1.0% or less, Mo: more than 0% and 2.5% or less, and the remainder being Fe and unavoidable impurities, wherein the stainless steel satisfies the following Formula (1), wherein an impact toughness value at -253 °C is 70 J to 200 J. − 0.6 ≤ 1 + Ni − Mn − 2 * Mn − 2 − 0.5 * Cr − Mo + Si + 0.1 * Cu ≤ 1.0 (wherein Si, Mn, Cr, Ni, Cu, and Mo represent the content of each element in weight percent)2. The austenitic stainless steel of claim 1, wherein the austenitic stainless steel satisfies Formula (2) below: 12.4 ≤ 4.4 + 23 * C + N + 1.3 * Si + 0.24 * Cr + Ni + Mn ≤ 19.1 (wherein C, N, Si, Mn, Cr, and Ni represent the content of each element in weight percent)3. The austenitic stainless steel of claim 2, wherein the austenitic stainless steel has a yield strength of 205 MPa to 450 MPa.

4. The austenitic stainless steel of claim 1, wherein the austenitic stainless steel has a tensile strength of 515 MPa to 850 MPa.

5. The austenitic stainless steel of claim 1, wherein the austenitic stainless steel has an elongation of 50% to 80%.

6. A method for manufacturing an austenitic stainless steel having improved cryogenic impact toughness and strength, the method comprising: manufacturing a slab comprising, by weight percent: C: more than 0% and 0.03% or less, N: 0.10% to 0.30%, Si: more than 0% and 1.0% or less, Mn: more than 0% and 5.0% or less, Cr: 17.0% to 22.0%, Ni: 5% to 12.0%, Cu: more than 0% and 1.0% or less, Mo: more than 0% and 2.5% or less, and the remainder being of Fe and unavoidable impurities, and satisfying Formula (1) below; heating and extracting the said slab; and hot rolling and hot-rolled annealing the said slab to obtain a hot-rolled steel sheet, wherein an impact toughness value at a temperature of -253°C of the austenitic stainless steel is 70 J to 200 J. − 0.6 ≤ 1 + Ni − Mn − 2 * Mn − 2 − 0.5 * Cr − Mo + Si + 0.1 * Cu ≤ 1.0 (wherein Si, Mn, Cr, Ni, Cu, and Mo represent the content of each element in weight percent)7. The method of claim 6, wherein the slab satisfies Formula (2) below: 12.4 ≤ 4.4 + 23 * C + N + 1.3 * Si + 0.24 * Cr + Ni + Mn ≤ 19.1 (wherein C, N, Si, Mn, Cr, and Ni represent the content of each element in weight percent)8. The method of claim 7, wherein the austenitic stainless steel has a yield strength of 205 MPa to 450 MPa.

9. The method of claim 6, wherein the heating and extracting of the slab is performed at a temperature of 1080 °C to 1280 °C.

10. The method of claim 6, wherein the hot rolling is performed at a temperature of 800°C or more with a rolling reduction ratio of 70% or more.

11. The method of claim 6, wherein the hot-rolled annealing is performed at a temperature of 1000°C to 1200 °C for 0 minutes to 60 minutes.

12. The method of claim 6, further comprising cooling after the hot rolling and before the hot-rolled annealing, wherein the cooling is performed at a cooling rate of 0 °C / s to 50 °C / s for 0 minutes to 10 minutes.

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