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 embodiment of the present invention comprises, by wt%, 0% exclusive to 0.03% inclusive of C, 0.10% inclusive to 0.30% inclusive of N, 0% exclusive to 1.0% inclusive of Si, 0% exclusive to 5.0% inclusive of Mn, 17.0% inclusive to 22.0% inclusive of Cr, 5% inclusive to 12.0% inclusive of Ni, 0% exclusive to 1.0% inclusive of Cu, 0% exclusive to 2.5% inclusive of Mo, and the remainder being Fe and inevitable impurities, satisfies formula (1), and has an impact toughness value of 70-200 J at -253 ℃. The austenitic stainless steel has improved ultra-low temperature impact toughness and improved strength. Formula (1): -0.6 ≤ 1+Ni-(Mn-2)*(Mn-2)-0.5*Cr-Mo+Si+0.1*Cu ≤ 1.0 (where Si, Mn, Cr, Ni, Cu, and Mo represent the weight% of each element.).

Description

Austenitic stainless steel with improved ultra-low temperature impact toughness and strength and its manufacturing method

[0001] The present invention relates to austenitic stainless steel, and more particularly, to austenitic stainless steel with improved ultra-low temperature impact toughness and strength.

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

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

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

[0005] The purpose of the present invention to solve the above-described problems is to provide an austenitic stainless steel and a method for manufacturing the same that has excellent cost competitiveness while securing excellent ultra-low temperature impact toughness and strength.

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

[0007] As a means for achieving the above-described object, an austenitic stainless steel according to an example of the present invention may be an austenitic stainless steel having improved ultra-low temperature impact toughness and strength, which includes, in wt%, C: more than 0% to 0.03% or less, N: 0.10% to 0.30% or less, Si: more than 0% to 1.0% or less, Mn: more than 0% to 5.0% or less, Cr: 17.0% to 22.0% or less, Ni: 5% to 12.0% or less, Cu: more than 0% to 1.0% or less, Mo: more than 0% to 2.5% or less, the remainder being Fe and unavoidable impurities, and satisfying the following formula (1), and having an impact toughness value at -253°C of 70 J or more to 200 J or less.

[0008] Equation (1): -0.6 ≤ 1+Ni-(Mn-2)*(Mn-2)-0.5*Cr-Mo+Si+0.1*Cu ≤ 1.0

[0009] (Here, Si, Mn, Cr, Ni, Cu, Mo represent the weight% of each element)

[0010] In addition, the austenitic stainless steel according to an example of the present invention may be an austenitic stainless steel with improved ultra-low temperature impact toughness and strength, satisfying the following equation (2).

[0011] Formula (2): 12.4 ≤ 4.4 + 23*(C + N)+1.3*Si+0.24*(Cr + Ni + Mn) ≤ 19.1

[0012] (Here, C, N, Si, Mn, Cr, and Ni represent the weight percent of each element)

[0013] In addition, the austenitic stainless steel according to an example of the present invention may be an austenitic stainless steel with improved ultra-low temperature impact toughness and strength, having a yield strength of 205 MPa or more and 450 MPa or less.

[0014] In addition, the austenitic stainless steel according to an example of the present invention may be an austenitic stainless steel with improved ultra-low temperature impact toughness and strength, having a tensile strength of 515 MPa or more and 850 MPa or less.

[0015] In addition, the austenitic stainless steel according to an example of the present invention may be an austenitic stainless steel having improved ultra-low temperature impact toughness and strength, with an elongation of 50% or more and 80% or less.

[0016] In addition, a method for manufacturing an austenitic stainless steel according to an example of the present invention comprises the steps of manufacturing a slab that contains, in wt%, C: more than 0% to 0.03% or less, N: 0.10% to 0.30% or less, Si: more than 0% to 1.0% or less, Mn: more than 0% to 5.0% or less, Cr: 17.0% to 22.0% or less, Ni: 5% to 12.0% or less, Cu: more than 0% to 1.0% or less, Mo: more than 0% to 2.5% or less, the remainder being Fe and inevitable impurities, and satisfying the following formula (1), a step of heating and extracting the slab, a step of hot-rolling and hot-annealing the slab to obtain a hot-rolled steel sheet, and an impact toughness value at -253°C of 70J to 200J or less, and additionally, cold-rolling and hot-annealing after the hot-rolling It may be a method for manufacturing an austenitic stainless steel having improved ultra-low temperature impact toughness and strength, including a cold rolling annealing process.

[0017] Equation (1): -0.6 ≤ 1+Ni-(Mn-2)*(Mn-2)-0.5*Cr-Mo+Si+0.1*Cu ≤ 1.0

[0018] (Here, Si, Mn, Cr, Ni, Cu, Mo represent the weight% of each element)

[0019] In addition, a method for manufacturing an austenitic stainless steel according to an example of the present invention may be a method for manufacturing an austenitic stainless steel having improved ultra-low temperature impact toughness and strength, wherein the slab satisfies the following formula (2).

[0020] Formula (2): 12.4 ≤ 4.4 + 23*(C + N)+1.3*Si+0.24*(Cr + Ni + Mn) ≤ 19.1

[0021] (Here, C, N, Si, Mn, Cr, and Ni represent the weight percent of each element)

[0022] In addition, a method for manufacturing an austenitic stainless steel according to an example of the present invention may be a method for manufacturing an austenitic stainless steel having improved ultra-low temperature impact toughness and strength, wherein the austenitic stainless steel has a yield strength of 205 MPa or more and 450 MPa or less.

[0023] In addition, the method for manufacturing austenitic stainless steel according to an example of the present invention may be a method for manufacturing austenitic stainless steel with improved ultra-low temperature impact toughness and strength, in which the step of heating and extracting the slab is performed at 1080°C to 1280°C.

[0024] In addition, a method for manufacturing an austenitic stainless steel according to an example of the present invention may be a method for manufacturing an austenitic stainless steel with improved ultra-low temperature impact toughness and strength, in which the hot rolling is performed at a temperature of 800°C or higher and a reduction ratio of 70% or higher.

[0025] In addition, a method for manufacturing an austenitic stainless steel according to an example of the present invention may be a method for manufacturing an austenitic stainless steel with improved ultra-low temperature impact toughness and strength, in which the hot rolling annealing is performed at 1000°C to 1200°C for more than 0 minutes and less than 60 minutes.

[0026] In addition, a method for manufacturing an austenitic stainless steel according to an example of the present invention may be a method for manufacturing an austenitic stainless steel with improved ultra-low temperature impact toughness and strength, wherein the method further includes a step of cooling after the hot rolling and before the hot rolling annealing, and the step of cooling is performed at a cooling rate of more than 0°C / s and less than 50°C / s for more than 0 minutes and less than 10 minutes.

[0027] In addition, a method for manufacturing an austenitic stainless steel according to an example of the present invention may be a method for manufacturing an austenitic stainless steel with improved ultra-low temperature impact toughness and strength, in which the cold rolling is performed at a reduction ratio of 50% or more at room temperature.

[0028] In addition, a method for manufacturing austenitic stainless steel according to an example of the present invention may be a method for manufacturing high-strength austenitic stainless steel with improved low-temperature toughness, wherein the cold rolling annealing is performed at 1000°C to 1200°C for more than 0 minutes and less than 10 minutes.

[0029] According to an embodiment of the present invention, an austenitic stainless steel having improved impact toughness and yield strength at extremely low temperatures can be provided by securing high austenite phase stability and stacking fault energy, thereby preventing phase transformation due to low temperatures. A method for manufacturing the same can be provided.

[0030] Preferred embodiments of the present invention are described below. However, the embodiments of the present invention may be modified in various ways, and the technical concept of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more fully explain the present invention to those of ordinary skill in the art.

[0031] The terminology used in this application is solely for the purpose of describing specific examples. Therefore, for example, singular expressions include plural expressions unless the context clearly dictates otherwise. Additionally, it should be noted that terms such as "comprise" or "have" used in this application are used to clearly indicate the presence of features, steps, functions, components, or combinations thereof described in the specification, and are not used to preliminarily exclude the presence of other features, steps, functions, components, or combinations thereof.

[0032] Meanwhile, unless otherwise defined, all terms used herein should be considered to have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Therefore, unless explicitly defined herein, specific terms should not be interpreted in an overly idealistic or formal sense. For example, singular expressions herein include plural expressions unless the context clearly indicates otherwise.

[0033] In addition, the terms "about", "substantially", etc. in this specification are used in the sense of or close to the numerical value when manufacturing and material tolerances inherent to the meanings mentioned are presented, and are used to prevent unscrupulous infringers from unfairly using the disclosure contents in which exact or absolute numerical values ​​are mentioned to aid in the understanding of the present invention.

[0034] An austenitic stainless steel according to an example of the present invention may include, in wt%, C: more than 0% to 0.03% or less, N: 0.10% to 0.30% or less, Si: more than 0% to 1.0% or less, Mn: more than 0% to 5.0% or less, Cr: 17.0% to 22.0% or more, Ni: 5.0% to 12.0% or less, Cu: more than 0% to 1.0% or less, Mo: more than 0% to 2.5% or less, and the remainder may include Fe and unavoidable impurities.

[0035] The reasons for limiting the composition range of each alloy element are described below. Unless otherwise specified, the unit is weight percent.

[0036] The content of C may be greater than 0% and less than or equal to 0.03%.

[0037] C is an element that is effective in stabilizing the austenite phase and can be added to secure the yield strength of austenitic stainless steel. However, if the C content is excessive, it can induce grain boundary precipitation of chromium carbide, which can have a negative effect on ductility, toughness, corrosion resistance, etc. Therefore, it is preferable to set the C content to more than 0% and less than 0.03%. Most preferably, the C content can be 0.02% or more and 0.027% or less.

[0038] The content of N may be 0.10% or more and 0.30% or less.

[0039] Nitrogen is a strong austenite stabilizing element, and is an effective element for improving the yield strength of austenitic stainless steel. It is desirable to have an N content of 0.10% or more. However, if the N content is excessive, the stacking fault energy at extremely low temperatures may decrease, causing frequent changes from wavy slip to planar slip, or short range ordering may cause a decrease in impact toughness. In addition, if the N content is excessive, there is a problem that makes manufacturing difficult, such as the occurrence of pin holes. Therefore, the N content is preferably 0.10% or more and 0.30% or less. Most preferably, the N content may be 0.14% or more and 0.29% or less.

[0040] The Si content may be greater than 0% and less than or equal to 1.0%.

[0041] Silicon (Si) can be added as an effective element to improve the strength of materials while acting as a deoxidizer during the steelmaking process. Furthermore, Si can be added as an effective element to improve the stacking fault energy of materials. However, Si is an element effective in stabilizing the ferrite phase, and if added excessively, it can promote the formation of delta (δ) ferrite in the cast slab, thereby reducing manufacturability. Furthermore, Si can adversely affect the ductility and cryogenic impact properties of the material. Therefore, it is preferable that the Si content be between 0% and 1.0%. Most preferably, the Si content may be between 0.2% and 1%.

[0042] The content of Mn may be greater than 0% and less than or equal to 5.0%.

[0043] Manganese (Mn) is an austenite phase stabilizing element that can partially replace Ni in the present invention and can be added to improve austenite stability. However, if the Mn content is excessive, it may form excessive S-based inclusions (MnS), which may reduce the ductility, toughness, and corrosion resistance of the austenitic stainless steel. In addition, if the Mn content is excessive, it may generate Mn fumes during the steelmaking process, which may cause manufacturing risks and may cause grain boundary embrittlement, which may lead to a chain deterioration of the material. In addition, if Mn exceeds a certain range, it may deteriorate the ultra-low temperature toughness of the material. Therefore, the Mn content is preferably set to be more than 0% and less than 5.0%. Most preferably, the Mn content may be 0.8 to 4.1%.

[0044] The Cr content may be 17.0% or more and 22.0% or less.

[0045] Cr is a ferrite-stabilizing element, but it is also an effective element in suppressing martensite phase formation. In addition, Cr is a basic element that can secure the corrosion resistance required for stainless steel, and it is desirable to add 17.0% or more of Cr. However, if the Cr content is excessive, ferrite may be generated, and delta-ferrite may remain excessively in the slab. Therefore, if the Cr content is excessive, hot workability may be reduced. In addition, if the Cr content is excessive, austenite becomes unstable, and a large amount of nickel must be included for phase stability, which may cause an increase in cost. Therefore, it is desirable to set the Cr content to 22% or less. Most preferably, the Cr content may be 17.5 to 21.3%.

[0046] The content of Ni may be 5.0% or more and 12.0% or less.

[0047] Ni is an austenite stabilizing element, so adding more Ni is advantageous in terms of austenite stabilization effect and low-temperature toughness. In addition, it is desirable to add more than 5.0% of Ni to suppress delta-ferrite formation during the manufacturing process. Existing products and inventions are designed to have a content of expensive Ni exceeding 12% to improve austenite stabilization in order to prevent martensite phase transformation due to temperature or processing. However, Ni is an expensive element with unstable raw material supply and extreme price volatility, and excessive addition of Ni can lead to increased costs and increase the probability of surface defects occurring during the manufacturing process. Therefore, it is desirable to set the Ni content to 5.0% or more and 12.0% or less. Most preferably, the Ni content can be 8.5% or more and 11.2% or less.

[0048] The content of Cu may be greater than 0% and less than or equal to 1.0%.

[0049] Copper (Cu) is a useful element for stabilizing the austenite phase and can be used as a substitute for expensive nickel (Ni). Furthermore, Cu is an element that suppresses martensite formation during forming and increases austenite stabilization. However, excessive Cu content can lead to the formation of low-melting-point phases, which reduces hot workability and degrades surface quality. Therefore, it is preferable to keep the Cu content to 1.0% or less. Most preferably, the Cu content is 0.4% or more and 1.0% or less.

[0050] The Mo content may be greater than 0% and less than or equal to 2.5%.

[0051] Considering the use of the present invention, which is frequently used in an atmosphere such as seawater, Mo can be added to improve Cl-base corrosion resistance. However, Mo is an expensive element, and excessive Mo content may reduce cost competitiveness. In addition, Mo is a strong ferrite stabilizing element, and may form a large amount of delta (δ) ferrite in the slab, which may reduce hot workability and adversely affect material properties. Therefore, it is preferable that the Mo content be more than 0% and less than 2.5%. Most preferably, the Mo content may be 0.1% or more and 2.3% or less.

[0052] In addition, the austenitic stainless steel according to one embodiment of the present invention may further include at least one of P: 0% or more and 0.035% or less and S: 0% or more and 0.01% or less.

[0053] The content of P may be 0% or more and 0.035% or less.

[0054] Phosphorus (P) is an impurity that is inevitably contained in steel and is a major cause of intergranular corrosion and impaired hot workability. Therefore, it is desirable to control its content as low as possible. In the present invention, the upper limit of the P content can be controlled to 0.035% or less.

[0055] The content of S may be 0% or more and 0.01% or less.

[0056] S is an impurity that is inevitably contained in steel, and is a major element that hinders hot workability by segregating at grain boundaries. Therefore, it is desirable to control its content as low as possible. In the present invention, the upper limit of the S content can be controlled to 0.01% or less.

[0057] The remaining component of the present invention is iron. However, since unintended impurities from raw materials or the surrounding environment can inevitably be mixed in during the typical manufacturing process, this cannot be ruled out. Since these impurities are readily apparent to anyone skilled in the art of conventional manufacturing, their full details are not specifically discussed in this specification.

[0058] An austenitic stainless steel according to an example of the present invention can satisfy equation (1).

[0059] Equation (1): -0.6 ≤ 1+Ni-(Mn-2)*(Mn-2)-0.5*Cr-Mo+Si+0.1*Cu ≤ 1.0

[0060] (Here, Si, Mn, Cr, Ni, Cu, Mo represent the weight% of each element)

[0061] If the value of Equation (1) is less than -0.6, the material's stacking fault energy is low, so planar slip occurs mainly during deformation at cryogenic temperatures, which may cause a deterioration in cryogenic impact toughness. In addition, since excessive addition of ferrite stabilizing elements may lead to the formation of delta ferrite through segregation, the cryogenic impact toughness may decrease significantly due to the embrittlement of ferrite at cryogenic temperatures.

[0062] On the other hand, if the value of Equation (1) exceeds 1.0, the elongation may decrease, which may deteriorate the ultra-low temperature impact toughness. In addition, excessive addition of austenitic stabilizing elements may result in a decrease in cost competitiveness, and surface defects and internal cracks may occur due to the austenite solidification behavior during solidification, which may deteriorate the ultra-low temperature toughness.

[0063] An austenitic stainless steel according to an example of the present invention can satisfy equation (2).

[0064] Formula (2): 12.4 ≤ 4.4 + 23*(C + N)+1.3*Si+0.24*(Cr + Ni + Mn) ≤ 19.1

[0065] (Here, C, N, Si, Mn, Cr, and Ni represent the weight percent of each element)

[0066] In the present invention, in order to secure high yield strength of austenitic stainless steel, the value of the following equation (2) was derived by considering the improvement of yield strength due to the stress field of the steel.

[0067] As the value of Equation (2) increases, the stress field between the lattices may increase due to the difference in atomic size between alloying elements. Therefore, as the value of Equation (2) increases, the limit for withstanding plastic deformation against external stress may increase. If the value of Equation (2) is less than 12.4, it may be difficult to secure the yield strength required in the present invention. Accordingly, in the present invention, for high strength characteristics, the lower limit of the value of Equation (2) may be limited to 12.4, the upper limit to 19.1, and preferably the lower limit to 16 and the upper limit to 19.1.

[0068] According to one embodiment of the present invention, austenitic stainless steel maintains relatively excellent impact toughness when exposed to low temperatures, making it a suitable material for low-temperature steel. Due to these advantages, austenitic stainless steel can be used not only in cryogenic environments such as LNG, liquid ammonium, liquid nitrogen, and liquid carbon dioxide, but also in cryogenic environments below -253°C, such as liquid hydrogen. Therefore, it is an appropriate material for liquid hydrogen infrastructure in the era of rapidly growing hydrogen energy.

[0069] Furthermore, austenitic stainless steel according to an example of the present invention exhibits excellent corrosion resistance, formability, and elongation, allowing it to be used in shapes and environments tailored to various customer needs without any problems, and can be applied to various components, pipes, tanks, equipment, and structural materials directly exposed to liquefied hydrogen. In particular, austenitic stainless steel has the advantage of being aesthetically pleasing without requiring additional work due to its attractive appearance.

[0070] Conventional austenitic stainless steels that fall outside the scope of the present invention may exhibit rapid deterioration in ultra-low temperature impact toughness due to a martensite phase transformation phenomenon that occurs in a metastable state.

[0071] Additionally, according to an example of the present invention, the austenitic stainless steel can achieve an ultra-low temperature impact toughness of 70 J or more to 200 J or less, preferably 70 J or more to 140 J or less, and most preferably 70.8 J or more to 93.1 J or less, while simultaneously securing a yield strength of 205 MPa or more to 450 MPa or less, preferably 220 MPa or more to 450 MPa or less, and most preferably 244 MPa or more to 388 MPa or less. In this case, when used in an environment that withstands stress, there is an effect of using a thicker material or of improving applicability.

[0072] Meanwhile, theoretically, considering only the stability of the austenitic phase at equilibrium, martensite and ferrite phases may not occur. However, in actual environments, martensite and ferrite phases often exist in certain areas due to segregation, resulting in a decrease in ultra-low-temperature impact toughness.

[0073] According to one example of the present invention, an austenitic stainless steel can secure excellent impact toughness in practical use environments and at extremely low temperatures compared to existing products and inventions, and has the effect of cost competitiveness.

[0074] An austenitic stainless steel according to an example of the present invention may have a yield strength of 205 MPa or more to 450 MPa or less by controlling the value of formula (2) to be 12.4 or more to 19.1 or less, preferably 220 MPa or more to 450 MPa or less, and most preferably 244 MPa or more to 388 MPa or less.

[0075] In addition, the austenitic stainless steel according to an example of the present invention may have a tensile strength of 515 MPa or more and 850 MPa or less, preferably 635 MPa or more and 850 MPa or less, and most preferably 635 MPa or more and 728 MPa or less.

[0076] An austenitic stainless steel according to an example of the present invention may have an elongation of 50% or more to 80% or less, preferably 53% or more to 80% or less, and most preferably 53% or more to 64% or less.

[0077] According to an example of the present invention, an austenitic stainless steel 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 -0.6 or more and 1.0 or less.

[0078] Hereinafter, a method for manufacturing an austenitic stainless steel having the above-described alloy composition according to one embodiment of the present invention will be described.

[0079] The austenitic stainless steel of the present invention can be manufactured by heating and extracting a slab having the above-described alloy composition, followed by hot rolling and hot annealing. The process may include a step of cooling after hot rolling and before hot annealing. In addition, the process may include a step of cold rolling and cold annealing after hot annealing.

[0080] According to an example of the present invention, a method for manufacturing an austenitic stainless steel comprises the steps of manufacturing a slab that contains, in wt%, C: more than 0% to 0.03% or less, N: 0.10% to 0.30% or less, Si: more than 0% to 1.0% or less, Mn: more than 0% to 5.0% or less, Cr: 17.0% to 22.0% or less, Ni: 5.0% to 12.0% or more, Cu: more than 0% to 1.0% or less, Mo: more than 0% to 2.5% or less, the remainder being Fe and unavoidable impurities, and satisfying the following formula (1), a step of heating and extracting the slab, a step of hot-rolling and hot-rolling annealing the slab to obtain a hot-rolled steel sheet, and further a step of cold-rolling and cold-rolling annealing the hot-rolled steel sheet after the hot-rolling annealing, and an impact toughness value at -253°C It may be a method for manufacturing an austenitic stainless steel having improved ultra-low temperature impact toughness and strength, with an impact strength of 70 J or more and 200 J or less, preferably 70 J or more and 140 J or less, and most preferably 70.8 J or more and 93.1 J or more.

[0081] Equation (1): -0.6 ≤ 1+Ni-(Mn-2)*(Mn-2)-0.5*Cr-Mo+Si+0.1*Cu ≤ 1.0

[0082] (Here, Si, Mn, Cr, Ni, Cu, Mo represent the weight% of each element)

[0083] In addition, a method for manufacturing austenitic stainless steel according to an example of the present invention may be a method for manufacturing austenitic stainless steel with improved ultra-low temperature impact toughness and strength, satisfying the following formula (2).

[0084] Formula (2): 12.4 ≤ 4.4 + 23*(C + N)+1.3*Si+0.24*(Cr + Ni + Mn) ≤ 19.1

[0085] (Here, C, N, Si, Mn, Cr, and Ni represent the weight percent of each element)

[0086] In addition, a method for manufacturing an austenitic stainless steel according to an example of the present invention may be a method for manufacturing an austenitic stainless steel having improved ultra-low temperature impact toughness and strength, and having a yield strength of 205 MPa or more to 450 MPa or less, preferably 220 MPa or more to 450 MPa or less, and most preferably 244 MPa or more to 388 MPa or less.

[0087] After manufacturing a slab having the above-described alloy composition, the heating and extraction step may be performed at 1080°C to 1280°C. In addition, the hot rolling step may be performed at 800°C or higher with a reduction ratio of 70% or higher. In addition, the hot rolling annealing step may be performed at 1000°C to 1200°C for more than 0 minutes and less than 60 minutes.

[0088] In addition, a cooling step may be further included before hot rolling annealing after hot rolling. The cooling step may be a step performed at a cooling rate of more than 0°C / s and less than or equal to 50°C / s for more than 0 minutes and less than or equal to 10 minutes. In addition, the cold rolling step may be a step performed at room temperature with a reduction ratio of 50% or more. In addition, the cold rolling annealing step may be a step performed at 1000°C to 1200°C for more than 0 minutes and less than or equal to 10 minutes. By performing cold rolling and cold rolling annealing after hot rolling annealing, additional thickness reduction may be intended.

[0089] The austenitic stainless steel of the present invention and the austenitic stainless steel manufactured by the method for manufacturing the austenitic stainless steel of the present invention can secure austenite phase stability and high stacking fault energy compared to the cost by using Cr, Ni and Mn.

[0090] Therefore, the austenitic stainless steel manufactured by the method for manufacturing austenitic stainless steel of the present invention can secure impact toughness of 70 J or more to 200 J or less even at an extremely low service temperature condition, that is, an extremely low temperature of -253°C, since phase transformation due to low temperatures does not occur, and at the same time, an austenitic stainless steel having a yield strength of 205 MPa or more to 450 MPa or less can be provided.

[0091] {Example}

[0092] Hereinafter, the present invention will be described in more detail through examples.

[0093] After obtaining a slab with an alloy composition according to Table 1 below, it was heated at 1200°C and extracted. In addition, hot rolling was performed at a reduction ratio of 70% at 800°C or higher, cooling was performed at a cooling rate of 30°C / s, and then hot rolling annealing was performed at 1100°C for 30 minutes.

[0094] Table 1 shows the components of C, Si, Mn, Ni, Cr, Cu, Mo, and N of comparative examples and examples of austenitic stainless steel and the values ​​of formula (1) calculated according to the composition contents of each comparative example and example.

[0095] Composition formula (1) Formula (2) CSiMnNiCrCuMoN Comparative example 10.0200.40.89.321.30.80.60.20-1.9117.5 Comparative example 20.0250.30.69.019.80.70.50.18-1.9916.6 Comparative example 30.0290.53.28.018.90.60.10.18-1.4317.1 Comparative example 40.0240.44.09.318.50.80.10.20-2.5717.7 Comparative example 50.0240.41.910.520.10.80.40.201.5217.9 Comparative example 60.0301.01. 911.518.50.90.80.113.5316.6Comparative Example70.0210.81.712.019.50.80.10.133.9416.9Comparative Example80.0210.22.77.017.10.41.60.09-2.4013.6Comparative Example90.0230.33.410.322.00.50.70.25-2.0119.6Comparative Example100.0280.40.311.120.70.71.20.23-1.8718.6Comparative Example110.0270.65.011.819.50.70.10.27-5.3820.7Comparative Example120. 0300.44.57.517.00.92.50.14-8.2615.8Comparative Example 130.0250.42.29.918.80.80.10.301.8419.8Example 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 5 0.0210.43.18.518.50.80.10.20-0.5817.2Example 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

[0096] Table 2 shows the yield strength YS (MPa), tensile strength TS (MPa), elongation EL (%), and impact toughness at -253℃. The yield strength YS (MPa), tensile strength TS (MPa), and elongation EL (%) were measured by performing a tensile test at a crosshead speed of 10 mm / min to 20 mm / min according to the ASTM E8 test method. The results are shown in Table 2.

[0097] The impact toughness at -253℃ was measured using the drop weight method at a low temperature of -253℃ using an impact tester from R&B. The results are shown in Table 2.

[0098] Yield strength (MPa)Tensile strength (MPa)Elongation (%)-Impact toughness at 253℃ (J)Comparative Example 13516985538.1Comparative Example 23116315439.5Comparative Example 33336735645.3Comparative Example 43586815931.5Comparative Example 53656785165.0Comparative Example 63216454830.5Comparative Example 73306714935.3Comparative Example 82816346132.4Comparative Example 93797105143.5Comparative Example 103606775855.0Comparative Example 113907285525.1Comparative Example 12 3036316123.5 Comparative Example 133707055468.5 Embodiment 13506985890.1 Embodiment 23887285484.1 Embodiment 33206555888.8 Embodiment 43527015380.0 Embodiment 53456906077.9 Embodiment 63407125589.5 Embodiment 73797215670.8 Embodiment 83486895981.7 Embodiment 92576656383.9 Embodiment 102446356493.1

[0099] Referring to Tables 1 and 2, it was confirmed that Examples 1 to 10 all had a value of -0.6 or more and 1.0 or less in Formula (1), a value of 12.4 or more and 19.1 or less in Formula (2), a yield strength of 244 MPa or more and 388 MPa or less, an impact toughness value at -253°C of 70.8 J or more and 93.1 J or less, a tensile strength of 635 MPa or more and 728 MPa or less, and an elongation of 53% or more and 64% or less, making them austenitic stainless steels with excellent physical properties and cost competitiveness. Comparative Examples 1 to 4 and Comparative Examples 8 to 12 had an inferior value of 23.5 J to 55.0 J in impact toughness at -253°C, with a value of Formula (1) of -5.38 to -1.43, which is less than -0.6. That is, it was confirmed that Comparative Examples 1 to 4 and Comparative Examples 8 to 12 did not secure the impact toughness at -253°C required by the present invention because internal delta-ferrite was excessive and planar slip acted as the main deformation mechanism at extremely low temperatures.

[0100] In addition, Comparative Examples 5, 6, and 13 had values ​​of Equation (1) of 1.52, 3.53, and 1.84, respectively, exceeding 1.0, resulting in poor product quality or limited deformation behavior, and it was confirmed that the impact toughness values ​​at -253°C were 65.0 J, 30.5 J, and 23.5 J, respectively, failing to secure the impact toughness required by the present invention.

[0101] And it was found that comparative examples 6 and 7 had elongation values ​​of 48% to 54%, which did not satisfy the elongation value required by the present invention.

[0102] Although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and those skilled in the art will understand that various changes and modifications are possible within the scope and spirit of the claims set forth below.

Claims

1. In weight%, C: more than 0% to 0.03% or less, N: more than 0.10% to 0.30% or less, Si: more than 0% to 1.0% or less, Mn: more than 0% to 5.0% or less, Cr: more than 17.0% to 22.0% or less, Ni: more than 5% to 12.0% or less, Cu: more than 0% to 1.0% or less, Mo: more than 0% to 2.5% or less, the remainder including Fe and inevitable impurities, Satisfies the following equation (1), An austenitic stainless steel having improved ultra-low temperature impact toughness and strength, having an impact toughness value of 70 J or more and 200 J or less at -253°C. Equation (1): -0.6 ≤ 1+Ni-(Mn-2)*(Mn-2)-0.5*Cr-Mo+Si+0.1*Cu ≤ 1.0 (Here, Si, Mn, Cr, Ni, Cu, Mo represent the weight% of each element) 2. In claim 1, An austenitic stainless steel having improved ultra-low temperature impact toughness and strength, satisfying the following equation (2). Equation (2): 12.4 ≤ 4.4 + 23*(C + N)+1.3*Si+0.24*(Cr + Ni + Mn) ≤ 19.1 (Here, C, N, Si, Mn, Cr, Ni represent the weight% of each element) 3. In claim 2, An austenitic stainless steel having improved ultra-low temperature impact toughness and strength, with a yield strength of 205 MPa or more and 450 MPa or less.

4. In claim 1, An austenitic stainless steel having improved ultra-low temperature impact toughness and strength, with a tensile strength of 515 MPa or more and 850 MPa or less.

5. In claim 1, Austenitic stainless steel with improved ultra-low temperature impact toughness and strength, having an elongation of 50% or more and 80% or less.

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

9. In claim 6, The step of heating and extracting the above slab is, A method for manufacturing an austenitic stainless steel having improved ultra-low temperature impact toughness and strength, performed at 1080°C to 1280°C.

10. In claim 6, The above hot rolling is a method for manufacturing an austenitic stainless steel with improved ultra-low temperature impact toughness and strength, wherein the hot rolling is performed at a temperature of 800°C or higher and a reduction ratio of 70% or higher.

11. In claim 6, A method for manufacturing an austenitic stainless steel with improved ultra-low temperature impact toughness and strength, wherein the hot-rolled annealing is performed at 1000°C to 1200°C for more than 0 minutes and less than 60 minutes.

12. In claim 6, Further comprising a step of cooling after the hot rolling and before the hot rolling annealing, A method for manufacturing an austenitic stainless steel with improved ultra-low temperature impact toughness and strength, wherein the cooling step is performed at a cooling rate of more than 0°C / s and less than or equal to 50°C / s for more than 0 minutes and less than or equal to 10 minutes.