Austenitic stainless steel with improved strength and low-temperature impact toughness and method for manufacturing the same

Abstract

According to the present invention, an austenitic stainless steel can be provided, which, by weight%, comprises: C: less than 0.10% (excluding 0), Si: less than 1.50% (excluding 0), Cr: 16-23%, Ni: 5.1-12%, Mn: less than 15% (excluding 0), Cu: less than 1.2% (excluding 0), N: 0.1-0.4%, (Nb+V): less than 0.01%, Mo: less than 1.5%, the balance being Fe and unavoidable impurities, wherein the product of the strength index expressed by the following formula (1) and the stacking fault energy (SFE) expressed by the following formula (2) is 0 or more. Equation (1) Strength index: (Yield strength (MPa)) -0.9Cr + 2Ni - 12Si - 1.1Mn - 80N + 400C - 275; Equation (2) SFE: 25.7 + 2Ni + 410C - 0.9Cr - 77N - 13Si - 1.2Mn (In Equations (1) and (2), Cr, Ni, Si, Mn, N and C represent the content (weight %) of each element).

Description

Technical Field

[0001] This invention relates to an austenitic stainless steel with improved yield strength (MPa) and low-temperature impact toughness, and a method for manufacturing the same. Background Technology

[0002] In recent years, with the development and widespread adoption of hydrogen fuel cell vehicles, the demand for storage containers and components required for hydrogen storage and transportation is increasing. When gaseous hydrogen is filled into storage tanks, it is cooled to -40°C to -60°C for storage, taking into account the temperature rise during filling. Liquefied hydrogen is stored at -253°C. Therefore, the steel used for hydrogen storage containers and components must be able to withstand the entire temperature range from -253°C to room temperature to prevent a decrease in strength and toughness caused by hydrogen and extremely low temperatures.

[0003] Generally, austenitic structures are known to have advantages in low-temperature toughness compared to martensitic or ferritic structures, and are therefore used as steels for hydrogen storage containers and components. Austenitic stainless steels used for hydrogen storage containers and components are represented by the 300 series, primarily 304L and 316L. However, these commercially available stainless steels have relatively low strength and suffer from the disadvantage of requiring increased material thickness when applied to extremely low-temperature environments.

[0004] Furthermore, the impact toughness and other physical properties of austenitic stainless steel may decrease as the temperature decreases. When exposed to a hydrogen environment, hydrogen can penetrate the steel, potentially further reducing its physical properties. Therefore, it is necessary to consider both the temperature-induced and hydrogen-induced reductions in the physical properties of the steel, and to develop austenitic stainless steels that exhibit excellent strength and toughness even in extremely low-temperature hydrogen-containing environments. Summary of the Invention

[0005] (a) Technical problems to be solved The present invention aims to provide an austenitic stainless steel with excellent low-temperature impact toughness and excellent yield strength, even in extremely low-temperature environments such as liquefied hydrogen storage as described above, and a method for manufacturing the same.

[0006] The technical problems to be solved by the present invention are not limited to those mentioned above. Other technical problems not mentioned can be clearly understood by those skilled in the art from the following description.

[0007] (II) Technical Solution One aspect of the present invention relates to an austenitic stainless steel, comprising, by weight %, 0.10% or less of C (excluding 0), 1.50% or less of Si (excluding 0), 16-23% of Cr, 5.1-12% of Ni, 15% or less of Mn (excluding 0), 1.2% or less of Cu (excluding 0), 0.1-0.4% of N, (Nb+V) 0.01% or less, the balance of Fe and unavoidable impurities, wherein the product of the strength index expressed by formula (1) and the stacking fault energy (SFE) expressed by formula (2) is 0 or more.

[0008] Equation (1) Strength index: (Yield strength (MPa)) - 0.9Cr + 2Ni - 12Si - 1.1Mn - 80N + 400C - 275 Equation (2) SFE: 25.7 + 2Ni + 410C - 0.9Cr - 77N - 13Si - 1.2Mn (In equations (1) and (2), Cr, Ni, Si, Mn, N and C represent the content (weight %) of each element) According to one aspect of the invention, the austenitic stainless steel may further contain less than 1.5% (excluding 0) of Mo.

[0009] According to one aspect of the present invention, the value of formula (1) can be from 0 to 100 for austenitic stainless steel.

[0010] According to one aspect of the present invention, the value of formula (2) in the austenitic stainless steel system can be from 0 to 40.

[0011] According to one aspect of the present invention, the value of the following formula (3) in the austenitic stainless steel system can be 0 or higher.

[0012] Equation (3): (100N) + Mn-Ni (Where N, Mn, and Ni represent the content (by weight %) of each element) According to one aspect of the present invention, the value of the following formula (4) in the austenitic stainless steel system can be 0 or higher.

[0013] Formula (4): (Ni+0.52Cu+40(C+N)+0.7Mn+45)-(0.48(Cr+Mo+1.7Si))-51 (Where Ni, Cu, C, N, Mn, Cr, Mo, and Si represent the content (by weight %) of each element.) According to one aspect of the present invention, in austenitic stainless steel, the content of δ-ferrite can be less than 4% by area fraction.

[0014] According to one aspect of the present invention, the yield strength of the austenitic stainless steel can be above 280 MPa.

[0015] According to one aspect of the present invention, the austenitic stainless steel can have a Charpy impact energy of 73J or higher at -196°C.

[0016] According to one aspect of the present invention, the austenitic stainless steel can have a Charpy impact energy of 54 J or more at -253°C.

[0017] According to one aspect of the present invention, a method for manufacturing austenitic stainless steel may include the following steps: manufacturing a slab, in weight percent, the slab comprising: C: less than 0.10% (excluding 0), Si: less than 1.50% (excluding 0), Cr: 16-23%, Ni: 5.1-12%, Mn: less than 15% (excluding 0), Cu: less than 1.2% (excluding 0), N: 0.1-0.4%, (Nb+V): less than 0.01%, the balance being Fe and unavoidable impurities, wherein the product of the strength index represented by the following formula (1) and the stacking fault energy (SFE) represented by the following formula (2) of the slab is 0 or more; hot-rolling the slab at a temperature of 1050-1300°C to manufacture a hot-rolled steel sheet; and annealing the hot-rolled steel sheet at a temperature of 900-1200°C.

[0018] Equation (1) Strength index: (Yield strength (MPa)) - 0.9Cr + 2Ni - 12Si - 1.1Mn - 80N + 400C - 275 Equation (2) SFE: 25.7 + 2Ni + 410C - 0.9Cr - 77N - 13Si - 1.2Mn (In equations (1) and (2), Cr, Ni, Si, Mn, N and C represent the content (weight %) of each element) According to one aspect of the invention, in a method for manufacturing austenitic stainless steel, the slab may further contain less than 1.5% (excluding 0) of Mo.

[0019] According to one aspect of the present invention, in a method for manufacturing austenitic stainless steel, the formula (1) can be from 0 to 100.

[0020] According to one aspect of the present invention, in a method for manufacturing austenitic stainless steel, the formula (2) can be from 0 to 40.

[0021] According to one aspect of the present invention, in a method for manufacturing austenitic stainless steel, the following formula (3) can be 0 or higher.

[0022] Equation (3): (100N) + Mn-Ni (Where N, Mn, and Ni represent the content (by weight %) of each element) According to one aspect of the present invention, in a method for manufacturing austenitic stainless steel, the following formula (4) can be 0 or higher.

[0023] Formula (4): (Ni+0.52Cu+40(C+N)+0.7Mn+45)-(0.48(Cr+Mo+1.7Si))-51 (Where Ni, Cu, C, N, Mn, Cr, Mo, and Si represent the content (by weight %) of each element.) According to one aspect of the present invention, in a method for manufacturing austenitic stainless steel, the hot rolling time in the hot-rolled steel sheet manufacturing step is 1 hour to 3 hours, and the annealing time in the annealing step can be 5 minutes to 100 minutes.

[0024] (III) Beneficial Effects According to the present invention, an austenitic stainless steel with minimized hydrogen-induced degradation of physical properties even at extremely low temperatures and a method thereof can be provided.

[0025] According to the present invention, an austenitic stainless steel exhibiting excellent yield strength and low-temperature impact toughness even at extremely low temperatures, and a method thereof for manufacturing the same, can be provided.

[0026] According to the present invention, by providing an austenitic stainless steel with improved strength and a method for manufacturing the same, the amount of material used in hydrogen storage tanks can be reduced, thereby providing a reduction in manufacturing costs and a lightweight effect for hydrogen storage tanks.

[0027] The effects that can be obtained by the present invention are not limited to those mentioned above. Other effects not mentioned can be clearly understood by those skilled in the art from the following description. Detailed Implementation

[0028] The preferred embodiments of the present invention are described below. However, the embodiments of the present invention can be modified in many other forms, 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 skilled in the art.

[0029] The terminology used in this application is for illustrative purposes only. Therefore, for example, singular expressions include plural expressions unless explicitly required by the context. Furthermore, it should be noted that terms such as “comprising,” “including,” or “possessing” are used to explicitly indicate the presence of features, steps, functions, constituent elements, or combinations thereof described in the specification, and not to presuppose the presence of other features, steps, functions, constituent elements, or combinations thereof.

[0030] Furthermore, unless otherwise defined, all terms used in this specification should be considered to have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Therefore, unless expressly defined herein, particular terms should not be interpreted as having overly idealized or formalistic meanings.

[0031] Furthermore, the terms "about," "substantially," etc., used in this specification are used to mean the numerical value or close to the numerical value when referring to the inherent manufacturing and material tolerances, and are used to prevent unethical infringers from improperly using the invention content that mentions accurate or absolute values, which are used to help understand the invention.

[0032] Unless otherwise specified, the percentages of each element in this specification are based on weight.

[0033] According to one aspect of the present invention, the austenitic stainless steel comprises, by weight percent: C: less than 0.10% (excluding O), Si: less than 1.50% (excluding O), Cr: 16-23%, Ni: 5.1-12%, Mn: less than 15% (excluding O), Cu: less than 1.2% (excluding O), N: 0.1-0.4%, (Nb+V): less than 0.01%, Mo: less than 1.5%, the balance being Fe and unavoidable impurities.

[0034] The following explains the function and content of each alloy component in the austenitic stainless steel according to the present invention.

[0035] The content of C is less than 0.10% (excluding 0).

[0036] Carbon (C) is a low-cost austenite stabilizing element that effectively inhibits the formation of δ-ferrite, promotes solid solution strengthening in steel, and thus improves product strength. Therefore, it must be present in steel. When the C content exceeds 0.10%, it easily forms carbides with elements such as Cr, Ti, and Nb, which may reduce the corrosion resistance, ductility, and toughness of the steel. Therefore, its content is controlled to below 0.10% (excluding 0), preferably 0.02-0.05%, and more preferably 0.02-0.03%.

[0037] The Si content is less than 1.50% (excluding 0).

[0038] Si is an element used for deoxidation in steel and is effective in improving corrosion resistance. Furthermore, Si is effective for solid solution strengthening and beneficial for ensuring the excellent strength of steel; therefore, it must be present in steel. However, Si is a ferrite stabilizing element; therefore, when the Si content exceeds 1.50%, intermetallic compounds such as the σ phase will form, which may reduce the ductility and toughness of the steel. Therefore, the Si content is controlled below 1.50%, preferably between 0.2% and 1.50%, and more preferably between 0.38% and 0.43%.

[0039] The Cr content is 16-23%.

[0040] Cr is an essential element added to improve the corrosion resistance of stainless steel. Furthermore, Cr strengthens the steel through solid solution by increasing the solubility of nitrogen (N). This effect is not achieved when the Cr content is less than 16%. However, Cr can act as a ferrite-forming element; therefore, when the Cr content exceeds 23%, excessive δ-ferrite residue reduces the steel's hot workability, and the austenite phase becomes unstable. In this case, to stabilize the austenite phase, a large amount of Ni, a high-valence austenite phase stabilizing element, is added, thus increasing manufacturing costs. Therefore, the Cr content is controlled at 16-23%, preferably 16.1-22%, and more preferably 16.1-21.6%.

[0041] The Ni content is 5.1-12%.

[0042] Ni is an element that inhibits the formation of δ-ferrite and stabilizes the austenite phase. It also improves low-temperature impact toughness through grain refinement. To achieve these effects, Ni must be added at least 5.1%. However, when Ni exceeds 12%, the probability of surface defects increases, and the manufacturing cost increases excessively due to the large amount of Ni, which is a high-valence element. Therefore, its upper limit is controlled at 12%. Thus, the Ni content is controlled at 5.1-12%, preferably 5.1-11%, and more preferably 5.1-10.5%.

[0043] The Mn content is less than 15% (excluding 0).

[0044] Mn, along with Ni, acts as a strong austenitic phase stabilizer. Mn is a lower valence element compared to Ni and can substitute for the higher valence Ni. Furthermore, it must be present in steel because it inhibits deformation-induced martensitic transformation and improves low-temperature impact toughness. However, excessive addition of Mn without appropriate Ni addition reduces the stacking fault energy (SFE), potentially decreasing low-temperature impact toughness. Therefore, the amounts of Mn and Ni must be added in an appropriate ratio. Thus, the upper limit of Mn is controlled at 15%, preferably below 13.2% (excluding 0%), more preferably 0.4-13.2%, and even more preferably 0.8-13.2%.

[0045] The Cu content is less than 1.2% (excluding 0).

[0046] Cu is an element that inhibits the formation of martensite phase during forming and is useful for stabilizing the austenite phase; therefore, it must be present in steel. However, when the Cu content exceeds 1.2%, the hot workability is significantly reduced due to the formation of low-melting-point phases, which may contribute to surface defects during manufacturing. Therefore, its upper limit is controlled at 1.2%. Preferably, it is controlled at 0.1-1.2%, and more preferably, at 0.36-1.17%.

[0047] The nitrogen content is 0.1-0.4%.

[0048] Nitrogen (N) stabilizes the austenite phase, acts as an interstitial element, and contributes to the increase in strength through solid solution strengthening. When N is added at a level greater than 0.1%, the strength of the steel can be increased through N solid solution, and the reduction in low-temperature impact toughness can be minimized, thus providing an austenitic stainless steel with excellent strength and low-temperature impact toughness. However, when the N content exceeds 0.4%, it becomes a cause of reduced ductility, and surface defects may occur during the manufacturing process. Therefore, the N content is controlled at 0.1-0.4%, preferably 0.11-0.2%.

[0049] The sum of Nb and V content (Nb+V) is less than 0.01%.

[0050] Typically, Nb and V are elements that form precipitates, contributing to increased strength. However, precipitation strengthening methods based on precipitates can lead to a significant decrease in low-temperature impact toughness at extremely low temperatures, potentially becoming a major cause of this reduction. This invention does not employ conventional Nb and V-based precipitation strengthening methods, but instead achieves both high strength and extremely low-temperature impact toughness through solid solution strengthening and optimization of metallurgical factors. Therefore, the formation of Nb and V-induced precipitates is controlled to be substantially non-existent. However, since Nb and V are unavoidable impurities present in steel, their upper limit is controlled to below 0.01%.

[0051] According to one aspect of the invention, the austenitic stainless steel may further contain less than 1.5% Mo.

[0052] The Mo content can be less than 1.5%.

[0053] Mo is an effective element for improving corrosion resistance in stainless steel. However, when added in large quantities, the low-temperature impact toughness may decrease with the increase of ferrite fraction, and the manufacturing cost may increase with the increase of Mo content as a high-priced element. With this in mind, Mo can be controlled to 1.5% or less, preferably 0.4% to 1.0% or less, and more preferably 0.4% to 0.8% or less.

[0054] The remaining components besides those described are iron (Fe). However, unintentional impurities from raw materials or the surrounding environment are inevitably introduced during normal manufacturing processes, and these impurities cannot be eliminated. These impurities are well known to those skilled in the art in the ordinary course of manufacturing, and therefore, not all of them are specifically mentioned in this specification.

[0055] Austenitic stainless steels, which are advantageous in extremely low-temperature environments such as liquefied hydrogen, have relatively excellent low-temperature impact toughness but low strength. Therefore, when using austenitic stainless steels to manufacture tanks or structures for extremely low temperatures, there is a disadvantage of increased steel thickness.

[0056] To address this, methods for improving steel strength based on cold working or precipitation strengthening based on precipitates have been proposed. However, cold working-based strength improvement methods introduce martensitic structures during manufacturing, leading to hydrogen embrittlement or reduced low-temperature impact toughness even with increased strength. Furthermore, precipitation strengthening methods based on precipitates are unsuitable for extremely low-temperature hydrogen environments due to significant reduction in low-temperature impact toughness at extremely low temperatures.

[0057] The inventors of this invention have studied a method for obtaining stainless steel that simultaneously ensures high strength and excellent low-temperature impact toughness. The results show that, to ensure high strength, the relationship between yield strength and alloy composition must be understood to obtain the optimal composition; to ensure excellent low-temperature impact toughness, the stacking fault energy (SFE) must be derived from the relationship between alloy compositions and optimized.

[0058] With this in mind, in order to improve the strength within the above alloy composition, the inventors of this invention optimized the correlation between yield strength and alloy composition and derived a strength index defined by the following formula (1).

[0059] Equation (1) Strength index: (Yield strength (MPa)) - 0.9Cr + 2Ni - 12Si - 1.1Mn - 80N + 400C - 275 (In the formula (1), each element symbol represents the content (weight %) of each element) When the value of equation (1) is 0 or higher, a high strength characteristic of yield strength of 280 MPa or higher at room temperature can be ensured. When the value of equation (1) is excessively high, it means that the C content is too high, and carbides precipitate in the steel. Precipitation strengthening may reduce low-temperature impact toughness. Therefore, the value of equation (1) is controlled to be 0 or higher. The value of equation (1) is preferably controlled to be between 0 and 100, and more preferably between 0 and 74.

[0060] In order to improve the low-temperature impact toughness within the above alloy composition, the inventors of this invention derived the stacking fault energy (SFE) defined by equation (2) based on the correlation between the alloy compositions.

[0061] Equation (2) SFE: 25.7 + 2Ni + 410C - 0.9Cr - 77N - 13Si - 1.2Mn (In the formula (2), each element symbol represents the content (by weight %) of each element) When the value of Equation (2) is 0 or higher, the Charpy impact energy at -196℃ is above 73J, ensuring excellent low-temperature impact toughness. When the obtained value of Equation (2) is too high, surface defects are prone to occur in the manufacturing process, and the quality of the product may be reduced. In addition, due to the large amount of high-priced elements, the manufacturing cost will increase, and thus productivity may be reduced. Considering this, the value of Equation (2) is controlled to be 0 or higher. The value of Equation (2) is preferably 0 to 40, and more preferably 0 to 23 or lower.

[0062] The inventors of this invention discovered that, according to formula (1) Equation (2) can predict the strength level of steel and its applicability in extremely low temperature environments. Specifically, by controlling the product of Equations (1) and (2) to be above 0, austenitic stainless steel with a Charpy impact energy of over 73 J at -196℃ and a room temperature yield strength of over 280 MPa is obtained. When Equation (1) When the value of equation (2) is below 0, either of the two factors has a negative value, thus it is impossible to simultaneously ensure low-temperature impact toughness and high strength. However, when equation (1) When the value of equation (2) is too high, it means that Ni, N, etc., have been excessively added, which may lead to increased costs and reduced low-temperature impact toughness. Considering this, equation (1)... The value of formula (2) is controlled to be above 0, preferably 0 to 10000, and more preferably 20 to 1000.

[0063] In addition, Ni, Mn, and N in the above alloy composition are elements closely related to the stabilization of the austenite phase. These elements, when added to steel, play a role in stabilizing the austenite phase, but excessive addition can lead to surface defects in the manufacturing process, reducing product quality. In particular, excessive addition of N will significantly reduce low-temperature impact toughness, and excessive addition of Mn will produce Mn fume, which may reduce productivity. Considering the correlation between these elements, the following equation (3) is derived.

[0064] Equation (3): (100N) + Mn-Ni (In formula (3), the symbols of each element represent the content (by weight %) of each element.) When the value of equation (3) is 0 or higher, austenitic stainless steel with a Charpy impact energy of 73 J or higher at -196°C and a room temperature yield strength of 280 MPa or higher can be obtained. However, when the obtained value of equation (3) is too high, the low-temperature impact toughness may decrease due to the martensitic phase transformation. Taking this into consideration, the value of equation (3) can be controlled to be 0 or higher, preferably 0 to 40, and more preferably 0 to 21.

[0065] Furthermore, based on the theoretical calculation formula of δ-ferrite, the inventors of this invention optimized the correlation between the alloy composition and strength and derived the following formula (4).

[0066] Formula (4): (Ni+0.52Cu+40(C+N)+0.7Mn+45)-(0.48(Cr+Mo+1.7Si))-51 (In formula (4), the symbols of each element represent the content (by weight %) of each element.) When the value of equation (4) is 0 or higher, a room temperature yield strength of 280 MPa or higher can be ensured. However, when the obtained value of equation (4) is too high, it is difficult to ensure low temperature impact toughness. Taking this into consideration, equation (4) can be controlled to be 0 or higher, preferably 0 or higher and 30 or lower, and more preferably 0 or higher and 8 or lower.

[0067] According to one embodiment of the present invention, the austenitic stainless steel can be manufactured by the general manufacturing process of austenitic stainless steel.

[0068] According to one embodiment of the austenitic stainless steel of the present invention, the δ-ferrite content, in terms of area fraction, can be less than 4%. When the δ-ferrite content, in terms of area fraction, exceeds 4%, the austenite phase has low stability, and high strength and excellent low-temperature impact toughness cannot be achieved.

[0069] According to one embodiment of the present invention, the austenitic stainless steel has a yield strength of 280 MPa or higher. There is no upper limit; for example, it can be 800 MPa or lower, 600 MPa or lower, or 450 MPa or lower. Within the above range, it exhibits excellent low-temperature impact toughness while further improving strength. In this case, for example, it is possible to achieve machinability suitable for use in liquefied gas storage containers, while simultaneously achieving excellent low-temperature impact toughness and high strength.

[0070] According to one embodiment of the present invention, the austenitic stainless steel can have a Charpy impact energy of 73J or more at -196°C.

[0071] According to one embodiment of the present invention, the austenitic stainless steel can have a Charpy impact energy of 54 J or more at -253°C. The Charpy impact energy at -253°C can be used to evaluate the impact toughness at -253°C, which is the temperature of liquefied hydrogen, to indicate whether the material has excellent impact toughness characteristics in the practical application environment of liquefied hydrogen.

[0072] Next, a method for manufacturing austenitic stainless steel according to the present invention will be described.

[0073] A method for manufacturing austenitic stainless steel according to one embodiment of the present invention includes the following steps: manufacturing a slab, the slab comprising, by weight %,: C: less than 0.10% (excluding 0), Si: less than 1.50% (excluding 0), Cr: 16-23%, Ni: 5.1-12%, Mn: less than 15% (excluding 0), Cu: less than 1.2% (excluding 0), N: 0.1-0.4%, (Nb+V): less than 0.01%, the balance being Fe and unavoidable impurities, wherein the product of the strength index expressed by formula (1) and the stacking fault energy (SFE) expressed by formula (2) of the slab is 0 or more; hot-rolling the slab at a temperature of 1050-1300°C to manufacture a hot-rolled steel sheet; and annealing the hot-rolled steel sheet at a temperature of 900-1200°C.

[0074] Equation (1) Strength index: (Yield strength (MPa)) - 0.9Cr + 2Ni - 12Si - 1.1Mn - 80N + 400C - 275 Equation (2) SFE: 25.7 + 2Ni + 410C - 0.9Cr - 77N - 13Si - 1.2Mn (In equations (1) and (2), Cr, Ni, Si, Mn, N and C represent the content (weight %) of each element) The role and content of each alloy component, and the explanation of formulas (1) to (4) are as described above.

[0075] During the annealing process of hot-rolled steel sheets, the annealing temperature has a significant impact on the elimination of residual stress and the microstructure. For example, the annealing temperature can be 900-1200℃.

[0076] When the annealing temperature is below 900°C, coarse carbides are formed, resulting in an uneven microstructure or the formation of chromium carbide precipitates around grain boundaries, which may lead to grain boundary corrosion. However, when the annealing temperature exceeds 1200°C, the grains may become extremely coarse. Considering this, it is preferable to limit the annealing temperature to 900-1200°C.

[0077] According to a method for manufacturing austenitic stainless steel according to one embodiment of the present invention, the slab may further contain less than 1.5% Mo.

[0078] According to a method for manufacturing austenitic stainless steel according to an embodiment of the present invention, the value of formula (3) of the slab can be 0 or higher.

[0079] Equation (3): (100N) + Mn-Ni (Where N, Mn, and Ni represent the content (by weight %) of each element) According to a method for manufacturing austenitic stainless steel according to an embodiment of the present invention, the value of the slab in the following formula (4) can be 0 or higher.

[0080] Formula (4): (Ni+0.52Cu+40(C+N)+0.7Mn+45)-(0.48(Cr+Mo+1.7Si))-51 (Where Ni, Cu, C, N, Mn, Cr, Mo, and Si represent the content (by weight %) of each element.) According to one embodiment of the present invention, in the method for manufacturing austenitic stainless steel, the hot rolling time in the hot-rolled steel sheet manufacturing step is 1 hour to 3 hours, and the annealing time in the annealing step can be 5 minutes to 100 minutes.

[0081] Austenitic stainless steel of formula (1) manufactured according to the manufacturing conditions of the present invention. The values ​​of Equation (2), Equation (3), and Equation (4) can be above 0, the ferrite fraction can be below 4%, the yield strength at room temperature can be above 280 MPa, the Charpy impact energy at -196℃ can be above 73 J, and the Charpy impact energy at -253℃ can be above 54 J.

[0082] The following description, through preferred embodiments of the present invention, will provide a more detailed explanation of the structure and function of the invention. However, this is presented as a preferred example of the invention and should not be construed as limiting the invention in any way.

[0083] (Example) A slab with the composition shown in Table 1 was manufactured, heated, and then hot-rolled at 1250°C for 2 hours. The hot-rolled steel sheet was then annealed at 1000°C–1200°C for no more than 60 minutes to obtain test pieces.

[0084] Table 1 below shows the component composition (weight %) of each sample. Underlined sections in Table 1 indicate contents outside the scope of this invention.

[0085] [Table 1] The yield strength of the specimen was measured using the following method. [Yield Strength] Sub-dimensional tensile specimens with a thickness of 1.5 mm were prepared. Specimens were collected at 1 / 2t position along the thickness direction of each specimen and measured according to ASTM E8. The average of five measurements is shown, expressed as 0.2% yield strength (YS0.2).

[0086] Table 2 shows the measured yield strength and the equations (1), (2) to (4), and (1) calculated based on it. The value of equation (2). The underlined part indicates content that is outside the scope of this invention.

[0087] [Table 2] Next, the δ-ferrite content, Charpy impact energy at -196℃, and Charpy impact energy at -253℃ were measured on the specimen. The measurement methods are as follows. [δ-ferrite content] Five locations were randomly selected at the center of each hot-rolled test piece. The δ-ferrite content was measured using a ferrite measuring instrument, and the average value was taken. The center of the test piece refers to the position between 1 / 4 and 3 / 4t when the overall thickness length is set to t.

[0088] [Charpy impact energy at -196℃, Charpy impact energy at -253℃] After manufacturing ASTM E23A type specimens, the Charpy impact energy at -196℃ and -253℃ was measured using a Charpy impact testing machine. Five identical tests were performed on each specimen, and the average of the five measurements was taken.

[0089] Table 3 below shows the physical properties, and underlined items indicate contents that are outside the scope of this invention.

[0090] [Table 3] Examples 1 to 15 are alloy compositions that satisfy the disclosure of this invention, and simultaneously, formula (1) The values ​​of equations (2), (3), and (4) are positive, indicating that the specimens within the scope of this invention, as shown in Examples 1 to 15, have a yield strength of 280 MPa or higher, a δ-ferrite content of 4% or less, a Charpy impact energy of 73 J or higher at -196°C, and a Charpy impact energy of 54 J or higher at -253°C, exhibiting both high strength and excellent low-temperature impact toughness. Comparative Examples 1, 3, 4, and 6 are specimens with insufficient N content. Therefore, the solid solution strengthening effect caused by N is insufficient, and a yield strength of 280 MPa or higher cannot be ensured. Thus, equation (1) The values ​​of equations (2), (3), and (4) are negative, indicating that stainless steel with both excellent yield strength and low-temperature impact toughness cannot be obtained simultaneously. Equation (1) of Comparative Examples 2, 5, and 9 is also relevant. The value of Equation (2) is negative, and the δ ferrite fraction exceeds 4%, thus the low-temperature impact toughness deteriorates.

[0091] Compare equation (1) of Comparative Example 7 and Comparative Example 8. The value of equation (2) is negative, therefore the low-temperature impact toughness deteriorates.

[0092] These examples and comparative examples show that, in order to provide an austenitic stainless steel with excellent yield strength and low-temperature impact toughness, it is necessary to include alloying elements such as Ni and Mn while ensuring that N contributes to ensuring yield strength, and to optimize the alloying elements by controlling formulas (1) to (4).

[0093] The above description refers to the embodiments, but those skilled in the art will understand that various modifications and alterations can be made to the present invention without departing from the spirit and scope of the invention as set forth in the following claims.

Claims

1. An austenitic stainless steel, wherein, The austenitic stainless steel, by weight percent, comprises: C: less than 0.10% and excluding 0, Si: less than 1.50% and excluding 0, Cr: 16-23%, Ni: 5.1-12%, Mn: less than 15% and excluding 0, Cu: less than 1.2% and excluding 0, N: 0.1-0.4%, (Nb+V): less than 0.01%, with the balance being Fe and unavoidable impurities. The product of the strength index, expressed by equation (1), and the stacking fault energy (SFE), expressed by equation (2), is greater than or equal to 0. Equation (1) Strength index: (Yield strength (MPa)) - 0.9Cr + 2Ni - 12Si - 1.1Mn - 80N + 400C - 275 Equation (2) SFE: 25.7 + 2Ni + 410C - 0.9Cr - 77N - 13Si - 1.2Mn In equations (1) and (2), Cr, Ni, Si, Mn, N and C represent the content of each element, and their units are by weight.

2. The austenitic stainless steel according to claim 1, wherein, The austenitic stainless steel further contains less than 1.5% of Mo, excluding 0%.

3. The austenitic stainless steel according to claim 1, wherein, Equation (1) is from 0 to 100.

4. The austenitic stainless steel according to claim 1, wherein, Equation (2) is 0 to 40.

5. The austenitic stainless steel according to claim 1, wherein, If equation (3) is 0 or higher, Equation (3): (100N) + Mn-Ni Wherein, N, Mn, and Ni represent the content of each element, and their units are by weight.

6. The austenitic stainless steel according to claim 2, wherein, If equation (4) is 0 or higher, Formula (4): (Ni+0.52Cu+40(C+N)+0.7Mn+45)-(0.48(Cr+Mo+1.7Si))-51 Where Ni, Cu, C, N, Mn, Cr, Mo, and Si represent the content of each element, and the unit is by weight.

7. The austenitic stainless steel according to claim 1, wherein, The δ-ferrite content is less than 4% by area fraction.

8. The austenitic stainless steel according to claim 1, wherein, The yield strength is above 280 MPa.

9. The austenitic stainless steel according to claim 1, wherein, The Charpy impact energy at -196℃ is over 73J.

10. The austenitic stainless steel according to claim 1, wherein, The Charpy impact energy at -253℃ is above 54J.

11. A method for manufacturing austenitic stainless steel, wherein, The manufacturing method includes the following steps: The slab is manufactured in weight percent, comprising: C: less than 0.10% and excluding 0, Si: less than 1.50% and excluding 0, Cr: 16-23%, Ni: 5.1-12%, Mn: less than 15% and excluding 0, Cu: less than 1.2% and excluding 0, N: 0.1-0.4%, (Nb+V): less than 0.01%, the balance being Fe and unavoidable impurities, wherein the product of the strength index expressed by the following formula (1) and the stacking fault energy (SFE) expressed by the following formula (2) of the slab is 0 or more; The slab is hot-rolled at a temperature of 1050-1300°C to produce hot-rolled steel sheet; The hot-rolled steel sheet is annealed at a temperature of 900-1200℃. Equation (1) Strength index: (Yield strength (MPa)) - 0.9Cr + 2Ni - 12Si - 1.1Mn - 80N + 400C - 275 Equation (2) SFE: 25.7 + 2Ni + 410C - 0.9Cr - 77N - 13Si - 1.2Mn In equations (1) and (2), Cr, Ni, Si, Mn, N and C represent the content of each element, and their units are by weight.

12. The method for manufacturing austenitic stainless steel according to claim 11, wherein, The slab further contains less than 1.5% Mo, excluding 0%.

13. The method for manufacturing austenitic stainless steel according to claim 11, wherein, If equation (3) is 0 or higher, Equation (3): (100N) + Mn-Ni Wherein, N, Mn, and Ni represent the content of each element, and their units are by weight.

14. The method for manufacturing austenitic stainless steel according to claim 12, wherein, If equation (4) is 0 or higher, Formula (4): (Ni+0.52Cu+40(C+N)+0.7Mn+45)-(0.48(Cr+Mo+1.7Si))-51 Where Ni, Cu, C, N, Mn, Cr, Mo, and Si represent the content of each element, and the unit is by weight.

15. The method for manufacturing austenitic stainless steel according to claim 11, wherein, The hot rolling time in the manufacturing process of the hot-rolled steel plate is 1 to 3 hours, and the annealing time in the annealing process is 5 to 100 minutes.

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