Austenitic stainless steel with improved strength and low-temperature impact toughness, and manufacturing method therefor

Abstract

According to the present invention, an austenitic steel sheet can be provided, the sheet comprising, by wt%, 0.10% or less (excluding 0) of C, 1.50% or less (excluding 0) of Si, 16-23% of Cr, 5.1-12% of Ni, 15% or less (excluding 0) of Mn, 1.2% or less (excluding 0) of Cu, 0.1-0.4% of N, 0.01% or less of (Nb+V), 1.5% or less of Mo, and the balance of Fe and inevitable impurities, wherein the product of strength index represented by relation (1) and stacking-fault energy (SFE) represented by relation (2) is at least 0. Relation (1) Strength index: (yield strength(MPa))-0.9Cr+2Ni-12Si-1.1Mn-80N+400C-275, Relation (2) SFE: 25.7+2Ni+410C-0.9Cr-77N-13Si-1.2Mn (In relation (1) and relation (2), Cr, Ni, Si, Mn, N and C represent the amount (wt%) of each element.)

Description

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

[0001] The present invention relates to an austenitic stainless steel having improved yield strength (MPa) and low-temperature impact toughness and a method for manufacturing the same.

[0002] With the recent expansion of the development and widespread adoption of hydrogen-powered vehicles, demand is increasing for storage containers and components necessary for hydrogen storage and transportation. Gaseous hydrogen is stored in storage tanks at temperatures ranging from -40 to -60°C to account for the rise in gas temperature during refueling. Liquid hydrogen is stored at -253°C. Therefore, the steel used in hydrogen storage containers and components must be able to withstand hydrogen and cryogenic temperatures across the entire temperature range, from -253°C to room temperature, to prevent deterioration in strength and toughness.

[0003] Austenitic structures are generally known to have advantageous low-temperature toughness compared to martensitic or ferritic structures, and are thus used as steel for hydrogen storage vessels and components. The most representative austenitic stainless steel used for hydrogen storage vessels and components is 300 series stainless steel, with 304L and 316L being the most commonly used. However, these commercial stainless steels have a relatively low strength, which means that the material thickness increases when applied to cryogenic environments.

[0004] Furthermore, austenitic stainless steels can exhibit deteriorating properties, such as impact toughness, as temperatures decrease. Furthermore, exposure to hydrogen environments can lead to hydrogen infiltration, further degrading properties. Therefore, by simultaneously considering both temperature-related and hydrogen-related deterioration of steel properties, the development of austenitic stainless steels with superior strength and toughness, even in extremely low-temperature hydrogen environments, is required.

[0005] The present invention aims to provide an austenitic stainless steel having excellent low-temperature impact toughness and excellent yield strength even in an extremely low-temperature environment such as liquefied hydrogen storage, and a method for manufacturing the same.

[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] One aspect of the present invention relates to an austenitic stainless steel, which comprises, in wt%, C: 0.10% or less (excluding 0), Si: 1.50% or less (excluding 0), Cr: 16 to 23%, Ni: 5.1 to 12%, Mn: 15% or less (excluding 0), Cu: 1.2% or less (excluding 0), N: 0.1 to 0.4%, (Nb+V): 0.01% or less, the remainder being Fe and unavoidable impurities, and wherein the product of a strength index expressed by the following formula (1) and a stacking fault energy (SFE) expressed by the following formula (2) is 0 or more.

[0008] Equation (1) Strength index: (Yield strength (MPa))-0.9Cr+2Ni-12Si-1.1Mn-80N+400C-275

[0009] Formula (2) SFE: 25.7+2Ni+410C-0.9Cr-77N-13Si-1.2Mn

[0010] (In Equations (1) and (2), Cr, Ni, Si, Mn, N, and C represent the content (weight%) of each element.)

[0011] The austenitic stainless steel according to one aspect of the present invention may further include Mo: 1.5% or less (excluding 0).

[0012] The austenitic stainless steel according to one aspect of the present invention may have a value of formula (1) of 0 to 100.

[0013] The austenitic stainless steel according to one aspect of the present invention may have a value of formula (2) of 0 to 40.

[0014] The austenitic stainless steel according to one aspect of the present invention may have a value of the following formula (3) of 0 or greater.

[0015] Equation (3): (100N)+Mn-Ni

[0016] (Here, N, Mn and Ni represent the content (weight%) of each element)

[0017] The austenitic stainless steel according to one aspect of the present invention may have a value of the following formula (4) of 0 or greater.

[0018] Equation (4): (Ni+0.52Cu+40(C+N)+0.7Mn+45)-(0.48(Cr+Mo+1.7Si))-51

[0019] (Here, Ni, Cu, C, N, Mn, Cr, Mo and Si represent the content (weight%) of each element)

[0020] The austenitic stainless steel according to one aspect of the present invention may have a delta ferrite content of 4% or less in area fraction.

[0021] The austenitic stainless steel according to one aspect of the present invention may have a yield strength of 280 MPa or more.

[0022] An austenitic stainless steel according to one aspect of the present invention may have a Charpy impact energy of 73 J or more at -196°C.

[0023] An austenitic stainless steel according to one aspect of the present invention may have a Charpy impact energy of 54 J or more at -253°C.

[0024] According to one aspect of the present invention, a method for manufacturing an austenitic stainless steel may include the steps of manufacturing a slab containing, in wt%, C: 0.10% or less (excluding 0), Si: 1.50% or less (excluding 0), Cr: 16 to 23%, Ni: 5.1 to 12%, Mn: 15% or less (excluding 0), Cu: 1.2% or less (excluding 0), N: 0.1 to 0.4%, (Nb+V): 0.01% or less, the remainder Fe and unavoidable impurities, and having a product of a strength index expressed by the following formula (1) and a stacking fault energy (SFE) expressed by the following formula (2) of 0 or more, hot-rolling the slab at a temperature of 1050 to 1300°C to manufacture a hot-rolled steel sheet, and annealing the hot-rolled steel sheet at a temperature of 900 to 1,200°C. there is.

[0025] Equation (1) Strength index: (Yield strength (MPa))-0.9Cr+2Ni-12Si-1.1Mn-80N+400C-275

[0026] Formula (2) SFE: 25.7+2Ni+410C-0.9Cr-77N-13Si-1.2Mn

[0027] (In Equations (1) and (2), Cr, Ni, Si, Mn, N, and C represent the content (weight%) of each element.)

[0028] A method for manufacturing austenitic stainless steel according to one aspect of the present invention may further include Mo: 1.5% or less (excluding 0).

[0029] In a method for manufacturing an austenitic stainless steel according to one aspect of the present invention, the above formula (1) may be 0 to 100.

[0030] In a method for manufacturing an austenitic stainless steel according to one aspect of the present invention, the above formula (2) may be 0 to 40.

[0031] In a method for manufacturing an austenitic stainless steel according to one aspect of the present invention, the following formula (3) may be 0 or greater.

[0032] Equation (3): (100N)+Mn-Ni

[0033] (Here, N, Mn and Ni represent the content (weight%) of each element)

[0034] In a method for manufacturing an austenitic stainless steel according to one aspect of the present invention, the following formula (4) may be 0 or greater.

[0035] Equation (4): (Ni+0.52Cu+40(C+N)+0.7Mn+45)-(0.48(Cr+Mo+1.7Si))-51

[0036] (Here, Ni, Cu, C, N, Mn, Cr, Mo and Si represent the content (weight%) of each element)

[0037] In a method for manufacturing austenitic stainless steel according to one aspect of the present invention, the hot rolling time in the hot-rolled steel sheet manufacturing step may be 1 to 3 hours, and the annealing time in the annealing step may be 5 to 100 minutes.

[0038] According to the present invention, an austenitic stainless steel and a method for manufacturing the same can be provided in which the deterioration of the properties of the steel due to hydrogen is minimized even in an extremely low temperature environment.

[0039] According to the present invention, an austenitic stainless steel having excellent yield strength and low-temperature impact toughness even in an extremely low-temperature environment and a method for manufacturing the same can be provided.

[0040] 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 a hydrogen storage tank can be reduced, thereby providing a reduction in the manufacturing cost and weight reduction effect of the hydrogen storage tank.

[0041] The effects that can be obtained from the present invention are not limited to the effects mentioned above, and other effects not mentioned can be clearly understood by a person having ordinary skill in the art to which the present invention pertains from the description below.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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 exploiting the invention contents in which exact or absolute numerical values ​​are mentioned to aid understanding of the invention.

[0046] Unless otherwise specifically stated herein, percentages indicating the content of each element are based on weight.

[0047] An austenitic stainless steel according to one aspect of the present invention contains, in wt%, C: 0.10% or less (excluding 0), Si: 1.50% or less (excluding 0), Cr: 16 to 23%, Ni: 5.1 to 12%, Mn: 15% or less (excluding 0), Cu: 1.2% or less (excluding 0), N: 0.1 to 0.4%, (Nb+V): 0.01% or less, Mo: 1.5% or less, the remainder being Fe and unavoidable impurities.

[0048] Hereinafter, the role and content of each alloy component included in the austenitic stainless steel according to the present invention will be described.

[0049] The content of C is less than 0.10% (excluding 0).

[0050] C is a low-cost austenite stabilizing element that effectively suppresses the formation of delta (δ) ferrite and promotes solid solution strengthening of steel, thereby improving the strength of the product. Therefore, it is essential to contain it in steel. When the C content exceeds 0.10%, it can easily form carbides with elements such as Cr, Ti, and Nb, thereby reducing the corrosion resistance, ductility, and toughness of the steel. Therefore, the C content is controlled to 0.10% or less (excluding 0), preferably 0.02 to 0.05%, and more preferably 0.02 to 0.03%.

[0051] The Si content is 1.50% or less (excluding 0).

[0052] Si is an element used for deoxidation of steel and is an effective element for improving corrosion resistance. In addition, Si is an element effective for solid solution strengthening and is an element advantageous for securing excellent strength of steel, so it is essential to contain it in steel. However, since Si is a ferrite stabilizing element, when the Si content exceeds 1.50%, it may form intermetallic compounds such as σ phase, thereby reducing the ductility and toughness of steel. Therefore, the Si content is controlled to be 1.50% or less, preferably controlled to be 0.2 to 1.50%, and more preferably controlled to be 0.38 to 0.43%.

[0053] The Cr content is 16 to 23%.

[0054] Cr is an element that must be added to improve the corrosion resistance of stainless steel. It also acts as a solid solution strengthening agent by increasing the solubility of nitrogen in the steel. If the Cr content is less than 16%, the aforementioned effect cannot be obtained. However, since Cr can act as a ferrite-forming element, if the Cr content exceeds 23%, excessive delta ferrite remains, which reduces the hot workability of the steel and makes the austenite phase unstable. In this case, in order to stabilize the austenite phase in the steel, a large amount of nickel, an expensive austenite phase stabilizing element, is included, which causes an increase in manufacturing cost. Therefore, the Cr content is controlled to be 16 to 23%, preferably 16.1 to 22%, and more preferably 16.1 to 21.6%.

[0055] The Ni content is 5.1 to 12%.

[0056] Ni is an element that stabilizes the austenite phase by suppressing the formation of delta ferrite and plays a role in improving low-temperature impact toughness through grain refinement. In order to obtain the above-mentioned effect, Ni must be added in an amount of 5.1% or more. However, if Ni is added in an amount of 12% or more, the probability of surface defects increases, and since a large amount of Ni, an expensive element, is contained, the manufacturing cost excessively increases, so the upper limit is controlled to 12%. Therefore, the Ni content is controlled to 5.1 to 12%, preferably controlled to 5.1 to 11%, and more preferably controlled to 5.1 to 10.5%.

[0057] The Mn content is less than 15% (excluding 0).

[0058] Manganese, together with Ni, acts as a stabilizing element for a strong austenite phase. Mn is a low-cost element compared to Ni, making it an element that can replace expensive Ni. In addition, it plays a role in improving low-temperature impact toughness by suppressing strain-induced martensitic transformation, and therefore, it must be included in steel. However, if Mn is excessively added without an appropriate addition of Ni, the stacking fault energy (SFE) may decrease, which may actually lower the low-temperature impact toughness. Therefore, the amount of Mn and Ni added must be added in an appropriate ratio. Therefore, the upper limit of Mn is controlled to 15%, preferably controlled to 13.2% or less (excluding 0), preferably controlled to 0.4 to 13.2%, and more preferably controlled to 0.8 to 13.2%.

[0059] The Cu content is less than 1.2% (excluding 0).

[0060] Copper (Cu) is an essential element for suppressing the formation of martensite during forming and stabilizing the austenite phase, and therefore must be included in steel. However, if the Cu content exceeds 1.2%, a low melting point phase is formed, which significantly reduces hot workability and may cause surface defects during manufacturing. Therefore, the upper limit is controlled to 1.2%. Preferably, it is controlled to 0.1 to 1.2%, and more preferably, it is controlled to 0.36 to 1.17%.

[0061] The N content is 0.1 to 0.4%.

[0062] Nitrogen is an element that stabilizes the austenite phase and acts as an interstitial element, contributing to the improvement of strength through solid solution strengthening. When 0.1% or more of Ni is added, the strength of steel can be improved through N solid solution, and the decrease in low-temperature impact toughness can be minimized, thereby providing an austenitic stainless steel having excellent strength and low-temperature impact toughness. However, when Ni is added in excess of 0.4%, it causes a decrease in ductility, which can cause surface defects in the manufacturing process. Therefore, the Ni content is controlled to 0.1 to 0.4%, and preferably 0.11 to 0.2%. 0.25

[0063] The sum of Nb and V contents, Nb+V, is less than 0.01%.

[0064] In general, niobium (Nb) and vanadium (V) are elements that help improve strength by forming precipitates. However, precipitation strengthening using precipitates can significantly reduce low-temperature impact toughness in cryogenic environments, which can be a major cause of reduced low-temperature impact toughness. In the present invention, rather than the conventional precipitation strengthening using niobium and vanadium (Nb) and vanadium (V), we aim to simultaneously achieve high strength and cryogenic impact toughness through solid solution strengthening and optimization of metallurgical factors. Therefore, we control the formation of precipitates by niobium and vanadium (V). However, since niobium and vanadium (V) are unavoidable impurities contained in steel, their upper limit is controlled to be 0.01% or less.

[0065] The austenitic stainless steel according to one aspect of the present invention may further contain 1.5% or less of Mo.

[0066] The Mo content may be less than 1.5%.

[0067] Mo is an effective element for improving corrosion resistance in stainless steel. However, when added in large amounts, the low-temperature impact toughness may deteriorate due to the increased ferrite fraction, and the increased content of Mo, an expensive element, may increase manufacturing costs. Considering this, Mo may 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.

[0068] In addition to the above composition, the remaining component is iron (Fe). However, during the normal manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed in, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the normal manufacturing process, their full details are not specifically mentioned in this specification.

[0069] Austenitic stainless steels, known to be advantageous in cryogenic environments such as liquid hydrogen, have relatively high low-temperature impact toughness but low strength. Therefore, when used in the construction of cryogenic tanks or structures, austenitic stainless steels require increased thickness, a drawback.

[0070] Accordingly, methods for enhancing strength through cold working or precipitation strengthening using precipitates have been proposed to achieve high strength in steel. However, methods for enhancing strength through cold working suffer from the problem of hydrogen embrittlement or reduced low-temperature impact toughness due to the appearance of martensite structure during the manufacturing process, even though strength is improved. Furthermore, precipitation strengthening using precipitates significantly reduces low-temperature impact toughness in cryogenic environments, making it unsuitable for cryogenic hydrogen environments.

[0071] The inventors of the present invention studied a method for obtaining stainless steel that simultaneously achieves high strength and excellent low-temperature impact toughness. As a result, they discovered that to secure high strength, the correlation between yield strength and alloy composition must be identified to determine optimal composition. Furthermore, to secure excellent low-temperature impact toughness, the stacking bond energy (SFE) must be derived and optimized through the correlation between alloy compositions.

[0072] In view of this, the inventors of the present invention optimized the correlation between yield strength and alloy composition to improve strength within the above-described alloy composition, and derived a strength index defined by the following equation (1).

[0073] Equation (1) Strength index: (Yield strength (MPa))-0.9Cr+2Ni-12Si-1.1Mn-80N+400C-275

[0074] (In the above formula (1), each element symbol represents the content (weight%) of each element)

[0075] When the value of formula (1) is 0 or greater, high strength characteristics of room temperature yield strength of 280 MPa or greater can be secured. If the value of formula (1) is derived excessively high, it means that the C content is excessively high, and carbides in the steel may be precipitated, which may lower the low-temperature impact toughness due to precipitation strengthening. Therefore, the value of formula (1) is controlled to be 0 or greater. The value of formula (1) is preferably controlled to be 0 to 100, and more preferably controlled to be 0 to 74.

[0076] The inventors of the present invention derived the stacking bond energy (SFE) defined by Equation (2) through the correlation between alloy compositions in order to improve low-temperature impact toughness within the above-described alloy composition.

[0077] Formula (2) SFE: 25.7+2Ni+410C-0.9Cr-77N-13Si-1.2Mn

[0078] (In the above formula (2), each element symbol represents the content (weight%) of each element)

[0079] When the value of Equation (2) is 0 or more, the Charpy impact energy at -196℃ is 73J or more, so excellent low-temperature impact toughness characteristics can be secured. If the value of Equation (2) is derived excessively high, surface defects are likely to occur during the manufacturing process, which may lower the quality of the product. In addition, since it contains a large amount of expensive elements, it may lead to an increase in manufacturing cost, which may lower productivity. Considering this, the value of Equation (2) is controlled to be 0 or more. The value of Equation (2) is preferably 0 to 40, and more preferably 0 to 23 or less.

[0080] The inventors of the present invention have found that the strength level of steel and its usability in extremely low-temperature environments can be predicted according to Equation (1)*Equation (2). Specifically, by controlling the product of Equations (1) and (2) to be 0 or greater, an austenitic stainless steel having a Charpy impact energy of 73 J or more at -196℃ and a yield strength of 280 MPa or more at room temperature was secured. If the value of Equation (1)*Equation (2) is 0 or less, one of the two factors has a negative value, so that both low-temperature impact toughness and high strength cannot be secured at the same time. However, if the value of Equation (1)*Equation (2) is excessive, it means that Ni, N, etc. have been excessively added, which may cause problems of increased cost and decreased low-temperature impact toughness. In consideration of this, the value of Equation (1)*Equation (2) is controlled to be 0 or greater, preferably 0 to 10,000, and more preferably 20 to 1,000.

[0081] Meanwhile, among the alloy compositions described above, Ni, Mn, and N are elements closely related to the stabilization of the austenite phase. These elements are added to steel to stabilize the austenite phase, but if added excessively, they can cause surface defects during the manufacturing process, thereby reducing product quality. In particular, excessive addition of N can significantly reduce low-temperature impact toughness, and excessive addition of Mn can generate Mn fume, which can reduce productivity. Considering the correlation between these elements, the following equation (3) was derived.

[0082] Equation (3): (100N)+Mn-Ni

[0083] (In formula (3), each element symbol represents the content (weight%) of each element)

[0084] When the value of Equation (3) is 0 or greater, an austenitic stainless steel having a Charpy impact energy of 73 J or greater at -196°C and a room temperature yield strength of 280 MPa or greater can be obtained. However, when the value of Equation (3) is excessively derived, the low-temperature impact toughness may deteriorate due to martesitic transformation. Taking this into consideration, the value of Equation (3) can be controlled to be 0 or greater, preferably 0 to 40, and more preferably 0 to 21.

[0085] In addition, the inventors of the present invention optimized the correlation between alloy composition and strength based on the theoretical calculation formula of delta ferrite and derived the following formula (4).

[0086] Equation (4): (Ni+0.52Cu+40(C+N)+0.7Mn+45)-(0.48(Cr+Mo+1.7Si))-51

[0087] (In formula (4), each element symbol represents the content (weight%) of each element)

[0088] When the value of Equation (4) is 0 or greater, a room temperature yield strength of 280 MPa or greater can be secured. However, if the value of Equation (4) is derived excessively, it is difficult to secure low-temperature impact toughness. Considering this, Equation (4) can be controlled to be 0 or greater, preferably 0 or greater and 30 or less, and more preferably 0 or greater and 8 or less.

[0089] An austenitic stainless steel according to one embodiment of the present invention can be manufactured through a general manufacturing process for austenitic stainless steel.

[0090] An austenitic stainless steel according to one embodiment of the present invention may have a delta ferrite content of 4% or less in terms of area fraction. When the delta ferrite content exceeds 4% in terms of area fraction, the austenite phase stability is low, making it difficult to achieve high strength and excellent low-temperature impact toughness.

[0091] According to one embodiment of the present invention, an austenitic stainless steel may have a yield strength of 280 MPa or more. The upper limit is not limited, but may be, for example, 800 MPa or less, 600 MPa or less, or 450 MPa or less. Within the above range, excellent low-temperature impact toughness can be achieved while further improving strength. In this case, for example, excellent low-temperature impact toughness and high strength can be achieved while achieving workability suitable for use in liquefied gas storage containers.

[0092] An austenitic stainless steel according to one embodiment of the present invention may have a Charpy impact energy of 73 J or more at -196°C.

[0093] An austenitic stainless steel according to one embodiment of the present invention may have a Charpy impact energy of 54 J or more at -253°C. The Charpy impact energy at -253°C can evaluate impact toughness at -253°C below zero, which is the temperature of liquid hydrogen, and indicate whether the material has excellent impact toughness characteristics in a liquid hydrogen environment where it is actually applied.

[0094] Next, a method for manufacturing austenitic stainless steel according to the present invention is described.

[0095] According to one embodiment of the present invention, a method for manufacturing an austenitic stainless steel comprises the steps of: manufacturing a slab containing, in wt%, C: 0.10% or less (excluding 0), Si: 1.50% or less (excluding 0), Cr: 16 to 23%, Ni: 5.1 to 12%, Mn: 15% or less (excluding 0), Cu: 1.2% or less (excluding 0), N: 0.1 to 0.4%, (Nb+V): 0.01% or less, the remainder being Fe and unavoidable impurities, and having a product of a strength index expressed by the following formula (1) and a stacking fault energy (SFE) expressed by the following formula (2) of 0 or more; hot-rolling the slab at a temperature of 1050 to 1300°C to manufacture a hot-rolled steel sheet; and annealing the hot-rolled steel sheet at a temperature of 900 to 1,200°C.

[0096] Equation (1) Strength index: (Yield strength (MPa))-0.9Cr+2Ni-12Si-1.1Mn-80N+400C-275

[0097] Formula (2) SFE: 25.7+2Ni+410C-0.9Cr-77N-13Si-1.2Mn

[0098] (In Equations (1) and (2), Cr, Ni, Si, Mn, N, and C represent the content (weight%) of each element.)

[0099] The role of each alloy composition and its content, and the explanation of formulas (1) to (4) are as described above.

[0100] During the annealing process for hot-rolled steel sheets, the annealing temperature significantly affects residual stress relief and microstructure. For example, the annealing temperature can range from 900 to 1,200°C.

[0101] If the annealing temperature is below 900°C, coarse carbides may form, leading to a non-uniform structure, or chromium carbide precipitates may form around grain boundaries, potentially causing intergranular corrosion. However, if the annealing temperature exceeds 1,200°C, grains may become extremely coarsened. Considering this, it is recommended to limit the annealing temperature to between 900 and 1,200°C.

[0102] In a method for manufacturing austenitic stainless steel according to one embodiment of the present invention, the slab may further include 1.5% or less of Mo.

[0103] In a method for manufacturing austenitic stainless steel according to one embodiment of the present invention, the slab may have equation (3) equal to or greater than 0.

[0104] Equation (3): (100N)+Mn-Ni

[0105] (Here, N, Mn and Ni represent the content (weight%) of each element)

[0106] In a method for manufacturing austenitic stainless steel according to one embodiment of the present invention, the slab may have a value of the following formula (4) of 0 or greater.

[0107] Equation (4): (Ni+0.52Cu+40(C+N)+0.7Mn+45)-(0.48(Cr+Mo+1.7Si))-51

[0108] (Here, Ni, Cu, C, N, Mn, Cr, Mo and Si represent the content (weight%) of each element)

[0109] In a method for manufacturing austenitic stainless steel according to one embodiment of the present invention, the hot rolling time in the hot-rolled steel sheet manufacturing step may be 1 to 3 hours, and the annealing time in the annealing step may be 5 to 100 minutes.

[0110] The austenitic stainless steel manufactured under the manufacturing conditions according to the present invention may have the values ​​of formula (1)*formula (2), formula (3), and formula (4) of 0 or more, a delta ferrite fraction of 4% or less, a room temperature yield strength of 280 MPa or more, a -196°C Charpy impact energy of 73 J or more, and a -253°C Charpy impact energy of 54 J or more.

[0111] Hereinafter, the structure and operation of the present invention will be described in more detail through preferred embodiments of the present invention. However, these are presented as preferred examples of the present invention and should not be construed as limiting the present invention in any way.

[0112] (Example)

[0113] A slab having the composition shown in Table 1 was manufactured, heated, and then hot-rolled at 1,250°C for 2 hours. Thereafter, the hot-rolled steel sheet was annealed at 1,000°C to 1,200°C for 60 minutes or less to obtain a specimen.

[0114] Table 1 below shows the compositional content (weight %) of each sample. In Table 1, underlined items are outside the scope of the present invention.

[0115] Alloy Composition (Wt%) CSiMnCrNiMoCuNExample 10.0 20.4 33.6 18.8 8.8-0.38 0.17Example 20.0 20.4 2.4 20.28.6-0.8 10.19Example 30.0 20.3 9 7.5 17.25.9-0.39 0.19Example 40.0 20.4 8.6 16.6 5.1-0.39 0.17Example 50. 020.420.921.610.40.60.790.2Example 60.020.415.617.55.5-0.360.18Example 70.020.434.518.28.4-1.170.15Example 80.020.391.219.410.5-0.410.11Example 90.030.413.216.15.3-0.4 10.11Embodiment 100.020.380.920.89.20.20.790.15Embodiment 110.030.4111.416.55.1-0.80.14Embodiment 120.020.39720.110.1-0.40.19Embodiment 130.020.414.119.28.5-0.430.2Embodiment 140.020.410. 821.59.40.60.810.2Example 150.020.427.217.77.3-0.40.17Comparative Example 10.020.61.318.18.1-0.30.05Comparative Example 20.020.528.816.42.8-1.890.22Comparative Example 30.020.451.316.810.12.050.270.071Comparative Example 40.020.591.116.110.32.110.310.016Comparative Example 50.030.3814.116.12.5-0.30.25Comparative Example 60.020.471.616.79.6-0.40.06Comparative Example 70.020.453.517.26.20.80.10.28Comparative Example 80.020.41019.35.3-0.20.21Comparative Example 90.021.111.622.55.2-0.40.14

[0116] The yield strength of the above specimen was measured using the following method. [Yield strength]

[0117] 1.5 mm thick plate-shaped sub-size tensile specimens were manufactured, and specimens were taken from the 1 / 2t point in the thickness direction of each specimen and measured according to ASTM E8. The average value of five measurements is shown and is expressed as 0.2% yield strength (YS0.2).

[0118] Table 2 shows the measured yield strength and the values ​​of Equations (1), (2) to (4), and Equation (1)*(2) calculated based on the measured yield strength. Underlined values ​​indicate values ​​outside the scope of the present invention.

[0119] Yield strength (MPa) Formula (1) Formula (2) Formula (1) * Formula (2) Formula (3) Formula (4) Example 1 326.5 37.5 11.6 434 11.8 3.7 Example 2 338.5 47.9 10.2 489 12.83.1 Example 3 313.8 15.0 1.5 2320.6 5.2 Example 4 337.7 38.10.5 2120.54.6Embodiment 5361.573.813.398310.53.2Embodiment 6334.737.53.212118.12.9Embodiment 7318.830.111.835511.13.9Embodiment 82823.722.5841.71.1Embodiment 9333.638.64.6178 18.96.3 Example 10341.656.716.09086.70.7 Example 11324.528.23.610020.36.0 Example 12324.331.87.925215.97.6 Example 13329.837.18.029615.64.8 Example 14350.861.51 1.771711.42.2Example 15327.532.65.417516.95.3Comparative Example 1254.1-25.620.6-528-1.8-3.2Comparative Example 2374.164.4-9.5-61328.05.2Comparative Example 3270.6-3.826.1-100-1.7-0.6Comparative Example 4237.7-32.829.8-976-7.6-2.6Comparative example 537057.4-12.6-72436.69.7Comparative example 6263.4-11.625.4-296-2.0-0.3Comparative example 739391.3-0.8-7225.35.7Comparative example 837871.6-6.2-44725.76.0Comparative example 933521.0-15.0-31420.42.2

[0120] Next, the delta ferrite content, -196℃ Charpy impact energy, and -253℃ Charpy impact energy of the above specimen were measured. The measurement method is as follows. [Delta ferrite content]

[0121] After hot rolling, the delta-ferrite content was measured at five randomly selected locations in the center of each specimen using a ferrite scope, and the average value was taken. The center of the specimen refers to the point 1 / 4 to 3 / 4t, where t is the overall thickness length.

[0122] [-196℃ Charpy impact energy, -253℃ Charpy impact energy]

[0123] After manufacturing ASTM E23 type A specimens, the Charpy impact energy at -196℃ and -253℃ was measured using a Charpy impact tester. The same experiment was performed five times for each specimen, and the average value of the five measurements was taken.

[0124] Table 3 below shows the above properties, and underlined items indicate those outside the scope of the present invention.

[0125] Yield strength (MPa) Delta ferrite content (area %) - Charpy impact energy at 196°C (J) - Charpy impact energy at 253°C (J) Example 1 3 26.51 or less 128 111 Example 2 3 38.51 or less 130 115 Example 3 3 13.81 or less 113 109 Example 4 3 37.7 1.7 106 95 Example 5 3 61.5 1.2 126 110 Example 6 3 34.71 or less 106 104 Example 7 3 18.81 or less 121 108 Example 8 2 821.5 188 136 Example 9 3 33.6 3.1 8667 Example 10 341.6 2.7 121 106 Example 11 324.5 3.4 7 354 Example 12 324.31 Below 128109 Example 13329.81 Below 151117 Example 14350.82.3126118 Example 15327.51 Below 131116 Comparative Example 1254.11.111095 Comparative Example 2374.14.23518 Comparative Example 3270.61 Below 156111 Comparative Example 4237.71.2165120 Comparative Example 53704.43220 Comparative Example 6263.41 Below 135118 Comparative Example 73931 Below 4225 Comparative Example 83781 Below 4719 Comparative Example 93354.63822

[0126] Examples 1 to 15 are specimens that satisfy the alloy composition disclosed in the present invention and in which the values ​​of Equation (1) * Equation (2), Equation (3), and Equation (4) are positive, thereby satisfying the scope of the present invention. Examples 1 to 15 have a yield strength of 280 MPa or more, a delta ferrite content of 4% or less, a Charpy impact energy of -196°C of 73 J or more, and a Charpy impact energy of 253°C of 54 J or more, thereby having high strength and excellent low-temperature impact toughness. Comparative Examples 1, 3, 4, and 6 are specimens in which the N content is insufficient. Accordingly, the solid solution strengthening effect by N was not sufficient, and a yield strength of 280 MPa or more could not be secured. Accordingly, the values ​​of Equation (1) * Equation (2), Equation (3), and Equation (4) were negative, so that a stainless steel having excellent yield strength and low-temperature impact toughness at the same time could not be obtained. Comparative examples 2, 5, and 9 showed that the values ​​of Equation (1) * Equation (2) were negative, and the delta ferrite fraction exceeded 4%, resulting in deterioration of low-temperature impact toughness.

[0127] In comparative examples 7 and 8, the values ​​of equation (1) * equation (2) were negative, indicating that the low-temperature impact toughness was deteriorated.

[0128] Through this example and comparative example, it can be seen that in order to provide an austenitic stainless steel having excellent yield strength and low-temperature impact toughness, it is necessary to optimize the alloy elements by controlling equations (1) to (4) while essentially including N, which contributes to securing the yield strength, and taking into account alloy elements such as Ni and Mn.

[0129] Although the present invention has been described with reference to the above embodiments, it will be understood by those skilled in the art that various modifications and changes can be made to the present invention without departing from the spirit and scope of the present invention as set forth in the claims below.

Claims

1. In weight%, C: 0.10% or less (excluding 0), Si: 1.50% or less (excluding 0), Cr: 16 to 23%, Ni: 5.1 to 12%, Mn: 15% or less (excluding 0), Cu: 1.2% or less (excluding 0), N: 0.1 to 0.4%, (Nb+V): 0.01% or less, the remainder including Fe and inevitable impurities, An austenitic stainless steel in which the product of the strength index expressed by the following equation (1) and the stacking fault energy (SFE) expressed by the following equation (2) is 0 or greater. Equation (1) Strength index: (Yield strength (MPa))-0.9Cr+2Ni-12Si-1.1Mn-80N+400C-275 Formula (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.) 2. In claim 1, Mo: Austenitic stainless steel containing not more than 1.5% (excluding 0).

3. In claim 1, The above formula (1) is an austenitic stainless steel having a value of 0 to 100.

4. In claim 1, The above formula (2) is an austenitic stainless steel having a value of 0 to 40.

5. In claim 1, Austenitic stainless steel in which the following equation (3) is 0 or greater. Equation (3): (100N)+Mn-Ni (Here, N, Mn and Ni represent the content (weight%) of each element) 6. In claim 2, Austenitic stainless steel in which the following equation (4) is 0 or greater. Equation (4): (Ni+0.52Cu+40(C+N)+0.7Mn+45)-(0.48(Cr+Mo+1.7Si))-51 (Here, Ni, Cu, C, N, Mn, Cr, Mo and Si represent the content (weight%) of each element) 7. In claim 1, Austenitic stainless steel with a delta ferrite content of 4% or less in area fraction.

8. In claim 1, Austenitic stainless steel with a yield strength of 280 MPa or more.

9. In claim 1, - Austenitic stainless steel having a Charpy impact energy of 73 J or more at -196℃.

10. In claim 1, - Austenitic stainless steel having a Charpy impact energy of 54 J or more at -253℃.

11. A step for manufacturing a slab, which contains, by weight%, C: 0.10% or less (excluding 0), Si: 1.50% or less (excluding 0), Cr: 16 to 23%, Ni: 5.1 to 12%, Mn: 15% or less (excluding 0), Cu: 1.2% or less (excluding 0), N: 0.1 to 0.4%, (Nb+V): 0.01% or less, the remainder being Fe and unavoidable impurities, and in which 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; A step of manufacturing a hot-rolled steel sheet by hot-rolling the above slab at a temperature of 1050 to 1300℃; A method for manufacturing austenitic stainless steel, comprising the step of annealing the hot-rolled steel plate at a temperature of 900 to 1,200°C. Equation (1) Strength index: (Yield strength (MPa))-0.9Cr+2Ni-12Si-1.1Mn-80N+400C-275 Formula (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.) 12. In claim 11, Mo: A method for manufacturing an austenitic stainless steel further comprising 1.5% or less (excluding 0).

13. In claim 11, A method for manufacturing an austenitic stainless steel in which the following formula (3) is 0 or greater. Equation (3): (100N)+Mn-Ni (Here, N, Mn and Ni represent the content (weight%) of each element) 14. In claim 12, A method for manufacturing an austenitic stainless steel in which the following formula (4) is 0 or greater. Equation (4): (Ni+0.52Cu+40(C+N)+0.7Mn+45)-(0.48(Cr+Mo+1.7Si))-51 (Here, Ni, Cu, C, N, Mn, Cr, Mo and Si represent the content (weight%) of each element) 15. In claim 11, In the above hot rolled steel plate manufacturing step, the hot rolling time is 1 to 3 hours, A method for manufacturing austenitic stainless steel, wherein the annealing time in the above annealing step is 5 to 100 minutes.