Austenitic stainless steel with excellent low-temperature impact toughness and manufacturing method thereof

Abstract

The present invention provides an austenitic stainless steel that can ensure high yield strength and excellent impact properties while ensuring the stability of the austenite phase at low cost, and a method for producing the same. [Solution] According to the present invention, it is possible to provide an austenitic stainless steel characterized by containing, by weight%, C: 0.03% or less (excluding 0), N: 0.15 to 0.25%, Si: 1.0% or less (excluding 0), Mn: 3.3 to 7.5%, Cr: 17.0 to 22.0%, Ni: 6.5 to 9.5%, Cu: 1.2% or less (excluding 0), Mo: 0.8% or less (excluding 0), and the balance being Fe and unavoidable impurities, satisfying formula (1): 70≦(100-ASP) / (Ni / Mn)≦170, and having a −196°C Charpy impact energy of 120 J or more. (Here, ASP means austenite phase stabilization degree, and ASP is calculated as 551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)-18.5Mo, where Ni and Mn mean the weight percentage of each element.)

Description

[Technical Field]

[0001] The present invention relates to an austenitic stainless steel, and more particularly to an austenitic stainless steel having high strength and excellent low-temperature impact properties that can be used for parts, equipment, and tanks intended for the storage, transportation, and use of LNG, liquefied ammonia, liquid nitrogen, liquefied CO2, liquefied hydrogen, and the like. [Background technology]

[0002] Stainless steel, with its excellent corrosion resistance, does not require additional capital investment for improved corrosion resistance and is an excellent material for various parts, equipment, and structural materials that are directly exposed to the atmospheric environment. Austenitic stainless steel, in particular, has excellent formability and elongation, making it easy to manufacture shapes to meet various customer requirements and offering the advantage of a beautiful aesthetic appearance. Furthermore, austenitic stainless steel does not become brittle at low temperatures due to its material properties, ensuring excellent impact properties at low temperatures. It is therefore widely used in industry as a material suitable for use in cryogenic environments, such as with LNG, liquefied ammonia, liquid nitrogen, liquefied CO2, and liquefied hydrogen.

[0003] However, the yield strength of typical austenitic stainless steels remains at 250 MPa or less, limiting their applicability to various applications. In addition, the martensitic phase transformation phenomenon that occurs in some metastable austenitic stainless steels causes a deterioration in impact properties, which acts as a factor preventing their use in cryogenic environments.

[0004] Conventional products use expensive elements such as Ni to improve the stability of the austenite phase and prevent martensite transformation, but excessive addition of Ni, an expensive element with unstable supply and demand for raw materials and subject to severe price fluctuations, places limitations on cost competitiveness.

[0005] Therefore, there is a need to develop an austenitic stainless steel that can improve the problems of conventional general-purpose austenitic stainless steels, while ensuring cost-effectiveness and austenitic phase stability, and also ensuring high yield strength and excellent impact properties. Summary of the Invention [Problem to be solved by the invention]

[0006] An object of the present invention is to provide an austenitic stainless steel that can ensure high yield strength and excellent impact properties while ensuring cost-effectiveness and austenitic phase stability, and a method for producing the same.

[0007] The problems to be solved by the present invention are not limited to the above problems, and other problems not mentioned will also be clearly understood by those skilled in the art from the following description. [Means for solving the problem]

[0008] As a means for achieving the above object, the austenitic stainless steel of the present invention contains, by weight%, C: 0.03% or less (excluding 0), N: 0.15 to 0.25%, Si: 1.0% or less (excluding 0), Mn: 3.3 to 7.5%, Cr: 17.0 to 22.0%, Ni: 6.5 to 9.5% or less, Cu: 1.2% or less (excluding 0), Mo: 0.8% or less (excluding 0), with the balance being Fe and unavoidable impurities, and is characterized by satisfying the following formula (1) and having a -196°C Charpy impact energy of 120 J or more.

[0009] Formula (1): 70≦(100-ASP) / (Ni / Mn)≦170 (where ASP stands for austenite phase stabilization degree, and is calculated as 551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)-18.5Mo, where Ni and Mn represent the weight percentage of each element.)

[0010] The austenitic stainless steel of the present invention may also be an austenitic stainless steel that satisfies the following formula (2).

[0011] Equation (2): 1.45Mn + 10Ni - 9.5Cu - 175N + 0.32 (CVN@25°C) ≥ 120 (where Mn, Ni, Cu, and N represent the weight percentage of each element, and CVN@25°C represents the Charpy impact energy value at 25°C.)

[0012] Furthermore, the austenitic stainless steel of the present invention may be an austenitic stainless steel that satisfies the following formula (3).

[0013] Equation (3): 4.4 + 23 (C + N) + 1.3 Si + 0.24 (Cr + Ni + Mn) ≧ 16 (where C, N, Si, Cr, Ni, and Mn represent the weight percentages of each element.)

[0014] The austenitic stainless steel of the present invention may be an austenitic stainless steel having a yield strength of 300 MPa or more.

[0015] The method for producing an austenitic stainless steel of the present invention includes the steps of producing a slab containing, by weight %, C: 0.03% or less (excluding 0), N: 0.15 to 0.25%, Si: 1.0% or less (excluding 0), Mn: 3.3 to 7.5%, Cr: 17.0 to 22.0%, Ni: 6.5 to 9.5%, Cu: 1.2% or less (excluding 0), Mo: 0.8% or less (excluding 0), with the balance being Fe and unavoidable impurities, and satisfying the following formula (1): heating and extracting the slab; hot rolling and hot-rolling annealing the extracted slab to obtain a hot-rolled steel sheet; and cold rolling and cold-rolling annealing the hot-rolled steel sheet, wherein the resulting steel has a -196°C Charpy impact energy of 120 J or more.

[0016] Formula (1): 70≦(100-ASP) / (Ni / Mn)≦170 (Here, ASP means austenite phase stabilization degree, and ASP is calculated as 551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)-18.5Mo, where Ni and Mn mean the weight percentage of each element.)

[0017] The method for producing an austenitic stainless steel of the present invention may also be a method for producing an austenitic stainless steel that satisfies the following formula (2).

[0018] Equation (2): 1.45Mn + 10Ni - 9.5Cu - 175N + 0.32 (CVN@25°C) ≥ 120 (where Mn, Ni, Cu, and N represent the weight percentage of each element, and CVN@25°C represents the Charpy impact energy value at 25°C.)

[0019] The method for producing an austenitic stainless steel of the present invention may also be a method for producing an austenitic stainless steel that satisfies the following formula (3) and has a yield strength of 300 MPa or more.

[0020] Equation (3): 4.4 + 23 (C + N) + 1.3 Si + 0.24 (Cr + Ni + Mn) ≧ 16 (where C, N, Si, Cr, Ni, and Mn represent the weight percentages of each element.)

[0021] In the method for producing an austenitic stainless steel of the present invention, the step of heating the slab and extracting may be carried out at 1080 to 1280°C.

[0022] The method for producing an austenitic stainless steel of the present invention may also be such that the hot rolling is carried out at 800° C. or higher and at a reduction rate of 70% or higher.

[0023] The method for producing an austenitic stainless steel of the present invention may further comprise the step of performing the hot rolling annealing at 1000 to 1200° C. for 60 minutes or less.

[0024] In addition, the method for producing an austenitic stainless steel according to the present invention may further include a step of cooling after the hot rolling and before the hot rolling annealing, wherein the cooling step is performed at a cooling rate of 50°C / s or less.

[0025] The method for producing an austenitic stainless steel of the present invention may also be such that the cold rolling is carried out at room temperature with a reduction rate of 50% or more.

[0026] The method for producing an austenitic stainless steel of the present invention may also include a step in which the cold rolling annealing is carried out at 1000 to 1200° C. for 10 minutes or less. [Effects of the Invention]

[0027] According to the present invention, by ensuring cost-effective austenitic phase stabilization with Ni and Mn, it is possible to provide an austenitic stainless steel that does not undergo low-temperature phase transformation and has excellent impact toughness from room temperature to cryogenic temperatures, and a method for producing the same.Furthermore, according to the present invention, it is also possible to provide an austenitic stainless steel that also has excellent yield strength, and a method for producing the same. DETAILED DESCRIPTION OF THE INVENTION

[0028] The austenitic stainless steel of the present invention contains, by weight%, C: 0.03% or less (excluding 0), N: 0.15 to 0.25%, Si: 1.0% or less (excluding 0), Mn: 3.3 to 7.5%, Cr: 17.0 to 22.0%, Ni: 6.5 to 9.5% or less, Cu: 1.2% or less (excluding 0), Mo: 0.8% or less (excluding 0), with the balance being Fe and unavoidable impurities, and satisfies the following formula (1), and has a -196°C Charpy impact energy of 120 J or more.

[0029] Formula (1): 70≦(100-ASP) / (Ni / Mn)≦170

[0030] (Here, ASP means austenite phase stabilization degree, and ASP is calculated as 551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)-18.5Mo, where Ni and Mn mean the weight percentage of each element.)

[0031] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various 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 completely explain the present invention to those having average knowledge in the art.

[0032] The terms used in this application are merely used to describe specific examples. Thus, for example, singular expressions include plural expressions unless the context clearly dictates otherwise. In addition, the terms "comprise" or "provide" as used in this application are used to explicitly indicate the presence of features, steps, functions, components, or combinations thereof described in the specification, and are not intended to preclude the presence of other features, steps, functions, components, or combinations thereof.

[0033] Furthermore, unless otherwise defined, all terms used herein should be understood to have the meanings commonly understood by a person of ordinary skill in the art to which the present invention pertains. Therefore, unless clearly defined herein, specific terms should not be interpreted in an overly ideal or formal sense. For example, singular expressions in this specification encompass plural expressions unless there is a clear exception in the context.

[0034] Furthermore, in this specification, the terms "about," "substantially," and the like are used to mean from or near the numerical value when the manufacturing and material tolerances inherent in the stated meaning are given, and are used to prevent unscrupulous infringers from unfairly taking advantage of disclosures that state precise or absolute numerical values ​​to aid in the understanding of the present invention.

[0035] The austenitic stainless steel of the present invention contains, by weight percent, C: 0.03% or less (excluding 0), N: 0.15 to 0.25%, Si: 1.0% or less (excluding 0), Mn: 3.3 to 7.5%, Cr: 17.0 to 22.0%, Ni: 6.5 to 9.5%, Cu: 1.2% or less (excluding 0), Mo: 0.8% or less (excluding 0), with the remainder consisting of Fe and unavoidable impurities.

[0036] The reasons for limiting the range of each alloying element are as follows.

[0037] The C content may be 0.03% by weight or less (0 excluded).

[0038] Carbon (C) is an element effective in stabilizing the austenite phase and can be added to ensure the yield strength of austenitic stainless steel. However, excessive C content can induce grain boundary precipitation of Cr carbides, adversely affecting ductility, toughness, corrosion resistance, etc. Therefore, the upper limit of the C content can be set to 0.03%. More preferably, C can be 0.010 to 0.025%.

[0039] The N content may be 0.15 to 0.25% by weight.

[0040] N is a strong austenite-stabilizing element and can be added in amounts of 0.15% or more as an element effective in improving the yield strength of austenitic stainless steel. However, excessive N content can cause a decrease in cryogenic impact toughness. It can also cause problems such as the occurrence of pinholes, making manufacturability difficult. Therefore, the upper limit of N content can be set at 0.25%.

[0041] The Si content may be 1.0 wt % or less (excluding 0).

[0042] Silicon (Si) acts as a deoxidizer during the steelmaking process and can be added as an effective element for improving the strength of the material. However, because Si is an effective element for stabilizing the ferrite phase, excessive addition of Si not only promotes the formation of delta (δ) ferrite in the cast slab, reducing manufacturability, but also potentially adversely affecting the ductility and impact properties of the material. Therefore, the upper limit of the Si content can be set to 1.0%. More preferably, the Si content can be 0.3 to 0.8%.

[0043] The Mn content may be 3.3 to 7.5 wt %.

[0044] In the present invention, Mn is an austenite phase stabilizer added in place of Ni. It can be added in an amount of 3.3% or more to improve austenite stability. However, excessive Mn content can reduce the ductility, toughness, and corrosion resistance of austenitic stainless steel by forming excessive amounts of S-based inclusions (MnS). It can also generate Mn fumes during the steelmaking process, creating manufacturing hazards. Excessive Mn addition can also induce planar slip behavior, potentially reducing cryogenic impact toughness. Therefore, the upper limit of Mn content can be set at 7.5%.

[0045] The Cr content may be 17.0 to 22.0 wt %.

[0046] Cr is a ferrite stabilizing element that is effective in suppressing the formation of martensite phase. It is a basic element that ensures the corrosion resistance required of stainless steel, and can be added in an amount of 17.0% or more. However, excessive Cr content increases manufacturing costs and can form a large amount of delta (δ) ferrite in the slab, which can reduce hot workability and have adverse effects on material properties. Therefore, the upper limit of Cr content can be set to 22.0%.

[0047] The Ni content may be 6.5 to 9.5 wt %.

[0048] Ni is a strong austenite phase stabilizing element and is essential for ensuring good workability. However, Ni is an expensive element, and adding a large amount of Ni increases raw material costs. Therefore, taking into consideration both the cost and efficiency of the steel, the upper limit of the Ni content can be limited to 9.5%. More preferably, Ni can be 6.5 to 9.1%.

[0049] The Cu content may be up to 1.2 wt % (excluding 0).

[0050] Cu is an austenite phase stabilizer and is added in place of Ni in the present invention. Cu can also be added as an element to improve corrosion resistance in a reducing environment. However, excessive Cu content can lead to problems such as deterioration of corrosion resistance, strength, and material properties, resulting in reduced productivity. Therefore, taking into consideration the efficiency and material properties of the steel, the upper limit of the Cu content can be set to 1.2%.

[0051] The Mo content may be 0.8 wt % or less (0 excluded).

[0052] Mo, along with Cr, is an element effective in ensuring corrosion resistance and significantly contributes to solid solution strengthening. However, excessive Mo content may not only cause a decrease in hot workability, but also increase manufacturing costs due to its high cost. Therefore, the upper limit of Mo content can be set to 0.8%. More preferably, the upper limit of Mo can be set to 0.6%.

[0053] The austenitic stainless steel of the present invention may further contain, as unavoidable impurities, one or more of P: 0.035% or less and S: 0.01% or less.

[0054] The P content may be 0.035% or less.

[0055] P is an impurity that is inevitably contained in steel and is the main cause of intergranular corrosion and impaired hot workability, so it is desirable to control its content as low as possible. In the present invention, the upper limit of the P content is controlled to 0.035% or less.

[0056] The S content may be 0.01% or less.

[0057] S is an impurity that is inevitably contained in steel and is an element that segregates at grain boundaries and is the main cause of impairing hot workability, so it is desirable to control the S content as low as possible. In the present invention, the upper limit of the S content is controlled to 0.01% or less.

[0058] The remaining component of the present invention is Fe. However, it cannot be excluded that unintended impurities may be inevitably mixed in from raw materials or the surrounding environment during the normal manufacturing process. Since these impurities are known to anyone skilled in the art of normal manufacturing processes, all of the details of these impurities will not be specifically mentioned in this specification.

[0059] The austenitic stainless steel of the present invention can satisfy formula (1).

[0060] Formula (1): 70≦(100-ASP) / (Ni / Mn)≦170

[0061] Here, ASP means the austenite phase stabilization degree, and Ni and Mn mean the weight percentage of each element.

[0062] Here, ASP can be obtained as 551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)-18.5Mo.

[0063] ASP is a value that represents the austenite phase stability of austenitic stainless steel. The lower the ASP value, the less likely martensite phase transformation occurs even at low temperatures, preventing brittleness at cryogenic temperatures. Ni and Mn are two representative elements that can increase austenite phase stability. For the same phase stability, the lower the Ni / Mn value, the more cost-competitive the material. Equation (1) is an index that utilizes these ASP and Ni / Mn values.

[0064] If the value of formula (1) is less than 70, excessive Ni is added within the austenite phase stability of the same grade, which may result in poor cost competitiveness. If the value of formula (1) is more than 170, the austenite phase stability may be low or excessive Mn may be added, which may result in poor material properties. Therefore, in the present invention, formula (1) can be limited to 70 to 170.

[0065] The present invention can ensure a high degree of austenite phase stabilization at low cost by controlling the value of formula (1) to 70 to 170. By achieving a high degree of austenite phase stabilization and preventing martensite phase transformation, it is possible to ensure impact properties even in cryogenic environments.

[0066] In the present invention, a lower ASP indicates a higher austenite phase stabilization. In the present invention, the ASP may be -170 to -40, but is not limited thereto. By controlling the alloy composition and formula (1), the present invention can ensure a low Ni / Mn ratio and excellent austenite phase stabilization at a low cost, even if the same ASP is maintained.

[0067] The austenitic stainless steel of the present invention may have a Charpy impact energy value of 145 J or more at −150° C. Also, in the present invention, the Charpy impact energy value of 120 J or more at −196° C.

[0068] Furthermore, the austenitic stainless steel of the present invention can satisfy the following formula (2), which corresponds to an index of cryogenic impact toughness.

[0069] Formula (2): 1.45Mn+10Ni-9.5Cu-175N+0.32(CVN@25℃)≧120

[0070] Here, Mn, Ni, Cu, and N represent the weight percentage of each element, and CVN@25°C represents the Charpy impact energy value at 25°C.

[0071] If the value of formula (2) is less than 120, the impact toughness at room temperature may be high, but the ratio of alloying elements may affect the potential transfer, which may result in a rapid decrease in impact toughness as the ambient temperature decreases. Alternatively, if the value of formula (2) is less than 120, it may be difficult to ensure basic room-temperature impact toughness, and in such cases, there is a problem that sufficient impact toughness cannot be ensured even at cryogenic temperatures. Therefore, in the present invention, formula (2) can be limited to 120 or more.

[0072] In the present invention, by controlling the value of formula (2) to 120 or more, it is possible to predict cryogenic temperature impact toughness by measuring the 25°C Charpy impact energy value and then measuring the impact toughness value at room temperature. By expressing the cryogenic temperature impact toughness index using formula (2), it is possible to provide an austenitic stainless steel that ensures impact properties in a cryogenic environment.

[0073] Furthermore, the austenitic stainless steel of the present invention can satisfy the following formula (3) in order to improve the yield strength.

[0074] Formula (3): 4.4+23(C+N)+1.3Si+0.24(Cr+Ni+Mn)≧16

[0075] Here, C, N, Si, Cr, Ni, and Mn represent the weight percentage of each element.

[0076] In the present invention, in order to ensure high yield strength of austenitic stainless steel, the formula (3) was derived in consideration of the improvement in yield strength due to the stress field of the steel material.

[0077] If the value of formula (3) is less than 16, it is difficult to ensure the yield strength required in the present invention. Therefore, in the present invention, formula (3) can be limited to 16 or more.

[0078] The larger the value of equation (3), the greater the interstitial stress field due to the atomic size difference between alloying elements, and the greater the limit of withstanding plastic deformation while resisting external stress.

[0079] In the present invention, by controlling the value of formula (3) to 16 or more, it is possible to obtain an austenitic stainless steel that ensures high strength properties.

[0080] The austenitic stainless steel of the present invention may have a yield strength of 300 MPa or more.

[0081] A method for producing the austenitic stainless steel of the present invention having the above-mentioned alloy composition will be described below.

[0082] 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, hot rolling annealing, cold rolling, and cold rolling annealing. After hot rolling, a cooling step may be included before hot rolling annealing.

[0083] The method for producing an austenitic stainless steel of the present invention may include the steps of producing a slab containing, by weight %, C: 0.03% or less (excluding 0), N: 0.15 to 0.25%, Si: 1.0% or less (excluding 0), Mn: 3.3 to 7.5%, Cr: 17.0 to 22.0%, Ni: 6.5 to 9.5%, Cu: 1.2% or less (excluding 0), Mo: 0.8% or less (excluding 0), and the balance being Fe and unavoidable impurities, and satisfying the following formula (1): heating and extracting the slab; hot rolling and hot-rolling annealing the extracted slab to obtain a hot-rolled steel sheet; and cold rolling and cold-rolling annealing the hot-rolled steel sheet, wherein the austenitic stainless steel has a -196°C Charpy impact energy of 120 J or more.

[0084] Formula (1) is 70≦(100-ASP) / (Ni / Mn)≦170 The explanations for the alloy composition, ASP, and formula (1) are the same as those for the austenitic stainless steels.

[0085] The method for producing an austenitic stainless steel of the present invention may also be a method for producing an austenitic stainless steel that satisfies the following formula (2).

[0086] Formula (2) is 1.45Mn+10Ni-9.5Cu-175N+0.32(CVN@25°C)≧120, and the explanations for the alloy composition, CVN@25°C, and formula (2) are the same as those for austenitic stainless steels above.

[0087] The method for producing an austenitic stainless steel of the present invention may also be a method for producing an austenitic stainless steel that satisfies the following formula (3).

[0088] Formula (3) is 4.4 + 23 (C + N) + 1.3 Si + 0.24 (Cr + Ni + Mn) ≥ 16, and the explanation for the alloy composition and formula (3) is the same as that for austenitic stainless steels above.

[0089] After producing a slab having the above-described alloy composition, the heating and extraction step may be performed at 1080 to 1280°C. The hot rolling step may be performed at 800°C or higher with a reduction of 70% or more. The hot rolling annealing step may be performed at 1000 to 1200°C for 60 minutes or less. After the hot rolling and before the hot rolling annealing, a cooling step may be further included. The cooling step may be performed at a cooling rate of 50°C / s or less. The cold rolling step may be performed at room temperature with a reduction of 50% or more. The cold rolling annealing step may be performed at 1000 to 1200°C for 10 minutes or less. By performing cold rolling and cold rolling annealing after the hot rolling annealing, an additional thickness reduction can be achieved.

[0090] The austenitic stainless steel produced by the method for producing austenitic stainless steel of the present invention may have a Charpy impact energy value of 145 J or more at -150°C and a Charpy impact energy value of 120 J or more at -196°C.

[0091] Furthermore, the austenitic stainless steel produced by the method for producing austenitic stainless steel of the present invention can have a yield strength of 300 MPa or more.

[0092] The austenitic stainless steel of the present invention and the austenitic stainless steel produced by the production method of the present invention can provide an austenitic stainless steel that can ensure high austenite phase stabilization at low cost by controlling the ratio of Ni and Mn, and that can ensure low-temperature impact toughness through the ensured austenite phase stabilization while simultaneously ensuring strength. [Example]

[0093] After obtaining a slab with the alloy composition shown in Table 1 below, it was heated at 1200°C and extracted. It was also hot rolled at 800°C with a reduction of 70%, cooled at a cooling rate of 50°C / s, and then hot-rolled and annealed at 1100°C for 60 minutes. It was then cold-rolled at room temperature with a reduction of 50%, and cold-rolled and annealed at 1100°C for 10 minutes.

[0094] Tensile tests were conducted on JIS13B tensile test pieces at room temperature with a crosshead speed ranging from 10 mm / min to 20 mm / min. The results show the yield strength YS (MPa), tensile strength TS (MPa), elongation EL (%), and cryogenic (-150°C, -196°C) impact toughness (Charpy V notch test) values ​​measured according to ASTM standards. CVN@25°C is the value measured for the Charpy impact energy at 25°C.

[0095] Table 1 shows the alloy composition, ASP, Ni / Mn, -196°C Charpy impact energy value, and formula (1).

[0096] [Table 1]

[0097] Table 2 shows formula (2), formula (3) and mechanical properties.

[0098] [Table 2]

[0099] Tables 1 and 2 show that Examples 1 to 7 satisfy the alloy composition of the present invention, have excellent cost-to-austenite phase stabilization ratios with formula (1) of 70 to 170, and have cryogenic impact toughness, corresponding to a Charpy impact energy value of 120 J or more at -196°C. Furthermore, formula (2) is 120 or more. Cryogenic impact toughness can be predicted by checking room temperature impact toughness using the Charpy impact energy at 25°C. It can be seen that cryogenic impact toughness is ensured with a Charpy impact energy value of 145 J or more at -150°C and a Charpy impact energy value of 120 J or more at -196°C. Furthermore, formula (3) is 16 or more, and yield strength is 300 MPa or more, confirming that strength is ensured.

[0100] Although Comparative Examples 1 to 5 do not satisfy the alloy composition of the present invention, they are able to ensure a Charpy impact energy value of 120 J or more at -196°C. However, it can be seen that the formula (1) is less than 70, and Ni is added in excess compared to the austenite phase stabilization degree of the same grade, resulting in a large Ni / Mn value of 7.88 or more. This shows that if the alloy composition and the lower limit of formula (1) are not satisfied, it is not possible to ensure austenite phase stabilization degree with excellent cost performance.

[0101] Furthermore, Comparative Examples 1 to 3 correspond to cases where formula (3) is less than 16. Comparative Examples 1 to 3 have yield strengths of less than 300 MPa, which do not satisfy the range of the present invention. This confirms that strength is poor when formula (3) is not satisfied.

[0102] In Comparative Examples 6 to 10, it can be confirmed that the formula (1) exceeds 170, and that the Mn content exceeds 7.5%. It can also be confirmed that the Charpy impact energy at -196°C is less than 120 J, and that low-temperature impact properties could not be ensured. In Comparative Examples 6 to 10, the formula (2) corresponds to less than 120, and it can be predicted that low-temperature impact toughness cannot be ensured. The Charpy impact energy at -150°C and the Charpy impact energy at -196°C do not satisfy the ranges of the present invention. This confirms that excellent low-temperature impact toughness cannot be ensured when the alloy composition, the upper limit of formula (1), and formula (2) are not satisfied.

[0103] Comparative Example 11 satisfies formula (1), but formula (2) is less than 120, and the Charpy impact energy at −150°C and the Charpy impact energy at −196°C do not satisfy the ranges of the present invention. This confirms that even if formula (1) is satisfied and excellent austenite phase stabilization relative to cost can be obtained, if the alloy composition and formula (2) are not satisfied, it is not possible to ensure excellent low-temperature impact toughness while ensuring excellent austenite phase stabilization relative to cost.

[0104] Comparative Example 12 satisfies formula (1), but formula (2) is less than 120, resulting in the -150°C Charpy impact energy value and the -196°C Charpy impact energy value not satisfying the range of the present invention. Furthermore, formula (3) is less than 16, resulting in a yield strength of less than 300 MPa, which does not satisfy the range of the present invention. This confirms that even if formula (1) is satisfied and excellent austenite phase stabilization relative to cost can be obtained, if the alloy composition, formulas (2), and formula (3) are not satisfied, it is not possible to ensure excellent low-temperature impact toughness and strength while also ensuring excellent austenite phase stabilization relative to cost.

[0105] In Comparative Examples 13 and 14, the Ni content does not satisfy the range of the present invention. The ASP values ​​correspond to -2.08 and -19.59, respectively, confirming that the austenite phase stabilization is low. This confirms that if the alloy composition does not satisfy the above criteria, excellent phase stabilization cannot be ensured even if formula (1) is satisfied. Furthermore, in Comparative Examples 13 and 14, formula (2) corresponds to less than 120. The -150°C Charpy impact energy value and the -196°C Charpy impact energy value do not satisfy the range of the present invention. This confirms that excellent low-temperature impact toughness cannot be ensured if formula (2) is not satisfied.

[0106] In Comparative Example 15, the formula (2) is 120 or greater, ensuring a Charpy impact energy of 145 J or greater at -150°C and a Charpy impact energy of 120 J or greater at -196°C. However, the formula (1) is less than 70, resulting in inferior cost competitiveness compared to alloys with the same austenite phase stabilization. The ASP of Comparative Example 15 is -122.48, and the Ni / Mn ratio is 3.44. In contrast, Example 5, which has the most similar ASP to Comparative Example 15, has an ASP of -123.12 and an Ni / Mn ratio of 2.33, both of which are smaller than those of Comparative Example 15. Furthermore, Example 4, which has a higher ASP than Comparative Example 15, has an ASP of -109.67 and an Ni / Mn ratio of 2.07, both of which are smaller than those of Comparative Example 15. This confirms that excellent austenite phase stabilization at low cost cannot be achieved unless the alloy composition and the lower limit of formula (1) are satisfied.

[0107] In Comparative Example 16, the formula (2) is 120 or greater, ensuring a Charpy impact energy of 145 J or greater at -150°C and a Charpy impact energy of 120 J or greater at -196°C. However, the formula (1) is less than 70, resulting in inferior cost competitiveness compared to alloys with the same austenite phase stabilization. Comparative Example 16 has an ASP of -103.31 and a Ni / Mn ratio of 7.58. In contrast, Example 3, which has the most similar ASP to Comparative Example 16, has an ASP of -99.70 and a Ni / Mn ratio of 2.59, both of which are smaller than those of Comparative Example 16. Furthermore, Example 4, which has an even smaller ASP, has an ASP of -109.67 and a Ni / Mn ratio of 2.07, both of which are smaller than those of Comparative Example 16. This confirms that excellent austenite phase stabilization at low cost cannot be achieved unless the alloy composition and the lower limit of formula (1) are satisfied.

[0108] In Comparative Example 17, Equation (2) is 120 or greater, ensuring a -150°C Charpy impact energy of 145 J or greater and a -196°C Charpy impact energy of 120 J or greater. However, Equation (1) is less than 70, confirming that excessive Ni is added compared to the same grade of austenite phase stabilization, resulting in a large Ni / Mn value equivalent to 8.73. Furthermore, Equation (1) is less than 70 in Comparative Example 17, resulting in inferior cost competitiveness compared to the same grade of austenite phase stabilization. The ASP value of Comparative Example 17 is -100.61, and the Ni / Mn ratio is 8.73. In contrast, Inventive Example 3, which has the most similar ASP to Comparative Example 17, has an ASP equivalent to -99.70 and an Ni / Mn ratio of 2.59, both of which are smaller than those of Comparative Example 17. Furthermore, Example 4 had an even smaller ASP of -109.67, and the Ni / Mn ratio was 2.07, which was smaller than that of Comparative Example 17. This confirms that if the alloy composition and the lower limit of formula (1) are not satisfied, it is not possible to ensure excellent austenite phase stabilization at low cost.

[0109] In addition, in Comparative Example 17, the formula (3) is less than 16, and the yield strength is less than 300 MPa, which does not satisfy the range of the present invention. This confirms that when formula (3) is not satisfied, the strength is poor.

[0110] Although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and it will be understood by those skilled in the art that various changes and modifications can be made without departing from the concept and scope of the claims set forth below. [Industrial Applicability]

[0111] The austenitic stainless steel of the present invention ensures austenite phase stabilization relative to cost, exhibits excellent impact toughness from room temperature to cryogenic temperatures, and at the same time, has excellent yield strength, and is therefore found to be industrially applicable.

Claims

1. The alloy contains, by weight, C: 0.03% or less (excluding 0), N: 0.15 to 0.25%, Si: 1.0% or less (excluding 0), Mn: 3.3 to 7.5%, Cr: 17.0 to 22.0%, Ni: 6.5 to 9.5%, Cu: 1.2% or less (excluding 0), Mo: 0.8% or less (excluding 0), and the balance being Fe and unavoidable impurities; An austenitic stainless steel characterized by satisfying the following formula (1) and having a -196°C Charpy impact energy of 120 J or more. Formula (1): 70≦(100-ASP) / (Ni / Mn)≦170 (Here, ASP means austenite phase stabilization degree, and ASP is calculated as 551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)-18.5Mo, where Ni and Mn mean the weight percentage of each element.)

2. 2. The austenitic stainless steel according to claim 1, wherein the following formula (2) is satisfied: Formula (2): 1.45Mn+10Ni-9.5Cu-175N+0.32 (CVN@25℃)≧120 (Here, Mn, Ni, Cu, and N mean the weight percentage of each element, and CVN@25°C means the Charpy impact energy value at 25°C.)

3. 2. The austenitic stainless steel according to claim 1, wherein the following formula (3) is satisfied: Formula (3): 4.4+23(C+N)+1.3Si+0.24(Cr+Ni+Mn)≧16 (Here, C, N, Si, Cr, Ni, and Mn mean the weight percentage of each element.)

4. 2. The austenitic stainless steel according to claim 1, wherein the yield strength is 300 MPa or more.

5. a step of producing a slab containing, by weight percent, C: 0.03% or less (excluding 0), N: 0.15 to 0.25%, Si: 1.0% or less (excluding 0), Mn: 3.3 to 7.5%, Cr: 17.0 to 22.0%, Ni: 6.5 to 9.5%, Cu: 1.2% or less (excluding 0), Mo: 0.8% or less (excluding 0), with the balance being Fe and unavoidable impurities, and satisfying the following formula (1): heating and extracting said slab; hot rolling and hot rolling annealing the extracted slab to obtain a hot rolled steel sheet; and cold rolling and cold rolling annealing the hot-rolled steel sheet; A method for producing austenitic stainless steel, characterized in that the -196°C Charpy impact energy is 120 J or more. Formula (1): 70≦(100-ASP) / (Ni / Mn)≦170 (Here, ASP means austenite phase stabilization degree, and ASP is calculated as 551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)-18.5Mo, where Ni and Mn mean the weight percentage of each element.)

6. 6. The method for producing an austenitic stainless steel according to claim 5, wherein the following formula (2) is satisfied: Formula (2): 1.45Mn+10Ni-9.5Cu-175N+0.32 (CVN@25℃)≧120 (Here, Mn, Ni, Cu, and N mean the weight percentage of each element, and CVN@25°C means the Charpy impact energy value at 25°C.)

7. 6. The method for producing an austenitic stainless steel according to claim 5, wherein the following formula (3) is satisfied and the yield strength is 300 MPa or more: Formula (3): 4.4+23(C+N)+1.3Si+0.24(Cr+Ni+Mn)≧16 (Here, C, N, Si, Cr, Ni, and Mn mean the weight percentage of each element.)

8. 6. The method of claim 5, wherein the step of heating and extracting the slab is performed at 1080 to 1280°C.

9. 6. The method for producing austenitic stainless steel according to claim 5, wherein the hot rolling is carried out at a temperature of 800° C. or higher and a rolling reduction of 70% or higher.

10. 6. The method for producing austenitic stainless steel according to claim 5, wherein the hot rolling annealing is carried out at 1000 to 1200° C. for 60 minutes or less.

11. After the hot rolling, the steel sheet is cooled before the hot rolling annealing.

6. The method for producing austenitic stainless steel according to claim 5, wherein the cooling step is performed at a cooling rate of 50[deg.] C. / s or less.

12. 6. The method for producing austenitic stainless steel according to claim 5, wherein the cold rolling is carried out at room temperature with a reduction ratio of 50% or more.

13. 6. The method for producing austenitic stainless steel according to claim 5, wherein the cold rolling annealing is carried out at 1000 to 1200° C. for 10 minutes or less.

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