Austenitic stainless steel having excellent low-temperature impact toughness and method for producing same

Abstract

According to one example of the present invention, it is possible to provide an austenitic stainless steel containing, in wt%, 0.03% or less of C (excluding 0), 0.15-0.25% of N, 1.0% or less of Si (excluding 0), 3.3-7.5% of Mn, 17.0-22.0% of Cr, 6.5-9.5% of Ni, 1.2% or less of Cu (excluding 0), 0.8% or less of Mo (excluding 0), and the balance Fe and unavoidable impurities, the austenitic stainless steel satisfying formula (1): 70 < = (100-ASP) / (Ni / Mn) < = 170, and having a Charpy impact energy at-196 DEG C of 120 J or more. ASP refers to austenite phase stability, ASP is calculated according to 551-462 (C + N)-9.2, Si-8.1, Mn-13.7, Cr-29 (Ni + Cu)-18.5, Mo, and Ni and Mn refer to the weight percent of all the elements.

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. The austenitic stainless steel can be applied to components, equipment and tanks for the purpose of storage, transportation and use of LNG, liquid ammonia, liquid nitrogen, liquefied CO2, liquefied hydrogen and the like. Background Art

[0002] Stainless steel with excellent corrosion resistance does not require additional equipment investment to improve corrosion resistance, so it is a material with advantages for various parts, equipment and structural materials directly exposed to the external environment. In particular, in the case of austenitic stainless steel, the formability and elongation are excellent, so there is no problem in making shapes that meet various customer requirements, and it has the advantage of beautiful appearance from an aesthetic point of view. In addition, due to the characteristics of the material, austenitic stainless steel does not become brittle at low temperatures, so it can ensure excellent impact properties at low temperatures, and is used in industry as a material suitable for use in extremely low temperature environments such as LNG, liquid ammonia, liquid nitrogen, liquefied CO2, liquefied hydrogen, etc.

[0003] However, the yield strength of conventional austenitic stainless steel is below 250 MPa, which limits its application in various applications. The martensitic transformation phenomenon occurring in some metastable austenitic stainless steels causes a decrease in impact properties, which is a factor that hinders its use in extremely low temperature environments.

[0004] Existing products use expensive elements to improve the stability of the austenite phase and prevent martensite transformation, and actively use Ni to improve the stability of the austenite phase. However, Ni is an expensive element with unstable raw material supply and great price fluctuations, and excessive addition of Ni has limitations in cost competitiveness.

[0005] Therefore, there is a need to develop an austenitic stainless steel which improves the problems of existing general-purpose austenitic stainless steels, thereby ensuring austenite phase stability relative to cost, and can ensure high yield strength and excellent impact properties. Summary of the invention

[0006] 1. Technical issues to be resolved

[0007] An object of the present invention to solve the above-mentioned problems is to provide an austenitic stainless steel which can ensure austenite phase stability relative to cost and can ensure high yield strength and excellent impact properties, and a method for producing the same.

[0008] The technical problems to be solved by the present invention are not limited to the above-mentioned technical problems, and those skilled in the art can clearly understand other technical problems not mentioned from the following description.

[0009] (II) Technical solution

[0010] As a means for achieving the above object, an austenitic stainless steel according to one example of the present invention contains, in weight %, C: 0.03% or less (excluding 0), N: 0.15-0.25%, Si: 1.0% or less (excluding 0), Mn: 3.3-7.5%, Cr: 17.0-22.0%, Ni: 6.5% to 9.5% or less, Cu: 1.2% or less (excluding 0), Mo: 0.8% or less (excluding 0), and the balance Fe and inevitable impurities, the austenitic stainless steel satisfying the following formula (1) and having a Charpy impact energy at -196°C of 120 J or more.

[0011] Formula (1): 70≤(100-ASP) / (Ni / Mn)≤170 (wherein ASP refers to austenite phase stability, ASP is calculated by 551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)-18.5Mo, and Ni and Mn refer to the weight % of each element.)

[0012] Furthermore, in the austenitic stainless steel according to one example of the present invention, the austenitic stainless steel may satisfy the following formula (2).

[0013] Formula (2): 1.45Mn+10Ni-9.5Cu-175N+0.32(CVN at 25°C)≥120 (wherein, Mn, Ni, Cu, and N refer to the weight % of each element, and CVN at 25°C refers to the Charpy impact energy value at 25°C.)

[0014] Furthermore, in the austenitic stainless steel according to one example of the present invention, the austenitic stainless steel may satisfy the following formula (3).

[0015] Formula (3): 4.4+23(C+N)+1.3Si+0.24(Cr+Ni+Mn)≥16 (where C, N, Si, Cr, Ni, and Mn refer to the weight % of each element.)

[0016] Furthermore, in the austenitic stainless steel according to one example of the present invention, the yield strength of the austenitic stainless steel may be 300 MPa or more.

[0017] According to an example of the present invention, a method for manufacturing austenitic stainless steel comprises the following steps: manufacturing a slab, which comprises, by weight%, C: less than 0.03% (excluding 0), N: 0.15-0.25%, Si: less than 1.0% (excluding 0), Mn: 3.3-7.5%, Cr: 17.0-22.0%, Ni: 6.5-9.5%, Cu: less than 1.2% (excluding 0), Mo: less than 0.8% (excluding 0), the remainder of Fe and unavoidable impurities, and the slab satisfies 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 Charpy impact energy at -196°C is greater than 120J.

[0018] Formula (1): 70≤(100-ASP) / (Ni / Mn)≤170 (wherein ASP refers to austenite phase stability, ASP is calculated by 551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)-18.5Mo, and Ni and Mn refer to the weight % of each element.)

[0019] Furthermore, in the method for producing austenitic stainless steel according to one example of the present invention, the austenitic stainless steel may satisfy the following formula (2).

[0020] Formula (2): 1.45Mn+10Ni-9.5Cu-175N+0.32(CVN at 25°C)≥120 (wherein, Mn, Ni, Cu, and N refer to the weight % of each element, and CVN at 25°C refers to the Charpy impact energy value at 25°C.)

[0021] Furthermore, in the method for producing austenitic stainless steel according to one example of the present invention, the austenitic stainless steel may satisfy the following formula (3) and the yield strength of the austenitic stainless steel may be 300 MPa or more.

[0022] Formula (3): 4.4+23(C+N)+1.3Si+0.24(Cr+Ni+Mn)≥16 (where C, N, Si, Cr, Ni, and Mn refer to the weight % of each element.)

[0023] Furthermore, in the method for manufacturing austenitic stainless steel according to an example of the present invention, the step of heating and extracting the slab may be performed at 1080-1280°C.

[0024] Furthermore, in the method for producing austenitic stainless steel according to an example of the present invention, the hot rolling may be performed at a temperature of 800° C. or higher and at a reduction ratio of 70% or higher.

[0025] Furthermore, in the method for manufacturing austenitic stainless steel according to an example of the present invention, the hot rolling annealing may be performed at 1000-1200° C. for 60 minutes or less.

[0026] Furthermore, in the method for producing austenitic stainless steel according to one example of the present invention, a cooling step may be further included after the hot rolling and before the hot rolling annealing, and the cooling step may be performed at a cooling rate of 50° C. / sec or less.

[0027] Furthermore, in the method for producing austenitic stainless steel according to an example of the present invention, the cold rolling may be performed at a reduction ratio of 50% or more at room temperature.

[0028] Furthermore, in the method for manufacturing austenitic stainless steel according to an example of the present invention, the cold rolling annealing may be performed at 1000-1200° C. for 10 minutes or less.

[0029] (III) Beneficial effects

[0030] According to one embodiment of the present invention, an austenitic stainless steel and a method for manufacturing the same can be provided, wherein the austenitic stainless steel uses Ni and Mn to ensure austenite phase stability relative to cost, thereby not causing phase transformation due to low temperature, and thus having excellent impact toughness from room temperature to extremely low temperature. In addition, according to one embodiment of the present invention, an austenitic stainless steel having excellent yield strength and a method for manufacturing the same can be provided at the same time. Best Mode for Carrying Out the Invention

[0031] An austenitic stainless steel according to an example of the present invention comprises, in weight %, C: 0.03% or less (excluding 0), N: 0.15-0.25%, Si: 1.0% or less (excluding 0), Mn: 3.3-7.5%, Cr: 17.0-22.0%, Ni: 6.5% to 9.5% or less, Cu: 1.2% or less (excluding 0), Mo: 0.8% or less (excluding 0), and the balance Fe and inevitable impurities, wherein the austenitic stainless steel satisfies the following formula (1) and has a Charpy impact energy of 120 J or more at -196°C.

[0032] Formula (1): 70 ≤ (100-ASP) / (Ni / Mn) ≤ 170

[0033] (ASP refers to austenite phase stability, and ASP is calculated by 551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)-18.5Mo, where Ni and Mn refer to the weight % of each element.) DETAILED DESCRIPTION

[0034] The preferred embodiments of the present invention are described below. However, the embodiments of the present invention can be modified into various other forms, and the technical ideas of the present invention are not limited to the embodiments described below. In addition, the embodiments of the present invention are provided to more completely describe the present invention to those skilled in the art.

[0035] The terms used in this application are only used to illustrate specific examples. Therefore, unless the context clearly indicates that it must be singular, the singular expression includes the plural expression. In addition, it should be noted that the terms "comprise", "include" or "have" used in this application are used to clearly specify the existence of the features, steps, functions, constituent elements or their combinations recorded in the specification, rather than excluding the existence of other features or steps, functions, constituent elements or their combinations in advance.

[0036] In addition, unless otherwise defined, all terms used in this specification should be deemed to have the same meaning as those generally understood by those skilled in the art. Therefore, in this specification, unless otherwise clearly defined, specific terms should not be interpreted in an overly ideal or formal sense. In this specification, unless otherwise clearly stated in the context, a singular expression includes a plural expression.

[0037] In addition, the words "approximately", "substantially", etc. in this specification are used to mean numerical values ​​or values ​​close to them when there are inherent manufacturing and material tolerances in the meanings involved, and are used to prevent the disclosure of precise or absolute numerical values ​​to aid understanding of the present invention from being unfairly used by unscrupulous infringers.

[0038] The austenitic stainless steel according to one example of the present invention may contain, by weight%, C: 0.03% or less (except 0), N: 0.15-0.25%, Si: 1.0% or less (except 0), Mn: 3.3-7.5%, Cr: 17.0-22.0%, Ni: 6.5-9.5%, Cu: 1.2% or less (except 0), Mo: 0.8% or less (except 0), and the remainder of Fe and unavoidable impurities.

[0039] The reasons for limiting the composition range of each alloy element will be described below.

[0040] The C content may be 0.03 wt % or less (excluding 0).

[0041] C is an element effective for stabilizing the austenite phase, and C can be added to ensure the yield strength of austenitic stainless steel. However, when the content of C is too high, it induces the precipitation of Cr carbides at the grain boundaries, which may have an adverse effect on ductility, toughness, corrosion resistance, etc. Therefore, the upper limit of the content of C can be limited to 0.03%. More preferably, C can be 0.010-0.025%.

[0042] The N content may be 0.15-0.25 wt %.

[0043] N is a strong austenite stabilizing element and is an effective element for improving the yield strength of austenitic stainless steel. More than 0.15% N can be added. However, when the N content is too high, a decrease in extremely low temperature impact toughness may occur. In addition, there is a problem that manufacturing becomes difficult, for example, pin holes are generated. Therefore, the upper limit of the N content can be limited to 0.25%.

[0044] The Si content may be 1.0 wt % or less (excluding 0).

[0045] Si plays the role of a deoxidizer in the steelmaking process and can be added as an element that effectively improves the strength of the material. However, Si is an element that is effective in stabilizing the ferrite phase. When silicon is added in excess, it promotes the formation of delta ferrite in the cast slab, thereby reducing manufacturability and may have an adverse effect on the ductility and impact properties of the material. Therefore, the upper limit of the Si content can be limited to 1.0%. More preferably, Si can be 0.3-0.8%.

[0046] The content of Mn may be 3.3-7.5 wt %.

[0047] Mn is an austenite phase stabilizing element added in place of Ni in the present invention, and 3.3% or more of Mn may be added to improve the stability of austenite. However, when the content of Mn is too high, excessive S-based inclusions (MnS) are formed, which may reduce the ductility, toughness and corrosion resistance of austenitic stainless steel, generate Mn fumes during the steelmaking process, which is accompanied by manufacturing hazards, and excessive addition may induce planar slip behavior, thereby reducing the extremely low temperature impact toughness. Therefore, the upper limit of the Mn content can be limited to 7.5%.

[0048] The content of Cr may be 17.0-22.0 wt %.

[0049] Cr is a ferrite stabilizing element, but it effectively suppresses the formation of martensite phase. As a basic element to ensure the corrosion resistance required for stainless steel, 17.0% or more of Cr can be added. However, when the Cr content is too high, the manufacturing cost increases, and a large amount of delta ferrite is formed in the slab, which may reduce hot workability and have an adverse effect on material properties. Therefore, the upper limit of the Cr content can be limited to 22.0%.

[0050] The Ni content may be 6.5-9.5 wt %.

[0051] Ni is a strong austenite phase stabilizing element and is essential for ensuring good processability. However, Ni is an expensive element, so adding a large amount will cause an increase in raw material costs. Therefore, taking into account 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 6.5-9.1%.

[0052] The Cu content may be 1.2 wt % or less (excluding 0).

[0053] Cu is an austenite phase stabilizing element, and copper is an element added to replace Ni in the present invention. Cu can be added as an element to improve corrosion resistance in a reducing environment. However, when the content of Cu is too high, there is a problem of deterioration of corrosion resistance, strength and material properties, and reduced productivity. Therefore, considering the efficiency and material properties of the steel, the upper limit of the content of Cu can be limited to 1.2%.

[0054] The content of Mo may be 0.8 wt % or less (excluding 0).

[0055] Mo is an element that is effective for ensuring corrosion resistance together with Cr, and contributes greatly to the solid solution strengthening effect. However, when the content of Mo is too high, not only the hot workability may be reduced, but also since Mo is an expensive element, the problem of increased manufacturing cost may be caused. Therefore, the upper limit of the content of Mo can be limited to 0.8%. A more preferred upper limit of Mo can be 0.6%.

[0056] Furthermore, the austenitic stainless steel according to one embodiment of the present invention may further contain one or more of P: 0.035% or less and S: 0.01% or less as unavoidable impurities.

[0057] The P content may be 0.035% or less.

[0058] P is an impurity inevitably contained in steel and is an element that mainly causes intergranular corrosion or hinders hot workability, so it is preferred to control the content of P to a level as low as possible. In the present invention, the upper limit of the content of P is controlled to be below 0.035%.

[0059] The S content may be 0.01% or less.

[0060] S is an impurity inevitably contained in steel and is an element that segregates at grain boundaries and is the main cause of hindering hot workability, so it is preferable to control the content of S to the lowest possible level. In the present invention, the upper limit of the content of S is controlled to be 0.01% or less.

[0061] The remaining component of the present invention is Fe. However, in the conventional manufacturing process, undesirable impurities are inevitably mixed from the raw materials or the surrounding environment, so these impurities cannot be eliminated. These impurities are well known to technicians in the conventional manufacturing process, so all of them are not specifically mentioned in this specification.

[0062] According to an embodiment of the present invention, the austenitic stainless steel may satisfy the formula (1).

[0063] Formula (1): 70 ≤ (100-ASP) / (Ni / Mn) ≤ 170

[0064] Here, ASP refers to austenite phase stability, and Ni and Mn refer to the weight % of each element.

[0065] Among them, ASP can be obtained from 551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)-18.5Mo.

[0066] ASP is a value representing the austenite phase stability of austenitic stainless steel. As the ASP value decreases, martensitic transformation is less likely to occur even at low temperatures, thereby preventing brittleness at extremely low temperatures. Ni and Mn are two representative elements that can increase austenite phase stability. When the phase stability is the same, the lower the Ni / Mn value, the better the cost competitiveness. Formula (1) corresponds to an index using such ASP and Ni / Mn values.

[0067] When the value of formula (1) is less than 70, this is the case where excessive Ni is added at the same level of austenite phase stability, and cost competitiveness may be poor. When the value of formula (1) exceeds 170, the austenite phase stability is low, or the material properties may be poor due to excessive addition of Mn. Therefore, in the present invention, formula (1) can be limited to 70 to 170.

[0068] The present invention can ensure high austenite phase stability relative to cost by controlling the formula (1) to 70 to 170. By obtaining high austenite phase stability to prevent martensitic transformation, impact properties can be ensured even in an extremely low temperature environment.

[0069] In the present invention, the lower the ASP, the higher the austenite phase stability. In the present invention, the ASP may be -170 to -40. However, it is not limited thereto. The present invention controls the alloy composition and formula (1) so that when the ASP is the same, the Ni / Mn value is low, thereby ensuring excellent austenite phase stability relative to the cost.

[0070] The Charpy impact energy value of the austenitic stainless steel according to one example of the present invention at -150°C may be greater than 145 J. In addition, the Charpy impact energy value of the present invention at -196°C may be greater than 120 J.

[0071] Furthermore, the austenitic stainless steel according to one example of the present invention can satisfy the formula (2) corresponding to the index of cryogenic impact toughness.

[0072] Formula (2): 1.45Mn+10Ni-9.5Cu-175N+0.32 (CVN at 25°C) ≥ 120

[0073] Here, Mn, Ni, Cu, and N refer to the weight % of each element, and CVN at 25° C. refers to the Charpy impact energy value at 25° C.

[0074] When the value of formula (2) is less than 120, the impact toughness at room temperature can be improved, but the movement of dislocations that vary depending on the ratio of alloy elements is affected, and the impact toughness may decrease sharply as the atmosphere temperature decreases. Alternatively, when the value of formula (2) is less than 120, it may be difficult to ensure basic room-temperature impact toughness. In this case, there may be a problem that sufficient impact toughness cannot be ensured even at extremely low temperatures. Therefore, in the present invention, formula (2) can be limited to 120 or more.

[0075] The present invention can predict the extremely low temperature impact toughness by controlling the formula (2) to be above 120, measuring the Charpy impact energy value at 25°C, and measuring the impact toughness value at room temperature. By expressing the extremely low temperature impact toughness index as the formula (2), an austenitic stainless steel having guaranteed impact properties in an extremely low temperature environment can be provided.

[0076] Furthermore, in order to consider the improvement of the yield strength, the austenitic stainless steel according to one example of the present invention may satisfy the formula (3).

[0077] Formula (3): 4.4+23(C+N)+1.3Si+0.24(Cr+Ni+Mn)≥16

[0078] Here, C, N, Si, Cr, Ni, and Mn refer to the weight % of each element.

[0079] In order to consider the stress and tension (stress field) of the steel material to increase the yield strength and ensure the high yield strength of the austenitic stainless steel, the present invention obtains equation (3).

[0080] When the value of the formula (3) is less than 16, it is difficult to ensure the yield strength required by the present invention. Therefore, in the present invention, the value of the formula (3) may be limited to 16 or more.

[0081] As the value of formula (3) increases, due to the difference in atomic size between alloying elements, the stress and tension between the lattices increase, and the tolerance limit to plastic deformation while resisting external stress increases.

[0082] The present invention controls the formula (3) to be 16 or more, thereby obtaining an austenitic stainless steel having high strength characteristics.

[0083] The yield strength of the austenitic stainless steel according to an example of the present invention may be 300 MPa or more.

[0084] Hereinafter, a method for producing the austenitic stainless steel according to one embodiment of the present invention having the above-mentioned alloy composition will be described.

[0085] The austenitic stainless steel of the present invention can be produced by heating and extracting a slab having the above alloy composition, and then subjecting it to a process of hot rolling-hot rolling annealing-cold rolling-cold rolling annealing. A cooling step may be included after hot rolling and before hot rolling annealing.

[0086] The manufacturing method of austenitic stainless steel of the present invention may include the following steps: manufacturing a slab, which contains, by weight%, C: less than 0.03% (excluding 0), N: 0.15-0.25%, Si: less than 1.0% (excluding 0), Mn: 3.3-7.5%, Cr: 17.0-22.0%, Ni: 6.5-9.5%, Cu: less than 1.2% (excluding 0), Mo: less than 0.8% (excluding 0), the remainder of Fe and inevitable impurities, and the slab satisfies the following formula (1); heating and extracting the slab; hot rolling and hot rolling annealing the extracted slab to obtain a hot-rolled steel plate; and cold rolling and cold rolling annealing the hot-rolled steel plate, wherein the Charpy impact energy at -196°C is greater than 120J.

[0087] The formula (1) is 70≤(100-ASP) / (Ni / Mn)≤170, and the description of the alloy composition, ASP and the formula (1) is the same as that of the above-mentioned austenitic stainless steel.

[0088] Furthermore, the method for producing austenitic stainless steel of the present invention may be a method for producing austenitic stainless steel satisfying the following formula (2).

[0089] Formula (2) is 1.45Mn+10Ni-9.5Cu-175N+0.32(CVN at 25°C)≥120. The description of the alloy composition, CVN at 25°C and formula (2) is the same as that of the above-mentioned austenitic stainless steel.

[0090] Furthermore, the method for producing austenitic stainless steel of the present invention may be a method for producing austenitic stainless steel satisfying the following formula (3).

[0091] Formula (3) is 4.4+23(C+N)+1.3Si+0.24(Cr+Ni+Mn)≥16. The description of the alloy composition and formula (3) is the same as that of the above-mentioned austenitic stainless steel.

[0092] After manufacturing the slab having the above alloy composition, the step of heating and extracting can be performed at 1080-1280°C. In addition, the hot rolling step can be performed at a temperature of 800°C or more with a reduction rate of 70% or more. In addition, the hot rolling annealing step can be performed at 1000-1200°C for less than 60 minutes. In addition, a cooling step may be further included after hot rolling and before hot rolling annealing. The cooling step may be performed at a cooling rate of 50°C / second or less. In addition, the cold rolling step may be performed at room temperature with a reduction rate of 50% or more. In addition, the cold rolling annealing step may be performed at 1000-1200°C for less than 10 minutes. Further thickness reduction can be achieved by performing cold rolling and cold rolling annealing after hot rolling annealing.

[0093] 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.

[0094] Furthermore, the yield strength of the austenitic stainless steel produced by the method for producing austenitic stainless steel of the present invention can be 300 MPa or more.

[0095] The austenitic stainless steel of the present invention and the austenitic stainless steel produced by the method for producing austenitic stainless steel of the present invention can provide an austenitic stainless steel which can ensure high austenite phase stability relative to cost by controlling the ratio of Ni and Mn, and can ensure low-temperature impact toughness and strength by the ensured austenite phase stability.

[0096] {Example}

[0097] The slab was obtained according to the alloy composition according to the following Table 1, and then heated and extracted at 1200°C. In addition, hot rolling was performed at 800°C with a reduction rate of 70%, and cooled at a cooling rate of 50°C / second, and then hot rolling annealing was performed at 1100°C for 60 minutes. In addition, cold rolling was performed at a reduction rate of 50% at room temperature, and cold rolling annealing was performed at 1100°C for 10 minutes.

[0098] For JIS13B tensile test pieces, the yield strength YS (MPa), tensile strength TS (MPa), elongation EL (%) obtained after a tensile test at room temperature in a crosshead range of 10 mm / min to 20 mm / min, and the extremely low temperature (-150°C, -196°C) impact toughness (Charpy Vnotch test) value measured according to ASTM standards are shown. CVN at 25°C is a value measuring the Charpy impact energy value at 25°C.

[0099] Table 1 shows alloy composition, ASP, Ni / Mn, Charpy impact energy value at 196°C and formula (1).

[0100] [Table 1]

[0101]

[0102] Table 2 shows equation (2), equation (3) and mechanical properties.

[0103] [Table 2]

[0104]

[0105] According to Tables 1 and 2, it can be confirmed that Invention Examples 1 to Invention Examples 7 satisfy the alloy composition of the present invention, formula (1) is 70 to 170, has excellent austenite phase stability relative to cost, and the Charpy impact energy value at -196°C is 120J or more, so that the extremely low temperature impact toughness is ensured. In addition, formula (2) satisfies 120 or more. By confirming the room temperature impact toughness at the Charpy impact energy at 25°C, the extremely low temperature impact toughness can be predicted, the Charpy impact energy value at -150°C is 145J or more, and the Charpy impact energy value at -196°C is 120J or more, so it can be confirmed that the extremely low temperature impact toughness is ensured. In addition, formula (3) is 16 or more, so it can be confirmed that the yield strength of 300MPa or more is ensured.

[0106] Although Comparative Examples 1 to 5 do not satisfy the alloy composition of the present invention, the Charpy impact energy value at -196°C can be ensured to be 120 J or more. However, it can be confirmed that formula (1) is less than 70, and Ni is added too much compared to the austenite phase stability of the same level, so Ni / Mn has a large value of 7.88 or more. It can be confirmed that when the alloy composition and the lower limit value of formula (1) are not satisfied, the austenite phase stability excellent in cost cannot be ensured.

[0107] In Comparative Examples 1 to 3, the formula (3) is less than 16. Since the yield strength of Comparative Examples 1 to 3 is less than 300 MPa, it cannot satisfy the scope of the present invention. It can be confirmed that when the formula (3) cannot be satisfied, the strength is poor.

[0108] It can be confirmed that in Comparative Examples 6 to 10, formula (1) exceeds 170, and more than 7.5% of Mn is added. In addition, it can be confirmed that since the Charpy impact energy value at -196°C is less than 120J, the low-temperature impact properties cannot be ensured. In Comparative Examples 6 to 10, formula (2) is less than 120, so it can be predicted that low-temperature impact toughness cannot be ensured, and the Charpy impact energy value at -150°C and the Charpy impact energy value at -196°C cannot meet the range of the present invention. It can be confirmed that when the alloy composition, the upper limit value of formula (1) and formula (2) are not satisfied, excellent low-temperature impact toughness cannot be ensured.

[0109] Although Comparative Example 11 satisfies formula (1), formula (2) is less than 120, so the Charpy impact energy value at -150°C and the Charpy impact energy value at -196°C cannot satisfy the range of the present invention. From this, it can be confirmed that even if excellent austenite phase stability relative to cost is obtained by satisfying formula (1), when the alloy composition and formula (2) are not satisfied, it is impossible to ensure excellent low-temperature impact toughness while ensuring excellent austenite phase stability relative to cost.

[0110] Although Comparative Example 12 satisfies formula (1), formula (2) is less than 120, so the Charpy impact energy value at -150°C and the Charpy impact energy value at -196°C cannot meet the scope of the present invention. In addition, in Comparative Example 12, formula (3) is less than 16, and the yield strength is less than 300 MPa, so it cannot meet the scope of the present invention. Even if formula (1) is satisfied and austenite phase stability excellent in cost can be obtained, when the alloy composition, formula (2) and formula (3) cannot be satisfied, it can be confirmed that it is impossible to ensure austenite phase stability excellent in cost while ensuring excellent low-temperature impact toughness and strength.

[0111] In Comparative Examples 13 and 14, Ni cannot satisfy the scope of the present invention. The ASP values ​​are -2.08 and -19.59, respectively, so it can be confirmed that the austenite phase stability is low. It can be confirmed that when the alloy composition is not satisfied, even if formula (1) is satisfied, excellent phase stability cannot be ensured. In addition, formula (2) in Comparative Examples 13 and 14 is less than 120. The Charpy impact energy value at -150°C and the Charpy impact energy value at -196°C cannot satisfy the scope of the present invention. It can be confirmed that when formula (2) is not satisfied, excellent low-temperature impact toughness cannot be ensured.

[0112] In Comparative Example 15, formula (2) is 120 or more, and the Charpy impact energy value at -150°C can be ensured to be 145J or more, and the Charpy impact energy value at -196°C can be ensured to be 120J or more. However, formula (1) is less than 70, and the cost competitiveness is poor compared with the austenite phase stability of the same level. The ASP of Comparative Example 15 is -122.48, and the Ni / Mn ratio is 3.44. In contrast, the ASP of Inventive Example 5, which has the closest ASP to Comparative Example 15, is -123.12, and the Ni / Mn ratio is 2.33, which is smaller than Comparative Example 15. The ASP of Inventive Example 4, which has a higher ASP than Comparative Example 15, is -109.67, and the Ni / Mn ratio is 2.07, which is smaller than Comparative Example 15. It can be confirmed that when the alloy composition and the lower limit of formula (1) cannot be satisfied, the austenite phase stability excellent in terms of cost cannot be ensured.

[0113] In Comparative Example 16, formula (2) is 120 or more, which can ensure that the Charpy impact energy value at -150°C is 145J or more, and the Charpy impact energy value at -196°C is 120J or more. However, formula (1) of Comparative Example 16 is less than 70, and its cost competitiveness is poor compared with the austenite phase stability of the same level. The ASP of Comparative Example 16 is -103.31, and the Ni / Mn ratio is 7.58. In comparison, the ASP of Invention Example 3, which is closest to Comparative Example 16, is -99.70, and the Ni / Mn ratio is 2.59, which is smaller than Comparative Example 16. The ASP of Invention Example 4, which has an even smaller ASP, is -109.67, and the Ni / Mn ratio is 2.07, which is smaller than Comparative Example 16. It can be confirmed that when the alloy composition and the lower limit of formula (1) cannot be met, the austenite phase stability that is excellent relative to the cost cannot be ensured.

[0114] The formula (2) of Comparative Example 17 is 120 or more, which can ensure that the Charpy impact energy value at -150°C is 145J or more, and the Charpy impact energy value at -196°C is 120J or more. However, the formula (1) is less than 70, and too much Ni is added compared to the same level of austenite phase stability, so it can be confirmed that the Ni / Mn value is 8.73 and is sufficiently large. In addition, the formula (1) of Comparative Example 17 is less than 70, and the cost competitiveness is poor compared to the same level of austenite phase stability. The ASP of Comparative Example 17 is -100.61, and the Ni / Mn ratio is 8.73. In comparison, the ASP of Invention Example 3, which is closest to Comparative Example 17, is -99.70, and the Ni / Mn ratio is 2.59, which is less than Comparative Example 18. The ASP of Invention Example 4, which has an even smaller ASP, is -109.67, and the Ni / Mn ratio is 2.07, which is less than Comparative Example 17. This confirmed that, when the alloy composition and the lower limit of the formula (1) are not satisfied, it is impossible to ensure austenite phase stability that is excellent relative to the cost.

[0115] In Comparative Example 17, the formula (3) was less than 16, and the yield strength was less than 300 MPa, which did not satisfy the range of the present invention. This confirmed that the strength was poor when the formula (3) was not satisfied.

[0116] The exemplary embodiments of the present invention are described above, but the present invention is not limited thereto, and those skilled in the art will appreciate that various changes and modifications may be made without departing from the concept and scope of the claims.

[0117] Industrial Applicability

[0118] The austenitic stainless steel according to one embodiment of the present invention ensures austenite phase stability relative to cost and exhibits excellent impact toughness from room temperature to extremely low temperature while having excellent yield strength, and is therefore considered to have industrial applicability.

Claims

1. An austenitic stainless steel, comprising, in weight %, C: 0.03% or less and excluding 0, N: 0.15-0.25%, Si: 1.0% or less and excluding 0, Mn: 3.3-7.5%, Cr: 17.0-22.0%, Ni: 6.5-9.5%, Cu: 1.2% or less and excluding 0, Mo: 0.8% or less and excluding 0, and the balance of Fe and unavoidable impurities, The austenitic stainless steel satisfies the following formula (1), has a Charpy impact energy of 120 J or more at -196°C, Formula (1): 70 ≤ (100-ASP) / (Ni / Mn) ≤ 170 in, ASP refers to austenite phase stability, and ASP is calculated by 551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)-18.5Mo, where Ni and Mn refer to the weight % of each element.

2. The austenitic stainless steel according to claim 1, wherein: The austenitic stainless steel satisfies the following formula (2): Formula (2): 1.45Mn+10Ni-9.5Cu-175N+0.32 (CVN at 25°C) ≥ 120 Here, Mn, Ni, Cu, and N refer to the weight % of each element, and CVN at 25°C refers to the Charpy impact energy value at 25°C.

3. The austenitic stainless steel according to claim 1, wherein: The austenitic stainless steel satisfies the following formula (3): Formula (3): 4.4+23(C+N)+1.3Si+0.24(Cr+Ni+Mn)≥16 Here, C, N, Si, Cr, Ni, and Mn refer to the weight % of each element.

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

5. A method for manufacturing austenitic stainless steel, the method comprising the following steps: A slab is manufactured, wherein the slab comprises, in terms of weight %, C: 0.03% or less and excluding 0, N: 0.15-0.25%, Si: 1.0% or less and excluding 0, Mn: 3.3-7.5%, Cr: 17.0-22.0%, Ni: 6.5-9.5%, Cu: 1.2% or less and excluding 0, Mo: 0.8% or less and excluding 0, and the balance is Fe and unavoidable impurities, and the slab satisfies the following formula (1); The slab is heated and extracted; hot rolling and hot rolling annealing the extracted slab to obtain a hot rolled steel sheet; as well as The hot rolled steel sheet is cold rolled and cold annealed, Among them, the Charpy impact energy at -196℃ is more than 120J. Formula (1): 70 ≤ (100-ASP) / (Ni / Mn) ≤ 170 Herein, ASP refers to austenite phase stability, and ASP is calculated by 551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)-18.5Mo, where Ni and Mn refer to the weight % of each element.

6. The method for producing austenitic stainless steel according to claim 5, wherein: The austenitic stainless steel satisfies the following formula (2): Formula (2): 1.45Mn+10Ni-9.5Cu-175N+0.32 (CVN at 25°C) ≥ 120 Here, Mn, Ni, Cu, and N refer to the weight % of each element, and CVN at 25° C. refers to the Charpy impact energy value at 25° C.

7. The method for producing austenitic stainless steel according to claim 5, wherein: The austenitic stainless steel satisfies the following formula (3), and the yield strength of the austenitic stainless steel 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 refer to the weight % of each element.

8. The method for producing austenitic stainless steel according to claim 5, wherein: The step of heating and extracting the slab is performed at 1080-1280°C.

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

10. The method for producing austenitic stainless steel according to claim 5, wherein: The hot rolling annealing is performed at 1000-1200° C. for less than 60 minutes.

11. The method for producing austenitic stainless steel according to claim 5, wherein: The method further includes a cooling step after the hot rolling and before the hot rolling annealing, wherein the cooling rate of the cooling step is 50° C. / second or less.

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

13. The method for producing austenitic stainless steel according to claim 5, wherein: The cold rolling annealing is performed at 1000-1200° C. for less than 10 minutes.

Images