Austenitic steel material with excellent ultra-low temperature toughness in the heat-affected zone of welds, and method for manufacturing the same.

Abstract

The present invention provides an austenitic steel material that has excellent ultra-low temperature toughness in the weld heat affected zone and can be used as a structural material in ultra-low temperature environments such as liquefied gas storage tanks and liquefied gas transport facilities, and a method for producing the same. [Solution] The above-mentioned austenitic steel material is composed, by weight, of 10 to 45% manganese (Mn), carbon (C) in a range that satisfies 24 x [C] + [Mn] ≧ 25 and 33.5 x [C] - [Mn] ≦ 18, 10% or less (excluding 0%) chromium (Cr), with the remainder being iron (Fe) and unavoidable impurities, and has austenite as a base structure, and is characterized in that when a -253°C standard Charpy impact test is performed on the weld heat-affected zone (HAZ), the lateral expansion in the weld heat-affected zone is 0.32 mm or more. In the formula, [C] and [Mn] mean the contents (wt%) of carbon (C) and manganese (Mn) contained in the steel material.

Description

[Technical Field]

[0001] The present invention relates to an austenitic steel material and a method for producing the same, and more particularly to an austenitic high-manganese steel material with excellent ultra-low temperature toughness in the heat-affected zone of the weld, and a method for producing the same. [Background technology]

[0002] Liquefied gases such as liquefied hydrogen (boiling point: -253°C), liquefied natural gas (LNG, boiling point: -164°C), liquid oxygen (boiling point: -183°C), and liquid nitrogen (boiling point: -196°C) require cryogenic storage. Therefore, structures such as pressure vessels made of materials with sufficient toughness and strength at cryogenic temperatures are necessary to store these gases.

[0003] Materials usable at low temperatures in a liquefied gas atmosphere have included Cr-Ni stainless steel alloys such as AISI304, 9% Ni steel, and 5000 series aluminum alloys. However, aluminum alloys have limitations due to their high alloy cost, low strength which increases the design thickness of structures, and poor weldability. While Cr-Ni stainless steel and 9% nickel (Ni) steel significantly improve upon the physical properties of aluminum, they are undesirable from an economic standpoint because they contain a large amount of expensive nickel (Ni). [Overview of the project] [Problems that the invention aims to solve]

[0004] The object of the present invention is to provide an austenitic steel material and a method for manufacturing the same, which exhibits excellent ultra-low temperature toughness in the heat-affected zone of the weld and can be used as a structural material in ultra-low temperature environments such as liquefied gas storage tanks and liquefied gas transport equipment. The problems that the present invention addresses are not limited to those described above. A person of ordinary skill should have no difficulty understanding further problems that the present invention addresses from the overall content of this specification. [Means for solving the problem]

[0005] The austenitic steel material of the present invention is characterized by comprising, by weight %, manganese (Mn): 10-45%, carbon (C): within the range satisfying 24×[C]+[Mn]≧25 and 33.5×[C]-[Mn]≦18, chromium (Cr): 10% or less (excluding 0%), the remainder being iron (Fe) and unavoidable impurities, having austenite as its matrix structure, and when a Charpy impact test is performed on the heat-affected zone (HAZ) of the weld at a reference temperature of -253°C, the lateral expansion in the heat-affected zone of the weld is 0.32 mm or more. In the above formula, [C] and [Mn] represent the carbon (C) and manganese (Mn) content (by weight) of the steel material.

[0006] The yield strength at room temperature of the above-mentioned steel material should preferably be between 245 MPa and less than 400 MPa. The above-mentioned heat-affected zone of the weld may contain 95% or more (including 100% area) of austenite and 5% or less (including 0% area) of grain boundary carbides as part of its microstructure.

[0007] The average grain size of the heat-affected zone during welding should preferably be between 5 and 200 μm. The average grain aspect ratio of the heat-affected zone during welding can be between 1.0 and 5.0. The dislocation density of the above steel material is 2.3 × 10⁻⁶. 15 ~3.3×10 15 / mm 2 It is preferable that this be the case.

[0008] The present invention relates to a method for producing austenitic steel, characterized by comprising the steps of: preparing a slab consisting of, by weight percent, manganese (Mn): 10-45%, carbon (C): within the range satisfying 24×[C]+[Mn]≧25 and 33.5×[C]-[Mn]≦18, chromium (Cr): 10% or less (excluding 0%), and the remaining iron (Fe) and unavoidable impurities; and heating the slab and then hot-rolling it to a rolling finish temperature of 800°C or higher. In the above formula, [C] and [Mn] represent the carbon (C) and manganese (Mn) content (by weight) of the slab.

[0009] The means of solving the above problems do not enumerate all of the features of the present invention. The various features of the present invention and the advantages and effects associated therewith can be understood in more detail by referring to the following specific examples and embodiments. [Effects of the Invention]

[0010] According to one aspect of the present invention, the austenitic steel material and method for manufacturing the same of the present invention provide an austenitic steel material and method for manufacturing the same that exhibit excellent ultra-low temperature toughness in the heat-affected zone of the weld, and are particularly suitable as a structural material in ultra-low temperature environments such as liquefied gas storage tanks and liquefied gas transport equipment. The effects of the present invention are not limited to those described above and can be interpreted as a concept that includes effects that can be reasonably inferred from the matters described herein. [Brief explanation of the drawing]

[0011] [Figure 1] This figure shows the correlation between the carbon content and manganese content of austenitic steel according to one aspect of the present invention. [Figure 2] This figure schematically illustrates a method for measuring the lateral expansion value in the heat-affected zone of an austenitic steel material according to one aspect of the present invention. [Modes for carrying out the invention]

[0012] The present invention relates to austenitic steel and a method for producing the same, and preferred embodiments of the present invention are described below. These embodiments can be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. These embodiments are provided to explain the present invention in more detail to those who have ordinary skill in the art to which the invention pertains. The austenitic steel material relating to one aspect of the present invention will be described in more detail below.

[0013] An austenitic steel material according to one aspect of the present invention consists of, by weight %, manganese (Mn): 10-45%, carbon (C): within the range satisfying 24×[C]+[Mn]≧25 and 33.5×[C]-[Mn]≦18, chromium (Cr): 10% or less (excluding 0%), the remainder being iron (Fe) and unavoidable impurities, and when a Charpy impact test is performed on the weld heat-affected zone at -253°C, it is preferable that the lateral expansion in the weld heat-affected zone is 0.32 mm or more. The following describes in more detail the steel composition contained in austenitic steel according to one aspect of the present invention. Unless otherwise specified, the percentages ("%) indicating the content of each element are based on weight.

[0014] Manganese (Mn): 10-45% Manganese is an element that plays an important role in stabilizing austenite. To stabilize austenite at ultra-low temperatures, it is preferable to have a manganese (Mn) content of 10% or more. If the manganese (Mn) content is less than this, epsilon martensite, a metastable phase, is formed and easily transforms into alpha martensite through processing-induced transformation at ultra-low temperatures, making it impossible to ensure toughness. There is a method to stabilize austenite by increasing the carbon (C) content to suppress the formation of epsilon martensite, but in this case, a large amount of carbides may precipitate, and the physical properties may deteriorate rapidly. Therefore, a manganese (Mn) content of 10% or more is preferable. A preferable manganese (Mn) content is 15% or more, and a more preferable manganese (Mn) content is 18% or more. If the manganese (Mn) content is excessive, it may not only reduce the corrosion rate of the steel but is also undesirable from an economic standpoint. Therefore, a manganese (Mn) content of 45% or less is preferable. A preferred manganese (Mn) content is often 40% or less, and a more preferred manganese (Mn) content is 35% or less.

[0015] The range of carbon (C) that satisfies 24 × [C] + [Mn] ≥ 25 and 33.5 × [C] - [Mn] ≤ 18. Carbon (C) is an element that stabilizes austenite and increases its strength. In particular, carbon (C) plays a role in lowering the Ms or Md transformation point from austenite to epsilon or alpha-martensite during cooling or processing. Therefore, carbon (C) is a component that effectively contributes to the stabilization of austenite. If the carbon (C) content is insufficient, the stability of the austenite will be insufficient, and stable austenite cannot be obtained at ultra-low temperatures. This may lead to processing-induced transformation to epsilon or alpha-martensite easily under external stress, reducing the toughness of the steel or decreasing its strength. On the other hand, if the carbon (C) content is excessive, the toughness of the steel may rapidly deteriorate due to carbide precipitation, and the strength of the steel may increase excessively, potentially reducing its workability.

[0016] The inventor of the present invention has conducted intensive research on the relative behavior between the contents of carbon (C) and manganese (Mn) in relation to the formation of carbides. As a result, as shown in FIG. 1, by determining the relative content relationship between carbon (C) and manganese (Mn), it has been concluded that while effectively achieving the stabilization of austenite, the precipitation amount of carbides can be effectively controlled. Carbides are formed by carbon (C), but carbon (C) does not independently affect the formation of carbides. Instead, it acts in combination with manganese (Mn) to affect the formation of carbides.

[0017] In order to achieve the stabilization of austenite, on the premise that other components meet the ranges defined in the present invention, it is preferable to control the value of 24×[C]+[Mn] (where [C] and [Mn] mean that the contents of each component are expressed in units of weight %) to be 25 or more. This boundary means the inclined left boundary of the parallelogram region shown in FIG. 1. When 24×[C]+[Mn] is less than 25, the stability of austenite decreases, and processing-induced transformation occurs due to impact at ultra-low temperatures, which may reduce the impact toughness of the steel material. On the other hand, in order to suppress the formation of carbides, on the premise that other components meet the ranges defined in the present invention, it is preferable to control the value of 33.5×[C]-[Mn] (where [C] and [Mn] mean that the contents of each component are expressed in units of weight %) to be 18 or less. When 33.5×[C]-[Mn] exceeds 18, carbides precipitate due to the addition of excessive carbon (C), which may reduce the low-temperature impact toughness of the steel material. Therefore, in the present invention, it is preferable to add carbon (C) so as to satisfy 24×[C]+[Mn]≧25 and 33.5×[C]-[Mn]≦18. As can be seen from FIG. 1, the lowest limit of the carbon (C) content within the range satisfying the above formulas is 0%.

[0018] Chromium (Cr): 10% or less (excluding 0%) Chromium (Cr) is also an austenite stabilizing element. Within an appropriate addition amount range, it stabilizes austenite, improves the low-temperature impact toughness of steel materials, and plays a role in increasing the strength of the steel by dissolving in austenite. Additionally, chromium (Cr) is also a component that effectively contributes to improving the corrosion resistance of steel materials. Therefore, the present invention adds chromium (Cr) as an essential component. The lower limit of the preferable chromium (Cr) content is preferably 1%, and the lower limit of the more preferable chromium (Cr) content is 2%. However, chromium (Cr) is a carbide-forming element, and there is a risk of forming carbides particularly at the austenite grain boundaries, which may reduce the low-temperature impact toughness of the steel material. Also, when the addition amount of chromium (Cr) exceeds a certain level, excessive carbides may precipitate in the heat-affected zone (HAZ) during welding, and there is a risk of inferior ultra-low temperature toughness. Therefore, in the present invention, the upper limit of chromium (Cr) can be limited to 10%. The upper limit of the preferable chromium (Cr) content is preferably 8%, and the upper limit of the more preferable chromium (Cr) content is 7%.

[0019] The austenitic steel material according to one aspect of the present invention can contain the remaining Fe and other inevitable impurities in addition to the above-described components. However, in the normal manufacturing process, it is inevitable that unintended impurities may be mixed in from raw materials or the surrounding environment, and thus it is impossible to completely exclude them. These impurities are understandable to anyone with ordinary knowledge in the technical field, and thus the present specification does not particularly mention all of their details. Furthermore, in addition to the above-described components, further addition of effective components is not completely excluded.

[0020] The austenitic steel material according to one aspect of the present invention can contain 95% or more by area of austenite as a microstructure from the viewpoint of ensuring the target physical properties. The preferable fraction of austenite is preferably 97% or more by area, and can include the case where the fraction of austenite is 100% by area. On the other hand, the austenitic steel material according to one aspect of the present invention can actively suppress the fraction of carbides to 5% or less by area in order to prevent the decrease in ultra-low temperature impact toughness. The preferable fraction of carbides is preferably 3% or less by area, and can include the case where the fraction of carbides is 0% by area. In the present invention, the measurement methods for the fraction of austenite and the fraction of carbides are not particularly limited, and can be easily confirmed by the measurement methods usually used by ordinary technicians in the technical field to which the present invention belongs for measuring the microstructure and carbides.

[0021] The dislocation density of the austenitic steel material according to one aspect of the present invention is 2.3×10 15 ~3.3×10 15 / mm 2 can satisfy the range. The dislocation density of the steel material can be measured by using X-ray diffraction to measure the intensity of a specific plane of the steel material and then using the Williamson-Hall method or the like. An ordinary technician in the technical field to which the present invention belongs can measure the dislocation density of the steel material without any special technical difficulties. If the dislocation density of the steel material does not reach a certain level, it is impossible to ensure the strength suitable as a material for a structure. Therefore, in the present invention, the lower limit of the dislocation density of the steel material is limited to 2.3×10 15 / mm 2 . On the other hand, when the dislocation density is excessively high, it is advantageous in terms of ensuring the strength of the steel material, but it is not preferable in terms of ensuring ultra-low temperature toughness. Therefore, in the present invention, the upper limit of the dislocation density of the steel material can be limited to 3.3×10 15 / mm 2 .

[0022] The room-temperature yield strength of the austenitic steel material according to one aspect of the present invention can satisfy a range of 245 MPa or more and less than 400 MPa. When the strength of the steel material is high, the low-temperature impact toughness decreases, and in particular, for steel materials used in ultra-low temperature applications of -253°C, such as the present invention, if the yield strength is excessively high, there is a high possibility that the desired impact toughness cannot be secured. In addition, commercially available austenitic welding materials generally do not easily exceed the strength of the base material, so if the strength of the base material is kept high, a strength difference will occur between the weld and the base material, which may reduce structural stability. Therefore, it is preferable that the room-temperature yield strength of the austenitic steel material according to one aspect of the present invention is at a level of less than 400 MPa. On the other hand, if the room-temperature yield strength of the steel material is excessively low, the thickness of the base material may be excessively increased in order to ensure the stability of the structure, which may result in an excessive increase in the weight of the structure. Therefore, the lower limit of the room-temperature yield strength of the austenitic steel material according to one aspect of the present invention is limited to 245 MPa.

[0023] Since structures are generally provided by processing and welding steel materials, even if the ultra-low temperature impact toughness of the base material itself is ensured, if the ultra-low temperature impact toughness of the weld is not ensured, the safety of the structure itself may be greatly reduced. Therefore, the austenitic steel material according to one aspect of the present invention aims to ensure not only the ultra-low temperature impact toughness of the base material itself, but also the ultra-low temperature impact toughness of the heat-affected zone (HAZ) of the weld. Accordingly, the present invention controls not only the microstructure of the base material, but also the fraction and shape of the microstructure of the heat-affected zone of the weld to a specific range.

[0024] When welding is performed using an austenitic steel material according to one aspect of the present invention as the base material, using a covered arc welding rod, flux-cored arc welding wire, TIG welding rod and wire, submerged arc welding wire and flux, etc., under normal welding conditions applied to welding structures for ultra-low temperature applications, the heat-affected zone (HAZ) may contain 95 area% or more of austenite and 5 area% or less of carbides. As previously explained regarding the microstructure of the base material, the fraction of austenite in the heat-affected zone (HAZ) is often 97 area% or more, and may include cases where the fraction of austenite is 100 area%. Furthermore, in order to prevent a decrease in ultra-low temperature impact toughness in the weld, the fraction of carbides in the heat-affected zone (HAZ) is often limited to 3 area% or less, and may include cases where the fraction of carbides in the heat-affected zone (HAZ) is 0 area%.

[0025] The average austenite grain size in the heat-affected zone (HAZ) of a weld can be within the range of 5 to 200 μm. If the average austenite grain size in the heat-affected zone (HAZ) is excessively small, the strength of the weld will improve, but a localized decrease in ultra-low temperature impact toughness may occur in the HAZ. Therefore, in the austenitic steel material according to one aspect of the present invention, the average austenite grain size in the HAZ is limited to 5 μm or more. On the other hand, while a larger average austenite grain size in the HAZ is advantageous for ensuring the ultra-low temperature impact toughness of the weld, a localized decrease in strength may occur in the HAZ. Therefore, in the present invention, the average austenite grain size in the HAZ is limited to 200 μm or less.

[0026] In terms of ensuring the physical properties of the heat-affected zone (HAZ) during welding, not only the austenite fraction and average grain size, but also the average aspect ratio of the austenite grains are influential factors. If the average aspect ratio of the austenite grains present in the HAZ is excessively small, it is advantageous in ensuring the ultra-low temperature impact toughness of the HAZ, but disadvantageous in ensuring the strength of the HAZ. Therefore, the present invention limits the average aspect ratio of the austenite grains present in the HAZ to a level of 1.0 or higher. On the other hand, if the aspect ratio of the average grain size of austenite present in the heat-affected zone (HAZ) is excessively large, it is advantageous in terms of ensuring the strength of the heat-affected zone (HAZ), but disadvantageous in terms of ensuring the ultra-low temperature impact toughness of the heat-affected zone (HAZ). Therefore, in this invention, the aspect ratio of the average grain size of austenite present in the heat-affected zone (HAZ) is limited to a level of 5.0 or less.

[0027] When welding is performed using an austenitic steel material according to one aspect of the present invention as the base material under normal welding conditions applied to welding structures for ultra-low temperature applications, it is preferable that the lateral expansion value in the heat-affected zone (HAZ) of the test piece subjected to a -253°C standard Charpy impact test be 0.32 mm or more. The inventors of this invention have recognized that, in the case of steel materials applied to ultra-low temperature environments, plastic deformation characteristics are a major factor from the viewpoint of ensuring safety. Specifically, as a result of diligent research, the inventors of this invention have confirmed that, in the case of steel materials satisfying the component system presented in this invention, the lateral expansion value (mm) in the heat-affected zone (HAZ) of the weld is a more important factor than the Charpy impact energy value (J) in the heat-affected zone (HAZ) from the viewpoint of ensuring the safety of the weld.

[0028] The lateral expansion value in the heat-affected zone (HAZ) of the weld refers to the average value of the lateral plastic deformation of a specimen subjected to a Charpy impact test based on -253°C. Figure 2 shows a photograph of a specimen subjected to a Charpy impact test based on -253°C. As shown in Figure 2, the lateral expansion value can be calculated by calculating the increase in lateral length (△X1 + △X2) near the fracture surface. If the lateral expansion value in the heat-affected zone (HAZ) is 0.32 mm or more, it can be determined that the structure possesses the minimum low-temperature safety required for ultra-low temperature structures.

[0029] According to the inventors' research results, the relationship between the Charpy impact energy (J) at -253°C and the lateral expansion value (mm) of the test specimen generally shows a similar trend to the relationship shown in equation 1 below, and it was confirmed that the lateral expansion value is 0.32 mm or greater. It was found that the larger the lateral expansion value (mm), the better the low-temperature impact toughness, and a lateral expansion value of 0.72 to 1.4 mm is more effective. [Relationship 1] Lateral expansion value (mm) = 0.0088 × Charpy impact energy value (J) + 0.0893 In one aspect of the present invention, when the austenitic steel material is welded under normal welding conditions for welding cryogenic structures using the steel material as the base material, the lateral expansion value in the heat-affected zone (HAZ) of the test piece subjected to a -253°C Charpy impact test is at a level of 0.32 mm or higher. Therefore, when a cryogenic structure is fabricated using this steel material, excellent structural safety can be ensured.

[0030] The following describes in more detail a method for manufacturing austenitic steel according to one aspect of the present invention. A method for producing austenitic steel according to one aspect of the present invention may include the steps of: preparing a slab consisting of, by weight %, manganese (Mn): 10-45%, carbon (C): within the range satisfying 24×[C]+[Mn]≧25 and 33.5×[C]-[Mn]≦18, chromium (Cr): 10% or less (excluding 0%), and the remainder being iron (Fe) and unavoidable impurities; and heating the slab and then hot-rolling it to a rolling finish temperature of 850°C or higher. In the above formula, [C] and [Mn] represent the carbon (C) and manganese (Mn) content (by weight %) contained in the slab.

[0031] Slab preparation A steel slab having a predetermined alloy composition is prepared. Since the steel slab of the present invention has a steel composition corresponding to the austenitic steel material described above, the explanation regarding the alloy composition of the steel slab is replaced by the explanation regarding the steel composition of the austenitic steel material described above. The thickness of the steel slab is not particularly limited, and steel slabs with a thickness suitable for producing structural materials for low-temperature or ultra-low-temperature applications can be used.

[0032] Slab heating and hot rolling After heating the prepared steel slab, it can be hot-rolled to the desired thickness. The heating temperature of the steel slab is not particularly limited, but a preferred heating temperature is 1000-1300°C. If the finishing temperature of hot rolling is excessively low, excess internal deformation energy may remain in the final steel material, potentially reducing its ultra-low temperature impact toughness. Therefore, in this invention, the lower limit of the finishing temperature of hot rolling is limited to 800°C. On the other hand, if the upper limit of the finishing temperature of hot rolling is excessively high, the microstructure of the final steel material may grow excessively, potentially leading to a deterioration of its strength properties. Therefore, in this invention, the upper limit of the finishing temperature of hot rolling is limited to 1050°C. [Examples]

[0033] The following describes in more detail, through specific examples, an austenitic steel material and its manufacturing method relating to one aspect of the present invention. It should be noted that the following examples are for the purpose of understanding the present invention and not for defining the scope of the rights of the present invention. The scope of the rights of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom. (Examples) After preparing 250 mm thick steel slabs with the alloy compositions shown in Table 1 below, each test specimen was fabricated by applying the process conditions described in Table 2 below. Each steel slab consists of iron (Fe) and other unavoidable impurities in addition to the alloy components listed in Table 1.

[0034] [Table 1]

[0035] [Table 2]

[0036] The microstructure of each example and comparative example listed in Table 2 was observed using an optical microscope, and the results are shown in Table 3 below. Furthermore, the dislocation density of each example and comparative example was measured using X-ray diffraction analysis, and the yield strength at room temperature was measured using a tensile testing machine; the results are also shown in Table 3. Subsequently, welding was performed on each example and comparative example using the usual welding conditions applied to the welding of cryogenic structures, and the results are shown in Table 3. At this time, an optical microscope was used to observe the microstructure of the heat-affected zone during welding, and the impact energy on the heat-affected zone was - 253 Measurements were taken at °C using a Charpy impact tester. In addition, the transverse expansion value at the impact test fracture surface of each specimen was measured, and the results are also listed in Table 3.

[0037] [Table 3]

[0038] As shown in Tables 1 to 3, the examples that satisfy the alloy composition and process conditions restricted by the present invention satisfy the target room temperature yield strength and lateral expansion value in the heat-affected zone (HAZ), whereas the comparative examples that do not satisfy one or more of the alloy composition or process conditions restricted by the present invention do not satisfy one or more of the target room temperature yield strength or lateral expansion value in the heat-affected zone (HAZ).

[0039] Although the present invention has been described in detail above with reference to examples, other forms of embodiments are also possible. Therefore, the technical idea and scope of the claims described below are not limited to the examples. [Explanation of Symbols]

[0040] ΔX1, ΔX2: Increase in transverse length near the fracture surface

Claims

1. In weight percent, it consists of manganese (Mn): 18-45%, carbon (C): within the range satisfying 24 × [C] + [Mn] ≥ 25 and 33.5 × [C] - [Mn] ≤ 18, chromium (Cr): 10% or less (excluding 0%), and the remainder being iron (Fe) and unavoidable impurities. It contains austenite in an area fraction of 95% or more. An austenitic steel material characterized in that, when a Charpy impact test is performed on the heat-affected zone (HAZ) of the weld based on -253°C, the lateral expansion in the heat-affected zone of the weld is 0.32 mm or more. (In the above formula, [C] and [Mn] represent the carbon (C) and manganese (Mn) content (by weight %) contained in the steel material.)

2. The austenitic steel material according to claim 1, characterized in that the yield strength of the steel material at room temperature is 245 MPa or more and less than 400 MPa.

3. The austenitic steel material according to claim 1, characterized in that the heat-affected zone of the weld contains, as a microstructure, 95 area percent or more (including 100 area percent) of austenite and 5 area percent or less (including 0 area percent) of grain boundary carbides.

4. The austenitic steel material according to claim 1, characterized in that the average grain size of the heat-affected zone during welding is 5 to 200 μm.

5. The austenitic steel material according to claim 1, characterized in that the aspect ratio of the average crystal grains in the heat-affected zone of the weld is 1.0 to 5.

0.

6. The dislocation density of the aforementioned steel material is 2.3 × 10⁻⁶. １５ ~3.3 x 10 １５ / mm ２ The austenitic steel material according to claim 1, characterized in that it is the same as described in claim 1.

7. The process involves preparing a slab consisting of, by weight percent, manganese (Mn): 18-45%, carbon (C): within the range satisfying 24 × [C] + [Mn] ≥ 25 and 33.5 × [C] - [Mn] ≤ 18, chromium (Cr): 10% or less (excluding 0%), and the remaining iron (Fe) and unavoidable impurities. A method for producing an austenitic steel material, characterized by comprising the step of heating the slab and then hot-rolling it to a rolling finish temperature of 800°C to 1050°C. (In the above formula, [C] and [Mn] represent the carbon (C) and manganese (Mn) content (by weight %) contained in the slab.)

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