Austenitic steel material and method for producing same

Abstract

The present invention relates to an austenitic steel material and a method for manufacturing the same, and more particularly, to an austenitic steel material and a method for manufacturing the same, which are preferably useful as structural materials used in low-temperature environments, such as structural materials of liquefied gas storage tanks and liquefied gas transportation equipment.

Description

Technical Field

[0001] The present disclosure relates to an austenitic steel material and a method for manufacturing the same, and more particularly, to an austenitic steel material and a method for manufacturing the same, which can preferably be used as a structural material used in a low-temperature environment, such as a structural material for a liquefied gas storage tank, a liquefied gas transportation equipment, etc. Background Art

[0002] Liquefied gases such as liquefied hydrogen (boiling point: -253°C), liquefied natural gas (LNG, boiling point: -164°C), liquefied oxygen (boiling point: -183°C), and liquefied nitrogen (boiling point: -196°C) require ultra-low temperature storage. Therefore, in order to store these gases, structures such as pressure vessels made of materials that have sufficient toughness and strength at ultra-low temperatures are required.

[0003] As materials that can be used at low temperatures in liquefied gas atmospheres, Cr-Ni stainless steel alloys such as AISI 304, 9% Ni steel, and 5000 series aluminum alloys have been used. However, in the case of aluminum alloys, the alloy cost is high, the design thickness of the structure increases due to low strength, and weldability is also poor, so its use is limited. Cr-Ni stainless steel, 9% nickel (Ni) steel, etc. greatly improve the physical properties of aluminum, but are not economically desirable because they contain a large amount of expensive nickel (Ni). Summary of the Invention

[0004] Technical issues

[0005] One aspect of the present disclosure is to provide an austenitic steel material and a method of making the same.

[0006] A preferred aspect of the present disclosure is to provide an austenitic steel material having excellent ultra-low temperature toughness and a method for manufacturing the same.

[0007] Technical Solution

[0008] According to one aspect of the present disclosure, an austenitic steel material comprises, in terms of weight %, manganese (Mn): 10% to 45%, carbon (C): in the range of 24×[C]+[Mn]≥25 and 33.5×[C]-[Mn]≤18, chromium (Cr): 10% or less (excluding 0%), and at least one of the following: Cu: 5% or less (excluding 0%), Si: 5% or less (excluding 0%), Al: 5% or less (excluding 0%), Mo: 5% or less (excluding 0%), and B: 0.5% or less (excluding 0%), and the remainder is iron (Fe) and unavoidable impurities, wherein the microstructure in a weld heat affected zone of the austenitic steel material comprises, in terms of area %, 95% or more (including 100%) of austenite and 5% or less (including 0%) of carbides, and the content of solid-dissolved C in the austenite is 60% or more of the average C content of the steel material.

[0009] The average grain size of austenite may be 10 μm to 200 μm.

[0010] After Charpy impact testing at -253°C, the lateral expansion of the weld heat affected zone may be 0.32 mm or more.

[0011] The Charpy impact energy of the weld heat affected zone at -253°C may be 27 J or greater.

[0012] According to another aspect of the present disclosure, a method for manufacturing an austenitic steel material includes: an operation of heating a slab at 1000° C. to 1300° C., the slab containing, in terms of weight %, manganese (Mn): 10% to 45%, carbon (C): in the range of 24×[C]+[Mn]≥25 and 33.5×[C]-[Mn]≤18, chromium (Cr): 10% or less (excluding 0%), and at least one of the following: Cu: 5% or less (excluding 0%), Si: 5% or less (excluding 0%), Al: 5% or less (excluding 0%), Mo: 5% or less (excluding 0%), and B: 0.5% or less (excluding 0%), and the remainder being iron (Fe) and unavoidable impurities; an operation of obtaining a hot-rolled steel plate by finish hot-rolling the heated slab at 800° C. to 1050° C.; and an operation of welding the hot-rolled steel plate.

[0013] Beneficial effects

[0014] According to one aspect of the present disclosure, an austenitic steel material and a method for manufacturing the same may be provided.

[0015] According to a preferred aspect of the present disclosure, an austenitic steel material having excellent ultra-low temperature toughness and a method for manufacturing the same can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is a graph showing the relationship between carbon content and manganese content of an austenitic steel material according to one aspect of the present disclosure.

[0017] Figure 2 is a diagram schematically illustrating a method for measuring a lateral expansion value of an austenitic steel material according to one aspect of the present disclosure. DETAILED DESCRIPTION

[0018] Hereinafter, an austenitic steel material according to an embodiment of the present disclosure will be described. First, the alloy composition will be described. Unless otherwise specified, the content of the alloy composition described below refers to weight %.

[0019] Manganese (Mn): 10% to 45%

[0020] Manganese is an element that plays an important role in stabilizing austenite. In order to stabilize austenite at ultra-low temperatures, it is preferred to contain 10% or more manganese (Mn). If the manganese (Mn) content is less than this value, ε martensite, a metastable phase, is formed, and this ε martensite can easily transform into α martensite through processing-induced transformation at ultra-low temperatures, making it impossible to ensure toughness. To suppress the formation of ε martensite, there is a method of increasing the carbon (C) content to stabilize austenite, but in this case, a large amount of carbides may precipitate, causing physical properties to deteriorate rapidly. Therefore, the manganese (Mn) content is preferably 10% or more. The preferred manganese (Mn) content can be 15% or more, and the more preferred manganese (Mn) content can be 18% or more. If the manganese (Mn) content is too high, it may not only reduce the corrosion rate of the steel material, but is also undesirable from an economic perspective. Therefore, the manganese (Mn) content is preferably 45% or less. The preferred manganese (Mn) content can be 40% or less, and the more preferred manganese (Mn) content can be 35% or less.

[0021] Carbon (C): 24×[C]+[Mn]≥25 and 33.5×[C]-[Mn]≤18

[0022] Carbon (C) is an element that stabilizes austenite and improves strength. In particular, carbon (C) plays a role in reducing Ms or Md during the cooling process, processing, etc., which are the transition points from austenite to ε or α martensite. Therefore, carbon (C) is a component that effectively contributes to the stability of austenite. If the carbon (C) content is insufficient, the stability of austenite is insufficient, and therefore stable austenite cannot be obtained at ultra-low temperatures, and external stress may easily cause processing-induced transformation into ε or α martensite, which may reduce the toughness of the steel material or reduce the strength of the steel material. On the other hand, if the carbon (C) content is too much, the toughness of the steel material may deteriorate rapidly due to carbide precipitation, and the strength of the steel material may increase excessively, thereby reducing workability.

[0023] The inventors of the present disclosure have conducted intensive research on the correlation behavior between the carbon (C) content and the manganese (Mn) content with respect to the formation of carbides, and as a result, as shown in FIG. Figure 1 As shown, it is concluded that determining the relative content relationship between carbon (C) and manganese (Mn) can effectively promote the stabilization of austenite and effectively control the amount of carbide precipitation. Carbides are formed by carbon (C), but carbon (C) does not affect the formation of carbides independently, but rather acts in combination with manganese (Mn) to affect the formation of carbides.

[0024] In order to stabilize austenite, assuming that other components satisfy the ranges specified in the present disclosure, it is preferred to control the value of 24×[C]+[Mn] (wherein [C] and [Mn] mean the contents of the respective components expressed in wt %) to 25 or more. Figure 1 The inclined left boundary of the parallelogram region shown in . When 24×[C]+[Mn] is less than 25, the stability of austenite is reduced, resulting in a processing-induced transformation caused by impact at ultra-low temperatures, and thus the impact toughness of the steel material may be deteriorated. On the other hand, in order to suppress the formation of carbides, on the premise that other components meet the ranges specified in the present disclosure, it is preferred to control the value of 33.5×[C]-[Mn] (wherein, [C] and [Mn] mean the content of each component expressed in weight %) to 18 or less. When 33.5×[C]-[Mn] exceeds 18, carbides may precipitate due to excessive addition of carbon (C), which may deteriorate the low-temperature impact toughness of the steel material. Therefore, in the present disclosure, it is preferred to add carbon (C) to satisfy 24×[C]+[Mn]≥25 and 33.5×[C]-[Mn]≤18. As in Figure 1 As can be seen from the figure, within the range satisfying the above formula, the minimum limit of the carbon (C) content is 0%.

[0025] Chromium (Cr): 10% or less (excluding 0%)

[0026] Chromium (Cr) is also an austenite stabilizing element, and stabilizes austenite up to a suitable addition range, thereby improving the low-temperature impact toughness of the steel material, and acts to improve the strength of the steel material by dissolving in the austenite. In addition, chromium (Cr) is also an element that effectively helps improve the corrosion resistance of the steel material. Therefore, in the present disclosure, chromium (Cr) is added as an essential element. The preferred lower limit of the chromium (Cr) content can be 1%, and the more preferred lower limit of the chromium (Cr) content can be 2%. However, chromium (Cr) is a carbide-forming element, and in particular, it can form carbides at the austenite grain boundaries and reduce the low-temperature impact toughness of the steel material. In addition, when the addition amount of chromium (Cr) exceeds a certain level, excessive carbides may precipitate in the weld heat-affected zone (HAZ), which may lead to poor ultra-low temperature toughness. Therefore, the present disclosure can limit the upper limit of chromium (Cr) to 10%. The preferred upper limit of the chromium (Cr) content can be 8%, and the more preferred upper limit of the chromium (Cr) content can be 7%.

[0027] At least one of the following: Cu: 5% or less (excluding 0%), Si: 5% or less (excluding 0%), Al: 5% or less (excluding 0%), Mo: 5% or less (excluding 0%), and B: 0.5% or less (excluding 0%)

[0028] Cu: 5% or less (excluding 0%)

[0029] The solubility of Cu in carbide is very low, and it diffuses slowly in austenite, and is therefore concentrated at the interface between austenite and nucleated carbide. Therefore, it effectively slows down the growth of carbide by hindering the diffusion of carbon, and finally has the effect of suppressing the formation of carbide. In addition, copper has the effect of stabilizing austenite and improving ultra-low temperature toughness. However, if the content of Cu exceeds 5%, there is the problem of reducing the hot workability of steel material, therefore preferably the upper limit is limited to 5 weight %. The preferred upper limit of copper (Cu) content can be 3%, and the preferred upper limit of copper (Cu) content can be 2%. The preferred lower limit of copper (Cu) content can be 0.1%, and the more preferred lower limit of copper (Cu) content can be 0.3%.

[0030] Si: 5% or less (excluding 0%)

[0031] Silicon (Si) is an element that improves the castability of molten steel, and in particular, when added to an austenitic steel material, effectively improves the strength by dissolving inside the steel material. It is also an element that affects the activity of carbon in the steel material and effectively inhibits the formation of carbides, thereby improving toughness. However, if added in an amount exceeding 5%, the stacking fault energy is reduced, twin formation is promoted, and there is a problem that toughness may be reduced due to high strength, so it is preferred to limit the upper limit to 5%. The preferred upper limit of the silicon (Si) content may be 3%, and the more preferred upper limit of the silicon (Si) content may be 2.5%. The preferred lower limit of the silicon (Si) content may be 0.1%, and the more preferred lower limit of the silicon (Si) content may be 0.3%.

[0032] Al: 5% or less (excluding 0%)

[0033] Al is an element that stabilizes austenite and reduces Ms and Md within an appropriate addition range, thereby improving the toughness of the steel material, wherein Ms and Md are the transformation points from austenite to ε or α martensite by cooling or processing. It is also an element that dissolves in the steel material to increase strength, and in particular, it is an element that effectively suppresses the formation of carbides by affecting the activity of carbon in the steel material, thereby improving toughness. In particular, it is well known as an element that effectively increases stacking fault energy and promotes slip. However, if it is added in an amount exceeding 5%, there is a problem of deteriorating the castability and surface quality of the steel by forming oxides and nitrides, so it is preferred to limit the upper limit to 5% by weight. The preferred upper limit of the aluminum (Al) content can be 3%, and the more preferred upper limit of the aluminum (Al) content can be 2.5%. The preferred lower limit of the aluminum (Al) content can be 0.2%, and the more preferred lower limit of the aluminum (Al) content can be 0.3%.

[0034] Mo: 5% or less (excluding 0%)

[0035] Molybdenum (Mo) is an element that stabilizes austenite within an appropriate addition range and reduces the transformation points Ms and Md from austenite to ε or α martensite by cooling or processing, thereby improving the toughness of the steel material. In addition, it is an element that dissolves in the steel material to increase strength, and in particular, it is an element that segregates in the austenite grain boundaries to increase the stability of the grain boundaries, thereby reducing energy and thus inhibiting the precipitation of carbonitrides at the grain boundaries. In addition, molybdenum (Mo) is well known as an element that effectively increases stacking fault energy and promotes slip. However, if it is added in an amount exceeding 5% by weight, it is an expensive element, so there is a problem that economic efficiency is reduced and toughness may be reduced due to high strength. Therefore, it is preferably limited to 5% by weight. The preferred upper limit of molybdenum (Mo) content can be 4.5%, and the more preferred upper limit of molybdenum (Mo) content can be 4%. The preferred lower limit of molybdenum (Mo) content can be 0.1%, and the more preferred lower limit of molybdenum (Mo) content can be 0.3%.

[0036] B: 0.5% or less (excluding 0%)

[0037] Boron (B) is a representative element that improves the hardenability of steel. It preferentially segregates at the grain boundaries of austenite at high temperatures, thereby reducing the energy of austenite grain boundaries and stabilizing them. Therefore, it improves hardenability so that during the quenching of carbon steel, undesirable structures such as ferrite, pearlite, etc. will not be produced at the austenite grain boundaries. Boron (B) also segregates in the austenite grain boundaries of high manganese steel, which reduces the energy of austenite grain boundaries, thereby preventing carbon diffusion, and is an effective element in suppressing the formation of carbides. However, if it is added in an amount exceeding 0.5 wt %, there is a problem of forming coarse boron nitride, which reduces physical properties, so it is preferably limited to 0.5 wt %. The preferred upper limit of boron (B) content can be 0.4%, and the more preferred upper limit of boron (B) content can be 0.3%. The preferred lower limit of boron (B) content can be 0.0005%, and the more preferred lower limit of boron (B) content can be 0.001%.

[0038] Since structures are typically created 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 significantly reduced. Therefore, according to one aspect of the present disclosure, an austenitic steel material is provided that ensures not only the ultra-low temperature impact toughness of the base material itself but also the ultra-low temperature impact toughness of the weld heat-affected zone (HAZ). Therefore, in the present disclosure, not only the microstructure of the base material but also the microstructure of the weld heat-affected zone are controlled within specific ranges.

[0039] According to one aspect of the present disclosure, in terms of ensuring the desired physical properties, the austenitic steel material can contain 95 area % or more of austenite not only in the microstructure of the substrate but also in the microstructure of the weld heat affected zone (HAZ). The preferred austenite fraction can be 97 area % or higher, and can include a case where the austenite fraction is 100 area %. At the same time, the austenitic steel material according to one aspect of the present disclosure can actively suppress the carbide fraction to 5 area % or less to prevent a decrease in ultra-low temperature impact toughness. The preferred carbide fraction can be 3 area % or less, and can include a case where the carbide fraction is 0 area %. In the present disclosure, the method for measuring the austenite fraction and the carbide fraction is not particularly limited, and can be easily determined by the measurement method for measuring the microstructure and carbide commonly used by those skilled in the art to which the present disclosure belongs.

[0040] Preferably, the amount of C dissolved in austenite is 60% or more of the average C content of the steel material. When all the C contained in the steel material is dissolved in the austenite, the amount of C dissolved in the austenite is 100%, and when the C contained in the steel material precipitates as carbides, the amount of C dissolved in the austenite is less than 100%. By dissolving a large amount of C in the austenite in this manner, the precipitation of carbides can be suppressed, and at the same time, deterioration of impact toughness can be prevented. More preferably, the amount of C dissolved in the austenite is 70% or more, more preferably 85%, and most preferably 100% relative to the average C content of the steel material. The method for measuring the amount of C dissolved in austenite relative to the average C content of the steel material is not particularly limited and can be easily determined using measurement methods commonly used by those skilled in the art for measuring microstructure and carbides.

[0041] The average grain size of austenite is preferably 10 μm to 200 μm. If the average grain size of austenite in the weld heat-affected zone (HAZ) is too small, the strength of the weld is improved, but the local ultra-low temperature impact toughness in the weld heat-affected zone (HAZ) may deteriorate. In addition, since the main precipitation site of carbides is the austenite grain boundaries, if the grain size is too fine, carbides may precipitate excessively, resulting in reduced impact toughness. Therefore, the average grain size of austenite in the weld heat-affected zone can be limited to 10 μm or larger. At the same time, a larger average grain size of austenite in the weld heat-affected zone is beneficial for ensuring the ultra-low temperature impact toughness of the weld, but local strength reduction may occur in the weld heat-affected zone. Therefore, the average grain size of austenite can be limited to 200 μm or less. The lower limit of the average grain size of austenite is more preferably 15 μm, and more preferably 20 μm. The upper limit of the average grain size of austenite is more preferably 180 μm, and more preferably 150 μm.

[0042] When welding an ultra-low temperature structure using an austenitic steel material according to one aspect of the present disclosure as a base material under normal welding conditions, the lateral expansion value of the weld heat-affected zone after a Charpy impact test at -253°C can be 0.32 mm or greater. Meanwhile, in the present disclosure, since a higher lateral expansion value is more advantageous, its upper limit is not particularly limited. However, the upper limit of the lateral expansion value can be, for example, 2.30 mm.

[0043] The inventors of the present disclosure have discovered that, in the case of steel materials used in ultra-low temperature environments, plastic deformation characteristics are a major factor in ensuring safety. Specifically, after extensive research, the inventors of the present disclosure have confirmed that, in the case of steel materials meeting the component system suggested by the present disclosure, the lateral expansion value (mm) in the weld heat-affected zone is a more important factor in ensuring welding safety than the Charpy impact energy value (J) in the weld heat-affected zone.

[0044] The transverse expansion value in the weld heat affected zone means an average value of the transverse plastic deformation amount of a specimen subjected to a Charpy impact test at -253°C. Figure 2 A photograph of a specimen subjected to a Charpy impact test at -253°C is shown, and Figure 2 As shown, the transverse length increase (ΔX1+ΔX2) near the fracture surface can be calculated to obtain the transverse expansion value. If the transverse expansion value in the weld heat affected zone is 0.32 mm or greater, it can be determined that the minimum low-temperature safety required for the ultra-low temperature structure is provided.

[0045] According to the research results of the present inventors, it was determined that the Charpy impact energy (J) and transverse expansion value (mm) of the corresponding samples at -253°C generally showed a trend similar to the following Relationship 1, and it was determined that the transverse expansion value (mm) was preferably 0.32 mm or greater. It can be seen that the larger the transverse expansion value (mm), the better the low-temperature impact toughness, and 0.72 mm to 1.4 mm is more effective.

[0046] [Equation 1]

[0047] Transverse expansion value (mm) = 0.0088 × Charpy impact energy value (J) + 0.0893

[0048] When welding is performed under normal welding conditions for welding an ultra-low temperature structure using the austenitic steel material according to one aspect of the present disclosure as a base material, the lateral expansion value in the weld heat-affected zone of a specimen subjected to a Charpy impact test at -253°C is at a level of 0.32 mm or more, so that when the ultra-low temperature structure is manufactured using the steel material, excellent structural safety can be ensured.

[0049] When welding a cryogenic structure using the austenitic steel material of the present disclosure as a base material under normal welding conditions, the Charpy impact energy of the weld heat-affected zone at -253°C can be 27 J or greater. By ensuring such a high level of cryogenic Charpy impact energy, ductile fracture occurs when the structure is destroyed, thereby ensuring the failure stability of the cryogenic structure. Meanwhile, in the present disclosure, since the ultra-low temperature Charpy impact energy is relatively high, which is advantageous, its upper limit is not particularly limited. However, as an example, the upper limit of the ultra-low temperature Charpy impact energy can be 250 J.

[0050] Hereinafter, a method for manufacturing an austenitic steel material according to one embodiment of the present disclosure will be described.

[0051] First, a slab having the above alloy composition is heated at 1000°C to 1300°C. If the slab heating temperature is lower than 1100°C, there are disadvantages such as alloy components not being redissolved and homogenized, or a long time being required to reach the target temperature at the center of the slab. If the slab heating temperature exceeds 1300°C, there are disadvantages such as partial melting in areas where the slab alloy components are segregated, or severe surface oxidation. The lower limit of the slab heating temperature is more preferably 1030°C, even more preferably 1070°C, and most preferably 1100°C. The upper limit of the slab heating temperature is more preferably 1250°C, even more preferably 1230°C, and most preferably 1200°C.

[0052] Thereafter, the heated slab is subjected to finish hot rolling at 800°C to 1050°C to obtain a hot-rolled steel sheet. If the above-mentioned finish hot rolling temperature is lower than 800°C, rolling is not easy due to the high-temperature strength of the material, and since non-recrystallization rolling occurs, there is a disadvantage that the strength of the material is excessively increased, thereby reducing the impact toughness. If the finish hot rolling temperature exceeds 1050°C, there are disadvantages of austenite coarsening and reduced strength. The lower limit of the finish hot rolling temperature is more preferably 820°C, more preferably 850°C, and most preferably 870°C. The upper limit of the finish hot rolling temperature of the slab is more preferably 1030°C, more preferably 1000°C, and most preferably 980°C. At the same time, the reduction rate during hot rolling can be applied within a suitable range according to the target plate thickness, and as a non-limiting example, the final thickness of the hot-rolled steel sheet can be 5mm to 80mm.

[0053] Thereafter, the hot rolled steel sheet may be air cooled to room temperature.

[0054] In the present disclosure, in order to form a weld heat affected zone, welding can be performed using submerged arc welding rods, flux-cored arc welding wires, TIG welding rods and wires, submerged arc welding wires, fluxes, etc. under normal welding conditions for welding ultra-low temperature structures.

[0055] Embodiments of the invention

[0056] Hereinafter, an austenitic steel material and a method for manufacturing the same according to one aspect of the present disclosure will be described in more detail through detailed examples. It should be noted that the following examples are merely for understanding the present disclosure and are not intended to specify the scope of the present disclosure. The scope of the present disclosure can be determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0057] (Example)

[0058] After preparing slabs with a thickness of 250 mm having the alloy composition described in Table 1 below, the slabs were heated and hot rolled under the conditions described in Table 2 below, and subjected to submerged arc welding under normal conditions to produce steel materials having a welded heat affected zone.

[0059] The microstructure and physical properties of the weld heat-affected zone of the steel material manufactured in this manner were measured, and the results are then shown in Table 2 below.

[0060] The microstructure was measured using an optical microscope at room temperature.

[0061] The average grain size of austenite was measured by taking micrographs using an optical microscope and then using image analysis.

[0062] The content of C dissolved in austenite compared to the average C content of the steel material was measured by point analysis of the matrix and carbides using energy dispersive X-ray spectroscopy using a scanning electron microscope.

[0063] After the specimen was kept at -253°C for 15 minutes or longer, the Charpy impact energy was measured using a Charpy impact tester.

[0064] The above lateral expansion is calculated by calculating the average value of the amount of lateral plastic deformation of the specimens subjected to the Charpy impact test at -253°C.

[0065] [Table 1]

[0066]

[0067] [Table 2]

[0068]

[0069] As can be seen from Tables 1 and 2 above, in the case of Examples 1 to 6 satisfying the alloy composition and manufacturing conditions of the present disclosure, it can be seen that excellent physical properties are ensured by ensuring the microstructure desired by the present disclosure and the C content dissolved in austenite compared to the average C content of the steel material.

[0070] In the case of Comparative Examples 1 to 4 that do not satisfy the alloy composition of the present disclosure, it can be seen that the physical properties are low because the microstructure desired by the present disclosure or the C content dissolved in austenite compared with the average C content of the steel material is not ensured.

Claims

1. An austenitic steel material, comprising: In terms of weight %, manganese (Mn): 10% to 45%, carbon (C): in the range of 24×[C]+[Mn]≥25 and 33.5×[C]-[Mn]≤18, chromium (Cr): 10% or less (excluding 0%), and at least one of the following: Cu: 5% or less (excluding 0%), Si: 5% or less (excluding 0%), Al: 5% or less (excluding 0%), Mo: 5% or less (excluding 0%), and B: 0.5% or less (excluding 0%), and the remainder being iron (Fe) and unavoidable impurities, wherein the microstructure in the weld heat affected zone of the austenitic steel material comprises, in area %, 95% or more (including 100%) of austenite and 5% or less (including 0%) of carbide, and The content of solid-dissolved C in the austenite is 60% or more of the average C content of the steel material. 2 . The austenitic steel material according to claim 1 , wherein an average grain size of the austenite is 10 μm to 200 μm. 3 . The austenitic steel material according to claim 1 , wherein a lateral expansion of the weld heat affected zone after a Charpy impact test at −253° C. is 0.32 mm or more. 4 . The austenitic steel material according to claim 1 , wherein the Charpy impact energy of the weld heat affected zone at −253° C. is 27 J or greater.

5. A method for producing an austenitic steel material, comprising: An operation of heating a slab at 1000° C. to 1300° C., the slab comprising, in terms of weight %, manganese (Mn): 10% to 45%, carbon (C): in the range of 24×[C]+[Mn]≥25 and 33.5×[C]-[Mn]≤18, chromium (Cr): 10% or less (excluding 0%), and at least one of the following: Cu: 5% or less (excluding 0%), Si: 5% or less (excluding 0%), Al: 5% or less (excluding 0%), Mo: 5% or less (excluding 0%), and B: 0.5% or less (excluding 0%), and remainder being iron (Fe) and unavoidable impurities; An operation of obtaining a hot-rolled steel sheet by finish hot rolling the heated slab at 800° C. to 1050° C.; and The hot rolled steel plate is welded.

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