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 which has excellent cryogenic toughness in a weld heat affected zone and can be used as a structural material in cryogenic environments such as liquefied gas storage tanks and liquefied gas transport facilities, and a method for producing the same. [Solution] The austenitic steel material of the present invention contains, by weight, 10 to 45% manganese (Mn), carbon (C) in a range that satisfies 24 × [C] + [Mn] ≧ 25 and 33.5 × [C] - [Mn] ≦ 18, 10% or less (excluding 0%) chromium (Cr), 0.5% or less (excluding 0%) of one or more of titanium (Ti), niobium (Nb) and vanadium (V), with the remainder being iron (Fe) and unavoidable impurities, and when a -253°C standard Charpy impact test is performed on a weld heat affected zone, the lateral expansion of the weld heat affected zone is 0.32 mm or more. In the formula, [C] and [Mn] refer to 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 with excellent ultra-low temperature toughness in the heat-affected zone of a weld and a method for manufacturing the same, and more specifically, to an austenitic high-manganese steel material with excellent ultra-low temperature toughness in the heat-affected zone of a weld that can be used as a structural material in ultra-low temperature environments and a method for manufacturing 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 have significantly improved 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 present invention aims 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.

[0005] 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]

[0006] The austenitic steel of the present invention contains, 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 one or more of titanium (Ti), niobium (Nb), and vanadium (V): 0.5% or less (excluding 0%), with the remainder being iron (Fe) and unavoidable impurities. When a Charpy impact test is performed on the weld heat-affected zone at -253°C, the lateral expansion in the weld heat-affected zone may be 0.32 mm or more. In the above formula, [C] and [Mn] refer to the carbon (C) and manganese (Mn) content (by weight %) contained in the steel.

[0007] The yield strength at room temperature of the above steel material may be between 270 MPa and less than 400 MPa.

[0008] The above-mentioned heat-affected zone of the weld may contain, as part of its microstructure, 95 area-percent or more austenite and 5 area-percent or less (excluding 0%) grain boundary carbides.

[0009] The average grain size of the heat-affected zone during welding can range from 5 to 200 μm.

[0010] The average grain aspect ratio of the heat-affected zone during welding can range from 1.0 to 5.0.

[0011] The dislocation density of the above steel material is 2.3 × 10⁻⁶. 15 ~3.3×10 15 / mm 2 It is possible.

[0012] The present invention relates to a method for producing austenitic steel, which may include the steps of: preparing a slab containing, 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 one or more of titanium (Ti), niobium (Nb), and vanadium (V): 0.5% or less (excluding 0%), with the remainder being iron (Fe) and unavoidable impurities; heating the slab and then hot-rolling it to provide a hot-rolled steel sheet; and heat-treating the hot-rolled steel sheet by heating it to a temperature range of 500-1000°C and holding it at that temperature range for a time of 1.3t (thickness of the hot-rolled steel sheet, mm) + 5 minutes or more. In the above formulas, [C] and [Mn] refer to the content (by weight %) of carbon (C) and manganese (Mn) contained in the slab.

[0013] 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]

[0014] According to the present invention, an austenitic steel material and a method for manufacturing the same can be provided, which exhibits excellent ultra-low temperature toughness in the heat-affected zone of the weld and is particularly suitable as a structural material in ultra-low temperature environments such as liquefied gas storage tanks and liquefied gas transport equipment.

[0015] 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]

[0016] [Figure 1] This figure shows the correlation between the carbon content and manganese content of the austenitic steel material of the present invention. [Figure 2] This figure schematically illustrates the method for measuring the lateral expansion value in the heat-affected zone of austenitic steel material according to the present invention. [Modes for carrying out the invention]

[0017] The present invention relates to an austenitic steel material and a method for manufacturing the same. Hereinafter, preferred embodiments of the present invention will be described. The embodiments of the present invention can be modified into various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. This embodiment is provided to explain the present invention in more detail to those having ordinary knowledge in the technical field to which the invention belongs.

[0018] Hereinafter, the austenitic steel material of the present invention will be described in more detail.

[0019] The austenitic steel material of the present invention contains, by weight %, manganese (Mn): 10 to 45%, carbon (C): in a range satisfying 24×[C]+[Mn]≧25 and 33.5×[C]-[Mn]≦18, chromium (Cr): 10% or less (excluding 0%), one or more of titanium (Ti), niobium (Nb) and vanadium (V): 0.5% or less (excluding 0%), and the balance is composed of iron (Fe) and inevitable impurities. When a -253°C standard Charpy impact test is performed on the weld heat-affected zone, the transverse expansion in the weld heat-affected zone can satisfy 0.32 mm or more.

[0020] Hereinafter, the steel composition contained in the austenitic steel material of the present invention will be described in more detail. Hereinafter, unless otherwise specified, “%” indicating the content of each element is based on weight.

[0021] Manganese (Mn): 10 to 45% Manganese is an element that plays an important role in stabilizing austenite. To stabilize austenite at ultra-low temperatures, it is preferable to contain 10% or more of manganese (Mn). If the manganese (Mn) content is less than this, epsilon martensite, which is a metastable phase, is formed and easily transforms into alpha martensite by processing-induced transformation at ultra-low temperatures, so toughness cannot be ensured. There is a scheme to increase the carbon (C) content to suppress the formation of epsilon martensite and achieve the stabilization of austenite. However, in this case, a large amount of carbides may precipitate, and the physical properties may deteriorate rapidly. Therefore, the manganese (Mn) content is preferably 10% or more. The preferable manganese (Mn) content may be 15% or more, and the more preferable manganese (Mn) content may be 18% or more. If the manganese (Mn) content is excessive, it may not only reduce the corrosion rate of the steel material but is not preferable from an economic perspective. Therefore, the manganese (Mn) content is preferably 45% or less. The preferable manganese (Mn) content may be 40% or less, and the more preferable manganese (Mn) content may be 35% or less.

[0022] Carbon (C): in the range satisfying 24×[C]+[Mn]≧25 and 33.5×[C]-[Mn]≦18 Carbon (C) is an element that stabilizes austenite and increases strength. In particular, carbon (C) is the transformation point M from austenite to epsilon or alpha martensite during processes such as the cooling process or processing. s or M d and plays a role in lowering it. Therefore, carbon (C) is a component that effectively contributes to the stabilization of austenite. When the carbon (C) content is insufficient, the stability of austenite is insufficient, and stable austenite at ultra-low temperatures cannot be obtained. It may easily cause processing-induced transformation into epsilon or alpha martensite by external stress, reducing the toughness of the steel material or potentially decreasing the strength of the steel material. On the other hand, when the carbon (C) content is excessive, the toughness of the steel material may deteriorate rapidly due to the precipitation of carbides, and the strength of the steel material may increase excessively, reducing the workability.

[0023] The inventors of this invention diligently studied the relative behavior of carbon (C) and manganese (Mn) content in relation to carbide formation. As a result, as shown in Figure 1, they concluded that by determining the relative content relationship of carbon (C) and manganese (Mn), it is possible to effectively stabilize austenite while effectively controlling the amount of carbide precipitation. Although carbides are formed by carbon (C), carbon (C) does not independently affect carbide formation; rather, it interacts with manganese (Mn) to influence carbide formation.

[0024] To stabilize austenite, it is preferable to control the value of 24×[C]+[Mn] (where [C] and [Mn] represent the content of each component in weight percent) to 25 or higher, assuming that the other components satisfy the range specified in this invention. This boundary refers to the inclined left boundary of the parallelogram region shown in Figure 1. If 24×[C]+[Mn] is less than 25, the stability of austenite decreases, and processing-induced transformation may occur due to impact at ultra-low temperatures, which may reduce the impact toughness of the steel. On the other hand, to suppress carbide formation, it is preferable to control the value of 33.5×[C]-[Mn] (where [C] and [Mn] represent the content of each component in weight percent) to 18 or lower, assuming that the other components satisfy the range specified in this invention. If 33.5 × [C] - [Mn] exceeds 18, the addition of excess carbon (C) may cause carbide precipitation, potentially reducing the low-temperature impact toughness of the steel. Therefore, in this invention, it is preferable that carbon (C) is added such that 24 × [C] + [Mn] ≥ 25 and 33.5 × [C] - [Mn] ≤ 18. As can be seen from Figure 1, the lower limit of carbon (C) content within the range that satisfies the above formula is 0%.

[0025] Chromium (Cr): 10% or less (excluding 0%) Chromium (Cr) is also an austenite-stabilizing element. Within an appropriate range of addition amounts, it stabilizes austenite, improving the low-temperature impact toughness of steel and increasing the strength of steel by solid dissolving within the austenite. Furthermore, chromium (Cr) is a component that effectively contributes to improving the corrosion resistance of steel. Therefore, this invention adds chromium (Cr) as an essential component. The lower limit of the preferred chromium (Cr) content may be 1%, and the lower limit of the more preferred chromium (Cr) content may be 2%. However, chromium (Cr) is a carbide-forming element, and it may form carbides, particularly at austenite grain boundaries, which can reduce the low-temperature impact toughness of steel. Also, if the amount of chromium (Cr) added exceeds a certain level, excessive carbides may precipitate in the heat-affected zone (HAZ) of the weld, potentially resulting in poor ultra-low temperature toughness. Therefore, in this invention, the upper limit of chromium (Cr) can be limited to 10%. The upper limit for a preferred chromium (Cr) content may be 8%, and the upper limit for a more preferred chromium (Cr) content may be 7%.

[0026] Titanium (Ti), niobium (Nb), and vanadium (V): 0.5% or less (excluding 0%) Titanium (Ti) is an element that increases strength through solid solution and precipitation hardening effects. In particular, it is an advantageous element that can suppress grain growth by titanium carbonitrides in the heat-affected zone of welding, thereby preventing deterioration of strength. It is also an element that can improve strength through the precipitation of carbonitrides during heat treatment. The present invention allows for the addition of titanium (Ti) for these effects. However, if titanium (Ti) is added in excess, coarse precipitates or crystals will be formed, inducing surface cracks during rolling and degrading the physical properties of the steel. Therefore, it is preferable to limit its content to a certain range.

[0027] Niobium (Nb) is an element that increases strength through solid solution and precipitation hardening effects. In particular, it can increase the recrystallization stopping temperature (Tnr) of steel, improve yield strength through grain refinement during low-temperature rolling, and improve strength through carbonitride precipitation during heat treatment. In this invention, niobium (Nb) can be added for these effects.

[0028] Vanadium (V) is an element that increases strength through solid solution and precipitation hardening effects; therefore, in this invention, vanadium (V) can be added for these effects.

[0029] However, if titanium (Ti), niobium (Nb), and vanadium (V) are added in excess, coarse precipitates may be formed, which may actually degrade the physical properties of the steel. Furthermore, the formation of coarse precipitates or crystals may induce surface cracks during hot rolling. Therefore, in this invention, the content of each of titanium (Ti), niobium (Nb), and vanadium (V) can be limited to 0.5% or less. Preferably, the total content of titanium (Ti), niobium (Nb), and vanadium (V) can be limited to 0.5% or less.

[0030] The austenitic steel of the present invention consists of the remaining Fe and other unavoidable impurities, in addition to the components described above. However, in the normal manufacturing process, unintended impurities may inevitably be mixed in from the raw materials or the surrounding environment, and therefore cannot be completely eliminated. Since these impurities are recognizable to anyone with ordinary skill in the art, not all of them are specifically mentioned in this specification. Furthermore, the addition of further effective components other than those described above is not entirely excluded.

[0031] The austenitic steel material 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 can be 97% or more by area, and can include the case where the fraction of austenite is close to 100% by area. On the other hand, in order to prevent the decrease in ultra-low temperature impact toughness, the fraction of carbide in the austenitic steel material of the present invention can be actively suppressed to 5% or less by area. The preferable fraction of carbide can be 3% or less by area. However, since the austenitic steel material of the present invention contains one or more selected from titanium (Ti), niobium (Nb), and vanadium (V) which are carbide-forming elements, 0% can be excluded from the lower limit of the carbide fraction. In the present invention, the method for measuring the fraction of austenite and the fraction of carbide is not particularly limited, and can be easily confirmed by a measurement method commonly used by ordinary technicians in the technical field to which the present invention belongs for measuring the microstructure and carbide.

[0032] The dislocation density of the austenitic steel material of the present invention can satisfy the range of 2.3×10 15 ~3.3×10 15 / mm 2 . The dislocation density of the steel material can be measured using the Williamson-Hall method or the like after measuring the intensity of a specific plane of the steel material using X-ray diffraction. Ordinary technicians in the technical field to which the present invention belongs can measure the dislocation density of the steel material without any special technical difficulties. When 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 can be 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 .

[0033] The room-temperature yield strength of the austenitic steel material of the present invention can satisfy a range of 270 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 of the present invention can be limited to 270 MPa.

[0034] 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 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.

[0035] When welding is performed using the austenitic steel material of the present invention as the base material, using covered arc welding rods, flux-cored arc welding wires, TIG welding rods and wires, submerged arc welding wires and flux, etc., under normal welding conditions applied to welding structures for ultra-low temperature applications, the heat-affected zone (HAZ) can 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 contained in the heat-affected zone (HAZ) can be 97 area% or more, and can include cases where the fraction of austenite is close to 100 area%. Furthermore, in order to prevent a decrease in ultra-low temperature impact toughness in the weld, the fraction of carbides contained in the heat-affected zone (HAZ) can be limited to 3 area% or less. However, since the austenitic steel material of the present invention contains one or more elements selected from titanium (Ti), niobium (Nb), and vanadium (V), which are carbide-forming elements, it is possible to exclude 0% from the carbide fraction in the heat-affected zone (HAZ) of the weld.

[0036] The average austenite grain size in the heat-affected zone (HAZ) of the weld can be within the range of 5 to 200 μm. If the average austenite grain size in the heat-affected zone is excessively small, although the strength of the weld will improve, a localized decrease in ultra-low temperature impact toughness may occur in the heat-affected zone. Therefore, the austenitic steel material of the present invention can limit the average austenite grain size in the heat-affected zone to 5 μm or more. On the other hand, while a larger mean austenite grain size in the heat-affected zone (HAZ) is advantageous for ensuring the ultra-low temperature impact toughness of the weld, it may cause a localized decrease in strength in the HAZ. Therefore, in the present invention, the mean austenite grain size in the heat-affected zone can be limited to 200 μm or less.

[0037] In terms of ensuring the physical properties of the heat-affected zone (HAZ) during welding, not only the fraction of austenite and the 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 heat-affected zone is excessively small, it is advantageous in terms of ensuring the ultra-low temperature impact toughness of the heat-affected zone, but disadvantageous in terms of ensuring the strength of the heat-affected zone. Therefore, the present invention can limit the average aspect ratio of the austenite grains present in the heat-affected zone 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, but disadvantageous in terms of ensuring the ultra-low temperature impact toughness of the heat-affected zone. Therefore, in this invention, the aspect ratio of the average grain size of austenite present in the heat-affected zone can be limited to a level of 5.0 or less.

[0038] When welding is performed using the austenitic steel material of the present invention as the base material under normal welding conditions applied to welding structures for ultra-low temperature applications, the lateral expansion value in the heat-affected zone (HAZ) of the test specimen subjected to a -253°C standard Charpy impact test may be 0.32 mm or more.

[0039] 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 composition system presented in this invention, the lateral expansion value (mm) in the heat-affected zone (HAZ) of the weld is a more important factor from the viewpoint of ensuring the safety of the weld than the Charpy impact energy value (J) in the heat-affected zone (HAZ).

[0040] The lateral expansion value in the heat-affected zone (HAZ) of a 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) of a weld 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.

[0041] 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.

[0042] [Relationship 1] Lateral expansion value (mm) = 0.0088 × Charpy impact energy value (J) + 0.0893

[0043] When the austenitic steel material of the present invention is welded under normal welding conditions for welding cryogenic structures, the lateral expansion value in the heat-affected zone (HAZ) of a test specimen subjected to a -253°C Charpy impact test is at a level of 0.32 mm or higher. Therefore, when cryogenic structures are fabricated using this steel material, excellent structural safety can be ensured.

[0044] The method for producing austenitic steel according to the present invention will be described in more detail below.

[0045] The present invention relates to a method for producing austenitic steel, which may include the steps of: preparing a slab containing, 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 one or more of titanium (Ti), niobium (Nb), and vanadium (V): 0.5% or less (excluding 0%), with the remainder being iron (Fe) and unavoidable impurities; heating the slab and then hot-rolling it to provide a hot-rolled steel sheet; and heat-treating the hot-rolled steel sheet by heating it to a temperature range of 500-1000°C and holding it at that temperature range for a time of 1.3t (thickness of the hot-rolled steel sheet, mm) + 5 minutes or more.

[0046] In the above formula, [C] and [Mn] represent the carbon (C) and manganese (Mn) content (by weight %) contained in the slab.

[0047] Slab preparation and hot rolling 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 a steel slab having a thickness suitable for producing structural materials for low temperature or ultra-low temperature applications can be used.

[0048] After heating the prepared slab, it can be hot-rolled to the desired thickness. The heating temperature of the slab and the conditions for hot rolling are not particularly limited, but as an example, the slab can be heated in the temperature range of 1000 to 1300°C, and the finish rolling can be performed in the temperature range of 800 to 1100°C. The reduction ratio during hot rolling can be applied within an appropriate range depending on the desired plate thickness, and as an example, the final thickness of the hot-rolled steel sheet can be in the range of 6 mm or more.

[0049] heat treatment After hot rolling, the hot-rolled steel sheet can be subjected to a heat treatment in which it is heated to a temperature range of 500 to 1000°C and held at that temperature range for a time of 1.3t (thickness of the hot-rolled steel sheet, mm) + 5 minutes or more.

[0050] Heat treatment after hot rolling is performed not only to appropriately control the grain size and shape of the final austenite, but also to remove the internal deformation energy present in the steel. If the heat treatment temperature does not fall within a certain range, the internal deformation energy may not be sufficiently removed; therefore, in this invention, the lower limit of the heat treatment temperature can be limited to 500°C. If the heat treatment temperature is low, the reduction in dislocation density due to heat treatment is not sufficient, the formation of precipitates is insufficient, and it is difficult to secure the desired strength characteristics; therefore, the heat treatment temperature can be limited to 500°C. A preferred lower limit of the heat treatment temperature may be 600°C. On the other hand, if the heat treatment temperature exceeds a certain range, excessive growth of the final microstructure may become a problem; therefore, in this invention, the upper limit of the heat treatment temperature can be limited to 1000°C. A preferred upper limit of the heat treatment temperature may be 950°C.

[0051] On the other hand, if the heat treatment time is insufficient compared to the thickness of the steel, it may be inadequate to remove the internal deformation energy in the center of the steel. Therefore, to ensure sufficient heating of the center of the steel, heat treatment should be performed for at least 1.3t (thickness of the hot-rolled steel sheet, mm) + 5 minutes. [Examples]

[0052] The austenitic steel material and its manufacturing method of the present invention will be described in more detail below through specific examples. 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 present invention. The scope of the present invention can be determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0053] (Examples) After preparing 250 mm thick steel slabs with the alloy compositions shown in Table 1 below, test specimens were prepared 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. Comparative Examples 2 and 4 are cases where no heat treatment was performed.

[0054] [Table 1]

[0055] [Table 2]

[0056] 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; these results are also shown in Table 3. Subsequently, welding was performed on each example and comparative example using standard welding conditions applied to cryogenic structures, and the results are shown in Table 3. During this process, an optical microscope was used to observe the microstructure of the heat-affected zone, and the impact energy on the heat-affected zone was measured at -235°C using a Charpy impact tester. Additionally, the transverse expansion values ​​at the impact test fracture surface of each specimen were measured, and these results are also shown in Table 3.

[0057] [Table 3]

[0058] As shown in Tables 1 to 3, the examples that satisfy the alloy composition and process conditions limited by the present invention satisfy the target yield strength at room temperature 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 limited by the present invention do not satisfy one or more of the target yield strength at room temperature or lateral expansion value in the heat-affected zone (HAZ). On the other hand, in the case of Comparative Example 2, since cracks occurred in the rolled material, observation of the microstructure and evaluation of physical properties were omitted.

[0059] 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.

Claims

1. In weight percent, it contains 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 one or more of titanium (Ti), niobium (Nb), and vanadium (V): 0.5% or less (excluding 0%), with the remainder being iron (Fe) and unavoidable impurities. The yield strength at room temperature is 270 MPa or higher and less than 400 MPa. Including the heat-affected zone of the weld, An austenitic steel material characterized in that, when a Charpy impact test is performed on the heat-affected zone 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 heat-affected zone of the weld contains 95 area percent or more of austenite and 5 area percent or less (excluding 0 area percent) of grain boundary carbides as part of its microstructure.

3. 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.

4. 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.

5. 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.

6. The process involves preparing a slab containing, 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 one or more of titanium (Ti), niobium (Nb), and vanadium (V): 0.5% or less (excluding 0%), with the remainder being iron (Fe) and unavoidable impurities. The steps include heating the slab and then hot-rolling it to provide a hot-rolled steel sheet, A heat treatment step in which the hot-rolled steel sheet is heated to a temperature range of 500 to 1000°C and held at that temperature range for a time of 1.3t (thickness of the hot-rolled steel sheet, mm) + 5 minutes or more, The step of welding the heat-treated hot-rolled steel sheet, A method for producing an austenitic steel material according to claim 1, characterized by including the following: (In the above formula, [C] and [Mn] represent the carbon (C) and manganese (Mn) content (by weight) of the slab.)

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