Austenitic steel with excellent ultra-low temperature toughness of welded heat affected zone and manufacturing method thereof

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 having excellent ultra-low temperature toughness in a weld heat affected zone and a manufacturing method thereof, and more particularly to an austenitic high manganese steel material having excellent ultra-low temperature toughness in a weld heat affected zone that can be used as a structural material in an ultra-low temperature environment and a manufacturing method thereof. [Background technology]

[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 cryogenic storage. Therefore, in order to store these gases, structures such as pressure vessels made of materials that have sufficient toughness and strength at cryogenic temperatures are required.

[0003] Materials that can be used at low temperatures in liquefied gas atmospheres include Cr-Ni stainless steel alloys such as AISI304, 9% Ni steel, and 5000 series aluminum alloys. However, aluminum alloys are limited in their use because of their high alloy costs, low strength that increases the design thickness of structures, and poor weldability. Although Cr-Ni stainless steel and 9% nickel (Ni) steel have significantly improved the physical property problems of aluminum, they contain a large amount of expensive nickel (Ni), making them undesirable from an economic standpoint. Summary of the Invention [Problem to be solved by the invention]

[0004] An object of the present invention is to provide an austenitic steel material which 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.

[0005] The object of the present invention is not limited to the above-mentioned contents, and a person skilled in the art will have no difficulty in understanding further object of the present invention from the overall contents of this specification. [Means for solving the problem]

[0006] The austenitic steel material of the present invention contains, in weight percent, manganese (Mn): 10-45%, carbon (C): in the 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 remainder is iron (Fe) and unavoidable impurities, and when a Charpy impact test based on −253° C. is performed on the welded heat affected zone, the lateral expansion of the welded heat affected zone can be 0.32 mm or more. [C] and [Mn] in the above formula mean the content (weight percent) of carbon (C) and manganese (Mn) contained in the steel material.

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

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

[0009] The average grain size of the heat affected zone may be 5 to 200 μm.

[0010] The average grain aspect ratio of the welded heat affected zone may be 1.0 to 5.0.

[0011] The dislocation density of the above steel is 2.3×10 15 ~3.3×10 15 / mm 2 It could be.

[0012] The method for producing the austenitic steel material of the present invention includes the steps of preparing a slab containing, in weight percent, manganese (Mn): 10-45%, carbon (C): in the range satisfying 24×[C]+[Mn]≧25 and 33.5×[C]-[Mn]≦18, chromium (Cr): 10% or less (excluding 0%), at least one of titanium (Ti), niobium (Nb) and vanadium (V): 0.5% or less (excluding 0%), and the remainder being iron (Fe) and unavoidable impurities, heating the slab and hot rolling it to obtain a hot-rolled steel sheet, and heating the hot-rolled steel sheet to a temperature range of 500-1000° C. and maintaining the temperature range for a time of 1.3t (thickness of the hot-rolled steel sheet, mm)+5 minutes or more. [C] and [Mn] in the above formulas refer to the contents (weight percent) of carbon (C) and manganese (Mn) contained in the slab.

[0013] The above solutions to the problems described above are not intended to be exhaustive, and the various features of the present invention and their associated advantages and benefits can be understood in more detail with reference to the following specific implementations and examples. Effect of the Invention

[0014] According to the present invention, it is possible to provide an austenitic steel material which has excellent ultra-low temperature toughness in the weld heat affected zone and is particularly suitable 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.

[0015] The effects of the present invention are not limited to the above-mentioned matters, and can be interpreted as a concept including effects that can be reasonably inferred from the matters described in this specification. [Brief description of the drawings]

[0016] [Figure 1] FIG. 2 is a diagram showing the correlation between the carbon content and the manganese content of the austenitic steel material of the present invention. [Diagram 2] FIG. 2 is a schematic diagram showing a method for measuring the transverse expansion value in a weld heat affected zone of an austenitic steel material according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] The present invention relates to an austenitic steel material and a manufacturing method thereof, and preferred embodiments of the present invention will be described below. The embodiments of the present invention 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. The embodiments are provided to explain the present invention in more detail to those skilled in the art to which the present invention pertains.

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

[0019] 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 the weld heat affected zone, the lateral expansion in the weld heat affected zone satisfies 0.32 mm or more.

[0020] The steel composition contained in the austenitic steel material of the present invention will be described in more detail below. Unless otherwise specified, "%" indicating the content of each element is based on weight.

[0021] Manganese (Mn): 10-45% Manganese is an element that plays an important role in stabilizing austenite. In order to stabilize austenite at ultra-low temperatures, it is preferable that manganese (Mn) is contained in an amount of 10% or more. If the manganese (Mn) content is less than this amount, epsilon martensite, which is a metastable phase, is formed and easily transformed into alpha martensite by processing-induced transformation at ultra-low temperatures, making it impossible to ensure toughness. There is a method of increasing the carbon (C) content to stabilize austenite in order to suppress the formation of epsilon martensite, but in this case, a large amount of carbides may precipitate, and physical properties may rapidly deteriorate. Therefore, the manganese (Mn) content is preferably 10% or more. The preferred manganese (Mn) content may be 15% or more, and the more preferred 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 also undesirable from the viewpoint of economic efficiency. Therefore, the manganese (Mn) content is preferably 45% or less. A preferred manganese (Mn) content may be 40% or less, and a more preferred manganese (Mn) content may be 35% or less.

[0022] Carbon (C): 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 from austenite to epsilon or alpha martensite during the cooling process or processing. s Or M d Therefore, carbon (C) is an element that effectively contributes to the stabilization of austenite, and if the carbon (C) content is insufficient, the stability of austenite is insufficient, making it impossible to obtain stable austenite at ultra-low temperatures, and external stress can easily cause processing-induced transformation to epsilon or alpha martensite, reducing the toughness of the steel or decreasing the strength of the steel. On the other hand, if the carbon (C) content is excessive, the toughness of the steel can be rapidly deteriorated due to the precipitation of carbides, and the strength of the steel can be excessively increased, decreasing the workability.

[0023] The inventors of the present invention have conducted extensive research into 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 Figure 1, they have come to the conclusion that by determining the relative content relationship between carbon (C) and manganese (Mn), it is possible to effectively control the amount of carbide precipitation while effectively stabilizing austenite. Carbide is formed by carbon (C), but carbon (C) does not affect the formation of carbides independently, but acts in combination with manganese (Mn) to affect the formation of carbides.

[0024] In order to stabilize austenite, it is preferable to control the value of 24×[C]+[Mn] (where [C] and [Mn] mean the contents of each component in units of weight%) to 25 or more, on the premise that the other components satisfy the ranges specified in the present invention. The boundary means the inclined left boundary of the parallelogram region shown in FIG. 1. If 24×[C]+[Mn] is less than 25, the stability of austenite decreases, and deformation-induced transformation occurs due to impact at ultra-low temperatures, which may result in a decrease in the impact toughness of the steel. On the other hand, in order to suppress the formation of carbides, it is preferable to control the value of 33.5×[C]-[Mn] (where [C] and [Mn] mean the contents of each component in units of weight%) to 18 or less, on the premise that the other components satisfy the ranges specified in the present invention. If 33.5×[C]-[Mn] exceeds 18, the addition of excess carbon (C) may cause carbides to precipitate, resulting in a decrease in the low-temperature impact toughness of the steel. Therefore, in the present invention, carbon (C) is preferably added 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 that satisfies the above formula is 0%.

[0025] Chromium (Cr): 10% or less (excluding 0%) Chromium (Cr) is also an austenite stabilizing element, and up to an appropriate addition amount, it stabilizes austenite to improve the low-temperature impact toughness of steel, and dissolves in austenite to increase the strength of steel. Chromium (Cr) is also an element that effectively contributes to improving the corrosion resistance of steel. Therefore, in the present invention, chromium (Cr) is added as an essential element. A preferable lower limit of the chromium (Cr) content may be 1%, and a more preferable lower limit of the chromium (Cr) content may be 2%. However, chromium (Cr) is a carbide forming element, and may form carbides, particularly at the austenite grain boundaries, thereby reducing the low-temperature impact toughness of the steel. In addition, if the amount of chromium (Cr) added exceeds a certain level, excessive carbides may precipitate in the heat-affected zone (HAZ), resulting in poor ultra-low temperature toughness. Therefore, in the present invention, the upper limit of chromium (Cr) may be limited to 10%. A preferred upper limit for the chromium (Cr) content may be 8%, and a more preferred upper limit for the chromium (Cr) content may be 7%.

[0026] One or more of titanium (Ti), niobium (Nb) and vanadium (V): 0.5% or less (excluding 0%) Titanium (Ti) is an element that increases strength through its solid solution and precipitation hardening effects, and is particularly advantageous in that it can inhibit grain growth in the heat-affected zone by titanium carbonitrides, thereby preventing deterioration of strength. It is also an element that can improve strength by precipitation of carbonitrides during heat treatment. In the present invention, titanium (Ti) can be added for this effect. However, if titanium (Ti) is added in excess, coarse precipitates or crystallized particles are generated, which induce surface cracks during rolling and deteriorate the physical properties of the steel, so it is preferable to limit the content to a certain range.

[0027] Niobium (Nb) is an element that increases strength by its solid solution and precipitation hardening effects, and in particular, it increases the recrystallization stop temperature (Tnr) of steel, can improve yield strength by refining crystal grains during low-temperature rolling, and can improve strength by precipitation of carbonitrides during heat treatment. In the present invention, niobium (Nb) can be added for these effects.

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

[0029] However, when titanium (Ti), niobium (Nb) and vanadium (V) are added in excess, coarse precipitates are generated, which may deteriorate the physical properties of the steel material, and the generation of coarse precipitates or crystallized products may induce surface cracks during hot rolling, so in the present invention, the content of each of titanium (Ti), niobium (Nb) and vanadium (V) may be limited to 0.5% or less. Preferably, the total content of titanium (Ti), niobium (Nb) and vanadium (V) may be limited to 0.5% or less.

[0030] The austenitic steel material of the present invention comprises the remaining Fe and other inevitable impurities in addition to the above-mentioned components. However, in a normal manufacturing process, there is a possibility that unintended impurities are inevitably mixed in from the raw materials or the surrounding environment, and therefore it is not possible to completely exclude them. Since these impurities are known to anyone having ordinary knowledge in this technical field, the entire contents of the impurities are not specifically mentioned in this specification. Furthermore, the addition of further effective components in addition to the above-mentioned components is not completely excluded.

[0031] The austenitic steel of the present invention may contain 95 area % or more of austenite as a microstructure in order to secure the desired physical properties. The preferred austenite fraction may be 97 area % or more, and may include a case where the austenite fraction is close to 100 area %. Meanwhile, the austenitic steel of the present invention may actively suppress the carbide fraction to 5 area % or less in order to prevent a decrease in the ultra-low temperature impact toughness. The preferred carbide fraction may be 3 area % or less. However, since the austenitic steel of the present invention contains one or more selected from titanium (Ti), niobium (Nb) and vanadium (V) which are carbide forming elements, the lower limit of the carbide fraction may exclude 0%. In the present invention, the method of measuring the austenite fraction and the carbide fraction is not particularly limited, and they may be easily confirmed by a measurement method that is usually used by a general engineer in the technical field to which the present invention belongs to measure a microstructure and carbides.

[0032] The dislocation density of the austenitic steel of the present invention is 2.3 × 10 15 ~3.3×10 15 / mm 2 The dislocation density of steel can be measured by measuring the strength of a specific surface of the steel using X-ray diffraction, followed by the Williamson-Hall method, and a person skilled in the art to which the present invention pertains can measure the dislocation density of steel without any special technical difficulties. If the dislocation density of steel does not reach a certain level, it is impossible to ensure a strength suitable for use as a structural material. Therefore, in the present invention, the lower limit of the dislocation density of steel is set to 2.3×10 15 / mm 2 On the other hand, if the dislocation density is excessively high, it is advantageous in terms of ensuring the strength of the steel material, but is undesirable in terms of ensuring the ultra-low temperature toughness. Therefore, in the present invention, the upper limit of the dislocation density of the steel material is set to 3.3 × 10 15 / mm 2 can be limited to.

[0033] The room temperature yield strength of the austenitic steel of the present invention may be 270 MPa or more and less than 400 MPa. When the strength of the steel is high, the low temperature impact toughness decreases, and in particular, when the yield strength of the steel for ultra-low temperature applications of -253°C as in the present invention is too high, the desired impact toughness may not be ensured. In addition, since it is difficult for austenitic welding materials that are generally commercially available to exceed the strength of the base material, when the strength of the base material is maintained high, a strength difference occurs between the welded part and the base material, which may reduce structural stability. Therefore, the room temperature yield strength of the austenitic steel according to one aspect of the present invention is preferably at a level of less than 400 MPa. On the other hand, when the room temperature yield strength of the steel is too low, the thickness of the base material increases excessively 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 of the present invention may be limited to 270 MPa.

[0034] Since structures are generally produced by processing and welding steel materials, even if the cryogenic impact toughness of the base material itself is ensured, if the cryogenic impact toughness of the welded portion is not ensured, the safety of the structure itself may be significantly reduced. Therefore, the austenitic steel of the present invention aims to ensure the cryogenic impact toughness of not only the base material itself but also the heat-affected zone (HAZ) of the weld. Therefore, the present invention controls the fraction and shape of the microstructure of the base material as well as the microstructure of the heat-affected zone of the weld within a specific range.

[0035] When welding is performed under normal welding conditions for welding of structures for cryogenic temperatures using the austenitic steel material of the present invention as a base material and a covered electrode, flux cored arc welding wire, TIG welding electrode and wire, submerged arc welding wire and flux, etc., the heat-affected zone (HAZ) of the weld can contain 95 area % or more of austenite and 5 area % or less of carbides. As previously described with respect to the microstructure of the base material, the fraction of austenite contained in the heat-affected zone (HAZ) of the weld can be 97 area % or more, and can include cases where the fraction of austenite is close to 100 area %. In addition, in order to prevent a decrease in the cryogenic impact toughness of the weld, the fraction of carbides contained in the heat-affected zone (HAZ) of the weld can be limited to 3 area % or less. However, since the austenitic steel of the present invention contains one or more carbide-forming elements selected from titanium (Ti), niobium (Nb), and vanadium (V), the carbide fraction in the weld heat-affected zone (HAZ) can be excluded from 0%.

[0036] The average austenite grain size in the heat-affected zone (HAZ) can be in 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 is improved, but there is a possibility that the ultra-low temperature impact toughness of the heat-affected zone (HAZ) may be locally decreased. Therefore, the austenitic steel of the present invention can limit the average austenite grain size in the heat-affected zone (HAZ) to 5 μm or more. On the other hand, a larger average austenite grain size in the weld heat-affected zone (HAZ) is more advantageous in ensuring ultra-low temperature impact toughness of the weld. However, there is a possibility that localized strength reduction may occur in the weld heat-affected zone (HAZ). Therefore, in the present invention, the average austenite grain size in the weld heat-affected zone (HAZ) can be limited to 200 μm or less.

[0037] In terms of ensuring physical properties in a heat-affected zone (HAZ), not only the austenite fraction and average grain size but also the average aspect ratio of austenite grains are influential factors. If the average grain aspect ratio of austenite present in a heat-affected zone (HAZ) is excessively small, it is advantageous in terms of ensuring the ultra-low temperature impact toughness of the heat-affected zone (HAZ), but is disadvantageous in terms of ensuring the strength of the heat-affected zone (HAZ). Therefore, the present invention can limit the average grain aspect ratio of austenite present in a heat-affected zone (HAZ) to a level of 1.0 or more. On the other hand, if the aspect ratio of the average grain 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 is disadvantageous in terms of ensuring the ultra-low temperature impact toughness of the heat-affected zone (HAZ). Therefore, in the present invention, the aspect ratio of the average grain of austenite present in the heat-affected zone (HAZ) can be limited to a level of 5.0 or less.

[0038] When the austenitic steel material of the present invention is used as a base material and welded under normal welding conditions applied to the welding of ultra-low temperature structures, the lateral expansion value of the weld heat-affected zone (HAZ) of a test piece subjected to a -253°C standard Charpy impact test can be 0.32 mm or more.

[0039] The inventors of the present invention have found that in the case of steel materials applied to ultra-low temperature environments, plastic deformation characteristics are a major factor in terms of ensuring safety. That is, as a result of extensive research, the inventors of the present invention have confirmed that in the case of steel materials that satisfy the composition system presented in the present invention, the lateral expansion value (mm) in the heat-affected zone (HAZ) is a more important factor in terms of ensuring the safety of the weld than the Charpy impact energy value (J) of the heat-affected zone (HAZ).

[0040] The transverse expansion value in the weld heat-affected zone (HAZ) refers to the average value of the transverse plastic deformation of a test piece that underwent a -253℃ standard Charpy impact test. Figure 2 shows a photograph of a test piece that underwent a -253℃ standard Charpy impact test, and as shown in Figure 2, the transverse expansion value can be calculated by calculating the increase in transverse length (△X1+△X2) near the fracture surface. If the transverse expansion value in the weld heat-affected zone (HAZ) is 0.32mm or more, it can be determined that the minimum low-temperature safety required for ultra-low temperature structures is met.

[0041] According to the research results of the present inventors, it was confirmed that the -253°C standard Charpy impact energy (J) and the lateral expansion value (mm) of the test piece generally show a tendency similar to that of the following relational expression 1, and that the lateral expansion value is 0.32 mm or more. It was found that the larger the lateral expansion value (mm), the better the low-temperature impact toughness, and it is more effective that the lateral expansion value is 0.72 to 1.4 mm.

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

[0043] When the austenitic steel material of the present invention is used as a base material and welded under normal welding conditions applied to welding of structures for cryogenic use, the lateral expansion value in the weld heat-affected zone (HAZ) of a test piece subjected to a -253°C standard Charpy impact test is at a level of 0.32 mm or more. Therefore, when the steel material is used to manufacture a structure for cryogenic use, excellent structural safety can be ensured.

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

[0045] The method for producing an austenitic steel material of the present invention includes the steps of preparing a slab containing, by weight, 10-45% manganese (Mn), 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, heating the slab and hot rolling it to provide a hot-rolled steel sheet, and heating the hot-rolled steel sheet to a temperature range of 500-1000°C and maintaining the hot-rolled steel sheet in the 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] refer to the contents (wt%) of carbon (C) and manganese (Mn) 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 above-mentioned austenitic steel material, the description of the alloy composition of the steel slab is replaced with the description of the steel composition of the above-mentioned austenitic steel material. The thickness of the steel slab is not particularly limited, and a steel slab having a thickness suitable for producing a structural material for low temperature or ultra-low temperature use can be used.

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

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

[0050] The 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 material. If the heat treatment temperature is not within a certain range, the internal deformation energy may not be sufficiently removed, so in the present invention, the lower limit of the heat treatment temperature may be limited to 500°C. If the heat treatment temperature is low, the reduction in dislocation density by heat treatment is not sufficiently performed, the generation of precipitates is insufficient, and it is difficult to ensure the desired strength characteristics, so the heat treatment temperature may 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 structure may become a problem, so in the present invention, the upper limit of the heat treatment temperature may 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 not long enough compared to the thickness of the steel, it may not be sufficient to remove the internal deformation energy at the center of the steel. Therefore, in order to sufficiently heat the center of the steel, heat treatment should be performed for 1.3t (thickness of the hot-rolled steel plate, mm) + 5 minutes or more. EXAMPLES

[0052] Hereinafter, the austenitic steel material and the manufacturing method thereof according to the present invention will be described in more detail with reference to specific examples. It should be noted that the following examples are for the purpose of understanding the present invention and are not intended to specify 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] (Example) After preparing a steel slab having a thickness of 250 mm and having the alloy composition shown in Table 1 below, each test piece was manufactured under the process conditions shown in Table 2 below. Each steel slab contained iron (Fe) and other unavoidable impurities in addition to the alloy components shown in Table 1. Comparative Examples 2 and 4 are cases where no heat treatment was performed.

[0054] [Table 1]

[0055] [Table 2]

[0056] The microstructures of the examples and comparative examples listed in Table 2 were observed using an optical microscope, and the results are listed in Table 3 below. In addition, the dislocation density of each example and comparative example was measured using X-ray diffraction analysis, and the room temperature yield strength was measured using a tensile tester, and the results are also listed in Table 3. Thereafter, welding was performed for each example and comparative example using normal welding conditions applied to welding of ultra-low temperature structures, and the results are also listed in Table 3. At this time, an optical microscope was used to observe the microstructure of the welded heat affected zone, and the impact energy for the welded heat affected zone was measured using a Charpy impact tester at -235 ° C. In addition, the lateral expansion value at the impact test fracture surface of each test piece was measured, and the results are also listed in Table 3.

[0057] [Table 3]

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

[0059] Although the present invention has been described in detail with reference to the embodiments, other embodiments are possible and the scope of the claims set forth below should not be limited to the embodiments.

Claims

1. The alloy contains, by weight, 10 to 45% manganese (Mn), 24 x [C] + [Mn] ≥ 25 and 33.5 x [C] - [Mn] ≤ 18 carbon, 10% or less chromium (Cr) (excluding 0%), and 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; An austenitic steel material characterized in that, 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. (The [C] and [Mn] in the above formula mean the contents (wt%) of carbon (C) and manganese (Mn) contained in the steel material.)

2. 2. The austenitic steel material according to claim 1, characterized in that the room temperature yield strength of the steel material is 270 MPa or more and less than 400 MPa.

3. 2. The austenitic steel material according to claim 1, wherein the weld heat affected zone contains, as a microstructure, 95 area % or more of austenite and 5 area % or less (excluding 0 area %) of grain boundary carbides.

4. 2. The austenitic steel material according to claim 1, wherein the average crystal grain size of the welded heat affected zone is 5 to 200 μm.

5. 2. The austenitic steel according to claim 1, wherein the average grain aspect ratio of the welded heat affected zone is 1.0 to 5.

0.

6. The dislocation density of the steel is 2.3×10 １５ ~3.3 x 10 １５ / mm ２ 2. The austenitic steel material according to claim 1,

7. preparing a slab containing, by weight, manganese (Mn): 10-45%, carbon (C): in the 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%), 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 a heat treatment step of heating the hot-rolled steel sheet to a temperature range of 500 to 1000°C and holding the hot-rolled steel sheet in the temperature range for a time of 1.3t (thickness of the hot-rolled steel sheet, mm) + 5 minutes or more. (In the formula, [C] and [Mn] refer to the contents (wt%) of carbon (C) and manganese (Mn) contained in the slab.)

Images