Austenitic steel having excellent cryogenic toughness in weld heat-affected zone, and manufacturing method therefor

Abstract

According to one aspect of the present invention, an austenitic high-manganese steel having excellent cryogenic toughness in a weld heat-affected zone, and a manufacturing method therefor can be provided.

Description

Austenitic steel with excellent ultra-low temperature toughness in the weld heat-affected zone and method for manufacturing the same

[0001] The present invention relates to an austenitic steel and a method for manufacturing the same, and more specifically, to an austenitic high-manganese steel having excellent ultra-low temperature toughness in the weld heat-affected zone and a method for manufacturing the same.

[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, to store these gases, structures such as pressure vessels made of materials that possess sufficient toughness and strength at ultra-low temperatures are required.

[0003] Cr-Ni stainless alloys such as AISI 304, 9% Ni steel, or 5000 series aluminum alloys have been used as materials capable of being used at low temperatures in a liquefied gas atmosphere. However, the use of aluminum alloys is limited because their high alloy cost and low strength lead to increased design thickness of structures, and their weldability is poor. Although Cr-Ni stainless steel and 9% nickel (Ni) steel have significantly improved the physical properties of aluminum, they are not desirable from an economic perspective due to their high content of expensive nickel (Ni).

[0004] According to one embodiment of the present invention, an austenitic steel material and a method for manufacturing the same can be provided, which has excellent ultra-low temperature toughness in the weld heat-affected zone and can be used as a structural material for ultra-low temperature environments such as liquefied gas storage tanks and liquefied gas transport facilities.

[0005] The problems of the present invention are not limited to those described above. A person skilled in the art to which the present invention pertains will have no difficulty understanding additional problems of the present invention from the overall contents of this specification.

[0006] An austenitic steel according to one embodiment of the present invention comprises, in weight percent, manganese (Mn): 10.00~45.00%, carbon (C): in a range satisfying 24*[C]+[Mn]≥25.00 and 33.5*[C]-[Mn]≤18.00, and chromium (Cr): 0.50~10.00%, and the remainder being iron (Fe) and unavoidable impurities, satisfies the following equation 1, and may have a room temperature yield strength of 270 MPa or more and less than 400 MPa.

[0007] [Relationship 1] 2.00 x 10 17 ≤ A / B ≤ 50.00 x 10 17

[0008] Here, A is the potential density ( / m 3 ) means, and B means the area fraction (area %) of the microstructure having a KAM (Kernel Average Misorientation) value of 0 or more and 1 or less. Each unit is omitted.

[0009] The above-described austenitic steel may have an area fraction of a microstructure with a Kernel Average Misorientation (KAM) value of 0 or more and 1 or less, measured using EBSD, of 20 area% or more.

[0010] The above-described austenitic steel may contain 95 area% or more (including 100 area%) of austenite and 5 area% or less (including 0 area%) of grain boundary carbides in its microstructure.

[0011] The above-described austenitic steel can satisfy the following relationship 2.

[0012] [Relationship 2] 1.65 x 10 19≤ A / (B x C) ≤ 230 x 10 19

[0013] Here, A is the potential density ( / m 3 ...means ), B means the area fraction (area %) of the microstructure having a KAM (Kernel Average Misorientation) value of 0 or more and 1 or less, and C means the average grain size (μm) of austenite in the weld heat-affected zone of the austenitic steel. Each unit is omitted.

[0014] The weld heat-affected zone of the above-described austenitic steel may contain 95% or more (including 100% area) of austenite and 5% or less (including 0% area) of grain boundary carbides in its microstructure.

[0015] The average grain size of the weld heat-affected zone described above may be 5 to 200 μm.

[0016] The average grain aspect ratio of the weld heat-affected zone described above may be 1.0 to 5.0.

[0017] When a Charpy impact test is performed at -253℃ on the aforementioned weld heat-affected zone, the lateral expansion in the weld heat-affected zone may be 0.32 mm or more.

[0018] A method for manufacturing an austenitic steel according to another embodiment of the present invention comprises the steps of: preparing a slab comprising, in weight percent, manganese (Mn): 10.00~45.00%, carbon (C): in a range satisfying 24*[C]+[Mn]≥25.00 and 33.5*[C]-[Mn]≤18.00, and chromium (Cr): 0.50~10.00%, and the remainder being iron (Fe) and unavoidable impurities; providing a hot-rolled steel sheet by heating the slab and then hot-rolling it; and heating the hot-rolled steel sheet in a temperature range of 500~1000℃, wherein the holding time T in the heating step may satisfy the following equation 3.

[0019] [Relationship 3] (1.3t+5) min ≤ T ≤ (1.3t+80) min

[0020] Here, t represents the thickness (mm) of the hot-rolled steel sheet, and T represents the holding time (minutes) during the heating step.

[0021] The heating of the above-described slab can be carried out in a temperature range of 1000 to 1300°C, and the finishing rolling can be carried out in a temperature range of 800 to 1100°C.

[0022] The present invention can provide an austenitic steel material and a method for manufacturing the same, which is particularly suitable as a structural material for ultra-low temperature environments such as liquefied gas storage tanks and liquefied gas transport facilities due to its excellent ultra-low temperature toughness in the weld heat-affected zone.

[0023] FIG. 1 is a diagram illustrating the correlation between the carbon content and manganese content of an austenitic steel according to one aspect of the present invention.

[0024] FIG. 2 is a schematic diagram illustrating a method for measuring the lateral expansion value in the base material or weld heat-affected zone of an austenitic steel material according to one aspect of the present invention.

[0025] Figure 3 is a KAM analysis image of an austenitic steel according to one embodiment of the present invention.

[0026] Preferred embodiments of the present invention will be described below with reference to the attached drawings. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0027] In addition, embodiments of the present invention are provided to more fully explain the present invention to those with average knowledge in the relevant technical field.

[0028] In describing the embodiments of the present invention, if it is determined that a detailed description of known technology related to the present invention may unnecessarily obscure the essence of the present invention, such detailed description will be omitted. Furthermore, the terms described below are defined considering their functions in the present invention, and these may vary depending on the intentions or conventions of the user or operator. Therefore, such definitions should be based on the content throughout this specification. The terms used in the detailed description are merely for describing the embodiments of the present invention and should not be limited in any way. Unless explicitly stated otherwise, expressions in the singular form include the meaning of the plural form.

[0029] In this description, expressions such as “include” or “equipped” are intended to refer to certain characteristics, numbers, steps, actions, elements, parts or combinations thereof, and should not be interpreted to exclude the existence or possibility of one or more other characteristics, numbers, steps, actions, elements, parts or combinations thereof other than those described.

[0030] Unless otherwise specifically defined in the specification of the present invention, % units mean weight %.

[0031] The present invention will be described in detail below through each embodiment or example of the invention. It should be noted that each embodiment or example described in this specification is not limited to a single embodiment or example, but may also be combined with other embodiments or examples. Accordingly, the citation of claims in the patent claims is merely an example of an embodiment, and the technical concept of the present invention should not be interpreted as being limited only to a combination with the cited claims; rather, combinations with various claims are also included within the scope of the technical concept of the present invention.

[0032] First, the alloy composition of an austenitic steel according to one aspect of the present invention will be described in detail.

[0033] An austenitic steel according to one aspect of the present invention may comprise manganese (Mn): 10.00~45.00%, carbon (C): in a range satisfying 24*[C]+[Mn]≥25.00 and 33.5*[C]-[Mn]≤18.00, and chromium (Cr): 10.00% or less, and the remainder may consist of iron (Fe) and unavoidable impurities.

[0034] The content ranges and reasons for limitation of each element are explained in detail below. Unless otherwise specifically indicated, the % representing the content of each element is based on weight.

[0035] Manganese (Mn): 10.00~45.00%

[0036] Manganese is an element that plays an important role in stabilizing austenite. To stabilize austenite at ultra-low temperatures, it is desirable to include at least 10.00% manganese (Mn). If the manganese (Mn) content falls short of this, the metastable phase epsilon martensite is formed and easily transforms into alpha martensite through work-induced transformation at ultra-low temperatures, making it difficult to secure ultra-low temperature toughness. Although there is a method to stabilize austenite by increasing the carbon (C) content to suppress the formation of epsilon martensite, this may instead lead to the precipitation of a large amount of carbides, causing a rapid deterioration of physical properties. Therefore, a manganese (Mn) content of at least 10.00% is desirable. A desirable manganese (Mn) content may be at least 15.00%, and a more desirable manganese (Mn) content may be at least 18.00%. If the manganese (Mn) content is excessive, it can reduce the corrosion rate of the steel and is also undesirable in terms of economic feasibility. Therefore, the manganese (Mn) content is preferably 45.00% or less. The desirable manganese (Mn) content may be 40.00% or less, and the more desirable manganese (Mn) content may be 35.00% or less.

[0037] Carbon (C): Range satisfying 24*[C]+[Mn]≥25.00 and 33.5*[C]-[Mn]≤18.00

[0038] Carbon (C) is an element that stabilizes austenite and increases strength. In particular, carbon (C) plays a role in lowering the transformation points Ms or Md, which are the transition points from austenite to epsilon or alpha martensite, during processes such as cooling or processing. Therefore, carbon (C) is a component that effectively contributes to the stabilization of austenite; if the carbon (C) content is insufficient, the stability of the austenite is lacking, making it impossible to obtain austenite that is stable at ultra-low temperatures. Furthermore, external stress can easily induce work-induced transformations into epsilon or alpha martensite, which may reduce the toughness of the steel or lower its strength. On the other hand, if the carbon (C) content is excessive, the toughness of the steel may rapidly deteriorate due to carbide precipitation, and the strength may increase excessively, leading to reduced workability.

[0039] The inventor of the present invention conducted an in-depth study on the relative behavior between carbon (C) and manganese (Mn) content in relation to carbide formation, and as a result, reached the conclusion that determining the relative content relationship between carbon (C) and manganese (Mn), as shown in Fig. 1, can effectively promote the stabilization of austenite while effectively controlling the amount of carbide precipitation. Although carbides are formed by carbon (C), carbon (C) does not independently influence the formation of carbides, but rather interacts in combination with manganese (Mn) to influence the formation of carbides.

[0040] In order to promote the stabilization of austenite, it is desirable to control the value of 24*[C]+[Mn] (where [C] and [Mn] represent the content of each component in weight% units) to 25.00 or higher, provided that other components satisfy the range specified in the present invention. The corresponding boundary refers to the inclined left boundary of the parallelogram region shown in FIG. 1. If 24*[C]+[Mn] is less than 25.00, the stability of the austenite decreases, causing work-induced transformation due to impact at ultra-low temperatures, and consequently, the impact toughness of the steel may decrease. On the other hand, to suppress the formation of carbides, it is desirable to control the value of 33.5*[C]-[Mn] (where [C] and [Mn] represent the content of each component in weight% units) to 18.00 or lower, provided that other components satisfy the range specified in the present invention. If 33.5*[C]-[Mn] exceeds 18.00, carbides may precipitate due to the addition of excessive carbon (C), which may reduce the low-temperature impact toughness of the steel. Therefore, in the present invention, it is preferable to add carbon (C) such that 24*[C]+[Mn] ≥ 25.00 and 33.5*[C]-[Mn] ≤ 18.00. As can be seen in FIG. 1, the minimum limit of the carbon (C) content within the range satisfying the aforementioned formula is 0%.

[0041] Chrome (Cr): 0.50~10.00%

[0042] Chromium (Cr) is also an austenite-stabilizing element; within an appropriate amount, it stabilizes the austenite to improve the low-temperature impact toughness of the steel and acts as a solid solution within the austenite to increase the strength of the steel. In addition, chromium (Cr) is a component that effectively contributes to improving the corrosion resistance of the steel. Therefore, the present invention may add chromium (Cr) at a level of 0.50% or more. The lower limit of the preferred chromium (Cr) content may be 1.00%, and the lower limit of the more preferred chromium (Cr) content may be 2.00%. However, chromium (Cr) is a carbide-forming element and, in particular, can form carbides at the austenite grain boundaries, thereby reducing the low-temperature impact toughness of the steel. Furthermore, if the amount of chromium (Cr) added exceeds a certain level, excessive carbides may precipitate in the heat-affected zone (HAZ) of the weld, which may result in a decline in ultra-low temperature toughness. Accordingly, the upper limit of chromium (Cr) in the present invention may be limited to 10.00%. The upper limit of the preferred chromium (Cr) content may be 8.00%, and the upper limit of the more preferred chromium (Cr) content may be 7.00%.

[0043] An austenitic steel according to one aspect of the present invention may contain the remaining Fe and other unavoidable impurities in addition to the aforementioned components. However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the normal manufacturing process, they cannot be completely excluded. Since these impurities are known to anyone with ordinary knowledge in the art, all details thereof are not specifically mentioned in this specification. Furthermore, the addition of effective components other than the aforementioned components is not completely excluded.

[0044] An austenitic steel according to one aspect of the present invention may contain 95 area% or more of austenite in its microstructure to secure desired physical properties. A preferred austenite fraction may be 97 area% or more, and may include cases where the austenite fraction approaches 100 area%. Meanwhile, an austenitic steel according to one aspect of the present invention may actively suppress the carbide fraction to 5 area% or less to prevent a decrease in ultra-low temperature impact toughness. A preferred carbide fraction may be 3 area% or less, and may include cases where the carbide fraction is 0 area%. The method for measuring the austenite fraction and the carbide fraction in the present invention is not particularly limited and can be easily verified through measurement methods commonly used by a person skilled in the art to which the present invention belongs for measuring microstructure and carbides.

[0045] In one aspect of the present invention, an austenitic steel may have an area fraction of a microstructure having a Kernel Average Misorientation (KAM) value of 0 or more and 1 or less, measured using EBSD, of 20 area% or more. At this time, a person skilled in the art to which the present invention belongs can measure the KAM value through EBSD without any particular technical difficulty.

[0046] A portion with a KAM value of 0 or more and 1 or less can be considered as a recrystallized structure, and a portion with a value exceeding 1 can be considered as an unrecrystallized structure. Therefore, if the area fraction of the portion with a value of 0 or more and 1 or less is less than 20 area%, the unrecrystallized structure fraction is excessive, which may exceed the upper limit of the yield strength required by the present invention. In such cases where the yield strength becomes excessively high, a problem may arise in which the low-temperature impact toughness intended by the present invention cannot be secured.

[0047] Meanwhile, an austenitic steel according to one embodiment of the present invention can satisfy the following relationship 1.

[0048] [Relationship 1] 2.00 x 10 17 ≤ A / B ≤ 50.00 x 10 17

[0049] Here, A is the potential density ( / m 3 ) means, and B means the area fraction (area %) of the microstructure having a KAM (Kernel Average Misorientation) value of 0 or more and 1 or less. Each unit is omitted.

[0050] In other words, the inventors realized through repeated experiments that when controlling the relative relationship between the dislocation density of the steel and the area fraction of the microstructure in which the KAM value is 0 or greater and 1 or less, excellent low-temperature toughness can be secured along with appropriate strength.

[0051] To explain in detail, the above A / B value is 2.00 x 10 17 If it is less than, it may be difficult to secure the desired strength because the dislocation density is too low or the recrystallization structure is excessive. On the other hand, the above A / B value is 50.00 x 10 17 If exceeded, the dislocation density may be too high or the unrecrystallized structure excessive, which can lead to a rapid increase in strength and, as a result, degrade low-temperature impact toughness. As another example, the above A / B value is 2.50 x 10 17 More than 40.00 x 10 17 It may be less than.

[0052] Meanwhile, the inventors have discovered that, along with the dislocation density and the area fraction of the microstructure having a KAM value of 0 or more and 1 or less as described above, the grain size of the heat-affected zone is also an important factor in securing the desired physical properties.

[0053] Specifically, an austenitic steel according to one example of the present invention can satisfy the following relationship 2.

[0054] [Relationship 2] 1.65 x 10 19 ≤ A / (B x C) ≤ 230 x 10 19

[0055] Here, A is the potential density ( / m 3 ...means ), B means the area fraction (area %) of the microstructure having a KAM (Kernel Average Misorientation) value of 0 or more and 1 or less, and C means the average grain size (μm) of austenite in the weld heat-affected zone of the austenitic steel. Each unit is omitted.

[0056] If the above A / (B x C) value is 1.65 x 10 19 If it is less than, it may be difficult to ensure strength, and 230 x 10 19 If it exceeds a certain amount, impact toughness may deteriorate. As another example, the value of A / (B x C) derived from the above Equation 2 is 2.00 x 10 19 Up to 200 x 10 19 It could be.

[0057] At this time, the dislocation density of the steel can be measured by using X-ray diffraction to measure the intensity according to a specific surface of the steel and then using the Williamson-Hall method, and a person skilled in the art to which the present invention belongs can measure the dislocation density of the steel without any special technical difficulty.

[0058] According to one aspect of the present invention, the room temperature yield strength of the austenitic steel can satisfy a range of 270 MPa or more and less than 400 MPa. When the strength of the steel increases, the low-temperature impact toughness decreases; in particular, for steel intended for ultra-low temperature applications such as -253°C as in the present invention, if the yield strength is excessively high, there is a high possibility that the desired low-temperature impact toughness cannot be secured. Furthermore, since it is difficult for commonly used austenitic welding materials to exceed the strength of the base material, maintaining a high strength of the base material may result in a strength difference between the weldment and the base material, thereby reducing structural stability. Therefore, it is preferable that the room temperature yield strength of the austenitic steel according to one aspect of the present invention be at a level of less than 400 MPa. Meanwhile, if the room temperature yield strength of the steel is excessively low, the thickness of the base material is excessively increased to ensure the stability of the structure, and consequently, the weight of the structure may increase excessively; therefore, the austenitic steel according to one aspect of the present invention may limit the lower limit of the room temperature yield strength to 270 MPa.

[0059] Since structures are generally provided by processing and welding steel, even if the ultra-low temperature impact toughness of the base material itself is secured, the safety of the structure itself may be significantly reduced if the ultra-low temperature impact toughness of the weldment is not secured. Accordingly, the austenitic steel according to one aspect of the present invention aims to secure 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). Accordingly, the present invention can control the microstructure of the base material as well as the microstructure fraction and shape of the weld heat-affected zone within a specific range.

[0060] Throughout the present invention, the weld heat-affected zone may refer to the area surrounding the weld in a welded member obtained when steel is welded with a flux-cored arc welding wire under normal conditions.

[0061] In an austenitic steel material according to one embodiment of the present invention, the weld heat-affected zone may contain 95 area% or more (including 100 area%) of austenite and 5 area% or less (including 0 area%) of grain boundary carbides in its microstructure.

[0062] As previously explained regarding the microstructure of the base material, the area fraction of austenite included in the heat-affected zone (HAZ) may be 97 area% or more, and may include cases where the area fraction of austenite is 100 area%. In addition, to prevent a decrease in ultra-low temperature impact toughness in the weldment, the area fraction of carbides included in the heat-affected zone (HAZ) may be limited to 3 area% or less, and may include cases where the area fraction of carbides in the heat-affected zone (HAZ) is 0 area%.

[0063] As an example, the average grain size of the weld heat-affected zone may be 5 to 200 μm.

[0064] When the average austenite grain size in the heat-affected zone (HAZ) is excessively small, the strength of the weldment is improved, but localized reduction in ultra-low temperature impact toughness may occur in the heat-affected zone. Accordingly, an austenitic steel according to one aspect of the present invention may limit the average austenite grain size in the heat-affected zone to 5 μm or more. On the other hand, while increasing the average austenite grain size in the heat-affected zone is advantageous for securing ultra-low temperature impact toughness in the weldment, localized reduction in strength may occur in the heat-affected zone; therefore, the present invention may limit the average austenite grain size in the heat-affected zone to 200 μm or less.

[0065] In terms of securing material properties in the heat-affected zone (HAZ), the average aspect ratio of the austenite grains is a factor that influences the above-mentioned austenite area fraction and average grain size. When the average aspect ratio of the austenite present in the heat-affected zone (HAZ) is excessively small, it is advantageous in terms of securing ultra-low temperature impact toughness in the heat-affected zone (HAZ), but disadvantageous in terms of securing strength in the heat-affected zone (HAZ). Therefore, the present invention can limit the average aspect ratio of the austenite present in the heat-affected zone (HAZ) to a level of 1.0 or higher. On the other hand, when the average grain aspect ratio of austenite present in the heat-affected zone (HAZ) is excessively large, it is advantageous in terms of securing strength of the heat-affected zone (HAZ), but disadvantageous in terms of securing ultra-low temperature impact toughness of the heat-affected zone (HAZ). Therefore, the present invention can limit the average grain aspect ratio of austenite present in the heat-affected zone (HAZ) to a level of 5.0 or less.

[0066] When a Charpy impact test is performed on the above-mentioned weld heat-affected zone at -253℃, the lateral expansion in the above-mentioned weld heat-affected zone may be 0.32 mm or more.

[0067] The inventors of the present invention have identified that plastic deformation characteristics are a key factor in ensuring safety for steel materials applied in ultra-low temperature environments. That is, after in-depth research, the inventors of the present invention were able to confirm that for steel materials satisfying the compositional system presented in the present invention, the lateral expansion value (mm) in the heat-affected zone (HAZ) is a more important factor than the Charpy impact energy value (J) in the heat-affected zone in terms of ensuring the safety of the weld.

[0068] The transverse expansion value in the weld heat-affected zone (HAZ) refers to the average value of the transverse plastic deformation of a specimen subjected to a Charpy impact test at -253°C. Figure 2 shows a photograph of a specimen subjected to a Charpy impact test at -253°C, and as shown in Figure 2, the transverse expansion value can be calculated by calculating the transverse length increase (△X1+△X2) near the fracture surface. If the transverse expansion value in the weld heat-affected zone (HAZ) is 0.32 mm or greater, it can be determined that the minimum low-temperature safety required for ultra-low temperature structures is provided.

[0069] According to the results of the inventor's research, it was confirmed that the Charpy impact energy (J) at -253℃ and the lateral expansion value (mm) of the specimen generally show a trend similar to the relationship 4 below, indicating that the larger the lateral expansion value (mm), the better the low-temperature impact toughness.

[0070] [Relationship 4]

[0071] Lateral expansion value (mm) = 0.0088 * Charpy impact energy value (J) + 0.0893

[0072] According to one aspect of the present invention, when an austenitic steel is welded under normal welding conditions for welding a cryogenic structure using the steel as a base material, the lateral expansion value in the heat-affected zone (HAZ) of a specimen subjected to a Charpy impact test at -253°C is 0.32 mm or higher, so excellent structural safety can be secured when a cryogenic structure is manufactured using the steel.

[0073] Hereinafter, a method for manufacturing an austenitic steel according to one aspect of the present invention will be described in more detail.

[0074] A method for manufacturing an austenitic steel according to one aspect of the present invention may comprise the steps of: preparing a slab comprising, in weight percent, manganese (Mn): 10.00~45.00%, carbon (C): in a range satisfying 24*[C]+[Mn]≥25.00 and 33.5*[C]-[Mn]≤18.00, and chromium (Cr): 0.50~10.00%, and the remainder being iron (Fe) and unavoidable impurities; heating the slab and then hot-rolling it to provide a hot-rolled steel sheet; and heating the hot-rolled steel sheet in a temperature range of 500~1000℃.

[0075] Below, each step is explained in detail.

[0076] Slab preparation and hot rolling

[0077] The present invention can prepare a steel slab having a predetermined alloy composition. Since the steel slab of the present invention has a steel composition corresponding to the aforementioned austenitic steel, the description of the alloy composition of the steel slab is replaced by the description of the steel composition of the aforementioned austenitic steel. The thickness of the steel slab is also not particularly limited, and a steel slab having a thickness suitable for manufacturing structural materials for low or ultra-low temperatures may be used.

[0078] After heating the prepared slab, it can be hot-rolled into steel with a desired thickness. Although the heating temperature of the slab and the hot-rolling conditions are not specifically limited, as a non-limiting example, the heating of the slab may be carried out in a temperature range of 1000 to 1300°C, and the finishing rolling may be carried out in a temperature range of 800 to 1100°C. During hot-rolling, the reduction rate may be applied within an appropriate range depending on the desired plate thickness, and as a non-limiting example, the final thickness of the hot-rolled steel sheet may satisfy a range of 6 mm or more.

[0079] heat treatment

[0080] After hot rolling, the hot-rolled steel sheet can be heated to a temperature range of 500 to 1000°C.

[0081] Heat treatment after hot rolling not only appropriately controls the grain size and shape of the final austenite, but also removes internal strain energy present in the steel, thereby increasing the region with a low KAM value. Since internal strain energy is not sufficiently removed and the area fraction of the region with a low KAM value may decrease when the heat treatment temperature is below 500°C, the present invention may limit the lower limit of the heat treatment temperature to 500°C. A more preferable lower limit of the heat treatment temperature may be 600°C. Meanwhile, since excessive growth of the final microstructure may become a problem when the heat treatment temperature exceeds a certain range, the present invention may limit the upper limit of the heat treatment temperature to 1000°C. A preferred upper limit of the heat treatment temperature may be 950°C.

[0082] Meanwhile, the holding time T in the heating step above can satisfy the following relationship 3.

[0083] [Relationship 3] (1.3t+5) min ≤ T ≤ (1.3t+80) min

[0084] Here, t represents the thickness (mm) of the hot-rolled steel sheet, and T represents the holding time (minutes) during the heating step.

[0085] If the heat treatment time is insufficient relative to the thickness of the steel, the removal of internal deformation energy in the center of the steel may be insufficient; therefore, the present invention may perform heat treatment for a time of 1.3t (thickness of hot-rolled steel plate, mm) + 5 minutes or more to ensure sufficient heat treatment in the center of the steel. However, if the holding time exceeds 1.3t (thickness of hot-rolled steel plate, mm) + 80 minutes, the strength of the steel may decrease due to grain coarsening; therefore, the present invention may set the upper limit of the holding time to 1.3t (thickness of hot-rolled steel plate, mm) + 80 minutes. As another example, the holding time T may be (1.3t + 5) minutes or more and (1.3t + 70) minutes or less.

[0086] The steel of the present invention manufactured according to the above description can be welded under normal welding conditions used for welding structures for ultra-low temperature applications.

[0087] As an example, welding can be performed using a slit arc welding rod, flux-cored arc welding wire, TIG welding rod and wire, submerged arc welding wire and flux, etc., with an austenitic steel material according to one aspect of the present invention manufactured as described above as a base material.

[0088] Each welding method and heat input can be selected according to normal conditions, taking into account the thickness of the steel plate, etc.

[0089] The present invention will be described in detail below through examples. However, it should be noted that the examples described below are intended merely to illustrate and embody the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0090] After preparing a steel slab with a thickness of 250 mm with the alloy composition of Table 1 below, each specimen was fabricated by applying the process conditions listed in Table 2 below. Each steel slab contains iron (Fe) and other unavoidable impurities in addition to the alloy components listed in Table 1. The thickness (mm) of the specimens obtained according to the above is listed in Table 2 below.

[0091] Steel Grade Alloy Composition (wt%) Mn24*[C]+[Mn]33.5*[C]-[Mn]CrA36.8143.77-27.102.36B32.6538.89-23.941.32C25.0933.73-13.033.11D18.9425.42-9.906.18E23.4831.40- 12.432.94F24.2032.36-12.813.23G24.2032.84-12.142.51H33.6039.60-25.2 311.70I17.2023.44-8.490.80J15.3044.3425.241.12K25.0633.73-13.033.11

[0092] Classification Steel Grade Slab Heating Temperature (°C) Finish Rolling Temperature Temperature (°C) Steel Thickness (mm) Heat Treatment Temperature (°C) Heat Treatment Time (min) Example 1A 1190 883 329 4153 Example 2B 119 290 824 785 41 Example 3C 118 59 15 1390 346 Example 4D 120 188 2269 20 45 Example 5E 117 88 84 228 48 34 Comparative Example 1F 118 59 23 1210 20 25 Comparative Example 2G 13 20 87 225-- Comparative Example 3H 119 39 253 58 65 55 Comparative Example 4I 1200 865 185 28 48 Comparative Example 5J 118 98 57 24 4 12 42 Comparative Example 6K118591513918105

[0093] Then, the microstructure and physical properties of the manufactured steel were measured and are shown in Table 3 below.

[0094] Each measurement method is as follows.

[0095] (Microstructure area fraction)

[0096] The above microstructure types and fractions were measured by etching a specimen taken from the steel at 1 / 4 of its thickness and observing it with an optical microscope.

[0097] (Area fraction of the microstructure having a KAM value of 0 to 1)

[0098] The KAM value was measured using EBSD in a 450 μm x 450 μm field of view on a cross-sectional plane parallel to the rolling direction at the 1 / 4 thickness point of a specimen taken from the steel. During this measurement, the average value of the orientation difference between each pixel within the grain and the adjacent pixel was calculated and expressed as the KAM value.

[0099] Next, the area fraction of the microstructure with the measured KAM value being between 0 and 1 was calculated and shown in Table 3 below.

[0100] (Potential density)

[0101] The dislocation density of the steel was measured by the Williamson-Hall method after measuring the strength along a cross-sectional plane parallel to the rolling direction of the steel using X-ray diffraction on a specimen taken at 1 / 4 of the thickness of the X-ray specimen.

[0102] (Yield strength)

[0103] Among the mechanical properties, the room temperature yield strength was measured using a tensile testing machine according to the ASTM E8M test method.

[0104] Classification Steel Type Base Material Microstructure (Area %) Room Temperature Yield Strength (MPa) KAM Value (%) Dislocation Density (x10 15 / m 3 )Dislocation density / KAM area fraction(x10 17 / m 3Example 1A Austenite 100% 31 29 2.6 2.5 4 2.74 Example 2B Austenite 100% 33 29 3.2 2.8 3 3.04 Example 3C Austenite 100% 29 79 9.6 2.6 2 2.63 Example 4D Austenite 100% 37 8 74.1 2.6 0 3.51 Example 5E Austenite 100% 29 19 9.7 2.7 1 2.72 Comparative Example 1F Austenite 100% 26 59 9.9 2.3 1 2.31 Comparative Example 2G Rolled material crack---- Comparative Example 3H Austenite 91%, Carbide 9% 38 47 8.0 2.5 3 3.24 Comparative Example 4I Austenite 77%, Epsilon Martensite 23% 3659 2.6 3.10 3.35 Comparative Example 5J Austenite 78%, Carbide 22% 478 11.8 3.6 73 1.10 Comparative Example 6K Austenite 100% 2539 9.9 1.25 1.25

[0105] Subsequently, flux-cored arc welding was performed on the obtained steel under normal conditions, and the microstructural characteristics and mechanical properties of the weld heat-affected zone of the obtained welded member were measured and listed in Table 4 below. Each measurement method is as follows.

[0106] (Microstructure)

[0107] The types and fractions of the microstructure and the average aspect ratio of austenite in Table 4 below were measured by etching the specimen taken from the weld heat-affected zone and observing it with an optical microscope.

[0108] And for the collected specimens, the average grain size of the austenite was measured using EBSD based on a misorientation angle of 15° or more.

[0109] (Charpy impact energy at -253℃)

[0110] The specimens taken from the above-mentioned weld heat-affected zone were processed into V-notch specimens, and then three impact tests were performed at -253℃ using the test method of EN ISO 148-1, and the average of the values ​​is shown in Table 4 below.

[0111] (Transverse expansion value)

[0112] The average value of the transverse plastic deformation of specimens subjected to a Charpy impact test at -253°C in the weld heat-affected zone (HAZ) is shown as the transverse expansion value in Table 4 below.

[0113] More specifically, for a specimen subjected to a Charpy impact test at -253℃, the increase in lateral length (△X1+△X2) near the fracture surface was calculated and expressed as the lateral expansion value.

[0114] Separate Welding Heat-Affected Zone γ Fraction (Area %) γ Average Grain Size (㎛) γ Average Aspect Ratio Charpy Impact Energy (J) Transverse Expansion (mm) Dislocation Density / (KAM Area Fraction x Grain Size) (x10 19 / m 3 Example 1 Austenite 100% 48 1.01 138 1.46 5.71 Example 2 Austenite 100% 35 1.058 60.63 8.68 Example 3 Austenite 100% 50 1.061 38 1.45 5.26 Example 4 Austenite 100% 45 1.018 40.78 7.80 Example 5 Austenite 100% 42 1.071 121.08 6.47 Comparative Example 1 Austenite 100% 43 1.01 17 11.28 5.38 Comparative Example 2------ Comparative Example 3 Austenite 89%, Carbide 11% 32 1.11 10.00 10.14 Comparative Example 4 Austenite 73%, Epsilon martensite 27% 521.13 20.01 6.44 Comparative Example 5 Austenite 76%, Carbide 24% 381.09 10.008 1.85 Comparative Example 6 Austenite 100% 821.02 1621.64 1.53

[0115] In Comparative Example 1, the temperature during heat treatment after hot rolling exceeded the range of the present invention, so the desired level of strength could not be secured due to grain coarsening. In Comparative Example 2, cracks occurred during hot rolling due to the high slab heating temperature.

[0116] In Comparative Example 3, excessive carbides were precipitated due to the addition of excessive Cr. As a result, low-temperature impact toughness was reduced.

[0117] In Comparative Example 4, the 24*[C]+[Mn] value was less than 25.00, resulting in reduced austenite stability. Consequently, the area fraction of austenite was less than 95 area%, making it impossible to secure excellent low-temperature impact toughness in the weld heat-affected zone.

[0118] Comparative Example 5 had a 33.5*[C]-[Mn] value exceeding 18.00, resulting in excessive carbide formation and consequently poor low-temperature impact toughness after welding.

[0119] Finally, Comparative Example 6 did not satisfy Equations 1 and 2 because the holding time during heat treatment after hot rolling exceeded Equation 3 proposed in the present invention. As a result, it was difficult to provide a steel material having high strength and excellent low-temperature impact toughness.

[0120] On the other hand, Examples 1 to 5 satisfy both the alloy composition and the manufacturing method proposed in the present invention, so the room temperature yield strength is 270 MPa or more and less than 400 MPa, and when a Charpy impact test is performed at -253°C in the weld heat-affected zone, the Charpy impact energy is 50 J or more, and the lateral expansion in the weld heat-affected zone is 0.32 mm or more.

[0121] Figure 3 is a KAM analysis image of an austenitic steel according to one embodiment of the present invention. As shown in the figure, it can be confirmed that most of the microstructure of the austenitic steel of the present invention has a KAM value of 0 or higher and 1 or lower. As such, by including a microstructure having a KAM value of 0 or higher and 1 or lower in an area of ​​20% or more, the present invention can simultaneously secure an appropriate level of strength and excellent cryogenic impact toughness.

Claims

1. In weight percent, it comprises manganese (Mn): 10.00–45.00%, carbon (C): within the range satisfying 24*[C]+[Mn]≥25.00 and 33.5*[C]-[Mn]≤18.00, and chromium (Cr): 0.50–10.00%, and the remainder consists of iron (Fe) and unavoidable impurities, and Satisfying the following relation 1, and Austenitic steel with a room temperature yield strength of 270 MPa or more and less than 400 MPa. [Relationship 1] 2.00 x 10 17 ≤ A / B ≤ 50.00 x 10 17 Here, A is the potential density ( / m 3 ) means, and B means the area fraction (area %) of the microstructure having a KAM (Kernel Average Misorientation) value of 0 or more and 1 or less. Each unit is omitted.

2. In Paragraph 1, Austenitic steel having a microstructure area fraction of 20 area% or more with a Kernel Average Misorientation (KAM) value of 0 or more and 1 or less as measured by EBSD.

3. In Paragraph 1, Austenitic steel having a microstructure of 95 area% or more (including 100 area%) of austenite and 5 area% or less (including 0 area%) of grain boundary carbides.

4. In Paragraph 1, Austenitic steel satisfying the following relationship 2. [Relationship 2] 1.65 x 10 19 ≤ A / (B x C) ≤ 230 x 10 19 Here, A is the potential density ( / m 3 ...means ), B means the area fraction (area %) of the microstructure having a KAM (Kernel Average Misorientation) value of 0 or more and 1 or less, and C means the average grain size (μm) of austenite in the weld heat-affected zone of the austenitic steel. Each unit is omitted.

5. In Paragraph 1, The austenitic steel, wherein the weld heat-affected zone of the above austenitic steel comprises, in its microstructure, 95 area% or more (including 100 area%) of austenite and 5 area% or less (including 0 area%) of grain boundary carbides.

6. In Paragraph 1, An austenitic steel having an average grain size of 5 to 200 μm in the weld heat-affected zone of the above austenitic steel.

7. In Paragraph 1, The above austenitic steel has an average grain aspect ratio of 1.0 to 5.0 in the weld heat-affected zone.

8. In Paragraph 1, An austenitic steel having a lateral expansion of 0.32 mm or more in the weld heat-affected zone when a Charpy impact test is performed on the weld heat-affected zone of the austenitic steel.

9. A step of preparing a slab comprising, in weight percent, manganese (Mn): 10.00~45.00%, carbon (C): in a range satisfying 24*[C]+[Mn]≥25.00 and 33.5*[C]-[Mn]≤18.00, and chromium (Cr): 0.50~10.00%, and the remainder being iron (Fe) and unavoidable impurities; A step of providing a hot-rolled steel sheet by heating the above slab and then hot-rolling it; and The method includes the step of heating the hot-rolled steel sheet in a temperature range of 500 to 1000℃, and A method for manufacturing austenitic steel, wherein the holding time T in the heating step satisfies the following relationship 3. [Relationship 3] (1.3t+5) min ≤ T ≤ (1.3t+80) min Here, t represents the thickness (mm) of the hot-rolled steel sheet, and T represents the holding time (minutes) during the heating step.

10. In Paragraph 9, The heating of the above slab is carried out in a temperature range of 1000 to 1300℃, and A method for manufacturing austenitic steel, wherein finishing rolling is carried out in a temperature range of 800 to 1100°C.

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