Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C30/00—Alloys containing less than 50% by weight of each constituent
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite

Definitions

• the present disclosure relates to an austenitic steel material, and a manufacturing method therefor, and, more specifically, to an austenitic steel material and a method for manufacturing the same, the material preferably being applicable as a structural material used in a cryogenic environment such as that of a liquefied gas storage tank, liquefied gas transportation equipment, or the like.
• Liquefied gases such as liquefied hydrogen (boiling point: -253°C), liquefied natural gas (LNG, boiling point: - 164°C), liquefied oxygen (boiling point: -183°C), liquefied nitrogen (boiling point: -196°C) and the like require ultra-low temperature storage. Therefore, to store these gases, a structure such as a pressure vessel or the like made of a material with sufficient toughness and strength at ultra-low temperatures is required.
• Cr-Ni stainless steel alloys such as AISI 304 or the like, 9% Ni steel, 5000 series aluminum alloys, or the like have been used.
• the alloy cost is high, the design thickness of the structure increases due to low strength, and the weldability is also poor, so their use is limited.
• Cr-Ni stainless steel, 9% nickel (Ni) steel and the like have greatly improved the physical properties of aluminum, but it is not desirable from an economic perspective because it contains a large amount of expensive nickel (Ni).
• An aspect of the present disclosure is to provide an austenitic steel material and a method for manufacturing the same.
• a preferred aspect of the present disclosure is to provide an austenitic steel material having excellent ultra-low temperature toughness and a method for manufacturing the same.
• an austenitic steel material includes, in weight%, manganese (Mn): 10 to 45%, carbon (C): within a range satisfying 24 ⁇ [C]+[Mn] ⁇ 25 and 33.5 ⁇ [C]-[Mn] ⁇ 18, chromium (Cr): 10% or less (excluding 0%), and at least one of Ni: 5% or less (excluding 0%), Cu: 5% or less (excluding 0%), Si: 5% or less (excluding 0%), Al: 5% or less (excluding 0%), and N: 1% or less (excluding 0%), a remainder of iron (Fe) and unavoidable impurities, wherein a microstructure includes austenite of 95% or more (including 100%) and carbides of 5% or less (including 0%) in area%.
• the microstructure may be maintained after applying 20% strain thereto through a unilateral tensile test at room temperature, being maintained at -253°C, and then measured at room temperature.
• the austenite may have an average grain size of 5 to 200 ⁇ m.
• the austenitic steel material may have a transverse expansion of 0.32 mm or more after a Charpy impact test at - 253°C.
• the austenitic steel material may have a Charpy impact energy of 27 J or more at -253°C.
• the austenitic steel material may have a yield strength of 245 MPa or more and less than 400 MPa at room temperature.
• a method for manufacturing an austenitic steel material includes an operation of heating a slab at 1000 to 1300°C, the slab containing, in weight%, manganese (Mn): 10 to 45%, carbon (C): within a range of 24 ⁇ [C]+[Mn] ⁇ 25 and 33.5 ⁇ [C]-[Mn] ⁇ 18, chromium (Cr): 10% or less (excluding 0%), and at least one of , Ni: 5% or less (excluding 0%), Cu: 5% or less (excluding 0%), Si: 5% or less (excluding 0%), Al: 5% or less (excluding 0%), and N: 1% or less (excluding 0%), and a remainder of iron (Fe) and unavoidable impurities; and an operation of obtaining a hot-rolled steel sheet by finishing hot-rolling the heated slab at 800 to 1050°C.
• an austenitic steel material and a method for manufacturing the same may be provided.
• an austenitic steel material having excellent ultra-low temperature toughness and a method for manufacturing the same may be provided.
• an austenitic steel material according to an embodiment of the present disclosure will be described.
• the alloy composition will be described.
• the content of the alloy composition described below refers to weight % unless otherwise specified.
• Manganese is an element that plays an important role in stabilizing austenite. It is preferable to include 10% or more of manganese (Mn) to stabilize austenite at ultra-low temperatures. If the manganese (Mn) content is less than this, epsilon martensite that is a metastable phase is formed and may be easily transformed into alpha martensite by strain-induced transformation at ultra-low temperatures, so that toughness cannot be secured. There is a method to increase the carbon (C) content to stabilize austenite to suppress the formation of epsilon martensite, but in this case, a large amount of carbides may be precipitated, rapidly deteriorating the physical properties. Therefore, the manganese (Mn) content is preferable to be 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 also undesirable from an economical 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.
• Carbon (C) A range satisfying 24 ⁇ [C]+[Mn] ⁇ 25 and 33.5 ⁇ [C]-[Mn] ⁇ 18
• Carbon (C) is an element that stabilizes austenite and increases strength.
• carbon (C) plays a role in lowering Ms or Md, which is the transformation point from austenite to epsilon or alpha martensite, during the process of cooling, processing or the like. Therefore, carbon (C) is a component that effectively contributes to the stabilization of austenite. If the carbon (C) content is insufficient, the stability of austenite is insufficient, so stable austenite cannot be obtained at ultra-low temperatures, and external stress may easily cause a strain-induced transformation into epsilon or alpha martensite, which may reduce the toughness of the steel material or reduce the strength of the steel material. On the other hand, if the carbon (C) content is excessive, the toughness of the steel material may rapidly deteriorate due to carbide precipitation, and the strength of the steel material may excessively increase, thereby reducing the workability.
• the inventor of the present disclosure conducted an in-depth study on the relative behavior between the carbon (C) and manganese (Mn) contents in relation to the formation of carbides, and as a result, as illustrated in FIG. 1 , it was concluded that determining the relative content relationship between carbon (C) and manganese (Mn) may effectively promote the stabilization of austenite while effectively controlling the amount of carbide precipitation.
• Carbide is formed by carbon (C), but carbon (C) does not independently affect the formation of carbide, but acts in combination with manganese (Mn) to affect the formation of carbide.
• Chromium (Cr) is also an austenite stabilizing element, and stabilizes austenite up to the range of an appropriate amount of addition, thereby improving the low-temperature impact toughness of steel materials, and acts to increase the strength of steel materials by being dissolved in austenite.
• chromium (Cr) is also an element that effectively contributes to improving the corrosion resistance of steel materials. Therefore, in the present disclosure, chromium (Cr) is added as an essential element.
• the preferable lower limit of the chromium (Cr) content may be 1%, and the more preferable lower limit of the chromium (Cr) content may be 2%.
• chromium (Cr) is a carbide-forming element, and in particular, may form carbides at austenite grain boundaries to reduce the low-temperature impact toughness of steel materials.
• the addition amount of chromium (Cr) exceeds a certain level, excessive carbides may be precipitated in the welded heat-affected zone (HAZ), which may result in poor ultra-low-temperature toughness. Therefore, the present disclosure may limit the upper limit of chromium (Cr) to 10%.
• the preferable upper limit of the chromium (Cr) content may be 8%, and the more preferable upper limit of the chromium (Cr) content may be 7%.
• Ni 5% or less (excluding 0%)
• Nickel (Ni) is an effective austenite stabilizing element, and is an element that improves the toughness of steel material by lowering the Ms and Md, which are transformation points from austenite to epsilon or alpha martensite, due to the cooling process or processing. In particular, it is well known as an element that promotes slippage by increasing the stacking fault energy of steel material. However, there is a problem that economic efficiency decreases when added in excess of 5%.
• the preferred upper limit of the nickel (Ni) content may be 4%, and the more preferrable upper limit of the nickel (Ni) content may be 3.5%.
• the preferable lower limit of the nickel (Ni) content may be 0.3%, and the more preferable lower limit of the nickel (Ni) content may be 0.5%.
• Cu has a very low solubility in carbides and is slow in diffusion in austenite, and thus is concentrated at the interface between austenite and nucleated carbides. Accordingly, it effectively slows down the growth of carbides by hindering the diffusion of carbon, and ultimately has the effect of suppressing the formation of carbides.
• copper has the effect of stabilizing austenite and improving ultra-low temperature toughness.
• the preferable upper limit of the copper (Cu) content may be 3%, and the preferable upper limit of the copper (Cu) content may be 2%.
• the preferable lower limit of the copper (Cu) content may be 0.1%, and the more preferable lower limit of the copper (Cu) content may be 0.3%.
• Si 5% or less (excluding 0%)
• Silicon (Si) is an element that improves the castability of molten steel and, in particular, when added to an austenitic steel material, effectively increases the strength by being dissolved inside the steel material. It is also an element that affects the activity of carbon in the steel material and effectively suppresses the formation of carbides, thereby increasing the toughness. However, if added in excess of 5%, it reduces the stacking fault energy, promotes twinning, and there is a problem that the toughness may be lowered due to high strength, so it is preferable to limit the upper limit to 5%.
• the preferable upper limit of the silicon (Si) content may be 3%, and the more preferable upper limit of the silicon (Si) content may be 2.5%.
• the preferable lower limit of the silicon (Si) content may be 0.1%, and the more preferable lower limit of the silicon (Si) content may be 0.3%.
• Al is an element that stabilizes austenite within an appropriate addition range and lowers the Ms and Md, which are transformation points from austenite to epsilon or alpha martensite by cooling or processing, thereby improving the toughness of steel materials. It is also an element that is dissolved in the steel material to increase strength, and in particular, it is an element that effectively suppresses the formation of carbides by affecting the activity of carbon in the steel material, thereby increasing toughness. In particular, it is well known as an element that effectively increases the stacking fault energy and promotes slip.
• the preferable upper limit of the aluminum (Al) content may be 3%, and the more preferable upper limit of the aluminum (Al) content may be 2.5%.
• the preferable lower limit of the aluminum (Al) content may be 0.2%, and the more preferable lower limit of the aluminum (Al) content may be 0.3%.
• Nitrogen (N) is an element that stabilizes austenite and improves toughness together with carbon, and particularly, is significantly advantageous in improving strength through solid solution strengthening like carbon. In particular, it is well known as an element that effectively increases stacking fault energy and promotes slippage. However, if it is added in excess of 1%, coarse nitrides are formed, which deteriorates the surface quality and physical properties of the steel material, so it is preferable that the upper limit is limited to 1 wt%.
• the preferable upper limit of the nitrogen (N) content may be 0.5%, and the more preferable upper limit of the nitrogen (N) content may be 0.2%.
• the preferable lower limit of the nitrogen (N) content may be 0.005%, and the more preferable lower limit of the nitrogen (N) content may be 0.007%.
• the austenitic steel material according to an aspect of the present disclosure may include the remainder Fe and other unavoidable impurities in addition to the above-mentioned components.
• unintended impurities may inevitably be mixed in from raw materials or the surrounding environment during the normal manufacturing process, and thus cannot be completely excluded. Since these impurities are known to anyone with ordinary knowledge in this technical field, not all of the contents are specifically mentioned in this specification.
• additional effective ingredients other than the aforementioned ingredients is not completely excluded.
• An austenitic steel material may include 95 area% or more of austenite as a microstructure to secure required physical properties.
• the preferred austenite fraction may be 97 area% or more, and may include a case where the austenite fraction is 100 area%.
• the austenitic steel material according to an aspect of the present disclosure may actively suppress the carbide fraction to 5 area% or less to prevent a decrease in ultra-low temperature impact toughness.
• the preferred carbide fraction may be 3 area% or less, and may include a case in which the carbide fraction is 0 area%.
• a method for measuring the austenite fraction and the carbide fraction is not particularly limited, and may be easily confirmed through a measuring method commonly used by a person skilled in the art to which the present disclosure pertains for measuring microstructure and carbide.
• austenite In the iron steel material, austenite is an unstable structure at room temperature. To enable austenite, which is stable at high temperatures, to remain up to room temperature, austenite stabilizing elements are usually added. Even if austenite is obtained at room temperature, austenite may transform into epsilon martensite or alpha martensite through additional cooling or processing, and this varies depending on alloy composition, cooling temperature, processing amount, or the like. Steel materials used for liquefied gas storage tanks or the like undergo cold processing to form the structure, so even if the microstructure is austenite when the rolled material is produced, it may undergo phase transformation by deformation during structure production or by cooling when operated at cryogenic temperatures after deformation. If weak epsilon or alpha martensite is created through phase transformation, impact toughness is deteriorated and it may not be applicable as a liquefied gas container.
• austenite should be stable even after cold working at room temperature and then being maintained at the use temperature of -253°C.
• structures such as storage tanks and the like are processed within a range of up to 20% based on uniaxial strain during cold forming, so austenite should be maintained without the occurrence of epsilon or alpha martensite even after 20% cold working and then holding at - 253°C.
• carbides are not related to phase transformation, there is no problem if they are controlled within 5% as controlled in the present disclosure. That is, the microstructure of the present disclosure is maintained even after applying 20% of strain thereto through a unilateral tensile test at room temperature, then holding at -253°C, and then measurement at room temperature.
• the austenite may have an average grain size of 5 to 200 ⁇ m. If the average grain size of the austenite exceeds 200 ⁇ m, the strength decreases due to coarsening of the austenite. In addition, since the strain-induced transformation temperature increases, transformation into epsilon martensite or martensite easily occurs during processing, which may have the disadvantage of reducing impact toughness. On the other hand, if the average grain size of the austenite is less than 5 ⁇ m, there may be a disadvantage in that the strength increases excessively and the toughness decreases.
• the lower limit of the average grain size of the austenite is more preferably 7 ⁇ m, and more preferably 10 ⁇ m.
• the upper limit of the average grain size of the austenite is more preferably 180 ⁇ m, and more preferably 150 ⁇ m.
• the steel material of the present disclosure may have a transverse expansion of 0.32 mm or more after a Charpy impact test at -253°C. Meanwhile, in the present disclosure, since the higher the transverse expansion value, the more advantageous it is, the upper limit thereof is not particularly limited. However, the upper limit of the transverse expansion value may be 2.30 mm as an example.
• the inventor of the present disclosure has found that plastic deformation characteristics are a major factor in terms of securing safety in the case of steel materials applied to an ultra-low temperature environment. That is, the inventor of the present disclosure was able to confirm, after in-depth research, that in the case of steel materials satisfying the composition system suggested by the present disclosure, the transverse expansion value (mm) is a more important factor than the Charpy impact energy value (J) in terms of securing the safety of the base material.
• the transverse expansion value means the average value of the transverse plastic deformation amount of a specimen subjected to a Charpy impact test at -253°C.
• FIG. 2 illustrates a photograph of a specimen subjected to a Charpy impact test at -253°C, and as illustrated in FIG. 2 , the transverse length increase ( ⁇ X1+ ⁇ X2) near the fracture surface may be calculated to derive the transverse expansion value. If the transverse expansion value is 0.32 mm or more, it may be determined that the minimum low-temperature safety required for ultra-low-temperature structures is provided.
• the austenitic steel material according to an aspect of the present disclosure has a transverse expansion value of 0.32 mm or more of a specimen that was subjected to a Charpy impact test at -253°C using the steel material as a base material, so that excellent structural safety may be secured when an ultra-low temperature structure is manufactured using the steel material.
• the austenitic steel material of the present disclosure may have a Charpy impact energy of 27 J or more at -253°C.
• a Charpy impact energy 27 J or more at -253°C.
• the ultra-low temperature Charpy impact energy since the ultra-low temperature Charpy impact energy is higher, it is advantageous, and therefore the upper limit thereof is not particularly limited.
• the upper limit of the ultra-low temperature Charpy impact energy may be 250J as an example.
• the room temperature yield strength of the austenitic steel material according to an aspect of the present disclosure may satisfy 245 MPa or more and less than 400 MPa.
• the strength of the steel material increases, the low temperature impact toughness decreases, and in particular, in the case of the steel material for ultra-low temperature use of -253°C like the present disclosure, if the yield strength is excessively high, the possibility of not securing the required impact toughness increases.
• the strength of the base material is maintained high, a strength difference may occur between the weld and the base material, which may lower the structural stability.
• the room temperature yield strength of the austenitic steel material according to an aspect of the present disclosure is preferably less than 400 MPa. Meanwhile, if the room temperature yield strength of the steel material is excessively low, the thickness of the base material may increase excessively to secure the stability of the structure, and accordingly, the weight of the structure may increase excessively. Therefore, the austenitic steel material according to an aspect of the present disclosure may limit the lower limit of the room temperature yield strength to 245 MPa.
• a slab having the above-mentioned alloy composition is heated at 1000 to 1300°C. If the slab heating temperature is less than 1100°C, there is a disadvantage that the alloy components are not re-dissolved and homogenized, or that it takes a long time to reach the target temperature to the center of the slab. If the slab heating temperature exceeds 1300°C, there is a disadvantage that partial melting occurs in the slab alloy component segregation area or that surface oxidation occurs severely.
• the lower limit of the slab heating temperature is more preferably 1030°C, even more preferably 1070°C, and most preferably 1100°C.
• the upper limit of the slab heating temperature is more preferably 1250°C, even more preferably 1230°C, and most preferably 1200°C.
• the heated slab is finish hot-rolled at 800 to 1050°C to obtain a hot-rolled steel sheet.
• finish hot-rolling temperature is less than 800°C, rolling is not easy due to the high temperature strength of the material, and since non-recrystallized rolling occurs, there is a disadvantage in that the strength of the material increases excessively, reducing the impact toughness.
• finish hot-rolling temperature exceeds 1050°C, there is a disadvantage that austenite coarsens and the strength decreases.
• the lower limit of the finish hot rolling temperature is more preferably 820°C, more preferably 850°C, and most preferably 870°C.
• the upper limit of the slab finish hot rolling temperature is more preferably 1030°C, more preferably 1000°C, and most preferably 980°C. Meanwhile, the reduction ratio during the hot rolling may be applied within an appropriate range depending on the target plate thickness, and as a non-limiting example, the final thickness of the hot-rolled steel sheet may be 5 to 80 mm.
• the hot-rolled steel sheet may be air-cooled to room temperature.
• the microstructure was measured using an optical microscope at room temperature. In addition, after applying 20% of deformation through a unilateral tensile test at room temperature, it was maintained at -253°C for 15 minutes or more, and then it was observed whether the room temperature microstructure was maintained even after the measurement at room temperature.
• the microstructure described in Table 2 below is the microstructure at room temperature and, at the same time, is the microstructure at room temperature after deformation and ultra-low temperature maintenance.
• the average grain size of austenite was measured by taking microscopic photographs using an optical microscope and then using image analysis.
• the Charpy impact energy was measured using a Charpy impact tester after the specimen was maintained at -253°C for 15 minutes or more.
• the room temperature yield strength was measured using a uniaxial tensile test method.

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