Thick steel sheet with excellent cryogenic toughness in weld heat-affected zone

Abstract

The present invention relates to a thick steel sheet exhibiting excellent cryogenic toughness in the weld heat-affected zone. The steel sheet comprises, by weight %, carbon (C): 0.03–0.10%, manganese (Mn): 1.4–3.0%, aluminum (Al): 0.025–0.10%, nickel (Ni): 4.8–6.0%, chromium (Cr): 0.1–1.0%, molybdenum (Mo): 0.1–1.0%, phosphorus (P): 0.01% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), and nitrogen (N): 0.01% or less (excluding 0%), and the balance being iron (Fe) and other inevitable impurities.

Description

Thick steel plate with excellent ultra-low temperature toughness in the weld heat-affected zone

[0001] The present invention relates to a steel plate, and more particularly, to a steel plate having excellent ultra-low temperature toughness in a weld heat-affected zone at an ultra-low temperature of -196 degrees Celsius, even when the nickel content is reduced.

[0002] Thick steel plates are primarily applicable to liquefied natural gas (LNG) exposed to extremely low temperatures. Typical applications include LNG storage tanks for transportation and LNG fuel tanks. However, their scope is not limited to LNG; they can also be used as storage and fuel tanks for liquefied fuels with higher boiling points than LNG, such as liquefied ammonia (-33 degrees Celsius), which can be stored at higher temperatures.

[0003] In the past, the high yield strength due to the addition of large amounts of nickel could cause difficulties in manufacturing cryogenic tanks that require bending. When manufacturing welded structures using arc welding, the addition of large amounts of nickel easily causes magnetization at the weld joint, including the base material, resulting in phenomena such as arc drift, which can degrade the quality of the completed weld joint. Recently, thick steel plates containing approximately 9% nickel (9% nickel steel) have been used. However, as the cost of nickel continues to rise, the development of thick steel plates with lower nickel contents but superior cryogenic toughness is underway. However, reducing the nickel content makes it difficult to satisfy the cryogenic toughness characteristics. In particular, there was a problem that brittle fracture in the weld heat-affected zone was more likely to occur when the nickel content was reduced. A prior document of the present invention is Korean Patent Publication No. 10-2020-0140907.

[0004] The technical problem to be achieved by the technical idea of ​​the present invention is to provide a steel plate having improved brittle fracture in the heat-affected zone of welding and excellent ultra-low temperature toughness.

[0005] However, these tasks are exemplary and the technical idea of ​​the present invention is not limited thereto.

[0006] According to one aspect of the present invention, a steel plate having excellent ultra-low temperature toughness in a welded heat-affected zone, a method for manufacturing the same, and a welded structure are provided.

[0007] According to one embodiment of the present invention, the steel plate having excellent weld heat-affected zone ultra-low temperature toughness contains, in wt%, carbon (C): 0.03 to 0.10%, manganese (Mn): 1.4 to 3.0%, aluminum (Al): 0.025 to 0.10%, nickel (Ni): 4.8 to 6.0%, chromium (Cr): 0.1 to 1.0%, molybdenum (Mo): 0.1 to 1.0%, phosphorus (P): 0.01% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), and the remainder being iron (Fe) and other unavoidable impurities, and the steel plate can satisfy the following equation 1.

[0008] [Equation 1] 50 ≤ E = [Mn] / [Al] ≤ 67

[0009] (The above [Mn] and [Al] are the weight % values ​​of manganese and aluminum, respectively)

[0010] According to one embodiment of the present invention, the rear plate can satisfy the following equation 2.

[0011] [Equation 2] 9 ≤ F = ([C]+[Mn] / 6+[N]) / [Al] ≤ 13

[0012] (The above [Mn], [Al], [C], and [N] are the weight % values ​​of manganese, aluminum, carbon, and nitrogen, respectively)

[0013] According to one embodiment of the present invention, the steel plate may have a microstructure including 5 to 15% of retained austenite in a tempered martensite matrix structure.

[0014] According to one embodiment of the present invention, the retained austenite may have a volume fraction of 50% or more of the needle-shaped retained austenite having an average circle-equivalent diameter of 3 μm or less and an aspect ratio of the short axis / long axis of 1.5 or more with respect to the entire retained austenite.

[0015] According to one embodiment of the present invention, the steel plate may have an average impact toughness of 27 J or more at -196°C.

[0016] According to one embodiment of the present invention, in the above-described thick plate, the weld heat-affected zone formed by a heat input in the range of 10 kJ / cm to 30 kJ / cm may have an average impact toughness of 27 J or more at -196°C.

[0017] According to one embodiment of the present invention, the thick steel plate can satisfy a yield strength (YS): 640 MPa to 890 MPa, a tensile strength (TS): 710 MPa to 930 MPa, and an elongation (EL): 20% to 30%.

[0018] According to one embodiment of the present invention, a method for manufacturing a steel plate having excellent weld heat-affected zone ultra-low temperature toughness comprises the following steps: a first step of providing a steel material containing, in wt%, carbon (C): 0.03 to 0.10%, manganese (Mn): 1.4 to 3.0%, aluminum (Al): 0.025 to 0.10%, nickel (Ni): 4.8 to 6.0%, chromium (Cr): 0.1 to 1.0%, molybdenum (Mo): 0.1 to 1.0%, phosphorus (P): 0.01% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), and the remainder being iron (Fe) and other unavoidable impurities; The method may sequentially include a second step of hot-rolling the above steel material under conditions of a reheating temperature of 1000°C to 1200°C and a finishing rolling temperature of Ar3 or higher; a third step of heating the hot-rolled steel material to a temperature range of Ac1 to Ac3, maintaining the temperature, and then rapidly cooling it; and a fourth step of tempering the steel material at a temperature of Ac1 or lower. The steel plate manufactured by the method for manufacturing the above steel plate may satisfy the following equation 1.

[0019] [Equation 1] 50 ≤ E = [Mn] / [Al] ≤ 67

[0020] (The above [Mn] and [Al] are the weight % values ​​of manganese and aluminum, respectively)

[0021] According to one embodiment of the present invention, the rear plate can satisfy the following equation 2.

[0022] [Equation 2] 9 ≤ F = ([C]+[Mn] / 6+[N]) / [Al] ≤ 13

[0023] (The above [Mn], [Al], [C], and [N] are the weight % values ​​of manganese, aluminum, carbon, and nitrogen, respectively)

[0024] According to one embodiment of the present invention, the third step can be performed while maintaining the temperature range of Ac1 to Ac3 for 30 to 150 minutes.

[0025] According to one embodiment of the present invention, the fourth step may be performed at a temperature ranging from 550°C to 620°C for 30 minutes to 200 minutes.

[0026] According to one embodiment of the present invention, a welded structure manufactured by welding the above-described heat-affected zone ultra-low temperature toughness-excellent thick steel plate can satisfy the following equation 1.

[0027] [Equation 1] 50 ≤ E = [Mn] / [Al] ≤ 67

[0028] (The above [Mn] and [Al] are the weight % values ​​of manganese and aluminum, respectively)

[0029] According to one embodiment of the present invention, the welded structure can satisfy the following equation 2.

[0030] [Equation 2] 9 ≤ F = ([C]+[Mn] / 6+[N]) / [Al] ≤ 13

[0031] (The above [Mn], [Al], [C], and [N] are the weight % values ​​of manganese, aluminum, carbon, and nitrogen, respectively)

[0032] The welded heat affected zone of the above welded structure may have an average impact toughness of 27 J or more at -196°C.

[0033] According to one embodiment of the present invention, the welded heat-affected zone of the welded structure may have a microstructure in which the volume fraction of bainite and lath martensite combined is 95% or more.

[0034] According to the technical idea of ​​the present invention, it is possible to provide a steel plate capable of securing an ultra-low temperature toughness of -196℃ or more in the weld heat-affected zone in a wide heat input range of 10 kJ / cm to 30 kJ / cm by reducing nickel by about 3% to 4% compared to the conventional method, thereby securing economic feasibility, and optimizing other component systems.

[0035] The effects of the present invention described above are illustrative, and the scope of the present invention is not limited by these effects.

[0036] Figure 1 is a flowchart illustrating a method for manufacturing a steel plate having excellent ultra-low temperature toughness in a welded heat-affected zone according to one embodiment of the present invention.

[0037] FIG. 2 is a drawing illustrating a heat treatment concept in a method for manufacturing a steel plate having excellent ultra-low temperature toughness in a welded heat-affected zone according to one embodiment of the present invention.

[0038] Figure 3 is a scanning electron microscope (SEM) photograph of the microstructure of a steel plate according to Example 9 and Comparative Example 6 among experimental examples of the present invention.

[0039] Figure 4 shows the results of analyzing the microstructure of the steel plate according to Example 8 and Comparative Example 1 among the experimental examples of the present invention using electron backscatter diffraction (EBSD).

[0040] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. Embodiments of the present invention are provided to more completely explain the technical idea of ​​the present invention to those skilled in the art. The following embodiments may be modified in various different forms, and the scope of the technical idea of ​​the present invention is not limited to the following embodiments. Rather, these embodiments are provided to more faithfully and completely convey the technical idea of ​​the present invention to those skilled in the art. Like reference numerals throughout this specification denote like elements. Furthermore, various elements and areas in the drawings are schematically drawn. Therefore, the technical idea of ​​the present invention is not limited by the relative sizes or intervals drawn in the attached drawings.

[0041] Heavy plate

[0042] According to one embodiment of the present invention, a steel plate having excellent ultra-low temperature toughness in a welded heat-affected zone contains, in wt%, carbon (C): 0.03 to 0.10%, manganese (Mn): 1.4 to 3.0%, aluminum (Al): 0.025 to 0.10%, nickel (Ni): 4.8 to 6.0%, chromium (Cr): 0.1 to 1.0%, molybdenum (Mo): 0.1 to 1.0%, phosphorus (P): 0.01% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), and the remainder includes iron (Fe) and other unavoidable impurities.

[0043] Here, the steel plate means a steel plate having a thickness of about 6 mm to 50 mm.

[0044] Hereinafter, the role and content of each component included in the steel plate according to the present invention will be described. In this case, the content of the component elements all refers to weight%.

[0045] Carbon (C)

[0046] Carbon (C) is an essential element for imparting high strength to steel plates. It also enhances the hardenability of steel plates and is a key element that determines the strength after quenching. Depending on the carbon content and manufacturing method, it can form a solid solution within the material structure or form carbides by combining with elements that strongly bond with carbon. Furthermore, to achieve high toughness after quenching, it is desirable to include carbon in an amount of at least 0.03 wt% of the entire slab. However, excessive addition can result in excessive strength increase and reduced ultra-low temperature toughness, so the upper limit is set at 0.10 wt%. Furthermore, carbon is an austenite-stabilizing element and is essential for securing retained austenite after tempering. Therefore, the range of carbon addition may be 0.03 wt% to 0.10 wt% of the total weight of the steel plate.

[0047] Silicon (Si)

[0048] Silicon (Si) is a solid solution strengthening element that contributes to the strengthening of steel sheets and, together with aluminum, acts as a deoxidizer to separate inclusions in molten steel into slag. However, in the present invention, silicon is not added separately except as an unavoidable impurity. If silicon is added excessively, the area of ​​reduced impact toughness in the weld heat-affected zone expands due to the temper embrittlement phenomenon in the weld heat-affected zone. Therefore, the purpose of this invention is to solve this drawback and secure the cryogenic properties of the steel by using an aluminum-only deoxidation process. That is, in the thick steel sheet of the present invention, silicon is not artificially added to enhance the auto-tempering effect of the weld toe, and it is sufficient to utilize aluminum as a deoxidizing element to control inclusions in the molten steel. Therefore, it is preferable to exclude silicon rather than artificially adding it.

[0049] manganese (Mn)

[0050] Manganese (Mn) functions as a deoxidizer, but as a representative austenite stabilizing element, it is an element that contributes to increasing the amount of retained austenite by adding it as a substitute for, rather than reducing, expensive nickel. To achieve this effect, its lower limit is set at 1.4 wt%. However, if manganese is added in excess, it will segregate at grain boundaries, increasing the possibility of brittle fracture, causing temper embrittlement, and forming a large amount of coarse retained austenite, which will actually lower the ultra-low temperature toughness. Therefore, its upper limit is set at 3.0 wt% to ensure the stability of retained austenite in a solid solution state. Therefore, the addition range of manganese can be 1.4 wt% to 3.0 wt% of the total weight of the steel plate.

[0051] Aluminum (Al)

[0052] Aluminum (Al) is a representative ferrite stabilizing element, which has the advantage of increasing the volume fraction of ferrite and improving the mechanical strength of steel by being dissolved in ferrite. In addition, it is expected to have the effect of suppressing brittle crack propagation by refining the lath martensite structure, which is the main structure of the steel plate of the present invention. However, since aluminum is not suitable as a ferrite stabilizing element from the perspective of increasing the volume fraction of the retained austenite described above, its upper limit is set to 0.10 wt%. On the other hand, since the ferrite refinement effect is reduced when added less than 0.025 wt%, its lower limit is set to 0.025 wt%. Therefore, the addition range of aluminum may be 0.025 wt% to 0.1 wt% of the total weight of the steel plate.

[0053] Person (P)

[0054] Phosphorus (P) is a representative impurity element, and its absence is desirable because it induces grain boundary destruction. However, it inevitably remains during the production of molten steel, so its upper limit is set at 0.01 wt% to secure ultra-low temperature toughness. Accordingly, the range of phosphorus addition may be between 0 wt% and 0.01 wt% of the total weight of the steel plate.

[0055] Yellow (S)

[0056] Sulfur (S), like phosphorus (P), causes intergranular fracture and combines with manganese to form manganese sulfide (MnS), thereby reducing the toughness of the steel plate. Therefore, its upper limit is set at 0.01 wt%. Accordingly, the range of sulfur addition may be from more than 0 wt% to 0.01 wt% of the total weight of the steel plate.

[0057] Nickel (Ni)

[0058] Nickel (Ni) is a useful element that can simultaneously improve strength and toughness, and as the amount of nickel added increases, the fraction of retained austenite increases. In particular, it plays a role in stabilizing retained austenite at room temperature and cryogenic temperatures (-196℃), thereby having the effect of suppressing brittle fracture of steel. However, in the present invention, the upper limit is set to 6.0 wt% in order to reduce the expensive nickel. However, if the nickel is reduced excessively, the target level of retained austenite cannot be secured, so the lower limit is set to 4.8 wt%. Therefore, the range of nickel addition may be 4.8 wt% to 6.0 wt% of the total weight of the steel plate.

[0059] Previously, research was conducted on steel plates with nickel content reduced to 6 wt%. However, when the nickel content is controlled to be lower than 6 wt%, it is difficult to secure retained austenite, and thus there was a problem in that the effect of improving ultra-low temperature toughness by retained austenite structure could not be obtained. However, in order to solve this problem, the present invention has developed a method that can obtain the effect of improving ultra-low temperature toughness even when the nickel content is controlled to 6 wt% or less by deriving the optimal range of aluminum, manganese, etc. in addition to nickel.

[0060] chromium (Cr)

[0061] Chromium (Cr) is an element useful for securing the ultra-low temperature toughness of steel by increasing stability by being concentrated in the austenite formed in the lamellarizing heat treatment temperature range. It is preferable to add 0.1 wt% or more of chromium. However, excessive addition of chromium may excessively increase strength and cause temper embrittlement, so the upper limit of chromium is set at 1.0 wt%. Therefore, the range of chromium addition may be 0.1 wt% to 1.0 wt% of the total weight of the steel plate.

[0062] molybdenum (Mo)

[0063] Molybdenum (Mo), along with chromium (Cr), is a useful element for increasing the strength of steel and enhancing the stability of austenite. To maximize the effects of the present invention, it is preferable to add molybdenum in an amount of at least 0.1 wt%. However, excessive addition of molybdenum excessively increases the strength of the steel and, conversely, reduces the ultra-low temperature toughness. Therefore, the preferred upper limit of molybdenum is 1.0 wt%. Therefore, the range of molybdenum addition may be 0.1 wt% to 1.0 wt% of the total weight of the steel plate.

[0064] Nitrogen (N)

[0065] Nitrogen (N) is an impurity element with a very small atomic size, which easily moves to dislocations when steel is deformed, forming a Cottrell atmosphere and is a harmful element that inhibits plastic deformation of the steel. However, when manganese is added in a large amount as in the present invention, nitrogen in the slag is re-dissolved into the molten steel during the steelmaking process, increasing the nitrogen content. Conventional 9% nickel steel is added in small amounts together with silicon only as a deoxidizer, but in the present invention, aluminum is added in excess of what is generally added, combining with nitrogen to form aluminum nitride (AlN), which is evenly distributed within the steel, thereby having the effect of making lath martensite fine. Therefore, the range of nitrogen addition may be more than 0 wt% to 0.01 wt% of the total weight of the thick steel plate.

[0066] The remaining component of the above-mentioned steel plate is iron (Fe). However, during the typical steelmaking process, unintended impurities from raw materials or the surrounding environment can inevitably be mixed in, and thus cannot be ruled out. Since these impurities are readily apparent to anyone skilled in the typical manufacturing process, their full content is not specifically addressed in this specification.

[0067] Meanwhile, the steel plate according to a modified embodiment of the present invention may additionally include at least one element having the following composition range in addition to the above-described alloy elements.

[0068] The above-mentioned steel plate may further contain at least one selected from the group including, in wt%, Cu: 1.00% or less (excluding 0%), Ti: 0.025% or less (excluding 0%), Nb: 0.10% or less (excluding 0%), V: 0.50% or less (excluding 0%), B: 0.0050% or less (excluding 0%), Ca: 0.0030% or less (excluding 0%), REM: 0.0050% or less (excluding 0%), Zr: 0.0050% or less (excluding 0%).

[0069] copper (Cu)

[0070] Copper (Cu) is an austenite stabilizing element that contributes to increasing the amount of retained austenite. However, if copper is added excessively, the strength of the steel will increase excessively, and the target ultra-low temperature toughness effect will not be obtained. Therefore, the upper limit is preferably set to 1.00 wt% or less (excluding 0 wt%). A more preferable upper limit of the copper content is 0.8 wt% or less, and even more preferably 0.7 wt% or less. Therefore, the range of addition of copper may be 0 wt% to 1.0 wt% of the total weight of the steel plate.

[0071] Titanium (Ti), niobium (Nb), and vanadium (V)

[0072] Titanium (Ti), niobium (Nb), and vanadium (V) are all elements that precipitate as carbonitrides and increase the strength of steel. These elements may be added alone or in combination of two or more. However, excessive addition of these elements may excessively increase the strength of the steel, preventing the targeted ultra-low-temperature toughness from being achieved.

[0073] Therefore, the titanium content may be 0.025 wt% or less (excluding 0 wt%), more preferably 0.018 wt% or less, and even more preferably 0.015 wt% or less. The niobium content may be 0.10 wt% or less (excluding 0 wt%), more preferably 0.05 wt% or less, and even more preferably 0.02 wt% or less. The vanadium content may be 0.50 wt% or less (excluding 0 wt%), more preferably 0.3 wt% or less, and even more preferably 0.2 wt% or less.

[0074] Boron (B)

[0075] Boron (B) is an element that contributes to improving the strength of steel by improving hardenability. If the content of boron is excessively added, the strength of the steel will be excessively improved, making it impossible to secure the target ultra-low temperature toughness. Therefore, the content of boron may be 0.005 wt% or less (excluding 0 wt%), more preferably 0.003 wt% or less, and even more preferably 0.002 wt% or less. Accordingly, the range of addition of boron may be 0 wt% to 0.005 wt% of the total weight of the steel plate.

[0076] Calcium (Ca), rare earth metals (REM), zirconium (Zr)

[0077] Calcium (Ca) may be at least one selected from the group consisting of 0.003 wt% or less (excluding 0 wt%), rare earth metal (REM) 0.005 wt% or less (excluding 0 wt%), and zirconium (Zr) 0.005 wt% or less (excluding 0 wt%). Calcium, rare earth metal, and zirconium are all deoxidizing elements, and their addition lowers the oxygen concentration in the steel, thereby reducing the amount of oxides, thereby having a positive effect on the toughness. These elements may be added alone, or two or more may be used in combination. In order to effectively exert the above action, it is preferable that the calcium content be 0.0005 wt% or more, the rare earth metal content (when the rare earth metals described below are contained singly, it is the singular content, and when two or more types are contained, it is their sum. Hereinafter, the same applies to the rare earth metal content.) be 0.0005 wt% or more, and the zirconium content be 0.0005 wt% or more. However, if added excessively, the size of the oxide increases and the ultra-low temperature toughness deteriorates, so the preferable upper limit of the calcium content may be 0.003 wt% or less (more preferably 0.0025 wt% or less), the preferable upper limit of the rare earth metal content may be 0.0050 wt% or less (more preferably 0.004 wt% or less), and the preferable upper limit of the zirconium content may be 0.005 wt% or less (more preferably 0.004 wt% or less).

[0078] The above rare earth metal is a rare earth element, which is a group of elements in which Sc and Y are added to the lanthanoid elements (15 elements from La with atomic number 57 to Lu with atomic number 71 in the periodic table), and these can be used alone or in combination of two or more. Preferred rare earth elements are Ce and La. The form in which the rare earth metal is added is not particularly limited, and it can be added in the form of a mischmetal mainly containing Ce and La (for example, containing Ce: about 70 wt%, La: about 20 to 30 wt%), or it can be added as a single substance such as Ce or La.

[0079] The contents of the manganese and aluminum above can satisfy the following equation 1. The contents of the carbon, manganese, nitrogen, and aluminum above can satisfy the following equation 2. The contents of the elements in equations 1 and 2 are in weight%.

[0080] [Equation 1] 50 ≤ E = [Mn] / [Al] ≤ 67

[0081] [Equation 2] 9 ≤ F = ([C]+[Mn] / 6+[N]) / [Al] ≤ 13

[0082] (The above [Mn], [Al], [C], and [N] are the weight % values ​​of manganese, aluminum, carbon, and nitrogen, respectively)

[0083] The above formulas 1 and 2 are intended to satisfy the impact toughness of 27J at -196℃ in the weld heat affected zone.

[0084] When at least one of the above formulas 1 and 2 is satisfied, the following effects are exhibited. In order to compensate for the disadvantages of silicon or nitrogen that reduce temper embrittlement or impact toughness, specifically, except for trace impurities, silicon is not added separately, but aluminum is added to secure a deoxidation effect within the steel, and nitrogen is trapped with aluminum to form nitrides and be evenly distributed within the structure.

[0085] To see the above-mentioned effect, the lower limit of aluminum should be 0.025 wt% or more. However, if it is added excessively exceeding 0.1 wt%, the ultra-low temperature toughness of the base material and the weld heat-affected zone deteriorates due to the formation of coarsely coalesced bainite due to the dissolution of aluminum, so the upper limit is limited to 0.1 wt%. In the present invention, it was confirmed that the ultra-low temperature toughness of the weld heat-affected zone tends to improve when an appropriate amount of aluminum nitride (AlN) is distributed. Based on this, it was confirmed through numerous basic experiments that the E value represented by the above equation (1) and the F value represented by the equation (2) can be utilized as useful parameters for evaluating the ultra-low temperature toughness of the weld heat-affected zone.

[0086] Figure 1 is a flowchart illustrating a method for manufacturing a steel plate having excellent ultra-low temperature toughness in a welded heat-affected zone according to one embodiment of the present invention.

[0087] FIG. 2 is a drawing illustrating a heat treatment concept in a method for manufacturing a steel plate having excellent ultra-low temperature toughness in a welded heat-affected zone according to one embodiment of the present invention.

[0088] Referring to FIGS. 1 and 2, a method for manufacturing a steel plate having excellent weld heat-affected zone toughness at extremely low temperatures according to an embodiment of the present invention comprises: a first step (S10) of providing a steel material containing, in wt%, carbon (C): 0.03 to 0.10%, manganese (Mn): 1.4 to 3.0%, aluminum (Al): 0.025 to 0.10%, nickel (Ni): 4.8 to 6.0%, chromium (Cr): 0.1 to 1.0%, molybdenum (Mo): 0.1 to 1.0%, phosphorus (P): 0.01% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), and the remainder being iron (Fe) and other unavoidable impurities; It sequentially includes a second step (S20) of hot-rolling the above steel material under conditions of a reheating temperature of 1000℃ to 1200℃ and a finishing rolling temperature of Ar3 or higher; a third step (S30) of heating the hot-rolled steel material to a temperature range of Ac1 to Ac3, maintaining the temperature, and then rapidly cooling it; and a fourth step (S40) of tempering the steel material at a temperature of Ac1 or lower.

[0089] Steel provision stage (S10)

[0090] A steel material having the composition described above is provided. The semi-finished product of the steel material may be, for example, a slab. The slab in a semi-finished state can be obtained by obtaining molten steel of a predetermined composition through a steelmaking process and then through a continuous casting process.

[0091] Hot rolling stage (S20)

[0092] The second step (S20) of hot rolling the above steel material is performed under the conditions of a reheating temperature of 1000℃ to 1200℃ and a finishing rolling temperature of Ar3 or higher.

[0093] Reheating is performed at a temperature ranging from 1000°C to 1200°C. If the reheating temperature is lower than 1000°C, homogenization in the austenite single-phase region may not be sufficient, the rolling start temperature may be low, which may cause equipment load, it may be difficult to achieve the finishing rolling temperature, and the austenite may be excessively refined, making it difficult to secure sufficient martensite after hot rolling. The reheating time may be, for example, 1 to 4 hours, but this is exemplary and the technical idea of ​​the present invention is not limited thereto.

[0094] If the reheating temperature exceeds 1200℃, excessive austenite growth may occur, resulting in coarse packets after final tempering and a decrease in the ultra-low temperature toughness of the base metal and heat-affected zone.

[0095] During hot rolling, the finishing rolling temperature must be limited to Ar3 or higher. If finishing rolling is performed at a temperature lower than Ar3, proeutectoid ferrite is formed, making it difficult to secure the target level of retained austenite after tempering (T3) described below, resulting in a decrease in ultra-low temperature toughness.

[0096] After final rolling, cooling should be performed rapidly enough to secure the martensitic matrix structure. Preferably, the steel sheet is cooled by water at a rate of 5°C / sec or more.

[0097] Lamellarizing heat treatment step (S30)

[0098] The hot-rolled steel is heated to a temperature range of Ac1 to Ac3, maintained, and then rapidly cooled in the third step (S30). After hot rolling, it is heated to a temperature range (TL) of Ac1 to Ac3, maintained, and then quenched. The purpose of performing lamelling heat treatment is to concentrate alloying elements such as Mn, Ni, Cr, and Mo in the austenite phase formed in the temperature range (TL) of Ac1 to Ac3 to obtain an austenite phase that exists metastable at room temperature. When lamelling heat treatment is performed below Ac1, alloying element distribution (partitioning) is not smooth, so the intended amount of retained austenite cannot be secured. When lamelling heat treatment is performed above Ac3, it enters the austenite single-phase region, so the alloying element concentration distribution within the structure becomes uniform, and the expected alloying element partitioning effect cannot be achieved. The temperature range of the above lamelling heat treatment may be, for example, 620°C to 790°C.

[0099] The holding time in the Ac1 to Ac3 region may be, for example, 30 to 150 minutes. If it is less than 30 minutes, alloying elements such as Ni, Cr, and Mo are not sufficiently concentrated in the austenite phase, resulting in the inability to secure the retained austenite phase to the target level. If it exceeds 150 minutes, the retained austenite phase coarsens, making it impossible to sufficiently suppress crack propagation during the cryogenic toughness test. The preferable holding time may be limited to 30 to 150 minutes. However, this is exemplary and the technical idea of ​​the present invention is not limited thereto.

[0100] Tempering stage (S40)

[0101] A fourth step (S40) of tempering the above steel at a temperature lower than Ac1 can be performed. After the lamelling heat treatment (L in FIG. 2), tempering (T in FIG. 2) is performed. The tempering heat treatment is performed at a temperature lower than Ac1, and specifically, for example, in a range (T3) of 550°C to 620°C, for example, 30 minutes to 200 minutes (t3). Through the tempering heat treatment, carbon (C) is concentrated in the metastable austenite generated during the lamelling heat treatment, thereby increasing the stability of the retained austenite, and thus, the ultra-low temperature toughness can be improved. When the tempering temperature is lower than 550°C, the metastable retained austenite phase may be decomposed into an α phase and cementite (Fe3C), which may deteriorate the ultra-low temperature toughness. If the tempering temperature exceeds Ac1 or 620°C, the retained austenite coarsens, reducing cryogenic cracking resistance. Tempering for longer periods, exceeding 200 minutes, can also result in a similar deterioration in cryogenic toughness. Cooling after tempering can be performed by water or air cooling.

[0102] The microstructure of the steel plate having excellent ultra-low temperature toughness in the welded heat-affected zone realized by performing the above-described steps may include retained austenite dispersed in the tempered martensite matrix structure and the grain boundaries of the old austenite and packets in a volume fraction of 5% to 15%. Here, the old austenite may refer to the initially formed austenite.

[0103] The above-mentioned retained austenite may have an average circle diameter of 3 μm or less. The above-mentioned retained austenite may have a volume fraction of 50% or more, and may be 50% to 90%, of the entire retained austenite in the form of needles having an aspect ratio of the short axis / long axis of 1.5 or more.

[0104] The above-mentioned welded heat-affected zone ultra-low temperature toughness excellent steel plate can have an average impact toughness of 27J or more at -196℃.

[0105] In the above-mentioned thick plate, the weld heat affected zone formed by a heat input in the range of 10 kJ / cm to 30 kJ / cm can have an average impact toughness of 27 J or more at -196°C.

[0106] The above-mentioned thick steel plate can satisfy the following: yield strength (YS): 640 MPa to 890 MPa, tensile strength (TS): 710 MPa to 930 MPa, and elongation (EL): 20% to 30%.

[0107] A welded structure manufactured using a steel plate having excellent ultra-low temperature toughness in a welded heat-affected zone according to one embodiment of the present invention may include a steel plate containing carbon (C): 0.03 to 0.10%, manganese (Mn): 1.4 to 3.0%, aluminum (Al): 0.025 to 0.10%, nickel (Ni): 4.8 to 6.0%, chromium (Cr): 0.1 to 1.0%, molybdenum (Mo): 0.1 to 1.0%, phosphorus (P): 0.01% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), and the remainder being iron (Fe) and other unavoidable impurities.

[0108] In addition, the steel plate may further contain at least one selected from the group including, in wt%, Cu: 1.00% or less (excluding 0%), Ti: 0.025% or less (excluding 0%), Nb: 0.10% or less (excluding 0%), V: 0.50% or less (excluding 0%), B: 0.0050% or less (excluding 0%), Ca: 0.0030% or less (excluding 0%), REM: 0.0050% or less (excluding 0%), Zr: 0.0050% or less (excluding 0%).

[0109] In the above welded structure, the microstructure of the parent material may include retained austenite dispersed in the tempered martensite matrix structure and the packet boundary in a volume fraction of 5% to 15%.

[0110] The weld heat-affected zone of the above welded structure may have a microstructure comprising a combined volume fraction of bainite and lath martensite of 95% or more. The remaining microstructure may include at least one of cementite and martensite-austenite (MA).

[0111] The above welded structure can be manufactured using the above steel plate, for example, by an arc welding method.

[0112] Experimental example

[0113] Below, experimental examples are described to aid understanding of the present invention. The following experimental examples are presented to aid understanding of the invention, and are not limited to the following experimental examples of the present invention.

[0114] Using a vacuum melting furnace, the elements shown in Table 1 were melted and cast to produce an ingot, and then each steel plate sample according to an experimental example of the present invention was manufactured. In addition to the component elements, the remainder is iron (Fe).

[0115] Classification CMnPSAlNiCrMoNExample 10.0572.190.00280.00210.0424.880.30.30.0039Example 20.0572.190.00280.00210.0424.880.30.30.0039Example 30.0541.460.00320.0020.0254.960.40.2070.0053Example 40.052.910.00430.00220.0514.950.610.450.0043Example Example 50.05 52.15 0.00 30.00 170.03 34.9 50.4 10.20 50.00 55 Example 60.04 8 52.21 60.00 510.00 220.03 9 8 5.50 40.30 7 30.20 36 0.00 78 Example 70.04 18 1.7 1 5 60.00 35 0.00 18 0.02 5 9 5.25 6 60.28 8 9 0.19 2 8 0.00 38 Example 80.03 5 2.7 60.00 29 0.00 19 0.04 5 5.37 0.30 20.00 31 Example 90.05 12.18 0.00 120.00 37 0.03 5 5.40 32 0.20 00 47 Example 100.03 12.21 0.00 26 0.00 28 0.04 5.20 30.20 00 82 Comparative Example 10.05 8 1.14 0.00 32 0.00 20.03 14.96 0.40 7 0.20 7 0.00 7 Comparative Example 20.06 21.10.00 33 0.00 20.03 24.98 0.40 20.21 10.018 Comparative Example 30.05 7 1.48 0.0 030.00090.0354.960.40.20.0035Comparative Example40.030.440.00310.00150.0255.910.60.20.0038Comparative Example50.061.50.00250.00210.0550.40.20.0041Comparative Example60.0512.710.0020.00170.0295.120.30.20.0036Comparative Example70.0450.750.00330.00190.0215.370.30.20.0028

[0116] Referring to Table 1, Examples 1 to 10 contain, in wt%, carbon (C): 0.03 to 0.10%, manganese (Mn): 1.4 to 3.0%, aluminum (Al): 0.025 to 0.10%, nickel (Ni): 4.8 to 6.0%, chromium (Cr): 0.1 to 1.0%, molybdenum (Mo): 0.1 to 1.0%, phosphorus (P): 0.01% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), and the remainder satisfies the composition range of iron (Fe) and other inevitable impurities.

[0117] In Comparative Example 1, manganese is outside the scope of the present invention. In Comparative Example 2, manganese and nitrogen are outside the scope of the present invention. In Comparative Example 3, the composition satisfies the range. In Comparative Example 4, manganese is outside the scope of the present invention. In Comparative Example 5, the composition satisfies the above. In Comparative Example 6, manganese is outside the scope of the present invention. In Comparative Example 7, manganese and aluminum are outside the scope of the present invention.

[0118] Table 2 shows the parameter values ​​and heat treatment temperature according to the composition in the experimental examples of the present invention.

[0119] Classification Parameter 1 Parameter 2 Heat treatment temperature and time Mn / Al (C + Mn / 6 + N) / Al TL (℃) tL (min) T3 (℃) t3 (min) Example 1 52.110.170 8556 1042 Example 2 52.110.168 1626 0959 Example 3 58.4 12.170 77 060 8111 Example 4 57.110.67 10386 1037 Example 5 65.212.77 11676 0572 Example 6 55.710.768 8556 1088 Example 7 66.212.86 70986 1091 Example 8 61.3 11.165 272591101 Example 962.312.06996661155 Example 1055.310.2711100618172 Comparative Example 136.88.269010260199 Comparative Example 234.48.270985611109 Comparative Example 342.38.88216159584 Comparative Example 417.64.364912160971 Comparative Example 530.06.37115660862 Comparative Example 693.417.56615450949 Comparative Example 735.78.2668100620150

[0120] In Table 2, “TL” is the lamelling heat treatment temperature, “tL” is the lamelling heat treatment time, “T3” is the tempering heat treatment temperature, and “t3” is the tempering heat treatment time.

[0121] In Table 2, parameter 1 is the E value represented by Equation 1 below, and parameter 2 is the F value represented by Equation 2 below.

[0122] [Equation 1] 50 ≤ E = [Mn] / [Al] ≤ 67

[0123] [Equation 2] 9 ≤ F = ([C]+[Mn] / 6+[N]) / [Al] ≤ 13

[0124] (The above [Mn], [Al], [C], and [N] are the weight % values ​​of manganese, aluminum, carbon, and nitrogen, respectively)

[0125] Referring to Table 2, Examples 1 to 10 satisfy the ranges of Equations 1 and 2. The comparative examples do not satisfy both Equations 1 and 2. Comparative Example 6 exceeds the upper limits of Equations 1 and 2, and the other comparative examples are found to be below the lower limits of Equations 1 and 2.

[0126] Table 3 shows the yield strength, tensile strength, and elongation of the parent material in the experimental examples of the present invention.

[0127] Classification Yield strength (MPa) Tensile strength (MPa) Elongation (%) Example 1 84 187 721 Example 2 79 486 624 Example 3 62 07 1826 Example 4 88 492 121 Example 5 67 07 74 25 Example 6 7 2 48 1224 Example 7 64 7 7 83 25 Example 8 82 58 6122 Example 9 7 3 2 7 9 822 Example 10 7 0 5 7 7 123 Comparative Example 1 63 5 7 2 7 24 Comparative Example 2 62 07 2 9 26 Comparative Example 3 66 27 4 125 Comparative Example 4 5 6 16 6 126 Comparative Example 5 63 37 2 125 Comparative Example 6 83 18 7 320 Comparative Example 7 5 5 36 6 126

[0128] Referring to Table 3, the above-mentioned thick steel plate can satisfy yield strength (YS): 640 MPa to 890 MPa, tensile strength (TS): 710 MPa to 930 MPa, and elongation (EL): 20% to 30%.

[0129] Table 4 shows the impact toughness of the base material composed of the above-mentioned steel plate at -196℃. The unit of the impact toughness is "J".

[0130] Classification 1st measurement 2nd measurement 3rd measurement Average Example 1 199 188 189 192 Example 2 188 181 187 185 Example 3 169 191 170 177 Example 4 127 169 131 142 Example 5 139 141 131 137 Example 6 127 106 116 116 Example 7 152 160 171 161 Example 8 158 163 162 161 Example 9 15 9145148151Example 10184153175171Comparative Example 1188152174171Comparative Example 2145138130138Comparative Example 3225221216221Comparative Example 4262321308297Comparative Example 5171159176169Comparative Example 6117143129130Comparative Example 7227196179201

[0131] Referring to Table 4, it can be confirmed that the examples and comparative examples all have an average impact toughness of 27J or more at -196°C.

[0132] In order to confirm the properties of the heat-affected zone of the weld, the ultra-low temperature (@-196℃) impact toughness (unit: J) under low heat input welding (10 kJ / cm) and high heat input welding (25 kJ / cm) conditions are summarized in Tables 5 and 6, respectively.

[0133] In Tables 5 and 6, FL represents the fusion line area, FL+1mm represents the area 1 mm away from the fusion line, FL+3mm represents the area 3 mm away from the fusion line, and FL+5mm represents the area 5 mm away from the fusion line. “Av” represents the average value.

[0134] Low heat input welding (10 kJ / cm) FLFL+1 FL+3 FL+5 1st 2nd 3rd Av 1st 2nd 3rd Av 1st 2nd 3rd Av 1st 2nd 3rd Av Example 16563495975388165126131115124171180173175 Example 25169686370816773133139117130191167186181 Example 34856555357786868215216203211216197196203 Example 46345394944 7162595160696010191132108Example 5193828289073717893575970149159151153Example 673576966102727784157161139152162173178171Example 75852535412999144124215190223209189223218210Example 84562384866587165121119101114165171149 162 Example 965815968132118141130179161187176181174186180 Example 10737790801259189102153141138144166172185174 Comparative Example 1172031232519352613511799117160149155155 Comparative Example 22619412926162924203176218199165200158174 Comparative Example 3355642442 47210212223220235219225243251207234Comparative Example 43129192651422539158181181173256271244257Comparative Example 547625454443847433834323558533147Comparative Example 613917131922161971905973131118124124Comparative Example 74539474461384949144128135136207225198210

[0135] Referring to Table 5, it can be confirmed that the average impact toughness at cryogenic temperatures for the heat affected zone (FL to FL + 5 mm) under low heat input welding (10 kJ / cm) conditions for Examples 1 to 10 is all 27 J or more.

[0136] On the other hand, Comparative Examples 1, 2, 4, and 6 can be confirmed to have an average cryogenic impact toughness of less than 27 J in at least a portion of the heat-affected zone (FL ~ FL + 5 mm). For example, Comparative Examples 1, 4, and 6 can be confirmed to have an average cryogenic impact toughness of less than 27 J in the fusion line, and Comparative Examples 1, 2, and 6 can be confirmed to have an average cryogenic impact toughness of less than 27 J in a region 1 mm away from the fusion line.

[0137] Classification High heat input welding (25 kJ / cm) FLFL+1FL+3FL+51st2nd3rdAv1st2nd3rdAv1st2nd3rdAv1st2nd3rdAvExample 1413529355582566498888791141162139147Example 24451635371889083103111108107171180162171Example 3514937464550444622869230176210209218212Example 43740 31365183726965887877121118127122Example 523345638827241654741354112275115104Example 660903562687810283158174159164162177181173Example 78443596252629068153172104143167185181178Example 8352941355427384098908992 135141119132Example97448646211110399104165162199175171188183181Example109189377264417761182150119150153187179173Comparative Example121151417241317183783706366578569Comparative Example2282932303353404272665263193127204175Comparative Example3162 2916602230377390446912317598132Comparative Example 4241417182012221816138259153218183223208Comparative Example 521121717131315142845443959567062Comparative Example 64329383765665161648810184127109116117Comparative Example 7111918162129182650616057831097689

[0138] Referring to Table 6, it can be confirmed that the average impact toughness at cryogenic temperatures in the heat affected zone (FL to FL + 5 mm) under high heat input welding (25 kJ / cm) conditions for Examples 1 to 10 is all 27 J or more.

[0139] On the other hand, Comparative Examples 1, 3, 4, 5, and 7 can be confirmed to have an average cryogenic impact toughness of less than 27 J in at least a portion of the heat-affected zone (FL ~ FL + 5 mm). For example, Comparative Examples 1, 3, 4, 5, and 7 can be confirmed to have an average cryogenic impact toughness of less than 27 J in the fusion line, and Comparative Examples 1, 4, 5, and 7 can be confirmed to have an average cryogenic impact toughness of less than 27 J in a region 1 mm away from the fusion line.

[0140] Looking at the results of the experimental examples described above, it can be understood that when the composition range of the present invention and the ranges required by parameters (1) and (2) are both satisfied, the average impact toughness at extremely low temperatures is 27 J or more throughout the entire heat-affected zone under low heat input welding and high heat input welding conditions.

[0141] On the other hand, according to the comparative examples, when the composition range of the present invention is not satisfied, or when the composition range of the present invention is satisfied as in Comparative Examples 3 and 5 but the range required by the parameter (1) or the parameter (2) is not satisfied, it can be confirmed that a region in which the cryogenic average impact toughness does not reach 27J appears within the weld heat-affected zone. Therefore, the parameter (1) and the parameter (2) can be important criteria for securing the cryogenic average impact toughness of the weld heat-affected zone of 27J or more.

[0142] Figure 3 is a scanning electron microscope (SEM) photograph of the microstructure of a steel plate according to Example 9 and Comparative Example 6 among experimental examples of the present invention.

[0143] Referring to Fig. 3, Example 9 secured the residual austenite fraction of the base material by adding 2.18 wt% of manganese, and obtained a fine lath martensite structure by adding 0.035 wt% of aluminum.

[0144] On the other hand, Comparative Example 6 does not satisfy Formula E because 2.71 wt% of manganese is added, but 0.029 wt% of aluminum is added, and it can be seen that lath martensite is formed coarsely.

[0145] Figure 4 shows the results of analyzing the microstructure of the steel plate according to Example 8 and Comparative Example 1 among the experimental examples of the present invention using electron backscatter diffraction (EBSD).

[0146] Referring to FIG. 4, Example 8 has a microstructure in which retained austenite is evenly distributed at the austenite grain boundaries and packet boundaries by sufficiently adding 2.76 wt% of manganese, and thus can secure excellent ultra-low temperature toughness characteristics.

[0147] On the other hand, since Comparative Example 1 was added at a lower amount of 1.14 wt% compared to the lower limit of the range of the present invention, a small amount of retained austenite was locally formed, and therefore, it may be difficult to effectively suppress brittle crack propagation at extremely low temperatures.

[0148] Referring to FIGS. 3 and 4, the microstructure of the steel plate according to the embodiment of the present invention includes retained austenite dispersed in the tempered martensite matrix structure, the old austenite grain boundaries, and packet boundaries, in a range of 5% to 15% in terms of volume fraction. The tempered martensite is contained in a range of 85% to 95% in terms of volume fraction. The retained austenite is evenly dispersed and distributed in the lath interface, packet interface, old austenite interface, etc., and includes needle-shaped retained austenite having an average circle-equivalent diameter of 3 μm or less and an aspect ratio of the minor axis / major axis of 1.5 or more in a volume fraction of 50% or more with respect to the entire retained austenite. It was confirmed that other examples also have similar microstructures.

[0149] In the comparative example, it can be confirmed that the cryogenic toughness cannot be secured above 27 J during high heat input welding due to the excessive formation of coarsely coalesced bainite. In the present invention, coarsely coalesced bainite can be defined as a structure in which the packet size is large and a large amount of cementite is distributed within the packet. In addition, in the comparative example, it can be confirmed that the cryogenic toughness of the base material and the weld heat affected zone deteriorates when the retained austenite grows excessively and takes a spherical shape.

[0150] It will be apparent to a person skilled in the art to which the technical idea of ​​the present invention pertains that the technical idea of ​​the present invention described above is not limited to the above-described embodiments and the attached drawings, and that various substitutions, modifications, and changes are possible within a scope that does not depart from the technical idea of ​​the present invention.

Claims

1. A steel plate containing, by weight%, carbon (C): 0.03 to 0.10%, manganese (Mn): 1.4 to 3.0%, aluminum (Al): 0.025 to 0.10%, nickel (Ni): 4.8 to 6.0%, chromium (Cr): 0.1 to 1.0%, molybdenum (Mo): 0.1 to 1.0%, phosphorus (P): 0.01% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), and the remainder being iron (Fe) and other unavoidable impurities. The above rear plate satisfies the following equation 1: [Equation 1] 50 ≤ E = [Mn] / [Al] ≤ 67 (The above [Mn] and [Al] are the weight % values ​​of manganese and aluminum, respectively) A thick steel plate with excellent weld heat-affected zone ultra-low temperature toughness.

2. In paragraph 1, The above rear plate satisfies the following equation 2: [Equation 2] 9 ≤ F = ([C]+[Mn] / 6+[N]) / [Al] ≤ 13 (The above [Mn], [Al], [C], and [N] are the weight % values ​​of manganese, aluminum, carbon, and nitrogen, respectively) A thick steel plate with excellent weld heat-affected zone ultra-low temperature toughness.

3. In paragraph 1, The above rear plate, A microstructure having a tempered martensite matrix structure and retained austenite dispersed in the grain boundaries and packet boundaries in a range of 5 to 15% by volume fraction. A thick steel plate with excellent weld heat-affected zone ultra-low temperature toughness.

4. In paragraph 3, The above-mentioned retained austenite has an average equivalent diameter of less than 3 μm, The retained austenite in the form of a needle having an aspect ratio of the short / long axis of 1.5 or more accounts for 50% or more in volume fraction with respect to the entire retained austenite. A thick steel plate with excellent weld heat-affected zone ultra-low temperature toughness.

5. In paragraph 1, The above rear plate, -An average impact toughness of 27J or more at -196℃ A thick steel plate with excellent weld heat-affected zone ultra-low temperature toughness.

6. In paragraph 1, In the above rear plate, The weld heat affected zone formed by a heat input in the range of 10 kJ / cm to 30 kJ / cm has an average impact toughness of 27 J or more at -196°C. A thick steel plate with excellent weld heat-affected zone ultra-low temperature toughness.

7. In paragraph 1, The above rear plate, Yield strength (YS): 640 MPa to 890 MPa, tensile strength (TS): 710 MPa to 930 MPa, and elongation (EL): 20% to 30%, A thick steel plate with excellent weld heat-affected zone ultra-low temperature toughness.

8. A first step of providing a steel containing, by weight%, carbon (C): 0.03 to 0.10%, manganese (Mn): 1.4 to 3.0%, aluminum (Al): 0.025 to 0.10%, nickel (Ni): 4.8 to 6.0%, chromium (Cr): 0.1 to 1.0%, molybdenum (Mo): 0.1 to 1.0%, phosphorus (P): 0.01% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), and the remainder being iron (Fe) and other unavoidable impurities; The second step of hot rolling the above steel material under the conditions of a reheating temperature of 1000 to 1200℃ and a finishing rolling temperature of Ar3 or higher; A third step of heating the hot-rolled steel to a temperature range of Ac1 to Ac3, maintaining it, and then rapidly cooling it; and A fourth step of sequentially tempering the above steel at a temperature below Ac1, The steel plate manufactured by the method for manufacturing the steel plate is Satisfying the following equation 1, [Equation 1] 50 ≤ E = [Mn] / [Al] ≤ 67 (The above [Mn] and [Al] are the weight % values ​​of manganese and aluminum, respectively) A method for manufacturing a steel plate having excellent ultra-low temperature toughness in the welded heat-affected zone.

9. In paragraph 8, The above rear plate satisfies the following equation 2: [Equation 2] 9 ≤ F = ([C]+[Mn] / 6+[N]) / [Al] ≤ 13 (The above [Mn], [Al], [C], and [N] are the weight % values ​​of manganese, aluminum, carbon, and nitrogen, respectively) A method for manufacturing a steel plate having excellent ultra-low temperature toughness in the welded heat-affected zone.

10. In paragraph 8, The third step above is, It is performed by maintaining the temperature range of Ac1 to Ac3 for 30 to 150 minutes. A method for manufacturing a steel plate having excellent ultra-low temperature toughness in the welded heat-affected zone.

11. In paragraph 8, The fourth step above is, Performed at a temperature ranging from 550℃ to 620℃ for 30 to 200 minutes, A method for manufacturing a steel plate having excellent ultra-low temperature toughness in the welded heat-affected zone.

12. A welded structure manufactured by welding a steel plate having excellent ultra-low temperature toughness in the welded heat-affected zone according to any one of clauses 1 to 7, Satisfies the following equation 1, [Equation 1] 50 ≤ E = [Mn] / [Al] ≤ 67 (The above [Mn] and [Al] are the weight % values ​​of manganese and aluminum, respectively) The weld heat affected zone of the above welded structure has an average impact toughness of 27J or more at -196℃. Welded structures.

13. In paragraph 12, The above welded structure, Satisfying the following equation 2, [Equation 2] 9 ≤ F = ([C]+[Mn] / 6+[N]) / [Al] ≤ 13 (The above [Mn], [Al], [C], and [N] are the weight % values ​​of manganese, aluminum, carbon, and nitrogen, respectively) Welded structures.

14. In paragraph 12, The above welded heat affected zone of the above welded structure is, A microstructure having a volume fraction of 95% or more of bainite and lath martensite combined. Welded structures.

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