Thick steel plate having excellent cryogenic impact toughness, and manufacturing method therefor

Abstract

The present invention provides a thick steel plate having excellent cryogenic impact toughness that can be applied to a liquid hydrogen storage tank and the like, and a manufacturing method therefor. According to one embodiment of the present invention, the thick steel sheet comprises, by wt%, 0.02-0.08% of carbon (C), 0.15-0.5% of silicon (Si), 0.5-2.0% of manganese (Mn), 0.001-0.06% of aluminum (Al), 13-16% of nickel (Ni), 0.001-0.5% of chrome (Cr), 0.01-0.5% of molybdenum (Mo), 0.001-0.01% of titanium (Ti), 0.001-0.05% of niobium (Nb), 0.001-0.01% of vanadium (V), 0.01% or less (excluding 0%) of phosphorus (P), 0.01% or less (excluding 0%) of sulfur (S), and the balance of iron (Fe) and other inevitable impurities.

Description

Thick steel plate with excellent ultra-low temperature impact toughness and method for manufacturing the same

[0001] The present invention relates to a steel material with excellent ultra-low temperature impact toughness and a method for manufacturing the same, and more specifically, to a thick steel plate with excellent ultra-low temperature impact toughness applicable to liquid hydrogen storage tanks, etc., and a method for manufacturing the same.

[0002] Various methods are being proposed and implemented under the name of carbon neutrality to realize a sustainable society. Among these, industries utilizing hydrogen are receiving significant attention. Applications of hydrogen energy include large-scale hydrogen mobility such as trains and ships, small-scale hydrogen mobility such as drones, passenger vehicles, and construction machinery, and hydrogen power generation including power plants and small generators. For the smooth application of hydrogen to these sectors, a hydrogen industry infrastructure encompassing production, storage, and transportation must be in place. In the field of storing and transporting produced hydrogen, tanks and cargo containers are utilized, and material assurance regarding the operating environment is considered critical.

[0003] For the transport and handling of gaseous hydrogen, high-pressure compression of over 200 bar must be maintained to reduce its large volume. On the other hand, liquid hydrogen is hydrogen existing in gaseous form at -253 o It is a form converted into a liquid by cooling below C, having 1 / 800th the volume of gaseous hydrogen and eliminating the need for high-pressure compression. However, in the case of liquid hydrogen, -253°C o Temperatures must be controlled below C, and since tanks and cargo holds are exposed to cryogenic operating temperatures, material assurance of steel at cryogenic temperatures is essential.

[0004] Currently, materials such as SUS 304, SUS 316L, and aluminum, which do not exhibit cryogenic brittleness, are primarily used for liquid hydrogen storage tanks. However, these materials are more expensive than general carbon steel, and due to the characteristics of their microstructure, achieving high strength is difficult; consequently, manufacturing costs increase as the material thickness is increased. On the other hand, a method can be applied to improve impact toughness in the cryogenic region by controlling the steel's microstructure to a soft austenite structure to facilitate deformation in the cryogenic range. However, forming a soft austenite structure at room temperature requires the addition of a large amount of alloy, and management of oxides and inclusions is necessary. Consequently, this results in the disadvantage of significantly high manufacturing costs for the steel.

[0005] The technical problem that the technical concept of the present invention aims to solve is to provide a thick steel plate with excellent ultra-low temperature impact toughness applicable to liquid hydrogen storage tanks, etc., and a method for manufacturing the same.

[0006] However, these tasks are exemplary, and the technical concept of the present invention is not limited thereto.

[0007] According to one aspect of the present invention, a thick steel plate with excellent ultra-low temperature impact toughness and a method for manufacturing the same are provided.

[0008] According to one embodiment of the present invention, the thick steel plate comprises, in weight percent, carbon (C): 0.02% to 0.08%, silicon (Si): 0.15% to 0.5%, manganese (Mn): 0.5% to 2.0%, aluminum (Al): 0.001% to 0.06%, nickel (Ni): 13% to 16%, chromium (Cr): 0.001% to 0.5%, molybdenum (Mo): 0.01% to 0.5%, titanium (Ti): 0.001% to 0.01%, niobium (Nb): 0.001% to 0.05%, vanadium (V): 0.001% to 0.01%, phosphorus (P): 0.01% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), and the remainder being iron (Fe) and other It may contain unavoidable impurities.

[0009] According to one embodiment of the present invention, the thick steel plate has a yield strength (YS): 600 MPa or more, -253 o Elongation (EL) at C: 17% or more, and -253 o Impact toughness at C: Can satisfy 90 J or more.

[0010] According to one embodiment of the present invention, the thick steel plate may have a microstructure including austenite, ferrite, and martensite.

[0011] According to one embodiment of the present invention, in the thick steel plate, the area fraction of the austenite is 17% or more and 50% or less, and the area fraction of the ferrite and the area fraction of the martensite may be greater than 50% and less than 83%.

[0012] According to one embodiment of the present invention, the thick steel plate may have a microstructure including austenite and ferrite.

[0013] According to one embodiment of the present invention, in the thick steel plate, the area fraction of the austenite is 17% or more and 50% or less, and the area fraction of the ferrite may be greater than 50% and less than 83%.

[0014] According to one embodiment of the present invention, the thick steel plate can satisfy the following equation.

[0015] 5[Ni] + 20[Cr]-300[C]-10[Mn]+[Mo] > 40

[0016] (Here, [Ni], [Cr], [C], [Mn], and [Mo] are the weight percent of nickel, chromium, carbon, manganese, and molybdenum in the composition constituting the thick steel plate)

[0017] According to one embodiment of the present invention, the thick steel plate may include at least one of cobalt (Co): 0% to 0.001%, boron (B): 0% to 0.001%, and tin (Sn): 0% to 0.01% as the unavoidable impurities.

[0018] According to one embodiment of the present invention, the method for manufacturing the thick steel plate comprises, in weight percent, carbon (C): 0.02% to 0.08%, silicon (Si): 0.15% to 0.5%, manganese (Mn): 0.5% to 2.0%, aluminum (Al): 0.001% to 0.06%, nickel (Ni): 13% to 16%, chromium (Cr): 0.001% to 0.5%, molybdenum (Mo): 0.01% to 0.5%, titanium (Ti): 0.001% to 0.01%, niobium (Nb): 0.001% to 0.05%, vanadium (V): 0.001% to 0.01%, phosphorus (P): 0.01% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), and the remainder being 1,100 steel materials containing iron (Fe) and other unavoidable impurities o C ~ 1,250 o Step of reheating at a temperature of C; the heated steel at 800 o C ~ 900 o The method may include: a step of hot rolling to be terminated at a temperature of C; a quenching step of maintaining the hot-rolled steel at a temperature of Ac3 or higher and then rapidly cooling it; a lamellating step of heat treating the quenched steel by maintaining it at a lamellating temperature (L) in the range of Ac1 to Ac3; and a tempering step of heat treating the lamellated steel by maintaining it at a temperature of Ac1 or lower.

[0019] According to one embodiment of the present invention, the method for manufacturing the thick steel plate can satisfy the following formula.

[0020] (L - Ac1) / (Ac3 - Ac1) < 0.41

[0021] According to one embodiment of the present invention, the quenching step comprises 750 steel o C ~ 950 o The process is performed by maintaining within the C range and then rapidly cooling, and the lamellating step is performed by the steel at 550 o C ~ 720 oPerformed while maintaining within the C range, and the tempering step is performed on the steel at 520 o C ~ 580 o It can be performed by maintaining the C range.

[0022] According to one embodiment of the present invention, the steel material can satisfy the following equation.

[0023] 5[Ni] + 20[Cr]-300[C]-10[Mn]+[Mo] > 40

[0024] (Here, [Ni], [Cr], [C], [Mn], and [Mo] are the weight percent of nickel, chromium, carbon, manganese, and molybdenum in the composition constituting the thick steel plate)

[0025] According to one embodiment of the present invention, the thick steel plate manufactured by the method for manufacturing the thick steel plate has a yield strength (YS): 600 MPa or more, -253 o Elongation (EL) at C: 17% or more, and -253 o Impact toughness at C: Can satisfy 90 J or more.

[0026] According to one embodiment of the present invention, a thick steel plate manufactured by the method for manufacturing a thick steel plate has a microstructure including austenite, ferrite, and martensite, wherein the area fraction of the austenite is 17% or more and 50% or less, and the area fraction of the ferrite and the area fraction of the martensite may be greater than 50% and less than 83%.

[0027] According to one embodiment of the present invention, the steel material is provided as a steel slab manufactured by charging iron scrap during the steelmaking process, and the steel slab can be manufactured using 80 weight% or less of blast furnace molten iron and 20 weight% or more of iron scrap.

[0028] A thick steel plate with excellent ultra-low temperature impact toughness according to the technical concept of the present invention is based on ferrite, and by nickel content and quenching-lamellarizing-tempering heat treatment, it is possible to secure ultra-low temperature impact toughness, contribute to improving manufacturing costs, and can be applied as a material for shipbuilding and land-based liquid hydrogen storage tanks.

[0029] The effects of the present invention described above are illustrative and the scope of the present invention is not limited by these effects.

[0030] FIG. 1 is a process flowchart schematically showing a method for manufacturing a thick steel plate according to an embodiment of the present invention.

[0031] FIG. 2 is a scanning electron microscope image showing the microstructure of a thick steel plate manufactured using a method for manufacturing a thick steel plate according to an embodiment of the present invention.

[0032] Figure 3 shows a schematic diagram of the structure of a thick steel plate manufactured according to an embodiment of the present invention.

[0033] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. The embodiments of the present invention are provided to more fully explain the technical concept of the present invention to those skilled in the art, and the following embodiments may be modified in various different forms, and the scope of the technical concept of the present invention is not limited to the following embodiments. Rather, these embodiments are provided to make the present disclosure more faithful and complete and to fully convey the technical concept of the present invention to those skilled in the art. In this specification, the same reference numerals denote the same elements throughout. Furthermore, various elements and areas in the drawings are depicted schematically. Accordingly, the technical concept of the present invention is not limited by the relative sizes or spacing depicted in the attached drawings.

[0034] Austenitic steel, with its austenite crystal phase, exhibits minimal low-temperature embrittlement and ensures stable ductility and toughness at low temperatures. In contrast, ferrite steel displays pronounced embrittlement at low temperatures, making it generally unsuitable for low-temperature use. However, since ferritic steel possesses higher yield strength compared to austenitic steel, allowing for reduced tank thickness, efforts to apply it to liquid hydrogen storage tanks are ongoing.

[0035] According to the technical concept of the present invention, a ferrite-based thick steel plate with excellent ultra-low temperature impact toughness is provided. Retained austenite is formed in the ferrite, and the low-temperature toughness and low-temperature ductility of the ferritic steel are improved. To form the retained austenite, 13% to 15% by weight of nickel, an austenite stabilizing element, is added. The ferritic steel with increased nickel content has a very high yield strength compared to the austenitic steel, and accordingly, the tank thickness can be reduced to save material, and it can be applied to ships and land-based equipment due to its high allowable stress.

[0036] Hereinafter, a thick steel plate with excellent ultra-low temperature impact toughness, which is one aspect of the present invention, will be described.

[0037] Thick steel plate with excellent ultra-low temperature impact toughness

[0038] A thick steel plate, which is one aspect of the present invention, comprises, in weight percent, carbon (C): 0.02% to 0.08%, silicon (Si): 0.15% to 0.5%, manganese (Mn): 0.5% to 2.0%, aluminum (Al): 0.001% to 0.06%, nickel (Ni): 13% to 16%, chromium (Cr): 0.001% to 0.5%, molybdenum (Mo): 0.01% to 0.5%, titanium (Ti): 0.001% to 0.01%, niobium (Nb): 0.001% to 0.05%, vanadium (V): 0.001% to 0.01%, phosphorus (P): 0.01% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), and the remainder being iron (Fe) and other unavoidable It contains impurities. The above-mentioned thick steel plate may be composed of the composition and content of the elements described above.

[0039] In addition, the above-mentioned thick steel plate may include at least one of cobalt (Co): 0% to 0.001%, boron (B): 0% to 0.001%, and tin (Sn): 0% to 0.01% as the above-mentioned unavoidable impurities.

[0040] The role and content of each component included in the thick steel plate according to the present invention are described below. In this case, the content of the component elements refers to weight percent.

[0041] Carbon (C): 0.02% ~0.08%

[0042] Carbon is an element that improves the strength of steel and stabilizes austenite. If the carbon content is less than 0.02%, it may be difficult to secure strength because martensite cannot be formed during the cooling process after heat treatment. If the carbon content exceeds 0.08%, austenite may be formed in excess of what is necessary, and ductility and toughness may decrease. Therefore, it is desirable to add carbon in an amount of 0.02% to 0.08% of the total weight of the thick steel plate.

[0043] Silicon (Si): 0.15% ~ 0.5%

[0044] Silicon is added as a deoxidizer to remove oxygen from steel and can improve the stability of residual austenite by suppressing the formation of carbides during tempering. If the silicon content is less than 0.15%, the effect of adding silicon is insufficient. If the silicon content exceeds 0.5%, weldability may be reduced, and toughness may be reduced by excessively forming non-metallic inclusions. Therefore, it is desirable to add silicon at a level of 0.15% to 0.5% of the total weight of the thick steel plate.

[0045] Manganese (Mn): 0.5% ~ 2.0%

[0046] Manganese is an effective solid solution strengthening element for increasing the strength of steel, and it is an element that stabilizes residual austenite by diffusing into grain boundaries and phase boundaries during tempering. If the manganese content is less than 0.5%, the effect of manganese addition is insufficient. If the manganese content exceeds 2.0%, austenite may be formed in excess of what is necessary, inclusions such as MnS may be formed, and reduced workability and resistance to delayed fracture due to segregation may occur, and ductility and toughness may be reduced. Therefore, it is desirable to add manganese in an amount of 0.5% to 2.0% of the total weight of the thick steel plate.

[0047] Aluminum (Al): 0.001% ~ 0.06%

[0048] Aluminum acts as a deoxidizer to remove oxygen from steel and can also help refine the metal structure or reduce dissolved nitrogen, which lowers cryogenic toughness, by forming AlN. If the aluminum content is less than 0.001%, the effect of adding aluminum is insufficient. If the aluminum content exceeds 0.06%, it may form aluminum oxide, a non-metallic inclusion, which can lower low-temperature impact toughness. Therefore, it is desirable to add aluminum in an amount of 0.001% to 0.06% of the total weight of the thick steel plate.

[0049] Nickel (Ni): 13% ~ 16%

[0050] Nickel is an essential element for stabilizing retained austenite and ensuring cryogenic toughness. When the nickel content is less than 13%, -253 o The effect of nickel addition is insufficient, such as a decrease in cryogenic toughness at C. If the nickel content exceeds 16%, it may reduce economic efficiency. Therefore, it is desirable to add nickel at 13% to 16% of the total weight of the thick steel plate.

[0051] Chrome (Cr): 0.001% ~ 0.5%

[0052] Chromium is an element that ensures hardenability and increases strength. If the chromium content is less than 0.001%, the effect of adding chromium is insufficient. If the chromium content exceeds 0.5%, it may reduce toughness and weldability. Therefore, it is desirable to add chromium in an amount of 0.001% to 0.5% of the total weight of the thick steel plate.

[0053] Molybdenum (Mo): 0.01% ~ 0.5%

[0054] Molybdenum is an element that increases strength and also has the effect of suppressing intergranular embrittlement. If the molybdenum content is less than 0.01%, the effect of adding molybdenum is insufficient. If the molybdenum content exceeds 0.5%, it may reduce toughness and weldability. Therefore, it is desirable to add molybdenum in an amount of 0.01% to 0.5% of the total weight of the thick steel plate.

[0055] Titanium (Ti): 0.001% ~ 0.01%

[0056] Titanium is an element that forms carbide precipitates, and strength can be improved through the precipitation of TiN and the grain refinement effect. If the titanium content is less than 0.001%, the effect of titanium addition is insufficient. If the titanium content exceeds 0.01%, inclusions may be over-formed, which can lower toughness. Therefore, it is desirable to add titanium (Ti) in an amount of 0.001% to 0.01% of the total weight of the thick steel plate.

[0057] Niobium (Nb): 0.001% ~ 0.05%

[0058] Niobium is a precipitation-strengthening element that improves strength by forming carbon and carbides when added. If the niobium content is less than 0.001%, the effect of adding niobium is insufficient. If the niobium content exceeds 0.05%, inclusions may be over-formed, which can reduce toughness. Therefore, it is desirable to add niobium in an amount of 0.001% to 0.05% of the total weight of the thick steel plate.

[0059] Vanadium (V): 0.001% ~ 0.01%

[0060] When added, vanadium forms carbon and carbides, thereby improving strength. If the vanadium content is less than 0.001%, the effect of vanadium addition is insufficient. If the vanadium content exceeds 0.01%, cryogenic toughness may be reduced. Therefore, it is desirable to add vanadium in an amount of 0.001% to 0.01% of the total weight of the thick steel plate.

[0061] Phosphorus (P): 0.01% or less (excluding 0%)

[0062] Since phosphorus segregates at grain boundaries during the slab reheating process and causes brittleness, it is better to have a lower content. If phosphorus is contained in an amount exceeding 0.01%, it segregates at crystal grain boundaries, causing work brittleness and potentially reducing toughness and weldability. In the present invention, phosphorus is not intentionally added but may be included as an unavoidable impurity. Therefore, it is desirable to control phosphorus to 0.01% or less (including 0%) of the total weight of the thick steel plate.

[0063] Sulfur (S): 0.01% or less (excluding 0%)

[0064] Sulfur is an element inevitably contained along with phosphorus during the manufacture of steel, and it can impair the toughness and weldability of the steel. If sulfur is contained in an amount exceeding 0.01%, it forms inclusions (MnS), which worsen resistance to stress corrosion cracking and may cause cracks during the processing of the steel, thereby reducing the corrosion resistance of the steel. In the present invention, sulfur is not intentionally added but may be contained as an unavoidable impurity. Therefore, it is desirable to control the sulfur content to 0.01% or less (including 0%) of the total weight of the thick steel plate.

[0065] The remaining component of the above-mentioned thick steel plate is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the normal manufacturing process, they cannot be excluded. As these impurities are known to any skilled person in the normal manufacturing process, all details thereof are not specifically mentioned in this specification.

[0066] By controlling the specific components of the aforementioned alloy composition and the content ranges thereof, the thick steel plate manufactured through the method for manufacturing a thick steel plate described below has a yield strength (YS): 600 MPa or higher, -253 o Elongation (EL) at C: 17% or more, and -253 oImpact toughness at C: Can satisfy 90 J or more. For example, the above-mentioned thick steel plate, yield strength (YS): 600 MPa ~ 780 MPa, -253 o Elongation (EL) at C: 17% to 35%, and -253 o Impact toughness at C: Can satisfy 90 J ~ 120 J.

[0067] The above-mentioned thick steel plate may have a thickness in the range of 10 mm to 50 mm.

[0068] The above-mentioned thick steel plate may have a microstructure including austenite, ferrite, and martensite. The area fraction of the austenite may be 17% or more and 50% or less. The area fraction of the ferrite and the area fraction of the martensite may be the sum of the remaining area fraction, and may be greater than 50% and less than 83%.

[0069] The technical scope of the present invention also includes cases where the above-mentioned thick steel plate comprises austenite and ferrite but does not comprise martensite, in which case the area fraction of the austenite may be 17% or more and 50% or less, and the area fraction of the ferrite may be greater than 50% and less than 83%. The above-mentioned area fraction refers to the area ratio derived from a microstructure photograph of the above-mentioned thick steel plate through an image analyzer.

[0070] The thick steel plate according to the technical concept of the present invention is a ferritic steel that may contain ferrite in a high area fraction and may contain a portion of austenite. The austenite is used metallurgically as a means to secure ultra-low temperature ductility and toughness. A low area fraction of austenite means that the proportion of phases capable of participating in deformation in an ultra-low temperature environment is small, which can have a significant effect on the ultra-low temperature elongation. Furthermore, as a material for storing liquid hydrogen, not only must it guarantee performance at ultra-low temperatures, but resistance to hydrogen embrittlement that may be caused by hydrogen must also be considered. That is, austenite has high solubility for hydrogen and a low diffusion rate, resulting in high resistance to hydrogen. If the austenite area fraction is less than 17%, it may induce hydrogen embrittlement.

[0071] As the above-mentioned thick steel plate is applied in an extreme environment of minus 253 degrees, the formation behavior of austenite around ferrite is very important. Austenite exhibits almost no embrittlement behavior at low temperatures, and accordingly, can ensure toughness and ductility at low temperatures. Therefore, the amount of the austenite phase formed is important. This austenite formation behavior is determined by the following lamelling or tempering steps, and the amount of residual austenite may vary depending on the temperature in the phase range. In addition, the amount of austenite formed may also vary depending on the heat treatment holding time during the lamelling or tempering steps.

[0072] However, a high austenite content does not necessarily guarantee superior ultra-low temperature toughness and ductility. In other words, differences in material properties at ultra-low temperatures can occur due to interactions between ferrite and austenite. Therefore, an optimal austenite fraction may exist.

[0073] Martensite is formed as carbon from the austenite is not expelled to the outside due to quenching. Subsequently, during the lamelling or tempering stage, the martensite can be transformed into ferrite by expelling carbon into the austenite. That is, ferrite is formed in the phase region, and furthermore, the martensite can transform to form more ferrite.

[0074] A method for manufacturing a thick steel plate according to the present invention will be described below with reference to the attached drawings.

[0075] Method for manufacturing thick steel plates

[0076] FIG. 1 is a process flowchart schematically showing a method for manufacturing a thick steel plate according to an embodiment of the present invention.

[0077] In the method for manufacturing a thick steel plate according to the present invention, the semi-finished product as the steel material subject to the hot rolling process may be, for example, a slab. The slab in the semi-finished product state can be obtained through a continuous casting process after obtaining molten steel of a predetermined composition through a steelmaking process.

[0078] The above steel comprises, in weight percent, carbon (C): 0.02% to 0.08%, silicon (Si): 0.15% to 0.5%, manganese (Mn): 0.5% to 2.0%, aluminum (Al): 0.001% to 0.06%, nickel (Ni): 13% to 16%, chromium (Cr): 0.001% to 0.5%, molybdenum (Mo): 0.01% to 0.5%, titanium (Ti): 0.001% to 0.01%, niobium (Nb): 0.001% to 0.05%, vanadium (V): 0.001% to 0.01%, phosphorus (P): 0.01% or less (excluding 0%), sulfur (S): 0.001% or less (excluding 0%), and the remainder being iron (Fe) and other unavoidable impurities.

[0079] In addition, the steel may contain at least one of cobalt (Co): 0% to 0.001%, boron (B): 0% to 0.001%, and tin (Sn): 0% to 0.01% as the unavoidable impurities.

[0080] Referring to FIG. 1, a method for manufacturing a thick steel plate according to an embodiment of the present invention includes a reheating step (S110), a hot rolling step (S120), a quenching step (S130), a lamelling step (S140), and a tempering step (S150).

[0081] Reheating step (S110)

[0082] In the reheating step (S110), steel having the above composition, for example, a slab plate, 1,100 o C ~ 1,250 o Reheating is performed at a Slab Reheating Temperature (SRT) of C. Through this reheating, the redissolution of components segregated during casting and the redissolution of precipitates may occur; in particular, a sufficiently high temperature is required to redissolve precipitation-hardening elements such as niobium, thereby forming fine precipitates upon subsequent cooling. These fine precipitates hinder grain growth, leading to grain refinement, which can consequently increase strength. The reheating temperature is 1,100 o If the value is less than C, maximum redissolution of precipitation-hardened elements such as niobium contained in the steel may occur, and segregated components may not be sufficiently evenly distributed during casting. The reheating temperature is 1,200 o If C is exceeded, the austenite grains may coarsen, which can lead to a decrease in yield strength. Furthermore, as the reheating temperature increases, there are problems such as rising manufacturing costs and reduced productivity due to increased heating costs and the time required to extract the heat to meet the hot rolling temperature.

[0083] Hot rolling step (S120)

[0084] In the hot rolling step (S120), the reheated steel is first hot-rolled to adjust its shape. The hot rolling can be performed continuously in width rolling, rough rolling, and finish rolling. Through the hot rolling step, the steel can form a hot-rolled steel sheet.

[0085] The above hot rolling, that is, the above finishing rolling, is 800 o C ~ 900 o It can be terminated at a finish rolling temperature (FRT) of C. The above finish rolling temperature is 800 o If the temperature is below C, rolling occurs in the abnormal region between austenite and ferrite, resulting in the incorporation of grains and non-uniform deformability, which may lead to a decrease in rollability. The above finish rolling end temperature is 900 o If C is exceeded, the grain size may coarsen, which may result in a decrease in the strength of the final steel. After the rolling is completed, the thickness of the steel can be controlled to 12 mm to 50 mm. After the hot rolling is completed, the steel can be air-cooled to room temperature.

[0086] Quenching step (S130)

[0087] In the quenching step (S130), the hot-rolled steel is quenched by heating and maintaining it at a constant temperature above Ac3, and then rapidly cooling it. That is, the hot-rolled steel is heated to a temperature above Ac3, which is above the austenite region temperature, i.e., to the austenite region temperature, and then cooled to room temperature, for example, 0 o C to 40 o Quenching heat treatment is performed by rapidly cooling to a temperature in the range of C using methods such as water cooling. Accordingly, the entire material is transformed into austenite by heating, and the austenite can be transformed into martensite by quenching.

[0088] In the present invention, 13 wt% to 15 wt% of nickel has an Ac3 temperature of 600o C ~ 750 o Since it is C, considering the margin, the heating temperature for the quenching step is, for example, 750 o C ~ 950 o You can set it to the C range, for example, 800 o C ~ 900 o It can be set to a range of C. For example, the above hot-rolled steel is 750 o C ~ 950 o It can be heated to C and maintained for 20 to 150 minutes. During quenching, the above steel is 8 o It can be cooled at a cooling rate of C / s or higher, and 8 o C / s to 100 o It can be cooled at a cooling rate in the range of C / s. The above cooling rate is 8 o If C / s is less than 1, a soft structure rather than martensite is mixed in, and a sufficient quenching effect cannot be achieved.

[0089] Lamelizing step (S140)

[0090] In the lamellarizing step (S140), the quenched steel is heat-treated at a lamellarizing temperature within the Ac1 to Ac3 range, which is an ideal temperature range. The lamellarizing heat treatment step is intended to form ferrite and austenite from a structure in which martensite is formed, and the temperature range may vary depending on the composition of the alloy. The Ac1 and Ac3 temperatures can be determined through dilatometer testing. Depending on the applied temperature within this ideal temperature range, the formation behavior of austenite, such as the austenite fraction, size, and distribution, can be significantly altered. After performing the lamellarizing step, the temperature is lowered to room temperature, for example, 0 o C to 40 oRapid cooling can be performed by means such as water cooling to a temperature in the range of C. Through such rapid cooling, the austenite can be transformed into martensite. The above lamellating step may be performed one or more times.

[0091] In the present invention, the temperature of the lamellating step is, for example, 550 o C ~ 720 o You can set it to the C range, for example, 580 o C ~ 680 o It can be set to a C range. The lamellating step can be performed while maintaining the temperature for, for example, 20 to 150 minutes. In addition, the temperature of the lamellating step is, for example, 590 o C or higher 630 o A range of less than C is desirable.

[0092] Tempering step (S150)

[0093] In the tempering step (S150), the lamelled steel is heat-treated at a temperature below Ac1, which is the lowest point of the abnormal temperature range. The tempering step can finely control the ferrite and austenite formed in the lamelling step, remove residual stress in the ferrite and austenite, and thereby increase the elongation. After performing the tempering step, the temperature is lowered to room temperature, for example, 0 o C to 40 o Cooling can be performed by means such as water cooling or air cooling to a temperature in the range of C. The above tempering step may be performed one or more times.

[0094] In the present invention, the heat treatment temperature for tempering is, for example, 580 o It can be set to a range of C or less, for example, 520 o C ~ 580 o It can be set to a C range. The tempering step can be performed, for example, while maintaining the temperature for 40 to 180 minutes.

[0095] A thick steel plate according to the technical concept of the present invention can satisfy the following Equation 1 with respect to composition content.

[0096] [Equation 1]

[0097] 5[Ni] + 20[Cr]-300[C]-10[Mn]+[Mo] > 40

[0098] Here, [Ni], [Cr], [C], [Mn], and [Mo] are the weight percent of nickel, chromium, carbon, manganese, and molybdenum in the composition constituting the thick steel plate.

[0099] Nickel (Ni) is an austenite-stabilizing element, but since its effect on elongation and impact toughness is relatively small when over-added, it was set to a positive value (+) in Equation 1 above.

[0100] Carbon (C) is an austenite stabilizing element, but if added in excess, it can form a Cottrell atmosphere within the ferrite and hinder the deformation of the austenite, so it was set to a negative value (-) in Equation 1 above.

[0101] Manganese (Mn) is an austenite stabilizing element, but if added in excess, it forms MnS inclusions and causes concentration or segregation at grain boundaries, which can reduce ductility and toughness; therefore, it was set to a negative value (-) in Equation 1 above.

[0102] Chromium (Cr) and molybdenum (Mo) are elements that improve strength and can provide an effect similar to that of nickel, so they were set as positive values ​​(+) in Equation 1.

[0103] In addition, the thick steel plate according to the technical concept of the present invention can satisfy the following Equation 2 with respect to the lamelling temperature.

[0104] [Equation 2]

[0105] (L - Ac1) / (Ac3 - Ac1) < 0.41

[0106] Here, L is the lamelling temperature, Ac1 is the Ac1 temperature, and Ac3 is the Ac3 temperature.

[0107] In the case satisfying the above Equation 2, austenite fraction, yield strength, ultra-low temperature (-253 o C) Elongation and ultra-low temperature (-253 o C) Impact toughness can be guaranteed within the scope of the present invention. If the lamelling step is performed under conditions that do not satisfy Equation 2, then if an additional lamelling step is performed under conditions that satisfy Equation 2, the physical properties can be guaranteed within the scope of the present invention.

[0108] In the above lamellating step, the austenite and ferrite fractions can be determined by the lever rule within the anomalous temperature range, and the elements constituting the austenite can also change. In this case, the roles of carbon, manganese, and nickel as austenite stabilizing elements, which maintain the stability of the austenite against thermal and mechanical environments, are important. If the lamellating heat treatment is performed at a relatively high temperature within the anomalous temperature range, the formed austenite has a low degree of stability and becomes unstable upon rapid cooling, potentially transforming back into martensite; consequently, the austenite fraction decreases.

[0109] If the lamelling step is performed at a high temperature, for example, a high temperature that does not satisfy Equation 2, and the target properties cannot be achieved, the lamelling step can be performed again at a low temperature, for example, a temperature that satisfies Equation 2, so the target properties can be achieved and the elongation can be further improved. On the other hand, if the austenite fraction is excessive, the elongation decreases, so it is analyzed that an optimal condition exists.

[0110] The above steel material can be manufactured by charging iron scrap during the steelmaking process. The above steel slab can be manufactured using 80 weight percent or less of blast furnace molten iron and 20 weight percent or more of iron scrap. The above steelmaking process can be carried out using an electric furnace. By this method, the amount of molten iron used in conventional blast furnace operations, which emit a large amount of carbon, is reduced, and the insufficient amount of molten iron is replaced with iron scrap that does not generate carbon during the manufacturing process, thereby reducing carbon emissions generated during the manufacturing process. By charging the above iron scrap, costs can be lowered, contributing to the prevention of environmental pollution, reducing the generation of greenhouse gases such as carbon dioxide during the steel product manufacturing process, and preventing problems such as increased costs due to carbon taxes and environmental burdens.

[0111] Experimental Example

[0112] Preferred experimental examples are presented below to aid in understanding the present invention. However, the following experimental examples are intended only to aid in understanding the present invention and do not limit the present invention. Details not described herein can be sufficiently technically inferred by those skilled in the art, so their description is omitted.

[0113] The characteristics of the thick steel plate manufactured in this experimental example were measured and determined as follows.

[0114] The cryogenic tensile test of the thick steel plate was performed according to the tensile test method for metallic materials specified in ISO 6892-4. Test specimens were taken at a point 1 / 4T thick of the thick steel plate, perpendicular to the rolling direction. Three tests were conducted for each thick steel plate, and the yield strength, tensile strength, and elongation are the average values ​​of the recorded data.

[0115] The impact toughness of the thick steel plate was measured by the Charpy impact test. Specimens were taken at a point 1 / 4T thick, perpendicular to the rolling direction, and the notches were machined parallel to the rolling direction. After immersing the specimens in liquid helium, the specimen temperature was -253°C (-253 o We waited until C was reached. Additionally, the waiting time was increased by 1 minute per 1T of specimen thickness. Three tests were conducted for each thick steel plate, and the impact toughness refers to the average value of the recorded data. The above impact toughness may be referred to as Charpy impact absorption energy.

[0116] The austenite fraction of the thick steel plate was measured by an X-ray diffractometer according to the standard test method for X-ray determination of residual austenite described in ISO E975-22.

[0117] The deterioration temperature range of the thick steel plate was determined through dilatometer measurements. The volume of a material increases linearly as temperature increases. However, in the case of iron, it begins to transform into austenite at temperatures above Ac1; since austenite and ferrite have different volumes, a rapid change in volume occurs at temperatures above Ac1. Subsequently, beyond the Ac3 region, the volume increases linearly again. In this technology, to determine the deterioration temperature range, the points where the volume changes abruptly were designated as Ac1 and Ac3, respectively.

[0118] A thick steel plate was formed according to the following composition and process conditions.

[0119] Table 1 shows the composition of the thick steel plates of the examples and comparative examples. The content of each component is in weight percent. The remainder consists of iron (Fe) and impurities inevitably contained in the steelmaking process.

[0120] Steel CsiMnAlNiCrMoTiA0.050.210.990.0213.00.00490.060.0021B0.060.210.950.0514.30.2040.300.0060C0.020.501.980.0315.00.00250.060.0025D0.040.080.950.0 29.90.00390.060.0045E0.060.271.030.0212.20.00440.060.0068F0.120.220.990.0214.90.00250.050.0051G0.060.222.310.0314.80.00210.010.0061 Steel NbVPSCoBSn type 1A0.00530.00360.00660.00270.00060.00030.001340.3B0.00200.00500.00500.003000.00060 .001448.4C0.00120.00210.00330.00330.00010.00070.001049.3D0.00440.00250.00940.0114 0.00090.00200.001728.1E0.00560.00290.01000.01350.00120.00170.001832.8F0.00680.002 90.00510.00330.00030.00220.002228.7G0.00420.00300.00490.00390.00040.00080.002033.0

[0121] Referring to Table 1, steel grades A, B, and C are compositional ranges according to embodiments of the present invention, in weight%, carbon (C): 0.02% to 0.08%, silicon (Si): 0.15% to 0.5%, manganese (Mn): 0.5% to 2.0%, aluminum (Al): 0.001% to 0.06%, nickel (Ni): 13% to 16%, chromium (Cr): 0.001% to 0.5%, molybdenum (Mo): 0.01% to 0.5%, titanium (Ti): 0.001% to 0.01%, niobium (Nb): 0.001% to 0.05%, vanadium (V): 0.001% to 0.01%, phosphorus (P): 0.01% or less (excluding 0%), sulfur (S): It satisfies a composition range of 0.001% or less (excluding 0%) and the remainder containing iron (Fe) and other unavoidable impurities.

[0122] On the other hand, steel grades D, E, F, and G represent compositional ranges according to the comparative examples of the present invention. Steel grade D has a silicon content lower than the lower limit of the present invention, a nickel content lower than the lower limit of the present invention, and a sulfur content higher than the upper limit of the present invention. Steel grade E has a nickel content lower than the lower limit of the present invention and a sulfur content higher than the upper limit of the present invention. Steel grade F has a carbon content higher than the upper limit of the present invention. Steel grade G has a manganese content higher than the upper limit of the present invention.

[0123] When evaluated based on the above Equation 1, steel grades A, B, and C exceed 40 and thus satisfy the scope of the present invention, while steel grades D, E, F, and G are 40 or less and thus do not satisfy the scope of the present invention.

[0124] For reference, in Table 1, cobalt (Co), boron (B), and tin (Sn) are added as unavoidable impurities, not as intentionally added elements.

[0125] Table 2 shows the process condition values ​​for forming the thick steel plates of the examples and comparative examples. In all cases, the reheat temperature is 1150 o It was C, and the rolling finish temperature was 850 oIt was C.

[0126] Classification Steel Type AC3 Temperature ( o C)AC1 temperature( o C)Quenching temperature (Q)( o C) Lamelling temperature (L)( o C) Tempering temperature (T) o C) Formula 2 Example 1 A6745768005905600.143 Example 2 A6745769005905600.143 Example 3 B 6645909006205600.405 Example 4 C 6515869005905800.062 Example 5 C 6515869006105800.36 9 Example 6A 6745769006155600.4 Example 7B 6645909005955600.068 Comparative Example 1D 7125748006805700.768 Comparative Example 2D 7125748006005700.188 Comparative Example 3E 6885518006605500.796 Comparative Example 4E6885518005805500.212 Comparative Example 5F6515878006005800.203 Comparative Example 6G6415808005905700.164 Comparative Example 7A6745768006555570.806 Comparative Example 8A6745768006405200.653 Comparative Example 9A6745768006805571.061 Comparative Example 10C6515869006305800.677 Comparative Example 11B6645909006255600.473 Comparative Example 12B6645909006355600.608 Comparative Example 13B6645909006855601.284

[0127] Referring to Table 2, 750 in all cases o C ~ 950 o Quenching heat treatment temperature of C, 580 o C ~ 680 o Lamelling temperature of C (excluding Comparative Example 13), and 520 o C ~ 580 o The tempering heat treatment temperature of C is satisfied. Comparative Examples 1, 3, 7, 8, 9, 10, 11, 12, and 13 do not satisfy Equation 2.

[0128] Table 3 shows the microstructure and physical properties of the thick steel plates of the examples and comparative examples.

[0129] Classification Austenitic Fraction (%) Yield Strength (MPa) - 253 o C Elongation Rate (%)-253 o C Impact Toughness (J) Target Value 17% or more 600 MPa or more 17% or more 90 J or more Example 1 17.6767 23.2104 Example 2 17.2745 21.3106 Example 3 4 1.3713 19.6105 Example 4 27.472124.1101 Example 5 22.174023.499 Example 6 30.7707 19.991 Example 7 32.8687 18.2110 Comparative Example 1 11.5696 15.049 Comparative Example 2 19.668115.251 Comparative Example 3 17.785326.0 40 Comparative Example 4 21.98 30 24.037 Comparative Example 5 21.98 43 9.825 Comparative Example 6 30.17 21 12.161 Comparative Example 7 8.97 81 16.193 Comparative Example 8 10.17 76 15.8 109 Comparative Example 9 3.97 80 12.8 193 Comparative Example 10 12.77 42 16.1 100 Comparative Example 11 14.67 41 16.898 Comparative Example 12 117 41 16.6 120 Comparative Example 13 2.97 99 14.1 194

[0130] Referring to Table 3, in all cases regarding yield strength, the range of the present invention of 600 MPa or more was satisfied.

[0131] Embodiments of the present invention include austenite fraction, yield strength, -253 o Elongation at C and -253 o The impact toughness in C satisfied the scope of the present invention.

[0132] On the other hand, Comparative Examples 1 to 6 do not satisfy the compositional range of the present invention and, together with this, do not satisfy Formula 1 regarding the compositional content, with -253 o The impact toughness at C is less than 90 J, so it does not satisfy the scope of the present invention. In addition, Comparative Examples 1, 2, 5, and 6 are -253 oThe elongation at C is less than 17%, so it does not satisfy the scope of the present invention. In addition, Comparative Example 1 has an austenite fraction of less than 17%, so it does not satisfy the scope of the present invention. Therefore, if the compositional range of the present invention and Formula 1 above are not satisfied, the target physical properties are not achieved, and in particular -253 o It can be seen that impact toughness in C is not achieved.

[0133] Comparative Examples 7, 8, and 10 are cases that satisfy the compositional range of the present invention but do not satisfy Equation 2 related to the lamelling temperature. Comparative Examples 7, 8, and 10 have yield strength and -253 o Although the impact toughness in C satisfied the scope of the present invention, the austenite fraction was less than 17% and the elongation was less than 17%, so it did not satisfy the scope of the present invention.

[0134] In addition, regarding the case of Comparative Example 7, 577 o A thick steel plate was manufactured by performing a tempering treatment after additionally performing a lamelling heat treatment at a temperature of C. The thick steel plate had an austenite fraction of 21.7%, a yield strength of 742 MPa, and 30.3% -253 o Elongation at C, and -253 of 103 J o It possessed impact toughness at C and satisfied all the scopes of the present invention. Therefore, in the case of a high lamelling temperature, additional lamelling heat treatment at a low temperature can be performed to satisfy the physical properties of the present invention.

[0135] Comparative Examples 11 to 13 are cases that satisfy the compositional range of the present invention but do not satisfy Equation 2 related to the lamelling temperature. Comparative Examples 11, 12, and 13 have yield strength and -253 o Although the impact toughness in C satisfied the scope of the present invention, the austenite fraction was less than 17% and the elongation was less than 17%, so it did not satisfy the scope of the present invention.

[0136] Therefore, the lamellating temperature satisfies Equation 2, and also, for example, 590 o C or higher 630 o A range of less than C is desirable.

[0137] For Example 3, 620 o As a result of additionally performing lamellating heat treatment at a temperature of C, the austenite fraction increased to 47.9%, but the yield strength and -253 o Elongation at C, and -253 o The impact toughness at C was reduced to 684 MPa, 17.9%, and 93 J, respectively. Therefore, since the above properties are not increased simply by increasing the austenite fraction, an optimal fraction range may be possible.

[0138] FIG. 2 is a scanning electron microscope image showing the microstructure of a thick steel plate manufactured using a method for manufacturing a thick steel plate according to an embodiment of the present invention.

[0139] Referring to FIG. 2, this is an EBSD analysis image of the microstructure of a thick steel plate according to the present invention. Blue represents austenite with a face-centered cubic (FCC) structure, and red represents ferrite and martensite with a body-centered cubic (BCC) structure. Since ferrite and martensite have a body-centered cubic (BCC) structure and a body-centered tetragonal (BCT) structure, respectively, it is difficult to distinguish between them. Martensite can have a large amount of internal dislocations as carbon is included in the ferrite crystal structure.

[0140] When analyzing ferrite and martensite based on ferrite information, they appear bright yellow as they approach the ferrite information and dark green as they move further away. Based on this analysis, the martensite is formed around the austenite. This suggests that during the anomalous temperature range, austenite and ferrite are intermingled; however, upon rapid cooling, the thermally unstable austenite transforms into martensite, while the stable austenite remains intact and untransformed even after cooling.

[0141] Figure 3 shows a schematic diagram of the structure of a thick steel plate manufactured according to an embodiment of the present invention.

[0142] Referring to FIG. 3, martensite is formed entirely during the quenching step, and during the lamelling step, the martensite is transformed into an abnormal structure of ferrite and austenite, and subsequently, during the tempering step, the ferrite and austenite are finely controlled. The ferrite is composed of martensite in a form containing carbon in the ferrite crystal structure, as described in FIG. 2.

[0143] It will be obvious to those skilled in the art that the technical concept of the present invention described above is not limited to the aforementioned embodiments and attached drawings, and that various substitutions, modifications, and changes are possible within the scope of the technical concept of the present invention. Accordingly, the scope of rights of the present invention should be determined by the claims set forth below.

Claims

1. In wt%, Carbon (C): 0.02% to 0.08%, Silicon (Si): 0.15% to 0.5%, Manganese (Mn): 0.5% to 2.0%, Aluminum (Al): 0.001% to 0.06%, Nickel (Ni): 13% to 16%, Chromium (Cr): 0.001% to 0.5%, Molybdenum (Mo): 0.01% to 0.5%, Titanium (Ti): 0.001% to 0.01%, Niobium (Nb): 0.001% to 0.05%, Vanadium (V): 0.001% to 0.01%, Phosphorus (P): 0.01% or less (excluding 0%), Sulfur (S): 0.01% or less (excluding 0%), and the remainder comprising Iron (Fe) and other unavoidable impurities, Thick steel plate.

2. In Paragraph 1, The above-mentioned thick steel plate is, Yield Strength (YS): 600 MPa or higher, -253 o Elongation (EL) at C: 17% or more, and -253 o Impact toughness at C: satisfying 90 J or more, Thick steel plate.

3. In Paragraph 1, The above-mentioned thick steel plate is, having a microstructure including austenite, ferrite, and martensite, Thick steel plate.

4. In Paragraph 3, In the above-mentioned thick steel plate, The area fraction of the above austenite is 17% or more and 50% or less, and The area fraction of the ferrite and the area fraction of the martensite combined are greater than 50% and less than 83%, Thick steel plate.

5. In Paragraph 1, The above-mentioned thick steel plate is, having a microstructure including austenite and ferrite, Thick steel plate.

6. In Paragraph 5, In the above-mentioned thick steel plate, The area fraction of the above austenite is 17% or more and 50% or less, and The area fraction of the above ferrite is greater than 50% and less than 83%, Thick steel plate.

7. In Paragraph 1, The above-mentioned thick steel plate is, Satisfying the following equation, 5[Ni] + 20[Cr]-300[C]-10[Mn]+[Mo] > 40 (Here, [Ni], [Cr], [C], [Mn], and [Mo] are the weight percent of nickel, chromium, carbon, manganese, and molybdenum in the composition constituting the thick steel plate) Thick steel plate.

8. In Paragraph 1, The above-mentioned thick steel plate is, As the above-mentioned unavoidable impurities, comprising at least one of cobalt (Co): 0% to 0.001%, boron (B): 0% to 0.001%, and tin (Sn): 0% to 0.01%, Thick steel plate.

9. Steel comprising, in wt%, Carbon (C): 0.02% ~ 0.08%, Silicon (Si): 0.15% ~ 0.5%, Manganese (Mn): 0.5% ~ 2.0%, Aluminum (Al): 0.001% ~ 0.06%, Nickel (Ni): 13% ~ 16%, Chromium (Cr): 0.001% ~ 0.5%, Molybdenum (Mo): 0.01% ~ 0.5%, Titanium (Ti): 0.001% ~ 0.01%, Niobium (Nb): 0.001% ~ 0.05%, Vanadium (V): 0.001% ~ 0.01%, Phosphorus (P): 0.01% or less (excluding 0%), Sulfur (S): 0.01% or less (excluding 0%), and the remainder being Iron (Fe) and other unavoidable impurities 1,100 o C ~ 1,250 o Step of reheating at a temperature of C; The above heated steel material 800 o C ~ 900 o A step of hot rolling to be terminated at a temperature of C; A quenching step of maintaining the above hot-rolled steel at a temperature of Ac3 or higher and then rapidly cooling it; A lamellating step of heat-treating the above-mentioned quenched steel by maintaining it at a lamellating temperature (L) in the range of Ac1 to Ac3; and A tempering step of heat-treating the above-mentioned lamellarized steel by maintaining it at a temperature of Ac1 or lower; comprising Method for manufacturing thick steel plates.

10. In Paragraph 9, The above method for manufacturing thick steel plates is, Satisfying the following equation, (L - Ac1) / (Ac3 - Ac1) < 0.41 Method for manufacturing thick steel plates.

11. In Paragraph 9, The above quenching step is 750 of the steel o C ~ 950 o Performed by maintaining within the C range and then rapidly cooling, The above lamellating step involves the steel material 550 o C ~ 720 o Perform while maintaining the C range, and The above tempering step involves the steel material 520 o C ~ 580 o Performed by maintaining the C range, Method for manufacturing thick steel plates.

12. In Paragraph 9, The above steel material satisfies the following formula, 5[Ni] + 20[Cr]-300[C]-10[Mn]+[Mo] > 40 (Here, [Ni], [Cr], [C], [Mn], and [Mo] are the weight percent of nickel, chromium, carbon, manganese, and molybdenum in the composition constituting the thick steel plate) Method for manufacturing thick steel plates.

13. In Paragraph 9, The thick steel plate manufactured by the above-mentioned method for manufacturing thick steel plates is, Yield Strength (YS): 600 MPa or higher, -253 o Elongation (EL) at C: 17% or more, and -253 o Impact toughness at C: satisfying 90 J or more, Method for manufacturing thick steel plates.

14. In Paragraph 9, The thick steel plate manufactured by the above-mentioned method for manufacturing thick steel plates is, Having a microstructure comprising austenite, ferrite, and martensite, wherein the area fraction of the austenite is 17% or more and 50% or less, and the combined area fraction of the ferrite and the martensite is greater than 50% and less than 83%. Method for manufacturing thick steel plates.

15. In Paragraph 9, The above steel material is, It is provided as a steel slab manufactured by charging iron scrap during the steelmaking process, and The above steel slab is manufactured using 80 weight% or less of blast furnace molten iron and 20 weight% or more of iron scrap, Method for manufacturing thick steel plates.

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