High-performance steel rod and manufacturing method thereof

Abstract

A high-performance steel rod according to an embodiment of the present invention comprises 0.03-0.12 wt% of carbon (C), 0.50 wt% or less of silicon (Si), 1.3-2.0 wt% of manganese (Mn), 0.02 wt% or less of phosphorus (P), 0.02 wt% or less of sulfur (S), 0.002-0.50 wt% of chromium (Cr), 0.5 wt% or less of copper (Cu), 0.35-0.65 wt% of nickel (Ni), 0.002-0.10 wt% of molybdenum (Mo), 0.005-0.04 wt% of aluminum (Al), 0.005-0.045 wt% of vanadium (V), 0.004-0.045 wt% of niobium (Nb), 0.001-0.015 wt% of titanium (Ti), and 0.015 wt% or less of nitrogen (N), with the remainder comprising iron (Fe) and inevitable impurities, and has a room-temperature yield strength (YS) of 500 MPa or more.

Description

[Technical Field]

[0001] The present invention relates to a high-performance steel bar and a method of manufacturing the same.[Background Art]

[0002] Deformed steel bars or rebars are long, slender steel products used for reinforcing concrete. Due to their high bonding strength with concrete and their ability to compensate for concrete's vulnerability to tension, they help reduce the width of cracks when concrete undergoes cracking. This makes them widely used in construction and civil engineering sites. Such deformed steel bars or rebars are now key materials for various civil engineering and construction structures, including bridges, large marine structures, underground structures, and storage tanks.

[0003] Recently, there has been growing global demand and interest in liquefied natural gas (LNG), driven by factors such as the Russia-Ukraine war, national policies for denuclearization, increasing demand for renewable energy sources such as wind and solar power to expand green energy sources, and increased recoverable reserves due to advances in natural gas extraction technologies. Specifically, the consumption of LNG is expected to significantly grow, from 2.9 billion TOU in 2013 to 4.2 billion TOU by 2040, a 46.1% increase in the global energy mix.

[0004] Natural gas undergoes refining and liquefaction before being transported to LNG terminals for storage. Liquefied at -170 °C (ultra-low temperature), natural gas becomes LNG, and materials used in LNG storage tanks must withstand such ultra-low temperatures.

[0005] An LNG tank typically consists of an inner shell and an outer shell, with the inner shell made of 9% nickel steel and the outer shell made of reinforced concrete. Earlier LNG tanks used a double-containment design with an external protective wall, but since the late 1980s, a full-containment design, where the inner and outer shells are closely attached, has become prevalent. This design ensures that even if the inner shell is damaged and LNG leaks, the outer shell prevents the liquid from leaking into the ground or the atmosphere.

[0006] Consequently, the deformed steel bars used in LNG storage tanks must have sufficient physical properties to maintain structural integrity and resist brittle fracture caused by sudden temperature drops that can occur due to LNG leaks, requiring them to withstand temperatures as low as -170 °C.[Disclosure][Technical Problem]

[0007] To solve the above problems of the related art, the present invention aims to provide a high-performance steel bar with excellent toughness and ductility even in cryogenic environments, and a method of manufacturing the same.

[0008] The present invention also aims to provide a high-performance steel bar that ensures stable performance in both room temperature and ultra-low temperature environments, and a method of manufacturing the same.

[0009] The objectives of the present invention are not limited to those described above, and other objectives not mentioned can be clearly understood by those skilled in the art from the descriptions below.[Technical Solution]

[0010] A high-performance steel bar according to an embodiment of the present invention includes: 0.03 to 0.12 wt% of carbon (C), 0.50 wt% or less of silicon (Si), 1.3 to 2.0 wt% of manganese (Mn), 0.02 wt% or less of phosphorus (P), 0.02 wt% or less of sulfur (S), 0.002 to 0.50 wt% of chromium (Cr), 0.5 wt% or less of copper (Cu), 0.35 to 0.65 wt% of nickel (Ni), 0.002 to 0.10 wt% of molybdenum (Mo), 0.005 to 0.04 wt% of aluminum (Al), 0.005 to 0.045 wt% of vanadium (V), 0.004 to 0.045 wt% of niobium (Nb), 0.001 to 0.015 wt% of titanium (Ti), 0.015 wt% or less of nitrogen (N), and the balance being iron (Fe) and unavoidable impurities, wherein the steel bar has a room-temperature yield strength (YS) of 500 MPa or greater.

[0011] In addition, the high-performance steel bar may further include one or more of 0.1 wt% or less of tin (Sn), 0.1 wt% or less of magnesium (Mg), and 0.05 wt% or less of calcium (Ca).

[0012] The total content of vanadium (V), niobium (Nb), and titanium (Ti) may be 0.1 wt% or less.

[0013] The microstructure may include bainite, acicular ferrite, and pearlite.

[0014] The average grain size of the acicular ferrite structure may be 13.2 µm or less.

[0015] The ductile-brittle transition temperature (DBTT) may be -48 °C or lower.

[0016] A method of manufacturing a high-performance steel bar, according to an embodiment, includes: (a) reheating steel including 0.03 to 0.12 wt% of carbon (C), 0.50 wt% or less of silicon (Si), 1.3 to 2.0 wt% of manganese (Mn), 0.02 wt% or less of phosphorus (P), 0.02 wt% or less of sulfur (S), 0.002 to 0.50 wt% of chromium (Cr), 0.5 wt% or less of copper (Cu), 0.35 to 0.65 wt% of nickel (Ni), 0.002 to 0.10 wt% of molybdenum (Mo), 0.005 to 0.04 wt% of aluminum (Al), 0.005 to 0.045 wt% of vanadium (V), 0.004 to 0.045 wt% of niobium (Nb), 0.001 to 0.015 wt% of titanium (Ti), 0.015 wt% or less of nitrogen (N), and the balance being iron (Fe) and unavoidable impurities; (b) hot rolling the steel by controlling the rolling start temperature to 950 to 1100 °C and the rolling finish temperature to 930 to 1050 °C; and (c) cooling the rolled steel.

[0017] In addition, the steel may further include one or more of 0.1 wt% or less of tin (Sn), 0.1 wt% or less of magnesium (Mg), and 0.05 wt% or less of calcium (Ca).

[0018] The total content of vanadium (V), niobium (Nb), and titanium (Ti) in the steel may be 0.1 wt% or less.

[0019] In step (a), the steel may have a recrystallization stop temperature of 952 °C or higher.

[0020] In step (b), the rolling finish temperature may be controlled at or below the recrystallization stop temperature of the steel.

[0021] Step (c) may be performed by accelerated controlled cooling (ACC) at a cooling rate of 50 to 400 °C / sec, and the steel may be cooled to a final cooling temperature of 500 to 650 °C.

[0022] After performing step (c), the steel may have an average ferrite grain size of 13.2 µm or less.

[0023] After performing step (c), the steel may have a ductile-brittle transition temperature (DBTT) of -48 °C or lower.

[0024] After performing step (c), the steel may have a microstructure including bainite, acicular ferrite, and pearlite.[Advantageous Effects]

[0025] According to an embodiment of the present invention, the high-performance steel bar and its manufacturing method provide excellent mechanical properties at both room and ultra-low temperatures while also ensuring superior seismic performance.

[0026] The advantageous effects of the present invention are not limited to those described above, and other effects not mentioned can be clearly understood by those skilled in the art from the descriptions in the claims.[Description of Drawings]

[0027] FIG. 1 is a flowchart illustrating the process steps for manufacturing a high-performance steel bar according to an embodiment of the present invention. FIG. 2 shows final microstructure photographs of the comparative material and the inventive material. [Modes of the Invention]

[0028] In this specification, when a component (or region, layer, part, etc.) is described as being "on," "connected to," or "coupled to" another component, it may be directly disposed, connected, or coupled to the other component, or a third component may be disposed therebetween.

[0029] The same reference numbers refer to the same components. Additionally, in the drawings, the thicknesses, proportions, and dimensions of the components are exaggerated for the purpose of effectively explaining the technical content.

[0030] The term "and / or" includes all possible combinations of the associated components that can be defined.

[0031] Terms such as "first" and "second" may be used to describe various components, but these components should not be limited by these terms. These terms are used solely for the purpose of distinguishing one component from another. For example, without departing from the scope of the present invention, the first component may be referred to as the second component, and similarly, the second component may be referred to as the first component. Singular expressions include plural expressions unless the context clearly indicates otherwise.

[0032] In addition, terms such as "below," "lower," "above," and "upper" are used to describe the relationship between the components depicted in the drawings. These terms are relative concepts and are explained based on the directions indicated in the drawings.

[0033] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as generally understood by those skilled in the art to which the present invention pertains. Additionally, terms that are defined in commonly used dictionaries should be interpreted as having meanings consistent with the context of the relevant technology, and are explicitly defined herein unless they are interpreted in an idealized or overly formal sense.

[0034] Terms such as "include" and "have" are intended to indicate the presence of features, numbers, steps, operations, components, parts, or combinations thereof described in the specification, and do not preclude the possibility of the presence or addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0035] Unless specified otherwise, the notation "A to B" for numbers A and B means "A or more and B or less." Where a unit is attached only to value B in such notation, the unit is also applied to A.

[0036] Hereinafter, embodiments of the present invention will be described in more detail with reference to the drawings.High-Performance Steel Bar

[0037] A high-performance steel bar according to an embodiment of the present invention includes 0.03 to 0.12 wt% of carbon (C), 0.50 wt% or less of silicon (Si), 1.3 to 2.0 wt% of manganese (Mn), 0.02 wt% or less of phosphorus (P), 0.02 wt% or less of sulfur (S), 0.002 to 0.50 wt% of chromium (Cr), 0.5 wt% or less of copper (Cu), 0.35 to 0.65 wt% of nickel (Ni), 0.002 to 0.10 wt% of molybdenum (Mo), 0.005 to 0.04 wt% of aluminum (Al), 0.005 to 0.045 wt% of vanadium (V), 0.004 to 0.045 wt% of niobium (Nb), 0.001 to 0.015 wt% of titanium (Ti), 0.015 wt% or less of nitrogen (N), and the balance being iron (Fe) and unavoidable impurities.

[0038] According to an embodiment of the present invention, the high-performance steel bar including the above-described alloy composition exhibits a room-temperature yield strength (YS) of 500 MPa or greater.

[0039] The high-performance steel bar according to the present invention may exhibit excellent mechanical properties in ultra-low temperature environments. More specifically, the high-performance steel bar according to the present invention has enhanced low-temperature toughness in addition to strength under ultra-low temperature environments, thereby preventing brittle fracture even during sudden temperature drops when applied to cryogenic structures, and thus exhibiting excellent product characteristics.

[0040] Furthermore, the enhanced seismic performance of the high-performance steel bar allows its application not only to the outer shells of LNG storage tanks, but also to civil engineering and architectural structures in polar regions, making it suitable for use as a safety reinforcing bar in various applications.

[0041] In addition, by securing sufficient spacing between reinforcing bars and preventing congestion in reinforcement arrangements, concrete placement is facilitated, resulting in shorter construction periods, reduced rebar usage, and lower overall construction costs.

[0042] Meanwhile, the high-performance steel bar according to an embodiment of the present invention may further include one or more of 0.1 wt% or less of tin (Sn), 0.1 wt% or less of magnesium (Mg), and 0.05 wt% or less of calcium (Ca).

[0043] Hereinafter, the roles and contents of each alloying element included in the high-performance steel bar according to an embodiment of the present invention will be described in detail.Carbon (C)

[0044] Carbon (C) is the most effective and important element for increasing the strength of steel. It dissolves in austenite and forms a martensitic structure upon quenching. As the carbon content increases, quenching hardness improves. However, deformation may occur during quenching or the elongation and low-temperature toughness of the steel may deteriorate. Additionally, carbon combines with elements such as iron (Fe), chromium (Cr), molybdenum (Mo), and vanadium (V) to form carbides, thereby enhancing strength and hardness.

[0045] The high-performance steel bar according to an embodiment of the present invention may contain 0.03 to 0.12 wt% of carbon.Silicon (Si)

[0046] Silicon (Si) is a residual element from pig iron and deoxidizers. Unless it forms compounds such as SiO 2 , silicon dissolves in ferrite and does not significantly affect the mechanical properties of steel. As a strong deoxidizer, adding up to 4.5 wt% of silicon enhances strength, but adding 2.0 wt% or more of silicon reduces toughness and impairs plastic workability, thereby limiting its allowable content. Silicon also improves resistance to softening during tempering.

[0047] In the high-performance steel bar according to an embodiment of the present invention, silicon may be contained in an amount of 0.50 wt% or less.Manganese (Mn)

[0048] Manganese (Mn) is generally contained in steel in an amount of 0.35 to 1.0 wt%. Part of it dissolves in the steel, while another part combines with sulfur (S) contained in the steel to form non-metallic inclusions such as MnS. MnS is ductile and elongates in the working direction during plastic processing. However, the formation of MnS reduces the sulfur content in the steel, weakening grain boundaries and suppressing the formation of low-melting-point compounds such as FeS.

[0049] Meanwhile, since manganese imparts toughness to steel, steel containing 1.0 to 1.5% of manganese is called tough steel. As an austenite stabilizing element, adding 1.3 to 2.0 wt% of manganese is advantageous for forming acicular ferrite and bainite.

[0050] In the high-performance steel bar according to an embodiment of the present invention, manganese is contained in an amount of 1.3 to 2.0 wt% to promote the formation of acicular ferrite and bainite microstructures, which are advantageous for improving toughness at extremely low temperatures, while reducing the content of expensive nickel (Ni).Phosphorus (P)

[0051] Phosphorus (P) is not problematic when it is uniformly distributed in steel, but it typically forms harmful compounds such as Fe 3 P. Fe 3 P is extremely brittle and segregated, remaining unhomogenized even after annealing, and elongates during processes such as forging and rolling. Phosphorus also reduces impact resistance and promotes temper embrittlement. Although it improves machinability in free-cutting steel, phosphorus is generally a harmful element in steel, necessitating compositional control.

[0052] In the high-performance steel bar according to an embodiment of the present invention, phosphorus may be contained in an amount of 0.02 wt% or less.Sulfur (S)

[0053] Sulfur (S) typically combines with manganese, zinc, titanium, molybdenum, etc., to improve the machinability of steel, and it forms MnS inclusions by combining with manganese. When the manganese content in steel is insufficient, sulfur combines with iron to form FeS. FeS is very brittle and has a low melting point, causing cracking during hot and cold working. In the present invention, the sulfur content is controlled to prevent the formation of FeS inclusions while improving elongation and ensuring low-temperature toughness.

[0054] In the high-performance steel bar according to an embodiment of the present invention, sulfur may be contained in an amount of 0.02 wt% or less.Chromium (Cr)

[0055] Chromium (Cr) is a ferrite stabilizing element. When added to C-Mn steel, chromium delays carbon diffusion through a solute drag effect, thereby affecting grain refinement and improving hardenability.

[0056] The high-performance steel bar according to an embodiment of the present invention may contain 0.002 to 0.5 wt% of chromium.Copper (Cu)

[0057] Copper (Cu) dissolves in ferrite up to 0.35 wt% at room temperature and exhibits a solid solution strengthening effect, thereby partially improving strength and hardness but reducing elongation. Steel containing copper has poor hot workability, and especially when more than 0.5 wt% of copper is contained, it causes hot shortness.

[0058] In the high-performance steel bar according to an embodiment of the present invention, copper may be contained in an amount of 0.5 wt% or less.Nickel (Ni)

[0059] Nickel (Ni) refines the steel microstructure and easily dissolves in both austenite and ferrite, making it useful for matrix strengthening. When coexisting with chromium (Cr) or molybdenum (Mo), nickel exhibits excellent hardenability and facilitates heat treatment of large steel products. It enhances the low-temperature toughness of steel without impairing weldability and malleability. However, as it is an expensive alloying element, its content must be controlled considering cost.

[0060] The high-performance steel bar according to an embodiment of the present invention may contain 0.35 to 0.65 wt% of nickel.Molybdenum (Mo)

[0061] Even when added at 0.1 to 0.3 wt%, molybdenum (Mo) can enhance hardenability up to ten times more effectively than nickel (Ni), and it prevents temper embrittlement, thereby providing excellent resistance to temper embrittlement. It also forms carbides, making it highly effective as an alloying element in high-grade cutting tools, and raises the grain coarsening temperature. However, for hardenability improvement, molybdenum is more effective when used in combination with chromium rather than alone, although its high cost remains a drawback.

[0062] Therefore, the high-performance steel bar according to an embodiment of the present invention may contain 0.002 to 0.10 wt% of molybdenum.Aluminum (Al)

[0063] Aluminum (Al) acts as a strong deoxidizer during steel production, controlling oxide-type non-metallic inclusions in the steel. However, excessive addition increases non-metallic inclusions, causing problems during manufacturing, such as nozzle clogging. Thus, the addition amount must be controlled.

[0064] The high-performance steel bar according to an embodiment of the present invention may contain aluminum in an amount of more than 0.005 wt% to 0.040 wt%.Vanadium (V), Niobium (Nb), and Titanium (Ti)

[0065] Vanadium (V), niobium (Nb), and titanium (Ti) are micro-alloying elements that form precipitates with small additions, contributing to strength enhancement. In particular, through its solute drag effect, niobium (Nb) delays recrystallization during high-temperature rolling, increasing the recrystallization stop temperature and contributing to final ferrite refinement.

[0066] In the case of titanium (Ti), in addition to its precipitation effect, titanium reduces dissolved nitrogen by forming TiN precipitates in the molten state.

[0067] In the high-performance steel bar according to an embodiment of the present invention, 0.005 to 0.045 wt% of vanadium (V), 0.004 to 0.045 wt% of niobium (Nb), and 0.001 to 0.015 wt% of titanium (Ti) may be contained. Additionally, the combined content of vanadium (V), niobium (Nb), and titanium (Ti) may be 0.10 wt% or less, and preferably 0.08 wt% or more and 0.10 wt% or less.Nitrogen (N)

[0068] Even in extremely small amounts, nitrogen (N) significantly affects the mechanical properties of steel, increasing tensile and yield strengths while reducing elongation. In particular, the reduction in impact value and the rise in (ductile-to-brittle) transition temperature are notable. Additionally, nitrogen refines austenite grains, enabling the production of fine-grained steel, and forms nitrides with titanium, zirconium, vanadium, niobium, etc., contributing to grain refinement. However, excessive nitrogen may reduce high-temperature toughness, cause grain boundary embrittlement through AlN precipitation at austenite grain boundaries, and lower high-temperature creep strength.

[0069] In the high-performance steel bar according to an embodiment of the present invention, nitrogen may be contained in an amount of 0.015 wt% or less.

[0070] The high-performance steel bar according to an embodiment of the present invention may further include one or more of 0.1 wt% or less of tin (Sn), 0.1 wt% or less of magnesium (Mg), and 0.05 wt% or less of calcium (Ca).Tin (Sn)

[0071] Tin (Sn) may increase the tensile and yield strengths of steel while reducing elongation and the impact value. Tin may cause hot shortness, temper embrittlement, and low-temperature embrittlement during hot working, and it may improve corrosion resistance.

[0072] To this end, in the high-performance steel bar according to an embodiment of the present invention, tin may be contained in an amount of 0.1 wt% or less.Magnesium (Mg)

[0073] Trace amounts of magnesium (Mg) may segregate to grain boundaries, reducing grain boundary energy and phase boundary energy. It may improve and refine the morphology of grain boundary carbides and other grain boundary precipitates, such as carbide blocking and spheroidization. It may effectively suppress grain boundary sliding, reduce grain boundary stress, and eliminate notch sensitivity. Additionally, as a trace element, magnesium enhances plasticity and increases impact toughness and fatigue strength.

[0074] To this end, in the high-performance steel bar according to an embodiment of the present invention, magnesium may be contained in an amount of 0.1 wt% or less.Calcium (Ca)

[0075] Calcium (Ca) is a strong deoxidizer. During steelmaking, it induces CaS formation from MnS inclusions, causing them to float and separate, or if they remain, they do not elongate like MnS during rolling, thereby preventing mechanical properties from deteriorating. Aluminum (Al) is also a strong deoxidizer, typically added for denitrification and deoxidation. If AlN, a nitride, is finely precipitated, effective grain refinement of the steel may be achieved.

[0076] To this end, in the high-performance steel bar according to an embodiment of the present invention, calcium may be contained in an amount of 0.05 wt% or less.

[0077] Meanwhile, in the high-performance steel bar according to an embodiment of the present invention, as described above, the combined content of vanadium (V), niobium (Nb), and titanium (Ti) may be 0.10 wt% or less.

[0078] This is to minimize the content of expensive nickel (Ni) while utilizing the solute drag effect of niobium (Nb) to delay recrystallization during high-temperature rolling, thereby increasing the recrystallization stop temperature. Ultimately, through grain refinement of final ferrite, a low ductile-brittle transition temperature (DBTT) may be secured, simultaneously satisfying stability and economic efficiency at ultra-low temperatures.

[0079] Therefore, in the high-performance steel bar according to an embodiment of the present invention, the recrystallization stop temperature, at which recrystallization caused by deformation does not occur, increases due to the solute drag effect of niobium (Nb), and the recrystallization stop temperature may be 952 °C or higher.

[0080] Additionally, in the high-performance steel bar according to an embodiment of the present invention, when hot deformation is performed below the recrystallization stop temperature, continuous dynamic recrystallization of austenite does not occur. Instead, the austenite grains become pancake-shaped, increasing the grain boundary fraction per unit area. As a result, fine ferrite may be formed due to an increased number of nucleation sites during phase transformation, and ultra-low temperature stability may be maximized through the ultrafine-grained ferrite formed thereby. At this time, the ferrite structure may be acicular ferrite, with an average grain size of 13.2 µm or less, and preferably 9.5 µm or less.

[0081] However, the high-performance steel bar according to an embodiment of the present invention may have a final microstructure that includes bainite and pearlite in addition to acicular ferrite.

[0082] Meanwhile, low-temperature toughness may be evaluated through the average ferrite grain size and ductile-brittle transition temperature (DBTT). According to an embodiment of the present invention, the average ferrite grain size may be 13.2 µm or less, and the DBTT may be -48 °C or lower.

[0083] Accordingly, the high-performance steel bar according to an embodiment of the present invention may secure a fine ferrite structure together with a remarkably low ductile-brittle transition temperature (DBTT), thereby exhibiting excellent low-temperature toughness.

[0084] Furthermore, the high-performance steel bar according to an embodiment of the present invention may exhibit a notch sensitive ratio (NSR), defined as the ratio of the tensile strength of a notched specimen (TS_notched) to the yield strength of an unnotched specimen (YS_unnotched) at -170 °C, of 1.0 or greater.Method of Manufacturing High-Performance Steel Bar

[0085] Referring to FIG. 1, the method of manufacturing a high-performance steel bar according to an embodiment of the present invention includes (a) reheating, (b) hot rolling, and (c) cooling.

[0086] More specifically, the method includes (a) reheating steel including 0.03 to 0.12 wt% of carbon (C), 0.50 wt% or less of silicon (Si), 1.3 to 2.0 wt% of manganese (Mn), 0.02 wt% or less of phosphorus (P), 0.02 wt% or less of sulfur (S), 0.002 to 0.50 wt% of chromium (Cr), 0.5 wt% or less of copper (Cu), 0.35 to 0.65 wt% of nickel (Ni), 0.002 to 0.10 wt% of molybdenum (Mo), 0.005 to 0.04 wt% of aluminum (Al), 0.005 to 0.045 wt% of vanadium (V), 0.004 to 0.045 wt% of niobium (Nb), 0.001 to 0.015 wt% of titanium (Ti), 0.015 wt% or less of nitrogen (N), with the balance being iron (Fe) and unavoidable impurities; (b) hot rolling the steel by controlling the rolling start temperature to 950 to 1100 °C and the rolling finish temperature to 930 to 1050 °C; and (c) cooling the steel.

[0087] Here, the steel may be an ingot or a billet produced by a continuous casting process conducted before the reheating step. Therefore, the alloying elements and their compositional ranges contained in the steel are understood to be identically included in the high-performance steel bar manufactured through the ingot or billet.

[0088] Additionally, the reason for controlling the rolling finish temperature, which is the final hot deformation finishing temperature range of the steel, to 930 to 1050 °C is to promote phase transformation through hot deformation at or below the recrystallization stop temperature, ultimately forming a fine ferrite structure.

[0089] That is, the recrystallization stop temperature, at which recrystallization caused by deformation does not occur, increases due to the solute drag effect of niobium (Nb). When deformation is applied below the recrystallization stop temperature, continuous dynamic recrystallization of austenite does not occur. Instead, the grains become pancake-shaped, increasing the grain boundary fraction per unit area. As a result, ferrite may be refined due to the increased number of nucleation sites during phase transformation of the steel, and ultra-low temperature stability may be maximized through the ultrafine-grained ferrite formed thereby.

[0090] Ultimately, when a fine grain size of ferrite is obtained, it is possible to significantly reduce the ductile-brittle transition temperature (DBTT).

[0091] In step (a), the recrystallization stop temperature (RST) of the steel may be 952 °C or higher.

[0092] Therefore, in step (b), the rolling finish temperature, which is the final hot deformation finishing temperature range of the steel, may be controlled to 952 °C or lower, which is the recrystallization stop temperature of the steel.

[0093] In step (c), the steel may be cooled by accelerated controlled cooling (ACC). During this process, accelerated controlled cooling (ACC) is a cooling process in which water, as the cooling medium, is uniformly sprayed at high pressure using a Tempcore cooling system during the intermediate stage between hot deformation and final cooling, thereby inducing structural changes in the internal and external microstructure of the steel.

[0094] In the method of manufacturing a high-performance steel bar according to an embodiment of the present invention, in step (c), the steel may be cooled at a cooling rate of 50 to 400 °C / sec and to a final cooling temperature (reheating temperature) of 500 to 650 °C.

[0095] Ultimately, in the method of manufacturing a high-performance steel bar according to an embodiment of the present invention, after hot deformation of the steel, a steel bar structure that satisfies high strength and low-temperature toughness even at ultra-low temperatures may be achieved through reheating and cooling temperature control using accelerated controlled cooling (ACC).

[0096] At this time, the steel bar after performing step (c) may exhibit ferrite refinement as the grain boundary fraction per unit area increases and nucleation sites are increased through phase transformation. Accordingly, the average grain size of the ferrite structure may be 13.2 µm or less, and preferably 9.5 µm or less. In addition, as shown in FIG. 2, the steel bar after performing step (c) may have a final microstructure including bainite, acicular ferrite, and pearlite.

[0097] Furthermore, the steel after performing step (c) may exhibit a ductile-brittle transition temperature (DBTT) of -48 °C or lower by realizing a fine-grained ferrite structure.Comparative Example and Experimental Example

[0098] Hereinafter, preferred comparative and experimental examples are presented to facilitate understanding of the present invention. However, the following experimental examples are provided merely to aid understanding of the present invention, and the present invention is not limited thereby.

[0099] Tables 1 and 2 below show the main alloy compositions (units: wt%) of the inventive materials of the present experimental example and the comparative material of the comparative example. Table 3 below shows the operating conditions for manufacturing each specimen of the present experimental and comparative examples. Tables 4 and 5 show the measured mechanical properties of the specimens prepared under the respective operating conditions of the present experimental and comparative examples.

[0100] The "YS" item refers to yield strength, the "TS" item refers to tensile strength, the "EL" item refers to elongation, the "UE" item refers to uniform elongation, and the "NSR" item refers to notch sensitive ratio, calculated as the ratio of the tensile strength of a notched specimen to the yield strength of an unnotched specimen. The symbol "un" indicates an unnotched specimen, and "n" indicates a notched specimen. Room temperature corresponds to 25 °C. [Table 1]ClassificationChemical Composition (wt%)CSiMnPSCuCrMoNiComparative material0.0700.151.630.010.010.150.200.020.57Inventive material 10.0660.171.520.0080.010.160.150.020.55Inventive material 20.0580.191.520.0090.0080.160.150.010.41 [Table 2] ClassificationChemical Composition (wt%)Ceq.RST (°C)VNbTiSnAlCaNComparative material0.0020.0030.0020.0070.0020.0020.0090.43842Inventive material 10.0390.0400.0120.0080.0200.0020.0100.40987Inventive material 20.0390.0350.0120.0100.0200.0020.0090.40952

[0101] Referring to Tables 1 and 2, the comparative material contains 0.57 wt% of nickel (Ni), while Inventive materials 1 and 2 contain reduced amounts of nickel, 0.55 wt% and 0.41 wt%, respectively.

[0102] Thus, although Inventive materials 1 and 2 contain significantly smaller amounts of expensive nickel compared to the comparative material, their low-temperature toughness is enhanced, while maintaining excellent weldability. As a result, Inventive materials 1 and 2 can achieve cost-effectiveness by reducing manufacturing costs compared to the comparative material, while securing mechanical properties at both room and ultra-low temperatures.

[0103] Furthermore, the comparative material is composed of 0.002 wt% of vanadium (V) + 0.003 wt% of niobium (Nb) + 0.002 wt% ≤ 0.007 wt% of titanium (Ti), whereas Inventive material 1 is composed of 0.039 wt% of vanadium (V) + 0.040 wt% of niobium (Nb) + 0.012 wt% ≤ 0.091 wt% of titanium (Ti), and Inventive material 2 is composed of 0.039 wt% of vanadium (V) + 0.035 wt% of niobium (Nb) + 0.012 wt% ≤ 0.086 wt% of titanium (Ti).

[0104] In particular, the comparative material contains 0.003 wt% of niobium (Nb), whereas Inventive materials 1 and 2 contain 0.040 wt% and 0.035 wt% of niobium, respectively.

[0105] This difference in niobium content affects the recrystallization stop temperature (RST). The comparative material exhibits an RST of only 842 °C, whereas Inventive materials 1 and 2 show dramatically higher RST values of 987 °C and 953 °C, respectively.

[0106] This is because the solute drag effect of niobium (Nb) raises the recrystallization stop temperature, at which recrystallization caused by hot deformation does not occur.

[0107] Therefore, the high-performance steel bar according to an embodiment of the present invention exhibits superior technical and economic aspects for property expression at ultra-low temperatures, as its properties within the specified alloy composition range are similar to or slightly enhanced than those of conventional materials. [Table 3]ClassificationDiameterOperating ConditionsRolling Start Temperature (°C)Rolling Finish Temperature (°C)Cooling Rate (°C / sec)Tempering Temperature (°C)Comparative materialD19960520D35982573Inventive material 1D19950-110096050-400525D35970569Inventive material 2D19940523D35947571 [Table 4] ClassificationDiameterRoom Temperature PropertiesGrain SizeLow-Temperature ToughnessYS (MPa)TS (MPa)EL (%)Ferrite Average Grain Size (µm)DBTT (°C)Comparative materialD1955363218.413.3-40D3554064517.4Inventive material 1D1958265720.39.1-53D3557465319.7Inventive material 2D1954461022.99.5-48D3556262917.2 [Table 5] ClassificationDiameterLow-Temperature Physical Properties (-170 °C)YS-un (MPa)UE-un (%) [3%↑]TS-n (MPa)NSR [1.00↑]Comparative materialD198099.28251.02D357767.17921.02Inventive material 1D1981310.88461.04D357859.78121.03Inventive material 2D1979710.48131.02D357628.27781.02

[0108] Referring to Tables 1 to 5 above, the comparative material and Inventive materials 1 and 2 will be specifically compared.

[0109] The room-temperature yield strengths (YS) of the comparative material were 553 MPa and 540 MPa, respectively, while those of Inventive materials 1 and 2 were 582 MPa and 574 MPa, and 544 MPa and 562 MPa, respectively.

[0110] Additionally, the room-temperature tensile strengths (TS) of the comparative material were 632 MPa and 645 MPa, respectively, while those of Inventive materials 1 and 2 were 657 MPa and 653 MPa, and 610 MPa and 629 MPa, respectively.

[0111] Moreover, the room-temperature elongations (EL) of the comparative material were only 18.4% and 17.4%, respectively, while those of Inventive materials 1 and 2 increased to 20.3% and 19.7%, and 22.9% and 17.2%, respectively.

[0112] Meanwhile, the comparative material contained 0.003 wt% of niobium (Nb) and had a recrystallization stop temperature (RST) of 842 °C, and final finish rolling was conducted at rolling finish temperatures of 960 °C and 982 °C, which are higher than the RST of 842 °C.

[0113] On the other hand, Inventive materials 1 and 2 contained 0.040 wt% and 0.035 wt% of niobium (Nb), respectively, resulting in increased recrystallization stop temperatures (RST) of 987 °C and 953 °C, respectively.

[0114] Inventive materials 1 and 2 underwent final finish rolling at rolling finish temperatures of 960 °C / 970 °C and 940 °C / 947 °C, respectively, which are lower than their respective recrystallization stop temperatures (RST) of 987 °C and 952 °C.

[0115] At this time, the comparative material, which underwent final finish rolling at rolling finish temperatures higher than the RST, formed a coarsened ferrite structure with an average grain size of 13.3 µm.

[0116] However, Inventive materials 1 and 2, which underwent final finish rolling at rolling finish temperatures lower than the RST, formed very fine ferrite structures with average grain sizes of 9.1 µm and 9.5 µm.

[0117] This can be confirmed in FIG. 2. Referring to FIG. 2, the final structure of the comparative material shows visibly coarse grains, whereas the final structure of the inventive materials shows much finer grains.

[0118] This difference in average grain size of the ferrite structure affects the low-temperature toughness of the steel bar. The comparative material showed a ductile-brittle transition temperature (DBTT) of only -40 °C.

[0119] However, Inventive materials 1 and 2 showed ductile-brittle transition temperatures (DBTT) of -53 °C and -48 °C, respectively, by forming fine-grained ferrite structures.

[0120] Thus, it can be seen that the finer the grain size of the ferrite structure, the lower the ductile-brittle transition temperature can be, and the lower the ductile-brittle transition temperature, the better the low-temperature properties of the steel bar are.

[0121] At the low temperature of -170 °C, the yield strengths (YS_un) of the comparative material were 809 MPa and 776 MPa, respectively, whereas those of Inventive materials 1 and 2 were 813 MPa and 785 MPa, and 797 MPa and 762 MPa, respectively.

[0122] At the low temperature of -170 °C, the tensile strengths (TS_n) of the comparative material were 825 MPa and 792 MPa, respectively, whereas those of Inventive materials 1 and 2 were 846 MPa and 812 MPa, and 813 MPa and 778 MPa, respectively.

[0123] Regarding the uniform elongation (UE_un) of unnotched specimens, the comparative material showed values of only 9.2% and 7.1%, respectively, whereas Inventive materials 1 and 2 showed increased values of 10.8% and 9.7%, and 10.4% and 8.2%.

[0124] Regarding the notch sensitivity ratio (NSR), defined as the ratio of the tensile strength (TS_n) to the yield strength (YS_un) at -170 °C, the comparative material recorded values of 1.02 and 1.02, whereas Inventive materials 1 and 2 exhibited values of 1.04 and 1.03, and 1.02 and 1.02.

[0125] As described above, preferred embodiments of the present invention have been described, and it is apparent to those skilled in the art that the present invention may be embodied in other specific forms without departing from the spirit or scope of the present invention, in addition to the embodiments described above. In other words, the above-described embodiments should be considered as illustrative rather than restrictive, and accordingly, the present invention is not limited to the above description and may be modified within the scope of the appended claims and their equivalents.

Claims

1. A high-performance steel bar comprising: 0.03 to 0.12 wt% of carbon (C); 0.50 wt% or less of silicon (Si); 1.3 to 2.0 wt% of manganese (Mn); 0.02 wt% or less of phosphorus (P); 0.02 wt% or less of sulfur (S); 0.002 to 0.50 wt% of chromium (Cr); 0.5 wt% or less of copper (Cu); 0.35 to 0.65 wt% of nickel (Ni); 0.002 to 0.10 wt% of molybdenum (Mo); 0.005 to 0.04 wt% of aluminum (Al); 0.005 to 0.045 wt% of vanadium (V); 0.004 to 0.045 wt% of niobium (Nb); 0.001 to 0.015 wt% of titanium (Ti); 0.015 wt% or less of nitrogen (N); and the balance being iron (Fe) and unavoidable impurities, wherein the steel bar has a room-temperature yield strength (YS) of 500 MPa or greater.

2. The high-performance steel bar of claim 1, further comprising one or more of 0.1 wt% or less of tin (Sn), 0.1 wt% or less of magnesium (Mg), and 0.05 wt% or less of calcium (Ca).

3. The high-performance steel bar of claim 1, wherein the total content of vanadium (V), niobium (Nb), and titanium (Ti) is 0.1 wt% or less.

4. The high-performance steel bar of claim 1, wherein the microstructure includes bainite, acicular ferrite, and pearlite.

5. The high-performance steel bar of claim 4, wherein the acicular ferrite structure has an average grain size of 13.2 µm or less.

6. The high-performance steel bar of claim 1, wherein the steel bar has a ductile-brittle transition temperature (DBTT) of -48 °C or lower.

7. A method of manufacturing a high-performance steel bar, comprising: (a) reheating steel including 0.03 to 0.12 wt% of carbon (C), 0.50 wt% or less of silicon (Si), 1.3 to 2.0 wt% of manganese (Mn), 0.02 wt% or less of phosphorus (P), 0.02 wt% or less of sulfur (S), 0.002 to 0.50 wt% of chromium (Cr), 0.5 wt% or less of copper (Cu), 0.35 to 0.65 wt% of nickel (Ni), 0.002 to 0.10 wt% of molybdenum (Mo), 0.005 to 0.04 wt% of aluminum (Al), 0.005 to 0.045 wt% of vanadium (V), 0.004 to 0.045 wt% of niobium (Nb), 0.001 to 0.015 wt% of titanium (Ti), 0.015 wt% or less of nitrogen (N), and the balance being iron (Fe) and unavoidable impurities; (b) hot rolling the steel by controlling the rolling start temperature to 950 to 1100 °C and the rolling finish temperature to 930 to 1050 °C; and (c) cooling the steel.

8. The method of claim 7, wherein the steel further includes one or more of 0.1 wt% or less of tin (Sn), 0.1 wt% or less of magnesium (Mg), and 0.05 wt% or less of calcium (Ca).

9. The method of claim 7, wherein the total content of vanadium (V), niobium (Nb), and titanium (Ti) in the steel is 0.1 wt% or less.

10. The method of claim 7, wherein in step (a), the recrystallization stop temperature of the steel is 952 °C or higher.

11. The method of claim 10, wherein in step (b), the rolling finish temperature is controlled to be at or below the recrystallization stop temperature of the steel.

12. The method of claim 7, wherein step (c) is performed by accelerated controlled cooling (ACC) at a cooling rate of 50 to 400 °C / sec, and the steel is cooled to a final cooling temperature of 500 to 650 °C.

13. The method of claim 7, wherein the steel obtained after performing step (c) has an average ferrite grain size of 13.2 µm or less.

14. The method of claim 7, wherein the steel obtained after performing step (c) has a ductile-brittle transition temperature (DBTT) of -48 °C or lower.

15. The method of claim 7, wherein the steel obtained after performing step (c) has a microstructure including bainite, acicular ferrite, and pearlite.

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