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 les

Description

High-performance steel bar and its manufacturing method

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

[0002] Deformed bars, or rebars, are thin, long steel reinforcements used for concrete reinforcement. They offer strong bonding to concrete, complementing its tensile strength and reducing the width of cracks in concrete. These advantages make them widely used in construction and civil engineering. Today, these deformed bars are a key component in a variety of civil engineering structures, including bridges, large offshore structures, underground structures, and storage facilities.

[0003] Recently, demand for and interest in liquefied natural gas (LNG) has been increasing globally due to the Russo-Ukrainian War, national nuclear phase-out policies, and the expansion of green energy sources. Furthermore, advances in natural gas extraction technology have led to increased recoverable reserves. Specifically, LNG's share of the overall energy mix is ​​projected to increase by 46.1%, from 2.9 billion TOU in 2013 to 4.2 billion TOU by 2040.

[0004] Meanwhile, natural gas undergoes a refining and liquefaction process before being transported to LNG terminals for storage. Natural gas is liquefied at temperatures below -170°C (ultra-low temperature) to become liquefied natural gas (LNG). LNG storage tanks, designed to store this ultra-low temperature LNG, require materials that can withstand ultra-low temperatures.

[0005] LNG tanks are largely composed of an inner shell and an outer shell, each constructed of 9% nickel steel and reinforced concrete, respectively. While LNG tanks were previously designed as a double-protection system with a protective wall built around the outside, the design evolved in the late 1980s to a fully protected system with the inner and outer shells sealed tightly together. This design prevents LNG leakage due to inner shell failure, while the outer shell prevents ground and air leakage.

[0006] Therefore, for the deformed steel bars used in LNG storage tanks, properties that can withstand -170℃ are required to maintain the structure without brittle fracture even when the temperature drops rapidly due to LNG leakage.

[0007] In order to solve the problems of the above-described prior art, the purpose of the present invention is to provide a high-performance steel bar having excellent properties of toughness and ductility even in an extremely low-temperature environment and a method for manufacturing the same.

[0008] In addition, the purpose of the present invention is to provide a high-performance steel bar having stable performance in room temperature and ultra-low temperature environments and a method for manufacturing the same.

[0009] The tasks of the present invention are not limited to the tasks mentioned above, and other tasks not mentioned will be clearly understood by those skilled in the art from the description below.

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

[0011] In addition, it may further include at least one of tin (Sn) 0.1 wt% or less, magnesium (Mg) 0.1 wt% or less, and calcium (Ca) 0.05 wt% or less.

[0012] Additionally, the sum of the vanadium (V), niobium (Nb), and titanium (Ti) contents may be 0.1 wt% or less.

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

[0014] Additionally, the average grain size of the above-mentioned needle-shaped ferrite structure may be 13.2 ㎛ or less.

[0015] Additionally, the ductile-brittle transition temperature (DBTT) may be below -48 ℃.

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

[0017] In addition, the steel may further include at least one 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] Additionally, the steel may have a sum of vanadium (V), niobium (Nb), and titanium (Ti) contents of 0.1 wt% or less.

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

[0020] In addition, the above step (b) can be controlled so that the rolling end temperature is below the recrystallization stop temperature of the steel.

[0021] In addition, the above step (c) is performed by accelerated control cooling (ACC), and can be cooled at a cooling rate of 50 to 400 ℃ / sec and at a final cooling temperature of 500 to 650 ℃.

[0022] In addition, the steel material that has undergone the above step (c) may have an average grain size of the ferrite structure of 13.2 ㎛ or less.

[0023] Additionally, the steel material that has undergone the above step (c) may have a ductile-brittle transition temperature (DBTT) of -48 ℃ or lower.

[0024] The steel material that has undergone the above step (c) may have a microstructure including bainite, needle-like ferrite, and pearlite.

[0025] According to a high-performance steel bar and a manufacturing method thereof according to one embodiment of the present invention, it is possible to obtain excellent mechanical properties in room temperature and ultra-low temperature environments, and also to secure earthquake-resistant performance.

[0026] The effects of the present invention are not limited to the effects mentioned above, and other effects not mentioned will be clearly understood by those skilled in the art from the description of the claims.

[0027] Figure 1 is a flowchart illustrating the sequence of a high-performance steel bar manufacturing method according to one embodiment of the present invention.

[0028] Figure 2 shows the final tissue photographs of the comparative and inventive materials.

[0029] In this specification, when it is said that a component (or region, layer, portion, etc.) is “on,” “connected to,” or “coupled to” another component, it means that it can be directly disposed / connected / coupled to the other component, or a third component may be disposed between them.

[0030] Identical drawing numbers indicate identical components. Furthermore, in the drawings, the thicknesses, proportions, and dimensions of components are exaggerated for the purpose of effectively illustrating the technical content.

[0031] “And / or” includes any combination of one or more of the associated constructs that can be defined.

[0032] While terms such as "first" and "second" may be used to describe various components, these components should not be limited by these terms. These terms are used solely to distinguish one component from another. For example, without departing from the scope of the present invention, a first component may be referred to as a "second component," and similarly, a second component may also be referred to as a "first component." Singular expressions include plural expressions unless the context clearly indicates otherwise.

[0033] Additionally, terms such as "below," "lower," "above," and "upper" are used to describe the relationships between components depicted in the drawings. These terms are relative concepts and are described based on the directions indicated in the drawings.

[0034] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by those skilled in the art to which this invention pertains. Furthermore, terms defined in commonly used dictionaries should be interpreted to have a meaning consistent with their meaning in the relevant technical context, and unless interpreted in an idealized or overly formal sense, they are explicitly defined herein.

[0035] Terms such as "include" or "have" should be understood to specify the presence of a feature, number, step, operation, component, part, or combination thereof described in the specification, but not to exclude in advance the possibility of the presence or addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0036] Unless otherwise specified, the notation 'A to B' for numerical values ​​A and B means 'A or more and B or less'. In such notation, if a unit is attached only to numerical value B, the unit shall be applied to numerical value A as well.

[0037] Hereinafter, one embodiment of the present invention will be described in more detail with reference to the drawings.

[0038]

[0039] High-performance steel bar

[0040] According to one embodiment of the present invention, a high-performance steel bar comprises carbon (C) 0.03 to 0.12 wt%, silicon (Si) 0.50 wt% or less, manganese (Mn) 1.3 to 2.0 wt%, phosphorus (P) 0.02 wt% or less, sulfur (S) 0.02 wt% or less, chromium (Cr) 0.002 to 0.50 wt%, copper (Cu) 0.5 wt% or less, nickel (Ni) 0.35 to 0.65 wt%, molybdenum (Mo) 0.002 to 0.10 wt%, aluminum (Al) 0.005 to 0.04 wt%, vanadium (V): 0.005 to 0.045 wt%, niobium (Nb): 0.004 to 0.045 wt%, titanium (Ti): 0.001 to 0.005 wt% Contains 0.015 wt% of iron (Fe), 0.015 wt% or less of nitrogen (N), and the remainder of iron (Fe) and unavoidable impurities.

[0041] A high-performance steel bar according to one embodiment of the present invention including the above-described alloy composition satisfies a room temperature yield strength (YS) of 500 MPa or more.

[0042] The high-performance steel bar according to the present invention can exhibit superior mechanical properties in ultra-low-temperature environments. More specifically, the high-performance steel bar according to the present invention exhibits enhanced low-temperature toughness along with strength in ultra-low-temperature environments, thereby preventing brittle fracture even when subjected to rapid temperature drops when applied to ultra-low-temperature structures, thereby exhibiting superior product characteristics.

[0043] In addition, because its seismic properties have been improved, it can be applied not only to the outer shell of LNG storage tanks but also to civil engineering and building structures in polar regions, and can be utilized in various ways as a safety steel.

[0044] In addition, by securing sufficient spacing between reinforcement bars and preventing over-concentration, concrete pouring becomes easier, shortening the construction period and reducing the amount of reinforcement used, thereby reducing construction costs.

[0045] Meanwhile, the high-performance steel bar according to one embodiment of the present invention may further include at least one 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).

[0046] Hereinafter, the role and content of each alloy element included in the high-performance steel bar according to one embodiment of the present invention will be described in detail.

[0047]

[0048] carbon (C)

[0049] Carbon (C) is the most effective and important element for increasing the strength of steel. It is dissolved in austenite and forms martensite during quenching. As carbon content increases, quenching hardness improves, but quenching can also cause deformation, reduced elongation, and low-temperature toughness. Furthermore, it combines with elements such as iron (Fe), chromium (Cr), molybdenum (Mo), and vanadium (V) to form carbides, enhancing strength and hardness.

[0050] A high-performance steel bar according to one embodiment of the present invention may contain 0.03 to 0.12 wt% of carbon (C).

[0051]

[0052] Silicon (Si)

[0053] Silicon (Si), a residue from pig iron and deoxidizers, is dissolved in ferrite and has little effect on the mechanical properties of steel unless it forms compounds such as SiO2. As a powerful deoxidizer, adding up to 4.5 wt% improves strength. However, adding more than 2.0 wt% reduces toughness and impairs plastic workability, so its use is limited. It also increases softening resistance during tempering.

[0054] A high-performance steel bar according to one embodiment of the present invention may contain silicon (Si) in an amount of 0.50 wt% or less.

[0055]

[0056] manganese (Mn)

[0057] Manganese (Mn) typically exists in steel at 0.35 to 1.0 wt%. Some of this manganese is dissolved in the steel, while the remaining portion combines with sulfur to form the non-metallic inclusion MnS. MnS is ductile and elongates in the working direction during plastic working. However, the formation of MnS reduces the sulfur (S) content in the steel, weakening grains and inhibiting the formation of FeS, a low-melting-point compound.

[0058] Meanwhile, manganese imparts viscosity to steel, so steel with 1.0 to 1.5% manganese added is called strong steel. It also acts as an austenite stabilizing element, and when added at 1.3 to 2.0 wt%, it is beneficial for the formation of acicular ferrite and bainite.

[0059] A high-performance steel bar according to one embodiment of the present invention has manganese (Mn) added in an amount of 1.3 to 2.0 wt% to induce the formation of needle-like ferrite and bainite, which are microstructures advantageous for improving toughness at extremely low temperatures, while reducing the expensive Ni content.

[0060]

[0061] Person (P)

[0062] Phosphorus (P) is not a significant problem if distributed uniformly throughout the steel, but it commonly forms harmful compounds of Fe3P. Fe3P is extremely brittle and segregates, so it does not homogenize even after annealing and elongates during processing such as forging and rolling. Furthermore, while phosphorus reduces impact resistance, promotes temper embrittlement, and improves machinability in free-cutting steels, it is generally considered a detrimental element to steel, and therefore its composition must be controlled.

[0063] A high-performance steel bar according to one embodiment of the present invention may contain phosphorus (P) in an amount of 0.02 wt% or less.

[0064]

[0065] Yellow (S)

[0066] Sulfur (S) usually improves the machinability of steel by combining with manganese, zinc, titanium, molybdenum, etc., and forms MnS inclusions by combining with manganese. If the amount of manganese in the steel is insufficient, it combines with iron to form FeS. FeS is very brittle and has a low melting point, so it causes cracks during hot and cold working. In the present invention, the compositional content of sulfur (S) was controlled to prevent the formation of FeS inclusions while improving elongation and securing low-temperature toughness.

[0067] A high-performance steel bar according to one embodiment of the present invention may contain sulfur (S) in an amount of 0.02 wt% or less.

[0068]

[0069] chromium (Cr)

[0070] Chromium (Cr) is a ferrite stabilizing element. When added to C-Mn steel, it delays the diffusion of carbon due to the solute interference effect, thereby affecting grain refinement and improving hardenability.

[0071] A high-performance steel bar according to one embodiment of the present invention may contain 0.002 to 0.5 wt% of chromium (Cr).

[0072]

[0073] copper (Cu)

[0074] Copper (Cu) dissolves in ferrite at room temperature in concentrations of up to 0.35 wt% and exhibits a solid-solution strengthening effect, which improves strength and hardness to some extent but reduces elongation. Steels containing copper have problems with hot workability, and copper content exceeding 0.5 wt% can cause red-hot embrittlement.

[0075] A high-performance steel bar according to one embodiment of the present invention may contain copper (Cu) in an amount of 0.5 wt% or less.

[0076]

[0077] Nickel (Ni)

[0078] Nickel (Ni) refines the steel structure and is readily incorporated into austenite and ferrite, making it useful for matrix strengthening. When combined with chromium (Cr) or molybdenum (Mo), it exhibits excellent hardenability and facilitates heat treatment of large steels. It enhances the low-temperature toughness of steel without compromising weldability or malleability. However, because it is a high-cost alloying element, its composition must be controlled to minimize cost.

[0079] A high-performance steel bar according to one embodiment of the present invention may contain 0.35 to 0.65 wt% of nickel (Ni).

[0080]

[0081] molybdenum (Mo)

[0082] Molybdenum (Mo) can improve hardenability by up to 10 times that of nickel (Ni) with just 0.1 to 0.3 wt% additions, and it imparts resistance to temper embrittlement by preventing it from forming. It also forms carbides, making it an effective alloying element for advanced cutting tools and increasing the grain coarsening temperature. However, while it is more effective when used in conjunction with chromium to enhance hardenability than on its own, its high price is a drawback.

[0083] Therefore, in a high-performance steel bar according to one embodiment of the present invention, molybdenum (Mo) may be included in an amount of 0.002 to 0.10 wt%.

[0084]

[0085] Aluminum (Al)

[0086] Aluminum (Al) acts as a powerful deoxidizer during the production of steel and controls non-metallic oxide inclusions in the steel. However, excessive addition can cause manufacturing problems such as nozzle clogging, so it is necessary to control the amount added.

[0087] A high-performance steel bar according to one embodiment of the present invention may contain aluminum (Al) in an amount of more than 0.005 wt% to 0.040 wt%.

[0088]

[0089] Vanadium (V), niobium (Nb), titanium (Ti)

[0090] Vanadium (V), niobium (Nb), and titanium (Ti) are microalloying elements that, when added in small amounts, form precipitates, contributing to increased strength. In particular, niobium (Nb) delays recrystallization during high-temperature rolling through its solute-attraction effect, thereby increasing the recrystallization stop temperature, contributing to the final refinement of ferrite.

[0091] In the case of titanium (Ti), it plays a role in reducing dissolved nitrogen by forming TiN crystals in the molten state in addition to the precipitation effect.

[0092] A high-performance steel bar according to one embodiment of the present invention may include 0.005 wt% to 0.045 wt% of vanadium (V), 0.004 wt% to 0.045 wt% of niobium (Nb), and 0.001 wt% to 0.015 wt% of titanium (Ti). In addition, the sum of the contents of vanadium (V), niobium (Nb), and titanium (Ti) may be 0.10 wt% or less, and preferably, the sum of the contents of vanadium (V), niobium (Nb), and titanium (Ti) may be 0.08 wt% or more and 0.10 wt% or less.

[0093]

[0094] Nitrogen (N)

[0095] Even trace amounts of nitrogen (N) significantly affect the mechanical properties of steel, increasing tensile strength and yield strength while decreasing elongation. In particular, the reduction in impact strength and the increase in transition temperature are notable. Furthermore, it refines austenite grains, enabling the production of fine-grained steel. It also forms nitrides with titanium, zirconium, vanadium, and niobium, further refining grains. However, its presence in large amounts can lead to decreased high-temperature toughness, intergranular embrittlement due to AlN precipitation at austenite grain boundaries, and decreased high-temperature creep strength.

[0096] A high-performance steel bar according to one embodiment of the present invention may contain nitrogen (N) in an amount of 0.015 wt% or less.

[0097]

[0098] In addition, in a high-performance steel bar according to one embodiment of the present invention, at least one 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) may be further included.

[0099]

[0100] Sn

[0101] It can increase the tensile strength and yield strength of steel and reduce elongation and impact strength. It can also cause red-hot embrittlement, tempering embrittlement, and low-temperature embrittlement during hot working, and improve corrosion resistance.

[0102] To this end, the high-performance steel bar according to one embodiment of the present invention may contain tin (Sn) in an amount of 0.1 wt% or less.

[0103]

[0104] Magnesium (Mg)

[0105] Trace amounts of magnesium can separate and reduce grain boundary energy and phase boundary energy at grain boundaries, improve and refine the morphology of grain carbides and other grain boundary precipitates, such as blocking and spheroidizing carbides, effectively suppress grain boundary sliding, reduce grain boundary stress, and eliminate notch sensitivity. Furthermore, trace elements of magnesium can enhance plasticity and increase impact toughness and fatigue strength.

[0106] To this end, the high-performance steel bar according to one embodiment of the present invention may contain magnesium (Mg) in an amount of 0.1 wt% or less.

[0107]

[0108] Calcium (Ca)

[0109] Calcium (Ca) is a powerful deoxidizer and induces the formation of CaS in the MnS inclusions formed during steelmaking, thereby preventing deterioration of mechanical properties by causing CaS to float and separate, or by preventing it from being elongated like MnS during rolling even if it remains. Aluminum (Al) is also a powerful deoxidizer and is commonly added for denitrification and deoxidation. When AlN, a nitride, is not precipitated, it can effectively achieve grain refinement in steel.

[0110] To this end, the high-performance steel according to one embodiment of the present invention may contain calcium (Ca) in an amount of 0.05 wt% or less.

[0111]

[0112] Meanwhile, in a high-performance steel bar according to one embodiment of the present invention, the sum of the respective contents of vanadium (V), niobium (Nb), and titanium (Ti) may be 0.10 wt% or less, as described above.

[0113] This is to minimize the content of nickel (Ni), an expensive alloy, while utilizing the solute attraction effect of niobium (Nb) to delay recrystallization during high-temperature rolling and increase the recrystallization arrest temperature. Ultimately, by refining the grain size of the final ferrite, a low ductile-brittle transition temperature (DBTT) can be secured, thereby satisfying both stability and economy at ultra-low temperatures.

[0114] Therefore, in a high-performance steel according to one embodiment of the present invention, the recrystallization stop temperature at which recrystallization due to deformation does not occur is increased due to the solute attraction effect of niobium (Nb), and the recrystallization stop temperature can be 952°C or higher.

[0115] In addition, in the high-performance steel bar according to one embodiment of the present invention, when hot deformation is applied below the recrystallization arrest temperature, instead of continuous dynamic recrystallization of austenite, pancake-shaped particles are formed, thereby increasing the grain boundary fraction per unit area. This allows for the formation of fine ferrite by increasing the number of nucleation sites during phase transformation, and the ultra-fine-grained ferrite thus formed can maximize ultra-low temperature stability. At this time, the ferrite structure may be acicular ferrite, and the average grain size may be 13.2 ㎛ or less, and preferably 9.5 ㎛ or less.

[0116] However, the high-performance steel according to one embodiment of the present invention may have a final microstructure that includes bainite and pearlite in addition to needle-like ferrite.

[0117] Meanwhile, low-temperature toughness can be confirmed through the average ferrite grain size and the ductile-brittle transition temperature (DBTT). According to one embodiment of the present invention, the average ferrite grain size may be 13.2 ㎛ or less, and the ductile-brittle transition temperature (DBTT) may be -48 ℃ or less.

[0118] That is, it can be confirmed that the high-performance steel bar according to one embodiment of the present invention has excellent low-temperature toughness by securing a significantly low ductile-brittle transition temperature (DBTT) along with a fine ferrite structure.

[0119] In addition, the high-performance steel bar according to one embodiment of the present invention may have a notch sensitivity ratio (NSR), which is a ratio of the tensile strength (TS_notched) of a notched specimen to the yield strength (YS_unnotched) of an unnotched specimen at -170°C, of ​​1.0 or more.

[0120]

[0121] Manufacturing method of high-performance steel bar

[0122] Referring to FIG. 1, a method for manufacturing a high-performance steel bar according to one embodiment of the present invention includes (a) a reheating step, (b) a hot rolling step, and (c) a cooling step.

[0123] More specifically, (a) carbon (C) 0.03 to 0.12 wt%, silicon (Si) 0.50 wt% or less, manganese (Mn) 1.3 to 2.0 wt%, phosphorus (P) 0.02 wt% or less, sulfur (S) 0.02 wt% or less, chromium (Cr) 0.002 to 0.50 wt%, copper (Cu) 0.5 wt% or less, nickel (Ni) 0.35 to 0.65 wt%, molybdenum (Mo) 0.002 to 0.10 wt%, aluminum (Al) 0.005 to 0.04 wt%, vanadium (V): 0.005 to 0.045 wt%, niobium (Nb): 0.004 to 0.045 wt%, titanium (Ti): 0.001 to A method for producing a steel product, comprising: (a) a step of reheating a steel product containing 0.015 wt% or less of nitrogen (N), 0.015 wt% or less of iron (Fe), and the remainder of iron (Fe) and unavoidable impurities; (b) a step of hot-rolling the steel product by controlling the rolling start temperature to 950 to 1100°C and the rolling end temperature to 930 to 1050°C; and (c) a step of cooling the steel product.

[0124] Here, the steel may be an ingot or a billet manufactured through a continuous casting process prior to the reheating step. Therefore, the alloying elements and compositional ranges contained in the steel can be understood to be identical to those contained in high-performance steel bars manufactured through ingots and billets.

[0125] In addition, the reason for controlling the rolling end temperature, which is the hot deformation finishing temperature range of steel, to 930 to 1050 ℃ is to promote phase transformation through hot deformation below the recrystallization stop temperature and ultimately form a fine ferrite structure.

[0126] That is, the recrystallization stop temperature, where no recrystallization occurs due to deformation, is raised by the solute attraction effect of niobium (Nb), and when deformation is applied below the recrystallization stop temperature, instead of continuous dynamic recrystallization of austenite, pancake-shaped particles form, increasing the grain boundary fraction per unit area. Through this, the number of nucleation sites increases during the phase transformation of steel, which can refine ferrite, and the ultra-fine-grained ferrite thus formed can maximize ultra-low temperature stability.

[0127] Ultimately, by obtaining a fine grain size of ferrite, the ductile-brittle transition temperature (DBTT) can be significantly lowered.

[0128] And, in the above step (a), the recrystallization stop temperature (RST) of the steel may be 952 ℃ or higher.

[0129] Accordingly, in the above step (b), the rolling end temperature, which is the hot deformation finishing temperature range of the steel, can be controlled to be 952 ℃ or lower, which is the recrystallization stop temperature of the steel.

[0130] And, in the step (c), the steel can be cooled by accelerated controlled cooling (ACC), wherein the accelerated controlled cooling (ACC) is a cooling process that induces internal and external structural changes in the steel by spraying a tempcore and water as a cooling medium at high pressure / uniformly in the middle of the final cooling completion stage immediately after hot deformation of the steel.

[0131] In the method for manufacturing a high-performance steel bar according to one embodiment of the present invention, in step (c), the steel material can be cooled at a cooling rate of 50 to 400 ℃ / sec and can be cooled to a final cooling temperature (recuperation temperature) of 500 to 650 ℃.

[0132] Ultimately, the method for manufacturing high-performance steel bars according to one embodiment of the present invention can realize steel bars having a structure that satisfies high strength and low-temperature toughness even at ultra-low temperatures by hot-deforming steel and then controlling the reheating and cooling temperature through accelerated controlled cooling (ACC).

[0133] At this time, the steel bar that has undergone the step (c) above can have the average grain size of the ferrite structure be 13.2 ㎛ or less, and preferably 9.5 ㎛ or less, as the grain boundary fraction per unit area increases and the number of nucleation sites increases through phase transformation, thereby enabling refinement of the ferrite. In addition, the steel bar that has undergone the step (c) above can have a final microstructure that includes bainite, acicular ferrite, and pearlite, as illustrated in Fig. 2.

[0134] In addition, the steel material that has performed the above step (c) can secure a ductile-brittle transition temperature (DBTT) of -48 ℃ or lower by implementing a fine-grained ferrite structure.

[0135]

[0136] Comparative and experimental examples

[0137] Hereinafter, preferred comparative examples and experimental examples are presented to aid understanding of the present invention. However, the following experimental examples are provided solely to aid understanding of the present invention, and the present invention is not limited to the following experimental examples.

[0138] Tables 1 and 2 below show the main alloy compositions (unit: weight%) that make up the invention material of this experimental example and the comparative material of the comparative example, Table 3 below shows the operating conditions for manufacturing each specimen of this experimental example and the comparative example, and Tables 4 and 5 show the results of measuring the mechanical properties of specimens implemented according to the operating conditions for manufacturing each specimen of this experimental example and the comparative example.

[0139] 'YS' stands for yield strength, 'TS' stands for tensile strength, 'EL' stands for elongation, 'UE' stands for uniform elongation, and 'NSR' stands for notch sensitivity ratio, which is calculated as the ratio of (tensile strength of notched specimen) to (yield strength of unnotched specimen). 'un' stands for unnotched specimen, and 'n' stands for notched specimen. Room temperature corresponds to 25℃.

[0140]

[0141] Classification Chemical composition (weight%) CSiMnPSCuCrMoNi Comparative material 0.0700.151.630.010.010.150.200.020.57 Inventive material 10.0660.171.520.0080.010.160.150.020.55 Inventive material 20.0580.191.520.0090.0080.160.150.010.41

[0142] Classification Chemical composition (weight%) Ceq.RST (℃) VNbTiSnAlCaN Comparative material 0.0020.0030.0020.0070.0020.0020.0090.43842 Inventor material 10.0390.0400.0120.0080.0200.0020.0100.40987 Inventor material 20.0390.0350.0120.0100.0200.0020.0090.40952

[0143]

[0144] Looking at Tables 1 and 2 above, the comparative material contains 0.57 wt% of nickel (Ni), but the inventive materials 1 and 2 contain nickel (Ni) in amounts lowered to 0.55 wt% and 0.41 wt%, respectively.

[0145] In this way, Inventive Materials 1 and 2, despite containing significantly less expensive nickel (Ni) than the comparative material, achieved enhanced low-temperature toughness and superior weldability. Ultimately, Inventive Materials 1 and 2 demonstrated economic feasibility, enabling lower manufacturing costs compared to the comparative material, while also securing mechanical properties at both room and ultra-low temperatures.

[0146] And, it can be seen that the comparative material is composed of 0.002 wt% vanadium (V) + 0.003 wt% niobium (Nb) + 0.002 wt% titanium (Ti) ≤ 0.007 wt%, but the inventive material 1 is composed of 0.039 wt% vanadium (V) + 0.040 wt% niobium (Nb) + 0.012 wt% titanium (Ti) ≤ 0.091 wt%, and the inventive material 2 is composed of 0.039 wt% vanadium (V) + 0.035 wt% niobium (Nb) + 0.012 wt% titanium (Ti) ≤ 0.086 wt%.

[0147] In particular, the comparative material is composed of 0.003 wt% niobium (Nb), but the invention material 1 and invention material 2 are composed of 0.040 wt% and 0.035 wt% niobium (Nb), respectively.

[0148] This difference in the content of niobium (Nb) affects the recrystallization stop temperature (RST). The recrystallization stop temperature (RST) of the comparative material is only 842℃, but the recrystallization stop temperature (RST) of the invention material 1 and invention material 2 increases rapidly to 987℃ and 953℃.

[0149] This is because the recrystallization stop temperature, at which recrystallization due to hot deformation does not occur, increases due to the solute attraction effect of niobium (Nb).

[0150] Therefore, the high-performance steel bar according to one embodiment of the present invention may be superior in terms of technical / economic aspects in terms of the expression of properties at ultra-low temperatures, in that the contrast in properties that can be expressed in the range of the alloy components formed is similar to or slightly increased.

[0151]

[0152] Classification DiameterOperating ConditionsRolling Start Temperature (℃)Rolling End Temperature (℃)Cooling Speed ​​(℃ / sec)Reheat Temperature (℃)Comparative Material D19950~110096050~400520D35982573Inventive Material 1D19960525D35970569Inventive Material 2D19940523D35947571

[0153] Classification DiameterRoom temperature propertyGrain sizeLow temperature toughnessYS(Mpa)TS(Mpa)EL(%)FerriteAverage grain size(㎛)DBTT(℃)Comparative materialD1955363218.413.3-40D3554064517.4Inventive material1D1958265720.39.1-53D3557465319.7Inventive material2D1954461022.99.5-48D3556262917.2

[0154] Classification Diameter Low temperature properties (-170℃) YS-un (Mpa) UE-un (%) [3%↑] TS-n (Mpa) NSR [1.00↑] Comparative material D198099.28251.02 D357767.17921.02 Inventive material 1 D1981310.88461.04 D357859.78121.03 Inventive material 2 D1979710.48131.02 D357628.27781.02

[0155]

[0156] With reference to Tables 1 to 5 above, let us examine the comparative materials and the invention materials 1 and 2 in detail.

[0157] The room temperature yield strengths (YS) of the comparative materials were 553 MPa and 540 MPa, respectively, and the room temperature yield strengths (YS) of the inventive materials 1 and 2 were 582 MPa, 574 MPa, 544 MPa, and 562 MPa, respectively.

[0158] In addition, it can be seen that the room temperature tensile strength (TS) of the comparative material was 632 MPa and 645 MPa, respectively, and the room temperature tensile strength (TS) of the inventive materials 1 and 2 was 657 MPa, 653 MPa, 610 MPa, and 629 MPa, respectively.

[0159] In addition, it can be confirmed that the room temperature elongation (EL) of the comparative materials is only 18.4% and 17.4%, respectively, but the room temperature elongation (EL) of the inventive materials 1 and 2 increases to 20.3%, 19.7%, 22.9%, and 17.2%, respectively.

[0160] Meanwhile, the comparative material was composed of 0.003 wt% niobium (Nb), had a recrystallization stop temperature (RST) of 842°C, and the final finishing rolling was performed at 960°C and 982°C, which are higher than the recrystallization stop temperature (RST) of 842°C, as the finishing temperature for hot deformation.

[0161] On the other hand, invention materials 1 and 2 are composed of niobium (Nb) at 0.040 wt% and 0.035 wt%, respectively, and as a result, the recrystallization stop temperature (RST) increased to 987°C and 953°C, respectively.

[0162] In addition, the final finishing rolling of invention materials 1 and 2 was performed at 960℃ / 970℃ and 940℃ / 947℃, which are lower than the recrystallization stop temperature (RST) of 987℃ and 952℃, respectively, which is the finishing temperature of hot deformation.

[0163] At this time, the comparative material was subjected to final finishing rolling at a rolling end temperature higher than the recrystallization stop temperature (RST), thereby forming a coarse ferrite structure with an average grain size of 13.3 ㎛.

[0164] However, the invention materials 1 and 2 were formed with a very fine average grain size of 9.1㎛ / 9.5㎛ of ferrite structure by performing the final finishing rolling at a rolling end temperature lower than the recrystallization stop temperature (RST).

[0165] This fact can be confirmed in Figure 1. Looking at Figure 1, it can be easily confirmed with the naked eye that the final structure of the comparative material has a coarse particle size, whereas the final structure of the inventive material has a very fine particle size.

[0166] This difference in average grain size of the ferrite structure affects the low-temperature toughness of the steel, and the comparative material was found to have a ductile-brittle transition temperature (DBTT) of only -40℃.

[0167] However, the invention materials 1 and 2 were found to have a ductile-brittle transition temperature (DBTT) of -53℃ and -48℃ by forming a fine-grained ferrite structure.

[0168] In this way, 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 can be realized.

[0169] At a low temperature of -170℃, the yield strengths (YS_un) of the comparative materials were 809 MPa and 776 MPa, respectively, and the yield strengths (YS_un) of the invention materials 1 and 2 were 813 MPa, 785 MPa, 797 MPa, and 762 MPa, respectively.

[0170] And, at a low temperature of -170℃, the tensile strength (TS_n) of the comparative material was 825MPa and 792MPa, respectively, and the tensile strength (TS_n) of the invention materials 1 and 2 was 846MPa, 812MPa, and 813MPa / 778MPa, respectively.

[0171] Un-notch - The uniform elongation (UE_un) of the specimens was only 9.2% and 7.1% for the comparative materials, respectively, but it was found to have increased to 10.8% and 9.7% and 10.4% and 8.2% for the inventive materials 1 and 2.

[0172] And, when looking at the notch sensitivity ratio (NSR), which is the ratio of tensile strength (TS_n) / yield strength (YS_un) at -170℃, the comparative material was 1.02 and 1.02, while the invention materials 1 and 2 were 1.04, 1.03, 1.02 and 1.02.

[0173] As described above, preferred embodiments of the present invention have been described. It will be apparent to those skilled in the art that the present invention can be embodied in other specific forms, in addition to the embodiments described above, without departing from the spirit or scope thereof. Therefore, the above-described embodiments should be considered illustrative rather than restrictive, and accordingly, the present invention is not limited to the above description, but may be modified within the scope of the appended claims and their equivalents.

Claims

1. Carbon (C) 0.03 to 0.12 wt%, silicon (Si) 0.50 wt% or less, manganese (Mn) 1.3 to 2.0 wt%, phosphorus (P) 0.02 wt% or less, sulfur (S) 0.02 wt% or less, chromium (Cr) 0.002 to 0.50 wt%, copper (Cu) 0.5 wt% or less, nickel (Ni) 0.35 to 0.65 wt%, molybdenum (Mo) 0.002 to 0.10 wt%, aluminum (Al) 0.005 to 0.04 wt%, vanadium (V): 0.005 to 0.045 wt%, niobium (Nb): 0.004 to 0.045 wt%, titanium (Ti): 0.001 to Contains 0.015 wt% of iron (Fe), 0.015 wt% or less of nitrogen (N), and the remainder of iron (Fe) and unavoidable impurities. The yield strength at room temperature (YS) is 500 MPa or more, High performance steel bar.

2. In paragraph 1, Containing at least one of tin (Sn) 0.1 wt% or less, magnesium (Mg) 0.1 wt% or less, and calcium (Ca) 0.05 wt% or less, High performance steel bar.

3. In paragraph 1, The sum of vanadium (V), niobium (Nb), and titanium (Ti) contents is 0.1 wt% or less, High performance steel bar.

4. In paragraph 1, The microstructure includes bainite, needle ferrite and pearlite. High performance steel bar.

5. In paragraph 4, The average grain size of the above-mentioned bed ferrite structure is 13.2 ㎛ or less, High performance steel bar.

6. In paragraph 1, The ductile-brittle transition temperature (DBTT) is below -48 ℃, High performance steel bar. 7.(a) Carbon (C) 0.03 to 0.12 wt%, silicon (Si) 0.50 wt% or less, manganese (Mn) 1.3 to 2.0 wt%, phosphorus (P) 0.02 wt% or less, sulfur (S) 0.02 wt% or less, chromium (Cr) 0.002 to 0.50 wt%, copper (Cu) 0.5 wt% or less, nickel (Ni) 0.35 to 0.65 wt%, molybdenum (Mo) 0.002 to 0.10 wt%, aluminum (Al) 0.005 to 0.04 wt%, vanadium (V): 0.005 to 0.045 wt%, niobium (Nb): 0.004 to 0.045 wt%, titanium (Ti): 0.001 to A step of reheating steel containing 0.015 wt% or less of iron (Fe), 0.015 wt% or less of nitrogen (N), and the remainder of iron (Fe) and unavoidable impurities; (b) a step of hot rolling the steel by controlling the rolling start temperature to 950 to 1100 ℃ and the rolling end temperature to 930 to 1050 ℃; and (c) a step of cooling the above steel material; including, Method for manufacturing high-performance steel bars.

8. In paragraph 7, The above steel is, Containing at least one of tin (Sn) 0.1 wt% or less, magnesium (Mg) 0.1 wt% or less, and calcium (Ca) 0.05 wt% or less, Method for manufacturing high-performance steel bars.

9. In paragraph 7, The above steel is, The sum of vanadium (V), niobium (Nb), and titanium (Ti) contents is 0.1 wt% or less, Method for manufacturing high-performance steel bars.

10. In paragraph 7, In step (a) above, The recrystallization stop temperature of the above steel is 952 ℃ or higher, Method for manufacturing high-performance steel bars.

11. In paragraph 10, Step (b) above, The rolling end temperature is controlled below the recrystallization stop temperature of the steel. Method for manufacturing high-performance steel bars.

12. In paragraph 7, Step (c) above, It is performed by Accelerate Control Cooling (ACC). Cooled at a cooling rate of 50 to 400 ℃ / sec and cooled to a final cooling temperature of 500 to 650 ℃. Method for manufacturing high-performance steel bars.

13. In paragraph 7, The steel material that has undergone the above step (c) is The average grain size of the ferrite structure is 13.2 ㎛ or less, Method for manufacturing high-performance steel bars.

14. In paragraph 7, The steel material that has undergone the above step (c) is The ductile-brittle transition temperature (DBTT) is below -48 ℃, Method for manufacturing high-performance steel bars.

15. In paragraph 7, The steel material that has undergone the above step (c) is The microstructure includes bainite, needle ferrite and pearlite. Method for manufacturing high-performance steel bars.

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